VIETNAM NATIONAL UNIVERSITY - HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY --- DOAN NGOC ANH DUC IRON – BASED ORGANIC FRAMEWORKS AS CATALYSTS FOR DEHALOGENATION OF ARYL..
Metal-organic frameworks
Introduction
Metal-organic frameworks (MOFs) are porous crystalline materials whose frameworks is constituted based on the self-assembly of organic linkers (bridging ligands) and organic nodes to form one-, two-, three-dimensional structure [1, 2]
These materials are synthesised by reaction of metal salts with linkers having nitrogen or oxygen as donating atoms to forms different porous crystalline materials structure, with high specific surface area and pore volume, due to the wide of transition metals and the rich number of linker’s structures [3] For example, MOF-5(IR-MOF-5) developed by the replacement of hydrogen atoms by polar or nonpolar substituents of benzene ring backbone without changing the original cubic topology that were more variety pore sizes [4] Several members of this series have pore sizes in the mesoporous range (>20 Å) as well as the lowest crystal density Besides, the polarity were change, more hydrophilic or hydrophobic, respectively.
Figure 1.1 Comparison of the cubic structures of IR-MOF-5 formed when linear aromatic dicarboxylic acids are reacted with Zn(II) ions [2]
2 MOF materials can be classified into different families according to the dimensionality of the inorganic framework Organic–inorganic hybrid materials in which inorganic moieties can be organized into either 1D chains (like MIL-53) or 2D layers (such as Zn 2 L) that are separated by organic pillars Open-framework coordination polymers, which are made from 0D ―inorganic‖ clusters or isolated metal ions connected by bridging organic polytopic ligands (MOP-1, MOF-5, and HKUST-1) This classification is not only conceptual, since it has implications on the properties observed 0D structures are more appropriate for photocatalysis applications and Lewis-type catalysis, whereas 1D may be appropriate for acid–base Brứnsted-type catalysis [1]
Figure 1.2 Examples of structural dimensionality From left to right 0D (MOP-1), 1D
Because of the diversity of structures, a variety of researches have been conducted to synthesized different kinds of MOFs There are many strategies to generate MOFs (Figure 1.2) and in all of them, solvothermal method is the most common and powerful method to accelerate discovery of new MOFs structures and optimize synthesis protocols However, many drawbacks existed in this method such as high temperature, longtime reaction, and being effected by concentrate of reactants
The negative effects are limited by using several supporting techniques such as microwave, ultrasound, electrical, etc [5]
3 Figure 1.3 Indicative summary of the percentage of MOFs synthesized using the various preparation routes [5]
Since Yaghi and co-workers successfully synthesized MOF-5 and applied in gas absorption [6], scientists have paid attention to potential applications of this porous materials Nowadays, applications of MOFs in gas storage, gas separation, size-, shape-, and enantioselective separation [7], luminescent and fluorescent materials [8], and drug storage and delivery [9] have been explored Along with consciousness and requirements for environmental protection are focused, heterogeneous catalysts in chemical processes have also attracted interest because of its advantages such as easy catalyst separation, simple recycling, reducing waste…
The first mentions of the use of MOFs in heterogeneous catalysis dated to the beginning of the 1990s [10] The catalytic properties of MOFs relate not only to the presence of metal cation frameworks, but also to the presence of functional groups on the inner surface of the MOFs voids and channels [11] MOFs have several advantages in comparison with another well-known heterogeneous catalyst and most commonly used in industry – zeolites Table 1 provides the comparison of some features that are relevant in catalysis for zeolites and MOFs [12] These applications concern to a wide range of MOFs structures and properties like pore dimension, pore shape, inside surface In addition, due to the possibilities for modification of structure by replacing difference ligands to receive expected structures (shape, size of pore), there may be more MOFs and their applications will be discovered in the future
4 Table 1.1 Comparison of zeolites with MOFs with some properties relevant to catalysis [12].
Iron based-MOFs (Fe-MOFs)
So far, most MOFs are based on divalent cations (e.g Zn 2+ ,Cu 2+ ,Ni 2+ ) Recent studies have shown that for a given carboxylate linker, when trivalentcations are used instead the chemical stability is improved, and toward hydrolysis also decreased
However, the list of existing porous M 3+ -based MOFs (e.g with
Surface area Pore volume Thermal stability Chemical stability Diffusion Basicity
Metal site density Framework defects
Active site environment Poisoning of active sites Chirality
High Bridging Si(OH)/Al hydroxyl groups
High for small molecules in the gas phase and slow in the liquid phase
Arises from the framework oxygens
Low Plays an important role in many reactions
Mostly hydrophilic but can also be made hydrophobic Reactivation by thermal treatment
Not possible or difficult to achieve
Introduced by post-synthetic modifications or functional groups present in the ligands not compromised in the structure Up to 5000 m 2 g -1
Limited and incompatible with certain solvents
Strongly influenced by the polarity of linkers
Introduced by post-synthetic modifications or directly through linkers
High Expected to play a minor role
Thermal treatment not valid for catalyst regeneration
Homochiral solids can be easily obtained from chiral linkers or postsynthetic modifications
5 Al 3+ ,Fe 3+ ,Cr 3+ ,In 3+ ,V 3+ ,Sc 3+ ,Ln 3+ ) is still short,[2] and structures based on M 4+ cations, such as Ti 4+ or Zr 4+ , are even more scarce[13].Among the possible trivalent metals, iron(III) is a highly promising candidate owing to its low toxicity, natural abundance, and redox properties In the presence of carboxylates, iron(III) often leads to the formation of specific building units, such as isolated octahedra, corner-sharing chains of octahedra [14] , helical chains [15], oxo-centered trimersof octahedra (Figure 1.4) [16], and sometimes tetramers of octahedra [17] Octahedral form in the majority of structures of Fe-MOFs which possess opening metal sites – an important factor for catalytic application
Figure 1.4 a) Oxo-centered trimer of FeO 6 octahedra in MIL-142A (Fe atoms: orange, O atoms: red, water molecules: blue, counteranion: purple); b) terephthalate (bdc); c) 1,3,5-benzenetrisbenzoate (btb); d) views of the hybrid super octahedra along the b (left) and c axis (right); e) view of the structure of MIL-142 along thecaxis; f) schematic representation of the interpenetrated ReO3 topology [16]
One important properties of iron MOFs is highly flexible structure For example, with the flexible porous structure, Fe-MIL-53 was used as a material for
6 adsorption and delivery of ibuprofen [18] The very slow and complete delivery of ibuprofen was achieved under physiological conditions after 3 weeks
Fe-MOFs were used as catalyst in many reactions such as aldol condensation reaction [19], ring opening epoxides [20], acetalization of aldehyde [21], amine oxidation [22], thiol [23] and photocatalysts [24] For example, A Dhakshinamoorthy et al used Fe(BTC) as Lewis acids for Claisen–Schmidt reaction to form chalcones (1,3-diarylpropenones) from benzaldehydes and acetophenones (Scheme 1.1) [19] In this study, various MOFs and homogeneous catalyst was employed to ascertain optimized condition
Scheme 1.1 Claisen–Schmidt condensation of benzaldehyde with acetophenone using
Fe-MOFs as heterogeneous catalyst [19]
Recently, in 2010, Dhakshinamoorthy and co-worker synthesized (dimethoxymethyl)benzene from benzaldehyde with Fe(BTC) as solid heterogeneous catalysts and various alcohols at room temperature, resulted 71 % conversion of benzaldehyde in 24h [21]
Scheme 1.2 Acetalization of benzaldehyde with methanol using Fe(BTC) [21]
In different study of Dhakshinamoorthy used Fe(BTC) for ring opening epoxides styren oxide with solvents as methanol which has conversion more than 99 % and higly selectivity up to 94 % [20]
7 Scheme 1.3 Ring opening of styrene oxide with methanol[20].
Dehalgenation of aromatic halides
Aryl halides are commonly used as solvents, insect repellents, fungicides, and organic intermediates [25] However, these aryl halides (especially some priority pollutants) are high risk to our health and environment due to their toxicity and strong bioaccumulation potential [26] Unfortunately, incineration is unattractive from the environmental standpoint that some products such as dioxins are even more toxic than the aryl halides, which are also not readily biodegradable [27] Therefore, great efforts have been devoted to developing methods for dehalogenating aryl halides, which leads to a drastic decrease in toxicity On the other hand halides have been applied as protecting and directing groups in organic synthesis Commonly, after the halide functionalities have fulfilled their obligations a removal is needed to create the final product Recent advances in this field have led to several new methods by employing palladium [28], rhodium [29], iron [30] , and nickel [31] catalysts Although numerous reagents or protocols are available for dehalogenation, many reported methods suffered from some limitations Functional group compatibility as well as selectivity is rarely addressed [32]
Jingbo Chen, Yushun Zhang, in 2007, described debromination reaction of aromatic halides and α-haloketones used Pd(OAc) 2 in 100 o C with alcohol as hydrogen donor, reaction utilization of bromine For this method, desired debromination product was obtained in 98% isolated yield at 100 0 C However, Pd(OAc) 2 can’t recycle after react [32]
8 Scheme 1.4 Pd catalyzed dehalogenation of aromatic halides with alcohol as hydrogen donor [32]
So, in 2009 Heike Hildebrand and co-worker used nanoscale Pd-on-magnetite catalyst (Pd/Fe 3 O 4 ) which catalysts can be recycle easy, without decrease yield of reaction [33]
Recently, in 2012, a preliminary study we have found that the system formed by the pair NaBH 4 and TMEDA (N,N,N′,N′-tetramethylethylenediamine) as a hydride source in combination with a palladium catalyst is an efficient system for the hydrodehalogenation of halopyridine derivatives The good results obtained in that work prompted us to verify thep otential of this system for the removal of halogens in less reactive heterocycles In these paper report that this system is able to hydrodehalogenate efficiently and selectively a variety of halogenated heteropentalenes with one or two heteroatoms at room temperature [34]
Scheme 1.5 Hydrodehalogenation of halogenated heteropentalenes [34]
In 2002, Desmarets and et al described dehalogenation of aryl halides was efficiently performed in refluxing THF using a catalytic combination composed of Ni(0)/N-heterocyclic carbene (NHC)/β-hydrogen-containing alkoxide IMes.HCl (1,3- bis(2,4,6-trimethylphenyl)imidazolium chloride) and Ni(acac) 2 used respectively as carbene and Ni(0) precursors associated to in situ generated i-PrONa were found to be the most effective for the dehalogenation of functionalized aryl chlorides, bromides, iodides, and polyhalogenated hydrocarbons The role of the in situ generated alkoxide is threefold: (1) it initially activates sodium hydride used to reduce Ni(acac) 2 into
9 Ni(0) and ensures the repeatability of this reaction, (2) it deprotonates the imidazolium chloride to form the carbene ligand which coordinates to the metal, and (3) it acts as a hydrogen donor since it possesses a β-hydrogen [35]
Scheme 1.6 Dehalogenation of aryl halides with Ni(0) [35]
In 2010, Marı´a L Buil and co-worker, using Rh-complex have been prepared by reaction of the dimer [Rh(μ-Cl)(η 2 -C 2 H 4 ) 2 ] 2 with the corresponding nitrogen donor ligand, under 1 atm of hydrogen, in 2-propanol as solvent, at 60 0 C, and in the presence of K t BuO (Brứnsted base) However, this method has disavantages that observed byproduct from hydrogenated [36]
Scheme 1.7 Dehalogenation of chloroarenes with K t BuO/H 2 in the presence of
In 2013, Weidauer and et al, described dehalogenation of organic halides, based on protocol for the nickel-catalyzed complex, with isopropyl zinc bromide
Result, a variety of aryl and alkyl halides were converted However, the yield reaction in such conditions is variable and low yield could be observed by many byproducts which complicated purification process of product, even by column chromatography and recrystallization [37]
10 Scheme 1.8 Nikel-catalyzed dehalogenation with i PrZnBr [37]
Iron-containing catalysts have attracted particular attention [38], since it is quite inexpensive and abundant, compared to late-transition metals, and it is less harmful to the environment Furthermore, iron(III) is a harder Lewis acid, compared to late-transition-metal cations, allowing better activation of carbon halide bonds [30]
Among different approaches for heterogenization, stabilization and incorporation of an active metal into insoluble microporous [39] and mesoporous inorganic solids, having high surface areas are extensively studied Thus, we have chosen the iron- containing, highly ordered, mesoporous materials with high surface area, good pore wall stability, as well as good mechanical, thermal, and chemical stability, which can be utilized in the dehalogenation reaction [30]
In 2000, Yue Xu were synthesized Subcolloidal (