NONOXIDATIVE DEHYDROGENATION AND AROMATIZATION OF METHANE OVER W HZSM-5-BASED CATALYSTS/

Highly active and heat-resisting W/HZSM-5-based catalysts for nonoxida tive dehydro-aromatization of methane (DHAM) have been developed and s tudied. It was found from the experiments that the W-H2SO4/HZSM-5 cata lyst prepared from a H2SO4-acidified solution of ammonium tungstate (w ith 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(W O6)(n-) groups constructing polytungstate ions in the acidified soluti on of ammonium tungstate. The H-2-TPR results showed that the reductio n of precursor of the 3% W-H2SO4/HZSM-5 catalyst may occur at temperat ures below 900 K, producing W species with mixed valence states, W5+ a nd W4+, whereas the reduction of the 3% W/HZSM-5 occurred mainly at te mperatures 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-) g roups with octahedral coordination as the precursor of catalytically a ctive species. Incorporation of Zn (or La) into the W-H2SO4/HZSM-5 cat alyst has been found to pronouncedly improve the activity and stabilit y 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 s electivity to benzene at similar to 96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used.