External acidity as performance descriptor in polyolefin cracking using zeolite-based materials

Abstract

Thermal pyrolysis is gaining industrial adoption to convert large volumes of plastic waste into hydrocarbon feedstock. However, it suffers from a high reaction temperature and relatively low selectivity. Utilizing a catalyst in the process, moving from thermal pyrolysis to catalytic cracking could help overcome both challenges. In order to develop efficient catalyst materials for this process, understanding structure-composition-performance relationships is critical. In this work, we show that in contrast to cracking of small molecules, plastic cracking activity using ultrastable zeolite Y materials does not depend on the bulk Brønsted acid site content, but rather on the concentration of acid sites located on the outer surface and in mesopores. This external acidity, however, fails to capture all the observed performance trends. Detailed kinetic experiments reveal that the scaling of the reaction rate with the catalyst loading differs drastically between highly similar catalyst materials. More specifically, doubling the catalyst loading leads to doubling of the reaction rate for one material, while for another it leads to more than fivefold increase. When very bulky reactants, such as polyolefins, are converted over microporous catalysts, structure-composition-performance relationships established for smaller molecules need to be revisited.

Fig. 1. Comparison of small molecule (2,4-dimethylpentane, DMP) and plastic (polypropylene, PP) cracking activity and external acid site characterization.

a First order rate constant of DMP cracking as a function of Brønsted acid site content determined by pyridine infrared (IR) spectroscopy for 5 steamed and 1 non steamed zeolite Y. The index in the zeolite label indicates the SiO₂/Al₂O₃ ratio. See Supplementary Fig. S1 for the IR spectra of adsorbed pyridine. Error bars taken from the error of the extinction coefficient. b Temperature of highest cracking rate T_max in cracking of PP determined by ramped thermogravimetric analysis (TGA) of the polymer-catalyst mixtures at different catalyst loadings. A lower T_max indicates higher cracking activity. See Supplementary Fig. S2 for full TGA profiles. ZY₅₆ was omitted for clarity, for all catalysts see Fig. S3. c IR spectra of tri-tert-butylpyridine (TTBP) adsorbed from the gas phase on ZY_x, showing changes in zeolite O-H vibrations and vibrations of the probe molecule. High-frequency (HF) and low-frequency (LF) indicate high and low-frequency O-H vibrations of acid sites. The spectra of the dried zeolites were subtracted. d Bulk and external Brønsted acidity of ZY_x probed by IR spectroscopy of adsorbed pyridine (ring stretch vibration mode at 1544 cm⁻¹) and TTBP (N-H stretch vibration mode at 3369 cm⁻¹), respectively with the pore diameter of ZY and the kinetic diameters of both probe molecules. No error for the extinction coefficient of TTBP was reported.

Fig. 2. Isothermal thermogravimetric kinetics for cracking of polypropylene (PP) using zeolite Y.

a Logarithm of normalized PP weight over time of cracking experiment using the ZY₁₄ zeolite material at different catalyst loadings (0.1–0.5) and temperatures (230–250 °C). The apparent first-order rate constant k’ was determined from the slope of a linear fit in a conversion regime of 25−75% (dashed line). See Supplementary Fig. S4 for all catalyst materials under study. b ln(k’) as a function of inverse temperature and logarithm of the catalyst loading. A plane was fitted to each dataset according to Eq. 2. The slopes yield the activation energy E_a and the pseudo-order in catalyst loading n. Data for the ZY₅₆ zeolite material are omitted for clarity. c Calculated k’ from fitting results according to Eq. 2 at 250 °C as a function of catalyst loading. d, e Constable plots showing the compensation relationship between E_a and ln(A) for PP and DMP cracking, respectively using ZY_x. See Supplementary Fig. S5 for Arrhenius plots of dimethylpentane (DMP) cracking. Linear fit using orthogonal distance regression was used to determine the characteristic temperature T* from the inverse slope. Confidence ellipses are drawn around one standard deviation determined from the covariance matrix of the fits. f n for all catalysts studied plotted as a function of framework aluminum content determined from inductively coupled plasma optical emission spectroscopy (ICP-OES) and ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). Error bars show one standard deviation determined from the covariance matrix of the fit.

Fig. 3. Rationalizing differences in pseudo-order in catalyst loading.

a Pore size distribution for 4 selected zeolites determined by non-local density functional theory (NL-DFT) of the adsorption branch of Ar physisorption at 87 K using a hybrid kernel of spherical micropores and cylindrical mesopores as well as Hg porosimetry. See Supplementary Fig. S6 for all catalyst materials under study, which were omitted for clarity. b Simulated weight-loss profiles for a plastic cracking reaction. The bonds to be cracked were selected randomly using different probability distributions: In blue, the probability follows a Gaussian distribution around the middle of the chain, in orange the probability decreases exponentially from the chain end. First-order kinetics in bonds broken were enforced. Insert shows schematically the steps of the simulation. c Cartoons depicting how different locations of acid sites could determine which bonds along the polymer backbone are cracked. The polymer chain is yellow, and the catalyst surface in blue. d Cartoon showing how acid sites in different locations could work in tandem by consecutive viscosity reduction leading to a self-accelerating effect. e Scanning electron micrograph of ZY₁₄ particles. Extended chains and ideal coil sizes for polypropylene (PP) with an M_w = 23,000 g/mol are drawn schematically to illustrate relative length scales. See Supplementary Fig. S7 for the scanning electron microscopy (SEM) images of all catalyst materials under study.

Fig. 4. Effect of zeolite acidity on cracking selectivity.

a Evolution of non-condensed hydrocarbons for cracking of polypropylene (PP) using different zeolite Y catalysts in a semi-batch reactor determined by on-line gas chromatography (GC). b Cumulative yield of the reaction. Gaseous products were probed by on-line GC, liquid products were characterized by ex-situ GCxGC with flame ionization detection (FID) and mass spectrometry (MS) (see Supplementary Fig. S12 for the corresponding 2D chromatograms), coke deposits were determined by thermogravimetric analysis (TGA). Deviations from 100% yield (2.5 g) are due to minor condensation in cold sections of the autoclave. c, d Total yield of aromatics, coke, propane, and propene as a function of bulk Brønsted acid site content determined by pyridine infrared (IR) spectroscopy. Error bars taken from the error of the extinction coefficient. Trend lines are drawn to guide the eye.