Pores for thought: Zeolite catalysts open wide

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  • Published: Aug 1, 2011
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Pores for thought: Zeolite catalysts open wide

Highly converting

Researchers in China have developed a new process for making zeolite catalysts with wide open pores. Their controllable synthesis leads to materials active in the conversion of methanol to propylene with a high, 98%, conversion rate.

Xufang Qian, Junming Du, Bin Li, Min Si, Yisu Yang, Yuanyuan Hu, Guoxing Niu, Yahong Zhang, Hualong Xu, Bo Tu, Yi Tang and Dongyuan Zhao of Fudan University, in Shanghai in China, have synthesised a range of zeolite core materials with ordered mesoporous silica shells using an ultra-dilute liquid-phase coating strategy in an acidic medium. The method is facile, reproducible and could be used for a wide range of different zeolite materials. They used Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS) and other analytical tools to verify the character and structure of their products.

The team explains how ordered, porous siliceous materials with pore sizes ranging from the micropore scale less than 2 nanometres to the mesopore, 2-50 nm, have wide potential as catalysts, adsorbents, carriers as well as advanced applications in medical diagnostics and microelectronics. As such, hierarchical pore structures in crystalline zeolites have been the focus of much attention during the last decade because of the possibilities of synergistic functionality derived from the different pore sizes available.

Core-shell solution

For example, core-shell composites beta-zeolite@Silicalite-1 and ZSM-5@Silicalite-1 have demonstrated remarkable utility in separation, storage, controlled release and catalysis. However, some of these materials have non-uniform, disordered, wormlike porosity, which is not the most useful structure. The development of a controlled synthesis that generates uniformity in the hierarchical pore structure would be of much greater utility. Various methods have been tried but in terms of synthesising cores for coating of MCM-41-type silica shells they have primarily been associated with noble metal and metal oxides and so lack af?nity towards the reactants for mesoporous silica coating.

The Fudan team chose submicro-sized zeolite single-crystals such as MFI-type Silicalite-1 or ZSM-5 with pseudohexagonal prism-shaped morphologies as core components and the tri-block copolymer Pluronic P123 as a template to construct mesoporous silica shells under acidic conditions. The team explains that the resulting materials have "a micro/mesoporous core-shell structure consists of intrinsically crystalline microporous frameworks and highly ordered SBA-15 mesopores (6.9 to 8.1 nm) with plenty of mesotunnels (2.8 to 3.6 nm) located in amorphous silica walls." The zeolite cores are connected directly with the large mesopores of SBA-15 resulting in a highly opened junction between micropore and mesopore frameworks. The high-resolution SEM images show the surfaces of the core-shell particles to have whorls resembling those of a fingerpring, although obviously on a much smaller scale!

The team adds that the unique crystal face-dependent shell-thickness of their products can be tailored to some extent (30-45/40-120 nm), which, they explain, gives them tuneable porosity. Preliminary tests show the specificity and catalytic potential for methanol conversion of these "opened" hierarchical porous materials in which surface acidity might be sacrificed in favour of better selectivity for C5 to C6 hydrocarbon products.

We asked Zhao about the next developmental steps and the potential commercialization of this catalyst. "This is the first time we have reported a facile method to prepare large pore core-shell structured composites using triblock copolymer as templates," he told us, "therefore, we can further develop the synthesis of core-shell composites with tunable mesostructures (e.g., cage-like mesoporous silica shells) to improve our synthesis system." He adds that to open up other potential applications, the team is working on the application of heavy petroleum oil hydrocracking. Tests are now being carried out in that regard. "We will also try to extend this synthesis method to other functional core materials other than zeolites to obtain new interesting nanomaterials," Zhao adds. Commercialization is expected but more exploration and cooperation is now needed. "We hope our research work can trigger new interest in core-shell material synthesis and get a big step forward in the future," adds Zhao. 

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Credit: Adapted from RSC Chemical Science - Researchers in China have developed a new process for making zeolite catalysts with wide open pores. Their controllable synthesis leads to materials active in the conversion of methanol to propylene with a high, 98%, conversion rate.
Hierarchical porosity and whorls

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