Teresa Curtin

Direct Air Capture of CO2 by Physisorbent Materials

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture, DAC), could mitigate carbon emissions. Here five materials are investigated for their ability to adsorb CO2 directly from air and other gas mixtures. The sorbents studied are benchmark materials that encompass four types of porous material, one chemisorbent, TEPA-SBA-15 (amine-modified mesoporous silica) and four physisorbents: Zeolite 13X (inorganic); HKUST-1 and Mg-MOF-74/Mg-dobdc (metal-organic frameworks, MOFs); SIFSIX-3-Ni, (hybrid ultramicroporous material). Temperature-programmed desorption (TPD) experiments afforded information about the contents of each sorbent under equilibrium conditions and their ease of recycling. Accelerated stability tests addressed projected shelf-life of the five sorbents. The four physisorbents were found to be capable of carbon capture from CO2-rich gas mixtures, but competition and reaction with atmospheric moisture significantly reduced their DAC performance.

The catalytic oxidation of ammonia : influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

Abstract The CuO/Al2O3 system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al2O3 is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N2 or N2O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N2O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N2 formation.

Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam

Abstract Boron oxide catalysts supported on alumina have been prepared with boria loadings in the range 0–20 wt.-%. These materials were characterized by B.E.T. surface area measurements, X-ray diffraction and temperature-programmed desorption of ammonia and were used as catalysts for the Beckmann rearrangement of cyclohexanone oxime to caprolactam by passing the oxime vapour through a reactor containing the catalysts at 300°C. The surface areas of the samples decreased as the boria loading was increased, and all catalysts tested for the rearrangement reaction diminished in activity with time-onstream. There was a direct relationship between the amount of coke which formed on the surface and the loss in catalytic activity. This loss was least for the highest boron oxide loading tested, and this catalyst also featured the least amount of coke formation. Selectivity to caprolactam was associated with the presence of surface acidic sites of intermediate strength. A correlation was observed between the concentration of these surface sites and the selectivity to caprolactam.

Factors affecting selectivity in the rearrangement of cyclohexanone oxime to caprolactam over modified aluminas

Abstract The Beckmann rearrangement of cyclohexanone oxime to caprolactam has been studied over a range of aluminas modified by the addition of chloride, phosphate, sulphate, sodium or boron oxide. The catalysts were characterized by temperature-programmed desorption of ammonia and carbon dioxide and by pyridine adsorption monitored by infrared spectroscopy. Conversion of the oxime declined with time-on-stream for all catalysts studied, but selectivity to caprolactam remained constant during the first five hours of operation. In this period a relationship was observed between the selectivity to cap- rolactam and the surface concentration of intermediate strength acid sites, wherein the selectivity increased as the surface concentration of these sites increased. The boron oxide modified alumina presented the largest concentration of intermediate strength acid sites, and the low level of coke formation on this material was associated with the total absence of surface basicity. A mechanism for the rear- rangement reaction on the modified aluminas is presented.

The relationship between acidity of aluminas and their selectivity in the conversion of cyclohexanone oxime into caprolactam

A range of aluminas modified by addition of sodium, phosphate, sulphate, chloride and boria have been tested for the Beckmann rearrangement of cyclohexanone oxime to caprolactam in the temperature range 300–350 °C. A relationship was observed between the selectivity to caprolactam and the surface concentration of acidic sites of intermediate strength.

Flue-gas and direct-air capture of CO2 by porous metal-organic materials.

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture), is a technological goal important to large-scale industrial processes such as gas purification and the mitigation of carbon emissions. Previously, we investigated five porous materials, three porous metal–organic materials (MOMs), a benchmark inorganic material, Zeolite 13X and a chemisorbent, TEPA-SBA-15, for their ability to adsorb CO2 directly from air and from simulated flue-gas. In this contribution, a further 10 physisorbent materials that exhibit strong interactions with CO2 have been evaluated by temperature-programmed desorption for their potential utility in carbon capture applications: four hybrid ultramicroporous materials, SIFSIX-3-Cu, DICRO-3-Ni-i, SIFSIX-2-Cu-i and MOOFOUR-1-Ni; five microporous MOMs, DMOF-1, ZIF-8, MIL-101, UiO-66 and UiO-66-NH2; an ultramicroporous MOM, Ni-4-PyC. The performance of these MOMs was found to be negatively impacted by moisture. Overall, we demonstrate that the incorporation of strong electrostatics from inorganic moieties combined with ultramicropores offers improved CO2 capture performance from even moist gas mixtures but not enough to compete with chemisorbents. This article is part of the themed issue ‘Coordination polymers and metal–organic frameworks: materials by design’.

Effectiveness of a constructed wetland for treating alkaline bauxite residue leachate: a 1-year field study

Increasing volumes of bauxite residues and their associated leachates represent a significant environmental challenge to the alumina industry. Constructed wetlands have been proposed as a potential approach for leachate treatment, but there is limited data on field-scale applications. The research presented here provides preliminary evaluation of a purpose-built constructed wetland to buffer leachate from a bauxite residue disposal site in Ireland. Data collected over a 1-year period demonstrated that the pH of bauxite residue leachates could be effectively reduced from ca. pH 10.3 to 8.1 but was influenced by influent variability and temporal changes. The wetland was also effective in decreasing elemental loading, and sequential extractions suggested that the bulk of the sediment-bound metal inventory was in hard-to-leach phases. Elemental analysis of Phragmites australis showed that although vegetation displayed seasonal variation, no trace elements were at concentrations of concern.

The potential for constructed wetlands to treat alkaline bauxite residue leachate: laboratory investigations

High alkalinity (pH > 12) of bauxite residue leachates presents challenges for the long-term storage and managements of the residue. Whilst the use of constructed wetlands is gaining in interest for its use in the treatment of alkaline waters, thus far, there is limited evidence of its suitability for treating NaOH dominated bauxite residue leachate. A series of batch trials were conducted to investigate the potential for constructed wetland conferred mechanisms (dilution water quality, contact with CO2, and substrate type) for treating NaOH solutions to levels permissible for discharge (p < 9). Results demonstrate that significant reductions in solution pH can be achieved depending on the diluting water quality. Levels achieved may not always be suitable for direct discharge (i.e. pH ≤ 9), but further reductions occur with carbonation and soil contact. The extent of pH decrease and the timeframe required are influenced by soil quality, with greater efficiency observed in soils with higher organic matter content. Decrease in solution pH to discharge permit values are possible through a combination of the mechanisms occurring in a constructed wetland. Formation of a calcite precipitate was observed in some treatments and further characterisation by XRD and XPS suggested surface coating with Na2CO3. It is therefore suggested that, under suitable conditions, constructed wetland technology can reduce leachate pH to <9 through mechanisms supporting the precipitation of sodium carbonate from solution. Further trials should investigate the activity under biological conditions representative of an operating constructed wetland.

Reactions of ketoximes and aldoximes over solid acid catalysts

Abstract The Beckmann rearrangement has been studied over a range of solid acids, including zeolites and modified aluminas, in the temperature range 250-380°C. Generally ketoximes rearrange to the corresponding lactams, but acetoximes dehydrate to the nitriles. Coke formation was a major problem associated with all solid acids tested, giving rise initially to lowering in conversion and later to lowering in selectivity to the lactam product. There was a direct relationship between levels of coke formation and the decline in catalytic activity. Because coke formation was most prevalent over those catalysts with the largest numbers of surface basic sites, it is proposed that caprolactam polymerization and condensation reactions of byproducts, such as between aniline and cyclohexanone, involve surface basic sites. By contrast, there was a direct relationship between the surface concentration of acidic sites of intermediate strength and the rate of caprolactam formation from cyclohexanone oxime.

Copper exchanged beta zeolites for the catalytic oxidation of ammonia

Copper exchanged on beta zeolites are extremely active and selective for the catalytic oxidation of ammonia to nitrogen and water and this activity correlates to the ease of reduction of the copper species.