In-Situ-Prepared Nanocopper-Catalyzed Hydrogenation–Liquefaction

Article pubs.acs.org/EF

In-Situ-Prepared Nanocopper-Catalyzed Hydrogenation− Liquefaction of Biomass in a Glycerol−Methanol Solvent for Biofuel Production Zheng Li, Xianhai Zeng,* Xing Tang, Yong Sun, and Lu Lin* College of Energy, Xiamen University, Xiamen, Fujian 361005, People’s Republic of China ABSTRACT: Hydrothermal or alcoholysis liquefaction are common pathways to produce biofuel in mild reaction conditions. However, the application was limited by the reliance of corrosive acid and base catalysts and high-cost hydrogen donors. In this research, in-situ-prepared nano-Cu was employed for the first time as a methanol decomposition catalyst in a glycerol− methanol−water solvent to generate hydrogen in situ for the hydrogenation−liquefaction of Miscanthus. Methanol was effectively decomposed to H2, CO, and CO2 by the catalysis of Cu. The percentage of conversion of biomass into liquid product was over 90% at 350 °C for 4 h. Bio-oil with main components, including alcohols, esters, ethers, alkyl phenolics, and other glycerolderived molecules, were obtained. This strategy also showed an excellent liquefaction capacity when other woody and herbaceous biomasses were selected as feedstock.

1. INTRODUCTION The world is facing a preponderant reliance problem with unrenewable fossil fuel, which has resulted in an endemic crisis, environment degradation, climate change, and extensive impacts on human health.1 Inedible biomass, including agriculture wastes, woody feedstock, municipal wastes, and energy crops, have recently been regarded as a desirable substitute of fossil resources to produce environmentally friendly and sustainable biofuels. Several common technologies have been developed and employed to convert the carbonaceous feedstock into renewable liquid biofuels, such as biological conversion,2 fast pyrolysis,3,4 and high-pressure liquefaction.5,6 Hydrothermal liquefaction has been investigated as an efficient pathway to convert biomass into liquid products directly with relatively less energy consumption in pretreatment, such as drying and milling at mild reaction conditions.7 Supercritical water has been applied as a remarkable solvent, in which biomass could be converted to low-molecular-weight liquid fuels.8 However, bio-oil produced by the traditional hydrothermal liquefaction route contains a considerable amount of water, which reduced the heat value of products and results in complicated and environmentally unfriendly separate processes, while alcoholic solvents, including methanol, ethanol, and glycerol, have attracted considerable attention because of their specific reaction activity with biomass.9,10 Crude glycerol, a byproduct from the biodiesel industry, has been used as an economical and effective solvent in the alcoholysis liquefaction of biomass to produce biooil10−16 in the presence of catalyst, such as sulfuric acid,10,12,16 which has given rise to the problem of corrosion. Xiu et al. reported that the crude glycerol could be separated into three layers (i, free fatty acids; ii, glycerol, methanol, and water; and iii, remaining catalyst) by the addition of 85% phosphoric acid.13,14 A glycerol-based solvent containing 66.95% glycerol, 26.2% methanol, and 6.85% water was obtained and used in the liquefaction of swine manure.13,14 Besides, reducing agents or © 2014 American Chemical Society

hydrogen donors, such as tetraline, hydrogen, syngas, and calcium formate, are necessary in the liquefaction, with the purpose of stabilizing the ions and radicals generated during the decomposition of biomass and preventing them from polymerizing and condensing to form cokes.7,17,18 The corrosive catalysts and high-cost reducing agents are deficiencies, limiting the development of alcoholysis liquefaction technology. In this study, we developed an attractive liquefaction strategy based on the glycerol−methanol−water solvent (water was supplied by the moisture in the feedstock) under a N2 atmosphere. The reducing agent hydrogen was prepared in situ via the decomposition of methanol over Cu, which has been recognized as an effective, cheap, heterogeneous catalyst for hydrogen production from methanol19 and has rarely been applied in biorefinery studies to produce highvalue chemicals20,21 or biofuel. Hydrogen produced by the decomposition of methanol catalyzed by various metal catalysts, including Cu, Zn, noble metals, and their oxides, has been performed in fuel cells,22−24 but there are thus far no reports about employing these in situ hydrogen supply systems to specific reactions other than fuel cells. In this paper, we studied the liquefaction efficiency of Miscanthus in a glycerol− methanol−water solvent with hydrogen supplied by metalcatalyzed decomposition of methanol. Several metal oxides, including CuO, MgO, Al2O3, Fe2O3, and CaO, were estimated to eliminate the reliance on externally supplied H2. The percentage of conversions of biomass obtained under different conditions were compared to each other to optimize the liquefaction process. Finally, the liquefaction effect on other species of biomass was tested to investigate the applicability of this strategy. Special Issue: International Biorefinery Conference Received: January 15, 2014 Revised: April 21, 2014 Published: April 21, 2014 4273

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Figure 1. Pathway for catalytic hydrogenation−liquefaction of biomass.

2. MATERIALS AND METHODS 2.1. Materials. The raw Miscanthus (C, 43.38%; H, 5.191%, N, 1.303%) was harvested in Gunong Farm in Zhangzhou, Fujian, China. The other feedstocks, including bamboo, manioc, pine, cedar, and bagasse, were collected in Xiamen, Fujian, China. The raw materials were naturally dried and grilled roughly and had a proper size (5 mm length and 0.5 mm diameter). The analytical reagent (AR) glycerol, methanol, and metal oxides were from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Experimental Section. As shown in Figure 1, the biomass, catalyst, and solvent were added to a 100 mL Hastelloy batch reactor. The weight ratios of feedstock, solvent, and catalyst are given in subsequent figures. The reactor was sealed and purged with nitrogen before reaction, but a little oxygen was left in the reactor [the amount of O2 was controlled by maintaining the flow rate (10 mL/s) and purging time (1 min) of nitrogen] to promote the catalytic decomposition of methanol on Cu, which will be discussed in section 3.4. The pressure in the reactor was atmospheric pressure at ambient temperature, which reached 4−10 MPa at the working temperature. The working pressure was recorded and given in the reaction conditions in Figures 2, 3, 4, 5, and 11. The reactor was heated from room temperature (RT, about 20 °C) to the working temperature (WT, 300−380 °C, mainly 350 °C) in an adjustable electric stove at a rate of 6 °C/min. The temperature of the reactor was monitored by a thermocouple connected to the reactor. The stirring began when it reached the working temperature, which was regarded as the zero reaction time. However, it should be emphasized that, during the heat process from RT to WT, a heat-up time (HUT) of about 1.5 h before the zero reaction time was necessary. The HUT was not contained in the reaction time annotated in a subsequent discussion, during which some Miscanthus was liquefied already. The heater and stirrer were shut down after reaction, and then the reactor was taken out of the stove and cooled to room temperature quickly to ensure that the reaction time was accurate and methanol vapor was condensed. Each experiment has been repeated 2 times. The gaseous product was collected in an aluminum foil bag for later analysis. The solid−liquid mixtures were filtrated by a vacuum filter, and then they were weighted and stored in the shadow to inhibit decomposition and evaporation. The percentage of conversion of biomass was deduced from the mass of solid residue by the following equation:

Figure 2. Liquefaction of Miscanthus with different atmospheres and catalysts. Reaction conditions: 1.5 g of Miscanthus, 0.2 g of catalyst, 9 mL of glycerol, 20 mL of methanol, N2 or H2 (working pressure: 5, 8.8, 4.8, 5.7, 5.5, 6.2, and 8 MPa, respectively), 700 rpm, 350 °C, and 4 h. The catalyst bars under the horizontal axis stand for the mcat/mmis ratio (approximately equal to 0.13). The recycle rate of Cu was 95.8%, which was measured in three blank experiments. The biomass converted to gaseous products was contained in X because of the difficulty in separating biogas with the gas derived from methanol. The solid residue was washed by acetone, which was later added to the liquid product. Then, the bio-oil was extracted by acetic ether. The residue, including unreacted feedstock, catalyst, and char, is named by the reaction temperature and time. For example, Cu-350-4 stands for the residue generated during the liquefaction at 350 °C in 4 h, in which Cu is the main component. All solid residues were dried at 65 °C for 48 h for subsequent analyses. 2.3. Characterization of Products and Catalysts. The elemental compositions (C/H/N) of Miscanthus and the residues were measured using a Vario EL III elemental analyzer, in which they were combusted at 900 °C in oxygen. The component of biogas was analyzed with a Shimadzu GC-2010 instrument. Bio-oil extracted by acetic ether was diluted by methanol with a volume ratio of 1:10 and determined by a Shimadzu GCMS-QP2010 SE instrument using helium as the carrier gas with electron ionization (EI). Thermogravimetric analysis (TGA) of the residue was conducted on a TG 209F1

percentage of conversion (%) = 1 − [(mresidue − mcatalyst × recycle rate)]/mfeedstock 4274

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Figure 3. Effect of the reaction temperature on conversion of liquefaction. Reaction conditions: 1.5 g of Miscanthus, 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, N2 (working pressure: 2.7, 3.5, 6, 8, and 7.4 MPa, respectively), 700 rpm, and 4 h. The catalyst bars under the horizontal axis stand for the mcat/mmis ratio (approximately equal to 0.13).

Figure 5. Effect of Miscanthus dosage on conversion of Miscanthus. Reaction conditions: 350 °C, N2, 700 rpm, 4 h, and (A) 0 g of Miscanthus, 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, and working pressure of 10 MPa (mmis/vsol, 0%; mmis/mcat, 0), (B) 1 g of Miscanthus, 0.2 g of CuO, 10 mL of glycerol, 20 mL of methanol, and working pressure of 3.6 MPa (mmis/vsol, 3.3%; mmis/mcat, 5), (C) 1.5 g of Miscanthus, 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, and working pressure of 5.5 MPa (mmis/vsol, 5.2%; mmis/mcat, 7.5), and (D) 2 g of Miscanthus, 0.15 g of CuO, 6 mL of glycerol, 15 mL of methanol, and working pressure of 6 MPa (mmis/vsol, 9.5%; mmis/mcat, 13.3).

Table 1. Concentrations (%, v/v) of Components in Gaseous Products substance

blank A (%)

blank B (%)

biogas-350-4 (%)a

H2 O2 N2 CO CH4 CO2

5.6 3.0 47.2 14.2 2.9 27.2

58.5 1.7 7.3 7.1 1.4 24.0

1.6 0.2 3.2 32.6 1.0 11.0

a

The total percentage composition of biogas-350-4 was less than 100% because of the existence of unknown gases.

Figure 4. Effect of the reaction time on the conversion of Miscanthus. The first bar was at 0 h because some Miscanthus was reacted during the HUT (1.5 h) before zero reaction time. Reaction conditions: 1.5 g of Miscanthus, 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, N2 (working pressure: 3.6, 5.2, 5, and 5.5 MPa, respectively), 700 rpm, and 350 °C. The catalyst bars under the horizontal axis stand for the mcat/mmis ratio (approximately equal to 0.13).

Table 2. Properties of Liquid Products density (g/mL) viscosity (mPa s) pHa

blank

bio-oil-350-4

1.075 13 4.5

1.135 16 4.5

a

Liquid products were mixed with water with a mole ratio of 1:9 and shook. Then, the supernatant was separated for the measurement of pH.

thermal analyzer under a dynamic N2 atmosphere (100 mL/min) at a temperature range of 25−800 °C with a heating rate of 20 °C/min. The residue after washing and drying was ground for XRD analysis on an X’pert PRO X-ray diffractometer to identify the crystal forms of catalyst and char. Morphological properties of the samples were observed on a JSM-6390 scanning electron microscope.

hard to filtrate and could only be dissolved in acetone, which was formed by the cyclization and repolymerization of lignin fragments and has been regarded as heavy oil in previous research.7,25,26 However, it was difficult to figure out the composition of heavy oil by gas chromatography−mass spectrometry (GC−MS) because of its complex composition, high molecular weight, and boiling point. As for the liquefaction experiment conducted under H2, the conversion was increased by nearly 34%, which suggested that the coking reaction was inhibited effectively. Although the percentage of conversion of biomass was increased by the addition of H2, it is uneconomic in the storage

3. RESULTS AND DISCUSSION 3.1. Selection of Atmosphere and Catalyst. The liquefaction of Miscanthus was initially preceded in a glycerol−methanol solvent under a H2 or N2 atmosphere (atmospheric pressure at ambient temperature) without a catalyst. As shown in Figure 2, liquefaction performed under a N2 atmosphere resulted in a rather low bio-oil yield. A lot of residues containing unreacted Miscanthus, coke, and a viscose jelly were observed at the bottom of the reactor. The jelly was 4275

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Table 3. Product Distribution of Blank Liquid Products and Bio-oil substance

residence time (min)

blank (%)

bio-oil-350-4 (%)

propylene glycol acetic acid, methoxy-, methyl ester propanoic acid, 2-methoxy-, methyl ester 2-propanol, 1,3-dimethoxypropane, 2-methoxy-2-methyl1,1,3-trimethoxypropane 1,2-propanediol, 3-methoxy1,4-butanediol, 2,3-dimethoxy2-(2-methoxyethoxy)ethyl acetate 1,3-propanediol, diacetate glycolaldehyde dimethyl acetal 4-heptanol, 4-methyl2-propanol, 1-(2-methylpropoxy)furan, tetrahydro-2-(methoxymethyl)butanoic acid, phenyl ester 1,3-dioxolane-4-methanol, 2-ethylpentanoic acid, 2,4-dioxo-, methyl ester 2-furanmethanol, tetrahydrobutanoic acid, 2-ethyl-3-oxo-, methyl ester 2-cyclopenten-1-one, 2,3-dimethyl2-cyclopenten-1-one, 2,3,4-trimethylphenol, 2-methylmequinol acetic acid, ethoxy-, ethyl ester 2-propanol, 1,1′-[(1-methyl-1,2-ethanediyl)bis(oxy)]bisbenzofuran, 2-methylphenol, 2,6-dimethylbutane, 1-chloro-4-ethoxyphenol, 4-ethyl3-pentanol, 3-ethyl-2-methyl4-methyl-2-hexanol oxalic acid, 2-isopropylphenyl octyl ester oxirane, (ethoxymethyl)carbomethoxy-2-[carboethoxy]ethyl disulfide 1,3,4-trimethoxy-butan-2-ol 1,3,6-trioxocane propanedioic acid, oxo-, dimethyl ester 5-O-methyl-D-gluconic acid dimethylamide naphthalene, 1,2,3,4-tetrahydro-5,6-dimethylbutanoic acid, 2-hydroxy-, ethyl ester, (±)D,L-erythro-O-methylthreonine phenol, 2,6-dimethoxyphenol, 2-methoxy-4-propyl2,4-hexadienedioic acid, 3,4-diethyl-, dimethyl ester, (Z,Z)1,2-benzenedicarboxylic acid, mono(2-ethylhexyl) ester

3.048 3.332 3.9 6.364 6.395 7.115 7.383 7.5 8.087 8.932 8.993 9.381 9.639 9.98 10.42 10.461 10.576 10.637 11.161 11.562 12.222 12.318 12.948 13.06 13.133 13.403 13.48 13.55 15.168 16.086 16.403 16.531 17.312 17.555 17.653 17.725 17.793 18.136 18.25 18.328 18.82 19.129 19.452 24.144 37.685

16.09 2.39 33.41

2.29 2.05 17.77 3.37 2.51 0.26 26.09 5.16 0.46 0.31 1.42 0.34 0.46 0.28 0.38 1.28 0.32 0.47 0.39 0.44 0.27 1.89 1.14 0.23 0.23 0.33 0.45 0.28 0.72 0.38 0.34 0.33 0.56 1.97 4.59 1.35 1.65 4.39 0.54 0.48 3.52 0.60 0.34 0.48 2.81

1.79 7.15 1.7 1.65

under a N2 atmosphere in the presence of Al2O3 but a little lower than the experiment performed in H2. The addition of Fe2O3, CuO, and CaO obviously increased the yield of bio-oil probably because of the generation and delivery of H2 derived from methanol, among which CaO and CuO were more effective. However, CaO was dissolved in the liquid phase after reaction and, thus, could not be recovered by filtration; therefore, CuO was chosen as the hydrogen-supply catalyst in subsequent experiments. It is noted that CuO was reduced to Cu quickly during the reaction, which will be discussed in section 3.4. 3.2. Effects of the Temperature, Reaction Time, and Feedstock Dosage. To optimize reaction conditions, CuO was chosen as the H2 supply catalyst precursor in the

and transportation for industrial-scale production of biofuel. On the other hand, various metal and metal oxides have been proven effective in the H 2 production from gaseous methanol.27−30 In consideration of the catalytic effect and cost, Al2O3, CaO, CuO (as the precursor of Cu), Fe2O3, and MgO were selected as the hydrogen-supply catalyst, which have not been used over 300 °C under a high pressure in a batch reactor before. The bio-oil yields in the presence of CuO, MgO, Fe2O3, Al2O3, and CaO are concluded in Figure 2. The catalyst addition ratio (mcat/mmis) was symbolized by the slash boxes under the horizontal axis. The yield of bio-oil was not increased by the addition of MgO. On the contrary, the amount of residue was even a little larger than that generated under a N2 atmosphere. The bio-oil yield was higher than that obtained 4276

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Figure 6. XRD spectra of catalyst: CuO and Cu in liquefaction with different temperature and time conditions.

Figure 7. SEM images of catalyst: (A) CuO precursor, (B) Cu generated in the blank experiment with no Miscanthus added, (C) Cu generated in the liquefaction of Miscanthus, and (D) CuO regenerated after the combustion of Cu-350-4. Panels B and C reaction conditions: 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, N2 (working pressure: 10 and 5.5 MPa), 700 rpm, and 350 °C.

temperature (HRT) increased to 300 and 325 °C, more Miscanthus dissolved in the solvent. At 350 °C, there was little residue, except the Cu catalyst. However, more coke generated when the HRT reached 365 °C. Considerable coke was formed at 380 °C, in which the weight of the residue was even larger than raw Miscanthus because of the polymerization between feedstock and glycerol. The effect of the liquefaction time performed similarly to the HRT. A shorter reaction time resulted in more unreacted Miscanthus, while a longer time promoted the generation of coke. Duration ranges from 3 to 4 h were proper for the liquefaction of Miscanthus.

liquefaction. The effects of the temperature, reaction time, and Miscanthus dosage in the liquefaction process were studied to make the method more effective and economical. Figure 3 showed that the proper reaction temperature was 350 °C, which provided the highest bio-oil yield. The reaction temperature was over 300 °C because the boiling point of glycerol was 290 °C, at which temperature glycerol would also decompose, polymerize with itself, or react with methanol. There was much residue containing a lot of unreacted Miscanthus that remained after the reaction proceeded under 290 °C, indicating that glycerol did not actively react with Miscanthus below its boiling point. As the highest reaction 4277

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Figure 8. Thermogravimetry (TG)−differential thermogravimetry (DTG) curve of Cu-350-2.

Figure 10. Decomposition mechanism of methanol on Cu.34

Figure 9. C and H contents in each used catalyst.

In experiments A−D in Figure 5, the m/v ratio of Miscanthus and solvent (mmis/vsol) ranged from 0 to 9.5%, while the m/m ratio of Miscanthus and catalyst (mmis/mcat) was from 0 to 13.3. A higher Miscanthus dosage led to more residues at the same reaction conditions. When mmis/vsol and mmis/mcat were increased to 9.5 and 13.3, the percentage of conversion of feedstock reduced from over 90% to less than 80%. Used catalyst was difficult to be separated with coke, which brought the necessity of combustion to regenerate the catalyst. 3.3. Analysis of the Liquefaction Product. The gaseous products generated during the liquefaction of Miscanthus at 350 °C in 4 h (biogas-350-4) and blank experiments A and B performed under the same conditions (A, glycerol + methanol; B, CuO + glycerol + methanol; 350 °C, 4 h, and 700 rpm) were detected by GC. The main components of biogas are listed in Table 1. The amount of gas generated in blank A was less than 0.1 g, in which there was 5.6% H2, implying that only a little methanol was decomposed to H2. On the contrary, about 600 mL of gas generated with the presence of Cu. There was nearly 60% H2 in the gas products in blank B, proving that Cu played a vital role in the decomposition of methanol. After Miscanthus was added as feedstock, the amount of gas reduced to about 0.2 g, in which only 1.63% was hydrogen, indicating that H2 was

Figure 11. Liquefaction efficiency of the in situ hydrogenation− liquefaction strategy on various biomasses. Reaction conditions: 1.5 g of raw material, 0.2 g of CuO, 9 mL of glycerol, 20 mL of methanol, N2 (working pressure: 5.5, 6.1, 6, 7.8, 4.3, and 5.1 MPa, respectively), 700 rpm, 350 °C, and 4 h.

consumed by secondary reactions, such as the quenching of radicals and the reduction of carboxylic acids and aldehydes. However, the amount of CO increased obviously, accompanied by some unknown gases, such as alkanes and alkenes. The liquid products derived from the liquefaction of Miscanthus and blank experiments were extracted, distilled, and diluted with methanol before analysis. The properties of bio-oil and liquid product, including density, pH, viscosity, and product distribution, were measured and concluded in Tables 2 4278

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The surface reaction mechanism for the decomposition of methanol on Cu has been conducted over decades, but researchers are still not sure whether the effective Cu state is Cu0,31 Cu+,32 or Cu0−Cu+33 couples. According to Reitz,31 CuO was reduced to Cu only after O2 in the system was totally consumed, which is in consistent with the observation of the quick reduction of CuO in our experiments performed in N2. Reitz also remarked that the reforming activity of methanol with principal selectivity to H2 is observed over Cu0, conforming to the increasing pressure observed during the reaction from 1 to 4 h. The probable decomposition mechanism of methanol on Cu was described in Figure 10. According to the conclusion by Fisher and Bell,34 there should be surface oxygen on Cu to react with methanol to form methoxide species and then formaldehyde generated via the dehydration of methoxide. Formaldehyde could subsequently react with adsorbed oxygen, methoxides, and polymerizes or desorb into the gas phase. Fisher and Bell also reported that most methoxide species were decomposed to CO2 and H2, while only a little formed formaldehyde (could be decomposed to CO and H2 later) and H2, leading to a CO2/CO ratio of 20.9:5.6, which was coincident with the CO2/CO ratio in blank B (24:7.08). However, the increase of the CO amount in the liquefaction of Miscanthus indicates that the decomposition of methanol proceeded according to the mechanism on the right side in Figure 10, which implies that there were not enough unoccupied reaction sites on the surface of Cu. The generation of CO may be restrained by the larger catalyst surface area and dosage. 3.5. Cu-Based Catalytic Applications for Biomass Liquefaction. To verify the applicability of this in situ hydrogenation−liquefaction strategy on other kinds of biomasses, herbaceous biomass, including bagasse, Miscanthus, and bamboo, and woody biomass, such as manioc, pine, and cedar, were chosen and liquefied via the same process. The percentage of conversions of various feedstocks shown in Figure 11 are quite high, indicating that the strategy worked efficiently in the liquefaction of herbaceous and woody biomasses. However, the liquid product from woody feedstock was more viscous than that from herbaceous biomass and rather difficult to filtrate (Figure 12), which corresponded with the “heavy oil” mentioned in reports by Zou et al. and Ye et al.9,15 The formation of heavy oil was attributed to the high lignin content of woody biomass. In conclusion, this strategy showed excellent liquefaction capacity on herbaceous and woody biomasses. Moreover, the in situ hydrogenation−liquefaction strategy does not need a highcost or manual desiccation process, except natural drying, which ensures its application on potential and water-rich biomass, such as microalgae.

Figure 12. Heavy oil generated during the liquefaction of cedar.

and 3. In comparison to the blank liquid product, the liquefaction of Miscanthus brought higher density and viscosity and similar pH values. The main components of blank liquid product were molecules, such as propylene glycol, methyl methoxyacetate, methyl 2-methoxypropionate, 2-methyl-3-oxobutanoic acid ethyl ester, which were derived from glycerol and methanol. The addition of Miscanthus led to the generation of many other kinds of alcohols, esters, and ethers. Besides, phenols, such as o-methylphenol, mequinol, 2,6-dimethylphenol, 2,6-dimethoxyphenol, and 2-methoxy-4-propylphenol, were also verified, which originated from the decomposition of lignin in Miscanthus. There was no glycerol detected in the non-extracted liquid product, suggesting that glycerol was consumed completely to react with feedstock and methanol to form glycerol-derived alcohols, ethers, and esters. Differently, most of the methanol still existed as solvent in the product, while only a little methanol was decomposed to H2 or reacted with glycerol and biomass. 3.4. Characterization of the Cu Catalyst. Unless there were other metal oxides, CuO was reduced to Cu quickly (in less than 1 h after zero reaction time) during the reaction, as shown in the XRD spectra in right panel of Figure 6. The characteristic peaks of CuO at 2θ = 37°, 39°, and 49° disappeared, while peaks at 2θ = 43°, 50°, and 74° appeared after reaction, which indicated the generation of Cu. The obvious broad peak between 2θ = 15° and 30° in Cu-380-4 is attributed to the amorphous carbon content adsorbed on the surface of the Cu catalyst. The scanning electron microscopy (SEM) image in Figure 7A shows that CuO was with a kind of clumpy and strip appearance. There were nano-Cu particles generated in the blank experiment with a diameter of 500 nm. However, Cu generated in the liquefaction reaction of Miscanthus was dispersed in coke, and there was no obvious spherical or nubby morphology. Cu-350-4 was combusted at 500 °C for 2 h to regenerate crystal CuO particles, as shown in Figures 6 and Figure 7D. The TGA results of Cu-350-2 indicate that the coke was mainly combusted at 320 °C (Figure 8). However, there was still some coke left after combustion (∼10%), which was calculated by TGA and elemental analysis data (Figures 8 and 9). The C and H contents in Cu-300-4, Cu325-4, Cu-350-4, Cu-365-4, Cu-blank-350-4, Cu-350-2, and CuO combusted are summarized in Figure 9. There were less C and H in Cu-325-4 and Cu-350-4 than others, which confirmed that the temperature between 325 and 350 °C was proper for the liquefaction.

4. CONCLUSION In this strategy, we tried to employ the nano-Cu particles generated in situ as a hydrogen generation catalyst for the hydrogenation−liquefaction of biomass. Cu could effectively catalyze the decomposition of methanol into hydrogen at 350 °C, which promoted the liquefaction of Miscanthus. The highest percentage of conversion of Miscanthus of about 95% was obtained at 350 °C with a reaction time of 4 h, much higher than that performed in external H2 or in the presence of other metal oxides. Glycerol played an active role in the decomposition of Miscanthus, leading to the generation of glycerol-derived esters, ethers, and other alkyl phenolics. Other 4279

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woody and herbaceous biomasses could also be effectively liquefied in the system. In conclusion, desirable results have been achieved by hydrogenation alcoholysis liquefaction of biomass via in situ hydrogen generation over a heterogeneous Cu catalyst. This strategy opens an attractive door to the environmentally friendly and economical thermochemical conversion of biomass under mild conditions.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-592-2880701. E-mail: xianhai.zeng@ xmu.edu.cn. *Telephone/Fax: +86-592-2880702. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2010CB732201), the Key Research Program from Science and Technology Bureau of Xiamen City in China (3502Z20131016), the National Natural Science Foundation of China (21106121), the Fundamental Research Funds for the Central Universities (2010121077), the Key Program for Cooperation between Universities and Enterprises in Fujian Province (2013N5011), and the Fundamental Research Funds for the Xiamen University (201312G009). The authors are also grateful to Dr. Michael K. Danquah for his help in refining the manuscript.

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