Enhanced Stability of MFI Zeolite Membranes for Separation of

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Enhanced stability of MFI zeolite membranes for separation of ethanol/water by eliminating surface Si-OH groups Zhengqi Wu, Chun Zhang, Li Peng, Xuerui Wang, Qingqing Kong, and Xuehong Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17191 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Applied Materials & Interfaces

Enhanced stability of MFI zeolite membranes for separation of ethanol/water by eliminating surface Si-OH groups Zhengqi Wu‡, Chun Zhang‡, Li Peng, Xuerui Wang, Qingqing Kong, and Xuehong Gu*

College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, P. R. China

ABSTRACT: The practical application of pure-silica MFI zeolite membranes for ethanol/water separation by pervaporation is limited by its poor stability. Herein, we present the Si-OH eliminated MFI membranes by a simple dopamine modification, which can effectively prevent the chemical reaction between Si-OH groups and components, endowing the long-term pervaporation stability.

KEYWORDS: MFI zeolite membrane; Pervaporation; Ethanol/water separation; Dopamine modification; Stability The ever-growing demands on energy and environmental sustainability have stimulated the industry of ethanol production from renewable biomass. Pervaporation is considered as one of the most promising energy-efficient process for ethanol extraction from the fermentation broth

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containing dilute alcohol (70% of the zeolite membrane price)13 as well as the decreasing separation performance during the long-term operation are the main concerns of MFI zeolite membranes for the practical applications in ethanol permselective permeation over water. Ceramic hollow fiber substrate is potential to take over other substrates to be used for separation for less cost.14 Recently, Wang and co-workers prepared MFI zeolite membrane on αAl2O3 hollow fibers at low template content.15, 16 The membrane exhibited a separation factor of 58 and a permeation flux of 9.8 kg·m-2·h-1 for the pervaporation of 5 wt% ethanol aqueous solution at 60 °C. However, Al element within the alumina substrate could be leached out and incorporated into the Si-based framework during hydrothermal synthesis, decreasing the hydrophobicity of MFI membrane and the separation selectivity. Thus, Al-free substrates, such as stainless steel17, silica18 and yttria-stabilized zirconia (YSZ)19, should be the most promising substrates to prepare pure-silica MFI membranes. Recently, we prepared thin silicalite-1 zeolite membranes on porous YSZ hollow fibers, which exhibited a high flux of 7.4 kg·m-2·h-1 with the separation factor (ethanol/water) of 47 for separating 5 wt% ethanol/water at 60 °C, due to low transport resistance and elimination of Al contamination for the hollow fiber substrates.19 Even though great efforts have been made on the membrane fabrication, the practical application of pure-silica membrane is significantly hindered by the dramatic decreasing separation performance during the long-term operation. Kunh et al. firstly discovered the


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inconsistent separation performance of MFI zeolite membrane during long-term pervaporation in ethanol/water mixture.20 The total flux and the ethanol separation factor decreased significantly at a feed temperature of 100 °C and the ethanol separation factor decreased from 40 to 10 within 177 h of operation at various conditions. However, they considered that the mechanism of deterioration required further investigation to enable the fabrication of more stable zeolite membranes for these processes. Our previous study showed that MFI zeolite membranes with improved stable performance can be prepared by using ammonium fluoride (NH4F) as mineralizing agent and tetrapropylammonium bromide (TPABr) as template.21 It was stable during 50 h pervaporation. It has been confirmed that, Si atoms in the skeleton of the MFI molecular sieves synthesized under the alkaline condition , are easily missing to generate Si-OH to compensate the charge balance.22 While for the MFI molecular sieves synthesized under nearly neutral condition with the present of fluoride, such groups can be effectively eliminated.23-25 Thus, we assumed that the presence of Si-OH in zeolite, which can react with ethanol to generate Si-OC2H5, could be the reason for the poor stability of the MFI zeolite membrane during pervaporation (Figure 1). Herein, we present a novel dopamine modification approach to eliminate surface Si-OH groups and suppress the membrane performance degradation during ethanol/water pervaporation separation. Meanwhile, the mechanism for the long-term stability of MFI membrane was explored.


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Figure 1. Schematic representation of elimination of surface Si-OH groups of MFI zeolite membrane by dopamine modification and the effects of ethanol/water mixture on the MFI zeolite membrane surfaces. The self-assembly of dopamine on MFI zeolite membranes was carried out by vertically immersing membranes in an open buffered aqueous solution (pH = 8.5) with dopamine for different time at 25 °C. Figure 2 (a) is the surface of the as-prepared MFI zeolite membrane. After dopamine modification, the surface of the membrane was brown (Figure 2 (b)), which indicated that the poly-dopamine (PD) layer had been attached on the surface of MFI zeolite membrane effectively. As shown in Figure 2 (b), the surface of MFI zeolite membrane has been completely covered with a rough PD layer after 24 h immersion. The EDX analysis also


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confirmed the successful modification (Figure S1). No obvious change of the thickness of MFI zeolite membrane can be observed after modification (Figure 2 (c), (d)). In addition, the measurement of the water contact angle showed that there was no difference for the static contact angle of MFI zeolite membrane before and after modification (Figure 2 (e), (f)). It meant that the hydrophobicity of the membrane surface was kept, still benefit to the separation of ethanol/water mixture.


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Figure 2. SEM images of MFI zeolite membrane before (a, c) and after (b, d) modification by dopamine; photographs of MFI zeolite membranes and water contact angle of the membranes before (e) and after (f) dopamine modification. The membranes before and after modification were applied in the separation of 5 wt% ethanol/water mixture at 60 °C. Without modification, the membranes had an initial high


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separation selectivity between 38-43 and permeation flux of 4.7 kg·m-2·h-1 (Table S1); however, the separation performances declined continuously during pervaporation. For an example, the permeation flux of an unmodified membrane declined obviously from 4.9 to 0.5 kg·m-2·h-1 within 180 h operation; meanwhile the separation factor declined continuously from 43 to 1, meaning the loss of separation ability (Figure 3 (a)). For dopamine modified zeolite membranes, the decline of pervaporation performance was slowed down with the extension of modification time, suggesting enhanced stability of membranes (Figure S2). For modification time of 24 h, the membrane showed a separation factor of 40, which was similar to the separation selectivity of unmodified membranes. However, it decreased obviously after 36 h modification. It was because of an increasing thickness of the PD layer. On the other hand, it might be partially attributed to the more hydroxy groups existing on PD layer surface with the further reaction. Figure 3 (b) shows pervaporation performance of hollow fiber MFI zeolite membrane modified by dopamine for 24 h. Compared with unmodified membranes, the initial flux of modified membrane decreased to 2.2 kg·m-2·h-1, due to the increased mass transfer resistance caused by the dopamine modification. However, it was interesting to find that the flux of the modified membrane was noticeably stable during long-term pervaporation. For the operation of 180 h, the modified membrane maintained a good separation performance with the separation factor of 44 and the flux of 2.6 kg·m-2·h-1.


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Figure 3. Pervaporation performance of MFI zeolite membrane before (a) and after (b) dopamine modification (Feed condition: 5 wt% ethanol/water mixture at 60 °C).


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To verify the mechanism proposed in Figure 1, we examinated the regeneration of separation ability of MFI zeolite membranes after separating ethanol/water mixture at 60 °C through thermo treatment firstly. The results showed that the attenuation of the membrane performance could be recovered by calcination of the membrane at 450 °C for 4 h (Figure S3). This indicated that the decline of the separation performance was not because of structural degradation of MFI zeolite membranes but related with component adsorption. We further carried out the pervaporation experiments using pure water and anhydrous ethanol as feeds, respectively. The permeation flux of MFI zeolite membrane decreased obviously in anhydrous ethanol, but not in the case of water (Figure S4, S5). It meant that ethanol affected the stability of MFI zeolite membrane significantly. Since it is hard to determine the property of the membrane directly, we collected the MFI zeolite particles generated from the bottom of the synthesis vessel for further study. TG result (Figure S6) of ethanol treated MFI zeolite particles in nitrogen atmosphere showed an obvious mass loss occurring above 400 °C. However, fresh MFI zeolite particles didn't give significant mass loss at this range. Hence, the mass loss for the ethanol treated MFI zeolite particles are due to the chemical reaction with ethanol molecules. An obvious characteristic peak at -101.3 ppm assigned to (Si–O)3(Si–OH) can be observed in the

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Si MAS NMR spectrogram for the fresh

MFI zeolite particles (Figure S7). This is because Si atoms in the skeleton of the MFI molecular sieves, synthesized under the alkaline condition, are easily missing to cause silicon defects. It led to Si-OH generating on the surface of the zeolite crystal.22 The intensity of this peak for the ethanol treated MFI zeolite particles was reduced, showing that Si-OH quantity of MFI molecular sieves decreased after ethanol treatment. This also indicated that some chemical reactions occurred.


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To illustrate what happened between molecular sieves and ethanol molecules, the MFI zeolite particles before and after treated by ethanol were characterized by FT-IR (Figure 4 (a), (b)). The particles after ethanol treatment were dried in oven at 120 °C over night. An absorption peak occurred at 960 cm-1 for the fresh MFI zeolite particles, assigned to Si-OH asymmetric stretching vibrations in the zeolite framework. However, this characteristic peak of the MFI zeolite particles after ethanol treatment was weaker than the fresh. For the ethanol treated MFI zeolite particles, the characteristic peaks of -CH3 and -OC2H5 were detected at 1400-1500, and 29003000 cm-1, respectively. It was proved that Si-OH in zeolite crystals reacted with ethanol to generate Si-OC2H5. For the FT-IR spectrum of the MFI zeolite after dopamine modification (Figure 4 (c)), the absorption peak at 960 cm-1 almost disappeared. This indicated that the silanol might react with PD during the modification. The peaks at 1602 and 1510 cm-1 were attributed to the overlap of the C=C resonance vibrations in the aromatic ring and the N-H bending vibrations, respectively. These results showed that PD had effectively grafted on the MFI zeolite surface. XRD pattern of MFI zeolite before and after modification are consistent (Figure S8), indicating no phase transition or degradation occurred. Due to the size of PD was larger than the pore of MFI zeolite, the PD only existed on the surface of the molecular sieves, neither in the channels of the molecular sieves nor in the crystal lattice.


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Figure 4. FT-IR spectrum of the as-synthesized (a), ethanol treated (b), and dopamine modified (c) MFI zeolite particles. Above mentioned results showed that, in the process of synthesis of MFI type zeolite membrane under conventional alkaline conditions, Si atom in MFI zeolite framework was easy lost to generate Si-OH. During separating ethanol/water mixture, the MFI zeolite membrane contacted with these two materials. Si-OH would react with ethanol, and then oxyethyl replaced hydroxyl on the membrane surface (Figure 1). The -OC2H5 mainly distributed on the orifice of the crystal of the membrane surface and the intercrystalline defects. The oxyethyl groups on the membrane surface narrowed the orifice diameter, leading to a more easily permeance through membrane for water than ethanol. However, the oxyethyl groups increased the membrane surface hydrophobicity, which was not beneficial to water penetration. As a result, both the separation factor and flux through the membranes declined significantly as the separation processing.


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Adopting dopamine as modification material to reduce the silanol amount on the membrane surface can enhance the stability of the MFI zeolite membrane. Figure 1 shows the reaction mechanism of dopamine and the possible reaction between poly-dopamine and MFI zeolite. Dopamine can spontaneously polymerize into poly-dopamine and adheres to the substrates in an aqueous solution. The catechol was easy to be oxidized and formed quinone structure. The primary roles of adhesive bonding and cross-link formation can be assigned to the reverse dismutation reaction between catechol and quinone and other catechol compounds molecule.26-28 The PD layer formed on the MFI zeolite membrane surface reacted with Si-OH instead of ethanol. Meanwhile, it played a role as protective layer to prevent the direct contact between the membrane surface and ethanol/water mixture. This could effectively reduce the effect of raw materials on MFI zeolite membrane surface. Moreover, poly-dopamine was not only deposited on the membrane surface, but also in the intercrystalline defects. The deposition of polydopamine in the defects could decrease flow through the defects and increase separation selectivity. For further verification, we also use amorphous SiO2 to eliminate the Si-OH groups distributed on the orifices of the zeolite crystals and non-zeolitic pores through the hydrolysis of tetraethylorthosilicate, a simple healing method for zeolite membrane we developed before (see supporting information for detail).29 Although the effect of SiO2 modification is not as good as that of poly-dopamine, the resulted membrane also showed improved stability (Figure S9). After SiO2 modification, abundant Si-OH groups should exist on amorphous SiO2 covering the membrane surface, but it seems that this kind of SiOH groups didn’t affect the performance stability of MFI zeolite membrane. This result further confirmed that the Si-OH groups, which evolved during the MFI zeolite


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membrane structure formation step and located on the orifices of zeolite crystals, is the principle threaten to the stability of zeolite membrane. Thereby, the surface modified MFI zeolite membranes could separate ethanol/water mixture stably for a long time. In summary, we investigated the stability of MFI zeolite membranes in ethanol/water separation by pervaporation intensively. MFI zeolite membranes synthesized in the alkaline condition were unstable in separating ethanol/water mixture due to the reaction between the surface Si-OH and ethanol. The resulting oxyethyl groups can block the entrance of zeolite channel and narrow the effective orifice size, so both flux and separation factor decreased. Modification of MFI zeolite membrane with dopamine can effectively eliminate the effect of silanol and improve the long-term separation stability. Our research presenting here endows the potential practical application of MFI zeolite membrane for the bioethanol production. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge. Experimental Procedures, pervaporation performance of MFI zeolite membranes (Table S1, and Figures S2, S3, S4 and S5), EDX results, TG profiles, 29Si MAS NMR spectra, XRD patterns. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (21490585, 21606126), National High-tech R&D Program of China (2015AA03A602), the “Six Top Talents” and “333 Talent Project” of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1) Miyazawa I.; Kokugan T. Effect of product removal by a pervaporation on ethanol fermentation. J. Ferment. Bioeng., 1998, 86, 488-493. (2) Chapman P. D.; Oliveira T.; Livingston A. G. and Li K. Membranes for the dehydration of solvents by pervaporation. J. Membr. Sci., 2008, 318, 5-37. (3) Vane L. M. Separation technologies for the recovery and dehydration of alcohols from fermentation broths. Biofuel. Bioprod. Biorefin., 2008, 2, 553-588. (4) Kondo M.; Komori M.; Kita H.; Okamoto K. Tubular-type pervaporation module with zeolite NaA membrane. J. Membr. Sci., 1997, 133, 133-141. (5) Lai Z.; Bonilla G.; Diaz I.; Nery J. G.; Sujaoti K.; Amat A. M.; Kokkoli E.; Terasaki O.; Thompson R. W.; Tsapatsis M. Microstructural optimization of a zeolite membrane for organic vapor separation. Science, 2003, 300, 456-460. (6) Drews T. O.; Tsapatsis M. Progress in manipulating zeolite morphology and related applications. Curr. Opin. Colloid In., 2005, 10, 233-238. (7) Caro J.; Noack M.; Kölsch P.; Schäfer R. Zeolite membranes-state of their development and perspective. Micropor. Mesopor. Mater., 2000, 38, 3-24.


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Table of Contents Graphic

Si-OH eliminated MFI membranes by dopamine modification, which can prevent the reaction between Si-OH and components, endows long-term pervaporation stability.


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Enhanced Stability of MFI Zeolite Membranes for Separation of

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