Effect Of Precursor On Cu-Zn-Al Catalysts By Complete Liquid-Phase Technology For Ethanol Synthesis

With high octane number,low toxicity,low sulfur content and ash content,ethanol（EtOH）is promoted as one kind of green high quality alternative liquid fuel and an excellent gasoline additive.Grain fermentation and ethylene hydration method are two traditional ethanol synthesis methods,which have the problems of shortage of raw materials and large energy consumption.In view of the relatively rich coal resources in China,the synthesis of ethanol from coal based syngas is a promising approach with great research value.In our previous work of methanol synthesis,it has been found that Cu-Zn-Al catalyst prepared by complete liquid-phase method had a certain capacity for ethanol synthesis.However,the highly selective ethanol catalyst was sensitive to the precursor structure and was difficult to be reproduced.In later studies,it was found that the precursor structure was affected by many factors such as sol preparation method,aging process,gelling agent and so on.In addition,we found that Al OOH can promote CO dissociation and carbon chain growth.Thus in this paper,Cu-Zn-Al catalysts were prepared by complete liquid phase method,and the effects of different precursors’ structure on catalytic performance of Cu based catalysts were studied for the direct synthesis of ethanol from syngas.The structure of precursor was adjusted by changing the preparation methods（alcoholysis,hydrolytic method）,the amount of Al OOH sol,aging method（mixed aging method,separated aging method）and gelling agent.The catalysts were characterized by XRD,N₂-adsorption-desorption,H₂-TPR,NH₃-TPD-MS,FT-IR,TG-DTG,27Al-MAS-NMR,XPS,TEM.The main conclusions were as follows:（1）A new method was invented by preparing Cu-Zn-Al sol（the first aluminium source）,AlOOH sol（the second aluminium source）separately to get the precursor and then synthesize the catalyst without other promoters.By this method,the stability of the catalysts were increased to 120 h and the CO conversation could reached to 60%.（2）Adding different amount of AlOOH sol,the initial selectivity of ethanol（Et OH）/ total alcohol（ROH）was the highest when the molar ratio of Cu:Zn:Al:AlOOH was 2:1:0.8:0.6 using the preparing method of conclusion1.Under this proportion,the EtOH/ ROH selectivity using alcoholysis method is higher than that of using hydrolysis method.Meanwhile,the EtOH/ ROH selectivity of mixed aging method is higher than that of separated aging method.When using the hydrolytic method,the initial selectivity of EtOH/ROH was as high as 70.3% for the catalyst using separated aging method and was 35.4% for the catalyst using mixed aging method.As for using the hydrolytic method,the initial EtOH/ROH selectivity could reach up to 27.1% when using mixed aging method and was 15.8% when using separated aging method.（3）Experimental results of the method from conclusion1 showed that aluminum species existed in the catalysts was in an amorphous form of AlOOH and there were five coordinated aluminum and octahedral aluminum before reaction.During the reaction,a proportion of Al species of the catalyst changed from AlOOH to Al2O3.Meanwhile,tetrahedral aluminum（AlOⅣ）and octahedral aluminum（AlOⅥ）presented in the catalyst and five coordinated aluminum（Al OV）disappeared after the reaction.Combined with the results of ethanol selectivity,it can be speculated that high EtOH/ ROH selectivity requires a appropriate proportion of AlOV and Al OⅥ.What’s more,when the proportion of AlOⅥ is much higher than Al OV and existing AlOⅣ,it is conducive to the formation of DME.（4）When using citric acid as gelling agent,the selectivity of EtOH/ROH decreased from 29.6% to 20.2% at 72 h and then increased to 42.6% at 96 h.Meanwhile,the CO conversion of the catalyst was up to 37.7% at 48 h,and then decreases after.In a word,the method which preparing Cu-Zn-Al collosol（the first aluminium source）and AlOOH collosol（the second aluminium source）separately could remarkably improve the stability and reproducibility of the Cu based catalysts.