The Mechanism of Methanol to Hydrocarbon Catalysis

Abstract

The process of converting methanol to hydrocarbons on the aluminosilicate zeolite HZSM-5 was originally developed as a route from natural gas to synthetic gasoline. Using other microporous catalysts that are selective for light olefins, methanol-to-olefin (MTO) catalysis may soon become central to the conversion of natural gas to polyolefins. The mechanism of methanol conversion proved to be an intellectually challenging problem; 25 years of fundamental study produced at least 20 distinct mechanisms, but most did not account for either the primary products or a kinetic induction period. Recent experimental and theoretical work has firmly established that methanol and dimethyl ether react on cyclic organic species contained in the cages or channels of the inorganic host. These organic reaction centers act as scaffolds for the assembly of light olefins so as to avoid the high high-energy intermediates required by all "direct" mechanisms. The rate of formation of the initial reaction centers, and hence the duration of the kinetic induction period, can be governed by impurity species. Secondary reactions of primary olefin products strongly reflect the topology and acid strength of the microporous catalyst. Reaction centers form continuously through some secondary pathways, and they age into polycyclic aromatic hydrocarbons, eventually deactivating the catalyst. It proves useful to consider each cage (or channel) with its included organic and inorganic species as a supramolecule that can react to form various species. This view allows us to identify structure-activity and structure selectivity relationships and to modify the catalyst with degrees of freedom that are more reminiscent of homogeneous catalysis than heterogeneous catalysis.

Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

“…Figure 4 depicts the induction periods (black) and active periods (red) of catalysts deactivated by the three different methanol purities as measured by on‐line mass spectrometry. Feedstock impurities clearly have an effect on the induction period as previously reported 40, 61, 62, 63, 64. The results summarized in Figure 4 show that methanol feedstock (U), containing very few impurities, has the longest induction and active periods.…”

A research into the thermodynamics of methanol to hydrocarbon (MTH): conflictions between simulated product distribution and experimental results

“…2b, c). Our simulation well supports the real experimental observations that higher pressure and a lower temperature may benefit the yield of aromatics [2,11,17,18].…”

“…Like other researches, in our experiments methanol conversion mainly yield large portion of gas alkanes, gas olefins, as well as aromatics [4,5,11]. This is owing to the continuous flow of methanol feedstock and the non-stop transportation of previous formed products to the outside of the system (before the catalyst deactivates); in other words, the reaction happens in an opening system, which greatly reduces the chances of coke formation.…”

“…Although the MTH mechanism is in a great complexity, thermodynamics are not affected by the reaction routes. The selected 25 reactions have covered all the directions from methanol to the major MTH products (C 1 -C 9 ) with essential product intra-conversions, and are supposed to happen in a real MTH system proceeding in timeon-stream [11,12]. Particularly, we have further separated the isomers of xylene and tri-methylbenzene, for a more comprehensive study than the previous works, and a consideration that para-xylene might be the most favourable product in the methanol to aromatics process via H-ZSM-5 zeolite [4,8,9].…”

“…However, our assumption is based on the 'hydrocarbon-pool' cycle, which has been considered as the origin of series of MTH reactions and gives C 2 -C 4 olefins and methyl-aromatics as the primary products [4]. Further consideration is given to the fact that latter stage hydrogen transfer may convert these primary products into either saturated hydrocarbons (alkanes) or graphite species (deeply dehydrogenated cokes), and these intra-conversions indeed play an important role in deciding the following catalytic process, and the final product yields [3,4,11,12]. This simulation also takes the advantage that all of these 'hydrocarbon-pool' primary products (C 2 -C 4 olefins and C 6 -C 9 aromatics) take a large portion in the product yield of short time (a few minutes to several hours) lab reactions and industrial applications [4,5].…”