從塑料到高價值金屬有機框架材料：使用無溶劑研磨和烘焙法開發用於PET/PC回收和BHET/PET轉化為MOF的高性能催化劑

Abstract

開發一個塑膠回收和高值化應用的技術，以解決在過去半個世紀中因塑膠產量的大幅提升而日益惡化的全球塑膠污染，已是目前非常重要的議題。在本論文中，我們提出了以異相觸媒進行乙二醇解聚對苯二甲酸乙二酯(PET)和甲醇解聚碳酸酯(PC)的技術，並將PET乙二醇解後的產物聚對苯二甲酸乙二酯（BHET)當做配體以合成高價值的金屬有機框架(MOFs)。 在論文的第一部分中，我們使用機械化學合成法，在沒有溶劑的情況下成功合成四種尖晶石肥粒鐵(MFe2O4, M= Co, Ni, Cu, and Zn)以做為異相觸媒在常壓下乙二醇解PET。這四種尖晶石肥粒鐵在190 °C下皆具有良好的反應性並產出高純度的BHET，其中，他們的反應活性因為金屬離子的路易斯酸強度而可以排出以下結果：ZnFe2O4 > CuFe2O4 > CoFe2O4 > NiFe2O4。儘管CoFe2O4 在這些觸媒中的反應活性僅是第三佳，但其擁有的最高飽和磁化強度使它非常容易即可使用磁鐵和反應物分離。此外，它的反應活化能188 kJ mol–1也和目前文獻中異相觸媒的反應活化能相近。在實驗室規模的批次反應器中我們已可連續使用此觸媒至少五次，並透過Aspen Plus®軟體模擬證明這個乙二醇解技術在被放大後依然可行。 在論文的第二部分中，我們開發出一種以低成本且容易取得的鋁酸鈉做為觸媒來進行PC的甲醇解技術以得到高純度和高結晶度的雙酚A(BPA)單體。我們首先篩選了一系列的有機溶劑如丙酮、乙腈、氯仿、環己烷、二氯甲烷、碳酸二甲酯、庚烷、四氫呋喃(THF)，其中THF由於和PC有相似的極性而有最好的催化效果。在溶劑是THF的系統時，鋁酸鈉做為固態鹼性觸媒，其反應活性和溶解度很高的氧化鍶相當，並遠高於氧化鎂和氧化鈣。在60 °C與常壓下，鋁酸鈉可在2小時內達到98.1%的PC轉化率和96.8%的BPA產率，並且可重複進行此反應至少四次。同時，我們證明了反應機制是由甲氧基途徑進行，並且THF會使PC快速膨脹和溶解以加速反應。此外，在動力學分析上，使用鋁酸鈉做為觸媒僅有75.1 kJ mol–1的反應活化能，這是目前發表過的異相催化反應文獻中最低的數值。 在第三部分中，我們使用在第一部分中經由PET乙二醇解得到的BHET做為配體，並在沒有溶劑的情況下合成金屬有機框架。透過我們研發的“研磨和加熱”法可在不需溶劑的條件下以130 °C和30分鐘的反應時間合成UiO-66(Zr)，這比目前所有發表過的機械化學加熱法和溶劑熱合成方法都要快。此反應的反應機制是透過BHET水解產生的對苯二甲酸（BDC2−）陰離子和Zr簇的形成以合成出UiO-66(Zr)。此外，當我們將配體改成合成UiO-66(Zr)常用的對苯二甲酸，並使用相同的合成手法時，我們驚訝的發現不管是使用四氯化鋯或是氯氧化鋯做為鋯前驅物皆無法合成具高結晶度的UiO-66(Zr)。同時，我們也使用此技術合成出其他種類的MOF，如UiO-66(Hf)、Cu-BDC和MIL-53(Al)。與現有技術相比，本技術不僅實現了從廢PET到MOF的高值化轉化，還在更溫和的反應條件下同時兼顧環保。 第四部分做為第三部分的延續，我們發現將PET浸泡在乙二醇後 ，即可當做配體，並透過我們第三部分提到的研磨和加熱法來合成UiO-66(Zr)。此發現進一步證明了將PET直接轉化為MOF的可能性，讓廢PET轉化到MOF有更多可能。

Owing to the rapid upsurge in plastic production in the last five decades, which has resulted in a dramatic increase in global plastic pollution, the development of sustainable techniques for plastic recycling and upcycling is indispensable in an attempt to decrease global plastic pollution and to attain a sustainable cycle. In response to this, the present author performed the development of heterogeneous catalysts for the chemical recycling of polyethylene terephthalate (PET) via glycolysis and polycarbonate (PC) via methanolysis, and the conversion of PET and the glycolysis product of PET, namely bis(2-hydroxyethyl) terephthalate (BHET), into valuable metal-organic frameworks (MOFs). The first section covers the use of solvent-free mechanochemically synthesized spinel ferrite (MFe2O4, M= Co, Ni, Cu, and Zn) as a solid catalyst for PET glycolysis under atmospheric pressure. While all catalysts were active for the reaction at 190 °C, producing highly pure BHET, the catalytic activity over ZnFe2O4 > CuFe2O4 > CoFe2O4 > NiFe2O4. The difference in the catalytic activity among the catalysts was revealed due to the difference in the Lewis acid strength of the M2+, whereas the catalyst with the higher Lewis acid strength possessed higher catalytic activity. Although CoFe2O4 was the third-best catalyst in terms of catalytic activity, it exhibited the highest saturation magnetization, which is a great advantage for the magnetic separation of the catalyst. Over CoFe2O4, the reaction possessed an apparent activation energy (Ea) of 188 kJ mol–1, which is comparable to most reported heterogeneous catalysts for the reaction. While the catalyst was reusable at least five times, a simulation using Aspen Plus® revealed that scale-up is feasible. In the second section, PC methanolysis by using a low-cost and readily available sodium aluminate (NaAlO2) is presented. NaAlO2 was a highly active solid base catalyst for the reaction in the presence of tetrahydrofuran (THF) as a solvent, producing highly pure and crystalline bisphenol A (BPA) monomer, with the catalytic performance comparable to soluble SrO and much higher than those of MgO and CaO. Among tested organic solvents, e.g., acetone, acetonitrile (ACN), chloroform, cyclohexane, dichloromethane (DCM), dimethyl carbonate (DMC), heptane, and THF, THF was the best one in aiding the reaction owing to the polarity similar to the PC according to the Hansen solubility parameters (HSPs). At 60 °C and under atmospheric pressure, 98.1 and 96.8% of PC conversion and BPA yield, respectively, were achieved within just 2 h. While the catalyst can be reused for at least four runs, the mechanistic study revealed that the reaction was governed by the methoxide pathway, with THF aiding in the faster swelling and dissolution of PC. Over NaAlO2, the reaction exhibited Ea of 75.1 kJ mol–1, which is the lowest ever reported for a reaction over a heterogeneous catalyst. In the third section, the use of BHET (the glycolysis product of PET obtained from the first section) as a new linker source for the solvent-free synthesis of MOFs is highlighted. Via the solvent-free “grind and bake” technique, UiO-66(Zr) was easily synthesized. It was found that the hydrolysis of BHET to terephthalate (BDC2−) anion by proton produced from the hydrolysis and clustering of Zr precursor was the key to the crystal growth of UiO-66(Zr). Surprisingly, the use of H2BDC, which is a typical linker source for UiO-66(Zr), resulted in no and poor diffraction patterns of UiO-66(Zr) when ZrCl4 and ZrOCl2•8H2O, respectively, were used as Zr precursors. In this work, UiO-66(Zr) can be synthesized within 30 min (at 130 °C), which is much shorter than any previously reported mechanochemical-heating and solvothermal synthesis of UiO-66(Zr). Some MOFs, namely UiO-66(Hf), Cu-BDC, and MIL-53(Al), have also been successfully synthesized. This work realizes the idea of PET waste-to-MOFs with more convenience and green compared to the prior arts. The fourth section is the follow-up of the third section, where it was found that when PET is treated with ethylene glycol (EG), it can be used as a linker source for the grind and bake synthesis of UiO-66(Zr). This work demonstrates that direct conversion of PET into MOF is possible, further pushing the limit of PET-to-MOF conversion.