Part 5a: Solvent chemistry: NMR ana lysis and studies

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mostly for vapor–liquid equilibrium model ana lysis, and for some other qualitative as well as kinetic .... parameters of the electrolyte NRTL model were care-.
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Review Series

Part 5a: Solvent chemistry: NMR ana­lysis and studies for amine–CO2–H2O systems with vapor–liquid equilibrium modeling for CO2 capture processes Carbon Management (2012) 3(2), 185–200

Huancong Shi1,2, Zhiwu Liang*1,2, Teerawat Sema2, Abdulaziz Naami2, Phairat Usubharatana2, Raphael Idem1,2, Chintana Saiwan2,3 & Paitoon Tontiwachwuthikul1,2 This is Part 5a of a special review series on recent progress and new developments of the post-combustion carbon-capture technology using reactive solvents. NMR spectroscopy is an important method used in analytical chemistry to test the functional groups and molecular structures of organic compounds. This methodology has been recently applied in CCS in the field of solvent chemistry for aiding the development of novel solvents for capturing CO2. NMR has been used for testing new amines and amine–CO2–H2O systems with 1H and 13C spectra for a number of years. This paper is a comprehensive review recent research papers that have applied NMR spectroscopy in the ana­lysis of a variety of amines and a systems for multiple purposes, mostly for vapor–liquid equilibrium model ana­lysis, and for some other qualitative as well as kinetic analyses.

Background NMR spectroscopy is an indispensable methodology in

organic chemistry for work with synthesized chemicals [1] . The functional groups, the carbon backbones, and C-C double and triple bonds can be detected directly, mostly with liquid phase 1H and 13C spectra. Details on T1 relaxation times (or relaxation delay) and NMR instrumentation operations can be referred to the specific ‘liquid NMR manual’ [201] . Moreover, in the field of CCS technology, NMR spectra have been applied into test the ion species in the complicated amine–CO2–H2O systems to develop the vapor–liquid equilibrium (VLE) models, which are discussed in detail in this review. This can be considered an important part in the field of solvent chemistry, besides absorption kinetics and solubility, which are discussed in Part 5b (also in this issue) and in a future part of this series. This review focuses on the chemical equilibrium and VLE of CO2 absorption into water soluble amines. The former involves Henry’s constant and the latter involves reaction thermodynamics [2] . In this research field, the ion speciation and the exact concentrations of cations, free molecules and anions at different CO2 loadings are

of research interest, and ion speciations were detected with NMR spectroscopy combined with other equilibrium methods [3,4, Shi H et al., 13C NMR spectroscopy of a novel amine species in the DEAB–CO2–H2O system: VLE model (2012), Submitted] .

These experiments generate very useful plots of ion speciation for different amines at different temperatures, which are the so-called VLE plots. VLE plots provided a very important database for the CO2–amine reaction kinetics study in Part 5b. Since the reactant’s (amine) rate of conversion X A was expressed as (C A0 -C A*)/C A0, where the C A* is the equilibrium concentration. However, the equilibrium concentrations of major ions were difficult to detect experimentally because the major ions have strong interactions (see the reaction scheme in Equations 1–13). With the experimental ‘VLE plot’ of certain CO2–amine–H2O systems generated with NMR ana­lysis, all the major ion concentrations were tested and grouped in one plot with respect to different CO2 loading from lean to rich amine. Based on a certain CO2 loading at certain temperature, the major ion concentrations can be obtained directly, which can facilitate the tests of main ion concentrations in industrial CO2-absorbent systems right away.

Joint International Center for CO2 Capture & Storage, Department of Chemical Engineering, Hunan University, Changsha, 410082, P.R. China International Test Centre for CO2 Capture, Faculty of Engineering, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada 3 Petroleum & Petrochemical College, Chulalongkorn University, Phya Thai Road, Patumwan, Bangkok 10330, Thailand *Author for correspondence: Tel.: +86 13618481627; E-mail: [email protected] 1 2

future science group

10.4155/CMT.12.12 © 2012 Future Science Ltd

ISSN 1758-3004

185

Review Series  Shi, Liang, Sema et al.

Key terms NMR spectroscopy: Research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of NMR and can provide detailed information about the structure, dynamics, reaction state and chemical environment of molecules.

ƒƒ NMR ana­lysis of amine–CO2–H2O systems

As mentioned above, NMR techniques were mostly applied to identify/verify the structures and functional groups of chemicals, for qualitative ana­lysis. However, the NMR spectra in this review are applied for the quantitative ana­ lysis of certain ion concentrations. Amine–CO2–H2O systems: Important To establish a useful relationship research objective of this article. As long between the chemical shift d (ppm) as CO2–amine interactions reach chemical equilibrium, there are eight to and concentration C (mol/l) of nine cations, neutral molecules and certain ions was the foundation anions in the solutions, which will draw of this research, which has to be a lot of research interest. NMR ana­lysis discussed first. was applied to this complex system to detect the equilibrium models. After several trials in early research work, NMR spectroscopy Vapor–liquid equilibrium model: Condition wherein a liquid and its vapor became a very effective technique (gas phase) are in equilibrium with each that can test ion concentrations other, or a condition or state where the directly due to several advantages. rate of evaporation equals the rate of The specific species in the solution condensation on a molecular level such that there is no net (overall) possess their own chemical shifts vapor–liquid interconversion. both in 1H and 13C NMR spectra Ion speciation: The changing [3,4] . The chemical shift of carconcentration of varying forms of an ion bamate and bicarbonate/carbon(cation or anion) as the pH of the ate in 13C NMR spectra are quite solution changes. In specific NMR ana­lysis of this article, ion speciation different, and they can be easily refers to the changing concentrations of distinguished. Moreover, from + varying ionic forms of amine/amine H , the 13C NMR spectra, the ratio carbamate, and bicarbonate/carbonate of HCO3 - /CO32- can be detected as the CO2 loading of the solution increase from lean to rich, with pH value directly based on the exact chemidecreasing from strong base to cal shift of the peak. The proton weak base. transfer phenomenon was too fast CO2 loadings: The molar loading, which to detect by NMR spectra, then is the exact amount of CO2 absorbed the peak of HCO3 - /CO32- is one per each mole of amine, with the units integral peak. However, the ‘posi‘mol CO2/mol amine’. The loading can be tested with analytical titration and tion’ (chemical shift) of the peaks Chittick apparatus. can itself illustrate the ratio of Chemical shift: In NMR spectroscopy, these two components directly [5] . the chemical shift is the resonant This is based on such equations as frequency of a nucleus relative to a the following: the chemical shifts standard. Often the position and number of chemical shifts are of pure bicarbonate and carbondiagnostic of the structure of a ate at 25.0°C were 160.33 and molecule. 168.09 ppm, respectively; then, the ratio of bicarbonate and carbonate were listed as: CO32-/HCO3- = (d - 160.33)/(168.09 - d), where d is the chemical shift of the bicarbonate/carbonate mixture (160.33