Nanoporous zeolitic imidazolate frameworks (ZIFs) form structural topologies equivalent to zeolites. ZIFs containing only one type of imidazole linker show separation capability for limited molecular ...pairs. We show that the effective pore size, hydrophilicity, and organophilicity of ZIFs can be continuously and drastically tuned using mixed-linker ZIFs containing two types of linkers, allowing their use as a more general molecular separation platform. We illustrate this remarkable behavior by adsorption and diffusion measurements of hydrocarbons, alcohols, and water in mixed-linker ZIF-8 x -90100–x materials with a large range of crystal sizes (338 nm to 120 μm), using volumetric, gravimetric, and PFG-NMR methods. NMR, powder FT-Raman, and micro-Raman spectroscopy unambiguously confirm the mixed-linker nature of individual ZIF crystals. Variation of the mixed-linker composition parameter (x) allows continuous control of n-butane, i-butane, butanol, and isobutanol diffusivities over 2–3 orders of magnitude and control of water and alcohol adsorption especially at low activities.
The nature of the surface species formed through the adsorption of CO2 on amine‐grafted mesoporous silica is investigated through in situ FTIR spectroscopy with the aid of 15N dynamic nuclear ...polarization (DNP) and 13C NMR spectroscopy. Primary, secondary, and tertiary amines are functionalized onto a mesoporous SBA‐15 silica. Both isotopically labeled 13CO2 and natural‐abundance CO2 are used for accurate FTIR peak assignments, which are compared with assignments reported previously. The results support the formation of monomeric and dimeric carbamic acid species on secondary amines that are stabilized differently to the monocarbamic acid species on primary amines. Furthermore, the results from isotopically labelled 13CO2 experiments suggest the existence of two carbamate species on primary amines, whereas only one species is observed predominantly on secondary amines. The analysis of the IR peak intensities and frequencies indicate that the second carbamate species on primary amines is probably more asymmetric in nature and forms in a relatively smaller amount. Only the formation of bicarbonate ions at a low concentration is observed on tertiary amines; therefore, physisorbed water on the surface plays a role in the hydrolysis of CO2 even if water is not added intentionally and dry gases are used. This suggests that a small amount of bicarbonate ions could be expected to form on primary and secondary amines, which are more hydrophilic than tertiary amines, and these low concentration species are difficult to observe on such samples.
Surface species revealed: The formation of CO2 surface species on amine‐grafted SBA‐15 is observed through in situ FTIR spectroscopy. The results show that the structures of carbamate and carbamic acid species are different between primary and secondary amines, whereas only a small concentration of bicarbonate ions is observed on tertiary amines under ostensibly dry conditions.
Zeolite membranes show great potential for gas and hydrocarbon separations, but high manufacturing cost has been one of the main hurdles in their industrial application. Here we demonstrate a method ...termed viscosity‐confined dry gel conversion (VCDGC) for zeolite hollow fiber membrane fabrication. We demonstrate in detail the VCDGC synthesis of small‐pore CHA zeolite membranes. Extensive permeation measurements reveal that dry gel‐processed CHA zeolite hollow fiber membranes have excellent gas and hydrocarbon separation characteristics well exceeding or comparable to current membranes. Medium‐pore MFI membranes are also fabricated, and their favorable hydrocarbon separation characteristics indicate the versatility and reliability of the VCDGC method.
A new approach (viscosity‐confined dry gel conversion, VCDGC) is demonstrated for versatile fabrication of zeolite membranes on inexpensive hollow fibers. VCDGC does not require liquid‐phase hydrothermal processing and is amenable to integration with coating processes. The VCDGC synthesis allows preparation of high‐quality CHA and MFI zeolite type membranes, which showed excellent performance in gas separations.
Supported catalysis is emerging as a cornerstone of transition metal catalysis, as environmental awareness necessitates “green” methodologies and transition metal resources become scarcer and more ...expensive. Although these supported systems are quite useful, especially in their capacity for transition metal catalyst recycling and recovery, higher activity and selectivity have been elusive compared with nonsupported catalysts. This Account describes recent developments in polymer-supported metal−salen complexes, which often surpass nonsupported analogues in catalytic activity and selectivity, demonstrating the effectiveness of a systematic, logical approach to designing supported catalysts from a detailed understanding of the catalytic reaction mechanism. Over the past few decades, a large number of transition metal complex catalysts have been supported on a variety of materials ranging from polymers to mesoporous silica. In particular, soluble polymer supports are advantageous because of the development of controlled and living polymerization methods that are tolerant to a wide variety of functional groups, including controlled radical polymerizations and ring-opening metathesis polymerization. These methods allow for tuning the density and structure of the catalyst sites along the polymer chain, thereby enabling the development of structure−property relationships between a catalyst and its polymer support. The fine-tuning of the catalyst−support interface, in combination with a detailed understanding of catalytic reaction mechanisms, not only permits the generation of reusable and recyclable polymer-supported catalysts but also facilitates the design and realization of supported catalysts that are significantly more active and selective than their nonsupported counterparts. These superior supported catalysts are accessible through the optimization of four basic variables in their design: (i) polymer backbone rigidity, (ii) the nature of the linker, (iii) catalyst site density, and (iv) the nature of the catalyst attachment. Herein, we describe the design of polymer supports tuned to enhance the catalytic activity or decrease, or even eliminate, decomposition pathways of salen-based transition metal catalysts that follow either a monometallic or a bimetallic reaction mechanism. These findings result in the creation of some of the most active and selective salen catalysts in the literature.
Direct CO2 capture from atmospheric air is gaining increased attention as one of the most scalable negative carbon approaches available to tackle climate change if coupled with the sequestration of ...CO2 geologically. Furthermore, it can also provide CO2 for further utilization from a globally uniform source, which is especially advantageous for economies without natural sources of carbon-based feedstocks. Solid-supported amine-based materials are effective for direct air capture (DAC) due to their high CO2 uptakes and acceptable sorption kinetics at ambient temperature. In this work, we describe the application of polymer/silica fiber sorbents functionalized with a primary amine-rich polymer, poly(ethylenimine) (PEI), for DAC. Monolithic fiber sorbents composed of cellulose acetate and SiO2 are synthesized via the dry-jet, wet quench spinning technique. These fibers are then functionalized with PEI (M w 800 Da) in a simple and scalable postspinning infusion step and tested for CO2 capture under pseudoequilibrium conditions as well as under breakthrough conditions. An investigation to study the effect of feed flow rate, adsorption temperature, and presence of moisture in the feed on the CO2 breakthrough performance of a densely packed fiber sorbent module is conducted to highlight the potential application of this class of structured contactors in direct air capture. The pressure drop of these contactors at high gas velocities is also evaluated. Finally, a vacuum-assisted desorption step is demonstrated for production of high-purity CO2 from both dry and humid ambient air mixtures.
Direct adsorption of CO2 from ambient air, also known as direct air capture (DAC), is gaining attention as a complementary approach to processes that capture CO2 from more concentrated sources such ...as flue gas. Oxide-supported amine materials are effective materials for CO2 capture from dilute gases, but less work has been done on metal organic framework (MOF) supported amine materials. Use of MOFs as supports for amines could be a versatile approach to the creation of effective amine adsorbents because of the tunability of MOF structures. In the present work, MIL-101(Cr) materials functionalized with amine species are evaluated for CO2 capture from simulated air. Two amines are loaded into the MOF pores, tris (2-amino ethyl) (TREN) and low molecular weight, branched poly(ethylene imine) (PEI-800), at different amine loadings. The MIL-101(Cr)-TREN composites showed high CO2 capacities for high loadings of TREN, but a significant loss of amines is observed over multicycle temperature swing adsorption experiments. In contrast, MIL-101(Cr)-PEI-800 shows better cyclic stability. The amine efficiency (mol CO2/mol amine) as a function of amine loading is used as a metric to characterize the adsorbents. The amine efficiency in MIL-101(Cr)-PEI-800 showed a strong dependence on the amine loading, with a step change to high amine efficiencies occurring at ∼0.8 mmol PEI/g MOF. The kinetics of CO2 capture, which have important implications for the working capacity of the adsorbent, are also examined, demonstrating that a MIL-101(Cr)-PEI-800 sample with a 1–1.1 mmol PEI/g MOF loading has an excellent balance of CO2 capacity and CO2 adsorption kinetics.
Cooperative interactions between aminoalkylsilanes and silanols on a silica surface can be controlled by varying the length of the alkyl linker attaching the amine to the silica surface from C1 ...(methyl) to C5 (pentyl). The linker length strongly affects the catalytic cooperativity of amines and silanols in aldol condensations as well as the adsorptive cooperativity for CO2 capture. The catalytic cooperativity increases with the linker length up to propyl (C3), with longer, more flexible linkers (up to C5) providing no additional benefit or hindrance. Short linkers (C1 and C2) limit the beneficial amine–silanol cooperativity in aldol condensations, resulting in lower catalytic rates than with the C3+ linkers. For the same materials, the adsorptive cooperativity exhibits similar trends for CO2 capture efficiency.
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