Optimization of a heterogeneous Pd-Cu/zeolite y Wacker catalyst for ethylene oxidation

Article abstract of DOI:10.1039/c9cc08835k

Exchanging low amounts of palladium with excess copper in a heterogeneous zeolite Y catalyst leads to high Wacker activity and mitigates activity loss by reducing the extent of palladium sintering. Employing periodic regenerative treatments in oxygen to remove carbon deposits and reoxidize palladium results in the partial (but reversible) recovery of the high initial activity.

Full text of DOI:10.1039/c9cc08835k

ChemComm
View Article Online
View Journal
COMMUNICATION
Optimization of a heterogeneous Pd–Cu/zeolite Y
Wacker catalyst for ethylene oxidation†
Cite this:DOI: 10.1039/c9cc08835k
ab
ab
a
Jerick Imbao,
Jeroen A. van Bokhoven
*
and Maarten Nachtegaal*
Received 12th November 2019,
Accepted 27th November 2019
DOI: 10.1039/c9cc08835k
rsc.li/chemcomm
Exchanging low amounts of palladium with excess copper in a deactivates through the precipitation of copper oxalate in the
1
,4
heterogeneous zeolite Y catalyst leads to high Wacker activity homogeneous Wacker system and the irreversible formation
and mitigates activity loss by reducing the extent of palladium of inactive zero-valent palladium in the heterogeneous Wacker
4
sintering. Employing periodic regenerative treatments in oxygen system. Overall, the homogeneous system is still widely utilized
to remove carbon deposits and reoxidize palladium results in the commercially because the catalyst solution can be regenerated
partial (but reversible) recovery of the high initial activity.
continuously and efficiently in high-purity oxygen (one-stage pro-
cess developed by Farbwerke Hoechst) or air (two-stage process
2,4,12
The homogeneous Wacker oxidation of olefins into synthetically developed by Wacker-Chemie).
Structural characterization of
versatile carbonyls is one of the most successful industrially Pd–Cu/zeolite Y for the heterogeneous Wacker process is limited,
1,2
applied processes after the Second World War. In particular, preventing the rational design of better catalysts. Here, we show
the palladium-catalyzed oxidation of ethylene remains industrially the influence of palladium and copper loading on the activity and
relevant for acetaldehyde production due to the continued growth stability of Pd–Cu/zeolite Y for Wacker oxidation of ethylene. We
3
in the demand for manufacturing its derivatives. However, the investigated the catalyst deactivation and reactivation mechanisms
excessive use of hydrochloric acid leads to high corrosivity and by determining the electronic and structural changes, deciphered
formation of unwanted chlorinated side-products. Moreover, by XAS and STEM, during the course of the reaction.
oxidizing longer-chained alkenes leads to products with boiling
The catalysts were prepared through aqueous ion exchange
points above that of water, rendering their separation from the of the sodium form of the zeolite Y (SiO /Al O = 5.1) denoted as
2
2 3
4
aqueous catalyst solution challenging. Consequently, there had Pd(a)Cu(b)–Y (a,b = wt%; Table S1, ESI†). The HAADF-STEM
been various attempts to heterogenize Wacker catalysts using image and the corresponding elemental maps of Pd1Cu5–Y
chloride-free systems with varying co-catalysts. Stobbe-Kreemers (Fig. S1, ESI†) illustrate that both palladium and copper are
5
et al. replaced the conventional copper co-catalyst with vanadia homogeneously distributed in the zeolite after ion exchange with-
6
while Barthos et al. optimized this system with monolayer out any evident particle formation. The catalysts were pre-treated
7
vanadia supported on silica and a-alumina. Nowinska et al.
in oxygen at 378 K and their activity for Wacker oxidation was
utilized the palladium salts of heteropolyacids supported on measured in a fixed-bed plug flow reactor at 378 K with a gas feed
silica for the selective oxidation of ethylene to acetaldehyde. consisting of ethylene, water and oxygen (1 : 7 : 10 molar ratio)
8
,9
10
ꢀ1
Espeel et al. and Arai et al. separately showed that palladium- diluted in helium at constant flow (W/F = 0.86 kg s mol ) to
0
cat
and copper-exchanged zeolite Y (Pd–Cu/zeolite Y) is one of the guarantee that ethylene conversion did not exceed 15%.
most active heterogeneous Wacker catalysts. The activities of Optimum Wacker activity and acetaldehyde selectivity were
these heterogeneous catalysts (0.5–0.8 mmol acetaldehyde mmol determined by testing catalysts containing varying amounts of
ꢀ1
ꢀ1
Pd min ) are similar to those of the homogeneous systems copper and palladium (Fig. 1). Without palladium, no Wacker
ꢀ
1
ꢀ1 11
(
0.3–1.0 mmol acetaldehyde mmol Pd min ). The catalyst conversion was observed over Na–Y (not shown) and Cu7–Y
(Fig. 1a). Without copper, low ethylene conversion was observed
over Pd2–Y (Fig. 1d) with a selectivity of roughly 80% for
acetaldehyde, which corresponds to an amount greater than
the stoichiometric amount of palladium present. This is indica-
tive of catalytic acetaldehyde formation in the absence of copper.
The remaining ethylene was oxidized to carbon dioxide. Fig. 1a
shows that the conversion of ethylene over Pd–Cu/zeolite Y with
a
b
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
E-mail: maarten.nachtegaal@psi.ch
Institute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1,
8
093 Zurich, Switzerland. E-mail: jeroen.vanbokhoven@chem.ethz.ch
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cc08835k
This journal is © The Royal Society of Chemistry 2019
Chem. Commun.

View Article Online
Communication
ChemComm
that there are enough copper ions in the proximity of each palladium
ion. At higher copper loading (ca. 6 wt%), the additional copper ions
participate in the total oxidation of ethylene.
Table S2 (ESI†) lists the properties of the catalysts having low
(Pd1Cu5–Y) and high (Pd4Cu5–Y) palladium loading. Both cata-
lysts exhibit lower surface area and total pore volume after 4 hours
of Wacker oxidation relative to the as-prepared catalysts, which
could be due to the accumulation of carbon deposits. The surface
area and total pore volume of Pd4Cu5–Y are much lower than that
of Pd1Cu5–Y after 4 hours under Wacker conditions, albeit having
nearly similar amount of accumulated carbon deposits. In situ
X-ray absorption spectroscopy (Methods, ESI†) was employed to
monitor the changes in the electronic and local atomic structures
of the catalysts under operating conditions. The XANES spectra of
both pre-treated Pd1Cu5–Y and Pd4Cu5–Y (Fig. 2) are nearly
similar suggesting that the difference in palladium loading does
not lead to significant differences in the oxidation state and
structure of palladium and copper. The Cu and Pd K-edge XANES
spectra of both as-prepared Pd1Cu5–Y and Pd4Cu5–Y indicate
that copper is present as hydrated Cu(II) (Fig. 2a: pre-edge feature
13
at 8977.5 eV resulting from the 1s - 3d transition; continuum
resonance peak at 8996 eV) and palladium as amminated Pd(II)
Fig. 1 Effect of different metal loadings on ethylene oxidation. (a and d)
Ethylene conversion, (b and e) rate of acetaldehyde formation and (c and f)
product selectivities over Pd–Cu/Y with different Pd (a–c: fixed 5 wt% Cu)
and Cu (d–f: fixed 1 wt% Pd) loadings. Hollow points in (a–c) represent
Cu7-Y (without Pd) while those in (d–f) refer to Pd2–Y (without Cu). All
ꢀ1
values were determined after 4 hours of reaction (378 K, 0.86 kgcat s mol ).
Gas feed consists of C : H O : O at a 1 : 7 : 10 molar ratio.
H
2 4
2
2
fixed copper loading (ca. 5 wt%) increased significantly from zero
to ca. 9% in the presence of palladium (1 wt%, Cu/Pd = 6.7).
Ethylene conversion continued to improve from 9% to 13.4%
while the selectivity for acetaldehyde decreased slightly (from 91%
to 85%, Fig. 1c) when the palladium loading was increased from
1
wt% (Cu/Pd = 6.7) to 4 wt% (Cu/Pd = 2.2). However, normalizing
the activity to the amount of palladium (Fig. 1b) reveals that the
rate of formation of acetaldehyde diminished as the palladium
loading was increased from 1 wt% to 4 wt%. Moreover, only
Pd1Cu5–Y reached steady-state activity after 4 hours while both
Pd2Cu5–Y and Pd4Cu5–Y continued to deactivate thereafter.
Next, we investigated the effect of varying the copper loading on
the activity of Pd–Cu/Y with a fixed palladium loading of 1 wt%
(Fig. 1d–f) since low palladium loading resulted in optimum Wacker
activity. In comparison with Pd2–Y, a small improvement in conver-
sion (from 1.6% to 2.2%) with a corresponding decrease in the
selectivity for acetaldehyde (from 80% to 69%) was observed when
the copper loading is 1 wt% (Cu/Pd = 1.7). When the copper loading
was increased from 1 wt% to 5 wt% (Cu/Pd = 6.7), the ethylene
conversion to acetaldehyde improved significantly, highlighting the
co-catalytic activity of copper. However, a further increase in the
copper loading from 5 wt% to 6 wt% (Cu/Pd = 9.4) did not lead to
any significant variation in ethylene conversion but instead resulted
in an increase in carbon dioxide selectivity at the expense of
acetaldehyde selectivity. These results suggest that excess copper
Fig. 2 Normalized (a) Cu and (b) Pd K-edge XANES spectra of Pd–Cu/Y.
ꢀ1
Wacker conditions: 378 K, 0.86 kgcat s mol . Gas feed consists of C
2
H
4
:
2 2
H O : O at a 1 : 7 : 10 molar ratio. Inset: Magnification of (b) Pd K-edge
(
Cu/Pd = 6.7) is needed for optimum Wacker activity as it ensures XANES spectra at half-edge height.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2019

View Article Online
ChemComm
Communication
(
Fig. 2b: whiteline peak at 24372 eV, similar to the spectrum of the Table 1 Pd K-edge EXAFS curve fitting results for Pd–Cu/zeolite Y in
2
+
ꢀ1
different reaction conditions
Pd(NH
)
3 4
; Fig. S2, ESI†: weak IR band at 1310 cm attributed to
9
symmetric N–H deformation ). Pre-treatment in oxygen at 378 K
resulted in the decrease of the whiteline intensity at 8996 eV
(
a
b
2
2 c
2
d
Catalyst Path
N
R (Å )
s
(Å )
R
fac
Pre-treated Pd1Cu5–Y Pd–O/N 3.5(0.2) 2.01(0.01) 0.004(0.001) 0.010
due to partial dehydration) in the Cu K-edge XANES spectra of
e
Wacker-eq Pd1Cu5–Y Pd–O/N 3.1(0.3) 2.01(0.01) 0.004(0.001) 0.009
Pd1Cu5–Y and Pd4Cu5–Y but there were no changes observed
in the Pd K-edge XANES spectra. The Pd K-edge XANES of the
Wacker-equilibrated Pd1Cu5–Y (Fig. 2b) exhibit a shift towards
lower absorption edge energy at half-height relative to the pre-
treated catalysts, which is indicative of the reduction of a fraction
Pd–Pd 1.5(0.9) 2.76(0.05) 0.018(0.003)
Pre-treated Pd4Cu5–Y Pd–O/N 3.6(0.2) 2.01(0.01) 0.004(0.001) 0.011
Wacker-eq Pd4Cu5–Y Pd–O/N 2.9(0.5) 2.04(0.01) 0.006(0.001) 0.017
e
Pd–Pd 2.6(0.7) 2.75(0.01) 0.006(0.001)
a
Number of nearest neighbors determined by fixing the amplitude
2
0
reduction factor (Sb
) values obtained from the fits to Cu (0.82) and Pd
c
of Pd(II) to Pd(0). The Pd K-edge XANES spectrum of the Wacker- (0.86) foil spectra. Interatomic distance. Pseudo Debye–Waller factor.
ꢂ
Nfit
Nfit
equilibrated Pd4Cu5–Y reveals a broader continuum resonance
peak centered at 24372 eV and the appearance of a shoulder at
ca. 24395 eV, suggesting the formation of more Pd(0) compared
to what was observed for the Wacker-equilibrated Pd1Cu5–Y.
d
P
ꢀ
ꢁ
2
P ꢀ
ꢁ
2
e
R-factor ¼
X im easured ꢀ X im odelðxÞ
X im easured
.
Equilibrated
i
i
ꢀ1
in Wacker conditions (378 K, 0.86 kgcat s mol ).
The Cu K-edge XANES spectra of Pd4Cu5–Y after 4 hours under Pd-normalized rate of acetaldehyde formation observed over
Wacker conditions (Fig. 2a) show the formation of a minor peak Pd4Cu5–Y.
at 8983 eV, which has a higher intensity than that in the spectra
of Wacker-equilibrated Pd1Cu5–Y. This indicates the formation tested for Wacker oxidation under differential conditions. Fig. 4
The stability of the optimized catalyst, Pd1Cu5–Y, was then
1
3
of more Cu(I) species in the catalyst with higher palladium depicts the first four hours of the reaction exhibiting two stages
loading after 4 hours under Wacker conditions.
of activity loss or deactivation versus time-on-stream (TOS).
EXAFS analyses (Fig. 3, Fig. S3, S4, S8 and Table 1, Table S3, Rapid deactivation was observed during the 1st hour of the
ESI†) reveal a decrease in the average Pd–O coordination number reaction followed by a more gradual loss of activity before
for Pd1Cu5–Y (from 3.5 to 3.1) and Pd4Cu5–Y (from 3.6 to 2.9) at reaching steady-state conditions. After 4 hours under Wacker
a bond distance of 2.01 Å after 4 hours under Wacker conditions. conditions, a partial loss of palladium–ammine coordination is
ꢀ
1
Pd(II) - Pd(0) reduction is evident in the Wacker-equilibrated evident from the decrease in the IR band intensity at 1310 cm
ꢀ
1
Pd4Cu5–Y due to the appearance of a Pd–Pd (metallic) coordina- and the appearance of a band at 1450 cm in the IR-ATR
tion shell at 2.76 Å with a higher coordination number (2.6 Pd) spectrum (Fig. S2, ESI†) of spent Pd1Cu5–Y. Fig. S6 (ESI†)
9
than that of the Wacker-equilibrated Pd1Cu5–Y (1.5 Pd). This shows that Pd1Cu5–Y is stable under Wacker conditions for up
signifies that the extent of Pd(II) reduction in Pd4Cu5–Y is higher to 20 hours on stream with mild deactivation and an acetalde-
than that in Pd1Cu5–Y after 4 hours under Wacker conditions. hyde selectivity of ca. 90%. If the catalyst deactivation is largely
The HAADF-STEM image and the corresponding EDX maps of spent induced by the accumulation of carbon deposits and the rever-
Pd4Cu5–Y (Fig. S5, ESI†) illustrate the formation of big palladium sible formation of Pd(0), the activity should be fully recovered
particles (ca. 5–10 nm), which corroborate the Pd K-edge XANES after performing calcination to burn off all the carbon deposits
and EXAFS spectra. Hence, high palladium loading leads to the and reoxidize Pd(0). The reactivation procedure involved heating
formation of more Cu(I) species and inactive palladium metallic
nanoparticles under Wacker conditions. This results in a lower
3
Fig. 3 k -Weighted Fourier transform magnitude (non-phase shift corrected)
of the Pd K-edge EXAFS spectra of (a and b) Pd1Cu5–Y and (c and d) Pd4Cu5–Y
in different reaction conditions (FT performed in the 3–11 Å range and fitted in
Fig. 4 Ethylene oxidation over Pd1Cu5–Y and the regenerated catalyst
(3 subsequent reactivation cycles) under standard Wacker conditions vs.
ꢀ1
ꢀ1
R space in the 1–3 Å range; black: magnitude of FT, red: best fit). The
time-on-stream (TOS). Wacker conditions: 378 K, 0.86 kgcat s mol . Gas
feed consists of C : H O : O at a 1 : 7 : 10 molar ratio.
3
corresponding k -weighted Pd K-edge EXAFS spectra are shown in Fig. S8, ESI.†
2
H
4
2
2
This journal is © The Royal Society of Chemistry 2019
Chem. Commun.

View Article Online
Communication
ChemComm
Table 2 Rates of acetaldehyde formation over Pd1Cu5–Y and after cycles, the loss of activity (ca. 10%) is reversible and can be
regeneration in the initial period (after 15 min) and after 4 h under Wacker
mainly, if not totally, attributed to the formation of carbon
deposits and zero-valent palladium. This regeneration proce-
ꢀ1
conditions (378 K, 0.86 kgcat s mol
)
Initial reaction rate
mmol AcH mmol
Reaction rate after 4 h dure could possibly still be optimized further by employing
ꢀ1
(
(mmol AcH mmol
periodic regeneration steps before the catalyst is maximally
deactivated to take advantage of the high initial activity of the
catalyst during the first 1 or 2 hours of the reaction before
reaching steady-state conditions.
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
Catalyst
Pd min
)
Pd min
)
Pd1Cu5–Y
Regeneration in O
30.8
, 623 K
24.8
16.5
15.4
10.2
2
1
2
3
st cycle
nd cycle
rd cycle
6.6
5.8
5.3
In summary, we show that optimum Wacker activity is
achieved by exchanging low loadings of palladium with excess
copper while high palladium loading was observed to induce
more palladium sintering. In addition, accumulation of carbon
deposits and formation of inactive Pd(0) were observed leading
to the deactivation of the catalyst under steady-state Wacker
conditions. The initial regeneration in oxygen at 623 K only
resulted in a partial recovery of initial activity but became
reversible in subsequent reactivation cycles. These results
suggest that although deactivation of Pd–Cu/zeolite Y catalyst
was not prevented, the heterogeneous process was optimized
by diminishing the activity loss by utilizing less palladium and
could be optimized further by including periodic regeneration
steps, improving the stability.
at a higher temperature (623 K) than what was used in the
standard activation and reaction conditions (378 K) because
calcination of the spent catalyst at lower temperatures did not
burn off all the carbon deposits. However, only 75% of the
initial activity of Pd1Cu5–Y was regenerated by the reactivation
procedure. Again, a fast and a slow deactivation of the catalyst
were observed through time (Fig. 4, 1st cycle), albeit with a
lower steady-state rate of acetaldehyde formation after 4 hours
under Wacker conditions (Table 2). At temperatures higher than
5
73 K, the majority, if not all, of ammine ligands that were initially
14
present are released from their coordination with Pd(II). Fig. S2
We thank the Swiss National Science Foundation (159555)
for the financial support and the SuperXAS beamline at the
Swiss Light Source for the allocation of beamtime.
ꢀ
1
(
ESI†) depicts the disappearance of the bands at 1310 and 1450 cm
in the IR-ATR spectrum of regenerated Pd1Cu5–Y, which is indicative
9
of the loss of Pd–ammine coordination and the subsequent release
1
4
and oxidation of ammonia at 623 K.
These lead to irreversible changes in the original catalyst as Conflicts of interest
observed in the Pd and Cu K-edge EXAFS spectra of Pd1Cu5–Y
after the 1st reactivation (Fig. S7, ESI†). EXAFS analyses (Table S3,
There are no conflicts to declare.
ESI†) reveal the recovery of the initial intensity of Pd–O (2.02 Å)
and Cu–O (1.95 Å) scattering peaks; the disappearance of metallic
Notes and references
Pd–Pd (2.75 Å) scattering peak; and the emergence of Cu–(O)–Cu
2.91 Å) and two weak Pd–(O)–Pd coordination shells at 3.04 and
.43 Å. These results signify that the reactivation procedure resulted
1 J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. R u¨ ttinger and
H. Kojer, Angew. Chem., 1959, 71, 176–182.
(
3
2
3
R. Jira, Angew. Chem., Int. Ed., 2009, 48, 9034–9037.
Acetaldehyde: A Global Strategic Business Report, 2016, Global
Industry Analysts, Inc., San Jose, California.
in the reoxidation of Pd(0) and Cu(I) with the corresponding
formation of palladium(II) oxide and copper(II) oxide particles.
After 4 hours under Wacker conditions, the intensity of Pd–O
and Cu–O scattering peaks decreased; the Pd–Pd (metallic)
coordination shell reappeared at 2.75 Å; and the intensity of
two Pd–(O)–Pd scattering peaks increased, which are indica-
tive of the reduction of Pd(II) and Cu(II) and the formation of
more palladium(II) oxide aggregates. Thus, we posit that the
irrecoverable loss of initial activity in the 1st cycle of reactiva-
tion is associated largely with the release of ammine ligands
from palladium coordination and the corresponding irreversible
formation of inactive palladium and copper(II) oxide nanoparticles.
4
M. Eckert, G. Fleischmann, R. Jira, H. M. Bolt and K. Golka,
Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag,
Weinheim, Germany, 2000, 1, 191–207.
5 A. W. Stobbe-Kreemers, M. Makkee and J. J. F. Scholten, Appl. Catal.,
A, 1997, 156, 219–238.
6
R. Barthos, G. Novod ´a rszki and J. Valyon, React. Kinet., Mech. Catal.,
016, 121, 17–29.
7 K. Nowi n´ ska, D. Dudko and R. Golon, Chem. Commun., 1996,
77–279.
P. H. Espeel, M. C. Tielen and P. Jacobs, Chem. Commun., 1991,
69–671.
9 P. H. Espeel, G. De Peuter, M. C. Tielen and P. Jacobs, J. Phys. Chem.,
994, 98, 11588–11596.
0 H. Arai, T. Yamashiro, T. Kubo and H. Tominaga, J. Jpn. Pet. Inst.,
976, 18, 39–44.
The reactivation procedure was done for a second time 11 O. Bander et al., US Pat., 3154586, 1964; L. Hornig, et al., US Pat.,
149167, 1964; W. Riemenschneider, et al., US Pat., 3301905, 1967.
2
2
8
6
1
1
1
3
(Fig. 4, 2nd cycle) resulting in a nearly similar steady-state rate
1
2 H. J. Hagemeyer and U. b. Staff, Kirk Othmer Encyclopedia of
of acetaldehyde formation but only 67% of the initial activity
Chemical Technology, John Wiley & Sons, Inc., 2014, pp. 1–16.
was recovered relative to what was measured in the first cycle 13 L. S. Kau, E. I. Solomon and K. O. Hodgson, J. Phys. Colloques, 1986,
7, C8-289–C8-292; L. S. Kau, D. J. Spira-Solomon, J. E. Penner-
Hahn, K. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 1987, 109,
433–6442.
observed in the second cycle. In the 2nd and 3rd reactivation 14 S. T. Homeyer and W. M. H. Sachtler, J. Catal., 1989, 117, 91–101.
4
(Table 2). The third cycle of reactivation led to nearly full
regeneration of Wacker activity in comparison with what was
6
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2019