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Catalysis and Kinetics 2

Impact of Carbon Properties on MoC/Carbon Catalysts for the Hydrodeoxygenation of 4-Methylphenol Shida Liu, Haiyan Wang, Robertus Dhimas Dhewangga Putra, Chang Soo Kim, and Kevin J. Smith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00531 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Impact of Carbon Properties on Mo2C/Carbon Catalysts for the Hydrodeoxygenation of 4-Methylphenol

Shida Liu, Haiyan Wang, Robertus Dhimas Dhewangga Putra , Chang Soo Kima,*, Kevin J. Smith* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, B.C., V6T 1Z3, Canada. aKorea

Institute of Science and Technology, Room 353B Lower Mall Research Station, 2259 Lower Mall, Vancouver, BC V6T1Z4

Keywords: Mesoporous carbon; Carbothermal hydrogen reduction; Hydrodeoxygenation; 4methylphenol

*Corresponding authors: [email protected] (K. J. Smith) Tel.: +1 6048223601; Fax: +1 6048226003. [email protected] (C.S. Kim) Tel: +1 6048228850; Fax: +1 6048226003 1

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Highlights: 

Mo2C prepared by carbothermal hydrogen reduction (CHR) of various carbons.

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Carbon reactivity and O-content impact Mo2C formation by CHR.

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Activated bio-char (ABC) is more reactive than activated petcoke (APC).

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ABC yields more active catalyst at lower CHR temperature than APC.

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ABC has higher selectivity than APC for direct deoxygenation (DDO).

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Graphical Abstract

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Abstract Petcoke and biochar were chemically activated with KOH at 800 and 700 °C, respectively to acquire microporous activated petcoke (APC) and activated bio-char (ABC) with large surface areas (~ 2000 m2/g). The O content of the petcoke increased while that of the biochar decreased following activation. Nevertheless, ABC had a higher O content on the surface and in the bulk compared to the APC. The APC and ABC precursors were wet impregnated with ammonium heptamolybdate (AHM), dried and reduced in H2 to yield Mo2C/APC and Mo2C/ABC catalysts with a nominal loading of 10 wt% Mo. These catalysts were assessed for the hydrodeoxygenation (HDO) of 4-methylphenol (4-MP). The Mo2C/ABC catalyst had higher activity (per gram Mo) and direct deoxygenation (DDO) selectivity than that of the Mo2C/APC due to the higher dispersion of Mo species and higher O content on the ABC surface. The 10%Mo2C/ABC_R650 catalyst prepared at CHR of 650 °C had the highest activity per gram Mo among all catalysts, whereas turnover frequencies on all catalysts were similar.

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1. Introduction Bio-oil derived from the pyrolysis of woody biomass contains a considerable amount of water (20 ~ 40 wt%) and oxygen that lowers the heating value of the oil and makes the oil highly unstable [1, 2]. To overcome these issues, catalytic hydrodeoxygenation (HDO) of the bio-oil is used to remove the oxygen, thereby stabilizing the oil and increasing its heating value [3, 4].

Known catalysts for HDO include conventional Co(Ni)MoS [5], noble metals [6], and transition metal carbides. CoMoS catalysts supported on conventional alumina supports suffer from coke formation and noble metals are expensive [7]. Transition metal carbides, on the other hand, have been identified in previous studies [8-10] as promising alternative catalysts for HDO. Catalyst supports used in HDO include acidic silica-alumina oxides as well as neutral activated carbons. Carbon is favored for deep HDO since polymerization reactions occur on the acidic silicaalumina surface, leading to coke formation [5, 6, 11, 12].

In previous work, Mo2C supported on an activated charcoal (Mo2C/AC) was examined for the HDO of 4-methylphenol [13]. Carbothermal hydrogen reduction (CHR) was used to prepare the carbon supported Mo2C catalysts [13-16]. CHR refers to a temperature programmed reduction in H2 of the catalyst precursor impregnated onto the carbon support, as first described by Mordenti et al. [17] CHR has been proven to effectively generate Mo2C and simultaneously enlarge the pore size of the carbon support [14]. Key to the formation of Mo2C is the hydrogenation of the carbon support to yield CH4. The formed Mo2C also contributes to the catalytic hydrogenation of the carbon and the generation of mesopores [14]. Consequently, the properties of the carbon used in the synthesis of Mo2C/carbon catalysts are expected to significantly impact catalyst properties. 5 / 36


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Santillan-Jiminez et al. [18] studied Mo2C supported on activated carbon, multi-walled carbon nanotubes and carbon nanofibers. They reported that the carbon nanofibers (CNF) yield mostly nano-sized Mo2C particles, smaller than the Mo2C particles on the carbon nanotubes, and that the CNF supported catalysts had the highest phenol yield from guaiacol conversion. Macedo et al. [19] also examined Mo2C supported on CNF but prepared cubic phase α-MoC1-x and the hexagonal phase β-Mo2C. The α-MoC1-x/CNF was more active than the β-Mo2C/CNF for the hydrodeoxygenation of stearic acid. The authors attributed the higher activity to a lower site density on the face-centered cubic α-MoC1-x/CNF, making the Mo atoms at the surface of the αMoC1-x phase more accessible for a large reactant. Mo2C supported on biochar for the HDO reaction has been reported Zhang et al. [20] who showed that non-polar solvents such as hexane enhance HDO more than polar solvents such as ethanol. Ochoa et al. [21] reported that high carburization temperature (650 °C) and low heating rates (1°C/min) gave the highest yield of Mo2C and the corresponding highest catalyst activity. Wang et al. [22] confirmed that Mo2C/AC has excellent performance in HDO of vegetable oil relative to conventional catalysts such as MoS2/Al2O3.

In the present study, activated carbon derived from petcoke (APC) and biochar (ABC) are used as carbon supports of Mo2C catalysts that are assessed for the HDO of the model reactant 4methylphenol (4-MP). The impact of the carbon on the catalyst properties and activity is compared with previous work that used activated charcoal as the carbon source [14]. Petroleum coke (or petcoke) is a by-product of crude oil refining. The high carbon content and low ash content make it a good candidate for gasification [23]. Similarly, biochar is the solid product from biomass pyrolysis [24], the properties of which are largely dependent on the pyrolysis

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processing conditions [25]. Biochar is much less dense than petroleum coke, and has significant potential as an adsorbent [26-28]. Both petcoke and biochar also have potential to be used as a catalyst support. The many O functional groups that are known to exist on the biochar surface [29] decrease hydrophobicity of the carbon surface, improving the surface wettability by aqueous solutions [30], thereby improving the dispersion of metal catalysts prepared by impregnation of the carbon support [31].

2. Experimental 2.1 Raw materials Raw petroleum coke (petcoke) from a delayed coking process of the Canadian oilsands was provided by Suncor Energy Inc. The biochar was prepared by the pyrolysis of SPF (spruce, pine, fir) pellets (moisture content 6 wt%; particle size of 0.5 ~ 1.0 mm) at 450-550 oC in a N2 flow of 5000 L(STP)/h. The raw petcoke (PC) and bio-char (BC) were ground and sieved to particles with diameters between 90-180 μm before thermochemical activation. Elemental analysis of the PC and BC is reported in Table 1.

2.2 Preparation of Mo2C/ABC and Mo2C/APC catalysts The as received coke samples were thermochemically activated in the presence of KOH. About 3.5 g of the coke was dry mixed with KOH in a KOH to coke mass ratio of 3:1. The samples were activated in N2 (200 mL(STP)/min) while heating in a tube furnace at a ramp rate of 5 oC/min

to the final temperature and then holding the final temperature for 2 h. The final

temperature in this study varied from 600 to 900 oC. After cooling to room temperature the 7 / 36


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samples were washed with 1M HCl solution and then dried at 110 oC for 8 h [14] to yield the activated carbon derived from petcoke (APC_XYZ) or from biochar (ABC_XYZ), where XYZ indicates the activation temperature.

The ABC prepared at 700 °C and the APC prepared at 800 °C were selected for use as the supports of the Mo2C catalysts since they had similar surface areas and pore volumes. Both APC and ABC were wet impregnated with ammonium heptamolybdate (AHM) to obtain a nominal 10 wt% Mo loading of the carbon supports. The AHM was dissolved in a 10 vol % acetone/water solvent and added to the carbon support. The mass ratio of solvent to ABC or APC was 5:1. The resulting slurry was transferred to a round bottom flask and sonicated for 1 h before removing the solvent using a rotary evaporator at 35 oC operated under vacuum. The resulting catalyst precursors were dried for 12 h prior to initiating carbothermal hydrogen reduction (CHR) by placing the sample in a tubular reactor under a H2 flow of 100 mL(STP)/min. The reactor temperature was increased from room temperature to 500 oC at a ramp rate of 10 oC/min, followed by increasing temperature at a ramp rate of 1 oC/min to the final temperature (600 ~ 800 oC), which was then held for a further 90 min before rapidly cooling the sample to room temperature. A mass spectrometer was used to monitor the exit gas from the CHR and quantify the yield of CH4 and CO during CHR. Helium (He) was used as the reference gas during this analysis. The relative pressure of CH4 to He (PCH4/PHe) and CO to He (PCO/PHe) were plotted as a function of reaction time to provide a comparison between the ABC and APC carbon hydrogenation that occurred during CHR to yield the supported Mo2C catalyst. The catalysts are identified as 10%Mo2C/APC_RXYZ and 10%Mo2C/ABC_RXYZ for the catalyst prepared on

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the activated petcoke and biochar, respectively, with XYZ representing the final temperature of the CHR process.

The loss of carbon during the thermochemical activation with KOH and the CHR is defined as the burn-off: Burn off % = 100 ×

(𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝑚𝑎𝑓𝑡𝑒𝑟) 𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙

where 𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 represents the initial mass of the catalyst precursor prior to the KOH activation or prior to the CHR; 𝑚𝑎𝑓𝑡𝑒𝑟 is the mass of the carbon or catalyst after the KOH activation or CHR treatment, respectively.

2.3 Characterization of ABC, APC, Mo2C/ABC and Mo2C/APC catalysts The ultimate analysis and CHNS content of the petcoke and biochar were measured using an elemental analyzer (Perkin-Elmer 2400 series II CHNS/O) operated at a combustion temperature of 975 oC. Elemental analysis of the samples was done by inductively coupled plasma atomic emission spectroscopy (ICP-OES). All samples were acid digested in aqua regia at 110 oC to extract all metals from the support prior to the ICP-OES analysis.

Textural properties of the carbons and catalysts were measured by N2 physisorption at -196 oC using a Micromeritics ASAP 2020 analyzer. The samples were degassed at 200 oC (100 μm Hg) for 4 h prior to the adsorption measurement. The textural properties, including surface area (SA), micropore volume (Vmicro), and mesopore volume (Vmeso), were calculated using 2D non-local density functional theory (2D-NLDFT) based on a N2-DFT model, assuming the pores have a slit 9 / 36


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geometry. The method is considered the most accurate to describe carbon materials, especially those containing mostly micropores. The surface area was calculated based on the adsorption isotherm acquired at relative pressure (p/po) between 0.01 ~ 0.30. The Vmicro is the accumulated N2 volume for pore diameter ≤ 2 nm. Vmeso is the pore volume for pore diameter between 2 ~ 50 nm.

A Horiba Jobin Yvon LabRAM HR800 was used for Raman analysis operated at a laser power of 10 mW, a grating of 1800 g/m, resolution of 0.65 cm-1, and laser wave length of 532 nm. The samples were analysed without any pre-treatment.

X-ray diffraction (XRD) patterns were obtained at 2θ = 10º ~ 90º using a Bruker D8 Focus (0-20, LynxEye detector) diffractometer. A Co Kα (λ=1.789 Å) source operated at 35 kV and 40 mA was used for the analysis. All samples were stored under a N2 blanket prior to sample loading. A quick transfer was conducted to ensure the sample exposure to air was less than 2 minutes. XPS analysis was done using A Leybold Max200 X-ray photoelectron spectrometer with Mg Kα as the photon source generated at 1253.6 eV. The C1s peak at 284.5 eV was taken as reference to account for charging effects. The pass energy was set at 192 eV for the survey scan and 48 eV for the narrow scan. XPSPEAK41 was used for peak deconvolution using a nonlinear leastsquare method with 20% Lorentzian and 80% Gaussian functions.

Pulsed CO uptake measurements were conducted using a Micromeritics AutoChem 2920 with a TCD detector. All samples were prepared in-situ under a 50 mL(STP)/min flow of 9.5 mol% H2/Ar and the same temperature program used for the corresponding catalyst preparation. After

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cooling to about 100 oC, the gas was switched to pure He for 2 h. Then the temperature was reduced to 60 oC waiting until the baseline stabilized. Subsequently, 0.5 mL pulses of CO were injected into a flow of He (50 mL(STP)/min) repeatedly until no further CO uptake was observed. The CO uptake is normalized per gram of Mo.

2.4 Catalytic performance of APC and ABC supported Mo2C catalysts 4-Methylphenol (4-MP) was chosen as model reactant to determine the catalytic activity of the Mo2C/APC and Mo2C/ABC catalysts for the hydrodeoxygenation (HDO) reaction. The reaction was conducted at 350 oC under 4.3 MPa H2 pressure in a batch reactor. Further details are provided in [13], in which activated charcoal supported Mo2C catalysts were assessed at the same reactions conditions [13]. As noted previously, the main products of 4-MP HDO at 350 oC and 4.3 MPa H2 are toluene, methylcyclohexane, 1-methylcyclohexene, and 4methylcyclohexene. The primary product is toluene which is produced by Ar-OH bond hydrogenolysis or direct deoxygenation (DDO). Hydrogenated products result from ring hydrogenation and rapid dehydration to produce 4-methylcyclohexene which is rapidly hydrogenated to methylcyclohexane (the hydrogenation route or HYD). The kinetics of the DDO and HYD reactions were assessed using the pseudo-1st order rate equations for the DDO and HYD parallel reaction paths, [13] as shown in Equations (1) and (2).

𝑟𝐷𝐷𝑂 = (𝑘𝑇𝐷𝐷𝑂 + 𝑘𝐷𝐷𝑂𝐶𝑐𝑎𝑡)𝐶4 ― 𝑀𝑃 𝑟HYD = (𝑘𝑇𝐻𝑌𝐷 + 𝑘𝐻𝑌𝐷𝐶𝑐𝑎𝑡)𝐶4 ― 𝑀𝑃

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(1) (2）

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where 𝑘𝑇𝐷𝐷𝑂 and 𝑘𝑇𝐻𝑌𝐷 (min-1) represent the 1st-oder rate constants of the thermal reactions (without catalyst), Ccat represents the catalyst concentration (gMo/mLfeed), kDDO and kHYD (mL/(gMo.min)) represent the 1st-order rate constants of the catalytic reaction and 𝐶4 ― 𝑀𝑃 is the concentration of 4-MP. The total kinetic constant kc is the sum of kDDO and kHYD. The kinetic rate constants were estimated using a Levenberg-Marquardt nonlinear regression method described previously, applied to the measured component concentration versus time batch reactor data. Separate experiments done without catalyst were used to determine 𝑘𝑇𝐷𝐷𝑂 and 𝑘𝑇𝐻𝑌𝐷. The catalyst turnover frequency (TOF) is calculated by Equation (3): 𝑇𝑂𝐹 =

𝑘𝑐 ∗ 𝐶4 ― 𝑀𝑃

(3)

𝐶𝑂 𝑢𝑝𝑡𝑎𝑘𝑒

3. Results Table 1 summarizes the bulk composition of the carbons before and after activation. Note the higher S content of the PC compared to the BC. The N content of the PC is ~ 10 x’s higher than the BC; whereas, the O content of the BC is 5x’s higher than that of PC. These differences reflect the origin of the materials. The petcoke, generated by delayed coking of bitumen that has high S and N content, has similar composition to petcoke derived from fluid coking of bitumen [32]; whereas, the bio-char is the solid product from the fast pyrolysis of a woody biomass that contains high concentrations of oxygen enriched organic matter. The data show that during activation with KOH, most of the S was removed from the PC, even at 600 oC, where the S content decreased from 6.6 wt% (PC) to 1.3 wt%. No S was detectable in the sample treated at  800 oC and the H decreased to below detection limits at temperatures  700 oC. The BC does not contain any S and the H content decreased to zero at 600 oC. N was not completely removed

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from any of the activated carbons, but the N content decreased to ~0.6 wt% for the APC and to ~0.4 wt% for the ABC.

The data of Table 1 show that KOH activation of the PC results in an increase in the O content of the activated carbon, as reflected in the O/C atomic ratio before and after activation of the raw PC. However, as the activation temperature increased to 800 oC, the O content decreased (Table 1, column 7), suggesting the removal of this O at high temperature (likely as CO2). For the BC, the KOH activation introduced O functional groups to the biochar, as shown by the increased O/C ratio for the BC versus the ABC_600 reported in Table 1. However, the data also show that the O content of the ABC begins to decline at much lower temperature (700 oC) compared to the APC (800 oC). The ability of KOH to add oxygen to PC during thermochemical activation is reported in other studies [33, 34] and a decrease in the O content of the ABC as temperature is increased above 600 °C has also been observed previously [35, 36].

The Raman spectra peak intensity ratio I(D)/I(G), where I(D) is the intensity of the D band peak at 1600 cm-1 and I(G) is peak intensity of the G band at 1350 cm-1, is an indicator of the degree of disorder of carbon materials [37, 38]. The ratio reported in Table 1 and the Raman spectra of Figure S1 (see Supporting Information) indicate that the raw PC had high crystallinity (I(D)/I(G) = 1.5), but the ratio increased gradually with increased activation temperature from 600 oC (I(D)/I(G) = 2.2) to 800 oC (I(D)/I(G) = 2.9), indicative of increased disorder. However, at 900 oC,

the ratio decreased (I(D)/I(G) = 1.9). The data indicate that at low temperature KOH

activation destroys the carbon crystal structure, yielding amorphous carbon, likely on the pore walls that were exposed to the KOH. However, at higher temperature, graphitization of the

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carbon dominates. The Raman spectra data of the raw BC and ABC show similar results. As presented in Table 1, despite the high oxygen content of the biochar, the crystallinity (I(D)/I(G) = 1.5) is similar to that of the raw petcoke. The degree of disorder of the ABC increased at an activation temperature of 600 to 800 °C and then decreased significantly (I(D)/I(G) = 0.4) as the activation temperature was increased to 900 oC.

Table 2 summarizes the textural properties of the APC and ABC, activated with KOH at different temperatures. The APC and ABC isotherms (Figure S2, Supporting Information) are of Type I, based on the IUPAC classification [39] and approach saturation at relative pressure 0.2 0.4, indicative of microporous materials. The data also show that the N2 adsorption capacity increased as the activation temperature increased, in agreement with Mochizuki et al. [40]. A hysteresis loop begins to appear at 700 oC for the APCs while for the ABCs the hysteresis appears at 900 oC. The APC has a marginally higher mesopore volume than the ABC when compared at the same activation temperatures. In general, the surface area and pore volume increased as the activation temperature increased for both APC and ABC.

To compare the two different activated carbons as catalyst supports, APC_800 and ABC_700 were chosen for the subsequent CHR synthesis of the Mo2C catalyst, since these two samples had similar surface areas and total pore volumes. The difference in mesoporosity between APC and ABC is considered of less significance since the CHR develops most of the final catalyst mesoporosity. The composition of the selected supports is reported in Table 1 and Table S1 (Supporting Information), showing that the APC_800 had significantly less O content than the ABC_700 but the APC contains more trace elements than ABC. The XPS data of Table S2 and

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Figure S3 show the surface O content is also higher for the ABC_700 than the APC_800 sample. This result is consistent with the FT-IR spectra of Figure S4, which also show the loss in O species following activation.

Figure 1 reports the CH4 concentration in the product gas during CHR to 700 °C for the 10%Mo2C/APC_R700 and 10%Mo2C/ABC_R700 catalysts [14]. The data show that the ABC support generates more CH4 during CHR than the APC at the same temperature, and the hydrogenation of the carbon is initiated at lower temperature compared to the APC sample. These differences are important because it has been reported previously that the formation of the Mo2C is related to the formation of CH4 during the CHR process [14]. Figure S5 also reports the CO produced during the CHR process that is associated with the conversion of MoO3 to Mo2C and the reduction of O-species associated with the activated carbon. The data show higher CO production from the ABC supported catalyst.

Figure 2 summarizes the XRD analysis of the 10%Mo2C/APC and 10%Mo2C/ABC samples, reduced at different temperatures. In the case of APC, distinct peaks at ~15o and ~28o representing graphitic carbon are identified at a CHR temperature of 600 oC. The peak at 46.07o assigned to Mo2C (101) can be observed at 650 oC for the APC supported catalysts, and at 700 oC,

the peak intensity increased. XRD patterns for the ABC supported catalysts at all three

reduction temperatures, on the other hand, did not show any peaks that could be assigned to Mo2C. The presence of SiO2 is associated with the silica sand used as heat medium in the biomass pyrolysis process used to prepare the biochar.

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Figure 3 presents the deconvoluted Mo 3d XPS spectra of the APC supported catalysts, using the same deconvolution method applied in previous work [41]. The deconvoluted peaks at binding energy (B.E) 228.4 eV, 229.4 eV and 232.4 eV are assigned to Mo2+, Mo4+ and Mo6+, respectively. Table 3 summarizes the detailed distribution of Mo species identified by XPS. From 600 oC to 650 oC, the Mo valence decreases from Mo6+ and Mo4+ to Mo2+. The effect is much more significant at a CHR temperature of 700 oC. Mo2+, which can be assigned to Mo2C, reaches 49.6%. The Mo 3d deconvoluted XPS spectra of the ABC supported catalysts are shown in Figure 4 with the same deconvolution applied. In this case the Mo2+ surface content is higher than that on APC, at all reduction temperatures, suggesting that the ABC supported catalysts form more Mo2C during the CHR process, compared to the APC supported catalysts. Also, the TEM image shown in Figure S6 identifies a d-spacing of 0.23 nm for the Mo particle, characteristic of the (101) plane of Mo2C supported on ABC.

Table 4 summarizes the kinetic data for the HDO of 4-MP obtained over the APC and ABC supported Mo2C catalysts, prepared at different reduction temperatures. Figure S7 reports the measured data and the fit to the 1st-order kinetic model described by Equations (1) and (2). The relevant data for AC supported Mo2C catalysts, reported previously [13], are also listed for comparison. Both the 10%Mo2C/ABC and the 10%Mo2C/APC catalysts show very high DDO selectivity compared to the 10%Mo2C/AC catalysts. Also, both the 10%Mo2C/ABC and the 10%Mo2C/APC catalysts have much higher 1st-order reaction rate constants than the 10%Mo2C/AC. For the Mo2C/APC catalysts, the rate constants (mL/(min. g Mo)) are not significantly different from each other as the CHR temperature increased from 600 to 700 oC. Among the Mo2C/ABC catalysts, the 10%Mo2C/ABC_R650 had the highest activity per gram of

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Mo and kc was approximately twice that measured on the Mo2C/ABC catalysts prepared at CHR temperatures of 600 and 700 oC. Finally, accounting for the CO uptake on all the catalysts, the turnover frequency (TOF) of the catalysts is reported in Table 4. These data suggest that the activity of the both Mo2C/ABC and Mo2C/APC catalysts for the HDO of 4-MP are approximately the same as the TOFs for the Mo2C/AC catalysts reported previously [13].

4. Discussion The raw petcoke generated by delayed coking where heavy feed (bitumen) is processed at high temperature [42] is of higher bulk density (Table S3) than the raw biochar generated in a rapid devolatilization process with high gas yields that generate porosity in the solid biochar [43]. The properties of the activated carbon derived from petcoke and biochar as well as the catalysts synthesized by the CHR process, are affected by the properties of the carbon. The O content of the raw biochar is much higher than the raw petcoke and the CHNS analysis of the activated carbons indicate that the O content of the ABC is higher than the APC. The S present in the raw petcoke is removed during the activation with KOH and consequently, the effect of S on Mo2C synthesis is negligible for both carbon supports investigated herein. The N content, on the other hand, is considerable for both APC and ABC. Although the N may be reduced to NH3 during CHR in H2 at elevated temperature [44] and the NH3 may in turn react with the Mo, the low concentration of N compared to C (N:C ratio is < 0.008) and O (N:O ratio is < 0.08) make such a reaction of much less significance than the direct carburization. The metals present in the raw petcoke and biochar may also impact the activity of the prepared catalyst. However, the metal concentrations are 103 times lower than the Mo, suggesting that the contribution of these metals to the catalyst activity would be minimal compared to the Mo. Also note that the characteristic 17 / 36


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feature of the raw biochar - the very high oxygen content - remained high after the chemical activation with KOH.

The Raman spectroscopy of the APC and ABC show that KOH chemical activation increased the amorphous carbon content of the support, with the degree of disorder higher for the APC than the ABC. The reaction between KOH and C begins at temperature > 400 oC [45, 46]. As the KOH etches through the carbon, pores develop, as indicated by the isotherm data. The H2O and CO2 generated during the activation with KOH may also react with the carbon surface leading to the formation of oxygen functional groups [47, 48]. At low temperature ( 700 °C for APC and  600 °C for ABC) the O content increased with increased activation temperature. At higher activation temperature (≥ 800 oC for APC and ≥ 700 oC for the ABC), as more carbon is reacted, the oxygen functional groups are also reacted, leading to a decrease in O content; whereas, graphitization also increases at these high temperatures. Note, however, that because the initial O content of the biochar is very high, after thermochemical activation the O content of the biochar remains relatively high.

The oxygen functional groups make the carbon surface more amorphous and less crystalline [49] and amorphous carbon is more readily hydrogenated in the CHR process [14]. The abundant oxygen in the ABC may also be reduced by H2 during CHR in the presence of Mo, forming surface vacancies that have the ability to activate H2 and further facilitate the CH4 formation reaction [50]. The higher O content of the ABC compared to the APC results in more CH4 generation on the Mo-impregnated ABC_700 than the APC_800. XRD results of the supported catalysts also suggest a higher dispersion of Mo species on the ABC than on APC. The

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formation mechanism of Mo2C on APC during CHR has been reported previously and the formation mechanism of Mo2C on ABC is also expected to be strongly dependent on the concentration of CH4 generated during CHR. With higher Mo dispersion and higher CH4 concentration observed from the CHR of the ABC, the generation of Mo2C can be expected at lower temperature. The XPS results confirm this by showing that the ABC supported catalysts contain more Mo2C species on the catalyst surface than the APC supported catalysts prepared at the same reduction temperature.

The data of Table 4 show that the Mo2C catalysts supported on the three different carbon supports have similar TOFs for the HDO of 4-MP. Although the characterization data show that the textural properties and Mo2C dispersion of the Mo2C/carbon catalysts are strongly dependent on the carbon support, similar TOFs are obtained on all three carbon supports. Thus, the activity of the catalysts, determined by the number of Mo2C active sites, is dependent upon the Mo2C loading and dispersion. The deconvoluted Mo 3d XPS analysis of the 10%Mo2C/APC catalysts show similar proportions of Mo2+ and Mo4+ to that reported for the 10%Mo2C/AC at different reduction temperatures [13]. Yet the 1st-order rate constants reported per gram of Mo (Table 4) were significantly different on each of the supports. The 10%Mo2C/APC catalysts have much higher activity per gram of Mo than the 10%Mo2C/AC catalysts because the APC support provides a higher surface area and better Mo2C dispersion (higher CO uptake, Table 4). The higher dispersion is a result of oxygen functional groups, such as –OH, on the carbon surface that ensure wetting of the carbon by the solvent during impregnation and anchoring of the metal during heat treatment that reduces sintering [51]. Thus, the properties of the APC support lead to a better dispersion of Mo2C compared to AC. The 10%Mo2C/ABC catalysts, with a higher

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proportion of Mo2+ (i.e. Mo2C, Table 3) and better Mo2C dispersion due to a higher concentration of oxygen surface groups (Table S4) also has a higher activity (per gram Mo) than the APC supported Mo2C. However, given the fact that very similar TOFs are obtained on all three carbon supports, we conclude that the carbon does not impact the active sites of the Mo2C catalyst, but rather simply determines the number of active sites, based on the activity of the carbon during CHR and the resistance to sintering of the formed Mo2C nanoparticles.

We also note that for the 10%Mo2C/APC catalyst, an increase in reduction temperature from 600 oC

to 650 oC increased the number of active sites and the porosity of the resulting catalyst, which

resulted in an increase in HDO conversion of 4-MP. However, a further increase in reduction temperature to 700 °C resulted in a drop in conversion (Table 4) despite a significant increase in Mo2+ on the catalyst surface (Table 3). In this case the loss in conversion is due to significant loss in surface area and pore volume at 700 oC (Table 5), which together with the increased burnoff, yields less dispersed Mo2C, as reflected in the CO uptake data of Table 4. Similar effects of increased CHR temperature are also apparent for the ABC supported Mo2C. In this case, although the low density and high oxygen content of the biochar results in the generation of more CH4 during CHR, the loss of pore walls means less mesopore growth (Figure S8). The narrow pore size could make some of the active sites very difficult to access. The kinetic rate constants of the best catalysts in this study (per gram of Mo) are ~5x’s higher (Table S5) than the previously reported Mo2C catalyst supported on AC [13].

The Mo2C/ABC and Mo2C/APC catalysts all have higher DDO selectivity (SDDO/HYD, Table 4 and Figure S7) compared with the Mo2C/AC catalysts [13]. For the ABC and APC supported

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catalysts kDDO is 80 – 90% of the magnitude of kC; whereas, for the AC the value is 70-80 % of kC. The increased DDO selectivity may be due to structural and/or size effects of the Mo2C, or the higher O content of the APC and ABC. During CHR, these oxygen functional groups are removed, creating O vacancies that can facilitate DDO selectivity [52, 53].

5. Conclusion Thermochemically activated petcoke and biochar were investigated as supports of Mo2C catalysts, prepared by CHR. The properties of the ABC (high surface O content, higher reactivity in H2) result in higher Mo2C formation and higher Mo2C dispersion on the ABC support compared to the APC support. Nevertheless, when accounting for the Mo2C dispersion, the TOF of the HDO of 4-MP is shown to be very similar on Mo2C supported on activated petcoke, biochar and charcoal. However, compared to the 10%Mo2C/AC catalyst, both 10%Mo2C/APC and 10%Mo2C/ABC catalysts have much higher DDO selectivity. The ABC supported catalysts also have higher DDO selectivity than APC supported catalysts, due in part to the high O content of the ABC surface. The DDO selectivity of ABC supported catalyst reached ~90%.

Acknowledgements The financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Korea Institute of Science and Technology (KIST) is gratefully acknowledged.

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[23] B.N. Murthy, A.N. Sawarkar, N.A. Deshmukh, T. Mathew, J.B. Joshi, Petroleum coke gasification: A review, The Canadian Journal of Chemical Engineering, 92 (2014) 441-468. [24] D.A. Laird, R.C. Brown, J.E. Amonette, J. Lehmann, Review of the pyrolysis platform for coproducing bio-oil and biochar, Biofuels, Bioproducts and Biorefining, 3 (2009) 547-562. [25] E.W. Bruun, P. Ambus, H. Egsgaard, H. Hauggaard-Nielsen, Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics, Soil Biology and Biochemistry, 46 (2012) 73-79. [26] D. Mohan, A. Sarswat, Y.S. Ok, C.U. Pittman, Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent–a critical review, Bioresource technology, 160 (2014) 191-202. [27] Z. Jiyi, P. Lijun, L. Gen, Preparation of biochar adsorbent from straw and its adsorption capability, Transactions of the Chinese Society of Agricultural Engineering, 2011 (2011). [28] Y. Ma, W.-J. Liu, N. Zhang, Y.-S. Li, H. Jiang, G.-P. Sheng, Polyethylenimine modified biochar adsorbent for hexavalent chromium removal from the aqueous solution, Bioresource technology, 169 (2014) 403-408. [29] M. Ahmad, A.U. Rajapaksha, J.E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage, S.S. Lee, Y.S. Ok, Biochar as a sorbent for contaminant management in soil and water: A review, Chemosphere, 99 (2014) 19-33. [30] C. Moreno-Castilla, Adsorption of organic molecules from aqueous solutions on carbon materials, Carbon, 42 (2004) 83-94. [31] S. Dong Jin, P. Tae-Jin, I. Son-Ki, Effect of surface oxygen groups of carbon supports on the characteristics of Pd/C catalysts, Carbon, 31 (1993) 427-435. [32] Y. Shi, J. Chen, J. Chen, R.A. Macleod, M. Malac, Preparation and evaluation of hydrotreating catalysts based on activated carbon derived from oil sand petroleum coke, Applied Catalysis A: General, 441 (2012) 99-107. [33] H. Zhang, J. Chen, S. Guo, Preparation of natural gas adsorbents from high-sulfur petroleum coke, Fuel, 87 (2008) 304-311. [34] T. Otowa, Y. Nojima, T. Miyazaki, Development of KOH activated high surface area carbon and its application to drinking water purification, Carbon, 35 (1997) 1315-1319. [35] T. Tay, S. Ucar, S. Karagöz, Preparation and characterization of activated carbon from waste biomass, Journal of Hazardous Materials, 165 (2009) 481-485. 24 / 36


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[49] I.D. Rosca, F. Watari, M. Uo, T. Akasaka, Oxidation of multiwalled carbon nanotubes by nitric acid, Carbon, 43 (2005) 3124-3131. [50] Y. Gao, D. Ma, C. Wang, J. Guan, X. Bao, Reduced graphene oxide as a catalyst for hydrogenation of nitrobenzene at room temperature, Chemical Communications, 47 (2011) 2432-2434. [51] C. Prado-Burguete, A. Linares-Solano, F. Rodriguez-Reinoso, C.S.-M. De Lecea, The effect of oxygen surface groups of the support on platinum dispersion in Pt/carbon catalysts, Journal of Catalysis, 115 (1989) 98-106. [52] T.N. Phan, C.H. Ko, Synergistic effects of Ru and Fe on titania-supported catalyst for enhanced anisole hydrodeoxygenation selectivity, Catalysis Today, 303 (2018) 219-226. [53] V.O. Gonçalves, C. Ciotonea, S. Arrii-Clacens, N. Guignard, C. Roudaut, J. Rousseau, J.-M. Clacens, S. Royer, F. Richard, Effect of the support on the hydrodeoxygenation of m-cresol over molybdenum oxide based catalysts, Applied Catalysis B: Environmental, 214 (2017) 57-66.

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Figures

Figure 1: Profile of detected relative intensity of PCH4/PHe as a function as time for 10%Mo2C/APC_R700 (represented by by

—) and 10%Mo C/ABC_R700 samples (represented 2

—).

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Figure 2: XRD patterns of 10%Mo2C/ABC and 10%Mo2C/APC reduced at different CHR temperatures. (♦ identifies C-graphite; * identifies Mo2C; • identifies SiO2)

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Figure 3: XPS spectra of Mo 3d of fresh 10%Mo2C/APC samples reduced at different CHR temperatures: (a) Survey scan of 10%Mo2C/APC_R600; narrow scans of: (b) 10%Mo2C/APC_R600, (c) 10% Mo2C/APC_R650, and (d) 10%Mo2C/APC_R700.

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Figure 4: XPS narrow scan spectra of Mo 3d of fresh 10%Mo2C/ABC samples reduced at different CHR temperatures. (a) 10%Mo2C/ABC_R600; (b) 10% Mo2C/ABC_R650; (c) 10%Mo2C/ABC_R700.

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List of Tables Table 1: Elemental analysis of the as-received raw carbons and the activated carbon supports.

CHNS Analysis

a.

Raman Analysis

Sample

C (wt%)a

H (wt%)a N (wt%)a

S (wt%)a

O (wt%)a

O/C (at.)

I(D)/I(G)c

Raw Petcoke (PC)

83.27

3.60

2.02

6.58

4.53b

0.04

1.5

APC_600

77.31

0.88

1.15

1.26

19.41

0.19

2.2

APC_700

71.21

0.00

0.58

0.15

28.07

0.30

2.6

APC_800

89.87

0.00

0.67

0.00

9.46

0.08

2.9

APC_900

92.91

0.00

0.61

0.00

6.48

0.05

1.9

Raw Biochar (BC)

76.20

0.75

0.54

0.00

22.52

0.30

1.5

ABC_600

73.02

0.00

0.30

0.00

26.69

0.37

2.1

ABC_700

80.90

0.00

0.39

0.00

18.72

0.23

2.0

ABC_800

83.23

0.00

0.46

0.00

16.32

0.20

2.4

ABC_900

85.58

0.00

0.39

0.00

14.04

0.16

0.4

The error associated with each element: C ± 0.03%; H ± 0.13%; N ± 0.18%; S ± 0.28%; O ± 0.06%.

b. O was calculated by 100-(C+H+N+S) (wt%). c.

Relative intensity of D (~1350 cm-1) band to G band (~1600 cm-1) for carbon materials. Error is < 0.1.

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Table 2: Textual properties of APC and ABC samples thermochemically activated at different temperatures.

Sample

Surface area (m2/g)

Pore volume (cm3/g) Micropore Mesopore volume volume

Total volume

1560 1817 2037 2088

0.53 0.62 0.78 0.77

0.01 0.02 0.07 0.22

0.54 0.63 0.84 0.99

27.2 31.2 37.5 48.2

1837 2096 2224 1951

0.70 0.84 0.96 0.82

0.00 0.00 0.03 0.17

0.70 0.84 0.99 0.99

28.1 39.9 42.5 68.8

Burn-off (%)

APC samples APC_600 APC_700 APC_800 APC_900

ABC samples ABC_600 ABC_700 ABC_800 ABC_900

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Table 3: XPS analysis of Mo 3d species for 10%Mo/APC and 10%Mo/ABC fresh catalysts at different CHR temperatures.

CHR Temperature

R600

R650

Mo species

10%Mo2C/APC 10%Mo2C/ABC

R700

10%Mo2C/APC

10%Mo2C/ABC

10%Mo2C/APC

10%Mo2C/ABC

%

%

%

%

%

%

Mo2+

15.9

46.3

30.0

49.5

49.6

63.9

Mo4+

25.7

27.1

21.0

27.1

11.7

7.9

Mo6+

58.4

32.2

49.0

23.4

38.7

18.2

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Table 4: Kinetic rate constants, selectivity, and conversion of 4-methylphenol (4-MP) on Mo2C/APC and Mo2C/ABC catalysts with different CHR temperatures at 350 oC and 4.3 MPa in hydrodeoxygenation (HDO). Mo content, Catalyst

wt%

Mo load in the reactor mgMo/100 mLfeed

X4-MPa

SDDO/HYDb

kDDO

%

kHYD

kc

mL/(min.gMo)

CO uptaked

TOF

μmol/g

s-1

Mo

x 10-2

10%Mo2C/APC samples 10%Mo2C/APC_R600

10.23

59

66.4

4.9

4.66±1.08e

0.95±0.84

5.62±1.92

661

3.6±1.1

10%Mo2C/APC_R650

12.59

58

77.2

6.9

5.40±0.76

0.79±0.58

6.19±1.32

797

3.3±0.5

10%Mo2C/APC_R700

13.79

46

70.2

11.0

6.69±1.06

0.61±0.80

7.30±1.86

693

4.5±0.9

10%Mo2C/ABC samples 10%Mo2C/ABC-R600

9.05

51

66.7

8.0

4.55±0.60

0.57±0.50

5.12±1.1

593

3.7±0.6

10%Mo2C/ABC-R650

13.25

49

84.0

9.7

9.30±1.38

0.96±0.94

10.26±2.32

844

5.2±0.9

10%Mo2C/ABC-R700

16.26

54

78.6

9.4

5.92±1.18

0.63±0.88

6.56±2.04

599

4.6±1.3

10%Mo2C/AC samples [13] 10%Mo/AC-600

11.36

72

43.2

2.9

1.260.12

0.430.11

1.730.23

147

5.0±1.1

10%Mo/AC-650

12.71

72

44.0

3.4

1.330.08

0.390.07

1.720.15

187

3.9±0.5

10%Mo/AC-700

17.55

72

36.3

4.0

1.000.12

0.250.11

1.250.23

156

3.4±1.0

a. The conversion of 4-methylphenol after 5 h’s HDO of 4-MP. b. The selectivity of DDO to HYD. c. kc=kDDO+kHYD; unit: (mL/(min.gMo)). 34


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Energy & Fuels

d. The associated error for CO uptake is ±5%. e. The associated error for all kinetic constants is reported with the 95% confidence interval (CI; CI= 2 × Std. dev).

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Page 36 of 37

Table 5. Physical properties of Mo2C catalyst supported on APC and ABC with different reduction temperatures. Surface area a

Sample

Pore volume b Burn off c Vtotal

m2/g

Vmeso cm3/g

%

APC prepared catalysts 10%Mo – AHM/APC precursor

1281

0.50

0.04

—

10%Mo2C/APC _R600

1372

0.66

0.07

18.3

10%Mo2C/APC_R650

1302

0.72

0.15

35.4

10%Mo2C/APC_R700

1098

0.62

0.21

53.7

10%Mo – AHM/ABC precursor

1565

0.66

< 0.01

—

10%Mo2C/ABC _R600

1631

0.73

< 0.01

20.1

10%Mo2C/ABC_R650

1630

0.79

0.04

38.1

10%Mo2C/ABC_R700

948

0.41

< 0.01

55.5

ABC prepared catalysts

a.

The specific surface area was calculated from the measured N2 adsorption isotherm using carbon-N2, 2DNLDFT-Heterogeneous surface applied in the P/Po range of 0.01~0.30.

b.

The pore volume data were calculated by 2D-NLDFT method, where the mesopore is defined as 2~50 nm and micropore is ≤ 2 nm.

c.

Burn-off%=100 ×

𝑚𝐼𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝑚𝑓𝑖𝑛𝑎𝑙 𝑚𝐼𝑛𝑖𝑡𝑖𝑎𝑙

.

36


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Energy & Fuels

Graphical Abstract


Carbon Catalysts for the

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