47 The Kinetics of Ammoxidation of Xylenes over Vanadium Catalysts MASATOMO ITO, KENICHI SANO, and MICHITOSHI KITABATAKE
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The Central Research Laboratory, Showa Denko Co., Ltd., Haramachi, Ohta-ku, Tokyo, Japan
The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface.
T^ecently, the kinetics of the ammoxidation of m-xylene ( I ) and xylene -•^ isomers (2) over vanadium catalyst, and of m-xylene over mixed vanadium catalysts (3) were reported. This paper summarizes the re sults concerning the specific rate constants for each reaction path obtained in the above studies and adds some data on physical properties of each catalyst. Kinetic Studies The data on the rate of reaction of o-, m-, and p-xylene over vanadium oxide catalyst and of m-xylene over mixed vanadium oxide catalysts (chromium-vanadium and antimony-vanadium) were correlated with the reaction scheme below by the following rate expressions, which are based on the Langmuir-Hinshelwood mechanisms where the adsorption of m-xylene is strong. 288
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
47.
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Ammoxidation of Xylenes
ITO E T A L .
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*5
1
rx = k r*i = fci — hiPu/Ρχ r — h + k P /Px TB = h + k PWPx x
D
2
M
4
The symbol r is the reaction rate of xylene, r , ri>, and r are the forma tion rates of m-tolunitrile, dicyanobenzene, and other products ( C 0 , H C N ) , respectively. P and P are the partial pressures of m-tolunitrile and xylene, respectively. The symbols k , k , k^ fc , and k are the specific rate constants of the reaction paths of xylene, m-tolunitrile, respectively. The values of the specific rate constants for each reaction path are listed in Table I. x
M
B
2
M
x
x
Table I.
m-Xylene m-Xylene o-Xylene p-Xylene p-Xylene m-Xylene m-Xylene
h
Specific Rate Constants for Reactions of Xylenes over Vanadium Catalysts Rate Constants
Temper Catalyst ature °C.
Xylene
4
2
V V V V V Cr-V Sb-V
420 400 400 400 380 400 400
k3/kz 4 6 4 6 8 2 1.2
0.55 0.59 0.56 0.79 0.80 0.61 0.72
0.20 0.16 0.12 0.08 0.06 0.11 0.08
Vk* 0.25 0.24 0.32 0.12 0.11 0.29 0.20
kj/kj,
kj/kjf
0.81 0.74 0.77 0.87 0.84 0.82 0.83
0.19 0.26 0.23 0.13 0.16 0.18 0.17
W h e n the vanadium oxide catalyst is used, &M is much smaller than k , while the specific rate constant of the reaction of m-tolunitrile i n the absence of xylene is larger than that of xylene. This seems to be caused by a strong adsorption of xylene on the catalyst surface. The great variations i n the reaction kinetics using a vanadium oxide catalyst for each xylene are shown by the relative ratios for the direct x
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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OXIDATION O F ORGANIC COMPOUNDS
formation of dicyanobenzenes from xylenes and for the formation of tolunitriles from xylenes—i.e., k Jk and k /k . The order fc tofc ,for the direct formation of dicyanobenzenes, is ortho > meta > para. Taking into account the steric effect, these results suggest that the adsorption of xylene on the catalyst surface occurs mainly by methyl groups. The ratio k Jk therefore indicates the degree to which the two methyl groups of xylene adsorb simultaneously on the catalyst surface. When a chromium-vanadium oxide and an antimony-vanadium oxide catalyst were used, the atomic ratio of chromium or antimony to vanadium was unity. The ratios of k to k for both mixed vanadium catalysts were much smaller than that for a vanadium catalyst, while the selectivities for the formation of isophthalonitrile from m-tolunitrile, k /ky for mixed vana dium catalysts were higher than that for a vanadium catalyst. :
:
x
x
3
x
x
x
x
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II
u
2
i9
In the absence of m-xylene, a higher selectivity for the formation of isophthalonitrile from m-tolunitrile was obtained for all catalysts. These results suggest that fc /&M decreases with a depression of the reaction rate of m-tolunitrile by m-xylene. The relative ratios for the formation of m-tolunitrile from m-xylene, k\/k was nearly the same for both a vanadium and a chromium-vana dium catalyst, but it was higher for an antimony-vanadium catalyst. T h e increase of k i / k , which indicates an increase of the degree of single methyl group adsorption for m-xylene, seems to be ascribable to the strength of adsorption and the surface structure of an antimony-vanadium catalyst. 2
x
x
"Physical Properties of Catalysts The fresh catalysts used for the ammoxidation of xylenes consisted essentially of ν Ο , S b V 0 , and z-phase for a vanadium catalyst respec tively. X-ray diffraction patterns for the catalysts showed that a vana dium catalyst consisted essentially of V 0 , while mixed catalysts re tained the original composition even after prolonged use. 2
δ
4
2
4
The infrared absorption spectrum of S b V 0 showed only weak absorption in the range 700-1200 cm." . S b V 0 also gave a single broad E S R absorption spectrum with a g value of 1.98 and was difficult to reduce with ammonia at 400°C. This suggests that the nature of vanadium ion in S b V 0 is similar to that in V 0 . The z-phase i n a chromium-vanadium system was quite different from C r V 0 with respect to the x-ray diffraction pattern, E S R absorption spectrum, and infrared absorption spectrum and could be reduced with ammonia at 4 0 0 ° C , thus differing from C r V 0 . Taking into account the infrared absorption spectra of a chromium-vanadium system, the 4
1
4
2
4
4
4
4
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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ιτο E T A L .
Ammoxidation of Xylenes
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z-phase seems to have a transitional structure with the polyanion of V ( V and is reduced partially under reaction condition. O n the other hand, a catalyst in which the C r V 0 was one of major constituents had little catalytic activity for the ammoxidation of xylene. These observations indicate that the nature and the distribution of metal ions and oxygen ion on the catalyst surface affect the catalytic activity and selectivity. It is difficult to predict the relationship between the adsorptivity of reactants and the physical properties of catalyst, but it may be assumed that adding more electronegative metal ions affects the electronic properties of the vanadium ion, which functions as an adsorp tion center. Further details on the physical properties of catalysts for the ammoxidation of xylenes w i l l be reported later.
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Acknowledgment The authors are grateful to Taijiro Oga, Hideo Ichinokawa, Hazime Tanaka, and Akira Matsumoto for their valuable discussions. Literature Cited (1) Ito, M., Sano, K., Bull Chem. Soc. Japan 40, 1307 (1967). (2) Ibid., p. 1315. (3) Ibid., p. 1321. RECEIVED October 16,
1967.
Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
