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Investigation of Thermochemistry Associated with the Carbon-Carbon Coupling Reactions of Furan and Furfural using ab initio Methods Cong Liu, Rajeev S. Assary, and Larry A Curtiss J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp503702t • Publication Date (Web): 05 Jun 2014 Downloaded from http://pubs.acs.org on June 10, 2014

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Investigation of Thermochemistry Associated with the Carbon-Carbon Coupling Reactions of Furan and Furfural using ab initio Methods Cong Liu,a Rajeev S. Assary,a Larry A. Curtiss*a a

Materials Sciences Division, Argonne National Laboratory, Lemont, IL, USA, 60439

Corresponding Author: [email protected] (LAC), Tel: 630-252-7380, Fax: 630-252-9555 Abstract Upgrading of furan and small oxygenates obtained from the decomposition of cellulosic materials via formation of carbon-carbon bonds is critical to effective conversion of biomass to liquid transportation fuels. Simulation-driven molecular level understanding of carbon-carbon bond formation is required to design efficient catalysts and processes. Accurate quantum chemical methods are utilized here to predict the reaction energetics for conversion of furan (C4H4O) to C5-C8 ethers and the transformation of furfural (C5H6O2) to C13-C26 alkanes. Furan, can be coupled with various C1 to C4 lower molecular weight carbohydrates obtained from the pyrolysis via Diels-Alder type reactions in the gas phase to produce C5-C8 cyclic ethers. The computed reaction barriers for these reactions (~25 kcal/mol) are lower than the cellulose activation or decomposition reactions (~50 kcal/mol). Cycloaddition of C5-C8 cyclo-ethers with furans can also occur in the gas phase, and the computed activation energy is similar to that of the first Diels-Alder reaction. Furfural, obtained from biomass, can be coupled with aldehydes or ketones with α-hydrogen atoms to form longer chain aldol products and these aldol products can undergo vapor phase hydrocycloaddition (activation barrier of ~20 kcal/mol) to form the precursors of C26 cyclic hydrocarbons. These thermochemical studies provide the basis for further vapor phase catalytic studies required for upgrading of furans/furfurals to longer chain hydrocarbons. Keywords: Furan upgrading, Thermochemistry, Thermal Cycloaddition reactions, G4MP2 1

Introduction

Efficient conversion of naturally abundant biomass to liquid transportation fuels is essential due to our heavy dependence on declining fossil fuel resources and ever increasing demands1-8. Developing domestic biomass as a clean, sustainable energy resource for transportation offers a range of benefits such as stimulation of economy, improvement of the petroleum trade balance, mitigation of climate impacts, and energy security. Cellulose, hemicellulose, and lignin are the three major components of lignocellulosic biomass9-10. Three major strategies are currently being developed to transform these biomass materials into potential fuels: (1) gasification to make syn-gas (i.e., CO and H2), (2) pyrolysis to produce bio-oils, which is a mixture of tars, acids, chars, alcohols, aldehydes, esters, ketones and aromatics, and (3) hydrolysis to make sugar derivatives and aromatic ethers11. Further chemical transformation of these first-generation products (syn-gas, bio-oil, and sugar derivatives) is essential to upgrade to useful industrial chemicals and liquid transportation fuels6, 12-16. 1 ACS Paragon Plus Environment

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Transformation of biomass via pyrolysis or acid hydrolysis produces significant amounts of furan or furfural17-19. These two platform compounds can be used in a broad range of applications including synthesis of transportation fuels20-21. A schematic of the conversion of biomass to liquid transportation fuels is shown in Figure 1. Pyrolysis of biomass results in the formation of furans (C4) and low molecular weight carbohydrates (LMWC, C1-C4). Table 1 lists examples of LMWCs derived from biomass pyrolysis reported in recent literatures22-35. These LMWCs are mainly oxygenated compounds with lower heating value and cause bio-oil instability29, therefore efficient utilization of these molecules is essential. On the other hand, acid hydrolysis of biomass often generates furfural (C5) and its derivatives. As shown in Figure 1, furans with LMWCs and furfural can go through C-C coupling to form longer chain ethers, which can then be converted to fuel range alkanes via hydrogenation/deoxygenation reactions11, 14, 24-27. Thus, C-C coupling is a key step to convert furan, LMWCs and furfural to longer chain hydrocarbons and is essential to successful bio-oil upgrading6, 11, 36-39. Transportation fuels are composed of hydrocarbons with a number of carbon atoms (Cn); for example, gasoline, jet fuels, and diesel fuels have C4-C8, C9-C14, and C9-C20 carbon chains, respectively40. A number of promising organic reactions have been reported for C-C coupling41, and the most common reactions include cycloaddition (e.g., Diels-Alder reaction), aldol condensation, and oliogomerization reactions (e.g., Michael addition)

37, 40, 42-45

. After the C-C coupling

reaction, catalytic hydrogenation or hydrodeoxygenation is required to produce diesel or jet fuels. Both furan and furfural (diene) present in bio-oil are capable of undergoing Diels-Alder reactions with LMWCs (dienophiles)36, 46-48. Therefore the C-C coupling can be performed in tandem with hydrogenation in ‘one pot’, known as hydrocycloaddition reactions, which results in the formation of complex mixtures of hydrocarbons with different numbers of carbon atoms. Considering the various possibilities for the C-C coupling reactions of furans with likely dienophiles, a molecular level understanding of these reactions using experimental measurements is challenging, whereas computational studies offer an efficient alternative to understand the thermodynamic and kinetic feasibility of these reactions. Detailed thermochemical studies for carbon-carbon coupling reactions involved in furan upgrading can provide important information and guidance for future development of efficient catalysts. Reaction mechanisms of C-C coupling reactions involve bond breaking and formation. These interactions often require sophisticated calculations that take well-balanced account of electron correlation49. Recent computational studies have frequently utilized density functional (DFT) methods, especially B3LYP50-51, to investigate the thermochemistry of C-C coupling reactions (e.g., Diels-Alder and aldol reactions)52-57. Although B3LYP is accurate for the prediction of ground-state geometric parameters, it has sometimes shown large errors in the prediction of relative energies of isomers of hydrocarbons and other main group element containing compounds58-60. Some recent studies49, 61 have also suggested that B3LYP gives poor reaction enthalpies for Diels-Alder reactions compared with high level G362 and CBS-QB363-66 methods. Both G3 and CBS-QB3 have shown good agreement on the experimental reaction enthalpies of Dials-Alder reactions61. Therefore, it is essential to evaluate the accuracy of the computational methods for the C-C coupling reactions in any computational studies of these reactions. Previously, our lab has reported on detailed thermochemistry regarding the decomposition of sugar molecules via dehydration, retro-aldol reactions and rehydration reactions using high-level quantum mechanical methods67-70. In this contribution, we report on a systematic computational study of accurate energetics including the reaction barriers for 2 ACS Paragon Plus Environment

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various carbon-carbon coupling reactions associated with furan and furfural with small oxygenates obtained from the pyrolysis. First, various levels of computational methods are used to calculate the reaction energetics of possible cycloaddition reactions of furan with C1-C4 LMWCs, to assess the accuracy of various computational methods. Second, the thermochemistry of various reactions of furan C1-C4 and LMWCs is presented in terms of reaction barriers, rate constants and temperature dependence of the rate constants. Finally, we present detailed mechanistic studies and conformational analysis of the reaction energetics of chemical conversion of furfural to C13-C26 hydrocarbons, including the production of tridecane (C13, component in jet and diesel fuel), C21 alkane (potential source for diesel fuel) and C26 cycloalkanes. 2

Computational Methods

The B3LYP71 density functional (DFT) method with 6-31+G(d) basis set was used for geometry optimizations and frequency calculations. Frequency calculations were performed to identify all of the stationary points as minima or transition states and provide enthalpies (H) and Gibbs free energies (G) at 298.15 K and 1 bar in the gas phase. Intrinsic reaction coordinate (IRC) calculations were also performed to confirm the transition states. For the furan reactions, reaction energies and barriers were calculated using both DFT and ab initio methods to assess the accuracy of various levels of theories. Single point energies were calculated at the CCSD(T)/cc-pVTZ, MP2/6-31+G(d) and B3LYP/631+G(d) levels with the geometries and thermal corrections determined at the B3LYP/6-31+G(d) level of theory. Highly accurate G4MP272 calculations, as a bench mark for the reaction energies, were also carried out based on B3LYP/631G(2df,p) geometries and thermal corrections. Reaction free energies of selected furan reaction (reaction with formaldehyde and acetaldehyde were computed at 25, 50, 100, 150, 200, 300, 400, and 500 ̊C to evaluate the influence of temperature in the reaction energetics. To model furfural cycloaddition reactions, we have employed the MP2/631+G(d)//B3LYP/6-31+G(d) level of theory, due to its accuracy and computational efficiency (explained in Section 3.1.1). Solvent effects were also considered for selected Diels-Alder reactions using the SMD solvation model73. The computed reaction enthalpies (H) and the free energies (G) of all reactions investigated here are reported either in the main text or in the Supporting Information. For all the cycloaddition reactions, the conformers of the products are the lowest energy conformers, which are determined based on the relative energies of endo and exo adducts. All the calculations presented in this paper were carried out using the Gaussian09 program74. 3

Results and Discussions

3.1 Thermochemistry of Possible Cycloaddition Reactions of Furan with C1-C4 Molecules 3.1.1 Assessment of Various Computational Methods on Reaction Enthalpies and Barriers The accuracy of three levels of theory (CCSD(T), MP2, and B3LYP) in predicting the reaction enthalpies (H) and activation enthalpies (barriers) was investigated for possible Diels-Alder reactions of furan with selected LMWCs from biomass pyrolysis. Reaction enthalpies and barriers calculated using the G4MP2 theory is considered as a benchmark. Eighteen Diels-Alder reactions (Table 2) were taken into account, which includes reactions of furan with H2 and fourteen different LMWCs. These LMWCs are alkenes, aldehydes and ketones ranging from C1 to C4 compounds. For ketones and 3 ACS Paragon Plus Environment

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aldehydes, their enol forms were also studied. For molecules with multiple C=O or C=C bonds, reactions of furan with each double bond was studied. The transition states for the Diels-Alder reactions were computed for synchronous concerted coupling between a diene and a dienophile74. The computed reaction enthalpies (∆H) and barriers (∆H‡) for all reactions (18 reactions, denoted as R1 to R18) using four different levels of theory are presented in Table 2. The computed deviations of reaction enthalpies and barrier with the G4MP2 level of theory are denoted as ∆∆H and ∆∆H‡, respectively. The calculated results show that B3LYP performs poorly in predicting the reaction enthalpies and the barriers for all reactions compared to the G4MP2 level of theory (Table 2). The mean absolute deviations (MAD) of activation energies and reaction enthalpies of the B3LYP/6-31+G(d) calculations are 7.3 and 11.7 kcal/mol, respectively. These large errors on reaction enthalpies also caused changes on the signs of the reaction enthalpies. Based on these results, the B3LYP is not suitable for investigating the thermodynamics of the cycloaddition reactions of furan with selected LWMCs. On the other hand, the computed energetics at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory show a reasonable accuracy, with a MAD of 3.5 kcal/mol for reaction barriers and 1.1 kcal/mol for reaction enthalpies, respectively. Note that, reactions R14 and R17 showed relatively larger deviations (more than 5 kcal/mol). In order to analyze the source of the deviations, additional MP2 calculations were performed using larger basis sets including 6311+G(d) and cc-pVTZ. The calculations suggest that the larger basis sets have little effect on the deviations of the MP2 energies. Computed energetics at the CCSD(T)/cc-pVTZ/B3LYP/6-31+G(d) level of theory show very good agreement with the reference G4MP2 values. The MADs of the CCSD(T) barriers and reaction enthalpies are 1.5 and 0.4 kcal/mol, respectively. This suggests that the larger deviations of MP2 results (especially for R14 and R17) are likely due to the intrinsic limitation of the MP2 method, rather than the geometry change caused by the two different basis sets (6-31+G(d) and 6-31G(2df,p)). Although CCSD(T)/cc-pVTZ results showed good accuracy, these calculations are computationally expensive, which would not be feasible for calculating larger systems. From this section, we have identified a suitable computational method, MP2/6-31+G(d), for investigating reaction energies and barriers of Diels-Alder reactions for furan with small molecules (C1-C4). Although a few DFT functionals, such as M05-2X75 and ωB97X-D76, were also identified previously49, 77 to give good accuracy on reaction enthalpies of Diels-Alder reactions, in this work we chose MP2 to calculate the thermochemistry of furfural related reactions in the later section (Section 3.2), based on its reliability and acceptable computational cost.

3.1.2 Thermodynamic Analysis of Cycloaddition Reactions of Furan with C1-C4 Molecules The thermochemical analysis of cycloaddition reactions of furan with C1-C4 molecules is based on the G4MP2 results in Table 2 including trends in the reaction energies and barriers for the furan Diels Alder reactions. The first reaction (R1) is hydrogenation reaction of furan via concerted synchronous addition. The G4MP2 energies indicate that this reaction is exothermic by 5.5 kcal/mol and the activation enthalpy required for this reaction is 42.7 kcal/mol. Compared to R1, a typical cycloaddition reaction of furan with ethylene (4+2 cycloaddition, reaction R2) has a reaction barrier of 22.5 kcal/mol and the reaction is exothermic by 11 kcal/mol.

The cycloaddition of furan with formaldehyde (R3),

acetaldehyde (R4) and oxaldehyde (R7) has reaction barriers of 22.9, 26.2 and 17.4 kcal/mol, respectively. These 4 ACS Paragon Plus Environment

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reactions are slightly more endothermic compared to R2 or R1. The computed activation barrier when using the enol from of oxaldehyde (R8) is 25.0 kcal/mol and it is exothermic compared to R7. The R9 reaction, cycloaddition of two furans, is endothermic by 4.1 kcal/mol. In comparison, the cycloaddition of furan with 2, 3-dihydrofuran (R10) requires a marginally lower reaction barrier, and results in an exothermic reaction (-12.3 kcal/mol). The difference between the enthalpies of R9 and R10 is 16.4 kcal/mol, which is close to the resonance energy of furan, 16 kcal/mol. Thus, the higher reaction enthalpy for R9 is predominantly due to the energy required to break the aromaticity of furan. The calculations suggest that, in most cases, the Diels-Alder reactions of furan with C=C sites (R5, R14, R17) are marginally more exothermic than the C=O sites (R4, R13, R18) of the dienophile. In terms of computed reaction barriers, a majority of cylcoaddition reactions (R2 to R18) require less than 30 kcal/mol, which is significantly lower than the activation barrier required for decomposition of cellulose or glucose fragmentation68, 78-80, retro-aldol, or dehydration reactions67, 70. The cycloaddition of a furan to another furan (R9) requires a marginally high barrier (∆H‡ = 28.6 kcal/mol) compared to many other cycloaddition reactions with C=C bond. The Diels-Alder reactions of furan (C4) with C1-C4 small oxygenates result in the formation of cyclo ethers (C5-C8) with two less π bonds (C5-C8 refer to the total numbers of the carbons in the molecules). Due to the existence of the C=C bonds, the C5-C8 cyclo-ethers may continue to interact with additional furans (or other dienes) in the system. Selected C5C8 cyclo ethers were considered for the interaction with an additional furan (Table S2 of the Supporting Information). The reaction enthalpies indicate that the formation of the second-generation cyclo ethers from C5-C8 ethers has either lower or similar reaction barriers and the reactions are exothermic. This suggests that these C5-C8 cyclo ethers, in the presence of additional furans, are likely to form larger cyclo ethers; thus, in order to produce C5-C8, the continuing C-C coupling reactions need to be prevented. Therefore, catalysts that activate dienes/ dienophiles and can control the continuing cycloaddition by providing porous confinement are necessary for selective upgrading of furan and LMWC to form gasoline range alkanes. 3.1.3

Temperature Dependence of Rate Constants

The reaction temperature plays a significant role in controlling the thermodynamic and kinetic feasibility of the energetics of the cycloaddition reactions. The free energies (∆G) for the cycloaddition reaction of furan with formaldehyde (R3) and acetaldehyde (R4) were calculated for temperatures ranging from 0 to 500 ̊C. The computed free energy profile at constant pressure (1 atm) is shown in Figure 2 (top). As expected, the ∆G’s of both systems increase as the temperature rises. The slopes of both lines (0.049 and 0.047) represent the negative entropy changes. At 0 ̊C the reaction free energy of the formaldehyde system is 16.2 kcal/mol, and that of the acetaldehyde system is 19.7 kcal/mol. The free energy barriers of the two systems have shown similar changes with respect to the temperature (Figure 2, right). At 0 ̊C the free energy activation barrier of the formaldehyde system is 33.7 kcal/mol, and that of the acetaldehyde system is 37.4 kcal/mol. The rate constant (k) can be computed using the following Eyring equation: ‡

݇=

௞ಳ ் ି∆ಸ ݁ ೃ೅ ௛

(1)

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where kB is the Boltzmann Constant, h is Planck’s constant, T is temperature, ∆G‡ is activation free energy, and R is the universal gas constant. Using Eqn (1), rate constants for all 18 reactions in Table 2 were computed at 298 K (Table S3, Supporting Information), and the calculated rate constants of reactions R3 and R4 at various temperatures (to 500 ̊C) are given in Table 3. At room temperature (25 ̊C), the rate constants for R3 and R4 are on the order of 10-13 and 10-15, respectively. At 300 ̊C, the rate constant of R3 is on the order of 10-5 M-1s-1, suggesting that activation can be attained at this temperature to facilitate the Diels-Alder reaction. Note that, a rate constant of a reaction, 10-5 M-1s-1 corresponds to a half-life (t1/2) of 28 hours based on second order reaction kinetics. At 500 ̊C, the computed rate constant of R3 is in the order of 10-3 M-1s-1, suggesting a fast reaction turn-over at this temperature (t1/2) = 0.28 hours. However, for R4, the computed rate constant at 500 ̊C is in the order of 10-5 M-1s-1 suggesting R4 is significantly slower than the R3 at identical reaction conditions. Based on the evaluation of rate constants of R3 and R4, the rest of the 16 reactions presented in Table 2 can be categorized based on their computed activation free energies (See Table S3 of the Supporting information). Based on classical transition state theory, the calculated free energy barriers of all the 18 reactions at room temperature correspond to specific rate constants at room temperature. Reactions R2, R5-R8, R10-R12 and R14-R17 (See Table 2) have quantitatively smaller free energy barriers than R3; and, therefore, are likely to occur at 300 ̊ C with enhanced kinetics at 500 ̊C. However, reactions, R1, R9, R13 and R18 (See Table 2) have larger free energy barriers compared to R3 and R4, and would, therefore, exhibit relatively lower reaction rates. Based on the analysis of the temperature dependence of these reactions, it is experimentally feasible to control the products of furan reacting with C1-C4 molecules by altering the temperature parameter. 3.2 Reactions for Conversion of Furfural to Hydrocarbon Fuels In this section, furfural (furan-2-carboxaldehyde), an important platform chemical from biomass conversion is taken as a model compound to investigate the energetics of stepwise conversion of furfural to longer hydrocarbon chains (C13C30). Aldol condensation is a convenient way to synthesize a functionalized larger molecule; the reaction joins two carbonyl containing molecules together forming a C-C bond between the α carbon of one molecule and the carbon of the carbonyl of the second molecule

36, 44

. Furfural is an ideal starting reagent for aldol reaction, which is not possible with

furan. A recent experimental study by Huber et al.45, reports that with a stepwise aldol and C-C coupling reaction (e.g., Michael additions and Diels-Alder type reactions) it is possible to convert furfural to longer hydrocarbon chains (C13-C30). Inspired by Huber’s work45, we focus on the thermochemistry including the reaction barriers of a network of likely reactions associated with the conversion of furfural to longer chains of hydrocarbons. Figure 3 presents a sequence of reactions that lead to hydrocarbon fuels from furfural (C5) through C-C couplings and subsequent hydrodeoxygenation. Four pathways are shown to explain the stepwise conversion of furfural to longer chain hydrocarbons. Pathways 1 to 4 result in the formation C8, C13, C26, and C21 hydrocarbons respectively. Additionally, Pathways 2 to 4 are initiated via the formation of FAF (C13, short for furfural–acetone (2:1) dimer). In Pathway 1, the furfural undergoes an aldol reaction with acetone (or any carbonyl compound with α-hydrogen atoms) to form an adduct FA (C8, short for furfural–acetone (1:1) dimer). Subsequent hydrogenation and hydrodeoxygenation of FA leads to octane (C8). Computations show that both hydrogenation and hydrodeoxygenation reactions are largely exothermic (>40 6 ACS Paragon Plus Environment

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kcal/mol). The FA molecule, which possesses an α-hydrogen atom essential for the aldol reaction, will react with another furfural to form another aldol product labeled as FAF (C13, see Figure 3). These two aldol condensation reactions, Furfural
FA and FA
FAF, are slightly endothermic and are catalyzed by bases (NaOH, proline, imines etc.)40, 81-84. The FAF is a very reactive intermediate due to the presence of olefinic and carbonyl functional groups. In Figure 3, three likely pathways that utilize FAF to convert to the longer chain hydrocarbons are shown (Pathways 2, 3 and 4). Pathway 2 produces tridecane (C13) from the direct hydrogenation and hydrodeoxygenation of FAF. It is reported that the FAF can be hydrogenated over a Ru/Al2O3 catalyst at relatively low temperatures (80-140 ̊C)36, 40. Fully hydrogenated FAF can then be converted to C13 hydrocarbon fuels through high temperature hydrodeoxygenation (200-300 ̊C) 85. In Pathway 3, the hydrocycloaddition reaction of FAF and subsequent hydrodeoxygenation is shown. Upon hydrogenation of FAF, the reaction system is likely to have up to 24 partially hydrogenated FAF intermediates and 1 fully hydrogenated FAF (These species are named as FAFm, where m is the index from 1 to 25). Highly reactive FAF and FAFm could undergo further cycloaddition reactions to form FAF dimers (FAFm-FAFn). These FAF dimers can then be fully hydrogenated to H-FAF-FAF’s, which go through hydrodeoxygenation to form C26 cyclo hydrocarbons (Figure 3). It is notable that the hydrogenation of FAF generates 24 possible FAFm intermediates. Detailed relative energetics of hydrogenated FAFm and FAFm-FAFn intermediates are described in the following sections. In Pathway 4, chemical conversion of FAF to C21 hydrocarbons via a Michael addition and hydrodeoxygenation is shown

81, 85

. In this route, the first reaction is 1,4 conjugate addition of FAF (C13) with the parent FA (C8) to form FA-

FAF enol. This reaction is marginally endothermic (ΔH = +2.8 kcal/mol). The enol form of FA-FAF is likely to rearrange to its more stable keto isomer (∆H = -12.7 kcal/mol). Complete hydrogenation and hydrodeoxygenation of the FA-FAF keto generates C21 hydrocarbons. 3.2.1

Partial Hydrogenation of FAF

Hydrogenation of FAF results in 24 partially hydrogenated FAF intermediates and 1 fully hydrogenated FAF, denoted here as FAFm. The computed relative enthalpy (H) of hydrogenation per H2 for all the 25 structures are computed at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory and presented in Figure 4. The enthalpy of the hydrogenation is computed using the following equation: ΔH(hydrogenation) = (H FAFm- HFAF)/nH2

(2)

where ‘n’ is the number of molecules of hydrogen added to FAF to form FAFm. All the hydrogenation reactions are exothermic, and the average enthalpy of reaction per H2 of each compound ranges from -2.1 to -24.5 kcal/mol, as shown in Figure 4. For the convenience of discussion, the C-O and C-C double bonds in FAF are labeled as four types including ‘a’ (the C-O double bond), ‘b’ (the C-C double bond on the linear chain), ‘c’ (the furan C-C double bond with less substituted hydrogen atoms) and ‘d’ (the furan C-C double bond with more hydrogen atoms), as shown in Figure 4. The hydrogenation of the four C=C bond types are represented by FAF1 (a), FAF2 (b), FAF6 (d) and FAF7 (c) in Figure 4. Among these compounds, hydrogenation of FAF to form FAF2 is computed to be the most exothermic reaction. From the enthalpy of hydrogenation, the relative feasibility of the hydrogenation of four double bonds of FAF are in the order of: b>d>a>c. From the chemistry point of view, breaking down the aromaticity of the furan structure requires extra energy, 7 ACS Paragon Plus Environment

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and hydrogenation of a ketone C-O double bond also requires more energy than hydrogenating an alkene C-C double bond because of the greater stability of the C-O double bond86. Thus, the hydrogenation of FAF is likely to first occur on the two C-C double bonds in the linear chain before the other double bonds including aromatic furan sites. Further, detailed experimental and theoretical studies are required to understand the preference of catalytic hydrogenation energetics as well as the reactivity and selectivity of various unsaturated sites.

3.2.2

Diels-Alder Cycloaddition of FAF and FAFm intermediates

In principle, Diels-Alder reactions of FAF and 24 partially hydrogenated FAF compounds, the FAFm species, can generate hundreds of FAFm-FAFn structures. In our investigation, we have identified over 200 FAFm-FAFn compounds, and their structures are shown in Table S1 of Supporting Information. Here, we have computed energies for 170 reactions and reaction barriers for 14 representative reactions. To begin with, the 170 calculated Diels-Alder reactions were ranked using the computed free energies (red lines) and reaction enthalpies (blue lines) as shown in Figure 5. Except for a small portion of the reactions that are exergonic (-10 kcal/mol < ∆G < 0), most of the reactions are slightly endergonic (0 < ∆G < 10 kcal/mol) in the gas phase at the standard temperature and pressure. Since all the reaction free energies are close to 0, these reactions are reversible at the standard conditions. As shown in Figure 5, the reaction enthalpies of all 170 reactions are exothermic (-25 kcal/mol < ∆H < 0). A schematic representation of 14 representative cyclo adducts among various FAFm-FAFn is shown in Figure 6. The computed energetics including the reaction barriers are shown in Table 4. Among the 170 Diels-Alder reactions, six general linkage types were found for FAFm-FAFn intermediates, based on the positions of reacted C-C double bonds (b, c and d types in Figure 4). The most common linkage, Linkage 1, shown in Figure 6 results from the cycloaddition between a furan group of one FAF /FAFm and the linear C-C double bond ( type b in Figure 4) of another. The Linkages 2 and 3 came from the cycloaddition between a furan group and type c and d C-C double bonds, respectively. Linkage 4, 5 and 6 formed 14 and 18 membered ring structures from the intramolecular DielsAlder reactions of a Linkage 2 or 3 compound (FAF3-FAF8-II, FAF3-FAF9-II, and FAF5-FAF5-II in Figure 6). The reaction energetics of the compounds in Figure 6 are listed in Table 4. The reaction enthalpies and free energies did not show any trends as a function of linkage, however the computed rate constants for the cycloaddition are in the following order: Linkage 1 > Linkage (4,5,6) > Linkage 2 > Linkage 3 (Table 4). The cycloaddition reactions to form Linkage 1 adducts requires relatively lower reaction barriers than the cycloaddition reactions between furan and formaldehyde (R3 in Table 2). Therefore they are more likely than other linkages. Based on the computed rate constants, the cycloaddition between FAF and partially hydrogenated FAF species (FAFm) are more likely to occur than the rest to form linkage 1 compounds. The cyclo adducts (C26) compounds contain a significant amount of C=C and C=O bonds, which are required to be hydrogenated prior to the hydrodeoxygenation process to form cyclic oils. Experimentally, the cycloaddition reactions of FAF and FAFm have been carried out in tetrahydrofuran40. We have performed calculations to include solvent effects in the thermochemical data. Both gas phase and solution phase calculations showed similar trends; linkage type 1 showed lower free energy barriers than the other types of reactions. Also in the solution the free energy barriers and reaction free energies are quantitatively more positive than in the gas phase (See Table S6 in the Supporting Information for details). On average, in tetrahydrofuran the free energy barriers 8 ACS Paragon Plus Environment

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were increased by about 6 kcal/mol and the reaction free energies were increased by about 2 kcal/mol. The comparable results for both phases suggest that non-polar solvents play a less significant role in the thermodynamics of the DielsAlder reactions described in this paper.

3.2.3

Energetics of Hydrodeoxygenation of the Diels-Alder Products

Diels Alder reactions of FAF and FAFm result in the formation of various FAFm-FAFn intermediates (C26). The energetics for hydrogenation of FAFm-FAFn intermediates are similar to that of the hydrogenation of FAF (Figure 4). The hydrogenation of all FAFm-FAFn intermediates are exothermic, and the hydrogenation of each linkage type results in the formation six representative cyclic hydrocarbons, as shown in Figure 7. For example, complete hydrogenation and subsequent hydrodeoxygenation of all six linkage 1 compounds results in the formation of C26. The hydrodeoxygenation of all FAFm-FAFn intermediates are exothermic and the reaction enthalpies per H2 did not show any significant trends. The resulting C26 hydrocarbons can be a potential source of fuel oil40.

4

Summary and Conclusions

One of the central challenges in the utilization of biomass is the efficient conversion of cellulosic materials to liquid transportation fuels. During the chemical transformation of biomass, it is essential to couple precursor compounds obtained from pyrolysis or aqueous phase reforming processes to form longer chain hydrocarbons via formation of a carbon-carbon bond. In this paper, we report on accurate quantum chemical calculations of the structures and energetics of carbon–carbon bond formation reactions of two model compounds, furan and furfural in the gas phase. On the basis of the detailed computational results, the following conclusions can be drawn: 1. Different levels of theory (CCSD(T), MP2 and B3LYP) were compared against highly accurate G4MP2 method to assess reliability for cycloaddition reactions of furan. B3LYP is shown to give significant errors in the prediction of reaction enthalpies and barriers. The MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory is a good choice for reactions described here based on the accuracy and computational cost. 2. Furan, can be coupled with various C1 to C4 lower molecular weight carbohydrates obtained from the pyrolysis via Diels Alder type reactions in the gas phase to produce C5-C8 cyclic ethers. The computed reaction barriers for these reactions (~25 kcal/mol) are lower than the cellulose activation or decomposition reactions (~50 kcal/mol). Cycloaddition of C5-C8 cyclo ethers with furans can also occur in the gas phase, and the computed activation energy is similar to that of the first Diels-Alder reaction. Catalytic confinement will probably be required to limit the degree of cycloaddition, and, hence, the chain length of the cyclo-adducts. 3. Furfural, obtained from biomass, can be coupled with aldehydes or ketones with α-hydrogen atoms to form longer chain aldol products and these aldol products can undergo vapor phase hydrocycloaddition or Michael addition reactions to form precursors for C13-C26 cyclo hydrocarbons. Aldol condensation of furfural with acetone to produce a C13 intermediates is an endothermic reaction, while hydrogenation of these aldol adducts is an exothermic process. 9 ACS Paragon Plus Environment

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4. Partially hydrogenated aldol products (23 compounds studied) can also be coupled with aldol products via DielsAlder reactions to form a plethora of cyclo adducts. Based on the reaction barriers, a handful of reactions are kinetically feasible among the 170 cycloadducts considered in this investigation. Hydrodeoxygenation of these cyclo-adducts can produce C26 cyclic hydrocarbons (2-butyl-1-heptyl-2-nonyl-cyclohexane). Chemical reactions associated with fast pyrolysis are extremely complex due to the bio-oil composition and large number of possible chemical reactions. Formation of furan (and derivatives) from bio-oil is now possible using catalytic fast pyrolysis, and subsequently upgrading these furans by utilizing C1-C4 fraction of bio-oil components is ideal to form distillate range carbon frameworks. The computations discussed in the present work provide an initial basis to understand such possible coupling reactions between furans and small oxygenates. Future computational studies of catalysts and experimentation will enable further understanding and optimization the vapor phase upgrading of bio-oil, with respect to the effect of catalytic sites and catalytic confinements on the selectivity and the extent of cycloaddition reactions between furan derivatives and small oxygenates obtained from biomass transformation.

Acknowledgements: This research was supported by the Bioenergy Technologies Office (BETO) program of Energy Efficiency & Renewable Energy (EERE). We gratefully acknowledge the computing resources provided on “Fusion”, a 320-node computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Supporting Information Available: Reaction free energies and free energy barriers of Diels-Alder reactions 1-18 are shown in Table S1. Reaction enthalpies of selected C5-C8 cyclo ethers reacting with additional furan are in Table S2. Rate constants and half-lives of reactions 118 are shown in Table S3. Enthalpies and free energies of hydrogenation per H2 of hydrogenated FAF’s are shown in Table S4. Reaction enthalpies and free energies of Diels-Alder reactions to form FAFm-FAFn’s are shown in Table S5. Reaction free energies and barriers (kcal/mol) of the Diels-Alder cycloaddition of FAF and selected FAFm’s in THF are shown in Table S6. Identified structures of FAFm-FAFn’s are shown in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org. 5

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Figure 1 Strategies of producing liquid transportation fuels from biomass

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50 45 40 35 30 25 20 15 10 5 0

Acetaldehyde y = 0.049x + 19.7 Formaldehyde y = 0.047x + 16.2

0

100

200

300

400

500

Free Energy Barriers ∆G‡ (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Reaction Free Energies ∆G (kcal/mol)

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70 Acetaldehyde y = 0.047x + 37.4

60 50 40

Formaldehyde y = 0.045x + 33.7

30 20 10 0 0

100

Temperature ( ̊C)

200

300

400

500

Temperature ( ̊C)

Figure 2. Effect of temperature on the reaction free energies (left) and activation free energies (right) for Diels Alder carbon-carbon coupling of furan with formaldehyde (R3 in Table 2) and acetaldehyde (R4 in Table 2) in the gas phase computed at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory

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Pathway 1 OH

H2

O

H= -61.4 H2

H-FA

C8

H= -41.2

Aldol Condensation O O

Furfural

O

O

O

Furfural

O - H2O H= +1.4

O

- H2O H= +2.8

FA

7H2 H= -114.1

w th a P

OH O

O FAF

ay

Pa th w

2

ay

H2

H= +2.8

4

Michael Addition

O

I

O

FA

O

O

O

OH O

Fully Hydrogenated FAF(FAF10)

O

O

Partially Hydrogenated FAF (FAFm) Diels-Alder Reaction

H2O Hydrodeoxyenation

II

H = -71.9

O

H2

FA-FAF (enol)

O

O

H= -12.7 O O C13

O

O O

FAFm-FAFn O

O

O

H2 III O

Hydrocycloaddition

O

HO

FA-FAF (keto isomer) H2

OH O

O

H= -113.0 OH OH

H-FAF-FAF O H2

O

O

Hydrodeoxygenation

H2O IV

H2 H2O

C26 Cyclo Oil

Hydrodeoxyenation H= -150.2

C21

Figure 3. Various reaction steps associated with the conversion of furfural to longer chain hydrocarbons (C8, C13, C21, C26). The computed reaction enthalpies (kcal/mol) at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level are shown. Twentyfour species ‘I’ involving hydrogenation of FAF are considered (referred to as FAFm, see Section 3.2.1), depending on which C=C bonds in FAF are hydrogenated. More than 150 structures with 6 typical linkages are possible for species ‘II’ (referred to as FAFm-FAFn, see section 3.3.2). The species ‘III’ are hydrogenated FAF dimers (referred to as H-FAFFAF), which have 6 major conformations. The species ‘IV’ are C26 cyclic hydrocarbons with 6 different conformations (see details in section 3.2.3). 16 ACS Paragon Plus Environment

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The Journal of Physical Chemistry

Figure 4. Schematic of hydrogenated FAF (FAFm, 25 structures) structures with various degrees of hydrogenation denoted by m and computed relative enthalpies of hydrogenation per H2 molecule at the MP2/6-31+G(d)//B3LYP/631+G(d) level of theory. All energies are given in kcal/mol.

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10

5

0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170

Reaction Enthalpies and Free Energies (kcal/mol)

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-5

-10

-15

-20 ∆H

∆G

Reaction Index -25

Figure 5. Computed reaction enthalpies and free energies of possible 170 Diels Alder Reactions of FAF and various FAFm intermediates at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory. The reaction indices are shown in the Xaxis (The indices refer to the reaction indices in Figure S3, Supporting Information). See Figure S1, and Table S4 of the supporting information for details of structures and energies, respectively, for all the 170 structures.

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The Journal of Physical Chemistry

Figure 6. Possible linkages (1 to 6) of FAFm-FAFn structures. The computed free energies of FAFm-FAFn intermediates (170 compounds are considered) are shown Table S5 of the Supporting Information.

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Figure 7. Schematics of FAFm-FAFn intermediates and C26 hydrocarbons obtained upon hydrodeoxygenation.

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The Journal of Physical Chemistry

Table 1. Examples of low molecular weight carbohydrates (LMWC, C1-C4) obtained from biomass pyrolysis Number of carbons

C1

C2

Compound Name

References

Formaldehyde

22-26

Number of carbons

Compound Name

References

Propionaldehyde

22, 24

Hydroxyacetone

23, 25, 27, 30

Methylglyoxal

23, 25

Acetaldehyde

24, 26-29

Methanol

22, 26, 28-29, 31

Formic acid

22, 28-29, 32

Acetone

22, 24, 26-29, 33

Glyoxal

23, 25, 29

Crotonaldehyde

29, 34-35

Acetic acid

22-23, 28-29, 32

2,3-Butanedione

25, 29

Glycolaldehyde

22, 25, 27, 29-32

1-Hydroxy-2-butanon

28-29

Ethylene

25-26

2-butene

26

(E)-ethene-1,2-diol

29

2-Hydroxy-3-oxobutanal

29, 31

C3

C4

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Table 2. Computed activation enthalpies (∆H‡) and reaction enthalpies (∆H) of cycloaddition reactions (18 reactions) of furan with various dienophiles at different levels of theory. The ∆∆H and ∆∆H‡ denotes the deviations of CCSD(T), MP2 and B3LYP methods from the G4MP2 reaction energies and barriers. All values are given in kcal/mol. ∆H‡ G4MP2

Reactions (Rn)

∆∆H‡ CCSD(T) /cc-pVTZ

∆∆H‡ MP2/ 6-31+G(d)

∆∆H‡ B3LYP/ 6-31+G(d)

∆H G4MP2

∆∆H CCSD(T) /cc-pVTZ

∆∆H MP2/ 6-31+G(d)

∆∆H B3LYP/ 6-31+G(d)

R1

42.7

-0.3

4.5

1.1

-5.5

-1.4

2.8

-1.0

R2

22.5

1.3

-3.4

5.3

-11.0

-0.1

-2.1

8.3

R3

22.9

1.1

-1.5

2.9

2.1

0.0

1.2

7.4

R4

26.2

1.2

-1.8

6.1

5.0

0.1

1.2

10.5

R5

25.4

1.1

-4.1

5.9

-8.2

0.1

-2.9

9.7

R6

23.9

1.9

-4.1

9.9

-10.4

0.5

-2.1

13.5

R7

17.4

1.3

-2.5

4.9

0.4

0.3

0.8

9.2

R8

25.0

1.9

-4.7

7.2

-12.0

0.4

-2.9

11.2

R9

28.6

2.2

-2.7

8.2

4.1

0.7

0.6

12.9

R10

24.2

1.7

-4.7

8.4

-12.3

0.3

-2.3

12.7

R11

17.4

1.4

-3.5

9.5

-0.2

0.7

0.2

14.2

18.0

1.5

-2.9

6.5

0.2

0.5

0.5

10.7

R13

33.0

1.3

-1.9

8.9

13.7

0.4

1.5

13.1

R14

11.6

3.1

-6.5

7.6

-11.9

0.8

-2.3

15.7

R15

16.0

1.3

-3.4

9.9

-2.5

0.1

-0.4

14.5

R16

14.3

0.8

-3.2

7.5

-5.1

-0.1

-0.6

12.3

R17

17.5

2.0

-5.2

11.7

-5.8

0.5

-2.4

19.5

O O

R12

2-Oxobutanal

O O

O

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The Journal of Physical Chemistry

R18

MADa a

32.3

1.3

-2.0

10.0

13.7

0.4

2.1

14.0

0

1.5

3.5

7.3

0

0.4

1.1

11.7

Denotes Mean Absolute Deviation (MAD)

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Table 3. Temperature dependance of the computed rate constants of (k, M-1s-1) for reactions of furan with formaldehyde (R3 in Table 2) and acetaldehyde (R4 in Table 2), computed using transition state theory at the MP2/631+G(d)//B3LYP/6-31+G(d) level of theory Temperature (̊C) 0 25 50 100 150 200 300 400 500 a

k(M-1s-1) R3a 6.2×10-15 1.9×10-13 3.4×10-12 3.3×10-10 1.1×10-8 1.8×10-7 1.2×10-5 2.3×10-4 2.1×10-3

k(M-1s-1) R4a 6.7×10-18 3.6×10-16 9.4×10-15 1.8×10-12 9.7×10-11 2.3×10-9 2.6×10-7 7.6×10-6 9.6×10-5

See Table 2 for the reactions

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The Journal of Physical Chemistry

Table 4. Computed reaction enthalpies (∆H), reaction free energies (∆G), activation enthalpies (∆H‡), activation free energies (∆G‡), and rate constants (in s-1, at 298.15K, 1atm) of the 14 representative Diels-Alder cycloaddition reactions involving FAF and FAFm intermediates at the MP2/6-31+G(d)//B3LYP/6-31+G(d) level of theory. Computed enthalpies of hydrogenation per hydrogen atom of the 14 representative FAFm-FAFn intermediates (See Figure 6) are also given. All energies are reported in kcal/mol. Linkage Typea

∆G‡

∆G

k(M-1s-1)b

∆H of Hydrogenation per H2

FAF + FAF → FAF-FAF

4.0 -10.8 18.9

4.5

8.7×10-2

-17.4

FAF + FAF2 → FAF-FAF2

4.3 -8.4 19.4

6.2

3.7×10-2

-17.0

FAF1 + FAF → FAF1-FAF

3.3 -11.2 19.0

3.5

7.3×10-2

-17.0

FAF2 + FAF2 → FAF2-FAF2

7.9 -11.2 22.8

3.9

1.2×10-4

-16.0

FAF3+ FAF1 → FAF3-FAF1

6.5 -21.8 22.0

-6.0

4.6×10-4

-16.3

FAF + FAF12 → FAF-FAF12 11.5 -15.6 26.6

0.5

2.0×10-7

-18.6

FAF3 + FAF8 → FAF3-FAF8-I 12.3 -17.7 28.3

1.3

1.1×10-8

-17.2

FAF5 + FAF8 → FAF5-FAF8 15.6 -17.0 30.6

0.0

2.3×10-10

-19.6

FAF + FAF14 → FAF-FAF14 16.5 -10.4 31.1

5.7

9.9×10-11

-18.5

FAF3 + FAF9 → FAF3-FAF9-I 16.5 -16.9 32.8

0.0

5.6×10-12

-17.8

FAF5 + FAF5 → FAF5-FAF5-I 17.2 -17.1 33.6

2.3

1.5×10-12

-17.7

4

FAF3-FAF8-I → FAF3-FAF8-II 18.5 -7.7 25.6

1.3

1.1×10-6

-5.1

5

FAF3-FAF9-I → FAF3-FAF9-II 19.2 -11.3 25.2

-1.5

2.1×10-6

-9.1

6

FAF5-FAF5-I → FAF5-FAF5-II 18.0 -10.3 24.7

-2.0

4.9×10-6

-9.2

1

2

3

∆H‡ ∆H

Reactions

a

: See Figure 6 for various linkage types.

b

: Computed using Equation 1.

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Table of Content:

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