Other Solid Acids

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Incomplete carbonization of sugar, starch or cellulose followed by sulfonation produces stable and very active catalysts [25]. These materials, denoted sugar catalysts , not only show higher activity than other solid acids in the esterification of waste oils, but also maintain more than 90% of their original activity even after 50 cycles and they are stable up to 275 °C [26]. 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

NMR has been extensively applied to carbonyl compounds in acidic zeolites and other solid acids. The unshared pairs of electrons on the oxygen can interact with either Brpnsted or Lewis sites, and aldol condensation reactions are commonly observed. Acetone was first studied on a zeolite by Bosacek and co-workers (146) followed by Haw and co-workers (147) and later by Gorte and co-workers (148). The conclusion of an earlier acetone paper of Gorte and co-workers (149) was that acetone forms a static complex on the Brdnsted site of HZSM-5 at room temperature, but this claim was later revised (150) upon the realization that molecular motion in the complex is not halted except at appreciably lower temperatures. 

Ellis and co-workers published a 13C MAS study of ethylamine on solid acids in 1981 (157). Maciel and Haw (158, 159) published NMR studies of pyridine as a probe molecule on solid acids in 1983. We have recently begun to reexamine the 15N spectrum of pyridine on zeolites and other solid acids (160). At low temperatures pyridine is remarkably sensitive to the kinds of acid sites present. Figure 28 shows 15N spectra of pyridine adsorbed on HY samples before and after dealumination. Dealumination in this case seems to make four kinds of Lewis sites distinguishable by NMR of adsorbed pyridine, suggesting pyridine as a good candidate for... 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of... 

A range of other solid acids was also tested and they usually exhibited good characteristics. These include graphite powders,64,65 NdCl3 supported on K10 montmorillonite,66 Ga203 and ln203 supported on MCM-41,67 and supported ionic liquids.68... 

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. 

Due to the high conversion obtained with SAC 40 and especially with SAC 80, only a small amount of catalyst contamination was observed and the catalyst could be reused several times or regenerated by a washing procedure with acetone and diluted nitric acid solutions. Compared with the conversion of resorcinol using lower amounts of other solid acid catalysts such as Amberlyst 15 (74%), H-BEA (36%), H-US-Y (10%) and H-ZSM-5 (3%), the SAC 40 and SAC 80 Nafion/silica nanocomposites are the favoured catalysts. 

The Pechmann reaction over Nafion/silica composites is an example that in some cases, zeolites show no advantage over other solid acids. Further progress, not only in the area of zeolites but also in the field of other solid acids like acidic ion-exchange resins, is unavoidable. 

In several cases, such as the MPV reduction, the specific pore structures of zeolites lead to selectivities not realizable with other catalysts. Nevertheless, in some cases, a zeolite shows no advantage in activity over other solid acids, except for the superior regenerability provided by their thermally stable inorganic matrix. 

Olefin alkylation, long carried out in solution with Bronsted acids, is now beginning to turn to the use of zeolites or other solid acid reagents under mild conditions. The main problem is removal of both the heat generated by the reaction and the carbon deposits. While the former problem can be solved by slurry reactor techniques, the latter requires a chemical treatment which is best carried out by metal-catalyzed hydrogenation at high temperature. 

There are many other examples of useful applications of zeolite and mesoporous materials, as well as other solid acid catalysts, for developing sustainable chemical processes. Several have been reviewed by Corma [233] and other authors [234—236]. 

Traffic fuels account for about one-third of oil use and thus development of improved and more sustainable traffic fuels is a priority worldwide. We will not discuss this problem here, but instead take only two examples to evidence that not only zeolite or mesoporous materials are used as solid acid catalysts but that other solid acid catalysts (ion-exchange resins in these specific cases) are also vhdely applied [218]. 

According to the open literature, other solid acid alkylation catalysts are generally susceptible to poisoning/deactivation by water and other common feed impurities (e.g., oxygenates, sulfur compounds, dienes, etc.), thus necessitating (potentially costly) feedstock pretreatment for their removal. In some cases, this requirement is further mandated by the potential corrosion problems associated with the use of halogens in the catalyst system. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

Figure 6. Relative performance of an engineered catalyst to other solid acid catalysts.

A remarkable synthesis of oxalate salts using a supercritical mixture of CO2 and CO under very drastic conditions (400 bar, 380°C) in the presence of solid 082(003) was reported [71]. Friedel-Crafts-type alkylations on zeolites [72] or other solid acid catalysts [73] have been studied using SCCO2 as the medium. Performing the reaction in the supercritical state was found to be superior to either liquid or gas phase processes. Using scCOj lead to enhanced catalyst life-... 

Using the catalyst 1, the alkylation reaction (3) which is a probe reaction for the synthesis of fragrances proceeded with better yields compared to various other solid acid catalysts (see figure 6). The drying method of the catalyst and the amount of residual water is critical for the reaction rate. 

The use of heterogeneous catalysts is almost unknown, although some patents claims the catalytic effect of silica [11], and only a few papers deal with the use of other solid acids like zeolites [12]. 

Sulfonated polysiloxanes have been tested in a variety of reactions. A typical reaction used to compare the acid strength of these catalysts with that of other solid acids is dehydration of isopropanol to yield propene [3]. The splitting of ethers, especially MTBE, at temperatures up to 200 °C has also been successfully demonstrated [14]. 

Nafion-silica composite has proved to be a promising catalyst in acid catalysis. Different studies with this catalyst revealed very high activities per weight of catalyst compared with the pure Nafion and with other solid-acid catalysts, e. g. zeo-... 

For continuous processing it is necessary that the catalyst does not deactivate and that it is possible to regenerate the catalyst for reuse. If not, catalyst disposal will lead to waste production and other solid-acid catalysts, e. g. zeolites, might be more attractive. This aspect must still be investigated in detail, as must the reproducibility of the production of the composites. Finally, it should be mentioned that the selectivity of the reactions can be affected by reducing the acidity of the acid sites, because of an interaction between the silica matrix and the polymer backbone. 

---