Kinetic features of the adsorption of water vapor by copper and nickel ammine complexes with polyoxometallate anions

Abstract

The rate of adsorption of water vapor by compounds obtained from cationic copper(II) and nickel(II) ammine complexes and polyoxometallate anions may be described by the linear driving force mass transfer model as one or two parallel processes. The differences observed were attributed to differences in the crystal structure of the adsorbents.Polyoxometallates (POM) are a large family of anionic metal-oxygen cluster species with highly variable composition, structure, and electronic structure. These features impart POM with many valuable properties and may permit their use in catalysis, materials science, medicine, etc. [1][2][3][4]. In light of their fixed nanodimensional size, POM anions are also suitable building blocks for obtaining structurized materials (organometallic framework materials, OMF), which may be expected to have specific adsorption properties. However, this aspect of the chemistry of these compounds has hardly been studied.Recent investigations have shown that ionic OMF with the general formula M n [Cr 3 O(OOCR) 6 (H 2 O) 3 ][XW 12 O 40 ]·mH 2 O (M = K + or Cs + , R = H or Et, X = Si IV or Co II ) [5-9] and the complex [Pd(en)(bpy)] 2 [SiW 12 O 40 ]·8DMSO·4DMF (en = ethylenediamine, bpy = 4,4¢-bipyridyl) [10] have a number of features distinguishing them from other adsorbents derived from organometallic compounds without POM.Firstly, the hydrophilic nature of the oxometallate building blocks leads to high selectivity of these adsorbents relative to polar molecules such as water, acetone, alcohols, and nitriles and their virtual inability to adsorb nonpolar adsorbates or molecules, which interact with the adsorbent by means of van der Waals forces [5-10]. Such molecules include nitrogen [7,8,10], which makes the use of the traditional adsorption chemistry methods for analyzing the porous structure of these compounds impossible. Secondly, as noted for compounds with M n [Cr 3 O(OOCR) 6 (H 2 O) 3 ] (n+1)+ cations, variation of the alkali metal and organic fragment permit fine control of the selectivity of the adsorption processes due to slight changes in the crystal lattice parameters. Thus, the complex Cs 5 [Cr 3 O(OOCH) 6 (H 2 O) 3 ][CoW 12 O 40 ] selectively adsorbs water in the presence of methanol and ethanol [6], while K 2 [Cr 3 O(OOCH) 6 (H 2 O) 3 ][CoW 12 O 40 ] also binds alcohols and nitriles with two or less carbon atoms in addition to water [7]. Similar behavior is found for K 2 [Cr 3 O(OOCC 2 H 5 ) 6 (H 2 O) 3 ][SiW 12 O 40 ] with the distinction that this compound also adsorbs alcohols with four or more carbon atoms [8]. The replacement of the potassium ions in this complex by cesium ions leads to the appearance of additional hydrophobic channels in its lattice, capable of binding dichloromethane molecules. This process occurs independently of the adsorption of water molecules in the hydrophilic channels [9].In light of the very limited number of compounds studied and their interesting properties, we investigated the adsorption characteristics of other compounds containing POM. For thi...

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

“…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].…”

“…Exposure of complexes 1 and 2 in a moist atmosphere also leads to an increase in their mass but is accompanied by "spreading" of the samples, indicating the condensation of water on the surface of the microcrystallites rather than its entry into the crystal lattice. Such a conclusion is consistent with the crystalline structure of the given isostructural compounds, in the structure of which there is no free space available for additional molecules of the adsorbate [11]. Moreover, the complexes proved inert to gaseous ammonia, which is clearly also explained by the above-mentioned features of their crystal structure.…”

“…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.…”

“…The synthesis of most of the compounds was described in [11]. The complexes (NH 4 ) 2 [Cu(L 1 )] 3 The amount of combined molecules of adsorbate per formula unit of initial crystal solvate during saturation.…”

“…The complexes (NH 4 ) 2 [Cu(L 1 )] 3 The amount of combined molecules of adsorbate per formula unit of initial crystal solvate during saturation. b Obtained in the present work by a method similar to that described in [11]. c For the partially dehydrated sample, containing 1. of the POM have been described before for other systems, and the structure of the final products in these cases were confirmed by X-ray crystallographic analysis [12,13].…”