Hydrogenation of ethylene by zinc oxide. II. Mechanism and active sites

Abstract

The rate of hydrogenation of ethylene over zinc oxide at room temperature has been determined for hydrogen pressures ranging from 23 to 583 mm and ethylene pressures ranging from 5 to 330 mm with the ratio of hydrogen to ethylene ranging from 1 to 50. Initial rates were proportional to the hydrogen pressure to the one-half power and showed a slight dependence on ethylene pressure ascribable to the extent of the surface covered with ethylene. The kinetic isotope effect was also determined for a fixed set of conditions. These measurements, supplemented by infrared studies of the sample either with chemisorbed gases (CZH~, 02, CO) or under reaction conditions and an examination of poisoning effects of water vapor provide the basis for a model of the active site consisting of zinc ions imbedded in a close-packed oxide layer. This tentative model leads to a mechanism consistent with the kinetics and provides a plausible explanation for most of the experimental observations. It further suggests that there should be striking similarities between hydrogenation over oxides and hydrogenation over homogeneous catalysts. IntroductionNearly 30 years ago, Woodman and Taylor' discovered that zinc oxide is an effective catalyst for the hydrogenation of ethylene. Since this discovery, a number of reports'-' have appeared dealing with this aspect of the chemistry of zinc oxide, but the only thorough studies of kinetics have been carried out by Teichner's g r~u p~-~J at elevated temperatures, Le., roughly 100 to 400". Under these conditions, the reaction is quite complex: orders and activation energies change from one temperature range to a n~t h e r ,~ and formation of ethylene residuess with consequent poisoning' is evident. Since these studies span a range of temperatures in which the adsorption of hydrogen is a complex phenomenon,*-1° complexities of the related hydrogenation kinetics are to be expected. Nevertheless, the results are such that they frustrate attempts to put forth a coherent picture for the mechanism of ethylene hydrogenation.

Single-site zinc on silica catalysts for propylene hydrogenation and propane dehydrogenation: Synthesis and reactivity evaluation using an integrated atomic layer deposition-catalysis instrument

“…19,21 In this paper, a complete zinc oxide monolayer is defined as the material generated from the reaction of one equivalent of DEZ with one equivalent of surface hydroxyl groups (-OH The zinc oxide-type sites generated from the ALD synthesis were characterized via diffuse-reflectance infrared Fourier-Transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS), and were found to be isolated tetracoordinate Zn(II) sites on silica (b, which are inactive for hydrogenation under the given conditions. [30][31][32][33][34] This is consistent with TEM images of Zn B which show the presence of ~20 nm ZnO particles (a, Figure 3). This mode of ZnO speciation is consistent with reported island-like nucleation of ZnO on oxide supports.…”

Hydrodechlorination reactions of 2-chloro-1,1,1,2-tetrafluoroethane and chloroethane in H2 and D2 on Pd catalysts

“…This result is consistent with our previous finding showing that the rate-determining step for a series of CF 3 CF 3−x Cl x (x = 1-3) chlorinated compounds involves C-Cl bond cleavage [11,12]. There are examples in the literature showing that when H 2 adsorption or C-H (or C-D) bond scission or formation is the rate determining step, such as hydrocarbon exchange reaction with D 2 [44,45] and ethylene hydrogenation [46][47][48][49], the reaction rate with H 2 is at least two times faster than the reaction rate with D 2 .…”

“…Additionally, for step 9, the C-H (or C-D) bond formation step will be affected by a kinetic isotope effect, since the zero point energy of C-H bond is 1.1 kJ mol −1 higher than a C-D bond [53]. Absolute reaction rate theory [41] predicts that under the same reaction conditions, C-H bond formation rate constant (k 1,H ) will be higher than C-D bond formation rate constant (k 1,D ) [49,54,55]. As a result of the combination of these two factors, the term K from Eq.…”

Nature and Estimation of Functional Groups on Solid Surfaces

“…The (110) plane was the favored crystal face for the assignment of OH frequencies on Ti0 2 (rutile) [79,81,90]; Jones and Hockey [91], however, consider all three low index planes (110), (101) and (100). Dent and Kokes [92] suggested a model for the surface of ZnO which assumes the (0001) and (0001) planes of the wurtzite structure to form the external crystal surface layer. Each of these two faces provides one possible configuration for surface OH groups, a singly coordinated OH in the (0001) plane and a triply bridging group in the (0001) plane as shown in Figure 4.…”