Hydrophobic Modified Molecular Sieves For Typical Automotive Painting VOCs Adsorption Performance

Volatile organic compounds（VOCs）are the key precursors of PM_2.5 and O₃ in the atmosphere.The exhaust gas of automobile painting workshop has the characteristics of large exhaust volume and low concentration of VOCs,so the process of"molecular sieve adsorption and concentration+combustion"is generally adopted to purify the exhaust gas.Due to the high moisture content of the paint shop exhaust gas in the automotive painting workshop,there is competition between moisture and VOCs in the adsorption process,which will affect the adsorption of VOCs on the adsorbent.Therefore,studying the adsorption of VOCs on molecular sieve under high humidity,exploring the mechanism of the influence of humidity on the adsorption of VOCs,and improving the adsorption performance of molecular sieve on VOCs under high humidity through effective hydrophobic modification methods,can provide theoretical basis and data support for the practical application of VOCs adsorption.The paper investigates the adsorption characteristics of VOCs on molecular sieve with different humidity by targeting xylene,toluene and butyl acetate generated from automotive painting process,and improves the adsorption of VOCs on molecular sieve at high humidity by two different hydrophobic modifications with hydrochloric acid and trimethylchlorosilane（TMCS）.The main findings are as follows:（1）At RH=0%,the adsorption performance of NaY molecular sieve on VOCs was the best.The order of adsorption size of NaY molecular sieve on three VOCs was:xylene>toluene>butyl acetate.In the binary adsorption system of water and VOCs,H₂O-xylene and H₂O-toluene had competing adsorption behaviors,while the two-component system of H₂O-butyl acetate showed co-adsorption because both H₂O and butyl acetate were polar molecules.Humidity had a significant effect on the adsorption performance of VOCs on the molecular sieve,and the adsorption of xylene,toluene and butyl acetate by NaY molecular sieve decreased from 327.64 mg/g,144.6 mg/g and 236.43 mg/g to 5.3mg/g,1.13 mg/g and 6.36 mg/g,respectively,when the humidity was increased from RH=0%to 85%.（2）The best adsorption performance of 100 mesh NaY molecular sieve was obtained by 1 mol/L hydrochloric acid impregnation for 12 hours at RH=65%for toluene,and its adsorption amount was 5.4 mg/g,which was 1.94 times of the adsorption amount of unmodified molecular sieve.It is shown that the molecular sieve impregnated with hydrochloric acid has better hydrophobicity,which is beneficial to the adsorption of VOCs under high humidity.The characterization results of BET,XRD,FTIR and water contact angle show that the specific surface area of NaY molecular sieve decreases with the increase of hydrochloric acid concentration,too high hydrochloric acid concentration makes the molecular sieve crystalline structure change,which affects the adsorption capacity;the surface hydrophobic functional group Si-O-Si increases and hydrophilic Si-OH functional group decreases;the water contact angle increases from the original 13.2°to 25.6°.The modified molecular sieve was subjected to five adsorption and desorption cycles,and the results showed that the adsorption capacity of the modified molecular sieve decreased slightly by 0.99%～1.90%at RH=0%and by 0.36%～2.22%at RH=65%,indicating that the adsorbent stability of the molecular sieve modified with 1 mol/L hydrochloric acid was better.（3）A mixture of ethyl orthosilicate,anhydrous ethanol and water aging solution was used to pretreat the NaY molecular sieve,followed by hydrophobic modification with different volume ratios of TMCS/hexane.The results showed that the best modification ratio was 1:7,and the adsorption amount of toluene at RH=65%was 7.15 mg/g,which was 2.58 times of the adsorption amount of unmodified molecular sieve.The characterization results of BET,XRD,FTIR and water contact angle show that the modification does not affect the crystalline structure of the molecular sieve,but it reduces its specific surface area,the increase of hydrophobic Si-CH₃ and Si-O-Si functional groups,and the water contact angle increases from 13.2°to 121.6°.The modified molecular sieve was subjected to five VOCs adsorption and desorption cycles,and the results showed that the adsorption performance of the modified molecular sieve was stable,with a decrease of 0.68%～1.24%at RH=0%and 1.20%～7.44%at RH=65%.Compared with hydrochloric acid modification,the TMCS modification causes less damage to the crystalline structure and significantly enhances its hydrophobicity.From the perspective of less damage to the original molecular sieve,TMCS modification was evaluated as the superior method for hydrophobic modification of molecular sieve.