Process Integration and Catalytic Cascades

Overall R1SH + R2NH2 + 2 NaOH + Cl2 ----------- R1SNHR2 + 2 NaCI + 2 HzO 

An elegant example is the Rhodia process for the manufacture of the flavor ingredient, vanillin [30]. The process involves four steps, all performed with a heterogeneous catalyst, starting from phenol (Fig. 1.48). Overall, one equivalent of phenol, H202, CH3OH, H2CO and 02 are converted to one equivalent of vanillin and three equivalents of water. 

Another pertinent example is provided by the manufacture of caprolactam [135]. Current processes are based on toluene or benzene as feedstock, which can be converted to cyclohexanone via cyclohexane or phenol. More recently, Asahi Chemical [136] developed a new process via ruthenium-catalysed selective hydrogenation to cyclohexene, followed by zeolite-catalysed hydration to cyclo-hexanol and dehydrogenation (Fig. 1.49). The cyclohexanone is then converted to caprolactam via ammoximation with NH3/H202 and zeolite-catalysed Beckmann rearrangement as developed by Sumitomo (see earlier). 

Alternatively, caprolactam can be produced from butadiene, via homogeneous nickel-catalysed addition of HCN (DuPont process) followed by selective catalytic hydrogenation of the adiponitrile product to the amino nitrile and vapor phase hydration over an alumina catalyst (Rhodia process) as shown in Fig. 1.49 [137]. 

Interestingly, the by-product in the above-described hydrocyanation of butadiene, 2-methylglutaronitrile, forms the raw material for the Lonza process for nicotinamide (see earlier) [123]. Four heterogeneous catalytic steps (hydrogenation, cyclisation, dehydrogenation and ammoxidation) are followed by an enzymatic hydration of a nitrile to an amide (Fig. 1.50). 

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