SITES, BR0NSTED

Lewis acid site Br0nsted acid site ... 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

The modification of the electronic and catalytic properties of metal clusters in zeolites has been reviewed by Gallezot. [108] He showed that in addition to changes in the electronic structure of metal clusters as a result of intrinsic size effects, the electronic structure of a metal cluster can also be modified by the cluster environment, for example, by electron transfer from the metal dusters to electron acceptor sites in the zeolite lattice. He considered Lewis add sites, Br0nsted add sites, and multivalent cations to be potential electron acceptors. Electron defident clusters appear to be resistant to poisoning by sulfur, which is advantageous in catalytic applications. The issues of duster size effects and electronic effects are complex and continue to be vigorously debated. 

Aluminum Site Br0nsted Site Relative Energy ... 

Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites.

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

The major addic sites on H-MOR are Bronsted sites determined by pyridine adsorption studies above 80 % of addic sites are Br0nsted sites and the rest are Lewis add sites [4,5]. After adsorption of NH3, 0.3 kPa of EA are admitted on H-EDTA-MOR at 473 K (Figure 6F) adsorbed NH3 is easily replaced by EA to produce deformation bands of NH3+ (1597 cm-i, 1497 cm-t), CH2 (1460 cm-i). This spectrum is quite the same as the spectrum in Figure 6A. The results suggest that adsorption of EA is much stronger than that of NH3. When adsorbed EA is heated up to 573 K (Figure 6G-6H), the spectra are almost the same as the spectra in Figure 6B and 6C. 

In two recent papers [8,10], we have initiated studies aimed at understanding the catalytic behavior of WZ and PtWZ. Our observations, which motivated the present study, can be summarized as follows, a) Water of reduction results in the formation of Br0nsted acid sites, as monitored by pulsed addition of pyridine to a DRIFTS chamber at room temperature [8,10]. In this paper, we have complemented those results with similar pyridine adsorption experiments at... 

Figure 2. The Hammett indicator p-fluoronitrobenzene on the Br0nsted site of zeolites. Left -Theoretical structure calculated with DFT at the BLYP/DNP level. Right - Experimental MAS NMR spectra on HY (top) and HZSM-5 (bottom).

Sulfate-doped zirconia was found to have both Lewis and Brdnsted add sites on the surface, where the sulfate groups were the carrier of the protonic site. The ratio of Lewis to Br0nsted sites was found to vary depending on heat treatment, but the percentage of sulfate groups associated with a proton was found to be constant at 12-17%. 

Two-Dimensional correlation analysis to study Br0nsted acid sites in zeolites... 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

For zirconia, the species in Scheme 11.4 have been proposed [67], accounting for both Br0nsted and Lewis acidity. These Br0nsted acid sites are proposed to be present only if sulfates, and possibly poly(sulfates), are also present on the top terminations of Zr02 crystallites [66]. 

Fig. 2 Br0nsted acidic and basic sites of BINOL phosphates...

According to Limtrakul and colleagues the mechanism of the Beckmann rearrangement of the oxime molecule on the Br0nsted acid site of a zeolite proceeds according to the following steps outlined in equation 73. 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

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