Non-templated intercrystalline mesoporosity in heteroatom-doped AlPO₄-5 using N-methyldicyclohexylamine as structure-directing agent

Non-doped and 11 (Si, Co, Mg, Zn, Mn, Sn(II), Sn(IV), Cr, Fe, V and Zr) heteroatom-doped AlPO₄-5 samples have been crystallized in a high purity using N-methyldicyclohexylamine as structure-directing agent by simply changing the heteroatom source. Needle- or rod-like crystals of nano size, arranged in cylindrical or spherical agglomerates containing intercrystalline mesopores, compose all samples, excepting VAPO-5, whose nanocrystals and agglomerates are shapeless, and ZnAPO-5 composed by larger crystals not defining any mesorpore in their arrangement. Crystal sizes and dimensions were evaluated by direct application of Scherrer equation to the most intense XRD peaks and by Williamson-Hall plots of h 0 0 and 0 0 l reflections, respectively. Scherrer equation systematically underestimated crystal sizes. Williamson-Hall method gives different crystal dimensions, which were much closer to those observed by SEM and TEM studies. Moreover, Williamson-Hall plots consider separately crystal dimensions and strain/stress contributions to XRD peaks broadening, which have been used as key information to rationally understand the role of different heteroatom ions in the sample morphology and their mesoporosity. Intercrystalline mesoporosity could be studied by N₂ adsorption/desorption isotherms in as-prepared samples, as well as in calcined ones, confirming the stability of mesopores under calcinations conditions. Both maxima of pore size distribution and external (extra-micropore) area well correlate with crystal size of MeAPO-5 with a given agglomerates morphology, suggesting different compact packing for each morphology.