Methyl iodide nickel-activated carbon

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

Table IV shows the reactivities of raw materials and products on a nickel-activated carbon catalyst and the effect of hydrogen on the reactions. When carbon monoxide and hydrogen were introduced into the catalyst, no product was formed. Thus, the hydrogenation of CO does not proceed at all. When methyl iodide was added to the above-mentioned feed, 43% of the methyl iodide was converted to methane. In the presence of methyl iodide small amounts of methane, methanol, and acetic acid were formed from methyl acetate, while small amounts of methane and acetic acid were also formed from acetic anhydride. Hydrogen fed with methyl acetate accelerated the formation of methane and acetic acid remarkably.

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

The formation of the active catalyst can be retarded with high carbon monoxide partial pressure. High CO partial pressure leads to more CO in solution which competes with the ligand over the tricarbonyl species, Ni(C0)3, and forms the inactive nickel tetracarbonyl. The active complex stability was retained by increasing the promoter concentration. The complex formed between nickel and promoters is more stable than Ni(C0)4. In addition, promoters may impart higher electron density to the central atom and increase its nucleophilic character towards methyl iodide. 

We have already reported that nickel supported on activated carbon exhibits an excellent activity for the vapor phase carbonylation of methanol in the presence of methyl iodide (Mel) at moderate pressures (14-16). In addition, corrosive attack of iodide compounds on reactors is expected to be minimized in the vapor phase system. 

Liquid phase carbonylation of methanol to acetic acid with a rhodium complex catalyst is a well known process (ref. 1). The authors have found that group 8 metals supported on carbonaceous materials exhibit excellent activity for the vapor phase carbonylation of methanol in the presence of iodide promoter(ref. 5). Especially, a nickel on active carbon catalyst gave acetic acid and methyl acetate with the selectivity of 95% or higher at 100% methanol conversion under 10 atm and 250 °C. In the present study it has been found that a small amount of hydrogen which is always contained in the commercially available CO and requires much cost for being removed completely, accelerates greatly the carbonylation reaction. 

The experiments were conducted in a fixed bed flow type reactor under pressurized conditions as have been reported in detail elsewhere (ref. 2). Methanol (MeOH) and methyl iodide (Mel) were mixed and fed with a high-pressure microfeeder. Catalysts were prepared by impregnating a commercially available granular active carbon (A.C., Takeda Shirasagi C, 20-40 mesh) with nickel acetate and drying at 120 °C for 12 h in an air oven. They were used without any further pretreatment. 

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