Hydrotreatment Upgrading of Bio-oil from Torrefaction of Pubescens in

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Article Cite This: ACS Omega 2018, 3, 4836−4846

Hydrotreatment Upgrading of Bio-oil from Torrefaction of Pubescens in Alcohol over Pd/NbOPO4 Jindong Li,† Xiaoyan Lv,‡ Yue Wang,‡ Qiuxing Li,‡ and Changwei Hu*,†,‡ †

College of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China Key Laboratory of Green Chemistry and Technology, MOE, College of Chemistry, Sichuan University, Chengdu 610064, P. R. China

‡

S Supporting Information *

ABSTRACT: Pd/NbOPO4 multifunctional catalyst was prepared and used for catalytic hydrotreatment upgrading of biooil, which was obtained from low-temperature torrefaction (LTT) of pubescens and contained mainly lignin oligomers. The upgrading temperatures were investigated at 220−280 °C. Pd/NbOPO4 exhibited good performance for the effective depolymerization of oligomers with increasing temperature. It was revealed that both Brönsted acid sites and Lewis acid sites existed on NbOx, which contributed to the cleavage of the C− O−C bond. Furthermore, esterification, hydrogenation, and O-alkylation of monomeric derivatives also occurred simultaneously during the depolymerization process. After hydrotreatment upgrading, the average molecular weight of bio-oil decreased from Mw = 320 Da (Mn = 298 Da) to Mw = 273 Da (Mn = 254 Da) and the bio-oil quality was improved dramatically. The oxygen content decreased from 29.53 to 9.78 wt %. The upgraded bio-oil obtained at 280 °C had a heating value of 40.48 MJ kg−1, which was much higher than that of the original bio-oil (26.96 MJ kg−1).

1. INTRODUCTION Biomass has been viewed as an alternative resource of fossil fuel and can be used to produce high-quality fuels and useful chemicals1−3 to overcome the rapid decrease of mineral resource. Pyrolysis is one of the effective methods to convert the low-fuel-density biomass to high-quality bio-oil. However, the bio-oil obtained from high-temperature pyrolysis generally contains hundreds of compounds and a large amount of oxygenated compounds, which results in undesirable properties such as low thermal and chemical stability, low heating value, immiscibility with the fossil fuels, and high acidity,4 thus making it difficult to be directly used. Therefore, several kinds of strategies, including pretreatment of the raw biomass,5 upgrading of the bio-oil obtained,6 catalytic pyrolysis,7 etc., are used to improve the quality of bio-oil obtained. Among them, pretreatment of raw biomass and upgrading of bio-oil are widely used. Torrefaction, carried out at a relatively lower temperature than that for pyrolysis,8 is generally used to pretreat the raw biomass material to change its composition and property and then to improve to a certain extent the quality of bio-oil obtained from further pyrolytic conversion.9,10 Zheng et al.11 studied the torrefaction effect on pyrolysis and found the following enhancements of bio-oil obtained after torrefaction: relatively low water content, slight increase in pH, and obvious increase of heating value. Chen et al.12 also reported that torrefaction led to a decrease in the water content of pyrolysis © 2018 American Chemical Society

bio-oil, and the heating value increased with lower acid content and higher phenol content. However, it is interesting to note that low-temperature torrefaction (LTT) also produces certain amount of bio-oil products composed of mainly oligomers,8 the utilization of which needs to be considered to avoid waste of resource and prevent pollution. The upgrading of crude bio-oil and different model compounds of bio-oil with various catalysts has been performed by catalytic cracking and hydrogenation. Generally, catalytic cracking using zeolites (HZSM-5, Al2O3-SiO2, SAPO-5, SAPO11) and some other compounds13,14 is being conducted under mild pressure and at high temperatures (>400 °C) in the absence of hydrogen. Unfortunately, this process often suffers from severe coke deposition and fast catalyst deactivation due to high-temperature operation. Hydrodeoxygenation (HDO) is found to be an efficient catalytic upgrading method, which can remove oxygen atoms in bio-oil via releasing CO, CO2, and H2O in the presence of H2 over heterogeneous catalyst and then improve the quality and stability of bio-oil.15 Heterogeneous catalysts generally included noble metals (Pt, Pd, Ru), transition metals (Ni, Mo, Co, W), and bimetallic catalysts (Pt−Pd, Rh−Pd, CuNi, NiMo, NiW, NiCo, CoMo) among many others supported on various carriers, such as Al2O3, ZrO2, Received: January 29, 2018 Accepted: April 24, 2018 Published: May 2, 2018 4836

DOI: 10.1021/acsomega.8b00180 ACS Omega 2018, 3, 4836−4846

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ACS Omega TiO2, MgO, mesoporous zeolites, and activated carbons,13,16,17 which all showed excellent performance in upgrading bio-oil. For example, sulfide CoMo/γ-Al2O 3 and NiMo/γ-Al2O3 catalysts, used for hydrodesulfurization (HDS) in conventional petroleum industry, are applied in HDO to upgrade bio-oil and effectively avoid excessive coking. Unfortunately, sulfide catalysts have disadvantages such as contamination of the final products due to the introduction of sulfur into the system.18,19 Furthermore, Al2O3 is also unsuitable for HDO because alumina supports tend to be transformed into boehmite by crystallization in the presence of water vapor at elevated temperature (140−380 °C)20 and then blocked the active sites. Generally, despite being expensive, noble-metal catalysts exhibit higher activities than non-noble-metal catalysts in HDO of bio-oil.21 Moreover, the acidity of supports on the catalysts is also crucial to deoxygenation of bio-oil.22 Strong acid sites are favorable for HDO via dehydration or ringopening reaction, and high Brönsted acid sites concentration on the solid acid catalyst would benefit C−O bond cleavage reaction rate.23,24 Furthermore, to reduce the formation of coke and prolong the lifetime of the catalyst, organic solvent was added to the bio-oil in HDO reaction.25,26 The solvent can not only facilitate the dissolution of relatively high-molecularweight reactants/intermediates but also act as a co-reactant, stabilizing the products and sometimes the reaction system. Especially, the use of ethanol as environmentally friendly solvent was widely reported in the literature. Peng et al.27 reported that the contents of carboxylic acids can effectively be reduced with ethanol as solvent. Oh et al.28 found that with a significant decrease of acidity, the heating value of the bio-oil increased from 17.3 to 27.8 MJ kg−1. That is to say, ethanol is a good agent to decrease the content of acids and produce esters by esterification under mild conditions.29 Because bio-oil has complicated compositions, including hundreds of compounds, most of which are oligomers, and hence the upgrading is proved to be very difficult.30,31 However, the bio-oil obtained from low-temperature torrefaction is relatively simple, upgrading of which might provide clues for the upgrading of high-temperature pyrolysis bio-oil. Nevertheless, the study of the upgrading of liquid products from torrefaction pretreatment is rarely reported. The purpose of this study is to upgrade the bio-oil obtained from the lowtemperature torrefaction (LTT) of pubescens with the mesoporous niobium phosphate-supported Pd multifunctional catalyst and investigate the depolymerization of oligomers in the hydrotreatment of bio-oil under mild conditions (220−280 °C and 500 psi hydrogen pressure) in alcohol in an attempt to provide way to utilize raw biomass materials to their fullest, avoiding the waste of raw biomass and preventing the possible pollution from biomass conversion.

Figure 1. N2 absorption−desorption isotherms and pore size distribution of NbOPO4 and Pd/NbOPO4.

meanwhile the Brunauer−Emmett−Teller (BET) surface decreased from 267.62 to 204.54 m2 g−1 (Table S1). The X-ray powder diffraction (XRD) patterns of samples are displayed in Figure 2. It can be seen that NbOPO4 only shows

Figure 2. XRD patterns of catalysts: (a) NbOPO4, (b) Pd/NbOPO4, (c) reduced Pd/NbOPO4, and (d) used Pd/NbOPO4.

two board peaks at 15−40 and 40−60° and no diffraction lines of crystallized Nb2O5 or NbOPO4 were detected, indicating the glassy nature of the mesoporous niobium phosphate. There was only one diffuse diffraction peak at 33.5° ascribed to PdO (1, 0, 1) on Pd/NbOPO4, and no other apparent diffraction peaks were seen, which indicated that the PdO particle was small. When the catalyst was reduced, the diffraction peak for PdO disappeared, whereas that for metal Pd appeared. However, after the catalyst was used, the intensity of the peaks ascribed to Pd became higher, which indicated that Pd particles aggregated after reaction. The temperature-programmed desorption of ammonia (NH3-TPD) profiles and the amounts of acid sites are shown in Figure 3 and Table 1, respectively. It can be seen that the total desorption curves and fitted curves for different-strength acid sites of the two samples exhibited similar shape (Figure S1). Pd/NbOPO4 showed a broad peak from 100 to 600 °C centered at 190 °C, which indicated that the surface acid strength has a wide distribution. Generally, the ammonia desorption peak below 350 °C was attributed to weak acid sites, whereas the part of temperature over 350 °C was attributed to stronger acid sites.32 This illustrated that there were mainly

2. RESULTS AND DISCUSSION 2.1. Characterization of the Catalysts. The textural properties of niobium phosphate and Pd/NbOPO4 were measured by N2 adsorption. The adsorption−desorption isotherms and pore size distribution are shown in Figure 1. Both samples exhibited representative type IV isotherm, and it can be observed that the niobium phosphate was a typical mesoporous material with hysteresis loops intermediated between typical H1- and H2-type in the P/P0 range of 0.40− 0.80. It can be seen that the pore volume showed a little decrease after Pd being supported, which indicated that the PdO particle was certainly deposited in the channels and 4837


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samples also certified the fact that mesoporous niobium phosphate has more strong acid sites (Brönsted and Lewis acid sites). X-ray photoelectron spectroscopy (XPS) results of the surface species on Pd/NbOPO4 catalyst before and after reaction (upgrading temperature, 260 °C) are shown in Figure 5. For Pd/NbOPO4, the peaks corresponding to Pd (3d5/2,3/2)

Figure 3. NH3-TPD spectra of NbOPO4 and Pd/NbOPO4.

Table 1. Amounts of Acid Sites with Different Acid Strengths mmol NH3 g−1 sample

total acid sites

weak acids

medium acid sites

strong acid sites

NbOPO4 Pd/NbOPO4

3.71 3.30

0.46 0.60

1.13 1.23

2.12 1.46

medium acid sites on the Pd/NbOPO4 catalysts. From Table 1, it can be observed that the total acidity of the Pd/NbOPO4 catalyst was 3.3 mmol NH3 g−1, lower than 3.71 mmol NH3 g−1 for NbOPO4. The weak acid sites and medium acid sites increased from 0.46 to 0.60 mmol NH3 g−1 and 1.13 to 1.23 mmol NH3 g−1, respectively. The results implied that the introduction of metal Pd on NbOPO4 support could increase the weak and medium acids but decrease the amount of strong acids. The types of acids on mesoporous niobium phosphate were identified by pyridine-Fourier transform infrared (PyFTIR) (Figure 4). It can be seen that the absorbance bands at

Figure 5. Pd 3d XPS images of catalysts: (a) Pd/NbOPO4 and (b) used Pd/NbOPO4.

at 336.6 and 341.86 eV can be assigned to Pd2+ and those at 337.7 and 342.96 eV can be attributed to Pd4+ after calcination.36 This implied that PdO was the primary oxidation state of Pd/NbOPO4. After hydrotreating reaction, the Pd (3d5/2,3/2) peaks at 334.76 and 340.06 eV can be ascribed to Pd0 37 and those at 336.6 and 341.86 eV can be attributed to Pd2+. It indicated that Pd2+ was clearly reduced after reaction and that Pd0 was the predominant species with a small amount of Pd2+ possibly due to the oxidation by air in the transfer process. 2.2. Properties of Upgraded Bio-oil (UBO). 2.2.1. Analysis of Elemental Composition. The effects of hydrotreating temperature were investigated in the range of 220−280 °C with the same reaction time of 8 h. Table 2 shows the yields of upgraded bio-oil and the corresponding content of organic elements. A small part of volatile products could be evaporated with ethanol, resulting in lower yield of bio-oil. The yield of bio-oil decreased with increasing temperature. However, the yield suddenly increased at 280 °C, which might be due to the fact that more ethanol reacted with small molecular compounds via different reactions such as etherification, which happened under all reaction conditions, and the tendency was more enhanced with elevated temperature. Furthermore, while using the support NbOPO4 as catalyst, there existed more insoluble compounds in solvents, which might be due to the repolymerization catalyzed by strong acid sites of supports, and lower yield was obtained. It implied that the loading of palladium not only provided active metal for hydrogenation, but also adjusted the acidity of the catalyst and then avoided repolymerization in the hydrotreating process. According to the literature,38−40 several possible reactions are shown in Scheme 1. The contents quantified by gas chromatography (GC) of parts of low-molecular-weight products obtained from hydrotreatment of bio-oil in ethanol are shown in Table S2. There were many unidentified peaks on the GC; however, because of

Figure 4. Pyridine-FTIR spectra of NbOPO4 at different temperatures: (a) 150 °C, (b) 300 °C, and (c) 400 °C.

1540 and 1450 cm−1 were the characteristic bands of Brönsted and Lewis acid sites, respectively. The band at 1490 cm−1 could be attributed to the adsorption of pyridine on both Brönsted and Lewis acid sites. The Brönsted (derived from Nb−OH and P−OH on the surface) and Lewis acid sites could be due to the presence of a certain number of octahedra NbO6 and tetrahedra NbO4 in the mesoporous niobium phosphate frameworks.33−35 Furthermore, the spectrum of different-temperature-evacuated 4838


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ACS Omega Table 2. Elemental Analysis of CBO and UBO Obtained at Different Hydrotreating Temperatures elemental analysis (wt %)

a

sample

yield (wt %)

C

H

N

S

Oa

molar H/C

Molar O/C

HHV (MJ kg−1)

CBO 220 °C 240 °C 260 °C 280 °C 260 °Cb

83.49 76.88 71.22 82.60 59.6

61.19 66.34 72.11 74.96 79.09 75.25

7.98 8.66 9.9 10.24 10.73 9.11

0.53 0.55 0.37 0.27 0.24 0.14

0.78 0.19 0.14 0.12 0.13 0.16

29.53 24.2 17.42 14.3 9.78 15.25

1.55 1.57 1.65 1.64 1.62 1.45

0.36 0.27 0.18 0.14 0.09 0.15

26.96 30.59 35.55 37.56 40.48 35.86

By difference. bCatalyst: NbOPO4.

Scheme 1. Possible Reactions in Pd/NbOPO4 Upgrading: (a) Esterification, (b) O-Alkylation, and (c) Etherificationa

a

R2: H, −CH3, and −CH2CH3.

the lack of standard sample, they were not quantified. The gas products were mainly CO, CO2, CH4, and C2H6, which may come from the organic acids through decarboxylation and aldehydes through decarboxylation and decarbonylation, as well as −OCH3 of methoxy phenol through demethylation.41 The carbon content of UBO ranged from 61.19 to 79.09 wt % by catalytic HDO, and the hydrogen content also apparently increased from 7.98 to 10.73 wt %, which implied that more stable C−H bonds were formed.15 Furthermore, the oxygen level was much lower than that of the initial bio-oil, demonstrating that oxygen was effectively removed. The atomic H/C and O/C ratios were important characters to evaluate the degree of deoxygenation and provide some information about the aromatic content of bio-oils, respectively.42 The atomic O/C ratio was effectively reduced from 0.36 in CBO to 0.09 in UBO obtained at 280 °C. Furthermore, the supports also showed excellent performance on the CBO upgrading and the carbon and hydrogen content in UBO reached 75.25 and 9.11%, respectively. The nitrogen and sulfur contents in the UBO also decreased, indicating that HDN and HDS simultaneously occurred with hydrogenation. The higher heating value (HHV), calculated based on the elemental composition by the Dulong equation, increased with elevated temperature, and the highest HHV (40.48 MJ kg−1) was obtained at 280 °C. It is illustrated that the catalytic HDO process accompanying loss of oxygen can effectively improve the energy density of the bio-oil. 2.2.2. GC−Mass Spectrometry (MS) Analysis. The chemical compositions of CBO and UBO obtained at different hydrotreating temperatures were identified by GC−MS (Figure S3). Table 3 shows the relative amounts of each component detected in the bio-oil. Each sample contained mainly phenols, esters, fatty acids, ketones, aldehydes, and other heteroatom (N, S) compounds. The amounts of small molecular products derived from bio-oil are low, including phenols, aldehydes, and

Table 3. Distribution of the Chemical Composition of CBO and UBO Obtained at Different Hydrotreating Temperatures Qualitatively Determined by GC−MS compounds

CBO

220 °C

240 °C

260 °C

280 °C

260 °Ca

phenol esters acids aldehydes alcohols ketones ethers N, S others

28.48 4.67 10.3 29.17 0.09

48.43 21.23 11.15 0.1 4.48 0.48 4.25 3.87 6.23

38.85 18.27 5.54

39.99 23.26 6.41

7.06 0.45 2.57 4.3 20.47

3.5 2.66 3.5 2.61 18.37

32.62 19.83 5.09 2.56 2.85 1.44 6.31 6.06 22.45

42.40 16.31 0.43 2.6 3.1

a

3.55 23.74

0.94 1.8 32.42

Catalyst: NbOPO4.

acids, and the main components were oligomers. Particularly, the small molecular phenolic compounds contained methoxy, unsaturated side-chain alkyl groups. It can be seen that the number of small molecular compounds increased dramatically after being hydrotreated. Esters and monomeric phenolic derivatives make up the largest part of the upgraded oil. Esterification could easily take place with the use of a strong acid catalyst at low reaction temperature.43 Brönsted acids and Lewis acids coexist on the surface of NbOPO4, which could catalyze the esterification reaction and promote the cleavage of main linkages (β-O-4 and 4-O-5 ether bonds) in lignin oligomers to monomeric phenolic units. There were certain amounts of monomeric phenolic compounds when the support was used as the catalyst. It could also be seen that there were more esters formed through esterification with ethanol, which was an effective approach to reduce the bio-oil’s acidity and enhance its stability.44 The content of esters increased dramatically from 4.67 to 23.26%. Most of esters existed in the form of aliphatic esters; however, the corresponding acids 4839


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Figure 6. GPC molecular-weight distribution of CBO and UBO obtained from hydrotreating at different temperatures. *Catalyst: NbOPO4.

for some aliphatic esters, such as ethyl hexadecanoate and ethyl octadecanoate, were not detected in CBO and thus the Guerbet reaction between ethanol and the aliphatic fragments of lignin might occur.45 Huang et al. suspected that these esters come from two parts: one was the transesterification between alcohols and triglycerides and the other was oxidative esterification reaction between the aliphatic −OH groups of lignin oligomers and ethanol.46 Notably, there still existed many acids, but majority of them were aromatic compounds that were not detected in CBO. This further certified that the lignin oligomers degraded during the process. Furthermore, the supported Pd metal was responsible for the hydrogenation of unsaturated alkyl side group of monomeric phenolic compounds in CBO.28 The monomeric phenolic derivatives increased from 28.48 to 39.99%. 4-Vinylguaiacol can be converted into 4-ethylguaiacol. 4-Vinylphenol as a representative monomer in CBO could be converted into 4-ethylphenol and ethylcyclohexanone via hydrogenation. Moreover, the category of monomeric phenolic compounds got more than that in CBO, and the newly observed ones, such as 4methylphenol, could be originated from the hydrogenolysis depolymerization of lignin oligomers. Nevertheless, the acid catalyst could also catalyze a number of other reactions, including transesterification, etherification, aldol condensation, oligomerization, and dehydration/decomposition of sugars/ furans.43 The O-alkylation of phenolic compounds with proper proportion was observed, which implied that ethyl group substitution occurred during the process and high reaction temperature enhanced the etherification. Furthermore, the diphenol, which might be originated from the removal of the methyl group of methoxy phenol, was also observed. These reactions were beneficial to retard repolymerization of depolymerized lignin derivatives.38 Although the series of reactions might account for the reasons why HHV increased with elevated temperature, hydrotreatment of bio-oil was the main reason for the HHV increase. It should be noted that aldehydes were the important parts identified in CBO and that the aldehyde compounds declined during the depolymerization

process by CO hydrogenation. Finally, it was worth noting that cyclic ketone and its derivatives were also detected in UBO, which might be originated from the following three aspects: (1) the monomeric phenol being deeply hydrogenated over the catalyst to form products such as 4-methylcyclohexanone and 4-ethylcyclohexanone; (2) hydrogenation of furfurals and its derivatives; and (3) degradation of the oligomers in the process. More specially, arenes or alkanes were hardly detected under all reaction conditions due to the existence of competed adsorption mechanism on catalyst surface between furfurals and phenolic compounds, which suppressed the deep hydrodeoxygenation of phenolic compounds.47 Ethanol also participated in the reaction, which also retarded phenolic hydrodeoxygenation. Moreover, the kinds of small molecular ester had decreased dramatically using NbOPO4 as catalyst, which might be caused by deploymerization of parts of sugar derivatives. The unsaturated side chains of phenolics were also still detected CC in UBO when NbOPO4 was used as catalyst. Notably, the O-alkylation of phenolic compounds and diphenol was detected and the results illustrated that the acid sites of support catalyzed these processes. 2.2.3. Effect of Catalysts on the Distribution of Molecular Weight (Mw) of Bio-oil. Gel permeation chromatography (GPC) analysis of the nonvolatile fractions (Figure 6) demonstrated that catalyst and temperature had significant effect on product distribution. Compared to the molecularweight distribution of CBO (Mw = 320; Mn = 298), the average molecular weight of UBO was distinctly reduced (Figure S4). It was demonstrated that the amounts of small molecular compounds significantly increased after hydrotreatment degrading process over Pd/NbOPO 4 in ethanol. The percentage of the species with a molecular weight range of 300−400 Da in CBO was 41.58% and that above 400 Da accounted for 22.16%, whereas that with low molecular weight, i.e., below 200 Da, was only 5.33%, which agreed well with the GC results. Furthermore, it should be noted that the species with molecular weight between 300 and 400 Da and above 400 4840


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Figure 7. ESI-MS oil analyses of each experiment. *Catalyst: NbOPO4.

which was difficult to be cleaved, and the other was that ethanol as reaction medium reacted with degraded products containing phenolic −OH and −OCH3 via the O-alkylation (alkylation on phenolic −OH groups), as well as etherification of phenolic compounds, leading to increase of the molecular weight. Furthermore, the phenolic −OH group and CαCβ unsaturated side chain were found to be the most reactive toward repolymerization due to the condensation between aromatic rings and the side-chain carbons.48 From the quantified monomers, we observed the decrease of phenolic compounds with evaluated temperature. Particularly, the support showed better prosperity: the species with molecular weight below 200

Da were dramatically reduced compared to the CBO with evaluated temperature. Particularly, when the hydrotreating temperature reached 280 °C, the average molecular weight (Mw = 275; Mn = 242) had an apparent decrease. The percentage of species with molecular weight below 200 Da was 18.87%, whereas those with molecular weight above 400 Da was only 3.34%. It implied that that the degraded products have lowmolecular-weight distribution. Notably, the species with molecular weight below 200 Da had not dramatically increased and most species centered at a molecular weight range of 200− 300 Da. Two reasons could account for these results: one was that there existed more C−C connection between dimers, 4841


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Figure 8. Comparison of the 1H−13C HSQC NMR spectra of CBO and UBO (260 °C).

Figure 9. Comparison of 1H−13C HSQC NMR spectra of (a) aromatic region and (b) side-chain region.

relative intensity distribution region ranging from approximately 100 to 650. However, the UBO after hydrotreating had a relatively narrow intensity molecular-weight distribution region and the relative intensities were concentrated in the range of 100−350. Most of the obtained products were mainly monomers and small dimers, and the largest molecular fragment m/z showed high intensities of 304 and 332. It could be noted that the results at different temperatures showed a similar distribution of m/z and that the supports also achieved oligomer depolymerization to some extent. Generally, the molecular fragments with m/z higher than 250 are assigned

Da was 17.80%, and those with molecular weight above 400 Da was also 16.59%. Furthermore, there existed a certain amount of high-molecular-weight polymers not detected due to their insolubility in solvents. To reveal the depolymerized products in detail, the molecular-weight distribution of fragments in CBO and UBO with increase of reaction temperature was detected by electrospray ionization (ESI)-MS. The ESI-MS images depicted in Figure 7 compared the absolute molecular mass distribution of the organic products obtained at different temperatures. It could be observed that the crude bio-oil covered a broad m/z 4842


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Figure 10. Proposed mechanism of oligomer depolymerization scheme.

to oligomers rather than to monophenolic compounds.49 There still existed a little amount of oligomers in UBO possibly due to partial depolymerization of large oligomers. 2.2.4. Possible Structure of Oligomers Formed. Twodimensional (2D) heteronuclear single quantum coherence (HSQC) NMR, as a useful method for detailed understanding of the lignin structure, was applied to the analysis of the lignin oligomers of bio-oil. Comparing the spectra of CBO and UBO (Figure 8), it can be observed that the NMR signals in aromatic

(δC/δH 90−160/4.6−8.4 ppm) and oxygenated side-chain regions (δC/δH 50−90/2.8−5.0 ppm) decreased, whereas those in the aliphatic side-chain region (δC/δH 10−40/0.5− 2.5 ppm) significantly increased. The different lignin unit cross signals from syringyl (S′), guaiacyl (G), hydroxyphenyl (H), ferulate (FA), and p-coumarate (PCA) can be clearly observed in the aromatic region in CBO (Figure 9a). In the side-chain region, methoxy cross signals were predominant and interunit linkages, such as β-O-4′ and 4-O-5, can also be observed, 4843


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ACS Omega whereas other linkages, such as 5-5, β-1, and β-β′ cannot be detected probably due to their relative low abundance.46 After being hydrotreated, the signals of G and S′ units were both decreased or even disappeared in the aromatic region, whereas the H units increased compared to G and S′ units after hydrotreatment. The initial signals corresponding to FA, S, G, Pβ, and PCA units completely or partly disappeared. Notably, in the side-chain region (Figure 9b), the newly appeared peaks can be assigned to ethyl and methyl groups of alkylated products and esters. Furthermore, the intensity of methoxy group signals decreased, which indicated that demethoxylation occurred, as proved by gas products. These results also illustrated that the oligomers could be depolymerized in a greater degree over Pd/NbOPO4 in ethanol. Combining the ESI-MS results and deploymerized monomers, structures of a few oligomers were proposed for the rich lignin oligomers of CBO. The lignin oligomers can be effectively depolymerized to monomers or dimers by cleavage of intraunit linkages. NbOPO4 exposed to strong Lewis acid Nb5+ sites generates an oxygen-deficient phase by losing its lattice oxygen and with a high propensity for the adsorption of oxygen,50 which benefits the polarization of C−O. The structures shown in Figure S2 were considered to be the major oligomers formed in bio-oil. The possible depolymerization of the oligomers in ethanol in the presence of catalyst is proposed schematically in Figure 10. 4-Methylphenol can be produced by cracking the oligomer 274 m/z (G unit), whereas 4-ethylphenol can be produced by cracking the oligomer 436 m/z (S′ unit). Furthermore, oligomers VII (480 m/z), VIII (496 m/z), and IX (524 m/z) can produce corresponding guaiacol by cleavage of C−O−C. However, only a negligible amount of guaiacol existed, confirming that transalkylation occurred with ethanol.

pubescens was purchased from Anji county of Zhejiang province in China. 4.2. Preparation of Bio-oil. Bio-oil (BO) was obtained from torrefaction of pubescens at 200 °C with a heating ramp of 2.5 °C min−1 in N2 atmosphere. Before being used in torrefaction, the pubescens was ground to get particles with particle size smaller than 80 meshes, washed six times with distilled water, and dried in an oven overnight at 110 °C. The use of low heating rate could avoid overheating on the one hand, since the final temperature of the torrefaction is controlled to be only 200 °C. On the other hand, because the heat-transfer capacity of the pubescens sample is poor, the low heating rate will help to achieve uniform heating of pubescens sample. The crude bio-oil (CBO) used for upgrading is obtained via reduced-pressure evaporation at 40 °C of BO for 2 h, where part of water was removed. According to our previous study,8 oligomers (mainly lignin oligomers) were the major torrefaction liquid products. 4.3. Catalyst Preparation. Mesoporous niobium phosphate support was synthesized according to the literature.52,53 The catalysts were prepared by incipient wetness impregnation of the support with 0.93 mol L−1 Pd(NO3)2·xH2O aqueous solution. After impregnated for 24 h, the sample was dried at 110 °C for 12 h, followed by calcination in air at 500 °C for 3 h with a linear heating ramp of 2 °C min−1. The loading of Pd on the catalyst is about 5% based on theoretical calculation according to the starting materials used. 4.4. Bio-oil Upgrading Test. The process of hydrotreating was carried out using ethanol as solvent. The CBO (0.2 g), Pd/ NbOPO4 (0.2 g), or NbOPO4 (0.2 g) catalyst and 25 mL of ethanol as solvent were added into a 50 mL of Hastelloy alloys in a high-pressure autoclave (SenLang MC50) equipped with a thermocouple and a mechanical stirrer. The reactor was purged six times by H2, with each time releasing the pressure back down to atmosphere pressure and then increasing the pressure to 500 psi with H2. The autoclave was heated to the target reaction temperature with a stirring rate of 500 rpm and then kept for 8 h. After reaction, the autoclave was cooled rapidly by electric fan to ambient temperature, the reaction mixture was filtered to remove the solid, the solid was washed with ethanol three times, and the filtrate and washing liquid were collected in a volumetric flask. The upgraded bio-oil (UBO) was obtained via reduced-pressure evaporation at 40 °C for 30 min to remove ethanol and part of water. For all catalytic runs, solid products were collected after reaction. Further analyses of the organic phase were carried out by GC−MS or FID, GPC, LCTOF-MS, and 2D HSQC NMR methods. The yields of organic products were calculated as

3. CONCLUSIONS Bio-oil from low-temperature torrefaction of pubescens was upgraded using ethanol as solvent with the Pd/NbOPO4 heterogeneous catalyst. The HHV increased from 26.96 to 40.48 MJ kg−1, and the oxygen content decreased from 29.53 to 9.78%. Compared to CBO, the acids, aldehydes, and furans were efficiently converted into esters and alcohol in UBO, where esters, ketones, and alkyl-substituted phenol compounds were the main components. The cleavage of aryl ether bond took place simultaneously with the dehydration, esterification, etherification, acetalization, and hydrogenation in the upgrading process, and the lignin oligomers were further depolymerized over the Pd/NbOPO4 catalyst. The oligomers were well depolymerized into small molecular compounds and mainly monomers. It is proved that Pd/NbOPO4 is an excellent catalyst in promoting bio-oil upgrading.

yield =

wt of products wt of starting bio‐oil

(1)

Elemental analysis was conducted by Vario MACRO cube (Elementar Analysensysteme, Hanau, Germany). The oxygen content was calculated by difference based on C, H, N, and S.54 The HHV was determined using the Dulong formula54

4. EXPERIMENTAL SECTION 4.1. Materials. Pd(NO3)2·xH2O, niobium pentoxide, and tartaric acid were purchased from J&K Chemicals. Hydrofluoric acid was purchased from Sigma-Aldrich. All other reagents were purchased from Kelong Chemical Reagent Co. Ltd., China. All chemicals were used as received without further purification. Pubescens,51 a kind of typical high-production and fastgrowing lignocelluloses biomass, mainly consisting of hemicellulose (∼18.8 wt %), cellulose (∼45.1 wt %), and lignin (∼24.2 wt %), was chosen as a representative of fast-growing renewable biomass resource in the study. The sample of

HHV (MJ kg −1) = 0.3383C + 1.443(H − (O/8)) + 0.0942S

(2)

where C, H, O, N, and S refer to the mass fractions of the elements. 4.5. Catalyst Characterizations. The Brunauer−Emmett−Teller (BET) surface area analysis of the catalysts was 4844


Article

ACS Omega carried out by N2 adsorption and desorption at 77 K using a Micromeritics TriStar II 3020 apparatus. The total pore volume was determined from the adsorption and desorption branches of the nitrogen isotherms at P/P0 = 0.99. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku (Japan) D/MAX-RC X-ray diffractometer set at 40 kV and 40 mA using Cu Kα radiation. Data were collected at steps of 1° min−1 in the 2θ range of 10−80°. Metallic surface and dispersion were determined by H2 chemisorption. The amounts of acid sites on the catalysts were determined by temperature-programmed desorption of ammonia (NH3-TPD), using the Auto Chem II 2920 instrument. Prior to TPD measurements, the samples were in situ reduced at 260 °C, pretreated in flowing He (45 mL min−1) for 1 h at 500 °C, and then cooled to 100 °C. NH3 was adsorbed onto the samples by flowing 5% NH3 in He gas mixture (50 mL min−1) for 4 h at 100 °C. Residual and physically adsorbed NH3 was removed by purging the samples with flowing 45 mL min−1 at 100 °C for 1 h. Desorption of NH3 was performed by heating the samples at a rate of 10 °C min−1 under flowing He (45 mL min−1) from 100 to 600 °C. Pyridine-FTIR spectra were used to identify the type of acid sites (Brönsted or Lewis) using a Vertex 70/80 apparatus equipped with a Spectra catalytic chamber. The chemical state and surface elemental composition were measured by X-ray photoelectron spectroscopy (XPS) in an AXIS Ultra DLD (KRATOS) spectrometer at 80 eV pass energy using monochromatic Al Kα radiation. The C 1s peak at 284.6 eV was used as reference to locate the other peaks. 4.6. Product Analysis. The small molecular liquid products were analyzed by GC−MS (Agilent 6820, Agilent Technologies) using an HP-INNOWAX capillary column. The oven temperature was maintained at 50 °C for 5 min and then increased to 250 °C at a ramp of 5 °C min−1 and maintained at 250 °C for 10 min. The identified small molecular products were further qualitatively quantified by peak area normalization method to calculate their relative contents. The identified small molecular liquid products were quantitatively determined by an internal standard method using GC-FID (Fuli 9750) equipped with a DB-5 capillary column, and the GC temperature program involved a hold at 50 °C for 5 min, rise to 250 °C at a rate of 5 °C min−1, and then holding at 250 °C for 10 min. The temperatures of both the injector and detector were maintained at 280 °C. Benzyl alcohol was used as an internal standard to quantify the content of the products. Meanwhile, gas products were analyzed by GC-TCD (PANNAA91) with C-2000 packed column. The temperatures of the GC oven and detector were maintained at 120 and 180 °C, respectively. Molecular weights of the crude bio-oil (CBO) and upgraded bio-oil (UBO) were measured by gel permeation chromatography (GPC) using Waters 1525-2487 system equipped with a manually packed column. The injection volume was 30 μL, the analysis time was 10 min, and rotary-evaporated sample was dissolved in tetrahydrofuran with a concentration of 2.5 mg mL−1. Polystyrene was used as the reference.

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in the HSQC spectra; acids distribution data, possible chemical structure of the oligomers; and GC−MS data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Changwei Hu: 0000-0002-4094-6605 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 program, No. 2013CB228103) and the Special Research Fund for the Doctoral Program of Higher Education of China (No. 20120181130014). The characterization by the Analytical and Testing Center of Sichuan University is greatly appreciated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00180. Additional details regarding the data of catalyst physicochemical properties, GC, and average molecular weight; assignment of main lignin 13C−1H cross signals 4845


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Hydrotreatment Upgrading of Bio-oil from Torrefaction of Pubescens in

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