This journal is©The Royal Society of Chemistry 2015 Chem. Soc. Rev.
Cite this:DOI: 10.1039/c5cs00859j
Potential and challenges of zeolite chemistry in the catalytic conversion of biomass
Thijs Ennaert, Joost Van Aelst, Jan Dijkmans, Rik De Clercq, Wouter Schutyser,
Michiel Dusselier, Danny Verboekend* and Bert F. Sels*
Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Brønsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.
The limited supply of cheap petroleum has renewed the interest towards more sustainable energy sources like wind, solar and hydro technology. While these renewable energy sources have the potential to constitute an important part of the energetic matrix, the production of chemicals and fuels requires a renewable carbon source, such as biomass (Fig. 1). Accordingly, these possibilities have sprouted a widely-varied research attention on the valorisation of various biomass resources (Fig. 2).
Major streams of biomass are lignocellulose, lipids containing triglycerides from animal fats, vegetable origin and microalgae, and turpentine streams. Of these, lignocellulose, composed of cellulose, hemicellulose and lignin, represents the most abundant one. Finally, there is a smaller group of proteinbased fractions of animal or vegetable origin (Fig. 1).1,2 The structural nature of these molecules determines the ultimate
Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering,
KU Leuven, Celestijnenlaan 200f, B-3001, Heverlee, Belgium.
E-mail: firstname.lastname@example.org, email@example.com;
Fax: +32 16 321 998; Tel: +32 16 321 610
Thijs Ennaert obtained his MSc in
Catalytic Technology (Bioscience
Engineering) at KU Leuven (Belgium) in 2012. He did his master thesis at the Center for
Surface Chemistry and Catalysis, where he explored the stability of
USY zeolites in hot liquid water.
He is currently doing a PhD as a
IWT-funded fellow, also under the guidance of Prof. Bert
F. Sels. His research focuses on the stability and application of zeolite catalysts in biomass transformations.
Danny Verboekend obtained his
PhD at ETH Zurich (Switzerland, 2012) on the preparation and exploitation of hierarchical zeolite catalysts. Afterwards, he continued to work at the same university in the context of a post-doctoral stay (2012–2014).
He is currently working at the
KU Leuven (Belgium) as a FWOfunded post-doctoral fellow on the topic of heterogeneous catalysis. His general interest focuses on combining applied and sustainable research with the highest degree of fundamental understanding.
Received 16th November 2015
DOI: 10.1039/c5cs00859j www.rsc.org/chemsocrev
Chem Soc Rev
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Chem. Soc. Rev. This journal is©The Royal Society of Chemistry 2015 transformation routes needed to convert them to useful fuels, chemicals and materials.
Three distinct modes of valorising the carbon content of biomass may be distinguished (Fig. 3). The first mode represents biological transformations, exemplified by microbial or yeast-based fermentation processes, such as the commercial conversion of carbohydrate biomass into ethanol or lactic acid.3–8
The second mode is thermal, ranging from burning biomass for heat generation over gasification to biogas or liquefaction and pyrolysis to bio-oils.9 The third mode encompasses all specific chemical and catalytic processes, which target a single chemical or narrow range of products, perhaps with the exception of catalytic fast pyrolysis (CFP).
Zeolite catalysis plays a significant role in many of the chemical biomass conversion routes (Fig. 2 and 3). First, there is the concept of blending-in biomass fractions, pyrolysis oils or platform molecules into existing petroleum refinery operations.10–12 Secondly, in CFP, either on platform compounds, such as glucose or furans, or raw biomass feedstock, zeolites have proven promising efficiency.13–16 Thirdly, specific liquid phase catalytic processes have been proposed, where zeolites contribute in converting raw biomass feedstock. The last, and most documented use of zeolites in biomass valorisation, is found one-step down from the raw or pretreated feedstock in
Fig. 1, namely in the upgrading of bio-derived platformmolecules to fuels, chemicals and materials by specific transformations. In general, these processes target the on-purpose synthesis of one final product or intermediate from a biomass-derived (platform) molecule.10,12,17–25
The wide-scale application of zeolites in biomass conversion can be explained by their numerous positive attributes. Zeolites are aluminosilicates with a crystalline, microporous framework built from oxide tetrahedra.26 Accordingly, zeolite crystals are highly porous, with precisely-defined micropores (0.4–1 nm).
Combined with the ability to load them with exchangeable cations makes them useful as adsorbents, molecular sieves, ion exchangers and catalysts. For example, since breakthrough work in the early 1960s, synthetic zeolites have become the most prominent heterogeneous catalysts in the refining and petrochemical industries.27–31