基于二氧化碳、水、太阳能的绿色化工精炼

化石资源为人类提供了不可或缺的化学品、材料以及燃料,但也造成了大量二氧化碳排放。生物质是可以生产低碳化工产品的可再生资源,但要占用有限的可耕地资源。提出了直接以二氧化碳、水和太阳能为原料的绿色化工精炼。采用光电板收集太阳能并转化为电能,电能通过膜式水电解池产生氢气,氢气通入新型生物反应器并在自养菌的作用下把二氧化碳还原为聚三羟基丁酸酯(PHB)。此新型生物反应器解决了因气体溶解性低而影响传质速度的关键技术,微生物干重产出达0.18 g·L^-1·h^-1,其中PHB质量分数约50%。PHB不仅是优良的可生物降解塑料,也是可用于生产C₃～C₄有机低分子和芳香烃的平台化合物。在磷酸催化作用下,PHB可转化为与汽油相当热值和元素组成的C₄～C₁₆燃料油。分离PHB后的细菌生物质残渣可水热分解获得生物油和富氮水相产物。此生物油具有比植物生物油更高的热值,而水相产物可作为营养物用于培养微生物。

Green refinery of carbon dioxide,water and solar energy

Refining and reforming of fossil feedstock produces indispensable materials, chemicals and fuels, but releasing a substantial amount of carbon dioxide (CO₂). Bio-refineries of renewable biomass produce bio-based products of low carbon footprint. The biomass feedstock originates from CO₂, water and sunlight, but relies on limited natural resources including arable land. This paper illustrates a green refinery that directly uses CO₂, water and solar energy as the feedstock. A laboratory facility was set up and operated for verification, including a photovoltaic assembly, a membrane water electrolyzer, and a novel bioreactor in which an autotrophic hydrogen-oxidizing bacterium fixed CO₂. Hydrogen was the only source of reducing agents and biological energy in microbial CO₂ fixation, and obtained from water electrolysis with solar electricity. The reduced carbon was stored in bacterial biomass (CH_1.74O_0.46N_0.19) and polyhydroxybutyrate (PHB, C₄H₆O₂). PHB is a thermoplastic that can find a variety of applications because of its genuine biodegradability and similar material properties to that of polypropylene. It is also a platform material from which C₃-C₄ chemicals and aromatic compounds can be derived. Under catalysis of phosphoric acid, PHB was reformed into a gasoline-like transportation fuel. In addition, the residual bacterial mass can be also liquefied under thermal hydrolysis conditions and separated into a hydrophobic bio-oil and hydrophilic nutrient-rich hydrolysates. The former has a higher heating value than bio-oils from lignocellulose, and the latter can be reused in the microbial CO₂ fixation as nutrients. A bioreactor of high mass transfer rate is the key to the technology because of the very low solubility of the gaseous substrates in the aqueous solution. A novel bioreactor was tested, exhibiting a high mass transfer rate even at a low gas feeding rate. The dry cell mass productivity reached 0.18 g·L^-1·h^-1 and the PHB content was about 50% (mass).