Properties and activity of Zn-Ta-TUD-1 in the Lebedev process

Article abstract of DOI:10.1039/d0gc00103a

A zinc and tantalum-containing mesoporous silica catalyst highly active and selective in the Lebedev process has been prepared using the one-pot TUD-1 methodology. Selectivity towards butadiene reached 60-70%, making Zn-Ta-TUD-1 one of the best performing catalysts in the literature. To rationalize these results and establish a structure-activity relationship, a series of similar catalysts was prepared and characterized. Nitrogen physisorption, XPS, ICP-AES, XRD, TEM, UV-vis spectroscopy, TGA NH₃-TPD, H₂-TPR and FT-IR techniques were used. The most active samples were found to possess a large specific surface area and highly dispersed metal oxide phase incorporated within the mesoporous silica matrix. In combination with catalytic testing, characterization also showed a direct correlation between the number of Lewis acid sites and butadiene yield, confirming the structure-activity relationship theory prevalent for the Lebedev process. Deactivation of Zn-Ta-TUD-1 was also studied using the same techniques to characterize the properties of spent catalysts. It was found that the accumulation of heavy carbonaceous species caused a reduction of specific surface area and pore size coinciding with the observed loss in activity. Nevertheless, the pores of TUD-1 were large enough to avoid total pore blockage and a high selectivity could be maintained for 72 hours.

Full text of DOI:10.1039/d0gc00103a

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Properties and activity of Zn–Ta-TUD-1 in the
Lebedev process†
Cite this: DOI: 10.1039/d0gc00103a
Guillaume Pomalaza,a Pardis Simon,a Ahmed Addad,b Mickaël Caprona and
a
Franck Dumeignil
*
A zinc and tantalum-containing mesoporous silica catalyst highly active and selective in the Lebedev
process has been prepared using the one-pot TUD-1 methodology. Selectivity towards butadiene
reached 60–70%, making Zn–Ta-TUD-1 one of the best performing catalysts in the literature. To rational-
ize these results and establish a structure–activity relationship, a series of similar catalysts was prepared
and characterized. Nitrogen physisorption, XPS, ICP-AES, XRD, TEM, UV-vis spectroscopy, TGA NH3-TPD,
H2-TPR and FT-IR techniques were used. The most active samples were found to possess a large specific
surface area and highly dispersed metal oxide phase incorporated within the mesoporous silica matrix. In
combination with catalytic testing, characterization also showed a direct correlation between the number
of Lewis acid sites and butadiene yield, confirming the structure–activity relationship theory prevalent for
the Lebedev process. Deactivation of Zn–Ta-TUD-1 was also studied using the same techniques to
characterize the properties of spent catalysts. It was found that the accumulation of heavy carbonaceous
species caused a reduction of specific surface area and pore size coinciding with the observed loss in
activity. Nevertheless, the pores of TUD-1 were large enough to avoid total pore blockage and a high
selectivity could be maintained for 72 hours.
Received 9th January 2020,
Accepted 27th March 2020
DOI: 10.1039/d0gc00103a
rsc.li/greenchem
minantly comes from the steam cracking of naphtha.8
However, this method was found unsustainable ecologically9
1. Introduction
Catalytic conversion of ethanol to value-added chemicals is a and economically,10 in part due to recent trends in the crack-
promising alternative to fossil-based processes. Because of the ing feedstock.11,12 This situation has spurred interest into the
availability of ethanol, which is produced in the 100 s of bil- Lebedev process, which produces butadiene from gaseous
lions of liters yearly via the fermentation of biomass, it is an ethanol via a catalytic reaction. In fact, it was an important
attractive renewable feedstock.1,2 Owing to its convertibility source of butadiene in the first half of the last century.5
into a wide range of chemical commodities,3 ethanol is However, to compete financially with fossil-based routes, the
expected to play an increasing role in replacing unsustainable Lebedev process must overcome performance limitations.13,14
hydrocarbon feedstocks.4 Sun and Wang, who compiled a list
Limitations to the Lebedev process are comparable to that
of valuable chemicals obtainable from ethanol, showed that of other ethanol conversion reactions.2,3 At relevant reaction
the development of catalytic processes is essential for making conditions, the high reactivity of ethanol not only forms the
ethanol a viable alternative to fossil feedstocks.3
desired butadiene and the intermediate species, but also to
The Lebedev process, the conversion of ethanol to large numbers of undesirable byproducts, including coke pre-
butadiene,5,6 has attracted attention as an on-purpose techno- cursors responsible for catalytic deactivation.14,15 In addition,
logy for producing the world’s most important conjugated low butadiene productivity could hinder the economic viability
diene in an environmentally sound fashion. Butadiene, which of the process,13 but attaining high butadiene space–time yield
is essential to the automotive industry as the main feedstock by increasing the ethanol flow rate was found to coincide with
for manufacturing the synthetic rubber used in tires,7 predo- lower selectivity.16 Catalyst design can help overcoming these
limitations by improving performances in the Lebedev
process. However, this requires a better understanding of the
aUniv. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181 – UCCS – Unité de
structure–activity relationship so that new materials are tai-
lored for optimal catalytic activity.5
Catalyse et Chimie du Solide, F-59000 Lille, France.
E-mail: franck.dumeignil@univ-lille.fr
It is generally recognized that the ethanol-to-butadiene
reaction mechanism depicted in Fig. 1 requires a multi-func-
tional catalyst.5,6 However, the relationship between the per-
bUniv. Lille, CNRS, UMR 8207 – UMET, F-59000 Lille, France
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc00103a
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loss of activity.24,34,35 Accordingly, highly active acidic or basic
sites are responsible for the formation polyaromatic carbon-
aceous species that, once deposited at the surface of the cata-
lyst, block the access to active sites. Alternatively, Li et al.
suggested that relatively lighter oxygenated cyclic species
formed by the condensation of aldehydes poison the sites
responsible for butadiene formation.15 Sintering and changes
in the active phase oxidation state have also been
proposed.31,32,36 As Taifan et al. proved,32 the deactivation
mechanism depends on the type of catalyst used. Hence, the
individual study of catalytic systems appears necessary.
In a previous issue of this journal, we published a short
communication on our early findings concerning a zinc–tanta-
lum catalyst introduced into TUD-1 mesoporous silica; it
exhibited unprecedented activity and stability in the Lebedev
process compared to the literature.37 The performances of Zn–
Ta-TUD-1 were compared with those of equivalent catalysts
Fig. 1 Toussaint–Kagan mechanism for the conversion of ethanol to
1,3-butadiene. Reaction steps: (i) Ethanol dehydrogenation; (ii) self-aldol
condensation; (iii) dehydration of acetaldol; (iv) Meerwin–Ponndorf–
Verley–Oppenaur (MPVO) reaction; (v) dehydration of crotyl alcohol.5
formances of catalysts and their properties remain under supported on dealuminated zeolite β and commercial silica,
debate.17–19 Although much insight has been acquired in the with TUD-1 found to be the best carrier to reach high activity
recent years, the rational design of catalysts for optimal per- with the Zn–Ta couple. Better suited morphological properties
formances in terms of butadiene productivity, selectivity and and the TUD-1 synthesis process were seen as potential contri-
stability.
butors to catalytic activity. In addition, a correlation between
Much attention has focused on the role of Lewis acid sites, the number of acid sites probed by NH3 and butadiene pro-
also known as Lewis acid–base pairs, which have been pro- ductivity of each catalyst tested was established, suggesting the
posed to catalyze the condensation of acetaldehyde to C4 inter- involvement of acid sites in the reactions.17,22 More than a
mediates that lead butadiene, depicted in step (ii) of highly active catalyst, Zn–Ta-TUD-1 has the advantage of using
Fig. 1.20,21 Ivanova et al. established a direct correlation an environmentally benign synthesis. Contrarily to other struc-
between the relative number of “open” Zr(IV) sites and initial ture-directing agents used for preparing mesoporous
butadiene formation using
a supported silver-zirconium materials, the agents used in TUD-1 synthesis—either tri-
catalyst.20,22,23 Since acetaldehyde condensation is often recog- ethanolamine or tetraethylene glycol—are non-toxic and
nized as the rate-determining step,20,24,25 their observation biodegradable.38
strongly supports the involvement of Lewis acid–base pairs in
The present work gives a detailed study of the Zn–Ta-TUD-1
the reaction. Kyriienko et al. also reported a direct relationship catalytic system and its activity in the Lebedev process, unra-
between butadiene productivity and the relative amount of velling fundamental aspects of the reaction by studying the
Lewis acid sites probed by CDCl3 on Cu-doped Zr-containing relationship between the chemical properties of this catalytic
zeolites.26 Similar correlations have yet to be established on system and its performances. Various techniques were
other materials, but there is strong evidence that Lewis acidity employed to characterize the catalyst: N2 physisorption, induc-
plays a role in other catalytic systems.27
tively coupled plasma atomic emission spectroscopy, X-ray
Another crucial component of catalysts active in the powder diffraction, X-ray photoelectron spectroscopy, UV-Vis
Lebedev process is their ability to convert ethanol to acet- diffuse reflectance spectroscopy, infrared spectroscopy tech-
aldehyde.28 Several dehydrogenation promoters have been niques, transmission electronic microscopy and temperature-
tested and their activity investigated: Ivanova et al. proposed a programmed experiments. The as-obtained results confirmed
mechanism for dehydrogenation on silica-supported metallic the existence of Zn–Ta-TUD-1 as a mesoporous material with
silver;20,29 Dagle et al. found silver particle size to be a crucial highly dispersed Zn(II) and Ta(V) phases.
parameter;30 Angelici et al. and Taifan et al. researched the
A set of catalysts with varying metal loading and synthesis
activity and deactivation of copper on MgO-SiO2;14,31,32 procedure were prepared and compared to investigate the
Kyriienko et al. compared the promoter effect of Ag, Zn and Cu structure–activity relationship. Notably, a direct correlation
on Ta-containing zeolites, identifying the latter as better suited between the number Lewis acid sites probed by pyridine and
for the Lebedev process.33 Because the results of mechanistic the initial butadiene formation rate was established. This is
studies differ from one promoter to another, it is likely that the first account of such a relationship on a non-Zr catalyst in
different mechanisms leading to acetaldehyde can take place the Lebedev reaction. In addition, deactivation of Zn–Ta-
depending on the catalytic system.
TUD-1 during the Lebedev process was investigated by catalytic
Neglected for long, the topic of deactivation has been the testing, surface-sensitive analytic techniques and other charac-
subject of recent articles, providing new information useful for terization methods. Reduction of the active phase was ruled
preparing catalysts with improved stability. Many authors out as a deactivation mechanism. Instead, deposition of car-
argue that coke deposition on the catalyst surface results in bonaceous species on the pore channels of the catalyst block-
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ing the access to active sites appear to cause the observed loss materials. Hemimorphite, a zinc silicate, was also synthesized
of activity with time on stream.
according to the method detailed in the patent of Teles et al.41
2.2. Catalyst characterization
N2 physisorption at −196 °C with a Micrometric Tristar II
instrument was used to study the morphological properties of
the ZTT series, spent ZTT-1 and monometallic TUD-1 samples.
Analysis was performed after outgassing 50–200 mg of powder
at 150 °C for 6 hours. The Brunauer–Emmett–Teller (BET) and
Barret–Joyner–Halenda (BJH) methods were used to calculate
specific surface area (SBET) as well as pore diameter (Dp) distri-
bution and pore volume (Pvol).
Elemental analysis of the as-synthesized ZTT samples was
performed using inductively coupled plasma atomic emission
spectroscopy (ICP-AES). 50 mg of catalyst were dissolved in a
heated mixture of HF, HNO3 and H3BO3 prior to analysis.
Catalysts were analysed with X-ray powder diffraction (XRD)
using a Brüker D8 apparatus using Cu-Kα1 a source (λ =
1.5406 Å). A step of 0.02° with an acquisition time of 0.5 s was
used.
2. Experimental
2.1. Catalyst preparation
Zn–Ta-TUD-1 with molar Si/Zn and Si/Ta ratios within 4–100
were synthesized using the procedure detailed in our previous
article,37 itself adapted from the TUD-1 methodology.39,40
Briefly, the TUD-1 synthesis process involves the gelation by
TEAOH of TEOS dissolved in ethanol with metal precursors
complexed by tetraethylene glycol to ensure their dispersion.
The resulting gel is dried and treated in a Teflon-lined auto-
clave at 180 °C, which creates the mesoporous morphology
using tetraethylene glycol as a structure-directing agent. The
resulting solid is calcined under air flow at 600 °C, ground in a
mortar and sieved to 125 μm, affording a white powder. The
precursor gel were prepared with the following reagents:
optical grade tantalum ethoxide (Alfa Aeser, 99.95%); zinc
acetate dehydrate (Acros Organics, 98+%); tetraethylene glycol
(or TEG, Agros Organics, 99.5%) was used as complexing
agent; tetratethyl orthosilicate, (or TEOS, Agros Organics, 98%)
was used silica precursor; tetraethyl ammonium hydroxide (or
TEAOH, Aldrich, 35 wt% in water) acted as the alkalizing
agent; absolute ethanol (Aldrich, 99.8%) was used as solvent.
In total, 5 catalysts labelled ZTT were prepared. To achieve
different properties, the synthesis method was modified with
regards to the thermal treatment duration and calcination
method. Table 1 lists the synthesis details of the ZTT catalysts
series. The final gel compositions before thermal treatment
were: 1.0 TEOS:x Zn:y Ta:0.5 TEAOH:1.0 TEG, where x and y
correspond the loadings listed in Table 1 for the respective
gels. As the best performing catalyst resulting for previous syn-
thesis studies,37 ZTT-1 was the main focus of our investigation,
and the primary subject of characterization. Other ZTT
samples were used for comparative purposes when the need
arose.
Thermogravimetric analysis (TGA) and differential scanning
calorimetric analysis (DSC) were performed with
a TA
Instrument SDT-Q600. Experiments proceeded under air flow
(100 mL min−1), where spent samples were heated up to
700 °C (10 °C min−1). Pure alumina was used a reference.
X-ray photoelectron spectroscopy (XPS) experiments were
carried out using an AXIS Ultra DLD Kratos spectrometer
equipped with a monochromatic AlKα radiation (1486.6 eV)
operating at 225 W (15 mA, 15 kV). Binding energies were cali-
brated according to the C 1s core level set at 284.8 eV. Spectra
of C 1s, O 1s, Zn 2p, Zn LMM and Ta 4d were analysed using
the CasaXPS software.42 Spectra decomposition was performed
via mixed Gaussian–Lorentzian peak fitting; semiquantitative
analysis was performed after
subtraction.
a Shirley-type background
UV-Vis diffuse reflectance spectra of the as-synthesized cata-
lysts were acquired at room temperature using a Lambda 650
PerkinElmer spectrophotometer equipped with an integrating
sphere. Recoding ranged between 200 and 800 nm at a step of
0.2 nm with a slit width of 1 nm. BaSO4 was used as standard.
Reflectance spectra were converted using the Kubelka–Munk
function f(R) = (1 − R)2/2R.43
To complete our study, TUD-1 materials containing exclu-
sively Zn or Ta were prepared in the same way with the follow-
ing loadings: Zn = 3.0 mol% and Ta = 2 mol% in two different
Attenuated total reflection infrared (ATR-IR) spectra of the
synthesized catalysts were recorded using a Nicolet iS50 FT-IR
spectrometer from Thermo-Fisher equipped with an iS50 ATR
sampling station. 50 scans over a scanning range of 4000 and
200 cm−1 with a resolution of 2 cm−1 were recorded.
Table 1 Synthesis detail of Zn–Ta-TUD-1 catalysts, including the silica-
metal molar ratio in the gel precursor and the concentration measured
in the final product by ICP-AES. The duration of the thermal treatment
in autoclave of the TUD-1 dried gel is also listed
High-resolution
transmission
electron
microscopy
(HR-TEM) images were obtained using a TECNAI electron
microscope operating at 200 kV. Samples were deposited onto
holey-carbon copper grids.
High-angle annular darkfield imaging during TEM has
been performed on a FEI Titan themis 300, equipped with a Cs
probe corrector and a High-efficiency Super-X detector (EDX).
At 300 kV in HRSTEM mode it is possible to reach a resolution
of 0.7 Å.
Zn loading
(mol%)
Ta (mol%)
Catalyst
Gel
Product
Gel
Product
Treatment time (h)
ZTT-1
ZTT-2
ZTT-3
ZTT-4
ZTT-5
3.0
2
2
2.9
2.1
2.3
2
1
1
4.2
4.2
1.9
1.1
0.9
4.9
4.5
24
6
48
24
24
25.2
6.3
22.2
8.7
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Infrared spectra were recorded during pyridine adsorption– tained at 200 °C, into which 30 mL min−1 of He was flown and
desorption experiments using a Nicolet Protege 460 infrared fed to the reactor, affording a WHSVEtOH of 0.3 h−1. After
spectrometer fitted with an MCT detector (4 cm−1). Outgassing 1 hour on stream, the feed was switched to an ethanol-pyridine
at 400 °C under vacuum (10−1 mbar) for 1 hour preceded each mixture containing 5 mol% of pyridine (Fisher, 99%); the
experiment. Pyridine (Fischer, general purpose grade) adsorp- reactor lines were kept above 125 °C to avoid condensation.
tion took place at room temperature up to saturation coverage. After 1 hour of pyridine co-feeding, the feed was returned to
Desorption under vacuum was performed at 150 °C, 250 °C, pure ethanol. The reactor output was monitored with an
350 °C and 450 °C. IR spectra were acquired before and after online GC-FID throughout the experiment.
every desorption step of the experiment. Acid sites were quan-
tified based on the integrated IR bands using an extinction product (Si, %), the molar yield of each product (Yi, %) and
coefficient found in the literature.44 the molar productivity of butadiene (PBD, mmolBD gcat−1 h−1)—
Ethanol conversion (X, %), the selectivity towards each
Acid sites number and strength were evaluated by tempera- eqn (1), (2), (3) and (4) respectively—were used to describe
ture-programmed desorption using ammonia as a probe (NH3- catalytic activity, where ci represents the number of carbon
TPD). The experiments were performed on a Micrometrics moles measured for a given compound i. When initial values
Autochem 2920 apparatus equipped with a Pfeiffer mass are concerned, such as the initial butadiene productivity, they
spectrometer used to monitor NH3 desorption. NH3 was represent the value obtained by extrapolating to zero the data
adsorbed over 100 mg of catalyst at room temperature for obtained over time on stream. The carbon balance (CB) for
30 minutes using a 50 mL min−1 flow of 5% NH3 in He. A each test was calculated by dividing the sum of carbon moles
ramp of 10 °C min−1 until 900 °C was used to desorb NH3.
detected with the molar amount of carbon introduced as
Temperature-programmed reduction with hydrogen (H2- ethanol and found to range between 95–105%.
TPR) was used to study the reducibility of selected catalysts
cEtOH;in ꢀ cEtOH;out
X¼
ꢁ 100
ꢁ 100
ð1Þ
ð2Þ
with a Micromeritics Autochem 2920 coupled with a with a
thermal conductivity detector. The samples were heated to
1100 °C (10 °C min−1) while being reduced using a 50 mL
min−1 flow of 5% H2 in argon.
cEtOH;in
ci;out
Si ¼
cEtOH;in ꢀ cEtOH;out
Yi ¼ X ꢁ Si
ð3Þ
ð4Þ
2.3. Catalyst, poisoning and stability tests
Catalytic testing was performed with a Multi-R® apparatus
from Teamcat Solutions SAS,45 which is a high-throughput
device for heterogeneous catalyst screening. Four parallel glass
reactors are used simultaneously with the gaseous reactant
feed calibrated by a splitter that ensured an equal inlet flow.
Reactor outputs were analysed online with an Agilent 7890 A
equipped with an FID detector. An independently controlled
valve selected the output of each reactor for analysis.
PBD ¼ X ꢁ SBD ꢁ WHSVEtOH ꢁ 0:1087
3. Results and discussion
3.1. Structural properties
Comparison the different catalysts in the ZTT series and N2 physisorption of ZTT-1, 2 and 3 showed Type IV physisorp-
monometallic TUD-1 was performed at 350 °C and a pressure tion isotherm with an H2 type hysteresis loop indicative of
of 1 atm. Each sample was ground and sieved to 120 mesh their mesoporous morphology (Fig. S1†);46 the nitrogen intake
granules. 30 mg of solid were loaded inside the glass reactors plateau above 0.9P/P0 further suggested an absence of macro-
and held in place with SiC. He was passed through a bubbler pores.47 ZTT-4 and 5 with high metal loading instead showed
containing ≥99.8% ethanol, set at a pressure and temperature nitrogen adsorption beyond 0.9P/P0 with H1 hysteresis loop,
adjusted to afford a gaseous ethanol concentration of 4.5% implying the presence of macropores and a different meso-
according to the Antoine’s law, which was fed into the reactors. porous morphology (Fig. S1†).47 At high metal loadings
Weighted hourly space velocity of ethanol (WHSVEtOH) was (10–60 wt%), M-TUD-1 materials are known to possess extra-
adjusted to 5.3 h−1 by tuning the inlet flow rate and catalyst framework metal nanoparticles that typically reduce pore size
mass.
Catalytic deactivation tests were performed at 400 °C at a
by obstruction.47,48
The morphological characteristics of Zn–Ta-TUD-1 evalu-
WHSVEtOH of 5.3 h−1 with ZTT-1 using the Multi-R® apparatus. ated with the BET and BJH methods are listed in Table 2.
Rather than interrupting the reaction to sample the spent cata- ZTT-1 possessed a large specific surface area of 658 m2 g−1, a
lyst for analysis, 5 reactions were scheduled and conducted in porous volume of 2.45 cm3 g−1 and a mesopore diameter aver-
parallel to provide catalysts with 1.5, 6, 24, 48 and 72 hours aging 11.9 nm; as illustrated in Fig. S2,† pore size distribution
spent on stream. Samples were kept in a sealed N2 atmosphere was narrow. Comparatively, ZTT-2 exhibited a larger surface
until characterization.
area, but smaller pores due to the shorter thermal treatment
A pyridine poison study was conducted in a steady-state time; the longer treatment duration of ZTT-3 resulted in the
fixed bed glass reactor at 350 °C. Ethanol was fed by pumping opposite effect.40 At higher metal loading, ZTT-4 and ZTT-5,
absolute ethanol with an HPLC pump into a vaporizer main- the treatment time had similar effect on the morphological
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Table 2 Morphological properties of Zn–Ta-TUD-1 catalysts obtained
by N2 physisorption
Catalyst
SBET (m2 g−1
)
Vp, BJH (cm3 g−1
)
DP, BJH (nm)
ZTT-1
ZTT-2
ZTT-3
ZTT-4
ZTT-5
ZTT-1a
658
702
394
486
601
208
2.45
1.49
2.47
0.79
1.29
0.88
11.9
7.0
27.7
12.6
11.9
11.4
a Spent catalyst after 72 hours on stream.
properties, but porous volume was overall smaller, possibly
due to the formation of mentioned extra-framework particles.
The presence and amounts of Zn and Ta into the series of
ZTT catalysts were confirmed and evaluated by ICP-AES
(Table 1). As evidenced by the observed metal loadings, the
TUD-1 synthesis method efficiently introduced the desired
amount of active phase at metal loading below 5.0 mol%. At
higher loadings, e.g., 27.2 and 13.3 mol% for ZTT-4 and 5,
respectively, metal content deviated significantly from the
target values. The TUD-1 preparation method is known,
depending on the type of metal and method used, to have
loading limits beyond which incorporation is less successful.40
XPS was used to determine the oxidation state of Zn and Ta
in ZTT-1; the results obtained are depicted in Fig. 2. The two
peaks of Fig. 2(a) at 1046.0 eV and 1022.8 eV in the Zn 2p1/2
and 2p3/2 range suggested the presence of Zn(II), as the
binding energy (BE) of the latter peak was close to the expected
value for oxidized zinc compounds, which lies between
1022.1–1022.7 eV.49,50 However, distinguishing Zn(II) from its
metallic form is ambiguous due to the small BE difference (<1
eV) between the two states. The 2+ oxidation state was con-
firmed by inspecting the X-ray induced Zn L3M4,5M4,5Auger
peak, for the larger kinetic energy (KE) shift that separates Zn
(II) from its metal form allowed a clear identification. As can
be seen in Fig. 2, the Auger electron peak of Zn had a value of
986.3 eV, lower than expected >990.0 eV of metallic Zn, and
lacked the distinctive double-peak shape of the latter.51
The chemical environment of Zn was further investigated
using the modified Auger parameter. Shifts in this parameter
—defined as the sum of the KE of an Auger transition invol-
ving electrons from a core level and the corresponding binding
energy—can be used to monitor changes in the chemical
environment of a given element. The KE of the Zn L3M4,5M4,5
Auger electron and the BE of the Zn 2p3/2 peak of ZTT-1 were
used to compute a modified Auger parameter of 2009.1 eV. A
Wagner plot, which provides rapid visualization of the modi-
fied Auger parameter by plotting the Auger electron KE in
function of the photoelectron BE, is illustrated in Fig. 3 and
compares the chemical environment of Zn in ZTT-1 to other
Zn samples available in the NIST database. As can be seen, the
modified Auger parameter of Zn in the TUD-1 material is
different from that of metallic Zn, but also of ZnO. Instead,
the Wagner plot indicates that the chemical environment of
Zn in ZTT-1 is closer to that of natural zinc minerals, such as
52
Fig. 2 XPS spectra of ZTT-1 for (a) Zn 2p, (b) Zn L3M4,5M4,5 Auger
peak and (c) Ta 4d regions.
of zinc silicates, notably of hemimorphite supported on SiO2,
or Zn(OH)2,53,54 suggesting it is incorporated within the silica
matrix of ZTT-1, rather than as extra-framework oxide particles
that may form during the TUD-1 synthesis.
Regarding the Ta 4d core level (Fig. 2(d)), the doublet peak
shown correspond to Ta 4d5/2 and 4d3/2 contributions. The Ta
4d5/2 BE (243.0 eV) is consistent with that of Ta2O5,55
suggesting the presence of Ta(V) in ZTT-1. Contrarily to recent
publications on similar materials,33,56 analysis and quantifi-
cation of Ta was not performed with Ta 4f peak to avoid the
interference by prominent O 2s signals due to the SiO2 matrix.
The Kerkhof–Moulijn (K–M) model was employed to study
the dispersion state of Zn(II) and Ta(V) of the entire ZTT series
by analyzing the XPS results.57–59 The K–M model predicts the
XPS relative intensity of a homogeneously supported phase,
e.g., the “promoters”, and its catalyst carrier according to eqn
(5):
ꢀ ꢁ
ꢂ ꢃ
1
2
2
Ip
Dp σp
ꢁ
p
s
β2 ð1 þ eꢀβ
Þ
Þ
ð1 ꢀ eꢀα
Þ
¼
ꢁ
ꢁ
ꢁ
ꢁ
ð5Þ
Is
Ds σs
2
ð1 ꢀ eꢀβ
α1
b
XPS
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Fig. 3 Wagner plot for Zn compounds comparing Zn–Ta-TUD-1 to
those found in the literature.53,54
Fig. 4 Comparison between the experimental and calculated intensity
ratios of (a) Zn/Si and (b) Ta/Si versus the bulk ratios according to the
model of Kerkhof and Moulijn.57 The dashed red lines represent a 10%
error on the calculated value.
where (p/s)b is the bulk atomic ratio between promoter and
support, σp/σs is the relative photoelectron cross section, β are
dimensionless support thickness parameters, D is the detector
efficiency for the given element and α1 is the dimensionless
particle size parameter. Cross sections for Zn 2p, Ta 4d and Si
2p were obtained from the relative sensitivity factor library
available from Kratos Analytical. The detector efficiency of
each parameter can be further defined as:
support.57 In addition, data points further below the theore-
tical line are indicative of particles with relatively larger
diameter.
Fig. 4(a) shows that only ZTT-4, with a 22.3% atomic
loading of Zn, was below the monolayer limit, indicating that
three-dimensional ZnO particles were formed. Concerning
ZTT-1 and the other catalysts of the ZTT-series, the experi-
mental ratio obtained by XPS was either within or above the
t
λss
t
λps
d
λpp
β1 ¼
;
β2 ¼
;
α1 ¼
;
ð6Þ
where t is the empirical thickness of the support, estimated monolayer limit for Zn. It is unclear why ratios over the
from its density and specific surface area as t = 2/ρs × S, and d maximum dispersion prediction were obtained. We suggest
is the average particle size, a number with the value of zero if the similitude between the photo-emitted electron escape path
homogeneous dispersion is assumed. The mean free path of of Zn and the thickness of SiO2 layers to be the source of this
escaping electrons (λ) was calculated according to the Tanuma, discrepancy, as it implies that not all the emitted Zn electrons
Powell and Penn formula60 using the QUASES-IMFP-TPP2 M could be seen by XPS. Nevertheless, the absence of extra-frame-
software.61
work ZnO particles as shown by UV-Vis spectroscopy
Fig. 4 represents the experimental XPS intensity ratios (a) (vide infra) suggests that Zn(II) in ZTT-1 is very highly
Zn/Si and (b) Ta/Si as a function of the bulk ratio determined dispersed.
by ICP-AES; the experimental points are compared to the red
In the case of Ta(V), Fig. 4(b) shows that ZTT-1, 2 and 3,
lines representing the theoretical intensity ratio predicted by with total atomic content of Ta below 2% were within the
the K–M model corresponding to the monolayer limit where (1 monolayer limit of the K–M, indicative of their homogeneous
− e−α1)/α1 = 1, that is to say the maximum dispersion of the dispersion. Interestingly, the case of ZTT-4 denotes how, in bi-
supported phase that can be measured with the XPS tech- metallic TUD-1 catalyst, the detrimental effect of high metal
nique. It should be noted that the monolayer limit implies loading on dispersion is not self-contained to each individual
only the monolayer thickness of the promoter phase, not its metal: the larger Zn content resulted in a greater deviation of
monatomic dispersion. According to León,58 monatomic and the Ta(V) points from the K–M model compared to ZTT-5
monolayered clusters give the same effect in the model. despite having similar Ta mol.% loading—4.6 and 4.9%,
Experimental XPS intensities below the monolayer limit indi- respectively. Optimization of α1—the dimensionless particle
cate that the promoter phase exists in three-dimensional par- size parameter—indicates that the effective particle size of the
ticles heterogeneously dispersed on the surface of the latter two is 3.3 and 0.9 nm, respectively. This observation
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further confirms that the TUD-1 synthesis method fails to
achieve a homogeneous dispersion at higher metal loadings.40
XRD patterns of as-prepared ZTT catalysts showed a broad
peak at 2θ around 25°, typical of amorphous silica (Fig. S3†).
Moreover, the diffractogram suggested the absence of crystal-
line ZnO or Ta2O5 particles, even at higher metal loadings.
This suggests that the metal oxides are either poorly crystalized
and/or highly dispersed in the silica phase.
The ATR FTIR spectra of ZTT-1 (Fig. 5) showed the signals
typical of silica can be found in the 1500–200 cm−1 region:
bands at 1043 and 1219 cm−1 are due to the asymmetric
stretching vibrations of Si–O–Si; symmetric stretching
vibration of Si–O–Si results in the band at 796 cm−1; the band
at 436 cm−1 is owed to O–Si–O bending vibrations. The absorp-
tion at 965 cm−1 could be generated by the stretching
vibrations of Si–O–M62,63 and/or terminal silanol groups.
DRS UV-Vis analysis was used to verify the incorporation of
Zn(II) and Ta(V) within the mesoporous silica framework. Fig. 6
represents the UV-Vis spectra of ZTT, which is characterized by
an intense band cantered at 233 nm. According to the litera-
ture, such a signal can be attributed to the charge transfer
between silica lattice oxygen and transition metals, notably
tetrahedral TaO4 species dispersed on silica,64,65 as well as
tetrahedral Zn(II) incorporated within silicates.66–68 This sug-
gestions is supported by the similar presence of an intense
band in the same UV region for monometallic Zn3%-TUD-1
and Ta2%-TUD-1 materials (Fig. S4†). The sharp band at
216–221 nm indicative of monoatomic Ta(V)33,69–71 or Zn(II)33
in silicates was not observed, but could be hidden in the
shoulder of the main signal. These results suggest that tanta-
lum and zinc exist in Zn–Ta-TUD-1 as single-atom sites or
small oxide domains contained within the mesoporous silica
framework, likely in their tetrahedral form. Bandgap tran-
sitions typical of ZnO and Ta2O5 at 360–390 nm72–74 and
Fig. 6 DR UV-Vis spectra of ZTT-1with metal loadings of 3.0 mol% Zn,
1.9 mol% Ta.
260–275 nm,21,56,65 respectively were absent excluding their
presence in the bulk form.
HR-TEM of ZTT-1 depicted in Fig. 7 confirmed the sponge-
like mesoporous morphology expected of TUD-1 materials.40
Moreover, no oxide particles could be detected, further con-
firming homogeneous incorporation of Zn(II) and Ta(V) inside
the catalyst carrier of ZTT-1.
HAADF-STEM was used to further investigate the structure
of metals inside Zn–Ta-TUD-1. As illustrated in Fig. 8, ZTT-1
consisted of mononuclear metal sites and small polymeric
oxide clusters around 1 nm in diameter. These were attributed
exclusively to Ta(V) as the Z contrast between Si and Zn was too
Fig. 5 ATR-FTIR of ZTT-1 between 4000 and 200 cm−1
.
Fig. 7 HR-TEM image of ZTT-1.
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Fig. 8 HAADF-STEM images of ZTT-1.
low to clearly identify the structure of Zn(II) within the meso- Moulijn model. HAADF-STEM indicated that Ta(V) in ZTT-1
porous silica. Nevertheless, STEM-EDX mapping (Fig. 9) con- existed in the form of mononuclear sites, along with polymeric
firmed the presence of Zn(II) and its strong degree of proximity oxide clusters no greater than 1 nm in diameter. UV-vis
with the Ta(V) phase.
showed Ta(V) to be incorporated within the silica matrix with a
Based on the combined characterization results, we can band at 233 nm. According to the literature, such signal
then describe the structure of our best-performing Zn–Ta- belongs to tetrahedral TaO4 sites, as octahedral species were
TUD-1 catalysts. The mesoporous sponge-like morphology not detected.65 The exact coordination of Zn(II) could not be
typical of TUD-1 materials was confirmed by HR-TEM. N2 phy- determined, but UV-vis analysis found zinc oxide to be incor-
sisorption further provided the dimensions of pore diameter porated within the carrier matrix. The modified Auger para-
and specific surface; uniform pores between 6–20 nm were meter further supported this conclusion, as the chemical
obtained together with high specific areas larger than 390 m2 environment of Zn was closer to that of zinc silicates than bulk
g−1, even reaching 658 m2 g−1 for the best performing catalyst. ZnO. Both phases were found to be in close proximity with one
IR spectroscopy showed signals typical of silica materials inter- another by EDX mapping.
acting with a metal oxide phase, in this case Zn and Ta.
Oxidation states of Zn(II) and Ta(V) were determined by XPS
3.2. Chemical properties
analysis. XPS further revealed that below 5.0 mol% of metal,
Zn–Ta-TUD-1 catalysts were homogeneously dispersed over the
silica surface in the form of metal oxides with the Kerkhof–
The surface acid properties of ZTT samples were evaluated
using pyridine chemisorption monitored by IR spectroscopy.
Fig. 10(a) depicts the results for the ZTT-1 sample at different
Fig. 9 STEM-EDX mapping of ZTT-1.
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Fig. 10 FTIR spectra of chemisorbed pyridine: (a) on ZTT-1 sample at different temperature; (b) on samples of the ZTT series for comparison.
desorption temperature. The bands detected at 1611, 1578 and Table 3 Quantification of Lewis acid sites on ZTT samples probed
1454 cm−1 are attributable to pyridine coordinatively bound
using pyridine-FTIR and calculated using the Beer–Lambert law
on metal cations, e.g., Lewis acid sites (LAS).44,75 No signal at
Catalyst
LAS (mmol g−1
)
Zn (mol%)
Ta (mol%)
1545 and 1638 cm−1 attributable to pyridinium ions formed
on Brønsted acid sites (BAS) were observed. However, their
presence on the surface of Zn–Ta-TUD-1 cannot be excluded,
as weak BAS have been detected using stronger basic probes
on catalysts highly active in the Lebedev process.23,33 Bands
located at 1490 cm−1 can be attributed to both LAS and BAS.
The progressive desorption of pyridine with increased temp-
eratures entails the existence of LAS with different strengths
ZTT-1
ZTT-2
ZTT-3
ZTT-4
ZTT-5
0.276
0.153
0.101
0.163
0.199
3.1
2.1
2.3
22.3
8.8
1.9
1.1
1.0
4.9
4.6
on the surface of ZTT-1. Our results are similar to those tration of LAS. This observation can be attributed to the high
observed with pyridine over silicate-supported transition degree of dispersion confirmed by TEM and XPS.
metals active in the Lebedev process, including the related Zn–
Ta-SiBEA catalyst prepared by Kyriienko et al.33
Using NH3-TPD, the acid strength distribution of ZTT-1 was
studied and compared to that of Zn3%-TUD-1 and Ta2%-TUD-1.
As Fig. 10(b) illustrates, the ZTT series also showed an As we previously reported, Zn–Ta-TUD-1 showed a single broad
absence of signal attributable to BAS. However, the intensity of and asymmetric desorption peak indicative of a somewhat
the bands present differed, implying different amounts of LAS. heterogeneous acid strength distribution (Fig. 11(c)). Similar
Their quantification was performed using the Beer–Lambert experiments on other bimetallic catalysts, e.g., Zn–Zr/SiO2 and
law (eqn (7)):
MgO-SiO2, for the ethanol-to-butadiene reaction have shown
comparable results,77,78 although some authors were able to
clearly identify acid sites of distinct strengths on other
materials.28,79 Gaussian decomposition of the NH3-TPD pro-
files, although purely a mathematical tool, showed at least
ε ꢁ W ꢁ Cw
A ¼
ð7Þ
S
where W (kg), Cw (mol kg−1) and S (m2) represent sample three hidden peaks centred at different temperatures, denoting
weight, probe concentration and disk area, respectively.44,75 As the heterogeneous strength distribution, which can be classi-
listed in Table 3, the samples prepared for our comparative fied as weak, medium and strong.80 When contrasted with
study had different numbers of LAS, which were not directly monometallic samples, the desorption profile of ZTT-1 more
correlated with their metal content, notably at high loading. closely resembled that of Zn-TUD-1 (Fig. 11(b)), revealing
Most likely, the larger metal oxide particles formed at higher almost identical “hidden” peaks. Ta-TUD-1 showed more dis-
metal contents resulted in fewer acid sites, a phenomenon cernible peaks with similar desorption temperatures, except
observed elsewhere.76 Despite having a lower metal content, for a second medium-strength acid site centred at 320 °C
ZTT-1, the most active catalyst, showed the highest concen- absent on ZTT-1 (Fig. 11(a)).
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electrons covalently bound with the oxygen atoms of the
framework, which becomes a Lewis basic site.85 The resulting
Lewis acid–base pairs can participate in a variety of organic
reactions by interacting with electron-rich compounds, often
via the cooperation between the acid and basic moiety.
Both zinc86–89 and, more rarely, tantalum oxide64 have been
described as amphoteric materials. In fact, the necessity of
basic or redox sites for converting ethanol first to acetaldehyde
before BD has often been highlighted.33,36,90 However, in our
previous publication, we reported that CO2-TPD of Zn and Ta-
containing catalysts revealed little correlation between the
basic properties of our samples and their performances;37 the
inadequacy of CO2 for probing sites active in the Lebedev
process reported by other scholars may be the cause.77,91
H2-TPR was also used to characterize the properties of cata-
lysts capable of dehydrogenation reactions, as their reducibility
can be related to activity.92,93 In accordance with the literature,
Ta-TUD-1 did not reduce in a hydrogen atmosphere even at
1100 °C.64
Fig. 12 compares the H2-TPR profiles of Zn-TUD-1 and
ZTT-1. In Zn-TUD-1, Zn(II) predominantly reduces at 764 °C,
with a secondary signal around 890 °C. With ZTT-1, the pres-
ence of Ta(V) significantly lowers the reducibility of Zn(II), the
Fig. 11 NH3-TPD profiles of (a) Ta-TUD-1, (b) Zn-TUD-1, and (c) ZTT-1.
Nevertheless, comparing the sum of surface acid sites
numbers on each monometallic sample suggested both Zn(II)
and Ta(V) contributed to the acid properties of ZTT-1 (Table 4).
Interestingly, we did not observe the passivation of stronger
acid sites caused by the presence of Zn as reported by other
scholars on Zn–Hf and Zn–Zr catalysts.28,36,81 Instead, the Zn-
TUD-1 possessed more strong sites than Ta-TUD-1. The signifi-
cantly different preparation methods used may explain this
discrepancy, as other works have generally added Zn sub-
sequently. It should be noted that the dissimilar number of
acid sites quantified by NH3-TPD and pyridine-FTIR is not an
uncommon observation, and the difference in probe size and
pKa may account for it.75,82
The observed LAS can be attributed to the Zn(II) and Ta(V)
species, both having demonstrated Lewis characteristics when
supported on silicates and studied with pyridine-FTIR
analysis.70,83,84 In Zn–Ta-TUD-1, Lewis acidity—the ability to
accept a pair of electron—originates from the partial positive
charge of the metal cations that comes about when its valence
Fig. 12 TPR profiles of Zn-TUD-1 and ZTT-1.
Table 4 Acid site strength distribution and quantification on selected catalyst according to their deconstructed NH3-TPD profile
Number of acid site (mmol g−1
)
Catalyst
Weak (205 5 °C)
Medium 1 (261 5 °C)
Medium 2 (320 °C)
Strong (382 2 °C)
Total
ZTT-1
Zn-TUD-1
Ta-TUD-1
0.112
0.068
0.036
0.231
0.134
0.058
n/a
n/a
0.061
0.418
0.276
0.043
0.772
0.478
0.198
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major species now reducing at 902 °C, with a smaller signal at were butadiene, acetaldehyde and ethylene—other compounds
738 °C.
such as diethyl ether, propylene and crotonaldehyde
accounted for less than 4% of products on a carbon basis.
Ethanol conversion and product selectivity remained stable for
the duration of catalytic test of 4 hours. Initial conversion and
BD selectivity, which were obtained by extrapolation at TOS =
0 h, were 67.0% and 68.1%, respectively. Although the result-
ing butadiene yield was lower than at 400 °C,37 BD selectivity
is comparable to that of other well-performing catalysts found
in the literature.5,6
Catalytic test results for the other samples of the ZTT series
are listed in Table 5. Obviously, performances differed from
one catalyst to another, notably in terms of butadiene yield.
This provided us with the opportunity to establish a relation-
ship between the properties characterized and catalytic
activity.
As previously mentioned, Ivanova et al. established a corre-
lation between the relative amount of ‘open’ Lewis acid sites on
Zr-containing dealuminated zeolites, and the initial formation
rate of BD.17 For the first time, we report a similar correlation
on zirconium-free catalyst.
As illustrated in Fig. 14, a linear correlation exists between
the number of Lewis acid sites quantified by pyridine-FTIR
(Table 3) and the initial productivity of BD (Table 5). The best
fit was obtained with the quantification after desorption at
150 °C, suggesting that strong acid sites were not exclusively
required, although their rapid deactivation may also explain
the lack of fit. The relationship observed suggests Lewis acid
sites catalyse the rate-limiting step, which is believed to be the
3.3. Acidity & catalytic activity
To perform the many steps in the conversion of ethanol to
butadiene (Fig. 1), catalysts require a combination of chemical
properties. As stipulated by Ivanova et al., this is often
achieved by combining metal oxides, each possessing part of
the desired activity, most notably a dehydrogenation function
and a condensation function.90 It is assumed that the necess-
ary dehydration steps are so thermodynamically favoured that
the aforementioned active phases suffice.5 In the case of our
catalyst, Zn(II) and Ta(V) are the active phases. Zinc oxide,
whether as a bulk phase or supported, can dehydrogenate and
dehydrate short-chain alcohols.87,88,93 Supported tantalum
oxide has long been known to catalyse the conversion of
ethanol-acetaldehyde
Consequently, simple explanation of the Zn–Ta-TUD-1
mixtures
to
butadiene.21,94–96
a
activity would be that the zinc phase provides the ability to
catalyst of forming acetaldehyde from ethanol and the tanta-
lum phase to condense it to precursors of butadiene.37
Catalytic testing combined with characterization was employed
to check this hypothesis.
The catalytic activity of Zn–Ta-TUD-1 under conditions
aimed at maximizing butadiene productivity have formerly
been reported.37 As we previously noted, increasing contact
time and temperature improves ethanol conversion and buta-
diene yield. In this work, temperature and ethanol space vel-
ocity were controlled allowing a better comparison between
the selected catalysts by avoiding total ethanol conversion. At
350 °C and a WHSVEtOH of 5.3 h−1, conversion did not exceed
67%. The performances exhibited by ZTT-1 under these con-
ditions are depicted in Fig. 13. It shows the main products
aldol condensation due to the accumulation of acetaldehyde at
20,24,37
high WHSVEtOH
.
To confirm the involvement of the
Lewis acid sites probed by pyridine in the conversion of
ethanol to butadiene, a poison study was conducted. As shown
in Fig. S5,† upon co-feeding pyridine with ethanol at shown at
350 °C, an important decrease in the butadiene yield and
increase in acetaldehyde yield was observed. Consequently, the
Lewis acid sites poisoned by pyridine are proposed to catalyse
the condensation of acetaldehyde to the C4 precursors that
ultimately lead to butadiene. This conclusion is in agreement
with the experiments conducted with Zr catalysts,17,20,23 as
Table 5 Initial catalytic performances in the Lebedev process of Zn–
Ta-TUD-1 samples at 350 °C and WHSVEtoH of 5.3 h−1
CB
(mol C
S. (%) %)
iBD Prod.
−1
XEtOH
Catalyst (%)
BD S. AcH S. C2 =
(mmol gcat
(%)
(%)
h−1
)
ZTT-1
ZTT-2
ZTT-3
ZTT-4
ZTT-5
67.0
53.6
36.2
40.6
57.3
68.1
42.8
49.3
50.6
58.8
21.9
24.6
24.3
40.0
30.5
6.5
24.1
18.1
7.1
96.5
91.5
91.7
97.7
95.8
26.3
13.2
10.3
11.8
19.4
6.5
Fig. 13 Conversion and selectivity towards major products of ethanol
XEtOH: ethanol conversion; BD S.: butadiene selectivity; AcH S.: acet-
aldehyde selectivity; C2 = S.: ethylene selectivity; CB: carbon balance;
iBD Prod.: initial butadiene productivity.
conversion on ZTT-1 over time. T = 350 °C, P = 1 atm, WHSVEtOH
5.3 h−1
=
.
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Ta2%-TUD-1 exclusively formed products of ethanol de-
hydration, e.g., ethylene and diethyl ether. Selectivity towards
dehydration products of short-chain alcohol are typical of
highly dispersed Ta2O5 supported on silica and associated
with primarily acidic properties.64 The absence of dehydro-
genation function to produce acetaldehyde explains why the
acid sites previously correlated with butadiene productivity are
inactive in the Lebedev process. It can be concluded that Ta(V)
in TUD-1 exclusively contributes to the condensation reaction
when alone.
Zn3%-TUD-1 primarily formed acetaldehyde, highlighting
the crucial role of Zn(II) in providing the necessary multi-func-
tionality to the catalyst. However, Zn3%-TUD-1 also displayed a
high selectivity towards ethylene that was not observed with
ZTT-1. Yet, despite possessing the both acid sites—as evi-
denced by NH3-TPD—and a redox functionality, no butadiene
was detected, suggesting that the acid sites present on Zn3%
TUD-1 do not contribute to the condensation.
-
The presence of synergism between Zn(II) and Ta(V) was
Fig. 14 Initial butadiene productivity versus the number of Lewis acid
sites on the ZTT series quantified by FTIR after pyridine desorption at investigated by performing a catalytic test with the mechanical
150 °C.
mixture of Zn3%-TUD-1 and Ta2%-TUD-1. This was done so as
to create a catalyst that contained both Zn(II) and Ta(V) phases
without the close proximity afforded by the TUD-1 preparation
well as the mechanistic studies performed with Ta-containing
method. As can be seen in Table 6, this mixture proved signifi-
catalysts using ethanol-acetaldehyde feeds.21
cantly more active in the Lebedev than either monometallic
Evidenced by the surface probing with NH3 (Fig. 11), both
catalyst. The high selectivity towards butadiene observed can
the Zn(II) and Ta(V) contribute to the to the surface acidity of
be explained by the presence of Ta(V) inside the catalyst.
Zn–Ta-TUD-1, which is predominantly Lewis acidic due the
However, the higher selectivity towards ethylene recorded
absence of pyridinium IR bands. As a result, it is unlikely that
when compared to the performances of ZTT-1 (Table 5), indi-
aldol condensation exclusively takes place on Lewis Ta(V) sites,
cate that properties resulting from the mechanical mixing of
despite its well-established condensation activity.5,21,94,95 In
the two monometallic TUD-1 samples. This suggests that the
agreement with Li et al.,97 who studied the Lebedev mecha-
TUD-1 method produces materials with properties favourable
nism on Zn–Y/SiBEA, Zn(II) is likely to contribute to the coup-
to the Lebedev process: enhanced dehydrogenation and con-
ling activity of the catalyst. This conclusion is further
densation activity, and suppression of dehydration activity.
reinforced by a linear correlation we previously observed
These favourable conditions may arise from the proximity
between the total number of acid sites probed by NH3 on Zn–
of the two oxide phases established using STEM-EDX mapping
Ta catalysts37 and BD selectivity.
(Fig. 9) that are unlikely to occur through mechanical mixing
at room temperature; such a proximity may result in a syner-
gism between the two phases. As illustrated with the NH3-TPD
3.4. Synergism between Zn & Ta
To test the presumed role of Zn(II) and Ta(V), monometallic
studies, no significant difference in acid strength could be dis-
TUD-1 materials were tested under reaction conditions com-
tinguished the monometallic samples and ZTT-1, it can be
parable to those used with the ZTT series. Table 6 details the
concluded that this phenomenon does not occur as a passiva-
catalytic performances of these monometallic samples.
tion of acid sites resulting in lower dehydration activity.
Another possibility is may be found in changes induced by the
presence of Ta(V) to the chemical state of Zn(II)—this factor is
Table 6 Catalytic performances in the Lebedev process of monometal-
lic TUD-1 catalysts and the material produced by their mechanical known to influence the activity of Zn.88,93 The change observed
mixture. Major byproducts unaccounted for where diethyl ether,
1-butanol, crotonaldehyde and ethyl acetate
in the reducibility of Zn(II) monitored by H2-TPR (Fig. 12)
further suggests that the high activity in the Lebedev process
of Zn–Ta-TUD-1 and its low selectivity to ethylene lies in
changes to the redox properties of Zn(II).
Catalyst
XEtOH (%)
BD S. (%)
AcH S. (%)
C2 = S. (%)
Zn3%-TUD-1
Ta2%-TUD-1
Zn3%-TUD-1+
Ta2%-TUD-1
20.7
24.5
35.1
0.0
0.0
47.5
34.7
2.0
19.8
31.0
45.9
33.6
Explaining the superior activity of Zn(II) in presence of Ta(V)
is difficult, in part because the parameters that favour alcohol
dehydrogenation over dehydration with Zn catalysts are not
fully understood. For bulk ZnO, Drouilly et al. attributed its
alcohol dehydrogenation activity to the presence of oxygen
vacancies.88,98 The structural properties of ZnO were also
XEtOH: ethanol conversion; BD S.: butadiene selectivity; AcH S.: acet-
aldehyde selectivity; C2 = S.: ethylene selectivity. TOS = 1 h; T = 350 °C;
WHSVEtOH = 5.3 h−1
.
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found to influence its activity with short-chain alcohol.87,93,99
Unfortunately, fewer studies have been conducted regarding
the activity of supported zinc oxide in dehydrogenation reac-
tions,100 and none could be found regarding ethanol
dehydrogenation.
The work of Perez-Lopez et al. showed that increasing the
reducibility of zinc oxide nanoparticles by tuning their syn-
thesis method favored the dehydrogenation of short-chain
alcohols over dehydration. Zn(II) reducilibty was also identified
as a parameter in the ability of zinc-containing catalysts to
dehydrogenate propanol87,99 and propane.92 We suspect that
the changes to the redox properties of Zn(II) in presence of Ta
(V) evidenced by H2-TPR may explain the superior activity in
ethanol dehydrogenation of Zn–Ta-TUD-1 in comparison to
Zn-TUD-1.
Judging from the results of the monometallic catalysts Ta(V)
contributes to the condensation activity and Zn(II) to the dehy-
drogenation activity. However, a synergism provided by the
TUD-1 synthesis both suppresses ethylene formation and
enhances ethanol dehydrogenation, improving the overall per-
formances in the Lebedev process. Clearly, the proximity with
the Ta(V) phase is needed to enhance the dehydrogenation
activity of Zn(II) and suppress dehydration reactions when
incorporated within TUD-1. Most likely, this effect is linked to
the redox properties of Zn, not the strength of acid sites.
Instead, Lewis acid sites attributable to both the Zn(II) and Ta
(V) phases have been found to directly correlate with the initial
productivity in butadiene. Maximizing the number of available
Lewis acid sites by increasing the active phase dispersion has
been identified as a key factor for achieving high performances
in the Lebedev process.
Fig. 15 Ethanol conversion and product selectivity during the Lebedev
process with ZTT-1 at 400 °C and WHSVEtOH of 5.3 h−1. The blue line
indicates the accumulation of heavy coke content. XEtOH: ethanol con-
version; BD S.: butadiene selectivity; AcH S.: acetaldehyde selectivity; C2
= S.: ethylene selectivity.
3.5. Deactivation
There are strong economic incentives to limit the deactivation
that occurs during the Lebedev process.5,101,102 Understanding
the deactivation mechanism would assist the design of more
resistant catalysts. While recent work has provided precious
insights,15,35,36 catalyst deactivation has not been fully under-
stood, in part due to the multiplicity of materials and reaction
conditions used.
Deactivation of Zn–Ta-TUD-1 was studied by testing ZTT-1
at 400 °C for a period of 70 hours. The higher temperature was
used to reflect reaction conditions more suitable for maximiz-
ing butadiene productivity. As shown in Fig. 15, ethanol con-
Fig. 16 Thermograms of spent ZTT-1 at different TOS in the Lebedev
process at 400 °C and WHSVEtOH of 5.3 h−1
.
version decreased in a linear fashion. Selectivity towards BD calcination under air regenerated the catalytic activity of Zn–
initially increased and stabilized for the first
hours. Ta-TUD-1—a possible sign of deactivation by coking.37,102
Interestingly, it was mirrored by a loss in ethylene selectivity.
Coke deposits occurring in the Lebedev process on ZTT-1
6
This phenomenon may be explained by the initial poisoning were quantified by the TGA of spent catalysts after
of acid sites favourable to ethanol dehydration. BD selectivity 1.5–72 hours on stream at 400 °C. The resulting thermograms
peaked after six hours on stream and gradually decreased. (Fig. 16) indicated the accumulation of removable matter with
This decline was compensated by an increase in acetaldehyde increased reaction time. Deposited substances were classified
selectivity.
according the methodology of Liu et al.103 into three tempera-
Coking has been identify major as source of deactivation ture regions. Weight loss in region I (T < 180 °C) was assigned
during the conversion of ethanol to BD over many different to water and volatile species, e.g., reactants, intermediates and
catalysts.19,24,34–36 Moreover, our team previously reported that products. Region II (180 °C ≤ T ≤ 330 °C) was attributed to the
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loss of soft cokes—mobile, yet heavier carbonaceous species
such as bulkier byproducts. The loss in region III (330 °C ≤ T)
corresponded to the combustion of heavy coke compounds,
further evidenced by the combined DSC-TGA analysis per-
formed with ZTT-1 after 72 hours, which indicated an exother-
mal process took place beginning near 330 °C (Fig. S6†). TGA
results summarized in Table S1† indicated that soft coke rep-
resented <1% weight loss, a proportion which decreased after
peaking at 6 h on stream. Heavy coke represented the greater
fraction of substances accumulated; its content in ZTT-1 as a
function of reaction time was plotted in Fig. 15. As shown,
heavy carbon content rapidly increased during the first
6 hours of the reaction, coinciding with the initial stabilization
of product selectivity mentioned previously. Beyond six hours,
heavy coke content slowly increased as catalyst activity simi-
larly decreased, implying the participation such specie in the
deactivation process.
XPS was used to characterize the evolution of the surface of
spent ZTT-1 as a function of time on stream. Bibby et al. devel-
oped a simple model for studying the coke deposition on cata-
lyst pores using XPS.104 For materials with surface area larger
Fig. 17 Measured C/Si ratio compared to the weight percent of carbon-
aceous species on spent ZTT-1 catalyst. The dashed line represents the
C/Si function calculated assuming internal coke filling from a solid solu-
tion of carbon in SiO2.104
than 200 m2
g
−1, the dispersion of carbonaceous species
within pore channels was proposed to proceed homogeneously
following eqn (8).
ZTT-1 sample was analysed. Although the ethanol atmosphere
of the Lebedev process can reduce metal oxides,31,105 Fig. 18
depicts how the oxidation state of Zn(II) in ZTT-1 was
unaffected after several hours on stream. As expected, neither
was Ta(V) (not shown). Consequently, we conclude that in situ
reduction of the active phase is not a source of deactivation in
Zn–Ta-TUD-1. Ostensibly, the low reducibility of Zn(II) in the
ꢀ
ꢁ
C
Si
wt 100 ꢀ wt
¼
ꢁ
ð8Þ
12
60
XPS
where wt is the theoretical carbonaceous compound weight
fraction, assuming the 100% silica support act as a solid solu-
tion; C and Si are the atomic percentages quantified by XPS
using the C 1s and Si 2p peaks, respectively. Fresh ZTT-1 was
used reference to estimate the quantity of pre-adsorbed atmos-
pheric carbon.
Fig. 17, the resulting plot is compared to the atomic con-
centration of C and Si on ZTT-1 (TOS of 1.5 to 48 hours) versus
TGA, where wt% was considered as the total weight loss
accounting for trapped reactants. As illustrated, the model
accurately predicted the relative XPS signal of carbon species
inside ZTT-1 pore channels—deposition on the outer surface
would have resulted in a drastic break-off from the theoretical
line. Accordingly, the observations indicated that mesoporous
structure of ZTT-1 accommodated the homogeneous depo-
sition of carbonaceous compounds formed during the conver-
sion of ethanol. Total pore blockage can be disregarded a sig-
nificant source of deactivation up to 48 hours, as no deviation
from the theoretical model was observed. N2 porosimetry con-
firmed that catalytic testing resulted in a reduction in average
pore diameter, porous volume and specific surface area
(Table 2).
Recently, Villanueva Perales et al. proposed that the de-
activation of their silica-supported hafnium-containing catalyst
was owed to the in situ reduction of Zn(II) sites containing
within hemimorphite—a zinc silicate used to promote ethanol
dehydrogenation.36 To verify whether this phenomenon took
Fig. 18 LMM Auger line of spent ZTT-1 with increasing time on stream
during the Lebedev process. Dotted lines represent the appearance of
place with Zn–Ta-TUD-1, the Zn LMM Auger line of spent Zn at different oxidation sate, as found in the literature.52
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presence of Ta(V) which was observed with H2-TPR increased Consequently, we propose that Lewis acid sites are the active
the resistance to deactivation of Zn–Ta-TUD-1. sites in this reaction step. Characterization of the surface
Considering that the accumulation of heavy carbon species acidity suggested that both the Zn(II) and Ta(V) phases contrib-
coincided with catalytic deactivation, we propose that the pre- uted to the condensation reactivity. Furthermore, synergy
dominant deactivation mechanism is the formation and depo- between Zn(II) and Ta(V) species was found necessary for
sition of carbonaceous species inside the pores of Zn–Ta- enabling both the dehydrogenation of ethanol to acetaldehyde
TUD-1. Initially, there appears to be a selective poisoning of and its subsequent condensation.
some active sites responsible for ethanol dehydration, as evi-
The deactivation of Zn–Ta-TUD-1 was studied under reac-
denced by the relatively faster change in selectivity during the tion conditions intended to maximize butadiene productivity,
first 6 hours (Fig. 18). However, the subsequent homogeneous e.g., 400 °C and WHSVEtOH of 5.3 h−1. Analysis of spent cata-
channel filling indicated by the XPS analysis suggested the lysts recovered after 72 h runs revealed a significant accumu-
carbon deposition was not selective beyond that point. It also lation of up to 16 wt% of heavy carbonaceous species
found that total pore blockage did not occur after 48 hours on coinciding with the loss of catalytic activity. XPS revealed the
stream. Rather, the surface of pore channels was progressively deposition of carbon species proceeded homogeneously
filled with heavy carbon species, reducing the average pore dia- within the channels of the catalyst, which reduced its specific
meter, porous volume and specific surface area. In this scen- surface area, pore volume and pore size. Consequently, the de-
ario, the loss of activity can be attributed to the physical inac- activation mechanism appears to be the deposition of heavy
cessibility of active sites. No change to the oxidation state of carbon species within the catalyst, hindering the access to
the metal oxide phase was observed by XPS, excluding it as a active sites. Total pore blockage and the reduction of the oxide
source of deactivation.
active phase were ruled out as a source of deactivation.
Conflicts of interest
4. Conclusion
There are no conflicts to declare.
Our previous study showed Zn–Ta-TUD-1 to be one of the best
performing catalyst in the production of butadiene from
ethanol.37 In the present study, a stable selectivity of 68%
towards butadiene was achieved with Zn3.1%–Ta1.9%-TUD-1 at
350 °C and 5.3 h−1, confirming the high performance of this
catalytic system. The TUD-1 preparation method allows for the
one-pot synthesis of mesoporous materials with highly dis-
persed metal oxide phases. To verify the success of the syn-
thesis method, 5 Zn–Ta-TUD-1 solids were synthesized with
different metal loadings parameters, characterized and com-
pared. For the best performing catalyst, N2 porosimetry and
TEM confirmed the foam-like mesoporous morphology
expected of TUD-1 material. A combination of spectroscopy
techniques revealed that the active phase consisted of highly
dispersed Zn(II) and Ta(V) species incorporated within the
silica matrix. TEM showed the Ta(V) phase to consist of mono-
nuclear species and small metal oxide domains around 1 nm
in diameter. This high degree of dispersion resulted in a
strong concentration of Lewis acid sites. Contrarily, the TUD-1
synthesis could not be highly dispersed when 13 mol% of
metal were loaded in the synthesis, indicative of the method’s
limit for introducing a highly dispersed active phase. Instead
extra-framework nanoparticles where formed, resulting in a
lower Lewis acid site concentration despite the higher metal
content.
Acknowledgements
Oliver Gardoll is acknowledged for performing the tempera-
ture-programmed experiments. Authors acknowledge the
support from the French National Research Agency (ANR-15-
CE07-0018-01). Chevreul Institute (FR 2638), Ministère de
l’Enseignement Supérieur, de la Recherche et de l’Innovation,
Région Hauts-de-France and FEDER are acknowledged for sup-
porting and funding partially this work.
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