Abstract
Mordenite (MOR) is a typical one-dimensional large-pore microporous zeolite with unique twelve-membered-ring straight channels and abundant strong Brønsted acid sites, which has been regarded as the most commercialized molecular sieve for C8 aromatic isomerization. Compared with BEA, MFI and FAU zeolites, H-type MOR zeolite possesses appropriate pore confinement effect, adjustable acidity and excellent thermal stability, which can effectively promote ethylbenzene conversion and xylene skeletal rearrangement, while inhibiting excessive aromatic cracking and heavy coke formation. This article focuses on the structural advantages, acidic regulation, catalytic mechanism, modification strategies and industrial application of MOR zeolite in xylene isomerization. Modified Pt/H-MOR bifunctional catalysts exhibit high para-xylene selectivity, low side reaction rate and long catalytic lifetime, which are widely applied in petrochemical aromatic production plants. The limitations and future optimization directions of MOR-based isomerization catalysts are also summarized in this paper.
Keywords: Mordenite; MOR zeolite; xylene isomerization; C8 aromatics; bifunctional catalyst; skeletal isomerization
1. Introduction
Para-xylene (PX) is an essential raw material for polyester, pharmaceutical and fine chemical industries, which is mainly produced via C8 aromatic isomerization in petrochemical enterprises. The C8 mixed aromatic fraction consists of para-xylene, meta-xylene, ortho-xylene and ethylbenzene (EB), and efficient catalytic isomerization is the core technology to break thermodynamic equilibrium and upgrade low-value xylene isomers.
Zeolite solid acid catalysts have replaced traditional corrosive liquid acid catalysts due to green performance and recyclability. As a classical MOR-topology zeolite, hydrogen-form mordenite (H-MOR) has irreplaceable superiority in aromatic isomerization. Different from cross-connected pore zeolites, MOR has regular one-dimensional straight 12-membered channels, matching the molecular kinetic diameter of C8 aromatic hydrocarbons perfectly. At present, noble metal modified Pt/H-MOR bifunctional catalysts have become the mainstream catalyst for low-temperature xylene isomerization industrial units worldwide.
2. Structural Characteristics of MOR Zeolite
2.1 Pore Structure Feature
Mordenite belongs to orthorhombic crystal system with typical MOR topological framework. Its pore structure is divided into two parts:
(1) Main channel: One-dimensional straight twelve-membered ring pore, pore size 0.65 nm × 0.70 nm, which allows free diffusion of xylene and ethylbenzene molecules;
(2) Side pocket: Small eight-membered recessed cavity with aperture of 0.28 nm, which limits macromolecular polycyclic aromatics generation and reduces coking precursors.
Industrial MOR zeolite has a specific surface area of 420–500 m²/g, micropore volume of 0.20–0.25 cm³/g. The non-interlaced straight channel greatly reduces diffusion resistance of aromatic isomers, avoiding molecular shape limitation existing in ZSM-5 medium-pore zeolite.
2.2 Acidic Properties for Isomerization
H-MOR zeolite owns high-density strong Brønsted acid sites generated by framework Si-OH-Al groups, which provide active sites for carbocation formation—the key step of aromatic skeletal isomerization. Moderate Lewis acid sites derived from extra-framework aluminum can promote ethylbenzene dehydrogenation-isomerization reaction. Compared with H-Beta and HY zeolite, H-MOR has moderate acid strength and narrow acid distribution, which effectively suppresses bimolecular cracking, transalkylation and aromatic disproportionation side reactions during C8 isomerization.
The framework SiO₂/Al₂O₃ ratio of industrial isomerization-used MOR is controlled at 15–25. This silicon-aluminum ratio balances catalytic activity, hydrothermal stability and anti-coking performance optimally.
3. Catalytic Mechanism of MOR for Xylene Isomerization
Industrial xylene isomerization adopts metal-acid bifunctional reaction mechanism over Pt/H-MOR catalyst under hydrogen atmosphere:
First, metallic Pt sites realize reversible dehydrogenation-hydrogenation: ethylbenzene is dehydrogenated to styrene, and xylene is transformed into cyclic olefin intermediate;
Second, strong B acid sites on H-MOR protonate cyclic olefins to form aromatic carbocations, realizing intramolecular methyl skeletal rearrangement;
Finally, the rearranged carbocation is deprotonated and hydrogenated to form equilibrium xylene mixtures, increasing the proportion of high-value para-xylene.
Benefiting from side pocket confinement, MOR inhibits bimolecular transalkylation, so the yield of C7/C9 by-products is lower than other acidic zeolites.
