Catalyst Development for Selective Hydrogenation of Functionalized Alkynes and Nitroarenes

Abstract

Catalyst design is of major importance for fine chemicals industry due to complexity of the synthesis and high product quality requirements. Moreover, the catalyst formulation has to comply with the standards of health, safety and environmental regulations. The conventional systems often do not satisfy the latter criteria and, therefore, have to be improved to become both efficient and eco-friendly. The advancing technologies of materials synthesis and characterization allow catalyst development from model systems (i.e. colloidal metal nanoparticles (NPs)) towards real heterogeneous catalysts for industrial implementation. The main approach taken in this thesis is based on surface and electronic structure design of the catalytically active NPs (Pd, Pt) by means of size control (2.1 – 9.8 nm) and modification by a second component (Ag, Cu, Zn) during the catalyst preparation. We analyzed the size effect of unsupported Pd NPs in the selective hydrogenation of alkynols with different alkyl chains, i.e. C16 in dehydroisophytol (to isophytol) and C1 in methylbutynol (to methylbutenol). Antipathetic structure sensitivity was established where larger Pd NPs exhibit intrinsically higher (up to ca. 2-fold) specific activity (per Pd surface atom). A higher Pd dispersion delivered greater yields to isophytol (89 -> 93%), while the product distribution in methylbutynol hydrogenation was insensitive to NP size. The distinct size effect on hydrogenation performance for both α-alkynols is associated with modifications in the adsorption strength linked to the hydrocarbon chain length. Furthermore, the incorporation of Ag or Cu in Pd NPs had a critical impact on yield to the target isophytol (up to 97%). This result is attributed to the dilution of the Pd surface sites by a second metal (Ag, Cu) and a modification of the Pd electronic properties. Pd-Ag NPs, having shown the highest selectivity, were further deposited on a structured support based on sintered metal fibers (SMF) coated with ZnO. The improved selectivity achieved over the unsupported Pd-Ag colloidal particles was retained over the structured catalytic system. The resulted Pd-Ag/ZnO/SMF (patent WO 2013/060821 A1) represents a viable alternative to the conventional Lindlar catalyst. Another bimetallic system was studied in selective 3-nitrostyrene hydrogenation, where Pt catalyst was modified by alloying with Zn through the reactive metal– support interactions. Pt/ZnO catalyst containing monodispersed Pt NPs (ca. 2.3 nm) favored 3-vinylaniline formation (97%) showing a remarkable activity (close to that of the benchmark Pt catalysts), i.e. the catalyst had superior combined selectivity/activity response as compared to the reported literature. Additionally, the catalytic tests were performed over bimetallic Pt-Zn NPs embeded within the hypercross-linked polystyrene (HPS) support. The controlled pore size of HPS (ca. 4 nm) allows immobilization and a NP size control by confining Pt within the nanocavities of the polymeric matrix. The catalytic results demonstrated boosted yields towards 3-vinylaniline (16 -> 97%) over Pt-Zn/HPS catalyst as compared to monometallic Pt/HPS. The findings presented here over monodispersed NPs establish the basis of catalyst development for the selective production of fine chemicals. The product distribution can be controlled on a nano-level by tuning the properties of the active phase through the particle size optimization and incorporation of a second metal.