A Quantum Chemical Study of Comparison of Various Propylene Epoxidation Mechanisms Using H₂O₂ and TS-1 Catalyst

Abstract

We present a comparison of the following prominent propylene epoxidation mechanisms using H2O2/TS-1 at a consistent density functional theory (DFT) method: (1) the Sinclair and Catlow mechanism on tripodal site through Ti-OOH species, (2) the Vayssilov and van Santen mechanism on tetrapodal site without Ti-OOH formation, (3) the Munakata et al. mechanism involving peroxy (Ti-O-O-Si) species, (4) the defect site mechanism with a partial silanol nest, and (5) the defect site mechanism with a full silanol nest. We have reproduced the previously published (ethylene epoxidation) pathways (1-3) for propylene epoxidation using larger and SiH3-terminated cluster models of the T-6 crystallographic site of TS-1. Mechanism 5 is a new mechanism reported here for the first time. The use of a consistent level of theory for all the pathways allows for the first time a meaningful comparison of the energetics representing the aforementioned pathways. We have rigorously identified the important reaction intermediates and transition states and carried out a detailed thermochemical analysis at 298.15 K and 1 atm. On the basis of the Gibbs free energy of activation, the Sinclair and Catlow mechanism (Delta G(act) = 7.9 kcal/mol) is the energetically most favorable mechanism, which is, however, likely to operate on the external surface of TS-1 due to the tripodal nature of the Ti site in their model. The newly reported defect site mechanism (with a full silanol nest) is a competitive propylene epoxidation mechanism. There are two main steps: (1) hydroperoxy formation (Delta G(act) = 8.9 kcal/mol) and (2) propylene epoxidation (Delta G(act) = 4.6 kcal/mol). This mechanism is likely to represent the chemistry occurring inside the TS-1 pores in the liquid-phase epoxidation (H2O2/TS-1) process and could operate in direct gas-phase epoxidation (H2/O2/Au/TS-1) as well. If only the propylene epoxidation step is considered, then the Munakata peroxo intermediate (Si-O-O-Ti) is the most reactive intermediate, which can epoxidize propylene with a negligible activation barrier. However, formation of the Munakata intermediate is a very activated step (Delta G(act) = 19.8 kcal/mol). We also explain the trends in the activation barriers in different mechanisms using geometric and electronic features such as orientation of adsorbed H2O2 and propylene, hydrogen bonding, O1-Ti bond distance in the Ti-O1-O2-H intermediate, and O1-O2 stretching in the transition state. Implications of different Ti site models are also discussed in light of the nature of external/internal and nondefect/defect sites of TS-1.

Switching off H2O2 Decomposition during TS-1 Catalysed Epoxidation via Post-Synthetic Active Site Modification

“…The Arrhenius barrier obtained for TS-1 is found to be in good agreement to previous theoretical studies [12,13]. Although this data indicates that TS-1B is a less active epoxidation catalyst-being less able to reduce the kinetic barrier-this decrease in activity is partially compensated for by an increase in the pre-exponential factor, which increases from 1 × 10 3 for TS-1, to 2 × 10 5 for TS-1B.…”

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

“…This mechanism, as well as related recently proposed mechanisms involving organic hydroperoxide (29), are directly at odds with one proposed mechanism based on experimental evidence (30-32) (and supported with DFT modeling) (31,32), which involves H−O-bond cleavage during hydrogen peroxide chemisorption and the open site (SI Appendix, Fig. 1S entry 1a) as a relevant intermediate.…”

“…For both the open and closed structures, the extra equivalent of hydroperoxide lowers the transition-state energy via hydrogen-bond-donating interactions to the hydroperoxide involved in oxygen transfer, as described previously (29), and this additional transition-state energy lowering corresponds to ∼0.6-1.0 kcal/mol. We note that the hydrogen bond donor within this context can in principle be either organic hydroperoxide, alcohol byproduct of organic hydroperoxide that is formed following oxygen transfer, or silanol (30)(31)(32)(33). Previous comparisons of trimethylsilyl (TMS)-capped and native silanolcontaining Ti-on-silica olefin epoxidation catalysts show a lack of appreciable activity or selectivity difference under dry conditions, when using organic hydroperoxide as oxidant (45).…”

Chemical diversity of zeolite catalytic sites

“…25 The mechanism of epoxidation appears to proceed through proximal oxygen abstraction (Figure 5), where the need for coordination to the metal to activate the hydrogen peroxide is evident (Wells et al, 2006). 26 The mechanism of other oxidations remains a matter of debate.…”