Micropore accessibility of large mordenite crystals

Large mordenite crystals are synthesized using silicic acid as a silica source. Heat treatment of the silicic acid is effective to grow large MOR crystals and the higher the heat-treatment temperature the larger the crystals. The micropore accessibility of the synthesized MOR zeolites with and without post-treatments has been examined by physicochemical characterization and the adsorption of butane isomers as probes. The lodgement of a fraction of the Na+ cations in the channels of the synthesized mordenites restricts the accessibility to the probe molecules. An oxalic acid treatment and the transformation from the starting Na-MOR samples into the H-form MOR do neither create mesopores nor change the ratio of Si to Al and the crystal size and morphology, but effectively remove this lodgement and consequently improve the accessibility. The Langmuir model appropriately describes the adsorption isotherms of butane isomers on the MOR adsorbents investigated. On the same MOR sample, the extracted saturation capacity for n-butane is larger than for isobutane, ascribed to the higher packing efficiency of the linear n-butane molecules inside the MOR channels, while the estimated heat of adsorption is slightly higher for isobutane because of the more intimate interactions between the branched isobutane and the MOR channel wall due to its larger molecular size compared to n-butane. Comparison with molecular modelling studies indicates that still ∼30% of the pore volume is inaccessible.