Epoxide ring opening with alcohols using heterogeneous Lewis acid catalysts: Regioselectivity and mechanism

Article abstract of DOI:10.1016/j.jcat.2018.11.038

Lewis acidic catalytic materials are investigated for the regioselective ring opening of epoxides with alcohols. For ring opening epichlorohydrin with methanol, the catalytic activity shows a strong dependence on the type of support and Lewis acidic species used. While Sn-SBA-15 is catalytically active, significantly higher catalytic activity can be achieved with hydrothermally synthesized zeolites of which Sn-Beta is 6 and 7 times more active than Zr-Beta or Hf-Beta, respectively. Sn-Beta is determined to be more active and more regioselective for epoxide ring opening of epichlorohydrin with methanol than Al-Beta. For Sn-Beta, the activation energy for the reaction between epichlorohydrin and methanol is determined to be 53 ± 7 kJ mol^?1. For epichlorohydrin, the activation energy barrier and experimentally observed regioselectivity are found using DFT to be consistent with a concerted reaction mechanism involving activation of the epoxide on an alcohol adsorbed on the catalytic site and nucleophilic attack by a second alcohol. The epoxide is shown to impact the regioselectivity and the mechanism since isobutylene oxide is selectively ring opened by methanol to form the terminal alcohol. DFT calculations indicate the mechanism for isobutylene ring opening involves epoxide activation and ring opening on an alcohol adsorbed onto the catalytic site. Finally, catalyst reuse testing indicates that Sn-Beta can be used for multiple reactions with no decrease in activity and limited to no leaching of the tin site, demonstrating Sn-Beta is a promising catalytic material for epoxide ring opening reactions with alcohols.

Full text of DOI:10.1016/j.jcat.2018.11.038

Journal of Catalysis 370 (2019) 46–54
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Epoxide ring opening with alcohols using heterogeneous Lewis acid
catalysts: Regioselectivity and mechanism
Nitish Deshpande a, Aamena Parulkar a, Rutuja Joshi a, Brian Diep a, Ambarish Kulkarni b,
Nicholas A. Brunelli a,
⇑
a Ohio State University, William G. Lowrie Department of Chemical and Biomolecular Engineering, 151 W. Woodruff Ave., Columbus, OH 43210, United States
b University of California Davis, Department of Chemical Engineering, 1 Shields Avenue, Davis, CA 95616, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Lewis acidic catalytic materials are investigated for the regioselective ring opening of epoxides with alco-
hols. For ring opening epichlorohydrin with methanol, the catalytic activity shows a strong dependence
on the type of support and Lewis acidic species used. While Sn-SBA-15 is catalytically active, significantly
higher catalytic activity can be achieved with hydrothermally synthesized zeolites of which Sn-Beta is 6
and 7 times more active than Zr-Beta or Hf-Beta, respectively. Sn-Beta is determined to be more active
and more regioselective for epoxide ring opening of epichlorohydrin with methanol than Al-Beta. For
Sn-Beta, the activation energy for the reaction between epichlorohydrin and methanol is determined
to be 53 7 kJ molÀ1. For epichlorohydrin, the activation energy barrier and experimentally observed
regioselectivity are found using DFT to be consistent with a concerted reaction mechanism involving acti-
vation of the epoxide on an alcohol adsorbed on the catalytic site and nucleophilic attack by a second
alcohol. The epoxide is shown to impact the regioselectivity and the mechanism since isobutylene oxide
is selectively ring opened by methanol to form the terminal alcohol. DFT calculations indicate the mech-
anism for isobutylene ring opening involves epoxide activation and ring opening on an alcohol adsorbed
onto the catalytic site. Finally, catalyst reuse testing indicates that Sn-Beta can be used for multiple reac-
tions with no decrease in activity and limited to no leaching of the tin site, demonstrating Sn-Beta is a
promising catalytic material for epoxide ring opening reactions with alcohols.
Received 11 September 2018
Revised 29 November 2018
Accepted 30 November 2018
Keywords:
Lewis acidic zeolites
Epoxide ring opening
Sn-Beta
Cluster calculations
Regioselectivity
Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction
stand factors controlling catalytic activity and selectivity for these
reactions.
Epoxides are versatile chemical intermediates that can be trans-
formed to produce many products. Indeed, epoxides are commonly
transformed through ring opening reactions with different nucle-
ophiles such as water [1,2], alcohols [3–12], amines [8,13–16],
and other substrates [17–20] to produce a broad range of bi-
functional products. In particular, alcohol ring opening products
are commonly encountered in compounds relevant to the pharma-
ceutical and solvents industry [21,22]. The key challenge for these
reactions is selectivity, because of the tendency of epoxides – espe-
cially terminal epoxides like epichlorohydrin – to polymerize [23].
This necessitates the reaction be carried out under mild conditions.
Additional selectivity challenges result from the fact that the ring
opening can occur with attack of the nucleophile on the carbon
in the 1 or 2 position (Scheme 1). Thus, it is important to under-
The ring opening reactions are commonly catalyzed using
strong acids [16,24,25], Lewis bases [26], or Lewis acids [4,10,27–
30]. While strong acids are important industrially, they tend to
result in low regioselectivity for the epoxide ring opening with
alcohols [16,24]. Recently, Lewis basic amines were investigated
for ring opening with phenolic nucleophiles, demonstrating that
immobilized tertiary amines could be used as heterogeneous cata-
lysts [26]. These catalysts were incompatible with substrates such
as epichlorohydrin since the nucleophile can displace the chloro
group, affording a low chemoselectivity. The importance of this
reaction has spurred investigations into using Lewis acids as cata-
lysts for these reactions. While alcohols are weak Lewis bases that
can adsorb on Lewis acidic sites, powerful catalysts such as cobalt-
salen and cobalt-porphyrin were demonstrated to be highly active
and could be made into a heterogeneous catalyst [10,28,29,31].
These catalysts tended to deactivate through loss of a counterion
that maintained the cobalt in a +3 oxidation state, which is the
catalytically active form of cobalt. Additional catalysts include
⇑
Corresponding author.
E-mail address: brunelli.2@osu.edu (N.A. Brunelli).
@OSUChemEProfBru (N.A. Brunelli)
https://doi.org/10.1016/j.jcat.2018.11.038
0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
47
dry tetrahydrofuran (THF) before addition of SnCl4Á5H2O in THF
(Si:Sn of 200:1). After stirring for 24 h, the mixture is rotovapped
to dryness before calcining under air at 550 °C.
2.1.3. Hydrothermal synthesis of Lewis acidic zeolite Beta
Scheme 1. Epoxide ring opening reaction converting a terminal epoxide (1) with an
alcohol (2) using a Lewis acidic catalyst (0.4 mol% Lewis acid - mostly Sn - and 60 °C
is used) to produce beta alkoxy alcohols with either a terminal ether (3) or a
secondary ether (4).
The synthesis of Sn-Beta is achieved through scaling up the pro-
cedure used in previous work [32,34,39] to produce larger quanti-
ties of material. Briefly, a mixture is made of TEOS and tetraethyl
ammonium hydroxide (TEAOH; 35 wt% aqueous solution) before
addition of SnCl4Á5H2O. The mixture is allowed to hydrolyze over-
night before concentrating using a rotovap, transferring to a
Teflon-lined 200 mL acid-digestion vessel (Parr Inst. Comp.), and
adding aqueous hydrofluoric acid (HF), and 350 mg (5 wt%) of cal-
cined Si-Beta seeds. The final gel composition is 1 SiO2/ 0.005 Sn/
0.54 F-/ 0.54 TEA/ 7.5 H2O (Si:Sn of 200:1). The acid digestion ves-
sel is sealed and placed in a preheated oven at 140 °C with rotation
at 35 RPM for 30 days. Materials formed are filtered and washed
with 1 L DI water before drying and calcining in air at 550 °C. A
similar procedure is used for Zr-Beta and Hf-Beta with the modifi-
cation that the materials are produced on the 0.9 g scale by scaling
the amounts of each component down by a factor of eight and sub-
stituting the tin source with either zirconyl chloride (ZrOCl2) and
hafnium tetrachloride (HfCl4) for Zr-Beta and Hf-Beta, respectively.
metal organic frameworks such as NU-1000, MOF-808, and Fe(BTC)
[12,27]. NU-1000 and MOF-808 can be activated for this reaction
through creating defects in the material that may limit stability.
Fe(BTC) can catalyze the reaction without any such activation,
but the crystal structure changes upon catalytic testing that could
impact catalyst reuse. Therefore, it is desirable to find alternative
Lewis acidic catalysts for this reaction that maintain activity upon
catalytic material reuse.
Lewis acidic Zr, Hf, or Sn substituted into the silica framework
offer another alternate class of important heterogeneous Lewis
acidic catalysts. These materials have higher thermal stability than
MOFs, making them a robust catalytic support. Indeed, microp-
orous Sn-Beta and Sn-MFI, and mesoporous Sn-SBA-15 have been
utilized for numerous reactions as Lewis acid catalysts [32–35].
While the Lewis acid catalyzed epoxide ring opening reaction with
amines [13] and water [1] has been studied, similar studies have
not examined epoxide ring opening with alcohols.
In this work, a suite of catalytic supports are examined that con-
tain Lewis acidic catalytic sites, ranging from mesoporous Sn-SBA-
15 [13] to zeolite Sn-Beta. The catalytic activity is investigated for
these different materials through studying the epoxide ring open-
ing of epichlorohydrin with methanol as a standard reaction. After
identifying a highly active catalyst, the scope of the reaction is
tested through reacting a range of alcohols and epoxides to deter-
mine the effect of substrate on the catalytic activity and regioselec-
tivity. The heterogeneous catalyst is further tested in recycle
experiments to determine the reusability of the catalytic materials.
Cluster calculations are utilized to examine the potential different
catalytic pathways, allowing differentiation between four potential
mechanisms. Overall, the work demonstrates a range of catalytic
materials that are highly active and selective for the epoxide ring
opening reaction.
2.1.4. Synthesis of Sn-MFI
A mixture is made of 0.177 g SnCl4Á5H2O and 6.75 g DI water in
a 200 mL Teflon-lined acid-digestion vessel (Parr Inst. Comp.). To
this, TEOS (21 g) is slowly added to this solution under stirring
(magnetic Teflon stir bar). After 30 min, a mixture of 22.25 g
tetrapropylammonium hydroxide (40% by wt.) and 22.25 g DI
water is added to this mixture slowly. After 1 h of mixing, 18.9 g
DI water is added, and the mixture is stirred for 30 min. The stir
bar is removed, the reactor sealed and placed in a pre-heated oven
at 160 °C under static conditions. After 5 days, the reactor is cooled
under flowing water, and the contents are centrifuged and washed
with DI water three times before drying at 80 °C overnight. The
dried material is calcined in air using the same conditions
described for Sn-Beta.
2.2. Material characterization
The materials are characterized using a battery of standard
techniques, including powder X-ray diffraction (PXRD), nitrogen
physisorption, scanning electron microscopy (SEM), diffuse reflec-
tance ultraviolet-visible spectroscopy (DRUVS), and diffuse reflec-
tance Fourier Transform Infrared spectroscopy (DRIFTS). PXRD is
recorded using a Bruker PXRD in standard reflection mode using
2. Experimental methods
2.1. Material syntheses
monochromatic Cu K radiation (k = 1.54 Å) at 40 kV and 50 mA
a
1
2.1.1. Co-condensation synthesis of mesoporous silica with tin (Co-Sn-
SBA-15)
to demonstrate the crystallinity of zeolites. Nitrogen physisorption
is measured using a Micromeritics 3Flex surface characterization
analyzer after degassing at 140 °C under vacuum to investigate
the textural properties (e.g., pore volume) of the materials. DRUVS
is collected using an Evolution 300 UV–Vis spectrometer with a
resolution of 2 nm at a scan rate of 10 nmꢀsÀ1 using pure silica
analogues of materials analyzed as a baseline to identify tin oxide
(SnO2) formation [40]. DRIFTS is performed using a Nicolet iS50
spectrometer equipped with a MCT-A liquid nitrogen cooled detec-
tor (32 scans at 2 cmÀ1 resolutions are collected for every spectrum
using the bare material (without CD3CN exposure) as the back-
ground reference) using a Harrick high temperature reaction
chamber with ZnSe windows. After degassing the material at
500 °C under a flow of nitrogen, deuterated acetonitrile is adsorbed
on the material at room temperature, and a spectra is recorded to
identificaty of open and closed sites [41]. SEM is performed on a FEI
Nova 400 NanoSEM scanning electron microscope on samples
prepared by dispersing the material (2 mg in 100 mL of methanol)
Co-Sn-SBA-15 is synthesized using a procedure reported in lit-
erature [36]. Briefly, a mixture is made of P-123 and DI water with
HCl. The solution is stirred at 40 °C to dissolve P-123 before adding
tetraethylorthosilicate (TEOS) and tin tetrachloride pentahydrate
(SnCl4Á5H2O) and allowing the mixture to stir for 24 h. The con-
tents are transferred to a 44 mL Teflon-lined acid-digestion vessel
(Parr Inst. Comp.) and aged at 100 °C under static conditions for
24 h. After cooling to room temperature, the reaction mixture is fil-
tered and washed with DI water. Recovered solids are calcined
under flowing air at 550 °C for 12 h.
2.1.2. Post-synthetic incorporation of mesoporous silica with tin (PS-
Sn-SBA-15)
Pure silica SBA-15 is synthesized using a procedure reported in
literature [37,38]. After calcination, the pure silica SBA-15 is dried
under vacuum overnight at 140 °C. The material is suspended in

48
N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
on carbon conductive tape and drying the sample before sputter
coating at 17 mA for 60 s with a gold–palladium alloy using Cress-
ington 108 Sputter coater or an 8 nm thick layer using EMS 150T-S
before analysis. TGA-DSC is performed using STA 449 F5 JupiterÒ
(NETZSCH instruments) under flowing air (20 mL/min) and nitro-
gen (20 mL/min) at a ramp rate of 10 °C min-1 from 30 to 900 °C
followed by a 5 min hold at 900 °C. Galbraith laboratories per-
formed elemental analysis using inductively coupled plasma opti-
cal emission spectrometry (ICP-OES) to determine the actual Sn, Zr,
and Hf content in the samples.
2.4. Computational methods
Density functional theory calculations are performed using the
Perdew–Burke–Ernzerhof (PBE [42]) functional with D3(BJ [43])
vdW corrections, as implemented in the Vienna ab-initio Simula-
tion Package (VASP). Starting from the BEA crystal structure from
the IZA database, the energetically favorable T5 site (using notation
used previously [44]) at the channel intersection is replaced by the
Sn atom and is used for all calculations. For initial insights on the
mechanism, the computational cost associated with evaluating dif-
ferent mechanisms, alcohols (i.e., methanol, n-, sec- and t-butanol)
and epoxides (i.e., epichlorohydrin and propylene oxide) is reduced
through performing calculations using a 5T atom cluster. The clus-
ter is terminated by hydrogen atoms and placed in a large
24 Â 24 Â 26 Å3 box. For a smaller subset of calculations, the pre-
dicted trends and energetics from the cluster model are verified
using the full periodic structure of BEA. All calculations (both clus-
2.3. Catalytic testing
2.3.1. Kinetic testing
In a septum-sealed 10 mL two neck (2N) round-bottom flask
equipped with a septum-sealed condenser, a solution (2 mL) of
0.4 M epoxide in alcohol is added with diethylene glycol dibuty-
ter and periodic) are performed using the
C-point with an energy
lether (DGDE) as the internal standard. A t0 sample (40 lL) is
cutoff of 500 eV and a Gaussian smearing of 0.05 eV. The atomic
positions are optimized until the forces are lower than 0.03 eV/Å.
The transition states are determined using the climbing image
nudged elastic band and the dimer method [45,46], and are verified
using a vibrational analysis. All energies are referenced to adsorbed
molecules within the zeolite pore.
drawn from this mixture before adding an amount of the catalyst
to achieve an epoxide:heteroatom (Sn, Zr, or Hf) ratio of 250:1 in
all tests. The round bottom flask is then lowered into a silicone
oil bath pre-heated to the reaction temperature using a stir plate
(600 RPM) equipped with temperature control (Heidolph). Cat-
alytic testing demonstrates that external mass transfer does not
impact the observed catalytic behavior (Fig. S1). Using a syringe-
The free energies are obtained using the harmonic approxima-
tion, where only the vibrational modes of the reacting atoms are
included. This implicitly assumes that the rotational and transla-
tional modes of the long, flexible substrate molecules are decou-
pled with the reaction coordinate. This assumption avoids the
use of the harmonic approximation for these modes [47], but
may lead to underestimation of the entropy of the low frequency
modes. The effect of the solvent molecules (i.e., the alcohol) is
not included, which assumes that the initial and transition states
in the liquid phase are stabilized to a similar extent. Although
the above two assumptions may lead to deviations in the absolute
value of the calculated barriers, the simplified model is expected to
provide qualitative trends and insights on the reaction mechanism.
Calculations examining the entropic contributions and solvent
effects necessitate the use of rare event sampling [48] methods
and are currently underway.
needle, 40 lL samples are drawn at different times from the
septum-sealed neck of the round bottom flask. These samples are
filtered using a silica plug in a cotton-plugged glass pipette and
diluted with acetone. These samples are then analyzed using Agi-
lent 7820A GC-FID equipped with Agilent HP-5 ms Ultra Inert col-
umn. The conversion is calculated using the internal standard
method. The regioselectivity is determined through comparing
integrated areas of GC-FID peaks that are identified utilizing GC–
MS by examining the fragmentation patterns of the species
(Table S1).
2.3.2. Hot-filtration test
Typical kinetic testing conditions are used to perform the hot
filtration test to analyze if leached Sn atoms are responsible for
the activity of Sn-Beta. After a short period of the reaction, the
reaction mixture is filtered using a small 2-piece filtration appara-
tus. Immediately after filtration, the reaction mixture is transferred
3. Results and discussion
to
a second 2N round bottom flask, which is subsequently
3.1. Synthesis and characterization of catalytic materials
immersed in a pre-heated silicone oil bath. The mixture is sampled
immediately and after an extended period of reaction time to
determine if further conversion of the epoxide occurs.
The Lewis acidic mesoporous and microporous materials are
successfully synthesized, as demonstrated using standard charac-
terization techniques. The PXRD data for pure Sn-Beta, Zr-Beta,
and Hf-Beta are shown in Fig. S2(a) while PXRD of Sn-MFI is shown
in Fig. S2(b). The data are consistent with crystalline zeolites. Anal-
ysis with nitrogen physisorption reveals that all zeolite samples
(Sn-Beta, Zr-Beta, Hf-Beta, and Sn-MFI) display reversible
adsorption-desorption isotherms (Type I), which is consistent with
isotherm behavior for microporous materials (Fig. S3(a)). Similarly,
nitrogen physisorption measurements of SBA-15, Co-Sn-SBA-15,
and pure Si-SBA-15 samples display Type IV isotherms characteris-
tic of mesoporous SBA-15 materials, as shown in Fig. S3(b). The
results are listed in Table 1. Also, these materials are investigated
with SEM to determine particle size (Table 1 and Fig. S4).
The materials are analyzed using elemental analysis demon-
strating the successful incorporation of the desired heteroatom in
the catalytic material, as shown in Table 1. Consistent with previ-
ous results, the synthesis methods achieve efficient heteroatom
incorporation into the zeolite. While heteroatoms can be incorpo-
rated into the framework or form metal oxide species on the
2.3.3. Reusability testing
Reusability of the catalyst is investigated using the standard
reaction of epichlorohydrin with methanol. The reactions are car-
ried out in 50 mL 2N round bottom flasks equipped with a
septum-sealed condenser. The first reaction is performed at five
times the typical scale described in the kinetic testing section. After
5 h at 60 °C, the reaction mixture is allowed to cool to room tem-
perature and filtered using a small 2-piece filtration apparatus. The
filter-cake is washed with 10 mL methanol twice and then oven-
dried at 80 °C overnight. The recovered catalyst is used under the
same conditions, scaling the reaction contents based on the
amount of catalyst recovered. This process is repeated to observe
any decrease in catalyst activity or selectivity. Between the third
and fourth use, the catalyst is analyzed using TGA and N2
physisorption. Following calcination under conditions similar to
Sn-Beta, the Sn-content is analyzed using ICP-OES, and then the
material is used for the fourth time.

N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
49
Table 1
Summary of catalysts synthesized and associated characterization data.
Material
SAphysi (m2/g)a
l
pore volume (cm3/g)b
Heteroatom (wt%)c
Si:Heteroatom
Particle size (l
m)d
Sn-Beta
Zr-Beta
Hf-Beta
Sn-MFI
468
432
432
407
902
897
796
0.20
0.18
0.18
0.14
–
0.827
0.597
1.91
0.773
1.64
–
237:1
253:1
153:1
254:1
119:1
–
3.3 1e
1.9 0.2e
3.2 0.9e
0.3 0.1
29 12
Co-Sn-SBA-15
SBA-15f
–
–
PS-Sn-SBA-15
0.921
212:1
1.4 0.3
–
Sn-Beta-re
0.763
224:1
a
b
c
d
e
f
Based on nitrogen physisorption, BET method.
Based on nitrogen physisorption, t-plot method.
Based on elemental analysis.
Obtained using SEM.
Base of the bipyramidal structure noted as the largest dimension.
Used to make PS-Sn-SBA-15.
surface, the hydrothermal methods utilized in this work are known
to produce primarily framework sites. This can be demonstrated
through a combination of spectroscopy methods, including DRUVS
and DRIFTS. Accordingly, the zeolites are characterized using
DRUVS, which has previously been used to identify the presence
of SnO2. DRUVS is used to detect the signature feature associated
with SnO2, which is observed as a shoulder around 280 nm, as
shown in Fig. S5. DRUVS analysis of Sn-Beta and Sn-MFI lack this
characteristic peak, suggesting that limited to no SnO2 is present
in these materials.
investigated for catalytic activity, as shown in Fig. 1. Co-Sn-SBA-
15 catalyzes the reaction, reaching ꢁ60% conversion in 24 h. While
it is certainly active, all of the tin catalytic sites may not be acces-
sible since the co-condensation synthesis method can cause the tin
atoms to be incorporated into the walls of the materials. Thus, to
create more accessible catalytic sites in SBA-15, Sn was incorpo-
rated post-synthetically in calcined pure Si-SBA-15 (PS-Sn-SBA-
15). Compared with Co-Sn-SBA-15, PS-Sn-SBA-15 has a slight
increase in catalytic activity achieving 65% in 24 h, as shown in
Fig. 1. This suggests that some of the catalytic sites in Co-Sn-
SBA-15 are inaccessible, but this is not a significant limitation.
While Sn-SBA-15 is active for this reaction, it is found that
higher catalytic activity can be achieved through incorporating
the catalytic sites in a zeolite framework. Sn-Beta is determined
to be the most active material followed by Sn-MFI. The observed
difference in rates between Sn-Beta and Sn-MFI could be associ-
ated with internal diffusion limitations since the smaller pore size
of MFI (10 member ring; 10 MR) would have greater diffusion lim-
itations than zeolite Beta (12 MR). Our recent work with Lewis
acidic nano-MFI [52] and nano-Beta [53] zeolite synthesis demon-
strates that diffusion does not affect catalytic rates for the reaction
The materials are also analyzed using DRIFTS to determine the
nature of the catalytic site in the material. DRIFTS analysis of
deuterated acetonitrile adsorbed onto Sn-MFI results in two peaks,
as shown in Fig. S6. The peak at 2274 cmÀ1 is commonly associated
with deuterated acetonitrile adsorption on a surface silanol while
the peak at 2309 cmÀ1 is consistent with a framework Sn species.
For Sn-Beta, DRIFTS analysis results in three distinct peaks
(Fig. S6). Consistent with previous DRIFTS analysis [41], Sn-Beta
has a peak at 2282 cmÀ1 associated with deuterated acetonitrile
adsorption on silanols and two peaks associated with tin sites with
one peak indicating open sites (2315 cmÀ1) and the other peak
indicative of closed sites (2309 cmÀ1). The PS-Sn-SBA-15 is also
analyzed with DRIFTS, revealing two peaks with one associated
with silanols (2273 cmÀ1
) and a single weak peak around
2306 cmÀ1 that can be attributed to Sn species. Similar analysis
for Zr-Beta produced a spectrum with three peaks that could be
assigned Àt1o deuterated acetonitrile interacting with silanols
(2277 cm ), closed Zr sites (2305 cmÀ1
) and open Zr sites
(2312 cmÀ1). The presence of open and closed zirconium sites is
consistent with previous studies using carbon monoxide adsorp-
tion DRIFTS analysis [49]. The relative intensity of the two peaks
associated with Zr sites suggests that most of the Zr sites are
closed. For Hf-Beta, DRIFTS analysis reveals two peaks for deuter-
ated acetonitrile adsorption with one peak associated with silanols
(2275 cmÀ1) while the other peak can be attributed to Hf species
(2308 cmÀ1). Recent work suggests that the hydrothermal synthe-
sis methods results in Hf species that are open catalytic sites [50]
located in the framework [51]. Overall, DRIFTS analysis indicates
that the hydrothermal zeolite synthesis methods produce Lewis
acidic framework catalytic sites.
3.2. Catalytic testing
The catalytic materials are all investigated for catalytic perfor-
mance for the epoxide ring opening reaction of epichlorohydrin
with methanol at 60 °C with 0.4 mol% Sn (or the Lewis acidic het-
eroatom). To evaluate the effect of support characteristics, differ-
ent supports (Beta, MFI, and SBA-15) substituted with tin are
Fig. 1. Comparison between the catalytic performance of different catalysts, Sn-
Beta
(
(
), Sn-MFI (black), Co-Sn-SBA-15
(
), and PS-Sn-SBA-15
), for epichlorohydrin (0.4 M) ring-opening with methanol at 60 °C using
0.4 mol% Sn.

50
N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
Table 2
of epichlorohydrin with methanol, indicating that observed differ-
ences in rates are not associated with diffusion limitations.
It is investigated if the catalytic activity could be associated
with SnO2 species. A material incorporating SnO2 into the frame-
work is made, as described in the SI (SnO2-Beta). This material is
characterized using DRUVS to demonstrate the successful incorpo-
ration of SnO2, as shown in Fig. S5c. Additionally, SnO2-Beta is
tested for catalytic activity in the epoxide ring opening of
epichlorohydrin with methanol. While Sn-Beta can achieve com-
plete conversion of the epoxide in four hours, SnO2-Beta only
achieves 18% conversion of the epoxide in 24 h and requires
approximately two weeks to attain complete conversion (Fig. S7).
These tests are consistent with the catalytic activity not being asso-
ciated with amorphous SnO2 species, but rather the activity being
associated with framework tin.
The Lewis acidity of catalytic materials is known to strongly
impact catalytic activity and selectivity. Indeed, stronger Lewis
acids are beneficial for epoxide ring opening with water, but mod-
erate strength Lewis acids such as Zr are more active than Sn for
epoxide ring opening with amines [13]. Accordingly, zeolites with
different Lewis acidity are investigated for the standard epoxide
ring opening reaction. Both Hf-Beta and Zr-Beta are active for the
epoxide ring opening reaction, as shown in Fig. 2. Compared to
Sn-Beta, both Hf-Beta and Zr-Beta are less active catalysts. This
can be attributed to the stronger Lewis acidity of Sn as compared
to Zr and Hf, which have similar Lewis acidities [54]. This order
of reactivity is in contrast with the order reported for epoxide ring
opening with amines using post synthetically prepared Sn and Zr
Beta [13]. On account of their higher nucleophilicity, amines bind
strongly to Sn-sites lowering the reaction rates for Sn-Beta. Indeed,
amines have previously been used as catalytic site poisons for Sn-
Beta [55]. Being weaker nucleophiles, alcohols do not exhibit sim-
ilar behavior. The stronger Lewis acidic sites result in faster rates
by facilitating the nucleophilic attack.
Comparison of the catalytic performance of different catalysts analyzed in terms of
turnover frequency (hÀ1
and regioselectivity for (%) for epichlorohydrin ring
opening with methanol. TOF0 reported for reactions performed at 60 °C.
)
3
Material
TOF0 (hÀ1
)
Regioselectivity for 3 (%)
Sn-Beta
Sn-MFI
Hf-Beta
Zr-Beta
193
124
33
96
97
97
98
26
tivity for Al-Beta is determined to be only 93% for the terminal
ether (3), indicating that the Lewis acid catalyzed mechanism is
more selective than the Brønsted acid catalyzed route. Addition-
ally, Al-Beta (epoxide:Al of 250:1; 0.4 mol% Al) only achieves
greater than 90% conversion after seven hours while Sn-Beta
exceeds 90% epoxide conversion in only four hours, demonstrating
that Sn-Beta is more active than Al-Beta.
Sn-Beta is highly efficient for the epoxide ring opening reaction.
Quantitative product yield is determined through scaling up the
reaction by a factor of 10 and analyzing the reaction mixture with
1H NMR with dimethyl sulfone as an NMR internal standard
(Fig. S8). The yield of the terminal ether (3) is calculated to be
91%. This corroborates that Sn-Beta is a highly efficient and selec-
tive catalyst for the epoxide ring opening reaction.
3.3. Reaction regioselectivity
The regioselectivity is investigated for a series of epoxides and
alcohols to determine the potential scope of these catalytic mate-
rials (Fig. S9 and Table S2). Sn-Beta is used as the catalyst, unless
stated otherwise. For epichlorohydrin, the regioselectivity for 3
(terminal-ether) is consistently high with values for different alco-
hols consistently around 97%. This spans a broad scope of alcohols,
including methanol and the different butanols (1-butanol, sec-
butanol, and t-butanol) tested. This indicates that the alcohol has
little effect on the regioselectivity when using epichlorohydrin.
Changing the epoxide while using methanol as the nucleophile
has a significant impact on the regioselectivity. Interestingly, react-
ing isobutylene oxide with methanol produces the terminal alcohol
(4) with high regioselectivity of >99%. The identity of the regiomer
is assigned through using GC–MS analysis (Table S1). Despite the
steric implications of the methyl group, methanol attacks the ter-
tiary carbon of the epoxide since this can better stabilize charge.
This change in regioselectivity is consistent with the regioselectiv-
ity being controlled by inductive effects.
In addition to catalytic activity, the Lewis acidic catalytic mate-
rials are highly regioselective. At complete conversion, Sn-Beta has
a regioselectivity for the terminal ether (3) as determined by GC-
FID analysis to be 96% (Table 2). For all Lewis acidic zeolites, the
regioselectivity is consistently high. Interestingly, the regioselec-
To further illustrate this point, two additional epoxides are
tested to determine the regioselectivity: (1) epoxyoctane and (2)
n-butyl glycidyl ether (Fig. S9). These epoxides have the same
length of the backbone chain, but n-butyl glycidyl ether has an
oxygen atom beta to the epoxide. For epoxyoctane, the regioselec-
tivity for the terminal ether (3) is determined to be 56%. While the
alkyl chain can stabilize the secondary carbocation, the effect of
stabilization is insufficient to overcome the steric limitations asso-
ciated with the formation of the terminal alcohol (4). For n-butyl
glycidyl ether, the regioselectivity for the terminal ether (3) is
determined to be 88%. Relative to the alkyl chain, the oxygen atom
is more electronegative than a carbon atom, reducing the stability
of the 2 position relative to the terminal carbon. Overall, the
observed regioselectivity can be attributed to the inductive effect
of the functional groups in the vicinity of the epoxide. This suggests
that the zeolite beta framework is beneficial to increase the cat-
alytic rate, but it has little to no impact on the regioselectivity
for the epoxides and alcohols that are tested in this work. To inves-
tigate if the heteroatom in zeolite Beta affects regioselectivity,
Sn-Beta, Hf-Beta, and Zr-Beta are analyzed for ring opening of
Fig. 2. Comparison of the catalytic performance of zeolite beta catalysts with
different heteroatoms, Sn-Beta (
), Zr-Beta (
), and Hf-Beta (
) for
epichlorohydrin (0.4 M) ring-opening with methanol at 60 °C using 0.4 mol% Lewis
acid.

N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
51
1,2-epoxybutane with methanol. The regioselectivity shows a
modest increase from 54% to 56% to 60% for Sn-Beta, Hf-Beta,
and Zr-Beta respectively (Fig. S9(e)).
3.4. Catalyst stability and reusability
The robustness of Sn-Beta is evaluated through a combination
of a hot-filtration test and catalyst reuse experiments. Recent
reports indicate deactivation of Sn-Beta in biomass conversion
reactions through leaching of tin species [56,57]. In addition to
decreasing the reusability of these materials, the leached Sn spe-
cies may be responsible for catalytic activity. Therefore, the
propensity for Sn sites to leach is evaluated for the standard epox-
ide ring opening reaction with methanol using a hot filtration test.
After 30 min of reaction time, the catalyst is filtered from the reac-
tion mixture using a two-piece filtration apparatus. The filtrate is
transferred to a round-bottom flask and allowed to react at 60 °C,
taking samples periodically to determine the conversion over time.
As shown in Fig. 3, the epoxide conversion effectively stops after
the catalyst is removed from the mixture. These results provide
evidence that homogeneous species are not responsible for the cat-
alytic activity and that the catalytic site does not leach from the
material. Additionally, these results indicate that the catalytic site
is likely a framework species, suggesting that the material could be
a robust and reusable catalyst for this reaction.
Fig. 4. Comparison between fresh (
), once used (
), and twice used (
)
Sn-Beta for epichlorohydrin ring opening with methanol shows no significant
differences in activity. Same can be concluded for the calcined twice used catalyst
(black).
The reusability of Sn-Beta for the epoxide ring opening reaction
is examined through recycling the catalyst for three catalytic tests
using the standard epoxide ring opening reaction. For the first test,
the reaction is scaled to five times the typical reaction volume to
ensure sufficient material could be recovered for multiple reuse
tests. Samples are withdrawn periodically to determine conversion
over time, as shown in Fig. 4. After five hours of reaction, the reac-
tion is stopped through removing the round-bottom flask from the
silicone oil bath. The reaction mixture is filtered, the catalyst
washed, dried, and weighed to determine the amount of catalyst
recovered, and the second cycle is performed through scaling the
amount of reaction mixture to the amount of catalyst to maintain
0.4 mol% Sn (epoxide:Sn of 250:1). As shown in Fig. 4, the conver-
sion is nearly identical for the first, second, and third cycle, indicat-
ing that the catalytic material can be readily reused without loss of
catalytic activity. After the third cycle, the catalyst is recovered
through filtration, calcined, and is analyzed for elemental composi-
tion. Elemental analysis (EA) reveals a decrease in the weight per-
centage of Sn from 0.827% to 0.763%. This difference is small
compared to the expected accuracy of EA. At the same time, the
Sn:Si does not change significantly from 237:1 before reaction to
224:1 after 3 cycles, indicating that the catalytic material remains
robust. Using the catalyst for a fourth time (labeled Sn-Beta-re in
Table 1) reveals that the catalytic activity is maintained even in a
fourth cycle. Combined these results indicate that Sn-Beta is a
promising and reusable catalyst for the epoxide ring opening reac-
tion with alcohols.
3.5. Catalytic reaction mechanism from DFT calculations
Inspired by the promising experimental results, DFT calcula-
tions are used to further understand the reaction mechanism. As
summarized in Fig. 5 and S11, three potential reaction mechanisms
involving a single alcohol and epoxide molecule are explored: (1)
activation of the epoxide by an alcohol that is dissociated and
bound to the Sn site; (2) activation of the epoxide by an alcohol
that is intact and bound to the Sn site; and (3) activation of the
epoxide bound by the Sn site for attack by the alcohol. In addition
to mechanisms involving
a single alcohol, a mechanism is
considered that involves the SN2-type activation of the epoxide
by a Sn-bound alcohol and simultaneous nucleophilic attack by a
2nd alcohol molecule. Each of these mechanisms involve several
steps, including the adsorption of the substrate on the catalytic site
and the coupling reaction. Using the periodic BEA model, the bind-
ing energy is calculated for non-dissociative adsorption of metha-
nol and epichlorohydrin (<5 kJ/mol difference; see SI and Fig. S11
for details). Similar calculations for water and ethanol dissociation
using a different functional and more sophisticated methods have
been recently reported for ethanol dehydration [47] that indicate
that the interaction energies for water and ethanol with the cat-
alytic site are similar. Accordingly, these calculations indicate that
the experimental conditions employed in these studies (i.e., 0.4 M
epoxide in alcohol) would result in the catalytic sites to be
Fig. 3. Hot filtration test for Sn-Beta in the epoxide ring opening reaction of
epichlorohydrin with methanol at 60 °C using 0.4 mol% Sn. The conversion is shown
as a function of time for the standard Sn-Beta (
) test and the Sn-Beta hot-
filtration test ( ). Hot filtration is performed at 30 min (marked by dotted line).

52
N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
Fig. 5. Activation of the epoxide by an alcohol that is dissociated and bound to the Sn site (Mechanism 1).
interacting with the alcohol. Even small amounts of water that
might be present in the hydrophobic framework would be
expected to be displaced from the catalytic site by methanol.
In addition to intact binding of substrates, the energy of the
transition state is calculated for dissociation of methanol and
epichlorohydrin over the Sn-site (Fig. S12). The calculated free
the epoxide along with higher calculated barriers (compared to
mechanism 1 and 4) indicate a negligible contribution of the bound
epoxide mechanism (mechanism 2).
At this stage, it is useful to compare mechanism 1 and 4 in more
detail, especially as they relate to the observed regioselectivity. The
regioselectivity is determined by which Cepoxide-Oepoxide bond is
cleaved during the reaction. Thus, calculations are performed
examining both modes of breaking the Cepoxide-Oepoxide bond. To
reduce the computational cost, a 5T cluster model are used for
the following analysis. The two epoxides considered are epichloro-
hydrin and isobutylene oxide since these epoxides provide distinct
regioselectivities. For epichlorohydrin, nucleophilic attack at the
primary Cepoxide leads to the terminal ether product (ꢁ97%), while
attack at the secondary C leads to the minor terminal alcohol pro-
duct (ꢁ3%). Mechanism 1 involves protonation of the Oepoxide by
the dissociated methanol, followed by front-side SN1-type nucle-
ophilic attack on the Cepoxide. As with SN1 reactions, a lower barrier
(76 kJ/mol) is observed for the formation of the more stable, sec-
ondary carbocation compared to the primary carbocation (82 kJ/-
mol) pathway. In contrast, the trends for the SN2, mechanism 4
are reversed, where the lowest barrier (62 kJ/mol) is observed for
reaction at the primary Cepoxide as determined by the low steric
hindrance. Thus, results from epichlorohydrin/methanol are con-
sistent with the two alcohol, SN2 mechanism (mechanism 4)
involving nucleophilic attack of the less sterically hindered pri-
mary Cepoxide being responsible for the experimentally observed
regioselectivity for the terminal ether product for the epichlorohy-
drin/methanol reaction.
energy barrier for methanol dissociation (
is significantly lower than the corresponding value for epichloro-
hydrin (
Gaepicholorhydrin;diss = 135 kJ/mol). The experimentally calcu-
lated activation energy (53 7 kJ/mol; details in SI and Fig. S13)
is significantly lower than
Gaepicholorhydrin;diss, indicating that the
D
Gamethanol;diss = 53 kJ/mol)
D
D
dominant reaction pathway does not involve the dissociative
adsorption of the epoxide. For all these different mechanisms,
the rate determining step involves the reaction between the alco-
hol and the epoxide and the activation barrier is associated with
breaking the CepoxideAOepoxide bond.
The activation barrier for CepoxideAOepoxide bond breaking is
examined for the different mechanisms for the reaction between
methanol and epichlorohydrin. Table 3 summarizes the free energy
barriers for the rate determining step of the four mechanisms
(transition states shown in Figs. S14–S19). The calculated barriers
for intact, bound methanol (
D
Gaintact;bound alcohol = 137 À 155 kJ/mol,
mechanism 2) and bound epoxide (
D
Gabound
= 114 À 120 kJ/mol,
epoxide
mechanism 3) pathways are higher than the dissociated methanol
(
D
Gadissociated;bound
= 76 À 82 kJ/mol, mechanism 1) and two
alcohol
methanol SN2 pathway (
D
Gatwo alcohol;SN2 = 74 À 62 kJ/mol, mecha-
nism 4). The high barrier for the intact, bound alcohol pathway
(mechanism 3) arises because of the strained transition state
geometry and is not discussed further as it is associated with an
inaccessible reaction barrier at reaction conditions. The relative
excess of the alcohol compared to the epoxide (i.e., 0.4 M) and
the similar binding energies of the epoxide and the alcohol to the
Sn site (difference < 5 kJ/mol) suggest that only a small fraction
of the Sn sites will be epoxide bound. Here, the low coverage of
From the above discussion, it is clear that the experimentally
observed regioselectivity depends on the energetics of the SN1
and SN2 pathway for the reactant epoxide. To examine this hypoth-
esis further, a similar analysis is performed for the reaction
between isobutylene oxide and methanol. In contrast to the
epichlorohydrin results above, the reaction of isobutylene oxide
and methanol results exclusively in the terminal alcohol (ꢁ100%)
Table 4
Table 3
DFT-calculated activation free energies (at 80 °C) for the reaction of isobutylene oxide
with methanol for different mechanisms.a
DFT-calculated activation free energies (at 80 °C) for the reaction of epichlorohydrin
with methanol for different mechanisms.
Product
Terminal alcohol
Terminal ether
Product
Terminal alcohol
Terminal ether
Reaction site
Mechanism 1
Mechanism 4
Tertiary Cepoxide
61
73
Primary Cepoxide
92
72
Reaction site
Mechanism 1
Mechanism 2
Mechanism 3
Mechanism 4
Secondary Cepoxide
76
137
114
74
Primary Cepoxide
82
155
120
62
a
Mechanism 2 and 3 are not evaluated for this reaction because of the high
barriers calculated previously for epichlorohydrin and methanol for these
mechanisms.

N. Deshpande et al. / Journal of Catalysis 370 (2019) 46–54
53
regiomer, which arises because of the nucleophilic attack at the
tertiary Cepoxide. The calculated free energy barriers for mechanism
1 and mechanism 4 are presented in Table 4. For the reaction
occurring at the tertiary Cepoxide, it is observed that SN1 mechanism
(61 kJ/mol, mechanism 1) is preferred compared to the SN2 path-
way (73 kJ/mol, mechanism 4) owing to the higher stability of
the tertiary carbocation. Further, the SN1 pathway is not favored
for the primary carbocation (92 kJ/mol) and a SN2 pathway
(72 kJ/mol) is preferred. Thus, in the case of isobutylene oxide, it
can be concluded that the high steric hinderance at the tertiary
Cepoxide along with the stability of a tertiary carbocation leads to
SN1 mechanism 1 as the dominant reaction pathway. Consistent
with the experimental trends, this leads to the formation of the
terminal alcohol product.
of Energy Office of Science User Facility operated under Contract
No. DE-AC02-05CH11231. We thank Prof. Justin Notestein and
Mihir Bhagat from Northwestern University for providing insight-
ful discussion and samples for comparative analysis to facilitate
identification of different regiomers.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2018.11.038.
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4. Summary
Lewis acidic materials are determined to be regioselective cata-
lysts for epoxide ring opening of epichlorohydrin with alcohols. Of
the mesoporous and microporous materials tested, kinetic testing
demonstrated that Sn-Beta has a higher catalytic activity than
other Sn, Hf, and Zr-substituted materials. For epichlorohydrin ring
opening with methanol, Sn-Beta is more active and achieves a
higher regioselectivity than Al-Beta. The regioselectivity is primar-
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material pore size. Indeed, the results for regioselectivity for most
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epichlorohydrin ring opening with methanol.
Acknowledgements
We gratefully acknowledge the American Chemical Society Pet-
roleum Research Fund (ACS-PRF 55946-DNI5), the National Science
Foundation (NSF Career 1653587), and the Ohio State University
Institute for Materials Research (OSU IMR FG0138) for their finan-
cial support. This research used resources of the National Energy
Research Scientific Computing Center (NERSC), a U.S. Department

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