【作者】 王健；

【导师】 马新宾；

【作者基本信息】 天津大学 ， 化学工艺， 2019， 博士

【摘要】 基于现阶段我国资源储备的特点,开发合成气经费托合成（FTS）工艺生产油品和低碳烯烃以满足日益增长的消费需求具有现实意义。Fe基和Co基FTS催化体系多物种共存且相变动态复杂,这使得研究特定物相和催化性能之间的关系变得尤其具有挑战性。本文围绕着活性中心的调控与构效关系解析,系统地开展了以下几方面研究:（1）调控反应气氛获得Fe基FTS催化剂样本研究物相演变并建立催化性能描述符;（2）开发了简单的甲醇预处理的方法用以调变活性中心,并揭示了其调控机制;（3）利用MOF介导方法进行N助剂修饰调控Fe活性相微观化学环境,研究N助剂对活性相的调控作用及其对FTS性能影响。合成不同Fe的纯相前驱体（Fe₂O₃、χ-Fe₅C₂、θ-Fe₃C和Fe⁰）,调变H₂/CO比例与反应压力,并对所得样品进行XRD、HRTEM和EELS等准原位体相表征,发现催化剂在反应中均形成核-壳结构,但各部分的组成不同且取决于前驱体物种的性质。通过准原位手段用软X射线吸收光谱对表面物种进行表征,证实了催化剂表面由氧化铁和碳化铁物种组成。基于此,对反应后的催化剂提出了两种催化剂模型,并对各催化剂的体相和表相进行了定量拟合,从而得到了催化剂的微观配位性质。XAS分析发现催化剂中Fe物种的配位环境决定了FTS的催化性能:催化活性（FTY）主要与Fe物种的体相平均氧化态（OS）数值呈线性正相关关系,而与表面Fe物种的平均OS关联不大;FTY_CH4随着Fe-C配位数（CN）的增加而线性增加。这为我们认识和解释Fe基催化剂提供了重要的描述符。采用甲醇为碳源分别预处理Co基和Fe基催化剂,以调控活性物种的还原和碳化程度。对Co基催化剂,XRD、化学吸附和XPS等表征的结果表明甲醇预处理有利于Co物种的还原,HRTEM和EELS表征结果表明,甲醇使Co物种表面形成了碳物种,从而促进了电子向Co物种的转移。In-situ FT-IR结果表明甲醇预促进了Co活性位对CO的解离吸附能力。对Fe基催化剂,甲醇预处理有利于碳化铁活性相的形成。TPH-MS结果发现,甲醇预处理有利于Co基和Fe基催化剂表面活泼的C_α物种的形成,因此使催化剂CO转化率提升:在240℃,2.0MPa,H₂/CO=2/1的反应条件下,甲醇预处理使X_CO从Co-4h催化剂的8.7%提升至17.1%,使X_CO从Fe-4h催化剂的6.0%提升至9.7%。此外,甲醇预处理使C₅⁺选择性由Co-4h的81.8%提升至91.9%,但甲醇预处理对Fe基催化剂的产物分布影响不大。采用Al-Mil-53材料为载体,通过表面氮修饰以调控负载型Fe基催化剂中碳化铁活性相的电子性质,并合成了具有不同N修饰量的Fe/AM53和Fe/NM53（x）催化剂。XRD和XPS结果证实N助剂作为电子型助剂有利于Fe物种的还原和碳化。CO-DRIFTS和CO-TPD表征结果表明,N助剂的给电子作用增强了Fe物种对CO的吸附强度。催化评价结果显示,N的电子助剂作用和金属–载体相互作用共同影响了催化性能;吡咯N和石墨N的含量和FTY密切相关,同时N助剂的加入可以抑制次级加氢反应,使O/P比提高。此外,适宜的氮掺杂（x=1）有利于Fe物种对CO的吸附量的提高,而过量的N修饰（x>1）会导致Fe Al₂O₄物种的生成,不利于CO的转化。在T=340℃,P=2.0 MPa,H₂/CO=1的反应条件下,Fe/NM53（1）催化剂展示出较好的FTY和低碳烯烃选择性,分别为677.0×10^-6 mol _CO·g _Fe^-1·s^-1和29.4%。更多还原

【Abstract】 Based on the current energy structure and natural reserves situation in China,developing the Fischer-Tropsch synthesis（FTS）technology which convert syngas to synthetic fuels and lower olefins is of great importance,especially considering the increasing domestic consumption demands.The structure and species of active phases for iron and cobalt catalysts are complex and dynamic under the FTS environment,which makes it challenging to identify the relationship between specific phase and catalytic performance.In this paper,we aimed at modulating active sites and investigating structure-performance relationship.We systematically discussed the following issues:（1）general descriptors were established to interpret the structure-performance relationship for Fe-based FTS catalysts,through the samples started from various Fe precursors and imposed to controlled reaction atmosphere;（2）a simple methanol pretreatment method is developed to modulate active sites and the modulation mechanism was elucidated;（3）the chemical environment of Fe active sites was modified by nitrogen doping through the MOF-mediated strategy,and the promotion mechanism of nitrogen doping on catalytic performance was further investigated.Starting from the various synthesized iron precursors（Fe₂O₃、χ-Fe₅C₂、θ-Fe₃C and Fe⁰）,we modulated the reactive atmosphere including H₂/CO molar ratio and pressure.Then by employing various bulk characterization techniques including XRD and HRTEM combined with EELS,we found that core-shell structures were formed for all catalysts irrespective of the catalyst precursors,while the compositions within the core and shell strongly depend on the intrinsic properties of the precursors.Quasi in-situ soft X-ray absorption techniques was used to determine the surface iron composition.The surface of catalysts consists of iron oxide（s）and carbide（s）.Further quantitative fitting for both the surface and bulk was conducted.It is found that the iron coordination environment mainly governs the performance during FTS.The FTY linearly correlates with the averaged bulk iron OS rather than the surface properties,and the CH₄ yield is related to the CN of the Fe-C bond.This study thus provides a general descriptor to interpret the structure-performance relationship of Fe-based FTS catalysts.Methanol was used as carbon source to modulate the reduction and carburization behavior of active sites for Co-based and Fe-based catalysts.For Co-based catalysts,XRD,chemisorption and XPS results show that methanol pretreatment is beneficial for the reduction of cobalt species;HRTEM and EELS results show the formed carbon species on cobalt phase after methanol pretreatment favors the electron donation to cobalt species.In-situ FT-IR results prove that methanol pretreatment can improve the dissociative adsorption of CO on Co active sites.For iron-based catalysts,methanol pretreatment is favorable for the formation of iron carbide active phase.TPH-MS results show that methanol pretreatment contribute to the formation of active Cαspecies on both the Co-based and Fe-based catalysts,thus improving the CO conversion.Co-Me OH catalyst possesses superior X_CO of 17.1%compared to that of 8.7%for Co-4h,and Fe-Me OH catalyst presents increased X_CO of 9.7%in relative to that of 6.0%for Fe-4h.Under the condition of 240℃,2.0 MPa and H₂/CO=2/1,the C₅⁺selectivity was found to increase from 81.8%for Co-4h catalyst to 91.9%for Co-Me OH catalyst,while the influences of methanol pretreatment on the products distribution is not obvious for Fe-based catalysts.Amino-group modification was achieved by the MOF-mediated strategy.Fe/AM53 and Fe/NM53（x）catalysts with various nitrogen amount were successfully synthesized.The electronic properties of iron carbides active phase were effectively modulated for the supported Fe-based catalysts.XRD and XPS results verified nitrogen promoter is favorable for the reduction and carburization of iron species.CO-DRIFTS and CO-TPD results evidenced nitrogen promoter plays as the electron donor to Fe species,which enhances the adsorption strength of CO on Fe species.Catalytic tests results show that the catalytic performance is dominated by both the electronic promoter effect and metal-support interaction.Meanwhile,the total content of pyrrolic N and graphitic N were found to correlate well with the FTY,and the addition of nitrogen can suppress the secondary hydrogenation ability and lead to higher O/P ratio.Moreover,proper amount of nitrogen doping（x=1）is beneficial for the enhancement of the CO uptake on Fe active sites.While excess nitrogen（x>1）doping would lead to the formation of inactive Fe Al₂O₄ species,which is unfavorable for CO conversion.The optimal FTY of 677.0×10^-6 mol _CO·g _Fe^-1·s^-1 and SC₂-C₄⁼of29.4%can be achieved on the Fe/NM53（1）catalyst under the evaluation condition of340℃,2.0 MPa,H₂/CO=1/1.更多还原