Theoretical study of adsorption complexes in H-ZSM-5

Catalytic conversions in acidic zeolites such as H-ZSM-5 find applications in a whole range of industrial production processes. Unraveling the actual network of reactions taking place inside the zeolite pores, however, can be a very challenging task. In recent years, theoretical modeling has proven to be a highly useful tool to complement experimental studies in gaining a deeper understanding of complex reaction mechanisms. Theoretical methods allow to establish if suggested reaction cycles can really occur in the zeolite pores, through the calculation of intrinsic barriers and rate coefficients for the elementary steps [1,2]. Although, before any reactions can take place, the reactants have to adsorb onto specific active centers inside the pores. With this preceding adsorption step, an additional enthalpy difference is associated, which is inevitably included in experimentally measured reaction barriers. Accordingly, to compare results from theoretical studies with experimental values, the heat of adsorption should be accounted for, and an accurate representation of the pre-reaction complexes is indispensable [3]. In this work, a series of adsorption complexes of alcohols and nitriles in H-ZSM-5, for which experimentally determined adsorption enthalpies are available in literature [4], is modeled using DFT-based quantum chemical techniques. We aim to verify whether these computational models (which have been successfully used to study elementary reaction steps at a reasonable computational cost) can qualitatively represent trends in adsorption enthalpies across a series of compounds, and if recently suggested additions to include long range dispersion interactions [5], succeed in improving accuracy of the theoretical results to the extent that prediction of experimentally observed values comes within reach

Efficient approach for the computational study of alcohol and nitrile adsorption in H-ZSM-5

Since many industrially important processes start with the adsorption of guest molecules inside the pores of an acidic zeolite catalyst, a proper estimate of the adsorption enthalpy is of paramount importance. In this contribution, we report ab initio calculations on the adsorption of water, alcohols, and nitriles at the bridging Bronsted sites of H-ZSM-5, using both cluster and periodic models to account for the zeolite environment. Stabilization of the adsorption complexes results from hydrogen bonding between the guest molecule and the framework, as well as from embedding, i.e., van der Waals interactions with the pore walls. Large-cluster calculations with different DFT methods, in particular B3LYP(-D), PBE(-D), M062X(-D), and omega B97X-D, are tested for their ability to reproduce the experimental heats of adsorption available in the literature (J. Phys. Chem. B 1997, 101, 3811-3817). A proper account of dispersion interactions is found to be crucial to describe the experimental trend across a series of adsorbates of increasing size, i.e., an increase in adsorption enthalpy by 10-15 kJ/mol for each additional carbon atom. The extended-cluster model is shown to offer an attractive alternative to periodic simulations on the entire H-ZSM-5 unit cell, resulting in virtually identical final adsorption enthalpies. Comparing calculated stretch frequencies of the zeolite acid sites and the adsorbate functional groups with experimental IR data additionally confirms that the cluster approach provides an appropriate representation of the adsorption complexes

Investigation of confinement effects on zeolite-catalyzed methylation reactions

Catalytic conversion of methanol to light olefins (MTO) over acidic zeolites is currently one of the most prominent alternatives to traditional crude oil cracking processes for the production of ethene and propene. The underlying reaction mechanisms have been under debate for decades, with current insight strongly supporting an indirect mechanism based on the hydrocarbon pool hypothesis: olefin formation is found to occur through repeated methylation and subsequent elimination and/or cracking reactions of organic co-catalysts inside the zeolite pores.[1] Depending on the characteristics of the zeolite material, the predominant hydrocarbon pool species vary from smaller alkenes to bulky polymethylbenzenes.[2] Theoretical studies showed that methylations are generally the rate-determining steps in the olefin producing catalytic cycles; therefore it is of utmost importance to gain an in-depth understanding of these reactions.[3,4] Quantum chemical calculations on extended cluster models that mimic the local environment of the active site were used in this work to model methylation reactions in a selection of zeolite frameworks. Activation barriers and rate constants are then mutually compared to assess the influence of confinement effects caused by different catalyst topologies. The balance between accuracy and computational efficiency signifies this approach as an important step toward routine study of reaction steps in heterogeneous catalysis.[5

Effect of temperature and branching on the nature and stability of alkene cracking intermediates in H-ZSM-5

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