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 Frie — del Crafts mechanism. Further addition of carbocations to mono-alkylated phenols generated bis-alkylated phenols.

In addition to reactions between bio-oil components and olefin/alco- hol 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-hydroxymethy furfu­ral (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-hydroxymeth — ylfuran via electrophilic aromatic substitution proceeded jointly with loss of formaldehyde to form oligomeric products. Simultaneous dimerization and isomerization of hydroxyacetone occurred, forming cyclic hydroxy — acetone dimers and propionic acid along with some 2-hydroxy-3-meth — ylcyclopent-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/alco — hol 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.

5.3 EXPERIMENTAL SECTION

All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA), and used without further purification unless otherwise noted.