ZHIJUN ZHANG, CHARLES U. PITTMAN, JR., SHUJUAN SUI, JIANPING SUN, and QINGWEN ^VANG

5.1 INTRODUCTION

Production of renewable fuels and chemicals from lignocellulosic biomass has attracted increasing attention because of decreasing oil reserves, en­hanced fuel demand worldwide, increased climate concerns, and the in­herent conflict between food prices and converting edible carbohydrates to ethanol or plant oils to bio-diesel [1-4]. Bio-oils, the liquid products obtained from biomass fast pyrolysis or liquefaction, are regarded as promising renewable energy sources by the virtue of their environmentally friendly potential [5,6]. Nonetheless, several drawbacks of bio-oil severe­ly limit its potential to replace or supplement high-grade transportation fuels. These include low heating values, high corrosiveness, high water

Catalytic Upgrading of Bio-Oil by Reacting with Olefins and Alcohols over Solid Acids: Reaction Paths via Model Compound Studies. © Zhang Z, Pittman, Jr CU, Sui S, Sun J, and Wang Q. Ener­gies 6 (2013), doi:10.3390/en6031568. Licensed under a Creative Commons Attribution 3.0 Unported License, http://creativecommons. org/licenses/by/3.0/.

content, thermal instability and immiscibility with hydrocarbon fuels etc. [7]. Thus, bio-oil has to be upgraded before using it as a fuel.

Numerous upgrading approaches to improve the bio-oil properties have been proposed, including hydrodeoxygenation, zeolite cracking, catalytic pyrolyses, steam reforming, and integrated catalytic processing such as combination of hydroprocessing and catalytic pyrolysis with zeo­lite catalysis [8-16]. However, these methods require temperatures from 300 to 800 °C where coke and tar easily form. This results in undesirable catalyst deactivation and reactor clogging. Hydrodeoxygenation can re­move most of the oxygen present in bio-oil; but it requires high pressures and substantial amounts of hydrogen, which would negatively affect the economics of this process.

Alternatively, bio-oil can be partially refined to less hydrophilic, more combustible and more stable oxygen-containing organic fuels, where oxy­gen is not fully removed. Hydrogen is not employed or consumed and car­bon is not lost as CO2 in this approach. Ideally, this process would retain all of the bio-oil’s original caloric value in the product. An example is es­terification of bio-oil’s carboxylic acids with alcohols, which also converts some ketones and aldehyde content to acetals. This can improve the chem­ical and physical properties of bio-oil. However, water is formed in the process. Excess alcohol use and water removal during reaction is required to drive these equilibrium reactions and their separation from the upgraded products should be considered [10,12]. We recently reported a promising approach, where bio-oil was converted into oxygen-containing fuels by reacting with added olefins over solid acid catalysts at low temperatures [17,18]. In this approach, acid-catalyzed esterification of bio-oil carbox­ylic acids by alcohols formed during olefin hydration, phenol alkylation, etherification, and hydration reactions of olefins occur simultaneously to convert carboxylic acids, phenolic compounds, alcohols and water into esters, alkylated phenols, ethers and alcohols, respectively. These prod­ucts are less hydrophilic and have a higher fuel value. Water is removed instead of being generated. The hydroxyl groups present in bio-oil were removed and the fuel value of the product was enhanced. By also adding a co-reagent alcohol, serious phase separation of the hydrophilic bio-oil and hydrophobic olefin was reduced. In addition, esterification and acetal formation occur and their equilibria are further driven by the removal of the product water from these reactions by its addition across the added olefins [19]. The alcohols selected, including ethanol and butanols, can be obtained by biomass fermentation [20,21], and they are fuels themselves. C-4 alcohols are now major industrial targets for carbohydrate or cellulose fermentation to fuels routes. If they become major commodity fuels, a portion of that production could be directed to and leveraged towards bio­oil to fuels manufacturing processes. While this future cannot be foreseen now, options should now be developed for the future.

Converting alcohols from gasoline additives to bio-oil refining re­agents, which end up in the fuel, does not change their ultimate caloric contribution for fuel use. Olefin mixtures can be used. So, although ole­fins are consumed that may have other uses, olefins or olefin mixtures, whatever is cheaper, can be applied. For example, cheaper olefin mixtures can be obtained by pyrolysis of waste polyolefin base plastics. The total caloric content of the combined olefin and alcohol reagents remain within the refined upgraded products together with all of the original caloric con­tent of the raw bio-oil. However, olefin/alcohol acid-catalyzed upgrading will not, and is not intended, to produce “drop in” fuels for use in gasoline or most diesel motors. Furthermore, the product is not primarily intended for subsequent feeding into refinery processes to make gasoline due to its considerable oxygen content. But the oxygenated products can be blended with petroleum fuels or biodiesel liquids and might have promise for ap­plication in low temperature/high compression diesel engines requiring low cetane number fuels someday.

In our earlier work, undesirable catalyst deactivation [18] and decom­position [17,19] occurred during these upgrading processes due to bio­oil’s complex composition, especially with substantial amounts of water present. A goal of this work is to develop and apply a more highly active catalyst with good hydrothermal stability for this process. A second goal is to more fully elucidate the complex competing reaction pathways in­volved in the acid-catalyzed refining of bio-oil with olefins plus alcohols. Bio-oil upgrading is exceptionally complex. This is because bio-oils are composed of a wide variety of oxygenated compounds (more than 300) and its chemical composition is feedstock and pyrolysis process depen­dent. However, all reported bio-oils are poorly defined mixtures of carbox­ylic acids, alcohols, aldehydes, esters, ketones, sugars or anhydrosugars, phenolic compounds, furans, water, a few cyclic hydrocarbons and multi­functional compounds such as hydroxyacetaldehyde, hydroxyacetic acid, hydroxyacetone, etc. [7]. Model compounds and their mixtures are often first employed to study the reaction steps involved in bio-oil upgrading processes [10,12,13]. For this work, we have selected phenol, water, acetic acid, acetaldehyde hydroxyacetone, D-glucose and 2-hydroxymethylfuran as typical bio-oil components and mixed them as a synthetic bio-oil. This composition contains a better representation of the types of compounds involved in the many reactions competing during refining and has al­lowed a more complete understanding of the reaction paths. Solid acid catalyzed reactions of 1-octene/1-butanol with this synthetic bio-oil were investigated in the liquid phase, respectively, over Dowex50WX2 (DX2), Amberlyst15 (A15), Amberlyst36 (A36), Cs25H05PW12O40 (an insoluble acidic heteropoly acid salt) and silica sulfuric acid (SSA) catalysts. All of these catalysts are reported water-tolerant strong acids. A short prelimi­nary communication in Bioresource Technology [22] on this effort has ap­peared. The full study is reported here.

SSA is a superior proton source compared with many acidic solid sup­ports, such as styrene/divinylbenzene sulfonic acid resins and Nafion-H [23]. SSA exhibited good activity and stability in preliminary catalytic upgrading of model bio-oils by simultaneous reactions with 1-butanol and 1-octene [22]. Reaction pathways were proposed, but, more system­atic research was needed to examine upgrading feasibility and elucidate the complicated reaction mechanism. Thus, various olefins and alcohols are investigated in reactions with phenol/water (1:1) mixtures in this pa­per. Also, reactions of 1-octene with phenol, phenol/water, phenol/wa — ter/acetic acid, phenol/water/1-butanol, phenol/water/2-hydroxymeth — ylfuran, phenol/water/D-glucose, phenol/water/hydroxyacetone and phenol/water/acetic acid/1-butanol are reported here. Herein, we pres­ent a more comprehensive reaction pathway and demonstrate coking/ catalyst poisoning caused by hydroxyacetone, 2-hydroxy-methylfuran and D-glucose.

Some researchers have proposed the use of alkaline and alkaline-earth ce­ramic oxides to prepare membranes that are able to separate CO2 at high temperatures via a different transport mechanism than those observed on porous membranes. Li2ZrO3 and Li4SiO4 based membranes are examples of the aforesaid. Permselectivity of CO2 through these membranes takes place not only due to the selective CO2 adsorption properties of ceramic phases but also via a mechanism of gas separation that involves the trans­port of CO32- and O2- ionic species through the electrolytes (carbonate-
metal oxide) phases formed by the reaction of the membrane with the CO2 [87-89].

Kawamura et al. [87] fabricated and characterized a membrane for CO2 separation at high temperatures. The membrane was made of lithi­um zirconate (Li2ZrO3), an alkaline ceramic oxide that reacts with CO2 to produced Li2CO3 and ZrO2. These two reaction products are electro­lyte materials produced in-situ when the membrane is exposed to the rich carbon dioxide atmosphere. The electrolytes formed thus are capable to transport both CO2 and O2 across the membrane via a dual ion conduc­tion mechanism. The prepared membrane exhibited a separation factor of

4.9 between CO2 and CH4 gas molecules at a temperature of 600 °C. The obtained separation factor is higher than the Knudsen diffusion limit, 0.6. Therefore, the results clearly suggest the potential use of this kind of mem­brane system for CO2 separation such as the case of CO2 removal from natural gas.

Yamaguchi et al. [88] investigated the concept of the dual-ion conduc­tion facilitated mechanism previously observed for the case of Li2ZrO3 membranes by focusing their efforts on the preparation of a CO2 permse­lective membrane based on lithium orthosilicate (Li4SiO4). The supported membrane was prepared via a dip coating technique by using Li4SiO4 sus­pensions. The coating process was repeated several times before impreg­nation of the membrane with a Li2CO3/K2CO3 carbonate mixture and final sintering at 750 °C. In this membrane system, Li4SiO4 reacts in-situ with CO2 to form Li2CO3 and Li2SiO3.

Gas separation studies were performed by using CO2/N2 mixtures as feed gas. The observed CO2 permeance values were about 1 x 10-8 mol m-2s-1Pa-1 in the temperature range of 525-625 °C. The CO2/N2 separation factor was estimated between four and six. Figure 3 shows a scheme of the dual-ion conduction mechanism explained as follows. In the feed side, carbon dioxide dissolves in the material and diffuses as carbonate ions through the molten carbonate electrolyte due to a concentration gradient. Then, in the downstream side of the membrane, the formation of gaseous CO2 implies the formation of oxygen ions which must diffuse back to the feed side across the membrane and apparently through the formed Li2SiO3 skeleton to obtain the charge balance.

FIGURE 3: Schematic representation of a membrane system for the CO2 separation via a dual-ion conduction mechanism.

Separation Factor (C02/N2)

o

00

P’co2> P”co2
Ceramic Oxide Phase
Vo	Vo
ch4 N2l
CO32
2
CO
cP
Molten Carbonate Phase

FIGURE 5: Schematic representation of a membrane system for the CO2 separation and SEM image of a ceramic oxide-carbonate dual-phase membrane

The proposed transport mechanisms supports the higher selectivity val­ues observed in the permeation test for both systems, Li2ZrO3 and Li4SiO4. Figure 4 shows the separation factor values (CO2/N2) obtained for differ­ent ceramic membranes described in the present report. The pure Knudsen value is written as baseline and separation factor of nonporous Li4SiO4 for comparison purposes. However, it is important to mention that the oxygen ion diffusion process is not totally clear. Indeed, there is no experimental study regarding the oxygen ionic conductivity properties of Li2SiO3 phase. On the other hand, pure ZrO2 exhibits poor bulk oxygen ion conductivity. In fact, good conduction properties are observed only in acceptor-doped ZrO2 based materials with oxygen vacancies being the predominant charge carriers [90]. Therefore, oxygen ion conduction through the membrane must be related to different transport paths, such as grain boundaries and interfacial regions formed between the ceramic and molten carbonate on the membrane.

More recently, the promising concept of ceramic oxide-carbonate dual­phase membranes has been proposed for carbon dioxide selective separa­tion at intermediate and high temperatures (450-900 °C) [91-97].

This concept involves the fabrication of nonporous membranes capable of selectively separating CO2 via its transport, as carbonate ions. Dual phase membranes are made of an oxygen ion conductive porous ceramic phase that hosts a molten carbonate phase. Rui et al. [98] proposed the CO2 separa­tion by the electrochemical conversion of CO2 molecules to carbonate ions (CO32-), which are subsequently transported across the membrane. Carbon­ate ionic species (CO32-) are formed by the surface reaction between CO2 and oxygen that comes from the ceramic oxide phase (feed side, Eq.(10)) and then transport of CO32- takes place through the molten carbonate.

CO2 + OOx ~ CO32- + VO" (10)

Once carbonate ions have reached the permeate side, molecular CO2 is released to the gas phase, delivering OOx species back to the ceramic ox­ide solid phase. This process takes place due to a chemical gradient of CO2 in the system (Figure 5). Here, it is important to emphasize that dual-phase membranes are nonporous and therefore exhibit high separation selectivi­ty as a result of the transport mechanism. Figure 5 also shows the SEM im­age of the cross section of a ceramic oxide-carbonate membrane prepared by pressing La06Sr04Co08Fe02O3-s powders and subsequent infiltration of the obtained porous ceramic (bright phase) with carbonate (dark phase).

Table 3 summarizes the different studies reported and certain advances that have been achieved so far regarding the dual-phase membrane con­cept. This table also includes the Li2ZrO3 and Li4SiO4 nonporous mem­branes previously described. Although the original reports do not clearly explain the operational mechanism [26-27], the dual-phase membrane concept gives a much better idea of the possible phenomenology involved [30,33,36].

What Needs to be Analyzed and Why?

DMITRY MURZIN and BJARNE HOLMBOM

3.1 INTRODUCTION

Today, the use of biomass is considered a promising way to diminish nega­tive environmental impact. Moreover, in some future scenarios, renewable raw materials are thought to be able to replace finite mineral-oil-based raw materials before 2050 [1]. This means that new synthetic routes, which should desirably adhere to the principles of green chemistry [2], need to be developed for the production of chemicals.

Lignocellulosic biomass, as a renewable source of energy and chemi­cals, has attracted a lot of attention recently [3-10]. Wood biomass consists of cellulose (40-50%), lignin (3-10%), hemicelluloses (15-30%) and a variety of extractives (1-10%). Cellulose is a linear polymer of D-glu — copyranose and can contain up to 10,000 units (C6H10O5), connected by glycosidic ether bonds, while the molecular mass for hemicelluloses is

Murzin D and Holmbom B (2013). “Analytical Approaches in the Catalytic Transformation of Bio­mass: What Needs to be Analyzed and Why?” in Catalysis for the Conversion of Biomass and Its Derivatives, Behrens M andDatye AK (Eds.), ISBN: 9783844242829, Reprinted with permisison from the authors.

lower. Hemicelluloses have a more heterogeneous structure than cellulose, consisting mainly of five-carbon (xylose, arabinose) and six-carbon sugars (galactose, glucose and mannose). Contrary to cellulose lignin is a co- niferyl alcohol polymer with coumaryl, coniferyl and sinapyl alcohols as monomers, which are heavily cross-linked, leading to complex structures of large lignin molecules [11].

Chemical treatment of lignocellulosic biomass in general, and wood in particular, can have several targets. One of the options is delignifi- cation of the biomass leading to cellulose and some residual hemicel — luloses, which are further applied in the production of paper or board, or derivatives of cellulose. Thermal (or catalytic) treatment of biomass, e. g., thermal or catalytic pyrolysis, is a route to bio-based synthesis gas and biofuels [12]. Depolymerization results in the formation of low — molecular-mass components (sugars, phenols, furfural, various aromatic and aliphatic hydrocarbons, etc.), e. g., unique building blocks for further chemical synthesis.

Wood biomass contains many valuable raw materials for producing fine and specialty chemicals (Figure 1). These raw materials are carbohy­drates, fatty acids, terpenoids and polyphenols, such as stilbenes, lignans, flavonoids and tannins. Some of them can be exuded directly from living trees, while others are extracted and purified via chemical methods.

In this context, applications of catalytic reagents, which are superior to stoichiometric reagents producing stoichiometric amounts of wastes, are worth mentioning. Well-known benefits in using heterogeneous catalysts are associated with easy catalyst separation, regeneration and reuse, as well as relatively low prices compared to homogeneous catalysts. The re­search regarding catalytic transformations of different wood-derived com­pounds is currently very active [13].

Because of the complexity associated with the processing of biomass per se or the transformation of biomass-derived chemicals, in-depth chemical analysis of all components and their reactions is difficult to perform. Therefore, most analytical methods will be a result of a com­promise between information depth and available resources. It is also obvious that in industrial processes only a limited number of rather fast analytical methods could be utilized since a large number of samples should be processed.

FIGURE 1: Chemical by-products from the forest industry

To have in-depth and molecular-level understanding of the chemical reactions occurring during the transformation of biomass not only ad­vanced analytical methods are required, but additionally, a broad spectrum of these methods needs to be applied. Let us consider, for example, the catalytic conversion of cellulose [14-17] in the presence of hydrogen lead­ing to sugar alcohols. During such a depolymerization reaction not only the concentration of carbohydrates and other products in the liquid phase should be measured, but also the crystallinity of cellulose, its morphology, molecular mass distribution and presence of sugar oligomers. The analysis is even more complicated if in this reaction wood is used directly instead of cellulose.

Analytical techniques have made a tremendous progress in recent years giving a possibility to utilize a wide range of modern instrumental methods, including advanced chromatography, microscopy and spectros­copy. It is apparently clear that all the methods currently available cannot
be treated in this review, thus a rational selection of them was done by the authors based on their experience, with an understanding that it might not cover all the analytical methods presently utilized in catalytic transforma­tions of biomass-derived chemicals, but focuses mostly on chromatography.

Al2O3-foams were tested as a possible support material for a suitable catalyst at the start of the experimental stage. For this purpose, two such foams were inserted into the walls of the combustion chamber. However, first of all it was important to calculate the pressure drop across the monoliths in order to ascertain the smooth operation of the stove after installing the monoliths. The pressure drop across the monoliths was found to be lower than 0.5 Pa which is sufficiently low and shows the applicability of the foams.

As observed, there is no negative effect on the combustion behavior of the stove after installing the uncoated Al2O3-foams, so it leads to the test­ing of the monoliths in the combustion chamber with Mixed Metal Oxide (MMO) as an active phase.

STELLA BEZERGIANNI

2.1 INTRODUCTION

The depletion of world petroleum reserves and the increased concern on climate change has stimulated the recent interest in biofuels. The most common biofuels are based on energy crops and their products, i. e. veg­etable oil for Fatty Acid Methyl Esters (FAME) biodiesel [1] and sugars/ starch for bioethanol. However these first generation biofuels and associ­ated production technologies face several considerations related to their economic and social implications regarding energy crops cultivation, by­products disposal, necessity for large investments to ensure competitive­ness and the “food versus fuel” debate.

As a result, second generation biofuel technologies have been devel­oped to overcome the limitations of first generation biofuels production [2]. The goal of second generation biofuel processes is to extend biofuel production capacity by incorporating residual biomass while increasing

Stella Bezergianni (2013). Catalytic Hydroprocessing of Liquid Biomass for Biofuels Production, Liq­uid, Gaseous and Solid Biofuels — Conversion Techniques, Prof. Zhen Fang (Ed.), ISBN: 978-953-51­1050-7, InTech, DOI: 10.5772/52649. Licensed under Creative Commons Attribution 3.0 Unported License, http://creativecommons. org/licenses/by/3.0/.

sustainability. This residual biomass consists of the non-food parts of food crops (such as stems, leaves and husks) as well as other non-food crops (such as switch grass, jatropha, miscanthus and cereals that bear little grain). Furthermore the residual biomass potential is further augmented by industrial and municipal organic waste such as skins and pulp from fruit pressing, waste cooking oil etc. One such technology is catalytic hydro­processing, which is an alternative conversion technology of liquid bio­mass to biofuels that is lately raising a lot of interest in both the academic and industrial world and is the proposed subject of this chapter.

Catalytic hydroprocessing is a key process in petrochemical industry for over a century enabling heteroatom (sulfur, nitrogen, oxygen, metals) removal, saturation of olefins and aromatics, as well as isomerization and cracking [3]. Due to the numerous applications of catalytic hydroprocess­ing, there are several catalytic hydroprocessing units in a typical refinery including distillate hydrotreaters and hydrocrackers (see Figure 1). As a result several refinery streams are treated with hydrogen in order to im­prove final product quality including straight-run naphtha, diesel, gas-oils etc. The catalytic hydroprocessing technology is evolving through the new catalytic materials that are being developed. Even though hydroprocess­ing catalysts development is well established [4], the growing demand of petroleum products and their specifications, which are continuously be­coming stricter, have created new horizons in the catalyst development in order to convert heavier and lower quality feedstocks [5]. Furthermore the expansion of the technology to bio-based feedstocks has also broadened the R&D spam of catalytic hydrotreatment.

Catalytic hydroprocessing of liquid biomass is a technology that offers great flexibility to the continuously increasing demands of the biofuels market, as it can convert a wide variety of liquid biomass including raw vegetable oils, waste cooking oils, animal fats as well as algal oils into biofuels with high conversion yields. In general this catalytic process tech­nology allows the conversion of triglycerides and lipids into paraffins and iso-paraffins within the naphtha, kerosene and diesel ranges. The products of this technology have improved characteristics as compared to both their fossil counterparts and the conventional biofuels including high heating value and cetane number, increased oxidation stability, negligible acid­ity and increased saturation level. Besides the application of this catalytic technology for the production of high quality paraffinic fuels, catalytic hydroprocessing is also an effective technology for upgrading intermedi­ate products of solid biomass conversion technologies such as pyrolysis oils and Fischer-Tropsch wax (Figure 2). The growing interest and in­vestments of the petrochemical, automotive and aviation industries to the biomass catalytic hydroprocessing technology shows that this technology will play an important role in the biofuels field in the immediate future.

In the sections that follow, the basic technical characteristics of cata­lytic hydrotreatment are presented including a description of the process, reactions, operating parameters and feedstock characteristics. Further­more key applications of catalytic hydroprocessing of liquid biomass are outlined based on different feedstocks including raw vegetable oils, waste cooking oils, pyrolysis oils, Fischer-Tropsch wax and algal oil, and some successful demonstration activities are also presented.

The current biofuel market is largely dominated by ethanol, which ac­counts for 90% of world biofuel production. [19] Indeed, the rate of etha­nol production around the world is increasing rapidly, from 13 billion gal­lons in 2007 to the current level of almost 20 billion gallons in 2009, [20] with the US (55%, corn-derived) and Brazil (33%, sugar cane-derived) being the main producers. [21] The ethanol industry has benefited from a mature and simple technology in which selected microorganisms (e. g., yeast, bacteria, and mold) transform aqueous sugars to the desired final ethanol product. Ethanol is added to gasoline to increase the octane num­ber of the mixture and, with it, improve the combustion characteristics of the fuel. The oxygen in ethanol allows low-temperature combustion with the subsequent reduction of pollutants such as CO and NOx. [19] Addi­tionally, apart from CO2 emission savings, blending gasoline with ethanol helps to reduce SOx emissions to the atmosphere, because ethanol contains a negligible amount of sulfur compared to petroleum. [18]

A key aspect that is responsible for expansion of the ethanol industry in recent years is the compatibility of ethanol with the existing infrastructure for gasoline. Thus, ethanol blended with conventional gasoline is currently used in many countries as a renewable fuel in existing spark-ignition en­gines. This compatibility, however, is not complete and the use of etha­nol is presently limited to low-concentration blends (5-10% by volume), namely E5-E10. Ethanol-enriched mixtures such as E85 require cars with specially designed engines, designated as flexible-fuel vehicles (FFVs), which are commonly used only in a few countries like Brazil and Sweden. E85 mixtures are not tolerated by conventional vehicles, because ethanol, especially in high-concentration blends, can cause corrosion of some me­tallic components in tanks and deterioration of rubbers and plastics used in internal combustion engines. [21] This constraint in ethanol blending represents the main issue of the growing ethanol industry. As outlined in Fig. 1, projections indicate that the US ethanol industry will approach the blending wall (i. e., the point at which blending 10% of ethanol in each gallon of gasoline will not be able to accommodate the rate of ethanol production) in 2010. [22] Furthermore, experts predict that the number of E85 fuelling stations and flexi-fuel vehicles will not grow sufficiently fast to accommodate the growing volumes of ethanol produced in the US. [23] A potential solution to overcome the blending wall is to raise the amount the ethanol allowed in gasoline to beyond 10% by commercializing inter­mediate ethanol blends (i. e., E15- E20) (Fig. 1). However, there are seri­ous issues in using blends with higher ethanol concentrations. In countries like the US, the utilization of E15-E20 blends in regular vehicles is still not authorized, since the effect of these mixtures on pollutant emissions, driving performance and materials compatibility (e. g., tanks, pipelines, dispensers) is not fully understood, [23] and current European standards allow only for E5 blends. [18]

Apart from the aforementioned blending issues, ethanol presents an­other important limitation as a transportation fuel. Ethanol contains less energy per volume (i. e., energy density) than conventional gasoline, which ultimately reduces the fuel mileage of the vehicles. In this sense, it has been estimated that cars running on ethanol rich mixtures like E85 operate with 30% lower fuel mileage. [24] This fact, along with the small price differential between E85 and regular gasoline, has discouraged drivers to purchase E85 cars or fuel so far. [22]

Conventional transportation fuels are composed of liquid hydrocar­bons with different molecular weights (e. g., C5-C12 for gasoline, C9-C16 for jet fuel, and C10-C20 for diesel applications) and chemical structures (e. g., branched for gasoline, linear for diesel). The entire transportation infrastructure (including engines, fueling stations, distribution networks, and storage tanks) has been developed to take advantage of the excellent properties of these compounds as fuels. Thus, the special composition of hydrocarbons fuels, based only on carbon and hydrogen, provides them with high energy-density and stability (allowing efficient storage at ambi­ent conditions) and superior combustion characteristics, properties highly desired for transportation liquids. Thus, instead of using biomass to pro­duce oxygenated fuels (such as ethanol) with new compositions, an attrac­tive alternative would be to utilize biomass to generate liquid fuels chemi­cally similar to those being used today derived from oil. [17,25] These new fuels would be denoted as green gasoline, green diesel and green jet fuel, and they would be essentially the same as those currently used in the transportation fleet, except that they would be synthesized from biomass instead of petroleum. When compared with ethanol, the production of hy­drocarbon fuels from biomass has important advantages. The main benefit would include full compatibility with the existing energy system. Since green hydrocarbon fuels would be essentially the same as those currently obtained from petroleum, it would not be necessary to modify engines, pumps or distribution networks to accommodate the new renewable liq­uids in the transportation sector.

Unlike ethanol, biomass-based hydrocarbons fuels are energy equiva­lent to fuels derived from petroleum. The heating value (i. e., the heat re­leased when a known quantity of fuel is burned under specific conditions) of ethanol is only two-thirds that of gasoline, which, as indicated above, penalizes the fuel mileage of the vehicles running on gasoline-ethanol mixtures. The use of renewable hydrocarbon fuels would additionally help to meet the increased standards of fuel economy imposed by governments to the automobile industry. In the case of the US, these standards establish a mandatory increase in average fuel economy from the current 25 miles per gallon (mpg) to 35 mpg by 2022. [26]

The addition of oxygenated components to conventional fuels in­creases the water solubility of the mixture. This increase is particularly marked in the case of gasoline-ethanol blends, since pure ethanol is high­ly hygroscopic and completely miscible in water. Thus, adding 10% of ethanol to regular gasoline raises the water solubility of the blend more than 30 times (from 150 ppm v/v of regular gasoline to 5000 ppm for E10). [27] Once the water contamination reaches the saturation level, additional water separates from the mixture, removing the ethanol from gasoline and leading to phase separation. In fact, when phase separation occurs in the storage tank, the ethanol-water layer may combust in the engine at higher temperatures causing damage to it. [27] The water tol­erance of a gasoline-ethanol blend (i. e., fraction of water that the mix­ture can contain without phase separation) decreases with temperature and increases with ethanol content (Fig. 2). Consequently, phase separa­tion is an important concern in countries with cooler climates and when low-concentration blends such as E5 are used. Water can be absorbed by the ethanol-gasoline mixture from the atmosphere (in the form of moisture), from the air trapped in the tank (by condensation of water when temperature decreases), or even from the ethanol itself which typ­ically carries traces of water when delivered from the biorefinery. In this respect, many countries have regulated the maximum amount of water allowed in fuel-grade ethanol to the level of 1% (v/v), to avoid phase separation issues. [28] Ethanol affinity for water has important implica­tions for distribution logistics as well. Pipelines, considered to be the least expensive means of safely transporting bulk fuel shipments, [23] are not suited to transport ethanol or gasoline-ethanol blends on a com­mercial scale, because apart from corrosion issues, ethanol can pick up water in the pipeline with the potential result of phase-separation. Con­sequently, ethanol has to be distributed by other fossil fuel-consuming transportation modes such as rail, truck and barge. The hydrophobic character of biomass-derived hydrocarbons eliminates these problems, since these molecules are immiscible in water. Additionally, the ability of liquid hydrocarbons to self-separate from water, as represented in Fig. 3, is highly beneficial in that it eliminates the need for expensive and energy-consuming distillation steps required in the ethanol purifica­tion process. In particular, ethanol is initially obtained in form of a di­lute aqueous solution (5-12% v/v), which is subsequently concentrated to 96-99% by distillation. It is estimated that this intense water removal step is responsible for 35-40% of the total energy required for ethanol

production, [29] and this energy is typically supplied by combustion of fossil fuels such as natural gas.

Any technology envisaged to convert biomass feedstocks into liquid fuels must address one important limitation of this resource: the low en­ergy-density of biomass compared to fossil fuels. Although the energy — density of biomass varies considerably depending on the source, an aver­age value for biomass (15-20 MJ kg-1) is well below that of crude oil (42 MJ kg-1). [30] Large amounts of biomass will thus be required to produce liquid fuels, leading to high costs for transporting the biomass source to the processing location. [31] Furthermore, if biomass transportation in­volves utilization of fossil fuels, then the overall CO2 emission savings of the bioprocess would be penalized. Consequently, for biomass conversion technologies to be cost-competitive and truly carbon-neutral, it is neces­sary to develop efficient processing units at small scale that can be distrib­uted close to the biomass source. [17] Even though ethanol plants achieve a significant size reduction compared to petrochemical refineries, the mild conditions employed and the low levels of ethanol achieved for bacterial fermentation (e. g., 30-50 oC, ethanol concentrations lower than 15% v/v) significantly limit the reaction rates and require reactors with a size large enough to make the process economically feasible. As will be described in following sections, biomass-based hydrocarbon fuels, in contrast, can be produced at high temperatures and using concentrated water solutions, [14] which allow for faster conversions in smaller reactors.

The utilization of edible biomass (such as corn or cane sugar) for the large-scale production of fuels can produce competition with food for land use. The so-called food-versus-fuel debate has arisen in many countries as a response to the sharp increase in food prices during 2007 and 2008. Although it has been pointed out that this rise in price was the result of several linked worldwide events, [22] some authors indicate that the increased demand for corn to produce ethanol had a direct impact on food prices, especially in food- insecure areas of the world where food is based on grain consumption. [32] These issues have driven researchers around the world to develop technolo­gies to process non-edible biomass (e. g., lignocellulosic biomass), thereby permitting sustainable production of a new generation of biofuels (so-called second generation of fuels), without affecting food supplies. Lignocellulosic biomass has two important advantages over edible biomass feedstocks: it is more abundant and can be grown faster and with lower costs. [33] In this respect, it is estimated that the US could sustainably produce more than 1 billion tons of non-edible biomass per year by 2050 with relatively modest changes in land use and agricultural and forestry practices. Once converted into biofuels, these lignocellulosic resources would have the potential to dis­place more than one third of the petroleum currently consumed by the trans­portation sector.34 Lignocellulosic feedstocks ($3 per GJ) are slightly less expensive than edible biomass (5$ per GJ), and potentially more economical than crude oil (10-15 $ per GJ) and vegetable oils (18-20 $ per GJ); however, due to its recalcitrant nature, lignocellulose is more difficult to convert and, consequently, processing costs increase for this resource. [35] Thus, accord­ing to recent analyses, the cost of producing lignocellulosic ethanol would be almost double that of corn-derived ethanol. [19] This fact represents the main limitation of lignocellulose as a renewable resource, and the lack of cost-competitive technologies for the generation of liquid fuels from non­edible sources has been identified by experts as the key bottleneck for the large-scale implementation of lignocellulose — derived biofuel industry. [36] Recalcitrance of lignocellulosic biomass can be explained in terms of its chemical structure, comprised of three major units: cellulose, hemi — cellulose and lignin. [13,37,38] Cellulose (40-50%) is a high molecular weight polymer of glucose units connected linearly via b-1,4-glycoside linkages. This arrangement allows for extensive hydrogen bonding be­tween cellulose chains, which confers this material with rigid crystallinity and, thus, high resistance to deconstruction. [39] Cellulose bundles are additionally attached together by hemicellulose (15-20%), an amorphous (and consequently more readily deconstructed) polymer of five different C5 and C6 sugars. Cellulose and hemicellulose, the carbohydrate frac­tion of lignocellulose, are protected by a surrounding three-dimensional polymer of propyl-phenol called lignin (15-25%), which provides extra rigidity to the lignocellulose structure. To overcome lignocellulose recal­citrance, a variety of physical and chemical methods have been developed, and a comprehensive description of such technologies can be found else­where. [40-42] The approach most commonly used involves pretreatment of lignocellulose (with the aim of breaking/ weakening the lignin protec­tion and increasing the susceptibility of crystalline cellulose to degrada­tion), followed by hydrolysis to depolymerize hemicellulose and cellulose and, thus, isolate the sugars from the lignin fraction.

In a typical synthesis, reported by our group [45], a total amount of 1.00 g of P123 was placed in a flask and 47.80 mL of distilled H2O, 3.42 mL con­centrated HCl and 2.45 mL 1-butanol were added. The solution was stirred for 1.5 h at room temperature and subsequently 1.86 mL of E-BTSE was added to the mixture. Next, the mixture was vigorously stirred at 35 °C. After 4 h, the temperature was raised to 90 °C and aged for 16 h. After­wards, the mixture was left to cool down, and the white solid was filtered and washed with acetone and distilled water. Finally, P123 was removed via Soxhlet extraction with acetone for 5 h. The white solid, denoted as EP, was dried at 120 °C under vacuum (~0.1 Pa).