Perspectives on NMR studies of CO2 adsorption Bernin, Diana; Hedin, Niklas
Current opinion in colloid & interface science,
January 2018, 2018-01-00, 2018, Letnik:
33
Journal Article
Recenzirano
There is a consensus about a long-term goal of a carbon-neutral energy cycle, but the CO2 emissions to the atmosphere are currently very large. Carbon Capture and Storage (CCS) technologies could ...allow a transformation of the global energy system into a carbon-neutral one and simultaneously keeping the temperature rises within agreed bounds. The CO2 separation step of CCS is, however, very expensive, and adsorption-driven technologies have been put forward as alternatives. Hence, a recent focus has been on studying solid adsorbents for CO2, which include activated carbons, zeolites, metal-organic frameworks, and amine-modified silica. In this context, we summarize the literature concerning CO2 sorption studied with Nuclear Magnetic Resonance (NMR), outline selected NMR methods, and present an outlook for further studies.
Display omitted
•NMR studies on CO2 sorbents are reviewed.•Emerging NMR methodologies are briefly discussed.•Experimental specifics are highlighted for NMR studies of CO2 adsorption.•An outlook for further physi- and chemisorption studies is presented.
Microporous polyureas can be highly stable, but isocyanates or phosgene are normally used for the synthesis. Here, it was postulated and demonstrated that 1,1′-carbonyl diimidazole (CDI) could be ...used for the synthesis. By reacting tetrakis(4-aminophenyl)methane with CDI, a series of new polyureas with ultramicropores (pores <0.7 nm) were synthesized. To tailor thermal properties and porosity, the ratio of tetraamine-to-CDI and the reaction temperature were varied. The CO 2 adsorption capacities (with values up to 0.74 mmol/g at 0.15 bar/273 K and 1.91 mmol/g at 1 bar/273 K) were ascribed to the ultramicroporosity. The CO 2 -based Dubinin-Radushkevich surface areas reached 395 m 2 /g (at 273 K), while the N 2 -based BET surface areas (at 77 K) were small. The apparent CO 2 -over-N 2 selectivity was also high for the polymers at 273 K with estimated values of 31–92 for 15/85 v/v mixtures of CO 2 and N 2 . This high selectivity was ascribed to the kinetic hindrance of N 2 diffusion. It was noted that one of the polymers changed color irreversibly upon heating. In conclusion, it was shown that CDI and amines could be used to synthesize ultramicroporous polyureas, and that these polymers can exhibit irreversible thermochromism. This thermal effect was attributed to the electron-rich urea moieties, aromatic units, and conjugation.
Display omitted
•Commercial and potential adsorbents were investigated for separation of CO2 and CH4.•Zeolite |Na12−xKx|-LTA (1.8⩽x⩽3.2) has good CO2 but almost no CH4 uptake.•SAPO-56 has a high CO2 ...working capacity and high CO2/CH4 selectivity.
Several commercial and potential adsorbents were investigated for the separation of CO2 from CH4, which is relevant for the upgrading of raw biogas. The main focus of the paper was on the working capacities and selectivities of the adsorbents for a generic vacuum swing adsorption (VSA) process. Zeolites 4A and 13X had good estimated CO2-over-CH4 selectivities and reasonably high working capacities for the removal of CO2. A variant of zeolite A – |Na12−xKx|-LTA (with 1.8⩽x⩽3.2), had at least the same working capacity as zeolite 4A but with a significantly improved selectivity. Hence, the environmentally important “CH4 slip” can be minimized with this |Na12−xKx|-LTA sorbent. If a high working capacity for CO2 removal is the most important characteristic for a VSA process, then silicoaluminum phosphate, specifically SAPO-56, appeared to be the best candidate among the studied sorbents. In addition, SAPO-56 had a substantially high estimated CO2-over-CH4 selectivity with a value between ∼20 and 30.
Wilkinson's catalyst RhCl(PPh3)3 was heterogenized on common silica by the use of a grafting/anchoring technique. The immobilized catalyst showed high activity and selectivity in transfer ...hydrogenation reactions of a range of carbonyl compounds in 2‐propanol. Reactions carried out in 2‐propanol at reflux afforded the corresponding alcohols in high yields in short reaction times. The heterogeneous feature of the catalyst allows for easy recovery and efficient reuse in the same reaction up to 5 times without any detectible loss of catalytic activity.
Wilkinson's catalyst RhCl(PPh3)3 has been immobilized on silica through a monodentate phosphane ligand and used in the transfer hydrogenation of alkyl and aryl carbonyl compounds. The use of 2‐propanol as both hydrogen donor and solvent allows for high yields of the corresponding alcohols under mild reaction conditions. The recyclability of this heterogenized Rh catalyst is also demonstrated.
The structure of a novel molecularly ordered two-dimensional (2D) silicate framework in a surfactant-templated mesophase has been established by using a combination of solid-state nuclear magnetic ...resonance (NMR) spectroscopy, X-ray diffraction, and quantum chemical and empirical force-field modeling. These materials are unusual in their combination of headgroup-directed 2D crystalline framework ordering, zeolite-like ring structures within the layers, and long-range mesoscopic organization without three-dimensional (3D) atomic periodicity. The absence of registry between the silicate sheets, resulting from the liquidlike disorder of the alkyl surfactant chains, has presented significant challenges to the determination of framework structures in these and similar materials lacking 3D crystalline order. Double-quantum 29Si NMR correlation experiments establish the interactions and connectivities between distinct intra-sheet silicon sites from which the structure of the molecularly ordered inorganic framework is determined.
Mechanically strong monoliths of zeolite NaKA with a hierarchy of pores displayed very high CO2-over-N-2 selectivity. The zeolite monoliths were produced by pulsed current processing (PCP) without ...the use of added binders and with a preserved microporous crystal structure. Adsorption isotherms of CO2 and N-2 were determined and used to predict the co-adsorption of CO2 and N-2 using ideal adsorbed solution theory (IAST). The IAST predictions showed that monolithic adsorbents of NaKA could reach an extraordinarily high CO2-over-N-2 selectivity in a binary mixture with a composition similar to flue gas (15 mol% CO2 and 85 mol% N2 at 25 degrees C and 101 kPa). Structured NaKA monoliths with a K+ content of 9.9 at% combined a CO2-over-N-2 selectivity of >1100 with a high CO2 adsorption capacity (4 mmol g(-1)) and a fast adsorption kinetics (on the order of one minute). Estimates of a figure of merit (F) based on IAST CO2-over-N-2 selectivity, and time-dependent CO2 uptake capacity, suggest that PCP-produced structured NaKA with a K+ content of 9.9 at% offers a performance far superior to 13X adsorbents, in particular at short cycle times.
Nanostructured hydrated vanadium oxides (V2O5·nH2O) are actively being researched for applications in energy storage, catalysis, and gas sensors. Recently, a one-step exfoliation technique for ...fabricating V2O5·nH2O nanosheets in aqueous media was reported; however, the underlying mechanism of exfoliation has been challenging to study. Herein, we followed the synthesis of V2O5·nH2O nanosheets from the V2O5 and VO2 precursors in real time using solution- and solid-state 51V NMR. Solution-state 51V NMR showed that the aqueous solution contained mostly the decavanadate anion H2V10O284– and the hydrated dioxovanadate cation VO2·4H2O+, and during the exfoliation process, decavanadate was formed, while the amount of VO2·4H2O+ remained constant. The conversion of the solid precursor V2O5, which was monitored with solid-state 51V NMR, was initiated when VO2 was in its monoclinic forms. The dried V2O5·nH2O nanosheets were weakly paramagnetic because of a minor content of isolated V4+. Its solid-state 51V signal was less than 20% of V2O5 and arose from diamagnetic V4+ or V5+.This study demonstrates the use of real-time NMR techniques as a powerful analysis tool for the exfoliation of bulk materials into nanosheets. A deeper understanding of this process will pave the way to tailor these important materials.
Tuning the charge transfer processes through a built-in electric field is an effective way to accelerate the dynamics of electro- and photocatalytic reactions. However, the coupling of the built-in ...electric field of p–n heterojunctions and the microstrain-induced polarization on the impact of piezocatalysis has not been fully explored. Herein, we demonstrate the role of the built-in electric field of p-type BiOI/n-type BiVO4 heterojunctions in enhancing their piezocatalytic behaviors. The highly crystalline p–n heterojunction is synthesized by using a coprecipitation method under ambient aqueous conditions. Under ultrasonic irradiation in water exposed to air, the p–n heterojunctions exhibit significantly higher production rates of reactive species (·OH, ·O2 –, and 1O2) as compared to isolated BiVO4 and BiOI. Also, the piezocatalytic rate of H2O2 production with the BiOI/BiVO4 heterojunction reaches 480 μmol g–1 h–1, which is 1.6- and 12-fold higher than those of BiVO4 and BiOI, respectively. Furthermore, the p–n heterojunction maintains a highly stable H2O2 production rate under ultrasonic irradiation for up to 5 h. The results from the experiments and equation-driven simulations of the strain and piezoelectric potential distributions indicate that the piezocatalytic reactivity of the p–n heterojunction resulted from the polarization intensity induced by periodic ultrasound, which is enhanced by the built-in electric field of the p–n heterojunctions. This study provides new insights into the design of piezocatalysts and opens up new prospects for applications in medicine, environmental remediation, and sonochemical sensors.
Highly porous and hyper‐cross‐linked polymers (HCPs) have a range of applications and are typically synthesized in an unsustainable manner. Herein, HCPs were synthesized from abundant biobased or ...biorelated compounds in sulfolane with iron(III) chloride as Lewis acid catalyst. As reactants, quercetin, tannic acid, phenol, 1,4‐dimethoxybenzene, glucose, and a commercial bark extract were used. The HCPs had high CO2 uptake (up to 3.94 mmol g−1 at 0 °C and 1 bar), total pore volumes (up to 1.86 cm3 g−1), and specific surface areas (up to 1440 m2 g−1). 1H NMR, 13C NMR, and IR spectroscopy, wide‐angle X‐ray scattering, elemental analysis, and SEM revealed, for example, that the HCPs consisted of amorphous and cross‐linked aromatic and phenolic structures with significant contents of aliphatics, oxygen, and sulfur.
Greener HCPs: A wide range of biomass‐based aromatic compounds are used as precursors for highly porous hyper‐cross‐linked polymers (HCPs) in a sustainable synthesis system. The HCPs have large pore volumes and high CO2 uptakes, which are useful in sustainability‐related applications.
Display omitted
► Hydrothermally carbonized biomass were used to produce activated carbons. ► Physical activation with CO2 were compared with chemical activation with H3PO4. ► The activated carbons ...had small pores and displayed a high capacity to adsorb CO2. ► The activated carbons adsorbed CO2 rapidly and had excellent cyclability. ► A chemically activated carbon showed a high CO2-over-N2 selectivity.
Activated carbons prepared from hydrothermally carbonized (HTC) waste biomass were studied with respect to the adsorption of carbon dioxide. The physically activated carbons (PAC) exhibited a large adsorption of CO2 of 1.45mmol/g at a small partial pressure of CO2 (10kPa and a temperature of 0°C). These PACs were prepared by activation in a stream of CO2 and had significant amounts of ultramicropores, which were established by analyzing the adsorption of CO2 with a density functional theory. The uptake at such low pressures of CO2 is of most importance for an adsorption-driven CO2 capture from flue gas at large power stations, as it is difficult to imagine a pressurization of the flue gas. The capacities to adsorb CO2 of the different activated carbons were compared with both the micropore volumes as established by N2 adsorption, and the ultramicropore volumes as established by CO2 adsorption. The ultramicropore volume is of crucial importance for the capture of CO2 from flue gas. PAC from HTC grass cuttings and from horse manure had the largest ultramicropore volumes. In general, the PAC showed excellent cyclability of adsorption/desorption of CO2 and a minimal capacity loss after subsequent cycles. In addition, the PAC showed a rapid adsorption of CO2. Both characteristics are essential for the eventual use of such PACs in the adsorption driven separation of CO2 from flue gas. A chemically activated carbon (CAC) was prepared by treating hydrothermally carbonized beer waste with H3PO4 and a heat treatment in a flow of N2. This CAC showed a significant amount of mesopores in the range of 5nm, in addition to micropores. The apparent selectivity for the activated carbons for CO2-over-N2 adsorption was determined at 0°C and 10kPa.
PDF
