Title of Invention	"PROCESS FOR THE PREPARATION OF A MOLECULAR SIEVE ADSORBENT FOR THE SIZE/SHAPE SELECTIVE SEPARATION OF AIR"
Abstract	A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air The present invention relates to a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air. The invention relates to the use of pore engineered zeolites as size / shape selective adsorbents in the separation of gases having closely related physical properties. In the present invention inventive features to obtain the molecular sieve adsorbent is by the control of the pore mouth of the zeolite (i) by the deposition of silica by chemically reacting an alkoxide with silanol groups present on the external surface of the zeolite followed by calcination at 450-650°C, (ii) by liquid phase chemical reaction of tetra alkyl orthosilicate in a moisture free solvent to ensure uniform deposition of silica on the surface of the zeolite at ambient conditions, (iii) enhancement of thermal and hydrothermal stability of the adsorbent by silica deposition on the external surface of the zeolite, (iv) to prepare a zeolite based oxygen selective adsorbent based on shape /size selectivity by a method other than conventionally used cation exchange

Title of Invention

"PROCESS FOR THE PREPARATION OF A MOLECULAR SIEVE ADSORBENT FOR THE SIZE/SHAPE SELECTIVE SEPARATION OF AIR"

Abstract

A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air The present invention relates to a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air. The invention relates to the use of pore engineered zeolites as size / shape selective adsorbents in the separation of gases having closely related physical properties. In the present invention inventive features to obtain the molecular sieve adsorbent is by the control of the pore mouth of the zeolite (i) by the deposition of silica by chemically reacting an alkoxide with silanol groups present on the external surface of the zeolite followed by calcination at 450-650°C, (ii) by liquid phase chemical reaction of tetra alkyl orthosilicate in a moisture free solvent to ensure uniform deposition of silica on the surface of the zeolite at ambient conditions, (iii) enhancement of thermal and hydrothermal stability of the adsorbent by silica deposition on the external surface of the zeolite, (iv) to prepare a zeolite based oxygen selective adsorbent based on shape /size selectivity by a method other than conventionally used cation exchange

Full Text	Process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air The present invention relates to a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air. The invention relates to the use of pore engineered zeolites as size / shape selective adsorbents in the separation of gases having closely related physical properties. More specifically, the invention relates to the preparation and use of a molecular sieve adsorbent, which is selective towards oxygen from its gaseous mixture with nitrogen and / or argon. FIELD OF THE INVENTION The use of adsorption techniques to separate a gaseous component from a gaseous stream was initially developed for the removal of carbon dioxide and water from air. Gas adsorption techniques are now conventionally employed in processes for the enrichment of hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen and nitrogen. Adsorbents most often effect separations by adsorbing one (or more) component more strongly than another. The various interaction forces engaged in adsorption process are van der Waals interactions, acid-base interactions, hydrogen bond, electrostatic, chelation, clathration and covalent bond. Two important separation mechanisms are exclusion of certain molecules in the feed because they are too large to fit into the pores of the adsorbent (molecular sieving effect, size / shape selective separation) and differences in the diffusion rates of the adsorbing species in the pores of the adsorbent. The four types of adsorbents which are dominantly used are activated carbon, zeolite molecular sieves, silica gel and activated alumina. Carbon molecular sieves (CMS), which exhibit very narrow pore size distribution, facilitate separation based on different inter particle diffusion rates. The efficient separation of air to recover nitrogen has provided a secure and somewhat growing market for carbon molecular sieves. Adsorption processes for the separation of oxygen and nitrogen from air have been increasingly used tor commercial purposes for the last three decades. Oxygen requirements in sewage treatment, fermentation, cutting and welding, fish breeding, electric iurnaces, pulp bleaching, glass blowing, medical purposes and in the steel industries, particularly when the required oxygen purity is between 90 to 95%, are being largely met by adsorption based pressure swing or vacuum swing processes. It is estimated that at present, around 20% of the world's oxygen demand is met by the adsorptive separation of air. However, the maximum attainable purity by adsorption processes is around 95% with separation of 0.934mole percent argon present in the air being a limiting factor to achieve 100% oxygen purity. Furthermore, the adsorption-based production of oxygen from air is economically not competitive with the cryogenic fractionation of air for production levels of more than 200 tonne per day. The adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for the desired components the better is the adsorbent as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of a particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process. The adsorption selectivity of a component over others is calculated as the ratio of the volumes of gas adsorbed at any given pressure and temperature. The adsorption selectivity of a component results from steric factors such as the differences in the size and shape of the adsorbate molecules; equilibrium effect, i.e. when the adsorption isotherms of components of a gas mixture differ appreciably; kinetic effect, when the components have substantially different adsorption rates. BACKGROUND OF THE INVENTION The principal characteristic of the separation, removal or concentration of oxygen from the air is that usually there is no cost for the starting material, which is air. The cost of the oxygen produced or removed, depends essentially upon the following factors. (a) Costs of equipment necessary for separating or concentrating oxygen, (b) Costs of energy necessary for operating the equipment. (c) When purified oxygen is needed, the cost of the purification step has to be taken into account. Another characteristic is that separation or concentration of oxygen can be achieved either by separating oxygen or by separating nitrogen from air as a starting material. Taking into consideration the above-described factors, various economically advantageous processes have heretofore been proposed. These include, for example, the process in which air is liquefied at low temperatures to separate oxygen or nitrogen, making use of the difference in the boiling point between liquid oxygen (-182.9°C) and liquid nitrogen (-195.8°C). The apparatus employed is suited for producing large amounts of oxygen and the production of most of the oxygen and nitrogen in the world is based on this procedure. One disadvantage of the process is that it requires large amounts of power. Another is that large-scale equipment is necessarily site specific and portability is low. Another is that it takes hours to switch on and switch off the plant. In another approach the membrane separation system is employed for the separation of oxygen and nitrogen from air [US patent 5a 091, 216 (1992) to Hayes et.al. US patent 5,004,482 (1991) to Haas et.al, US patent Application 20020038602 (2002), to Katz; ct al.]. The main drawback, of this method is the thin polymeric films used in the separation process are too weak to withstand the high differential gas pressures required for the separation and the purity of the product gas is only around 50%. In the prior art, adsorbents which are selective for nitrogen from a mixture with oxygen and argon have been reported [US patent 4,481,018 (1984) to Coe et.al, US patent 4,557,736 (1985) to Sircar et.al, US patent 4,859,217 (1989) to Chao; Chien-Chung, US patent 4,943,304 (1990) to Coe etal, US patent 4,964,889 (1990) to Chao; Chien-Chung, US patent 5,114,440 (1992)to Reiss; Gerhard, US patent 5,152,813 (1992) to Coe etal, US patent 5,174,979 (1992) to Chao; Chien-Chung ct.al, US patent 5,454,857 (1995) to Chao; Chien-Chung, US patent 5,464,467 (1995) to Fitch et.al., US patent 5,698,013 (1997)to Chao; Chien-Chung., US patent 5,868,818 (1999) to Ogawa et.aU US patent 6,030,916 (2000) to Choudary et.al., US patent 6,231,644 (2001)] to Jain et. al.] wherein the zeolites of type A, faujasite, mordenite, clinoptilites, chabazite and monolith have been used. Efforts to enhance the adsorption capacity and selectivity have been reported by exchanging the extra framework cations with alkali and / or alkaline earth metal cations and increasing the number of extra framework cations in the zeolite structure by modifying the chemical composition. The adsorption selectivity for nitrogen has also been substantially enhanced by exchanging the zeolite with cations like lithium and / or calcium in some zeolite types. They have been employed in processes for the separation or concentration of oxygen by removing nitrogen selectively from the air. However, the molecular sieves of these types have an isotherm, which follows the Langmuir adsorption isotherm. As a result, when the pressure reaches 1.5 atmospheres absolute (ata) the increase in the adsorptivity is not large compared with the increase in the pressure. Moreover, a very large amount of nitrogen must be separated since the molar ratio of N2/Q* in the air is 4. Therefore, the advantage achieved by enlargement of the apparatus to permit the use of high pressure is rather small. This limits the application of this process to small volume installations. The maximum attainable oxygen purity by adsorption processes is around 95%, with separation of 0.934-molc percent argon present in the air being a limiting factor to achieve 100% oxygen purity. These adsorbents are highly moisture sensitive and the adsorption capacity and selectivity will decay in the presence of moisture. The chromatpgraphic separation of oxygen and argon is also possible by using these adsorbents. US patent 4,4S3,952 (1984) to Izmi et.al. discloses the manufacture of an oxygen selective adsorbent by substituting the Na cations of zeolite A with K and Fe (II). The adsorbent shows oxygen selectivity only at low temperature and its preparation requires multi-stage cation exchange, adding to the cost of preparation. Cation exchange is carried out at around 8Q°C using aqueous salt solutions of the metal ions to be exchanged. This results in a higher energy requirement as well as the generation of effluents during the exchange process. Furthermore, potassium exchange in zeolite leads to lower thermal and hydrothermal stability of the adsorbent. Carbon molecular sieves arc effective for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen. The difference in rates of adsorption is due to the difference in size of the oxygen and nitrogen molecules. Since the difference in size is quite small, approximately 0.2 A°, the pore structure of the carbon mobcular sieve must be tightly controlled in order to effectively separate the two molecules. In order to improve the performance of carbon molecular sieves, various techniques have been used to modify pore size. The most common method is the deposit of carbon on carbon molecular sieves. For example, US Patent 3,979,330 to Munzner etal discloses the preparation of carbon containing molecular sieves in which coke containing up to 5% volatile components is treated at 600°C-900°C in order to split off carbon from a hydrocarbon. The split-off carbon is deposited in the carbon framework of the coke to narrow the existing pores. US Patents 4,528,281; 4,540,678; 4,627,857 and 4,629,476 to Jr. Robert, S.F. disclose various preparations of carbon molecular sieves for use in the separation of gases. US Patent 4,742,040 to Ohsaki etal. discloses a process tor making a carbon molecular sieve having increased adsorption capacity and selectivity by pelletising powder coconut shell charcoal containing small amounts of coal tar as a binder, carbonising, washing in mineral acid solution to remove soluble ingredients, adding specified amounts of creosote or other aromatic compounds, heating at 950°C -1000°C, and then cooling in an inert gas. US Patent 4,880,765 to Knoblauch ct.al., discloses a process for producing carbon molecular sieves with uniform quality and good separating properties by treating a carbonaceous product with inert gas and steam in a vibrating oven and further treating it with benzene at high temperatures to thereby narrow existing pores. Preparation of a carbon molecular sieve is a multi-step process requiring the utmost care at each stage to get a totally reproducible carbon molecular sieve. Additionally, the process is a very high temperature process, which results in a higher cost of the adsorbent. US Patent 5,081,097 to Sharma et.al., discloses copper modified carbon molecular sieves for the selective removal of oxygen. The sieve is prepared by pyrolysis of a mixture of a copper-containing material and a polyfunctional alcohol to form a sorbent precursor. The sorbent precursor is then heated and reduced to produce a copper modified Carbon molecular sieve. Pyrolysis is a high temperature process making the whole process of preparation of the adsorbent an energy intensive process. Another process uses a transient metal-based organic complex capable of selectively absorbing oxygen [US patent 4,477,418 (1984) to Mullhaupt Joseph ct.al.; US patent 5,126,466(1992) to Ramprasad et.al.; US patent 5,141,725(1992) toRamprasad ct.al.; US patent 5,294,418(1994) to Ramprasad et.al.; US patent 5,945,079 (1999) to Mullhaupt Joseph et.al; US patent Application 20010003950 (2001), to Zhang, Delang et al.]. The absorption by these complexes is reversible with changes in temperature and pressure so that it is theoretically possible to achieve separation or concentration of oxygen by means of a temperature swing or a pressure swing cycle of the air. However, in practice, severe deterioration of the organic complex occurs with repeated cycles of absorption and liberation of oxygen. Moreover, the organic complex itself is expensive. Therefore, the use of this process is limited to special situations. The main drawback of this process lies in the air and moisture sensitivity of the metal complexes used, which lowers the stability of the adsorbent produced. Additionally, the cost of the metal complexes used in the preparation of the adsorbent is very high. US patent 6,087,289 (2000) to Choudary et al. discloses a process for the preparation of a zeolte-based adsorbent containing cerium cations for the selective adsorption of oxygen from a gas mixture. Cerium exchange into the zeolite is carried out under reflux conditions using an aqueous solution of cerium salt at around 80°C for 4-8 hours and repeating the exchange process several times. The main drawbacks of this adsorbent lie in the observation of oxygen selectivity only in the low-pressure region. Furthermore, the adsorbent preparation is a multi-step ion exchange process, which also generates liquid effluent. European Patent 0,218,403 to Greenbank discloses a dense gas pack of coarse and fine adsorbent particles wherein the size of the largest fine particles is less than one-third of the coarse particles and sixty percent of all particles are larger than sixty mesh. Although not specifically stated, it is evident from the examples that these percentages are by volume. This system is designed primarily for enhancing gas volume to be stored in a storage cylinder. It is mentioned, however, that it can be utilized for molecular sieves. There is nothing in this application, however, which would give an insight into the fact that significantly enhanced PSA efficiency could be obtained by combining coarse and fine particles of a kinetically-selective sieve material in a single bed. It has been found in accordance with the present invention that, within certain limits as will be defined herein, a mixture of coarse and fine kinctically selective sieve particles will unexpectedly give enhanced PSA performance. In another approach, the chemical vapour deposition technique was used for controlling the pore opening size of the zeolites by the deposition of silicon alkoxide [M. Niwa ct al., JCS Varaday Trans. I, 1984, 80, 3135-3145; M. Niwa et al., M. Niwa ct al., J. Phys. Chem., 1986, 90, 6233-6237; Chemistry Letters, 1989, 441-442; M. Niwa et al., Ind. Eng. Chem. Res., 1991, 30, 38-42; D. Ohayon et al., Applied Catalysis A- General, 2001, 217, 241-251]. Chemical vapour deposition is carried out by taking a requisite quantity of zeolite in a glass reactor, which is thermally activated at 450°C in situ under inert gas e.g. nitrogen flow. The vapours of silicon alkoxide are continuously injected into the inert gas stream, which carries the vapours to the zeolite surface where the alkoxide chemically reacts with the silanol groups of the zeolite. Once the desired quantity of alkoxide is deposited on the zeolite, the sample is heated to 550 °C in air for 4-6 hours after which it is brought down to ambient temperature and used for adsorption. The major disadvantages of this technique arc (i) Chemical vapour deposition, which leads to a non-uniform coating of alkoxide which in turn results in non-uniform pore mouth closure, (u) The process has to be carried out at elevated temperature where the alkoxide gets vaporised, (Hi) The deposition of the alkoxide requires to be done at a slow rate for better diffusion and (iv)This method is expensive and the lack of a commercial level at higher scale will be difficult. At present nitrogen and argon containing less than lOppm Oxygen are produced by using a deoxo hybrid system in which the oxygen is removed by reducing it to water over a catalyst with hydrogen. OBJECT OF THE INVENTION The main object of the present invention is to provide a process for the preparation of a molecular sieve adsorbent for the size/ shape selective separation of air, which obviates the drawbacks as detailed above. Still another object of the present invention is to provide an oxygen selective zeolite based adsorbent from its gaseous mixture with nitrogen and argon. Still another object of the present invention is to provide an adsorbent, which can be prepared by the external surface modification of the zeolite A. Yet another object of the present invention is to provide an oxygen selective adsorbent by a simple liquid phase surface modification of zeolite A. Yet another object of the present invention is to have a uniform deposition of alkoxide on the surface of zeolite A. Yet another object of the present invention is to provide an adsorbent with high thermal and hydrothermal stability. Yet another object of the present invention is to provide an adsorbent, which is selective to oxygen over nitrogen and argon and can be used commercially for the separation and purification of nitrogen and argon. In the drawings accompanying this specification, Figure 1 represents the adsorption isotherms of nitrogen, argon and oxygen at 15°C on the adsorbent obtained from example-1. Figure I represents the adsorption isotherms of nitrogen, argon and oxygen at 15°C on the adsorbent obtained from example-6. SUMMARY OF THE INVENTION Accordingly, the present invention provides a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air, which comprises a molecular sieve adsorbent represented by the general formula, (Na2O)6.(Al2O3)r,.(SiO2)i2+x.wH2O where the values of x varies from 0.001 to 0.1, w being the number of moles of water, which comprises (1) activating the commercially available zeolite A in the temperature range of 350 to 450 ° Celsius to eliminate physically adsorbed water, for a period ranging from 3 to 6 hours; (2) cooling the activated zeolite in a desiccator under vacuum in the range of IX 10'2to IXlO^mm Hg; (3) treating the cooled zeolite with tetra allcyl ortho silicate dissolved in a dry solvent in the concentration range of 0.1'to-1.0 wt%/volume for a specified period in the range of 4 to 8 hours under continuous stirring; (4) recovering the solvent by conventional techniques for re-use; (5) drying the treated zeolite in air in a static condition at ambient temperature in the range of 20 to 35° Celsius; (6) heating the modified zeolite in the temperature range of 450 to 600°Celsius for a period ranging from 3 to 8 hours; (7) cooling the zeolite at ambient temperature in a static condition; (8) activating the zeolite samples at the temperature range of 350 to 450°Celsius prior to measuring the adsorption of oxygen, nitrogen and argon by a static volumetric system. In an embodiment of the present invention commercially available zeolite A may used for the preparation of the molecular sieve adsorbent. In another embodiment of the present invention the zeolitc-A was activated at 350 to 550° C for 3-6 hours followed by cooling under inert or vacuum conditions. In another embodiment of the present invention the tctra alkyl ortho silicate was dissolved in dry solvent, which may be selected from e.g. toluene, benzene, xylene and cyclohexane. In another embodiment of the present invention 0.10 to 1.00 weight percentage of tetra alkyl ortho silicate may be deposited onto the zeolite in a single step by treating the activated zeolite with a solution Of tetra alkyl ortho silicate in a dry solvent for 4 to 8 hours. In still another embodiment of the present invention ■• said tetra alkyl ortho silicate may be deposited in the range discussed above at tetra alkyl ortho silicate concentration of 0.10 to 1.00% by weight of the zeolite. In still another embodiment of Lhe present invention the alkoxide deposition may be carried out in liquid phase for a period ranging from 4 to 8 hours under continuous stirring at ambient temperature. In still another embodiment of the present invention the alkoxide deposition may be uniform on the surface of the zeolite. In still another embodiment of the present invention the solvent was recovered by the distillation method, preferably under vacuum distillation and can be re-used. In still another embodiment of the present invention the adsorbents are dried in air or under vacuum conditions. In Still another embodiment of the present invention the adsorbent is calcined in the temperature range 500 to 600°C preferably at 550°C. DESCRIPTION OF THE INVENTION The present invention relates to a novel process to control the pore size of zeolite A, which has oxygen adsorption selectivity over nitrogen and argon. Furthermore this adsorbent displays high thermal and hydrothermal stability. Zeolites, which are microporous crystalline alumino-silicates, are finding increased applications as adsorbents for separating mixtures of closely related compounds. Zeolites have a three dimensional network of basic structural units consisting of SiO4 and A1O4 tetrahedrons linked to each other by sharing apical oxygen atoms. Silicon and aluminium atoms lie in the centre of the tetrahedron. The resulting alumino silicate structure, which is generally highly porous, possesses three-dimensional pores accessed through molecular sized windows. In a hydrated form, the preferred zeolites are generally represented by the following formula, M2/nCXAl203.xSiC)2.wH2O where M is a cation, which balances the electrovalence of the tetrahedron and is generally referred to as extra framework exchangeable cation, n represents the valency of the cation and x and w represent the moles of SiO2 and water respectively. The attributes which make the zeolites attractive for separation include, an unusually high thermal and hydrothermai stability, uniform pore structure, easy pore aperture modification and substantial adsorption capacity even at low adsorbate pressures. Furthermore, zeolites can be produced synthetically under relatively moderate hydrothermal conditions. Structural analysis of the samples was done by X-ray diffraction, wherein the crystallinity of the zeolite is measured from the intensity of the well-defined peaks. The in situ X-ray powder diffraction measurements at 30°C, I00°C, 200°C, 300°C, 400°C, SQO'C, 600°C, 650°C, 7O0C, 750°C, 800°C and 85O°C show that the newly developed adsorbent has high thermal stability. X-ray powder diffraction was measured using a PHILIPS X'pert MPD system equipped with a XRK. 900 reaction chamber. The zeolite NaA powder [Nan (AIO2) u. (S1O2) 12.WH2O] was used as the starting material. X-ray diffraction data showed that the starting material was highly crystalline. A known amount of the zeolite NaA powder [Nan (A1O;>) i2. (SiC>2) i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and mixed thoroughly with a solution having a known amount of tetra alkyl orthosilicate in 100ml dry solvent, the sample was dried by evaporating solvent under reduced pressure and the tetra alkyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C Oxygen, nitrogen and argon adsorption at 15°C was measured using a static volumetric system (Micromeritics ASAP 2010), after activating the sample at 350°C to 450°C under vacuum for 4 hours as described in the Examples. Addition of the adsorbate gas was made at volumes required to achieve a targeted set of pressures ranging from 0.5 to 850 mmHg. A minimum equilibrium interval of 5 seconds with a relative target tolerance of 5.0% of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine the equilibrium for each measurement point. The selectivity of pure components of two gases A and B is given by the equation, O:A/B=[VA/VB]P>T where VA and VB are the volumes of gas A and B adsorbed at any given pressure P and temperature T. The important inventive steps involved in the present invention are that the molecular sieve adsorbent obtained by the control of the pore mouth of the zeolite (i) by the deposition of silica by chemically reacting an alkoxide with silanol groups present on the external surface of the zeolite followed by calcination at 500 - 600 °C, (ii) by liquid phase chemical reaction of tetra alkyl orthosilicate in a moisture free solvent to ensure uniform deposition of silica on the surface of the zeolite at ambient conditions, (iii) enhancement of thermal and hydrothermai stability of the adsorbent by silica deposition on the external surface of the zeolite, (iv) to prepare a zeolite based oxygen selective adsorbent based on shape /size selectivity by a method other than conventionally used cation exchange The following examples are given by way of illustration and therefore should not be constructed to limit the scope of the present invention. EXAMPLE-1 A known amount of zeolite NaA, [(Na2O) «.(Al2O3)6.(SiO2)i2.wI-feO], was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. The adsorption capacity for oxygen is 3.48cc/g at 15°C and 765mmHg and selectivity for nitrogen over oxygen is around 3 to 5 in the pressure range studied, the values are given in table 1. EXAMPLE-2 lO.Og of the zeolite NaA powder [Na,2 (AlO2)i2.(SiO2)i2-wH2O] was activated at 40CPC to remove the adsorbed water in the zeolite and stirred with 0.1 Og tetra methyl orthosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating solvent under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothennal stability. The adsorption capacity for oxygen is 3.50cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 1.5 to 0.95 and selectivity for oxygen over argon is 1.3 to 2.1 in the pressure range studied, the values are given in table-1. EXAMPLE-3 lO.Og of the zeolite NaA powder [Nai2 (AiO2)i2.(SiO2)]2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with 0.1 Og tetra ethyl orthosilicate in 100ml dry solvent. The sample was dried after 5 hrs by evaporating the solvent under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination; of the zeolite at 5506C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrotherrnal stability. The adsorption capacity for oxygen is 3.53 cc/g at 15°C and 765mm Hg, selectivity for oxygen over nitrogen is 1.6 to 1.1 and selectivity for oxygen over argon isl .3 to 2.2 in the pressure range studied, the values are given in table-1. EXAMPLE-4 lO.Og of the zeolite NaA powder \|Nai2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.15g tetra ethyl orthosilicate inlOQml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.15cc/g at 15°C and 765mmHg, selectivity tor oxygen over nitrogen is 1.8 to 0.97 and selectivity for oxygen over argon is 2.8 to 3.2 in the pressure range studied, the values are given in table 1. EXAMPLES lO.Og of the zeolite NaA powder [Na^ (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with 0.20g tetra ethyl orlhosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550uC. A known amount of the sample was activated at 350°C uoder vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.78cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2 to 1.1 and selectivity for oxygen over argon is 3.0 to 3.4 in the pressure range studied, the values are given in table-.1. EXAMPLE-6 lO.Og of the zeolite NaA powder \|Na]2 (A102)i2.(SiO2)i2.wn20] was activated at 40CPC to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicatc in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tctra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 500°G.; A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.42 cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is around 2.1 to 1.5 and selectivity for oxygen over argon is 3.5 to 3.8 in the pressure range studied, the values are given in tablel. EXAMPLE-7 lO.Og of the zeolite NaA powder [Nai2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(PC to remove the water adsorbed in the zeolite and stirred with 0.30g tetra ethyl orthosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination; of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.63cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.5 to 1.6 and selectivity for oxygen over argon is 4.6 to 4.7 in the pressure range studied, the values are given in tablc-1. EXAMPLE-8 lO.Og of the zeolite NaA powder [Na]2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with l.OOg tetra ethyl orthosiUcatc in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl orllio silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show . that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 1.32 cc/g at 159C and 765mmHg, selectivity for oxygen over nitrogen is 2.5 to 1.4 and selectivity for oxygen over argon is 2.5 to 3.5 in the pressure range studied, the values are given in table-1. EXAMPLE-9 lO.Og of the zeolite NaA powder [Nai2 (AlO2)i2-(SiO2)i2-wH2O] was activated at 40CPC to remove the water adsorbed in the zeolite and stirred with 0.20g tetra methyl orthosilicate in lOOml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 85CfC show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.77cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.6 to 1.4 and selectivity for oxygen over argon is 3.3 to 4.1 in the pressure range studied, the values are given in table-1. EXAMPLE-10 lO.Og of the zeolite NaA powder [Nai2(AlO2)i2-(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicate in 100ml dry benzene. The sample was dried after 5 hrs by evaporating benzene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 500°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 85(fC show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.85cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.4 to 1.3 and selectivity for oxygen over argon is 4.0 to 4.3 in the pressure range studied, the values are given in table-1. EXAMPLE-11 lO.Og of the zeolite NaA powder [Na^AlC^n.CSiQO^.wl-kO] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.20g tetra ethyl orthosilicate in 100ml dry benzene. The sample was dried after 5 hrs by evaporating benzene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 35O°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity tor oxygen is 3.79cc/g at 15°C and 765mmV\g, selectivity for oxygen over nitrogen is 2.4 to 1.2 and selectivity for oxygen over argon is 3.6 to 4.0 in the pressure range studied, the values are given in table-1. EXAMPLE-12 lO.Og of the zeolite NaA powder [Nai2(AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicate in 100ml dry cyclohcxane. The sample was dried after 5 hrs by evaporating cyclohexane under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination! of the zeolite at 600°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrolhermal stability. The adsorption capacity for oxygen is 2.92cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen 2.6 to 1.5 and selectivity for oxygen over argon is 4.5 to 4.8 in the pressure range studied, the values arc given in table-1. EXAMPLE-13 lO.Og of the zeolite NaA powder [Nat2(AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicate in 100ml dry cyclohexane. The sample was dried after 5 hrs by evaporating: cyclohcxane under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at SSO't. A known amount of the sample was activated at 35O°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption cqpacity for oxygen is 2.87cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.8 to 1.5 and selectivity for oxygen over argon is 4.7 to 4.9 in the pressure range studied, the values are given in table-1. EXAMPLE-14 lO.Og of the zeolite NaA powder tNai2(AlO2)i2.(SiO2)i2-wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicate in 100ml dry xylene. The sample was dried after 5 hrs by evaporating xylene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination: of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.93cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.4 to 1.4and selectivity for oxygen over argon is 4.3 to 4.5 in the pressure range studied, the values are given in table-1. EXAMPLES 15 lO.Og of the zeolite NaA powder [Nai2(AlO2)i2.(SiO2)i2.wH2O] was activated at 40()°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicatc in 100ml dry xylene. The sample was dried after 5 hrs by evaporating xylene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was convened into silica by calcination of the zeolite at 650°C. A known amount of the sample was .activated at 350°C under vacuum and adsorption measurements were carried out as described earlier.In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.77cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.3 to 1.3 and selectivity for oxygen over argon is 4.7 to 4.8 in the pressure range studied, the values are given in table-1. The adsorption capacity and selectivity of all the 15 samples are enumerated in the Table-1. The main advantages of the present invention include: 1. The adsorbent, prepared by the modification of zeolite A shows oxygen selectivity over nitrogen and argon. 2. The adsorbent is prepared by a simple liquid phase alkoxide deposition. 3. The alkoxide deposition is uniform on the zeolite surface. 4. The alkoxide deposition is carried out at ambient temperature and pressure. 5. The solvent used for the alkoxide deposition can be recovered by distillation methods. 6. The adsorbent shows very high thermal and hydrothermal stability. 7. The adsorbent shows oxygen / argon selectivity of nearly 4.9 in the pressure range studied. 8. The adsorbent is useful in the commercial separation and purification of nitrogen and argon from a mixture with oxygen. 9. The adsorbent is useful for the chromatographic separation of oxygen nitrogen and argon. TABLE-1 Oxygen, Nitrogen and Argon Adsorption Capacities and Selectivity for Various Adsorbents atl5°C&765mmHg We Claim: 1. A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air, the said molecular sieve adsorbent represented by the general formula (Na2O)6 (Al2O3)6 (SiO2) 12+x WH2O where x varies from 0.001 TO 0.1, w being the number of moles of water, which comprises which comprises (1) activating a commercially available zeolite A comprising a molecular sieve at a temperature in the range of 350 to 450 degree C. to eliminate physically adsorbed water, for a period of time ranging from 3 to 6 hours; (2) cooling the activated zeolite in a desiccator under vacuum in the range of 1X 10-2 to 1X10-4 mm Hg; (3) treating the cooled zeolite with tetra alkyl ortho silicate dissolved in a dry solvent at a concentration ranging from 0.1 to 1.0 wt. %/volume for a specified period of 4 to 8 hours under continuous stirring; (4) recovering the solvent by conventional techniques for re-use; (5) drying the treated zeolite in air in static condition at an ambient temperature in the range of from 20 to 35 degree. Celsius to provide a modified zeolite; (6) calcining the modified zeolite to a temperature in the range of 450 to 650.degree Celsius for a period of time 3 to 8 hours; and (7) cooling the zeolite at ambient temperature in static conditions to provide the molecular sieve adsorbent having a preferential oxygen adsorption selectivity over nitrogen and argon. 2. A process as claimed in claim 1, wherein 0.10 to 1.00 weight percent of tetra alkyl ortho silicate was deposited uniformly on the zeolite surface from its dry solution in the dry solvent selected from toluene, benzene, xylene and cyclohexane. 3. A process as claimed in claim 2, wherein the said tetra alkyl ortho silicate deposition on the zeolite surface was carried out in a liquid phase reaction at ambient temperature and pressure. 4. A process as claimed in claim 1, wherein the tetra alkyl ortho silicate deposited on the zeolite surface was converted into silica by calcination in air at 500 to 650 degree. C for 3 to 6 hours. 5. The process as claimed in claim 1, wherein the tetra alkyl ortho silicate deposited on the zeolite surface was converted into silica by calcination in air at 550.degree. C. for 4 hours. 6. A process as claimed in claim 1 , wherein the adsorbent as prepared is useful for the separation and purification of nitrogen and argon from its mixture with oxygen. 7. A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air substantially as herein described with reference to examples accompanying this specification.

Full Text

Process for the preparation of a molecular sieve adsorbent for the size/shape
selective separation of air
The present invention relates to a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air.
The invention relates to the use of pore engineered zeolites as size / shape selective adsorbents in the separation of gases having closely related physical properties. More specifically, the invention relates to the preparation and use of a molecular sieve adsorbent, which is selective towards oxygen from its gaseous mixture with nitrogen and / or argon.
FIELD OF THE INVENTION
The use of adsorption techniques to separate a gaseous component from a gaseous stream was initially developed for the removal of carbon dioxide and water from air. Gas adsorption techniques are now conventionally employed in processes for the enrichment of hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen and nitrogen.
Adsorbents most often effect separations by adsorbing one (or more) component more strongly than another. The various interaction forces engaged in adsorption process are van der Waals interactions, acid-base interactions, hydrogen bond, electrostatic, chelation, clathration and covalent bond. Two important separation mechanisms are exclusion of certain molecules in the feed because they are too large to fit into the pores of the adsorbent (molecular sieving effect, size / shape selective separation) and differences in the diffusion rates of the adsorbing species in the pores of the adsorbent.
The four types of adsorbents which are dominantly used are activated carbon, zeolite molecular sieves, silica gel and activated alumina. Carbon molecular sieves (CMS), which exhibit very narrow pore size distribution, facilitate separation based on different inter particle diffusion rates. The efficient separation of air to recover nitrogen has provided a secure and somewhat growing market for carbon molecular sieves.

Adsorption processes for the separation of oxygen and nitrogen from air have been increasingly used tor commercial purposes for the last three decades. Oxygen requirements in sewage treatment, fermentation, cutting and welding, fish breeding, electric iurnaces, pulp bleaching, glass blowing, medical purposes and in the steel industries, particularly when the required oxygen purity is between 90 to 95%, are being largely met by adsorption based pressure swing or vacuum swing processes. It is estimated that at present, around 20% of the world's oxygen demand is met by the adsorptive separation of air. However, the maximum attainable purity by adsorption processes is around 95% with separation of 0.934mole percent argon present in the air being a limiting factor to achieve 100% oxygen purity. Furthermore, the adsorption-based production of oxygen from air is economically not competitive with the cryogenic fractionation of air for production levels of more than 200 tonne per day.
The adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for the desired components the better is the adsorbent as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of a particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process.
The adsorption selectivity of a component over others is calculated as the ratio of the volumes of gas adsorbed at any given pressure and temperature. The adsorption selectivity of a component results from steric factors such as the differences in the size and shape of the adsorbate molecules; equilibrium effect, i.e. when the adsorption isotherms of components of a gas mixture differ appreciably; kinetic effect, when the components have substantially different adsorption rates.

BACKGROUND OF THE INVENTION
The principal characteristic of the separation, removal or concentration of oxygen from the air is that usually there is no cost for the starting material, which is air. The cost of the oxygen produced or removed, depends essentially upon the following factors.
(a) Costs of equipment necessary for separating or concentrating oxygen,
(b) Costs of energy necessary for operating the equipment.
(c) When purified oxygen is needed, the cost of the purification step has to be taken into
account.
Another characteristic is that separation or concentration of oxygen can be achieved either by separating oxygen or by separating nitrogen from air as a starting material.
Taking into consideration the above-described factors, various economically advantageous processes have heretofore been proposed. These include, for example, the process in which air is liquefied at low temperatures to separate oxygen or nitrogen, making use of the difference in the boiling point between liquid oxygen (-182.9°C) and liquid nitrogen (-195.8°C). The apparatus employed is suited for producing large amounts of oxygen and the production of most of the oxygen and nitrogen in the world is based on this procedure. One disadvantage of the process is that it requires large amounts of power. Another is that large-scale equipment is
necessarily site specific and portability is low. Another is that it takes hours to switch on
and switch off the plant.
In another approach the membrane separation system is employed for the separation of oxygen and nitrogen from air [US patent 5a 091, 216 (1992) to Hayes et.al. US patent 5,004,482 (1991) to Haas et.al, US patent Application 20020038602 (2002), to Katz; ct al.]. The main drawback,

of this method is the thin polymeric films used in the separation process are too weak to withstand the high differential gas pressures required for the separation and the purity of the product gas is only around 50%.
In the prior art, adsorbents which are selective for nitrogen from a mixture with oxygen and argon have been reported [US patent 4,481,018 (1984) to Coe et.al, US patent 4,557,736 (1985) to Sircar et.al, US patent 4,859,217 (1989) to Chao; Chien-Chung, US patent 4,943,304 (1990) to Coe etal, US patent 4,964,889 (1990) to Chao; Chien-Chung, US patent 5,114,440 (1992)to Reiss; Gerhard, US patent 5,152,813 (1992) to Coe etal, US patent 5,174,979 (1992) to Chao; Chien-Chung ct.al, US patent 5,454,857 (1995) to Chao; Chien-Chung, US patent 5,464,467 (1995) to Fitch et.al., US patent 5,698,013 (1997)to Chao; Chien-Chung., US patent 5,868,818 (1999) to Ogawa et.aU US patent 6,030,916 (2000) to Choudary et.al., US patent 6,231,644 (2001)] to Jain et. al.] wherein the zeolites of type A, faujasite, mordenite, clinoptilites, chabazite and monolith have been used. Efforts to enhance the adsorption capacity and selectivity have been reported by exchanging the extra framework cations with alkali and / or alkaline earth metal cations and increasing the number of extra framework cations in the zeolite structure by modifying the chemical composition. The adsorption selectivity for nitrogen has also been substantially enhanced by exchanging the zeolite with cations like lithium and / or calcium in some zeolite types. They have been employed in processes for the separation or concentration of oxygen by removing nitrogen selectively from the air. However, the molecular sieves of these types have an isotherm, which follows the Langmuir adsorption isotherm. As a result, when the pressure reaches 1.5 atmospheres absolute (ata) the increase in the adsorptivity is not large compared with the increase in the pressure. Moreover, a very large amount of nitrogen must be separated since the molar ratio of N2/Q* in the air is 4. Therefore, the advantage achieved by enlargement of the apparatus to permit the use of high pressure is rather small. This limits the application of this process to small volume installations. The maximum attainable oxygen purity

by adsorption processes is around 95%, with separation of 0.934-molc percent argon present in the air being a limiting factor to achieve 100% oxygen purity. These adsorbents are highly moisture
sensitive and the adsorption capacity and selectivity will decay in the presence of moisture. The chromatpgraphic separation of oxygen and argon is also possible by using these adsorbents.
US patent 4,4S3,952 (1984) to Izmi et.al. discloses the manufacture of an oxygen selective adsorbent by substituting the Na cations of zeolite A with K and Fe (II). The adsorbent shows oxygen selectivity only at low temperature and its preparation requires multi-stage cation exchange, adding to the cost of preparation. Cation exchange is carried out at around 8Q°C using aqueous salt solutions of the metal ions to be exchanged. This results in a higher energy requirement as well as the generation of effluents during the exchange process. Furthermore, potassium exchange in zeolite leads to lower thermal and hydrothermal stability of the adsorbent.
Carbon molecular sieves arc effective for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen. The difference in rates of adsorption is due to the difference in size of the oxygen and nitrogen molecules. Since the difference in size is quite small, approximately 0.2 A°, the pore structure of the carbon mobcular sieve must be tightly controlled in order to effectively separate the two molecules. In order to improve the performance of carbon molecular sieves, various techniques have been used to modify pore size. The most common method is the deposit of carbon on carbon molecular sieves. For example, US Patent 3,979,330 to Munzner etal discloses the preparation of carbon containing molecular sieves in which coke containing up to 5% volatile components is treated at 600°C-900°C in order to split off carbon from a hydrocarbon. The split-off carbon is deposited in the carbon framework of the coke to narrow the existing pores. US Patents 4,528,281; 4,540,678; 4,627,857 and 4,629,476 to Jr. Robert, S.F. disclose various preparations of carbon molecular sieves for use in the separation of gases.

US Patent 4,742,040 to Ohsaki etal. discloses a process tor making a carbon molecular sieve having increased adsorption capacity and selectivity by pelletising powder coconut shell charcoal containing small amounts of coal tar as a binder, carbonising, washing in mineral acid solution to remove soluble ingredients, adding specified amounts of creosote or other aromatic compounds, heating at 950°C -1000°C, and then cooling in an inert gas. US Patent 4,880,765 to Knoblauch ct.al., discloses a process for producing carbon molecular sieves with uniform quality and good separating properties by treating a carbonaceous product with inert gas and steam in a vibrating oven and further treating it with benzene at high temperatures to thereby narrow
existing pores. Preparation of a carbon molecular sieve is a multi-step process requiring the utmost care at each stage to get a totally reproducible carbon molecular sieve. Additionally, the process is a very high temperature process, which results in a higher cost of the adsorbent.
US Patent 5,081,097 to Sharma et.al., discloses copper modified carbon molecular sieves for the selective removal of oxygen. The sieve is prepared by pyrolysis of a mixture of a copper-containing material and a polyfunctional alcohol to form a sorbent precursor. The sorbent precursor is then heated and reduced to produce a copper modified Carbon molecular sieve. Pyrolysis is a high temperature process making the whole process of preparation of the adsorbent an energy intensive process.
Another process uses a transient metal-based organic complex capable of selectively absorbing oxygen [US patent 4,477,418 (1984) to Mullhaupt Joseph ct.al.; US patent 5,126,466(1992) to Ramprasad et.al.; US patent 5,141,725(1992) toRamprasad ct.al.; US patent 5,294,418(1994) to Ramprasad et.al.; US patent 5,945,079 (1999) to Mullhaupt Joseph et.al; US patent Application 20010003950 (2001), to Zhang, Delang et al.]. The absorption by these complexes is reversible with changes in temperature and pressure so that it is theoretically possible to achieve separation or concentration of oxygen by means of a temperature swing or a pressure swing cycle of the air.

However, in practice, severe deterioration of the organic complex occurs with repeated cycles of absorption and liberation of oxygen. Moreover, the organic complex itself is expensive. Therefore, the use of this process is limited to special situations. The main drawback of this process lies in the air and moisture sensitivity of the metal complexes used, which lowers the stability of the adsorbent produced. Additionally, the cost of the metal complexes used in the preparation of the adsorbent is very high.
US patent 6,087,289 (2000) to Choudary et al. discloses a process for the preparation of a zeolte-based adsorbent containing cerium cations for the selective adsorption of oxygen from a gas mixture. Cerium exchange into the zeolite is carried out under reflux conditions using an aqueous solution of cerium salt at around 80°C for 4-8 hours and repeating the exchange process several times. The main drawbacks of this adsorbent lie in the observation of oxygen selectivity only in the low-pressure region. Furthermore, the adsorbent preparation is a multi-step ion exchange process,
which also generates liquid effluent.
European Patent 0,218,403 to Greenbank discloses a dense gas pack of coarse and fine adsorbent particles wherein the size of the largest fine particles is less than one-third of the coarse particles and sixty percent of all particles are larger than sixty mesh. Although not specifically stated, it is evident from the examples that these percentages are by volume. This system is designed primarily for enhancing gas volume to be stored in a storage cylinder. It is mentioned, however, that it can be utilized for molecular sieves. There is nothing in this application, however, which would give an insight into the fact that significantly enhanced PSA efficiency could be obtained by combining coarse and fine particles of a kinetically-selective sieve material in a single bed. It has been found in accordance with the present invention that, within certain limits as will be defined herein,
a mixture of coarse and fine kinctically selective sieve particles will unexpectedly give enhanced PSA performance.

In another approach, the chemical vapour deposition technique was used for controlling the pore
opening size of the zeolites by the deposition of silicon alkoxide [M. Niwa ct al., JCS Varaday Trans. I, 1984, 80, 3135-3145; M. Niwa et al., M. Niwa ct al., J. Phys. Chem., 1986, 90, 6233-6237; Chemistry Letters, 1989, 441-442; M. Niwa et al., Ind. Eng. Chem. Res., 1991, 30, 38-42; D. Ohayon et al., Applied Catalysis A- General, 2001, 217, 241-251]. Chemical vapour deposition is carried out by taking a requisite quantity of zeolite in a glass reactor, which is thermally activated at 450°C in situ under inert gas e.g. nitrogen flow. The vapours of silicon alkoxide are continuously injected into the inert gas stream, which carries the vapours to the zeolite surface where the alkoxide chemically reacts with the silanol groups of the zeolite. Once the desired quantity of alkoxide is deposited on the zeolite, the sample is heated to 550 °C in air for 4-6 hours
after which it is brought down to ambient temperature and used for adsorption. The major disadvantages of this technique arc (i) Chemical vapour deposition, which leads to a non-uniform coating of alkoxide which in turn results in non-uniform pore mouth closure, (u) The process has to be carried out at elevated temperature where the alkoxide gets vaporised, (Hi) The deposition of the alkoxide requires to be done at a slow rate for better diffusion and (iv)This method is expensive and the lack of a commercial level at higher scale will be difficult.
At present nitrogen and argon containing less than lOppm Oxygen are produced by using a deoxo hybrid system in which the oxygen is removed by reducing it to water over a catalyst with hydrogen.
OBJECT OF THE INVENTION
The main object of the present invention is to provide a process for the preparation of a molecular sieve adsorbent for the size/ shape selective separation of air, which obviates the drawbacks as detailed above.

Still another object of the present invention is to provide an oxygen selective zeolite based adsorbent from its gaseous mixture with nitrogen and argon.
Still another object of the present invention is to provide an adsorbent, which can be prepared by the external surface modification of the zeolite A.
Yet another object of the present invention is to provide an oxygen selective adsorbent by a simple liquid phase surface modification of zeolite A.
Yet another object of the present invention is to have a uniform deposition of alkoxide on the surface of zeolite A.
Yet another object of the present invention is to provide an adsorbent with high thermal and hydrothermal stability.
Yet another object of the present invention is to provide an adsorbent, which is selective to oxygen over nitrogen and argon and can be used commercially for the separation and purification of nitrogen and argon.
In the drawings accompanying this specification,
Figure 1 represents the adsorption isotherms of nitrogen, argon and oxygen at 15°C on the
adsorbent obtained from example-1.
Figure I represents the adsorption isotherms of nitrogen, argon and oxygen at 15°C on the
adsorbent obtained from example-6.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for the preparation of a molecular sieve adsorbent for the size / shape selective separation of air, which comprises a molecular sieve adsorbent represented by the general formula, (Na2O)6.(Al2O3)r,.(SiO2)i2+x.wH2O

where the values of x varies from 0.001 to 0.1, w being the number of moles of water, which comprises (1) activating the commercially available zeolite A in the temperature range of 350 to 450 ° Celsius to eliminate physically adsorbed water, for a period ranging from 3 to 6 hours; (2) cooling the activated zeolite in a desiccator under vacuum in the range of IX 10'2to IXlO^mm Hg; (3) treating the cooled zeolite with tetra allcyl ortho silicate dissolved in a dry solvent in the concentration range of 0.1'to-1.0 wt%/volume for a specified period in the range of 4 to 8 hours under continuous stirring; (4) recovering the solvent by conventional techniques for re-use; (5) drying the treated zeolite in air in a static condition at ambient temperature in the range of 20 to 35° Celsius; (6) heating the modified zeolite in the temperature range of 450 to 600°Celsius for a period ranging from 3 to 8 hours; (7) cooling the zeolite at ambient temperature in a static condition; (8) activating the zeolite samples at the temperature range of 350 to 450°Celsius prior to measuring the adsorption of oxygen, nitrogen and argon by a static volumetric system.
In an embodiment of the present invention commercially available zeolite A may used for the preparation of the molecular sieve adsorbent.
In another embodiment of the present invention the zeolitc-A was activated at 350 to 550° C for 3-6 hours followed by cooling under inert or vacuum conditions.
In another embodiment of the present invention the tctra alkyl ortho silicate was dissolved in dry solvent, which may be selected from e.g. toluene, benzene, xylene and cyclohexane.
In another embodiment of the present invention 0.10 to 1.00 weight percentage of tetra alkyl ortho silicate may be deposited onto the zeolite in a single step by treating the activated zeolite with a solution Of tetra alkyl ortho silicate in a dry solvent for 4 to 8 hours.

In still another embodiment of the present invention ■• said tetra alkyl ortho silicate may be deposited in the range discussed above at tetra alkyl ortho silicate concentration of 0.10 to 1.00% by weight of the zeolite.
In still another embodiment of Lhe present invention the alkoxide deposition may be carried out in liquid phase for a period ranging from 4 to 8 hours under continuous stirring at ambient temperature.
In still another embodiment of the present invention the alkoxide deposition may be uniform on the surface of the zeolite.
In still another embodiment of the present invention the solvent was recovered by the distillation method, preferably under vacuum distillation and can be re-used.
In still another embodiment of the present invention the adsorbents are dried in air or under vacuum conditions.
In Still another embodiment of the present invention the adsorbent is calcined in the temperature range 500 to 600°C preferably at 550°C.
DESCRIPTION OF THE INVENTION
The present invention relates to a novel process to control the pore size of zeolite A, which
has oxygen adsorption selectivity over nitrogen and argon. Furthermore this adsorbent displays high thermal and hydrothermal stability.
Zeolites, which are microporous crystalline alumino-silicates, are finding increased applications as adsorbents for separating mixtures of closely related compounds. Zeolites have a three dimensional network of basic structural units consisting of SiO4 and A1O4 tetrahedrons linked to

each other by sharing apical oxygen atoms. Silicon and aluminium atoms lie in the centre of the tetrahedron. The resulting alumino silicate structure, which is generally highly porous, possesses three-dimensional pores accessed through molecular sized windows. In a hydrated form, the preferred zeolites are generally represented by the following formula, M2/nCXAl203.xSiC)2.wH2O where M is a cation, which balances the electrovalence of the tetrahedron and is generally referred to as extra framework exchangeable cation, n represents the valency of the cation and x and w represent the moles of SiO2 and water respectively.
The attributes which make the zeolites attractive for separation include, an unusually high thermal and hydrothermai stability, uniform pore structure, easy pore aperture modification and substantial adsorption capacity even at low adsorbate pressures. Furthermore, zeolites can be produced synthetically under relatively moderate hydrothermal conditions.
Structural analysis of the samples was done by X-ray diffraction, wherein the crystallinity of the zeolite is measured from the intensity of the well-defined peaks. The in situ X-ray powder
diffraction measurements at 30°C, I00°C, 200°C, 300°C, 400°C, SQO'C, 600°C, 650°C, 7O0C,
750°C, 800°C and 85O°C show that the newly developed adsorbent has high thermal stability.
X-ray powder diffraction was measured using a PHILIPS X'pert MPD system equipped with a
XRK. 900 reaction chamber.
The zeolite NaA powder [Nan (AIO2) u. (S1O2) 12.WH2O] was used as the starting material. X-ray diffraction data showed that the starting material was highly crystalline. A known amount of the zeolite NaA powder [Nan (A1O;>) i2. (SiC>2) i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and mixed thoroughly with a solution having a known amount of tetra alkyl orthosilicate in 100ml dry solvent, the sample was dried by evaporating solvent under reduced pressure and the tetra alkyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C

Oxygen, nitrogen and argon adsorption at 15°C was measured using a static volumetric system (Micromeritics ASAP 2010), after activating the sample at 350°C to 450°C under vacuum for 4 hours as described in the Examples. Addition of the adsorbate gas was made at volumes required
to achieve a targeted set of pressures ranging from 0.5 to 850 mmHg. A minimum equilibrium interval of 5 seconds with a relative target tolerance of 5.0% of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine the equilibrium for each measurement point.
The selectivity of pure components of two gases A and B is given by the equation,
O:A/B=[VA/VB]P>T
where VA and VB are the volumes of gas A and B adsorbed at any given pressure P and temperature T.
The important inventive steps involved in the present invention are that the molecular sieve adsorbent obtained by the control of the pore mouth of the zeolite (i) by the deposition of silica by chemically reacting an alkoxide with silanol groups present on the external surface of the zeolite followed by calcination at 500 - 600 °C, (ii) by liquid phase chemical reaction of tetra alkyl orthosilicate in a moisture free solvent to ensure uniform deposition of silica on the surface of the zeolite at ambient conditions, (iii) enhancement of thermal and hydrothermai stability of the adsorbent by silica deposition on the external surface of the zeolite, (iv) to prepare a zeolite based oxygen selective adsorbent based on shape /size selectivity by a method other than conventionally used cation exchange
The following examples are given by way of illustration and therefore should not be constructed to limit the scope of the present invention.

EXAMPLE-1
A known amount of zeolite NaA, [(Na2O) «.(Al2O3)6.(SiO2)i2.wI-feO], was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. The adsorption capacity for oxygen is 3.48cc/g at 15°C and 765mmHg and selectivity for nitrogen over oxygen is around 3 to 5 in the pressure range studied, the values are given in table 1.
EXAMPLE-2
lO.Og of the zeolite NaA powder [Na,2 (AlO2)i2.(SiO2)i2-wH2O] was activated at 40CPC to remove the adsorbed water in the zeolite and stirred with 0.1 Og tetra methyl orthosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating solvent under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothennal stability. The adsorption capacity for oxygen is 3.50cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 1.5 to 0.95 and selectivity for oxygen over argon is 1.3 to 2.1 in the pressure range studied, the values are given in table-1.
EXAMPLE-3
lO.Og of the zeolite NaA powder [Nai2 (AiO2)i2.(SiO2)]2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with 0.1 Og tetra ethyl orthosilicate in 100ml dry solvent. The sample was dried after 5 hrs by evaporating the solvent under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination; of the zeolite at 5506C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent

has high thermal and hydrotherrnal stability. The adsorption capacity for oxygen is 3.53 cc/g at 15°C and 765mm Hg, selectivity for oxygen over nitrogen is 1.6 to 1.1 and selectivity for oxygen over argon isl .3 to 2.2 in the pressure range studied, the values are given in table-1.
EXAMPLE-4
lO.Og of the zeolite NaA powder |Nai2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.15g tetra ethyl orthosilicate inlOQml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.15cc/g at 15°C and 765mmHg, selectivity tor oxygen over nitrogen is 1.8 to 0.97 and selectivity for oxygen over argon is 2.8 to 3.2 in the pressure range studied, the values are given in table 1.
EXAMPLES
lO.Og of the zeolite NaA powder [Na^ (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with 0.20g tetra ethyl orlhosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550uC. A known amount of the sample was activated at 350°C uoder vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.78cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2 to 1.1 and selectivity for oxygen over argon is 3.0 to 3.4 in the pressure range studied, the values are given in table-.1.

EXAMPLE-6
lO.Og of the zeolite NaA powder |Na]2 (A102)i2.(SiO2)i2.wn20] was activated at 40CPC to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicatc in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tctra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 500°G.; A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.42 cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is around 2.1 to 1.5 and selectivity for oxygen over argon is 3.5 to 3.8 in the pressure range studied, the values are given in tablel.
EXAMPLE-7
lO.Og of the zeolite NaA powder [Nai2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(PC to remove the water adsorbed in the zeolite and stirred with 0.30g tetra ethyl orthosilicate in 100ml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination; of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.63cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.5 to 1.6 and selectivity for oxygen over argon is 4.6 to 4.7 in the pressure range studied, the values are given in tablc-1.
EXAMPLE-8
lO.Og of the zeolite NaA powder [Na]2 (AlO2)i2.(SiO2)i2.wH2O] was activated at 40(fC to remove the water adsorbed in the zeolite and stirred with l.OOg tetra ethyl orthosiUcatc in 100ml

dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra ethyl orllio silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show . that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 1.32 cc/g at 159C and 765mmHg, selectivity for oxygen over nitrogen is 2.5 to 1.4 and selectivity for oxygen over argon is 2.5 to 3.5 in the pressure range studied, the values are given in table-1.
EXAMPLE-9
lO.Og of the zeolite NaA powder [Nai2 (AlO2)i2-(SiO2)i2-wH2O] was activated at 40CPC to remove the water adsorbed in the zeolite and stirred with 0.20g tetra methyl orthosilicate in lOOml dry toluene. The sample was dried after 5 hrs by evaporating toluene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 85CfC show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 3.77cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.6 to 1.4 and selectivity for oxygen over argon is 3.3 to 4.1 in the pressure range studied, the values are given in table-1.
EXAMPLE-10
lO.Og of the zeolite NaA powder [Nai2(AlO2)i2-(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicate in 100ml dry benzene. The sample was dried after 5 hrs by evaporating benzene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 500°C. A known amount of the sample was activated

at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 85(fC show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.85cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.4 to 1.3 and selectivity for oxygen over argon is 4.0 to 4.3 in the pressure range studied, the values are given in table-1.
EXAMPLE-11
lO.Og of the zeolite NaA powder [Na^AlC^n.CSiQO^.wl-kO] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.20g tetra ethyl orthosilicate in 100ml dry benzene. The sample was dried after 5 hrs by evaporating benzene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at 550°C. A known amount of the sample was activated at 35O°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity tor oxygen is 3.79cc/g at 15°C and 765mmV\g, selectivity for oxygen over nitrogen is 2.4 to 1.2 and selectivity for oxygen over argon is 3.6 to 4.0 in the pressure range studied, the values are given in table-1.
EXAMPLE-12
lO.Og of the zeolite NaA powder [Nai2(AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicate in 100ml dry cyclohcxane. The sample was dried after 5 hrs by evaporating cyclohexane under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination! of the zeolite at 600°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrolhermal stability. The adsorption capacity for oxygen is

2.92cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen 2.6 to 1.5 and selectivity for oxygen over argon is 4.5 to 4.8 in the pressure range studied, the values arc given in table-1.
EXAMPLE-13
lO.Og of the zeolite NaA powder [Nat2(AlO2)i2.(SiO2)i2.wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicate in 100ml dry cyclohexane. The sample was dried after 5 hrs by evaporating: cyclohcxane under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination of the zeolite at SSO't. A known amount of the sample was activated at 35O°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption cqpacity for oxygen is 2.87cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.8 to 1.5 and selectivity for oxygen over argon is 4.7 to 4.9 in the pressure range studied, the values are given in table-1.
EXAMPLE-14
lO.Og of the zeolite NaA powder tNai2(AlO2)i2.(SiO2)i2-wH2O] was activated at 400°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra ethyl orthosilicate in 100ml dry xylene. The sample was dried after 5 hrs by evaporating xylene under reduced pressure. The tetra ethyl ortho silicate species deposited on the zeolite surface was converted into silica by calcination: of the zeolite at 550°C. A known amount of the sample was activated at 350°C under vacuum and adsorption measurements were carried out as described earlier. In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.93cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.4 to 1.4and selectivity for oxygen over argon is 4.3 to 4.5 in the pressure range studied, the values are given in table-1.

EXAMPLES 15
lO.Og of the zeolite NaA powder [Nai2(AlO2)i2.(SiO2)i2.wH2O] was activated at 40()°C to remove the water adsorbed in the zeolite and stirred with 0.25g tetra methyl orthosilicatc in 100ml dry xylene. The sample was dried after 5 hrs by evaporating xylene under reduced pressure. The tetra methyl ortho silicate species deposited on the zeolite surface was convened into silica by calcination of the zeolite at 650°C. A known amount of the sample was .activated at 350°C under vacuum and adsorption measurements were carried out as described earlier.In situ X-ray powder diffraction measurements at various temperatures up to 850°C show that the adsorbent has high thermal and hydrothermal stability. The adsorption capacity for oxygen is 2.77cc/g at 15°C and 765mmHg, selectivity for oxygen over nitrogen is 2.3 to 1.3 and selectivity for oxygen over argon is 4.7 to 4.8 in the pressure range studied, the values are given in table-1.
The adsorption capacity and selectivity of all the 15 samples are enumerated in the Table-1. The main advantages of the present invention include:
1. The adsorbent, prepared by the modification of zeolite A shows oxygen selectivity over
nitrogen and argon.
2. The adsorbent is prepared by a simple liquid phase alkoxide deposition.
3. The alkoxide deposition is uniform on the zeolite surface.
4. The alkoxide deposition is carried out at ambient temperature and pressure.
5. The solvent used for the alkoxide deposition can be recovered by distillation methods.
6. The adsorbent shows very high thermal and hydrothermal stability.
7. The adsorbent shows oxygen / argon selectivity of nearly 4.9 in the pressure range studied.

8. The adsorbent is useful in the commercial separation and purification of nitrogen and
argon from a mixture with oxygen.
9. The adsorbent is useful for the chromatographic separation of oxygen nitrogen and
argon.

TABLE-1
Oxygen, Nitrogen and Argon Adsorption Capacities and Selectivity for Various Adsorbents atl5°C&765mmHg

We Claim:
1. A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air, the said molecular sieve adsorbent represented by the general formula (Na2O)6 (Al2O3)6 (SiO2) 12+x WH2O where x varies from 0.001 TO 0.1, w being the number of moles of water, which comprises which comprises (1) activating a commercially available zeolite A comprising a molecular sieve at a temperature in the range of 350 to 450 degree C. to eliminate physically adsorbed water, for a period of time ranging from 3 to 6 hours; (2) cooling the activated zeolite in a desiccator under vacuum in the range of 1X 10-2 to 1X10-4 mm Hg; (3) treating the cooled zeolite with tetra alkyl ortho silicate dissolved in a dry solvent at a concentration ranging from 0.1 to 1.0 wt. %/volume for a specified period of 4 to 8 hours under continuous stirring; (4) recovering the solvent by conventional techniques for re-use; (5) drying the treated zeolite in air in static condition at an ambient temperature in the range of from 20 to 35 degree. Celsius to provide a modified zeolite; (6) calcining the modified zeolite to a temperature in the range of 450 to 650.degree Celsius for a period of time 3 to 8 hours; and (7) cooling the zeolite at ambient temperature in static conditions to provide the molecular sieve adsorbent having a preferential oxygen adsorption selectivity over nitrogen and argon.
2. A process as claimed in claim 1, wherein 0.10 to 1.00 weight percent of tetra alkyl ortho silicate was deposited uniformly on the zeolite surface from its dry solution in the dry solvent selected from toluene, benzene, xylene and cyclohexane.
3. A process as claimed in claim 2, wherein the said tetra alkyl ortho silicate deposition on the zeolite surface was carried out in a liquid phase reaction at ambient temperature and pressure.
4. A process as claimed in claim 1, wherein the tetra alkyl ortho silicate deposited on the zeolite surface was converted into silica by calcination in air at 500 to 650 degree. C for 3 to 6 hours.
5. The process as claimed in claim 1, wherein the tetra alkyl ortho silicate deposited on the zeolite surface was converted into silica by calcination in air at 550.degree. C. for 4 hours.
6. A process as claimed in claim 1 , wherein the adsorbent as prepared is useful for the separation and purification of nitrogen and argon from its mixture with oxygen.
7. A process for the preparation of a molecular sieve adsorbent for the size/shape selective separation of air substantially as herein described with reference to examples accompanying this specification.

Documents:

564-DEL-2004-Abstract (22-01-2010).pdf

564-del-2004-Abstract-(22-03-2004).pdf

564-del-2004-abstract.pdf

564-DEL-2004-Claims (22-01-2010).pdf

564-del-2004-Claims-(22-03-2004).pdf

564-del-2004-claims.pdf

564-DEL-2004-Correspondence-Others (22-01-2010).pdf

564-DEL-2004-Correspondence-Others-(12-03-2010).pdf

564-del-2004-correspondence-others.pdf

564-del-2004-correspondence-po.pdf

564-del-2004-correspondence.pdf

564-DEL-2004-Description (Complete) (22-01-2010).pdf

564-del-2004-description.pdf

564-del-2004-drawings.pdf

564-DEL-2004-Form-1 (22-01-2010).pdf

564-del-2004-form-18.pdf

564-DEL-2004-Form-3 (22-01-2010).pdf

564-del-2004-form1.pdf

564-del-2004-form2.pdf

564-del-2004-form3.pdf

564-del-2004-form5.pdf

564-DEL-2004-Petition 137-(12-03-2010).pdf

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Patent Number

250597

Indian Patent Application Number

564/DEL/2004

PG Journal Number

02/2012

Publication Date

13-Jan-2012

Grant Date

11-Jan-2012

Date of Filing

22-Mar-2004

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	JINCE SEBASTIAN	CENTRAL SALT AND MARINE CHEMICALS RESEARCH INSTITUTE, GIJUBHAI BADHEKA MARG, BHAVNAGAR - 364 002, GUJARAT (INDIA)
2	RAKSH VIR JASRA	CENTRAL SALT AND MARINE CHEMICALS RESEARCH INSTITUTE, GIJUBHAI BADHEKA MARG, BHAVNAGAR - 364 002, GUJARAT (INDIA)
3	CHINTANSINH DHARMENDRASINH CHUDASAMA	CENTRAL SALT AND MARINE CHEMICALS RESEARCH INSTITUTE, GIJUBHAI BADHEKA MARG, BHAVNAGAR - 364 002, GUJARAT (INDIA)

PCT International Classification Number

C07C7/13

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	03072071	2003-03-28	U.K.