Theoretical and Experimental Chemistry, Vol. 47, No. 1, March, 2011 (Russian Original Vol. 47, No. 1, January-February, 2011)
INFLUENCE OF MECHANICAL ACTIVATION OF MOLYBDENUM OXIDE ON ITS CATALYTIC ACTIVITY IN THE REACTION OF THE PARTIAL OXIDATION OF METHANOL
R. N. Rumyantsev, A. A. Il’in, A. P. Il’in, and I. V. Pazukhin
UDC 546.773
The influence of mechanical activation on the structure and catalytic activity of MoO3 was studied. It was established that as a result of mechanical treatment grinding, secondary aggregation of particles and accumulation of materials from the grinding machine occurred. An increase in the catalytic activity was observed in the oxidation of methanol to formaldehyde.
Key words: mechanical activation, molybdenum oxide, oxidation of methanol.
Molybdenum oxide, MoO3, is used as a catalyst in organic synthesis and is mainly a component for the development of catalytic systems for the partial oxidation of hydrocarbons [1, 2]. There have been a negligible number of publications on the problem of the mechanical activation of molybdenum oxide. For example, the reactivity of molybdenum trioxide after mechanical effects and its effect on the mechanism of sintering has been studied [3, 4]. In papers [5, 6] the structure, the granulometric and phase composition of MoO3 were investigated. The authors of paper [7] observed the influence of mechanical activation of molybdenum trioxide in various media with various different mechanical stresses on its activity in the oxidation of benzene to maleic anhydride. However, there are no publications on the dependence of the catalytic properties of molybdenum(IV) oxide on mechanical activation in the oxidation of methanol to formaldehyde. In this connection the present work investigated the influence of mechanical activation in a medium power density devices on the structure and catalytic properties of MoO3 on the oxidative dehydrogenation of methanol to formaldehyde.
EXPERIMENTAL In this work molybdenum oxide MoO3 “chemically pure” was used. Mechanical activation was carried out in air in a laboratory roller-circular vibration mill VM-4 (Czechoslovakia) with a rate of vibration of 930 min–1 and an achieved acceleration of 3g. X-ray structural analysis of samples was carried out with a DRON-3M diffractometer with CuKá radiation. The specific surface was measured by the BET method using low temperature adsorption of argon. Determination of the dimensions of the secondary particles was carried out with an Analysette-22 laser analyzer. The iron content in the activated samples was determined by the atomic absorption method using a Saturn apparatus. IR spectroscopic data in the 400-4000 cm–1 range were obtained with a Specord M-80 spectrometer. The catalytic activity of samples using the test reaction of oxidation of methanol to formaldehyde was measured in a flow reactor. An alcohol–air mixture was passed though a quartz reactor, then the ___________________________________________________________________________________________________ Ivanovo State University of Chemical Technology, Prospekt Fridrikha Engelsa, 7, Ivanovo 153000, Russian Federation. E-mail:
[email protected]. Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 47, No. 1, pp. 37-40, January-February, 2011. Original article received December 15, 2010; revision submitted January 26, 2011. 0040-5760/11/4701-0041 ©2011 Springer Science+Business Media, Inc.
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Fig. 1. Results of laser analysis of molybdenum oxide powder: 1) initial MoO3; 2) 15 min of mechanical activation; 3) 45 min of mechanical activation; 4) 90 min of mechanical activation.
reaction products were passed through a sparging column, the formaldehyde in the water produced was determined by a photometric methods. No other oxidation products were observed in the water produced by either chemical or chromatographic methods. The gases exiting from the wash column were analyzed for carbon dioxide and carbon monoxide by a chromatographic method. The methanol content in the starting mixture was 7.5%, the weight of catalyst was 0.1 g, the fractional composition was 0.5-0.25 mm, the volume gas flow was 10000 h–1, and the carrier gas was helium. The determination of the substructural parameters of the samples was carried using programs produced by the GAFRL method.
RESULTS AND DISCUSSION It was established by laser analysis that the MoO3 starting material consisted of large particles up to 120 ìm in size (Fig. 1) with about 1% of particles less than 1 ìm in size. Mechanical activation in the roller-circular vibration mill for 15 min decreased the particle sizes to up to 10 ìm, with 50% of the particles with sizes of less than 1mm. Increasing the activation time to 45 min or more led to aggregation of the particles. Apparently this is due to the fact that when dispersed using mechanical disruption minimal size of the particles is achieved sufficiently rapid that stops the process of grinding and the process of secondary aggregation prevails which is explained by the tendency of the system to decrease excess free energy. So the TABLE 1. Kinetics of Changes of the Physico-Chemical Properties of MoO3 during Mechanical Activation Time of mechanical activation, min Parameter measured 0
15
30
45
60
90
1.5
15
30
55
24
18
Femet content, %
–
0.1
0.19
0.23
0.31
0.38
Size of coherent scattering region Dcsr, nm
34
31
28
27
25
22
0.26
0.45
0.52
0.55
0.56
0.57
Surface area, m2/g
Amount of microdeformation, %
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Fig. 2. Dependence of the productivity with respect to formaldehyde on the time of mechanical activation: 1) initial sample; 2) 45 min of mechanical activation; 3) 60 min of mechanical activation; 4) 90 min of mechanical activation.
microstructure of the products of treatment of the samples in the milling apparatus results from two conflicting processes: decrease of size of the individual particles by decomposition and formation of rather large aggregates from these particles. It should be noted that the time of mechanical activation has an important influence on the quantitative characteristic of the surface area of the samples (Table 1). So the MoO3 starting material has a relatively low specific surface – about 1.5 m2/g. Mechanical activation for 45 min led to a sharp increase in the specific surface to 55 m2/g. Further increase in the treatment time led to a decrease in its value of 18 m2/g after 90 min of mechanical activation. This is explained by union of the highly dispersed particles into aggregates with so much packing that the interior interparticle surface pores become inadequate for adsorption of nitrogen and other gases used as adsorbates for measuring the specific surface. On mechanical activation of various materials the mechanical influence affects not only the refined substance, but also the milling bodies and working surface of the mill [8], so that maximal stress occurs at the working surface of the apparatus and the milling bodies in the zone of its contact with the milled material. It is evident that all of the latter, arise with intense mechanical effects, including high loading, pressure, and temperature, which are significant for the material of the mill. It has been established that during grinding part of the material, from which the mill is made, passes from its working surface to the activated substance itself. Thus the content of iron Femet in the activated molybdenum oxide MoO3 increased with increasing activation time and reached 0.38% (Table 1) after 90 min. Investigation of the sample by X-ray structural analysis showed that MoO3 starting material had the following characteristics: Dcsr = 34 nm, microdeformation value 0.26%. As a result of mechanical activation for 90 min the coherent scattering region was reduced to 22 nm, and value of microdeformation increased to 0.57% (Table 1). Analysis of the X-ray photograph showed that, as a result of the mechanical activation, a step-wise decrease in the relative integral intensity and a broadening of the characteristic reflections of molybdenum oxides occurred, brought about by grinding and amorphization. According to IR spectroscopic data, in samples of MoO3 subjected to mechanical activation, intense absorptions were observed in the region 1400-1600 cm–1, characteristic for carbonate structures of the monodentate type. These bands may be explained first of all by surface anion modification of activated oxide since the spectra were obtained in the diffuse reflection regime. The activated samples evidently contain hydroxide anions as is evident from the presence of characteristic absorption bands in the 3400-3500 cm–1 region in the spectra of the activated powders. So in the process of mechanical activation interaction of the surface of the milled material occurs with water vapor and CO2 in the air. We have studied the catalytic activity of molybdenum oxide in the oxidative dehydrogenation of methanol to formaldehyde over the temperature range 250-400 °C and have estimated the specific productivity of the microreactor (ìmol CH2O/(gcat·s)). After mechanical activation the reactivity of the molybdenum oxide had changed considerably. It was 43
established that the catalytic activity of the samples in the initial stage of activation (15-45 min) increased and then began to decrease (Fig. 2). The increase in activity is explained by the increase in the specific surface. However there is an increase in the content of metallic iron, which is partially oxidized to its oxide and reacts with the molybdenum oxide during the course of the reaction with the formation of a significant amount of amorphous Fe2(MoO4)3, which is an active component. The decrease in activity with increasing mechanical activation time beyond 45 min occurs as a result of decrease in the surface area from 55 to 18 m2/g. So it has been shown in this work that in the process of mechanical activation of molybdenum oxide there is a decrease in the size of the aggregates of particles and a decrease in the size of the microblocks, composed of crystals, and also an increase in the amount of microdeformation. The size of the surface area passes through a maximum, after which secondary aggregation of the particles takes place with increase in the time of mechanical activation beyond 45 min. It has been established that in the course of mechanical activation carbonization and hydration of the surface layer of the oxide occurs. A considerable increase in the catalytic activity of MoO3 in the oxidative reaction of methanol to formaldehyde was noted.
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