Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway

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Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway Xia Yu , Yueying Chu , Lei Zhang , Hui Shi , Mingjiang Xie , Luming Peng , Xuefeng Guo , Wei Li , Nianhua Xue , Weiping Ding PII: DOI: Reference:

S2095-4956(19)30911-8 https://doi.org/10.1016/j.jechem.2019.11.020 JECHEM 1016

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

12 August 2019 18 November 2019 20 November 2019

Please cite this article as: Xia Yu , Yueying Chu , Lei Zhang , Hui Shi , Mingjiang Xie , Luming Peng , Xuefeng Guo , Wei Li , Nianhua Xue , Weiping Ding , Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.11.020

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Highlights The adjacent Brønsted sites lead to co-interaction on fructose. The co-interaction results in more stable transition state in the reaction. Faster HMF formation rate is detected on adjacent Brønsted sites. The catalysts with adjacent active sites are expected in future.

Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway Xia Yua, Yueying Chub, Lei Zhangc, Hui Shid, Mingjiang Xiea, Luming Penga, Xuefeng Guoa, Wei Lic, Nianhua Xue*,a, Weiping Ding*,a a

Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China b

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,

Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, China c

Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China d

Department of Chemistry and Catalysis Research Center,

TechnischeUniversitätMünchen, Lichtenbergstr. 4, 85747 Garching, Germany Abstract To study the effect of adjacent hydroxyl to the active sites, several acid catalysts, i.e. substituted benzoic acids with adjacent carboxyl are employed in the fructose dehydration to 5-hydroxymethylfurfural (HMF). Experimental results reveal that Brønsted acid sites with adjacent carboxyl present higher catalytic ability than isolated ones. Computational results suggest that the adjacent sites lead to co-interaction on fructose, corresponding more stable transition state and faster HMF formation rate. Based on the enhancement from the adjacent sites, a novel ordered *

Corresponding authors: [email protected] (N. Xue), [email protected] (DWP).Phone: +86 25 83686115, Fax: +86 25 83317761;

mesoporous carbon (OMC) full of carboxyls in surface is prepared and turns out to be an effective solid catalyst for HMF production from fructose derived from biomass.

Keywords:

Fructose

5-Hydroxymethylfurfural.

dehydration,

adjacent

sites,

cooperative

catalysis,

1. Introduction Production of 5-hydroxymethylfurfural (HMF), an important platform molecule, from fructose has attracted high attention. Previous studies were primarily focused on solvent effects, reaction mechanism, as well as design and optimization of homogeneous and heterogeneous catalytic systems. Due to environmental benignity and easy recycling, among other reasons, solid catalysts are preferable in the process of fructose dehydration to HMF. They includes zeolites [1-4],metallic oxides [5-12], sulfonic oxides [13], resins [14-16],modified silica [17-21], mesoporous zirconium phosphate [22],phosphoric acid polysilsesquioxane [23],functionalized (sulfonic or phosphoric acid-functionalized) carbon materials, HSO3-H2PO3-grafted polyethylene fiber [24-26] and heteropolyacids [27-29]. Recently, carbon materials, e.g., carbonaceous microspheres prepared by hydrothermal carbonization of glucose, magnetic lignin-derived carbonaceous catalyst, graphene oxide and mesoporous carbon based sulfonic acid have also been explored as effective catalysts [30-33]. Sulfonic acid-functionalized metal-organic framework is also a good heterogeneous catalyst giving a high HMF yield (90%) in DMSO [34]. Furthermore, to improve the yield of HMF, reactive adsorption with carbon black adsorbents was used to prevent consecutive reactions of the produced HMF [35]. Studies mentioned above mostly focused on the relationship between the properties of catalysts (acid strength, concentration of acid sites) and the catalytic performance (fructose conversion and HMF yield), while more recent work started to place an emphasis on understanding how the dehydration process can be influenced by subtle

changes in the electronic environment around the active sites. For example, a new poly-benzyl ammonium chloride (PBnNH3Cl) resin has been developed and used as solid catalysts for fructose dehydration to HMF [36]. In this case, the multiple BnCl groups bind fructose onto the resin surface by hydrogen bonding, while BnNH3Cl acts as the catalytic site; the synergistic action of BnCl and BnNH3Cl leads to the superior catalytic performance for HMF production. It was also shown that triazaheterocyclic compounds with electron-withdrawing substituents (Cl, SH, etc.), together with the electropositive center (P and C atom), offer strong interactions with fructose through hydrogen bonds which are responsible for the efficient dehydration of fructose to HMF [37]. In a most recent example, a series of cyclopentanone-based acidic resins were prepared and proved to be effective fructose-to-HMF catalysts in which the presence of hydroxyl in proximity to the SO3H group promotes the adsorption of fructose [38]. Despite advances in understanding solvation, mechanisms, and cooperativity of active centers in acid catalyzed sugar chemistry, further scientific guidance on how to seek and develop more effective catalysts is still lacking. Adjacent acid sites are common on solid surfaces with high acid site concentrations. We demonstrated previously that the simultaneous interaction of two adjacent Brønsted acid sites with one fructose molecule in MFI-zeolites increased the dehydration rate for HMF production [39]. Similarly, it was established that adjacent Brønsted acid sites in zeolites cooperatively activate alkanes to change the reaction pathways and rates of protolytic alkane cracking [40]. Inspired by the different examples of synergistic activation of molecules mentioned above, we hypothesized

that the dehydration rates and selectivities of fructose, which is abundant in hydroxyls, can be altered by placing adjacent functional groups capable of H-bonding with fructose in the vicinity of catalytically active sites. In the present work, we first used a series of molecular acid catalysts, i.e., substituted benzoic acids with a carboxyl or a hydroxyl located at different distances (ortho-, meta- and para-) from the carboxyl, for fructose dehydration in DMSO solvent. The influence of functional groups in the proximity of Brønsted acid sites (carboxyl in benzoic acids) has been studied through experiments and theoretical calculations, with a focus on the hydrogen bonding effects for this model system. In light of the observed rate enhancement arising from site adjacency and cooperative catalysis, a novel ordered mesoporous carbon (OMC) with high surface concentrations of carboxyls has been prepared and used as an effective solid catalyst for HMF production from fructose. This work opens a new avenue for rational design of more effective acid catalysts for biomass utilization.

2. Experimental 2.1 Materials D-(-)-Fructose(≥99%,Sigma-Aldrich), isophthalic

acid(≥99%,Sigma-Aldrich),

grade,Sigma-Aldrich),

salicylic

acid(98%,Sigma-Aldrich),

phthalic

acid(≥99.5%,Sigma-Aldrich),

terephthalic

acid(vetec

acid(≥99%,Sigma-Aldrich),

4-hydroxybenzoic

reagent

3-hydroxybenzoic

acid(≥99%,Sigma-Aldrich)

and

DMSO(99.7%,J&KSeal)were used without further purification. HBEA-19 (Si/Al=19) and HBEA-150 (Si/Al=150) were both purchased from Zeolyst.

To get carboxyl grafted carbon catalysts with different density of acid sites, 1g of ordered mesoporous carbon (OMC), synthesized according to the method in reference [41], was oxidized with 150mL of 0.5M and 5M of nitric acid at 353 K for 5h, cooled to room temperature, filtered and washed several times with distilled water until the value of pH was 7, and dried at 323 K in a vacuum dryer. The resultant samples were denoted as OMC-0.5NAT and OMC-5NAT, respectively.

2.2 Catalytic reaction and analytical methods The dehydration of fructose was carried out in the glove-box under a nitrogen flow. A certain amount of fructose (for substituted benzoic acids and solid acids as catalysts, 0.6 g of fructose)dissolved in DMSO (for substituted benzoic acids and solid acids as catalysts, 10mL)mixed with the catalyst are placed in round-bottomed flask in an oil bath with a magnetic stir bar. When the oil bath was heated to 373 K, the mixture including reactant and catalyst dissolved in DMSO in flask was put in oil bath for 1 hour. After reaction, quantitative analysis of products was performed on an Agilent HPLC using an aminex ion exclusion column HPX-87 (300 mm ×7.8 mm). HCOOH (0.001 M) was used as the mobile phase at a flow rate of 0.5 mL min-1. The column temperature was maintained at 308 K, the detector’s temperature was set on 308 K, and the UV detector maintained at λ=320 nm. 2.3 MD Simulations To study the interaction between benzoic acids in DMSO, molecular dynamics simulations were performed with the Tinker program using the OPLSAA force field.

The simulations were carried out in the NVT ensemble at 400 K. The density of the solution was assumed to be 1.1g mL-3. The equation of the motion was integrated with the time step of 1fs and the trajectories were collected every 1ps. Two sizes of simulation systems were used: the small one with 20 solute molecules and 60 solvent molecules in a cell, and a larger one with 20 solute molecules and 120 solvent molecules. The simulations were carried out for 10and 5ns for the two simulation systems, respectively. 2.4 DFT calculations A polarized continuum model (PCM-SMD) was utilized to simulate the solvent effect of DMSO [42-43]. The geometries were optimized by means of the density functional theory (DFT) method at the B3LYP/6-31G (2df, p) level. The B3LYP/6-31G (2df, p) method combined with the SMD model has been successfully employed in the decomposition of fructose in solution [44]. Based on the optimized structures, the frequency calculations were performed at the same level as geometry optimizations to check whether the stationary points found exhibited the proper number of negative frequencies. Only one negative frequency would be observed for transition state points and none for minima. The natural charges for fructose have been analyzed with the help of the natural bond orbital (NBO) analysis. All of the calculations in this work have been performed by the Gaussian 09 program package [45].

3. Results and discussion

3.1 Catalysis by substituted benzoic acids: the effect of distance between adjacent carboxyls fructose dehydration to HMF

The role of the adjacent groups on the activity of active sites in fructose dehydration was studied and the results are shown in Figures 1. The reaction temperature was kept to be no higher than 373 K to avoid solvent-induced blank reactions (significant blank activities in DMSO at 383 K and above). In the present work, the turnover frequency (TOF) is defined as the mole of converted fructose (or HMF produced) per second per mole of organic acid (as the catalyst). Among phthalic acid (PA), isophthalic acid (IPA) and terephthalic acid (TPA), phthalic acid showed the highest TOF of 8.80×10-3 s-1 for fructose conversion and 1.34×10-4 s-1 for HMF formation rate. It is worth mentioning that there is comparative fructose conversion rate (4.36 ×10-4 s-1) detected but with much lower HMF formation rate (2.02×10-6 s-1) on benzonic acid. The synergy of neighboring carboxyls groups should be considered during fructose dehydration [46]. Upon the enhancement, a hypothesis of fructose activation on these catalysts is proposed. On the one hand, the acid group offers proton to protonated fructose and the other adjacent OH group also interacts with the adsorbed molecule by hydrogen bonding. The additional polarization from the adjacent groups contributes to the enhancement of dehydration rate of the reactant with several hydroxyls. On the other hand, the formation of hydrogen bond between adjacent OH groups makes the acidity stronger and reaction faster. The cooperative effect from the adjacent carboxyl or hydroxyl on the active center is leading to the

enhancement of HMF formation. To prove this hypothesis, the apparent activation energy of the reaction was measured firstly. Figure 2 shows the Arrhenius relation between the HMF formation rate constant and temperature. For benzoic acid, the lower activation energies for the HMF formation is present as the two carboxyls become closer. The apparent activation energy of HMF formation from fructose dehydration is 161.5 kJ mol-1 on phthalic acid. However, the activation energies are measured as 400.9 and 323.8 kJ mol-1 for isophthalic acid and terephthalic acid, respectively. The results are consistent with the highest catalytic performance of the PA in the reaction at 373 K discussed above (in Figures 1). In order to further study the catalytic reactivity of the three acids, the DFT calculations are carried out to explore the catalysis process. The HMF could be produced by 3 H2O molecules released from the reactant fructose. As shown in Figure 3, there are five kinds of −OH groups, and the nature charge analysis shows that the O atom at the 2OH site possessing much more negative charge (-0.480|e|). It is indicated that the H proton of the acids catalyst interacts preferentially with the −2OH in fructose. Then, the subsequent dehydration reaction should start at the 2OH site.

The fructose dehydration catalyzed by liquid acid has been widely explored in the previous work. Caratzoulas et al. proposed the single H+ was the active site and the reaction followed dehydration, hydride and proton transfer steps [47]. Recently, Ren et al. pointed out the protonated dimethyl sulfoxide (TMSOH+) were the catalytically

active species for the acid-catalyzed dehydration of fructose to HMF in DMSO solution and they demonstrated the catalytic performance of DMSOH+ originated from the valence unsaturation of both S and O atoms, as well as the H-mediated effect of −SOH group. However, due to the similar acidity, the higher catalytic reactivity of PA than IPA and TPA in the experiment cannot be explained by the single H+ and TMSOH+ mechanisms proposed in the previous work. Thus, a new mechanism is designed to explore the intrinsic character of the high reactivity of PA. As shown in scheme 1, the HMF formation contains three elementary steps. Different from the previous works, a H2O molecule and the H+ transfer to conjugated base site of the phthalic acid (COO-) occurs at every elementary step. The optimized transition state (TS) geometric structures and the energies profiles of different phthalic acids catalyze the fructose dehydration to HMF are depicted in Figure 3.For the first H2O molecule release over PA, the proton of PA completely shifted to the –2OH with the O2-H bond length 0.968 Å, and the formed H2O molecule separated from fructose with the C-OH2length elongatedto3.597 Å. Simultaneously, the catalyst was regenerated through the COO- conjugate base site obtaining a H+ from the fructose (rO-H=1.386 Å; rC-H =1.270 Å, see TS1 over PA in Figure 6a).It is worth to note that the adjacent carboxylic acid of PA has a hydrogen bond with the basic O atom of the fructose with the O-H length 2.082 Å. The calculated energy barrier for the first step is 149.4 kJ mol-1. For the second H2O molecule release, the TS2 is similar with TS1. The formed H2O molecule release with the C-OH2 length elongated to 1.929 Å, and the catalyst was regenerated through the COO- conjugate base site obtaining a H+

from the fructose (rO-H=1.014 Å; rC-H =1.522 Å, see TS2over PA in Figure 6a).The calculated energy barrier for the second step is 58.6 kJmol-1, much smaller than that in the first step which could be ascribed to the weak O-H bond compared with C-H bond in the catalyst regenerating process. The calculated energy barrier for the third H2O molecule release is 106.7 kJ mol-1, and the TS3 are similar with TS1 and TS2, with the C-OH2 length elongated to 2.185 Å (See TS3 over PA in Figure 4a). The calculated energy profile shows the first step is the rate determination step of the fructose dehydration reaction over PA, and the PA exhibits similar catalytic performance with H+ in the previous work, 31.8 kcal mol-1. Just like the PA, the first H2O release is also the rate determining step for the fructose dehydration over IPA and TPA (See Figure 4). As shown in Figure 4a, the activation energy of the rate-determining step (ΔEact1) decreases from 167.4 kJ mol-1 over IPA and 166.9 kJ mol-1 over TPA to 149.4 kJ mol-1over PA, indicating the high reactivity of the HMF formation from fructose dehydration catalyzed by PA. The optimized transition state (TS) structures revealed that IPA acid and TPA acid interact with the fructose molecule from one carboxyl of acid and one hydroxyl of fructose for all of the three TSs (See Figure 4a). It is interesting that the adjacent carboxyls in PA couldinter act on the same fructose molecule by hydrogen bonding, especially for TS1. This reveals that different catalytic form of fructose on the catalyst with two adjacent acidic groups and the catalyst with two distant ones (which could not provide additional stabilization factor with space permission). And, this kind of additional hydrogen

bond by the adjacent acid group could stabilize the TS structure more effectively, and then result in the lower activation energy for the rate-determining step. 3.2 An effective carbon catalyst with abundant carboxyls The prepared OMC and HNO3 treated OMC has been titrated carefully and the quantitative results are included in Table 1. After HNO3 treatment, carboxyl concentration in OMC is dramatically increased from 79 to 272 µmol g-1. The fructose was solved in DMSO and mixed with the OMC materials. After then the reaction was performed at 373 K for 20 min and the reaction results are compiled in the table. For turnover number of the HMF formation on the carboxyls in OMC, HNO3 treated OMC presented about 7 times faster than that on parent OMC. This is consistent with the cooperative effect of the adjacent acid sites aforementioned. This means an effective catalyst could be reached to control the surface concentration of acidic groups for fructose dehydration to HMF. This result is similar with the role of defect sites (OH groups) in silica or alumina surface for glucan hydrolysis to some extent [48]. From the table, the density of acid sites in the catalyst were reduced from 272 µmol g-1 of OMC-0.5NAT to 150 µmol g-1 of OMC-5NAT. However, they shows close TOF in HMF formation. This means the detected HMF formation rate on OMC-0.5NAT is about 1.8 times of the HMF formation rate on OMC-5NAT. The close TOF means the type of acid sites in both OMC are with similar acidity and proximity. The detailed distribution in treated OMC surface by further study is expected.

Furthermore, the fructose dehydration was performed on different acid catalysts with longer reaction time at 373 K and the results are shown in Figure 5. H-Beta zeolite with Si/Al ratio as 150 (HBEA-150) presented the lowest conversion and HMF selectivity. HNO3 treated OMC (OMC-0.5NAT) offered the highest performance, as 100% conversion and 63% yield of HMF, which means HNO3 treated OMC, full of carboxyls in surface, is an excellent acid catalysts for HMF production in heterogeneous process of fructose dehydration.

4. Conclusions The adjacent hydroxyl to the Brønsted acid sites leads to enhanced dehydration of fructose to HMF due to the co-interaction on the fructose from the adjacent sites. The stronger adsorption and lower reaction activation over the adjacent sites are contributed faster reaction rate. The hydrogen bonding between substituted benzoic acids enhanced the chance of vicinity of active sites and results in the acceleration of dehydration. On the basis of this effect, a novel and effective OMC material full of carboxyls in the surface which is introduced by HNO3 treatment has been fabricated. Over this catalyst, HMF yield reaches 63% after 4 h reaction at 373 K. The catalyst full of H-bonds would offer a new approach towards effective solid acid catalysts for biomass utilization.

Declaration of Interest Statement There are no conflicts to declare.

Acknowledgments

The project was supported by Natural Science Foundation of Jiangsu Province (BK20151380) and NSF of China (21103087 and 21872067), and also supported by the Fundamental Research Funds for the Central Universities (020514380116).

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Figure 1. Fructose dehydration to HMF catalyzed by phthalic acid (PA), isophthalic acid (IPA) and terephthalic acid (TPA) at 373 K. Reaction conditions: fructose 0.6 g, catalyst 0.1 mmol, DMSO 10 mL.

Figure 2. The Arrhenius relation between the HMF formation rate constant and temperature from fructose catalyzed by phthalic acid, isophthalic acid and terephthalic acid Reaction reactions: Fructose 0.6 g, Catalyst 0.01 mmol, DMSO 10 mL, reaction

temperatures: 363 K~369 K. The fructose conversion was carefully controlled as low as 1.2~15%.

Figure 3. The optimized structure of the fructose. The O atoms nature charges of the five OH group are labeled.

Figure 4. The optimized structures of the TSs (a) and the energy profiles for the fructose transformation process on acids in DMSO. The main geometric parameters

(in Å), the corresponding negative frequency (in cm-1) for the TS and the activation energies are labeled (in kJ mol-1).

Figure 5. The catalytic performance of HBEA and OMC catalysts. Reaction conditions: Fructose 0.6 g, Catalyst 0.1 g, DMSO 10 mL, reaction temperature373 K.

Table 1. The catalytic performance of the OMC and HNO3 treated OMC for fructose dehydration (0.6 g of fructose and 10 mL DMSO, catalyst 0.2 g, 20 min reaction). Catalyst

-

-1 a

-5

-1 b

[-COOH ] (μmol g )

TOF (10 s )

OMC

79

2.2

OMC-0.5NAT

272

15.8

OMC-5NAT

150

a

The concentration of carboxyls in OMC materials.

b

The average rate of HMF formation at 373 K.

14.9

Scheme 1. The proposed mechanism of HMF formation through fructose dehydration catalyzed by phthalic acid.