## 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; 2; 3; 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 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; 9; 10; 11; 12; 13; 14; 15; 16]. However, these methods require temperatures from 300 to 800 \({}^{\circ}\)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 CO\({}_{2}\) 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 obtainedby 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 bio-oil to fuels manufacturing processes. While this future cannot be foreseen now, options should now be developed for the future.