Antimony-Based Catalysts

Research and Application of Antimony-Based Catalysts

Polyester (PET) fiber is the largest variety of synthetic fiber. Clothing made of polyester fiber is comfortable, crisp, easy to wash, and quick to dry. Polyester is also widely used as a raw material for packaging, industrial yarns, and engineering plastics. As a result, polyester has developed rapidly worldwide, increasing at an average annual rate of 7% and with a large output.

Polyester production can be divided into dimethyl terephthalate (DMT) route and terephthalic acid (PTA) route in terms of process route and can be divided into intermittent process and continuous process in terms of operation. Regardless of the production process route adopted, the polycondensation reaction requires the use of metal compounds as catalysts. The polycondensation reaction is a key step in the polyester production process, and the polycondensation time is the bottleneck for improving the yield. The improvement of the catalyst system is an important factor in improving the quality of polyester and shortening the polycondensation time.

UrbanMines Tech. Limited is a leading Chinese company specializing in the R&D, production, and supply of polyester catalyst-grade antimony trioxide, antimony acetate, and antimony glycol. We have conducted in-depth research on these products—the R&D department of UrbanMines now summarizes the research and application of antimony catalysts in this article to help our customers flexibly apply, optimize production processes, and provide comprehensive competitiveness of polyester fiber products.

Domestic and foreign scholars generally believe that polyester polycondensation is a chain extension reaction, and the catalytic mechanism belongs to chelation coordination, which requires the catalyst metal atom to provide empty orbitals to coordinate with the arc pair of electrons of carbonyl oxygen to achieve the purpose of catalysis. For polycondensation, since the electron cloud density of carbonyl oxygen in the hydroxyethyl ester group is relatively low, the electronegativity of metal ions is relatively high during coordination, to facilitate coordination and chain extension.

The following can be used as polyester catalysts: Li, Na, K, Be, Mg, Ca, Sr, B, Al, Ga, Ge, Sn, Pb, Sb, Bi, Ti, Nb, Cr, Mo, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Zn, Cd, Hg and other metal oxides, alcoholates, carboxylates, borates, halides and amines, ureas, guanidines, sulfur-containing organic compounds. However, the catalysts that are currently used and studied in industrial production are mainly Sb, Ge, and Ti series compounds. A large number of studies have shown that: Ge-based catalysts have fewer side reactions and produce high-quality PET, but their activity is not high, and they have few resources and are expensive; Ti-based catalysts have high activity and fast reaction speed, but their catalytic side reactions are more obvious, resulting in poor thermal stability and yellow color of the product, and they can generally only be used for the synthesis of PBT, PTT, PCT, etc.; Sb-based catalysts are not only more active. The product quality is high because Sb-based catalysts are more active, have fewer side reactions, and are cheaper. Therefore, they have been widely used. Among them, the most commonly used Sb-based catalysts are antimony trioxide (Sb₂O₃), antimony acetate (Sb(CH₃COO)₃), etc.

Looking at the development history of the polyester industry, we can find that more than 90% of the polyester plants in the world use antimony compounds as catalysts. By 2000, China had introduced several polyester plants, all of which used antimony compounds as catalysts, mainly Sb₂O₃ and Sb(CH₃COO)₃. Through the joint efforts of Chinese scientific research, universities, and production departments, these two catalysts have now been fully domestically produced.

Since 1999, French chemical company Elf has launched an antimony glycol [Sb₂ (OCH₂CH₂CO) ₃] catalyst as an upgraded product of traditional catalysts. The polyester chips produced have high whiteness and good spinnability, which has attracted great attention from domestic catalyst research institutions, enterprises, and polyester manufacturers in China.

I. Research and application of antimony trioxide

The United States is one of the earliest countries to produce and apply Sb₂O₃. In 1961, the consumption of Sb2O3 in the United States reached 4,943 tons. In the 1970s, five companies in Japan produced Sb₂O₃ with a total production capacity of 6,360 tons per year.

China's main Sb₂O₃ research and development units are mainly concentrated in former state-owned enterprises in Hunan Province and Shanghai. UrbanMines Tech. Limited also has established a professional production line in Hunan Province.

(I). Method for producing antimony trioxide

The manufacture of Sb₂O₃ usually uses antimony sulfide ore as raw material. Metal antimony is first prepared, and then Sb2O3 is produced using metal antimony as raw material.

There are two main methods for producing Sb₂O₃ from metallic antimony: direct oxidation and nitrogen decomposition.

1. Direct oxidation method

Metal antimony reacts with oxygen under heating to form Sb2O3. The reaction process is as follows:

4Sb＋3O₂＝＝2Sb₂O₃

2. Ammonolysis

Antimony metal reacts with chlorine to synthesize antimony trichloride, which is then distilled, hydrolyzed, ammonolyzed, washed, and dried to obtain the finished Sb2O3 product. The basic reaction equation is:

2Sb＋3Cl₂＝＝2SbCl₃

SbCl₃＋H₂O＝＝SbOCl＋2HCl

4SbOCl＋H₂O＝＝Sb₂O₃·2SbOCl＋2HCl

Sb₂O₃·2SbOCl＋OH＝＝2Sb₂O₃＋2NH₄Cl＋H₂O

(II). Uses of antimony trioxide 

The main use of antimony trioxide is as a catalyst for polymerase and a flame retardant for synthetic materials.

In the polyester industry, Sb₂O₃ was first used as a catalyst. Sb₂O₃ is mainly used as a polycondensation catalyst for the DMT route and the early PTA route and is generally used in combination with H₃PO₄ or its enzymes.

(III). Problems with antimony trioxide 

Sb₂O₃ has poor solubility in ethylene glycol, with a solubility of only 4.04% at 150°C. Therefore, when ethylene glycol is used to prepare the catalyst, Sb₂O₃ has poor dispersibility, which can easily cause excessive catalyst in the polymerization system, generate high-melting-point cyclic trimers, and bring difficulties to spinning. To improve the solubility and dispersibility of Sb₂O₃ in ethylene glycol, it is generally adopted to use excessive ethylene glycol or increase the dissolution temperature to above 150°C. However, above 120°C, Sb₂O₃ and ethylene glycol may produce ethylene glycol antimony precipitation when they act together for a long time, and Sb2O3 may be reduced to metallic antimony in the polycondensation reaction, which can cause "fog" in polyester chips and affect product quality.

II. Research and application of antimony acetate

Preparation method of antimony acetate

At first, antimony acetate was prepared by reacting antimony trioxide with acetic acid, and acetic anhydride was used as a dehydrating agent to absorb the water generated by the reaction. The quality of the finished product obtained by this method was not high, and it took more than 30 hours for antimony trioxide to dissolve in acetic acid. Later, antimony acetate was prepared by reacting metal antimony, antimony trichloride, or antimony trioxide with acetic anhydride, without the need for a dehydrating agent.

1. Antimony trichloride method

In 1947, H. Schmidt et al. in West Germany prepared Sb(CH₃COO)₃ by reacting SbCl₃ with acetic anhydride. The reaction formula is as follows:

SbCl₃+3(CH₃CO)₂O==Sb(CH₃COO)₃+3CH₃COCl

2. Antimony metal method

In 1954, TAPaybea of the former Soviet Union prepared Sb(CH₃COO)₃ by reacting metallic antimony and peroxyacetyl in a benzene solution. The reaction formula is:

Sb＋(CH₃COO)₂==Sb(CH₃COO)₃

3. Antimony trioxide method

In 1957, F. Nerdel of West Germany used Sb₂O₃ to react with acetic anhydride to produce Sb(CH₃COO)₃.

Sb₂O₃＋3（CH₃CO）₂O＝＝2Sb（CH₃COO）₃

The disadvantage of this method is that the crystals tend to aggregate into large pieces and stick firmly to the inner wall of the reactor, resulting in poor product quality and color.

4. Antimony trioxide solvent method

To overcome the shortcomings of the above method, a neutral solvent is usually added during the reaction of Sb₂O₃ and acetic anhydride. The specific preparation method is as follows:

(1) In 1968, R. Thoms of the American Mosun Chemical Company published a patent on the preparation of antimony acetate. The patent used xylene (o-, m-, p-xylene, or a mixture thereof) as a neutral solvent to produce fine crystals of antimony acetate.

(2) In 1973, the Czech Republic invented a method for producing fine antimony acetate using toluene as a solvent.

III. Comparison of three antimony-based catalysts