EFFECT OF PRESSURE ON THE PHASE COMPOSITION
913
the hydrolysis, several drops of nitric acid (50%) were of catalyst per hour. Methane conversion was deterꢀ
added. The resultant gel was left to stand at room temꢀ mined from gasꢀchromatographic analysis data as the
perature for 12 h and then dried in a muffle furnace ratio of the total methane converted to the total
while the temperature was raised from 70 to 100
Next, the mixture was subjected to hydrothermal the ratio of the amount of methane converted to a parꢀ
treatment at 180 in a Teflonꢀlined autoclave. In the ticular product to the total methane converted.
final step, the material was placed in a porcelain cup,
heated in air to 600 C at a constant rate of C/min,
°
C.
amount of methane, and selectivity was determined as
°
С
°
1
°
RESULTS AND DISCUSSION
and then calcined at this temperature for at least 10 h,
until complete decomposition of the template.
The freshly prepared Naꢀcontaining catalyst conꢀ
sisted of metastable cristobalite with a trace amount of
tridymite [9–11], cubic Na2WO4, and cubic Mn2O3
Pressure was generated by a Pꢀ250 laboratoryꢀscale
hydraulic press. The contact tools used were heated
3Kh2V8F steel dies (RF State Standard GOST 5950ꢀ
73). Nichrome resistors ensured that the tools and
sample were heated (with the dies compressed) at a
(Cꢀbixbyite). As shown earlier [4, 7], during a proꢀ
longed OCM (500 or 1000 h) the SiO2 in the composꢀ
ite reaches a stable phase state (quartz + tridymite)
and Na2WO4 sublimes from Na/W/Mn/SiO2 previꢀ
ously forming of Na2W2O7 [6, 12]. In the presence of
Li+, the amorphous silica crystallizes predominantly
into quartz, and the content of metastable cristobalite
drops sharply even after shortꢀtime OCM [6, 7, 13]. In
addition to bixbyite, several Li2WO4 polymorphs
(rhombohedral, tetragonal, and orthorhombic) can be
formed [14], as well as Li6WO6 [6] and Li6W2O9 [14].
Pure cristobalite annealed with Na2WO4 does not
exhibit catalytic properties [10]; that is, the activity of
cocrystallized Na(Li)/W/Mn/SiO2 composites for
OCM seems to be due to the possibility of coexistence
of several crystalline SiO2 polymorphs in a wide temꢀ
perature range in a multicomponent system. On the
one hand, the polymorphic transformations of SiO2
enable alkali cation exchange between cristobalite and
tridymite or cristobalite and quartz. In this process,
rate of 500 C/h. To prevent contamination during
°
pressing, the Li/W/Mn/SiO2 and Na/W/Mn/SiO2
powders were poured into Ni foil cans (10 mm in
diameter). The powder was degassed by heating for
20 min and then pressed. The pressure in the pellet–
die zone reached 2.5 GPa and, because of the pellet
pressingꢀout, decreased to 1 GPa. The sample was
held under pressure for 60 min at 500 C. Next, the
°
samples were ground into powder for phase analysis
and catalytic tests.
The phase composition of Li/W/Mn/SiO2 and
Na/W/Mn/SiO2 before and after exposure to pressure
and after OCM experiments was determined by
Rigaku MiniFlex 600 Xꢀray diffractometer (Cu
K radiꢀ
α
ation,
λ
= 1.54187 Å). Intensity data were collected in
° to 100° at a scan step of
the angular range
2 = 5
θ
0.05° and counting time per data point of 0.2 s. In
determining the phase composition of the catalysts, we
used the International Centre for Diffraction Data
(ICDD) PDF database.
To run a continuous OCM process, we used methꢀ
ane (Purity Standard TU 51ꢀ841ꢀ87, volume fraction
of 99.99% in terms of dry matter) and oxygen (extraꢀ
pure grade, Purity Standard TU 6ꢀ21ꢀ10ꢀ83, volume
fraction of 99.999% in terms of dry matter) produced
at the Moscow Gas Refinery Plant.
The catalytic properties of the composites for
OCM before and after exposure to pressure were studꢀ
ied in a flowꢀtype differential quartz reactor (650 mm
length, 8 mm inner diameter, 4 mm outer diameter of
the thermocouple pocket). Various molar ratios of
methane and oxygen (CH4 : O2 = 1.5 : 1 to 4.6 : 1) were
introduced into the reactor. The catalyst was placed in
the hot zone of the reactor, about 500 mm from its
inlet. To reduce the volume of the precatalyst and
aftercatalyst zones, the reactor was filled with quartz
glass insets, which were tightly put on the thermocouꢀ
ple pocket. A quartz wool layer (mixer/filter) was
placed at the reactor inlet, as well as in front of and
tungstates, which form no solid solutions with SiO2
,
act as Na+ and Li+ sources [7]. On the other hand,
Na2WO4 has long been used as a mineralizer to accelꢀ
erate quartz–tridymite (870°C) and tridymite–cristoꢀ
balite (1470 ) solidꢀstate transformations at atmoꢀ
°C
spheric pressure [15]. In the presence of mineralizers,
no tridymite is formed, and quartz first transforms into
cristobalite and then gradually converts to tridymite
[16]. In addition to quartz, tridymite, and cristobalite,
there are SiO2 polymorphs obtained in experiments at
high pressures: coesite [17], stishovite [18], and others
[19]. According to T–p phase diagrams [19], at 500°C
and 2.5 GPa the composites are in the region of the
quartz–coesite polymorphic transformation. It was
expected that a crystalline SiO2 polymorph experiencꢀ
ing highꢀpressure structural changes can be stabilized
in a metastable state by reducing the pressure to atmoꢀ
spheric one, thereby influencing the catalytic activity
of the Li(Na)/W/Mn/SiO2 composites.
Comparison of the Xꢀray diffraction patterns in
Fig. 1 and data in the table demonstrates the following:
The phase composition of the asꢀprepared
behind the catalyst. The Li(Na)/W/Mn/SiO2 catalyst Li/W/Mn/SiO2 (Fig. 1b, scan
charge was 0.1 g, the particle size was within 0.1 mm, (Fig. 1a, scan ) composites remains unchanged upon
and the catalyst layer was 1–2 mm in height. The temꢀ exposure to high pressures (scans ): The phases perꢀ
perature in our experiments was varied from 730 to sisting in the Liꢀcontaining catalyst are cristobalite,
910 , and the gas flow rate was 10 to 70 L per gram quartz, bixbyite (Mn2O3), Li2WO4 (tetragonal), and
3) and Na/W/Mn/SiO2
3
2
°C
INORGANIC MATERIALS Vol. 50 No. 9 2014
Nipan
