Robert G. Bowman

The reaction between hydrogen and carbon monoxide on catalysts derived from Mo(CO)6Al2O3

Abstract If Mo(CO) 6 /(dehydroxylated alumina, γ− + δ-Al 2 O 3 ) is activated in helium to 500 °C, clusters of Mo about 4 nm in diameter are formed. They are contaminated with some retained carbon which is essentially completely removed as methane by hydrogen at 650 °C. The average oxidation number of the molybdenum is about +0.3 and it is suggested that the clusters consist of Mo 2+ plus Mo(0) in which the charge is delocalized and balanced by AlO − groups at the surface. The clusters are not identical with metallic molybdenum since they do not chemisorb nitrogen. Materials so prepared exhibit a turnover frequency for formation of methane at 300 °C of about 0.04 sec −1 per surface atom of molybdenum in flowing hydrogen plus carbon monoxide (mole ratio = 5). Smaller amounts of ethane and propane are also formed. Activation at 300–500 °C in helium or hydrogen leads to similar turnover frequencies and selectivities. Activation of Mo(CO) 6 /(alumina dehydroxylated at 475 °C) at 300 °C in hydrogen or helium results in a material in which Mo is ionic and highly dispersed. Turnover frequencies for formation of methane at 300 °C are about 0.001 sec −1 per atom of molybdenum and selectivities are about the same as for the catalyst on dehydroxylated alumina. Activation in hydrogen results in formation of about 1.4 molecules of methane per molecule of Mo(CO) 6 and the resulting water, in essence, further oxidizes the molybdenum beyond the Mo 2+ formed initially by reaction of Mo(0) with surface hydroxyl groups, Mo + 2AlOH 8 → Mo 2+ + H 2 + 2AlO 8 post− .

The hydrogenolysis of propane on Mo/Al2O3 catalysts

Metallic molybdenum on dehydroxylated alumina (prepared by impregnating Mo(CO)6 onto alumina which had been dehydroxylated in flowing helium at 950 °C and then treating the Mo(CO)6Al2O3 by He,300–500 °;H2,650–950 °) is one of the more active catalysts for the hydrogenolysis of propane. Its turnover frequency per surface Mo atom at 250 °C is ~0.2 sec−1 at P = l atm and C3H8H2 = 0.038. Both single and double hydrogenolyses occur simultaneously, the latter having the larger apparent activation energy. MoAl2O3 catalysts with higher oxidation numbers of molybdenum are less active. Mo(0)Al2O3 is easily poisoned for hydrogenolysis by O2, CO, H2O, and even to some extent by N2. Initial activity is restored by H2 at ~950 °C. MoO3/hydroxylated alumina is reduced by H2 at 650 °C to an average ON (oxidation number) of Mo of +2 and by 800–950 °C to an ON of zero. The catalytic activities of these materials for the hydrogenolysis of propane at 200–300 °C, the hydrogenation of propylene at −46 °C, the metathesis of propylene at 65 °C, and the hydrogenolysis of cyclopropane at 0–100 °C are very nearly the same as those of Mo2+Al2O3 and Mo(O)Al2O3 made from Mo(CO)6Al2O3.

Hydrogenation of propylene and hydrogenolysis of cyclopropane on activated Mo(CO)6/Alumina catalysts

Abstract The hydrogenation of propylene and the hydrogenolysis of cyclopropane has been studied on Mo(CO) 6 Al 2 O 3 activated in flowing He or H 2 . The γ-alumina was either partially dehydroxylated in flowing He at 475 °C (PDA) or nearly completely dehydroxylated at 950 °C (DA). In a pulse reactor with hydrogen carrier, Mo(CO) 3 PDA converted propylene both to ethylene + 2-butene (metathesis) and to propane (hydrogenation). Activation in He or in H 2 to 300 °C (average ON ∼- +2 and +3, respectively) greatly enhanced the activity of Mo(CO) 3 PDA for hydrogenation which had to be measured in a flow reactor at −46 °C. Mo(CO) 6 DA activated to 300–500 °C in He or in H 2 (average ON near zero) was an even more active catalyst and heating DA;He,300 ° in H 2 to 500 °C led to a still more active catalyst. Hydrogenation was approximately zero order in propylene and first order in hydrogen. The same materials catalyzed the hydrogenolysis of cyclopropane at 0–100 °C. DA;He,300 ° and PDA;He,300 ° were the most active catalysts. Both single and double hydrogenolysis occurred as initial reactions to form propane and methane + ethane, respectively. Double hydrogenolysis was favored by molybdenum in lower ON.