Oligomeric ricinoleic acid preparation promoted by an efficient and recoverable Brønsted acidic ionic liquid

Raw material from biomass and green preparation processes are the two key features for the development of green products. As a bio-lubricant in metalworking fluids, estolides of ricinoleic acid are considered as the promising substitute to mineral oil with a favorable viscosity and viscosity index. Thus, an efficient and sustainable synthesis protocol is urgently needed to make the product really green. In this work, an environment-friendly Brønsted acidic ionic liquid (IL) 1-butanesulfonic acid diazabicyclo[5.4.0]undec-7-ene dihydrogen phosphate ([HSO3-BDBU]H2PO4) was developed as the efficient catalyst for the production of oligomeric ricinoleic acid from ricinoleic acid under solvent-free conditions. The reaction parameters containing reaction temperature, vacuum degree, amount of catalyst and reaction time were optimized and it was found that the reaction under the conditions of 190 °C and 50 kPa with 15 wt % of the [HSO3-BDBU]H2PO4 related to ricinoleic acid can afford a qualified product with an acid value of 51 mg KOH/g (which corresponds to the oligomerization degree of 4) after 6 h. Furthermore, the acid value of the product can be adjusted by regulating the reaction time, implying this protocol can serve as a versatile method to prepare the products with different oligomerization degree and different applications. The other merit of this protocol is the facile product separation by stratification and decantation ascribed to the immiscibility of the product and catalyst at room temperature. It is also worth mentioning that the IL catalyst can be used at least for five cycles with high catalytic activity. As a result, the protocol based on the IL catalyst, i.e. [HSO3-BDBU]H2PO4 shows great potential in industrial production of oligomeric ricinoleic acid from ricinoleic acid.


Introduction
In recent years, the biomass has attracted much attention due to its abundance, renewability and potential of conversion to useful chemicals. It can't be denied that the diverse transformation and utilization of biomass provides an alternative avenue to liberate us from the reliance on petroleum resource [1]. Nowadays, fuels, fine chemicals and functional molecules/materials can be derived from biomass such as lignocellulose and plant oils [2,3], wherein the plant oils play an important role in the polymer industry [4]. Especially, the characteristics of nonvolatility and biodegradability make plant oil the most promising material to develop functional polymeric materials with superior performance [5][6][7].
Castor oil is one kind of nonedible oil and can be extracted from the seeds of the castor bean plant. Globally, around one million tons of castor seeds are produced every year and the leading producing areas are India, China, and Brazil [8]. The castor oil has long been used as purgative or laxative to counter constipation and nowadays it is used commercially as a highly renewable resource for the chemical industry [9,10]. Numerous platform chemicals such as ricinoleic acid and undecylenic acid can be prepared from castor oil, wherein ricinoleic acid is a crucial platform chemical for derivation of useful chemicals [11][12][13][14].
Ricinoleic acid can be easily prepared by hydrolysis of castor oil [15]. The presence of both hydroxy and carboxy groups in the molecule of ricinoleic acid enables it to undergo intermolecular esterification, thus resulting in the formation of the oligomeric ricinoleic acid. The oligomeric ricinoleic acid with different acid value (which is an indirect index for oligomerization degree) has different applications. For example, as an additive in shampoos, the oligomeric ricinoleic acid with an acid value of 60-90 mg KOH/g is required while for cosmetic formulations, an acid value of 20-40 mg KOH/g is suitable [16][17][18][19]. The oligomeric ricinoleic acid can also be used as lubricant for metal cutting oils due to its appropriate viscosity, good adsorptivity and film formation ability on metal surfaces. Furthermore, the biodegradability of these estolides in the environment makes them attractive as green products.
In parallel with the increasing demand for high-quality oligomeric ricinoleic acid, the synthetic method has kept on developing. Traditionally, p-toluenesulfonic or sulfuric acid are used as catalysts for the preparation of oligomeric ricinoleic acid. However, the equipment corrosion and the tedious workup process are inevitable, which reduce the process efficiency. Moreover, the byproduct formation and the resulting product coloration reduce the product quality. To address the above issues, the enzyme catalysis is proposed accordingly. Nevertheless, the high cost, low efficiency and operational unstability of enzymatic reaction make it difficult to industrial production [20][21][22][23][24][25]. Very recently, tin(II) 2-ethylhexanoate has been reported as an efficient catalyst for the synthesis of oligomeric ricinoleic acid [19]. Unfortunately, the separation of oligomeric ricinoleic acid and the recovery of the catalyst still encounters difficulties. Therefore, it is urgently desirable to develop an efficient, green and recyclable catalyst and design simple operating procedures for the preparation of estolides oligomeric ricinoleic acid.
Ionic liquids (ILs) have been proved to be efficient catalysts in various organic syntheses due to the designable structure, tunable properties as well as superior solubility [26,27]. Furthermore, the thermal stability and negligible vapor pressure of ILs can facilitate the product separation after reaction. To the best of our knowledge, the IL 1-butylsulfonic-3methylimidazolium tosylate ([HSO 3 -BMim]TS) can be used as catalyst in the synthesis of oligomeric ricinoleic acid, the estolide with an acid value of 48 ± 2.5 mg KOH/g was obtained after 14 h [28]. Nevertheless, the product separation and catalyst reusability have not yet been investigated until now.
Based on the fact that Brønsted acids present excellent catalytic activity for this intermolecular esterification reaction and Brønsted acidic ionic liquids have been successfully used as catalyst in organic syntheses [29][30][31], we designed a series of Brønsted acidic ionic liquids and applied them as catalysts for the preparation of oligomeric ricinoleic acid from ricinoleic acid to develop a facile synthesis and separation protocol. Gratifyingly, the IL [HSO 3 -BDBU]H 2 PO 4 showed to be efficient both in synthesis and product separation [32]. Through process parameter selection, it was found that a product with different acid value can be obtained by changing the reaction time at 190 °C and 50 kPa with 15 wt % IL as catalyst. The viscosity characterization showed the product derived from 6 h of reaction, whose acid value is 51 mg KOH/g and the corresponding average oligomerization degree is 4, meets the requirement of lubricant additive index. Notably, after reaction, the product and IL catalyst can be easily separated by phase separation and the recovered catalyst can be used at least for five cycles without obvious deactivation, which shows tremendous advantage for large-scale industrial application. The reaction and separation procedures are depicted in Scheme 1.

Results and Discussion
Considering protic acids have catalytic effects on the esterification reaction, we designed and synthesized a series of ILs containing protic anions as shown in Scheme 2. With the synthesized ILs as catalyst, the ricinoleic acid esterification was performed and the catalytic activity of these functional ILs was evaluated with the acid value of the product as it is a convenient index to monitor the degree of oligomerization by reflecting the concentration of carboxy groups in the system, which has been cross verified by HPLC method [19,28].
The results showed that the protic anion is necessary for the intermolecular esterification as no catalytic activity can be ob-  4 showed a much more attractive feature in product separation as stratification of product and catalyst was observed in the flask within a few minutes after reaction (Table 1, entry 9). Therefore, the IL [HSO 3 -BDBU]H 2 PO 4 not only acts as an efficient catalyst but also provides a facile protocol for  Having selected the IL [HSO 3 -BDBU]H 2 PO 4 as catalyst, the process optimization was performed to further promote the intermolecular dehydration esterification. The reaction temperature, catalyst loading, and vacuum degree on the reaction outcome were in detail investigated and the acid value of the product was used to evaluate the reaction results (Table 2). It was easily found that a higher temperature was favorable for the formation of oligomeric ricinoleic acid ( Table 2, entries 1-5). The vacuum degree is another factor to affect the intermolecular esterification as different vacuum degree can result in a different water removal rate, which may lead to a different equilibrium toward estolides product. Five different levels of vacuum degrees were applied to the reaction system, the results showed the oligomerization degree increased with increasing vacuum degree (  (Table 3, entries 2-10). Simultaneously, the 1 H NMR spectra of the samples in CDCl 3 were also examined. In the 1 H NMR study, a peak found at 3.62 ppm gradually disappeared and a new peak at 4.88 ppm appeared ( Figure 1). This is associated with the changes of the chemical environment for the C 12 -H bond. That is, in ricinoleic acid (l 1 , Figure 1), the C 12 is attached to the hydroxy group while in the corresponding ester product (l 2-10 , Figure 1), C 12 is linked to the ester bond, thereby resulting in a change in the chemical shift of H connected with C 12 . As the chemical shift of H in methyl at 0.87 ppm does not change before and after the reaction, it is used as reference and the peak integral ratio of C 12 -H (A n ) to methyl-H (A m ) in the 1 H NMR spectra is used to determine the oligomerization degree. Theoretically, oligomeric ricinoleic acids with different oligomerization degree have their characteristic A n /A m values as listed in Table 3. For a specific sample, both the peaks for C 12 -H and for methyl-H were integrated and the ratio of A n /A m was calculated. Then the resulting value was compared with the theoretical ones to determine the oligomerization degree of the product. It should be noted that this calculation is based on the assumption that the oligomerization degree of the product is monodisperse. Actually, the obtained oligomerization degree is the average oligomerization degree. By adopting the index A n /A m , the relationship of acid value and oligomerization degree can be constructed. According to the 1 H NMR results, the value A n /A m increased as the reaction proceeded (Table 3, entries 2-10 vs entry 1), which means that the average oligomerization degree of oligomeric ricinoleic acid can be adjusted by changing the reaction time.
For oligomeric ricinoleic acid, its physicochemical properties depend on the oligomerization degree. In order to be used as lubriant in metal-working fluid, the viscosity and the viscosity index of oligomeric ricinoleic acid should fall into the acceptable scope according to Hostagliss L4 oil soluble lubricant additive product index. To determine the suitable reaction time and obtain the qualified product, the viscosity and viscosity index of samples derived from 4 h, 6 h and 8 h of reaction (i.e. NK-A, NK-B and NK-C) were further measured and the results are compared with the commercial product Hostagliss L4 (Table 4).

Reusability of [HSO 3 -BDBU]H 2 PO 4
As the IL catalyst [HSO 3 -BDBU]H 2 PO 4 is immiscible with the product at room temperature, stratification occurs after reaction, thus renders an easy catalyst recyclability and product separation. After decanting the upper product and washing the remaining IL with a small amount of dichloromethane, the weight of [HSO 3 -BDBU]H 2 PO 4 was examined and then the fresh ricinoleic acid was added for the next cycle synthesis of oligomeric ricinoleic acid. The IL catalyst was used in five cycles of reaction and the resulting acid value of the product in each cycle was presented in Figure 2 along with the weight of    Figure 3. It can be seen that the IL catalyst was stable during the reaction as the spectrum remained basically unchanged compared with the fresh catalyst.

Proposed mechanism
According to our experimental results, the protic acid is crucial for the reaction. Therefore, it is inferred that the reaction undergoes proton-promoted intermolecular esterification and the reaction mechanism with catalyst [HSO 3

Conclusion
To conclude, a highly efficient Brønsted acidic IL catalyst [HSO 3 -BDBU]H 2 PO 4 was developed for the esterification of ricinoleic acid. The reaction performed well under solvent-free conditions, the qualified biolubricant oligomeric ricinoleic acid can be prepared at 190 ºC and under vacuum degree of 50 kPa with 15 wt % IL as catalyst in 6 h. Both the acid value and average oligomerization degree of the product were determined and it was found the acid value was 51 mg KOH/g and the average oligomerization degree was 4. The reaction has excellent selectivity and no other byproducts were detected except water. More remarkably, a simple stratifying at room temperature will cause the separation of catalyst and product, which means the additional solvent extraction or distillation separation employed in the traditional IL catalysis was unnecessary in this system. Besides, the catalyst can be reused at least for five cycles without significant activity lost. Therefore, this protocol employing the Brønsted acidic ionic liquid as catalyst represents a green synthesis method for oligomeric ricinoleic acid and can be run in environmentally manner. It is a promising candidate for the commercial production of oligomeric ricinoleic acid from ricinoleic acid. For the preparation of other ILs in this paper and for full experimental data see Supporting Information File 1.

Catalytic dehydration esterification of ricinoleic acid and catalyst recycling
The dehydration esterification of ricinoleic acid was investigated using ILs as catalyst. In a typical run, 10 g (30 mmol) ricinoleic acid and 1 g (2.6 mmol) [HSO 3 -BDBU]H 2 PO 4 were added into a 100 mL glass flask equipped with magnetic stirrer, a reflux condenser and connected with vacuum line. Then, under a stirrer rate of 500 r/min, the temperature was increased to 190 °C to promote the esterification reaction. Simultaneously, the reaction run at 50 kPa to remove the generated water. After 2 h of reaction, the reaction mixture was cooled to room temperature for stratifying. The supernatant was oligomeric ricinoleic acid and it could be decanted directly for further treatment and acid value analysis. The IL catalyst deposited in the lower layer can be washed with a small amount of dichloromethane to remove the residual oligoester. After that, the fresh ricinoleic acid was added for the second run reaction and the recovered catalyst can be used repeatedly.

Product characterization
The oligomeric ricinoleic acid is a yellow oily liquid, and all experiments have yields greater than 90%, which are determined by weighing. The spectral results identified the product. 1

Acid value determination
The acid value of product was determined using a modified ASTM D664 method [33], in which about 1 g sample was dissolved in 30 mL CH 2 Cl 2 and the resulting solution was titrated against standard 0.1 N isopropanol KOH. Acid value (mg KOH/g sample) was then calculated as follows: where V (mL) and C (mol/L) are the consumed volume and concentration of KOH solution, respectively, W (g) is the weight of the sample.
To exclude the presence of IL in supernatant and ensure the reliability of acid value in oligomerization degree determination, additional treatment was performed for the supernatant before acid value determination. That is, the supernatant was diluted with dichloromethane, and the putative IL in the sample was extracted three times with distilled water. Then the remaining organic phase was dried with Na 2 SO 4 for 24 h followed by filtration. After that, the filtrate was collected and the dichloromethane solvent was removed by vacuum evaporation to obtain oligomeric ricinoleic acid for acid value measurement (Table 5, entries 1, 3 and 5). On the other hand, the acid value of supernatant without any treatment was also determined for comparison (Table 5, entries 2, 4 and 6). The acid values of the two treatments were identical, indicating that IL settled well after the reaction and was absent in the product.