Acidic sites on alumina

The basic and acid sites on alumina surfaces have been represented graphically (43, 72). In order for the acid and the basic sites of the alumina to attack tra s-l,4-cyclohexanediol from different planes of the catalyst surface it is necessary for the dehydration to be restricted to submicroscopical holes or crevices or to occur between channels of those particles. Since the basic and acid sites of the alumina have to surround the cyclohexanediol, as in the solvolytic reaction, the alumina therefore can be considered as a pseudosolvent for such dehydration reactions. 

Rate increases of orders of magnitude can be obtained by addition of Lewis acids like BF3 or A1C13, or in the heterogeneous systems, by Lewis acid sites on alumina that coordinate to the acyl group and thus drive the reaction, probably via steps of the type,... 

Although it is clear that surface Lewis acid sites on alumina are due to coordi-natively unsaturated AP ions, it is not fully clear what is the coordination of such surface ions. Most authors agree that at least three different types of Lewis acid sites (weak, medium, strong) exist on transitional aluminas, arising in some way from the two or three coordinations of the ions in the bulk spinel-type structure. 

The sequence involves dehydrogenation of n-pentane on Pt particles to form intermediate n-pentene (N ), which then migrates to the acidic alumina and reacts to i-pentene (H) in a rate-determining step. The i-pentene subsequently migrates back to the Pt particles where it is hydrogenated to the product i-pentane. The following sequence describes these processes (where represents an acid site on alumina) ... 

H2 reduction. This may be due to interaction with Lewis acid sites on alumina. The IR spectra of cluster-derived platinum catalysts indicate bridged CO chemisorption, which is not seen with conventional Pt catalysts (Fig. 12). This is attributed to the morphological situation on the alumina-bound Pt clusters, in which coordinatively unsaturated faces are exposed. Moreover, it is interesting to find that the Pts-Ptis clusters did not undergo oxidative fragmentation in CO chemisorption even on the hydrated alumina and MgO (66a) unlike the case of Rh carbonyl cluster analog such as Rh4(CO)i2 and Rh,(CO)i5(Fig. 11). 

Pearson has contributed an elegant paper in his study of Bronsted acid sites on alumina. Although Lewis sites are regarded by many as the main contributors to acidity, the Bronsted sites can be measured as follows. When physically adsorbed water has been removed under mild conditions and deuteri-ated pyridine is added to the surface then reaction occurs on the surface as shown (7). Under the right conditions the proton associated with the pyridine appears as the sole narrow line in the n.m.r. spectrum. It can be measured quantitatively. 

In literature the two types of acidic sites on alumina sur ce have been discusse (33,34) as follows ... 

Surface science studies have generated much insight into how hydrocarbons react on the surfaces of platinum single crystals. We refer to Somorjai [G.A. Somor-jai. Introduction to Surface Chemistry and Catalysis (1994), Wiley, New York] for a detailed overview. Also, the reactions of hydrocarbons on acidic sites of alumina or on zeolites have been studied in great detail [H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice (1991), Elsevier, Amsterdam],... 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

In this reaction, an acidic site on the catalyst is shown as an aluminum atom bound to three oxygen atoms, and this is a site where an alcohol molecule can bind. If the acidic sites on the alumina are shielded by reacting it with a base, the oxide is no longer an effective acid catalyst. In other words, the acidity is removed by attaching molecules such as Nff3 to the acidic sites. 

More recent studies in this field have revealed the importance of the intrinsic acidic sites on the aluminas in directing the course of the dehydration. Recently the consideration of stereochemical factors involved in the dehydration and the use of gas chromatography as an analytical tool has led to a better understanding of this reaction, with a resultant better appreciation of the reaction mechanism. 

Concerning the nature of the acid sites on dehydrated alumina, the chemical evidence is more in favor of Lewis-type than of Bronsted-type acids. Trambouze and Perrin (343) estimated the content of Lewis acid sites by thermometric titration of a slurry in benzene with dioxane. The authors stated that Lew is acidity was not observed in boehmite and its dehydration products, only in the products obtained from hydrargillite. As mentioned earlier, Webb (339) found no indication of Bronsted acidity ammonia did not form ammonium salts. The quantity of ammonia chemisoi-bed per unit area in the range from 175° to 500° was not a function of the hydrogen content. After exten.sive dehydration, more ammonia was chemisorbed at 100 mm pressure, even at 500°, than corresponded to the hydrogen present (341b). 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

Surface molybdenum species and acid sites on nitrided molybdena-alumina catalysts... 

The surface structure and acid sites of alumina-supported molybdenum nitride catalysts have been studied using temperature-programed desorption (TPD), and reduction (TPR), diffuse reflectance infrared spectroscopy, and X-ray diffraction (XRD) analysis. The nitride catalysts were prepared by the temperature-programmed reaction of alumina-supported molybdenum oxide (12.5% and 97.1%) with NH3 at temperatures of 773, 973, and 1173 K. TPR and XRD analyses showed that y-Mo2N was already formed at 973 K. On the basis of NH3-TPD measurements and IR spectroscopy, it was found that Lewis acid sites were predominant over Bronsted acid sites on the surface of Mo2N/A1203. 

A recent study by Yamamura and Nakatomi (126) shows that phospho-tungstic acid impregnated on alumina is more active than alumina for codimerization of ethylene with propylene. The authors concluded that the newly formed acid sites (H0 s - 5.6) increased the yield of pentenes. 

A recent infrared study of the interrelationship of silica-alumina acidity and activity for olefin polymerization was carried out by Mizuno et al. (134). They showed that Lewis acid sites on silica-alumina could be selectively poisoned with pyridine in an infrared cell by a three-step... 

Take et al. (69) extended the acid strength range of the n-butylamine titration method. They employed indicators such as 4-nitrotoluene (pKa = - 10.5) and 2,4-dinitrotoluene (pKa = - 12.8), which are considerably less basic than the other Hammett indicators used to measure surface acidity. Endpoints were determined spectrophotometrically. These authors found that the acid sites on silica-alumina catalyst had an acid strength corresponding to an H0 between - 10.5 and - 12.8 a few sites had even higher acid strengths (H0 < 12.8). Strong acid sites were eliminated when silica-alumina was poisoned with sodium ions. 

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 relationship between the two catalytic components is quite complex. Interactions between the support and the hydrogenation component can alter this relationship. For example, Larson et- al. (6) showed that, with platinum on silica-alumina, a selective adsorption of platinum by acid sites causes a reduction in catalyst acidity. Similarly, nickel reacts with the acid sites on silica-alumina forming nickel salts of the silica-alumina acid sites. It has been suggested (J) that one of the effects of sulfiding a nickel on... 

However it is now clear that acid strength is not a singlevalued function, that different kinds of bases will tend to rank acid strengths differently, depending on their "hardness" or "softness". This is illustrated by the work of Kobayashi (13), who used a spectroscopic method of determining [B]/[BH+] on a sample of silica-alumina titrated with n-butylamine and one titrated with pyridine (Figure 2). Evidently the acidity of indicators and of individual acid sites on solids needs at least two constants for complete characterization of acid strength. 

Beltramini and Trimm (67) utilized Pt-, Sn- and Pt-Sn- supported on y-alumina for the conversion of n-heptane at 500°C and 5 bar. They observed that during six hours less coke per mole of heptane converted was deposited on the Pt-Sn-alumina catalyst than on Pt-alumina however, the total amount of coke formed during six hours was much greater on Pt-Sn-alumina than on Pt-alumina. The addition of tin increased the selectivity of dehydrocyclization. Since hydrocracking and isomerization activity of a Sn-alumina catalyst remained high in spite of coke formation, the authors concluded that there was little support for the suggestion that tin poisons most of the acid sites on the catalyst. These authors (68) also measured activity, selectivity and coking over a number of alumina supported catalysts Pt, Pt-Re, Pt-Ir, Pt-Sn and Pt-... 

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... 

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