Regina Palkovits

Hydrogenolysis Goes Bio: From Carbohydrates and Sugar Alcohols to Platform Chemicals

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 C-C and C-O bonds by hydrogen and allow direct access to valuable platform chemicals already integrated in todays 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.

Development of Heterogeneous Catalysts for the Conversion of Levulinic Acid to γ‐Valerolactone

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.

Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition

The possible role of ammonia in a future energy infrastructure is discussed. The review is focused on the catalytic decomposition of ammonia as a key step. Other aspects, such as the catalytic removal of ammonia from gasification product gas or direct ammonia fuel cells, are highlighted as well. The more general question of the integration of ammonia in an infrastructure is also covered.

Isosorbide as a Renewable Platform chemical for Versatile ApplicationsuQuo Vadis

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.

Which Controls the Depolymerization of Cellulose in Ionic Liquids: The Solid Acid Catalyst or Cellulose?

Cellulose is a renewable and widely available feedstock. It is a biopolymer that is typically found in wood, straw, grass, municipal solid waste, and crop residues. Its use as raw material for biofuel production opens up the possibility of sustainable biorefinery schemes that do not compete with food supply. Tapping into this feedstock for the production of biofuels and chemicals requires--as the first-step--its depolymerization or its hydrolysis into intermediates that are more susceptible to chemical and/or biological transformations. We have shown earlier that solid acids selectively catalyze the depolymerization of cellulose solubilized in 1-butyl-3-methylimidazolium chloride (BMIMCl) at 100 degrees C. Here, we address the factors responsible for the control of this reaction. Both cellulose and solid acid catalysts have distinct and important roles in the process. Describing the depolymerization of cellulose by the equivalent number of scissions occurring in the cellulosic chains allows a direct correlation between the product yields and the extent of the polymer breakdown. The effect of the acid strength on the depolymerization of cellulose is discussed in detail. Practical aspects of the reaction, concerning the homogeneous nature of the catalysis in spite of the use of a solid acid catalyst, are thoroughly addressed. The effect of impurities present in the imidazolium-based ionic liquids on the reaction performance, the suitability of different ionic liquids as solvents, and the recyclability of Amberlyst 15DRY and BMIMCl are also presented.

Exploring the ruthenium catalysed synthesis of γ-valerolactone in alcohols and utilisation of mild solvent-free reaction conditions

Levulinic acid and alkyl-levulinates have been hydrogenated using a range of supported catalysts. The different reaction outcomes obtained in alternate solvents have been rationalized and the influence of varying catalyst supports examined. A range of solvent free conditions have been investigated with complete LA conversion obtained at temperatures as low as 25 °C.

Cellulose and hemicellulose valorisation: an integrated challenge of catalysis and reaction engineering

Platform molecules have recently been in the focus of numerous investigations as intermediates for transformation of lignocellulosic biomass into fuels and chemicals. Herein we focus on challenges associated with technical implementation of the production of selected platform molecules. 5-(Hydroxymethyl)-furfural, furfural and levulinic acid were chosen to illustrate pitfalls and limitations of currently available catalytic reaction systems. Owing to the prominent reactivity, high polarity and high boiling points of most biomass-derived platform chemicals, the design of efficient, economic and environmentally benign chemical processes offers a number of difficulties. Challenges include not only a selective synthesis of such platform chemicals and their further transformation into potential products but also methods to allow an energy efficient product separation and sufficient catalyst stability under the applied reaction conditions. In this review we discuss three approaches to tackle the integration of catalytic transformations, reaction engineering and product separation. First, extraction-assisted synthesis methods are discussed. In the next step the integration of such extraction-assisted approaches into reaction cascades is considered. Finally, one-pot transformations of lignocellulose-derived carbohydrates into value-added products without isolation of the intermediate platform chemicals are outlined. The present status of predictive tools facilitating the selection of suitable solvent systems for extraction systems is discussed to outline current opportunities and constraints of a theoretical process design and optimization. In summary, this review provides an overview of recent progress with regard to strategies for process integration of chemo-catalytic biomass valorisation and highlights challenges associated with such approaches.

Pentenoic Acid Pathways for Cellulosic Biofuels

The depletion of fossil fuels, climate change, growing world population, and future energy supplies are certainly important challenges to tackle these days. While several options exist to cover energy supplies of the future, including solar, wind, and water power, individual mobility, aviation, and heavy duty vehicles will for some time continue to require fuels of high energy density to guarantee sufficient drive capacity and cruising range. Bioethanol and biodiesel were the first biofuels and have certainly been valuable in developing the biofuel market. However, their production from sugars, starches, and vegetable oils induces competition with food production and can thus hardly deliver the large volumes required for worldwide transportation. Current expectations concentrate on lignocellulose, which is available in large amounts, potentially not in competition with the food chain, and could serve as an alternative feedstock for fuels and chemicals. Biomass gasification along with Fischer–Tropsch technology or pyrolysis of biomass to bio-oils would deliver hydrocarbons which could be integrated easily in today s refineries, but have rather high energy demands, mostly require hydrogen, and do not use the defined chemical structures of lignocellulose. The chemocatalytic synthesis of defined target molecules as building blocks for fuels and chemicals is an alternative approach and could be realized in a biorefinery concept to integrate complete value chains. Potential biofuels should ideally be suited to today s engines, exhibit a high energy density, require little energy in their production, be nontoxic, and result in reduced emissions during combustion. Fuel platforms based on glucose, 5-hydroxymethylfurfural (5HMF), and levulinic acid (LA) were described in previous publications. The latter two fuels may be derived by dehydration of hexoses to 5-HMF, followed by rehydration to yield LA along with formic acid (Scheme 1). Aldol condensation of 5-HMF with acetone and subsequent hydrogenation produces C9 or C15 alkanes, while hydrogenation of glucose to sorbitol followed by hydrodeoxygenation could yield hexane. 2] Both routes, however, require lots of external hydrogen, which is to a large extent “lost” as water. With regard to real “bio”fuels, esters of LA have been considered along with g-valerolactone (gVl) and methyltetrahydrofuran (mTHF), which can be obtained by hydrogenation of LA. Lactones, diols, and cyclic ethers could be thus obtained by controlled transformation of LA and itaconic acid by utilizing a single multifunctional molecular catalyst. Although suitable in terms of combustion properties and energy content, their polarity and high tendency to swell and dissolve conventional polymer materials complicate their application in today s combustion systems. Recently, two new directions have been proposed, both establishing a value chain based on pentenoic acid. 6] This acid can be obtained by hydrogenation of LA to gVl and subsequent ring opening catalyzed by solid acids. The hydrogen required for the production of gVl can be supplied by transfer hydrogenation from formic acid. 8] Dumesic and co-workers reported an integrated approach based on gVl for the synthesis of C8+ alkenes for fuel application without the need for external hydrogen (Scheme 2). gVl is converted into pentenoic acid, which Scheme 1. Dehydration of hexoses to 5-hydroxymethylfurfural and rehydration to yield formic and levulinic acid.

Cellulose-Based Sustainable Polymers: State of the Art and Future Trends

Nowadays, nearly all polymeric materials are produced from crude oil-derived monomers. With the steadily increasing demand for oil-based products and their decreasing availability in the near future, one of the main challenges of mankind is the replacement of crude oil as raw material by renewable resources such as biomass. So far, only a few polymers are available derived directly from cellulose as a main component of biomass by regeneration. On the other hand, a significant potential lies in the production of polymers from cellulose-derived monomers. A huge variety of different monomers is already available by convenient catalytic processes. This feature article focuses on the current status of mono- and resulting polymers derived either directly from cellulose processing and regeneration or by catalytic conversion to a number of monomers for the production of novel polymers and co-polymers.

Selective Aerobic Oxidation of HMF to 2,5‐Diformylfuran on Covalent Triazine Frameworks‐Supported Ru Catalysts

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-Al2 O3 , 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.