MAl-X [M-Zn, Mg, Ni; X- Cl, NO₃, CO₃] layered double hydroxides: Catalytic applicability in plastic waste recycling and wastewater treatment for the sustainable environment

Layered double hydroxides (LDHs) anionic clay materials have gained significant attention due to their versatile applicability in various fields because of their unique structure, lewis-bronsted basic nature, proton abstraction, active sites generation, efficient separation of photogenerated electrons and holes and features including high chemical composition flexibility like various combinations of metal (M²⁺, M³⁺ and M⁴⁺) ions, metal atomic ratios, nature of the interlayer anions, redox and electronic properties etc. MAl-X [M-Zn, Mg, Ni; X- Cl, NO₃, CO₃] [M^II:Al molar ratio = 3:1] were synthesized by one step co-precipitation method without any secondary anion exchange reaction. The structural and morphological characterization were carried out through microscopy and spectroscopic analysis of synthesized LDHs. XRD analysis revealed the presence of a brucite phase with the confirmation of MAl-Cl/NO₃/CO₃ LDHs formation. FTIR provided insights for the presence of different functional groups and particularly M-O bond in the LDHs. SEM-EDS confirmed the semi-spherical and bead-like morphology along with the chemical composition of the synthesized LDHs. Additionally, TEM image analysis revealed the plate-like, sheet-like shape with circumference ~ 30–43 nm and hexagonal brucite layer like structure of as-prepared LDHs samples. The synthesized LDHs have been utilized as catalytic materials in photodegradation of dyes (congo red and rhodamine blue) and glycolytic depolymerization of carbonyl group bonded heterochain polymer thermoplastics (polyethylene terephthalate and polycarbonate) waste into monomers as valuable materials. The effect of different anions intercalated LDH as catalysts and their vital role during the catalytical activity process have been studied. Among all synthesized LDHs, materials intercalated with carbonated anions showed the highest photocatalytic degradation because of the better proton abstraction and lewis-bronsted basic nature of CO₃²⁻ anion. However, the presence of acidic sites on surface of chloride and nitrate anions containing LDH along with weak basic nature of anions and less tendency to abstract proton. M^IIAl-CO₃ (Zn/Ni) showed more than 90% degradation of CR dye in 40 min (min) with 30 mg catalyst dosage. While the NiAl-CO₃ photocatalyst showed 91.3% degradation in case of RhB dye in 50 min of degradation time. The percentage yield of 79.34 and 82.63 was observed for BHET and BPA monomer units during glycolysis of PET and PC using ZnAl-CO₃ catalyst.