Postsynthesis modification

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],... 

For many applications, e.g. in the area of sensing, catalysis or separations, modifications of the inorganic backbone are required to provide a certain specific surface chemistry or active sites on the inner pore surface. Different routes, such as simple inclusion of molecules, or the covalent attachment of functional entities, are applied. Because of the specific synthesis strategy, the structure-directing agent can also be used for functionalization of the mesostructured material. Only in situ modifications are discussed postsynthesis modifications of the final porous material are, in principle, possible, but are beyond the scope of this article. 

Furthermore, with the advent of improved instriunentation and experimental techniques interesting in-situ investigations became possible which were related, for instance, to the synthesis of and heterogeneous catalysis on zeofites, catalyst deactivation, diffusion or solid-state ion exchange as well as other postsynthesis modifications. Combinations of IR spectroscopy with various characterization techniques such as, e.g., temperature-programmed desorption of probe molecules (TPD/IR, cf.[223,224]), electron spin resonance spectroscopy (ESR/IR, cf.[225,226]), UV-Vis spectroscopy [227,228], etc. were developed. 

Although tin is an important element in both adsorbent applications and catalysis science, procedures for the incorporation of Sn" into zeotype structures at framework positions were not readily evident (147). The postsynthesis modification of zeohtes using Snp4 or SnCl4 was attempted, but the crystalline structures of the zeohtes were damaged (4,148). However, Mai et al. (66a) incorporated Sn" into silicahte (MFI) and silicahte-2 (MEL) structures. Tin incorporation into the frameworks of MTW (66a, 149), beta (68), and MCM-41 (150) has also been accomplished. 

Li et al. (69) reported the preparation of nanosized Sn-beta particles by a postsynthesis modification. Highly dealuminated beta zeoHte underwent a gas-solid reaction with SnCl4 vapor at elevated temperature. The properties of the resultant Sn-beta sample were characterized by various techniques. It was inferred that the tin species were inserted into the framework via reaction of SnCl4 with sdanol groups in hydroxyl nests that had been created during the dealumination. The tin thus predominately occupied sites with tetrahedral coordination. The tin content that could be achieved by this postsynthesis method was as high as 6.2 wt%, corresponding to a Sn/(Si +Sn) ratio of 0.034. 

In conclusion, the epoxidation of propylene with bulky oxidants (such as cumene or TBHP) can be successfully achieved using titanium-containing mesoporous materials as catalysts. The catalytic chemistry of the active sites can be controlled via the synthesis conditions and postsynthesis modifications. The hydrophobicity of the catalyst is of great importance to achieve a highly selective catalyst. The Ti-MCM-41-based heterogeneous catalyst has demonstrated excellent performance in the commercial process for PO manufacture. 

A. Postsynthesis Modification of Pre-Formed Metal Oxide Clusters 59... 

OMTOC can be prepared by two strategies. The organic groups can either be grafted to a preformed cluster (postsynthesis modification method) or introduced during the cluster synthesis (in situ method). 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

BET = Brunauer-Emmett-Teller CMON = covalent metal-organic networks CTF = covalent triazine-based framework DCQNI = A,A -dicyanoquinodiimine DEF = A,A-diethylformamide DMA = A,A-dimethyl-acetamide FP-TRMC = flash-photolysis time-resolved microwave conductivity HHT = 2,3,6,7,10,11-hexa-hydroxytriphenylene M-CAT = metal-catecholate PSM = postsynthesis modifications PXRD = powder X-ray diffraction TCNQ = tetracyanoquinodimethane TOF = time-of-flight TTF = tetrathiafulvalene. 

Morao, A., I. C. Escobar, M. T. P. De Amorim, A. Lopes, and I. C. Gonsalves. 2005. Postsynthesis modification of a cellulose acetate ultrafiltration membrane for applications in water and wastewater treatment. Environ. Prog. 24 367-382. 

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