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The conversion of methanol-to-hydrocarbons (MTH) over an acidic zeolite catalyst represents the final step in upgrading any gasifiable carbon feedstock to gasoline-range hydrocarbons (methanol-to-gasoline, MTG) and light olefins (methanol-to-olefins, MTO). MTH proceeds through a complex mechanism in which there is interaction between the inorganic zeolite catalyst and an organic co-catalyst occluded within the zeolite pores, which serves as a reaction center for carbon chain growth. The active site in MTH catalysis therefore, is not the Brønsted acid site alone, but an inorganic-organic hybrid reaction center, consisting of a nanometer sized inorganic zeolite cavity and an essential organic compound. The control of selectivity in MTH has been of key interest industrially, with most researchers focusing on the effect of the inorganic catalyst on product selectivity by testing different zeolite and zeotype frameworks. Rather than changing the inorganic catalyst to affect selectivity, we report that selectivity for MTH can be systematically controlled by changing the organic co-catalyst through the introduction of small amounts of an olefin and aromatic co-feed.

 The MTH process was first invented in 1976 by Mobil and was commercialized in 1986, however, the mechanism of methanol conversion over acid zeolite catalysts was not well understood until much later, when Dahl and Kolboe proposed a “hydrocarbon pool” mechanism. In this mechanism, which is now widely accepted as the mechanism for MTH, polymethyl benzenes act as organic co-catalysts that dealkylate to form light olefins. Additionally, a second reaction cycle, in which olefins are methylated and larger olefins either crack and re-enter the methylation reaction cycle or participate in hydrogen transfer reactions to produce aromatics and alkanes, which are terminal products of MTH.

 We examined the effect on product selectivity of changing the organic catalyst by co-feeding olefin and aromatic compounds with dimethyl ether (DME) on H-ZSM-5. By systematically changing the olefin:aromatic ratio of the co-feed, we can modulate the relative contribution of the olefin and aromatic methylation cycles in MTH. We find that by co-processing propylene and toluene (total 4 kPa) with dimethyl ether (65 kPa) on H-ZSM-5, we can tune the selectivity to C₄–C₈ hydrocarbons at iso-conversion.

 Four different compositions were tested under these conditions: DME only, DME + propene, DME + toluene, and DME + 1:1 ratio of propene:toluene. Table 1 shows the carbon selectivity for each of these feed compositions. The results reported are shown for similar DME conversions (4.4-7.2%), where methanol is considered to be unreacted feed. Additionally, the olefin and aromatic co-feeds are not included in the assessment of carbon selectivity. The selectivity to larger hydrocarbons is increased when propene is co-fed compared to when DME alone is reacted over H-ZSM-5. In contrast, toluene co-feed results in a lower selectivity to C₄-C₇ hydrocarbons.

The data reported in Table 1 show that carbon selectivity for C₄-C₇ increases as the partial pressure of propene in the co-feed increases. This is also true for the C₈ fraction if only aliphatic hydrocarbons are considered. The total selectivity to C₄-C₈ aliphatic hydrocarbons changes monotonically as the olefin:aromatic ratio of the co-feed is changed, with the carbon selectivity of C₄-C₈ aliphatic hydrocarbons equaling 16%, 41%, and 63% for toluene, propene + toluene, and propene co-feed, respectively.

 Table 1. Carbon selectivity of 65 kPa DME with varying co-feeds at 548 K, WHSV=72 g DME (g catalyst h)^-1, 4.4-7.2% conversion

 The identity of the co-feed most likely influences the degree to which the aromatics-based cycle and the olefin-based cycle contribute to the overall product selectivity. The addition of propene increases the propagation of the olefin-based cycle relative to that of the aromatic methylation cycle while the addition of toluene increases the relative propagation of the aromatics based cycle. This is evidenced by the high C₄ selectivity when propene is co-fed, as butene is the primary methylation product of propene. Similarly, C₈ selectivity is highest when toluene is co-fed, as xylene is the primary methylation product of toluene.

 In summary, the organic hydrocarbon pool co-catalyst can be used a parameter to develop selective MTH catalysts. We show that by co-processing alkene or aromatic co-reactants, we can control the composition of the organic hydrocarbon pool and therefore its catalytic consequences on MTH selectivity on H-ZSM-5. In this work, the addition of propene resulted in a four-fold increase in carbon selectivity for C₄-C₈ aliphatics compared to the addition of toluene. A mixed co-feed with a 1:1 ratio of propene:toluene resulted in a selectivity between the single co-feed cases. The results provide the basis for an alternative to changing the inorganic zeolite catalyst to control the selectivity for MTH. Rather than changing the inorganic catalyst, the identity and reactivity of the organic co-catalyst can be manipulated to tune the selectivity of MTH on H-ZSM-5 in a way that can be both predicted and controlled.