Sulfuric Acid-Catalyzed Processes

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

Acid-catalyzed dimerization of styrene usually yields a linear unsaturated dimer I and a cyclic saturated dimer 2 (Eq. (6)) along with higher oligomers. Methods for selective synthesis of linear dimer 1 have not been established thus far the best method previously reported is a sulfuric acid-catalyzed process yielding / in a rather high but the reaction has a poor selectivity (with isomer 2 and higher oligostyrenes) Synthesis of 2, on the other hand, is not difficult, because I can readily be converted to 2 under acidic conditions... 

In general, the O-alkylation of benzoxepinones is accomplished via the anion. Alternatively, an acid-catalyzed process employing ortho esters may be used. For the acid-catalyzed formal O-alkylation of l-chloro-8-methoxydibenz[ft,/]oxepin-10(ll//)-ones with triethyl orthoformate rather drastic conditions are required (hot concentrated sulfuric acid) to give the 10-ethoxy derivative 12 in excellent yield.109... 

Another example of a transannular cyclization that occurs in the solid state is provided by the epoxy alcohol 31. This compound is stable when dissolved in organic solvents and in 0.25N sulfuric acid. However, the crystals transform rapidly to 32. Although the process is accompanied by partial melting, it appears to be a true solid-state one. Interestingly, the reaction is slowed down appreciably when the dry crystals are covered with ether. Hydrogen bromide is eliminated in the reaction and it may be that an acid-catalyzed process is also occurring in the presence of solvent this process may be slowed down by the dissolution of the decomposition products in the solvent (77). 

Xylose. Catalytic dehydration of xylose, which is the most abundantly available pentose monomer in hemicellulose, has been known for a long time (Scheme 5). In fact, as early as 1922, an industrial process involving sulfuric acid catalyzed dehydration of xylose to produce furfural was developed by the Quaker Oats Co. 

Polymerization with Sulfuric Acid. In the early 1930 s development work on sulfuric acid-catalyzed polymerization was undertaken by a number of research organizations. The most widely used process is that developed by the Shell companies (IS). 

The 2-butanol feedstock is conventionally obtained by the sulfuric acid-catalyzed addition of water to -butenes. This is a two-step reaction involving sulfation and hydrolysis in which the conversion of -butenes is 90% and selectivity to 2-butanol is 95% (15). During operation the sulfuric acid becomes diluted and must be reconcentrated before reuse. In 1983 Deutsche Texaco commercialized a single-step route in which 2-butanol is formed by the hydration of -butenes in the presence of a strongly acidic ion-exchange resin containing sulfonic acid groups (16—18). The direct reaction is carried out at 150—160°C and 7 MPa. Virtually anhydrous 2-butanol is recovered in this process (19). Direct hydration requires lower utilities and investment costs, operates at 99% selectivity to 2-butanol, but is hindered by low (5—15%) -butene conversion per pass. 

The high pentose yields obtained in the current study are favorable when compared to those previously reported for dilute-acid hydrolysis in batch processes (28-31). However, the hemicellulosic sugar yield reported in the literature is often expressed as xylose equivalents, which is difficult to compare to monosaccharide recovery. The highest monosaccharide recovery was obtained with low CS values, which is in agreement with previous findings for sulfuric acid-catalyzed steam explosion, in which also better hemicellulosic sugar recoveries were obtained at relatively low severities (28,32) of <2.2 (22). 

A problem that is characteristic of sulfuric acid-catalyzed alkylation is its capabihty to oxidize hydrocarbons. H2SO4 decomposes in the presence of isoalkanes to form water, SO2, and alkenes. This is a slow process, and so it occurs predominantly when the acid is in contact with hydrocarbons for a longer period. Higher temperatures favor the formation of SO2 (10). Some irreversible reactions between acid and hydrocarbons also take place during alkylation. Sulfone, sulfonic acid, and hydroxy groups have been detected in conjunct polymers produced with H2SO4 as the catalyst (8,96). Kramer (97) reported that... 

Ipatieff and coworkers carried out the first alkylation with alkenes and branched and normal chain alkanes (except methane and ethane) in the presence of AlCb as the catalyst. The sulfuric acid catalyzed alkylation reaction of arenes and isoalkanes, developed in 1938, is a still widely used industrial process to produce alkylates with high octane numbers. For synthetic applications, however, Friedel-Crafts-type alkylations of alkenes and alkanes have limited value since they tend to give mixtures of products, including oligomers of alkenes. ... 

These common features suggest that carbocations are key intermediates in alcohol dehydration, just as they are in the conversion of alcohols to alkyl halides. Figure 5.6 portrays a three-step mechanism for the sulfuric acid-catalyzed dehydration of ieri-butyl alcohol. Steps 1 and 2 describe the generation of tert-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ieri-butyl alcohol with hydrogen chloride. Step 3 in Figure 5.6, however, is new to us and is the step in which the double bond is formed. 

In the conventional dilute sulfuric acid-catalyzed percolation process, the plant raw material containing pentosans is mixed with dilute sulfuric acid and, in the first step, pentosans are hydrolyzed to pentoses (C6Hio05)n, which are then cyclodehydrated to furfural in the second step. The frirfiirai formed is recovered by steam distillation and fractionation. The chemical reaction is shown in Figure I. The mechanism of the reaction (4) is shown in Figure 2. 

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