Preparation Of Coffee Shell Activated Carbon And Its Water Vapor Adsorption Properties And Mechanism

In this paper,coffee shell was selected as raw material for activated carbon preparation by a two-step activation method including carbonization process and potassium hydroxide(KOH)activation process.The influences of carbonization temperature,activation conditions(activation temperature,activation time,mass ratio of KOH to char)and modification conditions of sodium dodecyl benzene sulfonate(mass fraction,calcination temperature,calcination time)on coffee shell activated carbon water vapor adsorption performance were investigated by single factor design,and the optimal preparation conditions of coffee shell activated carbon were determined.The physical and chemical properties of coffee shell activated carbon were characterized by thermogravimetric analysis,automated gas sorption analyzer,elemental analysis,fourier transform infrared spectroscopy(FT-IR),pH,Boehm titration,and scanning electron microscopy(SEM).Non-equilibrium water vapor adsorption experiment of coffee shell activated carbon was performed in a programmable constant temperature and humidity test box to research the relationship between pore structure and surface chemical properties of activated carbon and adsorption performance.Water vapor adsorption equilibrium data of coffee shell activated carbon was tested in dynamic adsorption experimental system.The water vapor adsorption isotherm was simulated by the finite simplification model of Do-Do adsorption model.The Clausius-Clapeyron equation was used to calculate adsorption thermodynamics of different samples.Water vapor adsorption kinetic was also measured by dynamic adsorption apparatus,and kinetic curves were fitted by pseudo-first-order model,pseudo-second-order model and Bangham model.The main research conclusions are as follows:(1)The optimum preparation conditions of coffee shell activated carbon are carbonization temperature of 650°C,activation temperature of 800°C,activation time of 10 min and mass ratio of 3:1.BET specific surface area and total pore volume of coffee shell activated carbon prepared under these conditions are 2817 m~2/g and 1.41cm~3/g,respectively.Water vapor adsorption capacity of coffee shell activated carbon reaches 665.0 mg/g,when adsorption time is just 6 h.And coffee shell activated carbon with high BET specific surface area and large water vapor adsorption capacity is identified.(2)The experimental results of sodium dodecyl benzene sulfonate(SDBS)modified coffee shell activated carbon show that the optimum modification conditions are mass fraction of 2%,calcination temperature of 500°C,and calcination time of 30min.Coffee shell activated carbon prepared under these conditions has the highest water vapor adsorption capacity of 833.2 mg/g,the biggest BET specific surface area of 2018 m~2/g and total pore volume of 0.985 cm~3/g.It is found that BET specific surface area and total pore volume are reduced after modification process but surface functional group content of coffee shell activated carbon is increased.(3)The limited simplified model for water vapor adsorption of coffee shell activated carbon is based on Do-Do adsorption model which believes that there is no upper limit for growth of water molecule clusters on surface functional groups adsorption sites.However,as a matter of fact,when water molecule clusters grow to a certain finite value,micropore filling behavior will be occurred in micropores of activated carbon.Therefore,assuming that growth of water molecule clusters on surface functional groups is finite,the Do-Do model is finitely simplified.The finite simplified model is used to fit relationship between relative pressure and water vapor adsorption,and a preliminary analysis of water vapor adsorption mechanism of coffee shell activated carbon is done.According to the classification of IUPAC,water vapor adsorption isotherms belong to common types-type V.In experimental temperature range,the cumulative water vapor adsorption capacity of coffee shell activated carbon decreases with increase of adsorption temperature.This experimental result shows that the higher adsorption temperature,the more detrimental it is to adsorption behavior of water vapor on coffee shell activated carbon.And the fitting result of finite simplified model shows that fitting correlation coefficient is very high,indicating that water vapor adsorption isotherms can be fitted by the model more accurately.(4)The results of equivalent adsorption heat show that water vapor adsorption process of coffee shell activated carbon is an exothermic process.In low range of water vapor adsorption capacity,the higher adsorption capacity,the smaller equivalent adsorption heat.However,in higher range of water vapor adsorption capacity,the higher adsorption capacity,equivalent adsorption heat increases slightly with the increase of adsorption capacity.This may be due to the presence of chemisorption during adsorption process,which requires a higher activation energy.Only with adsorption progress,the chemisorption capacity continues to increase.Thus,equivalent adsorption heat slightly increases.This indicates that physisorption and chemisorption coexist in water vapor adsorption process of coffee shell activated carbon.(5)The fitting results of adsorption kinetic show that the correlation coefficient R~2value of the Bangham model is the highest,and predicted adsorption capacity of the model is closest to the experimentally obtained adsorption capacity.This indicates that the dynamic water vapor adsorption process of coffee shell activated carbon can be described by Bangham model.At initial stage of water vapor adsorption,adsorption capacity increases rapidly with a faster adsorption rate.As adsorption process progresses,adsorption rate of coffee shell activated carbon drops sharply.After a certain adsorption capacity,the decline begins to slow down,and water vapor adsorption capacity increases slowly until equilibrium state of water vapor adsorption is reached.(6)Water vapor adsorption capacity of coffee-shell activated carbon is related to pore structure,surface chemical properties and oxygen-containing functional groups.Under low relative pressure,surface adsorption center site of coffee-shell activated carbon plays a dominant role in water vapor adsorption process,which induces movement of water molecules and forms hydrogen bonds between the two,and then water vapor adsorption behavior is occurred in coffee-shell activated carbon.As relative pressure increases,water molecules form clusters around the water molecules previously adsorbed at the active site through hydrogen-bond.When the clusters are large enough,the water molecules begin to fill pores of activated carbon until water vapor adsorption becomes saturated.