Functionalized ceria–niobium supported nickel catalysts for gas phase semi-hydrogenation of phenylacetylene to styrene

Abstract

The presence of phenylacetylene (PA) in the styrene (ST) stream poisons the catalyst during the polymerization reaction, increasing the operational cost. Preparation of low cost and active hydrogenation catalysts that are stable and highly selective to ST is expedient. To this end, different loadings of nickel and niobium metals supported on cerium(IV) oxide were prepared via the incipient wet impregnation approach. Potential reaction pathways were modelled using density functional theory (DFT) calculations. The catalysts were characterized to examine their physical and chemical properties. The results show a substantial influence of Nb on the NiCe catalysts. H₂-TPR reveals that Nb offers high interaction with the CeO₂ support that promoted O vacancies necessary to initiate facile switch in the Ce oxidation states from +4 to +3. XPS analysis confirms that the surface of the Nb doped sample encompasses abundantly adsorbed O initiated by the vacant sites on the surface. The Ni doped catalysts are active towards PA hydrogenation if the Ni content on the catalysts, determined by ICP-MS, is fixed at 5%. Ni loading above this value decreases the BET surface area and increases the particle crystallite size, leading to poor activity performance. Gas phase PA hydrogenation shows a steady increase in the conversion with the rise in temperature (150–300 °C). The observed products are limited to styrene and ethylbenzene, indicating the absence of oligomer species. The Nb catalysts display the highest conversion at the optimum value of 5% Nb content. The catalyst selectivity towards ST was highest on the Nb catalysts reaching 96% and 91% on the Ni–5% NbCe and Ni–10% NbCe catalysts, respectively. The high performance of Nb catalysts stems from the hydrogen spill over mechanism that affords the dissociated activated hydrogen on Ni active sites to spill to the neighboring Nb sites, thus, limiting the ensemble effect of Ni–Ni interaction. Analysis of the reaction pathway reveals that the transfer of hydrogen to PA is exothermic by 124.6 kJ mol⁻¹. Desorption of ST from the catalyst surface requires a mere energy value of 15 kJ mol⁻¹. The facile desorption of ST formed from the catalyst surface enhances the selectivity due to the electron transfer effect of Nb atoms to the CeO₂ support. Reported results compare favorably in terms of performance with commonly employed selective catalysts for PA hydrogenation.