Effect of CO2 in the reaction of oxidative dehydrogenation of

Effect of CO2 in the reaction of oxidative dehydrogenation of
propane over Cr/ZrO2 catalysts
João F. S. de Oliveira1, Diogo P. Volanti2, José M. C. Bueno1, Adriana P. Ferreira1
1 DEQ
- UFSCar, São Carlos-SP, 13.565-905, Brazil
IQ - UNESP, São José do Rio Preto-SP, Brazil
*Corresponding author: [email protected])
2
Keywords: Chromium-Zirconium oxides; Oxidative dehydrogenation of Propane; Basic sites; CO2; microwave
1. Introduction
Propene is a major raw material for the petrochemical
industry mainly used in polymer and rubber
industry and its growing demand requires new
production processes, as the transformation of
alkanes in their respective alkenes. Since a recent
past, an alternative route largely explored has been
the catalytic non-oxidative dehydrogenation of
propane (CDP) [1], which operates at high
temperatures (above 800 K) to overcome
thermodynamics restrains, with consequent catalyst
deactivation by propane/propene cracking with coke
deposition [2] and a loss in the propene selectivity
and yield. The oxidative dehydrogenation of propane
(ODP) becomes attractive due to the fact the reaction
is exothermic and can operate at lower temperatures,
suppressing coke formation [3]. However this
reaction favors complete oxidation of alkanes and
olefins and the reactants mixture hydrocarbonoxygen can be explosive, affecting drastically the
olefins yield due to low concentrations of propane. A
technological advancement can be achieved
increasing propene yield via higher propane
concentrations in process and/or coke gasification by
CO2 [4]. Furthermore CO2 must shift the equilibrium
to the product side and/or promote the
dehydrogenation through reaction coupling between
a simple dehydrogenation of propane and the reverse
water–gas shift reaction [4]. Herein, we analyze the
catalytic measurements in presence and absence of
CO2 using
Cr/ZrO2 catalysts obtained by
conventional and microwave assisted hydrothermal
treatments to address three major issues: (i) what is
the effect of CO2 on performance Cr/ZrO2 catalyst in
the dehydrogenation of propane; (ii) what is the
effect of structural properties of Cr/ZrO2, obtained
via different methods and Cr contents, on activity
and selectivity; (iii) the CO2 is effective in the
reactivation of Cr/ZrO2 catalyst used in ODP reaction.
2. Experimental Part
Cr/ZrO2 samples were prepared by hydrothermal
synthesis
with
two
methods: conventional
(autoclaves with solutions were heated a stove
with static atmosphere at 180oC during 24h) and
assisted by microwave (reactors with solutions
were coupled into the adapted microwave and
heated at 150oC during 2.5h). Aqueous solution of
zirconium nitrate (Aldrich) and specific amounts of
chromium nitrate (Aldrich) were used to prepare
samples containing 2.5, 5, 10 and 15 % (wt.) of Cr.
Precipitates were washed, dried and calcined in air
at 600oC, during 4h. These catalysts were named
y-xCZ, where "y” means c-Conventional or m–
Microwave assisted; “x” is the chromium load in
the catalysts. The catalysts were characterized by
nitrogen physisorption, X-ray diffraction, Raman
spectroscopy, temperature programmed reduction
and desorption of CO2 (TPD-CO2). The catalytic
performance tests were recorded in temperatureprogrammed on stream reactions (TPSR) during
300 minutes, flowing 20 mL/min of feed gas in ODP
(2.5 % (v/v) propane, 6.5 % (v/v) CO2 and balance
with He) or CDP (2.5 % (v/v) propane and He
balance). For used c-5CZ catalyst was flowing 20
mL/min of air synthetic for 5 minutes or pure CO2 for
30 minutes, at 600oC. At each catalytic cycle of 45
minutes on stream in CDP reaction conditions, a
regeneration procedure was applied and, then, a new
cycle was performed.
3. Results and discussion
The Cr/ZrO2 catalysts containing various chromium
contents between 2.5 and 15 wt. % of Cr were
prepared by hydrothermal conventional and
microwave assisted methods. The tetragonal ZrO2 9
t-ZrO2) is formed decreasing the crystallites sizes
with increases of Cr contents, the amorphous phase is
achieved for samples containing 10 and 15 wt% of Cr.
Smaller t-ZrO2 crystallites sizes are obtained by
microwave assisted method. TPR-H2 profiles indicate
CO2 was not efficient to promote re-oxidation of Cr+3
into Cr+6 species in reduced samples.
Figure
2.
Propene yield
of the fresh (A)
conventional
and
(B)
microwave
assisted y-xCZ
(y= c- or m-; x
= 2.5 to 15 %
wt. of Cr)
catalysts under
CDP and ODPCO2 reaction
conditions.
Figure 1. Mass spectra of CO2 (M.M.= 44 g.mol-1)
registered during TPD-CO2 from the y-xCZ (x = 0 to 15 %
wt. of Cr) catalysts (left) and from the c-10CZ catalyst
fresh and previously reduced (right). Factors between
parenthesis indicate that curve signal is multiplied by it.
Figure 1 presents TPD-CO2 results from y-xCZ
catalysts. Cr addition on ZrO2 changes drastically its
basicity (Fig. 1b). The strong basic sites are formed
and a maximum of density this sites is observed in
samples containing about 10 wt % of chromium. A
fraction of CO2 is desorbed at temperatures higher
than 500oC and characteristic of strong basic site at
interface like Cr6+-ZrO2. The presence and density of
these sites depends on Cr contents and preparation
methods. Figure 2 presents propene yield of the fresh yxCZ catalysts. In CDP the selectivity to propene
decreases with decreasing of Cr contents and
crystallization of t-ZrO2. The effects of t-ZrO2 on
selectivity decrease with increasing Cr contents,
which reflect in formation of ZrO2 amorphous phase.
The selectivity to propene decreases with presence of
CO2 and becomes less sensitive to chromium contents
than in absences of CO2 in CDP reaction. The
decreasing of initial activity depends on presence
of strong basic sites, which desorbs CO2 at high
temperature. The strong basic property is assigned to
interfacial sites like Cr6+-O-Zr. The maximum of
strong is observed in catalysts with about 10 wt. % of
chromium. On basis of catalytic measurements in
presence of CO2, the selectivity to propene decrease
with presence of CO2 and indicates that a relationship
between stability of activity with time on stream
and propane conversion is strongly dependent of
presence of strong basic sites. The catalysts that
exhibit only weak basic sites, the activity is
insensitive to CO2 and the deactivation occur by
reduction of Cr+6 species and by deposition of
carbonaceous species. The catalysts that exhibit
strong basic sites the activity decreases by presence
of CO2, caused through of strong interaction of CO2
site with activity sites like Cr6+-O-Zr. The CO2 is
more effective to maintain the activity with time on
stream for catalyst that contains strong basic sites.
The reactivation with CO2 indicates that the
mechanism is not self-sustained trough modulating
propane and CO2 in feed, the activity decrease with
number of cycles due a deposition of carbonaceous
species, which is effectively reactivated in presence
of O2.
4. Conclusions
The Cr/ZrO2 catalysts containing various chromium
contents between 2.5 and 15 wt % of Cr were
prepared by hydrothermal conventional and
microwave assisted methods. The tetragonal ZrO2
is formed, decreasing the crystallites sizes with
increases of Cr contents and smaller crystallites sizes
are obtained in microwave assisted method. The
TPD-CO2 results show that a fraction of CO2 is
desorbed at temperatures high than 500oC and
characteristic of strong basic site at interface like
Cr6+-ZrO2. The presence and density of these sites
depends on Cr contents and preparation methods. On
basis of catalytic measurements in presence of CO2,
the selectivity to propene decrease with presence
of CO2. The stability with time on stream depends
on presence strong basic sites. The catalyst that
shows only weak basic sites the activity decreases
with time on stream by reduction of Cr6+ species
and accumulation of carbonaceous species. The
catalyst with strong basic site the activity slight
decreases with time on stream in presence of CO2.
The initial activity decreases with number of cycles
upper than 2. The thermal treatment of used catalysts
in presence of O2 reestablishes the activity.
Acknowledgments
We acknowledge the financial support from CNPq and CAPES.
References (Time new roman, 9 pts)
[1] M. S. Kumar, J.C. Walmsley, A. Holmen, Catal. Commun,
2008, 9, 747.
[2] E. Daniel, E. Resasco, G.L. Haller, in: J.J. Spivey, S.K.
Agarwal (Eds.), Catalysis, vol. 11, Royal Society of Chemistry,
Cambridge, 1994, p. 379.
[3] R. Grabowski, Catal. Rev., 2006, 48, 199–268.
[4] C. Boucetta, M. Kacimi, A. Ensuqu, J.Y, Piquemal, F. BozonVerduraz, M. Ziyad. Applied Catalysis A: Gen, 2009, 356, 201–
210.