Novel Process For1,3-dihydroxyacetone (DNA)Production From Glycerol:Technological Feasibility Study And Highly Efficient Catalyst Synthesis

As the recent escalating production and utilization of biodiesel worldwide; glycerol, a byproduct from biodiesel manufacturing, becomes a versatile building block for the potential production of biofuels and fine chemicals. Towards this end, catalytic oxidation of glycerol to produce the high-value chemical dihydroxyacetone (DHA) is a promising approach. This paper mainly involves two aspects:the technological and economical feasibility study of the conversion glycerol to DHA through a group protecting technology; and the design and preparation of corresponding catalyst for enhancing triphasic oxidations. The main content is described as follows:(1) A novel and efficient conversion process of glycerol to DHA via indirect oxidation was developed. Due to the tri-hydroxyl structure of glycerol and rapid over-oxidation of DHA, the good selectivity to DHA at high glycerol conversion is difficult to achieve. Therefore, we describe a three-step conversion process capable of achieving DHA in a high yield without low-efficient and complicated purification. The first step is to protect the first and third hydroxyl groups of glycerol through acetalization with benzaldehyde to produce1,3-benzylidene glycerol (HPD). The second step is to oxidize the second hydroxyl group to give1,3-benzylidene dihydroxylacetone (PDO). Finally, the purified PDO is deprotected to produce DHA, and the released benzaldehyde is recycled. In this process, the oxidation of HPD is the key step. In our work, the Anelli (TEMPO/NaBr/NaClO) oxidation system was successfully examined to verify the feasibility of this new process. The overall yield of DHA was>90%with>99%purity, and>80.0%recovery as crystals after recrystallization using cold ethanol as anti-solvent. Based on the experimental results, the process of this conversion is preliminarily designed, and its material cost is also briefly evaluated.(2) A magnetic polystyrene nanosphere immobilized TEMPO catalyst was designed and prepared. Although the effectiveness of three-step conversion of glycerol to synthesize DHA catalyzed by Anelli oxidation system was successfully verified; the limitation of the isolation and reuse of the expensive TEMPO still hindered the scaled-up of this conversion process. Thereby, we immobilized the4-hydroxyl-TEMPO onto magnetic polystyrene nanospheres (Fe3O4/PS) through Williamson reaction to synthesize catalyst Fe3O4/PS-TEMPO, and firstly applied as a magnetic recyclable catalyst in the oxidation of various alcohols. The new and simple prepared heterogeneous Fe3O4/PS-TEMPO exhibits similar versatilability and efficiency to homogeneous TEMPO under Anelli conditions. The excellent stability of the Fe3O4/PS enables the Fe3O4/PS-TEMPO to be recycled more than20times in Anelli oxidation of benzyl alcohol without significant leaching of immobilized TEMPO radicals or degradation of Fe3O4nanoparticles. Finally, the catalyst Fe3O4/PS-TEMPO was applied for HPD oxidation to produce PDO; it was found that under the optimized reaction condition:3.0mol%immobilized TEMPO,1.5equiv NaCIO,10mol%NaBr,10℃, pH9.1and10min, the yield of PDO reached98.1%.(3) A novel and efficient Pickering-Anelli emulsion catalysis was established. To enhance the efficiency of the biphasic Anelli oxidation system, we apply the Pickering emulsion concept to develop a novel Pickering-Anelli oxidation system. The strategy involves converting the biphasic Anelli system into a Pickering emulsion phase with micrometer-scale droplets, thus ensuring a large reactive surface area of the oil-water interface. Several Pickering emulsifiers were designed and prepared by immobilizing ionic liquids [im-Cn]Cl (n=1,4,6and10) onto Fe3O4/PS surface to improve the emulsion formation. Based on this, we immobilize the TEMPO onto Fe3O4/PS surface using [im-C2]Cl as linkers to synthesize the catalyst Fe3O4/PS[im-C2TEMPO]Cl, and used as Pickering emulsifiers for potential emulsion catalysis to enhance the efficiency of triphasic Anelli oxidation. The catalytic rate-accelerating mechanism of Pickering emulsion catalysis in this triphasic oxidation system was also established. The catalyst can be recycled at least10times in Anelli oxidation of benzyl alcohol under solvent-free conditions.(4) A magnetic fixed micelle immobilized TEMPO catalyst was designed and prepared. Inspired by the result that the miceller catalysis can provide an accelerating effect through the sqlubilisation of hydrophobic reagent into water, we described the first application of magnetic superhydrophobic polymer nanosphere cage derived from Pickering emulsion as framework for potential miceller catalysis in Anelli biphasic catalysis. This system was designed by the covalent stabilization of active site (TEMPO) grafted-amphiphiles [im-CnTEMPO]Cl (n=4,6and10) onto magnetic hydrophobic core to form a micelle-like architecture, and used it as Pickering emulsifier to develop a new kind of Pickering emulsion based and micelle-catalysed biphasic system. The catalysis results demonstrate that these fixed micelle tethered TEMPO exhibit much higher activity than soluble counterparts in biphasic Anelli oxidation of alcohols. It is attributed to the unique properties of Pickering emulsion and miceller catalysis. Additionally, the excellent magnetic response ensures the efficient recycling of catalyst by magnetic decantation after ultrasonic demulsification. The catalyst can be recycled at least10times without significant loss of activity or degradation of magnetic susceptibility.