Robert Rioux, PhD, Pennsylvania State University
We are examining earth-abundant base metal catalysts as replacements for transition metal catalysts for the selective conversion of petroleum-derived hydrocarbons and oxygenates to higher-value products. To this end, we have focused on a particular class of base metal intermetallic catalysts containing Ni and d-block or p-block metals for the chemoselective hydrogenation of mixed hydrocarbon feeds (acetylene and ethylene) and a,b-unsaturated aldehydes/ketones to the corresponding unsaturated primary and secondary alcohols. We are particularly interested in the chemoselective conversion of crotonaldehyde to crotyl alcohol, as a model reaction because it represents a challenging yet industrially important selective hydrogenation process. The chemoselective hydrogenation of crotonaldehyde is conducted via stoichiometric reactions because the best heterogeneous catalysts consists of large Pt particles that achieve selectivity to crotyl alcohol of ~ 40% at 10-15% conversion (this amounts to a crotyl alcohol yield of 4-6%). The industrial catalyst of choice for the selective conversion of acetylene in excess ethylene is Pd-Ag. Pd is alloyed with Ag to break up Pd ensembles which promote unselective hydrogenation.
Although the two chemistries have notable differences, we have chosen to utilize Ni-Zn based intermetallics because we hypothesize through careful composition control, we can modulate the adsorption energy of acetylene/ethylene and the adsorption configuration of crotonaldehyde on the Ni-Zn intermetallics. We initially synthesized a series of size-controlled Ni nanoparticles utilizing a colloidal method in which nickel(II)acetylacetonate was decomposed and Ni(II) ions reduced in the presence of trioctylphosphine and oleyamine at 220°C. The size of the resulting particles could be controlled by the ratio of trioctylphosphine and oleyamine from 3 nm up to 15 nm. Particles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). In combination these techniques demonstrated that the Ni was in an fcc structure and composed entirely of Ni(0) with no indication of the formation of a NiO phase. After synthesis, the particles were dispersed on SBA-15 and examined for their activity and selectivity for the hydrogenation of acetylene. In the absence of excess ethylene, larger particles (>10 nm) demonstrated a higher selectivity to ethane (~5 mol %) compared with small Ni nanoparticles (3 nm). With the addition of ethylene to the feed, we found that the amount of ethane produced did not change, but we are currently performing isotope experiments with labeled 13C2H4 that will allow us to determine whether the ethane in the case of an acetylene feed in the presence of excess ethylene is due to the alkyne or alkene. Our calorimetry measurements and DFT calculations demonstrates acetylene binds to the surface Ni with a much higher heat of adsorption than ethylene, therefore, we believe that the surface coverage of ethylene is low under reaction conditions and the resulting ethane formation derives from acetylene. Future isotope exchange experiments will confirm the origin of ethylene.
Upon the conversion of Ni nanoparticles into what were believed to be NiZn nanoparticles via the injection of an organozinc complex (diethylzinc) into a dispersion of Ni nanoparticles (synthesized by the method described previously), the particles were characterized by the techniques already listed above, in addition to energy-dispersive x-ray analysis to determine the elemental composition. The elemental analysis demonstrated Zn was incorporated into the Ni nanoparticles, but XRD demonstrated there was significant ZnO found in the system. At this point in time, the origin of the ZnO is unknown, it may be due to the exposure of the bimetallic particles during work-up and XRD analysis, or it may formed during the synthesis due to the presence of oxygen in the acetylacetonate ligands. Upon supporting these particles on mesoporous SBA-15 silica, the Ni-Zn(O) nanoparticles demonstrated reduced activity but increased selectivity relative to the same size (10 nm) Ni-only nanoparticles. This is a promising result but the origin of the enhanced selectivity is unknown due to the presence a phase impure material. At this point, we do not know what amount of Zn has been incorporated into the Ni, but it is believed to be low (doping levels) because of the significant amount of ZnO formed during synthesis. Additionally, ZnO probably in the form of small islands on the surface of the Ni nanoparticles may be the reason for selectivity enhancement and this scenario would also account for the reduced acetylene conversion activity since Ni surface area would be lost due to the formation of ZnO islands. We are developing a new synthesis of Ni nanoparticles involving an oxygen-free Ni precursor and there eventually conversion to Ni-Zn nanoparticles via diethylzinc addition.
In an attempt to differentiate between the role of Zn and ZnO in the selective hydrogenation of acetylene in the presence of excess ethylene, we have synthesized a series of NiZn intermetallic compounds via a solid-state diffusion process, including Ni4Zn, a substitutional random alloy with an fcc structure, NiZn with a tetragonal unit cell and Ni5Zn21 with a cubic g-brass structure. After synthesis, the materials were ground to powders with surface areas of ~0.5-1 m2/g via ball milling. All samples were characterized by x-ray diffraction to confirm structure and synchrotron-based x-ray absorption spectroscopy to determine the oxidation state of Ni and Zn and the possible contribution of charge transfer between Ni and Zn in these bulk structures. The materials were determined to be fully metallic with no indication of bulk oxidation of any sort. Preliminary results on the series of bulk samples demonstrated the Ni4Zn and NiZn are both active for acetylene hydrogenation in the presence of excess ethylene, while Ni5Zn21 is inactive over the temperature range (140-180°C) examined. With respect to selectivity to ethane, it appears that Ni4Zn shows lower production of ethane compared with bulk Ni, while Ni-Zn produces more ethane than bulk Ni. The origin of this non-linear behavior is not well understood, but may be the results of the ordering of Zn within each of these lattices. In the second year of the PRF, we will focus on understanding the origin of selectivity in NiZn nanoparticles, and examine their catalytic activity/selectivity behavior in crotonaldehyde hydrogenation.