Controlled Hydrodeoxygenation of Phenolic Components in Pyrolysis

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Controlled Hydrodeoxygenation of Phenolic Components in Pyrolysis Bio-oil to Arenes Guangce Jiang, Yinghui Hu, Guoqiang Xu, Xindong Mu, and Huizhou Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03276 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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ACS Sustainable Chemistry & Engineering

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Controlled Hydrodeoxygenation of Phenolic

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Components in Pyrolysis Bio-oil to Arenes

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Guangce Jiang #†‡, Yinghui Hu # †, Guoqiang Xu *†, Xindong Mu *†, Huizhou Liu †

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† Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

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Sciences, No.189 Songling Road, Qingdao, Shandong, 266101, PR China. * Email:

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[email protected]; [email protected]

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‡ Department of Applied Chemistry, School of Science, Henan Agricultural University, NO. 63

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Nongye Road, Zhengzhou, Henan, 450002, PR China

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KEYWORDS: controlled hydrodeoxygenation, phenolic compounds, Ru-WOx/ZrO2 catalyst,

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pyrolysis bio-oil

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ABSTRACT: Hydrodeoxygenation of phenolic components in pyrolysis bio-oil is considered to

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be a potential strategy for producing renewable aromatic chemicals. The key issue of this process

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is the establishment of an effective catalytic system which can cleave the CAr–O bonds without

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affecting the aromatic structure. To achieve this goal, an efficient heterogeneous catalyst with

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solid acid support (WOx/ZrO2) and active metal (Ru) was prepared by this study. The Ru-

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WOx/ZrO2 catalyst can effectively convert model phenolic compounds into aromatic

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hydrocarbons. For a mixed phenolic sample, the conversion and selectivity to arenes were all

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around 90 %. The good selectivity was proved to be strongly related to the surface adsorption

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and acid properties of the catalyst as well as the reaction pathway. Moreover, the

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hydrodeoxygenation of a pretreated bio-oil was also conducted and presented a satisfactory yield

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of arenes at 240 oC with 1 MPa H2 reacted for 5 h. The depolymerization of high-molecular-

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weight phenolic oligomers was found to be significant during the catalytic process, which further

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enhanced the yield of aromatic monomers.

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Introduction:

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Pyrolysis integrated with upgrading is considered to be an applicable way to produce 1-5

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renewable fuel and chemicals from lignocellulosic biomass

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product of biomass pyrolysis, contains abundant compound resources. Particularly, the phenolic

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component (lignin derivatives) in the pyrolysis bio-oil can be used to produce renewable arenes

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via hydrodeoxygenation6-7. The crux of this issue is to establish a controlled catalytic system

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which can selectively cleave the “CAr–O” bonds without hydrogenation of the aromatic rings 8-10.

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Due to the high bond dissociation energies of C-O bonds in aryl ethers and phenols, harsh

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reaction conditions are usually required, e.g. temperature more than 250 °C and/or pressure of

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hydrogen more than 30 bar 11-12. Severe reaction conditions would reduce the economic benefits

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and cause significant hydrogenation on aromatic rings. Besides, the complex composition of the

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pyrolysis bio-oil leads multiple side reactions, such as esterification, repolymerization etc., to

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take place in the HDO process 13,14. The products of those side reactions would cause irreversible

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adsorption and coke deposition on the catalyst surface, which shall be adverse to activity and

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selectivity of the catalyst system

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chemicals from pyrolysis bio-oil remain a challenge. 16,17.

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. Bio-oil, which is the liquid

.Those facts make the production of renewable aromatic


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Several different types of catalysts have been investigated in the HDO of phenolic

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compounds. Catalysts with reactive metal and functional support have presented great potential

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on the cleavage of C–O bonds. Most of those catalytic systems need severe reaction conditions

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and cycloalkanes instead of aromatic hydrocarbons were the major products 9, 18-23. Among those

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attempts, an impressing research shows that RANEY Ni with zeolite can convert phenols into

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aromatic hydrocarbons using isopropanol as H-donor 24, 25, although a high usage amount of the

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catalyst was needed. Recently, some research indicated that catalysts of solid acid supported Ru,

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e.g. Ru-sulfate zirconia, Ru-WOx/ SiO2–Al2O3, exhibited both good activity and high selectivity

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on the controlled hydrodeoxygenation process of phenolic model compounds 26, 27. Water was

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used as solvent media, and C6-C9 aromatic hydrocarbons were the main products of the

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corresponding phenolic reactants. Those illuminating works demonstrated that producing

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aromatic hydrocarbon from a pretreated bio-oil which is mainly made up by guaiacol, syringol

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and their derivatives is with practicability.

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In this study, an efficient heterogeneous catalyst of Ru-WOx/ZrO2 was prepared and used in

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the controlled hydrodeoxygenation of both phenolic model compounds and a pretreated bio-oil.

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As a typical mixed oxide catalyst, WOx/ZrO2 could be more stable under hydrothermal

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environment than SO42-/MxOy solid superacid catalysts28. The added WOx can not only provide

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oxophilic sites, but also strongly affected the formation process of zirconium oxyhydroxide

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crystallization into ZrO2, and enhance the amount of t-ZrO2 during the calcination process,

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which is proved to be indispensable for the generation of active acid sites for isomerization,

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dehydration, and cracking, etc. The tungsten in WOx/ZrO2 can be in the form of surface mono-

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and poly-tungstate species and subnanometre clusters29. By changing the content and/or existing

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form of tungsten on the catalyst surface, an adjustable surface acidity property can be obtained.


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The acidity strength of WOx/ZrO2 can shift from near-neutral to solid superacid, and various

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changes in acid types can also be achieved, which promised a larger adjustable range for the

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selectivity properties of the catalyst. Thus, an enhancement on stability and activity/selectivity

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based on acidity change is expected to be obtained. Satisfactory yield and selectivity was

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achieved in hydrodeoxygenation of model compounds. An attempt to clarify the selectivity

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properties of the catalytic system was conducted from the perspectives of adsorption capacity

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and acid properties of the catalyst and the reaction pathway. In the hydrodeoxygenation of a

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pretreated bio-oil, mono-phenols in the feedstock can be majorly converted into arenes by a 5 h

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reaction with 1 MPa H2 at 240 oC. The depolymerization of high-molecular-weight phenolic

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oligomers was found to be significant, which further enhanced the yield of aromatic monomers.

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Experimental Section

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Materials. The phenolic model compounds (guaiacol, 2-methoxy-4-methylphenol, eugenol

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and syringol) were purchased from TCI. They were chosen by the composition of the pretreated

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bio-oil. Zirconium hydroxide and ammonium paratungstate were supplied by Aladdin Reagent

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Co. The other reagents or solvents were purchased from Sinopharm Chemical Reagent Co.

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The bio-oil used in this study is prepared from rice husk by flash pyrolysis (Shandong

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Toroyal Group Co., Ltd), which contained about ca. 7 wt. % detected phenols. Before it was used

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in the catalytic reaction, a water elution process (1.5 mL water per 1 mL bio-oil, 50 oC, eluted for

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3 times) was conducted to remove the water-soluble oxygen-containing compounds, and then the

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ethanol insoluble compounds were removed by centrifugation (2 mL ethanol per 1mL water-

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eluted bio-oil). About 5.9 g pretreated bio-oil can be obtained from 10g bio-oil feedstock, in

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which the content of detected phenolic compounds rise to ca. 18 wt. %. The composition of the


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bio-oil feedstock and the eluted bio-oil was presented in the supporting information (Part I, Fig.

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S1 and Table S1).

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The catalyst. The WOx/ZrO2 support was prepared by impregnation method. Zr(OH)4 was

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immerged in ammonium paratungstate solution. The support was dried at 110 oC for 12 h, and

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then calcined at the chosen temperature in a muffle furnace for 5 h. RuCl3 was impregnated onto

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the WOx/ZrO2 support by the incipient wetness method. The catalysts were dried and reduced in

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a H2 flow at 200 oC for 5 h prior to use. The prepared catalysts were designated as x%Ru-

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y%WOx/ZrO2, where x% and y% are the mass percentage of the loading Ru and W, respectively.

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XRD, SEM, XPS, Py-FTIR and NH3-TPD, etc. were used to study the structure and properties of

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the catalysts, and the details were listed in supporting information, Part II.

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Catalytic reaction. Typically, phenolic model components (5 mmol), Ru-WOx/ZrO2

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catalyst (0.1 g) and 35 mL water were placed in a 100 mL stainless steel reactor with magnetic

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stirring. After removing the air by purging the reactor with hydrogen several times, the reactor

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was charged with the chosen hydrogen pressure and heated to the experimental temperature.

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When the experiment finished, the reactor was dipped into cold water. The product was extracted

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with ethyl acetate and analyzed by GC (Shimadzu, 2010 plus) and GC-MS (Shimadzu, GCMS-

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QP 2010 Ultra). An internal standard n-dodecane was added to achieve quantitative analysis. The

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process for hydrodeoxygenation of the pretreated bio-oil was similar as that of model

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compounds. The selectivity of the catalyst was estimated by the following equation:

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S - a % = na × 100 % / nr

(1)

In equation (1), S-a % means the selectivity to arenes; na and nr are mole number of the generated arenes and converted guaiacol, respectively.


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Results and Discussion

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Hydrodeoxygenation of the model phenols. The activities of a series of Ru-WOx/ZrO2

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catalysts for the selective cleavage of CAr–OH and CAr–OMe bonds were investigated by using

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guaiacol and syringol as model compounds (Table 1).

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Table 1 Performances of the Ru-WOx/ZrO2 catalysts for controlled hydrodeoxygenation of guaiacol and syringol (0.1 g catalyst, 5 mmol reactant, 1 MPa hydrogen, 240 oC)

Catalyst

Substrate

Con. %

3%Ru-15%WOx/ZrO2 (550 oC) 3%Ru-15%WOx/ZrO2 (650 oC) 3%Ru-15%WOx/ZrO2 (700 oC) 3%Ru-15%WOx/ZrO2 (750 oC) 3%Ru-15%WOx/ZrO2 (850 oC) 3%Ru-10%WOx/ZrO2 (700 oC) 3%Ru-30%WOx/ZrO2 (700 oC) 5%Ru-15%WOx/ZrO2 (700 oC) 7%Ru-15%WOx/ZrO2 (700 oC) 10%Ru-15%WOx/ZrO2 (700 oC)

A B A B A B A B A B A B A B A B A B A B

77.1 66.2 79.3 72.8 93 83.7 86.4 81.3 68 45.1 84.1 75.1 61 44.3 97 85.7 98.2 94.7 98 95.2

1 62.9 35.5 59.8 30.1 80 76.2 77.5 72.1 70.6 67.2 56 21.1 41.2 17.4 85 80.1 90.2 85.8 90.5 83.9

2 19.4 48.5 20.1 47.9 7 2.6 8.1 10.2 12.7 8.9 26.1 61.1 36.6 62.3 2.1 3.8 4 5.7 2 3.6

3 5.4 5.6 5.1 9.1 4.4 2.2 3.3 3.1 4.6 6.2 7.2 5.3 7.5 5.2 3 0.1 3.2 1 3 0

S% 4 5.3 3.7 8.6 5.2 1.3 2.3 2.3 1.7 5.2 3.5 9 5.7 8.9 4.1 2 1 1.7 1 1.3 0

5 1.7 1.2 1.5 1.1 1.1 1.2 1.1 1.2 0.9 1.1 0.5 0.6 1.4 1.1 1.1 1.3 0 1 0.6 2

6 2 1.6 1.5 1.5 0.2 1.3 1.4 1.1 0.8 0.9 0.7 0.4 1.6 1.8 1.2 1.1 0.8 1.2 1.4 2.2

7 3.3 3.9 3.4 5.1 6 14.2 6.3 10.6 5.2 12.2 0.5 5.8 2.8 8.1 5.6 12.6 0.1 9.3 1.2 11.3

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7%Ru-15%WOx/ZrO2 turned to be the optimized catalyst since it achieved the highest yield

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of benzene. The performances of the optimized catalyst for more model compounds (others of

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the typical lignin-derived phenols, dimeric phenyl ethers) and a mixed phenol sample are

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presented in Fig. 1 and Table S2. All model components can be converted into aromatic


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hydrocarbons with high conversion and selectivity. For the mixed phenol sample, the conversion

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and selectivity to aromatic products were all around 90 %.

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Fig. 1. The controlled hydrodeoxygenation of other model compounds (5 mmol reactant for each reaction) and a mixed phenols sample (mono-phenolic compounds were reacted for 2 h; the mixed phenols sample was reacted for 2.5 h)

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similar to that of WOx/ZrO2 support (Fig. 2). A small peak of metal (Ru) was found at 44 o. The

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addition of tungsten would restrain transformation of the t-ZrO2 phase to the more

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thermodynamically stable m-ZrO2 phase, and thus only characteristic peaks of tetragonal-ZrO2

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(t-ZrO2) were clearly observed. The characteristic peak of WO3 was not found in either the

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optimized catalyst or its support 30-33. It is mainly because the WO3 is distributed on top of the

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ZrO2 phase until multilayers can form leading to crystalline WO3. When excessive tungsten was

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loaded, e.g. 30%WOx/ZrO2, the characteristic peaks of WO3 appeared, and the color of the

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support turned to yellow from white. Besides, the particles of Ru and W uniformly distributed on

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the catalyst surface, which confirmed by The STEM spectra and the corresponding elemental

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mapping analysis (Fig 3).

Characterization of the optimized catalyst. The XRD graph of the optimized catalyst is


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1 2 3

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Fig. 2 The XRD analyses of the Ru-WOx/ZrO2 catalysts

Fig. 3 STEM spectra and the corresponding elemental mapping analyses for the Ru and W particles of the optimized catalyst: (a) STEM spectra of the optimized catalyst; (b) elemental mapping analysis of Ru particles; (c) elemental mapping analysis of W particles; (d) elemental mapping analysis of Ru + W particles Fig. 4a shows the X-ray photoelectron spectroscopy (XPS) of Ru particles. The Ru 3d5/2

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peak at 280.8 eV is attributed to Ru0, which is much larger than the Ru 3d5/2 peak at 282.4 eV

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corresponding to RuO2. The peak area ratio indicated that about 79 % of the total Ru particles in

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the form of Ru0. The W 4f spectrum of the optimized catalyst is presented in Fig. 4b. Peaks at


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35.7 and 34.9 were characteristics of W6+ 4f7/2 and W5+ 4f7/2, respectively. The W6+ %

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calculated by the area ratio of corresponding peak is around 63 %. The peaks at 31.7 and 30.1 are

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mostly attributed to Zr4+ 22.

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5 6 7

Fig. 4 The XPS spectra of the optimized catalyst: (a) Ru 3d; (b) W 4f

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Influence of preparation conditions for the catalyst. A series of catalysts with different

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W contents and calcination temperatures were prepared, and their properties were listed in Table

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2.

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From table 2, it can be found that the surface area would be decrease with the rising of the

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calcination temperature. Besides, increasing W and Ru content would lead a decrease on the

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dispersion of Ru. When the W % is constant, the dispersion of Ru was increased with the

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concentration of medium acid sites (NH3 desorption temperature from 200 oC to 400 oC,

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Supporting Information Part II).

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Table 2 Influence of preparation conditions on the catalyst properties a

Item.

2 3 4 5

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Ru %

W %

o

T-c C

b

surface area m2/g

W W6+ % c

Ru W5+ % c

Ru0 % c

Ru4+ (RuO2) % c

1 3 15 550 57.2 81 19 82 18 2 3 15 650 47.3 83 17 81 19 3 3 15 700 44.6 69 31 83 17 4 3 15 750 40.4 70 30 82 18 5 3 15 850 33.6 71 29 84 16 6 3 10 700 43.1 83 17 80 20 7 3 30 700 42.8 89 11 85 14 8 5 15 700 45.6 65 35 81 19 9 7 15 700 46.8 63 37 79 21 10 10 15 700 48.1 64 36 72 28 a : the catalysts were reduced in H2 flow at 200 oC for 5 h; b : calcination temperature; c : the content of W or Ru with different valences in total W or Ru on catalyst surface. d : the partical size of Ru was detected by TEM

partical dispersion size nm d degree 11.1 0.091 10.8 0.093 9.7 0.121 9.5 0.114 13.2 0.085 7.3 0.142 14 0.079 9.6 0.102 10 0.089 13.3 0.062

Acid concentration mmol/g (NH3-TPD) B/L value (Py-IR) 100-200 200-400 sum o o C C 0.09164 0.17424 0.26588 0.586465 0.15576 0.19344 0.34920 0.494429 0.14844 0.28420 0.43264 0.938025 0.10892 0.25656 0.36548 0.927892 0.09380 0.1482 0.24200 0.836939 0.11140 0.22808 0.33948 0.445417 0.09376 0.14748 0.24124 0.394404 0.13641 0.28010 0.41651 0.939601 0.15927 0.27135 0.43062 0.942332 0.16824 0.27443 0.44267 0.946800


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The concentration ratio of Brønsted acid sites to Lewis acid sites (B/L) was initially

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increased with the calcination temperature and reached the maximum value at 700 oC, and so did

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the concentration of total acid and medium acid sites (NH3 desorption temperature from 200 oC

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to 400 oC)

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temperature is less than 500 oC, the ZrO2 was majorly in the form of amorphous; with the

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calcination temperature rising, the crystal form of ZrO2 was changed to t-ZrO2 then to m-ZrO2

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(>800 oC)

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sites. Besides, the W content also greatly influenced the acid properties of the catalyst. The

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existence form of tungsten significantly affected the acidity properties of Ru-WOx/ZrO2 catalyst.

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The B/L value was increased with the content of W5+ %, because Brønsted acid sites would form

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on WOx clusters when W6+ centers reduce slightly 36. When the total W content (W%) was low,

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the surface mono-tungstate species would have strong interaction with ZrO2 and thus are hard to

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be reduced in the H2 flow. On the other hand, if superfluous W was loaded on ZrO2, an

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obvious agglomeration would occur, and the agglomerate WO3 was also easy to be reduced into

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WO2, leading to a decrease on acidity acid strength 37. In consequence, an appropriate W content

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(15% in this study) was needed to obtain optimized acidity properties of the catalyst for the HDO

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process. It is reasonable to assume that the Brønsted acid sites generated from slightly reduced W

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species, which proved to be active in isomerization, dehydration, and cracking reactions etc.,

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majorly have middle acidity strength

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B/L value with the change on existence form of tungsten.

34, 35

32

. It mainly caused by the change on crystal form of ZrO2. When the calcination

. The t-ZrO2 has been proved to be indispensable for the formation of strong acid

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, causing the similar tendencies of acidity strength and

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Influence of the experimental conditions. The influences of temperature and hydrogen

22

pressure were investigated by uniform design and regression analysis using the mixed phenol

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sample as reactant (Fig. 5). Increasing hydrogen pressure would enhance the conversion, but


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cause a decrease in selectivity to arenes. Phenols in the mixed sample can be effectively

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converted into cyclohexanol/cyclohexane and their derivatives when hydrogen pressure was

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more than 1.5 MPa even at a low temperature of 160 oC. The cleavage of “CAr-OCH3” and “CAr-

4

OH” required a high temperature because of their high bonding energy. Thus,

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hydrodeoxygenation was the major reaction only when reaction temperature was more than 200

6

o

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high temperature (240 oC) and relatively low hydrogen pressure (1 MPa) was suggested.

C. In consideration of maintaining high conversion and selectivity, experimental condition with

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Fig. 5 The influences of temperature and hydrogen pressure for hydrodeoxygenation of the mixed phenols sample (a) experimental design and results;(b)change tendency of conversion; (c) change tendency of selectivity to arenes (0.1 g catalyst, 5mmol reactant, reacted for 2.5 h)

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its derivatives can be selectivity convert into benzene within 2 h; while for syringol, it took at

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least 2.5 h to reach a Con. % of more than 80%.

The Influence of reaction times was studied at 240 oC with 1MPa hydrogen. Guaiacol and

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Table 3 shows the influence of different solvents in the hydrodeoxygenation of guaiacol.

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Aromatic hydrocarbons can only be selectively produced in water. When 10 % of methanol was

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added, the conversion was decreased by 31.8 % and a rising on phenol content in the product

19

was observed. The conversion and selectivity to benzene were low if methanol or isopropanol

20

was used as the only solvent. When using alcohols as solvent, more light alkanes (n

Controlled Hydrodeoxygenation of Phenolic Components in Pyrolysis

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