Research Project Thermal Decomposition Of Lanthanum, Neodymium And Praseodymium Nitrates

Introduction

In recent years there has been increased interest in using lanthanides and rare earths elements in biomaterials, mainly as biological tracers in spectroscopic probes to follow the path taken by drugs in biological systems, markers in immunology (fluoroimunoassays) and as contrast agents in non-invasive diagnosis of tissue pathologies by Nuclear Magnetic Resonance imaging. In view of such applications, it is necessary to know the thermal behavior of their precursors, especially in the synthetic aspect. On the other hand, monitoring the final thermal decomposition process, verifying possible intermediate products and understanding the process of forming compounds can provide information about the formation of the final oxide that occurs at certain temperatures. Recently the group led by the proponent of this project has carried out a series of investigations regarding hydrated nitrates of gallium, gadolinium, yttrium, aluminum, chromium, iron, dysprosium, samarium, europium, gadolinium, lutetium and scandium [1-9], thus obtaining information on the most probable and simple mechanisms of their pyrolysis.

In all cases, the mass losses in the intermediate stages were shown to be in perfect agreement with the proposed mechanisms and the structural arrangements of the intermediate products. In 2017, in studies conducted by the same research group it was found that decomposition mechanisms can occur differently for europium and lutetium nitrates. In the aforementioned lanthanide series there have been no indications of redox processes since these elements maintain mainly the oxidation state 3+. At the same time, the decomposition processes for chromium and iron nitrates were shown also to be very different.

For example, in the case of chromium Cr (III), there have been oxudation to Cr (IV) with the formation of the Cr4O8 dimer. Iron nitrate, under these conditions, forms oxyhydroxide Fe4O4(OH)4, without evidence of oxidation up to Fe (IV), although iron dioxide really exists. As a general rule, rare earth hydrated nitrates are coordinated with O and Oˊ-bidentate ligands, so that the series can be represented as Ln(NO3)3∙xH2O where x = 5-6. Figure 1 shows the structural arrangement of europium nitrate hexahydrate illustrating the particularities of the internal coordination shell. The scheme shows that in this molecule the 4NO3 groups are coordinated in a bidentate manner, the 4 water molecules are located in the internal coordination sphere and the two remaining H2O are quite far apart. Fig. 2 Coordination scheme of Lu(NO3)3*3H2O, from with modifications.

Hydrogen atoms are not shown

The two aforementioned variants allow us to conclude that the decomposition of europium hexahydrate must occur differently than its analogue with lutetium, which actually takes place. In any case, the decomposition of most hydrated nitrates is a complex process which begins with the condensation of the initial molecules forming clusters which may contain from 4 to 6 starting monomers. With increasing temperature, the clusters lose water, then the azeotrope HNO3 + H2O and are transformed into heterocycles containing tetrahedral or hexahedral units. At even higher temperatures, the tetramers and hexamers lose nitrogen dioxide, the rest of intercalated water, and are converted into Ln2O3 oxides. Its composition may vary from one lanthanide to another, but generally the following oxynitrates are detected: Ln4(NO3)12, Ln4O2(NO3)8, Ln4O4(NO3)4 and Ln4O5(NO3)2. The solid phases formed during heating are amorphous. Therefore, the only technique that could provide any idea of their inner arrangements is molecular modeling that was shown to be a powerful instrument to elucidate composition in order to understand mechanisms of nitrates thermolysis. Figures 3 and 4 present the structural arrangements that were elaborated from the computational simulation for the case of lutetium, containing six NO3- groups.

As can be seen, the base of the tetramer/hexamer is formed by an inorganic heterocycle composed of six atoms of lanthanide, alternating with four/six atoms of oxygen which, to simplify things, are numbered from 1 to 4/6. Together they form a rather symmetric crown-like cycle, wherein six lanthanides and six oxygens sit in slightly different parallel planes, one anterior and the other posterior. The calculation of minimal potential energies for the aforementioned oxynitrates models shows that, in an arbitrary scale, their numerical values increase considerably with the number of oxygen bridges. This is a clear indication that the stability of the related structures decreases from the original tetramer/hexamer to the final product devoid of nitrogen.

In the case of the "light" lanthanides, which are lanthanum, neodymium and praseodymium, with the exception of the unconvincing results from the older sources, their thermal properties are often unknown. Meanwhile praseodymium nitrate is used as a starting material for the synthesis of organic chelates. Their future usage may provide an opportunity to prepare magnetic images contrasts in medicine. One of them is a praseodymium chelate 2-methoxyethyl-Pr-MOE-DO3A, which proved useful to generate in vivo temperature maps with sufficient spatial and temporal resolution.

The structure of praseodymium hexahydrate trihydrate [tetraaquatris(nitrato - κ22O,O')praseodymium(III)dihydrate], is isotypic with other [Ln(NO3)3(H2O)4]∙2H2O analogues. The asymmetric unit consists of Pr(III) cation, three nitrate anions, and in total six water molecules. The Pr(III) cation is ten-coordinated by the oxygen atoms of three bidentate nitrate anions and four water molecules. Additionally, two lattice water molecules are included in the second coordination shell. This remote water can be expected to be the first to be eliminated during the heat treatment. Figure 1 Structure of Pr(NO3)3∙6H2O. From with modifications. The thermal decomposition of Pr(NO3)3∙6H2O has been the subject of recent research in the study of the genesis and characterization of praseodymium oxide obtained from this hydrate. Although the general conclusions of the study are unobjectionable, it does not explain the mechanism involved, that is, how stoichiometric Pr6O11 with six Pr atoms could have been formed from PrONO3 containing only one Pr atom, without going through the condensation process. So it is possible, in our opinion, that a simpler and more convincing mechanism of thermal decomposition than that proposed by the authors might be suggested and applied to lanthanum and neodymium.

As is known, the decomposition of rare earth nitrates Ln(NO3)3∙6H2O is not a simple process of water loss. The stability of octahedral complex cations [Ln(H2O)6]3+ is so high that part of NO3 groups present in these compounds are eliminated before a complete dehydration is achieved, or at least simultaneously with this process. In this context, the conclusions drawn from the thermal decomposition kinetics of the alleged “dehydrated” nitrates are questionable when the compounds are obtained using a method that, in principle, would not have allowed preparing anhydrous salts. The aim of the present study is to revert to the thermolysis of the hydrates La(NO3)3*xH2O, Nd(NO3)3*xH2O and Pr(NO3)3*6H2O and offer a different and more realistic scheme of events, by applying the hypothesis of tetramer/hexamer structures and building up structural models for amorphous intermediate oxynitrates.

Objectives

General objective

To study thermal decomposition of lanthanum, neodymium and praseodymium nitrates

Specific objectives:

1. To carry out thermoanalytical measurements on title compounds.
2. To identify volatile compounds.
3. To study the dynamics of H2O, HNO3 and nitrogen oxides elimination.
4. To build Gram-Shmidt curves.
5. To suggest decomposition mechanisms.
6. To contribute to articles, books
7. To enroll chemistry students for scientific work.

Materials and Methods

The initial reagents employed will be lanthanum, neodymium and praseodymium nitrate hydrates Ln(NO3)3∙xH2O, of analytical grade purity, purchased preferentially from Sigma–Aldrich. Direct heating will be conducted in order to establish “water number”. Thermal gravimetric analysis (TG) and differential scanning calorimetry (DSC) wille used to study thermal behavior, in both cases employing a Netsch STA Jupiter 449 C Instrumentation Test specimens of the starting material (5 mg) will be heated in a flux of argon (50 mL min-1, 99. 998 % purity; oxygen content <5 ppm) in a 30–500 °C temperature range, at a heating rate of 10 °C min-1. Platinum microreceptacle will be used for heat treatment. Mass losses during heating will be analyzed and compared with previously calculated values.

Visual observations will be carried out in air, using a platinum receptacle, from room temperature to 300 °C. The samples will be sealed in glass ampoules in a hot condition in order to avoid the impact of water vapors from the air. The evolution of volatiles will be measured using a Tensor 27 Bruker FTIR spectrometer attached to the aforementioned Netsch STA Jupiter 449C instrumentation. The spectra will be detected in a 700–4,000 cm-1 range. Temperature of the transport gas line should be 240 °C. The IR spectra will be taken for 12 s at a frequency accuracy of 1 cm-1. The identification of the spectra will be done on the basis of NIST Chemistry WebBook. The chemical structures of all compounds will be built using Hyperchemic software, version 7. 5 (Hypercub Inc. 2003). Molecular mechanics calculations will be carried out using MM+ force field. Structures will be evaluated by minimizing the energy with respect to all geometrical variables, no assumptions being made other than that of appropriate symmetry. Interatomic distances and angles will be collected by using special features of the program.