G.B. Marin

Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.

Theoretical calculations are presented on elementary reactions which are important during coke formation in a thermal cracking unit. This process is known to proceed through a free radical chain mechanism. The elementary reaction steps that lead to the growth of the coke surface can be divided into five classes of reversible reactions: hydrogen abstraction, substitution, gas phase olefin addition to radical surface species, gas phase radical addition to olefinic bonds and cyclization. To identify the elementary reaction classes that determine the coking rate, all microscopic routes that start from benzene and lead to naphthalene have been investigated. It is found that initial creation of surface radicals, either by hydrogen abstraction or substitution and subsequent hydrogen abstractions, determines the global coking rate. The influence of the local polyaromatic structure on the kinetics of the hydrogen abstraction reactions is determined by performing calculations on a large set of polyaromatic hydrocarbons (PAHs). On basis of the BDE values six types of possible reactive sites at the coke surface can be distinguished. For the initial hydrogen abstraction the local polyaromatic structure strongly influences the reaction kinetics and abstraction is preferred from less congested sites of the polyaromatic.

The kind of olefins obtained from methanol in zeolites is strongly dependent on specific combinations of the intermediate organic hydrocarbon-pool species and zeolite topology (see picture). If the cage is too large, neutral species are favored over reactive cations. If the cage is too small, transition-state-shape selectivity poses severe limitations on the reactivity of bulkier species.

Following the recent boost of papers reporting synthesis of organic functionalized microporous and mesoporous materials, a detailed theoretical study was performed to probe the effect of organic functionalizations on certain fundamental properties in organosilicas from a microscopic viewpoint. The simplest functionalization of a bridging methylene unit was modeled in a zeolite MFI-type framework to serve as a model system for more complex organic moieties and other structures. Calculated adsorption energies for H2O and NH3 in methylenesilica reveal that the methylene functionalization increases the strength of the interaction of both probe molecules with the zeolite framework. Investigation of the combination of an ion-exchanged aluminum site containing a CH2-bridge demonstrates how the methylene moiety creates a steric obstruction for adsorbed alkali metal ions such as Li, Na and K, resulting in a weaker bond between these ions and the aluminum site. Finally, a study of proton mobility from a Brønsted acid site to a neighboring methylene bridge reveals that the acid proton will most likely migrate from the basic oxygen bridge to the methylene substitution. This implies that the combination of methylene moieties with aluminum impurities will lead to terminally bound methyl groups and cleavage of the hybrid organic–inorganic lattice.

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 CC bond can be formed directly.