Research Article pubs.acs.org/journal/ascecg
Liquid CO2 as a Safe and Benign Solvent for the Ozonolysis of Fatty Acid Methyl Esters Michael D. Lundin,† Andrew M. Danby,† Geoffrey R. Akien,† Thomas P. Binder,‡ Daryle H. Busch,†,§ and Bala Subramaniam*,†,∥ †
Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States Archer Daniels Midland (ADM) Company, Decatur, Illinois 62521, United States § Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States ∥ Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W. 15th Street, Lawrence, Kansas 66045, United States ‡
S Supporting Information *
ABSTRACT: We have shown that liquid CO2 dissolves O3 in higher concentrations compared to conventional solvents and is therefore an effective medium for the ozonolysis of unsaturated compounds. The Beer−Lambert law extinction coefficient for ozone in liquid CO2 is determined to be 3.2 ± 0.3 M−1 cm−1. Further, the Henry’s law coefficient for ozone dissolution in dense phase CO2 is determined to be on the order of 0.25 mol L−1 bar−1 in the temperature range −10 to +4 °C. Rapid ozonolysis of methyl oleate in liquid CO2 has been successfully demonstrated with the 1,2,4-trioxolanes (secondary ozonides) being the major stable product, as confirmed by NMR analysis. The product distribution is consistent with a lack of solvent cage effects due to the low polarity of liquid CO2. In addition to the inertness of liquid CO2 to ozone attack, the dominant presence of dense CO2 in the vapor phase promotes inherent process safety. KEYWORDS: Liquid carbon dioxide, Ozone, Ozonolysis, Henry’s law, Extinction coefficient, FAME, NMR
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INTRODUCTION Given its high oxidation potential, there has been extensive interest in the use of ozone as an oxidant in the synthesis of various chemicals. Currently, its widespread application is limited to water purification and to the industrial Emery process, which uses ozone to affect CC scission of biomassderived fatty acid methyl esters to make long chain acids and alcohols.1 Safe ozonolysis has long been a goal of the pharmaceutical industry due to its lack of residual toxicity compared to alternative oxidants such as osmium tetroxide.2 Surging interest in making chemicals from biomass feedstocks has generated renewed interest in ozone as an oxidant. The ability to produce ozone on site is well aligned with the distributed biorefinery concept and eliminates problems associated with biomass transport and storage.3−5 However, wider application of ozone in industrial processes has been tempered by safety concerns often associated with ozonolysis: pure ozone as a liquid or a gas is unstable and potentially © XXXX American Chemical Society
explosive above liquid nitrogen temperatures. Concentrated ozone/oxygen mixtures are also toxic. In industrial processes, ozone streams that typically contain a few wt % of ozone in oxygen are used to mitigate these safety and toxicity concerns. The trioxolanes (i.e., secondary ozonides, SOZ) and other light species formed during ozonolysis of unsaturated compounds have, in some cases, been reported to be unstable, often undergoing runaway thermal decomposition.6,7 These safety concerns have generated interest in developing novel flow reactors that incorporate inherently safe design.8 Most common organic solvents are susceptible to oxidation by ozone and therefore unsuitable for use in ozonolysis processes. Solvents that are more resistant to oxidation by ozone, such as CCl4 or various fluorocarbons, pose environReceived: August 21, 2015 Revised: November 7, 2015
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DOI: 10.1021/acssuschemeng.5b00913 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
some degradation by ozone and do not exhibit particularly high ozone solubility, with Henry’s law coefficients of 0.0737 mol L−1 bar−1 at 25 °C and 0.0613 mol L−1 bar−1 at 25 °C, respectively.20 These studies nevertheless serve as useful benchmarks to compare the experimental measurements reported in this work. The mechanism of the ozonolysis of olefinic compounds was first proposed by Criegee,23 and subsequently verified in numerous studies by Bailey,24 Murray,25 Kuczkowski,26 and Geletneky and Berger.27 According to this mechanism (Scheme 1), the primary ozonide (2), formed by the 1,3-dipolar
mental and/or toxicity concerns and are therefore avoided in practice. Water promotes the decomposition of ozone to form secondary oxidants, such as ·OH, which then react indiscriminately with any available organic substrate resulting in total oxidation to H2O and CO2. It is precisely for this reason that ozone is widely used for water treatment purposes. As previously reported,9 dense CO2 (Pc = 73.8 bar, Tc = 31.1 °C) in either liquid or supercritical state overcomes these challenges by offering a benign alternative medium for performing ozonolysis. Being completely oxidized, CO2 has no potential for reaction with ozone and, as a weakly polar solvent, has many potential applications in organic synthesis. Previous studies of ozonolysis in supercritical CO2 were either aimed at total oxidation of the organics10 or otherwise unsuccessful.11 To minimize ozone decomposition, subambient temperatures at which CO2 exists in a liquid state are preferred.12 This work is aimed at investigating the efficacy of performing ozonolysis in liquid CO2 and determining properties of the system required for engineering larger scale processes. Toward this end, the equilibrium solubility of ozone in liquid CO2 was measured at various temperatures and pressures. Guided by the solubility studies, we performed a study of the ozonolysis of methyl oleate in liquid CO2 with detailed and reliable analysis of the various types of products formed. Such fundamental data and insights are essential for the rational design and demonstration of ozonolysis processes. To quantify ozone solubility in liquids, spectrophotometric methods are preferred over titration methods, as discussed by Razumovskii and Kaikov.13 Several researchers have reported spectrometric measurements of absorption coefficients and cross sections, with the majority of measurements made in the Hartley and Huggins bands (195−350 nm) due to the high absorbance by ozone in this range. Most commercially available devices for measuring ozone concentration in gas mixtures rely on absorbance in this band (specifically at λMAX = 254 nm). For higher concentrations of ozone, the Hartley and Huggins bands absorb too intensely. Hence, a lesser absorbing, yet sufficiently sensitive to be reliably quantitative, wavelength is desired, such as those found in the Chappuis band (440−850 nm). Chappuis band absorption coefficient and cross section measurements have been measured for both pure and nearly pure ozone in the gas phase.14−17 Despite extensive debate surrounding the measurement procedure and the exact value of the extinction coefficient, the reported gas phase values (ε = 1.37 M−1 cm−1 at λMAX = 603 nm) in these reports are nevertheless in good agreement. In contrast, the reported values of extinction coefficient in aqueous solution (approximately, ε = 5.11 M−1 cm−1 at λMAX = 590 nm) by different investigators (Hart,18 Taube19), despite being in agreement with one another, differ significantly from those estimated from gas phase measurements. This suggests that the Beer−Lambert extinction coefficient in liquid CO2 could also be different, warranting its measurement as reported herein. The interest in ozone as an oxidant has also motivated experimental investigations into its solubility in a variety of common solvents (Aleksandrov,20 Biń21). Besides water, perhaps the most studied solvent for ozone solubility is CCl4, with Alexandrov,20 Teramoto,22 and Razumovskii13 providing Henry’s law coefficients at temperatures ranging from 0 to 30 °C. However, the reported data are not in good agreement, with Ostwald coefficients (gas/liquid) ranging from 1.87 to 2.93 at 15 °C. Solvents such as acetic acid and hexane, although not as toxic or environmentally harmful as CCl4, are subject to
Scheme 1. Mechanism for the Reaction of Ozone with Methyl Oleate
cycloaddition of ozone across the olefin double bond, cleaves to yield a carbonyl oxide (3) and a carbonyl compound (4). These species may either recombine within the same solvent cage to form a 1,2,4-trioxolane (“normal” secondary ozonide, 5) or the carbonyl oxide may leave the solvent cage to react with a different carbonyl partner (“crossed” product, 4 and 6) or alternatively dimerize or polymerize with other species present, e.g., with another carbonyl oxide to form the tetroxane 8. Other common side-products are carboxylic acids (9). These studies provide useful guidance to the first investigations of ozonolysis in liquid CO2 reported herein.
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MATERIALS AND METHODS
Determination of Beer−Lambert Extinction Coefficient. Early experiments indicated that utilizing light absorbance at the wavelength of 254 nm, commonly used for measuring ozone concentration in the gas phase, is unsuitable for measuring ozone concentrations in dense phase CO2. This is because of the rapid saturation of the light source by the high ozone concentrations in the 2.0 cm path-length highpressure cell, resulting in unreliable measurements. Consequently, the Beer−Lambert extinction coefficient, ε, was measured at 595 nm, a wavelength that is readily accessible but less intensely absorbed by ozone than 295 nm. A schematic of the experimental setup is shown in Figure 1. Liquid CO2 is introduced from a high-pressure cylinder with a dip-tube to a custom-made high pressure view cell constructed of grade 4 titanium with sapphire windows. Ozone was produced in a Praxair-Trailigaz Uniozone LO ozone generator using extra dry grade oxygen (99.6%, dew point < −50 °C, supplied by Matheson) and pumped into the view-cell using a Teledyne ISCO 500D syringe pump. The titanium view cell was constructed so that the sample path length for light absorbance measurements was precisely 20 mm. Light was provided by an Ocean Optics DH-2000 UV−vis-NIR light source with deuterium and halogen bulbs through 600 μm fiber optic cables and collimating lenses, and measured by an Ocean Optics USB2000 spectrometer with a linear silicon CCD array detector. Data at 595 and 850 nm (baseline) were recorded using the Ocean Optics SpectraSuite software. A MicroPump GA-X21 series gear pump (IDEX Corp) was used to circulate the dense phase CO2 through a 1 mL sample loop connected to a 6-way VALCO valve. This circulation not only facilitates rapid attainment of equilibrium but also allows sampling of the liquid phase B
DOI: 10.1021/acssuschemeng.5b00913 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 1. Schematic of high pressure view-cell with sampling loop and KI trap for online sampling.
Figure 2. Schematic of setup for determining Henry’s law coefficients for ozone solubility in liquid CO2. exceeded 40 °C. However, such adverse effects were found to be negligible when the compressor was operated for less than 1 h and also allowed to cool between runs. The compressed ozone was stored in a high pressure cylinder equipped with a back pressure regulator. Excess oxygen/ozone leaves the high pressure accumulator through the back pressure regulator with its concentration monitored by a Teledyne API model 452 ozone analyzer. Ozone and oxygen enter an Omega FMX 8400 series static mixer through a Brooks SLA mass flow controller. Liquid CO2 is pumped into the static mixer by a Teledyne ISCO 500D dual syringe pump. The combined stream of oxygen, ozone, and CO2 enters a view cell where the gas and liquid phases separate, with the gas stream exiting through a back pressure regulator before passing through a Teledyne API model 452 ozone analyzer. Liquid CO2, containing dissolved ozone and oxygen, is drawn out of the view cell by a gear pump into the bottom of a second view cell that is filled with the liquid mixture. The excess liquid exits the top of the view cell and mixes with the inlet stream past the static mixer and recirculated to the gas/liquid separator. Pressure is measured by an Omega PX329 series pressure transducer calibrated with a dead weight tester. Temperature is measured by Omega HTQ series type T thermocouples. Light absorption at 595 nm by dissolved ozone in the liquid phase within the view cell was continuously monitored as described previously. An external chiller was used to provide the cooling needed for the lines and devices. Data were recorded every 5 s by National Instruments FieldPoint data acquisition system (DAQ) and a suitably interfaced PC.
for analysis of the ozone concentration. When the temperature, pressure, and absorbance values had stabilized, the valve was switched to allow the sample to be purged into a high pressure trap containing an excess of the KI solution. A known volume of this KI solution was then titrated with sodium thiosulfate solution to determine the ozone concentration in the liquid CO2 phase. Each titration was performed in triplicate with the values averaged to determine the extinction coefficient and standard deviation at the specific conditions. Determination of Henry’s Law Coefficient. Early experiments in our laboratory indicated that ozone was highly soluble in liquid CO2 as inferred from the liquid phase developing a deep blue or purple color at low to moderate ozone partial pressures. However, maintaining a stable vapor−liquid equilibrium long enough to obtain reliable Henry’s law data in a batch cell containing CO2, ozone, and oxygen proved to be difficult. Thus, an alternative approach was developed whereby ozone is continually passed through recirculating liquid CO2, with both the liquid CO2 and exiting gas composition monitored for ozone composition. A schematic of the setup is shown in Figure 2. Ozone is generated as previously described and compressed using a RIX Microboost oxygen compressor, modified with 316 SS and Kalrez perfluoroelastomer components to tolerate high ozone concentrations. Experience has shown that Viton and similar fluoroelastomers are insufficiently compatible with ozone at the required conditions, often depolymerizing before failure and contaminating the reactant streams. It must also be noted that the compression of the oxygen/ozone stream to pressures as high as 105 bar did not adversely impact the quality of the ozone until the compressor surface temperature C
DOI: 10.1021/acssuschemeng.5b00913 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering After the ozone/oxygen and CO2 flows are established, the temperature, pressure and inlet ozone concentration were allowed to stabilize as the circulation of the liquid phase facilitated its saturation by ozone. The feed of the ozone/oxygen stream was set to 1500 std cm3 min−1, whereas CO2 was fed at a rate sufficient to maintain a fixed level in the gas/liquid separator. System equilibrium was usually attained within 20−30 min. After stabilization, a series of measurements were taken during the next 5−10 min. From these measurements, the Henry’s law coefficient is estimated from the following well-known equations, included here to show clearly how the various experimental measurements are used. The ozone partial pressure in the gas phase is estimated using eq 1:
PO3 = PtotyO
3
Baselines were sparingly corrected using the Bernstein polynomial method in MNova. In samples with deuterated solvents, referencing was carried out with respect to internal TMS at 0 ppm, or residual protonated solvent. The “100%” method was used for routine quantitative analysis, specifically using the peak area for the methyl ester resonance (50 ppm) as the internal standard. In general, this also agreed with the methyl resonance (15 ppm), but in thermally degraded reaction mixtures the methyl group concentration tended to be slightly larger than that of the ester groups so this should not be regarded as a universal method. This method was validated by the use of added 1,4dioxane as an internal standard, and/or the use of coaxial inserts containing 5% v/v 1,4-dioxane in D2O as an external standard (the latter also verifying the stability of 1,4-dioxane with the reaction mixtures). For routine analysis, the need to keep the NMR tube/insert assemblies paired and calibrated due to variability in the dimensions of the coaxial inserts outweighed any potential minor benefits compared to the 100% method. Note on Safety. The unstable or explosive nature of the products from the ozonolysis of olefins has mixed evidence in the literature. Medard7 noted, without elaboration, that reactions involving ozone should be conducted with caution citing laboratory experiments that resulted in spontaneous explosions. Patnaik6 suggests that it is the low molecular weight ozonides that are highly unstable and explosive. In contrast, Davies28 notes that the explosiveness of ozonides is exaggerated because they are not differentiated from impurities, most notably the 1,2,4,5-tetroxanes (8). Swern29 and Hawkins30 seem to agree with Davies, noting that many ozonides are stable enough to withstand vacuum distillation. It has been our experience that the ozonolysis products of long chain fatty acid methyl esters are extremely stable, exhibiting no tendency for spontaneous thermal decomposition upon exposure to percussive or thermal shock.31 Exposure of ozonolysis products to the flame of a Bunsen burner did not yield significantly different results from those of the original fatty acid methyl esters. 13C NMR analysis (discussed later) shows the absence of significant quantities of 8, although the presence of small amounts cannot be completely ruled out. Utmost care should be taken when performing these reactions, keeping in mind the safety precautions noted in the literature.
(1)
where PO3 is the partial pressure of ozone, Ptot is the total pressure of the system measured by the pressure transducer, and yO3 is the mole fraction of ozone in the gas as measured by the ozone analyzer. The ozone concentration in the liquid phase is estimated using eq 2:
c=
A εl
(2)
where c is the concentration of ozone in the liquid phase, A is the absorbance of light at 595 nm, l is the path length of the cell in cm, and ε is the previously determined Beer−Lambert extinction coefficient in M−1 cm−1. The Henry’s law coefficient was estimated using eq 3, with kH expressed in mol L−1 bar−1.
kH =
c PO3
(3)
Ozonolysis of Methyl Oleate. In a typical experiment, after 0.573 g of methyl oleate (Alfa Aesar, 96+%, 1) was placed in the previously described titanium view cell, the cell was filled with liquid CO2 from a tank with a dip tube until the cell was approximately half full (approximately 2.5 mL). The cell was then cooled until the internal temperature, as measured by a Type K thermocouple, reached a steady state value of 0.2 °C. Separately, ozone was compressed into a sample cylinder to a pressure of ∼97.5 bar with an approximate concentration of 3.6 mol % O3 in O2. The contents of the sample cylinder were then used to pressurize the methyl oleate/CO2 mixture in the titanium view cell to a total pressure of 94.6 bar. The cell was gently shaken to ensure mixing, allowed to sit for approximately 10 min, and the pressure was gently released via a needle valve. Upon depressurization, the cell was opened, and the resulting products were extracted with a syringe. The starting material and recovered product solution were analyzed by GC−MS and GC-FID by diluting 2 μL of recovered product with 1 mL of hexane. The GC method uses an HP-INNOWAX column on an Agilent 7890A GC coupled to a 5975C MS and uses a carrier gas flow of 1 std cm3 min−1, an inlet temperature of 250 °C, and an injection volume of 1 μL; the oven temperature was initially held at 40 °C for 5 min, then ramped at 10 °C per minute to 220 °C and held at this temperature for a further 20 min. Masses were scanned from 20 to 500 Da. The starting mixture and recovered products were also analyzed by NMR on a Bruker Avance III 500 MHz instrument at 25.0 ± 0.1 °C equipped with a 13C-observe cryoprobe (CPDUL) using the procedure as described later. 13C T1 measurements were carried out using the inversion recovery method on a Bruker DRX 500 equipped with a broadband observe probe (BBO). For experiments with external standards or protonated solvents, Wilmad-Labglass 528-PP-7 5 mm tubes were used with WGS-5BL stem coaxial inserts for 5 mm tubes. Quantitative 13C spectra were obtained using 90° pulses and inverse-gated decoupling, with recycle delays set to ≥5T1 of the resonances of interest. In particular, recycle delays of 28.5 s were chosen for routine analysis, with the limiting factor being that of 1,4dioxane in external D2O solutions. The next longest T1 was that of the carboxylic acids at ca. 4.2 s so that, in principle, it is possible to lower analysis times further. However, the longer recycle delay was used for routine analysis to cover all eventualities.
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RESULTS AND DISCUSSION Beer−Lambert Law Extinction Coefficient. Extinction coefficient data were acquired at temperatures ranging from −1.4 to +17.7 °C and at total pressures ranging from 54.9 to 99.9 bar. A typical plot of ozone absorbance in CO2 in the Chappuis band is shown in Figure 3. The peak absorbance in
Figure 3. Typical Chappuis band absorbance of ozone in CO2, with maxima at 595 nm. D
DOI: 10.1021/acssuschemeng.5b00913 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Absorbance and Extinction Coefficient of Ozone in CO2 at Various Temperatures and Pressures temperature (°C)
pressure (bar)
ρCO2 (mol L−1) [NIST data]32
absorbance (λ = 595 nm)
O3 concentration (M) ± 0.0004
2.4 16.9 17.2 17.7 12.6 0.3 −0.1
72.7 77.5 82.2 75.5 99.9 67.4 59.0
21.440 19.321 19.415 19.100 20.562 21.627 21.542
0.110 0.080 0.133 0.094 0.106 0.134 0.163
0.0162 0.0136 0.0226 0.0150 0.0165 0.0190 0.0220
this band, observed in pure ozone at 603 nm, experiences a blue shift to 595 nm in CO2. The half-life of ozone in CO2 was measured at 11.9 °C and 100 bar total pressure and found to be 592 min. Representative data under the various conditions are presented in Table 1, with an overall average extinction coefficient of 3.2 ± 0.3 M−1 cm−1 with the standard deviation representing the error. Burkholder and Talukdar33 found that ozone absorption of light did not depend on temperature in the Chappuis band. Our results also indicate that temperature has a negligible effect within the limits of experimental error. However, the molar density of liquid CO2 depends on temperature and pressure, ranging from 19.1 to 21.6 mol L−1 at our experimental conditions. In that range, a slight increase in extinction coefficient with an increase in CO2 density is detectable. A similar value of extinction coefficient (3.07 ± 0.06 M−1 cm−1) was estimated for SF6, a substance that is also resistant to ozone and has similar critical properties (Pc = 37.5 bar, Tc = 45.6 °C) as CO2. Henry’s Law Coefficients. Henry’s law coefficients for ozone in dense phase CO2 were generally found to be approximately 0.25 mol L−1 bar−1 in the temperature range of −10 to +4 °C. Data collection at higher temperatures was either impractical or impossible because the oxygen passing through the system would strip the CO2 from the liquid at too high a rate to maintain a stable vapor−liquid equilibrium. The stripping of CO2 from the liquid phase by oxygen, although minimal at the temperatures and pressures observed, produced some evaporative cooling. As such, with the feedback control of the system off, temperature stability was used as a criterion for monitoring the dynamics of the system and determining when steady state had been achieved. When fully developed, this experimental technique produced highly reproducible data. To benchmark the data acquired with the present setup with those reported by previous researchers in organic solvents, ozone solubility in CCl4 (among the most studied system) was measured using the same procedure. The results are shown in Table 2. A selection of raw data demonstrating the linear relationship between liquid phase ozone concentration and ozone partial pressure is available in the Supporting Information (Figure S1). The ozone solubility in CCl4 estimated in the present study is greater than the previously reported values (see last three rows of Table 2). Unfortunately, the lack of adequate experimental details in previously reported studies renders it difficult to make a reliable comparison. The kH value for ozone solubility in liquid SF6 (∼0.14 mol−1 bar−1) is roughly half of that estimated in liquid CO2 (see Table S2 in the Supporting Information). This suggests that SF6 may in principle be used as a medium for ozonolysis. However, the much higher cost of SF6 makes CO2 the clear choice as solvent.
ε (M−1 cm−1) 3.25 3.07 2.78 2.94 3.18 3.53 3.71
± ± ± ± ± ± ±
0.01 0.02 0.01 0.02 0.02 0.03 0.02
Table 2. Summary of Henry’s Law Coefficients in CO2 and CCl4 at Various Temperatures solvent
T (°C)
PTotal (bar)
ρCO2 (kg L−1)
kH (mol L−1 bar−1)
CO2 CO2 CO2 CO2 CO2 CO2 CCl4 CCl4 CCl4
−10.00 −7.29 −4.19 2.15 2.26 3.80 15.30 15 15
59.5 59.6 59.6 59.6 59.8 59.8 7.9 unknown unknown
0.720 0.636 0.565 0.432 0.468 0.427 n/a n/a n/a
0.29 0.28 0.26 0.23 0.22 0.22 0.35 0.12222 0.077713
Ozonolysis of Methyl Oleate. Methyl oleate was used as the preferred substrate for ozonolysis because it is asymmetrical and allows for identification of individual daughter products, permitting some insight into the underlying chemistry. The reaction was lean in ozone, which was totally consumed during the 10 min batch run. Thus, as expected, the ozonolysis reaction was quite rapid. GC−MS analysis of the products (approximately 0.378 g) recovered from a batch run indicated significant quantities of unreacted methyl oleate as well as nonanal, nonanoic acid, methyl 9-oxononanoate, and methoxy-9-oxononanoic acid. These compounds were identified using either comparison of retention times with those of standard samples or the NIST Mass Spectral Library. Saturated compounds appearing as impurities in the starting material (e.g., octadecanoic acid methyl ester) did not appear to be oxidized. All compounds identified by GC−MS could not be quantified due to the lack of availability of some standards, most notably the methoxy-9oxononanoic acid. GC-FID analysis found aldehydes in quantities 2.3 times greater than acids on the basis of raw peak area. The presence of secondary ozonides (SOZ) such as the 1,2,4-trioxolane and tetroxanes formed from the dimerization of carbonyl oxides (Figure 4), routinely observed by Bailey,24 Murray,25 and Criegee,23 were not observed by either GC−MS or GC-FID analysis. This is to be expected given previous work on lower molecular weight species required injection and oven temperatures of
