Catalysis of Organic..

482 Reductive AlkylationSince the preparation of a catalyst can be a complex process involving a number ofvariables, a thorough examination of all these variables would involve preparation ofhundreds of catalysts. Platinum loading was a natural choice as one of the variables.It is well known that acids catalyze the formation of imine (2,4). It is also knownthat treatment of carbon with acids lead to the formation of acidic surface groups,hence pH during catalyst preparation was chosen as another variable (5). In the caseof reductive alkylation, Thakur et. al. (6) showed that the reaction rate increases withsulfur loading therefore, sulfur loading was chosen as the third variable. Table 1shows the range of values for each of the chosen parameters.Experimental SectionReactions were carried out in liquid phase in a well-stirred (1000 rpm) high-pressurereactor (Parr Instruments, 300 mL) at 30 bar and 150°C. The reaction mixtureconsisted of 61 g of ADPA (Acros Chemicals), 53 g MIBK (Acros Chemicals) and370 mg of catalyst. The test procedures used here is similar to that described earlierby Bartels et al. (7). The reactor was operated at a constant pressure with the liquidphase in batch mode and the hydrogen fed in at a rate proportional to itsconsumption. The reaction was monitored by hydrogen uptake and the product yieldwas determined from gas chromatographic (Agilent Technologies, 6890N) analysis.The catalysts were commercial catalysts from Degussa with Pt-loading from1%-5%, S-loading varied between 0.1 and 0.5%. The pH of the catalyst duringpreparation was varied from 2-6. The dispersion of Pt was 52+/-5% for all thecatalysts tested.Results and DiscussionThe reactions were conducted according to a two factorial design with threevariables, which contains experimental points at the edges and the center of a facecenteredcube leading to 9 different experiments. Typically, the experiment at thecenter point is conducted at least 3 times to add degrees of freedom that allow theestimation of experimental error. Hence a total of 11 experiments are needed topredict the reaction rate within the parameter space. The parameter space for thecatalysts to be prepared is shown in columns 2-4 in Table 1.The reactions were conducted in the liquid phase at conditions described in theexperimental section. Test reactions were conducted to establish that the reactionswere kinetically limited. In cases where the rate of reaction was >5 mmol/(g*min),the selectivity to 6-PPD was >97% and the yield of 6-PPD was >96%. Hence, therate of hydrogen uptake was taken to be directly proportional to the formation of 6-PPD. This rate calculated at constant temperature and conversion was normalized tothe amount of catalyst used and is shown in column 6 of Table1. The two caseswhere Pt/S ratio was high (Run 3 & 4), hydrogenation of the ketone (MIBK) to thealcohol, methyl isobutyl carbinol (MIBC), was observed. In cases where the Pt/Sratio was low (Run 5 & 6), significant amounts of the imine was detected in the GC.