Liquid-phase glycerol hydrogenolysis by formic acid over Ni–Cu/Al2O3 catalysts
Graphical abstract
Highlights
► Glycerol hydrogenolysis over Ni–Cu/Al2O3, N2 pressure, and formic acid as H-donor. ► Ni–Cu alloy formation decreases C–C bond cleavage and increases 1,2-PDO selectivity. ► Acid sites and metal sites play a role in H-transfer from formic acid to glycerol. ► The OH groups of 1,2-PDO and glycerol compete for adsorption in the acid sites. ► A 90% glycerol conversion and 80% selectivity to 1,2-PDO were reported after 24 h.
Introduction
It is widely assumed that the dependence of human well-being on fossil fuels should be reduced, mainly due to geopolitical, natural resource scarcity, and environmental factors. Biomass appears as the only renewable source for liquid fuels and plastics [1]. Main biomass conversion processes that are being currently developed, like bio-oils from the pyrolysis or high-pressure liquefaction of biomass [2], [3], sorbitol valorization to polyols [4], or glycerol hydrogenolysis to propanediols (PDO) [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], require oxygen removal reactions. In these hydrogenation/hydrogenolysis reactions, high hydrogen pressures are required to reach acceptable conversions and selectivities. Molecular hydrogen is easily ignited in contact with air and shows high diffusivity; therefore, it presents considerable hazards when working at high pressures. In addition, most of the nowadays available hydrogen gas is produced from fossil fuels by energy intensive processes [18]. In situ generation of the hydrogen can be a promising alternative to avoid the inherent drawbacks of working with molecular hydrogen. Nevertheless, the so far published results on liquid-phase glycerol hydrogenolysis generating the hydrogen by simultaneous glycerol reforming [19], [20], [21] are still far to the best results reported under H2 pressure [12], [15] (see Table 1).
Another interesting option that allows working under inert atmosphere is to use hydrogen donor molecules. However, catalytic transfer hydrogenation reactions have not been extensively reported in the glycerol hydrogenolysis process. Musolino et al. studied 2-propanol and ethanol as solvents and hydrogen donor molecules for the process under inert atmosphere [22] or low hydrogen pressure [23]. It was recently reported that the reaction mechanism is different regarding the origin of the active hydrogen is molecular hydrogen or a hydrogen donor molecule. Under H2 pressure, glycerol is first dehydrated to acetol, which subsequently undergoes a hydrogenation process to give 1,2-PDO [24] (Scheme 1A). On the other hand, when the hydrogen atoms are produced from 2-propanol dehydrogenation, glycerol is directly converted to 1,2-PDO through intermediate alkoxide formation (Scheme 1B). Moreover, it was also observed that the hydrogen donor and glycerol compete for the same active sites [25]. In a previous work, not shown here, a semi-continuous process was developed in which the hydrogen donor molecule (2-propanol, methanol, or formic acid) was continuously fed into the autoclave containing the glycerol aqueous solution and Ni–Cu/Al2O3 catalyst. It was observed that there was an optimum feeding rate for each donor that maximized 1,2-PDO production, and that the bests results in terms of glycerol conversion and 1,2-PDO selectivity were obtained using formic acid.
An additional advantage of using formic acid as hydrogen donor is that it can be obtained from renewable resources through the Biofine Process. This process transforms non-food biomass feedstock by acid-catalyzed hydrolysis to give levulinic acid and formic acid, together with some furfural and a char residue [26]. Formic acid is also considered a promising H2 storage as it can be obtained through CO2 hydrogenation [27], [28]. Formic acid can be catalytically dehydrogenated to give hydrogen and CO2. A parallel dehydration reaction gives water and CO, which can be further converted to CO2 and hydrogen through water–gas shift reaction [29]. The selectivity to CO2 and hydrogen depends on the operating condition and the catalyst used.
This paper studies the role of supported Ni, Cu, and acid catalytic sites in the liquid-phase glycerol hydrogenolysis to 1,2-PDO when formic acid is used as the hydrogen donor molecule.
Section snippets
Catalysts preparation
Ni–Cu/Al2O3 catalysts were prepared by the sol–gel method. Aluminium isopropoxide (Aldrich) was dissolved in deionized water (9 mL of H2O per gram of aluminium isopropoxide) by vigorous stirring of the solution at 313 K. The pH was measured and kept between 3.8 and 4.2 adding the required amounts of HNO3 (1.0 M). Simultaneously, nickel (II) nitrate hexahydrate (Aldrich) and/or copper (II) nitrate hemi pentahydrate (Alfa Aesar) were dissolved in ethanol. The precursor solution was slowly added to
Optimization of the Cu/Ni ratio
In order to optimize the Cu/Ni ratio, six different Ni–Cu/Al2O3 catalysts were prepared by sol–gel method with a constant total nominal metal content of 35 wt.% and different proportions of Ni and Cu. These catalysts were deeply characterized and tested in the glycerol hydrogenolysis reaction using formic acid as hydrogen donor molecule.
Conclusions
In the present work, the performance of Ni–Cu/Al2O3 bimetallic catalysts on the glycerol hydrogenolysis to 1,2-PDO under inert atmosphere and using formic acid as the source of hydrogen was studied. It was observed that there is an optimum Cu/Ni ratio and also an optimum in the ratio and distribution of metal and acid sites that maximize the yield of 1,2-PDO. The glycerol hydrogenolysis occurs when glycerol is adsorbed on an acid site of Al2O3 to form a secondary alkoxide, and when this
Acknowledgments
This work was supported by funds from the Spanish Ministry of Science and Innovation ENE2009-12743-C04-04, and from the Basque Government (Researcher Training Programme of the Department of Education, Universities and Research). The authors greatly acknowledge Drs Schneider, Pohl, Radnik, Mr. Eckelt and Ms Evert for the work done in the characterization of the catalysts, and the Inorganic Chemistry Department at the University of Malaga for their technical support.
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