1,4-butanediol Selective Dehydration To3-Buten-l-ol Over Modified ZrO₂-Based Catalyst

3-Buten-1-ol (BTO) is a valuable chemical that is widely used in food flavoring, agricultural products, and especially in the medical field due to its active character. In addition, served as a valuable intermediate for synthesizing heterocyclic drugs, the market demand for BTO has been increasing year by year. The method used for BTO synthesis from1,4-butanediol (BDO) selective dehydration offers some advantages that include mild reaction conditions, a simple experimental procedure, and environmental friendliness. However, this approach has two favorable thermodynamic pathways. One is the intramolecular cyclodehydration of BDO to give tetrahydrofuran (THF), which in this context is a byproduct, while the other is the dehydration of BDO involving the terminal hydroxyl group and a β-H atom on the adjacent carbon atom. To increase BTO yield by this reaction pathway, particular attention were paid to prepare appropriate catalysts. Thus, it is a challenging task to obtain a catalyst that favors producting BTO by dehydration of BDO via regulating and controlling the texture, the structure, and surface properties of the materials.On the basis of our previous studies, a varity of single-component oxide catalysts were prepared. The effects of catalysts surface properties on catalytic activity were investigated. Then, a series of modified ZrO2catalysts were prepared by the impregnation method and the acid-base properties of the catalysts can be controlled by the amount of introduction component and calcination temperature. Through the synergistic interaction of the acid and base sites, the yield of BTO can be increased effectively. Combined with the catalytic performances evaluation, the thorough study of the BDO dehydration to BTO was investigated to infer and clarify the catalytic mechanism. The main results are listed as follows:1. A variety of single-component oxide catalyst (Al2O3, MgO, CaO, ZrO2and SiO2) were investigated in BDO selective dehydration reaction. The results show that the acid-base property of the catalysts plays an important role in the catalytic activity and the product distribution. The catalysts as Al2O3, whose surface mainly consists of acid sites, show the high BDO conversion but the low BTO selectivity; while the catalysts (MgO, CaO), whose surface mainly consists of base sites, exhibit the high BTO selectivity but the low catalytic activity. Among the single-component oxide catalysts studies, the ZrO2catalysts have the best catalytic activity in BDO selective dehydration reaction due to their acid-base bifunctional properties on the surface.2. The acid-base bifunctional ZrO2catalysts modified by different alkaline earth metal (CaO, SrO, BaO) were prepared and characterized using N2physisorption, XRD, FT-IR spectra, NH3-TPD and CO2-TPD techniques to obtain the texture, the structure, and surface properties of the catalysts. On basis of characterizations, the catalytic activities of BDO selective dehydration to BTO over the acid-base bifunctional catalysts were studied in details. The result indicates that the alkaline earth metals make a great impact on the acid-base properties of ZrO2-based catalysts. The Ca2+ion can enter into the ZrO2crystal lattice to form Ca-O-Zr hetero-linkages easily. And the hetero-linkages in binary complex oxides lead to the uneven distribution of charges and generate new acid sites. That is to say, the introduction of appropriate CaO not only increases alkali density, but also keeps acidic sites on ZrO2surface. The synergistic effect of the acid-base sites shows the high catalytic activity and BTO selectivity. Under the same conditions, the Sr2+and Ba2+ions with the large radius is difficult to enter into the ZrO2crystal lattice, and then to form zirconate on ZrO2surface. The zirconate decreases the amounts of acid and base sites in comparison to unmodified ZrO2, and induces the low catalytic activity and BTO selectivity.3. Based on the above work, the effect of different CaO content and calcination temperature of CaO-ZrO2catalyst on the acid-base properties and catalytic performance were investigated. With CaO content lower than12.5%, Ca2+replace the Zr4+to form the Ca-O-Zr structure that not only increase alkali density dramaticlly, but also keep acidic sites on ZrO2surface, which exhibits the high catalytic activity and BTO selectivity. With the further increase in CaO content, the CaO begins to aggregate and generate CaZrO3, and then the total amount of acidic and basic sites decreases significantly. Thus, the BDO conversion and the BTO selectivity decrease obviously due to the decrease of acid-base active center. Combining the studies of the structure of active center and the poisoning experiments used NH3or CO2as cattier gases, it is concluded that the acid-base properties of the CaO-ZrO2catalysts have significant influence in the dehydration of BDO. Every active center forming BTO is composed of two acidic sites and one basic site. The acidic sites act as anchoring sites for OH group to activate BDO while the basic sites interacted with β-hydrogen to form BTO. In order to determine the relationship between the basic property of the catalyst and the catalytic dehydration performance, the mild basicity, MgO, is further introduced to ZrO2surface, and the result shows that the weakly basic sites is favorable to the acid-base synergetic catalytic activity.4. After we summarized the conclusions from the acid-base synergetic catalytic activity, a series of SiO2-ZrO2catalys modified by CaO were prepared in BDO selective dehydration reaction. The results show that the formation of Zr-O-Si hetero-linkages can increase the acidic sites greatly, and the further CaO addition promotes the formation of acid-base synergetic active centers. These catslysts not only exhibit the high BDO conversion and BTO selectivity, but also decrease the complete conversion temperature of BDO. When the reacted temperature is325℃, the highest yield of BTO attains to58%.