A Novel Application of Methanesulfonic Acid as Catalyst for the

Oct 2, 2012 - ABSTRACT: Methanesulfonic acid (MSA) was used as the catalyst to remove trace olefins from aromatics (referred to as oil). The reactions...
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A Novel Application of Methanesulfonic Acid as Catalyst for the Alkylation of Olefins with Aromatics Ying Tian, Xuan Meng, Ji-yun Duan, and Li Shi* The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P.R. China ABSTRACT: Methanesulfonic acid (MSA) was used as the catalyst to remove trace olefins from aromatics (referred to as oil). The reactions were conducted in a round-bottom flask equipped with a reflux condenser and oil-bath under atmospheric pressure. The influences of the reaction temperature, the mass ratio of MSA to oil, and the reaction time were investigated. The experimental results showed that after a 40 min reaction time at 413 K, the olefins conversion of 89.45% was obtained with a mass ratio of MSA to oil of 6%. The catalyst was reused five times without a reactivation treatment, and the olefins conversion reached 66.02% the fifth time. More importantly, the catalytic mechanism was also proposed in this work. In addition, as MSA is biodegradable, this method represents an environmentally benign route to remove trace olefins from aromatics.

1. INTRODUCTION In petroleum processing, aromatic streams, which can be used as important feedstocks, are mainly produced from processes such as thermal cracking and naphtha reforming. However, these aromatic streams often contain some or all of the following compounds: mono-olefins, dienes, styrenes, and heavy aromatic compounds.1 In general, these olefins can cause some adverse effects on the purity standards of aromatics and certain petrochemical processes which are sensitive to olefins even in very low concentrations.2 Thus, currently a variety of methods are available to remove the trace olefins from aromatic streams, such as the hydrotreating process and the clay treatment process. Nonetheless, the hydrotreating process requiring high temperature will cause a loss of aromatics, while the clay treatment process has the problems of very rapid deactivation and environmental pollution. Many efforts on the modified clay also have been done by our research team,3,4 but the environmental pollution of the modified clay is still a serious problem in aromatics treatment. Consequently, here an innovative process for removal of trace olefins from aromatics using methanesulfonic acid as the catalyst was reported. Methanesulfonic acid (MSA), which has unlimited solubility in water, is a strong organic acid with no oxidizing properties.5 It is the best choice in a wide variety of application areas (i.e., esterification catalysts,6 alkylation catalysts,7 the Biginelli reaction catalysts8 and the Fries rearrangement catalysts,9 etc.). Meanwhile, it has high thermal stability and does not generate dangerous volatiles even at high concentration, making it convenient and safe to handle. It is extremely important that MSA is considered readily biodegradable and it only decomposes to form sulfate, carbon dioxide, water, and biomass, making it an environmental catalyst.10 In addition, in comparison with the usual alkylation catalysts, such as H2SO4, HF, and AlCl3, MSA is relatively noncorrosive and nontoxic. Ultimately, MSA has the advantage, as will be shown, that it can be separated readily from the reactants. In this paper, the influences of various reaction parameters, such as the mass ratio of MSA to oil, reaction temperature, reaction time, and the reusability of MSA, were studied. Furthermore, with the help of gas chromatography− © 2012 American Chemical Society

mass spectroscopy (GC−MS) and Fourier transform infrared spectrometer (FT−IR), the reaction mechanism was discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Methanesolfonic acid (AR) was provided by BASF-The Chemical Company. Benzene (AR) and n-octane (AR) were purchased from Shanghai Chemical Reagent Co., Ltd., Shanghai, China. 1-Octene (AR) was purchased from the Aladdin Chemistry Co., Ltd. CaCl2 (AR) was purchased from Sinopharm Chemical Reagent Company, Shanghai, China. The model oil, which was used to discuss the reaction mechanism, was prepared by adding a certain amount of benzene and 1-octene to n-octane resulting in a mass fraction of benzene of 15% and that of 1-octene of 5%. The experimental raw materials were aromatic hydrocarbons that were obtained from the catalytic reforming unit of Sinopec Zhenhai Refining & Chemical Company at different stages. The aromatic hydrocarbons consisted of two kinds of bromine index (BI). One BI was about 900−1000, and the other one was about 1100−1200. In addition, the compositions of the aromatic hydrocarbons with different BI were similar, and the main trace olefins were C8−C10 long-chain olefins. Moreover, the components of the aromatic hydrocarbons with BI about 1100−1200 are shown in Table 1.11 2.2. Experiments for Removal of Trace Olefins. All reactions were carried out in a 100 mL round-bottom flask Table 1. Components of the Aromatic Hydrocarbons with a BI about 1100−1200 component average content (wt %)

Received: Revised: Accepted: Published: 13627

ethyl toluene benzene 0.238

7.885

xylene

C9 aromatics

C10 aromatics

others

52.207

32.622

6.81

0.238

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fitted with a reflux condenser and oil-bath under atmospheric pressure. The temperature was kept stable by a silicon oil bath equipped with a thermostat and a magnetic stirrer, and the temperature was measured in the oil bath. The stirring speed was 600 rpm in all of the experiments. The aromatic feedstock (10 g) and a certain amount of the catalyst were added to the flask, and the mixture reacted for a period of time (0−50 min) at a certain temperature (298−443 K). After the reaction, the biphasic reaction mixture was put aside for 2 h at room temperature, and then the upper oil phase containing less olefins was simply separated from the catalyst in the bottom by decantation. 2.3. Methods for Analysis. The quality of aromatic products was quantified by the bromine index (BI). The BI was defined as the number of milligrams of bromine that would react with trace olefins present in 100 g of an aromatic sample.12 Therefore, the value of the bromine index was an indication of the relative amount of olefins, which were doublebounded hydrocarbons. In this study, the LC-2 bromine index detector was used to analyze all the aromatics samples. Then, the olefins conversion (Y) was obtained by the following equation: Y = (BI 0 − BI)/BI 0 × 100%

Figure 2. Effect of the reaction temperature on olefins conversion. Reaction conditions: oil =10 g, t = 30 min, P = atmospheric pressure.

(1)

where BI0 stands for the bromine index of the aromatic feedstock, and BI stands for the bromine index of the aromatic hydrocarbons after the reaction. The LC-2 bromine index detector was purchased from Jiangyan Electric Analysis Instrument Company. The operating principle of the detector is based on the microcoulometric titration principle. The bromine generated by electrolyzation will react with the sample which is added directly to the titration cell. After the reaction, the computer will automatically calculate the bromine index on the basis of the electricity consumption of the generated bromine.

3. RESULTS AND DISCUSSION 3.1. Effect of the Bromine Index and the Mass Ratio of MSA/Oil. The influences of the bromine index and the mass ratio of MSA to oil were studied (Figure 1). As shown in Figure

Figure 3. Effect of the reaction time on olefins conversion. Reaction conditions: oil = 10 g, MSA/oil = 6%, P = atmospheric pressure.

Figure 4. Effect of reusability of the catalyst. Reaction conditions: oil = 10 g, MSA/oil = 6%, T = 413 K, t = 40 min, P = atmospheric pressure. Figure 1. Effects of the bromine index and the mass ratio of MSA/oil on olefins conversion. Reaction conditions: oil = 10 g, T = 298 K, t = 30 min, P = atmospheric pressure.

1, when the mass ratio of MSA to oil increased from 0.2% to 6%, the olefins conversion went up significantly. The reason for 13628

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bromine index (BI) rose, the conversion of olefins became higher. This was mainly because there were more olefins, which were readily removed from the aromatics, in the aromatic feedstock with a higher BI. In China, the bromine index of the oil (aromatic hydrocarbons) used in most petroleum industries is less than 1100. Thus, in the subsequent experiments, the aromatic feedstock with a BI of about 900−1000 was used as the experimental raw material. 3.2. Effect of the Reaction Temperature. Figure 2 displays the effect of the reaction temperature on olefins conversion in the temperature range of 298−443 K. As expected, the reaction activity increased with increasing temperature, and this was attributed to the enhancement of the molecular thermal motion, which played an important role in increasing the reaction activity. As can be seen from Figure 2, when the mass ratio of MSA/oil was 0.6%, the olefins conversion increased significantly as the temperature increased from 298 to 383 K, and then it became constant as the reaction proceeded. At the temperature of 383 K, the olefins conversion of 45.32% was obtained. However, this olefins conversion was much lower than that of MSA/oil of 6% at the same temperature. On the other hand, it is obvious that when the mass ratio of MSA/oil was 6%, an increase in the reaction temperature from 298 to 413 K led to a remarkable increase in the conversion of olefins from 54.55% to 88.31%. Then, the further increase in the temperature from 413 to 443 K would not cause an obvious increase in the conversion of olefins. The reason for this behavior was probably that the reaction equilibrium was nearly approached at the temperature of 413 K, and then the influence of the reaction temperature on the olefins conversion was not evident. Thus, the results indicate that 413 K was an appropriate reaction temperature for this experiment, at the optimal MSA/oil ratio of 6%. 3.3. Effect of the Reaction Time. The effect of the reaction time on olefins conversion was investigated at different temperatures (353, 383, and 413 K) in the time range from 2 to 50 min. As illustrated in Figure 3, an increase of the reaction time caused an obvious increase in the olefins conversion. When the reaction proceeded for 20 min, at the temperature of 353, 383, and 413 K, the conversion was 70.95%, 74.41%, and 87.81%, respectively. In the case of 413 K, the olefins conversion hardly continued improving after 20 min. However, for 353 and 383 K, the conversion continued gradually

Figure 5. FT-IR of MSA: (a) fresh; (b) after the reaction. Reaction conditions: oil = 10 g, MSA/oil = 6%, T = 413 K, t = 40 min, P = atmospheric pressure.

Figure 6. A brief description of the catalytic process.

this was that with an increasing mass ratio of MSA/oil, the amount of MSA in the reaction system increased, so that MSA might collide and aggregate with olefins more often. But only a slight change in the olefins conversion was observed, when the MSA/oil ratio increased from 6% to 10%. Therefore, the optimum mass ratio of MSA to oil was found to be 6%. In addition, with the same mass ratio of MSA to oil, as the

Figure 7. Mechanism of this catalytic reaction. 13629

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Figure 8. GC−MS of the model oil separated after the reaction.

corresponding ion pair or polarized complex. The alkylcarbenium ions undergo a rapid secondary to secondary rearrangement in varying degrees, and result in the formation of isomeric alkylcarbenium ions. Finally, these isomeric alkylcarbenium ions attack the nucleus of the aromatics to form the isomeric alkylates. To confirm this mechanism, the model oil was prepared and GC−MS was used to analyze the model oil separated after reaction. The results are presented in Figure 8. 1-Octene was completely converted, the peaks of 10.19, 10.31, and 10.59 min were confirmed to be alkylates whose structures are shown respectively in Figure 8. As the reaction followed the Markovnikov’s rule, the 1-phenyl isomer was not formed. In addition, all these results are in agreement with the previous study conducted by Luong et al.14 Therefore, the mechanism shown in Figure 7 is reasonable. Finally, according to this mechanism, the catalyst with strong acidity will be advantageous to the reaction. Most importantly, this reaction mechanism provides us with the principles of choosing and creating the most effective catalyst.

increasing after 20 min, and varied slightly after 40 min. Hence, 40 min was chosen as the suitable reaction time in the present experimental conditions. 3.4. Effect of Reusability of the Catalyst. The possibility of reusing the catalyst was also investigated (Figure 4). After the reaction, the reaction system was still a biphasic system in which the catalyst phase was the lower layer, so the oil phase could be separated from the biphasic system by simple phase separation. Afterward, the lower catalyst was directly used in the next reaction run with fresh oil, under the identical reaction conditions to the first run. Owing to the trace residue of the catalyst in the oil phase, the catalyst could not be recycled completely by decantation. In addition, as the mass ratio of MSA to oil was only 6%, the dosage of the catalyst was small. To avoid an obvious loss of the catalyst, a trace amount of the oil still existed after the decantation. The residue of the oil might affect the catalytic reaction in the next run. On this account, from the results shown in Figure 4, it is found that as the recycle times increased, there was a decrease in the olefins conversion. However, the olefins conversion of 66.02% could still be obtained in the fifth run. 3.5. Mechanism of Olefin Removal. The experimental results demonstrated that MSA was an active substance. To find out if this process for removal of trace olefins from aromatics was a catalytic process, the FT-IR was employed to characterize fresh MSA and MSA after reaction. From Figure 5, it is worth noting that the FT-IR of MSA after the reaction showed a similar pattern to that of the fresh MSA. In other words, the properties of MSA did not change, and MSA had the catalytic activity according to the definition of ″catalyst″. As a result, it demonstrates that the process for removal of trace olefins from aromatics using methanesulfonic acid was a catalytic process. This catalytic process can be shown in Figure 6. The catalytic mechanism of olefin removal from aromatics is an alkylation reaction, when MSA is used as the catalyst. As MSA is a pure proton acid such as HF, the isomers of the alkylated product can be formed during this alkylation reaction. de Almeida et al.13 reported that with HF and H2SO4 as acid catalysts, the isomerization was assumed to occur by a repeated addition and elimination of a molecule of acid, giving a mixture of all possible secondary alkylcarbenium ions, which, in turn, alkylate the benzene to yield isomeric phenylalkanes. Consequently, on the basis of the above discussion, the mechanism of this catalytic reaction was proposed (Figure 7). As shown in Figure 7, the mechanism for alkylation of olefins with aromatics in the presence of MSA involves interaction of the olefin with the MSA to form an alkylcarbenium ion, a

4. CONCLUSIONS Trace olefins could be removed from the aromatics via an alkylation reaction using methanesulfonic acid as the catalyst. With a mass ratio of MSA/oil of 6%, the olefins conversion could reach 89.45% at 413 K in 40 min. Under the optimal conditions, the catalyst could be reused five times and the olefins conversion could achieve 66.02% in the fifth time without reactivation treatments. The strong acidity of the catalyst would be favorable to the conversion of olefins, as demonstrated by the proposed reaction mechanism. As this process was based on the noncorrosive, easily biodegradable MSA, it could be developed into an environmentally benign method for removal of trace olefins from aromatics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.021-64252274. Notes

The authors declare no competing financial interest.

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REFERENCES

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