Size control of SAPO-34 crystals and their catalyst lifetime in the methanol-to-olefin reaction

Crystals of SAPO-34 molecular sieves were synthesized under hydrothermal conditions by using tetraethylammonium hydroxide (TEAOH), morpholine (Mor), N,N,N',N'-tetraethyletliane-1,2-diamine (TEEDA), or a mixture of TEAOH and Mor as structure-directing agents (SDA). The crystal growth of SAPO-34 was studied by X-ray diffraction and by scanning electron microscopy and field-emission scanning electron microscopy. The crystal size of SAPO-34 could be controlled between 1.5 and 7 mu m by hydrothermal synthesis using a mixture of TEAOH and Mor as the SDA. Small SAPO-34 crystals (800 nm in size) were formed when TEAOH was used as the SDA. The smaller SAPO-34 crystals showed a longer catalyst lifetime in the methanol-to-olefins reactions. The amount of coke deposited on deactivated SAPO-34 catalyst increased with decreasing crystal size, indicating that the effectiveness of the catalyst could be improved by reducing the crystal size of SAPO-34. (C) 2009 Elsevier B.V. All rights reserved.