Methanol-to-hydrocarbons reaction over SAPO-34. Molecules confined in the catalyst cavities at short time on stream

In order to obtain more insight into the methanol-to-hydrocarbons (MTH) reaction the organic molecules confined in a working SAPO-34 catalyst have been studied. The reaction was run for varying times, 30 s to 30 min. At a predetermined time the reaction was stopped and the catalyst dissolved in 1 M HCl. The gas phase above the solution as well as a CCl₄ extract were analyzed by gas chromatography (MS detector). The gas phase consists mainly of isoalkanes C_4–6. The less volatile organic molecules were concentrated in the CCl₄ extract. More than 200 different species are present, but polymethylbenzenes, with one to six methyl groups, are always dominating. They constitute 30–50% of the samples. Penta- and hexamethylbenzene easily split off small hydrocarbons and turn into di- and trimethylbenzenes. It is speculated that methylations of arenes which thereupon split off small alkenes and then are remethylated again may be an essential part of the catalytic activity in the MTH reaction.