I
E. P. PREVIC, E. B. HOTELLING, and M. 8. NEUWORTH Research and Development Division, Consolidation Coal Co., Library, Pa.
A N e w Synthesis Method f o r . r
..
A Gasoline Antioxidant Production of 6-tert-butyl 2,4-xylenol may now be attractive, using o-cresol as the r a w material numerous references ( 5 ) to the potency of 6-tert-butyl 2,4-xylenol as a gasoline antioxidant. Commercial production has not been achieved, primarily because of the lack of an adequate supply of 2,4-xylenol, the preferred raw material. 2,4-Xylenol occurs in limited quantities in cresylic acids and is recovered as a constant boiling mixture with 30 to 55% 2,5-xylenol. Processing this mixture requires the difficult separation of these compounds and utilization of the large quantity of byproduct 2,5-xylenol. The synthesis of 6-tert-butyl-o-cresol from o-cresol in high yields ( 4 ) suggested o-cresol as an alternative raw material. Pure o-cresol is available in adequate volume a t an attractive price. Production of the desired 6-tert-butyl 2,4-xylenol would be accomplished by ring methylation of the intermediate 6-tert-butyl-ocresol. A two-step process has been developed starting with 6-tert-butyl-a-cresol, involving Mannich base synthesis and hydrogenation to produce 6-tert-butyl 2,4-xylenol in a n over-all yield of 94 mole %. The reaction sequence is illustrated : T H E R E ARE
on
tert-butyl
OH
6
CHI
CHeNRe
t n t
-
+wt-bulYlGCH,
CH~NRE
+
RINH
CHI
Since recovery and recycle of secondary amine was contemplated, the addition of a methyl group requires only the consumption of formaldehyde and hydrogen.
Experimental
Mannich Base Synthesis. For the preparation of the Mannich base, 4-dimethylaminomethyl 6 - tert butyl ocresol, the reaction was carried out in water-immiscible solvents such as toluene or xylene with efficient stirring to obtain good contact between the aqueous and
-
-
-
Literature Background Subject Methylation of hindered phenols effected by hydrogenation of intermediate methoxymethyl ether Methylation of hindered phenols by reaction with formaldehyde, methanol, and caustic using high pressures and high temperatures General methylation procedure involving Mannich base synthesis and hydrogenolysis with copper chromite yields methyl derivatives in moderate to low yields Hydrogenation of Mannich bases with MoSz catalyst produced the desired compound in yields of 60 to 7570
organic layers. T h e reactants were charged to a stirred vessel, equipped with a bottom withdrawal valve, in the following proportions : one mole of 6-tert-butyl-o-cresolplus sufficient toluene or xylene to make about a 40 wt. yo solution; one mole of dimethylamine as a 2570 aqueous solution; and one mole of formaldehyde as a 37y0 aqueous solution. Addition of the formaldehyde resulted in a temperature rise from room temperature to 40' to 45' C. Heating was then employed to maintain the 40 'to 45 ' C. temperature for 3 hours, followed by heating a t 90' C. for 1 hour to complete the reaction. The aqueous layer was separated and discarded; the organic layer was washed at 90' C. with 3 to 5 portions of hot water following which it was dried azeotropically a t reduced pressure so that the solution temperature did not exceed 100" C. This precaution was necessary to prevent pyrolysis of the Mannich base to methylene bis derivatives. T h e solution was then cooled with stirring to precipitate the base, which was collected by filtration to yield 88 to 93 mole 7 ' in a form suitable for the hydrogenolysis. T h e filtrate liquors contained an additional 4 to 9% of base. T h e recovered
Mannich base is a white crystalline solid, m.p. 117-119' C.; analysis, calcd.: C, 78.10; H, 10.42; N, 5.36; found: C, 78.22; H , 10.42; N, 5.27. Hydrogenolysis. Hydrogenolysis of the Mannich base was studied extensively with particular regard to amount and type of catalyst employed. Best performance was obtained with a copper chromite catalyst containing 54% cupric oxide and 41% chromic oxide designated CU-2OOOP (all catalysts were purchased from Harshaw Chemical Co.). T h e crystalline Mannich base was charged with an equal weight of xylene to a stirred high-pressure autoclave along with lY0-by weight based on the Mannich base-of the copper chromite catalyst. Hydrogenolysis for 5 hours a t 160' C. and 1000 p.s.i.g. of hydrogen pressure resulted in 95,5-9770 yield of 6-tert-butyl 2,4-xylenol. Distillation of the hydrogenolysis mixture a t atmospheric pressure yield 97.5 to 9870 of the liberated dimethylamine. T h e xylene solution containing 6-tert-butyl 2,4xylenol (b.p. 131' C./20 mm. Hg) was fractionated a t reduced pressure to yield a product of 984-70 purity, the major contaminant being 6-tert-butyl-ocresol. A small amount, 170, of nonvolatile residue remained. As mentioned previously, the Mannich base synthesis results in an over-all yield of 97y0,of which only 88 to 93% is available as first crop crystalline base suitable for hydrogenolysis. T h e remainder was utilized by hydrogenolysis of the filtrate liquor in the presence of copper chromite catalyst recovered from hydrogenolysis of the crystalline base. An additional 2% yield of 6-tert-butyl 2,4-xylenol and 4.5% recovery of 6-tertbutyl-o-cresol was possible by this procedure. T h e over-all yield of 6-tertbutyl 2,4-xyIenol from consumed 6-tertbutyl-o-cresol was therefore 94 to 9570.
Discussion of Results Mannich base derivatives of 6-tertbutyl-0-cresol can be produced in very VOL. 53, NO. 6
JUNE 1961
469
high yields from a variety of secondary amines including dimethylamine, piperidine, and morpholine. Dimethylamine was preferred on the basis of cost and favorable molecular weight. ,4n evaluation of the hydrogenolysis step established the need for a recrystallized Mannich base product for maximum yield of 6-tert-butyl 2,4-xylenol. Synthesis of the base in a water immiscible solvent, like xylene, permitted recovery of the purified product in 927, yield by simply cooling the reaction mixture followed by filtration. The hydrogenolysis of the purified Mannich base (4-dimethylamino-methyl6-tert-butyl-o-cresol) was studied in some detail aimed a t establishing the optimum temperature, catalyst concentration, and catalyst composition. Three copper chromite catalysts containing different copper oxide-chromium oxide ratios were tested. The compositions of these catalysts arc as follows : Wt. % Cata-
lyst
CuO
Crz03
1 2 3
82 41 54
16.4 44 41
BaO
... 11 ...
cuo/
Crz03 5.0 0.9 1.3
T h e details of the hydrogenation study and results are shown in the table. Using 1070 concentration of Catalyst 1 (Experiment 1) with a high copper oxide ratio, a yield of 6-tert-butyl 2,4-xylenol of 71y0 was obtained. This yield has been obtained with MoSz ( I ) . Catalyst 1 suffered a measurable loss of activity after one cycle. This concentration of catalyst represents a significant cost item in the over-all process. Catalyst 2, with a lower ratio of C u O to Cr203, was evaluated varying the reaction temperature and catalyst concentration. Hydrogenolysis a t 160' C. and catalyst
concentration of 2.57, appears optimum (Experiment 6) producing the desired 6-tert-butyl 2,4-xylenol in 95Y0 yield with essentially complete conversjon of the Mannich base. If the catalyst concentration is reduced to 1% (Runs 7 and 8) or the temperature reduced to 150' C., less favorable results are obtained. Catalyst 3, containing a slightly higher C u O to Cr203 ratio, gave even better results. One per cent of this catalyst (Experiments 10-12) gave equivalent )ields to those obtained with 2.5% of Catalyst 2. Apparently the presence of barium oxide as a stabilizer in Catalyst 2 \vas not particularly effective in improving the potency or selectivity. Reducing the catalyst concentration to 0.5% (Experiment 13) reduced the conversion of Mannich base to 86y0. -4 considerable simplification in the process Mould be effected if the crude hIannich base dissolved in hot xylene could be hydrogenated directly. Crystallization and filtration would be eliminated. A batch of Mannich base prepared in xylene was hydrogenated without any purification. Hydrogenation with lOY0 of Catalyst 2 (Experiment 14) gave a conversion of 80Y0 and a yield of 6-tert-butyl 2,4-xylenol of 4997,. IVashing the xylene solution (Experiment 15) with hot water prior to hydrogenation and 'or azeotropic drying (Experiments 16 and 17) improved the conversion and yield of 6-tert-butyl 2.4xylenol. However, the yields did not approach those obtained with the recrystallized Mannich base even using 10% catalyst. Significant degradation to 6-tert-butyl-o-cresol and nondistillable residue occurred. Recrystallization leaves behind a high boiling acid soluble impurity in the xylene which, if present during the hydrogenation, affects the activity and selectivity of the hydrogena-
6-Tert-Butyl 2,4-Xylenol Yield Was Improved b y Catalyst Selection and Mannich Base Purification Product Distribution, (Mole %) Acid 6-tert6-tertExp. Treat- Temp., Catalyst soluble Butyl- Butyl 2,4- Residue No. nient C. Type Concn. % residue o-cresol xylenol loss 1 C 165 1 10.0 18.0 71.0 11.0 2 3 4
C C
5
C C
6 7 8 9 10 11 12 13 14 15 16 17
C C (7
C C
C C C
None A B A 4- B
+
...
170 160 150 150 160 160 160 160 160 160 160 160 170 160 160 160
2 2 2 2 2 2 2 3 3 3 3 3 2 2 2 2
10.0 10.0 5.0 2.5 2.5 1.0 1.0 2.5 1.0 1.0 1.0 0.5 10.0 10.0 10.0 10.0
1.0 1.0 0.5 6.5 1.5 63.0 75.0 0.5 0.5 0.5 0.5 14.0 20.0 3.0 25.0 8.0
4.0 1.0 1.0
2.0 1.0 4.5 1.5
1.0 1.0 2.0 2.0
93.0 96.0 94.0 91 .O 95.0 26.0 13.0 98.0 97.5 95.5 95.5
...
...
23.0 15.0 11.5 21.5
49.0 71.5 54.5 64.5
8.0 10.5 9.0 6.0
1.5 4.0 7.0
7.0
...
...
4.5 4.0 0.5 0.5 1.5 1.5
Treatment A , water wash xylene solution of Slannich base. Treatment B , dry xylene solution of RLannich base. Treatment C, treatments A B followed by recrystallization of Mannich base.
+
470
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion catalyst. Distillation of the xylene or acid \vashing eliminates this impurity permitting recycle of xylene. Catalyst consumption under optimum conditions has been reduced to lY0 eliminating the need for catalyst recycle. Since the xylene mother liquor contains from 4 to 9% dissolved hlannich base, a n attempt was made to obtain additional 6-tert-butyl 2,4-xylenol by hydrogenation of the mother liquor with used copper chromite catalyst. Hydrogenation a t 160" C. (catalyst concentration 67, based on xylene solution) produced an additional 27, of 6-tert-butyl 2,4xylenol and 4970 6-tert-butyl-o-cresol, calculated on starting 6-tert-butyl-ocresol charged to hlannich base synthesis. Thus the over-all yield of 6-tert-butyl 2,4-xylenol based on 6-tert-butyl-o-cresol consumed is 94y0. Recovery and recycle of dimcthylamine are required to minimize chemical costs. Two methods of recovery of dimethylamine, acid extraction and direct distillation, were explored. The acid extraction procedure involves scrubbing the vented hydrogen with sulfuric acid followed by extraction of the xylene solution with sulfuric acid. T h e recovery of dimethylamine was 96% based on Mannich base charged. T h e dimethylamine sulfate solution is neutralized with caustic to a p H of 9.0 to 11.0. T h e aqueous dimethylamine containing sodium sulfate was used to prepare another batch of Mannich base by reaction with 6-tert-butyl-o-cresol and formaldehyde. The yield of crystallized Mannich base was 92.570, which is identical to the value obtained with fresh dimethylamine. Hydrogenation of this product with lY0 of Catalyst 3 resulted in a conversion of 987, and a yield of 6-tert-butyl 2,4-xylenol of 947,. This yield agrees quite well with the yields obtained under optimum conditions. Direct distillation of dimethylamine from the hydrogenation product resulted in a recovery of dimethylamine of 97 to 9870;. literature Cited (1) Bruson, H. S., Covert? L. Mi. (to Resinous Products and Chemical Co.), U. S. Patent 2,194,215 (March 19, 1940). (2) Caldwell, W. T., Thompson, T. R., J . Am. Chem. SOC.61, 2354 (1939). (3) Hotelling, E. B., Previc, E. P., Neu-
worth, M. B., others (to Consolidation Coal Co.), U. S . Patent 2,882,319 (April 14, 1958). (4) Neuworth, M. B., Hotelling, E. B., Depp, E. A. (to Pittsburgh Consolidation Coal Co.) Zbid., 2,836,627 (May 27, 1958). (5) Nixon, A. C., Minor, H. B., Calhoun, G. M., I N D . ENC.CHEM. 48, 1874 (1956). (6) Norton, D. G., Morris, R. C. (to Shell Development Go.), U. S. Patent 2,841,624 (July 1: 1958). (7) N. V. DeBataafsche Petroleum Maatschappij, Brit. Patent 814,278 (June 3, 1959). RECEIVED for review October 3, 1960 ACCEPTED February 23, 1961
