Activity and sulfur resistance of co-impregnated bimetallic PdNi/γ-Al2O3 catalysts during hydrogenation of styrene

Abstract

Pyrolysis gasoline (PyGas) is an unstable byproduct of the pyrolysis of naphtha and other hydrocarbons for the production of olefins. PyGas is stabilized by hydrogenation at mild conditions. The hydrogenation of styrene to ethylbenzene is considered a model test reaction for studying the selectivity of the catalysts because it has the slowest rate of conversion. The catalytic activity and selectivity of two bimetallic co-impregnated Pd–Ni catalysts supported on γ-alumina were assessed and compared to those of a commercial monometallic Pd catalyst (AXENS LD265). The laboratory-prepared catalysts had different metal content and Pd:Ni atomic ratios (1:1 and 1:7). Catalytic tests of selective hydrogenation of styrene to ethylbenzene were made in a batch stirred tank reactor and in a continuous fixed bed trickle bed reactor. The sulfur resistance of the bimetallic catalysts in semicontinuous condition, was assessed by means of the same operational conditions using thiophene as a model poison compound. The support, Pd–Ni co-impregnated catalysts and LD265 commercial catalysts were further characterized by N2 chemisorption, EPMA, OM, ICP elemental analysis, temperature programmed reduction, XPS and X-Ray diffraction. The results indicated the presence of different metal species: Pd0, Pdδ+, Ni0 and NiO. The lab-prepared bimetallic catalysts were found to be active for the selective hydrogenation of styrene (both in the batch and continuous system). During the continuous evaluations, the commercial LD265 catalyst had an intermediate activity level that lied between the values corresponding to the Pd–Ni bimetallic catalysts: PdNi(1:1) ≫ LD265 > PdNi(1:7). During the poison free semicontinuous evaluations, the pattern of conversion as a function of contact time of co-impregnated Pd–Ni catalysts are quite similar to that of commercial LD265 catalyst, though the catalytic activity is slightly better for PdNi(1:1). After 90 min of contact time the order of conversion was: PdNi(1:1) > LD265 > PdNi(1:7). On the other hand, during the poison semicontinuous tests (with 300 pp of thiophene), the initial reaction rates of the co-impregnated catalysts decreased, pointing a poisoning of the active sites by thiophene. PdNi(1:1) had higher initial reaction rate than PdNi(1:7) during poison free or poisoned in both conditions. PdNi(1:7) was more sulfur resistant than PdNi(1:1). The higher activity of the PdNi(1:1) catalyst was attributed to the presence of Pd0 and Pdn+/Nim+ species that favored respectively, the homolytic cleavage of the H–H bond and the adsorption of styrene. The higher sulfur resistance of the PdNi(1:7) catalyst would be associated with the higher Cl/Al surface atomic ratio in this catalyst (electronic effect) and the presence of electrodeficient species of Pd (Pdxδ+OyClz) that prevented the adsorption of thiophene by steric hindrance (geometrical effect of the large species) or by electronic effects.