高分散铬基复合氧化物的制备及其在氢氟烃合成中的应用

With global warming becoming more severe, the search for hydrofluorocarbons (HFCs) of low global warming potentials (GWP) is more urgent than ever before. Chromium-based compounds are well-known as catalysts for the synthesis of HFCs via gas-phase fluorination, but they suffer from some drawbacks such as ready deactivation, difficulty in regeneration, and short life span. Layered double hydroxides (LDHs) and its calcined products (LDO) have unique layered structures, versatility of the metal cations, high dispersibility, adjustability of the acidic/basic centres, higher surface areas, stable layered structures, and "memory effects" of the structures, these characteristics provide ready access to LDHs and LDO as catalysts or catalyst supports. This research proposal is based on adjusting acidity/alkalinity, improving chromium dispersion, recycling and re-using, with LDHs as pro-structures, to prepare Cr-based LDO with different compositions, and initiated studies on the gas-phase fluorination with high selectivity and high conversion, and on regeneration of deactivated catalyst, and on their structure activity relationships. It focuses on the influence of controllable preparations on catalytic activity and stability, characterization of the catalyst before and after catalysis and catalyst recycling, to probe the mechanistic of LDO in gas-phase fluorination. The final goal is to provide a theory and application basis for LDO in catalytic gas-phase fluorination of various HFCs.

由于全球变暖问题日益严重，对低GWP的氢氟烃(HFCs)的开发非常迫切。铬基化合物作为气相氟化合成HFCs的典型催化剂，存在易团聚失活、再生困难和使用寿命短等问题。层状双金属复合氢氧化物（LDHs）及其焙烧物(LDO)所具有的独特层状结构、稳定的层板结构、金属阳离子组成的广泛性和高分散性、较大的比表面积和结构"记忆效应"等特点，可用于构筑高分散铬基复合氧化物体系，改善活性中心的分散状态及稳定性，以期解决失活问题，并实现简单再生循环利用。围绕气相氟化合成HFCs所涉及的构效关系，利用LDO可控制备进行调变铬基复合氧化物的组成、形貌和微观结构，进一步对气相氟化过程动力学和热力学深入研究，结合氟化过程、再生循环过程中LDO的微观结构变化，揭示LDO作用于催化氟化的机理，意图为LDO类无机材料在多种HFCs气相合成中的推广奠定相关理论和应用基础。

以LDHs为前驱体构筑高分散体系，调控Cr3+的分散状态及酸碱性中心，制备不同组成的MgCrAl-LDO-F，用于催化合成HFCs和HFOs，对其催化性能进行了研究。.对催化剂前体的制备和结构进行了研究，研究发现：.（1）所制备的MgCrAl-LDHs和MgCr-LDHs均具有典型水滑石的层状结构；在所制备的MgCrAl-LDHs中，晶化时间24h、晶化温度65℃、摩尔比（Mg:Cr:Al）为6:1:1的MgCrAl-LDHs的比表面积相对较大；在所制备的MgCr-LDHs中，晶化时间24 h、晶化温度65℃、摩尔比（Mg:Cr）为3:1的MgCr-LDHs的比表面积相对最大。.（2）经焙烧后，MgCrAl-LDHs转化为晶相的MgCrAl-LDO，450℃下焙烧得到的摩尔比（Mg:Cr:Al）为4:1:1的MgCrAl-LDO粒度最小，400℃下焙烧得到的摩尔比（Mg:Cr:Al）为4:1:1的MgCrAl-LDO具有最大的比表面积。.对制备的催化剂前体进行氟化活化处理得催化剂MgCrAl-LDO-F，对其结构进行分析，并以其为催化剂分别对气相氟化HCFC-133a合成HFC-134a、气相氟化HCFC-1233xf合成HFO-1234yf进行研究，结果如下：.（1）相同焙烧温度制备的MgCrAl-LDO-F和MgCrAl-LDO的结构相一致，即催化剂氟化过程没有影响MgCrAl-LDO的结构。.（2）以摩尔比为10:1的HF和HCFC-l33a通入装有经400℃焙烧、摩尔比（Mg:Cr:Al）为4:1:1的MgCrAl-LDO-F催化剂的反应器中，在常压和300℃下反应，HCFC-l33a的转化率为34.72%，HFC-134a选择性为98.50%。连续反应2160 h（90天）后，催化剂的性能有所下降，HCFC-l33a的转化率为13.20%，HFC-134a选择性为81.37%。.（3）以摩尔比为10:1的HF和HCFC-1233xf通入装有经400℃焙烧、摩尔比（Mg:Cr:Al）为4:1:1的MgCrAl-LDO-F催化剂的反应器中，在常压和300℃下反应，HCFC-1233xf的转化率为90.56%，HFO-1234yf选择性为85.50%。.由目前的研究结果来看，所制备的MgCrAl-LDO-F气相氟化催化剂好于现有的铬基催化剂。