Catalytic role, zeolites

Presence of zeolites with 5 A, which were used to dry ethylene under pressure and catalysed the polymerisation. The installation was destroyed. The zeolites with 3 A do not play any catalytic role. 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

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. 

It should be emphasized that the active sites located on the external surface, often in small amounts compared to the inner sites (<1% for crystallites of 1 //m), play a catalytic role. Generally, this leads to a selectivity decrease, the external surface lacking the shape selective properties of the inner pores.. However, recent results show that reactions which can occur only on the external surface of zeolites or just within the pore mouth are very selective, suggesting a shape selective influence of external surface depending on the nature of the substrate (Table 1.2). 

Ferrisilicates are considered for catalytic applications, primarily in their H-form, due to the acidic function on the Si-OH-Fe groups. In addition, the extra-framework ions may also play a catalytic role (e.g. exhibiting Lewis acidity). Further, in the case of comparable size of diameter of channels ancJ reaction components, shape selectivity is imposed by the zeolite structure. Thus, the overall catalytic performance is influenced by various sources. It should also be considered that in general case the structure, the distribution of iron components among the possible oxidation and coordination states may also change under catalytic conditions in a ferrisilicate. 

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. 

The addition of small amounts of certain oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate ect. in the synthesis mixture of zeolites and their metallosilicate / silicate analogues significantly enhances the nucleation and crystallization rates, thereby reducing the overall crystallization time by as much as five times. Sensitivity enhanced high resolution liquid state Si and P NMR studies, using a specially designed probe, on low temperature (358 K) synthesis of Silicalite-1 in the presence of NaH2P04 as promoter indicate a catalytic role of the promoter. 

Thus the significant change in P signal such as position, resolution and retention of original P structure after completion of the crystallization indicates a catalytic role played by the promoter in enhancing the zeolite crystallization. Phosphate was taken as representative example for NMR studies due to its P abundance and effectiveness as promoter. 

The addition of small amount of certain oxyanions of group V-VII elements (such as phosphate, arsenate, perchlorate, chlorate etc.) enhance the nucleation and crystallization of zeolites and related molecular sieves to a varying extent. The effect of is very general in nature and applicable to different zeolite structures. Liquid state Si and P NMR experiments indicate reversible interaction of these promoter oxyanions with template enclathrated Q silicate-water structure at the onset of crystallization, and a catalytic role of the promoter in enhancing the crystallization rate of zeolites and related molecular sieves. 

The decomposition occurs if ZSM-48 is formed, while no decomposition is observed if the reaction mixture leads to EU-1. It seems therefore that the zeolite plays a catalytic role in the decomposition of hexamethonium ions. 

In most cases, catalysis on zeolites occurs inside the intracrystalline voids. Nevertheless, a catalytic role is sometimes attributed to the external surface of the crystals, which for many crystallographic directions consists of a collection of pore mouths. The possibility of catalysis at pore mouths was first discussed by Venuto [41]. Catalytic sites at the pore mouths can have a strength and a structural environment different from those within the intracrystalline cavities. In principle, situations may occur where the concentration of reactants is totally different at the pore mouths compared to the crystal interior. When intracrystalline diffusion is slow compared to the rate of the chemical reaction, only active sites near the external surface of crystallites may be responsible for catalysis. The absence of intracrystalline diffusion restrictions doesnot necessarily imply that the pore mouths are a-selective catalytic enviromnents. They may provide a local geometry which is different from that available inside the crystals. 

Conversion of Biomass to Chemicals The Catalytic Role of Zeolites... 

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