Characterization and Thermal Decomposition Modes of SAPO-34 Molecular Sieves Synthesized with Different Templates

Triethylammine(Et3N), Morpholine, tetraethylammonium hydroxide(TEAOH), TEAOH-Et3N dual template and TEAOH-Morpholine dual template can all be used to synthesize pure SAPO-34 molecularsieve. The SAPO-34 molecular sieve samples are synthesized with the five kinds of templates mentioned above and characterized with XRD, SEM, FTIR, NMR and TGA techniques, etc. And the samples are also evaluated by methanol-to-olefins(MTO) reaction. The SAPO-34 sample synthesizedwith single Et3Nhas the defects in the crystal latticewhich could be proved bythe undistinguished low peaks at 2θvalue 26o, 31oand the lower characteristic peaks in the XRD pattern. These defects can be effectively improved by using the dual template TEAOH-Et3N system as is shown by the two more distinct double peaks at 2θvalue 26o, 31oand the higher characteristic peaks. The relative crystallinity of sample synthesized with the dual template TEAOH-Morpholine(Et3N) is between the sample with single TEAOH and that with single Morpholine (Et3N). This phenomenon shows an interesting“additional effect”. Differenttemplates could synthesize SAPO-34 molecular sieves with different crystal sizes. TEAOHis favourable to produce the small crystal SAPO-34, whileMorpholine would synthesize big crystals and Et3Nforms the crystalswhose size is betweenTEAOHone andMorpholine one. The crystal size can be adjusted by the dual template method effectively. The SAPO-34 molecular sieve sampleswith the use ofTEAOHhave far smaller crystal sizes than the sampleswith singleMorpholine or Et3NnomatterTEAOHis the sole template or a composite template. Framework vibrancy modes of SAPO-34 molecular sieve samples synthesized from different templates are similar, while their interactionmodes between templatesmolecules and framework are different. TEAOHismore effective in the formation of SAPO-34 structure, and it is more favourable to enter the pore systemof SAPO-34 and easier to react with the negative charge of the structure of SAPO-34 to formthe protonated moleculeswhich are thermally decomposed at a highertemperature. The amountofMorpholine and Et3N ismore than thatofTEAOHin the initial gel, sothe main role ofMorpholine or Et3N is filling the cavity of SAPO-34. The SAPO-34 synthesized from different templates has the same thermal stability as their structures all thermally collapse at ca. 1155℃. The SAPO-34 molecular sieve synthesized with the dual template system has higher (C2=+C3=) selectivity inMTO reaction than the sample with single Morpholine or single Et3N, while it is still lower than theTEAOHsample. This phenomenon also shows a kind of the additional effect.