Study On Cu-based Catalysts For CO₂ Hydrogenation To Methanol

With the continuous advancement of industrialization level and the improvement of people’s living standard,the concentration of CO₂ in the atmosphere has increased remarkably,causing environmental problem and serious waste of C1 resources,which brings obstacles to human survival and sustainable development.Methanol is basic chemical raw material but also serves as a clean fuel with an advantage in storage and transportation,which has wide prospects and huge potential.The catalytic hydrogenation of CO₂ to methanol can effectively reduce CO₂ concerntration,and in the meantime produce value-added chemicals and fuels,which can offer a promising strategy to alleviate environmental problems and energy crisis.Therefore,it becomes one of the most concerned research fields in the utilization of CO₂ resources.Among CO₂ hydrogenation to methanol catalysts,copper-based catalysts have attracted much attention.However,it is essential to rational design and optimize catalytic performance of copper-based catalysts for efficient utilization of CO₂.In addition,co-precipitation method has some advantages in simple operation,easy control,uniform structure and stable activity of catalyst,so it is the most commonly used preparation method.A modified co-precipitation method was used in this study to prepare CZA-NHN catalyst with（NH₄）₂CO₃+NH₃·H₂O mixed solution as precipitator,so as to avoid the influence of sodium ion residue on catalyst stability in the traditional coprecipitation method.Moreover,the formation of copper ammonia and zinc ammonia complex can be inhibited by negative pressure ammonia evaporation process,to improve the synthesis rate of catalyst.The catalytic performance of prepared catalyst was tested in a fixed bed reactor.Compared to traditional co-precipitation method,the selectivity of methanol is 72%and the space-time yield reach 0.138 g·g_cat^-1·h^-1 over CZA-NHN catalyst under the conditions of 230 ℃,3 MPa and gas space velocity 3000 h^-1.The structural characteristics and physicochemical properties of the catalysts were further studied by XRD and N₂ adsorption desorption.It was found that the pore structure of CZA-NHN catalyst was improved,and the main active substance in the precursor was more crystallinity,which was conducive to the activity of the catalysts.Copper-based catalyst is a structure sensitive catalyst,and the different preparation conditions have a significant impact on its catalytic activity.Therefore,the effects of different preparation conditions,such as aging temperature,aging time,roasting temperature and mixing method,on the catalyst activity were investigated.It was revealed that after the feeding by parallel flow method,aging at 65℃for 2 h,and calcined at 350℃for 4 h,the optimal catalytic performance was obtained over CZA-NHN catalyst.Meanwhile,the hydrogenation of CO₂ to methanol is an incomplete reaction.Reaction conditions will shift the movement of reaction equilibrium,and then affect the reaction effect.Therefore,this study also investigated the influence of reaction conditions such as temperature,pressure and space speed on catalytic performance.The results indicated that after the reduction at 250 ℃ for 6 h,the CZA-NHN catalyst activity was the best with the CH₃OH space-time yield of 0.138 g·g_cat^-1·h^-1 at 230 ℃,3 MPa and 3000 h^-1.In addition,on the basis of the above preparation process of catalyst,Zr O₂,Mg O,La₂O₃ and Ce O₂ were introduced in this study to explore their effects on the physicochemical properties and catalytic performance of the catalyst.Consequently,compared with unmodified CZA-NHN catalyst,the catalytic activity of CZA-NHN catalyst modified by Ce O₂ was significantly increased by 27.5%,and the space-time yield of CH₃OH reach 0.176 g·g_cat^-1·h^-1.