Selective catalysts having silica supports for naphtha hydrodesulfurization

PATENT
Selective catalysts having silica supports for naphtha hydrodesulfurization
PATENT number: 8216958
Issue date: Jul 10, 2012
Application number: 12/087,978
Inventors: Jason Wu, Chuansheng Bai, Thomas R. Halbert, Stuart L. Soled, Sabato Miseo, Jonathan M. McConnachie, Valery Sokolovskii, David M. Lowe, Anthony F. Volpe, Jr., Jun Han
Original Assignee: ExxonMobil Research and Engineering Company
Abstract
A method for hydrodesulfurizing FCC naphtha is described. More particularly, a Co/Mo metal hydrogenation component is loaded on a silica or modified silica support in the presence of organic ligand and sulfided to produce a catalyst which is then used for hydrodesulfurizing FCC naphtha. The silica support has a defined pore size distribution which minimizes olefin saturation.

FIELD OF THE INVENTION

This invention relates to a method for hydrodesulfurizing naphtha. More particularly, a Co/Mo metal hydrogenation component is loaded on a silica or modified silica support in the presence of an organic additive and then sulfided to produce a catalyst Which is then used for hydrodesulfurizing naphtha. The silica support has a defined pore size distribution Which minimizes 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 significant octane number loss due to extensive reduction of the olefin content in the naphtha as Well as excessive consumption of hydrogen during the hydrotreating process. Selective hydrotreating processes have recently been developed to avoid such olefin saturation and octane number loss. Unfortunately, in such processes, the liberated H2S reacts With retained olefins forming mercaptan sulfur by reversion. Unfortunately, the H2S liberated in the process 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 number during sulfur removal is to modify the olefin content of the feed using an olefin-modification catalyst folloWed by contact With an 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 Wt. % to about 10 Wt. % MoO3; and about 0.1 Wt. % to about 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 A to about 200 A.
 
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 method for making a catalyst and a method for the hydrodesulfurization (HDS) of naphtha. One embodiment relates to a method for making a catalyst suitable for the HDS of naphtha comprising: (i) impregnating a silica support that has a silica content of at least about 85 Wt. %, based on silica and has a pore volume betWeen about 0.6 cc/ g and about 2.0 cc/ g and median pore sizes in the range of about 150 A to about 2000 A With an aqueous solution of (a) a cobalt salt, (b) a molybdenum salt, and (c) at least one organic additive to form a catalyst precursor; (ii) drying the catalyst precursor at temperatures less than about 200° C. to form a dried catalyst precursor; and (iii) sulfiding the dried catalyst precursor to form the catalyst, provided that the dried catalyst precursor or catalyst is not calcined prior to sulfiding or use for HDS.
 
Another embodiment relates to a method for the HDS of 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 selective HDS catalyst under hydrodesulfurization conditions, Wherein the selective HDS catalyst is prepared by impregnating a silica support that has a silica content of at least about 85 Wt. %, based on the Weight of the silica, and has a pore volume betWeen about 0.6 cc/g and about 2.0 cc/g, and median pore sizes in the range of about 150 A to about 2000 A With an aqueous solution of (a) a cobalt salt, (b) a molybdenum salt, and (c) at least one organic additive to form a catalyst precursor; (ii) drying the catalyst precursor at temperatures less than about 200° C. to form a dried catalyst precursor; and (iii) sulfiding the dried catalyst precursor to form the catalyst, provided that the dried catalyst precursor or catalyst is not calcined prior to sulfiding or use for HDS.
 
The silica supported catalyst, When used for the HDS of a naphtha, shoWs improved selectivity toWards olefin saturation While maintaining a high level of HDS of the naphtha feed.
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