Research Article pubs.acs.org/journal/ascecg
Detailed Kinetic Investigations on the Selective Oxidation of Biomass to Formic Acid (OxFA Process) Using Model Substrates and Real Biomass Jenny Reichert and Jakob Albert* Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: A detailed kinetic analysis of the selective catalytic oxidation of different biogenic model substrates to formic acid (OxFA process) is presented using two different polyoxometalate catalysts, HPA2 (H5PV2Mo10O40) and HPA-5 (H8PV5Mo7O40). By using high oxygen pressures (30−60 bar) and temperatures of 80−90 °C in aqueous solution, we were able to investigate the rate-determining substrate oxidation step catalyzed by the oxidized form of the polyoxometalates (POMs) by keeping the concentration of the catalytic active species constant. Under these conditions, kinetic parameters like effective reaction order and reaction rate constants could be determined by the differential method for model substrates like glucose, fructose, sorbitol, and gluconic acid. Furthermore, kinetic experiments using the two different catalysts showed a different behavior for various functionalities within the carbon framework of the substrate with the tendency of higher reaction rates with the HPA-5 catalyst. This effect was more pronounced for disaccharides like cellobiose and sucrose than for monosaccharides. Finally, a detailed kinetic study for water-insoluble biomasses like beech and spruce wood showed differences in formic acid (FA) formation and product selectivities depending on the composition of the wood in terms of their ratios of lignin, cellulose, and hemicellulose. KEYWORDS: Kinetic study, Biomass oxidation, Polyoxometalates, OxFA process
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INTRODUCTION
Since 2011, our group has been active in exploring an alternative way to convert biomass into valuable chemicals which differs from the ones mentioned above.15−19 In this context, we first presented a catalytic reaction in 2011 that selectively converts biomass into formic acid (FA).15 Later, the name “OxFA process” was coined for it.16 This biomass oxidation process is mildly exothermic and operates under mild temperature conditions of typically below 373 K using molecular oxygen or synthetic air as environmentally benign oxidants. As water is applied as solvent, biomass of different origins, compositions, and humidities can be applied without drying. As FA is produced as the only product in the liquid phase, tedious separation processes can be avoided. The only side-product is pure carbon dioxide that is formed in the gas phase and can be reused for the production of a new biomass substrate, i.e., algae. As the OxFA process converts a very broad range of biogenic raw materials into only two products that separate nicely into gas and liquid phases, its simplicity and robustness are clear advantages compared to other biomass valorization technologies. The isolated FA product can be used in a broad range of applications, namely, in the chemical, agricultural, textile,
Nowadays, crude oil, gas, and coal are the world’s main energy resources. However, at some point those sources will be diminished since the reproduction of fossil fuels is much slower than its depletion. This fact makes the research for alternative renewable energy sources more and more essential.1 Renewable energy is defined as the energy obtained from the naturally repetitive and persistent flow of energy occurring in the local environment. Renewable energy systems include, for example, solar radiation, wind, biomass, rivers, tides, and geothermal heat.2 One promising approach is the energetic use of biomass. In this context, the conversion of biomass to energy carrier molecules and fuels is most relevant as all the carbon in biomass stems from CO2 that has been previously captured from the atmosphere.3−5 Biomass can be used as a future raw material for the generation of energy and for the production of basic chemicals, for example, in biomass gasification,6 biomass depolymerization,7 pyrolysis,8 and dehydrogenation.9 Moreover, the catalytic oxidation chemistry of biomass is a growing field of research.10−14 The main challenge is to overcome the recalcitrant nature of the inhomogeneous carbon framework in natural biogenic substrates. In a first reaction step, the biopolymers have to be depolymerized in an acidic environment followed by an oxidative cleavage of the carbon bonds in the biomolecule to lower molecular fragments. © 2017 American Chemical Society
Received: May 31, 2017 Revised: July 5, 2017 Published: July 11, 2017 7383
DOI: 10.1021/acssuschemeng.7b01723 ACS Sustainable Chem. Eng. 2017, 5, 7383−7392
Research Article
ACS Sustainable Chemistry & Engineering leather, pharmaceutical, and rubber industries.20 More recently, FA has been proposed as a suitable hydrogen storage compound because it can be easily and selectively decomposed to hydrogen and CO2 through metal-catalyzed processes under very mild conditions.21−24 FA contains 4.4 wt % hydrogen25 which can be simply released by metal-catalyzed processes under very mild conditions (
