Elimination over solid catalysts

Olefins can be prepared by the dehydrogenation of paraffins, dehydration of alcohols, or decomposition of ethers and halides, if vapours of these substrates are passed over metals or metal oxides at elevated temperatures (300-600°C). Dehydration reactions have been most widely studied and by careful selection of the catalyst and the reaction conditions the direction and stereochemistry of elimination can be controlled. However, dehydration often has to compete with dehydrogenation, and isomerisation of olefinic products by the acidic sites on the catalyst can reduce the synthetic utility of these reactions. Most frequently alumina has been used as the catalyst and the advantages and complexities of the method are amply illustrated by the dehydration of alcohols. Surface-catalysed eliminations have been the subject of several reviews .  

---

The kinetic preference for cis- over imns-olefin elimination from acyclic compounds is rare. Cope and co-workers 91) reported a slight preference for cis- over irans-2-butene and 2-pentene in the thermal decomposition of the quaternary ammonium hydroxides, and Andr u and co-workers 92,93) found a preponderance of cis- over trons-2-butene in the elimination of hydrogen chloride from 2-chlorobutane over solid catalysts. Neureiter and Bordwell 94) found the formation of cis-2-butene rather than alkene from a-chlorosulfone on treatment with alkali ... 

Over most catalysts, with the notable exception of thoria [130], the thermodynamically more stable olefins are formed (Saytzeff rule) as primary products when two elimination directions are possible. This is in agreement with the results concerning other elimination reactions, both in the liquid phase (cf. refs. 64 and 65) and over solid catalysts. The striking difference in the action of thoria has been explained on the basis of a different mechanism [68],... 

Much less information is available on the deamination and related reactions over solid catalysts than on some other elimination reactions but it suffices for comprehension of the general features. 

When some straight and branched-chain aliphatic alcohols, such as n-propanol, n-butanol and isopropanol, are subjected to high temperatures, dehydrogenation products predominate over dehydration (51). Presumably the eliminations take place via a six-membered transition state and are catalyzed by hydrogen halides in the homogeneous phase (52) to produce olefins. On the other hand, gas phase dehydration over solid catalysts is a valuable process for the preparation of olefins and ethers. 

To evaluate properties of basic catalysts, the Knoevenagel condensation over aluminophosphate oxynitrides was investigated [13]. In this reaction usually catalysed by amines, the solid catalysts function by abstraction of a proton from an acid methylene group, which is followed by nucleophilic attack on the carbonyl by the resultant carbanion, re-protonation of oxygen and elimination of water. The condensation between benzaldehyde and malononitrile is presented below. 

Homogeneous molecular catalysts, which have far greater connol over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of mass-transfer problems can be achieved. Supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry (Jessop et al., 1995). 

The introduction of solid catalysts into a traditionally non-catalytic free-radical process like combustion occurred in recent years under the influence of two pressures, the energy crisis and the increased awareness of atmospheric emissions. The major applications of catalytic combustion are twofold at low temperatures to eliminate VOC s and at high temperatures (>1000 C) to reduce NOx emission from gas turbines, jet motors, etc. Both these applications are briefly reviewed here. Some recent developments in high-temperature catalytic combustion are trend-setters in catalysis and hence of particular interest. For instance, novel materials are being developed for catalytic applications above 1000 C for sustained operation for over one year. Where material/catalyst developments are still inadequate, systems engineering is coming to the rescue by developing multiple-monolith catalyst systems and the so-called hybrid reactors. 

Another method is to measure the amount of a gaseous base, such as ammonia, pyridine, or quinoline, adsorbed at elevated temperatures under a specified set of conditions. This has the advantage of allowing the study of a catalyst under conditions more nearly similar to those of reaction. Ammonia has been used extensively. Catalysts may be compared in terms of the amount of ammonia adsorbed as a function of the temperature over a range such as ISO to SOO C. A minimum temperature of about 150 C is necessary to eliminate physical adsorption. By infrared spectra it is possible to distinguish between Bronsted and Lewis acid sites. More details on characterizing the acidity of solid catalysts are described in reviews by Formi (1974), and by Bensei and Winquist (1978). 

The dehydrohalogenation of 1- or 2-haloalkanes, in particular of l-bromo-2-phenylethane, has been studied in considerable detail [1-9]. Less active haloalkanes react only in the presence of specific quaternary ammonium salts and frequently require stoichiometric amounts of the catalyst, particularly when Triton B is used [ 1, 2]. Elimination follows zero order kinetics [7] and can take place in the absence of base, for example, styrene, equivalent in concentration to that of the added catalyst, is obtained when 1-bromo-2-phenylethane is heated at 100°C with tetra-n-butyl-ammonium bromide [8], The reaction is reversible and 1-bromo-l-phenylethane is detected at 145°C [8]. From this evidence it is postulated that the elimination follows a reverse transfer mechanism (see Chapter 1) [5]. The liquidrliquid two-phase p-elimination from 1-bromo-2-phenylethanes is low yielding and extremely slow, compared with the PEG-catalysed reaction [4]. In contrast, solid potassium hydroxide and tetra-n-butylammonium bromide in f-butanol effects a 73% conversion in 24 hours or, in the absence of a solvent, over 4 hours [3] extended reaction times lead to polymerization of the resulting styrene. 

Over time the operating efficiency of hydrofluoric acid alkylation units has improved in order to minimize waste streams as well as minimize the actual hydrofluoric acid inventory. However, the only way to eliminate these issues completely is to develop a process based on a heterogeneous acid catalyst. Such a process would eliminate the hydrofluoric acid transportation and inventory, result in no waste polymer product, and would not require fluoride scrubbing and concomitant disposal of fluoride-containing solids. 

Before their use, zeolite catalysts have always to undergo activation treatments. Part of these treatments depends on the nature of the active sites (e.g. metals supported over zeolites have to be reduced before reaction). However, the elimination of moisture from hygroscopic solids, as zeolites are, has always to be carried out. 

Although the sulfate superacids are stable enough because of preparatory heat treatment at elevated temperatures, elimination of the sulfate is sometimes observed during reaction as a result of catalyst deactivation, especially in a solid-liquid system. It is hoped to synthesize superacids with the system of metal oxides. We have succeeded in preparing another type of superacid, not containing any sulfate ion but consisting of metal oxides, which can be used at temperatures over 800°C (188-192). 

A blank experiment without the catalyst eliminated homogeneous radiolysis as an explanation for the increases observed over zinc oxide, and a calculation shows that even radiolysis with energy transfer from the solid would not be sufficient. For 8 x lO ev gm sec i to decompose 3 X IQi molecules sec gm i the yield must be about 4 x 10 molecules decomposed per 100 ev absorbed, a quite unreasonable figure for the decomposition of methanol. The enhancement must therefore be in the catalytic process. 

---