Hybrid Ultramicroporous Materials Research Articles

Boosting CO2 transport of poly (ethylene oxide) membranes by hollow Rubik-like “expressway” channels with anion pillared hybrid ultramicroporous materials

In order to overcome “trade-off” Robeson upper bound 2008 and simultaneously increase the CO2 permeability and selectivity of traditional poly (ethylene oxide) polymer membrane, three anion pillared hybrid ultramicroporous materials (SIFSIX-2-Cu-i, TIFSIX-2-Cu-i and GEFSIX-2-Cu-i) were incorporated into Pebax/PEGDME polymer to fabricate mixed matrix membranes (MMMs). DFT-D2 calculation revealed that the C···F distance (2.62 Å) between CO2 molecule and pillared anion (GeF62-) in GEFSIX-2-Cu-i was lower than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. The synergistic effects of C···F van der Waals (vdW) interactions and O···H hydrogen bonding interactions in GEFSIX-2-Cu-i gave a higher CO2 binding energy (34.5 kJ/mol) than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. It was verified that GEFSIX-2-Cu-i with higher CO2 binding energy and specific surface area exhibited higher CO2 adsorption capacity of 5.02 mmol/g than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. These anion-pillared hybrid ultramicroporous materials with suitable porosity and surface chemistry, enabling multiple host–guest interactions, created unique Rubik’s tube-like “expressway” channels for CO2 molecule passage through the membrane. Results showed that the MMMs with 1 wt% GEFSIX-2-Cu-i nanoparticles exhibited optimal CO2 separation performance in terms of CO2 permeability (460 Barrers) and CO2/H2 (17, an increase of 12.2%), CO2/CH4 (18, an increase of 24.1%), and CO2/N2 (57, an increase of 9.6%) selectivities, as compared with the pure Pebax/PEGDME membrane. As GEFSIX-2-Cu-i loading increase from 1 to 10 wt%, the overall CO2 separation performance decrease due to the aggregation of these nanoparticles. Positron annihilation lifetime spectroscopy (PALS) experiments revealed that the fractional free volume of MMMs with 1 wt% GEFSIX-2-Cu-i nanofillers (FFV = 3.63%) was higher than those of MMMs incorporated with a higher loading (2.5–10 wt%) of GEFSIX-2-Cu-i.

Geometry control of adsorption sites in sulfonate-pillared hybrid ultramicroporous materials for efficient C4 olefin separations

Efficient isolation of C4 olefins is of paramount importance in petrochemical industry yet a challenging task mainly realized via heat-driven processes owing to their similar physical properties. Herein, sulfonate anion-pillared hybrid ultramicroporous materials (HUMs), namely TMOF-1 and ZU-619 (BDS-1-Cu-i) were reported for the first time for the efficient separation of ternary C4 olefin mixtures (C4H6/n-C4H8/iso-C4H8). The geometry of adsorption sites was judiciously controlled via tuning the interpenetration direction of sulfonate anions through varying the length of anions. Interestingly, ethanedisulfonate anions in TMOF-1 exhibit the vertically interpenetrated direction, leading to TMOF-1 with pore size of 4.5 Å to exclude iso-C4H8. Furthermore, the vertical interpenetration makes the adjacent anions behave as synergetic adsorption sites for n-C4H8 and C4H6 trapping, thus endowing TMOF-1 with strong host–guest interactions and the benchmark Henry coefficient separation selectivities for C4H6/iso-C4H8 (519.2) and n-C4H8/iso-C4H8 (93.2) mixtures. In contrast, ZU-619 with butanedisulfonate anions as pillars exhibits normal interpenetration framework, giving rise to the anion-restricted pore size of 9.4 Å. This large pore size results in ZU-619 with weak host–guest interactions for C4 olefins through one sulfate anion. The specific host–guest interaction modes were unveiled by molecular simulations, presenting the efficiency of binding site geometry control for gas separation. The separation efficiency was confirmed by breakthrough experiments, the results presented that TMOF-1 showed excellent separation performances for C4H6/n-C4H8/iso-C4H8 mixtures and ZU-619 can be a good candidate for C4H6/n-C4H8 separation.

Amino‐Functionalised Hybrid Ultramicroporous Materials that Enable Single‐Step Ethylene Purification from a Ternary Mixture

AbstractPyrazine‐linked hybrid ultramicroporous (pore size <7 Å) materials (HUMs) offer benchmark performance for trace carbon capture thanks to strong selectivity for CO2 over small gas molecules, including light hydrocarbons. That the prototypal pyrazine‐linked HUMs are amenable to crystal engineering has enabled second generation HUMs to supersede the performance of the parent HUM, SIFSIX‐3‐Zn, mainly through substitution of the metal and/or the inorganic pillar. Herein, we report that two isostructural aminopyrazine‐linked HUMs, MFSIX‐17‐Ni (17=aminopyrazine; M=Si, Ti), which we had anticipated would offer even stronger affinity for CO2 than their pyrazine analogs, unexpectedly exhibit reduced CO2 affinity but enhanced C2H2 affinity. MFSIX‐17‐Ni are consequently the first physisorbents that enable single‐step production of polymer‐grade ethylene (>99.95 % for SIFSIX‐17‐Ni) from a ternary equimolar mixture of ethylene, acetylene and CO2 thanks to coadsorption of the latter two gases. We attribute this performance to the very different binding sites in MFSIX‐17‐Ni versus SIFSIX‐3‐Zn.

Amino-Functionalised Hybrid Ultramicroporous Materials that Enable Single-Step Ethylene Purification from a Ternary Mixture.

Pyrazine‐linked hybrid ultramicroporous (pore size <7 Å) materials (HUMs) offer benchmark performance for trace carbon capture thanks to strong selectivity for CO2 over small gas molecules, including light hydrocarbons. That the prototypal pyrazine‐linked HUMs are amenable to crystal engineering has enabled second generation HUMs to supersede the performance of the parent HUM, SIFSIX‐3‐Zn, mainly through substitution of the metal and/or the inorganic pillar. Herein, we report that two isostructural aminopyrazine‐linked HUMs, MFSIX‐17‐Ni (17=aminopyrazine; M=Si, Ti), which we had anticipated would offer even stronger affinity for CO2 than their pyrazine analogs, unexpectedly exhibit reduced CO2 affinity but enhanced C2H2 affinity. MFSIX‐17‐Ni are consequently the first physisorbents that enable single‐step production of polymer‐grade ethylene (>99.95 % for SIFSIX‐17‐Ni) from a ternary equimolar mixture of ethylene, acetylene and CO2 thanks to coadsorption of the latter two gases. We attribute this performance to the very different binding sites in MFSIX‐17‐Ni versus SIFSIX‐3‐Zn.

Mixed-metal hybrid ultramicroporous material (HUM) precursor to graphene-supported tetrataenite as a highly active and durable NPG catalyst for the OER.

Current interest in investigating non-precious group (NPG) metals for catalyzing the oxygen evolution reaction (OER) has revealed that doping of Ni hydroxides with Fe results in the dramatic enhancement of catalytic activity. Herein, a facile pathway to construct tetrataenite, an NiFe alloy of extraterrestrial origin and to address the limited electrical conductivity of metal oxides/hydroxides by directly constructing them atop graphene sheets is described. In this approach, a one-pot, bottom-up assembly of hybrid ultramicroporous materials (HUMs) was carried out, in the presence of suspended graphene (G), to homogeneously deposit the HUMs on unmodified graphene sheets, affording HUMs@G. Single metal (SIFSIX-3-Ni@G) and mixed metal (SIFSIX-3-NiFe@G) HUMs can be readily synthesized from their respective metal salts to afford a well-designed catalyst for the OER. The pyrolysis of SIFSIX-3-NiFe@G resulted in the deposition of the nanoalloy tetrataenite on G, demonstrating an exceptionally low OER onset potential of 1.44 V vs. RHE and reduced overpotential at 10 mA cm-2 (η10 = 266 mV). The synergy between the composition of the active catalyst and the electronically conductive support was attained by designing a reaction system encoding the self-assembly of a crystalline pre-catalyst on G sheets.

Computer simulations for the adsorption and separation of CH4/H2/CO2/N2 gases by hybrid ultramicroporous materials

The adsorption isotherms of two types of hybrid ultra-microporous materials SIFSIX-3-M (M = Fe, Cu, Zn) and M′FSIX-2-Cu-i (M′ = Si, Ti, Ge) for H2, CH4, CO2 and N2 are comparatively studied by grand canonical Monte Carlo simulation (GCMC), and their differences are explained from pore structure and isosteric heat of adsorption. Natural gas and biogas upgrading (CO2/CH4), synthesis gas separation (CO2/H2), flue gas separation (CO2/N2), and steam methane reforming hydrogen production (CH4/H2) are simulated equimolar binary mixture system. The related separation factors are elucidated from the adsorption selectivity and the difference of isosteric heat of adsorption. The microscopic adsorption mechanism of the material is further analyzed by the preferential adsorption sites of the gas on the framework. Our works provide new insights into the adsorption and separation mechanism of HUMs for the energy-related gas, as well as in industrial application.

Fine-Tuning Pore Dimension in Hybrid Ultramicroporous Materials Boosting Simultaneous Trapping of Trace Alkynes from Alkenes.

Removing trace amounts of alkynes from alkenes is one of the most critical and challenging steps to produce high-purity alkenes, the fundamental raw materials in petrochemical industry. Selective hydrogenation using noble metal catalysts under harsh conditions can convert trace alkynes to alkenes, but suffers from limited selectivity, over-hydrogenation, and energy-intensive consumption. Herein, the simultaneously adsorptive removal of trace propyne (C3 H4 ) and acetylene (C2 H2 ) from quaternary C2 H2 /C2 H4 /C3 H4 /C3 H6 mixture is reported for the first time using an anion-pillared hybrid ultramicroporous material ZU-16-Co (or TIFSIX-3-Co) by finely tuning the pore dimensions and introducing different binding sites to match the shape of alkynes. ZU-16-Co with contracted aperture size and judiciously extended cell dimension simultaneously exhibits superior trapping capacity for propyne under low concentration (2.45mmol g-1 at 5000ppm) and surprisingly high C2 H2 uptake (4.18 and 1.4mmol g-1 at 1.0 and 0.01bar, respectively) through synergistic host-guest and guest-guest interactions. Importantly, the ability of ZU-16-Co to capture trace alkynes (C2 H2 and C3 H4 ) in one step is confirmed by breakthrough experiments for quaternary C3 H4 /C2 H2 /C3 H6 /C2 H4 mixtures, presenting ZU-16-Co as a promising material for alkyne trapping.

Thin film synthesis of hybrid ultramicroporous materials (HUMs)- a comparative approach

The thin-film synthesis of the hybrid ultramicroporous material (HUM) TIFSIX-3-Ni on glass substrates are reported for the first time. Several methods of film formation are employed including dip-coating, seeding and secondary growth, vapour-assisted conversion, rapid thermal deposition and in-situ coating. Using the in-situ approach with dimethylformamide as solvent, we were able to grow homogeneous TIFSIX-3-Ni films at relatively low temperatures (85 °C) and short times (5 h) without substrate modification. During this study, we also significantly reduced the TIFSIX powder synthesis time to 15 h.

Thermal decomposition of hybrid ultramicroporous materials (HUMs).

Hybrid Ultramicroporous Materials (HUMs) are porous coordination materials with exemplary gas sorption and separation characteristics, but relatively poor thermal stability when compared to other porous coordination polymers or metal-organic frameworks (MOFs). The origin of this poor thermal stability has not yet been experimentally verified. Therefore, we investigate the thermal decomposition mechanisms of representative HUMs with the general formulae [M(SiF6)(L)2] or [M(SiF6)(L)(H2O)2], where M = Ni(ii), Cu(ii) or Zn(ii) and L = pyrazine or 4,4'-bipyridine. We find that two decomposition mechanisms dominate: (i) the fragmentation of the XF62- pillar into gaseous XF4 and fluoride, and (ii) direct sublimation of the N-donor ligand. The former process dictates the overall thermal stability of the material. We also demonstrate that HF is a possible decomposition product from certain hydrated HUM materials.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO2). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO2 from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO2 capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO2 selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.

Theoretical Investigation on Adsorption and Separation of CO2/N2 in Hybrid Ultramicroporous Materials

Theoretical Investigation on Adsorption and Separation of CO2/N2 in Hybrid Ultramicroporous Materials

Finding the Optimal Balance between the Pore Size and Pore Chemistry in Hybrid Ultramicroporous Materials for Trace Acetylene Capture

The development of porous coordination networks based on square grids pillared by inorganic anions has attracted attention thanks to their benchmark gas sorption properties (e.g., high uptake capac...

Michael J. Zaworotko

Angewandte Chemie International EditionVolume 57, Issue 46 p. 14978-14978 Author ProfileFree Access Michael J. Zaworotko First published: 19 June 2018 https://doi.org/10.1002/anie.201806413AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract “The most important thing I learned from my parents is integrity. My favorite place on earth is any Hawaiian island except Oahu …” Find out more about Michael J. Zaworotko in his Author Profile. Michael J. Zaworotko The author presented on this page has published more than 10 articles in Angewandte Chemie in the last 10 years, most recently: “Reversible Switching between Highly Porous and Nonporous Phases of an Interpenetrated Diamondoid Coordination Network That Exhibits Gate-Opening at Methane Storage Pressures”: Q.-Y. Yang et al., Angew. Chem. Int. Ed. 2018, 57, 5684; Angew. Chem. 2018, 130, 5786. Date of birth: August 14, 1956 Position: Bernal Chair of Crystal Engineering, University of Limerick E-mail: xtal@ul.ie Homepage: www.ul.ie/crystalengineering/michael_zaworotko ORCID: 0000-0002-1360-540X Education: 1977 Undergraduate degree, Imperial College London 1983 PhD supervised by Jerry L. Atwood, University of Alabama 1982–1985 Postdoctoral position with Stephen R. Stobart, University of Victoria Awards: 2017 Science Foundation of Ireland Researcher of the Year Current research interests: Fundamental and applied aspects of crystal engineering with particular emphasis upon porous materials (e.g., MOFs and hybrid ultramicroporous materials) and multicomponent molecular materials (e.g., pharmaceutical cocrystals) Hobbies: If only time permitted! The most important thing I learned from my parents is integrity. My favorite place on earth is any Hawaiian island except Oahu. My most exciting discovery to date has been learning that the right pore size and the right pore chemistry affords special properties. My worst nightmare is having to going back to school and take exams again. The best advice I have ever been given is that the title and abstract are the most important parts of a paper or proposal. If I could go back in time and do any experiment, it would be to have first studied gas adsorption in 1995 instead of 2007. I can never resist temptation (apologies to devotees of Oscar Wilde). I celebrate success by going on a pub night with my group. My top three films of all time are Cloud Atlas, Dr. Strangelove, The Matrix. My favorite food is anything with chili peppers, such as Mexican, Thai, Indian, Sichuan, Irish-Mex (e.g., Irish stew with chili peppers). My favorite saying is a very, very small piece of something very large is infinitely better than all of nothing. The most significant scientific advance of the last 100 years has been molecular biology enabled by structural biochemistry. My 5 top papers: 1“Air and water stable 1-ethyl-3-methylimidazolium based ionic liquids”: J. S. Wilkes, M. J. Zaworotko, J. Chem. Soc., Chem. Commun. 1992, 965. (Until this paper was published, ionic liquids were a niche area based mainly upon compounds that could not be exposed to moist atmospheres.) 2“Porous Solids by Design: [Zn(4,4’-bpy)2(SiF6)]n⋅xDMF, a Single Framework Octahedral Coordination Polymer with Large Square Channels”: S. Subramanian, M. J. Zaworotko, Angew. Chem. Int. Ed. 1995, 34, 2127; Angew. Chem. 1995, 107, 2295. (A crystal by design and the parent member of pillared square grid networks.) 3“X-ray crystal structure of C6H3(CO2H)3-1,3,5⋅1.5(4,4’-bipy): a ‘super trimesic acid’ chicken-wire grid”: C. V. K. Sharma, M. J. Zaworotko, Chem. Commun. 1996, 2655. (Our first realization that cocrystals are a) amenable to design and b) can be favored over single-component crystals.) 4“Enhanced CO2 Binding Affinity of a High-Uptake rht-type Metal–Organic Framework Decorated with Acylamide Groups”: B. Zheng, J. Bai, J. Duan, L. Wojtas, M. J. Zaworotko, J. Am. Chem. Soc. 2011, 133, 748. (Realization that systematic control over pore chemistry profoundly impacts gas-sorption performance.) 5“Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation”: P. Nugent et al., Nature 2013, 495, 80. (The right combination of pore size and pore chemistry can enable an order-of-magnitude improvement in CO2 capture performance.) Volume57, Issue46November 12, 2018Pages 14978-14978 ReferencesRelatedInformation

Hexafluorogermanate (GeFSIX) Anion-Functionalized Hybrid Ultramicroporous Materials for Efficiently Trapping Acetylene from Ethylene

A trace amount of acetylene (C2H2, 1%) removal from ethylene/acetylene mixture is crucial for the production of polymer-grade ethylene (C2H4), which is a highly challenging task and currently is mainly realized via highly energy-intensive technologies. Herein, we first report the highly efficient trapping of C2H2 from C2H2/C2H4 mixtures using two hexafluorogermanate (GeF62–, GeFSIX) anion-functionalized hybrid ultramicroporous materials, GeFSIX-2-Cu-i and GeFSIX-14-Cu-i (also termed ZU-32 and ZU-33), as novel adsorbents. These GeFSIX materials exhibit high thermal stability and tunable pore structures and have a high density of electronegative GeF62– anions decorated on the pore surface. ZU-32 with an aperture size of 4.5 A exhibits preferential binding ability for C2H2 molecules and thus offers an excellent separation selectivity of 67 for the C2H2/C2H4 (1/99) mixture. ZU-33 with 4,4-azopyridine (azpy, 9.0 A) as an organic linker exhibits a contracted pore window size with pyridine ring tilting of ca. 30...

Efficient CO2 Removal for Ultra‐Pure CO Production by Two Hybrid Ultramicroporous Materials

AbstractRemoval of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra‐pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX‐3‐Ni and TIFSIX‐2‐Cu‐i, which are known to exhibit strong affinity for CO2, were examined with respect to their performance for this separation. The single‐gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2/CO selectivity (&gt;4000 for SIFSIX‐3‐Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra‐pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one‐step physisorption‐based separation process.

Efficient CO2 Removal for Ultra-Pure CO Production by Two Hybrid Ultramicroporous Materials.

Removal of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra-pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX-3-Ni and TIFSIX-2-Cu-i, which are known to exhibit strong affinity for CO2 , were examined with respect to their performance for this separation. The single-gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2 /CO selectivity (>4000 for SIFSIX-3-Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra-pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one-step physisorption-based separation process.

A Single-Molecule Propyne Trap: Highly Efficient Removal of Propyne from Propylene with Anion-Pillared Ultramicroporous Materials.

Propyne/propylene (C3 H4 /C3 H6 ) separation is a critical process for the production of polymer-grade C3 H6 . However, optimization of the structure of porous materials for the highly efficient removal of C3 H4 from C3 H6 remains challenging due to their similar structures and ultralow C3 H4 concentration. Here, it is first reported that hybrid ultramicroporous materials with pillared inorganic anions (SiF62- = SIFSIX, NbOF52- = NbOFFIVE) can serve as highly selective C3 H4 traps for the removal of trace C3 H4 from C3 H6 . Especially, it is revealed that the pyrazine-based ultramicroporous material with square grid structure for which the pore shape and functional site disposition can be varied in 0.1-0.5 Å scale to match both the shape and interacting sites of guest molecule is an interesting single-molecule trap for C3 H4 molecule. The pyrazine-based single-molecule trap enables extremely high C3 H4 uptake under ultralow concentration (2.65 mmol g-1 at 3000 ppm, one C3 H4 per unit cell) and record selectivity over C3 H6 at 298 K (>250). The single-molecule binding mode for C3 H4 within ultramicroporous material is validated by X-ray diffraction experiments and modeling studies. The breakthrough experiments confirm that anion-pillared ultramicroporous materials set new benchmarks for the removal of ultralow concentration C3 H4 (1000 ppm on SIFSIX-3-Ni, and 10 000 ppm on SIFSIX-2-Cu-i) from C3 H6 .

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance.

Phases of a 2-fold pcu hybrid ultramicroporous material (HUM), SIFSIX-14-Cu-i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1 : 99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance.

Crystal engineering of dichromate pillared hybrid ultramicroporous materials incorporating pyrazole-based ligands

Short and sterically encumbered pyrazole ligands have led to the first examples of non-interpenetrated, dichromate pillared hybrid ultramicroporous materials.

Controlling the Uptake and Regulating the Release of Nitric Oxide in Microporous Solids.

Representative compounds from three classes of microporous solids, namely, metal-organic frameworks (MOFs), hybrid ultra-microporous materials (HUMs), and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low-pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine-functionalized parent. The HUMs demonstrated reversible physisorption within the low-pressure regime, but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium.