Ethanolysis of Kraft Lignin over a Reduction-Modified MoO3

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Ethanolysis of Kraft lignin over a reduction modified MoO3 catalyst Mengmeng Chen, Xiaolei Ma, Rui Ma, Zhe Wen, Fei Yan, Kai Cui, Hong Chen, and Yong-Dan Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03585 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Industrial & Engineering Chemistry Research

Ethanolysis of Kraft lignin over a reduction modified MoO3 catalyst Mengmeng Chen b, Xiaolei Ma b, Rui Ma b, Zhe Wen b, Fei Yan b, Kai Cui b, Hong Chen a∗, Yongdan Li b,c∗ a

School of Environmental Science and Engineering, Tianjin University, Tianjin

300072, China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin Key Laboratory of Applied Catalysis Science and Technology, State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin, 300072, China c

Aalto University, School of Chemical Engineering, Department of Chemical and

Metallurgical Engineering, Kemistintie 1, Espoo, P.O. Box 16100, FI-00076 Aalto Finland

∗ ∗

Corresponding author. Tel.: +86-22-27409405. E-mail address: [email protected] (H. Chen). Corresponding author. Tel.: +86-22-27405613; Fax: +86-22-27405243. E-mail address: [email protected] (Y. Li). 1

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Abstract A reduction modified MoO3 catalyst is employed in the ethanolysis of Kraft lignin in supercritical ethanol. The reduction pre-treatment under hydrogen atmosphere is carried out and shows a strong effect on the activity. A sample MoO3 @ 623 K, i.e., pre-treated at 623 K, achieves the highest overall yield of small molecules, being 1266 mg/g lignin. Compared with that using untreated MoO3 sample, the yield increases almost 15%, implying that the reduction pre-treatment is effective for the enhancement of product yields and a promising strategy for industrial application. The XRD patterns and XPS spectra indicate that a transformation of bulk MoO3 to MoO2 happens at around 773 K, therefore, a MoO3 @ 773 K catalyst gives a very low yield. The oxygen vacancies and Mo6+ together with Mo5+ formed during the partial reduction facilitate the formation of active species.

Keywords: Kraft lignin, Catalytic ethanolysis, Supercritical ethanol, Molybdenum oxide, Reduction modification

1. Introduction With the gradual depletion of fossil-based fuels, the shortage issues of energy and chemicals has been highlighted in recent decades. Furthermore, overexploitation and abuse of fossil feedstocks lead to a number of environmental problems, e.g., acid 2


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rain and global warming.1 Therefore, a variety of novel, renewable and eco-friendly resources such as biomass are expected to arise in the foreseeable future.2, 3 Lignin, as a natural carbonaceous three-dimensional biopolymer consisting of methoxylated phenylpropane units, accounts for 15-30% by weight and 40% by energy in lignocellulosic biomass.4 It is estimated that every year, plant growth through photosynthesis produces approximately 50 billion tons of lignin,5 and exceeding 50 million tons of extracted lignin are co-produced from the pulp industry alone.6 However, only 2% of the lignin available is utilized as a low-heat-value boiler fuel to provide heat or electricity for the commercial application.7-10 As the most common pulping process, the Kraft technique employs alkaline solutions at 150-170 oC to separate lignin from carbohydrates, i.e., cellulose and hemicellulose. In China, more than 8 million tons of Kraft lignin are obtained every year, and have been a highvolume waste by-product of the pulping and paper industry.11,

12

Furthermore,

complex structure and botanical origin make it resistant to chemical attack and digestion.13 Therefore, the efficient utilization of Kraft lignin still remains a great challenge.14-16 Over the past decade, many insightful strategies have been practiced for the catalytic valorization of Kraft lignin.11 Pyrolysis represents a rapid heating process of biomass in the absence of oxygen to obtain liquid products called bio-oils,14, 15, 17-19 which can be utilized as oil additives to enhance octane number after oil updating such as moisture removal and hydrodeoxygenation reaction.9, 20, 21 However, the high

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temperature above 500 oC results in the formation of char and tar, together with COx and H2O. The high energy consumption of bio-oil updating is also economically infeasible. Gasification of lignin aims at the production of syngas that is a mixture of CO and H2,22-25 which can be converted into liquid fuels by means of methanol synthesis26 or Fischer-Tropsch synthesis27. The gasification process tolerates lignin with high moisture content, and eliminates the demand for drying the biomass.11 Nevertheless, multi-step conversion route involves complex and high-cost processes.28 Hydroliquefaction is described as a way to obtain high yield of liquid and low quantity of char and tar with hydro-treating catalyst and hydrogen-donor solvent.29-31 This process favours the production of bulk or fine monomeric chemicals, e.g., phenols and BTX (benzene, toluene, and xylene), but the high hydrogen pressure is inadequate for practical significance.32-35 Biocatalysis is mainly used to transform lignin under a moderate condition. The main enzymes like manganese peroxidase, lignin peroxidase and laccase36 are well-designed for the applications of lignin depolymerisation with excellent selectivity. However, biocatalysis is a very slow process, and the biocatalysts become quite unavailable due to high specificity. The ethanolysis strategy of Kraft lignin has attracted increasing attention in the biorefinery field. Mason et al.37 obtained a liquid mixture of C2-C6 aliphatic alcohols and methylated derivatives from woody biomass solids in supercritical methanol under the conditions of 300-320 oC and 16-22 MPa over a copper-doped porous metal oxide catalyst. Cheng et al.38 found that the acetone-methanol soluble products

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obtained from lignin degradation in supercritical ethanol had a lower molecular weight than those obtained in water. Recently, we reported the catalytic ethanolysis of Kraft lignin over α-MoC1-x/AC and Mo/Al2O3 catalysts at 280 oC for 6 h, and obtained 1640 and 1390 mg/g lignin small-molecular products, respectively, without any char or tar formation.12, 39 Huang et al.40 reported the conversion of Kraft lignin into aromatics with 23 wt% yield and without the formation of char or tar using a CuMgAlOx catalyst at 300 oC under an inert atmosphere. Several other Mo-based catalysts, e.g., MoS2, Mo2C, NiMo/Al2O3, MoC1-x/Cu-MgAlOz and Mo2N/Al2O3, were reported to be active in the conversion of biomass in the hydrogen-donor solvents.29, 30, 41-43 Compared with other Mo-based catalysts, MoO3 is earth-abundant and low-cost, and has been demonstrated to exhibit a good activity towards the hydrodeoxygenation of a set of lignin-derived model compounds under low hydrogen pressure44 or nitrogen atmosphere45. Besides, MoO3 catalyst is highly selective for the cleavage of C-O linkages and possesses excellent regeneration property via calcination for removing carbon. However, so far, researches have not been reported on the ethanolysis of Kraft lignin over a pure or reduction modified MoO3 catalyst. The reduction pre-treatment may facilitate the formation of active species and further increase the yields of desired products, which endows MoO3 unique advantages for practical application of lignin ethanolysis due to the above characteristics. Herein, in this contribution, MoO3 is employed as a catalyst in the catalytic ethanolysis of Kraft lignin. A pre-treatment in hydrogen atmosphere at different

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temperatures exhibits a profound influence on the distribution and the yield of the liquid products. The XRD patterns and XPS spectra of the fresh catalysts illuminate the structure changes of the bulk and surface of MoO3. Finally, the nature of the actual active sites is discussed.

2. Experimental

2.1. Materials

The Kraft lignin (product number 471003) was purchased from Sigma-Aldrich. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the lignin are 60,000 and 10,000 g/mol, respectively. The original lignin contains 49.5 wt% C, 4.71 wt% H, 0.15 wt% N, 2.80 wt% S and 19.4 wt% ashes.12 The Kraft lignin was dried overnight at 100 oC prior to use. The weight loss was less than 0.2%. Solvents and chemicals, including ethanol, ammonia water, o-cresol and ammonium molybdate (NH4)6Mo7O24·4H2O (AR reagent grade, Tianjin Guangfu Technology Development Co. Ltd.) were used as provided. The deionized water was prepared with an Ulupure ultrapure water purification machine (UPH-1-10). All these materials were used as received without further purification.

2.2. Catalyst preparation

A pure MoO3 sample was synthesised via the calcination of (NH4)6Mo7O24·4H2O at 673 K for 4 h. The reduction modified MoO3 catalysts were prepared under hydrogen atmosphere (80 mL/min) at different temperatures 623-823 K for 12 h. The 6


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program was started from 293 K to desired temperature at a heating rate of 5 K/min, and held at the temperature for 12 h. The catalyst was self-cooled to ambient temperature, and was directly used in the ethanolysis reaction without exposure to air.

2.3. Ethanolysis reaction

The catalytic ethanolysis reaction of Kraft lignin was carried out in a 300 mL batch reactor (Parr 4566, made of Hastelloy) equipped with a pressure sensor and a temperature controller (Parr 4848). In a typical run, 1.00 g Kraft lignin, 0.50 g molybdenum oxide catalyst and 80 mL solvent were loaded into the reactor. The reactor was evacuated with high-purity hydrogen or nitrogen for five times, and sealed with an initial pressure (gauge) 0 MPa. Then the reactor was heated to desired reaction temperature and kept for 6 h. When the reaction was finished, the reactor was cooled to ambient temperature by immersing into a cold-water bath. After releasing the gas, the mixture in the reactor was filtrated into solid residue and ethanol-soluble portion prior to the analysis. The solid residue was washed using ethanol, and then dried at 120 oC for 12 h.

2.4. Catalyst characterization

The TG analysis was carried out using a Netzsch STA 449 F3 system. The sample was loaded into an alumina crucible, and heated from ambient temperature to 1273 K at a heating rate of 10 K/min under a flow of pure nitrogen or mixed gas (total flow: 25 ml/min, O2:N2 = 1:4). X-ray diffraction patterns (XRD) of the catalysts were 7



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measured with a powder diffractometer (Bruker AXS D8-S4), equipped with a Cu-Kα radiation source. It was operated at 40 kV and 40 mA, and the scanning was carried out in the 2θ range between 10

o

and 70

o

at a rate of 4 o/min. X-ray photoelectron

spectroscopy (XPS) measurements were carried out with a Perkin-Elmer PHI-1600 spectrometer using monochromatic Mg Kα radiation. Binding energy was calibrated with respect to the signal for C1s at 284.8 eV.

2.5. Product analysis

Agilent 6890-5973 GC-MS and Agilent 6890 GC-FID were used for qualitative and quantitative analysis of the liquid products, respectively. The compounds were confirmed with comparing the mass spectra obtained from the samples to the standard spectra in the system’s database (NIST 2.0). An internal standard method was adopted for the quantitative calculation of product yield. O-cresol was used as an internal standard. The working conditions and programs for both Agilent GCs were the same as reported in our previous work.12, 39 The oven temperature program was set from an initial temperature of 45 oC to a final temperature of 250 oC at a rate of 10 oC/min, and then kept at 250 oC for 2 min. Both chromatographic column used were HP-5 MS capillary column (30m × 0.25 mm × 0.25 µm). A split ratio of 50 was used for the GC-FID and GC-MS analysis. The scanning m/z range of mass spectrometer was set from 10 to 500.

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2.6. Data analysis

The selectivity of a product (Si (%)) was obtained from the product content calculated by the internal standard method. The percentages of Mo and O species (Pi (%)) on the catalyst surface were calculated by the corresponding fitting peak area in the XPS spectra. In equations (1), m represents the mass of solid sample or liquid product. In equation (2), A represents the fitting peak areas of related species. Si (% ) =

m(product)i ´ 100 å m(product)i

Pi (% ) =

AMoi ´ 100 Pi (% ) = å AMoi

(1)

AOi ´ 100 å AO i

(2)

The particle size of MoO3 samples were calculated by the Debye-Scherrer equation, as depicted in equation (3). D represents the particle size (nm). K represents the Scherrer constant (0.89). λ represents the X-ray wavelength (0.154056 nm). θ represents the Bragg diffraction angle. B represents the line broadening at half the maximum intensity. F represents the full width at half maximum (FWHM).

D=

Kl Fp B= B cos q 180

(3)

3. Results and discussion

3.1. Catalytic ethanolysis of Kraft lignin

The composition of ethanol-soluble compounds in liquid phase was confirmed with a GC-MS. Table 1 summaries the quantified products. 22 kinds of liquid products are classified into two categories, i.e., aliphatic compounds in the form of C6 9



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alcohols, C8 and C10 esters, and aromatic compounds in the form of benzyl alcohols and arenes. No phenols, dimers or oligomers were detected. It was observed that the alkylated, isomerized and unsaturated derivatives of the corresponding substrates, i.e., hexanol, hexanoic acid ethyl ester, benzyl alcohol and toluene, are dominant in the quantified products, which is consistent with the previous results.12

3.1.1. Effect of reaction temperature and atmosphere

Figure 1 depicts the effect of reaction temperature and atmosphere on the product yields and the overall selectivity of the aromatic compounds over the MoO3 @ 623 K catalyst. The overall yields in Figure 1(A) present a monotonic increase trend with the increase of the reaction temperature, irrespective of the reaction atmosphere. At 553 K, the overall yields obtained under hydrogen and nitrogen atmosphere were almost the same. With the further increase of reaction temperature, the overall yields reached the maximum value of 486 and 1266 mg/g lignin at 593 K, respectively for the two atmospheres. From 553 to 593 K nitrogen atmosphere is more conductive to the increase of the overall yields than the hydrogen atmosphere. The difference becomes more important as the increase of the reaction temperature. The different behaviour of the ethanolysis reaction under different atmospheres can be attributed to the inhibition effects of the gas-phase hydrogen molecule on the release of active hydrogen atom from ethanol33, and on the suppression of the chemisorption of ethanol on the catalyst active surface in the presence of hydrogen molecule12.

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The grouped yields and aromatic selectivity are also dependent on the reaction temperature and atmosphere, as shown in Figure 1(B). At 553 K, only a small amount of esters and aromatics were produced, and no alcohols were formed under both hydrogen and nitrogen atmospheres. The increase of the reaction temperature remarkably facilitates the increase of the grouped yields, particularly under nitrogen atmosphere. Despite of the reaction temperature and atmosphere, the esters account for the most in all the products. The maximum yield of esters reached 319 and 982 mg/g lignin at 593 K under hydrogen and nitrogen atmosphere, respectively. As for the aromatic selectivity, a monotonic increase tendency with the increase of reaction temperature is observed. The highest selectivity of 11.3 and 10.7 wt% are observed under hydrogen and nitrogen atmosphere, respectively. Furthermore, in the range examined, the selectivity observed under hydrogen atmosphere is higher than that under nitrogen atmosphere. Compared with H2, the N2 atmosphere exhibits a better performance on the formation of aliphatics and the increase of alcohol and ester yields, and thus results in a slight decrease of aromatic selectivity. It is likely that at N2 atmosphere, the ethanol molecule is more readily dehydrogenated into aldehyde that acts as a crucial active intermediate in the formation of alcohols and esters.

3.1.2. Effect of catalyst partial reduction

Table 2 lists the overall and grouped yields obtained at 593 K for 6 h under nitrogen atmosphere using different molybdenum oxide catalysts partially reduced at different temperatures in hydrogen. The overall yields present a sequence as: MoO3 @ 11



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623 K > MoO3 > MoO3 @ 823 K > MoO3 @ 673 K > MoO3 @ 723 K > MoO3 @ 773 K ≈ none. In the absence of a catalyst, the overall yield is only 488 mg/g lignin, including 386 mg/g lignin aliphatics and 102 mg/g lignin aromatics. This result is consistent with that in our previous report.12, 46 When MoO3 without pre-treatment is employed, the overall yield is 1104 mg/g lignin. The difference of the grouped yields of alcohol and ester products is remarkable, being 124 and 880 mg/g lignin, respectively for the case with MoO3 as the catalyst. The yield of arenes is slightly higher than the case without a catalyst while an inverse order occurs to those of benzyl alcohols. However, the yield of aromatic products, i.e., the sum of benzyl alcohols and arenes, almost remains the same for the two cases. The catalyst after partial reduction at 623 K exhibits further better activity towards the ethanolysis of Kraft lignin. 1266 mg/g lignin of overall yields were obtained. All yields of the four groups increase in different degrees compared to those obtained on the pure MoO3. The yields of alcohols and esters reached the maximum values of 149 and 982 mg/g lignin, respectively. Nevertheless, the further increase of the reduction pre-treatment temperature brought about a decrease of activity and drop of the yield of the desired products. The overall yields presented a drastic decline to 503 mg/g lignin when the reduction temperature was 773 K. It is noted that the grouped yields obtained using MoO3 @ 773 K as the catalyst are close to those obtained without a catalyst. It is speculated that MoO3 @ 773 K sample is inactive for the ethanolysis of Kraft lignin. Interestingly, the yield bounced back when the partial reduction temperature was

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increased to 823 K. With MoO3 @ 823 K sample as the catalyst, 1063 mg/g lignin of overall yields were obtained. Furthermore, the grouped yields of benzyl alcohols and arenes reached the maximum values of 95 and 170 mg/g lignin, respectively, and both of them are higher than the yields of alcohols. The detailed yields of alcohols and esters obtained with the catalyst samples are illustrated in Figure 2. The blank experiment without catalysts gives a low alcohol yield, including 6 mg/g lignin 2-ethyl butanol, 9 mg/g lignin 3-hexenol, 6 mg/g lignin 2-methyl-2-pentenol and 11 mg/g lignin hexanol. The catalysts through partial reduction at low temperatures (≤ 673 K) and the fresh MoO3 catalyst remarkably facilitate the formation of alcohol products. In particular, 78 mg/g lignin of 2-methyl2-pentenol were obtained with MoO3 @ 623 K as the catalyst. However, for all the samples examined, MoO3 @ 773 K presented the poorest activity towards the production of alcohols, with the formation of 5 mg/g lignin 2-ethyl butanol, 8 mg/g lignin 3-hexenol, 7 mg/g lignin 2-methyl-2-pentenol and 9 mg/g lignin hexanol. Figure 2(B) depicts the detailed yields of 7 kinds of esters with 8 and 10 carbons. Irrespective of the difference in catalyst pre-treatment, the yields of different esters follow the same order: 2-hexenoic acid ethyl ester ≈ 3-hexenoic acid ethyl ester > hexanoic acid ethyl ester > 3-methyl valeric acid ethyl ester > 2-octenoic acid ethyl ester > octanoic acid ethyl ester > 3-octenoic acid ethyl ester. No 3-octenoic acid ethyl ester was observed with MoO3 @ 773 K and MoO3 @ 823 K as the catalysts. Moreover, the yields of C8 esters are far higher than those of C10 esters, which

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indicates a chain growth mechanism. The distribution of alcohol and ester products obtained over MoO3 @ 773 K catalyst is similar to that observed without a catalyst, which provides evidence for the inactivity of MoO3 @ 773 K sample. Table 3 lists the detailed yields of aromatic products, that is, benzyl alcohols and arenes. In the absence of a catalyst, only 22 mg/g lignin of 2-methyl benzyl alcohol were formed and no other benzyl alcohols were detected. Toluene and (o- and p-) xylenes, being 19 and 38 mg/g lignin, respectively, accounted for the most in the arene products. With the presence of fresh MoO3 as the catalyst, the yield of 2-methyl benzyl alcohol showed a slight decrease to 10 mg/g lignin, accompanied with 35 mg/g lignin toluene and 35 mg/g lignin xylenes. After partial reduction at 623 K, the yields of benzyl alcohols had little change. Nevertheless, a significant increase of the yields of xylenes especially o-xylene compound was observed. The yields of dimethyl ethylbenzenes presented a large increase from 4 to 26 mg/g lignin. Besides, no allyl benzene was formed. With the increase of the reduction temperature to 723 K, the yields of benzyl alcohols showed a sudden rise and reached the value of 59 mg/g lignin. When MoO3 @ 773 K was used as the catalyst, the product distribution and yields were close to those values obtained without a catalyst, indicating that MoO3 @ 773 K sample has no catalytic activity in the reaction. Except for benzyl alcohol, all the other aromatic products reached their maximum yields over MoO3 @ 823 K sample (entry 7). The main aromatic products are o- and p- alkyl benzyl alcohols (55

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and 25 mg/g lignin), o- and p- xylenes (43 and 31 mg/g lignin) and toluene (41 mg/g lignin).

3.1.3. The solid residue

The TG analysis of solid residues obtained after reaction was carried out at nitrogen and air atmosphere, respectively, in order to attain the content of residual organic compounds and char to a certain extent via the comparison of the weight loss difference at different atmosphere. The results are depicted in Figure 3. It is interesting that, although MoO3 @ 773 K sample gives similar product yields and distribution as the case without a catalyst, distinct weight-loss is observed with this sample, indicating that reaction does happen in this case. The weight losses of the used catalyst in air atmosphere is much larger than those in nitrogen atmosphere, indicating the formation of non-volatile high-molecular-weight ethanol-insoluble compounds as residues. Notably, the fact that the weight loss is even more at nitrogen atmosphere than at air atmosphere, especially for the samples reduced at high temperatures (773 and 823K), can be attributed to the oxidation of the samples and thus weight increase.

3.2. Catalyst characterization

The TG curve of ammonium molybdate decomposing into MoO3 in air atmosphere is depicted in Figure 4. It can be seen that at temperatures below 600 K, there are three weight-loss peaks due to the elimination of ammonium ion and lattice 15



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water.47 In the temperature range from 600 to 1100 K, no significant weight loss is observed and 18.39% of weight loss at 673 K is close to the theoretical value of 18.45 wt% of water and ammonium, indicating that complete decomposition of ammonium molybdate into MoO3 was achieved at 673 K. The DTG curve shows that the first three peaks correspond to the removal of water and ammonium.47 The peak located at 1238 K may be assigned to the evaporation of MoO3.48 The XRD patterns of the molybdenum oxide catalysts before reaction are illustrated in Figure 5. It can be seen that via the calcination of (NH4)6Mo7O24·4H2O at 673 K for 4 h, MoO3 phase was formed and confirmed by the XRD pattern. At low reduction temperature (≤ 723 K), the bulk MoO3 phase showed no structural change. However, when the reduction temperature was above 723K, the phases of Mo4O11 and MoO2 were observed. Mo4O11 is not only an intermediate but is also formed in a parallel reaction during the reduction of MoO3 to MoO2.49 Furthermore, the diffraction peaks of MoO2 phase are more significant at further higher temperatures. It is noted that both MoO3 and MoO2 phases exist after reduction at temperatures above 773 K, indicating incomplete reduction of MoO3 to MoO2 in the range of the temperature examined. With the purpose of eliminating the effect of particle size caused by the reduction pre-treatment on the activity, Table 4 lists the particle size of the fresh MoO3 samples calculated by the Debye-Scherrer equation based on the XRD patterns. It can be observed that there are no distinct links between the activity and the

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particle size. Therefore, some other factors, e.g., surface Mo and O species, may be responsible for the activity. Figure 6 depicts the XPS spectra of the Mo 3d and O 1s energy region of different fresh molybdenum oxide catalysts. Mixed oxidation states of the Mo 3d doublet are contributed from Mo4+, Mo5+ and Mo6+ located at the 3d5/2 bands of 229.2, 231.2 and 232.2 eV, respectively.50, 51 The corresponding state percentages of Mo element are also given in the form of colorized numbers. It can be seen that for the samples reduced at low temperatures (≤ 723 K), no signals of Mo4+ were detected. Besides, in the same range, the percentage of Mo5+ showed a monotonic decrease with the increase of the reduction temperature. The further increase of the reduction temperature facilitates the formation of Mo6+ to Mo5+ and Mo4+. The oxygen species were recognized as three kinds: lattice oxygen (OL), hydroxyl oxygen (OH) and oxygen vacancies (OV), showing as the 1s bands of 530.2, 531.6 and 533.2 eV, respectively.52 Obviously, fresh MoO3 catalyst possesses a high percentage of OV. The fresh MoO3 @ 623 K catalyst contains the highest percentage of OV, being 17% while the lowest percentage of 6% was obtained with the fresh MoO3 @ 773 K sample.

3.3. General discussion

The catalytic performance strongly depends on the reduction temperature of the partial reduction pre-treatment. Among all the samples examined, MoO3 @ 623 K catalyst exhibited the best activity towards lignin ethanolysis while MoO3 @ 773 K 17



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sample did the poorest. In addition, untreated MoO3 sample also presented a remarkable performance. It is noted that the MoO3 @ 773 K catalyst gives almost the same result as the case of the absence of a catalyst. On the basis of the above-mentioned results, it is speculated that three-step reactions took place in the catalytic ethanolysis process of Kraft lignin. Firstly, some active molecules and radicals, e.g., hydrogen, aldehyde and ethyoxyl groups, were formed via the self-transformation reaction of ethanol.12 Under the action of ethanol solvent and ethanol-derived active species, Kraft lignin was depolymerized into middle molecular-weight fragments. Then, these lignin-derived fragments were converted into small-molecular compounds in the presence of Mo5+. Lignin biomacromolecule is incapable of diffusion into the pores due to steric-hinerance effect, and traditional theory of heterogeneous catalysis cannot apply to this reaction system. The involvement of Mo5+ in the form of dissociative molybdenum (V) ethoxide in lignin ethanolysis was proposed in our previous work.46 In the reaction, molybdenum (V) ethoxide is formed in the supercritical ethanol and enters the fluid phase dissociatively. The transformation of lignin-derived fragments into smallmolecular compounds was achieved in this homogeneous system. In this case, based on the XRD patterns and XPS spectra of different fresh molybdenum oxide samples, Mo6+ and Mo5+ may be responsible for the formation of molybdenum (V) ethoxide active species. Under the action of in-situ hydrogen derived from the dehydrogenation of hydrogen-donor ethanol solvent53, partial Mo6+ on the sample surface is reduced to

18


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Mo5+. This Mo5+ together with original Mo5+ on the sample surface dissolves into the solvent and acts on the lignin linkages in the form of molybdenum (V) ethoxide. Finally, some oxygenated aromatic compounds were converted into arene products via the hydrodeoxygenation reaction over MoO3 catalyst in the presence of in-situ hydrogen. The Mo (V) signal was detected by the EPR spectra of various used Mobased catalyst, and the performance of molybdenum (V) ethoxide in the lignin depolymerization was tested in our previous work46. The similar products and the activity of molybdenum (V) ethoxide have been verified. MoO3 catalyst was demonstrated to present a good activity towards the hydrodeoxygenation reaction of lignin-derived model compounds. Mo6+ and Mo5+ played a key role in this hydrodeoxygenation

reaction

while

Mo4+

is

completely

inactive

in

the

hydrodeoxygenation reaction of phenolic compound and cannot be converted into Mo5+ in this condition, indicating that Mo4+ is not conductive to the formation of desired products.44 The oxygen vacancies formed during the partial reduction act as a Lewis acid sites that are capable of weakening C-O-C and C-O ether and phenolic linkages in the lignin subunits and formed intermediate fragment44. These fragments involve various important active species, e.g., undetected phenolic compounds and aldehydes, which play a key role on the lignin ethanolysis46. In addition, the Lewis acidity contributes to the activity of the oxygen vacancies towards the hydrodeoxygenation of lignin-derived oxygenic aromatics.

19



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In general, Mo5+ and Mo6+ play a crucial role on the formation of active sites and species while oxygen vacancies has an important influence on the subsequent hydrodeoxygenation reaction of lignin-derived intermediates and phenolic monomers, which efficiently inhibits the repolymerization reaction of phenolic monomers into oligomers or high molecular-weight compounds and further enhances the yields of desired products. Untreated MoO3 exhibits a good activity towards the ethanolysis of Kraft lignin, and high reduction temperature can alter the composition and percentage of Mo5+, Mo6+ and oxygen vacancies, resulting in a low product yield. With respect to MoO3 @ 623 K sample, appropriate composition and percentage of Mo5+, Mo6+ and oxygen vacancies are obtained, which contributes to the highest overall yield in the samples investigated. In summary, the reduction modified MoO3 catalyst exhibits a better catalytic activity towards the ethanolysis of Kraft lignin compared with untreated MoO3. Besides, it is demonstrated that MoO3 possesses good regeneration property in the biomass conversion. All these advantages endow untreated and reduction modified MoO3 catalyst unique attributes for practical significance, specifically industrial application. It is still underway for MoO3 catalyst to be further optimized for maximum performance.

4. Conclusion MoO3 partially reduced in hydrogen has been used as the catalyst for the ethanolysis of Kraft lignin in supercritical ethanol. 22 kinds of small molecules in liquid products were confirmed and used as the desired products. The reaction 20


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atmosphere and temperature had a great influence on the activity of the MoO3 sample. Nitrogen atmosphere favoured higher yields of products while hydrogen atmosphere was conductive to higher aromatic selectivity. In the range from 553 to 593 K investigated, elevated reaction temperature facilitated the significant increase of desired products. Using MoO3 @ 623 K as the catalyst, the highest overall yield, being 1266 mg/g lignin, was obtained in nitrogen atmosphere at 593 K for 6 h. This yield increases almost 15% compared with that using untreated MoO3 sample, showing that the reduction modified

MoO3 @ 623 K catalyst is sufficient for

practical significance. However, MoO3 @ 773 K sample was demonstrated to be inactive for the ethanolysis of Kraft lignin. No bulk reduction of MoO3 at below 723 K was observed from the XRD patterns. The phases of MoO2 and Mo4O11 were observed at above 773 K. The XPS spectra presented the effect of reduction temperature on the valence state of surface Mo element and the category of surface O species. The catalytic activity of molybdenum oxide samples originates from the synergetic action of Mo5+, Mo6+ and oxygen vacancies.

Acknowledgements

The financial support from the Ministry of Science and Technology of China (2011DFA41000) and the National Natural Science Foundation of China (21336008 and 21690083) is gratefully acknowledged.

References

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27



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Table and Figure captions Table 1 Summary of quantified products obtained from the ethanolysis of Kraft lignin in supercritical ethanol over different molybdenum oxide samples.

Table 2 Overall and grouped yields obtained from the ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h under nitrogen atmosphere over different molybdenum oxide samples.

Table 3 Yields of aromatic products obtained from the ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h over different molybdenum oxide samples.

Table 4 The particle size of the fresh MoO3 samples calculated by the Debye-Scherrer equation based on the XRD patterns.

Figure 1 Overall yields (A), grouped yields and aromatic selectivity (B) obtained from the ethanolysis of Kraft lignin in supercritical ethanol over MoO3 @ 623 K sample at different reaction temperatures and atmospheres.

Figure 2 Detailed yields of alcohols (A) and esters (B) obtained from the

ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h over different molybdenum oxide samples.

Figure 3 TG analysis of solid residues obtained from the ethanolysis of Kraft

lignin in nitrogen (A) and air (B) atmosphere in supercritical ethanol over different molybdenum oxide samples. 28


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Figure 4 TG and DTG analysis of ammonium molybdate in air atmosphere.

Figure 5 XRD patterns of different fresh molybdenum oxide samples. Figure 6 XPS spectra of the Mo 3d (A) and O 1s (B) energy region of different fresh molybdenum oxide samples.

29



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Table 1 Summary of quantified products obtained from the ethanolysis of Kraft lignin in supercritical ethanol over different molybdenum oxide samples. C6 alcohols Aliphatics C8 and C10 esters Benzyl alcohols Aromatics Arenes

30


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Table 2 Overall and grouped yields obtained from the ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h under nitrogen atmosphere over different molybdenum oxide samples. The grouped yields (mg/g lignin) Overall yields Entry Catalyst a (mg/g lignin) Alcohols Esters Benzyl alcohols Arenes 1 None 488 32 354 22 80 2 MoO3 1104 124 880 10 90 3 MoO3 @ 623 K 1266 149 982 11 124 4 MoO3 @ 673 K 951 119 739 8 85 5 MoO3 @ 723 K 671 43 464 59 105 6 MoO3 @ 773 K 503 29 374 20 80 7 MoO3 @ 823 K 1063 65 733 95 170 a MoO3 @ X K: ‘X’ means reduction temperature.

31



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Page 32 of 40

Table 3 Yields of aromatic products obtained from the ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h over different molybdenum oxide samples. Benzyl alcohols

a

Entry

Catalyst

1

None

2

MoO3

3

MoO3 @ 623 K

4

MoO3 @ 673 K

5

MoO3 @ 723 K

6

MoO3 @ 773 K

7

MoO3 @ 823 K

Arenes

--- a

22

---

---

19

16

22

6

5

3

9

---

10

---

---

35

11

24

6

10

4

---

---

11

---

---

39

14

36

9

---

6

20

---

8

---

---

32

11

25

5

8

4

---

18

23

16

2

24

19

27

8

6

6

15

---

20

---

---

23

13

18

6

5

4

11

11

55

25

4

41

31

43

13

12

8

22

“---” means undetected

32


Page 33 of 40

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Table 4 The particle size of the fresh MoO3 samples calculated by the Debye-Scherrer equation based on the XRD patterns. Entry

Catalyst

2θ/rad

FWHM

Particle size/nm

1 2 3 4 5 6

MoO3 MoO3 @ 623 K MoO3 @ 673 K MoO3 @ 723 K MoO3 @ 773 K MoO3 @ 823 K

27.46 27.24 27.42 27.44 27.34 27.34

0.204 0.200 0.180 0.212 0.203 0.176

39.6 40.4 44.9 38.1 39.8 45.9

33



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Figure 1 Overall yields (A), grouped yields and aromatic selectivity (B) obtained from the ethanolysis of Kraft lignin in supercritical ethanol over MoO3 @ 623 K sample at different reaction temperatures and atmospheres.

34


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Figure 2 Detailed yields of alcohols (A) and esters (B) obtained from the ethanolysis of Kraft lignin in supercritical ethanol at 593 K for 6 h over different molybdenum oxide samples.

35



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Figure 3 TG analysis of solid residues obtained from the ethanolysis of Kraft lignin in nitrogen (A) and air (B) atmosphere in supercritical ethanol over different molybdenum oxide samples.

36


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Figure 4 TG and DTG analysis of ammonium molybdate in air atmosphere.

37



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Figure 5 XRD patterns of different fresh molybdenum oxide samples.

38


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Figure 6 XPS spectra of the Mo 3d (A) and O 1s (B) energy region of different fresh molybdenum oxide samples.

39



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Ethanolysis of Kraft Lignin over a Reduction-Modified MoO3

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