Skeletal isomerization of n-butenes to isobutene over acid-treated natural clinoptilolite zeolites

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Abstract

The proton form of the natural clinoptilolite zeolite (HNZ) was modified by treatment with various acids. Effects of modification on catalytic performance were investigated for the skeletal isomerization of n-butenes to isobutene. Among HNZs treated with various acids, only those modified with boric acid (B-HNZ) showed an improved catalytic performance of skeletal isomerization in terms of activity and stability. Furthermore, B-HNZ exhibited higher isobutene selectivity compared to unmodified HNZ at the same conversion of n-butenes. The performance was comparable to that of a widely known ferrierite catalyst at the same reaction conditions. The main effect of boric acid treatment on HNZ was the selective increase in the number of acid sites of moderate strength required for efficient catalysts in skeletal isomerization without collapse of its framework. These acid sites of moderate strength were considered to be selective and stable reaction sites for skeletal isomerization of n-butenes. Coke deposition poisoned non-selective strong acid sites and imposed a spatial restriction on zeolite channels, leading to suppression of undesired dimerization reactions. Factors affecting selectivity to isobutene are discussed.

Introduction

Isobutene is a main raw material for the manufacture of the methyl-tertiary-butyl ether (MTBE), widely used as an octane booster for reformulated gasoline [1]. There also are various applications of isobutene as a chemical intermediate for industrial products. The current primary source of isobutene is C4 raffinates of catalytic cracking fractions. Since the demand for MTBE has continuously increased, considerable interest has been devoted to finding a new, independent source of isobutene. The catalytic skeletal isomerization of n-butenes to isobutene is the most promising way for the production of isobutene that could satisfy the increased demand for isobutene [2].

Recently, various zeolite and molecular sieve catalysts have been applied to skeletal isomerization of n-butenes as efficient catalysts, and studied extensively [3], [4], [5], [6], [7], [8]. The common features shared by these efficient zeolite catalysts are proper acid strength, pore size and unique pore structure with a 10-membered ring [9], [10]. Clinoptilolite, employed as a catalyst in this study, has a pore structure of a monoclinic framework consisting of a 10-membered ring (7.6 × 3.0 Å) and an 8-membered ring (3.3 × 4.6 Å), and is a silica-rich member of the heulandite family [11].

In our previous study [12], we reported that natural clinoptilolite zeolite produced in the Youngil area of South Korea was an effective catalyst for skeletal isomerization.

In the present study, we modified the natural clinoptilolite zeolite by reflux and impregnation with various acids, such as boric acid, phosphoric acid and nitric acid, in order to identify the role of the acidity in skeletal isomerization of n-butenes to isobutene. We also compared modified natural clinoptilolite zeolites with ferrierite, known as the most efficient catalyst.

Section snippets

Catalyst preparation

A natural zeolite (NZ) occurring in the Youngil area of South Korea was employed to obtain the proton-form natural zeolite catalyst (HNZ) by an ion-exchange method. Thus the natural zeolite was ion-exchanged four times with 1 M aqueous solution of NH4Cl (25 ml solution per gram of NZ) at 80°C for 24 h. and the resulting catalyst, the ammonium form (NH4+) of the natural zeolite, was filtered, washed several times with clean distilled water, dried at 110°C for 20 h, and then calcined at 500°C for 4 h

Characterization of acid-treated HNZ

The proton form of the natural clinoptilolite zeolite (HNZ) was modified by various acid treatments in order to investigate the effect of acidic properties on skeletal isomerization of n-butenes into isobutene. Modified HNZ catalysts were prepared in two ways: impregnation and reflux methods. In those methods, only samples prepared by reflux showed good activity for skeletal isomerization.

The physicochemical properties of fresh HNZ and modified catalysts by reflux are shown in Table 1. As the

Discussion

In the present study, we have modified the proton form of the natural clinoptilolite zeolite (HNZ) by acid treatment and investigated its effects on catalytic properties for skeletal isomerization of n-butenes to isobutene. Among acid-treated HNZ, only B-HNZ showed improved activity and stability. Its performance was comparable to ferrierite, known as one of the best catalysts.

In general, it has been known that unique pore structure, proper acidity, and acid site distribution are essential for

Conclusions

Among various acid treatments of natural clinoptilolite, only boric acid treatment by the reflux method shows improved performance of skeletal isomerization of n-butenes to isobutene. The boric acid-treated natural zeolite shows higher selectivity to isobutene, as compared to the untreated zeolite at the same conversion of n-butenes. The performance was comparable to ferrierite at the same reaction conditions. Though ferrierite exhibits good activity, selectivity to isobutene is much lower than

Acknowledgements

The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1998.

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