The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
Graphical abstract
Introduction
In recent years increasing bio-diesel production resulted in an excess production of glycerol, which is the main by-product of this process [[1], [2], [3]]. The further utilization of biomass-based aqueous glycerol solutions to value-added products such as ethers [4,5], esters [[6], [7], [8]], propanediols [[9], [10], [11]] and acrolein [[12], [13], [14]] is of great interest to increase the economic efficiency and environmental sustainability of bio-refineries. A promising utilization route is the selective hydrodeoxygenation (HDO) of glycerol to 1,2-propanediol (1,2 PDO). HDO requires the selective cleavage of a terminal CO bond while preserving CC bonds, whereas hydrogenolysis refers in particular to the cleavage of CC bonds. Most previous studies mainly focused on developing suitable HDO catalysts such as supported noble (e.g., Pt, Ru) [[15], [16], [17]] and less active, but selective non-noble (e.g., Cu, Ni, Co) [[18], [19], [20]] metals and characterising their relevant properties such as acidity, surface areas and metal dispersion [[21], [22], [23]]. Among these two classes of catalysts, Cu-based catalysts were found to be optimum, combining high selectivity to 1,2PDO and moderate activity [[24], [25], [26]]. Owing to the mechanistic complexity of HDO, the number of studies focussing on reaction kinetics is rather low.
A kinetic analysis of glycerol HDO over Cu:Zn:Cr:Zr mixed metal oxide catalysts was conducted by Sharma et al. [27] using high initial glycerol concentrations up to 100%. The addition of Zn and Zr to the catalyst matrix significantly increased glycerol conversion and the selectivity to 1,2-PDO, which was ascribed to enhanced bifunctionality. Assuming a Langmuir-Hinshelwood-Hougen-Watson reaction mechanism following the dehydration-hydrogenation pathway, pseudo-first order kinetics for glycerol consumption and an activation energy of 132 kJ mol−1 were derived for the HDO route.
Vasiliadou et al. [28,29] studied the reaction kinetics of glycerol HDO over a 18 wt% Cu/SiO2 catalyst. Two parallel routes leading to the formation of 1,2-PDO (95% selectivity) and 1,3-PDO (main by-product) were considered, and the kinetic parameters were derived assuming a power-law kinetic model. A reaction order of 0.17 was found for the overall glycerol consumption, and a reaction order of 1.06 was derived considering the H2 concentration in the liquid phase. The activation energies for the 1,2-PDO and 1,3-PDO routes were found to be 94.3 and 135.3 kJ mol−1, respectively. However, this work was performed using 1-butanol as solvent. The consideration of possible solvent effects is an important factor for liquid-phase reactions, and water is usually preferred to any organic solvents. Bienholz et al. [30] reported that glycerol conversion increased from 5% to 55% over a Cu/ZnO catalyst when water was replaced by 1,2-butanediol as solvent. N2O reactive frontal chromatography measurements using the catalyst before and after reaction revealed that the use of 1,2-butanediol led to a lower decrease of the Cu surface area due to inhibited growth of the Cu particles. Wang et al. [31] studied solvent effects with a Cu/ZnO catalyst using water, methanol and ethanol. It was found that solvents with low surface tension such as methanol and ethanol tend to inhibit the agglomeration of Cu particles, thus resulting in higher degrees of Cu dispersion and higher glycerol conversion, while highly polar solvents such as water tend to increase the product selectivity to 1,2-PDO.
In our previous study, HDO of glycerol over Cu/ZrO2 in the aqueous phase was found to proceed via a two-step dehydration-hydrogenation reaction pathway, yielding 1,2-PDO as main product with a high selectivity of 95% at 31% conversion under moderate reaction conditions (200 °C, 25 bar H2, 8 h reaction time) [32]. A linear dependence of glycerol conversion on the specific Cu surface area was observed. Thus, metallic Cu was identified as the active site for both dehydration and hydrogenation, whereas the acidic sites of the ZrO2 support had no detectable effect on the catalytic activity under the applied aqueous reaction conditions.
The aim of the present work is to investigate glycerol HDO kinetics under aqueous conditions in the presence of an active, highly selective and rather stable Cu/ZrO2 catalyst. In contrast to other kinetic studies claiming acid sites of the support to catalyse dehydration followed by hydrogenation on metallic sites, here a kinetic study over Cu-based catalysts is reported, which provides no evidence for bifunctionality. The reaction kinetics of glycerol HDO was first studied over Cu/ZrO2 with 18 wt% CuO nominal loading by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) as well as the initial glycerol concentration (2–8 wt%). Unsupported pure metallic Cu, derived from the reduction of commercial CuO nanopowder, was used as reference catalyst in comparison to the Cu/ZrO2 catalyst to prove the dominant role of metallic Cu in the HDO of glycerol kinetically.
Section snippets
Materials
The Cu/ZrO2 catalyst was prepared using copper nitrate (Cu(NO3)2 ∙ 3 H2O, Sigma Aldrich, 99.999%), zirconium oxynitrate (ZrO(NO3)2 ∙ 3 H2O, Sigma Aldrich, 99.0%) and sodium hydroxide (NaOH, VWR Chemicals, 98.5–100.5%). Glycerol (VWR Chemicals, 99.5%), 1,2-propenediol (Fluka, 99.5%), ethylene glycol (Alfa Aesar, 99%), hydroxyacetone (Alfa Aesar, 95%) and methanol (Fischer Chemicals, 99.8%) were used for experiments and GC calibration. CuO powder (Sigma Aldrich, < 50 nm) was used as reference
Results and discussion
The HDO of glycerol to 1,2-PDO over Cu/ZrO2 proceeds via a two-step dehydration-hydrogenation reaction pathway forming acetol as dehydration intermediate, which is further hydrogenated to 1,2PDO with approximately 95% selectivity [32]. The parallel direct CC bonds hydrogenolysis leading to ethylene glycol and methanol is the main side reaction with about 5% selectivity. The formation of 1,3-PDO resulting from HDO of the secondary C–O bond has not been observed. Further degradation products such
Conclusions
The reaction kinetics of glycerol hydrodeoxygenation and the competing CC hydrogenolysis was studied over a co-precipitated Cu/ZrO2 catalyst under aqueous conditions. The kinetic experiments were performed by varying the reaction parameters such as temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). Glycerol conversion was strongly influenced by temperature and H2 pressure leading up to 93% conversion at 225 °C and 25 bar H2 pressure. The
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