Identifying the Unique Properties of α-Bi₂Mo₃O₁₂ for the Activation of Propene

Abstract

In order to understand the remarkable activity of α-Bi2Mo3O12 for selective oxidation and ammoxidation of propene, the propene activation ability of four molybdenum-based mixed metal oxides-Bi2Mo3O12, PbMoO4, Bi2Pb5Mo8O32, and MoO3-was investigated using Density Functional Theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C-H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5-or 6-coordinate geometry at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that in 5-and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ~ 15 kcal/mol more favorable than on 4-coordinate molybdate oxo atoms. Bi 3+ cations in Bi2Mo3O12 thus promote catalytic activity by providing both strong adsorption sites for propene, and by forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE.

Quasi-Catalytic Identification of Intermediates in the Oxidation of Propene to Acrolein over a Multicomponent Bi–Mo Catalyst

“…In general, our results are consistent with the mechanism proposed by Bell et al ,, On the basis of theoretical and spectral data, the authors assumed molybdenum ether of AAlc to be the intermediate (called “allyl alkoxy intermediate”) in the propene oxidation to ACR. The present study provides experimental evidence of the existence of such an intermediate.…”

“…The idea of Grasselli et al was widely accepted. − However, as was noted by Bell et al, “there is no decisive experimental evidence to support either view”. As one can see further, such evidence can be successfully obtained using the QC study of the reaction.…”

Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches

“…Bismuth molybdates mixed with large radii cations lead also to homogeneous phases with a scheelite structure. As shown by Licht et al, cations such as La 3+ , Sb 3+ , and Pb 2+ + H + can replace Bi 3+ and show a similar electronic perturbation of the neighboring molybdenum, facilitating the initial hydrogen abstraction within the mechanism. Another example is the scheelite-structured Bi 3 FeMo 2 O 12 …”

“…However, bismuth needs to be present in order to provide a suitable electronic and structural environment for propylene oxidation. As Licht et al 41 pointed out recently, the presence of bismuth in Bi 2 Mo 3 O 12 enables a certain five-coordinate Mo geometry, which is significantly more active for hydrogen abstraction. This explains why pure MoO 3 is barely active for propylene oxidation.…”

Energy Maps of Complex Catalyst Surfaces

“…The density of propane adsorption sites is comparable on all catalysts and scatters around 0.1 nm −2 (Figure S5b). In contrast, the amount of C 3 H 6 increases significantly with the specific surface area of the materials −1 = f(q diff ), (see eq 2), of (a) propane and (b) propene at 313 K determined by measuring the heat of adsorption by microcalorimetry as a function of coverage (Figure S1b), which was determined based on a simultaneously performed volumetric measurement of the adsorbed amount (determination of the adsorption isotherm n ads = f(p)); Bars denote the heat of adsorption taken from the literature and determined experimentally on single-crystal references and polycrystalline reference compounds for propane on V 2 O 5 (001), 51 α-V 2 O 5 , and α-MoO 3 , 47 and values calculated by DFT for propene adsorption on V 2 O 5 (001), 52,53 oxygen terminated planes of Bi 2 Mo 3 O 12 (010), 54 MoO 3 (010), 54 and clusters of Mo 3 O 9 ; 55 All spectra are shown to the energy of 25 kJ•mol −1 , which corresponds to the beginning of multilayer formation; The heats of adsorption of the molecules on an empty surface q 0 and on the most abundant adsorption sites q max (Scheme 1) are indicated; The numerical values are provided in Table 1; For the differential heat profiles please see Figure S2a,c. (Figure S5a, Table 1), which is consistent with more specific adsorption of the alkene.…”