Speciality Chemicals Magazine JAN / FEB 2021 | Page 33


Figure 1 – Synthetic pathways for the production of MBE and cinnamyl alcohol
Undesired side products are shown in red
engineering . Our patented coating technology allows uniform layers of versatile catalysts to be formed , while Taylor flow regimes ensure superior mixing . The technology has been demonstrated in several hydrogenation reactions where selectivity is key .
Selective hydrogenation
One particular case study is that of triple to double C-C selective hydrogenation to obtain MBE ( Figure 1 ). MBE is a precursor compound to the synthesis of Vitamin E and a commercially important chemical in the flavours and fragrances sector . Traditionally , the compound can be afforded in batch using Lindlar catalyst , followed by distillation from the solvent . A major drawback of this method is the application of leadpoisoned catalyst , which is toxic to the environment and to people . To circumvent this issue and also reduce the environmental impact of solvent and energy use , reactions can be performed in Stoli reactor tubes . 2 Thanks to the high level
of control over residence time in the reactors , the conversion of MBY on both poisoned ( bismuth ) and non-poisoned catalysts was 90 %, with over 90 % selectivity towards MBE . These results were also achieved without the need for solvent , reducing both waste and environmental impact . In order to assess both the batch and flow processes , the chemical sustainability was calculated for these processes via the DOZN * green chemistry evaluator . 2 , 3 DOZN produces an aggregate score based on 12 principles of green chemistry with lower scores being more environmentally sustainable than higher ones . Based on the current generation of Stoli reactor tubes at 1 kg-scale , the semi-hydrogenation has an aggregate score of 32 , compared with over 130 for the batch process . 3 The most striking score is energy-efficiency , where Stoli tubes are nearly five times more efficient than batch processes . This effect is demonstrated in the increased space-time yield in flow of
0.45 mol / hr-1 / g Pd-1 , compared with 0.12 in traditional batch synthesis . Stoli has also teamed up with Vapourtec to marry its R-series flow chemistry system with our catalytic tube reactors in the chemoselective hydrogenation of cinnamaldehyde ( Figure 1 ). 4 At optimal conditions , 93 % conversion was achieved . Selectivity was also comparable with commercial batch synthesis , at 90 % yield toward the desired product , cinnamyl alcohol . 5 The performance of the reactor was very interesting , as the hydrogenation can be carried out under significantly milder conditions than in batch processes . Strikingly , comparing the reactor performance alone ( keeping the same platinum catalyst ), batch selectivity was as low as 15 % compared with 90 % in the Stoli tubes and 60 % in fixed bed . 4 Combined with leaching of around 0.02 ppm platinum , demonstrating the excellent stability of the catalyst , the flow reactor is also greener than the comparable batch reaction . 4 , 5 Again , DOZN scores were calculated for both processes and the aggregate score for ‣