Preparation And CO Adsorptive Separation Performance Of Monoptychial CuCl/AC Adsorbent

Carbon monoxide?CO?is an important basic chemical in industry,which can be used to synthesize many important chemical products.At the same time,the presence of unsaturated?bonds in the CO molecule can provide electrons to the d orbitals of metal catalysts and lead to the poisoning of the catalysts,and CO can also prolong the retention time of CH₄,a greenhouse gas in the atmosphere.In addition,CO is toxic to human beings because it combines with hemoglobin in blood to form carboxy-hemoglobin that reduces the oxygen carrying capacity of blood,which leads to death.It is important to separate and purify CO.Usually,CO coexists with impurities such as H₂,N₂,CH₄ and CO₂,so the separation and purification of carbon monoxide is of great economic and social significance.Compared with other technologies,adsorptive separation technology has the advantages of easy operation,low energy consumption and matured in technology.Adsorptive separation technology has considerable competitive advantages in industrial application to solve practical problems.The core of this technology is adsorbent.Compared with the traditional physical adsorption,the?complexation force is much stronger than the ordinary van der Waals force or electrostatic force,so it shows higher selectivity.Compared with general chemisorption,its relative weak?complexation bond makes it easy in regeneration to achieve by simply applying heating or decompressing.Herein,CuCl/AC adsorbents with high CO adsorption capacity and selectivity were successfully prepared by three different methods.Method I:the adsorbent precursor was prepared using CuCl₂ and Cu?HCOO?₂ as raw materials and activated carbon as carrier by impregnation method on the surface of the activated carbon?AC?,and then the CuCl/AC adsorbent was prepared by calcination.Method II:the CuCl/AC adsorbent was prepared using CuCl₂,HCOOH and Cu?OH?₂CO₃ as raw materials and activated carbon as carrier,the preparation procedure is the same as method I.Method III:the adsorbent of CuCl/AC was prepared using CuCl₂ and Cu?HCOO?₂ as raw materials and activated carbon as carrier,by simple mechanical mixing and then calcination.The above methods have the advantages of simple synthesis steps,easy control without using equipment for recovering organic solvents and anticorrosive manufacturing equipment during the preparation process.Under the atmosphere of air,divalent copper salts can be loaded on the surface of activated carbon?AC?to prepare the precursor of CuCl/AC adsorbent.Because of the stable chemical properties of CuCl₂ and Cu?HCOO?₂,adsorbent precursors can be preserved in air for a long time without deterioration,which facilitate to long-term storage and long-distance transportation.However,once the precursor is heated to obtain CuCl/AC adsorbent,it is necessary to seal and store in dark in a desiccator for use.Therefore,the adsorbent can be stored and transported in the form of precursor in industrial application,and a large amount of CuCl/AC adsorbents can be easily prepared by heating in inert atmosphere or vacuum environment when needed.AC has a significant effect on the CO adsorptive separation performance of the CuCl/AC adsorbent.It is found in this study that the CO adsorptive separation performance of the CuCl/AC adsorbent is affected by the pore size and specific surface area of AC.On one hand,when the pore size of AC is<0.7nm,due to the limitation of pore size,CuCl can not be dispersed well on the surface of the AC;on the other hand,the pore size of the CuCl/AC adsorbent is too small,and the dispersed CuCl on the surface of the AC can not be fully approached by CO to form?-complexation.When the pore size of AC is>0.7nm,the CuCl can be dispersed in the monolayer state on the surface of AC,which can be approached completely by CO,and the larger the pore size,the more favorable the dispersion of CuCl on the surface of the AC.In addition,large specific surface area of the AC benefits the dispersion of CuCl,and the adsorption capacity of CO.However,there is a balance between the specific surface area and pore size of AC.When the pore size of activated carbon increases to a certain extent,the specific surface area decreases.Therefore,the optimum pore size range of AC is0.7-2.5 nm,in order to prepare a robust CuCl/AC adsorbent.In Method I,the CO adsorption capacity of the optimum CuCl/AC adsorbent sample prepared in vacuum is 52.71 cm³·g^-1,while the CO adsorption capacity is about 46.00 cm³·g^-1when the adsorbent is prepared in inert atmosphere.This is because the inert atmosphere is favorable for removing a large amount of weakly bonded adsorbed water from the surface,but it need a long period of time for the removing of adsorbed water in the pores.CuCl is easy to be hydrolyzed and metamorphosed in high temperature steam,which requires a short processing time.The optimized raw material ratio is CuCl₂:Cu?HCOO?₂:AC=2 mmol:2 mmol:1 g for Method I and CuCl₂:HCOOH:Cu?OH?₂CO₃:AC=2 mmol:2.2 mL:1 mmol:1 g for Method II.The best calcination temperature and time are 220?and 4 h,and the pre-drying temperature is 120?for 2 h before calcination.Too high calcination temperature and too long calcination time will lead to the reduction of CuCl to copper.The best raw material ratio of the Method III is the same as that of the Method I,but the cupric salts and the activated carbon is simply mechanically mixed,so the aggregation of the cupric salts on the surface of the activated carbon is higher than that of the other two methods.Therefore,the calcination requires a higher temperature or longer time for the formation of CuCl and the dispersion on the surface of AC.In addition,compared with Method I,the Method II greatly reduces the production cost for the industrial production of CuCl/AC adsorbent,and Method III avoids the participation of H₂O as a solvent in the preparation process.To a certain extent,the absence of H₂O prevents CuCl from deteriorating by H₂O,which is important for the industrial application of CuCl/AC adsorbent.The selectivity of CO/N₂,CO/CO₂ and CO/CH₄ on CuCl/AC adsorbent increased with the increase of CuCl loading,and the lower concentration CO in CO/N₂,CO/CO₂ and CO/CH₄ mixtures,the higher CO selectivity can be achieved.The CO adsorption capacity and selectivity of CuCl/AC adsorbent prepared by Method I is close to that by Method II,and slightly higher than that by Method III.After 4 cycles of in situ adsorption and desorption of CO on CuCl/AC adsorbent,the CO adsorption capacity decreased by about 16%,and getting stable afterwards.If the adsorbent was regenerated by heating at 80?,the CO adsorption capacities were almost identical in the six adsorption/desorption cycles.The isosteric heat of adsorption of CO on CuCl/AC adsorbent was 46 kJ/mol.These energy can be recycled to heat the same mass adsorbent to about 100?,so the CO adsorption/desorption can be carried in the adsorption cycles without external heat imput,and keeping the CO adsorption capacity.CuCl/AC adsorbents can adsorb CO rapidly,reaching 90%of its adsorption capacity within 20 seconds,which can meet the requirement of less than 1 minute for the industrial adsorption process.