Effects of Zeolite Confinement and Solvent - ACS Publications

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution 2

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Mechanistic Insight into Propylene Epoxidation with HO over TS-1: Effects of Zeolite Confinement and Solvent Xiaowa Nie, Xianxuan Ren, Xiaojing Ji, Yonggang Chen, Michael J. Janik, Xinwen Guo, and Chunshan Song

J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b04439 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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The Journal of Physical Chemistry

JPC-B-R1_July2019

Mechanistic Insight into Propylene Epoxidation with H2O2 over TS-1: Effects of Zeolite Confinement and Solvent Xiaowa Niea,*, Xianxuan Rena, Xiaojing Jia, Yonggang Chend, Michael J. Janikc, Xinwen Guoa,*, and Chunshan Songa,b,c a State

Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research,

School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China b EMS

Energy Institute, PSU-DUT Joint Center for Energy Research and Department of

Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA c

Department of Chemical Engineering and PSU-DUT Joint Center for Energy Research, Pennsylvania State University, University Park, PA 16802, USA

d Network

and Information Center, Dalian University of Technology, Dalian 116024, P. R. China

Corresponding Author Dr. Xiaowa Nie Email: [email protected] Dr. Xinwen Guo Email: [email protected]

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Abstract Density functional theory (DFT) calculations were performed to investigate the effects of zeolite confinement and solvent on propylene epoxidation with H2O2 over TS-1 catalyst. The 144T and 143T cluster models containing typical 10MR channels of TS-1 were constructed to represent the tripodal(2I) and Ti/defect sites. It was found that the confinement of zeolite pore channel not only impacts the adsorption stability of guest molecules but also alters reaction barriers, as compared to the results obtained based on small cluster models. When dispersion corrections were considered, an enhancement of the adsorption stability of guest molecules was observed due to the important contribution from van der Waals interactions, especially for propylene adsorption. An explicit protic methanol molecule was introduced into the catalytic system to probe the influence of solvent on propylene epoxidation, based on which a significant enhancement of CH3OH-H2O2 co-adsorption strength was obtained owing to H-bonds formation. More importantly, the energy barrier for H2O2 dissociation is largely reduced by ~13 kcal/mol due to the participation of methanol molecule in H-transfer process and the formation of H-bond network, resulting an altering of the rate-limiting step. By comparison, adding an aprotic acetonitrile solvent does not have substantial effect on reaction path and kinetics. The calculation results clearly demonstrate the important role of protic methanol solvent, which not only strengthens the adsorption of guest molecules but also speeds up the kinetics for propylene epoxidation with H2O2 over TS-1 catalyst.

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1. Introduction Titanium silicalite-1 (TS-1) demonstrates superior catalytic performance towards selective oxidation such as aromatic hydroxylation,1-3 alkene epoxidation,4-6 and alkane oxidation,7-8 owing to the high selectivity and activity achieved under mild conditions. The epoxidation of propylene (PL) to propylene oxide (PO) is one of the typical reactions that applies to TS-1 selective oxidation.9-12 Alkenes epoxidation by using hydrogen peroxide (H2O2) oxidant is regarded as an eco-friendly technique due to its simple operation and less pollution, which has attracted increasing attention over the years.13-15 Rapid epoxidation kinetics can be achieved with TS-1 zeolites in H2O2 aqueous solution. Shin and Chadwick performed experimental studies to investigate the catalytic kinetics of propylene epoxidation with H2O2/TS-1 and a solvent mixture of methanol/water. This catalytic system demonstrated fast kinetics towards propylene epoxidation and a low apparent activation barrier of only 25.8 kJ/mol was obtained.13 The solvent mixture had a complicated influence on epoxidation rate and selectivity, showing a promotion on the selectivity at the cost of decreasing the reaction rate by increasing the content of methanol. In addition, the competitive adsorption of methanol with reactant molecules was found to be against the epoxidation reaction.13 Zuo et al. conducted propylene epoxidation experiments with H2O2/TS-1 at reaction temperature of 40 ℃, and through tuning the crystal size of TS-1 zeolites, a 98% selectivity of PO was achieved at a H2O2 conversion of 74%.9 When smaller crystal size (600 nm×400nm×250nm) of TS-1 catalysts was used, the conversion of H2O2 and selectivity of PO can reach to 92 and 98 mol%, respectively.16 According to the experimental results reported in literature, the epoxidation performance can be tuned/improved by adjusting coordination structure of Ti center 3



(coordination state, solvent coordination, defect formation, and hydrolysis process),17-20 tuning the crystal size,9, 16 modifying pore structures,21 solvent addition,22-24 and etc. Despite extensive experimental10-14, 21, 25-26 and theoretical studies19, 27-37 have been conducted on this subject, the impact factors on the formation of active intermediate species, reaction pathways, kinetics, and selectivity for alkenes epoxidation with H2O2 over TS-1 catalysts have not been clearly elucidated to date. These effects can be resulted from the structure and nature of Ti active center, the pore structure and confinement of TS-1, as well as solvent influence, and etc. The experimental studies have shown that the solvent has an important effect on the activity and selectivity of TS-1 catalysts in liquid phase epoxidation, however, this influence is very complicated, due to its dependence on both the catalyst properties and reaction conditions.22-23 In Ti-containing catalytic systems, the epoxidation rates of alkenes are higher in protic solvents (e.g. methanol and ethanol) than that in aprotic solvents such as acetonitrile and acetone.22 Commonly, the direct participation of solvent molecules in reaction can tune the adsorption stability of guest molecules and impact the pathways and kinetics for alkenes epoxidation with H2O2 over TS-1 catalysts.29-30, 37-38 Clerici and Ingallina, at the first time, proposed a five-membered ring (5MR) titanium hydroperoxy (Ti-OOH) active intermediate under solvent condition, Ti-(η1-OOH)-ROH, in which the protic solvent ROH forms a hydrogen bond with the distal oxygen atom of Ti-OOH species while coordinating with the Ti center.38 It was considered that the hydrogen bond network formed within the 5MR configuration of Ti(η1-OOH)-ROH can stabilize the Ti-OOH or increase the electrophilicity of this species, thereby enhancing the epoxidation rate.38 Khouw et al. proposed that a deprotonated peroxide is the active intermediate species from H2O2 dissociative adsorption, which gains additional 4


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stability from the formation of a 5MR complex with co-adsorbed solvent molecules or silanol group.39 Motivated by these previous studies,38-39 Vayssilov and van Santen utilized 5T cluster model, Ti(OSiH)4, to examine the catalytic activation of H2O2 on TS-1.37 They calculated the formation energies of various 5MR adsorption complexes formed by Ti-OOH species interacting with methanol, water, or a silanol group. The results showed that methanol participation facilitates the formation of 5MR co-adsorption complex than H2O and the silanol group, however, formation of such configurations are still highly endothermic with reaction energies of 90~130 kJ/mol.37 Later on, Sever and Root investigated the effect of protic solvents on the structure and formation of Ti-OOH species using density functional theory (DFT) calculations.29-30 On one hand, the protic solvent molecule can coordinate with the Ti center. However, since a H2O2 molecule has been already adsorbed at the Ti center, such coordination is thermodynamically unstable, and in this case, the TiOOH-H2O intermediate is mainly formed. On the other hand, the protic solvent can form hydrogen bonds with Ti-OH group and the H2O2 molecule, and this way the solvent molecules can promote proton transfer from H2O2 to form the Ti-OOH intermediate and result in a lowering of H2O2 dissociation barrier by 5-6 kcal/mol. In this mechanism, the active intermediate was considered to be a TiOOH-(H2O, ROH) species. Furthermore, the stability and epoxidation reactivity of the 5MR intermediate species (Ti-(η1OOH)-ROH) proposed by Clerici and Ingallina were found not to be further improved, thus could not explain the increased epoxidation rate in protic solvents based on experimental studies. Although computational efforts were put into forward to understand the role of solvent in H2O2 activation, dissociation and the formation of active Ti-OOH intermediate, the knowledge of the effect of different types of solvent such as protic vs. aprotic one, how solvent impacts the 5



pathways for propylene oxide production from propylene epoxidation with H2O2, and how the cluster size of TS-1 model and computational functional influence the results of solvent effect are still lacking on the way from investigation. With respect to zeolite catalysis, when the guest molecule is placed in the pore channel of the zeolite, its electronic structure will be changed accordingly, that is caused by the so-called confinement effect of zeolite. This effect not only can tune the adsorption and diffusion of reactants in the pore, but also impact the stability of intermediates and transition states , thus changing reaction rate and selectivity.40-44 Song et al. studied the interactions between different reactants, intermediates and the framework of zeolites through energy decomposition analysis (EDA),40 and found that the electrostatic interaction plays a major role in stabilizing cation intermediates, the orbital overlap and electrostatic interaction contribute significantly to stabilize alkoxy species forming covalent bonds, and the dispersion effect is beneficial to the adsorption of neutral hydrocarbons. They believed that the pore confinement effect mainly comes from two aspects, including the size of reactant, intermediate, transition state and product, as well as the charge properties of guest molecules. Lesthaeghe et al. discussed the effect of zeolite confinement on bulky intermediate molecules in the reaction of methanol conversion to olefins,45 which were mainly reflected by four aspects: electrostatic stability, spatial restriction and shape selectivity, rotation, and framework breath. Spanó and co-workers studied peroxo species generation in H2O2 aqueous solution over Ti-containing zeolite using Car−Parrinello method,36 and they proposed that the framework oxygen of zeolite acts as active oxygen medium species, however, the influence of zeolite pore channel on the epoxidation of olefin was not discussed in the work. Panyaburapa et al. used 9T/65T ONIOM2 hierarchical model to 6


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examine the mechanism of epoxidation of unsaturated hydrocarbons using H2O2 catalyzed by defective TS-1 zeolites.46 The computational model used in this work contains the straight and zigzag channels of TS-1, which can well describe the effect of zeolite framework and pore structure on reaction sites, but the authors did not discuss how the confinement effect of TS-1 impacts the adsorption and reaction of guest molecules inside the pore. Therefore, the confinement effect of zeolite on the adsorption and reaction of guest molecules using TS-1 catalyst for propylene epoxidation with H2O2 has not been understood so far, especially under the consideration of long-range dispersion corrections. In this work, we performed comprehensive DFT calculations to investigate the effects of zeolite confinement and solvent on the epoxidation of propylene with H2O2 over TS-1 catalyst. Comparisons between the results based on large cluster models containing the main pore channels of TS-1 and small clusters without pore structures were made to discuss the confinement effect of TS-1 on the adsorption and reaction of guest molecules within the zeolite pore. The effect of long-range dispersion corrections was examined which was found to play an important role in enhancing adsorption stabilities of reactants, especially for propylene adsorption. A methanol molecule (protic solvent) was included into the catalytic system as explicit solvent and it was found that the direct participation of methanol in reaction can strengthen H2O2 adsorption via H-bonds formation and significantly lower the activation barrier for H2O2 dissociation through H-transfer within the H-bond network including methanol, the Ti-OH group and H2O2 molecule. While slightly influencing the adsorption stability of propylene, methanol solvent inclusion increases the barrier for epoxidation step due to the competitive adsorption with reactants as well as the steric effect on the formation of transition 7



state inside TS-1 pore. For comparison, an aprotic solvent (acetonitrile) was introduced into the system to investigate its effect on H2O2 activation, dissociation and subsequent epoxidation to produce the propylene oxide. The results demonstrated that adding acetonitrile solvent does not change the reaction mechanism and has a small impact on the kinetics for propylene epoxidation with H2O2 over TS-1, exhibiting a very different role as compared to the protic methanol solvent. These studies provide fundamental and mechanistic insight into the effect of different types of solvents (methanol and acetonitrile) on propylene epoxidation chemistry and kinetics with H2O2/TS-1, and reveal the important influence of zeolite confinement towards the adsorption of guest molecules and stabilization of intermediates inside the pore channels, especially by taking into account of van der Waals interactions. 2. Computational Details 2.1 Computational methods In this work, we used Gaussian 09 software47 for all calculations, within the framework of density functional theory. The ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) computational approach developed by Morokuma and co-workers48-51 was used for geometry optimization and frequency calculations. The zeolite system was divided into two regions (reaction region, RR and surrounding environment, SE), as represented by the ONIOM2 scheme. The central reaction region was assigned as high layers and computed with the B3LYP (Becke, three-parameter, Lee-Yang-Parr) hybrid functional52 to describe the exchange-correlation energies. The effective core pseudopotentials (ECPs) were used to treat the core electrons. Two basis sets were employed for high level B3LYP calculations, with the Los Alamos LANL2DZ53,54 applied for titanium atom while the basis set at 6-31G(d,p)55 was 8


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adopted for other atoms including silicon, carbon, oxygen, and hydrogen. The remaining extended zeolite framework was treated with the universal force field (UFF) method.56 Vibrational frequency calculations were conducted to confirm minimas and transition states, and we confirmed that each transition state only has a single imaginary vibrational frequency along the reaction coordinate. The intrinsic reaction coordinate (IRC)57-58 analysis was also performed to verify the transition state along the minimum energy pathway. To obtain accurate energy evaluation, pure DFT single point energy calculations at the B3LYP level on the whole zeolite models were carried out based on the optimized geometries from the ONIOM2 scheme. To take into account of the dispersion effect of van der Waals interactions, the ωB97X-D functional was utilized to examine the impact of different levels of electronic structure theory on the adsorption and reaction of propylene epoxidation with H2O2/TS-1. 2.2 Computational models In this work, we constructed a hydrolyzed and a defective model to represent TS-1 catalysts, as shown in Fig. 1 and 2, respectively. The hydrolyzed TS-1 model was created from the hydrolysis and inversion of the original tetrahedral Ti site in TS-1 framework, and two different tripodal structures can be produced in this process, as illustrated in Scheme S1. In the structure of tripodal(1I), only the Ti-OH group rotates to the opposite direction while the SiOH group retains at the original position from hydrolysis. Whereas in the structure of tripodal(2I), both the Ti-OH and Si-OH groups rotate to opposite orientations to reduce the repulsion from hydrolysis. In our previous work,59 the tripodal(2I) model was found to be kinetically more favorable than tripodal(1I) for H2O2 activation and dissociation calculated with a 16T quantum cluster, and therefore in this work, we selected tripodal(2I) model to represent 9



the hydrolyzed TS-1 catalyst. Here, a large cluster of 144T was used, containing the intersection framework of the 10-membered ring straight pore channel (5.4 ×5.6 Å) and the zigzag pore channel (5.1 ×5.5 Å), which is more appropriate in size to investigate the confinement effect on the adsorption and reaction inside TS-1 pores. The T10 site was selected for Ti location, which has been identified as the preferential site for Ti location in previous studies,60-62 and a nearby T9 site was chosen as the Si-OH formation site from hydrolysis. As shown in Fig. 1, the central 20T of the 144T cluster was chosen as high layers (atoms shown with balls) in the ONIOM2 scheme and calculated with high level DFT method. While the rest of the extended framework was assigned as low layers (atoms shown in sticks) and computed using the UFF method. To construct the defective TS-1 model, we also started with the 144T cluster with Ti located at T10 site, and then the Si atom at another adjacent T10 site was removed, creating a 143T cluster model including a Si-vacancy, as illustrated in Scheme S2. The Ti-OH specie together with the three Si-OH groups form a full silanol nest at the Si-vacancy site, as shown in Fig. 2. Formation of such defective sites of TS-1 catalysts was reported in relevant studies.19, 46, 60 The central 19T of the 143T cluster model was treated with high level of theory in calculation while the rest framework was computed with the molecular force field method. During the construction of these cluster models, deleting O atoms from the outermost -Si-O bonds resulted in the formation of dangling -Si- bonds which were saturated by adding H atoms, whose positions were fixed along the original -Si-O- orientations in TS-1 framework. The Si-H bond lengths were kept to 1.47 Å. In geometry optimization, atoms belonged to central high layers together with the adsorbates were fully relaxed, while the rest framework atoms that treated with low level method were frozen at their original lattice positions. 10


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In our previous work,59 the 16T quantum cluster of hydrolyzed TS-1 with tripodal(2I) site was constructed based on the lattice of ZSM-5 with a Ti atom located at T10 site, and the framework contains four layers successively extended from the Ti center (TiSi15O20H28), as shown in Fig. S1. To construct the defective TS-1 model based on the 16T cluster, a Si atom at another T10 site was removed and a full silanol nest was formed with the Ti-OH and three SiOH groups at the Si-vacancy site (TiSi14O19H30), as illustrated in Fig. S2. Such small clusters can be used to model the internal active sites of TS-1, however, the confinement effect of the zeolite was not taken into account since the primary pore structures of the zeolite were not included. Therefore, comparative discussions were made between the large clusters (144T and 143T) in this work and the small ones (16T and 15T) in our previous work to reveal the important confinement effect of TS-1 on the adsorption and reaction of guest molecules inside zeolite pores.

Fig. 1. Structural illustration of the tripodal(2I) site of TS-1 modeled by a 144T cluster. The central 20T shown with balls was fully optimized and treated with high level DFT method, 11



while the rest framework shown with sticks was fixed at its original lattice position and calculated with the UFF method. (a) view along the straight channel and (b) view along the zigzag channel. (Ti=light grey, Si=green, O=red, H=white).

Fig. 2. Structural illustration of the Ti/defect site of TS-1 modeled by a 143T cluster. The central 19T shown with balls was fully optimized and treated with high level DFT method, while the rest framework shown with sticks was fixed at its original lattice position and calculated with the UFF method. (a) view along the straight channel and (b) view alng the zigzag channel. (Ti=light grey, Si=green, O=red, H=white). 3. Results and Discussion In our previous work based on small cluster models,59 two types of mechanisms were proposed for propylene epoxidation with H2O2/TS-1, which are stepwise and concerted mechanisms. The stepwise mechanism contains two main steps, namely, H2O2 dissociation and propylene epoxidation. While in the concerted mechanism, the adsorbed propylene molecule reacts with the co-adsorbed H2O2 in one step to accomplish the epoxidation process. These two types of mechanisms were fully examined over three categories of active sites of TS-1, which are tripodal(1I), tripodal(2I), and Ti/defect sites. By comparing the energy profiles of different pathways on these various Ti sites, the stepwise mechanism through a 5MR Ti-(η1-OOH) 12


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intermediate over the Ti/defect site was identified to be the energetically most favorable pathway for propylene epoxidation with H2O2 over TS-1 catalyst, which shows lower barriers for both H2O2 dissociation and the epoxidation step as compared to literature.27-28, 37, 63 In this work, we focused on the stepwise mechanism of propylene epoxidation with H2O2 over the tripodal(2I) and Ti/defect sites of TS-1 to investigate the effect of zeolite confinement and solvent effect. 3.1 Zeolite confinement effect on propylene epoxidation In this section, we calculated the stepwise mechanism over tripodal(2I) and Ti/defect sites with the 144T hydrolyzed and 143T Si-vacancy models to delve into the influence of zeolite confinement on propylene epoxidation pathways and energetics. The calculation results were comparatively discussed with those obtained using small cluster models in our previous work.59 3.1.1 Stepwise mechanism over the tripodal(2I) site The reaction network of the stepwise mechanism over the 144T hydrolyzed model is similar to that identified over the 16T model in our previous work.59 All structural details for adsorption states, reaction intermediates and transition states are given in Fig. 3 and key structural parameters of these states are provided in Supporting Information Table S1. The first step is H2O2 adsorption, as shown in Fig. 3a (Ads_H2O2), in which the H2O2 molecule is coordinated to the Ti center with the Ti-Oβ distance of 2.53 Å. The Hα and Hβ atoms are hydrogen-bonded with the Ti-OH group and framework O2 atom, with the interatomic distances being 1.74 and 2.62 Å, respectively, similar to the structural parameters obtained by the 16T model. The second step is the dissociation of H2O2 molecule to generate the Ti-hydroperoxy (Ti-OOH) intermediate. In this process, the transfer of Hα to the Ti-OH group occurs 13



concurrently with the formation of the bidentate –OOH moiety. In the transition state configuration (Fig. 3b, TS_Ti-(η2-OOH)-3MR), the interatomic distances of Ti-Oα and Ti-Oβ are 2.26 and 2.23 Å, respectively, and the O1-Hα distance decreases to 1.22 Å. Fig. 3c illustrates the three-membered ring Ti-OOH intermediate (Ti-(η2-OOH)-3MR), in which the Ti-Oα and Ti-Oβ distances contract to 1.93 and 2.15 Å while the Ti-O1 distance elongates from 2.05 Å in the TS to 2.19 Å in Ti-OOH intermediate. A H2O molecule is produced from H2O2 dissociation step, which remains coordinated to the Ti center. Therefore, the Ti center becomes 6-coordiated after the bidentate Ti-OOH formation, assembling in an octahedral-like configuration. Fig. 3d illustrates the co-adsorption complex of propylene with the Ti-OOH species (PL...Ti-(η2OOH)-3MR), in which the propylene molecule is weakly adsorbed nearby the Ti-OOH with the C1-Oα distance being 4.03 Å. The transition state (TS_PO) for propylene epoxidation is displayed in Fig. 3e, from which we observed that the Oα-Oβ bond perpendicularly incises the double bond of propylene equally, and the distances of C1-Oα and C1-Oβ are both 2.17 Å in this geometry. The Oα-Oβ bond length elongates to 1.78 Å, and the Ti-Oα bond length elongates from 1.93 Å in the co-adsorption complex to 2.02 Å in the TS while the Ti-Oβ bond length shortens from 2.15 to 2.00 Å. In this process, the cleavage of Oα-Oβ, Ti-Oα bonds and formation of C-Oα bonds take place simultaneously. Finally, the propylene oxide (PO) product, Ads_PO (Fig. 3f), is produced over the Ti site, with the Ti-Oα distance of 2.52 Å. The tripodal(2I) active site is regenerated after PO desorption which will proceed with the next catalytic cycle. The produced H2O molecule is coordinated to the Ti center with the Ti-O1 distance become 2.34 Å, which is 0.15 Å longer than that in the Ti-(η2-OOH)-3MR intermediate.

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Energy profiles obtained by B3LYP/6-31G(d,p) and ωB97XD/6-31G(d,p) theory methods are plotted in Fig. 4. Adsorption energies and activation barriers for propylene epoxidation with H2O2 over 144T and 16T tripodal(2I) models are summarized in Table 1 for comparison. At the B3LYP level, the adsorption energies of guest molecules over the 144T model are almost the same to those of the 16T model, with the H2O2 adsorption energy of -11.1 kcal/mol (144T), -10.0 kcal/mol (16T) and propylene adsorption energy of -1.1 kcal/mol (144T), -1.3 kcal/mol (16T). As shown in Fig. 4a and Table 1, the highest barrier step along the reaction path over 144T and 16T models both corresponds to H2O2 dissociation to form the hydroperoxy (-OOH) intermediate, with values of 24.2 and 17.9 kcal/mol for the large and small cluster models, respectively, calculated at B3LYP level of theory, which is considered to be the ratelimiting step (RLS) for the overall reaction. Including the confinement effect of TS-1 pore increases the barrier for H2O2 dissociation by 6.3 kcal/mol. The propylene epoxidation barrier over the 16T model, 14.5 kcal/mol, is 7.8 kcal/mol higher than that obtained over the 144T model, showing an obvious facilitation of epoxidation chemistry when zeolite pore structure is included in the calculation. Due to the bulky transition state associated with the propylene epoxidation step, the confinement effect is more pronounced in stabilizing the TS for this step than that for H2O2 dissociation. The overall reaction is exothermic, with the reaction energy of -58.5 kcal/mol over the 144T model and -58.7 kcal/mol over the 16T model. Since the energies for the 144T cluster were based on high level DFT single point energy calculations of the optimized structures from the ONIOM approach, the energy deviations resulted from the inaccurate treatment of electronic structures at the QM/MM boundary in ONIOM calculations could be compensated. Therefore, the observed differences in reaction potential energy surfaces 15



comparing 144T to 16T clusters shown in Fig. 4 would be mainly attributed to the effect of zeolite confinement. When dispersion corrections were taken into account by using the ωB97XD theory method, the adsorption energies of guest molecules are largely strengthened, 5.6 (144T) and 4.0 (16T) kcal/mol stronger than those obtained by B3LYP for H2O2 adsorption while are 15.5 (144T) and 6.3 (16T) kcal/mol stronger for propylene adsorption, more pronounced for propylene adsorption, as observed in Table 1. With respect to reactions, dispersion corrections have a slight impact (< 0.5 kcal/mol) on the activation barriers for H2O2 dissociation over the both models but have a visible influence on the kinetics for epoxidation step, especially for the large 144T model. The barrier is increased by 5.9 kcal/mol as compared to that calculated with B3LYP. However, the H2O2 dissociation is still the rate-limiting step calculated by ωB97XD. By comparing the small clusters, the confinement effect of TS-1 pore examined by ωB97XD is similar to that found by B3LYP, increasing the barrier of H2O2 dissociation by 5.6 kcal/mol but reduces the barrier for epoxidation step by 4.8 kcal/mol, which result from the impact on both the adsorption stability of guest molecules as well as the stability of transition states formed inside TS-1 pores. The overall reaction is 86. 3 (144T) and 77.4 (16T) kcal/mol exothermic, which is exothermic by 58.5 (144T) and 58.7 (16T) kcal/mol calculated with B3LYP functional, indicating that the binding energy of PO product over TS-1 becomes much stronger when dispersion corrections were taken into account. To precisely capture the adsorption properties of guest molecules inside zeolite pores, the van der Waals interactions need to be considered in calculations, which also can influence the reaction pathway and rate-limiting step determination.

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To examine the effect of zero point energies as well as the vibrational entropies, we provided the relevant energy results of ONIOM calculations at 0 K, ZPE corrected energies as well as free energies calculated at 313 K (experimental temperature)10,

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for propylene

epoxidation with H2O2 over the tripodal(2I) model of TS-1 in Table S2. The results show that ZPE corrections do not have substantial impact on the adsorption and reaction of guest molecules and the trend predicted is the same to that from ONIOM optimization and single point energy calculations, with the H2O2 dissociation step rate-limiting. The activation barriers calculated based on free energies at 313 K have tiny changes (ΔEact < 0.1 kcal/mol) comparing to the results from ONIOM optimization at 0 K.

Fig. 3. Optimized structures of states involved in propylene epoxidation with H2O2 over the 144T tripodal(2I) model in the stepwise mechanism. (a) H2O2 adsorption state, Ads_H2O2; 17



(b) transition state for Ti-hydroperoxy intermediate formation, TS_ Ti-(η2-OOH)-3MR; (c) Ti-hydroperoxy intermediate, Ti-(η2-OOH)-3MR; (d) propylene (PL) co-adsorption with TiOOH, PL...Ti-(η2-OOH)-3MR; (e) transition state for propylene epoxidation, TS_PO; and (f) the adsorbed propylene oxide (PO) product, Ads_PO. Only the central reaction region within the 144T cluster model is shown for clarity. Distances are in angstroms. (Ti=light grey, C=grey, O=red, Si=green, H= white).

Fig. 4. Energy profiles for propylene epoxidation with H2O2 over 144T and 16T tripodal(2I) models in the stepwise mechanism, calculated at (a) B3LYP/6-31G(d,p) level and (b) ωB97X-D/6-31G(d,p) level.

18


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Table 1. Adsorption energies and activation barriers (kcal/mol) for propylene epoxidation with H2O2 over 144T and 16T tripodal(2I) models in the stepwise mechanism. Active sites Theory Level Model H2O2 Adsorption Hydroperoxy Formation Propylene Adsorption Epoxidation

Tripodal(2I) B3LYP/6-31G(d,p) ωB97XD/6-31G(d,p) 144T 16T 144T 16T -11.1 -10.0 -16.7 -14.0 24.2 17.9 23.8 18.2 -1.1 -1.3 -16.6 -7.6 6.7 14.5 12.6 17.4

3.1.2 Stepwise mechanism over the Ti/defect site The stepwise mechanism of propylene epoxidation with H2O2 over the 143T Ti/defect site of TS-1 via a 5-membered ring monodentate intermediate Ti-(η1-OOH)-5MR was identified. All key geometries involved within the path are illustrated in Fig. 5, and relevant structural parameters are given in Table S3. First, the H2O2 is adsorbed over the active site by coordination of Oβ to the Ti center (Fig. 5a, Ads_H2O2). The interatomic distance of Ti-Oβ is 2.43 Å, the Hα is hydrogen bonded to O1 with the distance of 2.00 Å, and the Ti-O1 bond is measured to be 1.83 Å in the adsorption state. Fig. 5b illustrates the transition state (TS_ Ti(η1-OOH)-5MR) produced from H2O2 dissociation, in which the Ti-Oβ distance contracts to 2.20 Å, the Hα-Oα distance elongates to 1.31 from 0.98 Å in adsorbed H2O2, and the Hα already forms a bond with O1 of Ti-OH, as reflected by the Hα-O1 bond length of 1.13 Å. As a result, a H2O molecule is produced via this transition state and a 5MR collective was obtained in TS formation. The geometries and structural parameters produced from 143T are similar to those for the 15T model. In the Ti-(η1-OOH)-5MR complex (Fig. 5c), the –OOH species is Hbonded to the H2O molecule with the Hα-Oα distance of 1.54 Å and also coordinated to the Ti center with the Ti-Oβ distance being 2.13 Å. This way, the Ti center becomes 5-coordinated after formation of Ti-OOH species via H2O2 dissociation, unlike the case over tripodal(2I) 19



model where a 6-coordiated Ti center is generated after this reaction. Over the Ti/defect site, the Ti-OOH species involved in the 5MR collective is largely stabilized by the H-bond network between Ti-OH and surrounding silanol groups. As shown in Fig. 5b, the Hα-Oα, Ti-O1H1 and three Si hydroxyl groups collectively constitute the H-bond network during TS formation. In this process, the Hα in H2O2 directly migrates to the Ti-OH group to form a H2O molecule while the H1 in Ti-OH forms a H-bond with Oz-(Hz) (1.79Å), Hz forms a H-bond with Ox-(Hx) (1.73 Å), Hx forms a H-bond with Oy-(Hy) (1.70 Å), and Hy forms a H-bond with Oα (1.48 Å). This H-bond network created at the Ti/defect site would stabilize the transition state for H2O2 dissociation and thus lowering the barrier associated with this step. Fig. 5d illustrates the coadsorption complex of propylene with Ti-OOH (PL...Ti-(η1-OOH)-5MR), in which the nearest distance of C1 and Oα is calculated to be 3.24 Å, indicating a weak interaction between the propylene molecule and Ti-OOH species. Fig. 5e shows the transition state (TS_PO) for epoxidation step and the calculated interatomic distances of Oα-C1 and Oα-C2 are 2.21 and 2.34 Å, respectively. The H-bond network is maintained during propylene epoxidation process, which may stabilize the transition state formed over the Ti/defect site. Finally, the produced PO molecule (Ads_PO) is H-bonded to the Hα atom of the co-produced H2O, and further releasing of PO product can regenerate the Ti/defect site of TS-1 for next catalytic cycle. Fig. 6 plots the energy profiles of propylene epoxidation with H2O2 over 143T and 15T cluster models of Ti/defect sites, calculated at B3LYP and ωB97XD levels of theory. Adsorption energies and activation barriers are given in Table 2. As shown in Fig. 6a, H2O2 adsorption energies over the 15T and 143T cluster models are -16.1 and -11.1 kcal/mol, respectively, computed by B3LYP. Propylene adsorption energies over 143T and 15T models 20


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are almost equal, with the values of -2.5 and -2.0 kcal/mol, respectively, a little bit stronger than those obtained over the tripodal(2I) site. Catalyzed by the Ti/defect site, the activation barriers associated with H2O2 dissociation step are significantly reduced to only 5.6 (143T) and 5.1 (15T) kcal/mol, showing very fast formation of Ti-OOH intermediate. With respect to the subsequent epoxidation step, the Ti/defect site is also catalytically more active than the tripodal(2I) site due to the lowering of activation barriers over the two cluster models. More importantly, the confinement effect of TS-1 promotes propylene epoxidation by reducing the activation barrier from 10.4 to 5.5 kcal/mol. Comparing to the kinetic barriers obtained over tripodal(2I) models, the Ti/defect site exhibits superior catalytic activity towards both H2O2 dissociation and the epoxidation reaction. The overall reaction for propylene epoxidation over Ti/defect site is exothermic by 83.5 kcal/mol over the 143T model and 81.6 kcal/mol over the small 15T model, indicating much stronger binding of PO product over the Ti/defect site than the tripodal(2I) site. To probe the effect of van der Waals interactions on the adsorption and reaction properties, calculations at the ωB97XD level of theory were further conducted on the 143T and 15T models. Fig. 6b plots the energy profile and Table 2 includes the adsorption energies and activation barriers for relevant steps. It is observed that both H2O2 and propylene adsorptions are strengthened by dispersion corrections, especially for propylene, a significant enhancement of the adsorption stability was achieved over the 143T model containing pore channel of TS-1. For reactions, including dispersion corrections has a small impact on the activation barriers for H2O2 dissociation, which are slightly increased by ~1.5 kcal/mol over the two cluster models, due to the stabilization for both the adsorption and transition states. Whereas for the epoxidation step, the activation barriers are visibly increased, especially for the large 143T model, the 21



barrier over which is increased by 8.0 kcal/mol when dispersion correctios were considered. However, the barrier is only 3.1 kcal/mol increased of the epoxidation step over the 15T cluster model calculated by the ωB97XD functional. The large difference of dispersion effect on epoxidation kinetics between 143T and 15T models should be attributed to the significant stabilization of propylene adsorption obtained with the 143T cluster whereas the transition state gains less stabilization than reactant adsorption inside TS-1 pore with dispersion corrections. Therefore, the visible difference on epoxidation barriers between 143T and 15T calculated by B3LYP functional was largely reduced when van der Waals interactions were taken into account with a ωB97XD functional, as observed in Table 2. The RLS for the both models of Ti/defect site at the level of ωB97XD is propylene epoxidation, different from the result acquired on the tripodal(2I) models over which the H2O2 dissociation step has a larger barrier than epoxidation. The reaction energies over the two models of Ti/defect site are -111.2 (114T) and -97.3 (15T) kcal/mol, respectively, more exothermic than those obtained by B3LYP as well as those calculated at the same level of theory over tripodal(2I) models. These results indicate that the confinement effect of zeolite pore and van der Waals interactions also can facilitate the stabilization of PO product inside TS-1 pore.

22


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Fig. 5. Optimized structures of key states involved in propylene epoxidation with H2O2 over 143T Ti/defect model in the stepwise mechanism through the Ti-(η1-OOH)-5MR intermediate. (a) H2O2 adsorption state, Ads_H2O2; (b) transition state for of Ti-hydroperoxy intermediate formation, TS_ Ti-(η1-OOH)-5MR; (c) Ti-hydroperoxy intermediate, Ti-(η1OOH)-5MR; (d) propylene (PL) co-adsorption with Ti-OOH, PL...Ti-(η1-OOH)-5MR); (e) transition state for propylene epoxidation, TS_PO; and (f) the adsorbed propylene oxide (PO) product, Ads_PO. Only the central reaction region within the 143T cluster model is shown for clarity. Distances are in angstroms. (Ti=light grey, C=grey, O=red, Si=green, H= white).

23



Fig. 6. Energy profiles for propylene epoxidation with H2O2 over 143T and 15T Ti/defect models in the stepwise mechanism, calculated at (a) B3LYP/6-31G(d,p) level and (b) ωB97XD/6-31G(d,p) level.

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Table 2. Adsorption energies and energy barriers (kcal/mol) for propylene epoxidation with H2O2 over 143T and 15T Ti/defect models in the stepwise mechanism. Active sites Theory Level Model H2O2 Adsorption Hydroperoxy Formation Propylene Adsorption Epoxidation

B3LYP/6-31G(d,p) 143T 15T -11.1 -16.1 5.6 5.1 -2.5 -2.0 5.5 10.4

Ti/defect ωB97XD/6-31G(d,p) 143T 15T -16.6 -20.0 7.2 6.5 -21.7 -8.7 13.5 13.5

3.1.3 Confinement effect on propylene epoxidation over tripodal(2I) and Ti/defect sites The influence of zeolite confinement on propylene epoxidation with H2O2 was investigated by calculating the stepwise mechanism over tripodal(2I) and Ti/defect sites with 144T and 143T models, and compared to the results obtained based on small cluster models containing 16T and 15T. The results demonstrate that the confinement of zeolite pore indeed influences the adsorption stability and reaction kinetics. When dispersion corrections were considered, significant enhancement of the adsorption stability of propylene was achieved over the 144T tripodal(2I) and 143T Ti/defect sites due to the important contributions from van der Waals interactions between zeolite framework and guest molecules. With regard to reactions, the confinement effect increases the activation barrier for H2O2 dissociation, more pronounced for that occurred over the tripodal(2I) site. While for the epoxidation step, larger reduction of the energy barrier was observed over the tripodal(2I) site when the confinement effect was taken into account. However, it has a small impact on the Ti/defect site, especially when dispersion corrections were included in the calculation by using the ωB97XD method. The calculation results reveal that the Ti/defect site is catalytically more active towards propylene epoxidation with H2O2/TS-1. The dissociation of H2O2 proceeds very fast due to a facile H transfer within the created hydrogen bond network around the Ti/defect site, and thus the rate25



limiting step turns to epoxidation. Furthermore, the TS-1 pores can also stabilize the PO product, as reflected by the highly exothermic reaction energies (relative to small cluster models) obtained by ωB97XD, especially for the Ti/defect site, which shows advantage in both the adsorption strength and kinetics towards propylene epoxidation. Besides the important confinement effect identified for propylene epoxidation with H2O2 over TS-1 catalyst, it is also discerned that the long-range dispersion corrections also play a crucial role in describing the adsorption and reaction properties of guest molecules inside zeolite pores. 3.2 Solvent effect on propylene epoxidation It is well known that the solvent has a crucial impact on propylene epoxidation with H2O2 over TS-1 catalysts. The experimental studies by Wu et al. showed that methanol exhibits more significant promotion on the epoxidation activity than other solvents examined, especially superior to the aprotic solvent such as acetonitrile and tetrahydrofuran. Based on the studies on various aspects of solvent effect, it was found that the steric effect, electronic influence, as well as polarity mainly contribute to solvent effect on epoxidation reactivity.22 Methanol is commonly used as solvent for propylene epoxidation with H2O2 over TS-1 catalysts.9-10, 13, 22, 64-66

In this section, we investigated the effect of methanol solvent on propylene epoxidation

mechanisms and kinetics, by including an explicit methanol molecule into the catalytic system. To uncover the effect of the nature of solvent, the epoxidation property of propylene with H2O2 over TS-1 was also investigated in the presence of an aprotic acetonitrile solvent molecule, and compared with the results from the protic methanol solvent.

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H

H2O2+CH3OH

Ti O

O

Si

Si

O H

O

O

Si

Si

Si

H 3C

H2 C O

H H O

H H

O

Si

H

H2C

O

O

H Si

Si O H

C3H6 Adsorption

Si

O

H2 C O

Si H

H2 C

O

O

Si

Si

Si

O

O H

TiOOH Formation

O

H

Si

O

O

O

+

O Si

Si O

O Si

Si

PO Adsorption

CH3

O H

H

Tripodal (2I) Site Recovery

Scheme 1. Mechanistic illustration of propylene epoxidation with H2O2 over the 144T tripodal(2I) model in the stepwise mechanism with a methanol molecule included as explicit solvent. The mechanistic illustration of propylene epoxidation with H2O2 over the 144T tripodal(2I) model in the stepwise mechanism by including an explicit methanol molecule as solvent is provided in Scheme 1, and optimized geometries of all states involved in the reaction network are shown in Fig. 7, with relevant structural parameters summarized in Table S4. Similarly, the epoxidation occurs via two transition states, H2O2 dissociation and propylene epoxidation, however, the methanol molecule direct participation in H2O2 dissociation process through Htransfer alters the active intermediate and kinetics. As shown in Fig. 7a (Ads_H2O2_Me), the methanol and hydrogen peroxide molecules are co-adsorbed above the Ti center by forming a hydrogen-bond network together with the Ti-OH group. The methanol molecule forms hydrogen bonds with both the Ti-OH group and the H2O2, and the co-adsorption complex is stabilized in a 7MR configuration. The interatomic distances of Hα-Om and Hm-O1 are 1.70 and 1.71 Å, respectively, and the H2O2 coordinates to the Ti center via Oβ with the Ti-Oβ distance of 2.49 Å. Fig. 7b illustrates the transition state (TS_ Ti-(η2-OOH)-6MR_Me) produced in 27


H O

CH2 O HC

Ti

O Si

H

Si

H

TiOH...O-C3H6 (ts_2)

CH3

O

Ti O

Si

CH

C H

CH3

H

O

O Si

Si

O

H

H

O

Si

Ti

O

H2O2 Dissociation (ts_1)

H3C

H

O Ti

O

Si

Si

O

H

CH3

CH

O

H

O

O

H

CH3 H3C

H

H

O

H

Ti

Si

H

Si O

CH3OH and H2O2 Co-adsorption

O

Ti

O

Si

O

C H

O

O

O

O

H

Tripodal(2I) Site

O

H

O

Si

H

O

Ti O

H

H

O

O

Si

H

H

O

O

H O

O

Si

H 3C

H3C

H3C

CH3OH


formation of hydroperoxy intermediate from H2O2 dissociation, in which the Hα transfer from H2O2 to methanol concurrently occurs by the transfer of Hm of methanol to the O1 atom of TiOH group. The Hα-Oα and Hm-Om distances elongate to 1.28 and 1.11 Å while the Hα-Om and Hm-O1 distances contract to 1.15 and 1.31 Å, respectively, indicating that the methanol molecule mediates the H transfer between the Ti-OH group and H2O2. This step leads to the transformation of a hydrogen hydroxide molecule into a hydroperoxy intermediate and the transformation of the Ti-OH group into a water molecule that coordinates to the Ti center. The Ti-Oα and Ti-Oβ distances contract to 2.15 and 2.30 Å in the TS from 3.58 and 2.49 Å in the adsorption complex while the Ti-O1 bond length elongates to 1.95 from 1.84 Å. Fig. 7c shows the optimized geometry of the formed Ti-OOH intermediate (Ti-(η2-OOH)-6MR_Me) in which the Hα-Oα and Hm-Om bonds are completely cleaved (1.92 and 1.65 Å, respectively) while the new Hα-Om and Hm-O1 bonds are formed with bond lengths being 0.98 and 1.00 Å, respectively. The produced water molecule coordinates to the Ti center at a distance of 2.11 Å which then becomes 6-coordianted after H2O2 dissociation completion. The methanol molecule remains adsorbed around via H-bonding with both the -OOH intermediate and the produced H2O, and therefore, the active intermediate species is a six-membered ring TiOOH-(H2O, CH3OH) that assembled in a six-membered ring configuration, as shown in Fig. 7c. This structure identification agrees with the mechanism proposed by Sever and Root in which the protic solvent forms hydrogen bonds with the Ti-OH group and H2O2 molecule, and thus the solvent molecules promote proton transfer from H2O2 to form the Ti-OOH intermediate by participating in the reaction as proton transfer medium.29 Fig. 7d illustrates the co-adsorption complex upon addition of a propylene molecule (PL...Ti-(η2-OOH)-6MR_Me), the 28


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interatomic distance between Oα and C1 of which is calculated to be 4.31 Å, indicating a significant interaction between H2O2 and propylene upon co-adsorption. The methanol molecule still forms H-bonds with both the Ti-OOH and H2O, with the Om-Hm and Hα-Oα interatomic distances of 1.66 and 1.94 Å, almost equal to those (1.65 and 1.92 Å) calculated in the Ti-(η2-OOH)-6MR_Me intermediate. To form the second transition state (Fig.7e, TS_PO_Me), the propylene epoxidation step occurs by Oα atom attacking the C=C double bond, and the interatomic distances of C1-Oα and C2-Oα become 2.20 and 2.18 Å, respectively, in the TS configuration. The Ti-Oα bond length is elongated to 2.07 from 1.97 Å while the Ti-Oβ bond length is contracted to 2.00 in the TS from 2.20 Å in the co-adsorption complex, indicating that the cleavage of Ti-Oα bond occurs simultaneously with the formation of Ti-OβHβ group. In this TS formation process, the methanol molecule remains adsorbed nearby, with the Om-Hm and Hα-Oα distances change to 1.75 and 1.82 Å, respectively. In the meantime, the O1-Ti distance is elongated to 2.23 Å in the TS from 2.11 Å in the co-adsorbed complex. Both the H-bond structural parameters and Ti-H2O coordination distance indicate that the stability gained in transition state is less than that in reactant adsorption, which would result in an increased epoxidation barrier with methanol inclusion. The final state within the path is the adsorbed propylene oxide (Fig. 7f, Ads_PO_Me), which is adsorbed nearby via H-bonding with the OmHα group of methanol by Oα atom and hence the tripodal(2I) site is regenerated for next catalytic cycle. It turns out that the methanol molecule does not take part in the epoxidation step, which acts as a solvent to stabilize both the co-adsorption complex as well as the transition state. In the process of identifying the most stable co-adsorption configuration of CH3OH with H2O2, we did a careful searching and screening of possible adsorption sites and places for 29



CH3OH molecule. Several stational points were found, as illustrated in Fig. S3. The adsorption energies of these configurations of CH3OH-H2O2 were given in Table S5, from which it was found that all these structures are less stable than the one discussed above in Fig. 7(a). Therefore, we further calculated the reaction energy diagram based on the most stable adsorption structure of CH3OH-H2O2.

Fig. 7. Optimized structures of key states involved in propylene epoxidation with H2O2 over 144T tripodal(2I) model in the stepwise mechanism by including a methanol molecule as explicit protic solvent. (a) CH3OH and H2O2 co-adsorption complex, Ads_H2O2_Me; (b) transition state for Ti-hydroperoxy intermediate formation, TS_ Ti-(η2-OOH)-6MR_Me; (c) Ti-hydroperoxy intermediate, Ti-(η2-OOH)-6MR_Me; (d) propylene (PL) co-adsorption with Ti-OOH and methanol, PL...Ti-(η2-OOH)-6MR_Me; (e) transition state for propylene epoxidation, TS_PO_Me; and (f) the adsorbed propylene oxide (PO) product, Ads_PO_Me. Only the central reaction region within the 144T cluster model is shown for clarity. Distances are in angstroms. (Ti=light grey, C=grey, O=red, Si=green, H= white).

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Energy profiles for propylene epoxidation with H2O2 over the 144T cluster model of tripodal(2I) with and without methanol solvent are illustrated in Fig. 8, calculated at the B3LYP and ωB97XD levels of theory. Table 3 compares the adsorption energies and activation energies computed by using the two computational approaches for systems with and without methanol solvent. For the adsorption of H2O2, including the methanol solvent significantly enhances the adsorption stability, for the both cases without and with dispersion corrections. Moreover, the energy barriers for Ti-OOH formation from H2O2 dissociation are largely reduced by ~13 kcal/mol in the presence of methanol, calculated by B3LYP and ωB97XD, due to the direct participation of methanol in H-transfer process and the formation of H-bond network with the Ti-OH group and H2O2 molecule. This result is in good agreement with the finding by Sever and Root that a protic molecule as a bridging medium lowers the activation energy for formation of hydroperoxy intermediate by 5~6 kcal/mol through facilitating the proton transfer between the hydroxyl group and H2O2 molecule.29 However, in their study, a small Ti(OH)4 cluster was used to model the titanium-based epoxidation catalyst which does not include the confinement effect and van der Waals interactions which were found to be important to capture the adsorption and reaction properties inside zeolite pores. In our study, we found more significant reduction of the barrier for H2O2 dissociation in the presence of methanol solvent over the 144T cluster model that containing the main pore structures of TS1. The presence of methanol has a slight impact on propylene adsorption, but increases the barrier for epoxidation step by ~5 kcal/mol with the two levels of theory in calculation, as observed in Table 3. This negative effect of solvent may due to the steric effect of methanol molecule in the co-adsorption complex that can impair the stability of transition state formed 31



for propylene epoxidation within the TS-1 pore. In addition, the competitive adsorption of methanol molecule with propylene nearby the Ti-OOH active intermediate species could also impact the epoxidation kinetics. As demonstrated in the study by Shin and Chadwick, high concentration

of methanol in the solvent mixture impedes epoxidation reaction by the

competitive adsorption with reactant molecules at the active sites,13 consistent with the observation in this work. Due to the significant facilitation on H2O2 dissociation in the presence of methanol solvent, the rate-limiting step changes to epoxidation from H2O2 dissociation under solvent-free condition. The kinetics for propylene epoxidation with H2O2 is greatly promoted with solvent inclusion. These calculation results clearly demonstrate the crucial role of methanol solvent, not only enhancing the adsorption of guest molecules but also speeding up the kinetics for propylene epoxidation with H2O2 over TS-1 catalyst. To confirm the energy diagram identified was along the minimum energy pathway, we also calculated the energy barriers for H2O2 dissociation step with other two adsorption structures of CH3OH-H2O2 complex (Fig. S3: S#4 and S#6) and considered the CH3OH molecule acts only as a solvent without participating in H2O2 dissociation process. Optimized geometries of the initial, transition, and final states associated with these pathways are illustrated in Fig. S4 and Table S5 gives the activation barriers calculated with B3LYP and ωB97XD functionals. It shows that the dissociation barriers are much higher than the path with CH3OH directly participates in the H-transfer process for H2O2 dissociation discussed above. Therefore, the CH3OH molecule can promote the dissociation of H2O2 by forming H-bond network and taking part in the reaction through H-transfer, which is identified as the kinetically most favorable pathway.

32


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In the presence of an aprotic acetonitrile solvent molecule, the interaction between H2O2 and acetonitrile is not significant, as observed in Fig. 9(a). The N-Hα distance is calculated to be 3.61 Å and the adsorption energies of CH3CN-H2O2 calculated with the two computational functionals are stronger than those obtained on TS-1 without solvent but weaker than the case with CH3OH solvent, as observed in Table 3 for comparison. For H2O2 dissociation step, the acetonitrile molecule does not take part in the reaction and the calculated dissociation barriers of H2O2 are ~2 kcal/mol increased as compared to the case without solvent. Likewise, adding acetonitrile solvent does influence subsequent epoxidation step substantially, only increasing the barriers by 2.5 and 1.0 kcal/mol, calculated with B3LYP and ωB97XD functionals (Table 3). The overall reaction is still limited by the H2O2 dissociation step, same to the case without solvent. These calculation results demonstrate that the presence of an aprotic acetonitrile solvent does not change the mechanism for propylene epoxidation reaction nor facilitate the kinetics. Therefore, these two types of solvent exhibit different roles towards propylene epoxidation with H2O2 over TS-1, for which the presence of methanol can participate in the reaction and promote H2O2 dissociation significantly.

33



Fig. 8. Energy profiles for propylene epoxidation with H2O2 over the 144T tripodal(2I) model in the stepwise mechanism with and without methanol solvent, calculated at (a) B3LYP/6-31G(d,p) level and (b) ωB97XD/6-31G(d,p) level.

34


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Fig. 9. Optimized structures of key states involved in propylene epoxidation with H2O2 over 144T tripodal(2I) model in the stepwise mechanism by including an acetonitrile molecule as explicit aprotic solvent. (a) CH3CN and H2O2 co-adsorption complex, Ads_H2O2_Ac; (b) transition state for Ti-hydroperoxy intermediate formation, TS_ Ti-(η2OOH)_Ac; (c) Ti-hydroperoxy intermediate, Ti-(η2-OOH)_Ac; (d) propylene (PL) coadsorption with Ti-OOH and acetonitrile, PL...Ti-(η2-OOH)_Ac; (e) transition state for propylene epoxidation, TS_PO_Ac; and (f) the adsorbed propylene oxide (PO) product, Ads_PO_Ac. Only the central reaction region within the 144T cluster model is shown for clarity. Distances are in angstroms. (Ti=light grey, C=grey, O=red, Si=green, H= white, N=blue).

35



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Table 3. Adsorption energies and energy barriers (kcal/mol) for propylene epoxidation with H2O2 over the 144T tripodal(2I) model in the stepwise mechanism with and without methanol solvent, as well as the case with an acetonitrile solvent molecule. Active sites Tripodal(2I) Theory Level B3LYP/6-31G(d,p) ωB97XD/6-31G(d,p) Model 144T 144T_Me 144T_Ac 144T 144T_Me 144T_Ac H2O2 Adsorption -11.1 -20.8 -14.0 -16.7 -37.9 -32.3. Hydroperoxy 24.2 11.4 26.2 23.8 10.0 26.2 Formation Propylene Adsorption -1.1 -0.1 -1.4 -16.6 -16.3 -9.1 Epoxidation 6.7 12.2 9.2 12.6 17.9 13.6

4. Conclusions The confinement effect from zeolite pores and solvent effect play important roles in manipulating the chemical reactivity, reaction pathways, and kinetics. In this work, density functional theory calculations have been conducted to investigate the effects of zeolite confinement and solvent on propylene epoxidation with H2O2 over TS-1 catalyst. We find that the confinement of zeolite pore channel not only influences the adsorption stability of guest molecules but also alters reaction barriers and kinetics, as compared to the results obtained based on small cluster models without pore structures. When dispersion corrections are considered, substantial enhancement of the adsorption stability of guest molecules is observed over the 144T tripodal(2I) and 143T Ti/defect sites due to the important contributions from van der Waals interactions between zeolite framework and guest molecules, especially for propylene adsorption. With respect to reactions, the confinement effect of TS-1 increases the activation barrier for H2O2 dissociation, more pronounced for that occurs over the tripodal(2I) site. While for the epoxidation step, a visible lowering of the activation barrier is obtained over the 144T tripodal(2I) site by considering the confinement effect but it has a small impact for the 143T Ti/defect site, especially when dispersion corrections are included in the calculation by using the ωB97XD method. 36


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To probe the role of solvent in propylene epoxidation with H2O2, an explicit protic methanol molecule is introduced to the 144T tripodal(2I) model of TS-1, based on which a significant enhancement on CH3OH-H2O2 co-adsorption is observed due to the H-bond interactions within the system. Moreover, the energy barriers for Ti-OOH formation from H2O2 dissociation are largely reduced by ~13 kcal/mol, calculated by B3LYP and ωB97XD levels of theory, due to the direct participation of methanol molecule in H-transfer process and the formation of H-bond network with the Ti-OH group and H2O2 molecule. Nevertheless, the presence of methanol has a slight impact on propylene adsorption, and leads to an increase of the energy barrier for the epoxidation step by ~5 kcal/mol. This may be attributed to the steric effect of methanol molecule in co-adsorption complex that impairs the stability of transition state formed for propylene epoxidation within TS-1 pore. Due to the large facilitation on H2O2 dissociation in the presence of methanol solvent, the rate-limiting step changes to epoxidation from H2O2 dissociation under solvent-free condition, and that the reaction kinetics can be accelerated with methanol solvent inclusion. By comparison, adding an aprotic acetonitrile solvent does not change the reaction mechanism nor facilitate the kinetics for propylene epoxidation with H2O2, exhibiting a different role as compared to the protic CH3OH solvent. The calculation results in this work clearly demonstrate the important roles of zeolite confinement and solvent effect on propylene epoxidation with H2O2 over TS-1 catalyst, which not only can impact the adsorption stability of reactants, intermediates, transition states, and products, but also alter reaction pathways and kinetics. Besides, the long-range dispersion corrections also play a crucial role in describing the adsorption and reaction properties of guest molecules inside zeolite pores.

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Author information Corresponding Author *E-mail: [email protected] (X.W.N.) *E-mail: [email protected] (X.W.G.) Notes The authors declare no competing financial interest.

Associated context Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Schematic illustrations of formation of hydrolyzed and defective TS-1 models from the tetrahedral framework Ti sites in TS-1. Structural illustrations of the benchmark 16T and 15T cluster models of TS-1 used for comparative discussions. Other configurations identified for the co-adsorption of CH3OH-H2O2 complex. All states associated with H2O2 dissociation step along other pathways in the presence of CH3OH solvent at different adsorption sites. Key structural parameters associated with propylene epoxidation with H2O2 over various TS-1 models. The XYZ coordinates for each structure involved in this work.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (grant number 21872012), the National Key Research and Development Program of China 38


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(grant number 2016YFB0301704), and the Fundamental Research Funds for the Central Universities (grant number DUT18LK20). We acknowledge the Supercomputing Center of Dalian University of Technology for providing the computational resources for this work.

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