Hydrogenation of crude terephthalic acid by supported Pd and Pd-Sn catalysts on functionalized multiwall carbon nanotubes

Liquid phase hydro-purification of crude terephthalic acid (CTA) was performed using supported Pd and Pd-Sn catalysts on functionalized multi-wall carbon nanotubes (FMWCNT). Pd/FMWCNT catalysts were prepared by wet impregnation with Pd loadings of 0.05 to 0.6 wt.%. Pd-Sn/FMWCNT catalysts were prepared by co-impregnation (CI) and successive impregnation (SI) using 0.3 wt.% Pd loading and Sn/Pd molar ratios of 0.1 and 0.35 for the CI method and 0.05 to 0.35 for the SI method. Pd loading of 0.3 wt.% for Pd/FMWCNT was sufficient to decrease the 4-carboxybenzaldehyde (4-CBA) content of CTA from 2100 ppm to 5.5 ppm without excessive hydrogenation of terephthalic acid (TA). The commercial catalyst containing 0.5 wt.% Pd on activated carbon decreased the 4-CBA content to 23 ppm and showed excessive hydrogenation of TA leading to a lower selectivity. The Pd-Sn/FMWCNT had lower CTA hydrogenation activity compared with Pd/FMWCNT possibly due to alloy formation and palladium surface blocking due to Sn addition. Sn addition also led to a decrease in the decarbonylation rate of 4-CBA thus enhancing the product selectivity. Pd-Sn/FMWCNT prepared by the SI method and Sn/Pd ratio of 0.1 resulted in 99%+ removal of 4-CBA as well as enhanced selectivity compared with Pd/FMWCNT.