【作者】 谢冠群；

【导师】 李小年； 罗孟飞；

【作者基本信息】 浙江工业大学 ， 工业催化， 2009， 博士

【摘要】 α,β-不饱和醇具有重要的工业应用价值,主要通过α,β-不饱和醛选择性加氢来制备。然而,α,β-不饱和醛分子中含有共轭的C=C键和C=O键,且C=O的键能大于C=C键,在热力学上不利于C=O的选择性加氢生成α,β-不饱和醇,因此,提高α,β-不饱和醛中C=O的加氢选择性是催化领域中一项挑战性的课题。巴豆醛属于典型的α,β-不饱和醛,其选择性加氢生成巴豆醇具有广泛的代表意义。在巴豆醛选择加氢制备巴豆醇的研究中,负载型Au与Pt催化剂是近几年热点,尤其是Au与Pt负载在具有还原性载体上,可以具有很好的C=O加氢选择性。然而,负载型Au催化剂的热稳定性能有待提高,负载型Pt催化剂上的载体参与反应机理尚不清楚。本文主要以铈基氧化物为载体,采用沉积-沉淀法制备负载型Au与Pt催化剂,应用于巴豆醛选择性加氢反应,考察不同铈基载体对Au催化剂热稳定性能的影响,以及Pt催化剂上载体参与反应的情况。通过XRD、Raman、FT-IR、TEM、BET和CO化学吸附等表征手段,研究载体表面电子环境、载体组成对巴豆醛中共轭的C=O键和C=C键的选择性加氢的影响,揭示C=O键加氢的活性中心,探讨催化剂失活机理。1.铈基氧化物载体的制备与表征。采用一种改进的柠檬酸溶胶-凝胶法制备铈基固溶体,其主要过程为:将柠檬酸前躯体先在N₂保护下高温焙烧（碳化）,柠檬酸前躯体缓慢的分解为氧化物和无定形碳（原因是缺少O₂）,之后在较低温度下除碳得到纳米级铈基氧化物。通过改进的柠檬酸溶胶-凝胶法,我们得到晶粒小于10nm、比表面积高于90 m²/g的Ce_0.8Pr_0.2O_Y固溶体,并发现Ce_0.8Pr_0.2O_Y固溶体形成于N₂气氛高温预处理过程中,随后的空气中低温焙烧几乎没有对Ce_0.8Pr_0.2O_Y固溶体的本征特性产生影响。在改进的柠檬酸溶胶-凝胶过程中,碳化温度会对铈基固溶体的本征结构等性质产生重要影响。通过对Ce_0.8Zr_0.2O₂固溶体的研究发现:低碳化温度导致碳化中间体中的Ce-Zr氧化物为无定形或弱晶化,其在随后的除碳过程中发生团聚,只得到低比表面积的大颗粒Ce-Zr氧化物。高温碳化可以提供足够的热力学动力,其促使碳化中间体中的Ce-Zr氧化物发生高度晶化,使得其受随后除碳的影响较小,因而得到的Ce_0.8Zr_0.2O₂固溶体具有高比表面积和多孔结构。2.铈基氧化物负载Au催化巴豆醛选择性加氢。通过对Au/CeO₂催化巴豆醛选择性加氢的研究,我们发现:当采用较低比表面积的CeO₂（70 m²/g）作为载体时,Au负载量对催化性能有着极为重要的影响。当Au负载量为3%时,Au/CeO₂催化剂在反应稳态时巴豆醛转化率为12%,巴豆醇选择性为58%,远优于相关文献中催化效果。当Au负载量低于3%时,巴豆醛加氢效果显著下降,尤其1%以下Au负载量时,几乎没有催化活性和巴豆醇选择性。当Au负载量高于3%时,催化性能没有明显提高,接近于3%Au/CeO₂催化剂。为了更好提高Au催化剂的热稳定性,我们制备了Au/Ce_0.8Zr_0.2O₂催化剂,并应用于巴豆醛选择性加氢,发现其在稳态时巴豆醇选择性为62%,优于Au/CeO₂的实验结果,其原因是Ce-Zr复合氧化物中具有较多的氧缺位,促进了C=O吸附加氢,从而提高巴豆醇选择性。另外,Au/Ce_0.8Zr_0.2O₂催化剂在180℃反应下有较长寿命,使得其具有工业化前景。3.铈基氧化物负载Pt催化巴豆醛选择性加氢。我们利用原位技术对Pt/CeO₂催化剂进行一些特殊预处理（如,高温还原后原位低温再氧化）,保持Pt粒子大小和形貌不发生变化,只改变CeO₂载体的表面化学环境,考察高价Ce⁴⁺和低价Ce³⁺对巴豆醛加氢的影响,即CeO₂载体参与催化反应机理,得到如下结论:（1）Pt/CeO₂催化剂中低价的Ce³⁺离子可以向Pt提供电子,有利于C=O键的吸附和加氢,进而提高巴豆醇选择性,而高价态的Ce⁴⁺粒子不利于C=O键吸附加氢;（2）低价态的Ce³⁺离子也会引起催化剂表面积碳,从而导致催化剂失活。作为拓展,我们还考察了Pt/Ce_xSm_1-xO_2-δ催化剂对巴豆醛加氢的情况:（1）相对于Pt/CeO₂催化剂,Pt/Ce_xSm_1-xO_2-δ催化剂具有较好的催化活性和巴豆醇选择性;（2）Sm的掺杂量影响Pt的催化性能,其中Pt/Ce_0.8Sm_0.2O_2-δ催化剂具有很好的初始巴豆醛转化率（53.5%）和巴豆醇选择性（76.5%）。（3）高温处理Pt/Ce_0.8Sm_0.2O_2-δ催化剂可以提高巴豆醇选择性,可能原因是形成了较大的Pt粒子,有利于C=O的吸附和加氢。4.非铈基氧化物负载Au与Pt催化巴豆醛选择性加氢。作为对照实验,我们考察Au/TiO₂和Pt/Pr₆O₁₁催化巴豆醛选择性加氢情况。不同晶相的TiO₂对Au/TiO₂催化巴豆醛选择性加氢有重要影响:纯锐钛矿晶相比纯金红石相有利于提高巴豆醇选择性;而相对于纯相载体,复合相TiO₂具有最好的Au/TiO₂的催化活性和巴豆醇选择性。首次将Pt/Pr₆O₁₁催化剂应用于巴豆醛选择性加氢反应,发现700℃预还原处理后对巴豆醇的初始选择性达到75%以上。与Pt/CeO₂催化剂相似,高温还原后的Pt/Pr₆O₁₁催化剂中由于存在大量低价态Pr³⁺离子,在反应过程中为Pt提供电子,有利于C=O的吸附,从而提高巴豆醇选择性。但低价态Pr³⁺也可能是反应过程中催化剂表面积碳的重要原因,进而导致Pt/Pr₆O₁₁催化剂活性和巴豆醇选择性下降。更多还原

【Abstract】 α,β-Unsaturated alcohol is of very important industrial values and is produced mainly by selective hydrogenation ofα,β-unsaturated aldehyde.However,the hydrogenation of the C=C bond is thermodynamically preferable over the C=O bond, which leads to undesirable product such as saturated aldehydes.Therefore,it remains a challenge for the researchers in catalysis field to further increase the selectivity forα,β-unsaturated alcohol fromα,β-unsaturated aldehyde.Crotonaldehyde is a typitalα,β-unsaturated aldehyde.Recent studies on selective hyrogenation of crotonaldehyde to produce crotyl alcohol have been focused on the employment of supported gold and platinum catalysts.Currently,the thermodynamic stability of supported gold catalysts is still not satisfactory,and the mechanism of supported gold-and platinum-catalyzed hydrogenation requires further clarification.In this dissertation,Au and Pt catalysts supported on Ce-based oxides were prepared via deposition-precipitation method and applied in the selective hydrogenation of crotonaldehyde.Characterizations of the supported catalyst were achieved by XRD, Raman、FT-IR、TG-DTA、SEM、TEM、BET and CO chemical absorption.A series of experiments have been designed and carried out to examine the influence of electronic environment on the catalyst surface and the composition of the supports on the selective hydrogenation of the conjugated C=O and C=C bond,so as to elucidate the active center of the hydrogenation of the C=O bond and the reason for the deactivation of the catalyst. The contents of the dissertation are as follows:1.Preparation and characterization of Ce-based oxides.Ce-based solid solutions were prepared by an improved citrate sol-gel method,which mainly involved the following two steps:1)calcination（or carbonization）of the citrate precursor at high temperature under N₂ atmosphere（the step resulted in intermediate mixture of Ce-based oxides and carbon powder due to lack of oxygen）;2)removal of the carbon powder at low temperature in air.With the improved citrate sol-gel method,Ce_0.8Pr_0.2O_Y solid solutions with a cubic fluorine structure and more oxygen vacancies were synthesized, with particle size smaller than 10nm and specific surface area higher than 92.1m²/g.It can be concluded that the formation of the Ce_0.8Pr（0.2）O_Y solid solution occurred in the calcination process at 800℃in N₂ and its textural structure would not be influenced by subsequent removal of carbon powder at low temperature in air.Since carbonization process is a key step for the improved citrate sol-gel method,the carbonization temperature would have a vital effect on the textural structure of Ce-based solid solution. Low carbonization temperature resulted in weak crystallization of the Ce-Zr oxide, which would aggregate into big particles with low surface area in subsequent carbon removal process.While high carbonization temperature could provide sufficient thermodynamic force to afford high degree of crystallization of Ce-Zr oxide,and finally lead to the formation of Ce_0.8Zr_0.2O₂ solid solution with higher surface area and porous structure.2.Selective hydrogenation of crotonaldehyde over Au catalysts supported on Ce-based oxides.For the selective hydrogenation of crotonaldehyde over Au/CeO₂ catalysts,the catalytic properties are not only related to the surface area of CeO₂ support, but also to the gold loadings.Using CeO₂ with lower surface area（70m²/g）as the support,the gold loading of 3%in Au/CeO₂ catalysts gave satisfactory catalytic results: 12%conversion of crotonaldehyde and 58%selectivity of crotyl alcohol at steady state. When the gold loading is lower than 3%,the catalytic properties of Au/CeO₂ catalysts decreased linearly;while with higher gold loading,the catalytic properties did not increase noticeably.Besides,a series of Au/Ce_xZr_1-xO₂ were prepared with an attempt to furher improve the thermodynamic stability of Au catalyst.Studies found that among the Au/Ce_xZr_1-xO₂ examined,Au/Ce_0.8Zr_0.2O₂ catalyst could afford higher selectivity of crotyl alcohol（62%）than Au/CeO₂ in the steady state,The result may be attributed to the presence of more oxygen vacancies in the mixed oxides,which can promote the absorption of C=O bond,thus favoring the production of more crotyl alcohol.In addition,Au/Ce_0.8Zr_0.2O₂ exhibited longer catalyst life at 180℃,thus showing its promising prospect as a catalyst suitable for industry production.3.Selective hydrogenation of crotonaldehyde over Pt catalysts supported on Ce-based oxides.In this chapter,the in situ technique was used to pretreat the Pt/CeO₂ catalysts so that the particle size and morphology of platinum could remain intact but the surface chemical environment of CeO₂ support change.The pretreatment of the catalysts allowed us to investigate the effect of Ce⁴⁺or Ce³⁺on the selective hydrogenation of crotonaldehyde.It was found that Ce³⁺ions in Pt/CeO₂ catalyst can transfer electrons to Pt particles,thus enhancing the absorption of C=O bond and improving the selectivity of crotyl alcohol.On the other hand,Ce³⁺ions could also induce the coke deposition on the surface of Pt/CeO₂ catalyst,thus leading to the deactivation of the Pt catalysts.We further explored the hydrogenation of crotonaldehyde over Pt/Ce_xSm_1-xO_2-δcatalysts,and the main results could be concluded as:1)Pt/Ce_xSm_1-xO_2-δcatalysts have better catalytic properties than Pt/CeO₂ catalysts;2) the doped quantity of Sm has an important effect on the catalytic properties of Pt catalysts.For example,Pt/Ce_0.8Sm_0.2O_2-δhas the preferable initial conversion of crotonaldehyde（53.5%）and selectivity of crotyl alcohol（76.5%）;3)For Pt/Ce_0.8Sm_0.2O_2-δ,high temperature pretreatment can increase the selectivity of crotyl alcohol,which could be accounted for by the formation of big particle of platinum that favors the adsorption and hydrogenation of C=O bond.4.Selective hydrogenation of crotonaldehyde over Au and Pt catalysts supported on other oxides.As control experiments,Au/TiO₂ and Pt/Pr₆O₁₁catalysts were prepared and applied in the hydrogenation of crotonaldehyde.For the Au/TiO₂ catalysts,different crystal phases of TiO₂ can greatly affect the catalytic properties.The selectivity of crotyl alcohol on pure anatase support is higher than that on pure rutile support.Furthermore, the best catalytic properties of Au catalyst came from the use of mixed crystal phases as the support,where both the catalytic activity and the selectivity of crotyl alcohol were enhanced.For Pt/Pr₆O₁₁catalyst,the initial selectivity of crotyl alcohol can reach 75% provided that the catalyst was subjected to high temperature reduction at 700℃.High temperature reduction can produce plenty of Pr³⁺ions in Pt/Pr₆O₁₁catalysts.Such Pr³⁺ ions can donate electrons to Pt particles,thus favoring the adsorption of C=O bond and the generation of crotyl alcohol.Analogous to Pt/CeO₂ catalysts,low valent Pr³⁺ions can induce the coke deposition on the catalyst surface,and subsequently make the Pt/Pr₆O₁₁catalysts exhibit decreased catalytic properties.更多还原