Hydrodeoxygenation of Guaiacol Catalyzed by High-Loading Ni

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Kinetics, Catalysis, and Reaction Engineering

Hydrodeoxygenation of Guaiacol Catalyzed by HighLoading Ni Catalysts Supported on SiO2-TiO2 Binary Oxides Mohong Lu, Yu Sun, Peng Zhang, Jie Zhu, Mingshi Li, Yuhua Shan, Jianyi Shen, and Chunshan Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04517 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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Industrial & Engineering Chemistry Research

Hydrodeoxygenation of Guaiacol Catalyzed by High-Loading Ni Catalysts Supported on SiO2-TiO2 Binary Oxides

Mohong Lua,c, Yu Sun a, Peng Zhang a, Jie Zhua, Mingshi Lia,*, Yuhua Shana , Jianyi Shenb,* and Chunshan Songc

aJiangsu

Key Laboratory of Advanced Catalytic Materials and Technology, and

Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou, 213164, China b

Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing, 210093, China c

Clean Fuels and Catalysis Program, EMS Energy Institute, Departments of

Energy & Mineral Engineering and of Chemical Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, PA, 16802, USA

*Corresponding author: 1、Professor Mingshi Li School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu, 13164, China E-mail: [email protected] 2、Professor Jianyi Shen School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China E-mail: [email protected]

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Abstract A series of SiO2-TiO2 binary oxides supported high loading Ni catalysts were prepared

using

hydrodeoxygenation

the

co-precipitation

(HDO).

The

method

catalysts

were

and

tested

in

characterized

guaiacol using

N2

adsorption-desorption, XRD, TEM, FT-IR, XPS, H2-TPD and NH3-TPD. The formation of Si-O-Ti bond in the SiO2-TiO2 binary oxides was verified by XPS, which can increase the total amount of acidic sites and enhance the interaction between metal Ni and TiO2. The oxygen defect sites of TiO2 were formed near the perimeter of the metal-support interface, leading to generate Niδ--OV-Ti3+ interface sites, which play the role of active centers to catalyze guaiacol HDO. These positive factors promoted HDO activity over Ni catalysts supported on binary oxides, compared to Ni loaded on single oxide supports. The selectivity of products indicates relatively high temperature and low H2 pressure are beneficial for producing oxygen-free aromatics in HDO of guaiacol. Keywords: Ni, SiO2-TiO2 binary oxides, Hydrodeoxygenation, Guaiacol, Meal-support strong interaction

2


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1. Introduction

To meet the growing energy demand, bio-oils derived from biomass have gained significant attention due to its renewablity.1 However, these oxygen-rich bio-oils (10-45wt%) suffer from poor chemical stability, low heating value, undesirable viscosity and immiscibility with hydrocarbon fuels, preventing the liquid from being used as transportation fuels prior to upgrading.2 Consequently, the most important step of the upgrading is the process of hydrodeoxygenation (HDO) to make the pyrolysis oil more competitive substitute for fossil fuels.3 HDO is to remove the O-contained groups from phenolic compounds under hydrogen in the presence of catalysts. In a range of research work, guaiacol is often utilized as a model compound of bio-oil since its structure incorporates three typic C-O bonds, such as CAR-OH, CAR-OCH3 and CARO-CH3, which represent lignin and lignin-derived phenolic monomers.4, 5 Various catalysts have been investigated for the HDO of guaiacol. Traditional sulﬁded catalysts have been tested in HDO processes by many researchers.6 However, there are several drawbacks with using sulfide catalysts in the reaction: (1) for the purpose of maintaining the catalysts sufided state, sulfur (e.g., H2S) has to be introduced during the HDO processes, and simultaneously, the sulfur contamination of the product is inevitable.7, 8 (2) the γ-Al2O3 support may undergo rapid deactivation by carbon deposits and be unstable in the presence of water.9 To avoid the contamination of sulfur to products, noble metal-based catalysts have also been studied extensively.10, 11 They exhibit excellent HDO activity as it is easy for noble metal to absorb the reactants with moderate strength and can form active intermediate compounds.12 However, the cost of utilizing noble metal is greatly 3



high, which would limit its industrial application in a large scale. Transition metal phosphides, metal nitrides, metal carbides and borides catalysts have a better development prospect in industrial production as they are easy to prepare, inexpensive and exhibit a high HDO activity.2,

13-19

However, due to the drawbacks of these

catalysts such as poor stability and fast deactivation their catalytic performance requires further study.10 The more cost-effective monometallic Ni catalysts seem to be a promising candidate for HDO process.20 The loading of metallic nickel has significant effect on the activity of Ni catalysts. Although the reducibility of metallic nickel catalyst is improved as the Ni loading increases, the dispersion of the Ni particles would decrease, and the amount of surface active nickel sites and acidic sites will also be affected, which will in turn influence HDO activity of nickel catalyst. Thus, it is difﬁcult to improve loading, reducibility and dispersion simultaneously with a simple method.21 Xue et al. prepared Ni/SiO2 catalysts with high loading and dispersion by using the n-butanol as solvent to replace water. The low surface tension of n-butanol could reduce the interaction between grains and then inhibit the agglomeration of the precipitated small particles. In this way, highly dispersed Ni catalysts were obtained.22-24 The physicochemical properties of support are among the key factors which may have impacts on the HDO activity of supported metal. Jin et al. investigated the Ni catalysts supported over activated carbon, SiO2, SBA-15 and γ-Al2O3. The results showed that the silica materials possess a certain amount of acidic sites and high specific surface area which will promote the dispersion of active metal and display a better deoxygenation activity for anisole.7 However, as an inert support, the weak interaction between SiO2 and active metal is not conducive to the improvement of the 4


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HDO activity of the catalyst. Our previous work indicated that TiO2can be reduced partially to TiO2-X species, which can migrate to the surface of metal particles.10, 11 The new catalytic centers at the interface of metal and TiO2-X created can interact with the chemisorbed C-O bond and make them subject to the hydrogen attack, leading to a significant decrease of C-O bond scission energy. However, the relatively low surface area of TiO2 is unfavorable for the better metals dispersion. Amorphous silica-titania binary oxides have raised considerable concern because of their high homogeneity and dispersion of titanium. Various sorts of spectroscopic characterization techniques also demonstrated the existence of Ti-O-Si linkages in SiO2-TiO2 binary oxides which suggested the strong interaction of silica with titania and generated new BrØnsted acidic sites caused by charge imbalance of Ti-O-Si bridges.25, 26 In this work, we combined the two oxides, TiO2 and SiO2 as a support to modify catalysts for the guaiacol HDO. The detailed physicochemical properties of Ni supported over SiO2, TiO2, and SiO2-TiO2 binary oxides were obtained from XRD, TEM, FT-IR, XPS, NH3-TPD and H2-TPD. These characterization results demonstrated the strong interaction between Ni and partially reduced titanium species, which can improve the catalytic activity. The relatively higher activity achieved on Ni/SiO2-TiO2 catalysts compared with Ni/SiO2 and Ni/TiO2, confirmed that the high-loading Ni supported on SiO2-TiO2 is meaningful for the upgrading of lignin-derived oils.

2. Experimental

2.1. Catalyst Preparation

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All of the 60 wt% nickel metal catalysts were prepared by the co-precipitation method. Na2SiO39H2O (AR) and TiCl4 (99.0%) in ethanol were used as

Sisource

and Ti source, respectively. In brief, appropriate amounts of Ni(NO3)26H2O (AR) and 30 wt% TiCl4-ethanol solution were added into deionized water (100 ml) to obtain solution I. The desired amounts of Na2SiO39H2O and Na2CO3 (AR) were dissolved into 100 ml deionized water to obtain solution II. Both solutions were simultaneously added into 200 ml deionized water to form a mixture, which was stirred vigorously at 80 °C for 1 h to get a composite comprised of different Si/Ti molar ratios. Subsequently, the precipitate was washed using deionized water until no Cl- was detected. The precipitate was collected, and then 200 ml n-butanol (AR) was added into the precipitate to form a mixture. After evaporating the mixture at 80 °C for 12 h, the solid obtained was further dried at 120 °C for 12 h. The sample was denoted as Ni/SiO2-TiO2(x) (x represents the molar ratio of Si/Ti in binary oxides). For comparison, Ni/SiO2 and Ni/TiO2 were prepared with the similar method.

2.2. Catalyst characterization

BET surface area of catalysts was calculated according to nitrogen adsorption-desorption isothermsobtained by using ASAP 2020 automated system. XRD patterns of samples was tested suing a Rigaku D/Max2400 diffractometer with Cu-K radiation (λ=1.5418Å) to determine the crystalline structure. The samples were scanned in the 2θ range from 5° to 80° with a 0.02° step size. Scherer equation was used to calculate the crystallite size. TEM images were recorded with a JEOL-2010 at 200 kV. FT-IR study was carried out to examine the functional groups of the SiO2-TiO2 supports. The spectra (Bruker Tensor 27, Bruker, Germany) were 6


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recorded in a wave number region from 4000 to 400 cm-1. Prior to infrared analysis, the samples were prepared with KBr into a pellet form. XPS analysis using Mg-K X-ray (1253.6 eV) was conducted on a Multilab 2000 X-rayphotoelectron spectrometer. The binding energies of C 1s (284.8 eV) was used to calibrate. Prior to the characterization of XRD, TEM, FT-IR and XPS, the nickel catalyst precursor was reduced in flowing hydrogen (1 atm, 50 mL/min) at 450 °C for 3 h. After the reduced sample was cooled to the room temperature, it was passivated by 0.5% O2/Ar mixture for 24 h. NH3-TPD and H2-TPD was carried out on the ASAP 2020 analyzer. The sample (50 mg) was placed into a quartz U-tube reactor and treated by a 10% H2/Ar flow at 450 °C for 3 h. A 10% NH3/He stream was introduced into the sample for 1 h after it was outgassed in He at 100 °C for 1 h. Then the sample was outgassed in He flow (30 ml/min) to remove physisorbed NH3. The ammonia and hydrogen desorbed were detected using a thermal conductivity detector (TCD). For H2-TPD experiments, 50 mg samples were reduced in H2 for 3 h at 450 °C and was outgassed in He for 1 h. A 10% H2/Ar stream was introduced into the sample for 1 h after it was cooled to 50 °C. The ammonia and hydrogen desorbed were detected using a thermal conductivity detector (TCD).

2.3. Catalyst activity measurements

The HDO of guaiacol was carried out on a continuous down-flow microreactor with 6 mm inner diameter. Briefly, 0.20 g of the as-synthesized catalyst (20-40 meshes) was placed in the middle of the reactor filled by quartz sand at two ends. The nickel catalyst precursor was reduced in flowing hydrogen (1 atm, 50 mL/min) at 7



450 °C for 3 h. After cooling the microreactor to the designed temperature, a 3 wt% guaiacol solution in n-decane was pumped into the reactor. The activity evaluation was carried out under atmosphere (2 MPa and 80 mL/min) in the reange of temperature from 160 to 220 °C. When the reaction condition reached steady state for 3 h, the liquid reactant mixture was collected and analyzed using GCwith a flame ionization detector (FID) and a HP-5 column. Guaiacol conversion (XGUA) and product selectivities (Sproducts‑i) were defined as the two equations: XGUA =(MolGUA-in – MolGUA-out)/MolGUA-in; Sproducts-i = (Molproduct-i × ni)/(Molreacted guaiacol × 7), ni refers to the carbon number in product-i. The turnover frequency (TOF) for cyclohexane produced was calculated as the equation below. TOF (S -1) =

Reactant flow rate (µmol s) × conversion × selectivity of cyclohexane Quantity of sites (µmol g) × Catalyst weight (g)

Where the active sites of Ni catalyst were quantified based on the uptake of H2.

3. Results and discussion

3.1. Catalyst characterization

The N2 adsorption analysis was carried out to investigate the textural properties of nickel metal catalysts supported on SiO2, TiO2 and SiO2-TiO2 supports. Figure 1 depicts the isotherms for all the catalysts presenting characteristic type IV classification with different kinds of hysteresis loops which are associated with the 8


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presence of mesoporous nature.27 With increase of the TiO2 content, the hysteresis loop became broader and the position of the inflection point shifted to higher relative pressure, indicating the formation of larger mean pore size.28 In addition, the hysteresis loop type of Ni/SiO2 and Ni/SiO2-TiO2 (2:1) can be classified as H4, which indicates that narrow slit-like pores were formed. Ni/TiO2, Ni/SiO2-TiO2 (1:1) and Ni/SiO2-TiO2 (1:2) exhibited H3 type loops due to the presence of slit-shaped pores.27, 29

The textural properties of a series of 60 wt% Ni-containing catalysts are summarized in Table 1. Ni/TiO2 catalyst possesses the smallest surface area and total pore volume among the five catalysts. The surface area of catalysts increased from 191 to 223 m2/g increasing Si/Ti atomic ratio, while the pore volume slightly declined. The textural differences suggest that the addition of SiO2 increases the surface areas of catalysts, which promote the dispersion of metal nickel. Figure 2 depicts the XRD patterns of different catalysts. All the catalyst precursors (Figure 2a) displayed broad and weak peaks around 2θ = 24o, 36o and 63o, which can be assigned to Ni2(OH)2CO3H2O and nickel hydrosilicate.22 The XRD patterns for the reduced catalysts (Figure 2b) shows distinct diffraction peaks around 44°, 52° and 76°, which are assigned to the (111), (200), and (220) reflections of the Ni phase. It is obvious that relatively small metal Ni particles are formed for Ni/SiO2 and Ni/SiO2-TiO2 catalysts due to the broad and weak XRD peaks. For Ni/TiO2 the peaks assigned to anatase and rutile were observed. 27, 30 Moreover, it can be seen that all the catalysts except Ni/TiO2 showed a broad peak near 25° that is attributed to amorphous silica, and no peaks assigned to bulk TiO2 crystalline particles in SiO2-TiO2 binary oxides were observed, indicating that the presence of silica constricts the growth of the crystallinity of the titania species.28 The average diameter 9



of Ni obtained from Scherrer equation is given in Table 1. Ni/TiO2, Ni/SiO2-TiO2 (1:2), Ni/SiO2-TiO2 (1:1) Ni/SiO2-TiO2 (2:1) and Ni/SiO2 showed crystallite sizes of 14.9, 9.4, 9.0, 9.0 and 8.5 nm, respectively, suggesting the incorporation of silica could improve the dispersion of nickel metal, which will favor the HDO performance. TEM studies were used to characterize the microstructure of the highly loaded metallic Ni catalysts. Figure 3 displays representative TEM images for the catalysts reduced. For Ni/SiO2-TiO2 samples, the nickel particles size was quite uniform and the mean Ni particle size decreased with the increase of Si/Ti atomic ratio. And no lattice fringes attributed to titania particles were observed in the Ni/SiO2-TiO2 samples, indicating that the titania phase exists in the form of amorphous. This suggests that TiO2 is dispersed well in SiO2-TiO2 binary oxide due to the interconnection of silica and titania.29 The Ni particles apparently aggregated in Ni/TiO2 and the particle size distributed within the range of 12-18 nm. Compared with TiO2, SiO2-TiO2 binary oxide support promotes the dispersion of Ni according to the results from XRD and TEM. Figure 4a and Figure 4b present the FT-IR spectra of the catalyst precursors and reduced catalysts. The broad adsorption peaks located at 3473 cm-1 and 1632 cm-1 corresponding to the stretching vibration and bending mode of the hydroxyl groups of water molecular adsorbed in the catalyst, respectively.31, 32 It can be observed that the amount of -OH groups decreased significantly after the treatment in H2 flow at 450 °C. The band at about 475 cm-1 is ascribed to Si-O in SiO2 lattice or Ti-O bonds in TiO2 lattice, respectively.33 The broad and weak peaks at 1015 cm-1 could be attributed to the asymmetrical stretching vibration of Si-O-Si band in Ni/SiO2 and Ni/SiO2-TiO2 catalysts.25, 34 And a small shoulder near 915 cm-1 disappeared after the treatment in H2 at 450 °C, which is attributed to Si-OH groups.29, 35 10


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XPS technique was employed to investigate the surface composition and oxidation state of Ni, Ti, Si, and O elements. As shown in Figure 5a, the peaks at 530.0, 531.2, 530.9.0, 531.9 and 532.8 eV were attributed to the O1s in Ti-O-Ti bond, Ti-OH groups, Si-O-Ti bond, Si-O-Si bond and Si-OH groups, respectively,35-39 which is in agreement with the results from FT-IR (Figure 4b). Figure 5b shows the spectra of Si 2p. The peak with BE of ~102.8 eV is assigned to Si-O-Si bond.40, 41 And the Si 2p peak at ~101.9 eV may be assigned to Si-O-Ti bond.41 It is obvious that the BE of Si 2p peaks in SiO2-TiO2 mixed oxides decreased with the increase of Ti content, which turns out the occurrence of electron transform from Ti species to Si species. Figure 5c shows the Ni 2p core level spectra. it is clear that a proportion of unreduced Ni2+ species existed on the surface of the catalysts after 450 °C reduction, which are attributed to the passivation layer of NiO (855.2-855.9 eV) and the peaks at 861.1 eV could be assigned to Ni oxide.20 The peak at 852.1 and 853.2 eV can be attributed to Ni0 and Niδ+, respectively. 42 Notably, the deconvolution of the Ni 2p3/2 peak revealed that the binding energy peaks of Ni0 gradually negative shift increasing the Ti content, which suggests that the electron density gradually increases at the interface of the nickel atoms and forms Niδ- .43,

44

This phenomenon could be

explained by using the strong metal-support interaction (SMSI). The spillover hydrogen from the surface of metallic nickel diffused onto the reducible TiO2 surface to form partially reduced titania (TiO2-x). The transfer of electron from TiO2-x to the adjacent metal Ni atoms occurred to cause electron enrichment at the interface of Ni atoms. Meanwhile, the strong Ni-TiO2-x electronic interaction can induce the formation of the oxygen vacancies of titania species (i.e., OV-Ti3+ sites) near the perimeter of the metal-support interface and create the Niδ--OV-Ti3+ interface sites, 11



which play the role of active centers to catalyze the guaiacol HDO.43, 45-48 Compared with the percentage of bands assigned to Ni0 after the samples underwent the high-temperature reduction treatment (Table 2), the catalysts containing TiO2 exhibited a higher proportion of Ni0 than Ni/SiO2. This phenomenon can be ascribed to the partially reduced TiO2-x species, which may be beneficial in keeping the nickel species in the form of metallic Ni0, and can effectively mitigate the oxidation degree of nickel metal,10, 49, 50 which is supported by the formation of more Niδ+ species in the catalysts containing TiO2. The atomic ratios of Ni/(Si+Ti) for Ni/SiO2-TiO2 catalysts are lower than that for Ni/SiO2 and Ni/TiO2 (Table 2), indicating more Ni species are covered by SiO2 and TiO2, which is consistent with the trend of proportion of Ni0. The core level XPS spectra of Ti 2p (Figure 5d) can be deconvoluted to two peaks. The peak at about 458.9 and 458.4 eV are assigned to Ti4+ and Ti3+ species, respectively. 51-53 The value of Ti3+ 2p3/2 BE is higher than that (~457.5 eV) reported in the literatures.43,

52, 55

There are three main factors contributing to such a result.

Firstly, a part of the electron density of Ti3+ species could be transferred to Ni species through Niδ--OV-Ti3+ interface site during the formation of Niδ-.44 Secondly, since the electronegativity of Ti atoms are lower than Si atoms, and it is more polarizable than Si atoms, the positive charges could increase on the surface of Ti species. Moreover, the BE values of O 1s downward shift also indicated the negative charge enriched on O atoms and formed the Si-Oδ--Tiδ+,26 which is confirmed by the lower BE of Si 2p (Figure 5b). Thirdly, the catalytic activities of the highly loaded and dispersed Ni catalysts were so high that they were easily oxidized by oxygen in the air atmosphere after the high-temperature reduction in H2 flow. The partially reduced titanium species covered on the Ni atoms would be first oxidized by oxygen during passivation process. Considering the above factors, the BE values of Ti3+ 2p3/2 would obviously 12


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shift upward. According to the desorption peaks NH3 detected in the NH3-TPD profile (Figure 6), Ni/SiO2 catalyst had weak, medium and strong acid centers around 217 °C, 386 °C and 594 °C respectively, although many literatures reported that pure silica has neither BrØnsted acid sites nor Lewis acidity. The acidity of Ni/SiO2 would derive from two aspects: firstly, the unreduced nickel silicate confirmed by XPS can produce a certain amount of weak and medium-strong acidity ascribed to 217 °C, 386 °C, respectively.53 Secondly, the desorption peak appeared at 594 °C is probably due to the -OH groups on the surface of SiO2 support, which could be confirmed by the stronger acidity on Ni/ SiO2-TiO2 catalysts than that on Ni/TiO2. The amount of NH3 desorbed according to different temperature for all the catalysts was listed in Table 3. In general, the order corresponding to the amount of acid centers on the catalysts prepared by co-precipitation is as follows: Ni/SiO2-TiO2(2:1) > Ni/SiO2-TiO2(1:2) ≈ Ni/SiO2 > Ni/SiO2-TiO2(1:1) > Ni/TiO2. The adsorption of H2 was conducted to compare the dispersion of supported nickel and estimate the amounts of metal active sites. Figure7 depicts the H2-TPD curves for the catalysts. The relatively low temperature peak Ⅰ (300 °C) could be attributed to hydrogen spillover phenomena which occurred when the hydrogen species dissociated from the metallic Ni sites and migrated to the support.56, 57 The amount of H2 desorbed according to different temperature for all the catalysts was listed in Table 4. The value of desorbed hydrogen decreased obviously increasing the content of TiO2. The reason for this result could contribute to the small surface area of TiO2, which is unfavorable for the dispersion of metallic nickel. And more importantly, the morphological effects of SMSI should be considered. The 13



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titanium suboxide (TiO2-x) formed during high temperature H2 reduction migrates onto the surface of metal Ni particles, making more metallic Ni active centers covered by TiO2-x, which leads to the significant reduction of the amount of desorbed H2. Correspondingly, the coverage degree of metallic nickel increased,43,

58

which is

consistent with the result obtained from XPS.

3.2. Guaiacol HDO reaction

The guaiacol HDO was performed in the 160-220 °C temperature range under 2.0 MPa. Figure 8 shows guaiacol conversion on different catalysts. Ni/SiO2-TiO2 and Ni/SiO2 catalysts showed better HDO performance than Ni/TiO2 under the same reaction conditions. The trend of the HDO activity followed the order: Ni/SiO2-TiO2(2:1) > Ni/SiO2-TiO2(1:1) ≈

Ni/SiO2-TiO2(1:2) ≈

Ni/SiO2 >

Ni/TiO2. The HDO activity order could be attributed to four factors. Firstly, the surface area of TiO2 is the lowest, which caused the poor dispersion of nickel metal. Secondly, according to the results obtained from NH3-TPD (Table 4), the acid strength and overall acid amounts of Ni/SiO2-TiO2 and Ni/SiO2 were superior to Ni/TiO2, which favored the cleavage of C-O bond. Thirdly, the formation of TiO2-x covered on Ni particles decreased the number of Ni0 active sites. While increasing the content of SiO2 in SiO2-TiO2 binary oxides, the migration of TiO2-x was hindered and much more Ni0 active sites exposed to the reactants, thus leading to the higher catalytic activity.58 At last, the Si-O-Ti bond in the SiO2-TiO2 binary oxides verified by the XPS spectra suggests the presence of strong interaction between TiO2 and SiO2. This chemical bonding will restrain the growth of TiO2 particle and result in the increase in total effective surface area of TiO2 nanoparticles, which could promote the 14


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interaction between metal Ni and TiO2 support. Then more active centers are produced at the interface of Ni and TiO2, which would improve the HDO performance of Ni/SiO2-TiO2 catalysts. Moreover, plenty of new BrØnsted acidic centers generated on the surface of SiO2-TiO2 derived from Ti-OH or Si-OH (Figure 5), which favors the breakage of C-O bond. The product selectivity for HDO of guaiacol on the five Ni-based catalysts within

the

temperature

range

investigated

is

shown

in

Figure

9.

2-Methoxycyclohexanol was the main product at the reaction temperature lower than 200 °C for all the Ni catalysts. This result demonstrates that the high-loading nickel metallic catalysts have the strong hydrogenation ability. The amount of cyclohexane increased significantly at 220 °C for all catalysts except Ni/TiO2 and the selectivity of cyclohexane reached the maximum over the Ni/SiO2-TiO2 (2:1) catalyst. Ni/SiO2 exhibited relative low selectivity of cyclohexane within the temperature region examined compared with Ni catalysts supported on SiO2-TiO2 binary oxides, indicating the relative poor deoxygenation activity, even though acidity of Ni/SiO2 is not much inferior to the others. Moreover, based on the number of metallic Ni active sites carculated according to the uptake of H2, the turnover frequency (TOF) of the cyclohexane formation was calculated (Figure10). It can be seen that the TOF value for the formation of cyclohexane followed the trend: Ni/SiO2-TiO2 (1:2) > Ni/SiO2-TiO2 (2:1) ≈ Ni/SiO2-TiO2 (1:1) > Ni/TiO2 > Ni/SiO2. This result can be ascribed to the presence of Niδ--OV-Ti3+ interface sites on the surface of TiO2. The electron-enriched Niδ- active sites enhance the chemical adsorption of guaiacol molecule and the TiO2-x species activate the chemisorbed C-O bond and make the oxygenated molecules more easily attacked by the dissociated hydrogen on the Ni metal. Therefore, in the SiO2-TiO2 mixed oxides catalyst,the metal-support interface 15



sites serve as a new catalytically active center and enhance C-O bond cleavage ability of Ni/SiO2-TiO2.10, 11, 43 Unsaturated products, such as benzene, phenol, cyclohexene, anisole and toluene were not found under this reaction condition (2 MPa, 160-220 °C). It demonstrates that hydrogenating aromatic ring is the first step and then demethoxylation and demethylation reactions occur subsequently to result in cyclohexane.

3.3. Effects of temperature and pressure on HDO of guaiacol

Reaction conditions such as temperature and H2 pressure influence substantially the reaction pathway and product distribution of guaiacol HDO.7 At high hydrogen pressure and relatively low temperature, hydrogenation of

aromatic ring occurred on

the metal sites and the deoxygenation of 2-methoxycyclohexanol took place on acidic centers and Niδ--OV-Ti3+ interface sites as we discussed above. The conversion of guaiacol decreased slightly with the decrease of H2 pressure from 2.0 MPa to 0.5 MPa over the Ni/SiO2-TiO2 (2:1) catalyst (Figure11b). On the other hand, the selectivity of cyclohexanol raised significantly in the range of 160-220 °C, while the selectivity of cyclohexane did not change obviously. The phenomena could be demonstrated by the existence of two reaction pathways. The removal of -OCH3 from guaiacol to form phenol may be the first step under the deficient H2 pressure, followed by hydrogenation of phenol to produce cyclohexanol. Meanwhile, the HDO of guaiacol may occur following the reaction route at high pressure of H2, that is, the benzene ring is hydrogenated first to give 2-methoxycyclohexanol, and then the -OCH3 group is removed to form cyclohexanol. As a result, the simultaneous occurrence of the two different reaction pathways leads to an increase in the amount of cyclohexanol. 16


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At the elevated temperature of 340-360 °C and 0.5 MPa H2 pressure, main products are cyclohexane, cyclohexene, benzene and toluene. It can be noticed that a large amount oxygen-free unsaturated hydrogenation compounds are obtained (Figure11c). This result can be attributed to the fact that the high temperature is favorable for the hydrogenolysis of CAr-OCH3 and CAr-OH. Therefore, at low pressure and high temperature, the C-O bond breakage is main reaction rather than the aromatic ring hydrogenation, which is demonstrated by the emergence of phenol as an intermediate product (Figure 12). In general, we can get the conclusion that for the HDO of guaiacol on Ni catalysts, the high reaction temperature and low H2 pressure favor the production of benzene by the direct hydrogenolysis route, while at the low temperature and high hydrogen pressure, the hydrogenation of benzene ring in guaiacol and then the breakage of C-OCH3 and C-OH to form cyclohexane is the main reaction pathway. Based on the discussion above, the reaction network of guaiacol HDO catalyzed by Ni-based catalysts supported on SiO2-TiO2 binary oxides was proposed in Scheme 1.

4. Conclusion

Nickel supported on SiO2-TiO2 binary oxides prepared by the co-precipitation method exhibited better performance than Ni/TiO2. Meanwhile, the presence of partially reduced TiO2-x on the surface of SiO2-TiO2 binary oxides induced the formation of Niδ--OV-Ti3+ interface sites as new active sites for HDO of guaiacol, which can promote the breakage of C-O bond and result in a superior activity of Ni/SiO2 for deoxygenation. The results indicate that the Ni/SiO2-TiO2 catalysts exhibit excellent catalytic performance for the HDO of bio-oilunder the optimum 17



Page 18 of 44

conditions leading to production of cyclohexane-type HDO products. It is also noteworthy that the reaction temperature and H2 pressure affect the reaction pathway on supported Ni catalyst to a large extent for HDO of guaiacol.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21761132006 and 21676029). The authors also acknowledge the Natural Science

Foundation

of

Jiangsu

Higher

Education

Institutions

(grant

no.

12KJB530001), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for the financial support.

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22


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Page 23 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Surface areas and pore structures of all the catalysts reduced at 450 °C Catalysts

Surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter(nm)

Ni/TiO2

100

0.26

9.9

14.9

Ni/SiO2- TiO2 (1:2)

191

0.38

8.9

9.4

Ni/SiO2- TiO2 (1:1)

201

0.33

8.0

9.0

Ni/SiO2- TiO2 (2:1)

223

0.30

6.8

9.0

Ni/SiO2

334

0.35

5.3

8.5

23


Crystalline size of Ni (nm)


Page 24 of 44

Table 2. XPS data of catalysts Catalyst

Binding energy (eV) Ni 2p1/2

Ni 2p3/2

Ni0/Niall (%)

Ni/(Si+Ti)

Ti 2p3/2

Ni0

Niδ+

Ni2+

Ti3+

Ti4+

Ni/TiO2

851.7

853.3

855.2

458.0

458.5

1.7

0.93

Ni/SiO2-TiO2 (1:2)

852.0

852.8

855.4

458.1

458.7

1.8

0.48

Ni/SiO2-TiO2 (1:1)

852.2

853.1

855.6

458.3

458.9

2.1

0.47

Ni/SiO2-TiO2 (2:1)

852.2

853.7

856.0

458.4

458.9

1.4

0.56

Ni/SiO2

852.2

853.7

855.9

-

-

1.3

0.77

24


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Table 3. Weak, medium and strong acid sites concentration for all the catalysts. μmol(NH3) g-1

Catalyst Weak (450 °C)

Ni/TiO2

34

63

8

Ni/SiO2-TiO2 (1:2)

197

146

199

Ni/SiO2-TiO2 (1:1)

169

105

221

Ni/SiO2-TiO2 (2:1)

193

215

217

Ni/SiO2

166

129

233

25



Page 26 of 44

Table 4. Chemisorption of H2 on the catalysts reduced at 450 °C H2 (µmol/g)

H2 (µmol/g)

peak Ⅰ

peak Ⅱ

Ni/TiO2

48.9 (95 °C)

Ni/SiO2-TiO2 (1:2)

Da (%)

Db (%)

Coverage degree of Nic (%)

1.7 (554 °C)

1.0

7.5

86.7

84.9 (96 °C)

5.8 (520 °C)

1.7

11.5

85.2

Ni/SiO2-TiO2 (1:1)

170.2 (95 °C)

17.5 (550 °C)

3.3

12.0

72.5

Ni/SiO2-TiO2 (2:1)

209.6 (94 °C)

19.5 (525 °C)

4.1

12.0

65.7

Ni/SiO2

247.8 (93 °C)

29.2 (506 °C)

4.8

12.7

62.1

Catalyst

Note a: Metal dispersion (D) was calculated according to the uptake of H2. b: Ni dispersion was estimated based on XRD. c: Coverage degree of metallic Ni was defined as (1 - Da Db) × 100%. .

26


Page 27 of 44

300 9.9 nm

200 150 100

0

a

Ni/TiO2

Pore Volume ( cm3/g )

Volume ( cm3/g )

250

10

20

30

40

50

60

70

80

Pore diameter ( nm )

50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0 ) 300 8.9 nm

200 150

0

100

b

Ni/SiO2-TiO2 (1:2)


Volume ( cm3/g )

250

10

20

30

40

50

60

70

80


50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0 ) 300

200 150

0

10

100

c

Ni/SiO2-TiO2 (1:1)

8 nm

Pore Volume (cm3/g )

250

Volume ( cm3/g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

30

40

50


50 0 0.0

0.2

0.4

0.6

Relative pressure (

0.8 P/P0 )

1.0

27



300 6.8 nm


Volume ( cm3/g )

250 200 150

0

d

Ni/SiO2-TiO2 (2:1)

10

20

30

40

50


100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0 ) 300 5.3 nm

250 200 150

0

e

Ni/SiO2

Pore Volume (cm3/g)

Volume ( cm3/g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

20

30

40

50


100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0 )

Figure 1. N2 adsorption-desorption isotherms and pore size distributions of the five catalysts reduced in H2 at 450 °C.

28


Page 28 of 44

Page 29 of 44

Intensity ( a.u. )

 Ni2(OH)2CO3H2O

a

 Nickel hydrosilicate  

20









  Ni/SiO -TiO (1:1) 2 2





Ni/SiO2-TiO2 (2:1)

Ni/SiO2-TiO2 (1:2)





10

Ni/SiO2

30

40

  

Ni/TiO2



50

60

70

80

2 Theta ( Degree )

 Ni  Anatase  Rutile

b 

Intensity ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


    

10

20

30

Ni/SiO2



 Ni/SiO2-TiO2 (2:1)   Ni/SiO2-TiO2 (1:1)   Ni/SiO2-TiO2 (1:2)    



40



50



60

Ni/TiO2

70



80

2 Theta ( Degree )

Figure 2. XRD patterns of (a) Ni-based catalysts precursors and (b) reduced in H2 at 450 °C.

29



25

Ni/TiO2 Frequency (%)

20 15 10 5 0

0

2

4

6

8

10 12 14 16 18 20

Particle size ( nm )

35

Ni/SiO2-TiO2 (1:2) Frequency ( % )

30 25 20 15 10 5 0

Ni/SiO2-TiO2 (1:1)

0

2

4

0

2

4

6

8

10 12 14 16 18 20


30 25

Frequency ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 15 10 5 0

6

8

10 12 14 16 18 20


30


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Ni/SiO2-TiO2 (2:1) 40

Frequency ( % )

35 30 25 20 15 10 5 0

Ni/SiO2

0

2

4

0

2

4

6

8

10 12 14 16 18 20

6

8

10 12 14 16 18 20


35 30

Frequency ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10 5 0


Figure 3. TEM images and particle size distribution of the catalysts reduced at 450 °C.

31


a

3473

Ni/SiO2

1016

1632

475

915

Ni/SiO2-TiO2 (2:1) Ni/SiO2-TiO2 (1:1) Ni/SiO2-TiO2 (1:2) Ni/TiO2

4000 3500 3000 2500 2000 1500 1000

Wavenumber ( cm-1 )

500

b

Transmission ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Transmission ( a.u. )


Ni/SiO2 Ni/SiO2-TiO2 (2:1) Ni/SiO2-TiO2 (1:1) Ni/SiO2-TiO2 (1:2) Ni/TiO2

4000 3500 3000 2500 2000 1500 1000

500

Wavenumber ( cm ) -1

Figure 4. FT-IR spectra of (a) Ni-based catalysts precursors and (b) reduced in H2 at 450 °C.

32


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Ti-OH

O 1s

Ti-O-Ti

a Ni/TiO2

Si-O-Si

Ti-O-Si

Intensity ( a.u. )

Si-OH

Ni/SiO2-TiO2 (1:2)

Ni/SiO2-TiO2 (1:1)

Ni/SiO2-TiO2 (2:1)

Ni-O

536

534

532

530

Ni/SiO2

528

526

524

522

B.E.( eV ) Si 2p

Si-O-Si

Si-O-Ti

b

Intensity ( a.u. )

Ni/SiO2-TiO2 (1:2)

Ni/SiO2-TiO2 (1:1)

Ni/SiO2-TiO2 (2:1)

Ni/SiO2

108

106

104

102

100

Ni+

Ni+

B.E.( eV )

Ni 2p

98

Ni0

Ni/TiO2

Intensity ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96

c

Ni/SiO2-TiO2 (1:2)

Ni/SiO2-TiO2 (1:1)

Ni/SiO2-TiO2 (2:1)

Ni/SiO2

865

860

855

850

B.E.( eV )

33



Ti4+

Ti 2p

Ti3+

d

Ni/TiO2

Intensity ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni/SiO2-TiO2 (1:2)

Ni/SiO2-TiO2 (1:1)

Ni/SiO2-TiO2 (2:1)

468

466

464

462

460

B.E.( eV )

458

456

Figure 5. XPS spectra of (a) O 1s, (b) Si 2p (c) Ni 2p and (d) Ti 2p of Ni catalysts after reduction at 450 °C and passivation at room temperature.

34


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Page 35 of 44

Ni/TiO2

Intensity ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ni/SiO2-TiO2 (1:2)

Ni/SiO2-TiO2 (1:1)

Ni/SiO2-TiO2 (2:1)

Ni/SiO2

100

200

300

400

500 o

600

700

Temperature ( C )

Figure 6. NH3-TPD profiles of all the catalysts.

35



Ni/TiO2

Intensity ( a.u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni/SiO2-TiO2 (1:2) Ni/SiO2-TiO2 (1:1) Ni/SiO2-TiO2 (2:1) Ni/SiO2

100

200

300

400

500

600

Temperature ( oC )

700

Figure 7. H2-TPD profiles of all the catalysts.

36


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Page 37 of 44

100

Conversion ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 Ni/TiO2 Ni/SiO2-TiO2 (1:2) Ni/SiO2-TiO2 (1:1)

80

Ni/SiO2-TiO2 (2:1) Ni/SiO2

70 160

180

200

220

o

Temperature ( C )

Figure 8. Conversion of guaiacol as a function of temperature on different catalysts.

37



Selectivity ( % )

100 80

a

Cyclohexane Cyclohexanol 2-Methoxycyclohexanol

60 40 20 0 160

180

200

220

Temperature ( oC )

Selectivity ( % )

100 80


b

60 40 20 0 160

180

200

220

o

Temperature ( C ) 100

Selectivity ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

Cyclohexane Cycloexanol 2-Methoxycyclohexanol

c

60 40 20 0 160

180

200

220

o

Temperature ( C )

38


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Page 39 of 44

Selectivity ( % )

100 80


d

60 40 20 0 160

180

200

220

Temperature ( oC ) 100

Selectivity ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80


e

60 40 20 0 160

180

200

220

o

Temperature ( C )

Figure 9. Selectivity of products as a function of temperature over different catalysts (a) Ni/TiO2, (b) Ni/SiO2-TiO2 (1:2), (c) Ni/SiO2-TiO2 (1:1), (d) Ni/SiO2-TiO2 (2:1), (e) Ni/SiO2

39



0.007 0.006

TOF ( S-1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni/TiO2 Ni/SiO2-TiO2 (1:2)

0.005

Ni/SiO2-TiO2 (1:1)

0.004

Ni/SiO2

Ni/SiO2-TiO2 (2:1)

0.003 0.002 0.001 0.000 160

180

200

220

Temperature ( oC )

Figure 10. TOF of cyclohexane at different temperature.

40


Page 40 of 44

100

a


80 60 40 20 0

160

180

200

o

220

Conversion/Selectivity ( % )

Temperature ( C )


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 41 of 44

100

b Cyclohexane Cyclohexanol 2-Methoxycyclohexanol

80 60 40 20 0

160

180

200

Temperature ( oC )

220

100

c

Cyclohexane Cyclohexene Benzene Toluene

80 60 40 20 0

300

320

340

Temperature ( oC )

360

Figure 11. Conversion of guaiacol and selectivity of products at different reaction conditions. Reaction condition: (a) P = 2.0 MPa, T = 160-220 °C (b) P = 0.5 MPa, T = 160-220 °C (c) P = 0.5 MPa, T = 320-360 °C

41



100

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cyclohexane Cyclohexene Benzene Toluene Phenol

80 60 40 20 0 0.0

0.7

1.4

2.1

2.8

3.5

4.2

WHSV (h-1) Figure 12. Selectivity of products as a function of WHSV over Ni/SiO2-TiO2 (2:1) catalysts. Reaction condition: P = 0.5 MPa, T = 360 °C

42


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Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

OH

O OH

OH

Low temperature and high pressure

OH High temperature and low pressure

OH CH3

CH3

Scheme 1. Hydrodeoxygenation reaction network of guaiacol on Ni-based catalysts.

43



For Table of Contents Only

44


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Hydrodeoxygenation of Guaiacol Catalyzed by High-Loading Ni

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