اثرات حلالها بر اصلاح زئولیت SAPO-34 با استفاده از ۳-آمینوپروپیل تریدتروفسی سیلان برای تهیه غشای ماتریس مخلوط پلی سولفون نامتقارن در استفاده از جداسازی CO2 The effects of solvents on the modification of SAPO-34 zeolite using 3-aminopropyl trimethoxy silane for the preparation of asymmetric polysulfone mixed matrix membrane in the application of CO2 separation

Zeolites are inorganic aluminosilicate materials which have shown great potential in membrane gas separation due to their well-defined repeating pore structures. The accurate size and shape discrimination resulted from a narrow pore distribution ensures the superior selectivity in gas separation [1]. SAPO-34 zeolite is particularly suitable for gas separation because its chabazite (CHA) structure has a 0.38 nm framework pore size; which is nearly similar to the molecular sizes of various gases, especially in a natural gas application. Thus, SAPO-34 zeolite has been added as the selective fillers in mixed matrix membranes (MMMs) for carbon dioxide (CO2) separation [2,3]. In general, MMMs composed of polymer as the continuous phase and inorganic filler as the dispersed phase. MMMs exhibit the combined superior properties of both inorganic fillers and polymer to achieve excellent gas separation performance [4]. However, non-selective interfacial voids form in the MMMs due to the poor compatibility between the polymer and inorganic fillers, resulting profound impact on the separation performance [5]. Silane coupling agents were commonly proposed to improve the compatibility of zeolite in polymeric matrix. Silane coupling agents are silicon-based chemicals which contain two types of reactive groups, namely inorganic and organic groups in the same molecule. The general formula of a silane coupling agent is R-(CH2)n-Si-X3 in which X is a hydrolyzable group, such as methoxy, ethoxy or acetoxy, while R is an organo functional group, such as amino, methacryloxy and epoxy. After grafting on the zeolite, the silane coupling agent acts as an effective interface, which is a complex interaction of chemical and physical factors, such as concentration gradient, expansion coefficient and adhesion. The unique physical and chemical properties of silane coupling agent can even prevent debonding during composite aging [6]. Pechar et al. [7] reported the use of 3-aminopropyltrimethoxysilane (APTMS) to modify ZSM-2 zeolite before blending into polyimide membranes. Although SEM and TEM micrographs showed the absence of voids, the MMMs showed poor performance in terms of CO2 selectivity and permeability due to the pore blockage on ZSM-2 zeolite [8]. Pechar and co-workers further used silanated zeolite L filler modified with 3-aminopropyltriethoxysilane (APTES) to prepared polyimide MMMs. Both CO2 selectivity and permeability of the modified MMM dropped relatively to the neat membrane. The reduction of CO2 adsorption indicated the zeolite pores was also partially blocked by APTES. Instead of using toluene as the solvent in the silane modification, Ismail et al. [6] modified zeolite 4A using APTES in ethanol. Besides the good compatibility between the modified zeolite and polyethersulfone (PES) membrane was observed, a great improvement in the CO2 selectivity was achieved. However, the CO2 permeability was reduced nearly 80% compared to the neat PES membrane. The reduction of permeability can only be explained by the polymer rigidification [9]. In the preparation of PES MMMs, Li et al. [10] modified zeolite 3A, 4A and 5A using 3-aminopropylmethyldiethoxy silane (APMDES) in toluene. The modification reduced the rigidification of polymer chain and partial pore blockage which induced by the polymer matrix. As a result, both of the selectivity and permeability of CO2 via MMM blended with the modified zeolite were higher than those MMMs containing zeolite without the modification. Similar improvement in the gas separation was reported by Hillock et al. [9] who fabricated MMMs using SSZ-13 zeolite modified with (3-aminopropyl)-dimethyl-ethoxysilane (APDMES) in toluene as well. A group of researchers from Laval University [11], also synthesized 6FDA-ODA MMMs using intergrowth Faujasite and EMC-2 (FAU/EMT) zeolite grafted with APMDES. They reported that MMMs with 25 wt.% of zeolite grafted at optimum grafting condition (85 C, 24 h and 0.35 mmol APMDES/ml isopropanol (IPA)) exhibited great improvement in both CO2 selectivity and permeability compared to the neat polyimide membrane. However, modified MMMs possessed lower permeability, but higher selectivity as compared to unmodified MMM. Increasing the amount of APMDES chained on zeolite structure resulted rigidified interfacial phase [11]. Additionally, the reduction of surface area and micropore volume of the zeolites suggested the zeolite pores were also partially blocked by the silane coupling agent. In their subsequent study [5], FAU/EMT zeolite was grafted with APTES, APMDES and APDMES in solvents with varied polarity. The result revealed that all the modified MMMs, especially those MMMs containing modified zeolite using APTES with IPA as the grafting solvent, possessed higher CO2 selectivity and permeability compared to the unmodified MMMs and neat polymeric membranes. Besides improving filler/polymer interface, the grafting reaction further prevented the filler sedimentation, yielding improved homogenous dispersion of zeolite particles in polymeric matrix. The employment of polar solvent, such as IPA, in grafting reaction could generate more uniform distribution of grafting sites by increasing mobility of the aminosilane molecules. In addition, BET surface area was not much affected by changes on the amount of grafted aminosilane. To our best knowledge, not much research work focuses on the silane modification of SAPO zeolites to prepare MMMs for CO2 separation. Based on the detailed literature review, modification of SAPO zeolite using silane has a great potential to further enhance the CO2 separation of MMMs. In this work, 3-aminopropyl trimethoxy silane (APMS) was selected as silane coupling agent because methoxy silanes are more readily in hydrolysis reaction compared to ethoxy groups [12]. In addition, the effects of IPA and ethanol as the solvent for silane grafting were compared in order to achieve satisfactory CO2 separation.