Synthesis and characterization of new zeolite membranes and crystalline meta-organic frameworks

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Universidade de Aveiro 2018

Departamento de Química

Sebastião Melo

Refoios da Costa

Síntese e caracterização de novas membranas

zeolíticas

e

de

Redes

Metalo-Orgânicas

cristalinas

Synthesis and characterisation of new zeolite

membranes

and

crystalline

Metal-Organic

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i Universidade de Aveiro 2018 Departamento de Química

Sebastião Melo

Refoios da Costa

Síntese e caracterização de novas membranas

zeolíticas

e

de

Redes

Metalo-Orgânicas

cristalinas

Synthesis and characterisation of new zeolite

membranes

and

crystalline

Metal-Organic

Frameworks

Dissertação apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Engenharia Química, realizada sob a orientação científica do Doutor Zhi Lin, Investigador Principal no Departamento de Química da Universidade de Aveiro e co-orientação do Professor Doutor Carlos Manuel Santos da Silva, Professor Associado do Departamento de Química da Universidade de Aveiro.

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iii o júri

presidente Professora Doutora Maria Inês Purcell de Portugal Branco

Professora Auxiliar do Departamento de Química da Universidade de Aveiro

Doutor António Augusto Areosa Martins

Investigador de Pós-Doutoramento do Laboratório de Engenharia de Processos, Ambiente, Biotecnologia e Energia da Faculdade de Engenharia da Universidade do Porto

Professor Doutor Carlos Manuel Santos da Silva

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v

This Master Thesis is product of a research work conducted over the last year, and it is also the materialization of a five-year programme undertaken at the University of Aveiro, under the Integrated Master in Chemical Engineering. In the end of this long way, I would like to acknowledge all those that have been my support and source of inspiration.

I am indebted to my supervisor Dr. Zhi Lin for his supervision, guidance and stimulus. He was undoubtedly the hand that guided my research into the right direction.

I must also express my sincere gratitude to my co-supervisor, Professor Carlos Manuel Silva. His professionalism, knowledge and permanent availability, are, and always will be, traces of personality that I will carry on in my life. Without his support, this thesis would have never been possible. Thank You Professor!

To Simão, who acted as my third supervisor, and played central role in the laboratorial part of this thesis. I am sorry to bother you so often! To the EgiChem group I owe the privilege of had been integrated in a remarkable and professional research group, where knowledge percolates throughout membranes of friendship - thank you guys! To Jéssica, my girlfriend, I have to thank her presence, patience, and love. She has proved to be strong enough to stand up alongside my academic ups and downs.

To the friends that have been close to me during this process, in particular Chafarrica, Dani, Inês, João, Marco, Margarida, Milene, Pedro, Rita, Rute, São, Sónia, Tomás and Zé, I would like to thank their friendship, constant source of support, and nice dinners and parties! Research headaches are better overcome with nice people like you! Cheers!

To my mother, Graça, father, Carlos, and brother Lourenço, I owe the genes that bind my life. They represent the safe net and the harbour that provide me love and stimulus to move forward. To them, Filipa, Rebecca and to the rest of my family, I dedicate this thesis. agradecimentos

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vii palavras-chave Caracterização, Mecanismos de Transporte, Membranas,

Permeação Gasosa, Redes Metalo-Orgânicas Cristalinas, Síntese, Zeólitos

resumo A utilização de membranas para a separação de misturas gasosas tem ganho grande importância devido às suas vantagens quando comparadas com processos de separação convencionais, como o moderado consumo energético e simplicidade de operação. Apesar de as membranas poliméricas dominarem o mercado, estas são limitadas por temperaturas de operação e seletividades relativamente baixas seletividades. As membranas inorgânicas, por possuírem elevadas estabilidades térmica, química e mecânica, tornam-se particularmente interessantes para processos de separação. Recentemente, as redes metalo-orgânicas cristalinas (MOFs), têm sido alvo de grande investigação, já que a sua estrutura é composta por uma componente orgânica e outra inorgânica, revelando características típicas de ambos os elementos, como flexibilidade e maior controlo na distribuição do tamanho dos poros.

Na presente dissertação pretendeu-se sintetizar e, posteriormente, caracterizar, membranas inorgânicas microporosas de zeólitos e, paralelamente, MOFs. No total, quinze membranas foram preparadas. A estrutura e morfologia das membranas mais promissoras foram estudadas pelas técnicas de difração de raios-X (DRX) e de microscopia eletrónica de varrimento (SEM). A caracterização dinâmica foi realizada por ensaios de permeação de gases puros a temperatura fixa e programada, usando diferentes gases (i.e., moléculas com diâmetros cinéticos distintos).

Relativamente às membranas zeolíticas, suportadas em tubos de α-alumina, recorreu-se ao método de crescimento secundário, seguido de síntese hidrotérmica. Estudou-se o efeito do tempo de síntese e da presença de rotação durante a mesma. Concluiu-se que com tempos de síntese superiores são preparadas membranas melhores, enquanto a ausência de rotação origina defeitos na estrutura. Comprovou-se o caráter hidrofílico das redes cristalinas através de ensaios de aquecimento-arrefecimento preliminares. A melhor membrana (denominada por MFI93) foi estudada em maior detalhe na gama de temperaturas de 25-110 ºC utilizando He, N2 e O2. Confirmou-se que a permeância e o diâmetro cinético dos gases relacionam-se: hélio, que possui o menor diâmetro, obteve maiores valores de permeância. Pelos testes de permeação a temperatura programada, verificou-se que para baixas temperaturas o mecanismo de escoamento viscoso contribui para o transporte do gás.

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ix A sua contribuição diminui progressivamente com o aumento da temperatura, passando a difusão gasosa ativada a ser o mecanismo de transporte dominante. Através da caracterização por DRX e SEM provou-se a preparação de uma camada contínua de ZSM-5 no interior do suporte.

Em paralelo, procedeu-se à síntese de redes cristalinas de MOFs. A não deposição de uma camada uniforme foi evidenciada pela caracterização dinâmica das amostras, já que não se verificou qualquer separação. Porém, confirmou-se a formação de pequenos cristais de ZIF-8 na superfície interna do tubo, através de análises de DRX e SEM.

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xi keywords Characterisation, Gas Permeance, Membranes, Metal-Organic

Frameworks, Synthesis, Transport Mechanisms, Zeolites

abstract The use of membranes for the separation of gas mixtures has gained great importance owing to its advantages when compared to conventional separation processes, such as moderate energy consumption and simplicity of operation. Although polymer membranes dominate the market, they are limited by relatively low operating temperatures and selectivities. Inorganic membranes are characterised by high thermal, chemical and mechanical stabilities, and are particularly attractive for separation purposes. Recently, crystalline metal-organic frameworks (MOFs) have been subject of intense research, since their structure is composed of organic linkers and inorganic clusters, revealing characteristics of both elements, such as flexibility and better control of the size distribution of the pores.

A central aim of this dissertation was the synthesis and subsequent characterisation of inorganic microporous zeolite membranes and, in parallel, MOFs. In the whole, fifteen membranes have been synthesized. The structure and morphology of the most promising membranes were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. With regard to the dynamic characterisation, it was accomplished carrying out pure gas permeation assays at fixed and programmed temperature using different gases (i.e., with distinct kinetic diameters).

In what concerns the zeolite membranes supported on α-alumina tubes, a secondary growth method was implemented, followed by hydrothermal synthesis. The effect of the synthesis time and the presence of rotation were studied, being concluded that longer times give rise to better membranes, and the absence of rotation results in defects over the structure. The hydrophilic character of the crystalline layer was proved by preliminary heating-cooling cycles assays. The best membrane (labelled MFI93) was studied in detail in the temperature range of 25-110 °C using He, N2 and O2. It was possible to confirm that the permeance and the kinetic diameter of the gases are related: helium, which has the smallest diameter, attained higher permeance values. From the temperature-programmed permeation tests, it was found that at low temperatures the viscous flow mechanism has an important contribution to the gas transport. Its influence is progressively reduced with increasing temperature, becoming the activated gaseous diffusion the main transport mechanism. The subsequent characterisation of MFI93 membrane by XRD and SEM confirmed the preparation of a continuous ZSM-5 layer inside the support tube.

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xiii In parallel, the synthesis of MOFs membranes was also tested. The non-deposition of uniform coatings was evidenced by the permeance dynamic characterisation of such membranes, since no gas separation was observed. Nonetheless, the deposition of small crystals of ZIF-8 on the inner side of support was confirmed by XRD and SEM analysis.

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xv

List of Figures

Figure 2.1 - Representation of a membrane module [2]. ... 3

Figure 2.2 - Classification of synthetic membranes. ... 5

Figure 2.3 - Schematic representation of the three basic types of membranes: (from left to right) porous membrane; dense membrane; carrier membrane [3]. ... 6

Figure 2.4 - Facilitated transport through carrier membranes [15]. ... 7

Figure 2.5 - Classification of membranes according to their structure over the cross section [3]. .... 7

Figure 2.6 - Examples of zeolite and “zeotype” structures: CHA (Chabazite, SAPO-34, AlPO-34), MFI (ZSM-5, silicalite-1) and FAU (Zeolite-X, Zeolite-Y, SAPO-37). Oxygen atoms in red, T-atoms (Si, Al or P) in yellow and ring-opening formed by blue T-T-atoms bridging via Oxygen atoms. ... 11

Figure 2.7 - Representation of the pressure profile over the zeolite membrane and macroporous support [29]. ... 13

Figure 2.8 - Schematic representation of the MOFs structure [49]. ... 18

Figure 2.9 - Schematic representation of the ZIF-8 structure: (from left to right) network shown as a stick diagram; network shown as a tiling; largest cage of the network represented in yellow; representation of the pore aperture [56,57]. ... 19

Figure 2.10 - Preparation of a ZIF membrane by using APTES treatment over the support [61]. .. 19

Figure 3.1 - Schematic representation of Viscous Flow mechanism [66]. ... 22

Figure 3.2 - Schematic representation of Knudsen Flow mechanism [66]. ... 23

Figure 3.3 - Schematic representation of Activated Gaseous Diffusion mechanism [66]. ... 24

Figure 3.4 - Schematic representation of Surface Diffusion mechanism [66]. ... 25

Figure 3.5 - Schematic representation of Multicomponent Molecular Diffusion mechanism [66]. 26 Figure 3.6 - Permeance behaviour over temperature in the case of (a) non-adsorbable gases and (b) adsorbable gases [73]. ... 28

Figure 4.1 - Lab unit for permeance measurements including (a) an electric oven, (b) the membrane module, (c) the sealed membrane, and (d) an instrumentation and control box. ... 34

Figure 4.2 - Graphical interface: (a) input of data and (b) data collection. ... 34

Figure 4.3 - Block Diagram: input of the parameters resulting from the calibration of the MFM... 35

Figure 4.4 - Arrangement of the membrane module for the permeance measurements. ... 35

Figure 5.1 - Consecutive heating-cooling cycles. Assays accomplished in the range of 25-100 ºC using N2 and ∆𝑃 = 0.5 bar. ... 40

Figure 5.2 - Comparison of the permeance values obtained by fixed temperature measurements and during a heating-cooling cycle. Assays accomplished using N2 and ∆𝑃 = 0.5 bar. ... 41

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xviii

Figure 5.3 - Study of the effect of rotation during membrane synthesis on the permeance (SR - Slow Rotation, MFI93; NR - No Rotation, MFI92). Assays accomplished in the range of 25-100 ºC using He and ∆𝑃 = 0.5 bar. ... 42 Figure 5.4 - Study of the hydrothermal synthesis reproducibility. Assays accomplished in the range of 25-100 ºC using He and ∆𝑃 = 0.5 bar. ... 43 Figure 5.5 - Study of the effect of the number of depositions on the permeance. Assays

accomplished in the range of 25-100 ºC using He and ∆𝑃 = 0.5 bar. ... 44 Figure 5.6 - XRD patterns of pure ZSM-5 powder and a ZSM-5 membrane. The * peaks depict α-alumina support reflections. ... 45 Figure 5.7 - SEM images of (a) cross-section, (b) top view and (c) crystal details of a ZSM-5 membrane. ... 45 Figure 5.8 - Comparison of the permeance of several gases in MFI93 membrane: He, N2 and O2.

Assays accomplished in the range of 25-100 ºC and ∆𝑃 = 0.5 bar. ... 46 Figure 5.9 - Behaviour of the permeance with pressure for MFI93 membrane. Assays accomplished at room temperature (ca. 25 ºC) using N2. ... 47

Figure 5.10 - Comparison of the permeance at programmed temperature of several gases in MFI93 membrane: He, N2 and O2. Assays accomplished in the range of 25-110 ºC and ∆𝑃 = 0.5 bar. ... 47

Figure 5.11 - Experimental and modelled permeances of N2 in MFI93 membrane. Programmed

temperature measurement in the range of 25-110 ºC and ∆𝑃 = 0.5 bar. ... 48 Figure 5.12 - Experimental and modelled permeances of (a) He, (b) N2, and (c) O2 in MFI93

membrane, assuming viscous flow and activated gaseous diffusion. Programmed temperature measurements in the range of 25-110 ºC and ∆𝑃 = 0.5 bar. ... 49 Figure 5.13 - XRD patterns of pure ZIF-8 powder and CPm53 tube. The highlighted area resembles to ZIF-8 phase, the * peaks and the ? peak depict α-alumina support and contaminant reflections, respectively. ... 50 Figure 5.14 - SEM images of ZIF-8 on tube sample: CPm53 (a) top view, and (b) cross-section, and CPm54 (c) top view. ... 51 Figure 5.15 - Comparison of the permeate and feed flow rates of CPm53 sample when performing (a) the first and (b) the second heating-cooling cycles. Assays accomplished using N2 in the range

of ca. 20-55 ºC and 𝛥𝑃 = 0.5 bar. ... 52 Figure 5.16 - Thermogravimetric analysis curve of ZIF-8 [77]. ... 52 Figure A.1 - Graphical representation of the flow rate imposed on the program as function of the MFM voltage and respectively adjustment for He. ... 61 Figure A.2 - Bubble test procedure in order to calibrate the MFC... 62 Figure A.3 - Schematic representation of the experimental set-up used during the permeation assays [29]. ... 63

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xix

List of Tables

Table 2.1 - Examples of industrial applications of membrane technology and respective driving force [2,3,15]. ... 10 Table 2.2 - Examples of separation applications of different zeolite and “zeotype” structures [21,22]. ... 12 Table 2.3 - Summary of several techniques for membrane characterisation [3,45,46]. ... 16 Table 4.1 - Kinetic diameter and molar mass of the studied gases during the permeance

measurements [29]. ... 29 Table 4.2 - Overview of the different hydrothermal synthesis conditions studied... 30 Table 4.3 - Length and internal diameter of the synthesized zeolite membranes. ... 37 Table 5.1 - Permeance values before and after the preliminary heating-cooling (H/C) cycles. Assays accomplished at ca. 25 ºC and ∆𝑃 = 0.5 bar. ... 40 Table 5.2 - Model parameters for He, N2 and O2 permeation through MFI93 membrane. ... 49

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xx

Nomenclature

Symbols

𝐴𝐴𝑅𝐷 Average Absolute Relative Deviation (%)

𝐵 Mobility of the diffusing gas (mol m2_J-1_s-1₎

𝑏 Equilibrium adsorption constant (Pa-1₎

𝑐 Concentration of the diffusing species (mol m-3₎

𝐷 Diffusion coefficient (m2_s-1₎

𝐷_ij Interdiffusion coefficient (m2 s-1)

Ð Maxwell-Stefan Diffusivity (m2_s-1₎

𝑑i,M Support internal diameter (m)

𝑑p Pore diameter (m)

𝐸a Activation energy (J mol-1)

𝑔supp Geometric parameter of the support -

∆𝐻a Enthalpy of adsorption (J mol-1)

ℎM Membrane length (m)

𝐾 Henry’s constant (mol kg-1_Pa-1₎

𝐿 Onsager relation -

𝑙d Diffusion length (m)

𝑙M Membrane thickness (m)

𝑀 Molecular weight (kg mol-1₎

𝑁 Molar flux (mol m-2_s-1₎

𝑃 Total pressure (Pa)

𝑃̅ Mean pressure (Pa)

𝑃M Membrane Permeability (mol m-1 s-1 Pa-1)

𝑄 Mass flow (kg s-1₎

𝑞 Molar concentration of the adsorbed species in the solid (mol kg-1_{or mol m}-3₎

𝑅 Ideal gas constant (J mol-1_K-1₎

𝑆 Permeation area (m2₎

𝑇 Temperature (K)

𝑡hs Hydrothermal synthesis time (s)

𝑥 Molar fraction in the feed (mol mol-1₎

𝑦 Molar fraction in the permeate (mol mol-1₎

𝑍 Number of adjacent sites -

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xxi Greek Letters

𝛼i,j Selectivity of i over j -

𝛼_i,j∗ Ideal selectivity of i over j -

𝜀 Porosity -

𝛤 Thermodynamic factor -

𝜃 Fractional occupancy -

𝜂 Viscosity (Pa s)

𝜆 Mean free path (m)

𝜇 Chemical potential (J mol-1₎

𝛱 Permeance (mol m-2_s-1_Pa-1₎ 𝜌p Solid density (kg m-3) 𝜎 Kinetic diameter (m) 𝜏 Tortuosity - Subscripts 0 Reference condition F Feed

g Activated gaseous diffusion Kn Knudsen diffusion P Permeate s Surface diffusion sat Saturation tot Total V Viscous flow zeol Zeolite layer

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xxii

Abbreviations

AlPO Aluminophosphate

APTES Aminopropyltriethoxysilane BLMs Bulk Liquid Membranes BPR Back Pressure Regulator CVD Chemical Vapour Deposition ED Electrodialysis

ELMs Emulsion Liquid Membranes H/C Heating-Cooling Cycle

ILMs Immobilized Liquid Membranes MF Microfiltration

MFC Mass Flow Controller

MFI Mordenite Framework Inversed

MFM Mass Flow Meter

MMM Mixed Matrix Membranes MOFs Metal-Organic Frameworks

MS Maxwell-Stefan

mIm 2-methylimidazole NF Nanofiltration

PCNs Porous Coordination Networks PCPs Porous Coordination Polymers PSA Pressure Swing Adsorption PT Pressure Transducer

RO Reverse Osmosis

SAPO Silicoaluminophosphate SEM Scanning Electron Microscopy SILMs Supported Ionic Liquid Membranes SLMs Supported Liquid Membranes

SOD Sodalite

TC Temperature Controller UF Ultrafiltration

XRD X-Ray Diffraction

ZIFs Zeolitic Imidazolate Frameworks ZSM-5 Zeolite Socony Mobil-5

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CHAPTER 1 – MOTIVATION AND THESIS PLAN _______________________________________________________________________________

1

1 Motivation and Thesis Plan

Membrane technology is an increasingly important and versatile method for gas separation, when compared to other more conventional approaches like pressure swing adsorption (PSA) and cryogenic distillation. When compared to other methods, membranes provide the following advantages: (i) simplicity of operation, (ii) the possibility to be coupled to other industrial processes, (iii) low energy consumption (phase changes are not necessary, and the pump and/or compressor costs are relatively low), and (iv) various environment friendly features (no solvents are involved, no wastes are generated, no chemical reactions occur during the process, thus avoiding by-products). Therefore, this technique has already been applied in the industry for the production of high pure gases, water treatment and purification of bio-fuels.

Although the main applications of this technology use organic membranes, the utilization of inorganic membranes have been gaining growing importance since they present high thermal, mechanical and chemical stabilities, as well as a better control of their pore size distribution. The better ability than polymeric membranes, in terms of operating temperature and pressure conditions, results in an increasing number of studies on inorganic membranes.

In the class of inorganic materials used to prepare porous membranes, zeolites are particularly interesting. Taking into account their high thermal and mechanical stabilities and their crystalline structure consisting of channels in the molecular size range, these microporous aluminosilicate and related materials are favourable for gas phase separations. Recently, hybrid materials have been studied for separation application. Composed by organic linkers and inorganic ions, these structures balance characteristics of both type of solids: flexibility of organic materials, but also channels in the molecular size range and narrow pore size distribution of inorganic ones.

Hence, this work focuses the challenge of producing supported inorganic (ZSM-5) and hybrid (ZIF-8) membranes for gas separation. After the synthesis, a full characterisation of the materials was performed for identifying the crystalline phase of the prepared materials and their texture. The permeance measurements, at fixed and programmed temperature, lead to the examination of possible defects over the membrane layer and, at the same time, the study of the mass transport mechanisms dominant over the separation process.

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CHAPTER 1 – MOTIVATION AND THESIS PLAN

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2

This dissertation is organized into eight chapters.

Chapter 2 provides a review over the fundamentals of membrane technology, namely, the types of constituents used to prepare membranes, characteristics of different materials and applications of this technique. Then, zeolite and MOFs membranes are subject of a deeper survey.

Chapter 3 introduces the different mass transport mechanisms present in microporous materials, as well as in macro and mesodefects. The dependency of the permeance on temperature and pressure is also investigated.

Chapter 4 includes a full description of the methods used to prepare the membranes, the techniques applied to characterise the deposited materials, and the experimental set-up and measurements accomplished during the dynamic characterisation. In addition, the data treatment and the modelling procedure are also focused.

Chapter 5 presents the discussion of experimental and modelling results. The influence of synthesis conditions is studied, and the presence of defects and the main mass transport mechanisms are reported.

Chapter 6 unveils the main conclusions of the thesis.

Bibliography includes all the analysed material for the elaboration of the present work. Appendix A introduces some additional information regarding the accomplished work.

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CHAPTER 2 – MEMBRANE TECHNOLOGY _______________________________________________________________________________

3

2 Membrane Technology

2.1 Introduction

A membrane consists in a physical barrier that separates two fluids with different composition and/or concentration and controls the mass transfer between them under a certain driving force. A preferable permeation of one or more components of the feed is verified while the remaining ones are retained or transferred in less extension. The part of the feed that does pass through the membrane is called permeate, while the part of the mixture that does not pass through the membrane consists in the retentate [1]. In order to facilitate the removal of the permeate and, at the same time, to increase the driving force, an optional sweep fluid (liquid or gas) can be added. A generic membrane process is shown in Figure 2.1 [2].

To have an effective separation process the membrane must possess high permeance of species i. The permeance for a certain species diffusing through a membrane is equivalent to the flow rate of that species per unit of area of membrane and per unit of driving force. The molar transmembrane flux of species i, 𝑁_i, is given by:

𝑁_i = (𝑃Mi

𝑙_M) × (𝑑𝑟𝑖𝑣𝑖𝑛𝑔 𝑓𝑜𝑟𝑐𝑒) = 𝛱i× (𝑑𝑟𝑖𝑣𝑖𝑛𝑔 𝑓𝑜𝑟𝑐𝑒) (2.1) where 𝛱_i is the permeance of component i, which corresponds to the ratio of its permeability, 𝑃_Mi, and the membrane thickness, 𝑙_M.

At the same time, in order to have an efficient separation, a high molar flux ratio for the species i and the others is essential. Hence, it is important that a membrane has the ability to preferably transfer certain components and consequently provide a stream with a high composition of the species of interest. The parameter that describes the affinity of the membrane to a certain component of the feed is the selectivity, 𝛼i,j, and it is expressed by

Equation 2.2. It is important to notice that the selectivity of the process is highly dependent on both the membrane characteristics and the operating conditions.

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CHAPTER 2 – MEMBRANE TECHNOLOGY _______________________________________________________________________________ 4 𝛼i,j= (𝑦i 𝑥_i ⁄ ) (𝑦j⁄ )_𝑥_j (2.2)

Here 𝑥 corresponds to the molar fraction of the component in the feed, 𝑦 corresponds to the molar fraction of the component in the permeate, and the indexes i and j correspond to two different species involved in the separation process.

Taking as an example the gas separation of a binary mixture, where the pressure gradient between the feed and the permeate acts as the driving force of the process, the molar transmembrane flux for each species of the system is given by:

𝑁_i= (𝑃Mi

𝑙_M) × (𝑥i𝑃F− 𝑦i𝑃P) (2.3) 𝑁_j = (𝑃Mj

𝑙_M) × (𝑥j𝑃F− 𝑦j𝑃P) (2.4) where 𝑃F stands for the feed pressure and 𝑃P for the permeate pressure. If no sweep gas is

used, the ratio of molar fluxes is equal to the ratio of feed’s composition. 𝑁i 𝑁_j= 𝑦i 𝑦_j= (𝑃_𝑙Mi M) × (𝑥i𝑃F− 𝑦i𝑃P) (𝑃_𝑙Mj M) × (𝑥j𝑃F− 𝑦j𝑃P) = 𝛼_i,j𝑥i 𝑥_j (2.5)

Since the gradient of pressure acts as the driving force of the process, in the case of gas permeation, the pressure of the permeate is often much lower than the pressure of the feed. Therefore, 𝑥_i𝑃_F ≫ 𝑦_i𝑃_P and 𝑥_j𝑃_F≫ 𝑦_j𝑃_P, which leads Equation 2.5 to be rewritten in order to determine the ideal selectivity, 𝛼_i,j∗, expressed by:

𝛼_i,j∗ = 𝛱i

𝛱_j (2.6)

As Equation 2.6 shows, the ideal selectivity is calculated as a ratio of the permeance of the two pure gases at fixed temperature and pressure conditions. Clearly, this parameter only provides an estimation of the affinity of the membrane to a certain species and should not be assumed as the effective selectivity.

2.2 Classification of Membranes

Membranes may be, in a first place, classified as natural, i.e. biological, or synthetic. Natural membranes are subdivided into living and non-living membranes; the latter are increasingly important in areas such as medicine and biomedicine [3].

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5 Conversely, synthetic membranes may be classified in four main groups, depending on the nature of the material and/or its arrangement, as it is shown by Figure 2.2 [4].

Nowadays, organic membranes dominate the global membrane market due to their advantages [4] such as its low cost, good quality control, scalability and tunability [5]. However, these materials are often limited to a moderate temperature of operation, approximately 363-373 K due to their structural weakness. Moreover, polymeric membranes are sensitive to swelling, compaction and to certain chemicals like hydrochloric acid and sulphur oxides [6], and are limited to the intrinsic trade-off effect between permeability and selectivity.

Inorganic membranes, with a high thermal, mechanical and chemical stabilities, exceed the weakness of polymeric membranes not only in terms of operating temperature but also in pressure conditions, and consequently have been the target of an increasing number of studies. Yet, these materials are generally much more expensive than the organic ones, and the produced membranes may crack more easily since they are brittle and may also have low hydrothermal stability [7].

Hybrid materials, where the matrix contains organic and inorganic components, were developed in order to combine the benefits of both types of materials [8]. Therefore, thermal and mechanical stabilities are enhanced when compared to polymeric membranes due to the presence of the inorganic species, but at the same time, they do not exhibit the flexibility of organic materials [9].

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CHAPTER 2 – MEMBRANE TECHNOLOGY

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6

Liquid membranes consist of a liquid phase, e.g. a thin oil film, and are classified according to the design configuration, as supported (immobilized) liquid membranes (SLMs or ILMs), bulk liquid membranes (BLMs) and emulsion liquid membranes (ELMs) [10]. Although these membranes are highly selective and have the ability of recognizing specific molecules, their long-term performance is extremely limited due to their instability [11]. The stability of liquid membranes may be affected by the loss of organic solvents, due to evaporation or dispersion and, in the particular case of SLMs, by the low mechanical stability of the support [12]. In order to overcome the stability problems of liquid membranes, supported ionic liquid membranes (SILMs) have been developed where ionic liquids replace organic solvents, contributing to higher chemical and thermal stabilities and, at the same time, to lower liquid losses taking into account the (almost) null vapour pressure of ionic liquids [13].

Inorganic membranes can be categorized by their structure or morphology in two categories (see Figure 2.3 [3]): porous and non-porous (dense) membranes. Dense structures include both metallic (palladium, iron, etc.) and ceramic proton conducting membranes which take advantage of differences of molecules solubility and/or diffusivity. In the case of porous membranes, which include those made of carbon, amorphous silica or zeolites, the separation relies on variances in size, shape and affinity between the membrane and feed molecules. In conclusion, different morphologies and structures (shown in Figure 2.3 [3]) outcome in distinct mechanisms of transport through the membrane.

Porous materials are distinguished, following the IUPAC classification, by their pores size: macroporous (𝑑p > 50 nm), mesoporous (2 < 𝑑p < 50 nm) and microporous (𝑑p < 2

nm), where 𝑑p corresponds to the average pore diameter [14].

Besides dense and porous membranes, it is possible to identify a third category of membranes in terms of how transportation occurs: carrier membranes. In Figure 2.3 [3] the

Figure 2.3 - Schematic representation of the three basic types of membranes: (from left to right) porous membrane; dense membrane; carrier membrane [3].

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7 three types of membranes described before are represented. When compared to the other classes of barriers, the main difference one may single out is the fact that permselectivity towards a certain component depends on the specificity of the carrier molecule while the membrane has practically or even absolutely no influence on the transport. The carrier may be fixed to the membrane matrix or mobile if dissolved in liquid membranes as it is shown in Figure 2.4 [15] . The transport is facilitated since (i) the carrier chemically binds the specific solute, resulting a coordinated structure (solute-carrier complex), (ii) the complex diffuses through the membrane and (iii) then the bond between compounds is broken with the release of the transported species (decomplexation) [1,10].

Synthetic membranes may have a symmetric or an asymmetric structure. Symmetric membranes have an identic structure and transport properties over their entire cross section as it is shown in Figure 2.5 [3]. Therefore, the transmembrane flux depends on the thickness of the entire membrane. On the contrary, asymmetric membranes verify a variation of their structural and transport properties over the membrane cross section. In this case, a thin layer with smaller pores is deposited on other sublayer of macroporous material. The latter serves as a support, having almost none effect over the mass transfer process. Consequently, the “skin” layer represents the selective barrier and determines the mass flux. Symmetric and asymmetric membranes are usually referred as isotropic and anisotropic, respectively [16].

Figure 2.4 - Facilitated transport through carrier membranes [15].

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8

2.3 Applications of Membrane Technology

Membrane technology is already an important industrial separation approach, being used in a broad range of operations. In order to have a successful operation, there are several features which membranes must possess, such as a high selectivity and permeability, but also stability and durability. Therefore, regarding to the application, different types of membranes are required and chosen [7].

Depending on the type of process where membranes are involved, not only may the phase of the fluids/streams differ but also the driving force. More often, a gradient of pressure (partial pressure of one component or hydrostatic pressure) determines the flux; however, in some cases, differences of concentration or electrical potential are determinant to achieve the separation. Thus, the set of different membrane techniques may be classified as a function of the driving force applied [15].

In regard to microfiltration, ultrafiltration, nanofiltration and reverse osmosis, the processes are pressure-driven and take advantage of size exclusion, i.e. particles are concentrated in a retentate since they are not able to pass through the porous membrane. The main difference between these techniques is related with the size of the pores, which leads to significant variances on the operation conditions such as the hydrostatic pressure required to promote the separation.

In what concerns the gas permeation, both dense and porous membranes may be used to promote the separation of a compound under a pressure gradient of the permeating components. The transport of gases is described by diffusion or solution-diffusion, respectively, depending on the type of selective barrier used. In the case of pervaporation, the mechanism of separation is rather similar to the gas permeation; however, the separation process is combined with evaporation on the permeate side in order to obtain a vapour phase of the selective components from a liquid feed mixture. Therefore, the mechanism corresponds to a “permselective” separation coupled by an “evaporation”.

Dialysis, on the other hand, is operated under isobaric and isothermal conditions, being the differences of the components concentrations between the feed solution and the dialysate the driving force. Thus, each species diffuses through the membrane from the mixture to the dialysate and their flux is determined by the diffusion rate in the barrier.

Considering the case of the electrodialysis, it is verified the mass transport of charged components (anions and cations) under a gradient of an electrical potential across the barrier.

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9 Once there are two types of membranes (one carrying fixed positive charges and the other negative charges), these are selective to different types of ions, which leads cations and anions to move in contrary directions towards the opposite anode. Hence, a diluted solution is formed in parallel with alternating ion concentrated charged cells.

Finally, liquid membranes’ mass transport is based on solution-diffusion of the solute species due to a gradient of concentration. This type of barriers is often integrated with carriers (facilitators) in order to increase the transmembrane flux, possibly affecting as well the efficiency and selectivity towards the species of interest [10].

Some of the industrially membrane-separation operations are listed in Table 2.1 [2,3,15]. [17], [18]

As it was outlined before, membranes are widely used in the industry and appear most of the times in the form of large groups. These modules must: (i) possess an economically competitive cost of manufacture, (ii) provide support in order to keep the membranes’ integrity, (iii) guarantee sufficient mass transfer area, (iv) afford an easy exit of permeate and (v) permit the membrane to be cleaned [17,18]. Several sorts of equipment are nowadays integrated in the industry, depending on specific application:

-Hollow Fiber Membrane Modules are mainly used in ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), dialysis and gas separation;

-Plate and Frame Membrane Modules are able to perform microfiltration (MF), UF, RO and electrodialysis (ED);

-Spiral Wound Membrane Modules are mainly used for UF and RO, but are also found in gas separation and pervaporation.

The advantages of membrane separation make this method a target to continuously improve the current technology. In the first place, highly-permeable membranes are developed and, consequently, new designs of modules are produced so all the new properties are fully took as a gain.

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10

Table 2.1 - Examples of industrial applications of membrane technology and respective driving force [2,3,15].

Process Industrial Application Phases Driving force

Microfiltration (MF)

Water purification;

Sterilization and clarification of beverages and pharmaceuticals.

L – L 𝛥𝑃

Ultrafiltration (UF)

Colour removal from Kraft black liquor in papermaking;

Recovery of potato starch and proteins.

L – L 𝛥𝑃

Nanofiltration (NF)

Separation of proteins, nucleic acids and enantiomers of drugs;

Removal of divalent ions (Ca2+ and Mg2+_{) from water.}

L – L 𝛥𝑃

Reverse Osmosis (RO)

Removal of impurities from wastewater;

Desalination of brackish water. L – L 𝛥𝑃 Gas Permeation

Adjustment of H2/CO ratio in synthesis

gas;

Separation of H2 or CO2 from methane.

G – G 𝛥𝑝

Pervaporation

Separation of isotropic mixtures;

Removal of water from organic solvents and vice versa.

L – G 𝛥𝑝

Dialysis

Hemodialysis (removal of waste metabolites and excess of water); Removal of low-molecular-weight impurities from polymers.

L – L 𝛥𝑐

Electrodialysis (ED)

Production of salt from seawater;

Production of ultra-pure water. L – L 𝛥𝐸 Liquid

Membranes

Removal of zinc, phenol and cyanide from wastewater;

Removal of radioactive material from wastewater.

L – L 𝛥𝑐

2.4 Zeolite Membranes

Zeolites are microporous, crystalline, hydrated aluminosilicate materials constituted by AlO₄5− and SiO₄4−. The oxygen atoms act like bonds between these blocks, leading to a three-dimensional framework with well-defined channels. In order to compensate the negative charge of the structure, the cavities are occupied by cations from alkali or alkali-earth groups and water molecules. Therefore, featuring the pores or rings, it is possible to find singly- and/or doubly-charged cations like sodium (Na+), potassium (K+), magnesium (Mg2+) and

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11 calcium (Ca2+). Based on the crystal unit cell, the structural formula of zeolites is represented by [19]:

Ma_⁄_n∙ [(AlO₂)_a∙ (SiO₂)_b] ∙ wH₂O , (a ≤ b)

where M is the alkali or alkali-earth atom, a and b stand for the total numbers of tetrahedral Al and Si, respectively per unit cell, and w represents the number of water molecules per unit cell.

Owing to the arrangement of the tetrahedral building blocks, the dimensions of the shaped channels are close to molecular size, typically 0.3 – 1.0 nm [20]. This aspect, combined with the exceptional properties of zeolites, like their thermal, mechanical and chemical stabilities and unique catalytic, adsorption and ion exchange characteristics, makes them an outstanding choice rather than other microporous materials. Another particularity of zeolite materials is that, by changing the Si/Al ratio during synthesis, i.e. b/a ratio, it is possible to adapt the properties of the material in terms of what is more convenient for a certain application. Nowadays, this group of materials include over 200 structures, being MFI (Mordenite Framework Inverted), FAU (Faujacite), LTA (Linde Type A) and MOR (Mordenite) the most important frameworks.

Pores are characterised by the number of atoms that define the ring and consequently its size as: small (8-membered-ring, i.e. 8MR), medium (10MR) and large (12MR) [4].

Besides zeolites, other materials called zeolite-like structures or “zeotypes” were also developed. Thus, the term zeolite started to comprehend non-aluminosilicate frameworks, being silicoaluminophosphate (SAPO) and aluminophosphate (AlPO) the most noticed. Even though they have a similar structure to zeolites, they exhibit distinct chemical properties. Examples of common zeolite structures are depicted in Figure 2.6 where it is possible to compare zeolite and “zeotype” frameworks.

Figure 2.6 - Examples of zeolite and “zeotype” structures: CHA (Chabazite, SAPO-34, AlPO-34), MFI (ZSM-5, silicalite-1) and FAU (Zeolite-X, Zeolite-Y, SAPO-37). Oxygen atoms in red, T-atoms (Si, Al or P)

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12

As it was mentioned before, due to the outstanding characteristics of zeolites, these microporous materials are widely used in three fields of application:

i. Catalysis: zeolites are used as catalysts for a number of organic reactions, mainly on hydrocarbon conversion, such as processing of natural gas and crude oil;

ii. Ion exchange: zeolites promote the removal of radioactive and heavy cations from water and are used as water softening agents;

iii. Adsorption and membrane separation: zeolites are effective in purification processes and selective separation, based on adsorption selectivity and molecular sieving effect. They are particularly interesting for gas separation. [21], [22]

In Table 2.2 is presented an overview of separations promoted by zeolite and “zeotype” membranes. Taking into account a family of materials with so many distinct frameworks, there are several factors that determine whether a certain structure is suitable for the application, namely: (i) pore size, which fixes the size of the molecules that have the ability to diffuse through the barrier, (ii) crystalline system, which determines the diffusivity, and (iii) Si/Al ratio, which affects the polarity [21,22].

Even though a lot of research has been done concerning zeolite membranes, the only commercialized application is the de-watering of bio-ethanol by steam permeation using LTA membranes [23].

Table 2.2 - Examples of separation applications of different zeolite and “zeotype” structures [21,22].

Type Structure Si/Al Pore size (Å) Application

MFI ZSM-5 > 15.0 5.3 x 5.6 p-xylene/o-xylene

Silicalite-1 Ketones/water

LTA Zeolite A 1.0 4.1 Water/organics

FAU Zeolite X,Y 2.5 7.4 Propylene/propane

CO2/Air CHA SSZ-13 5-100 3.8 CO2/CH4, CO2/Air H2/CH4, H2/Air SAPO-34 0.01-0.3 AEI AlPO-18 0 3.8 CO2/CH4 2.4.1 Membrane Supports

Since the majority of zeolite membranes are synthesized as thin films, it is necessary to guarantee they are mechanically stable. Therefore, their preparation usually occurs on a porous support, being the latter an essential key to have a successful synthesis. The most common supports are made of ceramic materials (α-alumina, silica) or stainless steel. Nonetheless, examples of polymer [24], carbon [25] and even zeolite supports [26] are also

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13 reported. Regarding to the geometry, supports may be used in several configurations as flat, tubular and hollow fibbers, being the latter two more viable for industrial applications since they provide a higher surface-to-volume ratio [27]. Besides the mechanical stability conferred to the membrane, supports must have a high stability in hydrothermal and alkaline conditions, since high temperatures are usually required during the synthesis. Moreover, these materials must show a homogeneous surface, facilitating the formation of a zeolite layer free of defects, and should have a narrow pore size distribution, avoiding the appearance of the so-called “pinholes” in the top layer of the support [28].

Even though all these characteristics are important to be fulfilled, the most important feature supports must possess is a high porosity and large pores (macro and/or mesopores). This aspect is understandable taking into account the basic principles of membrane technology: adding a new layer of material (the support) corresponds to add a new mass transfer resistance. When operating membranes for gas separation, the driving force for permeation corresponds to a partial pressure difference (∆𝑃_tot) as shown in Figure 2.7 [29]. Thus, the diffusion resistance of the support should be the lowest as possible, so the pressure gradient across the zeolite layer (∆𝑃zeol) is the highest. When this requirement is not verified,

the driving force of two components through the whole membrane is lower than through the zeolite layer itself, reducing the overall flux and the selectivity.

In order to satisfy the purposes of the support during the operation, asymmetric supports seem to be the best solution. They are made of a thick bottom layer characterised for having large pores where the mass transport properties needed are provided, and a thin top layer with the wanted smoothness for the attachment of the zeolite membrane [3]. Yet, the better assets of asymmetric materials implicate higher production costs, those that need to be reduced for the application of zeolite membranes [30].

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14

2.4.2 Preparation Methods of Zeolite Membranes

Several strategies have been studied and applied to prepare supported polycrystalline zeolite layers. The most common methods reported are: in situ synthesis (synthesis without seeding) and secondary growth. Still, zeolite films may be prepared by pore-plugging, dry gel conversion methods or hydrothermal methods combined with microwave heating [20].

In situ techniques are characterised by offering direct crystallization on the support, i.e. zeolite crystals nucleate on the support surface and are inter-grown during crystallization. In this case, the support is directly immersed into the synthesis gel and the reaction occurs at high temperatures, often above 100 ºC [31].

Conversely, secondary growth methods are based on a very different principle: in the first step, nucleation sites (nanocrystals), i.e. seeds, are formed and attached on the surface of the support. Then, the zeolite layer is formed by hydrothermal synthesis in autoclave. Seeding techniques have the advantage of allowing a better control of the zeolite layer microstructure in terms of its thickness, orientation and crystal size, determining the quality of the membrane. In the case of zeolite crystal, the orientation of the crystal axes plays a key role in the overall performance since they are anisotropic [32]. Secondary growth methods have several variances in terms of how the seeds are deposited on the support, being dip-coating and rubbing the most common strategies. In the case of the dip-dip-coating method, the seeding is done by immersing the support into a colloidal solution of zeolite particles for a certain time, followed by drying. Since it is difficult to obtain a continuous and homogeneous layer of crystals on supports surface, the procedure may be repeated several times. One way of facilitating the attachment of the seed to the support is to add a calcination step after drying [33]. On the other hand, the rubbing technique consists of implanting the zeolite crystals to the surface of the support by using small brushes [34].

Pore-plugging is a technique that allows the preparation of supported infiltrated membranes. These type of membranes have a remarkable thermal and mechanical stability [30]. Yet, the barriers produced by this method may have a high thickness, which results in low permeance [31].

Dry gel conversion corresponds to a method of synthesizing membranes (quasi) in situ. A mixture of water and a volatile structure-directing agent is placed into the bottom of the autoclave, while a dry aluminosilicate gel is located in the middle. After heating, vapour and the organic template vaporize, reach the dry gel and lead to crystallization (vapour-phase

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15 transport). In the case of using non-volatile templates, these are mixed in the dry gel (steam-assisted conversion) [35]. [36] [4], [37]

Hydrothermal synthesis supported by microwave heating has also been used for the preparation of zeolite membranes, since this type of heating is often more efficient than the conventional one, i.e. the production of the films is faster. These microwave techniques may be combined with the most common synthesis strategies used, leading to new synthesis approaches, like in situ microwave synthesis and microwave assistant secondary growth. It is reported the preparation of MFI membranes within one hour [36]. The method of heating may also influence the properties of the zeolite layer in terms of the size and distribution of the material – small crystals are formed with narrow particle size – which consequently improves the performance of the membrane [4,37]. Higher heating rates, more uniform heating and enhanced dissolution of the precursor gel are possible explanation for the better results hydrothermal methods assisted with microwave heating show [38]. [39] [40] [4], [41] 2.4.3 Post-treatment Methods

2.4.3.1 Detemplation

During the synthesis of most zeolites, especially those with high silica content [39], it is required to use organic templates since they provide better control of zeolite crystallization. However, it is often observed that these templates remain into the pores after the preparation of the membrane [40]. As a result of this, it is necessary to remove these molecules. The conventional way of detemplation is calcination in air at 500-700 ºC. This method, on the other hand, may lead to appearance of cracks on the surface on the membrane, once the thermal expansion coefficients of the zeolite and the support are not the same [4,41]. In order to avoid the defects introduced by the conventional methods, many other procedures have been applied, such as heating and cooling at low heating rate [42] or detemplation with ozone at lower temperatures (ca. 250 ºC) [43].

2.4.3.2 Post-synthesis Modification

Post-synthesis techniques are often used after synthesis with the objective of decreasing the number of defects formed during the preparation of the membrane and calcination or to reducing the size of the pores. One of the most common approaches to tune the size of the pores is the chemical vapour deposition (CVD) method [20]. By using this technique, intercrystalline defects are eliminated and a uniform and fine pore size distribution is

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16

obtained. This is particularly important in the case of membranes for hydrogen separation [44]. Combining different zeolites and/or “zeotypes” in a unique structure, or preparing poly-layer zeolite membranes so the second poly-layer eliminates defects in the first one, are also effective post-synthesis approaches. Ionic exchange of the porous material can also change gas permeance; hence, it is considered a post-synthesis modification as well. [3], [45], [46]

2.4.4 Characterisation of Membranes Surface

When membranes are used as barriers to promote the separation of mixtures, it is of major interest to have the best properties possible to a certain application, leading to the preferable permeation. However, commonly the method of synthesis is likely to be improved and consequently are membranes’ characteristics. Thus, the optimization of the synthesis is undeniably a key step of this kind of separation method which is found by understanding the surface properties, morphology, transport properties and chemistry of membranes. There are rather instrumental techniques used to characterise membranes. Some of them are shown in Table 2.3 [3,45,46].

Table 2.3 - Summary of several techniques for membrane characterisation [3,45,46].

Technique Analysis

Fourier transform

infra-red spectroscopy FTIR

Assessment of the surface and its chemical functionality (membrane modification). Nuclear magnetic

resonance spectroscopy NMR

Investigation of structure relationships between different components of the membrane.

X-ray photon

spectroscopy XPS

Characterisation of the surface elemental composition in the parts-per-thousand range. X-ray diffraction XRD Study of the crystalline phases of the membrane;

investigation on possible contaminations. Electron dispersive

X-ray spectroscopy EDS

Analysis of surface (or individual points) elemental composition.

Scanning electron

microscopy SEM

Characterisation of the porous structure (top surface, cross-section and bottom surface); investigation on the symmetry.

Thermogravimetric

analysis TGA

Additional information to the study of morphology of membranes: analysis of the degradation of compounds with temperature.

Permeance

measurements -

Characterisation of the pore size by measuring the flux through the membrane (𝛥𝑃 constant);

investigation on the pore distribution by varying the driving force; analysis of possible defects.

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17 2.5 MOFs Membranes

Metal-organic frameworks (MOFs) are a relatively new but extensively studied class of materials, which has gained great interest for membrane applications in terms of gas and liquid separation [47]. They also offer promising results for gas storage, chemical sensors and optical devices [48].

MOFs, also known as porous coordination polymers (PCPs) or porous coordination networks (PCNs), comprise two main components: coordinated organic ligand molecules and metal ions (clusters). They are made of metal ligand complexes, which form the vertices of the structure, and are connected by the organic ligands, resulting in a porous structure (shown in Figure 2.8 [49]), being the pore size comparable to molecular dimensions [50]. In fact, the diversity of metal ions and organic linkers that may be used and consequently the number of structures that can be formed turn these materials even more interesting. Accordingly, by adapting the constituents of the framework it is possible to tune the properties of the material, enhancing its performance to a specific application [4]. The possibility of adapting the synthesis in order to control pores properties constitutes a key advantage of MOFs over zeolites, whose structures are more difficult to alter.

One of the most appealing characteristic of MOFs is the facility of its synthesis. They are produced by reticular synthesis where blocks of organic and inorganic molecules are linked by strong bonds, creating rough ordered networks [51]. These structures are characterised for having high porosity (as high as 50 % of the crystal volume), low density (0.2-1 g cm-3) and high surface area (1000-10000 m2 g-1) [52], generally higher values than those of zeolites. These materials offer reasonable thermal and mechanical stabilities.

Considering the application of MOFs in membrane technology, as singled out before, these materials are particularly interesting due to the possibility of tuning its pore size and shape. However, most of the advantages of MOFs, when compared with zeolites, are related with their chemistry and synthesis methods. Even though MOFs membranes may be prepared by the same techniques developed to improve the performance of zeolites such as seeding, microwave heating and the use of macroporous supports, MOFs synthesis does not require structure-directing agents, which represents an advantage when compared with inorganic membranes. Besides, MOFs appear to be less brittle and stiff than zeolites [50]. Because of the type of the network, these materials show a high degree of flexibility, resulting in effects as “breathing”, “gate opening” and “linker dynamics” [53]. In the same

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18

way of zeolite membranes, these hybrid membranes may also display pinholes or cracks, resulting from the synthesis of the membrane, which affect its performance, by reducing the selectivity.

In MOFs class, it is possible to highlight one of its subfamily: zeolitic imidazolate frameworks (ZIFs). Transition metals (Zn2+_{, Co}2+_{) and imidazolate linkers (with phenyl or}

methyl groups, as benzimidazolate, PhIM, and 2-methylimidazole, mIm, respectively) form these materials, leading to a 3D framework that very often resembles zeolite structures [54]. Some of these materials show hydrothermal and chemical stability but do not support high temperature (i.e. > 100 or 150 ºC) [55], which is due to the hydrophobic pores and strong metal-imidazole bonds. Once again, depending on the metal and organic constituents, different frameworks are obtained, being the pore size in the range of 0.2-1.5 nm. In some cases, like the ZIF-8 and ZIF-11, the pore sizes are twice as large as their respective zeolite counterparts, due to the longer imidazolate linking units [56]. [56], [57]

ZIFs, presenting sodalite topology (SOD), are particularly interesting for membrane applications, since they have large sodalite cages interconnected by small rings of 6 metal atoms [54]. In this cluster of frameworks (which include ZIF-7, ZIF-8, ZIF-9 and ZIF-90) ZIF-8 stands out as being the most studied structure. This material, which structure is presented in Figure 2.9 [56,57], is worth being used as selective membrane due its structural characteristics: it presents large cavities (1.16 nm) and small pore apertures (0.34 nm). These characteristics make this type of MOF an ideal material to promote several separations based on size such as propylene/propane [58] and H2 from CO2, N2 and CH4 [59].

On the other hand, despite all progress regarding MOFs, most of these materials including ZIFs still face some problems during the synthesis of defect-free membranes. The preparation of continuous MOFs membranes by in situ synthesis is difficult, being verified a poor nucleation of crystals on the support [60]. Thus, in order to have a direct nucleation and crystallization on the support surface, it is necessary to introduce chemical modifications

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19 on the surface of the macroporous material. This difficulty in the preparation of continuous membranes is related to the impossibility of the organic ligands to establish bonds with the OH groups of the support surface [61]. However, due to the high organic functionality and flexibility of the organic linkers, MOFs have the ability of being modified in order to improve the adherence to the support surface. One of the most common strategies, which is already applied in the preparation of some zeolite membranes, is the deposition of the membrane on 3-aminopropyltriethoxysilane (APTES)-functionalized supports [62], as it is shown in Figure 2.10 [61]. With the modification of the support, a thin and defect-free MOF membrane is formed, once the chemically modified particles act as stronger linkers but at the same time as fillers [63,64]. [63], [64]

Figure 2.9 - Schematic representation of the ZIF-8 structure: (from left to right) network shown as a stick diagram; network shown as a tiling; largest cage of the network represented in yellow; representation of the

pore aperture [56,57].

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CHAPTER 3 – TRANSPORT MECHANISMS _______________________________________________________________________________

21

3 Transport Mechanisms

When studying a new membrane for a certain separation method, it is important to understand the nature of mass transport mechanisms involved since they have high influence on both driving force and permeability [2]. [29], [65]

There are several mass transport mechanisms that might be present in gas separation. However, the contribution of each mechanism depends on the characteristics of the membrane such as pore diameter, geometry and interconnectivity or crystals orientation. Still, temperature and pressure influence the prevailing transport mechanisms [29,65].

The review of these mechanisms is a fundamental tool because allows not only to represent the permeation of gases across membranes, but also makes possible to analyse the existence of macro, meso and microdefects in the structure of membranes after performing experimental permeation measurements.

3.1 Single Gas Permeation 3.1.1 Viscous Flow

In macroporous structures (𝑑p > 50 nm), the pore diameter is much larger than the

component diameter. Consequently, intermolecular collisions are more dominant than molecule-surface collisions, as it is shown in Figure 3.1 [66]. In the presence of a pressure difference, the fluid fills and flows through the pore as a whole. This flow is generally undesirable because it is not permselective; in other words, it promotes no separation between species [2]. Convective flow may occur simultaneously with other transport mechanisms responsible for the gas separation.

In this case, since the flow is laminar, the molar flux might be described by a Hagen-Poiseuille type-law. Hence, the flow occurs in a porous structure and thus it is necessary to complement the latter law with a geometric factor 𝜀 𝜏⁄ , given by Equation 3.1. It is assumed the pores are cylindrical [67].

𝑁_V = −𝜀 𝜏 𝑑_p2 32𝜂 𝑃 𝑅𝑇 𝑑𝑃 𝑑𝑧 (3.1)

Here 𝑃 is the total pressure, 𝑅 is the ideal gas constant, 𝑇 is the absolute temperature, 𝑧 is the distance coordinate, 𝜀 is the porosity of the structure, 𝜂 is the gas viscosity and 𝜏 is the tortuosity.

Synthesis and characterization of new zeolite membranes and crystalline meta-organic frameworks

Sebastião Melo

Refoios da Costa

Síntese e caracterização de novas membranas

zeolíticas

e

de

Redes

Metalo-Orgânicas

cristalinas

Synthesis and characterisation of new zeolite

membranes

and

crystalline

Metal-Organic

Sebastião Melo

Refoios da Costa

Síntese e caracterização de novas membranas

zeolíticas

e

de

Redes

Metalo-Orgânicas

cristalinas

Synthesis and characterisation of new zeolite

membranes

and

crystalline

Metal-Organic

Frameworks

Contents

List of Figures

List of Tables

Nomenclature

1 Motivation and Thesis Plan

2 Membrane Technology

3 Transport Mechanisms