Optimizing PET Glycolysis with an Oyster Shell-Derived Catalyst Using Response Surface Methodology

Abstract

Polyethylene terephthalate (PET) waste was depolymerized into bis(2-hydroxyethyl) terephthalate (BHET) through glycolysis with the aid of oyster shell-derived catalysts. The equilibrium yield of BHET was as high as 68.6% under the reaction conditions of mass ratios (EG to PET = 5, catalyst to PET = 0.01) at 195 °C for 1 h. Although biomass-derived Ca-based catalysts were used for PET glycolysis to obtain BHET monomers, no statistical analysis was performed to optimize the reaction conditions. Thus, in this study, we applied response surface methodology (RSM) based on three-factor Box-Behnken design (BBD) to investigate the optimal conditions for glycolysis by analyzing the independent and interactive effects of the factors, respectively. Three independent factors of interest include reaction time, temperature, and mass ratio of catalyst to PET under a fixed amount of ethylene glycol (mass ratio of EG to PET = 5) due to the saturation of the yield above the mass ratio. The quadratic regression equation was calculated for predicting the yield of BHET, which was in good agreement with the experimental data (R² = 0.989). The contour and response surface plots showed the interaction effect between three variables and the BHET yield with the maximum average yield of monomer (64.98%) under reaction conditions of 1 wt% of mass ratio (catalyst to PET), 195 °C, and 45 min. Both the experimental results and the analyses of the response surfaces revealed that the interaction effects of reaction temperature vs. time and temperature vs. mass ratio of the catalyst to the PET were more prominent in comparison to reaction time vs. mass ratio of the catalyst to the PET.

Keywords: PET; box-behnken design; catalyst; depolymerization; glycolysis; oyster shell; response surface methodology.

Scheme 1

Typical procedure of the reaction.

Scheme 2

Depolymerization of poly (ethylene terephthalate) (PET) into bis (2-hydroxyehtyl) terephthalate (BHET) through glycolytic cleavage by using a biomass-derived CaO catalyst.

Figure 1

(a) XRD patterns of the oyster shell catalyst before and after calcination at 1000 °C for 5 h. (b) SEM image of oyster shell after calcination. (c) EDS spectrum of calcined oyster shell showing the presence of C, O, Na, Mg, Al, Si, Pt, and Ca. Note that the inset of (c) displays the magnified region in (b).

Figure 2

(a) Optical microscope of BHET crystal; the inset of (a) is image of white crystalline of purified products. (b) XRD patterns of the BHET. (c) FTIR spectra of BHET and PET. (d) ¹H-NMR and ¹³C-NMR spectrum of BHET in DMSO-d6 (500 MHz, 125 MHz).

Figure 3

Optimization of PET glycolysis using oyster-shell-derived catalyst. The effect on PET depolymerization and the BHET yield of (a) mass ratio EG to PET, (b) catalyst, (c) reaction time, and (d) temperature.

Figure 4

Plot of main effect for the BHET yield: (a) reaction temperature, (b) reaction time, (c) catalyst:PET (w/w).

Figure 5

Contour and response surface plots for interaction effect: (a) contour plot of X₁X₂, (b) 3D plot of X₁X₂, (c) contour plot of X₁X₃, (d) 3D plot of X₁X₃, (e) contour plot of X₂X₃, (f) 3D plot of X₂X₃.