S.Y. Lai

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 (Tc) of 500 or 700 ◦ C, the catalytic performance of 2 wt.% Mo/HZSM-5 for methane aromatization was compatible with that reported in the literature. When Tc was set at or above 750 ◦ C, the catalyst deactivated and ethylene was the dominant product. The results of our analytic studies suggested that at T c = 500 ◦ C, the Mo species were not uniformly distributed, but existed as microcrystalline MoO3, polymolybdate, and other Mo entities. At T c = 700 ◦ C, 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 ◦ C, 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.

Methane aromatization over 2 wt% Mo/HZSM-5 in the presence of O2 and NO

In the non-oxidative aromatization reaction (temperature = 770 C, flow rate = 34 ml min-1), 2 wt% Mo/HZSM-5 deactivated after 4 h due to severe coking. We observed that with a suitable amount of O2 (≤5.3 vol%) in the methane feed, the catalyst could last for more than 6 h with a ca. 4% yield of aromatics at 770 °C. Depending on the concentration of O2 or the reaction temperature, there are three reaction zones in the catalyst bed: (i) methane oxidation; (ii) methane reforming; and (iii) methane aromatization. CO and H2 produced in the first two zones are accountable for stability amelioration of the catalyst. The addition of NO exhibited similar effects on the reaction. Further increase in O2 (≥8.4 vol%) or NO (≥14.2 vol%) concentration would result in CO and CO2 being the predominant carbon-containing products; C2H4 and C2H6 were generated in small amounts and no aromatics were detected.

The oxidative coupling of methane overBa/CO3LaOCl catalysts

Abstract The performance of LaOCl at 800°C was promoted by BaCO 3 in OCM reaction. When 10 mol% BaCO 3 was added, there was little change in CH 4 conversion, but C 2 selectivity was increased from 37% to 66%. With the increase in BaCO 3 loading, the Ba/CO 3 LaOCl catalysts decreased in specific surface area. The improvement in C 2 selectivity is partly due to surface area diminution. The addition of BaCO 3 has also caused surface modification of LaOCl. In the absence of BaCO 3 , LaOCl in OCM reaction is capable of generating dioxygen species O 2 2− , O 2 n− and O 2 − , which can cause deep oxidation of CH 4 , C 2 H 6 and C 2 H 4 . With the addition of BaCO 3 , sites for the activation of oxygen molecules are depleted. The nearby Cl − ions have the ability of destabilizing the BaCO 3 , causing it to decompose at ca. 780°C, a decomposition temperature about 200°C lower than that of pure BaCO 3 . With the deprivation of dioxygen species, the CH 4 oxidative dehydrogenation process is enhanced, while deep oxidation processes are suppressed. Compared with the deep oxidation reaction processes, the number of sites required in the H-abstraction process is a lot fewer. Hence, although the specific surface area of LaOCl was reduced with the addition of BaCO 3 , the conversion of CH 4 over the Ba/CO 3 LaOCl catalysts did not drop significantly.