Vapor-phase isomerization of endo-tetrahydrodicyclopentadiene to its exo isomer over zeolite catalysts

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Abstract

Vapor-phase isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) to exo-THDCPD over zeolite catalysts (HY, H-USY, Hβ, HZSM-5 and HMOR) was studied as a green synthesis route for exo-THDCPD. The HY and H-USY catalysts showed catalytic activity while the Hβ, HZSM-5 and HMOR catalysts did not. Higher yield of exo-THDCPD was obtained over the H-USY catalyst than that over the HY catalyst, attributing to the presence of strong acid sites and mesopores in the H-USY catalyst. The yield of exo-THDCPD obtained via vapor-phase isomerization was better than that reported via liquid-phase isomerization. In addition to the properties of the catalyst itself, the vapor-phase isomerization process of endo-THDCPD was also affected by the reaction temperature, the concentration of endo-THDCPD, the contact time and the time on stream. Under the optimized conditions, high purity (>98 wt%) exo-THDCPD was directly obtained without any separation processes. Although deactivation of the catalyst with the time on stream was observed, the activity could be recovered easily via regeneration in air.

Highlights

▶ H-USY is an efficient catalyst for vapor-phase isomerization of endo- to exo-THDCPD. ▶ The conditions for isomerization in vapor phase are milder than those in liquid phase. ▶ Exo-THDCPD with purity of ≥98 wt% can be directly obtained from the reactor outlet.

Introduction

Due to its high volume energy density, extremely low freezing point and good stability for long period in storage, exo-tetrahydrodicyclopentadiene (exo-THDCPD) can be utilized for jet and rocket engines either as a high energy liquid fuel per se (designated as JP-10) or as a diluent for other high density fuels [1], [2], [3].

Exo-THDCPD was usually prepared by isomerizing its endo isomer (endo-THDCPD) with a strong acid catalyst. To date, various acid catalysts including Brönsted and Lewis acids have been studied for this purpose [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Among these acid catalysts, concentrated sulfuric acid (99.8% H2SO4) and aluminum trichloride (AlCl3) are the most studied ones and both of them are effective for catalytic isomerization of endo- to exo-THDCPD [2], [3], [4], [6]. These acid catalysts, however, are not benign to environments due to their corrosive nature and purification problems. In recent years solid acids have attracted more interests as green catalysts for isomerization of endo-THDCPD, among which zeolite catalysts are the most promising ones [8], [9], [10], [11], [12]. Most of these studies, however, were focused on isomerizing endo-THDCPD to adamantane rather than exo-THDCPD, which was regarded as a byproduct. Until very recent years, zeolites have not been used as catalysts for endo- to exo-THDCPD isomerization in the liquid phase [13], [14], [15]. In order to enhance the conversion rate of endo-THDCPD, high temperature was employed. The use of high temperature, however, is not preferred for conversion of endo- to exo-THDCPD due to the exothermic characteristics of this process [15]. In addition, exo-THDCPD is an intermediate in the consecutive reaction of endo-THDCPD to adamantane [9], [11], [15]. The control of reaction time is thus very important to maximize the yield of exo-THDCPD. From this point of view, a flow system is better than the batch one.

In this study, vapor-phase isomerization of endo- to exo-THDCPD over zeolite catalysts was investigated for the first time in a flow system, which is a green synthesis route for exo-THDCPD due to its simplicity, effectiveness and no use of any organic solvents.

Section snippets

Materials

Endo-THDCPD (99.3 wt%) and the HMOR zeolite sample were prepared in our laboratory. The NaY, NH4USY and Naβ zeolite samples were obtained from Xinnian Catalyst Co., China and the HZSM-5 zeolite sample was obtained from Nankai Catalyst Co., China. NaY and Naβ zeolites were first transformed into NH4Y and NH4β zeolites, respectively, by ion-exchange in 1 M NH4NO3 solution under stirring at 70 °C for 2 h for three times. NH4Y, NH4β and NH4USY zeolites were then transformed into acidic HY, Hβ and H-USY

Catalytic activities of different types of zeolites

Different types of zeolite catalysts (HY, H-USY, Hβ, HZSM-5 and HMOR) were tested for vapor-phase isomerization of endo-THDCPD. The results are summarized in Table 1. It can be seen that the Hβ, HZSM-5 and HMOR catalysts showed no activity, which is similar to the observed results for the liquid-phase isomerization [13], [14], [15]. On the other hand, the endo-conversion was 29.8% over the HY catalyst and 98.1% over the H-USY catalyst. The catalytic activity of the H-USY catalyst was also

Conclusions

High purity (>98 wt%) exo-THDCPD could be directly prepared via vapor-phase isomerization of endo-THDCPD. The H-USY catalyst is the most active catalyst for this process among HY, H-USY, Hβ, HZSM-5 and HMOR zeolite catalysts. The vapor-phase isomerization of endo-THDCPD could proceed effectively over the H-USY catalyst under milder conditions than the liquid-phase isomerization. In addition to the properties of the catalyst itself, the vapor-phase isomerization process was also affected by

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (grant no. 50976103) for financial support and Professor Zheng-bao Wang's group (Department of Chemical and Biological Engineering) for NH3-TPD measurement.

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