Catalysis of Organic..

Canning, Gamman, Jackson and Urquart 73At 673 K the nickel catalysts had low selectivity to MIBK (< 50 %) and asignificant contribution to this loss in selectivity was due to hydrogenation ofMIBK to MIBC. Indeed with the Ni-KOH/silica catalyst the yield of MIBC wasgreater than the yield of MIBK. This behaviour was to be expected as nickel isused industrially to hydrogenate 2-ethyl hexenal, the intermediate in the OXOalcoholprocess to make 2-ethyl hexanol (1), where both alkene and carbonyl arerequired to be hydrogenated. The ratio of MIBK:MIBC at 673 K was 1.4:1 forthe Ni-CsOH catalyst and 0.9:1 for the Ni-KOH catalyst compared to ~10:1 forboth catalysts at 473 K. This change in ratio with temperature is likely to be dueto different activation energies for C=C and C=O hydrogenation.It is noticeable that over the nickel catalysts, isophorone was a majorproduct. The production of isophorone and the small quantities of other byproductsonce again revealed formation of the three intermediate aldol productsof the reaction between MO and acetone. However it is clear that hydrogenationwas not as facile over the nickel catalysts as it was over the palladium catalysts.Hence there was more secondary and tertiary aldol condensation. To furtherinvestigate this a reaction was performed at 573 K using the Ni-KOH/silicacatalyst with nitrogen as the carrier gas rather than hydrogen. The results areshown in Table 1.Table 1. Comparison of selected reaction product yields (%) in N 2 carriercompared to H 2 carrier over Ni-KOH/silica at 573 K.Ni-KOH/SiO 2 MO MIBK MIBC IsophoroneNitrogen 2.1 0 0 1.2Hydrogen 0.1 2.8 0.1 0.2Clearly there is a switch from MO to MIBK when hydrogen was made thecarrier gas. The yield of isophorone dropped as at this temperature thehydrogenation intercepted the MO before it could undergo another aldolcondensation. At 673 K the aldol condensation reaction became kineticallymore competitive with the hydrogenation reaction.Over the temperature range studied the activity was controlled by the basecomponent of the catalyst. The conversion observed with the Ni-KOH catalystwas approximately the same as that observed with the Pd-KOH catalyst; whilethe CsOH catalysts were more active at both temperatures.In conclusion, four catalyst systems, Pd-KOH/silica, Pd-CsOH/silica, Ni-KOH/silica, and Ni-CsOH/silica, have been investigated for the conversion ofacetone to MIBK. Systems highly selective to MIBK have been obtained (Pd-CsOH/silica, 100 % at 473 K). Both the metal and the base affected the productdistributions. The nickel catalysts were generally less selective, with MIBKbeing further hydrogenated to MIBC and isophorone becoming a major productat high temperatures. Over Ni-CsOH/silica, a selectivity of around 30% was