|Title of Invention||
AN IMPROVED PROCESS FOR THE PRODUCTION OF DIFLUOROMETHANE (HFC-32)
|Abstract||An improved process for the production of difluorometharie (HFC-32) which comprises subjecting the dichlorodifluorornmethane (CFC-12) to conventional vapor phase hydrogenolysis such as herein described, in presence of a novel Pd supported carbon covered alumina and recovering HFC-32 by ccnventional distillation methods.|
|Full Text||The present invention relates to an improved process for the production of difluoromethane (HFC-32).
With the growing concern for ozone-depletion in the stratosphere, the production of CFCs has been banned and intensive search is going on for suitable substitutes from amongest hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and other hydrocarbons. Simultaneously, efforts are on to develop environmentally viable technologies to transform the CFCs into harmless products. Hydrogenolysis of CFCs, to produce HFCs and HCFCs is one of these technologies, in addition to processes based on fluorination. However, apart from some products which are not valuable, they produce hazardous byproducts. In this context, hydrogenolysis of CFC-12 to produce HFC-32 (a valuable product used as deep refrigerant) selectively, popularly called as selective hydrodechlorination, is being pursued vigorously. The present invention relates to an improved process for production of difluoromethane (HFC-32) from dichlorodifluoromethane (CFC-12) by the catalytic selective hydrodechlorination.
A number of possible methods of CFC transformation were reported in literature; important among them are destruction and catalytic transformation. The latter seems to be a better technique which produces HFCs/HCFCs. Catalytic transformation of CFC-12 to HFC-32 was disclosed in a set of patents Nos PIBR 92 01 323, JP WO 9411328, JP 05339182, JP 0601731 Pct. Int. WO 9617683. All these processes employ nobel metals as active components
(especially Palladium ) and activated carbon as support material. Most of these catalysts are associated with higher Palladium lodings i.e >10 wt.%. In a patent, No. EP 508660, a process for preparation of HFC-32 was described using HCFC-22 as starting material. In a Japanese patent, No. JP 0601731, a process was described using platinum on activated carbon as hydrodechlorination catalyst for the production of HFC-32 wherein the conversion of CFC-12 was reported as 80% with a HFC-32 yield of 20%. However, Palladium/C is reported to be a more selective catalyst compared to platinum on carbon (platinum/C) with respect to HFC-32 selectivity. In a UK patent, PCT.Int. Appl WO 9617683, a process for the production of HFC-32 with high selectivity (~80%) over Palladium-platinum/carbon catalyst was disclosed. However, all these processes involve activated carbon as support material, and for an industrial application, the activated carbon may not be suitable because of its poor mechanical strength and high surface area contributed by the micro pores present in it. Moreover, activated carbon possesses variety of oxygen functional groups and also contains lot of impurities, which cannot be removed by simple techniques. Other patents, like ENCSM,34053 Montpellier (CA. No. 119:94820t), No. ENSCM, 8 reu 1' [Ecole Normale Montpellier, Fr.] (CA. No. 119: 94820t) deal with Palladium supported on a metal oxide or metal fluoride (i.e., AI2O3 or AIF3). However, oxide supports are not resistant to HF/ HCI which are produced during the course of reaction. Under these corrosive reaction conditions the acidity of the support changes as a consequence of which there may be loss of catalytic activity. The support may also be converted to a mixture of hydroxyfluoride, oxyfluoride and fluoride during the reaction. The
fluoride supports catalyze side reactions in hydrogenolysis leading to low yield of the selected product ( The main object of the present invention is to provide a method for the production of difluoromethane from dichlorodifluoromethane by selective vapour phase hydrogenolysis of CFC-12 at atmospheric pressure using an improved catalyst.
Yet another object of the present invention is to propose a method wherein the activity and selectivity of the catalyst is higher compared to the conventionally prepared AI2O3_supported and also the carbon supported Palladium catalysts which are used under atmospheric operation.
Still another object of the present invention is to propose a process wherein the catalyst will have longer life compared to the alumina and the carbon supported catalysts.
These and other objects of this invention are apparent from the following paragraphs.
The catalysts used in this invention is a subject matter of our copending application No. 537/Det/99
In our intensive research and experimental studies we have found that by depositing carbon in its pure form by means of pyrolysis of hydrocarbons like cyclohexene and fluorobenzene on the surface of Al2O3, it is possible to prepare a support- (i) which possesses good mechanical strength, (ii) contains carbon in its purest form, (iii) avoids the presence of impurities normally associated with activated carbons and (iv) avoids micro-pores, normally present in activated carbon, which reduce the efficiency of the catalyst. This catalysts, prepared by the deposition of palladium on the carbon covered alumina and acidic carboncovered alumina supports, were found to be better active and selective than the Palladium catalyst prepared using alumina or carbon alone as support.
Further investigations have revealed that by a simultaneous carbon deposition and fluoride interaction, which is possible by pyrolysis of fluorobenzene, pure carbon can be deposited and the formation of an active fluoride can be avoided. The high temperature pyrolysis method produced an inactive fluoride which is required to stabilize the active species, namely Palladium, by not allowing it to diffuse into the bulk of the catalyst. Thus, the new support contains moderate acidity imparted by the pyrolysis of fluorine containing hydrocarbon on the alumina surface and is best suited for the
Deposition of Palladium. A detailed description of the preparation procedure to obtain the support mentioned above is given in a co-pending patent application. No. 537/Del/99
Accordingly, the present invention provides an improved process for the production of difluoromethane (HFC-32) which comprises subjecting the dichlorodifluorornmethane (CFC-12) to conventional vapor phase hydrogenolysis such as herein described, in presence of a novel Pd supported carbon covered alumina catalyst such as herein described and recovering HFC-32 by conventional distillation methods.
An embodiment of the present invention is to improved process wherein the GHSV during hydrogenylosis is in the range of 2000-5000 h1.
Another embodiment of present invention, is a process wherein the hydrogen partial pressure is in the range of 4-12.
Yet another embodiment of present invention, is a process wherein the hydrogenolysis is effected at 200 - 300°C.
The embodiment of the present invention, is a process wherein the hydrogen halides (HF & HCI) produced during the reaction have been scrubbed by bubbling the product mixture through a potassium hydroxide (KOH) saturator.
Another embodiment of the present invention is to describe a process wherein the drying of the product mixture is carried out by passing it through a molecular sieve drier.
Yet another embodiment is to describe a process wherein the reaction temperature is in the range 200 to 300°C, preferably 220 to 280°C which gives best results.
Still another embodiment of the invention is to describe a process wherein the ratios of H2/CFC-12 is in the range of 1 to 20, preferably 2-15, to get best results.
Supported Palladium catalysts were prepared by taking different commercial supports like Al2O3 and activated carbon. These catalysts are designated as CATALYST - A and CATALYST - B for Al2O3 and carbon supported catalysts respectively. Another two series of catalysts were prepared by using carbon-covered alumina and acidic carbon- covered alumina as supports and are designated as CATALYST - C and CATALYST - D respectively. The detailed preparation procedure is given in a co-pending application. In all these catalysts the Palladium loading was maintained in the range of 2-6 wt.%, preferably in the range of 2-4 wt% with respect to the support. These catalysts were evaluated in a micro-flow on-line reactor interfaced with a gas chromatograph (GC) equipped with thermal conductivity detector/flame ionisation detector ( TCD/FID). 0.5 - 4 g of the catalyst was placed in the micro flow reactor and reduced in hydrogen medium at a temperature of 300-500°C, preferably in the range of 300-400°C with a flow rate of 30-80 cc/min, preferably in the range of 30-40 cc/min for 4-6 h. After attaining the reaction temperature the feed containing CFC-12 and hydrogen was passed on to the catalyst bed at a GHSV of 4800 h-1. The product mixture,
before entering to the GC, was scrubbed with alkali to remove HCI/HF produced during the reaction. The HF/HCI free product mixture was analyzed by the GC at regular time intervals.
The present invention is described with reference to the following examples that are explained by way of illustrations only and should not therefore be construed to limit the scope of the present invention.
0.5 g of CATALYST - C was taken to study the effect of space velocity (GHSV) on the rate of hydrogenolysis of CFC-12 and the selectivity towards HFC-32. N2 gas was optionally used to change the GHSV with fixed feed ratios. The reaction temperature was maintained at 250°C. The results are given in Table!
Table 1: Effect of space velocity (GHSV) on the hydrogenolysis of CFC-12
In order to understand the effect of feed ratio (H2/CFC-12 ) on the activity and selectivity of the catalyst at a constant GHSV (4200 h*1), experiments were
carried out on CATALYST - C. The reaction temperature was maintained at 250°C ( Table 2).
Table 2: Effect of feed ratio on the activity and selectivity of the catalyst
The temperature effect was studied for all the catalysts i.e., CATALYST-A CATALYST-B, CATALYST-C, CATALYST-D under similar conditions (GHSV=4800 h"1 and H2 / CFC = 8). The results are given in Tables 3-6.
Table 3. Effect of temperature on the performance of CATALYST - A.
Hydrogenolysis of CFC-12 over CATALYST - D has been studied continuously for 12 h at 240 (C and the product mixture has been analyzed at regular intervals after scrubbing in KOH, followed by drying, by passing through molecular sieves. The time on stream analysis is given in Table 7. Table 7: Time on stream analysis
1. An improved process for the production of difluoromethane (HFC-32) which comprises subjecting the dichlorodifluorornmethane (CFC-12) to conventional vapor phase hydrogenolysis such as herein described, in presence of a novel Pd supported carbon covered .alumina catalyst such as herein described and recovering HFC-32 by conventional distillation methods.
2. An improved process as claimed in claim 1 wherein hydrogenylosis is carried out at GHSV (gas hourly, space velocity) in the range of 2000-5000 h-1.
3. An improved process as claimed in claims 1&2, wherein hydrogenylosis is carried out at a hydrogen partial pressure is in the range of 4-12.
4. An improved process as claimed in claims 1 to 3, wherein the hydrogenolysis is effected at 200 - 300°C.
5. An improved process for the production of difluoromethane (HFC-32) substantially as herein described with reference to the examples.
|Indian Patent Application Number||536/DEL/1999|
|PG Journal Number||10/2011|
|Date of Filing||08-Apr-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001,INDIA|
|PCT International Classification Number||C07C 17/10|
|PCT International Application Number||N/A|
|PCT International Filing date|