修飾碳材上之官能基對二氧化碳吸附的影響

Abstract

本研究旨在深入探討碳材料表面官能基對CO2吸附性能的影響，特別是利用介相瀝青碳(Mesophase pitch, MPC)作為前驅物，嘗試將石化副產品轉化為高效的碳捕獲材料，以回應減碳與直接空氣捕獲(DAC)對低能耗材料的需求。傳統的胺類吸附劑雖具高選擇性，卻伴隨高能耗的再生過程；若能採用強物理吸附特性的材料，助於突破此限制。過去文獻亦經理論計算指出磺酸基團相較其他種類官能基團對二氧化碳具有更高的非共價親和力，為本研究指明方向。 實驗中透過氧化、硝化、胺基化及磺酸化等多種化學方法對MPC進行修飾，並進一步製備了水熱處理的磺酸化碳材(MPC-SAHT1/2)及其共軛鹼形式MPC-SACB。材料結構與表面化學藉由傅立葉轉換紅外光譜、固態核磁共振光譜、拉曼光譜、酸鹼反滴定和元素分析加以確認；比表面積與孔徑結構由氬氣吸脫附測量，CO2吸附行為則透過等溫吸附曲線測量、維里方程計算等量吸附焓(ΔHads)以及原位吸脫附實驗來評估。 結果顯示，所有官能基化的MPC樣品吸附量皆優於原始MPC，值得注意的是，磺酸化MPC的吸附性能最佳，甚至超越胺基化樣品，挑戰傳統上認為鹼性位點主導CO2 捕獲的觀點。此外， CO2吸附量也與磺酸跟密度呈現正相關，在單個位點的基礎上，MPC-SAHT1的吸附效率量為MPC-NH2的7.7倍，凸顯了磺酸化樣品卓越的吸附效率。能量學分析指出，MPC-SA的等量吸附焓達到35.35 kJ/mol，與胺基化樣品相當，但於真空室溫下可完全脫附，表明其兼具強吸附與低能耗再生特性。轉化為共軛鹼形式(MPC-SACB)後，吸附量與ΔHads分別進一步提升至2.289 mmol/g和44.27 kJ/mol。13C NMR實驗提供了分子尺度的直接證據，顯示吸附在MPC-SACB上的13CO2 訊號往高場移動並明顯增寬，證實CO2中碳原子周圍的電子密度增加，與過去理論計算預測有一致的結果。除此之外，變溫與變壓的13C NMR實驗更進一步闡明了CO2在MPC-SACB表面的物理吸附行為，觀察到隨溫度升高其吸附能力減弱的趨勢，此結果能與等溫吸附曲線的觀察相符合，也為解析物理吸附物種提供一種新的分析途徑。 本研究成功展示透過官能基修飾，特別是磺化，可以顯著提升非多孔MPC材料中二氧化碳吸附的性能，結果挑戰傳統上認為鹼性位點在二氧化碳捕獲中主導地位的觀點，並凸顯了磺酸基團在強物理吸附中的重要作用，這項工作位開發基於石化副產品的高效、低能耗二氧化碳吸附劑提供了新的策略及見解。

This study systematically investigates the role of surface functional groups in enhancing CO2 adsorption performance, utilizing mesophase pitch carbon (MPC), a petrochemical byproduct, as the precursor. Given the urgency of global greenhouse gas emissions, the development of efficient adsorbents for Direct Air Capture (DAC) has become critical. Theoretical calculations suggest that sulfonic acid groups exhibit the strongest noncovalent affinity toward CO2 through hydrogen bonding and electrostatic interactions, thereby providing a compelling rationale for this research. Experimentally, MPC was modified via oxidation, nitration, amination, and sulfonation, with further preparation of hydrothermally treated sulfonated carbons (MPC-SAHT1/2) and their conjugate base form (MPC-SACB). Structural and surface chemical features were elucidated using Fourier-transform infrared spectroscopy, solid-state NMR, and Raman spectroscopy, acid-base back titration, and elemental analysis, while porosity was evaluated by argon adsorption-desorption. CO2 uptake was assessed through isotherms, isosteric enthalpy of adsorption (ΔHads), and in situ adsorption-desorption experiments. All functionalized samples demonstrated enhanced CO2 adsorption compared to pristine MPC. Significantly, sulfonated MPC exhibited the highest capacity, surpassing aminated MPC, which conventionally dominated CO2 capture at basic sites. The CO2 uptake correlated positively with sulfonic group density. Furthermore, on a per-site basis, MPC-SAHT1 achieved approximately 7.7 times higher uptake per site than aminated MPC, thereby demonstrating its exceptional adsorption efficiency. Thermodynamic analysis revealed strong yet reversible physisorption (ΔHads: 35.25 kJ/mol), while the conjugate base form further increased adsorption strength and capacity (2.289 mmol/g, 44.27 kJ/mol).13C NMR provided molecular-scale evidence of increased electron density around carbon atom of CO2, which is consistent with previous theoretical calculation studies. In addition, the variable-temperature and variable-pressure 13C NMR experiments further elucidate the physisorption behavior of the CO2 on MPC-SACB surface. The spectra reveal a clear temperature-dependent trend indicative of weakened adsorption at elevated temperatures. This observation is consistent with the behavior reflected in the macroscopic adsorption isotherms and provides a new analytical perspective for characterizing physisorbed CO2 species. These findings highlight sulfonation as a highly effective strategy for improving CO2 capture in nonporous carbons, challenging the conventional dominance of basic sites. We propose new design principles for low-regeneration-energy sorbent derived from petrochemical byproducts.