ZIF-71 as a potential ﬁ ller to prepare pervaporation membranes for bio-alcohol recovery

Metal – organic frameworks (MOFs) are an emerging class of materials for potential use in separation. ZIF-71 has super-hydrophobic pore channels and a very ﬂ exible pore architecture, which make ZIF-71 a potential candidate for recovering bio-alcohols from fermentation broths. In the present study, mixed matrix membranes (MMMs) based on PDMS and ZIF-71 were prepared for separation of alcohols (methanol, ethanol, isopropanol (IPA) or sec -butanol) from aqueous solutions. Experimental results reveal that the pervaporation performance of the PDMS membrane was improved in both ﬂ ux and separation factors upon embedment of ZIF-71. The pervaporation separation factors for alcohols of ZIF-71 ﬁ lled the PDMS membrane (PDMS : ZIF-71 ¼ 10 : 2) nearly double compared to those of un ﬁ lled PDMS membranes, suggesting that ZIF-71 is an excellent ﬁ ller to prepare organophilic pervaporation membranes for bio-alcohol recovery.


Introduction
Pervaporation is a membrane process for the separation of a liquid mixture by partial vaporization through a dense membrane, based on the solution-diffusion mechanism. 1pplication of pervaporation in bio-alcohol production attracts increasing scientic interest. 2,3An organophilic pervaporation unit coupled to fermentation can in situ recover bio-alcohols, meanwhile, reduce the product inhibition effect.The downstream bio-alcohol products can be puried by dehydration (hydrophilic pervaporation) aer distillation. 4To enhance the pervaporation performance of polymeric membranes, mixed matrix membranes (MMMs) were developed by incorporation of inorganic porous llers (zeolite, silica, silicalites.)][7][8][9][10] It was realized that the pervaporation performance can be enhanced by improving either the selective sorption or the selective diffusion or both via incorporation of appropriate llers.
Metal-organic frameworks (MOFs) are a large emerging class of crystalline hybrid inorganic-organic materials, which are formed by coordination of metal centres or clusters with organic ligand(s).MOFs offer a unique chemical versatility combined with a designable framework and permanent porosity.2][13] Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, are constructed from tetrahedral metal ions (M: e.g., Zn, Co) bridged by imidazolate (Im).The M-Im-M angle, which is similar to the Si-O-Si angle (145 ) in zeolites, has led to the synthesis of a large number of ZIFs with zeolite-type tetrahedral topologies. 14][21][22][23][24] [Cu II 2 (bza) 4 (pyz) n ] was the rst MOF applied in pervaporation as a ller. 20A loading of only 3 wt% in PDMS membranes already showed a separation factor increase from 2.0 and 2.3 to 6.5 and 6.2, as compared to an unlled PDMS membrane at room temperature for 5 wt% methanol and ethanol aqueous mixtures respectively.
ZIF-8 possesses a small pore window with a size of 0.34 nm and a relatively large cavity size of 1.16 nm.The sodalite (SOD) framework structure of ZIF-8 is highly exible.ZIF-8 was incorporated into a polymethylphenylsiloxane (PMPS) membrane to separate bio-alcohols. 21The 10 wt% ZIF-8 lled PMPS membrane showed almost doubled alcohol selectivities for 1 wt% aqueous alcohol solutions at 80 C. It was concluded that the hydrophobic channels and the gateopening effect made a signicant contribution to the selective permeation of alcohols, especially for iso-butanol.Interestingly and in contrast to the above, ZIF-8 was also used as a ller in polybenzimidazole (PBI) membranes for ethanol dehydration. 22The ZIF-8 lled PBI membranes showed a very high ux without much decrease in the water separation factor.This was explained by the molecular sieving effect of ZIF-8, leading to increased water diffusivity, hence an increased water ux.ZIF-7, with smaller windows (0.29 nm), has the same SOD topology and super-hydrophobicity as ZIF-8.Despite this hydrophobic character, ZIF-7 improved the dehydration efficiency of chitosan membranes. 25IF-71, having a RHO topology with lager cages (1.68 nm) interconnected through pore windows of 0.48 nm, is even more hydrophobic than ZIF-8.26,27 The rst ZIF-71 membrane was synthesized on a ZnO support by a reactive seeding method.28 Surprisingly, the ZIF-71 membrane has a higher separation factor of methanol (21.38) than ethanol (6.5) over water.In addition, the gate-opening effect was also found for ZIF-71.Later, ZIF-71 lled polyether-block-amide (PEBA) membranes were used for the recovery of bio-alcohols via pervaporation.23 The performance was improved compared to pure PEBA, but the obtained separation factors and uxes were low, probably mainly due to the choice of polymer.
Polydimethysiloxane (PDMS), which is a biocompatible, exible and hydrophobic polymer, has been widely used as a membrane material for organophilic pervaporation.In the present study, ZIF-71 particles were incorporated into PDMS membranes for pervaporation of aqueous alcohol solutions (methanol, ethanol, IPA, or sec-butanol).The inuence of ZIF-71 loading on (i) morphology and structure of PDMS membranes, (ii) membrane swelling and membrane contact angle, and (iii) ux and separation factors is investigated in order to fundamentally understand the ZIF-71 effect on the pervaporation performance of PDMS membranes.

ZIF-71 synthesis
A solution of zinc acetate (0.73 g) in 150 ml of methanol and a solution of 4,5-dichloroimidizole (2.2 g) in 150 ml of methanol were prepared separately.The two solutions were mixed under vigorous magnetic stirring for 30 min and le static at room temperature for 24 h.The methanol was removed aer the crystal precipitation and the remaining crystals were soaked in chloroform (2 Â 100 ml) for two days.To recover the crystals, the solution was centrifuged and the chloroform was decanted.The crystals were then dried under vacuum at 100 C for 24 h to remove the remaining solvents from the crystals.

Membrane synthesis
PVDF ultraltration membranes were used as supports.20 wt% of PVDF in NMP solutions was cast on a polypropylene non-woven support with a 200 mm casting knife and immersed in a room temperature water bath.The membranes were stored in water for two days and dried in air at room temperature.
A certain amount of ZIF-71 particles was dispersed in hexane by using a probe-type sonicator (Branson digital sonifer) in an ice bath.A PDMS solution (RTV615A : RTV15B ¼ 9 : 1) in hexane was pre-cross-linked at 60 C for 2 h.The above ZIF-71 solution was added into the pre-crosslinked PDMS solution to form a 10 wt% PDMS solution and the solution was sonicated for another 5 min.The ratio of PDMS and ZIF-71 in the solution was controlled from 10 : 0 to 10 : 4 (w/w).The composite membranes were prepared by coating the solutions on the PVDF support, which was taped onto a glass plate and placed at an angle of 60 .The coated membranes were cross-linked at 110 C. The solutions were poured in a Petri dish and crosslinked at 110 C to form lms for XRD, TGA characterization and solvent uptake experiments.
Characterization ZIF-71 and the MMMs were characterized by powder X-ray diffractometry (STOE StadiP diffractometer in high-throughput transmission mode employing Cu Ka1 radiation).
The morphologies of the ZIF-71 crystals and the membranes were characterized by scanning electron microscopy (SEM, JEOL-JSM-6010LV).To get sharp clear cross-sectional images of the membranes, the samples were fractured under liquid nitrogen.All samples were coated with a 1.5-2 nm Au layer to reduce sample charging under the electron beam using a Cressington HR208 high resolution sputter coater.
Attenuated Total Reection Infrared (ATR-IR, Bruker, Alpha) spectrometry was used to investigate the surface chemistry of the materials.
The thermal stability of the ZIF-71 lled PDMS membrane was investigated by thermo-gravimetric analysis (TGA, Q 500, TA Instruments, US) under nitrogen at a heating rate of 5 C min À1 from room temperature to 900 C.
The contact angle of the membranes was measured using a DSA 10 Mk2 (Krüss, Germany) system.

Membrane swelling
Dried pieces of the PDMS lm were immersed overnight in different solvents (water, methanol, ethanol, IPA or sec-butanol) at room temperature.Aer equilibrium was reached, the lms were weighed on a microbalance aer removing the solvent from the external surface by quickly wiping with a tissue paper.The amount of solvent uptake was recalculated to the amount of solvent sorbed per gram of lm.

Pervaporation
Pervaporation experiments were performed in a dead-end setup (Fig. 1).The dead-end cell has a feed volume of 1.5 l and an effective area of 12 cm 2 .The pervaporation was run at a constant temperature of 50 C.The permeate samples were collected using a cold trap immersed in liquid nitrogen aer stabilization for 2 h. 5 wt% aqueous alcohol solutions (methanol, ethanol, iso-propanol (IPA) or sec-butanol) were chosen as feeds to investigate the membrane performance.The concentration was measured by using an Automatic Digital Refractometer (RX-7000 a).Membrane ux and alcohol separation factors were calculated by: where J is the ux (g m À2 h À1 ), W the weight of the permeate sample (g), t the collecting sample time (h), A the membrane area (m 2 ), b i/j the separation factor, and C i,P and C i,F the component i concentration in the permeate and the feed (wt%).Pervaporation performance depends on both membrane properties and operating conditions.Therefore, normalizing the permeation ux to the driving force is very useful to understand the intrinsic membrane properties. 29,30The intrinsic membrane properties are the permeability P and the selectivity a i/j which are dened as follows: where J i is the permeation ux of component i, P vapour i,feed the equilibrium partial pressure of component i in the feed (kPa) calculated by soware Aspen Plus, n i,permeate the mole fraction of component i in the permeate side, P permeate the permeate side pressure (kPa), l the membrane thickness (m), P i and P j the permeability of component i and j (g m h À1 kPa À1 ), and M i and M j the mole mass of component i and j (g mol À1 ).

Membrane characterization
According to the SEM image (inset of Fig. 2), the ZIF-71 crystals have a uniform particle size of 1 mm.N 2 physisorption measurement of ZIF-71 shows a type I isotherm, characteristic for micro-porosity (Fig. 2).The calculated specic BET (Brunauer-Emmett-Teller) surface area, Langmuir surface area and micropore volume were 782 cm 2 g À1 , 1095 cm 2 g À1 and 0.37 cm 3 g À1 , respectively.XRD.The weak and broad peak in the XRD pattern of PDMS in Fig. 3 reects its amorphous property, as is known.The XRD patterns of ZIF-71 and ZIF-71 lled PDMS membranes match perfectly, suggesting that the polymer matrix does not alter the structure of ZIF-71.
ATR-IR.The ATR-IR spectra of PDMS, ZIF-71 and ZIF-71 lled PDMS are presented in Fig. 4. For PDMS, the peaks at 1259 cm À1 , 1076 cm À1 , 1018 cm À1 and 798 cm À1 can be assigned to the CH 3 symmetric bending in Si-CH 3 , Si-O-Si, and the CH 3 rocking in Si-CH 3 respectively.For ZIF-71, the characteristic peaks appear at 665 cm À1 (C-Cl vibration) and 1055 cm À1 (C-N vibration).The intensity of these peaks for the lled membranes increases with ZIF-71 loading.However, no new peaks or peak shis were observed for the ZIF-71 led PDMS.The results suggest that the ZIF-71 crystals are well preserved aer incorporation into the PDMS membrane.Both the measured ATR spectra and the XRD patterns are in good agreement with literature results. 26GA.The thermal decomposition temperature of ZIF-71 is about 450 C (Fig. 5), which is 50 C higher than for pure PDMS.The thermal stability of PDMS is improved to 470 C aer incorporation of ZIF-71.ATR-IR results reveal that there is no chemical interaction between ZIF-71 and PDMS.Therefore, the improved thermal stability is due to physical interaction only. 31ontact angle.For organophilic pervaporation, the hydrophobicity of the separation layer is very important.The solvent contact angles of ZIF-71 lled PDMS are slightly higher than for pure PDMS, as shown in Fig. 6, indicating that ZIF-71 improves the overall hydrophobicity of the membranes.Converting this into surface tensions, 32 the surface tension of PDMS changed from 21.24 mN m À1 to 18.56 mN m À1 for a PDMS : ZIF-71 ratio of 10 : 4.
Membrane morphology.ZIF-71 particles are well dispersed in the PDMS matrix even at higher ZIF-71 loadings, as shown in the cross-sectional images (Fig. 7).The thickness of the PDMS layer increases with increasing ZIF-71 loading as a result of increased viscosity of the coating solution.In addition, the cross-sectional images in Fig. 7 conrm that no visible interfacial voids exist between ZIF-71 and PDMS, suggesting good compatibility between them.The voids with very sharp edges present in the pictures were formed during fracturing of the membrane samples in liquid nitrogen.
ZIF-71 has a rather large crystal size of around 1 mm, as shown in Fig. 3, which facilitates settling down.Indeed, more ZIF-71 particles are observed at the bottom of the selective layers (Fig. 8).The surface of the pure PDMS membrane is very smooth and homogenous, as shown in Fig. 8.For ZIF-71 lled PDMS membranes, ZIF-71 particles are clearly visible as white spots.Undened edges around the crystals, in contrast to bulk ZIF-71 crystals, conrm a homogenous surface coverage by the PDMS polymer.

Membrane swelling
Pervaporation can be described by the solution-diffusion transport model.Pervaporation follows a three-step process: sorption-diffusion-desorption.The separation is based on   the selective sorption and diffusion.In the present study, membrane swelling was applied to investigate the ZIF-71 inuence on the properties of the lled membranes.As shown in Fig. 8 and as expected, the swelling of the unlled PDMS membrane is higher for longer chain alcohols.The Hildebrand solubility parameters for longer chain alcohols (29.6, 26.5, 23.6 and 22.1 MPa 0.5 for methanol, ethanol, IPA and sec-butanol, respectively) are much closer to those of PDMS (15.5 MPa 0.5 ), leading to a stronger affinity for the PDMS matrix. 33Methanol and ethanol uptakes in the PDMS lm increased upon lling with ZIF-71.ZIF-71 increases the solvent sorption capacity, thus actually playing the role of a solvent reservoir for these small alcohols.For IPA, the sorption capacity does not change substantially aer adding ZIF-71, and even decreases some what.This decrease is even more pronounced for sec-butanol.For the latter, this is in line with what could be theoretically calculated taking into account the full sorption capacity of the ller and the swelling of the lm (Fig. 9).
The PDMS swelling for this apolar alcohol is so high that it exceeds the maximum sorption capacity of the ller (i.e., the pore volume).Basu et al. studied the swelling behaviour of different types of MOF lled PDMS membranes by solvent uptake and also found a decrease in the uptake of IPA and toluene with increased ller loading. 19Indeed, for these hydrophobic alcohols, their major fraction is sorbed in the polymer matrix.Moreover, the overall uptake is also  inuenced by reduced mobility of the PDMS chain near the ller surface.The experimental uptake values indeed were lower than the theory values, which were calculated based on the pore volume of ZIF-71 and unlled PDMS uptakes.This evidences that the swelling of PDMS is reduced to some extent aer adding ZIF-71, due to the physical interactions between ZIF-71 and the PDMS matrix.

Pervaporation
The effect of ZIF-71 on the pervaporation performance of lled PDMS membranes was investigated by separation of 5 wt% aqueous alcohol solutions (methanol, ethanol, IPA, and sec-butanol) at four different ZIF-71 loadings at 50 C.
As shown in Fig. 10, the unlled PDMS membrane has separation factors of 5.3, 5.8, 9.0 and 16.3 for methanol, ethanol, IPA and sec-butanol, respectively.Indeed, the swelling results (Fig. 8) showed that PDMS had a higher swelling in longer chain alcohols.In addition, the alcohol vapour partial pressure difference for their aqueous solutions (3.3, 3.5, 5.3 and 4.7 kPa, respectively, at 5 wt%, 50 C) contributes to the difference in pervaporation driving force.This combined effect leads to the difference in both ux and separation factors.ZIF-71 contributes positively to the separation factor of the lled PDMS membranes, as shown in Fig. 10.The alcohol separation factors of ZIF-71 lled membranes increase quasi-linearly with ZIF-71 loading and reach maximum values of a meth ¼ 8.0, a eth ¼ 9.9, a IPA ¼ 13.6, and a sec-but ¼ 30.2.At higher ller loadings, separation factors mostly decrease, presumably due to the presence of membrane defects.It is very likely to form defects at higher ZIF loadings, especially if there is no chemical interaction between ZIF-71 and the PDMS matrix, as can be anticipated here.The maximum separation factors are almost twice as high as those for unlled PDMS membranes.
The cross-sectional SEM images in Fig. 7 show that the membrane thickness varies with ZIF-71 loading.The average thickness for the membranes is 7 mm (PDMS), 8 mm (10 : 1): 9 mm (10 : 2), 12 mm (10 : 3), and 15 mm (10 : 4).In order to understand the inuence of ZIF-71 on the transport process, all membrane uxes were normalized to 5 mm, as shown in Fig. 11.The alcohol and water uxes were calculated from the total ux and the permeate composition.In addition, in order to know the intrinsic membrane property, the membrane uxes were recalculated to permeabilities taking into account the respective driving force.
Aer correcting for this driving force contribution, it became obvious that all those membranes are more permeable to alcohols than to water, as all these membranes have higher alcohol permeabilities than water permeability.The methanol and ethanol uxes/permeabilities of ZIF-71 lled membranes increase linearly by increasing the ZIF-71 loading.As is known, the kinetic diameters 34,35 of methanol (0.39 nm) and ethanol (0.45 nm) are smaller than the window of ZIF-71 (0.48 nm).It is thus expected that the methanol and ethanol molecules can diffuse freely through ZIF-71 rather than through the dense PDMS, which is not very much swollen in these rather polar solvents (see Fig. 9).Although the kinetic diameter of IPA and sec-butanol 23 is in principle larger than the ZIF-71 pore window, increased permeabilities were also observed for IPA and sec-butanol upon adding the ller.This can probably be related to the gate opening effect of the exible structure of ZIF-71. 28The water permeabilities of ZIF-71 lled membranes were found to increase slightly as well with ZIF-71 loading.Water molecules are much smaller in size than the pore windows of ZIF-71.They might thus pass readily through the pores of ZIF-71, considering the high concentration of water compared to alcohols.This molecular sieving effect and the weak interactions between ZIF-71 and the PDMS matrix mentioned above can explain the unfavourable water ux increase.Conclusions ZIF-71 lled PDMS membranes were successfully prepared and applied in the pervaporation of 5 wt% aqueous alcohol (methanol, ethanol, IPA, sec-butanol) feed solutions.Adding ZIF-71 to the PDMS membranes signicantly improved both ux and separation factors of the membranes, thus demonstrating that ZIF-71 is an excellent ller for the fabrication of MMMs for organophilic pervaporation.However, this study also suggests that the use of micron-sized ZIF-71 particles is very likely to create defects in the membrane, especially at higher loadings.Therefore, nano-sized ZIF-71 could be highly desirable to prepare lled membranes with high loading but fewer defects.Organophilic pervaporation membranes with enhanced performance are expected via combination of such nano-sized ZIF-71 crystals and highly permeable polymers, such as PMPS or PTMSP.

Fig. 6
Fig. 6 Effect of ZIF-71 loading on water and alcohol contact angles of the PDMS membranes.

Fig. 7
Fig. 7 Cross-sectional images of filled PDMS membranes with different ZIF-71 loadings.The picture at the bottom right is a detailed image of the selective layer of the ZIF-71 filled membrane (PDMS : ZIF-71 ¼ 10 : 4).

Fig. 8 Fig. 9
Fig. 8 Surface images of the PDMS membrane and of the ZIF-71 filled PDMS membranes with different loadings.The picture at the bottom right shows the peeled-off selective layer of the ZIF-71 filled PDMS membrane (PDMS : ZIF-71 ¼ 10 : 1).

Fig. 10
Fig. 10 Effect of ZIF-71 loading in pervaporation on alcohol separation factors/selectivities of the PDMS membranes with different loadings.

Fig. 11
Fig. 11 Effect of ZIF-71 loading on the fluxes/permeabilities (fluxes were normalized to a 5 mm thickness) of the PDMS membranes.