1. Introduction
Mordenite, with the IZA topology code MOR, is a naturally occurring and artificially synthesized crystalline aluminosilicate zeolite. Compared with medium-pore pentasil zeolites and small-pore CHA-type zeolites, mordenite features large-size one-dimensional straight channels and rigid framework structure, showing superior mass transfer capacity for macromolecular organic reactions and ultra-high structural stability under high-temperature and hydrothermal conditions. Different from conventional 3D interconnected pore zeolites, MOR possesses a unique hierarchical pore system composed of open 12-MR main channels and confined 8-MR side pockets, which constructs differentiated spatial confinement environments for catalytic reactions.
The inherent strong Brønsted acidity and tunable pore structure make mordenite widely applicable in industrial catalysis. In recent years, with the rapid development of fine chemical industry and clean fuel upgrading, mordenite-based catalysts have achieved breakthrough progress in dimethyl ether carbonylation, light olefin selective conversion, alkane hydroisomerization and NOₓ removal. Targeted modification strategies including acid-base treatment, heteroatom doping and metal loading further optimize its acid distribution and anti-coking performance, effectively overcoming the defects of single one-dimensional channel diffusion limitation and rapid deactivation. This review comprehensively summarizes the structure-performance relationship, mainstream modification methods and typical industrial applications of mordenite zeolite, and analyzes its future development trends.
2. Structural Characteristics and Physicochemical Properties of Mordenite
2.1 Unique MOR Topological Structure
Mordenite has a typical one-dimensional anisotropic pore structure. Its framework is composed of interconnected 4-membered rings, 5-membered rings and 6-membered rings, forming straight 12-MR main channels with a pore size of 0.65 nm × 0.70 nm, accompanied by dense 8-MR closed side pockets distributed along the main channels. The two types of pore units present completely different spatial confinement effects: the open 12-MR channels facilitate the diffusion and mass transfer of large-molecule reactants and products, while the sterically confined 8-MR side pockets serve as high-precision shape-selective catalytic microreactors.
This unique dual-pore structure is the core advantage of mordenite distinguishing from other zeolites. Many high-selectivity reactions such as dimethyl ether carbonylation and ethylene methylation selectively occur in 8-MR side pockets, while excessive cracking and oligomerization side reactions requiring large space are effectively suppressed, realizing precise regulation of product distribution.
2.2 Core Physicochemical Properties
Adjustable silicon-aluminum ratio and strong acidity: Mordenite has a wide Si/Al adjustable range. Low-silicon mordenite possesses abundant concentrated strong Brønsted acid sites, which are far stronger than those of ZSM-5 and SAPO-34, providing efficient active centers for acid-catalyzed reactions. High-silicon mordenite exhibits moderate acidity and excellent hydrophobicity, suitable for high-selectivity organic synthesis reactions.
Excellent structural stability: The rigid MOR framework has ultra-high thermal stability, which can maintain complete crystal structure above 850 °C. It also shows outstanding resistance to high-temperature hydrothermal aging and structural collapse, adapting to harsh industrial reaction conditions.
Limitations of one-dimensional channels: The single straight channel structure leads to unidirectional molecular diffusion. Coke precursors and impurity deposits easily block pore channels, resulting in reduced mass transfer efficiency and accelerated catalyst deactivation, which is the main structural bottleneck restricting its long-cycle service life.
3. Common Modification Strategies for Mordenite Performance Optimization
3.1 Acid-Base Composite Post-Treatment
Alkali treatment can selectively etch the partial framework of mordenite, construct abundant intracrystalline mesopores, build hierarchical micro-mesopore structures, and effectively alleviate the diffusion limitation of one-dimensional channels. Acid washing can remove extra-framework aluminum impurities, optimize the distribution of framework aluminum, eliminate invalid strong acid sites, and precisely regulate the density and strength of effective Brønsted acid sites. The composite acid-base modified mordenite balances structural accessibility and acid site matching, significantly improving catalytic stability and anti-coking ability.
3.2 Metal Loading and Bifunctional Modification
Noble metals (Pt, Pd) and non-noble metals (Ni, W, Ce, La) are widely loaded on mordenite to construct bifunctional catalysts. Metal active sites undertake dehydrogenation-hydrogenation and redox reactions, while zeolite acid sites promote carbocation formation and skeletal rearrangement. The synergistic effect between metal and acid sites greatly improves the efficiency of hydroisomerization and hydrogenation upgrading reactions. Rare earth doping can further enhance the framework hydrothermal stability and anti-poisoning performance of mordenite.
3.3 Spatial Acid Site Regulation
Selective passivation of acid sites in 12-MR main channels while retaining active acid sites in 8-MR side pockets is a unique modification strategy for mordenite. Precisely regulating the spatial distribution of acid sites can maximize the shape-selective effect of confined pockets, achieve ultra-high selectivity of target products in carbonylation and olefin conversion reactions, and effectively inhibit invalid side reactions.
