Article pubs.acs.org/EF
Upgrading Bio-oil: Simultaneous Catalytic Esterification of Acetic Acid and Alkylation of Acetaldehyde Jun Ye, Chunjian Liu, Yan Fu, Shuai Peng, and Jie Chang* Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China ABSTRACT: This paper reports a simultaneous catalytic esterification of acetic acid and alkylation of acetaldehyde using a silica sulfuric acid catalyst from 40 to 140 °C. The results show that the esterification of acetic acid and acetalization of acetaldehyde with 1-butanol took place simultaneously, and both reactions were strongly affected by the temperature and catalyst dosage. The esterification of acetic acid was restrained by acetalization of acetaldehyde at a relatively low temperature (e.g., 90 °C). The increase of the reaction temperature, however, would result in the decomposition of acetals (1,1-dibutoxyethane). To solve these problems, 2-methyl furan, which can react with acetaldehyde, was introduced. Further work shows that the decomposition of 1,1dibutoxyethane was successfully resolved by the alkylation reaction of acetaldehyde, and yields of the ester (butyl acetate) decreased with the increase of the water content. The maximum yields of alkylation products [2,2′-ethylidenebis(5-methylfuran)] and butyl acetate were 84.5 and 74.4%, respectively.
1. INTRODUCTION Derived from fast pyrolysis of biomass, bio-oil has been recognized as a renewable feedstock for the production of transport fuels.1−4 However, the large-scale production of liquid fuels from bio-oil was limited because of its high acidity and thermal instability. Furthermore, bio-oil is a highly oxidized, compositionally complex mixture and contains almost all kinds of the oxygen-containing organics (e.g., acids, aldehydes, alcohols, phenols, sugars, and others with multiple functional groups).5 These oxygen-containing organics make bio-oil unstable, corrosive, and incompatible with conventional fuel and directly affect its commercial application.6−9 Therefore, refinement of bio-oil is required before it can be used as a substitute for fossil fuels. The oxygen-containing organic compounds, especially acids and aldehydes, are considered to be the major compounds contributing to the instability of bio-oil.10,11 To improve the stability of bio-oil, simultaneous esterification and acetalization were introduced to convert the reactive organic acids and aldehydes in bio-oil into esters and acetals.12,13 Alternatively, a number of studies have focused on simultaneous esterification and acetalization to upgrade crude bio-oil or model compounds with alcohols (e.g., methanol, ethanol, and 1-butanol).14−16 However, simultaneous esterification of acids and acetalization of aldehydes in bio-oil has revealed that some acetals would decompose at high operating temperature and high dosage of catalyst.6 To overcome this problem, an acid-catalytic alkylation reaction was introduced to replace the acetalization reaction of aldehydes. In a study on the acid-catalytic alkylation reaction for the production of diesel and jet-fuel precursors, 2methylfuran (2-MF) was reacted with furfural, acetone, 1butanal, hydroxymethylfurfural (HMF), and 5-methylfurfural.17 2-MF was obtained from selective hydrogenation of furfural and had an advantage over furan because one of the two reactive α positions was protected by an unreactive methyl group, which could reduce side reactions.18 © 2014 American Chemical Society
This study aims to increase the yield of desired products by altering and optimizing the chemical conversion pathway, in particular, shifting the acetalization pathway to alkylation to limit the decomposition of the acetal under conditions that simultaneously maximize ester production. Acetic acid and acetaldehyde were selected as model compounds of bio-oil. 2MF was applied to react with acetaldehyde to solve the decomposition of acetals. 1-Butanol was selected to react with acetic acid to produce stable esters. There are several advantages in the application of 1-butanol and 2-MF in simultaneous catalytic esterification of acetic acid and alkylation of acetaldehyde: (1) 1-butanol and 2-MF can react with acetic acid and acetaldehyde, respectively, to produce stable fuel precursors; (2) 1-butanol and 2-MF can be obtained by the fermentation of biomass and selective hydrogenation of furfural, respectively, and both of them can be directly used as gasoline additives; and (3) 1-butanol and 2-MF can improve the boiling point and carbon chain lengths of the oil product, producing an oil product with properties there are more similar to those of gasoline (boiling point, 30−205 °C; carbon chain lengths, C4− C12).18 Silica sulfuric acid (SSA) was an effective solid acid catalyst that exhibited great water tolerance and catalytic activity in many kinds of reactions (e.g., esterification and alkylation).19 In this work, SSA was selected as the catalyst in esterification, acetalization, simultaneous esterification and acetalization, and simultaneous esterification and alkylation.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Acetic acid (>99.8%), acetaldehyde (>99.5%), 2-MF (>98%), 1-butanol (>98%), CH2C12 (>99%), and silica gel (10 nm pore diameter, 380 m2/g) were purchased from Special Issue: International Biorefinery Conference Received: January 15, 2014 Revised: June 9, 2014 Published: June 10, 2014 4267
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Article
Figure 1. Reaction mechanism of esterification, acetalization, and alkylation. Aladdin Industrial Corporation. Chlorosulfonic acid (>99%) was purchased from Xiya Reagent. 2.2. Preparation and Characterization of SSA. Silica gel was dried at 120 °C for 3 h in air and further reacted with chlorosulfonic acid to produce SSA following the procedure of previous reports.19 The mass ratio of chlorosulfonic acid and silica gel was 1:0.962. SSA was identified by Fourier transform infrared spectroscopy (FTIR) analysis (Nexus 670, Nicolet), thermogravimetric (TG) analysis, derivative thermogravimetric (DTG) (TGAQ 5000) analysis, Brunauer−Emmett−Teller (BET) surface area analysis (ASAP 2020 V3.03 H, Micromeritics), and X-ray photoelectron spectroscopy (XPS) analysis (Kratos AXis Ultra). The hydrogen ion exchange capacity was estimated from the exchange of Na+ in aqueous NaCl solution.20 2.3. Reaction Procedure and Product Analysis. All of the experiments were performed in a 25 mL 316L autoclave reactor (Beijing Senlong Century Experimental Equipment Co., Ltd.) with a magnetic agitator. In a typical experiment, approximately 20 mL of the reaction mixture was loaded into the reactor at room temperature. The reactor was purged with nitrogen 4 times to displace the inside air. After that, the mixture heated to and then kept at the set temperature with a stirring rate of 700 rpm. Each reaction was maintained for 2 h and performed at a temperature from 40 to 140 °C. After the reaction, the composition of the oil products was identified by gas chromatography−mass spectrometry (GC−MS, Shimadu, QP 2010 Plus) analysis. Further, the yields of the main products were measured by the internal standard method using parachlorophenol as the internal standard substance. The reaction mechanism of esterification, acetalization, and alkylation is shown in Figure 1. The yields of butyl acetate, 1,1-dibutoxyethane, and 2,2′-ethylidenebis(5-methylfuran) were calculated by the following equations. The symbol “n” in these equations represents the mole content of substance of the corresponding substance. The “nacetic acid (initial)”, “nacetaldehyde (initial)”, and “n1‑butanol (initial)” represent the mole content of substance of the initial acetic acid (0.05 mol), acetaldehyde (0.025 mol), and 1-butanol (0.10 mol), respectively. nbutyl acetate Ybutyl acetate = × 100% nacetic acid (initial) (1)
Y1,1‐dibutoxyethane =
n1‐butanol (initial) − (n1‐butanol + nbutyl acetate) 2nacetaldehyde (initial) (2)
× 100% Y2,2′‐ethylidenebis(5‐methylfuran) =
n2‐methylfuran 2
× 100%
(3)
3. RESULTS AND DISCUSSION 3.1. Characterization of SSA. The FTIR spectra, TG/ DTG spectra, and N2 adsorption−desorption isotherms of silica gel and SSA are shown in Figure 2. From the FTIR spectra (Figure 2a), the peak of the Si−O rocking vibrations appears at approximately 475 cm−1. The band at 810 cm−1 is assigned to symmetric vibrations (Si−O−Si), and the band at approximately 1110 cm−1 is assigned to the asymmetric vibrations of (Si−O−Si) of silica gel. The band of the S−O vibrations appears at 600 cm−1. The bands at 1180 and 1280 cm−1 are
Figure 2. (a) FTIR spectra, (b) TG/DTG analysis, and (c) N2 adsorption−desorption isotherms, of silica gel and SSA. 4268
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assigned to symmetric and asymmetric stretching of SO, respectively. The spectrum also shows a broad −OH stretching absorption at approximately 3440 cm−1.6,19 Figure 3 shows the
Figure 4. Yields of butyl acetate in the esterification reaction [reagent ratio of acetic acid/1-butanol (mol), 0.05:0.10; catalyst dosage, 0.2, 0.4, and 0.6 g; temperature, 40−140 °C; and time, 2 h].
Figure 3. XPS spectra of SSA.
XPS spectra of SSA and indicated that S exists in the form of −SO3H groups (168.5 eV).21 In combination with the FTIR spectra of the silica gel and SSA, the existence of −SO3H groups on SSA indicates that sulfonation was successfully conducted. Figure 2b shows the TG/DTG curves of silica gel and SSA. In the TG/DTG curves of silica gel and SSA, the weight loss below 100 °C is most likely due to the desorption of water. After loading of the −SO3H group on silica gel, the amount of adsorbed water was obviously increased. SSA exhibits another obvious weight loss centered at 210 °C, which is assigned to the decomposition of the −SO3H groups.19 Results of nitrogen adsorption−desorption isotherms are shown in Figure 2c. The pore size distributions, specific surface area, pore volume, and hydrogen ion exchange capacity are listed in Table 1. After Table 1. Properties of Silica Gel and SSA sample
BET surface area (m2/g)
pore size (nm)
pore volume (cm3/g)
H+ exchange value (mmol/g)
silica gel SSA
379.5 147.2
8.39 11.02
0.96 0.41
5.16
Figure 5. Yields of 1,1-dibutoxyethane in the acetalization reaction [reagent ratio of acetaldehyde/1-butanol (mol), 0.025:0.10; catalyst dosage, 0.2, 0.4, and 0.6 g; temperature, 40−140 °C; and time, 2 h].
phenomena also appeared in the reactions with low catalyst dosages of 0.2 g at 110 °C and 0.4 g at 100 °C. As shown in Figure 1, both the esterification reaction of acetic acid and the acetalization reaction of acetaldehyde were equilibrium reaction processes and strongly affected by the reaction temperature and catalyst dosage.12 In comparison to the esterification reaction, the influence of the reaction temperature and catalyst dosage on the acetalization reaction was more remarkable. Yields of 1,1-dibutoxyethane were decreased with higher temperature (110 °C) and catalyst dosage (0.6 g). As previously shown, a high reaction temperature and high catalyst dosage could result in further decomposition of acetals.12 It is possibly the main reason that the decomposition of 1,1-dibutoxyethane. 3.3. Simultaneous Esterification and Acetalization Reaction. According to the reaction mechanism, as described in Figure 1, acetic acid and acetaldehyde were converted into butyl acetate and 1,1-dibutoxyethane, respectively.
loading the −SO3H group, the silica gel caused a significant decrease in surface area (147.2 m2/g) and pore volume (0.41 cm3/g), whereas the pore size (11.02 nm) increased. The hydrogen ion exchange capacity of SSA was 5.16 mmol/g. 3.2. Respective Esterification and Acetalization Reactions. Figure 4 shows the yields of butyl acetate obtained in the esterification reaction. As expected, the yields of butyl acetate increased with the increase of the temperature and the increase of the catalyst dosage. There is a rapid increase of the butyl acetate yield as the temperature increases from 40 to 70 °C. The maximum yield of butyl acetate (84.5%) was obtained at 130 °C. Figure 5 illustrates the yields of 1,1-dibutoxyethane produced in the acetalization reaction. Unlike the yield of butyl acetate, the yield of 1,1-dibutoxyethane was increased to a maximum value (59.8%) and then decreased once the temperature exceeded 90 °C in the presence of 0.6 g of catalyst. Similar 4269
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Although acetic acid and acetaldehyde were converted into esters and acetals, respectively, in the simultaneous esterification and acetalization reaction, there are still some problems. In comparison to the results shown in Figure 4, the yields of butyl acetate were reduced in the presence of acetaldehyde. This restraint was even more significant at lower temperatures (
