Solid acids acid site type

At this stage an important question has to be addressed from the NH probe point of view how do the (HT), (HTA1) solids compare with the (IS) materiaj. Restricting ourselves to solids with similar framework Si/Al ratios, it appears clearly that both type of solids possess acid sites with very similar... 

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in... 

These views naturally spill over into the discussion of solid acids. Acidity of Broensted sites in zeolites can be enhanced by modification of the local chemical environment of the framework and, since much evidence reveals the presence of both Broensted and Lewis sites, it is not unreasonable to suggest that enhanced acidity can arise fiom a synergistic interaction between bofli types of site (Scheme 3)... 

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... 

From Table 2 it can also be observed that the selectivity towards different hydrocarbon groups strongly depended on the acid properties of solids. Large amounts of C4 and C6 olefins were obtained for the mesoporous NiMCM-41 and NiMCM-48 catalysts with the lowest acid site concentration. In this case, a near Schulz-Flory-type product distribution (C4>C6>C8>Cio) was observed. The increase in acid site density (for the catalysts NiY, NiMCM-36, NiMCM-22) results in decrease of C 6/C8 ratio. These results are in agreement with the reaction network proposed in Scheme 1. 

The foregoing review of the alkylation mechanism and the influence of the catalyst type and reaction conditions show that, in essence, the chemistry is identical with all the examined acid catalysts, liquid and solid. Differences in the importance of individual reaction steps originate from the variety of possible structures and distributions of acid sites of solid catalysts. Changing process parameters induces similar effects with each of the catalysts however, the sensitivity to a particular parameter depends strongly on the catalyst. All the acids deactivate by the formation of unsaturated polymers, which are strongly bound to the acid. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

Although there are some important differences between what we describe as 3-connected aluminium sites in our bb-matrices and what the active sites are thought to be in zeolites, we have begun a preliminary study of the activities of the Al, Ti and V-containing bb-catalysts as solid acid catalysts in the dehydration of alcohols. For this type of bench marking reaction, there are two parameters that can be used as preliminary indicators of catalytic activity lightoff temperatures and product selectivity. A plot of conversion versus temperature produces what is known as a lightoff curve. The temperature at which 50% of the maximum... 

The spectral behavior of CO bonded to metal atoms (metal carbonyls) has been used to characterize the surface of solids (61). For instance, it is known that metal carbonyl interacts with surface site of metal oxides and zeolites to form a Lewis-type adduct where a CO ligand of the metal carbonyl interacts (via the oxygen atom) with surface OH groups or with co-ordinatively unsaturated metal ions (surface Lewis acid sites) (62,63). On the other hand, thermal treatment of the metal carbonyl support adducts lead to loss of CO with formation of subcarbonyls, which are anchored to the support (64,65). Papile et al. (66) reported the characterization... 

In a similar way, Mizota et al. grafted polymer chains functionalized with sulfonic sites over a polystyrene-type polymer. As observed above, the flexibility of the polymer chains allowed better accessibility of the catalytic sites and this solid acid catalyst was ten times more active than the conventionally used cross-linked resin in the hydrolysis of sucrose (Scheme 2) [27]. 

Since the negatively charged compound, naphthalenesulfonate, is not expected to interact with the solid support by specific interactions, the study by McCalley clearly indicates that, at least in this case, the tailing of the peaks has the electrostatic origin discussed here. Traditionally, the tailing of basic compounds has been attributed to interactions between the base and the solid support, e.g., silanol groups, acidic sites, etc. However, further quantitative studies in this area are needed to separate the electrostatic effect on peak asymmetry from other types of interactions. 

The fact that active and selective catalysts in general comprise two or more oxide components is certainly not a sufficient argument to assume bifunctionality the combination of oxides may also cause modification of sites or formation of one type of new sites which combine the specific properties required for a sequence of reaction steps. Such properties may concern the geometry, the type of oxygen bonding, oxygen and charge mobility in the solid, acidity, etc. 

This would indicate that (3 has stronger acid sites than dealuminated HYD samples, something which agrees with IR data of both solids, where the wavenumber of the accesible acidic hydroxy groups is 3620-3630 cm-1 on dealuminated HY, and 3612 cm- in HJ3 zeolites (ref. 8), A similar conclusion was reached by measuring the desorption of pyridine on these two types of large pore zeolites. 

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

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. 

K Tanabe Solid Acid and Base Catalysts This chapter deals with the types of solid acids and bases, the acidic and basic properties, and the structure ofacidic and basic sites. The chemical principles of the determination of acid-base properties and the mechanism for the generation of acidity and basicity are also described. Howacidid and basic properties are controlled chemically is discussed in connection with the preparation method of solid acids and bases. 

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