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ids heated up and pressurized over their critical constants acquire
outstanding properties. Thanks to its mild critical pressure and
temperature (Pc = 73.9 bar; Tc = 31 ◦C), supercritical carbon dioxide
(SCCO2) is the most widely used supercritical fluid. Supercriti-
cal water (SCW) requires more severe conditions (Pc = 221 bar;
Tc = 374 ◦C), but this fluid is especially interesting due to its high
density and diffusivity and its ability to form clusters or to dis-
solve gases and organic molecules. When both fluids are heated
up to an appropriate temperature, they are able to gasify carbon
based adsorbents [15–19], among other applications. The reports
concerning this topic are focused on the development of poros-
ity, but scarce attention is paid to the chemical properties of the
materials [20,21].
In this work, we report for the first time not only textural but
also chemical modifications suffered by a porous carbon material
upon exposure to SCCO2 and SCW. The raw material investigated
is an activated carbon fiber (ACF) cloth. The ability of the fibers
treated under supercritical conditions to catalyze such an impor-
tant reaction as the oxidative dehydrogenation of isobutane is
investigated. The performance of an ACF exposed to nitric acid is
also reported. Characterization of the catalytic activity and deter-
mination of possible relationships between the type of modification
and the production of isobutene are presented. Indeed, as far as we
know, no previous work delves into the catalytic ability of carbon
materials exposed to supercritical fluids.
isotherms were determined at −196 ◦C in a Quantachrome Instru-
ments NOVA 4200e. CO2 isotherms were determined at 0 ◦C in a
Quantachrome Instruments Autosorb-iQ-MP. Prior to analysis, each
sample (0.08–0.1 g) was outgassed under high vacuum at 150 ◦C
overnight. These isotherms were used to calculate the BET surface
area (SBET), the micropore volumes determined by N2 (V0(N2)) and
CO2 (V0(CO2)) and the mesopore (Vmeso) volume.
XPS measurements were made with a VG Scientific ESCALAB
200A spectrometer equipped with a Mg K␣ X-ray source, and SEM
images were taken using a FEI Quanta 400 FEG ESEM (15 keV) elec-
tron microscope. Both analyses were made at the Materials Center
of University of Porto (CEMUP).
Thermogravimetric (TG) analysis was performed using a STA
490 PC/4/H Luxx Netzsch thermal analyzer, by heating the sam-
ple in helium flow from 50 to 900 ◦C with a heating rate of 10 ◦C
min−1
.
Temperature programmed desorption (TPD) profiles were
obtained in an Altamira Instruments AMI-300 apparatus connected
to a Dycor Dimaxion Mass Spectrometer. The samples (0.1 g) were
heated at 5 ◦C min−1 from room temperature to 1100 ◦C under a
constantflowrateofhelium(25 cm3 min−1). TheamountsofCOand
CO2 released were monitored and the obtained TPD profiles were
deconvoluted in order to identify and quantify the surface oxygen
groups [23,24]. All the TPD spectra are shown in the Supplementary
data.
2.3. Catalytic tests
2. Experimental
The catalytic tests were performed by packing several round-
shaped layers of the cloth (0.2 g) inside a stainless steel tubular
reactor (Microactivity Reference, PID Eng & Tech). Mass flow con-
trollers were used to set the gas flow rates. The reaction mixture
was obtained by mixing 4 cm3 min−1 isobutane, 2 cm3 min−1 O2
and 24 cm3 min−1 N2. The reaction temperature, 375 ◦C, was mea-
sured by a thermocouple and maintained throughout the 300 min
of reaction. The gaseous products of the reaction were analyzed by
a GC 1000 Dani chromatograph equipped with a Chrompack Capi-
lary Column CP Sil 8CB low bleed/ms 30 m × 0.32 mm, 1 m, and
an online non-dispersive infrared (NDIR) analyzer for CO2.
The catalyst performance was evaluated in terms of the follow-
ing parameters:
2.1. Preparation of the catalysts
A commercial ACF cloth supplied by Kynol was used as raw
material. This ACF was prepared from the textile fiber Novoloid.
This carbon fiber was transformed into ACF by a one-step process
combining both carbonization and activation. This step is carried
out simply by exposing the textile structures to the products of
combustion of natural or liquefied petroleum gas at 900–1000 ◦C.
The as-received material is referred as Original. The fresh cloth was
subsequently exposed to different treatments.
Treatments with supercritical fluids were carried out inside a
tubular reactor made of Hastelloy. A fixed bed of approximately 5 g
of fresh cloth was introduced in the reactor. The maximum tem-
perature of the oven was 700 ◦C. The reactor was heated to the
desired temperature and then it was exposed to a flow of the cor-
responding supercritical fluid during 180 min. Water purified with
a Milli-Q device at ambient temperature, or liquefied CO2 at −5 ◦C,
were pumped at 3 cm3 min−1 by means of high pressure pumps.
Differenttreatmentsat two different temperatureswere performed
with each supercritical fluid. Samples SCW.500 and SCW.650 were
obtained by exposure of the cloth to a SCW flow at 290 bar and 500
obtained by exposure of the cloth to a SCCO2 flow at 100 bar and 250
or 700 ◦C, respectively. Further details concerning the experimen-
tal apparatus employed in supercritical treatments can be found
elsewhere [22].
Fisobutane,
− Fisobutane, outlet
inlet
Conversion Xisobu tan e, inlet
=
× 100
%
(
)
Fisobutane, inlet
Fisobutene,outlet
Isobutene yield Risobutene (%) =
× 100
Fisobutane,inlet
Fisobutene,outlet
Isobutene selectivity Sisobutene(%) =
× 100
Fisobutene,inlet − Fisobutene, outlet
FCO2,outlet/4
CO2 selectivity SCO2(%) =
× 100
Fisobutane,inlet − Fisobutane,outlet
Where X is the conversion, F is the molar flow rate, R is the yield
and S is the selectivity.
The as-received fiber was also subjected to an acidic treatment.
A sample modified with HNO3 was prepared by exposing 5 g of
fresh cloth to a boiling 1 M nitric acid solution in a Soxhlet during
180 min. Afterwards the cloth was repeatedly washed with deion-
ized water until any remaining acid was removed.
3. Results
3.1. Characterization of the catalysts
Table 1 shows the textural properties of the original and modi-
fied fibers. The raw material is essentially microporous, as reflected
by its high SBET and micropore volumes (Table 1). Nitric acid par-
tially destroys the microporosity due to its strong acidic character.
The micropore volume decreases while the mesoporosity and the
2.2. Characterization of the catalysts
The textural characterization of the catalysts was based on
their N2 adsorption-desorption and CO2 adsorption isotherms. N2