Stable Fe/ZSM-5 Nanosheet Zeolite Catalysts for the Oxidation of Benzene to Phenol

Copper-containing POM-based hybrid P2Mo22V4Cu4 nanocluster as heterogeneous catalyst for the light-driven hydroxylation of benzene to phenol

The current traditional phenol production process has many shortcomings, and the efficient and clean photocatalytic one-step oxidation to phenol is gradually attracting attention. Heteropolyacids (PMo10V2) with high-density Lewis acid active sites and excellent photoelectron transfer ability are ideal choices for catalytic reactions. In this study, a copper-modified isolated dimeric hybrid nanocluster, [Cu(pyim)2]2[Cu(pyim)2(P2MoVI20MoV2VIV4O82)]2·(H2O) (pyim = [2-(pyridin-2-yl)imidazole]), was synthesized by a convenient hydrothermal method. The structural analysis demonstrated that the compound was composed of metal-organic complexes containing pyim ligands, Keggin-type heteropolyacids, and transition metal copper ions. Remarkably, this not only solves the difficulty that the heteropolymeric acid cannot be recovered by dissolving in the solvent but also introduces the copper atom as a second active center. The catalyst exhibited a benzene conversion of 15.6% and a selectivity of 85.2% in a mixed solution of acetonitrile and acetic acid under optimal reaction conditions. After four catalytic cycles, the PXRD pattern proved that the catalyst was still stable. This study provides a good idea for photocatalytic reactions and other environmental applications.

Ferric Single-Site Catalyst Confined in a Zeolite Framework for Propane Dehydrogenation

Non-oxidative dehydrogenation of propane is a highly efficient approach for industrial preparation of propene that is commonly catalyzed by noble Pt or toxic Cr catalysts and suffers from coking. In this work, ferric catalyst confined in a zeolite framework was synthesized by a hydrothermal procedure. The isolated Fe in the framework formed distorted tetrahedra, which were beneficial for the selective dehydrogenation of propane and reached over 95 % propene selectivity and over 99 % total olefins selectivity. This catalyst had a silanol-free structure and was oxygen tolerant, hydrothermally stable, and coke free, with a deactivation constant of 0.01 h-1 . This study provided guidance for the synthesis of structural heteroatomic zeolite and efficient propane non-oxidative dehydrogenation over early transition metals.

Highly selective oxidation of benzene to phenol with air at room temperature promoted by water

Phenol is one of the most important fine chemical intermediates in the synthesis of plastics and drugs with a market size of ca. $30b¹ and the commercial production is via a two-step selective oxidation of benzene, requiring high energy input (high temperature and high pressure) in the presence of a corrosive acidic medium, and causing serious environmental issues^2-5. Here we present a four-phase interface strategy with well-designed Pd@Cu nanoarchitecture decorated TiO₂ as a catalyst in a suspension system. The optimised catalyst leads to a turnover number of 16,000-100,000 for phenol generation with respect to the active sites and an excellent selectivity of ca. 93%. Such unprecedented results are attributed to the efficient activation of benzene by the atomically Cu coated Pd nanoarchitecture, enhanced charge separation, and an oxidant-lean environment. The rational design of catalyst and reaction system provides a green pathway for the selective conversion of symmetric organic molecules.

Electrochemical Reduction and Oxidation of Chlorinated Aromatic Compounds Enhanced by the Fe-ZSM-5 Catalyst: Kinetics and Mechanisms

Devising cost-effective electrochemical catalyst system for the efficient degradation of chlorinated aromatic compounds is urgently needed for environmental pollution control. Herein, a Fe-ZSM-5 zeolite was used as a suspended catalyst to facilitate the degradation of lindane as a model chlorinated pesticide in an electrochemical system consisting of the commercial DSA (Ti/RuO₂-IrO₂) anode and graphite cathode. It was found that the Fe-ZSM-5 zeolite greatly accelerated the degradation of lindane, with the degradation rate constant more than 8 times higher than that without Fe-ZSM-5. In addition, the Fe-ZSM-5 zeolite widened the working pH range from 3 to 11, while efficient degradation of lindane in the absence of Fe-ZSM-5 was only obtained at pH ≤ 5. The degradation of lindane was primarily due to reductive dechlorination mediated by atomic H* followed by ^•OH oxidation. Fe-ZSM-5 zeolite could enrich lindane, H*, and ^•OH on its surface, thus provided a suitable local environment for lindane degradation. The Fe-ZSM-5 zeolite exhibited high stability and reusability, and reduced the energy consumption. This research provides a potential reduction-oxidation strategy for removing organochlorine compounds through a cost-efficient Fe-ZSM-5 catalytic electrochemical system.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Fluoride-free synthesis of beta zeolite with enrichment of polymorph B from a solvent-free route

Beta zeolite with enrichment of polymorph B is successfully synthesized in the absence of fluorine species under solvent-free conditions. The phase composition of polymorph B in the sample is about 70%.

Improvement of adsorption and catalytic properties of zeolites by precisely controlling their particle morphology

An aluminosilicate zeolite has a porous structure with openings comparable to the molecular size, which endows it with unique adsorptive and catalytic properties that are highly dependent on its chemical composition and crystal morphology. Thus, the precise control or rational design of zeolite's particle morphology has attracted much attention as it can greatly improve the adsorptive separation and catalytic properties by effectively adjusting the diffusion path of adsorbates, reactants and products. This paper reviews the recent progress made in the synthesis and application of zeolites with a specific crystal/particle morphology with emphasis on the control of the crystal size and facet exposure degree, oriented assembly of crystals, creation of hierarchical porous structures and synthesis of core-shell structures. It is shown that an appropriate decrease of the crystal size and/or an increase of the exposure degree of certain facets by adding seeds and optimizing the synthesis conditions enhances the catalytic stability and product selectivity in some reactions. This can also be achieved by introducing plenty of mesopores and/or macropores in zeolites as a result of significant alleviation of diffusion limitation. Assembly of zeolite crystals into membranes on porous substrates improves the adsorptive separation performance of zeolites, for e.g. alcohol/water mixture and xylene and butane isomers. Core-shell-structured composites with metal nanoparticles or subnanoparticles as the core and the zeolite, including its modified counterpart, as the shell show excellent catalytic performance in some hydrogenation, dehydrogenation and oxidation reactions. In addition, attempts to illustrate the relationship between zeolite's particle morphology and its catalytic performance are discussed and strategies for the rational design of zeolite's particle size and behavior are envisioned.

Advances in the synthesis and application of SSZ-39 zeolite

Zeolites, especially aluminosilicate zeolites, have been widely utilized in the process of petroleum refining, environmental protection, and fine chemicals. In the past decades, great attentions have been paid on the...

Synthesis of nanosheet epitaxial growth ZSM-5 zeolite with increased diffusivity and its catalytic cracking performance

The introduction of microporous substrate in the nanosheet zeolite reduces the “acid wall” barrier. The diffusional time constant of RP-120 is increased by 32%, and its TOF is increased by 54%.

Highly Efficient NO Abatement over Cu-ZSM-5 with Special Nanosheet Features

Conventional Cu-ZSM-5 and special Cu-ZSM-5 catalysts with diverse morphologies (nanoparticles, nanosheets, hollow spheres) were synthesized and comparatively investigated for their performances in the selective catalytic reduction (SCR) of NO to N₂ with ammonia. Significant differences in SCR behavior were observed, and nanosheet-like Cu-ZSM-5 showed the best SCR performance with the lowest T₅₀ of 130 °C and nearly complete conversion in the temperature range of 200-400 °C. It was found that Cu-ZSM-5 nanosheets [mainly exposed (0 1 0) crystal plane] with abundant mesopores and framework Al species were favorable for the formation of high external surface areas and Al pairs, which influenced the local environment of Cu. This motivated the preferential formation of active copper species and the rapid switch between Cu²⁺ and Cu⁺ species during NH₃-SCR, thus exhibiting the highest NO conversion. In situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that the Cu-ZSM-5 nanosheets were dominated by the Eley-Rideal (E-R) mechanism and the labile nitrite species (NH₄NO₂) were the crucial intermediates during the NH₃-SCR process, while the inert nitrates were more prone to generate on Cu-ZSM-5 nanoparticles and conventional one. The combined density functional theory (DFT) calculations revealed that the decomposition energy barrier of nitrosamide species (NH₂NO) on the (0 1 0) crystal plane of Cu-ZSM-5 was lower than those on (0 0 1) and (1 0 0) crystal planes. This study provides a strategy for the design of NH₃-SCR zeolite catalysts.

Bimetallic Fe–Cu/beta zeolite catalysts for direct hydroxylation of benzene to phenol: effect of the sequence of ion exchange for Fe and Cu cations

Recently, bimetallic cation-exchanged zeolite catalysts have received much attention.

Mesoporogen-free synthesis of hierarchical HZSM-5 for LDPE catalytic cracking

We show how Stöber silica spheres can be transformed into hierarchical HZSM-5 by a mesoporogen-free and modified SAC route.

Designing a Mesoporous Zeolite Catalyst for Products Optimizing in n-Decane Hydrocraking

Mesoporous ZSM-5 zeolite is developed to enhance the catalytic performance in a hydrocracking reaction. The generated mesopores and mesoporous channels in the new catalyst supply more opportunities for reactant accessing the active sites according to the better mass transfer and diffusion. Meanwhile, the acidity of the mesoporous catalyst is also weakened because of the removal of Si and Al species from its MFI structure, which makes the products distribution drift to more valued chemicals such as olefins. In the modified mesoporous ZSM-5 zeolites via different metallic promoters, the olefins’ selectivity increases as the alkalinity of the catalyst increases. The reason for this is that the formed olefins will be further hydrogenated into corresponding alkanes immediately over the extremely acidic zeolite catalyst. Hence, the moderate alkalinity will limit this process, while at the same time the remaining olefins products will too. Furthermore, the Pd-based mesoporous ZSM-5 zeolite shows an excellent n-decane conversion and high propane selectivity due to the occurrence of hydrogen spillover via the Pd promoter. The phenomenon of hydrogen spillover supplies more chemisorbed sites of hydrogen atoms for hydrocracking and hydrogenating in this reaction. In short, this study explores the important effect factors in n-decane hydrocracking reaction activity and products distribution. It also shows a potential for the further industrial application of petroleum-derived fuel hydrocracking according to the optimized products distribution under metallic promoted mesoporous zeolite.

The ZSM-5-Catalyzed Oxidation of Benzene to Phenol with N2O: Effect of Lewis Acid Sites

The oxidation of benzene to phenol (BTOP) with N2O as the oxidant has been studied with a variety of Fe/ZSM-5 catalysts. The literature has conclusively proven that Fe2+ sites are the active sites. However, some studies have suggested that the Lewis acidic sites (LAS) are responsible for the generation of the active chemisorbed oxygen. Nevertheless, there is no clear relationship between the LAS and the N2O selectivity to phenol. In an effort to elucidate the effects of LAS on BTOP with various ZSM-5 catalysts, we investigated the initial N2O selectivity to phenol. Here we show that the initial N2O selectivity to phenol is negative with the amount of LAS over a certain range. The catalyst H-ZSM-5-ST (H-ZSM-5 treated with water vapor) showed a remarkable initial N2O selectivity to phenol as high as 95.9% with a 0.021 mmol g−1 LAS concentration on the surface of the catalyst, while the Fe/ZSM-5 catalyst demonstrated the lowest initial N2O selectivity to phenol (11.7%) with the highest LAS concentration (0.137 mmol g−1). Another remarkable feature is that steaming was more effective than Fe ion exchange and high temperature calcining. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), N2-adsorption-desorption, UV-vis, NH3-TPD and pyridine Fourier transform infrared (FT-IR) techniques. Our results demonstrate how the concentration of LAS is likely to affect the initial N2O selectivity to phenol within a certain range (0.021–0.137 mmol g−1). This research has demonstrated the synergy between the active Fe2+ sites and LAS.

Solvent-free selective oxidation of toluene over metal-doped MCM-22

The synthesis of MCM-22, subsequent metal doping, and physicochemical characterization of the products are reported. The solvent-free catalytic oxidation of toluene using hydrogen peroxide is explored, and the reaction parameters are optimized. A possible reaction mechanism is also described.

Amorphous Cr-doped g-C3N4 as an efficient catalyst for the direct hydroxylation of benzene to phenol

The intrinsic correlations between catalyst structure and catalytic performance at different calcination temperatures were studied.

Zeolite acidity strongly influences hydrogen peroxide activation and oxygenate selectivity in the partial oxidation of methane over M,Fe-MFI (M: Ga, Al, B) zeolites

Brönsted acidity plays a crucial role in the partial oxidation of methane to oxygenated products.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

Immediate hydroxylation of arenes to phenols via V-containing all-silica ZSM-22 zeolite triggered non-radical mechanism

Hydroxylation of arenes via activation of aromatic C_sp2–H bond has attracted great attention for decades but remains a huge challenge. Herein, we achieve the ring hydroxylation of various arenes with stoichiometric hydrogen peroxide (H₂O₂) into the corresponding phenols on a robust heterogeneous catalyst series of V–Si–ZSM-22 (TON type vanadium silicalite zeolites) that is straightforward synthesized from an unusual ionic liquid involved dry-gel-conversion route. For benzene hydroxylation, the phenol yield is 30.8% (selectivity >99%). Ring hydroxylation of mono-/di-alkylbenzenes and halogenated aromatic hydrocarbons cause the yields up to 26.2% and selectivities above 90%. The reaction is completed within 30 s, the fastest occasion so far, resulting in ultra-high turnover frequencies (TOFs). Systematic characterization including ⁵¹V NMR and X-ray absorption fine structure (XAFS) analyses suggest that such high activity associates with the unique non-radical hydroxylation mechanism arising from the in situ created diperoxo V(IV) state.

Hydroxylation of arenes via activation of aromatic C_sp2–H bond remains a challenge. Here, the authors have managed to get various arenes hydroxylated to corresponding phenols using stoichiometric hydrogen peroxide and a series of robust V–Si–ZSM-22 catalysts synthesized via an ionic liquid involved dry-gel-conversion route.

Photochemical device for selective detection of phenol in aqueous solutions

We demonstrate that a lab-on-a-chip device (hereafter termed a photochemical phenol sensor) that integrates a photocatalytic long-period fiber grating (PLPFG), fiber Bragg grating (FBG), polymer membrane, ultraviolet (UV) visible light, and microchannels can be exploited to selectively detect phenol in aqueous solutions. The novel PLPFG consisted of a thinned long-period fiber grating (LPFG) and a UV-visible-light-driven Er3+:YAlO3/SiO2/TiO2 (EYST) coating. The polymer membrane with high phenol permselectivity was synthesized using PEBA2533 doped with β-cyclodextrin and was wrapped around the EYST surface, thus forming a microchannel between the membrane and PLPFG to enable the injection and outflow of standard analytes. Subsequently, a Z-shaped microchannel in a PMMA plate was fabricated and employed as a storage chamber for phenol analytes. To realize the EYST photocatalyst, UV-visible-light was irradiated using a tapered UV optical array. Thereafter, to eliminate the effect of temperature on the device, a FBG sensor as a temperature-compensating element was presented. To demonstrate the sensitivity and selectivity of the proposed device, we investigated the effects of the EYST coating's thickness, phenol-based analytes and temperature on the sensitivity and accuracy of the device for measuring phenol concentrations. The results of our present study suggest that the photochemical sensor is effective over a wide range of concentrations (7.5 μg L-1 to 100 mg L-1), pH values (2.0 to 14.0), and temperatures (10 to 48 °C) for selective detection of phenol in aqueous solutions. Thus, the proposed lab-on-a-chip device may be useful for accurate determination of phenol concentrations in real samples.

Nanosized MCM-22 zeolite using simple non-surfactant organic growth modifiers: synthesis and catalytic applications

Non surfactant cyclic alkylammonium can selectively decrease the rate of crystal growth along the x–y crystal axes during the synthesis of MWW zeolite.