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Hexagonal Boron Nitride for Adsorption of Saccharides Hirokazu Kobayashi, and Atsushi Fukuoka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05077 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017
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Hexagonal Boron Nitride for Adsorption of Saccharides Hirokazu Kobayashi*, Atsushi Fukuoka Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan.
ABSTRACT
Recognition of saccharides is crucial in their separation, purification, and catalytic conversion. In this work, we demonstrated that hexagonal boron nitride (h-BN) adsorbs saccharides in water. Controlled experiments and density functional theory calculations have indicated that the adsorption is mainly driven by dispersion force occurring between CH groups of saccharides and π electrons on basal plane of h-BN. Accordingly, h-BN can distinguish between different saccharides by the number of CH groups that can contact with the basal plane. Salt effect on the adsorption correlated with the Hofmeister series, which shows the presence of hydrophobic interactions in the adsorption of sugars. Moreover, conversion of glucose to fructose is accelerated by h-BN, possibly due to its acid/base catalysis.
INTRODUCTION
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Recognition of saccharides is crucial in their separation, purification, and catalytic conversion.1,2 Carbon-based materials show especially high affinity for cellulosic molecules in water, and have realized selective adsorption of cello-oligosaccharides3 and hydrolysis of cellulose.4-15 Such outstanding features may contribute to biorefinery processes using cellulose to sustain our future chemical and fuel demands.16 Mechanistic insight into how carbon attracts saccharides is useful to design new adsorbents and catalysts. In the interactions between cellulose and carbon materials, it has been proposed that the polycyclic aromatic surface of carbon attracts cellulose by CH−π interactions with CH groups existing on the axial face of cellulose.17,18 Hydrophobic interactions19 originating from the entropy increase of solvent water molecules further facilitate the adsorption.18 The combined forces lead to the strong adsorption of cellulose even in the microporous confinement of a carbon material in water.7 Furthermore, this strong adsorption facilitates hydrolysis of the adsorbed molecules by surface acidic functional groups on carbon at elevated temperature.6,7,20 Herein, we mainly focus on the development of a new adsorbent for sugars and hypothesize that the adsorption mechanism occurring on carbon is applicable for flat surfaces with extended π electrons as found in graphite analogues. Borazine (B3N3H6, Figure 1) is a benzene-like flat compound having six π electrons, satisfying Hückel’s rule for aromaticity.21,22 Indeed, all the B−N bond lengths are the same (1.43 Å).23 Planar extrapolation of the borazine structure followed by stacking of the sheets forms a hexagonal boron nitride (h-BN; Figure 1),24 which is an analogue of graphite. Hence, h-BN is expected to adsorb saccharides in a manner similar to that of carbon materials. A thickening agent, hydroxyethyl cellulose, disperses h-BN in water, likely due to the good affinity between h-BN and non-polar groups of hydroxyethyl cellulose.25
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Figure 1. Structures of borazine (left) and h-BN (right).
In this work, we demonstrate that h-BN adsorbs saccharides by CH−π interactions predominantly consisting of dispersion force and hydrophobic interactions in water using experimental and density functional theory (DFT) approaches. The adsorption strength greatly varies with the number of CH bonds locating on basal plane of h-BN and contact surface area. This study extrapolates the chemistry of carbon to those of other π systems with different elements, which offers new designs of adsorbents and catalysts for sugar biorefinery.
EXPERIMENTAL Materials. AP-170S (Maruka) was chosen as a h-BN material in this study because of the high surface area (163 m2 g−1), good availability, and low content of impurity (C 0.12%, Na 0.15%, Ca 0.02%). This sample was used after washing with the following procedure for all the experiments to remove soluble part, which was essential to obtain meaningful data. AP-170S of 2.00 g was dispersed in 45 mL of water, sonicated for 10 sec, and aged for 5 min at 298 K. The mixture was centrifuged at 5000 rpm for 5 min, followed by decantation. The remaining solid
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was further washed with the same manner for three times. The wet solid was dried using a rotary evaporator under decreased pressure of 30 hPa at 318 K until the slurry becomes particles, and subsequently at 333 K. Cellobiose was purchased from Kanto, cellotriose and cellotetraose were obtained from Megazyme, and N,N’,N’’-triacetylchototriose (denoted chitotriose) was from Tokyo Kasei. Other reagents were from Wako. Water was treated with activated carbon, deionized, distilled and filtrated with polytetrafluoroethylene (PTFE) membrane prior to use. Characterization. Boron nitride was characterized by IR (PerkinElmer, Spectrum 100; deuterated triglycine sulfate detector) in transmission mode. KBr disk (200 mg, ø10) containing 0.025 wt% BN was used for the analysis. XRD analysis was conducted with Ultima IV (Rigaku) equipped with a semiconductor array detector (D/teX Ultra). High-angle peaks were slightly weakened due to presence of a knife edge. N2 adsorption isotherm was measured at 77 K using Belsorp mini (BEL Japan) after drying at 383 K for 3 h under vacuum. We used a transmission electron microscope (TEM; JEOL, JEM-2100F) with an acceleration voltage of 200 kV. XP spectra was recorded with JPC-9010MC (JEOL; Al Kα, monochromatic mode, pass energy 20 eV). Adsorption. Sugar solution (2−4 mL) was added to boron nitride powder (2−80 mg), where the amounts were optimized to maximize the accuracy. The mixture was sonicated for 10 seconds to disperse boron nitride, and maintained at 298 K for >10 min for the adsorption. After centrifugation of the mixture, the supernatant was further filtrated with a polyvinylidene fluoride (PVDF) membrane (0.2 µm pore). The solution was analyzed by high-performance liquid chromatography (Shimadzu; refractive index detector) equipped with a SUGAR SH1011 column (Shodex; ø8×300 mm; eluent: water 0.5 mL min−1; 323 K). The sugar solution before adsorption
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was also analyzed by HPLC to precisely determine the sugar concentration. The error in HPLC analysis was less than 1% except for cellotetraose analysis at very low concentration. Computation. The energy of chemical structures was calculated using the Gaussian 09 program. The structure of the overall system was optimized using DFT calculations at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d,p) level.26-28 Dispersion interactions were included in an empirical manner using Grimme’s D3 damping function.29 The solvation effect was taken into account using the SMD model30 with the dielectric constant of bulk water. Basis set superposition error (BSSE) was corrected by the counterpoise method at the optimized adsorption structure.31 Conformation of h-BN was fixed in the calculation of the adsorption structure of glucose on h-BN to avoid large flection of h-BN due to the limited modelling size.
RESULTS AND DISCCUSION Characterization of h-BN material. We analyzed physical structure of the h-BN material employed in this study, AP-170S. N2 adsorption experiments indicated that the sample had a remarkably higher Brunauer−Emmett−Teller (BET) surface area (163 m2 g−1, Figure S1), compared to typical reagent-grade h-BN materials (
