Abstract
Mordenite (framework type MOR), a typical high-silica microporous aluminosilicate zeolite, possesses intrinsic one-dimensional hierarchical pore channels, excellent thermal stability, tunable Brønsted and Lewis acidity, and outstanding chemical corrosion resistance. As one of the earliest commercialized zeolites, natural and synthetic mordenite have been widely deployed in petrochemical catalysis, gas separation, environmental remediation and fine chemical synthesis. This paper systematically summarizes the crystal framework structure, classic synthetic routes, mainstream modification strategies, structure-performance correlation, and state-of-the-art industrial applications of mordenite. Meanwhile, this review analyzes the bottlenecks of conventional mordenite synthesis including high organic template cost, crystal diffusion limitation and environmental pollution, and prospects the development direction of low-cost, green organic-free synthesis and hierarchical porous mordenite for high-efficiency catalytic reactions. This work aims to provide theoretical reference for the rational design and scalable preparation of high-performance mordenite materials.
Keywords: Mordenite; Microporous Zeolite; Hydrothermal Synthesis; Hierarchical Modification; Acid Catalysis; Gas Adsorption
1. Introduction
Zeolites are crystalline microporous inorganic materials built by corner-sharing TO tetrahedra (T=Si, Al), featuring regular pore structures, large specific surface area and adjustable active sites. Mordenite, firstly discovered as natural mineral deposits in 1864 in Morden, Canada, is classified as a medium-to-high silica zeolite with fixed MOR topological framework authorized by the International Zeolite Association (IZA). Different from FAU, ZSM-5 and LTA zeolites, mordenite owns anisotropic pore systems with interconnected 12-membered ring (12-MR) main channels and 8-membered ring (8-MR) side pockets, which endows it with shape-selective catalytic properties distinct from other porous materials.
The ideal anhydrous chemical formula of sodium-type mordenite is NaAlSiO, and its common hydrated formula is NaAlSiO·nHO. The intrinsic Si/Al molar ratio of pristine mordenite ranges from 5 to 10, and the ratio can be further elevated to over 40 via post-synthetic dealumination treatment. Benefiting from high silica composition, mordenite can withstand thermal treatment above 600 °C and resist acidic and alkaline reaction media, which greatly expands its service life in harsh industrial reaction conditions. In recent decades, with the upgrading of green chemical processes, organic-free synthesis, waste-derived synthesis and pore structure regulation of mordenite have become core research hotspots in porous material chemistry.
2. Crystal Structure and Fundamental Physicochemical Properties of Mordenite
2.1 Framework Pore Structure
Mordenite adopts orthorhombic crystal system with Cmcm space group. Its framework is constructed by four-ring and five-ring aluminosilicate secondary building units (SBUs), forming two sets of independent pore channels: (1) Straight 12-MR main channels along the [001] direction with pore size of 6.5 Å × 7.0 Å, serving as the main mass transfer pathway for reactant and product molecules; (2) Torsional 8-MR side pockets along the [010] direction with narrow aperture of 2.6 Å × 5.7 Å, which are confined microcavities hosting most strong Brønsted acid sites. Notably, the 8-MR pockets are not interconnected with each other, leading to obvious intracrystalline diffusion resistance for macromolecular reactants, which is the major structural defect limiting the catalytic efficiency of conventional bulk mordenite crystals.
2.2 Core Physicochemical Properties
1) Acidity: Isomorphous substitution of Si by Al generates negative framework charges, balanced by exchangeable cations (Na, H, NH). Hydrogen-type mordenite (H-MOR) presents strong Brønsted acidity originating from bridging hydroxyls near framework Al atoms, and weak Lewis acidity from extra-framework aluminum species. The density, strength and distribution of acid sites can be precisely regulated by ion exchange, dealumination and metal doping.
2) Thermal and hydrothermal stability: High-silica mordenite maintains complete crystal framework after calcination at 650–700 °C. Under high-temperature steam atmosphere (400–550 °C), dealuminated mordenite still retains intact topological structure, superior to Y-type zeolite in hydrothermal stability.
3) Ion exchange performance: The extra-framework alkali metal cations in mordenite are reversible exchangeable, enabling functional modification via transition metal (Cu, Fe, Pt) and rare earth metal ion exchange for targeted catalytic reactions.
3. Main Synthetic Methods of Mordenite
3.1 Conventional Hydrothermal Synthesis
Hydrothermal synthesis is the most mature industrial preparation method for synthetic mordenite, which takes soluble silicon sources (silica sol, sodium silicate) and aluminum sources (sodium aluminate, aluminum nitrate) as raw materials, alkali hydroxide as mineralizer, and organic amines as structure-directing agents (SDAs). Tetraethylammonium cation (TEA) is the most widely used template for high-crystallinity mordenite synthesis, which can stabilize MOR framework and inhibit the generation of miscellaneous zeolite phases such as analcime and clinoptilolite. The typical reaction conditions are 150–200 °C autogenous pressure, 24–72 h crystallization time, and initial gel pH value controlled at 11–13. Nevertheless, organic TEA templates have obvious drawbacks: high raw material cost, toxic decomposition gas during calcination, and high chemical oxygen demand (COD) of synthetic wastewater, which fails to meet green manufacturing standards.
3.2 Green Organic-Free Synthesis
To eliminate organic template pollution, template-free hydrothermal synthesis has been developed in recent years, relying on alkalinity regulation and seed-induced crystallization to prepare pure-phase mordenite. Adding trace mordenite seed crystals can shorten crystallization time by 30%–50% and reduce reaction temperature to 120–160 °C. This route only uses inorganic sodium salts as mineralizers, without organic additives, which reduces production cost and calcination pollution, and has been applied in pilot-scale production of low-silica mordenite.
3.3 Solid-State Solvent-Free Synthesis
Solvent-free mechanochemical synthesis is an emerging low-carbon preparation technology. Solid silicon-aluminum raw materials, solid alkali and seed crystals are mixed and ground uniformly, then crystallized in a dry oven without solvent water. This method minimizes wastewater discharge, improves single-kettle yield, and effectively controls crystal grain size. The obtained solvent-free mordenite possesses richer micropore defects and higher specific surface area (320–350 m/g) than hydrothermal samples, showing better adsorption performance.
