Enhancing Propene Formation in the Metathesis of Ethylene with 2-Butene at Close to Room Temperature over MoO_x/SiO₂ through Support Promotion with P, Cl, or S

Abstract

The metathesis of ethylene with 2-butene to propene over catalysts with supported oxides of Mo, Re, or W is attractive both from industrial and fundamental viewpoints. The latter interest is the result of in situ transformation of metal oxides into metal carbenes, which are the active sites. In comparison with previous studies using metal oxide promoters for enhancing catalyst activity, the present study demonstrates that modification of the SiO2 support with P, S, or Cl is an efficient method for facilitating the ability of supported MoO x species to generate molybdenum carbenes at 50 °C and accordingly to produce propene from ethylene and 2-butene. The promoters are ordered in terms of their enhancing effect as follows: P ≫ S ∼ Cl. Modification with phosphorous results in the creation of a special type of hydroxyls, which assists in 2-butene adsorption and therefore facilitates the transformation of MoO x into Mo  CH–CH3. Ethylene cannot convert MoO x to any carbenes (particularly to MoCH2) at such low reaction temperatures. MoCH2 carbenes are, however, formed by ethylene reaction with MoCH–CH3. The presence of both of these carbene types is decisive for propene formation through the metathesis of ethylene with 2-butene.

Effect of AlSiOx support modification by alkali or alkaline earth metals on propene formation in the metathesis of C2H4 and 2-C4H8 over MoOx-based catalysts

“…[4][5][6] Their performance in the metathesis reaction depends on several factors including surface acidity, [7][8][9] reducibility of MoO x species, 10 dispersion of MoO x species, [11][12][13][14] catalyst pre-treatment conditions, 15,16 the type of support, 17 and the presence of surface hydroxyls. 18 All these factors are important for in situ formation of Mo-carbenes (MoCHR), which are the active sites in this reaction. Moreover, the kind of MoO x species, from which MoCHR are formed, was established to affect the overall scheme of propene formation.…”

Fundamentals and application potential of the synergy effect between ZnO and Mo/SiO2 for propene production in the metathesis of ethylene and trans-2-butene

“…2 This reaction is performed over relatively cheap and poison-resistant WO x /SiO 2 catalysts at temperatures above 300 °C. 3 Certain materials (MgO, Ca(OH) 2 , CaO, Y 2 O 3 , NaOH, NaNO 3 , KNO 3 and hydrotalcites) located upstream or admixed to WO x /SiO 2 are known to significantly increase the metathesis activity of the latter. 4 Noticeably, the additives are not active in the metathesis reaction and therefore defined as cocatalysts.…”

“…[6][7][8][9][10][11][12][13][14] MoO x /SiO 2 , MoO x /Al 2 O 3 , and MoO x /SiO 2 -Al 2 O 3 systems are considered as promising alternatives to the commercially applied WO x / SiO 2 . 3,8,15 The metathesis activity of MoO x /SiO 2 at low temperature can be significantly improved by treating the catalyst in methanol, 10 CO, 16 or hydrocarbons (methane, ethane, propane, ethene and propene) at high temperatures to create Mo-carbenes. 12,16,17 Similar to WO x /SiO 2 , the enhancing effect of Al 2 O 3 , AlSiO x or CaO cocatalysts was established by Goelden et al 5 for the above-mentioned Mo-containing catalysts in the metathesis of ethylene with 2-butene to propene above 150 °C.…”

“…2 This reaction is performed over relatively cheap and poison-resistant WO x /SiO 2 catalysts at temperatures above 300 °C. 3…”

“…6–14 MoO x /SiO 2 , MoO x /Al 2 O 3 , and MoO x /SiO 2 –Al 2 O 3 systems are considered as promising alternatives to the commercially applied WO x /SiO 2 . 3,8,15 The metathesis activity of MoO x /SiO 2 at low temperature can be significantly improved by treating the catalyst in methanol, 10 CO, 16 or hydrocarbons (methane, ethane, propane, ethene and propene) at high temperatures to create Mo–carbenes. 12,16,17…”