#
Estimating CO_{2}/N_{2} Permselectivity through Si/Al = 5 Small-Pore Zeolites/PTMSP Mixed Matrix Membranes: Influence of Temperature and Topology

^{1}

^{2}

^{*}

## Abstract

**:**

_{2}and N

_{2}permeability using zeolites of different topology (CHA, RHO, and LTA) in the same Si/Al = 5, embedded in poly(trimethylsilyl-1-propyne) (PTMSP) is evaluated with temperature. Several models are compared on the prediction of CO

_{2}/N

_{2}separation performance and then the modified Maxwell models are selected. The CO

_{2}and N

_{2}permeabilities through these membranes are predicted with an average absolute relative error (AARE) lower than 0.6% taking into account the temperature and zeolite loading and topology on non-idealities such as membrane rigidification, zeolite–polymer compatibility and sieve pore blockage. The evolution of this structure–performance relationship with temperature has also been predicted.

## 1. Introduction

_{2}emissions to the atmosphere from anthropogenic sources. Membrane separation technology is often presented as an energy efficient and economical alternative to conventional capture technologies although not yet passing through the stage of pilot plant scale [1]. Polymer membranes for CO

_{2}separation are especially constrained by a performance ‘upper bound’ trade-off between gas permeability and selectivity, which becomes especially significant for treating large volumes of flue gas. The simultaneous improvement on membrane permeability and selectivity is very attractive for industrial applications. Mixed matrix membranes (MMMs), which consist of the introduction of small amounts, usually below 30 wt %, of a special filler providing properties such as a molecular sieve, ion-exchange and robustness in a processable polymer matrix [2], are surpassing this upper bound [3,4,5,6,7]. More than homogenous distribution, the main challenge of MMM fabrication is achieving a good adhesion and compatibility between the inorganic filler and the polymer, avoiding the voids and defects that deteriorate separation performance [8].

_{2}separation [9]. The first and most widely used fillers are zeolites since the pioneering work of Zimmermann et al. [10]. Recently, zeolite 5A was introduced in Matrimid to prepare MMMs for CO

_{2}/CH

_{4}separation, after particle surface modification to obtain a defect-free membrane [11]. Amooghin et al. [12] reported the ion exchange effect of Ag

^{+}in zeolite Y-filled Matrimid MMMs led to a CO

_{2}permeability increase of 123% from 8.64 Barrer in pure Matrimid to 18 Barrer in 15% AgY-filled MMM, where 1 Barrer is defined as 10

^{−10}cm

^{3}(STP) cm cm

^{−2}s

^{−1}cmHg

^{−1}.

_{2}separation, we focused on the most permeable polymer, poly(trimethylsilyl-1-propyne), PTMSP, and observed that the adhesion with LTA fillers and therefore CO

_{2}/N

_{2}separation properties were best with a low Si/Al ratio even upon increasing temperature [14]. The strong influence of zeolite topology on CO

_{2}adsorption has also been acknowledged [15], giving the possibility to locally tune the energy interactions, promoting size and shape selectivity and clustering. However, this effect is not always straightforward because most zeolites cannot be synthesized in pure silica form or at similar Si/Al compositions. Exceptions to this rule are LTA (ITQ-29) [16] and CHA [17]. To avoid this and to see that the lower Si/Al favored the compatibility with glassy hydrophobic PTMSP [14], we fixed an intermediate value of the Si/Al ratio to 5, in order to study the influence of the zeolite filler topology using different small pore zeolites (LTA, CHA, RHO) in the CO

_{2}/N

_{2}separation of PTMSP-based MMMs in the temperature range 298–333 K [18]. These MMM surpassed the Robeson’s upper bound at 5 wt % loading even at increasing temperature, but the separation of CO

_{2}/N

_{2}mixtures with a 12.5 wt % CO

_{2}content resulted in a real separation factor much lower than the intrinsic selectivity of the membrane material.

## 2. Materials and Methods

^{3}.

_{2}and CO

_{2}was measured in that order, using a home-made constant volume set-up described elsewhere [14,18], in the temperature range 298 to 333 K and a feed pressure of 3–4 bar and atmospheric permeate pressure. The average values of the permeabilities and selectivities obtained previously and used in this work are collected in Table A1 in Appendix A.

## 3. Results and Discussion

#### 3.1. Comparison of Known Mixed-Matrix Membrane Model Predictions

_{2}and N

_{2}through MMMs, as

_{2}permeability values cannot be predicted by the series, parallel, Maxwell and Higuchi models with acceptable error in all the range of temperature under study. The prediction accuracy of CO

_{2}permeability varies as a function of the zeolite topology. Regarding CO

_{2}permeability, the series and parallel model approaches fit the 5 wt % CHA/PTMSP MMM performance at 323 K, with a lower average AARE for this membrane. The CO

_{2}permeability of LTA/PTMSP MMMs can be described by parallel, Maxwell and Higuchi models in the whole range of operating temperatures and LTA loadings, while the series model only fits the experimental data at low loading. As for the RHO/PTMSP MMM, this is only valid up to 10 wt % RHO loading in the PTMSP matrix. This agrees with the data reported for other MMMs prepared with dispersed fillers of RHO topology [36] where the Maxwell equation only describes the CO

_{2}permeability at low loading, as observed for the ZIF-20/Matrimid MMM, being ZIF-20 a zeolite imidazolate framework of RHO topology as well [36]. In the case of our RHO/PTMSP MMMs, all previous models overestimate the experimental permeabilities.

_{2}permeability. This overestimation is more significant at lower operation temperatures, as reported by Clarizia et al. [14]. In this work, this is true for CHA/PTMSP MMMs with the series model, Figure 2a, and the parallel and Maxwell model for LTA/PTMSP MMMs, Figure 2c. These are simplifications of the general Maxwell equation expressed by Equation (B1) to predict the overall steady-state permeability through an ideal defect-free MMM [26]. Those models provide a simple, quantitative framework to predict the transport properties of MMM when the transport properties of the constituent phases are known, especially at low dispersed phase loading. Only more advanced modifications of this Maxwell equation, such as Felske and Lewis–Nielsen, provide enough accuracy for the description of MMM performance, especially in the case of the slow permeating gas, N

_{2}, as reflected in Figure 2b,d,f.

#### 3.2. Reduced Mobility Modified Maxwell Model

_{eff}, is calculated first by

_{d}is the filler volume fraction in the polymer matrix, P

_{I}is the permeability through the rigidified continuous matrix, calculated as the ratio between the experimental permeability through a pure PTMSP membrane [18] and an adjustable parameter, β, as described in Figure 3a, and P

_{d}is the permeability through the zeolite. In this work, this value has been taken from literature data on CO

_{2}and N

_{2}permeation through pure zeolite membranes of similar Si/Al ratio and topology (Table 3) to avoid the usual dispersion on this parameter when calculated from experimental solubility isotherms [23].

_{I}acts as the permeability of the continuous phase, considering as such the interphase, assuming the bulk of the zeolite as the dispersed phase and the affected zeolite interphase with reduced permeability as the continuous phase [39], as represented in the scheme in Figure 3a. ϕ

_{s}is the volume fraction of the dispersed sieve phase in combined sieve and interphase, given by

_{I}is the volume fraction of the interface, and l

_{I}is the thickness of the ‘interface void’. The permeability of the whole MMM is thus estimated by applying the Maxwell equation again, as

_{d}+ ϕ

_{I}increases to one, the interphases of neighboring dispersed particles overlap and the overall mixed matrix is rigidified. This occurs preferentially as the zeolite particle loading is increased or the interphase void distance is increased, i.e., voids appear because embedding in the polymer chains becomes more difficult.

_{2}/N

_{2}separation, and including the influence of temperature. This model is thus based on three adjustable parameters, the interphase thickness, l

_{I}, and the chain immobilization factor, β, which depends on the permeating gas molecule [39], whose values are presented in Table 4, Table 5 and Table 6 for the CHA/PTMSP, LTA/PTMSP and RHO/PTMSP MMM, respectively.

_{2}than N

_{2}. This confirms that the polymer chain rigidification normally results in a larger resistance to the transport of the gas with larger molecular diameter [27]. The RHO/PTMSP MMM revealed a different trend, although only at 298 K, which may be attributed to the agglomeration of these larger crystal size and smaller pore size particles at the bottom of the MMM. Interestingly, β(CO

_{2}) and β(N

_{2}) of the three types of MMMs converge to similar values upon increasing temperature. This may be attributed to the compensating effects of polymer flexibility and chain rigidification of the polymer matrix, which are accentuated for the larger size of the RHO particles than LTA and CHA. This agrees with the current statement that in gas separation through MMMs there is not only an optimum in zeolite loading but also in operating temperature [40].

_{I}(μm), accounts for the compatibility between the zeolite and polymer phases, as well as the defects or voids due to poor compatibility between zeolites and polymer [25]. In this work, the void thickness decreases with increasing zeolite loading and is independent of the type of gas and temperature. It can also be observed that this parameter l

_{I}is influenced by the zeolite topology, in the following order: l

_{I}(LTA/PTMSP) < l

_{I}(CHA/PTMSP) < l

_{I}(RHO/PTMSP). This is attributed to the different interaction with the polymer matrix, and the decreasing particle size, in agreement with results obtained for zeolite-APTES/PES MMMs [27]. Those authors obtained as thickness of the rigidified region l

_{i}= 0.30 µm for a cubic zeolite A (Si/Al = 1) dispersed phase in PES, and values of the chain immobilization factor (β) of 3 and 4, for O

_{2}and N

_{2}, respectively. A rigidified thickness of 1.4 µm and chain immobilization factor was reported for ZIF-20/polysulfone MMMs, estimating a P

_{d}= 45 Barrer, in agreement with pore ZIF membranes of similar pore size and topology [41]. Therefore, the magnitude of the adjustable parameters obtained in this work are in the same order of magnitude.

#### 3.3. Extended Pore-Blockage Reduced Mobility Modified Maxwell Model

_{3rd}permeability calculated by Equation (5) is entered as the new dispersed phase, and the permeability of the rigidified region, P

_{rig}, is taken as the continuous phase, to calculate the new P

_{eff}, P

_{2nd}:

_{2nd}as the new permeability for the dispersed phase, turning the previous equations into

_{blo}and ϕ

_{rig}, the calculated volume fraction of the pore-blockage affected region, and the rigidified region, respectively, as well as β′ and β, whose values depend on the permeating gas, and identify the partial pore blockage affected and rigidified polymer region, respectively, as given in Figure 3b. Note that β is similar to the chain immobilization factor introduced by the previous reduced mobility modified Maxwell model, discussed in the previous section.

_{2}and N

_{2}permeability using both modified Maxwell models. The experimental results are well described for the Si/Al = 5 zeolites, indicating a good compatibility between intermediate Si/Al zeolites and the glassy PTMSP [14]. The optimized β value is higher for N

_{2}than CO

_{2}, for CHA and RHO/PTMSP MMMs. β(N

_{2}) values of 0.92 are obtained for the CHA/PTMSP MMMs, independently of zeolite loading, where as they increase from 0.66 to 1.40 for the RHO/PTMSP MMMs. β(CO

_{2}) gives smaller values than β(N

_{2}), as expected for smaller molecules. β(CO

_{2}) follows similar trends as β(N

_{2}), being constant for CHA and LTA/PTMSP MMMs, at values of 0.3 and 0.2, respectively, and increasing from 0.26 to 0.94 with increasing loading for RHO/PTMSP MMMs. These values are smaller than 1.6, the value recently published for Sigma-1/Matrimid MMMs, considering also the partial pore blockage effect [28]. The values of β′(CO

_{2}) are 0.06 for CHA and RHO/PTMSP MMMs, and below 0.03 for LTA/PTMSP MMMs. The β′(N

_{2}) are 70% higher in the LTA and RHO/PTMSP MMMs, and 30% higher than β′(CO

_{2}) in the case of CHA/PTMSP MMMs. These results reveal that, although the partial pore blockage is low in small–pore zeolites, it is more significant for the smaller pore size zeolite fillers as CHA or RHO, than LTA.

_{2}and N

_{2}permeability through the Si/Al = 5 zeolite/PTMSP MMMs as a function of zeolite loading, topology and temperature. The CO

_{2}permeability increases with temperature while the N

_{2}permeability slightly increases for CHA and RHO/PTMSP MMMs, behavior similar to pure zeolite membranes, as reflected by the activation energies derived from the Arrhenius equation in the previous work [18], in agreement with other works in literature [42]. The LTA/PTMSP MMMs show a maximum performance at 10 wt % zeolite loading and 323 K, losing permselectivity at higher loading and temperature. The worst AARE for the prediction of experimental permeabilities through the extended partial pore blockage reduced mobility model is 0.6%, for the 5 wt % CHA/MMM at 313 K, which were in some of the best agreement with the first modified Maxwell model. Partial pore blockage may be affecting permeability even with small-pore zeolite fillers in a glassy polymer matrix [28].

## 4. Conclusions

_{2}and N

_{2}permeabilities of Si/Al = 5 small-pore zeolites/PTMSP MMM has been compared with modified Maxwell model predictions as a function of zeolite topology (CHA, LTA, RHO), loading (0–20 wt %) and temperature (298–333 K). Three adjustable parameters accounting for the membrane rigidification, void interphase and partial pore-blockage have been optimized at values lower than reported in literature. They reveal the compatibility between Si/Al = 5 zeolites dispersed in the glassy polymer PTMSP, as well as a small influence of partial pore blockage in the case of the smaller pore size CHA and RHO. The CO

_{2}and N

_{2}permeabilities through these membranes are predicted with an AARE lower than 0.6% taking into account zeolite loading and topology on non-idealities such as membrane rigidification and sieve pore blockage and their influence on MMM performance. The evolution of this structure-performance relationship with temperature has also been predicted. The implementation of the Arrhenius dependency of the MMM permeability and the prediction studied in this work constitute a step further towards the understanding of the MMM performance in order to develop new membrane materials and module configurations with potential application in CO

_{2}separation, which will be addressed in a future work.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A

**Table A1.**Experimental data of the different MMMs with increasing order of particle size (LTA, 0.5 µm; CHA, 1 µm; RHO, 1.5 µm).

Filler and Loading [18] | T (K) | P(CO_{2}) (Barrer) | P(N_{2}) (Barrer) | α(CO_{2}/N_{2}) |
---|---|---|---|---|

5 wt % LTA | 298 | 7150 | 794 | 9 |

303 | 13,881 | 637 | 22 | |

313 | 12,448 | 816 | 15 | |

323 | 11,770 | 1208 | 10 | |

333 | 9026 | 3044 | 3 | |

10wt % LTA | 298 | 8813 | 951 | 9 |

303 | 12,921 | 865 | 15 | |

313 | 15,802 | 892 | 18 | |

323 | 16,648 | 1078 | 15 | |

333 | 11,029 | 4520 | 2.5 | |

20 wt % LTA | 298 | 10,587 | 1720 | 6 |

303 | 13,178 | 2585 | 5 | |

313 | 12,980 | 2519 | 5 | |

323 | 11,175 | 3966 | 3 | |

333 | 10,964 | 4316 | 2.5 | |

5 wt % CHA | 298 | 2274 | 292 | 8 |

303 | 3575 | 329 | 11 | |

313 | 5651 | 372 | 15 | |

323 | 11,772 | 409 | 29 | |

333 | 16,145 | 511 | 32 | |

10 wt % CHA | 298 | 3363 | 211 | 16 |

303 | 3620 | 262 | 14 | |

313 | 4351 | 216 | 20 | |

323 | 5892 | 241 | 24 | |

333 | 6485 | 330 | 20 | |

5 wt % RHO | 298 | 8205 | 1325 | 6 |

303 | 8383 | 1227 | 7 | |

313 | 11,722 | 1214 | 10 | |

323 | 12,726 | 1089 | 12 | |

333 | 13,324 | 1368 | 10 | |

10 wt % RHO | 298 | 3262 | 592 | 6 |

303 | 5996 | 509 | 12 | |

313 | 9111 | 712 | 13 | |

323 | 10,304 | 761 | 14 | |

333 | 11,114 | 1166 | 10 | |

20 wt % RHO | 298 | 4479 | 1229 | 4 |

303 | 8883 | 1173 | 8 | |

313 | 7784 | 1210 | 6 | |

323 | 9293 | 1341 | 7 | |

333 | 9498 | 1704 | 6 |

## Appendix B

_{d}is the dispersed phase volume fraction, calculated from the nominal weight fraction of the zeolite in the MMMs, using the density of the PTMSP polymer and the corresponding zeolite density (Table 1).

_{I}/r

_{d}. This model also needs three adjustable parameters, as in the reduced mobility modified Maxwell model.

_{d}< ϕ

_{m}. The solution diverges when ϕ

_{d}= ϕ

_{m}and it should be noted that when ϕ

_{m}→ 1, the Lewis–Nielsen model reduces to the Maxwell equation (Equation (A1)).

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**Figure 1.**Scanning electron microscope (SEM) images of the detailed contact between LTA (

**a**); CHA (

**b**); RHO (

**c**) and poly(trimethylsilyl-1-propyne) (PTMSP) in 5 wt % loaded mixed matrix membranes (MMMs). Bars correspond to 6 µm.

**Figure 2.**Comparison of CO

_{2}(left) and N

_{2}permeabilities through CHA (

**a**,

**b**), LTA (

**c**,

**d**) and RHO (

**e**,

**f**)/PTMSP MMMs with the predictions by the series (dashed lines), parallel (dotted lines), original Maxwell (dash-dot), Higuchi (dash dot dot), Felske (thin continuous line) and Lewis–Nielsen (thick continuous line) models, as a function of temperature. Zeolite loading: 5 wt % (black), 10 wt % (red), 20 wt % (green).

**Figure 4.**Effect of temperature and zeolite loading on the CO

_{2}(

**a**) and N

_{2}(

**b**) permeability through CHA/PTMSP MMMs: Thin lines correspond to the reduced mobility modified Maxwell model and thick lines to the extended modified Maxwell model. Dash, dot and continuous patterns, and void, half-filled and full symbols, refer to 5 wt %, 10 wt % and 20 wt % zeolite loading, respectively.

**Figure 5.**Effect of temperature and zeolite loading on the CO

_{2}(

**a**) and N

_{2}(

**b**) permeability through LTA/PTMSP MMMs: Thin lines correspond to the reduced mobility modified model and thick lines to the extended modified Maxwell model. Dash, dot and continuous patterns, and void, half-filled and full symbols, refer to 5 wt %, 10 wt % and 20 wt % zeolite loading, respectively.

**Figure 6.**Effect of temperature and zeolite loading on the CO

_{2}(

**a**) and N

_{2}(

**b**) permeability through RHO/PTMSP MMMs: Thin lines correspond to the reduced mobility modified model and thick lines to the extended modified Maxwell model. Dash, dot and continuous patterns, and void, half-filled and full symbols, refer to 5 wt %, 10 wt % and 20 wt % zeolite loading, respectively.

Filler | Crystal Size (µm) | Density (g/cm^{3}) | Pore Size ^{1} (nm) | Structure ^{2} |
---|---|---|---|---|

LTA | 0.5 | 1.498 [32] | 0.41 | |

CHA | 1.0 | 2.090 | 0.38 | |

RHO | 1.5 | 1.442 [33] | 0.36 |

^{1}From [18].

^{2}The crystallographic structures have been taken from the International Zeolite Database (http://www.iza-structure.org/databases/): View of the planes 100 for LTA and 001 for CHA and RHO, respectively.

**Table 2.**Percentage of average absolute relative error (AARE) for CO

_{2}and N

_{2}permeation (first and second values in every entry) prediction, highlighting those AARE values lower than 20%.

MMM | Series | Parallel | Maxwell | Higuchi | Felske | Lewis-Nielsen |
---|---|---|---|---|---|---|

5CHA/PTMSP | 17.32/370 | 108/2026 | 106/2006 | 146/2609 | 118/32.4 | 24.9/2.14 |

10CHA/PTMSP | 24.2/143 | 102/2966 | 99.7/2909 | 96.8/2854 | 80/936 | 10^{−4}/10^{−5} |

5LTA/PTMSP | 20.6/33.3 | 11.8/516 | 11.4/498 | 26.3/708 | 2.54/10^{−3} | 0.46/0.01 |

10LTA/PTMSP | 40.9/50.0 | 14.5/631 | 4.79/214 | 14.6/560 | 67.4/9.04 | 3.98/10^{−5} |

20LTA/PTMSP | 45.0 /50.0 | 7.11 /212 | 8.28/198 | 10.4/194 | 3.00/10^{−4} | 4.37/10^{−5} |

5RHO/PTMSP | 8.62/126 | 12.7/362 | 12.4/357 | 16.7/395 | 0.85/6·10^{−4} | 1.84/0.6·10^{−5} |

10RHO/PTMSP | 24.0/216 | 57.0/1030 | 54.5/1003 | 49.3/947 | 0.03/2·10^{−3} | 4.32/0.02 |

20RHO/PTMSP | 45.3/52.4 | 72.2/947 | 63.8/892 | 44.2/756 | 22.0/5·10^{−4} | 12.3/10^{−4} |

**Table 3.**Permeability data of the pure zeolite dispersed phase, P

_{d}, used for the model predictions.

Zeolite Dispersed Phase | P_{d}(CO_{2}) (Barrer) | P_{d}(N_{2}) (Barrer) | T (K) | Reference |
---|---|---|---|---|

CHA (Si/Al = 5) ^{1} | 88 | 0.59 | 293 | [37] |

CHA (pure silica) | 539 | 55 | 313 | [38] |

LTA (Si/Al = 1) | 139 | 0.048 | 298 | [25] |

RHO ^{2} | 623 | 260 | 298 | [33] |

^{1}Si/Al = 5 as the zeolites used in this work.

^{2}The CO

_{2}permeabilities reported for ZIF-8 composite values are considered as the Rho here, given the similar sodalite topology.

**Table 4.**Parameters estimated by the reduced mobility modified Maxwell model for the CHA/PTMSP MMMs.

T (K) | 5 wt % | 10 wt % | ||
---|---|---|---|---|

l_{I} (µm) = 1.39 | l_{I} (µm) = 0.98 | |||

β (CO_{2}) | β (N_{2}) | β (CO_{2}) | β (N_{2}) | |

298 | 7.42 | 61.2 | 4.90 | 86.61 |

303 | 4.56 | 53.28 | 3.48 | 64.0 |

313 | 2.25 | 42.8 | 2.87 | 70.5 |

323 | 1.01 | 31.41 | 1.97 | 50.4 |

333 | 0.73 | 20.5 | 1.00 | 10.2 |

**Table 5.**Parameters estimated by the reduced mobility modified Maxwell model for the LTA/PTMSP MMMs.

T (K) | 5 wt % | 10 wt % | 20 wt % | |||
---|---|---|---|---|---|---|

l_{I} (µm) = 0.60 | l_{I} (µm) = 0.56 ± 0.08 | l_{I} (µm) = 0.27 | ||||

β (CO_{2}) | β (N_{2}) | β (CO_{2}) | β (N_{2}) | β (CO_{2}) | β (N_{2}) | |

298 | 2.35 | 21.9 | 1.83 | 17.4 | 1.39 | 8.82 |

303 | 0.93 | 27.1 | 1.00 | 12.0 | 0.86 | 5.84 |

313 | 1.01 | 18.9 | 0.80 | 11.0 | 0.85 | 5.37 |

323 | 1.00 | 10.2 | 0.72 | 8.34 | 0.92 | 2.72 |

333 | 1.29 | 3.38 | 1.06 | 2.49 | 0.93 | 2.08 |

**Table 6.**Parameters estimated by the reduced mobility modified Maxwell model for the RHO/PTMSP MMMs.

T (K) | 5 wt % | 10 wt % | 20 wt % | |||
---|---|---|---|---|---|---|

l_{I} (µm) = 1.76 | l_{I} (µm) = 1.23 | l_{I} (µm) = 0.79 | ||||

β (CO_{2}) | β (N_{2}) | β (CO_{2}) | β (N_{2}) | β (CO_{2}) | β (N_{2}) | |

298 | 2.06 | 0.31 | 10.62 | 1.95 | 3.36 | 1.46 |

303 | 1.57 | 0.35 | 2.10 | 2.98 | 1.28 | 1.54 |

313 | 1.07 | 0.30 | 1.33 | 1.29 | 1.43 | 1.33 |

323 | 0.91 | 0.28 | 1.17 | 0.93 | 1.12 | 0.93 |

333 | 0.87 | 0.17 | 1.01 | 0.45 | 1.08 | 0.58 |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Casado-Coterillo, C.; Fernández-Barquín, A.; Valencia, S.; Irabien, Á.
Estimating CO_{2}/N_{2} Permselectivity through Si/Al = 5 Small-Pore Zeolites/PTMSP Mixed Matrix Membranes: Influence of Temperature and Topology. *Membranes* **2018**, *8*, 32.
https://doi.org/10.3390/membranes8020032

**AMA Style**

Casado-Coterillo C, Fernández-Barquín A, Valencia S, Irabien Á.
Estimating CO_{2}/N_{2} Permselectivity through Si/Al = 5 Small-Pore Zeolites/PTMSP Mixed Matrix Membranes: Influence of Temperature and Topology. *Membranes*. 2018; 8(2):32.
https://doi.org/10.3390/membranes8020032

**Chicago/Turabian Style**

Casado-Coterillo, Clara, Ana Fernández-Barquín, Susana Valencia, and Ángel Irabien.
2018. "Estimating CO_{2}/N_{2} Permselectivity through Si/Al = 5 Small-Pore Zeolites/PTMSP Mixed Matrix Membranes: Influence of Temperature and Topology" *Membranes* 8, no. 2: 32.
https://doi.org/10.3390/membranes8020032