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Article

Zeolitic Imidazolate Framework Decorated Molybdenum Carbide Catalysts for Hydrodeoxygenation of Guaiacol to Phenol

by
Jintu Francis Kurisingal
,
Shinjae Lee
,
Jun Gyeong Lee
and
Kwangjin An
*
School of Energy and Chemical Engineering, and Graduate School of Carbon Neutrality, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1605; https://doi.org/10.3390/catal12121605
Submission received: 7 November 2022 / Revised: 27 November 2022 / Accepted: 29 November 2022 / Published: 7 December 2022
(This article belongs to the Special Issue Catalytic Conversion of Biomass to Added Value Chemicals)
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Abstract

:
Bimetallic zeolitic imidazolate framework (BMZIF)-decorated Mo carbide catalysts were designed for the catalytic hydrodeoxygenation of guaiacol to produce phenol with high selectivity. A uniform layer of BMZIF was systematically coated onto the surface of the MoO3 nanorods. During carbonization at 700 °C for 4 h, BMZIF generated active species (ZnO, CoO) on highly dispersed N-doped carbons, creating a porous shell structure. Simultaneously, the MoO3 nanorod was transformed into the Mo2C phase. The resulting core@shell type Mo2C@BMZIF-700 °C (4 h) catalyst promoted a 97% guaiacol conversion and 70% phenol selectivity under 4 MPa of H2 at 330 °C for 4 h, which was not achieved by other supported catalysts. The catalyst also showed excellent selective cleavage of the methoxy group of lignin derivatives (syringol and vanillin), which makes it suitable for selective demethoxylation in future biomass catalysis. Moreover, it exhibits excellent recyclability and stability without changing the structure or active species.

Graphical Abstract

1. Introduction

For sustainable and green chemistry, the use of lignocellulosic biomass has received increasing attention owing to the increasing energy demand and environmental issues worldwide [1,2,3,4]. Lignocellulosic biomass primarily consists of cellulose (40–50%), hemicelluloses (20–30%), and lignin (18–28%). Despite the cross-linked complex and recalcitrant structure of lignin, aromatic building blocks make it a promising starting material for producing valuable aromatic compounds [5,6,7,8,9,10,11,12]. A feasible and effective route for lignin valorization is the depolymerization of lignin followed by hydrodeoxygenation (HDO) into lignin-derived phenolic compounds [13]. Guaiacol (2-methoxyphenol, C7H8O2), one of the main compounds in lignin-derived bio-oil, can be used in HDO reactions to produce value-added chemicals (Scheme 1a). Guaiacol contains three different types of C−O bonds: phenolic (Ar−OH) and methoxy (Ar−O−CH3) groups. Since product selectivity varies depending on how the C−O bond is broken, it is essential to develop highly selective HDO catalysts [14,15,16,17,18]. The phenolic group of guaiacol is known to be more thermodynamically stable than the methoxy group, indicating that the cleavage of Ar−OCH3 is more likely to produce phenol [19,20,21,22]. The production of phenol by selective HDO is highly valuable because of its high demand, rising market price, and low consumption of H2 for production. Phenol is used as a major industrial intermediate to produce pesticides and polymers. In particular, phenol has been used to produce adipic acid to synthesize industrially valuable polymers, such as nylon 6,6 [23,24,25].
Numerous studies have been conducted to develop effective catalysts with high conversion and high product selectivity in the HDO of guaiacol. These catalysts include noble metals [26,27], non-noble metals [28,29,30], metal sulfides [20], phosphides [31], nitrides [32], and carbides. Although noble metal catalysts have high conversion rates, most products are low-value saturated aromatics, such as cyclohexanol and cyclohexane, by complete hydrogenation of the aromatic ring with oxygen removal. Mo-based catalysts (MoO3, Mo2C, Mo2N, and MoS2) have shown excellent catalytic activity and selectivity in the HDO reaction of guaiacol to produce phenolic compounds [33,34,35,36,37,38,39,40,41]. The Roman–Leshkov group investigated the bond dissociation energy of the relevant phenolic C–O bonds of lignin-derived model compounds using MoO3. However, the MoO3-catalyzed HDO of guaiacol showed 29.3% selectivity towards phenol with a 97.5% conversion under low H2 pressure (1 bar) at 350 °C [42]. The Weckhuysen group reported phenol production from guaiacol over a sulfided CoMo/Al2O3 catalyst with a phenol selectivity of 34% at 300 °C and 50 bar of H2 [38]. They also reported Mo2C support on carbon nanofibers (CNFs) with enhanced phenol selectivity of more than 50% in 55 bars of H2 at 350 °C [43]. Chang et al. investigated various metal catalysts supported on carbon [44]. They found that the Mo/C catalyst exhibited the best selectivity toward phenol (76.5%) in the HDO of guaiacol at 40 bars of H2 at 400 °C. The partially reduced MoOx species were regarded as an active site for direct demethoxylation of guaiacol to form phenol. Cai et al. compared the HDO performance of guaiacol between activated carbon (AC)-supported MoO2 and Mo2C [45]. They found that MoO2/AC showed much higher selectivity toward phenol (72.0%) than Mo2C/AC (28.9%) in 30 bars of H2 at 300 °C. In contrast, Li et al. reported a graphite-encapsulated Mo-carbide core@shell catalyst (Mo2C@C), which showed 68.6% phenol selectivity in 28 bars of H2 at 300 °C [46]. The DeSisto and Escalona group investigated the HDO of guaiacol using various Mo2N-based catalysts. They revealed that the γ-Mo2N phase was the primary phase for the selective production of phenolic compounds [37,47,48].
Many previous studies confirmed that using Mo-based catalysts increases the selectivity of phenol production in the HDO reaction of guaiacol. Although the overall HDO yield of guaiacol increased as the reaction temperature and hydrogen pressure increased, the selectivity of phenol was significantly dependent on the type and preparation method of the catalyst. In the available literature, the debate about the active phase of the catalyst is still inconclusive. Recently, Baddour et al. reported transition-metal-modified β-Mo2C catalysts to alter the catalytic function in the HDO of guaiacol [49]. The HDO and hydrogenation performance was enhanced by tuning the H-site and acid-site densities by adding other metals to β-Mo2C. To further improve the performance of the existing Mo2C catalyst, research is being actively conducted to develop a bifunctional catalyst by adding other metals [50,51,52,53,54,55].
Recently, metal-organic framework (MOF)-derived porous carbon catalysts have attracted attention as highly dispersed metal-supported carbon catalysts because of their high specific surface areas, tunable compositions, and well-defined porous structure [15,56,57,58,59,60]. Zeolitic imidazolate frameworks (ZIFs) are a unique subclass of MOFs that can be directly converted into nitrogen (N)-doped graphitic carbon frameworks with a uniform distribution of metal/metal oxides by a simple pyrolysis process [61,62,63]. In particular, doping nitrogen atoms into the carbon network after carbonization of ZIFs not only increases the dispersion of active metals (or oxides) on the carbon, but also activates the deposited species [62]. Nitrogen-doped carbon induces charge delocalization of C atoms near N atoms, resulting in a highly positive charge density that is favorable for the adsorption of compounds containing nitro groups. Yan et al. reported that Co nanoparticles produced by calcination of Zn/Co-ZIF-9 were activated by N-doped carbons, because nitrogen atoms can act as electron withdrawing groups that change the electron density of the metal centers [62]. The electron transfer from Co nanoparticles to N-doped carbon further enhanced the interaction between Co and carbon. Bimetals or multimetals derived from ZIFs can further improve catalytic performance than single metals [63]. Kui et al. reported that mixed metal Zn/Co-ZIFs had improved physical and chemical properties compared to single metal ZIFs [63]. They found that Zn/Co-ZIFs with a sodalite pore structure similar to ZIF-8 and ZIF-67 had better CO2 conversion to cyclic carbonates as well as improved surface area and thermal stability compared to single-metal ZIFs (ZIF-8 and ZIF-67). ZIF-8 has a microporous structure and is easily transformed into carbon with a large surface area and uniformly distributed nitrogen after pyrolysis. However, ZIF-8 has disadvantages, such as a low degree of graphitization. Contrastingly, ZIF-67 exhibited a well-developed mesoporous structure but low nitrogen content and low specific surface area after carbonization. Thus, porous N-doped carbon catalysts derived from the combination of Zn-based ZIF-8 and Co-based ZIF-67 exhibit synergetic properties that combine the advantages of both ZIFs [64,65,66,67]. Hence, the resulting bimetallic ZIFs (BMZIFs) are expected to enhance the catalytic performance by utilizing two or more metal species that are highly dispersed in a well-defined carbon structure.
Inspired by the advantages of BMZIF, we designed BMZIF-decorated Mo carbide catalysts (Mo2C@BMZIF, MoO3@BMZIF) for the HDO of guaiacol to produce phenol at high conversion and selectivity. A uniform layer of BMZIF was controllably coated on the surface of MoO3 nanorods. During controlled carbonization at different temperatures, the structure of BMZIFs was changed to metal oxide (ZnO and CoO) on highly dispersed N-doped carbons, while the MoO3 nanorods were transformed into the Mo2C phase, creating a core@shell structure (Scheme 1b) [67]. By comparing the pristine and monometallic catalysts, the synergetic catalytic performance of Mo2C@BMZIF was demonstrated to produce phenol with high selectivity in the catalytic HDO of guaiacol.

2. Results and Discussion

2.1. Characterization of Catalysts

A BMZIF-based N-doped carbon composite was produced by the carbonization of BMZIF-coated MoO3 nanorods (Scheme 1b). Highly crystalline MoO3 nanorods with well-defined structures were synthesized via a simple hydrothermal method [68]. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images show a very smooth surface, and a rectangle-like cross-section of the as-prepared MoO3 nanorods with an average width of ca. 250 nm (Figure 1a,d). Then, the synthesized MoO3 nanorods were decorated with ZIF to produce an MoO3@ZIF core@shell structure. When two metal nitrate salts (20 molar ratio of Zn2+/Co2+) and 2-methylimidazole as an organic linker were mixed with MoO3 nanorods in the presence of polyvinylpyrrolidone (PVP), a MoO3@BMZIF core@shell structure was generated [67]. As shown in Figure 1b,e, FE-SEM and TEM images confirm the core@shell structure of the MoO3@BMZIF with a porous shell on the surface of the MoO3 nanorod. By carbonization at high temperatures (700–800 °C) under an N2 atmosphere, the core MoO3 nanorods were converted to Mo2C. In contrast, the ZIF shell was transformed into a porous N-doped carbon with highly dispersed ZnO and CoO particles. Figure 1c,f show that the distinct core@shell structure of the original MoO3@BMZIF was preserved even after high-temperature calcination at 700 °C for 4 h (denoted as Mo2C@BMZIF-700 °C (4 h)). In addition, it was confirmed that the rough, porous shell was transformed into a softer, curved surface through the carbonization process.
X-ray diffraction (XRD) analysis was used to confirm the structural evolution of MoO3@BMZIF to Mo2C@BMZIF. As-synthesized MoO3 nanorods showed a typical XRD pattern of MoO3 (JCPDS No. 89–5108) with high intensity, confirming the successful formation of the crystalline MoO3 phase (Figure 2a and Figure S1). When metal precursors containing Zn and Co were mixed with MoO3 nanorods in the presence of 2-methylimidazole, BMZIF shells were formed around the MoO3 surface. During carbonization at 700 °C, an ordered ZIF structure was formed and attached to the MoO3 surface. Depending on the carbonization time at 700 °C, the structural transformations in the core@shell composite were identified using XRD and TEM analyses. The XRD peaks in the range of 5−40 sharpened as the carbonization time of MoO3@BMZIF increased from 0.25 to 1 h, indicating that a well-defined ZIF structure was formed (Figure 2a). When the reaction time was increased beyond 5 h, the TEM results showed an overgrowth of BMZIF on the surface of the MoO3 nanorods. The well-covered BMZIF shells were detached from the MoO3 surface and collapsed to form aggregated clusters (Figure S2a–c). After carbonization at 700 °C for 4 h under N2 atmosphere, the produced Mo2C@BMZIF-700 °C (4 h) showed a distinct XRD peak at 34.5°, 39.5°, and 52.2°. These peaks correspond to the (002), (211), and (221) planes of orthorhombic Mo2C (JCPDS no. 77–0720) by eliminating the original MoO3 peaks (Figure 2b) [67]. In addition, the BMZIF shell was transformed into oxide species, such as ZnO and CoO, while the organic framework of the ZIF was converted into an N-doped carbon structure (Table S1). In addition to the Mo2C peaks, the XRD patterns also detected diffractions for metal oxides as active sites, such as ZnO and CoO, which were derived from the BMZIF.
Along with TEM and XRD analyses, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping results further demonstrated the well-defined core@shell structure of Mo2C@BMZIF with ZnO and CoO species dispersed in N-doped carbon and the transformed Mo2C nanorod core. Before carbonization, the pristine MoO3@BMZIF nanorods showed a well-defined core@shell structure with an outer ZIF shell containing a homogeneous distribution of Co and Zn species, as well as the MoO3 core (Figure S3). During carbonization, the internal MoO3 nanorods were reduced to form MoO2 in the presence of carbon in the following fashion (Equation (1)) [67]:
MoO3 + C → MoO2 + COx
As the reaction proceeded, MoO2 was further carbonized to create Mo2C (Equation (2)) [67]:
MoO2 + C → Mo2C + COx
Tian et al. reported a similar structural evolution of MoO3@BMZIF to Mo2C@BMZIF nanorods by carbonization [67]. The HR-TEM image of Mo2C@BMZIF-700 °C (4 h) shows lattice fringes of Mo2C with a lattice space of 0.22 nm, which is assigned to the (211) plane of orthorhombic Mo2C (Figure 3a). Figure 3b–d further demonstrate that the BMZIF shell generates ZnO and CoO nanoparticles with a lattice space of 0.33 nm and 0.25 nm, corresponding to the (002) and (111) planes, respectively [67]. The EDS mapping results show the core@shell structure of Mo2C@BMZIF-700 °C (4 h) with a uniform elemental distribution of Zn, Co, N, O, and C, which originated from the BMZIF shell coated on the core MoO3 (Figure 3e). It was also confirmed that the core@shell structure of Mo2C@BMZIF-700 °C (4 h) was maintained after carbonization at 700 °C for 4 h. The ZIF shell produced bimetallic elements of both ZnO and CoO, homogeneously dispersed in N-doped carbon. The change in porosity during carbonization was confirmed by N2 adsorption–desorption isotherms of MoO3@BMZIF before and after carbonization (Figure S4). The as-prepared MoO3@BMZIF exhibits a higher Brunauer–Emmett–Teller (BET) surface area of 1033 m2 g−1 and a total pore volume of 0.48 cm3 g−1 estimated at p/p0 = 0.99 (Figure S4a). In contrast, the N2 adsorption–desorption isotherm of Mo2C@BMZIF-700 °C (4 h) shows a type-IV hysteresis loop, revealing its mesoporous nature (Figure S4b). The measured BET surface area of Mo2C@BMZIF-700 °C (4 h) is 107 m2 g−1, a considerably decreased value compared to MoO3@BMZIF without carbonization. This indicates that the BMZIF has a reduced surface area due to shrinkage during carbonization at 700 °C; however, its porous nature is still maintained.
X-ray photoelectron spectroscopy (XPS) was performed to investigate the active species and their surface states. The main XPS spectrum of Mo2C@BMZIF-700 °C (4 h) verifies the presence of Mo, C, N, O, Zn, and Co elements (Figure 4a). To compare the Mo 3d spectrum of Mo2C@BMZIF-700 °C (4 h), pure Mo2C nanorods were prepared by carbonization at 700 °C for 4 h without adding ZIF. The Mo 3d spectra are split into 3d5/2 and 3d3/2 peaks because of the spin-orbital coupling effect (Figure 4b) [69,70]. For the converted Mo2C@BMZIF-700 °C (4 h), the shoulder of Mo 3d5/2 with a binding energy of 228.6 ± 0.1 eV can be attributed to Mo2+ species, which do not appear in MoO3 nanorods. On the other hand, the Mo 3d5/2 peak at 229.8 eV ± 0.1 eV is characteristic of Mo4+, indicating that the surface of Mo2C@BMZIF-700 °C (4 h) is highly oxidized to have both Mo4+ and Mo2+ species [67,71,72,73]. It is worth noting that the Mo2+ 3d5/2 (228.6 ± 0.1 eV) and Mo4+ 3d5/2 (229.8 ± 0.1 eV) peaks of Mo2C@BMZIF-700 °C (4 h) shift to lower binding energies compared to those of the bare Mo2C nanorod (Figure 4b) [72]. This result demonstrated an interaction between the metal atoms from the shell (ZnO and CoO) and the Mo atoms of the core nanorod. The deconvoluted Mo 3d peak reveals two oxidation states of Mo (Mo2+ and Mo4+) on the surface of Mo2C@BMZIF-700 °C (4 h) (Figure 4c) [73]. Although the original MoO3 nanorods display only Mo4+, characteristic XPS peaks corresponding to Mo2+ appear only in the converted Mo2C nanorods [67]. In addition, the Zn 2p3/2 peak of the Mo2C@BMZIF-700 °C (4 h) with a binding energy of 1021.81 eV shows a shift to higher binding energy compared to pure ZnO(II) (1021 eV) (Figure S5a). Similarly, the Co 2p3/2 peak with a binding energy of 781.57 eV shows a shift to a higher binding energy compared to pure CoO (780 eV) (Figure S5b). These results further demonstrate the interaction between the metals in the shell (ZnO and CoO) and Mo atoms in the core nanorods. The presence of nitrogen in the Mo2C@BMZIF-700 °C (4 h) is caused by the pyrolysis of the 2-methylimidazole organic linker. Essentially, the methylimidazole organic linker plays a vital role in generating N-doped carbon and homogeneously dispersing ZnO and CoO nanoparticles on the Mo2C@BMZIF-700 °C (4 h). Figure 4d shows the N 1s spectrum fitted into three peaks, corresponding to pyridinic nitrogen (398.6 eV), pyrrolic nitrogen (399.1 eV), and graphitic nitrogen (400.9 eV), revealing the production of N-doped carbons [73,74,75].

2.2. Hydrodeoxygenation of Guaiacol over Mo2C@BMZIF Catalysts

The catalytic HDO reaction of guaiacol was performed under optimized reaction conditions (330 °C, 4 MPa of H2 for 4 h). Different Mo2C@BMZIF catalysts produced at different carbonization temperatures and times were used. The uncatalyzed HDO reaction exhibited a very low guaiacol conversion. Figure 5a shows the guaiacol conversion, selectivity, and product distributions over Mo2C@BMZIF-700 °C (2 h), Mo2C@BMZIF-700 °C (4 h), and Mo2C@BMZIF-800 °C (2 h). For all the tested catalyzed reactions, phenol was the main product produced from guaiacol. Among the catalysts, Mo2C@BMZIF-700 °C (4 h) showed the best conversion of guaiacol (97%) and the highest phenol selectivity (70%). Mo2C@BMZIF-700 °C (2 h) and Mo2C@BMZIF-800 °C (2 h) catalysts showed 82% and 83% conversion, respectively. The guaiacol conversion and phenol selectivity proved to be highly affected by the distribution of active species (MoO3, Mo2C, ZnO, and CoO), which were generated by different carbonization processes at different temperatures and times. The role of each active species is discussed in the next section. The most effective and active catalyst for the HDO of guaiacol was Mo2C@BMZIF-700 °C (4 h). The least active catalyst had not only a lower guaiacol conversion but also a lower selectivity for phenol, causing the reaction to produce more anisole and cyclohexene as by-products.
The effects of H2 pressure (2–5 MPa), reaction temperature (300–350 °C), and time (1–4 h) were investigated over Mo2C@BMZIF-700 °C (4 h) since it was the most effective catalyst (Figure 5a). Figure 5b shows that the guaiacol conversion and phenol selectivity increased linearly as the H2 pressure was increased from 2 to 5 MPa at 330 °C. The highest phenol conversion (97%) and selectivity (70%) were obtained under 4 MPa of H2 pressure. Furthermore, increasing the H2 pressure to 5 MPa converted 100% of the guaiacol but had lower selectivity to phenol (62%).
As expected, the overall conversion increased linearly with the reaction temperature under the optimum H2 pressure of 4 MPa and reaction time of 4 h (Figure 5c). The phenol selectivity increased from 50% to 70% as the reaction temperature increased from 300 to 330 °C. A further increase in the reaction temperature to 350 °C resulted in the complete conversion of guaiacol. However, the selectivity towards phenol decreased substantially because further hydrogenation occurred, thus, producing cyclohexane at the expense of phenol. Therefore, continuously increasing the reaction temperature above 350 °C is not recommended for the Mo2C@BMZIF-700 °C (4 h) catalyzed HDO reaction because it promotes the hydrogenation of the aromatic ring and reduces the selectivity of phenol [76,77].
The influence of reaction time was investigated at a reaction temperature of 330 °C and an H2 pressure of 4 MPa, since these were the optimum reaction conditions. An increase in the guaiacol conversion was observed upon increasing the reaction time (Figure 5d). When the reaction was run for 1 h, a lower guaiacol conversion (43%) was obtained, while phenol selectivity was 76%. However, when the reaction time was increased to 4 h, the reaction showed the highest guaiacol conversion of 97%, and the phenol selectivity was reduced to 70%. It was deduced that the hydrogenation reaction conversion increased with increased reaction time but slightly decreased the phenol selectivity due to the formation of cyclohexane. These outcomes were also consistent with the findings of various researchers [78,79].
To understand the role of each active species, we prepared various supported catalysts and monometallic catalysts with and without Mo-based nanorods. When Mo-based nanorods were solely used without ZIF coating, the guaiacol conversion of MoO3 and Mo2C-700 °C (4 h) was 55% and 38%, respectively (Figure 6). Although MoO3 shows higher conversion than Mo2C in the present condition, both catalysts showed high catalytic selectivity toward phenol (>60%), demonstrating that Mo-based catalysts are suitable for high phenol production [18,80,81,82,83,84,85]. Both phases can further enhance guaiacol conversion by depositing additional metal species. When a Co or Zn precursor was impregnated on the Mo2C nanorods, which were carbonized at 700 °C for 4 h under an N2 atmosphere, the resulting Mo2C/Co-700 °C (4 h) and Mo2C/Zn-700 °C (4 h) showed higher conversion of guaiacol than the Mo2C-700 °C (4 h) nanorod (Figure 6). When two single metallic catalysts were physically mixed (Mo2C/Co+Zn-700 °C (4 h)) to be used for the HDO reaction, the conversion of guaiacol (70%) was higher than that of the individual catalysts. However, when two metallic precursors were formed as a BMZIF shell, the Mo2C@BMZIF-700 °C (4 h) catalyst exhibited the best performance. This outcome may be due to the uniform distribution of Co and Zn metals over the entire N-doped carbon network (Figure 6). Therefore, it was concluded that the enhanced catalytic performance of the Mo2C@BMZIF-700 °C (4 h) catalyst in the HDO of guaiacol for the selective production of phenol resulted from the synergistic effect of well-defined ZnO and CoO species dispersed in porous N-doped carbon and the core Mo2C nanorod. The ZnO and CoO species produced by the BMZIF shell were in intimate contact with the Mo2C surface. When Mo, Co, and Zn oxides were deposited on AC by impregnation, the resulting Mo+Co+Zn/AC catalyst showed an 82% guaiacol conversion while the phenol selectivity was 58% (Figure 6). Likewise, the AC-supported single metal oxide catalysts (Zn/AC and Co/AC) exhibited poor conversion (32% for Zn/AC and 28% for Co/AC) and phenol selectivity of less than 40%. From these results, it can be deduced that the well-defined Mo2C@BMZIF-700 °C (4 h) has distinct structural and catalytic properties. The BMZIF shells provide carbon matrices with nitrogen originating from the pyrolysis of organic linkers. The N-doped carbon anchors ZnO and CoO species to guarantee high dispersion in the porous carbon structure. It has been reported that graphitic nitrogen can donate extra free electrons to anchor active metal species, and pyrrolic and pyridinic nitrogen also provide more active sites [68]. The highly dispersed and stable ZnO and CoO species on porous N-doped carbon provide additional activity to the Mo2C nanorods. When the metals are supported on the surface of the Mo2C nanorods, the Zn and Co species are easily aggregated because the low surface area of Mo2C nanorods cannot effectively and stably disperse the metal. The TEM image of the impregnated Mo2C/Zn+Co-700 °C (4 h) catalyst shows aggregated metal species, which may have caused a decrease in catalytic activity (Figure S6a). Furthermore, the synergetic structural and catalytic effects of Zn, Co, and Mo species in the AC-supported catalyst Mo+Co+Zn/AC were not observed (Figure S6b).
The synergistic effect of the Mo2C@BMZIF-700 °C (4 h) catalyst derived from well-defined ZnO and CoO species dispersed in porous N-doped carbon in intimate contact with the core Mo2C nanorod was further confirmed by XPS results. In Figure 4b, the binding energies of the Mo2+ 3d5/2 and Mo4+ 3d5/2 peaks of Mo2C@BMZIF-700 °C (4 h) shifted to a lower binding energy as compared to the pristine Mo2C nanorod. The shift in the binding energy of the Mo spectra at Mo2C@BMZIF-700 °C (4 h) indicates a change in the electronic structure of Mo [86]. Tran et al. [87] reported that the shift in the binding energies of W and Mo in the MoWC catalyst could be attributed to the change in the electron density of W and Mo atoms. The chemical shift shown in XPS originates from the change in electron density due to the interaction between Mo and W atoms, enabling electron transfer from W to Mo. Liu et al. [88] reported that the valence distribution of Mo in the 20Cu20Mo2C/M41 catalyst was affected by the presence of Cu atoms. The XPS deconvolution results for Mo confirmed four molybdenum species in the 20Cu20Mo2C/M41 catalyst. From these results, it is speculated that the shift in the binding energy in the XPS profile is due to the charge transfer between the Mo atom of the core and the Zn and Co atoms of the shell consisting of N-doped carbon [86,89,90]. The collective interaction of well-dispersed Mo, Co, and Zn metal atoms, as well as the N atom in the porous carbon, synergistically promotes high phenol selectivity in the HDO of guaiacol, which was not achieved by other supported catalysts. The well-defined core@shell structure consisting of Mo2C@BMZIF nanorods with ZnO and CoO species dispersed in the N-doped porous carbon shell and the Mo2C nanorod core satisfied all these requirements to improve the guaiacol conversion and phenol selectivity.
To evaluate whether the Mo2C@BMZIF-700 °C (4 h) catalyst can be applied to the HDO reaction of various lignin derivatives other than guaiacol, we performed a catalytic HDO reaction using syringol and vanillin (Table S2) [85]. The HDO reaction using syringol showed an 82% conversion under the optimized reaction conditions (i.e., 330 °C and 4 MPa of H2 for 4 h). Syringol with two methoxy groups was deoxygenated into guaiacol and phenol, depending on the number of methoxy groups lost by H2. Although the HDO selectivity varied with reaction times, guaiacol and phenol were still the significant products, demonstrating the high HDO selectivity of the Mo2C@BMZIF-700 °C (4 h) catalyst (Table S2) [85]. For example, the guaiacol selectivity decreased from 36 to 12%, while that of phenol increased from 18 to 46% with an increase in reaction time (2–4 h). This result was attributed to the conversion of guaiacol to phenol, which improved the overall conversion from 60 to 82% due to the loss of two methoxy groups by the full HDO. When vanillin was used as a substrate for HDO, a 100% conversion was achieved with 79% toluene selectivity due to the selective HDO reaction by the Mo2C@BMZIF-700 °C (4 h) catalyst. The experiments confirmed that a well-designed Mo2C@BMZIF catalyst could be applied to the HDO reaction of various lignin derivatives.
The recyclability of the Mo2C@BMZIF-700 °C (4 h) catalyst was tested by three consecutive cycles of the guaiacol HDO reaction in 4 MPa of H2 at 330 °C for 4 h. There was no apparent loss of guaiacol conversion, even after three cycles (Figure 7a). In XRD, the spent Mo2C@BMZIF-700 °C (4 h) catalyst showed no change in the crystalline structure after three catalytic cycles (Figure 7b). In addition, the original morphology was preserved, as confirmed by FE-SEM and TEM images (Figure 7c,d), demonstrating the reusability and recyclability of the Mo2C@BMZIF catalyst without changing its structure or active species.

3. Materials and Methods

3.1. Chemicals

Ethanol (anhydrous 99.9%) and HNO3 (60%) were purchased from Samchun Chemicals. Ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), polyvinylpyrrolidone (300 K), Zn(NO3)2·6H2O, Co(NO3)2·6H2O, 2-methylimidazole, methanol (99.8%), and guaiacol (99%) were purchased from Sigma Aldrich.

3.2. Preparation of MoO3 Nanorods

MoO3 nanorods were synthesized by following a previously reported hydrothermal reaction [71]. Briefly, 1.4 g of ammonium heptamolybdate tetrahydrate was dissolved in a mixture of 33 mL of 60% HNO3 and 7 mL of deionized water. The solution was then heated at 200 °C for 20 h in a Teflon-lined stainless steel autoclave. The obtained product was washed with ethanol 2–3 times and dried at 80 °C overnight.

3.3. Preparation of Composites and core@Shell Nanorods

MoO3@BMZIF and Mo2C@BMZIF were prepared according to a previously reported method with slight modifications [70]. To decorate BMZIF shells on the surface of MoO3, 0.15 g of synthesized MoO3 nanorods (Section 2.1) were dispersed into 30 mL of methanol containing 0.5 g of polyvinylpyrrolidone (300 K). After vigorous stirring for 30 min, 15 mL of methanol solution containing 1.06 g of Zn(NO3)2·6H2O and 0.052 g of Co(NO3)2·6H2O were slowly added to the mixed solution and stirred for 2 h. Then, 2.5 g of 2-methylimidazole dissolved in 45 mL of methanol was slowly added to the above solution, followed by vigorous stirring for 30 min. The final product of MoO3@BMZIF was collected by centrifugation, washed with methanol 2–3 times, and dried at 60 °C overnight. The as-prepared MoO3@BMZIF was carbonized at 700–800 °C under N2 atmosphere for 2–4 h at a heating rate of 5 °C min−1 to obtain Mo2C@BMZIF core@shell nanorods.

3.4. Preparation of Reference Catalysts

To identify the active sites of the Mo2C@BMZIF core@shell nanorods, various reference catalysts were prepared with and without Mo-based nanorods, as well as monometallic-supported catalysts. To prepare monometallic ZIF-coated Mo2C nanorods, the as-synthesized MoO3 was slowly added to the Zn or Co precursor solution (the required number of metals in 5 mL of deionized water) in a similar manner to MoO3@BMZIF. After stirring at 25 °C for 2 h, 2-methylimidazole in methanol was added, then the solid product was separated and dried at 80 °C. Carbonization was then carried out to generate monometallic species, such as ZnO or CoO dispersed in N-doped carbon at 500 °C for 2 h under N2 flow. The obtained catalyst was further calcined at 700 °C for 4 h under 5% H2 to produce Mo2C@Zn-700 °C (4 h) and Mo2C@Co-700 °C (4 h). AC-supported catalysts were prepared using the impregnation method. Briefly, 1 g of AC (Vulcan XC 72R) was slowly added to the metal precursor solution (the required amount of Zn(NO3)2·6H2O (0.069 g) and/or Co(NO3)2·6H2O (0.091 g) in 5 mL of deionized water). After stirring at 25 °C for 2 h and drying at 80 °C for 4–6 h, calcination was carried out to impregnate the metal on AC at 500 °C for 2 h under N2 flow. The obtained catalyst was further calcined at 700 °C for 4 h under 5% H2 to produce the Zn/AC and Co/AC catalysts. When three precursors were added to AC, the Mo+Co+Zn/AC mixture catalyst was produced using the same method. The bimetallic-supported catalyst, Mo2C@Zn+Co-700 °C (4 h), was prepared by using the calcined Mo2C nanorods at 700 °C for 4 h by impregnating Zn and Co precursors. The total loading of Zn+Co to the Mo2C was 3.2 wt %.

3.5. Catalyst Characterization

The structural characterization of the prepared catalysts was performed using FE-SEM (Hitachi S-4800 microscope) and TEM (JEOL, JEM-2100F) with EDS (Oxford instrument, X-Max 80T), N2 physisorption by the BET (BELSorp-max) method, and powder XRD (PANalytical X’Pert Pro) using a Cu Kα radiation source (λ = 0.154056 nm). The surface chemical properties were characterized using the XPS (Thermo Fisher system) equipped with an Al Kα radiation source. The composition of metals in the catalyst was determined by inductively coupled plasma–optical emission spectrometry (ICP–OES, Varian, 700-ES). The concentration of Co (or Cu) was calibrated using a multistandard solution (inorganic ventures). The digestion of the catalyst was carried out by dissolving the catalyst in aqua regia for 3 days. The solution was diluted with DI water and filtered to remove the carbon support before measurement.

3.6. Reaction Procedure and Product Analysis

For the standard HDO reaction of guaiacol, 0.12 g of guaiacol was dispersed in 20 ml of n-decane, and 0.06 g of the catalyst was added to the mixture. This mixture was added to an autoclave reactor with a 100-mL inner volume. After purging the reactor with H2 gas, 4 MPa of H2 was added to the reactor. The reactor was heated to 330 °C; aliquots were collected from the solution every 2 h. For the controlled tests, the reaction temperature, H2 pressure, and reaction time varied from 300 to 350 °C, 2–5 MPa, and 1–4 h, respectively. After removing the solid from the mixture, the substrate conversion and product selectivity were quantified using GC FID. For the recycling test, the HDO reaction of guaiacol was carried out at the same initial condition. After each reaction, the catalyst was separated by filtration from the reaction mixture, washed with ethanol, and dried in the air at 100 °C for 2 h. Then the recovered catalyst was used for the next run. The mass balance of the catalytic result was approximately 94%, which was determined by comparing the conversion calculated from guaiacol loss to the products produced [91]. Carbon balance was calculated by Σ (carbon number × nproduct/nGUA × 7) and was approximately 96%.

4. Conclusions

We designed a BMZIF-decorated Mo carbide catalyst to develop a high-selectivity catalyst to produce phenols from HDO derived from guaiacol. When BMZIF was coated on the surface of the MoO3 nanorods, BMZIF-generated metal oxide (ZnO and CoO) species dispersed on N-doped carbon shells, whereas the core MoO3 nanorods were transformed into Mo2C. Different types of Mo2C@BMZIF core@shell nanorods were produced by controlling the carbonization temperature and time. The Mo2C@BMZIF-700 °C (4 h) catalyst produced by carbonization at 700 °C for 4 h, showed the highest guaiacol conversion (97%) and the highest phenol selectivity (70%) under the reaction condition of 330 °C, 4 h, 4 MPa using 60 mg of catalyst and 0.12 g of guaiacol in 20 mL n-decane. The improved catalytic performance of the Mo2C@BMZIF-700 °C (4 h) catalyst was attributed to the synergistic effect of the core Mo2C nanorod and the coated porous N-doped carbon shell containing highly dispersed ZnO and CoO species. It was found that the BMZIF shell generated the ZnO and CoO species in close contact with the Mo2C surface. The binding energy shift in the XPS profile demonstrated the charge transfer between the core Mo atom and the Zn and Co atoms of the shell consisting of N-doped carbon. The collective interaction of well-defined Mo, Co, and Zn metal atoms, as well as the N atom dispersed in the porous carbon, synergistically promotes the HDO performance of guaiacol with high phenol selectivity, which was not achieved by other supported catalysts. The Mo2C@BMZIF-700 °C (4 h) catalyst was also applied to the HDO reactions of syringol and vanillin as well as other lignin derivatives. The selective demethoxylation properties of Mo2C nanorods were further enhanced by the highly dispersed ZnO and CoO species on the porous N-doped carbon. The developed Mo2C@BMZIF catalyst has been proven to have excellent reusability and recyclability without changing the structure or active species. The unique core@shell structure of the Mo2C@BMZIF catalyst and its synergetic catalytic properties can be further applied to develop new HDO catalysts with high selective demethoxylation ability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121605/s1, Figure S1: XRD of as-synthesized MoO3 nanorods; Figure S2: TEM images of BMZIF-coated MoO3 nanorods obtained by different calcination time at 700 °C under N2 atmosphere: (a) 1, (b) 5, and (c) 24 h; Figure S3: A TEM image and corresponding elemental EDS maps of as-prepared MoO3@BMZIF nanorods without carbonization; Figure S4: N2 adsorption-desorption isotherms of (a) as-prepared MoO3@BMZIF and (b) Mo2C@BMZIF-700 °C (4 h) core@shell nanorods; Figure S5: XPS profiles of Mo2C@BMZIF-700 °C (4 h) showing deconvoluted (a) Zn 2p and (b) Co 2p spectra; Figure S6: (a) A TEM image of Mo2C/Zn+Co-700 °C (4 h) and (b) XRD pattern of Mo+Co+Zn/AC catalyst; Table S1: Quantitative elemental analysis of ZIF-decorated Mo carbide nanorods with a core@shell structure by ICP-OES measurement; Table S2: Catalytic HDO results of syringol and vanillin over BMZIF@Mo2C-700 °C (4 h).

Author Contributions

Conceptualization, methodology, experiment, data curation, J.F.K.; analysis, data collection, visualization, J.F.K., S.L. and J.G.L.; writing—original draft preparation, J.F.K. and S.L.; supervision, funding acquisition, writing—review and editing, K.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Basic Science Research Program (2021R1A2C2006713), the Engineering Research Center of Excellence Program (2020R1A5A1019631), the Climate Environment R&D Program (2022M3J1A1052840), the Regional Innovation Strategy (RIS) (2021RIS-003) of the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT, the Ministry of Education (MOE), and the Technology Innovation Program (20012971, 20010853) by the Ministry of Trade, Industry & Energy (MOTIE), and UNIST (1.220029.01).

Acknowledgments

This research was supported by the Basic Science Research Program (2021R1A2C2006713), the Engineering Research Center of Excellence Program (2020R1A5A1019631), the Climate Environment R&D Program (2022M3J1A1052840), the Regional Innovation Strategy (RIS) (2021RIS-003) of the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT, the Ministry of Education (MOE), and the Technology Innovation Program (20012971, 20010853) by the Ministry of Trade, Industry & Energy (MOTIE), and UNIST (1.220029.01).

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. (a) Schematic representation of the possible reaction pathways and products by catalytic HDO of guaiacol. (b) Schematic illustration of the synthesis of ZIF-decorated Mo carbide nanorods with a core@shell structure.
Scheme 1. (a) Schematic representation of the possible reaction pathways and products by catalytic HDO of guaiacol. (b) Schematic illustration of the synthesis of ZIF-decorated Mo carbide nanorods with a core@shell structure.
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Figure 1. Structural change in ZIF-decorated Mo carbide nanorods. (a–c) FE-SEM and (d–f) TEM images of (a,d) as-prepared MoO3 nanorods, (b,e) MoO3@BMZIF composite, and (c,f) Mo2C@BMZIF-700 °C (4 h) core@shell structure after carbonization under an N2 atmosphere at 700 °C for 4 h.
Figure 1. Structural change in ZIF-decorated Mo carbide nanorods. (a–c) FE-SEM and (d–f) TEM images of (a,d) as-prepared MoO3 nanorods, (b,e) MoO3@BMZIF composite, and (c,f) Mo2C@BMZIF-700 °C (4 h) core@shell structure after carbonization under an N2 atmosphere at 700 °C for 4 h.
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Figure 2. XRD patterns of (a) as-prepared MoO3 nanorods and MoO3@BMZIF catalysts obtained at different reaction times (0.25–24 h), and (b) Mo2C@BMZIF-700 °C (4 h).
Figure 2. XRD patterns of (a) as-prepared MoO3 nanorods and MoO3@BMZIF catalysts obtained at different reaction times (0.25–24 h), and (b) Mo2C@BMZIF-700 °C (4 h).
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Figure 3. (a–d) HR-TEM images of Mo2C@BMZIF-700 °C (4 h) showing Mo2C, ZnO, and CoO phases and (e) corresponding elemental EDS maps of Mo2C@BMZIF-700 °C (4 h).
Figure 3. (a–d) HR-TEM images of Mo2C@BMZIF-700 °C (4 h) showing Mo2C, ZnO, and CoO phases and (e) corresponding elemental EDS maps of Mo2C@BMZIF-700 °C (4 h).
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Figure 4. (a) XPS profiles of Mo2C@BMZIF-700 °C (4 h). (b) XPS Mo 3d spectra compared to a pure Mo2C-700 °C (4 h) nanorod and the Mo2C@BMZIF-700 °C (4 h) core@shell structure. XPS (c) Mo 3d and (d) N 1s spectra of Mo2C@BMZIF-700 °C (4 h).
Figure 4. (a) XPS profiles of Mo2C@BMZIF-700 °C (4 h). (b) XPS Mo 3d spectra compared to a pure Mo2C-700 °C (4 h) nanorod and the Mo2C@BMZIF-700 °C (4 h) core@shell structure. XPS (c) Mo 3d and (d) N 1s spectra of Mo2C@BMZIF-700 °C (4 h).
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Figure 5. Conversion and product selectivity of catalytic HDO of guaiacol over Mo2C@BMZIF catalysts as a function of (a) carbonization condition of the catalyst, (b) reaction pressure, (c) temperature, and (d) time (a standard reaction condition: catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa). Mo2C@BMZIF-700 °C (4 h) catalyst was used for b–d.
Figure 5. Conversion and product selectivity of catalytic HDO of guaiacol over Mo2C@BMZIF catalysts as a function of (a) carbonization condition of the catalyst, (b) reaction pressure, (c) temperature, and (d) time (a standard reaction condition: catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa). Mo2C@BMZIF-700 °C (4 h) catalyst was used for b–d.
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Figure 6. Comparison of conversion and product selectivity of catalytic HDO of guaiacol over various catalysts (reaction condition: catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa).
Figure 6. Comparison of conversion and product selectivity of catalytic HDO of guaiacol over various catalysts (reaction condition: catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa).
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Figure 7. (a) Catalytic recyclability test for 4 runs (catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa), (b) XRD, (c) TEM, and (d) FE-SEM results of the spent Mo2C@BMZIF-700 °C (4 h) catalyst after 4 runs.
Figure 7. (a) Catalytic recyclability test for 4 runs (catalyst 60 mg, guaiacol 0.12 g, n-decane 20 mL, 330 °C, 4 h, 4 MPa), (b) XRD, (c) TEM, and (d) FE-SEM results of the spent Mo2C@BMZIF-700 °C (4 h) catalyst after 4 runs.
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Kurisingal, J.F.; Lee, S.; Lee, J.G.; An, K. Zeolitic Imidazolate Framework Decorated Molybdenum Carbide Catalysts for Hydrodeoxygenation of Guaiacol to Phenol. Catalysts 2022, 12, 1605. https://doi.org/10.3390/catal12121605

AMA Style

Kurisingal JF, Lee S, Lee JG, An K. Zeolitic Imidazolate Framework Decorated Molybdenum Carbide Catalysts for Hydrodeoxygenation of Guaiacol to Phenol. Catalysts. 2022; 12(12):1605. https://doi.org/10.3390/catal12121605

Chicago/Turabian Style

Kurisingal, Jintu Francis, Shinjae Lee, Jun Gyeong Lee, and Kwangjin An. 2022. "Zeolitic Imidazolate Framework Decorated Molybdenum Carbide Catalysts for Hydrodeoxygenation of Guaiacol to Phenol" Catalysts 12, no. 12: 1605. https://doi.org/10.3390/catal12121605

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