Palladium nanoparticles incorporated within sulfonic acid-functionalized MIL-101(Cr) for efficient catalytic conversion of vanillin

Abstract

We report a highly efficient bifunctional catalyst, Pd/SO 3 H-MIL-101(Cr), consisting of Pd nanoparticles immobilized on a mesoporous sulfonic acid-functionalized metal-organic framework SO 3 H-MIL-101(Cr), which exhibits high catalytic performance in promoting biomass refining. The use of SO 3 H-MIL-101(Cr) as a support renders highly dispersed Pd nanoparticles with uniform size distribution, sufficient reactants contact in aqueous media, and rapid activation of the reactants induced by the Brønsted acid coordination sites (sulfonic acid groups from SO 3 H-MIL-101(Cr)). Thus, the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst exhibits novel synergy in the hydrodeoxygenation of vanillin (a typical model compound of lignin) at low H 2 pressure under mild conditions in aqueous media. Excellent catalytic results (100% conversion of vanillin with exclusive selectivity for the 2-methoxy-4-methylphenol product) could be achieved, and no loss of catalytic activity and selectivity were observed after seven recycles in succession.13 achieved over the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst including a 100% conversion of vanillin with a 100% selectivity for the 2-methoxy-4-methylphenol product within 120 min (entry 3 in Table 1). While in the presence of 2.0 wt.% Pd/MIL-101(Cr) catalyst, a rather low catalytic activity and selectivity were obtained when the reaction was carried out at the same conditions (entry 4). This is probably due to the lower acid strength of the MIL-101(Cr) as compared to that of the SO 3 H-MIL-101(Cr) support. Over the pure support SO 3 H-MIL-101(Cr) or MIL-101(Cr), however, no reaction took place, implying that Pd nanoparticles are inevitable for the vanillin hydrodeoxygenation (entries 1 and 2). Moreover, the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst was also compared with the commercially available Pd/C catalyst under the same conditions. The textural properties of the 2.0 wt.% Pd/C are shown in Table S1. The loading amounts of Pd within the Pd/SO 3 H-MIL-101(Cr), Pd/MIL-101(Cr) and Pd/C catalysts, determined by the ICP-AES analysis, were found to be 1.98 wt.%, 1.99 wt.% and 2.01 wt.%, respectively, very close to the nominal amount of 2.0 wt.%. The results (entry 5 in Table 1) clearly show that the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst gives significantly higher activity and 2-methoxy-4-methylphenol selectivity as compared to the Pd/C catalyst. It should be noted that the selectivity for 2-methoxy-4-methylphenol over the prepared 2.0 wt.% Pd/SO 3 H-MIL-101 catalyst is also significantly higher than that reported by Xiao et al [25] over 4.5 wt.% Pd/MSMF under the same reaction conditions with a similar conversion of vanillin (entry 6). The high selectivity over the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst is probably due to the readily accessible Brønsted acidic sites distributed throughout the framework as well as an abundance of mesoporous cages of MIL-101(Cr) thus, greatly facilitating the transfer of substrates [46,47].

Palladium Nanoparticles Supported on a Metal–Organic Framework‐Partially Reduced Graphene Oxide Hybrid for the Catalytic Hydrodeoxygenation of Vanillin as a Model for Biofuel Upgrade Reactions

“…The absence of the characteristic XRD peak of graphene oxide (GO) at around 2 θ =10.9° suggests the reduction of GO and the formation of the PRGO/Ce‐MOF composite. The characteristic Pd peaks are also not observed, which suggests the high dispersion of the Pd nanoparticles within the PRGO/Ce‐MOF composite and the low loading of the Pd nanoparticles as observed previously . The incorporation of Pd nanoparticles and PRGO with the Ce‐MOF creates a hybrid support system without compromising the integrity of the Ce‐MOF structure (Figure c).…”

“…Third, the wrapping of the Ce‐MOF crystals with the thin PRGO nanosheets prevents both the agglomeration of the Ce‐MOF nanocrystals and the stacking of the PRGO sheets, and thus enhances the dispersion of the larger Pd nanoparticles on the outer surfaces of the Ce‐MOF nanocrystals and on the PRGO nanosheets. Finally, the presence of oxygen functional groups within the PRGO domains of the hybrid PRGO/Ce‐MOF support may provide acidic sites for the Pd catalyst to accelerate the further deoxygenation of vanillin to produce the desired 2‐methoxy‐4‐methyl phenol product . Thus, the designed multifunctional Pd/PRGO/Ce‐MOF catalyst system is expected to exhibit new synergetic effects in the hydrodeoxygenation of vanillin to lead to a significantly enhanced selectivity.…”

“…The transformation of vanillin to 2‐methoxy‐4‐methylphenol can proceed through hydrogenation to form vanillyl alcohol followed by hydrogenolysis or by direct hydrogenolysis by two hydrogen molecules (Scheme ) …”

“…Although MOFs have been used successfully to disperse Pd and other metal nanoparticles for the hydrodeoxygenation of vanillin, to the best of our knowledge, this is the first example reported for a hybrid MOF‐PRGO support in which the reaction pathway and product selectivity can be controlled by small changes in the composition of the support system.…”

Influence of Carbon Supports on Palladium Nanoparticle Activity toward Hydrodeoxygenation and Aerobic Oxidation in Biomass Transformations

“…[5] Since different allotropic forms of carbon have become commercially available, and considering the influence that the support plays on the catalytic activity and stability of supported metal nanoparticles (MNPs), there have been an interest in comparing the performance of these MNPs supported on different carbon nanoforms. [9] In the literature there are precedents reporting Scheme 1. [7] In addition, carbon nanofibers (CNFs) consisting in 1D thick carbon materials with long aspect ratio can be advantageous also from the point of view of their availability.…”