Lewis acids sites

Chen and coworkers employed trimethylphosphine oxide (TMPO) as the probe molecule to characterize pure and sulfated zirconia both with and without a metal promoter [210]. On pure Zr02 resonances were detected at 62, 53, 41 and 34ppm. 

The peaks at 53 and 62ppm correspond to physisorbed TMPO while the peaks at 62 and 53 ppm can be assigned to TMPO adsorbed on Br0nsted (B) and/or Lewis (L) acid sites. Upon hydration the intensity of both these signals diminished, indicahng that they in fact correspond to Lewis acid sites. Had they been Br0nsted sites the interaction between TMPO and the catalyst would have been too strong to be influenced by hydration. Upon addihon of a metal promoter additional resonances are observed at 87, 76, 68 and 65 ppm. Hydrahon experiments confirm that these new sites are Bronsted acid sites. Furthermore, quantification of the distinct acid sites identified on the catalyst has been carried out, based on the assumption that each TMPO molecule can only be adsorbed on one acid site. 

In addihon to organic phosphines and phosphine oxides, other probe molecules which have been successfully employed in the study of Br0nsted sites include alkanenitriles [211-213] ( C and NMR), acetone [167] and CO [214] ( C NMR). 

As alluded to above, probe molecules such as organic phosphates can also be used to probe Lewis acid sites. However, the most common probe molecules employed are nitrogen-containing species, in parhcular ammonia, alkylamines and pyridine. For the latter two either or N NMR techniques may be applied however, N NMR is generally preferred as the resultant spectra appear less complex and, as adsorphon to the surface occurs via the nitrogen atom, the observed resonance shifts are larger than they would be for spectra. The development of this method to investigate Lewis acidity of aluminas and siUca-aluminas has previously been discussed in detail by Eckert [196] and by Maciel and Ellis [183] and will only briefly be recapped here. 

The initial studies demonstrating the feasibility of using probe molecules to study acid sites were conducted by Gay and coworkers from 1977 onwards [215, 216]. This approach was further developed by Dawson and coworkers who employed CP-MAS NMR to investigate the number and nature of acid sites on y-Al203 through the adsorption of n-butylamine and pyridine [217, 218]. In the case of 

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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... 

Lewis acid sites Lewis-base donors Lewis bases Lewisite... 

An additional effect of the use of an organic medium in the catalyst preparation is creation of mote defects in the crystalline lattice when compared to a catalyst made by the aqueous route (123). These defects persist in the active phase and are thought to result in creation of strong Lewis acid sites on the surface of the catalysts (123,127). These sites ate viewed as being responsible for the activation of butane on the catalyst surface by means of abstraction of a hydrogen atom. 

The mineral talc is extremely soft (Mohs hardness = 1), has good sHp, a density of 2.7 to 2.8 g/cm, and a refractive index of 1.58. It is relatively inert and nonreactive with conventional acids and bases. It is soluble in hydroduoric acid. Although it has a pH in water of 9.0 to 9.5, talc has Lewis acid sites on its surface and at elevated temperatures is a mild catalyst for oxidation, depolymerization, and cross-linking of polymers. 

Abstraction of a hydride ion by a Lewis acid site from a hydrocarbon... 

The next step is the ahstraction of a hydride ion hy a Lewis acid site from the zeolite surface to form the more stable allylic carhocation. This is again followed hy a proton elimination to form a cyclohexadiene intermediate. The same sequence is followed until the ring is completely aromatized. 

Catalyst acid properties depend on several parameters, including method of preparation, dehydration temperature, silica-to-alumina ratio, and the ratio of Bronsted to Lewis acid sites. 

Both the Bronsted and Lewis acid sites on the catalyst generate carbenium ions. The Bronsted site donates a proton to an olefin molecule and the Lewis site removes electrons from a paraffin molecule. In commercial units, olefins come in with the feed or are produced through thermal cracking reactions. 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

Figure 5.10. Defects consisting of oxygen vacancies constitute adsorption sites on a Ti02 (110) surface. Note how CO binds with its lone-pair electrons on a Ti ion (a Lewis acid site). O2 dissociating on a defect furnishes an O atom that locally repairs the defect. CO2 may adsorb by coordinating to an O atom, thus forming a carbonate group. [Figure adapted from W. Gopel, C. Rocher and R. Feierabend, Phys. Rev. B 28 (1983) 3427.]...

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Pt/Y-Al203 and indicates that Lewis acid sites are affiliated with the catalytic activity of the support to dissociate methane. 

For determination of the number of protonic sites and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-i (due to pyridine chemisorbed on Lewis acid sites, L-Py) and 1490 cm i (due to both the L-Py and pyridine chemisorbed on protonic acid sites, B-Py) were used with the tangent background for all samples. The values obtained were normalized to the weight of the sample wafer. To obtain the apparent absorption coefficients of the bands, a known amount of pyridine was adsorbed on the sample, and the absorbance of each band was measured. Then, a small quantity of water which is sufficient to convert all Lewis acid sites into protonic acid sites was introduced into the IR-... 

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...

The catalytic activity for NO oxidation [reaction(l)] was strongly inhibited by water vapor, because this reaction occurs on Lewis acid sites of zeolite as... 

At RT, NH3 adsorbed on Lewis acid sites, Zr and V. Accordingly, the intensity of bands from NH3 decreased little with the V-content, by 15% at most, as expected on account of the similar Lewis acid strengths of Zr V and VV. The symmetric bending... 

Pairs of Lewis acid and Lewis base sites are formed. The Lewis acid site is an ion that is co-ordinatively unsaturated. A water molecule will transform a Lewis site into a Bronsted site (Fig. 3.22). 

Figure 3.53. IR transmission absorption spectrum of pyridine adsorbed on partly dehydroxylated HY zeolite (Van Bekkum et al, 1991) B = Bronsted acid sites L = Lewis acid sites.

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

IR spectroscopy of two supports was used for the determination of their surface acidity. The presence of Lewis acid sites on the surface of sepiolite allowed the preparation of a catalyst able to transform citral into menthol in fairly good yield under veiy mild conditions (90°C, 1 barH2). 

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