You are the weakest link, goodbye! Many individual steps of the direct mechanisms in the methanol‐to‐olefin process are tied together in an integrated scheme, allowing a simple identification of the weakest links. Calculations show that a combined pathway from methanol directly to ethylene does not exist and no CC bond can be formed directly.

Unraveling the reaction mechanism of extremely complex catalytic processes can be a challenging task from a purely experimental viewpoint. For an industrially important process like the conversion of methanol into olefins (MTO), [1] this is especially the case, as secondary reactions often consume and mask the primary products. Methanol is easily and economically converted into olefins over solid acid zeolite catalysts, yet the ease of MTO conversion is in stark contrast to the difficulty of elucidating the underlying mechanism.[1] Instead of plainly following direct routes, [2][3][4][5] the MTO process has been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as co-catalysts inside the zeolite pores, adding a whole new level of complexity to this issue. [6][7][8][9] Therefore, a detailed understanding of the elementary reaction steps can best be obtained with the complementary assistance of theoretical modeling. Several experimental observables add to the theoreticians challenge: any full catalytic cycle should not only provide low-energy pathways towards olefin formation, but it should also explain the zeolite-specific product distribution. Furthermore, it should contain the cationic intermediates as observed by in situ NMR spectroscopic methods, [7,10,11] as well as an explanation for the scrambling of labeled carbon atoms into both the hydrocarbon pool species and the olefin products. [7, 9,12] Herein, we report a working catalytic cycle for the conversion of methanol into olefins, in full consistency with both experimental and theoretical observations. For each step, rate constants are presented which were obtained by quantum chemical simulations on a supramolecular model of both the HZSM-5 zeolite and the co-catalytic hydrocarbon pool species (see Methods section). This work not only represents the most robust computational analysis of a successful MTO route to date, but it also succeeds in tying together the many experimental clues.

Methylations of ethene, propene, and butene by methanol over the acidic microporous H-ZSM-5 catalyst are studied by means of state of the art computational techniques, to derive Arrhenius plots and rate constants from first principles that can directly be compared with the experimental data. For these key elementary reactions in the methanol to hydrocarbons (MTH) process, direct kinetic data became available only recently [J. Catal.2005, 224, 115-123; J. Catal.2005, 234, 385-400]. At 350 °C, apparent activation energies of 103, 69, and 45 kJ/mol and rate constants of 2.6 × 10(-4), 4.5 × 10(-3), and 1.3 × 10(-2) mol/(g h mbar) for ethene, propene, and butene were derived, giving following relative ratios for methylation k(ethene)/k(propene)/k(butene) = 1:17:50. In this work, rate constants including pre-exponential factors are calculated which give very good agreement with the experimental data: apparent activation energies of 94, 62, and 37 kJ/mol for ethene, propene, and butene are found, and relative ratios of methylation k(ethene)/k(propene)/k(butene) = 1:23:763. The entropies of gas phase alkenes are underestimated in the harmonic oscillator approximation due to the occurrence of internal rotations. These low vibrational modes were substituted by manually constructed partition functions. Overall, the absolute reaction rates can be calculated with near chemical accuracy, and qualitative trends are very well reproduced. In addition, the proposed scheme is computationally very efficient and constitutes significant progress in kinetic modeling of reactions in heterogeneous catalysis.

The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon-pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date-the paring mechanism and the side-chain mechanism-recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM-5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side-chain route) using both small and large zeolite cluster models of ZSM-5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side-chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de-alkylation to produce ethene. These results suggest that the current formulation of the side-chain route in ZSM-5 may actually be a deactivating route to coke precursors rather than an active ethene-producing hydrocarbon-pool route. Other routes may be operating in alternative zeotype materials like the silico-aluminophosphate SAPO-34.

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