Propylene Epoxidation Research Articles

Can lanthanum doping enhance catalytic performance of silver in direct propylene epoxidation over NaMoAg/SiO2?

In the attempt to reduce surface free energy of silica to improve interaction of silica with silver, silica was doped by different amounts of low surface energy lanthanum, La2O3, through impregnation. The doped and undoped silica were used as supports for preparation of Na/Ag/Mo/La2O3SiO2 catalysts. Catalytic performances of the catalysts were evaluated in direct epoxidation of propylene (DPO) using molecular oxygen under atmospheric pressure and without adding hydrogen. Adding 5 wt%La to the Na/Ag/Mo/SiO2 catalyst improves both the catalysts electivity in DPO and its stability over 20 h of time-on-stream. The characterization results show that La2O3 species interact strongly with silver particles on the silica surface which result in significant improvement in the dispersion profile of silver and marked decrease in the size of silver nanoparticles (AgNPs). The estimated mean diameter of AgNPs is ca. 4.0 nm in Na/Ag/Mo/5 wt%La2O3SiO2, which is smaller than that (53.9 nm) found in Na/Ag/SiO2. The presence of subnanometer AgNPs on Ag/La2O3SiO2 prior addition of MoO3 and NaCl to the sample can enhance the mutual electronic synergism between Ag, MoO3 and Na for selective production of propylene oxide.

Effective hierarchization of TS-1 and its catalytic performance in propene epoxidation

Abstract Hierarchical TS-1 zeolites have been synthesized by dry-gel conversion method employing conventional block copolymers F127 as scaffold. The proportion of the F127 added to the synthesis gel has a significant influence on the textural properties of the hierarchical TS-1 zeolites. Thus, by controlling the addition proportion of F127, the mesoporous pore volume and external surface area can be tailored. The functional mechanism of F127 was studied and proposed. The catalytic activity of this mesoporous TS-1 (denoted as MTS-1) was evaluated in propene epoxidation with H2 and O2. Well developed mesoporosity together with the Ti4+ atoms in tetrahedral coordination contributed to high catalytic stability than TS-1. After 120 h reaction at 200 °C, no significant decrement of catalytic activity can be observed. Moreover, the intrinsic reason for the enhanced stability is elucidated. The increased external surface and the better mass transfer ability results in the diminished coke formation and absence of heavier aromatic coke.

Olefin Epoxidation over Metal-Organic Frameworks Modified with Transition Metals

Mixed aluminum-containing metal-organic frameworks (MOF) modified with Fe, V, and Ti are synthesized in two steps. At the first step, the amorphous precursors of metal-organic frameworks containing various amounts of transition metals are obtained by the electrochemical method. At the second step, the precursors are crystallized under solvothermal conditions. Study of the effect of synthesis conditions shows that Fe-containing metal-organic frameworks crystallize in the presence of dimethylformamide; V- and Ti-containing counterparts crystallize in the presence of water. The time and temperature of crystallization are varied in the range from 150 to 200°С and 24 to 72 h depending on the concentration of the transition metals. The resulting samples are investigated by low-temperature nitrogen adsorption, scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and thermogravimetric and differential thermal analysis (TGA-DTA). Research into the influence of catalyst composition on its activity in 1-octene epoxidation by tert-butyl hydroperoxide suggests that that the titanium-containing samples are catalytically inactive, while the vanadium-containing samples are more active than the iron-containing ones. Testing of the vanadium-containing samples in propylene epoxidation demonstrates that their activity is comparable with the activity of a commercial titanium silicate catalyst.

Mechanism of Propylene Epoxidation via O2 with Co‐Oxidation of Aldehydes by Metalloporphyrins

Propylene epoxidation by O2 plus acrolein in the presence of metalloporphyrins were investigated. According to the difference in initiation time and the reaction rate, a hypothesis metalloporphyrins or its center ion played a principal role in the initiation of reaction (redox reaction) rather than in the catalysis of PO generation reaction have been proposed, which was verified by the cyclic voltammogram of metalloporphyrins and UV/Vis spectra of samples in the time course study. Furthermore, the propylene epoxidation experiments using aldehydes easier to be initiated and corresponding blank control without metalloporphyrins were also investigated, and the results showed the same conclusion. On the other hand, the mechanism that the active species are peroxyacyl radical and peroxy acid was proposed through density function theory calculations, and in peroxyacyl radical path the intermediate generation process was the free radical addition of propylene by peroxyacyl radical. Moreover, optimum reaction paths were also carried out by computational calculations, the results showed that peroxyacyl radical was with higher activity than peroxy acid. As for peroxyacyl radical, the main addition site was the C1 of propylene, which was proved by the condensed Fukui function.

Enhanced propylene oxide selectivity for gas phase direct propylene epoxidation by lattice expansion of silver atoms on nickel nanoparticles

A series of surfactant-free nickel-core and silver-shell (Ni@Ag) nanoparticles encapsulated within the mesopores of SBA-15 were synthesized and tested as catalysts for direct propylene oxidation by molecular oxygen. The influences of temperature, Gas Hour Space Velocity (GHSV) and Ni/Ag ratio on catalytic activity were systematically investigated. Among the prepared samples, Ni1Ag0.4/SBA-15 exhibited the best catalytic performance with selectivity of 70.7% and PO production rate of 4.4 nmol/g/s under 1 bar at 220 °C with GHSV of 192 h−1. High selectivity was attributed to longer Ag-Ag interatomic distance obtained by careful engineering the thickness of Ag shell over preformed Ni nanoparticles. In addition, all prepared new Ni@Ag core-shell catalysts presented excellent stability, which could maintain the conversion and selectivity for at least 10 h. These results suggest that new designs based on Ag surface atoms tailoring might pave the way to highly efficient and robust Ag catalysts for direct propylene oxidation using molecular oxygen as sole oxidant.

Mononuclear gold species anchored on TS-1 framework as catalyst precursor for selective epoxidation of propylene

Deposition of gold on supports can produce catalytically active forms of gold as well as spectators, but previous understanding of the nature of active immobilized precursors is poor. By using synchrotron X-ray powder diffraction (SXRD) and X-ray absorption spectroscopy (XAS) techniques, we report a novel synthesis and structural elucidation of atomically dispersed gold species anchored to the internal surface of TS-1 as K+Au(OH)2Na+(Of)3 (“f” signifies the framework atoms of TS-1 in the formula.). It is found that the choice of alkali ions plays a crucial role in nucleation and stabilization of the atomic precursor. These anchored single Au upon controlled reduction in H2 can form uniform gold clusters in direct contact with the TS-1 surface containing isolated Ti sites: their interface exhibits excellent specificity and stability towards epoxidation of propylene in H2/O2 due to synergetic effect.

The construction of Mo6−δO3−x-supported catalyst for low-temperature propylene gas-phase epoxidation by Cu modification

Selective epoxidation of propylene by molecular oxygen under mild conditions remains a great challenge to date. Here, we successfully designed and constructed molybdenum oxide in an electron-rich state by introducing the transition metal copper into MoO3–Bi2SiO5/SiO2 through a simple co-impregnation method. The new Mo species has the structure Mo6−δO3−x, and greatly improved propylene epoxidation performance at low temperatures. Propylene oxide formation rate increased from 106 to 336 g PO kgcat−1 h−1. X-ray photoelectron spectrometry (XPS) and UV–visible diffuse reflection (UV–vis DRS) characterizations suggest that the bridging oxygen bond between molybdenum and copper produces an oxygen vacancy, which facilitates the formation of the electron-rich state of molybdenum with lower 3d electronic binding energy. In addition, the incorporation of Cu effectively inhibits the formation of Bi2Mo3O12 and improves the dispersion of Mo species. Furthermore, kinetic studies show that Cu-modified catalysts maintain lower apparent activation energy for propylene epoxidation, which obviously reduced the selectivity of acrolein and then conspicuously improved the selectivity of propylene oxide. Our results not only offer a new solution for low temperature propylene epoxidation, but also provide a new strategy for the design of other types of low-temperature epoxidation catalysts.

Manipulating Gold Spatial Location on Titanium Silicalite-1 To Enhance the Catalytic Performance for Direct Propene Epoxidation with H2 and O2

Engineering Au spatial locations on support harbors tremendous potential to boost catalytic performance but still remains a scientific and technological challenge. Herein, we devise a strategy to manipulate Au spatial location inside micropores of TS-1 support by tuning Au hydrolysis process. By multitechniques (e.g., quench molecular dynamics simulations, aberration-corrected HAADF-STEM, in situ UV–vis, and quantitative model calculation), the correlation between Au dynamic location and average x in [AuClx(OH)4-x]− complexes is established. It is found that compositions and locations of Au complexes can be effectively manipulated using different preparation temperature. This results in more effective Au clusters in TS-1 microporous channels and smaller-sized Au NPs on external surfaces. Gratifyingly, the optimum Au/TS-1 catalyst shows the reported highest initial PO formation rate without adding promoters and high stable activity for direct propene epoxidation with H2 and O2. Moreover, single Au atoms an...

Theoretical Studies on the Direct Propylene Epoxidation Using Gold-Based Catalysts: A Mini-Review

Direct propylene epoxidation using Au-based catalysts is an important gas-phase reaction and is clearly a promising route for the future industrial production of propylene oxide (PO). For instance, gold nanoparticles or clusters that consist of a small number of atoms demonstrate unique and even unexpected properties, since the high ratio of surface to bulk atoms can provide new reaction pathways with lower activation barriers. Support materials can have a remarkable effect on Au nanoparticles or clusters due to charge transfer. Moreover, Au (or Au-based alloy, such as Au–Pd) can be loaded on supports to form active interfacial sites (or multiple interfaces). Model studies are needed to help probe the underlying mechanistic aspects and identify key factors controlling the activity and selectivity. The current theoretical/computational progress on this system is reviewed with respect to the molecular- and catalyst-level aspects (e.g., first-principles calculations and kinetic modeling) of propylene epoxidation over Au-based catalysts. This includes an analysis of H2 and O2 adsorption, H2O2 (OOH) species formation, epoxidation of propylene into PO, as well as possible byproduct formation. These studies have provided a better understanding of the nature of the active centers and the dominant reaction mechanisms, and thus, could potentially be used to design novel catalysts with improved efficiency.

Enhanced stability for propene epoxidation with H2 and O2 over wormhole-like hierarchical TS-1 supported Au nanocatalyst

Designing highly efficient Au/Ti-containing catalysts for propene epoxidation with H2 and O2 harbors tremendous scientific and industrial importance. In this work, novel hydrophobic hierarchical TS-1 (HTS-1) with wormhole-like mesopores (ca. 45 nm) and small crystal size (100 nm) is firstly synthesized by a two-step crystallization method using CTAB as template. Gratifyingly, the Au/HTS-1 catalyst shows simultaneously high PO formation rate of 150 gpoh−1kgCat−1 without any promoter additive, PO selectivity of 90% and impressive stability of 100 h, which are much better than traditional Au/TS-1 catalyst. Furthermore, the intrinsic reason for the enhanced performance is elucidated by multi-techniques such as N2 physisorption, HRTEM, TGA, FT-IR and 29Si NMR. Interestingly, it is found that the coke in 0.10 wt% Au/HTS-1 catalyst partly reside in mesopores, alleviating the deactivation of micropore blocking. Moreover, the enhanced mass transfer ability and higher hydrophobicity of Au/HTS-1 catalyst also lead to reduced coke weight and absence of aromatic coke.

Cost-efficient core-shell TS-1/silicalite-1 supported Au catalysts: Towards enhanced stability for propene epoxidation with H2 and O2

Designing cost-effective TS-1 with enhanced catalytic stability in direct propene epoxidation with H2 and O2 is attracting tremendous research interests. In this work, a novel hydrophobic TS-1/S-1 lamellar support with Ti-rich shell and Si-rich cores is first synthesized using trace amount of TPAOH template. The effect of TPAOH concentration on physico-chemical properties of catalyst is then investigated. Compared with conventional Au/TS-1, optimum Au/TS-1/S-1 catalyst exhibits fantastic catalytic stability for over 100 h and good PO formation rate of 126 gPOh−1kgCat−1. Furthermore, the intrinsic reason for the enhanced stability is elucidated by TGA, XPS, 29Si NMR and FT-IR. It is found that the coke weight for Au/TS-1/S-1 is only 0.99 wt%, much lower than 4.9 wt% for Au/TS-1 catalyst. The refractory aromatic coke is also not formed. On the one hand, this could be due to the unique core-shell structure and the presence of mesopores, which reduce the long diffusion path. On the other hand, it is because of enhanced hydrophobicity. This work sheds new light on the design and synthesis of highly effective and cost-efficient Au/Ti-containing catalysts.

Theoretical Study of Propylene Epoxidation over Cu2O(111) Surface: Activity of O2–, O–, and O2– Species

Propylene epoxidation catalyzed by cuprous oxide in the presence of oxygen species is important in both technology and scientific fields, and the active species—lattice oxygen (O2–), adsorbed atomic oxygen (O–), or adsorbed molecular oxygen (O2–)—plays a significant role in the catalytic reaction. In the present work, the mechanism of propylene epoxidation and dehydrogenation on a Cu2O(111) facet with different oxygen species has been studied through density functional theory (DFT) calculations with a Hubbard U correction in detail. The whole reaction processes of propylene oxidation includes two different routes: allylic hydrogen stripping (AHS) reactions and propylene epoxidation reactions. Acrolein can be generated by two H-stripping reactions in the AHS path, and propylene oxide (PO) is formed through the oxametallacycle propylene (OMP) intermediate. The calculated results show that the adsorbed atomic oxygen (O–) is the most active oxygen species for the selective oxidation of propylene because it has the strongest basicity among these three oxygen species, whereas the adsorbed molecular oxygen (O2–) has the highest selectivity for the PO formation among these three different oxygen species because of its relatively low basic properties and moderate oxidation compared to those of atomic oxygen (too active oxidation) or lattice oxygen (less oxidation due to the closed-shell nature of the oxide system). Moreover, a microkinetic simulation was used to confirm the above DFT calculation results. The aim of the present study is to give a guide in choosing the most efficient active oxygen species for PO formation, which should be the one with the moderate oxidant.

Influence of the Preparation Procedure of Vanadium-Containing SiBEA Zeolites on Their Catalytic Activity in Propene Epoxidation

Two series of V-containing BEA zeolite catalysts, VxSiBEA(I) and VxSiBEA(II), were prepared by a two-step postsynthesis preparation procedure which consists, in the first step, in the dealumination...

Hydrophobic and hierarchical modification of TS-1 and application for propylene epoxidation

Hierarchical and hydrophobic TS-1 was obtained by the desilication and silanization of TS-1. The modified TS-1 was characterized by adsorption–desorption isotherms analysis, transmission electron microscope, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy and contact angle measurement. It demonstrates that modified TS-1 has hierarchical channel structures consisting of micropores and mesopores. The average pore diameters of the micropores and mesopores are 0.55 and 5–10 nm, respectively. The contact angle measurements indicate that modified TS-1 is hydrophobic. The effects of the desilication and silanization of TS-1 on the catalyst structure and catalytic properties of propylene epoxidation were investigated. Modified TS-1 showed better catalytic activity and catalytic life than the TS-1 without modification. The H2O2 conversion, PO selectivity and PO concentration reached 99.72%, 91.33% and 0.85 mol L−1, respectively.

Enhanced Catalytic Performance for Propene Epoxidation with H2 and O2 over Bimetallic Au–Ag/Uncalcined Titanium Silicate-1 Catalysts

Gradually emerging environmental concerns have triggered the development of highly efficient catalysts for propene epoxidation with H2 and O2. Unfortunately, intensifying the activity and stability is still a challenging task. Herein, Au–Ag bimetallic catalysts were deposited on titanium silicate-1 with blocked pores (TS-1-B). Via density functional theory calculations together with multiple types of characterization (e.g., high-angle annular dark-field scanning transmission electron microscopy, ultraviolet–visible, and X-ray photoelectron spectrometry), we found that the synergy between Au and Ag significantly improved the catalytic performance via not only the decrease in the Au nanoparticle size but also the enhanced oxygen adsorption and electron transfer ability from Au to O2. Furthermore, a volcano-shaped relationship between the Au/Ag ratio and catalytic performance was also established, and the Au10–Ag1/TS-1-B catalyst showed an excellent stable PO formation rate (174–233 gPO h–1 gAu–1) and a H2 e...

Propylene oxide inhibits propylene epoxidation over Au/TS-1

The kinetics of propylene epoxidation over Au supported on titanium silicalite-1 (Au/TS-1) catalysts were measured in a continuous stirred tank reactor (CSTR) free from temperature and concentration gradients. Apparent reaction orders measured at 473 K for H2 (0.7 order), O2 (0.2), and propylene (0.2) for a series of Au/TS-1 with varied Au (0.02–0.09 wt%) and Ti (Si/Ti: 75–143) contents were consistent with those reported previously. Co-feeding propylene oxide enabled measurement of the apparent reaction order in propylene oxide (−0.4 to −0.8 order) and the determination that relevant pressures of propylene oxide reversibly inhibit propylene epoxidation over Au/TS-1, while co-feeding carbon dioxide and water had no effect on the propylene epoxidation rate. Analysis of previously proposed two-site reaction mechanisms in light of reaction orders for O2 (0.4), H2 (1), and C3H6 (0.4), corrected to account for propylene oxide inhibition, provides further evidence that propylene epoxidation over Au/TS-1 occurs via a simultaneous mechanism requiring two distinct, but adjacent, types of sites, and not a sequential mechanism that invokes migration of H2O2 formed on Au sites to PO forming Ti sites. H2 oxidation rates are not inhibited by propylene oxide, implying that the sites required for hydrogen oxidation are distinct from those required for propylene epoxidation.

Mechanistic insights into the direct propylene epoxidation using Au nanoparticles dispersed on TiO2/SiO2

Direct propylene epoxidation is an important gas-phase reaction for the future industrial production of propylene oxide (PO). However, the mechanistic understanding of this reaction system is still elusive, including the role of side reactions. Herein, kinetic Monte Carlo (KMC) simulations are applied to explore the underlying mechanistic aspects of propylene epoxidation over Au/TiO2/SiO2, and consistent results are obtained with respect to experimental benchmarks. The present study systematically probes the reaction mechanism, which involves acrolein formation on Au nanoparticles as well as the formation of PO and several byproducts at the Au/Ti dual interface sites, and it highlights the synergistic effect of interface sites in heterogeneous catalysis. The origin of the negative effect of rising reaction temperature on PO selectivity has been clarified, as it is correlated with decreasing atomic oxygen coverage on the Au surface. The varying feed concentration of H2/O2/C3H6 leads to different coverages on the catalyst surface, emphasizing the importance of an optimum H2/C3H6 inlet concentration ratio for enhancing PO selectivity and hydrogen utilization efficiency. Based on our model results, we propose that supports with acidic groups may promote hydroperoxy species transferring to the adjacent Ti sites, thereby decreasing its decomposition to atomic oxygen on the Au sites, which leads to byproduct formation.

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene Epoxidation with N2O

Vanadium modified mesocellular silica foams (MCF) materials (V content ca. 3 and 5 wt%) prepared by the impregnation method show mainly isolated or low-polymeric VOx species, which was confirmed by means of Raman spectroscopy and DR UV–Vis. Textural measurements, and also XRD and TEM results indicate that the characteristic mesocellular structural features of MCFs are preserved after vanadium incorporation. The MCF-supported vanadia catalysts exhibit much higher propene conversion and propene oxide productivity when compared to vanadium modified mesoporous silicas of 2D structure, demonstrating that apart from the presence of highly dispersed isolated vanadium species, internal molecular transport within three-dimensional ultra large pores of MCF materials also plays an important role in gas-phase propene epoxidation.Graphical

Propene epoxidation with H2 and O2 on Au/TS-1 catalyst: Cost-effective synthesis of small-sized mesoporous TS-1 and its unique performance

Designing cost-effective titanium silicalite-1 (TS-1) with enhanced mass transfer ability is of prime scientific and industrial importance. In this work, a novel small-sized mesoporous TS-1 (STS-1) material is first synthesized in cheap TPABr system using steam-assisted crystallization method. Compared with cheap microporous TS-1 prepared by classical hydrothermal method (HTS-1) with particle size of 950 nm, STS-1 shows mesoporous character of 13 nm and reduced average particle size of 590 nm. Moreover, effect of TPABr concentration is systematically investigated, and both of insufficient and overmuch TPABr concentrations are unfavorable because of low crystallinity and exorbitant price, respectively. Importantly, the catalytic performance of cost-effective Au/STS-1 for direct propene epoxidation with H2 and O2 is much better than that of Au/HTS-1, and comparable to the performance of the expensive Au/TS-1. By multi-characterizations and activation energy analysis, the underlying mechanism for the enhanced performance is further discussed. This work sheds new light on the design and synthesis of highly effective and cost-efficient TS-1 catalysts.

An efficient Titanium silicalite-1 catalyst for propylene epoxidation synthesized by a combination of aerosol-assisted hydrothermal synthesis and recrystallization

Reducing the crystal size and creating intra/inter-crystalline mesoporosity or macroporosity are two options to enhance the mass transportation efficiency in microporous zeolites. In this study, an improved aerosol-assisted hydrothermal method was used to synthesize small-crystal Titanium silicalite-1 (TS-1) with low template amount. By using a more concentrated template agent the particle size of the TS-1 crystal can be reduced from 350-800 nm to 230–270 nm. A well-dispersion of the Ti species can be achieved under optimized synthesis condition. A post-treatment with tetrapropyl ammonium hydroxide (TPAOH) was used to further improve the diffusion property. The mesopore volumes and external surface areas are significantly increased due to the formation of the intracrystalline voids. All the TS-1 samples were evaluated in propene epoxidation. The conversion of H2O2 and the selectivity of Propylene Oxide are 99.9% and 98%, which are improved as compared to those of the parent TS-1.