G.B. Marin

Resonance stabilization of the transition state is one of the key factors in modeling the kinetics of hydrogen abstraction reactions between hydrocarbons. A group additive model is developed which allows the prediction of rate coefficients for bimolecular hydrogen abstraction reactions over a broad range of hydrocarbons and hydrocarbon radicals between 300 and 1300 K. Group additive values for 50 groups are determined from rate coefficients determined using the high level CBS-QB3 ab initio method, corrected for tunneling and the hindered internal rotation around the transitional bond. Resonance and hyperconjugative stabilization of the transition state is accounted for by introducing 4 corrections based on the structure of the reactive moiety of the transition state. The corrections, fitted to a set of 28 reactions, are temperature-independent and reduce the mean absolute deviation on Ea to 0.7 kJ mol−1 and to 0.05 for log A. Tunneling contributions are accounted for by using a fourth order polynomial in the activation energy. Final validation for 19 reactions yields a mean factor of deviation between group additive prediction and ab initio calculation of 2.4 at 300 K and 1.8 at 1000 K. In comparison with 6 experimental rate coefficients (600–719 K), the mean factor of deviation is less than 3.

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 kethene/kpropene/kbutene = 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 kethene/kpropene/kbutene = 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.

Zeogrids and Zeotiles are hierarchical materials built from assembled MFI zeolite precursor units. Permanent secondary porosity in these materials is obtained through self assembly of nanoparticles encountered in MFI zeolite synthesis in the presence of supramolecular templates. Hereon, the aggregated species are termed nanoslabs. Zeogrids are layered materials with lateral spacings between nanoslabs creating galleries qualifying as supermicropores. Zeotiles present a diversity of tridimensional nanoslab assemblies with mesopores. Zeotile-1, -4 and -6 are hexagonal mesostructures. Zeotile-1 has triangular and hexagonal channels; Zeotile-4 has hexagonal channels interconnected via slits. Zeotile-2 has a cubic structure with gyroid type mesoporosity. The behavior of Zeogrids and Zeotiles in adsorption, membrane and chromatographic separation and catalysis has been characterized and compared with zeolites and mesoporous materials derived from unstructured silica sources. Shape selectivity was detected via adsorption of n- and iso-alkanes. The mesoporosity of Zeotiles can be exploited in chromatographic separation of biomolecules. Zeotiles present attractive separation properties relevant to CO2 sequestration. Because of its facile synthesis procedure without hydrothermal steps Zeogrid is convenient for membrane synthesis. The performance of Zeogrid membrane in gas separation, nanofiltration and pervaporation is reported. In the Beckmann rearrangement of cyclohexanone oxime Zeogrids and Zeotiles display a catalytic activity characteristic of silicalite-1 zeolites. Introduction of acidity and redox catalytic activity can be achieved via incorporation of Al and Ti atoms in the nanoslabs during synthesis. Zeogrids are active in hydrocracking, catalytic cracking, alkylation and epoxidation reactions. Zeogrids and Zeotiles often behave differently from ordered mesoporous materials as well as from zeolites and present a valuable extension of the family of hierarchical silicate based materials.