Georgios N. Kalantzopoulos

Intercalation and Retention of Carbon Dioxide in a Smectite Clay promoted by Interlayer Cations

A good material for CO2 capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO2, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO2 adsorption, and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO2 intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO2 was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li+, Na+ or Ni2+. Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO2 up to a temperature of 35°C at ambient pressure, and that the captured CO2 can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO2 comparable to other materials studied in this context.

In-situ flow MAS NMR and synchrotron PDF analyses of the local response of the Brønsted acidic site in SAPO-34 during hydration at elevated temperatures

In situ flow magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and synchrotron-based pair distribution function (PDF) analyses were applied to study waters interactions with the Brønsted acidic site and the surrounding framework in the SAPO-34 catalyst at temperatures up to 300 °C for NMR spectroscopy and 700 °C for PDF. 29 Si enrichment of the sample enabled detailed NMR spectroscopy investigations of the T-atom generating the Brønsted site. By NMR spectroscopy, we observed dehydration above 100 °C and a coalescence of Si peaks due to local framework adjustments. Towards 300 °C, the NMR spectroscopy data indicated highly mobile acidic protons. In situ total X-ray scattering measurements analyzed by PDF showed clear changes in the Al local environment in the 250-300 °C region, as the Al-O bond lengths showed a sudden change. This fell within the same temperature range as the increased Brønsted proton mobility. We suggest that the active site in this catalyst under industrial conditions comprises not only the Brønsted proton but also SiO4 . To the best of our knowledge, this is the first work proposing a structural model of a SAPO catalyst by atomic PDF analysis. The combination of synchrotron PDF analysis with in situ NMR spectroscopy is promising in revealing the dynamic features of a working catalyst.

SAPO-37 microporous catalysts: revealing the structural transformations during template removal

Abstract We have studied the structural behavior of SAPO-37 during calcination using simultaneous in situ powder X-ray diffraction (PXRD) and mass spectroscopy (MS) in addition to ex situ thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A spike in the unit cell volume corresponding to template removal (tracked using the occupancy of the crystallographic sites in the SAPO-37 cages) is revealed from the XRD data and is strongly correlated with the DSC curve. The occupancy of the different template molecules in the faujasite (FAU) and sodalite (SOD) cages is strongly related to the two mass loss steps observed in the TGA data. The templates act as a physical stabilizing agent, not allowing any substantial unit cell response to temperature changes until they are removed. The FAU cages and SOD cages have different thermal response to the combustion of each template. The FAU cages are mainly responsible for the unit cell volume expansion observed after the template combustion. This expansion seems to be related with residual coke from template combustion. We could differentiate between the thermal response of oxygen and T-atoms. The T–O–T angle between two double 6-rings and a neighboring T–O–T linkage shared by SOD and FAU had different response to the thermal events. We were able to monitor the changes in the positions of oxygen and T-atoms during the removal of TPA+ and TMA+. Large changes to the framework structure at the point of template removal may have a significant effect on the long-term stability of the material in its activated form.

A nano-silicate material with exceptional capacity for CO2 capture and storage at room temperature

In order to mitigate climate change driven by the observed high levels of carbon dioxide (CO2) in the atmosphere, many micro and nano-porous materials are being investigated for CO2 selectivity, capture and storage (CCS) purposes, including zeolites, metal organic frameworks (MOFs), functionalized polymers, activated carbons and nano-silicate clay minerals. Key properties include availability, non-toxicity, low cost, stability, energy of adsorption/desorption, sorbent regeneration, sorption kinetics and CO2 storage capacity. Here, we address the crucial point of the volumetric capture and storage capacity for CO2 in a low cost material which is natural, non-toxic, and stable. We show that the nano-silicate Nickel Fluorohectorite is able to capture 0.79 metric tons of CO2 per m3 of host material - one of the highest capacities ever achieved - and we compare volumetric and gravimetric capacity of the best CO2 sorbent materials reported to date. Our results suggest that the high capture capacity of this fluorohectorite clay is strongly coupled to the type and valence of the interlayer cation (here Ni2+) and the high charge density, which is almost twice that of montmorillonite, resulting in the highest reported CO2 uptake among clay minerals.