Polyethylene terephthalate (PET),
a chemically stable polyester
with multiple applications, has risen dramatically in manufacturing
and consumption in the past decades. The increase in PET use has resulted
in considerable volumes of waste PET pilling up and thus causing increased
health and environmental concerns. Apart from the landfills and incineration
solutions, the common waste PET recycling practices mainly focus on
low-value downstream products. All the abovementioned factors have
contributed to the lower waste PET recycling overall rate of less
than 30% in South Africa and created a need for alternative treatment
options. Our earlier work has proven the feasibility for converting
clear PET bottles into high value-added metal–organic frameworks
(MOFs) materials. The feedback from industries indicated that the
colorful PET bottles and food trays are currently considered problematic
to be recycled economically. In response, this work focuses on the
use of various types of PET wastes as sources of benzene dicarboxylic
acid (BDC) linker for the synthesis of the zirconium-based MOF UiO-66(Zr).
The BDC linker was extracted from food trays, green bottles, brown
bottles, and PETCO beads through glycolysis (depolymerization). Post-synthesis
characterization revealed that textural properties of the waste PET-derived
UiO-66(Zr) MOFs were comparable to those of the MOFs derived from
commercial chemicals as exemplified in the scanning electron microscope
images and X-ray diffraction patterns. The diffraction pattern peaks
typically observed for commercial grade BDC positioned at 2θ
= 17.21, 25.01, and 27.64° were observed for the PET-derived
BDC samples, confirming the crystalline nature of samples. However,
the MOFs synthesized from BDC derived from green and brown PET bottles
measured lower Brunauer–Emmett–Teller surface areas
in the range of 933–1085 m2/g compared to 1368 m2/g for MOFs synthesized from the commercial BDC linker. This
phenomenon is attributed to the presence of organic dyes contained
in the colored PET bottles residing in the MOF pores. This was further
confirmed by the infrared spectra of the postconsumer PET-derived
BDC showing a peak at 3158 cm–1 assigned to the
amine N–H functional group, as well as the much stronger C–H
bend. This study complements the business case development model of
“waste PET to value-added MOFs”.
The thermocatalytic hydrogenation of carbon dioxide (CO2) to methanol is considered as a potential route for green hydrogen storage as well as a mean for utilizing captured CO2, owing to the many established applications of methanol. Copper–zinc bimetallic catalysts supported on a zirconium-based UiO-66 metal–organic framework (MOF) were prepared via slurry phase impregnation and benchmarked against the promoted, co-precipitated, conventional ternary CuO/ZnO/Al2O3 (CZA) catalyst for the thermocatalytic hydrogenation of CO2 to methanol. A decrease in crystallinity and specific surface area of the UiO-66 support was observed using X-ray diffraction and N2-sorption isotherms, whereas hydrogen-temperature-programmed reduction and X-ray photoelectron spectroscopy revealed the presence of copper active sites after impregnation and thermal activation. Other characterisation techniques such as scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis were employed to assess the physicochemical properties of the resulting catalysts. The UiO-66 (Zr) MOF-supported catalyst exhibited a good CO2 conversion of 27 and 16% selectivity towards methanol, whereas the magnesium-promoted CZA catalyst had a CO2 conversion of 31% and methanol selectivity of 24%. The prepared catalysts performed similarly to a CZA commercial catalyst which exhibited a CO2 conversion and methanol selectivity of 30 and 15%. The study demonstrates the prospective use of Cu-Zn bimetallic catalysts supported on MOFs for direct CO2 hydrogenation to produce green methanol.
Waste plastics such as polyethylene terephthalate (w-PET) and stockpiled discard coal (d-coal) pose a global environmental threat as they are disposed of in large quantities as solid waste into landfills and are particularly hazardous due to spontaneous combustion of d-coal that produces greenhouse gases (GHG) and the non-biodegradability of w-PET plastic products. This study reports on the development of a composite material, prepared from w-PET and d-coal, with physical and chemical properties similar to that of metallurgical coke. The w-PET/d-coal composite was synthesized via a co-carbonization process at 700 °C under a constant flow of nitrogen gas. Proximate analysis results showed that a carbonized w-PET/d-coal composite could attain up to 35% improvement in fixed carbon content compared to its d-coal counterpart, such that an initial fixed carbon content of 14–75% in carbonized discard coal could be improved to 49–86% in carbonized w-PET/d-coal composites. The results clearly demonstrate the role of d-coal ash on the degree of thermo-catalytic conversion of w-PET to solid carbon, showing that the yield of carbon derived from w-PET (i.e., c-PET) was proportional to the ash content of d-coal. Furthermore, the chemical and physical characterization of the composition and structure of the c-PET/d-coal composite showed evidence of mainly graphitized carbon and a post-carbonization caking ability similar to that of metallurgical coke. The results obtained in this study show potential for the use of waste raw materials, w-PET and d-coal, towards the development of an eco-friendly reductant with comparable chemical and physical properties to metallurgical coke.
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