Hydrocarbon Oxidation

The conversion of hydrocarbons on oxides is being studied by adsorbing hydrocarbon moieties on model surfaces prepared in-situ under vacuum.  The selective oxidation of alkyl groups in particular is being explored by using alkyl iodides as suitable precursors.  For instance, the oxidation of 2-propyl moieties has been found to lead to some acetone production on Ni(100) surfaces partially covered with oxygen [N. R. Gleason and F. Zaera, J. Catal. 169 (1997) 365; F. Zaera, N. R. Gleason, B. Klingenberg, A. H. Ali, J. Mol. Catal. 146 (1999) 13].  ISS data strongly suggest preferential bonding of the hydrocarbon moiety to Ni sites [N. R. Gleason and F. Zaera, Surf. Sci. 385 (1997) 294], and RAIRS results point to a facile oxygen insertion into the metal-alkyl bond to generate 2-propoxide intermediates [F. Zaera, J. M. Guevremont and N. R. Gleason, J. Phys, Chem. B 105 (2001) 2257].

Evidence for the first of the two steps responsible for the oxidation of 2-propyl groups to acetone on an oxygen-treated Ni(100) single-crystal surface.  Reflection-absorption infrared spectroscopy (RAIRS) data is provided to highlight the early insertion of an oxygen atom into the metal-carbon bond to form an alkoxide intermediate; notice in particular the development of the C–O stretching vibration about 1100 cm-1.  This reaction appears to take place at temperatures as low as 200 K.  The data also indicate that the partial oxidation reaction can be enhanced by prior deposition of hydroxyls on the surface, as those groups lead to an increase in IR signal intensities (middle spectrum).  The top trace corresponds to the reactivity of 2-propanol on the same surface, and was used to help in the identification of the 2-propoxide intermediate.

The 2-propoxide moieties are stable on the surface up to ~325 K, at which point some undergo a rate-limiting beta-hydride elimination to yield acetone [N. Gleason, J. Guevremont and F. Zaera, J. Phys. Chem. B 107 (2003) 11133].  Other TPD experiments indicate that propene does not react directly to yield acetone on O/Ni(100) surfaces, a surprising result given that this is the pathway believed to be followed on many oxide catalysts.  The role of both subsurface oxygen and hydroxide groups has been probed as well [N. R. Gleason and F. Zaera, in 3rd World Congress on Oxidation Catalysis  (Elsevier Studies in Surface Science and Catalysis Series, Vol. 110), R. K. Grasselli, S. T. Oyama, A. M. Gaffney and J. E. Lyons, editors, Elsevier, Amsterdam, 1997, pp. 235-244; F. Zaera, Catal. Today 81 (2003), 149]. 

Evidence for the second of the two-step mechanism responsible for the oxidation of 2-propyl groups to acetone on an oxygen-treated Ni(100) single-crystal surface, namely, a beta-hydride elimination from 2-propoxide intermediates to yield acetone.  The production of acetone is indicated by its desorption above 300 K in these TPD traces.  Notice also the larger yield in the TPD results from the hydroxyl-covered surface shown in the middle trace.  The corresponding data for 2-propanol is again provided to illustrate the similar chemistry followed by the adsorbed alkyls and alcohols, an observation that suggests that both react via the formation of the same alkoxide intermediate.

Newer studies have extended some of the main conclusions reached with 2-propyl groups to other hydrocarbon moieties.  In particular, it has been clearly shown that both methyl [H. Guo and F. Zaera, J. Phys. Chem. B 108 (2004) 16226] and methylene [H. Guo and F. Zaera, Surf. Sci. 547 (2003) 299] groups can incorporate oxygen atoms on nickel surfaces to yield oxygenates.  In the case of methylene, the direct result is formaldehyde desorption.  With methyl moieties, on the other hand, surface methoxide groups form prior to their beta-hydride elimination to the aldehyde.  This work has also shown that both methyl and methylene groups not only couple directly on NiO surfaces, but also grow small amounts of C3 and C4 hydrocarbons, most likely via methylene insertion steps.  These results may have profound implications for methane oxidative coupling catalytic processes, as they suggest a possible Fisher-Tropsch mechanism to produce heavier hydrocarbons.

Schematic representation of the reaction mechanism for the thermal chemistry of diiodomethane coadsorbed with oxygen on Ni(110) surfaces.  Highlighted are the stepwise loss of iodine atoms in the adsorbed CH2I2, a chain growth mechanism based on methylene insertion into nickel-alkyl bonds followed by fast beta-hydride and reductive eliminations, the direct coupling of two surface methylene groups to yield ethylene, an oxygen insertion to produce formaldehyde, and a sequence of methylene insertions on CH2I(ads) intermediates to eventually produce C3H5· and C4H7· gas-phase radicals.

Typical kinetic data for the partial oxidation of alcohols by nickel catalysts. The data in this figure show the dependence of the rate of oxidation of 2-propanol on the partial pressure of oxygen over a Ni foil at 705 ± 2 K.  The main panel displays the raw data measured in a batch reactor, in the form of the accumulation of acetone  (in turnover numbers, TON = molecules/Ni surface atom·s) as a function of reaction time.  The inset provides a Ln - Ln plot of the initial rates of formation of acetone (in turnover frequencies, TOF = TON/time) versus oxygen pressure, as calculated from the raw kinetic data.  The order of the reaction with respect to oxygen was estimated to be 0.51 ± 0.05.  These experiments prove that the partial dehydrogenation of alcohols to aldehydes or ketones can be accomplished with good selectivity by using nickel foils and oxygen-rich mixtures.  Similar results were obtained on silica-supported nickel catalysts, but significant dehydration was seen on alumina-based solids because of the acidity of that oxide.

Additional kinetic and infrared characterization experiments have been carried out on supported nickel catalysts.  The kinetic behavior seen on nickel foils was by and large reproduced qualitatively on supported nickel catalysts.  Indeed, nickel particles deposited on neutral silica powders display similar high selectivities towards alcohol dehydrogenation to aldehydes or ketones.  When alumina is used instead, however, significant dehydration is seen as well.  For instance, when using a 2-propanol:oxygen 2:1 mixture, the 70% selectivity for 2-propanol dehydrogenation to acetone seen on a nickel foil goes down to only about 30% on Ni/alumina.  Additional transmission infrared (IR) spectroscopy characterization studies allow for the identification of most of the intermediates in these reactions.  The figure below shows a typical set of spectra for the thermal chemistry of 2-propanol on a 10% nickel/alumina catalyst in the presence of gas-phase oxygen.  In this particular instance, it is clear that the original alcohol decomposes to acetone between 440 and 450 K, but that some acetate surface species are also produced under certain conditions.  In fact, in the case of ethanol on Ni/SiO2, ethoxide, acetaldehyde, acetate, formate and CO2 were all sequentially identified with increasing temperature.  Finally, again, a thin and partially oxidized layer (as identified by CO infrared probing experiments) seems to optimize the partial oxidation.

Transmission infrared absorption spectra obtained during the thermal oxidation of 2-propanol on 10% Ni/Al2O3 after sequential exposures to 10 Torr of the alcohol and 10 Torr of oxygen.  A change in the nature of the adsorbed species is seen about 440 K, from molecular adsorption to metal-adsorbed acetone, with its characteristic IR bands around 1378, 1472, and 1590 cm-1 (the symmetric and asymmetric methyl deformations and the carbonyl stretching modes, respectively).  Experiments such as these allow for the identification of potential surface intermediates during catalysis.

The kinetics of the total oxidation of alkanes (methane, ethane, propane, n-butane and iso-butane) over Ni, Pd, and Pt foils was also studied under fuel-lean conditions by using the same recirculating-loop batch reactor with mass spectrometry detection [M. Aryafar and F. Zaera, Catal. Lett. 48 (1997) 173].