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Zeolite-Templated Carbon Catalysts for Adsorption and Hydrolysis of Cellulose-Derived Long-Chain Glucans: Effect of Post-Synthetic Surface Functionalization Mizuho Yabushita, Kota Techikawara, Hirokazu Kobayashi, Atsushi Fukuoka, and Alexander Katz ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01796 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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ACS Sustainable Chemistry & Engineering

Zeolite-Templated Carbon Catalysts for Adsorption and Hydrolysis of Cellulose-Derived Long-Chain Glucans: Effect of Post-Synthetic Surface Functionalization Mizuho Yabushita,†,‡ Kota Techikawara,‡,§ Hirokazu Kobayashi,‡,* Atsushi Fukuoka,‡,* and Alexander Katz†,* †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley,

California 94720, United States ‡

Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan

§

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo,

Hokkaido 060-0628, Japan

KEYWORDS Adsorption, Catalytic hydrolysis, Cellulosic biomass, Micropores, Post-synthetic modification, Weak acid-site catalysis, Zeolite-templated carbon

ACS Paragon Plus Environment

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ABSTRACT

This manuscript quantitatively investigates the effect of weak acid-site surface density on adsorption and catalytic hydrolysis of long-chain β-glucans, with post-synthetically modified zeolite-templated carbon (ZTC) catalysts. Our approach requires ZTC-surface modification and overcomes previous limitations of pore collapse in accomplishing this, which have previously necessitated electrochemical methods. We demonstrate that mild ZTC treatment in hydrogen peroxide preserves the 1.1 nm micropores of ZTC, which were previously shown to be ideal for β-glucan adsorption, while synthesizing surface-modified ZTC catalysts that hydrolyze adsorbed β-glucans to glucose in up to 87% yield. Our results demonstrate a direct increase in catalytic hydrolysis activity and glucose yield upon increasing acid-site density via surface functionalization. Upon investigating the mechanism of catalytic hydrolysis under buffered conditions, we rule out the synthesis of acid sites with stronger acidity as a result of possible greater anion delocalization as well as the possibility of a cooperative acid-base bifunctional mechanism. Our kinetic and spectroscopic data instead argue for the importance of a high density of surface carboxylic acid functionality as promoting the likelihood of pairing a surface acid site with a glycosidic oxygen of an adsorbed glucan, on a length scale that is commensurate with that required for general-acid catalysis.

INTRODUCTION Depolymerization of cellulose – the most abundant biomass-derived polymer on earth – is universally recognized as a central grand challenge for enabling the economic production of


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biofuels and value-added chemicals from biomass feedstocks.1–5 Solid catalysts for hydrolysis of β-1,4-glycosidic bonds offer a way for straightforward separation of glucose product from catalyst via filtration, and these have consisted of surface strong-acid functional groups6–14 as well as, more recently, weak-acid sites expressed on a surface.15–24 Although weak acids with pKa of larger than 3 are typically insufficient to hydrolyze β-1,4-glycosidic bonds,25 it has been demonstrated that the enthalpically- and entropically-favorable strong interaction between adsorbed β-glucan and the carbon-catalyst surface enables weak-acid sites on that surface to act as catalysts and activate β-1,4-glycosidic bonds for cleavage via hydrolysis.26–28 We recently demonstrated such adsorption of long-chain β-glucans in excess of 20,000 g mol-1 molecular weight, within 1.1 nm – 1.5 nm micropores of zeolite-templated carbon (ZTC), faster than we could measure experimentally, as well as the subsequent catalytic hydrolysis of these adsorbed glucans, in this ZTC catalyst, which lacked post-synthetic surface modification.23 A longstanding hypothesis is that this two-step adsorption-hydrolysis in our synthetic carbons could resemble the catalysis invoked for cellulase enzymes, which are thought to depolymerize adsorbed long-chain β-glucans by a pair of carboxylic acid and carboxylate within the confines of a microporous binding domain,29,30 and offers the prospect of higher selectivity relative to strong-acid sites. Yet our previous demonstration with unmodified ZTC catalyst23 lacks investigation on the possible role of surface acid sites on the catalytic hydrolysis mechanism. Here, our approach consists of post-synthetic functionalization of the microporous ZTC surface, in order to systematically study the role of acid site density on adsorbed β-glucan catalytic hydrolysis and quantitatively assess for the first time, the interdependence between surface acid-site density and rate of β-glucan catalytic hydrolysis, in a microporous carbon catalyst. This poses a difficult synthetic challenge in part because of the sensitivity of ZTC


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materials to surface functionalization, as a result of being prone to micropore collapse. In the past, such surface modification has required electrochemical approaches, for avoiding pore collapse.31,32 We demonstrate in this manuscript that this is now also possible to accomplish using non-electrochemical approaches, and characterize the resulting catalysts using N2 adsorption measurement, acid-base back titration,33 and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The synthesis of these materials permits us to investigate the effect of surface functional-group density on adsorption and catalytic hydrolysis of β-glucans, due to their almost identical microporous environment. Our results are amongst the first to rigorously demonstrate a direct relationship between catalytic activity and surface density of acid functional groups on carbon, when using the same approach for post-synthetic surface modification. We also investigate the possibility of acid-base bifunctional catalysis in this class of catalysts for glucan hydrolysis, since cellulase-enzyme mechanisms invoke acid-base bifunctional pairs at the active site,29,30 and also examine whether a higher density of acid functional groups results in stronger acid sites with lower pKa, by way of anion delocalization via hydrogen bonding to adjacent acid sites. Our data refute both of these possibilities and instead point to the benefit of a higher acid-site density on the surface as one that increases the likelihood of having the right distance between glycosidic oxygens of adsorbed glucan chains and surface acid sites. This explanation leverages on the crucial dependence of hydrolysis rate on this distance in prior intramolecular catalysis models,34 as well as prior explanations invoked in model glucan hydrolysis catalysts on inorganic-oxide supports.16


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EXPERIMENTAL SECTION Post-Synthetic Modification of ZTC. ZTC was synthesized by following previously reported procedures.23,35 The weak-acid sites were introduced to ZTC by post-synthetic modification using either hydrogen peroxide, nitric acid, or sodium hypochlorite as an oxidant.22,36 50 mg of ZTC was dispersed in 5 mL of either hydrogen peroxide (Sigma-Aldrich, 30 wt%), nitric acid (Sigma-Aldrich, 70 wt%), or sodium hypochlorite aqueous solution [SigmaAldrich, 1.5 wt% – 2 wt%. The pH was adjusted to 4–5 by the dropwise addition of hydrochloric acid (Sigma-Aldrich, 37 wt%)]. The suspension was stirred at room temperature for a certain time, typically 30 min, and then was filtered through a polytetrafluoroethylene (PTFE) membrane (0.2 µm mesh). The resulting solid was washed with at least 200 mL of Milli-Q water (18.2 MΩ cm), followed by drying under vacuum overnight (Labconco, FreeZone 12 Liter Console Freeze Dry Systems, ≤42.0 Pa). The samples thus prepared by hydrogen peroxide, nitric acid, and sodium hypochlorite are denoted as Z-H(t), Z-N(t), and Z-S(t), respectively, where t represents the treatment time in minutes. Characterization of ZTC. Textural properties of ZTC materials were characterized by N2 adsorption measurement at 77 K (Micromeritics, 3Flex Surface Characterization Analyzer, equipped with a turbo molecular pump). The t-plot method and non-local density functional theory (NLDFT) calculation were applied for the estimation of micropore area and pore-size distribution, respectively. In the NLDFT calculation, a kernel for N2 physisorption into carbon slit pores at 77 K was employed. The total pore volume was elucidated from the data point at P/P0 = 0.95.


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The number density of surface acid sites in ZTC materials was quantified by acid-base back titration.33 10 mg of ZTC was dispersed in 5 mL of sodium hydroxide aqueous solution (10 mM), and the suspension was vortexed at room temperature for 2 h. After filtration through a PTFE membrane (0.2 µm mesh) to remove the solid, 2 mL of the filtrate was mixed with 5 mL of hydrochloric acid aqueous solution (10 mM), followed by back titration using the sodium hydroxide aqueous solution (10 mM) in the presence of phenolphthalein as an indicator. The oxygenated functional groups of ZTC materials were characterized by DRIFT spectroscopy (Thermo Scientific, Nicolet 6700, deuterated triglycine sulfate detector, 1024 scans, 4 cm-1 resolution) without sample dilution. A baseline was recorded using potassium bromide (Acros Organics). Adsorption of β-Glucan on ZTC. Initially, long-chain β-glucan solution was prepared from cellulose by following a previously reported procedure.22,23,26,27 200 mg of Avicel (Fluka Analytical, PH-101) was dispersed in 10 mL of concentrated hydrochloric acid (Sigma-Aldrich, 37 wt%, room temperature) and was vortexed for 1 min. Then, 90 mL of cold hydrochloric acid (37 wt%, stored in a freezer and cooled by dry ice for 20 min before its use) was added into the vessel and was vortexed again for 1 min to dissolve Avicel completely. Afterwards, the vessel was placed at 297 K for 110 min to obtain a long-chain β-glucan solution. The accurate concentration of β-glucan based on mass of glucose was determined as follows. A part of the βglucan stock solution was placed at 297 K for 46 h to completely depolymerize β-glucan to glucose. After five-fold dilution in Milli-Q water, the amount of glucose in solution was quantified by high-performance liquid chromatography (HPLC, Shimadzu, Prominence HPLC System, equipped with a refractive index detector) with an Aminex HPX-87H column (Bio-Rad,


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ø7.8 × 300 mm, mobile phase 5 mM sulfuric acid aqueous solution 0.6 mL min-1, column temperature 323 K). The β-glucan stock solution thus prepared was used for adsorption experiments. Thus, 20 mg of ZTC was dispersed in 40 mL of stock solution and was vortexed at 277 K for 10 min. The suspension was filtered through a PTFE membrane (0.2 µm mesh) to obtain both solid and liquid fractions. The solid fraction was washed with 1 mL of Milli-Q water three times to extract any residual trace of hydrochloric acid and was subsequently dried under vacuum overnight (≤42.0 Pa). Afterwards, the dried sample was used for hydrolysis experiments (vide infra). To determine β-glucan adsorption uptake, the liquid-phase filtrate was treated at 297 K for 46 h to completely hydrolyze β-glucan to glucose. After performing a five-fold dilution in Milli-Q water, the amount of glucose in solution was quantified by HPLC (vide supra). The β-glucan uptake was determined from the difference between the amount of glucose in the stock solution and that in the liquid phase after the adsorption experiment. Hydrolysis of β-Glucan Adsorbed on ZTC. The hydrolysis of β-glucan adsorbed on ZTC was conducted in a small vial (Agilent Technologies, MS Analyzed Vials Kit, 1.5 mL, equipped with a seal). Typically, ca. 25 mg of the dried β-glucan/ZTC sample, 1 mL of Milli-Q water, and a magnetic stir bar were charged into the vial. The mixture was treated at 453 K for 3 h at a stirring speed of 440 rpm using a magnetic tumble stirrer (V&P Scientific, VP 710E-2HM-1). After cooling to room temperature, the suspension was filtered through a solid-phase extraction column (Thermo Scientific, HyperSep C18, silica bed 100 mg), and then the solid phase was washed with hot water (353 K – 363 K) to extract any adsorbed water-soluble compounds. The


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amounts of reaction products in the filtrate were subsequently quantified by HPLC (vide supra). The pH of the liquid fraction was measured by a SevenMulti pH meter (Mettler Toledo).

RESULTS AND DISCISSION In an attempt to introduce acidic functional groups on the edges of warped graphene-like planes of the ZTC network in a manner that obviates electrochemical approaches, several post-synthetic surface modification strategies of the parent ZTC material were investigated. Some of these were based on similar approaches for surface modification of conventional carbon materials previously, and consisted of either hydrogen peroxide, nitric acid, or sodium hypochlorite treatments,22,36 though treatment times were carefully varied in an attempt to avoid micropore collapse. The acid-site surface concentration as well as textural properties of the parent and surface-modified ZTC materials are summarized in Table 1. Upon prolonged hydrogen peroxide treatment time, up to 60 min, the amount of acid sites increased significantly from 1.05 mmol g-1 to 2.48 mmol g-1 (Entries 1–5), while for the same set of Z-H materials, the t-plot micropore area, pore size, and pore volume remain almost identical to the parent ZTC material (Table 1, Figures 1 and 2A). These data demonstrate a new and viable non-electrochemical method for introducing acidic sites to the edges of the ZTC network, with minimal change to its sensitive microporous structure. Such a result is significant for post-synthetic surface functionalization of ZTC materials, where it has been previously reported that electrochemical oxidation approaches are required in order to avoid ZTC micropore collapse.31,32 We indeed observe this pore collapse with other, more harsh approaches investigated here, consisting of nitric acid and sodium hypochlorite treatments. For these latter two treatments, though the number of surface acid sites


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in Z-N(30) (Entry 6) and Z-S(30) (Entry 7) are at least 3.7 times larger than that of the parent ZTC, the t-plot micropore area, pore size, and pore volume of Z-N(30) and Z-S(30) were halved after treatment. Pore-size distribution data in Figure 2B demonstrate pore collapse in the diameter range of 1.0 nm – 1.5 nm. This pore-size range is critical to preserve, because it has been previously shown to be ideally suited for β-glucan adsorption, in a manner that is consistent with principles of supramolecular chemistry.23 Data in Figure 2B indicate that nitric acid and sodium hypochlorite treatment convert these medium-sized micropores to smaller ones consisting of less than 0.7 nm in diameter, which we have previously postulated to be too small for β-glucan adsorption, based on the observed lack of such adsorption within the pore wall of mesoporous carbon nanoparticles (0.5 nm – 0.8 nm micropores) prior to silica template removal.27 Based on these observations, we hypothesized that Z-N(30) and Z-S(30) will have reduced performance to adsorb β-glucan relative to both the parent ZTC as well as Z-H materials.


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Table 1. Properties of Parent and Modified ZTC Materials and Their Performance in Adsorption and Hydrolysis of β-Glucan Entry Material

Acid sitea t-Plot micropore Pore sizeb Total pore

β-Glucan uptaked Glucose yielde

/mmol g-1 area /m2 g-1

/nm

volumec /cm3 g-1 /mgGlu eq g-1

/%

1

ZTC

1.05

3480

1.14

1.91

306

66

2

Z-H(5)

1.81

3700

1.09

1.90

390

72

3

Z-H(15)

2.10

3360

1.09

1.81

315

78

4

Z-H(30)

2.32

3400

1.09

1.82

328

85

5

Z-H(60)

2.48

3050

1.09

1.61

315

84

6

Z-N(30)

3.84

1550

0.53

0.74

138

24

7

Z-S(30)

5.14

1510

0.53

0.77

0

n.d.f

a

Quantified by acid-base back titration.33 bPore diameter, estimated by NLDFT calculation. The pore size

distribution is shown in Figure 2. cCalculated from the data point at P/P0 = 0.95. dBased on mass of glucose in βglucan per mass of adsorbent. Adsorption conditions: ca. 2 g L-1 of β-glucan solution 40 mL; adsorbent 20 mg; 277 K; 10 min. eGlucose yield was in hydrolysis of β-glucan adsorbed on ZTC materials. Reaction conditions: βglucan/ZTC ca. 25 mg; Milli-Q water 1 mL; 453 K; 3 h; 440 rpm. fNot determined, due to no adsorption of βglucan.

Figure 1. N2 adsorption isotherms of parent and modified ZTC materials.


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Figure 2. Pore-size distribution of parent and modified ZTC materials.

The β-glucan adsorption uptakes for all Z-H materials were 315 mgGlc eq g-1 – 390 mgGlc eq g1

(Table 1, Entries 2–5) and similar to that for the parent ZTC (306 mgGlc eq g-1, Entry 1). This

result is unexpected given the increase in the polarity of the carbon surface via its oxidation with hydrogen peroxide, and the fact that this polarity increase is not directly correlated with a decrease in β-glucan adsorption uptake. Such a decrease was previously observed and hypothesized as occurring as a result of decreased hydrophobic interactions (i.e., increased hydrophilicity of surface), when studying β-glucan and β-xylan adsorption on the surface of mesoporous carbon nanoparticles.19,20 In the current microporous ZTC materials, this increased hydrophilicity may be offset by favorable hydrogen bonding interactions with adsorbing sugars, as such interactions within microporous cavities have been previously shown to be crucial, in addition to hydrophobic CH–π interactions, when adsorbing sugars in protein hosts.37 The importance of the hydrophobic CH–π contacts is demonstrated by the direct scaling of the βglucan adsorption capacity with the t-plot micropore area in Table 1 (see also Figure S1 in the


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Supporting Information), as demonstrated previously for other carbon materials, including unfunctionalized ZTC.23,26–28 In stark contrast, the narrow pore size of Z-N(30) and Z-S(30) (vide supra) led to relatively poor performance, with no β-glucan adsorption observed for Z-S(30) as adsorbent. After β-glucan adsorption onto ZTC materials, catalytic hydrolysis of β-1,4-glycosidic linkages to synthesize glucose was investigated at 453 K for 3 h in water, and results are shown in Table 1. The parent ZTC catalyzed the synthesis of glucose in 66% yield (Entry 1). This is similar to the yield that was previously reported when using a slightly different ZTC batch under otherwise identical reaction conditions, which gave a turnover number value of 3.7.23 With ZH(30) as catalyst, the glucose yield increases up to 85% (Entry 4). We further conducted an optimization of the hydrolysis of β-glucan over Z-H(30) at 453 K for various reaction times, to maximize glucose yield (time course shown in Figure S2 in the Supporting Information). The optimum glucose yield occurred at a reaction time of 3 h – 4 h (85%), and then was decreased presumably due to degradation of released glucose in solution. Similar optimum timescales have been observed previously with carbon catalysts for adsorbed β-glucan hydrolysis.20,22,23 After an additional 3 h hydrolysis cycle involving partially hydrolyzed β-glucan/Z-H(30) composite, which consists of a small amount of unhydrolyzed β-glucan remaining on the Z-H(30) surface at 453 K (i.e., we conducted the second, additional hydrolysis cycle by adding only fresh hot water to avoid degradation of sugar that has been already released and in solution, without adding either fresh substrate or catalyst), in order to depolymerize the remaining β-glucan that still remained adsorbed after the first hydrolysis cycle. This second cycle increased the overall total glucose yield to 87% [89% when including all soluble products including 5hydroxymethylfurfural (HMF) released during sugar hydrolysis, see Figure S3 in the Supporting


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Information]. The small amount of soluble HMF detected is a sign of some side reactions also occurring, and this may be responsible for the lack of complete material balance closure, for example as a result of humin formation. Data in Table 1 (Entries 6 and 7) also demonstrate ZN(30) and Z-S(30) to be less active than the parent ZTC and Z-H materials, regardless of their high per-gram loading of acidic functional groups. We attribute this due to inaccessibility of surface sites as a result of the extensive pore collapse observed when performing ZTC surface modification with either nitric acid or sodium hypochlorite (vide supra).

Figure 3. DRIFT spectra of parent ZTC and Z-H(30). The inset represents a difference spectrum between Z-H(30) and parent ZTC in the range of 1600 cm-1 – 1800 cm-1. Assignments (cm-1): 3051–3716 [ν(O–H)]; 3003 [ν(C–H, aromatic)]; 1691, 1724 [ν(C=O)]; 1637 [ν(C=C) and δ(O– H)].

Given the observed productivity for the Z-H catalysts in Table 1, we wished to investigate the types of surface functional groups that hydrogen peroxide treatment synthesizes. The DRIFT


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spectra for ZTC and Z-H(30) are shown in Figure 3. The observed distinct bands at 1691 cm-1 and 1724 cm-1 are attributed to the C=O stretching mode of surface carboxylic-acid functionality.38–40 These data are consistent with previous reports that demonstrate an increase of surface carboxylic-acid functional groups following hydrogen peroxide treatment.36,41 Another possibility could be lactone functionality, which has similar bands in the infrared, and could in principle be included in acid site counts detected by the titration.33 However, lactones would be unable to account for the catalytic activity of hydrolysis of β-1,4-glycosidic bonds.42 The data in Table 1 provide a unique opportunity to quantitatively investigate how catalytic hydrolysis activity correlates with degree of acid-site functionalization of the carbon-catalyst surface. Figure 4 shows this correlation between glucose yield plotted versus the surface acid-site functional group density for all Z-H materials. The glucose yield increases in direct proportion with acid-site surface density for these materials. This is consistent with previous studies of β1,4-glycosidic bond hydrolysis with several various types of carbon catalysts, which demonstrated a qualitative trend consisting of increased glucose yield upon increasing the surface acid-site density, spanning these different catalysts.24,42 However, in these cases, a quantitative analysis was complicated by not having a single underivatized parent material as a common denominator for comparison (e.g., unfunctionalized ZTC, which in our case is functionalized to varying degrees). This was impossible because of the different types of carbon materials that were used to establish the trend in the prior studies,17,20,22,24,42 which makes it infeasible to deconvolute the effect of changes in acid-site surface density from other textural changes, when comparing different catalysts. In contrast, the post-synthetic nature of the modification of the same parent material for catalysts shown in Figure 4 allows a more direct correlation between acid-site density within the microporous ZTC network and β-glucan


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hydrolytic catalysis activity. The observed direct correlation between surface acid-site density and catalytic activity for adsorbed β-glucan hydrolysis in Figure 4 is consistent with similar trends when studying catalytic hydrolysis of anchored β-glucans on oxide supports.16 This direct correlation suggests the surface acid site to be the one responsible for catalytic hydrolysis. We partly rationalize the increased catalytic hydrolysis rate on acid-site density by a higher frequency of collisions, between glycosidic oxygens on the adsorbed glucan strand and acid sites on the surface. Below, we explore the possibility of other, more subtle effects on how acid-site density could influence catalysis, such as cooperative bifunctional mechanisms.

Figure 4. Glucose yield in hydrolysis of β-glucan adsorbed on parent ZTC and Z-H materials as a function of acid site density. The similar plot as a function of acid site amount is shown in Figure S4 in the Supporting Information. Reaction conditions: β-glucan/ZTC ca. 25 mg; water 1 mL; 453 K; 3 h; 440 rpm.


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We next turned to the possibility of cooperativity between acid sites. To investigate the possibility of acid-base bifunctional cooperative catalysis in our ZTC materials, where both the carboxylic acid as well as the conjugate base carboxylate function cooperatively as observed in glycosidase enzymes,29,30 we conducted adsorbed β-glucan hydrolysis on ZTC and Z-H(30) in buffer solutions of various pH (Figures 5 and S5 in the Supporting Information). In acetate buffer (pH 4.1), both ZTC and Z-H(30) were significantly less active, whereas in phosphate buffer (pH 2.2), the activity was similar to that reported in Table 1 in water. The latter similarity is accounted for by the similar final pH values whether reaction is performed in pH 2.2 phosphate buffer or water as solvent (Table S1 in the Supporting Information), possibly due to organic acids formed via side reactions of sugar in water. An intermediate pH of 3.1 in phosphate buffer did not produce a significant deviation from linearity in Figure 5A for both ZTC and Z-H(30) catalysts. This plot lacks a bell-shaped curve, which was observed in other established acid-base bifunctional catalysts,43 given the expected acidity of surface carboxylic-acid functionality (pKa = 4.2,44 Figure S6A in the Supporting Information). This suggests that an acid-base bifunctional mechanism (in this case, cooperative function of carboxylic acid and conjugate carboxylate) is not occurring in the observed ZTC catalysis. The lack of observed bifunctional catalysis here means that this remains an open challenge for future catalyst designers: to mimic the type of mechanism on a surface that is exhibited by hydrolytic enzymes. Along this line, the distance between acid groups needs to be decreased significantly and further on a synthetic carbon surface, in order to facilitate this type of bifunctionality. Previously, when dealing with bifunctional catalysts on surfaces,45,46 several groups per nm2 on the surface were required – significantly higher than densities achieved as represented on the abscissa of Figure 4.


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As an aside, the observed activity at pH 2.2 cannot be accounted for by specific-acid catalysis mechanisms involving H3O+, since the pH is still too high as shown by a lack of activity in the absence of catalyst, when dealing with amorphous cellulose as well as prior data that has shown pKa of acids required to be below 0 for specific-acid catalyzed mechanisms.20,25,47 To this effect, in an additional control, we previously observed glucose yields lower than 20% when hydrolyzing adsorbed glucans on a mesoporous-carbon-nanoparticle catalyst under similar phosphate-buffer reaction conditions, at a pH of 2.0,20 which further underscores the importance of post-synthetic surface functionalization and the microporous confines of ZTC investigated in catalysts here.


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Figure 5. Hydrolysis catalysis of adsorbed β-glucans on parent ZTC and Z-H(30) in aqueous solution, in which the pH during catalysis is controlled by buffer: (A) glucose yield versus initial pH; (B) k/kpH2.2 as a function of initial pH. The quantitative glucose yield and selectivity are shown in Figure S5 in the Supporting Information. Reaction conditions: β-glucan/ZTC ca. 25 mg; solvent 1 mL; 453 K; 3 h; 440 rpm. The reactions at pH 2.2 and 3.1 were conducted in 20 mM of phosphate buffer and that at pH 4.1 was in 20 mM of acetate buffer (see Table S1 in the Supporting Information).

We also investigated whether the increasing glucose yield with higher acid-site surface density could be possibly correlated with a stronger acid sites at those higher densities, by virtue of greater anion delocalization via hydrogen bonding to adjacent surface-acid functional groups. To demonstrate the latter, in a related system, Shinkai et al. measured a systematically decreased pKa of four pKa units for phenolic OH groups in calixarene molecules, owing to intramolecular hydrogen bonding between juxtaposed OH groups.48 Figure 5B correlates the normalized rate k relative to kpH2.2 (i.e., rate constant divided by maximum rate constant of catalytic hydrolysis at a


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buffered pH of 2.2) versus buffered pH value, in which the pseudo-first-order rate constant k is defined by eq. 1. Pseudo-first-order kinetics has been observed for similar acid-catalyzed β-1,4glycosidic bond hydrolysis reactions.18,49,50

k=−

ln(1 − X ) t hyd

(1)

where X is the glucose yield and thyd is the reaction time. The fact that both the parent ZTC and Z-H(30) catalysts exhibit the same slope in Figure 5B demonstrates that their effective pKa is similar; if it was shifted to lower pKa in Z-H(30) (see Figure S6B in the Supporting Information), the slope at lower pH would be higher. Our conclusions do not change if the reaction order for catalytic hydrolysis is instead assumed to be zero rather than first order (see Figure S7 in the Supporting Information). Similar approaches of using reactions to probe local pKa of acid sites in complex catalyst architectures have been used previously.51

Figure 6. Schematic of plausible active sites of ZTC catalysts.

When interpreting Figure 5A, the lower catalytic activity in acetate buffer media suggests that a higher density of surface carboxylic-acid sites is required for catalysis, but, given data


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above, this is not because of enforcing acid-base bifunctional cooperativity or a stronger acid site via conjugate-base anion delocalization via hydrogen bonding. Rather, the pH-controlled experiments above suggest that carboxylic acid in COOH form is required for hydrolysis (Figure 6), and there is no data to indicate any type of cooperativity between COOH groups given the linearity observed in Figure 5. At the same time, we cannot rule out possible COOH and phenolic OH cooperativity on the catalyst surface, because in the range of working pH range of Figure 5 (i.e., pH 2.2–4.1), the phenolic OH groups remain untitrated given their much higher pKa value (pKa ~ 10)44. This means that in principle they could still act cooperatively with surface COOH functional groups, in a manner previously demonstrated for organic glycosidicbond hydrolysis catalysts.52 However, given the linear decrease of hydrolysis rate and rate constant versus pH under catalysis conditions in Figure 5, the possible role of phenolic OH groups acting as the sole responsible catalytic site (i.e., in a non-cooperative fashion with COOH groups) can be ruled out, since, under such a scenario, Figure 5 would be expected to give a plateau or flat line rather than the observed decreasing trend, based on the pKa value of phenolic OH. To explain the trend in Figure 4, we argue that a higher density of acid sites on the surface makes it more likely that one of the surface acid sites will be in the correct position for hydrolytic catalysis, based on the crucial positioning aspect between glycosidic oxygen and acid site for facilitating hydrolytic catalysis. This is supported by observations in intramolecular catalysis systems of Capon et al.,34 as well as in the highly relevant related prior work by Fukuoka et al., in which organocatalysts possessing a high density of COOH/OH groups (i.e., vicinal COOH/OH groups) exhibited significantly greater catalytic activity for hydrolytic β-1,4glycosidic bond cleavage, when compared with organocatalysts consisting of more isolated


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COOH/OH functionalities.52 The work of Gazit and Katz also demonstrated a crucial aspect of pairing surface weak-acid sites surrounding a glycosidic oxygen of chemisorbed glucans, for achieving glycosidic bond hydrolysis on silica and alumina surfaces as model solid catalysts.16 Here, we hypothesize that in all of these systems, the high density of weak-acid surface functional groups on the catalyst leads to a greater probability of interaction with glycosidic oxygens, to activate and break β-1,4-glycosidic bonds.46,52

CONCLUSIONS We have demonstrated the post-synthetic modification of ZTC materials under carefullycontrolled oxidation conditions with hydrogen peroxide, which successfully introduces weakacid functional groups on the ZTC surface, without altering the microporous environment. The surface-modified ZTC catalysts show comparable β-glucan adsorption uptake relative to the parent ZTC catalyst prior to surface functionalization, due to retention of optimally sized micropores as well as a high microporous graphene-like surface area, which leads to strong CH– π interactions with the β-glucan adsorbate. In subsequent adsorbed β-glucan hydrolysis, the catalytic activity of the modified ZTC materials directly depends on the surface density of weakacid functional groups, which demonstrate synthesis of glucose in up to 87% yield. We demonstrate that acid-base bifunctional catalysis as well as mechanisms that invoke higher acid strength as a result of this proximity are unlikely under our conditions. We thus invoke that the high density of carboxylic acids in COOH form leads to preferential pairing with glycosidic oxygens of adsorbed β-glucan chains. This pairing is further facilitated by the microporous confinement offered by the ZTC catalyst interior, by enforcing the proximity of weak-acid


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functionalities in three-dimensions. This in turn results in high catalytic activity of modified ZTC materials in β-glucan hydrolysis, according to mechanisms of general-acid catalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.XXXXXXX. β-Glucan adsorption data, catalytic hydrolysis data, pH values after hydrolysis, and population of carboxylic acid and conjugated carboxylate (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Funding Sources This research was supported by Grant-in-Aid for postdoctoral fellows from Japan Society for the Promotion of Science (JSPS, 14J01171), funding from Japan Science and Technology Agency “Advanced Low Carbon Technology Research and Development Program” (JST-ALCA), and


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funding from the Office of Basic Energy Sciences of the Department of Energy (DE-FG0205ER15696). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to Prof. T. Kyotani and Dr. H. Nishihara (Tohoku University) for their fruitful advice for preparation and post-synthetic modification of ZTC.

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Table of Contents

Synopsis The post-synthetically modified zeolite-templated carbons with high density of carboxylic acid functionality exhibit high catalytic activity in β-glucan hydrolysis.


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Zeolite-Templated Carbon Catalysts for Adsorption ... - ACS Publications

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