Surface hydroxyl groups activation

Besides characterization of the zeolite structure and surface hydroxyl groups active in catalytic transformations of hydrocarbons, sohd-state NMR represents an indispensable tool for characterization of adsorbates in regard to their identification or establishing the pecuharities of intramolecular dynamics for molecules confined inside the pores of zeolites or other nanoporous materials. Identification of adsorbates, such as hydrocarbons formed in the course of the reaction and remained occluded in the zeohte pores due to impossibility to desorb for some reasons can be performed with or H MAS NMR. Establishing the intramolecular dynamics of the occluded molecules is performed by the analysis of the lineshape of the NMR spectrum of deuterated analogues of these adsorbed molecules. 

It was strongly suggested by the fact that 2,6 dimethyl pyridine detrimentally impacts the activity that surface hydroxyl groups participate in the water-gas shift reaction. 

Recent developments have been reported for imido-molybdenum systems. When X = 2,5-dimethylpyrrolyl, more stable surface species can be obtained (although a second minor species, formed by the addition of a surface hydroxyl group to the alkylidene ligand, is also present). Moreover, by tuning the imido ligand, the catalytic activity has been improved dramatically (Table 11.3, bottom rows) [20]. 

It was later found that stable organometallic compounds of transition metals exhibiting very low polymerization activity could be transformed into high-activity catalysts when deposited on silica, alumina, or silica-alumina.287-289 Interaction of surface hydroxyl groups with the organometallic compounds such as chromocenes, benzyl, and Tt-allyl complexes results in the formation of surface-bound organometallic complexes (41-43) 289-291... 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Recently, a new argument has been added on the basis of quantum chemical calculations [70] which has shown that the attack of an acid on the hydroxyl group activates the hydrogenes on for elimination whereas the loss of hydrogen from the hydroxyl or its substitution by a metal ion (corresponding to the formation of the surface alkoxide II) activates the Ca—H bond for dehydrogenation. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Although it was not suggested that defects are required for selective oxidation over other catalysts, the results indicated that defects and bismuth must be present for high activity and selectivity over scheelite-type catalysts. The authors concluded that the defects which were introduced into the bulk of these phases must manifest themselves in some manner at the surface. The question of how the introduction of defects into these phases affected their catalytic properties was not resolved. However, the active site for catalysis was suggested as a cation vacancy which could abstract a proton from an olefin to form the well-established allyl intermediate and should offer considerable stabilization to a surface hydroxyl group. 

The photocatalytic activity of MgO, too, is considered to be caused by surface hydroxyl groups. This is shown in an investigation of the hydrogen-deuterium exchange at room temperature 78>. Monochromatic light was used in the range between 1800 and 4000 A and with a band width of 96 A. The reaction is treat-... 

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