A novel Brönsted–Lewis acidic heteropoly organic–inorganic salt: preparation and catalysis for rosin dimerization

A novel Brönsted–Lewis acidic heteropoly organic–inorganic salt has been prepared via the replacement of protons in neat phosphotungstic acid with both organic and metal cations. This hybrid catalyst, Sm0.33[TEAPS]2PW12O40, exhibited satisfactory performance in the dimerization of rosin to prepare polymerized rosin Under optimum conditions (15.0 g rosin and 5.0 g Sm0.33[TEAPS]2PW12O40 catalyst in 18.0 mL toluene at 90 °C for 10 h), a polymerized rosin product with a softening point of 120.1 °C was obtained. In addition, the Sm0.33[TEAPS]2PW12O40 catalyst maintains excellent catalytic performance over five recycles.


Background
Polymerized rosin has a higher softening point, lighter color, and better stability than rosin, and is harder to oxidize. It is a key ingredient in oil paints, printing ink, adhesives, perfume, and more (Cheng et al. 1996;Chen 1992). The industrial preparation of polymerized rosin, employing aqueous mineral acids such as H 2 SO 4 or ZnCl 2 /HCl, suffers from various shortcomings, including corrosion, pollution, and difficult recovery. Some environmentally friendly catalysts, such as solid superacids (Luo and Wu 1999;Gao et al. 2007), have been used to realize the clean polymerization of rosin. However, despite their superior separation, solid superacids exhibit insufficient recycling performance due to their uneven and vulnerable active components.
Acid-functionalized ionic liquids, a class of catalyst with the advantages of both aqueous and solid acids, have been applied successfully in many acid-catalyzed reactions (Paun et al. 2008;Fang et al. 2008;Hoang et al. 2005), including the polymerization of rosin (Liu et al. 2005(Liu et al. , 2008. Abietic-type resin acids in gum rosin, which contain conjugated double bonds, undergo polymerization readily, but fir-type resin acids, having more steric hindrance, cannot. Brönsted acids are generally considered more apt to promote the isomerization of fir-type resin acids towards abietic-type resin acids (Scheme 1), while Lewis acids favor the dipolymerization of abietic-type resin acids (Scheme 2) (Cheng et al. 1996). As a result, acid catalysts comprising both Brönsted and Lewis acidity would exhibit a more outstanding catalytic performance in the preparation of polymerized rosin. We previously synthesized (3-sulfonic acid)-propyl-3-methylimidazoliume (and triethylammonium) chlorozincates and demonstrated their good catalytic efficiency and recycling performance in the polymerization of rosin, which can be attributed to the Brönsted-Lewis acidity of the ionic liquids and liquid-liquid two-phase process (Liu et al. 2008(Liu et al. , 2009). However, using these catalysts is inconvenient due to their lengthy synthetic cycles and high viscosity. Most importantly, the Lewis acidity of the metal chlorides in the anions would be difficult to maintain due to stability issues, similar to aluminum chloride acid salt ionic liquids.
For the past few years, types of heteropoly organic salt catalytic materials have called attention for their potential water tolerance, acidity and self-separation performance (Leng et al. 2009a(Leng et al. , b, 2012Li et al. 2011Li et al. , 2014Shimizu et al. 2009;Sun et al. 2012;Zhou et al. 2014). It has been found that heteropoly anions with high charge numbers Open Access *Correspondence: xiecongxia@126.com 1 State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Full list of author information is available at the end of the article in these materials lead to higher melting points than conventional ionic liquids (Yuan et al. 2014). Furthermore, based on the high charge numbers of heteropoly anions, sulfated organic cations with Brönsted acidity and metal cations with Lewis acidity can act together as counterions to heteropoly anions, establishing novel Brönsted-Lewis acidic heteropoly organic-inorganic salts (Yu ST 2013). Herein, we report a heteropoly organic-inorganic catalyst, Sm 0.33 [TEAPS] 2 PW 12 O 40 , with Brönsted-Lewis acidity, which has different performances for melting point, solubility and acidity with both heteropoly compounds and ionic liquids. Moreover, the dimerization of rosin catalyzed by Sm 0.33 [TEAPS] 2 PW 12 O 40 as a solid acid has been carried out to achieve an environmentally friendly process for polymerized rosin.

Materials and methods
Analytical grade H 3 PW 12 O 40 was dried at 180 °C. All other chemicals were of analytical grade and used without further purification. The 1 H-NMR spectra of the catalyst and intermediates were recorded with a 500 MHz Bruker spectrometer in D 2 O. FT-IR spectra for catalyst samples (the Py-IR sample was mixed with pyridine (2:1, v/v) for 24 h prior to measurement) on KBr discs were recorded on a Nicolet iS10 FT-IR instrument. Melting points were measured using a conventional method on an X-4 type micro melting point apparatus. TG analysis was performed with a NETZSCH-TG 209 F1 Libra instruments in dry N 2 at a heating rate of 20 °C/min from 30 to 800 °C.
The acidity of the prepared catalysts was determined by potentiometric titration (Shi and Pan 2008;Vazquez et al. 2000). A mixture containing the sample (0.5 g) and acetonitrile (30 mL) was mixed at the stable potential before being titrated with n-C 4 H 9 NH 2 solution (0.05 mol/L in acetonitrile). The initial and jump potential values were measured by a pH meter to identify the acid strength and total acid amount in catalyst samples.

Dimerization of rosin
In batch experiments, heteropoly organic-inorganic salt catalyst (5.0 g) was added to a round-bottomed flask contained toluene (18.0 mL) and dissolved rosin (15.0 g). The resulting reaction mixture was stirred vigorously at 90 °C for 10 h and then cooled to room temperature. The solid catalyst was removed by centrifugation and directly reused without further treatment. The reaction solution, from which the toluene solvent had been separated, was distilled under low pressure (2 mmHg) at 260-270 °C (system temperature) and 180-210 °C (steam outlet temperature) for 30 min to remove low softening point materials, such as rosinol and some unpolymerized rosin, and obtain the polymerized rosin product. The ring and ball softening points of the products were determined by SYD-2806G numerical control asphalt softening point tester.   Table 2 shows that a product with a similar softening point and acid value to gum rosin was obtained without catalyst. This was also observed in the reaction catalyzed by H 3 PW 12 O 40 , having high acid strength, indicating that Brönsted acidity derived from the antiprotons of H 3 PW 12 O 40 had almost no effect on the promotion of rosin dimerization under the solid acid catalyst conditions. In addition, the Brönsted acidity derived from sulfonic acid groups in the organic cations alone did not have the necessary catalytic ability for dimerization, resulting in a product with 103.7 °C softening point. When both Sm 3+ and organic cations containing sulfonic acid group [TEAPS] + were co-introduced to the structure of a heteropoly compound, Brönsted-Lewis double acidity (Fig. 2) and, consequently, a high catalytic performance were achieved (Table 2). In particular, a favorable polymerized rosin product with a softening point of 120.1 °C [higher than the result of Reference Liu et al. (2009) (Liu et al. 2008(Liu et al. , 2009 (Liu et al. 2008(Liu et al. , 2009 Table 3). The polymerized rosin products with high softening points (above 120 °C) were obtained even after reusing the catalyst five times, which was attributed to the concerted catalysis of Brönsted and Lewis acid sites and their covalent or ionic bonding pattern in the body of the catalyst in the solid-liquid catalytic system. The FT-IR spectrum of reused Sm 0.33 [TEAPS] 2 PW 12 O 40 , shown in Fig. 2, also indicates that no apparent structural change had taken place in the catalyst during use.

Conclusion
A novel heteropoly organic-inorganic salt with Brönsted-Lewis double acidity, Sm 0.33 [TEAPS] 2 PW 12 O 40 , was prepared via the replacement of protons in neat phosphotungstic acid with both organic cations containing sulfonic acid groups and metal Sm 3+ cations. As a solid acid catalyst, this environmentally benign Brönsted-Lewis double acidic hybrid enables an effective catalytic performance in the dimerization of rosin to afford polymerized rosin products with a softening point above 115 °C. Moreover, the catalyst also exhibited reasonable reuseability, demonstrated by a five-run recycling test.
Authors' contributions BY made the study desighs, did the data analysis, and drafted the manuscript. CXX, FLY, STY and JLZ participated in the design and coordination of the study and helped to draft the manuscript, XYY and XBC participated in the acquisition of data. All authors read and approved the final manuscript.