Method for converting vegetable material into chemicals
This invention relates to a method for converting vegetable material into chemicals and in particular to a method for converting complex biopolyesters of suberin and cutin type consisting of straight-chain hydroxy acid monomers containing 10 to 40 carbon atoms, into a plain mixture of organic acids and their salts, which contains only a few main components. 
     The purpose of the invention is, particularly, to produce from the side products of the forest industry, such as birch bark, oak cork and the similar suberin and cutin containing raw materials plain chemical mixtures containing as their main components valuable organic acids and their salts, of which the valuable main components can be recovered by separation methods known per se. 
     For the sake of simplicity, in the first place only birch bark is being dealt with in this context, but the treatment of other plant organs containing suberin and cutin, which have been dealt with e.g. in the publication of Kolattukudy (Science 208 (1980) 990) is analogous to the treatment of birch bark. 
     According to Kolattukudy suberin and cutin would have the following structural and monomer composition: 
     
         __________________________________________________________________________
Suberin                                                                   
Main monomers                                                             
__________________________________________________________________________
CH.sub.3 (CH.sub.2).sub.m COOH                                            
CH.sub.3 (CH.sub.2).sub.m CH.sub.2 OH                                     
 ##STR1##                                                                 
HOOC(CH.sub.2).sub.n COOH                                                 
(m = 18-30; n = 14-20)                                                    
Polymer                                                                   
 ##STR2##                                                                 
__________________________________________________________________________
Cutin                                                                     
C.sub.16 -Acids Main monomers C.sub.18 -Acids                             
__________________________________________________________________________
CH.sub.3 (CH.sub.2).sub.14 COOH                                           
                  CH.sub.3 (CH.sub.2).sub.7 CHCH(CH.sub.2).sub.7 COOH     
 ##STR3##                                                                 
                   ##STR4##                                               
 ##STR5##                                                                 
                   ##STR6##                                               
(y = 8, 7, 6 tai 5 x + y = 13)                                            
                   ##STR7##                                               
Polymer                                                                   
 ##STR8##                                                                 
__________________________________________________________________________
 
    
     The previous methods for the decomposition and separation of suberin into components comprise, for example, the alkaline hydrolysis, the alkaline or acid catalyzed exchange esterification or the reduction with aluminum lithium hydride (Holloway, P. J. in Plant Cuticle, Eds. D. F. Cutter, K. L. Ahein and C. E. Price, Linnean Society Symposium Series No. 10, Academic Press, London 1982). 
     By these methods it is possible to produce monomeric hydroxy and carboxylic acids from birch bark by separating them from the other main components of birch bark, i.e. betulin and the non-aliphatic part. The mixture of the organic acids obtained is, however, very pennached and it is difficult to separate it into components, which fact in general, is one of the greatest drawbacks of chemicals originating from nature in view of their economical utility. A good example of this are the attempts to utilize lignin by decomposing it to plain components. 
     Now it has unexpectedly been found that the suberin of birch bark can, as a new industrial raw material, be decomposed to a substantially simpler mixture of a few components, which can easily be separated to usable fractions. 
     According to the invention the suberin and cutin type biopolyester raw material mixture is converted by depolymerization with acid or alkali and by oxidation either with a strong oxidant or dehydration strong alkali at a high temperature either directly or then the raw material is first exposed to acidic or alkaline hydrolysis is a polar organic solution and/or water, whereafter the oxidation is carried out on the monomer mixture thus separated from the solids. 
     The dehydration oxidation can be carried out with sodium or potassium hydroxide or with their mixture at so high a temperature that the alkali is in molten state, preferably at 200° to 400° C., for example at 280° to 350° C., or in a strong alkali solution, whereby the raising of the temperature to a sufficient level may involve the use of excess pressure to prevent the evaporation of the water. 
     When suberin is first treated with alkali or acid in water, lower alcohol or their mixture, a complicated mixture of hydroxy and dicarboxylic acids or the esters of their lower alcohol is obtained and then, further, a simple mixture of carboxylic acids by oxidation in accordance with the invention with strong oxidants, such as potassium permanganate, nitric acid, ruthenium tetroxide, ozone, chromic acid, etc., or with dehydration oxidation in strong alkali at a high temperature. The mixture obtained by the method in accordance with the invention is, as will be shown later on, simple as to its composition and useful for the preparation of some mono- and dicarboxylic acids. In the following the method in accordance with the invention is described in further detail. 
     Great amounts of birch bark is formed in the various industrial fields in which birch is used as raw material of different kinds of refinement processes. Such industrial fields are the pulp, sawmill, chipboard, plywood and furniture industry, in which great amounts of birch bark is often gathered in the dumping areas. oak bark is formed respectively in industry using oak and as residue in the refinement of the cork oak&#39;s cork. 
     The main components of the outer layer of the bark of birch, the birch bark, are biopolyester (suberin), triterpene alcohols as well as a mixture comprising principally polymeric compounds containing among others phenolic components. Suberin and triterpene alcohols form together about 60% by weight of the birch bark. The triterpene alcohols can be removed from the birch bark by extracting with an organic solvent. As suberin is virtually insoluble in organic solvents, it can be concentrated into a solid extract residue. The amount of suberin thus increases to about 50% by weight of the extracted birch bark. It has been contended above that suberin is a polyester consisting of polymerized, straight-chain, even C 16  to C 24  ω-hydroxy fatty acids, some of which furthermore includes epoxy or hydroxyl groups in the middle of the carbon chain as well as of ω-dicarboxylic acids. The polar monomer component is the 9,10-epoxy-18-hydroxyoctadecane acid, which comprises about 30-43% of the monomer composition. 
     Suberin can be hydrolyzed to its monomeric components both with acids and with alkalis in an aqueous solution and in polar organic solvents. The selectivity of the hydrolysis can be adjusted by controlling the concentration of the acid or the alkali, the reaction circumstances and the solvent, in order to achieve the final result desired in each case. 
     An exhaustive hydrolysis of suberin with 0.5M alkali hydroxide in a 95% solution of a lower alcohol gives about 500 g monomers from one kilogram extracted birch bark. The composition of the monomer mixture obtained is disclosed in Table 1. As can be seen from the table, the 9,10-epoxy-18-hydroxyoctadecane acid and the 22-hydroxydocosane acid are the main components of the mixture. 
     A corresponding result is obtained by an acid hydrolysis in a solution of a lower alcohol with the difference that the products are in the form of lower alcohol esters and the 9,10-epoxy group has reacted forming 9(10)-methoxy-10(9)-hydroxy- and 9,10-dihydroxy derivatives as well as reproduction products of the epoxy group. The hydrolyses described above can be carried out also for unextracted birch bark with similar results. 
     The monomer mixture obtained by the above-described hydrolysis methods on extracted or unextracted birch bark can be treated further so that the terminal hydroxy groups comprised therein are converted into carboxylic acid groups and into dicarboxylic acid groups and the above-mentioned functional groups in the carbon chain are oxidized and the carbon chain broken forming dicarboxylic acids. The treatment methods that come into question can be selected from oxidations by various strong oxidizers, such as potassium permanganate, nitric acid, ruthenium tetroxide, ozone, or chromic acid or by dehydration in strong alkali at a high temperature (alkali melting). Chemically, oxidation of alcohols to carboxylic acids occur in these treatments. 
     The above-mentioned methods produce from a complex mixture of suberin monomers, depending on the method used, either only straight-chain saturated dicarboxylic acids (Table 4) or, if alkali is used as the oxidant, due to the mechanism, also short-chain monoacids (Table 3). As can be seen from Tables 3 and 4, the mixture has been considerably similified and concentrated in regard to certain acids and the mixture has been brought to such a state that further refining can be carried out by conventional means. 
     The acid mixture obtained by the method in accordance with the invention can be divided into its components by conventional methods, e.g. by crystallization from organic solvents or by liquid-liquid extraction. The short-chain fatty acids can also be distilled from mixtures of dicarboxylic acids at subatmospheric pressure and thereafter the dicarboxylic acids can be separated e.g. according to the chain length by fractional crystalliztion or by subatmospheric pressure distillation into two fractions, nonanediacid (azelaic acid) and the mixture of hexadecanedocosanediacid, which already as such are technically useful in the preparation of various derivatives. 
     A more detailed but non-limiting description of the invention is provided below by way of the following examples. 
    
    
     EXAMPLE 1 
     (Prior Art) 
     10 g of extracted, milled birch bark was charged to 500 ml column, which was equipped with a reflux cooler and a magnetic mixer. 250 ml of 0.5M sodium hydroxide solution of 90% ethanol was added to the column and the mixture was refluxed for 1 hour. After this the precipitate was filtered off and washed with hot ethanol and the monomers were analyzed from the solution after the acidification and diethyl ether extraction as silyl ether derivatives of their methyl esters with gas chromatography. The monomer yield was over 90% calculated from the suberin of the dry birch bark. The composition of the product obtained is disclosed in Table 1 and its treatment was continued in accordance with Example 4. 
     EXAMPLE 2 
     (Prior Art) 
     The hydrolysis was repeated as in the preceding example with the exception that water was used instead of ethanol and the reaction time was 3 hours. The monomer yield was over 95% calculated from the suberin of the dry birch bark. The suberin monomer composition of the hydrolysate obtained is disclosed in Table 1. 
     EXAMPLE 3 
     2 g of extracted birch bark was charged to 250 ml flask, which was equipped with a reflux cooler and a magnetic stirrer. 90 ml of methanol and 10 ml of 96% H 2  SO 4  were added to the flask and the mixture was refluxed for 3 hours. The methyl esters of the monomers of suberin were analyzed after silylation with a gas chromatograph. The yield of the methyl esters of monomers was about 90%. The composition of the product obtained is disclosed in Table 2. 
     EXAMPLE 4 
     To the dried salt mixture of monomers (0.5 g) obtained in the preceding example 1 was added 3 g of potassium hydroxide dissolved in 2 ml of water. The mixture was heated under agitation in a nickel crucible in a tin bath at a temperature of 330° C. for 15 minutes. The carboxylic acids obtained were analyzed as methyl esters with a gas chromatograph. The yield of the reaction calculated from the free monomers was 90%. The composition of the product obtained is disclosed in Table 3. If the batch is greater it is possible to operate with remarkably smaller, nearly equivalent amounts of alkali. 
     EXAMPLE 5 
     To the finely milled cork powder (0.5 g) was added 3 g of potassium hydroxide dissolved in 2 ml of water. The mixture was heated in a nickel crucible in a tin bath at a temperature of 330° C. for 15 minutes. The carboxylic acids obtained were analyzed after methylation gas chromatographically. The yield of the reaction calculated from the cork suberin was about 90%. The composition of the carboxylic acid obtained is disclosed in Table 3. When the batch is greater it is possible to operate with remarkably smaller, nearly equivalent amounts of alkali. 
     EXAMPLE 6 
     To the salt mixture (0.5 g) of the suberin monomer obtained in Example 1 was added 3 ml of 1N sodium hydroxide solution and the solution was refluxed for 16 hours. After this 15 ml of a 0.5M aqueous solution of potassium permanganate was added to the mixture which was heated at 50° C. in 0.5 hours. The solution was acidified with 3N sulphuric acid and extracted 5 times with diethyl ether and analyzed as in Example 5. The composition of the carboxylic acid mixture obtained is disclosed in Table 4. 
     EXAMPLE 7 
     3 ml of water was added to the dried salts of the suberin monomer mixture (9.2 g) obtained in Example 1 and the pH of the mixture was adjusted to 3.5 with 50% H SO. The suberin acids released were extracted with diethyl ether and the ether solution was evaporated to a dryness. The evaporation residue was dissolved in 5 ml acetone Jones-reagent was added dropwise until the colour of the oxidant remained. The Jones-reagent was prepared by dissolving 26.72 g of chromium trioxide in 100 ml of sulfuric acid, which was obtained by diluting 23 ml of concentrated H 2  SO 4  with water to 100 ml. 
     During the oxidation the reaction was monitored with a gas chromatograph as presented above. The compositions of the products obtained with strong oxidants are disclosed in Table 4. 
     TABLES 
     
                       TABLE 1                                                     
______________________________________                                    
The composition of products obtained in alkali hydroly-                   
sis of birch bark suberin                                                 
                     Example                                              
Carboxylic acid        1       2                                          
______________________________________                                    
Octadec-9-ene diacid   4.7     4.2                                        
18-hydroxyoctadec-9-ene acid                                              
                       12.2    11.7                                       
9,10-epoxy-18-hydroxyoctadecanoic acid                                    
                       39.2    19.8                                       
9,10,18-trihydroxyoctadecanoic acid                                       
                       8.4     28.8                                       
Docosane diacid        8.2     8.1                                        
22-hydroxydocasanoic acid                                                 
                       13.9    13.3                                       
Others (C.sub.16 -C.sub.24)                                               
                       13.4    14.1                                       
                       100.0   100.0                                      
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
The composition of the product obtained with acid                         
methanolysis of birch bark suberin                                        
Carboxylic acid ester Percentage of mixture                               
______________________________________                                    
Methyloctadec-9-enedioate                                                 
                      4.6                                                 
Methyl-18-hydroxyoctadec-9-enoate                                         
                      11.2                                                
Methyl-9(10)-methoxy-10(9),18-dihydroxy-                                  
                      29.8                                                
octadecanoate                                                             
Methyl-9,10,18-trihydroxyoctadecanoate                                    
                      13.8                                                
Methyldocosanedioate  8.0                                                 
Methyl-22-hydroxydocosanoate                                              
                      11.9                                                
Others C.sub.16 -C.sub.24 methyl esters                                   
                      20.7                                                
                      100.0                                               
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
The compositions of products obtained with alkali melting                 
of birch bark suberin monomers and cork suberin                           
                 Example                                                  
Carboxylic acid    4       5                                              
______________________________________                                    
octanoic acid      13.5    5.5                                            
Nonane diacid      24.4    16.9                                           
Decane diacid      2.3     2.0                                            
Hexadecane diacid  15.0    17.0                                           
Octadecane diacid  3.2     2.1                                            
Eicosane diacid    3.6     2.6                                            
Docosane diacid    20.8    24.1                                           
Others             17.2    29.8                                           
                   100.0   100.0                                          
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
The composition of products obtained with strong-oxidants                 
from birch bark suberin                                                   
Carboxylic acid Percentage of mixture                                     
Oxidant         KMnO.sub.4                                                
                         Chromic acid                                     
______________________________________                                    
Heptane diacid  9.3      1.0                                              
Octane diacid   46.2     12.9                                             
Nonane diacid   15.7     48.5                                             
Others.sup.(1)  24.9     37.6                                             
                100.0    100.0                                            
______________________________________                                    
 .sup.(1) Comprises eicosane diacid and docosane diacid as main components