Post-synthetic thermal and chemical treatments of H-BEA zeolite: effects on the catalytic activity
A beta zeolite sample was dealuminated by thermal and hydrothermal treatments. Structural and morphological changes were monitored by X-ray diffraction, nitrogen adsorption and FT-IR spectroscopy; the surface acidity was evaluated by temperature programmed desorption of ammonia, FT-IR of adsorbed pyridine and potentiometric titration with NaOH. The influence of the thermal treatments on the catalytic activity of the zeolite was tested in the anisole acylation and the Baeyer–Villiger oxidation of cyclohexanone.
Introduction
Catalytic processes are increasingly substituting, in the last years, stoichiometric processes in the production of fine and specialty chemicals [1].
The reactions that employ strong mineral acids, both of Brønsted and Lewis type, as reagents or catalysts, constitute a wide area of the organic industrial chemistry. The utilization of these reagents implies several problems connected to the danger of their use, the equipment corrosion and the disposal of the highly polluting exhausted mixtures. All this is promoting a wide research activity, both industrial and academic, addressed to the substitution of dangerous reagents with solid acids of safe use, non-corrosive for the equipment and recyclable.
Since the early sixties, acid zeolites, that found several applications in the petrochemical and refinery industry, were the first to also be used in fine chemicals and intermediates production, as documented by the patent and open literature [2], [3], [4], [5], [6], [7]. The reason for the success of these crystalline microporous materials originates from (i) their very high surface area, (ii) the possibility to tailor their adsorption properties and to control the strength and concentration of their active acid sites, (iii) the variety of structures and the dimensions of their pores that are similar in size to many organic molecules of interest (iv) their easy regenerability [8].
The access of the reacting molecules to the catalytically active internal sites of the zeolites is controlled by the dimensions of the pore openings that can be deduced from structural data. Nevertheless the surface morphology of several decationised zeolites can be modified by post-synthesis thermal, hydrothermal and chemical treatments. In fact medium and large pore zeolites such as Y-zeolite (H-Y), mordenite (HMOR), MFI (H-ZSM-5) undergo, upon high temperature thermal or hydro-thermal treatment, different levels of structural dealumination with formation of variable amounts of extraframework aluminum species (EFAL) with modification, not only of the amount and the nature of the surface acid sites, but also of their accessibility [9]. The large pore beta zeolite (BEA) is structurally disordered [10] and its framework structure [11], [12], [13] was described as a three-dimensional intersecting channel system. Two mutually perpendicular straight channels, each with a cross section of 0.76×0.64 nm2, run in the a- and b-directions. A sinusoidal channel of 0.55×0.55 nm2 runs parallel to the c-direction. The structure of ammonium exchanged and thermally treated beta zeolite (HBEA) is relatively fragile and is known to contain a substantial concentration of defects [12]. The unique acid properties, mainly related to local defects, and the optimal pore dimensions, make HBEA a very promising catalyst in shape-selective organic conversions [14]. Furthermore on this zeolite calcination or steaming above 673 K is known to cause more pronounced structural and chemical modifications [15], [16], [17] when compared with HY, HMOR and HZSM-5. We recently observed [18], by studying the HBEA catalyzed acylation of 2-methoxynaphthalene with propionic anhydride, that thermal treatments cause chemical and structural modifications of the zeolite catalyst leading to an improvement of the regio- and chemo-selectivity of the reaction. These changes have been attributed to structure dealumination followed by deposition of EFAL on the zeolite's channel walls. A successive EPR study [19], together with FT-IR, powder X-ray diffraction (XRD) and MAS-NMR determinations [20] allowed a more precise evaluation of the chemical and dimensional modifications of zeolite channels caused by the dealumination of the zeolite structure and EFAL formation. In this paper we describe how post-synthetic thermal and chemical treatments can influence the catalytic performances of a HBEA sample in two different reactions, carried out in condensed phase: the Friedel–Crafts acylation of anisole with propionic anhydride and the Baeyer–Villiger oxidation of cyclohexanone.
Section snippets
Materials
Cyclohexanone and ε-caprolactone were supplied by Aldrich Chimica. Hydrogen peroxide (35 wt.%) and biphenyl were obtained by Acros. 1,2 Dichloroethane is a Baker product. Propionic anhydride, anisole and p-methoxypropiophenone were supplied by Aldrich Chimica. Commercial beta zeolite in ammonium form (NH4-BEA) was the Zeolyst International product CP814E (SiO2/Al2O3=25).
Preparation of catalysts
The conversion of NH4-BEA in HBEA was carried out by calcination in air (16 h) at various temperatures: 773 K (HBEA773), 873 K
X-ray diffraction
Aluminum rich large pores protonic zeolites, such as HY, HMOR, HBEA, can be dealuminated by thermal treatment, acid leaching, steaming or by thermal treatment followed by acid leaching [9]. Acid leaching leads to total removal by dissolution of the EFAL from the zeolite surface and sometimes to partial realumination [17]. In this work the starting beta zeolite sample, with an initial SiO2/Al2O3=25, was subjected to calcination at different temperatures, 773 K (HBEA773), 873 K (HBEA873), 923 K
Conclusions
We can conclude that post-synthetic thermal treatments can variously affect the catalytic performances of aluminum rich large pores zeolites. Dealumination of the zeolite structure causes in fact modification of the number, strength and nature of the acid sites affecting the catalyst activity and selectivity. The transit of the reagents and products within the zeolite channels can be limited by the obstructions caused by the EFAL deposition, but EFAL can be removed by acid washing. Furthermore
Acknowledgements
Financial support from the Italian National Research Council (CNR, Rome (I) – Programma ‘Chimica’ L.95/95) is gratefully acknowledged.
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