Propylene Epoxidation Research Articles

Modulating Ti coordination environment in Ti-containing materials by sulfation for synergizing with Au sites to facilitate propylene epoxidation

Modulating the coordination environment of Ti sites in Ti-containing materials to synergize with Au sites is a crucial issue to boost the process of propylene epoxidation with H2 and O2 to produce propylene oxide (i.e., HOPO), but still remains a great challenge. Herein, we report a facile strategy to tailor the coordination states and electronic properties of Ti sites. The sulfur-decorated Ti-SiO2 (i.e., Ti sites anchored onto silica) with moderate sulfur loading (Ti-SiO2-S-M) can synergize well with Au sites for the HOPO process with high activity, propylene oxide (PO) selectivity, and H2 efficiency, which are superior to those achieved on the referenced Ti-SiO2 synergizing with Au sites. Multiple characterizations, in-situ diffuse reflectance infrared Fourier transform spectroscopy of propylene and PO, PO conversion experiment, and kinetics behaviors demonstrate that the superior advantages of Ti-SiO2-S-M against Ti-SiO2 result from the increased Ti binding energy, improved hydrophobicity, and weakened acidity, which simultaneously facilitate the adsorption of propylene and promote the desorption of PO. This insight reported here could guide the rational design and optimization of Ti-containing materials to synergize with Au catalysts for propylene epoxidation.

Characterization of Fresh, Deactivated, and Regenerated TS‐1 for Propylene Epoxidation: Investigating Surface Deactivation Species

AbstractTitanium silicalite‐1 (TS‐1) serves as an effective catalyst for propylene epoxidation in the hydrogen peroxide propylene oxide (HPPO) process, attracting wide attention from researchers and industry. Nonetheless, the TS‐1 catalyst experienced inevitable deactivation during industrial applications, which adversely impacts its lifespan. To enhance reaction stability of catalyst, the present work conducted a systematic investigation into the deactivation reason of the TS‐1 catalyst. The fresh, deactivated, and regenerated catalysts were characterized by using various analytical techniques to compare their physicochemical properties. The surface coke species of the deactivated catalysts were extracted by the Soxhlet extraction method and subsequently analyzed using GC–MS. Additionally, the pathways for by‐product formation were simulated through Gauss software. The results indicated that the so‐called coking deposits formed during the reaction predominantly consist of propylene glycol, ethers, and oligomers. These coke species covered the partial active sites on the catalyst surface, blocked the pores of the catalyst, and accordingly led to the deactivation of the catalyst, although the crystalline structure of the catalyst hardly changed after deactivation. These results provide a theoretical basis and novel insights for the rational design and development of efficient catalysts for the HPPO process.

Spatial‐coupled Ampere‐level Electrochemical Propylene Epoxidation over RuO2/Ti Hollow‐fiber Penetration Electrodes

AbstractThe electrochemical propylene epoxidation reaction (PER) provides a promising route for ecofriendly propylene oxide (PO) production, instantly generating active halogen/oxygen species to alleviate chloride contamination inherent in traditional PER. However, the complex processes and unsatisfactory PO yield for current electrochemical PER falls short of meeting industrial application requirements. Herein, a spatial‐coupling strategy over RuO2/Ti hollow‐fiber penetration electrode (HPE) is adopted to facilitate efficient PO production, significantly improving PER performance to ampere level (achieving over 80 % PO faradaic efficiency and a maximum PO current density of 859 mA cm−2). The synergetic combination of the penetration effect of HPE and the spatial‐coupled reaction sequence, enables the realization of ampere‐level PO production with high specificity, exhibiting significant potentials for economically viable PER applications.

Spatial-coupled Ampere-level Electrochemical Propylene Epoxidation over RuO2/Ti Hollow-fiber Penetration Electrodes.

The electrochemical propylene epoxidation reaction (PER) provides a promising route for ecofriendly propylene oxide (PO) production, instantly generating active halogen/oxygen species to alleviate chloride contamination inherent in traditional PER. However, the complex processes and unsatisfactory PO yield for current electrochemical PER falls short of meeting industrial application requirements. Herein, a spatial-coupling strategy over RuO2/Ti hollow-fiber penetration electrode (HPE) is adopted to facilitate efficient PO production, significantly improving PER performance to ampere level (achieving over 80 % PO faradaic efficiency and a maximum PO current density of 859 mA cm-2). The synergetic combination of the penetration effect of HPE and the spatial-coupled reaction sequence, enables the realization of ampere-level PO production with high specificity, exhibiting significant potentials for economically viable PER applications.

Insights into Cl effect for propylene epoxidation over Ag2O(111) surface: A periodic density functional theory study

Insights into Cl effect for propylene epoxidation over Ag2O(111) surface: A periodic density functional theory study

Isotopic Studies of Reaction Pathways within Propylene Epoxidation over Promoted Silver Catalysts

Isotopic Studies of Reaction Pathways within Propylene Epoxidation over Promoted Silver Catalysts

Direct propylene epoxidation with molecular oxygen over titanosilicate zeolites.

The direct epoxidation of propylene with molecular oxygen represents a desired route for propylene oxide (PO) production with 100% theoretical atomic economy. However, this aerobic epoxidation reaction suffers from the apparent trade-off between propylene conversion and PO selectivity, and remains a key challenge in catalysis. We report that Ti-Beta zeolites containing isolated framework Ti species can efficiently catalyze the aerobic epoxidation of propylene. Stable propylene conversion of 25% and PO selectivity of up to 90% are achieved at the same time, matching the levels of industrial ethylene aerobic epoxidation processes. H-terminated pentacoordinated Ti species in Beta zeolite frameworks are identified as the preferred active sites for propylene aerobic epoxidation and the reaction is initiated by the participation of lattice oxygen in Ti-OH. These results are expected to spark new technology for the industrial production of PO toward more sustainable chemistry and chemical engineering.

Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation

Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation

Enhanced catalytic performance for gas phase epoxidation of propylene with H2 and O2 by multiple pretreated Au/TS-1 catalysts

Direct epoxidation of propylene (C3H6) with hydrogen (H2) and oxygen (O2) over Au/TS-1 catalyst was promising way for the synthesis of propylene oxide (PO). However, high selectivity of PO (>95 %) and conversion of C3H6 (>5 %) cannot be achieved at the same time. The industrialization process was severely restricted. In this paper, Au/TS-1 catalysts were synthesized by deposition precipitation method with alkali metal promoter. And alkaline gases and silane coupling post-treatment were performed to promote the catalytic activity. The prepared samples exhibited a much better C3H6 conversion (6.5 %) and PO selectivity (95.1 %) compared with general catalyst using normal synthetic technology. Cs2CO3 was beneficial to increasing Au nanoparticles uptake efficiency. Both NH3 and triethoxyvinylsilane post-treatment were good for smaller and more uniform loading of Au nanoparticles. The decreasing in surface hydroxyl density effectively alleviated the occurrence of PO ring opening isomerization. The increased hydrophobicity of TS-1 surface was conducive to the rapid desorption of generated PO and significantly increased PO selectivity. Multiple pretreatment was also favorable for extending the lifetime of Au/TS-1 catalyst.

Theoretical Investigation of Propylene Epoxidation Using H2 and O2 Over Titanosilicate-Supported Au Catalysts

Titanosilicate-supported Au-cluster catalysts can be used to selectively synthesize propylene oxide from propylene using O2 and H2. However, the details of the catalytic reaction mechanism have not yet been elucidated. Thus, the reaction mechanism was investigated using density functional theory calculations. The calculation results revealed that active Ti-OOH forms on the surface Ti site, which is active as an oxidant and acts as an anchorage site for Au nanoclusters. The rate-determining step of propylene oxide synthesis on Au/titanosilicate is O insertion into propylene, with an activation energy of 1.37 eV. The propylene involved in this reaction is activated by adsorption on Au nanoclusters. Moreover, it was also found that the formation of Ti-OOH on Au/titanosilicate requires an activation energy of 0.48 eV, while it is barrierless on Au/anatase-TiO2. However, the decomposition energy of Ti-OOH on Au/titanosilicate is −0.16 eV, which is smaller than that on Au/anatase-TiO2 (−1.12 eV). The results indicate that Ti-OOH decomposes more readily on Au/titanosilicate than on Au/anatase-TiO2 but is easily regenerated because the reaction energy is significantly smaller than that on Au/anatase-TiO2. Therefore, these calculations are qualitatively in good agreement with the experimental results for Au/titanosilicate, which exhibited high catalytic activity at high temperatures.Graphical

Intracrystalline diffusion regulation of Au/hierarchical TS-1 for simultaneously enhanced stability and selectivity in propene epoxidation with H2 and O2

Regulation of intracrystalline diffusion plays a pivotal role in ensuring the catalytic stability of metal-supported zeolite catalysts. Herein, we successfully achieved precise modulation of intracrystalline diffusion in Au-loaded hierarchical TS-1 catalysts by two-step recrystallization approach. This modulation was achieved by manipulating the length of mesoporogens, characterized by the hierarchical factor (HF). The HF could be used as the descriptor to reveal the reaction stability and deactivation process including blocking degree (VAu: gold volume / Vna: volume not accessible for N2) by the Au nanoparticles and coke formation in fresh and used catalysts, respectively. In addition, the enhanced selectivity was attributed to the improved hydrophobicity of Au/hierarchical TS-1 with gradually decreased terminal silanols by two-step recrystallization approach. This work not only provides the new strategy for the regulation of zeolite intracrystalline diffusion, but also could shed new lights on the industrial process of direct propene epoxidation with H2 and O2.

The fabrication of potassium ion modification on Cu2O(111) for enhanced propylene oxide selectivity: Insights from density functional theory

Utilizing the alkali metal effect to enhance catalyst performance is an effective approach in the development of highly efficient catalysts. Herein, we report a K+ ion-modification strategy on Cu-based catalysts for the direct epoxidation of propylene (DEP) using molecular oxygen, a reaction deemed as one of the “dream reactions” in heterogeneous catalysis. By elaborately engineering of K+-modified Cu2O(111) catalyst, we unveil the versatile roles played by K+ ions in promoting catalytic performance for DEP. Firstly, K+ ions stabilize the surface coordination structures of Cu2O(111). Secondly, K+ ions make the dissociation of adsorbed O2* less likely, thus facilitating the reaction of propylene with O2*. Furthermore, both K+ and two-coordinated Cu+ serve as active centers in this reaction. Lastly, K+ ions create unique surface coordination structures favorable for propylene oxide (PO) formation. The density functional theory (DFT) calculations demonstrate that the effective utilization of the alkali metal effect can engineer highly efficient Cu-based catalysts for DEP.

Low Concentration Template Preparation of Microsized Au/TS-1 as an Efficient Catalyst for Gas-Phase Propylene Epoxidation with H2 and O2

Low Concentration Template Preparation of Microsized Au/TS-1 as an Efficient Catalyst for Gas-Phase Propylene Epoxidation with H₂ and O₂

Influence of Ti-incorporated Zeolite Topology and Pore Condensation on Vapor Phase Propylene Epoxidation Kinetics with Gaseous H2O2.

Vapor-phase propylene (C3H6) epoxidation kinetics with hydrogen peroxide (H2O2) strongly reflects the physical properties of Ti-incorporated zeolite catalysts and the presence of spectating molecules ("solvent") near active sites even without a bulk liquid phase. Steady-state turnover rates of C3H6 epoxidation and product selectivities vary by orders of magnitudes, depending on the zeolite silanol ((SiOH)x) density, pore topology (MFI, *BEA, FAU), and the quantity of condensed acetonitrile (CH3CN) molecules nearby active sites, under identical reaction mechanisms sharing activated H2O2 intermediates on Ti surfaces. Individual kinetic analyses for propylene oxide (PO) ring-opening, homogeneous diol oxidative cleavage, and homogeneous aldehyde oxidation reveal that secondary reaction kinetics following C3H6 epoxidation responds more sensitively to the changes in zeolite physical properties and pore condensation with CH3CN. Thus, higher PO selectivities achieved in hydrophilic Ti-MFI at steady-state reflect the preferential stabilization of transition states for C3H6 epoxidation (a primary reaction) relative to PO ring-opening and oxidative cleavage (secondary reactions) that solvation effects that reflect interactions among condensed CH3CN within pores and the extended pore structure.

Influence of Ti‐incorporated Zeolite Topology and Pore Condensation on Vapor Phase Propylene Epoxidation Kinetics with Gaseous H2O2

AbstractVapor‐phase propylene (C3H6) epoxidation kinetics with hydrogen peroxide (H2O2) strongly reflects the physical properties of Ti‐incorporated zeolite catalysts and the presence of spectating molecules (“solvent”) near active sites even without a bulk liquid phase. Steady‐state turnover rates of C3H6 epoxidation and product selectivities vary by orders of magnitudes, depending on the zeolite silanol ((SiOH)x) density, pore topology (MFI, *BEA, FAU), and the quantity of condensed acetonitrile (CH3CN) molecules nearby active sites, under identical reaction mechanisms sharing activated H2O2 intermediates on Ti surfaces. Individual kinetic analyses for propylene oxide (PO) ring‐opening, homogeneous diol oxidative cleavage, and homogeneous aldehyde oxidation reveal that secondary reaction kinetics following C3H6 epoxidation responds more sensitively to the changes in zeolite physical properties and pore condensation with CH3CN. Thus, higher PO selectivities achieved in hydrophilic Ti‐MFI at steady‐state reflect the preferential stabilization of transition states for C3H6 epoxidation (a primary reaction) relative to PO ring‐opening and oxidative cleavage (secondary reactions) that solvation effects that reflect interactions among condensed CH3CN within pores and the extended pore structure.

Efficient formation of propylene oxide under low hydrogen concentration in propylene epoxidation over Au nanoparticles supported on V-doped TS-1

The direct epoxidation of propylene to propylene oxide (PO) represents a unique feature of gold nanoparticle catalysts. Different amounts of vanadium were introduced into TS-1 using a hydrothermal method to give V-TS-1, and the effect of various vanadium loadings on the catalytic performance of Au/V-TS-1 was investigated in the epoxidation of propylene under different reaction conditions. The obtained characterization results indicate that the molecular sieve pore structure and crystalline phase structure of TS-1 were maintained after vanadium incorporation, and vanadium was incorporated into the molecular sieve framework. The introduction of an appropriate amount of vanadium resulted in the homogeneous dispersion of gold particles with a small average particle size, which facilitated the epoxidation reaction. When V was introduced into TS-1, the propylene conversion decreased under the condition of 10 % H2 concentration (C3H6/H2 = 1/1) in the reaction atmosphere, but the PO selectivity was maintained. Interestingly, when the H2 concentration in the reaction atmosphere was lowered to 2 % (C3H6/H2 = 3/1), the propylene conversion in Au/TS-1 was significantly decreased compared to 10 % H2 concentration condition, while in Au/V-TS-1, the propylene conversion was maintained in 2 % H2 concentration condition compared to 10 % H2. Au/V-TS-1 also maintained its selectivity for PO even when the H2 concentration was lowered. Therefore, under low hydrogen concentration conditions, the PO formation rate of Au/V-TS-1 was much higher than that of Au/TS-1. More, the same trend was observed when Mo was added in place of V. Our strategy will guide the design of future catalysts as a way to utilize hydrogen more effectively while maintaining PO productivity.

Post-treatment of Ti-MWW zeolite with potassium fluoride for propylene epoxidation

Post-treatment of Ti-MWW zeolite with potassium fluoride for propylene epoxidation

Aliphatic Olefin Epoxidation with Hydrogen Peroxide Catalyzed by an Integrated Mn/TS-1/N System

AbstractPropylene liquid-phase epoxidation with 50–75% H2O2 is an important process for the industrial production of propylene oxide (PO). To realize a propylene epoxidation process that proceeds with low hydrogen peroxide concentration, we developed an integrated Mn/TS-1/N catalytic system via in-situ reaction of Mn/TS-1 with an N-donor ligand, affording the PO product in excellent yield with only 30 wt% H2O2. Other long-chain aliphatic epoxides were also readily synthesized by this catalytic epoxidation system. Moreover, in addition to the standard micro-pressure reactor, a continuous-flow microreactor was developed that executed the hydrogen peroxide propylene oxide (HPPO) process with excellent efficiency for 1300 hours. This innovative Mn/TS-1/N catalyzed epoxidation represents a promising direction for advancing HPPO industrial processes, offering improved efficiency while minimizing the reliance on high concentrations of H2O2.

Synergizing tetra-coordinated Ti sites anchored onto micropore-free silica with Au sites to boost propylene hydro-oxidation

Constructing tetra-coordinated Ti sites on the micropore-free oxide surface as a substitute for the Ti-containing zeolites to synergize with Au sites for propylene epoxidation with H2 and O2 to propylene oxide (HOPO) can solve the problems resulting from micropores but remains a great challenge. Here, we report a facile strategy to anchor tetra-coordinated Ti sites onto micropore-free silica by precisely cleaving the surface hydroxyl and siloxane bridges of silica to generate under-coordinated O sites to bind Ti sites. Multiple spectroscopic characterizations, including ultraviolet Raman, UV–vis and Fourier transformation infrared and 29Si solid nuclear magnetic resonance spectroscopy, demonstrate the anchored Ti sites on the surface of silica are mainly the target tetra-coordinated ones. It is further unveiled that the removal of surface hydroxyl also improves the hydrophobicity of synthesized Ti-SiO2, which enhances the desorption of PO. Benefitting from the weak acidic properties, high hydrophobicity and micropore-free structures, the tetra-coordinated Ti sites on silica synergize well with Au sites in propylene epoxidation, demonstrating high activity, selectivity and stability, which are superior to those achieved on the most extensively researched Au/TS-1 catalysts.

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

The development of sustainable and anti-poisoning single-atom catalysts (SACs) is essential for advancing their research from laboratory to industry. Here, we present a proof-of-concept study on the poisoning of Au SACs, and the antidote of Au nanoparticles (NPs), with trace addition shown to reinforce and sustain propylene epoxidation. Multiple characterizations, kinetics investigations, and multiscale simulations reveal that Au SACs display remarkable epoxidation activity at a low propylene coverage, but become poisoned at higher coverages. Interestingly, Au NPs can synergistically cooperate with Au SACs by providing distinct active sites required for H2/O2 and C3H6 activations, as well as hydroperoxyl radical to restore poisoned SACs. The difference in reaction order between C3H6 and H2 (nC3H6-nH2) is identified as the descriptor for establishing the volcano curves, which can be fine-tuned by the intimacy and composition of SACs and NPs to achieve a rate-matching scenario for the formation, transfer, and consumption of hydroperoxyl. Consequently, only trace addition of Au NPs antidote (0.3% ratio of SACs) stimulates significant improvements in propylene oxide formation rate, selectivity, and H2 efficiency compared to SACs alone, offering a 56-fold, 3-fold, and 22-fold increase, respectively, whose performances can be maintained for 150 h.