Fe-MOFs Encapsulation And Synergistic Controlled Release Of Vanillin And Citral In Functional Low Volatile Aroma Compounds

Aiming to resolve the problem of low utilization rate of low volatile spices in the flavor industry,this paper put forward a strategy of"low boiling point molecular synergistic release"to ensure the efficient release of low volatile spices on porous materials,and to modify the adsorbent with dopamine.MIL-100(Fe)was used as an adsorbent for the high desorption efficiency of low volatile spices,and typical the low volatile spices,vanillin(boiling point:282°C)and citral(boiling point:228°C),were selected as target spices.Similarly,highly volatile ethanol(boiling point:78°C)was used as"carrying"molecules.The spices and ethanol were adsorbed into the MIL-100(Fe)pores to study the synergistic effect of highly volatile molecules on the low volatile aroma substances during the desorption process,so that MIL-100(Fe)can also achieve the efficient release of spices while ensuring a high adsorption of spices.The main contents and results of this thesis are as follows:(1)The adsorption and synergistic desorption properties of vanillin-ethanol system in MIL-100(Fe)were systematically investigated:Isothermal adsorption lines of vanillin on MIL-100(Fe)were measured and the equivalent adsorption heat fitting was performed according to the Clausius-Clapeyron equation.Under the condition of different ethanol loading,the effect of ethanol on the"carrying-volatile"of vanillin on MIL-100(Fe)was explored,and the carrying load condition was optimized to improve the synergistic volatilization effect of ethanol on vanillin on MIL-100(Fe).In order to further study the mechanism of vanillin ethanol co-volatile,the temperature programmed desorption curve of ethanol-vanillin mixed system on quartz sand(no adsorption)was determined to exclude the effect of adsorbent on the strong adsorption of vanillin.The synergistic volatilization/release relationship between ethanol and vanillin was further compared in the absence of adsorption force of the adsorbent.Finally,the cooperative carrying and release mechanism of vanillin and ethanol molecules on MIL-100(Fe)was explained by molecular simulation calculation.The results show that:As-prepared MIL-100(Fe)exhibited medium/micro double-channel structure,and a specific surface area of up to 1349.7 m~2/g with a high saturated adsorption capacity of 5.21 mmol/g for vanillin.The adsorption energy of vanillin on MIL-100(Fe)in the presence of ethanol was about-66.9k J/mol which was significantly lower than that of pure vanillin on MIL-100(Fe)(104.1 k J/mol).The reason is that the ether,aldehyde and hydroxyl groups of vanillin molecule can form hydrogen bonds with hydroxyl group of ethanol molecule.Strong hydrogen bond can weaken the adsorption binding energy of vanillin on MIL-100(Fe)and improve the desorption efficiency of vanillin.When the vanillin adsorption was about 3.99 mmol/g,the vanillin desorption rate increased from 39.6%to 59.1%.(2)The adsorption and synergistic desorption properties of citral-ethanol system in MIL-100(Fe)were systematically investigated:The isothermal adsorption line of citral on MIL-100(Fe)was measured,and the equivalent adsorption heat was estimated at different adsorption amounts.The temperature programmed desorption curve of citral on MIL-100(Fe)was measured in a pure citral system with varying amounts of citral/ethanol adsorption.The desorption activation energy on the MIL-100(Fe)was calculated by fitting the temperature programmed desorption curve of citral at different heating rates.At the same time,the temperature programmed desorption curve of ethanol-citral mixed system on quartz sand(no adsorption)was determined to exclude the effect of adsorbent on the strong adsorption of citral molecules.The synergistic volatilization/release relationship of ethanol to citral was further explored in the absence of adsorbent adsorption force.Finally,the mechanism of synergistic transport and release of citral and ethanol molecules on MIL-100(Fe)was explained by molecular simulation.The results show that:The saturated adsorption of citral on MIL-100(Fe)was 3.66 mmol/g,which was much lower than that of vanillin on MIL-100(Fe).This is due to the fact that the adsorption binding force between MIL-100(Fe)and citral was weaker than that of vanillin.The finding was supported by equivalent adsorption heat fitting.In the pure citral system,the citral desorption rate over MIL-100(Fe)increased by up to 64.2%with the increase of adsorption capacity,respectively.However,in the mixed system of citral/ethanol,the desorption rate of citral on MIL-100(Fe)increased first and then decreased with an increase in the amount of adsorption.If the adsorption capacity of citral was similar,the desorption rate of citral on the MIL-100(Fe)in the mixed system was higher than that in the pure citral system,and the maximum rate of desorption was 81.1%.Meanwhile compared to pure citral system,in citral/ethanol mixed system,the desorption activation energy of citral on MIL-100(Fe)was reduced from 82.3 k J/mol to 74.0 k J/mol.The reason is that the aldehyde group on citral can form a strong hydrogen bond with the hydroxyl group on ethanol(the length of hydrogen bond is 1.78 A).This strong hydrogen bond decreased the binding energy and adsorption of citral over MIL-100(Fe)and thus increased the desorption rate of citral.(3)The adsorption and synergistic desorption properties of the vanillin-ethanol system in the dopamine modified MIL-100(Fe)were systematically investigated:During hydrothermal synthesis of DA-MIL-100(Fe)via dopamine modification of MIL-100(Fe),the N of dopamine coordinated with metal sites of MIL-100(Fe)to form Fe-N bonds.These bonds weakened the polarity of metal sites,and the adsorption force of vanillin with high polarity was weakened and hence the desorption efficiency of vanillin was further improved.The formation of Fe-N bonds and the successful modification of dopamine were proved by a series of characterizations.Isothermal adsorption lines of vanillin on DA-MIL-100(Fe)at different temperatures were measured and equivalent adsorption heat fitting calculations were performed.Simultaneously,the desorption activation energy of vanillin on two materials was calculated using temperature programmed desorption curves at different heating rates.Finally,the effect of different vanillin adsorption by DA-MIL-100(Fe)over vanillin desorption rate was investigated.The results show that:The distribution of N elements in dopamine modified DA-MIL-100(Fe)was uniform with a proportion of 1.06 wt.%,and possessed good crystallinity,pore structure and thermal stability.Its specific surface area was high up to 1139.4 m~2/g.Furthermore,the saturated adsorption capacity of vanillin over DA-MIL-100(Fe)was 5.16 mmol/g.The adsorption binding force of vanillin on DA-MIL-100(Fe)was significantly lower than that on MIL-100(Fe)by fitting the equivalent adsorption heat.Meanwhile,the desorption activation energy of vanillin on MIL-100(Fe)before and after modification was reduced from 71.9 k J/mol to 65.6 k J/mol.As a result,the adsorption force between vanillin and MOFs was weakened after dopamine modification,which was beneficial for the desorption of vanillin on the adsorbent.When the adsorption capacity of DA-MIL-100(Fe)for vanillin was3.19 mmol/g,the highest desorption rate of vanillin over DA-MIL-100(Fe)reached 73.0%,which was far higher than the highest desorption rate of pure MIL-100(Fe)(59.1%).