纤维素超/亚临界水水解研究进展

纤维素作为天然可再生资源,由于其水解糖化技术的可行性和水解产物的重要性而受到人们的广泛关注。目前多种生物和化学技术应用于纤维素水解糖化的研究,其中超、亚临界水是纤维素水解的绿色新型水解技术之一,具有反应快、转化率高等特点。本文比较了纤维素在超、亚临界水中不同的水解机理和产物分布,归纳了反应温度、压力和时间以及催化剂和纤维素结构对超、亚临界水水解反应的影响,并且介绍了超、亚临界水水解技术联用在纤维素水解中的应用。     

超临界二氧化碳偶合超声预处理强化玉米秸秆酶水解(英)

提出了一个木质纤维素生物质预处理的全绿色加工过程.以玉米秸秆和玉米芯为原料,以超临界CO₂和超声偶合法对木质纤维素进行预处理.超临界CO₂预处理条件为:压力15-25 MPa,温度120₁70℃,含水量50%,反应时间0.5₄ h.超声场功率600W,温度80℃,作用时间2-8 h.用纤维素酶水解反应获得的还原糖总量来评价预处理效果.结果表明,单纯超临界CO₂和超临界CO₂偶合超声预处理都能够提高生物质水解反应还原糖产量.对于玉米芯,超临界CO₂预处理（170℃,20 MPa,3 0min）后,还原糖产率为62%（未预处理的为12%）.对于玉米秸秆（170℃,20 MPa,2.5 h）,还原糖产率为46.4%.对于玉米芯,超临界CO₂偶合超声预处理（600 W,80℃下超声处理6 h,然后用170℃,20 MPa超临界CO₂预处理30 min）后,还原糖产率为87%.对于玉米秸秆,超临界CO₂偶合超声预处理（600 W,80℃下超声处理8 h,然后用170℃,20 MPa超临界CO₂预处理1 h）后,还原糖产率为25.5%.与未处理生物质相比,X射线衍射结果表明玉米秸秆和玉米芯在超临界CO₂和超声预处理后其结晶度没有明显变化.扫描电镜分析则发现木质纤维素的表面积显著增加.     

PCBs的超（亚）临界水催化氧化及还原裂解*

简要分析了多氯联苯(PCBs)的来源及其对环境构成的危害,介绍了PCBs在超（亚）临界水中的反应及其处理效果。分别从超临界水氧化、超临界水裂解及亚临界水还原三个方面阐明了超临界反应过程中PCBs降解的反应路径和降解效率,解释了共溶剂（甲醇、苯）、碱催化剂（Na₂CO₃、NaOH）、氧化剂（NaNO₃、NaNO₂）等对PCBs脱氯和分解的增效作用机理。发现在超临界水氧化与超临界水裂解条件下CH3OH对PCBs降解反应的促进机制有所不同,碱催化剂通过中和反应过程中产生的HCl生成NaCl沉淀导致体系中Cl的含量降低,从而促进脱氯反应的进行。对反应器防腐、处理的经济性方面略作讨论,在总结上述研究工作的基础上提出了PCBs的超临界反应处理技术未来发展的若干研究方向。     

纤维素超临界水预处理与水解研究

利用超临界水解工艺进行生物质废弃物(秸秆)能源转化, 使其主要成分纤维素在超临界水中快速水解为低聚糖, 为其进一步葡萄糖转化和乙醇发酵解决技术瓶颈. 其中纤维素在超临界水中的溶解是预处理与水解过程的限速步骤. 研究表明, 反应温度达到380 ℃及以上时, 纤维素可迅速溶解并进行水解, 液化比例可达100%; 在374～386 ℃范围内反应温度对纤维素的转化率有明显作用, 低聚糖和六碳糖的总产率在临界点附近出现最大值. 超临界条件下, 低聚糖和六碳糖转化率在较短反应时间内出现峰值, 而后随反应时间的延长快速下降, 固液比对于纤维素的低聚糖和六碳糖转化也有显著影响. 最优水解条件研究显示, 在380 ℃, 40 mg纤维素/2.5 mL水条件下反应16 s可获得最大的低聚糖产率, 为29.3%, 在380 ℃, 80 mg纤维素/2.5 mL水条件下反应18 s可获得最大的六碳糖产率, 为39.2%.     

碱木质素超/亚临界乙醇体系解聚机理研究

采用微型高温高压反应釜,在超/亚临界乙醇体系,进行麦草碱木质素的解聚实验,通过扫描电子显微镜(SEM)、气相色谱/质谱联用仪(GC/MS)及红外光谱仪(FT-IR)对解聚产物进行分析,探讨大分子结构的解聚机理。结果表明,碱木质素在乙醇临界点条件(240℃,7.2 MPa)解聚获得最低残焦得率,数值为16.5%。碱木质素在亚临界乙醇体系解聚过程,碱木质素熔融形成直径1.0-2.0μm的微球分散于乙醇中,结构单体间少量醚键和苯环侧链Cα均裂断裂,形成酚类、酯类、酮类和酸类产物;碱木质素在超临界乙醇体系解聚过程,熔融微球直径明显缩小,解聚时发生大量结构单体间醚键、苯环侧链Cα断裂及酯类产物的二次分解反应,解聚产物中酯类产物含量(11.94%)降低,酚类产物得率(52.14%)提高。     

亚/超临界乙醇液化秸秆纤维素解聚反应研究与机理初探

利用间歇式高压反应釜,在反应温度200～330 ℃、乙醇用量0～150 mL条件下,考察了亚/超临界乙醇直接液化秸秆纤维素的解聚行为,并初步探讨了其液化机理。结果表明,反应温度、乙醇用量和反应停留时间对秸秆纤维素的液化均有显著影响,反应温度由200 ℃升高至330 ℃,重油和气体收率分别增加了12.55%、28.83%;乙醇用量增加,反应压力随之升高,乙醇进入超临界状态,残渣和气体收率相比单纯热裂解分别降低11.10%和8.44%。通过GC/MS、FT-IR分析生物油组分和残渣特性,表明秸秆纤维素在亚/超临界乙醇中断键裂解,且酮类和乙酯类化合物是生物油的主要成分。     

亚临界水萃取技术应用于环境样品预处理的研究

对近几年来亚临界水萃取（Sub-critical water extraction,SCWE）技术的研究进行了综述,介绍了亚临界水萃取技术的原理、特点及其在环境分析中的应用。与传统的样品预处理方法和一些新的样品预处理技术（如微波辅助萃取）相比,SCWE可以不使用有机溶剂,对环境造成的污染较小,是一种绿色的样品预处理技术。SCWE在萃取能力、回收率、精密度等方面具有相当的可靠性。方法具有较高的选择性分离预富集某些有机污染物（如多环芳烃等）的特点（引述文献22篇）。     

超（近）临界水中原位反应技术研究进展

水是一种环境友好的反应介质,超（近）临界水中的反应已成为目前研究的一个热点。原位反应技术是深入研究超（近）临界水中反应过程的重要手段之一,其中金刚石压腔和毛细管技术是目前最常用的主要方法,与拉曼光谱、红外光谱、质谱等分析方法联用,可以对超（近）临界水中反应机理进行研究。本文综述了超（近）临界水中的原位反应观测技术,介绍了金刚石压腔的结构和工作原理,金刚石压腔和毛细管的应用范围,阐述了金刚石压腔和毛细管技术在原位观测和反应机理研究方面的应用。最后,展望了超（近）临界水原位反应技术的应用前景。     

典型醇类物质超临界水氧化反应途径研究

采用自主设计的连续流动气封壁超临界水氧化反应装置,研究了典型醇类物质甲醇、乙醇和异丙醇在超临界水中氧化的反应途径,并归纳了醇类物质超临界水氧化反应的规律及特点。研究结果表明,甲醇超临界水氧化反应的主要中间产物为甲醛,同样条件下转化率较乙醇和异丙醇低;乙醇和异丙醇超临界水氧化反应的主要中间产物为丙酮、乙酸、乙醛和甲醇等。三种醇超临界水氧化过程中均涉及到大量活性自由基的相互作用,表现为脱氢、裂解和聚合等反应形式;产物包括碳链增长、不变、降低三种类型。总体来看,醇类物质超临界水氧化反应的趋势是向碳链降低的方向进行,即通过一系列中间产物最后生成CO₂和水。     

典型醇类物质超临界水氧化反应途径研究

采用自主设计的连续流动气封壁超临界水氧化反应装置,研究了典型醇类物质甲醇、乙醇和异丙醇在超临界水中氧化的反应途径,并归纳了醇类物质超临界水氧化反应的规律及特点。研究结果表明,甲醇超临界水氧化反应的主要中间产物为甲醛,同样条件下转化率较乙醇和异丙醇低;乙醇和异丙醇超临界水氧化反应的主要中间产物为丙酮、乙酸、乙醛和甲醇等。三种醇超临界水氧化过程中均涉及到大量活性自由基的相互作用,表现为脱氢、裂解和聚合等反应形式;产物包括碳链增长、不变、降低三种类型。总体来看,醇类物质超临界水氧化反应的趋势是向碳链降低的方向进行,即通过一系列中间产物最后生成CO2和水。     

超(近)临界水中的化学反应

水作为自然界最常用的绿色介质在化学反应中已经得到广泛应用.近年来,随 着超临界技术的发展,人们对超(近)临界水的认识逐渐深入,以超(近)临界水为介质进 行化学反应的研究引起了人们的极大兴趣,开展了许多卓有成效的工作.本文在介绍超(近)临界水性质的基础上,主要对近年来超(近)临界水中的有机合成、无机化学反应、生物质的转化反应及聚合物的降解等方面的研究进展进行了回顾总结.     

Mechanisms and kinetics of noncatalytic ether reaction in supercritical water. 1. Proton-transferred fragmentation of diethyl ether to acetaldehyde in competition with hydrolysis

Noncatalytic reaction pathways and rates of diethyl ether in supercritical water are determined in a quartz capillary by observing the liquid- and gas-phase 1H and 13C NMR spectra. The reaction is investigated at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.2-0.6 g/cm3, and in subcritical water at 300 and 350 degrees C. The neat reaction (in the absence of solvent) is also studied for comparison at 0.1 M and 400 degrees C. The ether is found to decompose through (i) the proton-transferred fragmentation to ethane and acetaldehyde and (ii) the hydrolysis to ethanol. Acetaldehyde from reaction (i) is consecutively subjected to the unimolecular and bimolecular redox reactions: (iii) the unimolecular proton-transferred decarbonylation forming methane and carbon monoxide, (iv) the bimolecular self-disproportionation producing ethanol and acetic acid, and (v) the bimolecular cross-disproportionation yielding ethanol and carbonic acid. Reactions (ii), (iv), and (v) proceed only in the presence of hot water. Ethanol is produced through the two types of disproportionations and the hydrolysis. The proton-transferred fragmentation is the characteristic reaction at high temperatures and is much more important than the hydrolysis at densities below 0.5 g/cm3. The proton-transferred fragmentation of ether and the decarbonylation of aldehyde are slightly suppressed by the presence of water. The hydrolysis is markedly accelerated by increasing the water density: the rate constant at 400 degrees C is 2.5 x 10(-7) s(-1) at 0.2 g/cm3 and 1.7 x 10(-5) s(-1) at 0.6 g/cm3. The hydrolysis becomes more important in the ether reaction than the proton-transferred fragmentation at 0.6 g/cm3. In subcritical water, the hydrolysis path is dominant at 300 degrees C (0.71 g/cm3), whereas it becomes less important at 350 degrees C (0.57 g/cm3). Acetic acid generated by the self-disproportionation autocatalyzes the hydrolysis at a higher concentration. Thus, the pathway preference can be controlled by the water density, reaction temperature, and initial concentration of diethyl ether.     

Conversion of polymers and biomass to chemical intermediates with supercritical water

Recent results are described on conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent. Reactions of cellulose in supercritical water are rapid (<50 ms) and proceed to 100% conversion with no char formation. They show a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. Further, there is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system. For polyethylene, higher yields of short-chain hydrocarbons, higher alkene/alkane ratios and higher conversions were obtained in supercritical water than those obtained by pyrolysis.     

Conversion of phenylacetonitrile in supercritical alcohols within a system containing small volume of water

The reaction of phenylacetonitrile in supercritical methanol and ethanol in a system containing a small volume of water was studied. The effects of various operating conditions, such as reaction temperature, reaction time, the mole ratio of phenylacetonitrile/water/methanol or ethanol on the product yield were systematically investigated. The optimal yield of methyl phenylacetate for phenylacetonitrile in supercritical methanol in a system containing a small volume of water was 70 % at 583 K and 2.5 h. The optimal yield of ethyl phenylacetate for phenylacetonitrile in supercritical ethanol with a small volume of water was 80 % at 583 K and 1.0 h. At the same time, a feasible mechanism was proposed for phenylacetonitrile in supercritical methanol and ethanol in a system containing a small volume of water.     

Prospects and Challenges of Microwave-Combined Technology for Biodiesel and Biolubricant Production through a Transesterification: A Review

Biodiesels and biolubricants are synthetic esters produced mainly via a transesterification of other esters from bio-based resources, such as plant-based oils or animal fats. Microwave heating has been used to enhance transesterification reaction by converting an electrical energy into a radiation, becoming part of the internal energy acquired by reactant molecules. This method leads to major energy savings and reduces the reaction time by at least 60% compared to a conventional heating via conduction and convection. However, the application of microwave heating technology alone still suffers from non-homogeneous electromagnetic field distribution, thermally unstable rising temperatures, and insufficient depth of microwave penetration, which reduces the mass transfer efficiency. The strategy of integrating multiple technologies for biodiesel and biolubricant production has gained a great deal of interest in applied chemistry. This review presents an advanced transesterification process that combines microwave heating with other technologies, namely an acoustic cavitation, a vacuum, ionic solvent, and a supercritical/subcritical approach to solve the limitations of the stand-alone microwave-assisted transesterification. The combined technologies allow for the improvement in the overall product yield and energy efficiency. This review provides insights into the broader prospects of microwave heating in the production of bio-based products.     

Protein Hydrolysis by Subcritical Water: A New Perspective on Obtaining Bioactive Peptides

The importance of bioactive peptides lies in their diverse applications in the pharmaceutical and food industries. In addition, they have been projected as allies in the control and prevention of certain diseases due to their associated antioxidant, antihypertensive, or hypoglycemic activities, just to mention a few. Obtaining these peptides has been performed traditionally by fermentation processes or enzymatic hydrolysis. In recent years, the use of supercritical fluid technology, specifically subcritical water (SW), has been positioned as an efficient and sustainable alternative to obtain peptides from various protein sources. This review presents and discusses updated research reports on the use of subcritical water to obtain bioactive peptides, its hydrolysis mechanism, and the experimental designs used for the study of effects from factors involved in the hydrolysis process. The aim was to promote obtaining peptides by green technology and to clarify perspectives that still need to be explored in the use of subcritical water in protein hydrolysis.     

Study on depolymerization of polycarbonate in supercritical ethanol

The characteristics of depolymerization of PC in supercritical ethanol were investigated in the range of 483-563 K by using a high-pressure batch autoclave reactor. Based on the qualitative and quantitative analyses of the products, a depolymerization-reaction model was proposed to explain the reaction mechanism, i.e. random scission and ester exchange reaction occurred simultaneously during the process of depolymerizaition of PC. It was suggested that the process of depolymerization consisted of subcritical region, transitional region and supercritical region. It was indicated that PC degraded with slow decrease of molecular weight determined by GPC and with the conversion of 7.5% at 513 K in subcritical region. While in the supercritical region, the molecular weight of PC decreased quickly and degraded completely in 30 min at 563 K. Continuous-distribution kinetics could be used to describe the mechanism of polymer degradation and the energy of activation for the random scission of PC in the supercritical region was 97.2 kJ/mol. Moreover, PC could be degraded completely into diethyl carbonate (DEC) and bisphenol A (BPA) with the yields of 89% and 90%, respectively, in supercritical region.