蒙脱石表面酸碱性“势可两立”: 可用于一锅内酸碱工业催化过程

2023-04-04 · 阅读

Qiming She^{a, b, c}, Minyang Qiu^{a, c}, Kejin Li^{a, c}, Jiahui Liu^{a, c}, Chunhui Zhou^{a, c, d, ⁎}

 
        酸碱双功能催化剂是一种同时具有酸性和碱性活性位点的多相固体催化剂。因其成本低、可重复使用的优点，在过去几十年中一直是各种有机反应研究的热点。然而，酸和碱催化反应都需要足够强的酸性和碱性强度，但在单一材料和均相体系中，酸碱活性位点常常“势不两立“，很容易相互失活，或在使用几次后催化活性显著降低，使得酸碱双功能多相催化剂的开发和一体化制备充满挑战。
       蒙脱石作为一种典型的粘土矿物，由于其表面的Brønsted和Lewis酸性，常作为固体酸催化剂应用于各类催化反应，且反应条件温和，产率高，选择性强，反应过程简单，只需要过滤去除催化剂和蒸发溶剂，易于回收和重复使用。然而，其表面碱性位点的存在及催化活性却很少被揭示，且蒙脱石表面酸碱活性位点的分布及构效关系目前仍缺乏深入的了解。
        近日，浙江工业大学化学工程学院、黄山学院和青阳非金属矿研究院等机构合作，在热活化蒙脱石表面发现了共同存在的酸和碱活性位点。蒙脱石独特的层状结构使其表面的酸和碱活性位点“势可两立”，作为催化活性中心，二者均独立发挥作用且不相互破坏，在甘油与碳酸二甲酯的酯交换反应中呈现优异的酸碱双功能催化性能。蒙脱石表面的碱性位点主要来源于端面的-M-OH (M = Mg, Si, Al) 基团，而酸性位点来源于层板结构端面上的五配位M^Ⅲ (M^Ⅱ)金属原子，酸碱活性位点在酯交换反应中呈现出了“双管齐下”的协同催化作用。在400 ℃下煅烧的钠基蒙脱石呈现出最大的碱性位点强度 (1.38 mmol/g) 和最高的的碳酸甘油酯收率(94.5%)。热活化温度在500 ℃以下时，钠基蒙脱石表面酸碱活性位点的变化并不明显，随着活化温度升至500 ℃及以上，钠基蒙脱石表面的酸性和碱性位点的强度逐渐减弱。当煅烧温度达到600 ℃及以上时，钠基蒙脱石的层间距从1.27 nm减小到了0.98 nm，阻碍了反应物和产物分子在层间的传质。钠基蒙脱石在高活化温度下较差的层间传质、表面脱羟基作用和结构重组综合导致了其催化活性的衰退。机理研究表明，蒙脱石端面的-M-OH基团衍生的碱性位点促进了甘油阴离子的生成，从而促进了甘油的转化。端面M^Ⅲ (M^Ⅱ)金属原子衍生的酸性位点有利于碳酸二甲酯的羰基活化，促进了甘油阴离子向碳酸甘油酯的转化，提高了碳酸甘油酯的选择性。通过以上对催化甘油酯交换生成碳酸甘油酯对热活化Na-Mt的酸和碱活性位点和构效关系的深入研究，可以促进蒙脱石基材料作为双功能催化剂在其他催化过程中的应用。该工作目前以“ Acidic and basic sites on the surface of sodium montmorillonite active for catalytic transesterification of glycerol to glycerol carbonate”为题发表于《Applied Clay Science》。
         该工作以甘油与碳酸二甲酯酯交换反应为探针反应，以端面和层间阳离子为切入点，研究了热活化对钠基蒙脱石的表面活性位点和酸/碱性质的影响，并探索了热活化的钠基蒙脱石在酯交换反应中的可能的催化机理，揭示了钠基蒙脱石表面酸碱活性位点的共存及其协同催化作用，为新型蒙脱石基酸碱双功能催化剂的构筑、精细结构调控和性能强化提供了理论基础。
   
 
Abstract
 
Montmorillonite (Mt) as a solid acid catalyst or support has been  widely used in catalytic reactions. However, the existence and catalytic activities of its surface basic sites have rarely been revealed. Here, the surface and structure of Na-Mt are modulated by thermal treatment, providing both acidic and basic sites for the trans-esterification of glycerol with dimethyl carbonate (DMC) to glycerol carbonate (GLC). The experimental results showed that the thermally activated Na-Mt exhibited bifunctional catalytic properties in glycerol trans-esterification. The Na-Mt calcined at 400 ◦C had a basic site density of 1.38 mmol/g and led to a glycerol conversion of 96.8% and a GLC yield of 94.5%. Edge surfaces of Na-Mt provided MII (MIII) atoms as Lewis acidic sites for facilitating the generation of glyceroxide anions from activated glycerol and -M-OH groups as Brønsted and Lewis basic sites for enhancing the carbonyl activation of DMC. This work revealed the co-existence of acidic and basic sites over thermally activated Na-Mt for synergetic catalysis in the transesterification of glycerol to GLC, making the development of Mt-based materials as bifunctional catalysts for one-pot acid-base catalytic processes possible. 
 
Graphical abstract
 

 
a
Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China
b
College of Chemistry and Chemical Engineering, Huangshan University, Huangshan, Anhui 245041, China
c
Qing Yang Institute for Industrial Minerals, You Hua, Qing Yang, Chi Zhou, 242804, China
d
Engineering Research Center of Non-metallic Minerals of Zhejiang Province, Zhejiang Institute of Geology and Mineral Resources, Hangzhou 310007, China
 
Received 16 January 2023; Received in revised form 12 March 2023; Accepted 13 March 2023