Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

Article abstract of DOI:10.1021/jacs.7b11467

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-Ti^IV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as suppo

Full text of DOI:10.1021/jacs.7b11467

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Outer-Sphere Control of Catalysis on Surfaces: A Comparative
Study of Ti(IV) Single-Sites Grafted on Amorphous
versus Crystalline Silicates for Alkene Epoxidation
Nicolás A Grosso-Giordano, Christian Schroeder, Alexander Okrut, Andrew Solovyov, Christian
Schöttle, Walter Chassé, Nebojsa Marinkovi#, Hubert Koller, Stacey I. Zones, and Alexander Katz
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11467 • Publication Date (Web): 22 Mar 2018
Downloaded from http://pubs.acs.org on March 22, 2018
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Outer-Sphere Control of Catalysis on Surfaces: A Comparative
Study of Ti(IV) Single-Sites Grafted on Amorphous versus
Crystalline Silicates for Alkene Epoxidation
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Nicolás A. Grosso-Giordano,§ Christian Schroeder,# Alexander Okrut,§ Andrew Solovyov,§ Christian
⊥
Schöttle,§ Walter Chassé,# Nebojša Marinković,‡ Hubert Koller,*,# Stacey I. Zones,*, and Alexander
Katz*,§
§Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California, 94720, United
States
#Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Münster, Germany
⊥Chevron Energy Technology Company, Richmond, California, 94804, United States
‡Department of Chemical Engineering, Columbia University, New York, New York, 10027, United States
Supporting Information Placeholder
have profound consequences on the reactivity of TiIV sites
ABSTRACT: The effect of outer-sphere environment on
supported on siliceous surfaces, the same type of active
alkene epoxidation catalysis using an organic
hydroperoxide oxidant is demonstrated for calix[4]arene-
TiIV single-sites grafted on amorphous vs. crystalline
delaminated zeotype (UCB-4) silicates as supports. A
chelating calix[4]arene macrocycle ligand enforces a
constant TiIV inner-sphere, as characterized by UV-visible
and X-ray absorption spectroscopies, thus enabling the
rigorous comparison of outer-sphere environments across
different siliceous supports. These outer-sphere
sites used in industrial propylene epoxidation catalysts.19–
21
Our approach enforces a constant inner-sphere
environment for grafted TiIV single-sites with a chelating
macrocyclic calix[4]arene ligand, while maintaining
active-site accessibility across crystalline zeolitic versus
amorphous outer-sphere environments of identical
composition. This enables us to unequivocally determine
that outer-sphere effects are responsible for rate
enhancements of up to 5-fold per TiIV site in crystalline
zeolitic supports during the catalytic epoxidation of
cyclohexene with t-butyl hydroperoxide. NMR
characterization reveals that these effects arise from
differences in the proximal organization of surface silanol
groups and their framework location relative to TiIV active
centers, demonstrating the importance of precisely
designing outer-sphere structures for active sites on
surfaces.
1
environments are characterized by solid-state H NMR
spectroscopy to comprise proximally organized silanols
confined within 12-membered-ring cups in crystalline
UCB-4, and are responsible for up to 5-fold
enhancements in rates of epoxidation by TiIV centers.
Supported catalytic active sites comprising isolated
cations are ubiquitous;1,2 however, their rational
molecular design is currently hampered by a lack of
understanding of how outer-sphere effects control their
reactivity. Outer-sphere effects are controlled by surface
moieties that are vicinal (non-coordinating) to the active
centers,3,4 as opposed to coordinating ligands of the inner-
sphere,5–9 and are commonly invoked in the reactivity of
soluble molecular catalysts10 and metalloenzymes.11
Their experimental observation on supported molecular
catalysts has been elusive, however, because of the
challenges associated with modifying the outer-sphere
without significantly affecting other aspects – such as
inner-sphere or steric accessibility – of the active site.12–
Our approach is shown in Scheme 1, and is based on a
calix[4]arene-Ti surface complex (highlighted in brown)
as a Lewis-acid catalytic TiIV center. This is grafted onto
a silanol (SiOH) group on the surface of three different
siliceous supports, consisting of a crystalline delaminated
zeotype UCB-4,22 and two amorphous silicas, which
comprise Aerosil® 200 fumed silica particles (denoted
SiO2) and MCM-41,23 a hexagonally ordered silicate with
uniform 2.6 nm mesopores (Supp. Info. 1 and 2).24 The
resulting catalysts comprising grafted calix[4]arene-Ti
complexes are denoted calix-Ti/SUP, where SUP =
UCB-4, SiO2, or MCM-41 indicates the support in
Scheme 1, with physicochemical properties summarized
18
Here, we demonstrate that outer-sphere environments
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in Table 1. Upon grafting, calix[4]arene-to-Ti ratios in all
(Scheme 1).28 These arrangements are synthesized by the
post-synthetic hydrolysis and removal of BIII from
zeolitic frameworks such as SSZ-70 studied here,
exposing a vacancy defect terminated by silanols (vide
infra).
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catalysts were measured to be near unity (calix:Ti in Table
1), confirming that the surface complex is intact and that
the calix[4]arene ligand remains bound to TiIV,25 as
depicted in Scheme 1. The macrocyclic dimensions of the
calix[4]arene-TiIV precursor prevents its diffusion into
zeotype micropores during grafting, ensuring that
grafting occurs on the external surface only, as also
indicated by analysis of grafting densities (Supp. Info. 3).
The location of all active sites on the external surface
ensures their equal accessibility for activation of organic
hydroperoxides in the absence of steric limitations
imposed by zeolitic micropores,26,27 enabling a direct and
rigorous comparison of intrinsic calix[4]arene-TiIV
reactivity across all materials.
Table 1. Physicochemical properties of supports and TiIV-
containing materials.
9
wt.%
Ti
LMCT
edge (eV)
a
b
Support
Calix:Ti
Arel
keff
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SiO2
0.35
0.70
0.37
1.0
1.0
1.1
2.29
2.21
2.24
0.92
8
MCM-41
UCB-4
0.89 10
0.93 42
aRatio of Ti K-edge XANES pre-edge area features, ±0.07
(Supp. Info. 1.3 and 6). bEffective epoxidation rate constant,
M-2s-1, ±1 (Supp. Info. 1.4 and 7).
Scheme 1. Grafted calix[4]arene-TiIV materials.
crystalline silicate
calix[4]arene-Ti
amorphous silicates
Given the role that silanol groups have on the grafting
and outer-sphere environments of calix[4]arene-TiIV sites,
we investigated their nature and local organization in both
proximal silanol
pairs in vacancy
defects
O
O
O
O
O
O
O
O
CH3
CH3
OH
Ti
1
Ti
crystalline and amorphous supports by solid-state H
Si
Si
Si
Si
O
HO
O
O
NMR spectroscopy, prior to calix[4]arene-TiIV grafting
OH
Si
Si
1
(in order to reduce interference of H signals from
SiO2
organics). We chose to examine the non-delaminated B-
SSZ-70, the calcined variant of the layered zeotype
precursor that leads to UCB-4 (Supp. Info. 1.1 and 2),
because delamination does not affect calix[4]arene-TiIV
grafting densities or catalytic activity (Table S2 and Fig.
S5). After BIII removal from the borosilicate framework
of B-SSZ-70 there is an appearance of resonances
corresponding to silanols in the 1H MAS NMR spectrum
delaminated
zeotype layers
non-porous
particles
2.6 nm
pores
Si
12-MR
pocket
OH
calix-Ti/UCB-4
calix-Ti/SiO2
calix-Ti/MCM-41
Schematic representation of grafted calix[4]arene-TiIV
structures (brown). Details of the crystalline SSZ-70
(MWW-type) zeotype framework of UCB-4 (left panel) are
shown along the a-c plane, with vertices representing SiO4/2
tetrahedra connected by Si-O-Si bonds. Silanol (SiOH)
species characterized by NMR on SSZ-70 are represented as
a proximal silanol pair in blue, and a crystal-terminating
isolated silanol in purple.
1
at 2-8 ppm (Fig. S2A). The H single-quantum double-
quantum (SQ-DQ) MAS NMR dipolar correlation
spectrum shows two well-resolved cross-correlation
peaks (Fig. 1A) for hydrogen-bonded silanols, at 3.3 and
4.0 ppm. This cross-correlation rigorously confirms two
silanols comprising different chemical shifts to be
adjacent in space, indicative of a well-ordered paired
silanol arrangement being formed in vacancies exposed
by BIII removal, as depicted in blue in Scheme 1.
Corresponding data for B-SSZ-70 where BIII remains part
of the crystalline framework (Fig. S2Ca) confirms that
this arrangement does not exist prior to BIII removal. Such
deboronation treatments are generally believed to
synthesize vacancy defect structures termed “silanol
nests” in zeotype frameworks, consisting of four
hydrogen-bonded silanols; however, no rigorous study
has determined the actual number of silanols involved in
such structures.30 Our 2Q NMR data unequivocally
demonstrates the existence of at least two such paired
silanols in SSZ-70, and all attempts by 1H NMR to show
the presence of more than two silanols employing 3Q or
4Q excitation have been unsuccessful. In contrast, the
amorphous SiO2 support (dehydroxylated at 823 K)
The outer-sphere environment as controlled by the
support surface surrounding the grafted calix[4]arene-
TiIV complex is compositionally identical in all siliceous
supports. However, it contrasts in (i) the ordered
crystalline versus random amorphous arrangement of
silicate framework, and (ii) the organization of surface
silanol (Si-OH) onto which calix[4]arene-Ti is grafted.28
This is respectively illustrated by (i) the presence of
twelve membered-ring pockets (12-MR pocket in
Scheme 1) on the external surface of crystalline UCB-4,29
which are absent in amorphous SiO2 and MCM-41;24 and
(ii) the random distribution of isolated silanol groups on
the surface of these partially dehydroxylated amorphous
silicates, in contrast to silanol organization into
cooperative proximal arrangements in crystalline UCB-4
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shows no signal for hydrogen-bonded silanols, and its
SQ-DQ MAS NMR spectrum shows only a weak
autocorrelation signal at 1.8 ppm (Fig. 1B),
corresponding to randomly paired surface silanols, whose
abundance is insignificant when compared to the paired
silanols in SSZ-70 (Supp. Info. 4).
Because of the close proximity of silanol pairs in UCB-
4, multidentate support Si-O-TiIV connectivity is possible
in principle, at the expense of breaking some Ti-O(calix)
bonds. To address this, we measured calix[4]arene-Ti
ligand-to-metal charge-transfer (LMCT) edge-energies
for calix-Ti/SUP materials, since this energy has been
previously reported to be sensitive to the number of
covalent connections between calix[4]arene oxygens and
grafted TiIV cations.33 These values are all measured to be
2.25 ± 0.05 eV (Table 1), consistent with tridentate
calix[4]arene connectivity and single-point attachment to
support, as two-point attachment of the calix[4]arene
would have resulted in a lower edge energy of 2.14 eV
(see Supp. Info. 5).33 This confirms the equivalent inner-
sphere coordination for TiIV in all catalysts, as depicted in
Scheme 1.
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To investigate possible variations in electronic
environment of TiIV centers grafted on different supports,
we performed Ti K-edge XANES,34,35 with results shown
in Fig. 2. Pre-edge features, highlighted in the insert of
Fig. 2, arise from electronic transitions of core electrons
into empty 3d orbitals of d0 TiIV, and are thus a descriptor
of 3d orbital availability and Lewis-acid strength.35 This
descriptor is quantified by the value of Arel in Table 1
(Supp. Info. 1.3 and 6), and its value of 0.9 together with
pre-edge positions at 4970 eV is consistent with site-
isolated 4-coordinate TiIV centers within grafted
calix[4]arene-Ti surface complexes.3,35,36 The lack of
changes in these values and pre-edge peak heights across
all materials (Fig. 2 and Table 1), within experimental
uncertainty, confirms the identical 3d orbital occupancy
and inner-sphere of TiIV centers across all catalysts.
Rate constants normalized by TiIV contents (keff) for the
Figure 1. 1H SQ DQ MAS NMR spectra acquired at a
magnetic field strength of 9.74 T and a MAS rate of 9000
Hz of (A) B-SSZ-70 after BIII removal, and (B) SiO2.
epoxidation
of
cyclohexene
with
tert-butyl
hydroperoxide (Scheme S3), a common probe reaction
for epoxidation activity, are summarized in Table 1 for all
calix-Ti/SUP catalysts, with epoxide yields uniformly
high (Fig. S6B). Values of keff are observed to be 4- and
5-fold higher in crystalline calix-Ti/UCB-4, as compared
to amorphous calix-Ti/MCM-41 and calix-Ti/SiO2,
respectively. Similar enhancements favoring calix-
Ti/UCB-4 are also observed when quantifying turnover
numbers for epoxide production (Fig. 3), irrespective of
whether the amorphous support contains a well-ordered
porous system in MCM-41 or not (SiO2). We surmise that,
on a single-site basis, all TiIV centers behave similarly
when grafted on amorphous supports, as described
previously,26 but with distinctly lower activity than on
crystalline calix-Ti/UCB-4. Control experiments were
performed to confirm that the high-activity in calix-
Ti/UCB-4 relative to the amorphous catalysts is not due
to artifacts of the delamination, acid washing treatment,
presence of BIII, or diffusional constraints, is independent
of calix[4]arene-TiIV surface density (Supp. Info. 3), and
is generalizable to other olefins and oxidants (Supp. Info.
7). Significantly, calix[4]arene-TiIV sites grafted on fully
hydroxylated SiO2 (calix-Ti/SiO2-AW in Supp. Info. 3)
In order to quantify the pairing and distances between
1
silanol groups, H DQ build-up curves were measured.31
These data on SSZ-70 reveal that 34% of silanols are
found within well-ordered pairs, with a 1H-1H distance of
2.5 Å (Fig. S2Db and Supp. Info. 4). This distance
indicates that considerable rearrangement of the local
structure is necessary upon BIII removal, as the average
edge length (between oxygen atoms) of a tetrahedral
center is 2.6 Å. A minority silanols in SSZ-70 are isolated
species on its external crystal-terminating surface, with
signal at 1.8 ppm,32 depicted in purple in Scheme 1. In
amorphous SiO2, the vast majority (82%) of silanols are
isolated, while the remaining protons exhibit an instant
rise to maximum double-quantum intensity at early
evolution times (Fig. S2Da), with an extremely large
dipolar coupling constant of at least 15 kHz (indicating a
H-H distance below 2.0 Å), unlikely to represent paired
silanols and instead typical of rigidly adsorbed water (H-
H distance of 1.6 Å).
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exhibit rates equivalent to those of calix-Ti/SiO2 (keff = 8
grafted on the crystalline framework of UCB-4, in
comparison to amorphous supports. We conclude that
these enhancements in rate are brought about by outer-
sphere effects intrinsic to the support in calix-Ti/UCB-4.
These effects specifically involve paired silanols located
within 12-MR pockets in UCB-4, distinct from those
present in fully hydroxylated amorphous SiO2, where
surface heterogeneity precludes a precise location of
silanol pairs with respect to active centers. Calix[4]arene-
TiIV grafting on UCB-4 may occur one of the two silanols
in the organized pair – thereby leaving one silanol in close
proximity to the site –, or on the exterior surface, adjacent
to the silanol pair. In either scenario, the existence of a
neighboring silanol may facilitate the activation of bound
intermediates (e.g. for hydroperoxide binding or
subsequent transfer of oxygen to epoxide),37,38 giving rise
to the observed outer-sphere effects. While these effects
have long been hypothesized,39 challenges in controlling
TiIV inner-spheres40 and steric accessibility across
supports have precluded their observation,26,27 despite
decades of seminal work.41,42 Our synthetic approach
overcomes these challenges, providing the first rigorous
experimental demonstration of outer-sphere effects on
per-site activity in titanosilicates.
M-2s-1), indicating that silanol pairing as achieved by high
local silanol densities on amorphous silicates is alone
insufficient for enhancing reactivity.
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Our findings evidence the role of organization and
location of outer-sphere functionalities for catalysis on
surfaces, analogous to outer-sphere residue cooperativity
within active sites of certain protease enzymes.43 By
exploiting delaminated zeotypes as accessible crystalline
supports with well-defined surface environments, optimal
active sites that incorporate outer-sphere functionalities
can be rationally designed. Such approaches may lead to
improvements in olefin epoxidation catalysts,44,45 where
state-of-the-art technologies still rely on amorphous
supports when using organic hydroperoxides as
oxidants,19–21 and in catalysis on surfaces in general,
where the control over the organization of structural
environments surrounding active centers is only
beginning to emerge.
Figure 2. Normalized Ti K-edge XANES spectra, with
insert detailing the pre-edge region, for calix[4]arene-TiIV
sites on the labeled support.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website.
PDF detailing experimental methods, support textural
properties, NMR characterization, analysis of LMCT edge-
energies, testing of control materials, details of Ti K-edge
spectra, kinetic analysis, and supplementary references.
Figure 3. Catalytic turnover numbers (TON) for epoxide
production as a function of time, for calix[4]arene-TiIV sites
on the labeled support.
AUTHOR INFORMATION
Corresponding Author
*askatz@berkeley.edu,
sizo@chevron.com,
Based on the identical electronic and inner-sphere
environments of grafted TiIV for all catalysts, active sites
would be expected to display similar epoxidation rates in
the absence of outer-sphere effects; however, they are
instead observed to be up to 5-fold more active when
hubert.koller@uni-muenster.de
Acknowledgement
Funding from the National Science Foundation (PFI:AIR-
TT 1542974) and the Deutsche Forschungsgemeinschaft
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Park, J. W.; Thomas, K.; Van Gestel, J.; Gilson, J. P.; Collet, C.;
(DFG, German Research Foundation – KO 1817/3-1) are
gratefully acknowledged. The authors are grateful to the
Management and Transfer of Hydrogen via Catalysis
Program funded by Chevron Corporation. XAS
measurements were performed on SSRL Beamline 2-2,
which was supported in part by the Synchrotron Catalysis
Consortium, US Department of Energy Grant No. DE-
SC0012335.
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Notes
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These authors declare the following financial interest(s): (1)
The funding for the research partially came from Chevron
Energy Technology Co. and (2) Stacey I. Zones is an
employee and stockholder in Chevron Corp.
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amorphous support
crystalline support
OH
O
keff
7
OH
8
9
O
O
O
O
O
CH3
Ti
O
OH
10
11
12
13
14
15
16
17
18
19
20
21
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23
24
25
26
27
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56
57
58
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Si
Si
Si
Si
HO
O
O
O
O
CH3
O
Ti
vs.
O
OH
Si
Si
Si
OH
SiO2
keff = 8-10 M-2s-1
keff = 42 M-2s-1
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