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Mechanisms and Energetics for Acid Catalyzed β-D-Glucose Conversion to 5-Hydroxymethylfurfurl Xianghong Qian Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States ABSTRACT: CarParrinello based ab initio molecular dynamics (CPMD) coupled with metadynamics (MTD) simulations were carried out to investigate the mechanism and energetics for acid-catalyzed β-D-glucose conversion to 5-hydroxymethylfurfurl (HMF) in water. HMF is a critical intermediate for biomass conversion to biofuels. It was found that protonation of the C2OH on glucose, the breakage of the C2O2 bond, and the formation of the C2O5 bond is the critical rate-limiting step for the direct glucose conversion to HMF without converting to fructose first, contrary to the wide-spread assumption in literature that fructose is the main intermediate for glucose conversion to HMF. The calculated reaction barrier of 30 35 kcal/mol appears to be solvent-induced and is in excellent agreement with experimental observations.
I. INTRODUCTION To replace up to 30% of the petroleum based transportation fuels with biofuels such as ethanol and liquid hydrocarbons by 2030,1 it is essential that efficient conversion of lignocellulosic biomass to biofuels is achieved. Even though cellulosic ethanol is currently the main candidate to replace petroleum based transportation fuels, it suffers from several limitations including low energy density, high separation cost, high volatility, and easy contamination by atmospheric water.2 Alternative potential liquid biofuels such as 2,5-dimethylfuran (DMF) and liquid alkanes produced from biomass can overcome many of these limitations. HMF is a critical intermediate for DMF and liquid hydrocarbons28 using liquid phase processing. However, HMF yields from biomass carbohydrates remain a critical issue, particularly from glucose, the most abundant monomer sugar from biomass. Production of HMF from fructose and glucose in water is not selective.5 If processed in dimethyl sulfoxide (DMSO), ionic liquids, or mixed solvents using a biphasic reactor,814 over 80% HMF yields from fructose have been achieved. However, HMF yields from catalytic conversion of glucose are limited to less than 80% and are extremely sensitive to processing conditions particularly with respect to the reaction media.57,10,12,1418 There are two mechanisms proposed in the literature for converting xylose and glucose from a six-member pyranose ring structure to furfural and HMF with a five-member furanose ring structure1921 for acid-catalyzed reactions. The mechanism proposed by Feather and Harris20 involves a series of open chains. Initially the open chain form of the glucose aldose structure isomerizes to form a fructose ketose structure. The open chain form of the fructose molecule loses two water molecules via acid-catalyzed dehydration. Finally, the open chain closes forming the five-member furanose ring structure. Based on r 2011 American Chemical Society
experimental evidence, Antal and co-workers19,21 proposed another mechanism involving direct transformation from a pyranose ring to a furanose ring without opening the ring structure. Furan aldehyde, B, shown in Figure 1, was proposed as a critical intermediate. HMF is formed by further dehydration of B by removing two more water molecules. Further, Antal and coworkers19 suggested that B is also the intermediate leading to the formation of HMF from fructose via protonation at C1OH. Our earlier ab initio molecular dynamics studies2225 using CPMD confirmed the mechanism proposed by Antal and coworkers. Further, our earlier results2225 show that protonation of C2OH on β-D-glucose, the breakage of the C2O2 bond and the formation of C2O5 bond is critical for protoncatalyzed glucose to HMF conversion as shown in Figure 1. CarParrinello26 based ab initio molecular dynamics (CPMD) simulations have been used successfully to study sugar reactions.2225,2732 Experimental evidence suggests that sugar loss due to the formation of various glucose disaccharides via condensation reactions is significant at high sugar concentrations.28,29,33 Our previous simulation results demonstrate that condensation reactions are initiated by the protonation of the C1OH on glucose, the breakage of the C1O1 bond and the formation of the C1 carbocation. In addition, it appears that solvent such as water plays a critical role in xylose and glucose condensation reactions due to the high proton affinity of the water molecules.25,28,29,34 The formation of the extensive hydrogen bonding network in water and its high mobility make protons remarkably stable in water. The reaction barriers for sugar condensation reactions and similarly Received: May 6, 2011 Revised: September 7, 2011 Published: September 14, 2011 11740
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Scheme 1. Collective Variables CV1, CV2, and CV3, Describing the Protonation of C2OH on β-D-Glucose, the Subsequent Breakage of the C2O2 Bond, and the Formation of the C2O5 Bond During Glucose Conversion to HMF
Figure 1. Conversion of glucose A to HMF C via a furan aldehyde intermediate B. Each atom on glucose molecule is numbered according to its position with respect to the ring O (atomic size in descending order C (blue), O (red), H (white)).
polysaccharide hydrolysis appear to be induced by the solvent water molecules. In addition, our studies show that partial proton desolvation due to the migration of proton from the bulk solvent to the neighborhood of the sugar molecules also contributes significantly to these reactions. CPMD and metadynamics (MTD) simulations3539 was used here to elucidate the effects of water on glucose to HMF conversion including the reaction mechanism(s) and the ratelimiting step(s). The reaction free energies and barriers were also determined. All simulations focused on β-D-glucose because it is the most significant of all glucose conformers40 and is essential for the conversion. The insights obtained here could then be applied to improve the glucose to HMF processes. Further our results could be applied to biomass pretreatment processes to maximize sugar yields with minimal sugar dehydration and sugar pathway engineering during thermochemical treatments of biomass. To investigate the effects of solvent, CPMD-MTD simulations of glucose reactions both in the absence and presence of the solvent molecules were studied. By comparing the reaction pathways and free energy surfaces of the reaction with and without the solvent, the influence of the solvent on the glucose reactions was elucidated. Moreover, CPMD-MTD simulations are especially computationally demanding, simulations carried out in the gas phase will help identify the potential pitfalls during the course of the study.
II. COMPUTATIONAL METHODS AND PROCEDURE The metadynamics (MTD) method was developed by Parrinello and co-workers38,39 for investigating chemical reactions and exploring associated free energy surfaces (FES). MTD is designed to enhance the probabilities of the energy-barrier crossing events during a chemical reaction or process.38,39 The MTD method is based on the ideas of extended Lagrangian26,38,41,42 and coarse-grained non-Markovian dynamics,38 which allows for very efficient exploration of the FES of the reactive system. This method assumes that several collective coordinates which distinguish reactants from products are able to characterize the reaction process. These collective coordinates (e.g., distances between atoms and coordination numbers) must include the relevant modes that cannot be sampled within the typical time scale of the ab initio MD simulations. The detailed description of the MTD method as well as its application to our glucose and
xylose systems could be found in our earlier publications.28,29,34 Before MTD simulations were conducted, CPMD simulations were conducted for several picoseconds (ps) so that the charges on the atoms were properly distributed for the initial reactant system. CPMDMTD allows for accelerated sampling of chemical reactions involving bond breaking and bond formation processes. The movement of water molecule due to diffusion is probably not going to alter the reaction mechanism. The selection of the collective variables for the protonation of the C2OH on glucose and the subsequent breakage of C2O2 bond and the formation of C2O5 bond to describe the conversion of a glucose molecule to HMF is shown in Scheme 1. The first CV (CV1) is the coordination number (CN) of C2 with respect to O2. The second CV (CV2) is the CN of C2 with respect to O5. The third CV (CV3) is the CN of O2 with respect to all the four H atoms (three from H3O+ and one from C1OH) assuming H atoms are indistinguishable. The equation of CN is given by CNði, jÞ ¼
1 ðdij =d0 Þp 1 ðdij =d0 Þq
ð1Þ
where dij is the distance between atoms i and j, d0 is the cutoff distance, and p and q are high power integers used to distinguish between the coordinated and noncoordinated states as used earlier in literature39,43 and our previous work.28,29 For CV1, CV2, and CV3, the values for d0 are chosen to be 2.0, 1.5, and 1.5 Å, respectively. The values p = 6 and q = 12 were routinely chosen for all the CVs. The dynamics of the CVs are controlled by the force constant k and fictitious mass m. In our simulations, k = 2.0 a.u. and m = 50 amu were used for all CVs. More details of the method and parameters used for investigating FES of sugar reactions can be found in our earlier work.28,29 All our calculations were carried out using the CPMD software package.36 The Becke, Lee, Yang, and Parr (BLYP) functional was used to describe valence and semicore electrons.44,45 The potentials exerted by the core electrons were approximated by the Goedecker pseudo potentials,46 which has been found to yield excellent properties for glucose, xylose and for their reactions in water.28,29 More details on the parameters can also be found in our previous publications.28,29 The MD simulations were carried out under NVT at 300 K with a Nose-Hoover chain thermostat.4749 In the gas phase, the system contains one glucose molecule and one H3O+. The simulation box has a 11741
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Figure 2. Fluctuations of the CVs during CPMDMTD simulations for glucose conversion to HMF intermediate (O2H CV3, green; C2O2 CV1, black; C2O5 CV2, red).
dimension of 15 15 15 in angstroms and was decoupled from its image by using the Hockney’s method with an extra 4 Å added to each dimension of the simulation box.50 For all the calculations in solution, periodic boundary condition was used. In pure water, the system contains one glucose molecule, one H3O+, one Cl counterion, and 76 H2O molecules. The simulation box size has a dimension of 14.4 14.4 14.4 Å and a corresponding density of 0.92 g/cm3. Ewald summation was used to integrate the long-range electrostatic interaction energies. Ewald summation51 remains the most accurate method for calculating electrostatic interactions in a periodic system. The size of our simulation box was determined by the concentration of the hydronium ion in the aqueous solution mimicking experimental conditions. Because ab initio molecular dynamics simulations were used to determine glucose conversion to a HMF intermediate involving bonding breaking and bond formation processes, any possible anisotropic effects as well as long-range correlation artifacts52,53 due to the application of periodic boundary conditions would probably not affect our simulation results and conclusions.
III. RESULTS AND DISCUSSION III.1. Glucose Conversion to HMF in the Gas Phase. A total of over 6000 MTD simulations steps were conducted for glucose conversion to HMF in vacuum. Because the reaction occurs rapidly in the gas phase even at room temperature, the simulations were conducted at 200 K to have sufficient sampling at the beginning of the reaction. At the beginning of the simulations as seen from Figure 2, the proton was transferred back and forth between the C2OH and the H2O molecule. When close to 80 MTD steps, the C2O2 bond breaks and C2O5 bond forms to produce a HMF intermediate B as shown in Figure 1 and one additional H2O molecule. The two H2O molecules thus produced form a hydrogen bonded complex with the C2 carbocation. At around 1000 MTD steps, the C2O2 bond forms again and the C2O5 bond breaks simultaneously, indicating the product was transformed back to the reactant glucose molecule. The regenerated H3O+ forms another hydrogen bonded complex with the glucose molecule. The sampling of this state takes place over 1400 MTD steps. At around 2400 MTD steps, the
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Figure 3. CPMD-MTD sampling along CV1CV3 trajectories for protonation of C2OH and the breaking of C2O2 bond during glucose conversion to the HMF intermediate.
Figure 4. Sampling along CV2CV3 for protonation of C2OH and the formation of C2O5 bond during glucose conversion to HMF intermediate.
glucose molecule again was transformed to the HMF intermediate. One H2O molecule takes away the proton from C1OH forming a CdO bond outside the ring structure. Subsequently, the two H2O molecules form a hydrogen complex with the CdO bond. This sampling takes place between 2400 MTD steps to 5000 MTD steps. At around 5000 MTD step, the C2O5 bond was again seen broken and the C1O5 bond was reformed. The system reverted back to the glucose structure. After additional 750 MTD step, the structure again was transformed to the HMF intermediate. As mentioned earlier, our choice of CVs was based on the reaction mechanism observed in the gas phase and in water using CPMD as well as deduced earlier by experimental work.21 As a result, the alternative CVs were not selected due to the high computing cost associated with using three CVs. During the CPMD-MTD simulations, the system went from the reactants to products then back to reactants twice. Therefore, we believe CPMD-MTD investigation of the back reaction is not necessary. Our MTD simulations were stopped when the CVs representing the C2O2 and C2O5 bonds become diffusive based on the criteria proposed by Liao and Gervasio54 to ensure the accuracy of the free energy sampling. 11742
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Figure 5. Projected two-dimensional CV1CV3 free energy contour plot (left panel) and three-dimensional FES (right panel) for protonation of C2OH and breakage of the C2O2 bond during glucose conversion to HMF intermediate.
Figure 6. Projected two-dimensional CV2CV3 free energy contour plot (left panel) and three-dimensional FES (right panel) for protonation of C2OH and the formation of C2O5 bond during glucose conversion to HMF intermediate.
Figures 3 and 4 show the sampling of the CV trajectories between CV1 (C2O2) and CV3 (O2H) and between CV2 (C2O5) and CV3, respectively. It can be seen that there are sufficient samplings along the reaction coordinates. What was different from the sampling of protonation of C1OH on glucose is that there are significant more Gaussians added to each site corresponding to the complex formed between the water molecules and the reaction intermediate. Figures 5 and 6 exhibit the projected 2-D free energy contour plots (left panels) between CV1CV3 and CV2CV3, respectively. The deep wells formed along certain sites indicate the stability of these hydrogen bonded complexes. It appears that the FESs obtained can be directly related to the proton affinities of these sites during the reaction process. The right panels in Figures 5 and 6 are the reconstructed three-dimensional free energy surfaces for CV1CV3 and CV2CV3, respectively. For sampling between CV1CV3, as shown in Figure 5, it can be seen that the C2OH on glucose molecule has a proton affinity close to 110 kcal/mol at
CV1 ≈ 0.8 and CV3 ≈ 1. After the C2O2 bond breaks during which the value of CV1 is reduced from about 0.8 to 0.2, the proton affinities on various sites of the HMF intermediate are increased dramatically to about 200240 kcal/mol depending on the location of the site. This seems to indicate that formation of HMF furan intermediate is very much favored and this intermediate appears to be more stable than the glucose molecule under the acidic condition. For sampling between CV2CV3, as shown in Figure 6, the results obtained are quite surprising. An opposite trend was observed. At CV2 ≈ 0.1 and CV3 ≈1 when the glucose molecule is the dominant structure, the proton affinity of various OH sites is close to 190 kcal/mol. This value is very close to the earlier experimental value for secondary alcohol 2-propanol55 with a proton affinity of 189.5 kcal/mol. When CV2 is increased to 0.75 at which the C2O5 bond has formed, the proton affinities of various sites on the HMF intermediate are close to 120140 kcal/mol. The results obtained from CV2CV3 plot are on average significantly lower 11743
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Figure 8. Short-lived unstable gemdiol structure during glucose conversion to a HMF intermediate.
Figure 7. Variation of CVs during the MTD simulations for glucose conversion to HMF intermediate (O2H CV3, green; C2O2 CV1, black; C2O5 CV2, red).
than those values obtained on CV1CV3 plot for the proton affinities on various sites of the HMF intermediate. On the contrary, the proton affinities obtained for the glucose molecule on CV2CV3 plot are significantly higher than the corresponding affinities observed on CV1CV3 plot. This is likely due to limited choice of collective variables to describe the reaction process. Some of the changes occurring during the reaction are not possible to distinguish. Increase the number of CVs will likely increase the simulation time tremendously, which is impossible with the current computing resources. As a consequence, the lower proton affinities likely reflect the true proton affinities of the various sites on the glucose molecule and HMF intermediate. Therefore, the proton affinities obtained from our current calculations are close to 100120 kcal/mol, significantly lower than the values obtained experimentally as well as the results from static quantum calculations.27 The reason for the lower proton affinity is most likely due to the fact that there are additional two H2O molecules around these sites forming hydrogen bonded complex structures, which reduces the proton affinities of the sites on the glucose molecule as well as on the HMF intermediate. It is still puzzling that protonation of C1OH does not involve these complex structures, whereas protonation of C2OH involves these complex structure formation. It is probably due to the fact that C1 carbocation is much more stable than C2 carbocation. Protonation of C1OH directly leads to the formation of C1 carbocation. The associated proton does not have the chance to sample other sites. In the case of protonation of C2OH, the proton has the opportunity to sample other sites due to the higher energy associated to create the C2 carbocation. III.2. Glucose Conversion to HMF in H2O. Here, glucose conversion to HMF intermediate was investigated using CPMDMTD for one glucose molecule surrounded by 76 H2O molecules representing the closest hydration shells. The same three CVs were selected to describe the reaction coordinates as in the gas phase. A total of 800 MTD steps were conducted for investigating the free energy surfaces for glucose conversion to HMF intermediate in water. Figure 7 exhibits the variation of the CVs during the course of CPMDMTD simulations. It can be seen that a proton
was transferred to C2OH at around 50 MTD steps and stayed on C2OH until about 80 MTD steps. Then the proton was transferred back and forth between H2O and C2OH. During the protonation of C2OH, Zundal-like complex described by earlier work56 was not observed, contrary to our previous observation during protonation of C1OH on glucose28 and xylose.29 However, once the intermediate of HMF was formed, Zundal-like structure was again seen after the formation of C1OH+. At around 230 MTD steps, the C2O2 bond breaks and C2O5 bond forms simultaneously. During the remaining simulation steps, the bond length of the C2O5 bond oscillates slightly but the bond remains intact. The C2O2 bond remains broken with the distance between C2 and O2 fluctuating particularly after 430 MTD steps. At around 730 MTD steps, there is one OH exchange occurring between the hydroxyl group of C1OH on HMF intermediate B and the neighboring H2O molecule. A gemdiol intermediate structure was formed as shown in Figure 8. After the OH exchange, the system started to sample another well on the FES. Because the product does not distinguish between these two different OH groups, a pair of symmetric product wells is expected. The first product well should reflect accurate enough FES for the product. As a result, the final free energy surface was reconstructed based on the first 800 MTD simulation steps. Figure 9 shows the CV trajectories between CV1CV3 (left panel) and between CV2CV3 (right panel). There appears to have sufficient sampling at the reactant and product wells, even though the crossing back to the reactant well has not been observed at the end of the simulation period due to the sampling of the other symmetric product well. However, the reaction barrier obtained should be reasonably accurate. Figures 10 and 11 exhibit the projected two-dimensional free energy contour plot and the corresponding 3-D free energy surface between CV1 CV3 respectively. From Figures 10 and 11 of the 2D free energy contour and 3-D FES plots between CV1 (C2O2 bond) and CV3 (O2H bond), respectively, the initial reactant well is located at position with coordinates around CV1 ≈ 0.9 and CV3 ≈ 1. As the proton was transferred to the C2OH on glucose, CV3 increases from about 1 to 1.8. Associated with this process, a reaction barrier of about 15 kcal/mol is observed. Subsequent processes involve the departure of the H2O molecule from protonated C2OH2 on the six-member pyranose ring structure and the formation of a C2 carbocation. Due to the unstable nature of the C2 carbocation, immediately the ring O5 forms a bond with C2 carbon and breaks the bond with C1 generating a five-member furanose ring structure. During this process, CV1 reduces from about 0.9 to 0.1. The final product for this process has a reaction coordinate CV1 ≈ 0.1 and CV3 ≈ 1.9. The reaction barrier to the product well from the protonated pyranose glucose structure to the furanose intermediate is another 1520 kcal/mol. The overall reaction barrier is around 3035 kcal/mol in excellent agreement with experimental data.33 It is interesting to observe that 11744
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Figure 9. Sampling of CV1CV3 trajectories (left) and CV2CV3 trajectories (right) for protonation of C2OH on glucose and the subsequent breakage of C2O2 bond during glucose conversion to HMF intermediate.
Figure 10. Projected two-dimensional free energy contour plot between CV1 (C2O2) and CV3 (O2H) for glucose conversion to a HMF intermediate initiated by protonation of C2OH.
the reaction barriers for protonation of C1OH to form a C1 carbocation and protonation of C2OH to form C2 carbocation are quite different, with the former at around 2530 kcal/mol,28 the latter at around 3035 kcal/mol. This is probably due to the relative stability of the carbocation. C1 carbocation is more stable and relatively easy to form whereas C2 carbocation is highly unstable leading to the formation of a HMF intermediate. Removing additional two water molecules from this intermediate via dehydration will lead to the formation of HMF. Energetically, this reaction appears to be very favorable with a significant
Figure 11. Three-dimensional free energy surface between CV1 (C2O2) and CV3 (O2H) for glucose conversion to HMF intermediate initiated by protonation of C2OH.
decrease in the product free energy. In addition, during xylose and glucose condensation reactions initiated by protonation of C1OH and formation of C1 carbocation, it appears that solvent plays a critical role in the reaction barrier.28,29 In fact, it appears that the barrier is entirely solvent induced. Partial proton desolvation due to proton migration to the neighborhood of the sugar molecule also contributes significantly to the reaction barrier. However, this is quite different for glucose conversion to HMF initiated by protonatin of C2OH. Partial desolvation is not the part of the rate-limiting step here. The protonation of C2OH and the breakage of C2O2 bond and the formation of C2O5 bond contribute almost entirely to the reaction barrier. 11745
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Figure 12. Projected two-dimensional free energy contour plot (left panel) and three-dimensional free energy surface (right panel) between CV2 (C2O5) and CV3 (O2H) for glucose conversion to HMF intermediate.
The differences between protonation of C1OH and C2OH for the glucose reaction will be discussed in more detail later. Figure 12 exhibit the projected 2-D free energy contour plot (left panel) and the 3-D free energy landscape (right) during the sampling of CV2 and CV3, respectively. Similar to the results in the gas phase, CV3 which represents the O2H bond oscillates between 0 and 2 with multiple stable or metastable structures observed resulting from the proton exchange with the water molecule and the formation of a gemdiol structure during sampling. However, the proton affinities of these sites are significantly smaller than the corresponding values in the gas phase. This is likely due to the fact that the glucose molecule is surrounded by the water molecules. Earlier results27 show that water clusters have a higher affinity for proton than the OH groups on glucose molecule. The presence of water molecules will therefore reduce the calculated proton affinities of various sites on the glucose and HMF intermediate. The extensive delocalized hydrogen bonding network formed will reduce the time that a proton samples each protonation site thus reducing the association energy. Figures 13 and 14 show the CV trajectory, projected twodimensional free energy contour plot (left panel on Figure 14) and three-dimensional free energy landscape (right panel on Figure 14) between CV1 (C2O2) and CV2 (C2O5) respectively. It seems that the energy barrier between the reactant and product well is very high at over 200 kcal/mol. The reactant and product wells are very deep with over 300 kcal/mol Gaussians added. This is likely due to the fact that between the glucose and HMF intermediate wells, there are actually many protonation sites visited by the proton. The FES between CV1 and CV2 represents the total energies added to the reactant and product wells without differentiating the different protonated complex structures in each well due to the choice of the two CVs used to describe the reaction. Because glucose reactions initiated by protonation of C1OH for the condensation reactions and protonation of C2OH for HMF conversion have different reaction barriers, it is worthwhile
Figure 13. Sampling of CV1CV2 trajectories for the breakage of C2O2 bond and the formation of C2O5 bond.
to investigate the role of solvent plays in these two different reactions. In both cases, protonation and the breaking of the CO bond appear to be the rate-limiting step. However, in the condensation reactions, the C1 carbocation is more stable than the C2 carbocation, with the positive charge on the C1 carbocation partially neutralized by the negative charge on the ring O resulting in the formation of the oxonium ion. Because the positive charge on C2 carbocation is not compensated by the neighboring atoms, the C2 carbocation is highly unstable and is hardly observed during the simulations. Once the C2O2 bond 11746
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Figure 14. Projected 2-D free energy surface contour plot (left panel) and three-dimensional free energy surface (right panel) between CV1 (C2O2) and CV2 (C2O5) during the glucose conversion to HMF intermediate.
Figure 15. Schematics for the contributions to the energy barriers for glucose condensation reaction and glucose to HMF formation (dehydration) reaction.
breaks, a C2O5 bond is formed simultaneously so that the positive charge is transferred immediately to the neighboring C1OH group outside the ring structure. This difference has significant consequences with respect to their energy barriers and the role of solvent. In the condensation reaction, our CPMD-MTD simulations give a reaction barrier of 15 kcal/mol with 5 kcal/mol arising from the protonation of C1OH and 10 kcal/mol from the breakage of the C1O1 bond. The remaining contribution to the reaction barrier comes from the partial proton migration to the neighborhood of the glucose molecule. The total reaction barrier of about 2530 kcal/mol agrees well with the experimental observations. This means that the proton partial desolvation, protonation of C1OH and breakage of the C1O1 bond are all rate-limiting steps. These three steps appear to be concerted. On the other hand, the energy
associated with protonation of C2OH followed by the breakage of C2O2 bond and the formation of C2O5 bond have a barrier almost close to the experimental value of 3035 kcal/mol already. Further, the instability of the C2 carbocation causes the breakage of C2O2 and the formation of C2O5 bonds at the same time. The partial desolvation with a barrier of about 15 kcal/mol appears to be rather small compared to the barrier associated with protonation of C2OH and the subsequent bond breaking and formation processes. Thus proton partial desolvation does not contribute to the overall reaction barrier. Quantum tunneling effects57 may play a relatively larger role in protonation of C2OH as the barrier for this proton transfer process is at around 10 kcal/mol, much higher than the protonation of C1OH with a barrier of 5 kcal/mol only and forming a Zundal-like complex intermediate. Figure 15 illustrates the differences between the two reactions. It appears that condensation reactions due to the C1 carbocation formation should be more affected by the solvent. Glucose to HMF conversion will also be affected by the solvent due to the critical step for protonation of the C2OH during the process. However, the effect is less. In both cases, solvent molecules will compete for proton with the OH groups on glucose. The higher the proton affinity of the solvent, the more difficult for protonation of the glucose OH groups, the higher the associated barrier.
IV. CONCLUSIONS The mechanisms and reaction barriers were determined for glucose to HMF conversion in the gas phase as well as in acidic aqueous solution. Glucose to HMF conversion involves protonation of the C2OH, the breakage of C2O2 bond, and the formation of the C2O5 bond, followed by the subsequent dehydration step. Glucose conversion to a HMF precusor is found to occur rapidly in the gas phase. Proton affinities on various protonation sites for both glucose and HMF intermediate 11747
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The Journal of Physical Chemistry A were obtained in the gas phase based on the free energy surface. The presence of one additional water molecule from H3O+ in the gas phase reduces the overall proton affinities of various protonation sites due to the competition from water with a strong affinity for proton. Protonation of C2OH, breakage of C2O2, and formation of C2O5 in aqueous solution is found to be the rate-limiting step, and the processes occur almost simultaneously. The estimated reaction barrier for glucose conversion to HMF is 3035 kcal/mol, which is in good agreement with experimental results. This is quite different from the glucose condensation reaction where proton partial desolvation resulting from its migration from the bulk solution to the neighborhood of the sugar molecule also contributes significantly to the reaction barrier.
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