Fabrication And Performance Study Of ZIF-8 Derived Porous Carbon Based On The Separation Of Acid-containing Furans Compounds

Furan compounds（furfural or 5-hydroxymethylfurfural）,important biological platform chemicals,are obtained from renewable biomass（rich in hemicellulose or cellulose）through sustainable biorefinery technology,and it can be directly used as fuel or synthesize a variety of high value-added chemicals.However,the industrially stock fluid of furfural or 5-hydroxymethylfurfural is composed of low-concentration aqueous solution and acidic by-products due to the characteristics of biorefinery technology.Therefore,how to efficiently separation of high-boiling furfural or 5-hydroxymethylfurfural from low-concentration multi-component and acid-containing aqueous solutions is a critical step in the biorefinery technology to prepare high-quality furan compounds.The traditional distillation separation process requires high investment and high energy consumption,so the adsorption with energy-saving has gradually drawn researchers’attention extensively.Zeolite imidazolate frameworks（ZIFs）exhibit high adsorption capacity for furfural and 5-hydroxymethylfurfural due to hydrophobic alkyl group on the pore surface and aromatic characteristics of imidazole ring on the pore structure.However,considering the acid-containing characteristics of the industrially stock fluid,the adsorbent should satisfy acid resistance,hydrophobicity and high selectivity contemporaneously.Therefore,ZIFs can be transformed ZIF-derived porous carbons with acid resistance,hydrophobicity and microporous structure,and the uniqueπ-πinteraction between porous and furan compounds can achieve high-efficiency enrichment of furfural or 5-hydroxymethylfurfural.The specific research contents are as follows:Firstly,ZIF-derived porous carbons with different surface chemistry/pore structure were prepared with typical ZIF-8 as the precursor,the relationship between the N species content/type and micro-mesoporous distribution of ZIF-8-derived porous carbon and water adsorption performance was established,which provides a basis for the preparation of hydrophobic porous carbon materials to enrich furfural or 5-hydroxymethylfurfural in low-concentration aqueous solutions.First,NC_ZIF-8x C materials with different N species content/type and pore structure were prepared by using ZIF-8 as the sole carbon source and nitrogen source at different carbonization temperatures.Then NC_ZIF-8x C-700A₄ materials with different micro-mesopore distributions were prepared by further KOH activation.The water vapor adsorption performance of two kinds of materials were tested to understand the hydrophilic and hydrophobic of ZIF-8-derived porous carbons.Investigations suggest that the hydrophilicity and hydrophobicity of ZIF-8-derived porous carbon is mainly related to the surface chemistry and pore structure.The water uptake at P/P₀<0.2 is mainly depends on the N species content/type.The hydrophilicity of porous carbon is positively correlated with the N species content of the material.The higher the N content,the stronger the hydrophilicity,and the pyridinic-N and pyrrolic-N species make the dominant contribution to the water uptake.As the pressure increases,the water adsorption process becomes highly dependent on the porous structure,and there is a critical pore width above which pore-filling occurs.The small micropores of<12（?）are filled with water molecules first at P/P₀<0.6,followed by the larger micropores of 12–22（?）at 0.6<P/P₀<0.8,and finally the mesopores of 22–40（?）at 0.8<P/P₀<0.9.In addition,the organic vapor adsorption was tested under the background of humidity proved that the NC_ZIF-81000C-700A₄ with low N content and large micropores has stronger hydrophobic and organophilic properties,and is more suitable for the adsorption and separation of furfural or 5-hydroxymethylfurfural from low-concentration acid-containing aqueous solutions.Secondly,the KOH activation mechanism of ZIF-8-derived porous carbon was studied.The relationship between the N species of the precursor(carbonization NC_ZIF-8x C)and the micro-mesoporous distribution of the activated sample NC_ZIF-8x C-y Az was established,which provides a basis for the preparation of microporous carbon materials for the enrichment and separation of furfural or 5-hydroxymethylfurfural in low-concentration aqueous solutions.The micro-mesoporous distribution of NC_ZIF-8x C-y Az samples were precisely controlled under different activation parameters（activation temperature and alkali-carbon ratio）by using rich-N NC_ZIF-8800C and less-N NC_ZIF-81000C as parent.The KOH activation mechanism of N-doped ZIF-derived porous carbon is proposed.For rich-N NC_ZIF-8800C,the reaction of the intermediate K₂CO₃/K₂O with the higher-reactivity C–N species（pyridine-N and pyrrolic-N）resulted in the formation of the intermediate product KOCN and decomposition product CO and NO,which led to the formation of abundant and larger mesopores of 30–60（?）.However,for less-N NC_ZIF-81000C,the intermediates K₂CO₃/K₂O reacted with the C species and released CO,which is beneficial for the construction of abundant and larger micropores of 10–20（?）,and a few mesopores of 20–30（?）.Thirdly,ZIF-8-derived porous carbon materials with different N content and micro-mesoporous distribution were used to study the adsorption performance of furfural/acetic acid by static adsorption,dynamic column adsorption,adsorption kinetics and desorption.And combined with DFT simulation to analyze the adsorption and separation mechanism of furfural/acetic acid.Using rich-N NC_ZIF-8800C,less-N NC_ZIF-81000C,microporous NC_ZIF-81000C-700A₄ and mesoporous NC_ZIF-8800C-1000A₄ as adsorbents,the relationship between the composition and structure of ZIF-derived porous carbon and the adsorption performance of furfural/acetic acid was established.The research results show that NC_ZIF-81000C-700A₄ with low N content and abundant micropores is a potential adsorbent for separating furfural from aqueous solutions.NC_ZIF-81000C-700A₄ can rapidly adsorb furfural in aqueous solution,and the adsorption equilibrium can be reached within 10 min.The static adsorption capacity of NC_ZIF-81000C-700A₄ for furfural is 1219.6 mg/g at the initial concentration is 5.0 wt.%,and the adsorption performance exceeds the values for reported adsorbents.The dynamic column adsorption capacity of NC_ZIF-81000C-700A₄ for furfural/acetic acid is 1253.9/5.5 mg/g at the simulation solution concentration of furfural/acetic is 5.0/2.0 wt.%,and the selectivity is as high as 91.8.In addition,thermal desorption and solvent desorption experiments proved that the material has good regeneration performance.Combined with the DFT simulation results,it can be seen thatπ-πinteractions,hydrophobicity and micropore confinement effect determine the strong furfural affinity of ZIF-8-derived porous carbon and makes the porous carbon have a high furfural adsorption capacity at high concentrations in the furfural/acetic acid adsorption process.Finally,ZIF-8-derived porous carbons with high microporosity and hydrophobicity were prepared by KOH direct activation using ZIF-8 as the precursor and KOH as the activator to simplify the preparation process.The adsorption and separation performance of 5-hydroxymethylfurfural/levulinic acid/formic acid in low concentration aqueous solution was further studied.Firstly,the activation mechanism of ZIF-8-derived porous carbons prepared by direct activation was proposed by studying the effects of activation temperature,alkali/carbon ratio and activation time on the pore structure.Then the NC_ZIF-81000CA₁ sample with the largest micropore volume and hydrophobic was used to study the separation performance of 5-hydroxymethylfurfural/levulinic acid/formic acid.The results showed that NC_ZIF-81000CA₁exhibited excellent adsorption capacity,adsorption kinetics and selectivity for 5-hydroxymethylfurfural.The static adsorption capacity of NC_ZIF-81000CA₁ for 5-hydroxymethylfurfural at 5.0 wt.%is as high as 961.1 mg/g,which exceeds the reported adsorbent.And the adsorption equilibrium can be reached within 10 min.In the three-component system,the dynamic column adsorption capacity of NC_ZIF-81000CA₁ for 5-hydroxymethylfurfural/levulinic acid/formic acid is 778.1/95.4/21.4 mg/g at the simulation solution concentration is 5.0/2.5/1.0 wt.%.And the selectivity of 5-hydroxymethylfurfural/levulinic acid and 5-hydroxymethylfurfural/formic acid is 4.6 and 35.5,respectively.NC_ZIF-81000CA₁ exhibited the highest adsorption capacity for 5-hydroxymethylfurfural due to theπ-πinteraction,followed by levulinic acid with hydrophobic alkyl groups,and finally the formic acid with hydrophilic.Combined with DFT simulation results,it can be seen that the interaction between NC_ZIF-81000CA₁ and the three adsorbates is:5-hydroxymethyl furfural>levulinic acid>formic acid.The NC_ZIF-81000CA₁ still maintains excellent adsorption performance after three dynamic column adsorption and desorption cycles.Therefore,theπ-πinteraction make NC_ZIF-81000CA₁ an excellent adsorbent for the separation of 5-hydroxymethylfurfural/levulinic acid/formic acid from acid-containing aqueous solutions.