КИНЕТИКА И КАТАЛИЗ, 2010, том 51, № 3, с. 418-422
УДК 541.127:547.313.4:546.763-31
A KINETIC STUDY OF OXIDATIVE DEHYDROGENATION OF ISOBUTANE TO ISOBUTYLENE OVER CHROMIUM OXIDE SUPPORTED ON LANTHANUM CARBONATE
© 2010 M. Hoang, J. F. Mathews*, K. C. Pratt**, Z. Xie
CSIRO Material Science & Engineering, Clayton South, Australia *Deparment of Chemical Engineering, Monash University, Clayton, Australia **Swinburne University of Technology, Hawthorn, Australia E-mail: manh.hoang@csiro.au Received 21.04.2009 г.
Kinetics of oxidative dehydrogenation of isobutane to isobutylene over chromium oxide supported on lanthanum carbonate was studied. The formation rate of isobutylene was found to be first order in isobutane and zero order in oxygen concentration and the rate limiting step was the regeneration of active sites by the gas phase oxygen. The formation of CO2 was due to the consecutive oxidation of isobutylene which occurred on the same active sites used for isobutylene formation, not by the parallel oxidation of isobutane.
Oxidative dehydrogenation of isobutane to isobutylene has received considerable attention in recent years. Many supported and unsupported metal oxides have been found to yield good conversion and selectivity in oxidative dehydrogenation of isobutane to isobutylene [1—18]. Among the large number of metal oxide catalysts being studied, supported chromium oxide was proven to be a promising catalyst for the oxidative dehydrogenation of isobutane to isobutylene. Chromium oxide catalysts on different supports (Al2O3, SiO2, TiO2, CeO2 and La2(CO3)3 ) were found to be active and selective at temperatures lower than 300°C [4, 5, 7, 9—13, 16]. Grabowski et al. [11] have studied the oxidative dehydrogenation of isobutane over chromium oxide supported on various metal oxides at temperatures between 200—400°C and reported selectivities to isobutylene of up to 73% at 5% of isobutane conversion. An isobutylene yield of 9% has been obtained on CrO^/liO2 and K-promoted CrO^/Al2O3. Moriceau et al. [9] reported 57% isobutylene selectivity at 10% of the isobutane conversion on a binary catalyst of Cr—Ce—O at 270°C. El-bashir et al. [16] have shown that alumina supported chromium oxide exhibited isobutane conversion of 10% and isobutylene yield of 6% at 250°C.
Supported chromium oxide is well known to possess high activity for the selective oxidation of hydrocarbons [1, 7, 19—21]. The catalytic performance of supported chromium catalysts are strongly affected by the acidity/basicity of the oxide support [22] and the nature of the support [11]. The reactivities of the surface chromium oxide species are controlled primarily by metal— support interactions as the bulk oxides such as CrO3 or Cr2O3 are believed to be inactive phases [23]. Moriceau et al. [18] have studied the oxidative dehydrogenation of
isobutane on Cr—Ce—O oxide catalyst and also found that agglomerates of Cr2O3 did not contribute to the reaction. It was found the activity of the catalyst was due to the well dispersed Cr6+Ox species on the ceria surface.
Chromium-manganese catalyst supported on silica has been studied by Mirzabekova and Mamedov for de-hydrogenation of isobutane in the presence of carbon dioxide [24]. Over this catalyst, the reaction of isobutane with carbon dioxide proceeds mainly through the dehydrogenation pathway with subsequent oxidation of hydrogen. Grzybowska et al. [8] found that the main reaction of isobutane with oxygen is the oxidative dehydro-genation to isobutylene in the presence of chromium oxide supported on alumina catalyst, with the selectivity at 10% and conversion higher than 50%.
The oxidative dehydrogenation of isobutane to isobutylene over chromium oxide supported on lanthanum carbonate has been previously reported [12—15]. This catalyst has been found active and selective for this reaction. In pulse reactor, selectivity ofgreater than 95% at reasonable activities has been achieved at temperatures below 250°C. Water and carbon dioxide are the only other reaction products observed. The performance of the catalyst was found to be a function ofchro-mium oxide loading and activation conditions. There is no detectable coke formation and carbon balances were routinely found to be better than 98%.
Reaction kinetics is an important aspect of catalysis. It can contribute to the understanding of reaction mechanisms, and thereby spark ideas for formulation of new catalytic systems. Despite great attention to the ox-idative dehydrogenation of isobutane to isobutylene, only limited research has been conducted on the kinetic study of this reaction [10, 25—28]. Madeira et al. [26,
r, prnol g 1 s 1
20
15
10
10
20
30 40
Isobutane, vol %
Fig. 1. Rate of isobutylene formation as a function of isobutane content (temperature 195°C, GHSV = 30000 h-1).
r, prnol g 1 s 1
10
0 10 20 30 40
Oxygen, vol %
Fig. 2. Rate of isobutylene formation as a function of oxygen content (temperature 195°C, GHSV = 30000 h-1).
8
6
5
0
27] studied the kinetics and mechanisms of the n-bu-tane oxidative dehydrogenation over a 3% Cs-doped a-NiMoO4 catalyst and proposed a kinetic model based on a redox (Mars—van Krevelen) mechanism.
This paper presents a study of the kinetics of oxidative dehydrogenation of isobutane over chromium oxide supported on lanthanum carbonate.
EXPERIMENTAL
Catalyst preparation
The mixed carbonate precursor was prepared by the drop-wise addition of a mixed solution of La(NO3)3 • • 6H2O and Cr(NO3)3 • 9H2O, in a predetermined mole ratios, to a stirred solution of NH4HCO3. The resultant hydro gel was separated by centrifugation, washed with water and then with acetone. The product was first dried at room temperature in an air stream and then at 110°C in air for 4 h. The final catalyst was obtained by calcination at 300°C in air for 2 h.
Kinetics measurement
Kinetic studies were carried out in a differentialmode reactor. Details of the testing reactor have been previously reported [12]. In order to eliminate mass and heat transfer effects, catalyst testing was carried out at a very short residence time (0.12 s) which was achieved by working at a space velocity of 3 x 104 h-1. The feed stream was also diluted with He.
The catalytic activity is expressed as the rate of product formation (isobutylene for the oxidative dehydroge-nation and CO2 for the combustion).
RESULTS AND DISCUSSION
Rate dependency on isobutane
The formation rate of isobutylene (r) as a function of isobutane concentration in feed gas is shown in Fig. 1. Oxidative dehydrogenation of isobutane is a two elec-
tron process, consuming only 0.5 oxygen molecules. Thus in this experiment, the oxygen concentration was kept constant at 20 vol % and the oxygen/isobutane ratio was >0.5. As can be seen from Fig. 1, the formation rate of isobutylene is first order in isobutane.
Rate dependency on oxygen
The influence of oxygen concentration on the formation rate of isobutylene is illustrated in Fig. 2. In this experiment, the isobutane concentration was kept constant at 2.9 vol %. Again, the oxygen/isobutane mole ratio was maintained >0.5. At oxygen concentrations >2vol%, and within the experimental error of <10%, the reaction was found to be zero order in oxygen. However, the non-zero intercept of the rate as a function of oxygen concentration is observed indicating a contribution of lattice oxygen from the catalyst surface to the reaction.
The reaction mechanism involving a paraffinic hydrocarbon at the catalyst surface involves an electron donating/accepting mechanism in which the hydrocarbon acts as an electron donor. In principle, the basic nature of the surface of the catalyst promotes hydrogen abstraction from hydrocarbons.
Oxidative dehydrogenation of isobutane possibly proceeds by the following path:
i-C4 —i-C4[S]—CO2
i-C=
Here [S] denotes a site for chemisorption.
Considering only the selective reaction for the formation of isobutylene, firstly an initialising reaction step takes place, assumed to be the formation of an adsorbed isobutane intermediate. The reduction of the active site [S] by isobutane is followed by the formation of isobutylene:
/-C4H!0 + [S]
■» I-
C4H8 + [S*] + H2O.
E = 76 kJ/mol
lnr [prnol g 1 s 3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1.90 1.95 2.00 2.05 2.10 2.15 2.20
1000/ T, K-1
Fig. 3. Arrhenius plot for isobutylene formation on 10% Cr2O3/La2(CO3)3.
The reduced site [S*] is then regenerated by oxygen supplied from the gas phase:
[S*] + 0.5O2 —— [S]. The overall reaction is
i-C4H1n + 0.502 ^ i-C4H8 + H2O.
In this equations kr and ko are the rate constants of the reduction and oxidation of active site respectively. This redox reaction seems to be the key for the oxidative de-hydrogenation mechanism.
At steady-state the overall rate of reaction can be expressed as:
к r P C4 ko P O,
r=-
k PCi + ko Po,
or
r ko Po2 kr Pc4 As suggested previously, at oxygen concentrations more than 2%, the reaction rate is independent of the oxygen partial pressure in the gas phase. From the experimental data, at 468 K,
ko = 1.4 x 10-
mol s 1 g 1 kPa
and
kr = 2.3 x 10-6 mol s-1 g-1 kPa-1.
From this analysis, the limiting step of the oxidative dehydrogenation of isobutane over supported chromium oxide/lanthanum carbonate catalyst appears to be the regeneration of active sites by gas phase oxygen.
In alkane oxidation, it is well known that the first step of the reaction involves the formation of a radical intermediate. It is reasonable to assume the same activation step for isobutane. Thus, isobutane is first activated by a catalyst that abstracts an H atom from the molecule to form an adsorbed isobutyl radical intermediate. This step requires the presence of a very reactive surface oxygen species such as Cr=O, and the catalytic activity can be related to the amount of this species at the catalyst
surface. In a previous study [15], we reported the presence of Cr=O species on the catalyst surface. The catalyt
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