Complete or partial selective hydrogenation of aromatic hydrocarbons is required in the production of multiple and diverse commodity and specialty chemicals. In the production of diesel fuels, hydrogenation of aromatic hydrocarbons is desirable because it increases the hydrogen to carbon ratio and, therefore, allows the fuel to burn cleaner and improves the cetane number of the fuel. In the production of jet fuels, hydrogenation of aromatic hydrocarbons is desirable because it improves the thermal stability and the smoke point of the fuel. Tetralin (C 10 H 12 , 1,2,3,4-tetrahydronaphthalene) is a useful probe molecule for studying hydrogenation of naphthalenes and other polyunsaturated hydrocarbons because hydrogenation of the last benzene ring is more difficult [1][2]. However, tetralin adsorption and reaction mechanisms at the molecular level on noble metals, which are common in hydrogenation catalyst formulations, are not well understood. This study addresses this issue by comparing adsorption and reactions of tetralin on noble metal catalysts with kinetic measurements, infrared (IR) and Raman spectroscopic measurements and density function theory (DFT) calculations.
Monometallic Pt, Pd, Ag, Au, Ru and Rh catalysts with 0.5-10 wt % metal loadings as well as bimetallics with a variable molar ratio were prepared using carbon and silica supports. In addition, the catalytic performance of bimetallics was compared to physical mixtures of monometallic catalysts with the same overall compositions. Tetralin hydrogenation to octalins (cycloolefins) and cis/trans decalins (cycloalkanes) was performed in liquid phase in a Parr batch reactor at 650 psi with an agitation speed of 700 rpm. The catalysts before and after the reaction were characterized with scanning electron microscopy (SEM), H 2 and tetralin temperature programmed desorption (TPD) measurements. IR spectra were collected using a Thermo Scientific Nicolet iN10 MX spectrometer. Raman spectra were collected with a 632.8nm wavelength laser using a custom-built setup. DFT calculations were performed with the DMol3 code in Materials Studio software by Dassault Systemes BIOVIA.
The microscopy results in Figure 1 show that the average particle sizes for 5 wt % Pd and 9.2 wt % Pt -catalysts with the same molar metal loading -were similar: 30-35 nm. The activity of the Pt catalyst, however, was higher. Furthermore, the activity of a 1:1 physical mixture of the monometallic catalysts was similar to that of the Pd catalyst. In contrast, the Pt-Pd bimetallic with the same equimolar composition exhibited a synergistic effect: the activity was significantly improved (Figure 1). In addition to activity differences, the Pt and Pd catalysts exhibited very different selectivities. The ratio of cis to trans decalins was consistently at ~3 for Pt catalysts with different metal loadings and, in contrast, much lower, at ~0.75 for all Pd formulations (Figure 2). The cis/trans decalin ratio is important because cis decalin in preferable in multiple chemical applications for selective ring opening and functionalization. DFT calculations and spectroscopic measurements demonstrate that although tetralin adsorbs similarly on Pt and Pd through its benzene ring on a bridge 30° site (Figure 3), the adsorption sites and stability of octalin intermediates are significantly different (Figure 4). For the octalins, bridge sites are preferable on Pt, while atop sites are preferable on Pd (Figure 4). Moreover, 9,10 octalin, which is the main intermediate in the formation of cis decalin, is significantly less stable on Pd (Figure 4), explaining the selectivity results in Figure 2. Based on additional experimental and computational results, the activity differences were assigned to differences in reaction intermediate structures and corresponding changes in reaction activation energies.
Reaction mechanisms for selective hydrogenation of aromatic hydrocarbons to cycloolefins and cycloalkanes over noble metal catalysts were studied at the molecular level. Stable adsorption structures and transient reaction intermediates were identified by combining experimental IR and Raman spectroscopic measurements with DFT calculations with vibrational analyses and related to activity and selectivity differences. Improved catalyst formulations were developed.