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Study on physicochemical properties of a microporous material and its derivatives: Demonstration of Mo-impregnated MWW type zeolites for methane dehydroaromatization reaction and zeolitic imidazolate framework-8 (ZIF-8) for CO2 cycloaddition reaction

  • Gihoon Lee (이기훈, 대학원 화공생명공학과 화공생명공학전공)

초록/요약

미세다공성 물질은 2 nm 이하의 기공 크기를 갖는 고체 물질을 말하며, 미세기공으로 인해 높은 비표면적을 가져 촉매, 지지체, 흡착제 등 다양한 분야에 활용된다. 다양한 미세다공성 물질 중 하나인 제올라이트는 규칙적인 미세 기공 구조와 산 특성으로 인해 석유 화학 분야에 촉매로 널리 활용된다. 250개 이상의 제올라이트 종류 중 하나인 MCM-22는 상업적으로 널리 사용되는 ZSM-5와 유사한 기공크기로 인해(MCM-22의 기공 크기는 5.5 × 4.1 nm2, ZSM-5의 기공 크기는 5.5 × 5.1 nm2) 많은 연구자들에 의해 연구되었으며, 이외에도 층상구조의 MCM-22는 MCM-22 전구체의 후처리를 통해 손쉽게 계층구조를 도입할 수 있어 다양한 방법을 통해 계층구조를 갖는 MWW 유형 제올라이트 합성에 대한 연구가 수행되었다. 이에 따라, 미세/중형 기공을 갖는 MCM-36, MCM-22 전구체의 박리를 통한 나노시트 형태의 ITQ-2 등 다양한 MWW 유형 제올라이트가 개발되었다. 이러한 MWW 유형 제올라이트들은 MCM-22의 층간구조를 공유한다는 점에서, 동일한 제올라이트 위상에서 제올라이트 구조의 영향을 입증할 수 있는 좋은 제올라이트로 활용될 수 있다. 제올라이트 외에도 MOF(Metal Organic Frameworks)는 미세다공성 물질로 잘 알려져 있으며, 높은 다공성, 구조 유연성과 같은 특징으로 센싱, 약물 전달, 가스 분리 및 촉매와 같은 다양한 분야에 활용되어진다. 다양한 MOF중 하나인 ZIF-8(Zeolitic imidazolate framework-8)은 아연과 2-메틸이미다졸간의 결합으로 이루어져 있는 금속유기골격체이다. 이때, ZIF-8 구조를 위해 아연과 2-메틸이미다졸이 결합되면, 산 특성이나 염기 특성을 나타 날 수 없어 ZIF-8을 활용한 촉매 반응에서 활성 성분은 여전히 모호하다. 비록 ZIF-8을 활용한 촉매 연구들은 ZIF-8내에 존재하는 결함에서 활성 성분이 나타 날 수 있다고 주장하고 있지만, 여전히 ZIF-8 구조내의 활성 성분에 대한 깊은 이해가 필요하다. 따라서, 본 연구에서는 두 가지 유형의 미세 다공성 물질(제올라이트 및 ZIF-8)을 활용하여 메탄 및 이산화탄소 전환 반응에 활용하였습니다. 이를 토대로 미세 다공성 물질의 물리 화학적 특성과 촉매 활성 간의 관계를 규명하고자, 다양한 특성 방법을 활용하였습니다. 제1장에서는 본 연구에서 사용된 MCM-22 제올라이트 및 계층 구조 제올라이트와 ZIF-8에 대하여 소개하였다. 제2장에서는 몰리브덴이 담지된 계층구조 제올라이트 촉매를 활용하여 메탄탈수소방향족화 반응을 수행하였으며, 이를 토대로 제올라이트 촉매의 특성이 메탄탈수소방향족화 반응의 성능과 촉매 안정성에 미치는 영향을 정량적으로 규명하였습니다. 이를 위해 4가지 형태의 계층구조 MWW 유형 제올라이트를 제조하고 몰리브덴을 담지하여 메탄탈수소방향족화 반응에 활용하였다. 반응 전·후 촉매의 특성 분석을 통해 제올라이트 촉매 내에 존재하는 제올라이트 기공 부피가 메탄탈수소방향족화 반응에 미치는 영향을 정량적으로 확인하였다. 또한, 반응 후 촉매의 열중량 분석 및 Ar 흡착 등온선의 상호 보완적인 특성 분석을 통해 제올라이트 촉매의 물리화학적 특성이 촉매 안정성에 미치는 영향을 조사하였다. 제3장에서는 세 가지 유형의 ZIF-8을 활용하여 CO2 고리화 첨가반응에 활용하여, ZIF-8 내에 존재하는 활성점을 규명하였다. 세 가지 유형의 ZIF-8을 위해 서로 다른 합성용액(메탄올(ZIF-8M) 및 수용액(ZIF-8W))을 활용하여 ZIF-8을 합성하였으며, 나머지 하나는 상업적으로 구매 가능한 ZIF-8을(ZIF-8C) 활용하였다. 세 가지 유형의 ZIF-8 중에 수용액에서 합성된 ZIF-8W만이 CO2 고리화 첨가 반응에 반응 활성을 보였으며, 나머지 두 형태의 ZIF-8(ZIF-8M 및 ZIF-8C)는 반응 활성을 나타내지 않았다. 반응 전 후 ZIF-8 촉매의 특성 분석을 통해 ZIF-8W 내에 존재하는 물 분자에 의해 ZIF-8 구조 내에 존재하는 Zn-N 결합이 분해 되었으며, 반응 활성이 없는 다른 두 ZIF-8 촉매들은 반응 후에도 ZIF-8 구조를 잘 이루는 것을 확인하였다. 이를 토대로, ZIF-8W내 존재하는 반응 활성점을 다양한 특성 분석을 통해 규명하였다. 제4장에서는 ZIF-8의 구조변화로 생성되는 새로운 입자(본 연구에서 DP로 지칭)를 합성하는 방법을 제안하고, 새로운 입자의 구조적 특성과 물리화학적 특성을 확인하고자 하였다. 새로운 입자 합성 방법을 제안하기 위해 네 가지 유형의 ZIF-8을 활용하여 수용액에 일정 시간 동안 침지 하였다. 이때 DMF 용액에서 합성된 ZIF-8만이(ZIF-8D) ZIF-8에서 DP 구조로 변화하는 것을 확인하였으며, 이를 토대로 단일 구조 DP 입자 합성을 위해 ZIF-8D의 합성 방법을 토대로 DP 합성 방법을 제안하였다. 본 연구에서 제안된 DP 합성 방법을 바탕으로 단일 구조의 DP 입자를 성공적으로 합성하였으며, 이를 바탕으로 실험 및 시뮬레이션을 활용하여 DP 입자의 구조적, 물리화학적 특성을 파악하였다.

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초록/요약

Microporous materials are solid materials containing pores with diameters of less than 2 nm. Due to the small size of pore diameter in their structure, microporous materials showed high specific surface area, which lead to the various applications as catalyst, support, and adsorbent. Among the various microporous materials, zeolites, consisted of silica and alumina, are widely employed for industrial applications as catalyst owing to its unique pore structure and acid property. Among the over 250 types of zeolites, MCM-22 (Mobil Composition Matter-22), a representative of MWW type zeolite, has attracted many researchers owing to its similar pore size to that of ZIF-5 (Zeolite Socony Mobil-5) (ca. 5.5 × 4.1 nm2 for MCM-22 and 5.5 × 5.1 nm2 for ZSM-5). Furthermore, the layered structure of MWW type zeolite allows the transformation of MWW type zeolite structure by modifying the MCM-22 precursors. Therefore, significant investigations into methods to modify the structure of MWW type zeolite have been conducted. Consequently, various MWW type zeolite derivatives such as microporous/mesoporous MCM-36 obtained by pillaring MCM-22 precursor and nanosheet ITQ-2 obtained by delaminating MCM-22 precursor were developed. Therefore, considering that those derivatives shared the MWW zeolite topology, MCM-22 and its derivatives can serve as a good candidate for demonstration of the effects of zeolite structure under the identical topology. Aside from zeolite, metal organic frameworks (MOFs) are known to be famous microporous materials. MOFs have potential utility in various applications such as sensing, drug delivery, separation, and catalysis on the basis of their physicochemical properties such as high porosity, structural flexibility, and diverse functionalities. Among the various MOFs, zeolitic imidazolate frameworks-8 (ZIF-8) ,which is composed of zinc cations and 2-methylimidaolate ligands, was widely used as gas separation, wastewater treatment, and heterogeneous catalysis. However, considering that ZIF-8 consisted of bonding of Zn and 2-methylimidazolate ligands, and fully saturated Zn and 2-methylimidazolate could not generate the active component such as acidic or basic properties, the active component in ZIF-8 is still ambiguous. Although previous literature insisted that unsaturated Zn or 2-methlimidazolate in the defective ZIF-8 structure might act as the active component, it is necessary to comprehend the active component in the ZIF-8 structure to properly utilize ZIF-8 as a catalyst. In this study, two types of microporous materials (i.e., zeolite and zeolitic imidazolate framework-8 (ZIF-8)) were applied as catalyst for CH4 and CO2 conversion, respectively. In addition, the physicochemical properties of microporous materials were investigated via comprehensive and supplemental characterization to elucidate the relationship between catalyst properties and catalytic activities. First of all, in Chapter 1, two types of microporous materials applied in this study were described; MCM-22 (Mobil Composition of Matter-22) and its derivative zeolite and zeolitic imidazolate framework-8 (ZIF-8). In Chapter 2, the effect of zeolitic pore in Mo-impregnated zeolite catalysts on catalytic performance and stability of methane dehydroraromatization (MDA) reaction was quantitively elucidated by using hierarchical zeolites. For this, four types of hierarchical MWW type zeolites (MCM-22 and its derivatives), which shared intra-layer structure, were precured and applied on the MDA reaction after Mo impregnation. The quantitative effects of zeolitic pore volumes in the Mo-impregnated catalysts on the MDA reaction were evaluated via comprehensive and complementary characterizations. Furthermore, TGA measurement and combined TGA and Ar physisorption analysis were performed to investigate the catalyst stability on the MFA reaction. In Chapter 3, three types of ZIF-8s (two types of ZIF-8s were synthesized in aqueous (ZIF-8W) and methanolic media (ZIF-8M), while the other was commercially available (ZIF-8C)) were employed to investigate the catalytic activity of the CO2 cycloaddition reaction, producing the cyclic carbonate. Among the three types of ZIF-8s, only ZIF-8W, which is synthesized in aqueous medium, showed a marked catalytic activity, while others were inactive in the CO2 cycloaddition reaction. Comprehensive and complementary characterization of fresh and spent ZIF-8 catalysts revealed that the water molecule occluded in the ZIF-8W pore triggered a cleavage of the Zn-N bond in the ZIF-8 structure during the reaction, while the other ZIF-8s (ZIF-8M and ZIF-8C) were maintained their original ZIF-8 structure. On the basis of these result, the active component in ZIF-8W were identified via complementary characterizations. In Chapter 4, preparation method of the newly formed structure (referred to as DP in this study) from the ZIF-8 structure was established. For this, four types of ZIF-8s (three types of ZIF-8s were synthesized under water (ZIF-8W), methanol (ZIF-8M), and dimethylformamide (ZIF-8D), while the other was commercially available(ZIF-8C)) were immersed in liquid water to elucidate the structure transformation of ZIF-8. Among the various types of ZIF-8s, only ZIF-8D showed the structure transformation of ZIF-8 to newly observed structure (referred to as DP, in this study). On the basis of these results, I established the preparation method for a single DP structure by modifying the synthetic recipe of ZIF-8D. To delve into the structural information of DP structure, Rietveld refinement was applied. Furthermore, physicochemical property of the DP particle was elucidated via experimental and simulation approaches.

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목차

Contents
ABSTRACT i
국문 요약 v
Contents ix
List of figures xii
List of schemes xxiii
List of tables xxiv
Chapter 1. Introduction 1
1.1 MWW zeolite and its derivatives 1
1.2 Zeolitic imidazolate frameworks 5
1.3 References 7
Chapter 2. Elucidation of Quantitative Effect of Zeolitic pore in Mo-Impregnated MWW Type Zeolites on Catalytic Activities and Stabilities of Methane Dehydroaromatization reaction 9
2.1. Introduction 9
2.2. Experimental 13
2.2.1. Synthesis of MWW type zeolites 13
2.2.2. Synthesis of Mo-impregnated MWW type zeolite catalysts 17
2.2.3. Characterizations 18
2.2.4. Methane dehydroaromatization reaction 21
2.3. Results and discussion 23
2.3.1. Structural and textural properties of MWW zeolite and its derivatives before and after Mo impregnation 23
2.3.2. Catalytic activities of Mo-impregnated zeolite catalysts on the methane dehydroaromatization 38
2.3.3. Disclosure of the effect of zeolitic pore for aromatic formations during the methane dehydroaromatization 47
2.3.4. Unravelling of origin of catalyst deactivation 53
2.3.5. Elucidation of the relationship between the zeolitic pore structures an methane dehydroaromatization 68
2.4. Conclusions 73
2.5. Reference 75
Chapter 3. Synthetic Origin-Dependent Catalytic Activity of Metal-Organic Frameworks: Unprecedented Demonstration with ZIF-8s on CO2 cycloaddition Reaction 79
3.1. Introduction 79
3.2. Experimental 81
3.2.1. Synthesis of ZIF-8 catalysts 81
3.2.2. CO2 cycloaddition reaction 83
3.2.3. Characterization 86
3.3. Results and discussion 89
3.3.1. Catalytic activities of three types of ZIF-8 and their physicochemical properties before and after reaction 89
3.3.2. Origin of the activity of ZIF-8W in the catalytic CO2 cycloaddition reaction 104
3.3.3. Disclosure of the origin of active sites of ZIF-8W for the catalytic CO2 cycloaddition reaction 113
3.3.4. Elucidation of chemical species on the outer surface of ZIF-8W 127
3.3.5. Effect of water molecules on the formation of active sites in ZIF-8W 137
3.3.6. Identification of the active sites in ZIF-8W for the catalytic CO2 cycloaddition reaction 143
3.4. Conclusions 153
3.5. References 155
Chapter 4. Elucidation of physicochemical properties of a dense phase derivable from zeolitic imidazolate framework-8 (ZIF-8) 160
4.1. Introduction 160
4.2. Experimental 163
4.2.1. Synthesis of ZIF-8 particles 163
4.2.2. Water stability test of ZIF-8s 166
4.2.3. Characterization 167
4.2.4. Calculation details for physicochemical properties of DP particles 168
4.3. Results and discussion 171
4.3.1. Water stabilities of ZIF-8s and dependency of precursor ratio for ZIF-8 and DP structure 171
4.3.2. Disclosure of physicochemical property of DP particle 185
4.3.3. Unravelling of the DP structure 191
4.4. Conclusions 210
4.5. References 212
Chapter 5. Summary and perspective 215
Acknowledgement 219

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