Article pubs.acs.org/EF
Cite This: Energy Fuels 2017, 31, 10866-10873
Quantitative In Situ Mass Spectrometry Analysis of Mannitol Decomposition Products under Hydrothermal Conditions Pattasuda Duangkaew,† Shuhei Inoue,† Tsunehiro Aki,‡ Yutaka Nakashimada,‡ Yoshiko Okamura,‡ Takahisa Tajima,‡ and Yukihiko Matsumura*,† †
Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan ‡ Department of Molecular Biotechnology, Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8530, Japan S Supporting Information *
ABSTRACT: In situ mass spectrometry analysis was conducted for the hydrothermal decomposition of mannitol at different temperatures and residence times in a batch reactor. The temperature was varied in the range of 140−220 °C, and the residence time was varied between 5 and 20 min. The products of mannitol decomposition included furfural, 5-hydroxymethylfurfural, phenol, formic acid, and levulinic acid. At high temperatures, the mass spectrum showed peaks at m/z 46 and 116 that corresponded to formic acid and levulinic acid, respectively. Conventional high-performance liquid chromatography could not detect these acids. Quantification of these products and the rate constants for the mannitol decomposition network were also obtained. The reaction rate coefficients were determined by fitting the experimental results.
1. INTRODUCTION An increase in energy demand leads to an increase in fossil fuel price, directly affecting the global economic activity1 and various environmental concerns, such as anthropogenic climate change. Therefore, development of sustainable, renewable energy sources is needed.2,3 One potential feedstock for a sustainable future society is the third-generation biofuels produced from marine macroalgae, which has attracted significant attention from researchers worldwide.4,5 Brown kelp macroalgae has high potential as a biomass energy resource because it is widely cultivated worldwide and hardly competes with food production. The main component of kelp is carbohydrates with low lignin and hemicellulose contents.6,7 Therefore, a bioenergy production process using kelp carbohydrates does not need a lignin removal step that is usually necessary to increase the digestibility of lignocellulosic materials.8 Among the carbohydrate compounds in kelp, mannitol, a sugar alcohol, is a major carbohydrate of brown kelp that accounts for approximately 30 wt % of total carbohydrates.9,10 Because mannitol can also be used as a feedstock to produce highly valuable lipids,11−13 the recovery and utilization of mannitol from kelp is of interest. To efficiently recover mannitol from kelp, proper pretreatment is needed. Among the various pretreatment processes, hydrothermal pretreatment is suitable to convert wet biomass, such as kelp. Because this pretreatment process is a thermochemical process using high-pressure and high-temperature water, removal of water from the wet biomass is not needed.14,15 Furthermore, it is a straightforward and economical process.16,17 Previous research demonstrated that one of the problems of the hydrothermal pretreatment of kelp18−21 was unwanted decomposition of mannitol. Thus, we further studied the decomposition of mannitol compared to glucose at various temperatures of the hydrothermal process (170−250 °C at 25 MPa for 80 s).22,23 © 2017 American Chemical Society
Mannitol decomposed faster than glucose possibly as a result of the difference in functional groups, and the addition of salt did not significantly change the decomposition rate of mannitol. However, details of the mechanism of mannitol decomposition, including the identity of the intermediates and corresponding reaction rate coefficients, have not been reported. Hence, understanding the hydrothermal decomposition of mannitol in more detail and elucidating its reaction kinetics are necessary. For this purpose, detection of the intermediate products using rapid and real-time analysis is required. Formerly, many researchers used gas chromatography/mass spectrometry (GC/MS) with a fast pyrolysis method to achieve real-time analysis of the products. Levoglucosan is widely belived to be a primary product of carbohydrates in fast pyrolysis, which degrades over time under the condensation process of the product bio-oil.24−28 Thus, direct analysis using GC/MS was found to be effective; however, GC/MS is still limited to the detection of only thermally stable and volatile compounds, and also carbohydrates larger than 162 Da cannot be detected without derivatization.29,30 To address these problems, in situ analysis using only mass spectrometry (MS) during the hydrothermal pretreatment process is required. MS is highly selective, sensitive, and affordable for high throughput. Furthermore, MS can be performed with small sample sizes without derivatization and can be combined with other instruments.25,27,31 However, in situ MS analysis for elucidating the intermediate decomposition products of hydrothermal reactions has not been well studied and developed. Our group succeeded in directly analyzing hydrothermally treated glucose used as a model compound of a terrestrial biomass with MS Received: June 2, 2017 Revised: September 1, 2017 Published: September 4, 2017 10866
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Figure 1. Mass spectral database for mannitol decomposition products.
analysis,32,33 but mannitol, which was used as the model compound of kelp, has not been well studied. In addition, quantitative analysis is needed for kinetic analysis but has not been developed. Consequently, the purpose of this study is to apply MS to determine the mannitol decomposition products, intermediates, and kinetics of the associated reactions during the hydrothermal pretreatment process.
mannitol (0.5 wt %) was fed into the reactor by the high-pressure pump with the MS valve closed until the desired pressure is achieved at a constant temperature. As a result of the high specific surface area of the tubular batch reactor, the mannitol solution could be heated rapidly to the reaction temperature. By keeping the mannitol solution in this reactor for a specific reaction time, mannitol was hydrothermally decomposed. After the desired reaction time, the valve at the end of the reactor was opened and the solution was released into the MS sampling chamber via a nozzle for analysis. The MS sampling chamber was tubing with an inner diameter of 16.6 mm, which was continuously evacuated by a rotary pump to 1 mPa or less. The temperature of the sampling line was maintained at the reactor temperature with an electric heater. Initially, the reaction temperature was set at 140, 180, and 220 °C while maintaining a reaction time of 10 min. Then, the reaction time was set at 5, 10, 15, and 20 min while maintaining a reaction temperature of 220 °C. The reaction pressure was held constant at 5 MPa for all experimental runs. A quadrupole mass spectrometer (ULVAC, Inc., Chigasaki, Japan) was used as a detector with an electron ionization energy of 70 eV. The mass spectral data were processed using the Qulee QCS version 3.0 software for gas analysis in full scanning mode in a mass range of m/z 0−300. Calibration curves for the MS analysis were obtained for the following six standard compounds: mannitol, 5-HMF, furfural, phenol, formic acid, and levulinic acid. First, an aqueous solution of each standard compound with a concentration of approximately 1 kg/m3 (1 g/L) was introduced into the reactor at room temperature, and the mass spectrum for each compound was determined. Then, the characteristic base or molecular ion peak of each compound was identified. For the calibration curve, aqueous solutions with concentrations of 12.5, 25, and 50 mol/m3 (12.5, 25, and 50 mM) and the previous 1 kg/m3 solution were introduced into the reactor
2. MATERIALS AND METHODS 2.1. Materials. Mannitol (extra pure reagent, CAS Registry Number 69-65-8), 5-hydroxymethylfurfural (5-HMF, ≥99% purity, CAS Registry Number 67-47-0), levulinic acid (98% purity, CAS Registry Number 123-76-2), and phenol (specially prepared reagent, CAS Registry Number 69-65-8) were the products of Sigma-Aldrich. Furfural (99% purity, CAS Registry Number 98-01-1), formic acid (>99.5% purity, CAS Registry Number 64-18-6), and acetonitrile (>99.5% purity, CAS Registry Number 75-05-8) were purchased from Nacalai Tesque. All chemicals, solvents, and distilled water (Nacalai Tesque) used in this study were used without further purification. 2.2. Experimental and Analytical Methods. The experimental apparatus has been reported elsewhere.32,33 In brief, rapidly heating mannitol for a specific time is important for a controlled hydrothermal decomposition. For this purpose, we employed a stainlesssteel (SS316) tubular batch reactor with an inner diameter of 4.35 mm and length of 15.7 m in an electric furnace to control its temperature. One end of the reactor was connected to a high-pressure pump to deliver the mannitol solution, and the other end was connected to the MS sampling chamber via a valve. After this reactor was heated with the electric furnace to the reaction temperature, an aqueous solution of 10867
DOI: 10.1021/acs.energyfuels.7b01558 Energy Fuels 2017, 31, 10866−10873
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Energy & Fuels and the ion current corresponding to the characteristic peaks of each compound was measured. For each calibration, the measurement was conducted in triplicate to evaluate the reproducibility and stability of the method. The MS analysis data of major fragment peaks of each standard compound given in Table S1 and Figure S1 of the Supporting Information illustrate how calculation of the regression linear equation was carried out. For comparison, conventional high-performance liquid chromatography (HPLC) was also used for product analysis after cooling the reaction products. The samples were filtered through a 0.2 μm syringe filter (Sartorius, Germany) before measurement. The SCR 102 HG
column from Shimadzu Corporation was used to quantify the amount of organic acids using an oven temperature of 40 °C, 5 mol/m3 (5 mM) perchloric acid (HClO4) as the eluent, a flow rate of 0.7 cm3/min, and a refractive index detector (Shimadzu RID 10-A). Cyclic compounds, such as 5-HMF, furfural, and phenol, were quantified using the DE413L column from Showa Denko K.K. using an oven temperature of 40 °C. HClO4 (5 mM) was mixed with acetonitrile at a ratio of 1:1 as the eluent at a flow rate of 0.7 cm3/min using an ultraviolet−visible (UV−vis) detector (Shimadzu, SPD 10-A).
3. RESULTS AND DISCUSSION Figure 1 shows the mass spectra of the six standard compounds. The summary of fragment ions and element compositions are presented in Table 1. Calibration curves were determined for these compounds using the characteristic peak as shown in Figure 2. Good linear relationships for each were obtained with the regression equation of each standard calibration summarized in Table 2. The effect of the reaction temperature on the mass spectrum of the effluent is presented in Figure 3. At 140 °C, the spectrum
Table 1. Summary of Major Fragmentation from the Mass Spectrum of Standard Compounds standard compound mannitol
5-HMF
furfural
phenol formic acid
levulinic acid
m/z
formula of ions
relative abundance (%)
73 61 43 126 97 41 96 67 39 94 77 46 45 29 116 57 43
C3H5O2 C2H5O2 C2H3O C6H6O3 C5H5O2 C2HO C5H4O2 C4H3O C3H3 C6H6O C6H5 CH2O2 CHO2 CHO C5H8O3 C3H5O C2H3O
100.0 67.3 49.0 56.7 100.0 65.1 100.0 27.8 65.6 100.0 17.7 57.3 39.9 100.0 5.7 20.3 100.0
Table 2. Linear Regression Equation of Mass Spectral Database for Mannitol Decomposition Product standard
m/z
elemental composition
mannitol 5-HMF furfural formic acid levulinic acid phenol
73 97 96 46 116 94
C3H5O2 C5H5O2 C5H4O2 CH2O2 C5H8O3 C6H6O
equation y y y y y y
= = = = = =
2.17 9.00 2.78 1.44 6.66 2.63
× × × × × ×
10−15x 10−15x 10−15x 10−14x 10−16x 10−15x
R2 0.988 0.937 0.922 0.897 0.873 0.963
Figure 2. Regression calibration for mannitol decomposition products. 10868
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Figure 3. Mass spectrum of mannitol decomposition under various temperatures of the hydrothermal pretreatment process in the range of 140−220 °C by in situ MS analysis.
using MS analysis and HPLC are listed in Table S2 of the Supporting Information. Surprisingly, phenol could be detected in small amounts in our experiments using both methods, and its concentration slightly increased with the reaction time. Previously, the formation of small amounts of hydroxylated aromatics from carbohydrates and derivatives has been reported for aqueous solutions.34−37 On the basis of our results, we confirm that phenol is formed from 5-HMF, which was a dehydration product of mannitol. A possible reaction pathway from 5-HMF to phenol is shown in Figure 5. Under high temperature and pressure of hydrothermal treatment, water participates as a reactant in several of the reaction steps.38 5-HMF is easily protonated and activated by water as a collision partner. The opening of the furan ring through hydration will give an intermediate tetrahydroxycyclohexadiene formed from the retro-aldol reaction that
is the same as original mannitol without any product peaks observed, indicating that decomposition has not started. At 180 °C and above, cyclic compounds, such as 5-HMF, phenol, and furfural, were observed on the basis of the ion peaks at m/z 97 (C5H5O2+), 94 (C6H6O+), and 96 (C5H4O2+), respectively. Additionally, formic acid and levulinic acid were observed at 220 °C based on fragment ions at m/z 46 (CH2O2+) and 116 (C5H8O3+), respectively. Notably, the ion current of the characteristic peak of mannitol decreased with the temperature, and the ion current of the cyclic compounds and acids increased with the reaction temperature, indicating that the decomposition of mannitol occurred. Figure 4 shows the corresponding HPLC results with only four compounds detected. The peak of mannitol decreased, and those of the cyclic compounds (5-HMF, phenol, and furfural) slightly increased, with a reaction temperature in agreement with in situ MS analysis. The concentrations of these compounds 10869
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Figure 4. HPLC results of mannitol decomposition products.
hydrothermal decomposition products at temperatures lower than 200 °C, where acids were not observed using HPLC. The effect of the reaction time on product yields determined by in situ MS and HPLC is shown in Figure 6. The mannitol concentration in the effluent decreased with the residence time, while the yield of the product increased with the residence time. The major product is 5-HMF for both measurements, but the detected amount is 2 orders of magnitude greater for in situ MS than for HPLC. Thus, in situ MS could detect 5-HMF effectively for the determination of the actual reaction kinetics. Here again, formic acid and levulinic acid were detected only by in situ MS. The change in the mannitol concentration in the effluent as a function of the reaction time is shown in Figure 7 in terms of ln(C/C0) versus residence time, where C and C0 denote the mannitol concentration in the effluent and feedstock, respectively. A linear relationship was obtained for ln(C/C0) using both methods, indicating that first-order kinetics can be applied to mannitol decomposition. The hydrothermal decomposition of mannitol can be expressed using eq 1
Figure 5. Possible reaction pathway for hydrothermal phenol formation from 5-HMF.
contains a conjugated π bond, hydroxyl groups, and carbonyl groups.37 This intermediate can undergo further isomerization (stereoisomers) and ring fusion, which may be catalyzed by the acid products of mannitol decomposition. Finally, phenol is produced by dehydration of the cyclohexadiene derivative. However, the formation of acids could not be detected by HPLC, showing the effectiveness of in situ MS. In a previous study, Matsumoto et al.22 reported similar results for mannitol
dC = − kC dt
(1)
where t and k denote the reaction time (s) and reaction rate constant (s−1), respectively. Equation 1 was integrated to obtain ⎛C⎞ − ln⎜ ⎟ = kt ⎝ C0 ⎠ 10870
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Figure 7. Overall kinetics of mannitol decomposition: ln(C/C0) versus time at 220 °C and 5 MPa using HPLC and in situ MS analysis.
concentration for each compound can be expressed with the following differential equations: d[mannitol] = −(k1 + k 2)[mannitol] dt
(3)
d[5‐HMF] = k1[mannitol] − k 3[5‐HMF] − k4[5‐HMF] dt − k5[5‐HMF]
(4)
d[furfural] = k 2[mannitol] − k6[furfural] dt
(5)
d[phenol] = k 3[5‐HMF] dt
(6)
d[formic acid] = k5[5‐HMF] + k6[furfural] dt
(7)
d[levulinic acid] = k4[5‐HMF] dt
(8)
where [A] is the concentration of compound A (kmol/m3), ki is the rate constant of reaction i (s−1), and t is the residence time (s). The rate constants were determined using the least squares method (LSM) to fit the model with the experimental data, so that the value shown in eq 9 was minimized. E=
∑ (Cexp − Ccal)2
(9)
Here, Cexp and Ccal are the experimental and calculated concentrations, respectively. In Table 3, the comparison of the resulting reaction rate constants between the two methods is summarized. Mannitol decomposition products were mannitol (remaining product), 5-HMF, phenol, furfural, formic acid, and levulinic acid, which were detected by in situ MS analysis. HPLC detected only four kinds of products: mannitol, 5-HMF, phenol, and furfural. For HPLC analysis, because formic acid and levulinic acid were not observed, the rate constants of the reactions to produce these compounds were not calculated. The kinetic parameters from the HPLC results and in situ MS analysis were different as a result of the difference in the quantitative amount of products that were obtained. Consequently, the change of the analysis method can cause significant variation in the results, especially
Figure 6. Concentration change of mannitol decomposition products: (a and b) in situ MS analysis and (c and d) HPLC at 220 °C and 5 MPa.
Thus, the slope in Figure 7 gives the reaction rate constant k. The rate constants obtained from in situ MS and HPLC are 3.66 × 10−4 and 2.22 × 10−4 s−1, respectively, and they agreed within an error of 65%. Assuming first-order reaction kinetics for all reactions in the reaction network shown in Figure 8, the change in the 10871
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Figure 8. Generalized reaction pathway for mannitol decomposition in hydrothermal pretreatment conditions.
calibration of major fragment peaks from each standard compound referred to in the Experimental and Analytical Methods (Table S1 and Figure S1) and quantitative results of mannitol decomposition under hydrothermal conditions by in situ MS and HPLC analyses referred to in the Results and Discussion (Table S2) (PDF)
Table 3. Reaction Rate Constants for Mannitol Decomposition under the Hydrothermal Reaction method rate constant (s−1)
in situ MS
HPLC
k1
3.79 × 10−4
6.02 × 10−7
k2
1.27 × 10−5
5.50 × 10−7
−7
−5
denotation
k3
6.73 × 10
6.46 × 10
k4
2.86 × 10−5
0.00 × 100
k5
1.98 × 10−5
0.00 × 100
k6
3.13 × 10−4
0.00 × 100
conversion of mannitol to 5-HMF conversion of mannitol to furfural conversion of 5-HMF to phenol conversion of 5-HMF to levulinic acid conversion of 5-HMF to formic acid conversion of furfural to formic acid
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Corresponding Author
*Telephone: +81-0-82-424-7561. Fax: +81-0-82-422-7193. E-mail:
[email protected]. ORCID
Yukihiko Matsumura: 0000-0002-1341-0493 Notes
The authors declare no competing financial interest.
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for low-concentration compounds, indicating the importance of in situ MS analysis.
ACKNOWLEDGMENTS This research was supported by the Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST).
4. CONCLUSION Mannitol was decomposed under hydrothermal conditions and analyzed with in situ MS and conventional HPLC analyses. In situ MS analysis could detect low-molecular-weight species, such as formic acid and levulinic acid, which was not observed by HPLC, illustrating the advantage of the rapid analysis, such as in situ MS, and this analysis system can be useful to several feedstocks, which would give more information on the reaction mechanism, particularly when more reactive derivatives, such as intermediate compounds, are formed under hydrothermal conditions. Further, the first-order reaction kinetics well expressed the overall decomposition of mannitol, and reaction rate constants for the reactions in the mannitol decomposition network were determined using the quantitative in situ MS analysis.
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AUTHOR INFORMATION
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REFERENCES
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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/acs.energyfuels.7b01558. Mass analysis data of each standard compound that was used to make the calibration curve and regression 10872
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