CH4 separation

Accepted Manuscript Title: Effect of mixed matrix membranes comprising a novel trinuclear zinc MOF, fumed silica nanoparticles and PES on CO2 /CH4 separation Authors: Hossein Mahdavi, Fereshteh Moradi-Garakani PII: DOI: Reference:

S0263-8762(17)30374-X http://dx.doi.org/doi:10.1016/j.cherd.2017.07.007 CHERD 2748

To appear in: Received date: Revised date: Accepted date:

17-12-2016 22-6-2017 4-7-2017

Please cite this article as: Mahdavi, Hossein, Moradi-Garakani, Fereshteh, Effect of mixed matrix membranes comprising a novel trinuclear zinc MOF, fumed silica nanoparticles and PES on CO2/CH4 separation.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2017.07.007 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 proof before it is published in its final 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.

Effect of mixed matrix membranes comprising a novel trinuclear zinc MOF, fumed silica nanoparticles and PES on CO2/CH4 separation Hossein Mahdavi*, Fereshteh Moradi-Garakani * School of Chemistry, College of Science, University of Tehran, Tehran, Iran P.O. Box 14155-6455, Tel/Fax: +98-21-66495291([email protected])

Highlights The novel Zn-based MOF has poorly affected CO2/CH4 separation parameters. The effect of two different fillers on membrane properties is considerable. The proper interaction of silica nanoparticles with polymer matrix is responsible for enhancement of permeability and selectivity coefficients.

Abstract The influence of binary and ternary component mixed matrix membranes containing a novel Znbased MOF, fumed silica and PES matrix was investigated for CO2/CH4 separation. The prepared membranes were characterized by XRD, FESEM, AFM, TGA and DSC techniques. The measured selectivity of 15 wt% loading of MOF in PES showed 39.21% decrease toward pure PES. However, the addition of nonporous fumed silica nanoparticles enhanced selectivity parameters by 20.04, 49.88 and 89.45% for 5, 10 and 15 wt% filler content in PES/silica/MOF MMMs, respectively. Keywords: mixed matrix membrane, gas separation, PES, fumed silica, MOF Introduction Application of the polymeric membranes in gas separation processes has been remarkable for many years due to their low cost and optimum energy consuming preparation methods beside their flexibility, modularity and facility to scale up. However, the industrial needs to improve permeation, selectivity and mechanical resistance have caused the recent efforts to incorporate different inorganic fillers in polymer matrices leading to development of mixed matrix membranes. For this reason, various types of fillers such as fumed and mesoporous silica [1-4], metal oxides [5-11], zeolites [12-18], metal organic frameworks [19-31] and sheet like materials [32] have been investigated as volunteers to change chain packing of polymers. Regarding to the far emplacement of inorganic membranes through Robeson upper-bound [33], the prepared mixed matrix membranes are expected to improve so called trade-off between permeability and selectivity of polymers. However, the consistency among different phases digested in filler-filler and polymer-filler interactions can play a key role because of their contrary effects on each other. Montazer-Rahmati et al. have extendedly discussed about MMMs four conceivable non-idealities formation due to inappropriate interfacial adhesion, dispersed phase blockage and stress formation [32]. They showed that to prepare a mixed matrix 1

membrane without improper properties, the four undesirable morphologies of “sieve-in-a-cage”, “leaky interface”, “matrix rigidification” and “plugged sieves” should be overcome. As a common case, poor interfacial adhesion can result in a weak membrane performance due to voids formation at polymer-filler interfacial regions [34].Therefore, most studies have been concentrated on different polymer-filler mixtures or membrane preparation methods to improve the compatibility of organic-inorganic phases and permselectivity properties of the membranes. Zhu et al. [35] used an asymmetric PES film as a microporous support for ZIF crystals, which are a subclass of MOFs consisting of inorganic metal ions or metal clusters being connected by organic imidazole/imidazolate ligands. The results showed an improvement in membrane preparation method due to the influence of trapped ZIF-8 crystals into the surface pores of polymer support and the favorable interactions among organic ligands of ZIF-8 and PES. Madaeni et al. [36] prepared three different types of PES/PDMS/TiO2 membranes to find the effective procedure of TiO2 nanoparticles embedment in composite membranes. On the other hand, application of ternary component mixed matrix membranes, comprising of two different fillers in a polymer matrix, has been noticeable in recent years due to significant interactions of different fillers leading to elimination of polymer/filler interfacial defects. Kalipcilar et al. [34] studied CO2/CH4 separation properties of PES/SAPO-34/HMA membranes. 2-hydroxy-5-methyl aniline (HMA) was used as a low molecular weight compatibilizer for PES/SAPO-34 membrane fabrication. The effect of zeolites and low molecular weight additives on permselectivity properties of polycarbonate based membranes was also investigated by Yilmaz et al. [18]. The combination of zeolites and MOFs in polysulfone membranes was explored to improve CO2/CH4 and CO2/N2 separation factors and CO2 permeability [16]. Coronas et al. indicated in this work that different nature of filler particles may produce synergetic effects to help their fine dispersion and disaggregation inside polymer matrix. The effect of in situ decoration of MOF on carbon nanotubes immerged in a polyimide matrix was studied by Zhu et al. [51]. The prepared MMMs exhibited not only a large CO2 permeability but also a high CO2/CH4 selectivity and a combined performance of permeability and selectivity factors even above the Robeson upper bound was shown. Such a favorable synergistic effect was also observable in combination of carbon nanotubes as a good highway to render high permeability and graphene oxide nanosheets as a selective barrier to present high selectivity in Matrimid MMMs [52]. In this work, nonporous fumed silica nanoparticles, as a well-known filler to improve membrane performance in CO2/CH4 separation properties [49], a novel type of zinc metal organic framework and their combination embedded in PES were used to prepare binary and ternary component mixed matrix membranes. Experimental Materials

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Polyethersulfone (PES) was purchased from Radel A-300, Solvay Advanced Polymers. It was dried overnight to remove adsorbed moisture right before the preparation of membranes. Fumed silica nanoparticles, N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) were supplied by Sigma-Aldrich and used as received. The gases were obtained from local companies and their purities were higher than 99.8%. Synthesis of {[Zn3(tp)4]-4H2O}n {[Zn3(tp)4]-4H2O}n nanoparticles were synthesized according to the method proposed by Abbasi et al. [37]: a mixture of Zn(NO3)2-6H2O (0.291 g, 1 mmol), C8H6O4 (0.324 g, 2 mmol) in 10 mL of DMF were sealed in Teflone-lined autoclave and heated under autogenous pressure to 120 °C for 4 days. In order to use nanoparticles for mixed matrix membrane preparation, the solution containing crystals of compound was ultrasonicated. Finally the white precipitates were centrifuged, washed with ethanol and dried at 50 °C for 24h. Pure PES membrane preparation Dehydrated PES pellets (0.3g) were solved in NMP (5.14 g). The solution was stirred at room temperature for 48h. After complete degassing, the solution was cast onto glass substrate and dried at 80 °C in oven for 12h. Any remainder solvent was removed by keeping the membrane in a vacuum oven for 24h at 120 °C. Binary component membranes The PES/silica and PES/MOF flat sheet mixed matrix membranes containing 15 wt% filler loading were prepared by applying priming technique. First, filler nanoparticles were mixed with NMP, stirred for 24 h and ultrasonicated for 60 minutes. Then PES was added to mixture in two or three parts (priming) to help a fine polymer-filler merging and to restrain nanoparticles agglomeration. In order to make a homogenous mixture, it was stirred and ultrasonicated alternatively up to four times. After degassing the solution, it was cast onto glass substrate and dried with the same treatment which was used for the pure PES membrane. Ternary component membranes The ternary component flat sheet membranes containing 50-50 proportion of silica and MOF as fillers and PES as matrix polymer were prepared at total filler content of 5, 10 and 15 wt%. First, silica nanoparticles were mixed with NMP, stirred for 24 h and ultrasonicated for 10 minutes. Then MOF was added gently to this mixture and after stirring and ultrasonication, the polymer pellets were added through two or three parts (priming). This procedure was followed by stirring and ultrasonication, up to four times, depending on the filler content. The final mixture was cast and treated in the same way of two component membranes. Characterization 3

The methods of Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) were used to calculate the specific surface area and the pore size distribution of the synthesized MOF. The volume of adsorbed-desorbed nitrogen was determined toward relative pressure at 70 K. The X-ray diffraction patterns of MOF powder and the ternary mixed matrix membranes were acquired by using an XRD instrument (Siemens D5000) with a Cu target (=0.1540 nm) at RT over a 2θ range from 5 to 50º. The instrument comprises a rotating anode generator which operated at 35 kV and 20 mA and the diffraction pattern was recorded using a scintillation counter detector. A field emission scanning electron microscope ((FESEM) HITACHI, Model S4160) equipped by an energy dispersive X-ray spectroscopy (EDX) was used to investigate the morphology and element mapping of membranes. The sections of membranes were prepared by cryogenically fracturing in liquid nitrogen. After breaking the samples, they were dried in vacuum oven at 35 ºC, then covered with a thin layer of Au and placed in microscope. An atomic force microscope (AFM, Digital Instruments, Veeco Metrology Group, USA) was employed to analyze the surface states of the sample membranes through its non-contact mode. The cantilever was made out of Si with a spring constant of 3.2 N/m and a nominal tip apex radius of 10 nm. Thermogravimetric analyses were performed using a PLTG analyzer (TGA 1000, UK) in order to investigate the thermal stability of prepared MOF and membranes. The TGA instrument worked over the temperature range of 30 °C to 800 °C with a heating rate of 10 °C/min under nitrogen stream. A differential scanning calorimeter ((Netzsch DSC 200F3) was used to measure T g of polymer and MMMs at different fillers loading. The DSC thermogram of membranes was recorded in the temperature range of 0 to 300 °C with a heating rate of 10 °C/min under N2 atmosphere. The density measurements of membranes were performed on a pycnometer (Micromeritics Accupyc 1330) according to the method reported by Nijmeijer [38]. Gas permeation measurements The permeation of gas molecules through dense membranes follows the principle of solutiondiffusion mechanism, whereby the gases are firstly absorbed at the membrane surface due to a high activity feed, then diffuse across the membrane thickness along a chemical potential gradient and finally are desorbed into a lower activity permeate [15,39]. In present work, the permeability of mixed matrix membranes for pure gases was measured through a variable pressure/constant volume method [40]. A fixed desired pressure of the penetrant was kept on the upstream chamber and the pressure change of downstream part was monitored by a high precise pressure transducer (0-100 mbar, BD sensors, Germany). Both upstream and downstream chambers were evacuated for 4-5 h before each measurement. The gas permeability is expressed through a simple thermo-kinetic equation as follows: 𝑃𝐼 = 𝐷𝑖 × 𝑆𝑖

(1) 4

where Si and Di are diffusivity and solubility coefficients of penetrant, i, respectively. In order to make an applicable relation among changes in vessel pressure due to permeate gas accumulation, equation (1) is rewritten to: 𝑃(𝐵𝑎𝑟𝑟𝑒𝑟) =

273.15𝑡1010 𝑉 𝑑𝑝 760×76𝐴𝑇𝑃

( 𝑑𝑡 )

(2)

where t is membrane thickness (cm), V is vessel volume (cm3), A is membrane surface area (cm2), T is experimental temperature (K), P is feed pressure and dp/dt is the pressure change gradient (mmHg/s) [41]. The selectivity factor is defined as the ratio of permeabilities of faster, i, and slower, j, permeating gases. 𝑃

𝛼𝑖𝑗 = 𝑃 𝑖

𝑗

(3).

Results and discussion MOF characterization The crystalline structure of synthesized MOF nanoparticles is shown in Fig. 1, which is in agreement with previously reported results [37]. The BET surface area and the mean pore diameter of MOF were determined as 2609 m2/g and 0.9 nm respectively. The SEM image of the MOF is indicated in

Fig. 2.

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Fig. 1. The XRD patterns of top: MOF nanoparticles, and down: PES/MOF/Silica (15 wt%).

Fig. 2. SEM image of zinc MOF.

Membranes characterization The SEM images of pure PES and mixed matrix membranes containing 15 wt% of silica or MOF and also their combination are represented in Fig. 3.

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Fig. 3. SEM images of (a) Pure PES and 15 wt% filler containing membranes, (b) PES/silica, (c) PES/MOF, (d) PES/silica/MOF from 1) top layer and 2) cross section.

These micrographs show changes in membrane morphology due to the incorporation of two different fillers. It is indicated that both nonporous silica and MOF nanoparticles are dispersed through membranes but appear to form some agglomerates in polymer matrix too. The SEM images of PES/silica MMMs cross sections (Fig. 3b-2) show good interfacial contact between the silica nanoparticles and polymer matrix, with no significant interfacial gap. However, there are some voids in the polymer/MOF interface (Fig. 3c-2), which are marked by circles. Similar phenomena were observed in previous literature considering the separated combination of polymer with SiO2 and a MOF too [53]. In contrast to PES/MOF MMMs, the increased plastic deformation of the polymer along with circular cavities and polymer veins are noticeable in cross sectional morphology of other membranes. This morphology, mostly observable in ternary component membranes (Fig. 3d-2) indicates a strong interaction between polymer matrix and the fillers. Such an observation reminds the existing explanation for the synergic effect of different fillers with different surface chemistries and chemical compositions to improve dispersion and disaggregation in the polymer matrix [16, 32]. It seems that the interaction of silanol groups on the surface of silica nanoparticles with existing oxygen atoms on polymer chains and MOF structure has an important role in preparation of homogeneous binary and ternary component membranes, in so far as the defects among MOF and polymer matrix has been lost.

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Fig. 4. Cross sectional X-ray mapping of a) PES/silica, b) PES/MOF and c) PES/silica/MOF mixed matrix membra nes containing 15 wt% filler.

The cross sectional X-ray mapping of binary and ternary component membranes containing totally 15 wt% of fillers, confirmed relatively homogenous dispersion in depth of polymer matrix (Fig. 4). The results of elemental analysis of membranes are shown in Table 1, the percentages of C, O and S atoms belong to PES matrix while silica and MOF analysis is indicated through atomic amounts of Si and Zn, respectively.

The AFM images obtained for 15 wt% filler containing PES/silica, PES/MOF and PES/silica/MOF membranes are indicated in Fig. 5. The obtained topographies are performed in non-contact mode and allow observation of pores distribution and surface roughness (Table 2). The dark spots on the membrane surface should be assigned to pores. The highest roughness among these three membranes belongs to PES/MOF, indicating MOF nanopartilces agglomeration.

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Fig. 5. The morphological AFM images of MMMs surface layer with 15 wt% filler content; (a) PES/silica, (b) PES/MOF and (c) PES/silica/MOF.

The XRD Pattern of 15 wt% filler containing ternary mixed matrix membrane is indicated in Fig. 1. The top pattern belongs to MOF, with peaks at 2θ = 5.14, 9.44, 10.42, 19.12 and 21.15° and the below one is that of MMM, with peaks at 2θ = 18.21, 20.24, 21.58 and 23.91° . A broad peak at 2θ = 18º can be detected as a characteristic peak of the pure polyethersulfone [36, 42]. The XRD patterns show that by embedment of MOF in mixed matrix membrane, its crystalline structure has been partially affected. There are two ideas to vindicate such an event; first is the MOF pore blockage due to the penetration of rigid polymer chains into the MOF pores and the second, relates such phenomenon to decomposition of some parts of MOF structure during membrane thermal treatment [37,43]. However, as the present mixed matrix membranes were not treated at high temperatures and due to TGA results which showed some kind of thermal stability of MOF after mixed matrix membrane preparation, it is more rational to cite the initial reasoning. The result of thermal degradation analysis of three different filler contents of mixed matrix membranes are indicated in comparison with pure PES, MOF and silica nanoparticles in Fig. 6. In contrary to silica nanoparticles, which are as expected mostly stable against thermal loads [44], the others show two steps of weight loss due to solvent removal and material degradation, respectively. As shown in this figure, the main weight loss of pure MOF occurs at about 450 °C which is compatible with previous reports [37]. The degradation process of mixed matrix membranes starts at higher temperature by an increase in filler contents and the micrographs of these membranes show higher decomposition temperature (Td) compared to the pure PES, resulting in good interaction among different kind of fillers and polymer matrix. The deterioration of pure PES started at 567 ºC while such a temperature for three component mixed matrix membranes with 5, 10 and 15 wt% of mixed fillers was 574, 576 and 579 ºC, respectively. 9

Fig. 6. Thermal gravimetric analysis (TGA) curves of PES, silica, MOF and ternary MMMs

The glass transition temperature of PES has been reported widely different in literature due to various molecular weights of this polymer [5,13,34,45-47]. The DSC result in present work indicates a Tg equal to 152 °C for pure PES, which is more than measured glass transition temperature of PES/MOF and ternary component MMMs (Table 5). However, the Tg of PES/silica mixed matrix membranes is slightly higher than that of pure PES indicating polymer chains rigidification due to stresses of membrane formation. The similar effects of fumed-silica nanoparticles on glass transition of mixed matrix membrane were observed in some previous works too [54-55]. As it is represented in Table 5, there is a significant decadence in Tg of PES/MOF mixed matrix membranes which can be related to interfacial voids at polymer-filler interface. In ternary MMMs, the addition of filler content from 5 to 10 wt% accompanies with no change in Tg. However, increasing the combination of silica and MOF loading to 15 wt% causes a small increase in Tg in comparison with other MMMs.

The measured density of 15% wt PES/silica/MOF membrane was 1.40 g/cm3 and shows an increase toward pure PES membrane (1.30 g/cm3). This phenomenon can be resulted from higher density of embedded fillers. Gas permeation results In order to investigate the effect of filler loading on gas separation performance, the permeability and selectivity of binary and ternary mixed matrix membranes with different silica and MOF contents were measured. The permeability of CO2 and CH4 was determined by taking an average 10

from three replicates permeation test at temperature and pressure of 25 °C and 3.5 bar, respectively and observed relative error among successive runs was about ±l-3%. The mechanism of gas permeation through polymeric membrane is solution-diffusion, indicating a slower diffusion for CH4 molecules, due to their larger molecular diameter than CO2 molecules [48]. Single gas permeability and ideal selectivity of PES/silica and PES/MOF membranes compared to pure PES, at 15 w% filler loading are shown in Table 4. The permeation study of PES/silica membrane indicates an increase in permeability and a notable improvement of selectivity which is in relevant to literature [49]. Such a result can be related to the disruptive effect of silica nanoparticles on polymer chain packing or the interaction between their surface silanol groups with polymer matrix or gas molecules.

It was expected that the porous structure of MOF results in a fair gas permselectivity. However, the perturbing effect of interfacial voids in PES/MOF membranes causes a significant increase in both CO2 and CH4 permeability and thereupon lowers the selectivity. This can be in relevant to nanoparticles agglomeration and interfacial voids as shown in SEM and AFM micrographs (Fig. 3 and Fig. 5) and proved by a highly decadence in calculated Tg results (table 3). According to previous works [32,50] this is a special type of “sieve-in-a-cage” named as “leaky interface” and appears when the polymer-filler interfacial void size is larger than the permeate gas molecule, so that permeability increases significantly but selectivity decreases. Additionally, the large pores of MOF are prone to be easily blocked by polymer chains so the inner pores may be leaved inaccessible. It is noteworthy that for ternary component mixed matrix membranes, the permeability increases in comparison with pure PES membrane and also by intensification of filler content. Such an increase in permeability could be related to the porous structure of MOF or the elongation of polymer segments due to a polymer-filler good interaction. The observed polymer veins in SEM images and the depression in glass transition temperature due to less rigidified polymer chains are two testimonies for this conclusion. The ideal selectivity coefficients of ternary mixed matrix membranes show an increasing trend by addition of filler content. As it is expected, the enhancement of CO2 permeability is more than CH4 and leads to 20.04, 49.88 and 89.45% improvement in CO2/CH4 selectivity for 5, 10 and 15 wt% filler loaded PES/silica, respectively. The selectivity of 15 wt% filler loaded PES/silica/MOF membranes represents an increase toward PES/silica and PES/MOF MMMs which would be inspired by better interaction between these two different fillers and PES matrix. As a matter of fact, the hydrogen bonding among silanol groups and oxygen atoms in two different kinds of fillers and polymer matrix plays an intermediating role to avoid agglomeration of MOF particles and to fill the filler-polymer interfacial gaps. Effect of MMM preparation on Robson upper-bounds

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The performance of pure PES and binary and ternary component mixed matrix membranes with reference to Robson upper-bounds is shown in Fig. 7 According to previous studies [34,50] three different cases are identified as Table 5 which relate polymer-filler interfacial morphology to gas permeation results. As it is indicated in Fig. 7, both permeability and selectivity of PES/silica and PES/silica/MOF MMMs have been improved and become closer to upper-bound toward pure PES, the state mentioned as case B in Table 5. However, a notable displacement to right is observed for PES/MOF which can be interpreted as case C. The non-selective void which is responsible for such state is caused by stresses frequently form at interfacial surface during mixed matrix membrane formation.

Fig. 7. The performance of studied mixed matrix membranes relative to Robeson upper-bounds

Conclusion A novel type of Zn-based metal organic framework was used to prepare binary component mixed matrix membranes with PES matrix. It was observed that some interfacial polymer-filler voids formed during preparation of this membrane that caused an increase in CO2 and CH4 permeability while decreasing the selectivity. In order to improve gas separation properties, the PES/silica/MOF MMMs containing 50-50 percent of fumed silica and MOF were prepared. The proper interaction of silica nanoparticles with polymer matrix caused an enhancement of permeability and selectivity coefficients insofar as a fair displacement toward Robeson upperbound was obtained for 15 %wt filler content MMM.

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Table 1. SEM–EDX cross sectional elemental analysis results of 15 wt% filler loading MMMs. Membrane

C content (at.%)

O content (at.%)

S content (at.%)

Si content (at.%)

Zn content (at.%)

PES/silica

73.12

21.91

3.48

1.49

-

PES/MOF

80.35

16.68

2.68

-

0.29

PES/silica/MOF

73.39

20.96

4.07

1.53

0.05

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Table 2. Surface roughness parameters of 15 wt% filler loading MMMs. MMMs type PES/silica PES/MOF PES/silica/MOF

Rms rough (nm) 2.463 6.302 1.883

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Mean rough (nm) 1.591 4.081 1.256

Table 3. The glass transition temperature of MMMs MMMs type

Total filler loading (wt%) 0 15 15 5 10 15

Pure PES PES/silica (15 wt%) PES/MOF (15 wt%) PES/silica/MOF (5 wt%) PES/silica/MOF (10 wt%) PES/silica/MOF (15 wt%)

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Tg (oC) 152 158 60 138 138 140

Table 4. Averaged single gas permeability and ideal selectivity of pure PES and MMMs at 25 °C and 3.5 bar Sample

Pure PES PES/silica (15 wt%) PES/MOF (15 wt%) PES/silica/MOF (5 wt%) PES/silica/MOF (10 wt%) PES/silica/MOF (15 wt%)

Permeability (Barrer) CO2 CH4 0.51 0.02 1.24 0.03 20.91 2.09 5.51 0.18 13.76 0.36 30.92 0.64

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Selectivity CO2/CH4 25.50 41.33 10.00 30.61 38.22 48.31

Table 5. The effect of different interfacial polymer-filler morphologies on gas separation properties Case A B C

Interfacial morphology matrix rigidification or pores partial blockage proper state non-selective voids

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Permeability decrease

Selectivity increase

increase increase

increase decrease