Zeolite Supported Pt for Depolymerization of Polyethylene by Induction Heating

Abstract

We demonstrate that for polyethylene depolymerization with induction heating (IH), using a bifunctional (Pt- or Pt-Sn-containing zeolite) hydrocracking catalyst, we can obtain high hydrocarbon product yields (up to 95 wt % in 2 h) at a relatively low surface temperature (375 °C) and with a tunable product distribution ranging from light gas products to gasoline- to diesel-range hydrocarbons. Four zeolite types [MFI, LTL, CHA(SSZ-13), and TON] were chosen as the supports due to their varying pore sizes and structures. These depolymerization results are obtained at atmospheric pressure and without the use of H₂ and result in an alkane/alkene mixture with virtually no methane, aromatics, or coke formation. We also demonstrate how IH helps overcome diffusional resistances associated with conventional thermal heating and thereby shortens reaction times.

Figure 1

(a) Desorption of 1-PA from LTL zeolites. Samples saturated with 1-PA at 50 °C prior to TPD. XPS spectra of: (b) Pt/Ba-K-L, Pt 4f_5/2,7/2; (c) PtSn/K-L, Pt 4f_5/2,7/2; and (d) PtSn/K-L, Sn 3d_5/2.

Figure 2

LDPE depolymerization with IH, 54 mT (500 A, 375 °C) input for 2 h at autogenous pressure. Liquid, gas, and coke conversions are on a weight basis.

Figure 3

Product distributions on carbon mol % basis for LDPE depolymerization at 500 A, 2 h.

Figure 4

(a) Alkene-to-alkane ratio of the product distributions. (b) Activation energy for IH depolymerization of LDPE with Fe₃O₄ and Pt-based LTL and MFI catalysts. The activation energy E_a was calculated at temperatures of 285 °C and higher. The R² values of these fits were 1, 0.99, 0.97, and 0.99 for PtSn/K-L, Fe₃O₄, Pt/K-MFI, and PtSn/K-MFI, respectively.

Figure 5

HRTEM images of: (a) fresh PtSn/K-L catalyst; (b) spent PtSn/K-L catalyst after depolymerization of LDPE at 375 °C for 2 h.

Figure 6

(a) Pt L_III XANES spectra for Pt–Sn/K-L catalysts (b) Fourier transformed XAFS spectra for fresh and used PtSn/K-L. Green and red dotted lines represent fits to bulk Pt and Pt₃Sn, respectively.