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The oxidation of 5-hydroxymethylfurfural (HMF) to 2, 5-diformylfuran (DFF) is an important reaction for the conversion of renewable biomass into feedstock chemicals. In this work, palygorskite (Pal)-supported ruthenium (Ru) catalysts were developed for the efficient selective oxidation of HMF to DFF. Using natural clay mineral palygorskite as the support enabled the immobilization and dispersion of Ru nanoparticles. A 100% HMF conversion, 98% DFF yield, and 98% DFF selectivity were obtained over Ru(III)/Pal catalyst after 4.5 h under the optimal reaction conditions at 110 °C and 10 bar of O 2 in toluene. Ru nanoparticles and palygorskite synergistically achieve the excellent catalytic performance of the catalyst. The Ru metal active sites facilitate the activation of the reactants or intermediates, accelerating the formation of DFF. Furthermore, due to their different particle sizes and Ru species, the Ru(III)/Pal catalyst exhibits higher catalytic activity than the Ru(0)/Pal catalyst for the oxidation of HMF to DFF. The palygorskite-enhanced dispersion effect on Ru nanoparticles augments the catalytic activity. Moreover, the palygorskite-supported Ru catalyst showed good stability and was reused for at least five times consecutive cycles without significant loss of its catalytic activity. • Natural palygorskite (Pal)-supported ruthenium (Ru) catalysts are synthesized. • Ru(III)/Pal exhibits higher activity with 98% 2,5-diformylfuran yield than Ru(0)/Pal. • Palygorskite enables high dispersion of Ru particles, enhancing catalytic activity. [ABSTRACT FROM AUTHOR]

Aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2, 5-furandicarboxylic acid (FDCA) is an important reaction for producing green and sustainable chemicals. Herein, natural clay mineral palygorskite (Pal)-supported bimetallic Pt–Pd catalyst for the oxidation of HMF to FDCA in water without the addition of any bases was developed. The 1Pt 9 Pd 1 /Pal catalyst with Pt/Pd molar ratio of 9/1 exhibited excellent catalytic performance, and 100% HMF conversion and 99% FDCA yield were obtained under optimum reaction conditions after 12 h, which was mainly attributed to the synergy between palygorskite and bimetallic Pt–Pd nanoparticles. Palygorskite used as support realized the immobilization and dispersion of bimetallic nanoparticles, increasing the catalytic activity. The basicity of palygorskite and alloying effects between Pt and Pd facilitated the formation of FDCA by synergistically catalyzing the oxidation of alcohol and aldehyde groups in the reactants and intermediates. Furthermore, this catalyst showed good stability and reusability. The present work provides a highly efficient, cost-effective, and environmentally benign catalyst for the base-free oxidation of HMF to FDCA in practical industrial applications. • Efficient palygorskite-supported bimetallic Pt–Pd catalyst is synthesized. • The catalyst exhibits high activity for the base-free oxidation of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid. • The basicity of palygorskite and the alloying effect facilitate the formation of 2, 5-furandicarboxylic acid. [ABSTRACT FROM AUTHOR]

Three different acidic ionic liquids were covalently grafted on halloysite to give heterogeneous catalysts. The acidity of the catalysts was higher than halloysite and differed based on the nature of ionic liquid. Investigation of the catalytic activity of the catalysts for the synthesis of 5-hydroxymethylfurfural indicates that imidazolium-based ionic liquid showed the highest activity. Using this catalyst (30 wt%), 5-hydroxymethylfurfural was achieved from dehydration of fructose in 88% yield at 85 °C in 73 min. Furthermore, the catalyst was recyclable up to six runs with slight loss of activity. Comparison of the performance of the catalyst with halloysite-free counterpart confirmed the role of halloysite in improvement of the catalytic activity and recyclability. [Display omitted] • Three halloysite-supported acidic ionic liquids have been prepared and used as acidic catalysts. • The Bronsted activity of the catalysts for conversion of carbohydrates to 5-hydroxymethylfurfural has been compared. • Using 30 wt% of the catalyst, 5-hydroxymethylfurfural could be obtained from fructose dehydration at 85 °C in 73 min. [ABSTRACT FROM AUTHOR]

• Halloysite nanoclay used as a natural support for the immobilization of an acidic ionic liquid by chlorosulfuric acid. • Catalytic conversion of various carbohydrates to 5-hydroxymethylfurfural. • Amino-functionalized halloysite o graft the ionic liquid on the halloysite. • 2,4,6-Trichloro-1,3,5-triazine and 2-aminopyrimidine to form an heterocyclic ligand on halloysite. • Catalyst working under mild reaction conditions, and of high recyclability. Halloysite nanoclay has been used as a natural support for the immobilization of an acidic ionic liquid with multiple acidic sites. To graft the ionic liquid on the halloysite, amino-functionalized halloysite was reacted with 2,4,6-trichloro-1,3,5-triazine and 2-aminopyrimidine successively to form heterocyclic ligand on halloysite, which was then converted to acidic ionic liquids through reaction with chlorosulfuric acid. The resultant catalyst was then characterized via SEM, EDS, and elemental mapping analysis, FTIR, XRD and TGA. Noteworthy, acid-base titration method confirmed that introduction of acidic ionic liquid significantly improved the Brønsted acidity of halloysite. The catalytic activity of the catalyst was investigated for conversion of various carbohydrates to 5-hydroxymethylfurfural. Gratifyingly, the results showed that the catalyst could promote the reaction of fructose, sucrose, glucose and galactose under mild reaction conditions (catalyst (0.03 g), at 85 °C in 70 min) to give the product in high to excellent yields. Computational DFT studies unveiled the reaction mechanism, pointing out the limiting steps kinetically, and in addition how important are the non-covalent interactions. Furthermore, the recycling tests confirmed high recyclability of the catalyst up to six runs. [ABSTRACT FROM AUTHOR]