Improving mass-transfer in controlled pore glasses as supports for the platinum-catalyzed aromatics hydrogenation

Abstract

The liquid-phase hydrogenation of toluene and other alkyl substituted benzene derivatives with different critical diameters was investigated over Pt-catalysts supported on spherical controlled pore glasses (CPGs) as model supports at 373 K in the batch mode. The effect of mass-transfer within the catalyst pores was studied by varying the pore width (4, 10, and 80 nm) and average grain size (18–150 μm) of the Pt/CPG catalysts. For toluene hydrogenation, internal mass-transfer limitations were absent (effectiveness factor >90%) only for catalysts with particle sizes below 25 μm and pore widths ≤10 nm or with a pore width of 80 nm and particle sizes around 75 μm, respectively. Effective diffusion coefficients obtained from initial reaction rates via the Thiele concept, e.g., 2.8 × 10⁻¹⁰ m² s⁻¹ for toluene over the catalyst with 10 nm pore width, were an order of magnitude lower than when determined by PFG-NMR. This difference was explained in terms of transport resistances such as surface barriers affecting the diffusivity assessment via the Thiele concept, while PFG-NMR measures intraparticle diffusion only.