2 research outputs found
Monoalkylcarbonate Formation in Methyldiethanolamine–H<sub>2</sub>O–CO<sub>2</sub>
In this work, the monoalkylcarbonate
((<i>N</i>-hydroxyethyl)Â(<i>N</i>-methyl)Â(2-aminoethyl)
hydrogen carbonate) formation in
the system methyldiethanolamine (MDEA)–water (H<sub>2</sub>O)–carbon dioxide (CO<sub>2</sub>) is investigated by nuclear
magnetic resonance (NMR) spectroscopy. Aqueous solutions containing
0.4 g/g of MDEA were loaded with CO<sub>2</sub> in valved NMR tubes,
and the composition of the liquid phase in equilibrium was determined <i>in situ</i> at 298 K at pressures up to 11 bar. By two-dimensional
NMR, the presence of monoalkylcarbonate was verified, which has been
widely overlooked in the literature so far. The experimental data
of this work and reevaluated NMR data obtained in previous work of
our group were used to calculate chemical equilibrium constants of
the proposed monoalkylcarbonate formation. A model taken from the
literature that describes the solubility of CO<sub>2</sub> in aqueous
solution of MDEA and the corresponding species distribution is extended
so that it can account for the monoalkylcarbonate in the liquid phase
as well. The extended model is validated using NMR data in the temperature
range 273–333 K. The study shows that more than 10 mol % of
the absorbed CO<sub>2</sub> is bound as monoalkylcarbonate under conditions
relevant for technical applications
An Investigation into the Hydrogen Storage Characteristics of Ca(BH<sub>4</sub>)<sub>2</sub>/LiNH<sub>2</sub> and Ca(BH<sub>4</sub>)<sub>2</sub>/NaNH<sub>2</sub>: Evidence of Intramolecular Destabilization
We report a study of the hydrogen
storage properties of materials that result from ball milling CaÂ(BH<sub>4</sub>)<sub>2</sub> and MNH<sub>2</sub> (M = Li or Na) in a 1:1
molar ratio. The reaction products were examined experimentally by
powder X-ray diffraction, thermogravimetric analysis and differential
scanning calorimetry (TGA/DSC), simultaneous thermogravimetric modulated
beam mass spectrometry (STMBMS), and temperature-programmed desorption
(TPD). The CaÂ(BH<sub>4</sub>)/LiNH<sub>2</sub> system produces a single
crystalline compound assigned to LiCaÂ(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>). In contrast, ball milling of the CaÂ(BH<sub>4</sub>)/NaNH<sub>2</sub> system leads to a mixture of NaBH<sub>4</sub> and CaÂ(NH<sub>2</sub>)<sub>2</sub> produced by a metathesis reaction and another
phase we assign to NaCaÂ(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>). Hydrogen desorption from the LiCaÂ(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>) compound starts around 150 °C, which is more than 160
°C lower than that from pure CaÂ(BH<sub>4</sub>)<sub>2</sub>.
Hydrogen is the major gaseous species released from these materials;
however various amounts of ammonia form as well. A comparison of the
TGA/DSC, STMBMS, and TPD data suggests that the amount of NH<sub>3</sub> released is lower when the desorption reaction is performed in a
closed vessel. There is no evidence for diborane (B<sub>2</sub>H<sub>6</sub>) release from LiCaÂ(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>), but traces of other volatile boron–nitrogen species (B<sub>2</sub>N<sub>2</sub>H<sub>4</sub> and BN<sub>3</sub>H<sub>3</sub>) are observed at 0.3 mol % of hydrogen released. Theoretical investigations
of the possible crystal structures and detailed phase diagrams of
the Li–Ca–B–N–H system were conducted
using the prototype electrostatic ground state (PEGS) method and multiple
gas canonical linear programming (MGCLP) approaches. The theory is
in qualitative agreement with the experiments and explains how ammonia
desorption in a closed volume can be suppressed. The reduced hydrogen
desorption temperature of LiCaÂ(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>) relative to CaÂ(BH<sub>4</sub>)<sub>2</sub> is believed to
originate from intramolecular destabilization