Hydrogenation Of Glycol Diester Of New Cu-based Catalysts In The Synthesis And Catalytic Performance Study

Great interest has been aroused due to the steadily growing demand for 1,4-butanediol(BDO) which is widely used as a starting material for polymers and solvents.The polybutylene terephthalate(PBT) and other polyurethances produced from BDO had superior properties as structural and engineering plastics,and play an important role in many fields.Tetrahydrofuran(THF),obtained through the dehydration of BDO,is an indispensable solvent for many polymers and a monomer in the manufacture of polytetramethylene ether glycol(PBT),etc.At present,Davy process has showed great advantage for producing BDO not only the lower operation pressure,but also because well-established fixed-bed reactor technology could be utilized.Moreover,the products distribution could be regulated to fit the market demand using dialkyl maleate as reactant.Ethylene glycol is also an important chemical used in polyesters manufacture or as antifreeze.The catalytic hydrogenation of dimethyl oxalate(DMO) to ethylene glycol(EG) is the second step of indirect synthesis of glycol from syngas,which is a new chemical process including the oxidative coupling reaction of carbon monoxide to diaikyl oxalate and the catalytic hydrogenation of dialkyl oxalate to glycol.In this work,catalytic hydrogenation of dimethyl maleate and dimethyl oxalate has been studied in details.The optimum copper-based catalysts were prepared by changing carriers,adding promoters as well as improving preparation methods.By means of N₂-adsorption,XRD,XPS&AES, SEM,TEM,TPR,FTIR,UV-Vis techniques,the elects of bulk structure and surface component of catalysts on the catalytic performance were investigated.The main contents of this paper are as follows:1.DMM hydrogenation over Cu/ZnO/Al₂O₃ catalystFor the purpose of obtaining higher yield of BDO,we adjusting the hydrogenation and dehydrogenation ability by preparation of Cu/ZnO/Al₂O₃ catalysts of different Zn:Al molar ratio.It is found that when Zn:Al is higher,the crystallinity of Cu species is also higher and the crystallite size larger,but its reduction temperature is smaller than the one with lower Zn:Al ratio,which is because of the promotion effect of ZnO to the reduction of CuO.When Zn:Al is lower,Cu species can be well dispersed,resulting in smaller CuO and Cu particles.However,because of the strong interaction between Cu species and Al₂O₃,the Cu species are reduced at higher temperatures and the catalysts display a lower Cu⁰ surface area and DMO hydrogenation activity;and also because of its higher acidic Al₂O₃ content,THF selectivity is higher over the catalysts with lower Zn:Al ratio.At a reaction temperature of 220℃,THF selectivity reaches 96%at CZA-14.On the contrary, higher Zn:Al ratio favor the selectivity of BDO.At 180℃,BDO selectivity reaches 73%over CZA-41.For the confirmation of the effect of acid center on BDO dehydration to THF,a small amount of K is added to CZA-14 catalyst by impregnation.It is found that K adding hinders reduction of Cu species,decreases both hydrogenation and dehydration activity and increases BDO selectivity.Moreover,the dehydration product distribution has been substantially changed after K modification.At higher reaction temperatures and higher K loadings,butanol becomes the main dehydration product,which may be because of the preferential covering of Br(o｜¨)nsted acid by K₂O, leaving Lewis acid uncovered.2.DMM hydrogenation over Cu/mesoporous SiO₂ catalyst10～80 wt%Cu was loaded on mesoporous MCM-41 by impregnation method. It was found that the long range ordered structure of MCM-41 was obviously deteriorated during impregnation,and Cu species could not be well dispersed.When this series of catalysts was used in DMO hydrogenation,it was found that 60Cu/MCM-41 catalyst showed the best catalytic performaces,with a BDO yield of 55.5%at a reaction temperature of 240℃.In order to increase copper dispersion and better retention of mesoporous structure,SBA-15 instead of MCM-41 is used as catalyst support and the method of impregnation is improved.It was found that over thus prepared Cu/SBA-15 catalyst, Cu species can be better dispersed yet not better enough,because an average CuO particle size of around 20 nm was detected over 20Cu/SBA-15.Compared with Cu/MCM-41 catalysts,DMO hydrogenation activity maximizes at a much lower Cu loading of 10 wt%over Cu/SBA-15.At 220℃,over 10Cu/SBA-15,a BDO yield of 56.4%was obtained.For the purpose of better understanding of the influence of Cu dispersion and valence state to DMM hydrogenation,Cu/SBA-15 prepared by different methods was carefully characterized and was related with DMM hydrogenation activity.It was found that deposition-precipitation(DP),homogeneous deposition-precipitation(HDP) and Grafting method can well disperse copper species before and after reduction, while incipient wetness impregnation(IWI) method can not.Small angle XRD pattern and N₂ adsorption-desorption data indicate that partial structure collapse was found over Cu/SBA-15 catalyst prepared by HDP method,however,the HDP catalyst shows the best DMM hydrogenation activity and the activity was found to be linearly correlated with the Cu⁰ surface area except for DP and Grafting Cu/SBA-15.The DMM hydrogenation activity of Grafting Cu/SBA-15 catalyst is comparable with DP Cu/SBA-15,while its Cu⁰ surface area was much lower.XPS-AES spectra indicate that after reduction and reaction,Cu⁺ predominates over Grafting Cu/SBA-15,while over other catalysts,Cu⁰ predominates.Combining the XPS-AES results,Cu⁰ surface areas and the catalytic performances,it can be deduced that Cu⁺ plays an important role in DMM hydrogenation.We suppose that it is Cu⁺/Cu⁰ cooperation that determines DMM hydrogenation activity.3.DMO hydrogenation over Cu/commercial SiO₂ catalystThe influence of ammonia evaporation temperature(T_AE) to the structure and DMO hydrogenation activity was studied over Cu/SiO2 catalysts prepared by ammonia evaporation(AE) method.It was found that a lower T_AE(333 K and 343 K) resulted in poor Cu dispersion and a weaker Cu-support interaction,while it was the opposite over Cu/SiO₂ catalysts prepared with higher T_AE.When T_AE was higher(353 K,363 K and 373 K),TEM,FTIR and N₂ adsorption results suggest the existence of copper phyllosilicate,whose amout maximized at T_AE of 363 K and XPS indicates a strongest Cu-support interaction in CuSiO-363 catalyst.As a result,after reduction, CuSi-363 contains the highest amour of Cu⁺.At lower T_AE,DMO hydrogenation activity was found to be linearly correlated with Cu⁰ surface area,while at higher T_AE, the activity was linearly correlated with Cu⁺ surface area,which means that Cu+ also plays an important role over DMO hydrogenation.So we also make a conclusion that Cu⁺/Cu⁰ cooperation determines the activity of DMO hydrogenation.Cu/SiO₂ catalysts was prepared by different methods including wetness impregnation(WI),chemisorption-hydrolysis(CH) and ammonia evaporation(AE), which will result in different Cu-support interactions,in order to better understand Cu⁺/Cu⁰ interaction.It was found that WI can not well disperse Cu species,thus resulted in lowest DMO hydrogenation activity.XPS-AES and N₂O titration results shows that although a higher Cu⁺/Cu⁰ ratio was found over CuSi-AE catalyst,Cu⁺ surface area was almost the same with that of CuSi-CH,however,a much larger Cu⁰ surface area was found over CuSi-CH catalyst,which resulted in a better DMO hydrogenation activity.It was also found that the Cu loading of the CH Cu/SiO₂ catalyst was determined by the SiO₂ support,further increase of Cu nominal loading could not increase the actually Cu loading.4.Di-ester hydrogenation over Cu/mesoporous SiO₂ catalystWe choose different mesoporous SiO₂ as support in order to increase Cu loading and DMO hydrogenation activity of CH CuSiO₂ catalyst.ICP-AES results show that Cu loading was substantially increased over Cu/mesoporous SiO₂ catalysts.Among SBA-15,MCM-41,MCF and HMS,it was SBA-15 supported Cu catalyst that shows the best DMO catalytic activity and the catalyst also best retains the mesoporous structure.Under optimized reaction conditions,an EG yield of 96.2%was obtained. The space time yield was calculated to be 0.758 g/(h·gcat),which was the highest among all the values reported by open literatures.Fenton method was used to remove the P123 surfacant in order to reduce the loss of suface silanols induced by calcination.However,it was found that when the Fenton SBA-15 was used as support,Cu loading and Cu⁰ surface area was not substantially increased when compared with calcined SBA-15,which may be because of the deterioration of SBA-15 support during Fenton and subsequentant CH process. Thus,calcined SBA-15 was again used as support for the preparation of CH Cu/SBA-15 catalyst and the catalyst was subjected to a stability test of 500 h under the optimized conditions.An average EG yield of 96%was obtained over 500 h time on stream,which proves the superior DMO hydrogenation activity and EG selectivity as well as stability of the CH Cu/SBA-15 catalyst.The CH Cu/SBA-15 catalyst was also used in DMM hydrogenation and it was found that it shows higher activity and BDO selectivity than HDP Cu/SBA-15 catalyst. A BDO yield of 74.6%was obtained under optimized conditions.These results indicate that CH Cu/SBA-15 shows superior catalytic performances in both DMO and DMM hydrogenation and is a very promising catalyst for industrial purpose.