Abstract
The synthesis of SSZ-13 was studied by evaluating the influence of two types of crystallization seeds with CHA structure (S1 seeds prepared in fluoride medium and S2 seeds by the conversion of zeolite Y), SiO2/Al2O3 (28, 40, 60 and 100) and OH/SiO2 (0.6 and 0.8) ratios. Choline chloride was used as the structure-directing agent and the crystallizations occurred at 140 ºC from 24 to 96 h. S1 seeds led to the coexistence of zeolites SSZ-13 and P, while S2 seeds promoted in all SiO2/Al2O3 ratios and OH/SiO2 = 0.8, the formation of pure SSZ-13 after 24 h. The SSZ-13 samples are formed by clusters of crystals with cuboid-like morphology and exhibited textural properties typical of CHA framework. SSZ-13 zeolite led to a decrease in the temperature and the activation energy required for the degradation of ultra-high molecular weight polyethylene polymer, demonstrating its efficiency as catalyst in this process.
Highlights
Zeolite SSZ-13 has a three-dimensional structure formed by 4, 6- and 8-membered ring (MR), containing large cages (0.73 nm) and intersecting channels with a diameter of 0.38 nm[1,2]
Due to some properties presented by SSZ-13, such as large specific surface area, high ion-exchange capacity, strong surface acidity, remarkable molecular-shape selectivity and thermal/hydrothermal stability, several studies report the application of this material in adsorption processes and CO2 separation[3,4,5], conversion of methanol to olefins (MTO), in which it can increase the yield of low-carbon olefins[6,7,8,9], in the selective catalytic reduction (SCR) of NOx10-13 and pyrolysis of low-density polyethylene (LDPE)[14]
The reaction mixture was prepared through the following steps: (i) dissolution of sodium hydroxide and sodium aluminate in 80% of the distilled water required for the synthesis (ii) addition of choline chloride in the solution obtained in step (i); (iii) addition of colloidal silica in the mixture resulting from step (ii); iv) The seeds (S1 or S2) were dispersed in the remaining water and were added to the mixture obtained in step (iii)
Summary
Zeolite SSZ-13 (framework type code CHA, with SiO2/ Al2O3 ratio > 6) has a three-dimensional structure formed by 4-, 6- and 8-membered ring (MR), containing large cages (0.73 nm) and intersecting channels with a diameter of 0.38 nm[1,2]. Martín et al.[25] described the combination of zeolite Y with TEAOH, resulting in chabazite being obtained after 72 h of crystallization. These routes present as a major disadvantage the long crystallization period, around 3 to 5 days. The use of crystallization seeds combined with a less expensive structure-directing agent is one of the alternatives to produce the SSZ-13 with shorter times, which makes the synthesis more economical and with greater potential for industrial application. The synthesis of zeolite SSZ-13 was investigated from reaction mixtures with different SiO2/ Al2O3 and OH/SiO2 ratios, using choline chloride as structuredirecting agent and two types of crystallization seeds (with CHA structure) obtained by different methodologies. The synthesized samples were evaluated in the thermocatalytic degradation of ultra-high molecular weight polyethylene (UHMWPE)
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