Dynamic atomic scale in situelectron microscopy in the development of an efficient heterogeneous catalytic process for pharmaceutical NSAIDS

Abstract

In heterogeneous catalysis the identification of the active site and crucially its location to prevent unwanted sintering and deactivation during the transformation of the precursor to active catalyst require the integration of dynamic in situ imaging at the atomic level and reactivity studies. We report nanostructural and physico-chemical studies towards an efficient low temperature heterogeneous catalytic process for nonsteroidal anti-inflammatory drugs (NSAIDS) such as N-acetyl-p-aminophenol (paracetamol or acetaminophen) on tungstated zirconia nanocatalysts of only a few nanometres in size. We directly visualised, in real-time, the dynamic precursor transformation to the active catalyst, which is of great significance in heterogeneous catalysis, using double aberration-corrected in situelectron microscopy at the atomic level under controlled conditions. We quantified the observations with catalytic activity studies for the NSAIDS. The observations using negative defocus imaging in AC-TEM combined with HAADF-STEM have provided the direct evidence for the presence of surface WO_x species with dimensions of ≤0.35 nm, nanoclusters and nanoparticles of WO_x from up to 0.6 to 1 nm, located at grain boundaries on the surface of the zirconia nanoparticles. The observations illustrate that the nanoparticles (NPs) are disordered (distorted) tungsten trioxide. The correlation between the nanostructure and activity of catalysts with different W loadings indicate that surface WO_x species, nanoclusters and distorted WO₃ nanoparticles create Brønsted acid sites highly active for the low temperature N-acetyl-p-aminophenol reaction, with distorted WO₃ NPs contributing to the activity. The results further elucidate that the anchoring of active sites at grain boundaries of the zirconia nanoparticles prevents undesirable coalescence of the active species and improves the catalyst stability and performance to make more product.