A Study on Novel Catalysts for Upgrading Polycyclic Aromatic Hydrocarbons and Characterization using XAFS Spectroscopy

국문 초록 (Abstract)

본 연구는 다환식 방향족 탄화수소의 고부가화를 주 대상으로 하며, 세부적으로 탈황, 수첨분해, 카르복실화의 세 촉매반응으로 구분된다. 촉매활성을 증진시키기 위해 낮은 산화수를 가지는 인 전구체, 나노크기의 촉매합성, 그리고 촉매와 반응물의 인큐베이션단계를 적용하였다. 다양한 조건 및 방법으로 제조한 촉매는 BET, CO uptake, TPR, TEM, ICP-AES, 그리고 XAFS 등의 분석기법을 이용하여 특성을 조사하였다. 연구를 통하여 얻은 결과를 종합하면 다음과 같다.
첫째, 리간드 안정화법으로 제조한 Ni2P 나노결정의 형성 메커니즘 및 TOP와 TOPO의 역할에 대해 연구하였다. XANES 및 EXAFS 결과를 통해 Ni2P 나노결정 형성과정 중의 Ni은 착화물형성단계(complexation), 핵성장단계(nucleation), 그리고 인화물형성단계(phosphidation)를 거치는 것을 확인하였다. 또한 Ni-TOPO 복합체로부터 형성된 Ni2P 나노입자의 크기가 약 11 nm인 것과 비교해서, Ni-TOP 복합체로부터 더 작은 크기인 5 nm의 나노입자가 형성된 것을 통해 Ni-TOP 간 상호작용이 더 강하다는 것을 확인하였다.
둘째, 종래의 고온 환원법 (773 K, Ni2P/SiO2-HT)을 개선하여 Ni2P 활성상의 분산도를 높이기 위하여 낮은 산화가를 갖는 인 전구체를 사용하여 Ni2P촉매를 제조하였다. 673 K이하의 낮은 온도에서 환원되는 방식을 통해 실리카 담체상에 Ni2P 입자들의 분산도를 높일 수 있었으며 (Ni2P/SiO2-LT), 그에 따른 전환율의 향상효과를 거두었다: Ni2P/SiO2-HT(54%) < Ni2P/SiO2-LT(67%). EXAFS 분석결과를 통해 담체상의Ni2P 입자 및 두 종류의 Ni (tetrahedral Ni(I), square pyramidal Ni(II) 이 존재함을 확인하였으며, Ni2P의 분산도가 증가함에 따라 두 종류의 Ni 배위 가운데, square pyramidal Ni(II) 의 배위수가 증가하여 4,6-DMDBT 전환을 위한 반응활성점으로 작용함을 확인하였다.
셋째, 리간드 안정화법을 사용하여 제조한 나노입자 단위의 MoS2 의 입자형성 메커니즘 및 형상특성 제어를 위한 다양한 합성조건을 확립하였다. EXAFS 및 TEM결과를 통하여 Mo은 핵성장단계 (nucleation)에 이어 황화물 형성단계 (sulfidation)를 거쳐 판상의MoS2구조가 얻어짐을 확인하였다. 결과적으로, 핵성장 초기단계에서는1 nm 이하의 구형의 나노입자가 형성되고 온도가 증가할수록 5-7 nm까지 길이 방향으로 증가하며, 황화단계를 통하여 단일 판상형태 MoS2상으로 발달되었으며(573 K), 황화온도를 593 K이상으로 승온시 판상이 여러측으로 적층되는 거동을 확인하였다.
넷째, 리간드 안정화법을 사용하여 제조한 MoS2 나노입자 촉매를 감압 잔사유의 수첨분해반응에 적용하여 나노입자 형상특성에 따른 촉매적 활성을 비교평가하였다. 이를 통해 단층형 MoS2 나노입자 촉매의 경우 종래 전구체기반의 분산촉매 대비 감압잔사유의 경질화에 대한 우수한 수율 (52.4 % vs. 78.2 %)을 보여 가능성을 입증하였다. 또한 EXAFS 및 TEM 분석결과를 통해10 nm이하의MoS2 나노촉매가 수첨분해반응 (673 K, 10.0 MPa) 중에도 안정하게 형상특성을 유지함을 확인하였다.
다섯째, 이산화탄소 고정화를 통한 방향족화합물의 고부가화 연구를 위하여 AlCl3루이스 산 촉매를 사용하여 toluene 및 2-naphthol의 직접 카르복실화 반응에 적용하였다. 반응 율속단계에 해당하는 이산화탄소의 활성화 단계를 촉진시키기 위하여 촉매전처리 (이산화탄소 및 루이스산촉매 인큐베이션 단계)를 도입하여 반응 카르복실산에 대한 수율 향상 효과를 확인하였다 (6.9 % vs. 2.5 %). 한편 각 반응의 주생성물에 해당하는 para-Toluic acid및 2-Hydroxyl, 1-naphthoic acid의 선택도는 거의 99.9 %에 가깝게 얻어져 알킬 기능기를 가지는 방향족은para위치로, 하이드록실 기능기를 가지는 방향족은 ortho 위치로의 카르복실화가 우세함을 입증하였다.

다국어 초록 (Multilingual Abstract)

This study includes the catalysis for upgrading polycyclic aromatic hydrocarbons. In detail, the catalysis was classified with three types of reaction including the hydrodesulfurization (HDS), the hydrocracking (HCK) and the carboxylation. In order to increase the catalytic activity, three approaches have been made by adopting less oxidic phosphorous precursor to lower reduction temperature, synthesizing nano-scaled catalysts to provide better accessibility, and undergoing incubation of non-reactive CO2 with the Lewis acid catalyst. Characterizations were made by BET, CO uptake, TPR, TEM, ICP-AES and XAFS spectroscopy.
First, a study has been conducted to demonstrate the formation mechanism of Ni2P nanocrystals and the role of coordinating solvents of TOP (trioctylphosphine) and TOPO (trioctylphosphine oxide). The Ni2P nanocrystals were successfully synthesized by a ligand stabilization method. The XANES and EXAFS results confirm that the Ni species undergo three consecutive steps of complexation, nucleation and phosphidation in the course of formation of Ni2P nanocrystals. The stronger interaction of Ni-TOP complex induced smaller Ni2P nanoparticles (5 nm) than those (11 nm) from Ni-TOPO complex.
Second, the Ni2P catalysts were prepared by a new synthetic method with use of less oxidic phosphorus precursor in order to achieve high dispersion on silica support, and their structural properties and catalytic activity in HDS of 4,6-DMDBT were studied. Low temperature reduction technique led to better dispersion of Ni2P particles on SiO2 support. EXAFS analysis of the Ni2P samples confirmed the formation of Ni2P phase present on the silica support and the presence of two types of sites, tetrahedral Ni(I) sites and square pyramidal Ni(II) sites, with the latter growing in number in the same order as the reactivity Ni2P/SiO2-LT(67 % HDS) > Ni2P/SiO2-HT(54 % HDS), as the dispersion of Ni2P phase increased. These results thus suggest that the HDS activity of the Ni2P catalysts highly depend on the dispersion of the Ni2P phase.
Third, MoS2 nanoparticles were synthesized by using the ligand stabilization method. Different temperature conditions for two consecutive synthesis steps of nucleation and sulfidation were applied to clarify the effect of synthetic conditions on the MoS2 nanoparticle formation. The EXAFS and TEM results confirm that the Mo species undergo the nucleation followed by the sulfidation in the course of formation of MoS2 particles. In the nucleation step, nuclei-like spherical nanoparticles below 1 nm in size were initially formed and were linked together into one-dimensional direction with 5-7 nm in length at a high temperature. In the sulfidation step, the development of MoS2 phase was promoted upon heating over 593 K with the formation of stacking structures comprising several layers of single slabs.
Fourth, the oil-dispersible nanoscaled-MoS2 catalysts synthesized by the ligand stabilization method were applied for the hydrocracking of vacuum residue. TEM and EXAFS results revealed that the MoS2 phase of less than 10 nm in length remained stable in the course of the hydrocracking. The nanoscaled-MoS2 catalysts gave a good activity for the hydrocracking of vacuum reside with a yield to the liquid oil product of 78.2 % at 673 K and 10.0 MPa, which was much higher than the bulk MoS2 catalyst of 52.4 %.
Fifth, the direct carboxylation of toluene and 2-naphthol was conducted by using AlCl3 as a Lewis acid catalyst. para-Toluic acid and 2-hydroxy,1-naphthoic acid were obtained with about both 100 % of selectivity and 16.9 % and 6.9 % of yield, respectively. Incubation step was beneficial to induce the CO2 activation and resulted in higher yield of carboxylic acid. This study verified that the aromatic compounds with alkyl group favor the para-carboxylation and those with hydroxyl group favor the ortho-carboxylation onto the adjacent site of hydroxyl group.

목차 (Table of Contents)

• Chapter I Introduction 1
• 1.1 Background 1
• 1.1.1. The source of polycyclic aromatic compounds 1
• 1.1.2. Upgrading of heavy oils 4
• 1.1.3. Upgrading of coal tar 6
• 1.2 Catalysts 7
• 1.2.1. Transition metal chalcogen catalysts 7
• 1.2.2. The need of nanosclaed catalysts for upgrading polycyclic aromaticjj compounds 11
• 1.2.3. Lewis acid catalyst for direct CO2 fixation onto aromatic compounds 12
• 1.3 Objectives 15
• 1.4 Overview 16
• 1.5 Experimental details 18
• 1.5.1. Synthesis of Ni2P catalysts 18
• 1.5.2. Synthesis of MoS2 catalysts 19
• 1.5.3. Characterization of catalysts 20
• Chapter II Insight of formation mechanisms of Ni2P nanocrystals using XANES and EXAFS spectroscopy 32
• 2.1 Background 32
• 2.2 Experimental 33
• 2.3 Results and Discussion 36
• 2.3.1. Complexation of Ni with coordinating agent 37
• 2.3.2. Nucleation and Phosphidation 40
• 2.3.3. Proposed mechanism on the formation of Ni2P nanocrystals 46
• 2.4 Summary 49
• Chapter III EXAFS as a Tool for Studies of Ni2P/SiO2 Hydrotreating Catalysts 52
• 3.1 Background 52
• 3.2 Experimental 52
• 3.2.1. Catalyst samples 52
• 3.2.2. Characterization of samples 53
• 3.2.3. Activity test for HDS 54
• 3.3 Results and Discussion 54
• 3.4 Summary 64
• Chapter IV EXAFS Studies on the Formation of MoS2 Nanoparticles 66
• 4.1 Background 66
• 4.2 Experimental 67
• 4.3 Results and Discussion 68
• 4.4 Summary 77
• Chapter V A Novel Nanoscaled-MoS2 Catalyst for Hydrocracking of Vacuum Residue 79
• 5.1 Background 79
• 5.2 Experimental 79
• 5.2.1. Preparation of catalysts 79
• 5.2.2. Characterization of catalysts 81
• 5.2.3. Activity test of catalysts in hydrocracking 81
• 5.3 Results and Discussion 82
• 5.4 Summary 91
• Chapter VI Direct carboxylation of toluene and naphthol using Lewis acid catalyst 93
• 6.1 Background 93
• 6.2 Experimental 94
• 6.3 Results and Discussion 94
• 6.4 Summary 99
• Chapter VII
• Conclusion 101
• Abstract (in Korean) 105