Confinement Effects in Lewis Acid-Catalyzed Sugar Conversion: Steering Toward Functional Polyester Building Blocks

Article abstract of DOI:10.1021/acscatal.5b01344

We report the use of solid Lewis acid catalysts for the conversion of tetrose sugars to four-carbon α-hydroxy acid esters (C₄-AHA), which are useful as functional polyester building blocks. Sn-β was by far the most active and selective catalyst, yielding up to 80% methyl vinyl glycolate (MVG), methyl-4-methoxy-2-hydroxybutanoate (MMHB), and α-hydroxy-γ-butyrolactone (HBL) combined at 95% conversion. A very high turnover frequency (TOF) of 330 mol_C4-AHA mol_Sn h^-1 was attained using Sn-β, a more than 6-fold increase compared with homogeneous SnCl₄·5H₂O. It is shown that, using different Sn-based catalysts with various pore sizes, the product distribution is strongly dependent on the size of the catalyst pores. Catalysts containing mainly mesopores, such as Sn-MCM-41 or Sn-SBA-15, prefer the production of the more bulky MMHB, whereas microporous catalysts such as Sn-β or Sn-MFI favor the production of MVG. This effect can be further enhanced by increasing the reaction temperature. At 363 K, only 20% MVG is attained using Sn-β, but at 433 K, this increases to 50%. Using a kinetic analysis, it was found that, in microporous catalysts, steric hindrance near the Sn active site in the catalyst pores plays a dominant role in favoring the reaction pathway toward MVG. Moreover, the selectivity toward both products is kinetically controlled.

Full text of DOI:10.1021/acscatal.5b01344

Research Article
pubs.acs.org/acscatalysis
Confinement Effects in Lewis Acid-Catalyzed Sugar Conversion:
Steering Toward Functional Polyester Building Blocks
†
,†,§
†
†
‡
Rik De Clercq, Michiel Dusselier,* Charles Christiaens, Jan Dijkmans, Remus Ion Iacobescu,
‡
,†
Yiannis Pontikes, and Bert F. Sels*
†Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium
‡Department of Materials Science, KU Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium
*
S Supporting Information
ABSTRACT: We report the use of solid Lewis acid catalysts
for the conversion of tetrose sugars to four-carbon α-hydroxy
acid esters (C -AHA), which are useful as functional polyester
4
building blocks. Sn-β was by far the most active and selective
catalyst, yielding up to 80% methyl vinyl glycolate (MVG),
methyl-4-methoxy-2-hydroxybutanoate (MMHB), and α-
hydroxy-γ-butyrolactone (HBL) combined at 95% conversion.
A very high turnover frequency (TOF) of 330 mol
molSn
C4‑AHA
−1
h
was attained using Sn-β, a more than 6-fold increase
compared with homogeneous SnCl ·5H O. It is shown that,
4
2
using different Sn-based catalysts with various pore sizes, the
product distribution is strongly dependent on the size of the
catalyst pores. Catalysts containing mainly mesopores, such as
Sn-MCM-41 or Sn-SBA-15, prefer the production of the more bulky MMHB, whereas microporous catalysts such as Sn-β or Sn-
MFI favor the production of MVG. This effect can be further enhanced by increasing the reaction temperature. At 363 K, only
2
0% MVG is attained using Sn-β, but at 433 K, this increases to 50%. Using a kinetic analysis, it was found that, in microporous
catalysts, steric hindrance near the Sn active site in the catalyst pores plays a dominant role in favoring the reaction pathway
toward MVG. Moreover, the selectivity toward both products is kinetically controlled.
KEYWORDS: solid Lewis acid, Sn-β, biomass conversion, α-hydroxy acids, polylactic acid (PLA), functional PLA,
methyl vinyl glycolate, α-hydroxy-γ-butyrolactone
INTRODUCTION
PLA suffers from a few limitations, however, which could
hamper its use for particular applications. More specifically,
■
51
Solid Lewis acids have attracted a lot of attention because of their
high catalytic activity in reactions such as the isomerization of
traditional, stereoregular L-PLA is very brittle and relatively
52,53
1
−10
(sometimes too) hydrophobic.
In addition, PLA is chemi-
carbohydrates;
Meerwein−Ponndorf−Verley reduc-
cally inert with few reactive centers, making surface and bulk
1
1−15
15−17
tions;
shifts;
Baeyer−Villiger oxidations;
1,2-hydride
and, more recently, aldol reactions. One of the key
reactions is the production of lactic acid or alkyl lactates from
53
modification a challenging task. By introducing functional or
1
8,19
20
adjustable chemical groups via homo- or copolymerization with
lactic acid, access to different properties in (PLA-based)
polyesters or anchor points for further modification can be
2
1−33
carbohydrates.
Lactic acid, a three-carbon (C ) α-hydroxy
3
acid, is considered one of the top carbohydrate-derived platform
54
obtained. During the conversion of hemicellulosic sugars to
3
4−36
chemicals
because it is a precursor to a wide range of useful
lactates with Sn-based catalysts, traces of four-carbon α-hydroxy
products, such as acrylic acid, pyruvic acid, propylene glycol, 2,3-
acid esters (C -AHAs), namely, methyl vinyl glycolate (MVG)
4
3
7−39
40
pentanedione,
and green solvents. Its most prominent use
and methyl-4-methoxy-2-hydroxybutanoate (MMHB), have
26,28,55
is as the renewable building block for the biodegradable plastic
been analyzed.
These compounds originate from tetrose
41,42
polylactic acid (PLA).
This polyester is considered a benign
sugars and glycolaldehyde (GA), the primary products of glucose
retroaldol. In contrast, lactic acid or methyl lactate (MLA)
originate from dihydroxyacetone (DHA) and glyceraldehyde
(GLY), the two triose retroaldol products of fructose.
synthetic substitute for petrochemical-based plastics. The latter
are usually nondegradable in nature, and this causes accumu-
lation of plastic fragments in marine environments, even at
4
3−47
remote locations.
PLA is of use in general applications such
as packaging (foils, containers, ...), coatings, and fibers, as well as
Received: June 26, 2015
Revised: August 20, 2015
48
49
in biomedicine (in vivo sutures, orthopedic devices, and drug
5
0
delivery ).
©
XXXX American Chemical Society
5803
DOI: 10.1021/acscatal.5b01344
ACS Catal. 2015, 5, 5803−5811

ACS Catalysis
Research Article
Scheme 1. Schematic Representation of the Conversion of Glucose and Fructose to α-Hydroxy Acid Esters and Their Use As
Functionalizable Polyester Building Units
Copolymerization of functional C -AHAs with lactic acid can
introduce functional groups in the PLA backbone, providing a
possible solution for some of the current limitations of PLA
microanalyzer (FEG EPMA) equipped with five wavelength
dispersive spectrometers (WDS) was used. Prior to conducting
the measurement, powder-like samples were embedded in resin
and allowed to harden. Then, the resin surface was ground,
polished, and coated with carbon. The microprobe was operated
4
(Scheme 1). As a proof of concept, our group successfully
introduced up to 12% of vinyl glycolic acid (hydrolyzed version
56
of MVG) in a PLA backbone. Subsequently, the incorporated
vinyl side groups in the polyester were found accessible to
functionalization via thiol−ene chemistry. The hydrophilicity of
the PLA-based polymer could be tuned by the choice of thiol via
at 15 kV and 50 nA. The Sn Lα -signal was detected using a
1
PETH crystal. The Sn and Si content were quantified using a
Cassiterite and Willemite standard, respectively. ZAF was used
for the matrix correction method. Ten different spots were
measured, and an average composition was calculated. N2
physisorption measurements were performed on a Micromeritics
Instruments Tristar 3000 at 77 K. All samples were degassed
5
6
this approach. We also elucidated the mechanistic details of C -
4
57
56
AHA formation from tetroses and glycolaldehyde using
homogeneous Sn halide catalysts in alcoholic media and showed
that MMHB could be synthesized in one pot and in high yields at
overnight at 673 K under a constant N flow prior to analysis.
2
3
63 K. However, (1) the selectivity toward MVG remained low
Specific surface areas were calculated by the Brunauer−
Emmett−Teller (BET) theory. Pore volumes were determined
by the t-plot method on the desorption branch.
and (2) homogeneous Sn catalysts are hard to recover for reuse.
The use of solid Lewis acid catalysts could provide a more robust
and environmentally sound solution for C -ester synthesis.
4
Catalytic Reactions. Reactions up to 363 K were performed
in 10 mL thick-walled glass vial reactors. In a typical reaction, 2.5
mmol of (S)-(+)-erythrulose hydrate (97%, Sigma-Aldrich), 100
mg of catalyst, and 50 mg of internal standard (1,4-dioxane,
Acros Organics) were weighed in a vial, and 5 mL of methanol
and a magnetic stirring bar were added. The glass vials were
closed by a crimp cap with a septum and placed in a multivial
heating block, preset at the desired temperature (usually 363 K).
Typically, the vials were allowed to react for 5 h under
continuous stirring. Afterward, the vials were quenched in an
ice bath. Reactions at temperatures over 363 K were performed
in stainless steel pressurized batch reactors (12 mL) in the same
Whereas the solid Lewis acid catalyzed conversion of
2
1−27,58−61
28
1,28,37,55
trioses,
pentoses, and hexoses
has been
extensively reported, that of tetroses remains largely unexplored.
Here, we systematically explore the conversion of tetrose sugars
to C -AHAs in alcoholic media with solid Lewis acid catalysts. A
4
selective route to MVG, the most versatile building block, is
unraveled by tuning catalyst design and process parameters on
the basis of insights in active site confinement effects.
EXPERIMENTAL SECTION
■
Catalysts. H-USY zeolites (CBV600, CBV712, and CBV720)
and Al-β (CP814E) were purchased from Zeolyst international.
The as-received catalysts were calcined in static air at 823 K (5 K
manner, except for an additional triple N -flushing procedure,
2
after which the reactor was filled with 20 bar of inert N . The
2
−1
pressurized reactors were placed in a preheated copper block at
temperatures ranging from 393 to 433 K. After reaction and ice
bath quenching to room temperature, excess pressure was
released. All reaction mixtures were centrifuged at 3000 rpm for 5
min to separate the catalyst prior to analysis.
min ) for 5 h prior to use to put the zeolites in their protonic
form. Si-β was purchased from Tosoh (HSZ-980HOA, Si/Al =
2
55). SnO was used as purchased (Alfa Aesar, <10 μm powder, 6
2
2
−1
m g ). SnO /Si-β was a physical mixture of SnO and Si-β. Sn-,
Ti-, and Zr-β; Sn-β deAl;
Sn-SBA-15 were synthesized according to methods known in
the art. A more detailed description is given in the Supporting
Information.
2
2
6
2
1,31
63
24
Sn-MFI; Sn-MCM-41; and
6
4
Analysis. MMHB and MVG were analyzed by GC−FID on a
3
0 m Agilent HP-5 column with N as carrier gas. HBL was
2
analyzed by GC−FID on a 30 m CP-SIL-5CB column with N as
2
Characterization. Powder X-ray diffraction patterns were
measured on a STOE Stadi P diffractometer using Cu Kα1
radiation (λ = 0.154 nm) equipped with a Ge monochromator
and image plate detector. Small-angle X-ray diffraction patterns
carrier gas. Product yields were calculated with internal
standards, taking into account the respective response factors
for each compound as determined via calibration curves with
commercial standards in the case of MVG (96%, TCI Europe)
65
(
for Sn-MCM-41 and Sn-SBA-15) were measured on a SAXSess
and HBL (>96%, TCI Europe) or by ECN-based calculations
2
mc instrument (Anton Paar) using line-collimated Cu Kα
radiation and a 2D imaging plate detector. For elemental analysis,
a Jeol Hyperprobe JXA-8530F field emission gun electron probe
for MMHB because of a lack of commercial standards. The
conversion of erythrulose was determined by measuring the
residual amount of erythrulose after reaction via HPLC (Agilent
5
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ACS Catalysis
200 Series). Residual amounts of other tetroses (i.e., threose
Research Article
1
Scheme 2. Proposed Reaction Scheme for the Conversion of
and erythrose) were not taken into account. For this, 1 mL of
Tetrose Sugars to C -AHAs
4
reaction mixture was left to evaporate under a mild N flow until a
2
dry syrup was obtained. Then, 1 mL of Millipore water was
added, and the solution was mixed and filtered over a 0.45 mm
PTFE filter and analyzed on a Varian Metacarb 67C column (300
×
6.5 mm) equipped with a refractive index detector using
Millipore water as the eluent. The conversion was calculated via a
calibration curve of (S)-(+)-erythrulose hydrate (97%, Sigma-
Aldrich). The column was kept at 358 K with a constant flow of
−1
0.5 mL min . Identification of compounds was performed by
GC/MS with electronic ionization (Agilent 6890 series equipped
with a 30 m HP-5MS capillary column with He as carrier gas),
1
3
C NMR, and by comparing GC retention times of the reaction
mixture with dilute solutions of commercial standards of
expected products.
RESULTS AND DISCUSSION
Catalyst Screening. Different Lewis acidic catalysts were
tested for the conversion of erythrulose (ERU) in methanol at
■
363 K (Table 1). Three main products were identified by GC,
Table 1. Catalyst Screening for the Conversion of ERU to C -
4
a
AHAs
yield of C -esters (%)
4
conversion
b
entry
catalyst
Si/M
MVG MMHB HBL total
(%)
1
2
3
4
5
6
7
8
9
CBV600
2.6
2.6
5.8
15
<1
2
3
11
12
5
<1
2
3
15
15
7
95
77
96
81
81
58
34
95
88
c
CBV600
CBV712
CBV720
Al-β
1
2
<1
<1
4
2
Lewis acidic extra-framework Al species yielded no significant
12.5
7
0
7
amounts of C -esters (entries 1−4), in contrast to their reported
4
d
Ti-β
500
500
480
480
480
480
62
17
2
3
24
7
21,22
activity for the conversion of trioses to alkyl lactates
aqueous tetrose isomerizations. A series of different β-zeolites
and
d
Zr-β
1
4
7
Sn-β
20
19
13
15
24
0
50
52
31
42
47
0
10
8
80
79
50
65
76
0
containing Lewis acidic heteroatoms (Al, Ti, Zr, and Sn, entries
e
Sn-β
5
−8) demonstrated the superior behavior of the Sn-β zeolite,
f
1
1
1
1
1
1
1
0
1
2
3
4
5
6
Sn-β
6
yielding a combined 80% of C -AHA product at 95% conversion.
4
g
Sn-β
8
96
93
37
7
This result exceeds the total yields obtained by using 5 mol % of
Sn-β deAl
5
homogeneous SnCl ·5H O (entry 16). Because negative
4
2
h
SnO /Si-β
38
0
2
controls (Si-β, SnO , and SnO /Si-β, entries 13−15) yield no
2
2
SnO2
1
0
0
1
products, the activity of Sn-β can be attributed to the presence of
Si-β (blank)
0
0
0
0
2
highly dispersed Sn species in the framework, in agreement with
SnCl ·
2
53
4
59
96
68−70
4
i
the literature.
The Sn-β catalyst retains its conversion and
5
H O
2
selectivity levels, even when doubling the substrate loading
(entry 9). Tripling the original substrate loading, however, results
in a significant decrease in yield (entry 10). Although this might
be due to an incomplete conversion, a prolonged reaction time
only slightly increased the product yields (entry 11). Because the
traditional bottom-up synthesis of Sn-β requires the use of
fluoride media and long crystallization times (≥10 days), we also
investigated the use of Sn-β prepared by a more accessible
a
Conditions: 1.25 mmol ERU, 100 mg of catalyst, 5 mL of methanol,
b
5
0 mg of 1,4-dioxane, 363 K, 5 h reaction time. Determined by
c
manufacturer (in the case of H-USY) or by EPMA analysis. Reaction
temperature of 393 K. Si/M ratio in the synthesis gel. 2.5 mmol of
d
e
f
g
ERU. 4.17 mmol of ERU. 4.17 mmol of ERU, 20 h reaction time.
h
i
Calculated from the amount of SnO and Si-β used. 2.5 mmol of
2
ERU, 5 mol % catalyst; 10 mol % of catalyst yielded 76% of MMHB
after 5 h.
method, that is, by grafting Sn onto a dealuminated commercial β
GC/MS, and 13C NMR (Supporting Information, Figure S1):
MMHB, MVG, and α-hydroxy-γ-butyrolactone (HBL) (struc-
tures are shown in Scheme 1 and 2). The latter is a cyclic α-
hydroxy acid that is an important intermediate in the production
according to a procedure proposed by our group
1,31,71
(further
denoted as Sn-β deAl). Its behavior matched that of the
hydrothermal Sn-β (entry 12).
Time Profile. The catalytic action of Sn-β was monitored in
time (Figure 1) and compared with literature values for
66,67
of herbicides and pharmaceutical compounds.
Erythrose and
5
7
threose (tetrose isomerization) were detected in trace amounts.
homogeneous SnCl ·5H O. The framework-incorporated Sn
4 2
The proposed reaction mechanisms for C -AHA formation, in
is more active than the dissolved Sn because the respective
turnover frequencies (TOF, h ) were 330 mol
4
56,57
−1
−1
line with our earlier reports
and the work on a C + C aldol
route toward HBL reported by Van de Vyver et al., are shown in
molSn
1
3
C4‑AHA
20
−1
h
for Sn-β (calculated from the initial linear part of total C4-
Scheme 2. H-USY zeolites with different Si/Al ratios containing
ester yield, see Figure 1; and Si/M ratio from Table 2) versus ±54
5
805
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Research Article
dimensional micropore system of 10-membered rings (5.5−6
Å) with straight channels interconnected by sinusoidal channels.
The BEA topology contains 12-membered straight channels
(
6.6−6.7 Å, along axis [100]) interconnected by 12-membered
73
zigzag channels (5.6 Å, along axis [001]). MCM-41 and SBA-
5, on the other hand, mainly consist of a regular arrangement of
1
cylindrical mesopores in a hexagonal array but can also contain
small amounts of micropores. Usually, mesopore diameters
range from 2 to 6.5 nm and from 2 to 30 nm for MCM-41 and
SBA-15 respectively, depending on their synthesis. The
structures of MFI, β, and mesoporous silica are shown
schematically in Figure 2.
Figure 1. Conversion and yield as a function of time for a typical
reaction of ERU (1.25 mmol) converted with Sn-β (100 mg) in
methanol (5 mL) at 363 K.
Figure 2. Schematic representation of the pore channels in (a) MFI, (b)
β (BEA), and (c) mesoporous MCM-41 and SBA-15.
molC4‑AHA molSn− h−1 for the salt. A similar trend, namely, a 10-
1
57
fold increase, has been reported for the conversion of trioses to
lactates.
6
1
The catalysts were analyzed by powder-XRD and N2-
physisorption (Table 2) and compared with their fully siliceous
analogues (Figures S2−S10). A high crystallinity was confirmed
for Sn-β and Sn-MFI, and the presence of long-range order was
verified for the mesoporous materials. The pore size distributions
for Sn-MCM-41 and Sn-SBA-15 showed the presence of uniform
mesopores with a pore width of 3.0 and 3.8 nm, respectively. For
Influence of Pore Geometry on Product Selectivity.
Remarkably, Sn incorporated in a zeolite matrix shows selectivity
patterns different from those of soluble Sn: with SnCl ·5H O,
4
2
MMHB is by far the major product (Table 1, entry 16), whereas
Sn-β produces significant amounts of MVG and HBL, as well
(
Table 1, entry 8). We investigated if this was due to differences
in Brønsted acidity in the reaction media, as dissolving SnCl4·
H O in methanol potentially introduces strong Brønsted acidity
all materials, no diffraction lines corresponding to SnO were
2
5
observed. To verify a successful Sn incorporation, the catalytic
activity of the series was asserted with the DHA to MLA reaction
under the literature conditions (Table S1). All catalysts gave high
MLA yields, proving the incorporation of catalytically active Sn
species in the framework or silica walls.
2
−
56
due to Cl dissociation and in situ HCl formation. When
reacting ERU with Sn-β in methanol in the presence of 1 mol %
HCl (relative to ERU), a strong decrease in product yields was
noticed (8 and 3% for MMHB and MVG, respectively, after 1 h).
Thus, the difference in selectivity does not seem to originate from
differences in Brønsted acidity. Another explanation might be
found in spatial limitations of the Sn active site inside the pores of
the catalyst. Such a trend was observed earlier for the cyclization
of citronellal into isopulegol, for which it was shown that the
available space around the catalytic Lewis center had an influence
The catalysts were then tested for the conversion of ERU to
C -AHAs in methanol (Table 3). Sn-MCM-41 acquires high
4
product yields after 5 h of reaction (entry 4), matching those of
SnCl ·5H O, Sn-β, and Sn-β deAl (entries 1−3). The low activity
4
2
of Sn-MFI (entry 5) could be due to diffusional limitations of the
product or reagents in the narrow channels of the crystals, in
agreement with empirical estimations of the kinetic molecule
diameters compared with the zeolite pores (Table S2). Sn-SBA-
15 is also less active than Sn-β or Sn-MCM-41 (entry 6), and
triple catalyst loadings were required to match Sn-β yields after 5
h of reaction (entry 7). Interestingly, the product distributions
70,72
on the diastereoselectivity of the reaction.
To assess whether
similar effects could explain the selectivities in C -AHA
4
formation, a series of solid Sn catalysts with varying pore sizes
was synthesized for catalytic testing: Sn-MFI, Sn-β, Sn-MCM-41,
and Sn-SBA-15. The MFI topology consists of a three-
Table 2. Sample Composition and Physicochemical Characteristics of Different Heterogeneous Sn Catalysts
BET surface area (m2g−1
)
c
pore volume (cm g−1)
3
a
d
e
d
e
f
entry
catalyst
Sn-MFI
Si/M
total
micropore
mesopore
total
micropore
mesopore
avg pore width (Å)
1
2
3
4
5
269
480
62
444
550
609
960
712
428
542
441
35
0
0
0.19
0.23
0.6
0.19
0.23
0.18
0.008
0.12
0
5.5−6
6.6−6.7
6.6−6.7
30
Sn-β
0
g
Sn-β deAl
Sn-MCM-41
Sn-SBA-15
168
925
468
0.42
0.69
0.41
36
0.69
0.53
b
50
244
38
a
b
c
Determined by EPMA analysis. Si/M ratio in the synthesis gel (not determined by EPMA). Ideally, the BET theory should not be applied to
zeolites, but for literature comparison, the calculated BET surface areas for Sn-β, Sn-β deAl, and Sn-MFI are also given, albeit possibly
7
4
d
e
underestimated. Determined using the t-plot method. Calculated by subtracting the micropore surface area or pore volume from the total surface
f
area or pore volume. Values obtained from literature (for Sn-MFI and Sn-β) or from the pore size distributions (for Sn-MCM-41 and Sn-SBA-15).
The large mesopore volume is likely due to intercrystalline porosity/voids.
g
1
5
806
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Research Article
Table 3. Conversion of ERU to C -AHAs with Different Sn-
Based Catalysts
disturb this 1,4-nucleophilic addition, increasing the selectivity to
MVG (and maybe HBL by equilibrium back-reactions). Such
selectivity control shows the characteristics of transition-state
4
a
yield (%)
75
shape selectivity, according to Weisz.
entry
catalyst
MVG
MMHB
HBL
conv (%)
A similar effect was recently discovered by Dusselier et al., who
have noticed a great increase in selectivity toward lactide in the
conversion of lactic acid using microporous H-β zeolites,
c
1
2
3
4
5
6
7
8
SnCl ·5H O
Sn-β
Sn-β deAl
2
20
24
3
53
50
47
61
12
26
57
4
10
5
96
95
93
95
87
79
95
86
4
2
76
compared with mesoporous or nonporous catalysts. To further
corroborate this hypothesis, we investigated the use of Sn-β in
ethanol for forming the ethyl analogues ethyl vinyl glycolate
Sn-MCM-41
7
d
Sn-MFI
9
8
Sn-SBA-15
3
6
(
EVG) and ethyl-4-ethoxy-2-hydroxybutanoate (EEHB) from
e
Sn-SBA-15
3
5
ERU. Previously, we noted that the use of ethanol slightly
f
f
f
Sn-β
33
11
8
increased the selectivity to EVG (relative to EEHB) with SnCl ·
4
a
56,57
Conditions: 1.25 mmol of ERU, 100 mg of catalyst, 5 mL of
5H O.
With Sn-β, however, the change in selectivity is far
2
c
methanol, 50 mg of 1,4-dioxane, 363 K, 5 h reaction time. 2.5 mmol
more pronounced in ethanol (Table 3, entry 8, and Figure 3).
The lower nucleophilic strength of ethanol combined with the
spatial limitations inside the pores significantly alters the product
selectivity toward EVG. In ethanol with Sn-β, EEHB comprises
d
e
of ERU, 5 mol % catalyst. 20 h reaction time. 300 mg of catalyst.
f
Reaction in ethanol for 20 h. Instead of MVG and MMHB, ethyl
analogues of ethyl vinyl glycolate (EVG) and ethyl-4-ethoxy-2-
hydroxy-butanoate (EEHB) are analyzed here.
only 21% of the total C -AHA fraction. This result is consistent
4
with our hypothesis that differences in selectivity between
with the mesoporous Sn-MCM-41 and Sn-SBA-15 catalysts are
homogeneous SnCl ·5H O and porous heterogeneous catalysts
very similar to that of homogeneous SnCl ·5H O, with MMHB
4
2
4
2
are primarily due to a constraint active site inside the pores of the
catalyst.
Influence of Temperature on Reaction Pathways. The
influence of temperature on the product distribution was
as the main product. As mentioned before, Sn-β produces
significantly more MVG, even though MMHB is still the major
product. Within the small pores of Sn-MFI, even more MVG and
HBL are made relative to MMHB, albeit with lower total yields.
The normalized product distributions in order of decreasing pore
size (Figure 3) suggest a direct correlation between the pore size
assessed with homogeneous SnCl ·5H O, mesoporous Sn-
4 2
MCM-41, and Sn-SBA-15, and microporous Sn-β (Table 4).
With SnCl ·5H O, no major differences are observed at higher
4
2
reaction temperatures (even up to 433 K, entries 1−5). Sn-
MCM-41 and Sn-SBA-15, on the other hand, show a slight
increase in MVG and concomitant decrease in MMHB at higher
temperatures (entries 6−15) with a maximum MVG yield of 10−
Table 4. Conversion of ERU to C -AHAs at Various
4
a
Temperatures
yield (%)
entry
catalyst
T (K) MVG MMHB HBL conv (%)
b
1
2
3
4
5
6
7
8
9
SnCl ·5H O
353
363
393
413
1
2
54
53
53
52
57
52
61
62
60
44
49
57
50
46
42
50
50
29
20
18
3
4
94
96
4
2
Figure 3. Influence of pore size of various Sn-based catalysts on the
normalized C -AHA distribution in the conversion of ERU in methanol.
2
4
95
4
2
2
95
c
433
353
363
393
413
7
3
96
of the catalyst and the C -AHA product distribution. The
4
Sn-MCM-41
2
4
94
selectivity to the bulkier MMHB increases with increasing pore
sizes, whereas the selectivity to MVG and HBL increases with
decreasing pore sizes.
3
7
>99
>99
>99
>99
95
6
7
10
10
2
8
These results suggest that confinement effects play a role in the
reaction pathways (Scheme 2). A crucial intermediate that is
assumed to largely influence the product selectivity in tetrose
c
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
433
353
363
393
413
6
d
Sn-SBA-15
5
3
4
92
5
6,57
conversion
is the hemiacetal of vinyl glyoxal (VG-HA,
7
5
94
Scheme 2). This intermediate can undergo two different reaction
pathways, each resulting in a different final product: A direct 1,2-
hydride shift of VG-HA produces MVG, trapping the vinyl bond
in the final product (route A). On the other hand, if a methanol
molecule is added to the terminal vinyl bond via a 1,4-
nucleophilic addition prior to this 1,2-hydride shift, the final
product is MMHB and the terminal vinyl group is replaced by a
10
13
13
20
25
34
50
5
95
c
433
353
363
393
413
7
92
Sn-β
2
90
10
6
95
>99
>99
>99
7
c
433
8
57
terminal methoxy group (route B). Homogeneous SnCl4·
5
H O and the mesoporous catalysts seem to prefer the latter
a
2
Conditions: 1.25 mmol of ERU, 100 mg of catalyst, 5 mL of
b
route, probably because of the high reactivity of the vinyl group in
conjugation with the carbonyl group in VG-HA. The spatial
constrictions in the narrow channels of Sn-β and Sn-MFI might
methanol, 50 mg of 1,4-dioxane, 5 h reaction time. 2.5 mmol of ERU,
c
5 mol % of catalyst. Sample taken after 1 h reaction time to avoid
d
product degradation. 300 mg of catalyst.
5
807
DOI: 10.1021/acscatal.5b01344
ACS Catal. 2015, 5, 5803−5811

ACS Catalysis
3% at 433 K. The microporous Sn-β catalyst shows a more
Research Article
1
temperature-dependent selectivity pattern (entries 16−20): at
lower reaction temperatures (353−363 K), MMHB is the major
product (50% versus 13−20% of MVG), but at 393 K, Sn-β
produces approximately equimolar amounts of MVG and
MMHB. At the highest temperatures (413−433 K), the main
product with Sn-β is MVG, with a yield up to 50% (versus 18% of
MMHB). The latter result is in line with an earlier report stating
that 56% of MVG was obtained when reacting erythrose with Sn-
55
β in methanol at 433 K for 20 h. The changing selectivities are
strikingly visualized in the corresponding chromatograms of the
reaction mixtures (Figure S11) and in Figure 4, which plots the
Figure 4. Influence of temperature on selectivity toward MVG,
compared with the total amount of MVG and MMHB produced, for
different Sn-based catalysts.
yields of MVG relative to the combined yields of MVG and
MMHB. These results suggest that temperature alone has little to
no isolated effect on the preferred reaction pathway; only in
combination with confinement effects inside the pores of the
heterogeneous catalyst is the selectivity altered.
Note that even though the mesopores of Sn-SBA-15 are
slightly bigger than those of Sn-MCM-41, the product selectivity
of the former is somewhat more sensitive to temperature. This
might be due to the more pronounced presence of micropores in
the SBA-15 structure (see Table 2, entries 4, 5). The
postsynthetic Sn-β deAl behaved very similarly with changing
temperature compared with hydrothermal Sn-β (Figure S12).
Kinetic Analysis. To further understand the selectivity
changes at higher temperatures, a kinetic study was performed on
Figure 5. (a−c) Arrhenius plots of the conversion of ERU to MVG (red
dots) and MMHB (black squares) with calculated activation energies
and pre-exponential factors, k (left), and accompanying rates of MVG
0
and MMHB formation for different temperatures (right) for (a) SnCl4·
5
H O, (b) Sn-MCM-41, and (c) Sn-β. (d) Ratio of the initial rate at
2
which MVG is formed relative to the initial production rate of MVG and
MMHB combined (from the right side of parts a−c) as a function of the
temperature.
the conversion of ERU with SnCl ·5H O, Sn-MCM-41, and Sn-β
4
2
as representatives of for soluble, mesoporous, and microporous
catalysts, respectively. Several reactions in the range of 353−373
K were carried out, and the product formation rates were
determined from the initial, linear parts in the kinetic plots
intercept with the y-axis. Using the experimental values of E and
a
k0, a theoretical evolution of the reaction rate as a function of the
reaction temperature was plotted for both compounds according
to the formula k = k exp(−E /RT) (Figure 5, right). For SnCl ·
(
details, see Figure S13) and used for the construction of
Arrhenius plots (Figure 5 (left)). Note that average Si/M values
were used to calculate the corresponding TOFs according to
EPMA (Table 2), which might also include catalytically inactive
Sn (e.g., Sn incorporated in the silica walls of Sn-MCM-41).
0
a
4
5H O, the formation of MMHB and MVG have comparable
2
−
1
apparent activation energies (80.8 and 70.3 kJ mol ,
respectively) but significantly different values for k , that is,
0
3
Apparent activation energies (E ) for the formation of MVG
that of MMHB is approximately a factor of 10 higher (Figure 5a,
a
and MMHB were derived from the slopes of these plots, whereas
left). Therefore, despite similar activation energies, the rate of
MMHB formation exceeds that of MVG at all calculated
temperatures (Figure 5a, right).
the pre-exponential factor k (which is correlated to entropic
0
77
factors, according to Eyring ) was derived from the calculated
5
808
DOI: 10.1021/acscatal.5b01344
ACS Catal. 2015, 5, 5803−5811

ACS Catalysis
Research Article
(−98.4 J mol− K ). This suggests that the active Sn site in the
micropores of the Sn-β catalyst forms the required substrate
orientation to convert VG-HA to MVG more efficiently than
toward MMHB. This strongly indicates that steric hindrance
inside the catalyst pores plays a dominant role in steering the
preferred reaction route and, thus, also the product selectivity.
Although the interpretations made in this section are based on
the assumption that true kinetics were measured, diffusional
limitations for MMHB in Sn-β cannot be conclusively excluded.
1
−1
For the mesoporous Sn-MCM-41, the activation energy for
MMHB remains about the same as for SnCl ·5H O (83 kJ
4
2
−1
−1
mol ), whereas that of MVG is higher (101 kJ mol ). However,
here, k for MVG formation slightly exceeds that of MMHB
0
(approximately by a factor of 50; see Figure 5b, left). Although
the rate of both compounds increases with increasing temper-
ature, the rate of MVG formation does not exceed that of MMHB
at the studied temperatures (Figure 5b, right).
The microporous Sn-β behaves very differently compared with
SnCl ·5H O or Sn-MCM-41. Here, MMHB formation has a
−1
The apparent E of 42.3 kJ mol with Sn-β is nearly half of the Ea
4
2
a
much lower activation energy than the production of MVG (42.3
calculated for the nonmicroporous catalysts (80.8−83.3 kJ
−
1
−1
and 79.2 kJ mol , respectively), but the value of k for MVG
mol ), which would be consistent with diffusionally limited
0
5
significantly exceeds that of MMHB by a factor 10 (Figure 5c,
transport inside the catalyst pores. Therefore, one could
hypothesize that diffusional limitation of MMHB might steer
the product selectivity toward MVG through product-type shape
selectivity rather than transition state selectivity control. In that
case, however, one would expect significant differences in the
product distribution for different crystal sizes of Sn-β. Yet, similar
results were obtained at different temperatures for the post-
treated Sn-β deAl, of which the crystal sizes are orders of
magnitude smaller than that of Sn-β synthesized in fluoride1
medium (approximately 10−30 nm vs 1−1.5 μm, respectively)
(Figure S12 and Table S3). This strongly suggests that the
observed product selectivity of Sn-β is not governed by a physical
phenomenon, but rather, by a chemical phenomenon such as
left). The effect of such a high k for MVG expresses itself in the
0
model (Figure 5c right): at low temperatures (<393 K), the rate
of MMHB formation is higher than that of MVG, whereas at high
temperatures (>393 K), the rate of MVG increases quickly,
eventually surpassing the rate of MMHB formation. These
theoretical calculations of both reaction rates derived from k and
0
Ea correspond very well to our experimental observed product
distributions. Indeed, Figure 5c (left) predicts an intersection
point at 383 K, meaning that at this temperature, the rates of
MVG and MMHB production with Sn-β should be equal. Close
to this temperature, at 393 K, nearly equimolar yields of MVG
and MMHB were obtained after 5 h (25 and 29% for MVG and
MMHB, respectively; Table 4, entry 18).
Combined, the data suggest that the product selectivity for this
reaction is kinetically controlled, that is, the product with the
fastest production rate will be the favored product in the final
mixture. To visualize this, we plotted the ratio of the initial rate at
which MVG is formed relative to the total production rate of
MVG and MMHB as a function of the temperature (Figure 5d).
A clear similarity with the final product yield distribution shown
active site confinement. Moreover, for Sn-β deAl, the E for
a
−1
MMHB formation is 57.2 kJ mol , still less than for the
nonmicroporous catalysts, but no longer following the E /2 rule
a
(Figure S15). Similar to conventional Sn-β, a strong decrease in
−1
−1
activation entropy (−71.0 J mol K ) was noticed for MMHB
(Figure S14). Thus, the results obtained with the much smaller
Sn-β deAl crystals, in which the presence of diffusional
limitations is less likely, are in line with our hypothesis of
steering product selectivity through steric hindrance of the
transition state around the Sn active site, in the absence of
diffusional limitations.
in Figure 4 is noticed. The pre-exponential factor, k , is usually
0
strongly correlated to entropic factors, which can be calculated
through Eyring−Polanyi plots (Figure S14). From the slopes and
‡
intercepts of these plots, we calculated the entropy (ΔS ),
Catalyst Recycling. Finally, we have assessed the reusability
of the best-performing micro- and mesoporous catalysts, that is,
Sn-β and Sn-MCM-41. For this, we performed five consecutive
reactions with the same catalyst (which was separated from the
reaction mixture via centrifugation) using an intermediate
calcination step. The results are given in Figure 6, which shows
that the Sn-β catalyst can be reused without significant losses in
conversion or selectivities for 5 consecutive runs. For Sn-MCM-
‡
‡
enthalpy (ΔH ), and Gibbs energy (ΔG ) of activation for both
‡
products for different Sn catalysts (Table 5). Here, ΔS
‡
‡
Table 5. Entropy (ΔS ), Enthalpy (ΔH ), and Gibbs Energy
‡
(
ΔG ) of Activation for MVG and MMHB Formation with
Different Sn Catalysts
SnCl ·5H O
Sn-MCM-41
Sn-β
4
2
4
1, a slight decrease in the conversion is noticed, although total
MVG
ΔS‡
ΔH
ΔG
ΔS
ΔH
ΔG
a
−44.4
67.4
83.5
10.4
77.9
74.1
−2.3
97.9
−1.7
76.2
product yields did not change significantly after 5 runs.
‡
b
‡
b
,
c
c
98.7
76.8
CONCLUSIONS
Sn-based solid Lewis acids are highly active catalysts for the one-
step conversion of tetrose sugars toward four-carbon α-hydroxy
■
‡
a
MMHB
−34.3
79.5
−98.4
39.3
‡
b
‡ ,
b
92.0
75.0
acid esters (C -AHA), useful as functional polyester building
4
a
Units: J mol−1 K . Units: kJ mol . Calculated at 363 K.
−1
b
−1
c
blocks. Sn incorporated in the zeolite matrix of Sn-β was
significantly more active than soluble SnCl ·5H O, achieving an
4
2
−1
−1
represents the change in entropy upon forming an activated
transition state complex with the active site, which is generally
correlated to changes in degrees of freedom (of translation,
rotation, vibration). For SnCl ·5H O, the formation of MMHB is
initial TOF of 330 mol
molSn h , as compared with ±54
C4‑AHA
−1
−1
molC4‑AHA molSn h , respectively. We have shown that Lewis
acid confinement effects strongly control the product selectivity,
that is, microporous Sn catalysts produce significantly more
MVG than mesoporous or soluble catalysts, which favor MMHB
formation. This effect can be further magnified at higher reaction
temperatures. By increasing the temperature from 353 to 433 K
using a Sn-β catalyst, the MVG yield could be increased from
20% to 50%. It is likely that, in microporous catalysts, the
formation of MMHB is sterically hampered inside the catalyst
4
2
−1
more entropically favored than MVG (10.4 versus −44.4 J mol
−1
‡
K , respectively; a negative ΔS means a loss of entropy and,
thus, a more ordered transition state complex). Using Sn-β,
however, the formation of an activated complex toward MVG
−1
−1
requires little loss in entropy (−1.7 J mol K ), whereas the
activation toward MMHB requires a strong decrease in entropy
5
809
DOI: 10.1021/acscatal.5b01344
ACS Catal. 2015, 5, 5803−5811

ACS Catalysis
Research Article
Present Address
§
Chemical engineering, California institute of Technology,
Pasadena, California 91125, United States
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.D.C. thanks IWT (Agency for Innovation by Science and
Technology, project number 131404) for financial support. M.D.
acknowledges FWO Vlaanderen for postdoctoral funding and
the Belgian American Educational Foundation (BAEF) for his
honorary fellowship. M.D. and B.F.S thank the Research Counsil
of KU Leuven, IDO KP/14/003. The EPMA-WDS work has
been feasible due to the support of the Hercules Foundation
(
project ZW09-09). We thank professor Johan Martens and Stef
Kerkhofs for measurement of small-angle X-ray diffraction
patterns.
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E-mail: bert.sels@biw.kuleuven.be.
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