Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of Catalysis

Article abstract of DOI:10.1021/jacs.5b13436

Mild calcination in ozone of a (POSS)-Sn-(POSS) complex grafted on silica generated a heterogenized catalyst that mostly retained the tetrahedral coordination of its homogeneous precursor, as evidenced by spectroscopic characterizations using EXAFS, NMR, UV-vis, and DRIFT. The Sn centers are accessible and uniform and can be quantified by stoichiometric pyridine poisoning. This Sn-catalyst is active in hydride transfer reactions as a typical solid Lewis acid. However, the Sn centers can also create Br?nsted acidity with alcohol by binding the alcohol strongly as alkoxide and transferring the hydroxyl H to the neighboring Sn-O-Si bond. The resulting acidic silanol is active in epoxide ring opening and acetalization reactions.

Full text of DOI:10.1021/jacs.5b13436

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Communication
Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of Catalysis
Evgeny V Beletskiy, Xianliang Hou, Zhongliang Shen, James R Gallagher,
Jeffrey T Miller, Yuyang Wu, Tiehu Li, Mayfair C Kung, and Harold H Kung
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13436 • Publication Date (Web): 17 Mar 2016
Downloaded from http://pubs.acs.org on March 17, 2016
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Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of
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Evgeny V. Beletskiya, Xianliang Hou,a,e Zhongliang Shen,a James R. Gallagher,c Jeffrey T. Miller,d
Yuyang Wu,b Tiehu Li,e Mayfair C. Kung,a* and Harold H. Kung.a*
a Chemical and Biological Engineering and b Chemistry Department, Northwestern University, Evanston, IL 60208
c Chemical Sciences Division, Argonne National Laboratory, Naperville, IL 60429
d Chemical Engineering Department, Purdue University, West Lafayette, IN 47907
e Northwestern Polytechnical University, Xi'an, Shaanxi 710072, P. R. China
Supporting Information Placeholder
lite channel16 and breaking it down into individual monosac-
ABSTRACT: Mild calcination in ozone of a (POSS)-Sn-
(POSS) complex grafted on silica generated a heterogenized
charides with the aid of HCl17 or high temperature18 is typically
required prior to a Sn-catalyzed transformation. Mechanistic
investigations utilizing poisoning experiments, DRUV-Vis,
DRIFTS and solid state NMR revealed that the reactions pro-
ceed via either Lewis acid catalysis or a less typical Lewis acid-
assisted Brønsted acid catalysis, thus demonstrating an unu-
sual case of dual-mode catalytic reactivity of a single site cat-
alyst.
catalyst which mostly retained the tetrahedral coordination of
its homogeneous precursor, as evidenced by spectroscopic
characterizations using EXAFS, NMR, UV-vis, and DRIFT. The
Sn centers are accessible and uniform, and can be quantified
by stoichiometric pyridine poisoning. This Sn-catalyst is ac-
tive in hydride transfer reactions as a typical solid Lewis acid.
However, the Sn centers can also create Brønsted acidity with
alcohol by binding the alcohol strongly as alkoxide and trans-
ferring the hydroxyl H to the neighboring Sn-O-Si bond. The
resulting acidic silanol is active in epoxide ring opening and
acetalization reactions.
The catalysts were prepared (Scheme I) by hydrosilylating a
Td Sn complex I, [(c-C6H11)7Si7O9(OSi(CH3)2(C2H3))O2]2Sn
(abbreviated as (POSS)-Sn-(POSS)), synthesized according to
our previous work,19 onto dimethylsilane-modified EH-5
fumed silica. The hydrosilylation reaction conditions under
which I retained its structural integrity were determined using
Many important industrial Lewis acid-catalyzed reactions
use homogeneous Lewis acids, although solid Lewis acids offer
advantages in the ease of handling, separation and regenera-
tion.1-4 Among solid Lewis acids, Sn-Beta zeolite is heavily in-
vestigated because of its thermal stability, water tolerance,
and ability to catalyze many selective transformations, even in
aqueous media.5-10 The active sites in Sn-Beta are isolated, tet-
rahedrally coordinated (Td) Sn,11 and possess properties dis-
tinct from the octahedral (Oh) Sn in small SnO2 clusters that
were generated in the channels and on the outer surface of the
beta zeolite.12,8,13 Size constraint due to the small channels of
zeolite has led to effort to incorporate Sn into mesoporous sil-
ica, but often there is a distribution of Sn species in these ma-
terials, especially at higher metal loadings.14,15
Scheme 1. Preparation of silica-supported catalysts II
and III
R
R
R
R
R
Si
R
R
Si
R
Si
O
Si
O
O
Si
O
O
Si
O
O
Si
O
R
O
R
O
R
O
R
O
O
R
O
R
O
R
O
R
Si
O
Si
O
Si
O
Si
O
Si
H
Silica-SiMe2
Pt catalyst
O
O
O
O
Si
Si
Si
Si
Sn
Sn
O
R
O
R
O
R
O
R
O
O
O
O
O
O
O
O
Si
R
Si
Si
R
Si
Si
Si
Si
Si
O
O
O
O
R
R
O
O
O
O
Si
R
Si
R
Si
R
Si
R
O
O
O
O
PhMe, 100°C
Si
Si
Si
Si
II
Si
O
Si
O
I
POSS-Sn-POSS
c
=
-C
R
6H11
OH
OH
OH
OH
Here we report the synthesis of a solid Lewis acid with uni-
form Td Sn(IV) centers supported on a high surface area non-
porous silica. These Sn centers were Lewis acids that cata-
lyzed many reactions including a Meerwein–Ponndorf–Verley
reaction, epoxide ring opening, acetal formation and glucose
isomerization. Because the support was nonporous, the cata-
lyst was also active in converting cellobiose to hydroxymethyl-
furfural (HMF). This is a reaction in which the reactant mol-
ecule is too large to be processed efficiently inside a beta zeo-
structure
POSS
represent
after
were converted
Squares
ozone
treatment. All R
groups
groups
O
O
O
O
Sn
into OH
III
NMR with a model reaction of I with 1,1,3,3,5,5-hexamethyl-
trisiloxane which served as a surrogate for silica (Figure S1).
CPMAS 119Sn NMR spectrum of II exhibited a single resonance
at -439 ppm (Figure S2a) characteristic of Td Sn, although the
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signal to noise ratio was low. Successful retention of the POSS-
the catalyst by filtration, and observing that the reaction
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Sn-POSS structure was also supported by EXAFS analysis (Ta-
ble S1 and Figure S2b), which showed a Sn coordination of
4.1±0.4 in II. The DRIFT spectrum showed sharp peaks in the
C‒H stretch region of 2800-3000 cm-1 from the cyclohexyl
groups of POSS and the methylsilyl groups on SiO2 (Figure
S3a).
stopped completely (Fig. S9). If benzyl alcohol was used as the
nucleophile instead of 2-propanol, the difference between II
and III was even larger. The conversion was complete within
10 min at room temperature using III (TON = 950), but was
only 50% after 3 hours at 80 °C with II.
Py poisoning of the reaction of styrene oxide with 2-
propanol was used to quantify and probe the uniformity of Sn.
The reaction remianed pseudo-first order in styrene oxide up
to high conversions even in the presence of py (Tables S5, S6
and Figure S8a). The exception was at the highest py/Sn ratio
of 54%, when the data deviated from pseudo first order
kinetics after 30 min for reasons yet to be investigated (Figure
S8a) and only initial data were used in calculating ke. The rate
constants decreased linearly with increasing py, as shown in
Figure 1, suggesting homogeneity of the Sn centers. Extrapola-
tion of the data showed that complete poisoning would occur
at py/Sn~80%, indicating that most of the Sn were active,
which was consistent with the 4.5 average coordination from
the EXAFS data and the fact that a large majority of Sn was
present as Td Sn. The higher than expected ke at ~15% py/Sn
was repeatable, and further work is needed to understand this.
III was formed by calcination of II in a flow of O2 and O3
mixture under mild temperature conditions to avoid possible
decomposition or rearrangement of the Sn centers. After cal-
cination, all organic functionalities attached to Si were oxi-
dized to SiOH as indicated by DRIFT (Figure S3a). EXAFS of
III showed that the Sn-O scattering increased slightly relative
to II and the average coordination number became 4.5±0.4
(Figure S3b and Table S1). There was a trace amount of higher
shells scattering, which may be due to SnOx clusters resulting
from “opening” of the tin sites with the water vapors forming
during the calcination process. This indicated that there
might have existed ca. 25% octahedral Sn as isolated center or
SnO2-type material, although the majority of the species were
predominantly single site, Td Sn. The EXAFS result was con-
sistent with the UV-vis spectrum, which showed an absorp-
tion peak at ~210 nm (Figure S7d), characteristic of charge
transfer from O2− to Td Sn4+.20 It should be noted that the ab-
sorption in the 283 nm region typical for Oh Sn was not de-
tectable for either II or III.21,22 The Sn loading of III was 1.1
wt.% as determined by ICP, translating into 90 μmol I/g SiO2
grafted, in line with the amount of I that disappeared from the
solution after hydrosilylation. Based on the footprint of I of 2.5
nm2 (from the X-Ray single crystal data)19 and a surface area
of silica of 190 m2/g, the surface coverage of II was estimated
to be 70%.
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+
III Py
O
O
OH
+
Ph
OH
Ph
50°C
150
100
50
0
The accessibility of Sn in II and III to pyridine (py) was in-
vestigated with DRIFT (Fig. S4 and S5). After the sample was
exposed to py vapor and He purged at 150 °C at a low flow rate,
peaks ascribed to physisorbed/hydrogen-bonded py (1445 and
1597 cm-1) and py coordinated to Lewis acid sites (1456, 1491,
1577 and 1613 cm-1)23 were detected for both samples. Trace
amounts of py adsorbed on Brønsted acid sites (1550 and 1640
cm-1) were also detected for III. The physisorbed and hydro-
gen bonded species were associated with the silica support
and could be removed with a high purging rate at 150 °C (Fig.
S5a). The remaining py bonded to Sn could be displaced by
aqueous NaOH, extracted into CDCl3, and quantified with 1H
NMR. The ratio of py to Sn was ca. 1:1 at 150°C, but decreased
at higher temperature or prolonged heating due to desorption
from the Sn Lewis acid site (Table S2).
0
20
40
60
80
100
Py/Sn (%)
Figure 1. Effect of the pyridine poisoning on the appar-
ent rate constant of the styrene oxide ring opening with 2-
propanol (from Table S5).
The quantitaive poisoning of Sn by py was surprising in view
of the 105 excess of alcohol to py and the results of the
following competitive binding experiment. The 13C CPMAS
NMR of 2-propanol adsorbed on III showed two broadened
resonances at 67 and 22.5 ppm (Fig. S8b), which were similar
to those of Sn 2-propoxide (Fig. S8c) but shifted from those of
2-propanol on silica. They did not appear to change upon
addition of py, implying that the alkoxide was not displaced
by py. Interestingly, py bound to the alkoxide-covered surface
at a ~1.1 Sn:py ratio (SI section Vc). DRIFT spectra revealed
that this py was bound to H+ as a pyridium ion with
characteristic peaks at 1491, 1546, 1617 and 1638 cm-1 (Figure 2,
top). UV-vis data showed that the original peak of III shifted
from ~210 nm to ~233 nm when 2-propanol was adsorbed and
The catalytic properites were examined using styrene oxide
ring opening with a 15-fold excess of 2-propanol. For III, the
reaction proceeded readily at 50°C with an apparent first order
kinetics in epoxide concentration. The apparent rate constant
was 2.0x10-2 min-1 which was equivalent to 2.2x105 min-1 (mol
Sn)-1 and TON = 490 at 40 min(Table S5-6), while II showed
only moderate reactivity at 80°C with ke= 8.6x102 min-1 (mol
Sn)-1. The effect of Sn loading was investigated with a lower Sn
loading sample (III-low with 0.21 wt.% Sn). Its activity per Sn
(ke= 1.6x105 min-1 (mol Sn)-1) was only 25% lower than III,
implying within uncertainties isolated Sn centers in III. This
was consistent with the structure of Sn center sandwiched
between bulky POSS ligands. Contribution of leached Sn to
the reaction of III was excluded by conducting the reaction at
room temperature until 64% conversion and then removing
dried at 150°C, consistent with formation of
a
pentacoordinated geometry of a Sn-2-propanol complex
(Figure 2, bottom).24 A small amount of SnO2 clusters at 280
nm was also observed, presumably due to hydrolysis of Sn
center at higher temperatures. Taken together, these results
indicated that 2-propanol adsorbed on III as 2-propoxide,
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generating a proton and expanding the coordination sphere
without cleaving any of the Sn‒O‒Si bonds.
benzyl alcohol was examined (Scheme 3, upper path). As es-
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4
5
6
7
8
tablished, alcohol binding to Sn in a form of alkoxide was
strong for III and it was not displaced by py or weaker bases
such as aldehyde. Thus, for III, the reaction would proceed via
the Sn bound alkoxide.25,26 The inhibitive effect of py was
small, as confirmed by the experiments (Table S7), which was
also consistent with similar observations reported for solid Zr
Lewis acids.27 II also catalyzed hydride transfer (TON ≈ 150
after 16 h for both II and III in toluene), even though the al-
cohol binding was weak. Interestingly, the reaction was accel-
erated in the presence of py (Table S9), and this was presum-
ably due to py shifting the alkoxide binding equilibrium via its
deprotonation. Aldehyde and alcohol can also undergo acetal
formation (Scheme 3, lower path) but Brønsted acids typically
catalyze this reaction. Thus, it was catalyzed by III and poi-
soned by py (Table S8), but not by II, similar to the epoxide
ring opening reaction.
PyH+
1638, 1617, 1546 & 1491 cm-1
Py
1598
cm-1
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1700
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1)
Scheme 3. Reaction of p-nitrobenzaldehyde with benzyl
alcohol via Meerwein–Ponndorf–Verley hydride-transfer
(upper path) and acetal formation (lower path).
Ar
O
200
225
250
275
300
325
H
Ph
O
HC
Ph
H
λ (nm)
H
O
+
O
+
Si
Si
O
O Si
or
II
III
Sn
Figure 2. Elucidation of the mechanism of styrene oxide
ring opening with 2-propanol. (Top) DRIFT of pyridine ad-
sorbed on III-2-propanol. (Bottom) DRUV-vis of III (red)
and III-2-propanol (green).
Ar
H
Ar
OH
PhMe
O
Si
O
100 °C
Ph
OH
III
Ar
H
Ar2
Ar3
O
Ph
O
O
Ar1
H
-
O
=
Ar p NO2C6H4
Si
Si
O
O
O Si
-
Ar1,2,3 Ph p NO2C6H4
or
=
We propose the mechanism for the epoxide ring opening
with 2-propanol and the effect of py on this reaction as follows
(Scheme 2). 2-Propanol was adsorbed onto a Sn center in III
dissociatively as 2-propoxide, while the hydroxyl hydrogen
was bound to an oxygen of a Sn‒O‒Si bond. This Sn Lewis
acid-activated acidic silanol was the active center in the acti-
vation of the epoxide oxygen and its binding to py via depro-
tonation poisoned the reaction. It should be mentioned that
under the same conditions, neither I nor II formed any stable
alkoxide adducts detectable by NMR, presumably due to their
steric and hydrophobic properties, and this accounted for
their low activity in the epoxide ring opening reaction.
Sn
Si
O
Interestingly, formation of metal alkoxides via addition of
alcohols to M‒OSi(H) fragments has previously been observed
or proposed,28-30 however we could not find a report where it
was linked to Brønsted acidity. Instead, the alcohol proton was
proposed to react with M‒OH to form H2O.29,30 Thus, it is
quite possible that the conclusion of formation of Brønsted
acidity observed in this study also applies to other metal zeo-
lites. For example, Román-Leshkov et al. have reported30 two
different deactivation profiles for a Sn-Beta zeolite catalyst in
hydride transfer and etherification reactions, which were
thought to proceed via Lewis and Brønsted acid catalyzed
pathways, respectively. If the Brønsted acid catalytic activity
were originating in Sn-Beta by SiOH binding to Sn in a same
way as in III, this could explain the observed drastic deactiva-
tion of the catalyst in the etherification process after the
“opening” of the Sn centers, which was detected by 119Sn DNP
NMR experiments.
Scheme 2. 2-Propanol and pyridine binding to III and a
proposed mechanism of the epoxide ring opening reac-
tion and its poisoning by pyridine
OH
Ph
OH
H
Si
Si
O
O
O Si
O Si
O
O
Si
O
O Si
Sn
Ph
O
Sn
Unlike Sn-Beta zeolites, where the Sn sites are located in-
side the hydrophobic pores, the active centers of III are ex-
posed on the surface of the hydrophilic support. Despite this,
III was active in styrene oxide ring opening by water (TON =
900 after 2h, 50°C, SI section VII) and in the presence of water
generated during acetal production. The TOF in the former
reaction was ≥450 h-1 for III at 50⁰C, which was comparable to
the literature TOF of 26-400 h-1 for ring-opening hydration of
various epoxides catalyzed by Sn-Beta catalysts at 40⁰C.31
Thus, the activity of III was comparable to Sn-Beta in this
epoxide hydration reaction. III also catalyzed glucose isomer-
ization in γ-valerolactone at 200°C (SI section VIII), although
H
Si
O
O Si
O
III
Si
Si
O
O
O Si
nm
UV-vis 235
Py
nm
Sn
UV-vis 210
O Si
Py
N
H
N
O
Si
Si
O
O
O Si
Si
Si
O
O
O Si
O
Sn
Sn
O Si
cm-1
O Si
cm-1
OH
Ph
IR 1456
IR 1546
To verify this proposal, the Meerwein–Ponndorf–Verley hy-
dride-transfer reaction between p-nitrobenzaldehyde and
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it was less active than Sn-Beta which was effective at 80⁰C for
for Atom-efficient Chemical Transformations (IACT), an En-
this Lewis acid-catalyzed hydride transfer reaction.7 The more
open configuration of the Sn center in III also catalyzed the
reaction of large substrates such as cellobiose. In a γ-valerolac-
tone-DMSO mixture at 180 °C, 5-(hydroxymethyl)-furfural
was formed with up to 30% yield and TON = 125 (Scheme 4),
after cleaving the glucoside bond and isomerizing and dehy-
drating the glucose. In contrast, the reported TON for Sn-Beta
in the isomerization of the disaccharide lactose was less than
3.16
ergy Frontier Research Center funded by US DOE Office of
Basic Energy Sciences. JTM and JRG were supported by DOE,
Office of Basic Energy Sciences, DE-AC-02-06CH11357.
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Scheme 4. Cellobiose conversion into (5-hydroxyme-
thyl)furfural catalyzed by III
OH
OH
III
O
HO
HO
CH2OH
O
HO
O
O
GVL, DMSO
180 °C
OH
OHC
OH
OH
Cellobiose
HMF, 30%
In summary, we have successfully synthesized a uniform, Td
Sn-based Lewis acid catalyst on a nonporous support utilizing
a Sn-precursor stabilized by a bulky silsesquioxane ligand. The
supported Sn catalyst was capable of catalyzing typical reac-
tions such as hydride transfer, but could also mediate
Brønsted acid catalyzed reactions such as epoxide ring open-
ing, acetal formation and glucoside bond cleavage. The
Brønsted acid pathway was suppressed in presence of bulky
hydrophobic substituents and pyridine additive, while Lewis
acid pathway remained intact, at least in simple hydride trans-
fer reactions. Thus, two tunable modes of activation are pos-
sible for a single catalytic site, and this offers a new avenue in
Sn-mediated catalysis.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, synthesis of materials, EXAFS,
NMR, DRIFT and UV-vis spectra, catalyst testing and poison-
ing experiment details are included in the supporting infor-
mation. This material is available free of charge via the inter-
net at http://pubs.acs.org.
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B.; Zhao, D. Adv. Funct. Mater. 2005, 15, 1377.
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AUTHOR INFORMATION
Corresponding Author
Mayfair C. Kung, Harold H. Kung
E-mail: m-kung@northwestern.edu, hkung@northwest-
ern.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
DOE Office of Basic Energy Sciences, DE-FG02-01ER15184 for
support of this work; the NUANCE facilities at Northwestern
University, the MR-CAT at the APS at the Argonne National
Laboratory (DE-AC02-06CH11357) for characterization; Cabot
Corporation for EH-5 silica sample, XH acknowledged support
by Chinese Scholarship Fund and partial funding by Institute
(31) Tang, B.; Dai, W.; Wu, G.; Guan, N.; Li, L.; Hunger, M. ACS
Catal. 2014, 4, 2801.
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