Lewis metal oxide catalysts

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Concerning the nature of Lewis basic sites, little work has been done to establish general rules and models, except for alkaline earth metal oxides and zeolites. With respect to the former, i.e., the nature of oxygen Lewis basic sites on alkaline earth metal oxide catalysts, a charge-density model predicts that the strength of the sites decreases in the order > OH > H2O > H30. ... 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Adsorption complexes of methane at MgO are interesting because they relate to the conversion of methane to ethylene and methanol. In particular, oxidative coupling of methane on metal-oxide catalysts attracted great attention [119]. Usage of methane as a probe to identify and characterize adsorption sites of different acid strength on oxide catalysts is another important aspect. Because CH4 is not easily captured by surfaces of metal oxides, the nature of the interaction of methane with surface sites was little understood until recently. A FTIR spectroscopy investigation of methane on MgO at 173 K revealed adsorbed molecular species preferentially bound at Lewis basic sites CH4 adsorption on a Lewis acid-base pair has also been proposed [120]. 

By far the largest outlet for benzene (approx. 60%) is styrene (phenyl-ethene), produced by the reaction of benzene with ethylene a variety of liquid and gas phase processes, with mineral or Lewis acid catalysts, are used. The ethylbenzene is then dehydrogenated to styrene at 600-650°C over iron or other metal oxide catalysts in over 90% selectivity. Co-production with propylene oxide (section 12.8.2) also requires ethylbenzene, but a route involving the cyclodimerization of 1,3-butadiene to 4-vinyl-(ethenyl-) cyclohexene, for (oxidative) dehydrogenation to styrene, is being developed by both DSM (in Holland) and Dow. 60-70% of all styrene is used for homopolymers, the remainder for co-polymer resins. Other major uses of benzene are cumene (20%, see phenol), cyclohexane (13%) and nitrobenzene (5%). Major outlets for toluene (over 2 5 Mt per annum) are for solvent use and conversion to dinitrotoluene. 

Fig. 2.4C) and S04 /Al203 (5 P -49 and -3 ppm Fig. 2.4D). Likewise, the additional weak peak appearing at 5 P 34 ppm (Fig. 2.4C) has been ascribed due to the presence of TMPO. On the basis of earlier mentioned results obtained firom the P-TMP NMR approach, it is indicative while metal oxides such as Z1O2, Ti02, and AI2O3 possess mosdy Lewis acidity, further sulphonation treatment of these metal oxide catalysts tends to not only increase the Lewis acidic strength (as indicated by the downfield shift of the P resonance) but also promote formation of new Bronsted acid sites.

The likely role of amide species produced by oxidation of Lewis-site coordinated ammonia finds support from the studies concerning ammonia activation and oxidation on metal oxide catalysts. 

The chemical and mechanistic aspects of the SCR-NH3 process over metal oxide catalysts have been discussed in details by Busca et al. [38]. Their conclusions are that the mechanism on V(W, Mo)-Ti02 catalysts is based on the dissociative chemisorption of ammonia on Lewis (vanadium) acid sites and that NO reacts with the amide chemisorbed species to form a nitrosamide key intermediate which then decomposes to N2 and H2O. The catalytic cycle is closed by reoxidation of the reduced catalyst by gas-phase oxygen. 

It is seen that metal and metal oxide catalysts have significant roles in the catalyHc process for the production of fuels and chemicals. There are several catalysts that are used commercially in the biorefinery system to produce fuels and chemicals. Metal oxides are composed of cations possessing Lewis acid sites and anions wifh Br0nsted base sites. They are classified into single metal oxides and mixed metal oxides. Transition metal oxides have catalytic activity for cellulose hydrolysis, and when used as solid acid catalysts, they are reusable and thus may be easily separated from the reaction mixture. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

The catalysts of epoxide polymerisation are strong bases, acids (H2SO4), Lewis acids (AICI3, FeCl3), metals (K, Al, Zn), and metal oxides (AI2O3, Fe203). 

Another important factor affecting carbon deposition is the catalyst surface basicity. In particular, it was demonstrated that carbon formation can be diminished or even suppressed when the metal is supported on a metal oxide carrier with a strong Lewis basicity [47]. This effect can be attributed to the fact that high Lewis basicity of the support enhances the C02 chemisorption on the catalyst surface resulting in the removal of carbon (by surface gasification reactions). According to Rostrup-Nielsen and Hansen [12], the amount of carbon deposited on the metal catalysts decreases in the following order ... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

The establishment of the equilibrium is often accelerated by acidic or basic catalysts, for example, by strong acids (p-toluenesulfonic acid), metal oxides (antimony trioxide), Lewis acids (titanium tetrabutoxide, tin acetates or tin oc-toates), weak acid salts of alkali metals or alkaline earth metals (acetates, benzoates), or by alcoholates. 

We focus attention here on titania (Ti02) for the following reasons. The first is that titania is a widely used oxide support for both metal particles and metal oxides, and used in some cases also directly as catalyst (Claus reaction, for example). The second is that it possesses multifunctional properties, such as Lewis and Bronsted sites, redox centres, etc. The third is that it has several applications both as a catalyst and an advanced material for coating, sensors, functional films, etc. The fourth is its high photocatalytic activity which make titania unique materials. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

A large variety of catalysts, both homogeneous and heterogeneous, has been found active for dehydrohalogenation. The catalysts include a number of Br nsted and Lewis acids (liquid or soluble, as well as solid), metal oxides, active carbon, carbides, nitrides and some metals. However, in the latter case, the actual catalysts are most probably surface metal halides... 

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