Ethanol to Butanol Conversion over Bifunctional Zeotype Catalysts Containing Palladium and Zirconium

Article abstract of DOI:10.1134/S0965544119080097

Abstract: A study of the kinetics of ethanol conversion in the presence of Zr-containing zeolites BEA doped with palladium particles has revealed the order of formation of the main reaction products. It has been shown that the primary processes are ethanol dehydrogenation to acetaldehyde on Pd sites and ethanol dehydration to diethyl ether on the acid sites of the catalyst. After that, acetaldehyde undergoes the aldol–croton condensation reaction to form crotonal, which is hydrogenated to butanol on the metal sites. Butanol, in turn, is dehydrated into butenes, which undergo hydrogenation to butane. The presence of hydrogen in the gas phase leads to the displacement of ethanol from the metal surface and prevents the formation of surface carbonates and acetates. It has been found that hydrogen significantly accelerates ethanol dehydration owing to a decrease in the activation energy, which can be attributed to hydrogen spillover to the zeolite. The addition of water inhibits all acid-catalyzed reactions owing to competitive adsorption on acid sites and thereby decreases the butanol yield and the ethanol conversion.

Full text of DOI:10.1134/S0965544119080097

ISSN 0965-5441, Petroleum Chemistry, 2019, Vol. 59, No. 8, pp. 925–934. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Sovremennye Molekulyarnye Sita, 2019.
Ethanol to Butanol Conversion over Bifunctional Zeotype Catalysts
Containing Palladium and Zirconium
a,
a
b
a
P. A . Kots *, A. V. Zabilska , Yu. V. Grigor’ev , and I. I. Ivanova
a
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
b
Crystallography and Photonics Federal Research Center, Russian Academy of Sciences, Moscow, 19333 Russia
*
e-mail: pavelkots@gmail.com
Received February 28, 2019
Abstract—A study of the kinetics of ethanol conversion in the presence of Zr-containing zeolites BEA doped
with palladium particles has revealed the order of formation of the main reaction products. It has been shown
that the primary processes are ethanol dehydrogenation to acetaldehyde on Pd sites and ethanol dehydration
to diethyl ether on the acid sites of the catalyst. After that, acetaldehyde undergoes the aldol–croton conden-
sation reaction to form crotonal, which is hydrogenated to butanol on the metal sites. Butanol, in turn, is
dehydrated into butenes, which undergo hydrogenation to butane. The presence of hydrogen in the gas phase
leads to the displacement of ethanol from the metal surface and prevents the formation of surface carbonates
and acetates. It has been found that hydrogen significantly accelerates ethanol dehydration owing to a
decrease in the activation energy, which can be attributed to hydrogen spillover to the zeolite. The addition of
water inhibits all acid-catalyzed reactions owing to competitive adsorption on acid sites and thereby decreases
the butanol yield and the ethanol conversion.
Keywords: ethanol, Guerbet–Markovnikov reaction, butanol, IR spectroscopy, spillover, bifunctional catal-
ysis
DOI: 10.1134/S0965544119080097
At present, the catalytic conversion of ethanol to conversion to butanol-1 is promising [5]. Unlike etha-
valuable chemicals is the focus of considerable atten- nol, butanol can be mixed with motor fuel in unlimited
tion in the scientific and patent literature [1]. This fact
is associated with the tendency to redirecting the pet-
rochemical industry to renewable raw materials. In
recent decades, the global production of bioethanol
from inedible biomass has increased; therefore, etha-
nol conversion is an alternative to the use of petroleum
products. In the presence of solid acid catalysts, etha-
nol is converted to ethylene [2], butadiene, acetic acid,
acetone, propylene [3], and isobutylene [4]. Many of
these products are sought-after feedstocks in the pro-
duction of elastomers, plastics, and other valuable
materials. The global production of ethanol from bio-
mass and cellulose is increasing annually; therefore,
quantities; the energy characteristics of butanol are
close to those of gasoline; therefore, butanol can be
thought of as a full-fledged biofuel. In addition, buta-
nol is commonly used in industry as a solvent, a per-
fumery component, and a perfume agent.
It is known that ethanol reacts with base catalysts to
be converted to butanol and other higher alcohols
С H2n+1OH (n = 2, 4, etc.). For ethanol, this conver-
2n
sion, which is characteristic of other alcohols too, was
first discovered by V.V. Markovnikov [6] and later
studied by M. Guerbet [7]. Ethanol conversion to
the problem of further ethanol conversion is extremely butanol can be described through a series of reactions
relevant in terms of sustainable development. Ethanol (Scheme 1).
O
OH
O
O
1
2
OH
OH
O
OH
O
OH
3
4
5
Scheme 1. Sequence of reactions in ethanol conversion to butanol according to Guerbet and Markovnikov [8, 9].
25
9

9
26
KOTS et al.
At the first stage (1), ethanol is converted to etha-
EXPERIMENTAL
nal via a dehydrogenation reaction; after that, ethanal
undergoes aldol–croton condensation (2) to form
crotonal. The reduction of the carbonyl group in cro-
tonal leads to the formation of crotyl alcohol (3),
which undergoes isomerization to butanal (4). The
second reduction of the carbonyl group leads to the
formation of butanol (5) [10]. It is reported that the
Catalyst Synthesis Procedure
The zirconium-containing BEA zeolite was syn-
thesized by hydrothermal synthesis in a fluoride-ion
medium as described in [15]. The following gel com-
position was maintained constant: 1SiO
:
2
0
.54TEAOH : 0.54HF : 0.005ZrO : 5.6H O, (Si/Zr
2 2
Guebret–Markovnikov process is implemented using 200), where TEAOH is tetraethylammonium hydrox-
bifunctional catalysts containing both noble metals ide. Tetraethoxysilane and zirconium oxychloride
(
e.g., gold [11]) and an acid component (alumina), ZrOCl ⋅ 8H O were used as a source of silicon and zir-
2 2
which is required for the occurrence of ethanal con- conium, respectively. Bifunctional catalysts were syn-
densation. There are reports on catalysts Au/Al O
thesized by incipient wetness impregnation. The orig-
2
3
and Ni/Al O
[10] and bimetallic catalysts inal Zr–BEA zeolite powder was impregnated with a
2
3
solution with a given concentration of Pd(NO ) in
AuCu/Al O [11]. Tests of a large number of metals
3 2
2
3
ethanol. After impregnation, the samples were dried in
air for 2 h and then at 100°C for 10 h; after that, they
were calcined in an air stream at 300°C for 5 h to
decompose the nitrate.
dispersed on a support suggest that the activity of
М/Al O catalysts decreases in the following order:
2
3
Ni > Pt > Au ~ Rh > Ru @ Ag. The best parameters
(
ethanol conversion of 25%, butanol selectivity of
8
2
2
0%) in the liquid-phase process are achieved at
50°С under a high pressure in the presence of a
0%Ni/Al O catalyst. The authors of [9], in studying
Instrumental Investigation Procedures
2
3
X-ray diffraction (XRD) analysis data were
obtained on a Bruker D2 Phaser instrument. Crystal
size and morphology were studied by scanning elec-
tron microscopy (SEM) on a Hitachi TM3030 micro-
scope at an accelerating voltage of 5 kV. Palladium par-
ticle size distribution was studied by transmission elec-
tron microscopy (TEM) on an FEI Osiris instrument
(source with a field emission of 200 keV); images of
basic oxides, showed that palladium-doped MgAlO
mixed oxides provide a high selectivity of ≥70% at
x
2
00°C in a closed system. The cited authors stated that
Pd nanoparticles are the most optimum hydrogena-
tion–dehydrogenation component of a bifunctional
catalyst. It was found that, in the case of copper sup-
ported on mixed basic oxides in the Cu/MgAlO sys-
x
tem, the strong base sites are active in butanol synthe- about 100 particles were used for analysis. The pore
sis, while the acid sites mostly catalyze ethanol dehy- structure of the catalysts was characterized by low-
dration to ethylene and diethyl ether. However, Ho et temperature nitrogen sorption. Isotherms were
al. [8] showed that calcium phosphate is an active cat- recorded at –196°C on a Micromeritics ASAP 2000
alyst for butanol synthesis. The authors of [12] showed instrument after pre-evacuation at 350°C in a vacuum.
that the selective conversion of ethanol to butanol Surface compounds were studied by IR spectroscopy.
requires the presence of optimum-strength acid sites Infrared spectra were recorded in the transmission
in Ca (PO ) . Thus, some acid catalysts can be active mode on a Thermo Scientific Nicolet iS-10 Fourier
3
4 2
in the Guerbet process.
transform IR spectrometer equipped with an MCT
detector cooled with liquid nitrogen. The spectra were
Lewis acid zeolites, in particular, zeolites of the
BEA framework type, are commonly used as catalysts
for a broad range of organic synthesis reactions [13–
–1
recorded at a resolution of 4 cm in a range of 4000–
–
1
6
50 cm . Before recording, the sample was com-
pressed into a disk without a binder and placed in a
special quartz cell with KBr windows, which was con-
nected to a vacuum line. The sample was heated in a
separate portion of the cell at 400°C for 1 h to a vac-
1
5]. Acid sites in the zeolite structure are formed via
the isomorphous substitution of a silicon atom in the
4+
4+
tetrahedral framework position by a Zr [15] or Sn
atom [16], while the total electroneutrality of the
framework is maintained. The sites formed in this way
can catalyze stages 2–4 of ethanol conversion to buta-
nol (Scheme 1). The doping of these molecular sieves
with nanoparticles of a metal that is active in dehydro-
genation/hydrogenation will also lead to an accelera-
tion of stages 1 and 5 in the presence of a bifunctional
catalyst. Similar metal-containing systems were used
previously [9]; however, the base component of those
catalysts was magnesium–aluminum mixed oxide.
–5
uum of 10 torr; after that, the spectrum of the pure
sample was recorded. After ethanol adsorption, the
spectra were subtracted from the spectrum of the pure
sample. The sample was heated to a required tempera-
ture (50–250°C), held for 15 min, and then cooled to
2
5°C to record a spectrum.
Temperature-programmed desorption (TPD) of
ethanol was studied in a quartz microreactor (internal
diameter of 3 mm). A 0.6%Pd/Zr–BEA sample in the
form of a fraction of 0.25–0.5 mm was placed between
This study is focused on ethanol conversion in the quartz wool layers and heated in a helium stream at
presence of a zirconium-containing Zr–BEA zeolite 300°C for 1 h. After that, the sample was cooled to
doped with palladium.
60°C and saturated with ethanol fed through a syringe
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ETHANOL TO BUTANOL CONVERSION
927
dispenser. Next, weakly sorbed ethanol was blown off
(a)
for 2 h; the sample was heated to 500°C at a rate of
1
0 deg/min. Desorbed compounds were recorded on-
line at the exit of the reactor using a mass-selective
detector.
Catalytic Testing Procedure
Ethanol conversion was studied in a flow reactor at
atmospheric pressure and an ethanol feed space veloc-
5
b)
15
25
θ, deg
35
45
ity of 0.15–6.5 g
/(g h). The catalyst in the form
cat
ethanol
2
of a fraction of 0.5–1 mm was placed in a quartz reac-
tor and preconditioned in a nitrogen stream at 300°C
for 1 h; next, it was cooled to 200°C, and hydrogen was
fed for 1 h to reduce palladium particles; after that, the
temperature was set at a required level. The tests were
conducted at a constant stream of nitrogen or hydro-
gen mixed with nitrogen.
(
RESULTS AND DISCUSSION
Physicochemical Properties of the Catalysts
According to chemical analysis, the Zr–BEA zeo-
lite sample prepared by hydrothermal synthesis had a
Si/Zr molar ratio of 200 ± 20, which is close to a value
set during synthesis. According to low-temperature
nitrogen adsorption, the sample is a microporous
5
μm
3
material with a pore volume of 0.25 cm /g, which cor-
responds to highly crystalline zeolite BEA [15].
According to XRD (Fig. 1), the sample comprises only
the BEA zeolite phase with a high degree of crystallin-
ity. According to SEM, the zeolite crystal size is about
Fig. 1. (a) Diffraction pattern and (b) SEM image of the
Zr–BEA sample.
2
–3 μm, which is characteristic of zeolites synthesized
in an anion fluoride medium.
were detected. To determine the order of formation of
the products, curves of the dependence of the
ith product yield on ethanol conversion were plotted
The state of palladium in the samples was studied
by TEM; the results are shown in Fig. 2. The image
shows the BEA zeolite crystallographic faces with an
interplanar spacing of about 0.6 nm. Palladium parti-
cles (mostly of a spherical shape) with a diameter of
about 3–4 nm are localized near the zeolite crystal; in
addition, it was found that the sample contains larger
palladium particles localized separately from the zeo-
lite crystals.
(
Fig. 3).
The dependence of the diethyl ether yield on con-
version corresponds to the primary stable product.
Diethyl ether is formed during the bimolecular dehy-
dration of ethanol; it is not involved in secondary pro-
cesses under these conditions. The curve of the acetal-
dehyde yield also corresponds to the primary product,
The Pd particle size distribution shows that the selectivity for which (i.e., the slope of the curve in
approximately 50% of the particles have a diameter of Fig. 3) decreases with increasing conversion. Acetal-
3
–6 nm. These particles are localized on the outer sur- dehyde is formed in the ethanol dehydrogenation
face of the zeolite crystals. The other 50% of the palla- reaction and then undergoes several secondary pro-
dium particles have a diameter larger than 10 nm and cesses, the main of which is condensation to crotonal.
are localized separately in the form of aggregates.
Products of the aldol–croton condensation of acetal-
dehyde were not detected in the catalyzate; this fact is
associated with the vigorous hydrogenation of the
C=C and C=O bonds. With an increase in conversion,
synchronously with a decrease in acetaldehyde selec-
Ethanol Conversion Products and Order
of Their Formation
In the study of ethanol conversion in the presence tivity, butanol and butane appear in the products.
of a catalyst containing 1 wt % Pd, the following reac- Apparently, at a short contact time, butanol and
tion products were found: ethane, butane, acetalde- butane are formed with comparable selectivities; how-
hyde, diethyl ether, butanol, and ethyl butyl ether; in ever, with an increase in conversion, the contribution
addition, traces of butanal, butenes, and ethyl acetate of butanol conversion to butane increases. This find-
PETROLEUM CHEMISTRY Vol. 59 No. 8 2019

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28
KOTS et al.
The sample was saturated with ethanol vapors at 25°C
(a)
and then heated to 250°C with a stepwise increase in
temperature; IR spectra of the surface compounds
were recorded after cooling to 25°C (Fig. 5).
During ethanol adsorption, a “negative” band at
–1
3
732 cm appears in the spectra owing to the forma-
tion of a hydrogen bond between the OH group of eth-
…
anol and the Si-OH O(H)-C H silanol group. The
2
5
–1
broad band at ~3400 cm corresponds to the same
complex. In the region of the C–H stretching vibra-
tions, the spectrum exhibits bands at 2980 (ν (CH )),
20 nm
as
3
–1
2
937 (ν (CH )), 2904 (ν (CH )), and 2880 cm
as 2 s 3
(b)
(ν (CH )), which are attributed to the ethyl group of
s 2
0.6
0.5
0.4
0.3
0.2
ethanol molecules [17]. In the region of bending vibra-
tions, bands characteristic of ethanol are located at
–1
1
(
483 (δ (CH )), 1451 (δ (CH )), and 1395 cm
as 2 as 3
δ (CH). Upon heating to 50°C, the spectra exhibit a
s
band of the carbonyl group vibrations ν(C=O) at
1724 cm– and less intense broad bands at 1686 and
1
0.1
–1
0
1641 cm . This finding indicates the occurrence of
dehydrogenation reactions to form acetaldehyde,
which in turn, even at 50°C, can undergo the aldol–
croton condensation reaction to form α,β-unsaturated
carbonyl compounds. In addition, a broad band at
1
2
3
4
5
6
7
8
Palladium particle diameter, nm
1
–
1
619 cm– corresponds to bicarbonates [(HO)CO ]
2 ,
which are also formed at a low temperature [18]. Fur-
ther heating leads to an increase in the intensity of the
bands corresponding to acetaldehyde. At 150°C, the
spectrum exhibits intense bands at 1754 (ν(С=О) in
1
0
30 50 70 90 110 130 150 170 190
Palladium particle diameter, nm
Fig. 2. (a) Representative TEM image (Pd particles are
marked with arrows) and (b) Pd particle size distribution
for the 0.6%Pd/Zr–BEA sample according to TEM.
COOH), 1717 (ν(С=О)), 1561 (δ (ОСО)),
as
–1
1427 (δ (ОСО)), and 1323 cm (δ(СН )), which are
s
3
associated with the formation of acetic acid and sur-
face acetates. These compounds can be formed owing
to acetaldehyde disproportionation to acetic acid and
ethanol via the Cannizzaro reaction. In addition, the
formation of ethyl acetate from two acetaldehyde mol-
ecules via the Tishchenko reaction can be observed.
Further heating to 200–250°C leads to the decompo-
sition of acetates and acetic acid. In this case, the spec-
ing can be attributed to butanol dehydration to butene
on the acid sites of the catalyst and the subsequent
butene hydrogenation to butane on the metal sites.
The formation of butane at a low conversion can indi-
cate that crotonal can undergo direct hydrogenation
over Pd particles to hydrolyze the C–O bond. The
curve of the ethane yield corresponds to the secondary
product; this fact suggests that ethane is formed via the
intermediate monomolecular dehydration of ethanol
to ethylene, rather than from ethanol. The overall
reaction scheme is shown in Fig. 4.
It should be noted that, under the process condi-
tions, the hydrogenation reaction equilibrium is sig-
nificantly shifted toward saturated hydrocarbons. A
key requirement for acid sites of a bifunctional catalyst
in this order of conversions is that they should exhibit
high activity in acetaldehyde condensation and low
activity in the dehydration of ethanol and butanol.
tra exhibit bands of carbonates and bicarbonates at
–1
1
620, 1460, and ~1340 cm . Schematic formation of
the surface compounds is shown in Fig. 6.
At a low temperature, the Pd sites of the catalyst
mediate the dehydrogenation of adsorbed ethanol to
acetaldehyde, which in turn undergoes condensation
to crotonal and C aldehydes. In addition, acetalde-
6+
hyde is converted to surface acetates and acetic acid,
which, under heating, undergo decomposition to car-
bonates. It is significant that, under real catalysis con-
ditions, metal sites can be blocked by substantially
sorbed acetates and carbonates. Therefore, ethanol
can mostly react on acid sites to form diethyl ether.
Surface Compounds According to IR Data
On the other hand, the IR spectroscopy data show
The structure and concentration of surface com- that ethanol dehydrogenation occurs even at 50–
pounds formed in the reaction of ethanol and a 100°C without the formation of acetates and carbon-
Pd/Zr–BEA catalyst were studied by IR spectroscopy. ates. This finding is in agreement with the data
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ETHANOL TO BUTANOL CONVERSION
(a)
929
3.0
2.5
2.0
O
Butanol
1
.5
.0
.5
1
0
Ethane
5
10
15
20
Ethanol conversion, %
(b)
7
3
2
2
.0
.5
.0
6
5
4
3
2
1
0
O
1
.5
.0
.5
1
Butane
0
5
10
Ethanol conversion, %
15
20
Fig. 3. Dependence of the yield of the main ethanol conversion products on conversion in the presence of the 1%Pd/Zr–BEA
–1
catalyst. Conditions: temperature, 235°C; feed space velocity, 4.3–0.7 h ; N /H /ethanol molar ratio, 2.9/1.6/1.0; and form of
2
2
fed ethanol, a 50% aqueous solution.
М
C H
C2H6
2
4
К
М
М
К
O
O
OH
OH
O
К
К
M denotes a metal
site (Pd)
K denotes an acid site (Zr)
М
Fig. 4. Schematic formation of the main ethanol conversion products in the presence of a Pd/Zr–BEA catalyst.
PETROLEUM CHEMISTRY Vol. 59 No. 8 2019

9
30
KOTS et al.
2
980
1396
1451
1723
1483
2904
1619
1561
50°C
3732
100°C
150°C
2
00°C
50°C
2
3
800 3600 3400 3200 3000 2800 2600
1800
1700
1600
1500
1400
–
1
Wave number, cm
Fig. 5. Infrared spectra recorded after ethanol adsorption at 25°C and heating to 50–250°C in the presence of the 1%Pd/Zr–BEA
sample.
CH3
OH
CH3
CH3
OH –CH4
O
–H2
O
O–
O–
OH
O
Fig. 6. Schematic formation of surface compounds according to IR spectroscopy of adsorbed ethanol.
obtained in the flow mode, where the acetaldehyde the surface acetates and carbonates, which are readily
selectivity was 15–17%. Apparently, acetaldehyde formed from ethanol; this reduction can contribute to
undergoes condensation quite slowly; it is this factor the purification of the Pd surface. On the other hand,
that limits the formation of butanol over the catalyst, hydrogen additionally fed to the system can undergo
whereas the formation of acetaldehyde occurs quite butanol hydrogenation to butane via the intermediate
rapidly.
formation of butenes. To verify these assumptions,
comparative tests with and without hydrogen feeding
were conducted (Table 1).
Effect of Hydrogen on Ethanol Conversion
In the case of reaction without the addition of
In terms of the Guerbet reaction stoichiometry, the hydrogen, the main products formed over the catalyst
addition of hydrogen is not necessary, because hydro- are acetaldehyde, diethyl ether, and butenes. Appar-
gen is evolved during in situ ethanol dehydrogenation. ently, ethanol dehydrogenation occurs quite rapidly;
In this case, hydrogen from the gas phase can reduce afterwards, the resulting acetaldehyde slowly under-
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ETHANOL TO BUTANOL CONVERSION
931
goes condensation to crotonal, which, in turn, without Table 1. Effect of hydrogen on the rate of formation of the
main ethanol conversion reaction products
intermediate desorption into the gas phase, is con-
verted to butanol on the metal sites of the catalyst. The
resulting butanol undergoes rigorous dehydration to
form butenes.
Formation rate, Hydrogen feeding (partial Without
μmol/(gcat h)
pressure of 0.29 atm)
hydrogen
Diethyl ether
335
146
100
21
169
444
3
83
1
Upon the addition of hydrogen to the stream, the
diethyl ether formation rate increases twofold. In Acetaldehyde
addition, the rate of formation of butanol and butane Butanol
increases. The acetaldehyde formation rate decreases
owing to an increase in the rate of further conversion
of acetaldehyde. The amount of butenes significantly
decreases, because they undergo hydrogenation to
Butenes
Butane
Total amount of
150
171
84
С hydrocarbons
butanes. In general, the amount of C hydrocarbons
4
4
increases twofold; this finding suggests that hydrogen Conditions: catalyst, 0.6%Pd/Zr–BEA; temperature, 235°C; feed
space velocity, 0.54 h– ; partial pressure of ethanol, 0.18 atm; and
1
has a promoting effect on the formation of crotonal as
a precursor of butanol, butenes, and butane.
form of fed ethanol, a 50% aqueous solution.
Hydrogen can significantly affect the state of the
active sites of the catalyst, particularly the state of the
palladium particles [19]. The effect of hydrogen was
studied by the ethanol-TPD method (Fig. 7).
while the desorption maximum is shifted to lower tem-
peratures by ~113°C. This finding is attributed to a
decrease in the apparent activation energy for dehy-
dration; a rough estimate made using the first-order
The TPD experiment revealed that diethyl ether
formed during ethanol dehydration without the addi-
tion of hydrogen to the stream is desorbed from the reaction equation shows a decrease by 8–9 kJ/mol.
catalyst in the form of a broad asymmetric peak with a The presence of hydrogen leads to the displacement of
maximum at about 303°C; the onset of desorption is
observed at 200°C. Most probably, ethanol dehydra-
tion occurs at lower temperatures; however, owing to
strong binding [20], diethyl ether is desorbed from the
acid site at a higher temperature. In the case of the
TPD experiment using dilute hydrogen, the intensity
ethanol from the palladium sites; therefore, the
adsorption capacity of the entire catalyst decreases. It
can be assumed that, for the same reason, the
chemisorbed ethanol displaced from the Pd surface
moves to the acid sites, where it rapidly undergoes
of the peak of diethyl ether significantly decreases, dehydration to diethyl ether.
303
190
(b)
(a)
5
× 10–11
2
× 10–10
100
200
300
400
500
Temperature, °C
Fig. 7. Desorption of diethyl ether (m/z = 74) from the 0.6%Pd/Zr–BEA sample (a) in a H /He stream (partial pressure of hydro-
2
gen of 0.134 atm) and (b) in a pure He stream.
PETROLEUM CHEMISTRY Vol. 59 No. 8 2019

9
32
KOTS et al.
(
Scheme 2). In this case, atomic hydrogen migrates to
the acid sites of the zeolite, where the adsorbed reac-
tants and products of the acid-catalyzed stages are
located [21].
5
× 10–12
(b)
H2
O
Zr4+
O−
Zr4+
H+
Pd
2H
Scheme 2. Formation of new acid sites on the Zr–BEA sur-
face owing to hydrogen spillover on metal particles and sub-
sequent hydrogenation.
The reaction of hydrogen atoms with adsorbed cro-
tonal leads to the hydrogenation of the C=O bond to
form a surface enolate and a new acid site. By analogy,
the hydrogenation of the C=C bond in the enolate can
occur. In this case, the new acid sites are localized on
the siloxane bridges of the zeolite (Scheme 3).
× 10–11
5
(a)
H
Si O Si + H+
Si O Si
+
1
00
200
300
400
500
Scheme 3. Formation of new acid sites.
Temperature, °C
The new acid sites are involved in both ethanol
esterification and acetaldehyde condensation. These
reactions can be accelerated owing to both an increase
in the number of acid sites and the fact that the hydro-
genation occurs directly on the acid site, without cro-
tonal diffusion to the Pd particle.
Fig. 8. Desorption of butanol (m/z = 56) from the
.6%Pd/Zr–BEA sample (a) in a pure He stream and
b) in a H /He stream (partial pressure of hydrogen of
0
(
2
0.134 atm).
Small amounts of butanol were recorded during
ethanol-TPD at m/z = 56 (Fig. 8). According to a sep-
arate experiment with gas chromatographic analysis,
an increase in temperature to ≥400°С led to the forma-
tion of butenes, while butanol desorption did not
occur any longer in this temperature range.
Effect of Palladium Loading
on Catalytic Properties
To develop a selective ethanol conversion catalyst,
it is necessary to provide a balance between the acidic
and hydrogenation/dehydrogenation functions of the
catalyst. To this end, samples with different palladium
concentrations were synthesized. The samples were
compared according to the initial rates of formation of
the key reaction products under low conversion condi-
tions (≤10%); the results are shown in Table 2.
The sample with the lowest palladium content
exhibited a fairly low activity in the formation of both
acetaldehyde and diethyl ether; no butanol was formed
in the presence of this catalyst. For the other samples,
an increase in the diethyl ether formation rate is asso-
ciated with the above effect found by the ethanol-TPD
It is evident from Fig. 8 that butanol desorption
occurs at a temperature of ≥180°С with a maximum at
275°C. Experiments in a dilute hydrogen stream
showed that the amount of desorbed butanol decreases
by about 2 orders of magnitude; however, the process
occurs in approximately the same temperature range.
Most probably, under TPD conditions, butanol is
quite rapidly converted to butane, which in turn was
detected at m/z = 43. By analogy with ethanol conver-
sion to diethyl ether, butanol dehydration can occur
many times faster in the presence of hydrogen.
The addition of hydrogen to the reactant stream method. Hydrogen spillover is capable of generating
leads to a significant increase in the ethanol dehydra- acid groups in a zeolite that mediate ethanol dehydra-
tion rate and a decrease in the apparent activation tion. The addition of 0.3% Pd to Zr–BEA leads to a
energy for this process. Therefore, the diethyl ether fivefold increase in the diethyl ether formation rate. A
selectivity considerably increases. The aldol–croton further increase in the palladium content apparently
condensation rate also significantly increases upon the leads to a change in the degree of dispersion of Pd and
addition of hydrogen, which is accompanied by an the coarsening of the particles. Owing to this effect,
increase in the yield of butanol and C hydrocarbons. the acceleration of dehydration becomes less pro-
4
Apparently, in the presence of hydrogen, a hydrogen nounced, because the large particles are localized sep-
spillover occurs on the surface of the Pd particles arately from the acid sites of the zeolite.
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ETHANOL TO BUTANOL CONVERSION
933
Table 2. Comparison of the rates of formation of products in the presence of x%Pd/Zr–BEA samples
Product formation rate, μmol/(gcat h)
Butanol selectivity,
mol %
Sample
butanol
butane
diethyl ether
acetaldehyde
1
.5 Pd
186
191
193
0
181
281
311
4
697
1331
1736
364
363
510
349
51
8.6
6.3
6.3
0.5
1.0 Pd
0.3 Pd
0.1 Pd
Conditions: feed space velocity, 1.1–1.2 h–1; temperature, 235°C; and feedstock, 50% ethanol/water.
Table 3. Effect of water additives on the rate of formation of products from ethanol in the presence of the 0.3%Pd/Zr–BEA
catalyst
Product formation rate, μmol/(gcat h)
Mass fraction of water
in a mixture with ethanol, %
Butanol selectivity,
mol %
butanol
butane
diethyl ether
acetaldehyde
5
25.4
1.3
8.0
56.9
13.8
15.0
1746.2
117.7
162.0
56.9
57.9
44.2
1.3
0.7
3.6
1
0
50
Conditions: T = 235°C, WHSV = 0.4–0.8 h–1
.
The acetaldehyde formation rate increases syn- Table 4. Comparison of experimental data with thermody-
namic calculation data
chronously with an increase in the palladium content
in the samples. For the 1.5%Pd/Zr–BEA sample, a
decrease in the rate is observed; it can be attributed to
the coarsening of the particles. The butane formation
rate decreases with an increase in the palladium con-
tent. Since butane is formed during the hydrogenation
of butenes, which are the butanol dehydration prod-
ucts, the butanol formation rate directly affects the
formation of butane. With allowance for this fact, it is
clear that an increase in the number of Pd sites at a
constant number of Zr sites leads to a decrease in
the butanol dehydration rate and the rate of ethanol
dehydration to diethyl ether. Comparison of butanol
selectivities for catalysts with different palladium con-
centrations shows that the 1%Pd/Zr–BEA sample is
optimal.
Diethyl ether/ethanol molar ratio
after reaction
Mole fraction
of water in ethanol
before reaction, %
thermodynamic
calculation
experiment
1
2
2.20
1.95
0.85
0.21
0.05
0.02
22
72
Temperature, 235°C; feed space velocity, 1.08 h–1
.
diethyl ether in equilibrium should be approximately
an order of magnitude higher than the experimentally
obtained value. Apparently, dehydration occurs in the
kinetic region, and equilibrium is not established. In
addition, according to calculations, an increase in the
amount of water in the feed ethanol has a weaker effect
on the relative content of diethyl ether than the effect
found in experiments. Thus, it can be assumed that the
effect of inhibition of dehydration with water is
attributed to the blocking of acid sites. These sites can
be localized both in the vicinity of the Pd nanoparti-
cles and in the zeolite pores. However, it is known that
Zr–BEA is a hydrophobic molecular sieve and water
sorption in the pores is limited [23].
Effect of Water on Catalytic Properties
Ethanol conversion in the presence of Pd/Zr–BEA
led to the formation of a large amount of diethyl ether.
It can be assumed that the addition of water to ethanol
can lead to a decrease in the dehydration reaction rate
owing to the displacement of equilibrium and owing to
adsorption on acid sites and their partial poisoning
[
22]. Results of tests with the addition of water are
shown in Tables 3 and 4.
The diethyl ether formation rate decreases 14-fold
upon the addition of water to the reactant stream. The
The addition of water has little effect on the appar-
butane formation rate also decreases owing to the ent acetaldehyde formation rate, because the addition
slowing down of the dehydration of both ethanol and of water slows down the further condensation of acet-
butanol upon the addition of water. Thermodynamic aldehyde and thereby leads to a decrease in the con-
calculations show that, at 235°C, the amount of sumption of this material for secondary products.
PETROLEUM CHEMISTRY Vol. 59 No. 8 2019

9
34
KOTS et al.
Since the apparent acetaldehyde formation rate
remains unchanged, it can be assumed that the etha-
nol dehydrogenation rate decreases with an increase in
the amount of water. The butanol formation rate also
decreases; this fact suggests that acetaldehyde conden-
sation is slowed down. Thus, the addition of water
inhibits all acid-catalyzed stages of the process and
slows down the dehydrogenation and hydrogenation
reactions over palladium particles.
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Translated by M. Timoshinina
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Vol. 59
No. 8
2019