Researchers in Spain have obtained high-quality transportation fuels (gasoline with RON >80 and diesel with cetane number >70) from oil obtained by the thermal cracking of low-density polyethylene (LDPE) using a bifunctional catalyst comprising Ni (7 wt %) deposited on a hierarchical Beta zeolite (Ni/h-Beta). A paper on their work is published in the ACS journal Energy & Fuels.
LDPE—the first grade of polyethylene, discovered in 1933 in the course of fundamental research at Imperial Chemical Industries in England— is created by the high pressure polymerization of ethylene. LDPE (SPI resin code “4” for recycling) is currently mainly used in the packaging, construction and automotive sectors, which accounted for 80% of the global LDPE demand in 2010, according to a 2011 market research report.
Demand for LDPE is growing at a slow pace with a compound annual growth rate (CAGR) of 2.5% in the period 2000-2010. The growth of LDPE demand can be attributed solely to the Chinese market which is the largest consumer of LDPE and accounted for 32% of the demand in 2010.
In previous works of our group, hierarchical zeolites have been successfully applied in the polyolefin [low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP)] cracking, showing quite higher activities than conventional zeolites because of the lower steric constraints and enhanced diffusion of the bulky plastic macromolecules. However, the obtained liquids contained high amounts of olefins, which are harmful for their usage in the formulation of fuels because they might cause the formation of gums in the car injectors and engines.
...To decrease the olefins content, we have applied recently bifunctional hierarchical zeolites (Ni/h-ZSM-5 and Ni/h-Beta) for the hydroreforming of the oil coming from LDPE thermal cracking to attain fuels for transportation, mostly gasoline and diesel. Although in the literature several works can be found wherein the plastics are directly hydrocracked, in our approach, a previous thermal cracking stage was included because several stages are required with real plastic wastes to avoid catalyst deactivation by the impurities present in the waste feed.
This combination of treatments is of remarkable interest because it could be a reasonable route for dealing with waste plastics. This is particularly true in the current context of searching for ways to divert waste plastics from landfills, different from the traditional incineration with energy recovery and mechanical recycling.—Escola et al.
Bifunctional catalysts contain both acid and metal sites. The acid sites are usually provided by an aluminosilicate carrier (zeolites, Al-MCM-41, and amorphous SiO2−Al2O3), while the metal sites come from a supported metal, such as Pt, Pd, or Ni. These catalysts are widely applied in hydro-reforming reactions, such as hydro-cracking, hydro-isomerization, hydro-desulfurization, and hydro-denitrification, the team notes.
In the reported study, the team investigated the influence of the main reaction variables (temperature, hydrogen pressure, catalyst/feed mass ratio, and reaction time) on the optimization of the production of high-quality transportation fuels within the gasoline and diesel range.
Among their findings were:
The presence of nickel in the catalysts significantly reduces the olefin content of the feed, avoiding oligomerization reactions and largely increasing the yield of gasolines in the products. The enhancement in the reaction temperature led toward increased hydrocracking because the amount of gasolines augmented from 250 to 350 °C.
Light diesel (C13–C18) was cracked more deeply than heavy diesel (C19−C40), likely because of their faster diffusion into the zeolite micropores. Higher temperatures promoted the extension of aromatization reactions, especially when working above 300 °C.
An increase in the hydrogen pressure up to 40 bar was very effective in hydrogenating olefins and also promoted the hydrocracking, giving rise to more gasolines, although it suppressed in a large proportion the formation of aromatic hydrocarbons.
The increase in the catalyst/feed mass ratio from 1:100 to 1:10 enhanced the extent of hydrocracking with increasing yields of gasoline.
The extent of hydroisomerization reactions increased with the reaction time, leading toward the highest isoparaffin yields (53% share) after 180 min of reaction.
From this study, it can be concluded that the Ni/h-Beta catalyst is especially appropriate for obtaining gasolines, with the most adequate conditions to maximize its selectivity (up to 68.7%) being the hydroreforming at 40 bar of hydrogen pressure. In terms of the amount and quality of the fuels obtained, the most adequate conditions are the following conditions: T = 310 °C; t = 45 min; catalyst/feed ratio = 1:30; and hydrogen pressure = 20 bar. On the other hand, regardless of the experimental conditions used, more than 80% of the olefins present in the raw oil were saturated during the hydroreforming treatment. In addition, the RON of the gasolines and cetane indexes of the diesel confirmed the good quality of the obtained fuels.
Therefore, Ni/h-Beta constitutes a potential catalyst for the upgrading by hydroreforming of the hydrocarbons attained in the LDPE thermal cracking for its usage as transportation fuels.—Escola et al.
J. M. Escola, J. Aguado, D. P. Serrano, L. Briones, J. L. Díaz de Tuesta, R. Calvo, and E. Fernandez (2012) Conversion of Polyethylene into Transportation Fuels by the Combination of Thermal Cracking and Catalytic Hydroreforming over Ni-Supported Hierarchical Beta Zeolite. Energy & Fuels doi: 10.1021/ef300938r