INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011
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Characteristics of Oxidation and Oxidative
Dehydrogenation Catalysts for Gas Phase
Reactions: A Review
C.R. Mistry, R.K. Mewada, V.K. Srivastava, R.V. Jasra
Abstract—Applications for catalytic selective oxidation and
oxidative dehydrogenation for production of various
petrochemicals are increasing. However due to series-parallel
side reactions in such processes emphasis mainly on higher
selectivity of the desired product. Selection and synthesis of
suitable catalyst with required composition to achieve the
desired selectivity is a very crucial task. For that proper
understanding of the catalyst behavior is essential. Each
catalytic material has its own characteristics. Behavior of each
molecule with that catalytic surface is also going to be
completely different. Thus it is highly complex phenomena.
Type of promoter, acidity and basicity of the catalysts plays an
important role in the reaction mechanism and selectivity of
desired product.
In this paper various processes based on either oxidation or
oxidative dehydrogenation were studied.
Compared to
dehydrogenation process, oxidative dehydrogenation process
offers advantage of complete conversion at low temperatures
and high pressures. Substantial improvement in yield of desired
product was achieved by optimizing the principal element for
the active phase like the presence of molybdenum oxide in FeMo catalyst for Oxidative Dehydrogenation of Methanol to
Formaldehyde and Methacrylate from Iso-butylene by using
Multi-Component Bismuth Molybdate catalysts.
Keywords--Oxidation Catalysts; Oxidative Dehydrogenation
Processes; Redox mechanism; Ethyl Benzene to Styrene;
Methanol to Formaldehyde; Methyl Methacrylate from IsoButylene
I.
S
INTRODUCTION
elective Oxidation of hydro-carbons has been considered
to be an important subject for the production of various
petro-chemical derivatives. About 50% of the principal
chemical products and over 80% of monomers are
synthesized by means of at least one stage of selective
heterogeneous catalytic oxidation [1]-[4]. The processes that
use solid (heterogeneous) catalysts are increasingly replacing
the homogeneous type, in order to reduce separation costs
and the environmental impact and/or to use new raw
materials.
Compared with the conventional steam-cracking method
of dehydrogenating alkanes to olefins and current catalytic
dehydrogenation processes, Oxidative Dehydrogenation
could reduce costs, lower greenhouse gas emissions, and save
energy. Capital and operational efficiencies are gained by
eliminating the need for a furnace and for decoking
shutdowns, lowering operating temperatures, lessening
material demands, conducting fewer maintenance operations,
and using a greater proportion of the alkanes in the olefin
conversion process. The processes for oxidative
dehydrogenation of alkanes are increasingly more
competitive than those for dehydrogenation of alkenes [5][6].
The action of solid catalysts in oxidation processes had
already been noted by the beginning of the Nineteenth
century. The first processes to be developed industrially
were: oxidation and ammonia oxidation (oxidation in the
presence of ammonia) of propylene to produce acrolein and
acrylonitrile respectively, oxidation of ethylene to ethylene
oxide and the oxidation of aromatics to form anhydrides
(maleic and phthalic anhydrides) [5]-[7] A major event in the
history of oxidation catalysis was the discovery of bismuth
molybdate (Bi+ MoO3) as a selective catalyst for the partial
oxidation of propene and also, in a one step operation, for the
ammoxidation of propene.
The current fourth generation catalysts, containing up to 25
elements, allow yields in excess of 80% to be obtained. The
development of new catalysts has brought about a
comparable evolution in the type of catalytic reactors used,
initially fixed bed, then „bubbling‟ fluid bed and finally
„braked‟ fluid bed. There has been an increasingly wider use
of alkanes as raw materials, instead of aromatics and alkenes;
for example, the synthesis of acrylonitrile from propane
instead of propylene and the synthesis of maleic anhydride
from n-butane instead of benzene, aimed at reducing costs
and/or improving the eco-sustainability of the process.
II.
DEVELOPMENT OF NEW CLASSES OF CATALYSTS AND PROCESSES
Selective catalytic oxidation processes can be divided into
three categories. The first relates to oxidation of inorganic
molecules (for example, oxidation of ammonia to NO and of
H2S by sulphur) [6][7]. The second class relates to synthesis
of basic chemical products (for example, ammonia oxidation
of methane by HCN or the partial oxidation of methane by
syngas; CO/H2 mixtures). Finally, the third category relates to
conversion of hydrocarbons by processing in the liquid phase
(principally in the homogeneous phase even if there is a
growing interest in the use of heterogeneous catalysts) and
processing in the gas phase, which is the most commonly
used industrially [7][8].
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TABLE 1.
PRINCIPAL PROCESSES OF SELECTIVE OXIDATION OF
HYDROCARBONS USING SOLID CATALYSTS AND TYPICAL
RESULTS OBTAINED (ARPENTINIER ET AL., 2001; CENTI ET AL.,
2002
Reagent
Principal
product
Types of
catalysts
Con.
(%)
Selectivit
y (%)
Methanol/air
Formaldehyd
e
Ag on aAl2O3, or FeMo oxides
97-99
91-98
Ethylene/O2/
acetic acid
Vinyl acetate
Pd-Cu-K on aAl2O3
8-12
92
n-butane/air
Maleic
anhydride
V-P oxides
75-80
67-72
Propylene/ai
r
/NH3
Acrylonitrile
97-99
75-83
n-butane/air
Butenes
/butadiene
55-65
93-95
> 97
85-90
97-99
95-98
97-99
81-87
Isobutene/
air
Methacrolei
n/air
o-xylene/air
Methacrolein
Methacrylic
acid
Phthalic
anhydride
Bi-Mo-Fe-CoK supported
oxides
Bi-Mo-P
oxides
Bi-Mo-Fe-CoK oxides
V-Mo-W
oxides
Oxides of VP-Cs-Sb on
TiO2
TABLE 2.
DIFFERENT CLASSES OF GAS PHASE SELECTIVE OXIDATION
PROCESSES (ON SOLID CATALYSTS) AND THE RELATIVE
INDUSTRIAL REACTIONS (ARPENTINIER ET AL., 2001; CENTI ET
AL., 2002)
Type of reaction
Examples
1) Propylene to Acrolein or Acrylic acid
2) Isobutene to Methacrolein or Methacrylic acid
Synthesis of the acids can be carried out in a single
Allylic oxidation
stage from the alkene,
but commercially it is preferred to use two stages for
the best possible
selectivities
1) Butenes to Butadiene and Isopentenes to
Isoprenes
Oxidative
2) Methanol to Formaldehyde
dehydrogenation
3) Isobutyric acid to Methacrylic acid
4) Ethylbenzene to Styrene
1) Epoxidation of ethylene to Ethylene oxide with
Electrophilic
O2
insertion of an
2) Direct synthesis of Phenol from Benzene with
oxygen atom
N2O Epoxidation of ethylene to Ethylene oxide
with O2
Synthesis of Vinyl Acetate from Ethylene and Acetic
Acetoxylation
acid
Synthesis of 1,2-dichloroethane from Ethylene and
Oxychlorination
HCl in the presence of O2
1) Propylene to Acrylonitrile
Ammonia
2) Isobutene to Methacrylonitrile
oxidation
3) α-Methylstyrene to Atroponitrile
Synthesis of
anhydrides
1)
2)
n-Butane to Maleic anhydride
o-Xylene to Phthalic anhydride
a)
Allylic oxidation: Mixed oxides of transition metals are
used as catalyst. These catalysts are capable of
selectively extracting hydrogen atom by breaking a C-H
bond in the allyl position and if necessary replacing it
with an oxygen atom [8]-[10]. These reactions are
characterized by a common first stage of allylic
oxidation where the extraction of a hydrogen atom in the
allyl position gives rise to a chemisorbed p-allylic
complex on the transition metal. The nature of the
subsequent stages determines the type of reaction and
product that is obtained. Oxidation and ammonia
oxidation of a side chain of alkyl aromatics in principle
follow a similar reaction mechanism, but the interaction
of the aromatic ring with the surface is different and
therefore different types of catalysts are used, such as
vanadium oxides supported on TiO2 or catalysts based on
molybdate of Fe-(V, P, K).
b) Addition of oxygen to the aromatic nucleus, with ring
opening: The electrophilic attack of oxygen on
hydrocarbon substrates typically leads to the formation
of carbon oxides, however in the case of the oxidation of
benzene; selective oxidation to maleic anhydride is
obtained [9]. This process, which employs catalysts
based on mixed vanadium and molybdenum oxides, has
been partially replaced by the synthesis by oxidation of
n-butane. Similar catalysts are used in the selective
oxidation of polyaromatic compounds.
c) Oxidation (or ammonia oxidation) of alkanes: In this
case the slow stage is the initial selective activation of
the alkane, for example for the concerted extraction of a
hydrogen atom by a surface Lewis site (a transition
metal) and of a second hydrogen atom by a base site
(oxygen atoms) to give an alkene, which is immediately
converted into an oxygenated product through oxidation
or allylic ammonia oxidation mechanisms [9][10].
Catalysts with properties which differ from those of
catalysts belonging to the Allylic oxidation reaction
category are necessary because of the weak interaction of
the substrate with the surface, and the activation
mechanism. For example, catalysts based on vanadyl
pyrophosphate are used for the oxidation of n-butane to
maleic anhydride, or those based on vanadium
antimonates are used for the ammonia oxidation of
propane. Antimony oxide is active in the ammonia
oxidation of propylene, but is not able to activate the
propane molecule; the addition of V gives the system the
capability of oxidizing the alkane.
d) Non-classic oxidation mechanisms: Ethyl benzene can
be oxidatively dehydrogenated, with high selectivity, to
styrene on various catalysts such as oxides and
phosphates, but the active phase is constituted by the
formation of a thin surface layer of carbon containing the
active sites of the reaction [10].
III.
MARS-VAN KREVELEN MECHANISM OF SELECTIVE OXIDATION OF
HYDROCARBONS ON OXIDE BASED CATALYSTS.
The catalysts used for these reactions can be classified on the
basis of their characteristic reaction mechanisms.
In this mechanism the catalyst undergoes an oxidationreduction cycle during the reaction [11]. In this mechanism a
hydro-carbon reactant molecule interacts strongly with the
3
INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011
catalyst at site containing M1n+ during the course of reaction,
lattice oxygen is consumed to form products, and the M 1
cation is reduced [13]. The active site and cation M1 are
reoxidized by the migration of lattice oxygen from M 2, which
in turn is reoxidized by the gas phase oxygen. The M 2 may or
may not be distinct from M1. According to Kung the
oxidation can be schematically represented as
would have existed in the absence of the support [3]. It is
clear, therefore, that the support must have surface area
characteristics suitable for the reaction of interest. In selective
oxidation, where the selectivity in the heavily formation of
the partially oxidized product is dependent on the subsequent
reactions to undesired products (for example, carbon oxides,
which are thermodynamically favoured), a support is needed
with a surface area which is not too large. In this way the rate
of the undesired secondary reactions, which are also
dependent on the time needed by the product to diffuse from
the active centre into the gas phase, is limited [8].
In some cases the support serves to alter the
characteristics of the intrinsic chemical reactivity of the
active phase, through the effects of the interaction between
the latter and the support itself. This comes about when the
support presents functional groups on its surface which can
lead to the formation of chemical bonds with the elements of
the active phase, or it takes place as a result of particular
crystallographic similarities between the surface and the
support.
V.
Fig.1.Mars-van Krevelen mechanism of selective oxidation of
hydrocarbons on oxide based catalysts
IV.
CHARACTERISTICS OF OXIDATION CATALYSTS
Oxidation catalysts belong to a wider class of materials
having redox or oxidoreductive type characteristics; systems
which catalyse reactions of hydrogenation, dehydrogenation,
halogenation and dehalogenation also belong to this class.
The most important catalysts in the field of petrochemicals
for the oxidation of hydrocarbons for processes carried out in
the gas phase. Presence of a transition metal as the principal
active component (V, Mo, Cu, Fe, Pd, Pt, Rh, Ag). Often in
these cases, a second element is also present which can be
transition or post transition (for example, P, Sb or Bi), which
contributes to establishing the reactive characteristics of the
catalyst [6][13][16]. This effect can be explained by the
formation of a „mixed oxide‟ (that is, of a specific compound,
such as for example Bi2Mo2O9, possibly only on the surface
of another oxide, of a solid solution or of an oxide doped with
the other element), with reactive characteristics different from
those of the single elements, if present in distinct phases. In
some cases the element is initially present in a metallic form,
but under reaction conditions it can generate the
corresponding oxide (or chlorides or oxychlorides). Presence
of small quantities of „promoter‟ (or „doping‟) elements can
optimize the performance of the principal active elements
[13]. The nature of the promoters can vary and they can
therefore play different roles in the transformation of the
reagents. The active elements and the promoter elements
constitute the active phase that is the phase directly involved
in the transformation of the reagents into products. Presence
of a support (usually silica, alumina or titanium oxide) in the
catalyst‟s formulation can fulfil a variety of tasks. A primary
task is that of dispersing the active elements, conferring a
larger surface area to the active phase compared with what
OPTIMIZATION OF THE REDOX CHARACTERISTICS OR OF THE
ACIDITY OR BASICITY PROPERTIES OF THE CATALYST.
The promoters (or doping agents) can play a fundamental
role in the control of these properties. For example, catalysts
used for oxidation or allylic ammonia oxidation always
contain Mo as the principal element for the active phase,
while catalysts for the synthesis of anhydrides or of acids
almost always contain V. Promoters with base-type
characteristics (alkaline or alkaline earth metal oxides) can
reduce the surface acidity of the active phase, with a
consequent improvement of selectivity through the
suppression of the acid-catalysed reactions (cracking,
formation of oligomers of unsaturated compounds)
[2][3][13]. Promoters with acid-type characteristics can
reduce the interaction between the active phase and
intermediates of the reaction which have acid-type
characteristics, thus favouring their desorption into gas phase
and limiting the contribution of the subsequent undesired
reactions.
VI.
REACTION SCHEME
The presence of consecutive reactions (typically,
combustion reactions of the desired product, or reactions
which lead from the reagent to the desired product through
the formation of intermediate products with an increasing
state of oxidation) involves the use of a catalyst with
characteristics such as to limit the contribution of these
reactions [16]. This can be achieved not only by control of
the intrinsic activity of the catalyst, but also by a modification
of the porosity of the active phase. High surface area and
porosity values entail effective intra-particle residential times
which are much higher than those calculable from the feeding
capacity of the reactor, and therefore a significant
contribution from the consecutive reactions for a given
conversion of the reagent. This can have a considerable
influence on the selectivity of the desired product.
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VII.
APPLICATIONS
1) Ethyl Benzene to Styrene:
Styrene can be produced by two processes:
a) Dehydrogenation of Ethyl benzene:
b)
∆H° = 84 kJ/mol
PRINCIPAL INDUSTRIAL PROCESSES AND RELEVANT
∆H° = 129.4 kJ/mol
Oxidative dehydrogenation of Ethyl benzene
∆H° = -126.04 kJ/mol
The former process (a), accounts for more than 90% of
the worldwide capacity. The catalytic dehydrogenation route,
in which the potassium promoted iron oxide catalyst is
typically used since 1957, produces most of the St. The
process can be run industrially either adiabatically or
isothermally over a fixed bed reactor in which the reactants
are passed over the catalyst bed employing radial or axial
flow [8][11]. The dehydrogenation reaction of Ethyl benzene
is endothermic (∆H = 129.4 kJ/mol) and equilibrium limited.
A large amount of super-heated steam is used as the additive
to provide heat needed for the reaction, to reduce the partial
pressure of ethyl benzene and hydrogen, and to keep the
catalyst clean and active.
Oxidative dehydrogenation of Ethyl benzene (b) is
exothermic reaction and there is no limitation by the
equilibrium. This can lower the reaction temperature and feed
of super-heated steam is, theoretically not required.
Operations nearly under isothermal conditions could aid the
yield and conversion [14]. The use of halogen, SO2 and S as
oxidants is also possible and good results have been reported.
However, high cost of hydrogen acceptors such as halogen
and SO2, the corrosion of the apparatus, and the incorporation
of hydrogen acceptors in to the products such as sulphides
and halides make it difficult to use them in the large scale
production.
2) Oxidative dehydrogenation of methanol to
formaldehyde:
Methanol can be converted into formaldehyde both by direct
oxidative dehydrogenation:
CH3OH + ½O2 → HCHO + H2O
∆H° = -155 kJ/mol
or by dehydrogenation combined with oxidation of the H 2
product:
CH3OH → HCHO + H2
H2+ ½O2 → H2O
∆H° = -238 kJ/mol
The two processes differ in their operating conditions and
type of catalysts.
Fig. 2. Schematic of oxide process (Centi et al., 2002)
In the first process low concentrations of methanol are
used in the feed, in order to avoid the formation of explosive
mixtures and to control the temperature of the reaction.
Commercial catalysts are based on iron molybdate, but also
contain an excess of molybdenum (Fe2 (MoO4)3+MoO3),
since the presence of molybdenum oxide is a necessary
condition for high selectivity. Typically a ratio of Mo/Fe
within the range of 1.5-3.0 is used; occasionally oxides of Co
and Cr are added as promoters [12][15][17].
Fig. 3.Reaction mechanism of the oxidation of methanol on
oxide based catalysts (Centi et al., 2002).
The excess of molybdenum is also necessary because the
sublimation of the oxide (particularly at the points of greatest
overheating) cause the progressive depletion of the Mo in the
catalyst and the condensation of MoO3 in the coldest parts of
the reactor. This induces, not just the deactivation of the
catalyst, but also a progressive increase in pressure loss.
Reaction temperatures are typically within the range of 310340°C, with conversions in excess of 98% and selectivity
equal to 92-95% at atmospheric pressure. Multi-tubular fixedbed reactors are generally used [13][15]. A recent
development has seen the introduction of a final (postreactor) adiabatic stage.
In dehydrogenation combined with partial combustion of H 2
(the overall process turns out to be partially exothermic)
under stoichiometric oxygen current is fed in, to operate in
the upper region at the limit of flammability [12]-[18].
INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011
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Due to the thermodynamic limits of dehydrogenation, it is Catalytic dehydrogenation has several draw backs. Thus,
necessary to operate at higher reaction temperatures than ODH is very attractive mainly due to the absence of
those for oxidative dehydrogenation. Selectivity to thermodynamic limitations (concerning the equilibrium) and
formaldehyde of 98-99% is obtained, with the formation of the need for heat transfer, allowing the operation at much
the following by-products: dimethylether ((CH 3)2O), whose lower reaction temperatures. An intense research effort has
formation is due to the presence of acidic sites in the catalyst; been directed to look for highly efficient catalysts. However,
methyl
formate
(HCOOCH3)
obtained
through the complexity of the reactions, associated with the large
disproportionation of the formaldehyde on basic sites; carbon number of factors that determine their behaviour, has
oxides, derived from both parallel and serial reactions. To seriously complicated the task. Moreover, and despite the
limit the formation of carbon oxides, rapid cooling of the large number of studies found in the open literature, some
reaction products is necessary when they leave the catalyst important aspects, such as the nature of the active sites, the
bed. High selectivity is achieved through the optimization of kinetics and mechanism of the reaction, the hydrocarbon
the acid-base properties of the catalyst, limitation of the activation process and the factors determine the selectivity,
oxidation of the formaldehyde to formic acid (a product are not sufficiently clear. It nevertheless seems evident that
which decomposes easily) and control of the redox properties the acid-base surface properties of the mixed oxides are an
of the catalyst.
important selectivity-determining factor.
This has led to the use of promoters, especially alkali or
3) Synthesis of methyl methacrylate:
Methyl ester of methacrylic acid, CH2=C (CH3) COOCH3, is alkaline-earth metals for the purpose of modifying the surface
used in the production of vinyl polymers. The acetone basicity in the strong favour of products. It is possible that in
cyanohydrin process (which consists of a reaction between the near future the combination of both factors, i.e., a highly
acetone and HCN, followed by a reaction of acetone efficient and economical reactor configuration with a catalyst
cyanohydrin with sulphuric acid and a final hydrolysis of the of excellent performance level, will provide the key for
adduct in the presence of methanol) is the most widely used definitive industrial implementation of oxidation processes as
for the synthesis of methyl methacrylate and has many an alternative to conventional direct dehydrogenation.
disadvantages, linked to the toxicity of HCN and the coformation of high quantities of ammonium sulphate, which is
IX.
ACKNOWLEDGMENTS
generated in the ratio of 2:1 with methyl methacrylate. The This work was supported by Reliance Industries Ltd,
alternative process is direct oxidation of isobutene in the gas Vadodara Manufacturing Division (as a part of M.Tech. final
phase (with subsequent or integrated stages of esterification); year Project Work).
however, this process gives yields and selectivity which are
too low to be competitive [8][12][13].
X.
REFERENCES
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