Zeolite cyclodimerization, butadiene

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

The Cu+/zeolite-catalyzed cyclodimerization of 1,3-butadiene at 100°C and 7 atm was found to give 4-vinylcyclohexene [Eq. (13.12)] with high (>99%) selectivity. Subsequent oxidative dehydrogenation over an oxide catalyst in the presence of steam gives styrene. The overall process developed by Dow Chemical113 offers an alternative to usual styrene processes based on ethylation of benzene (see Section 5.5.2). 

The first work in this area appeared in the form of two patents assigned to Union Carbide in 1969/1970 (172,173). These patents described methods of preparation of monovalent copper-containing zeolites which were claimed to be active and selective catalysts for the cyclodimerization of butadiene to 4-vinylcyclohexene (VCH), i.e.,... 

Maxwell et al. 177, 178) studied the deactivation of reduced Cu2+Y catalysts for butadiene cyclodimerization in some detail. This work showed that the catalyst stability could be markedly improved by using NH3 as a reducing agent and choosing the activation conditions such that excess NH3 remains selectively chemisorbed on the zeolite acidic sites. Further, the Cu2+Y-derived catalyst was thermally stable to 850°C and was therefore able to withstand a regeneration procedure which involved a polymer burn-off at 550°C. By contrast, the catalysts prepared by direct exchange with monovalent copper, i.e., Cu+Y, formed CuO irreversibly when heated above 330°C. 

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont-morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH (>95%). Preexchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange. 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

The first step of the process involves the cyclodimerization of butadiene to 4-vinylcyclohexene. The reaction is exothermic and can be catalyzed by either a copper-containing zeolite catalyst or an iron dinitrosyl chloride catalyst complex. Although both vapor-phase and liquid-phase processes have been studied, it appears that liquid-phase reactions are preferred because they achieve higher butadiene conversion levels. The second step is oxidative dehydrogenation of the 4-vinylcyclohexene to produce styrene. Dow has led the research effort in this area and has... 

Zeolites are clays with rather large internal pore structures which have the property of concentrating nonpolar organic compounds within their cavities. Measurements of gaseous hydrocarbon equilibria have shown enhancements of several orders of magnitude within zeolite pores relative to the vapor phase. Cyclodimerization of butadiene to 4-vinylcyclohexene (Equation 7.5) at 250°C was catalyzed by large-pore zeolites in the sodium form (Dessau, 1986). Zeolites in the Cu(I) form also promoted Diels-Alder addition of furan and other dienes with electron-deficient dienophiles such as methyl vinyl ketone (Equation 7.6). Dichloro-methane was the solvent in these reactions, which usually were carried out at 0°C or lower (Ipaktschi, 1986). 

Cyclodimerization of 1,3-butadiene over Cu-exchanged zeolites FAU and EMT was followed in situ by DRIFT in a study by Voskoboinikov et al. [906]. The reaction was Brqnsted acid-catalyzed. Even though the deactivation was rather Uttle, it could be also monitored in situ through the IR spectra showing the formation of polycyclic naphthenes, which then in part consecutively transformed into aromatics. 

Dow Chemical has developed a two-step zeolite-based process to produce styrene from butadiene contained in crude C4 streams. As shown in the following scheme, 1,3-butadiene (in the mixed C4 stream) undergoes a liquid-phase cyclodimerization (Diels-Alder reaction) over a proprietary copper-loaded zeolite catalyst at moderate temperature and pressure, to give 4-vinyl-l-cyclohexene (4-VCH) with 99% selectivity. In the second step, the 4-VCH is catalytically oxidized (in the presence of steam) to styrene over one of Dow s proprietary oxide catalysts. The overall yield of styrene is greater than 90%. This process was originally tested in a 40-lb-per-hour pilot plant, and is now in commercialization. 

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