(560aq) Sustainable Solutions for Selective Glycerol Oxidation into Dihydroxyacetone Using Commercial Catalysts
AIChE Annual Meeting
2019
2019 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Poster Session: Catalysis and Reaction Engineering (CRE) Division
Wednesday, November 13, 2019 - 3:30pm to 5:00pm
DHA production by catalytic glycerol aerobic oxidation was firstly reported in 1993 using carbon supported Pt/C and Pt-Bi/C catalysts and, since then, the scientific community focus has been mainly focused on the development of catalyst to increase the process selectivity towards DHA and glycerol conversion. Simultaneously, two main operating conditions were established for glycerol oxidation: basic or base free conditions. Although working under strong basic conditions (typically by adding NaOH) greatly increases reaction rate, Pt group catalysts are unstable and metal leaching may occur, DHA is unstable, leading to the formation of glyceric acid (GCA) and organic acids salts, which may precipitate, leading to process complications at industrial scale.
In the Lab-scale tests, three main catalysts groups were constituted to efficiently convert glycerol into DHA: Pt based, Au based and noble bimetallic catalysts. For Pt based catalysts, base free conditions were usually applied and the highest DHA yields were obtained with Bi and Sb promoted Pt catalysts supported in carbon materials, namely carbon nanotubes, and mesoporous silicas, due to its well defined structure and medium sized porous, favoring glycerolâs secondary hydroxyl group oxidation and avoiding DHA overoxidation. For Au catalyzed glycerol oxidation, the mandatory presence of strong bases, which resulted in poor or null DHA selectivity, was overcome in 2010, by using specific support materials, which increased catalysts activity under base free conditions. In 2014 high DHA yields were firstly reported using Au catalysts supported in late transition metal oxides as CuO, and later using Au/ZnO, which are so far the most promising results, opening new possibilities for reaction process design as Au catalysts are far more resistant to overoxidation compared with Pt group metals. Bimetallic catalysts, such as AuPt, showed enhanced performance compared with the monometallic catalysts, namely in terms of stability and activity, but only activated carbon (AC) supported AuPt-Bi catalysts achieved moderate DHA yields.
It is generally difficult to achieve very high DHA yields in batch reactors, due to catalysts deactivation and because with increasing reaction times, glycerol conversion increases but DHA overoxidation into compounds like hydroxypyruvic acid occurs, decreasing DHA selectivity. Concerning catalysts deactivation, although poisoning by surface overoxidation, metal leaching and sintering processes may occurs under certain operating conditions, the major cause of catalysts deactivation is the strong adsorption of reaction products onto catalysts surface, namely GCA, as most of the produced organic acids are well-known chelating agents. Catalyst washing with water revealed to be insufficient to restore the performance of the majority catalysts tested, whereas thermal treatment resulted for some catalysts.
To the best of our knowledge, about twenty-five years after the first work about selective glycerol aerobic oxidation into DHA, the process is still in laboratorial scale and far from reach commercial application in industry, mainly due to the unsatisfactory DHA yields achieved, the use of expensive noble metal catalysts and catalyst deactivation problems and also due to the fact that most of the catalyst tested have been synthesized by research groups and were not available at commercial scale.
Based on the above-identified problems, some of the most promising commercial catalysts available for selective glycerol aerobic oxidation into DHA were tested, to evaluate their performance and reactions kinetics. The commercial catalysts Pt5%/AC (Aldrich®), Pt5%-Bi1.5%/AC (Johnson Matthey®) and Pt5%-Bi5%/AC (Evonik®) were purchased and tested in a semi-batch reactor under base free conditions. The main operating conditions as temperature, initial glycerol concentration, oxygen pressure and oxygen inlet flow rate were screened. High glycerol conversions were obtained, namely Pt5%/AC and Pt5%-Bi5%/AC catalysts achieved glycerol conversions above 90% within the first 6h of reaction. It was possible to verify that the performance of Pt-Bi catalysts outcomes the performance of Pt catalysts in terms of DHA selectivity. The best results within the platinum catalysts tested in terms of DHA yield was attained with the Pt5%-Bi1.5%/AC from Johnson Matthey®.
Hence, this work demonstrates that commercially available Pt-Bi/AC catalysts may represent a potential solution for the scale up of DHA synthesis through glycerolâs aerobic oxidation.