纤维结构化纳米Au（Pd）催化剂的原电池反应制备及其催化气相草酸二甲酯加氢性能

The synthesis of ethylene glycol (EG) from syngas, though catalytic coupling of CO with methyl nitrite to dimethyl oxalate (DMO) and subsequent gas-phase hydrogenation of DMO, is a hot subject in the chemical processing of coal. However, effective and efficient solutions are urgently required to overcome the difficult problems associated with strong exothermicity of the hydrogenation process as well as the poor durability of traditional catalysts and their weak thermal conductivity. One promising way is to use and develop novel microstructured catalytic bed reactors, which can provide unique combination of high thermal conductivity, excellent hydrogenation activity/selectivity and long-term stability. Regarding the strong exothermicity, it is strongly desirable to enhance the intrebed heat transfer, which is quite helpful to quickly dissipate a great quantity of reaction heat liberated rapidly thereby making the bed temperature close to isothermal. This will not only suppress deep hydrogenation but also retard the catalyst sintering deactivation. Our strategy in this effort is to devolop a new-type microfibrous structured nano-Au (or Pd) catalysts with excellent heat transfer ability for the gas-phase hydrogenation of DMO. To accomplish this goal, a new carrier class of thin-sheet sinter-locked metal fibers (such as 2 to 20 um Ni, Cu, and Fe fibers) will be devopped firstly, being characterized with a special three-dimensional network, open structure and especially high thermal conductivity. The Au and/or Pd will be then firmly embedded into the thin-sheet metal-fiberous matrixes with the aid of a simple galvanic exchange reaction deposition method (e.g., 3Ni + 2HAuCl4 = 2Au + 3NiCl2 + 2HCl, driving by the large electrode potential difference between Ni2+/Ni0 (-0.22 V) and Au3+/Au0 (1.5 V) pairs). This new approach allows for one-step fabrication of such microfibrous Au (or Pd) nanocatalysts in reactor scale together with uniform geography design of catalyst bed in meso-scale and chemically and morphologically tuning of the active sites in nano-scale. The effects of metal-fiber surface properties, galvanic reaction conditions, and Au (or Pd) precursors, will be investigated systematically, on the Au (or Pd) particles in terms of size/morphology, surface chemical properties, embedment strength and catalytic hydrognation performance. The galvanic exchange reaction deposition method will be established for a new-class transient-metal-supported precious metal catalyst system with textural and nano-structural controllability. The relationship between the catalytic performace and the special nano-structure of Au (or Pd) particles galvanically deposited onto the metal fibers will be deeply revealed. Computational fluid dynamics (CFD) code FLUENT will be employed to simulate the temperature and reaction distributions inside such microfibrous-structured catalytic bed, demonstrating the process intensification benefitted form enhanced heat/mass transfer.

煤经由合成气的草酸二甲酯（DMO）气相加氢合成乙二醇（EG），是煤化工领域的热点之一。针对该气相加氢反应的强放热特点及现有催化剂低导热和稳定性差的问题，提出新型微结构化催化剂研制新策略，以实现优异的催化加氢活性和良好导热性的统一，消除高温热点以抑制产物深度加氢和减缓催化剂烧结失活，同时又能及时移走反应热。基于三维开放网络的烧结金属纤维（2-20微米Cu、Ni等）结构载体，利用HAuCl4及其盐或(NH4)2PdCl6与金属纤维表面的自发原电池反应，对Au（Pd）催化剂进行宏（反应器尺度制备）-微（织构均一调控）-纳（纳米分散和锚定）设计合成。研究金属纤维表面性质、原电池反应条件和Au（Pd）前体等，对Au（Pd）颗粒尺寸/形貌、表面与化学环境和锚定强度及催化加氢性能的影响，阐明纤维结构化催化剂的构筑和织构/晶构的调控规律，揭示表面特性-催化性能规律和织构-流动与传递-反应过程强化构效关系。

煤制乙二醇是煤化工的热点之一，其中的草酸二甲酯加氢制备乙二醇技术相对不够成熟。研制高效、稳定的非传统氧化物负载型草酸二甲酯加氢催化剂，是一个兼具学术和应用价值的富有挑战性的课题。.开展了烧结金属纤维结构化Pd、Au和Au-Pd 催化剂的原电池制备及其催化性能的研究，结果表明：Pd、Au 离子可被Cu-fiber 高效还原置换且均匀分散于纤维表面。Pd 能够有效地活化氢，对反应有利，但会促进Cu2O还原，对稳定性不利。Au 能够有效地抑制催化剂表面Cu 氧化物的还原、对催化剂稳定性有利，但该催化剂活性和EG 选择性较差。Au-Pd 共置换沉积形成了Au-Pd-Cu+三元活性位结构，AuPd 以合金的形式存在，对Cu+的DMO 加氢活性起重要的促进作用，同时抑制了Cu+在反应条件下因Pd解离活化H2而导致的还原；因而，Au-Pd/Cu-fiber催化剂具有良好的催化活性、乙二醇选择性和反应稳定性。还考察了金属纤维基体、Pd和Au前驱体及含量、焙烧气氛、焙烧温度等催化剂制备因素以及反应条件对纤维结构化纳米Pd、Au和Au-Pd 催化剂草酸二甲酯加氢制乙二醇催化性能的影响关系。.基于上述了Pd-Au-Cu+三元活性位结构的确定，以Pd-Au-Cu/Cu-fiber为模型体系，系统考察了稀土氧化物改性对催化剂低温活性、EG选择性和反应稳定性的影响，确定了La2O3为优选助剂并明确了催化剂的优化组成，阐明了La2O3的助催化本质在于：1) 其Lewis酸性的氢溢流促进作用；2) 其拉电子作用导致PdAu合金缺电子效应对H2活化的促进作用；3) 其部分还原物种参与活化酯基；4) 其对Cu+物种的稳定作用。在230 oC下，2La2O3-0.1Pd-0.5Au-Cu/ Cu-fiber的乙二醇收率高达93.4%，500 h内未见失活迹象。.通过水热方法，制得了FeNi-LDHs/Ni-foam催化剂，在290 oC、0.5 g催化剂装量、0.031 mL/min的13 wt%DMO甲醇溶液进料、2.5 MPa和180的氢酯比条件下，DMO可完全转化，乙醇选择性高达97.3%。FeNi水滑石催化作用本质在于FeOx的Lewis强/弱酸与Ni0协同作用，促进了酯基加氢以及C-OH加氢脱水，使得DMO一步加氢到乙醇，同时也促进了MG加氢到MA、HAc再加氢到乙醇。