An Efficient One-pot Synthesis Of Micro/mesoporous Zeolite Composites By Single Structure Directing Agent

Micro-mesoporous composite has the advantages of zeolites as well as mesoporous molecular sieves,including high hydrothermal stability,high acidity and large pore size,which make it a research hotspot.An efficient one-pot synthesis of micro-mesoporous composite by single structure directing agent was studied in this paper.Long-chain alkyl quaternary ammonium salts were used as organic structure directing agents（OSDA）and tetraethyl orthosilicate（TEOS）was used as silicon source in the synthesis with a composition of 1 OSDA:8.31 SiO₂:3.36 NaOH:1054 H₂O.The OSDAs included hexadecyl trimethyl ammonium bromide（CTAB）,[C₁₈H₃₇-N⁺（CH₃）₂-C₆H₁₂-N（CH₃）₂]Br^-(C₁₈N),[C₂₂H₄₅-N⁺（CH₃）₂-C₆H₁₂-N（CH₃）₂]Br^-(C₂₂N),[C₁₈H₃₇-N⁺（CH₃）₂-C₆H₁₂-N⁺（CH₃）₂-C₆H₁₃]Br₂^-(C₁₈N₂),[C₂₂H₄₅-N⁺（CH₃）₂-C₆H₁₂-N⁺（CH₃）₂-C₆H₁₃]Br₂^-(C₂₂N₂).By setting the condition of hydrothermal synthesis as 3 days at 100 ^oC and then two or four days at 150 ^oC,samples with different properties were obtained.These samples were characterized by powder X-ray diffraction（PXRD）,Fourier transform infrared spectroscopy（FT-IR）,field emission scanning electron microscope（SEM）,field emission transmission electron microscopy（TEM）and nitrogen sorption.Their hydrothermal stabilities were tested too.Results show that gemini quaternary ammonium salt surfactants C₁₈N₂ and C₂₂N₂exhibit better abilities of directing the formation of MCM-41 structure than CTAB at100 ^oC and the products will gradually crystallize to lamellar MFI structures at 150 ^oC.It is possible to get a micro-mesoporous zeolite composite by carefully controlling the crystallization condition.While their corresponding single quaternary ammonium salts C₁₈N and C₂₂N direct lamellar M41S products at 100 ^oC,which were further transformed into MCM-41 at 150 ^oC,although no crystallization was observed in this condition.Good MCM-41 mesoporous molecular sieves with a large specific surface area and micro-mesoporous zeolite composites with a high catalytic potential were synthesized,which provide new ideas for preparing micro-mesoporous composites.