Confined-space synthesis of hierarchical MgAPO-11 molecular sieves with good hydroisomerization performance
Hierarchical MgAPO-11 molecular sieves with large BET surface areas and high pore volumes have been synthesized via dry-gel conversion of the magnesioaluminophosphate-carbon (MAP-C) composites. The carbon film coated on the mesopores of amorphous magnesioaluminophosphate (MAP) provides a confined-space to restrict the overgrowth of the crystals. The products were well characterized using XRD , SEM, TEM , N 2 adsorption-desorption, infrared spectroscopy of pyridine adsorption (Py-IR) and solid-state NMR techniques. The results show that highly mesoporous MgAPO-11 with good crystallinity can be obtained at an appropriate carbon loading amount. The time-dependent study indicates that the formation process of hierarchical MgAPO-11 obeys a reversed crystal growth route: crystallization started on the outer surface of the amorphous precursor, then extended to inward, and finally hierarchical molecular sieves assembled by nanosized crystals with a size of about 50 nm were obtained. The catalyst based on hierarchical MgAPO-11 exhibits a significant higher activity and selectivity than the conventional Pt/MgAPO-11 catalyst in the hydroisomerization of n -dodecane, attributing to its higher acidity and excellent diffusion properties resulting from the hierarchical pore architecture.
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