Synthesis method of alcohol ether carboxylates for oilfield enhanced oil recovery

in high-temperature, high-salinity oil reservoirs requires surfactants and polymers to possess excellent temperature and salt resistance. Therefore, alcohol ether carboxylates (AECs) with nonionic-anionic hydrophilic complex groups have broad application prospects and are widely used as oilfield emulsifiers, viscosity reducers, solubilizers, or dispersants. However, these complex surfactants only contain EO groups. In recent years, with ongoing research, researchers have found that introducing polyoxypropylene groups into conventional nonionic-anionic complex surfactant molecules can further improve the interfacial properties, emulsifying ability, water solubility, and salt resistance of the products. However, further research is needed to optimize the molecular structure and synthesis process of these products, as well as the structure-activity relationship.

Synthesis of alcohol ether carboxylates:

The main synthetic methods reported to date for alcohol ether carboxylates include carboxymethylation, nitrile radical catalytic oxidation, and noble metal catalytic oxidation. These methods are briefly introduced below:

(1) Carboxymethylation method

Carboxymethylation is the earliest developed method for synthesizing alcohol ether carboxylates and is also the main method for industrial production of alcohol ether carboxylates both domestically and internationally. It typically uses alcohol ethers as raw materials, reacting them with chloroacetic acid in the presence of NaOH to obtain the product. The reaction process generally involves two steps: the first step is the protonation of the alcohol ether, i.e., the reaction of NaOH with the alcohol ether to generate sodium alkoxide; the second step is the carboxymethylation reaction of sodium alkoxide with chloroacetic acid to generate the target product, the alcohol ether carboxylate. In this reaction, if reactants are added according to the stoichiometric ratio, the synthesis conversion rate is often unsatisfactory; therefore, excess chloroacetic acid and NaOH are usually added. However, this results in a higher content of inorganic salts in the product, requiring further desalination to reduce the inorganic salt content. Because the conditions for carboxymethylation are relatively mild and the synthesis process is relatively simple, it is not only less waste-generating than oxidation, propylene, and acrylate methods, but also more economically feasible.

(2) Nitrogen oxide radical catalytic oxidation method

In 1971, Rozautsev et al. proposed a method for synthesizing alcohol ether carboxylates using nitroxide radicals as catalysts. This method directly oxidizes the terminal hydroxymethyl group in the alcohol ether to a carboxyl group using nitroxide radicals generated by strong oxidizing acids such as nitric acid. Commonly used nitroxide radicals include 2,2,6,6-tetramethylpiperidine, and Fe³⁺ and Cu²⁺ are usually added as catalysts to accelerate the reaction. The conversion rate is high, theoretically reaching 95%. However, due to the high viscosity of the reaction system, a solvent is often required. The presence of nitroxide radicals limits the choice of solvent, and the solvent is difficult to remove after the reaction. Furthermore, the nitric acid used in this reaction easily produces harmful byproducts such as aldehydes, which are difficult to separate from the product using simple processes, increasing costs and complicating the production process. Therefore, the nitroxide radical catalytic oxidation method is currently not suitable for large-scale industrial production.

(3) Noble metal catalytic oxidation method

The noble metal catalytic oxidation method utilizes air or oxygen as an oxidant to oxidize the terminal hydroxymethyl group of an alcohol ether to a carboxyl group under the action of a noble metal catalyst such as palladium or platinum. The noble metal used in this reaction needs to have high catalytic activity and selectivity, such as palladium and platinum. Activated carbon is generally used as a support to improve catalytic efficiency and enable catalyst recovery. Since this reaction involves three phases (gas, solid, and liquid), the requirements for the reaction apparatus and reaction conditions are high. On the other hand, as the reaction proceeds, the introduction of air into the reaction system generates a significant amount of foam, increasing the difficulty of the reaction. Therefore, it is necessary to continuously vent the air, which inevitably carries away some reactants, causing environmental pollution.