Synthesis and Hydrodeoxygenation Activity of Carbon Supported Molybdenum Carbide and Oxycarbide Catalysts

Abstract

Carbothermal hydrogen reduction (CHR) of ammonium heptamolybdate impregnated activated charcoal (AC) yields a mixed Mo 2 C/MoO x C y catalyst. As the CHR temperature increases (from 600 to 800 °C) the Mo 2 C content increases. At 675 °C graphite networks are generated that attach to the β-Mo 2 C particles, and at ≥700 °C agglomeration and sintering occur, all of which decrease catalyst activity. An optimal CHR temperature of ∼650 °C is identified based on the catalyst activity for the hydrodeoxygenation (HDO) of 4-methylphenol (4-MP) at 350 °C and 4.3 MPa H 2 and the high selectivity for direct deoxygenation (DDO: to yield toluene) versus hydrogenation (HYD: to yield cyclohexane). The Mo 2 C/MoO x C y catalysts have higher DDO selectivity than MoP, MoO 2 , or MoS 2 when operated at similar conditions. The apparent activation energies for DDO (125 kJ/mol) and HYD (89 kJ/mol) are invariant among the catalysts with varying Mo 2 C content, but the rate per g Mo correlates with the CO uptake. The fact that the kinetics are not strong functions of the CHR reduction temperature and hence relative content of Mo 2 C versus MoO x C y suggests that the active site of the catalyst is a result of O adsorption and/or exchange with the catalyst during reaction, and these active sites occur on both Mo 2 C and MoO x C y during the HDO reaction.

Preparation of Ni-Mo 2 C/carbon catalysts and their stability in the HDS of dibenzothiophene

“…An alternative preparation method uses carbothermal hydrogen reduction (CHR), employing a carbon support and pure H2 to generate the metal carbide at high reduction temperatures (700-800 o C). This method yields nanoparticles of Mo2C, as first demonstrated by Mordenti et al [22] and Liang et al [23] and also reported previously by the authors [24]. Thus, CHR is a relatively facile method to synthesize carbon supported Mo2C catalysts, in which the carbon support may also suppress coke formation during residue oil upgrading [25].…”

“…The calcined precursor (ca. 1.8 g) was placed in a U-tube reactor and subsequently converted to the Ni-Mo2C/AC catalyst by CHR in a continuous H2 flow (200 mL (STP)/min), while increasing temperature from room temperature to 500 o C at 10 o C /min, followed by increasing temperature at 1 o C/min to 650 o C and holding the final temperature for 90 min before quenching to room temperature in N2 [24]. This catalyst (designated as 10%Mo2C/AC-650) was compared with Ni-Mo2C/AC catalysts, prepared similarly but with successive impregnations of the AC with solutions of AHM and Ni(NO3)2, respectively, to obtain precursors with different Ni:Mo ratios.…”

“…However, CHR has a limited capacity to achieve complete reduction/carburization of the catalyst precursor, even at high reduction temperatures. For example, in previous work [24], CHR of ammonium heptamolybdate impregnated activated charcoal (AC) yielded a mixed Mo2C/MoOxCy phase with a relatively high portion of Mo2C (~50%) obtained at 700 o C. However, almost half of the carbon support was hydrogenated (mostly to CH4) during the CHR process, which decreased the support area available to achieve high dispersion of the Mo2C.…”

Task‐Specific Catalyst Development for Lignin‐First Biorefinery toward Hemicellulose Retention or Feedstock Extension

“…The carbon loss of the catalyst increases from 14 to 50 and to 70 wt %a st he temperature increases from 600 to 700 and to 800 8C, respectively.T his is mainly caused by the formation of C x H y in the reduction gas (2xC + yH 2 !2C x H y )and the replacemento fOspecies in the Mo-based catalyst by C species. When the temperature rises to 700 8C, it is observed that most particles (4-6 nm) were inside the tubes (Figure 2e,f), although af ew particles were distributed outside the tubes and slightly aggregated (15)(16)(17)(18)(19)(20)(21)(22) nm, see Figure S1 ai nt he Supporting Information). As shown in Figure 2a,b,t he initial prepared particlesa re dispersed on the internal surfaces of the tubes for the AHM/CNTs.…”

“…[19a, 20] All Mo-based catalysts were furthera nalyzed by TEM. The enlarged TEM images of these particles show a d-spacing of 0.228 nm assigned to the (1 01)p lane of the hexagonal phase of Mo 2 C, [20] as shown in Figure S2 (in the SupportingI nformation),p roving the particles both inside and outside the CNTsa re Mo 2 C. However,a ss hown in Figure 2g,h,t he formation of Mo 2 Ca nd destruction of the CNTst akep lace simultaneously at 800 8C. In the case of the Mo x C/CNTst reated at 600 8C, al arge number of particles with as ize range of 4-6 nm are uniformly distributed inside of CNTs ( Figure 2c,d).…”