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基于碳柱微通道反应器的风味酯连续流催化制备

Continuous-flow catalytic synthesis of flavor esters based on the carbonized-wood monolithic microreactor

  • 摘要: 针对传统搅拌反应存在的传质差、耗时长、物料返混严重、底物抑制等问题,该研究以具有天然三维微通道结构的椴木柱为主体,经700 ℃氮气氛围碳化后构建了碳柱微通道连续流反应器,用于肉桂酸乙酯的连续流催化制备。结果表明,在肉桂酸与乙醇摩尔比为1:20,催化剂浓硫酸的添加量肉桂酸质量的30%,反应温度100 ℃,底物流速为5 mL/min,反应器出口压力为0.2 MPa时,肉桂酸转化率可达96.5%。该碳柱微通道反应器显示出优异的机械强度和酸碱耐受性,连续运行10次仍能保持90%以上肉桂酸转化率,也可用于乙酸乙酯、己酸乙酯等多种风味酯的制备。研究结果为食品香精香料中风味化合物的连续化制备提供新方案。

     

    Abstract: Due to the poor mass transfer, long reaction time and severe material back mixing in traditional stirring reactions, the microfluidic reaction system with the advantages of enhanced mass transfer, fast reaction speed and mitigated substrate inhibition have received much attention. Natural wood is a cheap, renewable and earth-abundant material, which is regarded as the ideal model for monolithic reactors due to the existing 3D hierarchical structures. On this basis, carbonized wood with superior electrical conductivity, chemical and mechanical stability, tunable multifunctionality endows them as the monolithic reactors objects to synthesize advanced materials for multiple purposes. This paper constructed a carbonized monolithic microreactor for the continuous-flow catalytic synthesis of ethyl cinnamate, based on the basswood column with a natural three-dimensional microchannel structure which were carbonized in a nitrogen atmosphere at 700 ℃. The peristaltic pump tube is used to connect the metal coil and the carbonized monolithic microreactor in turn. The peristaltic pump sends the reaction liquid to the metal coil, and the oil bath pan heats the metal coil to preheat the reaction liquid. Subsequently, the reaction liquid enters the carbonized monolithic microreactor, and the oil bath circulation device heats the reactor to ensure the reaction temperature. The results indicate that the length and diameter of carbonized-wood columns were reduced from 200 mm and 40 mm to 165 mm and 29.6 mm, respectively, due to the pyrolysis of lignin, hemicellulose and cellulose at high temperature. The resulting material not only preserves the well-aligned microchannel topology of the original wood, but also exhibits significantly enhanced properties, including high chemical stability, robust mechanical strength, and exceptional mass and heat transfer performance—laying a solid foundation for efficient continuous-flow catalytic processes. SEM characterization demonstrated the regular and hierarchical porous structures of carbonized column with abundant tubular channels (5-50 µm in diameter) in the wood growth direction and micro-sized pores (0.5-1 µm) insides tubular channels. The micro-sized pores on the tubular channels allowed the liquid substrates to enter the adjacent channels and generate fluid disturbance for improved mass transfer and enhanced catalytic capacity. Then, 96.5% of cinnamic acid conversion was reached with the molar ratio of cinnamic acid to ethanol at 1:20, the catalyst addition of concentrated H2SO4 (98 wt%) being 30 % of the mass of cinnamic acid, the reaction temperature of 100 ℃, substrate flowrate of 5 mL/min and the outlet pressure at 0.2 MPa. Under the continuous-flow reaction mode, carbonized-wood monolithic microreactor induced maximum TOF of 42.4 h−1 for the catalyst of sulfuric acid, which was 11.7-22.3 times higher than those in batch-mode reaction. This carbonized monolithic microreactor exhibited excellent mechanical strength (4538 N in load, 31.2 MPa in compressive strength, 3839 MPa in elastic modulus) and acid--base tolerance, which could maintain over 90% of cinnamic acid conversion after 10 consecutive runs. The microchannel reactor was subjected to immersion tests in both acidic and alkaline solutions of varying concentrations for 24 hours. After drying, its structural morphology remained fully intact, demonstrating exceptional resistance to corrosive chemical environments. These properties ensure not only long-term chemical stability under continuous operation, but also structural integrity against collapse or deformation caused by reactive fluid flow under process conditions. Besides, it can also be used for the efficient preparation of various flavor esters such as ethyl acetate (93.5%), hexyl hexanoate (95.7%), iso-amyl p-methoxycinnamate (87.6%), ethyl hexanoate (78.9%), ethyl butyrate (92.0%) and cinnamic acid methylester (92.4%). Hence, the research developed a carbonized-wood monolithic microreactor with basswood as raw material, which was filled into metal casing after high temperature carbonization. The reactor not only exhibited high mass and heat transfer efficiency but also presented good mechanical properties and acid and alkali resistance. It provides a new strategy for the efficient synthesis of flavor esters by combining continuous flow reaction and acid catalysis, showing great potential in industrial applications in the field of food and cosmetics.

     

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