| The producing of nylon6and nylon66from the selective hydrogenation of benzene to cyclohexene is attracting increasing attention since it is a low-carbon, eco-friendly, energy-saving and safe process. However, thermodynamics make it difficult to obtain cyclohexene from selective hydrogenation to cyclohexene. Thus developing a catalyst and a catalytic system with high selectivity to cyclohexene is the key of this technology.This article is divided into eight chapters to address the following four problems.First, the influence of transition metals (Cr, Mn, Fe, Co, Ni, Cu and Zn) as the promoters on the performance of Ru catalyst was investigated in the first three chapters. It is found that monometallic Ru catalyst showed the little selectivity to cyclohexene in the absence of ZnSO4. However, it gave a high selectivity to cyclohexene in the presence of ZnSO4. It is confirmed that the Zn species on the catalyst after hydrogenation mainly existed as ZnO and a (Zn(OH)2)3(ZnSO4)(H2O)x (x=1,3,5or7) salt, which were formed by the hydrolysis of ZnSO4. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt enhanced the selectivity to cyclohexene of the monometallic Ru catalyst. The promoters Cr, Co, Ni, Cu, Mn, Fe and Zn in the corresponding catalysts were mainly present in Cr2O3, Co3O4, Ni(OH)2, CuO (Cu2+2(OH)3Cl), Mn3O4, Fe3O4and ZnO respectively. It is found that these promoters could not improve the selectivity to cyclohexene of the Ru catalyst in the absence of ZnSO4. The amount of the chemisorbed (Zn(OH)2)3(ZnS04)(H2O)x decreased with the increase of the amount of Cr2O3, leading to the decrease of the selectivity to cyclohexene. The amount of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x slightly increased with the increase of the amount of Co3O4and Ni(OH)2, slightly improving the selectivity to cyclohexene. The promoters Mn3O4, Fe3O4and ZnO could react with the ZnSO4in the slurry to the (Zn(OH)2)3(ZnSO4)(H2O)x salt. Thus the amount of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x remarkably increased with the increase of the amount of Mn3O4,Fe3O4and ZnO, resulting in the significant decrease of the activity and the significant increase of the selectivity to cyclohexene. The Ru-Mn(0.23) catalyst, the Ru-Fe(0.47) catalyst and the Ru-Zn(0.27) catalyst gave the maximum cyclohexene yields of55.3%,56.7%and53.4%respectively. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt played a key role in improving the selectivity to cyclohexene of the Ru catalyst. The roles might be attributed to the following four reasons.(1) The (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) could be uniformly dispersed on Ru particles. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt could selectively cover the most reactive Ru sites, which could reduce the active sites for the chemisorption of cyclohexene and suppress the further hydrogenation of cyclohexene to cyclohexane.(2) The (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) chemisorbed on Ru surface caused Ru catalyst to be surrounded by a firm stagnant water layer. The existence of the stagnant water layer on the surface of the Ru catalyst could accelerate the desorption and hinder the re-adsorption of cyclohexene for further hydrogenation to cyclohexane due to the very low solubility of cyclohexene, resulting in the improvement of the selectivity to cyclohexene.(3) The Zn2+of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) could form loosely bound adducts with cyclohexene, which could stabilize the formed cyclohexene on the surface of Ru catalyst and improve the selectivity to cyclohexene of Ru catalyst.(4) The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) could modify the electronic structure of the active Ru to suit the selective hydrogenation to cyclohexene.Therefore, the activity of the Ru catalyst monotonically decreased and the selectivity to cyclohexene monotonically increased with the amount of the chemisorbed (Zn(OH)2)3(ZnSO4)(H20)x salt (x=1,3,5or7).The promoter CuO could react with ZnSO4to from a Cu4(SO4)(OH)6(H2O) salt.The synergistic effect of the chemisorbed Cu4(SO4)(OH)6(H2O) salt and the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) improved the selectivity to cyclohexene of Ru-Cu(0.49) catalyst.Besides, the influence of the pretreatment on the performance of the Ru-Zn(8.6%) catalyst was also investigated in the chapter2. In the process of the pretreatment, more ZnO could react with ZnSO4to form more (Zn(OH)2)3(ZnSO4)(H2O)5salt. Moreover, the pretreatment of the catalyst in the presence of ZnSO4made the (Zn(OH)2)3(ZnSO4)(H2O)s salt be stably chemisorbed on the surface. Therefore, the pretreatment made the Ru-Zn(8.6%) catalyst exhibit the excellent selectivity to cyclohexene.Second, the influence of the rare-earth elements (La and Ce) on the performance of the Ru catalyst was investigated in the chapter5and6. It is found that La and Ce mainly existed as La(OH)3and CeO2in the corresponding catalysts. La(OH)3and CeO2alone could not improve the selectivity to cyclohexene of the Ru catalyst. However, almost all of the La(OH)3and CeO2could react with the ZnSO4to form the (Zn(OH)2)3(ZnSO4)(H2O)3salt. Thus the amount of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3salt significantly increased with the amount of La(OH)3and CeO2, resulting the decrease of the activity and the increase of the selectivity to cyclohexene.Third, the influence of the alcohols on the performance of the Ru-Zn(2.8%) catalyst was investigated in the chapter7. It is found the Ru-Zn(2.8%) catalyst prepared by co-precipitation exhibited the excellent selectivity to cyclohexene in liquid phase hydrogenation of benzene due to the nano-size effect and the chemisorbed ZnSO4·3Zn(OH)2·7H2O. The cyclohexene yield of the Ru-Zn(2.8%) catalyst generally increased with the molecular weight of PEG in the presence of ZnSO4. PEG-10000as a reaction modifier made the Ru-Zn(2.8%) catalyst give the highest yield of62.3%in the presence of ZnSO4for the following two reasons.(1) The addition of PEG-10000in the presence of ZnSO4could make more Zn2+chemisorbed on the surface of the catalyst. The Zn2+ions chemisorbed could be beneficial for stabilizing the formed cyclohexene, accelerating the desorption and hindering the re-adsorption of cyclohexene on the surface of the catalyst.(2) The PEG-10000added might form two molecular hydrogen bonds in eight-numbered rings with the cyclohexene in the liquid phase, which could stabilize and hinder re-adsorption of the cyclohexene in the liquid phase.2g of this catalyst with0.2g PEG-10000as a reaction modifier pretreated in the presence of ZnSO4gave a selectivity to cyclohexene of78.6%and the best yield of64.5%at the benzene conversion of82.0%at15min. The very small Carberry numbers and Wheeler-Weisz groups indicated that the liquid-solid mass transfer and the pore diffusion resistance were negligible. This suggests that the achievement of the best yield of cyclohexene could not be simply ascribed to physical effects such as hydrogen or cyclohexene mass transfer limitation. Thus the actual compositions of the pretreated Ru-Zn(2.8%) catalyst during hydrogenation and the liquid phase could strongly affect the performance of the catalyst. Moreover, this catalyst had an excellent stability since ZrO2and PEG-10000could effectively suppress the agglomeration of the catalyst.Fourth, the influence of diethanolamine as a reaction modifier on the performance of Ru-Zn catalyst for selective hydrogenation of benzene to cyclohexene was investigated. It is found that diethanolamine could react with ZnSO4to form (Zn(OH)2)3(ZnSO4)(H2O)3and diethanolamine sulfate. The amount of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3increased with the amount of diethanolamine added. The synergism of (Zn(OH)2)3(ZnSO4)(H2O)3and diethanolamine sulfate enhanced the selectivity to cyclohexene of the Ru-Zn(4.9%) catalyst. When the dosage of diethanolamine was0.3g,(Zn(OH)2)3(ZnSO4)(H2O)3was highly dispersed on the surface of the catalyst, and the Ru-Zn(4.9%) catalyst gave a selectivity to cyclohexene of75.5%and a cyclohexene yield of63.6%at the benzene conversion of84.3%in the third reused time. Moreover, the benzene conversion and the selectivity to cyclohexene were stable above75%and cyclohexene yield was above58%in the fourth reused time.In a word, the (Zn(OH)2)3(ZnSO4)(H2O)x salt (x=1,3,5or7) chemisorbed on the Ru surface played a very key in improving the selectivity to cyclohexene of the Ru catalyst. |
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