In the present investigation, the authors explored the depolymerization of waste poly(ethylene terephthalate) (PET) beverage bottles using glycolysis (transesterification) catalyzed by Al3+, Fe3+ and Zn2+ impregnated bentonite catalysts. Heterogeneous catalysts such as clay, zeolite, alumina etc. preferred over homogeneous catalysts, thus we focused on the development of solid acid catalysts based on economically viable clay catalysts. The desired Lewis acidic nature was introduced by wet impregnation method at variable metal ion to clay ratio such as 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 wt. %.
The effect of metal-ion loading on the surface area, pore volume and crystalline nature of the bentonite has been evaluated by nitrogen adsorption and desorption studies and XRD. The PET wastes collected from post-consumer beverage bottles were depolymerized using ethylene glycol as a glycolyting agent afforded bis(2-hydroxy ethyl)terephthalate (BHET) in 85-90 % yield under optimized conditions. The yield of BHET was observed to be increasing with increase in Lewis acidic metal ions loading from 0.5 to 4.0 wt. % as well as increase in catalysts to PET ratio. Among the catalysts, Al3+ and Zn2+-bentonite catalysts showed higher yield of BHET up to 90 % at 4 and 5 wt. % loading of metal-ion.
In this investigation, the use of phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) as well as Zn 2+ containing kaolin and bentonite explored for chemical recycling of post-consumer poly(ethyleneterephathalate) (PET) wastes have been explored. The clay supported catalysts containing 5wt% of the metals and heteropolyacids (HPAs) are synthesized using wet impregnation method. The effect of metal ions and HPAs loading on the surface area, pore volume, elemental composition and crystalline nature of the kaolin and bentonite has been evaluated by nitrogen adsorption and desorption studies, SEM-EDX mapping, powder XRD, FTIR and XPS analysis. The total surface area of BET increased with a loading of 5 wt% of Zn 2+ , PWA and PMA on kaolin and bentonite, while the pore volume and pore diameter remain unchanged. SEM and EDAX mapping images showed that the heteropolyacids crystals are well dispersed on the surface and occupied interlayer spaces of the clay support. SEM-EDX showed that bentonite showed a better loading of PWA and PMA compared with kaolin. PET waste water bottles collected from the local market used for the chemical recycling process. The aminolysis reaction using Zn 2+ and PWA loaded on bentonite showed complete depolymerisation of PET wastes to produce 87-98% of BHETA. The glycolysis reaction using the above catalysts showed complete depolymerisation at 180-210 °C and yielded 78-90% of BHET. When comparing the clay, bentonite performed well in terms of heteropolyacid loading and afforded a higher yield of BHET and BHETA due to higher loading of Zn and HPA as supported by SEM-EDX and XPS. Reusability of the catalysts were also examined for glycolysis.
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