合成气制乙醇Rh-Nb2O5/SiO2催化剂中的SMPI和助催剂作用本质的研究

使用负载型催化剂由合成气制乙醇是一碳化学研究的一个重要发展。本实验室前已报道用化学反应法检验出Rh-Nb_2O_5/SiO_2表面上除了铑位外同时还存在氧化铌位。本文为兰部分工作所组成: (1)进一步用氢还原过的Nb_2O_5/SiO_2催化乙炔聚合成聚乙炔的化学方法推断Rh-Nb_2O_5/SiO_2上可能存在着Nb—H键。(2)用FTIR法检测上述催化剂的红外光谱吸收带, 1740(w) cm~(-1)为V_(Rh-H), 1560 cm~(-1)(broad, m)、1269 cm~(-1)(s)为与Nb—H有关的吸收, 后者可能属于桥式Rh—H—Nb的吸收。(3)XPS检测出合成气处理的Rh-Nb_2O_5/SiO_2表面上存在Rh~0、Rh~Ⅰ、Nb~Ⅴ、Nb~Ⅳ和两种不同的沉积碳。根据这些结果, 提出活性中心可能为A(参见正文图3)简写为B(见图3), CO转化的主要途径是而后C—O还原断裂生成=CH_2***, 再与CO偶联为CH_2=C=O, 最后还原为乙醇或乙醛。根据实验结果对本体系催化剂中SMPI和助催剂作用的实质作了讨论。

STUDIES ON THE NATURE OF SMPI AND PROMOTER ACTION IN Rh-Nb2O5/SiO2 CATALYSTS FOR SYNGAS CONVERSION TO ETHANOL

Syngas conversion to ethanol by supported catalysts is an important progress in C_1 chemistry. It has been reported by our laboratory that Rh sites and niobia sites coexisted on Rh-Nb_2O_5/SiO_2 catalysts as detected by means of reaction chemi- stry involving the catalytic aromatization and polymerization of acetylene on such catalysts. This paper reports a further study consisting of the following three parts: (1)The probable existence of Nb-H bonds on hydrogen-reduced Rh-Nb_2O_5/ SiO_2 was deduced from the catalytic activity toward polymerization of acetylene to polyacetylene and the loss of catalytic activity toward cyclotrimerization of acetylene of such catalysts. (2) The FTIR spectra of the above mentioned catalysts were studied. The peak at 1740 cm~(-1) (vw) may be assigned to v_(Ph-H), that at 1560 cm~(-1) (m, broad) to v_(Nh-H), while that at 1269 cm~(-1) (s) most probably to a bridging species, Rh—H—Nb. (3) The existence of Ph~0, Rh~Ⅰ, Nb~Ⅴ, Nb~Ⅳ and two types of carboneceous deposits on syngas treated Rh-Nb_2O5/SiO_2 catalysts was detected by XPS; A model of active center, A (cf. Fig.3) (abbr. B), formed by partial reduction of niobia through hydrogen spillover and “wetting” or partial coating of the surfaces of rhodium particles (Rh_x~0Rh_y~I) by the partially reduced niobia, has been proposed. By analogy with known organometallic chemistry, the major reaction pathway might involve migratory insertion of chemisorbed species C to form E, hydrogenation to F cis-coupling with CO to form coordinated ketene, and further hydrogenation to ethanol or aldehyde with simultaneous regeneration of the active site by hydrogenation and elimination of H_2O; while methane CH_4 was produced by hydrogenation of the coordinated carbene CH_2 in a secondary reaction pathway. The true nature of SMPI in the present system has also been discussed.