-Aminoterephthalate Metal-Organic Frameworks: Synthesis, Characterization and Applications-
Pablo Serra Crespo
1. It is increasingly ignored that fundamental understanding in science is the key for new advances and developments.
Chapter 7 - The optimization of the NH2-MIL-53(Al) synthesis.
2. In contrast to the common believe that metal-organic frameworks are unstable, NH2-MIL-53(Al) is the exception to this rule.
Chapters 3 and 6.
3. Even though flexibility in porous materials may lead to interesting advantages in gas separation, some unexpected drawbacks may be related to this property.
Chapter 8.
4. The different ways of calculating gas separation selectivities do not allow an objective comparison of adsorbents.
Li, J.-R.; Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H.-K.; Balbuena, P. B.; Zhou, H.-C. Coordination Chemistry Reviews 2011, 255, 1791.
Liu, Y.; Wang, Z. U.; Zhou, H.-C. Greenhouse Gases: Science and Technology 2012, 2, 239.
5. Authors of scientific publications must be aware that the reader should be the one receiving the greatest benefit from that contribution in being able to fully reproduce the work.
6. Scientific publishers should compensate peer reviewers for their time and expertise; free open-access to their upcoming publications could be an example.
7. Low levels of investment and commitment condemn science to detained and slow advances.
8. Co-existence of different cultures in the same territory is the opportunity to build bridges, not to destroy them.
9. People’s fears and lack of knowledge should not be an impediment for research in fields involving genetically modified organisms, stem cells and nuclear energy. Stimulation through a better education would avoid this situation in the future.
10. If universities and research centres want to attract and hold the most qualified and talented young people, their job offers should be competitive with industry and not only rely on vocation.
These propositions are regarded and defendable, and have been approved as such by the supervisors, Prof.dr. F. Kapteijn and Prof.dr. J. Gascon.
-Aminotereftalaat Metal-Organic Frameworks: Synthese, Karakterisering en Toepassingen-
Pablo Serra Crespo
1. Het wordt in toenemende mate genegeerd dat fundamenteel begrip in de wetenschap de sleutel is voor nieuwe ontwikkelingen en vooruitgang.
Hoofdstuk 7 – De optimalisering van de synthese van NH2-MIL-53(Al).
2. NH2-MIL-53(Al) is de uitzondering op de algemene perceptie dat metal-organic frameworks instabiel zijn.
Hoofdstuk 3 en 6.
3. Hoewel flexibiliteit in poreuze materialen interessante voordelen in gasscheiding kan opleveren, kan een aantal onverwachte nadelen ook worden gerelateerd aan deze eigenschap.
Hoofdstuk 8.
4. De verschillende manieren waarop selectiviteit in gasscheiding kan worden berekend staan geen objectieve vergelijking van adsorbentia toe.
Li, J.-R.; Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H.-K.; Balbuena, P. B.; Zhou, H.-C. Coordination Chemistry Reviews 2011, 255, 1791.
Liu, Y.; Wang, Z. U.; Zhou, H.-C. Greenhouse Gases: Science and Technology 2012, 2, 239.
5. Auteurs van wetenschappelijke artikelen moeten zich ervan bewust zijn dat de lezer degene zou moeten zijn die het grootste voordeel van die bijdrage geniet door middel van de mogelijkheid om het werk volledig te reproduceren.
6. Uitgevers van wetenschappelijke artikelen moeten peer reviewers voor hun tijd en expertise compenseren; gratis open access voor hun aankomende publicaties zou een voorbeeld kunnen zijn.
7. Terughoudendheid en langzame vooruitgang in de wetenschap komen voort uit het lage niveau van investeringen en toewijding.
8. Het samenleven van verschillende culturen in hetzelfde gebied is een buitenkans om bruggen te bouwen, niet om hen te vernietigen.
9. Angst en gebrek aan kennis bij mensen zou geen belemmering moeten zijn voor onderzoek naar genetisch gemodificeerde organismen, stamcellen en kernenergie. Het is beter om dit te stimuleren door middel van beter onderwijs zodat deze situatie in de toekomst kan worden vermeden.
10. Als universiteiten en onderzoekscentra de meest gekwalificeerde en getalenteerde jongeren willen aantrekken en vasthouden, moeten ze hun arbeidsvoorwaarden op hetzelfde niveau brengen als de industrie en niet alleen vertrouwen op de roeping van onderzoekers.
Deze stellingen worden opponeerbaar en verdedigbaar geacht en zijn als zodanig goedgekeurd door de promotoren, Prof.dr. F. Kapteijn and Prof.dr. J. Gascon.
Pablo Serra Crespo
Aminoterephthalate Metal-Organic
Framework: Synthesis,
Author: Pablo Serra Crespo
PhD Thesis, Delft University of Technology The Netherlands, October 2014
Aminoterephthalate Metal-Organic Framework:
Synthesis, Characterization and Applications
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen
op maandag 31 october 2014 om 12.30 uur
door
Pablo SERRA CRESPO
Ingeniero Químico Universidad de Alicante
Prof. dr. F. Kapteijn Prof. dr. J. Gascon
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. dr. F. Kapteijn Technische Universiteit Delft, promotor Prof. dr. J. Gascon Technische Universiteit Delft, copromotor Prof. dr. ir. T.J.H. Vlugt Technische Universiteit Delft
Prof. dr. G. Mul University of Twente Prof. dr. P.A. Wright University of St Andrews Prof. dr. ir. G.V. Baron Vrije Universiteit Brussel
Dr. C.A. Grande SINTEF
Prof. dr. ir. M. Makkee Politecnico di Torino/Technische Universiteit Delft, reservelid
The research reported in this thesis was conducted in the Catalysis Engineering section of the Chemical Engineering department, Faculty of Applied Sciences (TNW) of the Delft University of Technology and received funding from the Dutch National Science Foundation (NWO-CW) / VENI Grant Agreement n.700.59.404.
Proefschrift, Technische Universiteit Delft Met samenvatting in het Nederlands
ISBN 978-94-6186-383-6
Copyright © 2014 by Pablo Serra Crespo
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form of by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without the prior permission of the author.
Chapter 1
Introduction
1
Chapter 2
Synthesis and Characterization of an Amino Functionalized
MIL-101(Al): Separation and Catalytic Properties
35
Chapter 3
Adsorption and Separation of Light Gases on an
Amino-Functionalized Metal–Organic Framework: An Adsorption and In
Situ XRD Study
57
Chapter 4
Interplay of Metal Node and Amine Functionality in NH
2-MIL-53:
Modulating Breathing Behaviour through Intra-Framework
Interactions
87
Chapter 5
NH
2-MIL-53(Al): A High Contrast Reversible Solid State
Nonlinear Optical Switch
107
Chapter 6
Experimental Evidence of Negative Linear Compressibility in the
MIL-53 Metal-Organic Framework Family
123
Chapter 7
Kinetic Control of Metal-Organic Framework Crystallization
Investigated by Time Resolved In Situ X-ray Scattering
137
Chapter 8
Separation of CO
2/CH
4mixtures over NH
2-MIL-53 – An
Experimental and Modelling Study
155
Chapter 9
Conceptual Design of a Vacuum Pressure Swing Adsorption
Process for Natural Gas Upgrading Based on Amino
Functionalized MIL-53
189
Appendix A
215
Appendix B
225
Appendix C
233
Appendix D
243
Summary and Outlook
259
Samenvatting en vooruitzichten
265
About the author
271
List of publications
273
Introduction
Abstract
The main technologies in the separation of carbon dioxide are reviewed with special emphasis on adsorption-based technologies. Metal-organic frameworks are presented as potential candidates for these technologies considering especially its sub-family of soft-porous crystals, which possesses extra features that may be beneficial for their final application as gas adsorbents. This chapter concludes with an outlook on the investigated topics described in this thesis.1.1.
INTRODUCTION
Fossil fuels have been consumed in very large amounts to power human society over the past 200 years. Nowadays, 85 % of the global energy demand is satisfied by fossil fuels (see figure 1-1) [1]. The high energy density and their abundance made them the primary energy source and created a strong dependency on their acquisition and trade.
The international energy demand prediction shows a 1.7% average growth for the period 2005-2020 [2]. Even though all energy sources are considered in this estimation, fossil fuels are expected to still rule the energy scene in the coming decades [3-6].
Figure 1 - 1 Energy production by source over the past 6 decades. Extracted from [7].
As a result of the increment in energy demand due to global population increase and the industrialization of more countries, the Earth’s atmosphere composition has been modified with a drastic rise in greenhouses gases (GHG), being carbon dioxide the main product of fossil fuels combustion and the main contribution to global warming. The most recent studies predict an increase in surface air temperature of more than 5 °C between the years 1861 and 2100 [2, 8].
A solution to this problem needs a worldwide collaborative effort that combines the participation of politicians, economists, scientists and engineers to find new solutions and to move from a fossil fuels based economy to a different energy source. Meanwhile new technologies capable of supply the demanded energy are developed; new strategies are crucial with a lower GHG emission as an essential
target. The combination of using renewable energy resources and the migration to less carbon-intensive fuel seems to be the most reasonable energetic path for the coming decades [7, 9].
Methane is the hydrocarbon with the lowest emission factor, 57.3 tons of CO2 per TJ of energy
produced. This fact, together with its high abundance, makes methane a very attractive source of energy. Methane is mainly found as the main component of natural gas, which also includes higher alkanes, carbon dioxide, nitrogen and hydrogen sulphide [10].
Natural gas demand is experimenting the highest growth rate and it is expected that in 2020 its extraction will exceed that of coal, promoted by the increasing restrictions in pollutant emissions. Novel transportation technologies, the remarkable reserves found, the lower overall costs and the environmental sustainability, they all point to natural gas as the primary energy source in the near future.
Natural gas contains the above-mentioned gases in variable concentrations, minor amounts of helium and mercury are usually found. Furthermore, in the case of natural gas associated with an oil reservoir, enhanced hydrocarbon recovery operations by nitrogen and/or carbon dioxide injection modify the chemical composition of the produced gas, decreasing its content in methane.
Even though natural gas is classified in several ways, the most common designations are dry or wet and sweet or sour. Wet gas stands for a higher content in C2+ hydrocarbons than 10 vol.% and sour gas
is applied when the concentration of hydrogen sulphide is higher than 1 vol.% or the concentration of carbon dioxide exceeds a 2 vol.%. Chemical composition determines the operation requested to reach the specifications needed for natural gas transportation or for its final processing. As an example, pipeline gas quality cannot contain a higher concentration of CO2 than 3-4 vol.% and 5.7-22.9 mg m-3
(STP) of H2S. The restrictions are much higher when natural gas is going to be liquefied for a more
efficient transportation, in that case the concentration of CO2 should be lower than 50 ppmv and 4
ppmv is the maximum value admissible in the case of H2S [11]. On the other hand, the water content
has to be reduced to levels that prevent corrosion, hydrate formation and freezing in cryogenic facilities [12, 13].
Nitrogen and carbon dioxide are the most common major contaminants in natural gas, both are considered inert because of their zero heating value and they have to be removed to low levels. Nitrogen, together with helium, is usually removed by cryogenic fractional distillation. Several approaches have been proposed to separate CO2 such as absorption, cryogenic distillation, membrane
separation and adsorption. Current technologies involve liquid amine absorbents reaching high selectivity but at the cost of expensive regeneration and the use of toxic compounds [13].
A gas mixture with similar composition is found in biogas. Biogas comprises gas produced by breakdown of organic matter in the absence of oxygen. Biogas is a renewable source of energy since it can be produced from organic waste. Biogas contains mainly methane and carbon dioxide, and may contain traces of sulphur compounds and water [3].
Either from natural gas or from biogas, a separation and purification process is needed before methane is obtained in the desired quality. After that, energy can be directly produced from methane combustion or it can be utilized as a raw material for the synthesis of liquid fuels. The most challenging and energy intensive step is the carbon dioxide removal, which represents the largest industrial gas separation process [14, 15]. In the coming section a brief summary of the different possibilities for carbon dioxide separation is given.
1.2.
TECHNOLOGIES IN CO
2SEPARATION
The most critical step in natural gas and biogas upgrading is the removal of carbon dioxide. Several approaches have been proposed to separate CO2 from gas mixtures such as absorption, cryogenic
distillation, membrane separation and adsorption (a summary of the applied technologies in carbon dioxide separation and their associated materials is given in figure 1-2).
Figure 1 - 2. Different technologies and associated materials for CO2 separation and capture. Reproduced
from [9].
The dominant technology for CO2 removal in industry is amine scrubbing, liquid phase absorption by
amine solutions. Solutions of mono-ethanolamine (MEA) and di-ethanolamine (DEA) are widely applied, reaching high selectivity but at the cost of a large energy input for regeneration.
At a temperature of around 40 oC, the reaction of CO2 with the amine (primary and secondary) occurs
through a zwitterion mechanism to form carbamates. A fraction of the carbamate species is hydrolyzed to from hydrogen carbonates see figure 1-3a. In the case of tertiary amines, the reaction with carbon
dioxide to form carbamates cannot proceed, leading to a base-catalyzed hydration of CO2 to form
hydrogen carbonates (see figure 1-3b) [16-18].
Figure 1 - 3. General reaction schemes for the chemical absorption of CO2 by a) primary or secondary
and b) tertiary amine-containing solvents. Reproduced from [17].
A typical liquid-phase gas treating process includes an absorber unit and a regenerator unit as well as accessory equipment. In the absorber, the down-flowing amine solution absorbs CO2 and other gases
like H2S from the up-flowing sour gas to produce a sweetened gas stream as a product and an amine
solution rich in the absorbed acid gases. The resultant saturated amine solution is then routed into the regenerator (a stripper with a re-boiler) to produce regenerated amine solution that is recycled for reuse in the absorber. The stripped overhead gas from the regenerator is concentrated on CO2 and the other
removed gases [19].
There are several disadvantages associated with this technology. First of all, the relatively high vapour pressure of the amines results in large losses during the regeneration step, what makes the process not only more expensive but also dangerous due to toxicity of the released amines. Many of these compounds show a high toxicity and a high corrosive power, complicating their implementation in environmentally friendly processes. Last, the regeneration of these compounds requires a tremendous amount of energy: 2.5 GJ/tonne of CO2 [20]. This energy penalty originates primarily from the need to
heat the large quantity of water in which the amine is dissolved as well as to break the C-N bond that is formed during the interaction between CO2 and the amine group. These disadvantages create the
Cryogenic distillation utilizes the principle of separation based on cooling and condensation. This technique is hypothetically very powerful for CO2 separation, but the substantial energy requirement
makes it less desirable for most industrial cases. Separation of CO2 from natural gas and biogas is
based on the fact that CO2, H2S and all other contaminants liquefy at different temperature-pressure
domains, making it easy to separate them from CH4. This separation process operates, however, at low
temperatures, near -100 °C, and at high pressures, almost 40 bars. The main advantage of cryogenic separation is both the large quantities as well as the high purity of the upgraded gas. The yield of this separation is 99% for CH4 [22]. The main disadvantage of cryogenic separation is that cryogenics
processes require the use of heavy process equipment, mainly compressors, turbines and heat exchangers. The need for the equipment causes this separation technique to be a process with large capital costs [23].
Membrane gas separation is used predominately to remove CO2, but only accounts for a 5% market
share for this operation. However, a range of very large natural gas sweetening projects using membrane technology have been announced in the last few years [24, 25]. Gas separation membranes offer a number of benefits over other gas separation technologies. Conventional technologies such as the cryogenic distillation, condensation and amine absorption require a gas-liquid phase change of the gas mixture that is to be separated. The phase change adds a significant energy cost to the separation cost. Membrane gas separation, on the other hand, does not require a phase change. In addition, gas separation membrane units are smaller than other types of plants, like amine stripping plants, and therefore have relatively small footprints. The lack of mechanical complexity in membrane systems is another advantage. Membrane devices for gas or vapour separation usually operate under continuous, steady-state conditions. The feed stream passes along one side of the membrane. The non-permeating molecules that remain at the feed-stream side exit the membrane as the retentate stream. A (partial) pressure difference across the membrane drives the permeation process. The mechanism of permeation (sorption of molecules and diffusion) depends on the membrane material. In case of membranes with well-defined pores (i.e. zeolites, metal organic frameworks, carbon molecular sieves (CMS)) adsorption, diffusion and eventually molecular sieving dominate membrane performance: in case of polymeric membranes permeation takes place mostly through a solution-diffusion mechanism [9, 26]. Many different types of membranes have been developed or are under development for industrial separations, but for CO2 removal, the industry standard is presently cellulose acetate. These polymeric
membranes consist of a thin layer (0.1 to 0.5 µm) of cellulose acetate is on top of a thicker layer of a porous support material, also polymeric. Permeable compounds dissolve into the membrane, diffuse across it, and then travel through the inactive support material. The membranes are thin to maximize
mass transfer and, thus, minimize surface area and cost, so the support layer is necessary to provide the needed mechanical strength [27, 28].
Membrane technology can be powerful for CO2 capture by decreasing equipment size and lowering
energy requirement. The inherent simplicity of membrane CO2 separation can bring about many
advantages compared with the conventional solvent absorption technology. Reduced equipment size, anticipated lower energy requirements, absence of potentially hazardous chemicals practice (which is required in other technologies such as above mentioned solvent absorption technology) are the main advantages of membrane separation. Membrane gas separation is currently an established technology, but the compromise between permeability and selectivity impedes the application of membrane technology on a large-scale. Furthermore, impurities in the gas stream usually plug the membranes, shortening their life and creating difficulties in their operation. For these reasons, new materials need to be developed to fabricate highly effective membranes for the CO2 separation before they can be
widely applied in this process [10, 13, 14, 25, 26, 29].
Gas separation based on adsorption has proven itself to be a promising method for CO2 separation.
Adsorption-based separations rely on the fact that gases reversibly adsorb in the pores at densities that far exceed the density of gases in equilibrium with the porous solid. As an example, the density of adsorbed CO2 in zeolite 13X in equilibrium with CO2 at 1 atm and 298 K is 4.7 mmol g-1; what means
that at these conditions the density of CO2 in the pores of zeolite 13X is 73.7 times larger than the gas
density [30]. The most important factor in this method is to develop and apply an effective adsorbent with high adsorptive capacity, high selectivity and moderate or low heat of adsorption [17]. Although materials for gas adsorptive separation have been established and a diverse range of useful sorbents are available for CO2 separation, there is still plenty of room to optimize the performance of these
materials. Conventional solid adsorbents include activated carbons, silica gel, ion-exchange resins, zeolites, mesoporous silicates, activated alumina, metal oxides, and other surface-modified porous media [9, 31, 32].
Depending on the regeneration methods, several adsorption processes can been adopted to achieve CO2
separation, including vacuum and pressure swing adsorption (VPSA and PSA), temperature swing adsorption (TSA), electric swing adsorption (ESA), simulated moving bed (SMB), and purge displacement [33-36]. Among them, PSA has attracted the attention from the markets because its intrinsic flexibility and low energy consumption.
As it is the case for other adsorption based separation processes, PSA involves two basic steps. In the adsorption step, certain components of a gaseous mixture are selectively adsorbed on a porous solid. This operation, performed at relatively high pressure by contacting the gaseous mixture with the
adsorbent in a packed column, produces a gas stream enriched in the less strongly adsorbed component of the feed mixture (the raffinate). After a given time of operation, the adsorbing bed approaches saturation and regeneration is required, which is the second basic step.
In the regeneration step, also known as desorption, the adsorbed components are released from the solid by lowering their gas phase partial pressures inside the column. After this operation, the adsorbent is ready to be employed in a further cycle. The gaseous mixture obtained from regeneration (the extract) is enriched in the more strongly adsorbed components of the feed.
Figure 1 - 4. Schematic diagram of the four-step main steps of a PSA cycle.
In industrial conditions, several columns are operated in a swing-mode to make the process continuous and additional steps are added to the basic cycle (see figure 1-4) in order to maximise productivity and energy saving. It is common practice to include other steps in the cycle in order to promote a more efficient and selective operation mode like a rinse, where the extract is used to evacuate the column, or a pressurization step, where the raffinate is fed to increase the pressure in the column. The whole cycle lasts minutes and is operated under approximately isothermal conditions. Natural gas is often available at high pressure. At least in principle, by using adsorbents able to capture contaminants, PSA processes can produce pure methane at high pressure as raffinate, thus reducing further compression work before transmission to downstream customers and therefore the overall energy consumption of the process [37, 38].
Carbon dioxide separation by adsorption in adsorbents like activated carbons and zeolites is much more energy-efficient than by chemical absorbents. The main reason for this difference is the absence of the formation of chemical bonds between the adsorbent and the adsorbate, therefore a significant
lower amount of energy is required for regeneration [39]. However, the traditional materials, such as activated carbons, are limited by low selectivities (ca. 10), while zeolites show significantly higher selectivities, but they suffer from lower CO2 loadings, unfavourable isotherms, and their performance
is compromised in the presence of water. As a consequence, there is an urgent need to develop advanced adsorbents with a high CO2 capacity and high selectivity which regeneration does not
consume high quantities of energy [4, 39].
Despite the fact that there are many adsorbents commercially available for PSA processes, there is still demand for highly stable, cheap and energy-efficient materials. Stability is needed since many contaminants may ruin the adsorbent, shortening its life, and a high mechanical stability could allow the processing of the adsorbent without the use of binders and would prevent any attrition once it is placed in the column. The adsorbent represents a significant part of the investments cots and low synthesis cost would represent a big decrease in the overall capital investment of such a process. Nonetheless, adsorbents with higher selectivity and higher capacity would boost the implementation, settling of this technology for carbon dioxide separation, and they would lead to a more efficient PSA system since they would limit the number of separation steps, reduce the cycle frequency thanks to their working capacity and avoid the use of external heating due to an easy regenerability [38, 40-42]. In recent years, a new family of adsorbents, known as metal-organic frameworks (MOFs), has been developed and resulted potentially applicable for gas separation. In the next section a summary of the newly developed MOFs is presented, focusing on the ones applied for carbon dioxide and methane adsorption and separation. In addition, a subclass of these porous materials exhibits changes in their porosity upon adsorption, this family will be covered with special emphasis on the effect of flexibility on gas separation.
1.3.
A NEW GENERATION OF POROUS ADSORBENTS FOR
CO
2SEPARATION
In the past two decades a new class of crystalline porous materials, commonly known as metal-organic frameworks (MOFs), has arisen [43, 44]. These hybrid materials are formed by the combination of inorganic subunits (metal cluster) and organic linker (ligand). Many of them exhibit colossal pore volumes, being nowadays the materials displaying the highest degree of porosity [45]. Due to the enormous number of possible combinations of different metals and ligands a plethora of structures have been synthesized [46, 47].
The first reports in literature on similar hybrid materials date from the late 50s and early 60s. At that time the term MOFs was not coined yet and they were referred to as coordination polymers [48-51]. At
the end of the 20th century several scientific groups worldwide rediscovered the field, mainly represented at first by Robson et al.[52, 53] and later by Kitagawa et al. [54, 55], Férey et al.[56] and Yaghi et al.[57]. This last group introduced the term MOF and which was widely spread mainly due to the material MOF-5, which is among the most studied MOFs and served as a standard in the field. Its structure can be found in figure 1-5 [58, 59].
Figure 1 - 5. Structure representation of MOF-5. On each of the corners is a cluster [OZn4(CO2)6] of an
oxygen-centered Zn4 tetrahedron that is bridged by six carboxylates of an organic linker (Zn, blue
polyhedron; O, red spheres; C, black spheres). The large yellow spheres represent the largest sphere that would fit in the cavities. Adapted from [60].
Initially, the research was almost focused on the discovery of new materials and structures. MOFs can be synthesized applying the classical hydro- (or solvo-)thermal route, in addition to new methods that include microwave, electrochemical, diffusion and ultrasonic techniques and even mechanical processes can lead to the formation of these porous materials [61-64]. Synthesis of MOFs also benefited from high-throughput techniques what helped to the isolation and purification of many compounds in a time effective way [65, 66]. Moreover, the mechanism of synthesis and crystallization of these porous materials has been studied in several reports [67-71].
Many possible applications in different fields have been proposed for MOFs. Due to the displayed huge porosity in many of the materials, adsorption was the first application that was thoroughly considered and there are many examples in literature of diverse materials for adsorption and separation of different gases, vapours and liquids. The adsorption and separation of gases, particularly carbon dioxide and methane, are going to be discussed in this chapter more in detail [3, 32, 72, 73].
In the field of gas separation, MOFs have not been only applied as adsorbents but also as membrane, in pure MOF layers and as fillers in mixed-matrix membranes. Both fields are still in their infancy but many promising results that point to a huge potential in these technologies have already been reported [74-81].
Catalytic applications have been explored extensively as well. In first place only proof of concept publications were available and a poor performance together with stability problems were observed. However, the field has rapidly developed during the last years and many innovative approaches have been followed from using the uncoordinated metal site to the post-functionalization of the linkers or the encapsulation of active species, just to mention few examples [82-88].
Other fields where applications for MOFs have been proposed are sensor devices and medicine. In the first case, either by their magnetic or luminescence properties which can be combined with different stimulus like adsorption to create sensors [89-92]. In the second field, MOFs have been proposed for applications in different direction within the medical field [93]. The most studied function of MOFs in medicine is their use as drug delivery vehicles. Férey and colleagues were the first to propose these materials as carriers for drugs. Drug release in some of their carboxylic acid based MOFs has been reported considering not only the evolution of the release with time but other factors like particle size and shape and the toxicity and excretion of the metals and linkers [94-98]. In the same direction, MOFs have been proposed for their use as releasing agent of nitric oxide. However, there are many points, like the stability of the materials and their biocompatibility, that need to be carefully studied before a real implementation can be envisaged [99-101]. Another use of MOFs that has been proposed in the field of medicine is their utilization as imaging agents, something quite interesting when combined with drug delivery [102].
In the two coming sections the current scientific bibliography in the application of MOFs in carbon dioxide separation, with emphasis in its removal in the presence of methane, is summarized, highlighting the most interesting and remarkable examples. First, a description of the methods applied to evaluate the potential of the adsorbents and some examples will be considered. After that, a sub-class of MOFs, the so-called soft-porous crystals, will be introduced and the effect of framework flexibility on gas separation will be discussed.
1.4.
ADSORPTIVE SEPARATION OF CARBON
DIOXIDE-METHANE MIXTURES IN MOFS
In many of the reported literature, selective adsorption is the primary tool used to give an indication on the potential for separation of a particular MOF. In selective adsorption, both the adsorption capacity and the selectivity are the main concerns, however how to define the selectivity can be matter of discussion. Two different methods are usually applied to estimate the selectivity, in terms of the single-component adsorption data. If adsorption species are presented at low loadings, within the Henry’s regime, the adsorption selectivity for an equimolar mixture is close to the ratio between the Henry’s constants for each species as if they were adsorbed as a pure component [103]. At non-dilute loadings, however, more information is required to estimate multi-component adsorption. One common approach is to use the well-developed Ideal Adsorbed Solution Theory (IAST) to predict multi-component adsorption isotherms and selectivity based on single-multi-component adsorption isotherms [104].
Since these calculations are based, in both cases, on the single-component adsorption isotherms, the selectivity factor from these methods does not consider the competition of gas molecules for the adsorption sites on the pore surface. For this reason they do not represent the actual selectivity from the dosing of a mixed gas. However, they provide a simple way of evaluating the performance of different MOFs in terms of selectivity [4, 9].
The selective adsorption of CO2 over CH4 in rigid MOFs can be attributed to two different factors, size
exclusion or a favourable gas-pore surface interaction. These factors are determined by the sizes and properties of both the adsorbate and the pores in a given MOF. If CH4 is compared with CO2 in terms
of kinetic diameter and polarity, it can be observed that CO2 is smaller and its quadrupole moment is
higher, which, in many cases, results in strong interactions with the pore surface of the adsorbents. As an example of a MOF that shows shape selectivity is Mn(HCO2)2, that was synthesized for the first
time by Dybtsev et al. This manganese formate selectively adsorbs H2 and CO2 but not N2, CH4 and
other gases with larger kinetic diameters, due to the small aperture of the channels [105].
In the case of selectivity based on a stronger interaction with carbon dioxide, the reported data are mainly based on single-component adsorption isotherms. Saha et al. calculated selectivity based on Henry’s constants for MOF-5 [60], one of the standards in the MOF community, and for MOF-177 [106], a zinc based MOF with benzene tribenzoate linkers with three dimensional pores and a huge porosity. The selectivity of these materials for CO2 over CH4 are 15.5 and 4.4 respectively [107].
Despite the fact that MOF-177 does not display an impressive value of selectivity, it is important to remark the fact that this material is capable of adsorbing 35 mmol g-1 of CO2 at 45 bar and room
temperature. The main drawback of these two zinc based MOFs is their poor stability, both easily hydrolyse in the presence of water, so are not relevant for practical application [108].
Using experimental single-component adsorption isotherms to calculate the adsorption selectivity of a multi-component mixture by IAST has been carried out for many MOFs in the selective adsorption and separation of CO2 over CH4. For the MOF Zn2(2,6-ndc)2(dpni), a selectivity of around 30 towards CO2
was calculated when the mixtures where were poor in carbon dioxide content [109]. Using the same approach, Mu et al. evaluated the selectivity of Cu2(Hbtb)2 using equimolar mixtures over a range of
pressure, obtaining a value of 12.4 at 1 bar [110].
There are examples in literature where even more simple approaches are used to estimate the selectivity, e.g. from Liang et al. The authors calculated adsorption selectivity of CO2/CH4 in
HKUST-1, a copper based MOF with benzenetricarboxylic acid ligands, by dividing the CO2 adsorption
capacity by the adsorption capacity of CH4 at each point. The results showed that selectivity towards
carbon dioxide at 25 °C decreases slowly when the pressure is increased [111].
The separation performance of an adsorbent can be evaluated in a simple and straightforward way for mixtures of gases using breakthrough experiments [37]. Even though the experimental results on the adsorptive separation of gas mixture containing CO2 in MOFs are not as common as results based on
single-gas adsorption isotherms, the information that they provide is more reliable because they take into account the competition of the different adsorbing gases. The breakthrough experiments consist of exposing a packed-bed with the adsorbent to a mixed-gas mixture and monitoring the evolution of the flow of the different gases leaving the column. When the column is saturated in one of the components the ‘breakthrough’ of the signal for that compound is detected. The difference in breakthrough times of two gases is representative of the adsorbent’s selectivity (see figure 1-6 for an schematic explanation of a breakthrough experiment). The adsorbed amounts of each gas can be estimated by integration of the breakthrough profiles. The selectivity is then defined as the ratio of the adsorbed amounts normalized by the composition of the gas mixture [112, 113].
Figure 1 - 6. Schematic representation of a breakthrough experiment of a binary mixture (green and purple) with the use of an inert for desorption (blue). The evolution of the concentration with time in the column is shown in colour gradient (top) and in a graph (bottom). Reproduced from [114].
The material Mg-MOF-74 is a magnesium based MOF with 2,5-dioxidoterephalate linkers. Britt el al. reported breakthrough experiments using this material as adsorbent for 20% CO2 in CH4 mixtures
[115]. The results showed a preferential CO2 adsorption over CH4 and a dynamic capacity of 8.9 wt%
carbon dioxide uptake at 1 bar and 298 K. Mg-MOF-74 was selected among a number of MOFs and compared with a currently applied zeolite, as the best material for the separation of CO2/CH4 mixtures
based on its high selectivity (exceeding values of 1000 at low pressures), frequency of regeneration and productivity by Krishna and Long [116]. The importance of the metal ion was evaluated by repeating the experiments with a material with the same topology but based on zinc, the Zn-MOF-74, which only adsorbed 0.35 wt% of carbon dioxide. The interaction between CO2 and Mg2+ gives
Mg-MOF-74 its high capacity and selectivity [115, 117].
As already mentioned, carbon dioxide is a quadrupolar molecule and the interaction with the porous surface will be quite different compared with the interaction of a non-polar molecule like methane.
This fact can be taken as advantage if the internal surface of the adsorbent is modified to give a more efficient interaction with CO2, what may lead to an enhanced adsorption and separation. Different
strategies have been followed to develop MOFs with a greater performance in this particular separation. Three approaches have shown to be effective: the generation of coordinatively unsaturated metal sites (CUS), the incorporation of a chemical functionalization and taking advantage of the flexibility of some MOFs.
The effect of CUS on MOFs with a similar topology was confirmed by Dietzel et al. by checking the performance of Ni-MOF-74 and finding similar results of preferential adsorption of CO2 [118]. Similar
behaviour was confirmed in MOFs with other topologies that include CUS like HKUST-1 and MIL-100 and MIL-101 [111, 119].
A typical example of functionalization to improve the affinity of certain MOFs towards CO2 is the
incorporation of amines into their structures to mimic CO2 stripping in industry by aqueous
amine-containing solutions. This is not a new concept, it has been already applied in other porous materials by impregnation or covalent bonding to the surface, being the preferred method to avoid leaching [39]. Some mesoporous solids like MCM-41 and MCM-48 and some zeolites like 13X and Y incorporated covalently linked amines [120-123]. All these adsorbents displayed a much higher capacity and selectivity, adsorbing CO2 with a low affinity for other components and improving their performance
when water was present.
In MOFs, the most common procedure for the immobilization of amines in the porous structure is the use of organic linkers containing amine groups in the synthesis. As a first example, Arstad et al. synthesized three MOFs based on three different metals (Al, Ni and In) using 2-aminoterephthalic acid as linker. In the three cases, the adsorption enthalpy and the CO2 capacity were higher for the amino
functionalized MOFs [124]. In a similar way, Vaidhyanathan et al. synthesized a zinc based MOF using aminotriazolatooxalate as ligand, which showed a preferential adsorption of carbon dioxide over gases at low pressures [125]. Via molecular simulations, they proved the existence of two different adsorption sites, one located in the free amines and the other near the oxalates. This cooperative binding of CO2 molecules together with the appropriate pore size gave a large CO2 uptake. Yuan and
co-workers reported the synthesis and characterization of an amino functionalized MOF, (Me2NH2)In(NH2BDC)2⋅DMF⋅H2O, which showed a gravimetric adsorption of CO2 of 25.3 wt% at 3
MPa and 25 oC and a high selectivity towards CO2 in mixtures with CH4 extracted from breakthrough
results [126].
Another possibility to incorporate free amines into MOFs is to post-synthetically modify either the linkers or the CUS. Demessence et al. modified the material Cu-BTTri grafting ethylenediamine to the
CUS of this MOF. The surface area decreased, however, the CO2 uptake increased at low pressures and
the heat of adsorption suffered an increment up to 90 kJ/mol [127]. A post-synthetic approach to incorporate amines in the linker can be found in the reported work by Bernt and co-workers. The chromium terephthalate MIL-101 is first synthesized using terephthalic acid, after that a nitration leads to the incorporation of nitro groups in the linkers and as last step they are reduced to form amino groups [128]. Even though that they did not report adsorption of carbon dioxide on this material, later Yan et al. utilized the amine functionalized mesoporous material for CO2 adsorption, adding extra
amines which were grafted at the CUS. It resulted in a much higher CO2/CH4 selectivity, in the order
of thousands, based on adsorption isotherms at low pressures [129].
The last reported manner for incorporation of amines is by linker exchange. Some of the first studies of this methodology were published by Karagiaridi et al. and by Kim et al. After synthesis, the crystals are immersed in a solution containing the linker that is desired to be incorporated in the material. Different examples of materials with different linkers and metals are demonstrated to show a successful exchange, including the exchange with amine functionalized linkers [130-132].
1.5.
FLEXIBLE MOFS AND THEIR ADVANTAGES IN GAS
SEPARATION
In 1998 Kitagawa and Kondo classified MOFs or porous coordination polymers (PCPs) into three different categories that were referred as first, second and third generation [133]. The first generation consists of materials that can be synthesized but after removal of the guest molecules they collapse and their porosity is lost. The second generation of materials can be evacuated and they maintain their porosity and crystallinity, therefore they can be used as adsorbents analogously to zeolites. The third generation is formed by compounds that respond to external stimuli, which can be physical or chemical, with a change in their porous framework. Those changes can vary from an increase or decrease in the pore volume to a rotational movement of the linkers. A schematic representation of the three MOF generations is displayed in figure 1-7.
The third generation of MOFs is widely known as soft porous crystals (SPCs) and they are defined as hybrid porous solid that possess a highly ordered network together with a structural transformability. In these materials the transformations are reversible and they must have porosity that can be occupied by guest molecules in at least one of their states [134].
In SPCs there are two types of transformations. In the most common of them, the transformation does not create a loss of crystallinity. In the other cases the transformation implies that the perfect
crystallinity is lost, which does not mean that the material becomes amorphous but that the long-range order of the framework is lost without a collapse of the structure [135].
Figure 1 - 7. Classification of porous coordination polymers into three categories. Adapted from [134].
In many cases the dynamic guest accommodation, due to the flexibility of the adsorbent, leads to adsorption isotherms with shapes that are not observed in other adsorbents and that cannot be classified according to the classical IUPAC types. Two examples of SPCs in which their isotherms do not show the profiles covered by the typical classifications are ZIF-7 and MIL-53.
ZIF-7 belongs to the MOF subfamily of zeolite imidazolate frameworks (ZIFs), which were named after the resemblance of the structures formed with that of zeolites. This is explained by the fact that metal-imidazolate-metal bond angles are very close to the Si-O-Si angles found in zeolites [66]. ZIF-7 is formed by Zn cations that are connected by benzimidazole linkers, leading to a sodalite topology with a six-membered ring pore opening. This material shows the unique property of being able to separate paraffin-olefin mixtures exhibiting the selectivity towards the paraffin. This unusual property is related to the soft porous nature of ZIF-7. Adsorption isotherms of short alkanes and alkenes show
no uptake at low pressures, and an abrupt increase in adsorption after a threshold pressure. This is a typical situation of a gate type adsorption profile and the pressure at which adsorption takes place is known as gate-opening pressure. The gate-opening pressure for the alkanes are lower than for the respective alkenes, what can lead to a selective separation if the experiments is carried out in the proper conditions of pressure and temperature.
The separation performance of ZIF-7 and the explanation for its reverse selectivity were elucidated by Gücüyener, van den Bergh and co-workers by a combination of breakthrough experiments and DFT calculations. The difference in gate-opening pressure is due to stronger adsorption of the alkenes on the external surface of ZIF-7’s windows, blocking the windows by pushing one of the benzene rings inwards (a representation of the window can be found in figure 1-8). This phenomenon occurs with the alkene rather than the alkane due to a more efficient interaction between the unsaturated hydrocarbon and the benzene ring. The difference of gate-opening pressure between the alkane and the alkene creates a window of operation that defines the breakthrough partial pressures that result in a highly selective separation [136, 137].
Figure 1 - 8. The main cavity entrance of ZIF-7 (left) together with the lateral (right) view of one of the
six-membered ring (6MR) pore openings, responsible of the gate-opening. Zn–N4clusters are represented
as polyhedra. Reproduced from [137].
A similar behaviour is found in ZIF-7 when the separation of CO2/CH4 is considered. The
gate-opening profile curve is found for the adsorption of carbon dioxide while no uptake of methane is observed. By means of in situ X-ray diffraction Aguado et al. were able to identify an expansion in the pore volume upon adsorption of CO2 what explains for this case the shape of the adsorption isotherm.
However, before the potential of this material for carbon dioxide separation can be considered, breakthrough experiments need to be carried out in order to confirm if the same behaviour is found in the presence of mixtures and under dynamic conditions [138].
MIL-53 is another SPC extensively studied. It is synthesized hydrothermally using terephthalic acid as linker. It was first synthesized with chromium but soon after materials based on other trivalent metals like aluminium, gallium and iron were reported as well. The framework is built up by the interconnection of infinite trans chains of corner-sharing (via OH groups) MO4(OH)2 octahedra by
terephthalate ligands that generate one-dimensional rhombic channels [139, 140].
Figure 1 - 9. Evolution of the different channel openings in the different members of MIL-53 (Al, Ga or Fe) as a function of temperature above RT (blue: hydrated form, green: low temperature form of the anhydrous solid; red: high temperature form; orange: intermediate form). Extracted from [141].
Its flexible behaviour was first observed in the chromium material when the material was activated after synthesis. Removal of solvent and unreacted terephthalic acid molecules led to a different X-ray diffraction pattern indicating a change in the pore configuration with a unit cell pore volume of approximately 1500 Å3. Furthermore, after exposure to ambient conditions and hydration, a different diffraction pattern was observed again reducing its unit cell pore volume to ca. 1000 Å3. This phase transition was named as breathing effect and it is a fully reversible process [139]. A schematic
representation of the breathing effect upon dehydration for MIL-53 based on different metals is shown in figure 1-9.
The same flexible behaviour was observed for the aluminium sample and a rational explanation for such phenomenon was addressed by a combination of in situ X-ray diffraction, modelling and NMR. The very large breathing effect observed during hydration/dehydration is explained by hydrogen bond between water molecule adsorbed in the pore and the bridging OH in the M-O-M chains [140, 142]. The breathing effect is influenced by the nature of the metal, the chromium and aluminium materials go to phase with larger pore volume while the gallium sample retains the small pore volume phases for a wider range of temperature and its phases transition takes places at around 200 oC [141, 143].
Figure 1 - 10. Adsorption isotherms of methane and carbon dioxide in MIL-53(Al) (left) and the corresponding enthalpies of adsorption (right). Extracted from [144].
The breathing effect phenomenon does not only occur when water is adsorbed, also other polar molecules can trigger the phase transition. The adsorption of carbon dioxide was studied and the isotherms exhibited an S-shape that became the fingerprint for the observed phase transition in MIL-53. In the samples made of aluminium and chromium the pores are initially in the large pore phase. However, when they are exposed to low pressures of CO2 the pore contraction takes places and the
material adsorbs CO2 until a first plateau is reached in the adsorption isotherm that corresponds to
around 1 molecule of CO2 per every 2 metal atoms present in the structure. If the pressure is increased
further the expansion of the pore takes place and the material comes back to the original pore size, allowing the uptake of more CO2 molecules and resulting in a second plateau in the adsorption
isotherm that is around 4 times higher. In contrast, adsorption isotherms of methane do show a typical type I isotherm indicating that the phase transitions do not occur when CH4 is adsorbed [144, 145].
Adsorption isotherms of carbon dioxide and methane and their heat of adsorption for MIL-53(Al) are represented in figure 1-10.
The co-adsorption of CO2-CH4 mixture together with breakthrough experiments were studied by
Hamon et al. for MIL-53 (Cr). The body of results shows that the co-adsorption of CO2 and CH4 leads
to a similar breathing of MIL-53(Cr) as with pure CO2. The breathing is mainly controlled by the
partial pressure of CO2, but increasing the CH4 content progressively decreases the transformation
from the large pore form to the narrow. Methane seems to be excluded from the narrow pore form, which is filled by CO2 molecules, and what leads under low methane partial pressures to selectivities
up to 15 [145].
The aluminium version of MIL-53 was also utilized as adsorbent in breakthrough experiments. The results reveal a preferential adsorption of CO2 compared to CH4 over the whole pressure and
concentration range. The separation selectivity was affected by total pressure; below 5 bar, a constant selectivity, with an average separation factor of about 7 was observed. Above 5 bar, the average separation factor decreases to about 4. The adsorption selectivity is affected by breathing of the framework and specific interaction of CO2 with framework hydroxyl groups [146].
As opposite to other adsorbents the presence of water during the separation of CO2/CH4 mixtures can
be beneficial for MIL-53. The presence of water brings the pores to the narrower configuration and hampers the adsorption of methane, lowering the amount adsorbed of this gas in comparison with the experiments under dry conditions and increasing the selectivity [147].
MIL-53 has been functionalized with many different functionalities, including hydroxyls, bromines and many other in the linkers [148]. One particular example that resulted to be very successful was the incorporation of amines. By replacing the terephthalic acid by 2-aminoterephthalic acid in the synthesis a material with the same topology but with uncoordinated amines is formed. In first place it was proposed as basic catalyst, however, it showed a huge potential as adsorbent for CO2 separation
from CH4 [149, 150].
Due to the resemblance of other materials where amines were immobilized the amino functionalized MIL-53, NH2-MIL-53(Al), was expected to capture carbon dioxide thanks to a direct interaction with
the amines in a similar fashion to what happens in liquid phase amines. Per contra, no direct interaction between the adsorbed carbon dioxide molecules and the free amines in the pore was observed in a series of in situ IR experiments. In addition, a set of DFT calculations pointed that the displayed selectivity was due to the presence of the narrow pore configuration in the absence of adsorbate that was the result of the interaction of the bridging OH and the free amines [151].
Figure 1 - 11. Comparison of selectivity and capacity of different representative adsorbents (▲
breakthrough data of equimolar mixtures, ■ simulated data and ● isotherm data. 1: NH2-MIL-53(Al) (a-
at 1 bar total pressure, b-15 bar total pressure) [Chapter 3][152]. 2: Zeolite 13X (a- at 1 bar total pressure, b-15 bar total pressure) [111]. 3: CuBTC (a- at 1 bar total pressure, b-15 bar total pressure)
[111]. 4: MIL-53(Al) (at 1 bar) [146]. 5: Cu2(Hbtb)2 (at 1 bar) [9]. 6: Mg-MOF-74 (from Langmuir
equation constants) [117]. 7: rho-ZMOF (at 1bar) [153]. 8: ZIF-100 (at 1 bar) [154]. 9: BPL carbon (at 1
bar) [154]. 10: Porous clay (at 1 bar) [155]. 11: MCM-48-NH2 (at 1 bar) [120]. 12: SiO2 (at 1 bar) [120].
1.6.
OUTLINE OF THE THESIS
This thesis focuses on the development of aminoterephthalic acid based MOFs, their application in carbon dioxide removal in the presence of methane, and on understanding their properties and features from a fundamental point of view. The thesis closes with an analysis of their practical application potential.
This Chapter 1 combines a short description of the state of the art methods in natural and biogas upgrading with a summary of a new family of adsorbents, MOFs, that open new possibilities in the field of adsorptive separation. Few examples of how the versatility, tuneability and flexibility of MOFs can be utilized to improve and enhance the performance of these materials in gas separation are included.
In Chapter 2, the synthesis and characterization of the mesoporous amine functionalized MIL-101(Al) are described. The effect of the addition of amines on the performance of this material in gas separation and catalysis is shown in this chapter as well.
Chapter 3 describes the performance of NH2-MIL-53(Al) in the adsorptive separation of light gases.
The chapter includes an in situ X-ray diffraction study that describes the phase transitions experienced by this adsorbent upon adsorption of carbon dioxide.
Chapter 4 documents the synthesis of NH2-MIL-53 with gallium and indium and compares how the
different metals (Al, Ga, In) influence the breathing behaviour of this material. A series of in situ techniques, including X-ray diffraction and different IR spectroscopy experiments were applied to understand the intra-framework interactions that led to the modulation of the flexible behaviour and the explanation for the high CO2 adsorption selectivity.
In Chapter 5, the MOF NH2-MIL-53(Al) is presented as the first reversible solid-state non-linear
optical switch with a high contrast. A detailed explanation on why the material exhibits such properties is included.
Chapter 6 reports a series of high pressure X-ray diffraction experiments in MIL-53(Al) and NH2
-MIL-53(Al) under different pressurization media. Both materials exhibit a negative linear compressibility and a high pressure resistance.
In Chapter 7 the crystallization of NH2-MIL-101(Al) and NH2-MIL-53(Al) was studied by in situ
X-ray scattering. The kinetics of formation and synthesis optimization were addressed.
Chapter 8 describes the performance of NH2-MIL-53 in the separation of CO2/CH4 mixtures under
Chapter 9 presents a conceptual design of a pressure swing adsorption process utilizing NH2
-MIL-53(Al) as adsorbent. The economic feasibility of such a process and a comparison with the currently applied technologies are evaluated based on this design.
Note that all the chapters have been written as individual publications and can be read independently. Because of this, some overlap in contents may be present.
26
REFERENCES
[1] S. Rackley, Carbon Capture and Storage, Elsevier Science, 2009.
[2] B. Metz, O. Davidson, H. De Coninck, M. Loos, L. Meyer, Carbon Dioxide Capture and Storage: Special Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 2005.
[3] S. Chaemchuen, N.A. Kabir, K. Zhou, F. Verpoort, Metal-organic frameworks for upgrading biogas via CO2 adsorption to biogas green energy, Chem. Soc. Rev., 42 (2013) 9304-9332.
[4] Y. Liu, Z.U. Wang, H.-C. Zhou, Recent advances in carbon dioxide capture with metal-organic frameworks, Greenhouse Gases: Science and Technology, 2 (2012) 239-259.
[5] Shell energy scenarios to 2050, in, http://www.shell.com/global/future-energy/scenarios/2050.html.
[6] Energy Outlook. ExxonMobil, in, http://corporate.exxonmobil.com/en/energy/energy-outlook. [7] S.D. Kenarsari, D. Yang, G. Jiang, S. Zhang, J. Wang, A.G. Russell, Q. Wei, M. Fan, Review
of recent advances in carbon dioxide separation and capture, RSC Advances, 3 (2013) 22739-22773.
[8] A.P. Sokolov, P.H. Stone, C.E. Forest, R. Prinn, M.C. Sarofim, M. Webster, S. Paltsev, C.A. Schlosser, D. Kicklighter, S. Dutkiewicz, J. Reilly, C. Wang, B. Felzer, J.M. Melillo, H.D. Jacoby, Probabilistic Forecast for Twenty-First-Century Climate Based on Uncertainties in Emissions (Without Policy) and Climate Parameters, Journal of Climate, 22 (2009) 5175-5204. [9] J.-R. Li, Y. Ma, M.C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P.B. Balbuena, H.-C. Zhou,
Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks, Coordination Chemistry Reviews, 255 (2011) 1791-1823.
[10] A.J. Kidnay, W.R. Parrish, Fundamentals of Natural Gas Processing, Taylor & Francis, 2006. [11] R.M. Flores, Coalbed methane: From hazard to resource, International Journal of Coal
Geology, 35 (1998) 3-26.
[12] M. Tagliabue, D. Farrusseng, S. Valencia, S. Aguado, U. Ravon, C. Rizzo, A. Corma, C. Mirodatos, Natural gas treating by selective adsorption: Material science and chemical engineering interplay, Chemical Engineering Journal, 155 (2009) 553-566.
[13] X. Wang, M. Economides, Advanced Natural Gas Engineering, Elsevier Science, 2013.
[14] E. Favre, R. Bounaceur, D. Roizard, Biogas, membranes and carbon dioxide capture, J. Membr. Sci., 328 (2009) 11-14.
[15] S.A. Stern, B. Krishnakumar, S.G. Charati, W.S. Amato, A.A. Friedman, D.J. Fuess, Performance of a bench-scale membrane pilot plant for the upgrading of biogas in a wastewater treatment plant, J. Membr. Sci., 151 (1998) 63-74.
[16] H.-B. Xie, Y. Zhou, Y. Zhang, J.K. Johnson, Reaction Mechanism of Monoethanolamine with CO2 in Aqueous Solution from Molecular Modeling, The Journal of Physical Chemistry A, 114 (2010) 11844-11852.
[17] D. D'Alessandro, B. Smit, J. Long, Carbon Dioxide Capture: Prospects for New Materials, Angewandte Chemie International Edition, 49 (2010) 6058-6082.
[18] P.D. Vaidya, E.Y. Kenig, CO2-Alkanolamine Reaction Kinetics: A Review of Recent Studies, Chemical Engineering & Technology, 30 (2007) 1467-1474.
[19] A.B. Rao, E.S. Rubin, A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control, Environmental
Science & Technology, 36 (2002) 4467-4475.
[20] E. Favre, Carbon dioxide recovery from post-combustion processes: Can gas permeation membranes compete with absorption?, J. Membr. Sci., 294 (2007) 50-59.
27 [21] M.R.M. Abu-Zahra, J.P.M. Niederer, P.H.M. Feron, G.F. Versteeg, CO2 capture from power
plants: Part II. A parametric study of the economical performance based on mono-ethanolamine, International Journal of Greenhouse Gas Control, 1 (2007) 135-142.
[22] A. Hart, N. Gnanendran, Cryogenic CO2 capture in natural gas, Energy Procedia, 1 (2009) 697-706.
[23] R. Agrawal, Synthesis of Distillation Column Configurations for a Multicomponent Separation, Industrial & Engineering Chemistry Research, 35 (1996) 1059-1071.
[24] J. Haggin, New generation of membranes developed for industrial separations, (1988).
[25] W.J. Schell, C.G. Wensley, M.S.K. Chen, K.G. Venugopal, B.D. Miller, J.A. Stuart, Recent advances in cellulosic membranes for gas separation and pervaporation, Gas Separation and Purification, 3 (1989) 162-169.
[26] J. Gascon, F. Kapteijn, B. Zornoza, V. Sebastián, C. Casado, J. Coronas, Practical Approach to Zeolitic Membranes and Coatings: State of the Art, Opportunities, Barriers, and Future Perspectives, Chem. Mat., 24 (2012) 2829-2844.
[27] R.W. Baker, Membrane Technology and Applications, 2012.
[28] D.F. Sanders, Z.P. Smith, R. Guo, L.M. Robeson, J.E. McGrath, D.R. Paul, B.D. Freeman, Energy-efficient polymeric gas separation membranes for a sustainable future: A review, Polymer, 54 (2013) 4729-4761.
[29] C.A. Scholes, G.W. Stevens, S.E. Kentish, Membrane gas separation applications in natural gas processing, Fuel, 96 (2012) 15-28.
[30] E.D. Akten, R. Siriwardane, D.S. Sholl, Monte Carlo simulation of single- and binary-component adsorption of CO2, N2, and H2 in zeolite Na-4A, Energy and Fuels, 17 (2003)
977-983.
[31] A.-H. Lu, G.-P. Hao, Porous materials for carbon dioxide capture, Annual Reports Section "A" (Inorganic Chemistry), 109 (2013) 484-503.
[32] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.-H. Bae, J.R. Long, Carbon Dioxide Capture in Metal–Organic Frameworks, Chemical Reviews, 112 (2011) 724-781.
[33] C. Costa, A. Rodrigues, Design of cyclic fixed-bed adsorption processes. Part I: Phenol adsorption on polymeric adsorbents, AIChE Journal, 31 (1985) 1645-1654.
[34] S. Cavenati, C.A. Grande, A.E. Rodrigues, Upgrade of Methane from Landfill Gas by Pressure Swing Adsorption, Energy & Fuels, 19 (2005) 2545-2555.
[35] S. Cavenati, C.A. Grande, A.E. Rodrigues, Removal of Carbon Dioxide from Natural Gas by Vacuum Pressure Swing Adsorption, Energy & Fuels, 20 (2006) 2648-2659.
[36] C.A. Grande, A.E. Rodrigues, Layered Vacuum Pressure-Swing Adsorption for Biogas Upgrading, Industrial & Engineering Chemistry Research, 46 (2007) 7844-7848.
[37] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, 1984.
[38] S. Sircar, Pressure Swing Adsorption, Industrial & Engineering Chemistry Research, 41 (2002) 1389-1392.
[39] S. Choi, J.H. Drese, C.W. Jones, Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources, ChemSusChem, 2 (2009) 796-854.
[40] S. Sircar, B.F. Hanley, Production of oxygen enriched air by rapid pressure swing adsorption, Adsorption, 1 (1995) 313-320.
[41] S. Sircar, Separation of Methane and Carbon Dioxide Gas Mixtures by Pressure Swing Adsorption, Separation Science and Technology, 23 (1988) 519-529.
[42] W.E. Waldron, S. Sircar, Parametric Study of a Pressure Swing Adsorption Process, Adsorption, 6 (2000) 179-188.
[43] C. Janiak, J.K. Vieth, MOFs, MILs and more: concepts, properties and applications for porous coordination networks (PCNs), New Journal of Chemistry, 34 (2010) 2366-2388.
