Mourad Mhamdi

Preparation of Co2+/ZSM5 catalysts by solid-state reaction : Influence of the precursor on cobalt speciation

Abstract Co2+/ZSM5 catalysts (Co/Al = 1, overstoichiometric ratio) have been prepared by solid-state reaction at 500 °C in argon from H-ZSM5 and four Co(II) precursors: formate, nitrate, chloride and acetate. Co3O4 particles supported by the zeolite grains are the only cobalt-containing phases detected when formate is used. When cobalt nitrate is the precursor, Co3O4 particles appear to be independent from the grains. A fraction of cobalt has nevertheless migrated into the zeolite; this is linked with [Co(H2O)6](NO3)2 low melting point and wetting of the zeolite grains by the molten salt at the beginning of the thermal treatment. Catalysts prepared from cobalt chloride contain both nanometric Co3O4 particles on the support grains and cobalt ions inside the zeolitic channels. Ion exchange is possible, at least partly, due to the fact that unlike nitrate and formate, cobalt chloride has not decomposed before reaching the temperature at which the exchange process is activated. This process is chemically driven by the release of HCl in the gas phase. Finally, the major phase detected when cobalt acetate is the precursor is a poorly organized cobalt phyllosilicate surrounding zeolite grains. Acetate ions attack the zeolitic framework from outside during the solid-state reaction; cobalt ions are inserted in the silicate matrix and progressively migrate deeper in the grains upon thermal activation.

Ammoxidation of ethylene into acetonitrile over Co-Zeolites catalysts

Acetonitrile can be produced from alkanes by ammoxidation over a variety of multidimensional 10 or 12 rings zeolites exchanged with cobalt. So as to find a suitable catalyst for ethylene ammoxiation into acetonitrile, Y and ZSM-5 zeolites were exchanged with cobalt, through aqueous ion exchange and solid-state ion exchange. The catalyst prepared from pentasil zeolites and cobalt acetate displayed high catalytic activity and selectivity to produce acetonitrile.

Synergy between vanadium and molybdenum in bimetallic ZSM-5 supported catalysts for ethylene ammoxidation

Ammoxidation of ethylene to acetonitrile was studied on V/ZSM-5, Mo/ZSM-5 and V–Mo/ZSM-5 catalysts prepared by a solid-state ion exchange method. The physico-chemical properties were investigated by means of XRD, N2 physisorption, 27Al and 29Si MAS NMR, UV-Vis DRS, XPS, pyridine-IR and FT-IR spectroscopies and H2-TPR/O2-TPO. Based on the characterization results, M–Ox (M = V or Mo) species react with zeolite protons during the exchange process and generate new Lewis acid sites, which act as redox centers. The M–Ox species are essentially, monomeric and dimeric/polymeric species or metal oxide crystallites (less than 4 nm) highly dispersed in the channel and/or on the external surface of zeolite. For the Mo/ZSM-5 sample, the formation of Al2(MoO4)3 nano-crystallites was observed. UV-Vis DRS and TPR results showed that V and Mo species in all catalysts are mainly in the highest oxidation states. The V–Mo/ZSM-5 catalyst exhibited a more reversible behavior of the M–Ox centers throughout the H2/O2 redox cycles than those in V/ZSM-5 and Mo/ZSM-5 catalysts. The best catalytic performance was achieved over the bimetallic V–Mo/ZSM-5 catalyst. These results revealed that the partial substitution of molybdenum with vanadium has a positive effect on the activity and the selectivity to acetonitrile. This implies clearly that a synergetic effect between V and Mo species plays an important role in the ammoxidation reaction. This synergetic effect is related to the existence of electronic interaction at short range order between the V and Mo species, which may influence the catalyst redox properties.

Ammoxidation of ethylene to acetonitrile over Cr-ZSM-5 catalysts

Abstract Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm −1 ), N 2 physisorption (BET), 27 Al MAS NMR, UV-Visible spectroscopy, NH 3 -TPD and H 2 -TPR. For all samples, UV-Visible spectroscopy and H 2 -TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, Cr 6+ species seem to play a key role in the ammoxidation reaction but Cr 2 O 3 oxide enhances the deep oxidation.

Physicochemical and catalytic properties of over- and low-exchanged Mo‒ZSM-5 ammoxidation catalysts

A series of Mo/ZSM-5 catalysts prepared by solid-state ion exchange at different Mo/Al molar ratios were characterized and tested in ethane and ethylene ammoxidation into acetonitrile. It has been concluded that the low-exchanged solid (Mo/Al = 0.2) stabilized MoO3, [Mo2O7]2− and [Mo7O24]6− species. However, besides these species, the solids prepared at Mo/Al = 0.5 and 1.5 stabilized [MoO4]2−. Nevertheless, only MoO3 and [Mo2O7]2− species were stabilized at Mo/Al = 1. The study performed by diffuse reflectance spectroscopy allowed the determination of the molar fraction relative to each Mo specie and, therefore, the calculation of the turnover frequency values. The catalytic activities of the various solids have been classified by taking into consideration the inefficiency of Al2(MoO3)4 phase, which inhibits the diffusion of reactants molecules towards the active sites, and amorphous MoO3 which catalyzes the undesired hydrocarbons’ combustion. However, [MoO4]2‒ species are efficient in the oxidative dehydrogenation of C2H6 into C2H4, while dimeric species catalyze the ammoxidation.

Selective catalytic reduction of NOx by NH3 on V-Mo-zeolite prepared by solid-state ion exchange method

Abstract V-Mo-Zeolite catalysts prepared by solid-state ion exchange were studied in the selective catalytic reduction of NOx by ammonia. The catalysts were characterized by chemical analysis, X-ray powder diffraction, N 2 adsorption (BET), DRIFT, UV-Vis and Raman, spectroscopy and H 2 TPR. Catalytic results show that upon addition of Mo to V-ZSM-5, catalytic performance was enhanced compared to V-ZSM-5.

Influence of the cobalt salt precursor on the catalytic properties of H-ZSM-5 modified with cobalt by solid-state ion exchange

Abstract Co-ZSM-5 catalysts prepared by either aqueous or solid-state ion exchange with different cobalt precursors (acetate and chloride) were studied in the selective ammoxidation of ethylene and ethane to acetonitrile. Co 2+ catalysts were characterized by chemical analysis, X-ray powder diffraction, N 2 adsorption (BET), FTTR, UV-Vis spectroscopy, TPD of ammonia and H2 TPR. The analysis of XRD and H2 TPR data indicated the presence of cobalt oxide when cobalt chloride is used. TPD of ammonia and FTIR of pyridine showed that solid-state exchange catalysts generated new Lewis acid sites. The catalyst prepared using solid-state exchanged cobalt acetate contained mostly isolated Co2+ ions in cationic exchange positions and showed high activity and selectivity toward acetonitrile.

10-P-08 - Preparation and characterization of H-ZSM-5 exchanged with cobalt by solid state ion exchange

Publisher Summary This chapter discusses the preparation and characterization of H-ZSM-5 exchanged with cobalt by solid-state ion exchange. It compares the physico-chemical and catalytic properties of Co-ZSM-5 zeolites prepared by solid-state exchange with different cobalt/aluminum (Co/Al) ratios and exchange temperatures. The catalysts are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-Vis) and temperature-programmed desorption (TPD) of ammonia. Solid-state ion exchange generated new Lewis acid sites and one proton is exchanged per Co 2+ species. UV-Visible spectra indicate the presence of some minor cobalt oxides not detected by XRD. Catalyst using Co/Al ratio equal to one and issued from exchange temperature equal to 500°C show high activity and selectivity toward acetonitrile by the ammoxidation of ethylene.