负载型Rh基催化剂上的CO加氢制乙醇反应:载体效应

CO Hydrogenation to Ethanol over Supported Rh-Based Catalyst:Effect of the Support

作为一种重要的基础化学品以及传统能源替代品,乙醇极具应用前景。当前乙醇的生产主要通过谷物发酵和乙烯水合法。然而效率低下的发酵过程和日益枯竭的原油资源限制了乙醇的大规模生产。因此,发展生产乙醇的可替代技术成为重要议题。由合成气(CO+H_(2))出发直接制备乙醇被认为是实现煤炭、天然气和生物质等含碳资源高值化、清洁化利用的新方式。负载型Rh基催化剂作为合成气直接制乙醇最有潜力且最有效的催化体系而受到广泛研究。对于负载型Rh基催化剂来讲,合适的助剂和载体通常可以有效地提升催化剂的活性和乙醇选择性。Fe作为一种提高乙醇选择性最有效的助剂之一而被广泛地应用在Rh基催化剂的研究中。在本文中,为了探究载体的作用,我们利用初湿浸渍法制备出不同载体(CeO_(2)、ZrO_(2)和TiO_(2))负载的Fe促进的Rh基催化剂用于合成气制乙醇反应中。结果表明,RhFe/TiO_(2)催化剂在反应条件为250℃,2 MPa时的CO转化率高达18.2%并且在醇分布中乙醇的选择性高达74.7%,远高于相同条件下RhFe/CeO_(2)和RhFe/ZrO_(2)催化剂的性能。表征结果表明,催化剂的比表面从小到大依次为RhFe/CeO_(2)<RhFe/ZrO_(2)<RhFe/TiO_(2),而Rh的分散度也依次增加、粒径依次减小。这是由于较大的比表面积可能有利于Rh物种的分散,高分散的Rh物种就意味着存在更多的活性位点。H_(2)-程序升温还原结果表明Rh与载体、Rh与Fe之间存在相互作用,并且在实验的还原条件下TiO_(2)会发生部分还原,而其它的载体则不会发生还原。X射线光电子能谱分析结果表明RhFe/TiO_(2)催化剂具有更多的乙醇生成(Rh_(x)^(0)-Rh_(y)^(+))-O-Fe^(δ+)活性位点,使得乙醇的选择性增加。CO-程序升温脱附被用来确认不同催化剂的CO吸附能力,结果表明TiO_(2)由于存在更多的O空穴和Ti^(3+)离子从而增强了对CO的吸附,因此有利于催化剂活性的提高。

Ethanol has great application prospects given it is an important essential chemical and a substitute for traditional energy sources.Currently,ethanol production is achieved through grain fermentation and petroleum-based ethylene hydration.However,the inefficient fermentation processes and increasingly depleted crude oil resources hinder the large-scale production of ethanol.Therefore,the development of alternative technologies for ethanol production has become an important issue.The direct production of ethanol from syngas(CO+H_(2))is considered to be a new strategy to acquire high value-added products and achieve clean utilization of carbonaceous resources such as coal,natural gas,and biomass.Supported Rh-based catalysts have been extensively studied as the most promising and effective systems for the direct production of ethanol from syngas.The use of promoters and supports is generally effective in increasing the activity and ethanol selectivity of supported Rh-based catalysts.Fe is widely used in the research on Rh-based catalysts,as it is one of the most effective promoters for enhancing ethanol selectivity.In this work,with the aim of exploring the role of the support,we used the incipient wetness impregnation method to prepare Fe-promoted Rh-based catalysts supported by CeO_(2),ZrO_(2),and TiO_(2) for the synthesis of ethanol from syngas.CO conversion of CO on the RhFe/TiO_(2) catalyst was as high as 18.2% under the reaction conditions of 250℃ and 2 MPa,and the selectivity to ethanol in the alcohol distribution was 74.7%,which was much higher than that observed with RhFe/CeO_(2) and RhFe/ZrO_(2) under the same conditions.The characterization results showed that the specific surface of the catalyst followed the order RhFe/CeO_(2)<RhFe/ZrO_(2)<RhFe/TiO_(2);the dispersion of Rh increased sequentially,and the particle size decreased in the same order.A larger specific surface area may favor the dispersion of the Rh species,and the highly dispersed Rh species would imply a greater number of active sites on the surface of the support.The results of H_(2)-temperature-programmed reduction indicated possible interactions between Rh and the support as well as between Rh and Fe,and partial reduction of TiO_(2) under the experimental reduction conditions;however,the other supports did not undergo reduction.The results of X-ray photoelectron spectroscopy indicated that the RhFe/TiO_(2) catalyst had the largest amount of Rh^(0) as well as Rh^(+)species.Thus,this catalyst has more(Rh_(x)^(0)-Rh_(y)^(+))-O-Fe^(δ+) active sites for the synthesis of ethanol,which greatly increases the ethanol selectivity.CO-temperature programmed desorption was used to confirm the CO adsorption capacity of different catalysts.The results showed that TiO_(2) enhances the adsorption of CO due to the presence of more O vacancies and Ti^(3+)ions,which is beneficial to the improvement of the catalyst activity.