Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium.
Inorg Chem. 2010 Apr 19;49(8):3573-83. doi: 10.1021/ic901814f.
Zeolites containing transition-metal ions (TMIs) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMIs Cu, Co, and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMIs preferably occupy exchange sites in six-membered oxygen rings (6MR), where the TMIs preferentially coordinate with the O atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extralattice O atoms during high-temperature pretreatments can result in autoreduction. Oxidation of reduced TMI sites often results in the formation of highly reactive oxygen species. In Cu-ZSM-5, calcination with O(2) results in the formation of a species, which was found to be a crucial intermediate in both the direct decomposition of NO and N(2)O and the selective oxidation of methane into methanol. An activated oxygen species, called alpha-O, is formed in Fe-ZSM5 and reported to be the active site in the partial oxidation of methane and benzene into methanol and phenol, respectively. However, this reactive alpha-O can only be formed with N(2)O, not with O(2). O(2)-activated Co intermediates in faujasite (FAU) zeolites can selectively oxidize alpha-pinene and epoxidize styrene. In Co-FAU, Co(III) superoxo and peroxo complexes are suggested to be the active cores, whereas in Cu and Fe-ZSM-5, various monomeric and dimeric sites have been proposed, but no consensus has been obtained. Very recently, the active site in Cu-ZSM-5 was identified as a bent Cu-O-Cu core (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 18908-18913). Overall, O(2) activation depends on the interplay of structural factors such as the type of zeolite and sizes of the channels and cages and chemical factors such as the Si/Al ratio and the nature, charge, and distribution of the charge-balancing cations. The presence of several different TMI sites hinders the direct study of the spectroscopic features of the active site. Spectroscopic techniques capable of selectively probing these sites, even if they only constitute a minor fraction of the total amount of TMI sites, are thus required. Fundamental knowledge of the geometric and electronic structures of the reactive active site can help in the design of novel selective oxidation catalysts.
沸石中含有过渡金属离子（TMIs），通常作为多相催化剂在污染减排和选择性氧化反应中表现出很有前景的活性。本文讨论了沸石中 TMIs（Cu、Co 和 Fe）的两个方面：（i）与晶格的配位；（ii）活化氧物种。在低负载下，TMIs 优先占据六元氧环（6MR）中的交换位置，在这些位置，TMIs 优先与 Al 四面体的 O 原子配位。高 TMI 负载会导致在沸石表面形成多种 TMI 物种。在高温预处理过程中去除晶格外的 O 原子会导致自还原。还原 TMI 位的氧化通常会导致形成高反应性的氧物种。在 Cu-ZSM-5 中，用 O(2) 煅烧会形成一种物种，该物种被发现是直接分解 NO 和 N(2)O 以及将甲烷选择性氧化成甲醇的关键中间体。在 Fe-ZSM5 中形成了一种称为 alpha-O 的活化氧物种，据报道它是甲烷和苯部分氧化成甲醇和苯酚的活性位点。然而，这种活性 alpha-O 只能与 N(2)O 形成，而不能与 O(2)形成。在丝光沸石（FAU）沸石中的 O(2) 活化 Co 中间体可以选择性地氧化α-蒎烯并环氧化苯乙烯。在 Co-FAU 中，建议 Co(III) 过氧和过氧配合物是活性核，而在 Cu 和 Fe-ZSM-5 中，提出了各种单体和二聚体位点，但尚未达成共识。最近，Cu-ZSM-5 中的活性位点被确定为弯曲的 Cu-O-Cu 核（Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 18908-18913）。总的来说，O(2) 的活化取决于结构因素的相互作用，例如沸石的类型以及通道和笼的大小，以及化学因素，如 Si/Al 比以及平衡电荷的阳离子的性质、电荷和分布。存在几种不同的 TMI 位点阻碍了对活性位点的光谱特征的直接研究。因此，需要能够选择性探测这些位点的光谱技术，即使它们仅构成 TMI 总位点的一小部分。对反应性活性位点的几何和电子结构的基本了解有助于设计新型选择性氧化催化剂。
