Energy Fuels 2010, 24, 95–99 Published on Web 08/11/2009
: DOI:10.1021/ef9005093
Decomposition of Formic Acid in Supercritical Water† Yongchun Zhang, Jun Zhang,* Liang Zhao, and Changdong Sheng School of Energy and Environment, Southeast University, Nanjing, 210096, People’s Republic of China Received May 22, 2009. Revised Manuscript Received July 16, 2009
The decomposition of 0.05-0.7 M formic acid in supercritical water was investigated in a temperature range of 550-650 °C and a pressure range of 24-30 MPa for residence times of 16-46 s. The gaseous products were composed of H2 and CO2 as major components, and CO as a minor one, which indicates that decarboxylation is the dominant reaction pathway and dehydration is secondary. High temperature increased hydrogen production. Compared with temperature, pressure had less effect on hydrogen production. Carbon gasification efficiency reached 94.5% at a residence time of 20 s, and extending the residence time had very little effect. High concentration of formic acid led to side-reactions, which caused a great decrease of hydrogen production. The mechanisms for formic acid decomposition were studied computationally using the GAUSSIAN 03 suite of programs. Results show that water takes part in the formic acid decomposition reaction as a catalyst, which promotes both the decarboxylation and dehydration, and the promoting effect on decarboxylation is more apparent.
then glyceraldehyde converts to dihydroxyacetone, and both glyceraldehyde and dihydroxyacetone dehydrate into pyruvaldehyde. Later pyruvaldehyde, erythrose, and glycolaldehyde are decomposed to smaller-molecular liquid species, and some of them are further transferred, producing gas. This means that for an understanding of the biomass transformation to H2 in supercritical water, it is better to study the transformation of the small molecular species to H2 first. Organic acids are one of the major small molecular species. It has been suggested that in the SCWG of biomass the formation of gaseous products;H2, CO, CO2, CH4, etc.;has a close relation with their decomposition.5,8,9 Formic acid is the simplest organic acid, and its conversion to H2 in supercritical condition was studied in this paper. The unimolecular decomposition of formic acid in the gas phase was investigated widely as a key intermediate in the oxidation of organic hydrocarbons in atmospheric chemistry.10,11 The experimental results showed that the gaseous products mainly consisted of CO, with minor H2 and CO2. Unimolecular formic acid was decomposed through two reaction pathways, dehydration (eq1) and decarboxylation (eq2), with the former being the major reaction pathway.
1. Introduction Current enthusiasm for the use of hydrogen as an alternative energy is founded on the expectation that hydrogen will be produced from renewable resources at a competitive price. Biomass, as a main renewable resource, is gaining increasing attention because it can be used to produce hydrogen by several methods, such as air gasification, steam gasification, and anaerobic fermentative biohydrogen production. Compared with other thermal-chemical methods, supercritical water gasification (SCWG) of biomass can directly deal with wet biomass without drying and has a higher yield of hydrogen, being one of the major technologies in research and development at present.1 SCWG of biomass is a very complex process, involving a large number of intermediate products. Knowledge of the gasification pathways and mechanisms is still superficial. Biomass is a combination of cellulose, hemicellulose, and lignin.2,3 Sasaki et al.4 conducted cellulose hydrolysis in subcritical and supercritical water and reported that glucose and oligomers as the hydrolysis products were formed. The decomposition of glucose in supercritical water has been investigated by a number of researchers.5-7 Glucose epimerizes to fructose or decomposes to erythrose plus glycolaldehyde or glyceraldehyde plus dihydroxyacetone first, and
ð1Þ
HCOOH f CO2 þ H2
ð2Þ
Formic acid is also an important intermediate product in the hydrothermal oxidation of a wide variety of organic compounds and wastes. Maiella and Brill12 reported 1 M formic acid decomposition under hydrothermal conditions of
† Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: junzhang@ seu.edu.cn. (1) Hao, X. H.; Guo, L. J. J. Chem. Eng. (China) 2002, 53 (3), 221–228. (2) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42 (2), 267–279. (3) Lee, I. G.; Kim, M. S.; Ihm, S. K. Ind. Eng. Chem. Res. 2002, 41 (5), 1182–1188. (4) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 1998, 13, 261–268. (5) Sına g, A.; Kruse, A.; Schwarzkopf, V. Ind. Eng. Chem. Res. 2003, 42 (15), 3516–3521. (6) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Ind. Eng. Chem. Res. 1999, 38 (8), 2888–2895. (7) Sina g, A.; Kruse, A.; Schwarzkopf, V. Eng. Life Sci. 2003, 3 (12), 469–473.
r 2009 American Chemical Society
HCOOH f CO þ H2 O
(8) Yoshida, K.; Wakai, C.; Matubayasi, N.; Nakahara, M. J. Phys. Chem. A 2004, 108 (37), 7481–7482. (9) Watanabe, M.; Inomata, H.; R., L. S., Jr; Arai, K. Appl. Catal., A 2001, 219 (3), 149–156. (10) Blake, P. G.; Davies, H. H.; Jackson, G. E. J. Chem. Soc. B 1971, 1923–1925. (11) Saito, K.; Kakumoto, T.; Kuroda, H.; Torii, S.; Imamura, A. J. Chem. Phys. 1984, 80, 4989–4996. (12) Maiella, P. G.; Brill, T. B. J. Chem. Phys. A 1998, 102 (29), 5886– 5891.
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Energy Fuels 2010, 24, 95–99
: DOI:10.1021/ef9005093
Zhang et al.
280-300 °C and 27.5 MPa. Yu et al. studied the decomposition of 0.02 M formic acid in a temperature range of 320-500 °C and a pressure range of 18.0-30.7 MPa. The gaseous products were also composed of CO, H2, and CO2, but reversely H2 and CO2 became the major products and the yield of CO was always at least an order of magnitude lower than those of H2 and CO2, which suggested that the major reaction pathway was decarboxylation (eq 2). For an effective gasification of biomass and its model compounds in supercritical water, the temperature adopted is usually over 500 °C with a pressure range of 2535 MPa,3,14,15 which are different from those conditions used in hydrothermal studies. Obviously the formic acid decomposition under these conditions should be known for a better understanding of the transfer routes of biomass to hydrogen in the SCWG. For this purpose the experimental investigations of the decomposition of 0.05-0.7 M formic acid in supercritical water in a temperature range of 550-650 °C were carried out with a pressure range of 24-30 MPa for residence times of 16-46 s. At the same time, the mechanisms of formic acid decomposition were also explored using quantum chemistry methods. 13
Figure 1. Schematic diagram of the experimental setup.
content by a total organic carbon analyzer (Shimadzu TOCVCPH /CPN).
3. Results and Discussion In all cases, the gaseous products were composed of H2 and CO2 as major components and CO as a minor one. The gas molar fractions (GMF, percentage of the moles of H2 (CO2, CO) in gaseous products) and the carbon gasification efficiency (CGE) depended greatly upon the reaction conditions. The CGE is defined as: MLOP 100% CGE ¼ 1 MF
2. Experimental System and Procedures A schematic figure of the system used in this experiment is shown in Figure 1. A 0.1 M solution of formic acid (Shanghai Jiuyi Chemical Company, 88%; the 12% impurity is just water.) was prepared with distilled, deionized water. The formic acid solution was fed by a high-pressure pump from a feed tank through a preheater tube and then into a reactor tube. The preheater and reactor were both constructed in 1Cr18Ni9Ti tubes (26 mm i.d. and 300 mm length) and heated by electric heaters. The temperature of the preheater was controlled using a K thermocouple installed on the outer wall at the outlet. The reactor axial temperature profiles along the reactor length were measured by three K-type thermocouples mounted on the reactor’s outer wall. All reactor temperatures mentioned in this paper were the values of the middle thermocouple. After leaving the reactor, the effluent flowed through a filter and then was rapidly cooled in a heat exchanger. A back-pressure regulator was employed between the pressure transducer and the gas-liquid separator to reduce the pressure to atmospheric pressure. The effluent was separated into gas and liquid phases in the gas-liquid separator at ambient conditions. The gas flow rate was measured by a wet test meter. Liquid product was collected at the bottom of the separator. To get a suitable preheated temperature, the pre-experiments were conducted, in which only the preheater was powered. A higher preheated temperature at which the decomposition of formic acid does not happen is beneficial to the study of the reaction occurring at the reactor. Finally, the preheated temperature of 300 °C was determined. As a consequence, the residence time was defined as the reactor volume divided by volumetric flow rate of water at the reactor temperature and pressure. The volumetric flow rate of water was given by mass flow rate of water divided by density of water at the reaction conditions. Gaseous products collected were analyzed with a gas chromatogragh (Agilent, model 6890N) with the thermal conductivity detector for the detection of hydrogen, carbon monoxide, and carbon dioxide. Helium was used as a carrier gas. Samples of the liquid phase were collected and analyzed for total organic carbon
where MLOP is molar amount of C atoms in liquid organic products, MF is molar amount of C atoms in feedstock. 3.1. Temperature. Panels a-c of Figure 2 show the results obtained at the temperature range of 550-650 °C and at the pressures of 24, 27, and 30 MPa, respectively. At all three pressures, the H2 and CO2 GMF values steadily increased with temperature, whereas the CO GMF steadily decreased. The CO GMF was always much lower than GMF of H2 and CO2, suggesting that the decarboxylation reaction producing H2 and CO2 was the dominant reaction pathway, and the dehydration reaction producing CO was the minor pathway. Figure 2a-c also shows the variations of the CGE with the temperature. The CGE increased sharply when the temperature increased from 550 to 600 °C and then increased steadily with the temperature increasing from 600 to 650 °C. Compared with the temperature, the pressure (or water density) had a small effect on the GMF and the CGE, especially at high temperature. It can been seen from Figure 2a-c that the CGE remained about 98.0% at all pressures when the temperature reached 650 °C. The reason might be that the formic acid had been decomposed almost completely at this temperature with low pressure, leading to little increase when increasing the pressure further. These results demonstrated that high temperature was beneficial to formic acid decomposition and hydrogen production. 3.2. Residence Time. The results obtained at different residence time are shown in Figure 3. It could be seen that the GMF of the three gaseous constituents had an obvious change before 20s and kept almost constant after that. The CGE increased dramatically from 58.9 to 94.5% when the reactor residence time increased from 16 to 20 s, reaching the highest value (98.0%) at 28 s, and further extending the residence time could not increase the CGE any more. These results demonstrated that there was an optimal reactor residence time for formic acid decomposition in supercritical water. 3.3. Feedstock Concentration. The experimental results of the effect of formic acid solution concentration on the GMF and the CGE are shown in Figure 4. With the increase of concentration from 0.05 to 0.7 M, the H2 GMF decreased
(13) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1998, 37 (1), 2–10. (14) Lu, Y. J.; Ji, C. M.; Guo, L. J. J. Xi’an Jiao Tong Univ. 2005, 39 (3), 239–242. (15) Kruse, A.; Henningsen, T.; Sınag, A.; Pfeiffer, J. Ind. Eng. Chem. Res. 2003, 42 (16), 3711–3717.
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: DOI:10.1021/ef9005093
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Figure 3. Effect of reactor residence time on 0.1 M formic acid decomposition in supercritical water at 600 °C and 30 MPa.
Figure 4. Effect of feedstock concentration on 0.1 M formic acid decomposition in supercritical water at 600 °C, 30 MPa, and a 20s residence time.
decomposition involved many side-reactions besides the decarboxylation and dehydration. These side-reactions16 may be as follows. ð3Þ 2HCOOH f HCOH þ CO2 þ H2 O HCOH þ HCOOH f CH3 OH þ CO2
ð4Þ
HCOH þ H2 f CH3 OH
ð5Þ
HCOH þ H2 þ CO
ð6Þ
CH3 OH f 2H2 þ CO
ð7Þ
The reactions 3, 4, and 5 might be the main reactions, which led to a great increase of CO2 and decrease of H2. The reactions 6 and 7 might be minor. As a result, the CO GMF only had a slight increase. In addition, Figure 4 also showed the CGE decreased rapidly from 99.2 to 53.6%. This was due to an incomplete decomposition of high-concentration formic acid at the residence time of 20 s. These results reveal that the high feedstock concentration is both unfavorable to the formic acid decomposition and the hydrogen production. Studies of SCWG of biomass and its model compounds17-19 also showed the similar problem, that is,
Figure 2. Effect of temperature on 0.1 M formic acid decomposition.
from 54.0 to 34.2%. On the other hand, the GMF of CO2 and CO increased steadily up to 51.1 and 13.3%, respectively. It can be found that the GMF of all three products had little change in a concentration range of 0.05-0.1 M. The CO2 GMF began to exceed the H2 GMF when the concentration was about 0.45 M. This suggested that low-concentration formic acid solution (e0.1 M) decomposition mainly consisted of the decarboxylation and dehydration. For high-concentration formic acid solution (>0.1 M), the
(17) Yu, D.; Aihara, M.; Antal, M. J. Energy Fuels 1993, 7 (5), 574– 577. (18) Hao, X. H.; Guo, L. J.; Mao, X.; Zhang, X. M.; Chen, X. J. Int. J. Hydrogen Energy 2003, 28, 55–64. (19) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Jerry Antal, M., Jr Biomass Bioenergy 2005, 29 (4), 269–292.
(16) Osada, M.; Watanabe, M.; Sue, K.; Adschiri, T.; Kunio, A. J. Supercrit. Fluids 2004, 28 (2), 219–224.
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: DOI:10.1021/ef9005093
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high feedstock concentration results in a low yield of hydrogen. 4. Mechanisms of Formic Acid Decomposition Our experimental results for low-concentration formic acid solution (e0.1 M) decomposition showed that the yields of H2 and CO2 were both much higher than CO. This suggested that formic acid decomposition in supercritical water also consisted of the decarboxylation and dehydration, but the decarboxylation was the dominant reaction pathway. It is noteworthy that the decarboxylation reaction produces H2 and CO2 in equal yields, but in our experiment the yield of H2 was higher than the yield of CO2. It was caused by the dissolved CO2 in the liquid phase. To explore the mechanisms of formic acid decomposition in the gas phase and in supercritical water and to explain the change of the major reaction pathways, we conducted computational studies using the Gaussian 03 suite of programs. The optimized geometries of the reactants, intermediates, and transition states were calculated at the B3LYP/6-311þG(3df,2p) level. The vibrational frequencies were also calculated at this level for the characterization of zero-point energy (ZPE) corrections to the stationary points. The calculated geometries of the reactants, intermediates, and transition states for formic acid decomposition in the gas phase and in supercritical water are summarized in Figures 5 and 6, respectively. The energies of all relevant species are illustrated in Figure 7. 4.1. Unimolecular Decomposition of Formic Acid in The Gas Phase. The possible mechanisms for unimolecular decomposition of formic acid in the gas phase are as follows:
Figure 5. Optimized geometries of intermediates and transition states of the unimolecular decomposition of formic acid in the gas phase calculated at the B3LYP/6-311þG(3df,2p) level. Bond distances are in angstroms, and angles are in degrees.
Figure 6. Optimized geometries of intermediates and transition states of the decomposition of formic acid in supercritical water calculated at the B3LYP/6-311þG(3df,2p) level. Bond distances are in angstroms, and angles are in degrees.
t-HCOOH f TS1:1 f c-HCOOH f TS1:2 f H2 þ CO2 ðdecarboxylation pathwayÞ f TS1:3 f H2 O þ CO ðdehydration pathwayÞ As shown in Figure 6, two configurations of formic acid were identified: the t-HCOOH and c-HCOOH. The t-HCOOH isomerizes to the c-HCOOH by O-H bond rotation via TS1.1 with a low barrier height of 11.2 kcal/mol. From c-HCOOH, the decarboxylation takes place via the fourcentered-ring transition state TS1.2 to generate H2 and CO2. The barrier height of this step is 63.0 kcal/mol. The dehydration reaction takes place from the t-HCOOH and produces H2O and CO through a three-centered-ring transition state TS1.3. This step needs to overcome a high barrier height of 66.1 kcal/mol. 4.2. Decomposition of Formic Acid in Supercritical Water. The possible mechanisms for formic acid decomposition in supercritical water are as follows: HCOOH þ H2 O f IM2:1 f TS2:1 f IM2:2
Figure 7. Schematic energy diagram for the decomposition reaction of formic acid calculated at the B3LYP/6-311þG(3df,2p) level, where energy is given in kcal/mol.
overcome a barrier of 41.7 kcal/mol. IM2.3 involves the t-HCOOH and H2O with one hydrogen bond. The dehydration reaction takes place from the IM2.3 and produces CO þ 2H2O via a five-centered-ring transition state, TS2.3. The barrier height of this step is 45.5 kcal/mol. Our computational results show that water takes part in the formic acid decomposition reaction as a catalyst. This makes the barrier heights for decarboxylation and dehydration of formic acid decomposition in supercritical water are 24.2 and 21.9 kcal/mol lower than that in the gas phase, respectively (Figure 7). The decarboxylation and
f TS2:2 f H2 þ CO2 þ H2 O ðdecarboxylation pathwayÞ f IM2:3 f TS2:3 f CO þ 2H2 O ðdehydration pathwayÞ The decarboxylation reaction starts by the formation of IM2.1. In IM2.1, the t-HCOOH and H2O are bound with two weak hydrogen bonds. The water molecule acts simultaneously as a proton donor and acceptor. IM2.1 isomerizes to IM2.2 via TS2.1, passing over a 12.7 kcal/mol barrier. IM2.2 decomposes to H2 þ CO2 þ H2O via a floppy sixcentered-ring transition state, TS2.1. This step needs to 98
Energy Fuels 2010, 24, 95–99
: DOI:10.1021/ef9005093
Zhang et al.
low-concentration formic acid (e0.1 M) in supercritical water included decarboxylation and dehydration. Decarboxylation was the dominant reaction pathway, which is different from the unimolecular formic acid decomposition in the gas phase. To elucidate the mechanisms from a miro viewpoint, the computational studies were performed using the Gaussian 03 suite of programs. Computational results show that, both in the gas phase and in supercritical water, formic acid decomposition contains two reaction pathways, decarboxylation and dehydration. In the decomposition of formic acid in supercritical water, water takes part in the decomposition reaction as a catalyst, which promotes both decarboxylation and dehydration, but the promoting effect on decarboxylation is more apparent. The decomposition of high-concentration formic acid involves side-reactions besides decarboxylation and dehydration. Thus, additional experimental and computational researches for high-concentration formic acid are necessary.
dehydration are both promoted by water. The promoting effect on decarboxylation is more apparent. As a result, the decarboxylation reaction becomes dominant for formic acid decomposition in supercritical water. This is the reason that the main reaction pathways are different for formic acid decompositions in the gas phase and in supercritical water. 5. Conclusions Formic acid decomposition in supercritical water was studied to explore the effects of operation parameters and the mechanisms. The main gaseous products were found to be H2 and CO2. CO also appeared as a product, but its yield was small. Formic acid decomposition was greatly affected by temperature, and high temperature was good for H2 production. Compared with the temperature, the pressure had a small effect on the GMF and the CGE, especially at high temperature. As the reactor residence time increased from 16 to 20 s, the H2 and CO2 GMF increased steadily and the CGE increased rapidly. Extending the residence time had no great effect on the GMF and CGE. High concentration led to a great decrease of the H2 GMF. The decomposition of
Acknowledgment. This work was supported by the Funds for Major State Basic Research Projects of China under project No. 2009CB220007.
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