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.