Lecture 11 Lignin Structure Wood Chemistry PSE 406 Lecture 11 Lignin Structure PSE 406 Lecture 11

Class Agenda Functional groups Lignin structures Methoxyl, phenolic hydroxyl, aliphatic hydroxyl, carbonyl Lignin structures Lignin – carbohydrate complexes Lignin analytical procedures Lignin trivial facts Appendix PSE 406 Lecture 11

Lignin Functional Groups Phenolic Hydroxyl 15-30 free phenolic hydroxyl/100C9: Softwood 10-15 free phenolic hydroxyl/100C9: Hardwood Reactivity Units containing free phenolic hydroxyl groups much more susceptible to cleavage reactions - hydrolysis of a and b aryl ether linkages Structures much more reactive towards modification reactions PSE 406 Lecture 11

Lignin Functional Groups Methoxyl ~0.95/C9 in softwoods ~1.5/C9 in hardwoods Generally resistant to acid and alkali HS cleaves to form thiols, mercaptans (Kraft mill odor) PSE 406 Lecture 11

Lignin Functional Groups Aliphatic Hydroxyl Majority of aliphatic hydroxyl groups are primary: on g carbon Site relatively non-reactive In some species, g carbon oxygen linked through ester linkage to r-coumaric acid, etc Benzyl alcohols Debated amount: 16-40/100 C9 in spruce Play dominant role in delignification reactions PSE 406 Lecture 11

Lignin Functional Groups Carbonyl Groups Total carbonyl groups 20/100C9 in spruce 1/2 Conjugated Structures Coniferaldehyde and a-keto structures Play important role in delignifcation reactions 1/2 Non-conjugated Structures Glyceraldehyde from b-1 coupling Larger amount in certain hardwoods and grasses due to esters. PSE 406 Lecture 11

Lignin Structure Sakakibara This structure was generated by Sakakibara over 20 years ago. He developed it by compiling linkage and functional group information obtained by a variety of researchers. Putting this information together into a structure is a time consuming and frustrating experience. Your intrepid lecturer did this himself 15 years ago generating Bob’s lignin. This structure is not included in these notes because of massive inaccuracies. If you go through this structure and compare the linkages to experimental numbers, you will see certain problems. For one thing, it is difficult to include all the linkages in a structure with 25 rings if the linkage frequency is less than 3-4%. For a second thing, it is known that lignin from different parts of the tree has different linkages and functional groups which this structure doesn’t address. Thirdly, this structure shows the possible linkages but not in any particular order. It is now known that there is sections of lignin that has multiple b-O-4 linkages. This structure takes none of this into account. There have been multiple other structures developed by other lignin chemists. Some examples of these structures are included in the appendix. Text PSE 406 Lecture 11

Lignin-Carbohydrate Complex All purified holocellulose materials contain a certain amount of lignin All purified lignin fractions contain a certain amount of monosaccharides LCCs have been enzymatically prepared from lignin and monosaccharide model compounds Significant work studying isolated LCCs No definitive information on exact covalent bonding patterns Generally accepted bonding patterns PSE 406 Lecture 11

Lignin-Carbohydrate Complex Proposed Linkages PSE 406 Lecture 11

Lignin-Carbohydrate Complex General Information Mw of isolated LCCs 600®15,000 LCC linkage stability Esters: alkali labile, acid labile Ethers: selectively alkali labile, mildly acid labile Glycosides: mildly alkali labile, acid labile Formation during pulping processes possible LCCs: residual lignin and bleaching Removal of that last little bit of lignin Enzyme assisted bleaching PSE 406 Lecture 11

Lignin Structure Analytical Procedures All analysis require model compound studies! Linkages Enzymatic dehydrogenation (test tube studies) Degradation studies (see appendix) NMR Functional groups Wet Chemistry techniques Spectroscopy PSE 406 Lecture 11

Lignin Trivial Facts I Solubility Molecular Weight Native lignins: limited/no solubility in all solvents without modification Molecular Weight Average Mw for softwood ~20,000, lower for hardwoods Polydispersity ~ 2.5-3.0 Mw measured for lignosulfonates as high as 1,000,000 PSE 406 Lecture 11

Lignin Trivial Facts II Elemental Composition MWL Spruce C9H7.92O2.40(OCH3)0.92 C9H8.83O2.37(OCH3)0.96 Beech C9H8.50O2.86(OCH3)1.43 C9H7.93O2.95(OCH3)1.46 UV Absorption Strong adsorption at 205 and 280nm Carbohydrates do not adsorb at 280nm Compression Wood (Softwoods) High % lignin (~40%), high % of r-hydroxy units (to 70%) Tension Wood (Hardwoods) Reduced lignin content PSE 406 Lecture 11

Lecture 11 Appendix PSE 406 Lecture 11

Lignin Structure Elucidation Studies Nitrobenzene Oxidation Text Nitrobenzene oxidation is one of oldest form of lignin analysis. The major product of the oxidation of softwoods is vanillin. With hardwoods, a mixture of syringaldehyde and vanillin is obtained. This procedure, therefore, gives an indication of the methoxyl content of the lignin. It was also an important procedure in early lignin studies as it gave another indication of the phenolic nature of lignin. An important point to note is that the total yield of vanillin (by far the major product) from this procedure is at the most 25%. Therefore, the majority of phenolic units do not react. PSE 406 Lecture 11

Lignin Structure Elucidation Studies Permanganate Oxidation The permanganate oxidation procedure has been used to determine the the abundance of most of the lignin linkages. The procedure is as follows: A lignin preparation is either methylated or ethylated with dimethyl or diethyl sulfate over a 3 day period at a pH of 11.0 +/- 01. Free and only free phenolic hydroxyl groups are alkylated. The derivitized lignin is reacted with potassium permanganate which chews up the lignin destroying most of the lignin linkages and rings. When the oxidation reaches a derivitized ring, the oxidation stops leaving an aromatic carboxylic acid structure. Quantification of these structures gives information on linkages. Dimeric structures and dicarboxylic acid monomeric structures allow the quantification of 5-5, 4-O-5, b-5, a-6, and other units. It is not possible to quantify b-O-4 linkages using this procedure because other linkages such as b-1 give the same product. This procedure will also give information on the syringyl/guaicyl ratio, the number of catechol functionalities, and the number of free phenolic hydroxyls. A problem with this analysis is that only those groups that originally contain a free phenolic hydroxyl group can be determined through this procedure. To get around this problem, lignin is often oxidized first with copper oxide to cleave b-O-4 linkages and increase the number of free phenolic hydroxyls. Text PSE 406 Lecture 11

Lignin Structure Elucidation Studies Acidolysis Acidolysis and thioaidolysis procedures are used to quantify the number of b-O-4 linkages in lignin. The product of acidolysis are a number of aromatic compounds known as Hibbert’s ketones. By running model compound studies, it is possible to determine the structures which will produce specific ketones under these reaction conditions. Like permanganate oxidation, this procedure is very time consuming. Text PSE 406 Lecture 11

Lignin Structure: Adler PSE 406 Lecture 11

Lignin Structure: Freudenberg PSE 406 Lecture 11

Lignin Structure: Nimz PSE 406 Lecture 11

Lignin Structure: Forss PSE 406 Lecture 11