| Ethylene glycol is an important organic raw material in petrochemical industry, mainly utilized for polyester fibers, antifreeze, unsaturated polyester resin, etc preparation. The traditional synthesis methodology of ethylene glycol is through the oil route, that is ethylene epoxidation then follow rehydrated route to prepare. But some problems exist in this synthesis route such as low selectivity of target products EG, with side products and high energy consumption, etc. Economical efficiency of epoxidation of preparation from ethylene to ethylene glycol is gradually reduced along with the increasing depletion of oil resources in the world. Combined with more coal but less oil of energy structure in our country, much attention was paid on environment friendly and high economic carbon synthetic route. In recently years, a large amount of research work is carried out on the synthesis of ethylene glycol through synthetic gas from dimethyl oxalate by the indirect method with Cu/SiO2 as catalyst to achieved great success. But Cu-based catalyst is deactivated easily, and the leaching of SiO2 and the inferior product quality is inevitable. This dissertation focuses on efficient, stable, constructed without SiO2 dimethyl oxalate hydrogenation catalyst system to carry out preparation of sintered metal fiber structure of Au-Pd catalysts of primary battery and its catalytic performance. The main results are given below:Firstly, this thesis primarily used galvanic exchange reaction method to prepare four Pd/X-fiber catalysts (X represent Cu, Ni, Al, stainless steel 316 L (SS) with diameter of 8-30μm). We found that under the same reaction condition, the Pd/Cu-fiber supported catalysts (10 vol%,90% vol% voidage) exhibit the best DMO hydrogenation activity. And then, catalyst preparation parameters such as palladium-precursors, palladium-contents, calcination atomosphere and temperature were investigated also reaction conditions effect on the Pd/Cu-fiber catalysts to act on DMO hydrogenation performance. The best preparation of catalyst of O.1Pd/Cu-fiber, the conversion of DMO and EG selectivity could be obtained as 98% and 90% respectively under the reaction conditions as following:270℃,2.5 MPa, liquid weight hourly space velocity (LWHSV) of 5.3 h-1 and H2/DMO molar ratio of 180. XRD, TEM and H2-TPR results indicated that Pd2+could be reduced by Cu-fiber and uniformly dispersed in the surface of the catalyst, Pd can be effectively activated hydrogen thereby leading to an activity promotion; but it also will promote Cu2O reduction, which was adverse to the catalyst stability.Secondly, we used monolithic sintered metal copper fiber as carrier (structure consisting of 5 vol% 30-μm Cu-fiber and 95 vol% voidage) with good heat and mass transfer performance to prepare Au/Cu-fiber catalyst with copper fiber structure used in catalytic hydrogenation DMO by throught HAuCl4-Cu galvanic replacement reaction to generate Au particles supported on copper-fiber surface. Through the Au content, calcination temperature and other factors investigated process, we prepare the catalyst 0.5Au/Cu-fiber with best performance of dimethyl oxalate hydrogenation, among them,0.5 wt% Au onto Cu-fiber and gained by calcining in air at 300℃. Over this catalyst, the DMO conversion of 92% was achievable with the EG selectivity of 60% under the optimal reaction conditions, i.e.,270℃,2.5 MPa, LWHSV of 5.3 h-1 and H2/DMO molar ratio of 180. This catalyst has better stability and no signs to show deactivation after 80 h. Result shows on the surface of catalyst to form Cu2O and CuO by original battery replacement reaction. Au nanoparticles were found to be dispersed on the surface of Cu-fiber, as evidenced by XRD, SEM and TEM/SEM. H2-TPR results indicated that Au could inhibit the reduction of Cu+species thereby leading to an improvement of catalyst stability. However, the activity and the EG selectivity of this catalyst was unacceptably low.Thirdly, in order to solve the problem such as the low stability of 0.5Pd/Cu-fiber catalyst and the low reactivity of 0.5Au/Cu-fiber catalyst, through the route of galvanic replacement reaction to load Au, Pd on the surface of Cu-fiber to prepare ternary Pd-Au-CuOx catalysts system, finally can be used for vapor-phase dimethyl oxalate (DMO) hydrogenation. As expected, the catalyst has excellent catalytic activity and good stability. The best catalyst 0.5Au-0.1Pd/Cu-fiber (Au:0.5 wt%, Pd: 0.1 wt%; calcination temperature:300℃) was capable of converting 98% DMO into EG product at a selectivity of 93% under the optimal reaction conditions, i.e.,270℃, 2.5 MPa, LWHSV of 5.3 h"1 and H2/DMO molar ratio of 180. In stability test during 200h, the catalyst activity and selectivity has remained stable, no signs show deactivation. The test result of ICP-AES, TEM, H2-TPR, XPS and STEM suggest that, Au、Pd can be replaced efficiently to deposited on the surface of Cu-fiber; Au、Pd and Cu+can form a Au-Pd-Cu+ternary active site on structure; The Pd-Au-CuOx synergistic effect promoted the hydrogenation activity and stabilized Cu+sites to suppress deep reduction deactivation. A ternary Pd-Au-Cu+complex was tentatively proposed as illustrated in Scheme, in which Cu+acted as an essential site for the gas-phase DMO hydrogenolysis, and Au-Pd alloy synergistically promoted the hydrogenolysis activity of Cu+sites as a result of strong Au-Pd electron interaction while Au played a key role in stabilizing Cu+sites to prevent deep reduction deactivation especially the Pd-catalyzed reduction. |
PDF Full Text Request