112
P. Cheung et al. / Journal of Catalysis 245 (2007) 110–123
6
0
890) equipped with a methyl-siloxane column (HP-1, 50 m ×
.32 mm × 1.05 µm) connected to a flame ionization detector
× 1.05 µm) using mass spectrometry (Agilent 5973) and flame
ionization detection. 12C and C isotopomer distributions in
DME and methyl acetate were measured from ion yields (DME:
46, 47, and 48 amu; methyl acetate: 43, 44, 45, 74, 75, and
76 amu) using deconvolution methods and mass fragmentation
patterns for unlabeled molecules [29]. Samples were treated in
13
and a Porapak Q column (80–100 mesh, 12 ft. × 1/8 in.) con-
nected to a thermal conductivity detector. These apparatus and
treatment protocols were also used for the studies described in
Sections 2.3–2.5.
3
−1 −1
g , zero grade, Praxair) at 773 K
flowing dry air (3.33 cm s
−
1
2
.3. Transient reaction studies involving DME–CO and CO
(0.0167 K s ) for 2 h and then cooled to reaction temperatures
before introducing reactants.
cycling
12
12
CH3O CH3 (99.5%, Praxair) and He (UHP, Praxair) mix-
Samples were sequentially contacted with DME–CO reac-
tures were recirculated over the catalysts held at 438 K for 1
h, followed by evacuation and He purge for ∼3.25 h before
the samples were exposed to isotopic mixtures to eliminate in-
tants and pure CO streams in transient studies designed to
probe the nature of carbonylation reactive intermediates and el-
ementary steps. Catalysts were treated in flowing dry air and
contacted first with DME–CO mixtures (930 kPa CO, 20 kPa
DME, 50 kPa Ar, 438 K) (as described in Section 2.2) for
12
12
duction periods. A mixture of CH3O CH3 (99.5%, Praxair)
13
13
13
and CH3O CH3 (99 at% C, Isotec) isotopomers was con-
tacted with CO (UHP, Praxair) on HMOR_9.8 to probe the
12
∼
5 h to obtain steady-state rates; the system was then brought
reversibility of C–O cleavage in DME during carbonylation
reactions. CH3OCH3 and CD3OCD3 (99 at%, Isotec) were re-
acted with CO either separately or as an equimolar mixture to
probe the reversibility of C–H bond dissociation steps and the
involvement of C–H bonds in kinetically relevant steps during
DME carbonylation.
−
1
−1 −1
to ambient pressure and treated in He (3.33 cm
s g ,
UHP, Praxair) until DME levels in the effluent (measured by
mass spectrometry) were below 0.02%. The system pressure
was then increased to 1 MPa in He before the samples were
exposed to either a 95% CO/Ar (UHP, Praxair) or pure CO
(
99.99%, Praxair) flow at 1 MPa and 438 K for various time
intervals, after which DME–CO mixtures were reintroduced
at 1 MPa total pressure (930 kPa CO, 20 kPa DME) and
2
.6. IR spectroscopic studies of DME, acetic anhydride, and
carbon monoxide adsorption
4
38 K. The transient evolution of methyl acetate (43 amu) and
DME (43 and 45 amu) was measured by on-line mass spec-
trometry with a time resolution of 10 s during these experi-
ments.
IR spectra were measured with 2 cm 1 resolution on self-
supporting wafers (∼20–40 mg) held within a quartz vacuum
cell with NaCl windows using a Nicolet NEXUS 670 IR spec-
trometer equipped with a Hg–Cd–Te (MCT) detector in the
−
2
.4. Acetic anhydride reactions with Brønsted acid sites
−
1
4
000–400 cm
frequency region. Samples were treated in
3
−1
HMOR_9.8 samples (∼0.5 g) treated in dry flowing air
flowing dry air (∼1.67 cm s , zero grade, Praxair) at 723 K
for 1 h, evacuated at 723 K for 2 h using a diffusion pump
(<0.01 Pa dynamic vacuum; Edwards E02) and cooled to 438 K
in vacuum before samples were contacted with DME (99.5%,
Praxair) or acetic anhydride (99%, EMD Chemicals) for 0.25 h.
Samples were treated similarly before cooling to 123 K (using
a constant flow of liquid N2) and exposing them to CO (UHP,
Praxair) at 123 K. IR spectra were collected for 120 s after each
CO dose without intervening evacuation.
(
(
as described in Section 2.2) were exposed to flowing He
3
−1 −1
g , UHP, Praxair) saturated with acetic anhy-
3.33 cm s
dride (99%, EMD Chemicals; 273 K, 0.11 kPa) and held at
4
38 K for ∼3.5 h. The reactor effluent was analyzed by gas
chromatography and mass spectrometry (as described in Sec-
tion 2.2) to determine the identity and the number of molecules
evolved in reactions of acetic anhydride molecules with zeolitic
protons. After contact with acetic anhydride, samples were ex-
posed to flowing He (3.33 cm s
remove physisorbed molecules before introducing DME/CO/Ar
3
−1 −1
g
) for ∼2 h at 438 K to
−1 −1
g ; 2/93/5 kPa; 438 K) and monitoring
2
.7. CO adsorption and temperature-programmed desorption
3
mixtures (3.33 cm s
studies
the products formed by mass spectrometry and gas chromatog-
raphy.
Samples (0.3–0.7 g, 125–250 µm pellet diameter) were
3
−1 −1
g , zero grade, Prax-
treated in flowing dry air (∼6.67 cm s
2
.5. Isotopic exchange and kinetic isotope effects
−
1
air) at 773 K for 1 h before cooling (at 0.167 K s ) to 253 K
using liquid nitrogen. A mixture of 1% CO/He (UHP, Praxair;
Isotopic-exchange experiments were carried out in a gradi-
3
3
−1 −1
g ) was introduced for 0.5–0.75 h on sam-
entless recirculating batch reactor (590 cm ) as described previ-
∼1.67 cm s
3
−1
)
ously [28]. The reactant stream was circulated (∼3.33 cm s
ples held at 253 K before flushing CO(g) with flowing He
3
−1 −1
g , UHP, Praxair) for 1.5 h to remove weakly
over the catalyst bed (∼0.5 g; 125–250 µm particle diam-
(∼1.67 cm s
eter) using a graphite gear micropump (Micropump, model
adsorbed CO. The temperature was then increased to 523 K
(0.167 K s ) and held at 523 K for 360 s. The concentration of
3
−1
1
82–000). Gas samples (1 cm ) were extracted from the circu-
lating gas stream and analyzed by gas chromatography (Agilent
890; HP-1 methyl siloxane capillary column; 50 m × 0.32 mm
CO (28 amu) in the He stream was monitored continuously by
mass spectrometry (MKS Orion Compact).
6