The effect of calcination temperature on the catalytic performance of 2 wt.% Mo/HZSM-5 in methane aromatization

We observed that, at a calcination temperature (T-c) of 500 or 700 degreesC, the catalytic performance of 2 wt.% Mo/HZSM-5 for methane aromatization was compatible with that reported in the literature. When T-c was set at or above 750 degreesC, the catalyst deactivated and ethylene was the dominant product. The results of our analytic studies suggested that at T-c = 500 degreesC, the Mo species were not uniformly distributed, but existed as microcrystalline MoO3, polymolybdate, and other Mo entities. At T-c = 700 degreesC, Mo species were dispersed on the external surface as well as diffused into the channels of the zeolite. At or above a calcination temperature of 750 degreesC, dealumination as well as reduction in crystallinity of the zeolite became considerable. The results of atomic absorption spectrophotometric analysis confirmed that the loss of Mo during calcination was insignificant. With the partial destruction of HZSM-5 zeolite and the disappearance of Bronsted acid sites, the 2 wt.% Mo/HZSM-5 material ceased to function as a catalyst for methane aromatization. (C) 2002 Elsevier Science B.V. All rights reserved.