A Glycerol-Derived Physical Solvent for CO - ACS Publications

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Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 911-921

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1,2,3-Trimethoxypropane: A Glycerol-Derived Physical Solvent for CO2 Absorption Brian S. Flowers,† Max S. Mittenthal,† Alexander H. Jenkins,† David A. Wallace,† John W. Whitley,† Grayson P. Dennis,† Mei Wang,‡ C. Heath Turner,† Vladimir N. Emel’yanenko,§,∥ Sergey P. Verevkin,*,§ and Jason E. Bara*,†

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†

Department of Chemical & Biological Engineering, University of Alabama, Box 870203, Tuscaloosa, Alabama 35487-0203, United States ‡ School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, People’s Republic of China § Department of Physical Chemistry, University of Rostock, Dr-Lorenz-Weg 1, D-18059 Rostock, Germany ∥ Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18, 420008 Kazan, Russia S Supporting Information *

ABSTRACT: 1,2,3-Trimethoxypropane (1,2,3-TMP) is the trimethyl ether of propane-1,2,3-triol, better known as glycerol, which can be derived from triglycerides originating from either plant or animal sources. Despite its simple structure and the ubiquity of its glycerol precursor, successful synthesis of 1,2,3-TMP was only recently reported in the literature, with studies suggesting it may be a “green” and nontoxic alternative to solvents such as diglyme, a constitutional isomer. However, no thermophysical properties of 1,2,3-TMP have yet been reported. Furthermore, the structure of 1,2,3-TMP is also analogous to polyether solvents used in the Selexol process for removal of CO2 and other “acid” gases from CH4, H2, etc. As such, examining the solubility of CO2 in 1,2,3-TMP is also of interest. This work details our initial studies and characterization of 1,2,3-TMP as a physical solvent for CO2 absorption, as well as the characterization of its density, viscosity, and vapor pressure with respect to temperature. 1,2,3-TMP exhibits favorable properties, and glycerol-derived triethers warrant deeper consideration as new solvents for CO2 absorption and other gas treating applications. KEYWORDS: Glycerol, Green chemistry, Physical solvent, Precombustion CO2 capture, Selexol

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INTRODUCTION

and future electric power generation from coal or other fuels via integrated gasification combined cycle (IGCC) processes.7 Absorptive (i.e., solvent-based) technologies are the most widely used and mature processes for CO2 removal from industrial gas streams. The selection of an appropriate solvent for CO2 removal is related to the inlet partial pressure and outlet purity specification.6 If the inlet partial pressure of CO2 is low (e.g., flue gas), then a reactive or “chemical” solvent such as aqueous monoethanolamine (MEA) will be required. The capacity of a chemical solvent is primarily limited by the

Most discussions of “CO2 capture and sequestration” (CCS) in recent years relate to the removal of CO2 from low partial pressure and low concentration postcombustion sources, such as the flue gas of coal- and natural-gas-fired power plants.1 However, the removal of carbon dioxide (CO2) from high concentration, high pressure sources is a well-established, ongoing, and important need within many chemical processes/ industries.2−6 Removal of CO2 from raw natural gas to levels below 2 vol% is necessary to limit pipeline corrosion and maintain heating value.5 Separation of CO2 is also a key component of water−gas shift reactions which produce H2 and CO2 from CO and H2O, which is important to NH3 synthesis © 2016 American Chemical Society

Received: September 15, 2016 Revised: October 20, 2016 Published: October 24, 2016 911

DOI: 10.1021/acssuschemeng.6b02231 ACS Sustainable Chem. Eng. 2017, 5, 911−921

Research Article

ACS Sustainable Chemistry & Engineering concentration (e.g., molarity) of active species in solution. However, once the active species has become saturated with CO2, increased CO2 pressure will result in minimal additional absorption in the chemical solvent. If the inlet pressure is high and only “bulk” (∼90%) removal of CO2 is required, then a nonreactive or “physical” solvent can be utilized.6 Physical solvents are typically polar (e.g., nonhydrocarbon) organic solvents with suitable thermophysical properties such as low viscosity, low vapor pressure, high CO2 absorption capacity, thermal stability, etc.3,8 The capacity of a physical solvent is dependent on the partial pressure of CO2 in the gas stream, typically showing a linear (i.e., Henry’s Law) relationship. Figure 1 presents a simple illustration of the relationship between CO2 partial pressure and CO2 absorption capacity for chemical and physical solvents.9

Figure 2. Diagram of the Selexol process applied to precombustion CO2 capture at an IGCC plant. Reproduced from ref 9.

N-methylpyrrolidinone (NMP), and mixtures of morpholine derivatives and propylene carbonate (PC) as the respective absorbents.2 A number of imidazole and imidazolium-based ionic liquid (IL) species have been shown to act as physical solvents for CO2 absorption by our group and others.11−17 These works have primarily focused on structure−property relationships that arise from modifying the substituent(s) attached to the imidazole/imidazolium ring,18−21 as well as the anion in the case of ILs.22−26 However, while both imidazoles and ILs possess some desirable qualities, the overall combination of properties and economics associated with these species does not appear to be competitive with existing commercial physical solvents such as DMPEG.11 The conventional unit operation for gas absorption is vertical packed or trayed-columns, where efficient mass transfer requires a balance between sufficient interfacial area (promoting gas−liquid contact) and a high void fraction (minimizing pressure drop).1 Although this technology has been in use for over a century, disadvantages include large profiles (e.g., height and footprint) and relatively low surface area to volume ratios ( 2000 mg/kg, and no acute toxicity to fish.39 Recently, 1,2,3-TMP was added to the GlaxoSmithKline (GSK) Solvent Sustainability Guide,42 and is thought to be a potentially green ether-based solvent, although it is noted that many thermophysical properties of 1,2,3-TMP have not yet been reported, and these early assessments are based on predictions and “nearest neighbor” analogies to chemicals of similar structure. Given the unique structure of 1,2,3-TMP and its potential as a green solvent, herein we report the characterization of 1,2,3TMP as a physical solvent for CO2 absorption with a comparison to diglyme (diethylene glycol dimethyl ether), a constitutional isomer. Solubilities of CO2 in 1,2,3-TMP and diglyme were measured over the range 30−75 °C at pressures from 2 to 10 atm. The density, viscosity, and vapor pressure of 1,2,3-TMP and diglyme were also measured with respect to temperature. Furthermore, we have employed the Gaussian09 and COSMOTherm software packages to estimate thermophysical properties and visualize the molecular-level electronic structures of 1,2,3-TMP and diglyme which illustrate the differences in conformations adopted by these two isomers. This initial study of 1,2,3-TMP indicates that glycerol-derived physical solvents may be of significant interest as alternatives to DMPEG for CO2 absorption applications.

include increased mass transfer coefficients, reduced solvent inventory, and a minimized process footprint.28 However, performance of HFMCs may be more sensitive to solvent viscosity in terms of pressure drop down the length of the fiber and reduced mass transfer rates which will necessitate more membrane area to achieve the desired level of gas absorption. These effects become more pronounced if the solvent is chilled. In this context, DMPEG, the absorption fluid of the Selexol process, may not be an ideal choice for HFMCs due to its relatively high viscosity compared to other physical solvents. Advanced physical solvents with low vapor pressure, low viscosity, and good CO2 absorption performance will greatly benefit the operation of HFMCs. Low surface tension and biocompatibility (or reduced toxicity) are other desirable properties. We have reconsidered our approach to the design of physical solvents for CO2 absorption, now focusing on substances that share some commonalities with DMPEG, but with the aim of reducing viscosity relative to DMPEG and other organic solvents. Glycerol (IUPAC: propane-1,2,3-triol) is a natural polyol that can be derived from both plant matter and animal fat.36 Although glycerol is a low cost liquid that has a very low volatility (Tb = 290 °C), it is too viscous in its pure form (μ > 1000 cP at 20 °C)37 to be considered as a viable physical solvent for CO2 absorption. Dilution with H2O to reduce viscosity would also likely have a detrimental impact on CO2 solubility in glycerol-containing solutions.8 However, synthetic modification of glycerol from a triol to triether is possible,38,39 and species such as trimethyl glycerol also known as 1,2,3trimethoxypropane (1,2,3-TMP) might be interesting to consider as physical solvents for CO2 absorption, given a similarity to the polyether structure of DMPEG.40 1,2,3-TMP is also a constitutional isomer of diglyme (diethylene glycol dimethyl ether), with both sharing the same molecular formula C6H14O3 and MW (134.18 g mol−1) (Figure 4).

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EXPERIMENTAL SECTION

Materials. Glycerol (>99.7%) and KOH (pellets, ACS grade) were purchased from BDH. DMS (>99%) was purchased from SigmaAldrich. TBAHS (98%) was purchased from Baker. CaH2 (90−95%) and diglyme (99%, < 0.5% H2O) were purchased from Alfa Aesar. Research grade CO2 (99.999%) was purchased from Airgas. Synthesis of 1,2,3-TMP. 1,2,3-TMP was synthesized from glycerol, KOH, and DMS in the presence of TBAHS, a phase transfer catalyst, in the absence of any additional solvent according to the procedure described by Sutter (Scheme 1).39

Scheme 1. Synthesis of 1,2,3-TMP.39

Figure 4. Molecular structures of DMPEG, 1,2,3-TMP, and diglyme. Following the reaction, pentane (1000 mL) was added to the solids, and the slurry was stirred for several hours via mechanical mixing. The solids were then separated by vacuum filtration, and the filtrate volume was reduced to ∼150 mL via rotary evaporation, followed by distillation of 1,2,3-TMP over CaH2. Sutter reports the boiling point of 1,2,3-TMP as 66−69 °C at 90 mbar as recorded during distillation to isolate pure 1,2,3-TMP. Using Karl Fischer titration, the H2O content of 1,2,3-TMP postdistillation was determined to be >1000 ppm, which was reduced to ∼75 ppm upon storage in crimped septum

Until very recently, there had been almost no studies on 1,2,3-TMP, a seemingly simple triether molecule based on a ubiquitous natural product, glycerol. Sutter et al. first reported the successful synthesis of pure 1,2,3-TMP in 2013.38,39 The authors note that their synthetic methods, although viable, are still far from ideal and “green”, and that alternative methods of preparing 1,2,3-TMP were under investigation.41 These works also found that 1,2,3-TMP is safer and less toxic than diglyme, 913


Research Article

ACS Sustainable Chemistry & Engineering vials over activated 3 Å carbon molecular sieves. Due the large volume of solid byproducts and the lack of suitably large equipment in our lab, the Soxhlet extraction step described by Sutter was not performed, and thus, the yield of 1,2,3-TMP was ∼90 g (∼50%). 1H NMR characterization data and spectra for 1,2,3-TMP were consistent with published results38 and are included as Supporting Information. As discussed by Sutter et al., the synthetic method in Scheme 1 and subsequent distillation is certainly viable to produce 1,2,3-TMP in a pure form, yet the use of the highly toxic DMS is an inconvenient aspect of this approach. However, there are but a few methylating agents suitable for doing this synthesis in a typical laboratory. From an “atom economy” standpoint,43 DMS reduces the mass of byproducts produced relative to other methylating agents such as methyl iodide (CH3I). While CH3Cl would further reduce the overall mass of byproducts, it is also a hazardous gas and does not represent a more desirable or practical option in the laboratory. We would also note that the solvent-free conditions result in a reaction medium that is difficult to homogenize, even with a powerful overhead mechanical mixer (RZR 2052 Control, Heidolph), leading to thermal heterogeneities or “hot spots” in the reactor despite being fully submerged in a cooling bath. Density Measurements. Using a procedure outlined in prior work from our group,13 density data were measured using a MettlerToledo DM45 DeltaRange density meter that operates using electromagnetically induced oscillation of a glass U-form tube, with automatic compensation for changes in atmospheric pressure. The density meter can measure samples in the liquid phase with densities between 0 and 3 g/cm3 and requires a minimum sample size of 1.2 cm3. Density measurements are accurate within ±0.00005 g/cm3 for all operating temperatures. Densities of 1,2,3-TMP were recorded at 10 °C intervals from 20 to 80 °C. Viscosity Measurements. Viscosity data were measured using a Brookfield DV-II+ Pro viscometer using a procedure reported in prior work from our group.13 The viscosity is based on a torque value and shear rate of a spindle in contact with a specified amount of fluid. For these experiments, the “ULA” spindle and jacketed sample cell were used because viscosities were 20 min). The pressure was recorded, and the vessel was disconnected from the pressure sensor, removed from the water bath, and dried; the mass of the vessel was recorded. The increase in the mass of the vessel relative to the initial state can be taken as the total mass of CO2 added to the vessel distributed between the vapor and liquid phases. The vessel was then returned to the water bath and reconnected to the pressure sensor and the bath heated to 45 °C while stirring. The system pressure was allowed to equilibrate at 45 °C and the pressure recorded. The process is repeated again at 60 and 75 °C. After this procedure was completed, the system was cooled to 30 °C, and the system was exposed to CO2 at an absolute pressure of ∼ 4 atm until equilibrium was reached, with the new mass of the system recorded. Equilibrium pressures were recorded at the same temperature intervals. The process was repeated a final time starting at 30 °C and an absolute pressure of CO2 at ∼6 atm. Measuring the mass increase of the system allows for the number of total moles of CO2 in the system to be calculated via eq 1, where nCO2 is the number of moles of CO2 present in the system at a given T,P equilibrium, mi is the measured mass of the system at this equilibrium in g, m0 is the initial mass of the degassed system in g, and MW is the molecular weight of CO2 (44.01 g mol−1). m − m0 nCO2 = i (1) MW Partitioning of CO2 between the liquid and vapor phases is calculated from eq 2, where P is the absolute pressure, z is the compressibility factor as calculated from the NIST REFPROP software, nCO2 vap is the number of moles of CO2 in the headspace, R is the gas constant, T is the absolute temperature, Vtotal is the volume of the empty reactor minus the known volume of the stir bar, Vliq is the volume of the solvent as calculated from the mass and density, Vcrit is the molar solubilized volume of CO2 (taken as the critical volume, 34 cm3/mol), and nCO2 is the total number of moles of CO2 in the system as calculated from the mass balance in eq 1.

P=

zn vapRT v Vtotal − Vliq − Vcrit(nCO2 − nCO ) 2

(2)

Solving for eq 2 for nvap at each equilibrium T, P condition gives the number of moles of CO2 absorbed into 1,2,3-TMP or diglyme, and the moles of CO2 dissolved in the liquid phase (nLCO2) can be determined via eq 3. 914


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ACS Sustainable Chemistry & Engineering Table 1. Density Data for 1,2,3-TMP and Diglyme from 20 to 80 °C density (g/cm3) at the relevant temperatures 1,2,3-TMP diglyme

20 °C

30 °C

40 °C

50 °C

60 °C

70 °C

80 °C

0.94169 0.94387

0.93171 0.93371

0.92168 0.92378

0.91155 0.91375

0.90133 0.90365

0.89099 0.89346

0.88050 0.88314

L V nCO = nCO2 − nCO 2 2

functional at the DFT level to generate optimized geometry .coord files. The .coord files were processed through COSMOTherm with calculations performed at the TZVP level of theory.

(3)

The mole fractions of CO2 (xLCO2) in the liquid phase at each given T, P condition are calculated from eq 4 L xCO = 2

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RESULTS AND DISCUSSION Density and Viscosity. The measured densities of 1,2,3TMP and diglyme from 20 to 80 °C are presented in Table 1. Equations 7a and 7b express the respective linear fits of density for 1,2,3-TMP and diglyme as functions of temperature, both with R2 > 0.9999.

L nCO2 L nCO2 + nS

(4)

where nS is the moles of solvent (1,2,3-TMP or diglyme) originally added to the vessel. The Henry’s constant (HCO2,1 (atm)) of CO2 at each temperature, where CO2 is the solute and 1 = solvent (1,2,3-TMP or diglyme), was calculated from the inverse linear relationship in eq 5:

HCO2,1 (atm) =

P (atm) L xCO 2

(5)

Vapor Pressure Measurement by the Transpiration Method. Absolute vapor pressures of 1,2,3-TMP and diglyme were measured by using the transpiration method,44−47 with a detailed experimental description provided as Supporting Information. Small glass beads were covered with the sample under study and placed in a U-shaped saturator. A well-defined N2 stream was passed through the saturator at a constant temperature (±0.1 K), and the transported material was collected in a cold trap. The amount of condensate was determined by gas chromatography (GC). The vapor pressure pi at each temperature Ti was calculated from the amount of the product collected within a definite period. Assuming the validity of Dalton’s Law applied to the N2 stream saturated with the substance i, values of pi were calculated with eq 6:

pi = miRTa /VMi ;

V = VN2 + Vi

(VN2 ≫ Vi )

⎛ g ⎞ ρ⎜ 3 ⎟ = −0.00101T (°C) + 0.96413 ⎝ cm ⎠

(7b)

Table 2. Measured Viscosity Data for 1,2,3-TMP and Diglyme viscosity (cP) at the relevant temperatures 1,2,3-TMP diglyme

−1

Here, the following abbreviations apply: R = 8.314462 J K mol ; mi is the mass of the transported compound, Mi is the molar mass of the compound, Vi its volume contribution to the gaseous phase, VN2 is the volume of the carrier gas, and Ta is the temperature of the soap bubble meter used for measurement of the gas flow. The volume of the carrier gas VN2 was determined from the flow rate and the time measurement.

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(7a)

The density values reported here for diglyme are highly consistent with those reported by Valtz,59 varying by only ±0.0001 g/cm−3. The measured viscosity values for 1,2,3-TMP from 20 to 80 °C are shown in Table 2.

(6) −1

⎛ g ⎞ ρ⎜ 3 ⎟ = −0.00102T (°C) + 0.96246 ⎝ cm ⎠

20 °C

30 °C

40 °C

50 °C

60 °C

70 °C

80 °C

1.07 1.25

1.05 1.23

1.03 1.16

1.01 1.12

0.95 1.07

0.89 1.02

0.83 0.98

It is clear from the table that the viscosities of 1,2,3-TMP and diglyme are comparable to that of H2O (∼1 cP), with 1,2,3TMP about 15% less viscous at each temperature. The reduction in viscosity for 1,2,3-TMP may be attributed to its branched structure which may experience fewer intermolecular interactions compared to the linear structure of diglyme (Figure 4). CO2 Solubility. The experimentally measured solubilities of CO2 in 1,2,3-TMP and diglyme in terms of mole fraction of CO2 with respect to equilibrium CO2 pressure at 30−75 °C are presented in Figure 6. Table 3 summarize the solubility of CO2 in 1,2,3-TMP and diglyme, respectively, in terms of three standard metrics: Henry’s constants (H (atm)), volumetric solubility (Sv [=] cm3 (STP) CO2 per (cm3 solvent) per atm CO2), and molality per pressure (Sm, mol CO2 (kg solvent)−1 per atm CO2). Furthermore, using the moles of CO2 solubilized in the 1,2,3TMP or diglyme, the enthalpy of absorption (ΔHabs) can be calculated using the van’t Hoff equation (eq 8).11

COMPUTATIONAL METHODS

Electrostatic properties of the molecules were calculated by using firstprinciples density-functional theory (DFT), as implemented in Gaussian09.48 Geometry optimizations were performed using the B3LYP functional49,50 with the 6-31g+(d,p) basis set. Following the geometry optimizations, the partial charges were calculated using the CHelpG analysis51 at the same level of theory, along with dipole moments and polarizability. To benchmark experimentally measured thermophysical properties of 1,2,3-TMP and diglyme against calculations and to aid in the visualization of their molecular structures, COSMOThermX (v. C30_1301, COSMOLogic GmbH) was employed. The .cosmo file for diglyme (8 conformers) was already available within the library purchased from the software provided. The .cosmo files for 1,2,3-TMP (8 conformers) were prepared by our group using the same techniques as our prior works.12,52−54 A 2-D structure was first produced in ChemDraw Professional 15 (PerkinElmer) which was then converted to a 3-D structure (.sdf file) using Chem3D Professional 15 (PerkinElmer). Eight conformers of 1,2,3-TMP were identified through the MM2 energy minimization routine in Chem3D, although more may be possible. The .sdf file of each conformer was then processed through TURBOMOLE,55 using the triple-ζ valence potential (TZVP) basis set56 with the Becke and Perdew (b-p)57,58

ΔHabs +B (8) RT 2 As shown in Figure 6, the isotherms are highly linear (R > 0.997 with y-intercept = 0). ΔHabs values for CO2 in 1,2,3-TMP and diglyme were calculated as −13.2 and −13.4 kJ/mol, L ln(xCO ) =− 2 P

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ACS Sustainable Chemistry & Engineering

Figure 6. Absorption isotherms relating the mole fraction of CO2 in 1,2,3-TMP (left) and diglyme (right) to the partial pressure of CO2. Circles = 30 °C, squares = 45 °C, diamonds = 60 °C, triangles = 75 °C. Dotted lines represent the linear least-squares regression with intercept at the origin.

Table 3. CO2 Solubility in 1,2,3-TMP (Top) and Diglyme (Bottom) Expressed as Henry’s Constant (atm) and as mol/volume and mol/massa Sv (cm3 (STP) cm−3 atm−1)

H (atm)

temp (°C)

Sm (mol kg−1 atm−1)

1,2,3-TMP 30 45 60 75

44.2 58.3 72.4 87.2

± ± ± ±

0.4 0.9 1.9 3.2

30 45 60 75

42.1 54.7 68.8 83.8

± ± ± ±

0.3 0.2 1.5 0.8

3.88 2.89 2.26 1.83

± ± ± ±

0.19 0.14 0.14 0.13

0.186 0.141 0.112 0.092

± ± ± ±

0.009 0.007 0.007 0.007

4.02 3.04 2.36 1.91

± ± ± ±

0.11 0.07 0.10 0.06

0.192 0.148 0.117 0.096

± ± ± ±

0.005 0.004 0.005 0.003

Diglyme

a

Uncertainty represents one standard deviation.

Table 4. Experimentally Measured Vapor Pressures for 1,2,3-TMP and Diglyme from This Work, along with Data from Stull and Li 1,2,3-TMP

diglyme (this work)

diglyme (Stull)

diglyme (Li)

T (K)

p* (Pa)

T (K)

p* (Pa)

T (K)

p* (Pa)

T (K)

p* (Pa)

277.7 281.0 281.6 284.0 285.3 287.7 288.2 289.4 291.1 292.5 293.2 294.1 298.1 301.5 304.7 308.0 311.3

101.4 129.9 139.2 165.8 185.3 219.5 226 244.3 276.5 309.5 325.2 351.4 465.9 586.8 737.8 916.3 1122.8

276.2 279.5 282.7 285.7 288.6 291.5 294.5 297.4 298.3 301.4 303.3 306.4 309.4 312.4

40.30 52.37 67.43 83.82 107.45 130.25 160.85 198.96 213.34 262.51 305.66 368.89 439.09 540.14

370.0 390.0 410.0 430.0 432.9 440.0 450.0

12 066 25 742 50 620 92 903 100 937 123 040 160 744

371.0 375.0 380.0 385.0 390.0 402.5 410.0 415.0 420.0 430.0 434.0 440.0 450.0

11 979 14 067 17 100 20 664 24 831 38 408 49 167 57 637 67 272 90 520 101 486 123 040 160 744

respectively. These values of ΔHabs are in line with many other physical solvents including ILs and 1-alkylimidazoles.17,60 Methane (CH4) exhibits a low solubility in ether-based solvents,8,40 and its small MW (16.04 g mol−1) made accurate determination of the influence of T and P on CH4 solubility in 1,2,3-TMP and diglyme challenging as the mass of CH4 absorbed in the liquid phase was outside of the resolution of

our balance (10 mg). Proceeding to higher pressures (>10 atm) to absorb a greater mass of CH4 was not an option given the rating of the pressure tube. However, this is an encouraging sign as it demonstrates 1,2,3-TMP and diglyme have a high selectivity for absorbing CO2 over CH4. In order to get a more accurate and reproducible reading for CH4 and other species 916


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ACS Sustainable Chemistry & Engineering with low solubilites (e.g., N2 and H2), a larger cell with a greater solvent volume is needed. Vapor Pressures of 1,2,3-TMP and Diglyme. This is the first report with experimental data for the vapor pressure of 1,2,3-TMP. Vapor pressure measurements for diglyme were previously reported by Stull61 and Li.62 The available vapor pressure data are compiled in Table 4. The vapor pressures of diglyme measured in this work at ambient temperatures seem to be in agreement with results measured with ebulliometry at significantly higher temperatures by Li,62 as well as with those in the high temperature range reported by Stull.61 A graphical comparison of these data, along with the results from COSMOTherm calculations, is presented in the section on Computational Results and Comparisons with Experimental Data. Taking this good agreement into account, all available experimental data (except for low temperature data from Stull) were additionally regressed together using Clarke and Glew’s equation63 (eq 9) to develop correlations accurately describing the vapor pressures of diglyme over a possibly broad temperature range (see Table 4): ⎛ p⎞ Δg G ◦ (θ ) ⎛1 1⎞ R ln⎜ 0 ⎟ = − l m + Δgl Hm◦ (θ )⎜ − ⎟ ⎝ θ θ T⎠ ⎝p ⎠ ⎡θ ⎛ T ⎞⎤ ◦ + Δgl Cp,m (θ )⎢ − 1 + ln⎜ ⎟⎥ ⎝ θ ⎠⎦ ⎣T

Experimental absolute vapor pressures measured by the transpiration method. The values of Δgl H°m(T) and Δgl S°m(T) are given in Table 5 and Table S1. Table 5. Compilation of Data on Enthalpies of Vaporization Δgl Hm ° of Polyethers

methoda T range/K

diglyme

N/A

277.7− 311.3 286−433

CGC

298

E

371−434

T

276.2− 312.4

51.6 ± 0.2 46.3 ± 2.0

45.3 ± 2.0 51.5 ± 0.2

ref

51.3 ± 0.3c

this work

51.3 ± 2.1

Stull61

(48.0 ± 0.6) 53.9 ± 2.1

Nichols64

51.1 ± 0.3

this work

51.2 ± 0.3

average

Li62

Methods: T = transpiration; CGC = correlation gas chromatography; E = ebulliometry. bVapor pressures available in the literature were treated using eq S1 and eq 6 in order to evaluate enthalpy of vaporization at 298.15 K in the same way as our own results in Table 5. cWeighted mean value. Value in parentheses was excluded from the calculation. Values in bold were recommended for further thermochemical calculations.

(9)

The thermodynamics of vaporization of 1,2,3-TMP were studied here for the first time. The enthalpy of vaporization of diglyme measured in this work is in good agreement with previous results by Stull and Li et al.,61,62 but in disagreement with the result by Nichols et al. derived from correlation gas chromatography.64 The latter method crucially depended on the network of reference compounds taken into correlation, which could be not always reliable enough. Computational Results and Comparisons with Experimental Data. Given that 1,2,3-TMP is just the first and simplest of many possible triether molecules that can be derived from glycerol,38,39 there is a need to benchmark computational methods against experimental data as a means of guiding future synthetic and experimental work and identify new derivatives with favorable thermodynamic and thermophysical properties. Using the Gaussian09 package, dipole moments of 1,2,3TMP and diglyme were calculated as 2.08 and 1.43 D, respectively. Additionally, the isotropic polarizabilities of 1,2,3TMP and diglyme were calculated as 88.16 Bohr3 and 89.72 Bohr3, respectively. COSMOTherm σ-surfaces and σ-profiles for all of the 1,2,3TMP and diglyme conformers are presented in the Supporting Information. The two molecules show very little difference in terms of their electron-rich/poor surface area distribution, although one conformer of diglyme (c6) does exhibit a significantly more electron-rich area than any other conformer of diglyme or 1,2,3-TMP. This would correlate to stronger Lewis basicity, which is seen in a comparison of the σ-potentials of 1,2,3-TMP and diglyme (see Supporting Information) and thus might explain the slightly higher solubility of CO2 in diglyme than 1,2,3-TMP. Table 6 presents a comparison of experimentally measured physical properties and those calculated from COSMOTherm. The normal boiling point (Tb) is defined as the temperature at

Δgl Gm◦ (298.15 K) = (15.2 ± 0.5) kJ mol−1 Δgl Hm◦ (298.15 K) = (52.6 ± 0.5) kJ mol−1

and ◦ Δgl Cp,m (298.15 K) = −(61 ± 8) J K−1mol−1

These parameters can be used for the thermodynamic calculations with diglyme. Thermodynamics of Vaporization from Transpiration Method. Enthalpies of vaporization of polyethers at temperature T were derived from the temperature dependences of vapor pressures (see Supporting Information) using eq 10: (10)

Entropies of vaporization at temperature T were also derived from the temperature dependence of vapor pressures using eq 11: Δgl Sm◦ (T ) = Δgl Hm◦ /T + R ln(pi /p0 )

T

Δgl H°m (298.15 K)b/ kJ mol−1

a

Here, the following abbreviations apply: p is the vapor pressure at temperature T, p0 is an arbitrary reference pressure (p0 = 1 Pa in this work), θ is an arbitrary reference temperature (in this work we use θ = 298.15 K or θ was an average temperature of the experimental range), R is the molar gas constant, Δgl Gm ° (θ) is the difference in the standard molar Gibbs energy between the gaseous and the liquid phases at the selected reference temperature, Δgl H°m(θ) is the difference in the standard molar enthalpy between the gas and the liquid phases, and Δgl Cp,m ° (θ) is the difference in the molar heat capacity at constant pressure between the gaseous and the liquid phase (see Table S2). An advantage of the Clarke and Glew equation is that the fitting coefficients are directly related to the thermodynamic functions of vaporization.63 The following thermodynamic values of vaporization were derived:

◦ Δgl Hm◦ (T ) = −b + Δgl Cp,m T

1,2,3-TMP

Δgl H°m (Tav)/ kJ mol−1

(11) 917


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Table 6. Comparison of Selected Experimental and Calculated Physical Property Data for 1,2,3-TMP and Diglyme at 25 °C 1,2,3-TMP diglyme a

expt COSMO expt COSMO

density (g/cm3)

viscosity (cP)

p* (mbar)

Δgl Hm ° (kJ/mol)

Tb (°C)

0.93696 0.93023 0.93888 0.95159

1.06 1.21 1.24 1.04

3.49 2.60 1.60 0.99

51.3 46.6 51.2 51.5

148a 179.60 162 193.75

Authors’ estimate based on relative difference between experimental and COSMO values for diglyme.

for diglyme by about 32 °C relative to the experimentally measured value (162 °C), thus leading us to estimate the Tb of 1,2,3-TMP as 148 °C. While 1,2,3-TMP and diglyme are constitutional isomers, their vaporization properties appear to vary significantly due strictly to the branched structure of 1,2,3TMP, which must diminish intermolecular cohesion. Relevance of Glycerol Triethers to DMPEG. 1,2,3-TMP possesses a lower viscosity and is less toxic and likely a “greener” solvent than diglyme. Both 1,2,3-TMP and diglyme are significantly less viscous than DMPEG at 25 °C (∼1 cP vs 5.8 cP) and exhibit about a 10−15% greater absorption capacity for CO2 than DMPEG at 25−30 °C on the basis of moles CO2 per volume of solvent at a given pressure.2 As physical solvents, 1,2,3-TMP and diglyme also compare favorably to sulfolane, having nearly twice the CO2 capacity at a given T, P condition, and being 90% less viscous at 30 °C.65 However, diglyme is not used industrially as a solvent for CO2 absorption in natural gas sweetening or other applications,40 likely due to its much higher vapor pressure compared to the longer polyethers that comprise DMPEG, which have an estimated boiling point of 275 °C and a vapor pressure of only 0.097 Pa at 25 °C. DMPEG has a maximum operating temperature of 175 °C as it will begin to degrade above this point.2 As mentioned previously, sulfolane is most useful in combination with water and amines in a “hybrid” physical + chemical solvent. Pure sulfolane has a melting point of 27.5 °C which is a disadvantage in terms of managing its potential to solidify within process equipment. Thus, it does appear that the low vapor pressure of DMPEG outweighs other thermophysical properties of smaller polyethers in selecting a physical solvent. 1,2,3-TMP and diglyme could be refrigerated to suppress their vapor pressure (similar to the use of MeOH in the Rectisol process). Chilling 1,2,3TMP or diglyme would certainly increase CO2 solubility based on the exothermic ΔHabs for CO2 in these solvents. The corresponding increase in viscosity (potentially lower mass transfer rates) at lower temperatures would be at least partially offset by further increased CO2 solubility. At 25 °C, 1,2,3-TMP is only ∼15% of the viscosity of DMPEG, and chilling 1,2,3TMP may be viable. It is important to note that 1,2,3-TMP it is just the first example of a glycerol-based triether that may be of interest for gas processing, as species such as 1,2,3-triethoxypropane (1,2,3TEP) and a number of others were recently synthesized by Lebeuf.66 One particularly interesting glycerol triether yet to be synthesized is 7-(2-methoxyethoxy)-2,5,9,12-tetraoxatridecane (Figure 8) which is a constitutional isomer of the prototypical DMPEG molecule with 6 ether linkages (cf., Figure 4).5,7

which the vapor pressure (p*) is equal to 1013.25 mbar (1 ° from Table 5 was atm). The weighted mean value for Δgl Hm used for diglyme. Table 6 shows that COSMOTherm provides an excellent estimation of density at 25 °C (within 1−2%) for 1,2,3-TMP and diglyme. COSMOTherm predicts the viscosity of diglyme to be less than that of 1,2,3-TMP although our experimental measurements illustrate the opposite. Nonetheless, the absolute errors are with 0.20 cP, which is more than acceptable for initial process engineering calculations. Additionally, the viscosity of these liquids is at or near the limitations of the spindle used, and more accurate viscosity measurements for 1,2,3-TMP will be performed in the future using a cone and plate viscometer. The experimental data we have obtained here confirm that 1,2,3-TMP is more volatile than diglyme, which is also predicted by COSMOTherm. Prior work from our group showed that COSMOTherm calculations for the vapor pressures of ether-functionalized imidazoles tended to underestimate experimental data,44 and the same trend is observed here as well. Figure 7 presents a plot of experimental vapor pressure data for 1,2,3-TMP and diglyme along with the COSMOTherm calculations as lines.

Figure 7. Comparison of vapor pressure data for 1,2,3-TMP (red) and diglyme (blue): 1,2,3-TMP vapor pressure measured in this work (filled diamonds); Sutter’s reported distillation condition for 1,2,3TMP (hollow diamond); diglyme vapor pressure measured in this work (filled circles); diglyme vapor pressure measured by Stull (hollow circles); COMSOTherm vapor pressure calculation for 1,2,3-TMP (dashed line); COSMOTherm vapor pressure calculation for diglyme (dashed line).

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Although COSMOTherm did provide an estimate for vaporization enthalpy of diglyme that was consistent with the experimentally determined value, the vaporization enthalpy of 1,2,3-TMP was underestimated by ∼5 kJ/mol, which was also observed in our prior work with ether-functionalized imidazoles.44 COSMOTherm simultaneously overestimates Tb

CONCLUSIONS Several thermophysical and thermodynamic properties of 1,2,3TMP were characterized and compared to its constitutional isomer, diglyme. 1,2,3-TMP and diglyme possess very similar properties with respect to CO2 solubility, density, and viscosity, 918


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also thanks Dr. Steven Kelley from University of Alabama Department of Chemistry for performing the Karl Fischer titrations.

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Figure 8. 7-(2-Methoxyethoxy)-2,5,9,12-tetraoxatridecane, a glycerolderived analogue of DMPEG.

(1) Rochelle, G. T. Amine Scrubbing for CO2 Capture. Science 2009, 325 (5948), 1652−1654. (2) Burr, B.; Lyddon, L. A Comparison of Physical Solvents for Acid Gas Removal. In Gas Processors’ Association Convention; Grapevine, TX, 2008. (3) Bucklin, R. W.; Schendel, R. L. Physical Solvent Processes Can Be Very Useful for Acid Gas Removal Applications. Energy Progress 1984, 4 (3), 137−142. (4) Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with Chemical Solvents; John Wiley & Sons: New York, 1983. (5) Kidnay, A. J.; Parrish, W. R. Fundamentals of Natural Gas Processing; CRC Press: Taylor & Francis Group: Boca Raton, FL, 2006. (6) Tennyson, R. N.; Schaaf, R. P. Guidelines can help choose proper process for gas-treating plants. Oil Gas J. 1977, 75 (2), 78−86. (7) Miller, B. G. 7Clean Coal Technologies for Advanced Power Generation. In Clean Coal Engineering Technology; ButterworthHeinemann: Boston, 2011; pp 251−300. (8) Lin, H. Q.; Freeman, B. D. Materials selection guidelines for membranes that remove CO2 from gas mixtures. J. Mol. Struct. 2005, 739 (1−3), 57−74. (9) NETL. DOE/NETL Advanced Carbon Dioxide Capture R&D Program: May 2013 Update. https://www.netl.doe.gov/ File%20Library/Research/Coal/carbon%20capture/handbook/CO2Capture-Tech-Update-2013.pdf (15 September 2016). (10) Murrieta-Guevara, F.; Romero-Martinez, A.; Trejo, A. Solubilities of carbon dioxide and hydrogen sulfide in propylene carbonate, N-methylpyrrolidone and sulfolane. Fluid Phase Equilib. 1988, 44 (1), 105−115. (11) Shannon, M. S.; Tedstone, J. M.; Danielsen, S. P. O.; Bara, J. E. Evaluation of Alkylimidazoles as Physical Solvents for CO2/CH4 Separation. Ind. Eng. Chem. Res. 2012, 51 (1), 515−522. (12) Shannon, M. S.; Tedstone, J. M.; Danielsen, S. P. O.; Hindman, M. S.; Irvin, A. C.; Bara, J. E. Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51 (15), 5565−5576. (13) Shannon, M. S.; Bara, J. E. Properties of Alkylimidazoles as Solvents for CO2 Capture and Comparisons to Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2011, 50 (14), 8665−8677. (14) Bara, J. E.; Camper, D. E.; Gin, D. L.; Noble, R. D. RoomTemperature Ionic Liquids and Composite Materials: Platform Technologies for CO2 Capture. Acc. Chem. Res. 2010, 43 (1), 152− 159. (15) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Guide to CO2 Separations in ImidazoliumBased Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48 (6), 2739−2751. (16) Carvalho, P. J.; Kurnia, K. A.; Coutinho, J. A. P. Dispelling some myths about the CO2 solubility in ionic liquids. Phys. Chem. Chem. Phys. 2016, 18 (22), 14757−14771. (17) Carvalho, P. J.; Coutinho, J. A. P. On the Nonideality of CO2 Solutions in Ionic Liquids and Other Low Volatile Solvents. J. Phys. Chem. Lett. 2010, 1 (4), 774−780. (18) Bara, J. E.; Gabriel, C. J.; Carlisle, T. K.; Camper, D. E.; Finotello, A.; Gin, D. L.; Noble, R. D. Gas separations in fluoroalkylfunctionalized room-temperature ionic liquids using supported liquid membranes. Chem. Eng. J. 2009, 147 (1), 43−50. (19) Carlisle, T. K.; Bara, J. E.; Gabriel, C. J.; Noble, R. D.; Gin, D. L. Interpretation of CO2 solubility and selectivity in nitrile-functionalized room-temperature ionic liquids using a group contribution approach. Ind. Eng. Chem. Res. 2008, 47 (18), 7005−7012. (20) Bara, J. E.; Gabriel, C. J.; Lessmann, S.; Carlisle, T. K.; Finotello, A.; Gin, D. L.; Noble, R. D. Enhanced CO2 separation selectivity in

although 1,2,3-TMP is somewhat more volatile. Computational methods were utilized to calculate the dipole moment of each molecule and visualize the molecular-level structures and electron distributions. Glycerol-based triethers are a unique class of molecules that are only recently emerging in the literature. Despite the current challenges associated with their synthesis, we see great promise for them as they are at least partially derived from renewable resources (glycerol) and may be much less toxic than conventional or “linear” triethers such as diglyme. Further studies in glycerol triethers for CO2 absorption (and removal of other acid gases) are of interest in that they share commonalities in their structure with the linear triethers found in the DMPEG solvent used in the Selexol process. Furthermore, we see potential for the study of 1,2,3-TMP and other glycerol-based triether derivatives for the solvation of metal cations (e.g., Li+, Na+, etc.) for use in battery electrolytes.67−70

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02231. 1 H NMR spectra and data for 1,2,3-TMP; COSMOTherm ball and stick representations, σ-surfaces, and σprofiles for 1,2,3-TMP and diglyme; experimental procedure for vapor pressure measurements; detailed experimental vapor pressure data for 1,2,3-TMP and diglyme; molar heat capacities for for 1,2,3-TMP and diglyme (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*For correspondence concerning the vapor pressure measurements and associated analysis. Phone: +49-381-498-6508. Fax: +49-381-498-6524. E-mail: [email protected]. *Phone: +1-205-348-6836. Fax: +1-205-348-7558. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.W. and J.E.B. acknowledge partial support from the National Science Foundation (REU supplement to CBET1139597). V.N.E. acknowledges partial support from the Russian Government Program of Competitive Growth of Kazan Federal University and Russian Foundation for Basic Research (15-03-07475). M.W. acknowledges partial support from the Hubei Provincial Department of Education and Wuhan Polytechnic University. J.E.B. thanks Dr. Kenneth A. Belmore from the University of Alabama Department of Chemistry for acquisition of 1H and 13C NMR spectra. J.E.B. 919


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