Mercaptan oxidation catalyst

A catalyst is disclosed for oxidizing mercaptans to disulfides, or inorganic sulfides to elemental sulfur. The catalytic agent is a compound consisting of a metal atom bonded to a chelate such as phthalocyanine, and also to axial ligands. The compound preferably is composited on an inert granular solid support. The catalyst is an improvement over existing catalysts in that its use does not require basic agents such as caustic.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the treatment of sour petroleum distillates. 
Specifically, the invention relates to a catalyst for sweetening sour 
petroleum distillates by using an oxometallo chelate catalyst to oxidize 
mercaptans in the distillates to disulfides. The catalyst may also be used 
to oxidize inorganic sulfides to elemental sulfur. 
2. Description of the Prior Art 
Several processes are currently used in the petroleum refining industry to 
sweeten sour petroleum distillates. Sweetening refers to the oxidation of 
offensive mercaptans contained in petroleum distillates to disulfides. The 
objectionable properties of mercaptans include their foul odors, 
toxicities, and corrosive behavior to metals. Sour petroleum distillates 
include straight-run gasoline, cracked gasoline, kerosene, jet fuel, 
naptha, fuel oil, gaseous fractions, and the like. 
One type of sweetening process utilizes an oxidizing agent, usually air, 
and an oxidation catalyst, usually a metal phthalocyanine to sweeten the 
sour distillates. A general feature of such process is the requirement of 
a strongly basic medium to effect the oxidation reaction as disclosed in 
U.S. Pat. No. 2,882,224. In such process, a metal phthalocyanine chelate 
catalyst is contained in an alkaline aqueous solution, which is contacted 
with a sour petroleum distillate and air. The oxidation catalyst can be 
retained in the alkaline aqueous phase by incorporating suitable 
substituent groups on the phthalocyanine chelate. Thus, suitable catalysts 
include mono- and polysulfonated metallo phthalocyanines. 
Another sweetening process is disclosed in U.S. Pat. No. 2,988,500, wherein 
a metallo phthalocyanine catalyst is composited on a solid support, such 
as charcoal, and an oxidizing agent, aqueous caustic solution, and sour 
petroleum distillate are passed over the composited catalyst. 
Another sweetening process is described in U.S. Pat. No. 4,207,173, wherein 
an organic base, such as tetra-alkyl guanidine, is employed as the basic 
medium. The guanidine is added to the sour petroleum feed stream, which 
admixed with air is passed over a metallo phthalocyanine catalyst 
composited on a carbon carrier. 
Another manner by which the sweetening of sour petroleum distillates can be 
achieved is disclosed by U.S. Pat. No. 4,260,479, wherein the basic medium 
is provided by a quaternary ammonium hydroxide, which is preferably 
composited with a metallo phthalocyanine catalyst on a carbon carrier. 
The chemistry of hydrogen sulfide, which may be found in the lower boiling 
or gaseous petroleum fractions, or of alkali-metal salts of sulfides in 
aqueous solutions, such as sodium sulfide in waste water, is sufficiently 
similar to mercaptan oxidation chemistry so that petroleum sweetening 
catalysts have been directly and easily applied to processes that oxidize 
these inorganic sulfides to elemental sulfur. Hydrogen sulfide and 
alkali-metal sulfide salts are objectionable for similar reasons as 
mercaptans, namely their toxicity, foul odor, corrosive tendency, and gum 
or sludge forming or causing tendency. 
In each of the petroleum sweetening processes of the prior art, the 
catalyst used consists of a metal-chelate compound. The preferred metal in 
the prior art is cobalt, while the preferred chelate is phthalocyanine or 
a structurally similar chelate such as porphyrin or 
tetrapyridinoporphyrazine. The use of porphyrins is described in U.S. Pat. 
No. 2,966,453 and the use of tetrapyridinoporphyrazines is described in 
U.S. Pat. No. 3,980,582. These chelates are similar in that all of the 
atoms of the chelate that are in close proximity to the metal atom are 
coplanar with the metal atom and each other. Thus, the metal atom is 
bonded by four nitrogen atoms in a square planar coordination environment. 
This environment results in catalytic activity of the metal atom. An 
examination of this square planar structure reveals that there are two 
additional coordination sites available to the metal atom, neither of 
which is in the metal-chelate plane. Instead these positions are above and 
below the metal chelate plane. Ligands at these positions form an axis, 
together with the metal atom, through and perpendicular to the 
metal-chelate plane. Such ligands will be referred to as axial ligands. 
##STR1## 
In the prior art the usual catalyst has been a cobalt phthalocyanine 
compound. This metal chelate generally does not bond to axial ligands, and 
no use of such axial ligands in petroleum sweetening catalysts has been 
considered in the prior art. 
There are a number of variations of the sweetening process of sour 
petroleum distillates using a metal phthalocyanine or similar catalysts 
and a basic environment. The use of a basic environment, however, has been 
the case of numerous problems such as disclosed in U.S. Pat. No. 
4,207,173. These problems include the formation of soaps which plug the 
charcoal catalyst bed; the contamination of the final distillate product 
with either sodium hydroxide or water, or both; formation of emulsions 
from sodium salt that carry water into the final product; and the cost of 
replacing and disposing of the caustic solution which is required when the 
solution eventually becomes contaminated with toxins or catalyst poisons 
extracted from the distillate. 
Other methods that employ organic bases to supply the basic medium have 
their own problems. Organic bases tend to be more expensive than aqueous 
caustic solutions due to the greater cost of such organics over aqueous 
caustic. If organic bases such as tetra-alkyl guanidines are used without 
an additional aqueous phase, they must be added in proportion to the 
amount of mercaptan contained in the sour distillate, increasing the cost 
of their use. When composited on a fixed bed with a metal chelate 
catalyst, the amount of base becomes depleted as the treatment of sour 
distillate proceeds. Alternatively, the organic base may be incorporated 
in aqueous solutions, in which case they are used as the caustic solution 
in practicing such methods. Thus, while the prior art has examined many 
ways to accommodate the problems caused by using organic or inorganic 
bases, no method is known that does not require the use of base in 
addition to the metal chelate catalyst. 
SUMMARY OF THE INVENTION 
It is the object of this invention to provide a novel catalyst for the 
sweetening of sour petroleum distillates that does not require the use of 
a basic agent with said catalyst. This novel oxidation catalyst consists 
of an oxometallo chelate compound, said compound having two axial ligands. 
The first axial ligand is an oxygen atom, and the second axial ligand can 
be any of several radical groups, but typically is an alkoxy radical. The 
first and second axial ligands occupy positions on the metal atom adjacent 
to said chelate and opposite to each other. Said ligands are referred to 
as "axial" ligands because their positions in said compound may be thought 
of as forming an axis passing through the plane of the metal atom and the 
chelate. The catalyst is preferably composited on a solid inert granular 
support such as charcoal or a refractory oxide such as alumina or silica. 
By use of this novel catalyst composite, it has been discovered that the 
addition of basic agents to the sweetening process becomes unnecessary, 
and all the aforesaid disadvantages of using basic agents are avoided. 
DETAILED DESCRIPTION OF THE INVENTION 
The oxometallo chelate compound of the present invention is represented by 
the formula MO(Pn)X, where: 
M represents a metal atom, 
O represents the first axial ligand and is an oxygen atom double-bonded to 
the metal atom, 
(Pn) represents a tetradentate chelate, and 
X represents the second axial ligand, which is a radical group 
single-bonded to the metal atom. 
Suitable metals for the compound are those from the group consisting of 
molybdenum, tungsten, chromium, vanadium, niobium, tantalum, manganese, 
rhenium, polonium, antimony, bismuth, praesodymium, neodymium, promethium, 
and uranium, as well as the metalloids selenium and tellurium. The 
preferred metals are molybdenum and tungsten, with molybdenum especially 
preferred. 
The chelate may be selected from a large variety of chelates well known to 
the art, such as phthalocyanines and substituted phthalocyanines as 
described in U.S. Pat. No. 2,988,500; or porphyrins or substituted 
porphyrins as described in U.S. Pat. No. 2,966,453; or 
tetrapyridinoporphyrazines as described in U.S. Pat. No. 3,980,582; 
corrinoid chelates as described in U.S. Pat. No. 3,252,892; or other 
macrocyclic chelates such as Schiff bases, and the like. Dimeric or 
polymeric chelates such as polyporphyrins may also be used. The preferred 
chelates are substituted phthalocyanines such as phthalocyanine sulfonate. 
Substituted porphyrins such as tetraphenyl porphyrin are also preferred 
chelates. 
The second axial ligand, X, is a radical group single-bonded to the metal 
such as an aliphatic or aromatic alkoxo radical (.OR); hydroxo radical 
(.OH); fluoro, chloro, bromo, or iodo radical; cyano, thiocyanato, 
isocyanato, or hydroperoxo radical; bisulfato, bicarbonato, nitrato, 
chlorato, perchlorato, or bisulfito radical; a primary aromatic or 
aliphatic amido radical; a secondary aromatic or aliphatic amido radical; 
a secondary aliphatic and aromatic radical; an acid phosphato radical; or 
an unsubstituted amido radical (.NH.sub.2). Imidazolo radicals, and 
substituted Imidazolo radicals may also be used. The preferred axial 
ligand X is a small chain aliphatic alkoxo radical such as methoxo, 
ethoxo, propoxo radicals and the like, or the simple hydroxo radical. 
The axial ligand X may also be bound to the chelate by means of suitable 
straight chain or other chemical group, e.g., an alkoxo ligand with a 
straight chain aliphatic group consisting of from 5 to 12 carbon atoms 
bound to the chelate such as on one of the phenyl rings of tetraphenyl 
porphyrin. It may be desirable to attach the ligand in this manner to the 
chelate since the ligand tends to be rather labile and may become 
displaced in the practice of the invention. In the event that the axial 
ligand X is displaced, the catalyst will still be able to function, 
although somewhat less effectively. In such case the catalyst will 
incorporate entrained impurities in the petroleum distillate such as 
alcohols, phenols, or water, and will convert them to alkoxo, phenoxo or 
hydroxo radical ligands in the axial position. This will occur even if the 
entrained impurity is present in very low concentrations. 
This, an example of the preferred oxometallo chelate compound would be 
MoO(C.sub.32 N.sub.8 H.sub.15 (SO.sub.3 H))OCH.sub.3, 
oxomethoxo(phthalocyanato-4-sulfonic acid)molybdenum. This is an example 
using a substituted phthalocyanine chelate, and has the following 
structure: 
##STR2## 
It should be remembered that the oxo and methoxo ligands are perpendicular 
to the plane of the molybdenum and the phthalocyanine atoms. 
The catalyst can be generated in situ from related compounds. Thus MO.sub.2 
(Pn) can be reduced to MO(Pn)X in the presence of mercaptans and entrained 
HX impurities. MO(Pn) can be oxidized to MO(Pn)X in the presence of air 
and entrained HX impurities. MO(Pn)-O-(Pn)MO can be cleaved to MO(Pn)OH 
and MO(Pn)X by entrained HX. Other similar compounds also will be 
converted to MO(Pn)X under the conditions prevalent in the sweetening 
process, namely the presence of air, mercaptans, and other impurities. The 
resting state of the compound in the presence of air is MO(Pn)X. 
The oxometallo chelate compound is best used as a catalyst for petroleum 
sweetening by first supporting it on an inert high surface area solid. 
This practice is well known in the prior art and is described in U.S. Pat. 
No. 2,988,500 and in U.S. Pat. No. 4,087,378. Suitable solid supports 
should be inert to and insoluble in the petroleum products being 
sweetened. A preferred type of support is activated charcoal, derived from 
the destructive distillation of wood, peat, lignite, or nut shells, etc., 
and treated by heat or chemicals so that it is highly porous with 
increased adsorbent capacity. Other preferred solid supports are the 
refractory inorganic oxides, which may be syntheticly prepared or obtained 
naturally. Examples of such supports are alumina, silica, boria, zirconia, 
zeolites, clays, pumice, kieselguhr, etc., and mixed supports such as 
alumina/silica and the like. The oxometallo chelate compound is 
impregnated upon the support by dissolving or dispersing it in a suitable 
solvent such as methanol or ethanol or other alcohol, and then causing the 
support to be immersed in the solution or dispersion until the chelate 
compound is absorbed. The solvent alcohol is then removed by evaporation, 
or decantation or other suitable means. 
The catalyst may be employed in the sweetening of petroleum the same manner 
as current oxidation catalysts discussed in the above description of the 
prior art, except that no additional basic agents are required. 
Such sweetening methods are well explained in U.S. Pat. No. 2,988,500. 
Other aspects of such prior sweetening methods such as the admixing of air 
or oxygen or oxygen--inert gas mixtures, or the use of mild temperatures 
from 20.degree. C. to 50.degree. C. or higher, or the use of mild 
pressures such as 1 atmosphere or higher, and related aspects will be 
unchanged in the practice of the present invention. The catalyst of the 
present invention works best when the sour petroleum feed stream is 
neither unduly acidic nor unduly basic, although the catalyst will 
function within a fairly broad range of acidity. The catalyst is effective 
in the presence of varying quantities of water, including the presence of 
a separate aqueous phase, provided it is not unduly acidic or basic.

EXAMPLE I 
MoO(TPP)OEt, oxoethoxotetraphenylporphinato molybdenum, can be prepared in 
good yield from MoOCl.sub.3 and H.sub.2 TPP as follows: MoOCl.sub.3 (5 
parts) and H.sub.2 TPP (5 parts) are placed in a dry flask fitted with a 
reflux condenser and adapters so that the reaction can be performed under 
insert atmosphere. Carefully dried mesitylene (300 parts) and 
2,6-dimethylpyridine (2 parts) are added and the reaction is refluxed for 
12 hours. The crude product is recovered in the presence of air. The 
mesitylene is recovered first by distillation under reduced pressure. The 
crude product residue is dissolved in dichloromethane and washed 
alternately with dilute aqueous KOH (ca 0.5N) and dilute aqueous HCl (ca 
1N). Ethanol (10 parts) is added to the solution, and the solvent is 
stripped by evaporation. The dark blue solid product is sufficiently pure 
for catalytic purposes, but can be purified further by chromatography on 
neutral alumina with dichloromethane as solvent. Yields are excellent, 
usually 70% to 80% based on unreacted H.sub.2 TPP. 
EXAMPLE II 
MoO(Pc)OEt, oxoethoxophthalocyanato molybdenum, can be prepared in good 
yield from MoOCl.sub.3 and o-C.sub.6 H.sub.4 (CN).sub.2 as follows: 
MoOCl.sub.3 (5 parts) and orthodicyanobenzene (10 parts) are placed in a 
dry flask fitted with a reflux condenser and adapters for an inert 
atmosphere. Distilled dimethylformamide (DMF) (400 parts) is added and the 
reaction is refluxed under inert gas for 12-16 hours. The crude product is 
recovered by vacuum distillation of the DMF, followed by washing the 
blue/black residue with water, ethanol, and dichloromethane. The product 
is sufficiently pure for catalytic purposes, but can be purified by 
chromatography on silica with dimethylsulfoxide/ethanol in ca 9:1 ratio. 
Yields are good, usually 50% based on dicyanobenzene. 
Particularly preferred chelates are derivatives of phthalocyanine, 
especially the mono and di-sulfonated derivatives. Such derivatives can be 
obtained from an unsubstituted phthalocyanine chelate by standard 
sulfonation techniques, such as treating the compound with fuming sulfuric 
acid. Extended treating with oleum will afford tri- and tetrasulfonated 
phthalocyanine derivatives. Another preferred phthalocyanine derivative is 
the carboxylated derivative, which can be obtained by treating the 
phthalocyanine with phosgene and aluminum chloride, followed by hydrolysis 
of the acid chloride initially obtained. 
EXAMPLE III 
MoO(Pc(SO.sub.3 Na).sub.4)OMe, 
oxomethoxo(tetrasulfophthalocyanato)molybdenum, tetrasodium salt, can be 
obtained from MoO(OH).sub.3, urea, and Na.sub.3 C.sub.6 H.sub.3 
(CO.sub.2).sub.2 SO.sub.3.2H.sub.2 O as follows: MoO(OH).sub.3 (7 parts) 
urea (30 parts) and 4-sulfophthalic acid, trisodium salt, (20 parts) are 
finely ground into powders and mixed thoroughly with each other. The 
mixture is placed in a large flask and is heated for 3 hours at sufficient 
temperature to melt the reactants, ca 160.degree. C. The crude product is 
recovered by dissolving the black residue in water and filtering it. 
Methanol is added to the filtrate, and the solvent is stripped by 
evaporation. 
EXAMPLE IV 
A catalyst is supported an a solid support and tested for petroleum 
sweetening as follows: MoO(Pc(SO.sub.3 H))OEt, 
oxoethoxo-(phthalocyanto-4-sulfonic acid) molybdenum (150 mg) is dissolved 
in 150 ml ethanol. The solution is passed through a column containing 
about 100 cc of activated charcoal particles. The granular charcoal has a 
particle size of 30-40 mesh, but smaller particles up to about 200 mesh 
may be used. The solution is collected off the bottom of the column and is 
reintroduced at the top of the column until the color of the solution 
dissipates, indicating that the compound has been adsorbed. The solvent is 
drained and the column is dried in an oven at 100.degree. C. for an hour. 
A synthetic solution of sour kerosene is prepared by adding butyl mercaptan 
(0.5 ml) to a commercially available sweet kerosene (500 ml) to give about 
350 ppm mercaptan sulfur. The kerosene is passed through the column with 
entrained air (saturated), at a LHSV of about 3-4. Samples taken at the 
bottom of the column throughout the run are doctor negative. 
While a number of particular forms of the invention have been disclosed, it 
will be apprent that various modifications and improvements thereto can be 
made without departing from the spirit and scope of the invention. 
Accordingly, it is not intended that the invention be limited by the above 
description, but that the invention comprehend all such modifications and 
improvements which are apparent to one skilled in the art from the above 
description.