Study Of Skeletal Isomerization Of 1-Butene On MCM-49 And MCM-22 Zeolite Catalysts

The demand of isobutene is continually increased every year because it is widely used in many chemistry industry processes as chemical raw material or reaction intermediates. Skeletal isomerization of 1-butene is of considerable interest as an alternative route to produce isobutene because of the cheapness of raw material and its simple technique. So far, various proton-form zeolites, including ZSM-22, ZSM-23, SAPO-11, and ferrierite (FER) were found to be active catalysts for the selective skeletal isomerization of 1-butene. However, a common problem is that most of the studied catalysts exhibited unsatisfactory selectivity and/or poor stability. Besides, many questions such as the nature of active sites, the role of coke, the reason of deactivation and the reaction mechanism are still unclear. Therefore, it is a highly attractive and challenging research subject to search new and effective catalysts and to solve the basic problems for the skeletal isomerization of 1-butene. In this work, MCM-49 and MCM-22, two new members of 10-member-ring zeolites, were selected as novel catalysts for the skeletal isomerization of 1-butene. The structure and the physiochemical properties of the zeolites catalysts were characterized by various characterization means. The relationship between the acidic properties of the catalysts and the catalytic performance was systematically studied. Moreover, the nature of active centers, role of surface acidity and the isomerization mechanism were discussed. The main experimental results and conclusions are list below: 1. Highly crystalline and pure MCM-49 and MCM-22 with high Si/Al ratio are synthesized by improved dynamic and static hydrothermal methods. Compared with the static method, the samples synthesized by dynamic method show higher crystalline level and larger specific surface area. The results of NH3-TPD and FT-IR suggest that the strength and the concentration of Bronsted and Lewis acidity in the sample synthesized by dynamic method were slightly higher than those synthesized by static method. 2. Both MCM-49 and MCM-22 are active for the skeletal isomerization of 1-butene to isobutene. The main side-product is propene, and other side-products like C6 (or above) are undetectable. The catalytic performance of the catalysts can be considerable influenced by various factors, such as preparation method, Si/Al ratio, reaction conditions (temperature, space velocity) etc. Under an optimized reaction conditions (i.e. T = 450oC, WHSV = 42 h-1, P1-C4== 0.01 Mpa, reaction-time = 1 h), the MCM-49 sample (Si/Al=15, dynamic method) shows a 56.5% conversion of 1-butene, and 51.4% selectivity of isobutene, while the MCM-22 sample (Si/Al=15, static method) exhibits a relatively high catalytic performances with 47.9% conversion of 1-butene and 68.3% selectivity of isobutene, which is comparable with the previously reported FER and SAPO-11 zeolites. By the analysis of the reaction data and related references, we believe that theformation of isobutene is mainly followed a mono-molecule mechanism, while the side-products are formed via a bi-molecule mechanism over the MCM-49 and MCM-22 catalysts. 3. The catalytic performance especially the selectivity to isobutene of MCM-49 can be considerably improved by the controlled treatment with phosphorous acid or boric acid. For example, the isobutene selectivity on the MCM-49 sample (Si/Al = 15, dynamic method) increase from 51.4% to 79.6%, together with a slightly decrease of n-butane conversion after the treatment with 2.5% phosphorous acid. Modification of MCM-22 zeolites (Si/Al = 30, static method) with different deposition agent, such as tetraethoxysilane (TEOS), silicon tetrachloride (SiCl4) etc., can also improve the selectivity of isobutene (i.e. from 67.2% to 82.3%). 4. The results of NH3-TPD, FT-IR and MAS-NMR suggest that the modification of MCM-49 and MCM-22 with different agents could mainly diminish the Br?nsted acidic sites located on the external surface of zeolites, which is believed non-shape selective to isomerization of 1-butene, thus increasing the selectivity to the main product of isobutene. Therefore, the isomerization of 1-butene mainly occurs inside of the microporous channel the zeolites. The Br?nsted acidic sites located on the internal surface of the zeolites are proved to be the main active centers for the selective skeletal isomerization of 1-butene to isobutene. And no obvious role of Lewis sites on the isomerization reaction is detectable over these catalysts. It can be conclude that the catalytic performance of both MCM-49 and MCM-22 could be influenced by the acidic types (L or B acid), acidic strength, acidic density and by the location of the acidic sites (related with the size and space microporous channel).