Fabrication and characterization of polycrystalline MFI membranes on coarse macroporous supports for the separation of butane isomers

Abstract

Separation of butane isomers holds substantial importance across various industrial sectors due to their divergent chemical characteristics and pervasive applications. Their efficient separation into high-purity iso-butane (i-C₄) and normal butane (n-C₄) allows their optimal utilization while maximizing economic benefit and minimizing waste, though their separation presents significant challenges due to their similar physicochemical properties and energy-intensive separation requirements. Polycrystalline zeolite-based membrane separation processes emerge as an energy-efficient approach for separating butane isomers by reducing the total operating energy consumption as compared to the conventional distillation process. However, the fabrication cost of polycrystalline zeolite membranes is high, predominantly dictated by the production cost of the support (≈ 50-70 %) rather than the polycrystalline zeolite layer. Herein, we report the fabrication of polycrystalline MFI membranes on inexpensive coarse macroporous α-Al₂O₃ tube supports (symmetrical supports) with an average pore size in the range of 2 ∼ 3μm for separating butane isomers. Macroporous α-Al₂O₃ tube supports are coated with single (sheet-like MFI crystals) and dual (sheet-like MFI crystals + MFI crystals) seed layers to achieve the desired separation performances. For coating a single seed layer, MFI nanocrystals were employed as precursors for synthesizing sheet-like MFI crystals with a high aspect ratio as compared to the conventional MFI crystals. Taking advantage of the high aspect ratio of sheet-like MFI crystals, single seed layer was fabricated on coarse macroporous α-Al₂O₃ tube support, eliminating the time-intensive steps to reduce the support pore size and reducing the problem of pore plugging. Subsequently, a dual seed layer (sheet-like MFI crystals + MFI crystals) was also created by hot dip coating the MFI crystals on the surface of a single seed layer with the aim of fabricating a more compact, continuous, and molecular sieving polycrystalline MFI membrane with enhanced separation performance for the separation butane isomers. After hydrothermal crystallization, polycrystalline MFI membrane (M-2) fabricated on a single seed layer exhibits a separation factor and n-butane permeance of 33.80 and 2.00x10^-7 mol/m².Pa.s, respectively. In contrast, the polycrystalline MFI membrane (D-3) formed on a dual seed layer demonstrates a separation factor and n-butane permanence of 40.60 and 1.47x10^-7 mol/m².Pa.s, respectively. Both these membranes demonstrate good stability and reproducibility. Fabrication of polycrystalline MFI membranes on inexpensive coarse macroporous tube supports is a crucial advancement in reducing fabrication costs and facilitating industrial implementation. However, substantial potential remains for further scaling up and enhancing the separation performance of current polycrystalline MFI membranes.