Aromatic Permeation through Crystalline Molecular Sieve Membranes

Abstract

The fluxes of aromatic molecules (p-xylene, o-xylene, and benzene) were measured as a function of temperature and feed partial pressure through several molecular sieve membranes (SAPO-5, SAPO-11, and mordenite) and three types of MFI membranes (silicalite-1, ZSM-5, and boron-substituted ZSM-5). Single-file diffusion appeared to control transport through the SAPO and mordenite membranes. Hence, those membranes showed ideal selectivities greater than 1 for benzene over the xylene isomers but no separation selectivities for the mixtures. Surface diffusion and activated gaseous transport were the controlling mechanisms for the MFI membranes. The highest p-xylene/o-xylene selectivities were obtained for a boron-substituted ZSM-5 membrane. At feed partial pressures of 2.1 kPa and at a temperature of 425 K, the best selectivities were 130 (ideal) and 60 (separation). Zeolite pores preferentially permeated p-xylene and took as long as 8 h to reach steady state. Nonzeolite pores preferentially permeated o-xylene after much shorter breakthrough times. Higher pressures of p-xylene distorted the membrane framework, resulting in increased o-xylene permeation and reduced selectivity. After reaching steady state, the flux of p-xylene through zeolite pores was stable for at least 10 h. The flux of o-xylene through nonzeolite pores was similarly stable at 373 K but continuously decreased for at least 12 h at 405 K.

Xylene isomerization in an extractor type catalytic membrane reactor

“…It is interesting to note this study did not provide any hint of further permeance increase at higher temperatures, as commonly quoted (gas activated transport) on different gases [8,9,[23][24][25] and also on xylenes [6,7,9,10]. This may be related to the support pore-plugging structure of the membrane, and will be detailed in another paper [26].…”

“…Recent research efforts focus on using MFI-zeolite membranes for xylene separation, a potentially more energy-efficient separation method [5][6][7][8]. The MFI zeolite pore structure consists of straight, circular pores (0.54 × 0.56 nm), interconnected with sinusoidal, elliptic pores (0.51 × 0.54 nm) [9]. These pore sizes are close to the kinetic diameter of para-xylene (d k = 0.58 nm), and it is expected that its bulkier isomers (d k = 0.68 nm) would diffuse at a slower rate, and adsorb to a lesser extent in the MFI framework due to their size and shape [6].…”

“…A maximum p-xylene flux at 400 K was said to be due to opposing effects of adsorption and diffusion. Gump et al [9] studied the fluxes of aromatic molecules (p-xylene, oxylene and benzene), through several molecular sieve membranes (SAPO-5, SAPO-11 and mordenite) as well as three types of MFI membranes (silicate-1, ZSM-5 and boron substituted ZSM-5), as a function of pressure and temperature. They found that surface diffusion and what they identified as activated gas transport were the controlling mass transfer mechanisms for MFI membranes.…”

Zeolite Catalysis

“…Several groups have studied the separation of xylenes with MFI zeolite membranes demonstrating high permselectivity for p-xylene over the other isomers, attributed mainly to the preferential permeation of p-xylene, since the zeolite pores expose sterical hindrance for the permeation of the bulkier m-and o-xylene isomer molecules [104,221,[223][224][225]. In an outstanding study, Lai et al [226] reported on dramatic improvement of the p-/o-xylene separation by b-oriented silicalite-1 membranes.…”

Synthesis of NaA zeolite membrane on a ceramic hollow fiber

“…MFI zeolite membrane showed promising performance on organic separations, e.g., butane isomers and xylene isomers [4][5][6][7][8][9][10], while X-type and Y-type zeolite membrane showed good performance on carbon dioxide separation [11] and 1,3-propanediol separation [12]. The most significant progress was achieved by the A-type zeolite membrane in the pervaporation dehydration [13][14][15][16][17].…”