polyurethane-silica nanocomposite membranes for separation of propane/methane and ethane/methane

Polyurethane-Silica Nanocomposite Membranes for Separation ofPropane/Methane and Ethane/MethaneAfsaneh Khosravi,† Morteza Sadeghi,* Hadi Zare Banadkohi, and Mohammad Mehdi Talakesh

Chemical Engineering Department, Isfahan University of Technology, Isfahan 84154-8311, Iran

ABSTRACT: This study examines the role that silica nanoparticles play on the permeation of methane, ethane, and propanegases through two types of polyurethane (PU) membranes: one based on polyether and the other based on polyester. These PUmembranes are synthesized from polycaprolactone (PCL225) polyester and polypropylene glycol (PPG) polyether in a 1−3−2mol ratio of polyol/hexamethylenediisocyanate/1,4-butane diol. The prepared PU-silica membranes are characterized usingFourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC),thermogravimetric analysis (TGA), and wide-angle X-ray diffraction (WAXD) analyses. The characterization analyses confirmedthe nanoscale distribution of silica particles within the polymer matrix. Permeation experiments reveal that in polyether-basedPU, permeability first increases by increasing silica content up to 2.5%, and then decreases. The permeability of gases inpolyester-based PU constantly decreases by increasing silica nanoparticle loading. The selectivity for C3H8 over methaneincreases with the inclusion of silica particles in the polyether-based PU membranes, while it decreases in polyester-based PUmembranes. Our results indicate high propane permeability and propane/methane selectivity of polyether-based mixed matrixmembranes (MMMs) containing 12.5% silica at 2 bar pressure up to, 118 barrer and 7.01, respectively.

■ INTRODUCTIONThe recovery of higher hydrocarbons from raw natural gas isdesirable due to a myriad of factors, including the high value ofthese hydrocarbons in chemical feedstocks and the need forprevention of partial dissolution/softening of plastic pipes andmeters by liquid slugs through higher-molecular-weight hydro-carbon condensation.1,2 Hydrocarbon separations are usuallyachieved by pressure swing adsorption, rectification, cryogenicdistillation,3 and/or membranes. The application of membranetechnology for gas separations has recently become a promisingalternative to other traditional separation techniques because ofits environmental safety and economic advantages, includingpotential energy saving capacity and low capital investment.4,5

The growing interest in membranes has stimulated extensivestudies of hydrocarbon transport properties in variousmembranes.6−13 Polymeric membranes tend to be moreeconomical than other membranes because of their low cost,good intrinsic properties, and their ease of manufacture intodesirable forms, such as asymmetric hollow fibers or spiral-wound modules.14,15

It is well-known in rubbery polymers that permeabilityincreases with an increase in permeate condensability. Ascondensability is often linked to the size of a molecule, largemolecule hydrocarbons permeate preferentially in rubberypolymers. As a result, rubbery membranes show highpermeability and selectivity in hydrocarbon separations andexhibit more efficient separation to remove higher hydro-carbons from methane in comparison to glassy polymers.16−18

The most attractive rubbery polymers for this particularmembrane separation are those having low glass transitiontemperatures (Tg).

3,7

Schultz et al.6 tested more than 40 polymers and found n-butane/methane selectivity to be below 10 in all cases exceptfor poly(octyl methyl siloxane) (POMS) and poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes, in which

selectivity reached values of 12 and 27, respectively. Arrueboand Coronas prepared silicalite membranes for the removal ofheavy hydrocarbons from natural gas and found the n-butane/methane selectivity to be 14.1 Starannikova et al. have shownthat cis-polypentenamer (cis-PPM), a polymer with a low glasstransition temperature, is able to separate hydrocarbons frommethane with a C3/C1 selectivity of 11.3 and a C4/C1 selectivityof 53.7 Pinnau et al. investigated hydrocarbon/methane andhydrocarbon/hydrogen separation properties of polydimethyl-siloxane membranes. They showed that an increase in the feedvapor concentration at constant feed pressure and temperatureleads to an increase in gas permeability and selectivity of allmixture components. In addition, their results demonstrate thatat constant feed pressure and feed composition, a decrease intemperature results in a significant increase in hydrocarbon/methane and hydrocarbon/hydrogen selectivity in PDMSmembrane.8 Toy et al. studied the pure-gas and vaporpermeation and sorption properties of poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA) compared tothose of PTMSP, poly(4-methyl-2-pentyne) (PMP) andpoly(1-phenyl-1-propyne) (PPP) and scrutinized the effect oftemperature on their permeation properties. They represented,as temperature increases, the permeability in PTMSDPAincreases for permanent gases and decreases for larger, morecondensable gases.9 Belousov et al. used impregnated liquidmembranes for separation of C1−C4 hydrocarbon gases.10

Kuraoka et al. described the effect of hydrocarbon, which wasused as a glassy membrane surface modifier, for the length of asingle gas permeation through the porous glass membranes.11

Received: April 5, 2013Revised: December 30, 2013Accepted: January 10, 2014Published: January 10, 2014

Article

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© 2014 American Chemical Society 2011 dx.doi.org/10.1021/ie403322w | Ind. Eng. Chem. Res. 2014, 53, 2011−2021

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Polyurethanes are multiblock copolymers usually consistingof hard and soft segments. The hard segments are based ondiisocyanate and/or a chain extender, while the soft segmentscan be composed largely of polyester or polyether.19 Polyur-ethanes, which have low glass transition in their soft segmentsand have strong bonds between their hard segments, could beconsidered as mechanically stable membranes for hydrocarbonseparation.19 The hard segments act as fillers and physicalcross-links while the soft segments act as flexible chains thatlead to increased chain mobility in the polymer.20 Because ofthe difference of polarity between these segments, microphaseseparation can occur and affect the final properties ofpolyurethanes.12 While there have been many reports ofusing PU membranes20−24 in gas separation, only a few studieson using PU membranes for hydrocarbon separation can befound in the literature.19

Despite many attempts at maximizing membrane perform-ance by varying the molecular structure, polymers still exhibit atrade-off between permeability and selectivity. To overcomesuch limitations, many researchers have attempted toincorporate inorganic materials like silica, alumina, zeolites,and carbon molecular sieves to enhance selectivity andpermeability simultaneously.25−32 The act of adding inorganicparticles to membranes is essentially an attempt to synergizethe effect of both components by combination of theadvantages of each phase: high selectivity and desirablemechanical properties of the dispersed fillers as well aseconomic advantages and ease of processing of poly-mers.14,25−32

In our previous work,28 the gas separation properties ofpolyether-based PU-silica nanocomposite membranes for pureCO2, CH4, N2, and O2 gases were studied. Because of the highgas separation performance of the prepared nanocompositemembranes, we tried to evaluate the performance of theprepared membranes for hydrocarbon separations. Hence, theobjective of this paper is to quantify the effects of silicananoparticles on C2H6/CH4 and C3H8/CH4 separation with aPU-based membrane. This is the first report of the hydrocarbonvapor permeability and selectivity properties of polyester andpolyether-based silica- PU membranes.

2. EXPERIMENTAL SECTION

2.1. Materials. Hexamethylene diisocyanate (HDI, MerckCo.), polycaprolactone (PCL225, Mw = 2000, InteroxChemical, England), and polypropylene glycol (PPG, Mw =2000, Sigma Aldrich), dried at 80 °C under vacuum for 48 h aswell as 1,4-butanediol (BDO, Merck), dried over 4 Å molecularsieves, are used for polymer synthesis. Dimethylformamide(DMF, Merck Co.) solvent was used for membranepreparation. Tetraethoxysilane (TEOS, Merck Co.), 3-glycidyloxypropyl trimethoxysilane (GOTMS, Merck Co.),hydrochloric acid (HCl, Merck Co.), and ethanol are used

for preparation of silica particles. The CH4, C2H6, and C3H8(purity 99.5%) used for gas permeation tests are purchasedfrom Technical Gas Service. The N2 gas (purity 99.99%), usedfor checking the plasticizing effect of hydrocarbons, waspurchased from Ardestan Gas Co., Isfahan, Iran.

2.2. Preparation of PU Membrane, Silica Nano-particles, and PU/Silica Composite Membrane. Thepolyurethanes are synthesized by two step polymerization,described in our previous work.28 These polyurethanemembranes were synthesized from polycaprolactone(PCL225) polyester and polypropylene glycol (PPG) polyetherin a 1−3−2 mol ratio of polyol/hexamethylenediisocyanate/1,4-butane diol. Table 1 shows the chemical structure of theraw materials used for polymer synthesis. The PU membrane isprepared by the thermal phase inversion method. Polyurethanewas dissolved in DMF at 10 wt % concentration, at 70 °C. Theprepared polymer solution was passed through a filter with 100μm pore sizes. Then, it was cast in a Petri dish and placed in anoven at 65 °C for 24 h. Then for complete removal of thesolvent, the prepared films were kept in a vacuum oven at 65°C for another 10 h.Silica nanoparticles are synthesized by hydrolysis of TEOS as

described in our previous works.25,29 PU/silica nanocompositemembranes are prepared by the same method following theaddition of the silica sol in different weight fractions to thepolymer solution. Table 2 shows the names of samples and the

amount of silica nanoparticles in prepared hybrid PU/silicamembranes. All the prepared pure and nanocompositemembranes have a thickness in the range of 100−120 μm.

2.3. Membrane Characterization. The obtained func-tional groups and their interactions in synthesized PUs areinvestigated with a Bio-Rad FTS-7 Fourier transform infraredspectrometer (FTIR) in the range of 500−4000 cm−1. All thefilms used for FTIR measurement were prepared by casting the2 wt % PU solution on KBr discs. X-ray diffraction patterns arerecorded by monitoring the diffraction angle 2θ from 5° to 60°on a Philips X’Pert instrument under a voltage of 40 kV and acurrent of 40 mA. The morphology of the membranes and thepresence of silica nanoparticles are observed using a scanning

Table 1. Chemical Structure of Raw Materials Used for Polymer Synthesis

Table 2. Amount of Silica Nanoparticle in Hybrid PU/SilicaMembranes

samplesilica content (% wt) in

membrane samplesilica content (% wt) in

membrane

PPG-S0 0 PCL-S0 0PPG-S2 2.6 PCL-S2 2.5PPG-S5 6 PCL-S5 5PPG-S10 11.5 PCL-S10 10

PCL-S20 20PCL-S30 30

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie403322w | Ind. Eng. Chem. Res. 2014, 53, 2011−20212012

electron microscope (SEM, Philips XL30) operated ataccelerating voltage of 17 kV. Prior to the scanning, membranesamples are fractured in liquid nitrogen to obtain sharp brittlefractures without altering the morphology. The membranes aremounted on an aluminum disk and then sputter-coated withgold using a sputter coater (SCDOOS, Bal-Tec). Thermalproperties of the membranes are measured by differentialscanning calorimetry (DSC) using a Mettler-Toledo DSC822eat a heating rate of 5 °C/min and the temperature range of−120 to 300 °C. The thermal stability of membranes has beenevaluated by thermogravimetric analysis (TGA) using PL at aheating rate of 10 °C/min and the temperature range ofambient to 800 °C.2.4. Gas Permeation. Gas permeation experiments are

carried out for CH4, C2H6, and C3H8. The method used tomeasure gas permeability is the constant pressure method.33

The feed side pressure of the membrane cell is kept at 2, 4, or 6bar and the effective membrane surface area is 11.64 cm2. Thegas permeability is determined by the following equation:

=−

Pq

A p p t( )1 2 (1)

where P is the gas permeability (1 barrer = 1 × 10−10 cm3

(STP)/cm2·s·cmHg), q/t is the volumetric flow rate of thepermeate (cm3/s), L is the membrane thickness (cm), p1 and p2are the absolute pressures of the feed side and permeate side,respectively (cmHg), and A is the effective membrane area(cm2).The ideal selectivity, αA/B, of membranes is calculated from

eq 2.

α =PPA/B

A

B (2)

All data are based on pure gas measurements at steady-stateconditions using dense films. The selectivity of gases at 2 barpressure are reliable because under these pressure feed streamconditions no plasticization effects occurred in the mem-branes.34 To prevent any plasticizing effect on membranes, thegas permeation tests are performed in the following order: (1)

Figure 1. FTIR spectroscopy of polyether-based polyurethane/silica hybrid membranes.

Figure 2. FTIR spectroscopy of polyester-based polyurethane/silica hybrid membranes.



methane, (2) ethane, and (3) propane. In other words, we usedeach membrane first to measure the permeability of methane,then for ethane and at the end for propane. In addition, toevaluate the plasticizing effect on membranes, the permeabilityof nitrogen before and after the propane test was measured. At2 bar pressure, no changes have occurred in the permeability ofnitrogen, which implies that plasticization does not occur.

■ RESULTS AND DISCUSSION3.1. Characterization. 3.1.1. FTIR Analysis. FTIR is an

effective tool for studying the composition of polyurethanes,especially their molecular bonds. The results of the FTIRanalysis of pure silica, pure PUs, and hybrid membranes areshown in Figures 1 and 2. In this case, special focus is given tochanges occurring in carbonyl strength (Figure 3). As shown in

Figure 3, all spectra appear to be composed of two bands at theurethane carbonyl stretching region. The band centered around1720−1740 cm−1 is assigned to stretching of free urethanecarbonyl groups, while the band at 1680−1700 cm−1 isattributed to hydrogen-bonded urethane carbonyl groups.35,36

The intensity of free carbonyl vibration slowly decreases, andthe intensity of bonded carbonyl vibration increases. Thereduction in hydrogen bonding between ether and ester groupsof soft segments and urethane groups of hard segments isevidence that the OH groups in a portion of the silicananoparticles distributed in soft segment domains and form H-bonds with the carbonyl or ether groups of polyol. This impliesthat the amount of carbonyl groups and ethereal groups of softsegments that are available for hydrogen bonding to the N−Hof the urethane linkage in hard segments decreases, and as aresult N−H groups of polyurethane bond more to carbonylgroups of hard segments. Taking into account theseobservations for the hybrid membranes spectra, it can beconcluded that the silica particles are distributed more in softsegment domains, and the OH groups of silica particles have

good interaction with ethereal and carbonyl groups of softsegments.28 In addition, a study has recently been conductedon polyurethane silica nanocomposite and confirmed that thesilica particles are more distributed in soft segments byconnection of the OH groups in silica and in ether and estergroups in soft segment domains of PU.37

3.1.2. Thermal Analysis. The thermal properties of polyur-ethane membranes composed of silica nanoparticles areevaluated by DSC and TGA analyses. Figure 4 shows the

DSC thermograms of prepared polyether based PU−silicasamples. The Tg transition that occurred around −60 °C isrelated to the PPG chains in the soft segments. The DSCresults show that the Tg of a soft segment does not change, andall of the Tg transitions are around −60 °C. Malay et al.37 alsoshowed in their research that, by addition of silica nanoparticlesto polyurethane, the Tg transition of soft segments do notchange. As mentioned in the FTIR study, by addition of thesilica particles into polymer network, the OH groups in silicawould connect to the ether groups. One may expect theincrement in Tg by connection of rigid silica particles to mobilepolyol chains. Although the rigid silica connected to polyolchains are mentioned in FTIR, the connection of the ethergroups to urethane NH groups of urea decrease, and so, thetotal mobility of the soft segments does not changesignificantly. Therefore as reported by Malay et al., Tg doesnot change significantly. The two Tm peaks, a wide peak in therange of 120−170 °C, and a peak at 50−60 °C, are the result ofthe hard segment crystallinity. As PPG alone could notcrystallize,38,39 the crystallization peaks could only be caused bycrystallization of the hard segments. Figure 4 also shows that byincreasing the silica particles the crystalline peak of the hardsegments does not change remarkably. This leads to theconclusion that most of the silica particles are distributed in softsegment domains and a small population of the silica particleshas been introduced into hard segments. If silica particles weredistributed more in hard segments, it would be expected thatthe order of the chains in hard segments would reduce. Sinceless regularity of chains in the polymer reduces the crystallinity,the lack of significance change in crystallinity of hard segmentsshows the change in chain order at hard segments is small.Therefore, it would be concluded that silica particles could notinteract to hard domains significantly and the chain order inhard segment domains has remained unchanged.

Figure 3. FTIR spectroscopy of carbonyl strength at polyester-basedpolyurethane/silica hybrid membranes.

Figure 4. DSC thermograms of polyether based PU−silicamembranes.



The thermal stability of polyurethane−silica compositemembranes is studied using TGA. Figure 5 and 6 show the

weight reduction of prepared composite membranes by heating.Two different slopes of reduction are observed in this region.The first one is related to degradation of urethane bonds, whilethe second step is related to the thermal decomposition ofpolyol.40 As shown in Figure 5, the first step of degradation ofpolyether-based PU occurs around 270 °C, and the second stepof degradation starts around 370 °C, while nanocompositemembranes based on polyether degrade at about 20−30 °Chigher than the pure polymer degradation temperature. Thistwo-step degradation is not clearly apparent in polyester-basedPU. This may be due to the greater connection of the hard andsoft segments. In addition, as shown in Figures 5 and 6 innanocomposite membranes, the range of weight reduction,especially in the second step, is broadened by the presence ofsilica in the polymers. This phenomenon shows that thethermal stability of the polymer increases. The increasedbroadening in the slope of the reduction weights ofnanocomposites in the polyol step shows more interaction ofthe silica particles to these domains. Given this data, it could beconcluded that silica particles distribute to soft segments more.

3.1.3. WAXD Analysis. Wide angle X-ray diffraction(WAXD) is performed on the pure polymer and hybridmembranes in an attempt to observe any morphologicalchanges. Figures 7 and 8 illustrate the XRD pattern of the

polyether-based PU−silica and polyester-based PU−silicahybrid membranes, respectively. As shown, the crystal peaksappear at a 2θ value of 24.5° in both the pure polymers andhybrid membranes. These peaks are related to hard segmentcrystallization of polyether-based and polyester-based PUs. It iswell established in polyether based PU that polypropyleneglycol does not crystallize.38,39 So the resulting crystal peakmust be related to hard segment domains. Also, the samecrystal peak has been observed in the polyester-based PU and ismore intense than the peak in the polyether-based PU. Thiscrystal peak corresponds to the overlap of crystallized regions inpolyester-based PU. Figure 9 shows the XRD pattern of

polycaprolactone used in polyester-based PU. As shown inFigure 9, PCL has two crystal peaks at 2θ values of 21 and 24°.The more intense crystal peak in the polyester-based PU maybe due to a synergistic effect of the polycaprolactone and hardsegment crystals. Accordingly, the crystal peak in all PUs wouldlikely be related to hard segment crystallization which is theresult of the microphase separation of hard and soft segments.41

Figure 5. TGA analysis of PPG-based polyurethane−silica hybridmembranes.

Figure 6. TGA analysis of PCL-based polyurethane−silica hybridmembranes.

Figure 7. XRD pattern obtained for polyether based polyurethane−silica hybrid membranes.

Figure 8. XRD pattern obtained for polyester based polyurethane−silica hybrid membranes.

Figure 9. XRD pattern of the polycaprolactone monomer.



Furthermore, it is clear that the presence of silica nanoparticlesdoes not have a significant influence on the crystalline structureof the hard segment except for a mild reduction in the intensityof the broad peak. Therefore, it may be concluded that the silicananoparticles are most likely distributed in soft segments.On the basis of the reported FTIR, WXAD, DSC, and TGA

results, it could be concluded that most of the silicananoparticles have been distributed in soft segment domainsand interacted with the active groups present in the softsegment domains via hydrogen bonding.3.1.4. Scanning Electron Microscopy (SEM). SEM micro-

graphs of nanocomposite cross sections are used to observe thedistribution of particles in polymer matrix and compatibilitybetween the nanoparticles and the polymer matrix. In addition,the particle size distributions of the silica nanoparticles innanocomposites are verified. The cross-sectional morphologyof polyether and polyester-based PU nanocomposite mem-branes is shown in Figure 10. As demonstrated in the SEMimages, there are two types of dispersion of silica particles inthe polymers. There are some particles with no aggregationwhich indicate effective nanoscale mixing and homogeneousdispersion into the polymer matrix. Some of the particles,however, aggregated together to form larger particles within thepolymer. As shown in the SEM images most of the aggregatedparticles are smaller than 200 nm in size. It is also clear fromthese images that all membranes have a nonporous, densestructure and there are no pinholes, connected pores, or cracks.3.2. Gas Permeation Results. In this study, the

permeation of CH4, C2H6, and C3H8 gases through polyetherand polyester-based PU−silica hybrid membranes are inves-tigated at 25 °C and pressures of 2, 4, and 6 bar.

3.2.1. Evaluation of Silica Effect on Permeation. Thepermeability coefficient of gases through the polyether andpolyester based PU−silica hybrid membrane versus the weightfraction (wt %) of silica is reported in Figures 11 and 12. Asshown in these figures, in both polyether- and polyester-basedhybrid PU membranes, the permeability coefficient of gasesdecreases in the following order:

> >P P P(C H ) (C H ) (CH )3 8 2 6 4

The permeation of gases through the polymeric membranesis well explained via the solution−diffusion mechanism.33 Thesynthesized PUs in this study exhibit typical rubbery polymerproperties. As previously mentioned, solubility is the dominantmechanism in permeation of gases through a rubbery polymermatrix; consequently, the permeability of these polymers issolubility controlled. The solubility is dependent on thecondensability of the permeate gases within the membrane,and the condensability itself is related to the criticaltemperature of the gases.4 In other words, solubility coefficientsof gases well correlate with their critical temperature. Aspredicted by the critical temperature of propane, ethane, andmethane (TC CH4 = 190.4 K, TC C2H6 = 305.4 K, and TC C3H8 =369.8 K),9 polyurethane and other rubbery polymers are alwaysmore permeable to propane than to methane. Therefore, C3H8with the highest condensability would be expected to have thehighest permeability and CH4 with the lowest condensabilitywould be expected to have the lowest permeability.The obtained results also show that by increasing silica

nanoparticle content, the permeability of gases changesdifferently in the two types of polymers. In polyether-basedPU, increasing silica content up to 2.5% causes the permeability

Figure 10. SEM micrographs of cross section of hybrid membranes: (a) PPG-S2, (b) PPG-S10, (c) PCL-S5, (d) PCL-S20.



first to increase, and then to decrease. The permeability of gasesin polyester-based PU constantly decreases by increasing silicananoparticle loading. On the basis of the silica loading and typeof polymers, incorporating silica into the polymer matrix cancause two different consequences which control gas perme-ation: formation of absorption sites and formation of tortuouspaths in the polymer matrix. These two consequences havebeen described in detailed as follows:Previous studies have confirmed the formation of new phases

at the polymer−particle interface. The interface between thepolymer and silica particles could possibly be sites such asvoids, or a new phase with different morphology, which couldoffer suitable locations for gas sorption in hybrid membranes.42

By increasing silica nanoparticle loading up to 2.5% wt, thesenew phases can possibly provide suitable locations to absorb

the condensable gases within the polymer. Consequently, thepresence of these absorbing sites would explain the rise in gaspermeability which is seen in polyether-based PUs.WAXD and reported DSC studies28 clearly indicate there is

not a significant difference between crystallinity of the hardsegments of polyurethanes before and after adding silicananoparticles in polyether- and polyester-based hybrid PU/silica membranes. Furthermore, it has been concluded fromcharacterization studies that the silica nanoparticles are mostlikely distributed throughout soft segments. Therefore the silicananoparticles act as a gas barrier and change the gas pathway toa tortuous path, resulting in stronger effect of diffusivity on gastransport and also a decrease of diffusivity as the silica loadingincreases.Therefore, in up to a certain content of silica in polyether-

based PUs, the solubility contribution of the particlesdominates. In higher loading of silica particles, the diffusivityreduces due to silica particle barrier properties and diffusivitybecomes the dominant mechanism for permeation of thesecondensable gases.In contrast to polyether-based PUs, the less rubbery

character of polyester membranes offers more appropriatelocations to adsorb the silica particles in the polymer. Theseabsorption locations cause a stronger interaction between thesilica and polymer, which leads to the formation of fewer voidsor other suitable sites for gas sorption in the polymer.Therefore, the reduction of diffusivity due to the presence ofnanoparticles has an adverse effect, and consequently, the

Figure 11. Permeability of CH4, C2H6, and C3H8 in polyether basedPU and PU/silica hybrid membranes at 25 °C (a) at 2 bar pressure,(b) at 4 bar pressure, and (c) at 6 bar pressure.

Figure 12. Permeability of CH4, C2H6, and C3H8 in polyester basedPU and PU/silica hybrid membranes at 25 °C (a) at 2 bar pressureand (b) at 6 bar pressure.



permeability does not show any significant increase as a resultof the addition of silica particles.The order of the reduction in the gas permeability is

>

> ‐

propane (86.05%) ethane (74.62%)

methane (44.17%) (In PCL S10 at 2 bar)

>

‐

ethane (17.31%) methane (5.81%)

(In PPG S10 at 2 bar)

The above-mentioned permeability reduction is demonstratedin that the gas permeation of PCL-S10 samples has decreasedenormously. Indeed, in the polyester-based PU−silica nano-composites the diffusion mechanism dominates in gas transferthroughout the polymer. This results from more phase mixingof hard and soft segments. Therefore, by increasing the numberof silica nanoparticles, diffusion of gas molecules becomeslimited. Consequently, the gas permeation rate is significantlyreduced.In addition, based on domination of the diffusion mechanism

in controlling the changes occurred in gas permeation ofpolyester-based PUs, the larger molecular size gases arerestricted more in transport through the membrane by theaddition of nonpermeable particles in the polymer matrix.28

In the case of the PPG-S10 sample, the permeability of thepropane gas increases about 43.77% in comparison to the purepolymer. Furthermore, the order of gas permeability reductionis less than that of PCL-S10. Due to higher phase separation inpolyether-based PUs, the solubility mechanism is the dominantmechanism for gas permeation. Therefore, the addition of silicaparticles in the polymer cannot cause a marked change inpermeability.3.2.2. Effect of Feed Pressure on Permeation. The variation

of the permeation of studied gases by pressure change from 2 to6 bar is as follows:

>

> − ‐

propane (168.69%) ethane (15.10%)

methane ( 54.70%) (PCL S10)

>

> − ‐

propane (329.11%) ethane (20.85%)

methane ( 2.06%) (PPG S10)

Comparison of the permeabilities of penetrants at 2 barpressure to at 6 bar pressure indicates that raising the partialpressure of the feed side increases gas permeability. This mightbe due to an increase in the driving force and the enhancementof plasticization of the polymer by the penetrant. A possibleexplanation for the increase in the permeability afterplasticization is that hydrocarbons may loosen the entangle-ment of the polymer network and allow production ofinterstitial space of during intermolecular and intramolecularchain rearrangements.43−45 For this reason, the rate of gasdissolution increases within the polymer matrix by increasingthe pressure of condensable gases.

As the above-mentioned comparison shows, the permeabilityof this highly condensable gas, propane, increases more incomparison to other gases due to greater plasticization effects.The reduction in the permeation of methane with an increasein feed pressure may result from an increase in the glassyproperty of these two polymers by the addition of silicananoparticles. As demonstrated in the characterization analyses,the silica nanoparticles have good interaction with anddistribution within the soft segment phases in the polymer.Therefore, by adding silica, the mobility of the chains decreasesand the glassy-like behavior appears, causing variation in thepermeability of the poorly condensable methane gas as feedpressure is changed.A comparison of the propane permeation changes in two

kinds of polymer with a variation in feed pressure show that thepermeation increase in polyether-based PUs is significantlyhigher than that in polyester-based PUs. This phenomenonconfirms the greater rubbery nature and domination of thesolubility mechanism in polyether-based PUs in comparison topolyester-based PUs.

3.2.3. Evaluation of Plasticization Effect. Since the studiedgases (methane, ethane, and propane) in this research arehighly condensable, sorption of these gases in the polymer mayhave a significant effect on the nature of the polymer. In thecase of polymer softening, the ideal selectivity would varygreatly from the real selectivity. Therefore, in order to ensurethe accuracy of the ideal selectivity and to present it as an indexfor the actual behavior of membrane, it is necessary to definethe pressure at which no plasticizing occurs in the polymer.The test of membrane plasticizing is carried out by using

pure nitrogen gas, since nitrogen has no effect on polymerchains. The samples are tested by propane under 2 bar pressureand 25 °C. The nitrogen permeability before and afterpermeation of condensable gases indicated no softening occursin the polymer chains at 2 bar pressure (Table 3).Therefore, the results obtained at 2 bar pressure and 25 °C

are reasonable to report as actual properties of the studiedmembranes during hydrocarbon separation.

3.2.4. Evaluation of Selectivity. The C3H8/CH4 and C2H6/CH4 ideal selectivity values of PUs and PU−silica hybridmembranes based on polyether and polyester soft segments at25 °C and 2 and 6 bar feed pressure can be observed in Tables4 and 5, respectively. As reported in Tables 4 and 5, with theinclusion of silica nanoparticles, the gas selectivity ofmembranes does not follow in the same pattern.The C3H8/CH4 selectivity in polyether-based PUs increases

gradually with addition of silica nanoparticles. The increase inthe gas selectivity of polyether-based PUs by adding the silicananoparticles may be related to the increase in sorption sites inthe interface of the polymer and silica, which are suitable placesfor sorption of condensable gases in hybrid membranes.Therefore, the solubility of the condensable gases increases inthe polymer, and consequently, the selectivity of propane tomethane increases.

Table 3. Nitrogen Permeability before and after Applying Propane Permeation through Prepared Membranes at 2 bar Pressureand 25 °C

polyether-based membranes polyester-based membranes

N2 permeability (barrer) PPG-S0 PPG-S2 PPG-S5 PPG-S10 PCL-S0 PCL-S2 PCL-S5 PCL-S10 PCL-S20 PCL-S30

before 6.9 5.1 4.3 3.9 1.9 1.25 1.15 1.03 0.82 0.53after 7 5.05 4.3 3.8 1.91 1.25 1.18 1.01 0.81 0.53



As reported in the tables, the selectivity of propane/methaneand ethane/methane in polyether-based PUs and theirnanocomposites is more than that of polyester-based PUs. Asmentioned previously, due to greater phase separation,polyether-based polyurethane exhibits more rubber-like behav-ior than the polyester-based polyurethane. Therefore, inpolymers with greater rubbery characteristics, the sorption ofpropane is greater than that of “noncondensable” methane gas.Hence, the selectivity values of these gases are higher.Comparison of the obtained results with other studied results

for separation of propane and ethane from methane is reportedin Table 6. The results presented here indicate thatpolyurethane (PPG-HDI-BDO)−silica hybrid membraneshave reasonable permeability and selectivity among reported

polymers, and they have potential to be commercialized in thefield of C3H8/CH4 separation.

■ CONCLUSIONIn this research, the effect of silica nanoparticles on the gaspermeation properties of polyester- and polyether-based PUmembranes for the purpose of separation of methane, ethane,and propane has been investigated. The SEM, FTIR, DSC,TGA, and WAXD analyses have been selected to characterizethe hybrid membranes. With combined characterization results,it has been determined that most of the silica nanoparticleshave been distributed in the soft segment domains and haveinteracted with active groups in soft segment domains viahydrogen bonding. Furthermore, polyether-based PU has amore rubbery nature that polyester-based PU. The results ofthese gas permeability experiments indicate that by adding silicaparticles in polyether-based PU, permeability first increases, andthen decreases. The increase in permeability by increasing silicacontent may be the result of formation of active sites at thepolymer−silica interface that are suitable sites for adsorptioncondensable gases, and the reduction of permeability byincreasing silica content may be the result of the introductionof tortuous paths in membranes and reduction of gas diffusivity.Therefore, with the addition of a small amount of silica to themembrane, solubility enhancement is dominant, while at highersilica loadings the diffusivity reduction is dominant inpermeation of these condensable gases through PU−silicamembranes. In polyester-based PU, by increasing the amountof silica particles, permeability decreases.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +98311 3915645. Fax: +98311 3912677. E-mail address:[email protected].

Present Address†A.K.: Chemical Engineering, School for Engineering of Matter,Transport and Energy, Arizona State University, Tempe, AZ85287.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to Ryan P. Lively, Brian Kraftschik, andHeather Johnson for their English editing. This research wassupported by National Iranian Gas Company (NIGC).

■ ABBREVIATIONSαA/B = ideal gas selectivity1 Barrer = 1 ×10−10 cm3 (STP)/cm2·s·cmHgA = effective membrane area (cm2)L = membrane thickness (cm)p1 = absolute pressures of the feed side (cmHg)p2 = absolute pressures of the permeate side (cmHg)q/t = volumetric flow rate of the gas permeation (cm3/s)BDO = 1,4-butanediolDMF = dimethylformamideHCL = hydrochloric acidHDI = hexamethylene diisocyanatePCL225 = polycaprolactonePPG = polypropylene glycolTEOS = tetraethoxysilane

Table 4. Pure Gas Selectivity in Polyether-Based PUs andPU−Silica Hybrid Membranes at 25 °C at 2 and 6 barPressure

Selectivity

2 bar 6 bar

sample C2H6/CH4 C3H8/CH4 C2H6/CH4 C3H8/CH4

PPG-S0 3.02 4.61 3.45 18.96PPG-S2 2.71 5.66 3.15 29.07PPG-S5 2.61 6.86 3.22 30.05PPG-S10 2.65 7.03 3.44 31.41

Table 5. Pure Gas Selectivity in Polyester-Based PUs andPU−Silica Hybrid Membranes at 25 °C at 2 and 6 barPressure

Selectivity

2 bar 6 bar

sample C2H6/CH4 C3H8/CH4 C2H6/CH4 C3H8/CH4

PCL-S0 2.33 4.81 2.21 6.08PCL-S2 1.15 1.26 2.69 8.61PCL-S5 1.11 1.16 2.30 7.56PCL-S10 1.06 1.20 1.98 7.12PCL-S20 1.09 1.46 1.90 7.07PCL-S30 1.03 1.30 1.82 7.00

Table 6. Comparison of Obtained Result with Other StudiedResult for Separation of Propane and Ethane from Methane

permeability selectivity

membrane conditionC2H6/CH4

C3H8/CH4

silicalite1 temperature: 30 °C 2.1 2sweep gas: He, 50 mL (STP)/min

silicalite1 pressure: 1 bar Temperature: 30 °C 2.1 5.8sweep gas: He, 50 mL (STP)/min

PPM46 pressure: 1.01 bar 4.81 11.41temperature: 20 °C

PDMS47 Pressure: 1 ba 2.63 4.32Temperature: 30 °C

PDMS48 pressure: 4.46 bar 3.00 5.70temperature: 35 °C

PPG-S10 pressure: 2 bar 2.65 7.03temperature: 30 °C

PPG-S10 pressure: 6 bar 3.44 31.41temperature: 30 °C



mailto:[email protected]

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polyurethane-silica nanocomposite membranes for separation of propane/methane and ethane/methane

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