Xavier Rozanska

Catalytic isomerization of 2-pentene in H-ZSM-22—A DFT investigation

The skeletal isomerization of a 2-pentene molecule catalyzed by acidic ZSM-22 was investigated by ab initio DFT studies. Two different scenarios proposed in the literature were tested. First a reaction including an alkyl shift was considered: a methyl or ethyl group is detached from the carbenium ion chain and reattached at another site in the residual hydrocarbon chain. However, this mechanism is rather unlikely, since the alkyl ion is a high-energy species, so its detachment from the carbenium ion induces a high activation energy. We find that the more likely pathway for skeletal isomerization inside the channels of ZSM-22 involves the rearrangement of the carbenium ion into a protonated dimethylcyclopropane and implies the formation of relatively stable secondary carbenium ions as transient intermediates.

Characterization of Ga/HZSM-5 and Ga/HMOR synthesized by chemical vapor deposition of trimethylgallium

Chemical vapor deposition (CVD) of trimethylgallium (TMG) has been studied as a method to disperse extraframework Ga in acidic ZSM-5 and mordenite zeolite. Various samples were extensively characterized by ICP, XPS, NMR, and FTIR. Silylation with tetramethyldisilazane is explored as a method for deactivating the external zeolite surface. The deposition of TMG in silylated ZSM-5 results in a gallium-to-aluminum ratio close to unity, which indicates a homogeneous metal distribution in the micropore space. However, pore blockage in the one-dimensional channels of mordenite results in a inhomogeneous distribution and a low Ga loading. Upon exposure to moistened air, the adsorbed methylgallium species decompose and alkoxy groups are formed. Subsequent oxidation or reduction leads to the complete removal of methyl groups. The reductive route is the preferred one resulting in a better dispersion of Ga, since oxidation of the methyl groups leads to water formation and hydrolysis of cationic Ga species.

A periodic density functional theory study of thiophenic derivative cracking catalyzed by mordenite

A periodic density functional theory study of thiophenic derivative cracking catalyzed by proton- or lithium-exchanged mordenite has been performed. The same qualitative trends in activation energies as those described employing the cluster approach method have been obtained. However, the zeolite framework appears not to stabilize the transition state complexes. This is explained by the zwitterionic nature of the thiophenic derivatives cracking transition state complexes. The zeolite framework has more subtle effect on reactivity, as shown by the alteration of the ionic nature of the transition state complex in the case of better fit with the zeolite cavity. Notwithstanding, thiophenic derivative cracking catalyzed by zeolites remains difficult. General comments concerning the use of zeolite catalysts in hydrodesulfurization are made. Predictions on zeolite-based catalysts more suitable to achieve hydrodesulfurization are described.

Theory of zeolite catalysis

Abstract The reactivity of acidic zeolites to the activation of hydrocarbons is used to illustrate different modeling approaches applied to catalysis. Quantum-chemical calculations of transition-state and ground-state energies can be used to determine elementary rate constants. But to predict overall kinetics, quantum-mechanical studies have to be complemented with statistical methods to compute adsorption isotherms and diffusion constants as a function of micropore occupation. The relatively low turnover frequencies of zeolite-catalyzed reactions compared to superacid-catalyzed reactions are due mainly to high activation energies of the elementary rate constants of the proton-activated reactions. These high values are counteracted by the significant interaction energies of hydrocarbons with the zeolite micropore wall dominated by van der Waals interactions.

Synthesis of New Tetraethyl(N-alkyl-1-aminoethan-1,1-diyl)bisphosphona-tes and ESR Analysis of Chemical Exchange of Derived Nitroxides of Acyclic Aminobisphosphonates

The synthesis and the full characterization of two new linear bisphosphonates (tetraethyl(N-tert-butyl-1-aminoethan-1,1-diyl)bisphosphonate and tetraethyl(N-sec-butyl-1-aminoethan-1,1-diyl)bisphosphonate), and the first analysis of the ESR spectra of the corresponding nitroxides is reported. The preliminary results of theoretical calculations on model compounds suggest a small B0 (in the McConnell equation). The results of bisphosphonate ester and bisphosphonic acid are similar. The discrepancies of P coupling for the diphosphorus compound stems from B2 that is different when the dihedral angle is larger than 90°.

Theoretical Study of Reactions Catalyzed by Acidic Zeolite

After a general introduction on zeolites and their properties, we will show how the current quantum chemical methods allow nowadays the simulation of catalytic models that become more and more realistic. Predictability of activity, selectivity and stability based on known structures of catalysts can be considered the main aim of the theoretical approach to catalysis. Here for a particular class of heterogeneous catalysts, the acidic zeolites, and for a particular class of reactions, the aromatics isomerization reactions, we will describe a quantum mechanic study of a reaction pathway investigation. The cluster approach method as well as electronic structure periodic method will be used for this purpose.

Reaction mechanisms in protonic zeolites

Publisher Summary This chapter emphasizes on the application of quantum mechanical methods to the study of microporous materials with a strong focus on mechanistic concepts related to hydrocarbon activation catalyzed by acidic zeolites. A strong emphasis is also placed on the relationship between transition-state (TS) structures and their energies as a function of the zeolite micropore structure. The well-defined microporous structure is essential to the molecular sieve capability of zeolites, and that when used as a catalyst or catalyst support, it will induce product selectivity when reactions are diffusion limited. Because of their very limited solvation power, the enthalpies of reaction catalyzed by zeolites compare more closely to gas-phase reactions than to reactions catalyzed by homogeneous high-dipolar acids. Proton-exchanged zeolites can induce alkylation, transalkylation, isomerization, and cracking reactions. Moreover, they are also applied to achieve organic fine-chemical reactions. Some of the mechanistic aspects related to the reactions induced by proton-exchanged zeolites are the carbocationic nature of the transition states and complexity of reaction mechanisms. Short-range polarization of the zeolite-framework oxygen atoms is essential for the stabilization of TS complexes. The importance of matching TS structure size and shape with the dimensions of the zeolite micropores supports the idea that improved zeolite catalysts can be developed with greater specificity. The lock and key principle also controls TS selectivity in zeolite catalysis.

Control of the radical polymerization by 2,2,15,15-tetramethyl-1-aza-4,7,10,13-tetraoxacyclopentadecan-1-oxyl and its sodium salt

The control of the radical polymerization of styrene by 2,2,15,15-tetramethyl-1-aza-4,7,10,13-tetraoxacyclopentadecan-1-oxyl is reported here in bulk at 90degreesC, 120degreesC and in miniemulsion. Similarly, the control by its sodium complex is reported in bulk at 90degreesC.

15-O-03 - A theoretical study of the methylation of toluene by methanol over acid mordenite

Publisher Summary This chapter presents a theoretical study of the alkylation of toluene by methanol catalyzed over acid mordenite. Cluster density functional theory (DFT) as well as periodic structure DFT calculations have been performed to obtain full reaction energy diagrams of the elementary reaction steps leading to the formation of the three xylene isomers. The use of periodic structure calculations allows taking into account the framework electrostatic contributions and steric constraints that are important in zeolite catalysis.

A DFT study of the isomerization reactions of aromatics catalyzed by acidic zeolites.

Publisher Summary This chapter presents a density functional theory (DFT) study of the isomerization reactions of toluene catalyzed by acidic zeolite. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed. The uses of periodical structure calculations and small cluster approach method calculations allow analyzing the effect of zeolite electrostatic contributions and steric constraints on reactivity.