Catalytic hydrogenation of sulfolenes

A sulfolene compound, especially 3-sulfolene, containing dissolved sulfur dioxide as impurity is contacted with an oxidizing agent selected from the group of (a) peroxomonosulfuric acid, (b) peroxodisulfuric acid, (c) peroxomonosulfates and an acid having a pKa less than 4, and (d) peroxodisulfates and an acid having a pKa of less than 4, so as to remove at least a portion of dissolved SO.sub.2. Preferred agents are KHSO.sub.5 and H.sub.2 SO.sub.4. The thus purified sulfolene compound, particularly 3-sulfolene, can be hydrogenated to a sulfolane compound over a suitable catalyst, e.g., Raney nickel.

BACKGROUND OF THE INVENTION 
This invention relates to a process for catalytically hydrogenating 
sulfolenes to sulfolanes. In another aspect, this invention relates to a 
process for hydrogenating sulfolenes to sulfolanes over a nickel catalyst. 
In a further aspect, this invention relates to pretreating sulfolenes so 
as to remove impurities, particularly SO.sub.2, which interfere with the 
subsequent catalytic hydrogenation of sulfolenes. 
The catalytic hydrogenation of sulfolenes to sulfolanes is well known. 
Generally a supported nickel catalyst is employed. Also the removal of 
sulfur dioxide and other impurities from sulfolenes by means of oxidizing 
agents has been disclosed, e.g., in U.S. Pat. Nos. 3,622,958; 3,514,469; 
3,417,103 and 3,152,144. However, there is an ever present need to find 
still more effective methods of removing impurities from sulfolenes so as 
to speed up the subsequent hydrogenation of sulfolenes to sulfolanes and 
to reduce the amount of catalyst used. 
SUMMARY OF THE INVENTION 
It is an object of this invention to remove dissolved SO.sub.2 and other 
impurities from a sulfolene. It is another object of this invention to 
remove SO.sub.2 and other hydrogenation-retarding impurities from a 
sulfolene by treatment with an oxidizing agent. It is a further object to 
remove said impurities by said treatment prior to the hydrogenation of a 
sulfolene to a sulfolane over a nickel catalyst. Further objects and 
advantages will become apparent from the following disclosure and appended 
claims. 
In accordance with this invention, a sulfolene compound containing 
dissolved SO.sub.2 is contacted with at least one oxidizing agent selected 
from the group consisting of peroxomonosulfuric acid, peroxodisulfuric 
acid, peroxomonosulfates plus at least one acid having a pKa of less than 
4, and peroxodisulfates plus an acid having a pKa of less than 4, under 
such conditions as to produce a sulfolene compound having a reduced level 
of dissolved SO.sub.2. In one embodiment, an acid having a pKa of less 
than 4 and an alkali metal peroxomonosulfate is added to 3-sulfolene 
(C.sub.4 H.sub.6 SO.sub.2) prior to its being catalytically hydrogenated 
to sulfolane (C.sub.4 H.sub.8 SO.sub.2). 
DETAILED DESCRIPTION OF THE INVENTION 
The term "a sulfolene" (somtimes also referred to as "sulfolenes" and 
"sulfolene compounds") as employed herein is defined in U.S. Pat. No. 
3,622,598, herein incorporated by reference. This term includes 
substituted and unsubstituted 3-sulfolenes and 2-sulfolenes. The preferred 
sulfolene compound employed in this invention is unsubstituted 
3-sulfolene, which is commercially available and is produced by reaction 
of 1,3-butadiene and sulfur dioxide. The terms "sulfolane" or "sulfolane 
compounds" are also defined in U.S. Pat. No. 3,622,598. 
The term pKa is defined as the negative logarithm of the ionization 
constrant Ka of an acid (pKa=-.sub.10 log Ka). The determination of the 
ionization constant Ka and its definition is explained in "Physical 
Chemistry", F. Daniels and R. Alberty, Second Edition, 1961, John Wiley 
and Sons, Inc., pages 364, 365, 428-430, herein incorporated by reference. 
Thus an acid having a pKa of less than 4 has an ionization constant 
(H.sup.+)(A.sup.-)/(HA) of greater than 10.sup.-4. If a polyprotic acid is 
employed, Ka refers to the dissociation of the first hydrogen. It is 
understood that the term acid includes acid salts such as KHSO.sub.4, 
wherein the HSO.sub.4.sup.- ion can further ionize to H.sup.+ and 
SO.sub.4.sup.2-. 
In the process of this invention, at least one sulfolene compound, 
preferably 3-sulfolene (C.sub.4 H.sub.6 SO.sub.2), is treated with at 
least one oxidizing agent selected from the group consisting of (a) 
peroxomonosulfuric acid (H.sub.2 SO.sub.5), (b) peroxodisulfuric acid 
(H.sub.2 S.sub.2 O.sub.8), (c) peroxomonosulfates (i.e., compounds 
containing the HSO.sub.5.sup.- or SO.sub.5.sup.2- ion) and at least one 
acid having a pKa value of less than 4, and (d) peroxodisulfates (e.g., 
compounds containing the HS.sub.2 O.sub.8.sup.- or S.sub.2 O.sub.8.sup.2- 
ion) and at least one acid having a pKa value of less than 4, so as to 
reduce the level of sulfur dioxide dissolved in said sulfolene compound. 
It is understood that the acid in (c) and (d) can be (a) or (b). The 
treatment of a sulfolene compound with said oxidizing agent results in the 
oxidation of dissolved SO.sub.2 to compounds having a higher oxidiation 
state of sulfur. It is believed that the reduced concentration of SO.sub.2 
in the sulfolene results in an enhanced rate of catalytic hydrogenation to 
sulfolane. 
Non-limiting examples of peroxomonosulfates are LiHSO.sub.5, Li.sub.2 
SO.sub.5, NaHSO.sub.5, Na.sub.2 SO.sub.5, KHSO.sub.5, K.sub.2 SO.sub.5, 
NH.sub.4 HSO.sub.5, (NH.sub.4).sub.2 SO.sub.5, Mg(HSO.sub.5).sub.2, 
MgSO.sub.5, Ca(HSO.sub.5).sub.2, CaSO.sub.5, Ba(HSO.sub.5).sub.2, 
BaSO.sub.5, Zn(HSO.sub.5).sub.2, ZnSO.sub.5. Presently preferred are 
ammonium and alkali metal peroxomonosulfates, most preferably KHSO.sub.5. 
KHSO.sub.5 is marketed under the trademark Oxone.RTM. by E. I. DuPont de 
Nemours and Company, Willmington, Del. Oxone.RTM. is a complex salt of the 
approximate formula 2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4. 
Non-limiting examples of peroxodisulfates are LiHS.sub.2 O.sub.8, Li.sub.2 
S.sub.2 O.sub.8, NaHS.sub.2 O.sub.8, Na.sub.2 S.sub.2 O.sub.8, KHS.sub.2 
O.sub.8, K.sub.2 S.sub.2 O.sub.8, NH.sub.4 HS.sub.2 O.sub.8, 
(NH.sub.4).sub.2 S.sub.2 O.sub.8, NH.sub.4 KS.sub.2 O.sub.8, MgHS.sub.2 
O.sub.8, MgS.sub.2 O.sub.8, CaHS.sub.2 O.sub.8, Ca.sub.2 S.sub.2 O.sub.8, 
Zn.sub.2 S.sub.2 O.sub.8. (NH.sub.4).sub.2 S.sub.2 O.sub.8 and K.sub.2 
S.sub.2 O.sub.8 are the most readily available peroxodisulfates. Presently 
the peroxidisulfoates are not the preferred oxidants in the process of 
this invention. 
Non-limiting examples of acids having a pKa value of less than 4 are 
H.sub.2 SO.sub.4, KHSO.sub.4, NaHSO.sub.4, H.sub.2 SO.sub.5, H.sub.2 
S.sub.2 O.sub.8, HCl, HClO.sub.3, HClO.sub.4, HNO.sub.3, H.sub.3 PO.sub.4, 
oxalic acid, trichloroacetic acid. Presently preferred is H.sub.2 
SO.sub.4, which is generally added in form of a dilute aqueous solution to 
the sulfolene compound. 
When a peroxomonosulfate or peroxodisulfate and an acid are added to a 
sulfolene, said peroxosulfate compound and acid can be charged to the 
sulfolene compound in any order, either sequentially or essentially 
simultaneously. It is presently preferred to add the acid first and then 
the peroxosulfate compound, preferably an alkali metal peroxomonosulfate, 
most preferably KHSO.sub.5. The mixing of the sulfolene, the acid, and the 
peroxodi- or peroxomonosulfate can be carried out in any vessel. Some 
agitation is generally preferred so as to afford intimate contact of these 
three ingredients and to facilitate the removal of gaseous SO.sub.2 from 
the liquid mixture. This process can be carried out batchwise or 
continuously. Also a suitable solvent such a water can be present. 
In a preferred embodiment, the acid is added first to a sulfolene, 
preferably in such an amount as to attain a pH of the sulfolene ranging 
from about 1 to 4. Then a gas such as oxygen, nitrogen, air, helium, argon 
and the like is introduced near the bottom of the vessel containing the 
sulfolene compound and bubbled through the sulfolene compound, so as to 
sweep out a portion of dissolved sulfur dioxide. Thereafter, the 
peroxosulfate compound is added. The flow rate of the introduced gas 
greatly depends on the amount of sulfolene to be purified, the 
concentration of SO.sub.2 in sulfolene, the desired rate of SO.sub.2 
removal, and the configuration of reactor and gas inlet means. Generally 
the above described purging with a gas is carried out for a time period 
ranging from about 1 minute to about 1 hour. 
The temperature during the intimate contact of the sulfolene compound and 
the peroxosulfate compound, preferably a peroxomonosulfate plus an acid 
having a pKa of less than 4, is not believed to be critical. Generally, 
the temperature during said contacting ranges from about 10.degree. C. to 
about 80.degree. C., preferably from about 20.degree. C. to about 
50.degree. C. The pressure during the contacting of the sulfolene compound 
and peroxosulfur compound can be atmospheric, subatmospheric (i.e., under 
vacuum conditions) and superatmospheric (i.e., above 1 atm). Generally the 
pressure is atmospheric (i.e., approximately 1 atm or 15 psia). The ratio 
of the amount of peroxosulfate compound (e.g., KHSO.sub.5) to the amount 
of sulfolene in the process of this invention generally ranges from about 
0.1:1 to about 20:1, preferably about 0.5:1 to about 5:1, millimoles of 
HSO.sub.5.sup.- or SO.sub.5.sup.-2 (e.g., KHSO.sub.5) per 100 g 
sulfolene. The pH of the sulfolene compound generally ranges from about 1 
to about 4 (after addition of the acid). 
The sulfolene compound, preferably 3-sulfolene, which has been purified by 
the above described treatment with a peroxosulfate compound, preferably an 
alkali metal peroxomonosulfate and an acid having a pKa of less than 4, 
can thereafter be catalytically hydrogenated to a sulfolane compound. 
Processes for hydrogenating sulfolenes, e.g., in the presence of a nickel 
containing hydrogenation catalyst such as Raney nickel, and for recovering 
sulfolanes from the reaction mixture have been described in U.S. Pat. Nos. 
3,152,144, 3,417,103, 3,514,469; 3,622,598 and 4,188,327, herein 
incorporated by reference. Presently preferred hydrogenation conditions 
include a Raney catalyst, an initial hydrogen pressure of about 500-2000 
psig, a reaction temperature of about 100.degree.-150.degree. C., and a 
reaction time ranging from about 10 minutes to about 2 hours. The 
previously added oxidizing agent of this invention, ingredients, e.g, a 
peroxosulfate compound and an acid is generally not separated from the 
sulfolene compound prior to the hydrogenation and is thus present during 
the catalytic hydrogenation reaction. 
The following examples are presented to further illustrate this invention 
without unduly limiting the scope of this invention.

EXAMPLE I 
This example illustrates the use of hydrogen peroxide for the purification 
and subsequent catalytic hydrogenation of 3-sulfolene, substantially in 
accordance with U.S. Pat. No. 3,152,144. The hydrogenation catalyst was a 
commercial Raney nickel catalyst, obtained from Strem Chemicals, Inc., 
Newburyport, MA and was identified as a fine Raney nickel, catalog number 
28-1890. The catalyst material used in the experiments of Examples I and 
II was a settled mixture of this Raney nickel catalyst and about 20-30 
weight-% water. 3-Sulfolene was manufactured by Phillips Chemical Company, 
Bartlesville, Okla.; and hydrogen peroxide was employed as a 10 weight-% 
aqueous solution. A 300 mL autoclave equipped with stirrer, pressure gauge 
and thermocouple was used for the catalytic hydrogenation tests. The 
decrease in pressure during the reaction was considered a measure of the 
rate of hydrogen consumption and thus a measure of the rate of the 
hydrogenation. 
The reaction temperature of the hydrogenation in all runs ranged from about 
110.degree. to 120.degree. F. In several runs, dilute sulfuric acid was 
added to sulfolene before the hydrogenation so as to lower the pH of 
sulfolene. In addition, in the same runs the acidified sulfolene was 
purged by passing flowing air through sulfolene at room temperature for 
about 30 minutes so as to sweep out dissolved SO.sub.2, which retards the 
subsequent catalytic hydrogenation. Pertinent test conditions and results 
of these control runs are summarized in Table I. 
TABLE I 
__________________________________________________________________________ 
Wt. of Wt. of Millimoles Reaction 
Psig Drop 
Sulfolane 
Catalyst.sup.1 
pH of 
Air- 
of Added 
Reaction 
Pressure (Psig) 
Time per Minute per 
Run 
(g) (g) Sulfolene 
Purged 
H.sub.2 O.sub.2 
Temp (.degree.F.) 
Initial 
Final 
(min.) 
100 g Sulfolene 
__________________________________________________________________________ 
1 115 1.45 -- No 2.9 107-119 
1000 
225 105 6.4 
2 122 1.45 -- No 1.5 108-116 
1000 
215 80 8.0 
3 126 1.45 1.6 Yes 0.9 110-121 
1000 
240 85 7.1 
4 121 1.45 1.6 Yes 0.6 112-118 
1000 
190 90 7.4 
5 120 1.45 1.6 Yes 0.3 111-117 
1000 
240 95 7.5 
__________________________________________________________________________ 
.sup.1 A settled mixture of Raney nickel and about 20-30 weight % H.sub.2 
O. 
Data in Table I show that the average rate of the hydrogen pressure drop (a 
measure of the rate of sulfolene hydrogenation) in control runs 1-5 was 
about 7.3 psig per minute per 100 g sulfolene, at a temperature of about 
110.degree.-120.degree. F., in the presence of 1.45 grams of wet Raney 
nickel. Air purging did not have a significant effect on the hydrogenation 
rate. 
EXAMPLE II 
This example illustrates the use of an acidified monoperoxosulfate solution 
for the purification and subsequent catalytic hydrogenation of sulfolane. 
The experimental procedure was essentially the same as the one described 
in Example I, with the exception that Oxone.RTM. (marketed by Du Pont de 
Nemours & Company; Wilmington, Del.), a monoperoxosulfate of the formula 
2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4 (molecular weight: 614), was used 
in lieu of H.sub.2 O.sub.2. Pertinent test conditions and results of 
invention runs are summarized in Table II. 
TABLE II 
__________________________________________________________________________ 
Wt. of Wt. of Millimoles Reaction 
Psig Drop 
Sulfolane 
Catalyst.sup.1 
pH of 
Air- 
of Added 
Reaction 
Pressure (Psig) 
Time per Minute per 
Run 
(g) (g) Sulfolene 
Purged 
HSO.sub.5.sup.- 
Temp (.degree.F.) 
Initial 
Final 
(min.) 
100 g Sulfolene 
__________________________________________________________________________ 
6 117 1.42 -- No 1.6 110-116 
1000 
100 49 15.7 
7 80 1.42 -- No 1.3 114 1000 
148 80 13.3 
8 110 1.45 2.0 No 1.6 110-118 
1000 
165 60 12.7 
9 70 1.45 3.0 No 1.0 111-113 
1000 
830.sup.1 
60 5.7.sup.2 
10 127 1.45 2.5 Yes 2.0 115-119 
1000 
125 25 27.8 
11 127 1.45 2.5 Yes 1.6 113-119 
1000 
125 25 27.6 
12 126 1.45 2.0 Yes 1.3 114-124 
1000 
185 44 14.7 
13 124 1.45 3.0 Yes 1.0 113-116 
1000 
150 57 12.0 
__________________________________________________________________________ 
.sup.1 A settled mixture of Raney nickel and about 20-30 weight % H.sub.2 
O. 
.sup.2 Believed to be erroneous. 
A comparison of the data in Tables II and I shows that the hydrogen 
pressure drop per minute per 100 grams of sulfolene was significantly 
higher when Oxone.RTM. peroxymonosulfate was employed in lieu of H.sub.2 
O.sub.2. The average pressure drop rate per 100 gram of sulfolene of 
invention runs 6-13 employing 1.0-2.0 millimoles of SO.sub.5.sup.- (Table 
II) was 16.2, compared with 7.3 of control runs 1-5 employing 0.3-2.9 
millimoles of H.sub.2 O.sub.2 (Table I). One mole of both H.sub.2 O.sub.2 
and HSO.sub.5.sup.- can donate one gram-atom (i.e., 2 gram equivalent) of 
oxygen in oxidation reactions. 
Therefore, unexpectedly the rate of hydrogen consumption, and thus the rate 
of hydrogenation of sulfolene, in the presence of acidified 
peroxomonosulfate was more than twice the rate of hydrogenation in the 
presence of of an equivalent amount of hydrogen peroxide. In addition, air 
purging generally had a beneficial effect on the rate of reaction when 
acidified peroxomonosulfate was employed, whereas air purging did not 
exhibit such a beneficial effect when H.sub.2 O.sub.2 was used (see Table 
I). 
Reasonable variations and modifications can be made in this invention 
without departing from the limit and scope thereof.