PRETREATMENT EFFECT ON THE PROPERTIES OF THE Cr–Mg CATALYST
393
The Cr content of catalysts was determined by
I
inductively coupled plasma atomic emission spectros-
12
copy (ICP-AES) on an emission spectrometer (Baird,
United States); the Cl content, by measuring the
residual Ag concentration after AgCl precipitation.
2
II
11
V
Catalytic activity was estimated in PCE hydrofluori-
nation to pentafluoroethane (ozone-friendly refrigerant
R-125). The reaction was conducted in a catalytic
fixed-bed flow reactor. The design of the unit allowed
us to analyze reaction products containing corrosive
gases (HF and HCl) in the online mode. This feature
distinguished the unit from common laboratory units
operating in the product accumulation mode.
1
3
10
7
III
IV
8
9
5
Over ten intermediates and by-products form in
the course of the reaction [12, 13]. The hydrofluorina-
tion of PCE (CCl =CCl ) to pentafluoroethane
6
4
2
2
(
CF CHF ) is described by the following empirical
3 2
Fig. 1. Schematic diagram of the catalytic PCE hydroflu-
reaction:
orination setup: (1) wessel with HF, (2) heated zone,
(
(
3) saturator, (4) thermostat, (5) U-shaped reactor,
6) furnace with air blown around it, (7) HF and HCl
CCl =CCl + 5HF = CF CHF + 4HCl.
(I)
2
2
3
2
absorber, (8) condenser, (9) collector, (10) flow meter,
Our laboratory setup is schematized in Fig. 1.
Compressed nitrogen (flow I) forced liquid hydrogen
fluoride (flow II) out of a vessel (1) placed on a bal-
ance. Through a capillary, HF flew to a zone pre-
heated to 120°С (2), where it evaporated. Gaseous HF
in a saturator (3) was saturated with PCE (flow III).
(
11) chromatograph, and (12) data processing unit. Flows:
I, N ; II, HF; III, HF + PCE; IV, HF + HCl + PCE +
2
organic products of the reaction: V, CO + H O + organic
2
2
products of the reaction.
The degree of saturation was regulated by varying the constant, a = −ln(1 − х)/τ, where х is the PCE con-
temperature of water supplied into the jacket of the version and τ is the residence time (s).
saturator from a thermostat (4). The reactant mixture
(
flow III) was fed into U-shaped a nickel reactor (5)
3
RESULTS AND DISCUSSION
2
−8 cm in volume and 6 mm in diameter packed with
a catalyst, which was heated with an electric furnace (6)
According to ICP-AES data, the initial precursor
with air blown past it. The reaction mixture (flow IV) contained 5.08 wt % Cr and 6.81 wt % Cl; that is, the
that had passed through the catalyst bed was returned Cl : Cr ratio was ~2 : 1, not 3 : 1 as in the initial
into the heated zone 2, and was then directed at atmo- hydrated salt CrCl ⋅ 6Н О. A similar deviation was
3
2
spheric pressure through a metering valve into an
absorber (7) filled with an alkali metal carbonate. In the
absorber, the corrosive gases (unreacted HF and result-
ing HCl) were replaced in the product stream by water
and carbon dioxide (flow V). After the reaction mixture
passed the absorber, gaseous samples were taken out
noted in an earlier work dealing with a chromium–
magnesium sample prepared in the same way [14].
This is apparently due to the fact that, as a result of the
prolonged stirring of the wet paste followed by heat
treatment at 70−90°C and then at 150°С, CrCl
3
undergoes partial hydrolysis to yield HCl, which
passes readily into the gas phase:
(flow V), which were analyzed online on a chromato-
graph (11). Thereafter, the reaction mixture from the
heated zone was passed through a condenser (8) to con-
dense the unreacted PCE and liquid reaction prod-
ucts. The condensate flew down into a collector (9),
and the mixture of carbon dioxide and gaseous prod-
ucts passed through a flow meter (10). The reaction
products were analyzed on a Saturn-2000 GC–MS
system (Varian, United States). The online analysis of
the reaction mixture was carried out on a Tsvet-500
gas chromatograph (Tsvet, Russia) with a thermal-
conductivity detector.
СrCl + H O = Cr
(
OH
)
Cl + HCl ↑.
(II)
3
2
2
The composition of the precursor containing a
hydrate of the Cr(OH)Cl salt is described by the for-
2
mula Cr(OH)Cl ⋅ nH O/MgF . The number of water
2
2
2
molecules, n, can be determined by chemical analysis
for Cr and Cl. At n = 7, the calculated chromium con-
tent is 5.02 wt %, and the calculated chlorine content
is 6.86 wt %. These data coincide with the chemical
analysis data within the measurement error. Thus, the
composition of the initial precursor is Cr(OH)Cl ⋅
2
All experiments were performed under kinetic con-
trol at temperatures of 320−350°С, a pressure of
7
H O/MgF .
2 2
0
.4 MPa, HF : PCE molar ratios of 10 : 1 to 15 : 1, and
The dynamics of the variation of the precursor
residence times of 1−3 s. Catalytic activity was esti- composition under heat treatment in air was studied
mated in terms of the first-order PCE conversion rate by TA. Figure 2 presents the thermogravimetric (TG)
KINETICS AND CATALYSIS Vol. 58 No. 4 2017