An Insight into the Role of Reactant Structure Effect in Pd/C Catalysed Aldehyde Hydrogenation

Abstract

The different activity of a 1% Pd/carbon catalyst towards aromatic and aliphatic aldehydes hydrogenation has been explored by 13C NMR relaxation. The ratio between T1 relaxation times of adsorbed (ads) and free diffusing (bulk) molecules (T1ads/T1bulk) can be used as an indicator of the relative strength of interaction between the reactant and the catalytic surface, where the lower the T1ads/T1bulk, the higher the adsorption strength. It can be seen that 1% Pd/carbon showed a reverse catalytic behaviour towards benzaldehyde and octanal hydrogenation, which can be explained by analysing the T1 relaxation times related to each substrate in the presence of the catalyst. Comparing and correlating the different T1ads/T1bulk values, we were able to prove that the different catalytic results mainly depend on the contrasting adsorption behaviour of substrates on the catalyst. Moreover, the role of the solvent has been disclosed, as NMR results revealed that the adsorption of the reactants was strongly affected by the choice of solvent, which is revealed to be critical in modulating catalytic activity. As a consequence, T1ads/T1bulk measurements can provide a guide to the selection of appropriate reaction conditions for improving catalytic activity.

Pt‐WOx/C Catalysts for α, β‐Unsaturated Aldehydes Hydrogenation: An NMR Study of the Effect of the Reactant Adsorption on Activity and Selectivity

“…NMR relaxation can be used to probe the interaction between an organic substrate and the surface of a catalyst. [20] In particular, from 13 C NMR relaxation spectra T1 relaxation times can be derived for each carbon atom of the molecule. The T1 bulk is calculated from the spectrum acquired on the free molecule in a solvent (conventionally CDCl 3 ), while the T1 ads is calculated from the spectrum acquired in the presence of the catalyst.…”

“…On the contrary, values lower than 0.9 indicates that there is an interaction, which is stronger the more the value decreases. [20] As the catalytic activity is strongly affected by the adsorption/desorption equilibrium of reactants and products on the catalyst surface, [32] the determination of the interaction between the substrate with the catalytic surface in the real reaction medium is crucial. 13 C NMR could provide useful data that could be correlated with the catalytic activity.…”

“…The different activity and selectivity were conventionally correlated to the physical-chemical characteristic of the materials through XRD, HAADF-STEM, XEDS mapping and XPS characterization, but also by the whole system point of view using NMR relaxometry, recently reported as an innovative technique to study the effect of the reactant adsorption in the presence of a solvent on activity and selectivity. [20] The adsorption/desorption of the reactant (R) and the product (P), as well as diffusion to the active site, are all important factors which determine the activity of a catalyst. Too weak adsorption often results in low catalytic activity, whereas a too strong interaction could result in irreversible adsorption and consequent deactivation of the active site.…”

“…[21][22][23] Spin relaxation can be considered a sensitive probe for molecular dynamics as the relaxation times like T1 (longitudinal) and T2 (transverse) may be employed to detect molecular dynamics processes. [24] In particular, 13 C NMR relaxation was successfully applied to study the role of the reactant structure effect in aldehydes hydrogenation using carbon based catalysts, [20] where the large chemical shift range of 13 C allows for the discrimination of individual resonances and the measurement of the interaction of each individual carbon atom with the catalyst surface. In particular, the T1 ads /T1 bulk ratio is used as an adsorption strength signal, where the T1 ads is the relaxation time for the surface adsorbed (ads) molecule and the T1 bulk is the relaxation time for the free diffusing (bulk) molecule.…”

“…In this work Pt/C was modified by WOx using different approaches, resulting in different chemical and physical surface properties. The different activity and selectivity were conventionally correlated to the physical-chemical characteristic of the materials through XRD, HAADF-STEM, XEDS mapping and XPS characterization, but also by the whole system point of view using NMR relaxometry, recently reported as an innovative technique to study the effect of the reactant adsorption in the presence of a solvent on activity and selectivity [20].…”