## 2 Results and Discussion
### Proposed Reaction Pathways for Model Bio-Oil Components  
A complicated but clear reaction pathway is postulated and shown in Figure 2 based on the detailed product analyses and discussions mentioned above. Under acid-catalyzed conditions, olefin protonation and subsequent proton loss and reprotonation steps generated the isomerized olefins and their cation intermediates. Simultaneously, a series of competing reactions occur, where bio-oil's components (water, carboxylic acids, phenols and alcohols) and the added olefins add to these cations. This leads to hydration, esterification, _O_-alkylation, etherification and oligomerization which forms alcohols, esters, phenol _O_-alkylates, ethers and olefin oligomers, respectively. Moreover, diene intermolecular cyclizations and branched olefin cracking into small fragments as well as reoligomerization of these small fragments occurred. Similarly, under acid catalyzed conditions, protonation of alcohols and subsequent dehydration of these protonated products occurred generating carbocations. Meanwhile, additional competing reactions among carboxylic acids, aldehydes, alcohols, phenols and levulinic acid with these carbocations occurred, generating esters, acetals, ethers, phenol O-alkylates and alkyl levulinates, respectively. _O_-Alkylated phenols isomerized to the thermodynamic _C_-alkylated phenol via a Friedel Crafts mechanism. Further addition of carbocations to mono-alkylated phenols generated bis-alkylated phenols.  
In addition to reactions between bio-oil components and olefin/alcohol reagents, the added alcohol adds across olefins to give intermolecular etherification. Self-etherification of the added alcohol reagent also occurs. These reactions occur simultaneously, generating corresponding ethers. Acid-catalyzed dehydration of d-glucose to levulinic acid [28] occurred. First, dehydration of d-glucose gives 5-hydroxymethylfurfural (HMF). Then, HMF hydration to its hemiacetal occurred followed by sequential rehydration, ring-opening, loss of both water and formic acid generating levulinic acid. Also, acid-catalyzed 2-hydroxymethylfuran rehydration and subsequent ring opening, dehydration and tautomerization formed levulinic acid [29]. Levulinic acid, in turn, is converted to alkyl levulinates by alcohols. Independently, polymerization of 2-hydroxymethylfuran via electrophilic aromatic substitution proceeded jointly with loss of formaldehyde to form oligomeric products. Simultaneous dimerization and isomerization of hydroxyacetone occurred, forming cyclic hydroxyacetone dimers and propionic acid along with some 2-hydroxy-3-methylcyclopent-2-enone. This latter product was likely formed by aldol condensation of hydroxyacetone to an open hydroxyacetone dimer and its subsequent dehydration and cyclodehydration reactions.  
Clearly, bio-oil upgrading by simultaneous reactions with olefin/alcohol over solid acids is complex, involving many simultaneous equilibria and competing reactions. However, the key reason for the success of this upgrading process is the role of acid-catalyzed olefin hydration. Olefin hydration removes water. As water concentration drops, esterification and acetal formation equilibria shift toward ester and acetal products.  
Figure 2: Reaction pathways of the model bio-oil components during upgrading with olefin/alcohol over solid acid catalysts.  
Figure 1: Reaction pathways of the model bio-oil components during upgrading with olefin/alcohol over solid acid catalysts.