JUAN CARLOS SERRANO-RUIZ, ANTONIO PINEDA,

ALINA MARIANA BALU, RAFAEL LUQUE,

JUAN MANUEL CAMPELO, ANTONIO ANGEL ROMERO, and JOSE MANUEL RAMOS-FERNANDEZ

7.1 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] and [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] and [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-oxo — pentanoic 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] and [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] and [16]. Thus, well known reactions such as aldol condensation of carbonyl compounds, catalytic ketonic decarboxylation or ketonization of carboxylic acids,

C0X, H20

H2

FIGURE 1: Scheme of the catalytic routes required to convert biomass-derived acids such as lactic acid and levulinic acid into advanced biofuels.

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] and [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] and [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] and [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.