Elsevier

Fuel

Volume 177, 1 August 2016, Pages 28-38
Fuel

Upgrading of biomass-derived 2-hexanol to liquid transportation fuels on Cu–Mg–Al mixed oxides. Effect of Cu content

https://doi.org/10.1016/j.fuel.2016.02.084Get rights and content

Highlights

  • Cu–Mg–Al mixed oxides promote 2-hexanol upgrading to liquid fuels.

  • C9–C24 oxygenates and hydrocarbons are main products.

  • ∼70% of the products are C9–C15 compounds for jet fuel applications.

  • Product quality depends on the Cu content.

  • Reaction mechanism changes with the base/metal site ratio.

Abstract

The gas-phase synthesis of high molecular weight compounds of application as liquid transportation fuels from 2-hexanol was studied on Cu–Mg–Al mixed oxides with different copper content (0.3–61.2%) and a Mg/Al = 1.5 M ratio. Catalysts were prepared by coprecipitation and characterized by several techniques such as BET surface area, XRD, TPD of CO2, TPR and N2O decomposition. Yields of up to 87% were obtained for compounds in the C9–C24 range, ≈80% of which were suitable as jet fuels and the rest as diesel substitutes. This product pool was a hydrophobic mixture of ketones, alcohols and hydrocarbons with 160–200 g/mol average molecular weight and an O/C atomic ratio as low as 0.04. Because low copper content catalysts are hard to reduce, on these materials the reaction occurs via a base-catalyzed mechanism involving consecutive dehydrogenation, Csingle bondC bond formation, dehydration and hydrogenation steps, that forms mainly even carbon atom number products. Partially reduced Cun+ atoms contribute to promote a distinct pathway toward odd products. In contrast, on high copper content oxides the reaction yields similar amounts of even and odd products and proceeds by a bifunctional Cu0-base mechanism in which the Csingle bondC coupling is rate-limiting.

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 Csingle bondC 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 Csingle bondC 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 Csingle bondC 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/Csingle bondC 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 Csingle bondC 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.

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