The production of functional nanostructured materials starting from cheap natural precursors using environmentally friendly processes is a highly attractive subject in material chemistry today. Recently, much attention has been focused on the use of plant biomass to produce functional carbonaceous materials, encompassing economic, environmental and social issues. Besides the classical route to produce activated carbons from agricultural side products, the hydrothermal carbonization (HTC) process shows clear advantages in that it can generate a variety of cheap and sustainable carbonaceous materials with attractive nanostructure and functionalization patterns for a wide range of applications. In this tutorial review we present the latest developments in this traditional but recently invigorated technique. It will be shown that HTC does not only access carbonaceous materials under comparatively mild hydrothermal conditions, but also replaces the more technical and structurally well-defined charring by a controlled chemical process. It will be shown that this makes it possible to tailor the final structure with the tools of colloid and polymer science, leading to very different morphologies with miscellaneous applications, including modern carbon nanocomposites and hybrids.

Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization

Many efforts have been made to develop new catalysts to oxidize cyclohexane under mild conditions. Herein, we review the most interesting systems for this process with different oxidants such as hydrogen peroxide, tert -butyl hydroperoxide and molecular oxygen. Using H 2 O 2 , Na-GeX has been shown to be a most stable and active catalyst. Mesoporous TS-1 and Ti-MCM-41 are also stable, but the use of other metals such as Cr, V, Fe and Mo leads to leaching of the metal. Homogeneous systems based on binuclear manganese(IV) complexes have also been shown to be interesting. When t -BuOOH is used, the active systems are those phthalocyanines based on Ru, Co and Cu and polyoxometalates of dinuclear ruthenium and palladium. Microporous metallosilicates containing different transition metals showed leaching of the metal during the reactions. Molecular oxygen can be used directly as an oxidant and decreases the leaching of active species in comparison to hydrogen peroxide and tert -butyl hydroperoxide. Metal aluminophosphates (metal: Mn, Fe, Co, Cu, Cr V) are active and relatively stable under such conditions. Mn-AlPO-36 yields directly adipic acid, but large amounts of carboxylic acids should be avoided, as they cause metal leaching from the catalysts. Rare earth exchanged zeolite Y also shows good selectivity and activity. In the last part of the review, novel alternative strategies for the production of cyclohexanol and cyclohexanone and the direct synthesis of adipic acid are discussed.

Cyclohexane oxidation continues to be a challenge

Hydrotalcite-like anionic clays (HTs; or layered double hydroxides, LDHs) can be used as precursors to mixed oxides, but their catalytic potential reaches much further. This review demonstrates that HTs can be successfully applied in a broad spectrum of organic reactions, with advantages such as improved activity, selectivity, metal dispersion, less waste production, and an improved recuperation of immobilized catalysts. HTs can be used in the as-synthesized form or after different pretreatments. The HT as such is a solid base or, depending on the elemental composition of its octahedral layers, may have redox properties. Specific metals can be incorporated either as a cation in the octahedral layer or as an anion via exchange. The anion-exchange strategy is especially successful in the context of heterogenizing homogeneous catalysts. A particular advantage for base catalysis is that the number and strength of the basic sites can be tuned precisely to a specific reaction. HTs are excellent materials to des...

Hydrotalcite-like anionic clays in catalytic organic reactions

IR spectroscopy in catalysis

Organic reactions catalyzed over solid acids

Hydrotalcites (HTs) are prepared by low supersaturation and high supersaturation methods, containing CO 2− 3 , Cl − , SO 2− 4 interlayer anions and with different Al(III)/Al(III)+Mg(II) ( x =0.33 and 0.25) ratios. Palladium hydrotalcite catalysts are obtained by impregnation of PdCl 2 aqueous solution on HTs. Different precursors of palladium are used for loading the metal onto HT prepared at high supersaturation. HT and palladium HTs are well characterized by XRD, surface area, IR, SEM, DTA. CO chemisorption is used to estimate palladium dispersion and metal area. The activity is evaluated for phenol hydrogenation reaction. The selectivity to cyclohexanone is always ≥90% even though phenol conversion varies with metal area.

Hydrotalcite-supported palladium catalysts: Part I: Preparation, characterization of hydrotalcites and palladium on uncalcined hydrotalcites for CO chemisorption and phenol hydrogenation

Structure activity relationship in Pd/hydrotalcite: effect of calcination of hydrotalcite on palladium dispersion and phenol hydrogenation

Hydrogenation of phenol was studied over uncalcined and calcined hydrotalcite supported palladium catalysts and compared with the conventional Γ-Al2O3 and MgO supported catalysts. Pd on uncalcined hydrotalcite support (Pd/UHT) showed very high conversions (≥ 95%) and high selectivities (≥ 85%) towards cyclohexanone. These catalysts maintained their activity for nearly 4 h and the results are presented.

Highly active hydrotalcite supported palladium catalyst for selective synthesis of cyclohexanone from phenol

ZSM-5 zeolite containing SiO2/Al2O3 = 28 was synthesised by hydrothermal process with and without template. Characterisation of the zeolites was done using XRD, IR, NMR, SEM and BET surface area measurements. Acidity of the zeolites was measured by ammonia adsorption-desorption. Catalytic activity of HZSM-5 zeolites was evaluated by aniline alkylation, cumene dealkylation, alkylation of benzene with ethylene and propylene and isomerisation ofm-xylene. Catalytic activity is discussed from the point of view of zeolite acidity and morphology.

K. Krishna

Papers

Hydrotalcite-supported palladium catalysts: Part I: Preparation, characterization of hydrotalcites and palladium on uncalcined hydrotalcites for CO chemisorption and phenol hydrogenation

Structure activity relationship in Pd/hydrotalcite: effect of calcination of hydrotalcite on palladium dispersion and phenol hydrogenation

Highly active hydrotalcite supported palladium catalyst for selective synthesis of cyclohexanone from phenol

Synthesis of ZSM-5 type zeolites with and without template and evaluation of physicochemical properties and aniline alkylation activity

Structural influence of hydrotalcite on Pd dispersion and phenol hydrogenation