Brdnsted acids sites

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

Fig. 13. Dehydration of methanol on a Brdnsted acid site (a) shows the side-on complex, (b) the transition state, and (c) the dissociative complex of surface methoxy and water. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

Fluoride addition promotes the cracking (108-110) and isomerization (108, 111) activity of alumina, presumably, because of the formation of Brdnsted acid sites. In a comprehensive study of fluorided aluminas, Antipina et al. (110) demonstrated that there is a close parallelism between generation of cumene cracking activity at 430°C and surface acidity when fluoride content is increased from 0 to 7% wt (see Fig. 15). Acidity was measured by n-butylamine titration endpoints were determined by means of an arylcarbinol indicator that detected Br0nsted acids stronger than those corresponding to a pKa of — 13.3. In a separate article (112), Anti-... 

Sugioka and Aomura (133) provide kinetic evidence indicating that the rate-determining step in the hydrocracking of aliphatic sulfur compounds over silica-alumina is catalyzed by Brdnsted acid sites. Conversions of reactants were measured by use of a pulse reactor hydrogen was used as the carrier gas. They found that reactivities of mercaptans are in the following order ... 

The observed catalytic behavior in the case of l-methyl-2-ethylbenzene isomerization (158) was not so straightforwardly related to the Brdnsted site concentration. The maximum activity was observed with samples activated at temperatures where significant dehydroxylation had occurred. This reaction occurs more readily over acid catalysts than toluene disproportionation or xylene isomerization and may require fewer Brdnsted acid sites, or the reaction mechanism may involve Lewis sites. 

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. 

Spectra of adsorbed pyridine have been recorded for the MoCo-124 catalysts, for which the final calcination temperature after the cobalt impregnation has been varied. It turns out that the 400 and 500°C calcined samples and the 650 and 700°C calcined samples show very similar spectra. Therefore we show only the spectra of the 400°C (low calcined) and the 650°C (high calcined) samples. Figure 4 shows spectra after desorption at 150 and 250°C. Few Brdnsted acid sites are observed in the low calcined MoCo-124 samples. The reflection spectra (Figure 1) indicate for these low calcined samples the presence of cobalt on the catalyst surface, because no cobalt aluminate phase could be detected. The high calcined samples do show the presence of Brdnsted acid sites the presence of a cobalt aluminate phase is concluded from the reflection spectra (Figure 1) for these samples. 

These experiments indicate that at low calcination temperatures the cobalt ions are present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate surface layer. At the higher calcination temperatures, the cobalt ions move into the alumina lattice. The BrGnsted acid sites reappear, indicating that the situation on the molybdate surface is restored. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

The reappearance of Brdnsted acid sites has been observed for the high calcined nickel-molybdenum-alumina catalysts. The presence of a nickel aluminate phase has been concluded from the reflectance spectra. The second Lewis band (1612 cm l) has a very low intensity, in comparison with the cobalt containing catalysts of a same composition and after the same calcination conditions. 

Kiviat and Petrakis (19) have shown that the cobalt and nickel ions influence the spectra of pyridine adsorbed on molybdenum-alumina. They have concluded that the introduction of these ions results in a change of the ratio of the intensities of the bands of the Lewis and Brdnsted acid sites. Our observations show that this... 

A picture has been formed of the way in which the promotor ions are built in the M0O3-AI2O3 system. The neutralization of the Brdnsted acid sites, as originally present in M0O3-AI2O3 systems by the cobalt ions for the catalysts calcined at low temperatures ( 500°C) indicates that the cobalt ions are present on the catalyst surface. The liberation of these sites in catalysts calcined at high temperatures ( 650°C) and the observation of the characteristic reflectance spectrum of C0AI2O4 show that the cobalt ions enter the alumina lattice. However the interaction between cobalt and molybdenum, as indicated by the second Lewis band remains present. This leads to the conclusion that the cobalt ions are present in the surface layers of the alumina lattice. 

For the high temperature calcined cobalt-molybdenum-alumina catalysts, the presence of a cobalt alumlnate phase has been concluded from the reflection spectra. The Brdnsted acid sites reappear In the Spectrum of absorbed pyridine. Indicating that the... 

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/Al ratios calculated using equation (23) and their comparison with bulk values [14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Brdnsted acid sites on the external surface of the zeolite crystals. The measurement of... 

In this respect it is interesting to note that from the Nj3 binding energies of chemisorbed pyridine it was indeed found that the Brdnsted acid sites are stronger in Al-ZSM-22 and Fe-ZSM-22 [44,45] and weaker in Fe-ZSM-5 and B-ZSM-5 [46] than in Al-ZSM-5. 

B/L)32 under the assumption that peak 2 was for a Brdnsted acid site. Obviously the first hypothesis is very far from being verified whereas the second hypothesis yields B/L estimates in good agreement with the IR data, specially for samples B, C and D. These three samples are the ones which were found to have surface properties more representative of the bulk in the discussion of Table 4. 

The FR results are not correlated with the OH bands observed in the FTIR spectra of the numerous zeolites studied (second column in Fig. 1). For exanq>le X- and Y-faujasites have the same structure and OH-bands, but their FR spectra are quite strikingly different. The FR method seems to be more sensitive to distinguidiing Brdnsted acid sites with different strengths especially when the dependence on temperatme of the FR spectra is taken into consideration. 

IR spectra of H-boralites show four hydroxyl bands at 3450, 3680, 3720 and 3740 cm. These bands were assigned [7,11] to Si—OH—O, B—OH, Si—OH -B, and terminal Si—OH respectively. In the case of H-ZSM-5, two bands were observed 3610 (Si—OH— Al) and 3740 cm (terminal Si—OH). The sorption of pyridine resulted in a reaction of Si—OH - B and Si—OH—A1 groups and the formation of PyH ions. The maximal intensity of PyH band was used to calculate the content of Brdnsted acid sites in the boralites and zeolites studied. These concentrations are presented in Table 2. After the neutralization of all Bronsted acid sites pyridine was desorbed. This was accompanied by the diminishing of PyH and reappearance of acidic hydroxyl bands (3720 cm in the case of boralites or 3610 cm in the case of H-ZSM-5). The intensities of both PylT and OH bands are presented in Figure 1. 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

Pt/HZSM-5 showed high and stable catalytic activity for the hydrodesulfurization of thiophene at 400 C and its catalytic activity was higher than that of commercial C0M0/AI2O3 catalyst. It is concluded that the Brdnsted acid site and spillover hydrogen formed on Pt particle in Pt/HZSM-5 catalyst play an important role for the hydrodesulfurization of thiophene. 

As mentioned above, Pt/HZSM-5 showed higher activity than Pt/NaZSM-5 in the hydrodesulfurization of thiophene. This indicates that the BrSnsted acid site of HZSM-5 in Pt/HZSM-5 catalyst plays an important role for the hydrodesulfurization of thiophene.lt can be assumed that the Brdnsted acid site in the Pt/HZSM-5 catalyst is one site active for the activation of thiophene, whereas R is an active site for the activation of hydrogen. In other words, Pt/HZSM-5 catalyst is a bifunctional catalyst for the hydrodesulfurization of thiophene, in which both Brdnsted acid and platinum metal sites are active sites. 

Acceding to this mechanism, thiophene is adsorbed on Brdnsted acid site of Pt/HZSM-5 and hydrogen is activated on R to form spillover hydrogen. The spillover hydrogen formed on R attacks the reaction intermediate, e.g., species such as S=C=CH-CH=CH2, formed by the decomposition of thiophene adsorbed on the strong Brdnsted acid site of the zeolites [16]. 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

Zeolites are widely used as solid acid catalysts for a number of organic transformations, such as the cracking of n-paraffins which are catalysed by Bronsted acid sites. " In the case of zeolites, the so-called bridging hydroxyl groups in the i-OH-Als configuration as shown in Eq. (1) are known to act as Brdnsted acid sites and they are responsible for the ability of zeolites to catalyse the reactions. Therefore, the characterization of acidic properties of solid acids is of great importance in discussing the catalytic properties of solid acids. 

Fig 1 gives IR spectra for pyridine adsorbed on previously dehydrated samples. The spectrum of starting zirconium dioxide obtained through an alcogel step lacks the band eharacteristic for Brdnsted acid sites. The addition of Cr203 into zirconium dioxide leads to acidic B-sites characteristic for pyridinium ions with a band at 1540 cm". This may be related to formation of structure such as [3] ... 

Both Lewis and Brdnsted acid sites exist on pillared clays. The acidity depends on the exchanged cations, the preparation method and the starting clay [8], It is known that surface acidity is important for SCR reaction of NO by NH3 [4, 5, 9]. Different pillared clays were synthesized and tested for their activities in the SCR NO [10, 11]. Research on titanium pillared clays was initiated by Sterte [12], who first reported the synthesis of titanium pillared montmorillonite using TiCU solution in hydrochloric acid. Bernier et al. 

Brdnsted acid sites) ratioed against the 1870 cm zeolite lattice overtone band. These results are shown in Table 1. 

Only the band of silanol groups absorbing at 3740 cm could be clearly observed on samples which were activated in vacuo or in D2 at 673 K. After rehydrating the samples thus pretreated then evacuated at 723 K show a broad OH band centered at 36DD cm in addition to the SiOH band. The a-cidity tests, in which pyridine was used as probe, showed practically only Lewis acidity. The band due to Brdnsted acidic sites at 1550 cm was detectable but not evaluable. From these results, it can be concluded that the concentration of Brdnsted acidic sites is negligible in the dehydrated samples independently of the pretreatment conditions. 

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