Cobalt-modified molybdenum carbide as a selective catalyst for hydrodeoxygenation of furfural
Graphical abstract
Introduction
The catalytic upgrading of biomass into chemicals and fuels is a promising method to address the current energy and environmental challenges [[1], [2], [3], [4], [5]]. Many biomass-derived molecules contain excess oxygen, which leads to low energy density and undesired reactivity to produce byproducts [6] when used directly in the fuel and chemical sectors. In order to convert these molecules into more valuable products, the excess oxygen needs to be selectively removed via the hydrodeoxygenation (HDO) reaction without carbon-carbon bond scission. Among the biomass derivatives, furfural is a promising platform chemical that can be produced from the dehydration of xylose, a derivative from hemicellulose [7]. Furfural is an example of the over-functionalized molecules that can be upgraded efficiently with the HDO reaction.
In this work, we focused on the conversion of furfural into 2-methylfuran (2-MF), which is a promising fuel additive due to its high blending research octane number [7]. In the conversion from furfural to 2-MF, CO bond cleavage of the external carbonyl group is required via the HDO reaction. To selectively break the CO bond, a catalyst should bind to oxygen strongly enough to cleave the CO bond to produce 2-MF and atomic oxygen, but weakly enough so that the oxygen atom can be easily removed to complete the catalytic cycle. Thus, the oxygen binding energy of the catalyst should play an important role in controlling the catalytic activity and stability. Previously, copper-based catalysts have been reported to be active for the furfural HDO reaction [8]. However, these reactions used copper chromite, which is toxic and non-environmental friendly. More recently, bimetallic catalysts FeNi/SiO2 and bimetallic surfaces Fe/Ni(111) and Co/Pt(111) were reported to selectively convert furfural to 2-MF [[9], [10], [11]]. Bimetallic catalysts typically possess different d-band characteristics and thus different binding energies from their parent metals. This unique property allows the tuning of the activity and selectivity of the monometallic catalysts with admetals [12]. However, this unique property is structurally dependent. For example, it was reported that the structure of a Co-overlayer deposited on Pt(111) showed higher 2-MF activity than the Pt/Co/Pt(111) subsurface structure [11]. At elevated temperatures, the Co-overlayer on Pt can easily diffuse into the subsurface, decreasing the HDO activity for 2-MF production.
On the other hand, molybdenum carbide (Mo2C) possesses similar bulk electronic properties to Pt [13] and can also serve as a diffusion barrier to prevent the metal overlayer from diffusing into the subsurface [14], making it a good substrate to maintain the metal overlayer structure. By replacing the Pt substrate with Mo2C, the cost of the catalyst can also be significantly reduced. In addition, Mo2C is a selective catalyst for the CO bond scission. For example, Mo2C has been shown as a highly selective catalyst for the HDO reaction of C3 molecules, such as propanol and propanal, to produce propylene [15]. Mo2C was also reported to selectively convert furfural to 2-MF but deactivated quickly [16]. The cause of the deactivation was considered to be poisoned by the strong bonding of atomic oxygen [17] and the polymeric derivatives of furfural [16] by blocking the active sites on Mo2C.
It has been reported that admetals can be used to modify the binding energies of carbon, oxygen and hydrogen on Mo2C [18]. This property can potentially be used to reduce the furfural and oxygen binding energies on Mo2C to enhance the stability of the catalyst. In order to solve the stability issues associated with the bimetallic and Mo2C catalysts, this work combines the advantages of both types of catalysts and uses Co overlayer to modify Mo2C. In this way, the Co overlayer reduces the oxygen and furfural binding energies on Mo2C to prevent the poison by these adsorbates to increase the catalyst stability, and the Mo2C substrate serves as a diffusion barrier to stabilize the Co overlayer structure. Density functional theory (DFT) was used to calculate the binding energies and bond lengths of furfural on Mo2C(0001) and Co/Mo(0001). The gas-phase products were determined using temperature-programmed desorption (TPD) and the surface intermediates and adsorption configurations were identified using high-resolution electron energy loss spectroscopy (HREELS) under ultra-high vacuum (UHV) conditions. Parallel flow reactor experiments were performed to verify the stability trend between Mo2C and Co/Mo2C powder catalysts at ambient pressure.
Section snippets
DFT calculations
All DFT calculations were preformed using the Vienna Ab initio Simulation Package (VASP) [[19], [20], [21]] with the Projector Augmented Wave (PAW) method [22]. Generalized Gradient Approximation (GGA) [23] and the PW91 functional [24] were used with a kinetic cutoff energy of 400 eV. The total energies were minimized self-consistently using a convergence threshold of 10–6 eV. The nuclear degrees of freedom were optimized to a force convergence threshold of 0.05 eV Å–1. A periodic 4 × 4 unit
DFT calculations
DFT calculations were performed to investigate the adsorption configurations of furfural on Mo2C(0001) and Co/Mo2C(0001). The atomic assignment is shown in Scheme 1 and the side views of adsorbed furfural are shown in Fig. 1. Table 1 summaries the C1C2 and C1O1 bond lengths and the binding energies of furfural on the two surfaces. Compared to gas phase furfural, the C1O1 bonds of adsorbed furfural on the Co/Mo2C(0001) and Mo2C(0001) surfaces are both elongated, consistent with a η2(C,O) bonding
Conclusions
Based on the results and discussion presented above, the following conclusions can be made regarding the potential utilization of Co/Mo2C as an active and stable HDO catalyst:
- (1)
DFT calculations predict that the external carbonyl CO bond in furfural is elongated on both Mo2C/Mo(110) and Co-Mo2C/Mo(110) surfaces due to the strong interaction between the carbonyl group and the surfaces, which should promote the CO bond scission to form 2-MF. DFT calculations also indicate that the binding energies
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
This work was sponsored by the National Science Foundation (Award Number 1565964). The DFT calculations were performed using computational resources at the Center for Functional Nanomaterials, a user facility at Brookhaven National Laboratory, supported by US Department of Energy under Contract No. DE-AC02-05CH11231. We also acknowledge the assistance of Dr. Binhang Yan for the flow reactor experiments, Baohuai Zhao for the XRD analysis and Dr. Zhifeng Jiang for the surface science experiments.
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