CH4 separation

CH4 separation

Author's Accepted Manuscript Mixed matrix membranes incorporated with amine-functionalized titanium-based metalorganic framework for CO2/CH4 separati...

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Author's Accepted Manuscript

Mixed matrix membranes incorporated with amine-functionalized titanium-based metalorganic framework for CO2/CH4 separation Xiangyu Guo, Hongliang Huang, Yujie Ban, Qingyuan Yang, Yuanlong Xiao, Yanshuo Li, Weishen Yang, Chongli Zhong

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PII: DOI: Reference:

S0376-7388(15)00023-X http://dx.doi.org/10.1016/j.memsci.2015.01.007 MEMSCI13408

To appear in:

Journal of Membrane Science

Received date: 6 October 2014 Revised date: 26 December 2014 Accepted date: 6 January 2015 Cite this article as: Xiangyu Guo, Hongliang Huang, Yujie Ban, Qingyuan Yang, Yuanlong Xiao, Yanshuo Li, Weishen Yang, Chongli Zhong, Mixed matrix membranes incorporated with amine-functionalized titanium-based metalorganic framework for CO2/CH4 separation, Journal of Membrane Science, http: //dx.doi.org/10.1016/j.memsci.2015.01.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mixed matrix membranes incorporated with amine-functionalized titanium-based metal-organic framework for CO2/CH4 separation Xiangyu Guoa, Hongliang Huanga, Yujie Banb, Qingyuan Yanga,*, Yuanlong Xiaoa, Yanshuo Lib,*, Weishen Yangb, Chongli Zhonga a

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

* Corresponding author. Email address: [email protected], [email protected]

Abstract. High performance mixed matrix membranes arise from a targeted selection of constituent materials, especially the choice of fillers. An amine-functionalized metal-organic framework (MOF), NH2-MIL-125(Ti), was used in this work to prepare polysulfone-based mixed matrix membranes (MMMs). Permeation measurements on CO2/CH4 gas mixture demonstrate that the incorporation of NH2-MIL-125(Ti) particles can significantly improve the CO2 permeability compared to the pure polymer membrane, along with slightly enhanced CO2/CH4 separation factor. This work also shows that the separation factor at high pressures can remain almost unchanged for the prepared MMMs with the MOF loadings up to 20 wt.%. The obtained results may provide useful information in facilitating the applications of promising MOF-containing MMMs for the practical membrane-based natural gas purification.

Keywords: Mixed matrix membranes, polysulfone, NH2-MIL-125(Ti), natural gas upgrade

1

1. Introduction Despite the significant progress in the development of new renewable energy in recent years, fossil fuels likely still occupy a dominant position in the worldwide energy supply for the foreseeable future [1]. Along with the shale gas revolution, natural gas is one of the fastest growing fossil fuels during the past decades. Natural gas has a relatively large storage capacity around the world and is much cleaner than petroleum oil and coal. However, before being transported into pipelines or used as vehicle fuel, natural gas must meet strict specifications for the undesired components. As the major impurity in natural gas, CO2 will decrease the energy content and lead to pipeline corrosion [2-6]. So far, pre-consumption removal of CO2 from natural gas was usually realized by cryogenic distillation, amine absorption and pressure swing adsorption (PSA) processes [7, 8]. Compare with these traditional methods, membrane-based separation process has advantages in energy efficient and environmentally friendly aspects and has been recognized as an important technology for CO2 capture and gas separation [9-12]. Due to the excellent mechanical strength, chemical and creep resistances and thermal stability [13-15], polysulfone (PSF) is one of the most popular amorphous polymers and has been widely used in preparing membranes for gas separation, acting as selective layer on rigid porous substrate or being as the supported membranes. The first commercial membrane system for industrial gas separation is the PRISMTM membrane separator designed by Monsanto Company. In this system, thin PSF membrane is used as the selective layer with the surfaces coated using silicon to eliminate the nonselective voids. The separator has been applied to remove the acid gases in natural gas, as well as recycle H2 from purge gas during ammonia synthesis. However, PSF membrane has a disadvantage in gas permeability, which affects its wide application in the field of membrane-based gas separation. Great efforts have 2

been devoted to improving the performance of PSF membranes; one of which is to incorporate fillers into the PSF matrices to fabricate mixed matrix membranes (MMMs) [4, 10, 16-18]. For this purpose, various kinds of fillers have been considered, including porous silica [19-22], zeolite [23-26], carbon nanotube [27], graphite [28] and metal oxide [29]. However, the particles of these traditional inorganic additives often exhibit poor affinities with the polymer chains, which may induce nonselective defects in the resulting MMMs. Thus, it remains a challenge to improve gas permeability of the membranes without a sacrifice of selectivity. Metal-organic frameworks (MOFs) are a new class of nanoporous crystalline materials that are built up from inorganic metal-ion subunits connected by polytropic organic ligands [30, 31]. The regular micropores with tunable surface chemical properties at the molecular level make MOFs ideal candidates for gas separation [32], as well as potential additives for the fabrication of MMMs [16, 33-42]. During the past several years, MOFs-containing MMMs have attracted tremendous interest in the membrane-based gas separation. For example, Zornoza et al. [43] fabricated MMMs based on PSF and ZIF-8, HKUST-1, Silicate-1 as well as the mixture of MOF and Silicate-1. They found that incorporation of all these porous fillers can increase the gas permeabilities at the cost of decreasing CO2/CH4 separation factor in different degrees. Gascon and co-workers [44] investigated the CO2/CH4 separation performance of NH2-MIL-53(Al)/PSF MMMs and found an increase of the separation factor with the addition of the MOF. Sorribas et al. [45] studied the effect of incorporating meso-microporous silica (MSS)/ZIF-8 composite into PSF matrix on CO2/CH4 separation. They showed that the gas permeability of the fabricated MMMs was significantly improved and the selectivity remains almost unchanged even at a filler loading of 32 wt.%. For the two MMMs prepared by Car et al. [46] using PSF with Cu3(BTC)2 and Mn(HCOO)2 3

as fillers at the loading of 10 wt.%, the CO2/CH4 ideal selectivities decrease by factors of 58% and 50%, respectively. Rodenas et al. [47] found that the CO2 permeability and the CO2/CH4 separation factor of NH2-MIL-101(Al)/PSF MMMs with a filler loading of 25 wt.% can be enhanced by factors of 63% and 22%, respectively. Musselman and co-workers investigated the gas separation performance of the Matrimid-based MMMs with embedded MOF-5 [48] and ZIF-8 [49]. Compared to the pure polymer membrane, they found that the CO2 permeability increases with the addition of MOF-5 particles, while the influence on the CO2/CH4 ideal selectivity is negligible. In contrast, the selectivity is significantly enhanced due to molecular sieve effect existing in the ZIF-8-containing MMMs. Zhang et al. [50] reported that the addition of Cu-BPY-HFS into Matrimid can increase the CO2 permeability but slightly decline the CO2/CH4 ideal selectivity. Fe(BTC) was also studied by Shahid et al. [51] as fillers for Matrimid membranes, showing that both the permeability and selectivity are slightly enhanced. It has been well-recognized that the water-insensitive property of MOFs is one of the vital prerequisites related to their practical utility [52]. Recently, Zlotea et al. [53] reported a novel amine-functionalized MOF named NH2-MIL-125(Ti) (MIL= Material Institut Lavoisier), which exhibits excellent water and thermal stabilities as well as high porosity. As shown in Fig. 1, this material is built up from the octameric Ti8O4(OH)4 oxoclusters bounded to twelve 2-amino-terephthalate ligands [53-55], leading to a quasi-cubic tetragonal structure in which the octahedral (10.7 Å) and tetrahedral (4.7 Å) cages are accessible through a window of about 5-7 Å. With a combination of experimental and computational methods, Vaesen et al. [56] recently investigated the adsorption behaviors of CO2 and CH4 in NH2-MIL-125(Ti). They found that this material exhibits an almost constant difference of adsorption heat between CO2 and CH4 in a wide range of pressure up to 20 bar (~12 kJ/mol), 4

which was attributed to the presence of accessible -OH and -NH2 sites in the framework. Considering these facts, it would be expected to improve the gas permeability as well as the CO2/CH4 selectivity of polymeric membranes after incorporation of NH2-MIL-125(Ti). Therefore, in the present work, this MOF was for the first time used as porous filler to fabricate PSF/MOF composite membranes. By surveying the literature, many studies available so far have used single-component gas permeation data to characterize the separation properties of polymer/MOF composite membranes. Such a treatment is helpful in primarily screening the promising MMMs for a targeted separation system. However, out of the consideration for practical applications, it is more useful to provide the information of the permeation properties toward real mixtures. Thus, the permeation experiments were directly performed on CO2/CH4 gas mixture to characterize the separation performance of the MMMs prepared in this work.

2. Experimental 2.1. Materials Titanium(IV) isopropoxide was purchased from J&K Scientific (China), and 2-aminoterephthalic

acid

(NH2-BDC)

was

obtained

from

Alfa

Aesar

(China).

Dimethylformamide (DMF) was supplied by Sinopharm Chemical Reagent Co., Ltd. and methanol (CH3OH) and acetic acid (HAc) were purchased from Beijing Chemical Works. The received solvent was further purified to remove the containing water. The PSF pellet (Ultrason® S 6010, see Fig. 2) was provided by BASF (China) and was thermal treated at 393 K in a vacuum oven overnight before the use. 2.2. Synthesis of NH2-MIL-125(Ti) NH2-MIL-125(Ti) was synthesized using the method of Zlotea et al. [53] with slight modifications. Typically, 6 mmol NH2-BDC, 25 ml CH3OH and 25 ml DMF were mixed and 5

10 ml HAc was added as the modulator for the purpose of retarding the hydrolysis of titanium isopropoxide and tuning the MOF particle sizes [55]. Then, 3 mmol titanium isopropoxide was further added under agitation. The slurry was introduced into a 125 ml Teflon liner within a stainless-steel autoclave. The reactor was heated at 433 K for 48 h. The resulting light yellow product was filtered off and washed with DMF to remove the excess of unreacted ligands, followed by using methanol to exchange the DMF. Finally, the sample was activated at 393 K under vacuum overnight, and the obtained powder was reserved for further characterization. 2.3. Membrane fabrication The membrane samples were fabricated through a mixing-casting process. Specifically, for pure PSF membrane fabrication, the polymer was dissolved into chloroform portion-wise to form a 25 wt.% solution. For MMMs preparation, the NH2-MIL-125(Ti) powder after activation was ground sufficiently in an agate mortar to reduce the particle sizes, and was then dispersed in chloroform. The suspension was placed in ultrasonic bath for 30 min before adding PSF pellets. The proportion of the MOF in the MMMs was calculated as

  wt. MOF MOF weight percentage (wt.%) =   × 100%  wt. MOF + wt. polymer 

Eq. 1

To obtain a homogeneous dispersion of the MOF particles, all the suspensions were stirred and ultrasonically treated three times in an interval of 10 min after each addition of PSF. The obtained solution and suspensions were further stirred overnight. Before membrane casting, the casting solution was ultrasonically treated for 30 min so as to remove the trapped bubbles. Then, the pure PSF membranes and the MMMs were casted on a flat glass plate with a casting knife and further statically placed for several minutes for solvent evaporation. Finally, the membranes were peeled off from the glass plate and annealed at 373 K in a vacuum oven 6

overnight to remove the residual solvents. 2.4. Characterizations The crystallinity of the as-synthesized NH2-MIL-125(Ti) sample and the prepared membranes were analyzed at room temperature by the Bruker D8 ADVANCE X-Ray Diffractometer (XRD) using Cu Kα radiation source with a wavelength of 1.54 Å. Powder sample was gently ground before analysis. High resolution images of cross-section of MMMs and NH2-MIL-125(Ti) powder were obtained using a Hitachi S-4700 scanning electron microscope (SEM) to investigate the distribution of MOF particles, the compatibility between MOF and polymer in the MMMs, and the structure of NH2-MIL-125(Ti) particles. For these measurements, the membrane samples were fractured in liquid nitrogen, and all the samples were placed on the SEM specimen holder through double-sided conductive tape. Thermal stability of NH2-MIL-125(Ti), pure PSF membrane and their MMMs were investigated by Thermal Gravimetric Analysis (TGA) using a METTLER TOLEDO TGA/DSC 1/1100 SF simultaneous thermal analyzer. The samples were heated at a rate of 10 K/min to 1073 K under a constant air flow of 10 ml/min. To measure BET surface area of the synthesized NH2-MIL-125(Ti), the sample was degassed overnight at 423 K and then characterized using nitrogen adsorption at 77 K measured by an Autosorb-iQ-MP (Quantachrome Instruments) automated gas sorption analyzer. 2.5. Permeation measurements A mixture of CO2 and CH4 (50/50 mol%) was used to characterize the gas separation

performance of the prepared MMMs. The experiments were realized with a custom-built permeation cell using the Wicke-Kallenbach technique. Detail of the measurements can be found elsewhere [57]. During the measurements, Helium was used as sweep gas in the permeate side and the fluxes were on-line monitored from this side by a well-calibrated gas 7

chromatograph with high precision (Agilent HP 7890B). The largest average relative error in the measurements on the same sample was found not more than 2.5%. For each MMM, the permeation data were averaged from the measurements performed on at least three samples. For each sample, the gas permeabilities and the separation factors of CO2 over CH4 were measured at 303 K and different feed pressures (up to 30 bar). Gas permeabilities were determined using the following equation:

Pperm, i =

dV l × i × 1010 ∆pi × A dt

Eq. 2

where Pperm,i is the permeability of component i (i = CH4 or CO2) in units of Barrer, l represents the thickness (in units of cm) of the membranes and A is the effective membrane area (in units of cm2). ∆pi and dVi / dt refer to the partial pressure drop (in units of cmHg) and the volume flux (in units of cm3/s) of component i through the membrane, respectively. It should be pointed out that the volume fluxes measured in this work were corrected to the standard conditions when calculating the permeabilities. The separation factors of the membranes for CO2/CH4 mixture ( α CO2

CH 4

) were calculated

by

α CO

2

CH 4

=

yCO2 yCH 4

Eq. 3

xCO2 xCH 4

where yCO2 and yCH4 are the mole fractions of CO2 and CH4 in the permeate stream, respectively, while xCO2 and xCH4 are the corresponding mole fractions in the feed stream, respectively. According to the IUPAC recommendations on the terminology for membranes a and membrane processes [58], the separation factor ( α CO 2

CH 4

) is defined as the ratio of the

compositions of components CO2 and CH4 in the permeate stream relative to the composition ratio of the two components in the reject (or called as retentate) stream a α CO

2

CH 4

=

yCO2 yCH 4 x0,CO2 x0,CH 4

Eq. 4

where x0,CO2 and x0,CH4 are the mole fractions of CO2 and CH4 in the reject stream, 8

respectively. In current work, the membrane stage cut, defined as the fraction of feed gas that permeates the membrane (that is, the flow-rate ratio of the permeate to feed streams), was always below 1%. Therefore, when calculating the separation factor, we used the mole factions in the feed stream instead of those in reject stream. In addition, the separation factor obtained using Eq. 3 is equivalent to the permselectivity which is calculated as the ratio of the permeability of CO2 to that of CH4. This can be proved from the binary-system mass balance on the basis of the model for complete-mixing flow pattern (i.e., Weller-Steiner Case I) [59].

3. Results and discussion 3.1. Characterizations The XRD patterns measured for pure PSF membrane, NH2-MIL-125(Ti) and the MMMs with three different MOF loadings are compared in Fig. 3. As expected, the pure PSF membrane exhibits an amorphous structure. The XRD pattern of NH2-MIL-125(Ti) is in good agreement with the data reported in the literature [53, 56]. The BET surface area of the MOF is around 1300 m2/g, which is also very close to the reference value (1245 m2/g) [56]. These observations demonstrate a success in the synthesis of MOF sample. With regard to the MMMs, the XRD patterns exhibit the characteristic peaks of NH2-MIL-125(Ti) structure, where the higher intensity of these peaks reflects a higher loading of the filler. These results indicate that the MOF particles have been successfully dispersed into the matrices of PSF. The SEM image of the activated NH2-MIL-125(Ti) sample is shown in Fig. 4. It can be evidenced from the scale that the size of most MOF particles is about 1 µm in length with 0.5 µm in width. The appearance of some relatively larger ones may be attributed to the aggregation of MOF particles during thermal treatment. The surface and cross-sectional SEM images of the prepared pure PSF membrane and the MMMs with different filler loadings are shown in Fig. 5 and Fig. 6. A uniform distribution of MOF particles can be found for each 9

MMM and there is no significant agglomeration even at the MOF loading up to 30 wt.%. The good compatibility between the MOF and the polymer phases can be revealed from the observation that the particles are well encapsulated in the polymer matrices. The prepared MMMs have a thickness of about 10 µm, and thus the membranes are thick enough to avoid the appearance of evident defects caused by the embedded particles with the sizes obtained in this work. This can be reflected from the good separation performance of the MMMs shown below. In addition, these figures also show that there are no MOF particles with the size as large as those observed in Fig. 4. This can be ascribed to the fact that the particle sizes can be further minified by the treatments of agitation and sonication during membrane fabrication. The relatively small particles make them easy to be dispersed efficiently in the solvent as well as in the polymer matrices, thus eliminating the defects that are caused by the incompletely encapsulated particles in the membranes. Fig. 7 shows a comparison of the TGA curves of NH2-MIL-125(Ti), pure PSF membrane and the composite membrane with a filler loading of 20 wt.%. For the MOF sample, the 10% weight loss at 373 K can be attributed to the removal of the adsorbed solvent molecules and the large weight loss at about 623 K indicates the decomposition of the MOF structure, which also agrees with the experimental observations obtained by others [56]. For the case of the MMM, the TGA result shows that the first stage of weight loss begins at 643 K, which is larger than that of the MOF sample. This indicates that the combination of NH2-MIL-125(Ti) with PSF can slightly enhance the thermal stability of the MOF.

3.2. Separation performance for CO2/CH4 mixture To investigate the effects of the addition of NH2-MIL-125(Ti) on the gas separation performance of the membranes, permeation experiments were firstly performed on the pure 10

PSF membrane and the MMMs at 303 K and 3 bar. The permeation results are shown in Fig. 8 and Table 1. Obviously, there is an enhancement on both the CO2 and CH4 permeabilities after incorporating NH2-MIL-125(Ti) into the pure PSF membrane. For polymeric membranes, the permeation behavior of gases can be described by a solution-diffusion mechanism [60]. For the MMMs, the presence of porous NH2-MIL-125(Ti) particles can generate additional transport channels in PSF matrices, which can facilitate the diffusion of gas molecules in the MMMs and thus lead to the increase in permeability [48]. In addition, Fig. 8 also discloses that there is a more significant increase in the permeability of CO2 than that of CH4 under the same MOF loading. For example, The CO2 permeability of pristine PSF membrane is enhanced from 9.5 to 40.0 Barrer (an increase of 320%) at a NH2-MIL-125(Ti) loading of 30 wt.%, while the permeability of CH4 is improved from 0.43 to 1.37 Barrer at the same loading (an increase of 219%). Such a larger enhancement with respect to CO2 can be explained by the adsorption-controlled permeation mechanism [61] through the pores of NH2-MIL-125(Ti). As pointed out by others [55] as well as in our previous study [56], -NH2 functional group present on the organic ligands of NH2-MIL-125 can greatly enhance the interactions between the CO2 molecules and the MOF framework. In addition, Janiak and co-workers [33] demonstrated that for Knudsen diffusion the diffusivities vary inversely with the square root of the molecular weights, and thus lighter gases will have higher permeabilities than heavier gases through the pores of MOFs under the same conditions. For the case of surface diffusion, one of the components in the gas mixture is preferentially adsorbed on the pore walls, and this component will become the preferentially permeating gas with a higher flux. Combing these facts with the results shown in Fig. 8, it can be reasonably deduced that the transport of more strongly adsorbed CO2 in the pores is favored via surface diffusion. At the same time, the results given in Table 1 indicate that the separation factor of CO2 over CH4 is also slightly 11

increased and reaches a value of 29.5 at the MOF loading of 20 wt.%. The improvement on the separation factor can be attributed to the preferential adsorption of CO2 over CH4 in NH2-MIL-125(Ti) due to the small cages and polar amino functional groups in the structure, as confirmed in previous work [56]. Table 2 compares the CO2/CH4 separation performance of the MMMs prepared in this work with those of some MMMs reported in the literature under similar conditions. Clearly, the addition of porous materials with low CO2/CH4 selectivity such as MCM-48 [21], Silica [62] and ZIF-71 [63] into polymers can increase both the permeabilities of CO2 and CH4, but reduce the separation factor with increasing filler loading. For instance, Ahn et al. [62] showed that the increase in gas permeability of the Silica/PSF MMMs can be mainly attributed to the significantly improved diffusion coefficients of CO2 and CH4, which facilitate the transport of penetrants through the membranes. The decrease in selectivity is caused by the decline of diffusion selectivity because the solubility coefficients of CO2 and CH4 as well as the solubility selectivity remain almost unchanged with the addition of silica particles. These facts demonstrate that using nonselective porous materials as fillers can only be helpful to increase the free volume and inter-molecular spacing in MMMs, thus resulting in the increase of gas permeabilities. In contrast, when CO2-selective materials such as NH2-MIL-53(Al) [44], NH2-MIL-101(Al) [47], ZIF-90 [64], Fe(BTC) [51] are dispersed into polymers, the separation factor and the gas permeability can be both enhanced. Shahid et al. [51] indicated that the increase of ideal selectivity is resulted from the higher solubility of CO2 because of the strong electrostatic interactions between CO2 molecules and the open metal sites of Fe(BTC) particles, while the increase of gas permeability is attributed to the extra pore network provided by the MOF particles for gas molecules. These results demonstrate the expectation that using CO2-selective materials as fillers is an effective 12

strategy to improve the permeability and selectivity simultaneously. Among the MMMs listed in Table 2, PSF-NH2-MIL-125(Ti) MMMs offer the largest improvement on gas separation performance compared to the pristine polymer membranes in terms of both separation factor and permeability. This mainly benefits from the large free volume provided by the MOF particles and the interaction between CO2 molecules and the existing accessible -OH and -NH2 functional group on the pore surface. The gas separation properties of the PSF-NH2-MIL-125(Ti) MMMs were further evaluated by correlating the gas permeability and selectivity for CO2/CH4 provided by Robeson in 2008 [65], as shown in Fig. 9. Pure PSF membrane has a relatively low permeability and high selectivity among the large amount of glassy polymer membranes. Thus, reinforcement of the transport of gas molecules through the membrane makes great senses in the industrial application of PSF-based membrane for gas separation. With increasing the content of NH2-MIL-125(Ti) in the MMMs, the CO2 permeability is enhanced significantly with slightly increase in separation factor for CO2 over CH4. This can be explained as follows. Within the range of MOF loadings considered in this work, adding more porous particles will make the MMMs have more transport channels for gases. At the same time, the transport of CO2 in the pores of MOF particles is favored via surface diffusion, as discussed above. Thus, the cooperation effect of the two factors results in the significant enhancement on the CO2 permeability, while the enhancement on that of CH4 with relatively smaller magnitude is mainly contributed from the former factor. Consequently, the more favored transport of CO2 through the membrane leads to the slight increase in the separation factor. Although the permeation performance of the MMMs prepared in current study does not overcome the Robeson upper bound, the behavior of the MMMs becomes much closer to the upper bound. Consequently, it can be concluded that our work can make contributions to the 13

application of PSF in the area of membrane-based gas separation. 3.3. Influence of feed pressure Membrane separation at high pressure is favored in industrial application, especially for

natural gas upgrade [66]. Therefore, mixed-gas permeation experiments were further performed on the MMMs prepared in this work at higher feed pressures. The related data at 10 bar are shown in Table 3. It can be found that within 20 wt.% MOF loadings, the permeabilities of CO2 and CH4 through the MMMs are slightly reduced at this pressure compared to the results observed at 3 bar (Fig. 8 and Table 1). Such a decreasing trend at elevated pressures has also been found in other MMMs [44, 47, 51, 63]. The CO2/CH4 separation factors of these MMMs at 10 bar are very close to those measured at 3 bar. Table 1 also indicates that the best separation performance can be achieved for the MMM with a MOF loading of 20 wt.%. The above pressure dependence of gas permeability can be explained by the dual-mode model. As demonstrated by Pandey and Chauhan [60], the partial immobilization dual-mode sorption model is the most useful in phenomenological description of gas transport in glassy polymers such as PSF used in our study, which is also applicable to the permeation of gas mixture. Thus, according to this model, the evolution of permeability with pressure can be described by [68] Pperm = k D DD +

CH' bDH 1 + bp

Eq. 5

where DD and DH are the diffusion coefficients in the continuous phase and Langmuir sites respectively, k D is the Henry’s law sorption coefficient, CH' is Langmuir capacity constant, b is gas-polymer affinity constant, and p is the pressure. For a given membrane, this model clearly indicates that gas permeability decreases with increasing pressure. In current work, the CO2 permeability of pristine PSF membrane decreases from 9.5 Barrer at 3 bar to 7.3 Barrer at 10 bar, which is similar to the results observed by Kentish and co-workers [69]. 14

In their study, the decreasing trend of the CO2 permeability of PSF membrane was well fitted by the above dual-mode model. Herein, we found that decrease in the permeability with increasing pressure for the MMMs is more significant than that for the pristine PSF membrane, and is proportional to the loading ratio of NH2-MIL-125(Ti). The explanation is that adsorption of CO2 in the MOF obeys Langmuir-type isotherm, i.e. the second term in Eq. 5, contributing to the decrease in the permeability with pressure. Therefore, the above analysis indicates that the dual-mode model could also explain the changes observed for the prepared MMMs. By comparing the results shown in Tables 1 and 3, it can be found that there is a large increase in the permeability of CH4 at 10 bar for the 30 wt.% MOF-containing MMM, together with a dramatic decrease in the separation factor. To explore the origin of this phenomenon, we retested the separation performance of this membrane at 3 bar using the same samples. It was discovered that the good separation performance observed initially cannot be recovered. The retesting experiments indicate that the phenomenon observed above may arise from the membrane failure, that is, this membrane cannot exhibit a good resistance to the pressure. The industrial natural gas purification may be operated far above 10 bar, and thus it is important for the membranes to have a good mechanical stability under such operation conditions. For this purpose, we have further conducted the permeation experiments on the MMMs at pressures up to 30 bar. Since the MMM with a MOF loading of 20 wt.% exhibits the best performance as observed above, Fig. 10 only presents a comparison of the results measured on this MMM and the pure PSF membrane. As can be seen from this figure, the CO2/CH4 separation factors of the MMM only change slightly, and the permeabilities of CO2 are also much higher than those through the pure PSF membrane. These results reveal that good separation performance can still be maintained at high pressures considered in this work. Thus, it can be deduced that the prepared MMMs have a good pressure-resistant property. In addition, these observations demonstrate that the PSF/NH2-MIL-125(Ti) membrane can be considered as a promising candidate for practical membrane-based natural gas purification in 15

which high operating pressures are required to maintain desired gas permeation fluxes.

4. Conclusion In this work, NH2-MIL-125(Ti) with cage-like pores and accessible -OH and -NH2 groups in the structure was used as fillers in polysulfone to prepare mixed matrix membranes. The SEM images show that the MOF particles can be well-dispersed into the polymer matrices due to good compatibility with the polymer chains. Gas mixture permeation measurements indicate that incorporating this MOF into the polymer can meet the expectation of improving permeability and separation factor of the membranes simultaneously. The MMM with a MOF loading of 20 wt.% exhibits the best separation performance. Within the pressure range up to 30 bar, the CO2/CH4 separation factor of this MMM only changes slightly. These observations demonstrate that PSF/NH2-MIL-125(Ti) membrane can be considered as a promising candidate for practical membrane-based natural gas purification.

16

Acknowledgement This work was supported by the National Key Basic Research Program of China (“973”) (2013CB733503), Natural Science Foundation of China (Nos. 21136001, 21276009 and 21322603, 21176231, 21276249 and 21361130018), and the Program for New Century Excellent Talents in University (No.NCET-12-0755).

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24

Figures and Tables Fig. 1. Illustration of the NH2-MIL-125(Ti) structure. Hydrogen atoms were omitted for clarity. The large yellow and cyan spheres represent the void regions inside the octahedral and tetrahedral cages, respectively (Ti, green polyhedral; O, red; C, black; N, blue).

Fig. 2. Chemical structure of PSF (Ultrason® S 6010). Fig. 3. XRD patterns of pure PSF membrane, NH2-MIL-125(Ti) and PSF-NH2-MIL-125(Ti) MMMs.

Fig. 4. SEM image of NH2-MIL-125(Ti) powder after activation. Fig. 5. Surface SEM images of (a) pure PSF, (b) PSF-NH2-MIL-125(Ti)-10%, (c) PSF-NH2-MIL-125(Ti)-20%, and (d) PSF-NH2-MIL-125(Ti)-30%.

Fig. 6. Cross-sectional SEM images of (a,b) pure PSF, (c,d) PSF-NH2-MIL-125(Ti)-10%, (e,f) PSF-NH2-MIL-125(Ti)-20%, and (g,h) PSF-NH2-MIL-125(Ti)-30%.

Fig. 7. TGA of pure PSF, PSF-NH2-MIL-125(Ti)-20%, and synthesized NH2-MIL-125(Ti). Fig. 8. Permeabilities of CO2 (a) and CH4 (b) through PSF-NH2-MIL-125(Ti) MMMs at 303 K. The increment magnitude relative to the pure PSF membrane is labeled on the top of each column. The feed pressure is 3 bar. Error bars correspond to the standard deviation.

Fig. 9. Evaluation of the CO2/CH4 separation performance of PSF-NH2-MIL-125(Ti) MMMs relative to the Robeson upper bound. The filled square symbols represent the data for the MMMs with different loadings.

Fig. 10. CO2/CH4 separation performance of (a) pristine PSF membrane and (b) PSF-NH2-MIL-125(Ti)-20% MMM, as a function of feed pressure. Operation conditions: gas feedstock mixture CO2:CH4=50:50 and T=303 K. Error bars represent the standard deviations.

Table 1. CO2/CH4 separation properties of MMMs at 303 K and 3 bar (The values in the brackets are the standard deviations).

Table 2. Comparison of CO2/CH4 separation performance of different MMMs under similar conditions.

Table 3. CO2/CH4 separation properties of the MMMs at 303 K and 10 bar (The values in the brackets are the standard deviations).

25

Fig. 1. Illustration of the NH2-MIL-125(Ti) structure. Hydrogen atomss were omitted for clarity. The large yellow and cyan spheres represent the void regions inside the octahedral and tetrahedral cages, respectively (Ti, Ti, green polyhedral; O, red; C, black; N, blue).

26

Fig. 2. Chemical structure of PSF (Ultrason® S 6010).

27

Intensity (a.u.)

NH2-MIL-125(Ti) PSF-NH2-MIL-125(Ti)-30% PSF-NH2-MIL-125(Ti)-20% PSF-NH2-MIL-125(Ti)-10% Pure PSF membrane

10

20

30

40

50

2 Theta (degree)

Fig. 3. XRD patterns of pure PSF membrane, NH2-MIL-125(Ti) and PSF-NH2-MIL-125(Ti) MMMs.

28

Fig. 4. SEM image of NH2-MIL-125(Ti) powder after activation.

29

Fig. 5. Surface SEM images of (a) pure PSF, (b) PSF-NH2-MIL-125(Ti) 125(Ti)-10%, (c) PSF-NH2-MIL-125(Ti)-20%, and (d) PSF-NH2-MIL-125(Ti)-30%.

30

Fig. 6. Cross-sectional SEM images of (a,b) pure PSF, (c,d) PSF-NH2-MIL-125(Ti)-10%, (e,f) PSF-NH2-MIL-125(Ti)-20%, and (g,h) PSF-NH2-MIL-125(Ti)-30%.

31

100

Weight loss (%)

80 60 40

Pure NH2-MIL-125(Ti) Pure PSF PSF-NH2-MIL-125(Ti)-20%

20 0

400

500

600

700

800

900

1000

1100

Temperature (K)

Fig. 7. TGA of pure PSF, PSF-NH2-MIL-125(Ti)-20%, and synthesized NH2-MIL-125(Ti).

32

320%

a) 208%

30 95%

20

10

0

0

10%

20%

1.6

CH4 permeability (Barrer)

CO2 permeability (Barrer)

40

219%

1.2

130%

0.8

51%

0.4

0.0

30%

b)

0

10%

20%

30%

NH2-MIL-125(Ti) loading (wt.%)

NH2-MIL-125(Ti) loading (wt.%)

Fig. 8. Permeabilities of CO2 (a) and CH4 (b) through PSF-NH2-MIL-125(Ti) MMMs at 303 K. The increment magnitude relative to the pure PSF membrane is labeled on the top of each column. The feed pressure is 3 bar. Error bars correspond to the standard deviation.

33

CO2/CH4 permselectivity

Robeson Upper Bound (2008)

100

(0.10)

10

(0.20) (0.30)

Pure PSF membrane PSF-NH2-MIL-125(Ti) MMMs

10

20

30

40 50

CO2 permeability (Barrer)

Fig. 9. Evaluation of the CO2/CH4 separation performance of PSF-NH2-MIL-125(Ti) MMMs relative to the Robeson upper bound. The filled square symbols represent the data for the MMMs with different loadings.

34

40

35

35

30

25

30

25

30

20

25

20

25

15

20

15

20

10

15

10

15

5 1.5 1.0 0.5 0.0

10

5 1.5 1.0 0.5 0.0

10

a)

5 3

10

15

20

25

30

0

40

b)

35

5 3

10

15

20

25

30

Separation Factor

Permeability/Barrer

30

Separation Factor Permeability/Barrer

35

0

Pressure/bar

Pressure/bar

Fig. 10. CO2/CH4 separation performance of (a) pristine PSF membrane and (b) PSF-NH2-MIL-125(Ti)-20% MMM, as a function of feed pressure. Operation conditions: gas feedstock mixture CO2:CH4=50:50 and T=303 K. Error bars represent the standard deviations.

35

Table 1. CO2/CH4 separation properties of MMMs at 303 K and 3 bar (The values in the brackets are the standard deviations).

Membranes PSF pristine PSF-NH2-MIL-125(Ti)-10% PSF-NH2-MIL-125(Ti)-20% PSF-NH2-MIL-125(Ti)-30%

Gas mixture permeability (Barrer)

Separation factor

CO2

CH4

CO2/CH4

9.5 (0.50) 18.5 (0.45) 29.3 (0.81) 40.0 (1.41)

0.43 (0.01) 0.65 (0.05) 0.99 (0.10) 1.37 (0.11)

22.0 (1.31) 28.3 (0.38) 29.5 (2.54) 29.2 (2.38)

36

Table 2. Comparison of CO2/CH4 separation performance of different MMMs under similar conditions. MMMs

NH2-MIL-125(Ti)/PSF

MCM-48/PSF*

Silica/PSF*

NH2-MIL-53(Al)/PSF

NH2-MIL-101(Al)/PSF

ZIF-90/6FDA-DAM

Fe(BTC)/Matrimid*

ZIF-71/6FDA-Durene

Filler loading

CO2 Permeability

Separation

(wt.%)

(Barrer)

Factor

0

9.5

22.0

303 K,

10

18.5

28.3

3.0 bar

20

29.3

29.5

30

40.0

29.2

Conditions

308 K, 4.0 bar

308 K, 4.4 bar

0

4.5

25.9

10

8.5

25.5

20

18.2

23.6

0

6.3

28.6

10

9.3

24.5

20

19.7

17.9

0

4.7

24.7

308 K,

16

5.2

32.5

3.0 bar

25

6

46.2

40

10.5

16.6

0

5.2

23.8

308 K,

8

5.5

24.0

3.0 bar

15

7.2

25.0

25

8.5

29.0

298 K

0

390

24.0

2.0 bar

15

720

37.0

0

8.5

24.0

308 K

10

9.5

27.0

5.0 bar

20

11

28.0

30

13.5

30.0

0

917

21.8

308 K

10

1606

20.0

3.5 bar

20

3435

16.0

30

5642

9.2

*Permeability data taken from single-gas measurements.

37

Refs.

This work

[21]

[62]

[44]

[47]

[64]

[51]

[63]

Table 3. CO2/CH4 separation properties of the MMMs at 303 K and 10 bar (The values in the brackets are the standard deviations).

Membranes PSF pristine PSF-NH2-MIL-125(Ti)-10% PSF-NH2-MIL-125(Ti)-20% PSF-NH2-MIL-125(Ti)-30%

Gas mixture permeability (Barrer)

Separation factor

CO2

CH4

CO2/CH4

7.3 (0.51) 15.0 (0.46) 22.8 (0.85) 36.8 (2.01)

0.27 (0.03) 0.53 (0.06) 0.78 (0.10) 6.48 (0.63)

27.0 (1.02) 28.5 (1.48) 29.5 (1.79) 5.7 (2.54)

*.

38

Highlights 1. Preparation of new mixed matrix membranes (MMMs) based on PSF and NH2-MIL-125(Ti). 2. Simultaneously enhanced gas permeability and separation factor with adding the MOF. 3. The MMMs can maintain the performance at high pressure for CO2/CH4 separation.

39