【作者】 曾凡龙；

【导师】 潘鼎；

【作者基本信息】 东华大学 ， 材料科学与工程， 2008， 博士

【摘要】 粘胶活性碳纤维（RACF）具有十分优异的吸附性能,已应用于化工、防护、制药、卫生等领域,在环保、新能源等新兴领域也显示出了巨大潜力。但其制备过程中反应复杂、产品强度低、孔结构单一、得率低、能耗大、价格高、应用开发缺乏等不利因素极大地限制了其广泛应用。针对这些不足,本论文在粘胶活性碳纤维催化体系、制备过程中分子的化学和物理结构转变机理、表面孔结构控制、低成本制备新技术、以及对大分子有机污染物吸附降解和高性能电极材料等应用方面,进行了全面系统的研究。借助热失重分析（TG）,比较了5种代表性粘胶纤维催化剂（磷酸铵（AP）、磷酸氢二铵（DAP）、磷酸二氢钱（ADP）、磷酸（PA）和硫酸铵（AS））的作用效能,分析了反应过程。催化剂能选择性地降低化学键的键能、降低反应温度,优先脱水,从而提高了碳化得率。DAP的催化能力最强,其具有最高的碳化、活化得率和比表面积。在DAP催化时的低温氧化阶段,空（氧）气的存在有利于粘胶纤维的脱水和热分解反应,并限制了左旋葡萄糖的产生,使得率提高:但空气对其它催化剂的影响不显著。发现和建立了将热分解峰形态特征与热解速度及得率之间相关联的新方法。用粘胶热失重曲线上主分解峰的峰高/半高宽之比(H/W_1/2)来描述热分解峰的形态特征。该值越小,表明分解反应越缓和,碳得率就越高:反之反应就激烈,得率就越低。可由该值的相对大小推断不同催化剂下粘胶的热分解行为和碳化得率,但得率与峰面积之间没有确定的关系。这一新方法将有利于了解热分解过程和快速评选新型高效的催化剂。运用傅立叶红外转换光谱（FTIR）、广角X衍射（WAXD）、元素分析（EA）、扫描电子显微镜（SEM）、原子力显微镜（AFM）、自动氮吸附仪等多种分析手段,分别研究了在磷酸氢二铵（DAP）作用下,粘胶纤维热处理过程中的化学结构、结晶结构、表面孔结构和形态的转变规律。DAP能显著促进粘胶大分子在热处理中的化学和物理结构转化。在150℃下纤维素大分子即已开始脱水反应,200℃时出现C=O基团,220℃纤维素大分子主链结构开始受到破坏,到250℃出现C=C基团,大分子结构完全被破坏。这些温度比纯粘胶相应的温度低50～100℃。脱水和分解反应主要集中在220-250℃之间,这是一个最重要的转变区间,纤维的分子结构、结晶结构、失重、碳含量、纤维直径和颜色等发生了突变。此时,纤维已完全失去纤维素的结构特征,转化为无定形的热解（脱水）纤维。红外光谱进一步证实,空气中氧的存在确实有助于粘胶纤维的脱水和碳化,使裂解和脱水反应温度降低了约30℃。热处理过程中DAP进入纤维素的晶区,破坏纤维素的结晶结构。但当DAP自身的结晶结构在150～175℃开始被破坏后,纤维素的结晶结构可以部分再恢复。DAP促进粘胶纤维同时脱水和碳化,含碳量提高。DAP不仅在加热过程中起到酸催化剂的作用,而且其含有的N也作为反应物参与了脱水反应。这不同于以往的认识,DAP既是催化剂,也是反应物。在升温碳化过程中官能团或非碳原子迅速减少直至绝大部分消失,只有C=C基团因芳构化结构的形成,吸收峰由1715cm^-1移至1584cm^-1。碳化活化过程中纤维直径变化不大,但失重一直持续增大。在450℃以后,DAP对碳化过程和化学结构的影响不十分显著;碳化到800℃时开始形成尺寸极小的类石墨微晶结构,其WAXD几乎显示为无定形的馒头峰。增加DAP的用量能提高粘胶纤维的碳化和活化得率提高。活化前后纤维中化学基团的变化规律与活化温度有关,低温（750℃）活化使含氧基团降低,而高温（950℃）活化使含氧基团增加。粘胶活性碳纤维的表面孔主要集中在2nm附近。活化温度不同,纤维表面刻蚀程度很不相同。850℃活化时,纤维表面刻蚀较少,能保持大致平整。950℃活化时,纤维表面被严重刻蚀,纤维变得十分粗糙。活化温度提高有利于提高比表面积和孔体积,但对提高中孔体积的比例不显著。AFM观察表明:由高温碳化后再活化的纤维表面微细结构与其它活性碳纤维的明显不同,存在着10～25nm的卵石状球形结构,其间的缝隙构成微孔。现有活性碳纤维（ACF）孔径很小,属微孔范围（2nm以下）,只适合于吸附小分子物质,如丙酮、甲苯等。而对中等大小的分子（如染料、农药等）和大分子（如洗涤剂、药物、聚合物、气溶胶、细菌和病毒等）,直径达到几个至几百纳米,就需要孔径更大的ACF。文献中偶见有中孔ACF的报道,但并不详细,大孔ACF国内外还未见报道。为了扩大ACF的应用面和吸附能力,探索了扩孔工艺,创新地制备出了大孔ACF。将TiO₂添加到粘胶纤维中,制备出了中孔ACF。中孔率依赖于活化时间和温度。活化温度升高、活化时间延长,都能够提高比表面积和中孔率,但产品得率迅速降低。当活化温度为950℃,活化时间为60分钟时,中孔率高达50.6%,孔径集中在2nm和4nm附近。为了进一步扩大孔径,探索了多种方法。通过将常规的微孔型粘胶ACF用复合金属钴盐处理,再二次活化,成功制备出了中孔率更高达91.5%、表面孔径达到20～150nm的大孔ACF。纤维手感柔软,强度适当,可以正常使用。制备过程中活化程度越深,ACF的孔径和和孔体积越大,微晶的晶粒尺寸和结晶度越小,晶面间距越大。微孔、中孔和大孔三种孔结构的ACF对亚甲基蓝的吸附能力表明:相同比表面积条件下,孔径越大,吸附速度越快,吸附量也越大;大孔型吸附量可比微孔型提高1倍。因而,对分子量较大的物质,大孔径ACF的吸附具有十分显著的优势。现有制备ACF的碳化活化工艺（二步法）路线长、消耗大、成本高,本论文研究了省去碳化的一步法低成本制备新工艺,以及产品结构和性能。用该工艺制备出了强度、手感、表面化学结构和孔结构等质量指标与两步法相近的活性碳纤维,且产品收缩率更稳定,但微晶大小有所不同。为了比较两种工艺质量的优劣,提出了一个质量新指标——有效得率（=活化得率×比表面积）,它综合平衡了得率与比表面积两个相互矛盾的工艺指标,比单一指标更客观全面。一步法要明显优于两步法,这一新工艺省去了碳化步骤和氮气,极大地减少了电力消耗和设备费用,有效得率更高,提高了效率,可降低生产成本1/3。最后,本文研究了RACF在环境净化和清洁能源材料等新兴领域的应用。采用静态吸附、模拟室内空气试验箱结合现场试验等方法,研究了ACF对气相和液相中单组分低分子有机溶剂和多组分大分子复合污染物的吸附和再生规律。结果表明:ACF对丙酮、四氯化碳、乙醇和甲苯等低分子有机溶剂蒸气的吸附速度非常快,10分钟就可达到吸附平衡,吸附量高达400～1500mg/g。脱附再生十分容易,二次吸附容量可达初次的95%以上。对极性较大的物质如水等也有较大的吸附容量,可达400mg/g,只是吸附速度要慢得多,约需2小时才达到平衡。但甲醛常温下难以被ACF吸附,其原因在于它的沸点（-19.5℃）远低于室温。但甲醛的存在能使对水汽的吸附量可提高10%。从中得到启示,在ACF表面负载胺类化合物,甲醛吸附量可高达450mg/g。ACF对多种大小分子的复合有机油类有优异的吸附能力,是活性炭的5倍。0.94%的ACF用量就能将PAN纺丝凝固浴中95%的油类杂质吸附掉。与对低分子的吸附规律不同,升高温度有利于吸附量的提高。吸油后的ACF可以通过溶剂加以再生,脱附率达79%,但二次吸油效率有所降低。水中低浓度大分子有机污染物的净化处理是一个前沿性的课题,还没有好办法。一是处理成本高,二是大分子容易粘附在过滤材料（如膜分离）上,使其难以再生而报废。本文研究了用ACF将大分子污染物富集在表面,再借助TiO₂将其光催化分解和无害化的新方法。在ACF上负载和制备出了具有很高光催化活性的TiO₂,进行了光催化降解的研究。以钛酸四正丁酯为前驱物,用溶胶—凝胶法,制备出了在ACF表面分散性良好、具有多孔和锐钛矿型结构的TiO₂。在紫外光的照射下,负载TiO₂的ACF能够有效地将柠檬酸、聚乙烯醇（PVA）等大分子有机物予以降解。常温下经过6小时,降解效率可达70～80%,远优于常规TiO₂的几十个小时。吸附——光降解能够反复多次进行,每经过一次循环,比表面积损失很小,仅1～2%,绝大部分比表面积能够获得再生。这一结果表明,用光催化结合ACF吸附技术进行大分子污染物的降解是可行的,很有应用前景:同时也为ACF对吸附大分子、高沸点物质后脱附再生困难提供了一个新的技术方法和思路。作为一种新颖高效的电极材料,ACF显示出了极大的前景。ACF作为超级电容器的电极,要求其强度高、比表面积大和有一定中孔分布,国内无法提供,也没有研究。为了使ACF能够满足这一高要求,研究了高比表面积、高强度ACF的制备方法。以高强度粘胶碳纤维为原料,在较高的活化温度（900～950℃）及较长的活化时间（40～60min）,制成了强度高（100N/5cm以上）、比表面积大（1400～2000m²/g）、中孔率32.68%的ACF,达到了先进水平;掌握了小试和小批量工业加工技术。试制出的高强度ACF能够满足超级电容器对强度的要求,可以连续化机械加工,并被制成了10000F和80000F的超大容量超级电容器。在模拟工况条件下,用Arbin电池测量仪测试了超级电容器的各项电性能指标。经过50A、100A的大电流测试,综合性能良好,比电容高达130～150F/g,与俄罗斯的先进水平接近。已可应用于太阳能、风能发电系统蓄电装置和辅助电源等。本论文围绕粘胶活性碳纤维制备机理和功能性应用展开了系列基础研究,在催化剂评价方法、大孔径和高强度ACF以及一步法低成本制备、光催化降解大分子有机物等方面取得了一定的进展和创新性研究结果,对理解RACF制备反应过程和机理、提高强度和降低成本、扩大应用领域等都具有重要的现实意义。更多还原

【Abstract】 Rayon-based activated carbon fiber（RACF） has excellent adsorption and can be used widely in chemical industry,personal protection,pharmacy and hygiene,ect., and has shown the vast potential in the new fields of environmental protection and clean energy resources,too.But,the applications are hindered by its disadvantages such as complicated reactions in processing,low strength and yield of resultant, mono-disperse micro-pore,high electric consumption and cost,and the lack of practically technical development.In order to overcome these disadvantages,this paper has investigated systematically the catalysts,the mechanism of chemical and physical structural transition of rayon macromolecule in fabrication,pore structure control,low-cost processing technique,the adsorption and photo-catalytic degradation to organic composite pollutants,and high performance electrode,etc.With the help of differential thermogravimetric analysis（DTG）,5 catalysts, ammonium phosphate（AP）,dibasic ammonium phosphate（DAP）,ammonium dibasic hydrogen phosphate（ADP）,phosphoric acid（PA） and ammonium sulfate（AS）,have been used to treat rayon fibers and make ACFs in parallel to compare their reactive ability with pure rayon as a control.At the same time,reactive mechanisms have also been guessed.The catalysts can reduce the bond energy selectively,and then lows reactive temperature,dehydrates rayon molecule first and so increases char yield. DAP has the highest catalytic activity among the catalysts basing on the yields of carbonization and activation,as well as specific surface area of ACF.During the low temperature treatment,the air or oxygen is favorable to the dehydration and pyrolysis of rayon,and so enhances the yield when DAP is used.But there are no notable differences on catalyses of other 4 catalysts in or not in air.The relation between the shape character of the DTG peaks of rayon and pyrolytic speed as well as char yields has been built first in this paper,and thus a method to choosing an effective catalyst of rayon-base ACF has been founded.The main pyrolytic peaks on the DTG curves can be characterized by the ratio(H/W_1/2) of the heights（H） to the half-high widths(W_1/2) of the peaks.The little the value of the ratio,indicating the more moderate the pyrolytic reaction is,the higher the char yield. On the contrary,a bigger ratio shows a lower yield certainly.There is no clear relation found between areas of peaks and yields.From the ratio a pyrolytic behavior and char yield can be deduced rapidly for a new catalyst.The chemical,crystalline,pore structures and their transitional mechanism of rayon catalyzed by DAP during the stepwise heat treatment were researched by Fourier transform infrared spectroscopy（FTIR）,wide angle X-ray diffraction （WAXD）,elemental analysis（EA）,scanning electric microscopy（SEM）,atomic force microscopy（AFM） and automatic nitrogen apparatus,etc.DAP can promote the structure transformations evidently,lower the starting dehydration temperature of rayon to 150℃.Group C=O is generated at 200℃while group C=C appears at 250℃. These temperatures are lower 50-100℃than the corresponding ones of pure rayon. 220-250℃is a key temperature range where the chain,ring and crystal structures of rayon macromolecules are damaged completely to lose the basic characteristics of cellulose by dehydration,companying by the drastically physical changes:weight-loss, discoloration and shrinkage,etc.At this time rayon is changed into amorphous thermo-cellulose.It has been confirmed again by FTIR that oxygen in air indeed accelerates the dehydration and carbonization of rayon,and lows the corresponding reaction temperature 30℃at least.DAP enters and breaks down the crystal region of rayon partly,the crystal structure of rayon,however,can restore when DAP’s crystal is cracked in the 150-175℃.DAP can not only behave like an acid catalyst,but also react with rayon.Differing from former view,DAP works as both a catalyst and a reactant.The bands of chemical groups decrease very fast and disappear during carbonization.The exception is that the band of bond C=C exists still and transfers from 1715 cm^-1 to 1584 cm^-1 resulting from aromatization.At this stage,the diameter of rayon fiber keeps unchangeable but its weight-loss increases steadily.DAP does not notably impact on the structures and carbonized rate while the temperature is over 450℃.Very tiny crystalline structures of turbostratic graphite begin to form near 800℃,leading to a much broad diffuse peak on WAXD.The increment of usage of DAP can get higher yields of oxidation,carbonization and resultant ACF.The activation at a lower temperature（750℃） decreases the contents of oxygen-containing groups,but increases them at a higher temperature（950℃）.The widths of micro-pores on ACF concentrate on 2nm.The roughness and burn-off of ACF quite depend on the activation temperature.The surface of ACF can keep even when activation carries out below 850℃,but become rugged when over 950℃.A higher activation temperature can enhance the specific surface area and pore volume,but not increase the ratio of m（?）so-pore effectively.ACF made from rayon-based carbon fiber carbonized at a high temperature 1500℃has close cobble-like or elongated spherical structures on its surface,and pores are formed among the spherical structures.The sizes of spherical structures and pores range from 10nm to 25nm observed by AFM.These images are much different from pitch- or phenol-based ACF.The ordinary micro-pore ACF is limited to use in dealing with low molecular weight organs such as acetone,toluene,etc.But in some cases,middle molecular weight substances（such as dyes and organic pesticides） and macromolecules（such as detergents, drugs polymers,aerosols,bacteria and viruses,etc.）,need elimination by adsorption.Their sizes can reach from several to hundreds of nanometers,micro-pore ACF is not suitable for them.The pores must be widened to the same order of magnitude in order for better adsorption.Occasionally meso-pore ACF was mentioned but not in detail,The fabrication techniques of meso-pore and macro-poreACF were pursued innovatively to enhance the adsorption to the bigger molecules in the new fields.TiO₂ powder,as an additive,was added in the rayon solution to produce meso-pore ACF with two-step routine as usual.The volume of meso-pores rather relies on the temperature and time of activation.A higher activation temperature and longer time can give higher meso-pore volume and specific surface area,and in turn a lower yield.The ratio of meso-pore volume is up to 50.6%and the peaks of pore distribution locate at 2nm and 4nm,when activation at 950℃for 60min. By secondary activation,the micro-pores of common ACF can be widened to macro-pores with the width of 20-150nm and the ratio of meso-pore volume as high as 91.5%after the impregnation with composite cobalt salts.The fiber with macro-pores,not reported before, still keeps the satisfactory handle,strength and feasibility.The deeper the activation extent is,the bigger the pore width and volume and inter-planar spacing do,and the smaller the crystalline size and crystallinity are.Methylene blue,a dye with a middle molecular weight（volume）,was adopted to test the adsorption of ACFs with micro-pore,meso-pore and macro-pore respectively.The bigger average pore width shows a faster rate and higher capacity of adsorption.Therefore,ACF with macro-pores is much favorable for the organic macro-molecular substances.The common two-step routine（TSR）（carbonization and activation） is both time and energy consuming,and higher cost,too.A novel technique—single-step routine（SSR）（no carbonization and inert gas N₂,only direct activation）,has been investigated to make low-cost rayon-based ACF.Comparing with the ACF from TSR,ACF from SSR has the similar strength,handle,surface chemical and pore structure,but somewhat different crystalline size.In order to evaluate the two routines,a new term,effective yield（EY） （=activation yield x specific surface area）,was put forward to balance the two contradict technical indexes in ACF process.SSR is superior to TSR in the removal of carbonization and inert gas N₂.It decreases electric consumption and investment of device,finally enhances processing efficiency and reduces the cost by 1/3.Moreover,SSR has a higher EY and more steady shrinkage for resultant ACF.Lastly,RACF has been researched for the applications in environmental purification and clean energy material.The samples of RACF in form of non-woven felt were put in a small air-tight glass container to adsorb the different gases of single volatile organic solvents and multiple pollutants of macro-molecular organs in a solution,or formaldehyde in an imitated indoor air cabinet（2.2m³） or a 20m² room in-situ（full of new furniture）,respectively.It is very fast for ACF to adsorb the vapors of non-polar organs with low molecular weight such as acetone,tetrachloromethane, ethanol and toluene.Generally only about 10min is spent to reach equilibrium with the amounts adsorbed from 400 up to 1500mg/g.Regenerations are rather easily by heat treatment in air,and secondary adsorption volume keeps more than 95%of the first time.But it is much slow to adsorb polar molecules like steam,needing 2hr to reach saturated volume of 400mg/g.Formaldehyde is an exception and adsorbed hardly by ACF in room temperature,which comes from its very low boiling point （-19.5℃）.The adsorption to formaldehyde would be increased greatly to 450mg/g by the surface modification of ACF through loading organic amines.5 times of adsorption of activated carbon,ACF also exhibits an outstanding adsorption to the complex oil from different organics with low-and macro- molecules.For the cycling salt bath of PAN fiber processing,containing a concentration of 1.3%oil,when the usage of ACF is only 0.94%of the bath,95%oil in the bath can be removed at room temperature after adsorption only one time.Higher temperature to 60-90℃will lead higher adsorption.The soaked ACF by oil can be regenerated by organic solvent such as ether,with a desorption of 79%.The regenerated ACF shows a relatively lower secondary adsorption to oil more or less.Dealing with dilute organic macromolecular pollutants is very challenging,there are no good solutions now because of high cost and hard regeneration of filters（such as filtering film）.They can be enriched on ACF and then broken down to no-harmful substances by photo-catalytic TiO₂ loaded on it.Tetra-n-butyl titanate was as a precursor to prepare porous nano-anatase on the surface of ACF,via sol-gel method and stepwise heat treatment under 550℃.When an ultraviolet lamp is used to supply energy to excite TiO₂,70～80%citric acid or poly（vinyl alcohol）（PVA） in aqueous solution is decomposed completely within 6hr,comparing to the tens of hrs while only TiO₂ powder is used.ACF loaded TiO₂ can be used repeatedly in the cycle of adsorption--photo regeneration--adsorption,with each cycle at a cost of 1-2%specific surface area.The results testify that the technique of ACF combining photo-catalysis is feasible and promising to resolve the problems of macromolecular pollutants with high boiling points,as well as ACF’s regeneration.ACF has shown a great potential as a novel high performance electrode in super capacitor.ACF electrode must be of both high strength and specific surface area （1200m²/g） with a ratio of meso-pores of 30%at least.This electrode has not been supplied domestically,and not developed,either.In order to meet these strict requirements for mechanical processing and good electrochemical properties,a high strong rayon-based ACF has been trial-produced in batch,and used to fabricate electric double layer capacitors with capacity as high as up to 1×10⁴F and 8×10⁴F successfully.This special ACF was activated at a higher temperature（900-950℃） for longer time（40-60min）.Its technical parameters have reached top level in China with a high specific surface area of 1400-2000m²/g,a strength of more than 100N/5cm and a meso-pore ratio of 32.68%.Under imitated operating mode,the capacitors have been detected systematically by Arbin battery tester.When the testing currents were up to 50A and 100A,they still behaved very well with a very high specific capacity of 130—150F/g,near Russian top level.So they can be applied in auxiliary electric storage system of sonar power and wind-power,etc.In this paper,many important aspects on the fabrication and functional applications of RACF have been comprehensively investigated,innovative results and certain progresses have been obtained including the method for the evaluation of the catalysts,the processing techniques of macro-pore and high-strong as well as low-cost ACFs,the photo-catalytic decomposition to macro-molecular organics,etc.These results will give great help to understand the thermal reactive mechanism of rayon, enhance the strength and decrease the cost of RACF,and enlarge the application in new fields.更多还原