Preparation Of Mordenite Membranes For Pervaporation Of

Desalination 148 (2002) 25–29

Preparation of mordenite membranes for pervaporation of water-ethanol mixtures Alberto Navajas, Reyes Mallada, Carlos Téllez, Joaquín Coronas, Miguel Menéndez, Jesús Santamaría* Department of Chemical Engineering, University of Zaragoza. Zaragoza 50009, Spain Tel. +34 (97) 6761153; Fax +34 (97) 6762142; e-mail: [email protected]

Received 30 January 2002; accepted 26 February 2002

Abstract A mordenite membrane was synthesized on a ceramic tubular support by seeded hydrothermal synthesis and tested in the dehydration of a water/ethanol mixture by pervaporation. The influence of synthesis conditions (time and gel composition) on the water/ethanol separation factor and water flux was investigated. Typical membranes yielded a water/ethanol separation factor of 150, at a water flux of 0.2 kg/m2·h. The XRD patterns showed that mordenite was the only zeolite material present in the membrane. Keywords: Zeolite membrane; Pervaporation; Zeolite synthesis; Mordenite; Alcohol dehydration

1. Introduction Zeolite membranes are widely used in a variety of applications, such as separation processes, catalytic reactors and sensors. Separations of azeotropic and close-boiling liquids mixtures and the dewatering of organic solvents, such as alcohols (methanol, ethanol…) by pervaporation using zeolite membranes are examples of relatively

recent developments. While polymeric membranes have been commercially developed for these processes, their use is limited because their low thermal and chemical stability [1]. Microporous inorganic membranes (silica, zeolites, etc.) have been presented as an alternative to broaden the field of membrane application. In particular, the area of zeolite membrane has been the subject of intense research during the last years and several reviews about the subject have been published

*Corresponding author. Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7–12, 2002. 0011-9164/02/$— See front matter © 2002 Elsevier Science B.V. All rights reserved 0011-9164/02/$–See front matter © 2002 Elsevier Science B.V. All rights reserved

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A. Navajas et al. / Desalination 148 (2002) 25–29

[1–4]. Hydrophilic zeolite membranes are good candidates for removing water from its mixtures with organic compounds using pervaporation, with zeolite A as the main candidate, due to his high Al content and small pore size [e.g. 5–10]. Recently, zeolite A membranes for solvent dewatering have been made commer-cially available (Mitsui Engineering & Shipbuilding Co., LTD and Smart Chemical), yielding water/ethanol separation factors above 10000. Unfor-tunately these membranes are very sensitive to acidic environments, which prevents their applica-tion in processes, such as esterifications. Since the stability of the zeolite to acid environments increases with its silica content, more siliceous zeolite materials, such as mordenite [11] and ZSM5 [12] haven been investigated as possible alternatives. In a previous work [13], we have prepared a mordenite membrane on a ceramic tubular support by seeded hydrothermal synthesis. These membranes have been used successfully in the dehydration by pervaporation of water/alcohol mixtures. It was studied the influence of operating conditions on water/alcohol separation factor and water flux, finding that the water/alcohol separation factor and water flux increase when the sweep gas, alcohol concentration in the feed and temperature were augmented. This work focuses on the influence of synthesis conditions (time and gel composition) in the water/ethanol separation factor and permeation flux in the pervaporation of water/ethanol mixtures. 2. Experimental Mordenite membranes were prepared by seeded hydrothermal synthesis onto symmetric aalumina tubular supports with 1.9 µm pores. The synthesis method was described in detail in [13]. A rubbing technique with commercial crystals (0.5 mm, Tosoh Co. H/Mordenite Si/Al=5.1) was used for seeding the outer surface of the support. This was followed by conventional hydrothermal synthesis at 180ºC for variable lengths of time,

with a homogeneous gel that was prepared with different content in water, starting from the composition previously used in [14]. The gel has the following molar composition: H2O:SiO2:Na2O:Al2O3 = 80:X·1:X·0.38:X·0.025 were X took values of 1, 2/3 and 1/2. The phases present in the mordenite membranes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. XRD analyses were carried out on a) commercial mordenite crystals seeds, b) crystals prepared by hydro-thermal synthesis using commercial crystals as seeds, and c) the mordenite membrane top layer. The N2 permeance and weight gain of the membranes were also measured. Pervaporation experiments were performed in an experimental set-up described previously [13], using sweep-gas assisted pervaporation. Pervaporation was carried out at 150ºC, feeding 200 mL/min of a mixture containing 10 wt% water in ethanol and using a sweep gas flow of 200 mL(STP)/min of N 2. The water/ethanol separation factor and water flux have been measured in all the experiments after 36 h under constant operation conditions, which attained a stable pervaporation performance [13]. 3. Results and discussion Fig. 1 shows the XRD patterns of commercial mordenite seed crystals (a), of unsupported mordenite crystals that were hydrothermally synthesized using commercial crystals as seeds (b–d), and of the mordenite membrane top layer (e). The XRD pattern of the crystals synthesized after 2 h (Fig. 1b) shows mordenite as the only crystalline phase, plus some amorphous material; the amount of the latter decreases as synthesis time increases, and practically disappears after 24 h of synthesis (Fig. 1c). Regarding the gel composition, the XRD pattern of the membrane synthesized after 24 h with X = 1/2 (Fig. 1d) shows mordenite and a large proportion of amorphous material, in comparison with Fig. 1c. Also, in the

A. Navajas et al. / Desalination 148 (2002) 25–29

Fig. 1. -XRD patterns of: a) commercial mordenite seeds. b-d) Hydrothermal synthesis on commercial crystals prepared with different synthesis times dilution factors (X); b) 2 h and X=1; c) 24 h and X=1; d) 24 h and X=2/3; e) Top layer of a mordenite membrane obtained at 8 h and with X=1. (+, α-alumina peaks).

XRD pattern of crystals extracted from the top layer of the membrane (Fig. 1e, synthesis time 8 h and dilution 1) there is only mordenite as crystalline material, (other peaks correspond to the alumina support). From the XRD results, the orientation of the crystals in this material is approximately the same as in the seeds. However, the SEM micrographs (not shown) show some borientation. A layer of densely packed mordenite crystals can be appreciated in these micrographs, even though it is believed that the underlying 40– 70 µm layer of mordenite crystals filling the voids among the α-alumina particles in the support contributes effectively to the separation performance observed [13]. Table 1 shows the N2 permeance, weight gain

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and pervaporation results (water/ethanol separation factor and water flux) for the membranes prepared in this work. The seeded support shows a high N2 permeance, which was expected given the small weight gain of the seeding step (the unseeded supports have approximately a N 2 permeance of 10–5 mol/m2·s·Pa). There is some scatter of N2 permeance with respect to synthesis time and gel composition. Since N2 permeance was measured after drying the membrane overnight at a low temperature (100ºC), some water could remain adsorbed on the hydrophilic membrane pores, affecting the reproducibility of the permeation measurements. Most of the membranes shown in Table 1 were synthesized with X = 1 in the gel. The membranes synthesized with more diluted gels showed a small weight gain, as expected because of the lower concentration of reactants, and displayed a poorer performance (see also Fig. 2). For instance, at the same synthesis time of 24 h, membranes prepared with X = 2/3 and 1/2 separated the water/ethanol mixture with smaller separation factors, (20 and 100 respectively), compared to the membrane prepared with X = 1(170). Both, the higher proportion of amorphous phase (as confirmed by XRD analysis) and the higher probability of connecting defects in thinner membranes would contribute to the lower separation factors for the membranes prepared with higher dilution. Fig. 2 shows the influence of synthesis time on the weight gain, water/ethanol separation factor and water flux. Initially, the weight gain increases with the synthesis time, until it reaches, a constant value at approximately 24 h due probably to the consumption of the reactants (crystals are also formed in the gel and collected at the bottom of the autoclave). After 24 h, a slight decrease of weight sometimes takes place, due to partial dissolution of the formed mordenite. The water/ ethanol separation factor in pervaporation shows a broad maximum with synthesis time between 8–24 h. The water flux remains at around 0.2 kg/ m 2h for most of the synthesis time interval explored. This means that membrane preparation

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A. Navajas et al. / Desalination 148 (2002) 25–29

Table 1 N2 permeance, weight gain and pervaporation results (water/ethanol separation factors and water fluxes) for membranes prepared at different synthesis times and gel dilution levels

Synthesis time Gel composition factor (X)

N2 permeance·107 (mol/m2·s·Pa)

Weight gain (mg/g)

Separation factor

Water flux (kg/m2·h)

48

1

0.86

15.7

20

0.15

24

1

0.88

18.0

170

0.16

24

Ҁ

1.16

8.4

100

0.10

24

½

6.70

8.6

20

0.20

18

1

1.22

12.7

160

0.20

12

1

0.93

13.4

160

0.15

12

Ҁ

0.80

10.7

60

0.16

8

1

0.30

10.2

120

0.18

8

1

0.49

11.1

310

0.33

6

1

1.00

13.6

60

0.28

4

1

0.22

8.6

110

0.18

2

1

0.76

11.1

125

0.25

1

1

0.88

7.6

95

0.15

0

—

0.8

—

100

is essentially complete after only 2 h of synthesis. Extending synthesis times to 18 h decreases the water flux and increases the water/ethanol separation factor, but both changes are of a small magnitude. Okamoto et al. [6] observed some similar trends when studying the influence of synthesis time of zeolite A membranes on the pervaporation results observed. They proposed the formation of an amorphous gel phase in contact with the seeds; subsequently this gel evolved to give zeolite A. A similar mechanism could be postulated in this case. The amorphous gel would accumulate onto the seeded surface of the support during the time required for heating to the reaction temperature (180ºC), at which the mordenite structure is formed with a considerable rate. The presence of this amorphous gel and the mordenite seeds was confirmed by XRD, as already discussed for the 2 h synthesis. The seed crystals grow inside the amorphous gel, to form a relatively dense

crystall-ine layer on top of the support, and crystalline deposits in its pores, given rise to an increase in the water/ethanol separation factor. 4. Conclusions The influence of synthesis conditions (synthesis time and level of dilution in the synthesis gel) during the preparation of mordenite membranes has been related to their performance in the dehydration of water/ethanol mixtures by pervaporation. XRD analysis indicates that mordenite was the only present zeolite in the membrane throughout the conditions investigates. Optimum synthesis times (in the range 8–24 h) and water dilution levels exist for this system. The best results were obtained with the following gel composition: H2O:SiO2:Na2O:Al2O3= 80:1:0.38:0.025 Under these synthesis conditions, the membranes

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Fig. 2. Weight gain (mg of zeolite/g of support) water/ ethanol separation factor and water pervaporation flux as a function of synthesis time and water content in the precursor gel.

formed had water/ethanol separation factors around 150 and water fluxes of approximately 0.20 kg/m2h, with maximum values of 0.33 kg/m2h and a separation factor of 310. Acknowledgement This work was financially supported by the European Union through the Project GRD1-199910320. References [1] M. Matsukata and E. Kikuchi, Zeolitic membranes: synthesis, properties, and prospects, Bull. Chem. Soc. Jap., 70 (1997) 2341–2356.

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[2] J. Coronas and J. Santamaria, Separations using zeolite membranes, Sep. Pur. Methods, 28 (1999) 127–177. [3] A. Tavolaro and E. Drioli, Zeolite Membranes, Adv. Mater., 11 (1999) 975–996. [4] J. Caro, M. Noack, P. Kölsch and R. Schäfer, Zeolite membranes-state of their development and perspective, Micro. Meso. Mater., 38 (2000) 3–24. [5] Y. Morigami, M. Kondo, J. Abe, H. Kita and K. Okamoto, The first large-scale pervaporation plant using tubular-type with zeolite NaA membrane, Sep. Pur. Tech., 25 (2001) 251–260. [6] M. Kondo, M. Komori, H. Kita and K. Okamoto, Tubular-type pervaporation module with zeolite NaA membrane, J. Membr. Sci., 133 (1997) 133–141. [7] K. Okamoto, H. Kita, K: Korii and K. Tanaka, Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation and vapor permeation of water/ organic liquid mixtures, Ind. Eng. Chem. Res., 40 (2001) 163–175. [8] I. Kumakiri, T. Yamaguchi and S Nakao, Preparation of zeolite A and faujasite membranes from a clear solution, Ind. Eng. Chem. Res., 38 (1999) 4682–4688. [9] J.J. Jafar and P.M. Budd, Separation of alcohol/water mixtures by pervaporation through zeolite A membranes, Micro. Mater., 12 (1997) 305–311. [10] D. Shah, K. Kissick, A. Ghorpade, R. Hannah and D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci., 179 (2000) 185–205. [11] X. Lin, E. Kikuchi and M. Matsukata, Preparation of mordenite membranes on α-alumina tubular supports for pervaporation of water–isopropyl alcohol mixtures, Chem. Commun., (2000) 957–958. [12] M. Noack, P. Kölsch, J. Caro, M. Schneider, P. Toussaint and I. Sieber, MFI membranes of different Si/Al ratios for pervaporation and steam permeation, Micro. Meso. Mater., 35(36) (2000) 253–265. [13] L. Casado, R. Mallada, C. Téllez, J. Coronas, M. Menéndez and J. Santamaría, Preparation of mordenite membranes for pervaporation of wateralcohol mixtures, submitted to Microp. Mesop. Mater. [14] K. Suzuki, Y. Kizoyumi, T. Sekine, K. Obata, Y. Shindo and S. Shin, Preparation and characterization of a zeolite layer, Chem. Express, 5 (1990) 793–796.