Controllable Synthesis of Titanium Silicon Molecular Zeolite Nanosheet with Short b-Axis Thickness and Application in Oxidative Desulfurization

Titanium silicon molecular zeolite (TS-1) plays an important role in catalytic reactions due to its unique nanostructure. The straight channel on TS-1 was parallel to the orientation of the short b-axis and directly exposed to the aperture of the 10-member ring with a diameter of 0.54 nm × 0.56 nm. This structure could effectively reduce the diffuse restriction of bulk organic compounds during the oxidative desulfurization process. As a kind of cationic polymer electrolyte, polydimethyldiallyl ammonium chloride (PDDA) contains continuous [C8H16N+Cl-] chain segments, in which the Cl- could function as an effective structure-directing agent in the synthesis of nanomaterials. The chain of PDDA could adequately interact with the [0 1 0] plane in the preparation process of zeolite, and then the TS-1 nanosheet with short b-axis thickness (6 nm) could be obtained. The pore structure of the TS-1 nanosheet is controlled by regulating the content of PDDA. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N2 physical adsorption analysis, infrared absorption spectrum and ultraviolet-visible spectrum were used to determine the TS-1. The thinner nanosheets exhibit excellent catalytic performance in oxidative desulfurization of dibenzothiophene (DBT), in which the removal rate could remain at 100% after three recycles. Here, the TS-1 nanosheet with short b-axis thickness has a promising future in catalytic reactions.

Disinfectant-Assisted Preparation of Hierarchical ZSM-5 Zeolite with Excellent Catalytic Stabilities in Propane Aromatization

A series of quaternary ammonium or phosphonium salts were applied as zeolite growth modifiers in the synthesis of hierarchical ZSM-5 zeolite. The results showed that the use of methyltriphenylphosphonium bromide (MTBBP) could yield nano-sized hierarchical ZSM-5 zeolite with a "rice crust" morphology feature, which demonstrates a better catalytic performance than other disinfect candidates. It was confirmed that the addition of MTBBP did not cause discernable adverse effects on the microstructures or acidities of ZSM-5, but it led to the creation of abundant meso- to marco- pores as a result of aligned tiny particle aggregations. Moreover, the generation of the special morphology was believed to be a result of the coordination and competition between MTBBP and Na+ cations. The as-synthesized hierarchical zeolite was loaded with Zn and utilized in the propane aromatization reaction, which displayed a prolonged lifetime (1430 min vs. 290 min compared with conventional ZSM-5) and an enhanced total turnover number that is four folds of the traditional one, owing to the attenuated hydride transfer reaction and slow coking rate. This work provides a new method to alter the morphological properties of zeolites with low-cost disinfectants, which is of great potential for industrial applications.

The Synergistic Impact of Crystal Seed and Fluoride Ion in the Synthesis of Silicalite-1 Zeolite in Low-Template Systems

Silicalite-1 zeolites are widely applied in gas adsorption, catalysis, and separation due to their excellent hydrothermal stability and unique pore structure. However, traditional preparation methods have inherent drawbacks such as high pollution, high cost, etc. Therefore, this work proposed a green and efficient route for preparing Silicalite-1 zeolite by adding NH4F (F/Si = 0.1) and seeds (10 wt%) in a much shorter time (8 h) in a low-template system (TPA+/Si = 0.007). It was found that NH4F is beneficial for inhibiting the formation of SiO2. The S-1 seeds could drastically induce the formation of the zeolite skeleton structure. Noteworthy, the morphology of zeolites was determined by the relative content of NH4F and seeds. The crystal morphology is determined by the higher content of the two substances; however, when the content is similar, the crystal morphology is determined by NH4F. The results showed that simultaneous control of NH4F and seeds can suppress SiO2 formation, can improve the relative crystallinity of products, and can be precisely regulated via the synergistic effect of both in zeolite morphology. This work not only provides new ideas for regulating the morphology of silicate-1 crystals but also offers a new path for industrial large-scale production of low-cost and efficient zeolites.

Design of an Organic Template for Synthesizing ITR Zeolites under Ge-Free Conditions

Germanosilicate zeolites with various structures have been extensively synthesized, but the syntheses of corresponding zeolite structures in the absence of germanium species remain a challenge. One such example is an ITR zeolite structure, which is a twin of the ITH zeolite structure. Through the modification of a classic organic template for synthesizing ITH zeolites and thus designing a new organic template with high compatibility to ITR zeolite assisted by theoretical simulation, we, for the first time, show the Ge-free synthesis of an ITR structure including pure silica, aluminosilicate, and borosilicate ITR zeolites. These materials have high crystallinity, corresponding to an ITR content of more than 95%. In the methanol-to-propylene (MTP) reaction, the obtained aluminosilicate ITR zeolite exhibits excellent propylene selectivity and a long lifetime compared with conventional aluminosilicate ZSM-5 zeolite. The strategy for the design of organic templates might offer a new opportunity for rational syntheses of novel zeolites and, thus, the development of highly efficient zeolite catalysts in the future.

Controllable Orientation Growth of ZSM-5 for Methanol to Hydrocarbon Conversion: Cooperative Effects of Seed Induction and Medium pH Control

The growth orientation of ZSM-5 zeolites strongly affects product selectivity in methanol conversion reaction. Here, we proposed a versatile synthetic strategy by introducing seeds and controlling medium pH to achieve controllable orientation growth of ZSM-5. The systematic analysis of the crystallization process indicated that the introduction of seeds ensured successful crystallization in a quasi-neutral solution and the dissolution rate of seeds and aluminosilicate determined the growth orientation of ZSM-5. In the quasi-neutral solution, the slow dissolution of seeds and aluminosilicate enhanced growth advantages along the c axis. The ratio between the length of the c axis and b axis (L_c/L_b) of the obtained ZSM-5 at pH of 7 could reach 8.1, much higher than 1.8 obtained at pH of 11. No obvious impact of seed added amount on growth orientation was found, while with increasing seed crystal size, the obtained ZSM-5 showed preferred growth along the c axis. The L_c/L_b of the sample adding seeds with a size of 355 nm reached 7.9, much higher than 2.1 of the sample adding seeds with a size of 70 nm. The obtained ZSM-5 with specific growth orientation exhibited potential shape selectivity in methanol to aromatics and olefin reaction. This work opens new possibilities to tailor the orientation growth of ZSM-5 based on the seed-induced strategy under mild conditions.

Constructing single-crystalline hierarchical plate-like ZSM-5 zeolites with short b-axis length in the synthesis for catalyzing MTO reaction

ZSM-5 zeolite with hierarchical and lamellar structure is highly desired in industrial application. This paper reports an efficient additive, tetramethylguanidine (TMG), modifying crystal growth of the zeolite to this morphology....

Plate-like MFI crystal growth achieved by guanidine compounds

The modifier not only changed the crystals’ morphology, but also enabled more Al or Ti to be incorporated into the MFI framework.

Protective dissolution: generating secondary pores in zeolite by mechanochemical reaction

Introduction of meso-/macropores into the intrinsic microporous framework of zeolites has raised substantial interest in catalytic reactions with bulky reactants. Herein, we report the formation of secondary meso-/macropores in Silicalite-1 zeolite by a solvent-free mechanochemical grinding process. The strategy allows the preservation of high crystallinity and microporosity of the pristine zeolite, and the generation of mesopores at room temperature and marcopores at higher temperatures. The roles of the tetrapropylammonium bromide (TPABr) and ammonium fluoride (NH₄F) have been proposed and demonstrated. A protective layer is formed by TPA⁺ ions bonded with the surficial defects to shield the outer surface from the direct attack by F⁻. Instead, F⁻ diffuses into the micropore system in a local aqueous environment within zeolite formed by the mechanochemical reaction. As a result, freely diffused F⁻ selectively dissolves zones with structural defects to form secondary pores inside the zeolite. Moreover, this strategy proves highly effective in encapsulation of nanoparticles (Pt, Co) in the meso-/macropores of Silicalite-1 zeolite, forming a yolk–shell composite catalyst for potential applications.

A novel strategy for meso-/macropores formation in zeolites by mechanochemical reaction.

Morphology-Controlled Synthesis of H-type MFI Zeolites with Unique Stacked Structures through a One-Pot Solvent-Free Strategy

H-type aluminosilicate zeolites are extensively used as solid-acid catalysts and support materials in industrial catalysis. However, the conventional synthesis methods involving hydrothermal syntheses and ion-exchange processes suffer from severe water pollution and toxic gas emissions. Herein, H-type MFI zeolite catalysts with a unique stacked structure were directly synthesized in the presence of NH₄ F and with the help of zeolite confinement through a solvent-free route without further ion-exchange procedures. A range of ex situ and in situ characterization procedures were used to provide evidence of the simultaneous use of pre-made ZSM-5 and NH₄ F as a confined Al source and mineralizer, respectively. The confined zeolite framework of ZSM-5 prevented the formation of AlF_x species between NH₄ F and Al atoms, ensuring that the prepared samples had desirable acidic properties. Moreover, the resulting morphology could be controlled by using different silica substrates. The obtained H-type MFI zeolites showed excellent catalytic performance in methanol-to-gasoline reactions owing to their unique structure and directly exposed acidic sites. The developed one-pot strategy provides an alternative method for the facile synthesis of H-type zeolites with defined morphology.

Boosting the turnover number of core–shell Al-ZSM-5@B-ZSM-5 zeolite for methanol to propylene reaction by modulating its gradient acid site distribution and low consumption diffusion

Hierarchical Al-ZSM-5@B-ZSM-5 core–shell zeolite was prepared to boost turnover number of MTP reaction.

Strategies to control zeolite particle morphology

Methods to synthesize zeolites with different crystal habits and assemble zeolite crystals into specific structures are reviewed for the rational design of zeolite particle morphologies.

Conversion of methanol to propylene over hierarchical HZSM-5: the effect of Al spatial distribution

Different silicon sources caused diverse Al spatial distribution in HZSM-5, and this affected the hierarchical structures and catalytic performance of desilicated zeolites. After being treated with 0.1 M NaOH, HZSM-5 zeolites synthesized with silica sol exhibited relatively widely distributed mesopores and channels, and possessed highly improved propylene selectivity and activity stability.

Catalytic test reactions for the evaluation of hierarchical zeolites

Hierarchical zeolites have received increasing attention in the last decade due to their outstanding catalytic performance. Several types of hierarchical zeolites can be prepared by a large number of different techniques. Hierarchical zeolites combine the intrinsic catalytic properties of conventional zeolites and the facilitated access and transport in the additional meso- or macropore system. In this tutorial review, we discuss several test reactions that have been explored to show the benefit of the hierarchical pore system with respect to their suitability to prove the positive effects of hierarchical porous zeolites. It is important to note that positive effects on activity, stability and less frequently selectivity observed for hierarchically structured catalysts not necessarily are only a consequence of the additional meso- or macropores but also the number, strength and location of active sites as well as defects and impurities. With regard to these aspects, the test reaction has to be chosen carefully and potential changes in the chemistry of the catalyst have to be considered as well. In addition to the determination of conversion, yield and selectivity, we will show that the calculation of the activation energy and the determination of the Thiele modulus and the effectiveness factor are good indicators of the presence or absence of diffusion limitations in hierarchical zeolites compared to their parent materials.

Conversion of methanol to propylene over nano-sized ZSM-5 zeolite aggregates synthesized by a modified seed-induced method with CTAB

Nano-sized ZSM-5 zeolite aggregates with mesopores and high catalytic activity were prepared by a modified seed-induced method using silicate-1 as seeds without additional templates in the presence of a trace amount of CTAB.

Aerosol-assisted synthesis of nano-sized ZSM-5 aggregates

An aerosol-assisted method is used to synthesize nano-sized ZSM-5 aggregates with good texture properties and Al species distribution.