Zeolites hydrophobic

Another approach involved encapsulation of a bulky guanidine, N,N,N-tricyclohexyl-guanidine, in the super-cages of hydrophobic zeolite Y (Sercheli et ai, 1997). The resulting ship-in-a-bottle guanidine catalysed the aldol reaction of benzaldehyde with acetone to give 4-phenyl-4-hydroxybutan-2-one. 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

Other important applications of adsorption are the control of greenhouse gases (CO, CH4, N20), the utilization of CH4, the flue gas treatment (SOx, N()x, Hg removal), and the recovery of the ozone-depleting CFCs (Dabrowski, 2001). Activated carbons and hydrophobic zeolites are used for the adsorption of HCFCs (Tsai, 2002). 

More complex rate expressions were observed when the reaction was carried out in the absence of solvents or when other solvents, such as H20 or ter/-butyl alcohol, were used. Complex rate expressions arise because a number of effects come into play, including preferential sorption in the hydrophobic zeolite, competition for adsorption at the titanium center, and competitive diffusion and reaction. It is therefore difficult to obtain unambiguous answers to mechanistic questions from kinetics. 

The high equilibrium selectivity for normal paraffins relative to aromatics observed for H-ZSM-5 and H-ZSM-11, so contrary to that reported for the lower silica/alumina ratio zeolites, may in part be due to the much higher silica/alumina ratio of these relatively hydrophobic zeolites, resulting in reduced polarity and ability to interact with polarizable molecules. However, other zeolites of comparable silica/alumina ratio, such as dealuminized H-morde-nite, exhibited no such enhanced preference for n-paraffins ] ) (see Table II). Clearly, silica/alumina ratio alone is insufficient to account for these differences. The structure of these zeolites, therefore, must play some role in the observed selectivity. 

Hydrophobic zeolites, as well as the all-silica zeolites or zeolites with a very small aluminum content, possess high capacity for adsorbing organic compounds dissolved in water. Some recent studies demonstrated that hydrophobic, dealuminated zeolites adsorbed organic compounds from water as effectively as activated carbon [2,37,88,89,214], The hydrophobicity of zeolites is controlled basically by changing the Si/Al ratio in the framework by synthesis conditions and postsynthesis modification treatments [215],... 

The most-tested hydrophobic zeolite in adsorption of organic compounds from water solutions is silicalite-1 [89], This material is a molecular sieve with an MFI structure composed of pure silica. The MFI framework has a 10-MR channel system with elliptical pores having diameters of 5.2 x 5.7 A [83], Additionally, other zeolites, as the all-silica P-zeolite [216] which possesses a three-dimensional, 12-numbered ring, interconnected channel system with pore diameters of 7.1 x 7.3 A [83] have been used in the elimination of methyl tert-butyl ether (MTBE) from water solutions [88],... 

In a variation on this theme, a bulky guanidine derivative, N,N,N"-tricyclohex-ylguanidine was encapsulated by assembly within the supercages of hydrophobic zeolite Y. The resulting ship-in-a-bottle catalyst was active in the aldol reaction of... 

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

Li J.-M. and Talu O., Effect of structural heto-ogeneity on multicomponent adsorption benzene and p-xylene mixture on silicalite, in M. Suzuki (ed.) Proc. IV Int. Corf, on Fundamentals of Adsorption, (Elsevio", Amsto dam, 1993) pp. 373-380. Meininghaus C. K.W. and Prins R., Sorption of volatile organic compounds on hydrophobic zeolites, Microporous and Mesoporous Materials 35-36 (2000) pp. 349-365. 

Block S. W., Hydrophobic Zeolite Adsorbents A Proven Advancement in Solvent Separation Technology. Environ. Prog. (1993), pp. 226-238. 

Chintawar, P. S. Greene, H. L. Adsorption and Catalytic Destruction of Trichloroethylene in Hydrophobic Zeolites. Appl CatB. 14 (1997) pp. 37-47. 

Surface of the zeolite crystal is fiiU of siloxane groups, and hydrophilic molecules can take advantage of this and also adsorb into the intracrystaUine spaces enhancing the permeance of the hydrophilic molecule. This special feature is an advantage for hydrophUic/hydrophobic separations, and makes the organic/water separations with hydrophobic zeolites more difficult. 

In the ensuing discussion we deal exclusively with hydrophobic (de-aluminated) zeolites. In these zeolites nearly all Al3+ ions have been substituted by Si ions, so that the stoichiometries of these solids are close to those of pure silica, Si02. The diameters of the pores in the framework are typically of the order of 5A, up to 7.sA (zeolite Y). (Recently, wider pore hydrophobic "zeolites" have been synthesised. These are discussed later in this chapter for now we restrict our attention to the "classical" zeolites.)... 

Wide-pore zeolites must have a low "framework density" (FD), equal to the number of tetrahedral "T" atoms (silicon or aluminium) per lOOOA. Two questions arise naturally in this context. What sets the framework density What is the minimum FD achievable for a hydrophobic zeolite These are difficult questions to answer within the standard three-dimensional Euclidean view of these frameworks. However, within a two-dimensional hyperbolic perspective, they are readily answered. 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

In recent years there have been attempts to utilize Si02-rich hydrophobic zeolites, which are obtained by intensive dealuminization (with SiCl4 or ammonium rephicement) of zeolite Y, for solvent recovery. 

There has been considerable interest on the influence of zeolites on several aspects of the photochemical behaviour of ketones. Turro et al. have carried out further work on the influence of zeolites on the photochemical activity of dibenzyl ketones. In this study pentasil zeolites were used. In other work Turro has reported the use of triplet-triplet energy transfer as a probe of surface diffusion rates. The luminescent properties of some aromatic ketones in the presence of the hydrophobic zeolite Silicalite have been examined and a spectral study of trsmsients formed by the irradiation of aryl ketones in zeolites has been reported. ... 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

A- HYDROPHILIC ZEOLITE MEMBRANE B- HYDROPHOBIC ZEOLITE MEMBRANE... 

Usually, the condensation between arenes and aldehydes is carried out in the liquid phase, and large pore zeolites are necessary in order to make the reaction occur at an acceptable rate in the condensed phase. When formaldehyde is the reactant, one main problem is the presence of water, since the aqueous solution of formaldehyde (formalin) is the simplest, cheapest and most widely available reactant for use on an industrial scale. This implies the need for hydrophobic zeolites, in order to avoid preferential filling of the pores by more polar water molecules rather than by the aromatic substrate [8,9]. The objective of the work reported here was to compare the performance of different solid acids, and to study the effect of operating parameters on catalytic performance in the hydroxymethylation of guaiacol with formaldehyde catalyzed by a commercial dealuminated, and thus more hydrophobic, H-mordenite zeolite. 

In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-Beta. These results can be explained by the well-known strong affinity of bulk HPA towards polar oxygenates, which would lead to the preferential adsorption of acetic anhydride on HPA, blocking access for toluene to the catalyst surface. It appears that the hydrophobic zeolites with high Si/Al ratios less strongly differentiate the adsorption than the hydrophilic HPA and, therefore, are more suitable catalysts for the acylation of nonpolar aromatics like toluene. 

Another technique used to desorb pollutants from adsorbent is heating by infiared radiation. Some examples have been published lor the regeneration of hydrophobic zeolites saturated by organic volatile compounds coming fiom the paint industry [94],... 

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