Bronsted silica-aluminas

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

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. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

Two such well studied systems are pyridine chemisorbed on alumina (15) and pyridine chemisorbed on silica-alumina (16). It had been previously shown that alumina contains only sites which adsorb pyridine in a Lewis acid-base fashion whereas silica-alumina has both Lewis and Bronsted acid sites. These two different kinds of sites are distinguishable by the characteristic vibrational bands of pyridine adducts at these sites (see Table I). Photoacoustic and transmission results are compared in Table II. Note that the PA signal strength depends on factors such as sample particle size and volumes of solid sample and transducing... 

Table I. Assignments of Pyridine Chemisorbed on Silica-Alumina As Lewis Acid Sites (LPY) and Bronsted Acid Sites (BPY)...

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

The silica-alumina surface is still more strongly acidic than the alumina surface. The acidity is less sensitive to poisoning by water. There has been much discussion whether the acidity of silica-alumina is caused by Bronsted or by Lewis acid sites. This matter has not been. settled definitely, although there is evidence that both types of acidity are present. This would explain the observation that the catalytic efficiency in different reactions may be selectively poisoned by different reagents. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

Maciel et al. (370,371,374) combined l3C and 15N CP/MAS NMR to study the adsorption of pyridine on silica-alumina. Hydrogen bonding was found to be the dominant interaction at high loading levels (0.5 to 1 monolayer). At lower coverages, a Lewis acid-base complex dominates and the pyridine is significantly less mobile. Bronsted complexes are found when the surface has been pretreated with HC1 gas. 

Fig. 72. I5N CP/MAS NMR spectra at 20.3 MHz spectra of I5N-enriched pyridine (Py) adsorbed on silica-alumina in the presence of varying amounts (in grams) of n-butylamine (NBA). B, L, and HB denote Bronsted, Lewis, and hydrogen-bonding sites, respectively. Chemical shifts are given in ppm from liquid ammonia (371).

The yield of the reaction (Table 1) clearly depends or the nature of the solid and on the experimental conditions (temperature, time). Thus, with silica and alumina, amorphous solids with a relatively low acidity, the acetophenone oxime molecule reacts with a very low yield, being the only reaction product the hydrolysis one, acetophenone. With the synthetic mixed oxide silica-alumina, that possesses simultaneously BrOnsted and Lewis acidic centres, the conversion is quantitative, being also the major product the hydrolysis one (3), when the reaction is carried out at 160°C. 

The question of the acidity of silica, alumina and silica-alumina surfaces has always been of great interest to catalytic scientists. Previously, transmision infrared spectroscopy, particularly of pyridine adsorption, has been used to distinguish the presence of Lewis and Bronsted acid sites on oxide surfaces (24). The frequency shift of the surface OH group during adsorption now... 

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28, 29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHt. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NHt near 3.2 and 7.0 p. 

The catalytic activity of amorphous silica-alumina ([Si—Al]) in reactions via carbonium ions is due to the existence of Bronsted acid sites on their surface. Consequently, amorphous [Si-Al] acid catalysts provide acid sites and transport to the active sites easily. As a result, amorphous [Si-Al] acid catalysts have been widely operated as cracking catalysts. Acid zeolites have been successfully applied as cracking catalysts. However, in some industrial applications of acid catalysts, for example, in the cracking of hydrocarbons of high molecular weight, zeolites are not useful, since... 

It therefore seems quite natural to choose silica, silica aluminas, and aluminium oxide as the objects of the first systematical quantum-chemical calculations. These compounds do not contain transition elements. They are built of the individual structural fragments primary, secondary, etc. This enables one to find the most suitable cluster models for quantum-chemical computations. The covalent nature of these structures again makes quite efficient a comparatively simple method of taking into account the boundary conditions in the cluster calculations. Finally, these systems demonstrate clearly defined Bronsted and Lewis acidity. This range of questions comprises the subject of the present review. This does not by any means imply that there are no quantum-chemical computations on the cluster models of the surface active sites of transition element oxides. It would be more proper to say that the few works of this type represent rather preliminary attempts, being far from systematic studies. Also, many of them unfortunately include some disputable points both in the statement of the problem and in the procedure of calculations. In our opinion, the situation is such that it is still unreasonable to try to summarize the results obtained, and therefore this matter is not reviewed in the present article. 

Alkylphenols may be produced via the alkylation of phenol with methanol. This reaction produces predominantly anisole and o-cresol with methylanisole and xylenol also being obtained. Strong Bronsted acids are not required to effect these transformations, as both amorphous aluminas and silica/aluminas are active catalysts. Often o-cresol can be produced in 100% isomeric selectivity, particularly when the reaction is run over amorphous alumina based catalysts. When zeolites are used isomer selectivities are changed. 

Bronsted acid sites can be directly probed through solid-state H NMR spectroscopy, as chemical shifts can be correlated with acid strength [195, 197, 198]. The precise chemical shift observed for any given Bronsted acid site is dependent on the material upon which it is located. For instance, on silica values of 1.6ppm are typically observed zirconia has two distinct OH sites, at 2.4 and 4.8ppm while on alumina a typical range may be -0.2 to 4.3 ppm. Early studies employing H NMR to study Bronsted acid sites focused on the characterization of the surface of amorphous silica-alumina materials [165, 199-201]. Extensive work, however. 

Depending on the preparation method, on the Si02/Al203 ratio and on the micro- or mesoporous structure, very different acidic properties have been observed for silica-alumina mixtures. Some samples contain both Bronsted and Lewis sites, while for others only acidity of the Lewis type was observed [86, 87]. 

The acidity of the catalyst arises due to interaction of the components (e.g., silica and alumina) during preparation. Pure silica and pure alumina have little or no cracking activity, but the presence of only a few hundredths of a per cent of alumina in silica is sufficient to produce an active catalyst (138,148,320,321). Activity and acidity both increase, up to a certain point, with increased alumina content (320,324). Infrared spectra of ammonia chemisorbed on silica-alumina catalyst indicate that most of the chemisorbed ammonia is in the NHs form, with only a relatively small amount of NH4+ (204). From this evidence it is concluded that most of the catalyst acid is of the Lewis type since, in reacting with a Bronsted acid or a hydrated Lewis acid, the ammonia would be converted to an ammonium ion. 

Liang used n.m.r. to characterize amines adsorbed on hydrated silica-alumina they found evidence for protonated species on sites which were not sterically hindered. Further work on the thermal desorption of pyridine and n-butylamine from silica-aluminas (13 and 25 wt % AI2O3) showed that the stronger acid sites, where pyridine was adsorbed, were of varied acid strength, and both number and strength of the sites were affected by alkali poisoning. An i.r. study of pyridine adsorbed on sodium-poisoned silica-alumina also showed both Lewis and Bronsted sites to be affected. 

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