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Catalyst Activation and Kinetics for Propylene Metathesis by Supported WO/SiO Catalysts x
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Soe Lwin, and Israel E. Wachs ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03097 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016
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ACS Catalysis
Catalyst Activation and Kinetics for Propylene Metathesis by Supported WOx/SiO2 Catalysts
Soe Lwin and Israel E. Wachs* Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA *Corresponding author:
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Abstract The activation and kinetics of propylene metathesis by well-defined supported WOx/SiO2 catalysts were investigated with transient temperature programmed surface reaction (TPSR) spectroscopy and steady-state olefin metathesis. The TPSR measurements revealed for the first time that catalyst activation with olefins creates three distinct activated sites (highly active (Tp~160oC), modestly active (Tp~450oC) and sluggishly active (Tp~600-750oC)). The number and reactivity of the activated surface WOx sites increase with activation temperature, olefin partial pressure and olefin size (C2= Kads[C3=]), and allows the kinetic equation to be further simplified rate = k Kads [C3=]
(11)
that accounts for the 1st-order kinetics observed for propylene metathesis by supported WO3/SiO2 catalysts at elevated temperatures in the present study and in prior publications. 3, 5, 11 The steady-state apparent rate constant is the product of k*Kads that yields an apparent activation energy (Eapp= E + Hads). The reported apparent activation energies for olefin metathesis by supported WOx/SiO2 catalysts vary from ~75-200 kJ/mol.3,5.11 In the current study, the apparent activation energy values varied from ~80-105 kJ/mol (see Figure S2). The reaction E values determined from the TPSR measurements indicate that the surface reaction activation energies vary from ~100-300 kJ/mol depending on which type of surface WOx sites are activated (see Table 2). This suggests that the wide range of reported Eapp values is most probably related to the distribution of different types of activated surface WOx sites in these propylene metathesis studies. The kinetic studies should be minimally affected by mass and heat transport since metathesis is an almost thermoneutral reaction with a slow reaction rate (TOF = 10-2 s-1 and even lower if more than 1% of the surface WOx sites are highly active).
5. Conclusions Activation of supported WOx/SiO2 catalysts by olefins generates several distinct activated sites demonstrating for the first time that multiple active sites are formed (highly active, modestly active and sluggishly active). The highly active sites can perform metathesis at ~160oC (k~103/s), the modestly active sites can conduct metathesis at ~450oC (k~10-4/s) and the sluggish active sites exhibit metathesis activity at ~600-750oC (k~10-9/s). Although ~2.5-4.5% of the total
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surface WOx sites are activated by olefins, the highly active sites only represent ~1% of the total surface WOx sites. The number of activated site increases with increasing activation temperature, olefin partial pressure and olefin size (C2=
