Carbon nanotube templated synthesis of hierarchical SAPO-34 catalysts with different structure directing agents for catalytic conversion of methanol to light olefins

Abstract

Crystalline silicoaluminophosphate of the CHA framework type (SAPO-34) with a hierarchical pore structure was hydrothermally synthesized using carbon nanotube as the mesopore template and TEAOH and/or morpholine as structure directing agents. The synthesized samples were characterized by XRD, SEM, TEM, FTIR, NH3-TPD, XRF and N2-physisorption techniques and their catalytic performance was tested in MTO reaction. N2-adsorption results showed that the population of mesopores created inside the SAPO-34 crystals could be controlled by tuning the amount of carbon nanotube applied. It was also found that the structure directing agents could lead to significant effects in the crystallinity and mesoporosity of hierarchical SAPO-34 samples. Increasing molar ratio of TEAOH in the initial gel increased the mesoporosity of SAPO-34, but decreased its crystallinity and particle size. The hierarchical SAPO-34 samples showed superior catalytic performance in the MTO reaction with longer lifetime compared to the conventional SAPO-34. The hierarchical SAPO-34 prepared by the mixture of TEAOH and morpholine exhibited longer catalyst lifetime compared to those of other hierarchical SAPO-34 obtained by only TEAOH or morpholine. The characterization of the spent catalysts indicated that both rate of coke formation and coke-susceptibility of hierarchical SAPO-34 was lower than conventional SAPO-34.