INTERACTION OF METHANE WITH SURFACES OF SILICA, ALUMINAS AND HZSM-5 ZEOLITE - A COMPARATIVE FT-IR STUDY

Infrared investigations on the interaction of methane with silica, alu minas (eta, gamma and alpha) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over thes e oxides and HZSM-5 zeolite. Our findings featured that the infrared i nactive upsilon(1) band (2917 cm(-1)) of a gaseous methane molecule be came active and shifted to lower frequencies (2900 and 2890 cm(-1)) wh en it adsorbed on the surfaces of these adsorbents. Our results also d emonstrate that hydroxyl groups played a very important role in methan e adsorption over the acidic oxides and the HZSM-5 zeolite. When inter action between the hydroxyl groups and methane took place, the band sh ift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-O H-Al > Al-OH > Si-OH, which is in accordance with the order of their a cidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, gamma-alumina and HZSM-5 zeolite could exc hange with CD4 at temperatures higher than 773 K, while those on eta-a lumina could exchange at a temperature as low as 573 K. Another intere sting observation was the formation of formate species over Al2O3 (bot h eta and gamma) at temperatures higher than 473 K. The formate specie s would decompose to CO2, or produce carbonate at much higher temperat ures. Formation of formate species was not observed over silica and HZ SM-5 under similar conditions. alpha-Al2O3 did not adsorb or react wit h methane in any case.