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Al ratio in the framework on the pervaporation properties of hollow fiber CHA zeolite membranes
Accepted Manuscript Effect of Si/Al ratio in the framework on the pervaporation properties of hollow fiber CHA zeolite membranes Ji Jiang, Li Peng, Xuerui Wang, Hao Qiu, Miaomiao Ji, Xuehong Gu PII:
S1387-1811(18)30390-1
DOI:
10.1016/j.micromeso.2018.07.015
Reference:
MICMAT 9025
To appear in:
Microporous and Mesoporous Materials
Received Date: 16 March 2018 Revised Date:
5 July 2018
Accepted Date: 7 July 2018
Please cite this article as: J. Jiang, L. Peng, X. Wang, H. Qiu, M. Ji, X. Gu, Effect of Si/Al ratio in the framework on the pervaporation properties of hollow fiber CHA zeolite membranes, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.07.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Si/Al ratio in the framework on the pervaporation
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properties of hollow fiber CHA zeolite membranes
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Ji Jiang, Li Peng, Xuerui Wang, Hao Qiu, Miaomiao Ji, Xuehong Gu*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Synergetic Innovation Center for Advanced Materials,
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Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China
*Corresponding author: Xuehong Gu
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Tel./Fax: (+86)25-83172268
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E-mail: [email protected] (X. Gu)
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Abstract Hollow fiber supported CHA zeolite membranes with different Si/Al ratios in the
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framework were successfully synthesized by changing precursor compositions. The
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influence of precursor composition on the membrane composition, microstructure
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evolution and dehydration performances of CHA zeolite membranes, were
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extensively investigated. Pervaporation (PV) results indicated that the membrane
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separation performances were strongly influenced by both the framework Si/Al ratio
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and the microstructure of the membranes. The membranes with Si/Al ratio of ca.
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2.7~2.8 composed of flake-like grains showed higher flux with relatively low
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selectivity, while the membrane with Si/Al ratio of >2.9 composed of block-shaped
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crystals exhibited higher separation factor of >10000. Both hydrothermal and acid
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stability of the membranes were dependent on the framework Si/Al ratio of CHA
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zeolite membranes.
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Keywords: CHA zeolite membrane; Si/Al ratio; Pervaporation; Microstructure
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evolution; Stability.
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1. Introduction Due to the well-defined channel structure and dimension, hydrophilic zeolite
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membranes show great potentials for pervaporation (PV) dehydration of organic
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mixtures with high permselectivity [1-4]. In particular, NaA zeolite membrane has
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been commercialized in the past decades [5-7], which exhibited excellent separation
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performance and low energy consumption in comparison with traditional separation
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technologies such as distillation and adsorption [8, 9]. However, the low Si/Al ratio of
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NaA zeolite framework limited its applications in near neutral solvents such as
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alcohols [10], acetonitrile [11], tetrahydrofuran [12], and ethylene glycol [13].
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In recent years, several types of acid-resistant zeolite membranes, such as T [14,
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15], FAU [16], MOR [17, 18], ZSM-5 [19] and CHA [20] have been prepared and
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applied for dehydration of acidic solvents or organic acids. Among these membranes,
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CHA zeolite membrane with the Si/Al ratio variable from 2 to ∞, have shown good
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dehydration performance and excellent acid stability for various organic solvent/water
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mixtures [20-34]. However, the industrial applications of CHA zeolite membrane are
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significantly limited by the use of the costly and toxic template [24-34]. The
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successful fabrication of high-quality CHA membranes from an individual
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K+-containing synthetic medium by Li et al. [23] as well as our group [20-22], further
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reduced the fabrication cost, offering the possibility of industrial applications of CHA
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zeolite membranes. Nevertheless, the high viscosity for milk-like gel precursor can
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easily lead to the formation of impure phase such as MER crystals as influencing the
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PV performance and stability of CHA zeolite membranes [21, 25-28]. Recently, we
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ACCEPTED MANUSCRIPT reported the synthesis of high-flux CHA zeolite membranes with Si/Al ratio of 2.7-2.8
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in clear solution [20]. The low viscosity of clear synthesis solution was benefit for the
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fabrication of pure-phase CHA zeolite membranes with high reproducibility. However,
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water permeation flux through the membranes decreased gradually in the long term
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PV dehydration of acidic ethanol solution with pH~3, which might be mainly due to
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the low Si/Al ratio in the membranes.
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Previous studies have shown that the stability of zeolite is highly related to Si/Al
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ratio in the membrane [35-38]. Zeolite membranes with high framework Si/Al ratio
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are relatively more stable in acid conditions. On the other hand, the increase of Si/Al
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ratio would lead to the decrease of hydrophilicity of zeolite and thus affect the
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separation performance of zeolite membranes [33]. Both of water permeation flux and
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separation factor decreased with the increase of Si/Al ratio in the zeolite membrane
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[33]. Thus, controllable Si/Al ratio for CHA zeolite membranes to dominate
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separation performance and stability is of great importance for practical applications.
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In this work, CHA zeolite membranes with varied Si/Al ratio were prepared by
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modulating the precursor compositions. The influence of precursor composition on
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the Si/Al ratio, microstructure evolution as well as PV performances of CHA zeolite
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membranes was extensively investigated. Moreover, the hydrothermal stability and
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acid stability of CHA zeolite membranes with different Si/Al ratio in the framework
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were evaluated.
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2. Experimental 4
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2.1 Membrane preparation CHA zeolite membranes were hydrothermally synthesized on homemade
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yttria-stabilized zirconia (YSZ) hollow fiber substrates with 80 mm long by vacuum
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seeding approach. The supports had outer diameter of 1.8 mm with a wall thickness of
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0.4 mm; the average pore size and porosity were 1.0 µm and 36%, respectively. For
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the vacuum-coating method, one end of the substrate was sealed with silicone cap and
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the other end was exposed to a pump with vacuum degree of 10 kPa. The substrate
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was immersed into 0.5 wt% ball-milled seed suspension for 5 s after being immersed
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into deionized water, unless otherwise stated. After seeding, both ends of the supports
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were sealed with silicone cap (ca. 3 mm in length) to avoid zeolite formation on the
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inner surfaces of the supports. The advantage of using ball-milled seeds for the
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fabrication of CHA zeolite membranes was described in our previous work [21]. After
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drying in an oven at 75 °C for 2 h, 4 seeded substrates were fixed onto a Teflon holder
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with uniform distribution and placed vertically in the 45 mL Teflon-lined autoclave
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(Teflon autoclave, I.D.: 24 mm; effective length: 100 mm). Then 40 mL synthesis
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solution was poured along the edge of the autoclave slowly. For the secondary growth,
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the synthesis precursor was prepared by dissolving potassium hydroxide (KOH, 85%,
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Shanghai Lingfeng Chemical Reagent Co., Ltd, China) and sodium aluminate
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(NaAlO2, 41 wt% Al2O3, Sinopharm Chemical Reagent Co., Ltd, China) into
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deionized water until a homogenous solution was obtained. Then colloidal silica
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(Ludox SM-30, Sigma-Aldrich) was dropwise added into the solution under stirring.
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The mixture was stirred at 20 °C for 1 h, and the corresponding molar ratio was 22
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1.375, respectively. Finally, the Teflon-lined autoclaves were placed vertically in the
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oven and the membranes were hydrothermally synthesized in static condition at
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140 °C for 16 h. The crystallized membranes were washed with deionized water and
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dried in an oven at 75 °C overnight before PV tests.
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2.2 PV experiments
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The as-synthesized membranes were tested for separation of 2.0 kg 90/10 wt%
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ethanol/water mixture or 2.0 kg 50/50 wt% acetic acid/water mixture by PV
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experiments. The PV apparatus was illustrated schematically in our previous paper
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[14]. The mixture was stirred with a high speed of 1000 rpm. Before separation test,
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one end of the membrane (ca. 3 mm in length) was blocked with silicone glue; the
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other end (ca. 7 mm in length) was mounted onto a glass tube by silicone glue, which
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was connected to vacuum line. The membrane, immersed in a feed solution, was
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evacuated by a vacuum pump through the lumen side, where the downstream pressure
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was maintained below 200 Pa throughout the test. The tested membranes had an
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effective length of ca. 7 cm (effective membrane area of 4.0 cm2). The permeated
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vapor was collected with cold traps cooled by liquid nitrogen. Both of the feed and
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permeate was analyzed by a gas chromatograph (GC, GC-2014A, Shimadzu)
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equipped with a thermal conductivity detector. The membrane performance was
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determined by separation factor (α) and permeation flux (J), which were respectively
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defined as follows:
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αi / j =
yi y j
(1)
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w A⋅ t
(2)
Where xi and xj are weight fractions of component i and component j in the feed;
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yi and yj correspond to weight fractions in the permeate; w is the total weight of the
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permeate water, kg; t represents the collecting time, h; A is the effective separation
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area of the membrane.
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2.3 Characterizations
The morphologies of CHA zeolite membranes were characterized by Field
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Emission Scanning Electron Microscopy (FE-SEM, S-4800, Hitachi). Elemental
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compositions of the as-synthesized CHA zeolite membranes were analyzed by EDX
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(EMAX x-act, Horiba). Prior to EDX test, all the samples were washed with DI
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water carefully. Elemental analyses with EDX were performed 4 times at different
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locations on each membrane surface and three membranes were tested for each
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condition. The crystal phases were determined by X-ray diffraction (MiniFlex 600,
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Rigaku) with Cu Kα radiation in the 2θ rang of 5-30°. The relative crystallinity of the
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fresh membranes was calculated by comparing the sum of intensities of the desired
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sample with the membrane prepared with precursor Si/Al ratio of 14 for the typical
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peaks at 2θ = 9.4° and 20.5°. While the relative crystallinity of the membrane after
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acetic acid dehydration was determined by comparing the sum of intensities of the
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treated sample with the corresponding fresh membrane.
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3. Results and discussion
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3.1 Membrane preparation and PV performance
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Al and Si sources are important for the fabrication of TO4 unit in zeolite growth,
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Therefore, the effect of precursor Si/Al ratio was evaluated for the preparation of
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CHA zeolite membranes. The Si/Al ratio was varied from 8 to 22 by changing Al
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content in the precursor. XRD results (Fig. 1) indicated that CHA zeolite membranes
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without any obvious impurities could be achieved in a wide precursor Si/Al ratio
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range of 9-21. However, the relative crystallinity of those membranes varied a lot.
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According to XRD results, the relative crystallinity of the membrane prepared with
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precursor Si/Al ratio of 9, 11, 13, 14, 18 and 21 were calculated to be 81%, 97%, 98%,
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100%, 96% and 35%, respectively. The low crystallinity of those membranes
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prepared with precursor Si/Al ratio of <11 or >18 might be related to the incomplete
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crystallization or dissolution of CHA zeolite crystals. SEM images for CHA zeolite
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membranes crystallized with various precursor Si/Al ratios were presented in Fig. 2.
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At lower precursor Si/Al ratio (Si/Al<10), the surface was mainly covered with
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dish-like crystals along with some block-shaped crystals (Fig. 2a). The membrane
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surface covered with polycrystalline flake-like grains (flake-like grains referred to
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those crystals that composed of cubic-like zeolite crystals smaller than 500 nm) was
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formed for Si/Al ratio of 11-13 (Fig. 2b and 2c), where the strip-shaped crystals was
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due to randomly-orientation of the membrane layer. With the increase in precursor
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Si/Al ratio, the morphology of CHA crystals gradually changed from flake-like grains
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to block-shaped crystals until a complete conversion at precursor Si/Al ratio above 14
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(Fig. 2d and 2e). Further increase of precursor Si/Al ratio to 21 resulted in a poor
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crystallinity and intergrowth of CHA zeolite crystals, as can be demonstrated by XRD
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results (Fig. 1f) and SEM observation (Fig. 2f). As can be seen from Fig. 2, the
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thicknesses of the membranes were between 5-8 µm. The variation of precursor Si/Al
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ratio mainly affected the morphology of the membranes. EDX analysis was used to analyze the Si/Al ratio of the CHA zeolite membrane
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and Fig. 3 presents the influence of precursor Si/Al ratio on the composition of the
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CHA zeolite membranes. The overall trend shows the increase of Si/Al ratio in the
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membrane layers as the Si/Al ratio in precursor increases. The Si/Al ratio of the
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membrane increased slightly from ca. 2.7 to ca. 2.8 when the precursor Si/Al ratio
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increased from 8 to 11, which could be related with morphology change of CHA
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zeolite crystals. In previous work, we noticed that most dish-like crystals were formed
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in the initial stage of CHA zeolite membrane crystallization [26, 27] or at lower
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crystallization temperature [20]. Therefore, the low crystallinity of dish-like crystals
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might have low Si/Al ratio in zeolite framework. The Si/Al ratio in the membranes
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maintained almost a constant of ca. 2.8 for precursor Si/Al ratio between 11 and 14,
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which generated flake-like grains in the membranes. The relatively high crystalline
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CHA zeolite crystals promote the slight increase in Si/Al ratio of the membrane.
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Further increase of Si/Al ratio in the precursor led to slightly increase of Si/Al ratio of
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the membranes before finally reaching a plateau of 3.2 for precursor Si/Al ratio above
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19. These results indicated that morphology evolution of CHA zeolite crystals from
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dish shape to block shape could be attributed to the increase of Si/Al ratio in the
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membrane layers.
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The effect of precursor Si/Al ratio on the PV performance of CHA zeolite
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membranes at each synthesis condition have been repeated for 4 times. The separation
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was operated for dehydration of 90 wt% ethanol solution at 75 °C. It was evident that
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water permeation flux increased significantly with increase in precursor Si/Al ratio
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from 8 to 11, where x changed from 1.375 to 1.0 for precursor composition 22 SiO2 :
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x Al2O3 : 15 K2O : x Na2O : 4400 H2O. The increase of water permeation flux might
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be related to the reduced membrane thickness from 8 µm to 5 µm (Fig. 2a and 2b), as
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well as the different morphology of membrane layer (dish and flake-like grains). The
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membrane layers composed of flake-like grains looked much denser, which could
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promote separation selectivity. It was noted that water permeation flux kept almost
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constant for the precursor Si/Al ratio between 11 and 13. The similar Si/Al ratio as
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well as similar membrane morphology of the membrane layers resulted in the similar
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flux of the membranes. However, due to the slight change in membrane morphology
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from flake-like grains to block-shaped crystals, the separation factor of the
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membranes increased from ca. 5000 to 9000 as precursor Si/Al ratio increased from
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11 to 13. Further increase of precursor Si/Al ratio to 14, water permeation flux
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decreased to ca. 12 kg·m-2·h-1 while the separation factor increased to >10000. The
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slight decrease in water permeation flux of the membranes should be related to the
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morphology change on membrane surface [20] and the slight increase in Si/Al ratio of
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membrane. While the significant increase in separation factor of CHA zeolite
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membranes should be related to the formation of much denser block-shaped crystals
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on the membrane surface, which has been demonstrated in previous work that
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with a higher separation factor [26, 39]. And the increase in Si/Al ratio of the
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membrane was helpful in the formation of block-shaped CHA zeolite crystals. Both
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permeation flux and separation factor deceased with further increases in precursor
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Si/Al ratio. The decrease of water permeation flux was mainly attributed to the
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increase of membrane thickness and Si/Al ratio in membrane layer, leading to the
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decrease of hydrophilicity of the membranes. The increase in Si/Al ratio of the
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membrane would make it more hydrophobic. Therefore, the separation factor
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decreased. Simultaneously, the poor intergrown CHA zeolite crystals at too higher
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precursor Si/Al ratio also contributed to the quick decrease in the separation factor of
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the membranes. It was concluded that the extremely low Al content was not easy for
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producing high-quality CHA zeolite membranes under such synthesis conditions.
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Finally, CHA zeolite membranes with higher separation factor of >10000 were
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achieved at precursor composition of 22 SiO2 : 0.786 Al2O3 : 15 K2O : 0.786 Na2O :
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4400 H2O. Furthermore, the membranes could be synthesized in a broad scope and
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showed higher reproducibility.
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3.2 Membranes stability
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3.2.1 Hydrothermal stability
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Hydrothermal stability of CHA zeolite membranes with different framework
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Si/Al ratio (MX-1: Si/Al=2.7, mainly composed of dish-like crystals; MX-2:
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Si/Al=2.8, mainly composed of flake-like grains and MX-3: Si/Al=3.2, mainly
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composed of block-shaped crystals) were investigated in continuous dehydration of
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framework structure of NaA zeolite membranes [40]. Fig. 5 showed the time-course
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dependence of flux and water content in permeate. As expected, the water contents in
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the permeate maintained above 99.8 wt% for all the three CHA zeolite membranes
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throughout the tests. Since MX-1 had lowest Si/Al ratio in the framework, it showed
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the highest initial water permeation flux of ca. 18 kg·m-2·h-1 while the membranes
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composed of flake-like grains and Si/Al ratio of 2.8 showed initial water flux of ca.
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17.3 kg·m-2·h-1. The membranes composed of block-shaped crystals only showed a
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lower initial water flux of ca. 15.1 kg·m-2·h-1, which might be related to the relatively
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higher framework Si/Al ratio (Si/Al~3.2). Nevertheless, water permeation flux of
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membrane MX-1 showed a quick decrease in the first 100 h from ca. 18.0 kg·m-2·h-1
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to ca. 10.8 kg·m-2·h-1 and then a slight decline afterward, which led to a total 50%
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decrement after 250 h. For membrane MX-2, water permeation flux also decreased
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from ca. 17.3 kg·m-2·h-1 to 14.0 kg·m-2·h-1 in the first 100 h; however, the decrement
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(about 34% in the final) was relatively smaller than that for the membrane MX-1. For
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membrane MX-3, a small decrement from ca. 15.1 kg·m-2·h-1 to 13.0 kg·m-2·h-1 in
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water flux occurred at the first 25 h, and the water flux maintained almost constant
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afterward. A 10% decline of water flux was observed after 250 h test. These results
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indicated that hydrothermal stability of CHA zeolite membranes in water-rich
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solutions might be related to Si/Al ratio in zeolite frameworks. A higher Si/Al ratio of
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≥ 3.1 might be benefit for improving the hydrothermal stability of CHA zeolite
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membranes in water-rich solvents.
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ACCEPTED MANUSCRIPT SEM images of CHA zeolite membranes after long-term hydrothermal stability
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tests are shown in Fig. 6. For MX-1 and MX-2, irregular shaped crystals with some
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caves were observed on the membrane surface, due to partial dissolution of zeolite
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crystals (Fig. 6a and 6b). However, no significant change in the zeolite crystals was
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observed on membrane MX-3 (Fig. 6c). EDX analysis was also applied for
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determining the stability of the membranes. The results indicated that the Si/Al ratio
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in the framework remained relatively stable for all the four samples (MX-1, Si/Al=2.8;
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MX-2, Si/Al=2.9; MX-3, Si/Al=3.3). The damage of zeolite crystals in MX-1 and
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MX-2 might be mainly due to the dissolution of low crystalline zeolite on membrane
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surfaces.
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3.2.2 Acid stability evaluation
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To further evaluate acid stability of CHA zeolite membranes, three CHA zeolite
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membranes with different Si/Al ratios (MX-4 to MX-6 had similar Si/Al ratio to that
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of MX-1 to MX-3 accordingly) were applied for dehydration of 50 wt% acetic
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acid/water mixtures. PV results of these membranes were listed in Fig. 7. Slight
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decline in water flux occurred for all the three membranes in the first 2 h, which
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might be due to blockage of zeolite pores by acetic acid [20, 41]. After that, a quick
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increase in permeation flux from ca. 9.8 kg·m-2·h-1 to >30 kg·m-2·h-1 was observed in
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2 h for membrane MX-4, which turned into to a slight decreasing tendency in the end.
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Besides, water contents in permeate decreased seriously from ca. 100 wt% to 55 wt%,
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indicating low acid stability of membrane MX-4 for acetic acid dehydration. Similar
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results were also observed for membrane MX-5 as the flux of MX-5 increased from
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wt% to 55 wt%. However, it took longer time to occur performance drop, indicating
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an increase of structural stability. Nevertheless, water permeation flux didn’t showed
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a rapid increase after 3 h, which could be related to the blockage of some zeolite pores
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by undissolved less crystalline zeolites. For membrane MX-6, water content in
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permeate showed a decline in the first 4 h from 98 wt% to 87 wt% and then both
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water content in permeate and water flux maintained constant (ca. 10 kg·m-2·h-1),
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suggesting relatively high acid stability of the membrane.
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SEM images for these membranes after PV dehydration in acetic acid solution
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for 5 h (membrane MX-6 for 7.5 h) were shown in Fig. 8. Similarly, dissolution of
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zeolite crystals were observed on membrane surfaces for MX-5 and MX-6.
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Furthermore, big cracks were generated on the membrane surfaces due to acid
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damage. Therefore, PV performance of these membranes decreased. The morphology
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of MX-6 maintained relatively stable. After PV experiments, the Si/Al ratio in the
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membrane layer increased from 2.7, 2.8 and 3.2 to 64.3, 60.3 and 9.3 for membrane
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MX-4, MX-5 and MX-6, respectively. This was due to the aluminum loss of zeolite
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framework in acid solution. The amount of alkali cation in the membrane layer also
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decreased because of charge compensation. However, the membrane with higher
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Si/Al showed lower loss of aluminum. Similarly, in our previous work, the CHA
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zeolite membranes with Si/Al ratio of ca. 3.6 only had minor change to 4.2 in Si/Al
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ratio after acetic acid dehydration [22]. XRD results of CHA zeolite membrane before
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and after dehydration test in 50 wt% acetic acid solution were shown in Fig. 9. We
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seriously after acetic acid treatment, especially for the membrane with lower Si/Al
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ratio of ca. 2.7 (MX-4). But for MX-6 with Si/Al ratio of 3.2, the peak intensity at 2θ
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= 9.4° decreased while the peak intensity at 2θ = 20.5° increased. Similar results were
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also observed in our previous work [22]. The relatively crystallinity of MX-4, MX-5
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and MX-6 after acetic acid dehydration were calculated to be 5%, 41% and 76%,
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respectively, indicating the serious damage of these membranes after acetic
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dehydration. These results indicated that Si/Al ratio in the framework had significant
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effect on acid stability of CHA zeolite membrane. Improvement of framework Si/Al
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ratio is vital in dehydration of acetic acid for practical application.
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4. Conclusions
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CHA zeolite membranes with different Si/Al ratio in the framework were
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successfully fabricated by changing precursor Si/Al ratio. The morphologies and
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stability of the membranes were related to the Si/Al ratio in CHA zeolite membranes.
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The membranes with lower Si/Al ratio (ca. 2.7~2.8) mainly composed of flake-like
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grains. The membrane with Si/Al ratio of >2.9 composed of block-shaped crystals,
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which exhibited higher separation factor of >10000. Both hydrothermal and acid
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stability of CHA zeolite membranes were affected by Si/Al ratio in the membranes.
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CHA zeolite membranes with higher Si/Al ratio had better stability in the high water
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content and acid environment. The understanding of CHA zeolite membrane stability
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is quite useful for practical applications.
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Acknowledgements This work is sponsored by the National Natural Science Foundation of China
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(21490585 and 21776128), the National High-tech R&D Program of China
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(2015AA03A602), the “Six Top Talents” and “333 Talent Project” of Jiangsu
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Province, State Key Laboratory of Materials-Oriented Chemical Engineering
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(ZK201602), the Priority Academic Program Development of Jiangsu Higher
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Education Institutions (PAPD).
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solution, J. Membr. Sci. 347 (2010) 193-196.
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of alcohol solutions, J. Membr. Sci. 364 (2010) 318-324. [27] Y. Hasegawa, C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka, Influence of
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performance of CHA-type zeolite membranes, J. Membr. Sci. 363 (2010)
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256-264.
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[28] R. Zhou, Y. Li, B. Liu, N. Hu, X. Chen, H. Kita, Preparation of chabazite
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Fluoride-mediated synthesis of high-flux chabazite membranes for pervaporation
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of ethanol using reusable macroporous stainless steel tubes, J. Membr. Sci. 510
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(2016) 91-100.
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[30] N. Hu, Y. Li, S. Zhong, B. Wang, F. Zhang, T. Wu, Z. Yang, R. Zhou, X. Chen,
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Microwave synthesis of zeolite CHA (chabazite) membranes with high
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pervaporation performance in absence of organic structure directing agents,
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Microporous Mesoporous Mater. 228 (2016) 22-29.
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[31] K. Sato, K. Sugimoto, N. Shimotsuma, T. Kikuchi, T. Kyotani, T. Kurata,
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Development of practically available up-scaled high-silica CHA-type zeolite
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membranes for industrial purpose in dehydration of N-methyl pyrrolidone solution, J. Membr. Sci. 409 (2012) 82-95.
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[32] N. Yamanaka, M. Itakura, Y. Kiyozumi, Y. Ide, M. Sadakane, T. Sano, Acid
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stability evaluation of CHA-type zeolites synthesized by interzeolite conversion
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acid aqueous solution, Microporous Mesoporous Mater. 158 (2012) 141-147.
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[33] N. Kosinov, C. Auffret, G.J. Borghuis, V.G. Sripathi, E.J. Hensen, Influence of
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the Si/Al ratio on the separation properties of SSZ-13 zeolite membranes, J.
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Membr. Sci. 484 (2015) 140-145.
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[34] H. Kalipcilar, T.C. Bowen, R.D. Noble, J.L. Falconer, Synthesis and separation
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performance of SSZ-13 zeolite membranes on tubular supports, Chem. Mater. 14
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(2002) 3458-3464.
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[35] R.L. Hartman, H.S. Fogler, Reaction kinetics and mechanisms of zeolite
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dissolution in hydrochloric acid, Ind. Eng. Chem. Res. 44 (2005) 7738-7745.
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[36] R.L. Hartman, H.S. Fogler, The unique mechanism of analcime dissolution by
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[37] R.L. Hartman, H.S. Fogler, Understanding the dissolution of zeolites, Langmuir 23 (2007) 5477-5484.
[38] K.V. Ragnarsdóttir, Dissolution kinetics of heulandite at pH 2-12 and 25 °C,
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hydrogen ion attack, Langmuir 22 (2006) 11163-11170.
Geochim Cosmochim Ac. 57(11) (1993) 2439-2449.
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[39] M. Matsukata, K. Sawamura, T. Shirai, M. Takada, Y. Sekine, E. Kikuchi,
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Controlled growth for synthesizing a compact mordenite membrane, J. Membr.
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Sci. 316 (2008) 18-27.
[40] Y. Li, H. Zhou, G. Zhu, J. Liu, W. Yang, Hydrothermal stability of LTA zeolite membranes in pervaporation, J. Menbr. Sci. 297 (2007) 10-15. [41] H. Zhou, Y. Li, G. Zhu, J. Liu, W. Yang, Microwave-assisted hydrothermal
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Sep. Sci. Technol. 65(2) (2009) 164-172.
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Figure Captions
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Fig. 1
ratios of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f). Fig. 2
ratios of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f).
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Fig. 3
Fig. 4
Effect of precursor Si/Al ratio on PV performance of CHA zeolite membranes for dehydration of 90 wt% ethanol solution at 75 °C (the dash
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lines were used for visual guidance only).
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membranes (the solid line was used for visual guidance only).
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Effect of precursor Si/Al ratio on the elemental composition of CHA zeolite
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SEM images for CHA zeolite membranes synthesized with precursor Si/Al
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XRD results of CHA zeolite membranes synthesized with precursor Si/Al
Fig. 5
Time dependency of PV performances of CHA zeolite membranes for dehydration of 50 wt% ethanol solution at 75 °C (MX-1: Si/Al~2.7; MX-2:
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Si/Al~2.8; MX-3: Si/Al~3.2). Fig. 6
stability test, MX-1 (a), MX-2 (b) and MX-3 (c).
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Fig. 7
PV dehydration of 50 wt% acetic acid solution for CHA zeolite membranes
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SEM images for CHA zeolite membranes after long-term hydrothermal
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with different Si/Al ratios (MX-1: Si/Al~2.7; MX-2: Si/Al~2.8; MX-3: Si/Al~3.2).
Fig. 8
SEM images for CHA zeolite membranes after acetic acid dehydration,
MX-4 (a), MX-5 (b) and MX-6 (c). Fig. 9 XRD patterns of CHA zeolite membrane before and after acetic acid dehydration for 5 h (MX-4 and MX-5) or 7.5 h (MX-6).
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Fig. 1 XRD results of CHA zeolite membranes synthesized with precursor Si/Al ratios
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of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f).
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Fig. 2 SEM images for CHA zeolite membranes synthesized with precursor Si/Al
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Fig. 3 Effect of precursor Si/Al ratio on the elemental composition of CHA zeolite
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membranes (the solid line was used for visual guidance only).
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Fig. 4 Effect of precursor Si/Al ratio on PV performance of CHA zeolite membranes
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for dehydration of 90 wt% ethanol solution at 75 °C (the dash lines were used for
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visual guidance only).
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Fig. 5 Time dependency of PV performances of CHA zeolite membranes for
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dehydration of 50 wt% ethanol solution at 75 °C (MX-1: Si/Al~2.7; MX-2: Si/Al~2.8;
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MX-3: Si/Al~3.2).
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Fig. 6 SEM images for CHA zeolite membranes after long-term hydrothermal
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stability test, MX-1 (a), MX-2 (b) and MX-3 (c).
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Fig. 7 PV dehydration of 50 wt% acetic acid solution for CHA zeolite membranes
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with different Si/Al ratios (MX-4: Si/Al~2.7; MX-5: Si/Al~2.8; MX-6: Si/Al~3.2).
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Fig. 8 SEM images for CHA zeolite membranes after acetic acid dehydration, MX-4
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Fig. 9 XRD patterns of CHA zeolite membrane before and after acetic acid
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Highlights CHA zeolite membranes with different Si/Al ratio were prepared Si/Al ratio affected the morphology of CHA zeolite membranes
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Si/Al ratio affected both hydrothermal and acid stability of the membrane
S1387-1811(18)30390-1
DOI:
10.1016/j.micromeso.2018.07.015
Reference:
MICMAT 9025
To appear in:
Microporous and Mesoporous Materials
Received Date: 16 March 2018 Revised Date:
5 July 2018
Accepted Date: 7 July 2018
Please cite this article as: J. Jiang, L. Peng, X. Wang, H. Qiu, M. Ji, X. Gu, Effect of Si/Al ratio in the framework on the pervaporation properties of hollow fiber CHA zeolite membranes, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.07.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Si/Al ratio in the framework on the pervaporation
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properties of hollow fiber CHA zeolite membranes
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Ji Jiang, Li Peng, Xuerui Wang, Hao Qiu, Miaomiao Ji, Xuehong Gu*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Synergetic Innovation Center for Advanced Materials,
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Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China
*Corresponding author: Xuehong Gu
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Tel./Fax: (+86)25-83172268
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E-mail: [email protected] (X. Gu)
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Abstract Hollow fiber supported CHA zeolite membranes with different Si/Al ratios in the
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framework were successfully synthesized by changing precursor compositions. The
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influence of precursor composition on the membrane composition, microstructure
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evolution and dehydration performances of CHA zeolite membranes, were
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extensively investigated. Pervaporation (PV) results indicated that the membrane
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separation performances were strongly influenced by both the framework Si/Al ratio
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and the microstructure of the membranes. The membranes with Si/Al ratio of ca.
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2.7~2.8 composed of flake-like grains showed higher flux with relatively low
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selectivity, while the membrane with Si/Al ratio of >2.9 composed of block-shaped
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crystals exhibited higher separation factor of >10000. Both hydrothermal and acid
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stability of the membranes were dependent on the framework Si/Al ratio of CHA
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zeolite membranes.
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Keywords: CHA zeolite membrane; Si/Al ratio; Pervaporation; Microstructure
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evolution; Stability.
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1. Introduction Due to the well-defined channel structure and dimension, hydrophilic zeolite
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membranes show great potentials for pervaporation (PV) dehydration of organic
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mixtures with high permselectivity [1-4]. In particular, NaA zeolite membrane has
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been commercialized in the past decades [5-7], which exhibited excellent separation
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performance and low energy consumption in comparison with traditional separation
7
technologies such as distillation and adsorption [8, 9]. However, the low Si/Al ratio of
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NaA zeolite framework limited its applications in near neutral solvents such as
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alcohols [10], acetonitrile [11], tetrahydrofuran [12], and ethylene glycol [13].
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In recent years, several types of acid-resistant zeolite membranes, such as T [14,
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15], FAU [16], MOR [17, 18], ZSM-5 [19] and CHA [20] have been prepared and
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applied for dehydration of acidic solvents or organic acids. Among these membranes,
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CHA zeolite membrane with the Si/Al ratio variable from 2 to ∞, have shown good
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dehydration performance and excellent acid stability for various organic solvent/water
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mixtures [20-34]. However, the industrial applications of CHA zeolite membrane are
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significantly limited by the use of the costly and toxic template [24-34]. The
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successful fabrication of high-quality CHA membranes from an individual
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K+-containing synthetic medium by Li et al. [23] as well as our group [20-22], further
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reduced the fabrication cost, offering the possibility of industrial applications of CHA
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zeolite membranes. Nevertheless, the high viscosity for milk-like gel precursor can
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easily lead to the formation of impure phase such as MER crystals as influencing the
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PV performance and stability of CHA zeolite membranes [21, 25-28]. Recently, we
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in clear solution [20]. The low viscosity of clear synthesis solution was benefit for the
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fabrication of pure-phase CHA zeolite membranes with high reproducibility. However,
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water permeation flux through the membranes decreased gradually in the long term
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PV dehydration of acidic ethanol solution with pH~3, which might be mainly due to
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the low Si/Al ratio in the membranes.
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Previous studies have shown that the stability of zeolite is highly related to Si/Al
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ratio in the membrane [35-38]. Zeolite membranes with high framework Si/Al ratio
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are relatively more stable in acid conditions. On the other hand, the increase of Si/Al
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ratio would lead to the decrease of hydrophilicity of zeolite and thus affect the
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separation performance of zeolite membranes [33]. Both of water permeation flux and
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separation factor decreased with the increase of Si/Al ratio in the zeolite membrane
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[33]. Thus, controllable Si/Al ratio for CHA zeolite membranes to dominate
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separation performance and stability is of great importance for practical applications.
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In this work, CHA zeolite membranes with varied Si/Al ratio were prepared by
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modulating the precursor compositions. The influence of precursor composition on
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the Si/Al ratio, microstructure evolution as well as PV performances of CHA zeolite
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membranes was extensively investigated. Moreover, the hydrothermal stability and
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acid stability of CHA zeolite membranes with different Si/Al ratio in the framework
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were evaluated.
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2. Experimental 4
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2.1 Membrane preparation CHA zeolite membranes were hydrothermally synthesized on homemade
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yttria-stabilized zirconia (YSZ) hollow fiber substrates with 80 mm long by vacuum
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seeding approach. The supports had outer diameter of 1.8 mm with a wall thickness of
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0.4 mm; the average pore size and porosity were 1.0 µm and 36%, respectively. For
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the vacuum-coating method, one end of the substrate was sealed with silicone cap and
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the other end was exposed to a pump with vacuum degree of 10 kPa. The substrate
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was immersed into 0.5 wt% ball-milled seed suspension for 5 s after being immersed
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into deionized water, unless otherwise stated. After seeding, both ends of the supports
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were sealed with silicone cap (ca. 3 mm in length) to avoid zeolite formation on the
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inner surfaces of the supports. The advantage of using ball-milled seeds for the
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fabrication of CHA zeolite membranes was described in our previous work [21]. After
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drying in an oven at 75 °C for 2 h, 4 seeded substrates were fixed onto a Teflon holder
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with uniform distribution and placed vertically in the 45 mL Teflon-lined autoclave
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(Teflon autoclave, I.D.: 24 mm; effective length: 100 mm). Then 40 mL synthesis
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solution was poured along the edge of the autoclave slowly. For the secondary growth,
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the synthesis precursor was prepared by dissolving potassium hydroxide (KOH, 85%,
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Shanghai Lingfeng Chemical Reagent Co., Ltd, China) and sodium aluminate
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(NaAlO2, 41 wt% Al2O3, Sinopharm Chemical Reagent Co., Ltd, China) into
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deionized water until a homogenous solution was obtained. Then colloidal silica
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(Ludox SM-30, Sigma-Aldrich) was dropwise added into the solution under stirring.
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The mixture was stirred at 20 °C for 1 h, and the corresponding molar ratio was 22
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1.375, respectively. Finally, the Teflon-lined autoclaves were placed vertically in the
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oven and the membranes were hydrothermally synthesized in static condition at
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140 °C for 16 h. The crystallized membranes were washed with deionized water and
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dried in an oven at 75 °C overnight before PV tests.
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2.2 PV experiments
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The as-synthesized membranes were tested for separation of 2.0 kg 90/10 wt%
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ethanol/water mixture or 2.0 kg 50/50 wt% acetic acid/water mixture by PV
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experiments. The PV apparatus was illustrated schematically in our previous paper
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[14]. The mixture was stirred with a high speed of 1000 rpm. Before separation test,
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one end of the membrane (ca. 3 mm in length) was blocked with silicone glue; the
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other end (ca. 7 mm in length) was mounted onto a glass tube by silicone glue, which
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was connected to vacuum line. The membrane, immersed in a feed solution, was
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evacuated by a vacuum pump through the lumen side, where the downstream pressure
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was maintained below 200 Pa throughout the test. The tested membranes had an
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effective length of ca. 7 cm (effective membrane area of 4.0 cm2). The permeated
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vapor was collected with cold traps cooled by liquid nitrogen. Both of the feed and
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permeate was analyzed by a gas chromatograph (GC, GC-2014A, Shimadzu)
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equipped with a thermal conductivity detector. The membrane performance was
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determined by separation factor (α) and permeation flux (J), which were respectively
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defined as follows:
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αi / j =
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w A⋅ t
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Where xi and xj are weight fractions of component i and component j in the feed;
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yi and yj correspond to weight fractions in the permeate; w is the total weight of the
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permeate water, kg; t represents the collecting time, h; A is the effective separation
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area of the membrane.
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2.3 Characterizations
The morphologies of CHA zeolite membranes were characterized by Field
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Emission Scanning Electron Microscopy (FE-SEM, S-4800, Hitachi). Elemental
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compositions of the as-synthesized CHA zeolite membranes were analyzed by EDX
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(EMAX x-act, Horiba). Prior to EDX test, all the samples were washed with DI
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water carefully. Elemental analyses with EDX were performed 4 times at different
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locations on each membrane surface and three membranes were tested for each
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condition. The crystal phases were determined by X-ray diffraction (MiniFlex 600,
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Rigaku) with Cu Kα radiation in the 2θ rang of 5-30°. The relative crystallinity of the
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fresh membranes was calculated by comparing the sum of intensities of the desired
16
sample with the membrane prepared with precursor Si/Al ratio of 14 for the typical
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peaks at 2θ = 9.4° and 20.5°. While the relative crystallinity of the membrane after
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acetic acid dehydration was determined by comparing the sum of intensities of the
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treated sample with the corresponding fresh membrane.
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3. Results and discussion
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3.1 Membrane preparation and PV performance
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Al and Si sources are important for the fabrication of TO4 unit in zeolite growth,
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Therefore, the effect of precursor Si/Al ratio was evaluated for the preparation of
3
CHA zeolite membranes. The Si/Al ratio was varied from 8 to 22 by changing Al
4
content in the precursor. XRD results (Fig. 1) indicated that CHA zeolite membranes
5
without any obvious impurities could be achieved in a wide precursor Si/Al ratio
6
range of 9-21. However, the relative crystallinity of those membranes varied a lot.
7
According to XRD results, the relative crystallinity of the membrane prepared with
8
precursor Si/Al ratio of 9, 11, 13, 14, 18 and 21 were calculated to be 81%, 97%, 98%,
9
100%, 96% and 35%, respectively. The low crystallinity of those membranes
10
prepared with precursor Si/Al ratio of <11 or >18 might be related to the incomplete
11
crystallization or dissolution of CHA zeolite crystals. SEM images for CHA zeolite
12
membranes crystallized with various precursor Si/Al ratios were presented in Fig. 2.
13
At lower precursor Si/Al ratio (Si/Al<10), the surface was mainly covered with
14
dish-like crystals along with some block-shaped crystals (Fig. 2a). The membrane
15
surface covered with polycrystalline flake-like grains (flake-like grains referred to
16
those crystals that composed of cubic-like zeolite crystals smaller than 500 nm) was
17
formed for Si/Al ratio of 11-13 (Fig. 2b and 2c), where the strip-shaped crystals was
18
due to randomly-orientation of the membrane layer. With the increase in precursor
19
Si/Al ratio, the morphology of CHA crystals gradually changed from flake-like grains
20
to block-shaped crystals until a complete conversion at precursor Si/Al ratio above 14
21
(Fig. 2d and 2e). Further increase of precursor Si/Al ratio to 21 resulted in a poor
22
crystallinity and intergrowth of CHA zeolite crystals, as can be demonstrated by XRD
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results (Fig. 1f) and SEM observation (Fig. 2f). As can be seen from Fig. 2, the
2
thicknesses of the membranes were between 5-8 µm. The variation of precursor Si/Al
3
ratio mainly affected the morphology of the membranes. EDX analysis was used to analyze the Si/Al ratio of the CHA zeolite membrane
5
and Fig. 3 presents the influence of precursor Si/Al ratio on the composition of the
6
CHA zeolite membranes. The overall trend shows the increase of Si/Al ratio in the
7
membrane layers as the Si/Al ratio in precursor increases. The Si/Al ratio of the
8
membrane increased slightly from ca. 2.7 to ca. 2.8 when the precursor Si/Al ratio
9
increased from 8 to 11, which could be related with morphology change of CHA
10
zeolite crystals. In previous work, we noticed that most dish-like crystals were formed
11
in the initial stage of CHA zeolite membrane crystallization [26, 27] or at lower
12
crystallization temperature [20]. Therefore, the low crystallinity of dish-like crystals
13
might have low Si/Al ratio in zeolite framework. The Si/Al ratio in the membranes
14
maintained almost a constant of ca. 2.8 for precursor Si/Al ratio between 11 and 14,
15
which generated flake-like grains in the membranes. The relatively high crystalline
16
CHA zeolite crystals promote the slight increase in Si/Al ratio of the membrane.
17
Further increase of Si/Al ratio in the precursor led to slightly increase of Si/Al ratio of
18
the membranes before finally reaching a plateau of 3.2 for precursor Si/Al ratio above
19
19. These results indicated that morphology evolution of CHA zeolite crystals from
20
dish shape to block shape could be attributed to the increase of Si/Al ratio in the
21
membrane layers.
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The effect of precursor Si/Al ratio on the PV performance of CHA zeolite
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2
membranes at each synthesis condition have been repeated for 4 times. The separation
3
was operated for dehydration of 90 wt% ethanol solution at 75 °C. It was evident that
4
water permeation flux increased significantly with increase in precursor Si/Al ratio
5
from 8 to 11, where x changed from 1.375 to 1.0 for precursor composition 22 SiO2 :
6
x Al2O3 : 15 K2O : x Na2O : 4400 H2O. The increase of water permeation flux might
7
be related to the reduced membrane thickness from 8 µm to 5 µm (Fig. 2a and 2b), as
8
well as the different morphology of membrane layer (dish and flake-like grains). The
9
membrane layers composed of flake-like grains looked much denser, which could
10
promote separation selectivity. It was noted that water permeation flux kept almost
11
constant for the precursor Si/Al ratio between 11 and 13. The similar Si/Al ratio as
12
well as similar membrane morphology of the membrane layers resulted in the similar
13
flux of the membranes. However, due to the slight change in membrane morphology
14
from flake-like grains to block-shaped crystals, the separation factor of the
15
membranes increased from ca. 5000 to 9000 as precursor Si/Al ratio increased from
16
11 to 13. Further increase of precursor Si/Al ratio to 14, water permeation flux
17
decreased to ca. 12 kg·m-2·h-1 while the separation factor increased to >10000. The
18
slight decrease in water permeation flux of the membranes should be related to the
19
morphology change on membrane surface [20] and the slight increase in Si/Al ratio of
20
membrane. While the significant increase in separation factor of CHA zeolite
21
membranes should be related to the formation of much denser block-shaped crystals
22
on the membrane surface, which has been demonstrated in previous work that
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2
with a higher separation factor [26, 39]. And the increase in Si/Al ratio of the
3
membrane was helpful in the formation of block-shaped CHA zeolite crystals. Both
4
permeation flux and separation factor deceased with further increases in precursor
5
Si/Al ratio. The decrease of water permeation flux was mainly attributed to the
6
increase of membrane thickness and Si/Al ratio in membrane layer, leading to the
7
decrease of hydrophilicity of the membranes. The increase in Si/Al ratio of the
8
membrane would make it more hydrophobic. Therefore, the separation factor
9
decreased. Simultaneously, the poor intergrown CHA zeolite crystals at too higher
10
precursor Si/Al ratio also contributed to the quick decrease in the separation factor of
11
the membranes. It was concluded that the extremely low Al content was not easy for
12
producing high-quality CHA zeolite membranes under such synthesis conditions.
13
Finally, CHA zeolite membranes with higher separation factor of >10000 were
14
achieved at precursor composition of 22 SiO2 : 0.786 Al2O3 : 15 K2O : 0.786 Na2O :
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4400 H2O. Furthermore, the membranes could be synthesized in a broad scope and
16
showed higher reproducibility.
17
3.2 Membranes stability
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3.2.1 Hydrothermal stability
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Hydrothermal stability of CHA zeolite membranes with different framework
20
Si/Al ratio (MX-1: Si/Al=2.7, mainly composed of dish-like crystals; MX-2:
21
Si/Al=2.8, mainly composed of flake-like grains and MX-3: Si/Al=3.2, mainly
22
composed of block-shaped crystals) were investigated in continuous dehydration of
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ACCEPTED MANUSCRIPT 50 wt% ethanol aqueous solution at 75 °C. Such a water-rich solution may destroy the
2
framework structure of NaA zeolite membranes [40]. Fig. 5 showed the time-course
3
dependence of flux and water content in permeate. As expected, the water contents in
4
the permeate maintained above 99.8 wt% for all the three CHA zeolite membranes
5
throughout the tests. Since MX-1 had lowest Si/Al ratio in the framework, it showed
6
the highest initial water permeation flux of ca. 18 kg·m-2·h-1 while the membranes
7
composed of flake-like grains and Si/Al ratio of 2.8 showed initial water flux of ca.
8
17.3 kg·m-2·h-1. The membranes composed of block-shaped crystals only showed a
9
lower initial water flux of ca. 15.1 kg·m-2·h-1, which might be related to the relatively
10
higher framework Si/Al ratio (Si/Al~3.2). Nevertheless, water permeation flux of
11
membrane MX-1 showed a quick decrease in the first 100 h from ca. 18.0 kg·m-2·h-1
12
to ca. 10.8 kg·m-2·h-1 and then a slight decline afterward, which led to a total 50%
13
decrement after 250 h. For membrane MX-2, water permeation flux also decreased
14
from ca. 17.3 kg·m-2·h-1 to 14.0 kg·m-2·h-1 in the first 100 h; however, the decrement
15
(about 34% in the final) was relatively smaller than that for the membrane MX-1. For
16
membrane MX-3, a small decrement from ca. 15.1 kg·m-2·h-1 to 13.0 kg·m-2·h-1 in
17
water flux occurred at the first 25 h, and the water flux maintained almost constant
18
afterward. A 10% decline of water flux was observed after 250 h test. These results
19
indicated that hydrothermal stability of CHA zeolite membranes in water-rich
20
solutions might be related to Si/Al ratio in zeolite frameworks. A higher Si/Al ratio of
21
≥ 3.1 might be benefit for improving the hydrothermal stability of CHA zeolite
22
membranes in water-rich solvents.
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2
tests are shown in Fig. 6. For MX-1 and MX-2, irregular shaped crystals with some
3
caves were observed on the membrane surface, due to partial dissolution of zeolite
4
crystals (Fig. 6a and 6b). However, no significant change in the zeolite crystals was
5
observed on membrane MX-3 (Fig. 6c). EDX analysis was also applied for
6
determining the stability of the membranes. The results indicated that the Si/Al ratio
7
in the framework remained relatively stable for all the four samples (MX-1, Si/Al=2.8;
8
MX-2, Si/Al=2.9; MX-3, Si/Al=3.3). The damage of zeolite crystals in MX-1 and
9
MX-2 might be mainly due to the dissolution of low crystalline zeolite on membrane
10
surfaces.
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3.2.2 Acid stability evaluation
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To further evaluate acid stability of CHA zeolite membranes, three CHA zeolite
13
membranes with different Si/Al ratios (MX-4 to MX-6 had similar Si/Al ratio to that
14
of MX-1 to MX-3 accordingly) were applied for dehydration of 50 wt% acetic
15
acid/water mixtures. PV results of these membranes were listed in Fig. 7. Slight
16
decline in water flux occurred for all the three membranes in the first 2 h, which
17
might be due to blockage of zeolite pores by acetic acid [20, 41]. After that, a quick
18
increase in permeation flux from ca. 9.8 kg·m-2·h-1 to >30 kg·m-2·h-1 was observed in
19
2 h for membrane MX-4, which turned into to a slight decreasing tendency in the end.
20
Besides, water contents in permeate decreased seriously from ca. 100 wt% to 55 wt%,
21
indicating low acid stability of membrane MX-4 for acetic acid dehydration. Similar
22
results were also observed for membrane MX-5 as the flux of MX-5 increased from
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2
wt% to 55 wt%. However, it took longer time to occur performance drop, indicating
3
an increase of structural stability. Nevertheless, water permeation flux didn’t showed
4
a rapid increase after 3 h, which could be related to the blockage of some zeolite pores
5
by undissolved less crystalline zeolites. For membrane MX-6, water content in
6
permeate showed a decline in the first 4 h from 98 wt% to 87 wt% and then both
7
water content in permeate and water flux maintained constant (ca. 10 kg·m-2·h-1),
8
suggesting relatively high acid stability of the membrane.
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SEM images for these membranes after PV dehydration in acetic acid solution
10
for 5 h (membrane MX-6 for 7.5 h) were shown in Fig. 8. Similarly, dissolution of
11
zeolite crystals were observed on membrane surfaces for MX-5 and MX-6.
12
Furthermore, big cracks were generated on the membrane surfaces due to acid
13
damage. Therefore, PV performance of these membranes decreased. The morphology
14
of MX-6 maintained relatively stable. After PV experiments, the Si/Al ratio in the
15
membrane layer increased from 2.7, 2.8 and 3.2 to 64.3, 60.3 and 9.3 for membrane
16
MX-4, MX-5 and MX-6, respectively. This was due to the aluminum loss of zeolite
17
framework in acid solution. The amount of alkali cation in the membrane layer also
18
decreased because of charge compensation. However, the membrane with higher
19
Si/Al showed lower loss of aluminum. Similarly, in our previous work, the CHA
20
zeolite membranes with Si/Al ratio of ca. 3.6 only had minor change to 4.2 in Si/Al
21
ratio after acetic acid dehydration [22]. XRD results of CHA zeolite membrane before
22
and after dehydration test in 50 wt% acetic acid solution were shown in Fig. 9. We
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2
seriously after acetic acid treatment, especially for the membrane with lower Si/Al
3
ratio of ca. 2.7 (MX-4). But for MX-6 with Si/Al ratio of 3.2, the peak intensity at 2θ
4
= 9.4° decreased while the peak intensity at 2θ = 20.5° increased. Similar results were
5
also observed in our previous work [22]. The relatively crystallinity of MX-4, MX-5
6
and MX-6 after acetic acid dehydration were calculated to be 5%, 41% and 76%,
7
respectively, indicating the serious damage of these membranes after acetic
8
dehydration. These results indicated that Si/Al ratio in the framework had significant
9
effect on acid stability of CHA zeolite membrane. Improvement of framework Si/Al
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ratio is vital in dehydration of acetic acid for practical application.
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4. Conclusions
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CHA zeolite membranes with different Si/Al ratio in the framework were
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successfully fabricated by changing precursor Si/Al ratio. The morphologies and
15
stability of the membranes were related to the Si/Al ratio in CHA zeolite membranes.
16
The membranes with lower Si/Al ratio (ca. 2.7~2.8) mainly composed of flake-like
17
grains. The membrane with Si/Al ratio of >2.9 composed of block-shaped crystals,
18
which exhibited higher separation factor of >10000. Both hydrothermal and acid
19
stability of CHA zeolite membranes were affected by Si/Al ratio in the membranes.
20
CHA zeolite membranes with higher Si/Al ratio had better stability in the high water
21
content and acid environment. The understanding of CHA zeolite membrane stability
22
is quite useful for practical applications.
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Acknowledgements This work is sponsored by the National Natural Science Foundation of China
3
(21490585 and 21776128), the National High-tech R&D Program of China
4
(2015AA03A602), the “Six Top Talents” and “333 Talent Project” of Jiangsu
5
Province, State Key Laboratory of Materials-Oriented Chemical Engineering
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(ZK201602), the Priority Academic Program Development of Jiangsu Higher
7
Education Institutions (PAPD).
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Figure Captions
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Fig. 1
ratios of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f). Fig. 2
ratios of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f).
5
Fig. 3
Fig. 4
Effect of precursor Si/Al ratio on PV performance of CHA zeolite membranes for dehydration of 90 wt% ethanol solution at 75 °C (the dash
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lines were used for visual guidance only).
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membranes (the solid line was used for visual guidance only).
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Effect of precursor Si/Al ratio on the elemental composition of CHA zeolite
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SEM images for CHA zeolite membranes synthesized with precursor Si/Al
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XRD results of CHA zeolite membranes synthesized with precursor Si/Al
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Time dependency of PV performances of CHA zeolite membranes for dehydration of 50 wt% ethanol solution at 75 °C (MX-1: Si/Al~2.7; MX-2:
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Si/Al~2.8; MX-3: Si/Al~3.2). Fig. 6
stability test, MX-1 (a), MX-2 (b) and MX-3 (c).
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Fig. 7
PV dehydration of 50 wt% acetic acid solution for CHA zeolite membranes
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SEM images for CHA zeolite membranes after long-term hydrothermal
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with different Si/Al ratios (MX-1: Si/Al~2.7; MX-2: Si/Al~2.8; MX-3: Si/Al~3.2).
Fig. 8
SEM images for CHA zeolite membranes after acetic acid dehydration,
MX-4 (a), MX-5 (b) and MX-6 (c). Fig. 9 XRD patterns of CHA zeolite membrane before and after acetic acid dehydration for 5 h (MX-4 and MX-5) or 7.5 h (MX-6).
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Fig. 1 XRD results of CHA zeolite membranes synthesized with precursor Si/Al ratios
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of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f).
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Fig. 2 SEM images for CHA zeolite membranes synthesized with precursor Si/Al
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ratios of 9 (a), 11 (b), 13 (c), 14 (d), 18 (e) and 21 (f).
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Fig. 3 Effect of precursor Si/Al ratio on the elemental composition of CHA zeolite
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membranes (the solid line was used for visual guidance only).
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Fig. 4 Effect of precursor Si/Al ratio on PV performance of CHA zeolite membranes
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for dehydration of 90 wt% ethanol solution at 75 °C (the dash lines were used for
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visual guidance only).
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Fig. 5 Time dependency of PV performances of CHA zeolite membranes for
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dehydration of 50 wt% ethanol solution at 75 °C (MX-1: Si/Al~2.7; MX-2: Si/Al~2.8;
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MX-3: Si/Al~3.2).
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Fig. 6 SEM images for CHA zeolite membranes after long-term hydrothermal
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stability test, MX-1 (a), MX-2 (b) and MX-3 (c).
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Fig. 7 PV dehydration of 50 wt% acetic acid solution for CHA zeolite membranes
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with different Si/Al ratios (MX-4: Si/Al~2.7; MX-5: Si/Al~2.8; MX-6: Si/Al~3.2).
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Fig. 8 SEM images for CHA zeolite membranes after acetic acid dehydration, MX-4
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(a), MX-5 (b) and MX-6 (c).
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Fig. 9 XRD patterns of CHA zeolite membrane before and after acetic acid
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dehydration for 5 h (MX-4 and MX-5) or 7.5 h (MX-6).
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Highlights CHA zeolite membranes with different Si/Al ratio were prepared Si/Al ratio affected the morphology of CHA zeolite membranes
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Si/Al ratio affected both hydrothermal and acid stability of the membrane