Crystal Growth Kinetics as a Tool for Controlling the Catalytic Performance of a FAU-Type Basic Catalyst

This study reports on the catalytic performance of nanosized zeolite X crystals and their precursors in the reaction of benzaldehyde with ethyl cyanoacetate. Crystal growth kinetics of FAU-type zeolite is studied at low temperature (35 °C) in order to discriminate different crystallization stages. First X-ray crystalline material is detected after 6 days of hydrothermal treatment. The formation of the crystalline phase is preceded by changes in the ring structure of an aluminosilicate precursor as revealed by the combined Raman–HEXRD–solid-state NMR analyses. The set of experimental data shows that these changes are related to the reorganization of the gel structure and the formation of zeolite units. Prior to the appearance of crystalline material, the apparently amorphous solid exhibits chemical composition and short-range order organization similar to that of a crystalline FAU-type zeolite. Knoevenagel condensation was used to test the catalytic activity of a series of zeolite intermediates and nanosized zeolite crystals. The amorphous precursor obtained after 5 days of hydrothermal treatment showed the highest yield of ethyl α-cyanocinnamate. Superior catalytic performance of this material was attributed to the combination of strong basic sites and less restricted and more accessible structure of the semicrystalline zeolite units. Thus, the crystal growth kinetics of FAU-type zeolite can be used as a tool to tune the properties of a catalyst used in Knoevenagel condensation

Isomorphous Substitution in a Flexible Metal–Organic Framework: Mixed-Metal, Mixed-Valent MIL-53 Type Materials

Mixed-metal iron–vanadium analogues of the 1,4-benzenedicarboxylate (BDC) metal–organic framework MIL-53 have been synthesized solvothermally in <i>N</i>,<i>N</i>′-dimethylformamide (DMF) from metal chlorides using initial Fe:V ratios of 2:1 and 1:1. At 200 °C and short reaction time (1 h), materials (Fe,V)^II/IIIBDC­(DMF_1–<i>x</i>F_<i>x</i>) crystallize directly, whereas the use of longer reaction times (3 days) at 170 °C yields phases of composition [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ (DMA = dimethylammonium). The identity of the materials is confirmed using high-resolution powder X-ray diffraction, with refined unit cell parameters compared to known pure iron analogues of the same phases. The oxidation states of iron and vanadium in all samples are verified using X-ray absorption near edge structure (XANES) spectroscopy at the metal K-edges. This shows that in the two sets of materials each of the vanadium and the iron centers are present in both +2 and +3 oxidation states. The local environment and oxidation state of iron is confirmed by ⁵⁷Fe Mössbauer spectrometry. Infrared and Raman spectroscopies as a function of temperature allowed the conditions for removal of extra-framework species to be identified, and the evolution of μ₂-hydroxyls to be monitored. Thus calcination of the mixed-valent, mixed-metal phases [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ yields single-phase MIL-53-type materials, (Fe,V)^III(BDC)­(OH,F). The iron-rich, mixed-metal MIL-53 shows structural flexibility that is distinct from either the pure Fe material or the pure V material, with a thermally induced pore opening upon heating that is reversible upon cooling. In contrast, the material with a Fe:V content of 1:1 shows an irreversible expansion upon heating, akin to the pure vanadium analogue, suggesting the presence of some domains of vanadium-rich regions that can be permanently oxidized to V­(IV)