Side-chain alkylation of toluene

Solid super bases, prepared by successive treatment of y-alumina with alkali metal hydroxide and alkali metal, are highly active catalysts for reactions involving reactive carbanions, and have been commercialised by Sumitomo (Suzukamo et al, 1997). For example, t.vobutylbenzene, the. starting material for ibuprofen (see earlier) is produced by side-chain alkylation of toluene with propylene over a K/KOH/AI2O3 catalyst (Eqn. (14)). 

A number of papers have appeared on the use of layered double hydroxides (e.g. Mg and Al containing oxides). A meixnerite-like catalyst has been reported to give 100% selectivity for diacetone alcohol from acetone at 0 C at close to thermodynamic equilibrium conversion of 23% (Tichit and Fajula, 1999). The side-chain alkylation of toluene with propylene to give isobutyl benzene (for ibuprofen) is a well-known example where Na/K alloy on Na2C03/K2C03 is used as the catalyst. 

No commercial process is offered at this time for side chain alkylation of toluene with methanol for styrene and ethylbenzene production. In the literature the reaction is typically carried out at toluene to methanol molar ratios from 1.0 7.5 to 5 1 from 350 to 450 °C at atmospheric pressures. In some cases inert gas is introduced to assist vaporizing the liquid feed. In other cases H2 is co-fed to improve activity, selectivity and stability. Exelus recently claimed 80% yields in their ExSyM process at full methanol conversion using a 9 4 toluene methanol feed ratio at 400-425 °C and latm (101 kPa) in a bench-scale operation. This performance appears to be... 

Itoh, H., Miyamoto, A., and Murakami, Y. (1980) Mechanism of the side-chain alkylation of toluene with methanol. 

Wieland, W.S., Davis, R.J., and Garces, J.M. (1996) Solid base catalysts for side-chain alkylation of toluene with methanol. Catal Today, 28, 443-450. 

Borgna, A., Magni, S., Speulveda, )., Padro, C.L., and Apesteguia, A.R. (2005) side chain alkylation of toluene with methanol on Cs-exchanged Na-Y zeolite effect of Cs loading. Catal. 

Fig. 25. Proposed mechanism of the side-chain alkylation of toluene by methanol on basic zeolites (a) and the surface species formed during the decomposition of methanol on basic zeolites (b).

A further possibility for side-chain alkylation of toluene is oxidative methylation with methane. Catalysts with occluded alkali metal oxides, prepared by impregnating zeolites with alkali metal hydroxides followed by calcination, usually exhibit better performance.441 Further enhancement was achieved by impregnating ion-exchanged zeolites 442 Significant improvements in stability and the yields of Cg hydrocarbons were also observed when NaX was impregnated with 13% MgO which was found to increase the amounts of active sites.443... 

Gas products from the alkylation of acetonitrile were regularly analysed using the same column as used for the side-chain alkylation of toluene. Liquid products were also collected every 30 minutes in an acetone-ice bath, but were analysed using a Porapak Q column at 150-180 °C with a helium carrier gas flow rate of 30 ml/min. To investigate the effect of carbonated catalysts, especially that with the excess cesium cation "clusters", carbon dioxide was introduced to the fresh CsNaX-CsOH at the reaction temperature, 350 °C, for 30 minutes before the alkylation of acetonitrile was carried out in a flow of helium. The cesium clusters of treated catalysts were presumed to be fully carbonated (CS2CO3) clusters and the activity of this catalyst was compared with the untreated CsNaX-CsOH. 

Figure I. Conversion of toluene and methanol with the intervention of carbon dioxide in the side-chain alkylation of toluene with methanol over CsNaX-CsOH...

Alkali metals supported by nanoporous carbons were proposed by Stevens et al. as non-pyrophoric solid-base catalysts with high activity in 1-butene isomerization and side-chain alkylation of toluene with propene [32, 33]. 

A material such as Na°/NaY catalyzes the aldol condensation of acetone, to form mesityl oxide and eventually isophorone. Another strong base catalyzed reaction is the side chain alkylation of toluene with ethylene. In contrast with acid catalysis, side chain reaction is strongly preferred over ring alkylation. With a Na°/NaX in the gas phase at 473 K, toluene reacts to give n-propylbenzene (66%) and the dialkylated product, 3-phenylpentane (32%) (41). 

Scheme 16. Side chain alkylation of toluene with methanol over basic zeolites.

Ethylbenzene can also be formed by side-chain alkylation of toluene with methanol over basic catalysts, such as alkali-exchanged X and Y zeolites [229,230]. The cross-coupling of toluene and methane to produce F.B/styrene at high temperatures (> 700 °C) in the presence of oxygen has also been attempted by using basic zeolites, such as CsX and BaO/NaX [231, 232,233],... 

The alkylation of toluene with nrethanol is readily catalyzed on synthetic zeolites. Previous work has shown that the aromatic-ring alkylation of toluene with methanol takes place over acid zeolites [1], while the side-chain alkylation occurs preferentialty over basic zeolites [2,3]. The side-chain alkylation of toluene with methanol, for producing a mbrtuie of styrene and ethylbenzene offers economical advantages conpared with the conventional homogeneously catalyzed Friedel-Crafts process, which use ethylene and benzene as reactants [4]. 

The experimental data were fitted employing the integrated form of Eq. (5). Figure 5 shows that the quality of fits was excellent when using this model to describe the poisoning effect of an acid compound on the side-chain alkylation of toluene with methanol. The kinetics parameters obtained Ifom this model were also summarized in Table 2. 

However, another type of reactions of benzene derivatives was studied by in-situ IR spectroscopy as well, viz. the side-chain alkylation of alkylbenzenes, for instance of toluene, over basic zeolite catalysts such as M -X zeolites (M=Na, K, Rb, Cs) [901,902]. The intermediate conversion of methanol to formaldehyde turned out to be crucial for the side-chain alkylation as well as a strong polarization of the methyl group of toluene, the preferential adsorption of toluene, and a sufficient basicity, i.e., base strength of the catalyst. Related to these IR studies of side-chain alkylation of toluene were in-situ IR spectroscopic investigations of the decomposition of methanol over basic zeolites (M+-X, M =Na+, K+, Rb, Cs+ Na-ZSM-5 and Cs-ZSM-5 [903]). It was shown that over weakly basic zeolites (Na-ZSM-5, Cs-ZSM-5) dimethyl ether was formed from methanol, whereas over more strongly basic X-type zeolites formaldehyde was produced, which is an indispensable intermediate for the side-chain reaction (vide supra). 

For example, isobutylbenzene as starting material for ibuprofen synthesis is produced by side-chain alkylation of toluene with solid super-base by Sumitomo (Eq. 8-28). 

The side-chain alkylation of toluene is also base-catalysed (scheme 7). Scheme 7... 

The CsAc/CsNa-zeolites, discussed above, are also effective in side-chain alkylation of toluene. However, in this reaction the weaker acidity and stnmger basicity, associated with the Y type materials, results in less formaldehyde and less dimethyl ether than is observed with the X catalysts [79]. 

This example of side-chain alkylation of toluene with methanol serves not only to demonstrate the interaction between basic and acidic centers in a zeolite but also to illustrate another type of zeolite catalysis or of catalysis in general. The key phrases "computer graphics" or "computer-aided catalyst design" are used to describe this novel catalyst research. With the aid of computer graphics, it is possible to simulate zeolite structures and produce images of these structures. Computer graphics can also be used to produce pictures of molecules such as toluene in the zeolite pores or cages. 

Modified Zeolites. As described above, alkali ion-exchanged zeolites are weak bases. Various efforts have been made to increase the base strength of alkali metal-ion exchanged zeolites. Metallic sodium particles in zeolites are formed by the decomposition of occluded sodium azide (59). These sodium particles are capable of performing base-catalyzed reactions. These catalysts catalyze the isomerization of butenes at 300 K and the side-chain alkylation of toluene with ethylene at 523 K. 

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