Elsevier

Catalysis Today

Volume 195, Issue 1, 15 November 2012, Pages 162-168
Catalysis Today

Catalytic transformations of biomass-derived acids into advanced biofuels

https://doi.org/10.1016/j.cattod.2012.01.009Get rights and content

Abstract

Biomass can efficiently replace petroleum in the production of fuels for the transportation sector. One effective strategy for the processing of complex biomass feedstocks involves previous conversion into simpler compounds (platform molecules) which are more easily transformed in subsequent upgrading reactions. Lactic acid and levulinic acid are two of these relevant biomass derivatives which can easily be derived from biomass sources by means of microbial and/or chemical routes. The present paper intends to cover the most relevant catalytic strategies designed today for the conversion of these molecules into advanced biofuels (e.g. higher alcohols, liquid hydrocarbon fuels) which are fully compatible with the existing hydrocarbons-based transportation infrastructure. The routes described herein involve: (i) deoxygenation reactions which are required for controlling reactivity and for increasing energy density of highly functionalized lactic and levulinic acid combined with (ii) Csingle bondC coupling reactions for increasing molecular weight of less-oxygenated reactive intermediates.

Highlights

► Biomass has the potential to replace petroleum in the production of transportation fuels. ► There is growing need for high-energy density, infrastructure-compatible biofuels (advanced biofuels). ► We explore potential of lactic and levulinic acids in the catalytic production of advanced biofuels. ► Catalytic routes involve oxygen removal combined with Csingle bondC coupling reactions.

Introduction

Fossil fuels are the primary source of energy, chemicals and materials for our modern society. Petroleum, natural gas and coal supply most of the energy consumed worldwide and their massive utilization has allowed our society to reach high levels of development in the past century. However, these natural resources are highly contaminant, unevenly distributed around the world and they are in diminishing supply. These important concerns have stimulated the search for new well-distributed and non-contaminant renewable sources of energy such as solar, wind, hydroelectric power, geothermal activity, and biomass. This shift toward a renewable-based economy is currently spurred by governments which have established ambitious targets to replace an important fraction of fossil fuels with renewable sources within next 20 years [1], [2]. In this sense, biomass is considered the only sustainable source of organic carbon currently available on earth and, consequently, it is the ideal substitute for petroleum in the production of fuels, chemicals and carbon-based materials [3].

Transportation sector of our society heavily relies on petroleum which accounts for essentially all (96%) of the transportation energy. This high reliance on petroleum is especially relevant since transportation is the largest and fastest growing energy sector, and it is responsible for almost one third of the total energy consumed in the world [4]. A large fraction of the extracted petroleum (70–80%) is consumed in form of transportation fuels (e.g. diesel, gasoline and jet fuels) in an attempt to cover this enormous demand for transportation energy. Consumption of petroleum is currently estimated to be around 80 millions of barrels per day, with projections to increase this amount by 30% within the next 20 years [4]. With these aspects in mind, an eventual displacement of petroleum by biomass will necessarily involve development of new technologies for large-scale production of fuels from this resource, the so-called biofuels.

The liquid biofuels most widely used today are bioethanol and biodiesel which are obtained from edible biomass sources such as sugar cane or corn and vegetable oils, respectively. An exponential increase in the consumption of such biofuels has taken place in the past few years [5]. Two are the main driven forces for this rapid expansion of bioethanol and biodiesel: (i) the simple and well-known technologies for their production (e.g. fermentation of sugars and transesterification of triglycerides with methanol) that has accelerated scale up of technologies and subsequent commercialization; and (ii) the partial compatibility of these biofuels with existing transportation infrastructure of diesel and gasoline which has allowed an easy penetration of these biofuels in the current fuel market.

Even though biodiesel and bioethanol (denoted as conventional biofuels) are produced by simple and mature technologies and are already commercially available, they possess a number of important drawbacks that seriously limit their further implementation in current transportation infrastructure [6]. For example, bioethanol is slightly corrosive and it has to be used in form of dilute mixtures with gasoline (e.g. E blends) in existing spark-ignition vehicles; it contains less energy per volume than gasoline (leading to lower fuel economy of vehicles running on E blends); and it induces water absorption in the fuel when added to gasoline thereby increasing risk of phase-separation episodes and engine damages. The corrosive nature of biodiesel also obligates to use it dilute with petrol-based diesel (e.g. B blends) and its higher cloud point compared to regular diesel increases the risk of plugging filters or small orifices at cold temperatures. Furthermore, this issue is compounded for the new generation of diesel engines which operate at higher injection pressures and with nozzles with a lower diameter.

These important limitations of conventional biofuels have stimulated the search for new technologies that allow production of high energy-density, infrastructure-compatible fuels (i.e. advanced biofuels) which could be easily implemented in the existing hydrocarbons-based transportation infrastructure (e.g. engines, fueling stations, distribution networks and petrochemical processes). In the past few years, these strong incentives have favored a dramatic change in funding directions from projects involving biodiesel and bioethanol to those aimed to the synthesis of advanced biofuels [7]. Relevant examples of advanced biofuels include higher alcohols (C4–C7) which possess energy density and polarity properties similar to gasoline [8]; and liquid hydrocarbon fuels (e.g. green hydrocarbons) which are chemically identical to those currently used in the transportation fleet [6], [9], [10].

When operating with biomass resources, the structural and chemical complexity of feedstocks is an important issue. One of the most common strategies to overcome biomass complexity involves previous conversion into simpler fractions that are more easily transformed in subsequent processes. Thus, complex biomass resources can be converted into simpler compounds or platform molecules, which can subsequently serve as starting materials for a number of valuable products [11]. These platform molecules are carefully selected in base of a number of indicators such as the availability of commercial technologies for their production from biomass sources, and the platform potential of these compounds for the simultaneous production of fuels and chemicals in biorefineries [11]. This selected group of biomass platform molecules include sugars (glucose, xylose), polyols (sorbitol, xylitol, glycerol), furans (hydoxymethyl furfural or HMF, furfural), acids (lactic acid, levulinic acid, succinic acid) and alcohols (ethanol).

In the present paper we explore the potential of two of these platform molecules (e.g. lactic acid and levulinic acid) for the production of advanced biofuels. As represented in Fig. 1, the very different chemical composition of these molecules compared to final products (e.g. hydrocarbons or higher alcohols) suggests that deep chemical transformations will be required during catalytic processing of these resources. As is common to all biomass derivatives, lactic acid (2-hydroxypropanoic acid) and levulinic acid (4-oxopentanoic acid) are highly oxygenated compounds and, consequently, their conversion into advanced biofuels will necessarily involve deoxygenation steps. This oxygen removal step increases energy density in the molecule and, simultaneously, achieves reduction of the chemical reactivity generating less-reactive intermediates that are more easily processed to final products with high yields. As a result, oxygen will be removed from biomass acids in form of H2O and/or COx species (CO and CO2) by hydrogenation, dehydration, Csingle bondO hydrogenolysis and decarboxylation/decarbonylation catalytic processes. This requisite deoxygenation step normally (although not always) involves consumption of large amounts of hydrogen which is expensive and typically derived from fossil sources (which negatively affects CO2 footprint of the bioprocess). As will be shown in subsequent sections, efforts are currently being made to drastically reduce external hydrogen consumption during deoxygenation of biomass platform molecules by, for example, utilization of renewable sources of this gas such as formic acid (a by-product of levulinic acid production industrial process) [12], [13].

The high oxygen content of biomass platform molecules (as compared to petroleum feedstocks) is not the only limitation to overcome when advanced fuel production technologies are envisaged. Platform molecules are typically derived (by chemical and biological routes) from biomass sugars which are compounds with a maximum number of carbon atoms limited to 6 (derived from glucose). Consequently, if targeted products are liquid hydrocarbon transportation fuels (e.g. C5–C12 for gasoline, C9–C16 for jet fuel, and C10–C20 for diesel applications) deoxygenation will necessarily have to be combined with additional reactions aimed to increase the molecular weight in the molecule (e.g. Csingle bondC coupling reactions) [14]. Among the numerous Csingle bondC bond forming routes that organic chemistry can offer us, there are some reactions with particular interest in biomass conversion processes [15], [16]. Thus, well known reactions such as aldol condensation of carbonyl compounds, catalytic ketonic decarboxylation or ketonization of carboxylic acids, oligomerization of alkenes, and alkylations of hydrocarbons are especially indicated to increase molecular weight and to adjust structure of final advanced biofuel products.

Aldol condensation and ketonization represents two of the most relevant reactions for Csingle bondC coupling of biomass derivatives [15]. The former achieves effective and high-yield coupling between carbonyl-containing biomass intermediates at moderate reaction conditions. With regard to advanced biofuels, aldol condensation has been shown to be effective for Csingle bondC coupling of biomass-derived furanics (containing an aldehyde group) such as HMF and furfural with ketones such as acetone to produce diesel and jet fuel green hydrocarbon components [17], [18], [19]. Ketonization, on the other hand, involves condensation of two molecules of carboxylic acid to produce a larger (2n  1) symmetric ketone [20]. This reaction possesses a great potential for the catalytic upgrading of biomass since Csingle bondC coupling takes place with simultaneous oxygen removal (i.e., the reaction involves the removal of CO2 and water) from carboxylic acids, the latter of which are common intermediates in biomass conversion processes [21], [22]. This reaction is typically catalyzed by inorganic oxides such as CeO2, TiO2, Al2O3 and ZrO2 at moderate temperatures (300–425° C).

In the case of oligomerization and alkylation, they are especially indicated for upgrading of deoxygenated petroleum feedstocks (e.g. alkenes and hydrocarbons). However, relevant works by Dumesic and Corma's groups have recently demonstrated that they can be successfully employed for advanced biofuels production in the last stages of biomass conversion processes, that is, when oxygen content in bio-derived feedstocks is very low [23], [24]. Next sections will provide examples on the application of the abovementioned Csingle bondC coupling processes for the transformation of lactic acid and levulinic acid into advanced biofuels for the transportation sector.

Section snippets

Lactic acid

Lactic acid (2-hydroxypropanoic acid) is the most widely occurring carboxylic acid in nature. Annual production reaches 120,000 tons per year, 90% of which is produced by bacterial fermentation of biomass sugars [25], including pentoses [26]. The bacterial route affords lactic acid in high yields (e.g. 90%) although it possesses important drawbacks namely low reaction rates and troublesome separation/purification from the reaction broth of the lactic acid product. Lactic acid is obtained in

Levulinic acid

Levulinic acid (4-oxopentanoic acid) is a high-boiling point, water-soluble biomass-derived acid that crystallizes at room temperature. Levulinic acid contains two reactive functional groups (single bondCdouble bondO and single bondCOOH) that provides, as in the case of lactic acid, a rich chemistry to this compound [37]. Levulinic acid occupies a prominent place in the selected list of biomass platform molecules [11] since it is simply and inexpensively produced from lignocellulose wastes (paper mill sludge, urban waste

Conclusions

Our society is highly dependent on fossil fuels, which are non-renewable and contribute to global warming. The conversion of biomass into fuels for the transportation sector can help to partially alleviate this reliance. Biodiesel and bioethanol, the main biofuels used today, present serious compatibility issues which can be overcome by the production of advanced biofuels such as higher alcohols and green hydrocarbons which are fully compatible with our existing hydrocarbons-based

Acknowledgments

Rafael Luque gratefully acknowledges Ministerio de Ciencia e Innovacion, Gobierno de España for the concession of a Ramon y Cajal contract (ref. RYC-2009-04199) and Consejeria de Ciencia e Innovacion, Junta de Andalucia for funding under project P10-FQM-6711. Funding from projects CTQ-2010-18126 and CTQ2011-2894-C02_02 (MICINN) is also gratefully acknowledged.

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