Acidic site characterization

The samples are named as Fe/Al-MCM41 (nSi/nAl). Pyridine as a molecule probe was adsorbed on dried samples for the acidic sites characterization by FTIR. MWCNTs were synthesized using the CCVD of ethylene as described elsewhere [7]. 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

The decrease in acidity is due to the selective elimination of Bronsted acid sited characterized by the band at 3600 cm in the infrared spectrum. The authors claim that at the same time new, strongly acidic sites are formed under these conditions. 

Infrared Spectra. Figure 2 shows the spectra of pyridine adsorbed on / -alumina. Two types of Lewis acid sites are present strong Lewis acid sites, which still bind pyridine on evacuation at 350°C and characterized by the 1622 and 1454 cm-1 bands and weak Lewis acid sites, characterized by the 1614 and 1450 cm- bands. Br nsted acid sites, which have characteristic bands around... 

M0O3) has been investigated for samples in both original and rehydroxylated form. The spectra are shown in Figure 3. It appears that Brdnsted acid sites, characterized by the 1636 and 1540 cm l bands are observed only, when the discs are rehydroxylated in wet air before the calcination under high vacuum in the IR cell takes place (Figure 3b). By consequence all spectra have been recorded for such rehydroxylated samples. Only one Lewis band is observed for the molybdenum-alumina sample, opposite to the observations of Kiviat and Petrakis (JL9), who have observed two Lewis bands for their samples. 

However, the molybdenum-alumina and the high calcined cobalt-molybdenum-alumina samples still show an important difference. The pyridine spectra of MoCo-124 indicate a second Lewis acid site, characterized by the 1612 cm-1 band. This band differs from the weak Lewis acid sites of the alumina support (1614 cm- ) because the position is significantly different. It also appears that the strength of the bond between pyridine and the catalyst is stronger, for the 1612 cm-1 band is still present after evacuation at 250°C, while the weak Lewis band (1614 cm-1) of the alumina has disappeared at this desorption temperature. Obviously the second Lewis band for the MoCo-124 catalyst is introduced by the interaction of cobalt with the surface molybdate layer. This interaction is... 

Armaroli, T., Busca, G., Carlini, C., Giuttari, M., Galletti, A. M. R., and Sbrana, G., Acid sites characterization of niobium phosphate catalysts and their activity in fructose dehydration to 5-hydroxymethyl-2-furaldehyde. J Molecular Catalysis A-Chem 2000, 151 (1-2), 233-243. 

Characterization of Surface Acid Sites. Characterization of surface acidity was one of the first successful applications of infrared in catalyst studies (1). Catalyst acidity is typically a function of surface hydration, and water is relatively easy to remove. 

In this approach it is assumed that the system under study consists of acidic sites characterized by their acidity constants, Ka It is also assumed that the population of sites can be described by a continuous pA a distribution, fipKa)-The experimental data can be transformed into a proton-binding isotherm, Q, representing the total amount of protonated sites, which is related to the pAT distribution by the following integral equation ... 

Recently, Tops e et al. have reconsidered the same system in a quite detailed study. Ammonia and pyridine adsorption have, in particular, been studied. The most active acidic site, characterized by the band at 3600 cm , are thought to be at the channel intersection. Weaker acidic sites, characterized by the band at 3720 cm" are, in contrast with previous assignments, related to terminal groups on the external surface of zeolite and possibly to non-zeolitic impurities. 

Table 3. Concentrations of Lewis acid sites characterized by different absorption bands in the FTIR spectrum of adsorbed CO...

The acidity of clays and pillared clays has been studied by infrared spectroscopy using pyridine as a probe molecule. Fig. 3.57 shows the infrared spectra of pyridine adsorbed on beidelite pillared with aluminum hydroxide oligomers after thermal treatment under vacuum at increasing temperatures. The intercalated beidellite contains Lewis acid sites and Bransted acid sites characterized by 1454 cm" and 1540 cm , respectively. The intense band at 1454 cm is associated to Lewis acid sites on the pillars, since the band does not show up in the spectra of proton exchange clays. 

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