Porous Tin-Organic Frameworks as Selective Epimerization Catalysts in Aqueous Solutions

Article abstract of DOI:10.1021/acscatal.7b00806

Epimerization of sugars is a carbon-efficient route not only to produce rare carbohydrates but also to extend the product scope for chemical production in future biorefineries. Industrially available catalysts for epimerization are limited mainly to soluble Mo(VI) species as well as substrate-specific epimerases. Here we report highly active and selective tin-organic frameworks (Sn-OF) as solid catalysts for the epimerization of aldoses at the C-2 position, such as the conversion of glucose to mannose. The reaction proceeds via a carbon skeleton rearrangement, that is, through breaking of a C-2/C-3 carbon bond and formation of a C-1/C-3 bond. Partially hydrolyzed Ph₃Sn-OH sites were found to be the catalytically active centers. Our results suggest that the high catalytic activity of Sn-OFs for the epimerization is determined by (1) Lewis acidity of tin; (2) free Sn-OH groups; and (3) the high hydrophobicity of organic linkers applied in the aqueous solutions.

Full text of DOI:10.1021/acscatal.7b00806

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Article
Porous Tin-Organic Frameworks as Selective
Epimerization Catalysts in Aqueous Solutions
Irina Delidovich, Andreas Hoffmann, Andrea Willms, and Marcus Rose
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Porous Tin-Organic Frameworks as Selective Epimerization
Catalysts in Aqueous Solutions
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Irina Delidovich, Andreas Hoffmann, Andrea Willms, Marcus Rose*
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen,
Germany.
ABSTRACT: Epimerization of sugars is a carbon-efficient route not only to produce rare carbohydrates but also to extend
the product scope for chemical production in future biorefineries. Industrially available catalysts for the epimerization are
limited mainly to soluble Mo(VI) species as well as substrate-specific epimerases. Here we report a highly active and se-
lective tin-organic frameworks (Sn-OF) as solid catalysts for the epimerization of aldoses at the C-2 position, such as the
conversion of glucose to mannose. The reaction proceeds via a carbon skeleton rearrangement, i.e., through breaking of a
C-2/C-3 carbon bond and formation of a C-1/C-3 bond. Partially hydrolyzed Ph3Sn-OH sites were found to be the catalyti-
cally active centers. Our results suggest that the high catalytic activity of Sn-OFs for the epimerization is determined by
(1) Lewis acidity of tin; (2) free Sn-OH groups; and (3) the high hydrophobicity of organic linkers applied in the aqueous
solutions.
KEYWORDS: Epimerization, Isomerization, Saccharides, Aldoses, Tin catalyst, Porous organic framework, Aqueous solu-
tion
silanol group participate in the transition state, as sup-
1. INTRODUCTION
ported by computational methods12-13 and experimental
Monosaccharides are of great importance in numerous
evidences.9 Interestingly, Rai et al. performed a computa-
technological sectors, including food industry, pharma-
tional study of Sn4+ as a sole active site of Sn-Beta, i.e.
ceutical production as well as the synthesis of fine chemi-
considering an adjacent silanol as a spectator, and pre-
cals.1 Moreover, recent developments in the field of bio-
dicted a different reaction mechanism (Scheme 1b).12
mass valorization enabled efficient chemical transfor-
According to their calculations, epimerization of an al-
mations of carbohydrates as a key factor for the produc-
dose via the ‘Bílik mechanism’ is energetically preferable
tion of bio-based fuels and commodity chemicals.2
in this case. The Bílik reaction3,
is an epimerization
14-15
Though many saccharides are highly available in nature,
the amount of abundant monosaccharides is limited to
seven, namely D-glucose, D-fructose, D-ribose, D-
galactose, D-xylose, L-arabinose, and D-mannose. The
portfolio of available carbohydrates can be significantly
extended by isomerizations as carbon-efficient reactions.
Therefore, recent research efforts have been focused on
the development of catalysts for the selective isomeriza-
tion.3 Lately, especially water-tolerant Lewis acidic cata-
lysts attracted great attention.4 Among the tested Lewis
acids, Sn-Beta zeolite exhibits the highest activity for the
isomerization due to optimal structure of the catalyst in
terms of high dispersion of the active sites: Tin ions are
embedded into a hydrophobic matrix of silicalite which
protects Sn4+ from hydrolysis in bulk water.4-7 Sn-Beta
demonstrated a high selectivity for the isomerization of
aldoses into corresponding ketoses, e.g., the transfor-
mation of glucose into fructose. Detailed investigations of
the reaction mechanism suggested that the isomerization
takes place at the open sites, i.e., partially hydrolyzed
sites, of Sn-Beta zeolites (Scheme 1a).8-10 It was shown that
the reaction proceeds via a hydride shift from the C-2 to
the C-1 atom of glucose yielding fructose as ketose iso-
mer.11 Importantly, both the Sn atom and the adjacent
accompanied by rearrangement of a carbon skeleton. It
involves a bond breaking between C-2 and C-3 carbon
atoms of a substrate and the formation of a new bond
between C-1 and C-3 atoms (Scheme 1b). Bermejo-Deval
et al. proved experimentally the different mechanism in
case adjacent silanol groups are not available.9 They per-
formed a post-synthetic ion exchange of protons with Na+
cations to obtain a material which does not possess free
OH-groups next to Sn4+ active sites. Remarkably, the Na-
exchanged Sn-Beta indeed demonstrated high selectivity
for epimerization of glucose into mannose via the ‘Bílik
mechanism’ (Scheme 1b). However, this material ap-
peared to be unstable under the reaction conditions due
to rapid leaching of Na+ into the solution. Consequently,
the initial selectivity for fructose was quickly restored in
the course of the experiment.9 More recently, Brand et al.
reported on molecular complexes of tin (tin silsesquiox-
anes) for the carbon shift also proving that the absence of
an adjacent silanol group promotes the selective epimeri-
zation of aldoses.16-17 Gunther et al. demonstrated that a
combination of Sn-Beta with soluble borate salts selec-
tively catalyzes the epimerization of aldoses, induced by
an in-situ formation of saccharide-borate complexes.18
However, this approach is hardly applicable in industry
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aldoses, namely for the epimeric pairs glucose-mannose,
ribose-arabinose, and xylose-lyxose.
1
2
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2. EXPERIMENTAL SECTION
2.1. Preparation of Catalysts
Syntheses of Sn-OFs were performed under argon atmos-
phere using Schlenk techniques. Sn-OF-1 was prepared
according to the protocol published by Fritsch et al.23 4,4’-
dibromobiphenyl (3.12 g, 10 mmol) was dissolved in 200
mL of dry THF. The solution was cooled down to 263K,
thereafter n-butyl lithium (8 mL of 2.5M in hexanes, 20
mmol) was added dropwise. The mixture was stirred for
30 minutes, next SnCl4 (1.3 g, 5 mmol) was added drop-
wise. The mixture was warmed up to the room tempera-
ture and stirred overnight. The obtained solid was recov-
ered using vacuum filtration, washed twice with THF (200
mL), distilled water (200 mL) and ethanol (200 mL). 1.63 g
of pale yellow powder were obtained.
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Scheme 1. Depending on the active sites of Sn-Beta
zeolite two different reaction routes of glucose are
known: (a) the isomerization in the presence of adja-
cent silanol groups and (b) the epimerization in-
duced by isolated stannanol groups.9, 12
Sn-OF-2 was synthesized following the protocol elaborat-
ed by Hausoul et al.25 for the preparation of phosphorous-
containing element organic frameworks. 1,3,5-Tri-(4-
bromophenyl)-benzene (2.74 g, 5 mmol) was dispersed in
140 mL Et2O. The solution was cooled down to 263 K and
n-butyl lithium (9.375 mL of 1.6M in hexanes, 15 mmol)
was added dropwise under stirring. After stirring the
suspension for 30 minutes, SnCl4 (0.98 g, 3.75 mmol) was
added dropwise. The mixture was warmed up to the room
temperature and stirred overnight. The obtained solid
was washed with Et2O (150 mL), THF (150 mL) distilled
water (150 mL) and ethanol (150 mL). White powder (1.01
g) was obtained after drying under vacuum at room tem-
perature.
due to cumbersome separation of the saccharides and
borates.19 To our knowledge, no solid Sn-containing Lewis
acidic catalyst has been reported for the selective epimer-
ization of aldoses so far, though heterogeneous catalysis
enables an easy catalyst separation and recycling. In gen-
eral, the number of selective catalysts for epimerization is
very limited. Thus, in addition to the aforementioned
examples, only Mo(VI)-based catalysts exhibit high selec-
tivity for the epimerization,20 though most of the reported
molybdenum catalysts are soluble15 or undergo leaching
of active species under reaction conditions.21 Moreover,
the majority of epimerases is efficient on saccharides
substituted with phosphate or nucleotide groups, but not
on free, non-activated carbohydrates.22 Consequently,
elaboration of a solid catalyst for epimerization is of great
advantage for the synthesis of rare saccharides as well as
to widen the scope in the production of novel biogenic
platform chemicals.
Sn-Beta catalyst was synthesized according to Dijkmans
et al.26 H-Beta zeolite in a powder form was supplied by
Clariant Produkte (SiO2:Al2O3 = 23 mol:mol; SBET = 583
m2g-1). The H-Beta (10.0 g) was subjected dealumination
by stirring in HNO3 (500 mL of 7M) for 20 hours at 80 °C.
After washing with of distilled water (10 L), dealuminated
zeolite was dried in air and at 80°C. Prior to tin grafting,
the dealuminated zeolite was kept at 150 °C in air over-
night to remove adsorbed water. The material was placed
in isopropanol (100 mL per g material) solution of SnCl4
(7.73 g, 30 mmol per g material) and refluxed for 7 hours.
The material was filtered, washed with isopropanol (100
mL per g), dried in air at 60 °C and calcined as follows:
ramping 3 °C min-1 to 200 °C, dwell for 6 hours, ramping 3
°C min-1 to 550 °C, dwell for 6 hours.
Herein we report the application of porous tin-organic
frameworks (Sn-OF) as solid catalysts that are able to
selectively catalyze the epimerization of monosaccha-
rides. These hydrophobic materials exhibit a superposi-
tion of partially hydrolyzed tin Lewis acidic centers that
are covalently connected by organic aromatic linkers. The
synthesis and characterization of the Sn-OF materials was
reported initially by Fritsch et al. using
a 4,4’-
dibromobiphenyl building block.23 The materials exhibit a
high thermal and hydrolytic stability due to covalent
bonds between Sn and the organic linker. The application
of Sn-OFs as solid catalysts with Lewis acidic centers was
so far only reported twice: the cyanosilylation of benzal-
dehyde23 and the esterification of oleic acid with
glycerol.24 Based on that, we synthesized the Sn-OFs con-
taining 4,4’-dibromobiphenyl (Sn-OF-1) as well as a new
one based on 1,3,5-tris-(4-bromophenyl)-benzene (Sn-OF-
2) and applied them successfully in the isomerization of
2.2. Characterization of Catalysts
The content of Sn in the materials was determined by
means of inductively coupled plasma optical emission
spectroscopy (ICP-OES; Spectro Analytical Instruments).
Prior to the analysis Sn-Beta was dissolved in a mixture of
HCl, HNO3 and HF (5:5:1). Elemental analyses (Sn, C, H)
were performed at micro analytical laboratory Kolbe
(Mikrolab Kolbe), Mühlheim/Ruhr, Germany. The cata-
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lysts were characterized by low-temperature sorption of
N2 using a Quadrasorb SI Automated Surface Area & Pore
1. n-BuLi
2. SnCl4
3. H2O
Br
1
2
Size Analyzer after preliminary outgassing under vacuum
at 100 °C for 20 hours. Brunauer-Emmett-Teller (BET)
theory was used for calculation of the specific surface area
(SBET). Micropore surface areas and micropore volumes
were determined using t-plot analysis. X-ray diffraction
analysis (XRD) was performed on a D5000 Siemens XRD
diffractometer with a CuKα X-Ray tube (λ=1.54056 Å).
The tube voltage and current were 45 kV and 40 mA,
respectively. Diffraction patterns were collected in the 3–
90° 2Θ range, with 0.02° intervals and a step time of 1 s.
FT-IR spectra were registered using Vertex 70 (Bruker)
spectrometer under diffuse reflectance mode (DRIFTS)
with a dried KCl powder as a reference. The spectra were
collected at room temperature in the range of 850-4000
cm-1. Scanning electronic microscopy (SEM) images and
energy-dispersive X-Ray (EDX) spectra were acquired
using a JEOL JSM-7000F microscope (accelerating energy
15 kV) combined with EDX/EBSD-System EDAX Pegasus.
The samples were coated with carbon prior to SEM inves-
tigation.
Sn
Sn
OH
Br
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Sn-OF-1
SnOH
Br
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1. n-BuLi
2. SnCl4
3. H2O
Sn
Sn
Br
Br
Sn-OF-2
Scheme 2. Synthesis of poly(arylstannanes) by an
organolithium route yields 3D-crosslinked porous
tin-organic frameworks with stannanol groups as
catalytically active structural defects.
3. RESULTS AND DISCUSSION
2.3. Catalytic Experiments
The Sn-OFs were synthesized based on an organolithium
route via a nucleophilic aryl lithium intermediate which is
crosslinked by a tin-based precursor under formation of a
Sn-C bond (Scheme 2). The product precipitates as white
powder and is filtered off, washed and dried accordingly.
Both Sn-OFs are X-ray amorphous and according to the
nitrogen physisorption isotherms exhibit a broad pore
size distribution among micro- and mesopores with spe-
cific surface areas of 350 and 506 m2g-1 for Sn-OF-1 and -2,
respectively (see SI). Importantly, in addition to carbon,
hydrogen and tin, the presence of oxygen was detected by
means of SEM-EDX (Fig. S2). Moreover, in line with the
data of Wee et al.,23 the vibrational band corresponding to
Sn-OH (νOH 3640 cm-1)28 was detected in the IR spectrum
(Fig. S3). These results suggest the presence of defects in
the structure of Sn-OFs, i.e., the tin-attached hydroxyl
groups due to partial hydrolysis during the synthesis and
work-up procedure (Scheme 2). For comparison, Sn-Beta
zeolite was prepared following the protocol proposed by
Dijkmans et al.26
The experiments were performed in a 20 mL autoclave
equipped with a glass inlet. In a typical catalytic experi-
ment the catalyst (100 mg) and 5 mL of 10wt.% solution of
a substrate were charged in the autoclave, sealed and
pressurized with 30 bar of nitrogen. The reactions were
carried out under stirring at 750 rpm. After the experi-
ments, the reaction mixture was cooled down in an ice
bath and filtered through a polyamide syringe filter (pore
size 0.2 μm). Commercial SnO2 (a powder of 30-60 nm
nanoparticles, 100 mg) as well as its mixture with a porous
hypercrosslinked polymer prepared according to Detoni
et al.27 (30 mg SnO2 and 70 mg HCP) were utilized in
control experiments. The filtrates were analyzed by HPLC
using a Shimadzu Prominence LC-20 system equipped
with a refractive index (RI) detector. Glucose-fructose-
mannose and ribose-ribulose-arabinose mixtures were
separated on Accucore Amid-Hilic 0.05M trimethylamine
column (Thermo Fischer, 100 mm × 4.6 mm) at 40 °C; the
eluent (90 vol.% acetonitrile + 10 vol.% 0.02 trimethyla-
mine in water) was supplied at 1 mL min-1 flow rate. Xy-
lose-xylulose-lyxose mixtures were analyzed using
Aminex HPX-87P column (Bio-Rad, 300 mm × 7.8 mm) at
80 °C using water as eluent supplied at 0.5 mL min-1 flow
rate. The experiment with D-(13C-1)glucose as substrate
was performed using standard reaction conditions for 1.5
h with EtOH+H2O as solvent and Sn-OF-1 as catalyst.
After the experiment, the solvent was evaporated under
reduced pressure at 30 °C, the obtained syrup was dis-
The prepared materials were first tested for the isomeriza-
tion of glucose in aqueous solution (Table 1). The product
distribution obtained in the presence of Sn-Beta is in
good agreement with the literature data showing prevail-
ing formation of fructose while mannose is obtained in
lower amount.3, 26 In contrast to the zeolite catalyst, Sn-
OFs give rise predominantly to mannose with only minor
production of fructose. The same trend was observed for
the temperature range of 80-120 °C, though selectivity
towards mannose somewhat decreases at temperatures
above 100 °C probably due to the moderate thermal stabil-
ity of saccharides (Fig. 1). Owing to the hydrophobic na-
ture of the aromatic pore walls, Sn-OFs exhibit a low
wettability in pure water and initially float on top of the
aqueous solution (Fig. S4). In order to improve the
13
13
solved in D2O and analyzed by C NMR. C NMR spectra
were recorded at Bruker AV-400 at room temperature.
CH3OH (δ 49.5 ppm) was used as internal reference for 13C
NMR spectra. The pH0 was adjusted using HNO3 with a
Titroline alpha titrator unit.
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Table 1. Results of catalytic experiments.
1
2
Entry Catalyst Substrate
Solvent
Time
Conv.
%
Ratio of monosaccharides / %
Balance
%
h
Aldose
Aldose Epim.
Ketose
3
4
Glucose
Mannose
Fructose
Sn-OF-1 Glucose[a]
Sn-OF-1 Glucose[a]
Sn-Beta Glucose[a]
Sn-Beta Glucose[a]
Sn-OF-1 Mannose[b]
Sn-OF-1 Mannose[a]
Sn-Beta Mannose[a]
EtOH+H2O
H2O
1.5
1.5
1.5
1.5
3
23
12
77
88
80
68
25
11
21
11
2
1
100
100
99
5
6
7
8
1
2
3
4
5
6
7
EtOH+H2O
H2O
20
33
29
14
2
17
27
5
5
98
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
EtOH+H2O
EtOH+H2O
EtOH+H2O
70
87
63
100
99
1.5
1.5
2
37
4
33
100
Arabinose
76
Ribose
20
Ribulose
4
8
Sn-OF-1 Arabinose[b]
EtOH+H2O
3
41
78
9
Sn-OF-1 Arabinose[a]
Sn-Beta Arabinose[a]
Sn-OF-1 Ribose[b]
Sn-OF-1 Ribose[a]
Sn-Beta Ribose[a]
EtOH+H2O
EtOH+H2O
EtOH+H2O
EtOH+H2O
EtOH+H2O
1.5
1.5
3
23
49
43
39
57
87
63
18
11
11
2
18
5
88
82
73
71
10
11
19
77
86
53
12
13
1.5
1.5
3
22
25
80
Xylose
74
Lyxose
25
Xylulose
14
15
16
17
18
19
Sn-OF-1 Xylose[b]
Sn-OF-1 Xylose[a]
Sn-Beta Xylose[a]
Sn-OF-1 Lyxose[b]
Sn-OF-1 Lyxose[a]
Sn-Beta Lyxose[a]
EtOH+H2O
EtOH+H2O
EtOH+H2O
EtOH+H2O
EtOH+H2O
EtOH+H2O
3
34
19
54
48
18
1
88
97
79
74
97
90
1.5
1.5
3
83
12
5
2
5
4
4
58
39
25
71
1.5
1.5
11
85
21
9
88
[a] Reaction conditions: 5 mL of 10 wt.% substrate solution, 100 mg catalyst, 100 °C, 750 rpm. [b] Reaction conditions: 5 mL of 5
wt.% substrate solution, 100 mg catalyst, 100 °C, 750 rpm. The saccharide distributions at equilibrium are: glucose:mannose =
70:30; xylose:lyxose = 58:42; arabinose:ribose = 67:33.15, 29
wettability of the Sn-OFs, mixtures of ethanol-water with
different compositions were used as solvent. Indeed, the
catalysts can be well dispersed in ethanol-water mixtures
and increase of the ethanol content improves the yields of
mannose (Fig. S5). The best results were obtained for a
solvent mixture that contains a high content of ethanol
although at concentrations above 50 wt.% the solubility of
glucose decreases tremendously. Hence, a mixture of
water and ethanol with a mass ratio of 1:1 was used as
solvent in this study. Importantly, the product selectivity
observed for Sn-OFs and Sn-Beta are independent of the
solvent composition. A question of great importance is
whether the catalysis with Sn-OFs is truly heterogeneous.
Therefore, the filtration test, i.e., removal of the catalyst
and continuation of the reaction, was carried out (Figure
1). It proved that the reaction proceeds heterogeneously
since no molecular or dissolved nano-sized active species
are leached into the solution. This was confirmed by ele-
mental analysis of the liquid phase which was negative for
tin. Additionally, SnO2 as well as a physical mixture of
SnO2 with a porous, hydrophobic hypercrosslinked poly-
mer were tested for the glucose isomerization. No conver-
sion of the substrate was observed under the typical reac-
tion conditions (100 °C, 1.5 h). This can be explained by a
low specific surface area of the used commercial SnO2,
since activity of the supported nano-SnO2 for isomeriza-
tion of glucose into fructose has been recently reported.30
Though the presence of SnO2 was not detected by XRD,
formation of a SnO2 supported amorphous phase on a Sn-
OF surface cannot be excluded. These SnO2 nanoparticles
can be responsible for the formation of fructose as a by-
product.
Reusability of the catalysts for the epimerization was
investigated in detail (Table 2). Firstly, the used Sn-OF-1
catalyst was washed with a mixture of EtOH-H2O and
dried before recycling. In the second run the glucose
conversion significantly dropped along with a dramatic
decrease of the specific surface area and the pore volume
of the material (Entry 1, Table 2). We hypothesized that
the observed collapse of the pore structure can occur due
to a contact of the material with a bulk liquid at high
temperature followed by a drying. Indeed, heating a Sn-
OF-1 suspension till 100 °C in a mixture of EtOH-water
(without adding glucose) with a subsequent drying led to
a similar textural degradation (Entry 2, Table 2). Alike
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breakage of pore structure was reported for silicagels or
a)
carbons due to capillary stress during the drying.31 Wash-
ing a material with a solvent exhibiting a low surface
tension prior to drying typically improves textural proper-
ties of the dried solids. In other words, a low interfacial
energy of the solvents facilitates reducing the capillary
pressure and helps maintaining the high porosity.31 There-
fore, the materials after a catalytic test were firstly rinsed
with a mixture of water (surface tension 72.8 mN m-1 at 20
°C) with ethanol (surface tension 22.1 mN m-1 at 20 °C) to
remove physically adsorbed polar molecules of saccha-
rides. Thereafter, the catalysts were washed either with
pure ethanol or with THF (surface tension 26.4 mN m-1 at
20 °C) to substitute the solvent in the pores and finally
dried (Entries 3, 4 and 6 in Table 2, respectively). As ex-
pected, washing with ethanol or THF results in enhancing
the robustness of the material upon drying as well as
improving the yield upon recycling. Additionally, the Sn-
OF-1 catalyst was rinsed after the first run with an etha-
nol-water mixture and recycled without drying. This pro-
cedure allowed maintaining the catalytic performance in
a subsequent cycle (Entry 5, Table 2). Apparently, drying
procedures are to be avoided to prevent partial structural
collapse of the organic framework with a loss of activity.
However, elaboration of a more suitable regeneration
procedure and further insight into the long-term stability
of the Sn-OF-1 catalysts is required.32-33
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3
4
5
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b)
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Inspired by the excellent performance in the glucose epi-
merization, we performed catalytic tests for other mono-
saccharides (Table 1). Remarkably, the formation of the
respective epimeric aldose clearly predominates in the
presence of Sn-OFs for all the tested substrates. In con-
trast, Sn-Beta exhibits higher selectivity for the corre-
sponding ketoses. Results of catalytic tests of Sn-OF-2
were similar to that of Sn-OF-1 (Table S3) proving that the
catalytic performance is determined by the open tin-sites
while the organic framework structure and the textural
parameters are only of minor influence on selectivity
when comparing different Sn-OFs. For a detailed under-
standing of the Sn-OF active sites and the differences in
selectivity compared to Sn-Beta the epimerization
Figure 1. a) Temperature-dependent conversion of glucose
and yield of mannose (reaction conditions: 5 mL of 10 wt.%
substrate solution in water, 100 mg Sn-OF-1, 1.5 h, 750 rpm)
and b) results of the filtration experiment to demonstrate
heterogeneity of the catalyst Sn-OF-1 (reaction conditions:
5 mL of 10 wt.% substrate solution, solvent 50 vol.% H2O +
50 vol.% EtOH, 100 mg catalyst Sn-OF-1, 100 °C, 1.5 h,
750 rpm).
Table 2. Results of Sn-OF-1 recycling and its properties as fresh (left) and treated/spent (right) catalyst.
Entry
Textural properties
Results 1st run[a]
Treatment[b]
Textural properties
Result 2nd run
SBET
Smicro
m2g-1
Vpore
CGlucose
%
YMannose
%
SBET
Smicro
m2g-1
Vpore
CGlucose
%
YMannose
m2g-1
350
350
350
350
350
350
cm3g-1
0.24
0.24
0.24
0.24
0.24
0.24
m2g-1
cm3g-1
0.075
0.073
0.14
0.14
-
%
4.6
7.5
8.3
9
1
228
228
228
228
228
228
12
-
11
-
EtOH/H2O; dried
dried
38
0
0
0
0
-
5
7.8
8.4
10
11
2[c]
22
3
12
12
12
-
11
11
11
-
EtOH/H2O, EtOH; dr.
EtOH/H2O,THF; dr.
EtOH/H2O; not dried
THF; dried
70
4
110
-
5
10
6[c]
122
0
0.12
-
-
[a] Reaction conditions: 5 mL of 10 wt.% substrate solution, solvent 50 wt.% H2O + 50 wt.% EtOH, 100 mg Sn-OF-1, 100 °C, 0.5 h,
750 rpm. [b] Treatment of the catalyst between the first and the second runs by washing with the respective solvent (mixture)
and drying. [c] Sn-OF-1 was charged into an autoclave and heated up in 50 wt.% H2O + 50 wt.% EtOH without glucose.
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H
H
1
2
3
R =
CHOH
C O
C OH
R
n
C O
C H
R
CH2OH
H
HO
OH
Sn
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8
H
O
Sn
R
C H
C
(d)
H
H
O
H
O
O
C O
C H
R
Sn
H O
(a)
H
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H
O
Sn
H
O
Sn
H
H
H
H
C
C H
R
O
O
C
C
H
O
O
R
(b)
(c)
Figure 2. 13C NMR spectra of D-(1-13C)glucose as a substrate
(bottom) and the reaction mixture after epimerization over
Sn-OF-1 (top) indicating the formation of mannose by a
rearrangement of the carbon skeleton.
Scheme 3. Proposed reaction mechanism for the
epimerization in the presence of Sn-OFs.
catalytic activity for the isomerization of glucose under
the applied reaction condition. This was expected since a
tetravalent tin substituted by four aromatic rings is steri-
cally hindered and should not exhibit accessible active
sites. Remarkably, mannose and fructose with 78 and 10 %
selectivities were obtained when using triphenyltin hy-
droxide as catalyst, though the substrate conversion was
lower than compared to Sn-OFs. This result highlights the
catalytic significance of a hydroxyl group attached to the
accessible tin-sites. Previous computational studies pre-
dicted an important role of a hydroxyl group attached to a
Lewis acid site partially functioning as Brønsted base and
hence, participating in activation of a substrate during the
carbon shift mechanism.8, 13, 35 Moreover, catalytic activity
of
partially
hydrolyzed
molecular
complexes
Figure 3. Dependence of the reaction rate (R) on the initial
pH of the reaction mixture.
M(OH)n(H2O)m, where M is Al or Cr, were proven exper-
imentally.36-38
mechanism was investigated using glucose as a substrate.
First, the epimerization of D-(1-13C)-glucose was studied.
Analysis of the obtained product mixture by 13C NMR
revealed the formation of D-(2-13C)-mannose as a major
product (Fig. 2). The solution of D-(1-13C)glucose demon-
strated two strong resonance peaks at 96.5 and 92.5 ppm
corresponding to C-1 of α-glucopyranose and β-
glucopyranose, respectively.34 Analyzing the reaction
solution after the experiment two additional 13C reso-
nance peaks were observed at 71.3 and 71.8 ppm related to
α-mannopyranose and β-mannopyranose (Fig. 2, Tab.
S4).34 This suggests that the reaction proceeds via rear-
rangement of the carbon skeleton owing to the carbon
shift as depicted in Scheme 1b. Molecular model com-
pounds of well-defined structure were investigated re-
garding their catalytic activity in order to gain more de-
tailed information on the type of active sites. Especially
tetraphenyltin and triphenyltin hydroxide were compared
as model compounds depicting segments of an ideal
crosslinked structure as well as partially hydrolyzed tin-
sites, respectively (Fig. S6). Tetraphenyltin showed no
Furthermore, investigations of the reaction rate of the
epimerization revealed a significant dependence on the
initial pH of the reaction mixture (Fig. 3). Increase of the
hydronium ion concentration results in protonation of
the hydroxyl group attached to the tin atoms and dimin-
ishes the concentration of Brønsted basic sites. Indeed,
the epimerization rate decreases when increasing the
acidity of the reaction medium. This proves that the OH-
group bound to Sn acts as base for the activation of aldos-
es. Hence, the mechanism for the epimerization based on
the carbon shift can be explained. For the hydride shift,
numerous computational and experimental evidences
also support the key role of a Lewis acidic atom and an
attached hydroxyl group,8, 13, 35-38 which is in line with our
findings. Nevertheless, the presence of a proton donor
nearby the coordination sphere of a Lewis acid site is
crucial for stabilization of the transition state for the hy-
dride shift mechanism. The examples of such proton
donors are an adjacent silanol for Sn-Beta12 or bulk water
for molecular salts.35 Apparently, the hydrophobic linkers
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experiments, 13C NMR chemical shifts of isotope labeled
of Sn-OFs disable any contacts of Sn-OH active sites with
1
2
3
4
5
6
7
8
sugars. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
bulk water and do not bear any proton donor themselves.
As a result, the absence of a proton donor next to the
coordination sphere of Sn makes the Sn-OFs unique solid
catalysts for the epimerization of aldoses. Hence, a reac-
tion mechanism is proposed based on the results for the
epimerization of glucose in the presence of Sn-OFs
(Scheme 3). The initial step is the interaction of the Sn-
OH group with the proton from the OH-group attached
to C-2 (a). Under elimination of water glucose undergoes
a bidentate coordination at the tin site (b). This transition
state (c) enables the carbon shift of C-3 bound to C-2
towards the formation of a new C-C bond with the initial
C-1 carbon. Finally, the formed epimeric aldose is de-
sorbed and replaced again by the initial hydroxyl group
resulting in a closed catalytic cycle. Based on the experi-
mental evidence and the feasible catalytic cycle three
peculiarities of Sn-OFs contribute to catalytic activation
of aldoses, namely (1) Lewis acidity of Sn; (2) Brønsted
basicity of the hydroxyl groups attached to Sn and (3)
highly hydrophobic environment of the Sn-OH active
sites due to aromatic organic linkers.
ACKNOWLEDGMENT
We gratefully acknowledge financial support by the German
Research Foundation (Deutsche Forschungsgemeinschaft,
DFG, RO 4757/5-1) and the excellence initiative. ID thanks
the Alexander von Humboldt Foundation and the Bayer
Foundation for the financial support. We also thank Noah
Avraham for the HPLC quantification, Heike Bergstein for
ICP OES analysis, and Karl-Josef Vaessen for TG and XRD
measurements. Regina Palkovits is gratefully acknowledged
for valuable discussions and the opportunity to carry out
independent research in her group.
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AUTHOR INFORMATION
Corresponding Author
* Dr. Marcus Rose (rose@itmc.rwth-aachen.de)
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
N2 physisorption isotherms, textural properties, elemental
analysis, SEM-EDX, DRIFTS, additional data of catalytic
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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).
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Porous tin-organic frameworks as solid catalysts enable a heterogeneously catalyzed epimerization of sugars in
aqueous solutions. The hydrophobic pores in combination with accessible and partially hydrolyzed tin sites are
responsible for an outstanding selectivity. This example demonstrates the potential of novel hybrid materials espe-
cially in the field of catalysis for tailor-made biogenic platform chemicals.
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For Table of Contents (TOC) only
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