Synergistic mechanism of isolated Fe3+ and highly dispersed Ptδ+ over zeolite for boosting propane dehydrogenation

Pt-based catalysts have been widely investigated in propane dehydrogenation (PDH) owing to their high activity in C H activation, while it suffers from Pt sintering and coke deposition. We develop a transition metal Fe and zeolite support synergistic-modified method to realize the highly dispersed and stable Pt species inside zeolite over Pt/Fe-silicate-1. And it shows the excellent PDH performance with propylene generation rate of 51.6 mol C₃H₆ g_Pt⁻¹ h⁻¹ and low deactivation rate constant k_d of 0.017 h⁻¹ as well as a high TOF_Pt of 37.6 s⁻¹ at 550°C. The systematic characterizations reveal the isolated Fe³⁺ species could significantly improve Pt dispersity and regulate Pt electronic density to realize a more positive Pt^δ+ species inside Silicalite-1 pore. And the further in situ DRIFTS experiments demonstrate that the synergistic effect between the appropriate acidic Fe sites and the highly dispersed Pt^δ+ species around Fe species are responsible for the superior PDH performance.     

Aromatization of propane over Ga/H-ZSM-5: An explanation of the synergy observed between Ga3+ and H+

The aromatization of propane is investigated for Ga₂O₃, H-ZSM-5 and Ga₂O₃/H-ZSM-5 catalysts, and the results are discussed for a series of ZSM-5 catalysts containing varying SiO₂/Al₂O₃ ratios. It is apparent that on addition of a gallium phase to H-ZSM-5, the yield of methane is significantly decreased. These results are discussed with respect to the mechanism of formation of the initial reaction product from propane. It is proposed that the synergy observed between the gallium compound and the zeolite can be explained in terms of a mechanism in which the role of the gallium phase is to induce C-H bond polarization in the propane, which leads to attack via the Bronsted acid sites of the zeolite, which leads to initial C-H bond cleavage occurring.     

Selective Oxidation of Propane to Acrolein over Ce-Doped Ag–Mo–P–O Catalysts: Influence of Ce Promoter

The selective oxidation of propane to acrolein was performed on Ce-doped Ag_0.3Mo_0.5P_0.3O _x catalysts. The maximal acrolein yield of 4.4% with 28.7% acrolein selectivity was obtained on Ce_0.1Ag_0.3Mo_0.5P_0.3O _y catalyst. The apparent activation energy of Ag_0.3Mo_0.5P_0.3O _x catalysts decreased with the addition of Ce. The addition of Ce facilitated the C-H activation of propane and enhanced conversion of intermediate propene to acrolein. The reducibility and the concentration of Mo⁵⁺ improved as the Ce content increased and was closely related to acrolein selectivity and propane conversion. The role of Ce in these catalysts was proposed: there was formation of the redox cycle Ce³⁺ + Mo⁶⁺ Ce⁴⁺ + Mo⁵⁺ in Ce-doped Ag_0.3Mo_0.5P_0.3O _x catalysts, leading to the modification of properties and catalytic performance of these catalysts.     

Physicochemical properties and catalytic performance of galloaluminosilicate in aromatization of lower alkanes: a comparative study with Ga/HZSM-5

A series of gallium-containing HZSM-5 zeolites with different Ga contents (Ga/(Al+Ga)?=?0.1?C0.6) were prepared by hydrothermal in situ synthesis and post synthesis. Their catalytic performance were compared in the aromatization of propane, butane and propane/butane mixture (1:1?molar). Galloaluminosilicate obtained via hydrothermal in situ synthesis exhibited high fraction of acidic framework Ga³⁺ with few dispersed extracrystalline Ga₂O₃. Ga/HZSM-5 obtained by post synthesis showed the presence of extracrystalline Ga₂O₃ and/or extra framework gallyl ions. The aromatization performance of Ga-containing HZSM-5 followed the following sequence; galloaluminosilicate > Ga/HZSM-5 (ion-exchange) > Ga/HZSM-5 (impregnation) ? HZSM-5. Optimum aromatization performance over galloaluminosilicate was achieved with Ga/(Al+Ga) ratio of 0.3. Propane conversion reached 50.9?wt% over galloaluminosilicate with Ga/(Al+Ga) of 0.3, as compared to 31.8 and 40.7?wt% for the corresponding Ga/HZSM-5 obtained by impregnation and ion-exchange, respectively, at gas hourly space velocity of 1,600?h^?1, and 540?°C. Comparison of aromatic selectivity at the same conversion level (~10.0?wt%) revealed that galloaluminosilicate is more selective than Ga/HZSM-5. The superior performance of galloaluminosilicate was attributed to the presence of highly dispersed-reducible extra-framework Ga₂O₃ (Lewis-dehydrogenating sites) formed by degalliation in close proximity to zeolitic Br?nsted sites. Thus, hydrothermal in situ approach can thus be considered as an effective method for improving the aromatization performance of HZSM-5.     

Enhanced Activity of Spinel-type Ga2O3–Al2O3 Mixed Oxide for the Dehydrogenation of Propane in the Presence of CO2

Catalytic performance of a series of Ga₂O₃–Al₂O₃ mixed oxides prepared by alcoholic-coprecipitation method for the dehydrogenation of propane in the presence of CO₂ was investigated. It is shown that the combination of Ga and Al oxides greatly improved the performance of the Ga₂O₃-based materials for catalytic dehydrogenation of propane, with the highest performance attainable at a Ga₂O₃–Al₂O₃ catalyst with a 20 mol% aluminum content. While the same tendency was observed for the specific activity normalized by BET surface area, significantly enhanced stability was achieved for Ga₂O₃–Al₂O₃ with higher aluminum content. X-ray diffraction (XRD) revealed that a homogeneous spinel-type Ga₂O₃–Al₂O₃ solid solution is uniformly formed by substitution of Ga³⁺ for Al³⁺ in the Al₂O₃ lattice. The enhanced activity of Ga₂O₃–Al₂O₃ mixed oxides was accounted for by the abundance of surface weak acid sites due to the synergetic interaction between Ga₂O₃ and Al₂O₃ in the solid solution systems.     

Dual-function catalysis in propane dehydrogenation over Pt1–Ga2O3 catalyst: Insights from a microkinetic analysis

The kinetics of propane dehydrogenation over single-Pt-atom-doped Ga₂O₃ catalyst has been examined by combining density functional theory calculations and microkinetic analysis. The doping of Pt not only can improve the selectivity of the Ga₂O₃ catalyst by hindering the deep dehydrogenation reactions but also helps to achieve a long-term stability by improving the resistance of Ga₂O₃ to hydrogen reduction. Microkinetic analysis indicates that upon Pt doping the turnover frequency for propane consumption is increased by a factor of 2.8 under typical operating conditions, as compared to the data on the pristine Ga₂O₃ surface. The calculated results suggest that the Pt₁–Ga₂O₃ catalyst shows a bifunctional character in this reaction where the Pt–O site brings about dehydrogenation while the Ga–O site is active for desorbing H₂, which provides a beautiful explanation for the previous experimental observation that even trace amounts of Pt can dramatically improve the catalytic performance of Ga₂O₃.     

Effects of adsorbed oxygen containing molecules on the XANES of Pt in supported Pt/SiO2 catalysts

Selective reduction of nitric oxide with propane in the presence of excess oxygen was investigated using gallium ion-exchanged zeolite catalysts. Gallium ion-exchanged ferrierite (Ga-ferrierite) showed extremely high selectivity for this reaction under oxygen-rich conditions (10%). The molar ratio of reacted NO to consumed C₃H₈ was found to be near 3 on Ga-ferrierite.     

Selective oxidation of propane over PMoV?M/α‐Al2O3 (M: Co,Fe, or Ni)

Alumina supported phosphovanodomolybdic acid and alumina supported phosphovanodomolybdic acid‐transition metal ions (M: Fe³⁺, Co²⁺, or Ni²⁺) were prepared by impregnation. The thermal decomposition, in situ at 400°C, of supported catalysts showed the formation of V₂O₅, P₂O₅, MoO₃ and MoO₃, CoMoO₄, (Mo_0.3V_0.7)₂O₅ phases, on the alumina surface, in the presence of H₄PMo₁₁VO₄₀/α‐Al₂O₃ and H₄PMo₁₁VO₄₀? Co/α‐Al₂O₃, respectively. The catalytic activity of alumina‐supported catalysts was evaluated in the reaction of propane oxidation at 380 and 400°C. The addition of transition metal increases the conversion and changes the reaction products distribution. The reaction conditions (temperature and propane/oxygen ratio) have also modified the behaviour of the studied catalysts.     

Modulation of ODH Propane Selectivity by Zeolite Support Desilication: Vanadium Species Anchored to Al-Rich Shell as Crucial Active Sites

The commercially available zeolite HY and its desilicated analogue were subjected to a classical wet impregnation procedure with NH₄VO₃ to produce catalysts differentiated in acidic and redox properties. Various spectroscopic techniques (in situ probe molecules adsorption and time-resolved propane transformation FT-IR studies, XAS, ⁵¹V MAS NMR, and 2D COS UV-vis) were employed to study speciation, local coordination, and reducibility of the vanadium species introduced into the hierarchical faujasite zeolite. The acid-based redox properties of V centres were linked to catalytic activity in the oxidative dehydrogenation of propane. The modification of zeolite via caustic treatment is an effective method of adjusting its basicity—a parameter that plays an important role in the ODH process. The developed mesopore surface ensured the attachment of vanadium species to silanol groups and formation of isolated (SiO)₂(HO)V=O and (SiO)₃V=O sites or polymeric, highly dispersed forms located in the zeolite micropores. The higher basicity of HY_deSi, due to the presence of the Al-rich shell, aided the activation of the C−H bond leading to a higher selectivity to propene. Its polymerisation and coke formation were inhibited by the lower acid strength of the protonic sites in desilicated zeolite. The Al-rich shell was also beneficial for anchoring V species and thus their reducibility. The operando UV-vis experiments revealed higher reactivity of the bridging oxygens V-O-V over the oxo-group V=O. The (SiO)₃V=O species were found to be ineffective in propane oxidation when temperature does not exceed 400 °C.     

Preparation of acidic or bifunctional catalysts by means of straightforward calcination of as-synthesized [Ga]-ZSM-5 zeolites obtained from alkali-free media. Propane aromatization

Propane aromatization (530°C, 1 atm) was used as a reaction model to evaluate the effect of the calcination temperature on the catalytic properties of an as-synthesized [Ga_1.3]-ZSM-5 zeolite obtained from alkali-free media and calcined at two different temperatures: 530°C (C-530) and 750°C (C-750). Results show that in spite of its lower acidity, C-750 is more active and selective toward aromatics than C-530. This is probably due to the fact that at higher temperature the decomposition of organic compounds used during the zeolite synthesis is accompanied by a partial degalliation of the zeolitic support leading to the production of a bifunctional xGa₂O₃ /H-[Ga_y-ZSM-5(2x+y=1.3)catalyst.     

Combined reforming of methane over supported Ni catalysts

The hydrogen exchange for propane-d ₈ adsorbed on zeolite Zn/H-MFI has been studied by¹H MAS NMR spectroscopy in situ within the temperature range of 420–490 K. Kinetic measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that only methyl groups of the alkane are involved in the exchange. Two mechanisms are proposed to rationalize the regioselectivity of the exchange: (i) propane dehydrogenation on Zn-sites followed by protonation of propene by acidic OH groups in accordance to the Markovnikov’s rule and abstraction of deuteride ion from another propane molecule; (ii) the reversible heterolytic dissociative adsorption of propane to form Zn-propyl species and acidic OH groups.     

Effect of Pt dispersion on the reduction of NO by propene over alumina-supported Pt catalysts under lean-burn conditions

The hydrogen exchange in the propane‐d ₈ loaded zeolite H‐ZSM‐5 was monitored by in situ ¹H MAS NMR spectroscopy within the temperature range 457–543 K. Measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that both methyl and methylene groups of alkane are involved in the exchange. The comparison of the experimentally obtained apparent activation energies for the exchange in methyl groups (108 ± 7 kJ mol^-1) and methylene groups (117 ± 7 kJ mol^-1) with theoretical values for methane and ethane supports the assumption that the H/D exchange for methyl and methylene groups takes place via a pentavalent transition state. This revised version was published online in July 2006 with corrections to the Cover Date.     

13C MAS NMR mechanistic study of the initial stages of propane activation over H-ZSM-5 zeolite

¹³C MAS NMR study of the early stages of propane 2-¹³C activation was performed over H-ZSM-5 catalysts with various content of protonic and aprotonic sites. The reaction mechanism was tested by addition of various probe-molecules (C₃H₆, C₆H₆, H₂, H₂O and CO). The results on tracing the fate of ¹³C label during this experiments conclude to a monofunctional mechanism involving propane protonation on the strong Brønsted sites of H-ZSM-5 and the formation of carbonium ion type transition states, which further evolve in four different ways leading to ¹³C scrambling in propane, cracking, dehydrogenation and disproportionation.     

The importance of site isolation and phase cooperation in propane ammoxidation on rutile-type vanadia catalysts

Propane ammoxidation to acrylonitrile over rutile-type vanadia catalysts is discussed regarding phase cooperation and site isolation effects. Compared with the pure phases, biphasic catalysts with both SbVO₄and -Sb₂O₄are considerably more selective to the formation of acrylonitrile. It is demonstrated that cooperation between the phases during the calcination of the catalyst and the use in propane ammoxidation results in spreading of antimony species from free antimony oxide to the surface of SbVO₄, forming Sb⁵⁺–V³⁺/V⁴⁺supra-surface sites being involved in the formation of acrylonitrile. Dilution and isolation of the vanadium centers in SbVO₄through the partial replacement with, e.g., Al, Ti and W improves the catalytic properties. Structure–reactivity correlations using data for a nominal Sb_0.9V_0.9-x Ti _x O _y series indicate that the activation of propane occurs on a V³⁺site and the activation of ammonia requires an Sb⁵⁺site.     

Methane aromatization on Zn-modified zeolite in the presence of a co-reactant higher alkane: How does it occur?

By using ¹³C solid-state NMR and GC–MS, the analysis of the ¹³C-label transfer from methane-¹³C into the products of methane and propane co-aromatization on Zn/H-BEA zeolite at 823–873 K has been performed. A high degree involvement of ¹³C-carbon atoms of methane into aromatic products (benzene, toluene, xylenes) has been demonstrated. The main pathway of methane conversion into aromatics has been determined to consist in the methylation of aromatics, which is produced exclusively from propane, by methane. The methoxy species formed by the dissociative adsorption of methane on ZnO species of the zeolite is responsible for the methylation.     

Effects of Pre-Treatment of a Silica-Supported Gallium Oxide Catalyst with H2 on Its Catalytic Performance for Dehydrogenation of Propane

The pre-treatment of a silica-supported gallium oxide catalyst with H₂ at 823 K increased the yield of aromatics and the selectivity to aromatics in the dehydrogenation of propane over the catalyst at 823 K. Gallium oxide in the catalyst was partially reduced with H₂ at 823 K. NH₃ desorption and DRIFTS studies on the gallium oxide catalyst suggest that the dehydrogenation of propane over a silica-supported gallium oxide catalyst would proceed in the following way: (1) the dehydrogenation of propane to produce propene would occur on Ga sites including Ga^δ+–H sites and (2) the aromatization of propene to aromatics on Ga–O–H acid sites.     

Transformation of propane and CO to carboxylic acid and ester by highly active CaCl2 catalyst

The functionalization reaction of propane with CO to afford carboxylic acids and an ester by CaCl₂ catalyst in the presence of K₂S₂O₈ and CF₃COOH has been studied. The reaction gave isobutyric acid as the main product and n-butyric acid and isopropyl trifluoroacetate as by-products. Atmospheric pressure of propane underwent the reaction with 30 atm of CO pressure at 80 °C for 24 h, giving about 95% total yield based on propane. The activation and thermodynamic parameters have been determined to be E_a = 130.3, 138.0 and 153.8 kJ/mol; A = 7.14 × 10¹³, 5.83 × 10¹⁴ and 5.80 × 10¹⁶ 1/s; ΔH ^‡ = 128.0, 134.7 and 150.5 kJ/mol; ΔS ^‡ = 10.3, 28.6 and 66.8 J/mol K and ΔG^‡ ₂₅₃ = 124.4, 123.9 and 126.9 kJ/mol for the products of isobutyric acid, n-butyric acid and isopropyl trifluoroacetate, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Liquid-Phase Cyclization of (Phenylthio)acetaldehyde Diethylacetal to Benzo[b]thiophene over Zn2+ Ion-Exchanged Zeolite BEA

Cyclization of (phenylthio)acetaldehyde diethylacetal was examined in 1,2-dichloroethane and chlorobenzene solvents in the presence of H-beta zeolite and Zn²⁺ ion-exchanged beta zeolite under reflux conditions. Zn²⁺ ion-exchanged beta zeolite showed better activity than its H-form counterpart owing to its higher Lewis acidity. The cyclization proceeds via two different pathways. At lower temperature the reaction proceeds predominantly via a pathway in which deacetalization of the reactant occurs first and then the resulting aldehyde cyclizes to benzo[b]thiophene.     

In situ encapsulated subnanometric CoO clusters within silicalite-1 zeolite for efficient propane dehydrogenation

Cobalt-based catalysts are promising alternatives to replace Pt- and Cr-based catalysts for propane dehydrogenation (PDH). However, the sintering and reduction of unstable Co sites cause fast deactivation. Herein, the ultrasmall cobalt oxide clusters encapsulated within silicalite-1 zeolites (CoO@S-1) has been obtained via a ligand assistance in situ crystallization method. This CoO@S-1 catalyst exhibits an attractive propylene formation rate of 13.66 mmol_C3H6·g_cat⁻¹·h⁻¹ with selectivity of >92% and is durable during 120-h PDH reaction with five successive regeneration cycles. The high PDH activity of CoO@S-1 is assigned to the encapsulated CoO clusters are favorable for propane adsorption and can better stabilize the detached H* species from propane, leading to the lower dehydrogenation barriers than framework Co²⁺ cations and Co₃O₄ nanoparticles. Additionally, the π-binding propylene on CoO clusters can prevent the over-dehydrogenation reaction compared with the di-σ binding propylene on metallic Co, leading to the superior propylene selectivity and catalytic stability.     

Initial activity/selectivity of H-gallosilicate (MFI) in propane aromatization: influence of H+ exchange and thermal/hydrothermal pretreatments

Initial activity/selectivity of H-gallosilicate (MFI) zeolite with different degrees of H⁺ exchange and pretreated under different thermal and hydrothermal conditions in propane aromatization (at 500C) has been determined using a pulse microreactor connected to GC. It is found to be strongly influenced by the degree of H⁺ exchange, calcination temperature and hydrothermal treatment at different temperatures and concentrations of steam. There exists a close relationship between the acidity (measured in terms of pyridine chemisorbed at 400 C) of the gallosilicate and its initial propane conversion and aromatization activity. Presence of strong acidic sites (attributed to FW Ga) at high concentration is essential for the well dispersed non-FW Ga oxide species to be active for dehydrogenation in the propane aromatization over the zeolite.