Invited for this month's cover are Irina Delidovich and Regina Palkovits from RWTH Aachen University. The mini-installation of sugar cubes and granulated sugar with chemical equations of isomerization reactions written on it represents a renewable feedstock for producing a broad range of valuable products. Our Review summarizes recent advances on chemocatalytic isomerization. The Review itself is available at 10.1002/cssc.201501577.

Selected aldohexoses (d-glucose, d-mannose, and d-galactose) and aldopentoses (d-xylose, l-arabinose, and d-ribose) are readily available components of biopolymers. Isomerization reactions of these substances are very attractive as carbon-efficient processes to broaden the portfolio of abundant monosaccharides. This review focuses on the chemocatalytic isomerization of aldoses into the corresponding ketoses as well as epimerization of aldoses at C2. Recent advances in the fields of catalysis by bases and Lewis acids are considered. The emphasis is laid on newly uncovered catalytic systems and mechanisms of carbohydrate transformations.

The co-production of formic acid during the conversion of cellulose to levulinic acid offers the possibility for on-site hydrogen production and reductive transformations. Phosphorus-based porous polymers loaded with Ru complexes exhibit high activity and selectivity in the base-free decomposition of formic acid to CO₂ and H₂. A polymeric analogue of 1,2-bis(diphenylphosphino)ethane (DPPE) gave the best results in terms of performance and stability. Recycling tests revealed low levels of leaching and only a gradual decrease in the activity over seven runs. An applicability study revealed that these catalysts even facilitate selective removal of formic acid from crude product mixtures arising from the synthesis of levulinic acid.

Lactic acid (LA) is a versatile platform molecule owing to the opportunity to transform this compound into useful chemicals and materials. Therefore, efficient production of LA based on inexpensive renewable feedstocks is of utmost importance for insuring its market availability. Herein, we report the efficient conversion of glycerol into LA catalyzed by heteropolyacids (HPAs) under mild base-free conditions. The catalytic performance of molecular HPAs appears to correlate with their redox potential and Brønsted acidity. Namely, H₃PMo₁₂O₄₀ (HPMo) exhibits the best selectivity towards LA (90 %) with 88 % conversion of glycerol. Loading of HPMo onto a carbon support (HPMo/C) further improves LA productivity resulting in 94 % selectivity at 98 % conversion under optimized reaction conditions. The reaction takes place through the formation of dihydroxyacetone/glyceraldehyde and pyruvaldehyde as intermediates. No leaching of HPMo was observed under the applied reaction conditions and HPMo/C could be recycled 5 times without significant loss of activity.

The selective aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran has been performed under mild conditions at 80 °C and 20 bar of synthetic air in methyl t-butyl ether. Ru clusters supported on covalent triazine frameworks (CTFs) allowed excellent selectivity and superior catalytic activity compared to other support materials such as activated carbon, γ-Al₂O₃, hydrotalcite, or MgO. CTFs with varying pore size, specific surface area, and N content could be prepared from different monomers. The structural properties of the CTF materials influence the catalytic activity of Ru/CTF significantly in the aerobic oxidation of HMF, which emphasizes the superior activity of mesoporous CTFs. Recycling of the catalysts is challenging, but promising methods to maintain high catalytic activity were developed that facilitate only minor deactivation in five consecutive recycling experiments.

In this series of articles, the board members of ChemSusChem discuss recent research that they consider of exceptional quality and importance for sustainability. In this contribution, Prof. Regina Palkovits describes some recent advances in heterogeneous catalytic transformations of lignocellulosic feedstocks into biofuels. While the recent works she highlights are of great significance, challenges with regard to catalyst stability, the associated deactivation mechanisms, and designs for aqueous-phase reactions remain.

The aqueous Ru/C-catalyzed hydrogenolysis of biomass-based polyols such as erythritol, xylitol, sorbitol, and cellobitol is studied under neutral and acidic conditions. For the first time, the complete product spectrum of C₂–C₆ polyols is identified and, based on a thorough analysis of the reaction mixtures, a comprehensive reaction mechanism is proposed, which consists of (de)hydrogenation, epimerization, decarbonylation, and deoxygenation reactions. The data reveal that the Ru-catalyzed deoxygenation reaction is highly selective for the cleavage of terminal hydroxyl groups. Changing from neutral to acidic conditions suppresses decarbonylation, consequently increasing the selectivity towards deoxygenation.

The base-free aqueous-phase oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxilic acid (FDCA) was performed at 140 °C and 20 bar of synthetic air as the oxidant. Ru clusters supported on covalent triazine frameworks (CTFs) enabled superior conversion (99.9 %) and FDCA yields in comparison to other support materials such as activated carbon and γ-Al₂O₃ after only 1 h. The properties of the CTFs such as pore volume, specific surface area, and polarity could be tuned by using different monomers. These material properties influence the catalytic activity of Ru/CTF significantly as mesoporous CTFs showed superior activity compared to microporous materials, whereas high polarities provide further beneficial effects. The recyclability of the prepared Ru/CTF catalysts was comparable to that of Ru/C at high conversions and product yields. Nevertheless, minor deactivation in five successive recycling experiments was observed.

A series of polyols, carbohydrates, and cellulose were tested in the aqueous, CuO/ZnO/Al₂O₃-catalyzed hydrogenolysis reaction at 245 °C and 50 bar H₂. The compositions of liquid-phase products were analyzed; based on these results a unified reaction mechanism is proposed that accounts for the observed product distribution. Elementary transformations such as dehydration, dehydrogenation/hydrogenation, Lobry de Bruyn–van Ekenstein isomerization and retro-aldol cleavage were identified as most important for controlling the selectivity of simple polyols and carbohydrates. For cellulose the product distribution is considerably different than for glucose or sorbitol, indicating a change in the reaction pathway. Therefore, next to the traditional hydrolysis of the glycosidic bond, an additional depolymerization mechanism involving only the reducing ends of cellulose oligomers is proposed to account for this observation.

The shortage of fossil energy carriers demands the use of renewable and sustainable energy resources. Biomass has the potential to fulfill these needs. The chemical transformation of cellulose, being a main part of plant biomass, allows the production of a widespread variety of chemicals. One especially promising candidate as platform molecule is 5-hydroxymethylfurfural (5-HMF). Until now 5-HMF was predominantly produced in batch reactions from fructose as substrate within high boiling solvents. A working setup for producing 5-HMF continuously out of fructose or in a coupled two-step reaction, out of glucose, in low boiling solvents is introduced.

Copper supported on mesoporous magnesium aluminate has been prepared as noble-metal-free solid catalyst for aldol condensation of 5-hydroxymethylfurfural with acetone, followed by hydrogenation of the aldol condensation products. The investigated mesoporous spinels possess high activity as solid-base catalysts. Magnesium aluminate exhibits superior activity compared to zinc and cobalt-based aluminates, reaching full conversion and up to 81 % yield of the 1:1 aldol product. The high activity can be correlated to a higher concentration of basic surface sites on magnesium aluminate. Applying continuous regeneration, the catalysts can be recycled without loss of activity. Focusing on the subsequent hydrogenation of aldol condensation products, Cu/MgAl₂O₄ allows a selective hydrogenation and CO bond cleavage, delivering 3-hydroxybutyl-5-methylfuran as the main product with up to 84 % selectivity avoiding ring saturation. Analysis of the hydrogenation activity reveals that the reaction proceeds in the following order: CC>CO>CO cleavage>ring hydrogenation. Comparable activity and selectivity can be also achieved utilizing 2-propanol as solvent in the transfer hydrogenation, providing the possibility for partial recycling of acetone and optimization of the hydrogen management.

In view of the diminishing oil resources and the ongoing climate change, the use of efficient and environmentally benign technologies for the utilization of renewable resources has become indispensible. Therein, hydrogenolysis reactions offer a promising possibility for future biorefinery concepts. These reactions result in the cleavage of CC and CO bonds by hydrogen and allow direct access to valuable platform chemicals already integrated in today’s value chains. Thus, hydrogenolysis bears the potential to bridge currently available technologies and future biomass-based refinery concepts. This Review highlights past and present developments in this field, with special emphasis on the direct utilization of cellulosic feedstocks.

γ-Valerolactone (GVL) has been identified as a potential intermediate for the production of fuels and chemicals based on renewable feedstocks. Numerous heterogeneous catalysts have been used for GVL production, alongside a range of reaction setups. This Minireview seeks to outline the development of heterogeneous catalysts for the targeted conversion of levulinic acid (LA) to GVL. Emphasis has been placed on discussing specific systems, including heterogeneous noble and base metal catalysts, transfer hydrogenation, and application of scCO₂ as reaction medium, with the aim of critically highlighting both the achievements and remaining challenges associated with this field.

Isosorbide is a platform chemical of considerable importance for the future replacement of fossil resource-based products. Applications as monomers and building blocks for new polymers and functional materials, new organic solvents, for medical and pharmaceutical applications, and even as fuels or fuel additives are conceivable. The conversion of isosorbide to valuable derivatives by functionalization or substitution of the hydroxyl groups is difficult because of the different configurations of the 2- and 5-positions and the resulting different reactivity and steric hindrance of the two hydroxyl groups. Although a substantial amount of work has been published using exclusively the endo or exo derivatives isomannide and isoidide, respectively, as starting material, a considerable effort is still necessary to transfer and adapt these methods for the efficient conversion of isosorbide. This Minireview deals with all aspects of isosorbide chemistry, which includes its production by catalytic processes, special properties, and chemical transformations for its utilization in biogenic polymers and other applications of interest.

Angesichts begrenzter Reserven fossiler Rohstoffe und einem Voranschreiten des Klimawandels werden effiziente und umweltfreundliche Technologien zur Nutzbarmachung erneuerbarer Rohstoffe unerlässlich. Dabei stellt die Hydrogenolyse eine vielversprechende Reaktion für zukünftige Bioraffineriekonzepte dar. Sie resultiert in einer C-C- und C-O-Bindungsspaltung mit Wasserstoff und erlaubt einen direkten Zugang zu nützlichen Plattformchemikalien, die bereits heute Teil der Wertschöpfungsketten der chemischen Industrie sind. Aus diesem Grund birgt die Hydrogenolyse das Potential, existierende Wertschöpfungsketten und zukünftige Bioraffineriekonzepte zu verbinden. Dieser Aufsatz stellt vorausgegangene und aktuelle Entwicklungen in diesem Bereich mit besonderem Augenmerk auf der direkten Umsetzung von Cellulose als Rohstoffe dar.

Die effiziente Nutzung nachwachsender Ressourcen gewinnt zunehmend an Bedeutung. Cellulose und Lignin stehen nicht in Konkurrenz zur Nahrungsmittelkette und bieten aufgrund ihrer hohen Funktionalisierungsdichte viele Möglichkeiten für gezielte Umwandlungen zu neuen Zielprodukten. Gleichzeitig stellt diese Überfunktionalisierung auch eine der Herausforderungen dar. Während die heutige Raffinerie von unpolaren Molekülen in Gasphasenprozessen bei vergleichsweise hohen Temperaturen geprägt ist, erfordert eine Bioraffinerie auf der Basis von Lignocellulose vorwiegend Flüssigphasenprozesse bei relativ niedrigen Temperaturen in polaren Lösungsmitteln. Damit ergeben sich Herausforderungen im Bereich der Prozess- und Trenntechnik sowie bei der Katalysatorentwicklung, um den Anforderungen dieser polaren Rohstoffe gerecht zu werden.

Lignocellulose presents a potential future carbon source for production of fuels and chemicals. The high density of functional groups opens numerous possibilities for tailored chemical transformations to novel target molecules. With regard to catalysis, however, this over-functionalization makes high demands on catalyst and process development. Today's refineries are based on gas phase processes at high temperatures introducing functionality in non-polar feedstocks. In contrast, transformations based on lignocellulose require liquid phase processes in polar solvents at rather low temperatures.