费托合成: 基于碳化钴中间体构建高α-Co位密度的Co@C催化剂

One of reasons for the limited performance especially concerning the C5+ space time yield of the cobalt (Co) Fischer-Tropsch catalysts lied in the disadvantages of β-Co (fcc phase) as active sites. The partial transition of β-Co site to advanced α-Co site via cobalt carbide (Co2C) intermediate (defined as the Phase Transformation Technique, PTT) was a solution, but it is difficult to achieve complete conversion of β-Co due to the constrain of surface energy. The carbon supported Co2C (Co2C@C), which was obtained after PTT was applied to the carbon supported Co materials (Co@C, derived from pyrolysis of metal organic frameworks), can form real catalysts with clear structure-function relationship and thus was an ideal model for investigating phase transformation mechanism. In order to fabricate the Co@C catalysts with α-Co as the sole site, efforts were made to systematically study the positive role of Co2C particle size tailored by etching method, gas adsorbate, or(and) metal oxide promoters in complete converting β-Co to α-Co, during which the influence of α-Co particle size and α-Co site density on catalysts’ activity, selectivity, and C5+ space time yield were also investigated. This paper aims to promote the development of PTT, and to reveal the relationship between the α-Co site density and the C5+ space time yield. This work can also laid the theoretical and technical foundation for the design of α-Co based catalysts with high C5+ space time yield.

费托合成Co催化剂的性能特别是C5+时空收率受到制约的原因之一在于β-Co(fcc晶相)活性位存在弊端。以碳化钴(Co2C)为中间体的相变技术可使β-Co部分转变为性能更好的α-Co(hcp晶相)，但表面能制约着β-Co的完全转变。相变技术与碳载高β-Co位密度材料(Co@C，由金属有机骨架热解而来)融合形成的碳载Co2C材料(Co2C@C)可形成构效关系明确的催化剂，是相变研究的理想模型。本课题通过刻蚀法调控Co2C颗粒尺寸，进而利用气体吸附剂或(和)金属氧化物助剂调控表面能以使β-Co向α-Co完全转变，系统研究Co@C催化剂上α-Co活性位的构建，以及考察α-Co颗粒尺寸、α-Co位密度对催化剂活性、选择性及C5+时空收率的影响。旨在促进相变技术发展，以及揭示α-Co位密度与C5+时空收率的关系，并为获得具有高C5+时空收率的费托合成α-Co基催化剂奠定理论和技术基础。

项目背景：.石油不仅是一种人们日常生活中的普通商品，而且是一种极其重要的战略资源。石油的安全供应不仅关系到人们的正常生活，也关系到一个国家的经济发展和社会稳定。从2017年到2019年，我国石油对外依存度已经从65%增加到72.7%。面对石油资源的严峻挑战，我国正在积极开发替代能源。立足我国富煤少油的先天性能源结构，发展煤经合成气制液体油品（费托合成技术）以缓解石油供需矛盾和保障国家能源安全是一种战略选择。催化材料的合理设计与研制是费托合成技术的关键之一。本项目将热解MOFs所得材料的优点与相变技术结合，设计了新型炭载碳化钴（Co2C@C）催化材料，并通过关键因素的调控以实现C5+时空收率的突破。.主要研究内容：.Co-MOF-71材料的合成优化；坍塌的Co-MOF-71材料的修复研究；含助剂的炭载碳化钴（Co2C@C）材料的制备与费托合成反应评价；由部分碳化的Co2C@C材料衍生纯Co2C@C材料的研究；负载型Co2C用于乙醇脱氢反应的研究；由Co-MOF-71的调变、热解、碳化制备适量N掺杂的Co2C@NC催化材料及其费托合成反应应用。.重要结果、关键数据.Co-MOF-71经调变后热解再碳化得到了Co2C@NC催化新材料。在合成工艺上，碳化过程所需压力从2MPa降到常压，大幅降低了设备门槛；在费托合成反应性能上，在3MPa、300oC、空速7200h-1条件下于1L浆态床反应器（装填系数为0.8，固含率0.05%）内评价时，可得1.004 kg/h/kg催化剂的C5+时空收率和26%的柴油组分（C12-C22烃）选择性（碳数，下同），特别是汽油组分（C5-C11烃）的选择性达到50%，综合指标达到世界领先水平。.科学意义.为煤经合成气直接制取汽油路线提供了高选择性、高收率的新技术方案，促进了费托合成催化剂的发展，推动了合成气定向转化的机理研究。