Brpnsted acidic surface

Unfortunately, this procedure was reported to be inadequate for metallocenes such as Cp2ZrMe2. The authors speculated that the low activity of the clay-supported metallocene was due to severe decomposition of the catalyst in contact with the strong Brpnsted acidic surface of the clay. Scott et al. [86] also showed that the clay dispersion in the polyolefin matrix is stable during annealing at 170°C for 30 min and related this behavior to the high molecular weight and high viscosity at the test temperature (170°C). 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

Krossner, M., Sauer, J., 1996, Interaction of Water With Brpnsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1 1 and 2 1 Surface Complexes , J. Phys. Chem., 100, 6199. 

Sato et al. [195] have studied the surface borate structures and the acidic properties of alumina-boria (3-20 wt.%) catalysts prepared by impregnation method using B(MAS)-NMR measurements and TPD of pyridine, as well as their catalytic properties for 1-butene isomerization. The number of Brpnsted acid sites was found to increase with increasing boria content, and the catalytic activity was explained by the strong Brpnsted acid sites generated by BO4 species on the surface of alumina. 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

On the other hand, Lewis acids are catalytically inactive for skeletal transformations unless proton donors are available at the same locality.1 Lewis acids consist of incompletely coordinated surface ions aluminum ion is the most frequently cited example. Since the relevance of acidity measurements for the prediction of catalytic activity is what we are trying to emphasize, we have concentrated on the determination of Brpnsted acidity in this critique. The problem of finding the most relevant method for acidity measurement has therefore been treated as an evolutionary process in which successive methods have been used more and more successfully for the characterization of a relatively small number of strong Br0nsted acids that are frequently accompanied by a multitude of other surface acids. 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

The fluorescence spectra of 9-anthrol (58) were studied in the sol-gel-xe-rogel transitions of Si-Al glasses of various composition prepared from tetraethyl orthosilicate and diisobutoxyaluminium triethylsilane [205], A biprotonic ESIPT between the anthrone and anthrol tautomers was observed in which a proton of the surface Brpnsted acid site on the —O—Si—O—A1—O— network was implicated. 

In the case of Brpnsted acidity, two reaction mechanisms have been suggested the formation of carbenium ion intermediate species (Scheme 6.2), which lead to the formation of branched oligomers[20] and the formation of a surface alkoxy structure intermediate, which leads to the formation of linear oligomers1191 (Scheme 6.3). In this case, the formation of alkoxy species has been recently... 

Although there are doubts about the existence of Brpnsted acid sites on TS-1 and related materials, there is strong evidence that Lewis acid sites are present on the surface of dehydrated TS-1. The significant activity of TS-1 and of Ti-MCM-41 in the cycloaddition of CO2 to epoxides to give cyclic carbonates (140), a reaction typically catalyzed by Lewis acids such as AICI3, SbFs, etc., lends strong support to the inference of the existence of Lewis acid sites on their surfaces. 

Shen et al. (219) reported on the photoluminescence and IR spectra of pyridine adsorbed on 7-AI2O3 to measure the surface acidities of alumina. They observed that neither the photoluminescence nor the IR spectra of adsorbed pyridine showed any evidence of Brpnsted acidity on y-AbOs pretreated at 673 K, but they showed four different weak OH bands when the alumina was treated at 873 K. 

Fig. 16. Schematic representation of the interaction between H3PO4 and surface OH groups of alumina, (a) Formation of two Brpnsted acid sites (B sites) from the adsorption of one H3PO4 on one OH site of alumina (b) formation of two B sites from two H3PO4 and two OH sites (interaction between neighboring P-OH groups) (c) formation of one B site from one H3PO4 and two OH sites (d) formation of one site with no B sites from one H3PO4 and three OH sites [adapted from Stanislaus et al. (46) and Petrakis et al. (47) reprinted with permission, copyright 1998 Elsevier Science and 1995 the Royal Society of Chemistry].

Sorption to mineral surfaces (as opposed to NOM) is generally viewed as more of a displacement than a dissolution phenomenon. Because mineral surfaces tend to be more polar than NOM, sorption to the former is more substantial for polar and ionic compounds than for those that are more hydrophobic (Curtis et al., 1986 Chiou, 1998). Furthermore, since most NOM and mineral surfaces exhibit either a neutral or negative charge, sorption to soils and sediments is considerably stronger for pesticide compounds that are positively charged in solution—such as paraquat or diquat—than for neutral species, and weaker still for anions. As a consequence, measured values in soils exhibit little dependence upon pH for pesticide compounds that are not Brpnsted acids or bases (Macalady and Wolfe, 1985 Haderlein and Schwarzenbach, 1993). 

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