Seed-assisted Syntheses Of HEU And CHA Zeolites And Their Applications In Environmental Protection

Various pollutants,such as sewage,waste gas and solid waste,are inevitably generated in the industrial production.Heavy metals cations,usually from mining,smelting,papermaking,and chemical industries,are closest to our daily life and the most harmful to human body.The adsorption or ion exchange is an efficient water purification route without secondary pollution,which has better performance and much lower cost for the removal of low-concentration heavy metal cations.Natural clinoptilolite with HEU topology has well-defined channel system and abundant ion-exchange sites and thus exhibits selective adsorption effects towards specific cationic pollutants,making it very suitable for the treatment of heavy metal containing sewage.However,the crystallinity,phase purity,and chemical composition of natural zeolites are different from deposit to deposit,resulting in different properties of natural zeolites with identical structure.Heulandite,possessing the same topology with clinoptilolite,has more ion-exchange sites due to its lower Si/Al ratio,which theoretically make it a better adsorption material for the removal of heavy metal cations compared with clinoptilolite.However,it is still a challenge to synthesize heulandite at present.Therefore,it is of great importance to develop a reliable and facile route for the synthesis of heulandite.Nitrogen oxides（NO_x）are major precursor air pollutants,resulting in severe air pollution including photochemical smog,acid rain,and haze.The transportation industry is one of the main sources of NO_x emissions,and diesel vehicles contribute more than 90%of vehicle NO_x emissions.At present,Cu-exchanged SSZ-13 zeolite is the sole commercial NH₃-SCR catalyst that meets the"China VI"standard for NO_xemissions.To reduce the cost of SSZ-13,a number of seed-assisted OSDA-free synthesis routes have been developed,but the resulting products are generally less hydrothermally stable due to low Si/Al ratio.The distribution of Al atoms in the CHA zeolites is a key factor affecting their hydrothermal stability.If the Al atoms in the low-silica CHA zeolites are distributed more in pairs,the hydrothermal stability of such zeolites will be greatly improved.Besides,from the perspective of industry production,since the seed crystal has a structure-directing effect similar to OSDA,it can replace part of TMAda OH and effectively reduce the amount of TMAda OH required for the synthesis of SSZ-13,which makes it possible to greatly reduce the cost of Cu-SSZ-13 catalyst.In this thesis,heulandite（S-Heu）with excellent performance in aqueous Cd²⁺capture was synthesized by a hetero-structural seed-assisted method using STI zeolite;low-silica CHA zeolite with abundant paired Al was synthesized using SAPO-18 seed and exhibits improved NH₃-SCR activity and hydrothermal stability;SSZ-13 zeolite with Si/Al close to 10 was synthesized with greatly reduced amount of TMAda OH.The resultant SSZ-13 has a better performance in NH₃-SCR.The main results are as follows:1.Based on the hetero-structural seed-assisted strategy,highly crystalline heulandite（S-Heu）was synthesized using stellerite（STI zeolite）with the same composite building units（CBUs）.This is the first report for the stellerite seed directed synthesis of a zeolite with different topology.The Si/Al ratio of S-Heu is 3.7,with Na⁺as the sole extra-framework cation.The thermal stability of S-Heu is typical for heulandite,which would loss most of the crystallinity under 400°C,but can be greatly improved by H⁺-exchange,making it a potential material for gas separation or thermo catalysis.As revealed by XRD and SEM,stellerite seed directs the formation of some STI phase at first,and then the STI phase gradually transformed into highly crystalline HEU phase.The aqueous Cd²⁺removal performance of S-Heu,natural clinoptilolite and synthetic clinoptilolite was investigated.It was found that the adsorption capacity of S-Heu for Cd²⁺is much higher than that of natural clinoptilolite and synthetic clinoptilolite.In addition,S-Heu can maintain over 80%Cd²⁺removal efficiency at p H=4～7 or in the coexistence of high concentrations of Na⁺,Ca²⁺or Mg²⁺as competing ions.After 6 adsorption-desorption cycles,the removal efficiency of S-Heu for Cd²⁺dropped slightly to 75%,indicating a good reusability.2.Inspired by the strict matching rule during the replication or translation of double-stranded DNA（also as duplex DNA）,aluminosilicate low-silica CHA with abundant Al pairs（CHA-SPAEI,Si/Al=3.7）was synthesized using SAPO-18containing N,N-diisopropylethylamine（SAPO-18-DIPEA）as seed.This is the first report that a silicoaluminophosphate zeolite seed was used as the sole structure-directing agent in the synthesis of an aluminosilicate zeolite.As revealed by ²⁷Al DQ-SQ 2D NMR spectra,P-O-Al-O-P fragments generated from the dissolution of SAPO-18-DIPEA seed induced the formation of a large number of Al-O-Si-O-Al fragments through Lowenstein role.These Al-O-Si-O-Al fragments exhibits structure-directing effect and eventually became part of the CHA framework to form Al pairs,making 78%of the framework Al in CHA-SPAEI arranged in pairs,which is much higher than that in SSZ-13 seed directed low-silica CHA（CHA-LS,percentage of Al pairs is 52%）.The NH₃-SCR performance of Cu exchanged CHA-SPAEI and CHA-LS was investigated.More Al pairs are favorable to the formation of Cu²⁺-2Z active center and thus improve the hydrothermal stability of Cu_4.3-CHA-SPAEI.Upon hydrothermal ageing at 800°C for 6 h,Cu_4.3-CHA-SPAEI maintains over 90%NO conversion within 225～550°C,while Cu_4.4-CHA-LS lost all the activity throughout the working temperature.3.From the perspective of industrial production,with the help of small amount of seed,the minimum dosage of OSDA（TMAda OH）required for the synthesis of SSZ-13 is decreased from TMAda OH/Si O₂=0.10 to 0.065.The crystallization process of SSZ-13 under the seeded route（SSZ-13-SD）and seed-free route（SSZ-13-SDF）was investigated by XRD,FT-IR and ICP-OES.It was found that the seed could1)significantly shorten the induction period and accelerate the crystallization 2)prevent most of the TMAda⁺from entering the solid phase during the induction period,so that the distribution of[Al O₄]^-is mainly regulated by Na⁺and thus more closely distributed to form Al pairs.Co²⁺exchange experiments showed that compared with SSZ-13-SDF,the proportion of paired Al in SSZ-13-SD was increased by 100%.Therefore,the NH₃-SCR activity and hydrothermal stability of SSZ-13-SD were significantly improved.Upon hydrothermal ageing at 800°C for 12 h,SSZ-13-SD maintained over 80%NO conversion within 225～550°C.The method of reducing OSDA dosage with seeds was further extended from the Na⁺+TMAda⁺system to the K⁺+TMAda⁺,and the resulting SSZ-13 also shows excellent NH₃-SCR performance.