Environment-friendly technical scheme for producing polylactic acid through one-step catalytic polymerization of lactic acid

Abstract

In the existing polylactic acid production, lactide is firstly prepared from lactic acid or purchased, and then lactide is subjected to ring-opening polymerization by using a relatively expensive catalyst such as metal zinc elementary substance, so that polylactic acid is prepared in two steps. According to the scheme, food-grade zinc acetate dihydrate is used as a catalyst for catalyzing lactic acid to accelerate dehydration condensation reaction, and lactic acid obtained by fermenting lactic acid bacteria after saccharification of starch is subjected to one-step catalysis is heated, stirred and polycondensed at a proper temperature to prepare polylactic acid. The whole process is free of disordered interference reaction, and all reactant products belong to food grade, are non-toxic and harmless, and are ecological and environment-friendly. Complex and expensive instruments, equipment and reagents are not needed. Zinc acetate dihydrate is easily dissolved in a lactic acid aqueous solution, ionized zinc ions have two valence states of + 1 valence and + 2 valence, an empty electron orbital and a unique Ar 3d10 electronic structure and have strong electronegativity, the + 1 valence zinc ions lose or obtain one electron, the surfaces of the zinc ions have rich catalytic active sites, and the catalytic activity of the zinc ions is improved. In an acid environment of a lactic acid solution, zinc ions catalyze hydroxyl carboxyl contained in lactic acid molecules to accelerate transition loss of electrons with different probabilities and probabilities or shift deflection of direction, angle and the like of shared electrons, so that water molecules are formed and removed, and dehydration condensation of hydroxyl carboxyl in lactic acid molecules and between lactic acid molecules is continuously catalyzed; and carrying out polycondensation on the oligomeric lactic acid to obtain polylactic acid. Meanwhile, a small amount of intermediate lactide formed by heating and catalyzing is also subjected to ring-opening polymerization reaction to form a linear polylactic acid chain.

Description

Technical Field

[0001] Environmentally friendly materials technology; catalyst technology. Background Technology

[0002] In existing polylactic acid (PLA) production processes, the most common method is to first obtain lactide from lactic acid, and then use relatively expensive catalysts, including elemental zinc, to perform ring-opening polymerization of the lactide, thus producing PLA in two steps. This proposed method uses food-grade zinc acetate dihydrate (Zn(CH3OO)2•2H2O) as a catalyst to accelerate the dehydration condensation reaction of lactic acid molecules. It can catalyze the condensation polymerization of lactic acid obtained from starch saccharification and subsequent lactic acid bacteria fermentation in a single step at a suitable temperature (between 80 and 100 degrees Celsius under normal pressure) to produce PLA. The entire reaction process is free from disruptive reactions, and all reactants and products are food-grade, non-toxic, harmless, and environmentally friendly. It eliminates the need for complex and expensive equipment and reagents, resulting in low overall cost. Summary of the Invention

[0003] Take 1 kg of starch at room temperature and pressure, gelatinize it with water, add sufficient colorless salivary amylase (to prevent the use of colored amylase from causing the polylactic acid to be colored and opaque, thus making agricultural film production impossible), heat the aqueous solution to between 35 and 37 degrees Celsius, pause heating and maintain the temperature to allow it to saccharify into glucose, then reheat to 45 degrees Celsius and maintain the temperature, add no less than sufficient live lactic acid fermentation bacteria to seal fermentation and obtain lactic acid, then heat the lactic acid solution to between 80 and 100 degrees Celsius (this heating temperature range is the most suitable catalytic polymerization temperature of zinc acetate dihydrate under normal pressure), add at least 1% by weight of food-grade zinc acetate dihydrate. 30 grams of zinc acetate (chemical formula Zn(CH3OO)2•2H2O) were stirred continuously in a clockwise or counterclockwise direction using a stirrer (in my experiment, I used chopsticks to continuously stir in both clockwise and counterclockwise directions in the inner pot of a rice cooker). After several minutes, a semi-fluid, viscous liquid phase of polylactic acid was obtained under heating and heat preservation conditions. After natural cooling at room temperature and pressure, it became a solid phase with a relatively hard texture, weighing 0.65 kg. Subtracting the weight of the catalyst, zinc acetate dihydrate, which remained constant due to mass conservation, a total of 0.62 kg of polylactic acid was obtained, with a yield of 62%, which is not low compared to the current yield of 50% to 70% in the mass production of industrial-grade polylactic acid. The added zinc acetate dihydrate powder has better solubility than anhydrous zinc acetate, and it easily dissolves completely in lactic acid solution at 80 to 100 degrees Celsius when stirred. Zinc acetate dihydrate is a strong electrolyte, and it is more likely to fully ionize into zinc ions and acetate ions than other zinc salts. Under continuous stirring and suitable working temperature conditions for catalytic polymerization, it can fully contact and combine with the substrate, and play a better role in catalyzing the dehydration and condensation of lactic acid molecules, and then the condensation polymerization reaction. This enables the production of polylactic acid at low cost, with widely available materials and in an environmentally friendly manner (the online retail price of each kilogram of domestic food-grade zinc acetate dihydrate is no more than RMB 20).

[0004] The reaction mechanism is as follows: Zinc acetate dihydrate dissolves in lactic acid aqueous solution, and the ionized zinc ions have two oxidation states, +1 and +2, as well as empty electron orbitals and unique [Ar]3d electron orbitals. 10 Due to its electronic structure, zinc ions possess strong electronegativity (the ability to gain electrons) and, being in the +1 valence state, can simultaneously gain or lose an electron. Furthermore, the zinc ion surface has abundant catalytically active sites. In the acidic environment of lactic acid aqueous solution, zinc ions can catalyze the acceleration of various electron transitions (gain / loss or deflection of shared electron pairs) between the hydroxyl (-[OH]) and carboxyl (-COOH) groups in the lactic acid molecule, resulting in electron transitions with different probabilities and probability densities. This facilitates the catalysis of the hydrogen ions (H+) generated by the ionization of each carboxyl-hydroxyl group in the lactic acid molecule. + ) and hydroxide ions (OH-) - Through the active movement of electrons and the deflection of electron pairs, the attraction between cations and anions due to their different charges promotes the formation and removal of water molecules. This process continuously catalyzes and promotes the dehydration condensation reaction between the hydroxyl and carboxyl groups within and between lactic acid molecules, thereby causing the oligomeric lactic acid mixture to continuously condense into polylactic acid. Meanwhile, a small amount of lactide molecules (cyclic esterified organic compounds, distinct from finished polylactic acid, and soft under normal temperature and pressure) formed in the substrate due to heating and catalysis are also catalyzed by zinc acetate dihydrate: under heating and acidic catalytic conditions, some lactic acid molecules in the substrate undergo dehydration esterification to form a small amount of lactide molecules with a cyclic ester structure. The epoxy groups in the cyclic ester structure of lactide undergo a ring-opening polymerization reaction with a coordination insertion mechanism under the catalysis of zinc acetate dihydrate (first, the lactide monomer coordinates with the zinc cation, then the acetate anion nucleophilically adds to the carbonyl carbon of the monomer, the carbonyl double bond opens and inserts into the chemical bond of zinc acetate, and finally the monomer acyl oxygen bond breaks to undergo ring-opening polymerization), forming a linear polylactic acid chain.

[0005] By utilizing the zinc acetate catalytic mechanism described above, polylactic acid (PLA) can be produced by one-step catalytic polycondensation of lactic acid. This replaces the existing industrial-grade PLA mass production technology, which mostly requires two steps: first, all lactic acid obtained from starch fermentation is converted into the intermediate lactide, or expensive lactide is purchased in stock, and then the lactide is catalytically ring-opened to produce PLA in the second step. This high-cost traditional production method saves costs and improves efficiency.

[0006] Compared to currently used elemental zinc (which is expensive and, due to its single valence state of 0, can only lose electrons and cannot gain them, thus its catalytic efficiency in terms of electron transitions is far inferior to zinc acetate dihydrate), other copper and aluminum salts (although copper and aluminum ions have good electronegativity, they are polluting and toxic), and other zinc salts (which are less ionized than zinc acetate dihydrate, being strong electrolytes that ionize more easily), food-grade zinc acetate dihydrate catalyzes the polymerization of lactic acid, offering advantages of high quality, low cost, and environmental friendliness. Furthermore, the entire catalytic polymerization process is free from interference from other random reactions, resulting in pure, unadulterated polylactic acid. Polylactic acid can be directly returned to the field and, after approximately 60 days, can completely and naturally degrade into CO2. 2、 H2O is pollution-free, eco-friendly, and environmentally friendly.

[0007] The catalyst, zinc acetate dihydrate, has a constant mass and remains in polylactic acid. After being used in products such as polylactic acid agricultural films and returned to the field, it is completely degraded without pollution and can also supplement the soil with zinc trace elements needed for crop growth.

Claims

1. Heat the lactic acid solution to between 80°C and 100°C (this heating temperature range is the most suitable catalytic polymerization temperature for zinc acetate dihydrate under normal pressure). Add 30 grams of sufficient food-grade zinc acetate dihydrate (chemical formula Zn(CH3OO)2•2H2O) (at least 1% of the total substrate mass). Stir continuously in a clockwise or counterclockwise direction using a stirrer (in my experiment, I used chopsticks to continuously stir in both clockwise and counterclockwise directions in the inner pot of a rice cooker). After several minutes, a semi-fluid, viscous liquid phase of polylactic acid was obtained under heated and kept warm conditions. After natural cooling at room temperature and pressure, it became a solid phase with a relatively hard texture, weighing 0.65 kg. Subtracting the 30 grams of zinc acetate dihydrate catalyst, which has a constant mass, a total of 0.62 kg of polylactic acid was obtained, with a yield of 62%, which is not low compared to the current yield of 50% to 70% in the mass production of industrial-grade polylactic acid. The added zinc acetate dihydrate powder has better solubility than anhydrous zinc acetate, and easily dissolves completely in lactic acid solution at 80-100 degrees Celsius when stirred. Zinc acetate dihydrate is a strong electrolyte, and it ionizes more readily than other zinc salts to release zinc ions and acetate ions. Under continuous stirring and suitable catalytic polymerization temperature conditions, it fully contacts and combines with the substrate, effectively catalyzing the dehydration and condensation reaction of lactic acid molecules, thus achieving low-cost, readily available, and environmentally friendly production of polylactic acid (the online retail price of domestic food-grade zinc acetate dihydrate is no more than twenty yuan per kilogram). Zinc acetate dihydrate dissolves in lactic acid aqueous solution, and the ionized zinc ions have two valence states (+1 and +2), empty electron orbitals, and a unique [Ar]3d... 10 Due to its electronic structure, zinc ions possess strong electronegativity (the ability to gain electrons) and, being in the +1 valence state, can simultaneously gain or lose an electron. Furthermore, the zinc ion surface has abundant catalytically active sites. In the acidic environment of lactic acid aqueous solution, zinc ions can catalyze the acceleration of various electron transitions (gain / loss or deflection of shared electron pairs) between the hydroxyl (-[OH]) and carboxyl (-COOH) groups in the lactic acid molecule, resulting in electron transitions with different probabilities and probability densities. This facilitates the catalysis of the hydrogen ions (H+) generated by the ionization of each carboxyl-hydroxyl group in the lactic acid molecule. + ) and hydroxide ions (OH-) - Through the active movement of electrons and the deflection of electron pairs, the attraction between cations and anions due to their different charges promotes the formation and removal of water molecules. This process continuously catalyzes and promotes the dehydration condensation reaction between the hydroxyl and carboxyl groups within and between lactic acid molecules, thereby causing the oligomeric lactic acid mixture to continuously condense into polylactic acid. Meanwhile, a small amount of lactide molecules (cyclic esterified organic compounds, distinct from finished polylactic acid, and soft under normal temperature and pressure) formed in the substrate due to heating and catalysis are also catalyzed by zinc acetate dihydrate: under heating and acidic catalytic conditions, some lactic acid molecules in the substrate lactic acid undergo esterification to form a small amount of lactide molecules with a cyclic ester structure. The epoxy groups in the cyclic ester structure of lactide undergo a ring-opening polymerization reaction with a coordination insertion mechanism under the catalysis of zinc acetate dihydrate (first, the lactide monomer coordinates with the zinc cation, then the acetate anion nucleophilically adds to the carbonyl carbon of the monomer, the carbonyl double bond opens and inserts into the chemical bond of zinc acetate, and finally the monomer acyl oxygen bond breaks to undergo ring-opening polymerization), forming a linear polylactic acid chain.

2. Utilizing the aforementioned zinc acetate catalytic mechanism, polylactic acid (PLA) can be produced from lactic acid through a one-step catalytic polymerization process. This replaces the existing industrial-grade PLA mass production technology, which typically involves two steps: first, all lactic acid obtained from starch fermentation is converted into the intermediate lactide, or expensive lactide is purchased in stock, and then the lactide is catalytically ring-opened to produce PLA in the second step. This high-cost traditional production method saves costs and improves efficiency. Compared to the currently used elemental zinc (which is expensive and has only one oxidation state, 0, meaning it can only lose electrons and cannot gain them, thus its catalytic efficiency based on electron movement and transitions is far inferior to zinc acetate dihydrate), other copper and aluminum salts (although copper and aluminum ions have good electronegativity, they are polluting and toxic), and other zinc salts (which are less ionized than zinc acetate dihydrate, being a strong electrolyte and more easily ionized), food-grade zinc acetate dihydrate catalytic polymerization of lactic acid has the advantages of high quality, low price, and environmental friendliness. Furthermore, the entire catalytic polymerization process is free from other disorderly reactions, resulting in pure PLA without any impurities. The catalyst, zinc acetate dihydrate, retains its own mass unchanged and remains in polylactic acid. When used in products such as polylactic acid agricultural films, it is returned to the field without pollution. It is completely degradable, environmentally friendly, and can also supplement the zinc trace element needed for crop growth in the soil.