Vapour phase hydrodeoxygenation of furfural into fuel grade compounds on NiPMoS catalyst: Synergetic effect of NiP and laponite support

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 4 9 6 8 e1 4 9 8 0

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Vapour phase hydrodeoxygenation of furfural into fuel grade compounds on NiPMoS catalyst: Synergetic effect of NiP and laponite support P. Santhana Krishnan**, P. Tamizhdurai, A. Alagarasi, K. Shanthi* Department of Chemistry, Anna University, Chennai 600 025, Tamil Nadu, India

article info

abstract

Article history:

The goal of the research would be to comprehend synergetic effect, additionally the Ni

Received 21 January 2019

promotion level in the unsupported and Laponite supported NiMoS & NiPMoS catalysts. In

Received in revised form

this process, the sulfide catalysts were synthesized by a soft chemical approach under

11 April 2019

hydrothermal condition and also the Ni promotion was enhanced with the addition of

Accepted 15 April 2019

phosphorus. The catalysts were characterized predicated on XRD, N2ephysisorption, TPR,

Available online 16 May 2019

H2 pulse chemisorption, and XPS analysis. The catalysts were evaluated and compared for HDO of furfural (Hemicellulosic model compound) on temperature ranges from 503 K to

Keywords:

583 K under 20 bar H2 pressurized reaction condition in the vapour phase reactor. The

Laponite clay

characterization of phosphorus added NiMoS catalysts exhibited improved textural prop-

NiPMoS

erties with a suitable surface morphology, extraordinary stability, metal-support interac-

Synergetic factor

tion, and metal dispersion on the support. The catalytic activity results revealed that the

Furfural

unsupported and Laponite supported NiPMoS catalyst performed better HDO conversion of

Vapour phase HDO

furfural with remarkable high stability, producing deoxygenated hydrocarbons. Thereby, an innovative new unsupported and Laponite supported NiPMoS catalysts were successfully developed. The synergetic factor, intrinsic rate associated with the catalysts were correlated with improved textural properties, surface morphology, percentage of sulfidation of NiMo species, a large amount of acid sites, and dissociated hydrogen species of the catalysts. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The future energy demand expedited the current research for an efficient conversion of abundant, sustainable and lignocellulosic biomass into value added chemicals and fuel additives [1]. The main components of lignocellulosic biomass, viz.

lignin, cellulose, and hemicellulose possess significantly higher content of oxygen than its derived products especially for energy-dense fuels or fuel additives. A well known hydro treating route, i.e. hydrodeoxygenation (HDO) was utilized to reduce the oxygen content from the biomass [2]. Cellulosic and hemicellulosic bio-mass derived chemicals furfural and 5-hydroxymethylfurfural has attracted great interest as an

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P.S. Krishnan), [email protected], [email protected] (K. Shanthi). https://doi.org/10.1016/j.ijhydene.2019.04.153 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 4 9 6 8 e1 4 9 8 0

important platform chemical in the production of useful green chemicals and transportation fuels [3]. HDO and hydrogenation of furfural were tried using various nano supported catalysts, and different reaction conditions and solvents [4e6]. Mostly, furfural production is carried out by use of agricultural residues as feedstock, like corncobs and bagasse, over solid acid catalysts [3,7]. In particular, furfural on hydrogenation can produce furfural alcohol via C]O bond cleavage, while selective hydrodeoxygenation (HDO) leads to 2-methyl furan (2-MF). Additionally, hydrogenolysis of the CeC bond can also take place to give furan and CO which results in the reduction of carbon chain length. Hence, a critical challenge for the conversion of 2-MF from furfural is based on the selective cleavage of the carbonyl oxygen by minimizing parallel hydrogenation reactions. It was reported that the size and shape of metal species, type of the support, and the reaction conditions are crucial for the vapour phase, selective hydrogenation, and decarbonylation of furfural over the supported Pt catalyst [8]. In addition, the presence of the Ni promoter and phosphorus additive under suitable reaction conditions was proved to be effective for the selective conversion of furfural into 2-MF [9]. Industrial hydrotreating, supported Ni (Co) promoted Mo or W sulfide catalysts were usually synthesized by post sulfidation of the oxide catalyst at high temperature with several sulfiding agents [10,11]. To avoid high exposure of H2S gas, several pre-sulfidation methods were carried out by thermal decomposition of precursors [12,13], mechanical activation [14,15], and hydrothermal method [16e19]. Among the methods investigated, hydrothermal aided synthesis of sulfidation of oxidic NieMo catalyst was found to be effective in terms of stability, homogeneous nano scale NieMo sulfide catalyst preparation, and textural properties [16]. For the activity of the Ni promoted MoS2 catalyst, models such as contact synergy and remote control were proposed for the hydrogen spill over for the activation of MoS2 phase from the promoter phase CoSx [20] and either NiSx or NiP [13,21]. Suitable modification of Laponite RD clay with ionic [22] and neutral surfactant [23,24] can enhance the surface properties, mechanical stability, and morphology such as specific surface area, average pore diameter, and pore size. The Laponite pillared clay-supported nickel, alumina, and zirconia catalysts were already studied by several groups for methane reforming with carbon dioxide [24,25]. Recently, it has been reported that Laponite supported MneAl hydrotalcite catalyst showed enhanced activity for the combustion of volatile organic compounds [26]. The aim of the work is to precisely comprehend the promotional effect of Ni, the synergetic effect between NiP and MoS2 in both the unsupported and Laponite supported catalysts. For that, hydrothermal synthesis of modified Laponite, NiPMoS, and NiPMoS/Laponite were adapted. To understand the synergetic effect and Ni promotion, the NiMoS, and NiMoS/Laponite catalysts were prepared under same reaction condition. Then, both the soft chemical approach of NiPMoS catalysts was prepared, separated and supported on the cost effective modified Laponite clay as catalysts which were tested for the hydrodeoxygenation of furfural. In the same context, the activity of unsupported and Laponite supported

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NiMoS and NiPMoS catalysts for the HDO of furfural was compared and the results were discussed.

Experimental Materials The chemicals are used for the synthesis of supports and catalysts without any further purification: Laponite RD (Rockwood additives), Tergitol surfactant (15-S-9) (Sigma Aldrich, 98%), Furfural (Sigma Aldrich, 99%), Sodium molybdate dihydrate (Merck, 99%), Nickel chloride hexahydrate (Merck, 98%), Sodium hypophosphite monohydrate (Spectrochem, 98%), Absolute ethanol (Hayman, 99%), Commercial N2, H2, He and the mixture of 10% H2/Ar, 10% NH3/He gases (ultra high purity) obtained from Indo gas pvt. Ltd. and purified by passing over dried silica gel.

Synthesis of Modified Laponite support using neutral surfactant Laponite RD clay was modified by the usage of neutral Tergitol surfactant (15-S-9). To enable this, 4 g of commercial Laponite RD clay (obtained from Rockwood additives) was dispersed in 200 ml of water and stirred to get a clear solution. Then, 20 g of Tergitol surfactant (15-S-9) [surfactant and clay wt. ratio 5:1] was added to the solution for an effective interaction. Stirring was continued up to 2 h for sufficient mixing [24]. The solution was then subjected to solvothermal treatment in an autoclave at 100
C for 2 days. Finally, the resulting solid was dried at 120
C for 3 h and calcined at 550
C for 10 h.

Catalyst synthesis The unsupported NiMoS catalyst was synthesized by using sodium molybdate and nickel chloride with 0.3 mol fraction of Ni/(Ni þ Mo). The unsupported NiPMoS catalyst was prepared by using the same metal precursors (sodium molybdate and nickel chloride) in addition to that, sodium hypophosphite was used as a phosphorous precursor with 0.3 mol fraction of NiP/(NiP þ Mo) [1.5 mol ratio of (Ni/P)]. After stirring the solution for 2 h, it was subjected to an autoclave condition for 24 h at 200
C. The resultant black solid was washed with absolute ethanol, filtered, and dried at 120
C under N2 atmosphere. Modified Laponite prepared earlier was used as a support for preparing NiMoS (15 wt%) and NiPMoS (15 wt%) catalysts. For that, calculated amount of NiMoS and NiPMoS precursor solutions prepared separately. To the above solution, 2 g of modified Laponite support was dispersed and stirred for 2 h. Then, the precursor solution containing Laponite support was subjected to the hydrothermal method under the same reaction conditions employed for synthesis of unsupported NiMoS and NiPMoS catalysts.

Catalyst characterization Bruker D8 diffractometer was used to record the diffraction patterns of the supports and catalysts at low (0.5
e5
) and high angle (10
e80
) with a scan rate of 0.02 s using Cu Ka

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radiation (l ¼ 1.548  A). The Quadrasorb SI automatic analyzer was used to measure N2 adsorption/desorption isotherms, pore volume, and pore size of the samples at liquid N2 temperature. Prior to the analysis, 0.05 g of the sample was degassed for 4 h at 300
C under N2 purging. The specific surface area was calculated by the BET method, and the pore volume (Vp) was determined by nitrogen adsorption at a relative pressure of 0.98. The pore size distributions was determined from the desorption branch of isotherms using BJH method. The temperature programmed reduction (TPR) and temperature programmed desorption (TPD) studies were carried out using Quadrasorb ChemBET TPR/TPD analyzer. Prior to the analysis, 0.05 g of the sample was loaded into the Ueshaped quartz tube and was degassed under N2 atmosphere at 200
C for 2 h and He atmosphere for 3 h at 450
C to clean the surface of the material. TPD was performed using 10% NH3/ 90% He as probe gas which was allowed to adsorb on the catalyst for 30 min, After that, the catalyst was flushed under He flow at a rate of 80 cm3/min to remove the physisorbed NH3 followed by heating up to 800
C under He atmosphere at a heating rate of 15
C/min (80 cm3/min). TPR was performed using 5% H2/95% Ar as a probe gas by heating up to 1000
C at a heating rate of 15
C/min with a 5% H2/95% Ar gas mixture (80 cm3/min). H2 pulse titration technique was carried out to measure H2 consumption, metal surface area, dispersion percentage and the crystal size of Mo with the help of TPRWin software. For this analysis, 0.05 g of the sample was loaded into the Ueshaped quartz tube and degassed under N2 purging at 200
C for 2 h. Then, the degassed sample was reduced under H2/Ar gas mixture with the flow rate of 15
C/min till 450
C and the temperature was maintained for 2 h. After reduction, the catalyst was allowed to cool, then, H2 pulse was injected in an automatic mode of 16 pulse of pure H2 (50 ml per pulse) into the catalyst at room temperature. The chemical environment of all the catalysts were analyzed by XPS (M/s. Omicron Nanotechnology, GmBH, Germany) with XM1000 monochromatic AlKa source (hn ¼ 1486.6 eV) operated at 300 W (20 mA and 15 kV) and a hemispherical electron energy analyser. Spectral fitting of Ni and Mo species were adopted using CasaXPS software, the percentage of Ni and Mo were calculated in both unsupported and Laponite supported catalysts. The percentage of sulfidation of Ni and Mo, Ni promotional rate, and (Ni: Mo) slabs were calculated and discussed thoroughly. The percentage of carbon deposition on the spent catalyst was analyzed using thermogravimetric (Shimadzue50) from room temperature to 800
C with a heating rate of 10
C/min in the presence of ultrapure oxygen with a flow rate of 20 ml/min.

Catalytic activity Catalytic activity of both unsupported, modified Laponite supported NiMoS and NiPMoS catalysts was performed for HDO of 2.5 wt % furfural dissolved in toluene (TOL) and isopropyl alcohol (IPA) in stainless steel (SS 316) high pressure fixed bed (Vapour Phase) fabricated by Amar Equipment Pvt. Ltd. Mumbai, India. A flow diagram of the reactor is provided in Scheme S1. The reaction parameters such as the effect of temperature, solvent, reactant feed rate, and time on stream

Fig. 1 e (A & B) High angle XRD patterns of support and catalysts: Laponite RD, modified Laponite support, NiMoS, NiPMoS, NiMoS/modified Laponite, NiPMoS/modified Laponite catalysts.

were optimized for high conversion and product selectivity. The catalyst was pre-treated by passing N2 gas and activated using ultra-pure H2 at 400
C for 3 h. The liquid product formed during the course of the reaction was collected every hour and analyzed with a GCe17 A Shimadzu gas chromatograph using an RTXe5 column and a flame ionization detector. The product distribution was analyzed using JEOL GCMATE II GCeMS. The assumed kinetic model is a first order reaction according to the integral reactor model [10], the rate constant of the reaction can be calculated according to the following equation. k¼ 

F lnðl  tÞ wC

(1)

The specific reaction rate and intrinsic rate of the catalysts can be expressed by the following expression (Equations (2) and (3)).

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Table 1 e Textural properties of support and catalysts: modified Laponite, NiMoS, NiPMoS, NiMoS/modified Laponite and NiPMoS/modified Laponite. Catalysts

Surface Area (m2/g)a

Pore volume (cm3/g)b

Pore Diameter (nm)c

11 13 511 183

0.09 0.07 1.065 0.67

4.3 4 9.6 8.3

II II IV IV

0.61 0.63 e 0.61

e e 2.6 2.6

93

0.25

3.8

IV

0.62

2.1

NiMoS NiPMoS Modified Laponite NiMoS/modified Laponite NiPMoS/modified Laponite

Isotherm d-spacing of MoS2 Laponite octahedral (060) Type (002)d planee (nm) (nm)

a, b & c

Surface area, Pore volume & Pore diameter(Dp) (desorption branch by BJH method) obtained from N2 physisorption study. from high angle XRD.

rs ¼ kC ¼ F=wlnðl  tÞ

(2) 1

Where, F is total molar flow of reactant (mol s ), t is total conversion, C is the initial concentration of reactant (mol L1) and w is the weight of the catalyst. The intrinsic reaction rate ri can be expressed by the following expression (Equation (3)).

d&e

Obtained

r ri ¼ *N n

(3)

Where, n is the number of Mo atoms per gram of the catalyst and N is Avogadro number. Vapour Phase (VP) reaction conditions: Reactant: 2.5 wt% of Furfural (FUR) in isopropyl alcohol (IPA) and Toluene (TOL), Reactant Feed: 2.0 ml/h; Reaction time: 6e8 h; Temperature:

Fig. 2 e N2 adsorption-desorption isotherms (A & B) and pore size distribution curves of (C & D) support and catalysts: modified Laponite support, NiMoS, NiPMoS, NiMoS/modified Laponite and NiPMoS/modified Laponite.

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Fig. 3 e H2-TPR profiles of catalysts: NiMoS, NiPMoS, NiMoS/modified Laponite and NiPMoS/modified Laponite. 503 Ke583 K; 20 bar H2 pressure, H2 flow rate ¼ 50 cm3/min at WHSV: 1.98 h1.

Results and discussion High angle X-ray diffraction (XRD) High angle XRD patterns of NiMoS, NiPMoS and Laponite supported catalysts are depicted in Fig. 1(A & B). The d-spacing and crystal size of an octahedral plane (060) present in the modified Laponite clay of all the catalysts was calculated and tabulated in Table 1. The diffraction pattern of NiMoS catalyst exhibits peak belongs to the mixture of MoS2 and NiS2 crystal phases. The peaks at 2q ¼ 14.6
, 33.1
, 59.6
can be assigned to MoS2 [27] and at 2q ¼ 21.6
, 26.4
, 30.5
, 37.8
, 46.3
, and 54.9
corresponds to NiS2 phase [28]. An additional feature observed at 2q ¼ 39.58
was assigned to NiMoS4 phase [28]. However, in case of NiPMoS catalyst a slight shift in a peak position and an increase in intensity was observed compared to NiMoS catalyst. XRD pattern of Laponite RD and modified Laponite are shown in Fig. 1 B. Pure Laponite exhibits peaks at 2q values of

20.43
, 28.09
, 35.95
, 53.91
, 61.24
, and 72.44
which is in accordance with the published literature report [29]. The peaks correspond to the modified Laponite are retained with the slight drop in intensity and shift in a peak position as Laponite. This is attributed to the delamination of the Laponite sheet that occurred during mild hydrothermal condition (100
C). The modified Laponite was used as support for NiMoS and NiPMoS catalysts, which was synthesized under same hydrothermal condition (200
C for 24 h). The synthesis condition favours the intensification of tetrahedral silica and trioctahedral silica of Laponite which are confirmed from the peaks at 2q value of 20.16
and 60.9
respectively. In addition, this type of drastic hydrothermal condition favorss formation of amorphous silica and trioctahedral plane (060) of Laponite leads to better interaction with well dispersion of NiMo active species. The characteristic peaks of NiMo sulfides were also observed in the Laponite supported catalysts. Further, the peaks observed at 2q ¼ 43.3, 47.4, and 53.6 confirm the formation of Ni2P phase [30]. In the case of NiPMoS catalyst, the intensity of multi stacked slabs (0 0 2) of MoS2 was weak while the intensity of layered slabs (1 1 0) of MoS2 became strong compared to NiMoS catalyst. The XRD pattern of P incorporated catalysts confirms the co-existence of NiSx, MoS2, NiP, and NiMoS4 phases. The reduction in characteristic peak intensity of the active species of MoS2, NiS2, NiP, and NiMoS4 on the support as compared to unsupported catalysts may be due to the interaction of the active species over the support. The reduction in peak intensity can be considered due to dispersion of the active species on the supports, as reported in literature [31]. Further, a small hump at 2q ¼ 37.23
in the NiPMoS catalyst confirms the formation of NiePeMoeS phase.

Nitrogen sorption analysis Textural properties of the catalysts obtained from N2 adsorption  desorption study are given in Table 1 and the corresponding isotherms are portrayed in Fig. 2(A & B). Unsupported NiMoS and NiPMoS exhibited type II isotherm (Fig. 2(A)), with surface area of 11 & 13 m2/g and pore size in the range of 4e4.3 nm. The addition of P into NiMoS catalyst, slightly reduced the pore volume and pore size as compared to the NiMoS catalyst. Modified Laponite supported NiMoS and NiPMoS catalysts exhibit type IV isotherm (Fig. 2(B)). The surface area and pore

Table 2 e H2 Chemisorption, total acidity, TPR profile and Edge energy values of the catalysts. Catalysts

Average metal (Mo) surface area(m2/g)a

NiMoS NiPMoS NiMoS/ modified Laponite NiPMoS/ modified Laponite

15 18 22

54 56 58

2.6 2.3 1.9

477, 623 & 706 365, 650 & 801 437 & 806

1.65 1.02 1.24

3.0 3.0 3.6

25

69

1.7

588 & 820

1.57

3.9

a, b & c

Metal (Mo) Average MoS2 dispersion (%)b Crystallite size (nm)c

Reduction Temperature (
C)d

Total Acidity Edge (mmol NH3/g cat)e energy (Eg eV)f

Obtained from H2-Pulse chemisorption analysis dH2 TPR & eNH3-TPD results, fCalculated from UVeVis DRS spectra result.

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Fig. 4 e (A) XP survey spectra, (BeE) Mo 3d and (FeI) Ni 2p region in NiMoS, NiPMoS, NiMoS/modified Laponite and NiPMoS/ modified Laponite catalysts. Fig. 2(C & D). In NiPMoS catalyst, multi pore size distribution was observed with an average pore size of 4 nm (Fig. 2C). Only one type of pore is present with peak maxima at 9.6 nm for the modified Laponite support, whereas the NiMoS deposition on the modified Laponite shifts the single peak maxima value to 9 nm. P added catalysts, a shift in peak maximum was observed to be 3.6 nm (Fig. 2D). This shift in the peak position was due to the blockage of pores by the formation of NiP, NiS2 species on the support along with MoS2, and NiMoS4 species. This type of pore blockage was reported due to NiS and NiP species [33,34].

size of the support and catalysts are measured to be in the range of 93e511 m2/g and 3.8e9.6 nm, respectively. It is observed that the surface area coverage of NiMoS on the support is more than 50% of the modified Laponite support. However, in the case of NiPMoS/modified Laponite catalyst, the surface area coverage is more than 80%. There is a significant drop in pore volume of NiPMoS/modified Laponite than NiMoS/modified Laponite catalyst. This is due to the deposition of the excess amount of MoS2, NiS2, and NiP species (also identified from XRD result, Fig. 2(B)) on the support. This result concludes that the addition of P to NiMoS supported catalyst has a remarkable reduction in surface area compared to that of NiMoS supported catalyst. Even though, the unsupported catalysts displayed type II isotherm, the catalysts on the Laponite support has type IV isotherm which is in consistent with the report [32]. The pore size distribution of all the catalysts obtained by BJH (desorption branch of an isotherm) analysis is shown in

Hydrogen temperature programmed reduction (H2-TPR) & hydrogen pulse chemisorption The typical TPR profile of unsupported and Laponite supported NiMoS and NiPMoS catalysts are shown in Fig. 3. It is obvious that TPR profiles of the unsupported and supported

Table 3 e (A). XPS Binding Energies (eV) of (A) Mo 3d, Ni 2p and S 2s region present in all the catalysts (B) the distribution of element (%), atomic ratios, % sulfidation, Ni promotional rate and Ni: Mo slabs in unsupported and Laponite supported NiMoS, NiPMoS catalysts. Mo4þ eV

Catalysts

NiMoS NiPMoS NiMoS/modified Laponite NiPMoS/modified Laponite

Catalysts

NiMoS NiPMoS NiMoS/modified Laponite NiPMoS/modified Laponite a

Mo (%)

Ni (%)

Mo5þ eV

Mo6þ eV

NiS eV

S2s eV

3d5/2

3d3/2

3d5/2

3d3/2

3d5/2

3d3/2

2p 3/2

2p 1/2

229.7 229.8 228.9 229.4

230.5 230.6 231.5 232.4

232.8 233.0 230.4 230.8

233.6 233.9 235.1 234.8

e e 232.8 231.8

e e 237.6 237.5

854.6 854.5 853.4 854.6

871.9 872.2 877.2 878.6

S P Si (%) (%) (%)

O Mo/ Mo/ (%) Ni Si

% of Mo sulfidation by hydrothermala

NiS/ % Ni NiT Nisulfidation Promotional rate

226.9 227.1 226.3 227.0

(Ni: Mo) slabs

55.09 11.12 21.43 e e 12.37 4.9 54.46 9.77 19.26 3.58 e 12.93 5.5 6.34 3.65 5.94 e 12.16 71.91 1.7

e e 0.52

86.8 86.2 38.4

0.58 0.46 0.44

81 76 69

23 32 25

0.33 0.46 0.75

6.48

0.50

35.1

0.40

71

31

1.03

2.90 4.34 1.43 12.74 72.11 2.2

% sulfidation obtained from Mo4þ/Mo total, Mo total (total molybdenum) ¼ Mo4þ þ Mo5þ þ Mo6þspecies, NiT ¼ NiS þ NiMoS þ NiO, Nisulfidation ¼ (NiS þ NiMoS)/NiT  100, Promotional rate ¼ (NiMoS/NiT)  100, (Ni: Mo)slabs ¼ promoter ratio ¼ NiMoS: Mo4þ.

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Fig. 5 e Influence of reaction temperature on conversion and HDO of furfural over NiPMoS and NiPMoS/modified Laponite catalysts in IPA (A & C) and TOL solvents (B & D).

catalysts are different. In NiMoS catalyst, the first reduction peak was observed at 477
C due to the partial reduction of amorphous, highly defective and multi-layered Mo sulfide. The second reduction peak at 623
C was due to the deep reduction of all Mo (Td) species. The third reduction, a low intense peak appeared at 706
C may be due to the reduction of Ni in NiS2 phase or sintering of NiMoS4 to Ni3S2 and MoS2 [35]. The reduction peak of NiPMoS catalyst at low temperature shifts downward as compare to NiMoS. However, the second and third reduction peak maximum was found to be at 650
C and 801
C, respectively. The increase in the temperature of the reduction peak can be attributed to the reduction of Ni from NiS2 and NiP. The reduction of NiP always higher than the NiS2 and NiO in line with published reports [33,36]. Such a high temperature reduction attributed to the highly thermodynamic stability of P]O bond and the surface diffusion of the H atoms [37]. In NiMoS/modified Laponite catalyst, the reduction of Mo species started at 365
C and ended at 650
C due to the deep reduction of Mo species in the NiMoS supported catalyst. It was also observed that in the NiPMoS/modified Laponite catalyst showed a broad peak which started around 250
C and

Fig. 6 e Influence of TOL and IPA solvents on furfural conversion and HDO over the catalysts.

intensified at 518
C. The shift in peak to the lower temperature may be due to the interaction of NiP with MoS2. A peak observed at 820
C was due to reduction of NiP and NiS2 species. Modified Laponite supported NiMoS/NiPMoS catalysts experienced the reduction of the active species at high temperature suggesting that the active species were stabilized on the Laponite surface due to their strong metal support interaction [38]. The average Mo surface area and dispersion of MoS2 on the unsupported and supported catalysts were measured by H2 pulse chemisorption. The values for the unsupported catalysts were in the range of 15e18 m2/g and 54e56%, whereas for supported catalysts were in the range of 22e25 m2/g and 58e69% respectively (Table 2). Among all the catalysts, supported NiPMoS possessed high Mo surface area and dispersion percentage of MoS2 which was due to the co-existence of MoS2, NiS2 and NiP phases on the support accounting for its promotional effect in the NiPMoS and NiPMoS/modified Laponite catalysts.

X-ray photoelectron spectroscopy (XPS) The survey spectra of Mo 3d, Ni 2p was recorded, deconvoluted, and presented in Fig. 4(AeI). The percentage of sulfidation of Mo, Ni promotional rate, Ni:Mo slabs, and dispersion of NiMo species on the Laponite support were calculated (Table 3(A & B)). The splitting of oxygen species measured at 533.4 eV confirms the presence of partial surface oxidation of MoS2 species and physisorbed water moisture present in the unsupported catalysts (Fig. 4A). The presence of O 1s species may be attributed to a silanol group of the support or may be due to oxysulfidic Mo species formed by the partial oxidation of Mo layer. Mo 3d doublets (3d5/2 and 3d3/2) species confirms different oxidation states of Mo (Mo4þ, Mo5þ, Mo6þ) and chemical environment (sulfidic and oxysulfidic species) of Mo atom [39]. From the ratio of Mo4þ species to the total amount of Mo4þ, Mo5þ, and Mo6þ species, the percentage of MoO3 converted to MoS2 was calculated (Table 3(A)). In NiMoS and NiPMoS catalysts, the percentages of Mo sulphidation are 86.8 and 86.2 respectively. Hence, it was clear that addition of P has no considerable influence on percentage sulfidation. However, on

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Fig. 7 e (A) WHSV, (B) Time on stream (TOS) study of NiPMoS and NiPMoS/modified Laponite catalysts, (C) Conversion and Product yield of all the catalysts, and (D) Intrinsic HDO activity and synergetic factor of NiPMoS and NiPMoS/modified Laponite catalysts (in TOL solvent at 583 K under 20 bar H2 pressure).

modified Laponite supported NiMoS catalyst, the percentage of Mo sulphide values was much lower than that of unsupported catalysts (38.4 and 35.1). This can be attributed to metal support interaction. The surface atomic percentage of Mo, Ni, S, P, Si, O, and surface atomic ratio of Mo/Ni, Mo/NiP or Mo/Si of all the catalysts were measured which is presented in Table 3(A). The addition of P was found to have the marked influence on the Mo/Ni or and Mo/Si ratio values of both unsupported and supported catalysts. A large value of atomic ratio Mo/Si and Mo/Ni signifies good dispersion [40] with the formation of the small sized MoS2 species on supported catalysts [38]. Mo/Ni ratio was higher in both unsupported and supported NiPMoS catalysts. The small size of MoS2 was beneficial for the reactant molecule to approach the active site of a catalyst to turn into a product molecule [38]. It was concluded from the XPS analysis that added P to NieMo may be helpful to maintain the Ni atom economy and utilization of Laponite as support significantly enhanced the dispersion with the suitable size of MoS2 species. Ni 2p XPS spectra of all the catalysts are presented in Fig. 4 (FeI). NiS, NiO, and NiMoS phases were detected and percentage of Ni sulfidation, Ni promotional rate and (Ni: Mo) slabs [41] were calculated and summarised in Table 3(B). Ni promotional rate and Ni: Mo slabs were observed to be high for NiPMoS as compared to NiMoS. Further, it is worth observing a large increase in the value of Ni:Mo slabs on the Laponite

supported NiPMoS catalyst due to the availability of a large number of Mo4þ and NiMoS species on the support.

Catalytic activity Influence of temperature Over 583 K, decrease in product selectivity (loss in product yield) observed. Hence, HDO of furfural was studied till 583 K, in vapour phase using SS fixed bed reactor. The furfural HDO activity of NiPMoS and NiMoS/modified Laponite catalysts was measured under steady state condition in the vapour phase (Fig. 5A & B) at different reaction temperatures (503 K  583 K) under 20 bar H2 pressure with H2 flow rate of 50 ml/min and at WHSV: 1.98 h1 using Toluene (TOL) and 2-propanol (IPA) as solvents. As the reaction temperature was raised from 503 K to 583 K, the conversion and HDO of furfural were increased in both the solvents. The products of furfural HDO were represented by percentage of 2-methyl furan (MF), tetrahydrofuran (THF), and methyl tetrahydrofuran (MTHF) formed.

Influence of solvent To study the influence of solvent on the conversion and product yield (%), the furfural HDO was carried out using toluene (TOL) and isopropanol (IPA) as solvents for vapour at

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Fig. 8 e Reaction pathways of furfural HDO in 2-propanol and Toluene.

583 K under 20 bar H2 pressure. It was observed (Fig. 6) that in IPA solvent the furfural conversion and product yield was low compared to TOL solvent. It may be attributed to the formation of isopropoxy methyl furan when IPA was used as the solvent, a major product formed by the dehydration of furfural alcohol (FOL) and IPA solvent thereby suppressed the formation of 2-Methyl Furan. The etherification product was minimum in TOL under the same reaction condition. Hence, TOL can be stated as the better choice as a solvent for HDO of furfural in vapour phase reaction conditions.

Influence of reactant feed rate & time on stream (TOS) The conversion (%) and HDO of furfural were measured under vapour phase conditions as a function of reactant feed rate using TOL as solvent at 583 K under steady-state conditions at a space velocity (WHSV, h1) within the range of 1.78 h1 to 2.18 h1 (Fig. 7A). A very high conversion of furfural (99%) with 26% of 2-MF was observed at a space velocity of 1.98 h1 in the vapour phase HDO of furfural on NiPMoS/Laponite. Further increasing the reactant feed rate (beyond 1.98 h1), the

Table 4 e H2 uptake values, Specific reaction rate, Intrinsic rate of all the catalysts on Furfural HDO. Catalysts

NiMoS NiPMoS NiMoS/modified Laponite NiPMoS/modified Laponite a b c d

r(Fur)b Volume of H2 consumption (mmol/g)a 107 mol g1 s1

r(Furri(Fur) (ri(Fur -HDO))d 105molec. HDO)c108 Mo at1 s1 1 1 mol g s

Synergetic Factor

290 380 170

1.9 4.3 2.5

7.3 11.5 2.8

1.3 2.0 4.3

e 1.5 e

230

6.6

4.3

6.4

1.5

Obtained from H2 pulse Chemisorption method. Specific rates:r(Fur), total transformation rate of furfural. r(Fur-HDO), rate of formation of 2-MF. Intrinsic rate of furfural conversion & HDO (% Mo obtained from XPS analysis). Synergetic factor for Furfural HDO ¼ ri(Fur-HDO) (NiPMoS)/ ri(Fur-HDO) (NiMoS) in both unsupported and supported NiPMoS catalysts under reaction conditions.

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remained steady after 4 h of reaction time in the vapour phase condition. Hence, performance of the catalysts was compared under steady state reaction conditions.

Comparison of catalytic activity

Fig. 9 e Reusability and stability of NiPMoS and NiPMoS/ modified Laponite catalysts in TOL solvent.

conversion decreased gradually. This may be due to unsaturation of the active sites of the catalysts by the reaction molecule. However, 2-MF yield slightly decreased beyond high space velocity of 1.98 h1 over NiPMoS catalyst. In unsupported NiPMoS catalyst, conversion, and 2-MF yield (%) were 95 and 55, respectively at a space velocity of 1.98 h1. Furfural conversion (%) and 2-MF yield (%) were measured as a function of time-on-stream at 583 K under vapour phase reaction condition (Fig. 7B). It was observed that the conversion and product yield increased gradually up to 4 h and

The conversion of furfural (%) and product distribution (%) obtained for all the catalysts under optimized reaction conditions are presented in Fig. 7C. The liquid products of furfural HDO identified by GC-MS. From the Fig. 7C, it was obvious that the presence of P in NiMoS showed enhanced conversion (%) and 2-MF yield (%) on both supported and unsupported catalysts. Furfural conversion (%), FOL and 2-pentanol yield were remarkably high on the supported catalysts. However, 2-MF yield was distinctively high on the unsupported catalyst. In addition to these three major products, formation of the considerable quantity of condensation products such as 2(butoxymethyl)furan isomers, 2-methyl-1,6-dioxaspiro[4.4] nona-2,7-diene, and 2-(furan-2-ylmethyl)-5-methylfuran were also observed. Under the vapour phase reaction condition, the hydrogenation of furfural to furfuryl alcohol (FOL), followed by hydrogenolysis to 2-MF appeared to be the common route on this catalyst. In another route, Furfural can undergo hydrogenation to FOL and then form 2-pentanol by ring-opening reaction, as seen predominately on the supported catalysts. However, hydrogenolysis of FOL to 2-MF was the predominantly occurred path on the unsupported catalysts. It could be seen that the highest conversion of furfural was coupled with the highest yield of 2-methylfuran. However,

Fig. 10 e XRD pattern (A & B) and N2 adsorption e desorption isotherm (C & D) of fresh and spent unsupported and supported NiPMoS catalysts.

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ring opening of FOL was facile which may be responsible for the decrease in the yield of 2-MF. The results further suggest that TOL solvent may enhance the inner atmosphere for inhibition of undesired products, thus avoiding etherification reaction (Fig. 8). Table S1 shows a comparison of Furfural HDO of NiPMoS catalysts with previously reported catalysts. The result suggested that the catalysts expressed excellent conversion & HDO yield (%) under optimized reaction conditions.

HDO rate and synergetic factor

the observed enhancement in the HDO activity of the catalysts.

Scheme for hydrodeoxygenation of furfural in TOL and IPA The possible reaction pathway of HDO of furfural on the catalyst is shown in Fig. 8. The HDO of furfural occurred through the following sequential steps. First, hydrogenation of FUR to FOL carried out, and followed by hydrogenolysis to MF. Then, MF underwent ring opening to form 2-pentanol. MF and 2-pentanol were exclusively formed in TOL solvent. However, with 2-propanol solvent, 2-(isopropoxymethyl) furan was formed as a major product by etherification between FOL and IPA which suppressed the formation of MF. This type of ether compound was observed earlier under conditions of furfural hydrogenation over Pt, Rh, and Ni based catalysts [42]. The formation of compounds (1, 2 & 3 in Fig. 8) identified from GC-MS data (Figure S1) (compound 1 & 3; dimers) possibly happened either via FA dimerization, or through acid catalyzed dehydration and/or via hydroxyalkylation/alkylation of MF. Compound 2 formation possibly proceeded through condensation of a hydroxyl group of FOL with butanol.

In order to understand the promotional effect of phosphorous addition to NiMoS, the specific reaction, HDO, and intrinsic HDO rates of all the catalysts were calculated and the results are presented in Table 4. The specific reaction rate, HDO rate, and intrinsic HDO rate on unsupported and supported NiPMoS were significantly higher than other catalysts by several folds. The result confirmed the beneficial effect of phosphorous in terms of NiP promotional level on both the catalysts. The specific reaction rate and HDO rate of unsupported NiPMoS were 5e6 times higher than supported NiPMoS catalysts. The intrinsic HDO rate of the supported catalysts was found to be higher than the unsupported catalysts owing to the enhanced furfural HDO per Mo atom per gram of the catalyst (Table 4). The intrinsic HDO rate measured on NiPMoS catalyst was nearly 1.5 times higher than the NiMoS catalyst in both unsupported and supported catalysts confirming that the presence of NiP and NiS2 phases in both unsupported and supported catalysts facilitate the creation of MoS2 active sites. However, a maximum intrinsic HDO rate value was observed for NiPMoS/modified Laponite catalyst can be attributed to the ease of availability of Mo active sites on the catalyst. In another way, the promotional effect of additive P (NiP) is understood on NiMoS catalysts, by calculating the synergetic factor for furfural HDO using the formula, Furfural HDO ¼ ri(Fur-HDO) (NiPMoS)/ri(Fur-HDO) (NiMoS) and the values are given in Table 4 and Fig. 7D. Synergetic factor on NiPMoS catalysts was found to be more than one indicating that the P addition showed a positive synergism. In conclusion, the P addition on NiMoS supported and unsupported showed positive synergism when HDO reaction was carried out under vapour phase reaction conditions.

The reusability and stability of NiPMoS and NiPMoS/modified Laponite catalysts were studied for three cycles for HDO of furfural under optimized vapour phase reaction conditions. The recyclability test was conducted after pre-treating the catalyst with N2 followed by activation before each run using pure H2 for 3 h at 400
C. Both the catalysts exhibited a negligible reduction in intrinsic HDO activity up to three cycles (Fig. 9) which indicated that the catalysts were stable under the present experimental conditions. A reduction in surface area of the spent catalyst may be due to coke formation on the surface as indicated in Table S2. XRD pattern recorded for the used catalyst showed that the NiMo active species were retained on the spent catalyst. However, the pore volume and pore diameter values did not change significantly (Fig. 10 & Table S2). The results indicated the stability of the catalysts.

Correlation of HDO activity with physicochemical properties of the catalysts

Conclusions

The specific reaction, HDO, and intrinsic rates of the phosphorus added NiMoS catalysts were higher than other catalysts (Table 4). The performance of these catalysts was correlated with the physicochemical properties. On the modification of the catalysts with the phosphorous, the textural properties were favorably affected (N2 sorption studies) (Fig. 2). Further, an influence on the dispersion percentage of active metal species and the promotional effect of mixed NiS2 and NiP phases on the support were seen (XRD, H2 pulse Chemisorption and H2 uptake value (Fig. 1, Tables 2 and 4)). In addition to this, upon phosphorous addition, an increase in the percentage of Ni sulfidation, Ni promotional rate, the number of (Ni:Mo) slabs and Mo/Ni ratio was evident from XPS analysis (Table 3(B)). These factors clearly accounted for

Laponite RD clay was successfully modified by hydrothermal method with high surface area and pore size. Further, the hydrothermal addition of phosphorus into NiMoS on both unsupported and supported NiMoS catalysts significantly improved the textural properties which enhanced the Ni promotional level in terms of percentage of Ni sulfidation, Ni promotional rate, and the number of (Ni:Mo) slabs. Moreover, the modified mesoporous Laponite clay aided the better dispersion of MoS2 fringes. In addition, HDO of furfural over the unsupported NiPMoS catalyst exclusively took place by the hydrogenolysis and resulted in a high yield of 2-MF. In contrast, supported NiPMoS catalyst favours hydrogenolysis in addition to ring opening results in the broader product distribution. As the temperature increased, the conversion

Recyclability and stability of spent catalysts

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and product yields also increased in the vapour phase reaction conditions. The study revealed that the phosphorus addition on unsupported and Laponite supported NiMoS catalysts was responsible for high intrinsic rate and positive synergetic factor. Toluene, a nonpolar solvent seems to be a suitable solvent for HDO of furfural on all the catalysts under optimized vapour phase reaction condition whereas, IPA solvent suppress the formation of 2-MF by etherification. Therefore, addition of phosphorus on both unsupported and supported NiMoS catalysts is essential for the production of 2-MF from furfural because of its high efficiency in the removal of oxygen by the hydrodeoxygenation pathway. The recyclability and stability of the catalysts were also excellent under the experimental conditions.

Acknowledgements The authors are grateful for the financial support provided by University Grants Commission (UGC)- Basic Scientific Research Fellowship (Award Lr.No. F.4-1/2006 (BSR)/7-7/2007 (BSR), dt. 13.03.2012). We are thankful to DRDO, UGC-DRS and DST-FIST for providing an instrumentation facility in the Department of Chemistry, Anna University, Chennai, India.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.153.

[16]

references

[17]

[1] Zhang Z, Song J, Han B. Catalytic transformation of lignocellulose into chemicals and fuel products in ionic liquids. Chem Rev 2016;117:6834e80. [2] Gilkey MJ, Xu B. Heterogeneous catalytic transfer hydrogenation as an effective pathway in biomass upgrading. ACS Catal 2016;6:1420e36. [3] Zhou P, Zhang Z. One-pot catalytic conversion of carbohydrates into furfural and 5-hydroxymethylfurfural. Catal Sci Technol 2016;6:3694e712. [4] Gilkey MJ, Panagiotopoulou P, Mironenko AV, Jenness GR, Vlachos DG, Xu B. Mechanistic insights into metal lewis acidmediated catalytic transfer hydrogenation of furfural to 2methylfuran. ACS Catal 2015;5:3988e94. [5] Lee W-S, Wang Z, Zheng W, Vlachos DG, Bhan A. Vapor phase hydrodeoxygenation of furfural to 2-methylfuran on molybdenum carbide catalysts. Catalysis Science & Technology 2014;4:2340e52. [6] Liu L, Lou H, Chen M. Selective hydrogenation of furfural to tetrahydrofurfuryl alcohol over Ni/CNTs and bimetallic CuNi/CNTs catalysts. Int J Hydrogen Energy 2016;41:14721e31. [7] Li H, Deng A, Ren J, Liu C, Lu Q, Zhong L, Peng F, Sun R. Catalytic hydrothermal pretreatment of corncob into xylose and furfural via solid acid catalyst. Bioresour Technol 2014;158:313e20. [8] Pushkarev VV, Musselwhite N, An K, Alayoglu S, Somorjai GA. High structure sensitivity of vapor-phase furfural decarbonylation/hydrogenation reaction network as

[18]

[19]

[20]

[21]

[22]

[23]

[24]

14979

a function of size and shape of Pt nanoparticles. Nano Lett 2012;12:5196e201. Sitthisa S, An W, Resasco DE. Selective conversion of furfural to methylfuran over silica-supported NiFe bimetallic catalysts. J Catal 2011;284:90e101. re P, Geantet C. Bui VN, Laurenti D, Deliche Hydrodeoxygenation of guaiacol: Part II: support effect for CoMoS catalysts on HDO activity and selectivity. Appl Catal B Environ 2011;101:246e55. Licea YE, Grau-Crespo R, Palacio LA, Faro Jr AC. Unsupported trimetallic Ni (Co)-Mo-W sulphide catalysts prepared from mixed oxides: characterisation and catalytic tests for simultaneous tetralin HDA and dibenzothiophene HDS reactions. Catal Today 2017;292:84e96. Yi Y, Zhang B, Jin X, Wang L, Williams CT, Xiong G, Su D, Liang C. Unsupported NiMoW sulfide catalysts for hydrodesulfurization of dibenzothiophene by thermal decomposition of thiosalts. J Mol Catal A Chem 2011;351:120e7. Santhana Krishnan P, Ramya R, Umasankar S, Shanthi K. Promotional effect of Ni2P on mixed and separated phase MoS2/Al-SBA-15 (10) catalyst for hydrodenitrogenation of ortho-Propylaniline. Microporous Mesoporous Mater 2017;242:208e20. Wu Z, Whiffen VM, Zhu W, Wang D, Smith KJ. Effect of annealing temperature on CoeMoS2 nanosheets for hydrodesulfurization of dibenzothiophene. Catal Lett 2014;144:261e7. Wang C, Wu Z, Tang C, Li L, Wang D. The effect of nickel content on the hydrodeoxygenation of 4-methylphenol over unsupported NiMoW sulfide catalysts. Catal Commun 2013;32:76e80. Wang W, Li L, Wu K, Zhang K, Jie J, Yang Y. Preparation of NiMo-S catalysts by hydrothermal method and their hydrodeoxygenation properties. Appl Catal Gen 2015;495:8e16. Wang W, Zhang K, Li L, Wu K, Liu P, Yang Y. Synthesis of highly active Co  Mo  S unsupported catalysts by a onestep hydrothermal method for p - cresol hydrodeoxygenation. 2014. He H-Y, He Z, Shen Q. Efficient hydrogen evolution catalytic activity of graphene/metallic MoS2 nanosheet heterostructures synthesized by a one-step hydrothermal process. Int J Hydrogen Energy 2018;43:21835e43. Zhang Y, Jin Z, Wang G. Charge transfer behaviors over MOF5@ g-C3N4 with NixMo1 xS2 modification. Int J Hydrogen Energy 2018;43:9914e23. Chen J, Zhang J, Mi J, Garcia ED, Cao Y, Jiang L, Oliviero L,  F. Hydrogen production by water-gas shift reaction Mauge over Co-promoted MoS2/Al2O3 catalyst: the intrinsic activities of Co-promoted and unprompted sites. Int J Hydrogen Energy 2018;43:7405e10. Li-Hua L, Shu-Qun L, Chai Y-M, Yun-Qi L, Liu C-G. Synergetic effect between Ni 2 P/g-Al 2 O 3 and MoS 2/g-Al 2 O 3 catalysts on their performance in hydrodenitrogenation of quinoline. J Fuel Chem Technol 2013;41:698e702.  lkova  H, Madejova  J, Zimowska M, Bielan  ska E, Pa  ska-Dobrzyn  ska L, Serwicka EM. LaponiteOlejniczak Z, Lityn derived porous clay heterostructures: I. Synthesis and physicochemical characterization. Microporous Mesoporous Mater 2010;127:228e36. Wu CJ, Gaharwar AK, Chan BK, Schmidt G. Mechanically tough Pluronic F127/Laponite nanocomposite hydrogels from covalently and physically cross-linked networks. Macromolecules 2011;44:8215e24. Hao Z, Zhu H, Lu G. Zr-Laponite pillared clay-based nickel catalysts for methane reforming with carbon dioxide. Appl Catal Gen 2003;242:275e86.

14980

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 4 9 6 8 e1 4 9 8 0

[25] Hwang K-S, Zhu H, Lu G. New nickel catalysts supported on highly porous alumina intercalated laponite for methane reforming with CO 2. Catal Today 2001;68:183e90.  ska E, [26] Napruszewska BD, Michalik-Zym A, Dula R, Bielan  ska-Dobrzyn  ska L, Rojek W, Machej T, Socha RP, Lityn Bahranowski K, Serwicka EM. Composites derived from exfoliated Laponite and Mn-Al hydrotalcite prepared in inverse microemulsion: a new strategy for design of robust VOCs combustion catalysts. Appl Catal B Environ 2017;211:46e56. [27] Polyakov M, Poisot M, van den Berg MW, Drescher T, Lotnyk A, Kienle L, Bensch W, Muhler M, Gru¨nert W. Carbonstabilized mesoporous MoS 2dstructural and surface characterization with spectroscopic and catalytic tools. Catal Commun 2010;12:231e7. [28] Mondal D, Villemure G, Li G, Song C, Zhang J, Hui R, Chen J, Fairbridge C. Synthesis, characterization and evaluation of unsupported porous NiS 2 sub-micrometer spheres as a potential hydrodesulfurization catalyst. Appl Catal Gen 2013;450:230e6. [29] Bippus L, Jaber M, Lebeau B. Laponite and hybrid surfactant/ laponite particles processed as spheres by spray-drying. New J Chem 2009;33:1116e26. [30] Guan Q, Li W, Zhang M, Tao K. Alternative synthesis of bulk and supported nickel phosphide from the thermal decomposition of hypophosphites. J Catal 2009;263:1e3. [31] Guan Q, Li W. Catalysis Science & Technology the synthesis and evaluation of highly active Ni 2 P e MoS 2 catalysts using the decomposition of hypophosphites. 2012. p. 2356e60. [32] Hao Z, Zhu HY, Lu GQ. Zr-Laponite pillared clay-based nickel catalysts for methane reforming with carbon dioxide. Appl Catal Gen 2003;242:275e86. [33] Yoosuk B, Kim JH, Song C, Ngamcharussrivichai C, Prasassarakich P. Highly active MoS 2, CoMoS 2 and NiMoS 2

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

unsupported catalysts prepared by hydrothermal synthesis for hydrodesulfurization of 4, 6-dimethyldibenzothiophene. Catal Today 2008;130:14e23. Yoosuk B, Tumnantong D, Prasassarakich P. Unsupported MoS 2 and CoMoS 2 catalysts for hydrodeoxygenation of phenol. Chem Eng Sci 2012;79:1e7. Mangnus P, Bos A, Moulijn J. Temperature-programmed reduction of oxidic and sulfidic alumina-supported NiO, WO3, and NiO-WO3 catalysts. J Catal 1994;146:437e48. Chen J, Ci D, Yang Q, Li K. Deactivation of Ni 2 P/SiO 2 catalyst in hydrodechlorination of chlorobenzene. Appl Surf Sci 2014;320:643e52. Yang S, Liang C, Prins R. A novel approach to synthesizing highly active Ni2P/SiO 2 hydrotreating catalysts. J Catal 2006;237:118e30. Fan Y, Xiao H, Shi G, Liu H, Qian Y, Wang T, Gong G, Bao X. Citric acid-assisted hydrothermal method for preparing NiW/USYeAl 2 O 3 ultradeep hydrodesulfurization catalysts. J Catal 2011;279:27e35. Beccat P, Da Silva P, Huiban Y, Kasztelan S. Quantitative surface analysis by XPS (X-ray photoelectron spectroscopy): application to hydrotreating catalysts. Oil Gas Sci Technol 1999;54:487e96. Braun S, Appel LG, Camorim VL, Schmal M. Thermal spreading of MoO3 onto silica supports. J Phys Chem B 2000;104:6584e90. Cao Z, Duan A, Zhao Z, Li J, Wei Y, Jiang G, Liu J. A simple two-step method to synthesize the well-ordered mesoporous composite Ti-FDU-12 and its application in the hydrodesulfurization of DBT and 4,6-DMDBT. J Mater Chem A 2014;2:19738e49. Merlo AB, Vetere V, Ruggera JF, Casella ML. Bimetallic PtSn catalyst for the selective hydrogenation of furfural to furfuryl alcohol in liquid-phase. Catal Commun 2009;10:1665e9.