Hydrodeoxygenation of Guaiacol on Ru Catalysts: Influence of TiO2

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Hydrodeoxygenation of Guaiacol on Ru Catalysts: Influence of TiO2-ZrO2 Composite Oxides Supports Mohong Lu, Hu Du, Bin Wei, Jie Zhu, Mingshi Li, Yuhua Shan, Jianyi Shen, and Chunshan Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02569 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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Industrial & Engineering Chemistry Research

Hydrodeoxygenation of Guaiacol on Ru Catalysts: Influence of TiO2-ZrO2 Composite Oxides Supports

Mohong Lua,c, Hu Dua, Bin Weia, Jie Zhua, Mingshi Lia,*, Yuhua Shana , Jianyi Shenb,* and Chunshan Songc

a

Jiangsu 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, Department of

Energy & Mineral Engineering and Department 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, P. R. China E-mail: [email protected] 2、Professor Jianyi Shen School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China E-mail: [email protected] 1

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Abstract TiO2-ZrO2 composite oxides with a series of different Ti/Zr atomic ratios were synthesized by using deposition precipitation method and employed as supports to prepare Ru catalysts. The physicochemical characteristics of Ru catalysts were tested by N2 adsorption, CO chemisorption, XRD, H2-TPR, SEM, TEM, NH3 adsorption and XPS and their catalytic hydrodeoxygenation (HDO) performance was evaluated by using guaiacol as a model compound. Ru/TiO2 catalyst exhibits low HDO activity due to the surface modification of Ru particles with Ti3+ species formed during the reduction of Ru/TiO2. The HDO activity of Ru catalysts was significantly promoted by ZrO2 in the supports because it can hinder the migration of Ti3+ species onto the surface of Ru particles and thus more Ru active sites are exposed. Higher selectivity of benzene for Ru catalysts supported on TiO2-ZrO2 composite oxides suggests that they are the promising supports for Ru catalysts in the HDO reaction of guaiacol. Keywords: Ru, TiO2-ZrO2 composite oxides, Hydrodeoxygenation, Guaiacol, Ti3+ species, Metal support interaction

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1. INTRODUCTION Biomass has attracted increasing attention as a type of renewable and sustainable energy resource in order to replace liquid fuel due to the depletion of petroleum-base reserve and global environmental problems.1 The thermal cracking of biomass to produce pyrolysis oil followed by upgrading of the resulting bio-oil is involved in the utilization of biomass.2 Unfortunately, high oxygen containing content in bio-oil (35-40 wt %) results in undesirable properties including high viscosity, low heating value, low thermal and chemical stabilities, corrosiveness and immiscibility with fossil fuels that make bio-oil unfitted to use as a standard fuel.3-4 A promising technology to upgrade the bio-oil is hydrodeoxygenation (HDO)，by which the oxygen in the bio-oil can be lowered in the presence of catalysts at high pressures of hydrogen. A number of attempts have been carried out to develop HDO catalysts with high activity and selectivity to hydrocarbons. In earlier studies, conventional hydrotreating sulfide catalysts, NiMoS and CoMoS, were employed to conduct HDO and exhibited good catalytic activity.5-9 But additional sulfur needs to be supplied to maintain the catalysts in the active phase due to the low sulfide content in bio-oil.10-11 Transition metal phosphides,4, 12-14 carbides,15-17 and nitrides18-20 have been widely investigated in HDO due to their catalytic activities similar to noble metal for some reactions.21-23 Although their excellent HDO performance made them be considered as the potential catalysts for HDO, suffering from fast deactivation during HDO reaction is still a challenge for their application.15 Recently supported noble metal catalysts, such as Pt, Pd, Ru have been the focus of bio-oil HDO due to their high activity and stability.24-34 Ruthenium metal catalysts have been demonstrated to be a type of promising catalysts 3



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for the bio-oil HDO due to their excellent HDO performance30, 35-38 and much lower price on comparison with Pd and Pt. It is clear that supports have played a key role in improving the HDO activity of catalysts. Catalytic HDO performance of different catalysts supported on γ-Al2O3, 39 SiO2,40 activated carbon, 41 TiO234-35, 38 and ZrO27, 42 have been investigated. TiO2 is considered to be a superior Ru catalyst support due to its redox activity.34-35,

38

Newman et al. have investigated several supported Ru catalysts for HDO of liquefied phenol and found that Ru/TiO2 catalysts showed the best activity and direct deoxygenation selectivity among all catalysts investigated.38 Nelson et al. demonstrated that Ru supported on TiO2 was a highly efficient HDO catalyst in the presence of water and the interfacial sites between Ru nanoparticles and TiO2 were involved in the heterolysis of H2. H proton accepted by TiO2 facilitates the breakage of C-O bond in phenol and the aromatic OH was replaced directly with H atom on ruthenium nanoparticles.30 TiO2-ZrO2 has been of increasing interest as a support of catalysts due to its outstanding feature, such as high specific area and high thermal stability compared to the relative single oxide.43 Zhang et al. investigated the effect of mixed oxides on the Ni catalyst for bio-oil HDO and demonstrated that Ni/TiO2-ZrO2 catalyst showed an excellent HDO performance, suggesting that TiO2-ZrO2 mixed oxides supports are promising for the improvement of catalytic HDO.44 Although TiO2-ZrO2 composite oxides possess more outstanding feature as supports of catalysts in some aspects than TiO2, few studies on Ru catalysts supported on them for bio-oil HDO have been reported. In this work, we put focus on the effect of TiO2-ZrO2 composite oxides supports on catalytic performance of Ru for the HDO of guaiacol as a model component of 4


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bio-oil. Ru catalysts supported on TiO2, ZrO2, and TiO2-ZrO2 composite oxides with different Ti/Zr atomic ratios have been characterized by XRD, TEM, H2-TPR, CO chemisorption, NH3 adsorption and XPS. The effect of composition of mixed oxides on the properties of Ru metal nanoparticles was explored. The highest activity of guaiacol HDO and selectivity to benzene were achieved on Ru/TiO2-ZrO2 with the Ti/Zr atomic ratio of 1:3, suggesting that TiO2-ZrO2 composite oxides are the promising supports for Ru catalysts in hydrodeoxygenation of bio-oil.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis All of the oxides supports were synthesized using deposition precipitation method. The solution of TiCl4 (≥99.0%, Guoyao) in ethanol was used as Ti source. Ti-containing solution and ZrOCl2 (≥99.0%, Guoyao) with appropriate quantities were added to the deionized water to get the desired Ti/Zr atomic ratios. After the solution was stirred for 1 hour, aqueous solution of 25% NH3 was added dropwise into the solution under stirring until the pH increased to 10. The precipitate was filtered and washed with deionized water until no Cl- was detected. Then the wet solid was dried at 120 oC for 12 h and calcined at 500 oC for 5 h to obtain oxide supports. Catalysts were prepared by incipient wetness impregnation method. Oxide support (3.0 g) was impregnated with RuCl3 solution containing predetermined quantity of Ru to achieve the final loading of Ru of 5 wt %. The impregnated sample was then kept overnight for 12 h at the room temperature followed dried at 120 oC for 12 h. Finally, the obtained solid was calcined at 500 oC for 5 h. The as-synthesized catalysts were denoted as Ru/TiO2, Ru/ZrO2, and Ru/TiO2-ZrO2(x), where x represents the atomic ratio of Ti/Zr in mixed oxides. 5



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2.2. Catalyst Characterization The specific surface areas of supports and catalysts were determined by N2 adsorption using ASAP 2020 analyzer. The amount of CO-chemisorption was measured on the ASAP 2020 Chemisorption system to investigate the interaction of metal and supports. A sample of 100 mg was put into a U-shaped quartz tube and then reduced at 200 oC for 2 hours by a flow of hydrogen. The reduced sample was then degassed by He for 2 hours at 200 oC and cooled down to 50 oC. The CO uptake was then determined at 50 oC by injecting CO pulses into the system. Powder X-ray diffraction (XRD) of supports and catalysts was conducted with a Rigaku D/Max2400 diffractometer, using Cu-Kα radiation at 40 kV and 100 mA to determine the crystalline structure. Transmission electron microscopy (TEM) images were obtained with a JEOL-2010 at 200 kV. The reducibility of catalysts was investigated by H2 temperature-programmed reduction (TPR) on a Micromeritics 2910 analyzer equipped with a thermal conductivity detector (TCD). A calcined sample (50 mg) loaded into a U-shaped quartz tube was heated from 20 oC to 600 oC at a heating rate of 10 oC/min under 5% H2 in Ar with a flow of 20 mL/min. The measurement of NH3 adsorption at 30 oC was carried out on the Tian-Calvet Type heat-flux microcalorimeter analyzer. Prior to the adsorption of NH3, the sample (100 mg) was treated by H2 (5ml/min) at 200 oC for 2 h, then evacuated at 200 oC for 1 hour. After the temperature desired was reached, the microcalorimetric measurement was performed by introducing NH3 with small doses sequentially into the sample till the saturated adsorption was obtained and the related data were collected. The heat 6


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released was determined by integrating the heat response data and the amount of NH3 adsorbed was calculated according the volume of NH3 adsorbed, which was employed to determine the differential heat for each dose of NH3. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Multilab2000 X-ray photoelectron spectrometer with Mg-Kα as the photon source. The binding energies (BEs) reference was taken by using the C1s peak at 284.8 eV. 2.3. Catalyst Activity Measurement The activity of catalysts for guaiacol HDO was measured by using a stainless steel fixed-bed reactor with 6 mm inner diameter. A sample of the evaluated catalyst (0.2 g, 20-30 meshes) was placed in the middle of the reactor. Prior to feed the solution of guaiacol (3.0 wt %) in n-dodecane, the catalyst was reduced using H2 at 200 oC for 3 h. The HDO reaction was performed in the temperature range 200

o

C-260 oC, H2 pressure of 2 MPa and H2 flow rate of 150 mL/min (at atmospheric pressure), WHSV 24 h-1. The liquid products at different temperature were collected by separating them from gas phase after the reaction reached the desired temperature and was hold for 6 h. The composition of liquid products was analyzed by off-line gas chromatograph equipped with a HP-5 column using flame ionization detector. Guaiacol conversion (CONguaiacol), products selectivities (Sproducts-i) were calculated as follows: CONguaiacol = (Molguaiacol-in-Molguaiacol-out)/Molguaiacol-in; Sproducts-i = (Molproduct-i × ni)/(Molreacted guaiacol × 7), ni refers to the carbon number in product-i.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization The physical properties of supports and catalysts as-synthesized are summarized in Table 1. The surface area of TiO2 was close to ZrO2. It is clear that the BET surface 7



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areas of TiO2-ZrO2 composite oxides were much larger than that of the single oxide TiO2 or ZrO2, which is consistent with the results reported by Manríquez et al.24 Especially the BET surface areas of TiO2-ZrO2(1:3) and TiO2-ZrO2(3:1) were almost twice that of TiO2 or ZrO2. All of the supports showed mesoporous properties and the pore diameters decreased with the decrease in Ti/Zr atomic ratio. The pore volume of supports showed the similar trend as the surface area. In comparison with the supports, the loading of Ru got rise to a slight decrease in the surface area, pore volume and pore diameter for all the catalysts. The XRD patterns of supports and catalysts are shown in Figure 1. The diffraction peaks assigned to anatase TiO2 were observed at 25.5o, 37.1 o, 38.7 o, 39.9 o, 48.2o, 54.0 o, and 55.3 o and the peaks attributed to ZrO2 were identified at 28.6o and 31.8o.45-46 The intensity of diffraction peaks of both TiO2 and ZrO2 decreased for the mixed oxides. The peak at 44o assigned to metallic Ru (JCPDS 06-0663) can be seen for all catalysts. The weak intensity of Ru diffraction peak suggested the small Ru particles well dispersed on the supports. The crystalline sizes of metallic Ru for all catalysts, calculated by using Scherrer equation, were listed in Table 1. The values of crystalline sizes of Ru on TiO2 and ZrO2 are 6.5 nm and 12.1 nm, respectively. Although the TiO2-ZrO2 composite oxides show higher surface areas than TiO2, the crystalline sizes of Ru on the TiO2-ZrO2 composite oxides are larger than on TiO2 and increase with the decrease in the amount of Ti in the catalysts, suggesting that TiO2 is favorable for the dispersion of metallic Ru. The morphology of all the catalysts can be investigated by SEM images (Figure 2). It can be seen that grain size of supports is influenced by the composition of supports. The TiO2 support shows larger grain size compared to the ZrO2 sample. Two types of grain sizes present in composite oxide supports. The average grain size of 8


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composite oxide supports decreases slightly with the decrease in Ti/Zr atomic ratios. The TEM photographs of all the catalysts are shown in Figure 3. It can be seen that the particle sizes of Ru supported on TiO2 are the smallest and those are the largest for Ru/ZrO2. The Ru particle sizes for the catalysts supported on TiO2-ZrO2 composite oxides increase with the content of ZrO2 in the supports, indicating TiO2 has the stronger interaction with Ru compared to ZrO2 so that TiO2 could enhance the dispersion of Ru on the supports. The results obtained from TEM are in good accordance with the values from XRD (Figure 1 and Table 1). The reduction properties of Ru catalysts were conducted by H2-TPR. The TPR profiles of Ru catalysts as prepared are shown in Figure 4. It can be observed that the reduction properties of Ru catalysts are influenced by the supports. The peak below 200 oC represents the H2 consumption attributed to the reduction of RuO2 to Ru. 34, 47 A broad H2 consumption peak was identified on TiO2 support while a shark peak was observed on ZrO2 support, indicating that the interaction between RuO2 and TiO2 is stronger than that between RuO2 and ZrO2. The H2 consumption peak altered gradually from broad to shark with the increase of ZrO2 content in the TiO2-ZrO2 composite oxides supports. It can be noticed that the reduction temperature related to the H2 consumption peak shifted to lower value on TiO2-ZrO2 composite oxides supports by comparison with TiO2 and ZrO2 supports，indicating the TiO2-ZrO2 composite oxides supports are benefit to the reduction of RuO2. The H2 uptakes of catalysts and reduction extent of RuO2 are listed in Table 2. For all catalysts experimental hydrogen consumptions are almost equal to the theoretical values, suggesting that RuO2 supported on all oxides is ready to be reduced to metallic Ru. 9



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The XPS was conducted to determine the oxidation state of Ru, Ti, and Zr elements on the surface of all reduced and passivated catalysts. Prior to the conduction of XPS analysis, the catalysts were treated in O2 (1% v/v in He) at room temperature after they were reduced in H2 at 200 oC for 3 h. The Ru 3d core lever spectra from Ru supported on all oxides are shown in Figure 5a. It is reported that the peak of Ru 3d is overlapped with C 1s peak around at 285 eV, which makes it difficult to analyze the Ru 3d species and correct the charge of Ru species. For Ru/TiO2 and Ru/TiO2-ZrO2(3:1) catalysts, only metallic Ru0 species with 3d5/2 B.E. at 280.0 eV48-50 was observed. While for Ru/TiO2-ZrO2(1:1) and Ru/TiO2-ZrO2(1:3) catalysts, the intensity of Ru 3d5/2 peak at 280.0 eV became weak and the sign of 3d5/2 binding energy at 281.2 eV and 383.1eV appeared, which assigned to Ruδ+ and Ru4+ species,48-50 respectively. Especially, no peak at 280.0 eV but the peak at 281.2 eV was observed in Ru/ZrO2 catalyst. The results suggested the presence of strong interaction between Ru and TiO2 on the surface of catalysts containing TiO2, in agreement with the results obtained by TPR (Figure 4), which can make Ru maintain the state of metallic Ru0 in the passivation process. In other word, Ru in the ZrO2 containing catalysts is easy to be oxidized, and easy to be reduced as well (as indicated in Figure 4). Figure 5b showed the Ti 2p XP spectra of Ru catalysts on supports containing TiO2. The presence of two Ti 2p sign peaks for the supports containing TiO2 can be observed on the surface of TiO2 and TiO2-ZrO2 composite oxides. The peak at 457.9 eV is assigned to Ti3+ species51, suggesting that Ti4+ was partially reduced to Ti3+ when the Ru catalysts was reduced at 200 oC. Generally speaking, the partial reduction of TiO2 to Ti3+ leading to strong metal-support interaction (SMSI) occurs by treating at high temperature (500 oC) for TiO2 supported noble metal system.52 It was 10


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suggested that the reduction of TiO2 to Ti3+ was promoted by the hydrogen spillover from Ru.34, 48, 53-54 Ftouni et al. reported that a significant amount of Ti3+ formed for Ru/TiO2 catalyst at the hydrogenation reaction condition (150 oC, 30 bar).50 The other peak at 458.4-459.0 eV is assigned to Ti4+ in TiO2.55-56 It is noted that the value of Ti4+ signal peak shifted to lower binding energy from 459.0 eV for TiO2 to 458.4 eV for TiO2-ZrO2(1:3). The negative shift of Ti4+ binding energy with the increase in ZrO2 in composite oxides may suggest the presence of electrons transformation from ZrO2 to TiO2 during the reduction process, which may promote the reduction of Ti4+ to Ti3+. In contrast to the change of Ti species in the supports containing TiO2, no change was observed for Zr species (Figure 5c). One peak at about 182.4 eV assigned to ZrO257 for all supports containing ZrO2 indicated that no reduction of ZrO2 happened during the reduction process. The interaction between Ru metal with supports was demonstrated by conducting CO chemisorption measurements. The results of CO chemisorption for all the catalysts are listed in Table 1. Ru supported on composite oxides showed higher CO chemisorption uptakes than that of TiO2 and ZrO2. Ru/ZrO2 catalyst exhibited the lowest CO chemisorption uptake due to the low Ru dispersion on the surface of ZrO2 support (Table 1). The phenomena, however, of low CO chemisorption for Ru/TiO2 catalyst with the highest Ru dispersion and the highest CO chemisorption on Ru/TiO2ZrO2(3:1) catalyst with relative low dispersion of Ru cannot be explained well using the same principle. The interaction between Ru and supports should be taken into account. It is well known that the SMSI between noble metal and TiO2 results in the migration of partially reduced TiO2-x (x

Hydrodeoxygenation of Guaiacol on Ru Catalysts: Influence of TiO2

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