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。     

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。     

超声协同TEMPO氧化制备阔叶木氧化纳米纤维素

采用超声协同的TEMPO/NaBr/NaClO催化氧化体系制备阔叶木氧化纳米纤维素,探究氧化过程中,随氧化程度增大,纤维素的形貌、尺寸、羧基含量、Zeta电位、得率的变化规律。结果表明,超声作用显著提高了氧化反应速率及羧基含量,且粒径明显低于TEMPO氧化。借助SEM扫描电镜可以看出,超声波的空化作用可以破坏阔叶木纤维的层状结构,提高了纤维的比表面积,对TEMPO氧化起到了促进作用。     

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。     

纤维素纳米晶体及其复合物的制备与应用研究

研究了纤维素纳米晶体的制备及其功能化改性,并以制得的纤维素纳米晶体为模板,分别制备了纳米Ag、Ag-Pd合金和Fe2O3与纤维素纳米晶体的复合物,对复合物作为高分子材料的多功能填料、DNA电化学生物传感器标记物及水处理吸附材料进行了详细研究。主要创新点如下:采用碱溶胀法对纤维素进行预处理,通过TEMPO/NaClO/NaBr氧化制     

氧化甲壳素的合成

在TEMPO（2，2，6，6-四甲基哌啶氮氧自由基）和NaBr的催化作用下用NaClO溶液氧化甲壳素得到氧化甲壳素；产品纯化后，经^13C-NMR和红外分析鉴定出糖基C6位羧基的存在；同时对原料的选择和反应的工艺备件作了初步的探索，得到了合适的反应温度（0-5℃）和pH值（10-11），产率迭到45％左右。     

TEMPO抑制K_2S_2O_8氧化甲苯生成苯甲醛过程中的过度氧化

采用TEMPO/Cu(OAc)2为催化剂,探究其在抑制苯甲醛过度氧化成苯甲酸方面的能力,考察反应时间、反应温度、催化剂用量等因素对苯甲醛收率的影响。研究结果表明,TEMPO以及TEMPO衍生物在Cu(OAc)2催化下,可以有效地抑制甲苯合成苯甲醛过程中苯甲醛进一步氧化成苯甲酸。优化条件下,甲苯氧化生成苯甲醛的收率高达78%,而且没有苯甲醇和苯甲酸生成。     

2,6-二氨基-4,5,6,7-四氢苯并噻唑的合成工艺改进

2,6-二氨基-4,5,6,7-四氢苯并噻唑是合成抗帕金森药物盐酸普拉克索的重要中间体,以4-苯二甲酰亚胺环己醇为起始原料,经Ca(ClO)2/TEMPO/NaBr体系氧化,HBr/H2O2体系溴代,Hantzsch环合,水解合成2,6-二氨基-4,5,6,7-四氢苯并噻唑。较佳的反应条件是:(1)氧化反应,Ca(ClO)2作为氧化剂采用催化氧化;(2)溴代反应,氢溴酸作为溴源,采用氧化溴化;(3)Hantzsch环合反应,碳酸氢钠为缚酸剂;(4)脱保护,氮气保护下,甲醇中水合肼回流。重要中间体结构经核磁氢谱确认。所用原料廉价易得,反应高效、绿色环保,后处理简单方便。     

氧化纤维对纳米纤维素气凝胶微球的影响

采用TEMPO/NaBr/NaClO体系氧化全漂硫酸盐针叶浆制备纳米纤维素,并以纳米纤维素为基体制备纳米纤维素气凝胶微球。研究了纤维尺寸及纤维羧基含量大小对纳米纤维素气凝胶微球的影响。结果表明:羧基含量相同时,随着超声波处理氧化纤维的时间增加,纤维尺寸越小,制备得到的纳米纤维素气凝胶微球粒径越小;羧基含量不同时,羧基含量越高,纤维越容易被解离,且在相同的超声波处理时间条件下,得到的纤维尺寸较小,气凝胶微球颗粒越小。     

电喷雾质谱对TiO2光催化选择性氧化苄胺反应机理研究

电喷雾电离质谱技术(ESI-MS)无需样品预处理，直接对反应进行实时取样、离子化并进行分析，可以快速、准确地捕捉反应过程中寿命较短的中间体和产物，并进行在线实时监测。基于此，以光催化选择性氧化苄胺生成亚胺反应为例，通过搭建常压质谱在线反应监测装置，对2,2,6,6-四甲基哌啶氧化物(TEMPO)辅助的TiO₂光催化苄胺氧化反应进行了原位、在线的监测。结果表明，常压质谱技术可以准确识别出TEMPO参与反应过程中的中间体，从而揭示TEMPO辅助TiO₂光催化苄胺氧化反应的反应机理。TiO₂纳米材料可以利用光能激发电荷对，在反应中产生活性氧化物种，从而促进氧化还原反应的进行。TEMPO作为助催化剂，在反应中可以作为氧化剂参与反应过程，促进光催化反应中的电子转移过程，从而提高反应效率和选择性。     

Polymorphism of LiGa5O8 and of LiGa5O8-MgGa2O4Solid Solutions

High-temperature X-ray diffraction and differential thermal analyses showed that LiGa₅O₈ exists in two polymorphs related by the first-order transition at 1138°±3°C of the low-temperature simple-cubic form, space group (probably) O⁷, to the high-temperature spinel (fcc) form, space group O _h ⁷. The transition is rapid, and the high-temperature form in pure LiGa₅O₈ could not be quenched to room temperature under the conditions used. However, the high-temperature polymorph can be quenched under equilibrium conditions when 40 mol% or more MgGa₂O₄ is present. The subsolidus equilibrium relations in the system MgGa₂O₄-LiGa₅O₈ are discussed.     

Magnetoresistance of CuxCo1-xFe2O4 Ferrites

Magnetoresistance measurements (Δ/R) were carried out on Cu _x Co_{1- x} Fe₂O₄ samples with x =1, 0.75, 0.50, 0.25 and cobalt ferrite in the range of magnetic intensity (H) from 1.0 to 4.5 kG. Linear plots were obtained between log (Δ/R) and log H for all compositions except that of cobalt ferrite. The values of n at x =0.5 do not agree with the results of some authors. The discrepancy may be attributed to the value of magnetic field intensity at which such measurements were carried out. The similarity of features of n and μ_D with composition leads us to believe that the magnetoresistance may arise from the scattering of conduction electrons by localized electrons involving the s-d exchange interaction.     

Sintering of Si3N4-Y2O3-Al2O3

Sintering kinetics of the system Si₃N4-Y2O₃-Al₂O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si₃N₄ were studied with additions of 4 to 17 wt% Y₂O₃ and 4 wt% A1₂O₃. Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si₃N₄ and glass or β '-Si₃N₄, α '-Si₃N₄, and glass. The compositions and amounts of the residual glassy phases are estimated.     

Expansive properties of the mixture C4ASH12 − 2CS III. Effects of temperature and restraint

The effects of temperature and restraint upon the hydration and the expansion of $\text{C}{}_4\text{A}\text{S}\text{H}{}_{12}\text{? 2}\text{C}\text{S}$ mixture compacts in different contact solutions have been investigated. Temperatures above 20°C do apparently hinder the formation of an impervious felt-like layer of ettringite around the $\text{C}{}_4\text{A}\text{S}\text{H}{}_{12}$ particles, thus greatly reducing the retarding effect of the lime. An uniaxial restraint of 1 Kg/cm² is enough to reduce sensibly the expansions which remain however high (about 100%). The results can be satisfactorily interpreted by the reaction and expansion mechanism hypothesized in our previous papers.     

Raman Study of Glasses of Ba2TiSi2O8 and Ba2TiGe2O8

Raman spectra are reported for fresnoite (Ba₂Ti(Si,Ge)₂O₈ glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba₂TiSi₂O₈ and Ba₂TiGe₂O₈. The Ba₂TiGe₂O₈ glass spectra show correspondence with the Ba₂TiGe₂O₈ crystalline Raman spectra; the v _s(Ge–O–Ge) mode occurs at 518 cm⁻¹ in the glass and at 521 cm⁻¹ in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba₂TiGe₂O₈ glass as revealed by a strong band at 824 cm⁻¹ in the I glass spectrum. The Ba₂TiSi₂O₈ glass spectra are similar to the Ba₂TiSi₂O₈ crystalline spectrum; the strongest band is found at 836 cm⁻¹ in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba₂TiSi₂O₈ glass has a structure similar to the crystalline phase.     

Preparation and properties of poly(Nε,Nε-dicarboxymethyl-l-lysine)

A new ampholytic homopolypeptide, $\text{poly}\text{(N}{}^{\mathrm{\epsilon }}\text{,N}{}^{\mathrm{\epsilon }}\text{-}\text{dicarboxy-methyl-}\text{l}\text{-lysine}\text{)}$, which has one tertiary amino and two carboxyl groups in the side chain has been derived from a hydrochloride salt of poly(L-lysine). The polymer in aqueous solution seems to be in the coil form with locally extended structure (LES) at neutral pH. In both the acidic and alkaline regions, the molar ellipticity of the polymer changes as a result of change in net charge on the side chain. The conformational changes may be from the coil with LES to other coiled forms. 5–7 M NaClO₄ and 80% aqueous methanol induce the α-helix in the polymer at neutral pH. Divalent cations, Cu²⁺ and Ca²⁺, do not induce any remarkably ordered structures such as α-helix or β-structure in the polymer in aqueous solution at any pH. Ultraviolet absorption studies show an absorption peak of the polymer-Cu²⁺ complex near 240 nm. Dependence of the peak intensity on pH at various q values ($\text{q =}\text{[}\text{Cu}{}^{\mathrm{2+}}\text{]}\text{[}\text{residue}\text{]}$) indicates the two steps of the complex formation. At q less than 0.64, the formation is described only with the first step. An average coordination number for Cu²⁺ at the first step was calculated to be about 2 by the method of Mandel and Leyte. The association constant of Cu²⁺ with the residue at the step was determined from the absorption data to be far smaller than that for the Cu²⁺-EDTA complex. The second step of the formation occurs in the case of large q but the absorption data for the second step cannot be obtained exactly due to precipitation.     

Effects of metal-support interactions on the hydrogenation of CO over PdSiO2 and PdLa2O3

A study of CO hydrogenation over $\text{Pd}\text{SiO}{}_2$ and $\text{Pd}\text{La}{}_2\text{O}{}_3$ has been carried out for the purpose of identifying the effects of Pd dispersion, Pd morphology, and support composition on the catalytic activity of supported Pd. The specific activity of each catalyst for methanol and methane synthesis was determined from microreactor studies carried out at a fixed set of reaction conditions. Palladium dispersion was measured by H₂O₂ titration, and the morphology of the Pd crystallites, as expressed by the distribution of Pd(100) and Pd(111) planes, was determined from in situ infrared spectra of adsorbed CO. The crystallite morphology of the $\text{Pd}\text{SiO}{}_2$ catalysts is the same, independent of Pd weight loading: 90% of the surface is comprised of Pd(100) planes and 10% of the surface is comprised of Pd(111) planes. By contrast, the crystallite morphology of the $\text{Pd}\text{La}{}_2\text{O}{}_3$ catalysts changes with Pd loading. Primarily Pd(100) planes are exposed at low-weight loadings while Pd(111) planes are exposed at high-weight loadings. The Pd dispersion has little effect on the methanol turnover frequency over both $\text{Pd}\text{SiO}{}_2$ and $\text{Pd}\text{La}{}_2\text{O}{}_3$, for dispersions between 10 and 20%. On the other hand, the methane turnover frequency is independent of Pd dispersion over $\text{Pd}\text{SiO}{}_2$, but increases with decreasing dispersion over $\text{Pd}\text{La}{}_2\text{O}{}_3$. It is further observed that the Pd morphology influences the specific activity of $\text{Pd}\text{La}{}_2\text{O}{}_3$ for methanol synthesis: Pd(100) is nearly threefold more active than Pd(111). For a fixed morphology, the specific methanol synthesis activity of $\text{Pd}\text{La}{}_2\text{O}{}_3$ is a factor of 7.5 greater than that of $\text{Pd}\text{SiO}{}_2$.     

In situ laser raman spectroscopy of the sulfiding of Moγ-Al2O3 catalysts

Raman spectra of sulfided $\text{Mo}\text{γ-}\text{Al}{}_2\text{O}{}_3$ catalysts were obtained using in situ techniques for two sulfiding methods. For samples sulfided by 10% $\text{H}{}_2\text{S}\text{H}{}_2$ at 400 °C, MoS₂ structures were observed. A stepwise sulfiding using 10% $\text{H}{}_2\text{S}\text{H}{}_2$, with spectra recorded at 150, 250, and 350 °C, resulted in observation of molybdenum oxysulfide, reduced molybdate, and surface “MoS₂” phases. Reexposure of these samples to air led to radical modification of the oxysulfide structures as well as transformation of some sulfide phases. A model incorporating terminal and bridging MoS bonding and anion vacancies is proposed. This model is based on the conversion of isolated and aggregated molybdate and MoO₃ species to oxysulfide and reduced molybdenum phases. Conversion of reduced molybdenum phases to sulfides is observed to be slow.     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.