Dehydration of alcohols by pervaporation using hydrophilic polyether urethane membranes

Transcription

1 Journal of Scientific DAS & Industrial et al: PERVAPORATION Research OF ALCOHOLS USING POLYETHER URETHANE MEMBRANES 219 Vol. 67, March 2008, pp Dehydration of alcohols by pervaporation using hydrophilic polyether urethane membranes Swatilekha Das, Suparna Sarkar, Piyali Basak and Basudam Adhikari* Materials Science Centre, Indian Institute of Technology, Kharagpur Received 22 September 2006; revised 09 January 2008; accepted 15 January 2008 This study presents pervaporation separation of methanol-water (MW), ethanol-water (EW) and isopropanol-water (IW) mixtures through a series of polyethylene glycol (PEG) based polyether urethane membranes. Five polyether urethane (PUPEG) membranes were developed by using PEGs of increasing molecular weight (PEG 200, PEG 400, PEG 600, PEG 1000 and PEG 4000 ). PUPEG membranes showed selectivity towards water, which increased with increasing molecular weight as well as increasing polymer chain length of the membrane. PUPEG membrane prepared using PEG 4000 showed highest separation performance in dehydrating water from three alcohol-water mixtures. A pervaporation flux of 0.48 kg/m 2.h was achieved with 94.5% water in permeates from 20% water present in isopropanol-water mixture using membrane. All PUPEG membranes showed water separation performances; flux and separation factor for water varied in the order: MW <EW<IW mixtures. Keywords: Flux, Pervaporation, Polyether urethane, Separation factor Introduction Purification of organic solvents such as methanolwater (MW), ethanol-water (EW) and isopropanol-water (IW) is of vital significance in organic synthesis. These solvents are widely used in chemical industries and in pharmaceutical laboratories 1. Among separation processes (extractive and azeotropic distillation, vacuum distillation, solvent extraction etc.), pervaporation (PV) offers high separation performance and low operation cost 2,3. Preparation of membranes with good PV performance is very important 4. Methods such as polymer blending 5, copolymerization 6, and introduction of bulky pendent group into polymer backbone 7 have been developed to fabricate a good membrane for selective removal of water by PV. Polymeric membranes (PUs) such as cellulose acetate 8, silicone imide 9, polyvinyl alcohol 10,11, asymmetric nylon 4 12 and chitosan 13 have been used to dehydrate alcohols by PV. Huang et al 14 utilized membranes of hydroxyl terminated polybutadiene (HTPB) and 4,4'-dicyclohexylmethane diisocyanate *Author for correspondence Tel: ; Fax: ba@matsc.iitkgp.ernet.in (H 12 MDI) based PU crosslinked with benzoyl peroxide (BPO) and then formed into an interpenetrating network (IPN) with 4-vinyl pyridine (4-VP) for PV separation of water-ethanol mixtures. Wang et al 15 synthesized a hydrophilic PU membrane with a H 12 MDI and grafted PU by 2,3-(epoxy propyl)-methacrylate (PU-g-EPMA). Being hydrophilic in nature, polyether based PUs were used 16 for dehydration of alcohols by hydrophilic PV. In present study, polyethylene glycol (PEG) based PUs have been prepared to investigate effects of molecular weight of PEG on PV separation performances of methanol-water (MW), ethanol-water (EW) and isopropanol-water (IW) mixures. Experimental Set up Materials Materials used include PEGs (PEG 200, PEG 400, PEG 600, PEG 1000, PEG 4000) (E. Merck, India), dibutyltindilaurate (DBTDL, used without purification) (Fluka), toluene diisocyanate (TDI, used as received) (E. Merck, Germany), tetrahydrofuran (THF, dried over metallic sodium) (E. Merck, India), sodium hydroxide pellet (Qualigans, India), and methanol, ethanol and isopropanol (E. Merck, India).

2 220 J SCI IND RES VOL 67 MARCH 2008 Synthesis of Polyethylene Glycol based Polyurethanes Five polyether based polyurethanes or polyether urethanes (PUPEG) were prepared in a round bottomed flask by reacting PEG 200, PEG 400, PEG 600, PEG 1000, PEG 4000 with TDI (NCO:OH = 2:1) in dry THF (50 wt% of reactants) using DBTDL (0.05 wt%) as catalyst with adequate precautionary measure of moisture guard and controlled stirring. Entrapped air during polymer formation was removed by applying vacuum. Resultant viscous solution of PU in THF was cast on flat-based glass petridish and allowed overnight for moisture curing (at ~50% RH) at room temperature followed by thermal curing at 80 C for 5 h. In order to remove residual unreacted TDI in the bulk interior, if any, PUPEG sheets were boiled in water for 1 h. Thin transparent slightly yellow sheets ( mm) of PUPEGs were dried and stored under vacuum in a desiccator. PUs synthesized from PEG 200, PEG 400, PEG 600, PEG 1000, PEG 4000 designated as, PUPEG 400, PUPEG 600, PUPEG 1000 and respectively. FTIR and XRD Analysis Synthesized PUPEG sheets were cleaned thoroughly with acetone, dried and kept in vacuum desiccators for 24 h. These films were used for FTIR (frequency, cm -1 ) analysis using Thermo Nicolet FTIR, Model Nexus 870, and XRD study using Phillips XRD Analyzer (Model PW 1729), Holland for the range 2θ = using Cu as target. Mechanical Properties Cured PUPEG films were kept in a vacuum desiccator overnight. According to ASTM specifications, dumbbell specimens (width, 5 mm) were cut out from polymer films and were subjected to test for mechanical properties like tensile strength, elongation at break, modulus using a Hounsfield H10KS Universal Tensile Testing Machine. Contact angle Measurement For hydrophilicity of synthesized PUPEGs, contact angle of polymer sheets were measured by using Rame Hart Goniometer (model ). Pervaporation (PV) PV cell was assembled from two cylindrical halfcells made of stainless steel fastened together by nuts and bolts. Membrane was supported on a sintered (perforated) stainless steel plate placed at the junction of two cells. For all measurements, downstream pressure was maintained at Pa (5 mm Hg) by applying vacuum. Effective membrane area for PV was m 2. The experiment was run for 3 h with no change in flux with time, confirming the steady-state operation. Permeate vapor-containing alcohol and water was collected in a glass condenser suspended inside a cryogenic trap kept at -40 C. Condensed permeate was warmed up to ambient temperature and then collected for analysis by refracting index measurement using Abbe refractometer. Permeate composition was analyzed by measurement of refractive index against a standard refractive index calibration curve for a wide range of alcohol water composition. Separation performances of these membranes were studied for a range of feed composition (water, 20-80%). All PV runs were repeated thrice for each of the feed compositions studied with % variations in flux data. Flux (J) was calculated from permeate weight collected after PV run using Eq. (1). Q J =...(1) A. t where, Q is total weight of permeates collected in time t and A is effective membrane area. Separation factor ± ij was calculated as y i/y j α ij =...(2) x /x i j where, x i and x j are weight fractions of components in feed, and y j and y i are weight fractions of components in permeate. Component i stands for water and j for alcohol. Separation effectiveness was characterized by separation selectivity of water compared to alcohols. Results and Discussion Membrane Preparation In synthesis of PUPEGs (, PUPEG 400, PUPEG 600, PUPEG 1000, ), when NCO to OH ratio was kept at 2.5:1 or above, polymer obtained was brittle in nature, and when the same was kept below 2:1, polymer obtained was a low viscous and sticky material. This observation dictated an optimization of mole ratio of NCO to OH so that polymer neither became brittle nor sticky in nature. With less TDI, some hydroxyl groups of

3 DAS et al: PERVAPORATION OF ALCOHOLS USING POLYETHER URETHANE MEMBRANES 221 OCN HO CH 2 CH 2 O H n O PEG C H 3 CH 2 CH 2 O CH 2 CH 2 O C n-1 + DBTDL Cat, THF, Room Temp. O Polyether urethane TDI N H H3 NCO C O NC H O Scheme 1 Scheme 2 (NH 2 )/ OCN^^^^NH CO NH^^^^NH 2 /( NCO) + PU oligomers Thermal curing through allophanate link formation OCN^^^^^^^^NCO PU oligomers ^^^^^^N CO NH^^^^^^^^^ C=O NH^^^^^^NH C=O ^^^^N CO NH^^^^^^^^ Scheme 3 PEG remain unreacted and results in sticky nature of the membrane. When more amount of TDI is present, all hydroxyl groups are reacted and a highly crosslinked structure is formed giving a brittle membrane. In synthesis of PUPEGs, PEGs were reacted with TDI in dry THF medium at room temperature with DBTDL as a catalyst (Scheme 1). Reaction continued for 2 h to obtain a viscous solution of PUPEGs in THF. Different PUPEG compositions were obtained by reacting PEG: TDI in mole ratios of 1:2 and 1:2.5 (Table 1). Before casting of these PUPEG membranes, major THF was removed under vacuum to avoid bubble formation. Subsequent heating in an oven at 80 C for 5 h carried out the residual curing of PUPEG membranes (Schemes 2 & 3). Curing of PUPEG was effected in two steps: i) Membrane was subjected to slow evaporation of residual solvent in the membrane with simultaneous reaction in presence of atmospheric moisture for 24 h; and ii) Isocyanate terminal groups of prepolyether urethane is converted to amine groups in presence of moisture. Amine groups further react with remaining terminal NCO groups for extension of PUPEG chain length. Concentration and rates of formation of the intermediates A, B and C etc. are dependent on moisture content and concentration of terminal NCO groups in PU oligomer. Polyurea structures B and C so formed participate in crosslinked PU structure formation (Scheme 3). FTIR Analysis In FTIR spectra of PUPEGs (Table 2), strong characteristic N-H stretching absorption peaks at cm -1 indicate the presence of hydrogen bonded urethane linkage. Shift of N-H stretching band towards lower value is attributed to the occurrence of hydrogen bonding. There is a strong peak at 1714 cm -1 due to carbonyl group of urethane bond. Another peak

4 222 J SCI IND RES VOL 67 MARCH 2008 Table 1 Compositions and physical properties of PEG based polyether urethane membranes Sample PEG PEG NCO Contact Tensile Elongation at angle, strength break, % Wt % Mole % Wt % Mole % MPa PEG PUPEG 400 PEG PUPEG 600 PEG PUPEG 1000 PEG PEG Table 2 FTIR peak assignment of PEG based polyether urethane membranes Membrane Wavenumber (cm -1 ) N-H streching >C=O peak of H-bonded >C=O peak C-O-C ether urethane of urethane linkage peak PUPEG PUPEG PUPEG observed at 1642 cm -1 is due to hydrogen-bonded carbonyl group of PU. There is a characteristic peak at 1110 cm -1, which confirms C-O-C aliphatic ether linkage. Since there is no characteristic peak at 2270 cm -1 for NCO group, it may be said that PUPEG does not contain free isocyanates. XRD Analysis From XRD patterns (Fig. 1), synthesized PUPEGs appear amorphous. Although PUs contained hard segments but due to lack of proper alignment of molecules, no crystalline phase was formed due to easy onset of crosslinking between segments of molecules. Molecular orientation was hindered on airside of cast film due to rapid solvent evaporation and moisture curing. If such reactions could be avoided during moisture curing, inter segmental alignment would have been possible to develop a crystalline phase in polymer matrix. Peak intensity decreased and peak at 22 broadened from to because soft segment content increased from to. Peak width (Fig. 1) at half height decreased gradually from to is attributed to the decrease in crystallite size. This indicates that PUPEG is more 4000 amorphous than other PUPEGs. Mechanical Properties Mechanical properties of PUPEGs such as tensile strength and elongation at break were measured (Table 1). In the morphology of PU, following two types of domain are seen: i) Hard domain formed by aromatic isocyanate group; and ii) Soft domain formed by PEG. shows highest tensile strength and lowest elongation at break and shows lowest tensile strength and highest elongation at break because soft segment is higher in. Contact Angle Surface hydrophilicity of PUPEG membranes is very important for water selectivity. Contact angle decreased with increasing length of PEG soft segment. showed highest contact angle value and showed lowest contact angle value (Table 1).

5 DAS et al: PERVAPORATION OF ALCOHOLS USING POLYETHER URETHANE MEMBRANES 223 Intensity (arbitrary units) PUPEG PUPEG PUPEG Fig. 1 XRD of PEG based polyether urethanes Swelling Properties Swelling behavior of PUPEG membranes is highly dependent on hydrophilicity of membranes (Fig. 2). Degree of swelling of all PUPEG membranes in EW increased with increase in water (40-50%) followed by a decrease at higher water concentration. Increasing molecular weights, hydrophilicity of PEGs increases due to more number of ether linkages. Except, degree of swelling of all other membranes in pure water has been found higher than that in pure alcohol and degree of swelling in pure water increased with increase in molecular weight of PEG in PU (Fig. 2). Degree of swelling of membranes increased due to flexible nature of molecular chains as well as due to increase of molecular weight of soft domain, which develops more interaction with water than alcohol. In a diffusion controlled swelling, molecules in the outer layer of membrane first undergo relaxation followed by diffusion of solvent molecules into the bulk 19. Although degree of swelling of membranes in pure alcohol is minimum but when a small proportion of water is present in alcohol, water molecules help relaxation of polymer molecules of membrane having hydrophilic polar atoms. This is the major reason for a jump of degree of swelling of the membranes in pure alcohol to that in alcohol water mixture. In general, region of maximum degree of swelling of all the membranes except has occurred for 30-60% water in alcohol, attributed to a combined effect of both alcohol and water. Up to 50% water in EW mixture, both water and alcohol permeate through the membrane at a comparable rate. After that, as concentration of alcohol decreases, degree of swelling decreases after 50% water in feed. As membrane swelling increases with increasing water content, more ethanol molecules are sorbed together with water molecules due to sorption coupling 20. Swollen polymer reduces resistance to alcohol permeation because of relaxation of polymeric chain. In IW mixture, all membranes swelled more by water than in MW and EW solutions of same concentration (Fig. 2). Highest degree of swelling of IW mixture is due to more influence of water than that of alcohol, which will be evident from the lower flux of isopropanol. Molecular sizes of methanol, ethanol and isopropanol are bigger than that of water. Polarity of isopropanol is lower than those of methanol and ethanol. Polar interaction with water decreases from methanol to isopropanol. Thus, in IW mixture, polar coupling

6 224 J SCI IND RES VOL 67 MARCH 2008 phenomenon is least and more water molecules are available for swelling. Effect of Feed Concentration on Pervaporation Parameters Flux of water is found to increase with the increase in its concentrations in feed (Fig. 3). PUPEG membranes are more selective towards water than alcohols. Separation factor of water (Fig. 4) decreased with increase in feed concentration. As water concentration in feed is increased, membrane is more swollen due to polymer chain flexibility and increased interaction with membrane polymer matrix. Molecular size of alcohols is larger than water. As the amount of alcohol increases in feed mixture, it becomes difficult for alcohol molecule to diffuse through swollen membrane. So, flux and separation factor of water are higher than that of alcohol. Fig. 2 Swelling of PUPEG membranes in (a) methanol-water, (b) ethanol-water and (c) isopropanol-water solutions at room temperature (30 C) for 48 h Effect of Membrane Composition on Pervaporation Parameters PV separation performance in dehydration of alcohols by PUPEG membranes increased from to (Figs 3 & 4). Dehydration performances of methanol, ethanol and isopropanol depend upon distribution of soft/hard segments as well as that of hydrophobic and hydrophilic domains of membranes. With increase in polymer chain length from to, flexible nature of molecular chains increased due to increase in soft segment. Soft domain proportion (wt% of PEG moiety) increases from to (Table 3). By using PEG of higher molecular weight as well as longer chain, molecular weight and chain length of prepared polyether urethane increased. Water selectivity of membranes increased with increasing molecular weight as well as increasing polymer chain length of membrane as evident from water flux and separation factor shown (Table 3, Figs 3 & 4). showed highest separation performance in dehydrating water from three alcoholwater mixtures. XRD patterns indicate that crystallinity of membrane is least among prepared membranes. More amorphous nature of membrane makes it more selective towards water molecule. Contact angle decreased in the order <PUPEG 400 <PUPEG 600 <PUPEG 1000 <. Membrane hydrophilicity increased in the order,>pupeg 400,>PUPEG 600,>PUPEG 1000 >. Hydrophilic behavior of membranes reflected on their PV separation of water from three different alcoholwater mixtures.

7 DAS et al: PERVAPORATION OF ALCOHOLS USING POLYETHER URETHANE MEMBRANES 225 Fig. 3 Variation of water flux in dehydration of (a) methanolwater, (b) ethanol-water and (c) isopropanol-water with feed concentration for PUPEG membranes Fig. 4 Variation of water separation factor in dehydration of (a) methanol-water, (b) ethanol-water and (c) isopropanol-water with feed concentration for PUPEG membranes at 30 C

8 226 J SCI IND RES VOL 67 MARCH 2008 Table 3 PUPEG composition and their pervaporation separation flux and separation factor for alcohol water mixture at a feed composition of 20% water in feed at 30 0 C Sample PEG PEG NCO Alcohols Water flux Alcohol flux Separation kg/m 2.h X 10 3 kg/m 2.h X 10 3 factor Wt % Wt % PEG Methanol Ethanol Isopropanol PUPEG 400 PEG Methanol Ethanol Isopropanol PUPEG 600 PEG Methanol Ethanol Isopropanol PUPEG 1000 PEG Methanol Ethanol Isopropanol PEG Methanol Ethanol Isopropanol Table 4. Pervaporation separation of water from alcohol-water mixtures using PUPEG membranes at 30 0 C Water-alcohol system Water in Water in permeate, % feed, % PUPEG 400 PUPEG 600 PUPEG 1000 Methanol-water Ethanol-water Isopropanol-water Effect of Different Feed Types on Pervaporation Parameters Separation efficiency of water in dehydration of three aqueous alcohols by all five membranes varied in the order MW, EW and IW mixtures (Table 4). PV dehydration of IW mixtures proceeds more easily than that of MW and EW mixtures due to larger molecular size of isopropanol and relatively weak coupling phenomenon 21 (less polar than methanol and ethanol) with water molecules and hydrophilic membranes. Higher permeation fluxes were observed for IW mixtures compared to MW and EW attributed to less polarity and less coupling in IW mixtures. Again, for the same reasons membranes are more efficient in dehydration of EW mixture compared to MW mixture. Conclusions Dehydration performances of three different alcohol-water mixtures by a series of PEG based PUPEG membranes were compared by membrane pervaporation. showed maximum hydrophilicity and minimum crystallinity. Thus membrane is best among five prepared membranes, which is also evident from degree of swelling values. Higher water permeation compared to alcohol suggests that developed

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