Ind. Eng. Chem. Res. 2010, 49, 2573–2580
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Catalytic Upgrading of Biomass Fast Pyrolysis Vapors with Pd/SBA-15 Catalysts Qiang Lu, Zhe Tang, Ying Zhang,* and Xi-feng Zhu* Anhui ProVince Key Laboratory of Biomass Clean Energy, UniVersity of Science and Technology of China, Hefei 230026, P.R. China
Palladium supported on SBA-15 catalysts were developed and employed for catalytic cracking of biomass fast pyrolysis vapors using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The Pd/SBA-15 catalysts displayed prominent capabilities to crack the lignin-derived oligomers to monomeric phenolic compounds and further convert them to phenols without the carbonyl group and unsaturated C-C bond on the side chain. Moreover, the catalysts almost completely eliminated the anhydrosugar products and decarbonylated the furan compounds. They also significantly decreased the linear aldehydes and dehydroxylated the linear ketones. In addition, the catalysts slightly decreased the acids, while methanol and hydrocarbons were increased. The above catalytic capabilities of the Pd/SBA-15 catalyst were enhanced with the increase of Pd content from 0.79 wt % to 3.01 wt %. 1. Introduction Fast pyrolysis of biomass is one of the most promising technologies for the utilization of renewable biomass resources, and it has gained extensive attention in recent years.1-4 The pyrolysis liquids, known as bio-oils, are regarded as new liquid fuels or resources of various valuable chemicals, but the successful application of bio-oils is difficult because of their unique chemical composition.5,6 Components in bio-oils, such as the water, acids, aldehydes, unsaturated compounds, and oligomers, result in the various undesirable properties of biooils.7 Therefore, it is necessary to upgrade bio-oils to eliminate those undesirable compounds or convert them to desirable ones. Many catalysts and methods have been developed for the upgrade of volatile compounds,8 but much less attention has been given to the large molecular oligomers in bio-oils. The oligomers can be derived from both holocellulose (cellulose and hemicellulose) and lignin.9-12 The lignin-derived oligomers account for 13.5-27.7% of bio-oils on a water-free basis,13,14 and they are known as pyrolytic lignins. The average molecular weight of the pyrolytic lignins was between 650 and 1300 g/mol.15,16 It is believed that pyrolytic lignins contribute significantly to the instability and high viscosity of bio-oils.17-19 Moreover, they could bring great difficulty to the processing of bio-oils because of their nonvolatility and thermal instability. During the catalytic upgrading process, at a temperature above 100 °C, they would rapidly polymerize to form coke on the catalyst surface and subsequently cause the deactivation of the catalyst. However, it is not appropriate to simply remove the oligomers because they significantly contribute to the heating value of bio-oils due to their low oxygen content; hence, it is better to convert them to stable monomeric compounds, which seems to be the primary task for catalytic upgrading of biooils. Several techniques have been applied to upgrade bio-oils. One of them is the in situ catalytic cracking of pyrolysis vapors immediately after the fast pyrolysis of biomass. In recent years, mesoporous catalysts, whose pore sizes are much larger than that of traditional zeolites, have attracted great interest for their potential to convert large molecular oligomers. Since the * To whom correspondence should be addressed. Tel.: 86-551-3603463. Fax: 86-551-360-6689. E-mail:
[email protected]; xfzhu@ ustc.edu.cn.
discovery of M41S by Mobil’s scientists in 1992, a series of ordered mesoporous silicate materials (M41S, SBA, MSU, HMS, etc.) have been successfully synthesized. However, these mesoporous materials are pure silica and lack catalytic activity. To create catalytic active sites, typically an appropriate metal is incorporated into the framework by doping (one-pot synthesis) or postgrafting. The MCM-41 catalysts as well their derivatives modified by incorporation of various metals (Al, Fe, Zn, Cu, etc.) have been investigated in a series of studies.20-24 The mesoporous SBA-15 and MSU have also been tested.21,25-27 To test the catalytic activity of various catalysts, the Py-GC/ MS experiment is a powerful tool, as it allows direct analysis of catalytic products. Several Py-GC/MS studies have already been carried out using different mesoporous catalysts. According to Adam et al.,20 the utilization of Al/MCM-41 catalysts eliminated levoglucosan and decreased large phenols, while increasing acetic acid, furans, small phenols, and hydrocarbons. Pattiya et al. found that the use of Al/MCM-41 and Al/MSU-F catalysts decreased lignin-derived products and increased aromatic hydrocarbons and acetic acid.26 It was confirmed in our previous study that Al/SBA-15 exhibited catalytic effects similar to that of Al/MCM-41, and moreover, the catalytic activity of the Al/SBA-15 was enhanced with the decrease of Si/Al ratios.27 Among the various mesoporous silicate materials, SBA-15 is by far the largest pore size mesoporous material with highly ordered hexagonally arranged mesochannels, thick walls, adjustable pore size from 3 to 30 nm, and high hydrothermal and thermal stability. In this study, palladium supported on SBA15 was developed for the upgrading of the biomass fast pyrolysis products. The catalytic and noncatalytic products from the PyGC/MS experiments were compared to reveal the catalytic capabilities of the Pd/SBA-15 catalysts. 2. Experimental Section 2.1. Biomass Material. The biomass material used in the study was poplar wood. Prior to experiments, the poplar wood was dried and ground in a high speed rotary cutting mill. The particles with a size of 0.125-0.3 mm were selected for experiments. The component composition of the poplar wood was analyzed according to the method proposed by Ranganathan et al.,28 and the results were cellulose 49.70%, hemicellulose 24.10%, lignin 23.55%, extractive 2.22%, and ash 0.43%.
10.1021/ie901198s 2010 American Chemical Society Published on Web 02/08/2010
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Figure 1. The placement of the catalyst and poplar wood in the pyrolysis quartz tube.
2.2. Catalyst Preparation. The SBA-15 catalyst used in this study was synthesized according to the reference using triblock copolymer EO20PO70EO20 (Pluronic P123) as organic template.29 In a typical procedure, 8.0 g of P123 (MW ) 5800, Aldrich) was dispersed in 60.0 g of water and 240 g of 2 M HCl solution at 40 °C with stirring, followed by the addition of 17.0 g of tetraethyl orthosilicate (TEOS, Aldrich). The mixture was continuously stirred at 40 °C for 24 h and then crystallized in a Teflon-lined autoclave at 100 °C for 2 days. Afterward, the solid product was centrifuged, filtered, washed with deionized water, and then dried in air at room temperature. Finally, the solids were calcined at 550 °C in air for 5 h to remove template and get the final SBA-15 catalyst. Pd/SBA-15 was prepared by the following procedure. Palladium(II) nitrate hydrate (99.9%, Strem Chemicals) was dissolved in deionized water at two different concentrations, followed by the addition of a certain amount of SBA-15 with stirring. Aqueous ammonia (25 wt %) was added dropwise to the mixture to a final pH around 9,30 and the mixture was stirred for 12 h. The resulting solid was filtered and dried at 100 °C for 3 h. Afterward, the dried product was reduced in hydrogen and nitrogen atmosphere at 280 °C for 3 h using a heating rate 1.0 °C/min to obtain the ultimate two Pd/SBA-15 catalysts. The flow rate of hydrogen and nitrogen was 10 mL/min and 100 mL/min, respectively. 2.3. Catalyst Characterization. The Pd content of the catalysts was measured by inductively coupled plasma and an atomic emission spectroscopy (ICP-AES) system (Atomscan Advantage of Thermo Jarrell Ash Corporation, Franklin, MA). X-ray diffraction (XRD) analysis was conducted with a Philips X’pert PRO X-ray diffractometer, which employed Cu KR radiation (λ ) 0.15418 nm). The data were recorded over the 2θ ranges of 0.5-5° and 10-70°. Nitrogen adsorption/desorption isotherms at 77 K were measured using a Micromeritics ASAP 2020 analyzer. The surface areas were determined using the Barrett-Emmett-Teller (BET) method. The pore volumes and pore size distribution were determined by the Barrett-Joyner-Halenda (BJH) method from the adsorption branch of the isotherms. 2.4. Py-GC/MS Experiments. Pyrolysis was performed using a CDS Pyroprobe 5250 pyrolyser (Chemical Data Systems). During the preparation of the experimental samples, the pyrolysis tube was successively filled with a quartz rod, some quartz wool, 0.30 mg of catalyst, some quartz wool, 0.20 mg of poplar wood, some quartz wool, 0.30 mg of catalyst, and some quartz wool. The placement of the catalyst and the poplar wood in the quartz filler tube is shown in Figure 1. An analytical balance with a readability of 0.01 mg was used, and the weight of biomass and catalyst (a layer) was strictly controlled to be exactly 0.20 mg and 0.30 mg, respectively. The
catalyst was placed at both sides of the poplar wood and functioned as a fixed bed, so that all the pyrolysis vapors would pass through the catalyst layer. The poplar wood and catalyst were separated by the quartz wool, to ensure that the pyrolysis of the poplar wood would not be influenced by the catalysts. The pyrolysis temperature was set at 600 °C and held for 10 s, with a heating rate of 20 °C/ms. Because of the poor thermal conductivity of biomass materials, the actual biomass pyrolysis temperature would be lower than the set value. Fortunately, the quantity of biomass and catalyst was very small, so they could be heated very rapidly and uniformly. It was reported that the actual pyrolysis temperature of the biomass was around 500 °C.26 The pyrolysis vapors were analyzed by GC/MS (Thermo Scientific, Trace DSQ II). The injector temperature was kept at 300 °C. The chromatographic separation was performed using a TR-35MS capillary column (30 m × 0.25 m i.d., 0.25 µm film thickness). Helium (99.999%) was used as the carrier gas with a constant flow rate of 1 mL/min and a 1:80 split ratio. The oven temperature was programmed from 40 °C (3 min) to 200 °C with a heating rate of 4 °C/min and then to 280 °C (4 min) with a heating rate of 12 °C/min. The temperature of the GC/MS interface was held at 280 °C, and the mass spectrometer was operated in EI mode at 70 eV. The mass spectra were obtained from m/z 20 to 400 with a scan rate of 500 amu/s. Identification of chromatographic peaks were achieved according to the NIST MS library and the literature data of bio-oils. For each catalyst, the experiments were conducted at least three times to confirm the reproducibility of the reported procedures. For each identified product, the average values of the peak area and peak area% were calculated and used for analysis. In addition, the standard deviation values were also calculated. It is well-known that the GC/MS technique could not give the quantitative analysis of the compounds. However, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area% is linear with its content. Therefore, for each product, its average peak area value obtained under different reaction conditions can be compared to reveal the changing of its yields, and the peak area% value can be compared to show the changing of its relative content among the detected products. 3. Results and Discussion 3.1. Catalyst Properties. Figure 2 shows the N2 adsorptiondesorption isotherms of the SBA-15 and Pd/SBA-15 catalysts. The isotherm of the SBA-15 illustrated typical irreversible type IV adsorption isotherms with a H1 hysteresis loop. A welldefined step occurred at the relative pressure (p/p0) range of 0.6-0.8, corresponding to capillary condensation of N2, implying this material had very regular mesoporous channels. The two Pd/SBA-15 catalysts also exhibited type IV adsorption isotherms like those of SBA-15, suggesting that they retained ordered structure after the postincorporation process. According to the N2 adsorption-desorption isotherms, the textural properties of the SBA-15 and Pd/SBA-15 catalysts can be calculated and are given in Table 1. On the basis of the Pd content, the two Pd/SBA-15 catalysts are designated as Pd/SBA-15(1) and Pd/SBA-15(3), respectively. After the Pd incorporation, the surface area and pore volume were decreased because of the occupation of Pd particles in the pore channels. The small-angle and wide-angle XRD patterns of the SBA15 and Pd/SBA-15 catalysts are shown in Figure 3. In the smallangle range, the XRD pattern of SBA-15 exhibited a prominent diffraction peak at 2θ ) 0.90° and two other weak peaks at 2θ
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Figure 2. N2 adsorption-desorption isotherms of the SBA-15 and Pd/SBA15 catalysts. Table 1. Textural Properties and Composition of Catalysts
catalyst SBA-15 Pd/SBA-15(1) Pd/SBA-15(3)
average BET pore pore surface micropore volume diameter 2 2 3 area (m /g) are (m /g) (cm /g) (nm) 642.3 427.8 397.2
55.9 28.3 28.2
1.04 1.01 0.98
6.9 8.8 9.2
composition pure silica 0.79 wt % Pd 3.01 wt % Pd
) 1.54° and 1.77°, which could be indexed to (100), (110), and (200) diffraction peaks, representing a 2D hexagonal mesostructure with space group p6 mm. The peak intensity of the two Pd/SBA-15 catalysts decreased significantly. Because the structure of the mesoporous molecular was intact after modification, the decrease of the diffraction peaks should be caused by incorporation and blocking of the channels with Pd crystallites. Moreover, the diffraction peaks of the Pd/SBA-15 catalysts shifted to higher 2θ degrees, which might result from the constriction of the frameworks.31 In the wide-angle range, Pd/SBA-15(1) only showed a broad diffuse peak of the amorphous SiO2; no diffraction peaks from Pd were detected, which might be due to too low Pd content and the good dispersion of Pd over the parent SBA-15. Pd/ SBA-15(3) showed a broad diffraction peak at around 2θ ) 40°, indexed as the (111) reflection of the cubic Pd phase. The crystallite size of Pd could be calculated by the Scherrer formula, to be about 2 nm. 3.2. Catalytic Effects on the Distribution of the Pyrolytic Products. Biomass fast pyrolysis vapors were composed of volatile compounds and nonvolatile oligomers, and GC/MS was only able to determine the volatile compounds. It is to be noted that the GC-detectable compounds should account for more than 50 wt % of the total organics of bio-oil.12,32 Figure 4 shows the typical ion chromatograms from catalytic and noncatalytic experiments. More than 100 peaks were displayed on the ion chromatograms, the identified pyrolytic products of the poplar wood were similar to those reported in the literature about the chemical composition of bio-oils,32-34 and detailed results have been reported previously.20,26,27,35 Some important products were numbered on the ion chromatograms and listed in Table 2.
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For a general overview of the ion chromatograms, the composition and yield of the pyrolytic products were altered considerably after catalysis. In order to show the changes of the product composition, the products were classified into seven groups, and the result is presented in Table 3. It is seen that the phenolic compounds were greatly increased by the two Pd/SBA15 catalysts, while the anhydrosugar products were almost completely eliminated by all the three catalysts. One of the drawbacks of the Py-GC/MS experiment is that it does not allow product collection, and thus the exact bio-oil yield could not be determined. However, it does allow a primary estimate of the yield changes of all detected products, through the comparison of the total chromatographic peak area obtained under different reaction conditions, and the result is shown in Figure 5. Among all the detected products, the phenolic products were mainly derived from lignin, while the other products were mainly derived from holocellulose. In order to characterize the yield changes of the lignin-derived products and holocellulosederived products, the total peak area of all the phenolic products is also given in Figure 5. During the catalytic process, the yield of volatile products would be influenced by the catalysts through the following two ways. On the one hand, the catalysts would cause the cracking of products to permanent gases or polymerize them to cokes or chars, which decreased the volatiles. On the other hand, the catalysts would promote the cracking of nonvolatile oligomers into monomeric volatile compounds, which increased the volatiles. The final volatile yield was determined by the above two ways. According to Figure 5, the total peak area was decreased by all three catalysts, indicating that the yield of the catalytic bio-oils would be reduced. To be more specific, the product yields from both the lignin and the holocellulose decreased after catalysis by SBA-15. However, this was not the case for the two Pd/SBA-15 catalysts. The phenolic products were increased considerably after catalysis, while the holocellulose-derived products were decreased more significantly than that of SBA-15. Fast pyrolysis of lignin generated volatile monomeric phenols and nonvolatile pyrolytic lignins. During the catalytic process, it is impossible that the monomeric phenols were deeply cracked to form permanent gases or deoxygenated to form aromatic hydrocarbons, and therefore the reduction of the phenols by SBA-15 might be attributed to their polymerization to form cokes or chars. The increase of phenols by the two Pd/SBA-15 catalysts should be attributed to pyrolytic lignin conversion to monomeric phenols, which clearly indicated the cracking capability of the Pd/SBA-15 catalysts. In the following two sections, the changes of the specific products will be discussed in detail to reveal the catalytic activity of the Pd/SBA-15 catalysts. 3.2.1. Catalytic Effects on the Phenolic Compounds. The catalytic effects on major phenolic products are shown in Figures 6-9. According to the changes of these compounds, they were classified into three groups. The first group was the phenolic compounds that contained no carbonyl group and no unsaturated C-C bond on the side chain, including all the compounds shown in Figure 6 and Figure 7a, as well as some compounds in Figures 8 and 9. These compounds were all increased after catalysis, and Pd/SBA-15(3) performed much better to produce these products than Pd/SBA-15(1). Notably, some of the products, which were not detected in noncatalytic pyrolytic products, were formed after catalysis, such as 4-ethyl-phenol, 4-propyl-phenol, and 2-methoxy-4-propyl-phenol. The second group was the phenolic compounds that contained no carbonyl group but an
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Figure 3. Small-angle and wide-angle XRD patterns of SBA-15 and Pd/SBA-15 catalysts.
Figure 4. Ion chromatograms from fast pyrolysis of the poplar wood and catalytic cracking of the pyrolysis vapors by different catalysts.
unsaturated C-C bond on the side chain, including all the compounds shown in Figure 7b and some compounds in Figures 8 and 9. They were all increased greatly by the Pd/SBA-15(1) catalyst but decreased with the increase of Pd content from 0.79 wt % to 3.01 wt %. The third group was the phenolic compounds that contained the carbonyl group, including all the compounds shown in Figure 7c and some compounds in Figures 8 and 9. They were all significantly decreased or completely eliminated after catalysis, and Pd/SBA-15(3) was more effective to reduce these products than Pd/SBA-15(1).
On the basis of the above results, it is seen that the two Pd/ SBA-15 catalysts were effective to remove the carbonyl group from the phenolic compounds, indicative of their decarbonylation activity. Moreover, with the increase of the Pd content from 0.79 wt % to 3.01 wt %, the phenolic compounds of the second group were decreased to very low content, while the compounds of the first group were increased remarkably and became predominant. Many reactions might contribute to the catalytic effects, and the reduction of the unsaturated C-C bond suggested that Pd/SBA-15(3) might have some hydrotreating capability. Further studies are required to reveal the mechanism involved in this process. In regard to previous Py-GC/MS experiments,20,26,27 none of the mesoporous catalysts showed such promising effects as the Pd/SBA-15 catalysts, for the upgrade of lignin-derived products. Phenols are valuable and useful chemicals, because they are widely used for the production of adhesives, pharmaceutical, dyes, and food additives. Therefore, catalytic bio-oils might be used for the recovery of these phenols. In addition, catalysis would also improve the fuel properties of the catalytic bio-oils. Because the viscosity would be reduced due to the conversion of pyrolytic lignins to monomeric phenols, the oxygen content would be decreased owing to the decarbonylation effect, and the stability would be improved owing to cracking, decarbonylation, and reduction of the unsaturated C-C bond. 3.2.2. Catalytic Effects on Other Products. Besides the phenolic products, the catalytic effects on other major products are shown in Figures 10-12. In these figures, only the peak area results are given. It is well-known that two competing pathways are mainly responsible for the primary decomposition of holocellulose, the depolymerization, and the ring scission.36 The depolymerization process formed various anhydrosugar products, and levoglucosan was predominant. It is clearly shown in Figure 10 that levoglucosan was significantly decreased after catalysis and agreed well with previous studies that levoglucosan was easily converted by catalysts.20,27 Alot of furans were detected in the pyrolytic products, and they are known to be formed as dehydration products of
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Table 2. Main Pyrolytic Products from Fast Pyrolysis of Poplar Wood no.
RT
compound
no.
RT
compound
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
2.10 2.13 2.39 2.61 3.00 3.36 3.45 3.51 3.58 4.28 4.78 4.99 5.56 6.91 8.63 9.22 10.24 11.53 15.15 16.32 18.33 19.27 20.16 21.14
methanol acetaldehyde furan acetone 2-methyl furan acetic acid 2,3-butanedione 3-buten-2-one hydroxyacetaldehyde benzene propanoic acid 2-butenal 1-hydroxy-2-propanone toluene 1-hydroxy-2-butanone acetoxyacetic acid methyl pyruvate furfural 1,2-cyclopentanedione phenol 2H-pyran-2,6-(3H)-dione 2-methyl-phenol 4-methyl-phenol 2-methoxy-phenol
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
23.66 24.78 26.68 26.86 27.64 29.51 30.48 30.52 31.75 32.46 34.00 34.07 34.44 36.59 36.61 38.26 38.34 42.25 42.50 44.06 44.10 44.22 44.71 48.06
4-ethyl-phenol 2-methoxy-4-methyl-phenol 3-methyl-2,4-(3H,5H)-furandione 4-propyl-phenol 2-methoxy-4-ethyl-phenol 2-methoxy-4-vinylphenol 2-methoxy-4-propyl-phenol 2-methoxy-4-(2-propenyl)-phenol 2,6-dimethoxy-phenol 2-methoxy-4-propenyl-phenol (E)-2-methoxy-4-propenyl-phenol 4-hydroxy-3-methoxy-benzaldehyde 1,2,4-trimethoxy-benzene 5-tert-butyl-1,2,3-benzenetriol 1-(3-hydroxy-4-methoxyphenyl)-ethanone levoglucosan 3,5-dimethoxyacetophenone 4-allyl-2,6-dimethoxy-phenol 4-hydroxy-3,5-dimethoxy-benzaldehyde 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol 4-hydroxy-2-methoxycinnamaldehyde 1-(2,6-dihydroxy-4-methylphenyl)-1-butanone 3,5-dimethoxy-4-hydroxycinnamaldehyde
Table 3. Composition of Pyrolytic Products before and after Catalysis (peak area%) catalyst
phenols
carbonyls
acids
furans
anhydrosugars
hydrocarbons
others
SBA-15 Pd/SBA-15(1) Pd/SBA-15(3)
29.7 27.8 49.9 55.9
15.4 20.1 10.2 8.3
9.6 14.4 9.7 8.3
8.9 12.9 9.7 7.4
11.4 0.5 0.9 0.5
0.1 0.4 1.5 5.5
6.6 9.1 7.8 4.5
carbohydrates.37-39 After catalysis, the compounds that contained the carbonyl groups, especially the aldehyde group, were decreased dramatically. For example, the furfural was decreased from 2.1% in the noncatalytic pyrolytic products to 0.5% in the Pd/SBA-15(3)-catalyzed products. Some other compounds such as 2,5-furandicarboxaldehyde and 5-hydroxymethyl-furfural were completely eliminated. Meanwhile, the two lightest furan compounds, furan and 2-methylfuran, were increased considerably. The peak area% of furan reached as high as 4.2% after catalysis by Pd/SBA-15(3). The results suggested that the Pd/SBA-15 catalysts cracked the furan compounds through decarbonylation to produce light ones, and Pd/SBA-15(3) performed better than Pd/SBA-15(1). The pyrolytic ring scission of holocellulose formed various light products, mainly linear carbonyls (Figure 11). The catalysts displayed different effects on the aldehydes and ketones. In regard to the aldehydes, hydroxyacetaldehyde and other alde-
Figure 5. Total chromatographic peak area of all the identified pyrolytic products and all the phenolic products.
hydes were significantly decreased by Pd/SBA-15(1), but acetaldehyde (2.0%) was increased. Whereas only a small amount of acetaldehyde (0.6%) was detected after catalysis by Pd/SBA-15(3), all the other aldehydes were completely eliminated. As a result, the linear aldehydes were decreased greatly after catalysis. In the case of the ketones, the compounds that contained the hydroxyl group, such as the 1-hydroxy-2-
Figure 6. The catalytic effects on the light phenolic products.
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Figure 7. The catalytic effects on the guaiacyl-lignin products.
Figure 8. The catalytic effects on the syringyl-lignin products.
propanone and 1-hydroxy-2-butanone, were decreased dramatically. Meanwhile, some other ketones without the hydroxyl group, such as acetone and 3-buten-2-one, were increased considerably. On the whole, the content of the linear ketones was increased after catalysis. Attention should be given to the carbonyls, because carbonyls, especially aldehydes, are mainly responsible for the aging reactions and instability of bio-oils. In addition to the linear aldehydes, some furan and phenolic products also contained the aldehyde group. It is clearly indicated in the above results that the total aldehydes were decreased to a very small content after catalysis, which is obviously beneficial for improving the stability of the catalytic bio-oils. Fast pyrolysis of poplar wood produced many acids, especially acetic acid. The presence of acids will negatively affect the fuel properties of bio-oils.40,41 It is seen from Figure 12 that acetoxyacetic acid was decreased remarkably after catalysis, while acetic acid was only slightly decreased. Moreover, propanoic acid (not shown) was increased by the two Pd/SBA15 catalysts, but its content was low (
