With Open Arms: Open Sites of ZrBEA Zeolite Facilitate Selective Synthesis of Butadiene from Ethanol

Article abstract of DOI:10.1021/acscatal.5b01024

Fourier transform infrared spectroscopy and density functional theory calculations have been used to elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of butadiene. We show that the content of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three -O-Si linkages and one OH group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene. The higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeolite-based catalysts. (Figure Presented).

Full text of DOI:10.1021/acscatal.5b01024

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ACS Catalysis
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With Open Arms: Open Sites of ZrBEA Zeolite Facilitate Se-
lective Synthesis of Butadiene from Ethanol
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Vitaly L. Sushkevich,1 Dennis Palagin,2 and Irina I. Ivanova1,*
1 Department of Chemistry, Lomonosov Moscow State University, 119991, Leninskye Gory 1, bld. 3, Moscow, Russia
2 Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxꢀ
ford, OX1 3QZ, United Kingdom
ABSTRACT: Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations have been used to
elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of butadiene. We show that the content
of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure conꢀ
nected to three ꢀOꢀSi linkages and one OHꢀgroup, correlates with the catalytic activity in the process of conversion of ethanol into
butadiene. Higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study
suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeoliteꢀbased
catalysts.
KEYWORDS Zr-Beta, zeolite, open sites, DFT, FTIR
Zeoliteꢀbased materials constitute an important new class of
solid catalysts that attract attention due to their wellꢀdefined
active sites exhibiting high activity in a wide variety of imꢀ
portant reactions.[1] The discovery of SnBEA zeolite as a cataꢀ
lyst for a range of reactions was an important breakthrough in
the use of Lewis acids as heterogeneous catalysts.[2] A more
recent example is a ZrBEA, which contains Zr(IV) sites introꢀ
duced in the βꢀzeolite framework.[3] The uniform distribution
of isolated Zr Lewis acid sites, in combination with unique
porous structure of the material, results in an unrivalled cataꢀ
lytic performance in a number of industrially important proꢀ
cesses,[4–10] including the synthesis of butadiene from ethaꢀ
nol.[8]
drogenation step rendering acetaldehyde condensation and
MPVO steps to be rate limiting.[8,17]
OH
1
2
OH
O
O
-H2
3
-H2O
5
+EtOH
OH
O
-H2O
4
The latter reaction is very important for the diversification
of the industrial routes of synthesis of butadiene, which is an
important monomer for the production of rubbers and elastoꢀ
mers. Isolation of butadiene from naphtha steam cracker fracꢀ
tions of paraffinic hydrocarbons is the major production
route,[11] which however lacks efficiency especially in an enviꢀ
ronmental perspective.[11ꢀ12] The alternative ethanol conversion
process is a promising sustainable substitute for the dominant
naphthaꢀbased method, capable of contributing to the decrease
in use of fossil fuel reserves, which stimulated a new wave of
research over the last five years.[13ꢀ21]
Chart 1. The main reaction pathway of ethanol to butadiene
conversion.
The investigation of the latter steps over Zrꢀcontaining cataꢀ
lysts suggested that both reactions are governed by the Lewis
acidity,[7,22] with the overall activity of the metal promoted Zrꢀ
based molecular sieves correlated with Lewis acidity of the
catalyst. Recent studies based on the FTIR spectroscopy alꢀ
lowed identification of two distinctive types of Lewis sites,
designated as closed and open Zr(IV) sites[23] in analogy with
the data obtained for the SnBEA zeolite[24] (Chart 2). Howevꢀ
er, the real configuration of the present sites, as well as their
corresponding catalytic activity remains unknown.
Recent developments render the ZrBEA zeolite as a material
with very high potential as a catalyst for the conversion of
ethanol into butadiene.[8] However, the nature of ZrBEA active
sites, which is a necessary preꢀrequisite for understanding
mechanisms of reactions and design of new catalysts, has not
been fully understood. The kinetic studies performed over Zrꢀ
containing catalysts[8,17] revealed that of the five reaction steps
that constitute the target reaction pathway leading to butadiene
(Chart 1), steps 1, 2 and 4, namely, ethanol dehydrogenation,
acetaldehyde condensation and Meerwein–Ponndorf–Verley–
Oppenauer (MPVO) reduction of crotonaldehyde with ethanol,
are the key reaction steps.[15] Addition of metal promoter, such
as silver, copper or nickel,[16,17] accelerates the ethanol dehyꢀ
Si
Si
O
O
O
O
O
O
Si
Zr
Si
Si
Zr
H
O
O
Si
Si
Chart 2. Configurations of closed (left) and open (right) sites
of the ZrBEA zeolite.
These challenges motivate the present study of the nature of
the active sites of the ZrBEA zeolite. ZrBEA zeolites with a
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Si/Zr molar ratio within 100ꢀ800 promoted with silver were reaction, and their content may be strongly influenced by the
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tested for the ethanol conversion into butadiene and showed
excellent catalytic activity in terms of butadiene yield and
selectivity. Combining spectroscopic and computational chemꢀ
istry approaches along with the studies of reaction kinetics, we
show that the open Zr(IV) Lewis acid sites are mainly responꢀ
sible for the catalytic activity of ZrBEA in the conversion of
ethanol to butadiene, and suggest that the control over the
number of such sites by varying the Zr loading and distribuꢀ
tion is the key for a rational design of novel multifunctional
zeoliteꢀbased catalysts.
Si/Zr ratio in the zeolite.
The product distribution is shown in Table S2. The results
suggest that butadiene is the main reaction product over all the
catalysts studied. The main side reactions involve ethanol deꢀ
hydration into ethylene and diethyl ether, ethyl acetate forꢀ
mation via Tischenko reaction of acetaldehyde, unselective
reduction of crotyl alcohol leading to 1ꢀbutanol, which dehyꢀ
dration gives butenes mixture.[8]
The comparison of the selectivity towards butadiene over
different catalysts at the same conversion level of 30% reveals
that the selectivity slightly decreases with increasing Zr conꢀ
tent. This observation can be due to higher density of the acꢀ
tive sites, which provides for the higher contribution of conꢀ
densation reactions and therefore higher amount of heavy byꢀ
products (Table S2). Selectivity towards other side products
does not change from Ag/ZrBEA(130) to Ag/ZrBEA(850).
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To study the nature of active Zr sites, a series of ZrBEA
catalysts with different Si/Zr ratio was prepared. The samples
are designated thereafter as ZrBEA(x), where x is a Si/Zr ratio.
All materials were characterized by XꢀRay diffraction (XRD),
XꢀRay photoelectron spectroscopy (XPS), 29Si magicꢀangle
spinning nuclear magnetic resonance (29Si MAS NMR), nitroꢀ
gen adsorptionꢀdesorption, and elemental analysis. The data
are summarized in Supporting information.
The catalytic activity of Ag/ZrBEA catalysts in the time
course of the reaction slightly decreases without changes in
products selectivity indicating gradual poisoning of active
sites by coke deposits.[8,17] But the loss of activity during 6 h
on stream does not exceed 10%.
For the catalytic tests ZrBEA materials were doped with 1
wt% of silver to promote ethanol dehydrogenation reaction
into acetaldehyde. The content of silver promoter was optiꢀ
mized previously [8,17,25] to accelerate the rate of dehydrogenaꢀ
tion step (step 1, Chart 1) with respect to condensation and
MPVO (steps 2,4 Chart 1) and to ensure that the overall reacꢀ
tion rate is controlled by Lewis acid sites. The examination of
silver distribution and particle size by TEM and SEM/EDX
(Fig. S5, S6) shows the uniform distribution of silver particles
of 2ꢀ5 nm on the surface of the ZrBEA which is required for
ethanol dehydrogenation.[25]
To establish the nature of the active sites responsible for the
catalytic activity of ZrBEA, samples were studied by FTIR
spectroscopy of the adsorbed CO (Figure 1).
The conversion of ethanol into butadiene over ZrBEA promotꢀ
ed with silver catalysts was studied at 593K, atmospheric presꢀ
sure and weight hour space velocity within 0.1–0.6 hꢀ1. These
reaction conditions were found to be optimal for the selective
synthesis of butadiene over Zrꢀcontaining catalysts [17]
.
The results of the catalytic activity tests presented in Table 1
show that the increase of the Zr content in ZrBEA from 20 to
122 ꢁmol/g leads to significant enhancement of the initial rates
of the reaction from 0.72 to 2.64 ꢁmol/gꢂs. However, no direct
correlation is observed. Thus, for the samples of ZrBEA(260)
and ZrBEA(460) ethanol conversion rates are almost identical,
whereas the Zr content differs by a factor of two.
Table 1. Catalytic properties of ZrBEA samples in ethanol
conversion into butadiene.
Initial rate,
Zr
ꢁmol/g
content,
Butadiene selectivꢀ
ity, mol%
Catalyst
ꢁmol/gꢂs
Figure 1. FTIR spectra of CO adsorbed on ZrBEA(130) collected
with the increase of CO coverage. Color boxes show the configuꢀ
ration of adsorption sites assigned to each peak.
Ag/ZrBEA(130) 122
Ag/ZrBEA(260) 63
Ag/ZrBEA(460) 36
Ag/ZrBEA(610) 27
Ag/ZrBEA(850) 20
2.64
1.30
1.34
0.90
0.72
64
65
66
68
68
This technique was proven to be appropriate to distinguish
Lewis sites of different types, and to measure their relative
amounts.[23] The adsorption of CO at low temperature (~100
K) leads to the formation of Hꢀbonds with OH groups and to
the coordination of CO to Lewis acid sites through sigmaꢀ
donation. The vibration band of the adsorbed CO subsequently
shifts to higher wavenumbers with respect to the band of
pseudoꢀliquid CO (2138 cm−1). This shift is characteristic for
the nature and strength of the site.
Such behavior of ZrBEA catalysts suggests that Lewis sites
generated by the incorporation of Zr atoms into the zeolite
framework are not equal in catalytic activity. In particular,
open and closed Zr sites may possess different activity in the
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Carbon monoxide calibrated aliquots were gradually introꢀ
ZrBEA. The calculated values are very close to the experimenꢀ
tally obtained ones (2176 and 2185 cm−1, respectively). Phyꢀ
sisorbed CO (simulated by adsorbing a second CO molecule to
an already “occupied” adsorption site) yields the CꢀO vibraꢀ
tional frequency of 2137 cm−1, which agrees well with experꢀ
imentally measured value of 2138 cm−1 of pseudoꢀliquid CO.
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duced into the cell cooled with liquid nitrogen, and the IR
spectra were subsequently recorded (Figure 1). With increase
of CO pressure, the bands at 2185, 2163, 2156, 2176 and 2138
cm−1 were successively observed in spectra.
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In accordance with Ref. 23, the high frequency bands (2185
and 2176 cm−1) were attributed to the CO adsorption on Lewis
sites, whereas the low frequency bands (2163 and 2156 cm−1)
were assigned to the CO interacting with OH groups. The last
band observed at 2138 cm−1 was due to pseudo liquid CO viꢀ
brations. The actual configuration of the sites detected was
further verified by DFT calculations, which have been carried
out using four cluster models, corresponding to the adsorption
of CO molecule on metal centers of the closed and open sites
of the ZrBEA, as well as on the OH groups of the Zr and Si
centered open sites (Figure 2). The structures obtained by adꢀ
sorption of CO on the proposed cluster models were optimized
within the DFT framework at the hybrid functional level. Viꢀ
brational spectra have been calculated within the harmonic
approximation.
Calculated adsorption energies support the above assignꢀ
ment of the IR bands: in the case of direct adsorption of CO
molecule on the metal center the adsorption energy is ~1.7
kcal/mol higher than in the case of the interaction with the OH
group, which agrees well with the higher vibrational frequenꢀ
cies observed in experiment.
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In further experiments ZrBEA catalysts with different Zr
loading were compared and similar set of bands were observed
for all samples (Figures S7, S8). The normalized intensities of
the bands at 2185 (open sites) and 2176 cm−1 (closed sites)
were plotted versus the Zr content in the samples as shown in
Figure 3a.
Figure 2. DFTꢀoptimized structures of CO adsorbed on closed (a)
and open (b) sites of ZrBEA, as well as on OH groups of Si (c)
and Zr (d) centered open sites. Atomic coordinates of each strucꢀ
ture are available in the Supporting Information.
Figure 3. Relative amount of open and closed Lewis sites deterꢀ
mined by FTIR spectroscopy of adsorbed CO versus Zr content in
silver promoted ZrBEA samples (a) and versus initial rates of
ethanol conversion (b).
Calculated vibrational frequencies of the adsorbed CꢀO
group allowed unambiguous identification of the four experiꢀ
mentally observed bands. Two lower frequency bands belong
to the CO adsorption on the OH groups: the COꢀHOSi(OSi)3
complex yields 2158 cm−1 vibration, while the COꢀ
HOZr(OSi)3 is featured at 2165 cm−1. These results are in good
agreement with experimental values of 2156 cm−1 and 2163
cm−1, respectively. Two higher vibrational frequencies are due
to CO interaction with Zr atoms of the closed (COꢀZr(OSi)4,
2177 cm−1) and open (COꢀZr(OSi)3OH, 2181 cm−1) sites of the
The results indicate that the relative amount of closed sites
is linearly correlated with Zr content. The band corresponding
to open sites has different behavior: at low Zr contents, the
relative amount of open sites decreases much slower than the
amount of closed sites. Thus, ZrBEA(260) and ZrBEA(460)
show nearly the same amounts of open sites, as noted above.
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(6) Liu, S. H.; Jaenicke, S.; Chuah, G.ꢀK. J. Catal. 2002,
The comparison of the results obtained over different
ZrBEA catalysts in ethanol conversion into butadiene with the
results of infrared spectroscopy of adsorbed CO points to lineꢀ
ar correlation between the catalyst activity and the relative
content of open sites (Figure 3b). The amount of the closed
sites, on the other hand, does not show direct correlation with
activity. All these observations suggest that the activity of the
ZrBEA catalyst depends on the content of open Zr(IV) Lewis
sites, whereas closed Zr(IV) sites seem to be inactive or less
active in this reaction. Higher activity of open sites could be
attributed to their high acid strength, as well as to better steric
accessibility compared to the closed sites.
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In summary, we have systematically studied the nature of
the activity of Ag/ZrBEA zeolite catalysts in the reaction of
the conversion of ethanol into butadiene. Our results suggest
that the Zr(IV) Lewis acid sites in such catalyst are representꢀ
ed by isolated Zr atoms in tetrahedral positions of the zeolite
crystalline structure linked with four (closed site) and three
(open site) silicon atoms. Combining the results of Fourier
transform infrared spectroscopy and density functional theory
calculations revealed that the open sites of ZrBEA are the
most efficient in the reaction of butadiene synthesis, which can
be rationalized in terms of higher acid strength and better steꢀ
ric accessibility of such sites. Our study suggests that by varyꢀ
ing the amount of Zr, it is possible to control the amount of the
open sites and thus the catalytic activity of the ZrBEA zeolite.
These findings illustrate the diversity and flexibility available
to zeolites as catalytic materials due to the tunability of their
properties via adjusting the nature and the amount of their
adsorption sites, and facilitate further design of the optimal
multifunctional catalysts.
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AUTHOR INFORMATION
Corresponding Author
* iiivanova@phys.chem.msu.ru
Author Contributions
The manuscript was written through contributions of all authors.
ACKNOWLEDGMENT
V.L.S. and I.I.I. thank the Russian Science Foundation for the
financial support (Grant №14ꢀ23ꢀ00094).
(25) Sushkevich, V.; Ivanova, I.; Taarning, ChemCatChem,
2013, 5, 2367ꢀ2373.
ASSOCIATED CONTENT
Supporting Information
Detailed synthetic procedures and characterization, nitrogen adꢀ
sorptionꢀdesorption isotherms, XRD, XPS, 29Si MAS NMR, addiꢀ
tional FTIR spectra, (by)ꢀproducts distribution for all Ag promotꢀ
ed ZrBEA catalysts, and details of the DFT calculations together
with atomic coordinates of all discussed structures are provided in
the Supporting Information. This information is available free of
charge via the intenet at http://pubs/acs/org/.
REFERENCES
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2003, 21, 2734–2735.
(4) Wang, J.; Jaenicke, S.; Chuah, G.ꢀK. RSC Adv. 2014,
26, 1348–13489.
(5) Corma, A.; Renz, M. Angew. Chem. 2006, 46, 298–300.
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