Catalytic upgrading of phenolic compounds using ceria-based materials
Aaron D. Sadow
This dissertation describes the catalytic upgrading of phenolic compounds over ceria-based materials. The work was motivated by the need to find alternative industrial uses for the light rare earth elements (e.g. cerium) as part of the Critical Materials Institute. Cerium is typically encountered in the +3 or +4 oxidation state and its dioxide is the most technologically advanced cerium compound. Cerium dioxide (ceria) is best known for its ability to release and store oxygen, thereby acting as an oxygen buffer in many catalytic systems. The oxygen storage and release capability has been exploited in three-way catalysis and is one of the most industrialized uses of cerium. Traditionally, ceria has been explored for small molecule (e.g. CO) oxidation catalysis, while its utility as a catalyst for organic oxidations has been gaining attention. The unique oxygen-activating property of ceria has delayed its study as a catalyst component for reductive transformations.
Phenolics represent an interesting class of oxygenates for transformation over ceria-based materials. The chemistry of hydroxyl-containing compounds over ceria is well-studied and is often preceded by reactive adsorption to form activated intermediates. The economic importance of phenolics as precursors for nylon production and the oxygenate-activating ability of ceria-based materials provide a good starting point for determining alternative industrial uses for cerium. Furthermore, phenolics can be derived from renewable resources, an industrial sector that is expected to see large growth during the current century. Identifying active, selective, and environmentally friendly catalytic systems that convert biorenewable compounds into commodity chemicals is thus critical for ushering in the biorefinery era. The catalytic systems described here demonstrate ceria-based materials are active for the reductive transformation of phenolics using molecular hydrogen and aliphatic alcohols.
Palladium supported on high-surface-area ceria was found to effectively catalyze the hydrogenation of phenol to cyclohexanone at atmospheric pressure and room temperature under batch conditions. The activity was highly dependent on the catalyst prereduction temperature which led to optimization of Pd dispersion and metal-support interactions. Analogous to alkyl alcohols, phenol underwent dissociative adsorption on ceria to yield cerium-bound phenoxy and water. The phenoxy species were activated toward dearomatization by molecular hydrogen.
Palladium supported on high-surface-area ceria (Pd/CeO2) and sodium-modified ceria (Pd/Ce-Na) were used as catalysts for the aqueous-phase transfer hydrogenation of phenol using 2-propanol under flow conditions. Both catalysts were active and showed constant activity for 7 days on-stream. Pd/Ce-Na showed a marked increase (6x) in transfer hydrogenation activity over Pd/CeO2. Modification of ceria by sodium provided more 2-propanol adsorption sites and redox active sites (i.e. defects) for 2-propanol dehydrogenation which resulted in higher phenol transfer hydrogenation activity. For primary alcohols, reduction of the ceria support to from cerium hydroxy carbonate occurred and led to irreversible deactivation of the catalyst. A deactivation mechanism involving C-C scission of acyl and carboxylate intermediates to form CO was proposed.
Deposition of trimethylphosphate onto ceria followed by thermal treatment resulted in formation of surface phosphates with retention of redox activity. The introduction of phosphates generated BrÃ Â¸nsted acidic sites and decreased the number of Lewis acidic sites on the surface. Upon deposition of Pd, the multifunctional material showed enhanced activity for phenolic hydrogenolysis compared to Pd on the unmodified ceria support. This was attributed to the cooperativity between the Lewis acid sites, which activate phenolics for dearomatization, and the redox/acid property, which catalyzes hydrogenolysis.