Direct Vapor Phase Carbonylation of Methanol over NiCl2/C Catalyst

Vol.16No. 22000Chemical Research in Chinese Universities188 ~ 190Direct V apor Phase Carbonylation ofMethanol over NiCl2/ C CatalystPENG Feng( Department of Chernical Engineering , South China University of Technology，Guangzhou 510640 ,P. R. China ) .( Received Dec. 20 , 1999 )Keywords Methanol , Vapor phase carbonylation , Catalyst , Nickel chloride , Methyl acetateArticle ID 1005- 9040( 2000 )-02-188-03IntroductionThe carbonylation of alcohols via homogenous catalysis is important in manufacturingacetic acid and higher carboxylic acids and their esters 121. The main route to produce aceticacid is to make methanol carbonylated by means of the Monsanto and BP process in which ahomogeneous rhodium catalyst is used. Although the homogeneous carbonylation of methanolis a highly selective process ,it is affected by the disadvantages associated with a highly corro-sive reaction medium due to the use of methyl iodide as the promoter ,and the difficulty of theproduct separationt. The use of a heterogeneous catalyst seems very interesting and attrac-tive to us 41 , especially the direct vapor phase carbonylation of methanol without a halide pro-moter is of considerable importance and is strong incentive economically. There has , however ,been very little success in finding either heterogeneous or homogeneous catalysts that can cat-alyze the reaction effectively without the addition of a promoter 561. According to the knowncarbonylation mechanisnt781 ，the methyl iodide directly carbonylates with CO to fromMeCOI which interacts with methanol( MeOH ) to produce methyl acetate( MeCOOMe ) andHI ,and then MeOH reacts with HI to from CH3I. In fact , this carbonylation reaction is theindirect catalytic carbonylation of methanot9]. In this work , a novel catalyst for the direct va-por phase carbonylation of methanol without the addition of any halide in the feed as a promot-er was investigated. Compared to the known liquid phase methanol carbonylation process ,some advantages of this vapor phase reaction are as follows :( 1 )corrosive attack to reactors and pipes of iodide acid solution may be avoided ;(2 ) the separation of the products from the c中国煤化工H by a gas- solid sys-tem ;YHCNMHG( 3 ) the reaction can be carried out under the moderately pressurized condition ;* Supported by the National Natural Science Foundation of Chin( No.29903003 )and Guangdong Provincial Natural Sci-ence Found际市数据in( No.970438 ).No.2PENG Feng .189(4 ) the catalyst is cheap.ExperimentalNickel chloride was supported on activated carbon ( DaTong A. C. , coal base , specific .surface area 1 000 m2/g ,particle size 20- -40 mesh ) by incipient wetness impregnation with aloading of 10%. Prior to catalytic tests , the dried NiCl2/C catalyst was treated with nitrogenat 400 C for 2 h. Methanol carbonylation was carried out in a continuous flow system with afixed-bed reactor at a total operation pressure of 1.013X 105 Pa. The amount of the catalystused was 2 mL. The reaction products were analyzed by gas chromatography.Results and DiscussionTable 1 shows the effects of the reaction temperature on the activity and the selectivity.Table 1 The effect of reaction temperature on the methanol carbonylationReaction temperatureMeOH conversionSelectivitie( % )/C( %)CH4DMEMeCOOMe__ <( mol (kgCat)1 hl)20016.401.598.50.7922025. 49.591.4 ..1.1424030.80.116.383.71.2726035.80.324.675. 11.3328040.233.5.65.41.3030044.96.437.656.0 .1.24* Space time yield of methyl acetate. Reaction conditions: p= 1.013X 105 Pa ,GHSV=1200 L/h. LHSVMoH=0.20 .L/h.The data were collected with the catalyst on stream for 4 h. A maximum of carbonylationactivity was obtained at 260 C. The selectivity for methyl acetate was 75.1 % , and the space-time yield was 1.33 mol/( kg Cat h ). Higher temperature led to a sharp decrease of themethyl acetate selectivity and an increase of the selectivities of methane and DME( dimethylether ). Decreasing the temperature to 220 C，the methanol conversion was lowered to25.4% , but the methyl acetate selectivity was> 90%. In this case , only trace amounts ofmethane was formed and the space- time yield100of methyl acetate was 1.14 mol/( kg Cat h )The result is better than that of sulfied CoMo/C catalyst used in the literature[( 0.15 mol/子50(kgCat h)j6].Fig.1 demonstrates that the catalyst ac-S 25Rtivity remained a constant over 36 h at 250 Cwhile at this temperature the methyl acetate中国煤化工43036yield dropped rapidly to <1% over 6 h for theTYHCNMHGreported RhW 12PO4o/SiO2 catalyst 10].Fig. 1 The stability of NiCl/C catalyst.A series of NiCl2 catalysts was prepareda. X MeOH);b. S MeOMe);with alumina , silica and zeolitd( 3A ) supports.c. S MeCOOMe ) .Table 2瓦方数握rates that all the prepared cat-t=250 C ;GHSV=1 000 L/(kgcat h).190Chemical Research in Chinese UniversitiesVol.16alysts are significantly inferior to the activated carbon-supported catalyst , and the unsupportedNiCl2 catalyst is also inactive. The Ni/C catalyst was prepared by the reduction of NiC2/Cwith H2at 400 C for 2 h and the tests for methanol carbonylation activity were carried out asmentioned above. The results show that the methanol conversion was only 3% and methanewas the main product ,and there was no methyl acetate detected in the product mixture. Simi-lar results were obtained if the Ni/C catalyst was prepared with NK NO3 ) 6H2O.Table 2 The actities of the prepared catalysts for the methanol carbonylation'MeOH conversionSelectivitie( % )Catalyst(% )H DME MeCOOMeCH4 DME MeCOOMeNiC2/C35.80.3 24. 675. 1| NiC21.534.6 65.40NiCI2/AI2O32.841.755. 31.0| Ni/C95.2 4. 8NiC2/SiO21.778.221.8|NiCl2-Na/C23. 5010.289.8NiC2/3A2.550.9 48.50.6* Reaction conditions:t= 260 C ,p=1.013X 105 Pa ,GHSV=1 200 L/h, LHSVmoH=0.20 L/h.DME is the main by-product , demonstrating the acidic character of the NiCl2/C catalyst.The dehydration of methanol to DME is a well known reaction and occurs readily in the pres-ence of an acidic catalyst. In this work a NiCl2-Na/C catalyst was prepared by adding NaOHinto the NiCl/C catalys( n( Na )n(Ni)=2/3 ). From Table 2 we can see that NiCl2-Na/Chas a significantly higher selectivity of methyl acetate than NiCl2/C. The dehydration reactionof methanol to DME is substantially retarded due to the basicity of NaOH , but the catalyst ac-tivity decreases.At present , very few researches dealing with methanol carbonylation without a promoterhave been reported. The detailed mechanism of the methanol carbonylation reaction is notknown. The special effect of supporters on the methanol carbonylation activity is unclear. It ispossible that activated carbon is a conductive supporter which increases ion( electron ) transferaction within the catalyst. We have demonstrated that the mechanism of the direct vaporphase carbonylation of methanol was not similar to the mechanism in the homogeneous processcatalyzed by Rh in the presence of an iodide promoter. The further research is still going on.References[ 1 ] Dekleva,T. W. and Forster ,D. ,J. Mol. Catal. ,33 ,269 1985 )[ 2 ] Peng,F. and Huang,Z. T. ,ChineseJ. Chem. Eng. ,5 ,261( 1997)[ 3 ] Howard ,M. J. ,Jones ,M. D and Robberts ,M. S. ,Catal. 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