Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

Abstract

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have...

Propane to olefins tandem catalysis: a selective route towards light olefins production

“…115 This is in accordance to SOMC insights showing that distortion of metal complexes using different ligands resulted in unfavored MCB isomerization, and thus less off-cycle square planar MCBs. 97 These results further suggest that mixed oxides of WO x may also result in better Met activity, which may be especially relevant in the case of intimate contact state between PDH and Met catalysts in PTO technologies.…”

“…28,109 Advances in SS-NMR and operando vibrational and electronic spectroscopy, on the other hand, can provide deep insights on reaction intermediates and help unravel activation, reaction and deactivation mechanisms. 97 For the metathesis reaction, WO x /SiO 2 is the optimal catalyst due to its resistance to trace quantities of oxygenates in the feedstock, the long catalyst lifetime compared to MoO x and ReO x supported metathesis catalysts, the easy regeneration to remove coke deposits and the absence of catalyst embrittlement upon periodic regeneration. 56,95 However, about 80 wt% of W worldwide is mined in China, which makes tungsten a strategic metal, and its sustainability critical.…”

“…MoO x catalysts are somewhat intermediate between rhenium and tungsten, can operate at 50-250 1C and are mainly used in the Shell Higher Olefin Process (SHOP). 97 However, they also tend to deactivate due to poisoning and high mobility of Mo oxides. 98 Despite being the less active of the three (conventional operating temperatures are 300-500 1C), WO x -based supported catalysts (typically WO 3 /SiO 2 ) are the most used in the combined dehydrogenation and metathesis PTO industrial technologies, due to their resistance to deactivation and to small oxygenates concentration in the feedstock, fast regeneration and high resistance to embrittlement over time.…”

“…95,105 Several mechanisms of active site formation have been proposed, which will not be discussed herein. 97,107 The in situ activation, however, results in active sites accounting for only a few percentage (5-10%) of the total metal sites, which is due to many factors and makes the investigation of active site formation challenging. First of all, the formation of alkylidenes is slightly thermodynamically unfavored, and second, the initiation is believed to occur with participation by proton transfer from the acid OH groups of the support.…”

“…Selectivity is a central issue in metathesis catalysis. 97 In the case of propene metathesis, especially at temperatures typical of the PTO reaction, the main issue is related to C 4 isomerization, possibly coupled with further metathesis on the internal olefin, and C 2 di-or oligomerization to higher alkanes. The Lewis and Brønsted acid character of the surface WO x sites and WO 3 NPs, respectively, is responsible for the byproducts formed by C 2 Q dimerization to C 4 Q and oligomerization to C 4 -C 6 alkanes.…”