Upgrading of the Acid-Rich Fraction of Bio-oil by Catalytic

Research Article pubs.acs.org/journal/ascecg

Upgrading of the Acid-Rich Fraction of Bio-oil by Catalytic Hydrogenation-Esterification Junhao Chen,† Qinjie Cai,‡ Liang Lu,† Furong Leng,† and Shurong Wang*,† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou 310027, China School of Resources and Environmental Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China

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ABSTRACT: The complex composition, high degree of unsaturation, and strong corrosiveness of bio-oil make direct catalytic upgrading problematic due to coking and device corrosion, and therefore, proper pretreatment is required. In this study, catalytic hydrogenation-esterification was identified as an efficient pretreatment method for bio-oil upgrading, and the hydrogenation-esterification behavior of typical compounds found in the acid-rich fraction of bio-oil over Cu/ SBA-15 catalyst was investigated. It was found that furfural, hydroxyacetone, and guaiacol could be synergistically transformed with acetic acid (AcOH) during hydrogenationesterification. Meanwhile, a higher reaction temperature promoted AcOH conversion and increased the degree of hydrogenation. On the basis of the quantification of the main products, the reaction pathways in the presence of furfural, hydroxyacetone, or guaiacol were proposed. Finally, the hydrogenation-esterification of simulated bio-oil was performed at 300 °C. The results showed that furfural, hydroxyacetone, and guaiacol were efficiently hydrogenated, and AcOH was almost completely converted into ethanol and esters, with the fraction of acid compounds drastically declining from 25.0 wt % to 0.4 wt %. Thus, a high quality pretreated bio-oil fraction, mainly consisting of alcohols and esters and ready for subsequent upgrading, was obtained in a long-term test. KEYWORDS: Bio-oil, Acid-rich fraction, Hydrogenation-esterification, Model compound mixture, Reaction pathway

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INTRODUCTION The bio-oil produced by fast pyrolysis of biomass has been considered to be an inexpensive renewable liquid fuel and a potential substitute for conventional fossil fuel. However, some inferior properties of crude bio-oil, including its low heating value, high oxygen and water contents, chemical instability, and strong corrosiveness, severely limit its direct utilization.1−4 Thus, upgrading technology is required for the high-grade utilization of bio-oil. Various upgrading techniques, including catalytic cracking, catalytic hydrogenation, catalytic esterification, steam reforming and emulsification, etc., have been developed recently to obtain hydrocarbon fuels, hydrogen, and emulsion fuels.1,4−10 However, the bio-oil composition is extremely complicated, and different chemical families have different reactivities. Thus, a single upgrading technique could not achieve the integral and efficient conversion of bio-oil. Previous studies on the cracking of bio-oil model compounds have shown that sugars and largemolecular-weight phenolic oligomers tended to undergo the thermal condensation reaction to produce coke,11−13 which was responsible for rapid catalyst deactivation. Meanwhile, the high degree of unsaturation of bio-oil components also promoted coke formation.14,15 In addition, the bio-oil exhibited strong corrosiveness, especially at high temperatures,2,5 due to the high © 2016 American Chemical Society

content of carboxyl acids (mainly acetic and formic acid), and this will increase the cost if bio-oil is directly applied to certain high-temperature processes such as catalytic cracking and steam reforming. Therefore, proper pretreatment is required for the efficient conversion of bio-oil and the following points could be addressed: (1) removal of sugars and large-molecular-weight phenolic oligomers, (2) improvement of bio-oil degree of saturation, and (3) relief of bio-oil corrosiveness under relatively mild conditions. Molecular distillation is an efficient bio-oil separation technique through which acids and ketones can be enriched in the distilled fraction while sugars and phenolic oligomers are reserved in the residual fraction.16 Consequently, the largemolecular-weight compounds were successfully removed from the thermally sensitive bio-oil. We have successfully upgraded the distilled fraction by catalytic cracking and steam reforming.8,17,18 However, because the degree of unsaturation of the distilled fraction was still high, a relatively high proportion of alcohol was introduced to achieve the effective Received: September 30, 2016 Revised: November 11, 2016 Published: November 20, 2016 1073

DOI: 10.1021/acssuschemeng.6b02366 ACS Sustainable Chem. Eng. 2017, 5, 1073−1081

Research Article

ACS Sustainable Chemistry & Engineering

separated by filtration, washed with deionized water until neutral, dried at 110 °C overnight, and then calcined at 450 °C for 4 h. Finally, the catalyst was sieved to 40−60 mesh. According to our previous study on the composition of the bio-oil fraction from molecular distillation,16,51 some typical bio-oil model compounds, AcOH, furfural, hydroxyacetone, and guaiacol, were chosen. The detailed weight ratio was set at 5:3:1:1, based on the results of the quantitative research of the bio-oil composition.52−55 Methanol was introduced as the coreactant, with an alcohol/acid weight ratio of 2:1. To investigate the pathway of catalytic hydrogenation and esterification, five model compound mixtures (MCM) were employed; their detailed compositions are listed in Table 1.

conversion of bio-oil to liquid hydrocarbons, and this increased the operation cost.8,19 Although the distilled fraction was directly steam reformed to produce hydrogen, corrosion and coking problems might still occur during long-term operation. Therefore, even for the bio-oil distilled fraction, improving the degree of saturation and converting acids prior to the cracking or reforming are still required. Considering the cost, techniques under mild conditions are preferred. The bio-oil degree of saturation can be improved by mild hydrogenation under relatively low pressures,7,20 which can saturate double bonds and aromatic rings. Typical hydrogenation catalysts include noble metal catalysts,7,20 metal phosphide catalysts,21 Mo-based sulfide catalysts,22 and other metal catalysts (Cu, Mg, Ni, etc.).23,24 In addition, Al2O3, TiO2, SiO2, ZrO2, activated carbon, and zeolites are usually employed as the support materials.25 In addition, methanol, ethanol, toluene, and water, etc. are commonly used as solvents.25 However, for the effective hydrogenation of acids, high temperatures, above 350 °C, are often required,26−28 which may lead to the decomposition of other bio-oil components. On the other hand, efficient conversion of acids can be achieved via catalytic esterification to produce neutral esters.29,30 However, phenols and ketones do not behave in the same way, and their unsaturation properties remain unchanged. Some researchers have combined catalytic hydrogenation and esterification to convert AcOH and furfural into alcohols and esters.31−34 Chen et al.35 achieved the simultaneous conversion of AcOH, furfural, and phenol using a similar method, and the yield of alcohols and esters reached 95%. Nevertheless, AcOH could not be converted sufficiently by this one-step hydrogenation-esterification method. In this study, an improved catalytic hydrogenationesterification process was developed to achieve a high conversion of acids, in which methanol was introduced as a solvent and coreactant. This method markedly improves the bio-oil saturation and decreases the corrosiveness, thus benefiting the subsequent cracking or reforming process. Cubased catalysts have been widely employed in the catalytic hydrogenation of acids,27,36,37 esters,38−41 ketones,42 aldehydes,23,43−46 and phenols47 due to their low cost and high activity. In addition, Cu+ and Cu0 could coexist on the surface of reduced Cu-based catalysts and display a synergetic effect in the hydrogenation.39 In addition, mesoporous silica SBA-15 materials possess a large specific surface area, high hydrothermal stability, and structured pores.48,49 Hence, they could be used as the catalyst support for catalytic hydrogenation and esterification.33,39,43,50 Therefore, we prepared Cu/SBA-15 as catalysts and investigated hydrogenation-esterification of AcOH and methanol, followed by the synergetic conversion behaviors in the presence of furfural, hydroxyacetone, or guaiacol. The corresponding reaction pathways were revealed. Finally, the hydrogenation-esterification of simulated bio-oil and methanol was studied.

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Table 1. Composition of the MCMs model compound mixture MCM-1 MCM-2 MCM-3 MCM-4 simulated bio-oil

composition (wt %) 33% AcOH + 67% methanol 31.5% AcOH + 62.5% methanol +6% furfural 28% AcOH + 56% methanol +16% hydroxyacetone 31.5% AcOH + 62.5% methanol +6% guaiacol 25% AcOH + 5% furfural +15% hydroxyacetone +5% guaiacol +50% methanol

The catalytic reactions were carried out in a fixed-bed system. The reactor was a stainless steel tube with an inner diameter of 8 mm. The Cu/SBA-15 (2 g) was placed in the reactor and supported on quartz wool. The catalyst was reduced at 360 °C for 3 h at a H2 flow rate of 50 mL/min prior to the reaction. The reactants were introduced by a high-performance liquid chromatography (HPLC) pump and then were nebulized with H2 and fed into the reactor. The reaction pressures were set at 3 MPa for the catalytic hydrogenationesterification of MCM-1−4 and 0.1−4.5 MPa for the simulated biooil, respectively. The ratio of n(H2)/n(hydrogenation reactants) was set at 10. The weight hourly space velocity (WHSV) of the reactants was 1 h−1. The outlet gas was cooled through the condenser and separated into noncondensable gases and liquid products. The reaction temperature varied from 220 to 320 °C. An online gas chromatograph (GC, Huaai GC 9560), a gas chromatograph−mass spectrometer (GC-MS; TraceDSQ II) system, and a gas chromatograph (Agilent 7890A) were employed to identify the structure of compounds and quantify the unconverted reactants and products. The details about the analysis were mentioned in our previous study.56 The reactant conversion (Xi) and product carbon selectivity (Sj, Sk) are defined by eq 1−3. The symbols “m” and “n” in the equation represents the mass of corresponding substances and the mole number of carbon, respectively.

Xi = [(m i )in − (m i )out ]/(m i )in × 100%

(1)

Sj (j = liquid products) = nj/[(nreactants)in − (nreactants)out ] × 100%

(2)

S k (k = C1 ∼ 4hydrocarbons, CO, CH3OCH3) = nk /[(nreactants)in − (nreactants)out ] × 100%

EXPERIMENTAL SECTION

The Cu/SBA-15 catalyst was prepared by homogeneous deposition precipitation, and the loading amount of Cu was 20 wt %. Cu(NO3)2· 3H2O and SBA-15 zeolite were purchased from the Sinopharm Chemical Reagent Co., Ltd. and Nanjing XFnano Materials Tech Co., Ltd., respectively. To prepare the catalyst, 3.80 g of Cu(NO3)2·3H2O and 13.4 g of urea were first dissolved in the requisite amount of deionized water at ambient temperature. Afterward, 4 g of SBA-15 was added to form a suspension through ultrasonication. The suspension was then heated in an oil bath at 90 °C for 7 h. The precipitate was

(3)

The Cu particle size and distribution were determined by highresolution transmission electron microscopy (TEM, Philips-FEI, Tecnai G2F30). Powder X-ray diffraction (XRD) analysis was performed on a PANalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation, operated at 40 kV and 40 mA. The scanning regions of the diffraction angle were 2θ = 10−80°. The BET surface areas and pore sizes of the prepared catalysts were determined by N 2 adsorption−desorption using the automated surface area and pore size analyzer (Quantachrome; Autosorb-1). 1074


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RESULTS AND DISCUSSION Catalyst Characterization. TEM. Figure 1 shows TEM images of the Cu/SBA-15 catalyst after reduction. From Figure

Figure 3. Hydrogenation-esterification of pure AcOH.

Figure 4. Hydrogenation-esterification of MCM-1.

Figure 1. TEM images of the catalyst prepared after reduction.

Figure 5. Reaction pathway of AcOH: (a) pure AcOH and (b) in the presence of methanol (green line, hydrogenation; blue line, esterification; red line, other reaction types).

Figure 2. XRD pattern of the catalyst prepared after reduction.

1a, it can be seen that the active components were well dispersed, with calculated average particle sizes as small as 3.8 nm, indicating high catalytic activity. The crystal planes of Cu and Cu2O are depicted in Figure 1b. XRD. Figure 2 shows the XRD pattern of the Cu/SBA-15 catalyst after reduction. The diffraction peaks detected at 2θ = 43.3°, 50.5°, and 74.3° were assigned to Cu. In addition, the crystal phase assigned to Cu2O with a 2θ of 36.5° was also

found. The results are in good accordance with TEM images above. BET. The specific surface area of the SBA-15 was 711 m2/g, with an average pore size of 6.0 nm. After loading, the specific surface area obviously decreased. However, the loaded catalyst still possessed a large specific surface area, 487 m2/g. Compared with the blank SBA-15, the average pore size of the Cu/SBA-15 1075


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EA, was used to evaluate the direct hydrogenation activity of AcOH. According to the experimental results, the reaction intensity was weak at 220 °C with a S(EtOH+EA) of 84.9%, and the corresponding AcOH conversion was just 16.3%. As the reaction temperature increased, the reaction intensity was significantly enhanced. The AcOH had a conversion of more than 70% at 320 °C, while the S(EtOH+EA) exhibited a slight decline because of the enhanced gaseous products formation. It could be concluded that the AcOH conversion was improved by the elevated temperature to a great extent and that the main products were consistently EtOH and EA. Furthermore, a small amount of aldehyde was observed, and this was identified as the intermediate of AcOH hydrogenation.36,37 Afterward, we performed the hydrogenation-esterification of MCM-1 under the same conditions. The reactant conversion and liquid product selectivity are shown in Figure 4. Similar to the above experimental results, the AcOH conversion increased with increasing reaction temperature, reaching 91.5% at 220 °C and more than 99% at 300 °C. Because the unconverted AcOH could participate in the esterification to produce esters during the hydrogenation process in the presence of methanol, the conversion was highly improved. For the liquid products, a small amount of ethanol was generated at low temperatures. At higher temperatures, the ethanol carbon selectivity reached a maximum of 59.5% at 300 °C and then decreased to 57.3% at 320 °C due to the generation of more gaseous products. Methyl acetate (MA) can be converted into ethanol via hydrogenation over Cu-based catalysts.39,40 However, based on our previous study,39 the ethanol selectivity began to decrease at 220 °C over Cu/SBA-15. Considering the hydrogenation behavior of pure AcOH, in the reaction process of MCM-1, the notable increase of ethanol selectivity could be mainly ascribed to the promotion of AcOH direct hydrogenation rather than ester hydrogenation. Meanwhile, the esterification of AcOH and methanol was consequently suppressed, which was responsible for the rapid decline of MA selectivity. The secondary esterification of AcOH and ethanol and the transesterification of MA and ethanol contributed to the EA formation.38,40 It was noteworthy that aldehyde was not detected. This was reasonable since the efficient conversion and sufficient hydrogenation of AcOH suppressed the byproduct formation. The formation of gaseous products was negligible at a low temperature, whereas it became obvious at 320 °C, reaching a total carbon selectivity of 18.6%. The main gaseous products contained dimethyl ether, CO, and C1−C4 hydrocarbons, which were produced by the ether reaction of

Figure 6. Hydrogenation-esterification of MCM-2 (furfural was totally converted).

Figure 7. Reaction pathway of MCM-2 (green line, hydrogenation; blue line, esterification; red line, other reaction types).

catalyst was larger, 8.5 nm. This result could be ascribed to the formation of larger pores by the alkali treatment during catalyst preparation. Hydrogenation-Esterification of MCM-1. To investigate the conversion pathway of AcOH at different temperatures, the hydrogenation-esterification of pure AcOH was carried out, and the results are shown in Figure 3. The hydrogenation of AcOH could produce ethanol, which underwent a secondary esterification with AcOH to generate ethyl acetate (EA). Therefore, S(EtOH+EA), the total selectivity of ethanol and 1076


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ACS Sustainable Chemistry & Engineering Table 2. Product Distribution of Hydroxyacetone Conversion in the Hydrogenation-Esterification of MCM-3 products carbon selectivity (%)

temperature (°C) 220

1,2-propanediol 1,2-propanediol, diacetate butanediol 2-propanol 1-propanol 2-butanol 3-hexanol 2-methyl-1-pentanol 1-methoxyl-2-propanol 2- methoxyl-1-propanol

11.71 1.16 3.51 0.27

(±0.62) (±0.09) (±0.16) (±0.02)

240 13.24 0.85 3.57 0.60 0.55

(±1.19) (±0.01) (±0.08) (±0.02) (±0.03)

260 12.87 0.45 2.55 2.15 1.79

(±0.91) (±0.07) (±0.21) (±0.14) (±0.02)

0.44 (±0.02) 0.56 (±0.03)

0.66 (±0.04) 0.28 (±0.01)

0.63 (±0.03) 0.28 (±0.01)

280 2.74 0.21 1.12 3.50 3.40 0.30 0.46 0.52 0.40 0.27

(±0.25) (±0.01) (±0.05) (±0.33) (±0.26) (±0.02) (±0.03) (±0.03) (±0.00) (±0.01)

300

320

0.93 (±0.07) 0.10 (±0.01)

0.47 (±0.02) 0.09 (±0.01)

13.73 1.50 1.23 3.28 1.00 0.54

(±1.04) (±0.10) (±0.06) (±0.09) (±0.05) (±0.04)

13.69 2.31 1.30 3.97 0.65 0.40

(±0.53) (±0.02) (±0.03) (±0.08) (±0.02) (±0.01)

Hydrogenation-Esterification of Other Model Compounds. To investigate the synergetic pathway in the hydrogenation-esterification of AcOH and methanol with the addition of other typical compounds, experiments on the hydrogenation-esterification of MCM-2, MCM-3, and MCM-4 were performed. Hydrogenation-Esterification of MCM-2. Figure 6 presents the reactant conversion and liquid product selectivity of MCM2. Furfural was completely converted in the temperature range of 220−320 °C. This result might be attributed to the high reactivity of the aldehyde group over Cu-based catalysts.45 Compared with an AcOH conversion of 91.5% for MCM-1, a higher conversion of 96.1% was observed for MCM-2 at 220 °C. In addition, furfuryl acetate, cyclopentyl acetate, and pentyl acetate were detected by GC-MS, indicating that the esterification of AcOH and the alcohols from furfural hydrogenation promoted the AcOH conversion. However, when the reaction temperature exceeded 280 °C, the direct hydrogenation of AcOH dominated and the effect of furfural addition was not obvious. As shown in Figure 6a, the maximum carbon selectivity to ethanol was 38.3% at 300 °C. Figure 6b displays the variation of the typical products of furfural hydrogenation as a function of temperature. As the reaction temperature increased to 260 °C, the carbon selectivity to furfuryl alcohol and cyclopentanol decreased, contrary to the results seen for 1-pentanol and 2-pentanol. On the basis of the product distribution of the hydrogenation-esterification of MCM-1, the reaction pathways are shown in Figure 7. The main hydrogenation products of furfural at low temperatures were alcohols, such as furfuryl alcohol and cyclopentanol, while 1-pentanol and 2-pentanol were the primary products at high temperatures. On the basis of the TEM and XRD analyses, after the reduction of Cu/SBA15, the two active components observed in the catalyst were Cu0 and Cu+. The Cu0 therein could absorb and dissociate hydrogen molecules, which played an important role in the hydrogenation of the CO group;46,57 thus the high selectivity to furfuryl alcohol was found in the hydrogenation of furfural over Cu-based catalysts.43,58 The rearrangement reaction of furfuryl alcohol will generate cyclopentanone and then form cyclopentanol in an acid-catalyzed reaction.59 Correspondingly, unconverted AcOH could be regarded as the acid catalyst, and Cu+ may also function as a Lewis acid or an electrophilic site.39,60 The carbon selectivity of furfuryl alcohol and cyclopentanol gradually decreased with increasing reaction temperature. Instead, more 1-pentanol and 2-pentanol were detected because of the breakage of the furan ring. The AcOH participated in the esterification reaction with alcohols to



methanol and the decomposition of AcOH. The detailed pathway is shown in Figure 5. Furthermore, a blank test of MCM-1 using SBA-15 was carried out to investigate the effect of Cu loading. It was observed that an AcOH conversion of 93.5% and an MA carbon selectivity of 89.8% were achieved under 300 °C, respectively, while the carbon selectivity to ethanol was only 0.9%. The experimental results revealed that the hydrogenation of AcOH hardly occur over SBA-15, indicating that Cu played an important role in the hydrogenation of AcOH. 1077


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ACS Sustainable Chemistry & Engineering Table 3. Product Distribution of Guaiacol Conversion in the Hydrogenation-Esterification of MCM-4 temperature (°C) products carbon selectivity (%)

220

240

260

cyclohexanol phenol cyclohexyl acetate cyclohexanone 2-methyl cyclohexanol 1-methyl-1,2-cyclohexanediol cyclohexane methyl-cyclohexane methoxy-cyclohexane

1.20 (±0.02) 0.84 (±0.02) 0.18 (±0.02)

2.74 (±0.06) 1.80 (±0.05) 0.31 (±0.01)

3.35 (±0.05) 2.23 (±0.13) 1.78 (±0.02)

1.28 (±0.05) 0.27 (±0.01)

280 5.70 1.40 2.80 1.32 0.69 0.72 1.37

(±0.14) (±0.02) (±0.03) (±0.01) (±0.01) (±0.01) (±0.03)

1.33 (±0.00)

300 3.23 1.14 0.49 0.58 0.45 0.10 3.56 2.82 1.54

(±0.22) (±0.03) (±0.01) (±0.00) (±0.01) (±0.00) (±0.10) (±0.08) (±0.02)

320 0.19 (±0.02) 0.06 (±0.00) 0.09 (±0.01)

6.37 (±0.18) 3.57 (±0.03) 1.25 (±0.01)

Figure 13. Composition of upgraded product under different pressures.


Figure 14. Stability of Cu/SBA-15 in the hydrogenation of simulated bio-oil (3 MPa/300 °C).

shown in Figure 8 and Table 2. The hydroxyacetone conversion exceeded 99% and finally reached 100%, which could be ascribed to the high activity of Cu0 for CO group hydrogenation.46 Similar to the effect of furfural addition, the AcOH conversion was also promoted under low temperatures in the presence of hydroxyacetone, with conversion rates as high as 97.3% at 220 °C. At 300 °C, a higher carbon selectivity, 29.2%, to ethanol was achieved. The transformation process of hydroxyacetone was rather complex, with different mechanisms dominant at different reaction temperatures. The 1,2-propanediol from direct hydrogenation was the dominant product below 240 °C, followed by butanediol and 1,2-propanediol diacetate, via alkylation and esterification, respectively. With increasing reaction temperature, the carbon selectivity of 1,2-

Figure 12. Comparison of the acid-rich fraction of bio-oil and upgraded product in composition.

generate esters. Some furfuryl acetate, cyclopentyl acetate, and pentyl acetate, which were derived from furfural hydrogenation, were also produced via esterification with unconverted AcOH. This synergetic effect further promoted the AcOH conversion. Moreover, the etherification of furfuryl alcohol and methanol also occurred to generate furfuryl methyl ether. Hydrogenation-Esterification of MCM-3. The experimental results of the hydrogenation-esterification of MCM-3 are 1078


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results. Accordingly, the detailed reaction pathway is presented in Figure 11. Hydrogenation-Esterification of Simulated Bio-oil. On the basis of the results presented in the preceding sections, the reactant conversion was maximized at approximately 300 °C, as well as the carbon selectivity of ethanol, indicating the best hydrogenation-esterification performance. Therefore, the hydrogenation-esterification of simulated bio-oil was performed at 300 °C. The conversions of AcOH, furfural, hydroxyacetone, and guaiacol were 98.4%, 100%, 98.2%, and 95.2%, respectively. As shown in Figure 12, the liquid composition was significantly improved via hydrogenation-esterification. In detail, the acids were almost completely converted, with the acid fraction drastically decreased from 25.0 wt % to 0.4 wt %. A notable decrease of the fraction of phenols, from 5.0 wt % to 0.6 wt %, was also observed. Moreover, the aldehydes and ketones were almost completely converted. The fraction of alcohols and esters increased to 60.6 and 16.6 wt %, respectively. Therefore, the degree of saturation was improved, and the corrosiveness was relieved to a significant extent after the hydrogenationesterification. In the hydrogenation of simulated bio-oil, the composition of the main products varied at different reaction pressures. The corresponding data are presented in Figure 13 (the other chemical families and undetected compounds were not listed). It could be concluded that elevated pressure facilitated the conversion of phenols and the production of alcohols. Only 0.2 wt % of acids and 0.4 wt % of phenols were detected in the upgraded product at 4.5 MPa. However, the operation cost should be taken into consideration when choosing the reaction pressure. The stability of the Cu/SBA-15 catalyst was tested at 300 °C and 3 MPa, as shown in Figure 14. Cu/SBA-15 catalyst exhibited a high stability for the hydrogenation of simulated bio-oil. Even after 40 h, the alcohol content was still maintained at 60.9 wt % in the liquid products. The integral reaction pathway was proposed in Figure 15. A large amount of AcOH could be converted into ethanol via direct hydrogenation or secondary hydrogenation of MA. In addition, the phenols, aldehydes, and ketones were hydrogenated into corresponding alcohols, which would subsequently react with unconverted AcOH. Consequently, the acidrich fraction could be transformed into a liquid product rich in alcohols and esters, with a high heating value. The upgraded product obtained is ready for the subsequent catalytic cracking and steam reforming processes.14,15,64

Figure 15. Integral reaction pathway of the acid-rich fraction of bio-oil.

propanediol sharply decreased, which agreed with the experimental results obtained by Akiyama et al.42 As a result, the carbon selectivities of butanediol and 1,2-propanediol diacetate were then reduced. Conversely, above 300 °C, more 1-propanol and 2-propanol were generated, resulting in a carbon selectivity of 1-propanol up to 13%. The formation of 2butanol, 3-hexanol, and 2-methyl-1-pentanol could be ascribed to the alkylation reaction on the different carbon atoms, which could be catalyzed by Cu+. In addition, the etherification reaction of 1,2-propanediol and methanol also occurred to generate a small amount of propylene glycol monomethyl ether (1-methoxyl-2-propanol and 2-methoxyl-1-propanol). According to the aforementioned results, the corresponding reaction pathway is displayed in Figure 9. Aside from MA and EA, the only other ester obtained was 1,2-propanediol diacetate, indicating that 1,2-propanediol had high esterification activity with AcOH. The formation of these esters contributed to the improved AcOH conversion at low temperatures. Although a large amount of propanol was produced, the propyl acetate was not detected, indicating the strong tendency of AcOH hydrogenation at elevated temperatures. Hydrogenation-Esterification of MCM-4. As shown in Figure 10, the addition of guaiacol could also promote AcOH conversion at low temperatures. However, the conversion of guaiacol itself was quite difficult, which was only 46.3% at 220 °C, while it exceeded 90% at 300 °C. Grange et al.61 also investigated the hydrogenation reaction of different compounds, finding that an efficient conversion of guaiacol could be achieved at temperatures above 300 °C. In addition, the highest carbon selectivity to ethanol, 35.4%, was achieved. Table 3 lists the product distribution from the hydrogenation-esterification of guaiacol. The guaiacol has a methoxyl side chain and a phenolic hydroxyl group; thus, the corresponding products were complex. Low temperatures favored demethoxylation and benzene ring hydrogenation to produce phenol and cyclohexanol. Afterward, cyclohexanol underwent esterification with AcOH to generate cyclohexyl acetate. At 260 °C, deoxygenation started to occur and cyclohexane was detected, which was the main product above 300 °C. Cyclohexanone was identified as the intermediate in the hydrogenation process of phenol and guaiacol.47,62,63 During the reaction process, the derivatives of cyclohexane and cyclohexanol were detected, which were derived from the demethylation and methylation catalyzed by Cu+,47 as shown in Table 3. Deutsch et al.47 carried out a study on the guaiacol hydrogenation over Cu−Cr catalysts, and the main products detected were consistent with our experimental

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CONCLUSIONS In this study, the upgrading of typical compounds in the acidrich fraction of bio-oil over Cu/SBA-15 catalyst was carried out via catalytic hydrogenation-esterification. The AcOH, together with furfural, hydroxyacetone, and guaiacol, could be converted synergistically. On the basis of the quantification of the main products, the reaction pathways of these typical compounds were studied. Higher reaction temperatures promoted AcOH conversion and increased the degree of hydrogenation. Afterward, the hydrogenation-esterification of simulated biooil was performed at 300 °C, and the liquid product mainly consisted of alcohols and esters, which is favorable for subsequent catalytic cracking and steam reforming. Finally, the catalyst stability and the effects of reaction pressure were also investigated. 1079


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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87952801. Fax: +86-571-87951616. E-mail: [email protected]. ORCID

Shurong Wang: 0000-0001-6733-3027 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Science and Technology Supporting Plan Through Contract (2015BAD15B06) and the National Natural Science Foundation of China (51276166).

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REFERENCES

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