(379j) Hydrogenation of Pyrolysis Fuel Oil over Pt/MCM-41 Catalysts

Steam cracking of naphtha is one of the most important petrochemical processes during the production of light olefins such as ethylene, propylene, butene, and butadiene. Pyrolysis fuel oil (PFO) is a heavy residue obtained from naphtha steam cracking plants. It is generally used in petrochemical plants as a process fuel. It is also noteworthy that the number of carbon atoms (C₈-C₁₅) in PFO is similar to that of aviation fuel. Nevertheless, the high aromatic content in PFO reduces the fuel quality and leads to the formation of unwanted emissions in the exhaust gas. The hydrogenation of PFO is an important process to upgrade its properties and make it more suitable for use as a component of aviation fuel.

Recently, platinum catalysts have been widely adapted for use in the hydrogenation of PFO due to their high activity and good selectivity in this reaction. These catalysts are designed to increase the active surface area and promote efficient mass transfers, leading to improved reaction rates and better product selectivity. This work aims to investigate the effect of Pt/MCM-41 catalysts on the hydrogenation of the polycyclic aromatic hydrocarbons included in PFO for the production of aviation fuel.

MCM-41 was synthesized using cetyltrimethylammonium bromide (CTABr) and sodium silicate solution. After dissolving 12.15 g of CTABr in 140 g of distilled water, the 50 g of sodium silicate solution (20 wt.% SiO₂) was added. The reaction mixture was heated to 100 °C in an oven for 24 h. After adjusting the pH to 10 with 50% acetic acid solution, the mixture was reacted again at 100 °C for 24 h. The product was washed with HCl/EtOH mixture, dried at 100 °C and finally calcined at 550 °C for 4 h.

The bead-type Pt/MCM-41 catalysts were prepared using two methods. In the first method, a mixture of MCM-41 powder, the organic binder and inorganic binder was initially formed into a bead (2 mm in diameter) through bead maker. Then, chloroplatinum hydrate (Aldrich) was impregnated into the MCM-41 bead using the incipient wetness method. The prepared bead-type catalyst was calcined at 400 °C for four hours, and it was termed 'Pt/MCM-41(AI) catalyst'. In the second method, chloroplatinum hydrate (Aldrich) was impregnated on the MCM-41 powder by incipient wetness method. Subsequently, catalyst forming was carried out after adding the organic and inorganic binders to prepare the bead-type catalyst (2 mm in diameter). The prepared bead-type catalyst was calcined at 400 °C for four hours to complete the catalysis of the bead-type Pt/MCM-41(BI) catalyst. The Pt loading amount of the of catalysts is 1.0 wt%.

Nitrogen adsorption analysis was performed using BELL SORP-mini â…¡ manufactured by BEL Japan. A 0.1 g sample was loaded into the cell, and after pretreatment at 200 °C for 8 h, adsorption-desorption isotherm was obtained at -196 °C. The specific surface area of the sample was calculated using the BET method. To measure the amount of acid cites and acid strength of the catalyst, ammonia-temperature programmed desorption (NH₃-TPD) analysis was performed using a BEL-CAT-B device (BEL Japan). Temperature programmed reduction of hydrogen (H₂-TPR) was performed using a BEL-CAT-B device from BEL JAPAN. Platinum dispersion was determined by CO-chemisorption experiments using Autochem â…¡ 2920 device (Micromeritics).

Crude PFO was received from Yeochun NCC Co., Ltd (Rep. of Korea). This crude PFO needs to distil out because it contains heavier polyaromatic hydrocarbons, which can lead to coke generation and catalyst deactivation during subsequent reaction. A total amount of 50 wt% PFO (PFO-cut) was obtained through distillation at 170 °C under vacuum of 20 mbar, which was used as a feedstock for the hydrogenation reaction. The majority of PFO-cut consists of aromatic hydrocarbons, with alkanes and cycloalkanes scarcely found. Naphthalene and its derivatives are presents as a main component (49.7%) along with alkylated benzene (19.1%), indene and its derivatives (16.3%), biphenyl (5.6%), fluorene (1.9%), and phenanthrene (1.3%). When the components are classified based on the number of rings, the fraction of bicyclic aromatic hydrocarbons is most prevalent at 72.5%, while the fractions of monocyclic aromatic hydrocarbons and tricyclic aromatic hydrocarbons account for 19.1% and 3.5%, respectively.

The hydrogenation of PFO-cut was carried out in a trickle-bed continuous-flow reactor. The reactor was manufactured using a SUS tube with an inner diameter of 1.3 cm and a length of 10 cm, which was used as a trickle-bed reactor. A 1 mm glass bead was filled in the upper and lower parts of the reactor, and 5 cc of the catalyst was filled between the glass beads and loaded into the reactor. The catalyst was reduced at 400°C under hydrogen flow. PFO-cut was injected using a high-pressure metering pump, and the hydrogen flow rate was controlled using a mass flow controller. The reactor pressure was constantly regulated through a back pressure regulator.

Liquid product samples were taken at regular intervals, and analyzed by gas chromatography (ACME 6100 GC, Young In Chromas) equipped with HP-5 column (30 m × 0.32 mm × 0.25 μm) and FID. In addition, a qualitative analysis of liquid sample was carried out using a gas chromatography-mass spectrometer detector (GC-MSD, Agilent 5937, Agilent Technologies, Inc.) equipped with an HP-5 column (30 m × 0.32 mm × 0.25 μm).

The yield of saturated cyclic compounds in PFO-cut hydrogenation did not decrease considerably over Pt-MCM-41 catalyst until ten hours of time-on-stream had elapsed, indicating that catalyst deactivation did not occur. As the time-on-stream increased from two to ten hours, the change in the carbon number distribution was negligible.

When the reaction temperature was increased from 200 °C to 300 °C, the yield of the saturated cyclic hydrocarbons increased monotonically, but when the temperature was raised further, the hydrogenation of PFO-cut instead decreased. Because the hydrogenation reaction of aromatic compounds is an exothermic reaction, the higher the temperature becomes, the less thermodynamically favorable the reaction becomes, and the equilibrium conversion rate is lowered. As a result, the optimal reaction temperature to maximize the conversion of PFO-cut under the given reaction conditions was determined to be 300 °C.

The Pt/MCM-41(BI) catalyst consistently maintained an average yield of about 37% of saturated cyclic compounds during the ten-hour hydrogenation reaction, representing higher yield than that of the Pt/MCM-41(AI) catalyst. This is attributed to the fact that the Pt/MCM-41(BI) catalyst has larger pore volume and pore size than those of Pt/MCM-41(AI) catalyst. In addition, the Pt/MCM-41(BI) catalyst has a larger number of acidic sites and an better reduction ability at low temperatures compared to the Pt/MCM-41 powder catalyst, resulting in an increased yield of aviation fuel.

The carbon number distribution of PFO-cut, the reaction raw material, was distributed between C₁₀ and C₁₅. It was found that a cracking reaction occurred during the hydrogenation reaction, with a shift toward light hydrocarbons. The most abundant compound among the PFO-cut hydrogenation reaction products was a cycloalkane with a two-ring structure, and C₈-C₁₄ hydrocarbons were included as the main products. The cyclic saturated hydrocarbon fraction with 12 carbon atoms was the most abundant in the PFO-cut hydrogenation product; this form is known to be suitable as the main fraction of aviation fuel.