Towards the control of intercrystalline mesoporosity in inorganic microporous materials: The case of CoAPO-5

The generation of mesoporosity in microporous materials has been postulated as a valid strategy to limit the diffusional problems of organic substrates within the pores of these materials, without affecting their intrinsic catalytic behavior. This article shows the effect of systematic variations of different synthesis parameters on the nature of the intercrystalline mesoporosity in CoAPO-5 materials. The hard impositions of this approach, including a structure-directing agent (SDA) possessing enough structural specificity towards AFI materials under a very wide range of synthesis conditions, and at the same time having the ability of generating intercrystalline mesoporosity, severely reduces the possible choices. Fortunately, N-methyldicyclohexylamine (MCHA) simultaneously obeys both conditions. We have examined some parameters conventionally affecting the crystallization process such as time, temperature and dilution, and some other parameters with particular effect on the results of the crystallization of microporous materials such as heteroatom content, heteroatom source, pH of the starting gel and heating source (conventional or assisted by microwave radiation). Except Co source, the rest of the parameters did have a significant influence on the nature and/or magnitude of intracrystalline mesoporosity. The effect of pH and dilution was particularly relevant, so that ordered intracrystalline mesoporosity is favored when CoAPO-5 is crystallized from relatively diluted and acidic gels. In addition, high Co content produces a higher and better-defined mesoporosity, although it implies a modification of the catalytic activity as the number of active centers is inherently varied. Finally, a relevant improvement in the mesoporosity is obtained when heating is assisted by microwave radiation, since the resultant CoAPO-5 is composed by smaller crystals efficiently and orderly agglomerated in pseudospherical particles with a well-defined mesoporosity of narrower pores.

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► MCHA is an excellent SDA for microporous AFI structures and intracrystalline mesoporosity.
► Intracrystalline mesoporosity in CoAPO-5 by modification of synthesis parameters.
► More ordered mesoporosity at low pH, short synthesis times, high Co content and diluted gels.
► Microwave-assisted heating produced much better intracrystalline mesoporosity than hydrothermal treatment.