(604s) Synthesis of AlPO4-18 Membrane for Water/Acetic Acid Separation

Recently, it has been increasing attention of membrane separation technologies for saving energy consumption in chemical industries, especially distillation processes. Acetic acid is used for acetate compounds production, which is an important intermediate for chemical industries. The volatility of acetic acid has very close with that of water, especially at high acetic acid concentrations. Therefore, a large energy is required for separation of water/acetic acid mixture by distillation. Many researchers have reported on the separation of water from water/acetic acid mixture using organic polymer or inorganic membranes. Inorganic membranes have some advantages, such as heat and chemical resistance and mechanical strength, compared with organic polymer membranes. Zeolite membranes categorized in one of the inorganic membranes have uniform micropores and specific adsorption properties, and these unique characters are different from those of other inorganic membranes. Microporous aluminophosphates (AlPO_n) are categorized with zeolite, and among them AlPO₄-18 has the AEI framework topology. Its framework possesses three dimensional channels with 8-membered oxygen rings. Though there have been few reports on AlPO₄-18 powder synthesis, AlPO₄-18 membrane synthesis for separation was hardly reported. AlPO₄-18 membrane for CO₂/CH₄ separation was studied by Carreon et al [1]. They successfully demonstrated the membrane permeation properties with 6.6 ´ 10^-8 mol m^-2 s^-1 Pa^-1 of CO₂ permeance, and 52-60 of CO₂/CH₄ selectivity at 295 K. While water permeation properties for acetic acid purification for AlPO₄-18 membrane has not been reported yet, AlPO₄ is anticipated to be water-selective membrane since this types of materials shows strong hydrophilicity.  We investigated AlPO₄-18 membrane synthesis and membrane permeation properties for water separation from water/acetic acid mixture.

We employed seed-assisted hydrothermal synthesis for AlPO₄-18 membrane on the outer surface of a-alumina tubular support. This a-alumina support had 30 mm in length, 10 mm of outer diameter and 7.0 mm of inner diameter. The AlPO₄-18 seed crystals were coated on the a-alumina support by a dip-coating method. The seed-assisted hydrothermal synthesis was carried out. Synthesis time ranged from 0 hour to 48 h at 453 K. The molar composition of the synthesis gel for the seed crystals was Al₂O₃ : P₂O₅ : 1.8DIPEA : 50H₂O [2], and that for the membrane was Al₂O₃ : P₂O₅ : 1.8DIPEA : 100H₂O. These synthesis gels were prepared by using Al(OH)₃, H₃PO₄, N,N-diisopropylethylamine (DIPEA) and distilled water at room temperature. After the membrane synthesis, the outer surface and cross-section of the membrane was observed by scanning electron microscope (FE-SEM), and the crystal structure was identified by X-ray diffraction (XRD). In addition, the membrane water permeation property was evaluated by vapor permeation tests for water/acetic acid mixture at 398 K. The X-ray reflection peaks corresponding to AEI structure were observed after the synthesis period of 1.5 h. The relative intensities calculated with the peaks for AEI structure and a-alumina support rapidly increased with prolonged synthesis from 1.5 to 6 h.  Growth of membrane continued slowly after 6 h of synthesis. According to the observations with FE-SEM, AlPO₄-18 crystals was identified after 1.5 h of synthesis. The membrane thickness was also rapidly increased from 1.5 (1.3 mm) to 6 h (6.2 mm).

The total membrane weight was rapidly increased up to 6 h. The membrane weight formed on the support surface and that inside the support pores was also rapidly increased with synthesis period (0 - 6 h). After 6 h of synthesis, the membrane weight formed on the support surface was increased slowly, but the membrane weight formed inside the pores of support was almost the same regardless of synthesis period. Based on these results, we consider that AlPO₄-18 membrane formed by way of following two steps. From 0 to 6 h of synthesis, AlPO₄-18 crystals grew on the support surface to cover there and micro-crystals was formed inside the support pores.

We evaluated water permeation properties by vapor permeation tests using AlPO₄-18 membranes for the water/acetic acid mixture (50/50 kPa) at 398 K. In this study, the membrane of 4 h-synthesis had excellent water permeance of ca. 1.0x10^-6 mol m^-2 s^-1 Pa^-1 and water/acetic acid separation factor of ca. 300.

References

[1] M.L. Carreon et al, Chem. Commun. 48 (2012), 2310-2312.

[2] J. Chen et al, Catal. Lett. 28 (1994), 241-248.