Zeolites with isolated-framework and oligomeric-extraframework hafnium species characterized with pair distribution function analysis

Article abstract of DOI:10.1039/c8cp00464a

Zeolites containing framework heteroatoms (e.g., Ti, Sn, and Hf) with open coordination sites behave as solid-state Lewis acids and exhibit remarkable catalytic properties unachievable with bulk oxides. However, direct evidence confirming the incorporation of such heteroatom species into the zeolite framework is difficult to obtain because of the limited number of analytical methods capable of discerning framework incorporation from extraframework species. In this work, the structural environments of hafnium (Hf) framework and extraframework species added post-synthetically into ?BEA zeolites were analyzed using coupled pair distribution function (PDF) and diffuse reflectance (DR) UV-vis measurements. PDF analysis enabled the visualization and identification of framework and extraframework HfO_x species, both of which were undetectable by traditional X-ray and neutron diffraction methods. Reactivity data from the aldol condensation of benzaldehyde and acetone confirmed that framework Hf species are responsible for catalytic activity.

Full text of DOI:10.1039/c8cp00464a

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Title
Zeolites with isolatedꢀframework and oligomericꢀextraframework hafnium species
characterized with pair distribution function analysis
Takayuki Iida1,2, Koji Ohara3, Yuriy RománꢀLeshkov2* and Toru Wakihara1*
1) Department of Chemical System Engineering, The University of Tokyo, 7ꢀ3ꢀ1 Hongo,
Bunkyoꢀku, Tokyo 113ꢀ8656, Japan
2) Department of Chemical Engineering, Massachusetts Institute of Technology, 25
Ames Street, Cambridge, Massachusetts 02139, United States of America
3) Japan Synchrotron Radiation Research Institute/SPringꢀ8, Kouto 1ꢀ1ꢀ1, Sayoꢀgun,
Hyogo 679ꢀ5198, Japan
1

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Abstract (100–150 words)
Zeolites containing framework heteroatoms (e.g., Ti, Sn, and Hf) with open
coordination sites behave as solidꢀstate Lewis acids and exhibit remarkable catalytic
properties unachievable with bulk oxides. However, direct evidence confirming the
incorporation of such heteroatom species into the zeolite framework is difficult to obtain
because of the limited number of analytical methods capable of discerning framework
incorporation from extraframework species. In this work, the structural environments of
hafnium (Hf) framework and extraframework species added postꢀsynthetically into
*BEA zeolites were analyzed using coupled pair distribution function (PDF) and diffuse
reflectance (DR) UV–vis measurements. PDF analysis enabled the visualization and
identification of framework and extraframework HfOx species, both of which were
undetectable by traditional Xꢀray and neutron diffraction methods. Reactivity data from
the aldol condensation of benzaldehyde and acetone confirmed that framework Hf
species are responsible for catalytic activity.
2

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Main text
The substitution of tetrahedral groupꢀIV metal cations (such as Ti4+, Sn4+, Zr4+, and
Hf4+) into the crystallographic framework vertex sites (or Tꢀsites) of zeolites, a group of
inorganic microporous crystals composed of tetrahedral metal oxide units, leads to
functional heterogeneous Lewis acid catalysts1. Among these catalysts, Hfꢀcontaining
zeolites exhibit unique reactivity, as demonstrated for aldol condensation2 and
Meerwein–Ponndorf–Verley reduction3 reactions, where Hfꢀcontaining zeolites
outperform their Zrꢀ, Tiꢀ, and even Snꢀcontaining zeolite counterparts both in terms of
reactivity and selectivity for some substrates. However, conclusive structural evidence
confirming that Hf cations are incorporated into and retained in the framework, and
demonstrating that these isolated species are the active Lewis acid sites has not been
reported.
The aforementioned challenge is pervasive for most heteroatomꢀcontaining zeolites,
especially in cases where the metal cations are inactive toward atomsꢀspecific
spectroscopic tools such as nuclear magnetic resonance (NMR) and electron spin
resonance (ESR)4–6. Titration techniques with various probe molecules7,8 provide
valuable information about the different types active sites, but several challenges
including the possible interaction with the unintended adsorption sites (e.g., structure
3

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defect sites), supraꢀstoichiometric adsorption, or the need to obtain unique molar
extinction coefficients, in many cases, increase the complexity to interpret the results9.
Extended Xꢀray absorption fine structure (EXAFS) analysis has emerged as an
alternative tool to investigate the local structure of heteroatoms inside a solid10.
However, the limited resolution in the radial distribution function calculated from
EXAFS oscillation can be problematic for differentiating between the framework and
the extraframework heteroatom species, as exemplified in a previous report on
Zrꢀcontaining zeolites11. Reactivity measurements are also often used to verify the
incorporation of heteroatoms; however, the possibility of the reaction being catalyzed
by the extraframework species inside the zeolite micropores is sometimes overlooked.
For example, in the isomerization of glucose, extraframework SnOx clusters inside the
hydrophobic zeolite micropores can also act as a solid base to drive the reaction via a
different reaction mechanism12 than the mechanism associated with the framework
Lewis acid sites in Snꢀcontaining *BEA zeolites13. Diffuseꢀreflectance (DR) UV–vis
measurements are also widely used to gain qualitative understanding of the presence of
extraframework heteroatom species12; however, the difficulty in correlating with the
absorption by nanosized oligomeric extraframework species makes full assignment of
the observed absorption spectra on the basis of the framework heteroatom species
4

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difficult. Lastly, extra care must be taken when using the lattice parameter calculated by
diffraction techniques to suggest the incorporation of heteroatoms because the
extraframework species can expand the pore diameter and thus disturb the
crystallographic information available by diffraction techniques14.
The pair distribution function (PDF), G(r), is an alternative methodology for
describing the structure of an analyte by presenting the probability of finding a certain
interatomic distance within a unit volume15. An important difference between the PDF
method and diffraction techniques is the ability to obtain structural information from the
scattering of Xꢀrays (or neutrons) instead of being limited to the information obtained
from diffraction. Because of such features, nanostructures with a periodicity no larger
than the unitꢀcell parameter can be measured, whereas diffraction can detect only
structures with larger dimensions of ordering (multitude of unit cells in size)16.
Therefore, not only can PDFs depict structures from a distinctly different perspective
than crystallography, but also can be applied to both crystalline and disordered materials,
such as the amorphous intermediates present before zeolite crystallization17,18. An
applied analysis method called the differential pair distribution function (dꢀPDF)19 has
been used to investigate the structure of some small nanoclusters/nanoparticles present
on catalyst supports. The features of this analytical tool are expected to be useful for
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examining the extraframework heteroatom species that do not generate diffraction
patterns as well as for discerning the structure of the heteroatom species inside the
zeolite framework.
One challenge considered beforehand was that the signal from the isolatedꢀframework
sites would be too weak to provide sufficient scattering data for analysis. Therefore, in
this work, Hfꢀcontaining *BEA zeolite was used as the model material because of the
large Xꢀray scattering factor of Hf (the Xꢀray scattering factor of an element is
proportional to its atomic number). Hf, with atomic number 72, has atomic properties
similar to those of Zr because of the lanthanide contraction of atomic size (crystal
atomic radii: Zr4+, 73 pm; Hf4+, 72 pm)20. The *BEA zeolite, a crystal intermixture of
BEA and BEB polymorphs (further details in Figure S1), is commonly used for catalytic
reactions involving bulky substrates because of its large, threeꢀdimensional pores and
was chosen in the present work as the base topology for postꢀsynthesis modifications.
In the present study, Hfꢀcontaining *BEA zeolites were prepared postꢀsynthetically
using a modified version of a previously reported method21. The postꢀsynthesis route
can produce zeolite catalysts with greater flexibility in the Hf composition as well as in
the degree of incorporation21, when compared to the direct synthesis route2 . Hf was
first inserted into the dealuminated *BEA zeolites (DeAlꢀbeta) using
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bis(cyclopentadienyl)hafnium(IV) dichloride (Hf(Cp)2Cl2) as a precursor, and the solid
was subsequently calcined to remove the organic groups (product denoted as
Hfꢀbeta(post)). Dealumination was performed by acid treatment of the commercial
aluminosilicate *BEA zeolite (Si/Al = 12.5) using concentrated (60 wt%) nitric acid.
Although similar methods have been used to produce various heteroatomꢀcontaining
zeolites21, the possibility of forming extraframework species is always present.
Therefore, a second acid treatment was then performed on the asꢀprepared Hfꢀbeta(post)
using concentrated nitric acid (60 wt%) to remove possible extraframework HfO2
species (samples denoted with suffix “_AT”). To prepare a sample that exclusively
contained extraframework HfOx species, we used Hf(Cp)2Cl2 to impregnate a pure silica,
defectꢀfree *BEA zeolite prepared in fluoride media, denoted as HfOx/Siꢀbeta(F).
Further synthetic and characterization details are summarized in the ESI.
A comparison of the XRD patterns of Hfꢀbeta(post), Hfꢀbeta(post)_AT, and
HfOx/Siꢀbeta(F) with the patterns of their parent zeolites confirmed the presence of
*BEA crystals with crystallinities similar to those of the original crystals (Figure 1). The
HfO2 phase was not detected via diffraction. The Si/Hf molar ratio is also shown in
Figure 1, and details of the elemental compositions of these catalysts are summarized in
Table S1. N2 physisorption measurements also verified that the crystallinity in terms of
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micropore volume was maintained throughout the treatment procedures (Table S2;
isotherms in Figure S2).
Figure 1. XRD patterns of Hfꢀbeta(post), Hfꢀbeta(post)_AT, dealuminated *BEA
zeolite (DeAlꢀbeta), HfOx/Siꢀbeta(F), and Siꢀbeta(F) in reference to HfO2. The
intensity values for HfO2 were reduced to oneꢀfourth of their original values.
Diffraction peaks confirmed in the zeolite samples all originated from the *BEA
zeolite crystal structure. The elemental compositions (Si/Hf) calculated from ICP–
AES measurements are also shown in brackets [ ].
Having confirmed the difficulty in differentiating these crystals by diffraction analyses,
we carried out structural investigations based on PDF analyses. In addition to using
monochromic synchrotron Xꢀray (61.4 keV), the obtained scattering data are normalized
by the incident Xꢀray intensity during the calculation process to remove the influence of
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fluctuation in the beam intensity. Also, to assure highꢀquality data, for all samples,
measurements were performed using long dwell times (for example, in the range
where diffraction does not appear i.e. > 4º, dwell time > 20 s with ꢁ = 0.05º). Thus, it
was confirmed from the Faber–Ziman total structure factors, ), used for the
θ
θ
θ
S(Q
calculation of PDFs (summarized in Figure S3) that the measurements are conducted
under conditions where there is little influence on the error range by the dwell time. The
PDFs of HfOx/Siꢀbeta(F) and Siꢀbeta(F) are shown in Figure 2 a). The peaks in the PDF
of Siꢀbeta(F), ꢀꢁꢂꢃꢄꢅꢆꢇ(ꢈ)(ꢉ), depict the probability of finding the given atomic distance
in the siliceous *BEA crystal structure. For example, the correlation peaks at
r = 1.61,
2.6, and 3.1 Å correspond to the nearestꢀneighbor Si–O, O–Si–O, and Si–O–Si
interatomic distances. We carried out a dꢀPDF analysis (shown in Figure 2 a)) to
highlight some of the subtle differences between the PDFs of HfOx/Siꢀbeta(F) and
Siꢀbeta(F). The dꢀPDF method is based on the linear feature of PDF, which enables
differential calculations between the PDFs,
G(r), to extract the structure of a minority
element in a binary phase admixture (a detailed explanation of the method is
summarized in the ESI). For example, in the context of this work, the PDF describing
the structure of oligomeric HfOx species in HfOx/Siꢀbeta(F), ꢀꢊꢋꢃ(ꢉ), can be calculated
from equation (1), which corresponds to the difference between the PDFs of
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HfOx/Siꢀbeta(F) and Siꢀbeta(F):
( )
ꢌꢍꢀꢊꢋꢃ ꢉ = ꢀꢊꢋꢎꢏ/ꢁꢂꢃꢄꢅꢆꢇ(ꢈ)(ꢉ) – ꢌꢐꢀꢁꢂꢃꢄꢅꢆꢇ(ꢈ)(ꢉ)
(1)
where ꢌꢐ and ꢌꢍ are coefficients. theoretically, the same calculation should provide
the structural environment around Hf in the zeolite framework Tꢀsites when the
calculation is performed for zeolites with Hf inside their framework. The calculated
dꢀPDF results for all of the samples are provided in Figure 2 b).
As anticipated, HfOx species were identified in the dꢀPDF of HfOx/Siꢀbeta(F). Notably,
the PDF is capable of distinguishing various HfO2 phases (e.g., monoclinic, cubic, and
orthorhombic) on the basis of the differences in the relative positions of Hf and O atoms
reflected in distinct PDFs (Figure S4). The dꢀPDF of HfOx/Siꢀbeta(F) was referenced
with the theoretical PDF of monoclinic HfO2 (mꢀHfO2) for “phase” identification
(Figure 2 c)) after the peak positions and the relative intensities were compared with the
theoretical PDFs of other HfO2 phases. Further, the deconvolution of the theoretical
PDF of
mꢀHfO2 clarified that most correlations originate from Hf–Hf and Hf–O
correlations because of the high Xꢀray scattering factor of Hf (Figure S5).
Likewise, when the dꢀPDF calculation was performed for Hfꢀbeta(post) (Figure 2 b)),
the presence of correlations corresponding to
m
ꢀHfO2 was confirmed in the region 3 ≤
r
≤ 6 Å. The lack of correlations corresponding to
10
m
ꢀHfO2 beyond 6 Å shows that the size

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of the nanoclusters is smaller than this value. This result implies that the
extraframework HfOx nanoclusters can be formed and stabilized inside the zeolite
pores22. Such correlation peaks were also confirmed in the dꢀPDF of Hfꢀbeta(post)_AT;
however, the areas of the peaks were smaller than those of Hfꢀbeta(post) signifying that
the acid treatment was effective at removing extraframework clusters. Interestingly the
dꢀPDF of Hfꢀbeta(post)_AT showed correlations different from those of HfOx/Siꢀbeta(F).
Specifically, the correlation at approximately
r = 2.0 Å, which corresponds to the
nearestꢀneighbor Hf–O correlation (shown with an arrow in Figure 2 b)), was at a
shorter distance than in the dꢀPDF of HfOx/Siꢀbeta(F). Although this correlation can be
influenced by the termination ripple artifacts formed during the Fourier transformation
used to obtain the PDFs23, the fact that the position of the nearestꢀneighbor oxygen
appears at a shorter distance implies the presence of tetrahedral Hf species in
comparison to the octahedral Hf in
mꢀHfO2. Overall, these findings indicate that the
correlations observed in the dꢀPDF of Hfꢀbeta(post) and Hfꢀbeta(post)_AT can originate
from the interatomic correlations between framework Hf and the surrounding Si and O
atoms in the zeolite structure and/or from the components of the extraframework HfOx
nanoclusters. The ratio between the framework and the extraframework Hf cannot be
fully determined only from the dꢀPDF results of these zeolite samples because of the
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possible overlap of the correlation peaks.
Figure 2. a) The dꢀPDF calculation of HfOx/Siꢀbeta(F) (pair distribution function,
G(r), of HfOx/Siꢀbeta(F) and Siꢀbeta(F), and the difference between these two PDFs
(dꢀPDF); b) dꢀPDF results for Hfꢀbeta(post)_AT, Hfꢀbeta(post), and
HfOx/Siꢀbeta(F); c) theoretical PDF of monoclinic HfO2, as calculated using the
PDFgui software24.
To justify the correlation between framework Hf and the zeolite in the positions
observed in the dꢀPDF of Hfꢀbeta(post) and Hfꢀbeta(post)_AT, the dꢀPDF was compared
with that of a different heteroatomꢀcontaining *BEA zeolite. An identical postꢀsynthetic
treatment was used to prepare *BEA zeolites with Sn incorporated inside the framework
(including the acid treatment, the product was designated as Snꢀbeta(post)_AT), and the
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dꢀPDF is shown in reference to that of Hfꢀbeta(post)_AT (Figure 3). A comparison with
the theoretical PDF of SnO2 (cassiterite, shown in Figure S6) showed that the
correlations of Snꢀbeta(post)_AT were completely different from the correlations of
SnO2. Notably, the positions of the dꢀPDF correlations for Hfꢀbeta(post)_AT and
Snꢀbeta(post)_AT were observed at similar distances (the crystal atomic radii of
tetrahedral Sn4+ and Hf4+ are similar, 69 and 72 pm, respectively)20, providing further
evidence that the dꢀPDF can represent the peripheral environment of the heteroatoms
incorporated inside the zeolite framework. Again, because of the possible overlap in the
correlations, the ratio of the framework and the extraframework Hf species cannot be
fully determined even at this stage.
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Figure 3. Comparison of the dꢀPDF between Hfꢀbeta(post)_AT and
Snꢀbeta(post)_AT. The dꢀPDF of Hfꢀbeta(post)_AT was offset, and the
corresponding peaks in the region between 1.8 ≤
lines as visual guides.
r ≤ 6 Å are shown with dotted
Further characterization based on DR UV–vis measurements, another bulk
characterization tool that can investigate the different Hf species, was performed (Figure
4). The presence of a broad absorption band between 250 and 500 nm was only
confirmed in the spectra of Hfꢀbeta(post) and HfOx/Siꢀbeta(F), whereas such absorption
was not present in the spectrum of Hfꢀbeta(post)_AT. Because HfOx/Siꢀbeta(F) is a
sample that exclusively possesses extraframework HfOx, and because such
extraframework species can be expected to be present in Hfꢀbeta(post), this peak was
assigned to the absorption by the oligomeric extraframework HfOx species. This
assignment is in agreement with the dꢀPDF results. Indeed, bulk HfO2 is known to show
an absorption peak at 210 nm (Figure S7), and the reason for the shift to the 250–500
nm region remains a topic for future investigations. By contrast, only the absorption
between 200 and 240 nm was confirmed in the spectrum of Hfꢀbeta(post)_AT. Because
Hfꢀbeta(post)_AT was the product obtained after an acid treatment that removed the
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oligomeric HfOx from Hfꢀbeta(post), as confirmed from the DR UV–vis spectra, the
absorption between 200 and 240 nm was assigned to the framework Hf species. These
assignments substantiate the report by Lewis et al.2 and Mahmound et al.25, which
makes the assignments of the DR UVꢀvis based on the spectra of zeolite catalysts that
were prepared differently having different activities for Lewis acidꢀcatalyzed
reactions2,25. The small shouldering peak at 225 nm was also confirmed and was
tentatively assigned to the octahedral Hf that partially bonds with the zeolite framework
because this peak was still present after the acid treatment that removed the HfOx
nanoclusters. Thus, in conjunction with the DR UV–vis results, the correlations
observed in the dꢀPDF of Hfꢀbeta(post)_AT can be conclusively assigned to Hf atoms
incorporated into the zeolite framework.
Figure 4. DR UV–vis spectra of various Hf zeolites.
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RomanꢀLeshkov et al. demonstrated that the aldol condensation between
benzaldehyde and acetone (Figure 5), can be catalyzed by the Lewis acid sites of Hf
inside the zeolite following a softꢀenolization route that results in a high selectivity
values toward the monocondensed product (product a) over the bicondensed product
(product b)2. We conducted this reaction at identical catalyst loadings and reaction times
to determine whether the framework Hf or the extraframework HfOx nanoclusters are
the active sites (Table 1). Despite the fact that Hfꢀbeta(post) possessed five times more
Hf than Hfꢀbeta(post)_AT, both catalysts were equally selective and active toward the
formation of the monocondensed product (product a). Because the dꢀPDF results
indicated that only nanoclusters with particle sizes smaller than 6 Å are present in
Hfꢀbeta(post), the formation of bulk extraframework HfOx species with small surface
areas and few active sites can be ruled out. These results confirm, as suggested in the
original paper2, that the framework Hf atoms are indeed the active site for the Lewis
catalyzed aldol condensation.
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Figure 5. Reaction scheme for the aldol condensation between benzaldehyde and
acetone to form benzylacetone (product a, monocondensed product) and
diphenylpentaꢀ3ꢀnone (product b, bicondensed product).
Table 1. Results for the aldol condensation between benzaldehyde and acetone
(conversion and yield of product a (monocondensed product))
Catalyst
Hf/(Hf+Si) [mol%]
Conversion [%]
Yield [%]
Hfꢀbeta(post)
2.6
20
25
14
16
Hfꢀbeta(post)_AT
0.54
Reaction conditions: Reaction solution: 1 g (benzaldehyde 1 mmol/L) and 3 equiv.
acetone; reaction temperature: 80°C; reaction period: 30 min; catalyst: 40 mg.
Trace formation of the bicondensed product was confirmed.
In summary, we applied PDF analysis to postꢀsynthetically prepared Hfꢀcontaining
*BEA zeolites to discriminate the framework and the extraframework species in
Hfꢀcontaining zeolites. The results provided unequivocal evidence of Hf incorporation
into the crystallographic framework Tꢀsites, as well as the visualization of the
extraframework HfOx species. Further, coherency and complementarity with the DR
UV–vis results were confirmed, enabling the assignment of absorption peaks based on
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visualized Hf structures. Finally, experimental results to assign the framework Hf
species to be the main active site for aldol condensation was acquired by comparing the
catalytic activity of a zeolite that contains both framework and extraframework Hf, and
another in which the latter was removed. This work is the first to shed light on how
extraframework heteroatom oxide species that are not detected by Xꢀray diffraction
actually exist inside the admixture with the *BEA zeolite crystals. Complementing
existing reports of the incorporation of different metals into the zeolite framework, the
current work provides a unique, powerful tool for articulating the catalytically active
structure inside zeolite catalysts.
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Acknowledgement
This work was supported by the U.S. Department of Energy, Office of Basic Energy
Sciences under Award No. DEꢀSC0016214. The HighꢀEnergy Total Xꢀray Scattering
experiments conducted at SPringꢀ8 were approved by the Japan Synchrotron Radiation
Research Institute (Proposal Nos. 2016B0115 and 2017A0115). T. I. thanks the Japan
Society for the Promotion of Science for a Grantꢀinꢀaid for Scientific research (This
work was supported by JSPS KAKENHI Grant Number 15J07161), and the Program
for Leading Graduate Schools, “Global Leader Program for Social Design and
Management (GSDM),” by the Ministry of Education, Culture, Sports, Science and
Technology, for financial support.
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TOC
PDF analysis enabled the visualization and identification of framework and
extraframework HfOx species in the postꢀsynthetically produced *BEA zeolites.
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