PREPARATION, CHARACTERISATION AND THERMAL STUDIES ON SOME OXOMOLYBDENUM(VI) OXALATE SYSTEMS

Abstract

The wide applicability of mixed oxides in different fields such as ceramics, optics, lasers, electronics, acoustics and in catalysis stimulated the interest of chemists to prepare them by the thermal decomposition of precursor compounds, a method which is not only economical but gives the product in a pure, state. A good deal of lit erature is available on the thermal decomposition of the salts of various carboxylic acids, but oxalates are perticularly useful as synthetic intermediates in the potential production of a number of simple and complex oxides. They offer many advantages in preparing homogeneous precursor compounds which have desired stoichiometry and their thermal decomposition yields finely divided highly reactive oxides. Although several mixed metal zirconates, titanates, cobaltates, chromates, aluminates, thallates, niobates, cuprates and vanadates have been prepared by the thermal decomposition of precursor oxalato complexes, but the preparation of mixed molybdenum(Vl) oxides by the thermal decomposition of oxalato molybdates(Vl) has received only a scant attention. Keeping in view the immense importance of molybdenum and its compounds in various fields such as lubricants, paints and corrosion inhibitors, flame retardants, catalysis and in agriculture, an attempt was made to synthesise some molybdates by the thermal decomposition of precursor oxalato complexes. The main problem in the conventional method of preparation of mixed molybdenum(Vl) oxides is the volatility of Mo0~ which puts a restriction on heating a mixture of the constituent oxides at high (ii) temperatures to make sure that the components react thoroughly. However, we prepared them successfully in a pure state and at a much lower temperature by the thermal decomposition of precursor oxalato compounds. The complexes were characterised by chemical analysis, thermogravimetry (TG), differential thermal analysis (DTA), derivative thermogravimetry (DTG), infrared spectroscopy (IR), magnetic measurements and X-ray diffraction studies. The decompos ition products were also characterised by chemical analysis, IR and X-ray diffraction studies. The intermediate compoy unds formed at various stages of decomposition were assigned the tentative compositions on the basis of weight loss measurements which were further verified by the IR spectral studies of the samples obtained isothermally. For the sake of comprehensiveness, the work embodied in this thesis is divided into five chapters described as follows : CHAPTER - I GENERAL INTRODUCTION A survey of the literature regarding solid state decomposition reactions is given. The techniques like TG, DTA and DTG are also discussed in brief. CHAPTER - II THERMAL DECOMPOSITION OF LITHIUM OXOMOLYBDENUM(VI) OXALATES Three complexes Li2[Mo03(C204)]. 1.5H20 (LMO-1), Li2[Mo206(C204)J. 2H20 (LMO-2) and Li2[Mo4012(C204)J. 3H20 (LMO-3) are prepared in aquous medium and characterised by chemical analysis and IR studies, their thermal decompos ition yielding Li^oO^ Li2Mo207 and Li2Mo4013 as the end products respectively. The decomposition is completed (iii) at or below 390°C. All the three complexes lose their water in a single step. The oxalate decomposition in LMO-1 proceeds in three stages via the formation of an oxalatecarbonate and a carbonate intermediate. LMO-2 also exhibits a three stage oxalate decomposition and two oxalate inter mediates are formed. This happens in LMO-3 in two stages via the formation of an oxalate intermediate. A tentative mechanism of the thermal decomposition is proposed. CHAPTER - III THERMAL DECOMPOSITION OF SODIUM, POTASSIUM AND AMMONIUM OXOMOLYBDENUM(Vl) OXALATES For the sake of convenience, this chapter is divided in two sections as follows: SECTION - A The preparation, characterisation and thermal decomposition of Na2[Mo03(C2O4)J. 3H20 (SMO-l), K2[Mo03(C204)(H20)J (PMO-1) and (NH4)2[Mo03(C204)J (AMO-1) to give NagMoO^ K2Mo04 and Mo03, respectively, as the end products forms the subject of the studies. In addition to the dehydration which takes place in two stages in SMO-l and in a single stage in PMO-l, the oxalate decomposition in both SMO-l and PMO-1 takes place in two stages via the formation of Na6[Mo3010(C204)(C03)] and K4Mo20Q. C02 as the intermediate compounds respectively. AMO-1 decomposes in a single step to give the end product. The decomposition is completed at or below 455°C. A tentative mechanism of the thermal decomposition is also proposed. (iv) SECTION - B It deals with the preparation, characterisation and and thermal decomposition of Na2[Mo206(C204)J. 4H20 (SMO-2), K2[Mo206(C204)J (PMO-2) and (NH4)2[Mo206(C204)J (AMO-2). No molybdenum(Vl) oxalato complex with a molybdenum:oxalate ratio of 2:1 appeares to have been reported so far. The complexes again decompose well below 450°C to give Na2Mo207, K2Mo207 and MoO~ as the end products respectively. This offers an economical and easy method for the prepar ation of dimolybdates. SMO-2 dehydrates in two stages and a single-stage oxalate decomposition is observed in all the cases. CHAPTER - IV THERMAL DECOMPOSITION OF RUBIDIUM AND CESIUM OXOMOLYBDENUM(VI) OXALATES This chapter deals with the thermal studies on Rb2[Mo205(C204)2(H20)2] (RMO), Cs2[Mo205(C204)2(H20)2] (CMO-1), Cs2[Mo2O6(C204)]. H20 (CMO-2) and Cs2[Mo03(C204)]. H20 (CMO-3). The thermogram reveales that the single-step, dehydration of both RMO and CMO-1 is immediately followed by the decomposition of the anhydrous compounds and proceeds in three similar stages via the formation of an oxalate and an oxalate-carbonate as the intermediate compounds to finally give Rb2Mo207 and CsJVkuO- at 340 C as the end products respectively. CMO-2 also loses its water in a single step and then decomposes in three stages to form the two oxalatecarbonates as the intermediates and thus finally yields Cs2Mo207 at 360°C as the end product. The single-step (v) dehydration of CMO-3 is followed by a single-step decompo sition of the anhydrous compound to give Cs2Mo04 as the end product at 390°C. The X-ray diffraction pattern of the sample obtained by heating CMO-3 isothermally at 400 C revealed the presence of a new Debye-Scherrer pattern. The mechanistic paths of the thermal decomposition of the complexes are given. CHAPTER - V THERMAL DECOMPOSITION OF BARIUM AND STRONTIUM OXOMOLYBDENUM(VI) OXALATES This chapter describes the preparation, characteris ation and thermal behaviour of Ba[Mo03(C204)] . 3H20 (BMO-l), Ba[Mo205(C204)2(H20)2]. 2H20 (BMO-2) and Sr[Mo205(C204)2(H20)2J. 2H20 (SMO). BMO-l loses its water in three stages and then decomposes in a single stage to give BaMo04 at 435°c as the end product. BMO-2 exhibits a two stage dehydration followed by the decomposition of oxalate which proceeds in three stages via the formation of Ba[Mo205(C204)(C03)j and BaMo207. 002 as the intermediates and a mixture of BaMo04, BaMo207 and some other unidentified phase is obtained at 360°C as the decomposition product. SMO also dehydrates in a similar faishon but the oxalate decomp osition takes place in two stages via the formation of an oxalate intermediate with carbon dioxide adsorbed on the solid. A mixture of SrMo04 and Mo03 is obtained at 360 C while SrMo04 is obtained at 830°C as the end product. The tentative mechanisms of the thermal decomposition of the complexes are given. CHAPTER - I GENERAL INTRODUCTION The complexity involved in the mechanism of reactions in the solid state has necessitated a systematic approach to the problem. The reactions in the gaseous state or fluid media differ from those in the solid state in the respect that in the first two cases the kinetic effects ensure the availability of the reactant molecules to one another and the reaction conditions may be defined by simple statist ical laws while the solid-state reactions generally occur between regular crystal lattices in which the position of atoms of one or both the reactants is either completely fixed or fixed within relatively small limits. In a solidstate reaction, the motion of the lattice unit is rather restricted and depends in a complex manner on the presence of lattice defects. The more perfect a crystal is, the smaller is its reactivity and the seats of reactivity are found at those places where there is a defect in the perfect order. Moreover the interaction can occur only at points of inti mate contact between the reactant phases. This reduces the scope of reactions in the solid state mostly to surface reactions since only the atomic groupings which are not entirely sourrounded by others are able to participate in the reaction. A solid-solid reaction is generally visualized as consisting of two reactant phases initially in close contact with only a single phase boundary separating them. Once the reaction starts, a layer of product is formed between the two reactant phases giving rise to two 2 . i different phase boundaries. The product layer progressively becomes thicker until the original reactant crystal lattices are completely consumed. The two important processes involved in this reaction are (i) diffusion of the reactant molecules through the product layer, and (ii) the phase boundary reaction. The relative rates determine the overall rate of the reaction. A solid-state reaction may be of two types. One in which no material crosses the boundary and only a change in the atomic arrangement takes place and the other which is accompanied by the loss of some material to give the product. The first type of solid-state reaction is, for example, a polymorphic transformation. Any change inside a solid can not involve the movement of any of its parts beyond the range of interatomic forces of its neighbours. Consequently, one of the main characteristics of solid-state phase transformations is the tendency of the solid to retain the original configur ation and changes which leave this only slightly altered, will occur much more readily than those which require serious alterations [1J. This is clearly seen in polymorphic transformations which are accompanied by slight changes in spatial arrangement of different atoms in the crystal lattice. A typical example of the second type of reactions is the thermal decomposition of solids. Garner [2] classi fied the decomposition of solids as endothermic and exothermic reactions and discussed the importance of the interface between the decomposed and the undecomposed portions of the solid. A broad defenition of the term ♦decomposition reactions of solids' would include all chemical transformations which involve a redistribution of the bonding forces of a crystalline reactant [3]. Such studies are intended to identify the sequence of steps by which a crystalline reactant is converted into products and also the parameters which control the reactivity of the participating species, including any intermediates identified. The bond redistribution process(es) by which the constituents of the reactant solid are transformed into product(s), starts occuring at certain discrete and energetically favoured positions in the reaction matrix [4], These positions are called nucleus forming sites and the process is known as nucleation. A number of sites having abnormal environment such as crystal imperfections or sites of highly localized decomposition may favour the further decomposition of the solid due to the relatively low energy of activation required at those sites. The low energy of activation may be the consequence of the enormous strain energy associated with such sites. These sites called germ nuclei, may either revert back to the original phase or may become stable and then grow to become growth nuclei as the decomposition proceeds. The extent of decomposition versus time curves frequently assume a familiar sigmoid shape which arises as a result of (i) the production of nuclei during an induction period, (ii) their growth during an acceleratory period, and (iii) the decay of the reaction as the nuclei overlap and the interface between the reactant and the product phases decreases. The concentration of germ nuclei and hence the rate of nucleus formation can be remarkably altered by changing the conditions of preparation of the material or by irradiating the sample with ultraviolet light or high energy radiations [4j. Conventional thermoanalytical techniques have played an important role in elucidating the bonding and structure of the complex compounds. Coupled with a variety of other analytical techniques, they can help to define the stoichi- ometry, kinetics and mechanisms of decompositions of these materials. Various thermoanalytical techniques, such as thermogravimetry (TG), differential thermal analysis (DTA), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), evolved gas analysis (EGA), thermomechanical analysis (TMA), electrothermal analysis (ETA) and thermomagnetometry have been used to investigate the reactions in solids. DTA, TG and DTG have been employed in the present work and hence it will be worthwhile to discuss some of the basic principles of these techniques. DIFFERENTIAL THERMAL ANALYSIS (DTA) Differential thermal analysis, abbreviated as DTA, is a technique to record the difference in temperature (AT) between a sample and an inert reference material (usually calcined a-alumina) as these two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate and the record of AT versus furnace temperature (T) is called a DTA curve. When no physical or chemical change occur in a region, a straight line is obtained which is known as base line. However, if the substance is thermally active, a series of peaks are obtained. A peak below and above the base line represents an endothermic and an exothermic reaction respectively. The position of a peak depends on the nature and the composition of the solid and the peak area is related to the energy involved in the change. Vaporization, fusion, dehydration, sublimation, boiling and decomposition reactions generally produce endothermic effects where as the crystallization and the oxidation reactions produce exothermic effects. A survey of literature revealed that the technique has been successfully used to study various types of reactions such as phase transitions [5-7], phase diagrams in condensed systems [8], thermal decomposition of solids [9-14], catalysis [15], polymers [16,17], solid fuels [18], textiles [19], glass making materials [20,21], explosives [22], in the study of reaction kinetics [23-25] and for a precise determination of melting and boiling points [26]. A number of books [27-30] and reviews [31,32] dealing with the different aspects of the technique are also available. THERMOGRAVIMETRY (TG) Thermogravimetry is another dynamic method which is used to study the change in mass of a substance as it is heated or cooled in a thermobalance at a constant rate. The resulting mass change versus temperature curve is known as thermogram or pyrolysis curve. It gives the information regarding the thermal stability and the composition of the starting material and the intermediate compounds as well as the composition of the end product. TG has proved to be a valuable technique to study the solid-state decomposition reactions. Perhaps the widest use of the technique has been to characterise the best weighing form and optimum drying temperature for precipitates in gravimetric analysis [33]. However, the appearance of a horizontal plateau does not essentially imply that the weighing form is isothermally heat stable over any or whole of the temperature range of the plateau and other tests should also be applied before drawing a final conclusion [34]. TG can also be used for the analysis of binary mixtures [35] and also to derive kinetic informations [36,37], Like DTA, TG is also a dynamic technique and a number of factors may affect the shape of the TG curve, although its quantitative character may not be affected [38]. Several books [28-30,33] and the review by Coats and Redfern [38] deal in depth with the use of this technique in the field of inorganic and organic chemistry. DERIVATIVE THERMOGRAVIMETRY (DTG) It refers to a technique in which the first derivative of the TG curve is plotted against time or temperature. The curve is similar to a DTA trace and is called a derivative thermogravimetric (DTG) curve. Most of the recent thermobalances have a DTG attachment and both the TG and DTG curves are simultaneously plotted. The DTG curve can also be obtained from TG data by plotting the rate of weight change with temperature against time or temperature. Changes following each other very closely on the TG curve can be easily distinguished by a sharp maxima on the DTG curve. The area under a DTG curve gives the change in weight precisely and so the DTG curves have also been used to deduce the kinetic information [39]. The object of any basic study of the decomposition of a solid is the formulation of a mechanism for the reaction, but only the thermoanalytical techniques can not be sufficient to deduce a detailed mechanism and they should be supported by chemical analysis data, infrared spectroscopy (IR) and X-ray diffraction studies. Infrared spectroscopy enables one to establish the presence of a structural or functional group of atoms contained in the molecule while X-ray diffraction studies are extensively used for structural studies and identification of different phases. A survey of literature revealed that a considerable amount of work has been done on solid state thermal decom positions of various salts such as perchlorates [40,41], chlorates [42], chromates [43], azides [44] and sulphides [45]. In addition to these, the decomposition of the salts of many organic acids such as formates [46,47], carbonates [48,49], oxalates [10-14], malonates [50] and other carboxylates [51,52] has also been studied. The thermal decomposition studies of mixed metal oxalates are of immense interest because of their importance in the production of various mixed oxides of industrial importance [10-14]. The preparation of mixed oxides by the conventional method of heating the constituent oxides at a high temperature presents certain problems in connection with the stoichiometry and homogeneity of the compound. High temperature 8 synthesis leads to the impure products in some instances and oxygen deficiency has also been observed in a number of such preparations [53]. Hence attempts were made to synthesise these materials at low temperatures from precursor compounds. Oxalates are perticularly useful as synthetic intermediates in the potential production of a number of mixed metal oxides [54]. They are advantageous in preparing homogeneous precursor compounds which have desired stoichiometry [53] and their thermal decomposition yields finely divided highly reactive oxides which are sometimes obtained at a much lower temperature than that required in the conventional method and thus offer an economical method of the preparation of mixed oxides. The volatility of molybdenum trioxide (MoO.J> which is widely used as one of the constituent oxides in the high temperature synthesis of mixed molybdenum oxides [55-60], prevents a mixture from being heated at temperatures high enough to make sure that the components react thoroughly. In addition to this, there is an increasing interest in the chemistry of oxo-species of molybdenum. All this and the wide applicability of molybdates inspired the author to prepare and characterise the oxalato molybdates(VI) of univalent and bivalent metals and to investigate their thermal behaviour systematically. The object of choosing the problem is two fold. One is the systematic investigation of the thermal decomposition of mixed metal oxalates and the other is to synthesise the important mixed oxides by a simpler method and with a high purity. In addition to the 9 present one (Chapter - I), the work embodied in this thesis is divided into four more chapters which are as follows: Chapter - II Thermal decomposition of lithium oxomolybdenum(Vl) oxalates. Chapter - III Thermal decomposition of sodium, potassium and ammonium oxomolybdenum(Vl) oxalates. Chapter - IV Thermal decomposition of rubidium and cesium oxomolybdenum(VI) oxalates. Chapter - V Thermal decomposition of barium and strontium oxomolybdenum(Vl) oxalates. The literature on the related work has been carefully surveyed and presented in the appropriate chapter. Due care has been taken to give proper credit to the work of earlier researchers in the field. The author would apologise if any contribution is omitted because of less importance, over sight, nonavailability of information or error in judgement.