【作者】 朱艺涵；

【导师】 陆维敏；

【作者基本信息】 浙江大学 ， 物理化学， 2010， 博士

【摘要】 丙烷选择氧化是利用储量丰富的天然气资源替代石油产品的化工生产途径之一,在天然气工业中有着极其重要的意义,所以对该反应催化剂的结构以及构效关系的研究成为催化氧化领域基础研究的热点。并且在对该课题的研究过程中,不仅合成了许多新型材料,还揭示了很多催化学科领域的新概念、新现象。虽然MoVTe(Nb)O复合金属氧化物是目前丙烷选择氧化制丙烯酸和丙烯睛最高效的催化体系,但对其晶体结构以及催化剂的构效关系尚待进一步的研究。例如,催化剂中多活性中心的组合作用、活性中心中d电子耦合作用以及M1相与M2相之间的协同作用等。值得关注的是,丙烷选择氧化高选择性地制丙烯醛至今仍然是一个悬而未决的难题。因而,开发新型高活性并有高丙烯醛选择性的催化剂是当前关注的热点。本论文在总结大量文献工作的基础上,开展了以下内容的研究工作。1.主要研究了M1相的电子结构和晶体结构的关系。发现M1相骨架中绝大多数以Mo/V为中心的八面体单元在沿着c轴方向形成的O-M…O桥键明显呈现键长交替的规律,并导致了d电子的耦合作用。相对于Mo活性中心而言,相互耦合的d电子更易分布在V活性中心上。并且,对同样的元素,M3、M7、M8、M11活性中心更能稳定局域化的d电子。V5+/V4+离子对具有非常高的氧化还原速率和可逆性,这导致了其在催化剂表面所形成的活性中心(VOx)易产生深度氧化产物。另一方面,高V掺杂含量引起的氧缺位会大大增加催化剂的体相氧扩散速率,从而导致催化剂的体相晶格氧扩散速率很高,并导致产物过氧化。此外,M1晶相结构的M4、M5、M9、M10活性中心趋向于生成氧缺位,这是产生大量C-C键断裂产物(甲醛和乙醛)的主要原因。2.详细研究了M2相的晶体结构和表面重构,本征氧缺位与体相氧扩散途径的关系及其对催化性能的影响,阐明了M1和M2相的相协同作用。研究发现M2相在表面重构为VOMoO4纳米晶,其原因在于M2相表面通过相邻两个TeO3E四面体单元进行的氧传递,使Te4+从Mo(V)O6相互连接八面体骨架得到两个电子而岐化为Te6+和Te0物种。前者偏析富集于M2相的最顶层,后者在反应条件下以金属Te和合金MoTe2的形式升华脱离M2相的表面,并通过晶界转移或者气相沉积对M1相中的Mo/Te进行补充。在M2相纳米晶重构最顶层富集着大量高氧化性的Te6+物种,它拥有高的丙烯α位C-H键的活化能力,并且能够更为有效地将M1相脱附而来的丙烯中间产物经过氧化或氨氧化转化为丙烯酸或丙烯腈。这就是M1相和M2向协同作用的本质。与此同时,M2相亚表层中的Te以Te0物种的形式发生了流失,因而结构非常不稳定,并重构形成了VOMoO4的纳米晶层。研究发现,利用硝酸对M2相中本征氧缺位的调制或利用氧化性气氛处理M2相能够非常有效地改变不同配位环境本征氧缺位的含量。这种调制可以控制M2相晶格中存在三条氧扩散途径：i)畸变八面体(也可认为四方锥)顶点之间,即12f Wyckoff端基03位置的晶格氧沿着[0110]方向的氧扩散；ii)在ab晶面上,即12f Wyckoff桥键02位置参与的氧缺位-晶格氧摆动形式的扩散；iii)在八面体内(四方锥),即赤道面02和端基03位置之间的氧缺位-晶格氧“齿轮转动”形式的扩散。临近V的氧缺位含量控制了所有氧扩散途径的速率,而位于端基的氧缺位含量则可以控制第iii)项扩散途径,并改变表面竞争的反应途径从C-H键的活化转变成为C-C键的断裂。3.研究了单斜与正交TeMo5O16相结构及其与催化性能的关系。由于V5+/V4+离子对的强氧化还原能力,因而在催化剂中大量存在V时就不可避免地引起丙烯醛的过氧化。我们通过引入微量V对丙烯选择氧化制丙烯醛的单斜TeMo5O16活性相进行结构弛豫。这种结构弛豫改变了单斜TeMo5O16相晶体结构中的Mo-O键长分布与d电子的空间分布和耦合性质。骨架结构中的d电子一方面局域在Mo(3)和Mo(5)为中心的八面体单元中,另一方面配对填充在能带隙下方的t2g轨道上这种结构调制还促进了Mo(3)-O-Mo(5)和Mo(2)-O-Mo(4)中氧缺位的生成。并且这种结构调制不仅仅发生在催化剂体相,而且也发生在表面,并产生了具有电子授-受和高速氧流动能力的(Mo5+-O-Mo5+)/(Mo6+-O-Mo6+)氧化还原活性中心,极大地促进了表面氧化还原反应的进行。4.研究发现,较高含量的V对单斜TeMo5O16活性相的掺杂可以诱导其相变为正交结构的TeMo5O16。根据TeMo5O16的相变温度首次合成了高活性粉晶正交TeMo5O16相。研究还发现了正交TeMo5O16相结构中孔道储氧现象及其对丙烯醛的选择性和产率的影响,利用高分辨X射线衍射研究了孔道氧物种的结构并提出了孔道储氧机理。根据所得到的研究结果,我们基于i)增加正交TeMo5O16相孔道储氧量；ii)通过M2相协同作用增加丙烯醛选择性这两条思路,成功地设计并合成了TeCrδMo5-δO16单相和M2/TeMo5O16混相催化剂,并都取得了很高的丙烯醛得率(20.1%和21.4%)。更多还原

【Abstract】 Propane selective oxidation is of high importance in nature gas industry. The studies in crystal structure and related properties are the hot topics and "seven pillars" in fundamental research of catalysis. The researches that focused on propane selective oxidation not only provide novel catalytic materials but also elucidate some new phenomenon and concepts in catalysis.There are still many ambiguities in crystal structure of MoVTe(Nb)O mixed oxides as the most efficient catalysts, such as the interaction between M1 and M2 phase:The symbiosis between M1 and M2 phase, that is the presence of M2 phase promotes the production of acrylic acid (AA) or acrylonitrile (ACN) of M1 phase; M2 phase replenishes the Mo/Te loss in Ml phase and alleviates its deactivation. On the other side, propane selective oxidation to acrolein in high selectivity is a difficult research topic in this century.Firstly, this work studied the relationship between the electronic structure and crystal structure of M1 phase and found out that there’s strong O-M…O alteration along the c axis which resulted in the coupling of d electrons. The coupled d electrons were prone to locate at V sites rather than Mo sites. M3, M7, M8 and M11 sites preferred the location of d electrons. The high redox activity and reversibility of V5+/V4+pair resulted in the formation of surface VOX active sites for deep oxidation. On the other side, the doping of V emerged oxygen vacancy in the bulk and boosted the bulk oxygen diffusion rate that caused the over-oxidation. While M4, M5, M9 and M10 sites were prone to present oxygen vacancy and acted as C-C active sites to produce formaldehyde and acetaldehyde.Secondly, the crystal structure and surface reconstruction of M2 phase were studied to interpret the interaction between M1 and M2 phases. Results showed that the surface of M2 phase reconstructed to nanocrystalline VOMoO4, because Te4+ species in the neighboring two TeO3E tetrahedral units accepted two electron from the Mo(V)O6 interconnections and disproportionated to Te6+and Te0 species on the surface. Te6+species are enriched on the topmost surface and Te0 species sublimed from M2 grains in the form of Te and MoTe2 to replenish Mo/Te loss in M1 phase by either spillover through grain boundaries or solid-gas-solid deposition. While those Te6+species should be more active than Te4+species activating a C-H bond and efficiently converted propylene intermediate that migrated from M1 grains and formed AA and ACN by (amm)oxidation. This was interpreted as the nature of M1/M2 phases’symbiosis. At the same time, the Te depleted in the sublayer and reconstructed to nanocrystalline VOMoO4.Having known the decent crystal structure of M2 phase, the content of intrinsic oxygen vacancy was intended to be tuned to study the roles of oxygen diffusion pathways in the catalytic performance. Results showed that the addition of nitric acid and oxidative atmosphere could tune the content of intrinsic oxygen vacancy with different coordination modes. These methods controlled three anisotropic oxygen diffusion pathways in the bulk lattice of M2 phase:ⅰ) inter-pyramid vertex anion hopping between fluctuated 12f Wyckoff 03 terminal sites along [0110] direction; ii) ab-plane intra/inter-pyramid anion-vacancy wagging involving 12f Wyckoff 02 bridge sites; iii) intra-pyramid "cog-wheel" type pitching between 02 and 03 sites. The content of oxygen vacancy neighboring to V controlled the rates of all anisotropic diffusion pathways, and content of oxygen vacancy at terminal sites controlled the diffusion rate of pathway iii) that can change surface reaction pathway from C-H activation to C-C bond cracking-oxidation.Due to the strong redox ability of V5+/V4+pair, the large content of V in the catalyst may result in the over-oxidation of acrolein. Hence, we introduce traces of V into the lattice of monoclinic TeMo5O16 to induce the structural relaxation. The structural relaxation changed the distribution of Mo-O bond lengths and the d electron as well as its coupling. The d electrons in the framework on one side localized at Mo(3) and Mo(5) centered octahedral units, and filled the lower t2g levels from Eg of 0.45 eV to 0.23 eV. The relaxation on the other side facilitated the formation of oxygen vacancy in Mo(3)-O-Mo(5) and Mo(2)-O-Mo(4) bonds. The relaxation occurred not only in bulk but also on the surface, and formed the redox sites of (Mo5+-O-Mo5+)/(Mo6+-O-Mo6+) with electron donor-acceptor and fast oxygen transfer abilities. Those active sites on the surface highly promoted the surface redox reactions.We also found high content of V doping in monoclinic TeMo5O16 active phase induced a phase transition to orthorhombic TeMo5O16. After investigating the phase transition temperature of TeMo5O16, we succeeded in synthesizing pure orthorhombic TeMo5O16. Studies also showed that the channel structure of orthorhombic TeMo5O16 acted as an oxygen reservior and played an important role producing acrolein. We found out the nature of the oxygen species in the channel and proposed a mechanism of oxygen transfer and reservation. Upon those ideas derived from previous studies, we designed novel catalyst in two ways:i) increasing the oxygen reservation;ⅱ) symbiosis between M2 and orthorhombic TeMo5O16 phases. The catalytic performance of as-synthesized TeCrδMo5-δO16 and M2/TeMo5O16 catalysts reached a world class yield of acrolein (20.1% and 21.4%).更多还原