High acid strength of H-MOR for cracking heavy hydrocarbons
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High Acid Strength of H-MOR for Cracking Heavy Hydrocarbons
H-MOR (proton-form mordenite, framework type MOR) is a widely used solid acid catalyst in petrochemical and refining processes, particularly valued for its strong Brønsted acidity and one-dimensional 12-membered ring pore system, which make it highly effective for the cracking of heavy hydrocarbons—including long-chain alkanes, cycloalkanes, and even some polyaromatics found in vacuum gas oil (VGO) or deasphalted oils.
1. Origin of High Acid Strength in H-MOR
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Framework Composition:
H-MOR is an aluminosilicate zeolite where tetrahedral aluminum (Al³⁺) substitutes for silicon in the framework. Each Al site generates a charge-balancing proton (H⁺), forming a bridging Si–OH–Al Brønsted acid site.
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Stronger Acidity than Many Zeolites:
Compared to FAU (Y zeolite) or BEA (beta), H-MOR exhibits higher intrinsic Brønsted acid strength, attributed to:
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The local geometry and bond angles around the Al site in the MOR framework.
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Lower framework dielectric screening, leading to less stabilization of the conjugate base and thus a more acidic proton.
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A relatively high Si/Al ratio (often 5–20 in commercial forms), which reduces Al–Al interactions and enhances acid strength per site (in line with the “next-nearest-neighbor” model).
2. Structural Features Enabling Heavy Hydrocarbon Cracking
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Pore Architecture:
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Main 12-membered ring channels: ～6.5 × 7.0 Å (0.65 × 0.70 nm) along the c-axis—large enough to admit bulky molecules like C₂₀⁺ alkanes or alkylated aromatics.
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Side pockets (8-membered rings, ～2.6 × 5.7 Å) are inaccessible to large molecules but host additional acid sites that contribute to overall acidity.
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Confinement Effect:
The narrow, tubular pores enhance van der Waals interactions between hydrocarbon chains and the pore walls, stabilizing carbocationic transition states and lowering activation barriers for C–C bond scission.
3. Catalytic Advantages in Heavy Feedstock Processing
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Efficient C–C Bond Cleavage:
Strong Brønsted acid sites readily protonate alkanes (via σ-bond protonation) or alkenes (to form carbenium ions), initiating β-scission—the key step in catalytic cracking.
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Resistance to Nitrogen Poisoning:
H-MOR shows better tolerance to basic nitrogen compounds (e.g., quinoline) than Y zeolite, partly due to its stronger acid sites that can compete more effectively for protonation.
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Selectivity to Light Olefins and Middle Distillates:
Under optimized conditions, H-MOR favors selective cracking of long chains into valuable C₃–C₅ olefins and naphtha-range products.
