G. Tavčar and T. Skapin
JournalofFluorineChemistry222–223(2019)81–89
concurrent processes, like gradual chlorination of the solid with the
associated reduction of Lewis acidity, partial crystallization and re-
duction of the surface area, and coke formation.
hydrofluoric acid, Riedel-de Haën) into a 40% aqueous solution of N2H4
(diluted 80% solution of hydrazine hydrate, Riedel-de Haën). White
crystalline product was obtained which was dried at 333 K (Calculated
for [N2H6]F2: N2H4, 44.4%; Ft, 52.8%; Found: N2H4, 43.5%; Ft, 52.0%).
Metal chromium (powder 99.99%, Alfa Aesar) was dissolved in a 40%
aqueous solution of HF. Solution of [N2H6]F2 in water was added into
the acidic chromium fluoride solution in a stoichiometrical ratio of 1:1.
The clear green solution was evaporated on water bath at 313 K, after
that the crystalline product was dried at 323 K (Calculated for [N2H6]
[CrF5]·H2O: N2H4, 16.1%; Ft, 47.7%; Cr, 26.1%; Found: N2H4, 16.2%;
Ft, 47.0%; Cr, 25.6%;).
3. Conclusions
Decomposition of hydrazinium fluorometallates(III) with F2 in aHF
medium, which was originally developed as an entirely inorganic route
to HS-AlF3 materials [24,25], was successfully applied also in the pre-
paration of HS-CrF3 materials with unprecedented characteristics. Al-
though the basic reaction mechanisms may appear similar, the two
reaction systems differ significantly.
In contrast to HS-AlF3, which was obtained from an anhydrous
precursor, the HS-CrF3 products are attainable exclusively from the
hydrated precursor, [N2H6][CrF5]·H2O. Reaction of the latter with F2 in
aHF medium proceeds in at least two consecutive steps: (i) in the initial
stage, oxidative decomposition of the cationic part in the precursor,
[N2H6]2+, with F2 takes place and gives a CrF3 intermediate with low
surface area, (ii) the following stage includes oxidation of some Cr3+ to
Cr> 3+, reaction with residual H2O/[H3O]+ species, and formation of
chromium fluoride oxides, CrO2F2 and, very likely, CrOF3. The final HS-
CrF3 with surface areas in the range of 180–420 m2 g−1 is obtained after
removal of residual volatile species, mainly HF, CrO2F2, and [NH4]F, at
moderate conditions. By this means, the nanoscopic character of the
basic particles is retained, and microporosity is formed.
4.1.2. Preparation of HS-CrF3
CAUTION! The following preparation procedures include the use of
anhydrous hydrogen fluoride (aHF) and fluorine (F2). Both chemi-
cals are highly toxic and potentially dangerous. They must be han-
dled with high precaution by using appropriate apparatus and pro-
tective clothing.
Volatile materials (F2, aHF) were handled in an all-PTFE vacuum
line equipped with PTFE valves. Manipulation of the non–volatile ma-
terials was done in a dry box at H2O levels below 1 ppm (M. Braun).
Small scale reactions (up to 4 g of starting material) were carried out in
fluorinated ethylene propylene (FEP, copolymer of hexa-
fluoropropylene and tetrafluoroethylene) reaction vessels (height
250–300 mm, ID/OD of 15.5/18.7 mm, Fig. 2) equipped with PTFE
valves and PTFE coated stirring bars. Larger scale reactions (over 4 g of
starting material) were carried out in modified round bottom 500 ml
flasks made of FEP, which were equipped with PTFE valves and PTFE
coated stirring bars. Prolonged use of the latter led to some con-
tamination with PTFE shavings (e.g. Table 2), especially in larger bat-
ches which were stirred for several weeks. In some experiments, the
stirring bars were therefore not used all the time. In these cases, ade-
quate agitation within the reaction vessel was attained on a laboratory
shaker. Prior to use, all lines and vessels were passivated with elemental
fluorine. Fluorine was used as supplied (98%, Solvay). Anhydrous HF
(Purum, Fluka) was pre-treated with K2NiF6 (Ozark Mahoning) for
several hours.
Despite its nanoscopic nature, exceptionally high surface area, and
pronounced microporosity, the bulk composition of the newly prepared
HS-fluoride is remarkably close to the stoichiometric CrF3. The main
residual by-product is supposed to be the non-volatile fluoride oxide,
CrOF3, which is the main source of Cr > 3+ and oxygen in the final
product. Other residual species, like H2O, [H3O]+, OH−, and [NH4]+
,
are present in much lower amounts. Although the exact mass balance of
oxygen could not be defined, the results indicate that the water from
the hydrated precursor is almost quantitatively decomposed to volatile
products and effectively removed from the reaction system as O2 (4),
and, partly, in the form of volatile chromium fluoride oxides, e.g. as
CrO2F2 (6). In addition, removal of volatile chromium species from the
amorphous CrF3 matrix generates microporosity. This appears to be the
key step in the formation of HS-CrF3.
In a typical preparation, the solid precursor, [N2H6][CrF5] · H2O,
was weighted in a dry box into a FEP reaction vessel. The vessel was
connected to a vacuum line, evacuated, and cooled to 77 K with liquid
N2. The required amount of aHF, employed in the current preparations
as a solvent and a medium, was condensed into the cooled vessel. After
aHF addition, the vessel was warmed-up to room temperature. The
green colour of the formed liquid phase (Fig. 2 left) indicates that the
starting material is slightly soluble in liquid aHF. Relatively large
quantities of the aHF solvent were employed to assure homogeneity, to
increase the heath capacity of the reaction medium, and to prevent
potentially dangerous local overheating, which may result from the
strongly exothermic reaction between F2 and the unwetted precursor
particles. Thereafter, F2 was dosed in several portions into the reaction
vessel kept at room temperature. In the initial, strongly exothermic
stages of the reaction (5), smaller portions of F2 were added (up to
120 kPa of F2 at room temperature). The course of reaction was mon-
itored by measuring the pressure inside the reaction vessel after cooling
it to 77 K. When the pressure in the cooled vessel was different from
that of F2 at this temperature, i.e. approx. 36 kPa, the reaction was
considered to be not completed and the volatiles were pumped out at
77 K. Afterwards, the vessel was warmed-up to room temperature again
and a new portion of F2 was added at room temperature. In the final
stages, larger doses of F2 were added, but not higher than 320 kPa of F2
at room temperature. Each F2 portion was allowed to react with the
strongly agitated reaction mixture for at least two days before a new
portion of F2 was added. After the reaction was completed, the raw
product was isolated at room temperature by pumping out the residual
gasses and aHF. Altogether, an approximate 33% excess of F2 was
The newly prepared HS-CrF3 materials exhibit Lewis acidity and are
consequently very active catalysts in the investigated model reactions
with some CFCs. High reactivity of these solids towards CFCs is evi-
denced by a substantial incorporation of chlorine because of the F/Cl
exchange reactions between the CFC molecules and the solid fluoride.
Observed reactivity of HS-CrF3 materials is the result of their ex-
ceptionally high surface area and high purity. These materials consist
mainly of CrF3, and minor amounts of the CrOF3 by-product. Presence
of the latter is apparently not problematic, since the in-situ reduction
and halogenation of the related Cr > 3+ sites with CFCs very likely
produce additional Lewis acidic sites. On the other side, content of
species, which could block or interfere with the active sites, like H2O/
[H3O]+ or [NH4]+, is comparatively very low. In addition, these ma-
terials, although metastable, can be employed at temperatures up to
623 K without losing their nanoscopic character. For these reasons, the
HS-CrF3 could be employed as an interesting new benchmark material
for mechanistic and catalytic investigations in fluorocarbon chemistry.
4. Experimental
4.1. Preparation procedures
4.1.1. Synthesis of the [N2H6][CrF5]·H2O precursor
Synthesis of the starting materials was performed according to a
modified procedure described earlier [44]. All preparation steps were
carried out in polyethylene beakers. The [N2H6]F2 was synthesized by
slow addition of
a 20% aqueous solution of HF (diluted 40%
87