Kinetic Modeling of Propane Oxidative ... - ACS Publications

Article pubs.acs.org/IECR

Kinetic Modeling of Propane Oxidative Dehydrogenation over VOx/γAl2O3 Catalysts in the Chemical Reactor Engineering Center Riser Reactor Simulator S. Al-Ghamdi,†,‡ J. Moreira,† and H. de Lasa*,† †

Chemical Reactor Engineering Centre, University of Western Ontario, London, Ontario, Canada Research & Development Center, Saudi Aramco Oil Company, Dhahran, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: This study reports kinetic modeling of propane oxidative dehydrogenation (ODH) employing a new VOx/γAl2O3 catalyst especially designed for propane ODH with a controlled acidity. This catalyst is prepared with different vanadium loadings (5−10 wt %). Kinetic experiments are carried out under an oxygen-free atmosphere in the Chemical Reactor Engineering Center fluidized bed riser simulator at 475−550 °C and atmospheric pressure. Successive-injection propane ODH experiments (without catalyst regeneration) over partially reduced catalysts show good propane conversions (11.73%-15.11%) and promising propylene selectivity (67.65−85.89%). Regarding propylene selectivity, it increases while that for COx decreases as the catalyst degree of reduction augments with the consecutive propane injections. This suggests that a controlled degree of catalyst reduction is needed for high propylene selectivity. Under such oxygen-free conditions, the lattice oxygen of the catalyst is consumed via the ODH reaction. On the basis of the data obtained, a kinetic model is proposed. In this model, reaction rates are related to the degree of catalyst reduction using an exponential decay function. The kinetic and decay model parameters are estimated using nonlinear regression analysis. Activation energies and Arrhenius pre-exponential constants are calculated with their respective confidence intervals. The proposed parallel-series kinetic model satisfactorily predicts the ODH reaction of propane under the selected reaction conditions.

1. INTRODUCTION

studies have been based on experiments involving cofeeding of hydrocarbon species and molecular oxygen to the reaction unit. However, only a few studies among the above-mentioned research have considered propane ODH in the absence of gas phase oxygen, and only a few of them reported data on kinetic investigations.45,50,52,58−65 In all these studies, the proposed kinetic models involved hydrocarbon molecules reacting with the lattice oxygen of the supported metal oxides, giving olefins and H2O. The reduced metal oxides are reoxidized by gas phase molecular oxygen to regenerate the active metal oxides and replenish the lattice oxygen. Moreover, there is limited information that deals with light olefins ODH in dense fluidized beds, risers, or downers reactors. These studies are being confined mainly to butane ODH.35,66−68 There is also the motivation of using fluidizedbed reactors in light paraffins ODH. This is due to their controlled isothermal conditions, uniform residence time distributions, and the absence of mass transfer limitations. Hence, one can circumvent with these units, the potential issues with nonisothermal conditions in fixed-bed catalytic reactors. Moreover, fluidized reactors with periodic catalyst reoxidation offer the means that enables the transport of the reduced catalyst from the oxidative dehydrogenation reaction unit to the

With the increasing worldwide demand for light olefins, the conversion of light paraffins, ethane, propane, and butane, to their corresponding olefins is a topic of current commercial interest.1−20 Light olefins, such as ethylene and propylene are important raw materials for the synthesis of polymers and various petrochemical products. Oxidative dehydrogenation (ODH) of light paraffins has been the most investigated route for olefin production, overcoming many limitations of the current olefin production processes, namely, high energy requirements, coke formation, and thermodynamic constraints.21−27 However, the low selectivity toward the desired olefin products is the main problem of light paraffin ODH process preventing it from an industrial application.2,28−32 Light paraffin oxidative dehydrogenation involves a complex reaction network. This reaction network involves competitive reactions taking place simultaneously. In such system, oxygen supply plays an important role. This is the case as the reaction rates for both olefin production and reactants/products combustion are strongly influenced by oxygen concentration. Thus, optimized oxygen concentration is required in order to maximize selectivity toward the desired olefin products. Kinetic studies for short chain alkane (ethane, propane, and butane) ODH have been the subject of a significant number of papers in the open literature.14,16,30,33−57 These contributions use different kinetic models such as the Langmuir−Hinshelwood, the Eley−Rideal, or the Mars van Krevelen models. It is important to mention that the proposed kinetic models in these © XXXX American Chemical Society

Special Issue: Alirio Rodrigues Festschrift Received: December 1, 2013 Revised: January 29, 2014 Accepted: February 3, 2014

A

dx.doi.org/10.1021/ie404064j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

3. KINETIC MODELING Propane ODH involves a network of consecutive and parallel reactions, namely, the ODH of propane, the undesired combustion of propane feed, and the secondary combustion of propylene product. These reactions can be described stochiometrically as 1 C3H8 + O2 → C3H6 + H 2O (1) 2

reoxidation unit. Thus, the operation of twin reactors, one for the ODH reaction and one for catalyst regeneration appears to be a requirement for the implementation of this technology at an industrial scale. In this regard, ethane ODH over VOx/γAl2O3 has been successfully tested in the Chemical Reactor Engineering Center (CREC) riser simulator in the absence of molecular gas oxygen. Promising ethane conversions and ethylene selectivities have already been published.69,70 Thus, it is in our interest and given the significant prospects for industrial applications to extend our research and explore propane ODH in the CREC riser simulator over VOx/γ-Al2O3 catalysts in an oxygen-free atmosphere. Given the above-described facts, the goal of this paper is to establish propane ODH kinetics over a new VOx/γ-Al2O3 fludizable catalyst for propane ODH in the absence of gas phase oxygen. To accomplish this, a kinetic model in which reaction rates are related to the degree of catalyst reduction using an exponential decay function is proposed. It is our understanding that such propane ODH kinetics in the absence of molecular oxygen gas together with a very selective vanadium-based catalyst and the demonstration of its performance in a fluidized bed laboratory reactor has not been reported before.

C3H8 +

1 (3x + 4)O2 → 3COx + 4H 2O 2

(2)

C3H6 +

1 (3x + 3)O2 → 3COx + 3H 2O 2

(3)

with the latter two reactions 2 and 3 being the limiting ones for propylene selectivity and yield during propane ODH. On the basis of the observed dependence between propane conversion and product selectivities for propane ODH, a parallel-series “triangular” reaction network is proposed in this study as described in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. The VOx/γ-Al2O3 catalysts used in this investigation were prepared by wet saturation-impregnation of a commercial porous γ-Al2O3 support with (SASOL, Catalox SCCa 5/200), with an aqueous solution of NH4VO3 (Sigma Aldrich, 99%) and oxalic acid (Sigma Aldrich, 99%) in a 1:2 weight ratio and at a pH of ∼2. The prepared samples were characterized by BET surface area, temperature programmed reduction (TPR), X-ray diffraction (XRD), and NH3 temperature programmed oxidation (NH3TPD), O2 pulse chemisorption, and laser Raman spectroscopy (LRS). A detailed description of the catalyst preparation and characterization techniques used are given in a recent publication by Al-Ghamdi et al.71 For the sake of avoiding already reported information, the specifics on the preparation and characterization techniques will not be given in the present manuscript. 2.2. Propane ODH Reaction Studies. The catalytic propane oxidative dehydrogenation experiments over various supported vanadium oxide catalysts were established at the CREC using the riser simulator. The CREC riser simulator is a bench-scale mini-fluidized bed reactor, invented by de Lasa.72 This unit is specially designed for catalyst evaluation and kinetic studies under fluidized bed (riser/downer) reactor conditions. Details of the CREC riser simulator as well as the experimental procedure and setup have already been reported elsewhere by Al-Ghamdi et al.70 Thermal and catalytic experiments of propane ODH were carried out at four temperatures (475, 500, 525, and 550 °C), one catalyst loading (0.76 g) and four contact times (5, 10, 15, and 20 s). All catalytic runs were repeated at three times to ensure reproducibility of the experimental results. The carbon mass balance closures, which considered all carbon-containing products such as CO, CO2, CH4, ethylene, ethane, propylene, propane, and carbon deposited over the catalyst were in the ± 7 % range. For each catalyst, 10 sequential propane injections were used at a particular temperature and contact time.. On the basis of the inlet and outlet concentrations, the conversion, selectivity, and yield were calculated.

Figure 1. Proposed reaction network for propane ODH over VOx/γAl2O3 catalysts in a CREC riser simulator.

The proposed reaction network assumes that propane feed reacts with lattice oxygen promoting two parallel reactions. One of the possible reactions is the formation of the desired propylene product via ODH, with a rate constant k1. Alternatively, propane can be competitively converted forming combustion products (COx), with a rate constant k2. Finally, the formed propylene may also follow a secondary reaction step leading to the formation of combustion products (COx), with rate constant k3. 3.1. Kinetic Model Development. Regarding kinetic model development and in cases such as the one of the present study, in which propane ODH is carried out in the absence of gas phase oxygen, it can be postulated that oxygen from the catalyst lattice is consumed via propane ODH reactions only. Under such conditions, the catalyst oxygen content has to be included in the kinetic model. Moreover, an allowance must be made for the catalyst’s time on stream or catalyst history in order to achieve a kinetic model capable of describing all transient observations during the course of the ODH reaction. To accomplish this, the catalyst oxygen content can be expressed using a time dependent degree of oxidation (% oxidation). This time-dependent degree of oxidation can be defined as the ratio of oxygen content left after the ODH reaction over the original oxygen content of the catalyst before the ODH run. The catalyst degree of oxidation is expected to decrease during the ODH reaction cycle. This is especially true, B



Article

as in the present study, if one does not allow catalyst reoxidation, or replenishment of the lattice oxygen by gas-phase molecular oxygen in between ODH reaction cycles. A possible approach to kinetic modeling based on the available lattice oxygen during oxygen-free ODH reactions is the use of an exponential decay function based on converted propane. This type of activity decay function is similar to the one initially proposed by de Lasa and Al-Khattaf73 for FCC catalytic activity decay, Hossain et al.74 for ethylbenzene dehydrogenation to styrene, and recently by Al-Ghamdi et al75 for ethane ODH reaction, and it can be written as follows:

ϕ = exp[−λ(XC3H8)]

C3H8 − [S2 ](s) + (4 + 3x)[S1O](s) k2

→ 3COx (g) + 4H 2O(g) + (4 + 3x)[S1R ](s) + [S2 ](s)

(v) Formation of COx from propene (r3) via further reaction of adsorbed propene with lattice oxygen in an oxidized site [S1O]: C3H6 − [S2 ](s) + (3 + 3x)[S1O](s) → 3COx (g) + 3H 2O(g) + (3 + 3x)[S1R ](s)

(4)

+ [S2 ](s)

where ϕ is the catalyst’s degree of oxidation, λ is a constant, and X is the propane conversion. The main advantage of this function is that it accounts for the effects of reaction conditions (temperature, concentration, and contact time) on the catalyst extent of oxidation. Furthermore, the choice of the kinetic model for propane ODH is of paramount importance. With this in mind, a Mars van Krevelen (MVK) mechanism is considered in the present study. This model is generally accepted for oxidative dehydrogenation reactions over vanadia catalysts involving VOx lattice oxygen species. On the basis of a MVK mechanism established in the context of two catalyst sites for propane ODH, the catalyst is reduced by the reaction of the adsorbed propane with catalyst lattice oxygen to produce propylene molecules and carbon oxides. Moreover, the adsorbed propylene product on the catalyst surface can further react with lattice oxygen producing carbon oxides. Gas phase oxygen can be intermittently introduced following several propane injections to replenish the lattice oxygen by reoxidation of the catalyst. As a result, the following six-step mechanism is proposed as shown below, with [S1O] representing lattice oxygen in an oxidized site-1, [S1R] denoting a surface oxygen vacancy in a reduced site-1, and [S2] representing alumina-based sites (i) Propane adsorption on a site type-2 [S2] on the catalyst surface: K C3H8

C3H8(g) + [S2 ](s) ←⎯⎯→ C3H8 − [S2 ](s)

O2 (g) + 2[S1R ](s) → [S1O](s) + [S1O](s)

r1 = k1θC3H8(1 − β)

(11)

r2 = k 2θC3H8(1 − β)

(12)

r3 = k 3θC3H6(1 − β)

(13)

where ri is the reaction rate (mol/g·sec), ki is the reaction rate constant (mol/g·sec), θi is the surface coverage of adsorbed species “i”, with β + γ = 1, in which β represents the reduced vanadium sites and γ = 1 − β represents the oxidized vanadium sites (site 1 type), and θC3H8 + θC3H6 + θv = 1 describes the alumina based sites (site 2 type). Thus, and as a result, the surface coverage θi is given by θC3H8 =

θC3H6 =

(5)

K C3H8CC3H8 (1 + K C3H8CC3H8 + K C3H6CC3H6)

(14)

K C3H6CC3H6 (1 + K C3H8CC3H8 + K C3H6CC3H6)

(15)

where Ki is the adsorption constant (cm3/mol) of species “i” and Ci is the concentration (mol/cm3) of species “i”. Moreover, given the proposed decay model for the catalyst’s degree of oxidation (ϕ) in eq 4, β can be related to ϕ as (1 − β) = φ

k1

→ C3H6 − [S2 ](s) + H 2O(g) + [S1O](s)

(16)

A substitution of eqs 14 to 16 into eqs. 11 to 13 gives the following rate equations based on the proposed reaction network in Figure 1:

(6)

(iii) Desorption of adsorbed propylene from a reduced site: K C3H6

(10)

On the basis of a MVK mechanism, the rate equations corresponding to each surface reaction in the above steps, (i.e., r1, r2, and r3) are expressed as follows:

C3H8 − [S2 ](s) + 2[S1O](s)

C3H6 − [S2 ](s) ←⎯⎯→ C3H8(g) + [S2](s)

(9)

(vi) Reoxidation of the reduced metallic center by molecular oxygen:

(ii) Formation of propylene (r1) via the surface reaction of adsorbed propane with lattice oxygen in an oxidized site [S1O] via H-abstraction:

+ [S1R ](s)

(8)

r1 =

k1K C3H8CC3H8 (1 + K C3H8CC3H8 + K C3H6CC3H6)

exp[−λ(XC3H8)]

(7)

(17)

(iv) Formation of COx from propane (r2) via the reaction of adsorbed propane with lattice oxygen in an oxidized site [S1O]:

r2 =

k 2K C3H8CC3H8 (1 + K C3H8CC3H8 + K C3H6CC3H6)

exp[−λ(XC3H8)] (18)

C


Industrial & Engineering Chemistry Research k 3K C3H6CC3H6

r3 =

(1 + K C3H8CC3H8 + K C3H6CC3H6)

Article

dyi

exp[−λ(XC3H8)]

=

dt (19)

Wc ⎛ MWi VR ⎞ ⎜ ⎟ri VR ⎝ Whc ⎠

This equation is appropriate under the conditions of the present study given that (a) the CREC riser simulator is a bench-scale isothermal well-mixed batch reactor unit with a high fluid gas recirculation and mixing times in the 20−30 ms scale77 and (b) the catalyst particles operate freely from intraparticle diffusional transport controls. The second condition is met given the small size of catalyst particles (90 μm) and the porous structure of the fludizable alumina used. By establishing one equation such as eq 25 for each component, one can obtain a set of differential equations to represent the propane ODH network. This set of ordinary differential equations is obtained by substituting the rate equation of each component given in eqs 20 to 22 into eq 25 and considering that

One can notice in eqs 17,18, and 19 that the rate of reaction is related to the adsorption constants, the catalyst extent of oxidation, and the intrinsic kinetic parameters. Furthermore, using these rate equations, one can establish the overall rate of consumption and/or formation of chemical species. This can be accomplished on the basis of the algebraic addition of reaction rates consistent with the reaction network given in Figure 1. On the basis of this, the following rate equations could be established for each chemical species involved in the propane ODH reaction network: Rate of Propane Consumption: rC3H8 = −(r1 + r2) =

−(k1 + k 2)K C3H8CC3H8 (1 + K C3H8CC3H8 + K C3H6CC3H6)

CC3H8 =

exp[−λ(XC3H8)]

Rate of propylene formation:

CC3H6 =

rC3H6 = r1 − r3 =

(1 + K C3H8CC3H8 + K C3H6CC3H6)

yC H Whc 3

exp[−λ(XC3H8)]

yC H Whc 3

dyC H 3

8

dt

=

×

exp[−λ(XC3H8)]

dyC H 3

6

dt

=

8

3

6

(28)

Wc ⎛ MWC3H6VR ⎞ ⎜ ⎟ VR ⎝ Whc ⎠ ×

k1αK C3H8yC H − k 3βK C3H6yC H 3

8

3

6

(1 + αK C3H8yC H + βK C3H6yC H ) 3

(23)

8

3

6

× exp[−λ(XC3H8)]

(29)

for COx: dyCO

x

dt

yW i hc MWi VR

8

for propylene:

where VR is the volume of riser simulator (cm3), Wc is the weight of the catalyst (g), Ci is the concentration of species “i” (mol/cm3), and t is the time (seconds). The concentration of any species (Ci) is related to its weight fraction (yi) as Ci =

3

(1 + αK C3H8yC H + βK C3H6yC H )


3.2. Kinetic Modeling in the CREC Riser Simulator. Kinetic modeling in the CREC riser simulator requires species balances. Such species balance equations can be established on the basis of a well mixed isothermal batch-scale reactor.76,77 Thus, the following CREC riser simulator equation can be considered: VR dCi Wc dt

(k1 + k 2)αK C3H8yC H 3

(22)

ri =

(27)

−Wc ⎛ MWC3H8 VR ⎞ ⎜ ⎟ VR ⎝ Whc ⎠

rCOx = r2 + r3 (1 + K C3H8CC3H8 + K C3H6CC3H6)

6

MWC3H6VR

for propane:

Rate of COx formation:

=

(26)

One can obtain

(21)

(k 2)K C3H8CC3H8 + (k 3)K C3H6CC3H6

8

MWC3H8VR

and

(20)

(k1)K C3H8CC3H8 − (k 3)K C3H6CC3H6

(25)

=

Wc ⎛ MWCOxVR ⎞ ⎜ ⎟ VR ⎝ Whc ⎠ ×

(24)

k 2αK C3H8yC H + k 3βK C3H6yC H 3

8

3


where Whc is the total mass of hydrocarbons injected into the riser simulator (g), MWi is the molecular weight of ith component (g/mol), and VR is the riser simulator volume (cm3). After replacing eq 24 into eq 23 and after the required algebraic steps, the CREC riser simulator design equation is obtained in terms of weight fractions chemical species:

3

6

(1 + αK C3H8yC H + βK C3H6yC H ) 8

3

6

(30)

where α = (Whc)/(MWC3H8VR) and β = (Whc)/(MWC3H6VR) The system of ODE given in eqs 28 to 30 can be solved with a carefully selected set of initial conditions. This allows describing the mass fraction changes of various chemical species (e.g., propane, propylene, and COx) with reaction time. D



Article

Table 1. Characteristics of Alumina Support and Supported Vanadia Catalyst sample γ-Al2O3 (activated) 5% V/γ-Al2O3 7% V/γ-Al2O3 10% V/γ-Al2O3

V contenta (wt %)

SBET (m2/ g)

H2 uptake (cm3 STP/g)

0

203.52

0.54

5 7 10

188.64 179.27 153.37

0.45 0.46 0.39

% reductionb

surface density (V/nm2)

NH3 uptake (cm3 STP/g)

Lewis/Brønsted ratio

% dispersionc (O/V)

1.8 4.2 5.1

63.26 59.81 43.25

16.26 81.28 83.06 99.18

3.1 4.6 7.7

10.94 11.50 13.02

V content, in wt % was determined by ICP. bBased on V2O5 + 2H2 → V2O3 + 2H2O. cDispersion = fraction of vanadium atoms at the surface, assuming Oads/Vsurf = 1 a

NH3-TPD and pyridine FTIR analysis showed that both the number and nature of the acid sites change with vanadium loading. The surface vanadia species (VOx) cover the exposed alumina surface Lewis acid sites, resulting in lower acidity than that of the bare support. Moreover, the number of surface Brønsted acid sites increased with increasing surface VOx. This indicated that polymeric surface VOx species possess more Brønsted acid character than the isolated surface VOx species. X-ray diffraction (XRD) demonstrated the absence of V2O5 bulk surface species and the high dispersion of VOx on the support surface. No evidence of AlV3O9 was detected in any of the samples suggesting that the reaction between VOx and Al2O3 is negligible during the treatment at 600 °C. Moreover, XRD analysis on spent catalysts after 10 consecutive ODH propane injections and at the most severe reaction conditions showed no diffraction patterns. This result was assigned to the structural changes other than those occurring in the bare γ-Al2O3 support. This can also be viewed as an indication of the high catalyst stability under the reaction conditions. 4.2. Propane ODH in CREC Riser Simulator. Table 2 reports propane conversion and product selectivities for the successive-injection propane ODH experiments over the various VOx/γ-Al2O3 catalysts at various contact times and different temperatures. One should also notice that the only identifiable carbon-containing products other than propylene were CO, CO2, CH4, C2H6, and C2H4. On the basis of the product distribution results shown in Table 2, the variation of product selectivities with propane conversion is shown in Figure 2, for the various VOx/γ-Al2O3 catalysts. It is apparent from Figure 2 that the propene selectivity is inversely related to the conversion for each catalyst. The decrease in propylene selectivity with a corresponding increase in COx selectivities shows that propylene is the primary reaction product of the propane ODH while COx is the secondary product of the consecutive oxidation of propane feed and propylene product. On the basis of these results, a parallelseries “triangular” reaction network with “direct alkane combustion” together with “alkene formation” and later “alkene combustion” could be proposed for propane ODH as given in Figure 1. Further discussion about the reaction network is given in the Kinetic Modeling section. Figure 3 reports the various gas species conversions/ selectivities as a function of a propane injection for two sets of propane ODH experiments over the various VOx/γ-Al2O3 catalysts. Figure 3a,b describes successive-propane injections during ODH runs. The catalyst is not regenerated in between propane injections (pulses). It can be observed in Figure 3a that the conversion of propane is highest after the first injection for all catalysts, and then decreases with the increasing number of propane injections.

Furthermore, to obtain the intrinsic kinetic parameters (activation energies (Ei) and pre-exponential factors (kio)), the temperature dependence of each (ki) parameter in eqs 28 to 30 can be represented using the Arrhenius’ equation as follows: ⎡ −E ⎛ 1 1 ⎞⎤ ki = kio exp⎢ i ⎜ − ⎟⎥ ⎢⎣ R ⎝ T Tm ⎠⎥⎦

(31)

koi

where is the pre-exponential factor (mol/g·sec) expressing ODH vanadium oxidize catalyst reactivity at reaction time zero and T = Tm, Ei is the activation energy (kJ/mol), R is the universal gas constant, and Tm is the average temperature (K). To reduce the cross-correlation between the pre-exponential factors koi and the activation energies Ei, the ki constants were reparameterized, as shown in eq 31. This was accomplished by centering the reaction temperature at an average value of Tm = 512.5 °C. This temperature corresponds to the average reaction temperatures used in the present study as recommended by Hossain and de Lasa.78 In the same way and on the basis of adsorption thermodynamics, one can relate the adsorption constant Ki with the reaction temperature T, as ⎡ −ΔH ⎛ 1 1 ⎞⎤ i K i = K io exp⎢ ⎟⎥ ⎜ − ⎢⎣ R ⎝ T Tm ⎠⎥⎦

(32)

where −ΔHi is the heat of adsorption (kJ/mol) and is the pre-exponential factor (cm3/mol). The substitution of eq 32 and eq 33 into eqs 29 to 31 gives a new set of ordinary differential equations with the intrinsic kinetic parameters (koi , Ei, Koi and −ΔHi) to be estimated. koi

4. RESULTS AND DISCUSSION 4.1. Catalyst Characterization. Catalyst characterization results are summarized in Table 1. Additional information regarding catalyst characterization can be found in our recent publication by Al-Ghamdi et al.71 BET surface area analysis of the prepared VOx/γ-Al2O3 catalysts showed a moderate decrease in the total surface area of the catalyst upon vanadium loading on the calcined γ-Al2O3 support. This decrease of the surface area was due to the plugging of some of the support pores by vanadium species. H2-TPR showed that all VOx/γ-Al2O3 catalysts exhibited a single low temperature reduction peak at around 450 °C which was due to the reduction of isolated and two-dimensional structures of surface vanadia. Moreover, the reducibility (% reduction) of the surface VOx species increased with surface VOx surface density in the submonolayer region. O2-chemisorption and Raman spectroscopy demonstrated that the structure and dispersion of the VOx surface species depend on their surface density. The surface vanadia species becomes more polymerized with increasing vanadium loading. E



Article

Table 2. Conversion and Product Distribution Results for Sequential-Injections Propane ODH Experiments over Various VOx/ γ-Al2O3 Catalystsa selectivity (%) catalyst

time (s)

T ( C)

XC3H8 (%)

C3H6

COx

CH4

C2H6

C2H4

C3H6 yield (%)

5% VOx/γ-Al2O3

5

475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550 475 500 525 550

2.35 3.08 3.83 4.08 3.24 3.74 4.60 5.06 3.73 4.22 5.26 6.16 3.29 4.59 6.32 6.08 4.18 5.60 6.55 8.41 4.67 6.27 7.24 10.50 4.19 5.77 7.99 9.12 5.14 6.89 9.77 11.21 5.90 7.68 10.79 12.93 5.38 7.32 8.88 11.73 6.42 8.11 11.88 13.36 7.16 9.14 13.41 15.05

70.89 73.37 75.44 76.38 60.73 64.75 65.29 66.20 55.12 57.24 58.79 60.64 76.86 77.53 79.92 81.24 67.20 68.94 70.60 71.49 61.99 63.43 64.75 66.18 80.35 81.89 83.28 84.52 67.68 71.58 71.98 74.46 61.67 64.64 65.78 66.81 84.42 84.70 85.79 85.94 71.20 73.09 73.79 75.34 64.13 65.73 67.29 67.77

86.49 86.26 88.01 89.66 35.81 29.00 24.36 20.01 41.52 36.74 30.29 23.79 89.88 89.96 91.09 91.86 29.30 24.78 19.46 15.64 34.59 30.05 25.05 18.79 91.53 91.89 92.90 93.88 28.76 22.00 19.11 11.67 34.82 29.27 25.89 16.13 94.45 95.22 95.52 96.90 25.19 21.03 17.51 10.43 32.45 28.66 23.58 15.01

2.76 4.73 6.14 6.30 2.57 4.45 6.31 7.12 2.48 4.27 6.60 7.83 2.97 4.20 5.67 5.49 2.61 4.29 5.79 6.60 2.49 4.44 6.04 7.51 2.97 4.98 5.58 5.05 2.64 4.37 5.00 6.73 2.60 4.13 4.85 8.53 2.94 4.15 4.84 5.41 2.73 4.02 4.78 7.21 2.53 3.72 5.21 8.72

0.24 0.60 1.51 1.54 0.25 0.65 1.76 2.43 0.27 0.71 2.06 3.18 0.27 0.69 2.10 2.20 0.30 0.89 2.20 2.98 0.33 1.03 2.28 3.68 0.29 0.94 2.60 3.30 0.32 1.01 2.31 3.92 0.34 1.03 2.07 4.75 0.31 0.89 2.30 3.62 0.34 0.99 2.40 4.44 0.36 1.07 2.48 4.99

0.70 1.30 2.26 3.78 0.64 1.16 2.28 4.24 0.61 1.04 2.26 4.56 0.67 1.06 2.03 2.76 0.59 1.11 1.94 3.30 0.59 1.06 1.88 3.84 0.67 1.12 1.99 2.44 0.61 1.03 1.61 3.22 0.57 0.92 1.41 3.78 0.61 0.96 1.75 1.55 0.55 0.87 1.51 2.58 0.53 0.82 1.43 3.51

1.63 2.18 2.85 3.13 1.62 2.14 2.63 3.18 1.48 2.01 2.47 3.49 2.50 3.46 5.03 4.98 2.59 3.53 4.34 5.90 2.57 3.42 4.10 6.64 3.35 4.71 6.67 7.73 3.29 4.69 6.78 8.36 3.32 4.52 6.56 8.53 4.53 6.21 7.64 10.11 4.36 5.82 8.45 10.06 4.24 5.70 8.32 10.16

7% VOx/γ-Al2O3

10% VOx/γ-Al2O3

5% VOx/γ-Al2O3

10

7% VOx/γ-Al2O3

10% VOx/γ-Al2O3

5% VOx/γ-Al2O3

15

7% VOx/γ-Al2O3

10% VOx/γ-Al2O3

5% VOx/γ-Al2O3

7% VOx/γ-Al2O3

10% VOx/γ-Al2O3

a

o

20

Reported values are the average of 10 consecutive C3H8 injections.

As no molecular oxygen is fed with the injection, the catalyst is believed to supply all the oxygen for the ODH reaction. Following the first injection, propane conversion declined drastically from 58.9, 76.5, and 81.9 to 12.5, 14.4 and 16.4%. Following this, there was a further gradual decrease to 11.5, 12.4, and 13.6% with subsequent propane injections over the 5%VOx/γ-Al2O3, 7%VOx/γ-Al2O3, and 10%VOx/γ-Al2O3 catalysts samples, respectively. Therefore, it is plausible to suggest

that the decrease of propane conversion over VOx/γ-Al2O3 catalysts mainly resulted from the consumption of the reactive lattice oxygen species present on the catalyst with propane injections. Moreover, it can be seen for all catalysts that the first injection also has very low propylene selectivity and high COx selectivity. Fully oxidized VOx/γ-Al2O3 catalysts are considered nonselective for the ODH reaction as seen from the results F



Article

Figure 2. Propylene selectivities (filled symbols) and COx selectivities (empty symbols) as a function of C3H8 conversion (C3H8 injected = 10 mL, contact time = 20 s, catalyst loaded = 0.76 g). Error bars correspond to standard deviation of three trials (data reported for 10 consecutive pulses without catalyst regeneration between the injections).

These findings demonstrate that in consecutive ODH reactions without catalyst regeneration in between (oxygenfree environment), propane can be converted selectively to propylene. This occurs due to the lattice oxygen involvement in the ODH reaction mechanism. One can also notice that following the first ethane injection (first pulse), the catalyst tested displays a stable performance in terms of ethane conversion and ethylene selectivity even after 10 successive reduction runs. On the other hand, Figure 3c,d reports a second set of experiments involving repeated ODH reaction-regeneration cycles at 550 °C over the various VOx/ γ-Al2O3 catalysts. Catalyst regeneration was conducted by flowing air at 550 °C over the catalyst after each propane injection (pulse). Results show that propane conversion and product selectivities (equivalent to a per-pass conversion and selectivity in a continuous riser/downer unit) remain stable during the different runs. It can be observed in Figure 3c,d that propane conversion remains at the 60, 73.2, and 80.1% values over the 5%VOx/γ-Al2O3, 7%VOx/γ-Al2O3, and 10%VOx/γ-Al2O3 catalysts samples, respectively. On the other hand, very low propylene selectivities in the 2.1, 0.93, and 0.57% levels are observed over the 5%VOx/γ-Al2O3, 7%VOx/γ-Al2O3, and 10%

obtained at the first injection. When the catalyst is in a fully oxidized state, there is a higher population of nonselective surface oxygen species which is more likely to promote the deep oxidation of hydrocarbons to carbon oxides. These nonselective oxygen species could be formed due to either the loosely bound lattice oxygen or the adsorbed oxygen from the regeneration step which could have remained on the VOx/γAl2O3 catalyst surface. However, it can be noticed in Figure 3b that following the first propane injection, the selectivities for propylene increase with propane injections showing stable values at around 92%, 82.2%, and 76.1% over the 5%VOx/γ-Al2O3, 7%VOx/γ-Al2O3 and 10%VOx/γ-Al2O3 catalysts samples, respectively. On the other hand, selectivities for COx decrease with propane injections and this seems to indicate that a certain degree of catalyst reduction is required in order to obtain good propylene selectivity. In this regard, a selective catalyst surface would be obtained only after the adsorbed oxygen had been consumed by the first propane injection. This is evident from the subsequent propane injections where the selectivity to propylene increases with the number of propane injections while that for combustion products (COx) decreases. G



Article

Figure 3. Propane conversion, propylene, and COx selectivities over various VOx/γ-Al2O3 catalysts in the CREC riser simulator. (T = 550 °C, C3H8 injected = 10 mL, reaction time = 20 s, catalyst loaded = 0.76g): (a,b) consecutive ODH runs without catalyst reoxidation in between propane pulses; (c,d) consecutive ODH runs with catalyst reoxidation in between propane pulses. Error bars correspond to standard deviation of three trials.

Table 3. Intrinsic Kinetic Parameters for the Proposed Kinetic Model. Kinetic Parameters Are Reported with Their 95% Confidence Interval over Various VOx/γ-Al2O3 catalysts 5% VOx/γ-Al2O3 parameter

7% VOx/γ-Al2O3

10% VOx/γ-Al2O3

value

95% CI

value

95% CI

value

95% CI

ko1a ko2 ko3 KoC3H8b

3.17 x10−3 9.28 x10−4 8.52 x10−5 76.72

±3.89 × 10−5 ±1.52 x10−5 ±6.78 x10−4 ±0.93

4.82 x10−3 3.04 x10−3 2.97 x10−4 51.14

±1.00 x10−4 ±6.76 x10−5 ±1.67 x10−3 ±1.01

6.34 x10−3 5.91 x10−3 6.33 x10−4 38.34

±2.68 x10−4 ±2.30 x10−4 ±3.06 x10−3 ±1.45

KoC3H6

29.95

±3.65

28.58

±5.37

26.12

±7.82

E1c E2 E3 −ΔHC3H8c

124.92 52.81 52.54 68.18

±1.33 ±1.71 ±8.99 ±1.41

115.08 51.07 52.73 50.47

±0.99 ±1.42 ±10.94 ±1.14

109.42 45.58 53.75 40.32

±2.52 ±2.34 ±17.55 ±2.18

−ΔHC3H6

54.59

±8.22

56.86

±10.80

41.60

±12.34

σd λe m DOF

2.15 × 10−4 0.01−0.053 960 950

2.65 × 10−4 0.017−0.056 960 950

3.90 × 10−4 0.015−0.047 960 950

a mol gcat−1 s−1. bcm3/mol. ckJ/mol. dσ = (∑(Xexperimental − Xestimated)2/(m − p))1/2, where m is the number of data points, p is the number of model parameters, and DOF is the degree of freedom. eDeactivation constant (λ) calculated by independent fitting of experimental data.

γ-Al2O3, 7%VOx/ γ-Al2O3, and 10%VOx/γ-Al2O3 catalysts samples, respectively.

VOx/γ-Al2O3 catalysts samples, respectively. In addition, COx selectivities remain stable at 88, 90, and 93% over the 5%VOx/ H



Article

Figure 4. Energy diagram for the ODH reaction: (a) propane ODH reaction forming propylene, (b) propane oxidation forming COx, (c) propylene oxidation forming COx.

directly; (b) only a small fraction of the catalyst stream is directed toward the regenerator. This operational strategy can allow the functioning of industrial scale ODH processes with high propylene selectivities, under conditions close to the ones reported in Figure 3a,b. 4.3. Kinetic Parameters Estimation. The postulated rate expressions in eqs 28 to eq 30 are nonlinear with respect to their parameters. This is due to the fact that the parameters appear both in the numerator and in the denominator of the rate expressions such as in the case of the adsorption constant (Ki). Therefore, the estimation of the 10 parameters (ko1, ko2, ko3, E1, E2, E3, KoC3H8, KoC3H6, −ΔHC3H8 and −ΔHC3H6) was developed using nonlinear least-squares regression. The MATLAB routine “LSQCURVEFIT” was used for the regression analysis. The numerical integration of the system of ODE given in eqs 29 to 31 and the determination of the 95% confidence intervals for each estimated parameter were performed using the MATLAB functions “ode113”; and “nlparci”, respectively. Experimental data at different reaction temperatures were used to evaluate reaction rate parameters. Moreover, an Arrhenius type of temperature-dependence function was used to express the specific rate constants. This was done by taking into account the temperature dependence of the experimental data as shown in eq 32. Moreover, the deactivation constant “λ” given in eq 4 was obtained through an independent fitting of the propane conversion data for every run (e.g., using the propane conversions obtained for injection 4 at 5 s, 10 s, 15 s, and 20 s). The ‘λ” parameter was calculated via nonlinear regression. Regression coefficients were, in all cases, in excess of 0.98. The obtained values of the deactivation constant “λ” were used as inputs in the proposed kinetic model along with all the mass fractions of the reactants/products. This approach was valuable to eliminate the high cross-correlation observed between λ and the other kinetic parameters The optimization criteria considered was established having the following bounds for the model parameters: (a) rate constants and activation energies for each reaction must be positive, (b) heats of adsorption (ΔHi) must be negative. On this basis, a minimum sum of squares of errors was calculated:

Table 4. Standard heat of Reaction for Propane and Propylene Combustion reaction

ΔHorxn (kJ/mol)

1 O2 → C3H6 + H 2O 2 7 C3H8 + O2 → 3CO + 4H 2O 2 C3H8 + 5O2 → 3CO2 + 4H 2O

−117.6

C3H8 +

C3H6 + 3O2 → 3CO + 3H 2O C3H6 +

9 O2 → 3CO2 + 3H 2O 2

−1195.1 −2043.9 −1077.5 −1926.4

Regarding propylene and COx selectivities during repeated ODH reaction−regeneration cycles, they are in clear contrast with the ones reported in Figure 3a,b for consecutive propane ODH runs. They show that the formation of the CO and CO2 is promoted by gas phase oxygen introduced for the catalysts regeneration. These low propylene selectivities and higher COx selectivities obtained at any given propane conversion during the repeated cycles of single injection ODH experiments can be related to both the amount and type of surface oxygen species available on the fresh catalyst surface. Excess amounts of surface oxygen as well as the existence of nonselective oxygen species on the fresh catalyst surfaces are believed to favor the conversion of propane and the produced propylene into combustion products. In view of the above, it can be concluded that fully oxidized (fresh) VOx/γ-Al2O3 catalysts are active but not selective for propane ODH reactions. This fact can be attributed to the excess (nonstoichiometric) amount of oxygen species on the catalysts surface. These species could be either loosely bound lattice oxygen or weakly adsorbed oxygen species produced from gas-phase O2 during pretreatment and the catalyst regeneration. Both types of oxygen species may be involved in the total oxidation of propane and propylene product. Thus, one can conclude that the performance of the various VOx/γ-Al2O3 catalysts of the present study is affected considerably by both ODH operation conditions and catalyst regeneration. It appears that consecutive reaction injections (pulses) are a preferred mode of operation for ODH. As a result, ODH catalyst regeneration shall only be allowed once this series of consecutive reaction cycles are completed. One can, for instance, envision that this is equivalent to operating a twin circulating fluidized reactor process (reactor− regenerator), where the following occurs: (a) most of the catalyst stream leaving the reactor is returned to the ODH unit

N

SSR =

∑ (xi ,exp − xi ,pred)2 i=1

(33)

where xi,exp and xi, pred are the mass fraction of component i obtained experimentally and predicted by the kinetic model, respectively. I



Article

Figure 5. Comparison between experimental data and model predictions () over a 5% VOx/γ-Al2O3 catalyst sample: (a) T = 475 °C; (b) T = 500 °C; (c) T = 525 °C; (d) T = 550 °C. (Data points are reported for the average of 2nd to 10th injections.)

increase with VOx surface density. This suggests that polyvanadate surface species, which are more dominant at higher VOx surface densities, are more active than monovanadate species. Similarly, the increase in the ko2 and ko3 pre-exponential factors with vanadium loading is also in agreement with the experimental results. It was also found that COx formation rates increase with VOx surface density. This can be attributed to the excess surface oxygen species available at higher loadings. This is also consistent with the decreasing propylene selectivity with VOx surface density as more COx are formed. Regarding the relative magnitude of the activation energies obtained as reported in Table 3, it can be noticed that COx formation activation energies (E2 and E3) are consistently smaller than E1. On the basis of this observation, one could argue that the obtained values are consistent with the energy diagrams given in Figure 4. One can observe in these diagrams, the major difference of the heats of reaction: the COx heat of reaction being much larger than the C3H6 heat of formation via ODH. Thus, on this basis, one can also envision the propane ODH reaction forming propylene with activation energy larger than the ones for the reactions leading to COx as reported in Table 4. These facts may contribute to explaining the limited ODH propylene selectivity at higher temperatures.

Furthermore, discrimination between possible models was based on (i) correlation coefficients (R2), (ii) lower SSR (sum of the squares of the residuals) criteria, (iii) lower crosscorrelation coefficient (γi), and (iv) smaller individual confidence intervals for the model parameters. The values of the 10 estimated parameters along with their corresponding 95% confidence intervals (CI) are reported in Table 3 for the different VOx/γ-Al2O3 catalysts. It can be noticed that all the estimated parameters display a reduced and acceptable 95% C.I. Moreover, the ability of establishing the 10 model parameters is consistent with the high DOF (degree of freedom) in this analysis with 960 experimental data points for each catalyst sample considering three trials for each run. One can also observe that all 10 parameters in Table 3 were calculated with reduced spans. In addition, several kinetic parameter trends regarding the effect of vanadium loading on the catalytic activity can be observed in Table 3. One can notice that the pre-exponential factors (ko1, ko2, and ko3) augment as the vanadium loading is increased. In view of these findings, one can consider correlating changes in the ko1, ko2, and ko3 values with the different vanadia structures present at specific vanadium loadings, though other factors may also be involved. In this respect, one can also notice that the increase in ko1 is in agreement with the experimental results. Specific initial rates of propane ODH were found to J



Article


related to physicochemical properties of the catalysts under study. On the basis discussed above, it can be concluded that the set of adsorption and kinetic parameters established as well as the kinetic model developed are adequate to predict propane ODH reaction rates in the CREC riser simulator in the range of the operating conditions studied. It is important to review the values of the activation energies obtained in the present study and to compare them with values already reported in the technical literature. Table 6 reports the activation energies for propane ODH on various supported VOx catalysts. Although the reported values for the three propane ODH catalysts of this study are consistently within the same range, there is some discrepancy between these energies of activation (± 20 kJ/mol) when compared with other values published in the technical literature. This discrepancy can be attributed to the dissimilarity in experimental systems, operating conditions, proposed ODH kinetic model, and catalysts studied. It should be stressed, however, that the values of activation energies for the propane ODH of this study are close to the ones reported by Rao and Deo, 200446 using VOx/γ-Al2O3 catalysts with similar vanadium loadings. For instance, for the propane ODH step, Rao and Deo, 2004 reported activation energies for propylene formation of 113.6, 100, and 98 kJ/mol

In addition, further insights into the validity of the proposed kinetic model and the estimated kinetic parameters could be obtained. This can be done by comparing the model predictions for products and reactant mass fractions with experimental data. This was, in fact, done for all the catalysts of this study as reported in Figures 5−7. It can be noted that, within the limits of experimental error, the model predictions compare very well with the experimental data. Thus, this comparison validates the adequacy of the proposed reaction model. Moreover, it can also be inferred from the parity plots reported in Figures 5−7 for model predictions, as compared to the experimental data, that the data is not clustered in horizontal or vertical lines. Horizontal bands may be the result of changes in the observed conversion caused by an independent variable which is not included in the kinetic model. On the other hand, vertical lines are an indication of the kinetic model overparameterization.79,80 The adequacy of the estimated parameters was checked by analyzing their dependence on each other through the crosscorrelation matrix as shown in Table 5. It can be observed, that in most cases, cross-correlation coefficients are below 0.90 with only three of them surpassing the 0.9 value. Hence, it can be concluded that the kinetic model as proposed is not overparameterized and that the defined parameters can be K



Article


support an ODH process using twin circulating fluidized beds with (a) an ODH reactor with oxygen being supplied from the catalyst lattice only and (b) a reoxidation reactor where the catalyst lattice oxygen is replenished. It is anticipated that in this type of ODH process, only a 1/23 fraction of the catalyst in the reactor outlet stream is going to be treated per pass in the reoxidation reactor. This is estimated considering up to 23 injections are required in the CREC riser simulator until oxygen is fully depleted from the VOx/γ-Al2O3 catalysts.71 The rest or most of the catalyst in the ODH reactor outlet stream is going to be partially cooled and recycled back directly to the reactor input stream. It is expected that such a twin and integrated ODH fluidized bed process will yield higher olefin selectivities as reported in the present study.

for catalysts with vanadium loadings of 7.5, 10, and 12.5 wt %, respectively. The values of activation energies observed in this study for the same step were 124.9, 115.1, and 82 KJ/mol for 5, 7, and 10 wt % catalysts, respectively. However, in the case of COx formation, the reported values by Rao and Deo, 200446 differ by ±20 kJ/mol from those obtained in the context of the present study. This discrepancy between activation energies of COx formation can be attributed to both the different reaction considered by Rao and Deo, 2004, where oxygen is being cofed with propane, and the different reaction network paths involved. Rao and Deo, 2004, assumed that CO and CO2 are formed from propylene instead of following the the parallel-series “triangular” reaction network of the present study. This proposed “triangular” reaction network of the present study is reinforced by the fact that COx species are observed even at very low propane conversions. Thus, a direct COx formation step from propane is required. As a result, on the basis of the experimental evidence gathered and on the adequacy of the parallel-series “triangular” kinetic model proposed, the following can be considered as reported in Figure 1: (i) There is an alkene formation step, (ii) there is a competitive direct alkane combustion step, and (iii) once alkene is formed, there is further alkene combustion. In summary, the catalytic runs of the present study together with the phenomenologically based kinetic model developed

5. CONCLUSIONS The following key points can be considered as the main conclusions of this study: 1. Propane was converted to propylene via ODH reaction over VOx/γ-Al2O3 catalysts utilizing the lattice oxygen of the catalysts. Good propane conversion (11.73%− 15.11%) and promising propylene selectivity (67.65− 85.89%) were achieved in successive-injection propane ODH experiments (without catalyst regeneration) in the CREC riser simulator at 475−550 °C. L



Article

Table 5. Cross-Correlation Coefficients for the Kinetic Model Optimized Parameters over Various VOx/ γ-Al2O3 Catalyst Samples ko1

ko2

ko3

KoC3H8

KoC3H6

E1

E2

E3

−ΔHC3H8

−ΔHC3H6

(a) 5% VOx/γ-Al2O3 Catalyst Sample ko1 ko2 ko3 KoC3H8

1.000 0.657 0.150 −0.975

1.000 −0.617 −0.803

1.000 0.061

1.000

KoC3H6

0.031

0.027

−0.019

−0.036

1.000

E1 E2 E3 −ΔHC3H8

−0.064 −0.127 0.009 0.064

0.039 0.232 0.020 −0.028

−0.112 −0.434 −0.020 0.103

0.027 0.029 −0.016 −0.032

0.099 0.082 −0.029 −0.101

1.000 0.871 0.101 −0.994

1.000 0.080 −0.888

1.000 −0.098

1.000

−ΔHC3H6

0.049

−0.047

0.119

−0.018

−0.497

−0.285

−0.291

−0.328

0.291

1.000

(b) 7% VOx/γ-Al2O3 Catalyst Sample ko1 ko2 ko3 KoC3H8

1.000 0.696 0.344 −0.949

1.000 −0.405 −0.881

KoC3H6

−0.062

E1 E2 E3 −ΔHC3H8

−0.521 −0.670 −0.043 0.625

−ΔHC3H6

0.129

1.000 −0.051

1.000

−0.093

0.014

0.083

1.000

−0.330 −0.255 0.016 0.474

−0.169 −0.507 −0.067 0.143

0.449 0.535 0.024 −0.588

0.096 0.063 −0.103 −0.104

1.000 0.705 −0.032 −0.928

1.000 −0.019 −0.823

1.000 0.039

1.000

0.121

0.017

−0.136

−0.477

−0.066

−0.076

−0.329

0.086

1.000

(c) 10% VOx/γ-Al2O3 Catalyst Sample ko1 ko2 ko3 KoC3H8

1.000 0.838 0.381 −0.965

1.000 −0.159 −0.949

1.000 −0.143

KoC3H6

−0.122

−0.118

−0.070

0.127

1.000

E1 E2 E3 −ΔHC3H8

0.696 0.591 0.091 −0.648

0.588 0.644 0.090 −0.516

0.290 −0.044 0.032 −0.293

−0.678 −0.639 −0.094 0.612

−0.096 −0.074 −0.503 0.083

1.000 0.834 0.046 −0.961

1.000 0.017 −0.892

1.000 −0.030

1.000

−ΔHC3H6

0.002

−0.023

0.060

0.008

−0.382

0.040

0.002

−0.196

−0.033

1.000

1.000

Table 6. Reported Activation Energies for Main Products Formation in Ethane ODH activation energy of formation (kJ/mol)

a

catalyst

C3H6

5% VOx/γ-Al2O3 7% VOx/γ-Al2O3 10% VOx/γ-Al2O3 10% VOx/γ-Al2O3 2.1%VOx/γ-Al2O3 7.5% V/γ-Al2O3 10% V/γ-Al2O3 12.5% V/γ-Al2O3 24%V/MgO 0.6%VOx/SiO2

124.9 115.1 109.5 82 113.6 100 98 92 114 147

carbon oxides (COx) 52.8a (COx) 51.1a (COx) 45.6a (CO) 35b (CO2) 87.5b (CO) 50b (CO) 55b (CO) 83b (CO) 72a (CO) 101b (CO2) 96b

reference (COx) (COx) (COx) (CO2)

52.53b 52.7b 53.7b 52.2b

(CO2) (CO2) (CO2) (CO2)

46b 49b 75b 42a (CO2) 92b

(present study) (present study) (present study) 81 82 46 46 46 43 82

Formation from C3H8. bFormation from C3H6.

oxygen-free conditions, the oxygen from the catalyst lattice is consumed by ODH reactions. 3. A parallel-series “triangular” reaction network was proposed for propane ODH over VOx/γ-Al2O3 catalysts where there is a propylene formation step, a competitive

2. The selectivity for propylene increased and that for COx decreased as the degree of reduction of the catalyst increased with propane injections. This shows that a controlled degree of catalyst reduction is needed in order to obtain higher propylene selectivity. Under such

M



4.

5.

6.

7.

8.

■

Article

XC3H8 = propane conversion, (%) Si = selectivity of component i (%) Yi = yield of product i SBET = Brunauer−Emmet−Teller specific surface area (cm2/ g) Nopropane = Initial moles of propane, (mols) Wcat = weight of catalyst, (g) Ei = activation energy of desorption, (kJ/mol) −ΔHi = heat of adsorption, (kJ/mol) koi = pre-exponential factor (mol gcat−1 s−1) Kd = adsorption constant (cm3/mol) Tm = centering temperature (K) R = the universal gas constant s = surface-adsorbed species g = gas phase species

direct propane combustion step, and propylene combustion step. This heterogeneous kinetic model based on a Marsvan Krevelen mechanism, allowed us to describe the effect of vanadia loading on propane ODH reaction over VOx/γAl2O3 catalysts. The proposed kinetic model accounts for all reactants (propane) and products (propylene and COx). On this basis, rate equations are developed including both reactant adsorption and catalytic reaction on the catalyst surface. The change in degree of oxidation of the catalyst during oxygen-free ODH reactions was effectively modeled and included in the rate equations by using a decay function based on reactant conversion. The proposed kinetic model was established with a DOF of 950. This model satisfactorily predicts the ODH reaction of propane under the selected reaction conditions over the various VOx/γ-Al2O3 catalysts with different vanadium loadings. The determined kinetic parameters included the preexponential factor, koi , the adsorption constant, Koi , the activation energies, Ei, for the propene formation, propane oxidation, and propene oxidation and desorption energies, −ΔHi. The intrinsic kinetic parameters were estimated using a nonlinear least-squares fit regression. The calculated kinetic parameters were able to predict the observed outlet mass fractions of all carbon containing compounds and were found to vary with vanadia loading. The calculated pre-exponential factors (ko1, ko2, and ko3) increased as the vanadium loading was augmented. In addition, the activation energies for COx formation (E2 and E3) were consistently smaller than the one for propylene formation (E1).

Greek Symbols

β = degree of reduction of catalysts, (%) θdes = surface coverage of adsorbed species ν = stoichiometric number λ = catalyst deactivation constant established via numerical for every injection in the consecutive injection series using propane conversion at 5 s, 10 s, 15 s, and 20 s ϕ = degree of oxidation of the catalyst (% oxidation) σ = standard deviation

Abbreviations

ASSOCIATED CONTENT

* Supporting Information S

Experimental results of propane ODH in the CREC riser simulator using the various VOx/γ-Al2O3 catalyst samples for 10 consecutive propane injections (pulse) without catalyst regeneration between injections; “λ” deactivations constants obtained via numerical regression for every propane injection and data at 5 s, 10 s, 15 s, and 20 s. This material is available free of charge via the Internet at http://pubs.acs.org.

■

■

ODH = oxidative dehydrogenation MW = molecular weight CREC = Chemical Reactor Engineering Center XRD = X-ray diffraction TPR = Temperature-programmed reduction TPO = temperature-programmed oxidation TPD = temperature-programmed desorption LRS = laser Raman spectroscopy XPS = X-ray photoelectron spectroscopy FID = flame ionization detector TCD = thermal conductivity detector AOS = average oxidation state

REFERENCES

(1) Blasco, T.; López-Nieto, J. M. Oxidative Dehydrogenation of Short Chain Alkanes on Supported Vanadium Oxide Catalysts. Appl. Catal. A Gen. 1997, 157, 117. (2) Mamedov, E. A.; Corberan, V. C. Oxidative Dehydrogenation of Lower Alkanes on Vanadium Oxide-Based Catalysts. The Present State of the Art and Outlooks. Appl. Catal. A Gen. 1995, 127, 1. (3) Baerns, M.; Buyevskaya, O.; Mulla, S. A. R. A Comparative Study on Non-Catalytic and Catalytic Oxidative Dehydrogenation of Ethane to Ethylene. Appl. Catal. A Gen. 2002, 226, 73. (4) Shen, Z.; Liu, J.; Xu, H.; Yue, Y.; Hua, W.; Shen, W. Dehydrogenation of Ethane to Ethylene over a Highly Efficient Ga2O3/HZSM-5 Catalyst in the Presence of CO2. Appl. Catal. A Gen. 2009, 356, 148. (5) Č apek, L.; Bulánek, R.; Adam, J.; Smoláková, L.; Sheng-Yang, H.; Č ičmanec, P. Oxidative Dehydrogenation of Ethane over VanadiumBased Hexagonal Mesoporous Silica Catalysts. Catal. Today 2009, 141, 282. (6) Shi, X.; Ji, S.; Wang, K. Oxidative Dehydrogenation of Ethane to Ethylene with Carbon Dioxide over Cr−Ce/SBA-15 Catalysts. Catal. Lett. 2008, 125, 331. (7) Č apek, L.; Adam, J.; Grygar, T.; Bulánek, R.; Vradman, L.; Košová-Kučerová, G.; Č ičmanec, P.; Knotek, P. Oxidative Dehydrogenation of Ethane over Vanadium Supported on Mesoporous Materials of M41S Family. Appl. Catal. A Gen. 2008, 342, 99. (8) Klose, F.; Wolff, T.; Lorenz, H.; Seidelmorgenstern, A.; Suchorski, Y.; Piorkowska, M.; Weiss, H. Active Species on Γ-

AUTHOR INFORMATION

Corresponding Author

*Tel: +1-519-661-2144. Fax: +1-519-850-2931. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors wish to acknowledge the National Sciences and Engineering Research Council of Canada (NSERC) for their financial support of this research and the Saudi Arabian Oil Company (Saudi ARAMCO), Saudi Arabia, for the scholarship awarded for the development of this Ph.D. research at the University of Western Ontario.

■

NOTATIONS ri = Rate of reaction i (mol/g.s) VOx = vanadium oxide surface species N



Article

Alumina-Supported Vanadia Catalysts: Nature and Reducibility. J. Catal. 2007, 247, 176. (9) Danica, B.; Desislava, A.; Mirko, P.; Stefan, H. An Experimental Study of the Partial Oxidation of Ethane to Ethylene in a Shallow Fluidized Bed Reactor. J. Serbian Chem. Soc. 2007, 72, 183. (10) Haddad, N.; Bordes Richard, E.; Hilaire, L.; Barama, A. Oxidative Dehydrogenation of Ethane to Ethene on AluminaSupported Molybdenum-Based Catalysts Modified by Vanadium and Phosphorus. Catal. Today 2007, 126, 256. (11) Karamullaoglu, G.; Dogu, T. Oxidative Dehydrogenation of Ethane over Chromium-Vanadium Mixed Oxide and Chromium Oxide Catalysts. Ind. Eng. Chem. Res. 2007, 46, 7079. (12) Tope, B.; Zhu, Y.; Lercher, J. A. Oxidative Dehydrogenation of Ethane over Dy2O3/MgO Supported LiCl Containing Eutectic Chloride Catalysts. Catal. Today 2007, 123, 113. (13) Cavani, F.; Ballarini, N.; Cericola, A. Oxidative Dehydrogenation of Ethane and Propane: How far from Commercial Implementation? Catal. Today 2007, 127, 113. (14) Lemonidou, A. A.; Heracleous, E. Reaction Pathways of Ethane Oxidative and Non-Oxidative Dehydrogenation on Γ-Al2O3 Studied by Temperature-Programmed Reaction (TP-Reaction). Catal. Today 2006, 112, 23. (15) Solsona, B.; Dejoz, A.; Garcia, T.; Concepcion, P.; López-Nieto, J. M.; Vazquez, M.; Navarro, M. T. Molybdenum−Vanadium Supported on Mesoporous Alumina Catalysts for the Oxidative Dehydrogenation of Ethane. Catal. Today 2006, 117, 228. (16) Heracleous, E.; Lemonidou, A. A. Ni−Nb−O Mixed Oxides as Highly Active and Selective Catalysts for Ethene Production via Ethane Oxidative Dehydrogenation. Part II: Mechanistic Aspects and Kinetic Modeling. J. Catal. 2006, 237, 175. (17) Heracleous, E.; Lemonidou, A. A. Ni−Nb−O Mixed Oxides as Highly Active and Selective Catalysts for Ethene Production via Ethane Oxidative Dehydrogenation. Part I: Characterization and Catalytic Performance. J. Catal. 2006, 237, 162. (18) Botella, P.; Dejoz, A.; López-Nieto, J. M.; Concepcion, P.; Vazquez, M. Selective Oxidative Dehydrogenation of Ethane over MoVSbO Mixed Oxide Catalysts. Appl. Catal. A Gen. 2006, 298, 16. (19) Nakagawa, K.; Miyake, T.; Konishi, T.; Suzuki, T. Oxidative Dehydrogenation of Ethane to Ethylene over NiO Loaded on High Surface Area MgO. J. Mol. Catal. A Chem. 2006, 260, 144. (20) López-Nieto, J. M.; Botella, P.; García-González, E.; Dejoz, A.; Vazquez, M.; González-Calbet, J. Selective Oxidative Dehydrogenation of Ethane on MoVTeNbO Mixed Metal Oxide Catalysts. J. Catal. 2004, 225, 428. (21) Anon, R. New Propylene Production Technologies Hold Great Promise. Oil Gas J. 2004, 102, 50. (22) Bhasin, M. Is True Ethane Oxydehydrogenation Feasible? Top. Catal. 2003, 23, 145. (23) Resasco, D.; Haller, G. Catalytic Dehydrogenation of Lower Alkanes. Catal. R. Soc. Chem. 1994, 11. (24) Chan, K. Y. G.; Inal, F.; Senkan, S. Suppression of Coke Formation in the Steam Cracking of Alkanes: Ethane and Propane. Ind. Eng. Chem. Res. 1998, 37, 901. (25) Dharia, D.; Letzsch, W.; Kim, H.; McCue, D.; Chapin, L. Increase Light Olefins Production. Hydrocarb. Process. 2004, 83, 61. (26) Ren, T.; Patel, M.; Blok, K. Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes. Energy 2006, 31, 425. (27) Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Industrial Catalytic Processes; 1st ed.; Chapman & Hall: London, 1997. (28) Chen, K.; Xie, S.; Bell, A. T.; Iglesia, E. Alkali Effects on Molybdenum Oxide Catalysts for the Oxidative Dehydrogenation of Propane. J. Catal. 2000, 195, 244. (29) Kung, H. Oxidative Dehydrogenation of Light (C2 to C4) Alkanes. Adv. Catal. 1994, 40, 1. (30) Chen, K.; Khodakov, A.; Yang, J.; Bell, A. T.; Iglesia, E. Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts. J. Catal. 1999, 333, 325.

(31) Bell, A. T.; Khodakov, A.; Olthof, B.; Iglesia, E. Structure and Catalytic Properties of Supported Vanadium Oxides: Support Effects on Oxidative Dehydrogenation Reactions. J. Catal. 1999, 181, 205. (32) Chen, K.; Xie, S.; Bell, A. T.; Iglesia, E. Structure and Properties of Oxidative Dehydrogenation Catalysts Based on MoO3/Al2O3. J. Catal. 2001, 198, 232. (33) Grabowski, R.; Sloczynski, J. Kinetics of Oxidative Dehydrogenation of Propane and Ethane on VOx/SiO2 Pure and with Potassium Additive. Chem. Eng. Process. 2005, 44, 1082. (34) Grabowski, R. Kinetics of Oxidative Dehydrogenation of C2-C3 Alkanes on Oxide Catalysts. Catal. Rev. 2006, 48, 199. (35) Lemonidou, A. A. Oxidative Dehydrogenation of C4 Hydrocarbons over VMgO Catalyst  Kinetic Investigations. Appl. Catal. A Gen. 2001, 216, 277. (36) Machli, M.; Boudouris, C.; Gaab, S.; Find, J.; Lemonidou, A. A.; Lercher, J. A. Kinetic Modelling of the Gas Phase Ethane and Propane Oxidative Dehydrogenation. Catal. Today 2006, 112, 53. (37) Klose, F.; Joshi, M.; Hamel, C.; Seidelmorgenstern, A. Selective Oxidation of Ethane over a VOx/γ-Al2O3 CatalystInvestigation of the Reaction Network. Appl. Catal. A Gen. 2004, 260, 101. (38) Rahman, F.; Loughlin, K. F.; Al-Saleh, M. A.; Saeed, M. R.; Tukur, N. M.; Hossain, M. M.; Karim, K.; Mamedov, E. A. Kinetics and Mechanism of Partial Oxidation of Ethane to Ethylene and Acetic Acid over MoV Type Catalysts. Appl. Catal. A Gen. 2010, 375, 17. (39) Chen, K.; Iglesia, E.; Bell, A. T. Isotopic Tracer Studies of Reaction Pathways for Propane Oxidative Dehydrogenation on Molybdenum Oxide Catalysts. J. Phys. Chem. B 2001, 105, 646. (40) Bell, A. T.; Iglesia, E.; Argyle, M. D.; Chen, K. Ethane Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts. J. Phys. Chem. B 2002, 106, 5421. (41) Bell, A. T.; Dinse, A.; Schomäcker, R. The Role of Lattice Oxygen in the Oxidative Dehydrogenation of Ethane on AluminaSupported Vanadium Oxide. Phys. Chem. Chem. Phys. 2009, 11, 6119. (42) Lemonidou, A. A.; Heracleous, E.; Lercher, J. A. Mechanistic Features of the Ethane Oxidative Dehydrogenation by in Situ FTIR Spectroscopy over a MoO3/Al2O3 Catalyst. Appl. Catal. A Gen. 2004, 264, 73. (43) Ramos, R.; Pinal, M. P.; Menendez, M.; Santamaria, J. Patience, G. S. Oxidative Dehydrogenation of Propane to Propene, 1: Kinetic Study on V/MgO. Can. J. Chem. Eng. 2001, 79, 891. (44) Creaser, D.; Andersson, B. Oxidative Dehydrogenation of Propane over V-Mg-O: Kinetic Investigation by Nonlinear Regression Analysis. Appl. Catal. A Gen. 1996, 141, 131. (45) Creaser, D.; Andersson, B.; Hudgins, R. R.; Silverston, P. L. Transient Kinetic Analysis of the Oxidative Dehydrogenation of Propane. J. Catal. 1999, 182, 264. (46) Rao, T. V. M; Deo, G. Kinetic Parameter Analysis for Propane ODH: V2O5/Al2O3 and MoO3/Al2O3 Catalysts. AIChE J. 2007, 53, 1538. (47) Chen, K.; Iglesia, E.; Bell, A. T. Kinetic Isotopic Effects in Oxidative Dehydrogenation of Propane on Vanadium Oxide Catalysts. J. Catal. 2000, 192, 197. (48) Creaser, D.; Andersson, B. Oxidative Dehydrogenation of Propane over V-Mg-O: Kinetic Investigation by Nonlinear Regression Analysis. Appl. Catal. A Gen. 1996, 141, 131. (49) Dinse, A.; Khennache, S.; Frank, B.; Hess, C.; Herbert, R.; Wrabetz, S.; Schlögl, R.; Schomäcker, R. Oxidative Dehydrogenation of Propane on Silica (SBA-15) Supported Vanadia Catalysts: A Kinetic Investigation. J. Mol. Catal. A Chem. 2009, 307, 43. (50) Balcaen, V.; Sack, I.; Olea, M.; Marin, G. B. Transient Kinetic Modeling of the Oxidative Dehydrogenation of Propane over a Vanadia-Based Catalyst in the Absence of O2. Appl. Catal. A Gen. 2009, 371, 31. (51) Bottino, A.; Capannelli, G.; Comite, A.; Storace, S.; Felice, R. D. Kinetic Investigations on the Oxidehydrogenation of Propane over Vanadium Supported on Γ-Al2O3. Chem. Eng. J. 2003, 94, 11. (52) Grabowski, R.; Pietrzyk, S.; Słoczy, J.; Genser, F.; Wcisło, K.; Swierkosz, B. G.-. Kinetics of the Propane Oxidative Dehydrogenation O



Article

on Vanadia/Titania Catalysts from Steady-State and Transient Experiments. Appl. Catal. A Gen. 2002, 232, 277. (53) Leveles, L. Oxidative Conversion of Propane over LithiumPromoted Magnesia Catalyst I. Kinetics and Mechanism. J. Catal. 2003, 218, 296. (54) Kumar, C. P.; Gaab, S.; Müller, T. E.; Lercher, J. A. Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts. Top. Catal. 2008, 50, 156. (55) Al-Zahrani, S. M.; Abasaeed, A. E.; Putra, M. Kinetics of Oxidehydrogenation of Propane over Alumina-Supported Sr−V−Mo Catalysts. Catal. Commun. 2012, 26, 98. (56) Late, L.; Blekkan, E. A. Kinetics of the Oxidative Dehydrogenation of Propane over a VMgO Catalyst. J. Nat. Gas Chem. 2002, 11, 33. (57) Gascón, J.; Valenciano, R.; Téllez, C.; Herguido, J.; Menendez, M. A Generalized Kinetic Model for the Partial Oxidation of N-Butane to Maleic Anhydride under Aerobic and Anaerobic Conditions. Chem. Eng. Sci. 2006, 61, 6385. (58) López-Nieto, J. M.; Soler, J.; Concepcion, P.; Herguido, J.; Menendez, M.; Santamaria, J. Oxidative Dehydrogenation of Alkanes over V-Based Catalysts: Influence of Redox Properties on Catalytic Performance. J. Catal. 1999, 332, 324. (59) Sadykov, V. A.; Pavlova, S.; Saputina, N.; Zolotarskii, I.; Pakhomov, N.; Moroz, E.; Kuzmin, V.; Kalinkin, A. Oxidative Dehydrogenation of Propane over Monoliths at Short Contact Times. Catal. Today 2000, 61, 93. (60) Leveles, L. Oxidative Conversion of Propane over LithiumPromoted Magnesia Catalyst II. Active Site Characterization and Hydrocarbon Activation. J. Catal. 2003, 218, 307. (61) Zhao, R.; Wang, J.; Dong, Q.; Liu, J. Selective Oxidation of Propane by Lattice Oxygen of Vanadium−Phosphorous Oxide in a Pulse Reactor. J. Nat. Gas Chem. 2005, 14, 88. (62) Fukudome, K.; Ikenaga, N.; Miyake, T.; Suzuki, T. Oxidative Dehydrogenation of Propane Using Lattice Oxygen of Vanadium Oxides on Silica. Catal. Sci. Technol. 2011, 1, 987. (63) Yoon, Y. S.; Ueda, W.; Moro-oka, Y. Oxidative Dehydrogenation of Propane over Magnesium Molybdate Catalysts. Catal. Lett. 1995, 35, 57. (64) Creaser, D.; Andersson, B.; Hudgins, R. R.; Silverston, P. L. Transient Study of Oxidative Dehydrogenation of Propane. Appl. Catal. A Gen. 1999, 187, 147. (65) Stern, D. L.; Grasselli, R. K. Propane Oxydehydrogenation over Molybdate-Based Catalysts. J. Catal. 1997, 167, 550. (66) Rubio, O.; Herguido, J.; Menendez, M. Oxidative Dehydrogenation of N-Butane on V/MgO Catalysts-Kinetic Study in Anaerobic Conditions. Chem. Eng. Sci. 2003, 58, 4619. (67) Soler, J.; López-Nieto, J. M.; Herguido, J.; Menendez, M.; Santamaria, J. Oxidative Dehydrogenation of N-Butane in a Two-Zone Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 1999, 38, 90. (68) Rubio, O.; Herguido, J.; Menendez, M. Oxidative Dehydrogenation of Butane in an Interconnected Fluidized-Bed Reactor. AIChE J. 2004, 50, 1510. (69) Al-Ghamdi, S. A.; Hossain, M. M.; Lasa, H. I. De. Kinetic Modeling of Ethane Oxidative Dehydrogenation over VOX/Al2O3 Catalyst in a Fluidized-Bed Riser Simulator. Ind. Eng. Chem. Res. 2013, 52, 5235−5244. (70) Al-Ghamdi, S. A.; Volpe, M.; Hossain, M. M.; de Lasa, H. I. VOx/c-Al2O3 Catalyst for Oxidative Dehydrogenation of Ethane to Ethylene: Desorption Kinetics and Catalytic Activity. Appl. Catal. A Gen. 2013, 450, 120. (71) Al-Ghamdi, S. A.; de Lasa, H. I. Propane Oxidative Dehydrogenation over a VOx/Al2O3 Catalyst: Characterization and Catalytic Performance. Fuel 2013, submitted. (72) De Lasa, H. I. Canadian Patent 1,284,017, (1991). USA Pat. 5,102,628, 1992. (73) De Lasa, H. I.; Al-Khattaf, S. Catalytic Cracking of Cumene in a Riser Simulator: A Catalyst Activity Decay Model. Ind. Eng. Chem. Res. 2001, 5398.

(74) Hossain, M. M.; Atanda, L.; Al-Yassir, N.; Al-Khattaf, S. Kinetics Modeling of Ethylbenzene Dehydrogenation to Styrene over a Mesoporous Alumina Supported Iron Catalyst. Chem. Eng. J. 2012, 207−208, 308. (75) Al-Ghamdi, S. A.; Volpe, M.; Hossain, M. M.; de Lasa, H. I. VOx/c-Al2O3 Catalyst for Oxidative Dehydrogenation of Ethane to Ethylene: Desorption Kinetics and Catalytic Activity. Appl. Catal. A Gen. 2013, 450, 120. (76) Ginsburg, J.; de Lasa, H. I. Catalytic Gasification of Biomass in CREC Fluidized Riser Simulator. Int. J. Chem. React. Eng. 2005, 3, A38. (77) Pekediz, A.; Kraemer, D. W.; Chabot, J.; de Lasa, H. I. Mixing Patterns in a Novel Riser Simulator. Appl. Sci. 1992, 133. (78) Hossain, M. M.; de Lasa, H. I. Reduction and Oxidation Kinetics of Co−Ni/Al2O3 Oxygen Carrier Involved in a Chemical-Looping Combustion Cycles. Chem. Eng. Sci. 2010, 65, 98. (79) Jarosch, K.; El Solh, T.; de Lasa, H. I. Modelling the Catalytic Steam Reforming of Methane: Discrimination between Kinetic Expressions Using Sequentially Designed Experiments. Chem. Eng. Sci. 2002, 57, 3439. (80) Moreira, J.; Serrano, B.; Ortiz, A.; de Lasa, H. I. A Unified Kinetic Model for Phenol Photocatalytic Degradation over TiO2 Photocatalysts. Chem. Eng. Sci. 2012, 78, 186. (81) Routray, K.; Reddy, K. R. S.; Deo, G. Oxidative Dehydrogenation of Propane on V2O5/Al2O3 and V2O5/TiO2 Catalysts: Understanding the Effect of Support by Parameter Estimation. Appl. Catal. A Gen. 2004, 265, 103. (82) Dinse, A.; Frank, B.; Hess, C.; Habel, D.; Schomäcker, R. Oxidative Dehydrogenation of Propane over Low-Loaded Vanadia Catalysts: Impact of the Support Material on Kinetics and Selectivity. J. Mol. Catal. A Chem. 2008, 289, 28.

P


Kinetic Modeling of Propane Oxidative ... - ACS Publications

Recommend Documents