Kinetics of Propylene Epoxidation over Extracrystalline Gold Active Sites on AU/TS-1 Catalysts

Abstract

To determine whether the catalytic roles of extracrystalline and intracrystalline gold nanoparticles supported on titanosilicate-1 (TS-1) for direct propylene epoxidation are intrinsically different, the kinetics of direct propylene epoxidation were measured in a gas-phase continuous stirred tank reactor (CSTR) over polyvinylpyrrolidone-coated (PVP) gold nanoparticles (Au-PVP/TS-1) deposited on TS-1. The as-made PVPcoated gold nanoparticles were too large to fit into the micropores of TS-1, even after ligands were removed in situ by a series of pretreatments, as confirmed by both TEM and TGA-DSC. The activation energy (51 kJ mol −1 ) and reaction orders for H2 (1.3), O 2 (0.4), propylene (0.4), propylene oxide (−0.6), carbon dioxide (0), and water (0) for propylene epoxidation measured on Au-PVP/TS-1 were consistent with those reported for Au/TS-1 prepared via deposition-precipitation (Au-DP/TS-1) (52 kJ mol −1 , H 2 , 1; O 2 , 0.4; C 3 H 6 , 0.4; C 3 H 6 O, −0.6; CO 2 , 0; H 2 O, 0). However, while the reaction orders for hydrogen oxidation on Au-PVP/TS-1 (H 2 , 0.8; O 2 , 0; C 3 H 6 , −0.3; C 3 H 6 O, −0.1; CO 2 , 0; H 2 O, −0.2) were similar to those measured on Au-DP/TS-1 (H 2 , 0.9; O 2 , 0.3; C 3 H 6 , −0.3; C 3 H 6 O, 0; CO 2 , 0; H 2 O, −0.1), a decrease in activation energy from approximately 30 kJ mol −1 for Au-DP/TS-1 to 4− 5 kJ mol −1 for Au-PVP/TS-1 suggests there is a change in rate-limiting step or active site for hydrogen oxidation. Additionally, an active site model was developed which determines the number of Ti within an interaction range of the perimeter of extracrystalline Au nanoparticles (i.e., the number of Au−Ti active site pairs). This model was used to estimate catalytic turnover frequencies over solely proximal Au−Ti pairs, assuming that hydrogen peroxide does not desorb and migrate from Au sites to Ti sites and instead propylene oxide forms in a concerted mechanism previously termed the "simultaneous" mechanism. Turnover frequencies estimated for this active site model for a data set containing both Au-DP/TS-1 and Au-PVP/TS-1 were ∼20× higher than the highest previous reported estimates (∼80 s −1 vs 1−5 s −1 at 473 K) for catalytic oxidation on noble metals, suggesting that the simultaneous mechanism occurring over proximal Au−Ti sites alone is incapable of explaining the observed rate of propylene epoxidation and that short-range migration of hydrogen peroxide is kinetically relevant. The agreement of reaction orders, activation energy, and active site model for propylene epoxidation on both Au-DP/TS-1 and Au-PVP/TS-1 suggests a common mechanism for propylene epoxidation on both catalysts containing small intraporous gold clusters and catalysts with exclusively larger extracrystalline gold nanoparticles. Rates of hydrogen oxidation were found to vary proportionally to the amount of surface gold atoms. This is also consistent with the hypothesis that the observed decrease in hydrogen efficiency and PO site-time-yield per gold mass with increasing gold loading are driven primarily by t...

Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H2 and O2 on Au–Ti Catalysts

“…Based on the semi-quantitative model of the Au/TS-1-B catalyst, the fraction of Au atoms at the Au–Ti proximity is relatively small (less than 5%), and the number of proximal Ti sites is also much less than that of the remote Ti sites. Thus, the PO formed by the simultaneous mechanism is limited because it requires the formation of H–Au–OOH species in proximal to Ti for epoxidation. , In addition, previous studies have shown that the adsorption of HOOH on the Au particle surface is weak, − and the energy barrier of the HOOH desorption step is relatively low . The HOOH species formed on Au sites are likely to desorb to the gas phase and adsorb on the remote Ti sites to form Ti-hydroperoxo, which is reactive for epoxidation.…”

“…Thus, the PO formed by the simultaneous mechanism is limited because it requires the formation of H−Au−OOH species in proximal to Ti for epoxidation. 49,64 In addition, previous studies have shown that the adsorption of HOOH on the Au particle surface is weak, 65−67 and the energy barrier of the HOOH desorption step is relatively low. 28 The HOOH species formed on Au sites are likely to desorb to the gas phase and adsorb on the remote Ti sites to form Ti-hydroperoxo, which is reactive for epoxidation.…”

Review and perspectives on TS-1 catalyzed propylene epoxidation

“…The CSTR facilitates the quantitative study of reaction orders, including product inhibition, by ensuring is concentration throughout the catalyst bed under all feed conditions and reactant conversions. 353 , 354 Commonly used reactors also include stainless steel reactors, 76 , 111 , 222 quartz tubular reactor 5 , 228 , 226 , 355 and continuous flow fixed-bed reactor. 334 , 356 …”

Nanoparticles as an antidote for poisoned gold single-atom catalysts in sustainable propylene epoxidation

“…The direct epoxidation of propylene with H 2 /O 2 to value-added propylene oxide (PO) using bifunctional Au-Ti catalysts (C 3 H 6 + H 2 + O 2 → C 3 H 6 O + H 2 O) is considered a dream reaction for PO production 17 , 18 . It is widely accepted that hydrogen peroxide or hydroperoxyl intermediate generated by H 2 and O 2 on Au sites would transfer and react with C 3 H 6 on adjacent Ti sites to generate PO, and neither Au nor Ti alone would give any significant PO activity 19 , 20 .…”