MWW Zeolites Research Articles

Catalytic pyrolysis of polystyrene over rice husk silica-derived traditional and hierarchical green MWW zeolites

This work aimed at the green synthesis of MWW zeolites using rice husk silica as an alternative raw material. It was also performed a post-synthesis desilication procedure with NaOH and CTABr aiming to obtain MWW zeolites with improved accessibility. The set of characterization techniques (XRD, 27Al MAS NMR, N2 physisorption, SEM, TEM, and Pyridine-FTIR) revealed the phase purity and combined structure of micro-mesopores after desilication. The zeolites were evaluated as catalysts for polystyrene pyrolysis, producing benzene, toluene and ethylbenzene, besides styrene monomers, and dimers. Without any catalyst, polystyrene produces only styrene monomers, dimers and trimers. The desilication increases the amount of Brønsted and Lewis sites and the external area, catalyzing the production of polyaromatics and naphthalene derivatives. The activation energy decreased for the catalyzed reactions, reflecting other reaction pathways.

Rapid Synthesis of Nine-Membered-Ring STT Aluminosilicate Zeolite from Chain Rearrangement of Lamella MWW Zeolite and Its Catalytic Performance in the Methanol-to-Olefin Reaction

Rapid Synthesis of Nine-Membered-Ring STT Aluminosilicate Zeolite from Chain Rearrangement of Lamella MWW Zeolite and Its Catalytic Performance in the Methanol-to-Olefin Reaction

Spectroscopic Characterization of Ti Sites in MWW Zeolite in Presence of Hydrogen Peroxide

AbstractTi‐MWW zeolite is a promising catalyst for partial oxidation reactions. In the present work, a Ti‐MWW sample with high TiO2 loading was synthesized. It was revealed that the Ti insertion in the purely siliceous MWW framework mainly occurs during the post treatment washing with HNO3, when the double structure directing agent synthetic method is used. By exploiting carbon monoxide, acetonitrile, pyridine and ammonia as probe molecules in infrared spectroscopy and diffuse reflectance ultraviolet spectroscopy, the acidic behavior of the Ti sites and the interaction of the Ti sites with hydrogen peroxide (H2O2) were revealed. The Ti Lewis acid sites showed stronger acidity than the one observed in Titanium Silicalite‐1 (TS‐1), with MFI framework. This is coherent with previously reported results, suggesting that a significative fraction of the Ti sites in Ti‐MWW are TiOH(SiO)3 instead of Ti(OSi)4. The Ti‐peroxo and ‐hydroperoxo complexes formed upon H2O2 were shown to be more labile than in TS‐1 and they were shown to be completely reversible upon calcination.

Production of Renewable p-Xylene from 2,5-Dimethylfuran and Ethylene over MWW Zeolite Catalysts

Production of Renewable <i>p</i>-Xylene from 2,5-Dimethylfuran and Ethylene over MWW Zeolite Catalysts

Solvent structure and dynamics over Brønsted acid MWW zeolite nanosheets

In the liquid phase of heterogeneous catalysis, solvent plays an important role and governs the kinetics and thermodynamics of a reaction. Although it is often difficult to quantify the role of the solvent, it becomes particularly challenging when a zeolite is used as the catalyst. This difficulty arises from the complex nature of the liquid/zeolite interface and the different solvation environments around catalytically active sites. Here, we use ab initio molecular dynamics simulations to probe the local solvation structure and dynamics of methanol and water over MWW zeolite nanosheets with varying Brønsted acidity. We find that the zeolite framework and the number and location of the acid sites in the zeolite influence the structure and dynamics of the solvent. In particular, methanol is more likely to be in the vicinity of the aluminum (Al3+) at the T4 site than at T1 due to easy accessibility. The methanol oxygen binds strongly to the Al at the T4 site, weakening the Al–O for the bridging acid site, which results in the formation of the silanol group, significantly reducing the acidity of the site. The behavior of methanol is in direct contrast to that of water, where protons can easily propagate from the zeolite to the solvent molecules regardless of the acid site location. Our work provides molecular-level insights into how solvent interacts with zeolite surfaces, leading to an improved understanding of the catalytic site in the MWW zeolite nanosheet.

Influence of Zn species in MWW zeolite on the ethane dehydroaromatization with the assistance of propane co-dehydroaromatization and CO2 reduction

The Zn-containing HMCM-22 samples with different zinc species were prepared and characterized to discriminate the influence of zinc species on the ethane activation by the initial C-H bond cleavage or the β-H cleavage dehydrogenation step of alkyl zinc species (Zn-C2H5) through the propane co-dehydroaromatization and CO2 reduction process. The synergistic effect of zinc species types with the Brønsted acid sites (BAS) of the parent zeolite HMCM-22 had a significant effect on the ethane dehydroaromatization. The synergistic effect between ZnO species and BAS had better ethane conversion and aromatics selectivity in ethane dehydrogenation, while the poor synergistic effect between ZnOH+ species and BAS was not conducive to the formation of aromatics, and ZnOH+ species was more conducive to ethane dehydrogenation. The ZnOH+ has better catalytic performance in CO2 activated ethane and promote the β-H desorption of Zn-C2H5 in ethane dehydrogenation. The synergetic effect of ZnO species and BAS in the zeolite facilitates the hydride transfer activation of ethane by propylene carbenium ions thus enhancing the ethane conversion through ethane-propane co-aromatization.

Zeolitic Pickering Emulsifier with Intrinsic Amphiphilicity.

The formation of oil-in-water Pickering emulsions stabilized by lamellar zeolite MWW (International Zeolite Association, three-letters code) emulsifier without surface grafting is investigated. The crucial emulsification factors are the oligolayer morphology and amphiphilicity developed upon acidic treatment (NH4+ exchange/calcination, HNO3 treatment). In contrast with the readily available/abundant hydrophilic ≡Si-OH group in layer MWW, the lipophilicity generated by strong acid sites is another key to the success of emulsification. Hydrocarbon-strong acid site interaction is long known in petrochemistry and superacid research. However, to the best of our knowledge, this interaction was first introduced to gain lipophilicity in emulsion formation. Finally, the Pd-loaded acidic form of the MWW zeolite successfully stabilized the toluene/H2O emulsion system. The biphasic interfacial nitroarene hydrogenation demonstrated excellent catalytic performance. Overall, this work provided not only a new kind of intrinsic solid to emulsify the organic-aqueous biphase system but also a new mechanism to generate lipophilicity. Both are important for the applications and designs of Pickering emulsion materials.

Epitaxial Growth of Two-Dimensional MWW Zeolite.

Two-dimensional (2D) zeolite, with a high aspect ratio, has more open skeletons and accessible active sites than its three-dimensional (3D) counterpart. However, traditional methods of obtaining 2D zeolites often cause structural damage and widespread skeleton defects, hindering efficient selectivity in molecular separation. In this study, we present, for the first time, a direct epitaxial synthesis of 2D zeolite (Epi-MWW) guided by hexagonal boron nitride (h-BN) with a coincidence matching of site lattices to MWW zeolite. The as-grown Epi-MWW zeolite possesses a high crystallinity and intact hexagonal 2D morphology, with an average thickness of 10 nm and an aspect ratio of over 50. Thanks to its excellent molecular accessibility, the diffusion time constants of o-xylene (OX) and p-xylene (PX) are as 12 and 133 times higher than those of conventional MCM-22, respectively; the PX/OX selectivity of Epi-MWW is 7.4 times better than MCM-22 as calculated by the ideal adsorbed solution theory.

Controllable interlayer hydroxyl condensation of MWW zeolite and its alkylation performance of benzene with ethylene

MWW zeolite holds great promise in the petrochemical industry. However, achieving precise control over its dimension and interlayer hydroxyl condensation remains challenging when using a single, low-toxicity, readily available template. In this study, the rational and targeted hydrothermal synthesis of two-dimensional (2D) MCM-22(P) and three-dimensional (3D) MCM-49 using low-toxic piperidine is realized. The synergistic effect of the organic template and inorganic ions plays a crucial role in the precise synthesis of 2D and 3D MWW zeolite. In contrast to the traditional hexamethyleneimine template, piperidine introduces a distinct characteristic whereby disordered MWW layers coexist with the synthesized products. Specifically, the higher concentration of Na+ species directs more Al species to locate in the T1 site, promoting the condensation of interlayer hydroxyl and leading to the formation of 3D MWW zeolite. In contrast, a system with abundant PI can suppress the condensation of interlayer hydroxyl, resulting in the formation of 2D MWW zeolite. H-MCM-49 synthesized exhibits higher ethylene conversion and ethylbenzene selectivity than H-MCM-22 in the liquid alkylation of benzene and ethylene.

Al Distribution of MWW Zeolites and Its Correlation with Catalytic Performance in Direct Syngas Conversion

Al Distribution of MWW Zeolites and Its Correlation with Catalytic Performance in Direct Syngas Conversion

Effect of Zn–Brønsted acid in zeolites on ethane dehydrogenation with tandem reverse water gas shift reaction

The abundant hydroxyl groups and tunable Brønsted acid sites (BAS) of MWW zeolites were used to introduce and anchor zinc active sites, preventing the volatilization and deactivation of ZnOx due to its reduction to Zn0. Fabricated Zn/MWW catalysts effectively promoted ethane dehydrogenation (EDH) and tandem complex reaction of dehydroaromatization and CO2 oxidative dehydrogenation at high temperatures. Brønsted Zn/MWW catalysts containing [ZnOH]+ were prepared via incipient wetness impregnation using liquid ion exchange whereas those containing [ZnOx]2+ were prepared via atom planting using the gas dechlorination reaction. Additionally, the different acidic sites in MWW zeolites effectively affected the interaction between ZnCl2 and hydroxyl groups during the dechlorination reaction. The catalytic EDH performance of [ZnOx]2+ active sites generated using stronger BASs was higher than that of [ZnOx]2+ active sites generated via weaker BASs and silanol. The synergetic effect of Zn and BASs in zeolites catalyzed dehydroaromatization can be suppress by control the [ZnOH]+ location in the external surface of zeolite or by homogenous consumption of the BAS by [ZnOx]2+. Furthermore, [ZnOH]+ and [ZnOx]2+ catalyzed the reverse water gas shift reaction, accelerated the rate-limiting step of EDH via the CO2 oxidative EDH reaction, inhibited the selectivity of aromatics, and consumed coke via the Boudouard reaction.

Advances in the green and controllable synthesis of MWW zeolite.

MWW zeolite is one of the commercialized zeolites that shows great promise in heterogeneous catalysis and other interdisciplinary application fields due to its coexisting multi-channel system. The green and controllable synthesis of MWW zeolite is conducive to its more efficient and broader application. Many researchers focus on precisely controlling the dimension, interlayer hydroxyl condensation, and aluminum siting, as well as obtaining MWW with low-toxicity, readily available organic structure directing agents (OSDAs) or without OSDAs. This review summarizes recent advancements in the synthesis and application of MWW zeolite, with a particular emphasis on selecting different OSDAs, controlling the interlayer condensation degree and adjusting aluminum distribution. Future research directions and development trends of MWW zeolite are also forecasted.

Elucidating the deactivation mechanism of beta zeolite catalyzed linear alkylbenzene production with oxygenated organic compound contaminated feedstocks.

Zeolite catalyzed alkylation of benzene with long-chain α-olefins is a promising method for the detergent industry. Considering the long-chain α-olefins from Fischer-Tropsch synthesis always contain some oxygenated organic compounds, the effect of which on the alkylation of benzene with 1-dodecene was comprehensively investigated over beta zeolite herein. n-heptanol, n-heptaldehyde and n-heptanoic acid were selected as the model oxygenated organic compounds, and it was revealed that an obvious decrease of lifetime occurred when only trace amount of oxygenated organic compounds were added into the feedstocks. The deactivated catalyst was difficult to regenerate by extraction with hot benzene or coke-burning. A series of characterization tests complementary with DFT calculations revealed that the deactivation was mainly caused by the firm adsorption of oxygenated organic compounds on the acid sites. Further, comparison with the open-framework MWW zeolite revealed a similar effect of oxygenated organic compounds and deactivation mechanism for both beta and MWW, but beta is less sensitive to the oxygenated organic compounds. The main reason lies in the three-dimensional framework of beta, wherein the much higher adsorption energy of 1-dodecene makes it difficult to be replaced by oxygenated organic compounds. Additionally, beta could be regenerated more easily by extraction with hot benzene compared with MWW. But coke-burning caused a sharp decrease of its lifetime, which is mainly due to the decreased acid sites after calcination.

Morphology Regulation of Zeolite MWW via Classical/Nonclassical Crystallization Pathways.

The morphology and porosity of zeolites have an important effect on adsorption and catalytic performance. In the work, simple inorganic salts, i.e., Na salts were used to synthesize MWW zeolite using the organic compound 1-Butyl-2,3-dimethyl-1H-imidazol-3-ium hydroxide as a structure-directing agent and the morphology was regulated by the alkali metals. The sample synthesized without Na salts shows a dense hexagon morphology, while different morphologies like ellipsoid, wool ball, and uniform hexagon appear when using NaOH, Na2CO3, and NaHCO3, respectively. Moreover, the impact of Na salts on the induction, nucleation, and the evolution of crystal growth was studied. Different kinds of Na salts have a different impact on the crystalline induction time in the order of NaHCO3 (36 h) < Na2CO3 (72 h) = NaOH (72 h). Meanwhile, the crystalline mechanism with the cooperation of inorganic salts and the organic SDAs is proposed. NaOH- and Na2CO3-MWW zeolite crystallized with a network of hydrogel via the nonclassical pathway in the system; however, the product is synthesized via a classical route in the NaHCO3 environment. This work provides information about MWW zeolite crystallization and modulating diverse morphologies by adjusting the process.

Accelerated Synthesis of Nanolayered MWW Zeolite by Interzeolite Transformation.

Hierarchical zeolites can offer substantial benefits over bulk zeolites in catalysis. A drawback towards practical implementation is their lengthy synthesis, often requiring complex organic templates. This work describes an accelerated synthesis of nanolayered MWW zeolite based on the combination of interzeolite transformation (IZT) with a dual-templating strategy. FAU zeolite, hexamethyleneimine (HMI), and cetyltrimethylammonium bromide (CTAB) were respectively employed as Al source and primary zeolite, structure directing agent, and exfoliating agent. This approach allowed to reduce the synthesis of nanolayered MWW to 48 h, which is a considerable advance over the state of the art. Tracking structural, textural, morphological, and chemical properties during crystallization showed that 4-membered-ring (4MR) units derived from the FAU precursor are involved in the faster formation of MWW in comparison to a synthesis procedure from amorphous precursor. CTAB restricts the growth of the zeolite in the c-direction, resulting in nanolayered MWW. Moreover, we show that this approach can speed up the synthesis of nanolayered FER. The merits of nanolayered MWW zeolites are demonstrated in terms of improved catalytic performance in the Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to p-xylene compared to bulk reference MWW sample.

Synthesis gold and jade type core shell structure Pt@Sn in deboronated MWW zeolite and its good performance for light alkane dehydrogenation

The Pt@Sn core–shell structure was generated in MWW zeolite catalysts prepared by atom-planting incorporated Sn and deposition–precipitation (DP) loaded Pt for direct dehydrogenation of ethane or propane and CO2 oxidative dehydrogenation. The introduced Sn species can immobilize cubic structure Pt nanoparticle with well dispersed particles by DP method through the charge’s attraction. The stronger charge transfer between Pt-Sn improves the catalyst activity for the C-H band breaking of alkane. The zeolite skeleton incoporated Sn formed Sn-O-Si bond and can be broken to be extracted from the zeolite structure as SnO2 under the dehydrogenation process. The extracted SnO2 in the ‘half-cup’ external supercage encapsulates the Pt nanoparticle and forms the gold and jade type core–shell structure, which reduces the Pt agglomeration and modifies the Pt nanoparticle crystal edge and corner active sites and reduced the C-C band cleavage to side products and coke formation, thus improving the dehydrogenation selectivity and stability.

Encapsulating Pt clusters during the crystallization of MWW zeolite

Directly encapsulation of Pt clusters within MWW zeolite was realized by precisely regulating the synthetic parameters, especially the alkalinity and the Pt content during the hydrothermal synthesis. Simple addition of Pt(NH3)4(NO3)2 to the routine recipe for MWW synthesis resulted in a mixture of MWW to FER zeolites, while appropriately lowering the alkalinity of the initial gel composition favored the formation of pure MWW phase and simultaneously the incorporation of Pt clusters into the micropores of MWW (Pt/Na/Al2O3/H2O/HMI/SiO2 = 0.0015/0.18/0.044/15/0.34/1). At high Pt loading (Pt/SiO2>0.00225), however, the crystallinity decreased considerably and the 3D MWW structure gradually evolved into disordered layers, while Pt particles located on the surface of the disordered layers and were prone to grow into larger ones. In short, addition of Pt(NH3)4(NO3)2 in the initial gel altered the crystallization process of MWW zeolite, and Pt-MWW could be obtained by appropriately lowering the alkalinity and accurately controlling the amount of platinum.

Influence of the Au–Ti Active Site of the Titanosilicate MWW Zeolite on the Catalytic Activity of Ethane Dehydrogenation in the Presence of O2

The titanosilicate zeolite with a MWW topology structure was synthesized by the atom-planting method through the dehydrochlorination of the hydroxyl group in the deboronated ERB-1 zeolite (D-ERB-1) and TiCl4, and Au was further loaded with the deposition precipitation method to apply for the ethane direct dehydrogenation (DH) and dehydrogenation of ethane in the presence of O2 (O2-DH). It was found that Au nanoparticles (NPs) below 5 nm exhibited good activity for ethane direct dehydrogenation and O2-DH. The addition of titanium can not only anchor more Au but also make Au have a more dispersed homogeneous distribution. The ethane O2-DH catalytic performances of Au-loaded Ti-incorporated D-ERB-1 (Ti-D-ERB-1) were compared to those of Au-loaded ZnO-D-ERB-1 and pure silicate D-ERB-1. The results confirm that ethane O2-DH catalyzed by Au-Ti paired active sites is a tandem reaction of catalytic ethane DH and selective H2 combustion (SHC) of generated H2. According to the experimental results and calculated kinetic parameters, such as the activation energy of DH and SHC reaction heat of O2-DH, SHC catalyzed by the Au/Ti-D-ERB-1 catalyst containing the Au-Ti active site can not only break the ethane dehydrogenation thermodynamic equilibrium limitation to improve the ethylene yield but also suppress the CO2 and CO selectivity.

Active precursor promoting nucleation/growth of MWW zeolite and controlling its morphology

MWW zeolites is an important catalyst in petrochemical industry. However, the efficient preparation of MWW zeolites still faces challenges, and the origin of influential factors for regulating its structure properties also remains obscure. Herein, we designed a nanoscale amorphous silica-alumina species denoted as active precursor (APS), and adopt the APS in the HMI mixture to synthesize MCM-22 zeolite (APS-MWW) successfully. To reveal the distinctive role of APS in promoting the crystallization of MWW zeolites, two crystal materials (ITQ-1 and MCM-22) and one mother liquor (ML) as seeds to synthesize three types of MWW zeolites. Typically, when adding APS in the synthetic mixture, the HMI amount was reduced to less than a quarter and crystallization time was reduced to 36 h. APS-MWW sample provides a smaller particle size (2–4 μm) and thinner stacked layer thickness (5–20 nm). Synchrotron radiation Small Angle X-ray Scattering (SAXS) shows each seed has a different impact on the species’ fractal structure and size distribution in the mixture, which is highly related to the nucleation and growth of MWW zeolites. APS shows a large number of 6 membered ring (MR) structure units which play a significant role in boosting the rapid nucleation and growth of APS-MWW zeolite. Among the synthesized MWW zeolites, the APS-MWW performs the highest ethylbenzene yield in the alkylation reaction of benzene-ethylene, which is attributed to its moderate flake thickness, appropriate texture properties and more external surface acidity. The results will provide a new perspective for producing MWW-types zeolites by using the available and effective active precursor.

Tailoring the accessibility and amphiphilicity of MWW zeolites for two-phase glycerol ketalization

Accessibility and amphiphilicity of catalysts play an important role in the ketalization of glycerol with acetone. Delamination and functionalization with organosilane were performed to improve the accessibility and amphiphilicity of MWW zeolites.