How does corn stover make polylactic acid?
Polylactic acid PLA has excellent degradation properties and is considered an environmentally friendly material. Currently, the popular degradable straws, tableware, and shopping bags on the market all contain polylactic acid PLA. However, the production capacity of polylactic acid PLA has been hindering its large-scale application, and the difficulty in the preparation of lactide, the intermediate synthetic raw material, indirectly increases the production cost of polylactic acid.
At present, the raw materials for producing polylactic acid PLA mainly include corn and cassava. In order to solve the problem of raw material sources, domestic polylactic acid production enterprises such as Jindan Technology even directly plant corn on a large scale to ensure the supply of polylactic acid.
However, many people have suggested that it is too "luxury" to use grain corn as the raw material for the production of polylactic acid PLA. If agricultural waste such as corn stalks can be used to produce polylactic acid. It not only saves food, but also realizes the utilization of waste, which kills two birds with one stone.
So what are the difficulties in the process of making polylactic acid from corn stalks, and what is the process?
Why can corn stalks produce polylactic acid?
The traditional process is to use starch from crops such as corn and cassava as raw materials for production. The starch is saccharified to obtain glucose, and then fermented by bacteria to produce high-purity lactic acid. The obtained lactic acid is synthesized into polylactic acid by direct polycondensation method or indirect polycondensation method (that is, ring-opening polymerization method of lactide), and the indirect polycondensation method is the most widely used in industry at present.
The main components of corn stalks are cellulose (32% to 36%), hemicellulose (35% to 40%) and lignin (17% to 20%). Cellulose is a macromolecule composed of glucose. Polysaccharide cellulose and hemicellulose can be processed to obtain fermentation raw materials—reducing sugars, which can not only replace the scarce traditional food resources, but also solve the pollution of straw burning problems and make full use of renewable resources.
Three steps in the production of lactic acid from straw
The main steps of fermentation of straw and other agricultural wastes to produce lactic acid are: pretreatment of plant fiber raw materials → hydrolysis of plant fiber raw materials → bacterial consumption of sugar to produce lactic acid → lactic acid crystallization and purification.
Preprocessing:
The natural plant cellulose raw materials that exist in nature are mainly composed of cellulose, hemicellulose and lignin, and their structure is very complex. Therefore, due to its own structure and the protective effect of lignin and hemicellulose on cellulose, it is generally difficult to hydrolyze and ferment cellulose. The purpose of pretreatment is to remove lignin and hemicellulose, reduce the crystallinity of cellulose, and increase the porosity of raw materials.
Pretreatment methods mainly include: physical method (such as mechanical crushing, pyrolysis), chemical method (acid treatment, alkali treatment, ozone method), biological method (using microorganisms to decompose lignin) and comprehensive method (such as carbon dioxide blasting method). Pretreatment of straw can effectively promote the production of sugar and improve the efficiency of enzymatic hydrolysis to generate sugar; effectively reduce the degradation and loss of sugar, which is one of the key steps in the production of lactic acid from corn straw.
Hydrolysis of lignocellulosic raw materials:
Commonly used catalysts for lignocellulosic raw materials are inorganic acids and enzymes. Concentrated sulfuric acid hydrolysis is carried out under normal temperature and pressure, and the sugar yield is relatively high. Enzymatic hydrolysis has the advantages of relatively single product and high sugar concentration, and is currently the main hydrolysis method commonly used. Due to the complex structure of lignocellulosic raw materials, in order to make the enzyme and cellulose contact effectively, lignocellulosic raw materials should be pretreated before enzymatic hydrolysis.
Enzyme reactions convert cellulose into sugars using specific processing enzymes. Cellulase works best when the temperature is 40°C~60°C and the pH value is 4~5. Some researchers used ion beam mutagenesis to select and bred Aspergillus niger, a high-yielding xylanase strain, to convert biomass stalks into L-lactic acid that can be used in industrial production. The saccharification rate of fiber material reached 79.5%.
Concentrated acid hydrolysis is carried out at a lower temperature. Generally, 72% sulfuric acid, 42% hydrochloric acid, and 77%~83% phosphoric acid solutions can dissolve crystalline cellulose, and the cellulose is also homogeneously hydrolyzed during the dissolution process. Hydrolysis of concentrated sulfuric acid can convert about 90% of hemicellulose and cellulose into fermentable sugar, and the recovery rate of sugar is also very high.
Production of lactic acid by fermentation:
There are direct fermentation method and simultaneous saccharification and fermentation method in fermentative preparation method of lactic acid.
The direct fermentation method is to directly ferment the lignocellulosic raw material without acid hydrolysis or enzymatic hydrolysis pretreatment. The yield of lactic acid during direct fermentation is low, and some by-products are easily formed. Experiments by some researchers have shown that the yield of direct fermentation is low. If the cellulose waste is pretreated with 0.3mol/L dilute phosphoric acid, and then enzymatically hydrolyzed with cellulase for 60 hours, the saccharification solution is fermented by Rhizopus oryzae, and the conversion of lactic acid The rate can reach about 20%.
The fermentation method of enzymatic hydrolysis and lactic acid fermentation of cellulose raw materials is a simultaneous saccharification fermentation method. The enzymatic hydrolysis product provides a carbon source for fermentation, and the five-carbon sugar and six-carbon sugar decomposed by cellulose can be consumed during fermentation, which can improve the overall Productivity. Although this method has higher requirements on strains, it shortens the production cycle, saves energy, and improves the utilization rate of strains.
