176
BOTHE-ALMQUIST ET AL.
samples used for the BET surface area measurements had at 680 C for 2 h in 4% high-purity oxygen (balance is he-
surface areas of less than 1 m2/g in most cases, which is out- lium). The heated samples were immediately immersed in
side the manufacturer’s recommended range to ensure high liquid nitrogen. Then, the samples were transferred into a
accuracy. The BET surface areas of the MgO supports and glove box under nitrogen purge before being loaded into
Li/MgO catalysts are presented in Table 1.
EPR quartz tubes. All the samples were prepared the same
way and were kept in a glove box under nitrogen before
analyzing them.
Chemical analyses. Lithium and magnesium concentra-
tions in each catalyst sample were determined using ICP
spectroscopy (Model 61E, Thermo Jerrell Ash Corp). Ap-
proximately 15 mg of each catalyst was dissolved in 2 vol%
HNO3 prior to the ICP analyses. The ICP instrument was
calibrated using prepared standards of MgO and LiCl in
2% nitric acid, ranging in concentrations from 0 to 160 ppm
by weight for lithium and from 0 to 650 ppm by weight for
magnesium. Chloride concentrations were measured using
a gravimetric method (6). This method requires the use of
silver nitrate and nitric acid. Dilute nitric acid (2% by vol-
ume) was used to dissolve samples of fresh catalysts. Silver
nitrate solution was added to aliquots of the dissolved cata-
lyst solutions. The silver ion from AgNO3 reacted with the
chloride ion to form AgCl which rapidly precipitated out
of solution. The precipitate was collected by filtration, and
the filtrate was subsequently dried and weighed. The chlo-
ride concentration in the catalyst was determined from the
collected weights of AgCl.
The EPR spectra were obtained on a Bruker (Billerica,
MA) ESP 300 spectrometer outfitted with a TM110 cavity
and a Wilmad 50-ml Dewar flask at 77 K. The instrument
parameters were 600-G sweep width, 5-mW microwave
power, 100-kHz modulation frequency, 1-G modulation
amplitude, 163-ms conversion time, and 20.5-ms time con-
stant. The g value of 2.0544 corresponding to the [Li+O ]
⊥
signal was determined relative to a paramagnetic Cr3+ stan-
dard prepared according to a published procedure (7). The
peak-to-peak height of the signal at g = 2.0544 was mea-
⊥
sured and then normalized with respect to the C2 sample
for determing the relative [Li+O ] amount in the various
catalyst samples.
Thermal stability experiments. Thermogravimetric ana-
lyses (TGA) were conducted on a Perkin–Elmer TAS 7
TGA apparatus to determine the thermal stability of the
catalysts and to quantify the weight loss at elevated tem-
peratures. The TGA analyses were conducted isothermally
at 730 C and atmospheric pressure for all catalysts. A con-
tinuous stream of nitrogen was used to purge off-gases from
the TGA electronics and sample region. The initial amount
of each catalyst used for the TGA analyses was approxi-
mately 15 mg.
XRD studies. The crystallinity of each catalyst was de-
termined using a Siemens X-ray diffractometer using a
CuK source. X-ray diffraction (XRD) spectra were ob-
tained for the lithium precursors and catalysts for compar-
ison. The d values of the main peaks were used to identify
the crystal phases and to compare the spectra of the cata-
lysts and lithium precursors. The XRD phases present in
the samples were identified with help of JCPDS files.
RESULTS AND DISCUSSION
XPS studies. X-ray photoelectron spectroscopy was
used to analyze the atomic surface concentrations on each
catalyst. The samples for the XPS studies were prepared
as follows. Prior to the XPS measurements all catalysts
were calcined at 650 C in dry air for 14 h. The calcined
samples were again heated at 680 C for 2 h in 4% oxy-
gen (balance helium). Then, the XPS analyses were con-
ducted on a Perkin–Elmer Model 5300 X-ray photoelec-
tron spectrometer with MgK radiation at 300 W. Typically,
89.45 and 35.75-eV pass energies were used for survey and
high-resolution spectra, respectively. The effects of sample
charging were eliminated by correcting the observed spec-
tra for a Mg 2p binding energy value of 50.3 eV. The pow-
dered catalysts were mounted onto the sample holder and
were degassed overnight at room temperature and pres-
sures on the order of 10 7 Torr. The binding energies and
atomic concentrations of the catalysts were calculated via
the XPS results using the total integrated peak areas of the
Mg 2p, Li 1s, O 1s, and Cl 2p regions.
The primary scope of this work was to study the effect
of the surface properties and effects of various magnesium
oxide and lithium precursors on the synthesis of Li/MgO
catalysts for the oxidative methylation of acetonitrile to
acrylonitrile.
Catalyst Performance in the Oxidative Methylation
of Acetonitrile to Acrylonitrile
Several experiments were conducted to assess the effects
of the magnesium oxide and lithium precursors and the
catalyst surface properties on the catalyst performance in
the oxidative methylation of acetonitrile. A summary of the
catalytic performance of all the samples after 2 h on stream
under identical operating conditions is shown in Table 2.
All catalysts have the same nominal Li composition
(20 wt%). The data presented in Table 2 represent aver-
ages of at least three replicate experiments. As shown in
Table 2, the performance of catalysts C1–C6 for the oxida-
tive methylation of acetonitrile is similar, suggesting that
EPR studies. The samples for the EPR studies were pre- catalyst performance is independent of the source and sur-
pared as follows. A known amount of catalyst was heated face area of the MgO support. A comparison of catalysts