Tuning Carbon Dioxide Adsorption Affinity of Zinc(II) MOFs by Mixing Bis(pyrazolate) Ligands with N-Containing Tags

Abstract

The four zinc(II) mixed-ligand MOFs (MIXMOFs) Zn(BPZ)x(BPZNO2)1-x, Zn(BPZ)x(BPZNH2)1-x, Zn(BPZNO2)x(BPZNH2)1-x and Zn(BPZ)x(BPZNO2)y(BPZNH2)1-x-y (H2BPZ = 4,4'-bipyrazole; H2BPZNO2 = 3-nitro-4,4'-bipyrazole; H2BPZNH2 = 3-amino-4,4'-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. Isoreticular to the endmembers Zn(BPZ) and Zn(BPZX) (X = NO2, NH2), they are the first examples ever reported of (pyr)azolate MIXMOFs. Their crystal structure is characterized by a 3-D open framework with 1-D square or rhombic channels decorated by the functional groups. Accurate information about ligand stoichiometric ratio was determined (for the first time on MIXMOFs) through integration of selected ligands skeleton resonances from 13 C CPMAS solid-state NMR spectra collected on the as-synthesized materials. Like other poly(pyrazolate) MOFs, the four MIXMOFs are thermally stable, with decomposition temperatures between 708 and 726 K. As disclosed by N2 adsorption at 77 K, they are micro-mesoporous materials with BET specific surface areas in the range 400-600 m 2 /g. A comparative study (involving also the single-ligand analogues) of CO2 adsorption capacity, CO2 isosteric heat of adsorption (Qst) and CO2/N2 selectivity in equimolar mixtures at p = 1 bar and T = 298 K casted light on interesting trends, depending on ligand tag nature or ligand stoichiometric ratio. In particular, the amino-decorated compounds show higher Qst values and CO2/N2 selectivity vs. the nitro functionalized analogues; in addition, tag "dilution"[upon passing from Zn(BPZX) to Zn(BPZ)x(BPZX)1-x] increases CO2 adsorption selectivity over N2. The simultaneous presence of amino and nitro groups is not beneficial for CO2 uptake. Among the compounds studied, the best compromise among uptake capacity, Qst and CO2/N2 selectivity is represented by Zn(BPZ)x(BPZNH2)1-x. KeywordsMixed-ligand Metal-Organic Frameworks (MIXMOFs)zinccarbon dioxideporous materialsnitrogen ligandscarbon capture and sequestration (CCS) -13 C CPMAS NMR spectroscopy -15 N CPMAS NMR spectroscopy

Carbon Dioxide Capture and Utilization with Isomeric Forms of Bis(amino)‐Tagged Zinc Bipyrazolate Metal–Organic Frameworks

“…As shown in Figure 3 a, both compounds show a type IV isotherm, typical of micro‐mesoporous materials. As expected, they are featured by comparable BET areas [463 and 504 m 2 g −1 for Zn(3,3′‐L) and Zn(3,5‐L) , respectively] that fall in the same range as those measured for Zn(BPZNH 2 ) (395 m 2 g −1 ) [4c] and for the MIXMOF Zn 2 (BPZ)(BPZNH 2 ) (502 m 2 g −1 ) [4b] . The t ‐plot analysis revealed that the main contribution to the total surface area comes from micropores for both MOFs [77 and 57 % for Zn(3,3′‐L) and Zn(3,5‐L) , respectively].…”

“…The isosteric heat of adsorption reflects the interaction strength between CO 2 and the inner pore walls of the two MOFs. Mirroring the trend observed for the CO 2 storage capacity, the values of 24.8 and 25.4 kJ mol −1 calculated at zero coverage for Zn(3,3′‐L) and Zn(3,5‐L) , respectively, are higher than that of their untagged isostructural analogue [Zn(BPZ); Q st = 23.7 kJ mol −1 ] [4b, 21] but much lower than those calculated for Zn(BPZNH 2 ) ( Q st = 35.6 kJ mol −1 ), [4c] Zn 2 (BPZ)(BPZNH 2 ) ( Q st = 30.8 kJ mol −1 ) [4b] and Zn(BDPNH 2 ) ( Q st = 28.1 kJ mol −1 ) [4d] . This is a further proof of evidence that the tag “dilution” in the solid matrix is of fundamental importance to minimize the intermolecular interactions between adjacent dangling tags in the lattice and consequently to improve the material thermodynamic response towards external guest gases.…”

“…% CO 2 , 4.8 mmol g −1 ) [4c] and of the MIXMOF Zn 2 (BPZ)(BPZNH 2 ) (22.3 wt. % CO 2 , 5.1 mmol g −1 ), [4b] but higher than that found for the longer linker parent Zn(BDPNH 2 ) (2.5 mmol g −1 at T = 273 K) [4d] …”

“…The reader is referred to the electronic Supporting Information for the description of the crystal structure of 3,3’‐H 2 L . [Epibromohydrin@ Zn(3,3’‐L) ] crystallizes in the orthorhombic space group Cccm and shares the same structural motif of other Zn bipyrazolate MOFs known from the literature like the monoamino‐tagged analogue Zn(BPZNH 2 ), [4c] the isomeric Zn(3,5‐L) , [4a] the MIXMOF Zn 2 (BPZ)(BPZNH 2 ) (BPZ 2− = 4,4′‐bipyrazolate) [4b] and the longer‐linker analogue Zn(BDPNH 2 ) (H 2 BDPNH 2 = 2‐amino‐1,4‐bis(1 H ‐pyrazol‐4‐yl)benzene) [4d] . The crystal structure will be briefly described hereafter for the sake of completeness.…”

Bivariate Metal–Organic Frameworks with Tunable Spin‐Crossover Properties

“…[10] Recently,m ultivariate MOFs (MTV-MOFs)c onstructed by virtue of multiple linkersw ith different functional groups have exhibited interesting properties such as enhanced gas adsorption capacity. [11] In MTV-MOFs, the coordination environmento f the metal centers can be precisely regulated by ligands, which will further change the electron distribution of the metal centers. Therefore, the multivariate MOFs approach introducing different ligands can meet the requirements of tuning the SCO properties.…”