Ru nanoparticles supported on N-doped reduced graphene oxide as valuable catalyst for the selective aerobic oxidation of benzyl alcohol
Graphical abstract
Introduction
The challenge of using biomass and its derivatives as a renewable feedstock to improve the sustainability of chemicals and fuels production is attracting significant industrial and academic interest. The conversion of biomass into chemicals is generally based in several transformation processes including thermal processes (e.g. gasification, pyrolysis and hydrothermal treatment) or biochemical platforms [1]. Lignocellulosic biomass is a widely available resource mostly composed of cellulose, hemicellulose and lignin, which when derived from the non-edible portion of biomass including bagasse, corn stover, grasses, forestry or agricultural waste, is viewed as a sustainable feedstock for use in bio-refining [2].
The selective oxidation of benzyl alcohol to benzaldehyde (Scheme 1) is an important transformation for the synthesis of precursors and intermediates in the agrochemical, pharmaceutical and perfumery industries, with benzaldehyde considered as the second most important aromatic molecule used after vanillin. While benzyl alcohol can be sourced as a natural product from plants, most benzyl alcohol is produced from petroleum-derived feedstocks via the hydrolysis of benzyl chloride. Recent reports of the application of engineered Escherichia-coli [3] in converting renewable glucose to bio-derived benzyl alcohol offers a promising sustainable route to produce a large number of industrially important derivatives such as benzaldehyde. However, the design of selective catalysts for benzyl alcohol oxidation are necessary to overcome challenges of by-product formation [4] from e.g. decarbonylation (benzene), hydrogenolysis (toluene) or esterification (benzyl benzoate).
Traditionally benzaldehyde is industrially produced via benzyl chloride hydrolysis derived from toluene chlorination or through toluene oxidation [5]. Extensive effort has been devoted to the production of benzaldehyde from benzyl alcohol using greener oxidation routes as replacements for conventional strong stoichiometric reagents like KMnO4 [6], chromites [7] or HNO3 [8]. It is well known, that these reagents are considered hazardous and generate a large amount of toxic waste, having a negative economic and environmental impact. Extensive research regarding the application of photocatalytic materials in the oxidation of benzyl alcohol has also been reported [5,9,10] as an alternative to stoichiometric oxidants. Other oxidants, like peroxides [[11], [12], [13]] have been proposed, however their production is not sustainable and present handling risks [14], thus the use of molecular oxygen as oxidant is preferable. The use of heterogeneous metallic catalysts has been extensively studied, but also often includes the use of undesirable additives such as NaOH [15], TEMPO [16,17], NaHCO3 [18], Na2CO3 [19] or K2CO3 [20,21].
The additive-free synthesis of benzaldehyde can be performed through the selective oxidation of benzyl alcohol either in vapour [22,23] or liquid phase using different types of metal catalysed processes. Choudary et al. [24] reported the application of hydrotalcite supported transition metal-based catalysts for the solvent free oxidation of benzyl alcohol, wherein Cu-Cr/HT systems exhibited the highest conversion (51%) with a 70% selectivity toward benzaldehyde. However, solvent-free oxidation processes have drawbacks from the high temperatures needed to carry out the reaction (210 °C) and extensive catalyst deactivation during recycling tests. The use of supported Au nanoparticles has also been explored for the selective solvent-free production of benzaldehyde using molecular oxygen [25], wherein Au/U3O8 and Au/MgO showed the best conversions of 53% and 51% respectively, however, selectivity’s were limited due to benzyl benzoate formation.
The liquid phase oxidation of benzyl alcohol to benzaldehyde over Pd catalysts has received significant attention, with Keresszegi and coworkers [26] reporting Pd/Al2O3 catalysts produce yields >30% after 1.5 h at 50 °C using cyclohexane as solvent. The catalytic intermediates formed at the material surface were monitored in situ by liquid phase ATR-IR spectroscopy, which revealed catalyst deactivation was attributed to strongly adsorbed CO, formed by decarbonylation of benzaldehyde, and the formation of surface water which block surface active sites. Decarbonylation results from metallic Pd sites [27], which can be minimized by maintaining high metal dispersions that stabilizes the active surface palladium oxide component critical for selective alcohol oxidation (selox) [28]. The influence of support architecture is also important for improving mass transport and increasing palladium dispersion, as demonstrated using interconnected porous KIT-6 silica frameworks [29]. Luque et al. [30] developed Pd nanoparticles supported on iron doped SBA15. Conversions of benzyl alcohol higher than 80% and high selectivity towards benzaldehyde were observed in the solvent free oxidation of benzyl alcohol at 85 °C after 9 h. Nitrogen doped carbon nanotubes (CNT) have been employed as supports for Pd selox catalysts, with a 54% benzyl alcohol conversion and 90% selectivity [31] obtained over 8.6% Pd/N-doped CNT after 3 h reaction at 120 °C. Investigations of a series of N-doped CNT supported Pd and Pd-Au nanoparticles for the selective oxidation of benzyl alcohol [32] revealed nitrogen functionalities incorporated by oxidation and further amination lead to an improvement in the TOF compared to pristine CNT. This promotion was attributed to the increased metal dispersion resulting from surface nitrogen groups, however, poor selectivity was observed toward benzaldehyde under solvent free conditions. When water was used as solvent the activity decreased significantly, but the selectivity toward benzaldehyde was higher than for the solvent-free conditions. Hutchings and coworkers [33] demonstrated that the incorporation of Pd into Au nanoparticles create synergistic effects improving the catalytic performance in the solvent-free oxidation of benzyl alcohol under mild conditions (100 °C and 10 bar of O2). 1%(Au-Pd)/TiO2 samples prepared by colloidal method were the more active, achieving a 92% selectivity and 29% conversion after 4 h. Liu et al. [34] developed Pt nanoparticles supported on graphitic TiO2 which exhibited 100% selectivity towards benzaldehyde at 77% benzyl alcohol conversion after 10 h at 26 °C, using water as a solvent. The authors claimed that anatase content was critical to improving the catalytic behaviour, however, very high catalyst:substrate ratios (0.1 g for 0.2 mmol) were employed to achieve this yield to benzaldehyde. Due to the high cost and scarcity of precious metals such as Pt, Au and Pd [35] and problems of poor stability of earth abundant transition metal catalysts, Ru has attracted significant attention as an alternative lower cost metal which can maintain high catalytic efficiency. Yamaguchi et al. reported full conversion of benzyl alcohol with 100% selectivity to benzaldehyde [36] using Al2O3 supported Ru catalysts in 1 h using trifluorotoluene solvent at 83 °C and atmospheric pressure of O2. However, again the catalyst: benzyl alcohol was very high and incomplete elimination of NaOH during washing between recycling tests could mask any catalyst deactivation. The deactivation and regeneration of RuO2/CNT has been described by Yu et al. [37], and while 75% conversion with excellent benzaldehyde selectivity (>99%) was obtained after 1 h, the samples showed significant deactivation in recycling tests. In a recent publication [38] RuO2 supported on NaY zeolites were presented as an alternative catalyst in the aerobic oxidation of benzyl alcohol to benzaldehyde. Samples exhibited only 12% conversion, but the reaction was only conducted for 3 h at 70 °C in toluene and atmospheric pressure of O2.
Despite the interest in supported Ru catalysts for selox reactions, to the best of our knowledge, there are no systematic reports on the application of graphenic supported Ru nanoparticles for the additive-free oxidation of benzyl alcohol into benzaldehyde. Here we report the effect of graphenic support materials and Ru precursors on the catalytic performance of Ru based catalysts in the selective oxidation of benzyl alcohol to yield benzaldehyde. Detailed characterization was performed to rationalize the observed differences in catalyst activity between samples, while in-depth evaluation of the recyclability of the prepared catalytic materials is reported.
Section snippets
Preparation of supports
Graphenic materials were prepared via thermal treatment of graphite oxide (GO), which was synthesized from natural graphite flake (325 mesh, Alfa Aesar, purity 99.8%), following a modified Brodie method, as described elsewhere [39]. Synthesized GO was dried to constant weight at room temperature under vacuum in a desiccator over P2O5. Exfoliation of the resulting GO was performed in a vertical quartz reactor under inert (N2) and reactive atmospheres (NH3) yielding reduced graphene oxide (rGO, S
Catalyst characterisation
The reduced catalysts used in this study are identical to those described in our previous work [39]. Table 1 compiles the Ru nanoparticle average diameter determined by TEM for the reduced catalysts, from which it can be seen that the particle size of Ru nanoparticles is in the range 1.4 to 2.4 nm, and is strongly dependent on the support used. When the rGO support is considered, the Ru particle size was found to be independent of the metal precursor used. When comparing the effect of different
Conclusions
The different supports strongly modify the catalytic behavior of Ru nanoparticles, the NrGO materials being those that under the experimental reaction conditions used produce the highest conversion of benzyl alcohol to the desired product. Thus, the catalytic performance is significantly enhanced by presence of N in the graphenic structure. These catalytic differences could be attributed to the interaction of the substrate and products of the reaction with the surface of N doped graphenic
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgment
CRB gratefully acknowledges financial support from Ministerio de Educacion, Cultura y Deporte of Spain, Grant Nº FPU15/01838. Also the financial support from the Spanish Ministerio de Economía, Industria y Competitividad of Spain under projects (CTQ2017-89443-C3-1-R and -3-R) is recognized. KW thanks the Royal Society for the award of an Industry Fellowship.
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