Effect of organic impurities on the hydrocarbon formation via the decomposition of surface methoxy groups on acidic zeolite catalysts

The possible effects of traces of organic impurities in methanol on the formation of hydrocarbons by surface methoxy groups on acidic microporous catalysts were studied under batch conditions. For this purpose, surface methoxy groups were prepared on zeolite H-Y and silicoaluminophosphate H-SAPO-34 from C-13-enriched methanol (C-13-enrichment of 99%, chemical purity of 98+%, with total organic impurities of ca. 1000 ppm) and highly purified nonenriched methanol (organic impurities < 30 ppm), respectively. The conversion of these surface methoxy groups on thermal treatments at 473-673 K was investigated by the combined C-13 MAS NMR-UV/vis spectroscopy. It was found that the formation of primary aromatics and carbenium ions on the methylated zeolite catalysts occurs at identical reaction temperatures from the conversion of both C-13-enriched and nonenriched surface methoxy groups. C-13 MAS NMR-UV/vis studies of H-SAPO-34 catalyst loaded with ethanol and acetone show that a coverage of at least 0.1 molecule per bridging OH group is essential to obtain aromatics and carbenium ions in a similar fashion as those formed by the conversion of surface methoxy groups; however, this coverage is at least two orders of magnitude higher than the ethanol and acetone content present as trace organic impurities in the aforementioned methanol feeds. These findings indicate that traces of organic impurities present in the methanol do not govern the formation of primary hydrocarbons from surface methoxy groups. (c) 2005 Elsevier Inc. All rights reserved.