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 °C to 460 °C over H-ZSM-5 catalyst with a Si/Al 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 C4=–C7= likely.

► Reaction conditions affect product distribution in MTO evidently at low conversions.
► The main reaction pathway in MTO changes with increasing methanol conversion.
► C3= selectivity is increased at higher methanol conversions through cracking of C6=.
► C2= selectivity is increased only at high space timevia over-cracking of olefins.