Abstract:
Vanillin is produced from alkaline oxidation treatment of acetovanillone or acetovanillone-rich internal waste streams of lignin processes without the need for utilizing nitrobenzene as an oxidizing agent.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to alkaline oxidation of p-hydroxy aromatic 1-carbonyl compounds to their corresponding aromatic aldehydes. In particular, this invention relates to the alkaline oxidation of acetovanillone to produce vanilla. 
     BACKGROUND OF THE INVENTION 
     Vanillin can be manufactured by a number of synthetic methods. One of the most utilized methods which involves oxidative alkaline hydrolysis of spent sulfite liquor, is called the lignin process. Lignin is an amorphous polymeric substance related to cellulose that together with cellulose forms the woody cell walls of plants and the cementing material between those cell walls. In the lignin process vanillin is produced by a copper catalyzed, alkaline air oxidation of lignosulfonates which are the predominant material of fermented spent-waste liquors from sulfite pulp mills. A major by-product of this lignin process is acetovanillone. Prior to the present invention, acetovanillone was treated as an impurity of the lignin process for vanillin. Acetovanillone was separated and either discarded or used as a pharmaceutical intermediate, food preservative or sun-screening agent. Crude vanillin containing small amounts of acetovanillone as an impurity has also been available commercially, e.g. as a substitute for pure vanillin for many chemical purposes. 
     Attempts have been made to convert acetovanillone into vanillin; however, heretofore, the only method of doing so involved the use of nitrobenzene. Such a method was first reported in an article entitled &#34;A New Vanillin Synthesis&#34; by Henry O. Mottern, Canadian Journal of Chemistry, 56:2107-2108(1934). Unfortunately, nitrobenzene reduces to nitrosobenzene which in turn reduces to phenylhydroxylamine which reduces to aniline along with certain ortho and para azo benzenes. Such compounds are considered undesirable because of their carcinogenic behavior. Accordingly, the nitrobenzene oxidation of acetovanillone to vanillin has not been commercially adopted. 
     Other methods have also been tried. See, the Canadian Journal of Chemistry, 45:3009-3011(1967) which reports the use of various oxidizing agents, particularly cupric oxide, in the oxidation of acetovanillone. This method, however, produced very little, if any, vanillin. 
     It is an object of the present invention to produce vanillin from acetovanillone without the need to introduce nitrobenzene as an oxidizing agent. 
     It is a further object of the present invention to provide a method for recycling internal plant streams of the lignin process where such streams are rich in acetovanillone and to produce additional vanillin from said internal streams. 
     These objects and others are attained by a process which provides for oxidation of acetovanillone with either air or air enriched with oxygen in the presence of an alkaline material. Such oxidation is effected in a medium substantially devoid of nitrobenzene. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the process of this invention, pure acetovanillone and/or waste streams from the vanillin process, which are composed substantially of acetovanillone, may be used as raw materials. The vanillin processes which have waste streams rich in acetovanillone may include processes which treat parts of coniferous trees. In such processes, coniferon may be extracted, then oxidized and finally hydrolyzed to vanillin, leaving certain waste streams rich in acetovanillone. 
     Alternatively, the process may begin with papermill waste liquors. This involves controlled oxidation of lignosulfonates, usually attained from the waste sulfite liquors. These waste sulfite liquors are oxidized by contacting with air and sodium hydroxide in a reactor, for example as taught in U.S. Pat. No. 2,692,291, incorporated herein by reference. 
     A crude product, containing very dilute vanillin, may then be extracted from lignin salts with a solvent, such as, for example 1-butanol. The crude product in butanol solution is then extracted with an aqueous solution of sodium bisulfite. The aqueous phase containing vanillin is then treated with sulfuric acid and air in a blow tower to produce crude vanillin. The crude vanillin is then subjected to vacuum distillation to remove high boiling impurities. Purified vanillin is obtained by multiple crystallization which provides acetovanillone in a waste stream. 
     More particularly, U.S. Pat. No. 3,600,442, incorporated herein by reference, teaches treating the crystallized liquors with an alkali metal hydroxide and a zinc or magnesium salt to precipitate vanillin and acetovanillone. The vanillin is then separated from the acetovanillone. 
     Alternatively, U.S. Pat. No. 3,686,322, incorporated herein by reference, discloses that hot hydrocarbon solvents, such as toluene, may be employed to extract vanillin from lignin liquor, after which the solvent is cooled to crystallize the vanillin. The residue after extraction is rich in acetovanillone. 
     Such streams containing residual acetovanillone after separation of the vanillin are preferred starting materials for the process of this invention. 
     The acetovanillone is subjected to alkaline oxidation with air or with air enriched with oxygen in the presence of a strong alkaline material as for example a hydroxide of an alkali or alkaline earth metal, preferably sodium hydroxide. We have found that by controlling the amount of alkaline material, e.g. as the equivalent weight ratio of sodium hydroxide to acetovanillone at a range of at least about 3.5 to 1 preferably between 3.5 to 1 and 4.0 to 1, commercially acceptable conversions of acetovanillone to vanillin may be accomplished. Ratios higher than 4.0 to 1 may be employed, however, we have not discovered any substantial improvement of the yield at such ratios. Ratios of sodium hydroxide to acetovanillone of lower than about 3.5 to 1 down to about 2.5 to 1 are useful but not as advantageous in bringing about commercially acceptable yields. 
     The conversion is preferably carried out at temperatures of at lease about 150° C. to as high as about 200° C. Additionally, the process of this invention is preferably conducted at a pressure of at least about 700 KPa, more preferably in the range of about 900-1200 KPa. Preferably, the time of conversion is at least about 2 hours and may range as high as about 4 hours. It is especially preferred to treat the material for 3 hours. 
     Catalysts such as sulfates and oxides of the transition elements, copper, manganese, iron, etc. may be employed; however, no appreciable catalytic affect has been observed to improve the conversion yields. 
     Air may be employed to oxidize the acetovanillone in this invention. Preferably air enriched with oxygen is utilized for purposes of economy and efficient conversion. More preferably, air enriched to about 40% (by volume) oxygen has been observed to improve the yield over the use of air alone by as much as 40 to 50%. 
     The following examples are intended to illustrate the practice of this invention without unduly limiting its scope. 
    
    
     EXAMPLES 1-25 
     Two-gram samples of acetovanillone were separately placed in a batch type reactor vessel with 100 gr of 2N NaOH so that the ratio of NaOH to acetovanillone was 4:1. The reactor vessel was heated to about 180° C. and pressurized with either air or air enriched to 40% oxygen to the pressures indicated in Table 1. The reaction was allowed to proceed for the time indicated in Table 1. In some cases, as indicated in Table 1, sulfates and oxides of the transition elements copper, manganese and iron were employed as catalyst and in one case sodium-m-nitrobenzene sulfonate abbreviated as NAMNO. The resulting oxidation products were analyzed by gas chromatography and yields of vanillin (Van), are reported in Table 1. 
     
                       TABLE 1______________________________________                          Vanil-      Vanil-Ex-                  Catalyst  lin   Oxidi-                                      linam-  Press.  Time    Type/Amount                          Yield zing  Yieldple  (KPa)   (Hour)  (Gm)      (gm)  Agent (%)______________________________________1    1140    3       None      0.31  Air   322    1140    3       MnSo.sub.4 /0.10                          0.29  Air   303    1140    3       CuSO.sub.4 0.10                          0.34  Air   354    1000    2       CuSO.sub.4 0.05                          0.16  Air   165    1000    2       CuSO.sub.4 0.10                          0.16  Air   176    1000    2       CuSO.sub.4 /0.10                          0.16  Air   167    1140     21/2   CuSO.sub.4 /0.10                          0.27  Air   278    1140    3       None      0.28  Air   299    1140    3       None      0.30  Air   3010   1140    3       None      0.27  Air   2711   1140    3       MnSO.sub.4 /0.10                          0.27  Air   2712   1140    3       CuSO.sub.4 /0.10                          0.29  Air   2913   1140    3       CuSO.sub.4 /0.10                          0.33  Air   3414   1140    3       CuSO.sub.4 /0.10                          0.43  40%   43                                O.sub.215   1140    3       None      0.43  40%   43                                O.sub.216   1140    3       None      0.44  40%   45                                O.sub. 217   1140     21/3   CuSO.sub.4 /0.10                          0.35  40%   35                                O.sub.218   1140    3       NAMNO/0.10                          0.31  Air   3119   1140    3       Fe.sub.2 SO.sub.4 /0.10                          0.24  Air   2420   1140    3       CuO/0.10  0.32  Air   3321   1000    7       None      0.24  Air   2422   1140    3       CuSO.sub.4 /0.10                          0.42  40%   42                                O.sub.223   1140    4       None      0.40  40%   40                                O.sub.224   1140    3       CuSO.sub.4 /0.50                          0.28  Air   2825   1275    3       None      0.36  Air   36______________________________________