Yu.A. Chesalov et al. / Catalysis Today 157 (2010) 39–43
43
VCrPO and V2O5-TiO2 catalysts consist in interpretation of gaseous
oxygen function. According to Takehira et al. [8], the effect of
the oxygen consist in filling the oxygen vacancies and further it
participation in surface complex formation (see Scheme 3 [8]).
The formation of oxygenated products in the absence of gaseous
oxygen [8] supports this idea. In the case of V2O5-TiO2 catalysts
gaseous oxygen reoxidates the reduced active cites with coordi-
nated surface complexes and thus promotes decomposition of the
intermediates with evolving target products.
4. Conclusion
The -picoline oxidation to nicotinic acid over V2O5-TiO2 fol-
lows by a parallel-consecutive reaction scheme directly from
-picoline and via intermediate product pyridine-3-carbaldehyde.
The formation of nicotinic acid from both -picoline and
pyridine-3-carbaldehyde proceeds via a consecutive transforma-
tion of the same surface complexes—aldehyde-like and carboxylate
stabilized at reduced vanadium. These complexes include the cat-
alyst oxygen.
Fig. 6. The dependence of the rate of -picoline consumption (1) and nico-
tinic acid formation (2) upon oxygen concentration. X = 60 1%; t = 270 ◦C;
-picoline:O2:H2O = 1:(9–23):20 mol%, balance N2. The points are experimental
data; lines are calculated according to kinetic equations.
Carboxylate complex is a direct precursor of nicotinic acid. It
turns into acid only in the presence of the gas-phase oxygen.
Two oxygen forms participate in nicotinic acid formation: cata-
lyst oxygen includes in formation of nicotinate and gaseous oxygen
includes in conjugated step of catalyst reoxidation–acid desorp-
tion. Effect of gas phase oxygen consists in decreasing the bond
strength of nicotinate with reduced active center during catalyst
reoxidation.
A complicated mechanism and the variety of oxygen functions
and of oxygen species require the maximum oxidized state of the
catalyst and explain the necessity of a high oxygen excess in the
reaction mixture.
Based on the proposed mechanism of the reaction kinetic model
graph theory. The description of kinetic model, procedure of kinetic
equations derivation in detail and calculated kinetic constants are
given in [11].
Fig. 6 shows the dependencies of rates of -picoline con-
sumption and nicotinic acid accumulation on the steady-state
concentration of oxygen. -Picoline conversion as well as concen-
trations of -picoline, water vapor and nicotinic acid were kept
constant. The oxygen concentration in the inlet reaction mixture
was changed from 9 to 23 mol%. The rate of -picoline consump-
tion and the rate of nicotinic acid formation reveal a tendency to
saturation as oxygen concentration increases. The curves are com-
ing to a plateau at above 16 mol%. The points are experimental
data; lines are calculated according to kinetic equations based on
forms in nicotinic acid formation. As one can see calculated curves
fit experimental points very well. A high concentration ratio of oxy-
gen to oxidizable reagent is typical in the selective oxidation of
heterocyclic compounds [12–14], including -picoline [15,16].
Effect of gas phase oxygen consists in decreasing the bond
strength of nicotinate with active center. The similar mechanism
of carboxyl acid formation over the same V2O5-TiO2 catalyst was
shown for reaction of formaldehyde oxidation [17]. During inter-
action of formaldehyde and lattice oxygen carboxylate (formate)
coordinated to reduced vanadium active center is formed. Formate
is the direct precursor of formic acid. Formic acid desorbs into the
gas phase in the presence of O2 only. According to DFT calculations
[18] the heat of desorption of the acid from reduced vanadium cite
is about 33 kcal mol−1. The heat of formic acid desorption from oxi-
dized cite is equal to 16 kcal mol−1. These values characterize the
bond strength of surface carboxylates. Strongly bound surface for-
mate is dead end complex and transforms into formic acid only in
the stage joined with catalyst reoxidation.
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