The Enhancing Effect of Brønsted Acidity of Supported MoO_x Species on their Activity and Selectivity in Ethylene/trans‐2‐Butene Metathesis

Abstract

Supported catalysts with a nominal Mo surface density of 0.15 and 1.5 Mo atoms nm−2 were synthesized by impregnation of alumina, silica, and alumina–silica supports with silica content between 1 and 70 wt %. They were tested for their activity and selectivity in the metathesis of ethylene and trans‐2‐butene to propene between 343 and 603 K at 125 kPa. The catalysts were characterized by UV/Vis, Raman, and IR spectroscopy, XRD and H2 temperature‐programmed reduction for elucidating the distribution, degree of polymerization, reducibility, and acidity of MoOx species. We established that Brønsted acidity of highly dispersed tetrahedral and polymerized octahedral MoOx species is required to ensure high metathesis activity. The acidic character of these species is influenced by their structure and support. Tetrahedral MoOx species with Brønsted acidic character are only formed on supports possessing such acidity, whereas Brønsted acidic octahedral MoOx is also created on supports without such acidic sites.

Active Sites in Olefin Metathesis over Supported Molybdena Catalysts

“…Though the general route of carbene generation was clarified, the structural identification of the relevant surface molybdenum oxide precursor species remains an elusive challenge, as it demands discrimination of a small minority (≈1 %) from the majority of spectator species. As for alumina‐supported molybdenum oxide catalysts the nature of the precursor structure has been controversially discussed, favoring both monomeric, as well as oligomeric surface MoO x species. Here, we study correlations between surface density of molybdenum oxide species, acid‐base properties and catalytic properties taking a series of silica‐supported molybdenum oxide catalysts with different Mo loading as basis.…”

The effect of supported MoOXstructures on the reaction pathways of propene formation in the metathesis of ethylene and 2-butene

“…Supports and catalysts were characterized by N 2 adsorption, ICP-OES, XRD, UV-vis and Raman spectroscopy, H 2 -TPR and IR pyridine adsorption. 8 The obtained results revealed that all catalysts with 0.15 Mo nm À2 possess highly dispersed tetrahedral MoO X species, while those with 1.5 Mo nm À2 contain polymerized octahedral MoO X . Catalytic tests were carried out in fixed-bed reactors at 423 K and 125 kPa.…”

“…This may be related to higher Brønsted acidity of the former catalyst. 8 Another important difference between isolated tetrahedral and polymerized octahedral MoO X species is their ability to engage ethylene in propene formation. This conclusion was derived from analysing the reaction rates of propene formation (r Mo (propene)) and ethylene consumption (r Mo (ethylene)) in catalytic tests using reaction feeds with an ethylene/trans-2-butene ratio between 0.1 and 2.8.…”

“…To the best of the authors' knowledge, such studies have not been reported up to now, while activity-determining factors have been thoroughly investigated. For example, the rate of propene formation over Mo-containing catalysts, which are promising alternatives to WO X /SiO 2 , is influenced by (i) the support 7,8 and (ii) the structure of surface MoO X species. [9][10][11] Recently, it has been demonstrated that Brønsted acidity of highly dispersed tetrahedral and polymerized octahedral MoO X species is also an essential activity-determining factor.…”

MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis

“…Molybdenum oxide on silica, alumina or silica-alumina belongs to the well-known and long-time used metathesis catalysts [ 1 ]. Albeit typical ill-defined catalysts they are still popular as relatively cheap catalysts finding industrial applications especially in the treatment of low olefins [ 2 – 5 ]. Their catalytic activity depends on many factors, especially on Mo loading, support acidity, and pre-reaction activations.…”