Oxide catalysts acidic character

A wide range of nonacidic metal oxides have been examined as catalysts for aromatization and skeletal isomerization. From a mechanistic point of view, chromium oxide catalysts have been, by far, the most thoroughly studied. Reactions over chromium oxide have been carried out either over the pure oxide, or over a catalyst consisting of chromium oxide supported on a carrier, usually alumina. Depending on its history, the alumina can have an acidic function, so that the catalyst as a whole then has a duel function character. However, in this section, we propose only briefly to outline, for comparison with the metal catalyzed reactions described in previous sections, those reactions where the acidic catalyst function is negligible. 

The one-stage conversion of propene to acrylic acid is much more difficult than the selective production of acrolein. The process is essentially a two-step process in which acrolein is the intermediate product. Further oxidation leads to acrylic acid. In fact, contrasting catalyst properties are required for these reaction steps. The acrylic acid production demands an acidic catalyst surface, while a basic, or only weakly acidic character is preferred for the selective acrolein formation. Therefore, enhanced combustion and by-product formation are unavoidable. 

Deeper oxidation products like furan and, particularly, maleic acid anhydride, can be produced by catalysts that have a stronger oxidative power than the above type-(a) catalysts, but, at the same time, have retained the capacity to transfer oxygen selectively to the organic molecule (a capacity which is absent in the type-(b) catalysts). Besides, a more acidic character of the catalyst surface is probably required to produce an acidic product like maleic anhydride effectively. The most interesting catalysts of this group are V2Os-based catalysts and certain molybdates and Mo03-based catalysts. 

H4SiMo12O40 on the support for catalytic oxidation of methanol (184). At coverages larger than 0.25 monolayer, the selectivities remain constant dimethyl ether is the main product (about 80%), showing the acidic character of the catalyst. Below 0.25 monolayer, the acidic character is lowered, and dimethyl ether rapidly disappears. 

Hydrogenation of flavone or flavanol over copper-chromium oxide at 145-165°C resulted in the formation of o-hydroxy-l,3-diphenylpropane as the chief product, which was isolated in yields of about 50%, and in no case flavan was found in a yield greater than 34%. In hydrogenation of flavonol (3-hydroxyflavone) (70) over copper-chromium oxide, the catalyst was deactivated rapidly because of a strong acidic character of flavonol, and only 17% of 3,4-dihydroxyflavan was obtained. Probably for a similar reason, quercetin (71) was not hydrogenated at all in dioxane over copper-chromium oxide or over Raney Ni, even at 200°C.233... 

A similar technique has been used to determine the acidic character of niobium oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane, toluene, methanol and isopropanol) using aniline and 2-phenyl-ethylamine as probe molecules [27, 28]. The heat evolved from the adsorption reaction derives from two different contributions the exothermic enthalpy of adsorption and the endothermic enthalpy of displacement of the solvent, while the enthalpy effects describing dilution and mixing phenomena can be neglected owing to the differential design and pre-heating of the probe solution. 

In this paper we present a comparative study on the ODH of Cg-C alkanes on VAPO-5, MgVAPO-5 and an active and selective V-Mg-0 mixed oxides catalyst. From these results, the importance of the acid-base character of the catalysts, in addition to the presence of redox sites, on the selectivity to olefins from the oxidative dehydrogenation of alkanes Is tentatively proposed. 

This behavior can be explained in terms of different oxide surface acidities. Whereas silica (pH 4) exhibits acidic properties, alumina surfaces shows a much more basic character (pH 8) [5] and, therefore, allows further hydrolysis of the alkoxysilanes. The influence of the surface pH on the characteristics of anchored molecular structures has also been reported by Deo and Wachs [6] for the preparation of supported vanadia catalysts via incipient wetness. 

The use of a class of pentafluorophenyl Pt(ll) complexes as catalysts allows the efficient epoxidation of simple terminal alkenes with environmentally benign hydrogen peroxide as the oxidant. Key features of this system are very high substrate selectivity, regioselectivity, and enantioselectivity, at least for this class of substrates. These properties are related to the soft Lewis acid character of the metal center that makes it relatively insensitive to water but, at the same time, capable of increasing the electrophilicity of the substrate by coordination. The reversal of the traditional electrophile/nucleophile roles in epoxidation helps explain the unprecedented reactivity observed. 

The physicochemical properties of potassium-, bismuth-, phosphorous- and molybdenum-doped (MeA7 atomic ratios of 0 to 1) V2O5/Y-AI2O3 catalysts and their catalytic behavior in the oxidative dehydrogenation of propane have been compared. The incorporation of metal oxides modifies the catalytic behavior of alumina-supported vanadia catalysts by changing both their redox and their acid-base properties. In this way, the addition of potassium leads to the best increase in the selectivity to propylene. This performance can be related to the modification of the acid character of the surface of the catalysts. The possible role of both redox and acid-base properties of catalysts on the selectivity to propylene during the oxidation of propane is also discussed. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

Figures 1, 2 and 3 summarize the results of the reactions of E2, E3 and E2M2 at 573 K, respectively. Yield was estimated after 60 min reaction. SA, SOa/ZrOz, alumina and modified clay minerals were active for the disproportionation. Other oxide catalysts such as TiOz, Zr02, Na-Y, MgO and niobic acid were inactive for the reaction, instead only a decomposition reaction took place. Si02 was totally inactive. Acidic catalysts showed good catalytic activity, while ones with weak or non-acidic character were inactive. Solid bases were inactive for the disproportionation reaction. Though W03/Ti02 and niobic acid have an acidic character and are excellent catalysts for the olefin isomerization [4] and the olefin-...

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions are usually mediated by metal alkoxides such as Al(0/-Pr)3. The activity of these catalysts is related to their Lewis-acidic character in combination with ligand exchangeability. The mechanism of these homogeneous MPVO reactions proceeds via a cyclic six-membered transition state in which both the reductant and the oxidant are co-ordinated to the metal center of the metal alkoxide catalyst (Scheme 1). The alcohol reactant is co-ordinated as alkoxide. Activation of the carbonyl by co-ordination to Al(III)-alkoxide initiates the hydride-transfer reaction from the alcoho-late to the carbonyl. The alkoxide formed leaves the catalyst via an alcoholysis reaction with another alcohol molecule, usually present in excess [Ij. 

As with enantioselective hydrogenation, we see that several factors are involved in the high efficacy of the Ti(OiPr)4-tartrate epoxidation catalysts. The metal ion has two essential functions. One is the assembly of the reactants, the allylic alcohol, and the hydroperoxide. The second is its Lewis acid character, which assists in the rupture of the 0—0 bond in the coordinated peroxide. In addition to providing the reactive oxidant, the r-butyl hydroperoxide contributes to enantioselectivity through its steric bulk. Finally, the tartrate ligands establish a chiral environment that leads to a preference for one of the diastereomeric TSs and results in enantioselectivity. 

Catalysts belonging to this group are less common, and their activity for redox reactions is relatively low, at least at low temperatures. The solid oxides of the third period Na20, MgO, AI2O3, Si02, and P2O5 are insulators, and they exemplify the transition from basic through amphoteric to acidic character. The oxides of the elements of other periods behave similarly. 

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