TEMPO及其衍生物制备纳米纤维素及其智能调节方法的研究进展

2,2,6,6-四甲基哌啶氧自由基(TEMPO)共氧化剂体系被广泛应用于选择性氧化纤维素的C6位伯醇羟基。在氧化过程中,纤维素的聚合度大幅度降低,因此被应用于制备纳米纤维素。随着TEMPO/NaClO/NaBr氧化技术的发展成熟,TEMPO/NaClO/NaClO2和4-乙酰胺基-TEMPO/NaClO/NaClO2体系都得到了广泛关注。随着研究的深入,TEMPO及其衍生物氧化体系已经成为一种高效且全pH范围适用的选择性氧化体系。TEMPO/NaClO/NaBr氧化体系在pH范围9～11活性最高,TEMPO/NaClO/NaClO2体系能够应用于pH中性的条件下,4-乙酰胺基-TEMPO/NaClO/NaClO2体系一般的pH使用范围为3.5～6.8。传统的TEMPO氧化过程需要持续手动控制pH恒定,操作繁琐,可控性差,应用缓冲溶液可控制TEMPO氧化过程中pH在一定范围内恒定,从而实现了TEMPO氧化体系的智能控制。文章综述了TEMPO及其衍生物制备纳米纤维素及其智能调节方法的研究进展。     

TEMPO体系对粘胶纤维的氧化过程研究

2,2,6,6-四甲基哌啶氧化物自由基(TEMPO)-NaClO-NaBr是一种新的选择性氧化体系,文章研究了该体系对粘胶纤维的氧化过程。CP/MAS13C-NMR反映该体系对粘胶纤维是C6位选择性氧化,并能完全氧化成纤维素糖醛酸;用CP/MAS13C-NMR、耗碱量法和酸碱中和法研究了反应过程中羧基含量的变化;粘度法对聚合度的研究显示,粘胶纤维的降聚现象在反应一开始就非常显著。     

纤维素选择性氧化研究进展

从纤维素氧化原理简单介绍了纤维素选择性氧化,综述了高碘酸盐、稳定的氮氧自由基、非稳定的氮氧自由基三种纤维素选择性氧化剂,以及它们选择性氧化纤维素的过程、原理和方法,对比说明高碘酸钠是目前选择性氧化纤维素C2和C3羟基最有效的氧化剂,稳定和非稳定的氮氧自由基是选择性氧化纤维素伯羟基最有效的氧化剂.展望了对于非选择性氧化的氧化剂,寻找合适的反应条件减少它们非选择性氧化部分,并减少氧化过程中对纤维素的降解是将来的研究方向.     

纤维素的选择性氧化反应及其体系

论述了纤维素的选择性氧化反应及不同选择性氧化反应适用的氧化体系。运用不同的选择性氧化体系。可以使纤维素单元上的伯羟基或仲羟基发生不同的选择性氧化反应。伯羟基的选择性氧化体系大致可以分为氧化氮类氧化体系，次氯酸盐氧化体系，ＴＥＭＰＯ氧化体系三类，近年ＴＥＭＰＯ类氧化体系作为新型的选择性氧化体系已成为热点。     

缓冲溶液对TEMPO/NaClO/NaBr选择性氧化纤维素的影响

TEMPO/NaClO/NaBr氧化体系被广泛应用于选择性氧化糖类C6位伯醇羟基,使其成为羧酸盐物质,从而获得更好的水溶性或其他特定功能。但在氧化反应过程中,pH随反应的进行下降,并且存在pH值持续控制繁琐等问题。实验比较分析TEMPO/NaClO/NaBr氧化纤维素在碳酸钠―碳酸氢钠缓冲溶液、硼砂―氢氧化钠缓冲溶液中的适用性,并以滴加的方式向体系中加入氧化剂NaClO溶液,探索两种缓冲体系中各自适宜的NaClO滴加速度。实验结果表明,碳酸钠―碳酸氢钠缓冲溶液和硼砂―氢氧化钠缓冲溶液都适用于TEMPO/NaClO/NaBr氧化体系。与传统方法相比,氧化纤维素的羧基含量可以提高20%～25%。使用缓冲溶液较单一氢氧化钠溶液对稳定pH值有更好的效果,并且方法简便。另外,硼砂―氢氧化钠缓冲溶液对NaClO滴加速度敏感,控制滴加速度可以有效地提高反应活性。     

漂白蔗渣浆中半纤维素含量对其TEMPO氧化性能的影响

以碱法提取半纤维素后的漂白蔗渣浆为原料,研究2,2,6,6-四甲基哌啶-1-氧自由基(TEMPO)氧化过程,探讨半纤维素含量对不同氧化程度漂白浆TEMPO氧化过程及氧化浆性能的影响。结果表明在氧化剂用量充足的情况下,减少纸浆中的半纤维素含量有利于加快氧化速率并延长氧化时间,从而促进羧基的生成,并且氧化程度越高促进作用越明显。半纤维素的存在一定程度上阻碍了氧化剂进入纤维内部,不利于TEMPO反应的进行。由于氧化降解反应,纸浆中半纤维素含量的减少以及氧化剂用量增加,会使氧化后浆料的聚合度和得率有所降低。     

壳聚糖C6位选择性氧化的研究进展

壳聚糖及其衍生物由于具有良好的生物活性、生物相容性、抗菌性和无毒等性能被广泛应用于众多领域。壳聚糖C6位选择性氧化为羧基的氧化剂主要有N2O4、NO2、CrO3、TEMPO/NaClO/NaBr体系和NaClO2/KBr/4,4’-二(2,2,5,5-四甲基咪唑-3-氧化-1-氧基自由基)苯基醚体系等。国内研究壳聚糖C6位选择性氧化的程度还远远不及国外,有待于在前人工作的基础上不断改进和开发氧化壳聚糖的方法。     

NaOH/硫脲/尿素预处理对棉纤维TEMPO选择性氧化的影响

采用NaOH/硫脲/尿素体系对棉纤维进行预处理,再进行选择性氧化,可以有效提高氧化棉纤维的羧基生成量。对比研究预处理棉纤维与普通棉纤维经2,2,6,6-四甲基哌啶氮氧自由基(TEMPO)选择性氧化后的羧基含量、纤维形态以及黏度。结果表明,经NaOH/硫脲/尿素体系预处理能够加快氧化反应速率,增加羧基生成量,但对纤维有一定的损伤。其中,羧基生成量随着纤维质量分数的增加呈先增加后减少的趋势,当纤维素质量分数为6%时,羧基生成量最大,棉纤维的可及度和反应性提高。纤维形态分析表明,经NaOH/硫脲/尿素体系预处理的棉纤维润胀溶解程度要大于未预处理的氧化棉纤维;在TEMPO的氧化条件下,氧化棉纤维的相对黏度随着纤维素质量分数增加而增加;当纤维素质量分数较高时,氧化过程中氧化棉纤维的羧基生成量和降解程度都近似于原纤维。     

TEMPO催化醇氧化反应进展

选择性催化氧化醇类化合物为相应的醛或酮是一类重要的官能团转化反应。四甲基哌啶氧化物(TEMPO)是一种含有稳定的氮氧自由基(NO·)的有机小分子催化剂,NO·可通过自身的强选择性,在加快醛或酮转化的同时不会过氧化成为羧酸。本文阐述了TEMPO催化体系催化醇选择性氧化反应的机理,在此基础上详述了过渡金属/TEMPO、非过渡金属/TEMPO、固载化TEMPO等体系催化醇选择性氧化反应的研究情况,并探讨了TEMPO在电催化以及光催化方面的应用。指出将高活性的TEMPO催化体系改进,在发挥其高选择性、高活性优点的同时,克服其本身价格昂贵的缺点,实现催化体系的重复利用,更符合生态、绿色理念,也是未来发展的趋势。     

羧基化改性纤维素凝胶球的制备和表征

以微晶纤维素为原料,利用NaOH/尿素体系对微晶纤维素进行溶解,得到再生纤维素溶液。采用滴定悬浮的方法制备纤维素水凝胶球,采用2,2,6,6-四甲基哌啶-1-氧基自由基（TEMPO）/NaBr/NaClO选择性氧化体系对纤维素水凝胶球进行氧化处理,获得羧基化改性纤维素水凝胶球,冷冻干燥得到羧基化改性纤维素气凝胶球。研究结果表明：羧基化改性纤维素水凝胶球的含水量为95.64%,吸附4h,亚甲基蓝的吸附量达到6.97mg/g。对羧基化改性纤维素气凝胶球进行傅里叶变换红外光谱（FT-IR）和扫描电子显微镜（SEM）表征分析,在1600cm^-1处出现了CO的伸缩振动峰,TEMPO的选择性氧化对样品起到羧基化改性作用,羧基化改性纤维素气凝胶增加了球形气凝胶的表面通透性,内部仍呈现网络结构,羧基化改性纤维素气凝胶球的密度为0.038g/cm³。     

Effect of thermal preoxidation and the addition of ammonium molybdate on the thermal conversion of cellulose and cellulose compositions

As found by thermogravimetric and differential thermal analysis and gas chromatography, the thermal oxidative pretreatment of cellulose and cellulose compositions with high-molecular-weight polycondensed aromatic systems of natural origin resulted in a shift of peaks due to intense thermal degradation to the region of lower temperatures and changes in the yields of carbon residues and in the quantitative composition of the main gaseous products of pyrolysis. In the presence of ammonium molybdate, which is a catalyst for cellulose oxidation and carbonization, the effect of the thermal oxidative treatment was even more enhanced. This was reflected in an increase in the low-temperature shifts of intense degradation processes and a considerable increase in the yields of carbon residues of cellulose and cellulose compositions.     

Studies in chemically modified celluloses. VII. Periodate oxidation of cellulose dyed with reactive dyes

Periodate oxidation of cellulose and cellulose dyed with reactive dyes at 32°C in the presence and absence of various concentrations of alkali metal chlorides and sulfates has been studied. The extent of oxidative degradation undergone by cellulose and assessed in terms of copper number and cuprammonium fluidity of the oxidized dyeing, the oxygen consumption during the oxidation, and the rate constant were shown to be markedly increased by the presence of the salts in the oxidizing solution. Borohydride and chlorous acid treatment were used to further modify the periodate oxycelluloses, and the effect of these treatments on the properties was studied.     

The stability of monochlorotriazinyl reactive dyes on cellulose films in aqueous alkaline solutions containing peroxide bleaching agents

The stability of seven reactive (one difluoromonochloropyrimidinyl and six monochlorotriazinyl) dyes on cellulose immersed in sodium peroxoborate (PB) solution (UK–TO solution) containing tetraacetylethylenediamine (TAED) was examined using cellulosic films at 60 °C. The extent of dye loss that occurred from the dyed cellulosic films which were immersed in the UK–TO solution without detergent correlated closely to the ratings obtained using the BS 1006 UK–TO wash test. The dye loss that occurred from the dyed cellophane films was attributed to three contributions, namely, alkaline hydrolysis of dye–fibre bonds, oxidative fading of the dye chromophore by peroxides and cellulose degradation accelerated by PB and TAED. The alkaline hydrolysis of the dye–fibre bond and the extent of cellulose degradation in the UK–TO solution were the main contributions to the dye loss; dye oxidation was a minor factor in the dye loss mechanism. The physical bonding of the dye molecules reinforced the covalent dye–fibre bond stability towards the UK–TO solution.     

Thermal,morphological and spectroscopic studies on cellulose modified with phosphorus,nitrogen, sulphur and halogens

The kinetics of the thermal degradation of cellulose and modified cellulose, namely, cellulose phosphate, cellulose carbanilate, cellulose tosylate, chlorodeoxycellulose, bromodeoxycellulose, and iododeoxycellulose in air were studied by thermogravimetry and differential thermal analysis from ambient temperature to 700°C. The various thermodynamic functions for different stages of thermal degradation had been obtained following the procedure of Broido. The activation energies for the oxidative decomposition of cellulose and modified celluloses were found to be in the range 30–399 kJ mol^?1. The infrared spectra of the residues of modified celluloses gave indication of formation of a compound containing P?O, P? O? P (only in the case of cellulose phosphate), C?C, and C?O groups in the final residual char. The EPR signals indicated the formation of trapped and stable free radicals in the thermal degradation of all the compounds, particularly halodeoxycelluloses showed generation of large amounts of trapped free radicals during the oxidative decomposition. Scanning electron micrographs of the thermally degraded cellulose derivatives show changes in the fibrillar structure, evolution of gasesous products, and film formation depending upon the nature of the substituent in the cellulose matrix. The mechanism of thermal degradation of these compounds has been proposed.     

TiO_2–SA–Arg nanoparticles stabilized Pickering emulsion for photocatalytic degradation of nitrobenzene in a rotating annular reactor

Pickering emulsions stabilized by salicylic acid and arginine modified titanium dioxide(Ti O2–SA–Arg) nanoparticles were prepared in this study for photocatalytic degradation of nitrobenzene in a rotating annular reactor,and the effects of various design parameters of the rotating annular reactor, initial nitrobenzene concentration,catalyst amount, and solution p H on the degradation rate of nitrobenzene were investigated. Meanwhile, the degradation mechanism of nitrobenzene was proposed. The results show that increasing the aeration rate, the rotational speed, and light intensity results in a higher photocatalytic degradation rate of nitrobenzene owing to the effective clearance of electrons and a high quantity of oxidative free radicals. The degradation of nitrobenzene in the rotating annular reactor follows the pseudo first-order kinetics, but it is not well described by the Langmuir–Hinshelwood equation. Aeration has a significant effect on the photocatalytic degradation pathway of nitrobenzene. Because nitrobenzene can undergo reduction reaction as electron acceptors and oxidative degradation initiated by hydroxyl free radicals, the photocatalytic degradation of nitrobenzene follows the reduction mechanism under no aeration, but the oxidation mechanism under aeration.     

纤维素/NMMO·H_2O溶液体系流变性能的研究

利用 Brookfield DV-Ⅱ型粘度计对纤维素/NMMO·H2O 溶液体系的流变性能进行了研究,讨论了温度、纤维素浓度、浆粕聚合度及添加剂等对溶液粘度的影响。结果表明,纤维素/NMMO·H2O 溶液的流动活化能较低,因此其表观粘度随温度的变化不大;纤维素浓度和浆粕聚合度的增加都可使溶液粘度增大,但纤维素浓度对溶液粘度的影响更显著;抗氧化剂没食子酸丙酯(GPE)的加入减缓了加热时溶液粘度的下降,降低了纤维素的氧化降解;二甲亚砜(DMSO)的加入可有效地控制溶液粘度,改善溶液的加工性能。     

Preparation and characterization of degradable oxidized bacterial cellulose reacted with nitrogen dioxide

Bacterial cellulose (BC) is regarded as one of the most attractive and promising biomaterials in medical applications owing to its high purity and mechanical properties and excellent biocompatibility. In this study, a new kind of degradable oxidized bacterial cellulose (OBC) was prepared by oxidizing BC in the presence of nitrogen dioxide (NO₂), which could be used as a scaffold for tissue engineering and tissue regeneration. The chemical structures, micromorphology, and in vitro degradability of OBC were characterized. The results demonstrated that the oxidation reaction was controllable for tailoring the degree of degradation of BC. After oxidation, the degradation of BC was accelerated, depending on the oxidation time. In the case of oxidation for 12 days, the mass loss rate of OBC increased sharply, up to 45% after being immersed in phosphate buffered saline (PBS) for 60 days, compared with only 10% for original BC in the same condition. The oxidation did not affect the crystal structure of BC, however, changed the morphology of its network. The original dense microfibril network of BC could be gradually degraded and disappeared within a desirable period of time in vitro through controlling the degree of the selective oxidation.     

The dilution of vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride, with aluminum phosphate: unexpected reactivity due to the contribution of the diluting agent

This paper describes the preparation, characterization and reactivity in n-butane oxidation of catalysts made of vanadyl pyrophosphate (VPP) diluted in aluminum phosphate. Catalysts were prepared by synthesizing at the same time the VPP precursor and AlPO₄, in order to develop mixed systems, suitable for the conglomeration into fluidizable particles by spray-drying. Al phosphate was chosen due to its presumed chemical inertness towards the active phase and towards n-butane. No evidence was obtained for the formation of mixed V/Al phosphates; an homogeneous reciprocal dispersion of the two compounds developed. However, Al phosphate showed an unexpected activity in n-butane oxidation; specifically, in the temperature range comprised between 340 and 400 °C, the hydrocarbon was converted to formaldehyde and carbon monoxide by radical-like reactions. The activity of Al phosphate became negligible above 400 °C, in the temperature range where instead VPP was active in hydrocarbon oxidation and selective to maleic anhydride. Moreover, Al phosphate was also responsible for the consecutive oxidative degradation of maleic anhydride, at high temperature.     

地膜稳定性研究进展

综述了影响地膜稳定性的因素、机理及稳定性指标评价等研究进展。对近几年有关地膜使用稳定性的研究进行了总结;结合地膜老化降解机理研究现状,从UV降解、生物降解、氧化降解等方面阐述了自由基氧化过程的作用,描述了研究地膜稳定性的方法及波谱学、热力学、流变学等现代分析手段在这一领域的具体应用;对影响地膜氧化降解的各种因素进行了全面概括,并介绍了抗氧剂对地膜氧化降解的研究现状及应用。