Anjaiah Nalaparaju

Modulated Hydrothermal Synthesis of UiO-66(Hf)-Type Metal–Organic Frameworks for Optimal Carbon Dioxide Separation

Recently, there has been growing interest in hafnium (Hf) metal-organic frameworks (MOFs). These MOFs may perform better as gas adsorbents than zirconium (Zr) MOFs due to the presence of Brønsted acid sites with high affinity toward adsorbates, together with the outstanding chemical and hydrothermal stabilities similar to their Zr analogues. However, Hf-MOFs have been rarely reported due to the lack of effective synthetic methods. We herein report a modulated hydrothermal synthesis of UiO-66(Hf)-type MOFs. Among these MOFs, UiO-66(Hf)-(OH)2 possesses a very high CO2 gravimetric uptake of 1.81 mmol g(-1) at 0.15 bar and 298 K, which is 400% higher than that of UiO-66(Hf) (0.36 mmol g(-1)). It also exhibits a record-high volumetric CO2 uptake of 167 v/v at 1 bar and 298 K. Ideal adsorbed solution theory calculations showed a CO2/N2 (molar ratio 15:85) selectivity of 93 and CO2/H2 (molar ratio 30:70) selectivity above 1700. Breakthrough simulations also confirmed its optimal CO2 separation attribute. Our results have demonstrated for the first time the strong potential of Hf-MOFs for advanced adsorbents for high-performance CO2-related separations.

CO2 Adsorption in Mono-, Di- and Trivalent Cation-Exchanged Metal–Organic Frameworks: A Molecular Simulation Study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henrys constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.

Biofuel purification by pervaporation and vapor permeation in metal–organic frameworks: a computational study

We report a computational study for the purification of biofuel (water–ethanol mixtures) in two metal–organic frameworks (MOFs), hydrophilic Na-rho-ZMOF and hydrophobic Zn4O(bdc)(bpz)2 at both pervaporation (PV) and vapor permeation (VP) conditions. In Na-rho-ZMOF, water is preferentially adsorbed over ethanol due to its strong interaction with nonframework Na+ ions and ionic framework, and the adsorption selectivity of water–ethanol is higher at a lower composition of water. With increasing water composition, water diffusivity in Na-rho-ZMOF increases but ethanol diffusivity decreases, and the diffusion selectivity of water–ethanol increases. In contrast, ethanol is adsorbed more in Zn4O(bdc)(bpz)2 as attributed to the favorable interaction with methyl groups on the pore surface, and ethanol–water adsorption selectivity is higher at a lower composition of ethanol. With increasing water composition, the diffusivities of water and ethanol in Zn4O(bdc)(bpz)2 increase and the diffusion selectivity of ethanol–water decreases slightly. The permselectivities in the two MOFs at both PV and VP conditions are largely determined by the adsorption selectivities. The maximum achievable permselectivity in Na-rho-ZMOF is approximately 12 at VP condition, and Na-rho-ZMOF is preferable to remove a small fraction of water from water–ethanol mixtures and enrich ethanol at the feed side. The maximum permselectivity in Zn4O(bdc)(bpz)2 is about 75 at PV condition, and Zn4O(bdc)(bpz)2 is promising to extract a small fraction of ethanol and enrich ethanol at the permeate side. This study presents microscopic insights into the separation of water–ethanol mixtures in hydrophilic and hydrophobic MOFs at both PV and VP conditions, and provides atomistic guidelines toward the selection of an appropriate MOF and operating condition for biofuel purification.

Atomistic insight into adsorption, mobility, and vibration of water in ion-exchanged zeolite-like metal-organic frameworks

The adsorption, mobility, and vibration of water in ion-exchanged rho-zeolite-like metal-organic frameworks (ZMOFs) are investigated using atomistic simulations. Because of the high affinity for the ionic framework and nonframework ions, water is strongly adsorbed in rho-ZMOFs with a three-step adsorption mechanism. At low pressures, water is preferentially adsorbed onto Na(+) ions, particularly at site II; with increasing pressure, adsorption occurs near the framework and finally in the large cage. Upon water adsorption, Na(+) ions are observed to redistribute from site I to site II and gradually hydrated with increasing pressure. In Li-, Na-, and Cs-exchanged rho-ZMOFs, the adsorption capacity and isosteric heat decrease with increasing ionic radius attributed to the reduced electrostatic interaction and free volume. The mobility of water in Na-rho-ZMOF increases at low pressures but decreases upon approaching saturation. With sufficient amount of water present, the mobility of Na(+) ions is promoted. The vibrational spectra of water in Na-rho-ZMOF exhibit distinct bands for librational motion, bending, and stretching. The librational motion has a frequency higher than bulk water due to confinement. With increasing loading and hence stronger coordinative attraction, the bending frequency shows a blue shift. Symmetric and asymmetric modes are observed in the stretching as a consequence of the strong water-ion interaction. This study provides a fundamental microscopic insight into the static and dynamic properties of water in charged ZMOFs and reveals the subtle interplay between water and nonframework ions.

Recovery of Dimethyl Sulfoxide from Aqueous Solutions by Highly Selective Adsorption in Hydrophobic Metal–Organic Frameworks

Metal-organic frameworks (MOFs) have emerged as a new family of nanoporous materials. While gas separation in MOFs has been extensively investigated, liquid separation is scarcely examined and lacks a microscopic understanding. A molecular simulation study is reported here for the recovery of dimethyl sulfoxide (DMSO) from aqueous solutions in three hydrophobic MOFs, namely, Zn(4)O(bdc)(bpz)(2), Zn(bdc)(ted)(0.5), and ZIF-71. Type I adsorption isotherms are observed for DMSO, while H(2)O exhibits type V adsorption isotherms with hysteresis. The saturation capacities of both DMSO and H(2)O decrease following the order of Zn(4)O(bdc)(bpz)(2) > Zn(bdc)(ted)(0.5) > ZIF-71, in accordance with the variation of free volume and porosity in the three MOFs. As attributed to hydrophobic frameworks, the three MOFs are highly selective toward DMSO adsorption from DMSO/H(2)O mixtures. The highest selectivity is predicted up to 1700 in ZIF-71. This simulation study provides molecular insight into the separation mechanism of DMSO/H(2)O liquid mixtures and suggests that hydrophobic MOFs are superior candidates for DMSO recovery.

CO2 capture in rht metal–organic frameworks: multiscale modeling from molecular simulation to breakthrough prediction

A multiscale modeling study is reported for CO2 capture in a series of metal–organic frameworks (Cu-TDPAT, PCN-61, -66, -68, NOTT-112, NU-111 and NU-110). These MOFs share the same rht topology, though with different ligands. The predicted adsorption isotherms of pure gases (CO2, N2, H2 and CH4) agree well with experimental data. The structure–property relationships between adsorption capacity and ligand size are established. With increasing ligand size, adsorption capacity drops at a low pressure, but increases near saturation conditions. Due to the presence of small ligands, unsaturated metals and amine groups, Cu-TDPAT exhibits the highest adsorption capacity and separation performance among the seven rht-MOFs. On this basis, a new structure, Cu-TDPAT-N, is designed by substituting the phenyl rings in Cu-TDPAT by pyridine rings. Compared to Cu-TDPAT, the N-rich Cu-TDPAT-N possesses higher capacity, isosteric heat and selectivity. From simulated results, the breakthrough profiles for CO2-containing mixtures (CO2/N2, CO2/H2 and CO2/CH4) are predicted, and the breakthrough time for CO2 in Cu-TDPAT-N is found to be extended by two-fold. This study provides microscopic insights into gas adsorption and separation in rht-MOFs, establishes quantitative relationships, and suggests that N-substitution is effective to enhance the performance for CO2 capture.

Functionalized metal–organic framework MIL-101 for CO2 capture: multi-scale modeling from ab initio calculation and molecular simulation to breakthrough prediction

By synergizing ab initio calculation, molecular simulation and breakthrough prediction, we investigate CO2 capture in metal–organic framework MIL-101 functionalized by a series of groups (–NH2, –CH3, –Cl, –NO2 and –CN). CO2 uptake and isosteric heat in a low-pressure regime increase in the order of MIL-101 < MIL-101-CN < MIL-101-NO2 < MIL-101-Cl < MIL-101-CH3 < MIL-101-NH2. This order follows the strength of the binding energies between CO2 and the functional groups. However, the effect of the functional groups is marginal for N2 adsorption. In terms of the separation of a CO2/N2 mixture, CO2/N2 selectivity is enhanced by functionalization following the order of MIL-101 < MIL-101-CN < MIL-101-CH3 < MIL-101-NO2 < MIL-101-Cl < MIL-101-NH2. At an infinite dilution, the enhancement of CO2/N2 selectivity is 2.5 times. The predicted breakthrough time is extended by functionalization, and the longest breakthrough time in MIL-101-NH2 is 2 times that in MIL-101. Furthermore, the working capacity of CO2 increases by approximately 40%. This multi-scale modeling study suggests that CO2 capture in MIL-101 can be considerably improved by functionalization, in terms of CO2 capacity, CO2/N2 selectivity, breakthrough time and working capacity.

Crucial role of blocking inaccessible cages in the simulation of gas adsorption in a paddle-wheel metal–organic framework

A molecular simulation study is reported to investigate the adsorption of CO2 and H2 in a recently synthesized paddle-wheel Cu-based metal–organic framework (Cu–MOF). The Cu–MOF consists of three types of cages; the type-III cages are restricted by narrow windows and inaccessible to gas molecules. By blocking the inaccessible type-III cages, the simulated adsorption isotherms for pure CO2 and H2 agree well with experimental data. Ideal-adsorbed solution theory (IAST) is used to predict the adsorption of a CO2/H2 mixture, and the predicted isotherms and selectivities are consistent with simulated results. Furthermore, the breakthrough profiles are evaluated for a CO2/H2 mixture in a fixed-bed packed with the Cu–MOF. The breakthrough times are estimated to be 2.8 and 85.2 for H2 and CO2, respectively, implying the efficient separation of the CO2/H2 mixture. The simulation study reveals the crucial role of blocking inaccessible cages in the proper simulation of gas adsorption in the Cu–MOF, and the capability of IAST applied to the Cu–MOF with inaccessible cages.