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Zeolitic imidazolate frameworks (ZIFs) have attracted great interest in recent years due to their high chemical and thermal stability with promising applications in gas storage and separations. We investigate the structures of three different crystalline ZIFs - ZIF-4, ZIF-8, ZIF-zni - and their amorphous counterparts using high field (13)C and (15)N CP MAS NMR. The high field (20 T) allows for the observation of all crystallographically independent carbon and nitrogen atoms in the crystalline ZIFs. Combining our experimental results with density functional theory calculations enabled the assignment of all chemical shifts. The crystalline spectra reveal the potential of high field NMR to distinguish between two ZIF polymorphs, ZIF-4 and ZIF-zni, with identical [Zn(C3H3N2)2] chemical compositions. (13)C and (15)N CP MAS NMR data obtained for the amorphous ZIFs clearly showed signal broadening upon amorphization, confirming the retention of chemical composition and the structural similarity of amorphous ZIF-4 and ZIF-zni. In the case of amorphous ZIF-8, we present evidence for the partial de-coordination of the 2-methyl imidazole linker.

Inorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal–organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3‐butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n‐butene, n‐butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations. [ABSTRACT FROM AUTHOR]

The evolution of metal-organic frameworks (MOFs) has been one of the most exciting aspects of materials chemistry over the last 20 years. In this Account, we discuss the development during this period in our understanding of the factors that control the crystallization of MOFs from solution. Both classical porous MOFs and dense MOF phases are considered. This is an opportune time at which to examine this complex area because the experimental tools now available to interrogate crystallization processes have matured significantly in the last 5 years, particularly with the use of in situ synchrotron X-ray diffraction. There have also been impressive developments in the use of density functional theory (DFT) to treat not only the energies of very complex structures but also their entropies. This is particularly important in MOF frameworks because of their much greater flexibility compared with inorganic structures such as zeolites. The first section of the Account describes how early empirical observations on the crystallization of dense MOFs pointed to a strong degree of thermodynamic control, with both enthalpic and entropic factors playing important roles. For example, reactions at higher temperatures tend to lead to denser structures with higher degrees of framework connectivity and lower levels of solvation, and polymorphs tend to form according to their thermodynamic stabilities. In the case of metal tartrates, these trends have been validated by calorimetric studies. It has been clear for more than a decade, however, that certain phases crystallize under kinetic control, especially when a change in conformation of the ligand or coordination around a metal center might be necessary to form the thermodynamically preferred product. We describe how this can lead to time-dependent crystallization processes that evolve according to the Ostwald rule of stages and can be observed by in situ methods. We then consider the crystallization of porous MOFs, which presents additional challenges because of solvation effects. In spite of these problems, much has been learned about the energetics of the underlying frameworks, where the relationship between porosity and stability initially seemed to mirror the behavior of zeolites, with more porous structures being less stable. Recently, however, this simple relationship has had to be reconsidered with the emergence of some very flexible structures wherein the open structures are more stable than their denser analogues at finite temperatures because of their large vibrational entropies. In the final section we describe how the concepts developed in the MOF work have been extended into the closely related area of hybrid organic-inorganic perovskites. We describe recent studies on polymorphism in hybrid perovskites, which is amenable to total free energy calculations using a combination of DFT and lattice dynamics methods. [ABSTRACT FROM AUTHOR]

X-ray diffraction has been used to investigate the kinetics of amorphization through ball-milling at 20 Hz, for five zeolitic imidazolate frameworks (ZIFs) – ZIF-8, ZIF-4, ZIF-zni, BIF-1-Li and CdIF-1. We find that the rates of amorphization for the zinc-containing ZIFs increase with increasing solvent accessible volume (SAV) in the sequence ZIF-8 ＞ ZIF-4 ＞ ZIF-zni. The Li–B analogue of the dense ZIF-zni amorphizes more slowly than the corresponding zinc phase, with the behaviour showing a correlation with their relative bulk moduli and SAVs. The cadmium analogue of ZIF-8 (CdIF-1) amorphizes more rapidly than the zinc counterpart, which we ascribe primarily to its relatively weak M–N bonds as well as the higher SAV. The results for the ZIFs are compared to three classical zeolites – Na-X, Na-Y and ZSM-5 – with these taking up to four times longer to amorphize. The presence of adsorbed solvent in the pores is found to render both ZIF and zeolite frameworks more resistant to amorphization. X-ray total scattering measurements show that amorphous ZIF-zni is structurally indistinguishable from amorphous ZIF-4 with both structures retaining the same short-range order that is present in their crystalline precursors. By contrast, both X-ray total scattering measurements and 113Cd NMR measurements point to changes in the local environment of amorphous CdIF-1 compared with its crystalline CdIF-1 precursor. [ABSTRACT FROM AUTHOR]

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K. [ABSTRACT FROM AUTHOR]

The I2-sorption and -retention properties of several existing zeolitic imidazolate frameworks (ZIF-4, -8, -69) and a novel framework, ZIF-mnIm ([Zn(mnIm)2]; mnIm=4-methyl-5-nitroimidazolate), have been characterised using microanalysis, thermogravimetric analysis and X-ray diffraction. The topologically identical ZIF-8 ([Zn(mIm)2]; mIm=2-methylimidazolate) and ZIF-mnIm display similar sorption abilities, though strikingly different guest-retention behaviour upon heating. We discover that this guest retention is greatly enhanced upon facile amorphisation by ball milling, particularly in the case of ZIF-mnIm, for which I2 loss is retarded by as much as 200 °C. It is anticipated that this general approach should be applicable to the wide range of available metal-organic framework-type materials for the permanent storage of harmful guest species. [ABSTRACT FROM AUTHOR]

The first thermochemical analysis by room-temperature aqueous solution calorimetry of a series of zeolite imidazolate frameworks (ZIFs) has been completed. The enthalpies of formation of the evacuated ZIFs—ZIF-zni, ZIF-1, ZIF-4, CoZIF-4, ZIF-7, and ZIF-8—along with as-synthesized ZIF-4 (ZIF-4·DMF) and ball-milling amorphized ZIF-4 (amZIF-4) were measured with respect to dense components: metal oxide (ZnO or CoO), the corresponding imidazole linker, and N,N dimethylformamide (DMF) in the case of ZIF-4·DMF. Enthalpies of formation of ZIFs from these components at 298 K are exothermic, but the ZIFs are metastable energetically with respect to hypothetical dense components in which zinc is bonded to nitrogen rather than oxygen. These enthalpic destabilizations increase with increasing porosity and span a narrow range from 13.0 to 27.1 kJ/mol, while the molar volumes extend from 135.9 to 248.8 cm3/mol; thus, almost doubling the molar volume results in only a modest energetic destabilization. The experimental results are supported by DFT calculations. The series of ZIFs studied tie in with previously studied MOF-5, creating a broader trend that mirrors a similar pattern by porous inorganic oxides, zeolites, zeotypes, and mesoporous silicas. These findings suggest that no immediate thermodynamic barrier precludes the further development of highly porous materials. [ABSTRACT FROM AUTHOR]

High-pressure synchrotron X-ray powder diffraction measurements of a sample of purely siliceous zeolite V (faujasite) were carried out up to 8.0 GPa at room temperature using a diamond anvil cell. Measurements using silicone oil as the pressure-transmitting medium show compression of the zeolite followed by a loss of long-range ordering at 2.2 GPa. The experimentally determined bulk modulus, 38(2) GPa, is, within experimental error, identical to that of quartz. When using a methanol:ethanol:water mixture (16:3:1) as the pressure-transmitting medium, two distinct compressibility regions are observed with a dramatic change in the compression mechanism at 4 GPa. Rietveld refinement analysis of the powder patterns provides a detailed description ot the underlying chemistry, with sequential pore filling the main response up to 4 GPa and framework distortions at higher pressures. [ABSTRACT FROM AUTHOR]

An extensive study of a highly crystalline of sodium maximum aluminum zeolite P (Na–MAP) ion exchanged with Mn2+, Cd2+, Pb2+, Sr2+, and Ba2+ cations and with a Si/Al ratio of 1.0, is presented. All the crystal structures were refined from conventional X-ray powder data in the monoclinic crystal system, space group P21/c, and have the GIS framework topology. The Cd–MAP and Mn–MAP structures are similar to that of Ca-gismondine and the positions of the Cd and Mn atoms match the cations positions in natural Ca-gismondine. The Ba–MAP, Sr–MAP and Pb–MAP structures, however, are quite different. The unit cells of Ba–MAP and Pb–MAP are metrically orthorhombic, but their structures were refined with the monoclinic space group P21/c. Sr–MAP, in contrast to the other structures, presents two sites for the extra-framework cations. The incorporation of Cd, Mn, Ba, Sr, and Pb cations produces different distortions in the GIS framework due to its high flexibility. Besides the distortions induced by the cation incorporation, the 29Si NMR data and the Si–O and Al–O bonds show that all structures present a strict ordering scheme of Si and Al. The number of water molecules per unit cell differs between the title compounds, as does their dehydration behaviour. Dehydration of Cd–MAP and Mn–MAP induces the formation of new phases. However, under rehydration conditions, these new phases are restored to the original ones. On the other hand, dehydration of Sr–MAP and Pb–MAP induces the formation of new phases that are unchanged under rehydration conditions. Dehydration of Ba–MAP at 140–180 °C induces the formation of a new phase that is restored to the original one upon rehydration. However, above this temperature range the new phase is formed irreversibly. The results indicate that the unusual reversibility and irreversibility of MAP zeolites are related to the cation-dominant media, because some cations act as a precursor for obtaining different natural zeolites whose frameworks are different from the GIS framework. [Copyright &y& Elsevier]

Examines the host-guest hydrogen bonding in the zeolite NaY. Determination of chloroform system by neutron scattering; Obtainment of differential H pair distribution function; Interaction of atoms.

Hydrogen sorption in the nanoporous nickel phosphates VSB-1 and VSB-5 has been studied with a combination of BET, temperature programmed desorption (TPD), and inelastic neutron scattering (INS) measurements. H[sub 2] BET isotherms for VSB-1 are similar to those seen in nanoporous zeolites, while VSB-5 adsorbs substantially more hydrogen due to a steep initial uptake at low partial pressures. TPD data show that hydrogen interacts strongly with VSB-5, with desorption peaks at 109 and 149 K in a nitrogen flow, whereas the absence of similar peaks for VSB-1 suggests a weaker interaction. INS spectra of the rotational tunnel transition of the adsorbed H[sub 2] also reveal a strong interaction with the VSB-5 host. These data strongly suggest the existence of coordinatively unsaturated Ni[sup 2+] sites accessible to H[sub 2] molecules in the pores of VSB-5. [ABSTRACT FROM AUTHOR]