Experimental and kinetic modelling studies of methanol conversion to hydrocarbons over zeolite catalysts

Methanol conversion to hydrocarbons (MTH) over zeolite catalysts is investigated using transient and steady-state kinetic experiments, FTIR spectroscopy and kinetic modelling to: (a) describe the formation of the first C-C bond during the induction period, (b) investigate factors governing steady-state product distribution and (c) improve product yields. ZSM-5 catalysts with Si/Al ratios of 11.5, 25, 36 and 135 were characterised by SEM, XRD, TGA, nitrogen sorption, EDX and FTIR. Temperature programmed adsorption and desorption studies were conducted in a temporal analysis of products (TAP) reactor to study the preferential adsorption of methanol or dimethyl ether (DME). Desorption profiles were deconvoluted into two adsorption sites over ZSM-5 (Si/Al=135) and three adsorption sites over ZSM-5 (Si/Al=25 and 36). Molecular adsorption on the low temperature sites and dissociative adsorption on the medium and high temperature sites were observed. Higher activation energies of desorption were observed for DME (121 kJ mol-1) compared to methanol (112 kJ mol-1) over high temperature sites as validated by a transient kinetic model that shows that DME is the key oxygenate. The transformation of DME to primary olefins is studied using a novel step-response methodology in the TAP reactor. Overshoots depicted by methanol and water, S-shaped propylene profiles and a rapid DME rise followed by a slower rise occur during a 44 min induction period in a first step response cycle at 300 °C. With temperature increase to 450 °C, methanol, water and DME increasingly exhibit monotonic profiles while ethylene and propylene retain their S-shaped behaviour. Precursors such as dimethoxymethane, carbon monoxide and hydrogen reduce the induction period and increase the autocatalytic rate of propylene formation according to a proposed crystal nucleation model. The transformation of the first C-C bond is rate-limiting according to a transient kinetic model. On subsequent step response cycles, the induction period is eliminated. Several reaction families describe the complex steady-state product distribution from methanol. The olefin cycle (methylation, oligomerisation and cracking) controls product distribution with DME being 3.5 and 2.5 times more effective than methanol for olefin methylation and aromatic methylation chemistries respectively. A novel form of a structured reactor called zeolite minilith improves gasoline yields compared to zeolite powder while keeping pressure drop low. The formation of the first C-C bond from methanol has been debated for over 40 years. This thesis provides evidence for the direct formation of primary olefins from methanol in which DME is the key oxygenate and the transformation of the first C-C bond is the major bottleneck during the induction period. At steady-state, the olefin cycle regulates product distribution and DME is the key methylating agent. Substantial operating cost reduction can be obtained using structured reactors such as zeolite miniliths while improving gasoline yield.