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

Abstract

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 (16–32C%), 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 C4+ 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 (10–40C%) of C3–6 olefins with 100C% methanol does not selectively suppress the catalytic cycle based on aromatic molecules. This led to unchanged selectivities to ethene and higher olefins (C3+). Within the C3+ 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 C3–6 olefins in the olefin-based cycle, the product distributions at high methanol conversion were identical when co-feeding C3–6 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.