Polyurethane-Silica Nanocomposite Membranes for Separation of

Article pubs.acs.org/IECR

Polyurethane-Silica Nanocomposite Membranes for Separation of Propane/Methane and Ethane/Methane Afsaneh 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 propane gases through two types of polyurethane (PU) membranes: one based on polyether and the other based on polyester. These PU membranes are synthesized from polycaprolactone (PCL225) polyester and polypropylene glycol (PPG) polyether in a 1−3−2 mol ratio of polyol/hexamethylenediisocyanate/1,4-butane diol. The prepared PU-silica membranes are characterized using Fourier 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 confirmed the nanoscale distribution of silica particles within the polymer matrix. Permeation experiments reveal that in polyether-based PU, permeability first increases by increasing silica content up to 2.5%, and then decreases. The permeability of gases in polyester-based PU constantly decreases by increasing silica nanoparticle loading. The selectivity for C3H8 over methane increases with the inclusion of silica particles in the polyether-based PU membranes, while it decreases in polyester-based PU membranes. Our results indicate high propane permeability and propane/methane selectivity of polyether-based mixed matrix membranes (MMMs) containing 12.5% silica at 2 bar pressure up to, 118 barrer and 7.01, respectively.

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INTRODUCTION The recovery of higher hydrocarbons from raw natural gas is desirable due to a myriad of factors, including the high value of these hydrocarbons in chemical feedstocks and the need for prevention of partial dissolution/softening of plastic pipes and meters by liquid slugs through higher-molecular-weight hydrocarbon condensation.1,2 Hydrocarbon separations are usually achieved by pressure swing adsorption, rectification, cryogenic distillation,3 and/or membranes. The application of membrane technology for gas separations has recently become a promising alternative to other traditional separation techniques because of its environmental safety and economic advantages, including potential energy saving capacity and low capital investment.4,5 The growing interest in membranes has stimulated extensive studies of hydrocarbon transport properties in various membranes.6−13 Polymeric membranes tend to be more economical than other membranes because of their low cost, good intrinsic properties, and their ease of manufacture into desirable forms, such as asymmetric hollow fibers or spiralwound modules.14,15 It is well-known in rubbery polymers that permeability increases with an increase in permeate condensability. As condensability is often linked to the size of a molecule, large molecule hydrocarbons permeate preferentially in rubbery polymers. As a result, rubbery membranes show high permeability and selectivity in hydrocarbon separations and exhibit more efficient separation to remove higher hydrocarbons from methane in comparison to glassy polymers.16−18 The most attractive rubbery polymers for this particular membrane separation are those having low glass transition temperatures (Tg).3,7 Schultz et al.6 tested more than 40 polymers and found nbutane/methane selectivity to be below 10 in all cases except for poly(octyl methyl siloxane) (POMS) and poly(1trimethylsilyl-1-propyne) (PTMSP) membranes, in which © 2014 American Chemical Society

selectivity reached values of 12 and 27, respectively. Arruebo and Coronas prepared silicalite membranes for the removal of heavy hydrocarbons from natural gas and found the n-butane/ methane selectivity to be 14.1 Starannikova et al. have shown that cis-polypentenamer (cis-PPM), a polymer with a low glass transition temperature, is able to separate hydrocarbons from methane with a C3/C1 selectivity of 11.3 and a C4/C1 selectivity of 53.7 Pinnau et al. investigated hydrocarbon/methane and hydrocarbon/hydrogen separation properties of polydimethylsiloxane membranes. They showed that an increase in the feed vapor concentration at constant feed pressure and temperature leads to an increase in gas permeability and selectivity of all mixture components. In addition, their results demonstrate that at constant feed pressure and feed composition, a decrease in temperature results in a significant increase in hydrocarbon/ methane and hydrocarbon/hydrogen selectivity in PDMS membrane.8 Toy et al. studied the pure-gas and vapor permeation and sorption properties of poly[1-phenyl-2-[p(trimethylsilyl)phenyl]acetylene] (PTMSDPA) compared to those of PTMSP, poly(4-methyl-2-pentyne) (PMP) and poly(1-phenyl-1-propyne) (PPP) and scrutinized the effect of temperature on their permeation properties. They represented, as temperature increases, the permeability in PTMSDPA increases for permanent gases and decreases for larger, more condensable gases.9 Belousov et al. used impregnated liquid membranes for separation of C1−C4 hydrocarbon gases.10 Kuraoka et al. described the effect of hydrocarbon, which was used as a glassy membrane surface modifier, for the length of a single gas permeation through the porous glass membranes.11 Received: Revised: Accepted: Published: 2011

April 5, 2013 December 30, 2013 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/ie403322w | Ind. Eng. Chem. Res. 2014, 53, 2011−2021

Industrial & Engineering Chemistry Research

Article

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

for preparation of silica particles. The CH4, C2H6, and C3H8 (purity 99.5%) used for gas permeation tests are purchased from Technical Gas Service. The N2 gas (purity 99.99%), used for checking the plasticizing effect of hydrocarbons, was purchased from Ardestan Gas Co., Isfahan, Iran. 2.2. Preparation of PU Membrane, Silica Nanoparticles, and PU/Silica Composite Membrane. The polyurethanes are synthesized by two step polymerization, described in our previous work.28 These polyurethane membranes were synthesized from polycaprolactone (PCL225) polyester and polypropylene glycol (PPG) polyether in a 1−3−2 mol ratio of polyol/hexamethylenediisocyanate/ 1,4-butane diol. Table 1 shows the chemical structure of the raw materials used for polymer synthesis. The PU membrane is prepared by the thermal phase inversion method. Polyurethane was dissolved in DMF at 10 wt % concentration, at 70 °C. The prepared polymer solution was passed through a filter with 100 μm pore sizes. Then, it was cast in a Petri dish and placed in an oven at 65 °C for 24 h. Then for complete removal of the solvent, 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 nanocomposite membranes are prepared by the same method following the addition of the silica sol in different weight fractions to the polymer solution. Table 2 shows the names of samples and the

Polyurethanes are multiblock copolymers usually consisting of hard and soft segments. The hard segments are based on diisocyanate and/or a chain extender, while the soft segments can be composed largely of polyester or polyether.19 Polyurethanes, which have low glass transition in their soft segments and have strong bonds between their hard segments, could be considered as mechanically stable membranes for hydrocarbon separation.19 The hard segments act as fillers and physical cross-links while the soft segments act as flexible chains that lead to increased chain mobility in the polymer.20 Because of the difference of polarity between these segments, microphase separation can occur and affect the final properties of polyurethanes.12 While there have been many reports of using PU membranes20−24 in gas separation, only a few studies on using PU membranes for hydrocarbon separation can be found in the literature.19 Despite many attempts at maximizing membrane performance by varying the molecular structure, polymers still exhibit a trade-off between permeability and selectivity. To overcome such limitations, many researchers have attempted to incorporate inorganic materials like silica, alumina, zeolites, and carbon molecular sieves to enhance selectivity and permeability simultaneously.25−32 The act of adding inorganic particles to membranes is essentially an attempt to synergize the effect of both components by combination of the advantages of each phase: high selectivity and desirable mechanical properties of the dispersed fillers as well as economic advantages and ease of processing of polymers.14,25−32 In our previous work,28 the gas separation properties of polyether-based PU-silica nanocomposite membranes for pure CO2, CH4, N2, and O2 gases were studied. Because of the high gas separation performance of the prepared nanocomposite membranes, we tried to evaluate the performance of the prepared membranes for hydrocarbon separations. Hence, the objective of this paper is to quantify the effects of silica nanoparticles on C2H6/CH4 and C3H8/CH4 separation with a PU-based membrane. This is the first report of the hydrocarbon vapor permeability and selectivity properties of polyester and polyether-based silica- PU membranes.

Table 2. Amount of Silica Nanoparticle in Hybrid PU/Silica Membranes sample

silica content (% wt) in membrane

PPG-S0 PPG-S2 PPG-S5 PPG-S10

0 2.6 6 11.5

sample

silica content (% wt) in membrane

PCL-S0 PCL-S2 PCL-S5 PCL-S10 PCL-S20 PCL-S30

0 2.5 5 10 20 30

amount of silica nanoparticles in prepared hybrid PU/silica membranes. All the prepared pure and nanocomposite membranes have a thickness in the range of 100−120 μm. 2.3. Membrane Characterization. The obtained functional groups and their interactions in synthesized PUs are investigated with a Bio-Rad FTS-7 Fourier transform infrared spectrometer (FTIR) in the range of 500−4000 cm−1. All the films used for FTIR measurement were prepared by casting the 2 wt % PU solution on KBr discs. X-ray diffraction patterns are recorded by monitoring the diffraction angle 2θ from 5° to 60° on a Philips X’Pert instrument under a voltage of 40 kV and a current of 40 mA. The morphology of the membranes and the presence of silica nanoparticles are observed using a scanning

2. EXPERIMENTAL SECTION 2.1. Materials. Hexamethylene diisocyanate (HDI, Merck Co.), polycaprolactone (PCL225, Mw = 2000, Interox Chemical, England), and polypropylene glycol (PPG, Mw = 2000, Sigma Aldrich), dried at 80 °C under vacuum for 48 h as well as 1,4-butanediol (BDO, Merck), dried over 4 Å molecular sieves, are used for polymer synthesis. Dimethylformamide (DMF, Merck Co.) solvent was used for membrane preparation. Tetraethoxysilane (TEOS, Merck Co.), 3glycidyloxypropyl trimethoxysilane (GOTMS, Merck Co.), hydrochloric acid (HCl, Merck Co.), and ethanol are used 2012

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


Article

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

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

electron microscope (SEM, Philips XL30) operated at accelerating voltage of 17 kV. Prior to the scanning, membrane samples are fractured in liquid nitrogen to obtain sharp brittle fractures without altering the morphology. The membranes are mounted on an aluminum disk and then sputter-coated with gold using a sputter coater (SCDOOS, Bal-Tec). Thermal properties of the membranes are measured by differential scanning calorimetry (DSC) using a Mettler-Toledo DSC822e at a heating rate of 5 °C/min and the temperature range of −120 to 300 °C. The thermal stability of membranes has been evaluated by thermogravimetric analysis (TGA) using PL at a heating rate of 10 °C/min and the temperature range of ambient to 800 °C. 2.4. Gas Permeation. Gas permeation experiments are carried out for CH4, C2H6, and C3H8. The method used to measure gas permeability is the constant pressure method.33 The feed side pressure of the membrane cell is kept at 2, 4, or 6 bar and the effective membrane surface area is 11.64 cm2. The gas permeability is determined by the following equation:

P=

qS A(p1 − p2 )t

(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 the permeate (cm3/s), L is the membrane thickness (cm), p1 and p2 are 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.

αA/B =

PA PB

(2)

All data are based on pure gas measurements at steady-state conditions using dense films. The selectivity of gases at 2 bar pressure are reliable because under these pressure feed stream conditions no plasticization effects occurred in the membranes.34 To prevent any plasticizing effect on membranes, the gas permeation tests are performed in the following order: (1) 2013



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good interaction with ethereal and carbonyl groups of soft segments.28 In addition, a study has recently been conducted on polyurethane silica nanocomposite and confirmed that the silica particles are more distributed in soft segments by connection of the OH groups in silica and in ether and ester groups in soft segment domains of PU.37 3.1.2. Thermal Analysis. The thermal properties of polyurethane membranes composed of silica nanoparticles are evaluated by DSC and TGA analyses. Figure 4 shows the

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

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RESULTS AND DISCUSSION 3.1. 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 FTIR analysis of pure silica, pure PUs, and hybrid membranes are shown in Figures 1 and 2. In this case, special focus is given to changes occurring in carbonyl strength (Figure 3). As shown in

Figure 4. DSC thermograms of polyether based PU−silica membranes.

DSC thermograms of prepared polyether based PU−silica samples. The Tg transition that occurred around −60 °C is related to the PPG chains in the soft segments. The DSC results show that the Tg of a soft segment does not change, and all of the Tg transitions are around −60 °C. Malay et al.37 also showed in their research that, by addition of silica nanoparticles to polyurethane, the Tg transition of soft segments do not change. As mentioned in the FTIR study, by addition of the silica particles into polymer network, the OH groups in silica would connect to the ether groups. One may expect the increment in Tg by connection of rigid silica particles to mobile polyol chains. Although the rigid silica connected to polyol chains are mentioned in FTIR, the connection of the ether groups to urethane NH groups of urea decrease, and so, the total mobility of the soft segments does not change significantly. Therefore as reported by Malay et al., Tg does not change significantly. The two Tm peaks, a wide peak in the range of 120−170 °C, and a peak at 50−60 °C, are the result of the hard segment crystallinity. As PPG alone could not crystallize,38,39 the crystallization peaks could only be caused by crystallization of the hard segments. Figure 4 also shows that by increasing the silica particles the crystalline peak of the hard segments does not change remarkably. This leads to the conclusion that most of the silica particles are distributed in soft segment domains and a small population of the silica particles has been introduced into hard segments. If silica particles were distributed more in hard segments, it would be expected that the order of the chains in hard segments would reduce. Since less regularity of chains in the polymer reduces the crystallinity, the lack of significance change in crystallinity of hard segments shows the change in chain order at hard segments is small. Therefore, it would be concluded that silica particles could not interact to hard domains significantly and the chain order in hard segment domains has remained unchanged.

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

Figure 3, all spectra appear to be composed of two bands at the urethane carbonyl stretching region. The band centered around 1720−1740 cm−1 is assigned to stretching of free urethane carbonyl groups, while the band at 1680−1700 cm−1 is attributed to hydrogen-bonded urethane carbonyl groups.35,36 The intensity of free carbonyl vibration slowly decreases, and the intensity of bonded carbonyl vibration increases. The reduction in hydrogen bonding between ether and ester groups of soft segments and urethane groups of hard segments is evidence that the OH groups in a portion of the silica nanoparticles distributed in soft segment domains and form Hbonds with the carbonyl or ether groups of polyol. This implies that the amount of carbonyl groups and ethereal groups of soft segments that are available for hydrogen bonding to the N−H of the urethane linkage in hard segments decreases, and as a result N−H groups of polyurethane bond more to carbonyl groups of hard segments. Taking into account these observations for the hybrid membranes spectra, it can be concluded that the silica particles are distributed more in soft segment domains, and the OH groups of silica particles have 2014



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

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

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

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

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

polyether-based PU−silica and polyester-based PU−silica hybrid membranes, respectively. As shown, the crystal peaks appear at a 2θ value of 24.5° in both the pure polymers and hybrid membranes. These peaks are related to hard segment crystallization of polyether-based and polyester-based PUs. It is well established in polyether based PU that polypropylene glycol does not crystallize.38,39 So the resulting crystal peak must be related to hard segment domains. Also, the same crystal peak has been observed in the polyester-based PU and is more intense than the peak in the polyether-based PU. This crystal peak corresponds to the overlap of crystallized regions in polyester-based PU. Figure 9 shows the XRD pattern of

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

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, while the second step is related to the thermal decomposition of polyol.40 As shown in Figure 5, the first step of degradation of polyether-based PU occurs around 270 °C, and the second step of degradation starts around 370 °C, while nanocomposite membranes based on polyether degrade at about 20−30 °C higher than the pure polymer degradation temperature. This two-step degradation is not clearly apparent in polyester-based PU. This may be due to the greater connection of the hard and soft segments. In addition, as shown in Figures 5 and 6 in nanocomposite membranes, the range of weight reduction, especially in the second step, is broadened by the presence of silica in the polymers. This phenomenon shows that the thermal stability of the polymer increases. The increased broadening in the slope of the reduction weights of nanocomposites in the polyol step shows more interaction of the silica particles to these domains. Given this data, it could be concluded that silica particles distribute to soft segments more.

Figure 9. XRD pattern of the polycaprolactone monomer.

polycaprolactone used in polyester-based PU. As shown in Figure 9, PCL has two crystal peaks at 2θ values of 21 and 24°. The more intense crystal peak in the polyester-based PU may be due to a synergistic effect of the polycaprolactone and hard segment crystals. Accordingly, the crystal peak in all PUs would likely be related to hard segment crystallization which is the result of the microphase separation of hard and soft segments.41 2015



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

3.2.1. Evaluation of Silica Effect on Permeation. The permeability coefficient of gases through the polyether and polyester based PU−silica hybrid membrane versus the weight fraction (wt %) of silica is reported in Figures 11 and 12. As shown in these figures, in both polyether- and polyester-based hybrid PU membranes, the permeability coefficient of gases decreases in the following order:

Furthermore, it is clear that the presence of silica nanoparticles does not have a significant influence on the crystalline structure of the hard segment except for a mild reduction in the intensity of the broad peak. Therefore, it may be concluded that the silica nanoparticles 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 silica nanoparticles have been distributed in soft segment domains and interacted with the active groups present in the soft segment domains via hydrogen bonding. 3.1.4. Scanning Electron Microscopy (SEM). SEM micrographs of nanocomposite cross sections are used to observe the distribution of particles in polymer matrix and compatibility between the nanoparticles and the polymer matrix. In addition, the particle size distributions of the silica nanoparticles in nanocomposites are verified. The cross-sectional morphology of polyether and polyester-based PU nanocomposite membranes is shown in Figure 10. As demonstrated in the SEM images, there are two types of dispersion of silica particles in the polymers. There are some particles with no aggregation which indicate effective nanoscale mixing and homogeneous dispersion into the polymer matrix. Some of the particles, however, aggregated together to form larger particles within the polymer. As shown in the SEM images most of the aggregated particles are smaller than 200 nm in size. It is also clear from these images that all membranes have a nonporous, dense structure 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 polyether and polyester-based PU−silica hybrid membranes are investigated at 25 °C and pressures of 2, 4, and 6 bar.

P(C3H8) > P(C2H6) > P(CH4)

The permeation of gases through the polymeric membranes is well explained via the solution−diffusion mechanism.33 The synthesized PUs in this study exhibit typical rubbery polymer properties. As previously mentioned, solubility is the dominant mechanism in permeation of gases through a rubbery polymer matrix; consequently, the permeability of these polymers is solubility controlled. The solubility is dependent on the condensability of the permeate gases within the membrane, and the condensability itself is related to the critical temperature of the gases.4 In other words, solubility coefficients of gases well correlate with their critical temperature. As predicted by the critical temperature of propane, ethane, and methane (TC CH4 = 190.4 K, TC C2H6 = 305.4 K, and TC C3H8 = 369.8 K),9 polyurethane and other rubbery polymers are always more permeable to propane than to methane. Therefore, C3H8 with the highest condensability would be expected to have the highest permeability and CH4 with the lowest condensability would be expected to have the lowest permeability. The obtained results also show that by increasing silica nanoparticle content, the permeability of gases changes differently in the two types of polymers. In polyether-based PU, increasing silica content up to 2.5% causes the permeability 2016



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

the condensable gases within the polymer. Consequently, the presence of these absorbing sites would explain the rise in gas permeability which is seen in polyether-based PUs. WAXD and reported DSC studies28 clearly indicate there is not a significant difference between crystallinity of the hard segments of polyurethanes before and after adding silica nanoparticles in polyether- and polyester-based hybrid PU/ silica membranes. Furthermore, it has been concluded from characterization studies that the silica nanoparticles are most likely distributed throughout soft segments. Therefore the silica nanoparticles act as a gas barrier and change the gas pathway to a tortuous path, resulting in stronger effect of diffusivity on gas transport and also a decrease of diffusivity as the silica loading increases. Therefore, in up to a certain content of silica in polyetherbased PUs, the solubility contribution of the particles dominates. In higher loading of silica particles, the diffusivity reduces due to silica particle barrier properties and diffusivity becomes the dominant mechanism for permeation of these condensable gases. In contrast to polyether-based PUs, the less rubbery character of polyester membranes offers more appropriate locations to adsorb the silica particles in the polymer. These absorption locations cause a stronger interaction between the silica and polymer, which leads to the formation of fewer voids or other suitable sites for gas sorption in the polymer. Therefore, the reduction of diffusivity due to the presence of nanoparticles has an adverse effect, and consequently, the

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

first to increase, and then to decrease. The permeability of gases in polyester-based PU constantly decreases by increasing silica nanoparticle loading. On the basis of the silica loading and type of polymers, incorporating silica into the polymer matrix can cause two different consequences which control gas permeation: formation of absorption sites and formation of tortuous paths in the polymer matrix. These two consequences have been described in detailed as follows: Previous studies have confirmed the formation of new phases at the polymer−particle interface. The interface between the polymer and silica particles could possibly be sites such as voids, or a new phase with different morphology, which could offer suitable locations for gas sorption in hybrid membranes.42 By increasing silica nanoparticle loading up to 2.5% wt, these new phases can possibly provide suitable locations to absorb 2017



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Table 3. Nitrogen Permeability before and after Applying Propane Permeation through Prepared Membranes at 2 bar Pressure and 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 after

6.9 7

5.1 5.05

4.3 4.3

3.9 3.8

1.9 1.91

1.25 1.25

1.15 1.18

1.03 1.01

0.82 0.81

0.53 0.53

As the above-mentioned comparison shows, the permeability of this highly condensable gas, propane, increases more in comparison to other gases due to greater plasticization effects. The reduction in the permeation of methane with an increase in feed pressure may result from an increase in the glassy property of these two polymers by the addition of silica nanoparticles. As demonstrated in the characterization analyses, the silica nanoparticles have good interaction with and distribution within the soft segment phases in the polymer. Therefore, by adding silica, the mobility of the chains decreases and the glassy-like behavior appears, causing variation in the permeability of the poorly condensable methane gas as feed pressure is changed. A comparison of the propane permeation changes in two kinds of polymer with a variation in feed pressure show that the permeation increase in polyether-based PUs is significantly higher than that in polyester-based PUs. This phenomenon confirms the greater rubbery nature and domination of the solubility mechanism in polyether-based PUs in comparison to polyester-based PUs. 3.2.3. Evaluation of Plasticization Effect. Since the studied gases (methane, ethane, and propane) in this research are highly condensable, sorption of these gases in the polymer may have a significant effect on the nature of the polymer. In the case of polymer softening, the ideal selectivity would vary greatly from the real selectivity. Therefore, in order to ensure the accuracy of the ideal selectivity and to present it as an index for the actual behavior of membrane, it is necessary to define the 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 polymer chains. The samples are tested by propane under 2 bar pressure and 25 °C. The nitrogen permeability before and after permeation of condensable gases indicated no softening occurs in 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 studied membranes during hydrocarbon separation. 3.2.4. Evaluation of Selectivity. The C3H8/CH4 and C2H6/ CH4 ideal selectivity values of PUs and PU−silica hybrid membranes based on polyether and polyester soft segments at 25 °C and 2 and 6 bar feed pressure can be observed in Tables 4 and 5, respectively. As reported in Tables 4 and 5, with the inclusion of silica nanoparticles, the gas selectivity of membranes does not follow in the same pattern. The C3H8/CH4 selectivity in polyether-based PUs increases gradually with addition of silica nanoparticles. The increase in the gas selectivity of polyether-based PUs by adding the silica nanoparticles may be related to the increase in sorption sites in the interface of the polymer and silica, which are suitable places for sorption of condensable gases in hybrid membranes. Therefore, the solubility of the condensable gases increases in the polymer, and consequently, the selectivity of propane to methane increases.

permeability does not show any significant increase as a result of 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 demonstrated in that the gas permeation of PCL-S10 samples has decreased enormously. Indeed, in the polyester-based PU−silica nanocomposites the diffusion mechanism dominates in gas transfer throughout the polymer. This results from more phase mixing of hard and soft segments. Therefore, by increasing the number of silica nanoparticles, diffusion of gas molecules becomes limited. Consequently, the gas permeation rate is significantly reduced. In addition, based on domination of the diffusion mechanism in controlling the changes occurred in gas permeation of polyester-based PUs, the larger molecular size gases are restricted more in transport through the membrane by the addition of nonpermeable particles in the polymer matrix.28 In the case of the PPG-S10 sample, the permeability of the propane gas increases about 43.77% in comparison to the pure polymer. Furthermore, the order of gas permeability reduction is less than that of PCL-S10. Due to higher phase separation in polyether-based PUs, the solubility mechanism is the dominant mechanism for gas permeation. Therefore, the addition of silica particles in the polymer cannot cause a marked change in permeability. 3.2.2. Effect of Feed Pressure on Permeation. The variation of the permeation of studied gases by pressure change from 2 to 6 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 bar pressure to at 6 bar pressure indicates that raising the partial pressure of the feed side increases gas permeability. This might be due to an increase in the driving force and the enhancement of plasticization of the polymer by the penetrant. A possible explanation for the increase in the permeability after plasticization is that hydrocarbons may loosen the entanglement of the polymer network and allow production of interstitial space of during intermolecular and intramolecular chain rearrangements.43−45 For this reason, the rate of gas dissolution increases within the polymer matrix by increasing the pressure of condensable gases. 2018



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

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

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CONCLUSION In this research, the effect of silica nanoparticles on the gas permeation properties of polyester- and polyether-based PU membranes 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 characterize the hybrid membranes. With combined characterization results, it has been determined that most of the silica nanoparticles have been distributed in the soft segment domains and have interacted with active groups in soft segment domains via hydrogen bonding. Furthermore, polyether-based PU has a more rubbery nature that polyester-based PU. The results of these gas permeability experiments indicate that by adding silica particles in polyether-based PU, permeability first increases, and then decreases. The increase in permeability by increasing silica content may be the result of formation of active sites at the polymer−silica interface that are suitable sites for adsorption condensable gases, and the reduction of permeability by increasing silica content may be the result of the introduction of tortuous paths in membranes and reduction of gas diffusivity. Therefore, with the addition of a small amount of silica to the membrane, solubility enhancement is dominant, while at higher silica loadings the diffusivity reduction is dominant in permeation of these condensable gases through PU−silica membranes. In polyester-based PU, by increasing the amount of silica particles, permeability decreases.

Selectivity 2 bar

6 bar

sample

C2H6/CH4

C3H8/CH4

C2H6/CH4

C3H8/CH4

PPG-S0 PPG-S2 PPG-S5 PPG-S10

3.02 2.71 2.61 2.65

4.61 5.66 6.86 7.03

3.45 3.15 3.22 3.44

18.96 29.07 30.05 31.41

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

6 bar

sample

C2H6/CH4

C3H8/CH4

C2H6/CH4

C3H8/CH4

PCL-S0 PCL-S2 PCL-S5 PCL-S10 PCL-S20 PCL-S30

2.33 1.15 1.11 1.06 1.09 1.03

4.81 1.26 1.16 1.20 1.46 1.30

2.21 2.69 2.30 1.98 1.90 1.82

6.08 8.61 7.56 7.12 7.07 7.00

As reported in the tables, the selectivity of propane/methane and ethane/methane in polyether-based PUs and their nanocomposites is more than that of polyester-based PUs. As mentioned previously, due to greater phase separation, polyether-based polyurethane exhibits more rubber-like behavior than the polyester-based polyurethane. Therefore, in polymers with greater rubbery characteristics, the sorption of propane 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 reported in Table 6. The results presented here indicate that polyurethane (PPG-HDI-BDO)−silica hybrid membranes have reasonable permeability and selectivity among reported

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Corresponding 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, AZ 85287. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ryan P. Lively, Brian Kraftschik, and Heather Johnson for their English editing. This research was supported by National Iranian Gas Company (NIGC).

Table 6. Comparison of Obtained Result with Other Studied Result for Separation of Propane and Ethane from Methane permeability selectivity membrane

condition

silicalite1

temperature: 30 °C sweep gas: He, 50 mL (STP)/min pressure: 1 bar Temperature: 30 °C sweep gas: He, 50 mL (STP)/min pressure: 1.01 bar temperature: 20 °C Pressure: 1 ba Temperature: 30 °C pressure: 4.46 bar temperature: 35 °C pressure: 2 bar temperature: 30 °C pressure: 6 bar temperature: 30 °C

silicalite1 PPM46 PDMS47 PDMS48 PPG-S10 PPG-S10

C2H6/ CH4

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C3H8/ CH4

2.1

2

2.1

5.8

4.81

11.41

2.63

4.32

3.00

5.70

2.65

7.03

3.44

31.41

AUTHOR INFORMATION

2019

ABBREVIATIONS αA/B = ideal gas selectivity 1 Barrer = 1 ×10−10 cm3 (STP)/cm2·s·cmHg A = 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-butanediol DMF = dimethylformamide HCL = hydrochloric acid HDI = hexamethylene diisocyanate PCL225 = polycaprolactone PPG = polypropylene glycol TEOS = tetraethoxysilane dx.doi.org/10.1021/ie403322w | Ind. Eng. Chem. Res. 2014, 53, 2011−2021


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