Catalysis Science & Technology MINIREVIEW Review of catalytic systems and thermodynamics for the Guerbet condensation reaction and Cite this: Catal. Sci. Technol.,2015, 5,3876 challenges for biomass valorization Dries Gabriëls,a Willinton Yesid Hernández,b Bert Sels,*a Pascal Van Der Voort*b and An Verberckmoes*c The Guerbet condensation reaction is an alcohol coupling reaction that has been known for more than a century. Because of the increasing availability of bio-based alcohol feedstock, this reaction is of growing importance and interest in terms of value chains of renewable chemical and biofuel production. Due to the specific branching pattern of the alcohol products, the Guerbet reaction has many interesting applica- Received 10th March 2015, tions. In comparison to their linear isomers, branched-chain Guerbet alcohols have extremely low melting Accepted 17th May 2015 points and excellent fluidity. This review provides thermodynamic insights and unravels the various mecha- nistic steps involved. A comprehensive overview of the homogeneous, heterogeneous and combined DOI: 10.1039/c5cy00359h homogeneous and heterogeneous catalytic systems described in published reports and patents is also www.rsc.org/catalysis given. Technological considerations, challenges and perspectives for the Guerbet chemistry are discussed. a Center for Surface Chemistry and Catalysis (COK), KU Leuven, Kasteelpark 1. General introduction Arenberg 23, 3001 Heverlee, Belgium. E-mail: [email protected] 1.1 Guerbet reaction b Center for Ordered Materials, Organometallics & Catalysis (COMOC), Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan The Guerbet reaction, as described in 1899 in the French 281-S3, 9000 Ghent, Belgium. E-mail: [email protected] journal Comptes Rendus,1 was named after its inventor Marcel c Industrial Catalysis and Adsorption Technology (INCAT), Department of Guerbet. The reaction is a condensation reaction of two alco- Industrial Technology and Construction, Faculty of Engineering & Architecture, ‘ ’ Ghent University, Valentin Vaerwyckweg 1, 9000 Ghent, Belgium. hols to the final Guerbet alcohol with the release of water, E-mail: [email protected] as shown in Scheme 1. A primary or secondary alcohol with a Dries Gabriëls obtained his MSc Willinton Y. Hernández studied in Engineering (Catalytic Tech- Chemistry at the National Uni- nology) at the Center for Surface versity of Colombia. He obtained Chemistry and Catalysis at KU his PhD in Chemistry at the Uni- Leuven in 2013. His master's the- versity of Seville in Spain (2010) sis was performed under the guid- by working on the design of anceofProf.B.Selsanddealt heterogeneous catalysts for the with the synthesis of hetero- preferential oxidation of CO in atomic zeolites and their appli- the presence of hydrogen. After a cation to acid catalysis in bio- post-doctoral period at the Uni- mass reactions. Currently, he is versity of Lyon-IRCELYON (Lyon, doing his PhD thesis investigat- France, from 2010 to 2013) and Dries Gabriëls ing heterogeneous catalysts for Willinton Yesid Hernández the ICIQ (Tarragona, Spain, the valorization of bio-alcohols from 2013 to 2014) working on by means of Guerbet condensation. automotive depollution systems and the water oxidation reaction, he joined the COMOC research group at Ghent University (Bel- gium). His current research is mostly focused on the valorization of bio-alcohols by the Guerbet reaction. 3876 | Catal. Sci. Technol.,2015,5, 3876–3902 This journal is © The Royal Society of Chemistry 2015 Catalysis Science & Technology Minireview methylene group adjacent to the hydroxylated carbon atom may be condensed with the same (self-condensation) or another alcohol (cross-condensation) resulting in a heavier and often branched alcohol containing the sum of the carbon atoms of the reactants. The rather complex Guerbet reaction requires the use of a catalytic system that exhibits at the same time acidic, basic and dehydrogenation/hydrogenation properties. A plurality of reactions is involved, thus the pro- cess is particularly sensitive and less predictable. Appropriate selection of reaction conditions and well-thought develop- ment of the catalytic system are of utmost importance as Scheme 1 Schematic representation of the Guerbet reaction for both the proportion and strength of each different site need primary alcohols. to be adequately tuned in order to maximize the yield of the target alcohols. This raises the question whether multifunc- tional catalysts should be developed, or whether the reaction should be executed in a multistage process using different catalysts, each of them bearing one catalytic function and iso- Prof. Bert F. Sels obtained his lated in different reactor types. PhD in 2000 at KU Leuven by In addition to higher alcohols formed by condensation working on oxidation chemistry, processes (self and/or cross-condensation ones), side reac- followed by a post-doc at BASF tions result in the formation of other compounds, such as until 2002 and post-doc for 3 esters and carboxylic acids, or salts thereof. Next to lowering years at the National Science the process efficiency, these unwanted products often poison Foundation. He became an assis- the catalytic system. It is therefore desirable to minimize the tant professor in 2003, teaching formation of such by-products. courses on analytical chemistry The investigation of Guerbet alcohols and their use in var- and heterogeneous catalysis. He ious fields is attractive due to the important advantages that is a full professor of the Faculty such alcohols have, as compared to their corresponding lin- of Bioengineering Sciences at ear isomers:2,3 Bert Sels Leuven since 2012. He has pub- 1. Branched Guerbet alcohols have a much lower melting lished 205 papers and 19 pat- point than their linear counterparts. They are used not only ents, and is a recipient of numerous awards including the DSM for the synthesis of functional liquids that have to remain Chemistry and ISGC Green Chemistry awards. His current fluid at very low temperatures, like some lubricants or research explores heterogeneous catalysis for renewables conver- hydraulic fluids for aircrafts, but also for their application as sion and its integration into bio-refinery concepts. jet fuel. Pascal Van Der Voort is a full An Verberckmoes obtained her professor at Ghent University MSc in Bio-engineering (Chemis- and the director of the research try) in 1994, her PhD in 1998 group COMOC that he founded (KU Leuven) and then joined the in 2007. The group consists of 5 R&D Center of ExxonMobil post-docs, 15 PhD students, 3 working in the area of catalyst technicians and typically 10 and process development which master students. His research resulted in 17 patents. In 2008, interests are mainly in the field she went to University College of porous materials, including Ghent and became an assistant MOFs, COFs, PMOs, oxides and professor in 2011. Since 2013, carbons for applications in chro- she has been an assistant profes- Pascal Van Der Voort matography, microelectronics, An Verberckmoes sor at Ghent University. She now urban mining, catalysis and teaches courses on organic chem- adsorption. He teaches general chemistry, solid state chemistry, istry and spectroscopy at the Faculty of Engineering and Architec- nanoporous materials and heterogeneous catalysis in several fac- ture. She co-heads the INCAT research group. Her current ulties of Ghent University. He has 172 papers indexed in the web research focuses on the synthesis optimization, scale-up and test- of science and holds several patents. ing of heterogeneous catalysts. This journal is © The Royal Society of Chemistry 2015 Catal. Sci. Technol.,2015,5, 3876–3902 | 3877 Minireview Catalysis Science & Technology 2. Cosmetic emollients made with Guerbet alcohols show technologies from low-cost and non-food biomass sources. good oxygen permeability due to branching of the alcohols. Promising results are reported for the production of ethanol This is an essential property for skin applications. Moreover, from cellulosic (second generation) and algal (third genera- – since the alcohols are completely saturated, they exhibit excel- tion) feedstocks.13 19 Next to carbohydrates, syngas can also – lent oxidation and color stability and do not become rancid. be fermented into alcohols.20 25 Syngas is a mixture 3. Because of branching, they are less viscous than their containing mainly CO, CO2,H2 and H2O that is obtained by linear equivalents, which is a desired property for surfactants gasification of coal or biomass, e.g. the thermochemical pro- in a number of detergent formulations where a low tempera- cess in which the carbon source reacts with air or oxygen.20 ture is desired. This allows for the utilization of a wider range of biomass 4. Finally, due to the unique position of the branches, types including low-cost organic waste without the need for Guerbet alcohols are more biodegradable than synthetic specific fractionation.26 The production of ethanol through branched alcohols, especially when the branches are made syngas fermentation is already available as a commercial pro- up of an even number of carbon atoms. cess, for instance in the INEOS Bio process.23,27 During the last three decades, numerous catalytic systems Next to ethanol, higher alcohols containing 3 to 5 carbon have been studied and proposed to promote the conversion atoms are also formed during fermentation. This mixture is and selectivity of the Guerbet reaction,