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Glucose-Assisted Preparation of Nickel-Molybdenum Carbide Bimetallic Catalyst for Chemoselective Hydrogenation of Nitroaromatics and Hydrodeoxygenation of m-Cresol Ting Zhang, Xinwen Guo, and Zhongkui Zhao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00735 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Nano Materials

Glucose-Assisted Preparation of Nickel-Molybdenum Carbide Bimetallic Catalyst for Chemoselective Hydrogenation of Nitroaromatics and Hydrodeoxygenation of m-Cresol Ting Zhang, Xinwen Guo and Zhongkui Zhao* State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, PR China.

*Corresponding Author: Zhongkui

Zhao.

Tel.:

+86

411

84986354.

Fax:

[email protected]

ACS Paragon Plus Environment

+86-411-84986354.

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

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ABSTRACT. Developing a safe, facile, clean, low-cost and scalable new method to replace conventional methane reductive carburization process for preparing carbon nanotubes coverage-free highly dispersed metal carbide-based bimetallic catalysts without a sacrificial metal loading is of great significance but remains a challenge. In this work, we develop a facile and robust strategy for successfully preparing highly dispersed supported nickel-molybdenum

carbide

(Ni-Mo2C)

bimetallic

catalyst

on

mesoporous

silica

(NiMo2C/SBA-15 (Glu.)), in which the renewable glucose is employed to serve as a assisting agent for high metal dispersion within the mesoporous channels of SBA-15 in the impregnation process and as carbon source to replace flammable methane for metal carbide formation through reductive carburization process. From diverse characterization results, the as-prepared NiMo2C/SBA-15 (Glu.) catalyst demonstrates a much higher metal dispersion (ca. 4.1 nm in this work vs. ca. 80 nm aggregates by conventional method) (proved by TEM, XRD, CO-chemosorption, et al) and promoted synergy effect between Ni and Mo2C than the NiMo2C/SBA-15 (Ref.) prepared by a conventional impregnation method followed by methane reductive carburization (proved by XPS, H2-TPR), besides the growth of carbon nanotubes is eliminated. As a consequence, the NiMo2C/SBA-15 (Glu.) catalyst shows unexpectedly 18 times higher catalytic specific activity for chemoselective hydrogenation of nitrobenzene and 12 times higher for hydrodeoxygenation of m-cresol than conventional NiMo2C/SBA-15 (Ref.). Moreover, NiMo2C/SBA-15 (Glu.) shows notably different product distribution for hydrodeoxygenation of m-cresol owing to the bi-functional effect of Mo2C. The as-prepared supported nickel molybdenum carbide catalyst can be extended to the other transformations, and also the developed method in this work can be extendible for preparing the other metal carbide catalysts towards diverse applications. KEY WORDS. molybdenum carbide, nickel, bimetallic catalyst, chemoselective hydrogenation, hydrodeoxygenation 1


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1. INTRODUCTION Owing to the similar properties to precious metals and low cost, metal carbides have attracted great attention and extensively used in many reactions including reforming,1 chemoselective hydrogenation,2 hydrodeoxygenation,3 water gas shift,4 reverse water gas shift,5 decomposition reactions of hydrazine, ammonia, formic acid and methanol,6 Fischer-Tropsch synthesis,7 CO2 reduction,8 and also the emerging applications in photocatalysis,9 electrocatalysis,10 and cathode materials for Li-O2 battery,11 etc. Concerning the limitation of single metallic catalyst, the bimetallic catalysts have been attracting the increasing interest in many catalytic processes.12-15 Typically, the catalytic activity and selectivity can be modified by geometric and electronic effects, while the catalytic stability can be efficiently improved by the stabilizing effects. The catalytic reaction rate can be promoted by the synergistic effect and bi-functional effects.16 Recently, the studies concerning the applications of metal carbide-based bimetallic catalysts, especially the Fe (or Co, Ni)-carbide bimetallic catalysts have attracted great attention in many catalytic processes.17-20 The conventional method for synthesizing Mo2C is as follows: The MoO3 is loaded on support, and then the supported MoO3 is subsequently carbonized at desired temperature (top to 1123 K) under 20% CH4/H2 flow, in which CH4 serves as carbon source and H2 serves as reduction gas.5,21,22 However, there are many intrinsic shortcomings existing in the conventional methane reductive carburization method: 1) large carbide particles owing to sintering in the carburization process at high temperature. 2) for Ni, Fe, Co-carbide bimetallic catalysts, the growth of carbon tubes on the catalysts is unavailable, which may cover the catalytic sites. 3) The flammable methane must be always introduced in the carburization process. The carburization process was generally very long, which led to the waste of methane, air contamination, besides the potential danger. These shortcomings deteriorate the

2



catalysis of carbides unavoidably. Therefore, to develop a new alternative preparation method to efficiently overcome the above issues is of great importance but remains a challenge. Some efforts have been made to overcome the aforementioned problems. Hexamethylenetetramine (HMT) has been applied as carbon/phosphorus source for the formation of metal-HMT-containing precursors, which subsequently were decomposed under inner atmosphere and metal carbide/phosphide obtained.23,24 Shu et al introduced Mo3O10(C6H8N)2·2H2O as precursor,25 which was obtained by hydrothermal treatment of (NH4)6Mo7O24·4H2O with aniline in the presence of HCl under high temperature. However, the synthesis of Mo3O10(C6H8N)2·2H2O precursor for carbide preparation process would release NOx, besides time and energy waste. Moreover, many new methods including the in situ O2 emission assisted synthesis method,26 plasma assisted carburization method,17 penchini method,27 brand-new method,28 ultrasonic spray pyrolysis method,29 have been unveiled. However, to the best of our knowledge, there is no report on the preparation of highly-dispersed carbide catalysts can be found, although the high dispersion is of quite significance for promoted catalysis. Now that the Fe (or Co, Ni)-carbide bimetallic catalysts have a great potential to become a practical catalyst for many catalytic processes, to develop a robust preparation for the synthesis of highly dispersed Fe (or Co, Ni)-carbide bimetallic catalysts with high metal loadings is highly desirable. Herein, we present a smart approach for preparing highly dispersed Ni-Mo2C bimetallic catalysts, in which sustainable glucose serves as the assisting agent in the metals-supporting process and as the renewable carbon resource in the synthesis process of carbide by reductive carburization. In comparison with the previously reported methods shown as above, by using the developed glucose assisted method, the continuous introduction of flammable methane was replaced by the required amount of sustainable glucose, which eliminates the potential explosive danger and air contamination, besides the preparation time was remarkably reduced.

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Moreover, this method also can improve the dispersion of supported carbides. Owing to high specific surface area and the ordered mesopores and the confinement effect, SBA-15 was chosen as a support so prepare supported highly dispersed Ni-Mo2C bimetallic catalysts. Glucose, Ni(NO3)2·6H2O and (NH4)6Mo7O24·4H2O were deposited into the mesoporous channel of SBA-15 by an incipient wetness impregnation (IWI) method, in which the glucose-metal ion complex can be formed due to the characteristic concerning polyhydroxyl groups. A hypothesis was proposed that, due to its complexation with metal ions, glucose could promote Ni2+ and Mo6+ to be more easily deposited into the mesoporous channels of SBA-15, and the discreting effect of glucose can enhance the metal dispersion. Then the confinement of mesoporous channel can limit the metal growth in the reductive carburization process at high temperature. The carbide can be formed via in-situ reductive carburization of glucose. The glucose would be a renewable alternative carbon source for the preparation of metal carbides, and the continuous introduction of methane can be avoided. The chemoselective hydrogenation of nitroaromatics to their corresponding aromatic amines is of great

importance

in

fine

chemicals

and

pharmaceutical

industries,

while

the

hydrodeoxygenation (HDO) is a key transformation process for biomass utilization. Therefore, the chemoselective hydrogenation of nitroaromatics and HDO reaction of m-cresol were employed as model reactions to evaluate our developed highly dispersed nickel-molybdenum carbide bimetallic catalysts. The highly dispersed metallic NPs and synergistic effect between Ni and Mo2C were deemed to endow the catalyst with excellent catalytic performance for chemoselective hydrogenation nitroaromatics and HDO reaction of m-cresol at a relative low temperature and H2 pressure. It has shown great potential for clean production of fine chemicals and the transformation of biomass. Moreover, the developed highly dispersed Ni-Mo2C can be definitely extended to diverse reactions, and the method of preparing metal carbide can be extended to prepare the other metal carbides.

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2. EXPERIMENTAL SECTION 2.1. Chemicals. Pluronic P123 (Sigma-Aldrich Co., average Mn ~5800), Tetraethyl orthosilicate (TEOS) (Sinopharm Chemical Reagent Co., Ltd, AR),Nickel nitrate (Damao Chemical Reagent Factory, AR), Ammonium molybdate (Tianjin Guangfu Technology Development Co., Ltd, AR), Glucose (Damao Chemical Reagent Factory, AR), Cyclohexane (Tianjin Fuyu Fine Chemical Co., Ltd, AR), n-heptane (Damao Chemical Reagent Factory, AR), m-cresol (Sinopharm Chemical Reagent Co., Ltd, AR), Nitrobenzene (Damao Chemical Reagent Factory, AR), n-dodecane (Damao Chemical Reagent Factory, AR). The chemicals were directly used without purification. 2.2. Synthesis of mesoporous silica SBA-15. Mesoporous silica SBA-15 was used as support. The SBA-15 was synthesized according to the reference by Zhao et al30 Typically, 4.0 g template (Pluronic P123,) was firstly dissolved into deionized water (30 mL) and 2 M HCl solution (120 g) with stirring at 313 K for 2 h. Then, 8.5 g TEOS was added into the above solution with stirring and aged for 24 h at 313 K. The resulting mixture was transferred to Teflon-line autoclave and heated at 375 K for 24 h. Finally, the obtained suspension was separated by filtration, and the solid was washed with ethanol, dried at 375 K overnight and calcined at 773 K for 6 h. 2.3. Preparation of supported nickel-molybdenum carbide bimetallic catalysts. The supported nickel and/or molybdenum carbide catalysts on SBA-15 were prepared via a facile incipient wetness impregnation (IWI) method. The required amounts of nickel nitrate (the nominal Ni loading of 10 wt%) and/or ammonium molybdate (the nominal Mo2C loading of 10 wt%) were dissolved into a glucose aqueous solution. After impregnation, obtained samples were dried at 378 K under air condition overnight, and thereafter thermal treatment was carried out in 10%H2/N2 atmosphere at 1123 K for 2 h in order to obtain Ni and/or Mo2C species. The obtained catalyst was passivated in 1% O2/Ar atmosphere at room temperature

5


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for 2 h. The as-prepared catalysts are designed as NiMo2C/SBA-15 (Glu), Ni/SBA-15 (Glu), or Mo2C/SBA-15 (Glu). Reference samples were prepared by conventional reductive carburization method according to our previous report.2 Aqueous solution of required amounts of nickel nitrate (the nominal Ni loading of 10 wt%) and/or ammonium molybdate (the nominal Mo2C loading of 10 wt%) was prepared. After impregnation, the obtained samples were dried at 378 K under air condition overnight, and then calcined at 813 K for 5 h. Finally, reductive carburization process was carried out in 20% CH4/H2 atmosphere at 1123 K for 0.5 h in order to obtain Ni and/or Mo2C species. The obtained catalyst was passivated by 1% O2/Ar atmosphere at room temperature for 2 h, and named as NiMo2C/SBA-15 (Ref.), Ni/SBA-15 (Ref.), or Mo2C/SBA-15 (Ref.). 2.4. Characterization: X-ray powder diffraction (XRD) patterns were taken on an X-ray diffractometer equipment of Rigaku Corporation SmartLab 9 using Cu Kα radiation to characterize the alloying texture. The wide-angle XRD data were collected from 5~80o for 2θ with a step size of 2o at 400 oC under H2 atmosphere. The small-angle XRD data were collected from 0.6~5o for 2θ with a step size of 0.02o at romm temperature. The transmission electron microscopy (TEM) images were obtained on a Tecnai F30 HRTEM instrument (FEI Corp.) at an acceleration voltage of 300 kV. X-ray photoelectron spectra (XPS) were collected on Thermo VG ESCALAB250 instrument using Al Kα radiation. The binding energy (BE) of the chemical species was calibrated using the C 1s peak at 284.8 eV as an internal standard. The percentages of the individual elements detected were determined by analyzing the areas of the respective peaks. The NH3 temperature programmed desorption (NH3-TPD) were measured by Builder PCA-1200 apparatus equipped with a thermal conductor detector (TCD), 100 mg of catalyst was loaded in a quartz tube and then pretreated in argon atmosphere at 300 oC for 30 min. After cooled down to ambient temperature, the sample was saturated with NH3 at 100 oC, and following flushed with Ar for 30 min. finally

6



the quartz tube was heating up to 800 oC at a heating rate of 10 oC·min-1. The H2 temperature programmed reduction (H2-TPR) were measured by Builder PCA-1200 apparatus equipped with a thermal conductor detector (TCD), 100 mg of catalyst was loaded in a quartz tube and then pretreated in argon atmosphere at 300 oC for 30 min. After cooled down to ambient temperature, 10 vol% H2/Ar (30 mL·min-1) was used for reduction by heating up to 800 oC at a heating rate of 10 oC·min-1. Nitrogen adsorption-desorption isotherms were recorded on a Beishide apparatus of model 3H-2000PSI system at 77 K. The porosity of as-prepared samples was obtained through Brunauaer-Emmetr-Teller (BET) analysis with the pore volume measured at P/P0 = 0.99 point. And BJH method was used for calculation of mesopore size distributions from adsorption branch. FTIR spectra on the metal ions-glucose complexes were collected in the wavenumber range of 4000-400 cm-1 on EQUINOX-55 Fourier Transform Infrared Spectrometer (BRUKER). The CO-Chemisorptions analysis of samples were performed by Builder PCA-1200 apparatus equipped with a thermal conductor detector (TCD), 100 mg of catalyst was loaded in a quartz tube and then pretreated in argon atmosphere at 300 oC for 30 min. After cooled down to ambient temperature, 10 vol% H2/Ar (30mL·min-1) was used for reduction at 800 oC for 1h. And then the quartz tube was cooled to ambient temperature in He. After that, a 10 vol% CO/He mixture (flowing rate: 30 mL·min-1) was introduced for CO-Chemisorptions at 50 oC. The activity sites dispersion was calculated as follow formula:

D=

VS × S F × M W × 100% SW × 22414

D: the dispersity of active sites, %. VS: the CO volume absorbed by tested samples, mL. SF: stoichiometric constant, 1 in this paper. SW: the weight of active metal, g.

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MW: the relative molecular weight of active metal, g·mol-1. 2.5. Catalytic performance evaluation: Chemoselective hydrogenation of nitroaromatics (CHN) and hydrodeoxygenation (HDO) of m-cresol were carried out as model reactions to evaluate the developed catalysts. The test process was performed as follows: the calculated amount of reduced catalyst and solvent were firstly charged into a batch autoclave equipped with mechanical agitator. Then, the reactor was flushed with Ar at ambient temperature for 3 times, and thereafter flushed with H2 at ambient temperature for 3 times. Subsequently, the autoclave was charged with H2 and conducted at 250 oC with a stirring speed of 500 rmp for a period time in order to eliminate the surface metal oxide. After the reduction of catalyst, nitrobenzene or m-cresol with internal standard (n-dodecane) was injected into the reaction system and following reacted under desired temperature. Finally, the products were analyzed by Fuli 9790II gas chromatograph equipped with a SE-54 capillary column (30 m × 0.32 mm × 0.50 µm) and a flame ionization detector (FID). The products were identified by retention times of their pure compounds at the same condition. Specific activity for (CHN) and HDO of m-cresol was calculated as follow formula:

r=

∆n mcat . × t × SBET

r: specific activity, µmol·h-1·m-2. ∆n: substrate consumed, µmol. mcat.: weight of catalyst, g. t: reaction time, h. SBET: specific surface area calculated by BET method, m2·g-1. 3. RESULTS AND DISCUSSION 3.1 Preparation and characterization of highly dispersed Ni-molybdenum carbide bimetallic catalyst. Herein, we present a smart approach for preparing highly dispersed

8



Ni-Mo2C bimetallic catalysts, in which the renewable glucose serves as discrete agent in the impregnation process to induce the metal ions into mesopores to get high metallic dispersion (one is inducing agent to induce metal ions into mesopores owing to its polyhydroxyl group characteristic, the other is discrete to inhibit metal ions from closing to each other) as a renewable carbon resource in the reductive carburization process. The assisting effect of glucose in the impregnation process leads to highly dispersed metals within mesopores, in combination of confinement of mesopores to prohibit the particle from growing in the high temperature carburization process, the the Ni and Mo can be highly dispersed and shows ca. 4 nm particle size although 20% of metal loading was used. The glucose also serves as renewable carbon resource for carbide formation without the introduction of methane. Moreover, no carbon nanotube can be formed. The method is also a robust method, the highly dispersed supported Ni-Mo2C can be obtained although the 30% of metal loading was used. The formation process is illustrated in Scheme 1.

Scheme 1. Schematic illustration showing the synthesis of highly dispersed NiMo2C/SBA-15 catalysts through the glucose assisted approach. Using the previously reported method,31,32 the NiII-glucose, MoVII-glucose and NiIIMoVII-glucose complexes were prepared. FTIR experiments were performed to confirm the formation of the aforementioned complexes. From Figure 1, the FTIR spectra of the

9


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complexes show extensive rearrangement of the hydrogen-bonding network due to ionization of the glucose, which is characterized by the presence of broad and merged bands in all the cases. The spectra of all the complexes show broad bands in the O-H and C-H stretching regions, indicating a merging of individual bands and thereby making the assignments of the individual vibrational modes difficult. The strongly coupled ring vibrational frequencies for bending modes -C-OH, -CH2 and –C-CH2 of the free-glucose (1500-1320 cm-1) show merging at ~1390 cm-1 upon complex formation. Similarly, the –C-O- and -C-C- stretching vibrations in the region 1140-940 cm-1 were also merged at ~1050 cm-1 upon complex formation, in contrast to the sharp bands observed for the free-glucose. Also, the anomeric region (950-500 cm-1) show very weak marker bands of complexes. The merging and broadening of bands indicate the successful formation of metal-glucose complex.

Figure 1. FTIR spectra of free-glucose, Ni-glucose, Mo-glucose and NiMo-glucose. XPS for Mo 3d and C 1s are depicted in Figure 2, all the binding energy (BE) values are corrected by using the C 1s signal at 284.6 eV as an internal standard. The existence of C, Ni, and Mo in the samples of NiMo2C/SBA-15 (Glu.) and NiMo2C/SBA-15 (Ref.) are confirmed from Figure 2a and 2b. The spectra of Mo 3d are shown in Figure 2c and Figure 2d, which 10



are mainly decomposed into four states in valency:33-36 Mo2+, Moδ+, Mo4+, and Mo6+.The peaks at 232.7 eV, 235.9 eV in Figure 2c and at 232.5 eV, 235.6 eV in Figure 2d can be assigned to the Mo6+ species, which are the predominant surface species. Besides, there are also contributions of Mo2+, Moδ+ and Mo4+ species (appearing at 228.4 and 231.5 eV, 229.7 and 232.8 eV in Figure 2c and at 228.6 and 231.7 eV, 229.4 and 232.5 eV in Figure 2d, respectively) for the two carbide-derivated catalyst. The high-resolution spectra of C 1s are displayed in Figure 2e and 1f. Three C species are observed with the binding energy at 283.3 eV, 284.8 eV and 286.8 eV assigned to C-Mo, C-C and C-O, respectively,37,38 indicating the molybdenum carbides are successfully prepared.

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Figure 2. XPS spectra of as-prepared samples. (a) Mo2C/SBA-15 (Ref.); (b) Mo2C/SBA-15 (Glu.); (c) and (e) Mo 3d and C 1s of NiMo2C/SBA-15 (Ref.); (d) and (f) Mo 3d and C 1s of NiMo2C/SBA-15 (Glu.). For further investigation, the XRD patterns of the as-prepared samples are collected. Figure 3 shows the XRD patterns. The peaks at 44.5o can be assigned to Ni, while the peak at 39.4o corresponds to β-Mo2C in Figure 3a.17 As can be seen, all the samples possess the typical peaks of Ni, indicating the successfully impregnation of Ni on SBA-15. A weak peak appears at 39.4o on NiMo2C/SBA-15 (Ref.), suggesting the existence of β-Mo2C species. Although no peak assigned to β-Mo2C on NiMo2C/SBA-15 (Glu.) with 10 wt% loading of Ni can be clearly resolved, when the loading of Mo2C increased to 80 wt%, the typical peak of β-Mo2C at 39.4o appears (Figure S1), implying the formation of β-Mo2C by our method, which is in line with the results of XPS. The absence of diffraction peak corresponding to Mo2C in the case of NiMo2C/SBA-15 (Glu.) with 10 wt% Mo2C loading might be resulted from the Mo2C well dispersion owing to the use of glucose as assisting agent in impregnation process (one is inducing agent to induce metal ions into mesopores owing to its polyhydroxyl group characteristic, the other is discrete to inhibit metal ions from closing to each other). The content of Mo6+ species in the two samples of NiMo2C/SBA-15 (Glu.) and NiMo2C/SBA-15 (Ref.) are obtained from the peak area ratio in Mo 3d XPS spectra (Figure 2c and Figure 2d). NiMo2C/SBA-15 (Glu.) shows 1.6 folds higher content of Mo6+ (63%) than NiMo2C/SBA-15 (Ref.) (40%), indicating that more metal surface is exposed because of the highly dispersion of metallic particles by our developed glucose assisted impregnation method. The peaks of Ni in patterns of NiMo2C/SBA-15 (Glu.) and NiMo2C/SBA-15 (Ref.) shift slightly to lower angle in comparison with the supported nickel catalysts (Figure S2), attributed to the expansion of Ni lattice cell by the insertion of Mo2C into Ni matrix.

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The XRD patterns in small-angle region (0.5~5o) of as-prepared samples are also performed to investigate the degree of order of mesopore (Figure 3b). The intensity of peaks assigned to mesopores of SBA-15 decreases after it suffers from impregnation process, implying the orderliness of mesopores in SBA-15, is disturbed resulting from the embedding of metallic particles into the channel. Moreover, the lower peak intensity of NiMo2C/SBA-15 (Glu.) compared to NiMo2C/SBA-15 (Ref.) indicating more metallic particles impregnated into the mesopores, but not the lowering ordering degree for pore channels. The confinement effect of mesopores can efficiently inhibit the metal particle from sintering, and therefore can maintain the higher dispersion of metallic particles when it suffers from high temperature for carburization process. The highly dispersed metals on SBA-15 could be intuitively observed from TEM images.

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Figure 3. XRD patterns of as-prepared samples. (a) wide-angle region XRD patterns; (b) small-angle region XRD patterns. TEM images (Figure 4) reveal that the bimetallic nanoparticles are uniformly dispersed in meso-channel throughout the SBA-15 support in the case of NiMo2C/SBA-15 (Glu) with a mean particle diameter of ca. 4.10 nm (Figure 4a and S3), while larger aggregates are clearly observed in the case of NiMo2C/SBA-15 (Ref.) (Figure 4d), which can lead to a lower catalytic activity. The metal dispersity (D (%)) of NiMo2C/SBA-15 (Glu.) is 8.57 times higher than that of NiMo2C/SBA-15 (Ref.) (Table 1), further proving the better dispersion of metallic NPs in the samples prepared by our method. Furthermore, even the metal loading increasing up to 30 wt%, the particles still highly dispersed within the mesopores of SBA-15 (Figure S4). HRTEM observation on NiMo2C/SBA-15 (Glu) reveals that only the lattice lines of Ni (111) crystal face are detected in the metallic NPs (Figure 4b and 4c), indicating the fine inserting between Ni and Mo2C and their dispersion. On the contrary, clearly phase separation in single nanoparticle is observed in the case of NiMo2C/SBA-15 (Ref.) (Figure 4e). Moreover, the as-formed carbon nanotubes during carburization process can be observed on the NiMo2C/SBA-15 (Ref.), while no visible carbon nanotubes can be observed on the developed NiMo2C/SBA-15 (Glu) catalyst.

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Page 16 of 34

Figure 4. TEM and HRTEM images of as-prepared samples. (a) TEM image of NiMo2C/SBA-15 (Glu.); (b), (c) HRTEM image of NiMo2C/SBA-15 (Glu.); (d) TEM image of NiMo2C/SBA-15 (Ref.); (e) HRTEM image of NiMo2C/SBA-15 (Ref.).

Table 1. Textural properties and metal dispersity of the as-prepared samples. VTotal

VBJH

3

3

SBET

SBJH

DBJH

Samples

D (%)

-1

-1

2

-1

2

-1

(cm ·g )

(cm ·g )

(m ·g )

(m ·g )

(nm)

SBA-15

1.42

1.38

708

631

7.9

-

NiMo2C/SBA-15 (Ref.)

1.06

1.06

459

459

7.2

3.8

NiMo2C/SBA-15 (Glu.)

1.15

0.91

318

174

6.4

23.8

N2 absorption-desorption experiment is one of the most direct characterizations for pore structure. Figure 5 showed the absorption-desorption isotherms and pore diameter

15


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distributions of as-synthesized samples (showing as inset). The isotherms of the three samples show an obvious difference. The bare SBA-15 features type IV isotherm with a sudden increase at ca. 0.7 of P/P0, suggesting the ordered mesoporous materials. However, the hysteresis loop and adsorption volume of samples minish after the support suffers from impregnation of metals, implying the decrease in pore volume (VTotal) and specific surface area (SBET). As depicted in Table 1, the pore volumes and specific surface area of NiMo2C/SBA-15 (Ref.) and NiMo2C/SBA-15 (Glu.) greatly decrease, indicating that the metallic NPs are inserted into the mesopores of SBA-15. Especially concerning the NiMo2C/SBA-15 (Glu.), the mesopore volume (VBJH) is ca. 29% lower than that of bare SBA-15, but only 20% lower in the case of NiMo2C/SBA-15 (Ref.). Accordingly, both of the above samples have a lower SBET compared with bare SBA-15, and the 55% and 37% of reduction degree for NiMo2C/SBA-15 (Glu.) and NiMo2C/SBA-15 (Ref.), respectively, can be observed. These phenomena can be related to the dispersion of the active metallic NPs inside the SBA-15 meso-channels for NiMo2C/SBA-15 (Glu.), which is in line with the observation on TEM. The trend of mesopore diameter varying with different preparation methods can be obtained from the pore size distribution curves (inset in Figure 5). All of the three samples possess uniformly pore diameter distribution, manifesting the meso-structure to be remained after metal particles being impregnated into SBA-15 meso-channels, in keep with the results of little angle XRD patterns. These also make clear that the metallic particles are highly dispersed through the SBA-15 channels, further proved from the observation on HRTEM. The most probable pore diameter (6.4 nm) concerning NiMo2C/SBA-15 (Glu.) significantly decreases compared with that of bare SBA-15 (7.9 nm), while 7.2 nm in the case of NiMo2C/SBA-15 (Ref.). This behaviour suggests that more metal particles should be well-dispersed embedding into SBA-15 meso-channels in the case of NiMo2C/SBA-15 (Glu.) than that for NiMo2C/SBA-15 (Ref.).

16



Page 18 of 34

Figure 5. N2 adsorption-desorption isotherms and pore-size distributions (insert figure, calculated

from

adsorption

branch)

of

SBA-15,

NiMo2C/SBA-15

(Glu.)

and

NiMo2C/SBA-15 (Ref.). The above results reveal that more metallic particles are impregnated into SBA-15 meso-channels and highly dispersed in the case of NiMo2C/SBA-15 (Glu.) than that of NiMo2C/SBA-15 (Ref.). The origin can be proposed that the hydroxyl-rich compound (glucose) can complex with metal ions with alterable valence state (Ni2+ and Mo6+).37,38 In this work, glucose, nickel nitrate and ammonium molybdate is dissolved in water and rested for a period of time to make sure the complexion happen. The resulted complexes are deposited into mesopores of supports by IWI, and the large hydrophilicity of glucose-metal ion complex induces the complex to penetrate into mesopores of supports. Therefore, the high metallic dispersion can be obtained through the assisting effect of glucose (one is inducing agent, the other is discrete agent). The growth of embedded metals within mesoporous channels can be suppressed during the carburization process at high temperature for carbide formation by the confinement effect of mesopores. As a consequence, the highly dispersed Ni-Mo2C bimetallic catalyst is successfully prepared.

17


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3.2. Intensified interaction between Ni and molybdenum carbide. The well compounded Ni and Mo2C species on NiMo2C/SBA-15 (Glu.) from TEM and the shift of peak corresponding to metallic Ni from XRD patterns enlighten us to envisage that the synergic effect may exist in the as-obtained NiMo2C/SBA-15 (Glu.) catalyst. H2-TPR and XPS experiments are performed to unveil the synergism between metallic Ni and Mo2C. The H2-TPR profiles of as-synthesized samples are shown in Figure S5. The monometallic Ni catalyst exhibits a strong reduction peak with a maximum at 326 oC corresponding to the reduction of NiO in the case of Ni/SBA-15 (Glu.) (Figure S5a and S5c). However, the reduction peak of Ni/SBA-15 (Ref.) is very small (Figure S5c). This can be explained that the abundant exposed metal surface is more easily oxidized because of the highly dispersion of metallic NPs in the case of Ni/SBA-15 (Glu.), which has been confirmed by CO-chemisorption results displayed in Table 2 column 5. The temperature for reducing NiO species increases and that for Mo2C species decreases for NiMo2C/SBA-15 (Glu.) compared with Ni/SBA-15 (Glu.) and Mo2C/SBA-15 (Glu.), respectively (Figure S5a), indicating the existence of synergic effect between Ni and Mo2C on NiMo2C/SBA-15 (Glu.). The higher reduction temperature of NiO and lower reduction temperature of Mo2C species in NiMo2C/SBA-15 (Glu.) compared to NiMo2C/SBA-15 (Ref.) implies the strengthened synergic effect in the former in comparison with the latter (Figure S5b). XPS is an efficient characterization to assess the chemical states, which can be used in the exploration of the interaction of Ni and Mo2C. The Ni 2p XPS spectra of as-prepared catalysts are shown in Figure 6. It can be deconvoluted according to two valence states of Ni0 and Ni2+. The peak appearing at ca. 852.6 eV (satellite peak at 857.9 eV) on Ni/SBA-15 (Ref.) can be assigned to Ni0 species, while the left two can be assigned to Ni2+ (NiO) forming via surface oxidation of metallic Ni during the passivation in the case of Ni/SBA-15 (Ref.) (Figure 6a).1 In comparison with Ni/SBA-15 (Ref.), the as-prepared Ni/SBA-15 (Glu.) by the

18



developed method in this work shows a lightly higher BEs corresponding to the two Ni species, indicating the strengthened electron transfer from Ni to SBA-15 owing to promoted Ni-support interaction by the high Ni dispersion.

Figure 6. Ni 2p XPS spectra of as-prepared samples. (a) Ni/SBA-15 (Ref.); (b) Ni/SBA-15 (Glu.); (c) NiMo2C/SBA-15 (Ref.); (d) NiMo2C/SBA-15 (Glu.). Moreover, in comparison with the supported single Ni catalysts, the increase in BEs of Ni for supported Ni-Mo2C bimetallic catalysts can be observed, suggesting the electron transfer from Ni to Mo by the interaction. The BEs of Ni concerningNiMo2C/SBA-15 (Glu.) increase in comparison with that towards NiMo2C/SBA-15 (Ref.), suggesting that the more electrons transfer from Ni to SBA-15 and Mo owing to strengthened interaction of Ni-support and Ni-Mo on NiMo2C/SBA-15 (Glu.) resulting from the increasing metals dispersion. Furthermore, the Mo 3p spectra have also been deconvoluted to reveal the interaction of Mo-SBA-15 and Mo-Ni (Figure 2 and S6). From Figure S6, Mo2C/SBA-15 (Glu.) shows similar BEs concerning Mo to Mo2C/SBA-15 (Ref.), implies the possibly similar interaction 19


Page 20 of 34

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degree of Mo-SBA-15 or no visible electron transferring between Mo and SBA-15. However, from Table S1, the Mo2C/SBA-15 (Glu.) shows the much higher Mo6+ percentage than Mo2C/SBA-15 (Ref.), clearly indicating the strengthened electron transferring from Mo to SBA-15 on Mo2C/SBA-15 (Glu.) in comparison with Mo2C/SBA-15 (Ref.). Correlated Figure S6 to Figure 2, the introduction of Ni leads to a decrease in BEs of Mo in comparison with the supported single metal Mo2C catalysts, suggesting the electron transferring from Ni to Mo by the interaction of Ni-Mo. This can be further confirmed by the increasing percentage of low-valent Mo species.2,39 Moreover, the more obviously increasing low-valent Mo percentage and the more visible decreasing BEs concerning Mo on the NiMo2C/SBA-15 (Glu.) than those on NiMo2C/SBA-15 (Ref.) in comparison with the supported Mo2C single metallic catalysts can be observed. From Table S1, the NiMo2C/SBA-15 (Glu.) shows much higher Mo2+ percentage by 2.8 times than NiMo2C/SBA-15 (Ref.), further indicating the strengthened electron transferring from Ni to Mo. The aforementioned analysis and discussion definitely demonstrate the strengthened interaction of Ni-Mo2C on the as-developed NiMo2C/SBA-15 (Glu.) in contrast to the NiMo2C/SBA-15 (Ref.) prepared by conventional co-impregnation followed by methane carburization method. 3.3. Catalytic performance for chemoselective hydrogenation. In this work, a highly dispersed bimetallic Ni-Mo2C catalyst supported on SBA-15 has been prepared, and the synergic effect between Ni and Mo2C is promoted compared to the conventional catalyst. Herein, chemoselective hydrogenation of nitroaromatics, one of the most important reactions in fine chemicals, is employed to explore the role of high bimetallic dispersion and strengthened Ni-Mo2C synergistic effect on the catalysis of the NiMo2C/SBA-15 (Glu.). From Figure 7, the developed highly dispersed NiMo2C/SBA-15 (Glu.) demonstrates much higher yield (99.3% vs. 14.8%) concerning aniline for 5 h with 6.7 folds higher conversion

20



Page 22 of 34

for transformation of nitrobenzene than NiMo2C/SBA-15 (Ref.), which distinguishes the unveiled preparation method of supported Ni-Mo2C bimetallic catalysts.

Figure 7. Effect of reaction time on catalytic performance for chemoselective nitrobenzene hydrogenation. Reaction conditions: nitrobenzene (0.50 g), catalyst (6 wt% of reactant), solvent (20 mL), PH2=2 MPa, Temp.=120 oC. Table 2. Results of chemoselective hydrogenation of nitrobenzene over as-prepared catalysts and metal dispersity measured by CO-chemisorptiona

Entry

Con.

Sel.

CO uptake

(%)

(%)

(µmol g-1 cat.)b

Catalyst

D (%)

1

Ni/SBA-15 (Ref.)

4.8

>99.9

39

2.3

2

Mo2C/SBA-15 (Ref.)

1.9

>99.9

—

—

3c

Ni/SBA-15 (Ref.)+ Mo2C/SBA-15 (Ref.)

7.5

>99.9

—

—

4

NiMo2C/SBA-15 (Ref.)

11.3

>99.9

128

3.8

5

Ni/SBA-15 (Glu.)

60.0

>99.9

479

28.1

6

Mo2C/SBA-15 (Glu.)

2.0

>99.9

—

—

7c

Ni/SBA-15 (Glu.)+ Mo2C/SBA-15 (Glu.)

63.1

>99.9

—

—

8

NiMo2C/SBA-15 (Glu.)

76.2

>99.9

405

23.8

21


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a

Reaction conditions: nitrobenzene (0.50 g), catalyst (6 wt% of reactant), solvent (20 mL),

PH2=2 MPa, Temp.=120 oC, t=4 h. b determined by CO chemisorption. c Physically mixed sample, 1:1 of mass ratio. The controlled experiments are performed to discover the effect of bimetallic dispersion and the strengthened interaction of Ni-Mo2C on the catalysis. From Table 2, either supported Ni catalyst or supported Ni-Mo2C catalyst prepared by the developed method shows much higher activity for chemoselective hydrogenation of nitrobenzene than the one prepared by conventional method, resulting from the promoted metallic dispersion. Especially, Ni/SBA-15 (Glu.) exhibits 12.5 times higher conversion than Ni/SBA-15 (Ref.). The supported Ni-Mo2C bimetallic catalysts show much higher conversion than the mixture composed of supported Ni and supported Mo2C with 1:1 of mass ratio, suggesting the existence of synergistic effect between Ni and Mo2C on the catalysis concerning chemoselective hydrogenation. From Table 2, NiMo2C/SBA-15 (Glu.) shows 1.27 times higher conversion than Ni/SBA-15 (Glu.), while NiMo2C/SBA-15 (Ref.) demonstrates 1.46 times folds higher conversion than Ni/SBA-15 (Ref.). It looks like that the strengthened Ni-Mo2C interaction doesn’t promote the conversion. Then the metallic dispersion is further measured. From Table 2, if the promoting effect by metallic dispersion is eliminated, NiMo2C/SBA-15 (Glu.) shows 1.27 times higher conversion than Ni/SBA-15 (Glu.); while the conventional NiMo2C/SBA-15 (Ref.) only exhibits 0.88 folds higher conversion. Therefore, the strengthened interaction between Ni and Mo2C concerning NiMo2C/SBA-15 (Glu.) definitely favors the catalytic transformation of nitroaromatics to their corresponding aromatic amines through the synergistic effect of Ni-Mo2C. Moreover, in comparison of NiMo2C/SBA-15 (Glu.) with Ni/SBA-15 (Glu.), the former shows a higher conversion (76.2%) than the latter (60.0%) even though it is with a lower dispersion (23.8%), which might be ascribed to the promoting of Mo2C since it has a Pt-like performance. The catalytic 22



Page 24 of 34

chemoselective hydrogenation of the other nitroaromatics is also investigated and the results are shown in Table 3, as can be seen, all of the selected nitroaromatics can be chemoselectively transformed into their corresponding aromatic amines with a 99.9% of high selectivity. In comparison with the recently reported result, the developed catalyst shows much higher catalytic activity, and much milder reaction conditions were used to obtain the similar conversion (using the same amount catalyst, the much more substrate, much lower reaction temperature, and shortened reaction times under the same pressure were used). Table 3. Chemoselective hydrogenation of various nitroarenes over NiMo2C/SBA-15 (Glu.) and the previously reported catalysts in reference. Selectivity/Yield (%) Entry

Substrate

Product This worka

Ref. [40]b

1

99.9/90.2

99.7/99.5

2

99.9/81.4

99.5/99.4

3

99.9/97.5

99.8/99.4

4

99.9/89.2

99.6/99.3

Reaction conditions:

a

substrates (4.1 mmol), catalyst (30 mg), PH2=2 MPa, Temp.=120 oC,

t=5 h. b substrates (0.81 mmol), catalyst (30 mg), PH2=2 MPa, Temp.=210 oC, t=7 h.

23


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3.4. Catalytic performance for hydrodeoxygenation. In comparison with fossil fuels, the essential characteristic of biomass is its higher oxygen content, which limits it to be directly used as transport fuel or intermediates for chemical industry.41-43 Therefore, the catalytic hydrodeoxygenation (HDO) reaction has been considered as the key process for biomass transformation. The HDO of m-cresol is used as model reaction for evaluating the developed supported Ni-Mo2C catalysts. From Figure 8a and Table 1, NiMo2C/SBA-15 (Glu.) exhibits notably higher catalytic activity for HDO of m-cresol than conventional NiMo2C/SBA-15 (Ref.), ascribed to the significantly improved metallic dispersion and the strengthened synergistic effect between Ni and Mo2C (similar to that for chemoselective hydrogenation of nitroaromatics). More interestingly, NiMo2C/SBA-15 (Glu.) shows higher catalytic activity for yielding methylcyclohexanol as the main product by the hydrogenation of aromatic ring if the reaction temperature is not more than 220 oC, and especially reaches the maximum at 180 oC, whereas the methylcyclohexane is the main product from HDO of m-cresol on the conventional NiMo2C/SBA-15 (Ref.) (Figure 8b). Furthermore, the methylcyclohexane can be formed as a main product on NiMo2C/SBA-15 (Glu.) if the reaction temperature increases up to 250 oC. The formation of methylcyclohexane comes from the dehydration of methylcyclohexanol along with the subsequential hydrogenation. From Figure S7, NiMo2C/SBA-15 (Glu.) features different acidic properties from NiMo2C/SBA-15 (Ref.). NiMo2C/SBA-15 (Glu.) possesses a large amount of medium acidic sites while the conventional NiMo2C/SBA-15 (Ref.) shows a large amount of strong acidic sites. As a result, the dehydration reaction of methylcyclohexanol on NiMo2C/SBA-15 (Ref.) occurs even at a low reaction temperature while it only happens at high reaction temperature on NiMo2C/SBA-15 (Glu). Reports mainly focused on producing methylcyclohexane or toluene from m-cresol,44,45 while methylcyclohexanol is also a crucial intermediate for chemical industry, especially fine chemical fields. In this work, the methylcyclohexanol with

24



high yield can be obtained over the developed catalyst under higher reaction temperatures (Figure 8).

Figure 8. Catalytic performance for HDO of m-cresol over the developed NiMo2C/SBA-15 (Glu) catalysts in this work and the traditional NiMo2C/SBA-15 (Ref). a) Effect of reaction time; b,c) Effect of reaction temperature. Reaction conditions: a) m-cresol (0.11g), catalyst (25 wt% of reactant), solvent (20 mL), Temp.=250 oC, PH2=2 Mpa, t=3 h; b) and c) m-cresol (0.11g), catalyst (25 wt% of reactant), solvent (20 mL), PH2=2 Mpa, t=3 h. In Figure 8b, A: methylcyclohexane, B: methylcyclohexanol, C: methylcyclohexene. 4. CONCLUSIONS In summary, we unveiled a facile, safe, low-cost and renewable strategy for the preparation of highly dispersed supported Ni-Mo2C bimetallic catalyst with ca. 4 nm of metallic particle size at 20% of high loading, in which glucose serves as assisting agent for promoting metallic dispersion in the impregnation process and as carbon resource to replace flammable methane gas for the formation of carbide from reductive carburization. The diverse characterization techniques indicate that the highly dispersed bimetallic Ni-Mo particles has been embedded within the mesopores of support owing to the high hydrophilicity of glucose-metal complex and the barrier effect of complex, and the growth of metal nanoparticles can be efficiently suppressed by the confinement effect of mesopores even if it suffers from high temperature calcination under reductive atmosphere. As a consequence, the highly dispersed Ni-Mo2C bimetallic catalyst is successfully prepared. It is

25


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also found that the high dispersion efficiently strengthen the interaction between Ni and Mo2C, which can improve the synergistic effect of Ni-Mo2C concerning the chemoselective hydrogenation of nitroaromatics and hydrodeoxygenation of m-cresol, a model molecule for lignocelluloses biomass. The developed NiMo2C/SBA-15 (Glu.) catalyst demonstrates 18 times higher specific activity than NiMo2C/SBA-15 (Ref.) for chemoslective hydrogenation of nitromatics and 12 times for hydrodeoxygenation of m-cresol. Moreover, the NiMo2C/SBA-15

(Glu.)

shows

different

product-distrtibution

from

conventional

NiMo2C/SBA-15 (Ref.) for hydrodeoxygenation of m-cresol, which presents a new concept for developing highly efficient HDO catalyst with the required product distribution. Moreover, the discovery in this work can be extended to the other carbide-containing bimetallic catalysts with high dispersion for diverse applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXX. XRD patterns of 10 wt% NiMo2C/SBA-15 (Ref.), 10 wt% NiMo2C/SBA-15 (Glu.) and 80 wt% NiMo2C/SBA-15 (Glu.); XRD patterns of Ni/SBA-15 (Ref.), Ni/SBA-15, NiMo2C/SBA-15 (Ref.) and NiMo2C/SBA-15; The particle size distribution; TEM image of the sample with 30 wt% Ni-Mo2C loading; H2-TPR patterns of as-prepared samples; Mo 3d XPS spectra of Mo2C/SBA-15 (Ref.) and Mo2C/SBA-15 (Glu.); NH3-TPD patterns of as-prepared samples; The percentage of Mo2+, Mo4+ and Mo6+ obtained from Mo 3d XPS spectra of as-prepared catalysts. AUTHOR INFORMATION Corresponding Author

26



*E-mail: [email protected] (Z.Z.). ORCID Zhongkui Zhao: 0000-0001-6529-5020. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work is financially supported by the National Natural Science Foundation of China (U1261104, 21676046, U1610104), and also sponsored by the Chinese Ministry of Education via the Program for New Century Excellent Talents in University (NCET-12-0079), and the Natural Science Foundation of Liaoning Province (grant no. 2015020200). REFERENCES (1) Li W. Z.; Zhao Z. K.; Ren P. P.; Wang G. R. Effect of molybdenum carbide concentration on the Ni/ZrO2 catalysts for steam-CO2 bi-reforming of methane. RSC Adv. 2015, 5, 100865-100872. (2) Zhao Z.; Yang H.; Li Y.; Guo X. Cobalt-modified molybdenum carbide as an efficient catalyst for chemoselective reduction of aromatic nitro compounds. Green Chem. 2014, 16, 1274-1281. (3) Chen C. J.; Bhan A. Mo2C Modification by CO2, H2O, and O2: Effects of Oxygen Content and Oxygen Source on Rates and Selectivity of m-Cresol Hydrodeoxygenation. ACS Catal. 2017, 7, 1113-1122. (4) Posada-Perez S.; Gutierrez R. A.; Zuo Z.; Ramirez P. J.; Vines F.; Liu P.; Illas F.; Rodriguez J. A. Highly active Au/δ-MoC and Au/β-Mo2C catalysts for the low-temperature water gas shift reaction: effects of the carbide metal/carbon ratio on the catalyst performance. Catal. Sci. Technol. 2017, 7, 5332-5342.

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