Mordenite adopts MOR topology constructed from 5-membered ring secondary building units. Its framework consists of large one-dimensional 12-membered-ring main channels with aperture of 0.65 nm × 0.70 nm, interconnected with smaller 8-membered-ring side pockets (0.26 nm × 0.57 nm). These side pockets are isolated and do not form continuous crossing channels, making mordenite essentially a 1D large-pore zeolite.
The simplified unit cell formula is \(\ce{|Na_{8}(H_{2}O)_{24}|[Al_{8}Si_{40}O_{96}]-MOR}\). The straight 12-MR channels allow diffusion of relatively large organic molecules, while the tiny 8-MR side pockets create confined microenvironments for selective reactions. The one-dimensional pore system also brings drawbacks: pore mouth blockage by coke easily occurs during reactions, which restricts mass transfer and accelerates catalyst deactivation. High-crystallinity mordenite has fewer lattice defects and better framework integrity under high-temperature reaction-regeneration cycles.
Framework Al atoms embedded in the MOR skeleton create negative framework charges. After ammonium exchange and calcination, H-mordenite generates abundant Brønsted acid sites. Mordenite generally exhibits strong Brønsted acidity compared with many other zeolites.
The Si/Al ratio of mordenite can be tuned via hydrothermal synthesis or post-synthetic dealumination. Lower Si/Al mordenite possesses high acid density; upon dealumination, partial framework Al is removed, reducing acid density while improving hydrothermal stability. Extra-framework aluminum species formed during steaming contribute Lewis acid sites.
Acid site distribution in MOR is non-uniform. Some acid sites reside inside 12-MR main channels, while others locate within the 8-MR side pockets. Acid sites in side pockets show stronger confinement and different catalytic selectivity. Excess strong acid sites promote cracking and coke formation, which is the primary cause of rapid deactivation for unmodified mordenite.
Mordenite is a benchmark catalyst for DME carbonylation to methyl acetate, a key step in syngas-to-ethanol routes. The 8-membered-ring side pockets of MOR provide the confined space required for selective carbonylation, suppressing side cracking reactions. This unique confinement effect makes mordenite superior to many other zeolites for this specific C1 conversion.
H-mordenite modified with noble metals forms bifunctional catalysts for n-alkane hydroisomerization. It is applied in light alkane isomerization to produce high-octane branched paraffins. Compared with ZSM‑22, mordenite’s larger 12-MR channels favor bulkier branched isomer products, but it is more prone to coking.
Mordenite catalyzes aromatic alkylation, transalkylation and disproportionation. Its large 12-MR pores can accommodate substituted aromatic molecules. It is also used for benzene alkylation and xylene isomerization in some refinery processes.
Na-type mordenite can be used for gas drying, selective adsorption of ammonia and VOC removal. It also shows selective separation performance for small gas molecules after ion exchange modification.
MOR-type mordenite combines straight 12-membered-ring main channels and isolated 8-membered-ring side pockets, accompanied by strong tunable Brønsted acidity. The unique confined environment inside the side pockets endows mordenite with outstanding selectivity for DME carbonylation. However, its intrinsic one-dimensional pore network is susceptible to pore plugging by carbon deposition. Rational acidity adjustment and hierarchical pore engineering are effective ways to improve mass transfer and prolong catalyst lifetime. Mordenite remains an indispensable zeolite in C1 chemical synthesis, hydrocarbon isomerization and aromatic conversion industries.
