Oxidation of bio-renewable glycerol to value-added chemicals through catalytic and electro-chemical processes

Due to its unique structure, characteristics, and bio-availability, glycerol transformation into value-added chemicals has been in the spotlight in recent years. This study provides a comprehensive review and critical analysis on catalytic and electro-chemical oxidation of glycerol into commodity chemicals, which have broad applications to the pharmaceutical, polymer, and food industries. Various synthesis methods (e.g. impregnation, sol-immobilization, incipient wetness, and deposition precipitation) for the preparation of the catalysts are discussed. Catalytic performance of mono-, bi-, multi-, and non-metal supported catalysts on carbon black, activated carbon, graphene, single- or multi wall-carbon nano-tubes, layered-double hydroxides, metal oxides, and polymers are evaluated. Among the methods, sol-immobilization is highly commended since fine metal NPs could be homogeneously distributed on the support, reported as an effective factor for controlling the selectivity of the desired product. In particular, the environmentally benign novel polymeric structures, illustrate significant breakthroughs in production of commodity chemicals compared to the conventional materials. Homogeneous oxidation of glycerol by enzymes and microorganisms also displayed acceptable performance particularly in production of DHA, but at the expense of long reaction time. Unlike the homogenous and heterogeneous catalytic processes, electro-chemical oxidation could be tuned for high product selectivity by controlling the nature, composition and structure of the electro-catalyst as well as the electrode potential. Most importantly, combination of electro-chemical oxidation of glycerol with oxygen or water reduction process in full- and electrolysis-cells, respectively could be the ultimate goal in this field. Simultaneous generation of value-added chemicals and electrical energy would have significant economical and environmental merits compared to the conventional processes. The current state-of-the-art of the glycerol oxidation process and recommendations for further research are also included.