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101.
生物质作为自然界唯一可再生的有机碳资源,其利用受到了越来越多的关注。特别是随着能源和环境危机的日益加重,将生物质中非可食用部分催化转化为燃料及具有高附加值的化学品被认为是高效、环保、原子经济的绿色过程。同时,多孔炭材料具有丰富的孔道结构、优异的水热稳定性和大比表面积,是生物质催化转化反应中最常用的载体材料之一。兼之炭材料表面极性、亲疏水性的可调变性,及对酸碱溶剂的反应惰性,也使其无论在学术研究还是在工业应用中都具有特殊的优势。另外,随着纳米炭材料科学的飞速发展,合成孔径、形貌、及表面官能团可控的介孔炭和具有多级孔道结构的多孔炭材料成为可能,将其应用到纤维素催化转化过程中,对深入理解孔道结构、表面官能团对纤维素转化的作用,揭示催化反应作用机制,指导炭基催化剂的设计合成,均具有重要意义。在本综述中,我们首先对纤维素转化中多孔炭的孔道结构和表面官能团性质的独特作用进行了阐述。由于商业活性炭的孔径一般在微孔尺度,但纤维素及可溶低聚糖的分子体积较大,因而其在活性炭中的传质受到了极大的限制。通过模板法获得的介孔炭材料,可实现孔径在2–10 nm的可控合成,大大提高了反应物的扩散速率,使之能与催化活性位有效接触。但孔道过于狭长,在反应过程中堵塞的可能性增高,进而导致催化剂失活;因此,在介孔孔道的基础上,建立互通的多级孔道结构对反应物、中间物、和产物的扩散,及催化活性的保持更为有利。另一方面,炭材料表面的含氧官能团不仅具有加强1,4-糖苷键吸附的作用,还可以作为酸性活性中心催化水解反应的进行;尤其是在传统的水相纤维素催化转化过程中,亲水表面对多孔炭催化剂与反应物的接触非常有利。本文以纤维素水解及纤维素水解加氢反应为例,展开讨论了多孔炭作为固体酸及双功能催化剂载体的应用。在水解反应中,纤维素首先在热水中降解为可溶低聚糖,之后再与活性炭表面官能团反应;其中多孔炭的比表面积、酸量、及酸强度均是促进水解发生的正向因素。在水解加氢反应中,炭载贵金属催化剂作为最常用的加氢催化剂,可获得以六元醇为主的纤维素转化产物。除了加氢作用之外,贵金属小颗粒被证实可以通过氢溢流作用提供水解所需的H+,同时,正价的贵金属也可促进反应过程中的氢转移。另一方面,由于钨物种可催化逆羟醛缩合反应的发生,因此在反应体系中引入钨物种时,水解加氢的主要产物由六元醇变为乙二醇。需要特别指出的是,在纤维素催化水解加氢的过程中,多孔炭材料作为载体同样具有非常重要的作用:一方面,三维介孔的孔道结构不仅有利于反应物、产物的扩散,也有利于加氢金属催化剂的分散,进而提高金属的催化加氢能力;另一方面,当炭材料的表面化学性质改变时,也会影响产物的选择性分布,例如当炭表面显碱性时,由于异构化作用,丙二醇成为主要产物。本文最后,我们列举了一些新型多孔炭材料,包括杂原子改性的多孔炭材料和金属氧化物-炭复合多孔材料的合成方法及其在纤维素催化转化乃至生物质转化中的潜在应用。 相似文献
102.
A series of cellulose 3,5-dimethylphenylcarbamates(CDMPCs) with different degrees of substitution(DS) and degrees of polymerization(DP) were homogeneously synthesized in 1-allyl-3-methylimidazolium chloride(Amim Cl). Then, the CDMPCs were coated on silica gel and used as chiral stationary phases(CSPs), and their chiral recognition abilities for seven racemates were evaluated by high performance liquid chromatography. The results showed that DS and DP of CDMPCs had a great influence on chiral recognition abilities of the CSPs. The CSPs with the DS ≈ 1 gives a low chiral recognition to most racemates. On the contrast, the CSPs with the DS ? 2 exhibited high chiral separation abilities. For example, six racemates could be separated on the CSP with CDMPC of DS ≈ 2(CSP-2). Especially, for the enantioseparation of 1-(2-naphthyl) ethanol and Tr?ger's base, CSP-2 gave the highest separation ability in all of CSPs. On the other hand, when the DP of cellulose was in a range from 39 to 220, the chiral separation abilities of CDMPCs increased as the DP increased. This work demonstrates that the structure of cellulose esters such as DS and DP has important effect on their chiral separation ability, and therefore provides a practical method to design and prepare desirable CSPs for different racemates. 相似文献
103.
104.
105.
Solar cell encapsulating film based on ethylene vinyl acetate copolymer (EVA) was modified by using bacterial cellulose (BC) nanofibres. Bacterial cellulose was chemically modified with propionic anhydride prior to compounding with EVA in a twin screw extruder. The effects of fibre content on the mechanical, thermal, optical and barrier properties of the EVA composite films were investigated. Better mechanical and barrier properties of the EVA films were obtained when the modified BC nanofibres were used. The results were ascribed to the different chemical functional groups on the fibre surface, as verified by FTIR spectra. Deacetylation of the EVA was delayed and visible light transparency of the EVA films above 75% was retained. Overall, our study showed that it was possible to improve the barrier properties of EVA film without sacrificing much transparency by using a suitable type and content of cellulose nanofibres. 相似文献
106.
Li W Zhang W Zhao Q Li Y Ma C Chen L Yin Y 《Analytical and bioanalytical chemistry》2004,378(5):1364-1368
The enantioseparation of seven novel chiral transition metal tetrahedral clusters has been achieved for the first time on cellulose derivatized with tris(3,5-dimethylphenylcarbamate) (CDMPC) as chiral stationary phase (CSP) and hexane containing different alcohols as modifiers as mobile phases. The effect of mobile-phase composition on enantioselectivity was studied, and the effect of structural variation of the solutes on their enantioseparation was also investigated. It was found that both the metal in the tetrahedral core and the ligand coordinated to the atom in the tetrahedral core had significant effects on the chromatographic behavior of the analytes. 相似文献
107.
Peroxynitrite, as a derivative of nitric oxide, is a potent oxidant. It reacts with several biological molecules, makes cellular and tissue damages, and is related with many diseases; therefore, it is of major concern in current medical research works. In this work, a special perm-selective cellulose acetate membrane sampler is used to implement flow injection analysis (FIA)/chemiluminscence (CL)-detection method for the detection of peroxynitrite with Luminol CL-reagent. Optimum detection conditions were established, and the permeability of peroxynitrite through cellulose acetate (CA) membrane, as well as the interference from matrix constituents were studied. The proposed method has the high sensitivity of the CL-detection and the selectivity of perm-selective membrane sampler. The obtained detection limit of 1×10−11 M (without dialysis membrane) and 1×10−10 M (with dialysis membrane), makes it possible to monitor the elusive peroxynitrite in biological samples. The mechanism of luminol CL-emission generated during oxidation by peroxynitrite and the kinetics of peroxynitrite decomposition were also studied using FIA/CL-detection set-up. 相似文献
108.
Sunkyu Park David K. Johnson Claudia I. Ishizawa Philip A. Parilla Mark F. Davis 《Cellulose (London, England)》2009,16(4):641-647
The crystallinity index of cellulose is an important parameter to establish because of the effect this property has on the
utilization of cellulose as a material and as a feedstock for biofuels production. However, it has been found that the crystallinity
index varies significantly depending on the choice of instrument and data analysis technique applied to the measurement. We
introduce in this study a simple and straightforward method to evaluate the crystallinity index of cellulose. This novel method
was developed using solid state 13C NMR and subtraction of the spectrum of a standard amorphous cellulose. The crystallinity indexes of twelve different celluloses
were measured and the values from this method were compared with the values obtained by other existing methods, including
methods based on X-ray diffraction. An interesting observation was that the hydration of the celluloses increased their crystallinity
indexes by about 5%, suggesting that addition of water increased cellulose order for all the cellulose samples studied. 相似文献
109.
Cellulose liquefaction in acidified ethylene glycol 总被引:3,自引:0,他引:3
Wood pulp cellulose was used in a study of its catalyzed liquefaction in the presence of ethylene glycol, p-toluene sulfonic acid monohydrate or sulphuric acid being the catalysts. For this study, microcrystalline cellulose, Whatman
filter paper no. 1 and cotton linters with molar masses of 76,000, 699,000 and 1,910,000 g mol−1, respectively were used. This liquefaction was studied by gravimetric determinations, by X-ray diffraction analysis of the
residual cellulose and by monitoring of the molar mass decrease over different time intervals, using size-exclusion chromatography.
The disordered regions, even of cellulose with the highest molar mass degraded in the initial minute of liquefaction. However,
the highly ordered cellulose regions remained relatively stable for a longer time. None the less, partial degradation of the
highly ordered regions of the cellulose was achieved. 相似文献
110.
An efficient strategy that comprised shorten,chain extension,active groups introducing and homogeneous reaction tactics,was adopted to modify multiwalled carbon nanotubes(MWNTs) with cellulose acetate(CA).Specially,by utilizing 2,4,6-trichloro- 1,3,5-triazine,a reactive intermediate of the MWNTs(MWNT-triazine) was obtained.Suitable solubility of the MWNT-triazine helps make the homogeneous modification become reality.Detailed characterizations further verified that reaction between chloride atoms in the ... 相似文献