Hexnuclear, octanuclear, dodecanuclear, and hexadecnuclear polyoxomolybdenum(v)-based porous materials for selective adsorption of gases

Abstract

Neutral hexnuclear and octanuclear polymolybdate-based porous complexes [Mo₆O₆(μ₂-O)₉(Hbtz)₆]·21H₂O (1, Hbtz = 1H-benzotriazole) and [Mo₈O₈(μ₂-O)₁₂(Hdatrz)₈]·28H₂O (2, Hdatrz = 1H-1,2,4-triazole-3,5-diamine), dodecanuclear and hexadecnuclear complexes Na₄[Mo₁₂O₁₂(μ₂-O)₄(μ₃-O)₁₂(3-trz)₈]·14H₂O (3, 3-trz = 1,2,4-triazol-3-amine) and Na₈[Mo₁₆O₁₆(μ₂-O)₁₆(μ₂-OH)₁₆(3-trz)₈]·29H₂O (4) have been successfully obtained and well characterized respectively. As the pH values of the solutions increase, the central metallic nuclei of 1–4 decrease gradually. Under basic conditions, isolated intrinsic microporous materials 1 and 2 can be constructed as six- and eight-membered rings with pore diameters of 3.2 and 4.8 Å, respectively, while dodecanuclear molybdenum(V) cluster 3 aggregates as an isopolyacidic anion. Interestingly, two {Mo₈} units in 4 are linked by strong intramolecular hydrogen bonds [2.692(5) and 2.691(5) Å] into a dimer between bridged hydroxy groups and bridged oxygen atoms. There are diversiform pores presented in 1–4, and the gas adsorption experiments show that 1–4 are obviously favorable for the adsorption of gases O₂ and CO₂ under different pressures, while little or no adsorption for gases N₂, CH₄ and H₂ respectively. The CO₂ adsorption capacities of 1–4 are attributed to the pore sizes and the functions of the amines and uncoordinated triazolate N-donor groups. The influences of neutral and anion complexes have been discussed on the skeletons and porous sizes.