Gas-phase carbonylation of methanol to dimethyl carbonate on chloride-free Cu-precipitated zeolite Y at normal pressure

Article abstract of DOI:10.1016/j.jcat.2006.09.009

Chloride-free Cu/zeolite Y catalysts with Cu loading of 2-14% were prepared by precipitation from aqueous copper(II) acetate solutions and inert activation with an Ar flow at 700-750 °C for 15 h. This inert activation resulted in a considerable activity of the catalyst for the oxidative carbonylation of methanol (MeOH) to dimethyl carbonate (DMC) under normal pressure at 140-160 °C at 10-12 wt% Cu loading. Space-time yields (STY) of DMC up to 100 g_DMC l^-1_Cat h^-1 were achieved with a feed composed of 36% MeOH, 48% CO, 6% O₂, and balance He at a gaseous hourly space velocity (GHSV) of 3000 h^-1. A threshold of copper loading (5-6 wt%) was found to exist before catalysts became active. This is associated with the preferential location of copper at ion-exchange positions of the zeolite structure Y not accessible for the reactants. After saturation of these sites, the placement of copper ions within the supercage led to active catalysts. Characterization of samples at various stages of preparation by N₂ adsorption, XRD, XPS, ESR, ²⁷Al-MAS-NMR, and TPR analysis revealed that the solid-state ion exchange during inert activation is accompanied by reduction of Cu²⁺ to Cu⁺. Copper ions exert a stabilizing effect on the crystallinity of the zeolite (in situ XRD, ²⁷Al-MAS-NMR). No crystalline metallic copper, cuprous oxide, or cupric oxide were formed (XRD), but melting occurred at 750 °C for catalysts with 14% copper loading, resulting in the formation of a glassy amorphous copper silicate/aluminate phase. The latter effect can be prevented by applying lower activation temperatures. The catalysts were prepared without using chloride, and the reaction did not require co-feeding of HCl for maintaining activity, as is needed for CuCl/zeolite catalyst formulations.

Full text of DOI:10.1016/j.jcat.2006.09.009

Journal of Catalysis 245 (2007) 11–24
www.elsevier.com/locate/jcat
Gas-phase carbonylation of methanol to dimethyl carbonate
on chloride-free Cu-precipitated zeolite Y at normal pressure
a,∗
a
a
a
a
b
a
M. Richter , M.J.G. Fait , R. Eckelt , M. Schneider , J. Radnik , D. Heidemann , R. Fricke
a
Leibniz Institute for Catalysis at the University of Rostock, Branch Berlin (former ACA), P.O. Box 96 11 56, D-12474 Berlin, Germany
b
Humboldt University Berlin, Faculty of Mathematics and Natural Sciences I, Department of Chemistry, Brook Taylor Str. 2, D-12489 Berlin, Germany
Received 1 March 2006; revised 9 August 2006; accepted 8 September 2006
Available online 18 October 2006
Abstract
Chloride-free Cu/zeolite Y catalysts with Cu loading of 2–14% were prepared by precipitation from aqueous copper(II) acetate solutions and
◦
inert activation with an Ar flow at 700–750 C for 15 h. This inert activation resulted in a considerable activity of the catalyst for the oxidative
◦
carbonylation of methanol (MeOH) to dimethyl carbonate (DMC) under normal pressure at 140–160 C at 10–12 wt% Cu loading. Space-time
−
1
−1
yields (STY) of DMC up to 100 g
l
h
were achieved with a feed composed of 36% MeOH, 48% CO, 6% O , and balance He at a
DMC
2
Cat
−
1
gaseous hourly space velocity (GHSV) of 3000 h . A threshold of copper loading (5–6 wt%) was found to exist before catalysts became active.
This is associated with the preferential location of copper at ion-exchange positions of the zeolite structure Y not accessible for the reactants.
After saturation of these sites, the placement of copper ions within the supercage led to active catalysts. Characterization of samples at various
stages of preparation by N adsorption, XRD, XPS, ESR, Al-MAS-NMR, and TPR analysis revealed that the solid-state ion exchange during
inert activation is accompanied by reduction of Cu to Cu . Copper ions exert a stabilizing effect on the crystallinity of the zeolite (in situ XRD,
2
7
2
2+
+
27
◦
Al-MAS-NMR). No crystalline metallic copper, cuprous oxide, or cupric oxide were formed (XRD), but melting occurred at 750 C for catalysts
with 14% copper loading, resulting in the formation of a glassy amorphous copper silicate/aluminate phase. The latter effect can be prevented by
applying lower activation temperatures. The catalysts were prepared without using chloride, and the reaction did not require co-feeding of HCl
for maintaining activity, as is needed for CuCl/zeolite catalyst formulations.
©
2006 Elsevier Inc. All rights reserved.
Keywords: Gas-phase carbonylation; MeOH; Dimethyl carbonate; Copper faujasite; Chloride-free
1
. Introduction
Synthesis routes to DMC have been reviewed by Pacheco
and Marshall [1] and by Delledonne et al. [2]. The conversion
of MeOH by phosgene is the classic but obsolete route for DMC
production,
Dimethyl carbonate (DMC) has increased in production vol-
ume because of its manifold applicability [1–4] as a monomer
in phosgene-free polycarbonate processes, as an environmen-
tally benign methylation agent, and as a possible gasoline-
blending component due to its high oxygen content, low tox-
icity, and rapid biodegradability. The latter application may be-
come attractive considering the current debate over potential
groundwater contamination by the gasoline oxygenate methyl
tert. butyl ether (MTBE).
COCl + 2CH OH → (CH O) CO + 2HCl.
2
3
3
2
(1)
A phosgene-free route is the two-stage process via methylni-
trite [5],
2
CH3OH + 2NO + (1/2)O2 → 2CH3ONO + H2O
and
CH3ONO + CO → (CH3O)2CO + 2NO.
(2)
2
(3)
This process runs in either a continuous or a batch mode over
a PdCl2/FeCl3 co-impregnated activated carbon (AC) catalyst.
A major disadvantage is the loss of chloride in form of HCl,
requiring regeneration steps and co-feeding of HCl.
*
Corresponding author. Fax: +49 30 6392 4392.
E-mail address: m.richter@catalysis.de (M. Richter).
URL: http://www.catalysis.de.
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.09.009

12
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
Alternatively, the direct oxidative carbonylation of MeOH,
CH3OH + CO + (1/2)O2 → (CH3O)2CO + H2O, (4)
measurements, XRD (in situ as well as ex situ), XPS, ESR,
Al-MAS-NMR spectroscopy, and temperature-programmed
reduction (TPR).
27
2
over a CuCl/zeolite catalyst is commercialized by ENICHEM
in a slurry phase process [1]. But again, loss of chloride is a
drawback, causing corrosion and decrement of activity. More-
over, chlorinated byproducts may affect product quality.
2. Experimental
2.1. Catalyst preparation
+
Several attempts have been reported for grafting Cu ions,
which are considered the active species on suitable supports
Preparation started with a commercial zeolite Y in its NH4
[
6–11], for a continuous gas-phase process of DMC produc-
form (Aldrich, Germany). For precipitation of Cu(OH) , a spec-
2
tion. In 1996, King [6,7] reported a catalyst preparation route
by solid-state ion exchange (SSIE) of Cu(I) into zeolite Y start-
ing from a physical mixture of CuCl or Cu2O and zeolite Y.
The maximum STY of DMC during continuous oxidative gas-
phase carbonylation of MeOH over 24% CuCl/zeolite HY af-
ified amount of dried NH -Y powder was suspended in dis-
4
tilled water and, in quick succession, a Cu(CH COO) solu-
3
2
tion (0.1 M) and a tetramethyl ammonium hydroxide solution
(10 wt%, pH = 13) were added. Thus, contact of the NH -Y ze-
4
olite and Cu(CH COO) solution for a longer time is avoided,
3
2
−
1
−1
◦
ter SSIE amounted to ca. 40 gDMC l
h
at 130 C (GHSV,
to minimize ion exchange at the acidic conditions (pH ≈ 4).
Cat
−
1
8
29 h ). Using Cu2O for SSIE resulted in less active cat-
The use of inorganic strong bases for pH increase is not pos-
+
alysts with rapid deactivation during time on stream, and a
sample made by liquid-phase ion exchange with Cu(NO3)2 so-
lution had no activity at all under the reaction conditions ap-
plied.
Other proposals to get active and stable catalysts for this
gas-phase process are numerous but do not convincingly im-
prove the STY of DMC. Anderson and Root [8,9] used zeo-
lites 13X and ZSM-5 to anchor Cu cations by SSIE starting
with a physical mixture of CuCl and the zeolite. A CuCl cat-
alyst heterogenized on diamide-immobilized copper on SBA-
sible because of competitive exchange of Na (in the case of
+
NaOH) or K (in the case of KOH) into the zeolite. Adding
ammonium hydroxide solution would lead to formation of sta-
ble copper tetraammine complexes and prevent precipitation.
Cu(OH)2 precipitated after addition of the organic base up to
◦
pH values of 8–9. Subsequent heating of the slurry to 80 C
caused a color change from blue to brown, indicating conver-
sion of Cu(OH)2 to dispersed CuO. Afterward, the slurry was
filtered, washed with distilled water three times, and dried at
◦
120 C overnight. There was no copper in the residual solution,
1
5 was reported by Cao et al. [10], and CuCl2 immobilized
so the copper content of the samples corresponded to the copper
concentration of the acetate solution.
on amino-functionalized MCM-41 and MCM-48 mesoporous
structures was used by Yang et al. [12]. Drake et al. [11]
proposed Cu/SiO2 prepared by precursor grafting (using Cu
alkoxides) or chemical vapor deposition (using CuCl) as a cat-
alyst for DMC production through gas-phase oxidative car-
bonylation of MeOH. Yang et al. [12] proposed a PdCl2–
CuCl2–CH3COOK co-impregnated AC catalyst for the gas-
phase oxidative carbonylation of MeOH with considerable STY
Samples are denoted as “synthesis forms” at this stage. Af-
◦
ter calcination at 400 C in static air for 2 h, the catalysts are
denoted as “precursors,” because all samples at this stage have
only marginal activity for the carbonylation reaction.
The activation was carried out by conducting an argon
3
−1
◦
stream (50 cm min ) through the catalyst bed at 700–750 C
for 15 h. The highest temperature required for activation de-
pends on the Cu loading. In a first series, the activation temper-
−
1
−1
h
Cat
of DMC (ca. 390 gDMC l
). However, the catalyst suf-
fered considerable deactivation already after a process time of
h. Obviously, chloride is essentially needed for stabilization
◦
ature was kept constant at 750 C (standard). A second series
6
applied lower activation temperatures for selected samples with
high Cu loading.
of active copper sites. Preparation routes using other copper
compounds (Cu2O, Cu(NO3)2) did not lead to active cata-
lysts.
Here we report a catalyst preparation route that does not
use any halogenide but nevertheless leads to catalysts with ap-
preciably high and stable STY of DMC under continuous-flow
conditions at normal pressure. The principal novelty of the ap-
proach involves the use of copper(II) salt solutions (although
Samples are designated as xxCu-Yzzz with the copper con-
tent xx in front (in wt%) and the pretreatment or activation
◦
temperature zzz as subscript of the zeolite (in C). The “syn-
◦
thesis” form (drying of the sample at 120 C overnight), the
“precursor” (calcination at 400 C in air for 2 h or 24 h), and
◦
the “activated” form (inert dry treatment under flow conditions
◦
at 700–750 C for 15 h) are differentiated. The overall exchange
+
Cu is finally required for catalysis) for preparation of a pre-
capacity of the parent zeolite Y was determined by decomposi-
tion of the ammonium form (see Section 2.2). Sample data are
summarized in Table 1.
2+
+
cursor that is activated by autoreduction of Cu to Cu under
inert conditions at elevated temperatures. The preparation is ac-
complished by removal of residual Brønsted acid sites, thus
tuning the redox system Cu2 /Cu .
+
+
2.2. Characterization
The route applied for preparing appropriate precursors con-
sists of precipitating copper hydroxide in the presence of NH4-
Y powder suspended in distilled water. The copper content was
varied over a range of 2–14 wt%. Interpretation of the catalytic
results is based on characterization data from N2 adsorption
Surface areas and pore volumes were determined from ni-
trogen adsorption isotherms at 77 K using the ASAP 2010M
characterization unit (Micromeritics). Samples were outgassed
◦
at 400 C for 2 h under vacuum; the surface area was calculated

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
13
Table 1
Sample characteristics
Ex situ X-ray diffraction measurements were carried out
using a STADI P (STOE) setup with Debye-Scherrer geome-
try and a Ge primary monochromator with CuKα1 radiation.
Precursor
SBET
VP
Catalyst
NH -Y
SBET
(m g
VP
2
−1
3
−1
2
−1
3
−1
)
◦
◦
(m g
)
(cm g
0.35
0.33
0.31
n.d.
)
)
(cm g
0.06
0.21
0.31
0.24
0.23
The XRD patterns were scanned in the 2θ range of 6 –60
◦
NH -Y
927
856
796
n.d.
n.d.
734
133
543
778
597
564
(step width, 0.5 ; 100 s per step) and recorded with a position-
sensitive detector (PSD). Data interpretation was carried out us-
ing the WinXpow software (STOE) and the Powder Diffraction
File database of the International Centre of Diffraction Data
4
120
4
750
2
4
6
8
1
Cu-Y
2Cu-Y
4
4
4
4
00
00
00
00
750
750
750
750
Cu-Y
Cu-Y
Cu-Y
4Cu-Y
6Cu-Y
8Cu-Y
a
(
ICDD). A Theta/Theta diffractometer (Seifert/FPM) with a
n.d.
Bühler HDKS1 heating chamber was used for the temperature-
programmed in situ investigations.
0Cu-Y
0.29
10Cu-Y
750
576
635
680
0.23
0.26
0.27
400
400
400
1
1
0Cu-Y
0Cu-Y
7
25
00
In situ X-ray powder patterns were scanned with CuKα ra-
diation at normal atmosphere in a temperature range of 150–
7
1
1
2Cu-Y
702
719
0.28
0.28
12Cu-Y
266
600
609
0.11
0.24
0.24
750
◦
1
1
2Cu-Y
2Cu-Y
850 C. The patterns were recorded in Bragg–Brentano geome-
7
25
00
7
try with a scintillation counter.
4Cu-Y
a
14Cu-Y
2
0.00
The XPS data were recorded with a VG ESCALAB 220iXL
unit using AlKα radiation (E = 1486.6 eV) at a base pressure
750
n.d.—not determined.
−
7
of the UHV chamber of ca. 1 × 10 Pa. The spectra were ref-
erenced to the Si2p peak of SiO2 at 103.3 eV. Peak position and
area were determined after satellite and background subtrac-
tion and fitting with Gaussian–Lorentzian curves. The amount
of a component and thus the composition of the near-surface
region was determined from the peak area after division by the
element-specific Scofield factor and the analyzer-specific trans-
mission function.
using the BET equation. Values used throughout the paper are
not corrected for the percentage of copper.
Temperature-programmed decomposition of the ammo-
nium zeolite form was performed in a He flow (flow rate,
3
−1
3
0 cm min ) heating 0.3 g of NH4-Y with a rate of 10 K
−
1
◦
min up to 600 C and maintaining the sample isothermally at
this temperature until the ammonia concentration reached the
baseline. The ammonia gas-phase concentration was recorded
continuously by a thermal conductivity detector (TCD). Before
detection, the gas flow was conducted through a KOH trap to re-
move desorbed water. The concentration of Brønsted acid sites
was calculated on the basis of the desorbed ammonia. In addi-
tion, desorbed ammonia was conducted through 0.05 N H2SO4
and back-titrated with 0.05 N NaOH after completion of the
run.
ESR spectra were recorded on a Bruker ELEXSYS 500-
1
0/12 spectrometer in X-band (about 9.5 GHz) at 293 K (room
temperature) and 77 K (liquid nitrogen temperature) after in situ
evacuation at room temperature for 1 h. The magnetic field was
measured with respect to a DPPH standard.
Solid-state 27Al-MAS-NMR spectroscopy was carried out
on a Bruker AVANCE 400 spectrometer at 104.27 MHz. The
2
7
Al-MAS-NMR spectra were obtained at a spinning speed of
2 kHz using a Bruker 4-mm MAS probe. All experiments were
1
TPR measurements were performed applying an AMI-100
catalyst characterization system (Zeton/Altamira) equipped
with a TCD for monitoring the H2 consumption from a feed
performed using single-pulse excitation with a pulse width of
µs (π/6-pulse) and an accumulation of 128 scans with a re-
laxation delay of 5 s. The isotropic chemical shifts, reported in
ppm, are relative to an external sample of 1.0 M AlCl3.
1
◦
stream containing 5 vol% H2 in Ar. TPR was started at 50 C
−
1
with a heating rate of 10 K min . One TPR run was carried out
with the precursor sample 10Cu-Y400 (sample weight, 0.06 g)
2.3. Catalytic measurements
without further pretreatment up to a maximum temperature of
◦
8
00 C. A second TPR run was taken from a fresh precursor
Catalytic measurements were performed in a parallel 16
channel flow microreactor system (Fig. 1) at normal pressure
◦
sample after inert in situ activation at 750 C with dry argon
flow for 10 h, and a third run was taken from a fresh sample
activated according to the second run but reoxidized in situ at
with a feed composed of 36% MeOH, 48% CO, 6% O , 1% Ar
2
(internal standard), and balance He. The GHSV amounted to
◦
3
−1
−1
5
00 C with air (flow rate, 50 cm min ) for 1 h. Results are
3000 h . MeOH was fed into the main stream by a syringe
graphically presented in form of H2 consumption (in arbitrary
units) versus temperature, the sample weight was normalized
to 0.1 g. Calibration of the peak areas of the TPR profiles was
performed according to the standard routine by introducing ar-
gon pulses into the 5 vol% H2/Ar flow. Average values from
seven pulses were taken to determine the calibration factor.
However, discussion is confined to qualitative features of the
TPR profiles, because already the precursor sample 10Cu-Y400
via an evaporator. Analysis was carried out by an on-line cou-
pled HP GCD system G18000 C (Agilent Technologies). Sep-
aration of the MeOH, dimethylether (DME), methylformate
(MF), dimethoxymethane (DMM), and DMC components was
achieved on a capillary column (HP-5MS, 30 m × 0.25 mm ×
◦
25 µm, split 25:1, carrier He) at 40 C. Detection was performed
by MS working in the single-ion mode (SIM) using m/e = 45
for identification.
+
contained small amounts of Cu ions (vide XPS), and the re-
MeOH conversion (XMeOH) was calculated from the prod-
uct balance taking into account the formation of DMC, DMM,
DME, MF (neither formaldehyde, formic acid, or any further
◦
duction was not completed at 800 C. These two peculiarities
do not allow a clear balance of the H2 consumption.

14
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
Fig. 1. Experimental setup for oxidative carbonylation of MeOH at normal pressure.
3
−1
C-containing products were detected), according to the stoi-
chiometry of the following reactions:
Maximum pore volumes amounted to 0.35 cm g , whereas
the dense phase had practically no pore volume at all.
Further data included in Table 1 for samples with 10 and
3
CH3OH + (1/2)O2 → (CH3O)2CH2 + 2H2O (DMM), (5)
CH3OH → (CH3)2O + H2O (DME),
1
2 wt% Cu loading demonstrate that a higher surface area was
retained when an activation temperature below 750 C was cho-
sen. This is particularly apparent for samples 12Cu-Y750 and
2Cu-Y725, where, after activation at 700 or 725 C, a surface
area of 609 or 600 m g was found, respectively, compared
with only 266 m g after activation at 750 C. Thermoana-
lytical investigation of sample 12 Cu-Y400 (not shown) found a
weak exothermic peak between 725 and 750 C. Taking into ac-
count that the thermoanalytical measurement was relatively fast
◦
2
(6)
and
◦
1
2
−1
2
CH OH + O → HCOOCH + 2H O (MF).
(7)
3
2
3
2
2
−1
◦
The product selectivities were calculated referring the particular
product concentration at the reactor outlet to the percentage of
MeOH converted, and the STY of DMC is given as grams of
◦
−
1
−1
).
DMC per liter of catalyst per hour (gDMC l
h
Cat
−1
(
temperature ramp 10 K min vs 15 h of isothermal standard
activation treatment), this exothermic peak is taken as indicat-
ing the start of thermal breakdown of the zeolite structure.
This underlines the fact that a balance must be found be-
tween the Cu loading and the appropriate activation temper-
ature. In fact, the thermal stability of the modified Y zeolite
depends on the Cu loading. Up to 10 wt%, the highest activa-
tion temperature possible (leaving the zeolite structure intact)
3
3
3
. Results
.1. Characterization
.1.1. N2 adsorption measurements
BET surface areas (SBET) of the precursors (not corrected for
2
−1
the Cu loading) indicate a systematic decrease from 927 m g
◦
2
−1
was near 750 C. Further enhancement of the Cu content re-
(
NH4-Y120) to 719 m g for 14Cu-Y400 (Table 1). Activation
◦
stricted the maximum temperature of activation as formation of
the dense copper silicate phase was accompanied by destruction
of the zeolite structure.
at standard temperature (750 C) resulted in a further loss of
surface area, depending on the Cu loading. The Cu-free NH4-Y
zeolite was nearly completely destroyed at 750 C; the surface
area was as low as 133 m g . Surprisingly, copper loading of
only 2 wt% maintained a surface area of 543 m g after acti-
vation (sample 2Cu-Y750, cf. Table 1), and at higher Cu loading
◦
2
−1
N adsorption isotherms are of type I, characteristic for mi-
2
2
−1
croporous zeolites. The type of isotherm is preserved during
the preparation steps (precursor and activated sample) with no
indication of mesoporosity, as shown for samples NH4-Y400,
(
up to 10 wt%), the resulting surface areas had an average loss
◦
1
0Cu-Y400, and 10Cu-Y750 in Fig. 2.
of about 20% after activation at 750 C compared with the cor-
responding precursors. However, copper loadings >10 wt% led
2
−1
to a dramatic decrease in surface area, reaching 2 m g at
4 wt% Cu. A dense phase was formed, characterized by a
3.1.2. Temperature-programmed conversion of NH4-Y to H-Y
Fig. 3 shows the profile of NH3 release from the parent
NH4-Y zeolite during temperature-programmed inert heating.
1
dark-brown color (intact samples are blue-green after activa-
tion). Pore volumes showed the same patterns, because they
were derived from the same experimental N2 uptake curves.
◦
NH3 release occurred between 200 and 500 C with a maxi-
◦
◦
mum centered at 278 C and a shoulder located around 385 C.

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
15
Fig. 3. Profile of NH release (arbitrary units) from NH -Y dur-
3
4
−
1
ing temperature-programmed inert heating. Heating rate 5 K min
sample weight 0.3 g, 30 cm min He flow.
,
Fig. 2. N adsorption isotherms for (1) NH -Y
(2), (2) precursor 10Cu-
3
−1
2
4
400
Y
("), and (3) activated sample 10Cu-Y
(Q).
400
750
Fig. 4. XRD patterns of samples 2Cu-Y
zoomed view of the 2θ range 5.5 –7 .
(1), 4Cu-Y
(2), 6Cu-Y
(3), 8Cu-Y
(4), 10Cu-Y
(5), 12Cu-Y
(6), and 14Cu-Y
(7). The inset shows a
750
750
750
750
750
750
750
◦
◦
The overall concentration of Brønsted acid sites amounted to
ing activation of sample 14Cu-Y400, but without formation of
any crystalline Cu-containing phase.
−1
2
.62 mmol g . This is equivalent to the exchange capacity
−
1
+
Cu
of the zeolite and allowed fixation of 2.62 mmol g
Two features of the XRD patterns need to be explained:
(i) the increased intensity of zeolitic diffraction lines associ-
ated with a smaller full width at half maximum (FWHM) of
the peaks, and (ii) the shift of the diffraction lines to lower 2θ
values with increasing copper content (cf. inset to Fig. 4).
Changes in intensity are associated with the degree of crys-
tallinity, whereas FWHM reflects variations in particle size.
Qualitatively, it can be concluded that the crystallinity of the
zeolite was poor for samples 2Cu-Y750 and 4Cu-Y750 but im-
proved with higher Cu loading up to 10 wt% (sample 10Cu-
Y750). Crystallinity of sample 12Cu-Y750 deteriorated, and
melting of the precursor 14Cu-Y400 during the activation proce-
dure destroyed the zeolite structure completely. This conclusion
−
1
2+
(
16.7 wt% Cu) or 1.31 mmol g Cu (8.4 wt%), assuming
strict electroneutrality.
3
.1.3. X-ray diffraction
X-ray diffraction patterns of the activated catalyst samples
(
2–14 wt% Cu) are shown in Fig. 4. All diffraction lines are
attributable to the zeolite structure Y. The samples contain nei-
ther crystalline copper oxides nor metallic copper. It can be seen
that the intensity of zeolite diffraction lines was low for samples
2
8
1
Cu-Y750 and 4Cu-Y750 but increased for samples 6Cu-Y750,
Cu-Y750, and 10Cu-Y750. Decreased intensity is observed for
2Cu-Y750. A breakdown of the zeolite structure occurred dur-

16
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
Fig. 5. Unit cell dimension of zeolite Y in dependence on Cu loading. (!) NH -
4
Y
from in situ XRD (cf. Fig. 6).
Fig. 6. In situ XRD. Unit cell constant of NH -Y
(!) and sample 2Cu-Y
120 120
400
4
(") vs temperature under ambient air.
is supported by the observed dependence of the surface area
on the Cu loading for the activated samples (cf. Table 1). In-
tact crystallinity of the zeolite is characterized by high surface
area and pore volume. The up and down of the crystallinity ver-
sus Cu loading as documented by XRD measurements for the
activated samples is fairly reflected by the surface areas/pore
volumes.
Smaller FWHM values correspond to larger crystallite sizes.
Whether larger crystallites were built during the preparation
process is not clear. But the variation of the FWHM of diffrac-
tion peaks paralleled the changes in intensity of the diffraction
lines and obviously was associated with the modification of ze-
olite crystallinity, as discussed earlier. Altogether, it is evident
that introduction of copper into the zeolite enhanced the re-
sistance against thermal deterioration up to a borderline value
that depends on the Cu loading and the activation tempera-
ture.
◦
Further temperature increase above 600 C initiated a strong
decrease in the unit cell dimension that might be attributed to
the thermal dehydroxylation of Brønsted acid sites with forma-
tion of Lewis acid sites. But simultaneously, the crystallinity of
the zeolite was destroyed, as followed from the low BET sur-
face area of the Cu-free NH4-Y120 zeolite after “activation” at
◦
750 C (cf. Table 1).
Sample 2Cu-Y120 had nearly the same unit cell dimension as
◦
the NH4-Y120 zeolite at 150 C (a = 24.66 Å vs a = 24.67 Å).
+
During further heating, replacement of NH ions (ion radius
4
1.43 Å) by Cu2 ions (radius 0.72 Å) occurred, accompanied
+
2+
+
by unit cell contraction. Reduction of Cu to Cu (ion ra-
dius 0.96 Å) was of minor importance, because an air flow was
used during the measurements. At higher temperatures (650–
850 C), the unit-cell dimension of sample 2Cu-Y1 was larger
than that of the Cu-free zeolite. Obviously, exchanged copper
ions prevent further unit cell contraction.
◦
20
The shift in 2θ values indicates characteristic changes of
unit cell parameters of the zeolite Y depending on the cop-
per content of the activated samples (Fig. 4, inset). A detailed
view is shown in Fig. 5. Zeolite NH4-Y (unit cell composi-
tion (NH4)58[Al58Si134O384]·240H2O) crystallizes in a cubic
space group with the lattice constant a = 24.645 Å [13]. The
Cu-free sample NH4-Y400 revealed a cell constant of 24.62 Å.
3.1.4. 27Al-MAS-NMR spectroscopy
Results of 27Al-MAS-NMR spectroscopy are shown in
120
zeolite had practically no octa-
hedrally coordinated Al (not shown). Calcination of NH -Y
Fig. 7. The parent NH -Y
4
4
◦
at 400 C in static air (to simulate the precursor state) led
◦
After thermal pretreatment of the ammonium form at 400 C
to dealumination with the characteristic signal of nonframe-
work Al at −1 ppm. However, the preponderant part of the Al
was on tetrahedrally coordinated framework positions (signal at
60 ppm). Penta-coordinated Al (signal at around 30 ppm) was
not present. Sample 2Cu-Y750 exhibited a part of nonframe-
work Al at 2 ppm, but this low percentage was further decreased
with higher Cu loading until exclusively tetrahedrally coordi-
nated framework Al was seen at 6 wt% Cu loading (sample
6Cu-Y750). This underlines the conclusions drawn from results
of XRD measurements. Copper ions fixed to the zeolite frame-
work improved the resistance of the zeolite framework against
thermal deterioration. Further correspondence between results
of XRD measurements and 27Al-MAS-NMR spectroscopy was
observed for samples 12Cu-Y750 and 14Cu-Y750. The destruc-
tion of the zeolite was also evident from the NMR spectra,
(
to simulate the precursor state), the zeolite was in a mixed
NH4-H form, and the unit cell size was correspondingly lower.
A contraction of unit cell size is known to occur during tran-
sition from NH4-Y to H-Y [14]. The zeolite unit cell size
was increased with increasing Cu loading (activated samples).
A nearly linear increase can be seen for samples 2Cu-Y750,
4
Cu-Y750, and 6Cu-Y750, whereas for samples 6Cu-Y750, 8Cu-
Y750, 10Cu-Y750, and 12Cu-Y750, the unit cell size of the zeo-
lite increased only marginally.
Subjecting zeolite NH4-Y120 a stepwise temperature in-
◦
crease by 100 K from 150 to 850 C clearly confirms that
+
+
replacement of NH ions by H during the thermal decompo-
sition of NH within the temperature range from 300 to 550 C
4
+
◦
4
(cf. Fig. 3) was accompanied by a unit cell contraction (Fig. 6).

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
17
because the signal of Al was asymmetrically broadened and
shifted to lower values. Most significant, for sample 14Cu-Y750,
aluminium was in a new environment due to the amorphous
phase formed (vide XRD).
3
.1.5. X-ray photoelectron spectroscopy
The normalized Cu2p spectra of selected samples with 2 and
0 wt% Cu loadings are shown in Figs. 8a and 8b, respectively.
1
A typical fingerprint with four peaks in the Cu spectrum indi-
2
p
2+
cating Cu is seen for sample 10Cu-Y120. The transition from
2
+
+
Cu to Cu species can be traced by the intensity of satel-
lite bands and by the chemical shift of the Cu2p3/2 peak. Thus,
Cu2 obviously was reduced during precursor preparation and
+
sample activation. These reduced species can clearly be iden-
+
tified as Cu due to the kinetic energy of the Cu LMM Auger
+
electrons (Table 2). The energies were lower for Cu than for
0
2+
2+
+
Cu or Cu . Al/Si ratios, Cu /Cu percentages, and Cu/(Al
Si) ratios in the near-surface region (the outermost 50–100 Å
of the samples) are summarized in Table 3. Samples 2Cu-Y
+
120
(synthesis form), 2Cu-Y400 (precursor), and 2Cu-Y750 (acti-
vated catalyst) show characteristic differences of the Cu valence
states and Al/Si ratios compared with corresponding samples
with 10 wt% Cu loading.
Samples 2Cu-Y120, 2Cu-Y400 and 2Cu-Y750 had nearly 50,
+
7
1
0, and 100% Cu , respectively, whereas samples 10Cu-Y120,
+
0Cu-Y400 and 10Cu-Y750 contained 0, 35, and 59% Cu , re-
2
7
Fig. 7. Al-MAS-NMR spectra (only the central range is displayed) of
NH -Y zeolite and of catalyst samples 2Cu-Y , 4Cu-Y , 6Cu-Y
spectively.
The synthesis form of samples exhibited a higher Cu/(Al +
Si) surface ratio than the precursors and activated catalysts, best
,
750
4
400
750
750
8
Cu-Y , 10Cu-Y , 12Cu-Y , and 14Cu-Y
(from bottom to top).
750
7
50
750
750
Fig. 8. XPS of samples 2Cu-Y , 2Cu-Y
and 2Cu-Y
(a), and of samples 10Cu-Y , 10Cu-Y , 10Cu-Y
and 10Cu-Y
(b).
750
1
20
400
750
120
400
400(24)

18
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
Table 2
XPS results of sample 10Cu-Y
Electron binding energy
Kinetic energy
Cu LMM
(eV)
Cu
2
p3/2
+
2+
Cu
Cu
Sample
1
1
1
1
0Cu-Y
934.4
935.2
935.2
935.4
916.6
914.4
914.4
913.7
1
4
4
7
20
0Cu-Y
0Cu-Y
0Cu-Y
932.5
932.6
932.7
00(2)
00(24)
50
References
0
Cu
932.7
932.5
933.6
918.6
916.2
918.1
Cu O
2
CuO
Fig. 9. TPR profile of sample 10Cu-Y
and 10Cu-Y750 after reoxidation in situ at 500 C with air for 1 h (3). H2 con-
sumption in arbitrary units, normalized to 0.10 g sample weight.
(1), 10Cu-Y
(activated in situ) (2),
750
400
Table 3
◦
2+
+
XPS data. Al/Si ratios, percentages of Cu /Cu and overall Cu/(Al + Si)
ratios
2
+
+
Sample
Al/Si
ratio
Cu
Cu
Cu /
2p
3
.1.6. TPR by H2
(Al + Si2s
)
2s
For demonstration how the activation procedure affects the
2Cu-Y
2Cu-Y
2Cu-Y
0.40
0.29
0.27
49
30
0
51
70
100
0.02
0.00
0.01
1
4
7
20
00
50
◦
state of reducible copper species, the inert activation at 750 C
was followed in situ, starting with the precursor 10Cu-Y400.
Results are shown in Fig. 9. Reduction of sample 10Cu-Y400
1
1
1
1
0Cu-Y
0Cu-Y
0Cu-Y
0Cu-Y
0.64
0.61
0.54
0.58
0.43
100
64
52
0
36
48
59
0.29
0.05
0.05
0.03
1
4
4
7
20
◦
occurred at about 180 C and the reduction proceeds in one step
00
◦
◦
centered at 255 C. (A faint shoulder at ca. 200 C can be distin-
guished on the low-temperature edge of the main peak.) After in
situ inert activation of a fresh sample 10Cu-Y400, no reducible
00(24)
41
50
a
Bulk ratio
a
◦
From ICP OES.
species were present up to 800 C. Inert activation of a further
sample 10Cu-Y400 and in situ reoxidation of the activated sam-
ple restored the initial TPR profile of the precursor only partly
(curve 3), because the main reduction peak at 255 C composed
seen for 10 wt% Cu loading. Already the preparation of the
precursors led to depletion of Cu from the outer surface. This
effect was further pronounced for the activated sample 10Cu-
◦
a significantly lower amount of reducible species than observed
for the precursor. Additional reduction peaks emerge at 220,
Y750. This means that migration and reduction of Cu2 occur
already during precursor preparation, but was further enhanced
during the activation process.
+
◦
470, and 770 C. The reduction onset was shifted to lower tem-
◦
peratures (ca. 150 C).
+
2+
The presence of Cu in sample 2Cu-Y120 is explained by
The reduction of Cu in Cu-Y zeolites occurs by a two-step
2+
+
a shift of the surface equilibrium between Cu and Cu in
the presence of a high proton concentration with preferential
stabilization of Cu [15],
mechanism [16],
2+
+
+
2
Cu + H2 ↔ 2Cu + 2H
(9)
+
and
+
+
−
+
[
CuOH] + H + e ↔ Cu + H2O.
(8)
+
0
+
2
Cu + H2 ↔ 2Cu + 2H .
(10)
+
The presence of Cu in the precursor samples despite the ox-
idative treatment may be due to reduction of Cu by ammonia
resulting from thermal decomposition of NH ions of the zeo-
2+
The reduction of Cu ions located at ion-exchange positions of
the zeolite to Cu and of dispersed bulk CuO to Cu occurred
in the same temperature region (200–300 C) and was difficult
to separate. Cu ions were stabilized on the zeolite and reduced
2+
+
0
+
◦
4
+
lite. However, it must be noted that the UHV conditions during
XPS also can initiate reduction of Cu2 ions.
+
at distinctly higher temperatures. The ease of reduction depends
on the location of the cations, in the order supercage > sodalite
cage > hexagonal prism [17].
The Al/Si molar ratio revealed a depletion of Al on the sur-
face (Al/Si ratio lower than the bulk ratio) for samples with
2
wt% Cu, along with enrichment of Al on the surface for the
The following conclusions can be derived from the TPR pro-
files:
sample with 10 wt% Cu (Al/Si ratio higher than the bulk ra-
tio). Obviously, some Al was solved during preparation by the
acetate solution at low Cu content, whereas precipitation of
2+
(i) The precursor sample 10Cu-Y400 contains Cu whose
◦
reduction occurs between 180 and 290 C (maximum,
1
0 wt% Cu shielded the surface of the zeolite crystallites and
◦
+
0
prevented dealumination. Whether a gradient of Al concentra-
tion within the zeolite crystallites was present a priori is not
clear.
255 C). A reduction of Cu to Cu occurs at tempera-
◦
tures above 800 C (outside the experimental temperature
range).

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
19
Fig. 10. ESR spectra of samples 2Cu-Y
temperature for 1 h.
and 2Cu-Y
(a), and of samples 12Cu-Y
and 12Cu-Y (b) recorded at 77 K after in situ evacuation at room
750
400
750
400
(
ii) After inert thermal treatment, practically no reducible
Table 4
2+
+
ESR parameters of samples 2Cu-Y and 12Cu-Y
Cu species were present, and reduction of Cu occurred
outside the temperature range (onset of H2 consumption
a
*
Sample
Signal A
Signal B
Signal B
◦
occurred above 700 C).
g
ꢀ
A
g
⊥
g
ꢀ
A
ꢀ
g
g
ꢀ
A
ꢀ
g
⊥
ꢀ
⊥
(
iii) Redistribution of copper ions occurred during activation,
because reoxidation cannot restore the TPR pattern of the
2Cu-Y
2.328 166.7 2.038 2.376 126.0 2.066
2.328 166.7 2.038 2.376 126.0 2.066
4
00
50
2Cu-Y
7
+
precursor. Furthermore, not all Cu ions are reoxidized to
12Cu-Y400
12Cu-Y
2.396 132.0 2.050
2.396 132.0 2.05
2+
◦
Cu , because the H2 consumption centered at 770 C in-
dicated reduction of Cu after reoxidation. Obviously, the
reducibility of Cu was now easier, because inert activa-
750
+
a
ESR spectra were recorded at 77 K after evacuation of the samples for 1 h
at room temperature; bold values indicate main signal.
+
tion removed Brønsted acid sites through dehydroxylation,
and thus reaction (10) was shifted to the right side.
2+
ues are collected in Table 4. All four samples contained Cu
species in square-pyramidal or distorted octahedral coordina-
tion according to the ESR parameters [21]. In addition, after
3
.1.7. ESR
2+
9
Cu ions are 3d paramagnetic ions that are readily ob-
◦
2+
inert vacuum treatment at 500 C, a part of Cu was retained
not shown). However, the ESR signal intensity decreased due
+
10
served by ESR, whereas Cu ions are diamagnetic (3d ) and
thus ESR-silent [18]. The ESR spectra of copper-loaded zeolite
Y were characterized by at least two overlapping axial signals at
(
2+
+
to reduction of Cu to Cu or to formation of ion pairs.
A comparison of spectra for samples 2Cu-Y400 and 2Cu-
Y750 (Fig. 10a) reveals changes in the ESR parameters (cf.
Table 4). Whereas signal A decreased drastically in intensity,
(
ꢀ
1)
(2)
ꢀ
g
= 2.36–2.41 and g = 2.30–2.34. As such, they reflected
2
+
the type of coordination of Cu in the structure. However, the
relative intensities of the two ESR signals varied with the Si/Al
ratio, type of co-cation, and copper loading and were indicative
signal B increased in intensity, showing a higher g value and
ꢀ
a distinctly lower hfs splitting Aꢀ. The overall intensity de-
2
+
of the site preference of Cu [19]. Most authors assign the two
creased slightly as a result of sample activation; that is, the
ESR signals to Cu2 ions located on different cationic sites or
+
concentration of Cu2 was diminished during activation, thus
+
in different Al surroundings [20].
confirming the autoreduction process qualitatively. According
ESR spectra of samples 2Cu-Y400, 2Cu-Y750, 12Cu-Y400,
to Yu and Kevan [22] the g tensor at 2.33 indicates the pres-
ꢀ
and 12Cu-Y750 are shown in Fig. 10. According to XPS, sample
2
Cu . This may be related to the different surface sensitivity
of these two methods. (XPS is sensitive for the near-surface re-
gion, whereas ESR is a bulk method.) Reduction of some Cu
to Cu in the UHV under X-ray irradiation during the XPS
measurements is also possible. Another reason could be the low
signal-to-noise ratio of the XP spectrum of sample 2Cu-Y750,
which does not allow resolution of the Cu2 features.
ence of Cu2 in hexagonal prisms of zeolite Y (site type I), and
+
+
Cu-Y750 contained exclusively Cu , whereas ESR detected
2+
2+
the gꢀ tensor at 2.39 was assigned to Cu in an almost planar
ꢁ
trigonal coordination in the six-membered ring site I . Because
ꢁ
2+
site types I and I are close to each other and could not be oc-
+
cupied simultaneously [23], the change in positions could be
27
attributed to dealumination, which was shown by Al-MAS-
NMR to proceed during activation of samples with low Cu
loading. This dealumination seemed to modify the surround-
+
2+
A comparison of ESR spectra (Fig. 10) from samples 2Cu-
Y400 and 12Cu-Y400 shows that at only 2 wt% Cu, the hyperfine
splitting (hfs) of the spectra was sufficiently resolved to obtain
ing of Cu and promote a change of position and coordination
of Cu2 ions.
+
2+
Under the same conditions, the Cu signal of sample 12Cu-
Y400 was characterized by a weak hfs resolution (Table 4, sig-
detailed information on the Cu2 ion state. The g and hfs val-
+

20
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
Fig. 11. Catalytic results at normal pressure, (a) MeOH conversion, (b) DMC selectivity, (c) DME selectivity, (d) DMM selectivity, (e) STY of DMC. Feed
−
1
composition 36% MeOH, 48% CO, 6% O , 1% Ar (He balance), GHSV 3000 h
2
.
*
nal B ). The estimated parameters differed only slightly from
3.2. Catalysis
those of signal B, possibly due to the low relative intensity of
the hfs signal. The signal was overlapped by an isotropic strong
Conversion of MeOH, selectivities of the main products
DMC, DMM, and DME, and the STY of DMC in dependence
on Cu loading are shown in Figs. 11a–11e for three selected
reaction temperatures: 140, 150, and 160 C. It follows from
Fig. 11a that a threshold of 6 wt% Cu loading must be passed
to achieve noticeable conversion of MeOH after inert activation.
2+
signal S, which was clearly caused by interacting Cu species
(
possibly Cu2 –O–Cu pairs). Sample 12Cu-Y750 exhibited
+ 2+
◦
decreased signal intensity by about one order of magnitude
compared with the precursor (12Cu-Y400) due to reduction of
2
+
Cu species during inert activation.

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
21
◦
◦
◦
◦
◦
◦
Fig. 12. Influence of activation temperature (750 C (Q), 725 C ("), 700 C
2)) on the catalytic behavior of sample 10Cu-Y . DMC selectivity ((- - -),
Fig. 13. Influence of activation temperature (750 C (Q), 725 C ("), 700 C
(2)) on the catalytic behavior of sample 12Cu-Y400. DMC selectivity ((- - -),
left) and STY of DMC ((—), right). Reaction conditions: feed composition
(
400
left) and STY of DMC ((—), right). Reaction conditions: feed composition
6% MeOH, 48% CO, 6% O , 1% Ar (He balance), GHSV 3000 h . Normal
−
1
−1
3
36% MeOH, 48% CO, 6% O2, 1% Ar (He balance), GHSV 3000 h . Normal
2
pressure.
pressure.
Maximum MeOH conversions of 9.0–11.5% were achieved at a
Cu loading of 12 wt%. At a Cu loading of 14 wt% and activation
◦
at 750 C, all activity was lost, due to the complete breakdown
of the zeolite structure as shown in Sections 3.1.1 and 3.1.3.
Selectivity to DMC (Fig. 11b) monotonically increased with
Cu loading and attained maximum values of 50–60% (sample
◦
1
2Cu-Y750) at reaction temperatures of 160 and 140 C, respec-
tively. This increase was mirrored by a corresponding decrease
in DME selectivity (Fig. 11c). DME formed nearly exclusively
over sample 2Cu-Y750, but its formation was progressively sup-
pressed at higher Cu loading, reaching values of 2–10% for
sample 12Cu-Y750. At a fixed Cu loading, the selectivity of
DME increased with higher reaction temperature. Two possible
routes of DME formation must be considered: (i) decomposi-
tion of DMC [24] and (ii) dehydration of MeOH. Anderson et
al. [24] found that the decomposition of DMC was highest on
H-Y but decreased with increasing Cu loading. These authors
concluded that the decomposition of DMC is an acid-catalyzed
reaction step. This conclusion is reconcilable with experimental
findings.
Fig. 14. Selectivities of DMC, DMM and DME vs temperature for sample
10Cu-Y . Reaction conditions: feed composition 36% MeOH, 48% CO,
6% O2, 1% Ar (He balance), GHSV 3000 h . Normal pressure.
750
−
1
that lower activation temperatures of precursor 10Cu-Y400 led
to lower DMC selectivities and STY values.
The dehydration of MeOH to DME is considered of minor
◦
importance at temperatures below 200 C according to accepted
In contrast, milder activation conditions had a benefi-
cial effect on the performance of sample 12Cu-Y (Fig. 13).
knowledge [25]. DMM selectivity passed through a maximum
at a Cu loading of 10% (Fig. 11d). Space-time yields of DMC
◦
When activated at 725 or 700 C, samples 12Cu-Y725 and
−
1
−1
h
Cat
revealed a maximum value of 55 gDMC l
1
for sample
12Cu-Y700 achieved STY values of DMC of approximately
−
1
−1 ◦
at 160 C despite the slightly lower DMC
◦
0Cu-Y750 at a reaction temperature of 160 C (Fig. 11e). For-
100 gDMC l
selectivity.
h
Cat
mation of MF was not observed.
This improvement is obviously due in part to the higher ac-
Based on the observation that the appropriate temperature
of inert activation depended on the copper loading, the ques-
tion whether the fixed activation routine at 750 C is adequate
2
−1
◦
cessible surface area, 266 m g after activation at 750 C and
2
−1 ◦
◦
6
09 m g after activation at 700 C (cf. Table 1). The de-
creased DMC selectivity at higher temperatures is attributable
to its thermal decomposition,
or not was addressed for samples 10Cu-Y and 12Cu-Y. Thus,
for precursors 10Cu-Y400 and 12Cu-Y400, two lower activation
temperatures—725 and 700 C—were applied for activation.
◦
(CH3O)2CO → (CH3)2O + CO2.
(11)
Catalytic results are presented in Figs. 13 and 14 for samples
1
0Cu-Y and 12Cu-Y, respectively, including results for sam-
This leads to the observed reversed pattern of DMC/DME
selectivity versus reaction temperature, as shown for sample
ples 10Cu-Y750 and 12Cu-Y750 for reference. Fig. 12 shows

22
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
monia [27]. Compared with SSIE starting from a physical mix-
ture of copper salts and zeolites, the dispersed state achieved
through the precipitation technique facilitates deaggregation of
copper oxide.
+
During inert activation, Cu is formed (the samples are grey-
white). TPR (after in situ activation without intermediate influx
of oxygen) of sample 10Cu-Y750 could not detect any further
2+
Cu ions, but XPS (taken after intermediate storage of the
activated sample at ambient atmosphere) detected nearly 40%
2+
Cu . This proves that contact of activated samples with ambi-
ent air (containing oxygen and moisture) at room temperature
+
2+
leads to oxidation of Cu and formation of Cu hydroxyl
+
2+
species ([CuOH] ). These Cu species are more easier re-
ducible than before (shoulder at 220 C in the TPR profile of
◦
sample 10Cu-Y750, reoxidized in situ). It should be mentioned
that reduction of [Cu–O–Cu]2 oxo-cations in zeolite ZSM-5
+
was reported to occur already at room temperature [29,30]. Be-
◦
cause TPR measurements started after equilibration at 50 C no
H2 consumption balance could be made. Therefore, the possi-
bility that formation of [Cu–O–Cu]2 oxo-cations occurred at
+
◦
at 150 C and nor-
Fig. 15. Time-on stream performance of sample 10Cu-Y
750
higher Cu loading could not be excluded.
mal pressure. Feed composition 36% MeOH, 48% CO, 6% O , 1% Ar (He
2
After the activation procedure, the copper was finely dis-
persed; XRD did not reveal any crystalline copper oxide phases
or metallic crystalline copper. Even the totally amorphous sam-
ple 14Cu-Y750 gave no reflection pattern.
−
1
balance), GHSV 3000 h . STY of DMC (left), MeOH conversion X
,
MeOH
and DMC selectivity S
(right).
MDC
1
0Cu-Y750 in Fig. 14. Dehydration of MeOH to DME, as well
as total oxidation of MeOH to CO2, played no decisive role
under these conditions, especially in the temperature range cho-
Thermal dehydroxylation of Brønsted acid sites can be ex-
pected to accompany the SSIE at 700–750 C, with formation
◦
−
1
−1
h at a reaction temperature
Cat
sen. An STY of ca. 60 gDMC l
of Lewis acidity. Lewis acid sites once formed at temperatures
◦
◦
of 150 C could be maintained over 100 h without significant
as high as 750 C were not reconverted into Brønsted acid sites
deactivation for sample 10Cu-Y750 (Fig. 15).
by adding water [28]. The concentration of protons influenced
the stability of Cu2 /Cu by the existing surface equilibrium
+
+
4
. Discussion
[Eq. (9)]. Removing protons shifted the equilibrium to the right
side, promoting Cu formation. Thus, high-temperature treat-
+
The investigated catalyst series was prepared without any
ment with an O2-purified inert gas flow actually destabilized the
2+
halogenide, starting with Cu(II) acetate solution, and forcing
precipitation of dispersed Cu(II) hydroxide onto suspended
NH4-Y zeolite by enhancing the pH value through addition
of a base. Catalysts showed acceptable productivity at copper
contents of 10–12 wt%, provided the preparation step was com-
pleted by inert activation at an appropriate temperature, which
depends on the copper loading. It turned out that a threshold
of Cu loading was required to get activity for the oxidative car-
bonylation of methanol in the gas phase at normal pressure. The
discussion that follows focuses on relevant aspects of active site
formation and catalytic reaction.
Cu oxidation state by removing protons (dehydroxylation of
Brønsted acid sites). This is thought to tune the redox properties
of Cu2 /Cu , which are preserved under reaction conditions,
+
+
because rehydroxylation of the zeolite is hampered.
The Cu/Al molar ratios of the catalyst series ranged from
0.31 (2 wt% Cu loading) to 0.85 (14 wt% Cu loading). Re-
+
ferring the exchange capacity of the zeolite to Cu , 31% was
achieved at 2 wt% and 85% of the available capacity is con-
sumed at 14% Cu loading.
4.2. Site occupancy
4
.1. Activation of the Cu-Y catalyst system
The fact that up to 5 wt% Cu loading on zeolite Y yielded no
noteworthy activity seems to contradict proposals that isolated,
coordinatively unsaturated Cu ions are the active sites. But the
specific site configuration and hence cation distribution within
the faujasite structure [31] must be taken into account. Favored
sites for fixation of transition metal cations are localized at po-
sitions within hexagonal prisms connecting the sodalite units
of zeolite Y. Copper ions located at these sites are not accessi-
ble for molecules such as CO or N2 and cannot take part in the
carbonylation reaction. Further sites face the supercage and are
available for the reaction.
The “activation” procedure initiated a SSIE of copper ions,
which become fixed to Brønsted acid sites. Replacement of
acidic protons by Cu /Cu protected the zeolite lattice against
dealumination ( Al-MAS NMR) and thermal deterioration
2+
+
27
2+
+
(
XRD, surface areas). A reduction of Cu to Cu was already
observed during preparation of the precursors and was further
promoted by the final inert treatment. It is known that Cu2
+
may be reduced during zeolite dehydration by oxide ions of
water molecules, by framework hydroxyl groups [26] or by am-

M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
23
The site capacity of the hexagonal prisms at temperatures
above 190 C amounts to ca. 15 copper cations per unit cell, ac-
gaseous CO led to carbo-methoxide (with some ambiguities re-
maining), and further reaction with MeOH/air led to DMC.
Although this investigation was performed for CuCl/zeoli-
te Y, the considerations are valid for Cu2O/zeolite Y as well,
because no interference of Cl in the reaction mechanism was
necessary. Catalysts were prepared by mechanical mixing of
CuCl or Cu2O with the zeolite and subsequent heating under
◦
cording to Coughlan and Keane [32], and 15.7 copper cations
per unit cell according to Maxwell and de Boer [33]. In both
cases, divalent copper is meant and calculation is based on the
stoichiometry 1 Cu2 :2 Brønsted acid sites for electroneutral-
ity. Relating 15 copper cations to a unit cell capacity of 58
exchangeable cations, approximately 25% of the exchange ca-
pacity was represented by these “hidden” sites.
+
◦
flowing helium at 650 C for 70 h, which is known to lead to a
solid-state ion exchange.
An estimation based on data in the present work shows that
5% of the exchange capacity of 2.62 mmol g corresponded
to a Cu loading of 4.1 wt%. The close proximity of this value to
the minimum loading of Cu (ca. 5–6 wt%) required to get activ-
ity supports the assumption that almost all copper of precursors
The same approach was applied by Anderson and Root [8,9]
for preparation of Cu/zeolite X (30% Cu) and Cu/zeolite
ZSM-5 (7% Cu). The mechanism involves the adsorption of
−
1
2
+
CO on Cu but suggests a Rideal–Eley type insertion of CO
into surface methoxide directly from the gas phase, because the
influence of CO gas-phase pressure was found to be of first or-
der.
2
Cu-Y400, 4Cu-Y400, and 6Cu-Y400 migrated to sites within the
hexagonal prisms of the zeolite structure and were not acces-
sible for reaction at low loading. Thus, first these sites must
be saturated before copper is forced to coordinate with Brøn-
sted acid sites within the supercage. This corresponds to the
variation of the unit cell parameters with copper loading. Unit
cell enlargement was observed as long as preferential location
of copper ions takes place at sites located within the restricted
environment of the hexagonal prisms, but was insignificantly
low as soon as location of copper ions within the supercage
continued. The catalysts with copper loading exceeding 6 wt%
acquired activity. However, a linear increase of active site con-
centration with further enhanced loading would imply a linear
increase of activity. This obviously was not the case, because
the activity (MeOH conversion, STY of DMC; Figs. 11a and
All preliminary characterization by DRIFTS measurements
[36] confirmed the formation of methoxide as well as the fix-
+
ation of CO to Cu , so that the same mechanism probably
proceeds. Thus, the high activity achieved through the specific
inert activation procedure is associated predominantly with the
creation of a favorable active site configuration.
4.4. Comparison of productivity
Comparison of activities is difficult, because catalysts are
evaluated mainly in fixed-bed integral-flow reactors for conver-
sion, selectivity, and productivity (STY). Conversion, selectiv-
ity, and productivity are largely dependent on the space velocity.
High conversion and selectivity at a low space velocity leads to
low STY, whereas high STY can be achieved even at low con-
version and selectivities if the space velocity is high. For DMC,
it has to be differentiated whether selectivity is referred to the
conversion of MeOH or to the conversion of CO. In the former
case, the stoichiometry has to consider that 2 mol of MeOH are
required to produce 1 mol of DMC [Eq. (4)].
1
1e) depended exponentially on the copper loading within the
range of 6–10 wt% Cu (and even up to 12 wt% Cu if the activa-
◦
tion temperature below 750 C was chosen).
4
.3. Mechanism
In 1970, Saegusa et al. [34] reported that the (nonoxidative)
reaction of cupric dimethoxide and carbon monoxide in an au-
toclave led to DMC. The results were explained by the insertion
of carbon monoxide into the copper–oxygen bond of cupric
dimethoxide to generate carbo-methoxy cupric species, which
then underwent coupling with an adjacent methoxyl group. The
coupling between the carbomethoxy and methoxyl ligands with
The highest values reported in the open literature for a
normal-pressure gas-phase oxidative carbonylation of MeOH
was given for a PdCl2–CuCl2–CH3COOK co-impregnated AC
catalyst (STY of DMC ca. 390 gDMC l
ever, the catalyst suffered considerable deactivation after a
−
1
−1
h ) [12]. How-
Cat
process time of 6 h, but still maintained an STY of DMC
release of DMC reduced Cu2 to Cu . The stoichiometric re-
+
+
> 100 gDMC l
−1 −1
h
with high DMC selectivity (>95%) and
Cat
◦
action ran to complete consumption of the methoxide.
MeOH conversions of 3–7% at 130 C. Values achievable from
For the slurry-phase reaction system with copper chloride, it
was suggested [35] that the oxidative carbonylation of MeOH
to form DMC proceeds in two steps, with the oxidation of CuCl
to (CH3O–Cu) Cl (cupric methoxy-chloride) reduced by CO
to DMC in a second step. Because chloride is actually not di-
rectly involved, it should be possible to replace chloride by the
negatively charged anionic zeolite lattice [6].
CuCl/zeolite systems (solid-state ion exchange) were in the
range of 40–80 gDMC l
−
1
−1
h [6–9].
Cat
The maximum STY of DMC values observed during the
+
−
present investigation were in the same range, but could be
−
1
−1
h by operating the sam-
Cat
enhanced to nearly 100 gDMC l
ple 12Cu-Y700 at slightly higher reaction temperatures (150–
◦
160 C). DMC selectivity was decreased to ca. 45% under these
King [6] studied the reaction steps (i) methoxide forma-
tion, (ii) insertion of CO into methoxide, and (iii) MeOH reac-
tion with carbo-methoxide over Cu/zeolite Y by infrared spec-
troscopy at 130 C. He showed that methoxide was formed
with copper (the zeolite itself gave no methoxide species), and
oxygen promoted formation of Cu-methoxide. Admission of
conditions, but the nearly exclusive side product was DMM.
For fuel blending, separation of both products is not really nec-
essary, although the O content of DMM is lower than that of
DMC. In any case, no chloride impurity needs to be feared.
As will be shown in a forthcoming paper [37] (see addi-
tional material), higher DMC selectivities and STY of DMC
◦

24
M. Richter et al. / Journal of Catalysis 245 (2007) 11–24
are accessible with copper-impregnated zeolite Y under flow
conditions at 0.8–1.2 MPa and optimized feed composition al-
lowing an STY of DMC of up to 560 gDMC l
selectivity levels of 70–80%.
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−
1
−1
h at DMC
Cat
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◦
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−
1
−1
◦
h
at 150 C and a GHSV of
Cat
−
1
3
000 h
The catalysts operate without any chloride admixture quite
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[
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−1
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Cat
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Additional material
The online version of this article contains additional mater-
[36] E. Schreier, unpublished results.
ial.
[37] M. Richter, M.J.G. Fait, R. Eckelt, E. Schreier, M. Schneider, M.-M. Pohl,
R. Fricke, Appl. Catal. B Environ., submitted for publication.
Please visit DOI: 10.1016/j.jcat.2006.09.009