Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation

调控含钛材料中钛位点的微环境与金位点协同促进丙烯环氧化

Au sites supported on Ti-containing materials(Au/Ti-containing catalyst)are currently considered as a promising catalyst for the propylene epoxidation owing to the synergistic effect that hydrogen peroxide species formed on Au sites diffuses to the Ti sites to form the Ti-hydroperoxo intermedi-ates and contributes to the formation of propylene oxide(PO).In principle,thermal treatment will significantly affect the chemical and physical structures of Ti-containing materials.Consequently,the synergy between tailored Ti sites with different surface properties and Au sites is highly expected to enhance the catalytic performance for the reaction.Herein,we systematically studied the intrinsic effects of different microenvironments around Ti sites on the PO adsorption/desorption and conversion,and then effectively improved the catalytic performance by tailoring the number of surface hydroxyl groups.The Ti^(Ⅵ) material with fewer hydroxyls stimulates a remarkable enhancement in PO selectivity and H_(2) efficiency compared to the Ti^(Ⅵ) material that possessed more hydroxyls,offering a 7-fold and 4-fold increase,respectively.As expected,the Ti^(Ⅵ+Ⅳ) and Ti^(Ⅳ) materials also exhibit a similar phenomenon to the Ti^(Ⅵ) materials through the same thermal treatment,which strongly supports that the Ti sites microenvironment is an important factor in suppressing PO con-version and enhancing catalytic performance.These insights could provide guidance for the rational preparation and optimization of Ti-containing materials synergizing with Au catalysts for propylene epoxidation.

环氧丙烷(PO)作为丙烯的重要下游产品,广泛用于丙二醇、聚醚多元醇、丙二醇醚等高附加值化工产品的合成.其中,70%以上的PO被用于合成聚醚多元醇,而这些聚醚多元醇又是蓬勃发展的聚氨酯工业的重要原料.与传统的氯醇法和共氧化法相比,丙烯在氢气和氧气氛围下直接气相环氧化反应生成PO(HOPO)的工艺因其环保和经济性优势,成为学术界与工业界关注的焦点.目前,高度分散的金纳米颗粒协同含有孤立四配位钛位点的钛材料(Au/Ti)被视为该反应最具潜力的催化剂,这主要是因为其独特的反应机理:在金位点上形成的过氧化氢(HOOH)物种与附近孤立的Ti4+位点作用,生成Ti-OOH中间物种,进而环氧化丙烯生成PO.因此,构筑和调控钛位点的微环境,建立其与丙烯及PO吸脱附的构效关系,对于高效利用金位点上的HOOH物种是一个重要且充满挑战的研究方向.本文深入探讨了具有不同钛配位状态(包括Ti^(Ⅵ),Ti^(Ⅵ+Ⅳ)和Ti^(Ⅳ))的含钛材料,通过统一的热处理策略调控钛位点的微环境,并与金位点协同作用,显著提升了HOPO过程中的PO选择性和氢气利用效率.以含Ti^(Ⅵ)材料为例,详细分析了钛位点的理化性质、PO进一步转化的潜能及其与HOPO催化性能之间的内在联系.X射线衍射(XRD)和拉曼光谱(Raman)分析显示含Ti^(Ⅵ)样品在经过热处理后,Ti^(Ⅵ)-L(500°C)和Ti^(Ⅵ)-M(800°C)仍然保持锐钛矿晶相,而Ti^(Ⅵ)-H(1000°C)则完全转变为金红石结构,高分辨率透射电镜观察进一步证实了这一结构转变.此外,氨气程序升温脱附(NH3-TPD)与吡啶红外光谱(Py-IR)测试结果表明,随着热处理温度的升高,含Ti^(Ⅵ)样品表面的酸性位点减少,这与表面羟基数量的减少趋势相吻合.傅里叶变换红外(FTIR)进一步证实,相较于Ti^(Ⅵ)-L和Ti^(Ⅵ)-M,Ti^(Ⅵ)-H含有更少的表面羟基.原位漫反射红外光谱(In-situ DRIFTS)实验揭示了表面羟基数量对PO吸附和转化行为的影响:Ti^(Ⅵ)-H样品因表面羟基和酸性位点较少,抑制了目标产物PO的吸附和后续转化;而Ti^(Ⅵ)-L上则发生PO的强吸附,并促进了其进一步转化为双齿基含碳物种.PO转化实验结果显示,Ti^(Ⅵ)-L样品具有高PO转化率(转化为丙醛和丙酮),约为66%,分别是Ti^(Ⅵ)-M和Ti^(Ⅵ)-H样品的11倍(6%)和13倍(5%).此外,动力学分析结果表明Ti^(Ⅵ)-M相较于Ti^(Ⅵ)-L具有更高的PO转化活化能,同样证实其更弱的PO转化能力;而Ti^(Ⅵ)-H具有更低的活化能,但其PO转化率却最低,这可能是由于其表面PO吸附转化的活性位点发生了改变.综合分析含Ti^(Ⅵ)样品的理化性质和PO吸附转化能力结果表明,高温热处理后的Ti^(Ⅵ)-H样品相比于Ti^(Ⅵ)-L和Ti^(Ⅵ)-M含有更少的表面羟基和酸性位,因此有效提升了PO的脱附能力.正因为PO在Ti^(Ⅵ)-H样品上的吸附和转化被抑制,所以相比Ti^(Ⅵ)-L,Ti^(Ⅵ)-H在协同金位点用于HOPO反应中表现出更好的催化性能,PO选择性和氢效分别提高了7倍和4倍.正如预期,含Ti^(Ⅵ)+Ⅳ和Ti^(Ⅳ)的材料在经过相同的热处理调控后,也表现出与Ti^(Ⅵ)样品一致的构效关系.综上所述,本文对含有不同钛配位的材料通过热处理的方式改变了钛位点周围的微环境.结果表明,含有较少表面羟基的钛材料和金位点共同用于HOPO过程时,获得了更高的PO选择性和氢气利用效率,实现了更高效的金钛协同作用.此外,本文进一步阐明了钛位点的微环境对PO转化的作用机制,为后续含钛材料的设计和优化提供了参考.