Enhanced CO₂ Adsorption and Selectivity of CO₂/N₂ on Amino-MIL-53(Al) Synthesized by Polar Co-solvents

Abstract

Amino-MIL-53(Al) was solvothermally synthesized using co-solvents, such as methanol (M), ethanol (E), methanol/acetic acid (MA), or ethanol/acetic acid (EA), as modulators with dimethylformamide (D). The effects of cosolvents on physicochemical properties of amino-MIL-53(Al) were investigated. It was found that the addition of co-solvents in the synthesis leads to the reduction of crystallinity and crystal size of the samples. The textural properties, such as specific surface area and porous structure, were manipulated. Amino-MIL-53-DMA exhibited the highest Brunauer−Emmett−Teller surface area of 632 m 2 /g as a result of the loss of the bridging hydroxyl group, while amino-MIL-53, amino-MIL-53-DE, amino-MIL-53-DEA, and amino-MIL-53-DM presented the surface areas of 400, 356, 321, and 348 m 2 /g, respectively. However, the primary amine groups were maintained on the surface of all of the amino-MIL-53 samples. The co-solvents enhanced CO 2 adsorption on modified amino-MIL-53. CO 2 adsorption capacities on amino-MIL-53, amino-MIL-53-DM, amino-MIL-53-DE, amino-MIL-53-DEA, and amino-MIL-53-DMA are 48, 71, 67, 54, and 75 cm 3 /g, respectively, at standard conditions (1 atm and 273 K). CO 2 adsorption heat could be reduced to 24 kJ/mol on amino-MIL-53-DMA, giving it as a promising adsorbent for carbon dioxide storage at ambient conditions. Besides, the selectivity of CO 2 /N 2 on amino-MIL-53-DEA and amino-MIL-53-DE demonstrates an unprecedentedly separating factor of 637 at 1 atm and 273 K, whereas the separating factors of CO 2 /N 2 on amino-MIL-53, amino-MIL-53-DMA, and amino-MIL-53-DM are only 43, 43, and 153, respectively. Amino-MIL-53-DEA and amino-MIL-53-DE impressively outperform other MOFs and exhibit as an auspicious adsorbent for CO 2 /N 2 separation.

Boosting CO2 adsorption and selectivity in metal–organic frameworks of MIL-96(Al) via second metal Ca coordination

“…Systematically, the CO 2 adsorption in microporous materials increases when the pore volume and BET surface area are augmented and vice versa. 55 It can be conrmed from Table 2 that the adsorption of CO 2 is signicantly boosted in most of the MOFs in this work and typically in MIL-96(Al)-Ca1 and MIL-96(Al)-Ca2. Although, almost all samples in the tabulated references have higher surface areas, they present lower CO 2 adsorption capacity than the present work listed samples.…”

Overcoming Crystallinity Limitations of Aluminium Metal–Organic Frameworks by Oxalic Acid Modulated Synthesis

“…[43] On the contrary,f luoride coordination to aluminium is fully compatible with the substitution of hydroxy groups in the framework since both anionsa re monodentate, have analogous size, and feature same charge and number of valence electron pairs (Scheme 2b). For similarr easons, until now the use of short-chain monocarboxylic acids as synthesis modulators for Al-MIL-53 as well as for other Al-MOFs focusedo n taking advantage of their tendency for linker replacement [44] to hamper the growth of specific crystal facets, [45,46] reduce the mean crystal size, [30] and introduce structural defects to modify the MOF's porosity. [29,47] The benefits of oxalic acid modulation are not limited to MIL-53, as they were also observed for its analogues with OH-, CH 3 O-, Br-, and NO 2 -functionalised linkers.…”

“…[26][27][28] Monocarboxylate modulators, commonly used in MOF synthesis, have shown to lead to defectf ormation [29] and even decreasei nc rystal size. [30] To date, the only effective approacht hat has been reported for the synthesis of highly crys-tallineA l-MOFs is based on the use of hydrofluorica cid, [31][32][33][34] borrowing from its well-documented use as mineralizer to improve the crystallinity of microporousi norganic materials. [35] Its widespreadu se, however,r aises concerns both on safety and on the possible fluoride inclusion in the framework architecture.…”

Overcoming Crystallinity Limitations of Aluminium Metal–Organic Frameworks by Oxalic Acid Modulated Synthesis

“…[43] On the contrary, fluoride coordination to aluminium is fully compatible with the substitution of hydroxy groups in the framework since both anions are monodentate, have analogous size, and feature same charge and number of valence electron pairs (Scheme 2 b). For similar reasons, until now the use of short-chain monocarboxylic acids as synthesis modulators for Al-MIL-53 as well as for other Al-MOFs focused on taking advantage of their tendency for linker replacement [44] to hamper the growth of specific crystal facets, [45,46] reduce the mean crystal size, [30] and introduce structural defects to modify the MOF's porosity. [29,47] The benefits of oxalic acid modulation are not limited to MIL-53, as they were also observed for its analogues with OH-, CH 3 O-, Br-, and NO 2 -functionalised linkers.…”

“…[26][27][28] Monocarboxylate modulators, commonly used in MOF synthesis, have shown to lead to defect formation [29] and even decrease in crystal size. [30] To date, the only effective approach that has been reported for the synthesis of highly crystalline Al-MOFs is based on the use of hydrofluoric acid, [31][32][33][34] borrowing from its well-documented use as mineralizer to improve the crystallinity of microporous inorganic materials. [35] Its widespread use, however, raises concerns both on safety and on the possible fluoride inclusion in the framework architecture.…”