Charanjit S. Paur

Control of zeolite framework flexibility and pore topology for separation of ethane and ethylene

Purifying ethylene with flexible zeolites Ethylene is a key feedstock for many chemicals and polymers, but its production requires cryogenic separation from ethane, an energy-consuming step. In theory, pure silica zeolites are well suited to separate olefins from paraffins. Bereciartua et al. synthesized a pure silica zeolite with very small pores, which, if static, would not adsorb either of these hydrocarbons. However, molecular dynamics suggested that the pores should be flexible. Indeed, in competitive adsorption experiments, the zeolite preferentially adsorbed ethylene from a mixed stream of ethylene and ethane. Science, this issue p. 1068 A pure silica zeolite has small, flexible pores that preferentially adsorb ethylene over ethane. The discovery of new materials for separating ethylene from ethane by adsorption, instead of using cryogenic distillation, is a key milestone for molecular separations because of the multiple and widely extended uses of these molecules in industry. This technique has the potential to provide tremendous energy savings when compared with the currently used cryogenic distillation process for ethylene produced through steam cracking. Here we describe the synthesis and structural determination of a flexible pure silica zeolite (ITQ-55). This material can kinetically separate ethylene from ethane with an unprecedented selectivity of ~100, owing to its distinctive pore topology with large heart-shaped cages and framework flexibility. Control of such properties extends the boundaries for applicability of zeolites to challenging separations.

New High- and Low-Temperature Phase Changes of ZIF-7: Elucidation and Prediction of the Thermodynamics of Transitions.

We have found that the 3D zeolitic imidazolate framework ZIF-7 exhibits far more complex behavior in response to the adsorption of guest molecules and changes in temperature than previously thought. We believe that this arises from the existence of different polymorphs and different types of adsorption sites. We report that ZIF-7 undergoes a displacive, nondestructive phase change upon heating to above ∼700 °C in vacuum, or to ∼500 °C in CO2 or N2. This is the first example of a temperature-driven phase change in 3D ZIF frameworks. We predicted the occurrence of the high-temperature transition on the basis of thermodynamic arguments and analyses of the solid free-energy differences obtained from CO2 and n-butane adsorption isotherms. In addition, we found that ZIF-7 exhibits complex behavior in response to the adsorption of CO2 manifesting in double transitions on adsorption isotherms and a doubling of the adsorption capacity. We report adsorption microcalorimetry, molecular simulations, and detailed XRD investigations of the changes in the crystal structure of ZIF-7. Our results highlight mechanistic details of the phase transitions in ZIF-7 that are driven by adsorption of guest molecules at low temperature and by entropic effects at high temperature. We derived a phase diagram of CO2 in ZIF-7, which exhibits surprisingly complex re-entrant behavior and agrees with our CO2 adsorption measurements over a wide range of temperatures and pressures. We predicted phase diagrams of CH4, C3H6, and C4H10. Finally, we modeled the temperature-induced transition in ZIF-7 using molecular dynamics simulations in the isobaric-isothermal ensemble, confirming our thermodynamic arguments.

Accurate structure determination of a borosilicate zeolite EMM-26 with two-dimensional 10 × 10 ring channels using rotation electron diffraction

A new borosilicate zeolite |N2H36C16|[Si22B2O48]·H2O, denoted as EMM-26, has been synthesized by employing a linear dicationic organic structure directing agent 1,6-bis(N-methylpyrrolidinium)hexane (OSDA). EMM-26 has a novel zeolite framework and contains two-dimensional (2D) intersecting 10 × 10-ring channels. Its structure was solved from sub-micrometer sized crystals using rotation electron diffraction (RED) and refined against both the RED and synchrotron powder diffraction data. We have shown for the first time that RED data alone can be used to accurately determine zeolite structures. The OSDAs can be removed from the framework generating permanent pores. EMM-26 shows good CO2 uptake and CO2/CH4 selectivity.

Loading‐Dependent Transport Properties of Zeolitic Imidazolate Frameworks Probed by In‐Situ PFG NMR

Zeolitic imidazolate frameworks (ZIFs), a unique subclass of metal‐organic frameworks (MOFs) recently reported by Yaghi group and others, are attracting worldwide attention. Our systematic work on the adsorption and molecular transport in ZIFs has revealed some unique properties of these new materials that could not have been anticipated from standard structural characterization results. More specifically, the rearrangement of the imidazolate linkers (and in some cases, the framework structure) driven by adsorbate‐adsorbent interaction causes the window sizes and accessible pore space of ZIFs to deviate (in some cases, drastically) from the values determined by low‐temperature single‐crystal X‐ray crystallography and typical physisorption experiments carried out at liquid nitrogen temperature (77 K) with H2, N2 and Ar. Here we show the high degree of structural flexibility of ZIFs at near ambient temperatures and report the first time their unique adsorption and transport characteristics.