In this work, we show a solvent-free “explosive” synthesis (SFES) method for the ultrafast and low-cost synthesis of metal-formate frameworks (MFFs). A combination of experiments and in-depth molecular modelling analysis – using grand canonical Monte Carlo (GCMC) simulations – of the adsorption performance of the synthesized nickel-formate framework (Ni-FA) revealed extremely high quality products with permanent porosity, prominent CH4/N2 selectivity (ca. 6.0), and good CH4 adsorption capacity (ca. 0.80 mmol g−1 or 33.97 cm3 cm−3) at 1 bar and 298 K. This performance is superior to those of many other state-of-the-art porous materials.

•Uniform [Ni3(HCOO)6] frameworks were prepared without additional solvent.•The frameworks preferably adsorb CH4 over N2 superior than most of conventional adsorbents.•PSA experiments showed that both CH4 purity and recovery could exceeded 90%.Adsorption-based separation of CH4/N2 mixture remains challenging. To this end, [Ni3(HCOO)6] frameworks were synthesized by a novel solvent-free method and used as adsorbent. CH4 and N2 adsorption performances of the samples were examined separately by static adsorption, breakthrough and two-bed pressure swing adsorption (PSA) experiments. CH4 adsorption capacity was 0.82 mmol/g at 298 K and 100 kPa. The CH4/N2 selectivity reached up to 6–7 at 298 K and 0.5 MPa, which was higher than that of most other adsorbents including activated carbon, zeolites, and metal-organic frameworks (MOFs). Two-bed lab-scale PSA experiments dealing with equimolar CH4/N2 mixture showed that both CH4 purity and recovery exceeded 90% under optimized operation conditions. The remarkable performance in benchmarking experiments confirmed [Ni3(HCOO)6] frameworks promising adsorbent for unconventional natural gas upgrading, owing to its high volume of uniform ultra-micropore and optimal polarizability.Download high-res image (165KB)Download full-size image

Metal–organic frameworks (MOFs), assembled by metal ions or their clusters and organic linkers, are one of the state-of-the-art crystalline materials. Their features such as ultra-high porosity, synthetic tailorability and relative ease of synthesis make them promising candidates for diversified applications. Controllable integration of MOFs and carbon-based materials not only leads to further enhancement of single-phase MOFs in terms of stability and electrical conductivity, but also surprisingly brings about a number of new functionalities like formation of new pores and template effects. These benefits allow the resultant MOF–carbon composites to be applied beyond the fields of single-phase MOFs. Increasing research interests have been aroused in this rapidly developing interdisciplinary area. This review aims to specifically group together the important reports focused on MOF–carbon composites till now. The methods used for composite synthesis and applications of the composites are investigated and categorized. The review also exclusively discusses the functionalities stemming from the synergistic effects of the two intriguing materials and pictures the future prospects at the end.

•A series of isostructural ultra-microporous MOFs are readily synthesized.•The pore size and internal surface properties are tuned by using different metal ions.•The adsorption behaviour of gas molecule in [M3(HCOO)6] is revealed by NH3-TPD.A series of isostructural ultra-microporous metal-organic framework (MOF) compounds [M3(HCOO)6] (M = Mg, Mn, Co and Ni) have been readily synthesized in large-scale, characterized, and evaluated for the separation of CH4 and N2. Results indicate that the metallic formates exhibit different CH4 adsorption capacities and distinct CH4/N2 selectivity in a sequence of Ni > Co > Mg > Mn analogue owing to their varied CH4 affinities. Thereinto, [Ni3(HCOO)6] shows the highest CH4 adsorption capacity of 1.09 mmol/g and CH4/N2 selectivity up to 6.5 at 0.4 MPa and 298 K in the dynamic experiments, which suggests the most suitable synergistic effect between constricted pores and surface properties among [M3(HCOO)6] frameworks. At the same time, the adsorption behaviour in [M3(HCOO)6] is investigated by NH3-TPD, revealing that the adsorbed NH3 molecules should have two different states. One is that gas molecules stay inside the pores, the other is that gas molecules are adsorbed on the adsorption sites induced by the coordinated metal ions or exposed oxygen of the [M3(HCOO)6], which directly affects the adsorption capacity, the ratio of two states of molecules and the final selectivity. These results confirm that alteration of metal ion plays an important role in the tuning of pore size and internal surface properties, thus providing new clues to design MOFs with different pore characteristics for enhanced gas sorption and separation.

•The uniform [Mg3(OOCH)6] crystals were synthesized with the modulation of HF.•By varying the amount of HF different crystals size and morphology were obtained.•The Mg2+-induced sites and ultra-micropore can effect the selectivity of CH4/N2.Ultra-microporous [Mg3(OOCH)6] frameworks with different uniform shapes, were synthesized via a facile coordination modulation method, in which HF was used as a modulator to promote the growth of [Mg3(OOCH)6] crystals. The as-prepared [Mg3(OOCH)6] frameworks were scrutinized and evaluated the CH4 adsorption capacity and selectivity over N2 by pure gas adsorption and breakthrough experiments. The [Mg3(OOCH)6] frameworks exhibit preferential adsorption of CH4 over N2, and much higher CH4 adsorption capacity (up to 0.74 mmol/g) by comparison with conventional zeolites. It has been confirmed that the equilibrium selectivities of [Mg3(OOCH)6] frameworks are shape dependent, and the uniform prism-like framework with size of 100 μm has the highest selectivity up to 5.5 at 298 K. These results suggest that achieving the optimal coupling of polarizability and structure of [Mg3(OOCH)6] framework is key factor to obtain a high selectivity for the separation of CH4/N2 mixture.

•Two ultra-microporous frameworks were synthesized in a feasible route to scale up.•The CH4/N2 separation performances for MOFs and zeolites were evaluated.•The highest selectivities ever reported for the separation of CH4 against N2 were achieved.•The mechanism explains the good separation performances for the two adsorbents.Two typical ultra-microporous adsorbents namely [Ni3(HCOO)6] and [Co3(HCOO)6] were successfully prepared in a feasible route to scale up using non-corrosive methyl methanoate instead of formic acid. Pure gas and binary gas mixture adsorption equilibria of CH4 and N2 were conducted on the two compounds to evaluate their CH4 adsorption capacities and selectivities against N2. The two compounds exhibit preferential adsorption of CH4 over N2 at 298 K in the pressure range 0.1–1.0 MPa, and the adsorption selectivities of CH4/N2 mixture are determined to be 6.0–6.5 for [Ni3(HCOO)6] and 5.1–5.8 for [Co3(HCOO)6], respectively. The uniform ultra-micropores and optimal polarizability resulted from multiple coordination modes play the essential roles in their preferential adsorption of CH4 with the highest selectivities up to date, making the two compounds as promising candidates of existing adsorbents for unconventional natural gas upgrading.

Separation of methane and nitrogen is an important issue in upgrading low-quality natural gas, and non-cryogenic, adsorption-based separation of CH4/N2 is particularly challenging. In this report, a metal–organic framework (MOF) adsorbent, namely a [Ni3(HCOO)6] framework, is comprehensively investigated for the separation of CH4–N2 mixture via pure gas adsorption and binary gas breakthrough experiments. All the prepared samples synthesized using different routes were also studied in detail by powder XRD, FT-IR, SEM, TGA/DSC and argon adsorption. The results show that the adsorptive separation performances can be improved significantly by optimizing the synthesis of the framework. The precursors play crucial roles in the crystallization of [Ni3(HCOO)6] frameworks, giving rise to a variability in ultra-micropore volume, surface area and pore size. Good crystallization can result in large ultra-micropore volume and furthermore brings about high separation selectivity. The [Ni3(HCOO)6] framework synthesized from nickel nitrate and methyl formate exhibits the best crystallization and the largest micropore volume, leading to the highest CH4/N2 separation selectivity of up to 7.5 in the pressure range of 2.0–10 bar, which is the highest value reported for MOFs. Moreover, this adsorbent presents uniform nanosized crystals (∼140 nm), permanent porosity and consistent separation performances, making the [Ni3(HCOO)6] framework a promising candidate for natural gas upgrading.

The mesostructured Al-BDC metal–organic frameworks (MOFs) with an average pore size of 2.58 nm were prepared via a simplified washing and drying process and applied to the separation of CO2/CH4 mixtures. The adsorption equilibrium and thermodynamics of CH4 and CO2 were studied in the dynamic processes by the volumetric–chromatographic and inverse gas chromatographic (IGC) methods. The experiments represent that the Al-BDC MOF with large pore size has a much higher CO2/CH4 selectivity of ca. 24 at 303 K in the pressure range 0–1.0 MPa and therefore appears to be a good candidate for the separation of CH4 from CO2. The initial heats of adsorption of CH4 and CO2 on the mesostructured Al-BDC MOFs were determined to be 11.5 and 25.2 kJ mol–1 by the IGC method, respectively, which are significantly reduced by ca. 25% compared with that on the microporous Al-BDC MOFs. The results indicate that the expanded pore size not only greatly increases the selectivity of CO2 adsorption over CH4 but also reduces the adsorption heat, revealing that it should be the desired method to obtain a satisfactory absorbent for CO2/CH4 separation.

Metal–organic frameworks (MOFs), assembled by metal ions or their clusters and organic linkers, are one of the state-of-the-art crystalline materials. Their features such as ultra-high porosity, synthetic tailorability and relative ease of synthesis make them promising candidates for diversified applications. Controllable integration of MOFs and carbon-based materials not only leads to further enhancement of single-phase MOFs in terms of stability and electrical conductivity, but also surprisingly brings about a number of new functionalities like formation of new pores and template effects. These benefits allow the resultant MOF–carbon composites to be applied beyond the fields of single-phase MOFs. Increasing research interests have been aroused in this rapidly developing interdisciplinary area. This review aims to specifically group together the important reports focused on MOF–carbon composites till now. The methods used for composite synthesis and applications of the composites are investigated and categorized. The review also exclusively discusses the functionalities stemming from the synergistic effects of the two intriguing materials and pictures the future prospects at the end.