12380 J. Phys. Chem. C 2009, 113, 12380–12386 Role of C-H Bond Strength in the Rate and Selectivity of Oxidative Dehydrogenation of Alkanes Michael Zboray, Alexis T. Bell,* and Enrique Iglesia* Chemical Sciences DiVision, E. O. Lawrence Berkeley National Laboratory and Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720-1462 ReceiVed: February 20, 2009; ReVised Manuscript ReceiVed: April 30, 2009 The oxidative dehydrogenation of alkanes (C2H6,C3H8, i-C4H10, and n-C4H10) was investigated on VOx supported on Al2O3. Rate constants for alkane dehydrogenation (k1), alkane combustion (k2), and alkene combustion (k3) were measured, and a model was developed to describe the effects of alkane composition on these rate constants. The proposed model accounts for the effects of the number of C-H bonds available for activation and the relative strengths of these bonds in both the reactant and the product molecules. The Brønsted-Evans-Polanyi (BEP) relationship is used to relate activation energies of secondary and tertiary C-H bonds to that of primary C-H bonds. The model gives a reasonable approximation of the relative order of alkane reactivity, expressed by k1 + k2, and the relative ranking of alkanes with respct to combustion versus oxidative dehydrogenation, expressed by k2/k1. The ratio of k2/k1 is described by the product of two components: one that depends on the nature, number, and relative strength of C-H bonds of surface alkoxides, and a second one that is independent of the alkoxide composition and structure but depends on the difference in the entropy of activation for COx precursor versus alkene formation. The model also explains the observed variation of k3 with alkene composition by considering two precursor states for alkenes. One is strongly bound through π-orbital interactions with Lewis acid centers, and the second weakly binds via H bonding and van der Waals interactions, similar to the binding of alkanes. As a result, the rate of alkene combustion depends strongly on the large heats of adsorption of alkenes and only slightly on the presence of weak allylic C-H bonds. The high rate of C2H4 combustion is thus a consequence of its high heat of adsorption. Introduction via similar interactions but also bind more strongly onto Lewis Oxidative dehydrogenation (ODH) provides an alternate route acid sites via their electron-rich π orbitals. The stronger binding for the conversion of alkanes to alkenes. It avoids the energy of alkenes can lead to reactivities much greater than those inefficiencies and ubiquitous deactivation of nonoxidative predicted from their weakest C-H bond, consistent with the ∼ 1 processes. Alkene yields are typically below 50%, even for modest yields ( 20%) often observed for C2H4 in spite of its - 1 - 1 C2H6, the alkane that leads to the most selective ODH reactions. strong C H bonds (464 kJ mol ; ref 11) relative to those in -1 - Yield limitations reflect the sequential nature of the pathways C2H6 reactants (421 kJ mol ; ref 11). Differences in C H bond involved and the higher reactivity of allylic C-H bonds, strength between the strongest and the weakest C-H bonds in - ubiquitous in alkenes, compared with C-H bonds in alkane alkanes are generally less than 20 kJ mol 1, and ubiquitous linear 12,13 reactants. Investigations carried out with VOx-based catalysts free energy relations suggest their differences in activation Downloaded by U OF CALIFORNIA BERKELEY on August 12, 2009 have shown that alkane ODH reaction rates are proportional to barriers are even smaller. As a result, stronger bonds increasingly Published on June 19, 2009 http://pubs.acs.org | doi: 10.1021/jp901595k 2 alkane pressure but independent of O2 pressure, consistent with contribute to measured rates as temperatures increase and must C-H bond activation as the sole kinetically relevant step. be considered in all relations between catalytic reactivity and Detailed kinetic and isotopic methods have confirmed these C-H bond energies. 3-8 conclusions for VOx and MoOx catalysts. The effects of C-H bond dissociation energy on activation Hodnett et al.9,10 proposed empirical relations between energies are typically described by Brønsted-Evans-Polanyi selectivity and the strength of C-H bonds in reactant and (BEP) relations;12,13 these relations express activation energies products for reactions involving the activation of these bonds. as a linear function of enthalpy changes for a given elementary These relations predict that achievable selectivities at a given step. This approach has proven useful, in spite of its empirical reactant conversion depend on the differences in dissociation basis, for homogeneous and catalytic reactions, such as in acid - energies between the weakest C H bonds in reactants and in catalysis,12 reactions of benzene derivatives,14 formate and 9 products for a broad range of catalytic oxidation reactions. This 15 methoxide decomposition on metals, CO, N2,O2, and NO approach does not include, however, any effects of the expected activation on metal surfaces,16 and C-H bond formation and differences in the adsorption of reactants and products or any dissociation on metals.17 For reactions limited by homolytic bond - contributions to reactivity from C H bonds other than the cleavage steps, ∆H will depend sensitively on C-H bond - R weakest one in each molecule. Low heats of adsorption ( ∆Hads) dissociation energies, thus allowing C-H bond activation for alkanes reflect predominant interactions via hydrogen- barriers to be related to the strength of individual C-H bonds. bonding or van der Waals forces. Alkenes interact with surfaces Here, we address apparent inconsistencies between measured - * To whom correspondence should be addressed. E-mail: bell@ activation energies and weakest C H bond energies and also cchem.berkeley.edu; [email protected]. the low C2H4 yields and preferential activation of the weakest 10.1021/jp901595k CCC: $40.75 2009 American Chemical Society Published on Web 06/19/2009 Oxidative Dehydrogenation of Alkanes J. Phys. Chem. C, Vol. 113, No. 28, 2009 12381 SCHEME 1: Pathways in the Oxidative catalyst mass ratios between 1 and 4. Space velocities were Dehydrogenation of Alkanes varied between 0.1 to 1.0 cm3 (g-cat s)-1 at constant inlet reactant pressures. Conversion data were corrected at all conversions greater than 5% using plug-flow formalisms and previously measured alkane ODH rate equations.3,6 Reactant and product concentrations were measured by gas chroma- tography (Agilent 6890) using a capillary column (HP-1, 50 m, 32 mm, 1.05 µm) connected to a flame ionization detector and a packed column (Hayesep-DB, 100/120, 30 ft × 1/8 in.) connected to a thermal conductivity detector. C-H bonds in hydrocarbons by measuring rate constants and Analysis of Reaction Data. Reaction rates and selectivities activation energies for C H ,CH , n-C H , and i-C H using 2 6 3 8 4 10 4 10 were measured as a function of reactor residence time, and these the reaction pathways depicted in Scheme 1. These alkanes data were used to estimate the rate constants shown in Scheme were selected because they and their alkene products differ 1. The rates of reactions 1-3 were assumed to be proportional significantly in the number and strength of their C-H bonds. to the pressure of the respective hydrocarbons and independent These data were obtained under strict kinetic control by the of O pressure.2,3,6 Values for k and k were obtained by deconvolution of primary and secondary pathways. Under 2 1 2 extrapolating measured rates to zero residence time. At low these conditions, C-H bond activation is the sole kinetically alkane conversions (k τ/3 < 1), alkene selectivities are given relevant step in the activation of alkanes and alkenes.3,6 We 3 by21 find that interpretation of the observed reactivity of alkanes and the distribution of products formed during ODH requires careful consideration of the nature, number, and relative k1 k3 - S ) (1 - τ) (1) strength of all C H bonds present in reactant alkanes and alkene k + k 2 product alkenes, as well as the modes by which alkenes can 1 2 adsorb on the catalyst. The accuracy of this approximate expression was checked by Experimental Methods also using the full solution to the corresponding differential Catalyst Synthesis. Vanadia domains supported on alumina equations describing mole balances in plug-flow reactors to were prepared via incipient-wetness impregnation of fumed estimate the rate constants in Scheme 1. This analysis gave the 2 -1 same values within experimental accuracy for all rate constants γ-Al2O3 (Degussa AG, 119 m g ) with a solution of vanadi- um(V) oxy-tri-isopropoxide (Sigma-Aldrich, 99%) in 2-propanol as those derived from extrapolation methods and eq 1. (Sigma-Aldrich, 99.9%). This sample contained ∼3% wt V O . 2 5 Results and Discussion Preparation details have been described previously18,19 and are included in the Supporting Information. Previous work9,10 has suggested that the rate of C-H Catalyst Characterization. Samples were characterized by activation for a given alkane is governed by the rate of N2 physisorption and Raman and UV-vis spectroscopies using activation of the weakest C-H bond. This implies that an methods reported in the Supporting Information. Surface areas Arrhenius plot of (k1 + k2)/nw, where nw is the number of (per mass of support) measured by N2 physisorption were weakest C-H bonds, would give a systematic trend between unchanged by impregnation and thermal treatment. Raman the apparent activation energy and the energy of the weakest spectra indicate that VOx species are predominantly present as C-H bond for various alkanes. Figure 1a shows an Arrhenius monovanadates, with traces of V-oxo oligomers but no plot of (k1 + k2)/nw for various alkanes; Table 1 reports detectable crystalline V2O5.