Zeolites hydrophobicity

The results indicate that the zeolite can selectively extract specific compounds from the reaction medium, due to the different affinity towards each of them. This makes possible to develop reactant concentrations inside pores which are different from the bulk ones. This property is a function of the zeolite hydrophobic characteristics, which are affected by the Si/Al ratio. The best zeolite is that one which does not interact too strongly neither with more polar molecules, so to allow activation of formaldehyde to proceed faster, nor with the least polar ones. The intermediate Si/Al ratio in H-mordenites is able to develop the optimal concentration ratio between reactants inside the pores, and to reach the highest yield to vanillols. 

We have seen that it is possible to design active sites with acidic or basic strength better adapted to the needs of a particular reaction. In comparison with progress made in acidic or basic catalysis with zeolites, redox catalysis with zeolites is in its infancy. We are just now understanding that properties such as zeolite hydrophobicity, pore dimensions, and active site location are determinant factors for this type of catalyst. Several reviews are recommended for those interested in organic chemistry [111-113]. 

Sikdar et al. (2000) developed adsorbent-filled PV membranes for removing VOCs from waste water. These membranes were prepared by dispersing at least one hydrophobic adsorbent uniformly into a polymer matrix. Polymeric membrane was made of rubbery polymer selected from the group consisting of PDMSs, PTMSP, PUs, polycarbonates (PCs), PE-block-polyamides, silicon PCs, styrene butadiene rubber, nitrile butadiene rubber, and ethane-propene terpolymer. The hydrophobic adsorbent was selected from the group consisting of hydrophobic zeolites, hydrophobic molecular sieves, activated carbon, hydrophobic polymer resin adsorbents, and mixtures thereof. 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

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. 

The concept of zeolite action was tested in a particular reaction where the enzyme is exposed from the beginning to an acidic environment the esterification of geraniol with acetic acid catalyzed by Candida antarctica lipase B immobilized on zeolite NaA [219]. Lipases have been used for the hydrolysis of triglycerides and due to their ambivalent hydrophobic/hydrophilic properties they are effective biocatalysts for the hydrolysis of hydrophobic substrates [220]. When water-soluble lipases are used in organic media they have to be immobilized on solid supports in order to exhibit significant catalytic activity. 

The epoxidation of hex-l-ene catalyzed by Ti-beta samples synthesized in the conventional, basic medium (Ti-beta(OH)) is compared in Table X with that catalyzed by a sample synthesized in a fluoride-containing medium (Ti-beta(F)) (13). The latter was more hydrophobic. Results for the reaction catalyzed by TS-1 are also included in Table X. Ti-beta(F) is superior to TS-1 for reaction in acetonitrile solvent. The most significant difference between Ti-beta(F) and Ti-beta(OH) is in their selectivities. Although the selectivity to the epoxide for reaction in acetonitrile is always very high, regardless of the zeolite for reaction in methanol, Ti-beta(F) is more selective than Ti-beta(OH) (76.6 vi. 54.9%, Table X). Both Ti-beta samples are, however, less selective than TS-1 for reaction in methanol. 

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

Along with hydrophobicity, large amounts of both water (to promote hydrolysis) and methanol employed as co-solvent in the catalyst preparation (to promote homogeneity) are needed to ensure optimal reactivity, showing the number of experimental parameters of the sol-gel synthesis which can be controlled independently to optimize the performance of the resulting catalyst. Finally, in contrast to zeolites and other crystalline porous materials, amorphous sol-gel materials show a distribution of porosity which does not restrict the scope of application of sol gel catalysts to substrates under a threshold molecular size. 

If one examines the evolution of new zeolite structures over the past decade the most interesting discoveries have been made with high silica compositions. Many of these phases can be prepared in essentially all silica forms. Purists would prefer to classify such molecular sieves as organosilicates or porosils (1), in part because the physical properties differ from more classical low Si/Al ratio zeolites. In particular, the high silica zeolites tend to be more thermally stable and chemically robust. Additionally, the higher the Si/Al ratio the more hydrophobic the zeolite. These features are desirable for catalysts that may be used in catalytic processes such as cracking (3). 

Alvarez-Cohen et al. [91] explicitly showed that microbial transformation rates of trichloroethylene (TCE) were proportional to the aqueous TCE concentrations and independent from zeolite-sorbed TCE concentrations. Apparently in contrast to these findings, Crocker et al. [92] reported on the direct bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay to Pseudomonas putida 17848, but not to Alcaligenes sp. strain NP-Alk. It should be noted that sorption to the hexadecyl chains of HDTMA resembles more the solubilisation by a surfactant than adsorption to a solid surface. Possibly, hydrophobic surface structures of strain 17848 allowed the close contact with HDTMA, thereby facilitating the uptake of naphthalene by a lipophilic pathway. 

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. 

DAY zeolites obtained by fluorination of Y zeolites are also hydrophobic (102). This is due to the low concentration of OH groups in these materials as shown by their I.R. spectra. 

He found that these zeolites were hydrophobic and attributed this to the absence of silanol groups and the formation of Si-O-Si bonds in the vacancies generated by dealumination. 

The formation of such bonds during the heat treatment of dealuminated mordenite has also been suggested by Rubinshtein et al. (72-74), in some instances without the intermediate formation of SiOH groups. The hydrophobic nature of the zeolite also increases with progressive dealumination. Chen (71) has shown that aluminum-deficient mordenite zeolites with SiO /Al O ratios over 80 absorb little or no water at low pressure. These highly silicious zeolites are truly hydrophobic and in this respect are similar to highly silicious zeolites prepared by direct synthesis (e.g. ZSM-5) (75). 

The zeolite ZSM-5 has the MFI-type structure and can be obtained with many different Si/Al ratios typically ranging from about 10 to If no aluminum is present (Si/Al = ), a pure siliceous structure is obtained and the resulting material is then called Silicalite-1. The unit cell composition of an MFI-type zeolite can be written as (Na+, H A Si, xOl92-/ H20 withx < 27 (in most cases <9) and n < 16 (the more siliceous a zeolite gets, the higher is its hydrophobicity). 

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