Birch bark suberin: A natural source of multifunctional chemicals

The European pulp mills produce large amounts of bark as a by-product, about 4.3 Mt/y. Additional amounts are produced at sawmills. Birch outer bark contains about 35% of suberin. Suberin - a natural polyester - can be easily separated and depolymerized leading to the corresponding fatty acids, which are mostly straight chain, even numbered C16-C24 hydroxy acids and diacids, with epoxy, hydroxy or olefinic functionalities in the middle of the chain. The dominant suberin monomer is 9,10-epoxy-18-hydroxyoctadecanoic acid. Suberin fatty acids provide a new potential starting material for various products, the driving force being to the replacement of polymers/materials obtained from petrochemical feedstock. At first, suberin from birch outer bark was degraded to the corresponding fatty acids by alkaline hydrolysis (NaOH/i-PrOH). A thorough analysis of suberin fatty acid mixtures was carried out by 1H and 13C NMR techniques. A variety of suberin intermediates were synthesized with good yield and conversion state by acylating a mixture of suberin acids in the presence of various carboxylic acid anhydrides. For example maleic, succinic, phthalic and acetic anhydrides were used as a reagent for suberin-based intermediates. Most of the syntheses were done in a melted state without solvents and additional catalysts. The intermediates were used further to prepare the corresponding esters with 2-ethylhexanol. Another class of products, methoxycarboxylic acid 2-ethylhexyl esters of suberin were prepared via the in situ transesterification reaction of methoxycarboxylic acid methyl esters with 2-ethylhexanol. These intermediates were prepared from hydroxy acid sodium salt of suberin by treating it with dimethyl sulfoxide. Structural characterization of the product mixtures was carried out by 1H and 13C NMR techniques. The boiling ranges and viscosity indices of suberin 2-ethylhexyl maleic esters and methoxycarbocylic acid 2-ethylhexyl esters were measured. The results met the requirements of lubricant properties, and the application research is in progress. The main interest in these suberin-derived materials resides in the fact that they are obtained from a renewable by-product, which is normally used by combustion for energy recovery. As a versatile chemical source, suberin is a potential starting material for various value-added products, such as environmentally benign polymers, lubricants, plasticizers, surfactants and for fine chemicals.