Synthesis And Modification Of SAPO Zeolites And Their Separation Performance On Small Molecules

The light hydrocarbon separation and the carbon dioxide capture have been highlighted as“Seven chemical separations to change the world”.The key is to develop the highly efficient adsorbents with excellent selectivity,high adsorption capacity,fast adsorption/desorption kinetics,and long durability.The traditional cryogenic rectification technology for olefins separation and amine solution absorption technology for carbon dioxide capture highly rely on thermal energy,which require large equipment investment with high operating costs.In general,the energy consumption in the separation and purification process accounts for about 10-15%of the world’s total energy consumption every year.Thus,developing highly efficient non-thermal alternative separation process could significantly reduce the energy consumption.Pressure swing adsorption process（PSA）is recognized to be the most potential alternative technology due to its short operating cycle,low energy consumption,simple and mature technical equipment,environment friendliness,and wide application range.The key in the PSA process is the adsorbent with extremely high selectivity,large adsorption capacity,fast adsorption/desorption kinetics,long durability,and low cost.Zeolites are a class of crystalline microporous oxides with well-defined pores and cavities of molecular dimensions,which have unique shape selectivity,uniform and adjustable pore system,rich and diverse framework elements composition and structure type,controllable acidity,polarity and internal electric field gradient,large specific surface area,excellent hydrothermal and chemical stability,low production costs,and easy preparation,making them wide application in gas separation,catalysis and ion exchange.In the past decade,the 8MR small pore zeolites,especially those presenting large cavities in their structure and those framework structure with flexible properties,have received much attention because of its unique separation mechanism in gas adsorption and separation regions,which show a rich variety of gas adsorption behavior and can be tuned to optimize kinetics and selectivity,making it possible to prepare zeolite-based adsorbent materials with excellent selectivity.In this thesis,the SAPO-RHO zeolite with high silicon content（Si/（Si+Al+P）=0.182）was synthesized by the seed-assisted method using DEA as organic structure directing agent（OSDA）;the SAPO-34 zeolite with a high framework Si fraction（Si/（Si+Al+P）=0.37）and high framework charge density（（Al–P）/（Si+Al+P）=0.36）was synthesized by the seed-assisted method under the completely inorganic environment in the presence of K⁺ions alone as the inorganic structure-directing agent（ISDA）.A series of M-SAPO-RHO（M=Li⁺,Na⁺,K⁺,Cs⁺）and M-SAPO-34（M=NH₄⁺,Li⁺,Na⁺,K⁺,Cs⁺）were prepared via ion-exchange process.The performance of M-SAPO-RHO and M-SAPO-34 in selective adsorption and separation of small molecular gases was systematically investigated.The main results of this thesis are as follows:1.Based on the seed-assisted synthesis strategy,SAPO-RHO zeolite with high Si content（Si/（Si+Al+P）=0.182）was successfully synthesized under traditional hydrothermal conditions.A series of M-SAPO-RHO（M=Li⁺,Na⁺,K⁺,Cs⁺）were prepared via ion-exchange process.The separation performance of M-SAPO-RHO for CO₂/CH₄ and CO₂/N₂ was systematically investigated.The result showed that:1)The Na-SAPO-RHO shows the best separation performance for CO₂/CH₄ and CO₂/N₂.2)Breakthrough experiments demonstrate that Na-SAPO-RHO can completely separation CO₂ from CH₄ or N₂.These superior features make Na-SAPO-RHO a promising candidate for CO₂ capture in biogas purification and flue gas separation via adsorption-based separation processes.3)The positions of the M⁺ions in M-SAPO-RHO were confirmed by the combination of powder XRD structure refinement（PXRD-Rietveld refinement）and density functional theory（DFT）,providing an insight into the excellent CO₂ separation performance.2.The separation performance of M-SAPO-RHO（M=Li⁺,Na⁺,K⁺,Cs⁺）for C₂H₄/C₂H₆ was systematically investigated.The results show that:1)The Li-SAPO-RHO has the best separation performance for C₂H₄/C₂H₆.The IAST（ideal adsorbed solution theory）was used for calculated the selectivity of Li-SAPO-RHO for the separation of C₂H₄/C₂H₆.The dynamic separation performance of the Li-SAPO-RHO is preliminarily evaluated through the C₂H₄/C₂H₆ two-component fixed bed breakthrough experiment.2)The positions of the Li⁺ions in Li-SAPO-RHO were investigated by combination of the high resolution scanning transmission electron microscopy（HR-STEM）and the experimental and theoretically calculated ⁷Li-MAS NMR spectra.3.SAPO-34 zeolite was successfully synthesized using K⁺ions as the inorganic structure-directing agents under conventional hydrothermal conditions.The coordination state of Si,Al,and P elements was confirmed by solid-state nuclear magnetic resonance（MAS NMR）.Because K⁺ion has a relatively higher charge density than quaternary ammonium salt,the as-synthesized K-SAPO-34 has notably higher framework charge density（（Al–P）/（Si+Al+P）=0.36）and content of Si（Si/（Si+Al+P）=0.37）than previously reported SAPO-34 zeolites.The selectivity of M-SAPO-34（M=NH₄⁺,Li⁺,Na⁺,K⁺,Cs⁺）for CO₂/CH₄ and CO₂/N₂ are explored via the single component equilibrium adsorption isotherm.The calculation results show that the as-synthesized K-SAPO-34 has the highest selectivity for CO₂/CH₄ and CO₂/N₂.