Gas Separation Applications Research Articles

PEEK-WC-Based Mixed Matrix Membranes Containing Polyimine Cages for Gas Separation.

Membrane-based processes are taking a more and more prominent position in the search for sustainable and energy-efficient gas separation applications. It is known that the separation performance of pure polymers may significantly be improved by the dispersion of suitable filler materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work, four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEK-WC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did not allow the formation of selective films. The H2, He, O2, N2, CH4, and CO2 pure gas permeability of the neat polymer and the MMMs were measured, and the effect of filler was compared with the maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion coefficient and improved the size-selectivity. The performance and properties of the membranes were discussed in relation to their composition and morphology.

Conjugate polymer-based membranes for gas separation applications: current status and future prospects

Conjugate polymers provide the possibility of exploiting both the chemical and physical attributes of the polymers for membrane-based gas separation. The presence of delocalized π electrons provides high chain stiffness with low packing density, thus making the membrane a rigid structure that favors facilitated transport. Historically, the polymeric membranes were constrained by the tradeoff relationship between gas permeability and gas selectivity. So, different methods were investigated to prepare the membranes that can overcome the limitation. In recent years, electroconductive polymeric membranes have gained attention with their enhanced transportation properties combining the separation behavior depending on both molecular size discrimination as well as the facilitated transport. They offer better selectivity toward polar gases such as CO2 because of the increased solubility. This review is aimed to provide a literature survey on gas separation using conjugate polymers such as polyaniline, polypyrrole, and some derivatives of polythiophenes. It contains various methods used by different researchers to enhance the gas separation properties of the membranes with improved mechanical and thermal stability such as changing the morphology and membrane preparation methods. In addition, it provides the pros and cons of various factors affecting the conjugate polymer membrane performance. The major challenges and future work that can be done in improving the transportation properties through the membrane to achieve viable membranes are also discussed so that they can be used for commercial and practical applications in the future.

Effects of Ionic Liquid Nanoconfinement on the CO2/CH4 Separation in Poly(vinylidene fluoride)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes.

A combined experimental and molecular dynamics (MD) simulation approach was used to investigate the effects of the nanoconfinement of a highly CO2/CH4-selective ionic liquid (IL), 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]), in porous poly(vinylidene fluoride) (PVDF) matrices on the gas separation performance of the resulting membranes. The observed experimental CO2/CH4 permselectivity increased by about 46% when the nominal pore diameter in PVDF, which is a measure of nanoconfinement, decreased from 450 to 100 nm, thus demonstrating nanoconfinement improvements of gas separation. MD simulations corroborated these experimental observations and indicated a suppression in the sorption of CH4 by [EMIM][SCN] when the IL nanoconfinement length decreased within the nonpolar PVDF surfaces. This is consistent with the experimental observation that the CH4 permeance through the IL confined in nonpolar PVDF is significantly less than the CH4 permeance through the IL confined in a water-wetting polar formulation of PVDF. The potential of mean force calculations further indicated that CO2 has more affinity to the nonpolar PVDF surface than CH4. Also, a charge/density distribution analysis of the IL in the PVDF-confined region revealed a layering of the IL into [EMIM]- and [SCN]-rich regions, where CH4 was preferentially distributed in the former and CO2 in the latter. These molecular insights into the nanoconfinement-driven mechanisms in polymer/IL membranes provide a framework for a better molecular design of such membranes for critical gas separation and CO2 capture applications.

CO2/CH4 mixed-gas separation in PIM-1 at high pressures: Bridging atomistic simulations with process modeling

Polymeric membranes with intrinsic microporosity have been at the center of attention for gas separation applications since the introduction of PIM-1. This study utilizes atomistic simulations to model and to understand the pure- and mixed-gas transport properties of PIM-1 for the CO2/CH4 gas pair. Monte Carlo and molecular dynamics methods were combined in the estimation of sorption and diffusion of CO2 and CH4 in PIM-1. Simulated sorption and permeability data compared very well with experimental reports. Mixed-gas adsorption simulations proved the existence of competitive adsorption, favoring CO2, hence resulting in an increase in solubility selectivities. However, in mixed-gas environment CH4 permeabilities increased significantly compared to pure gas conditions, overall decreasing perm-selectivities of the polymer. Plasticization of the polymer around 25 bar CO2 partial fugacity was apparent both in pure- and mixed-gas conditions. Simulations at different gas feed compositions proved the dependence of competitive sorption and CO2-induced swelling in partial feed gas fugacities. Simulation results were combined to obtain a macroscopic permeability model that relates the multicomponent permeability to the permeate pressure and composition. Accurate estimations of permeabilities by the model were achieved allowing future implementation of the model in process simulation tools.

Sustainable Hydrothermal and Solvothermal Synthesis of Advanced Carbon Materials in Multidimensional Applications: A Review.

The adoption of green technology is very important to protect the environment and thus there is a need for improving the existing methods for the fabrication of carbon materials. As such, this work proposes to discuss, interrogate, and propose viable hydrothermal, solvothermal, and other advanced carbon materials synthesis methods. The synthesis approaches for advanced carbon materials to be interrogated will include the synthesis of carbon dots, carbon nanotubes, nitrogen/titania-doped carbons, graphene quantum dots, and their nanocomposites with solid/polymeric/metal oxide supports. This will be performed with a particular focus on microwave-assisted solvothermal and hydrothermal synthesis due to their favourable properties such as rapidity, low cost, and being green/environmentally friendly. These methods are regarded as important for the current and future synthesis and modification of advanced carbon materials for application in energy, gas separation, sensing, and water treatment. Simultaneously, the work will take cognisance of methods reducing the fabrication costs and environmental impact while enhancing the properties as a direct result of the synthesis methods. As a direct result, the expectation is to impart a significant contribution to the scientific body of work regarding the improvement of the said fabrication methods.

Barium titanate-based thermistors: Past achievements, state of the art, and future perspectives

Barium titanate materials displaying a positive temperature coefficient of resistivity are ubiquitously employed as thermistors in electrical current and voltage control systems, as well as in gas and thermal sensing applications. The positive temperature coefficient of resistivity effect is widely accepted to be a grain boundary-based phenomenon, although detailed studies on grain boundary structure and chemistry, and their influence on the resulting electrical properties, are seriously lacking. Tailoring of the positive temperature coefficient of resistivity electrical characteristics, for specific high-value applications, will require improved understanding and control over grain boundary phenomenon. A comprehensive overview of the development of barium titanate-based positive temperature coefficient of resistivity ceramics is initially presented. We then advance to a discussion on emerging grain boundary characterization techniques, specifically, a stereographic analysis of electron backscatter diffraction data that could assist in enhancing control over BaTiO3 defect chemistry and microstructure, through characterization and subsequent manipulation of the population of grain boundary types. These techniques have great potential for increasing the understanding of the delicate interplay between processing conditions, chemistry, microstructure, and functional electrical properties, and are relevant to the development of advanced, high-performance ceramics and electroceramics in general. Contemporary advancements in the field, such as lead-free positive temperature coefficient of resistivity effect materials and multilayer miniaturized systems based on hypostoichiometric barium compositions, are reviewed. Finally, perspectives on future lines of thermistor research, with a focus on the energy sector, are presented including applications in gas separation and chemical sensing.

Metal-Organic Frameworks for Metal-Ion Batteries: Towards Scalability

Metal-organic frameworks (MOFs), being a family of highly crystalline and porous materials, have attracted particular attention in material science due to their unprecedented chemical and structural tunability. Next to their application in gas adsorption, separation, and storage, MOFs also can be utilized for energy transfer and storage in batteries and supercapacitors. Based on recent studies, this review describes the latest developments about MOFs as structural elements of metal-ion battery with a focus on their industry-oriented and large-scale production.

Molecular insights into the separation mechanism of imidazole-based ionic liquid supported membranes

Recently, imidazole-based ionic liquid supported membranes have found wide application in gas separation and purification, deriving of their good solubility for CO2 and their ultrathin membrane thickness. Our previous works preliminarily showed that three primary interactions, including the interaction between passing gas and ionic liquid, the interaction between anion and cation of ionic liquid, as well as the interaction between ionic liquid and supporting membrane, play crucial roles determining the gas permeability and CO2 selectivity over N2. However, a systematic discussion of these three interactions to disclose the underlying separation mechanism is lack. In this study, adopting molecular dynamic (MD) simulation and density functional theory (DFT) calculation, three ionic liquids including of [BMIM][BF4], [BMIM][PF6] and [BMIM][Tf2N] with high CO2 solubility are selected as research objects, these three interactions were discussed to reveal the gas separation mechanism in MoS2-SILM. Simulation results indicate that the interaction between gas molecules and ILs determines the gas solubility and selectivity, and the [BMIM][BF4] shows superior separation performance over other two ILs. In addition, the interaction between ILs and MoS2 sheets, together with the interaction between cation and anion of ILs, determine the layered structure of confined ILs, which is preferential for gas transport. Furthermore, the gas permeability difference between confined ILs and bulk ILs, and the influence of interlayer spacing were also discussed. Our study provides fundamental insight to the underlying gas separation mechanism and beneficial guidelines for device design.

Research and Development Journey and Future Trends of Hollow Fiber Membranes for Purification Applications (1970-2020): A Bibliometric Analysis.

Hollow fiber membrane (HFM) technology has received significant attention due to its broad range separation and purification applications in the industry. In the current study, we applied bibliometric analysis to evaluate the global research trends on key applications of HFMs by evaluating the global publication outputs. Results obtained from 5626 published articles (1970–2020) from the Scopus database were further manipulated using VOSviewer software through cartography analysis. The study emphasizes the performance of most influential annual publications covering mainstream journals, leading countries, institutions, leading authors and author’s keywords, as well as future research trends. The study found that 62% of the global HFM publications were contributed by China, USA, Singapore, Japan and Malaysia, followed by 77 other countries. This study will stimulate the researchers by showing the future-minded research directions when they select new research areas, particularly in those related to water treatment, biomedical and gas separation applications of HFM.

Various morphologies of clinoptilolites synthesized in alcohol-solvent hydrothermal system and their selective adsorption of CH4 and N2

Clinoptilolite (CP) with various morphologies, such as spherical, flower, cylindrical, and disc-shape, were successfully synthesized in the presence of alcohol solvents with different carbon chain lengths. The structural and textural properties of the resultant CPs were characterized via XRD, SEM, TG-DSC profiles, TEM, NMR, FT-IR spectra, and N 2 sorption isotherms. The effect of the additive amount of various alcohol solvents on the induction (nucleation) periods and growth stages of synthetic CPs were detailed investigated. Their activation energies of induction ( E n ) and growth ( E g ) periods during crystallization intervals were calculated according to the Arrhenius equation, where the former being larger than the latter, indicating that nucleation is the key step during the whole synthesis of CPs. Particularly, the E n value gradually increased as the increasing chain-length of the used alcohol, but smaller than that without additive alcohols. These demonstrations implied that the presence of alcohol solvents is beneficial to promote the nucleation process of synthetic CP. Furthermore, their CH 4 and N 2 adsorption capacity and selectivity were preliminary explored, showing the highest selective factor of 5.58 at 0 °C for CH 4 /N 2 separation using flower-like CP as an adsorbent. These results suggested a promising adsorbent for wide applications in gas separation. • Alcohol-modified hydrothermal method for synthesis of clinoptilolite was proposed. • Influence of aliphatic alcohols on morphology of clinoptilolites was explored. • Crystalline kinetic for induction and growth stage in clinoptilolite was studied. • Application of synthetic clinoptilolite in CH 4 /N 2 separation was investigated.

A triptycene derived hypercrosslinked polymer for gas capture and separation applications

AbstractThis work describes facile synthesis of a porous polymeric material (T‐HCP) using readily available reagents. Specifically, T‐HCP is a thermally stable and hypercrosslinked polymer (HCP) that is essentially microporous with a high BET specific surface area (940 m2 g−1). Triptycene based polymers are known to feature internal free volume. Thus, the incorporation of triptycene units and extensive crosslinking by an external cross‐linker in T‐HCP makes it a promising adsorbent for small gas capture applications. Experimental results show that T‐HCP demonstrated good CO2 capture capacity of 132 mg g−1 (273 K, 1 bar). Molecular hydrogen storage capacity of T‐HCP is estimated to be 17.7 mg g−1 (77 K, 1 bar). T‐HCP revealed high CO2/N2 selectivity (up to 63) as well as promising CO2/CH4 (up to 9.1) selectivity suggesting its potential applicability for CO2 separation from flue and natural gases.

Cephalopods-Inspired Rapid Self-Healing Nanoclay Composite Coatings with Oxygen Barrier and Super-Bubble-Phobic Properties.

Polymeric coatings with oxygen barrier properties are an important technology in food packaging that can extend the shelf life of food products and reduce waste. Although a typical technology in practical use is the deposition of metal or inorganic materials between multilayer films to reduce the oxygen transmission rate, once the film is damaged, oxygen permeates through the damaged area, damaging the packaged food. In addition, nanobrick wall structures consisting of nanoplatelet bricks have the potential to replace barrier films made of inorganic materials; however, they similarly lack repair performance or have slow repair speed despite having repair performance. Inspired by the rapid self-repair mechanism of cephalopods, the study develops a nanoclay-containing coating that can rapidly repair surface damage via water within 10 s. By introducing CaCl2-derived counterions and montmorillonite for nanobrick wall structures into polyelectrolyte multilayers stacked by layer-by-layer self-assembly, the noncovalent polymer network is increased, resulting in mimicking a strong cephalopod-derived β-sheet structure and noncovalent intermolecular interactions derived from cephalopods. The high water retention at the surface showed super-bubble-phobicity in water and inhibited gas permeation. The oxygen permeability of the coatings with more than a certain amount of montmorillonite was less than 1/100 of that of bare polyethylene. The ultrafast self-healing gas barrier coating has the potential to be used not only for food products but also for electronics and pharmaceutical packaging and gas separation applications. The key technology developed in this study provides novel insights into the construction of self-healing membranes made of composite materials and will contribute to the formation of a sustainable society.

An electrostatic repulsion strategy construct ZIFs based liquids with permanent porosity for efficient CO2 capture

Porous liquids (PLs), as a novel liquid sorbent with permanent porosity, still present challenges such as high viscosity and melting temperatures. Herein, an efficient electrostatic repulsion strategy is proposed to engineer zeolitic imidazolate frameworks (ZIFs) type III PLs employing ZIF-8-g-BPEI nanoparticle stability dispersed into BPEI. The viscosity and melting temperatures of ZIF-8-g-BPEI PLs could be tailored via control particle size, grafting polymer, and concentration of porous nanoparticles. The existence of BPEI on the surface of ZIF-8-g-BPEI facilitates the stable dispersion of ZIFs nanoparticles in BPEI via the electrostatic repulsion as well as enhances CO2 adsorption capacity of PLs. Moreover, the CO2 adsorption amount of ZIF-8-g-BPEI-30 PLs is 6.5 times high than that of neat BPEI, suggesting the permanent free volume of ZIFs is retained in their corresponding PLs. Benefiting from the branched structure of BPEI grafted onto the outer surface of ZIFs , ZIF-8-g-BPEI-10 PLs remain liquified down to -71.1 °C, together with viscosity as 1.7 Pa·s−1 at 25 °C. Therefore, the electrostatic repulsion strategy opens an effective avenue to prepare novel liquid materials and shows numerous potential applications in gas adsorption and separation.

Metal–Organic Framework Nanosheets: Programmable 2D Materials for Catalysis, Sensing, Electronics, and Separation Applications

Metal–organic framework nanosheets (MONs) have recently emerged as a distinct class of 2D materials with programmable structures that make them useful in diverse applications. In this review, the breadth of applications that have so far been investigated are surveyed, thanks to the distinct combination of properties afforded by MONs. How: 1) The high surface areas and readily accessible active sites of MONs mean they have been exploited for a variety of heterogeneous, photo‐, and electro‐catalytic applications; 2) their diverse surface chemistry and wide range of optical and electronic responses have been harnessed for the sensing of small molecules, biological molecules, and ions; 3) MONs tunable optoelectronic properties and nanoscopic dimensions have enabled them to be harnessed in light harvesting and emission, energy storage, and other electronic devices; 4) the anisotropic structure and porous nature of MONs mean they have shown great promise in a variety of gas separation and water purification applications; are discussed. The aim is to draw links between the uses of MONs in these different applications in order to highlight the common opportunities and challenges presented by this promising class of nanomaterials.

Efficient CO2 Separation Using a PIM‐PI‐Functionalized UiO‐66 MOF Incorporated Mixed Matrix Membrane in a PIM‐PI‐1 Polymer

AbstractMetal–organic framework (MOF) incorporated mixed–matrix membranes (MMMs) attract great interest for gas separation applications because they overcome limitations faced by typical polymer membranes, including permeability–selectivity trade‐off, aging effect, and plasticization phenomenon. However, optimal MOF–polymer interface compatibility is the key challenge in fabricating defect‐free high‐performance gas‐separation MMMs. Here, a surface modification strategy of the UiO‐66‐NH2 MOF using a covalently bound PIM‐PI‐oligomer is developed to engineer interface compatibility with the polymer that has an identical chemical structure (PIM‐PI‐1) in the MMMs. A series of MMMs are prepared with different loadings of homogeneously distributed PIM‐PI‐functionalized MOFs (PPM). Significant improvements in CO2/N2 and CO2/CH4 selectivity and permeability are achieved with these MMMs, ranging from 5 to 10 wt% of the PPM loadings. The MMM with 10 wt% loading (PPM‐10@MMM) shows a CO2 permeability of 3827.3 Barrer and a CO2/N2 and CO2/CH4 selectivity of 24 and 13.4, respectively. This surpasses the 2008 Robeson upper bound for CO2/N2 and is very close to the 2008 upper bound for CO2/CH4. The experimental results are further compared using Maxwell's equation for MMMs. The resulting MMMs show a plasticization resistance against CO2 up to 25 atm pressure and anti‐aging performance for 180 h.

Synthesis and gas separation properties of polyimide membranes derived from oxygencyclic pseudo-Tröger's base

Two neoteric oxygencyclic pseudo-Tröger's base (TB) -derived naphthalene diamines characterized by large π conjugated naphthalene rings and half-open cavity structure unit, 16H-8,16-methanodinaphtho [2,1-d:1′,2′-g] (Yampolskii, 2012; Maqsood et al., 2014) [1,3]dioxocine-3,13-diamine (OTB) and its furan-based bicyclic analogue, 7a, 14c-dihydronaphtho [( Xu et al., 2014; Yampolskii, 2012) 2,12,1-b]naphth [1′,2':4,5]furo [( Maqsood et al., 2014; Xu et al., 2014) 3,23,2-d] furan-3,12- diamine (FB), were successfully synthesized. Their 6FDA-based polyimides (6FDA-OTB and 6FDA-FB) were obtained for membrane-based gas separation study. 6FDA-OTB and 6FDA-FB exhibited good solubility and excellent thermal stability due to the presence of large π conjugated aromatic rings and half-open cavity structure unit. The two polymer membranes showed high gas pair selectivity and modest gas permeability as well as good physical aging properties. The 6FDA-FB membrane performed a high selectivity of H2/CH4 (104.0) and CO2/CH4 (49.1), which arises from the larger permeation activation energy difference of different gases. The performances of 100 days aged 6FDA-OTB and 6FDA-FB membranes are almost overlapped with the 1991 Robeson upper bounds for CO2/CH4 and H2/CH4. In short, introducing the novel rigid oxygencyclic pseudo-Tröger's base into the polyimide chains could make attractive membrane materials for various gas separation applications.

Amination of ether-linked polymers via the application of Ullmann-coupling reaction: synthesis, characterization, porosity, and thermal stability evaluation

In this report, an open-chain ether-linked polymer (PE-AF) has been prepared via one-step co-condensation polymerization of 1,4-dibromo-2,5-difluorobenzene with bisphenol AF. The new polymer has shown a good solubility in non-polar solvents, good thermal stability (up to 300 °C), high char residue with a limiting oxygen index (LOI) of 33.5, surface morphology with random fluffy-like texture, and a non-porosity nature with a surface area (SA) of 35 m2g−1. Tailoring these properties has been achieved by considering post-modification on the Bromo-sites of the polymer backbone. The post-modification was attained by applying Ullmann-coupling reaction of the Bromo-sites of PE-AF and aromatic diamines in presence of a catalytic amount of copper iodide. Upon inclusion of the aromatic diamines within the polymer’s backbone, the new polymers have depicted aggregated surface morphology (e.g. spherical particles ca. 0.3–0.1 µm), pronounced porosity nature (SA up to 354 m2g−1), varied range of pore size distribution, and higher thermal stability with lower char residue contents when compared with PE-AF. The successful production of the polymers was confirmed by (C, H, O, and N) elemental analysis, solid-state 13C-NMR, and infrared (IR) spectroscopy. The application of Ullmann-coupling raises the opportunity to create cavities and pores within the polymer’s framework. Though this modification lowered the LOI of the polymers and their flame-retardant tendency, it enhanced their opportunity for gas capture and separation applications.

Synthesis of ZIF-11 Membranes: The Influence of Preparation Technique and Support Type.

Due to its structural features, ZIF-11 is one of the most interesting materials for gas separation applications. Herein, we report a systematic study on the synthesis of ZIF-11 as a supported membrane. For this, we adapted optimized conditions for the ZIF-11 powder synthesis, identified in our previous works, to form ZIF layers on symmetric and asymmetric stainless-steel and asymmetric α-Al2O3 supports. Different techniques were investigated for the challenging layer formation, namely, in situ crystallization (ISC), multiple in situ crystallization (MISC), and the seeding and secondary growth (SSG) method. It was possible to deposit ZIF-11 on different supports by ISC and MISC, although it was difficult to obtain complete layers. SSG, in turn, was more effective in forming dense and well-intergrown ZIF-11 layers. This agrees well with the generally accepted fact that seeding considerably facilitates layer formation. Systematic studies of both individual steps of SSG (seeding and secondary growth) led to a basic understanding of layer formation of ZIF-11 on the different supports. The best membranes prepared by rub seeding and secondary growth achieved Knudsen selectivity. Improved gas separation performance is expected if the formation of defects can be avoided.

Unprecedented gas separation performance of ITTB/CNT nanocomposite membranes at low temperature by strong interfacial interaction enhanced rigidity

One of the biggest challenges in mixed matrix membranes (MMM) for gas separation applications is how to achieve homogeneous nanocomposite membranes with excellent separation performance. Here, for the first time, this goal can be achieved by novel MMMs fabricated from a highly microporous Tröger's base (TB) ladder polymer matrix (ITTB) containing triptycene as contortion center, and COOH functionalized single wall carbon nanotubes (CNT) as filler ranging from 0.6 to 2.0 wt%. By the strong acid-base interaction between COOH (from CNT) and tertiary-amine (from TB) as well as π-π interaction between the triptycene unit and CNT skeleton, the resulting MMMs with good compatibility were confirmed by Raman spectroscopy and SEM image analysis. The MMMs with 1.0 wt% CNT loading (P-CNT-1.0) exhibited both higher O2 permeability (701 vs 504 Barrer) and O2/N2 selectivity (5.79 vs 5.54) than the pristine ITTB, and the aO2/N2 further increased to 7.28 after 100 days aging. Additionally, the P-CNT-1.0 also showed an unprecedented pure-gas aO2/N2 of 11.7 combined with a PO2 of 129 Barrer at −20 °C. An exceptional high mixed-gas O2/N2 selectivity of 10.1 was also observed for P-CNT-1.0 at −20 °C and 20 bar upstream pressure, whereas the pristine ITTB plasticized at 2 bar. The higher performance of P-CNT-1.0 is due to the strong interaction between the CNT and ITTB further enhanced the rigidity of ITTB main chain. The 1.0 wt% CNT loading into ITTB simultaneously enhanced the permeability, selectivity and stability, which provides great potential for PIM-based MMMs in air separation applications.

Thermodynamic, Thermal, and Structural Stability of Bimetallic MIL-53 (Al1–xCrx)

Understanding the stability of porous materials, especially metal–organic frameworks (MOFs), is central to defining their applications in gas storage, separation, and catalysis. Herein, integrating high-temperature drop combustion calorimetry as well as simultaneous thermal and in situ structural analyses, we performed a comprehensive study on the thermodynamic, thermal, and structural stabilities of MOF in air. A family of MIL-53 (Al1–xCrx) with systematically tuned metal contents was intentionally chosen considering their unique property that H2BDC species serve as both coordinated linkers and guest species confined. The results suggest that as temperature increases, all samples underwent (1) a phase transition from the pore-filled Pnma to the Imma with empty pores, (2) structural degradation, and (3) complete oxidation (burning). At the same temperature, as the chromium (Cr) content increases, the thermal and structural stability of MIL-53 (Al1–xCrx) in air decreases. In contrast, interestingly, the intrinsic thermodynamic stability systematically increases as a function of Cr content, evidenced by the more exothermic enthalpies of formation, ranging from 92.8 ± 73.4 kJ/mol (slightly metastable) to −1593.2 ± 60.8 kJ/mol (stable). Such a phenomenon is likely due to enhanced H2BDC–MIL-53 guest–host interactions upon Cr substitution, which energetically neutralize the metastability of MIL-53 open frameworks. This study highlights that the thermal, structural, and energetic stabilities are different and have equal importance in governing the synthesis and applications of MOFs.