Dehydration of isobutane in fixed bed on the stable to carbon deposition chromia-alumina catalysts

Article abstract of DOI:10.1134/S1070427216110094

Study of chromia-alumina catalysts obtained from products of a thermochemical activation of aluminum trihydroxide for fixed-bed dehydrogenation of light paraffin hydrocarbons demonstrated that an increase in activity is observed in dehydrogenation of isobutane at a temperature of 590°C during the first 30 min of the catalytic process and then the activity slightly decreases. It was shown that the amount of carbon-deposition products on the surface of the synthesized catalysts after 120 min of isobutene dehydrogenation is 1–2 wt %. One of reasons for the high stability of chromia-alumina catalysts against carbon deposition is their porous structure. The use of the proposed catalysts in industry can improve the efficiency of the paraffin dehydrogenation processes by increasing the time of the reaction, while regeneration time is reduced.

Full text of DOI:10.1134/S1070427216110094

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 11, pp. 1791−1796. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © T.A. Bugrova, G.V. Mamontov, 2016, published in Zhurnal Prikladnoi Khimii, 2016, Vol. 89, No. 11, pp. 1422−1427.
CATALYSIS
Dehydration of Isobutane in Fixed Bed on the Stable to Carbon
Deposition Chromia-Alumina Catalysts
T. A. Bugrova and G. V. Mamontov*
National Research Tomsk State University, ul. Lenina 36, Tomsk, 634050 Russia
*
e-mail: GrigoriyMamontov@mail.ru
Received November 7, 2016
Abstract—Study of chromia-alumina catalysts obtained from products of a thermochemical activation of aluminum
trihydroxide for fixed-bed dehydrogenation of light paraffin hydrocarbons demonstrated that an increase in activ-
ity is observed in dehydrogenation of isobutane at a temperature of 590°C during the first 30 min of the catalytic
process and then the activity slightly decreases. It was shown that the amount of carbon-deposition products on
the surface of the synthesized catalysts after 120 min of isobutene dehydrogenation is 1–2 wt %. One of reasons
for the high stability of chromia-alumina catalysts against carbon deposition is their porous structure. The use
of the proposed catalysts in industry can improve the efficiency of the paraffin dehydrogenation processes by
increasing the time of the reaction, while regeneration time is reduced.
DOI: 10.1134/S1070427216110094
The processes of dehydrogenation of paraffin
hydrocarbons are of high industrial importance to obtain
a wide range of unsaturated hydrocarbons, including
with an inert gas [18]. According to data from different
sources, the dehydrogenation stage takes 4 to 12 min,
with the total duration of the activation–dehydrogenation–
regeneration cycle being 25–30 min [19]. One of
promising ways to improve the process efficiency is by
making longer the dehydrogenation stage. This can be
achieved by raising the catalyst stability against carbon
deposition, with the catalyst necessarily retaining high
conversion and selectivity parameters for providing a
high yield of the product, unsaturated hydrocarbons.
In industrial implementation of the dehydrogenation
process this will require some additional procedures
aimed to maintain a sufficiently high temperature of the
catalyst bed under the conditions of the endothermic
dehydrogenation reaction. This problem is partly solved
by using an additional heat-generating material, which is
charge in the Catofin process together with the catalyst
and the inert diluent [20, 21].
propylene, normal and iso-structural C –C olefins,
4
5
styrene, etc. [1]. Chromia-alumina catalysts have
found application in direct [2–7] and oxidative [8–10]
dehydrogenation of paraffins into the corresponding
olefins. In Russia, the dehydrogenation of paraffins is
mostly performed in a fluidized bed with microspherical
catalysts of AOK 73-24, IM-2201, KDM, and other
brands [11–14]. Performing the dehydrogenation in a
fixed catalyst bed is believed to be more efficient because
this provides a higher yield of unsaturated hydrocarbons,
the catalysts has a longer service life, and there is no
carryover of a substantial amount of the catalyst by the
gas flow of reagents in view of its zero abrasion [15, 16].
However, one of the main problems for the fixed-bed
catalyst, as also for a boiling-bed catalyst, is the loss
of activity due to the carbon deposition processes [17].
Therefore, the dehydrogenation in a fixed bed, including
the Catofin process, is performed in the cyclic mode,
which includes the reductive activation with hydrogen
In this regard, the purpose of this work was to develop
a chromia-alumina catalyst stable to formation of carbon
depositions to increase the time of the dehydrogenation
step. Chromia-alumina catalysts were prepared on the
basis of domestic raw materials, products formed in
thermochemical activation of aluminum trihydroxide
(occasionally, there is no such stage), dehydrogenation,
and oxidative regeneration. Between these stages, a
catalyst is subjected to a vacuum treatment, or blowing
1
791

1792
BUGROVA , MAMONTOV
(
TCAATH), and their catalytic properties and deactivation
technological procedures and involves only a single high-
temperature treatment, in contrast to the impregnation
method in which first the support is calcined and then
the catalyst.
in the reaction of isobutane dehydrogenation in a fixed
bed were examined.
EXPERIMENTAL
The characteristics of the porous structure were
studied by the method of low-temperature adsorption
of nitrogen on a Micromeritics Tristar 3020 instrument
For comparison, we studied two chromia-alumina
catalysts formed from products of thermochemical
activation of aluminum trihydroxide. According to [22],
the catalyst of the Catofin process contains the following
components (wt %): CrO 17–24, Na O 0.1–0.3, K O
(United States). The specific surface area was determined
by the BET method from the rectification of the adsorp-
tion isotherm at relative pressures in the range 0.05–0.30.
The pore size distribution was constructed by using the
BJH method from the desorption branch of the isotherm.
3
2
2
0
.1–2, and ZrO up to 1.5. We synthesized two catalysts
with chemical composition close to that of the Catofin
2
The catalytic properties of the catalysts synthesized
in the study were examined in the reaction of isobutene
dehydrogenation at a temperature of 590°C in a flow-
through catalytic installation with a quartz reactor
(inner diameter ~15 mm). The temperature was chosen
because the dehydrogenation process is performed in
the temperature range 540–600°C, but the processes of
catalyst deactivation as a result of the carbon deposition
have the highest intensity at 590°C and more. A 1-g
portion of the catalyst (0.5–1-mm fraction) was mixed
with an inert diluent, quartz glass of the same fraction in
a 1 : 1 ratio. The reaction products were analyzed with
a Khromatek-Kristall 5000.2 gas chromatograph. The
components formed in the dehydrogenation of isobutene
were separated on a Varian Capillary Column CP-Al O /
catalyst (wt %): Cr O 20, K O 2, and ZrO 1. Catalyst 1
was produced by the incipient wetness impregnation of
TCA ATH preliminarily molded as cylindrical granules
2
3
2
2
2
.7–3.0 mm in diameter and calcined at 700°C. The
impregnation was made with an aqueous solution of
chromium anhydride, potassium hydroxide, and zirconyl
nitrate, all of chemically pure grade. Then the samples
were dried at a temperature of 120°C for 4 h and calcined
at 700°C for 4 h.
Catalyst was produced by wet mixing. For this
purpose, TCAATH was mixed in a mixer with Z-shaped
blades with an aqueous solution containing a prescribed
amount of dissolved precursors of chromium oxide,
potassium, and zirconium. The resulting mass was molded
by extrusion to give granules of similar size. The catalyst
was dried at 120°C for 4 h and calcined at 700°C for 4 h.
This way to obtain the catalyst is interesting and more
profitable because of including the minimum number of
2
3
Na SO . The main activity parameters of the catalyst:
2
4
conversion X (vol %), selectivity S (%), and yield Y (%),
were calculated by the formulas
Athermal analysis was made on an STA449 F1 Jupiter
analyzer (Netzsch-Geraetebau GmbH) in the DSC–TG
mode in a flow of air (50 mL min ). The gas mixture
composition in the burning of carbon on the catalyst
surface was determined with a QMS 403D Aeölos mass
spectrometer.
RESULTS AND DISCUSSION
–
1
The porous structure of the catalysts obtained on the
basis of TCAATH products was examined by the method
of low-temperature adsorption of nitrogen. The table lists
values of the specific surface area and pore volume for the
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DEHYDRATION OF ISOBUTANE IN FIXED BED
1793
a, mmol g–1
Vpore, cm g nm
3
–1
–1
(a)
(b)
R, nm
Fig. 1. (a) Isotherms of adsorption–desorption a of nitrogen and (b) pore size R distribution for the catalysts under study. (P/P ) Relative
0
pressure and (Vpore) pore volume.
catalysts we synthesized. Catalysts 1 and 2 have specific
t-plot method is insignificant for both the catalysts (see
table), but is two times larger for catalyst 2. This may be
due both to the more significant pore size redistribution
for the impregnated catalyst in the repeated calcination
and reprecipitation of the support in the presence of
chromic acid (pH < 0) and to the distribution of chromium
oxide within micropores of the support.
2
–1
surface areas of 82 and 111 m g at pore volumes of
3
0
.25 and 0.24 cm , respectively. The nitrogen adsorption–
desorption isotherms and the pore size distributions for
the samples are shown in Fig. 1. The nitrogen-adsorption–
desorption isotherms (Fig. 1a) characteristically have a
hysteresis loop at relative pressures in the range 0.5–1.0,
which indicates that the catalysts have a mesoporous
structure. For the catalyst 1 obtained by the impregnation
method, the hysteresis loop is shifted to a large relative
pressure range, indicating a large mesopores contribution
to the porous structure of the sample. It can be seen from
the pore size distributions (Fig. 1b) that a bimodal pore
size distribution is observed for catalyst produced by
mixing: there are pores 2–8 nm in size and pores larger
than 8 nm. For catalyst 1, produced by impregnation of a
preliminarily molded and calcined aluminum oxide, the
pore size distribution is more strongly shifted to large
sizes (>8 nm), which may be due to the partial dissolution
of the alumina support in the course of impregnation with
the acid solution of chromic acid. Thus, the catalysts
based on TCAATH have a lager specific surface area and
pore volume. The micropore volume estimated by the
Figure 2 shows time dependences of the conversion
and selectivity of the catalysts in the reaction of isobutane
dehydrogenation at a temperature of 590°C. Catalysts 1
and 2 are characterized by an initial isobutane conversion
of about 53.9 and 56.4%, respectively, at a selectivity
of 93–95%. These data are close to the activity of the
industrial catalysts for the Catofin process characterized
by an initial isobutane conversion of 55.3–56.1% at a
selectivity with respect to isobutylene of 91.5–92.1%
for the process performed at a pressure of 0.33 atm [22].
Catalyst 2 produced by the mixing method is charac-
terized by high conversion of isobutane at 15th min of
dehydrogenation at a selectivity with respect to isobutyl-
ene of 93.5%. During the first 60 min of dehydrogenation,
an insignificant decrease in the conversion is observed
Texture characteristics of the catalysts under study
Volume of >8-nm pores
Vmicro(t-plot),
2
–1
3
–1
Catalyst
SBET, m g
Vpore, cm g
3
–1
cm g
3
–1
cm g
%
1
2
82
0.25
0.24
0.177
0.117
70.4
49.8
0.004
0.008
111
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 89 No. 11 2016

1794
BUGROVA , MAMONTOV
Catalyst 1, produced by the impregnation method,
exhibits in the initial period of time a lower conversion
of isobutane, compared with catalyst 2. However, its
activity (conversion of isobutane) slightly increases by
Catalyst 1
Catalyst 2
3
0th min of dehydrogenation and becomes comparable
with the conversion for catalyst 2. The increase in the
activity of the catalyst is indicative of its development
in the course of isobutane dehydrogenation, which may
be due to the after-reduction of Cr(VI) to Cr(III) by the
reaction mixture to give an additional number of active
centers. Catalyst 1 also has a high stability, and in its
subsequent dehydrogenation, the conversion of isobutane
decreases from 54.6 to 46%. The selectivity with respect
to formation of isobutylene is higher for the impregnated
catalyst, compared with catalyst 2, and insignificantly
grows during the dehydrogenation (from 94.9 to 95.5%).
τ, min
Fig. 2. Isobutane conversion X, selectivity S with respect to
isobutylene, and isobutylene yield Y vs. time τ for the catalysts
under study in the dehydrogenation of isobutane at 590°C.
Thus, catalysts based on TCAATH are characterized
by a high catalytic activity in the dehydrogenation of
isobutane, which is comparable with activity of industrial
catalysts, but have a higher selectivity and are weakly
deactivated in the course of a prolonged dehydrogenation.
and the selectivity increases from 93.5 to 95.6%, with
this value preserved during the second hour of dehydro-
genation. The increase in the selectivity may be due to
the blocking of a part of active centers of the catalyst,
supposedly strong acid centers, by carbon deposition
processes.As a result, the conversion of isobutane some-
what decreases, with the selectivity for the target process
increasing. During the second hour of dehydrogenation,
a more intense deactivation of the catalyst is observed,
with the conversion of isobutane decreasing from 56.4 to
To examine the carbon deposition products after
120 min of dehydrogenation, we cooled the catalysts,
instead of performing the subsequent oxidative treatment,
in the catalytic reactor to room temperature in a flow
of inert gas (nitrogen). Then, an approximately 10-mg
portion of a homogenized catalyst sample was examined
by the DSC–TG method, with mass-spectrometric
monitoring of gaseous products. Figure 3 shows the data
4
5.5%. The advisability of performing the dehydrogena-
tion during the second hour is determined by the yield of
isobutylene, which decreases from 50.8 to 43.4% during
the period from 60th to 120th min of dehydrogenation.
obtained with the catalysts heated in a flow of air at a rate
of 10 deg min–1. The catalysts exhibit a loss of mass in
the temperature range 250–450°C, accompanied by an
(b)
(a)
TF, %
I, A
.5 × 10‒11
Catalyst 1
Catalyst 2
Catalyst 1
Catalyst 2
6
5
.5 × 10‒11
4
.5 × 10‒11
T, °C
T, °C
Fig. 3. Data of (a) thermogravimetric analysis and (b) differential scanning calorimetry with ass spectrum of CO evolution for the
2
catalysts after 120 min of dehydrogenation with heating in a flow of air. (T) Temperature and (I) ionic current.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 89 No. 11 2016

DEHYDRATION OF ISOBUTANE IN FIXED BED
1795
exothermic effect and evolution of CO (Fig. 3b), which
One of the main reasons why the selectivity and
stability of the catalysts obtained in the study is the
special porous structure of the catalysts, produced because
products of a thermochemical activation of aluminum
trihydroxide are used as a precursor of the support. When
catalysts are synthesized by different methods, catalyst
1 with smaller specific surface area is comparable in
activity with catalyst 2 and is more stable due to the more
pronounced contribution of large mesopores (>8 nm),
which favor an effective delivery of reagents to the active
centers of the catalyst and removal of reaction products.
2
indicates that carbon burns on the catalyst surface. The
close temperature range and the nature of the loss of mass
point to the about the same nature of carbon deposits
formed on the catalysts under study.At the same time, the
amounts of mass lost by the catalysts differ significantly:
for catalyst 2, produced by the mixing method, the loss
of mass is 1.99%, and that for catalyst 1 is 1.05%. These
data are in good agreement with the data obtained in a
study of the operation stability of the catalysts in the
dehydrogenation of isobutane. Catalyst 1 shows the
highest selectivity and stability, and, accordingly, the
amount of carbon deposition products formed on its
surface is lower.
ACKNOWLEDGMENTS
The authors are grateful to V.V. Dutov (Toms State
University) for performing thermogravimetric analyses.
CONCLUSIONS
The study was supported by the FederalTarget Program
“
R & D activities in priority areas of development of the
Two chromia-alumina catalysts based on products
formed in thermochemical activation of aluminum
trihydroxides and close in chemical composition to the
industrial catalyst for the Catofin process were compared
in the reaction of isobutane dehydrogenation. The
catalysts have a high catalytic activity that is comparable
with the activity of the industrial catalyst, but show
a higher selectivity with respect to isobutylene. The
catalysts synthesized in the study are insignificantly
deactivated during 60 min of dehydrogenation, whereas
the industrial catalyst for the Catofin process is operative
during 4–12 min. The high stability of the catalysts can
be used to raise the efficiency of the process in which
light paraffin hydrocarbons and, in particular, isobutane
are dehydrogenated due to the longer duration of the
dehydrogenation stage.
scientific-technological complex of Russia for the years
of 2014–2020 (contract no. 14.578.21.0028, RFMEFI
5
7814X0028).
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