Mixed Matrix Membranes based on UiO-66 MOFs

in the Polymer of Intrinsic Microporosity PIM-1

Muhanned R. Khdhayyer1, Elisa Esposito2, Alessio Fuoco2, Marcello Monteleone2, Lidietta Giorno2, Johannes C. Jansen2*, Martin P. Attfield1, Peter M. Budd1*

1 School of Chemistry, University of Manchester, Manchester M13 9PL, UK. E-mail:

Peter.Budd@manchester.ac.uk

2 Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende (CS), Italy. E-

mail: johannescarolus.jansen@cnr.it

Abstract

This work presents a study of the gas transport properties for a novel class of mixed matrix membranes (MMMs) based on the polymer of intrinsic microporosity PIM-1 loaded with UiO-66 [Zr6O4(OH)4(O2CC6H4CO2)6] based metal-organic frameworks (MOFs). Three isoreticular MOFs were dispersed in the polymer matrix, standard UiO-66, UiO-66-NH2 (functionalized with an amino group) and UiO-66-(COOH)2 (functionalized with two carboxylic groups), in order to investigate the effect of the functionalization of the linker on the gas transport. The pure gas permeabilities of He, H2, O2, N2, CH4, CO2 were studied, for the as prepared membranes and after methanol treatment, focusing attention on the potential use of these membranes for CO2/CH4 separation. The pure gas transport of the MMMs was described on the basis of the Maxwell model. The predictions of the model are discussed and compared with the experimental permeability and selectivity of the MMMs and neat PIM-1. Mixed gas permeation tests were performed on a representative sample to investigate the actual separation performance with industrially relevant gas mixtures. These confirmed the good perspectives of these MMMs in applications like CO2 removal from biogas or from flue gas.

Keywords: Mixed Matrix Membranes, PIM-1, MOFs, UiO-66, gas transport, Maxwell Model.

1

1 Introduction

Mixed Matrix Membranes (MMMs) offer the opportunity to combine the benefits of easily

processable polymeric materials with the excellent transport performance of fillers [1]. The design

of these new materials for gas separation has the objective of producing innovative membranes with

enhanced permeability and selectivity that exceed the Robeson upper bound limit. In the case of

PIM-based MMMs, the aim is to combine the high gas permeability of the polymer and the good

selectivity of the filler materials. The choice of the polymer and of the fillers is the most important

parameter affecting the morphology and resulting performance of MMMs in gas separation [2]. In

the present work, the polymer used is the polymer of intrinsic microporosity arising from the step-

growth polymerization of 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane and

tetrafluoroterephtalonitrile (PIM-1). This novel polymer is characterized by a high intrinsic

microporosity. The polymers of intrinsic microporosity are recognized for their high intra-chain

rigidity and high free volume with interconnected voids [3]. The free volume elements are formed

as a direct consequence of the highly contorted shape and extreme rigidity of the polymer backbone

structure [4]. The presence of high free volume makes this polymer a successful candidate for the

development of highly permeable membranes [5]. PIM-1 has a gas separation performance that

helped to define the 2008 upper bound [6]. It is expected that the addition of fillers like Metal-

Organic Frameworks (MOFs) can improve further the performance of this polymer. For this reason,

there is an active research field that is developing MMMs using PIM-1 as the polymer matrix [7–

10]. MOFs are a new group of fillers and nanoporous materials made by linking organic units with

metal inorganic complexes [11], in which the inorganic units form the vertices of a framework and

the organic linkers form the porous structure with a definite size and shape [12]. MOFs present

advantages compared to inorganic fillers, such as a better compatibility with the polymer due to

their organic nature and a higher pore volume with a lower density [11,13]. The pre-defined

dimension of the cages and eventually the functionalization of the ligands can increase the affinity

of the MOFs for the organic polymer phase and for specific gases. This makes MOFs interesting for

the gas separation application [14].

Bushell et al. [15] prepared MMMs based on the polymer matrix of PIM-1 and a zeolitic

imidazolate framework, ZIF-8. They reported that an increase in MOF loading results in an

increased permeability coefficient and selectivity, but there was an exception for the CO2/CH4 pair,

where the selectivity is more or less independent of the ZIF-8 loading. Alentiev et al. [16] studied

the gas transport of composite membrane material based on PIM-1 and MIL-101 (chromium

terephthalate) MOFs. They demonstrated that the addition of MIL-101 leads to an increase in the

permeability and diffusion coefficient for He, O2, N2 and CO2 gases in accordance with the

2

behaviour observed by Bushell et al. for the PIM/ZIF-8 MMMs. Ma et al. [17] designed

nanoporous membranes combining a task-specific ionic liquid and MIL-101(Cr) loaded in PIM-1.

They found an improved selectivity, although their permeabilities are lower than those generally

reported in the literature for PIM-1.

Other MOFs of particular interest are the UiO-66-isoreticulars, Zr-based MOFs with different

functional linkers. One of the advantages of UiO-66 is that different linkers can be used to tailor the

affinity with the polymer matrix, which is one of the factors that affect the gas transport properties

of MMMs. Transport properties of various UiO-based MMMs have been reported. Nik et al. [18]

prepared MMMs with different types of isoreticular-UiO-66 in 6FDA–ODA polyimide. The

presence of –NH2 groups decreased the CO2 permeability and slightly increased the CO2/CH4

selectivity. Yang et al. [19] described the effect of ligand functionalization on the CO2/CH4

separation for the UiO-66 via a computational approach. They found CH4/CO2 adsorption

selectivity in the following order of linker groups –(COOH)2 > –NH2 > UiO-66, in accordance with

the results reported by Wu et al. [20]. Hu et al. [21] showed that the selectivity of the UiO-66-

(COOH)2 can be tailored by exchanging the hydrogen of the carboxylic group with a monovalent

cation. Recently, Smith et al. [22] described the use of Ti-exchanged UiO-66 to enhance the gas

permeability of PIM-1.

This work presents the ideal gas transport properties of novel MMMs based on isoreticular UiO-66

in PIM-1 for the six gases He, H2, O2, N2, CO2, CH4 determined in a fixed-volume pressure increase

instrument. The actual separation of the CO2/CH4 gas pair and the N2/CO2/O2 ternary mixture was

also tested under mixed gas conditions to investigate the membrane performance in simulated

industrial applications. Three different types of Zr-based isoreticular UiO-66 are used, with loadings

up to 27 wt-%: UiO-66, UiO-NH2 and UiO-66-(COOH)2. The effect of the functional groups on the

membrane performance is studied and the gas transport properties are described in terms of the

Maxwell equation. This model was already successfully used to describe the transport properties of

MMMs based on PIM-1 [23]. The membrane performance is compared with the state of art data and

is found to exceed the Robeson upper bound in many cases.

2 Theoretical description of gas transport in mixed-matrix membranes

Several models can be applied to describe the effect of fillers in polymer matrix [24]. Among them,

one of the simplest models that is usually used to describe the gas transport in mixed matrix

membranes is the Maxwell model [2,25–27]:

3

PMMM=Pc|Pd+2 Pc−2 Φd (Pc−Pd )Pd+2 Pc+Φd (Pc−Pd )

|11\* MERGEFORMAT (Eq. )

where the PMMM is the effective permeability of the mixed matrix membrane, Pc and Pd represent the

gas permeabilities in the continuous and dispersed phase, respectively and d is the volume fraction

of dispersed phase. Since it is remarkably accurate, this is the model that will be used in the present

work. The Maxwell model is valid for systems containing spherical fillers with a relatively low

loading [28]. The transport can occur in different ways depending on the type of nanoparticle

dispersion. The incorporation of fillers into the polymer phase can significantly alter the transport

properties of gases through the MMM. Several fundamentally different cases may occur.

First case

If the interaction between the polymer and filler particles is poor, the polymer chains do not

completely adhere to the surface of the fillers, giving rise to the formation of undesirable channels

between both phases. In this case, the permeability of the gas in the dispersed phase and around the

particles is much higher than that of the continuous phase: Pd » Pc. Thus, 1 becomes:

PMMM=Pc|1+2 Φd

1−Φd|

22\* MERGEFORMAT (Eq. )

The formation of these non-selective voids at the interface allows bypassing of the gas around the

filler particles. At low filler loadings this results in a higher permeability but constant selectivity. At

relatively high filler loadings, above the percolation threshold, continuous channels are formed

across the membrane, which deteriorates the selectivity of the MMM, and thus the gas separation

performance of the membrane [29].

Second case

Another phenomenon that can occur is the partial or complete blockage of the MOF cage by the

polymer phase. In this situation, the permeability of the gas in the dispersed phase is much lower

than that of the continuous phase: Pd « Pc. 1 reduces to:

33\* MERGEFORMAT (Eq. )

Thus, the fillers behave as impermeable obstacles to the transport and in this case no positive effects

of the addition of fillers on the transport properties of the membrane will be observed [7], although

4

even impermeable fillers could affect the packing of the polymer matrix, and thus influence the

permeability indirectly.

Third case

Generally, the permeability of the gases through the dispersed phase lies between the two extreme

cases discussed above, and 0 « Pd « ∞. The fillers exhibit an intrinsic finite permeability. This

permeability will be different from that of the polymer matrix and its behaviour must be described

by the complete Maxwell equation 1. This requires that the MOFs are well distributed within the

polymer matrix and adhere to the polymer without pore blockage. The three cases are schematically

indicated in Figure 1.

1000

10000

100000

0 10 20 30

Perm

eabi

lity (

Bar

rer)

Nanoparticols (wt %)

Pd =∞

Pd =0

0 < Pd < ∞

Filler content %wt

Case I

Case II

Case III

Perm

eabi

lity (

Bar

rer)

Figure 1: Graphic representation of gas transport properties of the previous cases as a function

of the filler concentration. The dashed lines delimit the range of permeabilities predicted by the

Maxwell model. The points indicate an example where the filler in the MMM slightly

increases the permeability of the polymer matrix.

Fourth case

This is a special example of the previous situation, in which the permeability of the continuous

phase is exactly equal to that of the dispersed phase, Pc=Pd. In this case, 1 simplifies to:

PMMM=Pc 44\* MERGEFORMAT (Eq. )

5

This means that the presence of the nanoparticles has no influence on the transport properties of the

membrane for a specific gas. In this case, the mixed matrix membrane will show an identical

permeability to the neat polymer membrane for this gas, but since different gases are probably

affected differently by the filler particles, the selectivity might change with respect to the neat

polymer.

3 Experimental

3.1 Materials

All starting materials and solvents were purchased from Sigma-Aldrich, except for chromium

nitrate nonahydrate (Cr (NO3)3.9H2O, Alfa, 98%) and sodium hydroxide (NaOH, Fisher Chemical).

All chemicals were used as received. Single gases were supplied by Pirossigeno at a minimum

purity of 99.9995%. Certified gas mixtures were supplied by Sapio at a purity of ±0.01% from the

certified concentration (CO2/CH4 mixture with 47.89 mol.% CH4 and N2/CO2/O2 mixture with 10.10

mol.% CO2 and 10.02 mol.% O2).

3.2 PIM-1 synthesis

PIM-1 (Figure 2a) was synthesized by the method of Du et al. [4,30]. To a dry 500 mL three-necked

round bottom flask equipped with a Dean-Stark trap, 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-

1,1'-spirobisindane (TTSBI) (17 g, 50 mmol), tetrafluoroterephthalonitrile (TFTPN) (10 g, 50

mmol), anhydrous potassium carbonate (20.7 g, 150 mmol), dimethylacetamide DMAc (100 mL)

and toluene (50 mL) were added under dry nitrogen gas. The reaction mixture was refluxed at 160 oC for 40 min. The product was then poured into methanol. The crude polymer was dissolved in

chloroform and reprecipitated from methanol. The product was refluxed for six hours in deionized

water and then dried at 100 oC for two days. Two batches were prepared following this procedure,

ground into powder and blended together. The blend was dissolved in chloroform with stirring for 2

h at room temperature, then reprecipitated from methanol, collected by filtration and dried under

vacuum at 100 C for two days. For the PIM-1 blend, average molar masses from multi-detector Gel

Permeation Chromatography (Viscotek 2001 instrument with two Polymer Laboratories mixed-B

columns, chloroform as eluent) were Mw = 112,000 and Mn = 48,600 g mol-1, polydispersity Mw/Mn

= 2.3.

6

a b

O OH

OOH

O OH

OOH

O

OH

O

OH

O OH

OOH

NH2

Figure 2: a) Chemical structure of PIM-1; b) Porous structure of UiO-66 MOFs,

tetrahedral cage (right) and octahedral cage (left); c) non-functionalized benzene-1,4-

dicarboxylates and functionalized with (-NH2) and (-COOH)2 ligands

3.3 MOF Synthesis

In this work, the synthesis of UiO-66(Zr) and UiO-66(Zr)-NH2 were adapted from previous work by

Katz et al. [33]. Carboxylic acid-functionalized UiO-66(Zr) was synthesized as described by Yang

et al. [34]. These MOFs are a family of Zr-terephthalate based metal–organic frameworks, namely

UiO-66. In UiO-66, each Zr6O4(OH)4 cluster is surrounded by maximally 12 terephthalate linkers

resulting in large octahedral and smaller tetrahedral cages with diameters of 11 Å and 8 Å,

respectively (Figure 2b) [31]. Triangular windows with a diameter of 6 Å guarantee the selective gas

passage [32]. Each of these UiO-66 MOFs differs from the others in terms of the functionalization

of the benzene-1,4-dicarboxylate linker, that allows a family to be obtained of isoreticular UiO-66.

The UiO-66-isoreticulars used in the present work are UiO-66-NH2 and UiO-66-(COOH)2,

functionalized with an amino group (-NH2) and with two carboxylic groups (-COOH)2, respectively.

3.3.1 UiO-66(Zr) Synthesis

Solutions of ZrCl4 (0.125 g, 0.5 mmol) dissolved in 5 mL DMF and 1 mL conc. HCl and

terephthalic acid (0.123 g, 0.75 mmol) dissolved in 10 mL DMF were mixed and sonicated for 20

min at room temperature. The whole mixture was heated in a glass jar (30 mL) at 80 oC overnight.

Upon cooling to room temperature, the solid product was filtered under vacuum, washed with DMF

to remove unreacted terephthalic acid, washed with methanol and dried at 100 oC. 7

c

3.3.2 UiO-66(Zr)-NH2 Synthesis

Solutions of ZrCl4 (0.125 g, 0.5 mmol) dissolved in 5 mL DMF and 1 mL conc. HCl and 2-

aminoterephthalic acid (0.134 g, 0.75 mmol) dissolved in 10 mL DMF were mixed and sonicated

for 20 min. The whole mixture was heated in a glass jar (30 mL) at 80 oC overnight. Once cooled to

room temperature, the solid product was separated under vacuum, washed with DMF, washed with

methanol and dried at 100 oC.

3.3.3 UiO-66(Zr)-(COOH)2 Synthesis

Pyromellitic acid (4.3 g, 0.017 mol) and ZrCl4 (2.3 g, 0.01 mol) were dissolved in 50 mL of distilled

water at room temperature with stirring and then heated under reflux for 24 h. The resulting solid

was filtered and washed several times with water to remove free acid. The white precipitate was

activated with water under reflux for 16 h. The UiO-66(Zr)-(COOH)2 was filtered and washed with

acetone and dried at 30 oC under vacuum.

3.4 Membrane preparation and conditioning

Membranes with different MOF loadings (9.1 wt.%, 16.6 wt.%, 23.1 wt.% and 28.6 wt.%) in PIM-1

were prepared. For preparation of the membranes, PIM-1 was dissolved in anhydrous chloroform

and stirred overnight. At the same time, the MOFs were dispersed in anhydrous chloroform and

stirred overnight in a vial. The particles were sonicated in an ultrasonic bath for 30 min before

filtering the PIM-1 solution through glass wool into the suspension. The resulting dispersion was

stirred overnight and then cast into a levelled flat-bottom glass petri-dish in a desiccator, and left to

evaporate over 3-5 days.

The residual solvent was removed and the casting history was cancelled by soaking the membrane

for 24 h in anhydrous methanol, followed by drying of the membrane for 24 h at 25°C and ambient

pressure.

3.5 Gas transport properties

3.5.1 Pure gas permeation

Pure gas permeation experiments were performed on a fixed volume/pressure increase instrument

constructed by Elektro & Elektronik Service Reuter (Geesthacht, Germany). Permeation tests were 8

carried out both on the as prepared MMMs and after methanol treatment. The feed gas pressure was

set at 1 bar (the actual value was read with a resolution of 0.1 mbar); the permeate pressure was

measured in the range from 0 to maximum, 13.3 mbar, with a resolution of 0.001 mbar. The gases

were always tested in the same order (He, H2, N2, O2, CH4, and CO2), although it was verified by

repeating a measurement cycle that if sufficiently long vacuum was applied to completely remove

the previous gas, the measurement order for these materials was irrelevant. Feed pressure, permeate

pressure, and temperature are continuously recorded during each measurement run. The temperature

was controlled at a constant temperature of 25 ± 1 °C. Before the first measurement, the membrane

cell was evacuated for sufficient time (at least 1 h) with a turbomolecular pump. Between two

subsequent measurements, the system was evacuated for a period of at least ten times the time lag

of the previous species in order to guarantee the complete removal of the previous gas. Circular

membranes, with an effective exposed surface area of 2.14 cm2 were used. The pressure increase on

the permeate side was recorded as a function of time from the moment that the membrane was

exposed to the feed gas. The whole permeation curve takes the following form:

55\*

MERGEFORMAT (Eq. )

in which pt is the permeate pressure at time t and p0 is the starting pressure, typically less than 0.05

mbar. The baseline slope (dp/dt)0 is usually negligible for a defect-free membrane. R is the universal

gas constant, T is the absolute temperature, A is the exposed membrane area, VP is the permeate

volume, Vm is the molar volume of a gas in standard conditions (0 °C and 1 atm), pf is the feed

pressure, S is the gas solubility, D the gas diffusion coefficient, and ℓ the membrane thickness. The

time lag method was applied to the recorded data to determine the gas diffusion coefficient [35].

The permeability coefficient, P, is calculated from the following equation, describing the steady

state permeation:

Pt=P0+(dp /dt )0⋅t+ RT⋅AV p⋅V m

⋅p f⋅p

ℓ ( t− ℓ2

6 D )66\* MERGEFORMAT (Eq. )

the last term in 6 corrects for the so-called permeation time lag, Θ, which is inversely proportional

to the diffusion coefficient of the gas:

77\* MERGEFORMAT (Eq. )

9

the approximate gas solubility coefficient, S, was obtained indirectly as the ratio of the permeability

to the diffusion coefficient by assuming the solution-diffusion transport mechanism:

S=P /D 88\* MERGEFORMAT (Eq. )

Permeabilities are reported in Barrer [1 Barrer=10-10 cm3 (STP) cm cm-2 s-1 cm Hg-1].

3.5.2 Mixed gas permeation

Mixed gas permeation experiments were carried out on a custom made constant pressure / variable

volume instrument, equipped with a modified Millipore permeation cell (diameter 47 mm). The

experiments were carried out at a feed flow rate of 100-200 cm3 min-1 and a feed pressure of 0-5 bar

(g), using EL-FLOW electronic Mass Flow Controllers (Bronkhorst) in the feed line and an EL-

PRESS electronic back pressure controller in the retentate line. Argon (30 cm3 min-1) was used as

the sweeping gas at ambient pressure. The actual temperature and pressure were recorded to convert

the measured flow rates to standard temperature and pressure conditions (STP, 1 atm at 0°C).

Highly permeable samples were masked with an adhesive aluminium tape with a smaller aperture to

limit the total permeate flow rate and to keep the stage cut close to or below 1%.

The permeate composition was determined with a Mass Spectrometric device equipped with a

quadrupole mass filter (Hiden Analytical, HPR-20 QIC Benchtop residual gas analysis system,

max. 200 AMU) and a sampling capillary with a typical flow rate of ca. 10-20 cm3 min-1 at ambient

pressure, depending on the gas sampled. The electron ionization energy was 70 eV and the gases

were detected with the SEM ion detector. Nitrogen was detected at 14 AMU to avoid overlap of N2

with the CO fragments from CO2 at 28 AMU in CO2/N2 mixtures; methane was detected at 15

AMU (as CH3) to avoid overlap of the molecular CH4 peak with the O fragment from CO2 at 16

AMU in the case of CO2/CH4 mixtures. All sensitivity ratios were previously calibrated against the

weaker 36Ar isotope at 36 AMU (ca. 0.3% abundancy). During the permeation experiments, this

signal was used as the internal standard for calculation of the gas concentrations in the

sweep/permeate.

Before each analysis, the membrane was flushed for at least 1 hour at both sides with two

independent argon streams until the MS signal was sufficiently stable. Subsequently, two

experiments were carried out. In the first experiment, the argon flux at the feed side was replaced by

the pure gas or gas mixture at atmospheric pressure (absolute pressure 1 bar (a)) to determine the

time needed to reach steady state permeation. In the second experiment, the feed pressure was

stepwise changed from 1-5 bar (g) and back, with sufficiently long time intervals to reach steady 10

state permeation in each step. The background signals were determined just before switching from

argon to the gas or gas mixture at the feed side, and were subtracted from the measured signal

during data processing.

All measured data were recorded with the MASsoft software package supplied with the mass

spectrometer and with the FlowPlot software supplied with the pressure and mass flow controllers.

The measured data were processed by a self-written elaboration program.

The mixed gas selectivity, i/j was calculated as the ratio of the individually calculated gas

permeances () in the mixture:

(Eq. 9)

(Eq. 10)

where the individual gas permeance, i, of the ith species in the mixture is obtained as the ratio of its

volumetric permeate flux, QPermeate, to the partial pressure difference between the feed and permeate

sides, pi:

(Eq. 11)

in which xi is the mole fraction of the ith species, pFeed and pPermeate are the total feed and permeate

pressures, respectively. The permeate flux is the flow rate per unit area, defined as:

(Eq. 12)

11

The volumetric permeate flow rate, JPermeate, is calculated from the known Argon sweep flow rate

and from the measured composition of the permeate/sweep mixture.

4 Results and discussion

4.1 MOF synthesis and membrane preparation

UiO-66 MOFs were synthesised successfully, as shown by the characterization data in the

Supplementary Information. BET surface areas determined by nitrogen adsorption at 77 K are

compared with published values in Table 1.

Table 1. Measured and literature values of BET surface area (m2 g-1) for UiO-66(Zr) MOFs.

MOFs This work Literature Refs.

UiO-66 1360 1110 1580 [33,36]

UiO-66-NH2 1120 1200 [33]

UiO-66-(COOH)2 423 415 [20]

Image analysis of the average particle size of the nanofillers dispersed in the polymeric matrix

revealed that their size is similar to that of the isolated nanofillers with a maximum in the particle

size distribution of ~200 nm. This indicates that there is no significant agglomeration (Fig. 2 and

Table S1 in the SI). This is clear from the homogeneous distribution of the MOFs in the SEM

images (See SI Figures S4, S5, S6). Moreover, the SEM images do not show any evidence of

macro-defects, indicating a good interaction between the surface of the fillers and the polymer

matrix.

4.2 Pure gas transport properties

The gas transport properties of the MMMs with the three UiO-66-isoreticular MOFs (UiO-66, UiO-

66-NH2, UiO-66-(COOH)2) all fall within the range predicted by the Maxwell model, indicating that

there are no anomalies 1. The permeabilities of one of the gas pairs of major interest (CO2/N2) are

given as a function of the MOF concentration in Figure 3. The increasing weight percentage of

MOFs tends to give more permeable membranes, proving the positive effect of the MOFs on the

12

permeability (Figure 3a,b,c). For all membranes, the methanol treatment increases the permeability

(Figure 3d,e,f). In neat PIM-1 this is attributed to a simultaneous removal of residual solvent and

increase in the excess free volume [4,37]. In the present case, part of the increase in permeability is

also a result of the removal of the casting solvent from within the MOF cages.

13

As cast MeOH treated

UiO

-66

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66 (wt %)

CO2 CO2 Pd=0 CO2 Pd=inf

N2 N2 Pd=0 N2 Pd=inf

CO2

N2

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66 (wt %)

CO2

N2

UiO

-66-

(CO

OH

) 2

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-(COOH)2 (wt %)

CO2

N2

CO2

N2

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-(COOH)2 (wt %)

CO2

N2

CO2

N2

CO2

N2

CO2

N2

CO2

N2

CO2

N2

UiO

-66-

(NH

2)

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-NH2 (wt %)

CO2

N2

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-NH2 (wt %)

CO2

N2

Figure 3: The CO2 and N2 permeabilities as a function of the MOFs concentration for a,d)

UiO-66(Zr)/PIM-1 b,e) UiO-66-(COOH)2/PIM-1 and c,f) UiO-66-(NH2)/PIM-1 membranes. On the left

“as cast” and on the right after methanol treatment. The lines correspond to the fit of the experimental data

14

a

b

c

d

e

f

with Maxwell equation for the lower limit and the upper limit with Pd=0 and Pd = ∞, respectively.

Figure 4 gives a complete overview of the individual transport properties, permeability, diffusivity

and solubility of six gases (CO2, H2, He, O2, CH4, N2), for the “as cast” membrane with UiO-

(COOH)2 fillers as a representative example of this family of membranes. The most permeable

species is CO2, confirming a solubility controlled transport, which is typical for CO2 in PIMs. The

permeation order of the different gases (CO2 > H2 >He > O2 > CH4 > N2) is not affected by the

presence of the MOFs. There is a progressive increase in permeability with increasing filler content.

The higher permeability of the MMMs with respect to the neat membranes is clearly due to the

enhanced diffusivity (Figure 4b), apparently because of the favourable diffusion path offered by the

cavities in the MOFs or in the MOF-polymer interface. In contrast, the apparent solubility,

indirectly calculated by 8, is hardly affected by the presence of the UiO fillers (Figure 4c).

a b c

100

1000

10000

0 10 20 30

Perm

eabi

lity (

Bar

rer)

UiO-66-(COOH)2 (wt %)

N2

CH4

O2

He

H2

CO2

1

10

100

1000

10000

0 10 20 30

Diff

usio

n (1

0-12

m2 /s

)

UiO-66-(COOH)2 (wt %)

N2

CH4

O2

He

H2

CO2

0

1

10

100

0 10 20 30

Solu

bilit

y (c

m3

(STP

)/(cm

3ba

r))

UiO-66-(COOH)2 (wt %)

N2

CH4

O2

He

H2

CO2

Figure 4: Gas permeability, diffusion coefficient and solubility coefficient for CO 2, H2, He, O2, CH4, N2 of UiO-66-

(COOH)2 loaded MMMs as function of the MOF concentration.

The gas permeabilities of the as-cast membranes and after methanol treatment are reported for

another gas pair of major interest, CO2/CH4, in Figure 5a,b,c, confirming the positive effect of the

MeOH soaking on the permeability. In most cases, the permeabilities after methanol-treatment

show a more monotonous trend than the “as cast” membranes because of the cancellation of the

thermomechanical history.

15

UiO

-66

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66 (wt %)

CH4

CO2

1

10

100

1000 10000 100000

Sele

ctiv

ity (

CO

2/CH

4)

CO2 Permeability (Barrer)

UiO

-66-

(CO

OH

) 2

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-(COOH)2 (wt %)

CH4

CO2

1

10

100

1000 10000 100000

Sele

ctiv

ity (

CO

2/CH

4)

CO2 Permeability (Barrer)

UiO

-66-

(NH

2)

100

1000

10000

100000

0 10 20 30

Perm

eabi

lity

(Bar

rer)

UiO-66-NH2 (wt %)

CH4

CO2

1

10

100

1000 10000 100000

Sele

ctiv

ity (

CO

2/CH

4)

CO2 Permeability (Barrer)

Figure 5: Left: CO2 and CH4 permeability of the as cast (filled symbols) and MeOH treated membranes (open

symbols) as a function of the UiO loading. a) UiO-66; b) UiO-66-(COOH)2 c) UiO-66-(NH2). Right: corresponding

Robeson diagrams with 9.1 wt % ( , ), 16.6 wt% ( , ), 23.1 wt% ( , ), 28.6 wt% (

16

a

b

c

d

e

f

, ); PIM-1 data are reported for comparison (orange symbols). ¿ Permeselectivity data for PIM-1/UiO-66-

(NH2) (9.1 wt%, 3 months aged after MeOH treatment) membrane, using the mixture CO2/CH4 (52.1:47.9). The red

line represents the Robeson 2008 upper bound and the black line Robeson 1991 upper bound [6].

For all MMMs the CO2/CH4 selectivity exceeds the upper bound in the region close to neat PIM-1.

The MMMs with UiO-66 and UiO-66-(COOH)2 show an increase in the permeabilities with the

MOF concentration and only a slight decrease of the CO2/CH4 selectivity. More interestingly, UiO-

66-(NH2) increases the permeability, maintaining the same selectivity, probably because the NH2

functionality enhances the affinity of the MOF for PIM-1.

4.3 Validation of the results via mixed gas permeation tests

Mixed gas permeability measurements were carried out in order to validate the performance of a

representative membrane for two industrially relevant separations, namely purification of CO2 rich

biogas and capture of CO2 from CO2 poor flue gas. For this purpose, two different gas mixtures

were used: a binary mixture of CO2/CH4 (52.1:47.9) and tertiary mixture of N2/CO2/O2 (80:10:10),

simulating the biogas stream and flue gas stream, respectively. The permeation tests were carried

out in the range from 1 to 6 bar at 25°C. Figure 6 shows the permeabilities and selectivities for the

selected PIM-1/UiO-66-(NH2) (9.1 wt%, 3 months aged after MeOH treatment) membrane, as

functions of the total feed pressure. For the N2/CO2/O2 mixture the CO2 permeability slightly

decreased with increasing pressure, just a little more than that of N2 (Figure 6a), and as a

consequence the CO2/N2 selectivity also slightly decreased (Figure 6b). At the same time, the O2

permeability seemed to be independent of the pressure and therefore also the CO2/O2 selectivity

decreased with pressure, but obviously the small amount of O2 is strongly affected by the much

higher amount of N2 and by the presence of highly permeable CO2.

Figure 6c and Figure 6d show equivalent data for CO2/CH4 mixture. The CO2 and CH4 permeabilities

exhibited a slightly stronger pressure drop with increasing pressure, due to the higher partial

pressure of CO2. This is typical for highly soluble gases that exhibit more evident dual mode

sorption behavior at relatively low pressures. Interestingly, during a cycle with increasing (closed

symbols) and decreasing feed pressure (open symbols), the permeability shows a weak hysteresis

effect, with somewhat higher permeability when returning from high to low pressure. Apparently,

the polymer matrix dilates upon exposure to high concentrations of the readily condensable CH4,

and especially CO2, and does not relax back quickly enough when releasing the pressure.

17

100

1000

10000

0 1 2 3 4 5 6 7

Perm

eabi

lity

(Bar

rer)

Total feed pressure (bar)

CO2

O2

N2

1

10

100

0 1 2 3 4 5 6 7

Sele

ctiv

ity (-

)

Total feed pressure (bar)

CO2/N2

O2/N2

CO2/O2

100

1000

10000

0 1 2 3 4 5 6 7

Perm

eabi

lity

(Bar

rer)

Total feed pressure (bar)

CO2

CH4

1

10

100

0 1 2 3 4 5 6 7

Sele

ctiv

ity (-

)

Total feed pressure (bar)

CO2/CH4

Figure 6: a) Pressure dependence of the CO2, O2 and N2 mixed gas permeabilities, and b) corresponding CO2/N2,

CO2/O2, O2/N2 selectivities, using the ternary N2/CO2/O2 (80:10:10) mixture and the PIM-1/UiO-66-(NH2) (9.1%wt)

membrane after MeOH treatment and 3 months aged; c) Pressure dependence of CO 2 and CH4 permeabilities with the

corresponding CO2/CH4 selectivity, using the mixture CO2/CH4 (52.1:47.9)

5 Conclusions

This work reports the gas transport properties of mixed matrix membranes of Zr-based UiO-66

metal-organic frameworks in the polymer of intrinsic microporosity PIM-1. The gas transport can

be described effectively by the Maxwell model, indicating a good dispersion of the fillers in the

organic polymer matrix. All three fillers used, UiO-66, UiO-66-NH2 and UiO-66(COOH)2, increase

18

a b

c d

the permeability of the neat polymer, with slightly different effects on the ideal CO2/CH4 selectivity.

For UiO-66 and UiO-66(COOH)2, the increasing CO2 permeability is accompanied by a decrease of

the ideal CO2/CH4 selectivity. In contrast, the UiO-66-NH2 based MMMs also have a higher

permeability than PIM-1, but they maintain the same selectivity. Only the NH2-functionalized UiO-

66 sample completely maintains its selectivity, while the standard sample and the COOH-

functionalized sample give a slight reduction of selectivity. This suggests that there might indeed be

a minor effect of the interface in the latter samples, and not in the UiO-66-NH2 sample, although

this is difficult to prove. As an overall result, the samples with UiO-66(COOH)2 follow the upper

bound, while the samples based on UiO-66 and UiO-66-NH2 exceed the Robeson 2008 upper

bound. The high pure gas permeability and selectivity is almost completely maintained during

mixed gas permeation experiments, highlighting the good separation performance of these

membranes.

Acknowledgements

The work leading to these results has received funding from the European Union’s Seventh

Framework Program (FP7/2007-2013) under grant agreement n° 608490, project M4CO2. MRK is

supported by the Iraqi Ministry of Higher Education and Scientific Research. EE and JJ received

financial support from the CNR/FCT Italian/Portuguese bilateral project 2015-2016. Dr. Ing. P.

Bernardo is gratefully acknowledged for some help in the elaboration of the permeability data.

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