(297b) Novel Oxidation Pathway for Low Temperature Conversion of Methane to Methanol: a Spectroscopic Investigation

Cobalt ZSM-5 had been reported to convert methane to methanol at temperatures 200C and below.  However, the products had to be extracted as they were purported to adsorb very strongly on the catalyst.¹   Here we report the use of DRIFT spectroscopy to understand the oxidation of CH₄ over Co-ZSM-5.   The catalyst was 2 wt.% Co impregnated onto Na-ZSM-5 ( Si/Al=25).  It was observed that at 200C the cobalt oxo species can react with methane to form surface adsorbed species even in the absence of gas phase oxygen.   However, the intensities of the IR bands of the adsorbed products increased with the inclusion of oxygen in the reaction feed.  These oxidation products included methoxide, formate and formaldehyde oligomers.  It was also observed that after CH₄ oxidation reaction, the prominent Co-formate band at 1578 cm^-1 continued to increase with time on stream in a He stream.   It was proposed that the formaldehyde oligomers underwent a self oxidation-reduction reaction (Cannizzaro reaction) in the presence of water to form adsorbed methoxide and formate.  The proposed reaction pathway was verified when a pulse of water after the CH₄ reaction markedly accelerated the formate formation rate.  Another observation consistent with this proposed pathway is that IR bands associated with adsorbed methoxide and formate were detected upon heating adsorbed formaldehyde to 200C.  The existence of an another viable pathway in addition to direct oxidation of CH₄ is very interesting as it opens up many possibilities both in catalyst formulation and reaction conditions to tune the selectivities of CH₄ oxidation reaction.

1. Benzis  et. al., Catal. Lett. 136 (2010) 52