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Selective MOF Catalysis of Glucose to 5Hydroxymethylfurfural Using Phosphate-Modified NU-1000 Mizuho Yabushita, Peng Li, Timur Islamoglu, Hirokazu Kobayashi, Atsushi Fukuoka, Omar K. Farha, and Alexander Katz Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Industrial & Engineering Chemistry Research

Selective MOF Catalysis of Glucose to 5Hydroxymethylfurfural Using PhosphateModified NU-1000 Mizuho Yabushita,*,†,‡ Peng Li,§ Timur Islamoglu,§ Hirokazu Kobayashi,‡ Atsushi Fukuoka,*,‡ Omar K. Farha,*,§,ǁ and Alexander Katz*,† †

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

Berkeley, California 94720, United States ‡

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

§

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

ǁ

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi

Arabia

ACS Paragon Plus Environment

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ABSTRACT

This manuscript demonstrates the synthesis of selective Lewis-acid sites in a metal-organic framework (MOF) for glucose transformation to 5-hydroxymethylfurfural (HMF). These sites are synthesized via partial phosphate modification of zirconia-cluster nodes in MOF NU-1000, which titrates strong Lewis-acid sites that would lead to undesired side reactions. Our mechanistic study using the isotope tracer analysis and kinetic isotope effect measurements reveals that an isomerization-dehydration mechanism mainly occurs on the MOF catalyst, where fructose is an intermediate. This mechanism suggests that dilute concentrations are favorable in order to suppress undesired intermolecular condensation of glucose/fructose/HMF and maximize HMF yield. We demonstrate both high yield and selectivity of HMF formation of 64% with the MOF catalyst, at an initial glucose concentration of 1 mM in water/2-propanol. In stark contrast, partial phosphate modification of a bulk zirconia still yields a catalyst that exhibits poor HMF selectivity, while possessing nearly identical Brønsted acidity to the selective NU-1000-based catalyst.

INTRODUCTION 5-Hydroxymethylfurfural (HMF) has been identified as a biomass-derived platform chemical with high-volume potential, with applications as a building block for polymers (e.g., 2,5furandicarboxylic acid, p-xylene, adipic acid, and 1,6-hexanediol),1–3 and a small-molecule clinical agent for sickle-cell disease.4 HMF is typically synthesized from hexoses via acidcatalyzed dehydration; in particular, most processes for fructose dehydration leading to HMF in


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stoichiometric yields consist of various homogeneous and heterogeneous Brønsted acid catalysts.4–7 Glucose is a more attractive reactant for conversion to HMF, due to its lower cost, but results in lower yields even in the presence of the best fructose-to-HMF catalysts. This has been previously ascribed to be a consequence of more stable pyranose form of glucose.4–7 While effective homogeneous catalysis has been developed for this transformation, the desire for ease of catalyst recovery has shifted focus to solid catalysts, and a variety of heterogeneous Lewis acids such as metal phosphates, metal oxides, and heteropolyacids have been investigated.4–7 One of the highest yields of glucose to HMF previously reported by Hara and coworkers involves the use of a phosphate-modified titania catalyst.8 The necessity of phosphate modification is the result of undesired side reactions that lead to humins, when using the unmodified titania as a catalyst. In a related work investigating a phosphate-modified niobia catalyst,9 the modification has been proposed to poison the unselective active sites, which were hypothesized to be Brønsted-acid sites, as supported by their consumption via infrared spectroscopy of bound CO before and after phosphate modification. This led to an optimized catalyst that synthesized HMF from glucose in 81% yield in water/tetrahydrofuran solution. Other inorganic oxides (i.e., niobia, alumina, and tin oxide) showed lower selectivity.9,10 In a separate elegant contribution, the same research group reported the mechanism of glucose conversion to HMF catalyzed by phosphate-modified titania.10 Deuterium labeling experiments demonstrate the mechanism to proceed through direct dehydration; that is, the open-chain form of glucose directly undergoes three consecutive dehydration reactions to form HMF. A key intermediate within this mechanism is 3-deoxyglucosone, which as a result of its high reactivity and undergoing intermolecular condensation may contribute to decreased HMF yield and selectivity.


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Metal-organic frameworks (MOFs) are crystalline materials consisting of metal nodes that are linked by organic linkers, and unlike conventional supported metal catalysts, this class of solid materials has completely isolated metal sites in their structure, which can be expected to show high catalytic activity.11–13 Besides, MOFs can be functionalized via pre/post-synthetic modification of linkers and post-incorporation to nodes with functionalized compounds.11–14 Such tunability of MOFs is one of their advantages and provides an attractive handle for the molecular design of catalytic active sites. The hydrothermally stable MOF material—MIL-101— has been reported to catalyze dehydration of both fructose and glucose to HMF, after postsynthetic modification, with yields below 45% when starting from glucose.15–19 Kitagawa et al. have also demonstrated the isomerization of glucose to fructose catalyzed by the bare Cr nodes in MIL-101 that function as Lewis acids.20 Zhao et al. have recently demonstrated another class of MOF material—NUS-6 consisting of Hf nodes and sulfonated linkers—to be an excellent catalyst for fructose dehydration to synthesize HMF in as high as 98% yield, without any postsynthetic modification.21 These previous reports have shed light on the potential applications of MOFs to biomass conversion, and indeed, other types of MOF-catalyzed reactions that transform biomass-derived compounds into useful chemicals have also been reported.22,23 Here, in this manuscript, we employ another chemically stable MOF material—NU-1000,24 which is built up from zirconia-cluster nodes isolated by pyrene linkers—as a catalyst for glucose transformation to HMF. Farha et al. have recently demonstrated that the open Zr sites in NU-1000 function as Lewis acids, to catalyze the destruction of chemical warfare agents.25 According to this as well as the results of Hara et al. discussed above,8–10 we hypothesized the Zr sites present in the isolated clusters of the NU-1000 framework to be active for glucose transformation as well, to synthesize HMF in good yields. In particular, when using the zirconia


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clusters present in NU-1000 as inorganic oxide catalysts for the conversion of glucose to HMF, we too found the need to poison unselective active sites with phosphoric acid; yet, we were able to demonstrate a distinctly different mechanism for the conversion of glucose to HMF – separate and orthogonal to the one observed by Hara and coworkers10 – as well as a catalyst that accomplishes the desired transformation in the highest yield and selectivity reported by a MOF catalyst to date.

EXPERIMENTAL SECTION Phosphate modification of NU-1000. NU-1000 was prepared by following the reported procedure for a large-scale synthesis.26 Then, phosphate species was immobilized on NU-1000 by a simple impregnation method. Thus, 50 mg of NU-1000 was dispersed in 5 mL of aqueous solution containing phosphoric acid (Fischer Scientific), the amount of which is equimolar or half-equimolar to OH groups of NU-1000 (total amount of OH groups is 5.34 mmol g-1)27. The mixture was ultrasonicated for 1 min to disperse NU-1000 powder well, and then water solvent was completely removed by a rotary evaporator at 313 K followed by drying under vacuum overnight (Labconco, FreeZone 12 Liter Console Freeze Dry Systems, ≤ 42.0 Pa). The materials thus prepared with equimolar and half-equimolar of phosphoric acid are denoted as PO4/NU(eq) and PO4/NU(half), respectively. As a reference, we also employed a bulk zirconia material (JRCZRO-5, supplied from the Reference Catalyst Division of the Catalysis Society of Japan, calcined at 573 K for 5 h in an electronic furnace). This bulk zirconia material was also treated with phosphoric acid in the same manner described above, to replace all, half, or 10% of OH groups present on the surface (total quantity of OH groups was speculated to be 0.7 mmol g-1,


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based on a previous report28) with phosphate species, and the resulting samples are denoted as PO4/ZrO2(eq), PO4/ZrO2(half), and PO4/ZrO2(10%), respectively. The Brønsted acidity of bare and modified materials was measured by the reported acid-base potentiometric titration technique.29 Glucose Transformation. The glucose conversion was conducted in a small glass vial (Agilent Technologies, MS Analyzed Vials Kit, 1.5 mL, equipped with a seal), into which 10 mg of catalyst, 1 mL of glucose aqueous solution (glucose was supplied from Sigma-Aldrich), and a magnetic stir bar were charged. For isotope tracer study and kinetic isotope effect (KIE) measurement, the reactant used was deuterium-labeled glucose (i.e., glucose-1-d1 from SigmaAldrich and glucose-2-d1 from Cambridge Isotope Laboratories). When conducting glucose conversion in a mixed solvent, 0.1 mL of glucose aqueous solution and 0.9 mL of organic liquid (1-propanol (1-PrOH), 2-propanol (2-PrOH), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), or N,N-dimethylformamide (DMF) from Fischer Scientific) were charged into the vial. In either case, the reactant mixture was initially subject to ultrasonication for 1 min to disperse the solid catalyst well in reaction solution, and then was treated at 413 K for a certain time 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 Silica, silica bed 100 mg). The solid phase was washed with 1 mL of hot water (353–363 K) five times, to extract any adsorbed water-soluble compounds. The amount of residual glucose and reaction products in the liquid filtrate was quantified by high-performance liquid chromatography (HPLC, Shimadzu, Prominence HPLC System, refractive index detector) equipped with an Aminex HPX-87H column (Bio-Rad, ø7.8 × 300 mm, mobile phase 5 mM sulfuric acid aqueous solution 0.6 mL min-1, column temperature


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323 K), where the amount of leached phosphate was simultaneously measured as phosphoric acid. The reaction mixture was also analyzed by liquid-chromatography/time-of-flight massspectrometry (LC/TOF-MS, Agilent Technologies 6520 Accurate-Mass Q-TOF LC/MS Instrument with 1200 Series LC System, mobile phase 0.1% formic acid aqueous solution 1.0 mL min-1, direct sample injection without column, negative ion mode). For isotope tracer study, after removing solvent by a rotary evaporator at 313 K, redissolving in 1 mL of water, and adding 20 µL of ethanol-d6 (Sigma-Aldrich) as an internal standard, the reaction mixture was analyzed by 2H nuclear magnetic resonance spectroscopy (NMR, Bruker, Avance III 600, 2H 92.1 MHz, 296 K, relaxation delay 1.00 s, acquisition time 1.11 s, 64 scans).

RESULTS AND DISCUSSION We initially conducted a study of HMF synthesis using unmodified NU-1000, in deionized water at 413 K for 5 h (similar to conditions previously reported for HMF synthesis)4–7. The results shown in Table 1 demonstrate that while NU-1000 as catalyst consumes 60% glucose under these conditions (entry 2), higher than the 16% conversion of the background reaction (entry 1), NU-1000 synthesized HMF in a low 2.3% yield, and instead produced fructose in 19% yield, with the remainder of glucose converted to by-products (i.e., 39%). The LC/TOF-MS data (Figure S1 in the Supporting Information) demonstrated compounds in this latter category to consist of partially dehydrated sugars, with a molecular weight of 162 g mol-1, as well as higher molecular weight (i.e., 216 g mol-1 and 270 g mol-1), which indicates the formation of dimers via intermolecular condensation and subsequent dehydration of sugars and/or their dehydrated compounds. The latter are inferred to be the precursors of larger and insoluble humin


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compounds, which are notoriously difficult to characterize.30 We also note that the color of the used NU-1000 catalyst turned from initially yellow to dark brown after catalysis, which is another indicator of humin formation. Altogether, these results confirm our initial hypothesis regarding the zirconia-cluster nodes of NU-1000 being able to act as Lewis acid sites for catalyzing glucose conversion with relatively high activity. However, from the standpoint of HMF synthesis, NU-1000 lacks desired selectivity for HMF formation and instead synthesizes undesired by-products, which lead to humins.4,5,7,31

Table 1. Glucose Conversion in Watera Yield /% Entry Catalyst

Conversion /% HMF

Fructose

1

None

16

1.6

11

2

NU-1000

60

2.3

19

3

H3PO4c

4

PO4/NU(eq)

5

PO4/NU(half) 50

7.8 17

Othersb 3.4 39

1.0

0.3

6.5

5.9

9.7

1.4

15

5.7

29

a

Reaction conditions: 100 mM glucose aqueous solution (1 mL); catalyst 10 mg; 413 K; 5 h; 440 rpm. bThe lack of material-balance closure (i.e., the reason why the glucose conversion does not equal the sum of product yields) is due to the formation of humin-related by-products. c53 µmol, equimolar to phosphate species on 10 mg of PO4/NU(eq).

Previously, titania acting as a Lewis-acid catalyst also exhibited a high level of glucose conversion activity (e.g., > 99% glucose conversion), under similar reaction conditions to those used in Table 1 for NU-1000; however, the degree of HMF yield was a low 8.5%.8,10 Hara et al.


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increased HMF yield up to 81% by poisoning some fraction of OH groups on titania via phosphoric-acid esterification, with maintaining the overall activity of glucose conversion. Based upon these observations, we hypothesized that the zirconia-cluster nodes of NU-1000 after phosphoric-acid treatment may also exhibit an enhanced HMF selectivity when converting glucose. Such a hypothesis relies upon the selective removal of the strongest Brønsted acids, as the ones expected to catalyze the unselective glucose transformations alluded to above. Thus, we synthesized two variants of NU-1000, in which both all (sample denoted as PO4/NU(eq)) and half (sample denoted as PO4/NU(half)) of the OH groups on the zirconia-cluster nodes were reacted with phosphate groups by treating NU-1000 with a controlled amount of phosphoric acid, followed by evacuation of water. Data in Table 1 show the phosphate-modified NU-1000 catalysts consisting of PO4/NU(eq) and PO4/NU(half) synthesize HMF in 5.9% and 15% yield (entries 4 and 5 of Table 1), respectively. The latter synthesizes HMF in a 6.5-fold higher yield relative to unmodified NU1000 (entry 2), whereas the activity of this catalyst (i.e., glucose conversion of 50% in Table 1) is only slightly lower than that for unmodified NU-1000. These results support our hypothesis regarding phosphate modification of unselective sites in NU-1000 alluded to above, though a more detailed mechanistic analysis (vide infra) demonstrates our catalysts to operate under a completely different reaction mechanism to those reported by Hara and coworkers.10 The fullcoverage phosphate-modified NU-1000 represented by PO4/NU(eq) is too highly poisoned with phosphate functionality to be an effective catalyst for glucose conversion, and produces a very low yield of HMF, at a glucose conversion of 17%. This indicates the importance of having some active sites remaining within the catalyst. All catalysts under the conditions of Table 1 synthesize


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a large amount of humin-related by-products, which have been characterized via LC/TOF-MS (vide supra) and are responsible for a lack of material-balance closure. To investigate any possible role of leached phosphate during catalysis, we measured the amount of this leaching under conditions of Table 1. The quantity of leached phosphate for the best catalyst, PO4/NU(half), amounts to less than 1% of the total quantity of phosphate content of the catalyst originally. An additional catalytic experiment was conducted to investigate the reactivity of soluble phosphate – represented as phosphoric acid. Data in Table 1 demonstrates phosphoric acid to be a poor catalyst for HMF synthesis (entry 3), as there was virtually no HMF synthesized and the glucose conversion was below background reaction level (entry 1, in the absence of phosphoric acid). A major previously reported obstacle to HMF synthesis is its instability in water at the high temperatures that glucose conversion requires. A mixture of water and miscible organic solvents have thus been intensively investigated to stabilize produced HMF in solution.4–7 Motivated by these results, we sought to further improve HMF yields relative to the values shown in Table 1, and conducted an investigation of mixed solvents for the reaction. Results shown in Table 2 demonstrate the use of a variety of mixed aqueous-organic solvent systems for our lead catalyst from Table 1, consisting of phosphate-modified NU-1000, PO4/NU(half). The aqueous mixtures based upon either 1-PrOH, 2-PrOH, or THF (entries 6–8 of Table 2) increase HMF yield up to 25%, higher than the 15% synthesized in water alone as solvent (entry 5 of Table 1). The beneficial effects of these three solvents as related to HMF synthesis yield are consistent with previous reports.4,6,7 In the case of the water/2-PrOH system (entry 7), the 9.7% of fructose synthesized and 24% HMF yield are indications that there may be room for further improvement in HMF yield by optimizing the reaction conditions (vide infra). When compared with high


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boiling-point solvents such as DMSO and DMF, the 2-PrOH co-solvent is advantageous due to its ease of removal via distillation. We also tested aqueous mixtures consisting of DMSO and DMF, which have also been reported to be good co-solvents for HMF synthesis;4–7 however, they show poor performance with the PO4/NU(half) catalyst (entries 9 and 10), with no HMF production observed whatsoever for the water/DMF solvent. Clearly, there may be multiple competing effects involving both HMF stability as well as a change of relative transition-state energies when tuning the solvent choice/composition, which all affect the HMF synthesis selectivity. The lack of material-balance closure in Table 2 is attributed to the same huminrelated by-products as discussed above within the context of Table 1 data.

Table 2. Solvent Effect on Glucose Conversion by PO4/NU(half)a Yield /% Entry

Solvent

Conversion /% HMF

Fructose

Othersb

6

Water/1-PrOH

95

22

4.8

68

7

Water/2-PrOH

94

24

9.7

60

8

Water/THF

97

25

5.0

67

9

Water/DMSO

53

2.8

10

Water/DMF

> 99

0

19 6.0

31 > 93

a

Reaction conditions: 100 mM glucose solution (1 mL); PO4/NU(half) 10 mg; 413 K; 5 h; 440 rpm. The volume ratio of water to organic solvent was 1:9. bThe lack of material-balance closure (i.e., the reason why the glucose conversion does not equal the sum of product yields) is due to the formation of humin-related by-products.

Before moving on to further improving HMF yield above values shown in Table 2, we wished to investigate the reaction mechanism occurring on the PO4/NU(half) catalyst, which was


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used to further guide rational optimization of the reaction conditions. Based on previously invoked mechanisms of Lewis-acid-catalyzed transformation of glucose to HMF,5,10,32 we envisioned two possible reaction mechanisms that could be responsible for HMF synthesis by phosphate-modified NU-1000 catalysts. These are both shown in Scheme 1 as an isomerizationdehydration mechanism (Scheme 1A), which includes an initial 1,2-hydride shift to synthesize fructose as an intermediate that is subsequently dehydrated, and a direct dehydration mechanism (Scheme 1B), where glucose in an open-chain form directly undergoes dehydration three times.

Scheme 1. Possible reaction mechanisms of glucose conversion to HMF, catalyzed by Lewis acids: (A) isomerization-dehydration mechanism and (B) direct dehydration mechanism.


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We use an isotope tracer study involving glucose-1-d1 and glucose-2-d1 in order to identify which of the mechanisms alluded to above are in operation with the PO4/NU(half) catalyst. The isomerization-dehydration stepwise mechanism forms fructose-1-d1 as a reaction intermediate via 1,2-hydride shift in the initial stage, which is subsequently dehydrated three times to form HMF, wherein the H and D atoms coordinating to C1 are eliminated with the same probability of 50%.10 This leads to an equimolar mixture of D-labeled and unlabeled HMF products for both glucose-1-d1 and glucose-2-d1 as reactants (see Scheme S1 in the Supporting Information). This is in stark contrast to the direct dehydration mechanism, which always synthesizes a single HMF species, HMF-1-d1 from glucose-1-d1 and unlabeled HMF from glucose-2-d1 (see Scheme S2 in the Supporting Information). Such a study was elegantly used previously by Hara et al. to prove mechanism of catalyst operation, when converting deuterated glucose substrates to HMF over a phosphate-modified titania catalyst.10 Data pertaining to our isotope tracer study use 2H NMR spectroscopy to quantify the amount of D-labeled HMF products (using ethanol-d6 as internal standard), which is compared relative to total HMF yield obtained for the same reaction mixture via HPLC analysis, when using our lead catalyst consisting of PO4/NU(half). With both D-labeled reactants—glucose-1-d1 and glucose-2-d1—the same HMF product consisting of a deuterium substituent on C1 is observed (Figure 1), together with the unlabeled HMF product, which is present relative to the former in a near-equimolar ratio. This ratio indicates that the PO4/NU(half) catalyst functions mainly according to the isomerization-dehydration mechanism of Scheme 1A (the detailed mechanism for both D-labeled glucose reactants are shown in Scheme S1 in the Supporting Information).


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Figure 1. 2H NMR spectra of reaction products synthesized from D-labeled glucose by PO4/NU(half). Reaction conditions: 100 mM D-labeled glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 413 K; 5 h; 440 rpm. The populations of D-labeled HMF in total HMF product are 42% for glucose-1-d1 and 53% for glucose-2-d1.

To further confirm this and elucidate the extent of the kinetic isotope effect (KIE), we investigated kinetics using glucose-1-d1 and glucose-2-d1. Based upon previously reported KIE values involving the latter reactant and Lewis-acid catalysts,33–36 a significant KIE value is expected for the isomerization-dehydration mechanism of Scheme 1A, since this mechanism involves C–H bond breakage at the C2 position as a rate-determining step (see also Scheme S1 in the Supporting Information). The converse of this last statement is that the direct dehydration mechanism of Scheme 1B (see also Scheme S2 of Supporting Information) is not expected to exhibit a large KIE.


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Figure 2 shows a pseudo-first-order analysis of the reaction kinetics for the conversion of D-labeled glucose as well as unlabeled glucose control reactants at 373 K, wherein the slope of the line directly corresponds to the first-order rate constant k. The ratio of k for unlabeled glucose to that for glucose-1-d1 (i.e., kH/kD) is 1.03 ± 0.10, which indicates that C–H scission at the C1 position is not involved in the rate-determining step. In stark contrast, for glucose-2-d1 as a reactant, a significant KIE of 1.76 ± 0.17 corresponding to kH/kD was observed, which is similar to previously reported KIE values.33–36 This clearly demonstrates that C–H bond breaking at the C2 position is involved in the rate-determining step, and this conclusion is consistent with the isomerization-dehydration mechanism, as discussed above. Altogether, our data of isotope tracer study and KIE demonstrate that glucose conversion to HMF as catalyzed by phosphate-modified NU-1000 proceeds via an isomerization-dehydration mechanism of Scheme 1A. This result is in stark contrast to that obtained by Hara et al. for a phosphate-modified titania catalyst, which operated via a direct dehydration mechanism of Scheme 1B.10 This comparison demonstrates that the mechanism of glucose conversion is highly dependent on the detailed characteristics of the surface-modified Lewis-acid catalyst, being different for the zirconia-cluster nodes of NU1000 versus titania.


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Figure 2. Kinetic isotope effect on glucose conversion by PO4/NU(half). Reaction conditions: 10 mM glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 373 K; 440 rpm. The ratios of rate constant (i.e., kH/kD) are calculated to be 1.03 ± 0.10 for glucose-1-d1 and 1.76 ± 0.17 for glucose-2-d1.

The isomerization-dehydration mechanism of Scheme 1A clearly involves glucose, fructose, and HMF in the reaction mixture. Based on previous reports demonstrating that glucose/fructose/HMF mixtures readily undergo intermolecular condensation to form insoluble humins in the presence of acid catalysts at high temperature,30,37,38 we postulated that one reason for our limited HMF yields in Tables 1 and 2 may be associated with such intermolecular reactions involved with humin formation. We have characterized the precursors to such humin formation via LC/TOF-MS (vide supra). We further hypothesized that by using lower concentration conditions for the reaction, we could improve HMF yields to be beyond that in entry 7 of Table 2, by decreasing the probability of intermolecular collision, which is an elementary step of any intermolecular reactions.31,39 We thus investigated the effect of glucose


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reactant concentration on HMF yield, and data shown in Figure 3 (see also Table S1 in the Supporting Information) demonstrate a maximum HMF yield of 64% at an initial glucose concentration of 1 mM upon complete conversion of glucose (> 99%; that is, HMF selectivity is calculated to be 64%). To the best of our knowledge, the highest HMF selectivity reported thus far using a MOF-based catalyst is 46%,15–19 and therefore, this HMF selectivity of 64% is the highest one reported for a MOF-based catalyst. In all cases in Figure 3, we attribute the lack of material-balance closure to the same humin-derived by-products already discussed within the context of data in Tables 1 and 2. Data in Figure 3 at higher initial glucose concentration demonstrates a rapid deterioration of

this

yield,

which

we

ascribe

due

to

increased

intermolecular

collision

of

glucose/fructose/HMF. These collisions are thought to lead to humin formation as a side reaction at higher concentrations. Indeed, we hypothesize that the observed deactivation of catalysts after 5 h of reaction time (regardless of the initial glucose concentration) is due to humin deposition covering the active sites of the PO4/NU(half) catalyst. The data in Figure 3 demonstrate the benefit of using a low concentration of glucose reactant. We expand our results above and generalize to provide guidelines for any heterogeneous catalyst involved with converting glucose to HMF. Given the fact that the reaction mixture will always contain a combination of glucose, fructose, and HMF, we infer based on our own experience here with our best catalyst, PO4/NU(half), that diluted initial glucose concentrations will help to suppress side reactions that consist of intermolecular condensation of glucose/fructose/HMF30,37,38 to undesired humin-related by-products.


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Figure 3. Initial glucose concentration effect on HMF yield. Reaction conditions: glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 413 K; 440 rpm. The detailed reaction results involving glucose conversion and fructose yield are summarized in Table S1 in the Supporting Information.

We investigated the possibility of synthesizing stronger Brønsted-acid sites upon grafting phosphate to the zirconia nodes, since it has been previously established that extended phosphate oligomerization contributes to enhanced Brønsted acidity40 and because of the previously reported slightly stronger electron-withdrawing effect of the zirconia-cluster node of NU-1000, relative to bulk zirconia.41 However, potentiometric titration data (summarized in Table S2 in the Supporting Information) demonstrate that the pKa values after phosphoric-acid modification do not change for NU-1000. We thus infer that the phosphoric-acid modification does not change strength of Brønsted acidity in the NU-1000 catalyst. This is consistent with the results of Hara et al., who suggested that the strongest acid on the phosphoric acid with pKa1 = 2.1 is consumed


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during condensation to the titania surface.10 Based on this, we conclude that Brønsted acidity is not what sets catalyst PO4/NU(half) apart in terms of its being selective. Instead, we focus on the Lewis-acid sites inherent in NU-1000, which have been previously discussed and identified.25 The strongest Lewis-acid sites, which could be unselective in glucose to HMF transformation, are titrated and removed from catalysis by Lewis basicity of lone electron pairs on the oxo substituents of grafted phosphate, as shown in Scheme 2. We posit that in the absence of these strong Lewis-acid sites, the nodes of NU-1000 become more selective catalysts for the glucose-to-HMF transformation, while preserving a similar degree of Brønsted acidity, which we deem to be crucial for converting fructose intermediate to HMF. Some amount of Lewis acidity is required and must remain even after phosphate surface modification, for the conversion of glucose to fructose, but this does not require the strongest Lewis-acid sites, which we posit are responsible for humin formation and which are titrated via phosphate surface modification.

Scheme 2. Plausible poisoning function of phosphate species grafted on zirconia-cluster node of NU-1000.


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Based on the optimized conditions found in Figure 3, we compared bulk zirconia as a catalyst, in order to better understand the importance of the cluster nodes of NU-1000 as being distinct in their catalytic role from that of a bulk material. We thus synthesized phosphatemodified bulk zirconia catalysts at levels corresponding to similar amounts of surface modification as used for NU-1000, corresponding to 10%, 50%, and 100% of maximum modification based on the expected number density of zirconols (see Experimental). Results shown in Figure 4 demonstrate all catalysts based on bulk zirconia to be catalytically unselective in terms of a lack of fructose and HMF production, while exhibiting significant glucose consumption. The HPLC trace of a reaction mixture catalyzed by phosphate-modified bulk zirconia does not show any peaks that could represent a single well-defined compound. We surmise that this lack of selectivity by the phosphate-modified bulk zirconia catalyst is the result of its catalyzing the conversion of glucose to almost exclusively humin precursors. These precursors were also observed via LC/TOF-MS for the MOF-based PO4/NU(half) catalyst (see Figure S1 in the Supporting Information), albeit in much reduced amounts according to material balance, since the PO4/NU(half) catalyst under the same conditions produces HMF in 64% yield.


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Figure 4. Glucose conversion to HMF by bare and phosphate-modified bulk zirconia materials. Reaction conditions: 1 mM glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); catalyst 10 mg; 413 K; 7 h; 440 rpm.

To understand why bulk zirconia has low catalytic selectivity, we investigated its Brønsted acidity via potentiometric titration, using both the unmodified oxide as well as the phosphatemodified variants (see Table S2 in the Supporting Information). In all of these catalysts, the observed Brønsted acidity is represented by a pKa1 of 3.17–3.34, which is similar to the Brønsted acidity of catalysts based on the NU-1000 (i.e., pKa1 = 3.41–3.59). We thus cannot rationalize the significant difference of catalytic selectivity between the cluster-based NU-1000 and bulk zirconia catalysts (see Figure 4) by Brønsted acidity. Instead, there may be many other possible reasons for the observed catalytic selectivity difference, including the possibility of a surrounding hydrophobic environment within NU-1000, since hydrophobic environments have been previously invoked to be important in Lewis-acid catalysts for glucose to fructose


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isomerization based on Sn sites in zeolites,42 along with possible differences in Lewis acidity/basicity43 as a result of the small size of the zirconia cluster node in NU-1000.

CONCLUSIONS We have investigated the synthesis of HMF from glucose using phosphate-modified Lewis-acidic zirconia-cluster nodes of NU-1000 as the active site. Our results demonstrate that partial phosphate modification of this catalyst is ideal in terms of maximizing HMF yield, which we interpret to be the result of poisoning of strong Lewis-acid sites by the oxo substituents of grafted phosphate species, which would otherwise facilitate side reactions. In contrast, fully poisoning Brønsted-acid sites diminishes catalytic performance of the zirconia-cluster nodes of NU-1000. Altogether, these observations clearly demonstrate a need for both reduced Lewis acidity as well as some Brønsted acidity within the NU-1000 catalyst. We posit the former to be required for glucose-to-fructose isomerization while the latter are necessary for fructose-to-HMF dehydration. Experiments aimed at elucidating the mechanism of catalysis involve isotope tracer studies, which employ 2H NMR spectroscopy, and KIE measurements. Data from these studies demonstrate the isomerization-dehydration mechanism as the major reaction pathway, in contrast to previous reports of a phosphate-modified titania and bare niobia,10,44 which instead proceed via the direct-dehydration mechanism. This mechanistic insight guided us to further optimize the reaction conditions to include higher levels of dilution in order to minimize intermolecular condensation processes involving glucose/fructose/HMF to form undesired humin-related byproducts. We have also investigated the solvent effect on HMF yield and have found that a mixture of water and 2-PrOH increases the HMF yield up to 64%, when conducting the reaction


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with an initial glucose concentration of 1 mM. Under the same conditions, the bulk zirconia is completely unselective in synthesizing HMF, even after phosphate modification at various levels. These data demonstrate a significant effect of the zirconia cluster in NU-1000 on catalytic performance in glucose conversion to HMF, which cannot be replicated with bulk zirconia. Though even our best catalyst is limited by humin-related by-product formation (deposition of humins on catalyst is responsible for darker coloration of catalyst after reaction), we anticipate that biphasic systems consisting of water and organic solvent, which allow in situ extraction of produced HMF from aqueous phase, may suppress intermolecular condensation of sugars with HMF, to minimize humin-related by-product formation.4–7 Such biphasic systems are planned to be investigated with the PO4/NU(half) catalyst in due course.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.XXXXXXX. LC/TOF-MS data of reaction mixture, detailed mechanisms of D-labeled glucose conversion to HMF, detailed reaction data of Figure 3, and pKa values of bare and phosphate-modified materials determined by potentiometric titration (PDF)


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This contribution was identified by Prof. Franklin Tao (The University of Kansas) as the Best Presentation in the session “Catalysts & Catalytic Technologies for Conversion of Biomass & Its Derivatives” of the 2016 ACS Fall National Meeting in Philadelphia, Pennsylvania. The authors are grateful to Dr. Kiyotaka Nakajima (Hokkaido University) for his fruitful advice for catalyst preparation and reaction mechanism, as well as Dr. Andrew Solovyov (University of California, Berkeley) for his help for 2H NMR analysis. This research was supported by the funding from the Office of Basic Energy Sciences of the Department of Energy (DE-FG02-05ER15696). O.K.F. gratefully acknowledges support from the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DESC0012702.


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Table of Contents


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Scheme 1. Possible reaction mechanisms of glucose conversion to HMF, catalyzed by Lewis acids: (A) isomerization-dehydration mechanism and (B) direct dehydration mechanism. 121x86mm (300 x 300 DPI)



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Figure 1. 2H NMR spectra of reaction products synthesized from D-labeled glucose by PO4/NU(half). Reaction conditions: 100 mM D-labeled glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 413 K; 5 h; 440 rpm. The populations of D-labeled HMF in total HMF product are 42% for glucose-1-d1 and 53% for glucose-2-d1. 67x54mm (300 x 300 DPI)


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Figure 2. Kinetic isotope effect on glucose conversion by PO4/NU(half). Reaction conditions: 10 mM glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 373 K; 440 rpm. The ratios of rate constant (i.e., kH/kD) are calculated to be 1.03 ± 0.10 for glucose-1-d1 and 1.76 ± 0.17 for glucose-2-d1. 64x48mm (300 x 300 DPI)



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Figure 3. Initial glucose concentration effect on HMF yield. Reaction conditions: glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); PO4/NU(half) 10 mg; 413 K; 440 rpm. The detailed reaction results involving glucose conversion and fructose yield are summarized in Table S1 in the Supporting Information. 73x63mm (300 x 300 DPI)


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Scheme 2. Plausible poisoning function of phosphate species grafted on zirconia-cluster node of NU-1000. 37x16mm (300 x 300 DPI)



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Figure 4. Glucose conversion to HMF by bare and phosphate-modified bulk zirconia materials. Reaction conditions: 1 mM glucose solution (1 mL, water/2-PrOH = 1:9 (vol/vol)); catalyst 10 mg; 413 K; 7 h; 440 rpm. 62x30mm (300 x 300 DPI)


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Table of Contents 38x17mm (300 x 300 DPI)


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