The effect of polyoxometallate complexes of nickel and copper on the rate of their heterophase reaction with ammonia

Abstract

It was established that the compounds formed by the cations in amine complexes of nickel(II) and copper (II) and the anions of polyoxometallates are capable of reacting stoichiometrically with gaseous ammonia when in the hydrated state. Some of the studied compounds are characterized by high sorption capacity (up to 40 molecules of adsorbate to a formula unit of the complex) and exhibit selectivity in the sorption of ammonia in the presence of water vapor.The anions of polyoxometallates (POM) are convenient building blocks for the creation of substances with promising oxidation-reduction, catalytic, magnetic, optical, and other functional characteristics [1][2][3][4]. In the light of their fixed dimensions, ranging from tenths of a nanometer to several nanometers, they can also be used for the design of materials with a porous structure, from which specific sorption characteristics can be expected. However, in spite of the obvious nature of this proposition, this aspect of the physical chemistry of polyoxometallate compounds has hardly been studied at all.Recent investigations have shown that materials constructed from the complex cations of chromium [5-9] or palladium(II) [10] and anions of the Keggin type [XW 12 O 40 ] n-(X = Si IV or Co II ) have interesting sorption behavior. This shows up primarily in their high and finely regulated (on account of the change in the size of the additional alkali-metal cation) selectivity toward polar molecules (water, acetone, alcohols, or nitriles). The materials do not interact with nonpolar adsorbates [5-10], including molecular nitrogen [7,8,10]. This feature makes it impossible to use traditional adsorption-chemical methods to analyze the porous structure of the compounds.We recently showed [11] that the crystal solvates formed by various molybdates (tungstates) and the amine complexes of copper(II) and nickel(II) undergo reversible and stoichiometric dehydration/rehydration. The observed fact that the kinetic curves for the hydration reactions are described by both one-and two-exponential relationships was explained by the presence of one or two types of energy barriers respectively for the diffusion of water molecules into the solid. The structural factors leading to the appearance of such barriers may be, for example, the pore sizes of the various bonding centers of the adsorbate. This suggestion agrees well with the fact that the two complexes having only one type of channel in the crystal are characterized by almost identical kinetic parameters for the water sorption processes [11].In order to obtain new information on the sorption characteristics of materials based on POM and to study their bonding to various molecules we studied the heterophase reaction of a series of such compounds with another polar adsorbate -gaseous ammonia. As subjects for the investigation we used the compounds, presented in Table 1, constructed on the basis of the cationic components [M(L n )] 2+ (M = Cu, Ni; L n = L 1 or L 2 ) and POM anions with various structures. 2040040-5760/07/4303-...

Structure of copper and nickel amine complexes with polyoxometallates and their adsorption properties relative to lower alcohols

“…The letter "a" denotes dehydrated samples of the corresponding compounds; a the amount of bound adsorbate molecules per formula unit of the starting crystal solvate at saturation, the amount of irreversibly adsorbed alcohol molecules is given in parentheses; b the amount of additional water molecules adsorbed by the crystal solvate at 100% relative humidity according to our previous data [12]; c samples "melt" under the experimental conditions; d the amount of EtOH molecules in addition to the crystal solvates.…”

“…As in previous cases, [M(L)] x [POM] y compounds where M is Cu 2+ or Ni 2+ , L is the tetradentate acyclic tetramine 1,9-diamino-3,7-diazononane (L 1 ) or macrocyclic ligand 2,4-diamino-1,3,5-triazin-6-yl-[3-(1,3,5,8,12-pentaazacyclotetradecane)] (L 2 ). The synthesis of the complex was carried out in water-alcohol media in accord with our previous procedures [11,12]. The elemental analysis data indicated that these compounds differ in the ratio of the cationic and anionic components (1 : 1 for 1, 2, and 11, 2 : 1 for 4, 6, and 7, 3 : 2 for 5, 8, 9, and 10) or contain two types of cations (3) ( Unfortunately, the low solubility of the compounds hinders the formation of monocrystals suitable for X-ray diffraction structural analysis.…”

“…However, this aspect of the chemistry of these compounds has not been studied extensively. Recent studies have shown that ionic compounds derived from chromium(III) or palladium(II) cation metal complexes and Keggin anions [XW 12 O 40 ] n-have a series of promising properties differentiating them from other adsorbents [5][6][7][8][9][10].In our previous work [11,12], we showed that the products of the reaction of copper(II) and nickel(II) amine complex perchlorate salts with various polyoxometallate anions (POM) in aqueous ethanol solution are crystalline materials, which, after drying in a desiccator over P 2 O 5 , contain stoichiometric amounts of solvent molecules of crystallization, mainly, water. The samples obtained may be dehydrated by heating.…”

“…This process is stoichiometric, i.e., in all cases, the mass increment curves reach saturation. We should note that the dehydration processes are completely reversible for all the compounds studied.A study of the reaction of the materials synthesized with gaseous ammonia showed that prior dehydration of the sample, while affecting the kinetic and thermodynamic characteristics of the binding process, is not necessary for their display of adsorption properties relative to a given adsorbate [12]. Furthermore, in many cases, the amount of bound ammonia molecules exceeds the amount of water of crystallization molecules removed during dehydration.…”

“…In our previous work [11,12], we showed that the products of the reaction of copper(II) and nickel(II) amine complex perchlorate salts with various polyoxometallate anions (POM) in aqueous ethanol solution are crystalline materials, which, after drying in a desiccator over P 2 O 5 , contain stoichiometric amounts of solvent molecules of crystallization, mainly, water. The samples obtained may be dehydrated by heating.…”