Solid acid catalysts distribution

The use of solid acid catalysts would eliminate waste disposal problems and allow for more advantageous control of product selectivities. However, rapid deactivation of these solid acid catalysts is a problem that significantly hinders the effective performance and selectivity of these catalysts. We have studied the performance of various solid acid catalysts for their activity/deactivation characteristics and also their shape selective effects. Specifically in the liquid phase system, unlike previous researchers, we have studied the activity/deactivation evolution with time on stream rather than rely on final product distribution only. This approach has allowed us to obtain unique data that clearly describe the deactivation pattern of these catalysts. 

AlkyClean Alkylation Process A True Solid Acid Catalyst (SAC) Process 493 Table 12.8 Product distribution and properties versus temperature [19]. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

Silica-occluded H3PW12O40 is a microporous material with a relatively sharp pore-size distribution with a peak at 0.55 nm (Figure 2). This microporous property seems to be favorable for molecular shape-selective reactions. Indeed, silica-occluded H3PW12O40 acts as a shape-selective solid-acid catalyst in the solvent-free alkylation of phenol with formaldehyde (Scheme 1). 

The Fries rearrangement of phenyl acetate (PA) over solid-acid catalysts was first studied in a fixed bed reactor at 400 °C by Pouilloux et al. [9,10]. o- and p-Hydro-xyacetophenone (o- and p-HAP), p-acetoxyacetophenone (p-AXAP), and phenol (P) were the main reaction products. Fluorinated alumina and H-FAU zeolites afforded approximately the same product distribution, o-HAP being highly favored over the para isomer. The reaction scheme proposed was that PA dissociates into phenol (P) and ketene and that o-HAP results partly from an intramolecular rearrangement of PA and partly from transacylation (Eq. 2) whereas p-HAP results from the latter reaction only [10]. 

HZSM-5 zeolite catalysts show high shape selectivity, because they have very fine micro pores within its crystallite [1], The pore diameter is almost equal to sizes of mono-aromatic molecules [2]. HZSM-5 catalysts are typical solid-acid catalysts, and their acid sites are distributed not only within but also on the outer surface of the crystallite [1,3]. Therefore, the shape selectivity of HZSM-5 zeolite is affected strongly by the size of the crystallite, the intracrystalline diffusivities of hydrocarbons and acidic properties within and on the outer surface of the crystallite [4-7],... 

In general, alcohols undergo dehydration to olefins and ethers over solid acid catalysts, and dehydrogenation to aldehydes and ketones over solid base catalysts. However, certain solid base catalysts promote dehydration in which the mechanisms and product distribution differ from those for acid-catalyzed dehydration. 

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

The transalkylation of toluene with trimethylbenzenes (TMB) takes place on solid acid catalysts and it is part of a complex network of reversible reactions. The transalkylation reaction is mainly in equilibrium with the disproportionation reaction and the isomerization of polymethylbenzenes also takes place [2,3]. The distribution of the several isomers is governed by kinetics factors like the reactivity of the... 

Recent developments on acidity characterization of solid acid catalysts, specifically those invoking P solid-state nuclear magnetic resonance (SSNMR) spectroscopy using phosphorus-containing molecules as probes, have been summarized. In particular, various P SSNMR approaches using trimethylphosphine, diphosphines, and trialkylphosphine oxides (R3PO) will be Introduced, and their practical applications for the characterization of important qualitative and quantitative features, namely, type, distribution, accessibility (location/proximity), concentration (amount), and strength of acid sites in various solid acids, will be illustrated. 

To materialise the promising catalytic properties of clay minerals, the textural characteristics which determine the transport properties, i.e., the dimensions of the porous catalyst bodies and the pore-size distribution, must be controlled. The acidity and the density of the acid sites usually has to be adapted to the organic reaction to be carried out. Another important characteristic of solid acid catalysts is therefore the nature and the number of acid sites. Besides the transport properties, it is thus required to be able to control also die nature and the density of the acid sites with the preparation procedure of the solid acid catalysts. 

It can be concluded that adsorbed ammonia has sufficient mobility at 423 K to equilibrate with the catalyst surface on the time scale of microcalori-metric measurements, and these measmements provide an effective method for quantifying acid site distributions of solid acid catalysts. However, when using pyridine it is preferable to choose a higher temperature, e.g., around 573 K. 

Functionahzed Periodic Mesoporous Organosilicas (PMOs) can act as a solid acid catalyst. PMOs exhibit large specific surface areas, pore diameters of 2-50 nm and narrow pore size distributions. They are mostly synthesized with bridged bis-silanes with a general stracture (EtO)3-Si-R-Si-(OEt)3 (Figure 1) This bis-silane polycondensates around a template such as the surfactant PI23 (EO)2o(PO)7o(EO)2o After formation of the PMO, the template is removed by an extraction to reveal the pores of the hybrid material. ... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

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