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Heterogeneous catalytic epimerization of D-glucose to D-mannose by a tin-organic framework

Valérie Toussainta, Loretta Pavlisa, Pia Großa, Samantha F. Ausmanb, Susannah L. Scottb, Karin Föttingerc and Irina Delidovich*a
aInstitute of Chemical, Environmental and Bioscience Engineering, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria. E-mail: irina.delidovich@tuwien.ac.at
bDepartment of Chemistry & Biochemistry and Department of Chemical Engineering, University of California, Santa Barbara, California 93106, USA
cInstitute of Materials Chemistry, TU Wien, Getreidemarkt 9, 1060, Vienna, Austria

Received 18th July 2025 , Accepted 16th September 2025

First published on 17th September 2025


Abstract

Monosaccharide D-mannose (D-Man) is of great interest in the food and pharmaceutical industries as a low-calorie sweetener and precursor for D-mannitol and medicaments. Nowadays, large-scale production of D-Man remains challenging due to a lack of efficient chemo-catalytic processes using D-glucose (D-Glc) as educt. In this work, heterogeneous catalytic epimerization of D-Glc to D-Man by a tin-organic framework (Sn-OF-1) was achieved. The reaction kinetics were explored using both conventional methods and time-resolved operando MAS 13C NMR spectroscopy. Under optimized reaction conditions (100 °C, 5 wt% D-Glc, 20 mg(cat) g−1(EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O)), epimerization yielded a 77[thin space (1/6-em)]:[thin space (1/6-em)]23 equilibrium mixture of D-Glc[thin space (1/6-em)]:[thin space (1/6-em)]D-Man after 1.5 h reaction time. Most D-Glc (ca. 73%) was recovered from the obtained mixture of saccharides by crystallization from EtOH[thin space (1/6-em)]:[thin space (1/6-em)]MeOH (4.7[thin space (1/6-em)]:[thin space (1/6-em)]1). The remaining mixture of D-Man and D-Glc was separated via adsorption on CaY zeolite, resulting in a stream containing D-Man at 70% purity.


1. Introduction

Processing of biomass-derived feedstocks is expected to expand in the near future, driven in part by the growing demand for specialty monosaccharides in the food and pharmaceutical industries. In this regard, the production of D-mannose (D-Man) is of considerable interest. D-Man can serve as a low-calorie sweetener, being 0.60 and 0.86 times as sweet as sucrose and D-glucose (D-Glc), respectively.1,2 It is also a precursor for antitumor and immunostimulating agents.1 Additionally, D-Man can be converted to D-mannitol, valued for its dental benefits and low hygroscopicity.1,3 D-Mannitol is industrially produced by hydrogenation of D-fructose-containing syrups, often a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of D-glucose and D-fructose yielding a 1[thin space (1/6-em)]:[thin space (1/6-em)]3 mixture of D-mannitol and D-sorbitol.4 Therefore, efficient and cost-effective large-scale production methods for pure D-Man, both in its own right and as a direct precursor for D-mannitol, could be industrially important.

Traditional extractions of D-Man from biomass components such as hemicelluloses or mannans via acid hydrolysis require large amounts of organic reagents and high temperatures. Enzymatic hydrolysis can be conducted under milder conditions, but face challenges such as enzyme deactivation and long reaction times, hindering commercial scalability.1,2,5 Chemo-catalytic epimerization of D-Glc could be a more efficient method for producing D-Man. The highly efficient homogeneous molybdate catalysts discovered by Bilík accomplish this epimerization, resulting in a 75[thin space (1/6-em)]:[thin space (1/6-em)]25 mixture of D-Glc to D-Man.6 Formation of D-Man from D-Glc in 17% yield in a phosphate buffer solution under subcritical conditions was reported.7 Homogeneous catalysis poses challenges for catalyst recycling and reuse. In heterogeneous systems, Kegging-type phosphomolybdate catalysts8 and commercial zeolite support molybdenum9 enable ca. 32.5% yield of D-Man with 94% selectivity. Although several solid molybdenum-based catalysts were developed, high leaching is observed, leading to low long-term stability and reduction of the active Mo species.10–12 In the presence of solid Ca–Al mixed oxides, 65% D-Man yield at 73% selectivity was reported. However, the mixed oxides exhibit leaching of the metal species, promoting degradation of monosaccharides.13 Porous tin-organic frameworks (Sn-OFs)5,6,11,14–18 with Lewis acidic Sn4+ sites connected by organic linkers via covalent tin-carbon bonds (Fig. 1) catalyze heterogeneous epimerization. For example, Delidovich et al. reported that hydrophobic Sn-OFs epimerize D-Glc to D-Man with 21% yield and 91% selectivity in a water-ethanol mixture.19 Sn-OFs catalyze efficient epimerization in water–ethanol solvent, with very low leaching of tin and minor structural changes during reaction. Results of the hot-filtration test support that Sn-OFs catalyzed the epimerization of D-Glc into D-Man heterogeneously.19 The structure of the Sn-OFs was comprehensively examined in the previous work.20 Sn-OFs also show high potential for epimerization of different substrates to produce rare sugars such as L-ribose, D-talose, D-lyxose, and L-quinovose with selectivities ranging from 67–95%.21


image file: d5re00314h-f1.tif
Fig. 1 Structure of Sn-OF-1.20

Separation of the resulting mixtures of D-Glc and D-Man presents a challenge.22 Purification of sugars is commonly achieved using ion chromatography (IC), generally performed with a cation-exchange resin and water as the mobile phase,23,24 but requires long columns and large amounts of eluent.19 Separation on cation exchange resins occurs due to formation of bidentate complexes (axeq) with alkaline earth cations (Kcompl. = 0.1 mol−1 in water) or, possibly, more stable tridentate complexes (axeqax) (Kcompl. = 1–5 mol−1 in water). CaY zeolites with large pores and high Ca2+ ion contents in the α-cages facilitate D-Man separation via complexation.25 In previous studies of chromatographic separation of sugars over sulfonated resins in their Ca2+ forms, bidentate coordination was suggested (Fig. 2).26,27 In addition, the β-D-mannopyranoside isomer (1C4) can form a tridentate complex with Ca2+ via its axeqax OH groups. Since D-Man has a higher number of axeq OH-groups that can coordinate to Ca2+, adsorptive separation over CaY zeolites is a promising alternative to chromatography over ion-exchange resins for purifying D-Glc/D-Man mixtures.


image file: d5re00314h-f2.tif
Fig. 2 Separation of D-Man via Ca2+ complexation in CaY zeolite.

Zeolites have been considered less frequently for separation of saccharides than ion-exchanged resins. Chromatography using zeolites as stationary phases was addressed for separation of D-Glc and D-Fru,28–30 a mixture of glucose-sucrose-sorbitol,31 or fructose oligosaccharides.32 Selective adsorption of sucrose on FAU-type zeolites was reported.33 The group of Buchholz systematically explored adsorption of sugars on zeolites of various frameworks and with different Al[thin space (1/6-em)]:[thin space (1/6-em)]Si ratios.34,35 Adsorption on KX zeolite was proposed for recovery of D-Glc from an aqueous ionic liquid.36 More recently, efficiency of BaX and BaY zeolites as adsorbents for separation of D-Glc and D-xylose was reported.37 Importantly, separation of D-Glc and D-Man has been rarely addressed. For instance, Sherman et al. reported the highest efficiency of CaY, BaX, and BaY zeolites as stationary phase for column separation of D-Glc and D-Man using aqueous eluent.38 To our knowledge, the influence of solvent on adsorption of D-Glc and D-Man onto zeolites has not been reported yet.

This study addresses the conversion of D-Glc to D-Man catalyzed by Sn-OF-1, describing the reaction kinetics and the separation of the resulting equilibrium mixture to recover the D-Man and recycle the D-Glc component.

2. Experimental

2.1. Chemicals

D-Glucose (>99.5%), NaY zeolite, n-BuLi (2.5 M in hexane), 4,4′-dibromobiphenyl, Dowex® 66 free base, anhydrous THF (99.9%), SnCl4 (98%), ethanol (>99.8%), 1-13C-D-glucose (99%), tetraphenyltin (SnPh4), and triphenyltin hydroxide (Ph3SnOH) were purchased from Sigma Aldrich. D-Mannose (>99%) was obtained from Acros Organics. CaCl2 (97%) and tetrahydrofuran (99.8%) were obtained from Fisher Scientific. Amberlyst® 15 (hydrogen form) was purchased from Supelco. Bis(triphenyltin) oxide ((Ph3Sn)2O) was synthesised as previously described.20

2.2. Characterization

X-ray diffraction (XRD) analysis was conducted on a PANalytical MPD Pro with a Cu tube, a BBHD mirror, 0.04 rad Soller collimator, and ½° divergence slit. An accelerator detector was used in scanning line mode for scans between 4 and 80° with a step size of 0.0201° and a counting time of 80.010 s per point. HighScore software was used for analysis.

N2 physisorption isotherms were recorded on a Micromeritics ASAP 2060 at 77 K, after degassing samples at 120 °C under vacuum for 24 h. Brunauer–Emmett–Teller (BET) and t-Plot models were used to determine surface areas. Micropore analysis was performed using the MicroActive software.

Scanning electron microscopy (SEM) images were acquired using a COXEM EM-30AX, with voltages of 5–15 kV. Samples were coated with Au using a COXEM sputter at 9 mA for 200 s.

X-ray fluorescence (XRF) was carried out on a PANalytical Axios. Samples were mixed with Ultra-Wax and pressed into pellets for analysis.

Adsorbed pyridine was used to characterize acid sites. Sn-OF-1 and each of the model tin compounds (Ph4Sn, Ph3SnOH and (Ph3Sn)2O) at 70 °C were heated under high vacuum (0.001 mbar) overnight at 120 °C before pyridine adsorption was carried out under static vacuum (5–10 mbar). The desorption was performed by applying a high vacuum of 0.001 mbar. IR spectra were recorded on a Vertex 70 FT-IR spectrometer equipped with an MCT detector (resolution 4 cm−1). Samples were prepared as self-supporting wafers (2 cm diameter, typically 7–8 mg cm−2).

2.3. Synthesis of Sn-OF-1

Sn-OF-1 was synthesized as previously reported.19 In brief, 4,4′-dibromobiphenyl (3.12 g, 10 mmol) was dissolved in anhydrous THF (200 mL). n-BuLi (8.0 mL, 20 mmol) was added dropwise while cooling the solution at −10 °C, then the solution was stirred for 30 min at −10 °C. SnCl4 (0.58 mL, 5.0 mmol) was added and stirring was continued for 30 min at −10 °C. The solution was allowed to warm slowly to room temperature, then stirred overnight. The white precipitate was filtered, and the solid was washed two times each with THF (200 mL), H2O (200 mL), and EtOH (200 mL). The solid was dried under high vacuum to give Sn-OF-1 as a white powder (96% yield, 2.02 g).

2.4. Catalytic tests and analysis of the liquid fraction

Catalytic epimerization was conducted in 9 mL pressure tubes, each containing 100 mg Sn-OF-1 and 5 mL D-Glc or D-Man solution in 50[thin space (1/6-em)]:[thin space (1/6-em)]50 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O. The reaction was initiated by placing the tubes in a pre-heated oil bath and stirring at 750 rpm. After the experiment, the tubes were cooled in an ice bath, then the reaction mixtures were filtered through a PTFE syringe filter (20/25), and the pH of the samples was measured with a pH electrode (WTW). Prior to HPLC analysis, samples were diluted 10-fold, stirred twice with Amberlyst® 15 (hydrogen form, 400 mg, 30 min) and Dowex® 66 free base (1000 mg, 60 min), then filtered again. HPLC analysis to quantify D-Glc, D-Man, and D-Fru was performed on an Agilent 1200 system with an RID detector and a COSMOSIL Sugar-D column (4.6 mm I.D. × 250 mm). The column was operated at 30 °C with an 80[thin space (1/6-em)]:[thin space (1/6-em)]20 v% acetonitrile eluent at 1 mL min−1.

2.5. Time-resolved operando MAS NMR spectroscopy

Epimerization of 1-13C-D-Glc was monitored using operando Magic Angle Spinning (MAS) NMR spectroscopy on a Bruker Avance NMR spectrometer equipped with an 11.7 T magnet and a triple resonance probe operating at 125.7788 MHz for 13C and 500.2185 MHz for 1H. In a typical setup, 9.4 mg Sn-OF-1, 90 mg H2O, 90 mg EtOH, and 25 mg 1-13C-D-Glc were placed in a 7.5-mm rotor (0.4 mL volume) equipped with Viton o-rings. This ratio of sugar to liquid was chosen to achieve a viscous mixture to ensure rotor stability during spinning with enough liquid for molecular mobility.39,40 After heating and establishing a stable 3 kHz MAS rate, quantitative (5 × T1) 13C MAS NMR spectra were recorded, acquiring 8 scans per spectrum. In 13C direct polarization experiments, a 35 kHz 1H decoupling field was used. The acquisition time was 30 ms for 13C, with a spectral width of 50 kHz. 13C chemical shifts were referenced to adamantane (38.5 ppm). Time-resolved NMR spectra were recorded over 5–6 h at 73, 83, 88, and 92 °C. Relative concentrations of D-Glc and D-Man were determined via signal integration. To estimate rate constants at each temperature, time-dependent changes were modeled by fitting a system of ordinary differential equations using nonlinear least-squares regression in Python.

2.6. Preparation of CaY zeolite and adsorption/desorption tests

Ion exchange of NaY zeolite. Na+ ions in NaY zeolite (15 g) were exchanged with Ca2+ by stirring the zeolite with a 0.2 M CaCl2 solution (300 mL) at 50 °C for 2 h in a 500 mL round-bottom flask. The solid was removed by centrifuge, washed four times with deionized water (200 mL) before drying in an oven at 80 °C overnight. The ion-exchange procedure was repeated once more. Approximately 77% of Na+ were substituted by Ca2+. The elemental composition of the CaY zeolite before and after exchange was determined by XRF analysis.
Adsorption–desorption experiments. Time-dependent adsorption curves were recorded for D-Glc and D-Man (50 mg g−1 sugar solution, 21 mL) in CaY zeolite (3.5 g). Various solvent mixtures (pure H2O, and EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O, 50[thin space (1/6-em)]:[thin space (1/6-em)]50 and 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt%) were screened. Each solution was stirred while aliquots were removed for analysis at 0, 1, 2, 4, 6, 8, 10, and 15 min. Adsorption isotherms were recorded for D-Glc and D-Man individually as well as their mixtures. To record the adsorption isotherms, CaY zeolite (500 mg) and 3 mL of the respective solution were stirred for 2 h at RT. Sugar concentrations in the range of 1 to 50 mg g−1 were screened. CaY zeolite was separated from the solution by filtration through a PTFE syringe filter (0.22 μm) and the supernatant was diluted for HPLC analysis.

CaY zeolite (500 mg) was used to adsorb D-Glc or D-Man (50 mg g−1) in 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (3 mL) at 500 rpm for 2 h at RT. After adsorption, the zeolite was separated via filtration through a PTFE filter (0.22 μm) and either directly desorbed or dried at 80 °C before desorption. Desorption was conducted in 3 mL water at RT upon stirring at 500 rpm for 1 h.

For competitive desorption experiments, sugars from a solution of D-Glc (60 mg g−1) and D-Man (30 mg g−1) in a 67[thin space (1/6-em)]:[thin space (1/6-em)]33 ratio were adsorbed by CaY zeolite (500 mg) suspended in EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt%). The zeolite was filtered then rinsed twice with water (3 mL). Sugars were desorbed by stirring with 3 mL water at 500 rpm and RT. Amounts of adsorbed and desorbed sugars were quantified by HPLC.

2.7. Synthesis and recovery of D-Man

Epimerization of D-Glc to D-Man catalyzed by Sn-OF-1. The catalytic experiment was conducted in a 50 mL round-bottom flask containing Sn-OF-1 (800 mg) and a D-Glc solution (40 g, 5 wt% in a 50[thin space (1/6-em)]:[thin space (1/6-em)]50 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O mixture). The reaction was initiated by heating the flask in a pre-heated oil bath at 100 °C while stirring at 750 rpm for 1.5 h. The flask was then cooled in an ice bath and the solution was filtered through a PTFE syringe filter (20/25). Prior to the HPLC analysis, ionic species were removed from the solution using the previously reported method.41 The resulting sugar solution was stirred twice with Amberlyst® 15 (hydrogen form, 400 mg, 30 min) and Dowex® 66 (free base, 1000 mg, 60 min). Finally, the solution was filtered through a syringe filter (PA 20/25).
Partial crystallization of D-Glc. The reaction solution resulting from epimerization catalyzed by Sn-OF-1 was freeze-dried to yield a sugar syrup. This syrup was dissolved in a mixture of ethanol (4.25 g) and methanol (0.9 g) by heating the solution at 70 °C for 15 min. Seed crystals of D-Glc were added, and the solution was allowed to stand at RT for one week. The crystallized sugar was separated from the solution by decantation, then washed with ethanol and dried under vacuum to obtain purified D-Glc.
Epimerization of crystallized D-Glc. Crystallized D-Glc recovered from the reaction solution was subjected to further epimerization using Sn-OF-1. A solution of D-Glc (3 g, 5 wt%) in a mixture of EtOH and H2O (50[thin space (1/6-em)]:[thin space (1/6-em)]50 wt%) containing Sn-OF-1 (60 mg) was heated at 100 °C for 1.5 h while stirring at 750 rpm. After reaction, the solution was filtered using a PTFE 20/25 syringe filter. D-Glc conversion and D-Man formation were quantified by HPLC. Before analysis, the product mixture was diluted and treated twice with Amberlyst® 15 (hydrogen form, 400 mg, 30 min) and Dowex® 66 (free base, 1000 mg, 60 min), followed by filtration through a PA 20/25 syringe filter.
Recovery of D-Man using CaY zeolite. After crystallization, the remaining EtOH[thin space (1/6-em)]:[thin space (1/6-em)]MeOH solution was removed using a rotary evaporator to yield a sugar syrup. This syrup was dissolved in a 70[thin space (1/6-em)]:[thin space (1/6-em)]30 mixture of ethanol (3.5 g) and water (1.5 g). CaY zeolite (500 mg) was added, and the mixture was stirred at RT for 2 h at 500 rpm. The zeolite was separated via filtration on a frit. The sugar-containing zeolite was rinsed twice with 3 mL water then stirred in 3 mL water for 1 h at 500 rpm and RT to desorb the sugar.

3. Results and discussion

3.1. Characterization of Sn-OF-1

Sn-OF-1 is an X-ray amorphous micro-/mesoporous material19 with a specific surface area SBET of 399 m2 g−1 and a pore volume Vt-Plot of 188 cm3 g−1 (Fig. S1). SEM analysis reveals a dendritic network of flat and rod-like particles with an average size of 300–400 nm (Fig. S2).

Our recent study suggested that the network consists mainly of tetraaryltin Ar4Sn structural units, where “Ar” designates a 4,4′-dibromobiphenyl linker, as well as oxygen-linked triaryltin sites Ar3Sn–O–SnAr3 (the hydrolysed Ar3Sn–OH sites were not detected) and alkylaryltin sites nBuxAr(4−x)Sn (incorporation of nBu groups is a result of the organometallic synthesis). A minor amount of SnO2 (corresponding to ca. 5 wt% of the total tin content) was also identified. Formation of D-Man takes place via the Bilik mechanism, which is accompanied by a 1,2-shift of the carbon skeleton. Our recent results suggest that Ar4Sn and Ar3Sn–O–SnAr3 structural units are the catalytically active sites for epimerization as shown by characterization of Sn-OF-1 in combination with catalytic data. Interestingly, the isomerization of D-Glc into D-Fru in the presence of Sn-OF-1 takes place following both the intramolecular hydride shift and the enediol formation mechanism. The carbon skeleton of D-Fru remains intact, which suggests isomerization of D-Glc into D-Fru and epimerization of D-Glc into D-Man catalyzed by Sn-OF-1 are parallel processes occurring via different mechanisms. SnO2 and nBuxAr(4−x)Sn present minor structural elements of Sn-OF-1 and catalyze the isomerization of D-Glc into D-Fru.20

In this study, the acidity of Sn-OF-1 was analyzed by pyridine adsorption (Fig. S3). The difference spectra, after desorption exhibited the bands at 1608, 1580, 1487, 1477, and 1448 cm−1 which can be attributed to pyridine interacting with Lewis acid sites or with weak Brønsted acid sites (to give hydrogen-bonded Py-H sites) (Table S1).42,43 No strong Brønsted acid sites are present, as judged by the absence of pyridinium bands at 1530–1550 cm−1 (Fig. S4). Molecular Sn(IV) compounds Ph4Sn and (Ph3Sn)2O structurally resembling previously uncovered catalytic active sites along with Ph3SnOH were exposed to pyridine for comparison (Figs. S5 and S6). Notably, Ph3SnOH shows a sharp negative band at 3600 cm−1 due to reaction of its Sn-OH sites.42 Interestingly, this band is not present in the difference spectra of Sn-OF-1. Instead, there is a broad negative band at slightly lower wavenumbers, likely due to residual solvent in the porous structure. Thus, pyridine adsorption corroborates previously reported MAS NMR spectra suggesting the presence of Lewis acidic Sn4+ sites and the absence of Sn-OH sites.20

3.2. Epimerization of D-Glc catalyzed by Sn-OF-1

Kinetic profiles for D-Glc epimerization to D-Man were obtained for various temperatures, catalyst loadings, and reactant concentrations (Fig. S7–S15). Temperatures in the range of 80–100 °C were considered since degradation of the saccharides and lower mass balance was observed at higher temperatures.19 Rates for reactant consumption (Tables S2 and S3) and product formation, determined from the initial slopes of the curves, are summarized in Table 1.
Table 1 Results of screening Sn-OF-1 for epimerizationa
Entry Reactant T [°C] csugar [wt%] mSn-OF-1 [mg] r0,Epi [mol L−1 min−1] TOFb [min−1] Ratio of monosaccharides MB [%]
D-Glc D-Man D-Fru
a r0,Epi is the initial rate of product formation; MB designates the mass balance. Reaction conditions: 5 mL EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solution, 1.5 h, 750 rpm.b Turnover frequencies (TOFs) were calculated using eqn S1.
1 D-Glc 80 10 100 2.5 × 10−3 0.07 88 12 0 93
2 D-Glc 90 10 100 2.9 × 10−3 0.08 85 15 1 95
3 D-Glc 100 10 100 4.8 × 10−3 0.13 86 13 1 94
4 D-Glc 100 2 100 2.0 × 10−3 0.06 71 26 4 91
5 D-Glc 100 5 100 3.6 × 10−3 0.10 76 22 2 92
6 D-Glc 100 15 100 6.3 × 10−3 0.18 82 17 1 89
7 D-Glc 100 10 70 3.9 × 10−3 0.16 81 18 1 91
8 D-Glc 100 10 120 6.3 × 10−3 0.25 80 17 3 92
9 D-Glc 100 10 150 7.5 × 10−3 0.30 80 19 2 90
10 D-Man 100 2 100 2.0 × 10−3 0.06 41 57 2 88
11 D-Man 100 5 100 3.1 × 10−3 0.09 39 59 2 89
12 D-Man 100 10 100 4.4 × 10−3 0.12 30 68 2 91
13 D-Man 100 15 100 5.0 × 10−3 0.14 22 77 1 97


Under all of these conditions, the major reaction is the epimerization. Its rate depends linearly on the catalyst mass (Fig. S16). Reverse epimerization of D-Man to D-Glc was also studied (Fig. S17–S20). Reaction orders for both the forward (D-Glc to D-Man) and reverse (D-Man to D-Glc) reactions were determined from the slopes of the plots of log (initial reaction rate) vs. log (reactant concentration) (Fig. S21). The fractional reaction orders 0.56 ± 0.01 (forward) and 0.47 ± 0.01 (reverse) were calculated. Based on these observations, we propose eqn (1)–(3) to describe the reaction kinetics, based on the network in Fig. 3. D-Glc epimerization to D-Man is represented as a reversible reaction with forward rate constant k1G→M and reverse rate constant k2M→G. Although one mole of D-Glc should yield one mole of D-Man, a gradual decrease in the mass balance over time indicates the formation of by-products (BP) (Fig. S22). Due to the higher concentration of D-Glc relative to D-Man, these by-products are assumed to originate predominantly from D-Glc, with reaction rate constant k3G→BP and a simple first-order dependence on the concentration of D-Glc. In addition to the formation of by-products, the adsorption of the saccharides can cause a decrease in mass balance. We explored adsorption of D-Glc on Sn-OF-1 in the substrate concentration range of 1 to 10 wt% and observed ca. 10–15% uptake at room temperature (results not shown). Since adsorption is normally exothermic, we expect less uptake of the saccharides at elevated reaction temperatures.

 
image file: d5re00314h-t1.tif(1)
 
image file: d5re00314h-t2.tif(2)
 
image file: d5re00314h-t3.tif(3)
The reaction was further explored by epimerizing 1-13C-D-Glc at different temperatures. The 13C label allows the reaction to be monitored using direct polarization solid-state NMR spectroscopy (Fig. 4 and S23–S25). Signals corresponding to α-C1- and β-C1-glucopyranose (GPy) at 92.4 and 96.4 ppm, respectively, decrease monotonically over time. Simultaneously, peaks at 71.3 ppm and 71.6 ppm, corresponding to α-C2- and β-C2-mannopyranose (MPy), emerged, indicating the formation of 2-13C-D-Man via a carbon shift from the C1 to the C2 position. Notably, the absence of signals at 94.1 and 93.7 ppm, typically associated with α-C1- and β-C1-MPy, excludes the possibility of a two-step isomerization via a D-fructose (D-Fru) intermediate.19 Moreover, D-Fru was not detected since the spectra lack the resonances at 64.2 and 65.4 ppm corresponding to β-C1-fructofuranose and β-C1-fructopyranose, respectively. The β-/α-MPy ratio remained constant throughout the reaction (Fig. S26), indicating rapid equilibration of the two anomeric forms of D-Man.


image file: d5re00314h-f3.tif
Fig. 3 Proposed reaction network for D-Glc (G) epimerization to D-Man (M) catalyzed by Sn-OF-1.

image file: d5re00314h-f4.tif
Fig. 4 Direct polarization MAS solid-state 13C NMR (11.7 T, 3 kHz MAS) of Sn-OF-1 (9.4 mg) in the presence of a solution of 1-13C-D-Glc in 50[thin space (1/6-em)]:[thin space (1/6-em)]50 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (7.0 mol L−1, 0.195 mL) at 92 °C. The time-resolved operando spectra show conversion of D-Glc to D-Man.

Kinetic profiles were derived from the NMR spectra by integrating the total area under the carbohydrate signals as a function of time (Fig. S27). By fitting these profiles with the kinetic models represented by eqn (1)–(3), the rate constants k1G→M, k2M→G, and k3G→BP were determined at different temperatures (Table 2), and equilibrium constants were calculated as Keq = k1G→M/k2M→G. Since the explored temperature range is narrow and the reaction exhibits a small reaction enthalpy change ΔH (Table 3), Keq values do not vary significantly. Notably, for all tested temperatures, the equilibrium constant is slightly higher than those reported for aqueous solutions, Keq346–366K = 0.34–0.38,44 likely due to the difference in solvent.

Table 2 Rate constants and equilibrium constants for epimerization at different temperatures,a calculated from time-resolved operando MAS NMR results
Entry T [°C] k1G→M [h−1] k2M→G [h−1] k3G→BP [h−1] Keq
a Reaction conditions: Sn-OF-1 (9.4 mg), 1-13C-D-Glc in 50[thin space (1/6-em)]:[thin space (1/6-em)]50 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (7.12 mol L−1, 0.195 mL).
1 73 0.13 ± 0.02 0.35 ± 0.04 0.007 ± 0.002 0.4 ± 0.1
2 83 0.25 ± 0.06 0.63 ± 0.08 0.024 ± 0.002 0.4 ± 0.1
3 88 0.31 ± 0.04 0.8 ± 0.2 0.033 ± 0.008 0.4 ± 0.1
4 92 0.47 ± 0.061 1.1 ± 0.3 0.033 ± 0.002 0.4 ± 0.2


Table 3 Activation energies and the epimerization enthalpy ΔH
Entry Reaction EA [kJ mol−1] ΔH = EA,1EA,2 [kJ mol−1]
1 D-Glc → D-Man 70 ± 5 11.8
2 D-Man → D-Glc 58 ± 5
3 By-product formation 88 ± 5  


Using the Arrhenius equation (eqn S2), the activation energy and pre-factors were derived from Arrhenius plots of ln(k) versus 1/T (Fig. S28), with results summarized in Table 3. The activation barrier for epimerization of D-Glc into D-Man in the forward direction (69.9 kJ mol−1) catalyzed by Sn-OF-1 is lower than those reported for molybdate catalysts (126 kJ mol−1 for ammonium heptamolybdate45 and 97 kJ mol−1 for molybdenum-based polyoxometalates12) and metal–organic frameworks (MOFs) containing Lewis acid sites (88 kJ mol−1 for ZrMOF-808 (ref. 46) and 83 kJ mol−1 for UiO-66(Zr)46). The activation energy of the side reaction, 88 kJ mol−1, exceeds both forward and reverse activation energies, implying that higher reaction temperatures reduce selectivity towards D-Man.47

The kinetic data offer insights into the optimal conditions for maximizing the yield and selectivity towards D-Man (Fig. S29 and S30). High selectivity was achieved across a range of reaction conditions (Fig. S30). However, at high D-Glc conversions, the selectivity for D-Man decreased, due to the increased prevalence of the by-product formation (Fig. S31). These changes correspond to the formation of by-products that are acidic, as confirmed by a slight decrease in pH (Fig. S32). Lower D-Glc concentrations favor increased selectivity for D-Man, while higher catalyst loadings reduce selectivity by accelerating the reaction rates for D-Glc consumption and D-Man formation, leading to a higher contribution of the side reactions (Table 2). The optimal conditions for maximizing both yield and selectivity of D-Man were identified as a D-Glc concentration of 5–10 wt%, 100 mg of Sn-OF-1 catalyst per 5 mL of solution, and a reaction temperature of 100 °C.

3.3. Purification of the D-Glc/D-Man mixture via adsorption by a CaY zeolite

The CaY zeolite was synthesized via ion exchange of commercial NaY zeolite using a CaCl2 solution. The surface area of the zeolite marginally decreased after the ion exchange from 652 to 610 m2 g−1 (Table S4). The micropore surface areas of CaY and NaY zeolites are 550 and 605 m2 g−1, respectively, indicating that the pore structure was preserved. The Si/Al ratio of the parent NaY zeolite, 2.8, remained unchanged after CaCl2 treatment. The chlorine content of 0.03 wt% confirmed the effective removal of CaCl2 by washing (Table S5). The ion-exchange process resulted in approximately 77% Na+ substitution by Ca2+. SEM analysis revealed well-defined square particles of 900–1000 nm, with no significant morphological changes upon ion exchange (Fig. S33). XRD analysis indicated that the crystal structure of the zeolite was retained (Fig. S34).

Individual isotherms for adsorption of D-Glc and D-Man on CaY zeolite, analyzed using the Langmuir model (eqn S6), are shown in Fig. 5. Table 4 summarizes the corresponding maximum loadings qmax, Langmuir constants KLangmuir, along with the Henry constants KHenry determined by linearization of the curves at low sugar concentrations (Fig. S35). It was not possible to explore higher sugar concentrations due to limited solubility of the substrates in water–ethanol mixtures.


image file: d5re00314h-f5.tif
Fig. 5 Adsorption isotherms for individually adsorbed D-Glc (left) and D-Man (right) on CaY zeolites at different EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O ratios. Adsorption conditions: 500 mg zeolite, 3 mL EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solution, 2 h, RT, 500 rpm.
Table 4 Results from individual sorption isotherms for D-Glc and D-Man, recorded for various solvent compositionsa
Entry Reactant EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O Henry model Langmuir model
KHenry qmax [mgsugar gzeolite−1] KLangmuir R2
a Adsorption conditions: 500 mg zeolite, 3 mL EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solution, 2 h, RT, 500 rpm.
1 D-Glc 50[thin space (1/6-em)]:[thin space (1/6-em)]50 254 141 2.3 0.93
2 D-Man 50[thin space (1/6-em)]:[thin space (1/6-em)]50 119 52 3.3 0.98
3 D-Glc 70[thin space (1/6-em)]:[thin space (1/6-em)]30 701 120 8.4 0.99
4 D-Man 70[thin space (1/6-em)]:[thin space (1/6-em)]30 2104 116 30 0.99


Adsorption is fast, with equilibrium being reached in less than 5 min (Fig. S36). The extent of adsorption depends significantly on the solvent composition: in pure water, nearly no sorption was observed, whereas an increase in fraction of ethanol resulted in a dramatic increase in the adsorption capacity. Fornefett et al. explored sucrose adsorption on Y zeolites from water–ethanol mixtures, reporting significantly higher sucrose loading from a water–ethanol mixture than from pure water. The effect was attributed to the hydrophilicity of the zeolite: by increasing the fraction of ethanol, the competition between water and saccharide for adsorption sites was reduced, allowing the zeolite to adsorb a significantly higher amount of sugar.33 In addition, the stability of Ca2+ complexes of saccharides increases upon addition of an organic co-solvent.23,48 Water efficiently desorbed the individually adsorbed D-Glc and D-Man: 78–95% of each adsorbed saccharide was recovered (Table 5). A somewhat lower D-Man desorption rate of 78% was observed when the material was dried after the adsorption step prior to the desorption (entry 3 in Table 5). When the drying process was omitted, 95% of D-Man was recovered by the adsorption (entry 4 in Table 5). This result can be explained by a partial degradation of D-Man coordinated to Ca2+ cation during the drying of the CaY zeolite at elevated temperature.

Table 5 Results of desorption experiments for individually adsorbed D-Glc and D-Mana
Entry Reactant q [mgsugar gzeolite−1] Adsorption [%] Desorption [%]
a Adsorption conditions: 500 mg CaY, 3 mL sugar solution (50 mg g−1) in 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O, 2 h, 500 rpm, RT. Desorption conditions: 3 mL H2O, 1 h, 500 rpm, RT.b After adsorption, the zeolite was dried in an oven at 80 °C.c Without drying the zeolite after adsorption.
1 D-Glcb 68 29 90
2 D-Glcc 66 28 85
3 D-Manb 96 43 78
4 D-Manc 94 41 95


Competitive adsorption of D-Glc and D-Man over CaY zeolite was studied in a 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solvent mixture. Fig. 6 shows the competitive adsorption isotherms over CaY zeolites, while Table 6 summarizes the adsorption parameters obtained by curve fitting as well as the separation factors (SF) calculated using eqn S9. Henry's Law constants for the adsorption of D-Man are significantly higher than for the adsorption of D-Glc at sugar ratios of 80[thin space (1/6-em)]:[thin space (1/6-em)]20 or 50[thin space (1/6-em)]:[thin space (1/6-em)]50 (Fig. S37). Interestingly, the selectivity for D-Man adsorption increases at the lower concentration of D-Glc, with SF values of 1.5 and 3.2 for solutions with 80[thin space (1/6-em)]:[thin space (1/6-em)]20 and 50[thin space (1/6-em)]:[thin space (1/6-em)]50 ratios, respectively.


image file: d5re00314h-f6.tif
Fig. 6 Competitive adsorption of D-Glc and D-Man by CaY zeolites. Adsorption conditions: 500 mg zeolite, 3 mL 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solution, 2 h, RT, 500 rpm.
Table 6 Results of competitive sorption isotherms for adsorption of sugars by CaY zeolitea
Entry Zeolite Sugar Sugar ratio D-Glc[thin space (1/6-em)]:[thin space (1/6-em)]D-Man Henry model Langmuir model SF
KHenry qmax [mgsugar gzeolite−1] KLangmuir R2
a Adsorption conditions: 500 mg zeolite, 3 mL 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% EtOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O solution, 2 h, RT, 500 rpm.
1 CaY D-Glc 50[thin space (1/6-em)]:[thin space (1/6-em)]50 796 40 26 0.99 3.2
D-Man 2955 83.2 43 1.00
2 CaY D-Glc 80[thin space (1/6-em)]:[thin space (1/6-em)]20 981 51.3 28.4 0.98 1.5
D-Man 2788 32.3 133.8 0.99


Water was used to desorb D-Glc and D-Man adsorbed from a mixture with 67[thin space (1/6-em)]:[thin space (1/6-em)]33 ratio of the saccharides. Rinsing the loaded zeolite twice with water resulted in recovery of 70% D-Man and 98% D-Glc. Stirring the zeolite after rinsing led to further recovery of 1% D-Man (Table S5). Thus, desorption with water is quick and efficient.

3.4. Combination of catalysis by Sn-OF-1 with crystallization and adsorption

Catalytic synthesis of D-Man over Sn-OF-1 was combined with separation of the product. D-Man was synthesized using Sn-OF-1 at 100 °C from a 5 wt% D-Glc solution under the optimized reaction conditions (Fig. 7). After the epimerization, the reaction solution contained 21% D-Glc and 68% D-Man estimated based on the carbon balance, which corresponded to a 77[thin space (1/6-em)]:[thin space (1/6-em)]23 ratio of D-Clc[thin space (1/6-em)]:[thin space (1/6-em)]D-Man. Next, a portion of D-Glc was removed from the solution via crystallization. Several solvents were reported in literature to achieve selective crystallization of D-Glc from its mixture with D-Man.6 The following liquids were tested in this study as antisolvents: 4.7[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ethanol-to-methanol as well as pure ethanol. The highest yield of crystallized D-Glc, obtained with the 4.7[thin space (1/6-em)]:[thin space (1/6-em)]1 ethanol-to-methanol mixture, corresponded to recovery of ca. 73% of the dissolved Glc with a purity of 96–98% (Table S7 and Fig. S38 and S39). The amount of D-Glc recovered by crystallization refers to ca. 50% of the initially loaded substrate. The obtained crystallized D-Glc was successfully epimerized in the presence of Sn-OF-1 (Table S8 and Fig. S40).
image file: d5re00314h-f7.tif
Fig. 7 Synthesis and isolation of D-Man via epimerization of D-Glc in the presence of Sn-OF-1 followed by recovery of D-Man by adsorptive separation over CaY zeolite. Percent values reflect the carbon balance.

After the crystallization step, the sugar solution contained 18% D-Glc and 21% D-Man when calculating the carbon balance to the initially loaded substrate. This mixture was dissolved in a 70[thin space (1/6-em)]:[thin space (1/6-em)]30 wt% mixture of ethanol and water for adsorption by CaY zeolite followed by desorption into water. Two liquid fractions were obtained: (1) the remaining supernatant, containing ca. 75% D-Man with a purity of ca. 50% (in Fig. 7, 16% D-Glc and 15% D-Man in EtOH[thin space (1/6-em)]:[thin space (1/6-em)]water 7[thin space (1/6-em)]:[thin space (1/6-em)]3), and (2) the desorption solution, containing ca. 25% D-Man with a purity of ca. 70% (in Fig. 7, 2% D-Glc and 5% D-Man in water). As Fig. 7 depicts, the approach results in recovery of crystalline D-Glc and two liquid fractions enriched with D-Man. The proposed separation method can be further developed by transferring into a continuous mode and performing a column separation. The data on adsorption in water and in water-ethanol mixtures will contribute to designing a gradient elution. Moreover, simulated moving bed (SMB) presents an attractive possibility to perform the separation of the binary mixture of D-Glc and D-Man in a continuous manner.49

4. Conclusions

This study demonstrates that Sn-OF-1 is a highly effective chemo-catalyst for the epimerization of D-Glc to D-Man, offering a feasible and efficient approach for producing D-Man under mild reaction conditions. The kinetic analyses reveal the optimal conditions—temperature, catalyst loading, and reactant concentration—to enhance selectivity towards D-Man. With an activation energy of 70 kJ mol−1, Sn-OF-1 exhibits an activity comparable to state-of-the-art catalysts, showing its potential for industrial applications.

Additionally, this work introduces a strategy for separating D-Glc and D-Man mixtures resulting from epimerization. The approach combines (1) partial recovery of unconverted D-Glc and (2) purification of D-Man. Adsorptive separation using CaY zeolite proved to be a simple and time-efficient method, employing water-ethanol and water as the adsorption and desorption solvents, respectively. Notably, the adsorbed sugars are desorbed rapidly without additional workup. The combination of Sn-OF-1 catalysis, crystallization, and adsorptive separation with CaY zeolites represents a promising pathway for the scalable synthesis of D-Man.

Author contributions

Valérie Toussaint: conceptualization, methodology, validation, investigation (synthesis and characterization, performing catalytic experiments, adsorption experiments), visualization, writing – original draft, writing – review & editing; Loretta Pavlis: investigation (performing adsorption and desorption experiments), writing – review & editing; Pia Groß: investigation (synthesis and characterization, performing catalytic experiments), writing – review & editing; Samantha Ausman: conceptualization, methodology (operando solid-state NMR spectroscopy), writing – review & editing; Karin Föttinger: instrumentation, writing – review & editing; Susannah Scott: methodology, supervision (operando solid-state NMR spectroscopy), writing – review & editing; Irina Delidovich: conceptualization, methodology, validation, funding acquisition, project administration, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: Supplementary information (SI) includes characterization data for Sn-OF-1 and CaY zeolite (N2 physisorption, XRD, XRF, IR spectra of pyridine adsorption, SEM), results of catalytic results of Sn-OF-1 for conversion of D-glucose, details of the kinetic study, adsorption/desorption profiles, details of the crystallization experiments, and all relevant calculations. See DOI: https://doi.org/10.1039/D5RE00314H.

We hereby confirm that the data underlying the reported research are available in the article itself and in the submitted SI.

Acknowledgements

We thank Jerry Hu and Jaya Nolt for their assistance with NMR spectroscopy and Jason Chalmers for his invaluable help and insightful discussions, particularly with data fitting. We acknowledge the support provided by Prof. Regina Palkovits and Dr. Tobias Riedl throughout this project. We thank Helga Seidelmann and Thomas Jung for HPLC analysis. We are grateful for the resources and facilities provided by the X-ray Center (XRC) at TU Wien.

Financial support of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project number 450360023 – is greatly appreciated. V. T. thanks the Austrian Marshall Plan Foundation for an academic exchange program scholarship. Financial support for the NMR measurements from the U.S. Department of Energy, Office of Science, Division of Basic Energy Sciences, under the Catalysis Science Initiative (DE-FG-02-03ER15467) is gratefully acknowledged. Part of this work was done in the MRL Shared Experimental Facilities, which are supported by the MRSEC Program of the NSF under Award No. DMR 2308708; a member of the NSF-funded Materials Research Facilities Network. The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme.

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