Upgrading of biomass-derived 2-hexanol to liquid transportation fuels on Cu–Mg–Al mixed oxides. Effect of Cu content
Graphical abstract
Introduction
In recent years, the increasing energy demand of the emerging economies and the environmental impacts associated with greenhouse gas emissions resulting from combustion of fossil fuels, among other factors, have motivated the search of new technologies for a sustainable production of transportation fuels. Thus, in the last decades several strategies have been postulated for the use of biomass as a renewable feedstock for this purpose. Unlike fossil fuels, the CO2 produced during combustion of biomass-derived liquid fuels is consumed during plant growth. However, the current biofuel production is still far from being considered a carbon neutral technology [1], [2]. In addition to the technical advantages, the use of liquid biofuels produced locally would generate geopolitical and social benefits such as the reduction of foreign oil dependency and the strengthening of the job market in the agricultural, forest and fuel sectors [3].
In particular, production of two biomass-derived liquid fuels (biodiesel and bioethanol), the so-called first generation biofuels, has been recently commercially implemented in the transportation sector. However, these solutions face technical limitations related to the blending properties of the resulting biofuel, they represent a small fraction of the total energy contained in the biomass and they require many energy-consuming separation/purification steps. In addition, production of first-generation biofuels entails ethical concerns because of the consumption of edible resources such as sucrose and triglycerides [4], [5]. Contrarily, lignocellulose, the most abundant carbohydrate in nature, can be obtained from non-edible sources and therefore, offers an attractive option as an inexpensive feedstock for the efficient liquid fuel production in a biorefinery [6], [7].
Lignocellulosic biomass can be treated by different procedures such as gasification, pyrolysis and hydrolysis to obtain by further processing, gasoline, gasoline components and fuels with specific molecular weights, respectively [4], [8]. In particular, chemical or biological hydrolysis of lignocellulose is used to separate lignin from sugars. The latter, in turn, can be converted in the biorefinery, resulting in the production of valuable chemicals and transportation fuels [3], [9].
Liquid transportation fuels consist of nonoxygenated hydrocarbons in the range of C5–C12 for gasoline, C9–C16 for jet fuel and C12–C20 for diesel [4]. Since sugars have a high O/C atom ratio (O:C = 1:1) which is undesirable for liquid fuel applications, the strategies to convert sugars must include oxygen removal as well as CC bond formation steps to obtain fuels with appropriate distillation, thermal stability and flashpoint properties as well as the suitable molecular weights for gasoline, diesel or jet fuel applications [4], [10].
Several strategies have been postulated for the conversion of carbohydrates such as glucose and xylose into liquid fuels. The primary conversion of these sugars affords formation of hydrogen and functional intermediates by a combination of reforming, reduction and dehydration reactions. These intermediates, so called “platform molecules”, comprise C4–C6 oxygenates such as secondary alcohols, ketones, furan-derivatives and acids [4], [10], [11]. For instance, West et al. [12] studied the transformation of sorbitol and glucose on a PtRe/C catalyst and found that at 100% sugar conversion the resulting organic phase contained ∼20% of a mixture of C4–C6 secondary alcohols with 2-hexanol being the main constituent (up to 44% of the total secondary C4–C6 alcohols). Scheme S1 summarizes the biorefinery processing of sugars toward platform molecules, in particular, toward formation of secondary C6 alcohols [10], [12]. These platform molecules can be then converted in liquid transportation fuels or fuel additives by different routes such as dehydration, alkylation and aromatization or by CC bond formation, hydrogenation and hydrodeoxygenation [4], [5], [11], [13]. In particular, Kunkes et al. [10] used bifunctional Cu–Mg–Al mixed oxides, that we had successfully developed years ago for 2-propanol conversion [14], for the upgrading of the C4–C6 fraction to C6–C12 compounds containing one or none oxygen atoms. We showed that Cu–Mg–Al catalysts promote also other reactions such as the gas-phase reduction of α,β-unsaturated ketones by hydrogen transfer reactions [15] and the upgrading of diols to valuable oxygenates [16]. In the last decade, other researchers have shown that similar Cu–Mg–Al mixed oxides are suitable catalysts for CC coupling of methanol and ethanol [17], aqueous-phase upgrading of bio-oil [18], total oxidation of methane and other VOCs [19], [20], [21], synthesis of 1,2-propanediol from glycerol [22], phenol hydroxylation [23] and cinnamaldehyde hydrogenation [24], among many other applications.
Following the approach of Kunkes et al. [10], we recently published our investigations on the synthesis of liquid transportation fuels using 2-hexanol as a model “platform molecule” derived from sugars [25], [26]. We reported that the gas-phase conversion of 2-hexanol carried out at 100 kPa and 573 K on Cu-containing mixed oxides with different acid-base properties produces high yields (≈90%) of a mixture of C9–C24 oxygenates and hydrocarbons either under inert or reducing atmosphere (gas-phase H2). Around 65–75% of the C9–C24 product pool is in the carbon atom range for jet fuel applications. The liquid product has an O/C atomic ratio as low as 0.025. We found that the main reaction pathway toward even carbon atom number products involves tandem dehydrogenation/CC coupling/dehydration/hydrogenation reactions. However, the reaction pathway shifts toward odd products under operating conditions deprived of gas-phase hydrogen. We elucidated the bifunctional nature of the catalytic process and the participation of Cu0 and acid-base sites in promoting the different reaction steps. Also, we demonstrated that the relative abundance of these sites determines the rate-limiting step.
In this work we continue our investigations on the 2-hexanol upgrading by oxygen removal and CC coupling reactions with the aim of stablishing the effect of Cu content on catalyst activity and selectivity. A set of Cu–Mg–Al mixed oxides with a wide range of copper content (0.3–61.2 wt.%) was prepared, characterized by several techniques and tested in the gas-phase conversion of 2-hexanol. The dependence of the reducibility and basic nature of the mixed oxides on the copper loading was studied, as well as how these properties affect the catalytic activity, selectivity and quality of the liquid products. In particular, we investigated how the odd/even carbon atom number product ratio, average molecular weight and oxygen content can be tuned by adjusting the catalyst formulation. The influence of the balance between metal and base sites on determining the rate-limiting step of the bifunctional metal/base mechanism operating at high copper loadings was demonstrated. Furthermore, evidence of the shift to a base-catalyzed mechanism at very low copper contents was provided.
Section snippets
Catalyst synthesis and characterization
Binary or ternary catalyst precursors of Cu-containing mixed oxides were prepared under similar conditions, as detailed elsewhere [14]. In brief, the catalyst precursors consisting of hydroxycarbonates of the metal cations were obtained by co-precipitation; an aqueous solution of the metal nitrates with a total cation concentration of 1.5 M was contacted with an aqueous solution of KOH and K2CO3 at a constant pH of 10. All the reagents were analytical grade. After filtering, washing and drying
Characterization of the ZCuMgAl mixed oxides
Catalysts combining a metallic function provided by Cu0 atoms and acid-base sites supplied by Lewis acid cations were prepared by coprecipitation and characterized by several techniques. They were denoted as ZCuMI(MII), where Z represents the copper content in wt.% and MI and MII are the accompanying cations (Mg2+, Al3+ or Ce4+). Binary Cu–Mg, Cu–Ce and Cu–Al, and ternary Cu–Mg–Ce mixed oxides containing similar copper loadings were denoted as 9.8CuMg, 7.4CuCe, 6.4CuAl and 6.9CuMgCe,
Conclusions
Cu–Mg–Al mixed oxides with a wide range of copper loadings (Z = 0.3–61.2 wt.% Cu) are active and selective for the upgrading of 2-hexanol toward liquid fuels, i.e., C9–C18+ compounds with low oxygen content. C9–C24 compounds are the main fraction with yields of up to 87%. This product pool is a hydrophobic mixture of compounds having one or none oxygen atom (ketones, alcohols and hydrocarbons), in which the O/C atom ratio is as low as 0.04.
Formation of C9–C24 compounds occurs through consecutive
Acknowledgements
Authors thank the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), Argentina (grant PICT 1888/10), CONICET, Argentina (grant PIP 11220090100203/10) and Universidad Nacional del Litoral, Santa Fe, Argentina (grant CAID PI 64-103/11) for financial support of this work.
References (54)
- et al.
An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates
Catal Today
(2007) - et al.
Catalytic conversion of biomass-derived carbohydrates to fuels and chemicals by formation and upgrading of mono-functional hydrocarbon intermediates
Catal Today
(2009) - et al.
One-step MIBK synthesis: a new process from 2-propanol
J Catal
(2002) - et al.
Vapor-phase methanol and ethanol coupling reactions on CuMgAl mixed metal oxides
Appl Catal A: Gen
(2013) - et al.
Aqueous-phase catalytic hydrogenation of furfural to cyclopentanol over Cu–Mg–Al hydrotalcites derived catalysts: Model reaction for upgrading of bio-oil
J Energy Chem
(2014) - et al.
New Cu-based mixed oxides obtained from LDH precursors, catalysts for methane total oxidation
Appl Catal A: Gen
(2009) - et al.
Total oxidation of selected mono-carbon VOCs over hydrotalcite originated metal oxide catalysts
Catal Commun
(2012) - et al.
Synthesis and characterization of CuMgAl ternary hydrotalcites as catalysts for the hydroxylation of phenol
J Catal
(2005) - et al.
On the catalytic properties of mixed oxides obtained from the Cu–Mg–Al LDH precursors in the process of hydrogenation of the cinnamaldehyde
Appl Catal A: Gen
(2004) - et al.
Liquid transportation fuels from biomass-derived oxygenates: gas-phase 2-hexanol upgrading on Cu-based mixed oxides
Appl Catal A: Gen
(2015)
Conversion of diols by dehydrogenation and dehydration reactions on silica-supported copper catalysts
Appl Catal A: Gen
Temperature-programmed reduction. Parametric sensitivity and estimation of kinetic parameters
J Catal
Distinction between surface and bulk oxidation of Cu through N2O decomposition
J Catal
Effect of the reduction treatment on the structure and reactivity of silica-supported copper particles
J Catal
Dispersion and surface states of copper catalysts by temperature-programmed-reduction of oxidized surfaces (s-TPR)
Appl Catal A: Gen
Characterization of Cu/SiO2 catalysts prepared by ion-exchange for methanol dehydrogenation
Appl Catal A: Gen
An improved procedure for estimating the metal surface area of supported copper catalysts
J Catal
Vapor-phase C–C coupling reactions of biomass-derived oxygenates over Pd/CeZrOx catalysts
J Catal
Gas-phase conversion of 1,3-butanediol on single acid-base and Cu-promoted oxides
Catal Today
Monoglyceride synthesis by glycerolysis of methyl oleate on solid acid-base catalysts
Chem Eng J
Effect of the chemical composition on the catalytic performance
J Catal
Structure and surface and catalytic properties of Mg–Al basic oxides
J Catal
Low temperature water gas-shift catalysts
J Mole Catal A: Chem
Kinetic study of the reduction of copper- zinc-aluminum mixed oxide catalysts
React Solids
One-step MIBK synthesis from 2-propanol: catalyst and reaction condition optimization
Appl Catal A: Gen
Effect of structural and acidity/basicity changes of CuO–CeO2 Catalysts on their activity for water–gas shift reaction
Catal Today
Direct synthesis of dimethyl ether from biomass-derived syngas over Cu–ZnO–Al2O3–ZrO2(x)/γ-Al2O3 bifunctional catalysts: effect of Zr-loading
Fuel Process Technol
Cited by (16)
Sustainable hydrogenation of limonene to value-added products using Cu–Ni catalysts supported on KIT-5
2024, Journal of Cleaner ProductionThe green-ol (green-alcohol) economy
2023, Nano EnergyThe effect of alkali metals doping on properties of CuMgCe for isobutanol synthesis from syngas
2022, Applied Catalysis A: GeneralModeling and optimization of bio-2-hexanol production from biomass derived dimethylfuran using Pt/K<inf>3</inf>PW<inf>12</inf>O<inf>40</inf> by response surface methodology
2021, Computers and Chemical EngineeringCitation Excerpt :The product pool was a mixture of ketones, alcohols and hydrocarbons with 160–200 g/mol average molecular weight, and up to 87% of the yield were obtained for compounds in the C9–C24 range. Around 70% of the products were C9–C15 compounds suitable for jet fuel applications and the rest as diesel substitutes (Luggren et al., 2016). HXL is also important for liquid phase C-C self-coupling that produces higher dimers with a yield of 79% and trimers with a yield of 11% at complete conversion (Shimura et al., 2013).
Deactivation of Cu–Mg–Al mixed oxide catalysts for liquid transportation fuel synthesis from biomass-derived resources
2020, Molecular CatalysisCitation Excerpt :At low Z values (catalysts with predominant basic properties and high nb values, Table 1) the contribution of the h.t.p. is notoriously enhanced at the expense of the l.t.p. As explained above, in these materials copper is diluted and buried inside the mixed oxide matrix making the catalyst reduction difficult under standard conditions [13]; coke therefore forms mainly on the abundant basic sites giving rise to the h.t.p. The shift of the latter to lower temperatures at higher copper loadings might be ascribed to a catalytic effect of the increasing amounts of the vicinal Cu° sites.