Pd/LDH低温催化CH_(4)-CO_(2)直接合成乙酸

Direct Synthesis of Acetic Acid from CH_(4)-CO_(2)over Pd/LDH at Low Temperature

利用CH_(4)和CO_(2)反应直接合成乙酸是一个100%原子经济同时转化CH_(4)并减排CO_(2)的有效方法.水滑石因具有比表面积大、热稳定性高、活性组分分散性好及适宜的酸-碱性被广泛用于CH_(4)和CO_(2)转化反应中,而Pd常被认为是C—C键偶联的活性中心.本文以Mg/Al摩尔比分别为1.5,3,5,7,9的镁铝水滑石(MgAl-LDH)为载体,采用离子交换法制备了Pd负载量(质量分数)为5%的Pd/LDH催化剂,并用于CH_(4)-CO_(2)两步梯阶转化直接合成乙酸;通过XRD,ICP,氮气吸附-脱附,XPS,NH_(3)-TPD,CO_(2)-TPD和in situ DRIFTS等表征方法对催化剂的结构和表面酸碱性进行了分析.结果表明,由Mg/Al摩尔比为5的LDH载体制备的MA5催化剂乙酸收率最高(61.8μmol·g_(cat)^(-1)·h^(-1)),且液相产物只有乙酸;其收率与催化剂中强酸量和表面Pd^(0)/(Pd^(2+)+Pd^(0))比呈正相关.催化剂的快速失活源于水滑石结构的坍塌,进而引起催化剂表面中强酸量大幅减少.原位红外漫反射光谱结果显示,CH_(4)在Pd/LDH催化剂表面解离生成CH_(x)^(*)中间体和H质子,CO_(2)直接插入CH_(x)^(*)形成CH_(x)COO^(*),之后进一步加氢生成乙酸,或CO_(2)与H质子结合生成COOH^(*)中间体,之后与CH_(x)^(*)发生偶联形成CH_(x)COOH^(*),最后再加氢得到乙酸.

The direct synthesis of acetic acid from CH_(4)and CO_(2)is an effective method for converting CH_(4)and reducing CO_(2)emissions in a 100%atom economy reaction.Hydrotalcite is widely used in CH_(4)and CO_(2)conversion reactions due to their large specific surface area,high thermal stability,good dispersion of active metals,and suitable acid-base properties,while Pd is often considered as the active center for C-C bond coupling.Herein,a series of Pd/LDH catalyst with Pd loading of 5%(mass fraction)was prepared by ion-exchange method using magnesium-aluminum hydrotalcite(MgAl-LDH)with Mg/Al molar ratios of 1.5,3,5,7 and 9 as the carrier,and applied for the direct synthesis of acetic acid from CH_(4)-CO_(2)by a two step-wise technique.The structure and surface acidity-alkalinity of the catalyst were analyzed by XRD,ICP,N2 adsorption-desorption,XPS,NH_(3)-TPD,CO_(2)-TPD and in situ DRIFTS.The results showed that the yield of acetic acid over MA5 catalyst with Mg/Al molar ratio of 5 was the highest(61.8μmol·g_(cat)^(-1)·h^(-1)),and acetic acid was the only product in the liquid phase product.Its yield was positively correlated with the amount of medium strong acid and surface Pd^(0)/(Pd^(2+)+Pd^(0))ratios.The rapid inactivation of the catalyst was due to the collapse of the hydrotalcite structure,which resulted in a significant decrease in the amount of medium strong acid.In situ DRIFTS experiments showed that CH_(4)was firstly dissociated to form CH_(x)^(*)intermediate and H proton on the catalyst surface,then CO_(2)was directly inserted into CH_(x)^(*)to form CH_(x)COO^(*),with further hydrogenated to yield acetic acid,or CO_(2)was combined with H protons to form COOH^(*)intermediates which were then coupled with CH_(x)^(*)to form CH_(x)COOH^(*),and finally hydrogenated to obtain acetic acid.