Article pubs.acs.org/EF
Catalytically Upgrading Bio-oil via Esterification Yichen Liu, Zhonglai Li, James J. Leahy, and Witold Kwapinski* Carbolea Research Group, Department of Chemical and Environmental Science, University of Limerick, Limerick, Ireland ABSTRACT: Sulfated ZrO2−TiO2 mixed oxides with different loading ratios were synthesized using a deposition precipitation approach. All catalysts were characterized by Brunauer−Emmett−Teller (BET), X-ray diffraction (XRD), infrared spectroscopy (IR), energy-dispersive X-ray (EDX) analysis, and transmission electron microscopy (TEM), and the activities for esterification were compared using acetic acid and ethanol as the model reaction. A total of 93.7% acetic acid was converted into ester when 50 wt % ZrO2 loading catalyst was applied at 100 °C. The characterizations of original and upgraded bio-oil via esterification by applying sulfated 50 wt % ZrO2−TiO2 loading ratio catalyst were investigated. After upgrading, fuel properties of pyrolysis oil improved. Acetic acid peak area percentage dropped remarkably from 15.87 to 0.774%, indicating that the properties of bio-oil were enhanced by SO42−/ZrO2−TiO2 catalytic esterification. Because of ethanol addition, the water content in bio-oil decreased from 40 to 16.9% and the high heating value increased from 8.08 to 22.7 kJ/g.
1. INTRODUCTION The fossil fuel shortage and serious environmental problems related to its consumption have led to a great interest in exploring a renewable and high-concentration energy source for transportation. The pyrolysis technique can convert lignocellulosic material by thermal decomposition into pyrolytic oil, combustible gases, and char. The liquid product of pryolysis is also called bio-oil, and it cannot be used directly as a fuel because of its physicochemical properties, such as low calorific value, high oxygen and water contents, low pH, high viscosity, and low stability. Extensive research is being conducted for improving the bio-oil properties, applying various upgrading techniques, such as hydrodexygenation,1−3 catalytic cracking of pyrolysis vapors,4,5 emulsification,6 steam reforming,7,8 and catalytic esterification.9−12 Most of the studies were undertaken on bio-oil from fast pyrolysis (heating rate of over 100 K/s), with a few studies on slow pyrolysis (heating rate of 5−20 K/s). However, the slow pyrolysis technique is more wildly applicable because of process and installation simplicity, and also, it is robust in handling different types and shapes of feedstock. In many installations, the vapors are directly combusted. The differences between bio-oil from the two types of pyrolyses are related to the rate of heat transfer into the biomass and vapor residence time in a reactor, which directly influences the quenched vapor (bio-oil) properties. Gozde et al.13 studied slow and fast pyrolyses of cherry seed; the fast-pyrolysis oil yield (44 wt %) was double that of slow pyrolysis (21 wt %) in most conditions. A long vapor residence time with a slow heating rate at a low temperature gives a higher char yield, while a high temperature preferably leads to gaseous production. Overall, short residence times and fast heating rates at moderate temperatures favor a high yield of bio-oil.14 Bio-oil is composed of more than 300 different compounds from simple low carbon chain length molecules to heavy molecular weight structures, which could be above 300 g/mol, some of which are reactive and, therefore, instable. Organic acids in bio-oil were, in particular, the cause of the low pH and instability, which are the main reasons that they prohibit its application. Catalytic esterification is a method focusing on © 2015 American Chemical Society
conversion of carboxylic acids into esters. Esters are more desired than acids in fuel and are less corrosive for the engine surface. Numerous solid catalysts have been applied in esterification for bio-oil upgrading. Song et al.12 carried out esterification on fast pyrolysis oil and ethanol with sulfated (SO42−) SiO2−TiO2. A higher heating value (HHV) of upgraded oil increased from 13.5 to 24.8 MJ/kg, and the percentage of organic acid identified by gas chromatography/ mass spectrometry (GC/MS) dropped from 11.5 to 3.23%. Xun et al.15 divided fast-pyrolysis bio-oil into water- and pastelike phases using a centrifuge and then carried out esterification between them on a proportional mixture of two phases with methanol and commercial Amberlyst-70 in an autoclave. They reported that the acetic acid (HAc) in the water and paste phases reduced from 5.4 to 1.1 wt % and from 3.5 to 0.38 wt %, respectively. Because of the bio-oil complexity, the number of studies is conducted on a mixture of only a few common constituents, mainly acetic acid16 and various phenols.9 Xiang et al.17 studied simultaneous catalytic esterification and acetalization using model compounds based on fast-pyrolysis bio-oil composition with methanol and commercial Amberlyst-70 in an autoclave and achieved a HAc conversion of 85% and a formaldehyde conversion of 90%, respectively. Mineral acids, such as sulfuric acid, hydrochloride acid, and phosphoric acid, have been usually used as a catalyst for esterification. However, corrosiveness and the inconvenience of separation from products have resulted in the need to develop a solid acid catalyst. TiO2 is suitable for the practical application as a metal oxide catalyst support because of the low cost, safety, and chemical stability, with only a minor catalytic effect. Metal oxides, such as TiO2-, ZrO2-, and Fe2O3-containing mixed oxides, become highly acidic on modification with anions, such as SO42−, PO43−, etc.,18 and these solid acid catalysts have been employed in several industrially important acid-catalyzed reactions at a low temperature, which is also the important Received: January 22, 2015 Revised: March 30, 2015 Published: May 5, 2015 3691
DOI: 10.1021/acs.energyfuels.5b00163 Energy Fuels 2015, 29, 3691−3698
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Energy & Fuels
500 °C. Catalysts with six different ZrO2/TiO2 loadings were prepared, which were 10, 20, 30, 50, 60, and 80 by mass ratio. Pure SO42−/ZrO2 and SO42−/TiO2 were prepared by the same method. 2.3. Catalyst Characterization. X-ray diffraction (XRD) patterns of the catalysts were obtained using Cu Kα radiation in a Philip X’Pert diffractometer to identify the different phases and the crystal structure. The scanning speed was 0.025°/min, and the range was between 5° and 80°. Infrared spectroscopy (IR) was performed from 4000 to 400 cm−1 using a Bomem spectrometer. The surface area was calculated with the Brunauer−Emmett−Teller (BET) equation using the Micromeritics Gemini surface analyzer. The concentrations of acid sites were measured by exchange in NaCl solution, followed by titration with NaOH solution. In a typical test, 0.4 g of catalyst was added to about 100 cm3 of 0.1 mol/dm3 NaCl aqueous solution. The solution was kept stirring to equilibrate for 24 h. The resulting white suspension was then titrated by the dropwise addition of 0.009 mol/dm3 NaOH solution. Transmission electron microscopy (TEM) was conducted at 200 kV using a JEOL-2010 equipped with Energy-dispersive X-ray (EDX) analysis. Samples for TEM analysis were prepared by drying sample material−isopropanol ink on an amorphous carbon-coated copper grid. 2.4. Model Reaction Procedure. In a typical model reaction, HAc and ethanol were mixed using a specific mass ratio of 1:1, 1:2, and 1:5 in a three-neck flask attached with a reflux condenser (−15 °C). The magnetic stirrer was set at 300 rpm. The catalyst was applied at a 1 wt % (2 or 5 wt %) of the reaction solution. The whole system was heated using a silicon−oil bath. The time started when the system reached the setup temperature. Samples were taken from the flask every 15 min from initial to 1 h and then every 60 min from 1 to 6 h and titrated using a NaOH-standardized solution. The equation for HAc conversion is
condition for bio-oil upgrading. More active and stable catalysts can be obtained by incorporating transition metals, especially noble metals, to the ZrO2 system.19 The SO42−/ZrO2−TiO2 solid is called superacid in the literature.20 It indicated that the incorporation of Zr and additional elements into TiO2 substantially enhanced catalytic performances by improving its water tolerance, which is very important for esterification. It also increases the presence of the sulfate group at the surface and decreases its crystallinity after calcination by retarding its crystallization from amorphous TiO2 to anatase TiO2.20 The modification is also responsible for increasing the catalystspecific surface area as well as its acidity, including the concentration and acid strength of the surface acidic sites of it.20,21 The esterification of linear alcohols with some aromatic acids has been carried out over PO43−/TiO2−ZrO2 as a catalyst, and it was less reactive and selective for esterification; however, when various amounts of PO43− were loaded on it, the catalyst showed very good activity and selectivity for this type of reaction without the removal of water.18 Hence, an increasing interest on the application of SO42−/ZrO2−TiO2 has been generated in reactions.22,23 Mao et al.24 tested the crystal structures and surface areas of pure TiO2, ZrO2, and ZrO2− TiO2 mixed oxides with various compositions prepared by the co-precipitation method. The mixed oxides exhibited much higher surface areas than the single oxide components, and the maximum surface area was attained at equimolar Ti/Zr composition. As a result, a catalyst with a high surface area and more functional groups for different purposes was obtained. Although it is known that TiO2−ZrO2 mixed oxides are very active for a variety of reactions, such as the esterification of lactic acid,22 the coupling of hydrolysis and dehydration reactions to produce hydroxymethylfurfural (HMF) and furfural forms of biomass,25 and the upgrading of biomass fast-pyrolysis vapors,26 the study of SO42−/ZrO2− TiO2 catalytic activity for upgrading of bio oil via esterification has not yet been performed. In this study, the focus is on slow-pyrolysis bio-oil upgrading by catalytic esterification. A pure SO42−/TiO2, SO42−/ZrO2 and mixed oxide SO42−/ZrO2−TiO2 with different Zr/Ti ratios have been characterized and tested. A catalytic model reaction was carried out to discover the most effective catalyst and the optimal conditions. On the basis of the optimum experimental conditions, the most effective catalyst was applied for slow pyrolysis bio-oil upgrading.
conversion = [1 − (M /V )/(Mo/Vo)] × 100%
(1)
where M and Mo are the weights of the sample analyzed every 1 h and entire HAc before the reaction, respectively, V and Vo are the volumes of NaOH standard solution consumed by the sample and entire HAc before the reaction, respectively. 2.5. Upgrading of Bio-oil. The raw bio-oil was upgraded by catalytic esterification. In a typical reaction, the mixture of a raw bio-oil and ethanol mass ratio of 2:1 was carried out in the same equipment as the model reaction. The catalyst was added as 2 wt % of the total mixture. Several properties of bio-oil before and after were tested. The HHV was measured using an oxygen bomb calorimeter (6200 number 442 m, Parr). The water content was determined by the Karl Fisher titration Mettler Toledo DL31 (Mason Technology). The pH was measured using a pH meter (Eutech Instruments). GC/MS (GC, Agilent 7890A; MS, Agilent 5975C) was used to analyze the composition of the bio-oil. The chromatographic separation was carried out using a HP-5MS capillary column (30 m × 0.25 mm, 0.25 μm film thickness). Helium [chemically pure (CP) grade] was used as the carrier gas with a constant flow rate of 4 cm3/min in spiltless mode. The column temperature was programmed from 30 °C (held for 10 min) to 280 °C (held for 10 min) at a heating rate of 8 °C/min. The mass analyzer was set up to scan from m/z 60 to 400 to ignore the solvent peak. Identification of chromatographic peaks was determined according to the National Institute of Standards and Technology (NIST) MS library, and identification was made starting from the top mass of each peak.
2. EXPERIMENTAL SECTION 2.1. Production of Bio-oil. Miscanthus pellets were pyrolyzed in a fixed bed using a quartz reactor (25 mm inner diameter), which was surrounded by heating tape (Omegalux STH102). First, the heating tape reached 600 °C; meanwhile, the reactor was continuously purged with nitrogen flow. Then, 100 g of Miscanthus, which had been dried in an oven (70 °C) for 24 h, was placed in the hot reactor. The pyrolysis vapor was quenched in a glycol-cooled condenser (−15 °C) and collected. The bio-oil was composed as two phases, an upper, aqueous light yellow phase and a lower, oil dark brown phase. Ethanol was added in a ratio of 2:1 in relation to bio-oil and was well-mixed to ensure that it was homogeneous. 2.2. Catalyst Preparation. All catalysts were prepared by deposition precipitation. Stoichiometric quantities of ZrOCl 2 (≥99.5%, Sigma-Aldrich) and TiO2 (powder, −325 mesh, anatase, Sigma-Aldrich) were added to distilled water. Aqueous ammonia solution was added dropwise to the slurry with constant stirring to adjust to pH 10. The precipitate was filtered and dried for 24 h, followed by suspension in H2SO4 for 4 h. The resulting white solid was dried overnight in an oven at 80 °C and calcined in a furnace for 5 h at
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Table 1 shows the surface area of the different catalysts. The surface areas of pure TiO2 and ZrO2 were 14.4 and 97.3 m2/g, respectively. It increased with the addition of ZrO2 into TiO2 and reached a maximum at 60% ZrO2 loading (49:50) in molar ratio; this was also ́ confirmed in results by Mao et al.24 and Manriquez et al.27. The 3692
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Energy & Fuels Table 1. Specific Surface Area and Acidity of Catalysts
pure TiO2 10% ZrO2 20% ZrO2 30% ZrO2 50% ZrO2 60% ZrO2 80% ZrO2 pure ZrO2
surface area (m2/g)
surface acidity (μmol/g)
surface acidity (μmol/m2)
pore volume (cm3/g)
pore diameter (nm)
14.4 31.4 48.1 79.2 98.6 102.5 82.2 97.3
58.2 141 223 378 488 493 401 469
4.04 4.49 4.64 4.77 4.95 4.81 4.88 4.82
0.112 0.254 0.256 0.234 0.257 0.262 0.251 0.255
8.3 7.1 5.1 5.3 4.5 4.9 5.2 6.2
surface acidity closely increased with the surface area. All of the SO42−/TiO2−ZrO2 catalyst had a much higher surface acidity than pure SO42−/TiO2. With the introduction of ZrO2, the catalyst surface area increased and, thus, accepted more SO42− as active sites, and also, the average surface acidity per square meter was raised. The surface acidity/m2 of pure TiO2 is 4.04 μmol/m2, and pure ZrO2 is 4.82 μmol/m2. The surface areas of 50, 60, and 80 wt % and pure ZrO2 were close to each other and were similar in surface acidity. The pore diameter decreased with the increase of ZrO2 loading; however, the pore volume of the ZrO2−TiO2 catalyst and pure ZrO2 catalyst were all higher than pure TiO2. As a result, 50 wt % ZrO2 loading was considered as the optimal catalyst with respect to price (high cost of Zr) and very close to the best surface properties from the investigated catalysts. The XRD patterns of the catalysts are shown in Figure 1. The peaks at 25.5°, 37.1°, 38.7°, 39.9°, 54.0°, and 55.3° are the
Figure 2. IR spectrum of catalysts: (a) pure TiO2, (b−g) 10−80 wt % ZrO2 loading catalysts, and (h) pure ZrO2.
vibration.31 The bands at 1044 and 990 cm−1 are assigned to the asymmetric and symmetric S−O stretching vibrations, respectively.32 The band around 1141.7 cm−1 is attributed to the symmetric stretching of the SO bond, and the band around 1232.5 cm−1 is assigned to the asymmetric stretching of the SO bond.33 The 3400 cm−1 band is associated with the hydroxyl group stretching mode of water associated with ZrO2/ TiO2, and the peak at 1630 cm−1 band is attributed to the combined symmetric and asymmetric stretching modes of molecular water associated with a sulfate group.34 The bands assigned to SO are observed for all catalysts, except pure TiO2. This is because the sulfate group content of pure TiO2 is much lower than that of others, which was confirmed by the surface acidity test. TEM can provide high-resolution information regarding the surface morphologies and particle sizes of the catalysts. Morphologies of pure TiO2 and ZrO2−TiO2 are shown in Figure 3. The length of the TiO2 ranges from 42 to 262 nm. The 50 wt % ZrO2 loading catalyst shows a significantly greater number of ZrO2 compared to the 20 wt % loading. An increase in the ZrO2 particle size was also discovered with increased loading. The corresponding EDX analysis of 60 wt % ZrO2 loading catalyst also implies the presence of Ti and Zr elements. The crystalline nature of ZrO2−TiO2 was also investigated by high-resolution TEM (HRTEM). The HRTEM image shows the surface of a catalyst with the lattice spacing of 0.35 nm being assigned to the (101) plane of the anatase phase [Joint Committee on Powder Diffraction Standards (JCPDS) 841286]. 3.2. Model Reaction Analysis and Kinetic Study. Esterification between HAc and ethanol was chosen as the model reaction to select the preliminary conditions and the best catalyst for bio-oil upgrading. The overall mass balance of the model reaction is over 95%. GC/MS also confirmed that ethyl acetate is the only product after the esterification, which indicated that all of the side reactions are negligible. 3.2.1. Effect of Different Catalysts. Figure 4 shows the conversion of HAc under different circumstances. Pure ZrO2 and 60 wt % ZrO2 catalysts showed the highest activity, with conversions of 83 and 84 wt % at 80 °C after 6 h, respectively. The 50 wt % ZrO2 catalyst is only slightly lower than those two. The overall catalyst effectiveness order is pure ZrO2 = 60 wt % ZrO2 > 50 wt % ZrO2 > 80 wt % ZrO2 > 30 wt % ZrO2 > 20 wt % ZrO2 > 10 wt % ZrO2 > pure TiO2. As we expected, this is
Figure 1. XRD patterns of catalysts: (a) pure TiO2, (b−g) 10−80 wt % ZrO2 loading catalysts, and (h) pure ZrO2.
diffractions of anatase TiO2, which appear in all of the catalysts, and the peaks at 28.6° and 31.8° are assigned to the ZrO2,28,29 which appear in the 30−80 wt % loading and pure ZrO2 catalyst. The 10 and 20 wt % catalysts only exhibit the TiO2 crystalline phase, and the formation of ZrO2 was observed when ZrO2 loading was raised to 30%. The reason for the absence for 10 and 20 wt % ZrO2 loading catalysts at the XRD spectra is the low ZrO2 loading and formation of the amorphous zirconia layer. The peak of ZrTiO4 was not found in the XRD patterns, because it only starts to form when the temperature is higher than 500 °C.30 The IR spectrum of SO42−/ZrO2−TiO2 is shown in Figure 2. The band around 585 cm−1 is attributed to the Ti−O bending 3693
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most common two different structures are bridge and chelate. The weakened O−H bonds, which were affected by neighboring SO42−, were Brønsted acid sites. The electronically deficient M4+ (Zr or Ti) was acting as a Lewis acid because of the electron-withdrawing nature. The acid is the key in the esterification reaction (Figure 5). Moreover, it is interesting that some Zr−O−Ti linkages were expected to be present in ZrO2 and TiO2 binary oxides.37 TEM showed us that TiO2 was surrounded by ZrO2 and the Zr−O−Ti linkages was expected on the surface of TiO2, thus enhancing the catalyst activity. The sulfate ion amount coordinates to the surface of the TiO2− ZrO2 binary oxide. This was confirmed by the BET surface area and catalyst surface total acidity. With the introduction of ZrO2, the catalyst surface area increased, thus accepting more SO42− as active sites. We can assume that the quantity of both Lewis acid sites and Brønsted acid sites will arise as a result. Therefore, a catalyst with higher activity was obtained. The strong acid sites of SO42−/ZrO2−TiO2 supply protons as a Brønsted acid, accelerating the initiation of esterification. The mechanism of the acidic activity is shown in Figure 5. The conversions of 50 and 60 wt % ZrO2 loading and pure ZrO2 catalysts are very similar. Considering the higher cost of zirconium, we decided to choose the 50 wt % ZrO2 loading catalyst as the optimal catalyst and applied it in the further study. 3.2.2. Effect of Catalyst Loading. It was found that the increase in catalyst loading resulted in an increase in fractional conversion because of the increase in the number of active sites. This result, presnted in Figure 6, indicated a large difference between 1 wt % loading and the others; however, the HAc converisons with 2 and 5 wt % catalyst loading are close to each other, especially after a longer time. This may be due to the fact that the mass transfer is high, and the additional amount of catalyst cannot influence the process so greatly any more. 3.2.3. Effect of Initial Mass Ratios of Ethanol/HAc. The catalytic esterifcation is a reversible reaction; thus, the use of an excess of one reagant will encourage the equilibium to the forward direction. Figure 7 shows that the highest yield was obtained with the initial mass ratio of 5:1 ethanol/HAc, followed by 2:1 and 1:1. This result is in agreement with the work of Calver et al.38 and Ismail et al.39 Nada et al.40 carried out the esterification with ethanol/HAc mass ratios of 10:1, 30:1, and 50:1. They reported that the highest conversion was received when the smallest amount of ethanol was used (10:1). This may due to the over excess of ethanol weakening the acidic environment; however, the esterification is catalyzed by an acid. 3.2.4. Temperature Effect. Figure 8 shows the testing of a 50% ZrO2 loading catalyst under different temperature conditions. The 100 °C was the optimal temperature for esterification because over 90% of the acetic acid was converted to ester after 6 h. The initial reaction rate constant is directly temperature-dependent and increases with the temperature. The HAc conversion for 120 °C up to 2 h was the highest, and it shortly reached equilibrium. However, the conversion for 120 °C after 6 h was even lower than 80 °C. This is attributed to the fact that esterification is an exothermic reaction. Higher temperatures would raise the rate of reaction, and the conversion at 120 °C before 3 h would be lower than 100 °C for a longer time. Equilibrium was pushed backward when the temperature reached 120 °C. This result indicates that the optimal temperature was 100 °C. Song et al.41 reported a similar result of the temperature effect.
Figure 3. TEM image of (a) pure TiO2, (b) pure ZrO2, (c) 20 wt % ZrO2 loading catalyst, and (d) 50 wt % ZrO2 loading catalyst. (e) HRTEM of pure TiO2. (f) EDX analysis of 50 wt % ZrO2 loading catalyst.
Figure 4. HAc conversion at 80 °C with every catalyst [catalyst loading of 2 wt %, with the initial reagent rate in 5:1 (mol/mol) ethanol/HAc].
because of the higher surface area of the catalyst, which would supply more acid sites and then enhance the catalyst activity. The incorporation of ZrO2 to TiO2 increases the catalyst surface area. Li et al.35 concluded that the catalysis by cooperation of acidic sites with basic sites is surprisingly powerful and both TiO2 and ZrO2 are crystalline solids and have small amounts of weak acid and basic sites and a large amount of new stronger acid/base sites. In sulfated metal oxides, several different structures have been assumed. Ward and Ko36 suggested that the structure contains both Brønsted acid sites and Lewis acid sites. The 3694
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Figure 5. Mechanism of acidic catalytic esterification.
Figure 8. HAc conversion at different temperatures [50 wt % ZrO2 catalyst loading 2 wt %, with the initial reagent rate in 5:1 (mol/mol) ethanol/HAc].
Figure 6. HAc conversion at 80 °C with different catalyst loadings [50 wt % ZrO2 catalyst, with the initial reagent rate in 5:1 (mol/mol) ethanol/HAc].
reaction system is assumed as an ideal solution. (4) The internal and external diffusion limitations of the solid acid catalyst are excluded. Under these conditions, the reactions were considered to be second-order in the forward and reverse directions. The overall rate of reaction follows second-order kinetics. −
d[CA ] = k1[CA ][C B] − k 2[C W ][C E] dt
(3)
Because the initial concentration of reactants were much higher than that of the other components in the experimental conditions, reaction rate k1 is far larger than k2. Equation 3 can be simplified in −
Figure 7. HAc conversion at 80 °C with different initial reagant mass ratios (50 wt % ZrO2 catalyst loading 2 wt %).
d[CA ] = k1[CA ][C B] dt
(4)
The molar ratio of M = [CB]/[CA] ≠ 1 was 5 in these experiments to favor the forward reaction. Equation 4 can be integrated to give eq 5 in terms of fractional conversion of acetic acid (XA).44
3.2.5. Kinetic Analysis. The esterification reaction of acetic acid (A) with ethanol (B) to produce ethyl acetate (E) and water (W) in the presence of a solid catalyst is given as follows:
ln 42,43
The kinetic model was built on the following assumptions: (1) The rate of non-catalyzed reactions is negligible compared to that of the catalyzed reations. (2) The catalytic activity of all sites on the solid catalyst surface is the same. (3) The whole
M − XA = CA0(M − 1)k1t M(1 − XA )
(5)
The plots were made of the left-hand side versus time for eq 5, and the values of k1 were calculated by the slope of the line. To consider the effect of the reaction temperature on the kinetic model, the Arrhenius equation (eq 6) was applied 3695
DOI: 10.1021/acs.energyfuels.5b00163 Energy Fuels 2015, 29, 3691−3698
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Energy & Fuels Table 2. Kinetic Parameters of the Esterification Reactions reaction temperature (°C)
k1 real (cm3 mol−1 s−1)
k1 ideal (cm3 mol−1 s−1)
Ea real (kJ/mol)
Ea ideal (kJ/mol)
Areal (s−1)
Aideal (s−1)
60 80 100 120
0.0093 0.0116 0.0156 0.0189
0.0103 0.0126 0.0166 0.0200
13.1
12.4
1.281
0.8807
Table 3. Properties of Bio-oil and Upgraded Bio-oil bio-oil bio-oil mixed with ethanol upgraded bio-oil
−ln k1 =
Ea − ln A RT
pH
water content (wt %)
N (wt %)
C (wt %)
H (wt %)
HHV (kJ/g)
2.5 4.5 5.3
40.1 16.7 16.8
0.92 0.8 0.73
34.8 47.9 48.7
15.8 16.0 16.1
8.08 22.4 22.7
converts more organic acids to esters, and the pH increased to 5.3. In addition, adding ethanol can improve the mobility of bio-oil, thus enhancing the property of transportation. Those results demonstrate that the entire process of esterification not only changed the elemental composition and HHV by adding ethanol but also generated the reactions between ethanol and components in bio-oil. These two steps together improve the fuel properties of the bio-oil. Organic acids in the bio-oil promote various reactions during storage and are also responsible for corrosiveness because of the low pH level of 2.3−3, as reported for bio-oil.11−13 HAc is one of the most abundant compounds containing the carboxylic group. In this work, efforts were made to convert HAc to ethyl acetate. The GC/MS spectra of the original bio-oil and upgraded bio-oil are shown in Figure 9. The chemical
(6)
where EA is the activation energy (J mol−1), A is the preexponential factor (s−1), T is the temperature (K), and R is the gas constant (J mol−1 K−1). Arrhenius law plots of ln k1 against 1/T give a straight line with a slop of Ea and a y intercept of A. For the non-ideal system, the slope will equal to [CA0][M − 1]k1 idealγAγB. As a result
k1 ideal =
k1 γAγB
(7)
where γA and γB are the activity coefficients for HAc and ethanol, respectively. The real initial reaction rate constant (from 15 to 60 min) was calculated on the basis of eq 5, which was calculated on the basis of UNIQUAC functional-group activity coefficients (UNIFAC)45 to account for the non-ideal thermodynamic behavior of reactants and products. Then, the ideal initial reaction rate constant was calculated on the basis of eq 7. The results are presented in Table 2 for the 50 wt % ZrO2 catalyst loading 2 wt % and initial reagent rate in 5:1 (mol/ mol) ethanol/HAc. Activation energies for the above reaction reported recently37,46 are over 28 kJ/mol for various catalysts. The values of the activation energy obtained from our experiments are relatively low and confirm that the applied catalyst was effective and with small mass transport limitation inside the porous material. 3.3. Bio-oil Upgrading. Bio-oil is a thermally unstable, complex mixture of organic acids, aldehydes, ketones, phenols, esters, and alcohols, which give rise to other unexpected reactions and coke forms in high temperatures, such as polymerization.47 As a result, 80 °C was chosen rather than 100 °C, which was determined to be more effective for the model reaction. The HHV of original bio-oil mixed with ethanol increases (Table 3). This is due to the higher carbon content (52.2%) and hydrogen content (13.1%) of ethanol. Moreover, the water was diluted to 16.7% by simply adding ethanol, with a tiny increase to 16.8% after catalytic esterification. The HHV of the mixture and upgraded bio-oil is 22.4 and 22.7 kJ/g, respectively. No significant change of the HHV takes place after esterification, which demonstrated that the HHV increased after adding ethanol in a physical way. The pH value is another crucial indicator for bio-oil. It is a measure of corrosiveness for metal reactors. The pH value before adding ethanol was 2.5, and after adding a double amount of ethanol by weight, it was 4.5. The upgrading by catalytic esterification
Figure 9. GC/MS spectra of bio-oil before and after esterfication.
compositions are shown in Table 4. A change of bio-oil composition after catalytic esterification can be observed. The peak area percentage at a retention time of 7.013 min, corresponding to HAc, fell greatly from 15.9 to 0.8%. In addition, the total peak area percentage of the carboxylic acid group decreased from 27.6 to 6.1%. On the contrary, ethyl acetate was formed as 16.2% of the peak area percentage in the upgraded bio-oil. This indicates that the carboxylic groups in the original bio-oil have been practically eliminated. This is due to the high catalytic activity of 50% SO42−/ZrO2−TiO2. The other components in the bio-oil also changed after catalytic esterification. Ketones reduced sharply from 14.9 to 3696
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Energy & Fuels Table 4. Bio-oil Composition before and after Catalytic Esterification acids (% area)
alcohols and esters (% area)
ketones (% area)
phenols (% area)
alkanes (% area)
furans and aldehydes (% area)
27.6 6.1
6.5 18.4
14.9 5.9
22.3 33.6
1.9 6.9
11.4 16.7
bio-oil upgraded bio-oil
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5.9%, and this is due to unsaturated CO bonds being converted into saturated compounds, thus increasing the stability of the bio-oil fraction.48 The total amount of alkanes and furans and aldehydes, especially phenols, showed an increasing tendency. As shown in Table 4, the amount of phenolic compounds increased from 22.3 to 33.6%, which is required because phenol and hydrocarbons have potential as platform chemicals as well as being useful fuel components. Moreover, only one hydroxyl group that conjugated with the benzene ring makes the structure stable. Furthermore, hydroxyl and methoxyl groups would not be activated in the presence of esterification. However, both the presence of the CC bonds and the carbonyl group in 2-methoxy-4-vinylphenol would allow for these compounds to be involved in the acid-catalyzed reaction, such as hydration.15 As a result, most of the phenolic compounds in upgraded bio-oil are derivatives of phenol, 2,6dimethoxy-. These results illustrate that the distribution of the bio-oil would change during esterification and is very difficult to predict and quantify.
4. CONCLUSION The main objective of the study was to prove that two-phase bio-oil from slow pyrolysis can be successfully upgraded by catalytic esterification using a sulfated Zr/Ti catalyst. The incorporation of ZrO2 to TiO2 increases the catalyst surface area, thus accepting more SO42− as active sites, and the average surface acidity per square meter was raised. The higher surface area of the catalyst, which would supply more acid sites, enhances the catalyst activity. The acetic acid and ethanol model reactions with a 50% SO42−/ZrO2−TiO2 catalyst were very close to the highest catalytic activity among four different ZrO2/TiO2 ratio catalysts, with conversions of 83 wt % at 80 °C after 6 h. The experiments conducted with slow-pyrolysis bio-oil obtained from Miscanthus showed that the two-phase mixture can be successfully upgraded to a single phase, and both the analysis of composition from GC/MS and the pH value illustrated that organic acids have been significantly reduced. The ethanol addition was the main reason for water content reduction in upgraded bio-oil, and the HHV rose to reach 22.7 kJ/g, which is nearly 3 times that of the original biooil.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +353-0-61234935. E-mail: witold.kwapinski@ul. ie. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Yichen Liu acknowledges the postgraduate grant received from the Chinese Scholarship Council. REFERENCES
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DOI: 10.1021/acs.energyfuels.5b00163 Energy Fuels 2015, 29, 3691−3698
