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Huen Lee

Publications and source records attributed to Huen Lee.

16 recordsLinked to original sources
PubMed2006

Critical guest concentration and complete tuning pattern appearing in the binary clathrate hydrates.

The concept of tuning phenomenon in binary hydrate systems has been suggested to enhance the gas storage capacity through molecular interactions. In this report, the existence of critical guest concentration (CGC) is investigated by means of spectroscopic methods. The existence of the critical guest concentration can act as a limiting factor in the application areas of binary hydrates. Therefore, it should be taken into account before applying this concept to application fields. In addition, further research on this concept using other hydrate systems is required to clarify the present findings.

Journal Article↗
PubMed2006

Structure transition and swapping pattern of clathrate hydrates driven by external guest molecules.

We first report here that under strong surrounding gas of external CH4 guest molecules the sII and sH methane hydrates are structurally transformed to the crystalline framework of sI, leading to a favorable change of the lattice dimension of the host-guest networks. The high power decoupling 13C NMR and Raman spectroscopies were used to identify structure transitions of the mixed CH4 + C2H6 hydrates (sII) and hydrocarbons (methylcyclohexane, isopentane) + CH4 hydrates (sH). The present findings might be expected to provide rational evidences regarding the preponderant occurrence of naturally occurring sI methane hydrates in marine sediments. More importantly, we note that the unique and cage-specific swapping pattern of multiguests is expected to provide a new insight for better understanding the inclusion phenomena of clathrate materials.

Journal Article↗
PubMed2006

Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates.

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. We explore here the swapping phenomenon occurring in structure I (sI) and structure II (sII) CH4 hydrate deposits through spectroscopic analyses and its potential application to CO2 sequestration at the preliminary phase. The present 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2+CO2 guests is surprising when compared with the rate of 64% for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2+CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII CH4 hydrates, we observe a spontaneous structure transition of sII to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension caused by structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications for the diverse hydrate-based inclusion phenomena as illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.

Carbon Dioxide↗
PubMed2005

Characterization of PVdF(HFP) gel electrolytes based on 1-(2-Hydroxyethyl)-3-methyl imidazolium ionic liquids.

Poly(vinylidenefluoride)-hexafluoropropylene (PVdF(HFP))-ionic liquid gel electrolytes were prepared using ionic liquids based on 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate and 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate. A conventional metathesis reaction was used to prepare these ionic liquids, which have high purity and exhibit a liquid state at room temperature. The prepared polymer-ionic liquid gel proved to be a free-standing and rubbery film in which the degree of transparency differed according to the ratio and type of ionic liquid used. TGA and FTIR analyses confirmed that the solvent, N,N-Dimethylacetamide (DMAC), used for mixing PVdF(HFP) polymer with ionic liquid was almost totally removed during the gelling and drying processes. SEM photographs were taken of the surface structure of the PVdF(HFP)-ionic liquid gel in order to evaluate the morphology of the film's surface according to the mixing ratio and the nature of the ionic liquid. The thermal behaviors of PVdF(HFP)-ionic liquid gels were observed to be similar to those of neat ionic liquids through DSC analysis, and the compatibility between the polymer and ionic liquid was investigated by XRD analysis. The ionic conductivities of all the gels were 10(-3)-10(-5) S cm(-1) in a temperature range of 20-70 degrees C.

Electrolytes↗
PubMed2005

Spectroscopic identification of the mixed hydrogen and carbon dioxide clathrate hydrate.

In this contribution, we first found the novel clathrate hydrate containing two gaseous guests of hydrogen and carbon dioxide by spectroscopic analysis. X-ray powder diffraction and NMR spectroscopy were used to identify structure and guest distribution of the mixed H2 + CO2 hydrate. X-ray diffraction result confirmed that the unit cell parameter was 11.8602 +/- 0.0010 A, and the formed hydrate was identified as structure I hydrate. 1H magic angle spinning (MAS) NMR and 13C cross-polarization (CP) NMR spectroscopy were used to examine the distribution of hydrogen and carbon dioxide molecules in the cages of structure I, respectively. These NMR spectra showed that carbon dioxide molecules occupied both small 512 cages and large 51262 cages, and hydrogen molecules only were occluded in small 512 cages of structure I. The new finding of the mixed hydrogen hydrate is expected to contribute toward the development of hydrogen production technology and, particularly, inclusion chemistry.

Journal Article↗
PubMed2005

Tuning clathrate hydrates for hydrogen storage.

The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies--where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements--have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approximately 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to approximately 4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.

Journal Article↗
PubMed2005

Efficient recovery of CO2 from flue gas by clathrate hydrate formation in porous silica gels.

Thermodynamic measurements and NMR spectroscopic analysis were used to show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixtures using water dispersed in the pores of silica gel. Kinetic studies with 1H NMR microimaging showed that the dispersed water in the silica gel pore system reacts readily with the gas, thus obviating the need for a stirred reactor and excess water. Hydrate phase equilibria for the ternary CO2-N2-water system in silica gel pores were measured, which show that the three-phase hydrate-water-rich liquid-vapor equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization NMR spectral analysis and direct measurement of the CO2 content in the hydrate phase suggested that the mixed hydrate is structure I at gas compositions of more than 10 mol % CO2, and that the CO2 molecules occupy mainly the more abundant 5(12)6(2) cages. This makes it possible to achieve concentrations of more than 96 mol % CO2 gas in the product after three cycles of hydrate formation and dissociation. 1H NMR microimaging showed that hydrate yields of better than 85%, based on the amount of water, could be obtained in 1 h when a steady state was reached, although approximately 90% of this yield was achieved after approximately 20 min of reaction time.

Air Pollutants↗
PubMed2004

Synthesis and antimicrobial properties of imidazolium and pyrrolidinonium salts.

For the purpose of developing new disinfectants and antiseptics, we searched for compounds having high bactericidal activity against gram-positive bacteria, gram-negative bacteria, and fungi. Three different series of quaternary imidazolium and pyrrolidinonium salts were synthesized: series A (1-alkyl-3-methylimidazolium chlorides and bromides); series B (1-alkyl-2-methyl-3-hydroxyethylimidazolium chlorides); and series C (N-alkyl-N-hydroxyethylpyrrolidinonium). Series B and C were newly designed. These three series were tested to evaluate their antibacterial and antifungal properties for the first time. Seven microbial strains were used in the study: Escherichia coli KCTC1924, Salmonella typhimurium KCTC1926, Staphylococcus aureus 209 KCTC1916, Staphylococcus aureus R209 KCTC1928, Bacillus subtilis KCTC1914, Candida albicans KCTC1940, and Chlorella regularis. The antimicrobial efficiency was measured by bacterial and fungal growth inhibition expressed as minimal inhibitory concentration (MIC) values. Series A and B imidazolium salts had very good antimicrobial activity against the examined Gram-negative bacteria, Gram-positive bacteria, and fungi. Also the pyrrolidinonium salt was found to have low MIC for some of tested microorganisms. The antibacterial and antifungal active properties of the salts depend upon the structure of functional groups and the alkyl chain length in the imidazolium and pyrrolidinonium ring. Among the synthesized quaternary imidazolium and pyrrolidinonium salts, the imidazolium salts containing a long alkyl chain and the introduction of a hydroxyethyl chain and methyl group into the imidazolium ring structure leads to broad spectrum active antimicrobial agents which not only have bacteriostatic properties but could be powerful bactericides.

Anti-Infective Agents↗
PubMed2004

Size-selective synthesis of gold and platinum nanoparticles using novel thiol-functionalized ionic liquids.

One-phase synthesis of gold and platinum nanoparticles using new thiol-functionalized ionic liquids (TFILs) is described for the first time. TFILs as stabilizing agents for gold and platinum nanoparticles were designed to have thiol groups on either the cation or anion and symmetrical or unsymmetrical positions only in the cation. Transmission electron microscopy, electron diffraction, and NMR were used for the characterization of nanoparticles. The metal nanoparticles formed using TFILs are crystalline structures with face-centered cubic packing arrangements and have small sizes (the average diameters are 3.5, 3.1, and 2.0 nm for Au and 3.2, 2.2, and 2.0 nm for Pt, respectively) and uniform distributions (the standard deviations are 0.7, 0.5, and 0.1 nm for Au and 1.1, 0.2, and 0.1 for Pt, respectively). It is believed that the nanoparticle size and distribution depend on the number and position of thiol groups in the IL.

Letter↗
PubMed2004

Prediction of the surface tension of mixed electrolyte solutions based on the equation of Patwardhan and Kumar and the fundamental Butler equations.

The predictive equation of Patwardhan and Kumar for the water activity of mixed electrolyte solutions has been used together with the fundamental Butler equations to establish a new simple predictive equation for the surface tension of mixed electrolyte solutions. This newly proposed equation can provide the surface tensions of multicomponent solutions using only the data of the corresponding binary subsystems of equal ionic strength. No binary interaction parameters are required. The predictive capability of the equation has been tested with the experimental data for 26 concentrated multicomponent electrolyte solutions at different temperatures and compared with the model of Li et al. Both equations agree well with the experimental results of systems examined over entire experimental composition ranges, but the new equation generally gives better predictions for most 1:1 electrolyte systems examined, and considerable improvement in predictions has been achieved for all the mixtures containing 1:2 and 2:2 electrolytes and for 1:1 electrolyte systems at higher temperatures.

Journal Article↗
PubMed2003

Effect of steric hindrance on carbon dioxide absorption into new amine solutions: thermodynamic and spectroscopic verification through solubility and NMR analysis.

Acid gas absorption technology is of great importance in these days for the prevention of global warming and the resulting worldwide climate change. More efficient process design and development for the removal of acid gases has become important, together with the development of new absorbents as one of urgent areas of research in addressing global-warming problems. In the present work, aqueous solutions of 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD), a sterically hindered amine, has been examined as a potential CO2 absorbent and compared with the most commonly used absorbent, monoethanolamine (MEA) solution, through equilibrium solubility measurements and 13C NMR spectroscopic analyses. The solubilities of CO2 in aqueous 10 mass % AHPD solutions were higher than those in aqueous 10 mass % MEA solutions above 4 kPa at 298.15 K, but below 4 kPa, the solubility behavior appeared to be the opposite. The solubility difference between these two solutions increased with the CO2 partial pressures above the crossover pressure. Equilibrated CO2-MEA-H2O and CO2-AHPD-H2O solutions at various CO2 partal pressures ranging from 0.01 to 3000 kPa were analyzed by 13C NMR spectroscopy to provide a more microscopic understanding of the reaction mechanisms in the two solutions. In the CO2-amine-H2O solutions, amine reacted with CO2 to form mainly the protonated amine (AMH+), bicarbonate ion (HCO3-), and carbamate anion (AMCO2-), where the quantitative ratio of bicarbonate ion to carbamate anion strongly influenced the CO2 loading in the amine solutions. A profusion of bicarbonate ions, but a very small amount of carbamate anions, was identified in the CO2-AHPD-H2O solution, whereas a considerable amount of carbamate anions was formed in the CO2-MEA-H2O solution. AHPD contains more hydroxyl groups than nonhindered MEA, and hence, the chemical shifts in its 13C NMR spectra were strongly influenced by the solution pH values. In contrast, MEA appeared to be insensitive to pH. The strong interrelations among CO2 solubility, CO2 partial pressure, bulkiness of the amine structure, and pH identified through the present experimental investigations can provide basic guidelines for finding new potential organic absorbents, including specifically designed amine chemicals.

Absorption↗
PubMed2002

Ionic liquids based on N-vinyl-gamma-butyrolactam: potential liquid electrolytes and green solvents.

The first examples of room temperature ionic liquids, containing N-alkyl-N-vinyl-2-pyrrolidinonium (N-alkyl-N-vinyl-gamma-butyrolactam) cations in combination with bromide and tetrafluoroborate anions, have been synthesized and the spectroscopic and physical characteristics of this family of ionic liquids have been investigated for intended use as liquid electrolytes and green solvents.

Journal Article↗