Industrial separation processes comprise approximately 10% of the global energy demand, driven largely by the utilization of thermal separation methods (e.g., distillation). Significant energy and cost savings can be realized using advanced separation techniques such as membranes and sorbents. One of the major barriers to acceptance of these techniques remains creating materials that are efficient and productive in the presence of aggressive industrial feeds. One promising class of emerging materials is zeolitic imidazolate frameworks (ZIFs), an important thermally and chemically stable subclass of metal organic frameworks (MOFs). The objectives of this paper are (i) to provide a current understanding of the synthetic methods that enable the immense tunability of ZIFs, (ii) to identify areas of success and areas for improvement when ZIFs are used as adsorbents, (iii) to identify areas of success and areas for improvement in ZIF membranes. A review is given of the state-of-the-art in ZIF synthesis procedures and novel ZIF formation pathways as well as their application in energy efficient separations.

The selective electrochemical reduction of CO2 to CO in water by a Re­(tBu-bpy)­(CO)3Cl catalyst incorporated into multi-walled carbon nanotubes (MWCNT) was investigated. Current densities of ∼4 mA/cm2 and selectivities (FECO) of 99% were achieved at −0.56 V vs RHE in CO2-saturated aqueous KHCO3 solutions. The Re­(tBu-bpy)­(CO)3Cl catalyst has been widely studied as a homogeneous catalyst in organic solvents. Supporting Re­(tBu-bpy)­(CO)3Cl on MWCNTs increases current densities, decreases overpotential, retains selectivity for reduction of CO2 to CO, and allows operation in water at pH = 7.3 compared to the molecular catalyst in acetonitrile solution. The Re/MWCNT electrocatalysts achieve TON > 5600 and TOF > 1.6 s–1. This electrocatalyst material is efficient, robust, simple to prepare, and scalable.

Recent increases in shale gas production provide an excellent opportunity for advancements in the energy-efficient separation of natural gas liquids because these are increasingly used as fuel sources and chemical feedstocks. Current fractionation schemes generally involve cryogenic distillation of C 1 −C 4 hydrocarbons and are extremely energyintensive. Here, we describe the first steps toward a lowerenergy, kinetic pressure-swing-adsorption cycle for the separation of ethane, propane, propylene, and butane using ZIF-8 as a diffusionally selective adsorbent. Crystal engineering techniques were employed to control the diffusive time scale of the separation, allowing for multiple separations using the same adsorbent within reasonable process times. Equimolar separation of ethane/propane mixtures at 293 K exhibited separation factors of 2.7 in the gas phase under nonoptimized conditions, which enhances the concentration of the feed mixture to 75 mol % propane. The separation performance was shown to improve to 3.8 at lower temperatures (81 mol % propane), which is attributed to differences in the activation energy of permeation of the two components. Propane/butane mixtures demonstrated a lower diffusive selectivity and almost negligible enhancement, while propylene/propane showed enhancement beyond ethane/propane due to a strong diffusive selectivity and sorption selectivities closer to unity. Single-component adsorption and diffusion results were incorporated into a computational model of the system and shown to be in relatively good agreement with the experimental values. The model was used to predict the separation system performance and recovery at various temperatures.

We report a synthetic route for the production of water-sensitive metal−organic frameworks (MOFs) in polymer fiber sorbents by use of metal oxides as seeding intermediates. Cellulose acetate/ZnO (48 wt %) fibers were spun via a dry-jet wet-quench method and converted through hydroxy double salt (HDS) intermediates into HKUST-1 and ZIF-8. MOF loadings within the fiber sorbent reached 85 and 66 wt %, respectively. We demonstrate this process on module-packaged fibers, in which ready-to-use fiber sorbents are synthesized in a moisture-free environment. Modules are then employed in proof-of-concept CO 2 /N 2 breakthrough experiments.

2017-08-25T16:28:54