Bronsted acid centers

Recrystallization procedure applied to the amorphous aluminosilicates of different chemical composition resulted in the formation of the dispersed zeolitic domains of the FAU and BEA structure in porous matrices. The structural transformation into the composite material was proved with TEM, XRD and 27Al and 29Si MAS NMR spectroscopies. The IR data revealed that strong Bronsted acid centers were main active sites generated in the composite materials, irrespectively of the Al content. 

TT-mordenite, which contained 11.2 wt % alumina, was dealuminated by A A heating for 4 hours at 700° C and then leaching under 6N HC1 reflux to 0.1 wt % alumina content (1) (aluminum-deficient H-mordenite). The intrinsic cracking activities of the parent H-mordenite and acid-leached H-mordenites are proportional to aluminum content (2) (i.e., the associated Bronsted acid centers). The deactivation rate of the aluminum-deficient H-mordenite is much lower than that of the parent H-mordenite. 

The most commonly used type of catalyst is a relatively small, bifunctional molecule that contains both a Lewis base and a Bronsted acid center, the catalytic properties being based on the activation of both the donor and the acceptor of the substrates. The majority of organocatalysts are chiral amines, e.g. amino acids or peptides. The acceleration of the reaction is either based on a charge-activated reaction (formation of an imminium ion 4), or involves the generalized enamine catalytic cycle (formation of an enamine 5). In an imminium ion, the electrophilicity compared to a keton or an oxo-Michael system is increased. If the imminium ion is deprotonated to form an enamine species, the nucleophilicity of the a-carbon is increased by the electron-donating properties of the nitrogen. ... 

The doubly-bonded (bridged) OH groups, formed according to Eq. (2), become thus Bronsted acid centers, while the singly-bonded (terminal) hydroxyls are expected to exhibit a predominantly basic character A large amount of reliable data regarding the interaction of water with titanium dioxide, are provided by infra-red spectroscopic studies associated with temperature-programmed desorption measurements. 

The acidity of bulk and supported Nb205 was probed by absorption of py. Raman and IR spectroscopy of the adsorbed pyridine indicated that highly distorted surface NbOfi octahedral sites corresponded to Lewis acid sites (the Raman band between 850 and 1,000 cm-1) and that slightly distorted surface Nb06 sites, as well as NbCF and NbOg sites, corresponded to Bronsted acid centers (Raman bands between 500 and 700 cm-1).690,691 The acidic character of hydrated niobia (niobic acid Nb205 H20) depended on calcination temperature. Niobic acid calcined at moderate temperatures of 373-573 K showed acidic character but it became almost neutral when calcined at 873 K.692 The presence or absence of water determines Lewis or Bronsted acidity of a given metal site (Scheme 50).693... 

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 selectivity for acylation as well as catalytic activity are lower at high temperatures the decrease in the yield of the acylated product at higher temperatures is due to the conversion of Bronsted acidic centers into Lewis centers. The acylation proceeds largely at the 2-position of the heteroaromatics to yield a somewhat sterically bulkier product. In fact, it is not shape selectivity but electronic and thermodynamic effects that mainly determine the position of the acylation in these substrates. 

Pyridine adsorption shows a dramatic increase of Bronsted acid centers paralleled by the decrease of Lewis acid centers (see upper spectrum of Fig. 2). This is in agreement with the observation of the growing amount of reduced Cu particles accompanied by a loss of Lewis acid sites. From our experiments a correlation between cluster size and the water and hydrogen partial pressure must be assumed. 

Martin et al. [429] used the IR/pyridine technique (cf. Sect. 5.5.2.6.2) to look at the elimination of Bronsted acid centers upon hydrothermal treatment of H-ZSM-5 zeolite and evidenced the condensation of extra-framework Al-containing species (Lewis sites). 

Reduction of reducible metal cations in exchanged zeolites with hydrogen resulted in the formation of structural OH groups (Bronsted acid centers), which... 

According to Equation 5-66, the Al center can form its fourth bond with a free electron pair of a hydroxide anion. At the same time, the proton can react with a free electron pair of a neighboring O atom, and the formation of a partial bond results in a Bronsted acid center. The Si" " center, which is more electopositive than Al, weakens the 0-H bond and increases the acidity. Experimentally it was found that maxumun acidity occurs at ca. 30 % AI2O3. This model also allows the chemisorption of ammonia on Bronsted centers to be explained (Eq. 5-67). 

A further finding was that only the moderately active Bronsted acid centers are responsible for dehydration, and that Lewis acid centers such as Al " are not involved. Evidence for this is that the addition of small amoimts of bases such as NH3 or pyridine does not inhibit the reaction. The formation of ether on AI2O3 is explained by a Langmuir-Hinshelwood mechanism, in which two adjacently adsorbed... 

Bases like pyridine are more strongly bound to such Lewis acid centers than to Bronsted acid centers, as can be shown by IR spectroscopy and temperature-controlled desorption. Figure 7-6 shows the transformation of Bronsted into Lewis acid centers on calcination of an HY zeolite, monitored by IR spectroscopic measurements on the adsorption of pyridine. 

Water adsorption isotherms were recorded in order to determine the impact of the alkaline treatments (i.e. changes in porosity) on water adsorptioa The adsorption of water within the pores of ZSM-5 is important to EtO filtratioa The surface of ZSM-5 is hydrophobic. Water adsorption on H-ZSM-5 occurs at silanol groups and Bronsted acid centers. Excessive water adsorption has the potential to reduce EtO filtration by firlly hydrating the Bronsted add site, thereby minirrrizing the adsorption arrd sirbsequerrt hydrolysis of EtO. Water adsorption isotherms are presented in Figirre 4. 

The reaction of NH3 with various kinds of acidic centers on Y zeolites (Si/Al = 2.4) at 573 K was also studied by Kapustin et al. [101] by adsorption microcalorimetry. It was determined that the heats of adsorption of ammonia on the Bronsted acid centers of Y zeolites were 110-90 kj moh. These values were closer to those obtained by Tsutsumi et al. [102,103], and can also be compared with those of Auroux et al. [23,104], Huang et al. [105], van San-ten [106], Stach et al. [107], and Lohse et al. [108], which can be found in Table 2. 

The most straightforward hypothesis about the Bronsted acidic center in H-[A1]-MTS, as advanced even recently [43,44], is to assume that it coincides with that of protonic zeolites, the hydroxyl group bridging between an Si and an A1 atom. This acidic group will be designated subsequently as bridging OH species and represented as Si(OH)Al a model is given in Scheme 1. 

A possible explanation is as follows. Because of the amorphous nature of the system and the flexibility of Si-O-Si bonds, Bronsted acidic centers responsible for the 3460 cm absorption after interaction may occur in a variety of coordinative environments, so that the corresponding O - H stretch is smeared out and not observed as a definite band in the spectra of the bare sample, nor is it as a negative band in difference spectra like those in section a of Fig. 8. Such is, for instance, the explanation proposed for the species Si(OH)Ga (see Sect. 7). Adsorption would restore homogeneity in the set of the heterogeneously broadened ensemble of acidic species. 

Following the same Une as liepold et al., Tfombetta et al. [55] and Busca et al. [60] proposed that Bronsted acidic centers of silica-alumina are due to terminal silanols which are somehow modified by neighboring Al cations. 

The presence of Bronsted acid centers was disadvantageous for the liquid-phase oxidation of cyclohexene. It induced the epoxide chemisorption resulting in ring opening and cyclohexanediol formation. 

The surface of E-glass fibers is complex but is believed to have Lewis acid centers and Bronsted acid centers depending upon the history of the surface. One of the useful methods to examine Lewis acid centers is the adsorption of hexachloroacetone.The... 

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