Study On The Adsorptive Separation And Recognition Mechanism Of Bio-based Alcohols By ZIFs With Hydrogen Bonding Sites

Based on the increasing reduction of fossil resources and the dual carbon target,renewable and low-carbon biomass resources have attracted attention.Biomass can convert into various important bio-based platform compounds by biorefinery.For example,bio-based alcohols we focus on:mono-alcohol biofuels,such as n-butanol and isobutanol;diols biochemicals,such as 1,3-propanediol and 2,3-butanediol.However,the raw solution of bio-based alcohols is a low concentration and multi-component aqueous solution.Based on the characteristics of the raw solution,distillation of traditional separation technology requires high energy consumption,while adsorption has the advantage of low energy consumption.Low-energy adsorptive separation technology requires the development of high-performance adsorbents.Traditional hydrophobic adsorbents have poor performance in identifying and separating multi-component bio-based alcohols through van der Waals interactions.Compared with zeolite,zeolitic imidazolate frameworks（ZIFs）with the same topology have large pore volume to ensure adsorption capacity,and aromaticity imidazole and rich imidazole groups on the framework can regulate their hydrophobicity and benefits of combining components.Bio-based alcohol components include similar carbon chain lengths with hydrophobic alkyl groups and similar polarity with hydrophilic oxygen-containing groups.Therefore,in this work,we construct microporous ZIFs with hydrogen（H）bonding sites to strengthen the H-bonding interactions through the response recognition of flexible ZIFs and the confined effect of the pore window/cage of ZIFs.The key pore property parameters for the adsorptive separation of bio-based alcohols are obtained,guiding the synthesis of separation materials with industrial application value.The main research content includes:（1）Establish the relationship between the vd W（van der Waals）,H-bonding recognition sites and confined structures and hydrophilicity and hydrophobicity of ZIFs.Deeply understanding of the water adsorption behavior of ZIFs is crucial for designing adsorbents that efficiently enrich bio-based alcohols in aqueous solution.Combining experimental-computational method,the results showed that the vd W interactions were negligible and that electrostatic interactions played a dominant role in the water adsorption of ZIFs.Therefore,at low relative pressure region（P/P₀<10%）,ZIFs are hydrophobic due to their aromaticity imidazole.At high relative pressure region（P/P₀>10%）,ZIFs with only non-polar or weak-polar groups（vd W recognition sites）adsorb water through vd W interactions,exhibiting hydrophobicity;ZIFs with polar groups（H-bond recognition sites）adsorbed water can form H-bonds,mainly through electrostatic interactions,exhibiting hydrophilicity.Meanwhile,ZIFs with H-bonding recognition sites can improve their hydrophilicity by reducing pore size.（2）Based on the demand for efficient adsorptive separation of bio-based mono-alcohols（components with similar polarity）,flexible ZIFs with H-bonding sites were constructed to identify and separate acetone/n-butanol and isobutanol/ethanol.We have systematically studied the synthesis and activation,structural analysis,guest（bio-based alcohols and alkanes）-responsive behavior of flexible ZIF-65（Zn）with H-bonding sites,as well as adsorptive separation of bio-based mono-alcohols in ZIF-65（Zn）.We discovered and analyzed a new structure ZIF-65（Zn）-II during the activation process of ZIF-65（Zn）-I by the removal of the guests.ZIF-65（Zn）exhibits a stepwise II→III→I expansion between the ellipsoidal cage（8.6?×15.9?for II）and the spherical cage（15.0?for I）.The adsorption of non-polar straight-chain alkanes and polar short/medium straight-chain mono-alcohols（n-butanol and ethanol）in ZIF-65（Zn）-II correspond to transform into ZIF-65（Zn）-III（10.4?×16.0?）,which induces the breathing behavior with small expansion magnitude（1.8?）;the adsorption of strongly polar diols,aldehydes and ketones（acetone）,and polar branched mono-alcohols（isobutanol）in ZIF-65（Zn）-II correspond to transform into ZIF-65（Zn）-I,which induces the breathing behavior with large expansion magnitude（6.4?）.The chemical responsive expansion magnitude of breathing behavior in ZIF-65（Zn）-II with unique ellipsoidal structure and H-bonding functional group（-NO₂）is selective depending on the polarity and shape of the guest molecule,mainly attributed to the differences in the synergistic effect of vd W interactions,and host-guest and guest-guest H-bonding interactions by molecular simulations revealed.Therefore,compared with rigid adsorbents with vd W recognition sites,we utilize the unique ellipsoidal breathing cage and H-bonding recognition sites of ZIF-65（Zn）to reversely preferentially adsorb acetone and achieve acetone/n-butanol separation,to strengthen the affinity of isobutanol and achieve excellent isobutanol/ethanol separation performance.（3）Based on the demand for efficient adsorptive separation of bio-diols（with differences in alkyl and hydroxyl positions）,ZIFs adsorbents with vd W and H-bonding recognition sites were constructed.By precisely adjusting the composition of H-bonding recognition sites in ZIFs,it can strengthen 2,3-butanediol adsorption to achieve effective separation of2,3-butanediol/1,3-propanediol in aqueous solutions.Firstly,a series of ZIF-71-93_x（x=0,13.3,21.2,32.8,58.1,and 100,mol%）with double ligand were synthesized using4,5-dichlorimidazole with vd W functional groups（-Cl）and 4-methyl-5-aldehydeimidazole with H-bonding functional groups（-CHO）.Secondly,the appropriate composition range of H-bonding recognition sites in ZIF-71-93_x with a certain degree of hydrophobicity was determined through hydrophilicity and hydrophobicity evaluation.Then,the appropriate composition range of H-bonding recognition sites in ZIF-71-93_x was determined for improving the static adsorption and enhancing the dynamic mass transfer of 2,3-butanediol through the static batch and dynamic column adsorption experiment.The results showed that ZIF-71-93_21.2 was the optimal material for adsorptive separation of2,3-butanediol/1,3-propanediol(50/50 g L^-1),with dynamic column adsorption capacity of2,3-butanediol and selectivity of 2,3-butanediol/1,3-propanediol being 136.7 mg g^-1 and 4.1,respectively.Molecular simulation reveals that 2,3-butanediol is selectively adsorbed on the pore window of ZIF-71-93_x which matches the distance with its hydroxyl and alkyl groups.There is one strong H-bond between one-OH group of 2,3-butanediol and-CHO group of ZIF-71-93_x,and vd W interactions between alkyl groups of 2,3-butanediol and-Cl groups of ZIF-71-93_x.Therefore,efficient adsorptive separation of 2,3-butanediol/1,3-propanediol can be achieved by precisely regulating the proportion of H-bonding recognition sites in ZIF-71-93_x and utilizing the synergistic effect of vd W and H-bonding interactions.（4）Inspired by the selective adsorption of 2,3-butanediol,the cage-like ZIFs with H-bonding recognition sites were constructed to match distance with two-OH groups of1,3-propanediol.Through controlling the confined effect of the cage-like ZIFs,the H-bonding interactions were enhanced to selectively adsorb 1,3-propanediol,thereby achieving effective separation of 1,3-propanediol/2,3-butanediol in aqueous solutions.By regulating the structure size,type of H-bonding recognition sites,and composition of H-bonding and vd W double recognition sites of the cage-like ZIFs,we studied the impact of these factors on the adsorptive separation of 1,3-propanediol/2,3-butanediol and finally obtained key pore structures and pore surface parameters for determining performance.The results showed that SIM-1（pore size:8.4?;H-bonding recognition site:-CHO）was the optimal material for adsorptive separation of 1,3-propanediol/2,3-butanediol(50/50 g L^-1),with the dynamic column adsorption capacity of 1,3-propanediol and selectivity of1,3-propanediol/2,3-butanediol being 93.3 mg g^-1 and 3.3,respectively.Molecular simulation reveals that 1,3-propanediol is selectively adsorbed in the sod cage which matches the distance with its two-OH groups.There are double strong H-bonds between two-OH groups of 1,3-propanediol and the ortho-CHO groups of SIM-1,thereby achieving efficient adsorptive separation of 1,3-propanediol/2,3-butanediol.