Effects of ultrasound on the synthesis of zeolites: a review

In this review, effects of ultrasound on the synthesis of different zeolites are studied. Ultrasound has been applied in crystallization because of its significant influences on the induction periods and nucleation. The effects of ultrasound assisted aging process for different zeolites compared with the effects of static aging, in some cases with microwave assisted aging, stirring aging and synthesis without any aging. Application of ultrasound can influence the size and morphology of the crystals, crystallization time and the crystallinity of the zeolites. The observations indicate that application of ultrasound can shorten the crystallization time. Application of ultrasound also improves crystallinity of the synthesized zeolites. Moreover, use of ultrasound assisted aging leads to altered morphologies and size of the particles.     

Sonochemical synthesis of silver nanoparticles in Y-zeolite substrate

Silver ions can be reduced by 24 kHz ultrasonic waves in ion-exchanged Ag⁺–Y zeolite. In this research, silver ions were introduced into the nano-porous (1.2 nm) zeolite lattice by ion-exchange route. After the reduction process, silver nanoparticles were placed in the cavities, with a size of about 1 nm and also on the external surfaces of the zeolite, with sizes about less than 10 nm. Fast and simple lab-scale reduction of silver ions in the zeolite is important for researchers who work on catalytic properties of metallic silver–zeolite. Several reduction methods have been reported but reduction by ultrasonic waves is a new, simple, and size-controllable method with a high practical value which does not need any complicated facilities. In a sonochemical process, a huge density of energy is provided by the collapse of bubbles which formed by ultrasonic waves. The released energy causes the formation of reducing radicals that consequently reduce the silver ions. It is concluded that the higher silver content may result in the formation of larger silver crystals on the external surface of zeolite crystals. Also, the addition of 1-propanol and 2-propanol to the aqueous reaction medium does not cause better reduction. In addition, increasing the irradiation time and ultrasonic power does not affect the silver crystal growth significantly but the extent of silver ion reduction increases when the power of ultrasonic waves increases. All samples were irradiated under the same ultrasonic conditions. The samples were analyzed by XRD, EDS, SEM, and TEM.     

Pore network model for catalytic dehydration of methanol at particle level

γ‐Alumina is used as a catalyst for converting methanol to dimethyl ether. The process takes place in a packed or fluidized bed reactor consisting of microporous particles with distributed pore sizes and interconnectivities. The efficiency of the process is, however, significantly affected by the pore space structure of the particles. All the previous attempts for modeling this phenomenon have used continuum formulation of the problem based on classical equations of mass transport and reaction, without any regards for the effect of pore space morphology. In this article, we study the catalyst's performance by developing a network model for the pore space, with distributed pore sizes and interconnectivities. The network model is used to study the effect of several parameters such as pore space morphology, concentration, and temperature on catalyst's effectiveness factor. The results will be used for reactor simulations. © 2008 American Institute of Chemical Engineers AIChE J, 2009