Conversion of Polyethylene into Transportation Fuels by the Combination of Thermal Cracking and Catalytic Hydroreforming over Ni-Supported Hierarchical Beta Zeolite

The oil obtained from the thermal cracking of low-density polyethylene (LDPE) is formed mainly by linear hydrocarbons with a high quantity of olefins, which hinders the possible application of this product in the formulation of transportation fuels. However, hydroreforming of this oil using bifunctional catalysts with high accessibility to the active sites would allow for the properties of the gasoline and diesel fractions to be significantly upgraded. This is the case of the catalyst employed here because it consists of hierarchical Beta zeolite (with a bimodal micro-mesoporosity) and containing 7 wt % Ni. The presence of nickel in the catalyst increased the share of gasolines with regard to the h-Beta support. The effect of the main variables involved in the hydroreforming process has been investigated and optimized, showing that the extent of hydrocracking is favored when increasing the temperature, the pressure, and the catalyst/feed ratio, leading to enhanced gasoline yields at the expense of heavy (C19–C40) and especially light (C13–C18) diesel fractions because of the faster diffusion of the latter. Ni/h-Beta proved to be an especially adequate catalyst for obtaining gasolines; therefore, a maximum in the selectivity toward gasoline (up to 68.7%) was found in the hydroreforming at 40 bar of hydrogen pressure. On the other hand, the values of the bromine index indicated that 80–100% of the olefins present in the raw oil were hydrogenated depending upon the reaction conditions. In addition, the Ni/h-Beta catalyst showed high activity for aromatization and, especially, hydroisomerization reactions. Thus, a 53% share of isoparaffins in the gasolines was obtained at long reaction times. The gasoline and diesel fractions obtained showed a high research octane number (RON) (>80) and cetane indexes above specifications (>70), respectively, which is indicative of their high quality as transportation fuels.