Introduction

Tributyl citrate (TBC) is a non-toxic plasticizer with high thermal stability (Figure 1). TBC is widely used in medical appliances, toys, printing inks and coatings, biodegradable polymers, cosmetics and other fields due to its good compatibility, high plasticization efficiency, low volatility, light resistance, water resistance and cold resistance. Moreover, tributyl citrate is a kind of environmental friendly plasticizer because the materials used in its synthesis are available from renewable resources by fermentation processes. TBC plasticizer would be an entirely bio-renewable and sustainable option because the reaction of citric acid (CA) and butanol from renewable sources has been extensively studied. Recent biotechnology and bioprocessing advancements on the production of CA and nbutanol have been reported in the literature.[1] 

Disadvantages of traditional catalysts

The traditional catalysts used in the synthesis of tributyl citrate were mainly sulfuric acid, titanate and solid acid.[2-3] The sulfuric acid has many disadvantages such as formation additional by product, corrosion of equipment, complicated manufacturing process and environmental protection issues. The sulfuric acid has one of the important disadvantage that the sulfuric acid cannot be reused. The titanate also has the disadvantages of higher cost, difficult separation from products, and high energy consumption. The production process of tributyl citrate synthesis mainly included the complex processes such as reactive distillation, three stage batch system and continuous water removal. Therefore, there was an urgent need to develop environmentally friendly and economical processes. Recently, the solid acid catalysts have been applied in the synthesis of tributyl citrate such as ionic liquids, HZSM-5, SA/MCM-41 and USY. It is well known that heteropoly acids (HPAs) as solid acid has been applied widely in various reaction because HPAs as economically and environmentally solid acid have the advantages of acidity and redox properties. However, industrial use of HPA is limited because of its solubility in polar solvents and small surface area. Hence, when used in heterogeneous catalysis, there is a need to support HPA on suitable solid, which will improve dispersion, acidity and stability.[4] 

Research on supported polyoxometalates catalyst

Polyaniline (PANI) can be obtained easily by chemical or electrochemical polymerization of aniline in aqueous or non-aqueous media. PANI doped with heteropolyacids as the catalysis has been used in many researchers due to the surface of the active phase of PANI supported catalysts exhibits properties signicantly different from those of crystalline catalysts.[4]

Polyoxometalates (polyacids) are a kinds of metal-oxygen clusters formed by the condensation of acid salts of V, Nb, Ta, Mo, W. They have the advantages of diverse composition and structure, high thermal stability and adjustable acidity. They have become the green catalysts for the synthesis of tributyl citrate.[1]

Limin Wang etal.[4]prepared supported HPA/PANI catalysts, which possesses high active and stable solid acid catalysis, and then used them in the synthesis of TBC by esterification of citric acid and butanol to develop sustainable, industrial benign and environmentally friendly catalytic process. The reaction conditions were molar ratio of CA to n-butanol of 1:5, CA of 0.2 mol, reaction temperature of 170℃ and reaction time of 6 h.The conversion of CA found to be increased from 67 to 78% with increase PTA loading from 10% to 20%. The conversion of CA increased to 80% when PTA loading was further increased up to 25%. The overall trend of TBC selectivity obtained was 10% PTA < 20% PTA < 25% PTA which the selectivity of TBC increased with increase the PTA loading. In view to maximize CA conversion, TBC selectivity, the utilization of PTA and the economical of catalyst, the most favourable parameters for esterification were achieved by process optimization over 20% PTA/PANI catalyst because with increase in PTA loading from 20% to 25% on PANI the conversion of CA increased only from 78 to 80%. Therefore, 20% PTA/PANI catalyst was applied in later reactions, which exhibited excellent recyclability up to 5 successive cycles. It could be observed that the conversion of citric acid and selectivity of TBC both decrease with increase catalyst recycle number. The conversion of citric acid decreased from 74% to 63% and the selectivity of tributyl citrate decreased only from 43% to 39%.

Meili Huang etal.[5] prepared polyacid@surfactant phase transfer catalysts, polyacid@graphene oxide and polyacid@zeolite immobilized catalysts were by two-phase electrostatic embedding technology, organic covalent modification and electrostatic self-assembly technology. The following method is relatively better. With polyacid anion as the inorganic nucleus and cationic surfactant as the organic shell, a series of different feeding ratio phase transfer composite catalysts CTAB@PW₁₂ 1:0.5, CTAB@PW₁₂ 1:1 and CTAB@PW₁₂ 1:1.5 were prepared by water/chloroform two-phase electrostatic embedding technology (CTAB is cetyl trimethyl ammonium bromide）; Through the comparison of the catalytic activities of composite catalysts for the catalyzing synthesis of tributyl citrate, the order of catalytic activity was as follows: CTAB@PW₁₂ 1:1.5 > CTAB@PW₁₂ 1:1 > CTAB@PW₁₂ 1:0.5; TBC activity by the composite catalyst, the order of catalytic activity is: SBA-15/Aptes@P₂W₁₈ 1:12 > SBA-15/Aptes@P₂W₁₈ 1:6 > SBA-15/Aptes@P₂W₁₈ 1:3; And the factors affecting the catalyzing synthesis of TBC were investigated by single factor and orthogonal experiments, the optimal process conditions for the catalying synthesis of tributyl citrate by supported heteropolyacid catalysts were as follows: reaction temperature of 160 °C, catalyst dosage of 0.15 g, the alcohol/acid molar ratio of 4:1, and highest esterification rate of tributyl citrate can reach 91.4%. After 5 times of reuse, the esterification rate can reach 87.84%.

Choosing a kind of high efficient catalyst for synthesis of TBC with citric acid and n-butyl alcohol as raw materials is an important step. More relevant research is expected to improve the production of tributyl citrate.

References

[1]Wang Zq. Preparation of supported polyacid catalysts and catalyticsynthesis of tributyl Citrate[D]. Inner Mongolia Minzu University, 2022.DOI:10.27228/d.cnki.gnmmu.2022.000003.

[2]Li Y, Hu S. Acidic ionic liquid-catalyzed esterification of oleic acid for biodiesel synthesis[J]. Chinese Journal of Catalysis, 2014,35(3): 396-406.

[3]Xu J, Jiang J. Synthesis of Tributyl Citrate Using Solid Acid as a Catalyst [J]. Chemical Engineering Communications, 2010, 198(4): 474-482.

[4]Wang L, Ding B, Zhang M. Preparation supported heteropoly (acid)/polyaniline catalysts and catalytic synthesis of tributyl citrate. RSC Adv. 2019;9(57):33124-33129.

[5]Huang Ml.Catalysic synthesis of tributyl citrate employing POMs-based compound catalysts[D].  Inner Mongolia Minzu University, 2024.DOI:10.27228/d.cnki.gnmmu.2024.000041.