Younès Ben Taarit

Oxidizing and acidic properties of copper-exchange Y zeolite

Abstract Copper Y zeolites have been studied by ESR and ir spectroscopy. At low activation temperatures, the zeolite exhibits Bronsted acidity. Pyridine is coordinatively adsorbed on cupric ions and gives rise to an ir band at 1451 cm −1 . No true Lewis acid sites are detected. However, CO-reduced samples acquire Lewis acid centers and cupric ions are reduced to cuprous ions. Hydrogen reduction results in the formation of metallic copper, and the solid acquires acidic hydroxyl groups which are more stable than those held on HY zeolites. The oxidizing properties of the cupric Y zeolite are attributed to the cupric ions while those of reduced samples are due to true Lewis acid sites.

EPR evidence for the isomorphous substitution of titanium in silicalite structure

TS-1 zeolite has been synthesized following the literature procedure and characterized via X-ray diffraction, IR spectrometry and 29Si NMR and shown to match previous findings. In addition titanium-exchanged HZSM-5 was prepared. Both solids were examined by EPR spectroscopy following CO reduction. Whereas the titanium-exchanged ZSM-5 exhibited the usual EPR signal consistent with trivalent titanium in tetragonally distorted octahedral symmetry with g⊥ = 1.97 and g∥ = 1.94, the Ti-substituted silicalite exhibited an axially symmetric signal with a split g∥ component (g∥)1 = 1.98, (g∥)2 = 1.97 and g⊥ = 1.913 best interpreted as arising from a trivalent titanium in a tetrahedral coordination, indicating that the precursor tetravalent ion was indeed in a T site. Reactivity towards O2 was also monitored and differences in the gzz values for O2− adsorbed on titanium confirm the different crystal fields prevailing at each titanium siting. It is also concluded that reduction of framework titanium with CO does not result in its removal from the lattice.

Effect of pore size and aluminium content on the production of linear alkylbenzenes over HY, H-ZSM-5 and H-ZSM-12 zeolites: Alkylation of benzene with 1-dodecene

Abstract H-ZSM-5 (Si/Al = 15 and 30), H-ZSM-12 (Si/Al = 80) and HY zeolites (Si/Al ratios in the range 2.7-26.4) were evaluated in the benzene alkylation reaction with 1-dodecene for linear alkyl-benzene-LAB production. A series of dealuminated Y-zeolites was prepared under strictly controlled conditions by treatment with silicon tetrachloride. The materials thus obtained were washed and exchanged with ammonium chloride. The catalysts were characterized by X-ray diffraction, infrared spectroscopy of the framework vibrations,29Si and27Al-nuclear magnetic resonance. Temperature-programmed desorption of ammonia and elemental analysis. H-ZSM-5 and H-ZSM-12 catalysts showed very low activity. H-ZSM-12 zeolite exhibits both product and transition state type shapeselectivity, favouring the formation of the least bulky 2-phenyl isomer. With HY-zeolites, the alkylation activity increased linearly with the number of lattice Al per unit cell over a range of Si/Al ratios from 2.7 to 26.4, so a practically constant turnover frequency is obtained. The HY-zeolites also gave 97–98% selectivity towards LAB and produced branched dodecene, traces of branched alkylates and dialkylbenzenes as by-products. Neither dialkyltetralins nor dialkylindanes were found among the products. HY-zeolites did not exhibit shape-selectivity effects. The apparent activation energy was 15 kcal/mol. Small changes in the low Na content of the zeolites were responsible for considerable increases in activity. Dealuminated HY zeolites showed the highest selectivity towards 2-phenyldo-decane. In this case we suggest that the alkylation reaction competed effectively with the migration of the double bond across the chain, so that some of the initial alkylcarbenium ions formed alkylated benzene without undergoing isomerization. The isomerization of the phenylalkane was not observed after alkylation.

Catalytic investigation of the dehydrogenation properties of pentasil type zeolites as compared with their cracking properties

Abstract The kinetics of propane conversion on acidic zeolites were investigated. The kinetic measurements were conducted under conditions that ensure the true kinetic rates for dehydrogenation and cracking to be determined. The ratio of dehydrogenation to cracking rates was shown to be independent of temperature. It was not affected by the acid strength or topology. These observations were rationalized in terms of a common rate-determining step involving the formation and decomposition of the carbonium ion to propene on the one hand and ethylene and methane on the other to restore the initial proton.

On the enhanced dehydrogenation versus cracking ability of an acidic gallium MFI: A tentative acid base interaction model

Abstract The hydrogenation and cracking properties of an authentic H(Ga)-MFI are investigated in a true kinetic regime. The cracking of propane over the H(Ga)-MFI decreased dramatically in comparison to results obtained over a comparable H(Al)-MFI. Conversely, the dehydrogenation of propane was very much increased. The Bronsted acid sites associated with the framework gallium seem to operate in a similar manner to those associated with framework aluminium in H(Al)-MFI. However, since their strength is attenuated, their greater dehydrogenation activity is ascribed to their preferred reaction with a C-H σ bond of propane to form the carbonium ion with respect to a C-C σ bond, while the reverse situation prevails with the stronger acid sites of H(Al)-MFI. A kind of soft-soft and hard-hard acid-base interaction is thought to govern the formation of carbonium ions having distinct structures which lead, respectively, to cracking or dehydrogenation.

On the influence of the mordenite acidity in the hydroconversion of linear alkanes over Pt-mordenite catalysts

Abstract Hydroconversion of n -octane and n -hexane was investigated on a series of platinum loaded mordenite samples with varied framework aluminium (FAl) content. In agreement with previous authors, it was observed that the rate of hydroconversion is first increased almost linearly with the number of FAl per unit cell, reached a maximum then decreased abruptly as the number of the FAl/unit cell is increased. Using dealuminaled and directly synthesised mordenite samples with very close FAl contents we reached the conclusion that the variation of the catalytic activity should neither be related to variation of the acid strength nor to changes in diffusion limitation but rather to the ability of the mordenite samples to favour adsorption of the paraffin.

The paramagentic resonance principles and fundamental parameters in powder spectra

Abstract The EPR principles are reviewed. The interaction of the unpaired electron with various local fields was shown to give rise to perturbations which help to define the environment of the electron spin. The principal magnetic parameters are defined and related to the physical parameters of the paramagnetic substances. The use of the g and A magnetic parameters in the case of a single unpaired electron is illustrated in various characteristic cases. The identification of the paramagnetic center and the determination of lattice crystal fields were shown to be possible through the use of g principal components. Further information about the nature and the structure of the paramegnetic center and its close environment is shown to be obtainable from the variation of spin-spin and spin-nuclear interactions. The cases where more than a single unpaired electron exist at a particular center are considered and the relevant additional parameters defined and their significance with respect to the symmetry of the center is examined.

Electron paramagnetic resonance studies of CO–2 radicals adsorbed on MgO. Identification and structure of the species using 13C and 17O labelling

A MgO surface containing either adsorbed O– ions or F+ centres irradiated with u.v. light reacts with CO or CO2 to form O—CO– or CO–2 species respectively. Using 13C and 17O labelling and X and Q band modes of the spectrometer, identification of the species and characterisation of the spin density distribution in the radicals were performed. Two types of species coexist for each preparation method in different relative amounts. Their e.p.r. parameters are not very different, which suggests that O– ions are sufficiently mobile on the surface of MgO to permit instantaneous reorientation of a (O—CO)– product, which has oxygen in a surface vacancy and the CO group projecting from the surface, into a symmetrical CO–2 radical with two approximately equivalent oxygen nuclei. Their structures are only slightly different suggesting that their chemical reactivities are similar.The sp hybridisation on the carbon atom indicates an OCO bond angle of about 125° for all species, which is slightly less than CO–2 trapped in formate matrices (129–130°). We suggest that the radicals are trapped at the MgO surface in a nearly symmetrical geometry with their oxygen ions stabilised by two Mg2+ ions and the carbon atom held away from the surface.

Applications of EPR to heterogeneous systems

Abstract The applications of EPR to heterogeneous systems are illustrated by examples which include the determination of oxidation state, the ion migration and the formation of ion pairs in solid-solid systems; the study of surface redox properties via radical formation in solid-liquid systems; the determination of the surface crystal field, the nature of the catalytic site and its coordination number and the mobility of the adsorbed species in solid-gas systems; the determination of the adsorption site in oxygen carriers in liquid-liquid systems and liquid-gas systems. The example of the coordination chemistry of paramagneticions in zeolites will be used to illustrate the case of the more complex solidliquid-gas system.

Catalytic Reactions Induced by Transition Metal Complexes Solvated in Zeolite Matrices: Oxidation, Carbonylation and Related Reactions

Publisher Summary This chapter discusses the catalytic reactions induced by transition metal complexes solvated in zeolite matrices such as oxidation, carbonylation, and related reactions. The electronic structure of the transition metal ion is certainly of importance even though its effects could not be correctly evaluated. The activation mode of both reagents as well as the Redox potential, which greatly affects the kinetic parameters, is primarily determined by the TMI electronic structure. In a number of studies, oxygen was shown to be activated associatively or dissociatively on various TMI and TMIZ where the cations were usually at a low oxidation state. Propylene also adsorbs in various ways on TMIZ, and thus equilibration of Pd II, Cu II, Ni II Y with propylene invariably resulted in a one electron reduction of the transition metal ion and presumably in the formation of the propylene positive radical. Despite the lack of experimental spectroscopic and kinetic data, as far as selective oxidation is concerned, general mechanistic schemes can be tentatively postulated, relying particularly on widely accepted schemes in homogeneous catalysis for soluble homologs of the transition metal complexes hosted in zeolites.