In Situ Generated Dispersed Catalysts Based on Molybdenum and Tungsten Phosphides in Hydroprocessing of Guaiacol

Amorphous catalysts based on molybdenum and tungsten phosphides were prepared in situ from oil-soluble precursors such as triphenylphosphine and carbonyls of the corresponding metals during hydrodeoxygenation of guaiacol. These catalysts were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy, and transmission electron microscopy. After 6 h of reaction at 320–380°C and an initial hydrogen pressure of 5 MPa, the guaiacol conversion amounted to 89–91% in the presence of the molybdenum phosphide catalyst and 80–86% with tungsten phosphide. The selectivity towards phenol as the main reaction product reached as high as 80% in the presence of molybdenum phosphide (360°C, 6 h) and 78% in the tungsten phosphide case (340°C, 1 h). In the presence of both catalytic systems, the reaction products also contained anisole, cresols, and toluene.


DOI: 10.1134/S0965544122110019
Guaiacol (2-methoxyphenol) and its derivatives are among the main products of lignocellulosic biomass processing [1]. The most promising and effective technique for biomass processing is hydrodeoxygenation over catalysts based on transition metals and their compounds [2,3]. In particular, phosphides of transition metals such as molybdenum and tungsten have been successfully used since the early 2010s for hydrodeoxygenation of various oxygenated classes [4].
Previously we proposed a method for the preparation of nickel, molybdenum, and tungsten phosphides that involved the formation of the active catalytic phase in a hydrodesulfurization reactor [5]. This phosphide synthesis technique was employed in the present study as well.
The purpose of this work was to prepare in situ molybdenum and tungsten phosphides during hydrodeoxygenation of guaiacol, and to investigate their catalytic activity.
Powder X-ray diffraction (XRD) of the catalyst samples was performed using a Rigaku Rotaflex D-MAX-RC diffractometer in the 2θ range of 10°-100°. The samples were further subjected to X-ray photoelectron spectroscopy (XPS) using a 2403M-T electron spectrometer (manufactured by the Institute for Analytical Instrumentation RAS, Russia) equipped with a SPECS PHOIBOS 100-5MCD energy analyzer and a SPECS XR50 X-ray source (MgK α ). A FEI Tecnai Osiris 200 kV transmission electron microscope (TEM) equipped with an energy-dispersive X-ray (EDX) analyzer was used to obtain micrographs and identify the chemical composition of the catalyst samples. The extended X-ray absorption fine structure (EXAFS) spectra of the catalysts and of the physical mixture of their precursors were recorded for pelletized samples in the transmission mode. EXAFS spectra around the W L 3 -edge were recorded at the Structural Materials Science station (Kurchatov Synchrotron Radiation Source, NRC "Kurchatov Institute," Russia). A Si(111) monochromator was used for energy scans. Higher harmonics were suppressed by mechanical adjustment of the monochromator. Mo K-edge spectra were recorded using the BM23 beamline of the European Synchrotron Radiation Facility (ESRF). Energy scans were performed in the continuous mode using a Si(311) double-crystal monochromator. Higher harmonics were suppressed by Rh-coated mirrors. The spectra were processed and analyzed using Demeter software. The operating principle of this package is similar to that of IFFEFIT [6]. To reduce the number of variables for the multiplescattering paths, a correction of initial energy (ΔE 0 ) was set to be common for all phases. Moreover, instead of independent simulations, the interatomic distances (ΔR i ) were estimated by proportionally increasing/decreasing all distances for each phase by the formula: where α is the proportional phase coefficient, modeled by the Demeter software until a good agreement between the model and experimental spectra was reached; and R 0i is the interatomic distance in the crystallographic structure.
The liquid reaction products were identified by GC/ MS using a Thermo Scientific ISQ 7000 instrument equipped with a Restek 5XI-17SIL MS CAP capillary column (30 m × 0.25 mm × 0.25 µm, helium as a carrier gas). The GC operating mode was as follows: injector temperature 300°C; initial thermostat temperature 50°C, followed by 5-minute holding, heating to 220°C at a rate of 15°/min, no holding, further heating to 270°C at a rate of 20°/min, and another 10-minute holding. The mass spectrometer operated in the following mode: ionization energy 70 eV; ion source temperature 200°C; transfer line temperature 250°C; scanning range 10-700 Da. NIST/EPA/NIH databases were used to identify components. The liquid products were quantitatively analyzed on a Crystal-Lux 4000M gas chromatograph (manufactured by Meta-Chrom, Russia) equipped with a flame ionization detector and an Optima-1 capillary column (25 m × 0.32 mm × 0.35 µm, helium as a carrier gas). The chromatograph operating mode was as follows: injector temperature 350°C; detector temperature 350°C; initial column temperature 40°C, followed by 2-minute holding, heating to 320°C at a rate of 5°/min, and another 10-minute holding. The chromatograph was calibrated in advance using model mixtures that contained guaiacol, phenol, toluene, o-cresol, and anisole in dodecane. NetChromWin software was used for the chromatographic analysis and interpretation of the chromatograms.

RESULTS AND DISCUSSION
To investigate the in situ generation of molybdenum and tungsten phosphides from carbonyls of the corresponding metals and triphenylphosphine during guaiacol hydrodeoxygenation at 360°C, XRD analysis was used (Fig. 1). Under these conditions, the catalysts remained amorphous as no crystals were generated. In our previous study, using similar catalyst precursors, amorphous molybdenum and tungsten phosphides were generated in situ in hydrotreating of light cycle oil [5].
The electronic states of the surface atoms of the catalyst were characterized by XPS ( Table 1). The MoP spectrum in the Mo3d region contains three doublets GOLUBEVA et al.
( Fig. 2a): they are attributed to Mo δ+ in MoP and to Mo 4+ and Mo 6+ in MoO 2 and MoO 3 or corresponding phosphates. 1 Likewise, the WP spectrum in the W4f region has three doublets (Fig. 2b): W δ+ in WP, W 4+ in WO 2 , and W 6+ in WO 3 or phosphates. In the P2p region, both phosphide spectra contain a main peak attributed to the oxidized phosphate phase (Figs. 2c, 2d). The formation of phosphates on the surface is presumably associated with the surface oxidation of phosphides or the oxidation of triphenylphosphine (a precursor) to triphenylphosphoxide.
The absence of crystalline particles in the MoP and WP samples (Figs. 3a, 3b and 3c, 3d, respectively), earlier revealed by XRD, was confirmed for both catalysts. The Mo/P and W/P molar ratios were determined by EDX to be 1.3 and 1.1, respectively; for WP, this proved to be closer to the stoichiometric ratio.
The EXAFS spectrum of the physical mixture of tungsten-based precursors (Fig. 4a) displays two distinct peaks with maxima at 1.6 and 2.7 Å without phase correction. These peaks are attributed to contributions of W-C and W-O, the interatomic distances of which (2.05±0.01 and 3.22±0.02 Å, respectively) were derived from an R-space model. The large amplitude of the second peak can be explained by the contribution of single and  [7,8]. The spectrum of the in situ prepared catalyst (Fig. 4b) lacks a carbonyl peak. The remaining peak of the first shell, typical of light atom contributions, is attributed to carbide W-C bonds. The shoulder of the first-shell peak (about 2 Å) corresponds to W-P interactions. This hypothesis was confirmed by approximating the experimental spectrum by the spectrum of two phases: hexagonal stoichiometric WC [9] and orthorhombic WP [10]. The  Like in the case of tungsten precursor, the absorption K-edges of the molybdenum precursor correspond to the molybdenum carbonyl structure (Fig. 4c) with 2.07±0.01 Å Mo-C bonds and 3.24±0.01 Å Mo-O bonds (the latter including multiple scattering paths). After the reaction, the carbonyl peak likewise disappeared (Fig. 4d). The right shoulder of the first-shell peak is attributed to 2.30±0.04 Å Mo-P bonds formed during the reaction. The modeling of relevant Mo-based catalyst parameters using the known MoC x and MoP x crystallographic structures did not satisfactorily fit the experimental data, presumably because of the amorphous form of the catalyst.
The catalytic activity of the in situ generated phosphides was tested in hydroprocessing of guaiacol. In the MoP case, the conversion was barely affected by heating: it remained at about 90% (Fig. 5a). Phenol, cresols, anisole, and toluene were identified as the reaction products. Phenol is known to be partly formed directly from guaiacol via demethoxylation, and to be also produced through demethylation of the anisole formed from deoxygenation of guaiacol [11][12][13]. Cresols are formed by isomerization of anisole [14]; they are further deoxygenated into toluene [13]. The main reaction product is phenol. Its selectivity increased under heating, to reach a maximum of 80% at 360°C. In the presence of the WP catalyst, the guaiacol conversion initially increased under heating, to reach a maximum of 86% at 340°C, after which it remained unchanged (Fig. 5b). Like in the MoP case, phenol was the main reaction product, the maximum phenol selectivity being 74% (at 380°C). Cresols and toluene were also identified as products, while no anisole was detected.
In the presence of both catalytic systems, the contribution of the reaction time to the guaiacol conversion rise was observed to be stronger at 340°C (Fig. 6). In the presence of both catalysts, the cresol selectivity was nearly independent of the reaction temperature; so was the toluene selectivity when catalyzed by MoP. Over the course of the reaction, the initial rise in the anisole selectivity was followed by its drop due to anisole conversion to phenol. After 6 h, anisole was detected only in the MoP catalyst case at 340°C. Over the entire time range and in the presence of both catalysts, the main reaction product was phenol. In the presence of MoP, the highest phenol selectivity (80%) was achieved after 6 h of reaction, whereas the WP catalyst provided the highest phenol selectivity (78%) as early as after 1 h.

CONCLUSIONS
In hydroprocessing of guaiacol at 320-380°C, the use of oil-soluble precursors, such as molybdenum and tungsten carbonyls and triphenylphosphine, promotes the formation of amorphous molybdenum and tungsten phosphides. The resultant phosphides exhibit catalytic activity in partial deoxygenation of guaiacol into phenol, anisole, and cresols and complete deoxygenation into toluene. Both in situ generated catalysts ensure selective production of phenol under various conditions. In the case of MoP, an extension of reaction time and temperature variations allow the phenol selectivity to be enhanced. In the case of WP, the reaction temperature has almost no effect on the phenol selectivity, and the highest selectivity is achievable within a lower reaction time.