Heterogeneous catalysts Friedel-Crafts alkylation

Thus, the combination of increased acidity and increased solvent power can be exploited for cleaner acid-catalysed reactions of organic compounds, for example Friedel-Crafts alkylation, [31] or hydrolysis of esters [9,28]. The acidity of high-temperature D20 can be used to deuterate organic compounds but the process is more efficient with an additional heterogeneous acid catalyst [32]. A related application, but not a catalytic one, exploits the acidity of SC-H2O for the in situ generation of H2 from metallic Zn the H2 can then be used for the selective reduction of organic nitro-compounds [29,30], see Figure 9.1-2. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Continuous Friedel-Crafts alkylation, with high selectivity, of mesitylene and ani-sole with propene or propan-2-ol in scCsHe or SCCO2 using a heterogeneous poly-siloxane solid acid catalyst (Deloxan, ASP 1/7) is described by Poliakoff et al. [52] (see Chapter 12). No comparison was made with the continuous alkylation in a conventional solvent and it is, therefore, diffiadt to judge the technical potential of this approach. 

Nafion resins in the acid form have been successfully used as catalysts in Friedel-Crafts alkylations and acylations60 as well as in gas-phase esterifications. In contrast to the batch process, which is slow and produces a low yield, the solid heterogeneous H-Nafion resin catalyst was used in a flow reactor, resulting in close to quantitative yields at contact times of several seconds only61. 

Poliakoff et al. introduced the supercritical phase to the Friedel-Crafts alkylation reaction by using SCCO2 or by making propene, one of the reactants, the supercritical fluid [56]. This heterogeneous supercritical phase reaction was conducted continuously and the selectivity was very high if a solid acid Deloxan catalyst was utilized. 

Examples here are Friedel-Crafts alkylations and acylations, in which the zeolite catalyst replaces the homogeneous Lewis acid. Zeolite catalysts can also be used in place of mineral acids and organic acids. However, these catalyst substitutions also involve changing the process from homogeneous to heterogeneous catalysis. 

ILs may themselves also act as catalyst. For heterogeneous catalysis, the IL has to be immobilized, for example, by covalent bonds on a porous support, as shown by Hoelderich et al. [40] for Friedel-Crafts alkylation reactions. Further examples of reactions tested with grafted ILs are acetylization [41], cycloaddition of CO2 [42, 43], and epoxidation of olefines [44]. 

SCCO2 has also been used as a solvent with a silica-immobilized catalyst in metathesis reactions [25]. A heterogeneous catalytic process is developed, in which catalyst leaching is avoided but the reactivity is lower than when using a homogeneous catalyst This application has also been extended to continuous-flow processes for hydrogenation [26], Friedel-Crafts alkylations [27], etherification [28], and hydroformylation [29] reactions. 

Hitzler et al. (316) report the Friedel-Crafts alkylation of mesitylene (C6H3Me3) and anisole with propene or 2-propanol using a heterogeneous polysiloxane-supported solid acid catalyst (Degussa s Deloxan) in a small fixed-bed continuous reactor (10-ml volume) using SCF propene or CO2 as the reaction solvent. For the alkylation of mesitylene with propene at 160-180°C and 200 bar, yield of the monoalkylated product (l-isopropyl-2,4,6-trimethylbenzene) was only approximately 25% due to the formation of the dialkylated product as well as dimers and trimers of propene. Selectivity to the monoalkylated product was significantly higher (40% yield) for alkylation with 2-propanol in scCOa. 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

In contrast, continuous flow reactors are already being used for hydrogenation reactions industrially (Licence el al., 2003). They are simple to construct and modify, and possess excellent mass- and heat-transfer properties. In academia, flow reactors have been used in conjunction with a variety of heterogeneous catalysts to carry out many reactions, including hydrogenations, dehydrogenations, hydroformylations, Friedel-Crafts acylations and alkylations, etherifications and oxidations (Hyde et al., 2001). 

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