Adsorption Properties And Sensing Effects Of Metal Organic Frameworks On Volatie Organic Compounds

Because of the prominent impact of volatile organic compounds(VOCs)on human health and environmental safety,it is of great significance to focus on the capture,monitoring and treating of VOCs.Metal organic frameworks(MOFs)have been used in adsorption and sensing of gas,due to their large specific surface area and specific channel size.The purpose of this study was to synthesize three MOFs:Cu-BTC,ZIF-8,and MIL-100(Al),use powdered MOFs to investigate their adsorption performance on the representative VOCs.The thin-film MOFs capacitive sensors were fabricated,and their sensing effects for different types of VOCs(isopropanol,ethyl acetate,para-xylene,n-decane,styrene,acetone,and acetaldehyde)were studied,and the effect of different relative humidity(RH)conditions on the response mechanism of sensors was also explored.The results of studies for equilibrium adsorption capacity of two representative VOCs,isopropanol and ethyl acetate,show that,ZIF-8 with the largest specific surface area and welldeveloped micropores had the largest equilibrium adsorption capacity for isopropanol and ethyl acetate,the theoretical maximum adsorption capacity fitted by Langmuir model reached 529.38 mg/g and 613.25 mg/g,respectively.In the breakthrough experiment,the adsorption capacities under(3±2)%RH,(30±2)%RH,(60±2)%RH,and(90±2)%RH were compared.The results show that with the increase of humidity,the adsorption capacity of three MOFs decreased.ZIF8 can maintain the best adsorption performance under high humidity conditions among the three MOFs.When humidity increased from(3±2)%RH to(90±2)%RH,the adsorption capacities of ZIF-8 for isopropanol and ethyl acetate only decreased by 10.9%and 22.7%,respectively.The adsorption performance of regenerated MOFs maintained well after reused for four times.Basically,all three MOFs capacitive sensors had sensing effects for 7 kinds of VOCs,which showed the effectiveness in sensing VOCs of MOFs capacitive sensors made in this study.Cu-BTC capacitive sensor had no response to ethyl acetate,and the sensitivity to the other six VOCs was isopropanol>acetaldehyde>acetone>n-decane>styrene>para-xylene;The sensitivity of MIL-100(Al)capacitive sensor the sensor to 7 kinds of VOCs was acetaldehyde>acetone>styrene>isopropanol>n-decane>para-xylene>ethyl acetate;the sensitivity of ZIF-8 capacitive sensor to 7 kinds of VOCs was acetaldehyde>acetone>isopropanol>n-decane>para-xylene>styrene>ethyl acetate.Its response sensitivity to acetaldehyde,acetone,para-xylene and ethyl acetate was the highest among three MOFs capacitive sensors,which was 16.1%,15.5%,7.3%and 4.4%respectively,this is because ZIF8 had excellent adsorption performance,so it still maintained the best sensing effect for some types of VOCs.The sensing effect of MOFs capacitive sensors was significantly affected by the type of VOCs,based on the calculated basic dielectric constant of the MOFs sensing layer before exposure to VOCs,when MOFs capacitive sensors were exposed to VOCs(acetaldehyde,acetone,isopropanol)whose dielectric constant higher than their basic dielectric constant,the capacitance value would increase,and vice versa(para-xylene,styrene,n-decane).Three MOFs capacitive sensors had the worst sensing effect for ethyl acetate,this is because the dielectric constant of ethyl acetate is too close to the basic dielectric constant of MOFs,it further illustrates that when the MOFs sensing layer senses VOCs with different dielectric constants,its own basic dielectric constant has a significant influence on the sensing effect.To illustrate the effect of humidity,taking isopropanol as example,a cycle sensing experiment under(3±2)%RH,(30±2)%RH,(60±2)%RH,and(90±2)%RH were performed.The results showed,both MIL-100(Al)and ZIF-8 capacitive sensors maintained the ability to recover capacitance values after re-exposure to air and could be used for the next cycle quickly,which reflects their convenience and fast reusability.The sensitivity of MOFs capacitive sensors for isopropanol decreased with increasing humidity.Under(90±2)%RH,when three MOFs capacitive sensors used for the fourth time,the sensitivity to 1700 ppm isopropanol was ZIF-8>MIL-100(Al)>Cu-BTC.Of the three materials,ZIF-8 was best able to maintain cycling sensing performance under high humidity conditions.Except for Cu-BTC capacitive sensors whose adsorption capacity and sensing effect under(90±2)%RH conditions was lost after two uses,MOFs capacitive sensors could maintain good reusability under other sensing conditions,and the changed capacitance value(minus the water vapor action part)can better indicate the concentration of isopropanol exposing to sensors.Except for(90±2)%RH conditions,the changed capacitance value(minus the water vapor action part)after stable had good linear relationship with the amount of static adsorption for isopropanol of sensing layer during sensing process(R2>0.9).Therefore,it can be speculated that when senses a certain type of VOCs,the adsorption amount by sensing layer has an important relationship with the sensing effect,that is,it is feasible to improve sensing effect by increasing the adsorption capacity of MOFs.