BASE-CATALYZED OXIDATION OF MERCAPTANS IN T H E PRESENCE OF INORGANIC TRANSITION METAL COMPLEXES T H O M A S J . W A L L A C E ,
A L A N S C H R I E S H E I M ,
H O W A R D
B. GLASER
H U R W I T Z , A N D M A R V I N
Process Resrarch Uirision, Esso Rvsrarrh nnd Engineeiiny Co., I m d p n . .IJ.
The base-catalyzed oxidation of n-butyl mercaptan by molecular oxygen in the presence of various inorganic transition metal complexes has been studied in aqueous sodium hydroxide at 2 3 . 5 " C. The results of this study indicate that the inorganic complex, cobalt pyrophosphate, has a marked catalytic effect on the oxidation of n-butyl mercaptan to n-butyl disulfide. Using an RSH-catalyst weight ratio of 180, essentially quantitative conversion to the disulfide was obtained in 120 minutes. In the absence of a d d e d catalyst, only 1870 conversion was observed in the same time period. The ability of the pyrophosphate ligand to form a stable six-membered ring with the cobalt(l1) cation i s probably responsible for the observed caialytic effect. Simulated process studies have indicated that a cobalt pyrophosphate system is feasible for the sweetening of actual feed stocks such as naphthas and kerosines. E I o~x I D r \ ' r I o N of mercaptans to disulfides (sweetening) is a Treaction that is of basic importance throughout the petroleum industry. T h e methods available for effecting this transformation may be loosel>- categorized as either chemical or catalytic in nature. Chemical oxidation techniques such as ..doctor" (sodium plumbite) ( I O . 7 / ) . lead sulfide (8).sulfur (5),and hypochlorite :2) sweetening are old. \vel1 established methods. Disulfide formation is accomplished by a reaction of the mercaptan with the reagent in question. T h e catalytic oxidation of mercaptans is a s\veetening technique that has reached practical fruirion in the last decade. This method of oxidation is carried out in aqueous sodium hydroxide using a n oxidation-reduction catalyst and air or oxygen as the oxidizing agent. If the catalyst employed is a n -V3-V'-dia1kylaromatic amine. the process is referred to as either "antioxidant" or "inhibitor" sxveetening (7. 73, 74). This method is usually restricted to olefinic feed stocks and gasolines \vhich contain relatively lovb. molecular Lveight mercaptans. but generall>- is not effective with heavier feed stocks such as kerosines and heating oils v;hich require a more efficient oxidationreduction catalyst. More recent studie:s on oxidation-reduction catal>-sts for sweetening have been concerned Lvith organic chelates such as bis-salicylaldehyde ethylenediimine cobalt(I1) and its derivatives? and the cobalt-histidine chelates (3. 9), These systems are well knoivn for their ability to adsorb oxygen ( 7 ) . and the transition metal cation is capable of being oxidized and reduced more efficiently than an amino anion. Gleim and Urban have recently demonstrated that transition metal phthalocyanines are more effective catalysts than the above chelates ( I ) . I n the inorganic counterpart of this area. the effect of inorganic sequestering agents on the catalytic activit!of transition metal cations in highly basic media \vas investigated (70). 'l'his paper summarizes the authors' exploratory studies aimed at defining the feasibility of s\veetening 1.rrroleurn fractions \vi1 h a n inorganic transition metal complex.
Theoretical Considerations
'The base-catalyzed oxidation of a mercaptan in the presence of a n oxidation-reduction catalyst occurs by an anion-radical mechanism. After initial ionization of the mercaptan has
occurred. ox!-gen can react Ivith thr cation by a onr-electron transfer reaction (Equation 1) to produce peroxide ion and the next highest oxidation state of the cation. Regeneration of the catalyst occurs by a one electron transfer reaction bettveen the mercaptide ion and the oxidized cation (Equation 2 ) . T h e resulting thiyl radicals dimerize, and the peroxid? ion is destroyed by reaction Lvith water (Equations 3 and 4).
+ o2 2RS.- + 2Mi3 2M+2
-+
+
2RS. 02-'
+ H?O
2x17-3
-+
-+
2
+ + 2RS.
(1)
0 - 2
(2)
RZS2
20€1-
(3)
+
(4)
202
Such a sequence Lvould require a n over-all stoichiometry of 0.2.5 mole of oxygen per mole of mercaptan converted to disulfide (Equation 5). T h e oxidation-rrduction steps are reasonable
2RSH
+ 0.502
-+
RSSR
+
(5)
H20
since the transition metal complex is present in catalytic amounts. the rate of oxygen consumption varies \vith the RSH catalyst ratio employed: and the same stoichiometry is observed in the absence of a catalyst ( 1 . 1 7 ) . It is reasonable to assume that the rate of these h z - 0 reactions will depend on the nature of the inorganic ligand and the specific cation employed. Experimental
Reagents. n-Rut>-l mercaptan (Matheson Coleman & Bell. b.p. 98' C,. nr,2" 1.441I ) was purified by distillation under nitrogen through a 14-inch silvered column rquipped Lvith a tantalum-\vir? spiral. T h e mercaptan u'as stored under nitrogen in a coldbox before use. Cobalt(1Ii chloi.idc. cobalt(I1) phosphate. copperiI) chloride. iron(I1) chloride: cobalt(I1) tungstatr. cobalt(I1) mol>-hdatr.nickrl(I1'i chloride. phosphomolybdic acid. and phosphotungstic acid were reagent grade materials (Baker .Analyzed Reagents). Pyrophosphoric acid \vas obtained from the C:ity Chcniical Co. ab thr reagrnt grade material. Cobalt Phthalocyanine. The chelate \vas prcpared h!- the reaction of anh) drouc cobalt( 11) chloridr \vith phthalic anhydride and urea in refluxing nitrobenzene according t o the mcthod of Linstrad and Robertson ( t i ) in 6.jf,4 yiyld. VOL. 3
NO. 3
JULY
1964
237
Preparation of Catalysts. Catalysts \cere prepared by heating equimolar mixtures of CoCl?. CuCI. YiClZ. and FeC12 \cith pyrophosphoric acid, phosphomolybdic acid and phosphotungstic acid a t 250' to 1250' F. under iiitrogen for 5 hours. Elemental analysis for chlorine \cas less than O . l % / , for all catalysts indicating reaction \vas complete. The catalysts \cere highly insoluble in water and aqueous sodium hydroxide. Similar properties \$-ere observed when metallic cations were treated Lcith sodium pyrophosphate in the pH region of 7 to 10 ( 7 2 ) . I n more acidic media, no precipitate forms ( 2 0 ) . Mercaptan Oxidation Technique for Exploratory Studies. n-Butyl mercaptan (0.2 mole, 18.036 grains) \cas added (under nitrogen) to 100 nil. of a 21t4 aqueous sodium hydroxide solution in a 500-ml. Erlenmeyx flask equipped \cith a side-arm. T h e catalyst (100 mg.) was added, the flask sealed. and then attached to the oxidation apparatus. Ox)-gen gas \$-assupplied to the reaction flask from a partially filled collapsible polyethylene gas balloon (ca. 14-liter capacitb-). T h e oxygen flowed through a \vet-test meter into a large drying tube packed \vith indicating Drierite and finally through an overhead Fredericks condenser and into the reaction flask which was thermostated a t 23.5' I 0.2' C. The reaction flask was first purged with oxygen through the reaction flask side-arm, the side-arm was sealed. a n equilibrium pressure\cas established. and the oxidation reaction was initiated by magnetic stirring a t 1300 r.p.m. T h e rate of oxygen consumption as a function of time was obtained from the wet-test meter. This apparatus and its applications have been described in detail previously ( 1 8 ) . I n several experiments, n-butyl disulfide \cas isolated by ether extraction and purified by distillation. Its infrared spectrum and physical properties were identical to n-butyI disulfide prepared from the reaction of n-butyl mercaptan with iodine in 207, aqueous sodium hydroxide (76). Mercaptan Oxidation Technique for Feasibility Study with Commercial Feed Stocks. The reactor used in this portion of the study consisted of a glass Buchner-type funnel with a fritted glass disk fused a t the bottom (Figure 1). Feed, caustic, and catalyst were placed in the vessel a t room temperature and air \cas introduced by forcing it through the fritted glass disk. T h e contents of the reactor were well mixed by means of a centrifugal stirrer Lvhich by imparting a lifting action was able to lift the heavy caustic and catalyst from the bottom of the reaction and disperse them to the higher oil phase. \.ortexing \cas eliminated by the use of baffles on the wall of the reactor. Samples of product were removed from the reactor at specific times during each run. This was accomplished by discontinuing the stirring and allowing the contents of the reactor to settle long enough to remove a sample of clean oil from the top of the reactor. Generally. this operation took about 1 to 2 minutes. T h e samples of oil removed from the reactor \cere immediately analyzed for mercaptan sulfur. T h e analyses were carried out using a Cenco coulometric automatic titrator (Cenco Catalog No. 20928). This instrument provides a rapid, accurate method of determining the mercaptan concentration of a sample. Results and Discussion
Catalytic Oxidation Studies with n-Butyl Mercaptan. n-Butyl mercaptan (0.2 mole) \cas oxidized at 23.5' 3~ 0.2" C . under a constant ou!-gen pressure of 1 atm. in the presence of several inorganic transition metal complexes. T h e disappearance o f mercaptan \cas folloxced by the moles of oxygen consumed as a function of time. This was possible since i t has previously been determined that the stoichiometry for disulfide formation in basic media requires 0.25 mole of 0 2 per mole of RSH ( 7 . 7 7 ) . Under these conditions, the catalytic oxidation of n-butyl mercaptan \vas studied in the presence of cobalt(I1) p>-rophosphate. cobalt(I1) phosphomolybdate. and cobalt(I1) phosphotungstate using an RSH--catalyst gram ratio of 180 (Figure 2 ) . Cobalt pyrophosphate \cas the most active catal>-stgiving a 98Yc yield of n-butyl disulfide in 120 minutes. I n the same time period. the phosphomolybdate and phosphotungstate catalysts afforded 57 and 267' yields. respectively. 238
I&EC
PROCESS DESIGN A N D DEVELOPMENT
CENTRIFUGAL STIRRER
FEED
-
CAUSTIC-
FRITTED GLASS
7
DISPERSE AIR
AIR
Figure 1. Apparatus used in the oxidation of actual feed stocks
of n-butyl disulfide. I n the absence of any added catalyst, an 187, yield of disulfide \cas obtained in 120 minutes and 98YG mercaptan conversion required about 21 hours. I n the absencr of base, no oxidation occurred over a 24-hour period. Thus. the mercaptide ion (RS-)is the active species ( 7 7 ) . '1comparison of cobalt pyrophosphate with cobalt phthalocyanine and cobalt phosphate \cas also made. Cobalt phthalocyanine is of interest since it is an organic chelate. Cobalt phosphate is of interest since it is an inorganic species in \chich the cobalt(I1) ion is not complexed. h s shown in Figure 3. cobalt pyrophosphate and cobalt phthalocyanine had essentially the same catalytic properties for the oxidation of n-butyl mercaptan. I n the case of cobalt phosphate. the rate of oxidation of n-butyl mercaptan \cas slightly greater than the uncatal!-zed reaction. Similar results \cere obtained with cobalt molybdate and cobalt tungstate. T h e importance of the inorganic ligand is demonstrated by the above results. T h e p)-rophosphate ligand is capable of forming a six-membered ring Lvith the cobalt cation (75). This
enhances the ability of the cation to donate a n electron to oxygen and stabilizes each oxidarion state of the cation. The same situation exists in the organic chelate. cobalt phthalocyanine. thus explaining the similar activity of the two catalyst systems. I n cobalt phosphate. chelation of the above type is not possible and this accounts for the relatively poor rate of oxidation observed \then this catalyst \\.as used. The ease of cation chelation \could be expected to contribute to the stability of the resulting inorganic complex \vhich in turn can influence the ease of cation oxidation-reduction. In the case of cobalt phosphomolybdate and cobalt phosphotungstate, the bulkiness of the ligand decreases in the order phosphotungstate > phosphomol>.bdate > pyrophosphate. I t is reasonable to assume that the steric factors for cation chelation would follow the same order. T h e resulting rates of oxidation in the presence of these systems \could then be pyophosphate > phosphomolybdate > phosphotungstate as \cas observed. This steric factor may be partially responsible for the de-
I i
100-
100
A
1
COPALT
>
5
-1
PYROPHOSPHATE
60-
60-
0
I
401 40
13
W A
+
20
40
60 TII;lE,
?I IN
Figure 2. Oxidation of n-butyl mercaptan catalyzed b y cobalt pyrophosphate, phosphomolybdate, and phosphotungstate Cobalt Cobalt Cobalt Sodium
40.20
0
160
TI I 1 E
2
80
100
120 1 4 0 1 6 0
hllN
0
Figure 3. Comparison of cobalt pyrophosphate with cobalt phthalocyanine and cobalt phosphate C o b a l t phthalocyanine C o b a l t phosphate
40
60
80
100
170 1 4 0
160
TIME, hllN
Figure 4. Effect of temperature on the activity of cobalt pyrophosphate 0 2 5 O 0 - 5 O O 0 F. calcination
W 750' F. calcination A 1250' F. calcinotion
pyrophosphate phosphomolybdate phosphotungstate hydroxide only
creased activity of cobalt pyrophosphate \cith increasing A s sho\cn in Figure 4: maximum calcination temperature. catal)-st activity is obtained at calcination temperatures of 2.50' to 500" F. \\'tien higher calcination temperatures \cere used--e.g.. 750" and '1250" F.---catal\.st activity \vas markedly decreased. Monomeric phosphoric acid derivatives can be polymerized to high molecular Iceight polyphosphoric acids that have not been \cell characterized (75). Decreased cation chelation \could result not only from increased steric factors but also from a decrease in the number of chelating sites per acid molecule. A series of other transition metal pyrophosphate catalysts \cere investigated under the above conditions. and these results are summarized in Figure 5. Copper(1). nickel(I1). and iron (11) pyrophosphates gave 77.48. and 35% yields. respectively. of n-butyl disulfide in 120 minutes. Thus. the over-all order of pyrophosphate activity is cobalt > copper > nickel > iron. It is impossible to make any definite statements on the variation of the rate of oxidation \cith the transition metal cation. Ho\vever. some speculative explanations on the observed order of pyrophosphate activity can be offered. 'Phe similarity between the cobalt(I1) pyrophosphate and cobalt(I1) phthalocyanine catalysts \cas noted above. The same similarities should be present in the Cu(1). S i ( I 1 ) . and Fc[II) pyrophosphate catalysts. T h k \vould allou oxidation-reduction of the cobalt(I1) and Cii(1) pyrophosphates to proceed readily but the Co(I1) catalyst (:3d9) \could be more easily oxidized than the copper(1) catalyst (3d1° or 4 9 ) \chich is also stable in the Cu(11) state. Oxidation of Si(I1) (3dI0) to S i ( I I 1 ) is possible
20
but probabl!. not as fawrable. 'I-he 12e[lll catalyst (3de) \could undergo oxidation to I:e(lII) (id') but rhr reduction step \could be difficult sincr the half-filled 3d shell \coiild be vcry stable. Clearl!.. magnetic susceptibilit! nicasurrinrnts are required to substantiate further this theor).. T h e dependencc of the rate of oxidation of n-biit\.I mercaptan on the gram ratio of mercaptan to cobalt pyrol)hosphatc has also been determined. As sho\\n in Figurr 6. the rate of oxidation of n-butyl mercaptan decreased as the gram ratio of RSHkatalj-st \\-as varied from 90 to 360. thus substantiating the catal! tic narure of cobalt pyrophosi~hate.
Sweetening Typical Petroleum Stocks with Cobalt Pyrophosphate. Basic stiidies had demonstrated that cobalt pyophosphate \cas highly effective as a catalybt for mercaptan oxidation. It \cas next drsiiable to determine die feasibilit), of s\veetening typical refinery petroleum fraction5 \cith t l i i i catalyst and to define the makeup of the catal!.st system for most effecrive siveetening. Thus. process variables siicli as [he effect of CaLlstic concentration. catalysr particle size. ;ind catalyst versatilit!. tcith various peiroleuni fraction5 1%CI'C invesrigared. T h e eRect of caustic strrngth and concentration 011 the rate of s\\-eetening Icith cobalt pyrophoslihatr \cas srudied \vith pure compounds and petroleum fractions. Caustic is essential for s\ceetening sincr the reactive mercaptide ion is formed in thc caustic phase. n-Butyl mercaptan in toluene ('3.35 mmoles per liter) \vas sweetened in the presence and absence of cobalt pyrophosphate using both 10yc ( 2 . 8 , I f i and 505; (19.2.U) N a O H (Figure '1. \ i h e n cobalt pyrophosphate \\as presrnt.
100 n W
tW
z VI
0
I
m VI
175-
40r
>
,1 2 5 -
20
;1 0 0 . 0
0
'0
150-
0
20
40
60
80
TREATING T'LlE
100
120
140
160
4
1'IY
Figure 5. Variation of catalyst activity with the transition metal cation
7 5 -~ 5 0 -
c w
25-
; 0---.L 0
0 C o b a l t pyrophosphate C o p p e r pyrophosphate W Nickel pyrophosphate A Iron pyrophosphate
+
50
GRAI.1 RATIO OF ii-C4HqSH/COBALT PYROPHOSPHATE
Figure 6. Rate of oxidation as a function of RSH-catalyst ratio VOL. 3
NO. 3
JULY
1964
239
20
10
20
40
I
l
l
100
120
140
160
TREATING TIkIE,
MIN.
60
80
180 TREATING TIME, MIN
Figure 7. Effect of catalyst and caustic concentration of the oxidation of n-butyl mercaptan in toluene
Figure 8. Sweetening of a light virgin naphtha (b.p. 110" to 310" F.) in the presence of cobalt pyrophosphate
10% NaOH, 2.8 moles H 50% NaOH, 19.2 moles 0.3 wt. cobalt pyrophosphate
A 0.3 gram of catalyst per 100 grams of feed
4.5 grams o f catalyst per 100 grams of feed
70
the concentration was 0.3 gram per 100 grams of total hydrocarbon. The volumetric hydrocarbon to caustic ratio was 6 to 1 and excess air \vas continuously supplied to the reactor by the method described in the experimental section. Increasing caustic strength increased the rate of s\ieetening of n-butyl mercaptan with or \vithout the catalyst. For example, when treating \iith cobalt pyrophosphate. increasing the caustic strength from 10 to SO% decreased the time required to remove 907, of the RSH by more than 10-fold. T h e sweetening activity of the catalyst bias demonstrated by comparing runs a t a given caustic concentration. Csing just 507, N a O H . it took 120 minutes to remove all of the mercaptan. Addition of 0.3 \it. Yo cobalt pyrophosphate reduced the sweetening time to 30 minutes. T h e initial disappearance of mercaptan in the noncatalytic system is attributed to the extraction of mercaptans from the oil phase into the caustic phase and some oxidation to the disulfide (27). To determine the feasibility of s\ieetening actual feed stocks. a light virgin naphtha boiling between 110' and 310' F. was treated in the above system using 307, (10M) Na0I-I. The feed contained 6.0 mg. of mercaptan sulfur per 100 ml. of feed. A ratio of 6 parts feed to 1 part N a O H solution \vas used, and the amount of catalyst used in the system was varied. T h e results, sho\vn in Figure 8, indicate that the amount of catalyst used in the system has a marked effect on the rate of oxidation and that \iith sufficient c a t a l y t (4.5 grams per 100 grams of hydrocarbon) all of the mercaptan can be converted in only 30 minutes. T h e use of high caustic strength \cas necessary to s i e e t e n a kerosine boiling between 364' and 548' F. and containing 11 mg. of mercaptan sulfur per 100 ml. of feed. As expected from the results \vith the naphtha. this feed sweetened very slowly \
