Catalysts For 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.

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.

Fabrication of gold nanoparticles supported on hollow microsphere of nanosized TS-1 for the epoxidation of propylene with H2 and O2

The TS-1 microsphere was facilely prepared via a direct spray drying of the mixture containing the nanocrystal TS-1 and mother liquor. The calcined TS-1 zeolites possess a hollow structure and numerous intercrystalline mesopores, which may facilitate the diffusion properties. Au nanoparticles were loaded on TS-1 support by utilizing a deposition–precipitation process to prepare the Au/TS-1-M catalysts for propylene epoxidation in the presence of H2 and O2 conditions. The effects of the Ti and Au loadings on the structure, porosity, morphology, and catalytic performances of the Au/TS-1 catalysts have been investigated by various techniques. Under the optimized synthesis conditions (Si/Ti molar ratio = 57, 0.49 wt% Au), a propylene conversion of 6.1 % and a PO selectivity of 91.7 % were obtained on the Au/TS-1 catalyst. A PO formation rate of 186.8 gPO·kgcat.−1·h−1 was achieved at the 20th hour.

Supported AgCuCl Nanoclusters as Bimetal Catalysts for Propylene Epoxidation with Molecular Oxygen

Direct epoxidation of propylene (DEP) into propylene oxide using molecular oxygen is a promising approach due to high atom utilization and environmental sustainability. However, the absence of highly selective catalysts has been a challenge in this field. Herein, the ligand protected AgCuCl nanocluster was synthesized via a one-pot method and loaded onto BaCO3 to prepare a small size bimetal catalyst (AgCuCl/BaCO3-N2) via calcination under N2 atmosphere. A PO selectivity of 85.29% at a relatively mild temperature of 275 °C can be obtained. Characterizations revealed that AgCuCl/BaCO3-N2 with low-valence Cu+ species facilitated the formation of electrophilic oxygen species that preferentially attacked the C═C bond of propylene, resulting in the production of PO instead of attacking the C–H bond for the formation of byproduct. This study provides a clear example of using supported bimetal catalysts to achieve propylene epoxidation and offers guidance for selecting supports for supported nanoclusters as an efficient catalyst.

Suppression of TS-1 Aggregation by Freeze-Drying to Enhance Au Dispersion for Propylene Epoxidation Using H2 and O2

Au particles immobilized on uncalcined TS-1 (i.e., TS-1-B) have emerged as highly active catalysts for propylene epoxidation using H2 and O2 owing to their good stability. However, it is challenging to enhance the dispersion of Au due to the limited exposed Ti sites for Au adsorption. In this work, we develop a simple method to improve the dispersion of Au particles by employing freeze-dried TS-1-B as a support. Compared with the generally used oven-drying method, the freeze-drying technology can significantly enhance the dispersion of TS-1-B particles, thus giving a small contact angle (indicating good wettability), which is beneficial to achieving the uniform adsorption of Au precursors on TS-1-B, hence forming highly dispersed Au particles. Thus, the as-prepared Au/TS-1-B catalyst shows a significantly improved propylene oxide (PO) formation rate (ca 240 gpo·h–1·kgcat–1) together with high PO selectivity (ca 92%) and hydrogen efficiency (ca 41%).

Using ammonia solution to fabricate highly active Au/ uncalcined TS‑1 catalyst for gas-phase epoxidation of propylene

Reducing the Au particle size and increasing the number of defective Ti4+ species have been considered effective strategies to boost the catalytic performance of Au–Ti bifunctional catalysts used for propylene epoxidation utilizing H2 and O2. Consequently, the development of an effective method that can simultaneously achieve these goals is of significant scientific and industrial importance. Herein, we demonstrate, for the first time, that the simultaneous immobilization of small Au particles (approximately 1.6 nm) onto uncalcined titanium silicate-1 (i.e., TS-1-B) and facilitation of the formation of defective Ti4+ species can be achieved using a modified deposition–precipitation (DP) method utilizing ammonia solution as the precipitant. The as-synthesized catalyst exhibits a significantly enhanced propylene oxide (PO) formation rate combined with fascinating PO selectivity and hydrogen efficiency, which were better than the reference catalyst prepared via the DP urea (DPU) method and other reported Au/uncalcined TS-1 catalysts. The catalyst structure–performance relationship was established using multiple characterization studies. the ammonia treatment facilitates the hydrolysis of some of the ≡Ti-O-Si≡ bonds into defective Ti4+ species (i.e., Ti(OSi)3OH), which not only provides more active Ti4+ sites but also favors the uniform deposition of Au. This methodology and the insights stated herein may also be applicable for enhancing the dispersion of Au in a variety of zeolite-supported catalysts.

Mechanistic insights into propylene oxidation to acrolein over gold catalysts

Direct epoxidation of propylene with H2/O2, being the dream reaction for propylene oxide (PO) production, has raised wide scientific and industrial interests. Fundamentally understanding the formation mechanism of acrolein, as the main by-product of this epoxidation process, is very important to achieve the high yield of PO. In this study, we perform the spin-polarized density functional theory (DFT) calculations to investigate the reaction pathway from propylene to acrolein over two representative Au surfaces, that is, Au(1 1 1) and Au(1 0 0), which incorporates propylene adsorption, methyl hydrogen activation and acrolein formation. The results show that the oxygenated species (mainly O*, OH* and OOH*) are able to stabilize the adsorption of propylene to decrease the energy barrier for its activation. It is demonstrated that the OOH* on Au(1 1 1) surface emerges as the most easily formed oxygenated species via the H-assisted O2 dissociation, which is also the most active for the cleavage of methyl CH bond in propylene. Furthermore, three pathways of acrolein formation activated by O*/OH*/OOH* are analyzed, in which O* is found as the key species to form acrolein. Finally, Bader charge analysis was conducted to explore the reasons behind the promotion effect of the oxygenated species. The insights reported here could be valuable in the design and optimization of gold catalysts for the direct epoxidation of propylene.

Insight into the effect of mass transfer channels and intrinsic reactivity in titanium silicalite catalyst for one-step epoxidation of propylene

One-step epoxidation of propylene to propylene oxide (PO) in TS-1/H2O2 system is a promising green and sustainable formation route. However, the TS-1 catalyst is limited by the small pore channels and inadequate intrinsic reactivity. Herein, tetrapropylammonium hydroxide (TPAOH) is used to modify the TS-1 skeleton structures to improve the mass transfer and intrinsic reactivity. Comparative catalytic performance tests indicate that the optimized temperature for TPAOH treatment to modify TS-1 was 90 °C, and the achieved H2O2 utilization efficiency and the PO selectivity can be over 80%. Different TPAOH treatment temperatures significantly influenced the dissolution and recrystallization behavior of TS-1, which altered the composition and microstructure in terms of the pore volume and size, surface properties, and active sites. Our findings indicated that the enlarged pore channels for favorable mass transfer, improved hydrophobic surfaces, enriched framework titanium, and reduced acid strength of titanium silicalite zeolite should be the primary reasons for the enhanced conversion and yields.

Enhanced oxygen transfer over bifunctional Mo-based oxametallacycle catalyst for epoxidation of propylene

Activation of inert propylene to produce propylene oxide (PO) is critical, but still faces some challenges in realizing higher PO selectivity and productivity. Herein, a temperature-controlled phase transfer catalyst (MoOO·DMF) is prepared for the liquid-phase epoxidation of propylene with tert-butyl hydroperoxide (TBHP) as oxidant, which exhibit the selectivity of 90.6% and the productivity of 1286.42·h−1 for PO (catalyst/propylene = 0.77 mol‰). Some experimental factors (solvent types, reaction temperature, contact time, the dosage of catalyst, TBHP and substrate) were investigated, and the reaction kinetics and thermodynamics are discussed. MoOO·DMF has the characteristic of both homogeneous and heterogeneous catalysts, which can be dissolved in the solvent at higher temperatures and separated from the solvent after reaction by lowering the temperature. Importantly, MoOO·DMF has a wonderful epoxidation performance for many olefins (e.g., light olefins, linear α-olefins, cyclic olefins and others). The mechanisms are proved by in-situ FT-IR, ESR and HRMS spectrum to be the selective oxygen transfer from tert-butyl peroxide radical and the MoOO bridge in MoOO·DMF to propylene. Density functional theory (DFT) calculations show that the MoOO bridge in catalyst is the key role for the activation of both the OH bond in TBHP and the CC bond in propylene, thus enhanced the epoxidation of propylene.

Combining trace Pt with surface silylation to boost Au/uncalcined TS‐1 catalyzed propylene epoxidation with H2 and O2

AbstractModification of bifunctional Au‐Ti catalysts for direct epoxidation of propylene with H2 and O2 is of burgeoning interest for potential commercialization requirements with desirable overall catalytic performance. Herein, we report a strategy by combining the addition of trace Pt into Au particles to enhance hydroperoxide species formation with the surface silylation to promote propylene oxide (PO) desorption. The resultant catalyst, that is, silylated Au20Pt1/uncalcined TS‐1, gives rise to significantly enhanced propylene conversion of 11%, PO selectivity of 88%, and H2 efficiency of 41%. The underlying nature of the above enhanced performance is elucidated by multiple techniques, such as high‐angle annular dark‐field scanning transmission electron microscopy, X‐ray photoelectron spectroscopy, Fourier transform infrared spectra, and theoretical calculations. The insights revealed here, the synergy of Au‐Pt‐Ti triple active sites together with the surface silylation, could guide the rational design of Au‐Ti catalysts for propylene epoxidation to boost PO production.

Competition of propylene hydrogenation and epoxidation over Au-Pd/TS-1 bimetallic catalysts for gas-phase epoxidation of propylene with O2 and H2

Au and Pd play a collaborative effect in the formation of H2O2 from H2 and O2, which is beneficial for the gas-phase epoxidation of propylene over Au-Pd/TS-1 (i.e., Au-Pd bimetallic nanoparticles supported on titanium silicalite-1). However, the introduction of Pd into Au/TS-1 makes this reaction more complicated. In this work, GC–MS results demonstrated the presence of propylene hydrogenation. A small amount of O2 was found to significantly promote the propylene hydrogenation over Au-Pd/TS-1, while O2 concentration had less effect. Four catalysts were designed to discuss the active sites on Au-Pd/TS-1. It was concluded that propylene hydrogenation preferentially occurred at Pd sites and propylene epoxidation occurred at Au-Ti sites. Based on the catalytic performance of Au-Pd/TS-1 at 100 °C, a four-stage model was proposed for the first time to reveal the competition between propylene hydrogenation and epoxidation. Moreover, the addition of CO could abate the propene hydrogenation over Au-Pd/TS-1.

Insight into the Effect of Copper Substitution on the Catalytic Performance of LaCoO3-Based Catalysts for Direct Epoxidation of Propylene with Molecular Oxygen

Direct epoxidation of propylene to propylene oxide (PO) with molecular oxygen as the most atomically economical route remains challenging due to the low PO yield. In this study, CuO supported on perovskites (LaCoO₃) modified with NaCl was found to be active for direct epoxidation of propylene with oxygen. LaCoₓCu₁–ₓO₃– modified with alkali metal salts were further fabricated to improve catalytic performance with the assistance of citric acid. Among the investigated catalysts, LaCo₀.₈Cu₀.₂O₃₋δ exhibited the best catalytic performance, with a PO formation rate of 60.4 gPO·kgcₐₜ⁻¹·h⁻¹ (PO selectivity of 10.2% and propylene conversion of 12.0%) at a lower temperature (250 °C). The characterization results indicate that Cu-doped LaCoO₃ catalysts with enhanced oxygen vacancies and abundant electrophilic oxygen species are crucial to selectively attack on C═C bond instead of C–H bond. Besides, LaCoₓCu₁–ₓO₃₋δ catalysts demonstrated higher propylene conversion and more stable catalytic performance. This work exemplifies the direct propylene epoxidation with molecular oxygen employing perovskite materials.

Novelty without nobility: Outstanding Ni/Ti-SiO2 catalysts for propylene epoxidation

An efficient gas-phase production of propylene oxide (PO) with noble metal-free catalysts could modify the industrial output of this valuable compound. We present a novel catalyst based on well-dispersed Ni nanoparticles loaded on a Ti-SiO2 support for the propylene epoxidation reaction using H2/O2 mixtures. XPS, High Resolution Transmission Electron Microscopy (HRTEM), and UV–Vis corroborate both the small size of Ni particles and the excellent dispersion and incorporation of Ti as tetrahedral single site species into the silica framework. The catalytic results under steady-state conditions at low temperature (200 °C) show high PO selectivity (around 85%) with a substantial propylene conversion (over 6%) and excellent H2 efficiency (~37%) using only 0.5 wt% of Ni on Ti-SiO2 (Ti/Si = 0.01 M ratio). In this work, we have also carried out a preliminary DFT study to gain understanding of the characteristics that make this nickel catalyst active and selective for the propene epoxidation reaction.

A DFT Study and Microkinetic Simulation in Propylene Oxidation on the “29” CuxO/Cu(111) Surface

The propylene epoxidation reaction in a rich variety of other industrially important catalytic processes was used for synthesis of many value-added products. As an attractive and environmentally friendly process for direct epoxidation of propylene by Cu-based catalysts, the active oxidation states of Cu (Cu0, Cu+, or Cu0/Cu+) in the epoxidation reaction were widely studied. Herein, “29” CuxO/Cu(111) was chosen to act as a precursor to bulk oxidation, as it can be observed in special oxide phases of Cu (Cux+). In our work, the crucial competitive reactions of dehydrogenation versus the epoxidation using the lattice oxygen (Olatt*) and adsorbed molecular oxygen as the oxidants were investigated on “29” CuxO/Cu(111) surface by virtue of the periodic density functional theory computational method. Because of the higher energy barriers of the key steps for the lattice oxygen (Olatt*) as the oxidant, there are two active species in the whole reaction processes of propylene oxidation with O2 as oxidant: the molecular O2* species and the atomic O* species. It was found that the molecular oxygen is difficult to dissociate on the surface directly, while the atomic O* species can be produced via the C3H6* + Olatt* + OO* = C3H5* + Olatt* + OOH* = C3H5Olatt* + O* + OH* mechanism or by the mechanism of C3H6* + OO* = dioxametallacycle = oxametallacycle + O*. It was found that the O* mechanism is preferred for the PO formation as compared to that of O2* on pure “29” CuxO/Cu(111) surface. Promising results in terms of selectivity were achieved for PO formation, which can reach 49% with the apparent activation energy of only 0.62 eV by microkinetic simulation. Therefore, “29” CuxO/Cu(111) surface involving the Cux+ active phase is expected to show an efficient PO selectivity, which is much higher that of Cu+ of Cu2O(111) and similar to that of metallic Cu0. It is hoped that the present study could provide a theoretical guide for the further development of catalysts for propylene epoxidation.

Synergistic Enhancement over Au‐Pd/TS‐1 Bimetallic Catalysts for Propylene Epoxidation with H2 and O2

AbstractAu−Pd bimetallic catalyst has attracted significant research interests due to its fascinating properties. Herein, Au−Pd nanoparticles (NPs) supported on titanium silicalite‐1 (denoted as Au−Pd/TS‐1) were prepared by the alcohol reduction method. The Au−Pd alloy structure was proven via multiple characterization. This Au−Pd/TS‐1 bimetallic catalyst showed improved activity compared with monometallic catalyst for propylene epoxidation with H2 and O2. The synergistic enhancement over Au−Pd/TS‐1 could be elaborated by H‐spillover process, which reduced the apparent activation energy significantly. The insights in this work are of referential importance to understanding the effect of interaction between bimetallic components on reaction system.

110th Anniversary: Near-Total Epoxidation Selectivity and Hydrogen Peroxide Utilization with Nb-EISA Catalysts for Propylene Epoxidation

The Nb-EISA catalyst with relatively low Nb loadings (∼2 wt %) shows exceptional propylene epoxidation performance with H2O2 as oxidant at 30–40 °C, 5–9 bar propylene pressure with nearly total pro...

Surface Engineering and Kinetics Behaviors of Au/Uncalcined TS-1 Catalysts for Propylene Epoxidation with H2 and O2

Uncalcined TS-1-immobilized Au bifunctional catalysts have been demonstrated to be highly active yet stable for the propylene epoxidation with H2 and O2. The objective of this study is to further e...

Structure and reactivity of single site Ti catalysts for propylene epoxidation

Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO2 atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO2 framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and TiO coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the TiO coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single TiSiO2 sites scale linearly as a function of the coordination number. Also, the predicted TiO bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships.

High Stability of Ruthenium–Copper-Based Catalysts for Epoxidation of Propylene

The catalytic performance of RuO2–CuO/SiO2-based materials for the direct gas-phase epoxidation of propylene using molecular oxygen at atmospheric pressure was investigated as a function of the Ru/Cu weight ratio and the addition of NaCl, TeO2, Cs2O, or TiO2. The best performing catalysts were then assessed for stability. Doping with NaCl delivered the highest PO yield, but its catalytic activity dropped quickly within a few hours due to the loss of chloride content. The same catalyst doped instead with TeO2, Cs2O, or TiO2 could maintain stable catalytic performance for 6 h. However, the addition of TeO2 presented the best improvement in terms of PO selectivity. Further optimization of PO selectivity for the RuO2–CuO–TeO2/SiO2 catalyst system was accomplished by varying reactor temperature and total feed gas flow rate. The highest PO selectivity could be achieved at 47% (255 gPO h−1 kgcat −1, 0.35% propylene conversion). The time-on-stream test revealed excellent catalytic stability within the 12 h of test period without any deactivation.

Confirmation of Gold Active Sites on Titanium-Silicalite-1-Supported Nano-Gold Catalysts for Gas-Phase Epoxidation of Propylene

The nature of gold active sites on Au–Ti catalysts dictates the reactivity and selectivity during propylene epoxidation reactions. The type of the sites and the location of Au clusters on the external surfaces of TS-1 or inside the TS-1 nanopores have remained the topic of interest. We synthesized and characterized dispersed Au clusters on titanium silicate-1 (Au/TS-1), uncalcined TS-1 (Au/un TS-1), and TS-1 coated with silicalite-1 (Au/S-1/TS-1) to probe the nature of gold active sites for gas-phase epoxidation of propylene. The results of Au/TS-1 and Au/S-1/TS-1 show the importance of Au nanoclusters (<1.0 nm) inside TS-1 nanopores. Propylene oxide (PO) rate of Au/un TS-1 suggests Au nanoclusters (<1.0 nm) on the external surfaces also play an important role. On the basis of this conclusion, better performance for direct propylene epoxidation can be developed over Au–Ti catalysts with Au nanoclusters (<1.0 nm) either inside TS-1 nanopores or on the external surfaces.