Palladium Supported Catalysts in CO + RONO Reactions By X.-Z. Jiang Department of Chemistry, University of Zhejiang, Hangzhou, P. R. China In the past decade much effort has been This article is a summary of our recent studies directed towards the synthesis of dialkyl on supported catalysts in CO + and dialkyl carbonates directly from carbon MeONO (or EtONO) reactions at atmospheric monoxide and alcohols over palladium supported pressure in the vapour phase (6 - 8). It was found catalysts, under mild reaction conditions (I -4). that the reactions of RONO, Although the mechanism of the reactions has not where R represents a methyl or ethyl group, been fully elucidated, the key reactions can be were very sensitive to the support, and that the illustrated by the following equations: main product was particularly dependent on the ,0-R nature of the support, as illustrated in Figure I. CO + 2RONO O=C + 2NO (i) -. Monocarbonylation ‘0- R 0 = C-OR The monocarbonylation of or ethanol 2CO + 2RONO- I + 2NO (ii) can be achieved by introducing the correspon- 0 = C-0-R ding nitrites (, MeONO, with b.p. (where R represents an alcohol residue). -12OC, or ethyl nitrite, EtONO, with b.p. The conditions required for these chemical 17OC) which were prepared according to reactions to take place are quite mild; for exam- established procedures (9). The nitrite was then ple, dimethyl or diethyl carbonate and able to react directly with to can be efficiently prepared in the vapour phase form or diethyl carbonate by bringing carbon monoxide into contact with over fured catalyst beds of palladium supported methyl nitrite or ethyl nitrite, respectively, on on active carbon catalysts, in the gas phase and a palladium fixed bed catalyst, at atmospheric at atmospheric pressure. The reaction pressure and at a reaction temperature between temperature varied from 80 to 12oOC. Because 80- 12oOC. The reason that this reaction can be the alkyl nitrites are fairly sensitive to heat and carried out under such mild conditions could be ultraviolet irradiation ( IO), they decompose rationalised on the basis of the iso-electronic rapidly above I~oOC,but below IIOOC they are structure of +NOand CO, leading to a strong quite stable for a long period of time. The op- synergistic catalytic effect on the active centre timum reaction temperature for the formation of of palladium (5). On the other hand, the nitrogen dimethyl carbonate or diethyl carbonate from oxide liberated from the reaction in Equations (i) MeONO or EtONO, respectively, is in the or (ii) will generate alkyl nitrite in the presence region of 90 to I IoOC, the selectivity being about of alcohol and , as indicated in the 90 per cent and with a carbon monoxide conver- chemical Equations (iii) and (iv): sion about 25 per cent in a flow system with a differential reactor. The formation of carbon 2NO + Y202 - N203 (iii) dioxide was extremely low in the vapour phase. N,O, + 2ROH - 2RONO + H20 (iv) The nature of the active carbon support, which As a result, there is no loss of alkyl nitrite, was made from various kinds of raw materials, which appears to play a role in circulating the significantly affects the catalytic activity for reagent in the reaction system. It is therefore of dimethyl carbonate or diethyl carbonate forma- great interest to investigate the applicability of tion (6, 7). Kinetic studies revealed that the 2 these reactions to industrial processes. per cent palladiumkarbon catalyst, in which the

Platinum Metals Rev., 1990, 34, (4), 178-180 178 /OR 2"10 Pd/C 1%Pd/a-Alz03 O=C-OR I '=',OR c80-120*C, 1 atm e0-120*~,latm* O=C-OR Dial kyl- Dialkyl- I carbonate oxa late Fig. 1 The main product of the RONO + CO reactions was found to be dependent on the I nature of the support. R represents a methyl or ethyl group active carbon was made from coconut shell, ex- be achieved in the monocarbonylation of hibited relatively &her formation activities for methanol and ethanol over the catalysts M + 2 dimethyl carbonate (DMC) or diethyl carbonate per cent palladiundcarbon (where M represents (DEC), for instance rDMC = 15 mmol/gcat.h; lithium, copper or iron) at a pressure of I at- while for an active carbon support made from mosphere and temperature of 80 to IIOOC. coal rDMC = 9 mmol/gcat.h, and for an active carbon support made from wood rDMC = 4 Dicarbonylation mmol/gcat.h, all under the same reaction con- An alternative reaction which formed dialkyl ditions. The main reason for these differences oxalates from CO + RONO over a palladium in the catalytic activity may be attributed to the catalyst supported on a-alumina under at- dispersion of the palladium particles exposed on mospheric pressure and at reaction temperatures the surface of the catalyst. The average palladium between 8o-12o0C in the vapour phase, is il- particle sizes measured by transmission electron lustrated in Figure I. Where carbon monoxide microscopy (TEM) can be ranked as follows: couplug occurred to form carbon-carbon bonds, dmM = 70 hi for the coconut shell-made active the occurrence of the dicarbonylation reaction carbon supported palladium catalyst; dmM = was mainly attributed to a function of the sup- 180 A for coal-made support and dnM = 340 port, therefore, a-alumina was utilised for the hi for wood-made active carbon support. From support instead of active carbon (8, I I). An im- a comparison of the dispersions of the palladium portant feature of palladium catalysts supported particles with the corresponding catalytic activi- on a-alumina is a fairly low dispersion of the ty, it can be concluded that active carbon sup- palladium particles exposed on the surface of the ported catalysts with higher dispersions of catalyst. It may be due to the very small surface palladium particles demonstrate higher catalytic area of the a-alumina support, for example, the activity for the formation of dimethyl carbonate surface area will be less than 8 m'lg, and the and diethyl carbonate from CO + MeONO, or crystallites of a-alumina are quite large. EtONO, under the mild gas phase reaction con- After a comparison of a-alumina-supported ditions mentioned above. and active carbon-supported palladium catalysts In some cases, however, the competitive for- a valuable conclusion could be drawn: a-alumina- mation of dimethyl (or diethyl) oxalate with car- supported palladium catalysts with a very low bonate would occur, especially at reaction dispersion of the palladium particles favoured temperatures above I I o°C. This competitive dicarbonylation reactions for the formation of reaction gives rise to a selectivity problem. An alkyl oxalates from CO + RONO; while active attempt was made to direct the selectivity carbon-supported palladium catalysts with a fair- towards monocarbonylation, rather than carbon ly high dispersion of palladium particles favoured monoxide coupling, by the inclusion of a small the monocarbonylation reaction, forming dialkyl amount of additive, such as a salt containing carbonates as the dominant product, under the lithium, copper or iron (6). As a result of this same reaction conditions. No doubt this is due additive selectivitiesgreater than 90 per cent can to the effect of the support.

Platinum Metals Rev., 1990, 34, (4) 179 The addition of a small amount of iron, gallium of attention for prospective industrial application. or titanium promoter will increase the catalytic At present the production of diethyl oxalate from activity for the formation of dimethyl (or diethyl) CO + EtONO over M + Pd/a-Al,O, catalyst oxalate by 3 or 4 times (11). Therefore the syn- is being developed in a plant in Shanghai. It will thesis of dimethyl and diethyl oxalates by car- undoubtedly result in more industrial utilisation bon monoxide coupling in the vapour phase can of palladium catalysts in the manufacture of be successfully achieved over catalysts of M + valuable organic compounds, perhaps replacing I per cent palladium/a-alumina, with the selec- existing production methods. tivity of about 85 per cent, and with 35 per cent carbon monoxide conversion and 60 per cent References RONO conversion in the integrated reactors. I K. Nishimura, S. Uehiumi, K. Fujii and K. Nishihira, Am. Chem. SOC.,Prep. Div. Pet. Chan., The heat of reaction of the reaction of (-AH) 1979, 24, (I), 355 CO + RONO, Equation (ii), was estimated to 2 K. Nishimura, S. Uehiumi, K. Fujii and K. be 47 kcal/mol at IOOOC,therefore the design of Nishihira, U.K.Patent 2,003,872; 1979 an integrated reactor on an industrial scale must 3 U. Romano, R. Tesel, M. Massi Mauri and P. L. Rebora, Id.Eng. Chem., Prod. Res. Dev., 1980, pay much attention to the heat effect. 19, 396 So far the mechanisms of the reactions, Equa- 4 S. G. David and M. S. Staines, European Appl. tions (i) and (ii), have not been established. We 134,668; 1985 5 S.-Y. Xu, B. Xue, Z.-F. Lin and X.-Z. Jiang, 3. have proposed that a synergistic effect of the iso- Catal. (Dalian, China), 1989,10, (2), 187 (Chinese) electronic structure of +NOand CO might play 6 X.-Z.Jiang, Y.-B. Zhu and S.-Y.Xu, 3. Catal. an important role in the reaction procedure (6, (Dalian, China), 1989, 10, (I), 75 (Chinese) Sci. 8, I I). Nishimura and colleagues assumed that 7 Y.-B. Zhu and X.-Z.Jiang, Chin. Bull., 1989, 34, (IO), 875 an intermediate of alkoxy palladium, such as 8 X.-Z.Jiang and Y. Chen, Fine Chemicals, 1989, Pd(N0) (OR) is formed during the reaction (I, 6, (I), 37 (Chinese) 2); moreover Rivetti and Romano isolated alkoxy 9 A. H. Blatt, “Organic Syntheses, Collective Volume”, Wiley, London, 1950, Vol.2, p.363 carbonyl complexes of palladium (IZ), such as I0 K. Kornblum and E. P. Oliveto, 3. Am. Chem. Pd(COOCH,),(PPh,),. They reported that this SOC.,1949,71, 226; J. B. Levy, Id.Eng. Chem. complex was stable for several hours in methanol (Id.Ed.), 1956, 48, 762 under carbon monoxide at room temperature, I1 X.-Z.Jiang, Chinese Patent Applicahn 90103093.7 but quickly decomposed at 5ooC, and then I2 F. Rivetti and U. Romano, 3. Orgammer. Chem., 1978, 154, 323 dimethyl oxalate was detected in the solution. Clearly further investigation will be required to Destruction of Organochlorines explore the mechanisms for mono- or dicar- bonylation of alcohols. Many organochlorine molecules are persis- tent environmental poisons, and after use they must be disposed of most carefully. Although Conclusions high temperature incineration is the generally This article has briefly described novel ways accepted method of destroying the for mono- and dicarbonylations of methanol polychlorinated biphenyls, for example, con- and/or ethanol in synthesising the corresponding cern about incomplete oxidation remains. carbonates and oxalates over active carbon or Now, a report from the University of Sydney a- outlines a relatively low cost process for the alumina supported palladium catalysts in both electrocatalytic oxidative destruction of academic and industrial research laboratories. organochlorines (J. K. Beattie, Pure Appl. The introduction of methyl or ethyl nitrite as a Chem., 1990, 62, (6), 1145-1146). Complete circulating reagent, which experienced almost no oxidation to carbonate and chloride is achieved loss during the process, was a key feature of the using oxygen in alkaline solution, however, ox- ygen is too weak an oxidant to regenerate the novel method. In addition the mild reaction con- ruthenium tetroxide which is the active ditions and high catalytic activity and selectivi- catalytic species, but this can be achieved using ty of these reactions have attracted a great deal a small applied voltage.

Platinum Metals Rev., 1990, 34, (4) 180