Synthesis Of Hierarchical Porous SAPO-34Molecular Sieves And Their Application In MTO Reaction

Inorganic porous materials have now been widely used in industrial productionand our daily life, involved in catalysis, adsorption, separation, and other fields. As akind of inorganic porous materials, hierarchical zeolite materials contain more thantwo levels of channels. The abundant porous structure affects the transportcharacteristics of molecules within the pore system. Reducing diffusion limitationsof molecules in the catalyst materials has attracted a widespread of interest.Hierarchical materials are still hot spot in research. Silicoaluminophosphate zeoliteSAPO-34with CHA topology is known as one of the best catalysts formethanol-to-olefin (MTO) conversion. But due to the pore size limitation and poormass transfer diffusion ability, conventional SAPO-34is easy to be deactivated, sothe synthesis hierarchical SAPO-34is of great significance.In this work, we demonstrate a facile one-step hydrothermal synthesis ofhierarchical macroporous SAPO-34through the etching effect of hydrofluoric acid.The highly crystalline hierarchical macroporous SAPO-34is prepared ascentral-holed rhombohedral crystals with particle size of ca.5–10μm that compriseintracrystalline parallel macrochannels of ca.100nm. The formation of macroporousstructures via in situ growth-etching can be directly imaged by SEM. Strikingly, a particular crystal configuration consisting of eight pyramidal parts has been firstobserved during the zeolite crystal growth process, which eventually grows to form aperfect rhombohedral shape. The HF etching effect is further elucidated by theanalysis of changes of pH values as well as of solid and liquid compositionsfollowing the evolution of crystallization. The texture properties, chemicalenvironments of framework atoms, and acidity of the synthesized SAPO-34arecharacterized by N2adsorption/desorption, MAS NMR and NH3-TPD measurements.The hierarchical macroporous SAPO-34shows larger micropore volume, slightlystronger acid strength, and lower external surface acidity than its conventionalcounterpart synthesized without using HF. Consequently, the hierarchicalmacroporous SAPO-34exhibits excellent MTO catalytic performance, showingmuch higher selectivity to ethylene and propylene as well as longer lifetime than theconventional counterpart. In comparison with previously reported methods for thegeneration of hierarchical porosity, this one-step HF-assisted in situ growth-etchingsynthetic route is simple, straightforward and cost-effective, which offers a newapproach to prepare hierarchical porous zeolites with improved catalytic activity.In an attempt to synthesize silicoaluminophosphate zeolite SAPO-34by meansof “green” chemistry, we study the syntheses of hierarchical silicoaluminophosphatemolecular sieves by a low cost approach through recycling waste mother liquid inthe SiO2-Al(OH)3–H3PO4–TEA–HF system (TEA: triethylamine). The textureproperties of the resulting hierarchical SAPO-34, chemical environments offramework atoms, and acidity of the synthesized SAPO-34are characterized by Aradsorption/desorption,29Si MAS NMR, and NH3-TPD measurements. Comparedwith the SAPO-34by conventional synthesis, the resulting SAPO-34synthesized byrecycling mother liquid demonstrates excellent MTO catalytic performance, andremains high selectivity to ethylene and propylene as well as the long lifetime ascompared with the initially synthesized hierarchical SAPO-34. The recycling motherliquid route is simple and cost-effective, which offers a green approach to prepare hierarchical porous zeolite catalysts with excellent catalytic activity.SAPO-34crystals synthesized by different templates are post etched by severalmethods. Crystal morphology and physicochemical characteristics of SAPO-34crystals are studied by complementary methods including X-ray diffraction (XRD)and scanning electron microscopy (SEM). Finally, these SAPO-34crystals aretreated by a variety of post methods, such as HF/NH4F post-treatment. The resultingof hierarchical SAPO-34zeolites has distinct morphologies. This indicates thatSAPO-34crystals synthesized by different templates have different defect domains,and post-treatment of HF/NH4F solution is a general method to treat SAPO-34crystals. Compared with the traditional silicoaluminate zeolites,silicoaluminumphosphate molecular sieve SAPO-34has much lower acid and alkaliresistance. HF/NH4F post-treatment will reduce the crystallinity of the crystals andshorten the lifetime of the catalyst. Usually, the selectivity of the catalyst shoulddecrease as its lifetime decreases. However, the HF/NH4F solution post-treatment inthis work can improve the selectivity of ethylene and propylene. This may guide thefurther research of improving molecular sieve catalytic properties through posttreatment.