Pyrolysis gasolines are by-products of ethylene industry. When producing ethylene by steam cracking from a liquid feed such as naphtha, gas oil, or the like, an output of the pyrolysis gasolines may be about 60 wt. % or more of the throughput of ethylene. Pyrolysis gasolines are typically subjected first to a first-stage selective hydrogenation to remove highly unsaturated olefins, such as dienes and styrene, and then to a second-stage hydrogenation to remove monoolefins as well as impurities containing sulfur, nitrogen, oxygen, or the like. The hydrotreated pyrolysis gasolines are then mainly used for producing aromatic hydrocarbons. Catalysts currently used in industry for the selective hydrogenation of pyrolysis gasolines are mainly Pd-based catalysts or Ni-based catalysts. These catalysts can be used in hydrogenation processes of middle distillates of pyrolysis gasoline, such as ones consisting of C6 to C8 hydrocarbons, and full boiling range pyrolysis gasolines consisting of C5 hydrocarbons up to hydrocarbons having an end boiling point of 204° C. The pyrolysis gasoline feeds to be hydrotreated from ethylene plants contain generally impurities and poisons, such as dienes, gums (i.e., polymers resulted from the polymerization of unsaturated components, such as dienes and styrene), As and heavy metals, although their contents may be quite different due to the differences of individual ethylene plants in cracking feedstock and cracking condition. These impurities and poisons will likely cause the deactivation of Pd-based catalysts. Ni-based catalysts have important use in the selective hydrogenation of pyrolysis gasolines, in particular full boiling range pyrolysis gasolines, because of their tolerance to As and stability at lower temperature.
Chinese Patent Application CN1644656A discloses a hydrogenation catalyst as well as relevant process and use. The catalyst contains 10 to 30 wt. % of NiO and 70 to 90 wt. % of Al2O3. This catalyst is suitable for the hydrogenation of distillate oils containing dienes and styrene as well as derivatives thereof, and the reaction conditions are as follows: temperature=50 to 200° C., pressure=2.0 to 4.0 MPa, liquid weight hourly space velocity=1 to 10 h−1, volume ratio of hydrogen to oil=100 to 300. With this catalyst and process, it is possible to produce directly a solvent oil having a high aromatic hydrocarbon content or a gasoline having a high octane value via hydrogenation.
Chinese Patent Application CN1218822A discloses a catalyst for selective hydrogenation. The catalyst consists of 5 to 25 wt. % of NiO, 0.1 to 2.0 wt. % of lithium or an alkaline earth metal (preferably magnesium), and the balance amount of alumina, and is suitable for the selective hydrogenation process of distillate oils containing diolefins, in particular full boiling range pyrolysis gasolines.
Chinese Patent Application CN1415413A discloses a nanonickel-based hydrogenation catalyst and the preparation thereof. This invention coats uniformly 20-50 nm sized powdery nickel on support strips via mechanical vibration, and then immobilizes them by sintering.
U.S. Pat. No. 6,686,308 discloses a supported nano-metal catalyst, comprising catalyst metal nanoparticles having an average particle size of 3 nm or less supported on support particles (for example, carbon). Typical catalyst metals include Ni, Pd, Pt, Ru, etc. A method of making the catalyst comprises the steps of: a) providing a solution of metal chlorides of one or more catalyst metals in solvent system containing at least one polyalcohol; b) forming a colloidal suspension of unprotected catalyst metal nanoparticles by raising the pH of the solution and heating the solution; c) adding support particles to the colloidal suspension; and d) depositing the unprotected catalyst metal nanoparticles on the support particles by lowering the pH of the suspension.
However, there still needs a Ni-based catalyst suitable for the hydrogenation of pyrolysis gasolines, which has better tolerance to water, heavy metals (such as Pb) and As, proper tolerance to gums, higher activity at lower temperature, and good selectivity, to prolong regeneration cycle length and service lifetime of the catalyst.