BACKGROUND OF THE INVENTION Mercaptan is a class of important chemicals. Lower alkyl mercaptans such as methyl mercaptan can be used as feedstock for synthesizing L-methionine, an important essential amino acid containing sulfur in its molecule, which is generally deficient or in low content in a variety of grain-derived foods; and ethyl mercaptan is widely used as odorant for natural gas. Higher mercaptans having 8 to 24 carbon atoms per molecule can be used in a variety of lubricants or the synthesis of polymers. For example, production of t-dodecyl mercaptan began in 1944 in association with synthetic rubber production for the World War II effort.

It is well known to one skilled in the art that direct addition of hydrogen sulfide to an olefinic compound produces mercaptan compounds.

Attempts to improve the productivity of mercaptan production have been concentrated on the catalysts, reaction temperatures, and reaction pressures.

Conceivably, these parameters (catalysts, reaction temperatures, and reaction pressures) could have been optimized. However, it is believed that the volume of mercaptans required in the future will increase by about 5 percent per year necessitating the development of a process for improving the production of mercaptans. Development of such an improved process would be a significant contribution to the art and the economy.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved process for the production of mercaptans. Another object of the invention is to effect the catalytic addition of hydrogen sulfide to certain types of olefinic compounds to produce mercaptans useful as synthetic rubber modifier. Other objects will become more apparent as the invention is more fully described hereinbelow.

According to the present invention, a process which can be used to produce a mercaptan is provided. The process comprises contacting an olefin or alcohol with hydrogen sulfide in which the process is catalyzed by an acidic catalyst.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows the relationship between the production of t-dodecyl mercaptan (TDM) and the ratio of the length of catalyst bed to the diameter of catalyst bed (L/D). WHSV denotes weight hourly space velocity in g of olefin per g of catalyst per hour and S/O denotes mole ratio of hydrogen sulfide to olefin.

DETAILED DESCRIPTION OF THE INVENTION The process of the invention can comprise, consist essentially of, or consists of contacting, in the presence of an acidic catalyst, an olefin or alcohol with hydrogen sulfide under a condition sufficient to effect the production of a mercaptan. Any olefins which can react with hydrogen sulfide to produce a mercaptan can be used. Generally, a suitable olefin has the formula of R(R)C=C(R)R in which each R can be the same or different and can be independently selected from the group consisting of hydrogen, alkyl groups, alkenyl groups, cycloalkyl groups, cycloalkenyl groups, aryl groups, alkyl aryl groups, aryl alkyl groups, and combinations of two or more thereof. If R is a hydrocarbyl group, it can contain 1 to about 20, preferably 1 to about 15, and most preferably 5 to about 12 carbon atoms per molecule. Each R can also be independently non- substituted or substituted with a functional group which is inert to the reaction with hydrogen sulfide. Such functional group can be a halogen, hydroxyl, nitro, alkoxy, amino, carbonyl, carboxylic, ester, amino, nitrile, or combinations of two or more thereof. Because R can be an alkenyl group, a polyolefin can also be used in the invention. Examples of suitable olefins include, but are not limited to, ethylene, propylene, butenes, pentenes, hexenes, cyclohexenes, heptenes, octenes, nonenes, decenes, undecenes, dodecenes, tridecenes, tetradecenes, dimers or trimers or tetramers thereof, and combinations of two or more thereof. The presently preferred olefin is a mixture of dodecylenes or a propylene tetramer. These olefins are commercially available.

Similarly, any alcohol that can react with hydrogen sulfide to produce a mercaptan can be used. Generally, a suitable alcohol has the formula of ROH in which R is the same as disclosed above. Examples of alcohols include, but are not limited to, methanol, ethanol, propanols, butanols, pentanols, hexanols, heptanols, octanols, decanols, dodecanols, and combinations of two or more thereof. The

alcohols illustrated hereinabove include all possible isomers. For example, butanols include n-butanol, isobutanol, and t-butanol.

Any acidic catalysts that catalyze the reaction of an olefin and hydrogen sulfide can be used in the invention process. Examples of suitable catalysts include, but are not limited to, acids, aluminas, clays, silicas, silica-alumina, zeolites, acidic ion exchange resins, and combinations of two or more thereof. Suitable catalysts also include metal oxide(s) supported on these acidic catalysts except the acids. Generally oxides of metals or elements of Group VIB and Group VIII of the Periodic Table of Elements, CRC Handbook of Chemistry and Physics, 67th edition, 1986-1987 (CRC Press, Boca Raton, Florida) can be used. Examples of suitable metals of the metal oxides include, but are not limited to, cobalt, molybdenum, nickel, and combinations of two or more thereof.

The presently preferred catalyst is a clay such as kaolinite, halloysite, vermiculite, chlorite, attapulgite, smectite, montmorillonite, illite, saconite, sepiolite, palygorskite, and combinations of two or more thereof. The presently most preferred catalyst is an acid-activated montmorillonite clay such as Filtrol Grade F-24 which is commercially available.

According to the invention, the process can be carried out under any suitable conditions that can effect the production of a mercaptan. Generally such conditions can include (1) a reaction temperature in the range of from about 50"C to 200"C, preferably about 70"C to about 175"C, more preferably about 90"C to about 1500C, and most preferably 90"C to 1250C; (2) feeding a feed that comprises, consists essentially of, or consists of an olefin, olefin oligomer, or combinations thereof, and hydrogen sulfide in which the mole ratio of hydrogen sulfide to olefin is in the range of from about 1 to about 20, preferably about 1.5 to about 10, and most preferably about 2 to about 6; and the weight hourly space velocity (WHSV) is in the range of from about 0.01 to about 20, preferably about 0.5 to about 10, and most preferably 1.0 to 5 g of olefin or alcohol per g of catalyst; (3) a pressure that can accommodate the temperatures disclosed above and generally be in the range of from about 200 to about 1,000, preferably about 300 to about 900, and most preferably 400 to 800 psig; and (4) carrying out the invention process in a reactor in which the ratio of length of the catalyst bed to the diameter of the catalyst

bed is in the range of from about 2 to about 50, preferably about 3 to about 40, and most preferably 4 to 30.

The reactor effluent or product stream, which can comprise a mercaptan, unreacted hydrogen sulfide and/or unreacted olefin, can be treated by any conventional means known to one skilled in the art for recovering hydrogen sulfide which generally can be separated and recovered by distillation under reduced pressure. Other means such as, for example, solvent extraction or chemical absorption can also be used. Hydrogen sulfide and any unconverted olefin are generally recycled to the feed stream.

The following examples are provided to further illustrate the invention process and are not to be construed as to unduly limit the scope of the invention.

EXAMPLES Conventional 316 stainless steel tubular reactors were used in the runs. One was 0.687 inch internal diameter, 0.152 inch wall thickness, 32 inches long and a thermocouple well constructed of 0.25 inch stainless steel tubing. It had a 3000 psig working pressure rating. The other was 1.5 inch internal diameter, 0.25 inch wall thickness, 29 inches long and a thermocouple well constructed of 0.25 inch stainless steel tubing. It had a 2400 psig working pressure rating. Stainless steel was also used in all piping, valves and gauges. The reactor was jacketed by three electric heaters which were controlled by individual thermocouples on the outside wall of the reactor.

The olefin feed was propylene tetramer known as tetramer K purchased from Exxon Canada. Hydrogen sulfide was a compressed liquefied gas of 99.5% purity obtained from Matheson.

These feed components were premixed at desired ratios in a stainless steel vessel and were metered down-flow to the reactor with a Pulsafeeder LS-30 diaphram metering pump. On the downstream side, pressure was controlled and products were removed by a Research Control valve operated by an air pressure transmitter and I/P interface.

In typical runs, the electric heaters were set to a predetermined temperature which also was the desired inlet temperature. The top portion of the

reactor was filled with glass beads and acted as a preheater. FiltrolX Grade F-24 catalyst was in the bottom portion of the reactor. The length of the catalyst bed was determined by guaging and with a ruler whereas the diameter of the catalyst bed was the inner diameter of the reactor. The reactor was pressured to the desired pressure with nitrogen and the feed pump started. The pumping rate was adjusted to the desired feed rate. Samples were taken every 0.5 to 1 hour downstream of the control valve after pressure was let down. The temperature of the reactor products was adjusted with a heat exchanger and tempered water. Any unreacted hydrogen sulfide was vented to flare.

The samples were analyzed with an HP-5890 GC with HP-1 column (0.53 mm X 5 m long) using helium as carrier gas and TCD detector at 3250C.

The initial temperature of the GC oven was 75"C with a one minute hold after injection, then ramping at 250C/min to a final temperature of 250"C, and holding there for 5 minutes. The results are shown in Tables I-III.

Table I shows results of runs at a ratio of the length of catalyst bed to the diameter of catalyst bed (hereinafter referred to as L/D) at 24 (L/D=24) using temperature as a variable.

Table I Product Stream (wt%) H2S/olefin Run WHSV" mole ratio "Cb Tetramer K TDMC Heaviesd 1 2.5 2.5 115 10.1 81.6 8.3 2 2.5 2.5 115 3.6 93.3 3.1 3 4 1 130 19.4 62 18.6 4 2.5 2.5 115 12.2 86.5 1.4 5 1 4 130 8.8 84.4 6.8 6 4 4 100 17.8 81.9 0.2 7 1 4 100 12.0 87.7 0.3 8 1 1 130 22.3 66.7 10.9 9 4 4 130 15.2 83.8 1.0 10 1 1 100 18.5 81.1 0.4 11 4 1 100 26.6 73.1 0.3 aWHSV, g of olefin per g of catalyst per hour. bReactor temperature. CTDM, t-dodecylmercaptan; also used as % conversion. dMostly C24 di sulfi des and mercaptides.

The hot spot temperature was at the top of the catalyst bed.

Generally, lower temperature, higher hydrogen sulfide/olefin mole ratio, and a higher WHSV produced higher TDM and lower heavies. Although lower temperature, higher hydrogen sulfide/olefin ratio, and higher feed rate (WHSV) had similar results, they did not appear synergistic.

To better simulate a commercial reactor, a 1.5 inch inner diameter reactor was installed and runs were done with a FiltrolZ Grade F-24 catalyst bed of L/D=2. The variables were reactor temperature, WHSV and hydrogen sulfide/olefin mole ratio as shown in Table II.

Table II" Product Stream (wt%) H2S/olefin Run WHSV mole ratio "C Tetramer K TDM Heavies 71 2 2 115 5.6 87.8 6.5 72 1 1 100 37.1 57.6 5.2 73 2 2 100 15.9 81.6 2.5 74 2 2 115 15.6 79.5 4.8 75 1 3 130 19.5 71.8 8.8 76 3 2 115 13.2 82.4 4.4 77 2 2 115 16.3 79.1 4.5 78 3 1 130 43.5 47.5 9.0 79 2 1 115 40.0 56.3 3.7 80 2 2 115 18.5 78.6 2.8 81 1 3 100 15.1 83.9 1.1 82 3 3 100 17.0 82.4 0.5 83 3 1 100 27.5 70.3 2.2 84 3 3 130 12.9 82.4 4.6 85 2 2 130 18.6 76.8 4.6 86 2 2 115 19.3 78.8 1.8 87 2 3 115 18.0 81 1.0 88 2 2 115 19.9 78.5 1.6 89 1 3 115 15.2 83.6 1.2 90 1 1 130 32.8 61.8 5.6 91 1 2 115 20.8 77.5 1.7 aSee Table I for footnotes.

Table III shows the results of runs at L/D=5.

The length of the catalyst bed in the 1.5 inch inner diameter reactor in Table II was increased to give an L/D of 5. The variables of reactor temperature, WHSV, and hydrogen sulfide/olefin mole ratio of the runs are shown in Table III.

Table III" Product Stream (wt%) H2S/olefin Run WHSV mole ratio "C Tetramer K TDM Heavies 101 1.5 1 100 23.7 75.7 0.6 102 1 3 115 7.3 86.5 6.2 103 0.5 1 100 16.5 77.3 6.2 104 1 3 100 11.8 86.8 1.4 105 1 3 115 9.0 84.8 6.2 106 0.5 5 130 4.9 91 4.1 107 1.5 3 115 13.4 85 1.7 108 1 3 115 7.2 89.6 3.2 109 1.5 1 130 19.8 58.9 21.3 110 1 1 115 23.3 65.5 11.2 111 1 3 115 13.5 85.7 0.8 112 0.5 5 100 20.1 79.7 0.3 113 1.5 5 100 14.4 85.5 0.1 114 1.4 5 130 10.8 88 1.2 115 1 3 130 10.2 83.1 6.7 116 1 3 115 7.7 90.3 3 2.0 117 1 5 115 12.1 87 0.9 118 1 3 115 8.3 90.7 1.0 119 0.5 3 115 7.4 90.6 1.9 120 0.5 1 130 18.8 59.3 21.9 121 1.5 5 100 18.1 81.8 0.1 See Table I for footnotes

Table I shows that increasing the hydrogen sulfide/olefin mole ratio to the range of 2 to 4, the conversion (TDM production) increased from about 75% to 90-95%. Table I also demonstrates that an increase in WHSV could significantly increase commercial throughput.

Tables II and III further show that increasing WHSV from 0.5 to the

range of 1.5 to 3 and the hydrogen sulfide/olefin mole ratio from 2.5 to 3, the conversion (wt% TDM) increased to 90%. Tables II and II also demonstrate that an increase of WHSV could increase throughput by a factor of 3.

These above results show that low temperatures always produced the highest conversion to TDM with the least heavies. These results also show not only did the excess hydrogen sulfide shift the equilibrium towards better conversion to TDM, but it provided more material for heat capacity and heat removal.

Comparative results for statistical analysis of the data presented in the above Tables I-III are shown in Table IV and plotted in FIG. 1. Table IV and FIG. 1 further suggest that the maximum conversion of olefin to TDM with the least heavies appeared to occur at L/D=5 or greater. Table IV Comparative Results from Statistical Analysisa of Tables I, II and III Reaction Conditions H2S/Olefin mole ratio 2 3 3 3 1 1.5 3 1.5 WHSV 1 1.5 3 1.5 Reactor Temperature, "C 100 100 100 100 Conversion to TDM % % % L/D 2 81 81 85 77 5 84 90 b 83 24 83 91 92 85 aStatistical Analysis done with JMP computer software from SAS Institute Inc. ingot determined.

The results shown in the above examples clearly demonstrate that the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those inherent therein. While modifications may be made by those skilled in the art, such modifications are encompassed within the spirit of the present invention as defined by the disclosure and the claims.