Catalytic role, zeolites production

In comparable reaction conditions as Pd +Cu +Y, Pd + and Cu2+ exchanged pentasil and ferrierite zeolites show a different type of activity [31]. The main products formed by propylene oxidation on these catalysts are acrolein and propionaldehyde below 120°C and 2-propanol above 120 C. Above 150°C consecutive oxidation of 2-propano1 to acetone is observed. The catalytic role of Pd and Cu in the 2-propanol synthesis is proposed to follow the Wacker concept. It is striking that when Pd + and Cu2+ are exchanged in 10-membered ring zeolites, oxidation of a primary carbon atoms seems possible, as acrolein and propionaldehyde are obtained from propylene. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

Of the three possible types of shape selectivity [1] - due again to the ZSM structure — the predominant (but not exclusive) formation of para-xylene may indicate a moderate product shape-selectivity. The appearance of meta- (and also ort/io-xylene) points to the non-negligible catalytic role of the outer surface of presumable larger zeolite crystallites [1] with some Pt particles present on them [19]. 

The positive effect of Na in FAU zeolites in the formation of oxidation products has already been reported in the literature. Thus, pyrene trapped in the supercages of HFAU zeolites was shown to be totally oxidised above 400°C over NaY and only above 550°C over HFAU [18]. Moreover, during coke oxidation over a series of NaHFAU zeolites, the CO/CO2 ratio was found to decrease with increasing Na amount in the zeolite, the authors concluding that the effect was probably due to a catalytic role of Na cations in CO oxidation [18]. The easier formation of oxidation products which is observed here with NaY seems to confirm the positive role of Na cations in oxidation. 

In this book, the main catalytic processes in oil refining and petrochemicals are reviewed with special emphasis on environmental issues they play a historic role in catalysis and illustrate nicely the interplay between chemistry, processes and products. The immense potential (hardly exploited) of zeolites in the clean synthesis of fine chemicals is demonstrated with various examples. 

All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on The Future Trends in Zeolite Applications . Special emphasis was placed on the role played by the sites at the external surface (pockets, etc.) or at the pore mouth, by mesopores, extraframework aluminum species, as well as by the polarity of reactant and product molecules. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts. It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. 

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

The catalytic properties of Pt-HZSM-5 can be explained by (a) its strong acidic sites (b) metallic sites (c) peculiar sites characteristic of the ZSM structure. Their role can he estimated hy comparing the product compositions with those obtained for other Pt-zeolites [13]. 

The role of catalysis in the petroleum industry has been equally revolutionary. Meta I-supported systems (e.g. of Topsoe and Shell) for catalytic reforming, hydrodesulfurization and hydrodenitrification, alkylation catalysts and shape selective systems (e.g. zeolites and pillared clays) for catalytic cracking (FCC) and production of gasoline from methanol (Mobil MTG) all represent significant technical and commercial achievements. 

Several conclusions may now be drawn from the present work. First of all, the crystallite size plays an important role in the diffusivity of reactant or product molecules, and subsequently in catalytic shape selectivity and deactivation rate. When the crystallite size increases, diffusion rate decreases, while shape selectivity and deactivation rate increase. These features obviously arise from an enhancement in the molecule pathway within the zeolite matrix, and from a decrease in the number of pore openings per unit weight when the crystallite size increases. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

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