Effect of MgO-doping on solid–solid interactions in MoO3/Al2O3 system

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Abstract

The effect of MgO-doping (2–10 mol%) on solid–solid interactions and phase transformation process in MoO3/Al2O3 system were studied using thermogravimetry and X-ray diffraction (TG, DTG and XRD) techniques. The proportions of molybdena expressed as weight percent were 12.36, 22.01 and 41.37. The results obtained showed that the MgO-doping promoted the solid–solid interaction taking place at 500°C between Al2O3 and MoO3 to produce Al2(MoO4)3. However, MgO interacts readily with MoO3 at temperatures starting from 500°C yielding MgMoO4 which remains stable even when heating at 1000°C. The produced Al2(MoO4)3 decomposed at temperatures starting from 800°C producing α-Al2O3 and MoO3, a portion of which sublimed and the other portion dissolved in alumina matrix forming MoO3–Al2O3 solid solution. MgO-doping decreased slightly the solubility of MoO3 in Al2O3. The promotion of Al2(MoO4)3 formation at 500°C and the decrease in the solubility of MoO3 in Al2O3 by MgO have been attributed to dissolution of a small portion of MgO in the MoO3 lattice with subsequent increase in the mobility of Mo6+ ions. The promotion effect of magnesia towards Al2(MoO4)3 formation and dissolution of MoO3 in Al2O3 solid are relatively small when compared to the observed effects reported in the case of Li2O-doping of MoO3/Al2O3 system. The limited effect of MgO-doping has been attributed to a limited solubility of MgO in MoO3/Al2O3 system due to the formation of MgMoO4.

Introduction

Molybdenum oxides loaded on an active Al2O3 support are one of the most important solid catalysts 1, 2, 3, 4, 5. The supported catalysts are usually prepared by impregnation of an alumina support from an aqueous solution of ammonium molybdate followed by thermal treatment at suitable temperatures.

The heating of physical mixture of crystalline MoO3 and γ-Al2O3 at 400°C for about 24 h resulted in the disappearance of all X-ray diffraction lines of MoO3 due to the formation of two–dimensional Al2(MoO4)3 film covering the surface of Al2O3 particles 6, 7, 8. The increase in calcination temperature of MoO3/Al2O3 to ≥500°C enhances the surface and bulk mobilities of MoO3, leading to well-crystallized Al2(MoO4)3 phase 9, 10.

The metal–support interactions in the MoO3/Al2O3 system could be influenced by doping with certain foreign cations such as Zn2+, Ga3+, Ge4+, Li+ and Na+ 3, 4, 11, 12. This influence may result from modification in the mobility of Mo6+ ions in the MoO3 lattice. It has been reported that doping of MoO3/Al2O3 mixed solids either with Li2O [11] or Na2O [12] enhanced metal–support interactions producing Al2(MoO4)3 phase.

The present investigation reports a study of MgO-doping on solid–solid interactions in the MoO3–Al2O3 system using TG and X-ray diffraction techniques. These techniques permitted us to clarify the effect of MgO-doping in the thermal behaviour of MoO3/Al2O3 mixed solids subjected to thermal treatment at temperatures up to 1000°C and to identify the different crystalline phases produced by heating the mixed solids at various temperatures.

Section snippets

Materials

A known mass of Al(OH)3, analytical grade supplied by BDH, was impregnated with ammonium paramolybdate (BDH) solutions containing three different proportions of (NH4)6Mo7O24·4H2O. The proportions of ammonium molybdate were calculated so that the molar compositions of the calcined materials were 0.1MoO3:Al2O3, 0.2MoO3:Al2O3 and 0.5MoO3:Al2O3. The impregnated materials were dried at 120°C, then calcined at 500°C, 700°C, 900°C and 1000°C. The magnesium oxide doping was effected by treating the

Thermal behaviour of pure and doped materials

TG and DTG curves of various pure and doped materials are summarized in Table 1. Representative TG and DTG curves are given in Fig. 1 and Fig. 2 for AlMo-II and AlMo-III doped with 10 mol% MgO. The recorded TG curves of the tested materials consist of seven successive mass loss processes. The first step extended between room temperature and 130°C and corresponds to departure of the physiosorbed water, while the second one extended between 130°C and 186°C indicates the removal of water of

Conclusions

The main conclusions that can be derived from the obtained results are as follows:

  • 1.

    Doping of MoO3/Al2O3 mixed solids with MgO enhance solid–solid interaction at 500°C to form Al2(MoO4)3.

  • 2.

    MgO interact readily with MoO3 at temperatures starting from 500°C producing MgMoO4 which remain thermally stable even by heating at 1000°C.

  • 3.

    Aluminium molybdate decomposed at temperature ≥800°C giving α-Al2O3 and MoO3, a portion of which sublimed and the other portion dissolved in aluminium lattice forming MoO3–Al2

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