Multi-functional rare-earth hybrid layered networks: photoluminescence and catalysis studies

Abstract

Hydrothermal reactions between rare-earth (RE) chloride salts and N-(carboxymethyl)iminodi(methylphosphonic acid) (H₅cmp) led to the isolation of a series of layered networks formulated as [RE(H₂cmp)(H₂O)] [RE³⁺ = Y³⁺, La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺]. All compounds were isolated as micro-crystalline powders (many of which were nano-sized in thickness), with the plate-like crystallites found to exhibit preferential growth perpendicular to the [002] vector, a feature which seems to favour catalytic performance. Full structural elucidation was attained by the combination of synchrotron radiation (micro-crystal and powder) diffraction data, solid-state NMR studies (¹H, ¹³C and ³¹P) and photophysical measurements. Materials consist of _∞²[RE(H₂cmp)(H₂O)] layers in the ab plane of the unit cell, constructed from a single RE³⁺ centre (in a highly distorted dodecahedral coordination environment with one water molecule in the first coordination sphere) and one H₂cmp³⁻ ligand present in a zwitterionic form. Connections between layers along the c-axis are assured by strong and highly directional O–H⋯O hydrogen bonds involving the protonated phosphonate group (donor) of one layer and one oxygen atom (acceptor) of the carboxylate group in the adjacent layer. The network is an unprecedented 12-connected uninodal plane net with total Schäfli symbol 3³⁰.4³⁴.5². The Eu³⁺ material is photoluminescent at room temperature and 12 K with ⁵D₀ lifetimes of 0.86 ± 0.01 ms and 0.89 ± 0.01 ms, respectively. Studies of the mixed-lanthanide diluted [(Gd_0.95Eu_0.05)(H₂cmp)(H₂O)] material showed that Gd³⁺-to-Eu³⁺ energy transfer occurs within the layers. The coordinated water molecule plays a decisive role in the non-radiative relaxation process of the Eu³⁺ emission. All synthesised materials were further tested in the cyclodehydration of xylose to furfural, with the observed results comparing quite favourably with those from other solid acid catalysts used in the same reaction under similar conditions. A detailed catalytic study was performed for [Y(H₂cmp)(H₂O)]: selectivity increased to 84% as the conversion reached 83%; this solid was also re-used successfully in three consecutive 4 h runs after separation from the liquid phase by centrifugation and regeneration using either thermal treatment at 280 °C or repeated washing with solvents. All materials have been routinely characterized using vibrational spectroscopy (ATR-FT-IR and FT-Raman), thermogravimetric analyses, SEM investigations and CHN elemental composition.