On the impact of co-feeding aromatics and olefins for the methanol-to-olefins reaction on HZSM-5

摘要

The impact of adding various aromatic molecules (benzene, toluene, and xylenes) or olefins (ethene, propene, 1-butene, 1-pentene, and 1-hexene) to methanol over a HZSM-5 catalyst on activity and selectivity was systematically studied. Addition of a low concentration of aromatic molecules (16-32 C%), which are free of diffusion constraints, significantly enhanced the aromatics-based catalytic cycle and greatly suppressed the olefin-based cycle. This led to enhanced methane and ethene formation and methylation of aromatic rings at the expense of propene and C_(4+) higher olefins. The ratio of propene to ethene is controlled by the concentration of the aromatic molecules added. Co-feeding the same molar concentration of benzene, toluene and p-xylene influenced the methanol conversion to a nearly identical extent, as none of them experience transport constraints and the methylation rapidly equilibrates the aromatic molecules retained in the pores. In stark contrast, addition of small concentrations (10-40 C%) of C_(3-6) olefins with 100 C% methanol does not selectively suppress the catalytic cycle based on aromatic molecules. This led to unchanged selectivities to ethene and higher olefins (C_(3+)). Within the C_(3+) fraction, the selectivity to propene decreased and the selectivity to butenes were enhanced with increasing concentration of the co-fed olefin. Because of the relatively fast rates in methylation and cracking of C_(3-6) olefins in the olefin-based cycle, the product distributions at high methanol conversion were identical when co-feeding C_(3-6) olefins with the same carbon concentrations. This work provides further insights into the two distinct catalytic cycles operating for the methanol conversion to produce ethene and propene over HZSM-5 catalysts.