Use of a metal oxide solid solution for sweetening a sour hydrocarbon fraction

An improved process for treating a sour hydrocarbon stream has been developed. This process involves contacting the sour hydrocarbon fraction with a metal oxide solid solution in the presence of an oxidizing agent such as air or oxygen. One example of a solid solution which can be used is a nickel oxide/magnesium oxide/aluminum oxide solid solution.

FIELD OF THE INVENTION 
This invention deals with a process for sweetening a sour hydrocarbon 
fraction. The process involves contacting a sour hydrocarbon fraction that 
contains mercaptans with a metal oxide solid solution in the presence of 
air or oxygen thereby converting the mercaptans to disulfides. 
BACKGROUND OF THE INVENTION 
Processes for the treatment of a sour hydrocarbon fraction where the 
fraction is treated by contacting it with an oxidation catalyst and an 
alkaline agent in the presence of an oxidizing agent at reaction 
conditions have become well known and widely practiced in the petroleum 
refining industry. These processes are typically designed to effect the 
oxidation of offensive mercaptans contained in a sour hydrocarbon fraction 
to innocuous disulfides--a process commonly referred to as sweetening. The 
oxidizing agent is most often air. Gasoline, including natural, straight 
run and cracked gasolines, is the most frequently treated sour hydrocarbon 
fraction. Other sour hydrocarbon fractions which can be treated include 
the normally gaseous petroleum fractions as well as naphtha, kerosene, jet 
fuel, fuel oil, and the like. 
A commonly used continuous process for treating sour hydrocarbon fractions 
entails contacting the fraction with a metal phthalocyanine catalyst 
dispersed in an aqueous caustic solution to yield a doctor sweet product. 
Doctor sweet means a mercaptan content in the product low enough to test 
"sweet" (as opposed to "sour") by the well known doctor test. The sour 
fraction and the catalyst containing aqueous caustic solution provide a 
liquid-liquid system wherein mercaptans are converted to disulfides at the 
interface of the immiscible solutions in the presence of an oxidizing 
agent-usually air. Sour hydrocarbon fractions containing more difficult to 
oxidize mercaptans are more effectively treated in contact with a metal 
chelate catalyst dispersed on a high surface area adsorptive 
support-usually a metal phthalocyanine on an activated charcoal. The 
fraction is treated by contacting it with the supported metal chelate 
catalyst at oxidation conditions in the presence of a soluble alkaline 
agent. One such process is described in U.S. Pat. No. 2,988,500. The 
oxidizing agent is most often air admixed with the fraction to be treated, 
and the alkaline agent is most often an aqueous caustic solution charged 
continuously to the process or intermittently as required to maintain the 
catalyst in the caustic-wetted state. 
The prior art shows that alkaline agents are necessary in order to 
catalytically oxidize mercaptans to disulfides. Thus, U.S. Pat. Nos. 
3,108,081 and 4,156,641 disclose the use of alkali hydroxides especially 
sodium hydroxide for oxidizing mercaptans. Further, U.S. Pat. No. 
4,913,802 discloses the use of ammonium hydroxide as the basic agent. The 
activity of the metal chelate systems can be improved by the use of 
quaternary ammonium compound as disclosed in U.S. Pat. Nos. 4,290,913 and 
4,337,147. 
It is also known that materials such as layered double hydroxides (LDH) or 
metal oxides solid solutions can be used as solid bases on which can be 
dispersed a metal chelate. These materials are described in U.S. Pat. No. 
5,232,887. This patent discloses the use of a solid solution of magnesium 
oxide and aluminum oxide as well as an LDH identified as hydrotalcite and 
having the formula 
EQU Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).multidot.4H.sub.2 O 
as solid bases. In order to obtain appreciable conversion of mercaptans to 
disulfides a polar compound such as water or methanol must be added. 
In Catalysis Letters, 11, pp. 55-62 (1991), the authors describe the 
oxidation of 1-decanethiol in water using an LDH in which cobalt 
phthalocyanine is intercalated between the LDH layers. The process also 
uses a borate buffer to maintain the pH at 9.25. 
In contrast to this art, applicants have discovered that solid solutions of 
metal oxides can catalyze the oxidation of mercaptans found in hydrocarbon 
fractions without the use of metal chelates or polar compounds or 
additional bases. The conditions necessary for oxidizing the mercaptans, 
i.e., sweetening the hydrocarbon fraction, are the same as those used in 
conventional sweetening processes. Thus, the instant process has the 
advantage that it does not introduce anything into the hydrocarbon stream 
which must later be removed. 
SUMMARY OF THE INVENTION 
As stated, this invention relates to a process for treating a sour 
hydrocarbon fraction containing mercaptans. Accordingly, one embodiment of 
the invention is a process for treating a sour hydrocarbon fraction 
containing mercaptans comprising contacting the hydrocarbon fraction with 
a catalyst effective in oxidizing mercaptans in the presence of an 
oxidizing agent under treating conditions thereby oxidizing the mercaptans 
to disulfides, the catalyst characterized in that it comprises a solid 
solution having the formula 
EQU M.sub.a (II)M.sub.b (III)O.sub.(a+b) (OH).sub.b 
where M(II) is at least one metal having a +2 oxidation state and selected 
from the group consisting of magnesium, nickel, zinc, copper, iron, 
cobalt, calcium and mixtures thereof, M(III) is at least one metal having 
a +3 oxidation state and is selected from the group consisting of 
aluminum, chromium, gallium, scandium, iron, lanthanum, cerium, yttrium, 
boron and mixtures thereof, and the ratio of a:b is greater than 1 to 
about 15. 
Other objects and embodiments of this invention will become apparent in the 
following detailed description. 
DETAILED DESCRIPTION OF THE INVENTION 
As stated, this invention relates to a process for treating a sour 
hydrocarbon fraction containing mercaptans. The process involves 
contacting the hydrocarbon fraction with a solid solution of metal oxides 
in the presence of an oxidizing agent. 
Thus, one necessary component of this invention is a solid solution of 
metal oxides. These solid solutions are described by the formula 
EQU M.sub.a (II)M.sub.b (III)O.sub.(a+b) (OH).sub.b 
where M(II) is at least one metal having a +2 oxidation state and selected 
from the group consisting of magnesium, nickel, zinc, copper, iron, 
cobalt, calcium and mixtures thereof, M(III) is at least one metal having 
a +3 oxidation state and is selected from the group consisting of 
aluminum, chromium, gallium, scandium, iron, lanthanum, cerium, yttrium, 
boron and mixtures thereof, and the ratio of a:b is greater than 1 to 
about 15. When M(II) is a mixture of two metals, the relative amount of 
each metal can range from 1 to 99 weight percent of the M(II) metal. That 
is, if M 1 and M2 represent the two metals making up M(II), then M1 and M2 
can vary from 1 to 99 weight percent of the amount of M(II) in the 
composition. Preferred solid solutions are Mg/Al oxides and Ni/Mg/Al 
oxides solid solution. 
These solid solutions are prepared by heating the corresponding layered 
double hydroxide (LDH) material at a temperature of about 300.degree. to 
about 750.degree. C. Layered double hydroxides (LDH) are basic materials 
that have the formula 
EQU M.sub.a (II)M.sub.b (III)(OH).sub.(2a+2b) (X.sup.-n).sub.b/n 
.cndot.zH.sub.2 O 
The M(II) and M(III) metals are the same as those described for the solid 
solution. The values of a and b are also as set forth above. X is an anion 
selected from the group consisting of carbonate, nitrate, and mixtures 
thereof, where n is the charge on the anion. Finally, z varies from about 
1 to about 50 and preferably from about 1 to about 15. These materials are 
referred to as layered double hydroxides because they are composed of 
octahedral layers, i.e., the metal cations are octahedrally surrounded by 
hydroxyl groups. These octahedra share edges to form infinite sheets. 
Interstitial anions such as carbonate are present to balance the positive 
charge in the octahedral layers. The preparation of layered double 
hydroxides is well known in the art and can be exemplified by the 
preparation of a nickel/magnesium/aluminum layered double hydroxide. This 
LDH can be prepared by coprecipitation of nickel, magnesium and aluminum 
carbonates at a high pH. Nickel nitrate, magnesium nitrate and aluminum 
nitrate (in the desired ratios) are added to an aqueous solution 
containing sodium hydroxide and sodium carbonate. The resultant slurry is 
heated at about 65.degree. C. to crystallize the compound and then the 
powder is isolated and dried. Extensive details for the preparation of 
various LDH materials may be found in J. Catalysis, 94, 547-557 (1985). 
As stated the LDH material is heated at a temperature of about 300.degree. 
to about 750.degree. C. to give the corresponding solid solution. The 
resultant solid solution is in the form of a powder which can be further 
processed by conventional means to form extrudates, spheres, pills, etc. 
The catalyst of this invention may optionally contain a secondary component 
selected from the group consisting of calcium oxide, magnesium hydroxide, 
magnesium oxide, calcium hydroxide and mixtures thereof. The secondary 
component can be combined with the solid base in an amount varying from 
about 0.1 to about 50 weight percent of the catalyst. 
Another necessary component of the instant process is an oxidizing agent. 
The oxidizing agent can be air, oxygen or other oxygen containing gases 
with air being preferred. The sour hydrocarbon fraction may contain 
sufficient entrained air, but generally added air is admixed with the 
fraction and charged to the treating zone concurrently therewith. In some 
cases, it may be advantageous to charge the air separately to the treating 
zone and countercurrent to the fraction separately charged thereto. 
The treating conditions, i.e., sweetening or mercaptan oxidation 
conditions, and specific methods used to carry out the present invention 
are those that have been disclosed in the prior art. Typically, the sour 
hydrocarbon fraction is contacted with the catalyst which is in the form 
of a fixed bed. The contacting is thus carried out in a continuous manner 
and the hydrocarbon fraction may be flowed upwardly or downwardly through 
the catalytic composite. The process is usually effected at ambient 
temperature conditions, although higher temperatures up to about 
105.degree. C. are suitably employed. Pressures of up to about 1,000 psi 
or more are operable although atmospheric or substantially atmospheric 
pressures are suitable. Contact times equivalent to a liquid hourly space 
velocity of from about 0.5 to about 50 hr.sup.-1 or more are effective to 
achieve a desired reduction in the mercaptan content of a sour hydrocarbon 
fraction, an optimum contact time being dependent on the size of the 
treating zone, the quantity of catalyst contained therein, and the 
character of the fraction being treated. Examples of specific arrangements 
to carry out the treating process may be found in U.S. Pat. Nos. 4,490,246 
and 4,753,722 which are incorporated by reference. 
It may also be desirable to add a polar compound to the hydrocarbon feed. 
The polar compound may be water or an alcohol such as methanol, ethanol, 
propanol, etc. The amount of polar compound which is added can vary from 
about 10 ppm to about 15,000 ppm based on hydrocarbon.

The following examples are presented in illustration of this invention and 
are not intended as undue limitations on the generally broad scope of the 
invention as set out in the appended claims. 
EXAMPLE 1 
Preparation of NiO/MgO/Al.sub.3 Solid Solution 
A 2L, 3-necked round bottomed flask was equipped with a reflux condenser, a 
thermometer, and a mechanical stirrer. To this flask there was added a 
solution containing 585 g of water, 60 g of Na.sub.2 CO.sub.3 
.cndot.H.sub.2 O and 71 g of NaOH and the flask was cooled to &lt;5.degree. 
C. An addition funnel containing 378 g water, 32.5 g of Mg(NO.sub.3).sub.2 
.cndot.6H.sub.2 O, 110 g of Ni(NO.sub.3).sub.2 .cndot.6H.sub.2 O, and 93 g 
Al(NO.sub.2).sub.3 .cndot.9H.sub.2 O was put in place of the reflux 
condensor and the solution added to the solution in the flask over a 
four(4) hour period while maintaining the temperature at &lt;5.degree. C. The 
resultant slurry was stirred for 1 hour at &lt;5.degree. C. after which the 
funnel was removed and the reflux condenser replaced. The flask was now 
placed in a Glass Col.RTM. heating mantle and was heated to 60.degree. C. 
.+-.5.degree. for 1 hour. The slurry was then cooled to room temperature, 
the solids recovered by filtration and washed with 10L of deionized water. 
These solids were then dried at 100.degree. C. for 16 hours. After 
crushing the solid was calcined at 450.degree. C. for 12 hours in a muffle 
furnace with air flow. X-ray diffraction analysis showed this product to 
be a solid solution of nickel, magnesium and aluminum oxides. This sample 
had a B.E.T. surface area of 199 m.sup.2 /g and was identified as sample 
A. 
EXAMPLE 2 
A reactor was loaded with 20 cc of sample A and heated to 38.degree. C. 
Over this catalyst there was flowed n-hexane containing 7,000 ppm water 
and 290 ppm (as sulfur) of thiophenol at a liquid hourly space velocity of 
1.2 hr.sup.-1. Air was injected to give a ratio of two times the 
stoichiometric amount required to oxidize the mercaptan to disulfide. The 
test was conducted for 48 hours during which time samples were withdrawn 
and analyzed to determine mercaptan conversion. The results from this test 
are presented in Table 1. 
TABLE 1 
______________________________________ 
Mercaptan Conversion Using a NiO/MgO/Al.sub.2 O.sub.3 Solid Solution 
Hours on Stream 
Mercaptan Conversion (%) 
______________________________________ 
8 100 
16 100 
20 100 
24 100 
28 99 
32 100 
36 100 
40 100 
44 100 
48 100 
______________________________________ 
EXAMPLE 3 
Preparation of NiO/Al.sub.2 O.sub.3 Solid Solution 
A 2L, 3-necked round bottomed flask was equipped with a reflux condenser, a 
thermometer, and a mechanical stirrer. To this flask there was added a 
solution containing 412 g of water, 38 g of Na.sub.2 CO.sub.3 
.cndot.H.sub.2 O and 48.1 g of NaOH and the flask was cooled to &lt;5.degree. 
C. An addition funnel containing 228.4 g water, 100.12 g of 
Ni(NO.sub.3).sub.2 .cndot.6H.sub.2 O and 64.12 g Al(NO.sub.2).sub.3 
.cndot.9H.sub.2 O was put in place of the reflux condensor and the 
solution added to the solution in the flask over a four (4) hour period 
while maintaining the temperature at &lt;5.degree. C. The resultant slurry 
was stirred for 1 hour at &lt;5.degree. C. after which the funnel was removed 
and the reflux condenser replaced. The flask was now placed in a Glass 
Col.RTM. heating mantle and was heated to 60.degree. C. .+-.5.degree. C. 
for 1 hour. The slurry was then cooled to room temperature, the solids 
recovered by filtration and washed with 10L of deionized water. These 
solids were then dried at 100.degree. C. for 16 hours. After crushing the 
solid was calcined at 450.degree. C. for 12 hours in a muffle furnace with 
air flow. X-ray diffraction analysis showed this product to be a solid 
solution of nickel and aluminum oxides. This sample was identified as 
sample B. 
EXAMPLE 4 
A reactor was loaded with 20 cc of sample B and heated to 38.degree. C. 
Over this catalyst there was flowed FCC gasoline containing 7,000 ppm 
water and 290 ppm (as sulfur) of mercaptans at a liquid hourly space 
velocity of either 1.2 hr.sup.-1 or 15 hr.sup.-1. Air was injected to give 
a ratio of two times the stoichiometric amount required to oxidize the 
mercaptan to disulfide. The test was conducted for 180 hours during which 
time samples were withdrawn and analyzed to determine mercaptan 
conversion. The results from this test are presented in Table 2. 
TABLE 2 
______________________________________ 
Sweetening of FCC Gasoline Using a NiO/Al.sub.2 O.sub.3 Solid Solution 
Mercaptan Conversion 
Hours on Stream 
LHSV (hr.sup.-1) 
(%) 
______________________________________ 
4 1.2 97 
8 1.2 99 
24 1.2 100 
36 15 99 
44 15 99 
64 15 100 
92 15 99 
112 15 99 
136 15 99 
148 15 99 
______________________________________ 
EXAMPLE 5 
A reactor was loaded with 10 cc of a MgO/Al.sub.2 O.sub.3 solid solution 
with excess MgO obtained from Alcoa Industrial Chemicals and identified as 
Sorbplus.TM.. Over this catalyst there was flowed a FCC gasoline stream 
containing 77 ppm (as sulfur) of mercaptans and 7,000 ppm water at a 
liquid hourly space velocity of 1.2 hr.sup.-1. Air was injected to give a 
ratio of two times the stoichiometric amount required to oxidize the 
mercaptan to disulfide. The test was conducted for 24 hours during which 
time samples were withdrawn and analyzed to determine mercaptan 
conversion. The results from this test are presented in Table 3. 
TABLE 3 
______________________________________ 
Sweetening of FCC Gasoline Using an MgO/Al.sub.2 O.sub.3 Solid 
Solution + MgO 
Hours on Stream 
Mercaptan Conversion (%) 
______________________________________ 
4 94 
8 99 
12 96 
16 99 
20 97 
24 99 
______________________________________ 
EXAMPLE 6 
In a container there were added 0.8 g of sample B and 50 grams of 
iso-octane containing 1,154 wppms of n-octanethiol. This mixture was 
stirred for a few minutes and then a sample was withdrawn to test for 
sulfur. The analysis showed that the iso-octane contained 356 wppm of 
mercaptan sulfur. This experiment shows that water is not essential for 
activity.