Crystallization Mechanism Of The Aluminophosphate Molecular Sieves By Solvent-Free Synthesis

Aluminophosphate(AlPO4-based)molecular sieves and zeolites are widely used in industry for separation and catalysis because of their unique frameworks with specific pores,channels,and cavities at molecular levels.Despite tremendous advances in the understanding of the principles underlying the formation of these materials under hydrothermal synthesis(HTS)conditions or dry gel conversion,it is an extremely complicated process with multiple reactions and equilibriums occurring simultaneously in solution and solid phases to figure out the crystallization mechanism from the HTS reaction system.Therefore,until now the crystallization mechanism of molecular sieves are still not well understood on a molecular level,and it is still difficult to design novel framework structures with desirable properties.Recently,a number of important zeolites and molecular sieves have been synthesized by solvent-free synthesis methodology.All of the reactive species are contained in the solid phase,which is favorable for deep understanding the molecular sieve crystallization.Advanced characterization techniques have been used,especially solid-state NMR technology[31P-{27Al} PT-J-HMQC and 2D 'H DQ-SQ MAS NMR],to character aluminophosphate molecular sieves synthesized by solvent-free method.The structural information of important species during the crystallization of aluminumphosphorus molecular sieves in spectroscopy ensures a better understanding of the entire crystallization process:how the initial one-dimensional parent chains are converted into two-dimensional layered structure,in turn transformed into three-dimensional molecular sieve framework.For the first time,the intermediates of the parent chains with 4-/6-membered rings in the initial system were clearly identified through investigation on AlPO4-5 molecular sieves crystallization processes.2D 31P-{27,Al} PT-J-HMQC and 2D 31P DQ-SQ NMR experimental results of AlPO4-11 molecular sieves demonstrate that the formed 4MR-6MR chains as the intermediate species has been used to assemble the 2D layered phase and finally to be transformed to 3D framework.2D 1H DQ-SQ NMR spectra indicate that DPA molecule promotes the formation of layered structures by the pronated DPA cations,while the transformation from 2D layered structures to 3D crystals is driven by the formation of a "twin" DPA molecule.2D 1H DQ-SQ NMR spectra provide important information on the crystallization process of AlPOs and SAPOs molecular sieves,especially on the structure directing roles of organic molecules during the crystallization process.It is found that the organic templating agent having an NH group exists as a "twin" molecule in the channels,and the organic amine having an N group is protonated to form an N+ cation.Moreover,amine-chains structures formed with templates and the intermediates,promoting mutual condensation with parent chains.After deep understanding of the crystallization mechanism of aluminumphosphorus molecular sieves,we successfully synthesize the hierarchical SAPO-11 molecular sieve by using new method.The solid DPA H3PO4 used as solid template is rapidly decomposed into DPA gas molecules as "porogen",forming a large number of worm-like mesopores without adding meso-sized templates.The hierarchical SAPO-11 molecular sieve acting as an excellent support in the long-chain alkane isomerization catalytic reaction,exhibits better isomerization selectivity in the catalytic reaction performance than conventional hydrothermal synthesis of SAPO-11 molecular sieve.