Title of Invention

"A PROCESS FOR THE PREPARATION OF SILICA AEROGEL."

Abstract

This invention relates to a process for the preparation of silica aerogel comprising reacting tetraethyl ortho silicate (TEOS) and a mixture of water and orgaric solvent selected from ethyl or isopropyl alcohor miscible with water and TEOS in the presence of a catalyst selected from citric acid, tartaric acid, hydrochloric acid, nitric acid, boric acid and sulphuric acid to form a gel wherein the said step of gelation is carried by hydrolysis and polymerization, removing the said solvent from the pores by the step of high temperature supercritical drying .

Full Text

FIELD OF INVENTION This invention relates to a process for the preparation of silica aerogel. BACKGROUND OF INVENTION Monolithic aerogels are classes of open cell porous materials derived from the supercritical drying process. The first aerogels were translucent pieces of silica glass made by S S Kistler (US Patent 2249767). Silica aerogel is a silica gel from which the liquid has been removed in such a way as to prevent significant collapse or change in the structure as liquid is removed. This is achieved by heating the alcogel in a suitable autoclave and extracting the solvent from the pores of the alcogel under supercritical conditions. The extraction is carried out above the critical temperature of the solvent used for the preparation of the gel. The critical temperature is the temperature above which it is impossible to liquefy a gas no matter how great a pressure is applied. At temperatures above the critical temperature the distinction between the liquid and gas phases disappears and hence no interface will be present. Since there is no interface between liquid and gas phase, there is no surface tension and hence no menisci will be formed. Collapsing of gel network will not take place because of this. OBJECTS FO THE INVENTION An object of this invention is to propose a novel process for the preparation of silica aerogel. A further object of this invention is to propose a novel process for the preparation of silica aerogel having uniform pore size distribution. A still further object of this invention is to propose a novel process for the preparation of silica aerogel which has porosity>90%. Further objects and advantages will be more apparent from the ensuing description. DESCRIPTION OF INENTION According to this invention there is provide a process for the preparation of silica aerogel comprising reacting tetraethyl ortho silicate (TEOS) and a mixture of water and orgaric solvent selected from ethyl or isopropyl alcohor miscible with water and TEOS in the presence of a catalyst selected from citric acid, tartaric acid, hydrochloric acid, nitric acid, boric acid and sulphuric acid to form a gel wherein the said step of gelation is carried by hydrolysis and polymerization, removing the said solvent from the pores by the step of high temperature supercritical drying. In accordance with this invention, silica aerogels having high porosity can be made through supercritical process using variety of catalysts. One embodiment of the invention is a method comprising reaction of Tetra ethyl ortho silicate (TEOS) and a mixture of water and organic solvent miscible with water and TEOS to form a gel. The mole ratio of TEOS to H2O in the liquid phase is kept at 1:5, the ratio of TEOS to solvent kept at 1:5 and the ratio of TEOS to catalyst varying in the range from 1:0.0005 to 1:01 as given in Table 1. The gelation time will vary depending on the catalyst concentration. Silica aerogel will be made by the removal of solvent from the pores through a high temperature supercritical drying process. Gelation studies were conducted by varying the mole % of the boric acid catalyst. Supercritical drying (SCD) experiments using gels The SCD experiments were conducted under varying conditions using the gels prepared by the present invention. For this, the gels in test tubes, beakers and petridishes were placed in the autoclave. Approximately 6 liters of highly pure isopropyl alcohol was added and the lid closed tightly. Prepressurisation to 50 bar of the autoclave was carried out using nitrogen gas. Supercritical drying conditions were achieved by heating the system to 245°C with a heating rate of 35°C per hour with a pressure of 100 bar. Soaking was carried out for 1 hour at the peak temperature. The autoclave valve was opened keeping the system in this temperature for the complete removal of the solvent from the autoclave. The pressure was allowed to diminish by about 1 bar per minute unit it reaches atmospheric pressure. At this pressure, N2 gas is flushed for ten minutes and the system was switched off and allowed to cool to ambient temperature. The aerogel samples are taken out at ambient temperature. The pressure-temperalure-time curve is given in Fig. 1 of the accompanying drawings. Preferred process Silica aerogels can be prepared using boric acid catalyst. The concentration of the catalyst can be varied in the range of 1:0.0005 to 1:0.01 in the TEOS/Catalyst Molar ratio. The silica aerogels produced are characterized by BET, TEM, SEM, FTIR, UV-visible spectroscopy etc. The silica aerogels with surface area -900 m2/g and pore size distribution having pore diameter in the range 7-10 nm. The BET isotherm is given in Fig.2. The cumulative pore size distribution is given in Fig.3. The BJH cumulative pore size distribution is given in Fig.4. The UV transparent spectra is given is Fig. 5. The SEM picture is given in Fig. 6, TEM picture is given in Fig. 7. The FTIR spectra is given in Fig.8. It is seen that the transmittance is more than 90% at 900nm. Silica alcogels from which the aerogels of the present invention may be formed by hydrolysing TEOS with water, organic solvents miscible with water and in presence of catalysts. The organic solvent miscible with water is preferably alcohol. The alcohol may be ethyl alcohol or more preferable isopropyl alcohol. The amount of alcohol may be varied considerably. The type of catalyst employed is a suitable acid catalyst. The acid catalyst used in this invention is boric acid. The amount of catalyst used may varied from 1:0.0005 to 1:0.01 by mole ratio with TEOS. Various other acid catalysts like citric acid, tartaric acid, hydrochloric acid, nitric acid, sulphuric acid etc. can also be employed. The TEOS and isopropyl alcohol are taken in container, preferably a glass beaker, and mixed thoroughly for 10 minutes. After that the catalyst is added and stirred again for 10 minutes. The solution mixture is added to test tubes, beakers or any other moulds and sealed properly. The moulds with solution are kept in vibrationless tables for aging without disturbing the solution during gelation. Gels will be formed within 7 to 10 days depending on the solution types, concentration, and nature and type of acid catalyst used in process. Hydrolysis and condensation will take place during gelation. The reaction can be represented (ideally) as: Hydrolysis (RO)3-Si-OR + H OH→(RO)3-Si-OH + ROH Polymerisation (RO)3-Si-OH + RO-Si(OR)3→ (RO)3-Si-O-Si-(RO)3+ ROH nSi (OR)4 + (2n+ (x-y)/2) H2O -» SiO2n-(x +y)/2 (OH)x (OR)y +(4n-y) ROH where R=methyl ethyl or Isopropyl and n is large number When the reaction is conducted in presence of isopropyl alcohol, the liquid of silica aerogel will be comprising of both isopropanol and ethanol. The sample are characterized as detailed below: BET Analysis The pore structure, surface area and the pore diameter of the aerogel samples were analysed by carrying out detailed BET analysis of aerogel samples. The results obtained for the aerogels prepared with boric acid catalyst are given below (Fig.2 to 4). The BET analysis shows that the isotherms are of type IV with type A hysterisis having mesopores embedded in the microstructure. The total surface area obtained for this sample is 687 m2/g. The total pore volume calculated is 2.57 cc/g, with an average pore diameter is 7.48nm. The aerogel with boric acid catalyst showed more micropores compared to the samples prepared using other catalysts. UV-visible studies Transparency studies of SCO silica aerogels were carried out using UV-Visible spectrometer. Transparency in the range 85-90% (at 900nm) was obtained for the aerogels having thickness ~lmm prepared using acid catalysts of PhSO-t, HCI, tartaric acid etc. However, better transparency was observed for very low (0.0005M) concentration of acid catalysts. The spectra obtained for various samples are given in Fig.5. FTIR studies FTIR analysis was carried out for the aerogel samples. A typical FTIR spectra of the aerogel sample prepared using boric acid as catalyst showed peaks at 1102, 466, 797, 970, 1633 and 3436 cm-1 (Fig 9). The strong absorbant band observed at 3436 cm-1 which may be due to O-H stretching vibrations. A weaker O-H bending vibration band is seen at 1633 cm-1. Both absorbed water and surface -OH groups contribute to these bands. Si-O-Si fundamental vibration gives a strong band at 1102 cm-1. There is region of high infrared transparency between 1900cm-1 and 3100 cm-1. This allows a certain amount of thermal radiation of pass through silica aerogels and lowers the thermal insulating performance. SEM The micrstructure was studies using SEM. SEM picture shows that particles are spherical oriel i itii-fi-kt-m i« rnQ+iit-a /I7irv *7and uniform in nature (Fig.7). TEM The aerogel sample was analysed using TEM and the picture taken for sample 46 is given in Fig. 8. Transmission Electron Microscopy studies confirmed the porous structure of the gels. The pore size observed from the TEM was >50nm. -3-The invention is further described in the following examples. Example 1. Silica aerogel is prepared according to an embodiment of present invention by mixing 10 ml of 98% pure TEOS (Aldrich), 17.2 ml of isopropyl alcohol (E Merck AR grade) 4ml of water and O.OO14g of boric acid. The mixture is kept for aging. Transparent alcogel is formed in ten days. This alcogel is supercritically dried in an autoclave. The aerogel thus formed will have a density of 0.15g/cc. The surface area (900m2/g and porosity >90% are measured by BET adsorption/desorption method. Example 2. Silica aerogel 1 using the same precursors as per example 1 and 0.0028g boric acid Alcogel was formed in 9 days. Semitransparent aerogel with slightly whitish in colour, with more strength was obtained Example 3. Silica aerogel 1 using the same precursors as per example 1 and 0.0056g boric acid content. Alcogel was formed in 9 days. Semitransparent aerogel with slightly whitish in colour, and more strength was obtained. Example 4. Silica aerogel 1 using the same precursors as per example 1 and 0.0084g boric acid content. Alcogel was formed in 9 days. Semitransparent aerogel was obtained. Example 5. Silica aerogel 1 using the same precursors as per example 1 and 0.00112 to 0.00140g boric acid content. Alcogel was formed in 8 days. Semitransparent aerogel was obtained. Example 6 Silica aerogel 1 using the same precursors as per example 1 and 0.0168 g boric acid content. Alcogel was formed in 7 days. Semitransparent aerogel was obtained. Example? Silica aerogel 1 using the same precursors as per example 1 and 0.0224 -0.0280g boric acid content. Alcogel was formed in 8 days. Semitransparent aerogel was obtained. Table 1: Effect of catalyst concentration (Table Removed) We Claim 1. A process for the preparation of silica aerogel comprising reacting tetraethyl ortho silicate (TEOS) and a mixture of water and orgaric solvent selected from ethyl or isopropyl alcohor miscible with water and TEOS in the presence of a catalyst selected from citric acid, tartaric acid, hydrochloric acid, nitric acid, boric acid and sulphuric acid to form a gel wherein the said step of gelation is carried by hydrolysis and polymerization, removing the said solvent from the pores by the step of high temperature supercritical drying 2. A process as claimed in claim 1, wherein said catalyst is boric acid. 3. A process as claimed in claim 1, wherein the molar ratio of . TEOS to catalyst is between 1; 0005 to 1: 01. 4. A process as claimed in claim 1, wherein the molar ratio of TEOS to alcohol is 1:5 and of TEOS to water is 1:5. 5. A process for the preparation of silica aerogel substantially as herein described and illustrated in the examples.

Documents:

442-DEL-2002-Abstract-(09-07-2009).pdf

442-DEL-2002-Abstract-(22-08-2008).pdf

442-del-2002-abstract.pdf

442-DEL-2002-Claims-(09-07-2009).pdf

442-DEL-2002-Claims-(22-08-2008).pdf

442-del-2002-claims.pdf

442-del-2002-correspondence others.pdf

442-DEL-2002-Correspondence-Others-(22-08-2008).pdf

442-del-2002-correspondence-po.pdf

442-DEL-2002-Corresponence-Others-(09-07-2009).pdf

442-DEL-2002-Description (Complete)-(09-07-2009).pdf

442-del-2002-description (complete)-22-08-2008.pdf

442-del-2002-description (complete).pdf

442-del-2002-drawings.pdf

442-DEL-2002-Form-1-(09-07-2009).pdf

442-del-2002-form-1.pdf

442-del-2002-form-18.pdf

442-DEL-2002-Form-2-(09-07-2009).pdf

442-DEL-2002-Form-2-(22-08-2008).pdf

442-del-2002-form-2.pdf

442-DEL-2002-Form-3-(22-08-2008).pdf

442-del-2002-form-3.pdf

442-DEL-2002-GPA-(09-07-2009).pdf

442-DEL-2002-Petition-137-(09-07-2009).pdf