Kinetics and mechanism of partial oxidation of ethane to ethylene and acetic acid over MoV type catalysts

Article abstract of DOI:10.1016/j.apcata.2009.11.026

A kinetics study for the partial oxidation of ethane to ethylene and acetic acid is performed over MoV type catalysts. It is established that ethylene is the primary product and acetic acid and carbon oxides are secondary products. Formation of acetic acid product increases significantly with the co-feeding of water into the reactor. The elementary steps of the reaction network are formulated using a two-site Eley-Rideal-Redox (ERR) model, which includes the participation of water in the reaction scheme through surface OH^- groups. The derived ERR model predicts the experimental data satisfactorily.

Full text of DOI:10.1016/j.apcata.2009.11.026

Applied Catalysis A: General 375 (2010) 17–25
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Kinetics and mechanism of partial oxidation of ethane to ethylene and acetic acid
over MoV type catalysts
Faizur Rahman a, , Kevin F. Loughlin b,d, Muhammad A. Al-Saleh b, Mian R. Saeed a, Nasir M. Tukur b,
*
Mohammad M. Hossain b, Khalid Karim c, Agaddin Mamedov c
a Center of Research Excellence in Petroleum Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
b Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
c SABIC Research and Technology, Riyadh, Saudi Arabia
d American University of Sharjah, United Arab Emirates
A R T I C L E I N F O
A B S T R A C T
Article history:
A kinetics study for the partial oxidation of ethane to ethylene and acetic acid is performed over MoV
type catalysts. It is established that ethylene is the primary product and acetic acid and carbon oxides are
secondary products. Formation of acetic acid product increases significantly with the co-feeding of water
into the reactor. The elementary steps of the reaction network are formulated using a two-site Eley-
Rideal-Redox (ERR) model, which includes the participation of water in the reaction scheme through
surface OHꢀ groups. The derived ERR model predicts the experimental data satisfactorily.
ß 2010 Published by Elsevier B.V.
Received 4 March 2009
Received in revised form 17 November 2009
Accepted 18 November 2009
Available online 24 November 2009
Keywords:
Ethane partial oxidation
Ethylene
Acetic acid
Reaction network
Kinetic model
Eley-Rideal-Redox model
MoV type catalyst
Reaction mechanism
1. Introduction
ethylene to acetic acid. The latter site arises from the formation of
highly dispersed Pd(II) sites into an Mo5O14 phase promoting the
In recent years the production of ethylene and acetic acid from
the partial oxidation of ethane has developed as an alternative
route to provide feedstock to the downstream petrochemical
industry. Mixed oxides catalysts containing molybdenum and
vanadium are the preferred catalyst for this investigation [1–15].
The key fact concerning these mixed oxide catalyst [16] is their
multiphase nature which strongly influences the mechanism and
the kinetics of the partial oxidation of ethane. Desponds et al. [3]
indicated that three oxide phases were necessary to have active
and selective catalysts.
Wacker hydroxylation reaction. Thornsteinson et al. [1] suggested
various mixed oxide catalysts containing Mo and V together with
transition metal oxides (Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta or Ce)
promoted partial oxidation reactions. The optimum composition
was observed to be Mo0.61V0.31Nb0.08 for reaction of ethane to
ethylene. The presence of niobium stabilizes the catalyst structure
against oxidation and reduction and permits a very strongly
oxidized or reduced catalyst to be recycled more readily to its
original value. Rouessel et al. [12] observed two phases, MoV0.4Ox
and MoV0.4Nb0.12Ox, and found that niobium is responsible for both
stabilization and nanosizing of MoO3 and (VNbMo)5O14 crystals.
Surface oxygen, OH groups, and oxygen vacancies are the most
abundant reactive sites during ethane oxidative dehydrogenation
(ODH) on active VOx domains of vanadium oxide (VOx/Al2O3 and
VOx/ZrO2) catalysts [20]. Ruth et al. [5] showed that the Mo6V9O40
phase in a multiphase catalyst led to the formation of acetic acid.
Catalytic experiments on pure VPO phases and titania supported
VPOx and VOx are discussed by Tessier et al. [23] for the selective
oxidation of ethane to acetic acid. The reaction mechanism is
Linke et al. [9,10] used a MoV0.25Nb0.12Pd0.0005Ox based catalyst
in their study of this reaction. They suggested the existence of two
different catalytic centers, one for the oxidative dehydrogenation
of ethane, and one for the heterogeneous Wacker oxidation of
* Corresponding author at: Box 1634, Dhahran 31261, Saudi Arabia.
Tel.: +96 6502662711; fax: +96 638694509.
E-mail addresses: rahman.faiz@gmail.com, frahman@kfupm.edu.sa (F. Rahman).
0926-860X/$ – see front matter ß 2010 Published by Elsevier B.V.
doi:10.1016/j.apcata.2009.11.026

18
F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
Linke et al. [9–11], Smejkal et al. [16] and Grabowski and
Nomenclature
Sloczynski [13] indicate that the presence of water increases the
formation of acetic acid but Desponds et al. [3] report that the
addition of water to the gas stream does not affect the product
distribution. The presence of niobium [1,3,15] or potassium [13] or
adsorbed water which promotes desorption of acetic acid [15]
inhibits the formation of carbon oxides.
The kinetics of reaction on these multiphase catalysts is
primarily redox and most kinetic models are based on this
principle. The kinetic models employed are redox [10], Eley-Rideal
[4,8,13], and Langmuir-Hinshelwood [1]. Most studies indicate
that the reaction order for ethane is close to 1, whereas for oxygen
it is close to zero [2,4,8]. Thornsteinson et al. [1] indicate that the
reaction rates to acetic acid and carbon oxides are dependent on
the ethylene concentration but are independent of the ethane
concentration.
Linke et al. [10] developed a set of elementary equations
based on the redox principle for their two-site catalyst. The
model has some deficiencies particularly in the elementary steps
of deep oxidation reactions. The rate equations for ethane
oxidation show no participation of water. However, acetic acid
formation from ethylene is promoted in the presence of the
oxydehydrogenation reaction. The elementary reaction steps 3,
4, 5, 9, 10 and 11 in their paper omit oxygen in their reaction
steps. Each of the reactions on the left contains O2, yet none of
the rate equations on the right side contain oxygen in their rate
expression. The activation energies for the reactions number 6
and 8 are negative. The redox reactions are also not balanced in
determining the fractions adsorbed.
In this paper, a comprehensive kinetics study is presented based
on mechanistic principles. In the literature, the effect of water on
the kinetics of ethane oxidation is not yet clearly described. Some
authors explain the physical adsorption of water on the surface of
the catalyst. Chemical participation of water in elementary steps
with the formation of surface intermediate has not been described.
This paper explains the kinetics investigation describing the
chemical interaction of water to the reaction mechanism.
Considering water as participating in the reaction, kinetics models
are derived explaining the effect of water to the different pathway
of ethane oxidation. An independent study1 using deuterated
water confirmed this effect resulting in its incorporation in the
kinetic model. The kinetic model based on mechanistic principles
is evaluated using experimental data.
A
Arrhenius pre-exponential factor typically (mmol/
g/s)
A1
A2
Ci
defined as equal to K1 ꢂ K2 in Table 4
defined as equal to K10 ꢂ K11 in Table 4
concentration of species i (
carbon oxides
m
mol/cm3)
COx
Ei
activation energy (kJ/mol)
Hi
enthalpy of species i (J/mmol)
ꢀD
H
heat of formation of products (J/
m
mol, transport
limitation)
(ꢀ
Ki
DH)Ri enthalpy of reaction of species i (J/mmol)
adsorption equilibrium constant (cm3/mmol)
equilibrium constant (dimensionless)
specific reaction rate constant (reaction order
units)
Ke
ki
LO
n
lattice oxygen
overall reaction order
P
reaction pressure (bar)
ri
rate of reaction of species i per unit volume (mmol/
g/s)
Ri
overall rate of reaction of species i per unit volume
mol/g/s)
(m
X
active site
XO
XOH
T
active oxygen center
hydroxylated X site
reaction temperature (K)
active site
Z
ZOH
hydroxylated Z site
Subscripts
i
species
Greek symbols
ui
n
fraction of the surface coverage by species i
Stoichiometric coefficient
strongly dependent on the multiphase nature of the catalyst, which
is supported by the two-site mechanisms [8,10]. The basic
mechanism is reduction of the catalyst by the adsorbed
hydrocarbon, reoxidation by oxygen [1,2,8,10,13,22] and surface
hydroxylation by water [1,10,13]. The role of lattice oxygen in the
reactions due to an active phase based on molybdenum and
vanadium oxides has been reported by several authors [2,4,10].
Panov et al. [24] pointed out that, strongly bonded lattice oxygen
having nucleophilic nature is responsible for selective oxidation
whereas weakly bonded reactive oxygen having electrophilic
nature contributes to complete oxidation. Sehgal [25] performed
oxidative dehydrogenation of ethane over transition metal oxides
and proposed that ethylene is formed by a redox mechanism of
Mars and van Krevelen with the involvement of lattice oxygen
species (O2ꢀ) and surface oxygen species (Oꢀ). The deep oxidation
of both ethane and ethylene to CO and CO2 is attributed to gas
phase oxygen species. Capek et al. [26] investigated the perfor-
mance of V-HMS, V-MCM41 and V-SBA-15 catalysts with varying
vanadium loadings for the oxidative dehydrogenation of ethane.
They concluded that the most active catalysts contained 2–4.5% V
finely dispersed over mesoporous support in the form of isolated
monomeric and oligomeric vanadium species and exhibited
comparable activity.
2. Experimental
The kinetic experiments were conducted using Mo-V based
catalysts in a fixed bed flow reactor under differential reaction
conditions. The details of catalyst preparation used in this study
are described in terms of compositions [14,21]. Systematic data
were generated to investigate the kinetics of C2H6 oxidation with
and without water in the feed.
The kinetic experiments were performed using 0.15 g of
catalyst diluted with inert material in a fixed bed flow reactor
(ID = 0.78 cm) under differential reaction conditions. The reactor
was maintained at temperatures ranging between 513 and 553 K,
pressures in the range of 150–200 psig and at different space
velocities. All the data are within the acceptable range of mass
balance ꢁ5% and the conversion of ethane is limited to less than 10%.
Transport limitations in the reactor, estimated using the procedure
described in references [17,19] showed that data generated for the
kinetic studies are free from internal and external heat and mass
transfer effects. For the kinetic investigation, a matrix of experiments
is performed solely in a differential reactor at three levels of
1
Personal Communication, SABIC, 2005.

F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
19
Table 1
Experimental conditions for kinetic parameter evaluation.
Parameter
Partial oxidation of ethane
Catalyst weight (g)
Temperature (K)
0.15
513, 533, 553
Pressure (bar)
14
Volumetric flow (cm3/min)
30
Space velocity (minꢀ1
)
205
Feed composition
Ethane (mol%)
Oxygen (mol%)
Steam (mol%)
30–60
6–12
0
Nitrogen (mol%)
Balance
temperature, ethane concentration and oxygen concentration as
illustrated in Table 1.
The products of ethane oxidation are C2H4, CH3COOH, and CO2.
The conversions are calculated from the outlet flow rate of un-
reacted C2H6 and the amount of C2H6 reacted to form products
C2H4, CH3COOH and CO2 considering the stoichiometry of the
individual reactions.
Fig. 2. Effect of variation in oxygen concentration on reaction rates at 553 K
(catalyst weight: 0.15 g, ethane to oxygen ratio: 3–6).
the selectivity to ethylene decreases. The selectivity to CH3COOH
and COx increases with temperature at the expense of C2H4. This
indicates C2H4 is a significant source of CH3COOH and COx. The
main products were found to be ethylene and acetic acid with
selectivity of 50% and 30%, respectively, at 553 K.
3. Results and discussions
3.1. Effect of reactant concentration on ethane oxidation
3.3. Effect of space velocity
The rate of ethane consumption and the rate of formation of
products ethylene and acetic acid increase almost linearly with
increasing concentration of ethane as illustrated in Fig. 1. On the
other hand, the effect of O2 concentration on the rate of C2H6
oxidation and on the rate of formation of products (C2H4, CH3COOH
and CO2) appears to be insensitive when oxygen to ethane ratio is
around 0.2 as can be observed in Fig. 2. Kinetic data implies that the
selective oxidation of ethane is approximately first order with
respect to ethane and is consistent with a mechanism in which the
breaking of a C–H bond is rate determining. The reaction was found
to be almost independent of the oxygen concentration, indicating
that at reaction conditions the surface of the catalyst is fully
oxidized, which is consistent with other reported data [2,5,20].
To elucidate the reaction network, experiments were per-
formed at various space velocities. The selectivity of products for
the partial oxidation of ethane without steam in the feed as a
function of conversion of ethane is illustrated in Fig. 4. Ethylene
appears as the primary product of reaction of the partial oxidation
of ethane. The decrease in ethylene concentration with corre-
sponding increase in CH3COOH and COx indicates that these are the
secondary products of consecutive oxidation of ethylene. Ethane
does not appear to be the source of the products CH3COOH, CO2,
and CO. The selectivity of products for the partial oxidation of
ethylene as a function of conversion of ethylene is illustrated in
Fig. 5. This figure clearly indicates that ethylene is the source of the
products CH3COOH, and COx and supports the results of Fig. 4.
3.2. Effect of temperature
A
selectivity study separately performed in the reactor
operating under integral mode (using catalyst = 7 g, ethane
(mol%) = 5–25, oxygen (mol%) = 16–20, steam = 0, T (K) = 483–
The conversion of C2H6 and product selectivities as a function of
temperature at constant space velocity, and its impact on the
product selectivity is illustrated in Fig. 3. As the reaction
temperature increases, the conversion of ethane increases while
Fig. 1. Effect of variation in ethane concentration on reaction rates at 553 K (catalyst
Fig. 3. Conversion of C2H6 and selectivity of products as a function of temperature at
constant space velocity (catalyst weight: 0.2 g, C2/O2 = 0.8, P: 14 bar).
weight: 0.15 g, ethane to oxygen ratio: 4–9).

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F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
Fig. 6. Product selectivity versus ethane conversion in an integral reactor without
steam in feed (solid lines represent trend lines; catalyst weight: 7 g, T: 483–553 K,
P: 14 bar, C2H6: 5–25 mol%, O2: 16–20 mol%, H2O: 0).
Fig. 4. Selectivity of products as a function of C2H6 conversion. Varying space
velocity (T = 538 K, C2H6 = 14 mol%, O2 = 17 mol%).
C2H6 þ 2:5O2 ! 2CO þ 3H2O
C2H6 þ 3:5O2 ! 2CO2 þ 3H2O
C2H4 þ O2 ! CH3COOH
(3)
(4)
(5)
(6)
563) presented in Fig. 6, confirms that as the conversion of ethane
increases, ethylene converts to acetic acid forming acetic acid and
carbon oxides. The acetic acid increases to a maximum and then
decreases due to decomposition of acetic acid to carbon oxides.
This increase of acetic acid selectivity with ethylene conversion
shows that acetic acid is produced from ethylene. Acetic acid
selectivity increase is in the range of 0–20% ethylene conversion as
may be observed in Fig. 5. The increase of acetic acid selectivity in
Fig. 5 can be related to the influence of water formed in the
reaction, i.e. the higher the degree of ethylene conversion, the
higher the partial pressure of water. The co-feeding of water
experiments also supports this explanation showing a significant
increase of acetic acid selectivity with a decrease of ethylene
selectivity. Therefore, Figs. 5 and 6 delineate all the direct and
consecutive reactions. CO in the case of ethylene conversion forms
in parallel reaction. That is why the selectivity extrapolation of CO
to zero conversion is not zero as it becomes an intermediate due to
its deep oxidation to CO2.
C2H4 þ 2O2 ! 2CO þ 2H2O
C2H4 þ 3O2 ! 2CO2 þ 2H2O
(7)
(8)
(9)
CH3COOH þ O2 ! 2CO þ 2H2O
CH3COOH þ 2O2 ! 2CO2 þ 2H2O
CO þ 0:5O2 ! CO2
(10)
The reaction network scheme shown in Fig. 7 illustrated that
ethane reacts only by route 1 to form ethylene is deduced from
Fig. 4. Reaction (1) is the primary reaction. No parallel reactions of
ethane occur implying reactions (2), (3) and (4) may be eliminated
from network stoichiometric reaction schemes.
Ethylene reacts in parallel mode simultaneously forming the
products CH3COOH, CO2, and CO as illustrated in Fig. 5 implying
that reactions (5), (6) and (7) exist in the network. Two parallel
reactions for the reaction of acetic acid to carbon monoxide and
carbon dioxide occur (steps 8 and 9). Finally, reaction (10), a
sequential reaction of carbon monoxide to carbon dioxide occurs
Based on the above observation the possible reactions of the
partial oxidation of C2H6 in the light of above discussion are:
C2H6 þ 0:5O2 ! C2H4 þ H2O
(1)
(2)
C2H6 þ 1:5O2 ! CH3COOH þ H2O
Fig. 5. Selectivity of products as a function of C2H4 conversion. Varying space
velocity (T: 538 K, constant feed composition C2H4 ꢃ 7 mol%, O2 ꢃ 19.5 mol%).
Fig. 7. Proposed reaction scheme for oxidation of C2H6 based on data without steam
in feed.

F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
21
Fig. 8. Product selectivity versus conversion of C2H6 with steam in feed at varying space velocity (T = 553 K, P = 14 bar, C2H6 = 40 mol%, O2 = 8 mol%, H2O = 20 mol%,
2 = 32 mol%).
N
based on an independent study carried out using carbon monoxide
alone as a feed.
to the increase of acetic acid selectivity. Basic idea of using two-site
mechanism is that acid product forms on acid sites and desorbs
faster from acid sites while the basic products (ethylene has much
more basic property than acetic acid) forms on basic site and
desorbs faster from basic site. Multicomponent catalyst studied in
our work has multiphase catalytic structure suggesting two types
of active sites, which is very common in oxidative catalysis with
separation of reaction steps between the active sites. This idea also
corresponds with the literature data that dehydrogenation
proceeds with participation of basic property nucleophilic centers,
while acid property products form with participation of electro-
philic oxygen centers [14].
Eventually, a two-site kinetic model based on combined Eley-
Rideal and Redox mechanisms was observed to render successful
regression results with the experimental data. Consequently, the
resulting model is named as an Eley-Rideal-Redox (ERR) model.
The ERR kinetic model is a comprehensive model considering the
The study of the effect of co-feeding of water on the product
distribution for partial oxidation of ethane showed a significant
increase of acetic acid selectivity with a decrease of ethylene
selectivity. The selectivity of products as a function of C2H6
conversion is illustrated in Fig. 8. The product distribution has
completely reversed in terms of C2H4 and CH3COOH as compared
to data without co-feed of water (Fig. 4). The selectivity to
CH3COOH has increased significantly at the expense of C2H4. It is
postulated that this increase in CH3COOH selectivity with addition
of water to the feed is due to oxy-hydoxylation reaction of C2H4
with water. Isotopic experiments using deuterated water (D2O) in
the feed confirmed that D2O is participating in the reaction to form
acetic acid.
Probably the increase of acetic acid selectivity with addition of
water is connected with the participation of OHꢀ group generated
from water in the elementary stage of the acetic acid formation. On
the basis of these experiments it has been suggested that acetic
acid formation proceeds through two routes: first nucleophilic
oxygen centers participate in the elementary step in the insertion
to ethylene molecule and in the second route OHꢀ groups lead to
the formation of hydroxyl containing ethylene fragments, which in
consecutive reactions form acetic acid.
Based on the selectivity versus ethane conversion data with
steam in the feed and from the information revealed by isotopic
experiments it is postulated that CH3COOH is formed from
ethylene via both an oxygen route and a hydroxyl route.
In view of above discussion, the reaction scheme proposed in
Fig. 7 is modified to that in Fig. 9 for the reaction network.
3.4. Kinetic Model
Several reaction mechanisms such as Langmuir-Hinshelwood,
Eley-Rideal, Redox, etc. were investigated to represent the
experimental data. The description of experimental data using
two types of active sites is based on the variation of catalyst
composition and acidic or basic properties: increase of basic
properties of the catalysts leads to increase of ethylene selectivity,
for example the results on ethane dehydrogenation to ethylene on
MoVMn containing catalyst [14]. Increase of acid properties leads
Fig. 9. Comprehensive proposed reaction scheme.

22
F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
Table 2
overall reaction scheme illustrated in Fig. 9. The following are the
general principles on which the model is based:
Elementary steps based on reaction scheme in Fig. 9.
Step
Reaction
I. Two sites, Z and X centers, are employed in the derivation of the
reaction kinetics for the network system. This is consistent with
the multiphase nature of the catalyst and literature work
supporting this concept [6,8,10].
II. In the reaction from ethane to ethylene, two routes are shown,
one via the oxide ZO type center and one via the hydroxyl ZOH
type center. Also two routes are indicated in the transformation
of ethylene to acetic acid, one via the oxide XO type center and
one via the hydroxyl XOH type center. Similarly, two routes are
indicated in the reaction of CO to CO2, one via the oxide XO type
center and one via the hydroxyl XOH type center.
III. In the presence of lattice oxygen centers, the introduction of
water leads to the formation of hydroxyl species. Acidic nature
of the Mo, V oxide phases of the mixed oxide catalyst may
contribute to the realization of reaction with the formation of a
surface OHꢀ center.
Reactions on Z type site
1
1
2
3
4
5
6
7
8
9
Z þ O2 ! ZO2
2
ZO2 þ Z !2ZO
3
Z þ ZO þ H2O !2ZOH
4
ZOH þ H2O ! ZOHðH2OÞ
L þ ZO $ Z þ LO
C2H6 þ LO6ꢀ;R!DSC2H5LOH
fast
C2H5LOHꢀ! C2H4 þ H2O þ L
C2H6 þ ZOH8ꢀ;R!DSC2H5Z þ H2O
fast
C2H5Z þ ZOHꢀ! C2H4 þ H2O þ 2Z
Reactions on X type site
10
10
11
12
13
14
15
16
17
18a
18b
18c
19
20
21
22
23
24
25
26
27
28
29
30
X þ O2 ! XO2
11
XO2 þ X ! 2XO
12
H2O þ X þ XO ! 2XOH
13
XOH þ H2O !XOHðH2OÞ
14
C2H4 þ XO ! C2H4XO
C2H4XO1ꢀ5;!RDSCH3CHOX
fast
CH3CHOX þ XOꢀ! CH3COOH þ 2X
C2H4XO þ XOH1ꢀ7;!RDSCH3CHXOH þ XO
CH3CHXOH þ XO ! CH3CXOH þ XOH
CH3CXOH þ XO ! CH3COOHX þ X
CH3COOHX $ CH3COOH þ X
O
2ꢀ þ HOH , 2OHꢀ
However at higher concentration of water, these centers are
blocked due to physical adsorption.
Menþ þ H2O þ OHꢀ , MenþOHꢀðH2OÞads
C2H4XO þ X1ꢀ9;!RDS½ꢄꢅ
fast
½ꢄꢅ þ XOꢀ!HCHOX þ X
fast
HCHOX þ XOꢀ! CO þ H2O þ 2X
C2H4XO þ XO2ꢀ2;!RDS2HCHOX
fast
HCHOX þ 2XOꢀ! CO2 þ H2O þ 3X
IV. On the X type site, ethylene reacts with either lattice oxygen or
reacts with the hydroxyl group center by an Eley-Rideal
mechanism. Again, a surfeit of water present on the site may
result in the blocking of sites.
24
CH3COOH þ XO ! CH3COOH ꢆ XO
CH3COOH:XO þ 2X2ꢀ5;!RDS2HCHOX þ XO
fast
HCHOX þ XOꢀ! CO þ H2O þ 2X
CH3COOH:XO þ XO2ꢀ7;!RDS2HCHOXO
fast
HCHOXO þ XOꢀ! CO2 þ H2O þ 2X
CO þ XO2ꢀ9;!RDSCO2 þ X
COXOH þ XOH3ꢀ0;!RDSCO2 þ H2O þ 2X
The shift in the behavior of catalyst in the formation of acetic
acid without water and with addition of water in the feed posed
difficulties during development of elementary steps. The increase
in acetic acid formation due to the addition of water in the feed was
solved by having two routes for the formation of CH3COOH
through the oxidation and hydroxylation of C2H4. The ERR Model
describes the participation of water as well.
The existence of parallel mechanism of acetic acid formation
with participation of oxide and OH groups is proved by isotopic
experiments: Water added to the reaction feed was replaced by
D2O. During similar experiments as performed with H2O, the acetic
acid product was observed to be containing deuterium, proving the
insertion of ODꢀ group in the acetic acid through surface oxygen
centers in accordance with the suggested mechanism:
Linke et al. [10] also consider that two catalytic centers are
required to explain the reaction pathways: oxidation of ethane to
ethylene, formation of acetic acid from ethane via a surface
intermediate and total oxidation is ascribed to one catalytic center.
The second center exclusively catalyzes the oxidation of ethylene
to acetic acid via the Wacker mechanism which is related to
hydroxyl groups forming the active center. Hence, they proposed
that formation of acetic acid from ethylene can be accelerated to a
certain degree with increase in water to the feed. For the
consumption of ethane, Thorsteinson [1] speculates that two sites
are involved in the ethane partial oxidation reaction and suggests
that the primary product of oxydehydrogenation of ethane is
ethylene only and that acetic acid and carbon oxides are formed by
the subsequent oxidation of ethylene. The present study suggests
that the adsorbed ethylene species on the XO center reacts in
parallel routes based on Eley-Rideal mechanism [steps 15, 17, 19
and 22] to produce CH3COOH and CO2 consistent with the
selectivity behavior shown in Fig. 5.
Acetic acid is formed by reaction of an oxygen center (XO) with
intermediate species of ethylene (step 16). An alternate route for
the production of acetic acid is the reaction of ethylene species
with the (XOH) oxygen center forming hydroxylated species as in
step 17. This step is considered as a rate determining step. The
hydroxylated species subsequently reacts with the XO center to
form acetic acid (steps 18a, 18b, 18c). This route accounts for
significant increase in acetic acid production with addition of
water in the feed. Parallel reactions of the ethylene C2H4XO species
produce COx (step 19 and 22). Acetic acid can further oxidize to CO
and CO2 (steps 25–30).
D2O þ O2ꢀ ! 2ODꢀ
The elementary steps of the reaction are presented in Table 2 using
the reaction scheme proposed in Fig. 9. The elementary steps are
based on two independent sites labeled Z-site center and X-site
center. Consider the Z-site first. Steps 1 and 2 are simply reversible
adsorption of oxygen and dissociation with formation of surface
oxygen species similar to that postulated by Linke et al. [10]. The
oxygen species forms a lattice oxygen species LO. In step 3, water
reacts with the ZO center to form surface hydroxyl species ZOH.
When the ZO centers are fully used up or limited by the lack of
ample oxygen, reversible physical adsorption of water can also
occur on these sites, which block the surface.
On the Z-site ethane reacts from the gas phase with lattice
oxygen sites to form a C2H5LOH, as shown in step 6, which is the
rate determining step (RDS). The C2H5LOH species then rapidly
decomposes in step 7 to produce ethylene. Similarly, it was
postulated that ethylene also forms through a hydroxyl species in
step 8 and 9. However, the formation of ethylene via steps 8 and 9
was later found insignificant or difficult to isolate through
modeling results and is excluded from further discussions.
Analysis of the ERR model showed that ethane reaction through
the ZOH route (Fig. 9) is not significant and no change in ethane
conversion was observed with variation in water in the feed. So,

F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
23
Table 4
Estimated parameters based on ERR Model.
Parameters
E or
D
H (kJ/mol)
A
95% confidence
for E or
DH
k1a
kb
108.2
91.20
1.46E+09
1.19E+09
3.68E+13
2.51E+11
8.84E+17
4.45E+13
1.39Eꢀ06
2.09Eꢀ05
2.49Eꢀ10
5.57Eꢀ12
4.62Eꢀ13
2.61E+09
2.82E+06
2.98Eꢀ04
ꢁ2.75
ꢁ6.92
ꢁ204
High
ꢁ122
ꢁ262
ꢁ66.3
ꢁ40.5
High
High
High
ꢁ46.4
High
High
k5a
k5b
k7
138.6
116.0
190.3
k9
141.9
K3
ꢀ56.80
ꢀ43.17
ꢀ96.56
ꢀ114.5
ꢀ124.0
99.37
K4
K12
K13
K14
A1
A2
70.42
K24
ꢀ35.44
data, the specific rate constants were expressed as Arrhenius type
of temperature-dependent functions. The optimization criteria are
that all the rate and equilibrium constants had to be positive, the
activation energies for reaction positive, and the heat of adsorption
Fig. 10. Modified reaction scheme based on non-steam differential reactor data.
(ꢀ
DH) positive, all consistent with physical principles. The
optimization criteria used was based on a minimum sum of
squares criteria defined by
reaction through route 1b has been excluded from the model
expressions for the ethane oxidation. The reactions ethylene to CO
and acetic acid to CO were also found to be insignificant at the low
conversions and at lower temperatures and hence reactions
through routes 6, 8 and 10a and 10b have been removed from
the network. The modified reaction scheme is shown in Fig. 10 and
the corresponding modified ERR Model rate equations are
presented in Table 3.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
N
uX
2
t
SSQ ¼
ðci;exp ꢀ ci;thÞ
i¼1
The model discrimination was carried out based on correlation
coefficients (R2), lowest sum of square (SSQ) residuals, smallest
individual parameter confidence intervals. The adequacy of each
model parameters was further established by analyzing their
physical significance and through statistical analysis such as
smallest confidence intervals and minimum values of the cross-
correlation coefficients. Estimated parameters with confidence
limits based on ERR Model for reaction scheme in Fig. 10 are given
in Table 4.
The Arrhenius/van’t Hoff plots for the model parameters are
shown in Fig. 11a and b, respectively, and it can be observed that
the figures follow the expected Arrhenius and van’t Hoff relation-
ships. The mean deviation between the experimental and
3.5. Estimation of model parameters
The equations of ERR (Eley-Rideal-Redox) model after exclud-
ing rate expressions for insignificant reactions are presented in
Table 3. The models were evaluated by using MATLAB (ODE 45 and
LSQCURVEFIT) least square fitting of the kinetic parameters. The
evaluation of reaction rate parameters was conducted using
experimental data at different reaction temperatures. In order to
take into account the temperature dependence of the experimental
Table 3
ERR Model rate equations for reaction network in Fig. 10.
At site Z: Ethane to ethylene
1=2
k k1aCC ðA1CO
Þ
H
b
2
6
2
n
h
i
o
r1a
¼
1=2
1=2
1=4
1=2
n
1k1aCC
1 þ ðA1CO
Þ
þ ðK3CH OÞ ðA1CO
Þ
ð1 þ K4CH OÞ þ kbðA1CO
Þ
H
2
6
2
2
2
2
2
where A1 =(K1K2)
At site X: Ethylene oxidation
1=2
k5aK13CC ðA2CO
Þ
H
2
4
2
h
i
i
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r5a
¼
1=2
1 þ ðA2CO
1 þ ðA2CO
Þ
Þ
1 þ K13CC
þ K24CCH
þ
K12CH OðA2CO Þꢀ1=4ð1 þ K14CH OÞ
H
COOH
2
2
4
3
2
2
2
1=2
1=2
k5bK13CC ðK12CH OÞ ðA2CO
Þ
H
2
4
2
2
h
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r5b
¼
1=2
1 þ K13CC
þ K24CCH
þ
K12CH OðA2CO Þꢀ1=4ð1 þ K14CH OÞ
H
COOH
2
2
4
3
2
2
2
r5 =r5a + r5b, where A2 = (K10K11
)
k7K13CC ðA2CO
Þ
H
2
4
2
r7
¼
¼
n
n
h
h
io
io
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
1=2
1 þ ðA2CO
1 þ ðA2CO
Þ
Þ
1 þ K13CC
þ K24CCH
þ
K12CH OðA2CO Þꢀ1=4ð1 þ K14CH OÞ
H
H
COOH
2
2
2
2
4
4
3
2
2
2
k9K24CCH COOHðA2CO
Þ
3
2
r9
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1=2
1 þ K13CC
þ K24CCH
þ
K12CH OðA2CO Þꢀ1=4ð1 þ K14CH OÞ
COOH
3
2
2
2

24
F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
Fig. 11. (a) Arrhenius plot—rate constants versus 1/T based on non-steam data. (b)
van’t Hoff plot—equilibrium constants versus 1/T based on non-steam data.
predicted reaction rates obtained is 17.2%. High deviations are
observed in prediction of CO2 data and this may be attributed to
error in the analysis of small quantities of CO2 in the product
stream. If the mean deviation is calculated excluding the error
involved due to CO2, the mean deviation reduces from 17.2% to
8.5%.
The parity plots for the rates of reactants and products are
illustrated in Fig. 12a and b, respectively. The predicted reactants
rates fit the experimental data in an excellent manner with even
distribution of error along the x-axis within a narrow range of ꢁ5%.
The fit is reasonable in case of products rates with distribution of error
along the x-axis within a range of ꢁ30%. Some deviations are
observed in predicted water and CO2 reaction rates.
The order of magnitude of the activation energies estimated
based on the ERR Model is similar to that of values reported in the
literature [1,4,9]. The frequency factors for all the rate constants
vary from 109 to 1017 typically in units of micromole/g cat/s. For
first order homogeneous reactions the value should typically are
around 1015 sꢀ1 or less [18]. Thus the order of magnitude of the
frequency factors appears reasonable.
Fig. 12. (a) Parity plot of reactants consumption rates using ERR Model. (b) Parity
plot of products formation rates using ERR Model.
rate of ethylene and acetic acid formation reactions is not
significant which shows that the surface of the catalyst at
investigated conditions is close to fully oxidized. Experimentally
observed significant positive effect of water to acetic acid
selectivity has been explained on the basis of surface OHꢀ groups
leading to the additional parallel route of CH3COOH formation.
Eley-Rideal-Redox kinetic model based on two-site reaction
mechanism has been evaluated for description of experimental
results on C2H6, O2 effect and chemical contribution of water to the
reaction pathways. The ERR model represents data adequately
with an overall average deviation of about 8.5%.
Chemical contribution of water to the reaction mechanism
through formation of surface OHꢀ groups show the existence of
acid base interaction of intermediate ethylene with surface. This
principal allows improving acetic acid formation selectivity
through modification of acidic properties of the catalyst. The
chemical contribution of water to kinetic behavior of the reaction
on MoV type catalyst through formation of surface hydroxyl
groups has been proved by isotopic experiments, which is the
subject of a later publication.
4. Conclusion
The kinetics and mechanism of C2H6 oxidation to C2H4 and
CH3COOH over MoV type catalyst has been studied. The primary
product of C2H6 oxidation is C2H4 which through consecutive
reactions transforms to acetic acid and to products of deep
oxidation such as COx. The reaction rate in both directions
proportionally increases with the increase of partial pressure of
C2H6, which shows that the reaction order for C2H6 in both
directions is close to 1. The effect of oxygen concentration to the
Acknowledgments
We would like to express our appreciations to SABIC for their
financial support under project # CRP2205. We also acknowledge
the support from Center of Research Excellence in Petroleum

F. Rahman et al. / Applied Catalysis A: General 375 (2010) 17–25
25
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