Nonoxidative dehydrogenation and aromatization of methane over W/HZSM-5-based catalysts
- 化学化工－已发表论文 
Highly active and heat-resisting W/HZSM-5-based catalysts for nonoxidative dehydro-aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W-H2SO4/HZSM-5 catalyst prepared from a H2SO4-acidified solution of ammonium tungstate (with a pH value at 2-3) displayed rather high DHAM activity at 973-1023 K, whereas the W/HZSM-5 catalyst prepared from an alkaline or neutral solution of (NH4)(2)WO4 showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of(WO6)(n-) groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H-2-TPR results showed that the reduction of precursor of the 3% W-H2SO4/HZSM-5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W5+ and W4+, whereas the reduction of the 3% W/HZSM-5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W5+. The results seem to imply that the observed high DHAM activity on the W-H2SO4/HZSM-5 catalyst was closely correlated with (WO6)(n-) groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W-H2SO4/HZSM-5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W-1.5% Zn-H2SO4/HZSM-5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV = 1500 ml/(hg-cat.), methane conversion (X-CH4) reached 23% with the selectivity to benzene at similar to 96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used.