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Selective Dehydration of Mannitol to Isomannide over H-Beta Zeolite Haruka Yokoyama, Hirokazu Kobayashi, Jun-ya Hasegawa, and Atsushi Fukuoka ACS Catal., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017
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Selective Dehydration of Mannitol to Isomannide over H-Beta Zeolite Haruka Yokoyama,†,‡ Hirokazu Kobayashi,† Jun-ya Hasegawa,*,† Atsushi Fukuoka*,† †
Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan
‡
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo,
Hokkaido 060-8628, Japan
KEYWORDS Sugar alcohol; Mannitol; Dehydration; Isomannide; Solid acid catalyst; Zeolite
ABSTRACT
Isomannide is a potential feedstock for the production of super engineering plastics. A prospective route to obtain isomannide is dehydration of mannitol derived from lignocellulosic biomass, but homogeneous acid catalysts reported in the literature produce a large amount of 2,5sorbitan as a by-product in the dehydration reaction. In this work, we initially studied the mechanism of proton-induced dehydration of mannitol by density functional theory (DFT) calculations, which suggested that local steric hindrance around acid sites designed at the
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Angstrom level can tune the selectivity toward isomannide formation. Based on this prediction, we found that the precisely-defined microporous confinement offered by Hß provides improved selectivity and high catalytic activity for the production of isomannide, where 1,4-dehydration is favored by 20 kJ mol-1 of activation energy. The optimization of the Si/Al ratio of Hß to balance the acid amount and hydrophobicity improved the catalytic activity and achieved 63% yield of isomannide, far exceeding the best result reported previously (35% yield).
INTRODUCTION Utilization of renewable biomass has recently attracted significant attention as a strategy to decrease CO2 emission and lessen the use of finite fossil resources.1,2 Woody biomass is an especially attractive feedstock for chemicals that does not compete with the use of biomass as food.3 Cellulose and hemicellulose, which are both sugar polymers (Scheme 1), account for ca. 70% of the dry weight of wood stems, thus indicating that synthesis and utilization of sugar should be the primary subject in biorefineries.4 Our group reported the hydrolytic hydrogenation of cellulose to sugar alcohols by supported platinum catalysts.5 The reaction affords sorbitol and mannitol in 25% and 6% yields, respectively. Mannitol is formed via fructose produced by the isomerization of a glucose intermediate.6 After substantial studies on the optimization of the reaction conditions, the yield of sorbitol exceeded 90% in the conversion of pure cellulose, and sorbitol can also be obtained from real biomass.7-9 Furthermore, production of mannitol in good yields is possible through the conversion of woody biomass. A niobium-molybdenum mixed oxide catalyst selectively epimerizes glucose to mannose,10 and the subsequent hydrogenation gives mannitol as the sole product.11 Softwood hemicellulose is also a promising feedstock for
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the synthesis of mannitol as it predominantly consists of glucomannan, a polymer of glucose and mannose. Shirai et al. succeeded in producing mannitol in 14% yield based on the total sugar content in cedar chips.12 Currently, 30,000 tons per year of mannitol is manufactured mostly from starch;13 however, the source may be replaced by lignocellulosic biomass.
Scheme 1. Production of diols from woody biomass.
Diols such as isosorbide and isomannide synthesized by the dehydration of sugar alcohols (Scheme 1) are precursors to plastics with outstanding properties. Elimination of two water molecules from sorbitol gives isosorbide, and isosorbide-derived polycarbonate has already been
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commercialized by Mitsubishi (DURABIO). This plastic shows excellent transparency, coloring properties, ultraviolet-resistance, impact-resistance, and surface hardness, which are useful for car coatings and surface protection of liquid crystal displays. Likewise, dehydration of mannitol gives isomannide, and isomannide-derived plastics exhibit similar remarkable properties found in isosorbide-based ones.14 Moreover, isomannide-based polyamides and polyethers possess higher thermal stability than isosorbide-based plastics.15,16 Therefore, isomannide is a potential precursor to engineering plastics. Studies on the dehydration of mannitol to isomannide have, however, been very limited in contrast to the intensive research on the conversion of sorbitol to isosorbide.17-24 In previous reports, dehydration of mannitol showed low selectivity toward the formation of isomannide. Isomannide is formed by 1,4-dehydration via 1,4-mannitan, but 2,5-dehydration of mannitol producing 2,5-sorbitan occurs in parallel (Scheme 2). For example, use of sulfuric acid resulted in 35% yield of isomannide and 40% yield of 2,5-sorbitan.25 Subcritical water converted mannitol to isomannide and 1,4-mannitan in 31% yield and 2,5-sorbitan in 41% yield.26 Therefore, selective dehydration of mannitol to isomannide is a challenge. In this work, we first studied the reaction mechanism of proton-induced dehydration of mannitol by density functional theory (DFT) calculations and then explored heterogeneous catalysts for selective synthesis of isomannide based on the clarified mechanism.
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Scheme 2. Reaction pathways of mannitol dehydration.
EXPERIMENTAL DFT Calculations. The potential energy profile of mannitol dehydration in the presence of a proton was calculated using the Gaussian 09 program. The structure was optimized by DFT calculations at the B3LYP/6-31G(d) level.27-29 The solvation effect was taken into account using a self-consistent reaction field (SCRF) method with a polarized continuum model,30 for which the dielectric constant of bulk methanol was used. Materials and Characterization. The zeolites used in this study were as follows: Hß (Si/Al = 12.5), Hß(25), Hß(75), and HZSM-5(45) from Clariant; Hß(50), Hß(250), and HUSY(250) from Tosoh; Hß(19), Hß(150), HY(2.55), and HUSY(40) from Zeolyst; and HMOR(45) from the Catalysis Society of Japan (JRC-HM-90). All the zeolite samples were calcined at 823 K for 8 h prior to use. N2 adsorption measurements were performed at 77 K with a BEL-SORP Mini (BEL Japan) to determine the BrunauerEmmetTeller (BET) surface areas and micropore volumes of the zeolites. The amounts of acid sites were quantified by NH3-temperature programmed desorption (NH3-TPD; BEL Japan, BELCAT-A, mass spectrometer, m/z = 16 and 15).
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Mannitol Dehydration. Mannitol dehydration was conducted in a three-necked flask (100 mL) with no solvent at 700 hPa (Figure S1). Reduced pressure was used to improve the reproducibility, although atmospheric pressure gave similar results. Mannitol (182 mg) and catalyst (50 mg) mixed for 5 min on a mortar were placed into the flask. When sulfuric acid was used as a catalyst, mannitol was impregnated with 0.42 wt% sulfuric acid and dried at room temperature under vacuum instead of mixing on a mortar. The flask was kept at 423 K, monitored by a Teflon-coated thermocouple sustained in the reaction mixture, in an oil bath for 1 h under stirring with a magnetic stir bar. After the reaction, water-soluble products were extracted with 20 mL of water. The solution was filtered to separate the solid catalyst and analyzed using a high-performance liquid chromatograph (HPLC; Shimadzu LC10-ATVP, refractive index detector) equipped with a Shodex Sugar SH-1011 column (⌀8 × 300 mm, mobile phase: water 0.5 mL min-1, 323 K). Products were identified with nuclear magnetic resonance spectroscopy (NMR; JEOL ECX-400, 1H 400 MHz). The coke content in the used zeolite was measured by a total organic carbon analyzer (Shimadzu TOC-V CSN).
RESULTS AND DISCUSSION Computation of the Mechanism for Mannitol Dehydration. Understanding the mechanistic details of mannitol dehydration is useful to design a catalytic system that selectively promotes the formation of isomannide. For this purpose, mannitol dehydration was simulated with DFT calculations in the presence of a proton as a model acid catalyst, where two pathways were hypothesized: 1,4-dehydration to 1,4-mannitan and 2,5-dehydration to 2,5-sorbitan (Scheme 2). The dehydration occurs via an SN2-type reaction, evidenced by steric inversion that formed only 2,5-sorbitan with no generation of 2,5-mannitan (see below). An SN2-type reaction also takes
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place in the dehydration of a similar sugar alcohol (sorbitol) by sulfuric acid.31 In Figure 1, the relative potential energy (E) is defined to be 0 kJ mol-1 for protonated mannitol (A). In the 1,4dehydration, an OH group at C4 attacks C1 bearing a protonated OH group via a transition state having a trigonal bipyramidal structure at C1 with E of 92 kJ mol-1 (D). Subsequently, elimination of a water molecule produces a stable protonated cyclic compound (E = 22 kJ mol1
) (E).
Figure 1. Energy diagram of mannitol dehydration in the presence of a proton. DFT calculations at the B3LYP/6-31G(d) level with SCRF: red, mannitol to 1,4-mannitan through 1,4dehydration; black, mannitol to 2,5-sorbitan through 2,5-dehydration.
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Mannitol conversion by 2,5-dehydration was compared with the 1,4-dehydration. Protonation of the C2 OH group gives E of 30 kJ mol-1 (F), which is more stable than the corresponding intermediate in 1,4-dehydration mainly due to an additional hydrogen bond between the proton and C1 OH group. Attack of the C5 OH group on C2 leads to a transition state with E of 52 kJ mol-1 (G). Thus, the transition state in the 2,5-dehydration is 40 kJ mol-1 more stable than that in the 1,4-dehydration. The higher stability of the transition state in 2,5-dehydration is due to the formation of hydrogen bonds and a trans configuration of the two hydroxyl groups at C3 and C4 in the five-membered ring, whereas 1,4-dehydration results in a more sterically hindered cis configuration at C2 and C3 in the ring (enlarged image, Figure S2). Next, the five-membered ring product with inversion of stereochemistry at the C2 position (protonated 2,5-sorbitan) has E of 4 kJ mol-1 (H). The final product of 2,5-dehydration has a higher potential energy than that of 1,4-dehydration despite the lower steric hindrance of the OH groups. This is attributed to the final configuration of 2,5-dehydration that enforces an eclipsed conformation (dihedral angle of CCCO: 2.5, Figure S3). The conformation causes steric repulsion and also undermines hyperconjugative resonance stabilization,32 thus resulting in higher potential energy. Note that the product (H) is less stable than intermediate (F), but 2,5-dehydration can occur due to irreversible removal of water and a negative G value in the total reaction (Table S1). The most important factor that determines the selectivity of the two pathways is the difference in the relative energies of the intermediates and transition states. Even taking into account errors in the DFT calculations,33 the estimated activation energies of the elementary steps (C to D for 1,4-dehydration and F to G for 2,5-dehydration) are very close to each other (both 82 kJ mol-1). However, the energy of the 2,5-dehydration pathway is relatively lower than that of the 1,4dehydration route, which gives a smaller apparent activation energy for the 2,5-dehydration (See
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Equations section in the Supporting Information). Hence, 2,5-dehydration mainly occurs in mannitol conversion. Accordingly, an additional controlling factor is necessary to change the selectivity toward 1,4dehydration, which could be a steric effect that selectively raises the energies of the intermediate F and transition state G. At the transition state, the peripheral structures of the reaction center are significantly different between the two reaction pathways (Figure 2). In the 2,5-dehydration, the reaction center C2 has a CH2OH group (dimensions 2.1 3.4 4.2 Å determined by the van der Waals radius) directed through the side of the leaving water molecule. In contrast, C1 in 1,4dehydration is terminated by only two hydrogen atoms (1.0 1.5 2.3 Å). Therefore, 2,5dehydration needs more space around the side of the reaction center. Based on this consideration, we propose that steric hindrance around acid sites may elevate the potential energies of the intermediate F and transition state G and suppress the 2,5-dehydration. As a result, the relative selectivity of less bulky 1,4-dehydration should increase. We assume that such steric confinement would be realized by a microporous acid catalyst with a precisely-defined structure at the Angstrom level.
Figure 2. Conformations of the transition states of mannitol dehydration.
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Screening of Zeolite Catalysts in Dehydration of Mannitol. The DFT results motivated us to use microporous zeolites as catalysts. Table 1 shows the results of mannitol dehydration over various zeolites at 423 K for 1 h. No reaction proceeded in the absence of catalyst under the conditions (entry 1). Hß(50) gave 96% conversion of mannitol and produced 60% yield of isomannide (entry 2). Other identified products were 1,4-mannitan (3.0% yield; an intermediate to isomannide) and 2,5-sorbitan (17% yield). Remaining part (16%) was not identified, but it might contain coke and inter-molecular dehydration products. It is notable that Hß gave 63% yield of 1,4-dehydration products in total, which was 3.7-fold higher than that of the 2,5dehydration product. In the 2,5-dehydration, the stereochemistry was inverted to form 2,5sorbitan and no 2,5-mannitan was observed, indicating an SN2 reaction for the dehydration. Note that the SN2 reaction is preferred in 1,4-dehydration due to the lower steric hindrance and higher energy barrier for the formation of a carbocation at the primary position. HMOR(45) converted only 14% of mannitol and the isomannide yield was 1.7% (entry 3). HZSM-5(45) showed as good selectivity for 1,4-dehydration as Hß, but the conversion was lower (entry 4). HUSY(40) provided a high conversion, 97%, but the main product was 2,5-sorbitan (52%) (entry 5), and the selectivity was similar to that by sulfuric acid (entry 6). The differences in the selectivities and activities among the zeolites are ascribed to structural characteristics of the zeolite pores. We have previously shown that dehydration of sorbitol predominantly takes place in the pores of zeolites.21 The *BEA framework of Hß has a pore dimension of 6.6 × 6.7 Å that is slightly larger than the molecular diameter of mannitol (6.3 Å). The steric confinement disfavors the bulky reaction center of 2,5-dehydration (Figure S4A), but accepts the compact center of 1,4-dehydration (Figure S4B) in the transition states. Indeed, 2,5dehydration was the primary reaction occurring in the pores of HUSY(40), which has super
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cages (12 Å) providing enough space for the dehydration reaction. In addition, mannitol and dehydration products can diffuse into the three-dimensional pores of *BEA. In contrast, the onedimensional pores of MOR and small pores of ZSM-5 (5.1 × 5.5 Å) lead to low catalytic activity probably due to slow diffusion of sugar molecules. Therefore, we conclude that the high activity and selectivity of Hß is attributed to the three-dimensional pore structure with a suitable size to preferably accommodate mannitol molecules proceeding to 1,4-dehydration. Hß shows high catalytic activity also for the dehydration of sorbitol,21,22 but the 1,4-dehydration selectivity of Hß in the sorbitol conversion was similar to that of H2SO4. Sorbitol easily undergoes 1,4dehydration without steric control by catalysts, which is in stark contrast to the mannitol dehydration. Therefore, the results represented here is one of the rare examples in which a zeolite changes regio-selectivity in biomass conversion reactions.34,35 Meanwhile, the acid strength of the zeolite has a minor effect; NH3 desorption enthalpy (HMOR: 140 kJ mol-1 > HZSM-5: 135 kJ mol-1; HUSY: 135 kJ mol-1 > Hß: 130 kJ mol-1)36 has no correlation with the catalytic activity or selectivity.
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Table 1. Catalytic dehydration of mannitol at 423 K for 1 ha
Entry Catalyst
Yield (%) SBETb Acid c d amount Conv. Sel. (m2 g1,42,51 ) (mmol g-1) (%) (%) Isomannide Mannitan Sorbitan Otherse
1
None
-
-
