Racing Crystallization Mechanism for Economical Design of Single-Crystal Hollow ZSM-5 with the Broken Limit of Si/Al Ratio and Improved Mass Transfer

Development of economic strategy to synthesize hollow zeolite with widely tunable Si/Al ratios providing variable acidity is of great significance in industry. Here, a one-step and low-cost strategy without mesoporogen was successfully developed to synthesize single-crystal hollow ZSM-5 containing mesopores/macropores, with variable Si/Al ratios of about 14-∞ and 114-∞ at critical TPA⁺/SiO₂ ratios of 0.05-0.1 and 0.05, respectively. This is the first time the usage of a large amount of TPAOH was avoided while breaking the traditional limitation of Si/Al ratio (25-50). The component of synthesis system and crystallization temperature acting as the vital roles in hollow structure has been confirmed by a series of characterization. Moreover, according to the investigation of the evolution process, a novel racing crystallization mechanism based on the competition relationship between surface crystallization and the internal dissolution rate was proposed for the first time. The racing crystallization mechanism and internal nonprotective aluminum become the crucial factors for synthesis. The prepared hollow ZSM-5 zeolites exhibit superior catalytic performance in the different acidity-catalyzed condensation involving large molecules between benzaldehyde and n-butyl alcohol as well as 2-hydroxyacetophenone, which is mainly attributed to the property acidity, more accessible active Al sites on the surface, and shorter diffusion path. By calculating, the effectiveness factor (η) of hollow zeolite is close to 1, further confirming its better mass transfer ability. The strategy has also been successfully extended to the synthesis of high-amount Fe-doped, Ga-doped, and B-doped hollow silicate-1.

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.

Effect of alkali-treated HZSM-5 zeolite on the production of aromatic hydrocarbons from microwave assisted catalytic fast pyrolysis (MACFP) of rice husk

We performed microwave-assisted catalytic fast pyrolysis (MACFP) of rice husk (RH) over an alkali-treated HZSM-5 zeolite, for production of hydrocarbons. The treatment consisted in the modification of the HZSM-5 by the organic base tetrapropylammonium hydroxide (TPAOH) solution at several concentrations. We characterized the resulting catalysts by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption, and temperature-programmed sorption of ammonia (NH₃-TPD). The results suggest that the TPAOH treatment generated mesoporous structures in the HZSM-5, while preserving its microporous structure and crystallinity. We obtained the highest yield (45.9%) of hydrocarbons from MACFP of rice husk (RH) at 550 °C. As the TPAOH concentration increases, the relative content of BTEX hydrocarbons (benzene, toluene, ethylbenzene, and xylene) reaches a maximum value of 22.9% at 2.0 mol/L. A comparison of results obtained over the organic base TPAOH (HZSM-5 modified by 2.0 mol/L TPAOH solution) with those obtained over an inorganic base (HZSM-5 modified by 2.0 mol/L NaOH solution) shows a 4.3% increase in the relative content of monocyclic aromatic hydrocarbons for the TPAOH. In addition, the TPAOH-treated catalyst shows excellent selectivity of BTEX (58.5%), which is higher than the selectivity obtained with the parent HZSM-5 (51.2%) and NaOH-treated HZSM-5 (53.9%). The TPAOH-modified HZSM-5 catalyst effectively reduced coke formation by 4.6% compared to MACFP over the parent HZSM-5, most likely because TPAOH decreases the concentration of strong acidic sites on the outer surface of the catalyst, creating a mesoporous structure while retaining the weak acidic sites on the HZSM-5 inner surface. The new catalyst generated in this work contains a moderate amount of mesopores structures, which allows for effective upgrading of pyrolysis vapors while simultaneously reducing coke formation, thereby addressing a significant problem in the development of the catalytic fast pyrolysis process.