Al2O3–TiO2 catalyst for hydrodeoxygenation of guaiacol: Effects of bimetallic composition and reduction temperature

Journal Pre-proof Spray pyrolysis synthesis of bimetallic NiMo/Al2 O3 -TiO2 catalyst for hydrodeoxygenation of guaiacol: Effects of bimetallic composition and reduction temperature Dieu-Phuong Phan, The Ky Vo, Van Nhieu Le, Jinsoo Kim, Eun Yeol Lee

PII:

S1226-086X(19)30647-1

DOI:

https://doi.org/10.1016/j.jiec.2019.12.008

Reference:

JIEC 4892

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

23 August 2019

Revised Date:

31 October 2019

Accepted Date:

6 December 2019

Please cite this article as: Phan D-Phuong, Vo TK, Le VN, Kim J, Lee EY, Spray pyrolysis synthesis of bimetallic NiMo/Al2 O3 -TiO2 catalyst for hydrodeoxygenation of guaiacol: Effects of bimetallic composition and reduction temperature, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.12.008

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Spray pyrolysis synthesis of bimetallic NiMo/Al2O3-TiO2 catalyst for hydrodeoxygenation of guaiacol: Effects of bimetallic composition and reduction temperature

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Dieu-Phuong Phan1a, The Ky Vo2a, Van Nhieu Le1, Jinsoo Kim1*, Eun Yeol Lee1*

Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero,

Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

Department of Chemical Engineering, Industrial University of Ho Chi Minh City,

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2

Equally contributed as the first author

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a

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12 Nguyen Van Bao, Go Vap, Ho Chi Minh City, Vietnam

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*Corresponding authors: TEL: +82-31-201-3839; Fax: +82-31-204-8114

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E-mail: [email protected] (E.Y. Lee), [email protected] (J. Kim)

Graphical abstract

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Abstract

Catalytic hydrodeoxygenation (HDO) is a process for removing of oxygen from oxygen-

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containing compounds using a catalyst. In this study, spherical bimetallic NiMo/Al2O3-TiO2 catalysts with different Ni/Mo ratios were successfully synthesized by combining sol-gel

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method and continuous flow spray pyrolysis process. The prepared catalysts were characterized

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by N2 adsorption-desorption, TEM, SEM, XRD, XPS, H2-TPR and NH3-TPD analyses. The catalysts were then applied for HDO of guaiacol as a model compound using a fixed-bed

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reactor. The effects of bimetallic compositions and reduction temperatures on HDO conversion of guaiacol as well as their product distributions were systematically investigated. The obtained

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results showed that bimetallic NiMo/Al2O3-TiO2 exhibited higher HDO conversion than monometallic catalysts (Ni/Al2O3-TiO2 or Mo/Al2O3-TiO2). The highest HDO conversion of

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98% with 100% hydrocarbon selectivity (85% cyclohexane, 13% methylcyclohexane and 2% toluene) was obtained over (10wt% Ni and 20wt% Mo)/Al2O3-TiO2 catalyst. In addition, the catalyst maintained a good catalytic stability for 24 h of reaction time, suggesting that spray pyrolysis derived NiMo/Al2O3-TiO2 catalyst can be a promising catalyst for HDO performance.

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Keywords: Hydrodeoxygenation, guaiacol, NiMo, bimetallic catalyst, spray pyrolysis, NiMo/Al2O3-TiO2

1. Introduction Currently, transportation fuel consumes nearly 30% of the world’s energy, in which almost 95% is originated in fossil fuel (gasoline and diesel)[1]. There needs many efforts to generate transportation fuels from renewable resources for dealing with the depletion of fossil

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source, as well as solving the environmental and economic issues. Conversion of

lignocellulosic biomass is one of the most efficient ways for producing the biofuel [2-5].

However, lignin-based biocrude usually contains high oxygen amount in its composition.

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Therefore, eliminating oxygen is required to enhance bio-oil properties by reducing its acidity

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and viscosity as well as increasing low heating value [6, 7]. Hydrodeoxygenation (HDO) is a process for removal of oxygen functionality by using particular catalysts to selective cleavage

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of C–O and C–C bonds in oxygen-containing compounds [8-15]. The development of a suitable catalyst for HDO reaction is not only a crucial factor but

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also a challenge. Noble metal catalysts have been studied for HDO process due to their high efficiencies in the activation of molecular hydrogen under mild conditions. However, its high

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cost could be a drawback for adapting in industrial scale [16-18]. Transition metals, such as Ni, Mo, Co, etc., have been extensively used as catalysts for HDO due to their high catalytic

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activities and reasonable cost. For these metals, supported monometallic species (Ni, Mo, Co, etc.) or bimetallic species (NiMo, CoMo, NiCo, etc.) can be used as catalyst for HDO reaction[12, 19-28] . It was reported that bimetallic catalyst exhibited a better catalytic activities compared to those of monometallic catalyst. Kordouli et al. [29] investigated the roles of metal species (Rh, Ni and Ni-Mo) supported by activated carbon (AC) for HDO conversion of phenol. The presence of Mo species in bimetallic NiMo/AC catalyst resulted in 3

enhancement of the stability of catalyst. Aqsha et al. found that the bimetallic NiMo/TiO2 had higher efficiency for HDO conversion of guaiacol than monometallic Ni/TiO2 and Mo/TiO2 [30]. Ruinart de Brimont et al. [31] and Robinson et al. [32] reported that the incorporation of Ni-Mo species could enhance HDO conversion over decarbonylation/decarboxylation or hydrogenation mechanism [31, 32]. Very recently, Kumar et al. [33] investigated the role of NiMo catalyst for the HDO of stearic acid. They found that the HDO activity was enhanced by the enrichment of highly active NiMo alloy on the catalyst surface with increasing Ni/Mo

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molar ratio. However, the detailed investigation in the roles of Ni and Mo species as well as Ni/Mo ratio on HDO of the phenolic compounds were not systematically investigated. The

sulfided NiMo and CoMo catalysts also have been employed for HDO performances [34-37].

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However, these materials can produce sulfur contamination in the final products and sulfur leaching can cause catalyst deactivation during HDO performance [38]. Therefore, non-

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sulfided transition metals have been considered as promising catalysts for HDO

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performances[20, 29, 39, 40].

Conventionally, impregnation method has been widely used for catalyst preparation.

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However, this approach has some inherent drawbacks such as non-uniform distribution of metal active sites in the support, as well as low density of active sites on support surface, which

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can reduce catalytic activity of the catalyst [41-43]. In our previous works, we prepared Mo/Al2O3-TiO2 catalyst by one step spray pyrolysis method and compared its catalytic activity

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for HDO reaction with that prepared by impregnation method. The obtained results showed that spray pyrolysis-derived catalyst exhibited much higher HDO conversion and product selectivity compared to the catalyst prepared by conventional impregnation method [44, 45]. It was attributed to high dispersion of metal active sites throughout the support for spray pyrolysis-derived catalysts. Moreover, spray pyrolysis can produce large amount of catalysts in a scalable continuous process [44-46]. Although bimetallic NiMo-based catalysts have shown 4

good performance for HDO, there is still no report on synthesizing bimetallic NiMo/Al2O3TiO2 catalyst by combining sol-gel and spray pyrolysis method. In this study, the bimetallic NiMo/Al2O3-TiO2 catalysts with various Ni/Mo ratios were prepared by combining the sol-gel process and spray pyrolysis method. The properties of catalysts were then characterized by N2 adsorption-desorption, SEM, XRD, XPS, H2-TPR and NH3 -TPD analyses. Catalytic activity for the HDO conversion of the prepared catalysts was evaluated in a fixed-bed reactor using guaiacol as a model compound of lignin. The effects of

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Ni/Mo ratios and reduction temperature on the HDO conversion and product selectivity were systematically investigated.

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2. Experimental

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2.1. Catalyst preparation

The Al2O3-TiO2 composite support and NiMo/Al2O3-TiO2 catalyst were prepared by

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combining the sol-gel process and spray pyrolysis method following our previous works [44, 45]. Briefly, a stable titania sol (0.1 M) and boehmite sol (0.2 M) were prepared separately by

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hydrolysis and condensation, followed by mixing under stirring condition for 1 h. In this work, the amount of boehmite sol and titania sol were fixed to obtain the composite support with

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weight ratios of Al2O3: TiO2 = 80:20, as this composition showed high acidity density [44].

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For catalyst metal loading, the mixture containing various amounts of nickel nitrate hexahydrate ((Ni(NO3)2.6H2O, Sigma Aldrich) and ammonium heptamolybdate ((NH4)6Mo7O24.4H2O, Sigma Aldrich) with citric acid was added drop-wise to the sol mixture under vigorous stirring condition. The resulting mixture was then used as precursor solution for spray pyrolysis by using a nebulizer with ultrasonic frequency of 1.7 MHz. The formed droplets were transferred into a quartz reactor heated at 600 oC by a constant flow N2 gas. The

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obtained spherical particles were collected in a Teflon bag settled at the bottom of the spray pyrolysis system [44]. The product samples were then calcined at 500 oC in the air atmosphere for 4 h in a muffle furnace. These calcined catalysts are denoted as xNiyMo, in which x and y are the weight percentages of Ni and Mo, respectively, and x + y = 30 wt.% of metal loaded on Al2O3-TiO2.

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2.2. Characterization The textural properties of the Al2O3-TiO2 support and NiMo/Al2O3-TiO2 catalysts were analyzed using N2 porosimetry (TriStar, Micromeritics, USA) at 77K. Before analysis, the

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samples were degassed at 150 oC for 12 h. The specific surface area and N2 adsorption-

desorption isotherms were determined by Brunaune-Emmet-Teller (BET) method. The total

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pore volume was determined by N2 adsorption at a relative pressure of 0.98. The pore size

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distribution was obtained by the Barrett-Joyner-Halenda (BJH) algorithm. X-ray diffraction patterns were obtained by powder X-ray diffractometer (PXRD; MAC-18XHF, Rigaku, Japan)

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using a Cu Kα radiation source (λ=1.54 Å). The morphology of catalysts was characterized using field-emission scanning electron microscopy (FE-SEM; Leo-Supra 55, Carl Zeiss STM,

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Germany).

Temperature-programmed reduction (H2-TPR) of all NiMo/Al2O3-TiO2 samples were

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analysed using a fixed-bed reactor. The calcined catalysts were first pretreated in-situ at 150 oC for 1 h in Ar stream to eliminate physisorbed contaminants and then cooled to room temperature. The reduction process was performed under a gas mixture stream of 10 vol% H2 in Ar with a flow rate of 50 cm3/min. The temperature was heated up to 900 oC with a heating rate of 2 oC/min and the consumption of H2 at different temperature was recorded by gas

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chromatography (6500GC, YL Instrument, Korea) equipped with a thermal conductivity detector (GC-TCD). The temperature-programmed desorption of ammonia (NH3-TPD) experiment was performed on a Micromeritics AutoChem II 2920 automatic analyser equipped with a thermal conductivity detector. First, the impurities on the surface of the samples were removed by pretreatment at 400 oC in He stream for 2 h. After cooling down to 100 oC, the samples were saturated by ammonia in a stream of 50 cm3/min of 10% NH3/He for 1 h. The ammonia

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desorption step was carried out in a 50 cm3/min of He flowing until 700 oC with a heating rate of 5 oC/min.

XPS measurement was conducted using a Thermo Scientific K-Alpha spectrometer

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instrument to determine oxide states of metal dopants (Ni, Mo). The binding energy was

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adjusted by using the carbon C (1s) line as a reference with a binding energy value at 284.6 eV. The reduced catalyst was passivated in a gas mixture of 3% air in Ar at room temperature for 2

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2.3. HDO of guaiacol

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h before exposing to the air.

HDO conversion of guaiacol was carried out in a continuous fixed-bed reactor (as shown

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in Fig. S1). The catalyst powder was placed in the middle of stainless steel reactor (I.D = 9.5 mm, L = 400 mm) and was held by quartz wool at both ends. The temperature of the reactor

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inside the electric furnace was monitored by a K-type thermocouple placed inside the reactor. The reaction pressure was controlled by a back pressure regulator. Before the reaction, the calcined catalysts were reduced in the mixture gas of H2/Ar (60/20 cm3/min) that were controlled by a set of mass flow controllers. 0.1 g of the calcined catalyst was first heated from room temperature to 300 oC at a rate of 5 oC/min, then from 300 to 500 oC at a rate of 2 oC /min and kept at 500 oC for 2 h. The temperature was then increased from 500 to 700 oC at a rate of 7

2 oC/min, and held at this temperature for 1h; finally, it was cooled down to desired reaction temperature. For each run, 3 wt% of guaiacol (99%, Aldrich) in dodecane (>99%, Aldrich) was fed into the reactor by using a high-pressure pump at a flow rate of 0.15 mL/min corresponding to the weight hourly space velocity (WHSV) of 57 h-1. To assure the complete evaporation, the reactant stream was preheated at 250 oC. The resulting mixture vapour was mixed with H2 flow (60 mL/min) before entering the reactor. The HDO reaction was conducted at 300 oC and hydrogen pressure of 2 MPa for 2 h. The vapour product was condensed into liquid phase at

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around -10 oC using an ethyl-glycol-cooled condenser system. The mass balance determined that the weight ratio of liquid product after condensation to reactant entering into the reactor was 88.7%  1%. The mass loss might be due to the uncondensed products, the liquid hold-up

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in the condenser, and coke deposit on the catalyst [47, 48]. The liquid product was collected at every 20 min interval and analysed by GC-MS (Agilent 7890B, USA) using a capillary column

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- HP 5MS (30m x 0.25mm x 0.25m). Its detector temperature was set to 280oC, the heating

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rate of GC analysis was initial at 60 oC, 10 oC/min to 90 oC, and followed by 60 oC /min to 250 o

C.

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The conversion of guaiacol, hydrodeoxygenation degree (XHDO) and product distribution were calculated by eqs. (1), (2) and (3), respectively [24, 40], where C0GUA is the

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initial guaiacol concentration (mol/L) and CGUA is guaiacol concentration at different reaction time (t, h), Ci is the concentration of product i in the HDO product stream, ai is the number of

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oxygen in each product i. XGUA (%) =

C0GUA −CGUA

XHDO (%) =

C0GUA ×XGUA ×2−∑i Ci ai

Ci

Si = ∑k

i=j Ci

C0GUA

× 100

CoGUA ×XGUA ×2

(1) × 100

(2)

× 100

(3)

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3. Results and Discussion 3.1. Characterization of catalysts The FE-SEM images of Al2O3-TiO2 support and NiMo/Al2O3-TiO2 catalysts with different Ni/Mo weight ratios prepared by spray pyrolysis were shown in Fig. S2 (Supporting Information). As doping 30 wt% metal into the Al2O3-TiO2 support did not change

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morphologies of catalyst samples, all support and catalyst samples were formed in spherical shapes without aggregation and all the samples maintained similar sizes in the range of 0.5 – 2 µm. Figs. 1(a-b) show TEM images and EDX dot mappings (Al, Ti, Mo, and Ni) of 10Ni20Mo

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and 15Ni15Mo catalysts. EDX dot mappings clearly show that doped metal species of Ni and

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Mo are uniformly dispersed throughout the Al2O3-TiO2 support matrix. Also, the Ni/Mo weight ratios of 10Ni20Mo and 15Ni15Mo samples were analyzed approximately 0.45 and 0.9,

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respectively, which are very close to the calculated values. Fig. S3 shows TEM images of the prepared catalyst at higher magnifications, revealing that metal dopants species were well -

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dispered throughtout the support with very small particle size (2 – 12 nm). This result suggests that spray pyrolysis method is very effective in doping high concentration of multi-metal

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species into the support.

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Fig. 2 shows the XRD patterns of reduced NiMo/Al2O3-TiO2 catalysts with various Ni/Mo weight ratios. Before analysis, all calcined NiMo-based catalysts were reduced following the reduction procedure described in Section 2.3. The reduced catalysts were cooled down to room temperature in the H2 flow and then passivated in a 0.5% vol O2/N2 flow (80 ml min-1) for 4 h before exposing to the air. The alumina phase was characterized by a set of peaks at 2θ= 37.6, 46.0 and 66.7o (JCPDS card No.01-080-1385), while the characterized peaks of anatase TiO2 phase was determined at 2θ = 25.4, 37.9, 48.0, 54.3, and 62.8o (JCPDS card No. 03-065-5714). 9

As shown in Fig. 2, there is no peak of molybdenum dopant on the XRD pattern of Mo/Al2O3TiO2, indicating that the Mo species are dispersed well throughout the support or the dispersion is under the monolayer dispersion amount [44]. For the monometallic Ni/Al2O3-TiO2 catalyst, however, the XRD pattern includes peaks at 2θ = 44.6, 51.9, 76.4 o, which are assigned to Ni metal (JCPDS card No. 00-004-0850), indicating the existence of Ni crystals on the support surface [49]. The peaks at 2θ = 37.5, 45.4, 66.3 and 67.1o are ascribed to nikel aluminum oxide (JCPDS card No. 00-020-0777). It can be seen that the XRD peaks of bimetallic NiMo/Al2O3-

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TiO2 slightly shifted towards lower 2θ values compared to those of monometallic samples, due to the formation of Ni-Mo alloy [50, 51].

The textural properties of Al2O3-TiO2 support and NiMo/Al2O3-TiO2 catalysts were

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shown in Fig. S4 (a-b) and Table 1. The spray pyrolysis-derived Al2O3-TiO2 composite has a specific area of 235 m2/g and the pore volume of 0.22 cm3/g. The loading of 30 wt% Mo

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resulted in decreasing of specific surface area to 178 m2/g due to the pore blockage with the

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presence of high metal loading. Interestingly, the monometallic 30Ni/Al2O3-TiO2 showed a specific surface area of 257 m2 and pore volume of 0.36 cm3/g, respectively, which were higher

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than those of pristine Al2O3-TiO2 support. This could be due to the suppression of TiO2 crystal growth by Ni as well as the generation of pores between Ni particle and support [44]. After

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loading 30wt% bimetallic NiMo into the support, the NiMO/Al2O3-TiO2 catalysts showed specific surface areas of 178- 246 m2/g and pore volumes of 0.24 - 0.31 cm3/g depending on

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their compositions. Pore size distribution data reveals that the introduction of metal species into the support matrix leads to significant shifts of pore size towards larger pore diameters (Fig. S4(b)).

Acidity of calcined Al2O3-TiO2 support and NiMo/Al2O3-TiO2 catalysts were determined by NH3-TPD analysis, as shown in Fig. 3. The types of acid sites can be categorized depending on the desorption temperature: weak acid site (< 250 oC), moderate acid site (250 – 500 oC) and 10

strong acid site (> 500 oC) (Table 2) [52]. As shown in Table 2, the loading of metal on Al2O3TiO2 resulted in the changes of acid sites distributions. The loading of monometallic (Ni or Mo) and bimetallic (NiMo) species resulted in an increase of weak acid sites and a decrease of strong acid sites. The amount of moderate acid sites decreased by the loading of monometallic Ni or Mo, but increased with the doping of bimetallic such as 25Ni5Mo and 15Ni15Mo catalysts. In general, total acid sites of catalyst increased with increasing Ni concentration in the bimetallic system. Meanwhile, the interaction between Mo dopant and support leads to a

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decrease of acid sites. In addition, the temperature peaks were slightly shifted towards lower temperature when high amount of Mo was used (Fig. 3). It was noticed that the acidity density of bimetallic catalysts of 5Ni25Mo and 10Ni20Mo were higher than those of the monometallic

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catalysts, suggesting that these bimetallic catalysts would have higher catalytic activities [53]. H2-TPR profiles of calcined NiMo/Al2O3-TiO2 catalysts were shown in Fig. 4. The

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30Ni catalyst exhibited two reduction peaks at 510 and 700 oC. The first peak at 510 oC was

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ascribed to the reduction of dispersed NiO species [54]. The larger peak at higher temperature (700 oC) was attributed to the reduction of nickel aluminate (NiAl2O4), indicating that Ni2+

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mainly exist in the form of NiAl2O4 spinel structure [55, 56]. This is consistent with the above XRD analyses. The monometallic 30Mo/Al2O3-TiO2 catalyst displays two main characteristic

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reduction peaks at 420 and 770 oC. The lower reduction peak was associated with the reduction of Mo6+ in the form of polymeric octahedral Mo oxide species to Mo4+[57], meanwhile the

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hydrogen consumption signal at high temperature region (700 – 900 oC) was ascribed to the second reduction step of Mo4+  Mo0 [44, 58]. The loading of bimetallic NiMo on Al2O3TiO2 resulted in the changes of reduction behaviors of catalysts depending on their compositions. For instance, the reduction of Mo6+ →Mo4+ of the bimetallic catalysts were shifted to lower temperature. In addition, the high temperature reduction peaks of the Ni rich-

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samples (25Ni5Mo, 20Ni10Mo, and 15Ni15Mo), which are assigned to the co-reduction of Mo4+ →Mo0 and Ni2+ →Ni0, were shifted to lower temperature. It could be due to the interaction between Ni and Mo, leading to weaken the interaction between these metals and support or to form NiMoO4 like-phases [29, 59, 60], which is in good agreement with the XRD analyses. Chemical states of Mo and Ni dopants of the calcined and reduced samples were

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examined through XPS analyses and presented in Figs. 5(a-b). As shown in Fig. 5(a), the Mo 3d core-level XPS of the calcined 30Mo/Al2O3-TiO2 shows two well-resolved peaks of 3d5/2 (232.8 eV) and 3d3/2 (235.8 eV )[44]. After reduction, these peaks were shifted towards the

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lower binding energy, indicating that Mo+6 species were reduced to its lower oxidation states. Specifically, the deconvolution of Mo 3d XPS spectrum after reduction showed the co-

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existence of Mo5+ (231.4 and 234.5 eV), Mo4+ (229.2 and 232.2 eV), and Mo0 (227.8 and 230.8 eV) [44, 61, 62]. As shown in Fig. 5(a), after reduction, the Mo6+ percentage in the reduced

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catalyst decreased from 24% to 15%, while the metallic Mo0 concentration increased from 10% to 27% when the amount of Ni increased from 0 to 25wt%. This suggests that the reducibility

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of Mo species increased with increasing Ni concentration in the bimetallic catalyst system. The Ni 2p core level XPS analyses are shown in Fig. 5(b). For calcined Ni/Al2O3-TiO2,

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two peaks at binding energy of 855 and 873 eV were assigned to Ni 2p3/2 and 2p1/2 of Ni 2+ [59,

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63]. The presence of a broad shake-up satellite at around 862 and 880 eV is due to a multiple electron excitation of typical Ni2+, including NiO, Ni2O3, spinel NiAl2O4 and NiMoO4 [39]. After reduction, two deconvoluted peaks at binding energy of 852.2 and 869.4 eV are ascribed to metallic Ni0 [64, 65]. It can be seen that there is not much difference in the Ni0/Ni2+ ratios with changing bimetallic compositions. These could be due to the strong interaction of Ni

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species with support, as well as the formation of the amorphous NiMoO4-like phase from Mo6+ and Ni2+ with oxygen vacancies [29].

3.2. Catalytic activity The catalytic HDO of guaiacol over the reduced NiMo/Al2O3-TiO2 catalysts with different Ni/Mo weight ratios was performed at 300 oC and hydrogen pressure of 2.0 MPa using a fixed-bed reactor. As shown in Fig. 6, the highest guaiacol conversion of 100% was

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observed for monometallic Ni/Al2O3-TiO2 catalyst, while the guaiacol conversion over Mo/Al2O3-TiO2 catalyst was only 60%. However, both of these monometallic catalysts showed low HDO conversion (< 5%). In contrast, HDO conversion was much improved over bimetallic

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NiMo/Al2O3-TiO2 catalysts, suggesting that bimetallic NiMo system facilitates the cleavage of C–O bonds during catalytic performance. The HDO conversion increased with increasing Ni

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concentration in the bimetallic NiMo system from 5 to 10 wt%, and reaching highest values of

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98%. With further increasing of Ni concentration (15, 20, and 25 wt.%), however, the HDO conversion decreased. The reason could be due to the lower acidic density of these catalyst samples as above-mentioned. The product distributions were also affected by catalyst

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compositions. Specifically, the Mo/Al2O3-TiO2 showed high selectivities of phenol and ocresol, while the Ni/Al2O3-TiO2 had high selectivities towards metylcyclohexanediol and

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cyclohexanol products. In contrast, the use of NiMo/Al2O3-TiO2 resulted in high selectivities of

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cyclohexane and methyl-cyclohexane. As shown in Fig. 6, the highest hydrocarbon selectivities of 100% (85% cyclohexane, 13% metylcyclohexane, 2% toluene) were obtained over the bimetallic catalysts with 10Ni20Mo, 15Ni15Mo and 20Ni10Mo. The higher catalytic activities of bimetallic NiMo catalysts compared to those of monometallic Ni or Mo catalysts could be due to the effects of the NiMo alloy formation[33]. It was noticed that the Ni-rich (25Ni5Mo)

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and Mo-rich (5Ni25Mo) samples had lower hydrocarbon selectivities of 31% and 76%, respectively (Fig. 6). The reaction pathways for guaiacol conversion can be proposed based on the production distribution. When Ni-rich catalysts (30Ni/Al2O3-TiO2 and 25Ni5Mo/l2O3-TiO2) are used, the main products are metylcyclohexanediol and cyclohexanol. This suggests that on the high hydrogenating Ni sites, the methyl group (-CH3) migrates to benzene ring, followed by the hydrogenation of benzene ring to produce 1-methyl-1,2-cyclohexanediol, thereafter the

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demethylation-dehydration to generate cyclohexanone and cyclohexanol. These compounds were finally converted to cyclohexane through the HDO step. These mechanisms were also observed for hydrodeoxygenation of guaiacol using Ni–based catalysts[66-68].

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For Mo-rich sites, however, phenol and o-cresol were mainly produced. Interestingly,

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there was no trace of catechol found with the prepared NiMo/Al2O3 –TiO2 catalyst. Some previous reports on the hydrodeoxygenation of guaiacol mechanism using catalysts

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Mo2C/CNF[12], K –modified NiMo/Al2O3[20], unsupported γ-Mo2N[69] or γ-Mo2N/C[70], catechol was generated via the demethylation of guaiacol during the hydrodeoxygenation

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process. Guaiacol might be directly converted into phenol by demethoxylation, followed by the methyl substitution to produce o-cresol [67, 71], and subsequent transformation into toluene

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and benzene through deoxygenation or demethylation. Afterwards, cyclohexane and methylcyclohexane can be produced as final products through the hydrogenation of aromatic ring. The

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proposed reaction pathways for guaiacol is demonstrated in Fig. 7. To gain insight into the effects of reduction temperature on guaiacol conversion and

product selectivity, the conversion of guaiacol over the 10Ni20Mo catalyst were performed after reduction at different temperatures (500, 600, 700 oC). As shown in Fig. 8, the HDO conversion of guaiacol increased with increasing reduction temperature. In addition, the

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product distributions were also changed with the change of reduction temperature. Specifically, at low reduction temperature of 500 oC, about 26% of oxygen-containing compounds (phenol and o-cresol) were produced. At reduction temperature of 700 oC, however, there were no traces of these products and the hydrocarbon selectivity reached up to 100%. To explain these catalytic behaviors, the reduced catalysts obtained at different temperatures were analyzed to determine the oxidation states of elements as well as their compositions. As shown in Fig. S5, the reducibility of catalyst increased with increasing reduction temperature, which are proved

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by the decrease of Mo6+ and Ni2+ concentration, and the increase of metallic Mo0 and Ni0 concentration with increasing the reduction temperature from 500 oC to 700 oC. The highest metallic concentration (Moo and Ni0) at high reduction temperature could be the reason for the

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highest HDO conversion of guaiacol.

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3.3. Stability test

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The catalytic stability of the prepared catalyst was investigated by the HDO test for 24 h using the 10Ni20Mo/Al2O3-TiO2 sample after being reduced at 700 oC. Fig. 9 shows the HDO

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conversion of guaiacol and product selectivity within reaction time of 24 h. As shown, the HDO conversion was still maintained at around 98% during 24 h of reaction time. During the

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reaction, cyclohexane and methyl-cyclohexane were the main products and their selectivities only slightly changed. It should be noticed that methyl cyclopentane appeared in the HDO

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product stream with selectivity of 2% after reaction time of 3 h, which could be produced from the further alkylation of cyclohexane. After 16 h of reaction, the cyclohexane selectivity was slightly decreased by 4%, and a slight increase of toluene and dimethyl-benzene selectivities, suggesting that the density of hydrogenating site can be slightly decreased for a long time of reaction. Chemical states of Mo and Ni of the spent catalyst were analyzed and compared to those of the freshly reduced sample (Fig. S6). It can be seen that there were only small changes 15

in the chemical states of these metal dopants after 24 h of reaction time, suggesting the prepared catalyst had good catalytic stability for HDO conversion of guaiacol under the investigated conditions.

4. Conclusion In this study, the spherical bimetallic NiMo /Al2O3-TiO2 catalysts with different Ni/Mo weight ratios were successfully synthesized by one-step spray pyrolysis. The prepared catalysts had particle size ranged 0.5 – 2m, high specific area surface (178 - 257 m2/g) and average pore

ro of

size (3.4 - 4.27 nm). HDO conversion of guaiacol showed that their catalytic activities were

strongly affected by Ni/Mo compositions and there were differences in the roles of Ni and Mo active species in orientating the HDO reaction pathways of guaiacol. The bimetallic

-p

NiMo/Al2O3-TiO2 had higher HDO conversion and hydrocarbon selectivity compared to those

re

of monometallic catalysts. The highest HDO conversion and hydrocarbon selectivity of 98% and 100%, respectively, were obtained over 10Ni20Mo/Al2O3-TiO2 catalyst. High reduction

lP

temperature of catalyst resulted in higher HDO activity due to its high concentration of metallic active sites of Ni0 and Mo0. The catalytic stability test showed that the HDO conversion and

na

product distribution were remained for 24 h of reaction time. These suggest that spray

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pyrolysis-derived NiMo/Al2O3-TiO2 catalyst can be a promising catalyst for HDO reaction.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement This work was supported by the R&D Program of the Ministry of Trade, Industry, and Energy (MOTIE)/ Korea Evaluation Institute of Industrial Technology (KEIT) (Project 16

No. 10049675). This research was also supported by the Basic Science Research Program (2017R1A2B4007648) through the National Research Foundation of Korea (NRF) funded by

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-p

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the Ministry of Science and ICT.

17

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19

(b)

Al

Ni

Ti

Mo

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(a)

Ni

lP

re

-p

Al

Mo

Jo

ur

na

Ti

Fig. 1. FE-TEM secondary electron (SE) images and EDX dot mappings of Al, Ti, Mo, and Ni species: (a) 10Ni20Mo/Al2O3-TiO2 catalyst, and (b) 15Ni15Mo/Al2O3-TiO2 catalyst.

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ro of -p

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Fig. 2. XRD patterns of (a) Al2O3-TiO2 support and (b-g) NiMo/Al2O3-TiO2 catalysts with various metal compositions: (b) 30Mo, (c) 5Ni25Mo, (d) 10Ni20Mo, (e) 15Ni15Mo, (f) 25Ni5Mo, (g) 30Ni.

30Mo 5Ni25Mo 10Ni20Mo 15Ni15Mo 25Ni5Mo 30Ni

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TCD signal (a.u)

Al2O3-TiO2

100

200

300

400

500

600

700

Temperature (°C) Fig. 3. NH3-TPD profiles of calcined Al2O3-TiO2 support and NiMo/Al2O3-TiO2 catalysts with various metal compositions.

21

TCD signal (a.u)

30Mo 5Ni25Mo 10Ni20Mo 15Ni15Mo 20Ni10Mo

30Ni

150

300

450

600

750

900

o

-p

Temperature ( C)

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25Ni5Mo

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Fig. 4. H2-TPR profiles of the prepared NiMo/Al2O3-TiO2 catalysts with various metal compositions.

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Fig. 5. XPS spectra of the reduced catalysts: (a) Mo 3d and (b) Ni 2p.

Fig. 6. Guaiacol conversion and product distributions obtained from HDO reaction over different reduced NiMo/Al2O3-TiO2 catalysts. Reaction conditions: 0.15mL/min of liquid feed, 60 mL/min of H2, 300 oC, 2MPa, reaction time of 2 hours.

23

ro of -p

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Fig. 7. Reaction pathways of HDO conversion of guaiacol over NiMo/Al2O3-TiO2 catalysts. HDO: Hydrodeoxygenation; HYD: Hydrogenation

na

80

60

HDO Conversion o-Cresol Phenol Toluen Met-Cyclohexane Cyclohexane

ur

Conversion/Selectivity (%)

100

40

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20

0

500

600

700 o

Reduction temperature ( C)

Fig. 8. HDO conversion of guaiacol and product distribution over reduced 10Ni20Mo/Al2O3TiO2 catalyst obtained at reduction temperature of 500, 600, and 700 oC.

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ro of -p

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Fig. 9. Guaiacol conversion and product distribution over 10Ni20Mo/Al2O3-TiO2 catalyst for 24 h of reaction time.

25

Table 1. Textual properties of support and calcined Ni-Mo catalysts BET, m2/g

Pore volume, cm3/g

Pore size, nm

80Al2O3 - 20TiO2

235

0.22

3.40

30Mo

178

0.25

4.22

5Ni25Mo

180

0.24

4.04

10Ni20Mo

185

0.24

4.06

15Ni-15Mo

217

0.27

4.10

20Ni-10Mo

225

0.30

25Ni-5Mo

246

0.31

30Ni

257

0.36

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Catalysts

4.15 4.04

-p

4.27

re

Table 2. Acidic property of calcined support and catalysts

Al2O3-TiO2 30Mo

Total amount of Aciditic acid sites density (µmol/g) (µmol/m2)

Weak (< 250 oC)

Moderate (250 - 500 oC)

Strong (>500 o C)

148.2

205.9

166.1

520.2

2.21

282.9

129.7

17.4

430.0

2.42

269.6

188.2

42.7

500.5

2.78

ur

5Ni25Mo

lP

Amount of acid site (µmol/g)

na

Sample

204.7

225.6

59.1

518.2

2.80

15Ni15Mo

178.5

279.1

81.7

539.4

2.49

25Ni5Mo

128.5

236.4

158.5

523.4

2.12

30Ni

182.1

184.7

163.8

530.6

2.06

Jo

10Ni20Mo

26