Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K

The direct transformation of CO2 into high-value-added hydrocarbons (i.e., olefins and aromatics) has the potential to make a decisive impact in our society. However, despite the efforts of the scientific community, no direct synthetic route exists today to synthesize olefins and aromatics from CO2 with high productivities and low undesired CO selectivity. Herein, we report the combination of a series of catalysts comprising potassium superoxide doped iron oxide and a highly acidic zeolite (ZSM-5 and MOR) that directly convert CO2 to either light olefins (in MOR) or aromatics (in ZSM-5) with high space–time yields (STYC2‑C4= = 11.4 mmol·g–1·h–1; STYAROM = 9.2 mmol·g–1·h–1) at CO selectivities as low as 12.8% and a CO2 conversion of 49.8% (reaction conditions: T = 375 °C, P = 30 bar, H2/CO2 = 3, and 5000 mL·g–1·h–1). Comprehensive solid-state nuclear magnetic resonance characterization of the zeolite component reveals that the key for the low CO selectivity is the formation of surface formate species on the zeolite framework. The remarkable difference in selectivity between the two zeolites is further rationalized by first-principles simulations, which show a difference in reactivity for crucial carbenium ion intermediates in MOR and ZSM-5.

The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host–guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C4–C8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene β-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3° → 3°) and B2 (3° → 2°) are expected to be dominant at operating conditions, whereas modes B1 (2° → 3°), C (2° → 2°), D2 (2° → 1°), and E2 (3° → 1°) are expected to be less important. All β-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for β-scission reactions and to assess the mobility of all species in the pores of the zeolite.

Herewith we have the pleasure to resubmit the paper entitled : "On the stability and nature of adsorbed pentene in Brønsted acid zeolite H-ZSM-5 at 323 K" by J. Hajek , J. Van der Mynsbrugge, K. De Wispelaere , P. Cnudde, L. Vanduyfhuys, M. Waroquier and V. Van Speybroeck, for publication in Journal of Catalysis.We have received the comments of two reviewers which were very positive and which suggest publication subject to some major/minor revisions. We have taken them all into account in the revised version. In separate documents we give a substantial reply to the reviewers that carefully addresses the issues raised in the reviewer's comments. We have added an author (Louis Vanduyfhuys) in the list. During the revision process he has given a very constructive contribution in a correct thermodynamic analysis of the metadynamics results. As requested by one of the reviewers, we re-analyzed all our MD results. We hope that you are willing to accept this slight change in the author list.We further hope that the manuscript is now suitable for publication. Reply to reviewer #1 :We thank the reviewer for his/her careful reading of the manuscript with manuscript number JCAT-16-151. We try to give a valuable reply to all comments which were all very constructive. Reviewer's comments are printed in blue.1. The DFT calculations have been carried out at 0 K by optimizing the geometry at the RPBE+D3 level and then conducting single point calculations with other functionals. While the general conclusions drawn from the calculations are independent of the functional chosen, it would be very useful to know whether geometry optimization with a better functional would change the conclusion about the relative stability of the -complex and alkoxide structures.The authors completely agree with the major concern of the reviewer regarding the reliability of the geometry optimization based on one level of theory (PBE+D3). This functional has proven to be reliable in many periodic static calculations on nanoporous materials, but to remove any doubt we performed new geometry optimizations and frequency calculations for 2-pentene π-complex and 2-pentoxide using BEEF-vdW functional .[1] Geometrical details of the selected structures with PBE-D3 and BEEF-vdW functionals are given in Table 1. The comparison of free energy ΔG and enthalpy ΔH differences between BEEF-vdW //PBE D3 and BEEF-vdW // BEEF-vdW is given in Table 2. The energy differences are very similar with each other, and support the conclusions made in the paper. These additional results have been included in *Revision NotesWe thank the reviewer for this remark and reformulated the sentence slightly. The origin of larger adsorption enthalpies for the -complex is indeed related to usage of optimized geometries, which take into account only one point on the potential energy surface. Our MD simulations show that in reality at finite temperatures, the -complex can adopt various geometries leading to a probability distribution as shown in Figure 3 of the main manuscript. This dis...