Oxidative addition Bronsted acids

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-... 

At an industrial scale, the esterification catalyst must fulfill several conditions that may not seem so important at lab-scale. This must be very active and selective as by-products are likely to render the process uneconomical, water-tolerant and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. In a previous study we investigated metal oxides with strong Bronsted acid sites and high thermal stability. Based on the literature reviews and our previous experimental screening, we focus here on application of metal oxide catalysts based on Zr, Ti, and Sn. 

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. 

The type of superacid sites on SO /metal oxides evacuated at 773 K is only a Lewis type according to the IR absorption bands of adsorbed pyridine [31]. Mortcrra ct al. [32] have shown that pyridine adsorbed on Lewis acid sites dominated the spectra of samples evacuated at 673 K and that the addition of water at 300 K significantly increased the amount of Bronsted acidity. Nascimcnto ct al. [8] report that both Bronsted and Lewis acid sites exist on SO4 /Zr02 treated at 723 K and the ratio of Bronsted to Lewis sites changes with the change of sulfur content. Recently. Lunsford ct al. revealed by use of 31P MAS NMR spectra of adsorbed trimcthylphosphinc that three types of Lewis... 

Ammonia (pfCi = 9.24, proton affinity in gas phase = 857.7kj moT ) and pyridine (pfti = 5.19, proton affinity in gas phase = 922.2kJ moT ) are the favored molecules for probing the overall solid acidity, since both Lewis and Bronsted acid sites retain these molecules. However the use of IR spectroscopy or XPS is necessary to distinguish qualitatively and unambiguously between these two types of sites. In addition to NH3 and pyridine, trimethylamine and triethylamine have also been used to probe the acidity of supported oxides. However, it has been mentioned [33] that these two molecules might not be able to equilibrate completely with the surface under typical experimental conditions. The use of substituted pyridines (2,6-dimethylpyridine) has also been considered in order to probe specifically the Bronsted sites [34]. 

Centi and coworkers [84] have suggested that, in addition to Lewis acidity, Br0nsted acidity plays an important role in the selective oxidation of butane to maleic anhydride. The surface phosphorus enrichment means that a number of P—OH groups are present on the catalyst surface. Centi offers three hypotheses for the role of Bronsted acidity the stabilization of reaction intermediates, the stabilization of an adsorbed oxygen species, or the generation of an organic surface species that is involved in oxygen activation or transport. 

A reaction that is catalyzed by a Bronsted acid site, or H, can often be accelerated by addition of a solid acid. Materials like ion-exchange resins, zeolites, and mixed metal oxides function as solid analogues of corrosive liquid acids (e.g., H2SO4 and HF) and can be used as acidic catalysts. For example, isobutylene (IB) reacts with itself to form dimers on cross-linked polyfstyrene-sulfonic acid), a strongly acidic solid polymer catalyst ... 

Figure 14 clearly shows the effect of the addition of potassium to the catalyst on the activity and of the effect of the presence of WO3 in the catalyst thereon. The poison resistance to both alkali metal oxides and arsenious oxide is significantly increased by the presence of WO3 in the WO3/V2O5 on Ti02 catalyst. The reason for this effect is that the Bronsted acid sites protect the catalyst for the attack of alkali. Moreover, WO3 inhibits the oxidation of ammonia and the oxidation of SO2 as well. 

Bidk vanadium pentaoxide is quite active but low selective catalyst of hydrocaibons partial oxidation. It was established by XRD that the higher content of the phosphorus additive in it [12] the weaker peaks attributed to V2O) in bulk catalysts and, simultaneously, P-VOPO4 phase reflections appeared. The latter became the major component of VPO catalyst at fp > 0.67. The constituents of the prepared sample were found to be also 6 0)2P207, VO(POj)2 and some amorphous compounds. At this takes place, all the cations were considered by authors [12] to be bonded in vanadyl groiqis V=0 and phosphorus atoms form Bronsted acidic center each. It has been found an increased concentration of phosphorus over the surface as compared to the biilk and the higher phosphorus content in the sample the grown bulk concentration of the reduced vanadium ions were observed. 

The Bronsted acidity of transition metal hydrides is a well known phenomenon which has been extensively studied in both mononuclear and polynuclear compounds. [50] The Bronsted acidity is a likely property of the dinuclear products resulting from H2 additions, especially for cationic species, since they often contain metal centers in relatively high formal oxidation states. In this respect, it... 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

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