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5DEPENDENCE OF THE ACTIVITY OF MOLYBDENUM-TUNGSTEN CATALYSTS ON THEIR
DEGREE OF CRYSTALLINITY
Aghayeva K.
PhD Student, Chemical technology faculty, Azerbaijan State Oil and Industry University, Baku, Azerbaijan
Abstract
In the work, the activity of 9 catalysts with an atomic ratio of elements from Mo-W = 1:9 to Mo-W = 9: 1 were studied in the reaction of oxidation of ethanol. The formation of two phases is observed in all samples, namely, the initial molybdenum and tungsten oxides It was found that in the Mo-W-O system, an increase in the degree of crystallinity of binary molybdenum-tungsten oxide catalysts accelerates the formation of ethylene and reduces the yields of acetic aldehyde.
Keywords: Ethanol, Ethylene, Acetic aldehyde, Tungsten oxide, Binary catalysts, Crystallinity.
Introduction.
Earlier provided researches shown that tungsten oxide catalysts are highly active in the reactions of eth-anol conversion [1]. It is known that catalysts based on molybdenum oxides are highly selective in the reactions of partial oxidation of organic compounds [2, 3]. Our researches shown that the activity of binary molybdenum-tungsten oxide catalysts strongly depend on their composition, which is apparently due to a change in the phase composition of the samples [4]. In this work we have studied influence the phase composition of binary molybdenum-tungsten oxide catalysts on its catalytic activity.
Experimental part.
Binary molybdenum-tungsten oxide catalysts of various compositions were prepared by coprecipitation from aqueous solutions of ammonium molybdate and ammonium tungstate. The obtained mixture was successively evaporated and dried at 100-120°C, decomposed at 250°C until nitrogen oxides were completely separated, and then calcined at a temperature of 600°C for 10 hours. Thus, 9 catalysts were synthesized with an atomic ratio of elements from Mo:W=1:9 to Cu:W=9:1. X-ray studies of binary molybdenum-tungsten oxide catalysts were carried out on a Bruker D2 Phaser automatic powder diffractometer (CuKa radiation, Ni filter, 3<29>80 °). We also degree of crystal-linity of studied samples calculated using the
DIFFRAC.EVA program. The activity of the synthesized catalysts was studied on a flow-through installation unit with a quartz reactor in the temperature range of 150-500°C. A mixture of ethanol with steam and air was passed through the catalyst loaded into the reactor with an ethanol: water: air ratio of 1:4:5. The space velocity of the feed mixture was 1200 h-1. Ethanol and its conversion products were determined by a chromato-graphic method.
Results and discussion.
Studies have shown that the activities of binary molybdenum-tungsten oxide catalysts strongly depend on their composition. Figure 1 shows the dependence of the activity of molybdenum-tungsten catalysts on their composition in the oxidation of ethanol at a temperature of 300°C. It can be seen that acetaldehyde is the main reaction product on all catalysts. The dependence of the acetaldehyde yield on the atomic ratio of molybdenum to tungsten has the form of a curve with two maxima for the samples Mo-W=3-7 and Mo-W=7-3. As can be seen from Figure 1, the yields of ethylene and acetic acid slightly depend on the atomic ratio of molybdenum to tungsten. Figure 1 also shows that the conversion of ethanol with a change in the atomic ratio of molybdenum to tungsten passes through a minimum on the sample Mo-W=5-5.
Fig. 1 Dependence of the activity of molybdenum-tungsten oxide catalysts in the ethanol oxidation reaction on
their composition. T = 300°C.
Another picture of the dependence of the yields of the reaction products on the atomic ratio of molybdenum to tungsten is observed at temperatures above 300°C (Fig. 2). It can be seen that with an increase in the content of molybdenum in the composition of the catalyst, the yield of ethylene decreases, while the yield of acetaldehyde increases. Thus, the ethylene yield decreases from 69% on the Mo-W=1-9 catalyst to 19.5% on the Mo-W=9-1 catalyst, and the acetaldehyde yield increases from 6.9% on the Mo-W=1-9 catalyst to 35.6% on
that the catalyst composition insignificantly affects the yields of acetic acid and carbon dioxide. It should also be noted that at a temperature of 450°C, ethanol with high selectivity is converted to ethylene on catalysts rich in tungsten.
Based on the obtained results, it can be said that on molybdenum-tungsten oxide catalysts rich in tungsten, the reaction of ethanol dehydration to ethylene proceeds, while the samples enriched in molybdenum are active in the oxidative dehydrogenation of ethanol
the Mo-W=9-1 catalyst. Figure 2 also shows to acetaldehyde. 100
90
80
70
60
50
40
30
20
10
0
0246 Mo/W ratio
D Q.
D
o
CH3CHO
C2H4
CO2
CH3COOH Conversion
10
Fig.2 Dependence of the activity of molybdenum-tungsten catalysts in the reaction of ethanol dehydrogenation
on their composition. T = 450°C
It is known that the phase composition of a catalyst and, consequently, its structural properties can strongly affect its activity [5, 6]. The X-ray phase studies of the samples synthesized by us showed that all samples consist of two phases (molybdenum and tungsten oxides) and in all the percentages of the components are preserved, which is evidenced by the regular change in the intensities of reflections in the diffraction patterns. Of course, one of the structural properties of a catalyst is crystallinity, which in turn depends on both the preparation conditions and the initial compounds used for preparation of the catalyst. Therefore, we studied the effect of the degree of crystallinity of the prepared catalysts on their activity in the conversion reaction of eth-anol. Figure 3 shows the dependence of the yield of ac-
etaldehyde, the selectivity of the process for acetalde-hyde and the conversion of ethanol on the degree of crystallinity of binary molybdenum-tungsten oxide catalysts. As can be seen from Fig. 3, with an increase in the crystallinity of molybdenum-tungsten oxide catalysts, the yield of acetaldehyde slightly decreases, and that of acetic acid and carbon dioxide is practically independent of the degree of crystallinity of the solid catalyst. It should also be noted that an increase in the degree of crystallinity leads to a sharp increase in the yield of ethylene and, as a result, the conversion of ethanol. This indicates that an increase in the crystallinity of molybdenum-tungsten oxide catalysts leads to an increase in the rate of the dehydration reaction and practically does not affect the reactions of partial and deep oxidation of ethanol.
100 90 80 70
v? 60
"5 50
a
S3
О 40
30 20 10 0
82
83
84
Crystallinity, %
85
86
Fig.3 Dependence of the activity of molybdenum-tungsten oxide catalysts in the ethanol oxidation reaction on
the degree of crystallinity.
Conclusion
Based on the conducted studies, it can be said that an increase in the degree of crystallinity of binary molybdenum-tungsten oxide catalysts accelerates the formation of ethylene and reduces the yields of acetic aldehyde.
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