【作者】 周敏；

【导师】 张鸿斌； 林国栋；

【作者基本信息】 厦门大学 ， 物理化学， 2006， 硕士

【摘要】 加氢精制是石油炼制和石油化工的重要工艺过程之一,多年来一直受到催化学界的重视。随着重油和成品油加工深度及环境保护要求的提高,加氢精制,诸如加氢脱硫（HDS）、加氢脱氮（HDN）和加氢脱芳（HDA）等催化剂的工作效率仍待改进。工业上现行的HDA催化剂多是以γ-Al₂O₃为载体的负载型贵金属催化剂。作为载体的γ-Al₂O₃,由于其与被担载的贵金属组分之间的相互作用较强,易形成比较稳定的、难于还原/活化的表面化合物种,不利于催化剂活性的提高。多壁碳纳米管（Multi-walled carbon nanotubes,简写为CNTs,下同）是一类新奇碳素纳米材料。典型的CNTs具有由sp²-C构成的类石墨平面按一定方式叠合而成的纳米级管状结构。鉴于这类新奇管状纳米碳材料具有独特的结构和物化性质,作为一种新型碳素催化剂载体或促进剂,较之一些常规载体材料（如Al₂O₃,SiO₂,AC等）更具特色,近年来引起国际催化学界的日益注意,所涉及用CNTs作为新型催化剂载体或促进剂的研究领域包括:选择加氢、氢甲酰化、选择脱氢、氨合成、FT合成、甲醇/低碳醇合成等。从化学催化角度考虑,多壁碳纳米管诱人的特性,除其高的机械强度、大而可修饰的表面、类石墨的管壁结构、以及纳米级的管腔外,其优良的电子传递性能、对氢强的吸附能力并可期产生的氢溢流效应也很值得注意。本文以自行制备的CNTs作为载体,经由等容浸渍法制备CNTs负载Pt加氢脱芳（HDA）催化剂,考察其对甲苯HDA的催化性能、并与常规载体（AC,γ-Al₂O₃）负载Pt参比体系作比较;利用TEM/SEM/EDX,XRD,XPS,H₂-TPR,H₂-TPD等多种物化表征方法对催化剂的制备过程及工作态催化剂体系进行表征研究,所获结果为深入了解CNTs的促进作用本质提供重要的实验和理论依据,对于一类高效新型Pt基加氢脱芳催化剂的设计和研发,也有重要现实意义。本文主要结果如下:1.碳纳米管负载/促进甲苯加氢脱芳Pt催化剂的制备和催化性能1)CNTs负载的铂催化剂（标记为:w%Pt/CNTs,W%为质量百分数）由等容浸渍法制备。Pt负载量的优化调查实验在0.4 MPa、373K、n_C7H8/n_H2=6/94（mol/mol）、GHSV=1.2×10⁵ ml_STP·h^-1·g_-cat.^-1的反应条件下进行,结果显示,甲苯HDA转化率随着Pt负载量由0逐步增加而上升,当Pt负载量为1.0%（质量百分数）时甲苯HDA转化率达100%,随后趋于平稳,表明CNTs负载催化剂的Pt负载量以1.0%为最佳。所考察8种不同Pt负载量催化剂上甲苯HDA反应活性高低顺序为:1.2%Pt/CNTs≈1.1%Pt/CNTs≈1.0%Pt/CNTs＞0.9%Pt/CNTs＞0.8%Pt/CNTs＞0.7%Pt/CNTs＞0.5%Pt/CNTs＞0.3%Pt/CNTs。对比制备实验表明,对于Al₂O₃和AC负载的催化剂,最佳Pt负载量分别为1.4%和2.4%。2)催化剂活性评价结果显示,在1.0%Pt/CNTs催化剂上,0.4 MPa、373 K、n_C7H8/n_H2=6/94（mol/mol）、GHSV=1.2×10⁵ ml_STP·h^-1·g_-cat.^-1的反应条件下,甲苯转化率达100%,相应的“比反应速率”[即:单位时间（秒）内单位质量Pt组分（以毫摩尔Pt表示）上甲苯HDA转化量（以毫摩尔C7H8表示）]达到1.74mmol_C7H8·S^-1·mmol_pt^-1,是γ-Al₂O₃和AC分别负载对应物（1.0%Pt/γ-Al₂O₃和1.0%Pt/AC）上这个值(分别为0.97和1.37 mmol_C7H8·S^-1·mmol_pt^-1的1.79和1.27倍,是γ-Al₂O₃和AC负载各自最佳Pt负载量催化剂（1.4%Pt/γ-Al₂O₃和2.4%Pt/AC）上这个值(分别为1.17和0.73mmol_C7H8·S^-1·mmol_pt^-1)的1.48和2.38倍。2.碳纳米管负载/促进甲苯加氢脱芳Pt催化剂的表征1)工作态催化剂的XRD图谱显示,对于CNTs负载的催化剂1.0%Pt/CNTs,除观测到归属于CNTs的XRD特征衍射峰之外,也出现归属于Pt微晶相的衍射峰,但其强度比γ-Al₂O₃负载的对应物体系相应XRD峰的强度来得弱,表明Pt组分在CNTs上高度分散,其金属Pt处于更小的微晶、甚至非晶状态;这与TEM/SEM观测到的绝大多数金属Pt颗粒粒径≤4nm的结果相一致。2)催化剂的氧化态前驱物的H₂-TPR测量结果显示,被调查的3种载体负载催化剂的可还原性高低顺序为:1.0%Pt/CNTs＞1.0%Pt/AC＞1.0%Pt/γ-Al₂O₃;前者的“比耗氢量”（即单位质量Pt还原的耗氢量）是后两者相应比耗氢量分别的1.28和1.61倍。这一顺序与这些催化剂上甲苯HDA的“比反应速率”高低顺序相一致。3)工作态催化剂的XPS表征揭示,两种碳素载体（CNTs和AC）负载催化剂的Pt（4f）-XPS谱峰的位置、峰形及其相对面积强度略有差别:在1.0%Pt/CNTs催化剂表面总Pt量中,Pt⁰-物种所占摩尔百分数达97.3 mol%,是AC负载对应体系上这个值（86.8 mol%）的1.12倍。由此可见,用CNTs代替AC作为Pt催化剂载体导致工作态催化剂表面催化活性Pt物种（Pt⁰）的浓度有所提高;这也是以CNTs为载体的催化剂具有较高可还原性（见H₂-TPR表征）的当然结果。与AC载体相比,具有高度石墨化表面的CNTs在缓和所负载Pt组分与载体之间的相互作用、以抑制难还原Pt物种的生成方面似乎更有效。4)预还原催化剂的H₂-TPD试样结果揭示,与AC或γ-Al₂O₃负载的参比体系相比,CNTs负载的Pt催化剂在室温至773K温度范围能可逆地吸附更大量的H₂。在3种催化剂试样上所观测H2-TPD谱峰在323～773K温度范围的相对面积强度比为:S_1.0%Pt/CNTs/S_1.0%Pt/AC/S_{1.0%Pt/γ-Al2O3}=100/30/21;这个顺序与这3种催化剂上甲苯HDA的反应活性高低顺序相一致。3.CNTs载体的促进作用本质本文结果表明,在所考察3种载体负载的催化剂中,CNTs负载催化剂1.0%Pt/CNTs对甲苯HDA反应具有最高的催化活性,无论是相对于γ-Al₂O₃和AC分别负载等Pt负载量的对应物、抑或各自最佳Pt负载量的催化剂。然而,对比实验却表明,用CNTs代替常规载体γ-Al₂O₃和AC并不引起所负载Pt催化剂上低碳醇合成反应的表观活化能发生明显变化,这暗示甲苯加氢脱芳的主要反应途径或速率决定步骤没有改变,。在另一方面,H₂-TPR和XPS的表征研究揭示,与γ-Al₂O₃和AC负载的对应体系相比,CNTs负载催化剂更易于被还原,工作态的1.0%Pt/CNTs催化剂表层催化活性Pt物种（Pt⁰）在总负载Pt量中所占份额较高;这无疑有助于以Pt负载量为计算基准的催化剂比活性的提高,但并非是导致不同载体负载催化剂活性差别的唯一原因。基于H₂-TPD实验调查结果,能够推断,在本文甲苯HDA反应条件下,在CNTs载体上存在着相当大量的氢吸附物种,这将有助于在工作态催化剂上营造较高稳态吸附氢浓度的表面氛围,这些活泼氢吸附物种通过“氢溢流”容易传输至pt⁰催化活性位,有助于加快甲苯加氢脱芳反应的进行。上述结果能导向如下结论:（1）CNTs是被担载Pt组分优良的分散剂,并能缓和/削弱载体与被担载Pt组分之间的相互作用,有助于后者在较低温度下还原活化、并产生较高表面浓度的催化活性Pt物种（Pt⁰）;（2）CNTs也是优良的H2吸附、活化、储存剂,这有助于在工作态催化剂表面营造高浓度活泼氢吸附物种的表面氛围,以提高表面加氢反应的速率。以上两方面的促进效应对催化剂活性的提高都有重要贡献。更多还原

【Abstract】 The saturation of aromatic compounds in distillate fractions and in particular in diesel fuel has received considerable attention in recent years.A high aromatic content is associated with poor fuel quality,giving a low cetane number in diesel fuel and a high smoke point in jet fuel.New legislation has been introduced to limit aromatics in diesel fuel and this has led to new catalyst and process developments.Industrial hydro-dearomatization catalytic processes have been largely carried out on noble metals/Al₂O₃ hydrogenation catalysts.Notable features of alumina supports include their ability to provide high dispersion of the active metal components.However,the results have also shown that numerous chemical interactions exist between the amorphous alumina and transition metal oxides in the precursor state.Multi-walled carbon-nanotubes（MWCNTs,simplified as CNTs）have been drawing increasing attention recently.This new form of nanostructured carbon has a much higher degree of structural perfection,and possesses a series of unique features, such as,highly graphitized tube-wall,nanosized channel and sp²-C-constructed surface.They also display high thermal and electrical conductivity,and excellent performance for adsorption and spillover of hydrogen,which make this kind of nanostructured carbon materials full of promise as a novel catalyst support or promoter.In this dissertation,highly active CNT-supported Pt catalysts were prepared by the conventional incipient wetness method.Their catalytic performance for hydro-dearomatization（HDA）of toluene was evaluated,and compared with the reference systems based onγ-Al₂O₃ and activated carbon（AC）.Using a number of physico-chemical methods（TEM/SEM/EDX,XRD,XPS,H₂-TPR,H₂-TPD,etc）, these catalyst systems were characterized.The results shed some light on understanding the nature of promoter action by the CNTs and the prospect of developing highly active catalysts.The progresses obtained in the present work were briefly described as follows: 1.Preparation and performance of CNT-supported/promoted Pt catalyst for HDA of toluene1)The optimization test of Pt-loading amount showed that over the eight catalyst samples with different Pt-loading amounts under reaction condition of 0.4 MPa, 373 K,C₇H₈/H₂ = 6/94（mol/mol）,GHSV = 1.2×10⁵ ml_STP·h^-1·g_-cat.^-1,the observed sequence of reactivity of toluene HDA is as follows:1.2%Pt/CNTs≈1.1%Pt/CNTs≈1.0%Pt/CNTs>0.9%Pt/CNTs>0.8%Pt/CNTs>0.7%Pt/CNTs> 0.5%Pt/CNTs>0.3%Pt/CNTs,with～1.0%（mass percentage）of Pt-loading amount optimal.2)It was experimentally demonstrated that the support could significantly affect the catalyst activity for the HDA reaction.Over 1.0%Pt/CNTs catalyst under the reaction conditions of 0.4 MPa,373 K,C₇H₈/H₂=6/94（mol/mol）,GHSV= 1.2×10⁵ml_STp·h^-1·g_-cat.^-1,the observed conversion of toluene HDA reached 100%, the corresponding specific reaction rate was 1.74 mmol_C7H8·s^-1·mmol_Pt^-1.This value was 1.79 and 1.27 times as high as that(0.97 and 1.37 mmol_C7H8·S^-1·mmol_Pt^-1)of the counterparts supported byγ-Al₂O₃ and AC, respectively,and was 1.48 and 2.38 times as high as that(1.17 and 0.73 mmol_C7H8·s^-1·mmol_Pt^-1)of the catalysts supported byγ-Al₂O₃ and AC with the respective optimal Pt-loading amount（1.4%Pt/γ-Al₂O₃ and 2.4%Pt/AC）, respectively.2.Characterization of CNT-supported/promoted Pt catalyst for HDA of toluene1)XRD measurement of the functioning catalyst of 1.0%Pt/CNTs catalyst showed that,except the features present at 2θ=26.1°,43.1°and 53.5°,which were due to the diffractions of（002）,（100）and（004）planes,respectively,of graphitized tube-wall of the CNTs,the peaks due to Pt were detected at 20 = 39.7°,46.3°,and 67.3°,which were due to the diffractions of（111）,（200）,and（220）planes of metallic Pt.Those peaks,especially the one at 2θ= 39.7°,were considerably weak,and obviously weaker than that of the counterpart based onγ-Al₂O₃, implying that the supported Pt component was highly dispersed at the surface of CNTs,and existed mostly in states of smaller micro-crystalline,even non-crystalline.2)The H₂-TPR profiles taken on the three precursors of catalysts supported by the CNTs,AC andγ-Al₂O₃,respectively,with the sane Pt-loading amount showed that the position（～498 K）of H₂-reduction peak of the 1.0%Pt/CNTs catalyst was somewhat lower than that（506 K or 531 K）of the system based on AC orγ-Al₂O₃,while its specific H₂-consumed amount（i.e.,the amount of hydrogen consumed due to reduction of unit mass of Pt）was 1.28 and 1.61 times as high as that of the systems based on AC andγ-Al₂O₃ respectively,indicating that the reducibility of the CNT-supported catalyst was much higher than that of the reference system supported by AC orγ-Al₂O₃.3)XPS measurements revealed that using the CNTs in place of AC orγ-Al₂O₃ as support of the catalyst led to a slight increase in concentration of catalytically active Pt-species（Pt⁰）at the surface of the functioning catalyst.On the functioning 1.0%Pt/CNTs catalyst,the observed molar percentage of Pt^o-species in the total surface Pt-amount reached 97.3%.This value is 1.12 times as high as that（86.8%）of the counterpart supported by AC.Compared to the AC support, the CNTs with highly graphitized surface seem to be more effective in mitigating chemical interactions between the supported Pt component and the support to inhibit formation of some of Pt species,which are very stable and difficult to be reduced.4)H₂-TPD tests of the pre-reduced catalysts revealed that the CNT-supported Pt-catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures from room temperature to 773 K,compared to the counterpart supported by AC orγ-Al₂O₃.The ratio of the relative area-intensities of the observed H₂-TPD profiles in the range of 323～773 K was as follows:S_1.0%Pt/CNTs/S_1.0%Pt/AC/S_{1.0%Pt/γ-Al2O3}=100/30/21;this sequence was in line with the observed reaction activity of toluene HDA over these catalysts. 3.Nature of the promoting action by the CNT-supportBased upon the aforementioned results about the preparation study of catalyst and the assay of HDA activity,as well as the characterization of the functioning catalysts,it has been demonstrated that the CNT-material could serve not only as cartier,but also as an excellent promoter,for the Pt-based catalyst for HDA of toluene. It was experimentally shown that using the CNTs in place of AC orγ-Al₂O₃ as support of the catalyst caused little change in the apparent activation energy for toluene HDA reaction,likely implying that the major reaction pathway of the toluene hydrogenation-conversion was unchanged,however,it led to a slight increase in concentration of catalytically active Pt-species（Pt⁰）at the surface of the functioning catalyst.In addition,the Pt/CNTs catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures from room temperature to 573 K.This unique feature would help to generate microenvironments with higher stationary state concentration of active hydrogen-adspecies at the surface of the functioning catalyst.Both factors mentioned above were favorable to increasing the rate of toluene HDA reaction.更多还原