Enhanced selectivity towards xylene isomers of a mixed ligand Ni(II) thiocyanato complex

Abstract

The separation of a mixture of structural isomers into their individual components using traditional methods is unfeasible should their physical properties be similar. An important commodity of the petrochemical industry, para-xylene, is one of the most valuable products obtained from crude oil and used as raw material for the production of polyethylene terephthalate (PET), fibres, and polyester films [1]. In the case of the three xylene isomers, ortho- (ox), meta- (mx) and para- (px), and ethylbenzene (eb), with respective boiling points of 144.4 °C, 139.1 °C, 138.4 °C and 136.5 °C, distillation is an ineffective undertaking for differentiation, especially in terms of the latter three aromatics. Other creative, but less conventional approaches, such as selective sorption or inclusion within a host material [2] have been broadly studied and significant success has been achieved. Contemporary inclusion resources such as metal–organic hosts [3] ; [4] demonstrate the ability for selective inclusion and prediction of crystal structures and their properties. If one of the guest isomers is preferentially selected, the crystalline material can be filtered and retrieved from the solution and extracted, allowing the host substance to be recycled. Werner complexes, of general formula MX2L4, where M is a divalent metal cation (Ni2+, Co2+, Fe2+, Cu2+ or Mn2+), X is an anionic ligand (NCS−, NCO−, CN−, NO3− or halide) and L is a substituted pyridine or α-arylalkylamine, can accommodate changes in the anion (X−) or ligand (L). Alterations in the “tunability” of these Werner hosts arise on transformation of the nature and size of the inclusion cavities formed by the packing in square-planar or octahedral geometries [5]. These Werner compounds, although similar in structure, lack attractive (such as H-bonding) chemical functionality, and rely on different architectures of the host suitable for the set of guest isomers [2]. In a previous work, the lack of preference of a Werner compound for one of the aromatic guest isomers was shown to be due to the rigidity of the isoquinoline ligand [6] leading to uniformity in the architecture of the structures. The enclathration properties of the host Ni(NCS)2(4-vinylpyridine)4 gave equivalent products on altering the methods of crystal formation with polyaromatic hydrocarbons [7]. However, the sorption properties of the clathrate Ni(NCS)2(4-phenylpyridine)4 were investigated recently [8] giving discriminatory results with respect to the xylene isomers.