Elsevier

Fuel

Volume 231, 1 November 2018, Pages 224-233
Fuel

Full Length Article
Enhancement of isobutane/butene alkylation by aromatic-compound additives in strong Brønsted acid

https://doi.org/10.1016/j.fuel.2018.05.081Get rights and content

Highlights

  • Trace amounts of thiophene additive enhanced significantly catalytic performance and quality of alkylate.

  • C8 selectivity and ratios of TMPs/DMHs reached 92.26 and 11.14, respectively.

  • RON increased markedly by 5.09 compared with that of H2SO4 alone.

  • The synergetic mechanism of thiophene-H2SO4 on isobutane/butene alkylation was speculated.

Abstract

The aromatic-compound additive in sulfuric acid (H2SO4) was found to promote isobutane/butene alkylation significantly. The selectivity of isooctane (C8) and the alkylate research octane number increased by 23.46% and 5.09, respectively, when thiophene was added to H2SO4, especially. The proportion of trimethylpentanes can be improved from 58.57% to 84.58%, and the ratio of trimethylpentanes to dimethylhexanes was doubled, compared with that in the presence of H2SO4 alone. The improved performance in the presence of thiophene additive in H2SO4 is attributed to the improvement in carbenium-ion stability and the acceleration of hydrogen proton release, because of the existence of electron-rich π-bond compounds, and the substitution of thiophene in the alpha site by the sulfonic group and isobutane/butene. Reaction parameters such as reaction temperature, reaction time, stirring speed, feed rate, and acid/hydrocarbon volumetric ratio were also investigated in detail. The synergetic mechanism of thiophene-H2SO4 on isobutane/butene alkylation was also speculated.

Introduction

Gasoline from isobutane/butene alkylation has a high octane number, a low vapor pressure and free of aromatics, sulfur and olefins [1], [2]. Much attention has been devoted to the alkylation of isobutane with butene in recent years [3], [4], [5]. Alkylates can be obtained only in the presence of strong acids, such as solid superacid, strong acid ionic liquids (ILs), hydrofluoric acid (HF) and sulfuric acid (H2SO4) [6], [7], [8].

Solid acid catalysts, especially zeolites, exhibit a high catalytic activity and selectivity initially, and it was hoped that they could substitute for traditional liquid acids, but they are deactivated rapidly because of the covering of acid sites and carbon deposition [9], [10], [11]. Liu et al. reported that the selectivity of trimethylpentanes (TMPs) is greater than 85% and the research octane number (RON) of alkylates is 98–101 in isobutane/butene alkylation when using acid chloroaluminate ILs [12], [13], [14]. However, this catalyst is extremely oxophilic and can lose hydrogen chloride easily, which results in catalyst deactivation and a reduction in halide content that is accompanied by an unfavorable environmental impact. Because of the high volatility of HF as a catalyst, toxic aerosol clouds can form in the case of gaseous HF leakage and be dispersed rapidly over several kilometers [8]. For safety, most industrial plants use H2SO4 as the catalyst in isobutane/butene alkylation.

Because of its high stability and excellent catalytic performance, H2SO4 has been used extensively as the preferred and commercial alkylation catalyst [15], [16], [17]. The main disadvantages of H2SO4 are associated with its high catalyst consumption (70–100 kg H2SO4 per metric ton of alkylate produced), which requires a regeneration plant near the alkylation unit and increases the operating cost because the regenerated H2SO4 is two to three times more expensive than fresh acid [18], [19]. Therefore, approaches for improved processing using H2SO4 have been investigated [20], [21].

Thus, many efforts were devoted to enhance the alkylate quality and to reduce the acid consumption in H2SO4 alkylation process. In the earlier stages, surfactant compounds were used extensively as additives in H2SO4 alkylation process. Rakow et al. reported that additives of dodecylbenzene sulfonic acid and p-phenylenediamine in H2SO4 enhanced the quality and yield of alkylates in isobutane/butene alkylation [22], [23]. McCoy et al. found that the selectivity of C8 and RON increased 7–10 and 2.4–2.6, respectively, in the presence of N,N′,N″-tri(alkyl) phosphoric triamide in alkylation of isobutane/butene as catalyzed by H2SO4 [24]. Chen et al. reported that the C9+ reduced from 43.59% to 25.46–32.48% and the TMPs increased from 37.57% to 44.31% compared with those of H2SO4, when anionic surfactants (amino hydroxynaphthalene disulfonic acid and naphthalenesulfonic acid) and cationic a surfactants (dimethylbenzylamine, dimethyl toluidine and dimethyldecylamine) was added to H2SO4 [25]. In the latter stage, owing to the tunable ability and excellent solubility of ILs, additives of imidazolium ILs with anions of HSO4, Tf2N and OTF had a significant effect on catalyst reactivity and the selectivity towards TMPs in isobutane/butene alkylation [26], [27]. In our previous work, when [Bmim][SbF6] was added to H2SO4, the TMPs and RON selectivity were enhanced slightly, whereas the catalyst lifetime was prolonged markedly [1], [28], [29]. Li et al. reported that caprolactam as a new additive in H2SO4 lead to a significant increase in C8 selectivity from 80% to 88% because of the improved solubility of isobutane in H2SO4 [30]. The results inspired us that special organic compounds can favor the selectivity of TMPs and the quality of alkylates in isobutane/butene alkylation. Especially, the use of aromatic additives of benzene and hexamethylbenzene in chloroaluminate ILs in isobutane/butene alkylation led to a 20% increase in C8 selectivity and an increase in RON from 88.4 to 95.6, which was attributed to the promoter formation of alkyl benzene through the electron-rich π-bond of benzene with coordination Lewis acid sites [31]. However, the aromatic additive in strong Brønsted acid such as H2SO4 have not been studied and its roles in strong Brønsted acid sites are also unclear, and more efforts were required to research their catalytic performance and mechanism in isobutane/butene alkylation.

Aiming at studying the aromatic additive in H2SO4, different electron-cloud densities of aromatic-compound additives, which contain different structure and electronic types of delocalized π-bonds, such as pyridine, benzene and thiophene, as shown in Fig. 1, were selected as additives to investigate the effect of aromatic compounds on Brønsted acid sites in isobutane/butene alkylation that was catalyzed by H2SO4. Particularly, for thiophene, which present excellent enhancement, we studied its catalytic properties in H2SO4 on isobutane/butene alkylation thoroughly. Key parameters that affect the alkylates, including the reaction temperature (T), reaction time (t), stirring speed, feed rate, and acid/hydrocarbon volumetric ratio (H/C) were investigated in detail with thiophene as a probe additive. Possible catalytic features that were based on the carbonium alkylation mechanism in isobutane/butene alkylation catalyzed by H2SO4 in the presence of thiophene were speculated and elaborated.

Section snippets

Materials and instruments

Sulfuric acid (H2SO4, ≥98 wt%) and benzene were from Beijing Chemical Works (Beijing, China). Thiophene, pyridine and D6-dimethyl sulfoxide (DMSO) were from Aladdin Chemistry Co. Ltd. (Beijing, China). 4-Methyl-3-pentene-2-one (mesityl oxide) was from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). All other chemicals were of analytical reagent grade and were from commercial sources unless indicated otherwise. Isobutane was from LingGas, Ltd. (Beijing, China). Butene was from Sinopec

Catalytic performance of different additives in H2SO4 in isobutane/butene alkylation

The effects of different additives in H2SO4 on the catalytic performance in the alkylation are listed in Table 2. When using H2SO4 (blank), the selectivity of C8 and the RON were 68.80% and 93.46, respectively, and consisted of a lower ratio (5.79) of TMPs to dimethylhexanes (DMHs). Same results were obtained in the presence of pyridine in isobutane/butene alkylation that was catalyzed by H2SO4, and may relate to a low electron density in the polarized delocalized π-bond because of the strong

Conclusions

The catalytic performance of aromatic-compound additives in H2SO4 on isobutane/butane alkylation was studied and its catalytic features were explored. In the presence of thiophene, an improved catalytic performance was obtained compared with that of H2SO4 only; selectivity of C8 and RON were 23.46% and 5.09 higher, respectively, compared with those of H2SO4, but the selectivity of C9+ was reduced by 13.45%. The selectivity of the TMPs increased significantly from 58.57% to 84.58%, whereas that

Acknowledgments

This work was supported by the National Natural Science Foundation of China Petroleum & Chemical Corporation Joint Fund (U1662129) and the One Hundred Talent Program of CAS. It was also supported by the Key Program of National Natural Science Foundation of China (91434203), Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-JSC0 11), Fund of State Key Laboratory of Multiphase Complex Systems, IPE, CAS (MPCS-2015-A-05).

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