Bifunctional catalysis of Mo/HZSM-5 in the dehydroaromatization of methane to benzene and naphthalene XAFS/TG/DTA/MASS/FTIR characterization and supporting effects

The direct conversion of methane to aromatics such as benzene and naphthalene has been studied on a series of Mo-supported catalysts using HZSM-5, FSM-16, mordenite, USY, SiO2, and Al2O3 as the supporting materials. Among all the supports used, the HZSM-5-supported Mo catalysts exhibit the highest yield of aromatic products, achieving over 70% total selectivity of the hydrocarbons on a carbon basis at 5-12% methane conversion at 973 K and 1 atm. By contrast, less than 20% of the converted methane is transformed to hydrocarbon products on the other Mo-supported catalysts, which are drastically deactivated, owing to serious coke formation. The XANES/EXAFS and TG/DTA/mass studies reveal that the zeolite-supported Mo oxide is endothermally converted with methane around 955 K to molybdenum carbide (Mo2C) cluster (Mo-C, C.N. = 1, R = 2.09 Angstrom; Mo-Mo, C.N. = 2.3-3.5; R = 2.98 Angstrom which initiates the methane aromatization yielding benzene and naphthalene at 873-1023 K. Although both Mo2C and HZSM-5 support alone have a very low activity for the reaction, physically mixed hybrid catalysts consisting of 3 wt% Mo/SiO2 + HZSM-5 and Mo2C + HZSM-5 exhibited a remarkable promotion to enhance the yields of benzene and naphthalene over 100-300 times more than either component alone. On the other hand, it was demonstrated by the IR measurement in pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO2/Al2O3 ratios, around 40, show maximum Bronsted acidity among the catalysts with SiO2/Al2O3 ratios of 20-1900. There is a close correlation between the activity of benzene formation in methane aromatization and the Bronsted acidity of Mo/HZSM-5, but not Lewis aciditiy. It was found that maximum benzene formation was obtained on the Mo/HZSM-5 having SiO2/Al2O3 ratios of 20-49, but substantially poor activities on those with SiO2/Al2O3 ratios smaller and higher than 40. The results suggest that methane is dissociated on the molybdenum carbide cluster supported on HZSM-5 having optimum Bronsted acidity to form CHx (x > 1) and C-2-species as the primary intermediates which are oligomerized subsequently to aromatics such as benzene and naphthalene at the interface of Mo2C and HZSM-5 zeolite having the optimum Bronsted acidity. The bifunctional catalysis of Mo/HZSM for methane conversion towards aromatics is discussed by analogy with the promotion mechanism on the Pt/Al2O3 catalyst for the dehydro-aromatization of alkanes. (C) 1999 Academic Press.