Mechanistic understanding of selectivity in methanol-to-hydrocarbons conversion on zeolites

University of Minnesota Ph.D. dissertation. May 2013. Major: Chemical Engineering. Advisor: Aditya Bhan. 1 computer file (PDF); xi, 126 pages, appendices A-B.The methanol-to-hydrocarbons (MTH) process over acid zeolite catalysts has seen renewed interest in recent years both for its ability to grow carbon chains and because methanol can be produced via a syngas intermediate from any gasifiable carbon-based feedstock. MTH proceeds through an indirect mechanism known as the "hydrocarbon pool" mechanism, involving two catalytic cycles: one in which olefins are repeatedly methylated to form branched species which are susceptible to cracking and another in which aromatics are repeatedly methylated and dealkylated to form light olefins. While the observed product distribution of MTH can be rationalized as an effect of the relative propagation of these two cycles, the current understanding of this chemistry is limited. In MTH conversion, the relationship between zeolite structure, its effect on the kinetics and mechanism, and observed product selectivity of MTH is not fully understood. In this dissertation, the link between the mechanism of MTH and the observed product selectivity is investigated with a focus on how selectivity can be controlled through manipulation of the mechanism and also give insight into the mechanism.The product selectivity of dimethyl ether (DME) conversion to hydrocarbons on H-ZSM-5, a medium-pore zeolite, was systematically tuned by co-feeding small amounts of 13C-propene and 13C-toluene (4 kPa) with 12C-DME (70 kPa) at iso-conversion conditions (20.8-22.7 C%) at 548 K. The selectivity to ethene (14.5-18 C%) and aromatics (7.1-33.7 C%) increased while selectivity to C4-C7 aliphatics (42.8-16.9 C%) decreased with increasing amounts of toluene (0-4 kPa) in the co-feed. Similar trends are also observed at lower conversions (4.6-5.1 C%) at 548 K and at higher temperatures (623 K) on H-ZSM-5. The relative propagation of the olefin- and aromatic-based cycle on H-BEA, a large-pore zeolite, was also tuned through the use of olefin and aromatic co-feeds. The addition of propene resulted in a four-fold increase in selectivity for C3-C7 aliphatics compared to the addition of 1,2,4-triMB at 623 K on H-BEA. By changing the ratio of olefins and aromatics in the co-feed, the selectivity of MTH over both H-ZSM-5 and H-BEA can be systematically tuned at iso-conversion, showing that the olefin:aromatic ratio can be used as a parameter to propagate the olefin- and aromatic-based carbon pools to varying extents within the range of conditions studied in this work. On H-SAPO-34, a zeotype material with large cages connected with small windows, the aromatic-based cycle dominates over the olefin-based cycle. Unlike H-ZSM-5 and H-BEA, however, the product selectivity of MTH conversion on H-SAPO-34 could not be tuned through the use of olefin co-feeds, most likely due to aromatics entrained within the catalyst blocking access to the active sites.The ratio of ethene to 2-methylbutane + 2-methyl-2-butene (ethene/2MB) yield can be used to describe the propagation of aromatic and olefin methylation/cracking cycles. At iso-conversion conditions at 548 K, propene is co-fed with DME to increase propagation of the olefin-based cycle and correspondingly a 1.7-fold decrease in the ethene/2MB yield is observed. Similarly, the co-reaction of toluene with DME increases propagation of the aromatic-based cycle and a 2.1-fold increase in the ethene/2MB yield is observed. The ethene/2MB yield also increased by a factor of 2 as DME conversion increased from 5-62%, which is consistent with the observed concurrent increase in selectivity to ethene and methylbenzenes. For the reaction of DME alone, increasing the temperature from 548 K to 723 K increases the propagation of the olefin-based cycle and a corresponding decrease in the ethene/2MB yield from 4.7 to 1.3 is also observed. The ethene/2MB yield varies systematically with feed composition, conversion, and temperature, showing that this ratio describes the relative propagation of the aromatic to olefin methylation/cracking cycles in MTH conversion on H-ZSM-5.Co-reactions of ~8 kPa of DME with 4 kPa of toluene, p-xylene, and 4-ethyltoluene on H-ZSM-5 at 523-723 K with varying isotopic feed compositions of 13C/12C show varying incorporation of 13C/12C-atoms into ethene and propene. Three distinct aromatic dealkylation mechanisms have previously been reported in the literature (paring, side-chain, and ring expansion mechanism) and were used to predict the 13C-contents of ethene and propene based on the experimentally observed isotopologue distribution of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, and 4-ethyltoluene. The predicted 13C-content of ethene and propene from 1,2,4-trimethylbenzene and 1,2,4,5-tetramethylbenzene from the paring mechanism most closely match the experimentally observed 13C-contents of ethene and propene, compared to the other mechanisms. This work, for the first time, quantitatively shows that aromatic dealkylation to form ethene and propene on H-ZSM-5, occurs through the paring mechanism