【作者】 郭峰波；

【导师】 张永发；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2013， 博士

【摘要】 CH4分子具有高度对称性和稳定性,实现CO2电子的转移需要高达20.4eV的能量,同样具有很强的稳定性。CH4-CO2重整制合成气研究是公认的20世纪催化和能源研究领域最具挑战性的研究方向,CH4-CO2反应涉及到高温反应性和选择性受热力学限制的问题,这些问题需要通过研制高效催化剂来解决。目前,甲烷二氧化碳重整制合成气催化剂的研究主要集中在贵金属和过渡金属,但贵金属受价格昂贵和资源有限等条件制约,而过渡金属做催化剂,其活性位容易被积碳所覆盖而降低催化活性或失活,已经成为CH4-CO2催化研究进展的瓶颈。本课题针对甲烷二氧化碳重整反应中催化剂存在易失活或价格昂贵的问题,创新地提出半焦催化CH4-CO2重整反应制合成气技术路线。以宏测褐煤半焦、神木烟煤半焦和晋城无烟煤半焦及高温高压H2O2和氨水改性半焦为催化剂,系统地开展了CH4-CO2重整制合成气研究,考察了反应条件对重整反应的影响；考察了高温高压H2O2和氨水改性对半焦催化CH4-CO2重整反应的影响规律；通过对半焦表面性质与结构变化情况分析,揭示了半焦催化剂在重整反应中的作用实质；考察了加压条件下半焦催化甲烷二氧化碳重整反应特性；探讨了半焦催化甲烷二氧化碳重整反应机理和动力学规律。经研究获得的主要成果和结论如下。(1)在不同煤种半焦催化下,甲烷二氧化碳重整转化表现出相同的变化规律,初始阶段转化率较高,随着反应的进行,转化率逐渐降低并趋于平稳,不同煤阶半焦在重整反应中具有相同作用机制。不同煤种半焦的催化活性顺序为：宏测褐煤半焦>神木烟煤半焦>晋城无烟煤半焦,即半焦的催化性能随煤阶的提高而降低,低阶褐煤半焦具有较高的比表面积和丰富的含氧官能团,是其催化活性高的主要原因。部分半焦催化剂在重整反应中与CO2发生气化反应,半焦催化剂是一种“消耗性催化剂”。(2)重整反应后,三种半焦在1450cm-1附近的硝基和-NH2吸收峰几乎消失,神木烟煤半焦和晋城无烟煤半焦在3444cm-1附近的羟基OH吸收峰有所减弱,在1023cm-1附近的吸收峰明显减弱,在1598cm-1处的羧基C=O官能团几乎消失,晋城无烟煤半焦在1087cm-1附近的含氮官能团-C-N-吸收峰减弱,半焦表面C=O、-OH、-NH2、脂肪族和环醚等官能团参与了重整反应。(3)高温高压H202改性后,初始催化效果低的晋城无烟煤半焦催化性能提升幅度较大,CH4和C02转化率分别提高了24.38%和21.73%,而初始催化效果较高的宏测褐煤半焦活性提升幅度相对较小,CH4和C02转化率仅提高了7.47%和1.28%。宏测褐煤半焦在3444cm-’处的-OH吸收峰明显增强,神木烟煤半焦和晋城无烟煤半焦在1598cm-1附近的羧基C=O和1023cm-1附近脂肪族和环醚等有机官能团含量都明显增加,但晋城无烟煤半焦在1598cm-1附近的羧基C=O吸收峰峰强增加幅度较大。活化后半焦表面的酸碱性官能团含量都有所增加,但碱性官能团含量提高幅度明显高于酸性基团含量的提高量,宏测褐煤半焦、神木烟煤半焦和晋城无烟煤半焦净碱量分别增加了0.161mmol/g、0.103mmol/g和0.102mmol/g,总结果是增加了半焦的净碱量,提高了对C02的吸附作用。(4)高温高压氨水改后,晋城无烟煤半焦催化性能提升幅度较大,CH4和C02转化率分别提高了30.42%和27.55%,宏测褐煤半焦活性提升幅度相对较小,CH4和CO2转化率仅提高了12.94%和3.79%。宏测褐煤半焦和神木烟煤半焦在3444cm-1附近的羟基-OH吸收峰,1598cm-1附近的羧基C=O吸收峰,1450cm-1附近由部分硝基、-NH2含氮官能团的吸收峰,1087cm-1附近的含氮官能团-C-N和797cm-1附近的C-H吸收峰都得到增强；晋城无烟煤半焦,在1598cm-1附近的羧基C=O吸收峰,1450cm-1附近的部分硝基、-NH2含氮官能团的吸收峰,1087cm1附近的含氮官能团-C-N吸收峰得到增强。活化后半焦表面碱性官能团含量明显提高,而酸性官能团含量则有所降低,宏测褐煤半焦、神木烟煤半焦和晋城无烟煤半焦酸性官能团含量分别降低了0.012mmol/g、0.014mmol/g和0.009mmol/g,而碱性官能团含量则分别增加了0.361mmol/g、0.311mmol/g和0.240mmol/g,氨水具有弱碱性,能够与半焦表面的酸性官能团发生中和反应,导致酸性基团含量减少。(5)完善了小型高温高压反应器基础,获得了加压条件下半焦催化甲烷二氧化碳重整反应规律。甲烷转化率随反应体系压力的增加而下降,而二氧化碳转化率却表现出先升后降的趋势；温度和气固接触时间对重整反应的影响与常压条件下变化规律基本一致,而产品气中CO/H2比值随CH4/CO2的增加而降低,且CO/H2>1,与常压变化规律不同。加压反应后,半焦催化剂在1700cm-1附近的C-O和O-H特征吸收峰,在压力为0.7MPa和1MPa时吸收峰明显减弱,在压力为2MPa和3MPa时与反应前变化不大。在2350cm-1附近出现了强吸收峰,该位置为C02的物理特征吸收峰,验证了催化重整反应,先是反应气在催化剂表面的吸附,然后再在催化剂表面发生反应。加压反应后半焦催化剂表面上覆盖了许多颗粒物,随着重整压力的增加,积碳在催化剂表面呈现出逐渐团聚的过程,即由分散的点的堆积渐变成面的覆盖,尤其是当压力为2MPa和3MPa时,催化剂表面的孔隙已经完全被积碳所覆盖,增加压力提高了积碳在催化剂表面的堆积密度,在催化剂表面形成致密层,阻挡了反应气与催化剂活性位接触,降低了催化剂的催化活性,甚至失活。(6)提出了半焦催化甲烷二氧化碳重整反应机理。半焦表面具有活性的含氧官能团首先解离出活性氧原子,同时CO2在高温下也能够解离出活性氧原子和CO,接着活性氧原子与CH4作用,生成活性物种CHxO,然后分解成CO和CH2；同时,CH4与半焦活性中心作用,逐步生成具有活性的碳原子和氢气,一部分碳原子与吸附在催化剂表面的C02发生反应,未及时反应的碳原子就沉积在催化剂表面形成积碳,从而阻碍了部分活性氧原子在半焦催化剂表面的解离,当上述相互作用达到动态平衡时,半焦催化剂就处于稳定状态。建立了半焦催化甲烷二氧化碳重整动力学方程,反应速率常数k=25.81exp(58.4±3.7kJ/RT)。更多还原

【Abstract】 The CH4molecules have high symmetry and stability, and the electron transfer of the CO2have needed the energy of up to20.4eV, which similarly have strong self-stability. The CH4-CO2reforming is one of the most challenging projects in catalysis and energy research areas. The high-temperature reactivity and selectivity of the CH4-CO2reforming limited by the thermodynamic are needed to be solved by developing efficient catalyst. Currently, studies on the catalysts used in CH4-CO2reforming mainly focused on the noble metals and transition metals used. However, the noble metals are expensive and these resources are limited while the transition metals are easily deactivated due to carbon deposition, which has become the bottleneck of its industrial application.Based on the existing problems, semi-cokes were creatively proposed to be used as the catalyst in the process of the CH4-CO2reforming. The effects of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes on the CH4-CO2reforming were investigated by using a fixed-bed reactor. The factors affecting the catalyst stability and activity were investigated. The semi-cokes were activated by the high-temperature and high-pressure during the hydrothermal reaction of ammonia and hydrogen peroxide, and the rules regarding the effect of activation conditions on the CH4-CO2reforming were investigated. The essence of the semi-cokes catalysts were revealed by analysing the changes of the surface properties and structure before and after reaction. The reaction characteristic of the CH4-CO2reforming was investigated under the condition of pressure. The mechanism of the CH4-CO2reforming over semi-cokes was studied, and the kinetics model was established. The conclusions are drawn as follows:(1) The CH4-CO2reforming has been studied over different semi-cokes. The CH4-CO2reforming shows the same conversion trend with different semi-cokes, where the CH4and CO2conversions are higher at the initial phase, and as the reaction progresses, the conversions gradually decrease and tend to be stable. Therefore it is shown that the reaction mechanism is the same in the CH4-CO2reforming over different semi-coke. When the reaction became stable, the order of the catalyst activity was Hongce lignite semi-cokes>Shenmu bituminous coal semi-cokes> Jincheng anthracite semi-cokes, indicating that the catalyst activity of the semi-cokes was lower with the increasing of coal rank. Hongce lignite semi-cokes have high catalyst activity because it has large specific surface area, low ash and rich oxygenic groups. The analysis of carbon balance shows part of semi-cokes catalyst participated in gasification reaction with CO2, and the semi-cokes catalyst is a consumptive catalyst.(2) The changes of functional groups and the surface structure of the semi-cokes before and after the reforming reaction have been analysed. After the reaction, the absorption peak near1450cm-1almost vanished for all three semi-cokes, the absorption peak at1023cm-1was obviously diminished for the Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes, and the absorption peak at1598cm-1for the carboxyl C=O function group almost disappeared. These changes indicate that the functional groups on the surface of the semi-cokes participated in the reforming reaction. The result shows that the coal rank and the BET are the main factors which influence the catalyst performance, and the functional groups on the surface of the semi-cokes and the types and amounts of alkaline metals have a certain type of influence on the activity of the reforming reaction. After480min of reaction, the diffraction peaks of the semi-cokes mainly emerge in carbon and silica. The peaks for the alkaline earth metals have almost completely diminished(3) The semi-cokes activated by H2O2under high temperature and high pressure have been carried out. The catalytic performance of Jincheng anthracite semi-cokes whose initial catalytic effect is poor increased significantly, with the conversions of CH4and CO2increasing by24.38%and21.73%, respectively. The catalytic performance of Hongce lignite semi-cokes whose initial catalytic effect is good increased little, with the conversions of CH4and CO2increasing by just7.47%and1.28%, respectively. After activated, the Hongce lignite semi-cokes absorption peak at3444cm-1is caused by OH increase significantly, The Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes absorption peak near1598cm-1is caused by C=O and the absorption peak near1023cm-1is caused by the organic functional groups such as fat clusters and alkylene oxides, etc. strengthen significantly. However, Jincheng anthracite semi-cokes absorption peak near1598cm-1caused by C=O increased more than others. At the same time, the content of acid and alkaline functional groups on the surface of semi-cokes has increased, and the content of alkaline functional groups increased higher than the acid functional groups, meaning that the content of alkaline has increased. The net amount of basicity of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes increased by0.161mmol/g,0.103mmol/g and0.102mmol/g, respectively.(4) The semi-cokes activated by ammonia under high temperature and high pressure have been carried out. After being activated, the Hongce lignite semi-cokes and the Shenmu bituminous coal semi-cokes absorption peak near3444cm-1are caused by-OH, the peak near1589cm-1is caused by OO, that near1450cm-1is caused by nitro and-NH2, that near1087cm-1is caused by-C-N, that near797cm-1is caused by C-H. These absorption peaks were increased significantly, Jincheng anthracite semi-cokes absorption peak near1589cm-1is caused by C=O, that near1450cm-1is caused by nitro and NH2, that near1087cm-1is caused by-C-N. These absorption peaks were also increased significantly. After ammonia hydrothermal reaction under high temperature and high pressure, the content of alkaline functional groups on the surface of semi-cokes have been increased greatly, but the content of acid functional groups have been decreased. The amounts of basicity of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes were increased by0.361mmol/g,0.311mmol/g and0.240mmol/g, respectively. But the amount of acidity was decreased by0.012mmol/g,0.014mmol/g and0.009mmol/g, respectively. On the one hand, ammonia has a weak alkaline, which can carry out a neutralization reaction with acid functional groups to reduce the content of acid functional groups. On the other hand, it becomes easy to be combined with phenol and at last it enhanced the content of alkaline functional groups on the surface of semi-cokes.(5) A small high temperature and high pressure reactor has been improved and built, and the rule of CH4-CO2reforming over semi-cokes under pressure has been gained. The conversion rate of methane decreased with the reaction pressure increased, but the conversion rate of carbon dioxide increased at the initial phase, then decreased and trend to stable at last; the reaction temperature and the gas-solid contact time have the same influence on the CH4-CO2reforming under atmospheric pressure, but the ratio of CO/H2decreased as the ratio of CH4/CO2increased and CO/H2>1, which is different from atmospheric pressure variation. After the reaction under pressure, the absorption peak of semi-coke at1700cm"1is caused by C-O and O-H declines significantly under pressure of0.7MPa and IMPa. Instead the absorption peak has changed little under pressure of2MPa and3MPa, which shows the catalyst activity of the semi-cokes decreased with increasing pressure; meanwhile it exhibited a strong absorption peak near2350cm-1which is just the physical characteristics of CO2absorption peak, and further validates the steps of catalytic reforming reaction. To be specific, at first reaction gas is adsorbed by the surface of catalyst, then it reacted on the surface of catalyst. The SEM figure shows a lot of granular objects are covered on the surface of semi-cokes catalyst after the pressurized reaction, and with the increase of reforming pressure, carbon deposit would agglomerate on the surface of semi-cokes catalyst, which means dispersed points accumulate gradually into the cover, especially under the pressure of2MPa and3MPa. The pores distributed in the catalyst surface have been completely covered by carbon deposition, and the pore structure on the surface of semi-cokes catalyst decreased progressively until it disappeared with the increase of reforming pressure, when pressure increased. The bulk density of carbon deposition on the surface of semi-cokes catalyst was improved, resulting in forming a dense layer on the surface of semi-cokes catalyst, which has prevented the reaction gas from contacting with the active sites of catalyst and reduced the activity of catalysts or even deactivated the catalysts.(6) The reaction mechanism of the CH4-CO2reforming over semi-cokes has been proposed. It is considered that the oxygenic functional groups on the surface of semi-cokes first dissociate activated oxygen atoms, and at the same time carbon dioxide can also dissociate activated oxygen atoms and carbon monoxide under high temperature; then the activated oxygen reacted with methane to form activated CHXO, and was decomposed into carbon monoxide and methylene CH2. Meanwhile methane reacted with semi-cokes active site to form gradually activated carbon atoms and H2. A part of carbon reacted with carbon dioxide which was adsorbed on the surface of catalyst, while the others which were too late to react would deposit on the surface of catalyst to form carbon deposition. It was not good for partially activated oxygen atoms to be free on the semi-cokes catalyst surface. When the above interaction reached dynamic balance, Semi-coke catalyst would reach steady state. The kinetic equation of the CH4-CO2reforming over semi-cokes has been built, and dynamics parameters have been gained, with the reaction rate constant. k=25.81exp(58.4±3.7kJ/RT).更多还原