Aluminum oxide surface hydroxyl groups

Figure 3.17 FTIR spectra of the surface hydroxyl groups of y-AhOs samples left aluminum oxide C from Degussa after outgassing at 673 K and 873 K right, samples from Akzo (A), Sud Chemie (B), Engelhardt (C) and Condea (D), all having surface area in the range 180-200m g". ...

We have already mentioned the existence of a chemical interaction of alcohols with a catalyst and the actual possibility of the formation of an intermediate compound on the surface of oxide catalysts. Numerous investigations by Soviet Scientists leave no doubt that the adsorption of alcohol on aluminum oxide and silica-alumina is accompanied by the formation of a surface compound of the ether type. In both cases, due to the similarity of their active centers which are surface hydroxyl groups connected to aluminum atoms /Al-OH, such a surface compound will ob-... 

Another possibility of redox reactions involving surface hydroxyl groups is the change of the oxidation state of the cations to which they are bound. In CoAPO-18 catalysts, a fraction of the framework aluminum is iso-... 

As with methanol ammoxidation, yield and selectivity to the nitrile product are relatively high compared to ammoxidation of an alkene or alkane substrate. Using alumina-supported V-P-Sb-0 catalysts, selectivity to acetonitrile of 96% is obtained at 84% conversion of ethanol (81% acetonitrile yield) at 400°C (101), whereas a silicoaluminophosphate molecular sieve gives a reported 99% yield of acetonitrile at complete conversion of acetonitrile at 350°C (102). Both catalysts possess a relatively high level of surface acidity, mainly because of the presence of phosphorus and aluminum oxide moieties. These are expected to promote the initial step of the reaction—surface alkoxy formation by, for example, an equilibrium with surface hydroxyl groups. 

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

Another fairly important stationary phase in straight phase PLC is aluminum oxide. Comparable with silica gel also in the case of aluminum oxides, hydroxyl groups at the surface of this adsorbent are responsible for the selective retention of sample molecules. The relevant physical parameters for the characterization of aluminum oxides suitable for straight phase PLC are the following ... 

Quite recently it was shown that phosphonic esters, trimethylsilyl [124, 143] and alkyl esters [124,143, 145] could also be used to modify the surface of titanium or aluminum oxide in organic solvents at moderate temperatures. Unlike Si-O-C bonds, P-O-C bonds are not easily hydrolyzed, and their cleavage on an oxide surface was unexpected. Most probably, coordination of the phosphoryl oxygen to the surface assists the condensation by increasing the electrophilicity of the P atom, thus facilitating the condensation of P-0-R groups with surface hydroxyls (Scheme 7) [124]. The chemisorption of... 

Most y-alumina powders placed under high vacuum and heated at moderate temperature (373-423 K) undergo complete water removal, and nearly all of the surface aluminum atoms are hydroxylated. Additionally, for hydroxylated alumina thin films prepared via aluminum evaporation/water oxidation cycles, although the surface density of OH groups would be the same as for powders, the specific surface area differs significantly [9]. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

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