Solid Acid Catalyzed Reactions

Acid catalyzed reactions have played a significant role in the synthesis of a variety of compounds. While most such reactions were initially run with simple protic or Lewis acids, the use of such materials now presents serious environmental problems concerning the disposal of the unused acids and their salts. To remedy this situation, solid acids have been developed to promote a number of different reactions. Not only are these solid acids easily separated from the product but they are also usually not destroyed in the reaction and can frequently be recycled for prolonged use. Generally, these solid acids are easy to handle and are non-toxic, non-volatile and non-corrosive. 

Not only do these acids present an economic advantage over conventional acid technology, but there are also frequent selectivity eidiancements and changes in reactivity associated with the use of these heterogeneous catalysts. A number of different types of solid acids have been involved in synthetically useful reactions. They range from the hydrogen forms of various ion exchange resins and the perflourinated resin sulfonic acid, Nafion-H, to the amorphous acidic oxides, silica and aluminum silicate,the crystalline zeolites and the natural clays. -9 

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M. Haouas, S. Walspurger, J. Sommer, Regioselective H/D isotope exchange and skeletal rearrangement reactions of propane over strong solid acids, J. Catal, 2003, 215, 122-128. M. Haouas, S. Walspurger, F. Taulelle, J. Sommer, The initial stages of solid acid-catalyzed reactions of adsorbed propane. A mechanistic study by in situ MAS NMR, J. Am. Chem. Soc., 2004, 126, 599-606. 

Ho et al. developed a correlation for the poisoning effects of nitrogen compounds on FCC catalysts and a scaling law for estimating the HDS reactivities of middle-distillates in terms of three key properties. " For solid acid catalyzed reactions, Sowerby and coworkers developed a method for estimating adsorption equilibrium constants from an integrated form of van Hofi s equation. 

Haouas M, Walspurger S, TauleUe F, Sommer J. The initial stages of solid acid-catalyzed reactions of adsorbed propane. A mechanistic study by in situ MAS NMR. J Am Chem Soc 2004 126 599-606. 

Vjunov A, Hu MY, Feng J, Camaioni DM, Mei D, Hu JZ, et al. Following solid-acid-catalyzed reactions by MAS NMR spectroscopy in liquid phase-zeolite-catalyzed conversion of cyclohexanol in water. Angew Chem Int Ed 2014 53 479-82. 

As a further illustration of the compensation effect, we use solid-acid-catalyzed hydrocarbon activation by microporous zeolites. A classical issue in zeolite catalysis is the relationship between overall rate of a catalytic reaction and the match of shape and size between adsorbate and zeolite micropore. 

It has been revealed that the formation of protonic acid sites from molecular hydrogen is observable for the catalysts other than Pt/S042--Zr02, and the protonic acid sites thus formed act as catalytically active sites for acid-catalyzed reaction. We propose the concept "molecular hydrogen-originated protonic acid site" as a widely applicable active sites for solid acid catalysts. 

Although this reaction was discussed earlier, it is mentioned here because it is catalyzed by solid acids such as NH4SCN, which provide H+ ions that bond to the pairs of electrons after breaking them loose from the metal. Over a rather wide range of catalyst concentrations, the rate is linearly dependent on the amount of solid acid. Once the NH4+ ion donates a proton, NH3 is lost and the protonated ethyl-enediamine molecule is the acid that remains and continues to catalyze the reaction. While base catalyzed reactions of complexes may be better known, there are many acid catalyzed reactions as well. 

The isopentene produced will either be protonated or be added to another carbenium ion. With a butyl cation, this would lead to a nonyl cation. The resultant carbenium ion fragment can accept a hydride and form a product heptane, or it can possibly add a butene to form a Cn cation. With hydride transfer, another alkane with an odd number of carbon atoms is produced. Just this example is sufficient to show the huge variety of possible reactions. By means of gas chromatographic analysis, Albright and Wood (82) found about 100-200 peaks in the C9-C16 region, regardless of the alkene and acid employed. A similar number of products can be observed for solid acid-catalyzed alkylation. 

Apart from metal cations, the negative framework can also be compensated by other positively charged cations such as NH4+ and H+. The protonated forms of zeolites have a high acidity because the proton can be easily removed from the zeolite and replaced by other positively charged species. This feature makes them very efficient solid-state acids, which can be used for acid-catalyzed reactions. 

Heterogenous reactions, Sh/Nu ratio, 27 64 Heteroligand complex, 32 260-262 Heteropolyacids defined, 41 117 heteroatoms, 41 118, 120, 121 Prins reaction, 41 156 supported, 41 149-150 Heteropolyanions, 41 113, 117, 119-121 Heteropoly blues, 41 191 Heteropoly compounds absorption, 41 179-180, 190-191 acid-catalyzed reactions heterogeneous, 41 161-178 liquid phase, 41 150-161 acidic properties in solid state, 41 141-150 in solution, 41 139—14] catalysis, 41 114, 116-117, 190-191 as catalyst, 41 113-116, 117, 223-232... 

The continuous reaction system could be combined with solid acid-catalyzed in situ racemization of the slow-reacting alcohol enantiomer [149]. The racemiza-tion catalyst and the lipase (Novozym 435) were coated with ionic liquid and kept physically separate in the reaction vessel. Another variation on this theme, which has yet to be used in combination with biocatalysis, involves the use of scC02 as an anti-solvent in a pressure-dependent miscibility switch [150]. 

This review starts with an introduction to the principles and techniques of solid-state NMR spectroscopy and the description of the most important experimental approaches for NMR investigations of solid catalysts in the working state (Sections II and III). Section IV is a summary of experimental approaches to the characterization of transition states of acid-catalyzed reactions under batch reaction conditions. 

Bulk type I catalysis was found in acid catalysis with the acid forms and some salts at relatively low temperatures. The reactant molecules are absorbed between the polyanions (not in a polyanion) in the ionic crystal by replacing water of crystallization or expanding the lattice, and reaction occurs there. The polyanion structure itself is usually intact. The solid behaves like a solution and the reaction medium is three-dimensional. This is called pseudoliquid catalysis (Sections l.A and VI). The reaction rate is proportional to the volume of the catalyst in the ideal case the rate of an acid-catalyzed reaction is proportional to the total number of acidic groups in the solid bulk. 

The pore diameter of zeolite beta is 7 A, larger than those of silicalite-1 and silicalite-2 (5.5 A). Titanium incorporated into zeolite beta reacts with molecules whose dimensions are too big to diffuse in the pores and be oxidized by TS-1 or TS-2. The drawback is that zeolite beta must contain Al3+ to crystallize, and this imparts strong protonic acidity to the solid, with the consequence that secondary acid-catalyzed reactions also take place. However, the acidic properties can be neutralized in several ways and highly selective oxidations can be carried out on Ti-beta (Section V.C.3.b). 

Okuhara, Mizuno, and Misono report the catalytic properties of heteropoly compounds as exemplified by H,PWl3O40 and the anion [PW,2O40p. Some of these compounds are strongly acidic, and some have redox properties the large-scale applications involve acid-catalyzed reactions. The heteropoly compounds are metal oxide clusters, used as both soluble and solid catalysts. Their molecular character provides excellent opportunities for incisive structural characterization and for tailoring of the catalytic properties. Physical properties also affect catalytic performance. Catalysis sometimes occurs on the surface of the solid material, and sometimes it occurs in the swellable bulk. 

Solid acid catalysts such as mixed oxides (chalcides) have been used extensively for many years in the petroleum industry and organic synthesis. Their main advantage compared with liquid acid catalysts is the ease of separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of the catalyst. Furthermore, the heterogeneous solid catalysts can lead to high selectivity or specific activity. Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. Up until now, the most useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions. 

Superacids Immobilized on Solid Supports. The considerable success of Magic Acid and related superacids in solution chemistry and interest to extend the scope and utility of acid-catalyzed reactions, particularly hydrocarbon transformations, logically led to the attempts to adopt this chemistry to solid systems allowing heterogeneous catalytic processes. 

In discussing superacids as catalysts for chemical reactions, we will review both liquid (Magic Acid, fluoroantimonic acid, etc.) and solid (Nafion-H, etc.) acid-catalyzed reactions, but not those of conventional Friedel-Crafts-type catalysts. The latter reactions have been extensively reviewed elsewhere (see G. A. Olah, Friedel-Crafts Chemistry, Wiley, New York, 1972 G. A. Olah, ed., Friedel-Crafts and Related Reactions, Vols. I-IV, Wiley-Interscience, New York, 1963-1965). 

About 10 years have passed since this study began to be seriously undertaken, but the usage of solid superacids as catalysts is still limited. Table IX summarizes the acid-catalyzed reactions on sulfated metal oxides, i.e., cracking, isomerization, alkylation, acylation, esterification,... 

Acid-Catalyzed Synthesis. The acid-catalyzed reaction of alkenes with hydrogen sulfide to prepare thiols can be accomplished using a strong acid (sulfuric or phosphoric acid) catalyst. Thiols can also be prepared continuously over a variety of solid acid catalysts, such as zeolites, sulfonic acid-containing resin catalysts, or aluminas (22). The continuous process is utilized commercially to manufacture the more important thiols (23,24). The acid-catalyzed reaction is commonly classed as a Markownikoff addition. Examples of two important industrial processes are 2-methyl-2-propanethiol and 2-propanethiol, given in equations 1 and 2, respectively. 

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

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