Matching the Activity of Homogeneous Sulfonic Acids: The Fructose-to

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Matching the activity of homogeneous sulfonic acids: the fructoseto-HMF conversion catalyzed by hierarchically porous sulfonicacid-functionalized porous organic polymer (POP) catalysts Matthew Du, Ananya M. Agrawal, Sanjiban Chakraborty, Sergio J. Garibay, Rungmai Limvorapitux, Baikleem Choi, Sherzod Madrahimov, and SonBinh T. Nguyen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05720 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Sustainable Chemistry & Engineering

Matching the activity of homogeneous sulfonic acids: the fructose-to-HMF conversion catalyzed by hierarchically porous sulfonic-acid-functionalized porous organic polymer (POP) catalysts Matthew Du, Ananya M. Agrawal, Sanjiban Chakraborty, Sergio J. Garibay, Rungmai Limvorapitux, Baikleem Choi, Sherzod T. Madrahimov, and SonBinh T. Nguyen* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 602083113, USA. Email: [email protected] KEYWORDS: 5-hydroxymethylfurfural (HMF), fructose, biomass, hierarchical porosity, porous organic polymers ABSTRACT

Three HO3S-functionalized porous organic polymers (HO3S-POPs) with high surface areas (500700 m2/g) and a broad range of porosity profiles were synthesized and tested against homogeneous-acid analogs and commercially available acid resins to evaluate their relative catalytic activities in the acid-catalyzed conversion of fructose to HMF. Comparison of fructose conversions and HMF yields demonstrates that the sulfonated POPs with hierarchical porosity

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can achieve catalytic activities that rival those of their homogeneous counterparts.

The

associated HMF selectivities represent optimized values that increase with higher temperature and faster heating, both of which can reduce the reaction time and limit product decomposition. Due to their intrinsically high mesoporosity and number of accessible acid sites, these HO3SPOPs also outperform the commercially available Amberlyst 15 resin catalyst and its crushed variant.

INTRODUCTION The negative consequences of mining fossil resources, such as contribution to climate change and lack of renewability, have accelerated the search for sustainable alternatives. As a result, a tremendous amount of research effort1-7 has been focused on the development of catalytic systems for the environmentally friendly and economically viable conversion of renewable biomass to value-added chemicals that can be transformed into everyday commodities.8 An example of such a chemical is 5-hydroxymethylfurfural (HMF), which is a versatile precursor to pharmaceuticals,9 plastics,10 and 2,5-dimethylfuran (2,5-DMF; a potential sustainable gasoline substitute that addresses the downsides of ethanol, the current market-leading alternative).11-12 To date, extensive progress has been made on designing methods for the efficient preparation of HMF via the acid-catalyzed dehydration of feedstocks such as monosaccharides, polysaccharides, and even raw biomass.9, 13-28 Among these starting materials, fructose has been the most commonly investigated as a model feedstock,18 and its conversion to HMF (Eq 1) has been tested against a plethora of acid catalysts.9,

13-14, 16-17, 19, 22-24, 26

While homogeneous

Brønsted acids, such as methanesulfonic acid (MSA) and p-toluenesulfonic acid (PTSA), generally afford excellent activity and selectivity for this reaction,14, 16 they can present major drawbacks including corrosion to reaction vessels and difficult product separation.16-17,

29

As


2

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such, recent studies have placed a significant focus on employing heterogeneous catalysts for reaction 1. HO

O

OH OH

acid catalyst

O

OH O

OH HO fructose

(1)

5-HMF

For the fructose-to-HMF conversion, the catalytic activities of solid acids have been positively correlated with the following factors: acid density,10, particle mesh size,35 surface area,10,

36-39

30-34

number of external acid sites,30-31

and distribution of larger pores.30,

37-38, 40-43

These

factors are essentially reflections of the accessibility of the acid sites to the fructose substrate.10, 30-31, 33, 38

Given the emergence of porous organic polymers (POPs) with high surface areas and

good accessibilities to a broad range of molecules,44-46 we were curious if their sulfonated analogs, sulfonic acid-functionalized porous organic polymers (HO3S-POPs), can be designed to match the activities of MSA and PTSA in reaction 1. While such materials have been shown to be good catalysts for HMF syntheses,39, 47 it is not clear that all of the acid sites are accessible for reaction 1, especially because the pore sizes of some of these materials48-49 are much smaller than the size of fructose (8.5 Å)30 and its hydrate (9.8 Å).50 Thus, we hypothesize that if POPs can be designed to possess a hierarchy of micro- and meso-pores, they will have activities that rival those for homogeneous sulfonic acids, arguably the best catalyst to date for these reactions. Herein, we report a study that positively correlates catalytic performance against acid-site accessibility in three aromatic-SO3H-functionalized POPs:

the hierarchically porous highly

crosslinked polymer (HCP) S1,51 the 1,4-diethynylbenzene-derived S2, which can be engineered to have high density of mesopores through an aerogel template approach,46 and the microporous aromatic framework (PAF) S352 (Figure 1). With high-temperature and short-time heating, which greatly reduces side reactions, all three HO3S-POPs afford superior catalytic activity in the


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fructose-to-HMF conversion than Amberlyst 15 (A15), a commercially available macroporous polystyrene-based cationic exchange resin.53 In addition, the most mesoporous POPs S1 and S2 were found to be as effective as the soluble-sulfonic-acid (MSA, PTSA, and benzenesulfonic acid (BSA)) positive controls. We attributed the high activities of these HO3S-POPs to the presence of hierarchical porosity, which renders their acid sites highly accessible, as verified by titration. While POP S3 is not hierarchically porous, its high surface area compensates and it can still outcompete the low-surface-area A15. These results suggest that hierarchical porosity and high surface area are important design parameters to allow for good accessibility of acid sites and thus the attainment of catalytic activities in the fructose-to-HMF conversion that are comparable to the high benchmarks observed for the homogeneous sulfonic acids.

SO3H

SO3H

HO3S SO3H

HO3S

SO3H

HO3S

HO3S

SO3H

HO3S

C

SO3H

HO3S

SO3H

HO3S

SO3H SO3H

HO3S

HO3S S1

S2

S3

A15

Figure 1. Schematic drawings of the pore structures of the HO3S-functionalized POPs that were used in this work and A15 resin. The molecular structures of these materials are shown at the bottom. EXPERIMENTAL SECTION


4

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Materials and instrumentation. See Section S1 in the Supplementary Information (SI) for descriptions of instrumentation, sources of materials, general procedures, and additional characterization data for the catalysts. Phenyl-functionalized silica aerogel (0.1 g) was presynthesized inside a 20 mL scintillation vial according to a literature procedure.46 Catalyst synthesis. See Section S2 in the SI for detailed descriptions of the syntheses of catalysts S1 and S2. S1. This POP was synthesized following a combination of literature protocols (Scheme 1).46, 54-55

Vinyl benzyl chloride, divinylbenzene, the initiator AIBN, and the porogenic solvent 1,4-

dioxane were combined and the copolymerization was carried out under air-free conditions inside a pre-synthesized phenyl-functionalized aerogel46 matrix. After the removal of the silica gel template through HF etching, the isolated hyper-crosslinked precursor copolymer was processed through a supercritical CO2 drying step.46 The resulting POPs was then sulfonated and reprocessed again under supercritical CO2 to afford S1. Scheme 1. Synthesis of POP S1.


5


+ Cl

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1) FeCl3, DCE, 80 °C 2) HF etching

AIBN 1,4-dioxane 60 °C Cl Cl

Cl

3) Supercritical CO2 processing

inside a phenylfunctionalized silica aerogel matrix

1) ClSO3H, DCM, rt 2) Supercritical CO2 processing

SO3H SO3H

SO3H

SO3H

HO3S

S1

S2. The synthesis of this POP (Scheme 2) comprises two steps: synthesis of the unsulfonated precursor over a silica aerogel matrix followed by sulfonation.

In the first step, the

polymerization of 1,4-diethynylbenzene inside a presynthesized phenyl-functionalized silica aerogel matrix was carried out as previously reported for the material named

scpPOP1|

46

a-Ph.

After the copolymerization, the Co catalyst was removed but the silica aerogel matrix was retained with the POP.

This composite was then combined with chlorosulfonic acid to

accomplish the sulfonation. The crude product was treated with HFaq to remove the aerogel and processed with supercritical CO2 to yield S2. Scheme 2. Synthesis of POP S2.

Co2(CO)8 1,4-dioxane reflux inside a phenylfunctionalized silica aerogel matrix

1) ClSO3H, DCM, rt 2) HF etching

SO3H

3) Supercritical CO2 processing HO3S

SO3H

HO3S S2


6

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S3. PAF-1, the unsulfonated form of S3, was synthesized according to a reported literature procedure.45 The sulfonation of PAF-1 was carried out using a modified version of the reported procedure for the material named PPN-6-SO3H.52 In a typical experiment, PAF-1 (0.1 g), chlorosulfonic acid (1 mL), and dichloromethane (15 mL) were combined together in a 30 mL vial equipped with a magnetic stir bar. The vial was capped and the reaction mixture was stirred at room temperature for 3 d before being filtered over a Büchner funnel. The collected solid was, washed over the funnel with water (25 mL × 3) and dichloromethane (10 mL × 3), and dried overnight under vacuum. Yield = 0.06 g. Catalytic reactions. Additional details of the experimental procedures can be found in the SI, Section S3. Dehydration of fructose to HMF. A broad range of reaction conditions (SI, Table S4) were explored for the conversion of fructose to HMF using 2-5 mL microwave vials equipped with appropriately sized magnetic stirbar. For each reaction, an aliquot of a premade aqueous fructose solution (see SI, Section S3 for preparation details) and an aliquot of an organic solvent (and ultrapure deionized water if needed) were combined in the 2-5 mL microwave vial to obtain a 3.75 mL volume of a mixture with the desired water:organic ratio and fructose concentration. Then, the catalyst (either an aliquot of the premade homogeneous acid solution or an amount heterogeneous acid catalyst) was added as necessary to achieve the desired catalyst loading. The vial was crimp-capped and the reaction was carried out at the desired temperature and time in either an oil bath that has been preheated to the desired temperature or in the microwave reactor. Depending on the catalyst phase (homogeneous or heterogeneous) and heating source (oil bath or microwave reactor), the appropriate protocol was applied to cool and sample (300 μL) the reaction mixture.


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Product analysis. An aliquot (~2 μL, excess in comparison to the amount of the acid catalyst) of triethylamine (TEA) was added to the 300 μL sample to neutralize the acid catalyst and the mixture was concentrated to dryness under vacuum at 50 °C using a rotary evaporator. The resulting residue was dissolved in an aliquot (1 mL) of a premade solution of dodecane in pyridine, charged with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) (100 μL), and heated in a sealed vial at 60 °C for 30 min to complete the derivatization of fructose, HMF, and byproducts.56 This mixture was then analyzed via GC. Fructose conversion and HMF yield were determined using an internal-standard/calibrationcurve method, with dodecane as the internal standard (SI, Figures S2b and S3b, respectively). For these calculations, we assume that the reaction volume does not change over the course of the experiment (i.e., no loss due to evaporation, and the amount of water generated from the dehydration of fructose (~0.13 mL maximum) does not significantly affect the volume). Titrations.

Back titration of the HO3S-functionalized solid catalysts (both POPs and

Amberlyst) were conducted in a non-aqueous acetonitrile (ACN) environment following a literature protocol.57 In a typical titration experiment, a mixture of the catalyst (~10 mg) and a [base + indicator] solution (20 mL) in ACN (25 mM base + 0.02 mM p-naphtholbenzein) was stirred vigorously at rt for 4 h. The resulting mixture was then filtered through a 0.2 µm PTFE syringe filter. Three aliquots (3 × 5 mL) of the filtrate were back-titrated using a 0.1 M solution of perchloric acid in acetic acid. RESULTS AND DISCUSSION As model catalysts, we selected HO3S-POPs S1, S2, and S3,46, 54, 58 which all have aromatic sulfonic acid moieties (Figure 1) but different porosity profiles (Table 1, cf. entries 1-3), to allow for comparison and correlation between activity and acid-site accessibility. As a solid-catalyst


8

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control, we employ A15, which has Brunauer-Emmett-Teller (BET) area that is much lower than those of the POPs (Table 1, cf. entries 1-3 vs. 4), as well as A15c, a 120-230 mesh crushed sample of A15. For homogeneous controls, we used MSA, PTSA, and BSA to cover the small range of pH differences among the sulfonic acids that are present in our experiments. Table 1. Porosity data and sulfonic-acid density for the HO3S-functionalized POPs, A15, and A15c. Total Micropore Micropore BET pore surface Entry Catalyst areaa volumec volumeb 3 -1 areac (cm g ) (m2g-1) (cm3g-1) (m2g-1)

External surface areac (m2g-1)

External surface area fraction

Sulfonicacid density (mmol/g)

1

S1

550

0.67

0.082

157

393

0.71

3.74

2

S2

693

0.84

0.124

271

422

0.61

3.35

3

S3

650

0.37

0.198

508

142

0.22

4.14

4

A15

40

0.12

0.002

4

36

0.90

4.7d

5

A15c (120230 mesh)

40

0.19

0.001

2

38

0.95

4.7d

aCalculated

using the Brunauer-Emmett-Teller (BET) model and adsorption data in the region P/P0 = 0.005-0.1. bCalculated as the sum of micropore volume and volume of larger pores, where the latter is calculated from adsorption data up to P/P0 = 1. cCalculated from N2 adsorption data using conventional t-plot methods (SI, Section S1). The values were selected over the t range of 3.5-5 Å by fitting the data to the Broekhoff-De Boer thickness equation that affords a physically sensible micropore volume while maintaining a correlation coefficient that is closest to 1. dBased on data from Rohm and Haas as quoted by Siril et al.53

Synthesis and characterization of HO3S-functionalized POPs. Each of the three POP catalysts (Figure 1) employed in reaction 1 was prepared by reacting the unsulfonated material with ClSO3H in DCM at room temp. The synthesis of the unsulfonated precursor45 to S3 as well as S3 itself52 have been reported previously and both materials were shown to be primarily


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microporous. Key to the synthesis of the unsulfonated precursor for S1 are silica-aerogeltemplating and supercritical-CO2-processing steps,46 whose combination was previously reported by us to result in POPs with large proportions of mesoporosity (i.e., large fractions of their total surface areas being external surface areas). While the unsulfonated precursor for S2 has also been made using either one or both of those steps,46 in this work we only employed the silicaaerogel-templating step to keep its proportion of mesoporosity below that of S1.

Post-

sulfonation, both S1 and S2 were subjected to supercritical-CO2 processing, which preserves the mesoporosity of both materials. As a result, while the BET area remains relatively constant for the three materials, the proportion of mesoporosity (e.g., fraction of external surface area), consistently decreases across the series (Table 1, cf. entries 1-3). The differential pore volume (DPV) profiles for these three materials (Figure 2) is consistent with this trend. The BET area and total pore volume (TPV) data for all three POPs clearly show that they are much more porous relative to A15 (Table 1, cf. entries 1-3 vs. 4). While our observed sulfonic acid loading for S3 is only slightly higher than that previously reported,52 its BET area and TPV are about ½ of the literature values (SI, Table S1).52 Nevertheless, S3 still has acid density and BET area that are comparable to those for S1 and S2, which allows us to assess the effect of decreasing mesoporosity in the catalysis of reaction 1 (Table 1, cf. entries 1 and 2 vs. 3). That the BET area of S1 is smaller than that reported for a similar material51 can be explained by its higher (3-4 fold) acid loading. In addition, the diminished micropore volume measured for S1 in this work is attributable to the use of a templating silica aerogel.46 As mentioned above, we also prepared A15c (SI, Section S1) as a control catalyst to verify that accessibility is positively correlated with HMF yields.35 The BET area of this crushed variant does not differ from that of the original material, consistent with previously reported


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measurements59 and its strong dependence on the degree of cross-linking31 (and not necessarily particle size).

Furthermore, the DPV profiles for both A15 and A15c show very little

microporosity, consistent with their macroreticular nature. It is also interesting to note that while the DPV profiles of S3, A15, and A15c clearly shows similar levels of “larger pores” (Figure 2B), S3 clearly has much more micropores (Figure 2A) and thus should be a better catalyst if its sulfonic acid moieties can be accessed. While the smaller particle sizes and larger interparticle pores in A15c should facilitate better accessibility of the substrate to the acid sites than those in A15,60 we do not expect them to be as effective as the intrinsic mesopores that exist in S1 and S2, both of which have external surface areas that are one order of magnitude larger than those for A15 and A15c (Table 1, cf. entries 1 and 2 vs. entries 4 and 5). A

5 4.5 Diff. Pore Vol. (dV/dW,(cm3/g·Å))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3.5 3

S1

2.5

S2

2

S3 A15

1.5

A15c

1 0.5 0 5

45

85

125

165

205

245

Pore width (Å)


11


B

0.8 0.7 Diff. Pore Vol. (dV/dW,(cm3/g·Å))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.5

S2

0.4

S1

0.3

A15 5 A15c

0.2

S3

0.1 0 5

45

85

125

165

205

245

Pore width (Å)

Figure 2. (A) Differential pore volume (DPV) plots for the HO3S-functionalized POPs, A15, and A15c. (B) A vertically expanded version of (A) to accentuate differences in the larger-pore region.

Catalytic reactions. Selection of solvent composition. To evaluate the catalytic performance of the POPs and control materials A15 and A15c, we chose to compare the resulting HMF yields and selectivities for the reaction in the presence of a 4:1 v/v water:dioxane mixture at high fructose conversion with all other reaction conditions being identical. Such a solvent system was selected after extensive screening (SI, Table S4), given the excellent miscibility and close boiling points (101 and 100 °C, respectively) of the two components. Moreover, this solvent mixture completely solubilizes fructose at room temperature, has low reactivity with the HMF product (in contrast to alcohols, which can react with HMF to form HMF ethers61-63) and can minimize the hydration of HMF to other common byproducts, such as levulinic acid and humins.64-65 These


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advantages can increase HMF selectivity and allow for a much simplified analysis of the reaction mixture. Indeed, when reaction 1 was carried out in water:dioxane mixtures, ~60 and ~80% HMF yields have been obtained from fructose66 and 90% high-fructose corn syrup (HFCS 90),67 respectively. Although the presence of water can partially facilitate the hydration of HMF to levulinic acid and humins,64-65, 68 carrying out the reaction in pure dioxane has been reported to afford low (~30%) HMF yields,69-73 presumably due to the low solubility of the fructose starting materials in this solvent.73,74 Optimized conditions. Before comparing the effectiveness of the solid acids in converting fructose to HMF, we optimized the reaction for MSA (pKa = -1.9)75 to find the conditions that would afford near-complete conversion of fructose.

MSA was selected as the benchmark

catalyst given its ready availability, strong acidity, low molecular weight, and excellent solubility in both organic solvents and water. We expected the homogeneous MSA to have the fastest kinetics in reaction 1 relative to those for any solid catalyst of comparable acidity. This would set the “upper limit” in the short reaction time that we imposed under microwave heating (see below) for complete conversion and avoid potential concerns about drops in selectivity due to side reactions of the HMF product upon either prolonged heating or past completion. Such a homogeneous positive control has been used by Schüth and coworkers as an upper limit in a systematic study of reaction 1 catalyzed by resin-bound sulfonic acids.31 Carrying reaction 1 to near-completion also greatly simplifies its analysis as HMF and fructose are the only two major species that need to be derivatized prior to gas-chromatographic separation. In this manner, the HMF selectivities for our solid catalysts can be directly compared against those of the homogeneous controls without the need to account for the various reactive intermediates, such as difructose anhydrides (SI, Scheme S1)67,

76-77

that interconvert with


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fructose under acidic conditions. Indeed, the HMF selectivity in our study linearly increases with respect to fructose conversion until the sugar has almost completely disappeared (SI, Figure S6), consistent with literature report for reactions occurring in mixtures of water:dioxane78 ,79 or water:THF.77 The parameters (temperature and time, solvent composition, catalyst loading, mode of heating, initial fructose concentration) for reaction 1 were first optimized with MSA to obtain the highest selectivity at near-complete conversion (Eq 2; see also SI, Table S4). Consistent with our expectation of reaching an upper limit with high-temperature and fast heating, the highest temperature experiments are the ones with the best combination of HMF selectivity and fructose conversion (SI, Figure S6). Microwave heating was selected over conventional oil-bath heating due to its well-known increased reaction rate80 and thus lower tendency for generating side products (SI, Table S4, cf. entries 20-26 vs. 31-37 and 27-30 vs. 38-44 for reactions carried out via oil-bath vs. microwave heating at 120 and 140 ˚C, respectively). Under the optimized conditions (15 min of microwave heating at 140 ˚C in the presence of 1 mol % of sulfonic acid, SI, Table S4, entry 49), we also carried out reaction 1 in the presence of PTSA (pKa = -2.55)81 and benzenesulfonic acid (BSA, pKa = -2.8),75 the homogeneous analogs of the heterogeneous catalysts studied in this paper. The observed reactivities and HMF selectivities for these other soluble acids are essentially the same as those for MSA (Figure 5), suggesting that small differences in acidities do not play a significant role in the reaction with our exact conditions. We note in passing that even under the best conditions, the HMF selectivity rarely rises above 70%, including at the highest yield (SI, Figure S7) and full conversion (SI, Figure S6), consistent with literature reports for dioxane:water mixture66-67 that this is an intrinsic problem with the sulfonic-acid-catalyzed fructose-to-HMF conversion. The yellow-brown color of the completed


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reaction mixtures also indicates humin formation although other low-molecular-weight products can be observed via GC analysis. Comparisons of solid catalysts. When reaction 1 was carried out in the presence of our HO3Sfunctionalized solid catalysts and under the optimized conditions (Eq. 2), we observed quantitative conversions with catalysts S1 and S2 (Figure 4). Notably, their HMF selectivities (and thus yields) are quite similar to those catalyzed by the homogeneous acids. POP S3 is slightly less effective (Figure 4), presumably due to the lower accessibility of its acid sites: it has much lower TPV than S1 and S2, and its BET area is mostly attributed to the presence of micropores (Table 1, cf. entries 3 vs 1 and 2). Indeed, the DPV plot for S3 (Figure 2b) shows a major peak ~9 Å, which is only slightly larger than fructose30 and smaller than its hydrated form.50 In contrast, S1 and S2 have pores primarily in the mesoporous regime (2-50 nm, Figure 2b) that should be more accessible to both fructose and its hydrate. Indeed, both fructose conversion (SI, Figure S4) and HMF yield (SI, Figure S5) appear to correlate well with the fraction of external surface area for the three POPs. In addition, with fast microwave heating at 140 °C, the HMF selectivities for our S1 and S2 catalysts become very similar to that of the homogeneous MSA catalyst (Figure 3). This is in contrast with S3, A15, and A15c, where the selectivity is successively reduced. Among these latter three materials, S3 is the best catalyst, presumably due to its high BET area, and A15c is much better than A15, consistent with its smaller particle size and thus better acid-site accessibility. Our results are consistent with a recent report by Schüth and coworkers where the resins with the most accessible acid sites, whether surface-sulfonated or swellable, can be very similar to PTSA in both fructose conversion and HMF selectivity.31


15


80 70

S1

S3 S2

HMF selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

60 A15c 50

A15 140 C, oil bath, homogeneous, 1 mol %

40

120 C, oil bath, homogeneous, 5 mol % 30 140 C, MW, heterogeneous, 1 mol % 20 10 0 0

20

40

60

80

100

Fructose conversion (%)

Figure 3.

Plot of HMF selectivity as a function of fructose conversion for reaction 1

contrasting the performance of the solid catalysts with that of the homogeneous MSA catalyst at 1 mol % loading and 140 °C heating. For comparison, data (blue squares) were also included for the MSA-catalyzed reaction at 5 mol % loading and 120 °C heating over a range of conversion.

For each data point associated with a

heterogeneously catalyzed reaction, the label indicates the name of the catalyst utilized and the error bar shown is derived from data for at least three trials; however, the error bars for some cases may be too small to be readily seen. Reaction conditions: volume = 3.75 mL of a 1:4 v/v mixture of water:dioxane, [fructose] = 112 mg/mL (0.6242 M), heating modes = either microwave (MW) or oil-bath. The black line is intended to serve only as a guide for the eyes.

HO

O

OH OH

OH HO fructose

140 °C, 15 min microwave heating 1 mol % -SO3H dioxane:H2O 4:1

O

OH O

(2)

5-HMF


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HMF Yield

HMF Selectivity

Fructose Conversion

100 90 Yield/Selectivity/Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50 40 30 20 10 0 MSA PTSA BSA

S1 S2 Catalyst

S3

A15c A15

Figure 4. Effect of catalyst choice on HMF yield and selectivity and fructose conversion for reaction 2. Temperature = 140 °C under microwave heating, reaction volume = 3.75 mL of a 1:4 v/v mixture of water:dioxane, [fructose] = 112 mg/mL (0.6242 M), [sulfonic acid] = 1 mol % (0.0062 M). For each catalyst, reaction 2 was stopped at 15 min, the near-completion point for the benchmark MSA-catalyzed reaction. Except for BSA, where only one measurement was obtained, the error bars for each catalyst were determined from the data obtained for at least three trials. We note in passing that given the high temperature and the short time of the reaction, as well as the relatively high pressure in the microwave reaction vessel, we were not able to carry out kinetic measurements to assess catalyst lifetime and productivity82 in a safe and practical manner. Moderate leaching due to aromatic desulfonation and humin formation did occur, both of which contribute to slight degradation of catalyst performance (SI, Section S3 and Figures S8-S9). Further supporting our hypothesis that the higher accessibility of the sulfonic acid sites leads to better catalytic activity for reaction 2 is the striking contrast in HMF selectivities, HMF yields, and fructose conversions in the reactions catalyzed by A15 and its crushed version A15c (Figure


17


4). Interestingly, while the porosity of A15 and A15c are quite similar (Table 1, cf. entries 4 vs. 5), the HMF yield for the A15c-catalyzed reaction is twice that of the A15-catalyzed control at only 20% excess conversion (80% vs. 60%, Figure 4). This performance trend is in agreement with a previous study of A15-catalyzed fructose-to-HMF conversion,35 where the HMF selectivity at full fructose conversion in DMSO was observed to increase as the particle size of the catalyst was reduced via crushing. The crushing of A15 must have made more acid moieties available in the very-large-pore regime, consistent with an increase in the hysteresis loop in the 0.9 ≤ P/Po ≤ 1 region (SI, Figure S1). That the HMF-selectivity/fructose-conversion data for all of the solid acids in this study follows the same trend as those observed for the MSA-catalyzed reactions (Figure 3) further supports the notion that the main difference between solid-state sulfonic acids and their homogeneous analogs is the level of substrate accessibility. We note in passing that a moderate loss of active species due the well-known aromatic desulfonation reaction does occur, as shown by titration experiments (SI, Section S3 and Figures S8-S9). Lutidine

Collidine

Pyridine

100.00

Acid Stie Accessibility (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80.00

60.00

40.00

20.00

0.00 S1

S2

S3 Catalyst

A15c

A15


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Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Accessibility of acid sites determined by back titration using either 2,6-lutidine, 2,4,6collidine, and pyridine. For each catalyst, the acid density corresponding to 100% acid-site accessibility is that obtained from the acid density data (Table 1). Error bars for each catalyst were determined from the data obtained for at least three trials but are too small to be clearly visible (see SI, Table S3 for average and standard deviation of acid-site accessibility for each catalyst). The difference in accessibility of the acid sites in our POPs can be qualitatively delineated through titration with pyridine, 2,6-lutidine, and 2,4,6-collidine (Figure 5).

As pyridine is

relatively small (kinetic diameter = 5.7 Å)83, it can access most of the acid sites in our POPs and Amberlyst controls. In contrast, the bulkier bases 2,6-lutidine (kinetic diameter = 6.7 Å)83 and 2,4,6-collidine (kinetic diameter = 7.4 Å)83 will not sample the most sterically hindered sites. That these bulkier bases sample about 90-95% of the acid sites for S1 and S2, but only 80-90% for S3 and the Amberlyst controls, are consistent with the observed trends in our catalysis data (Figure 4): S3, A15, and A15c are qualitatively less effective than S1 and S2 for reaction 1. Similar correlation of acid accessibility and catalytic activity have also been reached for a comparative study of several carbon84 and clay-based40 catalysts for the convert glucose to HMF.

CONCLUSION In summary, our study of the fructose-to-HMF reaction suggests that good catalytic activities and HMF selectivities can be achieved with HO3S-functionalized POPs. HO3S-POPs S1 and S2, with their intrinsically large total and external surface areas and large (> 20 Å) pores, afford catalytic performances comparable to those of the homogeneous sulfonic acids. While HO3SPOP S3 is slightly less effective, its catalytic performance is still superior to those of Amberlyst


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15 and its crushed variant. Ultimately, the key to good HMF yield and selectivity for HO3S-POP catalysts lies in the good accessibility of their aromatic sulfonic acid sites, as observed by Schüth and coworkers for macroreticular resins.31 If the intrinsic challenges with aromatic-sulfonic-acid catalysts in fructose-to-HMF conversion (desulfonation of the Ar-SO3H moiety at high temperature53 and humin formation64-65) can eventually be addressed, POP-based materials can be engineered to achieve catalyst activities that are comparable to those for homogeneous sulfonic acids.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. Materials and instrumentation, POPs characterization data (N2 isotherms, SEM), and catalysis data (GC, GC-MS). (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. M.D., A.M.A., S.T.M., S.C., and S.J.G. and S.T.N. conceived the experiments presented herein. M.D. carried out all catalysis experiments except for some homogeneous reactions, which were carried out by B.C. A.M.A. carried out the titration experiments. A.M.A., S.C., and S.J.G. carried out the synthesis of the POPs. A.M.A., S.C., S.J.G., and R.L. carried out the BET adsorption measurements. M.D. wrote the initial draft of the paper and received feedback from all co-authors. M.D. and S.T.N. finalized the paper. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This material is based upon work supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences. S.T.N. acknowledges additional support from DTRA (HDTRA1-14-1-0014). M.D. and A.M.A. thank Northwestern University for summer and academic-year research grants. R.L. was partially supported by the Chemical Sciences, Geosciences, and Biosciences Division, US Department of Energy through a grant (DE FG02-03ER15457) to the Institute of Catalysis for Energy Processes (ICEP) at Northwestern University (assistantship for R.L.). This work made use of the Clean Catalysis Facility of Northwestern University Center for Catalysis and Surface Science. Experimental facilities at the Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University are supported by the International Institute for Nanotechnology (IIN, partially supported by the NSF NSEC program, NSF EEC-0647560); the Keck Foundation; and the State of Illinois (through the IIN). ICP-OES analyses were carried out at the Quantitative Bio-element imaging center (QBIC) at NU. We thank Profs. Joseph T. Hupp, Omar K. Farha, and Mercouri Kanatzidis for the use of the adsorption instruments. SYNOPSIS. For the catalytic fructose-to-HMF conversion, hierarchically porous HO3Sfunctionalized POPs can achieve activities and selectivities equal to those for homogeneous sulfonic acids. REFERENCES


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TOC GRAPHIC HO

O

OH OH

HO OH fructose

POROUS ORGANIC POLYMER (POP)

OH O

SO3H SO3H HO3S

LARGE EXTERNAL HO3S SURFACE AREA O HO3S

FAST, SELECTIVE HMF

ACCESSIBLE ACID SITES SO3H

HO3S

For the catalytic fructose-to-HMF conversion, hierarchically porous HO3S-functionalized POPs can achieve activities and selectivities equal to those for homogeneous sulfonic acids.


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Matching the Activity of Homogeneous Sulfonic Acids: The Fructose-to

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