Influence of post-synthetic modifications on the composition, acidity and textural properties of ZSM-22 zeolite

Controlling Crystal Morphology of Anisotropic Zeolites with Elemental Composition

The morphology of zeolite crystals strongly affects their textural, catalytic, and mechanical attributes. However, controlling zeolite crystal morphology without using modifiers or structure-directing agents remains a challenging task because of our limited understanding of the relationships between zeolite crystal shape, crystallization mechanism, and composition of the starting synthesis mixture. In this study, we aimed at developing a general method for controlling the morphology of zeolites by assessing the impact of the Si/T molar ratio of the synthesis gel on the growth rate of zeolite crystals in various crystallographic directions and on the final crystal morphology of the UTL germanosilicate with a 2D system of intersecting 14- and 12-ring pores. Our results showed that flat UTL crystals progressively thicken with the Si/Ge molar ratio, demonstrating that Ge concentration controls the relative rate of crystal growth in the perpendicular direction to the pore system. The morphology of other zeolites and zeotypes with an anisotropic structure, including AFI (12R), IFR (12R), MWW (10-10R), and IWW (12-10-8R), can also be predicted based on their Si/T ratio, suggesting a systematic pattern across zeolite structures and in a wide range of zeolite framework elements. Combined, these findings introduce a facile and cost-efficient method for directly controlling crystal morphology of zeolites with anisotropic structures with a high potential for scale-up while providing further insights into the role of elemental composition in zeolite crystal growth.

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis and reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control and potential catalysts for partial methane oxidation, have in particular evidenced a wide variety of Cu structures that are observed to change with exposure conditions, zeolite composition, and topology. Here, we develop predictive models for Cu cation speciation and nuclearity in CHA, MOR, BEA, AFX, and FER zeolite topologies using interatomic potentials, quantum chemical calculations, and Monte Carlo simulations to interrogate this vast configurational and compositional space. Model predictions are used to rationalize experimentally observed differences between Cu-zeolites in a wide-body of literature, including nuclearity populations, structural variations, and methanol per Cu yields. Our results show that both topological features and commonly observed Al-siting biases in MOR zeolites increase the population of binuclear Cu sites, explaining the small population of mononuclear Cu sites observed in these materials relative to other zeolites such as CHA and BEA. Finally, we used a machine learning classification model to determine the preference to form mononuclear or binuclear Cu sites at different Al configurations in 200 zeolites in the international zeolite database. Model results reveal several zeolite topologies at extreme ends of the mononuclear vs binuclear spectrum, highlighting synthetic options for realization of zeolites with strong Cu nuclearity preferences.

Improved Catalytic Performances of the NaOH-Treated ZSM-22 Zeolite in the 1-Butene Skeletal Isomerization Reaction: Effect of External Acid Sites

In this paper, a series of alkaline-treated ZSM-22 zeolite samples were prepared by treating the parent ZSM-22 zeolite using NaOH aqueous solution with different concentrations. By investigating the effects of alkaline treatment on the parent ZSM-22 zeolite, we discovered that the alkaline treatment contributed to the reduction of Brønsted acid sites due to the coverage of extra-framework Al on its external surface. In addition, it was found that the alkaline-treated samples were favorable to the improvement of the isobutene yield and selectivity, while these features appeared to be low for the subsequent acid-washed counterparts in the skeletal isomerization reaction of 1-butene. These results indicate that the catalytic performance of ZSM-22 zeolite is related to reduced amounts of Brønsted acid sites in it. To further reveal the reasons for the promoted catalytic performances of the alkaline-treated ZSM-22 series zeolites, we studied the properties of coke deposited on the two series of samples using Raman spectroscopy and thermogravimetric analysis and mass spectrometry (TG/MS-TPO). It was shown that the carbon deposited on the alkaline-treated series samples was mainly distributed at the outer surface, while the coke was distributed to a relatively lesser extent at the exterior surface for the acid-washed series samples. Moreover, by partially passivating outer acid sites of the parent zeolite, the selected alkaline-treated zeolite, and acid-washed zeolite, their isobutene selectivities were all improved with the decrease in outer acid sites. These phenomena confirmed that the improved catalytic performances of the alkaline-treated samples are related to their decreased external Brønsted acid site density, which further demonstrated that the high isobutene yield and selectivity in the skeletal isomerization reaction of 1-butene is realized via the monomolecular reaction pathway of 1-butene.

Silica Coating of Metal-Loaded H-ZSM-22 to Form the Core-Shell Nanostructures: Characterization, Textural Properties, and Catalytic Potency in the Esterification of Oleic Acid

In this study, ZSM-22 was synthesized using N,N-diethylaniline as a template through a hydrothermal method. The proton and various metals such as zirconium, strontium, and iron were immobilized on the surface of obtained zeolites through the ion exchange method. The catalysts were studied by Fourier-Transform Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) adsorption isotherms, Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) elemental analysis, and Temperature-Programmed Desorption of ammonia (TPD-NH3) technique for determining the number of acid sites. In the esterification reaction of oleic acid, the operating conditions such as catalyst dosage, temperature, molar ratio of methanol to oil, and reaction time were optimized and adjusted at 11 wt%, 70°C, 10 : 1, and 48 h subsequently. The maximum yield% of 48.07% was achieved in the presence of Zr-H-ZSM-22 at optimum conditions. In order to improve the efficiency of three zeolites Zr-H-ZSM-22, Fe-H-ZSM-22, and Sr-H-ZSM-22, the core-shell structures with SiO2 coating were prepared. Zr-H-ZSM-22@SiO2 was less active than Zr-H-ZSM-22 due to the SiO2 coverage of Lewis active sites.

State of the Art and Perspectives of Hierarchical Zeolites: Practical Overview of Synthesis Methods and Use in Catalysis

Microporous zeolites have proven to be of great importance in many chemical processes. Yet, they often suffer from diffusion limitations causing inefficient use of the available catalytically active sites. To address this problem, hierarchical zeolites have been developed, which extensively improve the catalytic performance. There is a multitude of recent literature describing synthesis of and catalysis with these hierarchical zeolites. This review attempts to organize and overview this literature (of the last 5 years), with emphasis on the most important advances with regard to synthesis and application of such zeolites. Special attention is paid to the most common and important 10- and 12-membered ring zeolites (MTT, TON, FER, MFI, MOR, FAU, and *BEA). In contrast to previous reviews, the research per zeolite topology is brought together and discussed here. This allows the reader to instantly find the best synthesis method in accordance to the desired zeolite properties. A summarizing graph is made available to enable the reader to select suitable synthesis procedures based on zeolite acidity and mesoporosity, the two most important tunable properties.

Synthesis of ZSM-23 (MTT) zeolites with different crystal morphology and intergrowths: effects on the catalytic performance in the conversion of methanol to hydrocarbons

A series of zeolite ZSM-23 samples prepared with different organic structure-directing agents, gave rise to crystalline phases, which show a large variety of particle dimensions.