Solid acid catalysts reaction mechanism

The technology and chemistry of isoalkane-alkene alkylation have been thoroughly reviewed for both liquid and solid acid catalysts (15) and for solid acid catalysts alone (16). The intention of this review is to provide an up-to-date overview of the alkylation reaction with both liquid and solid acids as catalysts. The focus is on the similarities and differences between the liquid acid catalysts on one hand and solid acid catalysts, especially zeolites, on the other. Thus, the reaction mechanism, the physical properties of the individual catalysts, and their consequences for successful operation are reviewed. The final section is an overview of existing processes and new process developments utilizing solid acids. 

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

The requirement of small structural differences within the series of reactants for obtaining a LFER has its parallel in series of catalysts. Meaningful values of result only when the catalysts operate principally in the same way, that is, when the reaction mechanism is basically the same. This is most likely to occur when the catalysts differ only by minor modifications in the method of preparation or when their composition is only slightly modified by the addition of promoters. With chemically different catalysts the similarity is achieved when the active centers have as their decisive component a common species, for example, protons on solid acidic catalysts. 

With this purpose, several different types of solid acid catalysts have been investigated for the acylation of aromatics, but the best performances have been obtained with medium-pore and large-pore zeolites (3-9). In general, however, the use of acylating agents other then halides, e.g., anhydrides or acids, is limited to the transformation of aromatic substrates highly activated towards electrophilic substitution. In a previous work (10), we investigated the benzoylation of resorcinol (1,3-dihydroxybenzene), catalyzed by acid clays. It was found that the reaction mechanism consists of the direct 0-benzoylation with formation of resorcinol monobenzoate, while no primary formation of the product of C-benzoylation (2,4-dihydroxybenzophenone) occurred. The latter product formed exclusively by... 

There have been a number of reports of improved selectivity with sulfonic acid resin catalysts compared with conventional liquid acid catalysts[6—9]. Various explanations have also been proposed. If mechanisms usually postulated for condensation reactions with liquid Br0nsted acid [10] and solid acid catalysts [11] are adopted, the sequence of steps shown in Fig. 2 could be considered for the condensation of MFC. Both mechanisms incorporate the essential features of known carbenium ion chemistry, i.t., electrophilic attack on the aromatic ring by polar carbenium ion intermediates. Note that MDU is formed by this attack on the benzene ring of MPC, while the N—benzyl compound by the attack on nitrogen atom. 

Under very mild conditions (0-20°C, 200 Torr ethylene pressure), ethylene was shown to be selectively dimerized to n-butenes over RhY (140). As shown in Fig. 14, 1-butene was formed initially but further isomerized to an equilibrium composition of -butenes with increasing reaction time. In a comparative experiment using HY as a typical solid-acid catalyst, no ethylene conversion was measurable up to 200°C, and at higher temperatures unselective polymerization and cracking reactions occurred. This provided good evidence that the selective dimerization over RhY did not proceed via a carbenium ion mechanism. 

Volatile products derived from cracking PE with solid acid catalysts can be rationalized by carbenium ion mechanisms. Under steady-state conditions, hydrocarbon cracking processes that yield volatile prodncts can be represented by initiation, disproportionation, P-scission, and termination reactions [72, 73]. Initiation involves the protolysis of PE with Bronsted acid sites (H+ S ) to yield paraffins and surface carbenium ions ... 

The reformation of lower paraffins to aromatics has been studied for about 20 yr by using zeolite catalysts. Recently, an excellent review was published of lower alkane transformation to aromatics on ZSM-5 zeolites [2]. From the studies of the mechanism of this reaction, it has been suggested that the bifunctional catalysts, having solid acidity and dehydrogenation activity, can effectively promote the aromatization of lower paraffins[3-6]. It has been reported that ZSM-5 and ZSM-11 are excellent solid acid catalysts [7] and the transition metals [8], Ga [9] and Zn [9] show high dehydrogenation activity in this reaction. In the case of bifunctional... 

The mechanisms of acid-catalyzed DME formation from methanol and aromatiza-tion of olefins were widely investigated in the years before the discovery of the methanol-to-gasoline reaction. There is a consensus that the intermediate in DME formation from methan.ol over solid acid catalysts is a protonated surface methoxyl, which is subject to nucleophilic attack by methanol [2]. Aroma-tization of olefins is believed to proceed along classical carbenium pathways, with concurrent hydrogen transfer [3]. The mechanism of the crucial step of initial C-C bond formation from MeOH/DME is an unsolved problem, however, and is the subject of ongoing controversy. At last tally, there were some two dozen mechanistic proposals in the literature. It is not possible here to present a comprehensive review of the entire field. However, a number of common themes can be identified. This commonality is discussed and the concepts currently in vogue are critically reviewed. Another issue, whether ethylene is the "first" olefin, has been widely debated [2], but is beyond the scope of this survey. 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

Although many solid-acid catalysts have been reported for the vapor-phase Beckmann rearrangement [2], their performance has been less than satisfactory from an industrial standpoint and the heterogeneously catalyzed Beckmann rearrangement has not yet been commercialized. In this chapter heterogeneous catalysis of the Beckmann rearrangement, its mechanism, and acid properties and reaction conditions suitable for the reaction will be reviewed. 

The present study shows fundamental differences in the hydroxylation kinetics of phenol using strong acid catalysts or titanium silicalites. With the latter, the reaction occurs slowly but regularly, while, with the solid acids, the reaction shows an induction period followed by a very fast autocatalysis. These results cast doubt on the validity of the tests performed by stopping the reaction at a determined time. They also call into question the mechanism of the acid catalysis, the homogeneous as well as the heterogeneous contribution. Finally, taking into account that water is the best solvent for this reaction, solid acids should be considered as valuable catalysts for hydroxylation of phenol. 

Apart from discernment of acid type, qualitative and quantitative measures of acidic strengths in solid acids are also crucial for the detailed understanding of the mechanism and performance of the catalyst during catalytic reactions. Assorted illustrations for characterization of acidic strength in various solid acid catalysts invoking the aforementioned P SSNMR approaches wiU be discussed in this section. 

Solid acids Over Bronsted (protonic) solid acids, the reaction intermediate is a sec-butyl cations formed by the addition of a proton from the solid surface to butene. Hence, when solid acids are deuterated the deuterium of the catalyst is incorporated into both reactant and isomerized butenes. The same intermediate is often involved in the case of Lewis solid acids. In the latter case, protonic acid is induced by the reaction of butene on the Lewis acid site. For example, CH3CH = CH2 + L CH3CH — GHz — L (L Lewis site), where H at CH3 or CH2L acts as acidic proton. The intermediate is illustrated in Fig. 4.1 A. Fig. 4. IB shows the stereochemical reaction scheme for the case in which the deuterium atom (Da) is attached to cw-2-butene from below. If the Hb atom is removed downward from 1 or 2, the intermediate is transformed to (ranf-2-butene. The removal of D can produce tracer experiments based on this model reveals detailed information on the dynamic behavior of the intermediate. ... 

In transalkylation, one of the alkyl groups is transferred from one alkylaromatic molecule to another aromatic molecule. The mechanism of transalkylation was studied extensively in Friedel-Crafts chemistry. Though the reaction conditions are quite different from those of Friedel - Crafts catalysts, it seems quite probable that an essentially same mechanism is operative also in transalkylation with solid-acid catalysts. Thus, Kaeding et al. proposed the following mechanism for disproportionation of toluene over zeolites. ... 

The chloromethylated polystyrene resin used for Merrifteld solid-phase peptide synthesis is prepared by treatment of polystyrene with chloromethyl methyl ether and a Lewis acid catalyst. Propose a mechanism for the reaction. 

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. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

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. 

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