Chen Chen, Qiong Tang, Hong Xu, Lei Liu,*, Mingxing Tang, Xuekuan Li, Jinxiang Dong

1 College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

2 Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

Keywords:Naphthalene n-Butene Alkylation reaction Liquid coordination complexes Synthesis Lubricating base oils

A B S T R A C T With the development of coal che mical industry, large amounts of naphthalene and n-butene are produced, and converting them into high value-added products through alkylation has gained particular importance and interest. In this work, liquid coordination complexes (LCCs) were used as acid catalysts for the first time in the naphthalene alkylation reaction under mild conditions to obtain multibutylnaphthalenes with high yield. Various reaction conditions were thoroughly investigated. The LCC consisting of urea and AlCl3 showed excellent catalytic performance under optimal reaction conditions,giving 100% conversion of naphthalene and 99.66% selectivity towards multi-butylnaphthalenes.Combining the catalyst properties and catalytic results, a plausible reaction mechanism was proposed.The lubricating properties of the synthesized products were investigated for their potential application as lubricating base oils. The synthesized multi-butylnaphthalenes showed comparable physicochemical properties and tribological performances as the commercial cycloalkyl base oil.

1. Introduction

Alkylnaphthalenes with versatile application potential can be synthesized by Friedel-Crafts alkylation in the presence of acid catalysts [1-7]. Alkylnaphthalenes have excellent physical properties such as thermo-oxidative stability and good compatibility with additives, and their molecule structure is similar to that of typical cycloalkyl base oil derived from petroleum resources.Hence,alkylnaphthalenes may provide an alternative to cycloalkyl base oil,which is an important member in the family of lubricating oils.As a lubricating base oil [8-10], the performances of alkylnaphthalenes are closely related to the molecular structure of side chains and the number of alkyl groups attached to the naphthalene ring. Mono-substituted naphthalene with long-chain olefins have been investigated as lubricating base oils [11], but the cleaving of C—C bonds in the long-chain alkyl groups tends to become severe during harsh working conditions, leading to performance fluctuations. Generally, multi-substituted naphthalene exhibit high viscosity, low volatility at higher temperature and excellent thermo-oxidation stability[9].However,owing to steric hindrance of alkylnaphthalenes, it is difficult to further produce multialkylnaphthalenes by successive reaction.

The catalysts used for alkylnaphthalenes synthesis mainly include Bronsted acids[6,7]such as HF and H2SO4,and some Lewis acids represented by AlCl3[1], but they are highly toxic, corrosive and cause heavy pollution. Solid acid catalysts [2-4] such as zeolites and heteropoly acids are also used for alkylation,but the carbon deposition on catalyst usually deactivates them. Traditional ionic liquids (ILs) [5] as catalysts exhibit high catalytic performance for alkylation, but their complex preparation process and high cost limit their application.Recently,liquid coordination complexes (LCCs) have attracted considerable attention due to their Lewis acid activity and low cost. The acid property for LCCs can be adjusted over a wide range by changing the components of LCCs[12].

LCCs are homogeneous liquid mixtures [13-15] made up of metal halides(AlCl3,GaCl3,etc.)and donor molecules such as urea,N,N-dimethylacetamide, trioctylphosphine oxide, and thiourea.They show promising potential in various reactions including Friedel-Crafts acylation [16-18], oligomerization [19], sulfination[20] and isomerization [21]. In the case of alkylation, GaCl3-based LCCs have been reported as catalysts for the alkylation for benzene withn-decene, showing a high selectivity toward 2-phenyldecane[22]. Huet al. [23] reported that amide-AlCl3-based ionic liquid analogues could catalyze the isobutane alkylation with 2-butene to produce high-quality alkylate oils.To the best of our knowledge,there is no report on the alkylation of naphthalene with olefins with LCCs as catalyst.

With the rapid development of coal-to-olefins (C2-C3), a large amount ofn-butene is produced as the main by-product. Thus,the high value-added utilization of butene has become a key issue.Herein,n-butene from coal chemical industry was used to replace long-chain olefins for the naphthalene alkylation reaction with an attempt to obtain new alkylnaphthalenes lubricating base oil. The alkylation of naphthalene withn-butene was catalyzed by liquid coordination complexes under mild experimental conditions, and high selectivity of multi-butylnaphthalenes was realized by optimizing the reaction conditions. The synthesized products were investigated as lubricating base oils, and showed comparable performances to the commercial cycloalkyl base oils.

2. Materials and Methods

2.1. Materials

Anhydrous aluminum chloride (AlCl3; 99%), ferric chloride(FeCl3; 98%), urea (Ur; ≥99.5%), benzamide (98%),N,Ndimethylformamide (DMF; AR),N,N-dimethylacetamide (DMA;≥99.8%), naphthalene (99%),n-decane (＞99.0%) and ethyl acetate(99.9%) were purchased from Aladdin (Shanghai, China).n-Butene (99.5%) was obtained from Zhengzhou XingDao Chemical Technology Co. Ltd. (China),and 4010 cycloalkyl base oil was purchased from PetroChina Co.Ltd.(China).All reagents were used as received without further treatment.

2.2. Synthesis of LCCs

Liquid coordination complexes (LCCs) with different neutral donor ligands and anhydrous metal chlorides were prepared in a three-necked round-bottomed flask equipped with a magnetic stirring apparatus under argon gas protection. Urea (1.5 mmol),ndecane (30 mmol) as the solvent and anhydrous AlCl3(3 mmol)were added into the flask in turn, and stirred for 60 min at 80 °C to obtain a homogeneous liquid mixture. Two layers appeared in the mixture, the lower layer was LCCs and the upper layer was the solvent.Then-decane solvent was decanted,and the remaining LCCs were used as the catalyst for the subsequent alkylation reactions.

The ratioXAlCl3is defined as follows:XAlCl3＝nAlCl3/（nAlCl3+nurea）

where thenAlCl3andnureaare the molar quantity of AlCl3and urea,respectively.

2.3. Alkylation reaction

In a typical synthesis, alkylation of naphthalene withn-butene was carried out under argon atmosphere. First, 15 mmol of naphthalene, 15 mmol ofn-decane, and 1.5 mmol of LCC were placed in a 100 ml round-bottomed flask equipped with a magnetic stirring bar and a reflux condenser. Then, the flask was heated to the required temperature under vigorous stirring, andn-butene gas was bubbled into the bottom of the flask with a 3 mm inner diameter glass pipe to ensure thatn-butene gas was mixed into the reaction system. The surplusn-butene gas was released from the reaction system. The flow rate ofn-butene gas was controlled by a mass flowmeter. After the reaction was completed, the mixture was cooled to room temperature. The upper layer liquid was collected and washed by water to remove the residual catalyst.Finally,the obtained oil phase was subjected to vacuum distillation to removen-decane and determined quantitatively.The final products were further condensed by vacuum distillation and then decolorized by activated carbon.

2.4. Analyses

Pure acetonitrile was used as an FT-IR molecular probe to measure the Lewis acidities of LCCs [24] and characterized by IRAffinity-1 from Shimadzu. All the samples were prepared by mixing acetonitrile and LCCs in a volume ratio of 1:8, and then spreading into liquid films on KBr windows, fixed into the liquid cell for scanning test. The scan range was from 4000 cm-1to 400 cm-1with a resolution of 4 cm-1.

The conversion of naphthalene and the selectivity of products were determined by using gas chromatograph (Shimadzu GC-2010 Pro equipped with a Kromat PC-5HT column (30 m × 0.25 mm × 0.25 μm)), with pure argon gas as the carrier gas. The column injector temperature was 330°C and the FID detector temperature was 330 °C. The column temperature was initially 50 °C for 2 min,and increased to 200°C with a rate of 15°C·min-1,held with 2 min, then gradually increased to 300 °C at a rate of 5 °C·min-1,and maintained at this level for 10 min. Qualitative analysis of the products was performed by using GC-MS (Shimadzu GC-2010 Plus equipped with a Rtx-5 ms capillary column(30 m × 0.25 mm × 0.25 μm)). Electrospray ionization (EI) mode at 70 eV was employed for MS and the other conditions were the same as for GC.

The GC (Fig. S1) and GC-MS (Fig. S2 and Table S1 in Supplementary Material) results demonstrated that the multibutylnaphthalenes mainly included tri-, tetra-, penta- and hexa-butylnaphthalenes.

2.5. Lubricant properties and tribological test

The testing conditions for basic lubricant properties were based on the corresponding standards as follows: kinematic viscosity (KV,ASTM D445), density (ASTM 4052), flash point (ASTM D92), pour point (GB/T 3535), aniline point (GB/T 262), and color (ASTM D1500).Pressure differential scanning calorimetry(PDSC)test conditions were as follows:pressure:101.325 kPa,heat rate:10°C·min-1,O2flow rate: 100 ml·min-1, standard: SH/T 3950-2014.

The test for tribological properties were performed at a loading of 200 N,temperature of 50°C,time of 3600 s,frequency of 50 Hz,and stroke of 1 mm on an Optimol SRV-V oscillating reciprocating friction-and-wear system. The surface and depth of the grinding mark of the test plate were studied by using a three-dimensional(3D) non-contact optical surface profiler (Zygo, Zegage).

3. Results and Discussion

3.1. Effect of different LCCs on the alkylation

Some typical LCCs were investigated as catalysts for the alkylation of naphthalene andn-butene, and the catalytic results are summarized in Table 1. LCCs were composed of metal chlorides(AlCl3and FeCl3) and donor molecules (benzamide, DMF, DMA,and Ur). The different components lead to the difference in physical properties which is associated with the final catalytic performances [25]. The LCC consisting of urea and FeCl3showed a low conversion of naphthalene, and only mono-butylnaphthalenes were formed.In contrast,AlCl3-based LCCs exhibited high catalytic activity for alkylation as well as the high selectivity toward multibutylnaphthalenes.Among them,the LCC of Ur-AlCl3exhibited the highest total selectivity for multi-butylnaphthalenes (99.66%)including tri-, tetra-, penta-, and hexa-butylnaphthalenes with 100% conversion of naphthalene.

Table 1 Catalytic results of naphthalene alkylation with n-butene over various LCCs

Considering the relationship between acidities and catalytic performances of LCCs, acetonitrile was used as an FT-IR molecular probe to measure their Lewis acidities. As shown in Fig. 1, pure acetonitrile exhibited two IR bands at 2252 cm-1and 2293 cm-1due to its CN stretching vibrations. A new band appeared in the range of 2300-2400 cm-1when acetonitrile was mixed with LCCs.FT-IR spectra (Fig. 1) showed different acidities of various components.It was noted that the intensity of the new band for Ur-AlCl3is greater than that of Ur-FeCl3,indicating that Ur-AlCl3had stronger acidity. On the other hand, compared to benzamide-AlCl3,DMF-AlCl3and DMA-AlCl3,the second band(2293 cm-1)was more sensitive than others.The second band of Ur-AlCl3was blue shifted towards higher wavenumber, indicating that its Lewis acidity was stronger compared to the other three AlCl3-based catalysts. This acidity characterization result of LCCs was in good agreement with their reaction results,showing that stronger acidity was beneficial for the synthesis of multi-alkylnaphthalenes.

Fig. 1. FT-IR spectra of pure acetonitrile and different LCCs.

3.2. Acidities of LCCs

The different molar ratios between Ur and AlCl3influenced the catalyst performances in the alkylation reaction,and the results are presented in Table 2.AtXAlCl3＝0.5,no product was detected by GC analysis. When the proportion of AlCl3changed from 0.6 to 0.67,the conversion of naphthalene reached 100% and the selectivity of multi-butylnaphthalenes increased from 72.25% to 99.66%.However, with further increase in the molar ratio of AlCl3, the homogenous liquid was not formed. Hence, it was not studied further.Based on previous reports [25-28], the Ur-AlCl3catalyst has various dominant species in the presence of different AlCl3molar ratios (XAlCl3) of 0.5-0.67. In the Ur-AlCl3system, the neutral species of [AlCl3·Ur] is converted to the ionic species of [AlCl2Ur2]+and[AlCl4]-(1).WhenXAlCl3＝0.5,the neutral[AlCl4]-was the predominant anionic species.Meanwhile,more[AlCl4]-was converted into [Al2Cl7]-with increase in the AlCl3molar ratio. With further increase in the AlCl3molar ratios to 0.67, the catalyst system had more [Al2Cl7]-anionic species than at the molar ratio of 0.6. The catalyst system remained in dynamic equilibrium between[Al2Cl7]-and [AlCl4]-, as shown in Eq. (2).

Table 2 Catalytic results of LCCs with various molar ratios between Ur and AlCl3

Table 3 Evaluation of lubricant properties of multi-butylnaphthalenes and 4010 cycloalkyl base oil

The different AlCl3molar ratios affect its catalytic properties.Thus, the FT-IR spectra (Fig. 2) verified that more [Al2Cl7]-had stronger Lewis acidity. A third band was observed at 2330 cm-1whenXAlCl3＝0.5. Moreover, the wavenumbers shifted to higher values when the molar ratio of AlCl3increased from 0.6 to 0.67(2332 cm-1to 2335 cm-1).It also indicated that the acidity of catalyst increased with increasing the AlCl3content. Naphthalene could not be converted due to the weak Lewis acidity atXAlCl3＝0.5. WhenXAlCl3＝0.67, more [Al2Cl7]-was beneficial to the alkylation reaction,resulting in full conversion of naphthalene and high selectivity of multi-butylnaphthalenes (99.66%). Hence,the acidities of LCCs could be controlled by changing the amount of AlCl3. The suitable AlCl3molar ratio for the alkylation is suggested to be 0.67.

Fig. 2. FT-IR spectra of pure acetonitrile and Ur-AlCl3 LCCs containing different amounts of AlCl3.

3.3. Effect of reaction conditions

3.3.1. Molar ratios of naphthalene to LCCs

To obtain high selectivity of multi-butylnaphthalenes, alkylation of naphthalene withn-butene was investigated with various molar ratios of naphthalene to LCCs, as shown in Fig. 3. In the entire naphthalene/LCCs molar ratios range for this study, the naphthalene could be completely converted. However, the selectivity of multi-butylnaphthalenes increased from 72.31% to 99.66%by increasing the amount of LCCs,corresponding to change in naphthalene/LCCs ratio from 14 to 10. With further increase in the amount of LCCs, a substantial increase in multibutylnaphthalenes selectivity was not observed. Therefore, the naphthalene/LCCs molar ratio of 10 was chosen for the following study.

Fig.3. Catalytic results of naphthalene alkylation with n-butene with various molar ratios of naphthalene/LCCs (Reaction conditions: temperature, 60 °C; time,100 min; flow rate of n-butene, 20 ml·min-1; stirrer speed, 500 r·min-1).

3.3.2. Reaction temperature

The catalytic results over Ur-AlCl3were recorded at different reaction temperatures, as shown in Fig. 4. It can be seen that the conversion of naphthalene was 97.95% at 20 °C and naphthalene was completely converted at temperatures above 40 °C. The reaction temperature had significant influence on the selectivity of products at the temperature range of 20-60°C.When the temperature increased to 60°C,the selectivity of multi-butylnaphthalenes reached the highest level (99.66%). The selectivity decreased slightly by 0.46%at 100°C compared with that at the temperature of 60°C.The selectivity of multi-butylnaphthalenes was improved at high temperatures, while there were no significant changes in the composition of products at temperatures above 60 °C. Thus,the temperature of 60 °C was selected for synthesis of the final products.

Fig. 4. Effect of reaction temperature on conversion and selectivity (Reaction conditions: time, 100 min; flow rate of n-butene, 20 ml·min-1; naphthalene/LCCs ratio, 10; stirrer speed, 500 r·min-1).

Fig. 5. Naphthalene conversion and the distribution of products with different reaction time (Reaction conditions: temperature, 60 °C; flow rate of n-butene,20 ml·min-1; naphthalene/LCCs ratio, 10; stirrer speed, 500 r·min-1).

3.3.3. Reaction time

The effect of reaction time on the alkylation reaction of naphthalene withn-butene was investigated in the range of 20 min to 120 min and the results are shown in Fig. 5. After the first 20 min, the main product was mono-butylnaphthalenes and the selectivity was 56.12%.Then,the mono-products rapidly decreased as the reaction time was prolonged.The conversion of naphthalene reached 100% after 40 min, and the selectivity of multibutylnaphthalenes remained stable (99.66%) beyond 100 min.Thus,it can be seen that,alkylnaphthalenes with fewer side chains were synthesized at first during the alkylation process.Then,with the extension of time, the initial formed mono- and dibutylnaphthalenes continued to react withn-butene, and multibutylnaphthalenes were obtained. There was no effect on the selectivity of multi-butylnaphthalenes when the reaction was continued up to 120 min. Therefore, 100 min was selected as the optimum reaction time to ensure complete reaction.

Fig. 6. Influence of flow rate of n-butene on conversion and selectivity (Reaction conditions: temperature, 60 °C; time, 100 min; naphthalene/LCCs ratio, 10; stirrer speed, 500 r·min-1).

3.3.4. Flow rate of n-butene

The flow rate ofn-butene and the stirring speed are important factors in affecting of mass transfer during reaction. Firstly, the effect of stirring speed on the naphthalene conversion and the selectivity of products was investigated as shown in Fig.S4.When the stirring speed was 250 r·min-1,naphthalene could not be converted completely and the selectivity of multi-butylnaphthalenes was only 35.64%. Upon the stirring speed exceeded 500 r·min-1,naphthalene was fully converted and the selectivity of multibutylnaphthalenes achieved high level (99.66%). This result showed that the high stirring speed significantly improved the mass transfer in reaction process, which was closely associated with the final catalytic results.

The effects of the flow rate ofn-butene on the conversion of naphthalene and the selectivity of products were further investigated. As shown in Fig. 6, the flow rate ofn-butene had little influence on the conversion ratio of naphthalene, and naphthalene could be completely converted. When the flow rate ofn-butene was 10 ml·min-1, 79.88% selectivity of multibutylnaphthalenes was achieved. Then, with increase in the flow rate ofn-butene in the range of 20-40 ml·min-1, no significant changes in the selectivity of multi-butylnaphthalenes were observed. The high flow rate of the gas increased the possibility for contact betweenn-butene and naphthalene. However, when the flow rate ofn-butene was more than 20 ml·min-1, the selectivity of multi-butylnaphthalenes reached the maximum and the surplusn-butene gas was removed from the reaction system.Therefore, high flow rate ofn-butene gas into the reaction system was not significant to obtain high selectivity of multibutylnaphthalenes. The optimum flow rate ofn-butene was suggested to be 20 ml·min-1.

Fig. 7. Proposed catalytic mechanism for the alkylation of naphthalene with n-butene catalyzed by LCCs.

Fig.8. Dynamic friction coefficient of a steel disk lubricated with cycloalkyl base oil and multi-butylnaphthalenes.

3.4. Catalytic mechanism of LCCs in alkylation

A plausible mechanism was proposed for the alkylation of naphthalene in the presence of LCCs, as shown in Fig. 7. WhenXAlCl3＝0.67,in the dynamic equilibrium, [Al2Cl7]-(I)was the dominant active species of Ur-AlCl3[28], which led to excellent catalytic activity.n-Butene was polarized by the main Lewis acidic species (I) and formed unstable AlCl3adduct of butene (II). The butene adduct became the important intermediate (III) for further reactions.Then,the intermediate(III)attacked the π bond at different locations on the naphthalene ring to generate the naphthalene coordinated products (V). On a naphthalene ring, the H+is connected to the sp3hybridized carbon [29]. The H+replaced the unstable AlCl3, thereby generating mono-butylnaphthalenes (VI).Then,the process was repeated to obtain the final alkylnaphthalenes (VII, VIII) with more side-chains.

3.5. Multi-butylnaphthalenes as lubricating base oils

3.5.1. Lubricant properties test

It is of great significance to evaluate the basic physicochemical properties of alkylnaphthalenes for their possible application as lubricating base oils. The commercial cycloalkyl base oil having similar structure as the alkylnaphthalenes was selected as a reference in this study.Table 3 shows the physicochemical indexes corresponding to the two samples. The kinematic viscosity (KV) of synthesized products was lower than that of cycloalkyl base oil at 40°C and 100°C.The calculated viscosity index was below zero for the commercial cycloalkyl base oil and multibutylnaphthalenes according to the ASTM D2270, indicating that these kinds of base oils have a poor viscosity-temperature performance. However, the multi-butylnaphthalenes had lower pour point. The lower aniline point of multi-butylnaphthalenes(40.4 °C) indicated better solubility for additives than that of cycloalkyl base oil(101.6°C).The pressurized differential scanning calorimeter (PDSC) data (Fig. S3) demonstrated that the synthesized products had good thermo-oxidative stability under harsh conditions. Comprehensive comparison of the physicochemical properties indicated that the synthesized multibutylnaphthalenes had comparable proprieties compared to the cycloalkyl base oil.

Fig. 9. 3D micrographs and wear depth of disks for cycloalkyl base oil (a) and multi-butylnaphthalenes (b).

3.5.2. Tribological performance

The tribological behavior of cycloalkyl base oil and multibutylnaphthalenes was analyzed by using a SRV-V friction and wear tester. As shown in Fig. 8, the dynamic friction coefficient of multi-butylnaphthalenes remained relatively low and steady throughout the test period. The average friction coefficient of cycloalkyl base oil was 0.162, which was higher than that of multi-butylnaphthalenes (0.156). The wear surface morphologies of steel disks lubricated with multi-butylnaphthalenes and cycloalkyl base oil were measured by using 3D optical profiles.Fig. 9 shows the 3D microscopic images, it can be seen that the wear volumes (×10-4mm3) of the disks with multibutylnaphthalenes and cycloalkyl lubricating oil were 33.49 and 15.80, respectively. Moreover, the maximum wear width and depth obtained from the corresponding steel disks were 0.70 mm× 12.51 μm and 0.68 mm × 15.72 μm, respectively. Obviously,the wear volume of multi-butylnaphthalenes was more than that of cycloalkyl lubricating oil, but the differences in the track width and depth were not significant. Therefore, the tribological results showed that the synthesized multi-butylnaphthalenes could be used as an alternative to the commercial cycloalkyl lubricating oil.

4. Conclusions

In summary, multi-butylnaphthalenes were successfully synthesized in this work by the alkylation of naphthalene withnbutene under mild conditions. The LCC of Ur-AlCl3was found to be a highly efficient catalyst for the alkyaltion of naphthalene withn-butene, showing a naphthalene conversion as high as 100%, as well as high selectivity toward multi-butylnaphthalenes under the optimal reaction conditions.The structural composition of LCCs had a significant effect on the catalytic performances, as the dominant [Al2Cl7]-ions mainly contribute to the higher acidity. The synthesized multi-butylnaphthalenes were demonstrated to be suitable lubricating base oils with respect to their physicochemical properties and tribological performances, serving as an attractive alternative to commercial cycloalkyl lubricating oil.

CRediT authorship contribution statement

Chen Chen: Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft. Qiong Tang: Writing - Review& Editing, Methodology. Hong Xu: Writing - Review & Editing.Lei Liu: Conceptualization, Writing - Review & Editing, Supervision, Project administration, Funding acquisition. Mingxing Tang:Project administration, Funding acquisition. Xuekuan Li: Conceptualization, Project administration. Jinxiang Dong: Conceptualization, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U1910202, 21978194 and 21603256), the Natural Science Foundation of Shanxi Province(201801D121055), and Program for the Shanxi Young Sanjin Scholar.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.01.014.