Selective catalysts having high temperature alumina supports for naphtha hydrodesulfurization (Exxonmobil)

CATEGORY: HYDRODESULFURIZATION
PATENT
Selective catalysts having high temperature alumina supports for naphtha hydrodesulfurization (Exxonmobil)
PCT number PCT/US2007/001000
Publication date Jan 28, 2014
Also published as CA2636918A1
Inventors Jason Wu
Original Assignee Exxonmobil Research And Engineering Company
Abstract
This invention relates to a catalyst and method for hydrodesulfurizing naphtha. More particularly, a Co/Mo metal hydrogenation component is loaded on a high temperature alumina support in the presence of a dispersion aid to produce a catalyst that is then used for hydrodesulrurizing naphtha. The high temperature alumina support has a defined surface area that minimizes olefin saturation.
FIELD OF THE INVENTION
This invention relates to a catalyst and method for hydrodesulfurizing naphtha. More particularly, a CoMo metal hydrodesulfurization component is loaded on a high temperature alumina support in the presence of an organic additive to produce, after sulfidation, a catalyst that is then used for hydrodesulfurizing naphtha. The high temperature alumina support has defined properties that minimize olefin saturation.
BACKGROUND OF THE INVENTION
Environmental regulations mandate the lowering of sulfur levels in motor gasoline (mogas). For example, it is expected that regulations will require mogas sulfur levels of 30 ppm or less by 2006. In many cases, these sulfur levels will be achieved by hydrotreating naphtha produced from Fluid Catalytic Cracking (FCC cat naphtha), which is the largest contributor to sulfur in the mogas pool. Since sulfur in mogas can also lead to decreased performance of catalytic converters, a 30 ppm sulfur target is desirable even in cases where regulations would permit a higher level. As a result, techniques are required that reduce the sulfur in cat naphthas while at the same time minimizing the reduction of beneficial properties such as octane number.
Conventional fixed bed hydrotreating can reduce the sulfur level of cracked naphthas to very low levels. However, such hydrotreating also results in severe octane number loss due to extensive reduction of the olefin content as well as excessive consumption of hydrogen. Selective hydrotreating processes have recently been developed to avoid significant olefin saturation and octane number loss. Unfortunately, in such processes, the liberated H2S reacts with retained olefins forming mercaptan sulfur by reversion. Such processes can be conducted at severities which produce product within sulfur regulations. However, significant octane number loss also occurs.
One proposed approach for preserving octane during sulfur removal is to modify the olefin content of the feed using an olefin-modification catalyst followed by contact with a hydrodesulfurization (HDS) catalyst (U.S. Pat. No. 6,602,405). The olefin modification catalyst oligomerizes the olefins.
One recently developed method of HDS is SCANfining which is a process developed by Exxon Mobil Corporation. SCANfining is described in National Petroleum Refiners Association paper # AM-99-31 titled “Selective Cat Naphtha Hydrofining with Minimal Octane Loss” and U.S. Pat. Nos. 5,985,136 and 6,013,598. Typical SCANfining conditions include one and two-stage processes for hydrodesulfurizing a naphtha feedstock. The feedstock is contacted with a hydrodesulfurization catalyst comprised of about 1 to about 10 wt. % MoO3; and about 0.1 to abut 5 wt. % CoO; and a Co/Mo atomic ratio of about 0.1 to about 1.0; and a median pore diameter of about 60 Å to about 200 Å.
Even though SCANfining controls the degree of olefin saturation while achieving a high degree of HDS, there is still a need to improve the selectivity of the catalyst system to further reduce the degree of olefin saturation thereby further minimizing octane number loss.
SUMMARY OF THE INVENTION
This invention relates to a catalyst suitable for the hydrodesulfurization (HDS) of naphtha which is prepared by a process comprising: (i) heating an alumina precursor to a temperature above at least about 800° C. to form a high temperature alumina, impregnating the high temperature alumina with an aqueous solution of (a) a cobalt salt, (b) a molybdenum salt, and (c) at least one organic additive to form an impregnated high temperature alumina, provided that the high temperature alumina has a gamma, eta and chi alumina content less than about 50 wt. %, based on the total weight of the alumina; a median pore diameter in the range of about 100 Å to about 1000 Å; and a surface area between about 40 m2/g and about 200 m2/g; (ii) drying the impregnated high temperature alumina at temperatures less than about 200° C. to form a dried impregnated high temperature alumina catalyst precursor; and (iii) sulfiding the dried impregnated high temperature alumina catalyst precursor to form a HDS catalyst, provided that the HDS catalyst or catalyst precursor is not calcined prior to sulfiding or use for HDS.
In a preferred embodiment, the HDS catalyst precursor contains from about 1 wt. % to about 8 wt. % cobalt, based on catalyst support and from about 6 wt. % to about 20 wt. % molybdenum, based on catalyst support. The HDS catalyst may further contain alpha alumina in addition to other high temperature aluminas such as delta, theta and kappa.
Another embodiment relates to a method for making a HDS catalyst, with the catalyst prepared by: (i) heating an alumina precursor to a temperature above at least about 800° C. to form a high temperature alumina; (ii) impregnating the high temperature alumina with an aqueous solution of (a) a cobalt salt, (b) a molybdenum salt, and (c) at least one organic additive to form an impregnated high temperature alumina, provided that the high temperature alumina has a gamma, eta and chi alumina content less than about 50 wt. %, based on the total weight of the alumina; a median pore diameter in the range of about 100 Å to about 1000 Å; and a surface area between about 40 m2/g and about 200 m2/g; (iii) drying the impregnated high temperature alumina at temperatures less than about 200° C. to form a dried impregnated high temperature alumina catalyst precursor; and (iv) sulfiding the dried impregnated high temperature alumina catalyst precursor to form a HDS catalyst, provided that the HDS catalyst or catalyst precursor is not calcined prior to sulfiding or use for HDS.
Another embodiment relates to a method for the HDS of a naphtha having an olefin content of at least about 5 wt. %, based on the weight of the naphtha comprising: (i) contacting the naphtha with a HDS catalyst under hydrodesulfurization conditions, wherein the catalyst is prepared by heating an alumina precursor to a temperature above at least about 800° C. to form a high temperature alumina; impregnating the high temperature alumina with an aqueous solution of (a) a cobalt salt, (b) a molybdenum salt, and (c) at least one organic additive to form an impregnated high temperature alumina, provided that the high temperature alumina has a gamma, eta and chi alumina content less than about 50 wt. %, based on the total weight of the alumina; a median pore diameter in the range of about 100 Å to about 1000 Å; and a surface area between about 40 m2/g and about 200 m2/g; (ii) drying the impregnated high temperature alumina at temperatures less than about 200° C. to form a dried impregnated high temperature alumina catalyst precursor; and (iii) sulfiding the dried impregnated high temperature alumina catalyst precursor to form a HDS catalyst, provided that the HDS catalyst or catalyst precursor is not calcined prior to sulfiding or use for HDS.
The HDS catalyst, when used for the HDS of naphtha, shows improved selectivity towards olefin saturation while maintaining a high level of HDS of the naphtha feed.
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