Methanol conversion to olefins (MTO) over H-ZSM-5: Evidence of product distribution governed by methanol conversion

Methanol conversion to olefins (MTO) was investigated in an isotherm fixed-bed reactor at methanol partial pressures ranging from 5 to 50 kPa, water/methanol ratios from 0 to 9 and temperatures from 380 degrees C to 460 degrees C over H-ZSM-5 catalyst with a Si/AI ratio of 200. Product distribution was affected by operation conditions remarkably only at low methanol conversions (<80%). When methanol conversion approaches 100%, product distribution is hardly affected by operation conditions. In addition, it was always found that propylene selectivity is enhanced at the expense of hexenes at high methanol conversions. The dependence of product distribution on methanol conversion derives from the change of dominant reaction pathway from olefin methylation and cracking to oligomerization and cracking. Product distribution at low methanol conversions may be determined by the relative rate between olefin methylation and cracking, while at complete methanol conversion product distribution is determined by oligomerization and cracking of C-4(=)-C-7(=) likely. (C) 2012 Elsevier B.V. All rights reserved.