【作者】 李丹；

【导师】 唐思扬；

【作者基本信息】 四川大学 ， 化学工艺， 2022， 硕士

【摘要】 线性α-烯烃（LAOs）是重要的基础化学品,常用于生产高性能聚合物、高级醇等。商业LAOs的主要制备路线为乙烯齐聚、烯烃异位、脂肪醇脱水等,开发新的LAOs生产技术是学术界和工业界的共同研究兴趣。已有研究集中于烷烃脱氢领域,部分催化剂在热反应中具有较好的脱氢性能。前期的研究中,Pd/TiO₂可在常温下高选择性地光催化乙烷脱氢生产乙烯(C₂H₄选择性94.6%,C₂H₄614.9μmol·g-cata^-1·h^-1)。通过调节Pd的粒径尺寸,可以实现光催化乙烷氧化脱氢（PODHE）向光催化乙烷直接脱氢（PDHE）的转变。Cu/TiO₂可光催化丙烷脱氢制丙烯,丙烯选择性可达91.97%。因此本文提出探索金属负载型TiO₂（M/TiO₂）光催化长链烷烃脱氢制烯烃,通过合成不同金属M/TiO₂,考察其催化正丁烷、正己烷、正辛烷的光反应性能,解析产物分布特征和光催化烷烃脱氢制烯烃的选择性反应机理,探索了Cu/TiO₂光催化正丁烷制丁烯的调控技术,为开发新的光催化LAOs技术提供新的思路。合成不同的M/TiO₂（M=Pt、Rh、Ni、Cu、Pd和Au）用于光催化正丁烷和正己烷脱氢,不同的M/TiO₂产物分布不同。除了Rh/TiO₂外,M/TiO₂倾向于C-H键活化生成烯烃,而非C-C键活化生成裂解产物。Cu/TiO₂和Pd/TiO₂存在明显的α-烯烃选择性差异。进一步考察Cu/TiO₂和Pd/TiO₂光催化正辛烷脱氢,随着烷烃碳链的增加,α-烯烃的选择性降低,这与热力学平衡组成和无选择性吸附计算结果规律一致。Cu/TiO₂倾向于生成α-烯烃,Pd/TiO₂倾向于生成β-烯烃。Pd/TiO₂可光催化弱氧化剂CO₂氧化烷烃脱氢,Cu/TiO₂光催化烷烃直接脱氢。CO₂不参与烷烃脱氢过程,只与H₂发生逆水煤气变换生成CO,不影响烷烃脱氢产物分布。进一步解析Cu/TiO₂和Pd/TiO₂光催化烷烃脱氢的反应机理。金属纳米颗粒Cu和Pd的负载降低了TiO₂的带隙,提高了可见光响应,使导带（CB）和价带（VB）位置更负。Cu/TiO₂和Pd/TiO₂的VB位置低于正丁烷脱氢生成1-丁烯、反-2-丁烯和顺-2-丁烯的氧化还原电势,脱氢反应在热力学上可行,过程中只产生h⁺和·OH基团不具选择性。因此,Cu/TiO₂和Pd/TiO₂的VB位置不同不影响产物选择性差异。密度泛函理论（DFT）和原位红外（in situ FTIR）结果表明,非光照条件下,丁烷在Cu/TiO₂和Pd/TiO₂上的吸附构象相似,而光辐照下烷烃在Cu/TiO₂和Pd/TiO₂上的吸附构象不同。Cu/TiO₂倾向于吸附α-C形成α-烯烃,而Pd/TiO₂倾向于吸附β-C形成β-烯烃。通过负载金属调控光催化下催化剂的电子性质可以实现非极性烷烃分子的选择性吸附和反应。进一步探索Cu/TiO₂光催化正丁烷脱氢。Cu的电子特性是影响烷烃在光激发下在催化剂上吸附行为的关键因素,与负载量关系并不大。随反应温度增加,丁烯和H₂产量减少。温度升高,1-丁烯选择性减少,裂解选择性增加,温度升高利于裂解反应的发生。1-丁烯和2-丁烯的选择性不随反应时间增加而明显改变,未在光催化条件下发现烯烃间的异构化反应。本文发现了M/TiO₂存在光催化烷烃脱氢选择性差异;通过催化剂表征、理论计算和反应中间物种检测,发现在TiO₂上负载金属可以调控非极性烷烃分子在光激发下在催化剂表面的吸附行为,从而调控烯烃产物选择性。研究结果为开发光催化烷烃脱氢制LAOs路线提供新的思路。更多还原

【Abstract】 Linearα-Olefins（LAOs）are important basic chemicals commonly used in the production of high-performance polymers and higher alcohols,etc.Commercial LAOs are mainly obtained from ethylene oligomerization,olefin metathesis and fatty alcohol dehydration et.al.Developing new LAOs technology is attractive to the academia and industry.Many researches focus on n-alkane dehydrogenation.Some catalysts showed considerable dehydrogenation performance in thermal reactions.In our previous studies,photocatalytic ethane dehydrogenation was realized with a high selectivity of ethylene over Pd/TiO₂ at room temperature(C₂H₄ selectivity 94.6%,C₂H₄ 614.9μmol·g-cata^-1·h^-1).And the transformation from photocatalytic oxidation dehydrogenation of ethane（PODHE）to photocatalytic dehydrogenation of ethane（PDHE）can be realized by regulating the particle size of Pd.The photocatalytic dehydrogenation of propane to propylene（C₃H₆ selectivity 97.97%）was realized over Cu/TiO₂ catalyst.Therefore,this paper explored the photocatalytic dehydrogenation of n-alkanes over metal-supported TiO₂（M/TiO₂）.By synthesizing different M/TiO₂ for investigating the photoreaction performance of catalyzing n-butane,n-hexane and n-octane over M/TiO₂,the product distribution characterization and mechanism of selective photocatalytic n-alkane dehydrogenation were analyzed.The regulation technology of photocatalytic n-butane dehydrogenation over Cu/TiO₂ was explored.This work would provide new insight for the development of photocatalytic LAOs production technologies.M/TiO₂（M=Pt,Rh,Ni,Cu,Pd,and Au）was used for the photocatalytic reaction of n-butane and n-hexane system,it was found that there was a significant difference in product distribution between M/TiO₂.Except Rh/TiO₂,M/TiO₂ tended to be activated for C-H bond cleavage to produce olefin products rather than C-C bond cleavage to cracking produce.There was a clear difference inα-olefin selectivity between Cu/TiO₂and Pd/TiO₂.The photocatalytic dehydrogenation of n-octane over Cu/TiO₂ and Pd/TiO₂ was further investigated.With the increase of carbon chain,the selectivity ofα-olefin decreased,which was consistent with the thermodynamic equilibrium composition and non-selective calculation results.Cu/TiO₂ was more favorable for generatingα-olefins and Pd/TiO₂ was more favorable forβ-olefins.Photocatalytic oxidative n-alkane dehydrogenation mainly occurred on Pd/TiO₂ with CO₂ as a weak oxidant,while photocatalytic alkane dehydrogenation mainly occurred on Cu/TiO₂.CO₂ did not participate in the n-alkane dehydrogenation process,and only undergoes reverse water gas shift with H₂ to generate CO,which did not affect the distribution of alkane dehydrogenation products.The mechanism of photocatalytic n-alkanes dehydrogenation over Cu/TiO₂ and Pd/TiO₂ was further analyzed.The loading of metal nanoparticles Cu and Pd reduced the bandgap of TiO₂,improved the visible light response,and made the conduction band（CB）and valence band（VB）more negative.However,the VB positions of Cu/TiO₂ and Pd/TiO₂ are lower than the redox potential of the dehydrogenation of n-butane to1-butene,trans-2-butene and cis-2-butene,these reactions are thermodynamically feasible.The h⁺and·OH are generated,but they are non-selective.Therefore,the different VB positions of Cu/TiO₂ and Pd/TiO₂ were not responsible for the difference in photocatalytic dehydrogenation selectivity.Density functional theory（DFT）and in situ FTIR results showed that the adsorption behaviors of n-butane on Cu/TiO₂ and Pd/TiO₂were similar under non-irradiation,but different under irradiation.Cu/TiO₂ tended to adsorbα-C to formα-olefins,while Pd/TiO₂ tended to adsorbβ-C to formβ-olefins.The selective adsorption and reaction of non-polar alkane molecules over photocatalysts could be achieved by tunning the electronic properties of catalysts under irradiation by supporting metals.The photocatalytic dehydrogenation of n-butane over Cu/TiO₂ was further explored.The electronic property of Cu was the key factor affecting the adsorption behavior of n-butane on catalyst under irradiation,which had little relationship with the loading.As the reaction temperature increased,the yield of olefin and H₂ decreased,the selectivity of 1-butene decreased and the cracking selectivity increased,indicating that the increase of temperature was favorable for the cracking reaction.The 1-butene and 2-butene selectivity did not change substantially with the reaction time increased,indicating that the isomerization reaction between olefins did not occur.In this paper,it is found that there was a difference in the selectivity of photocatalytic alkane dehydrogenation between M/TiO₂.According to catalyst characterization,theoretical calculation and detection of reaction intermediate species,it is found that metal loading on TiO₂ can regulate the adsorption of non-polar alkane molecules on the catalyst surface under photoexcitation to control the olefin selectivity.The research results provide new insight for the development of photocatalytic alkane dehydrogenation to LAOs route.更多还原