Haw James F, Song Weiguo, Marcus David M, Nicholas John B
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, USA.
Acc Chem Res. 2003 May;36(5):317-26. doi: 10.1021/ar020006o.
The process of converting methanol to hydrocarbons on the aluminosilicate zeolite HZSM-5 was originally developed as a route from natural gas to synthetic gasoline. Using other microporous catalysts that are selective for light olefins, methanol-to-olefin (MTO) catalysis may soon become central to the conversion of natural gas to polyolefins. The mechanism of methanol conversion proved to be an intellectually challenging problem; 25 years of fundamental study produced at least 20 distinct mechanisms, but most did not account for either the primary products or a kinetic induction period. Recent experimental and theoretical work has firmly established that methanol and dimethyl ether react on cyclic organic species contained in the cages or channels of the inorganic host. These organic reaction centers act as scaffolds for the assembly of light olefins so as to avoid the high high-energy intermediates required by all "direct" mechanisms. The rate of formation of the initial reaction centers, and hence the duration of the kinetic induction period, can be governed by impurity species. Secondary reactions of primary olefin products strongly reflect the topology and acid strength of the microporous catalyst. Reaction centers form continuously through some secondary pathways, and they age into polycyclic aromatic hydrocarbons, eventually deactivating the catalyst. It proves useful to consider each cage (or channel) with its included organic and inorganic species as a supramolecule that can react to form various species. This view allows us to identify structure-activity and structure selectivity relationships and to modify the catalyst with degrees of freedom that are more reminiscent of homogeneous catalysis than heterogeneous catalysis.
在硅铝酸盐沸石HZSM-5上将甲醇转化为烃类的过程最初是作为一种从天然气制取合成汽油的途径而开发的。使用对轻质烯烃具有选择性的其他微孔催化剂，甲醇制烯烃（MTO）催化可能很快会成为天然气转化为聚烯烃的核心。甲醇转化的机理被证明是一个极具智力挑战性的问题；25年的基础研究产生了至少20种不同的机理，但大多数都无法解释主要产物或动力学诱导期。最近的实验和理论研究已经确凿地证实，甲醇和二甲醚在无机主体的笼或通道中所含的环状有机物种上发生反应。这些有机反应中心充当轻质烯烃组装的支架，从而避免了所有“直接”机理所需的高能量中间体。初始反应中心的形成速率，进而动力学诱导期的持续时间，可以由杂质物种控制。主要烯烃产物的二次反应强烈反映了微孔催化剂的拓扑结构和酸强度。反应中心通过一些二次途径持续形成，并且它们会老化成多环芳烃，最终使催化剂失活。将每个包含有机和无机物种的笼（或通道）视为一个可以反应形成各种物种的超分子是很有用的。这种观点使我们能够识别结构 - 活性和结构选择性关系，并以比多相催化更类似于均相催化的自由度来修饰催化剂。
