Low Adsorption Enthalpies Research Articles

Asymmetrical Modification of Cyclopentadienyl Cobalt in Eu-MOF for C2H2/CO2 Separation.

The development of novel adsorption materials is of significance for the efficient and low-energy purification of acetylene (C2H2). Emerging metal-organic framework (MOF) adsorbents demonstrate great application prospects in the field of gas adsorption and separation. Herein, we synthesized a Eu-MOF asymmetrically modified with cyclopentadienyl cobalt exhibiting two different types of cages, denoted as UPC-119. Adsorption isotherms and dynamic breakthrough curves confirm its potential in C2H2/CO2 separation, which is further evidenced by theoretical simulations. The high adsorption capacity and low adsorption enthalpy render UPC-119 as a promising adsorbent for C2H2/CO2 separation with ease of regeneration.

Solvent-Induced In(III)-MOFs with Controllable Interpenetration Degree Performing High-Efficiency Separation of CO2/N2 and CO2/CH4.

Herein, three In(III)-based metal-organic frameworks (In-MOFs) with different degrees of interpenetration (DOI), namely In-MOF-1, In-MOF-2, and In-MOF-3, constructed by In3+ and Y-shaped ligands 4,4',4″-s-triazine-2,4,6-triyltribenzoate (H3TATB), are successfully synthesized through the ionothermal/solvothermal method. Subsequently, three novel In-MOFs, including noninterpenetration polycatenation, 2-fold interpenetrated, and 4-fold interpenetrated structure, are employed as the platform for systematically investigating the separation efficiency of CO2/N2, CO2/CH4, and CO2/CH4/N2 mixture gas system. Among them, In-MOF-2 shows the highest CO2 uptake capacities at 298 K and simultaneously possesses the low adsorption enthalpy of CO2 (26.4 kJ/mol at low coverage), a feature desirable for low-energy-cost adsorbent regeneration. The CO2/N2 (v: v = 15/85) selectivity of In-MOF-2 reaches 37.6 (at 298 K and 1 bar), also revealing outstanding selective separation ability from flue gases and purifying natural gas, affording a unique robust separation material as it has moderate DOI and pore size. In-MOF-2 shows exceptional stability and feasibility to achieve reproducibility. Aperture adjustment makes In-MOF-2 a versatile platform for selectively capturing CO2 from flue gases or purifying natural gas.

Pore-structure control in ultramicroporous porous coordination polymers for efficient selective capture of acetylene

Physical adsorption separation of acetylene (C2H2) from carbon dioxide (CO2) on porous coordination polymers is a promising technique. However, it is still a particular challenge to realize the practical separation application of porous coordination polymers (PCPs) adsorbents with high separation performance, moderate adsorption heat, high stability and low cost. Herein, we demonstrate the efficient adsorption and separation of C2H2/CO2 from aspartate/malate anions columnar PCPs (namely PCPs-asp, PCPs-mal and PCPs-mal-n) exploiting the hydroxyl/amino sites within fine-tuning pore structures. Its one-dimensional (1D) chains and abundant hydroxyl/amino functional sites matched with target molecules, affording these anions columnar PCPs with not only high separation selectivity for C2H2/CO2, but also excellent gas capacity. Among them, PCPs-mal possesses an ultra-high C2H2 storage density of 655 g·L−1 under ambient conditions, and even close to the density of solid C2H2 at 189 K (729 g·L−1). PCPs-mal exhibits an IAST separation selectivity as high as 15.2 for the equimolar C2H2/CO2 binary mixture. The breakthrough experiments demonstrate that PCPs-mal realize efficient binary C2H2/CO2 separation with an acetylene dynamic adsorption capacity of 2.03 mmol·g−1. Theoretical calculations and GCMC simulations further provide critical insight into the separation mechanism at the molecular level. More importantly, the facile scaled synthesis, low adsorption enthalpy, and high separation performance make it cost-effective for real-world applications.

Fe-MOF with U-Shaped Channels for C2H2/CO2 and C2H2/C2H4 Separation.

The development of high-performance adsorbents is critical for the low-energy separation of acetylene. Herein, we synthesized an Fe-MOF (MOF, metal-organic framework) with U-shaped channels. The adsorption isotherms of C2H2, C2H4, and CO2 show that the adsorption capacity of acetylene is significantly larger than that of the other two gases. Meanwhile, the actual separation performance was verified by breakthrough experiments, indicating the potential to separate C2H2/CO2 and C2H2/C2H4 mixtures at normal temperatures. Grand Canonical Monte Carlo (GCMC) simulation demonstrates that the framework with U-shaped channels interacts more strongly with C2H2 than with C2H4 and CO2. The high C2H2 uptake and low adsorption enthalpy highlight Fe-MOF as a promising candidate for C2H2/CO2 separation with a low regeneration energy.

Optimizing Acetylene Sorption through Induced‐fit Transformations in a Chemically Stable Microporous Framework

AbstractDeveloping practical storage technologies for acetylene (C2H2) is important but challenging because C2H2 is useful but explosive. Here, a novel metal–organic framework (MOF) (FJI‐H36) with adaptive channels was prepared. It can effectively capture C2H2 (159.9 cm3 cm−3) at 1 atm and 298 K, possessing a record‐high storage density (561 g L−1) but a very low adsorption enthalpy (28 kJ mol−1) among all the reported MOFs. Structural analyses show that such excellent adsorption performance comes from the synergism of active sites, flexible framework, and matched pores; where the adsorbed‐C2H2 can drive FJI‐H36 to undergo induced‐fit transformations step by step, including deformation/reconstruction of channels, contraction of pores, and transformation of active sites, finally leading to dense packing of C2H2. Moreover, FJI‐H36 has excellent chemical stability and recyclability, and can be prepared on a large scale, enabling it as a practical adsorbent for C2H2. This will provide a useful strategy for developing practical and efficient adsorbents for C2H2 storage.

Optimizing Acetylene Sorption through Induced-fit Transformations in a Chemically Stable Microporous Framework.

Developing practical storage technologies for acetylene (C2 H2 ) is important but challenging because C2 H2 is useful but explosive. Here, a novel metal-organic framework (MOF) (FJI-H36) with adaptive channels was prepared. It can effectively capture C2 H2 (159.9 cm3 cm-3 ) at 1 atm and 298 K, possessing a record-high storage density (561 g L-1 ) but a very low adsorption enthalpy (28 kJ mol-1 ) among all the reported MOFs. Structural analyses show that such excellent adsorption performance comes from the synergism of active sites, flexible framework, and matched pores; where the adsorbed-C2 H2 can drive FJI-H36 to undergo induced-fit transformations step by step, including deformation/reconstruction of channels, contraction of pores, and transformation of active sites, finally leading to dense packing of C2 H2 . Moreover, FJI-H36 has excellent chemical stability and recyclability, and can be prepared on a large scale, enabling it as a practical adsorbent for C2 H2 . This will provide a useful strategy for developing practical and efficient adsorbents for C2 H2 storage.

The Consequences of Water Interactions with Nitrogen-Containing Carbonaceous Quantum Dots-The Mechanistic Studies.

Despite the importance of quantum dots in a wide range of biological, chemical, and physical processes, the structure of the molecular layers surrounding their surface in solution remains unknown. Thus, knowledge about the interaction mechanism of Nitrogen enriched Carbonaceous Quantum Dots' (N-CQDs) surface with water-their natural environment-is highly desirable. A diffusive and Stern layer over the N-CQDs, characterized in situ, reveals the presence of anionic water clusters [OH(H2O)n]-. Their existence explains new observations: (i) the unexpectedly low adsorption enthalpy (ΔHads) in a pressure range below 0.1 p/ps, and ΔHads being as high as 190 kJ/mol at 0.11 p/ps; (ii) the presence of a "conductive window" isolating nature-at p/ps below 0.45-connected to the formation of smaller clusters and increasing conductivity above 0.45 p/ps, (iii) Stern layer stability; and (iv) superhydrophilic properties of the tested material. These observables are the consequences of H2O dissociative adsorption on N-containing basic centers. The additional direct application of surfaces formed by N-CQDs spraying is the possibility of creating antistatic, antifogging, bio-friendly coatings.

A Microporous Metal-Organic Framework with Channels Constructed from Nonpolar Aromatic Rings for the Selective Separation of Ethane/Ethylene Mixtures.

The separation of ethane and ethylene is an important segment in the purification of chemical raw materials in industrial production. However, due to their similar physical and chemical properties, the separation of C2 H6 /C2 H4 is challenging. Herein, we report the selective adsorption of ethane over ethylene by a microporous metal-organic framework with nonpolar aromatic rings constructed channels, [Co1.5 (TATB)(H2 O)0.5 ] ⋅ 5DMA ⋅ 3H2 O (Co-TATB, H3 TATB=4,4',4''-(s-triazine-2,4,6-triyl) tribenzoic acid). This compound showed a higher ethane capacity than that of ethylene, and a low adsorption enthalpy of ethane only of 19.4 kJ mol-1 . Further, the dynamic breakthrough experimental confirmed that Co-TATB can selectively adsorb ethane from ethane/ethylene separation.

Adsorption properties of iodine on fused silica surfaces when evaporated from tellurium in various atmospheres

The evaporation of iodine containing species from tellurium has been investigated together with their adsorption behavior on a fused silica surface. In inert gas, the formation of two species was observed with adsorption enthalpies of around − 90 to − 100 and − 110 to − 120 kJ/mol, respectively. For reducing environments, a single species identified as monatomic iodine was observed with an adsorption enthalpy around − 95 kJ/mol. In oxidizing conditions, species with low adsorption enthalpies ranging from − 65 to − 80 kJ/mol were observed. Presumably, these are iodine oxides as well as oxo-acids of iodine (HIOx). The results of the thermochromatography experiments performed here prove the usefulness of the employed production method for carrier-free iodine isotopes and enhance the understanding of the evaporation and deposition behavior of iodine under various chemical conditions.

Adsorption properties, the pH-sensitive release of 5-fluorouracil and cytotoxicity studies of mesoporous silica drug delivery matrix

Mesoporous silica materials were investigated as the carriers for pH-sensitive drug delivery systems. Porous silica SBA-15 was first functionalized by anchoring N-[3-(trimethoxysilyl) propyl] aniline groups on the surface. After loading of an antineoplastic agent 5-fluorouracil, the pores were capped by β-cyclodextrin molecules. The studied samples were characterized by N2 adsorption/desorption measurements, thermal analysis, powder X-Ray Diffraction, and Transmission Electron Microscopy. Adsorption properties of 5-fluorouracil were explored via the construction of adsorption isotherms. The amount of 5-FU adsorbed on amine-functionalized SBA-15 was 60 mg per 1 g of solid. The adsorption of 5-fluorouracil on silica was also monitored by microcalorimetry, showing low adsorption enthalpies. Drug release properties from matrices were studied using UV–Visible spectroscopy with the un-blocked and blocked pores configuration to demonstrate the efficiency of the pH-responsive nanovalves and evaluated using different kinetic models. It was shown that no drug release occurred at neutral pH and that more than 80% of drug adsorbed amount was released at pH = 5. Working at the equilibrium, the initial burst of the drug from the silica surface, usually observed in other porous silica drug delivery systems, was avoided. The interaction between the β-cyclodextrin molecules and grafted amine functions were also studied as a function of pH. Finally, the cytotoxicity tests were performed using human glioma U87 MG cells.

Selective Low-Energy Carbon Dioxide Adsorption Using Monodisperse Nitrogen-Rich Hollow Carbon Submicron Spheres.

Monodisperse, nitrogen-doped hollow carbon spheres of submicron size were synthesized using hexamethoxymethylmelamine as both a carbon and nitrogen source in a short (1 h) microwave-assisted synthesis. After carbonization at 550 °C, porous carbon spheres with a remarkably high nitrogen content of 37.1% were obtained, which consisting mainly of highly basic pyridinic moieties. The synthesized hollow spheres exhibited high selectivity for carbon dioxide (CO2) over nitrogen and oxygen gases, with a capture capacity up to 1.56 mmol CO2 g-1. The low adsorption enthalpy of the synthesized hollow carbon spheres permits good adsorbent regeneration. Evaluation of the feasibility of scaling up shows their potential for large-scale applications.

Adsorption Performance Indicator to Screen Carbon Adsorbents for Post-combustion CO2 Capture

This work is the result of the collaboration between INCAR-CSIC (Spain) and CSIRO Energy (Australia) in the framework of the European Commission FP7 project HiPerCap (High Performance Capture) that started in January 2014. The HiPerCap project aims to develop novel post-combustion CO2 capture technologies and processes which are environmentally benign and have high potential to lead to breakthroughs in energy consumption and overall cost, involving all main separation technologies for post-combustion CO2 capture. The present contribution focuses on adsorption with low temperature solid sorbents as a promising technology to capture CO2 from the flue gas. More precisely, CSIRO and CSIC have developed CO2 adsorbents from different carbon precursors including petroleum pitch, phenolic resin and agricultural by-products.The performance of new materials can be evaluated either experimentally or by means of numerical simulations, but each methodology has drawbacks. Thus, it is of utmost importance to seek a simple method for evaluating and comparing materials on the basis of readily available adsorption data. It is well known that the ideal adsorbent has a high selectivity, a high working capacity, and a low adsorption enthalpy; however, in practice, it is very rare to find a material which combines all of these attributes. Therefore, the selection of the adsorbent will often involve a trade-off among these factors, making comparisons of varied adsorbents even more difficult. In this work, the potential of the activated carbons developed by INCAR-CSIC and CSIRO for a post-combustion CO2 capture process has been evaluated and compared by means of an adsorption performance indicator.

From Discrete Molecular Cages to a Network of Cages Exhibiting Enhanced CO2 Adsorption Capacity

AbstractWe have adopted the concept of “cage to frameworks” to successfully produce a Na–N connected coordination networked cage Na‐NC1 by using a [3+6] porous imine‐linked organic cage NC1 (Nanjing Cage 1) as the precursor. It is found that Na‐NC1 exhibits hierarchical porosity (inherent permanent voids and interconnected channel) and gas sorption measurements reveal a significantly enhanced CO2 uptake (1093 cm3 g−1 at 23 bar and 273 K) than that of NC1 (162 cm3 g−1 under the same conditions). In addition, Na‐NC1 exhibits very low CO2 adsorption enthalpy making it a good candidate for porous materials with both high CO2 storage and low adsorption enthalpy.

From Discrete Molecular Cages to a Network of Cages Exhibiting Enhanced CO2 Adsorption Capacity.

We have adopted the concept of "cage to frameworks" to successfully produce a Na-N connected coordination networked cage Na-NC1 by using a [3+6] porous imine-linked organic cage NC1 (Nanjing Cage 1) as the precursor. It is found that Na-NC1 exhibits hierarchical porosity (inherent permanent voids and interconnected channel) and gas sorption measurements reveal a significantly enhanced CO2 uptake (1093 cm3 g-1 at 23 bar and 273 K) than that of NC1 (162 cm3 g-1 under the same conditions). In addition, Na-NC1 exhibits very low CO2 adsorption enthalpy making it a good candidate for porous materials with both high CO2 storage and low adsorption enthalpy.

1D-2D-3D Transformation Synthesis of Hierarchical Metal-Organic Framework Adsorbent for Multicomponent Alkane Separation.

A new hierarchical MOF consisting of Cu(II) centers connected by benzene-tricarboxylates (BTC) is prepared by thermoinduced solid transformation of a dense CuBTC precursor phase. The mechanism of the material formation has been thoroughly elucidated and revealed a transformation of a ribbon-like 1D building unit into 2D layers and finally a 3D network. The new phase contains excess copper, charge compensated by systematic hydroxyl groups, which leads to an open microporous framework with tunable permanent mesoporosity. The new phase is particularly attractive for molecular separation. Energy consumption of adsorptive separation processes can be lowered by using adsorbents that discriminate molecules based on adsorption entropy rather than enthalpy differences. In separation of a 11-component mixture of C1-C6 alkanes, the hierarchical phase outperforms the structurally related microporous HKUST-1 as well as silicate-based hierarchical materials. Grand canonical Monte Carlo (GCMC) simulation provides microscopic insight into the structural host-guest interaction, confirming low adsorption enthalpies and significant entropic contributions to the molecular separation. The unique three-dimensional hierarchical structure as well as the systematic presence of Cu(II) unsaturated coordination sites cause this exceptional behavior.

Adsorption equilibria of CO2 and CH4 in cation-exchanged zeolites 13X

Ion-exchange with different cations (Na+, NH4 +, Li+, Ba2+ and Fe3+) was performed in binderless 13X zeolite pellets. Original and cation-exchanged samples were characterized by thermogravimetric analysis coupled with mass spectrometry (inert atmosphere), X-ray powder diffraction and N2 adsorption/desorption isotherms at 77 K. Despite the presence of other cations than Na (as revealed in TG-MS), crystalline structure and textural properties were not significantly altered upon ion-exchange. Single component equilibrium adsorption isotherms of carbon dioxide (CO2) and methane (CH4) were measured for all samples up to 10 bar at 298 and 348 K using a magnetic suspension balance. All of these isotherms are type Ia and maximum adsorption capacities decrease in the order Li > Na > NH4–Ba > Fe for CO2 and NH4–Na > Li > Ba for CH4. In addition to that, equilibrium adsorption data were measured for CO2/CH4 mixtures for representative compositions of biogas (50 % each gas, in vol.) and natural gas (30 %/70 %, in vol.) in order to assess CO2 selectivity in such scenarios. The application of the Extended Sips Model for samples BaX and NaX led to an overall better agreement with experimental data of binary gas adsorption as compared to the Extended Langmuir Model. Fresh sample LiX show promise to be a better adsorption than NaX for pressure swing separation (CO2/CH4), due to its higher working capacity, selectivity and lower adsorption enthalpy. Nevertheless, cation stability for both this samples and NH4X should be further investigated.

Methanol wetting enthalpy on few-layer graphene decorated hierarchical carbon foam for cooling applications

Vertical few-layer thick graphene petals are grown on macro-porous carbon foam surfaces having an intrinsic open porosity of 75%. This provides a hierarchical porous structure with a potential for surface adsorption/desorption or wetting/dewetting based thermal energy storage applications. Carbon foams have a combined advantage of large surface area and high thermal conductivity critical for thermal energy storage, but they are prone to oxidation and exhibit low adsorption enthalpies for lightweight hydrocarbons. Here we report graphene petal decoration of carbon foam surfaces and subsequent chemical modification through boron nitride incorporation in hexagonal carbon planes of both carbon foams and graphene petals. This chemically reactive hierarchical structure is characterized with FESEM, Raman, XRD, and XPS measurements. Methanol wetting enthalpy of this three-dimensional hierarchical material was measured with a solution calorimeter, and had shown a six fold increase (from 78 to 522J/g of foam) as compared to the carbon foam prior to the surface modification. Influences of petal decoration on the surface morphology of carbon foam, BN chemical modification, structure and stoichiometry of the hierarchical material surface, and methanol wetting enthalpy improvement are discussed in detail. The applicability of this hierarchical porous material for thermal energy applications is established.

High Performance Hydrogen Storage from Be-BTB Metal–Organic Framework at Room Temperature

The metal-organic framework beryllium benzene tribenzoate (Be-BTB) has recently been reported to have one of the highest gravimetric hydrogen uptakes at room temperature. Storage at room temperature is one of the key requirements for the practical viability of hydrogen-powered vehicles. Be-BTB has an exceptional 298 K storage capacity of 2.3 wt % hydrogen. This result is surprising given that the low adsorption enthalpy of 5.5 kJ mol(-1). In this work, a combination of atomistic simulation and continuum modeling reveals that the beryllium rings contribute strongly to the hydrogen interaction with the framework. These simulations are extended with a thermodynamic energy optimization (TEO) model to compare the performance of Be-BTB to a compressed H2 tank and benchmark materials MOF-5 and MOF-177 in a MOF-based fuel cell. Our investigation shows that none of the MOF-filled tanks satisfy the United States Department of Energy (DOE) storage targets within the required operating temperatures and pressures. However, the Be-BTB tank delivers the most energy per volume and mass compared to the other material-based storage tanks. The pore size and the framework mass are shown to be contributing factors responsible for the superior room temperature hydrogen adsorption of Be-BTB.

An rht type metal–organic framework based on small cubicuboctahedron supermolecular building blocks and its gas adsorption properties

A metal–organic framework NPC-5 was synthesized via reaction of a methyl-functionalized ligand 2,4,6-trimethyl benzene-1,3,5-triyl-isophthalate (TMBTI) with Co(NO3)2·6(H2O) under solvothermal conditions. The steric hindrance induced by the methyl groups on the central phenyl ring led to a non-planar configuration of the ligand and further resulted in a small cubicuboctahedron SBB sustained (3, 24)-connected rht network, which comprised three types of cages and exhibited high porosity. Experimental results showed that despite the use of different synthetic methods the same structure was obtained. Gas sorption study of this MOF revealed high CO2 and CH4 uptake capacities and relatively low adsorption enthalpies.

Adsorption of CO2, CO, CH4 and N2 on DABCO based metal organic frameworks

In this work, we report the gas adsorption properties of Ni2(bdc)2(dabco)(H2O)0.5(DMF)4 and Cu2(bdc)2(dabco)(H2O)0.5(DMF)4, which are more commonly known as NiDABCO and CuDABCO, respectively. Equilibrium adsorption measurements on these materials have been performed for four industrially relevant gases (CO2, CO, CH4 and N2), in the pressures range 0–25bar and at three different temperatures (294, 314 and 350K) using a standard gravimetric method. Virial and modified virial equations were used to model adsorption isotherms. Effect of adsorbate polarity and polarizability on adsorption characteristics of these MOFs is discussed. These compounds have similar adsorption characteristic as that of another DABCO based metal organic framework (MOF) viz. ZnDABCO. All DABCO compounds have saturated metal sites which result in small Henry’s constants, low adsorption enthalpies at zero coverage and therefore require milder regeneration condition compared to MOFs with open metal sites like MgDOBDC, MIL-101, CuBTC, etc. IAST was used to predict CO2 selectivity over other gases as well as that of CH4 over CO. As in case of ZnDABCO, for both NiDABCO and CuDABCO the selectivity of CO2 increases with increase in pressure and CO2 mole-fraction.