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

The direct conversion of methane to aromatics such as benzene and naphthale ne 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 y ield of aromatic products, achieving over 70% total selectivity of the hydr ocarbons 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 hydro carbon products on the other Mo-supported catalysts, which are drastically deactivated, owing to serious coke formation. The XANES/EXAFS and TG/DTA/ma ss studies reveal that the zeolite-supported Mo oxide is endothermally conv erted 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 whic h initiates the methane aromatization yielding benzene and naphthalene at 8 73-1023 K. Although both Mo2C and HZSM-5 support alone have a very low acti vity for the reaction, physically mixed hybrid catalysts consisting of 3 wt % Mo/SiO2 + HZSM-5 and Mo2C + HZSM-5 exhibited a remarkable promotion to en hance the yields of benzene and naphthalene over 100-300 times more than ei ther component alone. On the other hand, it was demonstrated by the IR meas urement in pyridine adsorption that the Mo/HZSM-5 catalysts having the opti mum SiO2/Al2O3 ratios, around 40, show maximum Bronsted acidity among the c atalysts with SiO2/Al2O3 ratios of 20-1900. There is a close correlation be tween the activity of benzene formation in methane aromatization and the Br onsted acidity of Mo/HZSM-5, but not Lewis aciditiy. It was found that maxi mum benzene formation was obtained on the Mo/HZSM-5 having SiO2/Al2O3 ratio s of 20-49, but substantially poor activities on those with SiO2/Al2O3 rati os smaller and higher than 40. The results suggest that methane is dissocia ted on the molybdenum carbide cluster supported on HZSM-5 having optimum Br onsted acidity to form CHx (x > 1) and C-2-species as the primary intermedi ates which are oligomerized subsequently to aromatics such as benzene and n aphthalene at the interface of Mo2C and HZSM-5 zeolite having the optimum B ronsted acidity. The bifunctional catalysis of Mo/HZSM for methane conversi on towards aromatics is discussed by analogy with the promotion mechanism o n the Pt/Al2O3 catalyst for the dehydro-aromatization of alkanes. (C) 1999 Academic Press.