Liquid-Phase Epoxidation of Light Olefins over W and Nb

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Liquid Phase Epoxidation of Light Olefins over W and Nb Nanocatalysts Wenjuan Yan, Guangyu Zhang, Hao Yan, Yibin Liu, Xiaobo Chen, Xiang Feng, Xin Jin, and Chaohe Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03101 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Sustainable Chemistry & Engineering

Liquid Phase Epoxidation of Light Olefins over W and Nb Nanocatalysts Wenjuan Yan*, Guangyu Zhang, Hao Yan, Yibin Liu, Xiaobo Chen, Xiang Feng, Xin Jin, Chaohe Yang* State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, No. 66 Changjiang West Road, Qingdao, Shandong Province 266580, China Corresponding authors: [email protected], [email protected] Key words: shale gas, heterogeneous catalysis, mesoporous materials, green chemistry, reaction mechanism

Abstract Epoxides are among most important essential building blocks with wide downstream applications in chemical industry. In this context, rational design of cost-effective catalysts is critical to achieve high atom efficiency during epoxidation processes. Therefore, in this perspective, recent research progress on heterogeneous W and Nb nanocatalysts for facile liquid phase epoxidation will be critically revised in terms of catalyst synthesis, surface characterization and structure-function relationship. Furthermore, plausible mechanisms for liquid phase epoxidation of ethylene, propylene, hexene, octene and other light olefins as well as deactivation

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mechanism of W and Nb catalysts will be systematically discussed with the aim to provide insights into fundamental understanding on novel epoxidation chemistry and improving activity, selectivity and stability nanocatalytic materials.

Introduction Interest in epoxidation of olefins has been amplified since it provides a straightforward access to various active epoxides as industrially important intermediates and essential building blocks for everyday chemicals. Epoxides are among the most versatile and tremendously useful precursors in organic synthesis.1-7 They are key components for the synthesis of a variety of valuable downstream products such as diols, aminoalcohols, allylic alcohols, ketones, polyethers, etc,8-32 which can be further transformed to surfactants, antistatic or corrosion protecting agents, plasticizers, and additives for laundry detergents, lubricating oils, textiles, cosmetics, epoxy resins, and complex molecules used to manufacture air-craft hulls, perfumes, and pharmaceuticals activities.33-40 Due to the strain associated with the three membered ring, epoxides are spring-loaded for reactions with different nucleophiles, leading to a wide range of multi-functional organic compounds.41 Therefore, catalysis community has been fascinated with prospects of selective synthesis of epoxides by olefin epoxidation.42-44 The catalytic epoxidation of oleochemicals has been one of the most popular subjects to many academic and industrial investigations. In this context, epoxidation 2


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of light olefins to epoxides are particularly important as it provides the essential chemicals for megaton products (Figure 1). For example, ethylene oxide (EO) is one of the widely used chemical intermediates, with applications in the production of detergents, thickeners, solvents, plastics and various organic chemicals. In 2013, the global EO production capacity was approximately 20.5 million metric tons. During the next decade, the EO demand is projected to grow at an annual rate of 6-7%.45 Propylene oxide (PO) is another value-added commodity intermediate that is used in the manufacture of polyurethane foams for the automobile and housing industries, polyester resins for the textile and construction sectors, and propylene glycol as additives in drugs, cosmetics, and heat transfer or hydraulic fluids.26 Other valuable epoxides such as hexene oxide (HO), octene oxide (OO), cyclohexene oxide (cyclo-HO), cyclooctene oxide and styrene oxide (SO). They all have important applications in polymers, food additives and pharmaceuticals. In addition, application of vegetable oils in the chemical industry has also become increasingly significant because of their accessibility from renewable resources.46 Epoxidized vegetable oils also play an important role as building blocks for the preparation of chemical intermediates for a wide variety of consumer products,47 including plasticizers and stabilizers, reactive diluents for paints, intermediates in the production of polyurethane-polyols, as well as components of lubricants and adhesives.48 The epoxidation of terpenes is another attractive chemical

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transformation as such epoxides are versatile building units.49 Particularly, epoxidation products are promising monomers for the synthesis of a new biodegradable “polylimonene carbonate” from biorenewable resources.50

Figure 1. Conversion of shale gas derivatives to value-added chemicals

Currently, epoxides are produced primarily by the reaction of olefins using stoichiometric amounts of organic peroxy acid51 or via chlorohydrin intermediates followed by strong alkaline treatments to form oxiranes.52 These processes not only generate stoichiometric amounts of corrosive organic acids and alcohols53 or chloride salts,52 respectively, and require subsequent energy-intensive separation, waste treatment, and regeneration, but also involve safety issues associated with handling of peracids.54. It is generally accepted that catalysis is the most important technology for “Green Epoxidation Chemistry”. More importantly, heterogeneous catalysis offers an additional advantage in facilitating separation at the end of the reaction and increasing catalyst life-time. Thus, the development of environmentally friendly solid catalysts for selective epoxidation is becoming an area of growing interests in the past decades. 4


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However, the development and implementation of catalytic processes that eliminate the use of hazardous reactants and reduce generation of wastes still remains a challenging goal of the modern organic synthesis stimulated by increasing environmental pressure. The selective oxidation of olefins into the corresponding oxygenated derivatives by heterogeneous catalysis is a key transformation in the fine chemicals industries. Among different oxidants, hydrogen peroxide (H2O2) is a safe, inexpensive, readily available and relatively non-toxic as it gives water as the only by-product.53,55-57 Epoxidation of light olefins in the presence of H2O2 can be realized under very mild condition. Therefore, the use of H2O2 as an efficient liquid oxidant is an attractive option from both environmental and economic perspectives.

Background of W and Nb Catalysts Tungsten (W) and niobium (Nb) based catalysts have shown promising performances for liquid phase oxidation of olefins (Figure 2). W based catalysts have attracted extensive interest because they have been widely employed in selective oxidation of alkenes58-66 and alcohols,67-70 olefin metathesis,71-78 oxidative cleavage of olefins,79 dehydration,80-87 oxidative desulfurization of sulfur-containing organics88 and acid catalyzed reactions such as isomerization.89-97 In order to achieve higher performance, recent efforts have been paid on proposing novel synthetic approaches for catalysts preparation, many of which focus on the formation of mesoporous and 5



nanostructured oxides and incorporation of active metal sites into inorganic matrices such as siliceous and non-siliceous mesostructured materials. For example, reported heterogeneous catalysts include tungsten oxide (WO3) containing mesoporous materials synthesized by means of a number of synthetic approaches,98 heterogenized polyoxotungstates and tungsten complexes, as well as tungsten exchanged layered double hydroxides. To this end, sol-gel techniques, hydrothermal procedures, and structure directing template approaches have been widely studied. 99,100

Figure 2. Application of W and Nb catalysts in organic synthesis

Niobium oxide (Nb2O5) species exhibit unique properties which have not well established compared with compounds of neighboring elements in the periodic table. Some of them, like strong metal support interaction (SMSI) or unique reversible 6


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interaction with several reagents are very important for the design of catalysts.101-103 The function of Nb compounds in catalysis can be that of promoter or active phase, support, solid acid catalyst, or redox material. Nb can be present in Nb2O5 in its bulk form as a catalyst, or in mixed oxides and as a heterogeneous support for other metal catalysts. It is generally believed that catalytic activity of Nb2O5 is largely determined by the surrounding of Nb and O species, namely surface redox properties.101 The presence of both Brønsted and Lewis acid sites has been reported in hydrated Nb2O5104 and niobium-silicon mixed oxides.105 The Lewis acid sites are due to the presence of an excess of effective positive charge in NbO4 tetrahedra, while the Brønsted ones are related to the presence of an excess of effective negative charge in NbO6 octahedra.106 Nb-based silica catalysts have also been taken into account as suitable heterogeneous catalysts. These solids have shown very promising catalytic performances in aqueous phase oxidation reactions, since they are highly stable and robust towards metal leaching and hydrolysis. Therefore, mesoporous Nb-SiO2 materials are widely considered effective systems for the epoxidation of cycloalkenes in the presence of aqueous H2O2 as the oxidant. In addition, it is also found that, compared with W catalysts, the utilization efficiency for Nb catalysts has been widely reported. Although several reviews have been published to summarize the catalyst performances of solid catalysts for epoxidation of a variety of olefins, there is a lack

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of comprehensive summary on the W and Nb catalyst design and plausible reaction mechanism for various olefins.25,107 Therefore, in this paper, the rational design of nanostructured W and Nb based catalysts and plausible mechanisms involved in epoxidation of ethylene, propylene, 1-hexene, 1-octene, 1-heptane, dodecene, norbornene, cyclohexene, cyclooctene, styrene, methyl oleate and geraniol will be critically revised with details on catalyst synthesis, surface characterization and kinetic aspects. The paper is divided into two sections. In the first part, W catalysts applied in heterogeneous liquid phase epoxidations will be systematically revised while in the second part, recent advances in Nb catalyst design will be covered in details. Tables 1-3 summarize experimental results on olefin epoxidation while Table 4 shows catalyst performances for epoxidation of other chemicals in recent literature. In addition, as the stability issues involving leaching of the active metal sites is frequently a problem affecting their stability, this perspective will provide insights into effective design of robust solid catalysts for liquid phase epoxidation reactions.

W-Based Epoxidation Catalysts Immobilization of homogeneous W species Immobilization of homogeneous W species is one of the most popular methods in preparing heterogeneous W catalysts (Figure 3). The sophisticated design on molecular level provides tunable W sites for selective epoxidation of olefins under mild conditions. Hoegaerts et al.108 studied peroxotungstate [PO4[WO(O2)2]4]-3 8


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species

(PW)

heterogenized

on

the

amberlite

support

(PW/Amberlite).

Peroxotungstate associations are known to be active and stable epoxidation catalysts.109 PW/Amberlite showed a cyclooctene conversion (X) of 85% and a selectivity (S) of 95+% with H2O2 as oxidant and acetonitrile (ACN) as solvent at 50 °C. More importantly, the H2O2 was consumed in a remarkably efficient way (utilization efficiency, U>95%). Another important fact is the heterogeneous character of the used catalyst. Hot filtration experiments (removing solid catalysts) have proved that leaching is negligible and that the catalyst could be reused several times without any loss of activity. Kim et al.110 modified PWA based complex catalysts with N-containing heterocycles, such as imidazole, pyrazole and 1,2,4-triazole for propylene epoxidation. The conversion of propylene as well as the selectivity to PO increased as the imidazole contents in complex catalysts increased, which is an obvious promotion effect of heterocycles on the modified PWA complex catalysts. The conversion of propylene and the selectivity to PO over (imidazole)3/PWA are 97% and 60%, respectively, improving (over 50%) the selectivity to propylene oxide compared with that over the raw PWA. Heterocycles played an important role in promotion of the catalytic activity and selectivity to PO, mostly due to the strong electronic interactions between heterocycles and terminal W-O on PWA. The complex catalysts maintained good thermal stability up to 450 °C mainly due to strong electronic interaction

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between heterocycle and terminal W-O bonds of PWA. The modified PWA catalysts can be recycled for five reaction runs without any significant loss of catalyst, suggesting that they are promising heterogeneous catalysts on propylene epoxidation.

Figure 3. Immobilization of homogeneous W catalysts on solid supports (a) on MCM-41 solid support (covalently anchored H6N3OP groups, redrawn from reference108), (b) Single-site WO4 species bound strongly to silica through W-O-Si covalent bonds (modified from reference111), (c, d) Schematic representation for the preparation of hybrid mesoporous SBA-15 (Santa Barbara Acid) materials66

Hoegaerts et al.108 developed a heterogeneous MCM-N+/PW catalyst with W coordinated on covalently immobilized phosphoramide (H6N3OP) groups grafted on

10


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MCM-41 support (Figure 3a). The catalyst was tested in cyclooctene epoxidation reactions using ACN as solvent, H2O2 as oxidant at 50 °C. The catalyst showed a considerable conversion of the cyclooctene in combination with a very high epoxide selectivity (X=70%, S>95%, U=90%, 12h). The oxidant was consumed in an extremely efficient manner and the epoxidation reaction occurred in a heterogeneous way.

The

leaching

was

with Inductively Coupled Plasma

less

than

(ICP)

1

analysis

ppm and

W, checked

as by

measured filtration

experiments. It is known that mesoporous MCM-41 material has uniform and hexagonally ordered pores with tunable dimensions and larger pore volumes. MCM-41 materials have thus been widely applied for variety of epoxidation reactions.112-124 W exchanged MCM-41 catalysts showed remarkable activity and selectivity for the epoxidation of a variety of bulky olefins, including cyclooctene (X=70%, S=95%, U>90%, 12h), norbornene (X=64%, S=93%, U>85%, 20h), geraniol (X=99%, S=95%, U>85%, 6h), and cyclohexene (X=30%, S=60%, U>80%, 20h). In addition, W leaching could be limited to less than 2%. Besides, it was also noticed that addition of an organic Lewis base, such as pyridine, led to a significant reduction of diol formation in cyclohexene epoxidation reaction (X=28%, S=85%, U>80%, 20h), maintaining nearly the same catalyst activity, suggesting hydrolysis reaction is crucial for cyclohexene epoxidation. Furthermore, oxidant consumption was very efficient.

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Figure 4. Schematic representation of the incorporation of tungsten species to the SBA-15 silica via random grafting and metal-templating method to form randomly grafted WO2

catalysts and templated WO2 catalysts , respectively: (a) random grafting of tungsten, (b) metalation with an excess of WO2Cl2 followed by washing with CH2Cl2, (c) formation of ZnII(tungsten species)2 followed by covalent attachment and (d) demetalation with CH3COOH125

Tang et al.66 discussed the synthesis of several hybrid mesoporous SBA-15 materials

containing

oxodiperoxo

tungsten

complexes

[WO(O2)2L]-SBA-15

(L=pyrazolylpyridine) by a post-grafting route (Figure 3c, d). The catalytic performances in the epoxidation of cyclooctene with H2O2 were investigated at 55 °C. It was found that all oxodiperoxo tungstate catalysts were very active (X=76%, S=100%). The catalyst could be successfully recycled for six times without significant decrease of activity or selectivity. The W content for the used catalysts 12


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only decreases slightly compared with the fresh one (7.7% W content leached). The remarkable stability against leaching is due to the special coordination of the pyrazolylpyridine ligand.126 Furthermore, the catalyst exhibits >85% H2O2 utilization. Based on surface characterization, it is found that the formation of a suitable coordination interaction between the chelate ligand and the WO(O2)2 fragment is key for

restraining

unwanted

H2O2

decomposition.

Yang

et

al.125

use

a

metal-template/metal exchange method to imprint covalently attached dioxotungsten complexes onto large surface-area, mesoporous SBA-15 silica to obtain single sited catalysts bearing different metal loadings by templated and grafted methods respectively (Figure 4). The random grafting of such complexes onto a support ensures the retention of the homogeneous coordination sphere and the catalytic reactivity due to the possible benefit of site isolation.127-129 The templated WO2 catalysts (X=96%, S=100%, TOF=97h-1) show superior performances to randomly grafted WO2 catalysts (X=85%, S=100%, TOF=61h-1) in the epoxidation of cyclooctene at 70 °C using t-BuOOH as oxidant and CHCl3 as solvent. Cai et al.60 synthesized a series of mesoporous-SBA-15-immobilized phosphotungstic acid (PWA), with various amounts of incorporated heteropoly acid, fabricated by directly immobilizing PWA on the channels of SBA-15 co-synthesized with (3-aminopropyl) triethoxysilane (APTES). The obtained materials were calcined at different temperatures, and the tungsten species were highly dispersed inside the

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SBA-15 channels (Figure 5a, b). The catalytic activities of these samples were tested in heterogeneous oxidation of cyclohexene in H2O2 and ACN medium at 80 °C. The samples exhibit remarkable catalytic activities. No other oxidation product was observed, indicating that the selectivity for cyclo-HO was almost 100%. The yields using the immobilized PWA/SBA-15 (X=95%, S=100%) samples are higher than i

those over Aerosil-Ti(O Pr), Ti-SBA-15 and Ti-MCM-41 reported in the literature.130-132 The effects of different loadings and treatment temperatures on the activities of the catalysts were also investigated. The calcination temperature plays an important role in increasing the catalytic activity. The yields over PWA/SBA-15 calcined at 100, 400, 600, 800 °C were chosen to compare the different catalytic behaviors. The catalysts calcined at 100 °C always gave relatively low yields (Y=80%), but the yields increased quickly for those calcined at 400 °C (Y=95%). When the calcination temperature was increased to 600 °C, the yields decreased significantly (Y=58%), and the catalysts calcined at 800 °C exhibited similar activities to those calcined at 600 °C (Y=65%). The catalysts calcined at 400 °C showed high reactivity and reusability. There was nearly no loss of activity after five cycles, indicating that the catalysts have high reusability. Heteropoly tungstic acids show unique surface physical and chemical properties during epoxidation processes as W-O groups in Keggin types of structure often interact strongly with surface functional groups of zeolite and N-containing solid supports.

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Dias

et

al.133

immobilized

tungsten

complex

[WI2(CO)3(L)2]

and

[WBr2(CO)3(L)2] synthesized by reaction of 2-amino-1,3,4-thiadiazole (L) with [WI2(CO)3(CH3CN)2] and [WBr2(CO)3(CH3CN)2] respectively, in MCM-41, yielding the materials MCM-C4 and MCM-C5. All the complexes and the functionalized materials were tested in catalytic oxidation reactions of 1-octene. The reaction was carried out in CH2Cl2, with tert-butyl hydroperoxide (t-BuOOH) as oxygen donor at 55 °C. The grafting of C4, and C5 on MCM modifies the results of the catalytic reactions. 1-Octene can be oxidized with almost 20% conversion and 79% selectivity by MCM-C4 catalyst and 10% conversion 17% selectivity by MCM-C5 catalyst.

MCM (Mobil Composition of Matter) Support MCM-48, an ordered silica-based 3D mesoporous material,134 has been widely applied for epoxidation reactions.99,120,135-139 Koo et al.99 prepared WO3 nanoparticles supported on MCM-48 template and evaluated the materials for cyclooctene epoxidation. Researchers have shown that nanosized WO3 particles supported on MCM-48 are highly active heterogeneous catalysts for epoxidation of norbornene (X=88%, S>99%, 12h), 1-hexene (yield, Y=52%, 12h), 1-octene (Y=91%, 12h), cyclooctene (X=99%, S>98%, 12h) using H2O2 in tert-butyl alcohol (TBA) medium at 40 °C. Recycle studies have shown that the catalysts could facilitate epoxidation of cyclooctene more than five cycles with excellent durability. H2O2 utilization efficiency was, however, not reported in this work. It was further found that the 15



activity of supported W catalysts was dependent on the nature of the supporting materials and the size of the WO3 nanoparticles. While bulk power WO3 exhibited a low surface catalytic activity during cyclooctene epoxidation, downsizing the particles (30-100nm) resulted in a significant enhancement in both activity and selectivity towards cylco-OO. More interestingly, it was seen that the catalytic activity was changed inversely to the metal content and size of the nanoparticles. Then solid catalyst was filtered out at the same reaction temperature after reaching 50% conversion, W was not detected from the filtrate by an ICP analysis. In addition, no further conversion was observed from the filtrate, thus strongly suggesting that the oxidation proceeds under the heterogeneous conditions.

SBA Support SBA based heterogeneous supports have also been extensively studied for immobilizing W catalysts. The main differences in the preparation of both mesoporous molecular sieves are the type of template used (ionic surfactant for MCM-41 and triblock copolymer in case of SBA-15) and the synthesis medium (basic MCM-41 and acidic, SBA-15).140Zhu et al.141 synthesized supported PWA on SBA-15 under hydrothermal conditions and detailed surface characterization using X-ray diffraction (XRD), N2 adsorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR). XRD and FTIR results indicate that the substitution of W occurs in the 16


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silicate framework structure of SBA-15. TEM and SEM investigations confirm the presence of

ordered

high

novel PWA/SBA-15 material.

degree

ordering

hexagonal

Their catalytic activity

structure

was

in

the

evaluated

in

the epoxidation of alpha-pinene (X=88%) and 2,3-epoxypinane (S=80%) was the main product. Bai et al.142 synthesized W-containing mesoporous molecular sieves (W-SBA-15) under conventional hydrothermal conditions in strong acidic solution using sodium tungstate (Na2WO4) as W source. Obtained W-SBA-15 materials were evaluated by catalytic epoxidation of styrene to styrene oxide using 30% H2O2 as oxidant in acetone. A conversion of 34.3% for styrene was achieved with the selectivity to styrene oxide being 37% at 70 oC. Synthesis of WO3 incorporated mesoporous molecular sieve WO3-SBA-15 via one-step hydrothermal process was also reported by Bera et al.143 using tetraethylorthosilicate (TEOS) as the silica source, Na2WO4 as the tungsten precursor, and pluronic P123 triblock polymer (EO20PO70EO20) as a structure-directing agent. The catalyst showed excellent catalytic efficiency in epoxidation reactions of various olefin feedstocks, such as styrene (X=88%, S=81%), cyclohexene (X=91%, S=91%), cyclooctene (X=94%, S=94%), 1-octene

(X=82%,

S=82%),

trans-β-methylstyrene

(X=78%,

S=100%)

and

cis-stilbene (X=58%, S=100%) using H2O2 as oxidant, with sodium bicarbonate (NaHCO3) as a co-catalyst and ACN as solvent. NaHCO3 facilitates catalytic turnovers due to the formation of bicarbonate-activated peroxide system as reported by Yao and Richardson.144 They described the epoxidation of alkenes by 17



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H2O2/NaHCO3 system in the mixture of ACN and deuterium oxide (D2O). They found that the presence of NaHCO3 will active H2O2 with bicarbonate ion and form bicarbonate-activated peroxide (BAP) system. In BAP system, the active oxidant peroxymonocarbonate ion, HCO4-, is formed presumably via the perhydration of CO2. Peroxymonocarbonate is an anionic peracid, a fast oxidant, with structure HOOCO2-.145 Ho et al.146 reported the epoxidation of alkenes using Mn2+ in the presence of H2O2 is electrogenerated in-situ in aqueous bicarbonate solutions. There are several other reports on epoxidation of alkenes using catalysts such as manganese meso-tetraphenylporphyrin

complex,147

MnSO4,148

oxodiperoxo

molybdenum

complex [MoO(O2)2(salox)] (salox=salicylaldoxime) mesoporous silica incorporated oxodiperoxo-8-hydroxyquinolinolato Mo6+,149 polymer supported Mo(CO)6150 and hexa-thiocyanatorhenate151 in the presence of H2O2/NaHCO3. For cyclooctene epoxidation, an olefin conversion of 94% with 100% epoxide selectivity were obtained at 30 °C using WO3-SBA-15 catalysts. The solid catalyst can be recovered after reaction and can be reused for several times in catalytic reactions without any significant loss of activity. Metal leaching analysis shows the presence of very little amount of tungsten ( SBA-16 (X = 36%, S = 56%) ≈ KIT-6 (X = 35%, S = 55%) > SBA-15 (X = 22%, S = 59%). These experimental results suggest that, 3D cubic mesostructured catalysts (KIT-6, KIT-5, and SBA-16) perform better than the 2D mesostructured SBA-15. However, the product distribution was found to be similar suggesting the nature and distribution of the W species are quite similar in these catalysts. They also observed that bulk WO3 material alone

does not catalyze

the

reaction,

which

implies

that the

framework-incorporated tungsten species and/or the polytungstate species are the key catalytic species for epoxidation reactions.

19



Figure 5. TEM images of mesoporous W catalysts. (a, b) PWA/SBA-15.60 (c, d) W-SBA-16152

WO3 Catalysts Studies by Hammond et al.153 combined spectroscopic and catalytic investigations and showed that not isolated WVI-species but isolated WO3 is the most catalytically active and stable phase for olefin epoxidation with H2O2. It appears that (bulk) WO3 is the active phase, as opposed to the mono- or polytungstate species proposed previously. This appears to be highly unusual, since bulk metal oxides are typically thought to be rather undesirable for Lewis acid-mediated transformations, such as H2O2-based epoxidation reactions. Typically, isolated Lewis acid centers are 20


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thought to be responsible for catalytic activity in such cases. For example, the epoxidation of olefins with H2O2 is typically catalyzed by Ti4+ species, isolated within inorganic matrixes.154,155 The activation of carbonyl groups by, for example, Sn4+, is also observed over isolated active sites.156 Optimal activity for cyclooctene epoxidation (X=86%, S=98%, TOF=140h-1) was subsequently found for a nanoparticulate WO3 prepared by flame aerosol technology. This material is characterized by a 50% increase in activity per W6+ site, and a 35-fold increase in space time yield, compared with the commercial WO3 (X=21%, S=98%, TOF=35h-1). It is suggested that this enhancement in activity is due to the following possible reasons: (i) increased surface area of WO3 nanoparticles, (ii) greater availability of WO3 due to removal of the SnO2 support, and (iii) possibility that some residual, isolated tungstate species remain in W-Zn/SnO2 framework.157,158 The procedure involves the pyrolysis of a continuous liquid feedstock containing predetermined amounts of metal precursors within a combustable solvent. In this way, the desired solid can be obtained in a single step, without need for high temperature calcination or multistage deposition. Such single-step preparation method is highly favorable on a large scale fabrication process.159 Siliceous mesostructured cellular foams (MCF) are porous materials with well-defined uniform ultralarge mesopores. Their structure is templated by oil in water microemulsions.160 Gao et al.111 synthesized WO3 containing mesocellular

21



silica foam WO3-MCF materials and compared the performance with WO3-SBA-15 and WO3-MCM-41 materials. The catalytic performance of these materials for the epoxidation of cycloocta-1,5-diene with aqueous H2O2 was investigated at 60 °C with TBA as solvent. They identified isolated tetrahedral WO4 species, condensed polymeric WO3 species and small particles of crystalline WO3 on the catalyst surface. A cycloocta-1,5-diene conversion of 26% and epoxide selectivity of 93 % was observed with WO3-MCF catalyst, which is much higher than WO3-SBA-15 (X=14.9%, S=88%) and WO3-MCM-41 (X=1%, S=81%) catalysts. The excellent catalytic performance in the selective oxidation of cycloocta-1,5-diene was attributed to the presence of isolated tetrahedral WO4 species anchored on the support through W-O-Si covalent bonds and the unique pore structure (Figure 3b). However, the H2O2 utilization efficiency on all of the catalysts are still low (~44%). This novel catalyst can be easily recycled after reaction and reused many times with no significant loss of activity and H2O2 utilization. No detectable leaching of tungsten species or obvious loss of W in the WO3-MCF-1 was observed. The good stability can be attributed to the presence of isolated W species anchored on the support through W-O-Si covalent bonds. They also investigated the effect of the two preparation methods. The in situ-derived WO3-MCF catalyst demonstrated better catalytic performance than the impregnation-derived WO3/MCF catalyst. This difference may be attributed to the different states of the W species over these two catalysts.

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Prasetyoko et al.161 studied the performances of WO3 supported on the titanium silicates (TS-1) by impregnation method for epoxidation of 1-octene in H2O2 and acetone medium at 70 °C. All samples show good activity (WO3/TS-1: 930h-1). In the epoxidation of 1-octene (WO3/TS-1: X=7%, S=96%) with aqueous H2O2 in acetone as a solvent, the WO3/TS-1 catalysts showed higher activity than the initial TS-1 due to higher hydrophilicity of the WO3/TS-1 catalysts. Brønsted acid sites have been generated in the WO3/TS-1 catalysts. It was found that all WO3/TS-1 catalysts with different amounts of W loading contain similar amount of Brønsted acid sites. It was suggested that the Brønsted acid sites were formed due to formation of Si-O-W bond in the WO3/TS-1 catalysts. They also studied the dependence of the TOF on the amount of WO3 loading in WO3/TS-1 for the epoxidation of 1-octene with aqueous H2O2 at 70 °C for 1h. All WO3/TS-1 catalysts showed higher TOF than the parent TS-1. Among the WO3/TS-1 catalysts, it is found that the TOF decreased sharply with an increasing amount of WO3 loading. This finding suggests that the capability of substrate to access the oxo-titanium active sites inside the pore of TS-1 is easier at lower amount of WO3 loading. Meanwhile, the higher activity observed in the sample with lower WO3 loading indicated that only small amount of WO3 is needed to increase the hydrophilicity of TS-1 for the formation of oxo-titanium species. In another study, a mesolamellar xerogel WO3 phase reported by Bolsoni et al.162 was

synthesized

using

cetyltrimethylammonium

23


bromide

(CTAB)

and


n-hexadecylamine (HDA) as template. The catalytic properties of the resulting material were investigated in the epoxidation of cyclooctene (Y=11%, Y=69%) and styrene (Y=26%, Y=2%) using H2O2 and t-BuOOH as oxygen transfer agents at 60 °C. In general, the catalytic results were comparable to those obtained with other W systems, thus suggesting the potential application of this material as catalyst for epoxidation reactions. Moreover, the synthetic route adopted herein is a simple and low-cost alternative for the preparation of ternary hybrid materials for catalytic purposes. Mixed metal oxides are also reported highly active for olefin epoxidation reactions. Kamata et al.59 reported a hybrid material consisting of W and Zn oxides on a tin dioxide (SnO2) support as effective and reusable solid catalysts for the epoxidation of propylene (X=94%, S=89%), 1-hexene (X=96%, S=95%), 1-octene (X=87%, S=93%), 1-dodecene (X=86%, S=93%) and cyclooctene (X=99%, S=99%) with H2O2. W-Zn/SnO2 catalysts showed a 94% conversion and 89% selectivity (using H2O2) in dimethyl carbonate (DMC) solvent at 60 °C. The catalyst could be recovered from reactions and reused four times, without an appreciable loss of its high catalytic performance. Negligible leaching of W and Zn species into the reaction mixture was confirmed by ICP (W< 0.01% and Zn< 0.02%). Experimental evidence suggests that polytungstates with dioxo groups in W-Zn/SnO2 play an important role in the epoxidation reaction.163,164

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Other Supports Other solid supports including KIT (Korea Institute of Technology), TUD (Technische Universiteit Delft), multi-wall carbon nanotubes (MWCNTs), etc have also been investigated for liquid phase epoxidation. Yan et al.45 observed significant ethylene epoxidation activity over W incorporated KIT-6 materials with aqueous H2O2 under mild operating conditions where CO2 formation is avoided. The EO productivity observed with these materials [30-800 mg EO/h/(g-W)] is of the same order of magnitude as that of the conventional Ag-based gas phase ethylene epoxidation process. The results reveal that the framework-incorporated metal species, rather than the extra-framework metal oxide species, are mainly responsible for the observed epoxidation activity. However, the tetrahedrally coordinated framework metal species also introduce Lewis acidity that promotes their solvolysis (which in turn results in their gradual leaching) as well as H2O2 decomposition. Recycle tests were carried out with selected catalyst samples up to two cycles. ICP analysis of the spent reaction mixture revealed that 32-75% of the metal in the catalyst had leached out during the first 5 h run. Interestingly, the productivity of the recycled W-KIT-6 catalysts increased with the recovered catalysts from the fresh run. This suggests that inactive metal species (such as oxides) initially leach out. However, the framework incorporated metal species also leach out in subsequent recycles resulting in a productivity decrease during the second recycle run, eventually causing complete catalyst deactivation. 25



Based on the observations, they postulate the following mechanism for the epoxidation, metal leaching and H2O2 decomposition over M-KIT-6 catalysts (Figure 6, M: metal). The tetrahedrally coordinated metal in the KIT-6 matrix forms a metal peroxo complex by reaction with H2O2.165-168 Experimental evidence for such complex formation on tungsten-grafted MCM-41 material has been previously reported.169,170 It is also commonly known that such peroxo species easily undergo reaction with olefinic substrates to form the corresponding epoxide. The peroxo species may either undergo reaction with ethylene leading to the formation of ethylene oxide or undergo solvolysis resulting in the formation of inactive metal oxide species that are easily leached. As shown in Figure 6, H2O2 also undergoes parallel decomposition. The experimental results indicate that both these deactivation pathways (involving the metal peroxo species and H2O2 decomposition) stem from the acidity of M-KIT-6 materials due to metal incorporation. These mechanistic insights therefore suggest that reducing the acidity could minimize, if not eliminate, these adverse side reactions.

Figure 6. Possible reaction mechanism of liquid phase ethylene epoxidation45 26


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Carbon nanotubes (CNTs) have attracted attention in synthesis, characterization, and other applications because of their unique structural, mechanical, thermal, optical, and electronic properties.171,172 Due to their high surface area, chemical stability, and insolubility in most solvents, CNTs can be used as catalyst supports. For example, Pt nanoparticles supported on CNTs have been used for methanol oxidation,173 manganese porphyrin and manganese salophen supported on MWCNTs for epoxidation of alkenes with sodium periodate (NaIO4),174,175 and molybdenum hexacarbonyl supported on MWCNTs for alkene epoxidation with t-BuOOH.176-178 Nooraeipour et al.179 180 reported highly efficient epoxidation of alkenes catalyzed by tungsten hexacarbonyl supported on MWCNTs modified with 1,2-diaminobenzene (Figure 7). The prepared catalyst was applied as efficient catalyst for facile epoxidation of cyclooctene (X=98%, S=100%), cyclohexene (X=100%, S=100%), styrene (X=76%, S=96%), heptane (X=100%, S=100%), dodecane (X=26%, S=100%). This heterogeneous metal carbonyl catalyst showed high stability and reusability in epoxidation without loss of its catalytic activity after six consecutive runs. A mechanism was proposed that CO ligands are eliminated and the WO4 species are produced.

27



Figure 7. Proposed mechanism for epoxidation of alkenes with H2O2 catalyzed by tungsten hexacarbonyl supported on multi-wall carbon nanotubes (redrawn from ref179)

WO3 can also be used as supports for olefin epoxidation reactions. Ghosh et al.181 developed a new synthesis strategy to prepare dispersed ultrasmall (2-5 nm) metallic Ag nanoparticles (Ag NPs) supported on WO3 nanorods in the presence of cationic surfactant CTAB, capping agent polyvinylpyrrolidone (PVP), and hydrazine. The synergy between the surface Ag and WO3 nanorods facilitates the dissociation of molecular oxygen on the metallic Ag surface to produce Ag2O, which then transfers its oxygen to the propylene to enhance PO selectively. The metallic Ag supported on the WO3 nanorods activated molecular oxygen, which can produce PO with very high 28


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selectivity directly from propylene without any additional reducing agent. The catalyst showed a propylene conversion of 16% with 83% PO selectivity at 250 °C using molecular oxygen. The catalyst exhibits a PO production rate of 6.1×10-2 mol/gcat/h. Ghosh et al.165,182 also prepared Ag-WO3 wafer-like nanoarchitectures for the first time for selective epoxidation of a wide range of alkenes, such as propylene (X=42%, S=99%, TOF=19h-1), cyclopentane (X=75%, S=91%, TOF=32h-1), cyclohexene (X=92%, S=95%, TOF=41h-1), cycloheptene (X=91%, S=90%, TOF=53h-1), cyclooctene(X=99%, S=97%, TOF=88h-1), 1-hexene (X=55%, S=99%, TOF=11h-1), styrene (X=58%, S=61%, TOF=9h-1), norbornene (X=88%, S=97%, TOF=18h-1) to their corresponding epoxides with high yield. The wafer-like nanostructure provides sufficient contact between the substrate/catalyst and overcomes the steric influence of the bulkier substrates (Figure 8a, b). Characterization studies prove that the formation of peroxo W species is responsible for epoxidation reaction. High stability and recyclability of the Ag-WO3 catalyst is also observed under the investigated conditions. The recyclability of the catalyst for the epoxidation of cyclooctene was carried out with the reused catalyst under the same reaction conditions. Activity of the recovered catalyst after five consecutive runs did not show any significant activity loss in terms of conversion or selectivity. Plausible reaction mechanism is shown in Figure 8e.

29



Amini et al.1 synthesized nanoparticles of MnO2 supported on WO3 (W1-xMnxO3) (x=0.048) by an impregnation method (Figure 8c, d). The catalytic oxidation of olefins and alcohols, in the presence of these materials and H2O2 as a green oxidant at room temperature was studied, including cyclooctene (X=68%, S=89%), cyclohexene (X=75%, S=83%), 1-hexene (X=41%, S=100%), 1-octene (X=39%, S=100%) and styrene (X=99%, S=91%). For cyclooctene epoxidation, an olefin conversion of 68% and epoxide selectivity of 89% were obtained at room temperature. Finally, catalyst leaching investigation of W1-xMnxO3 in the epoxidation of styrene as a model reaction showed that catalyst leaching is negligible under reaction condition and the catalytic system is truly heterogeneous catalysis.

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Figure 8. SEM and TEM images of (a, b) Ag-WO3 nanolayered catalyst182 catalysts; (c, d) W1-xMnxO31; (e) plausible reaction mechanism over Ag/WO3 catalysts165

W-based catalysts

Nb-Based Epoxidation Catalysts Nb has been used in catalysts formulations over the last few decades since many catalytic applications of Nb materials were discovered in the 1970s and 80s.24 Nb compounds and mesoporous materials now make an alternative to the customary Ti or Zr catalysts in many applications. Many efforts have been devoted to the synthesis, characterization and applications of the mesoporous materials containing Nb due to their attractive textural, structural, morphological and surface features; high surface areas and narrow pore size distributions; tunable pore sizes and pore structures; adjustable defects, holes and roughness in the fracture; unique topological properties; 31



special optical and electronic properties.9,23-25,107 Figure 9 summarizes general methodologies for immobilization Nb species on mesoporous materials.

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Figure 9. General methodologies for preparing Nb and W catalysts on mesoporous materials

Ziolek and Nowak have written a thorough review on Nb compound structures and their catalytic applications.107 Nowak summarized the progress on the characterization of physical and chemical properties for Nb materials and their application on gas and liquid phase oxidation reactions in one of the recent publications.25 Nowadays, the effort is continued in the catalytic research on Nb materials in the form of compounds/complexes, mesoporous molecular sieves, mixed bulk oxides, oxide supports and surface Nb2O5 phases.106,183-191 Within the past decades, Nb based materials have received increasing attention in many important chemical reactions such as dehydration of alcohols and sugars, condensation, oxidation, hydrolysis, esterification and isomerization.25,101,107,192-198 Compared with other reactions, studies of Nb-based materials for liquid-phase oxidation are very limited in literature. Experimental studies have confirmed that Nb-based catalysts 33



show a greater stability and robustness toward metal leaching and hydrolysis in comparison with V and Cr catalysts.186 This feature is a noteworthy advantage for potentially scalable applications for aqueous epoxidation under mild conditions. This part of review will present the most recent progress on the synthesis, characterization of Nb nanomaterials, and their catalytic performances in liquid phase epoxidation of light olefins. MCM Support Kilos et al.119 reported the incorporation of Nb, V and Mo during the synthesis of MCM-41 materials using hydrothermal method (Nb-MCM-41, Nb-V-MCM-41, Nb-V-Mo-MCM-41). The synthesized materials were characterized by high resolution transmission microscopy (HRTEM), ICP, N2 adsorption and XRD. ICP analysis proved that the use of the same atomic amount of metals in Nb, V, and Mo sources for preparing one transition metal containing MCM-41 lead to the following order for metal incorporation efficiency: Nb > V >> Mo. HRTEM micrographs confirm the presence of defects dispersed in materials containing Nb, which are absent in the samples without Nb incorporation. For Nb-MCM-41 materials prepared by impregnation method, the amount of defects decreases with the decrease of Nb loading. It is proposed that defect holes, generated by Nb are convenient for access of reagents to the active species and for making the diffusion easier (particularly important in the liquid-phase reactions). Cyclohexene conversion was approximately 34


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76 % and the cyclo-HO selectivity was 58 % at 45 °C. Hot filtration study showed that Nb leaching to the solution is negligible and it does not influence significantly the activity and selectivity. However, H2O2 utilization efficiency was not reported in this work. FTIR showed that the presence of active Mo, V and Nb increases the concentration of Lewis acid centers. They observed that the epoxide is more thermodynamically stabilized on Lewis acid centers, thus cyclohexene conversion can be enhanced. The linear relationship between cyclohexene conversion and the number of Lewis acid sites is observed for the sample containing almost the same amount of Nb. Furthermore, the introduction of water causes the blockage of Lewis acid sites and forms increasing numbers of Brønsted acid sites,199 believed to facilitate epoxide ring opening thereby increasing cyclohexanediol selectivity.200 Therefore, it is highly possible that Nb species in MCM-41 materials can play the role of electronic promoter, and/or structural promoter (creates defect holes), making the access of reagents to the active species easier. Gallo et al.201 synthesized well-ordered Nb-MCM-41 at room temperature with essentially all Nb sites substituted into the silica framework. The material was calcined and then silylated using hexamethyldisilazane. The FTIR spectra of the samples show the existence of organic moietiesbands reappeared in the silylated sample, confirming the success of the procedure. Thermogravimetry results showed that as for the silylated sample, the water loss was reduced to 0.1% which is less than

35



the loss of 2.6% of calcined sample, proving that the silylation was efficient in hydrophobizing the Nb-MCM-41. Catalytic performances of both calcined and silylated materials were further investigated in the epoxidation of cyclooctene with t-BuOOH with 70wt% aqueous H2O2 as oxidant to verify the influence of the silylation on the activity and selectivity. It was observed that the silylated material was more active in both cases, reaching 62% conversion and 94% selectivity after 48h with t-BuOOH, and 13% conversion and 80% selectivity after 5h with aqueous H2O2. The silylated Nb-MCM-41 show comparable performances (X=62%, S=94%, TOF=26h-1) with those obtained with Ti (X=98%, S=100%),202 Al (X=54%, S=100%),203 and Zr (X=41%, S=100%)203 -based catalysts. However, in the reactions with H2O2 as oxidant, silylated Nb-MCM-41 showed poor conversions compared with other catalysts. This is possibly because that the silylated sample degrades H2O2 more rapidly due to the presence of ammonium ion or amine residues generated during silylation processes. In other to study the synthesis method systematically, Gallo et al.204 reported Nb-MCM-41 catalysts synthesized at room temperature by varying the base [tetramethylammonium hydroxide (TMAOH), or ammonium hydroxide (NH3·H2O)], the silica source [TEOS, or tetramethyl orthosilicate (TMOS)], the Nb source [ammonium niobium oxalate, NH4[NbO(C2O4)2(H2O)2]·3H2O, or potassium niobate, K8Nb6O19] and the order of addition of the Nb source (before or after the silica source). These variations were determinant in the amount of Nb incorporated into the framework and in the structural order of the Nb-MCM-41 obtained. Only one 36


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method (TMAOH as base, TEOS as silica source and NH4[NbO(C2O4)2(H2O)2]·3H2O as Nb source, which was added after the silica source) led to the formation of Nb-MCM-41 with the desired characteristics and active in the epoxidation of cyclooctene with t-BuOOH leading to 19% conversion and 95% selectivity for cyclooctene oxide after 10h. Ziolek et al.101 investigated to the role of Nb, located in crystalline and amorphous catalysts (bulk Nb2O5: X=10%, S=9%; Nb-MCM-41: X=69%, S=36%; Nb-SBA-15: X=65%, S=50%; NbY: X=1%, S=51%) in liquid phase oxidation of cyclohexene with H2O2. The activity of the catalysts is significantly higher when Nb is dispersed in amorphous ordered silica. When H2O2 was applied as an oxidation agent the amorphous materials containing Nb species were the most effective catalysts because Nb in such catalysts strongly interacts with H2O2 resulting in the formation of active O=Nb5+-O2• radicals. In order to identify the species formed as a result of the interaction of the samples with H2O2, the sample were treated with H2O2 before activity tests. Such treatment leads to the change in colors from white to yellow, typical of metal peroxo species.205 The ESR (Electron Spin Resonance) studies were performed with the pretreated materials. All spectra exhibit the characteristic signal assigned to O=Nb5+-O2• species.206 They also proved that hexagonally ordered niobosilicates are also attractive supports for immobilizing metals (e.g. Au), cations (e.g. Cu2+) and binary oxides (e.g. Sb-V-Ox), where Nb

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species play a role of structural promoters. It was also evidenced that binary oxides strongly interact with Nb Lewis acid centers. Marin-Astorga et al.113 investigated metal incorporated MCM for selective catalytic oxidation of geraniol with H2O2 over La, Ti and Nb catalysts supported on mesoporous silica MCM-41. Among the various catalysts, Nb-MCM-41 catalyst showed an excellent selectivity (X=13%, S=96%, 4h) for allylic epoxide. In contrast, La-MCM-41 (X=7%, S=83%, t=4h) and Ti-MCM-41 (X=8%, S=59%, t=4h) catalysts exhibited high selectivity to citral. Surface characterization by FTIR and activity results clearly demonstrated that the differences on the acid sites generated with different strengths that could have an influence on the activity and selectivity. Surface modification and oxide used are the key for controlling the product distribution in the selective oxidation of geraniol.207 SBA Support Similar to MCM supports, SBA based materials also exhibit several synergistic features when immobilized with homogeneous Nb catalysts. Since the discovery of MCM-41, various modification techniques have been performed in the synthesis of mesostructured materials. Stucky et al.208 changed a basic synthesis medium for an acidic one and obtained materials denoted as SBA-3. Kilos et al.209 synthesized and characterized

Nb-SBA-3

mesostructured

materials-the

hexagonally

ordered

mesoporous molecular sieves without extra framework Nb2O5 phase. The catalyst exhibits a cyclohexene conversion of 82% and corresponding epoxide selectivity of 38


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70% at 45 °C. They observed that while the better isolation of Nb species improves the efficiency of cyclohexene epoxidation, high concentrations of the metal species could also lead to more unwanted H2O2 decomposition.210 Besides, the presence of Brønsted acid sites causes the ring opening of epoxide to form diols.199 Held et al.211 applied V, Nb and Ta doped SBA-3 mesoporous molecular sieves for selective oxidation of propylene towards PO in the presence of N2O as an oxidant. The catalytic tests have shown that propylene conversion and selectivity towards PO were affected both by the type and number of transition metal ions introduced into the system. Among the catalysts doped with transition metals of group V, the highest activity was observed over V-modified sample (propylene conversion achieved up to 10%, whereas on Nb/SBA-3 and Ta/SBA-3 samples propene conversion did not exceed 2%). Nowak et al.212 synthesized two different kinds of niobosilicate mesoporous molecular sieves. Both substituted derivatives of the silica-based mesoporous molecular sieves (Nb-MCM-41 and Nb-SBA-15) were prepared with the hexagonal structure and 1D mesopores. Niobosilicate mesoporous molecular sieves materials were evaluated for the epoxidation of cyclohexene in H2O2 and ACN medium. They studied the influence of the Nb source on catalytic performances. The highest activity was observed in the case of Nb-SBA-15 when niobium chloride (NbCl5) was used as a source of Nb (X=69%, S=77%) and in the case of Nb-MCM-41 when niobium

39



oxalate as a source of Nb. In the case of Nb-SBA-15, the samples prepared from ammonium trisoxalate complex (X=50%, S=66%) show higher activity than that prepared from niobium oxalate (X=13%, S=40%). It seems that the presence of chloride ions in the synthesis mixture during the incorporation of Nb into SBA-15 materials plays a very important role in obtaining active catalysts. The extra-framework positions contain mainly Nb dimers or oligomers, which might promote side product formation. It is possible to optimize the activity of the catalysts by adjusting the isolation of Nb species.

Figure 10. TEM images of (a) Nb-SBA-15(pH=2.2)213, (b) Nb-SBA-16 samples.188, (c)

Nb-KIT-5187 and (d) Nb-FDU-1 (Fairleigh Dickinson University) catalysts185 40


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Selvaraj et al.213 synthesized mesoporous Nb-SBA-15 materials using environmentally friendly and efficient hydrothermal method. To fine better hydrothermal stability, the synthesized Nb-SBA-15 materials have been treated in boiling water and water steam at different temperatures with various times. To investigate the effect of structural and textural properties with a higher Nb incorporation, Nb-SBA-15 materials were also synthesized at different pH in sol-gel solution. The results show that Nb-SBA-15 (pH=2.2) has superior hydrothermal stability compared to Nb-SBA-15 (pH=1.6).

diffuse reflectance UV-Vis

(DR-UV-Vis) and X-ray photoelectron spectroscopy (XPS) measurements show that Nb-SBA-15 (pH=2.2) material has a superior hydrothermal stability and a higher amount of tetrahedral Nb5+ species incorporated into SBA-15 mesoporous structure. TEM (Figure 10a) and SEM (Figure 11a) characterization further confirm that Nb-SBA-15 samples have a long rope-like morphology and uniform pore diameter, respectively. The Nb-SBA-15 materials have been used as the catalysts for epoxidation of cyclooctene using t-BtOOH and H2O2 as oxidant. Nb-SBA-15 (pH=2.2) catalyst has a relatively higher catalytic activity (X=66.5%, S=98%) and excellent hydrothermal stability. Trejda et al.214 synthesized hexagonally ordered mesoporous niobiosilicates of SBA-15 type by a new synthesis route in which the use of hydrochloric acid was avoided. This route allowed the incorporation of much higher amounts of Nb into the

41



mesoporous network as compared to the conventional method of preparation. The oxidation of cyclohexene was applied to examine the properties of the catalysts in liquid phase reactions. After 40h, the cyclohexene conversion was 49%, but the epoxide selectivity was only 14% and the main product for all the catalysts was the diol. They noticed that the activity systematically increased with the reaction time and it was accompanied by a significant decrease in epoxide and an increase in diol formation. The recycled catalysts were tested and the selectivity after 40h of reaction did not significantly change, implying the stability of the material. The epoxide formation did not depend only on Nb isolation in the samples but also on the surface acidity. In the presence of acid centers, epoxide can react further with another molecule of H2O2 or water to form a diol. Therefore, this factor is also important for considering the catalyst selectivity. Ramanathan et al.187,188 incorporated Nb into SBA-16 and KIT-5 by a direct hydrothermal synthesis technique. Nb species were successfully incorporated into SBA-16 material. Small-angle x-ray scattering (SAXS), XRD, N2 sorption, TEM (Figure 10b, c) results confirm the structural integrity of the material and the nature of Nb incorporation. DR-UV-Vis spectra and H2-TPR studies reveal that the Nb exists as mostly oligomeric tetrahedral NbO4 species and reducible Nb2O5 species. Temperature programmed desorption of ammonia (NH3-TPD) results reveal that Nb incorporation into the Nb-SBA-16 materials imparts acidity that increases with Nb

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content. The catalysts were evaluated in the epoxidation of cyclohexene and styrene with H2O2 as oxidant and ACN as solvent. The H2O2 utilization efficiency was found to be 20-25%. Batch reaction studies show that while the increased NbO4 content in the Nb-SBA-16 materials facilitates epoxidation at lower Nb loadings, further increased acidity at higher Nb loadings leads to unwanted side reactions such as ring opening of the epoxide and H2O2 decomposition. The results confirm the dependence of the structure of the mesoporous silicate support on catalyst activity with the Nb-SBA-16 materials showing superior performance for cyclohexene epoxidation compared to either Nb-KIT-6 or Nb-KIT-5. The catalysts were recycled and tested. No significant deactivation was observed after three recycles with only mild variation in the epoxide selectivity. Unlike Nb-KIT-5, Nb-SBA-16 showed clear evidence of reducible NbOx species at higher Nb loadings. Such species facilitated H2O2 decomposition and ring-opening reaction of epoxides, and were shown to contribute to enhanced Brønsted acidity.215 The results suggested that the nature of the oxides and the acidity stemming from the sites on the various supports is the key to tune catalyst activity and selectivity during epoxidation reactions. Nb2O5 Catalysts Nb2O5 represents the most studied oxide of Nb. Nb2O5 has been supported on different support materials for catalytic purposes. Different Nb surface species (isolated or bulk) may be formed depending on the nature and properties of the 43



support material, Nb precursors, Nb content and synthesis method.106 Turco et al.106 prepared a series of Nb base materials that containing the same amount of Nb2O5 by different methods (wet impregnation method, sol-gel method and commercial power, respectively) and compared in the epoxidation of methyl oleate with hydrogen peroxide as oxidant. Commercial and Nb2O5 prepared by impregnation method showed a good conversion (X=66% and 74% respectively) which is related with the presence of Lewis acid sites on the surface, but a moderate selectivity (S=28% and 9% respectively). It is generally believed that acid sites of moderate strength are involved in the epoxidation mechanism, while strong acidity activates the decomposition of H2O2, giving the epoxide ring opening.105 Particularly, Lewis acid sites of moderate strength promote the epoxidation, while Brønsted acid sites and/or strong Lewis sites facilitate the formation of diols.216 Gel-derived catalysts exhibit relatively lower conversion (X=15%), although selectivity is higher (X=73%) among all the investigated catalysts. This behavior can be related to unique characteristics of the gel-derived catalysts that are related to each other. The dispersion degree of the active phase (NbO4 tetrahedra) into the siloxane matrix and the distribution of the surface acid sites. According with recent results obtained for the epoxidation with H2O2 of limonene,217 cyclohexene and 1-methylcyclohexene,215 the presence of only Nb species in tetrahedral coordination seems to be critical to determinate the selectivity to epoxide. The catalytic performances of the catalyst prepared by impregnation do not differ substantially from the ones of Nb2O5. The performances of Nb catalysts 44


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prepared by sol-gel can be modulated by controlling the process parameters. The high dispersion of the active NbOx species obtained by this preparation method gives very higher selectivity than in the case of pure Nb2O5. Marin-Astorga et al.218 Nb2O5/SiO2 also studied the oxidation of geraniol over a mixed oxide of Nb2O5/SiO2 and mesoporous Nb2O5/SiO2 catalysts using H2O2 as oxidant in the absence of any organic solvent. High selectivity to the allylic C=C double bond was observed in all catalysts studied. However, for mesoporous Nb2O5/SiO2 and mixed oxide of Nb2O5/SiO2 the selectivity to epoxide decreases with the conversion, contrary to observed for Nb2O5 bulk. The differences in selectivity may be due to different routes for the activation of oxidant over the metallic center. This work proposes a useful application of Nb2O5 commercially available in selective organic reactions. Tanaka et al.219 prepared amorphous inorganic phase of an order amorphous mesoporous Nb2O5 with 2D hexagonal structure with maintaining the original well-arranged porous structure (Figure 11b). The difference in surface property between amorphous and crystalline Nb2O5 with similar ordered mesoporous structure was compared. It was found from water adsorption-desorption isotherms and observation by FTIR spectroscopy that the amorphous sample was hydrophilic and that the surface -OH groups were acidic. On the other hand, -OH groups on Nb2O5 surface were non-acidic and inside the pores was less hydrophilic. The surface property was also compared by oxidation of cyclohexene by H2O2. The high selectivity (S=95%) for cyclo-HO was obtained at 40 o

C for 2h in methanol (MeOH) solvent at 12% conversion. The same catalyst shows 45



good selectivity (S=68%) for 1,2-cyclohexanediol in ACN solvent at 60 oC for 2h at 22% conversion. The differences in selectivity and the optimal solvent between amorphous and crystalline samples were interpreted in terms of the acidic feature of surface -OH groups and hydrophilicity. While similar selectivity was observed over non-porous crystalline Nb2O5, much higher conversion over crystalline mesoporous Nb2O5 was attained at the same surface area. Thus, an advantage of mesoporous structure is attributed to the higher contact time of molecules inside the pores to the catalyst surface than those outside the particles.

Figure 11. SEM images of (a) calcined hexagonal mesoporous Nb-SBA-15 (pH=2.2)213 and (b) mesoporous Nb2O5 catalysts 219

Tiozzo et al.183 later reported two kinds of Nb-silica catalysts for the selective epoxidation. The catalysts were synthesized by post-synthesis modification of non-ordered mesoporous silica support, starting from niobocene dichloride via solvent less organometallic precursor dry impregnation (Nb/SiO2-DI) or conventional liquid-phase grafting technique (Nb/SiO2-Liq). Grafted Nb/SiO2 solids were used as 46


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catalysts, in the presence of aqueous H2O2, for the epoxidation of a broad series of unsaturated olefins, such as cyclohexene, 1-methylcyclohexene, limonene, carveol, terpineol, isopulegol, carvotanacetol, carvone, as well as squalene and isopulegyl acetate. These catalysts showed high yields (Y=73%) and excellent chemo-selectivity to the desired epoxides (Y=98%). Nb/SiO2-Liq, in particular, showed a remarkable higher activity than Nb/SiO2-DI. Such behavior is consistent with the one observed on other epoxidation catalysts105,219,220 and is attributed to a high fraction of evenly dispersed sites on the surface, and hence, to a better availability of Nb centers in the catalysts prepared via liquid phase grafting.217 In fact, in the sample Nb/SiO2-DI, in which some niobia-like aggregates were detected by spectroscopic investigation,215 a fraction of Nb sites can be buried within the aggregate and thus inaccessible, or less accessible, for the reactant. Recycle studies show that adsorption of the carbonaceous products on the catalyst surface or metal leaching might lead to deactivation of Nb/SiO2 catalysts. Tiozzo et al.215 obtained Nb containing silica materials by deposition via liquid phase grafting or dry impregnation of niobocene dichloride (Figure 12a). The Nb-silica catalysts were then tested in the epoxidation of cyclohexene and 1-methylcyclohexene using aqueous H2O2. Nb species was followed step by-step, and investigated using a combined DR-UV-Vis, NIR, Raman, XRD, XANES (X-ray absorption near edge structure) and EXAFS (Extended x-ray absorption fine structure) analyses. At the end of the grafting procedure, the nature of the surface active species can be described as an oxo Nb5+ site, tripodally grafted onto 47



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the silica surface in close proximity to other Nb5+ centers. The Nb complex undergoes nucleophilic attack by silanol groups on the silica surface. The liquid phase synthesis methodology yields catalysts with better dispersion of the metal sites onto the siliceous support in comparison with the dry-impregnation approach. All Nb-silica catalysts are active in the epoxidation of both cyclohexene and 1-methylcyclohexene. In general, the catalysts derived from liquid-phase grafting showed higher specific activities [i.e. Nb-SBA (by liquid phase synthesis method): X=73%, S=78% for cyclohexene epoxidation] than those prepared via dry impregnation [i.e. Nb-SBA (by dry-impregnation method): X=36%, S=98% for cyclohexene epoxidation]. This behavior is consistent with that observed for limonene epoxidation,217 and is attributed to a higher fraction of isolated sites and hence better availability of Nb5+ centers in catalysts prepared via liquid-phase grafting. Tiozzo et al.221 also prepared Nb-containing silica catalysts by deposition of niobocene dichloride onto the surface of high-surface area silica supports following a dry impregnation or a liquid-phase grafting approach. These materials were used as heterogeneous catalysts for conversion of unsaturated fatty acids, methyl esters, methyl oleate, methyl linoleate, and methyl ricinoleate. Catalysts are water-tolerant, recyclable, resistant to metal leaching, active and selective in the production of the related epoxidised fatty acid ethyl ester (FAME). Gallo et al.217 synthesized a series of Nb catalysts for epoxidation by post-synthesis modification of a commercial silica, starting from niobocene dichloride

through

solventless

organometallic

precursor

48


dry

impregnation

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(Nb/SiO2-DI) or conventional liquid-phase grafting technique (Nb/SiO2-Liq). DR-UV-Vis characterization results showed that for all Nb/SiO2 systems, no absorption bands were detected above 400 nm. No large domains of Nb2O5 or TiO2 were therefore present on the catalysts. All samples have charge-transfer transition between oxygen atoms and the metal centers Nb5+ in tetrahedral coordination, suggesting isolated metal centers. Nb-SiO2 catalysts which displayed an excellent performance in the epoxidation of limonene. In the case of Nb/SiO2-Liq, displayed up to 74% conversion and epoxide selectivity of 98% at 90 °C, Nb/SiO2-DI displayed up to 62% conversion and 97% selectivity The H2O2 utilization efficiency on average rather good (above 60%). The comparison of the two series of Nb catalysts, namely prepared via dry impregnation or via liquid phase grafting, shows that the dispersion of the metal sites is an important factor in terms of catalytic specific activity. Nb-SiO2-Liq, in fact, proved to be the most active catalyst, and this is consistent with the good site isolation of Nb centers observed by DR-UV-Vis spectroscopy. All the catalysts were easily recovered and reused in a second catalytic run.214,220,222 Nb2O5-SiO2 mixed oxides were synthesized by various of methods. Somma et al.207,220 synthesized Nb2O5-SiO2 mixed oxides under acidic conditions and dried the materials under supercritical conditions to yield meso-macroporous materials. The catalysts were evaluated in the epoxidation of geraniol and cyclooctene using H2O2 in MeOH medium. When Nb loading increases, the catalyst acidity increases, favoring

49



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the production of glycol with a significant drop in the epoxide selectivity. Besides, they

modified

the

catalyst

hydrophobic/hydrophilic

surface

properties

with of

methyl the

groups

surface

by

to change

the

addition

of

methyltriethoxysilane. They show that in the epoxidation of cyclooctene, methylation increases the conversion from 27% (non-methylated sample) to about 50% (methylated samples) and, at the same time, increases the efficiency in the use of the oxidant from 56% (non-methylated sample) to about 100% (methylated samples). Non-methylated catalysts can be recycled several times without changing the catalytic properties. The inactivity of the catalyst free liquid reaction mixtures was a clear indication of the absence of leaching of Nb active compounds during the catalytic run. They proposed the oxygen transfer step a mechanism similar to the one suggested by Kumar et al.223 for the epoxidation of geraniol with H2O2 catalyzed by TS-1. This mechanism is very similar to the classic one suggested by Sharpless et al.224 for soluble species and consists of (i) the activation of the oxidant at the Nb metal center, (ii) the pivotal role of the allylic alcoholic function in bringing the C=C double bond close to the peroxy oxygen, and (iii) the concerted transfer of oxygen via the so-called butterfly transition state, which is responsible for the stereochemistry of the epoxide. Aronne et al.105 synthesized Nb-SiO2 mixed oxides by a new sol-gel route and tested them for cyclooctene epoxidation with H2O2. The highest activity (X=39%, S=100%) was found for the catalyst with the lowest Nb content that also showed high

50


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stability in reuse. They noticed that catalytic properties were related to acid sites and to the presence of NbOx species with different coordination. It is believed that acid sites of moderate strength are involved in the mechanism of epoxidation with H2O2,207,225-229 while strong acidity sites decompose H2O2.228,229 No mention is made about either the presence or absence of metal leaching. The likely mechanism involves the reaction of an olefin molecule with a Nb-O-O-H group, which is formed by reaction of H2O2 with the acid sites of the catalysts. Specifically, the reaction involves Brønsted or Lewis type sites. The epoxide formation occurs through electrophilic transfer of oxygen favored by the high polarizing effect of Nb5+. Di Serio et al.216 evaluated the performance of Nb2O5-SiO2 catalysts prepared by sol-gel technique in the epoxidation of soybean oil with H2O2. The catalyst showed good activity in epoxidation reaction but poor selectivity to epoxide (X=31%, S=8%, 5h). The decrease of catalytic activity in epoxidation with Nb content agrees with the decrease of the concentration of Lewis acid sites of moderated strength.105 Therefore, a mechanism involving Lewis acid sites of moderate strength during epoxidation of alkenes with H2O2 is proposed207,230 while strong Lewis acidity activates the H2O2 decomposition reaction.231,232 The low selectivity has been attributed to the presence of strong Brønsted acid sites, which enhance the reaction of ring epoxide opening by hydrolysis.

51



Thornburg et al.189 grafted a variety of Nb precursors on mesoporous silica (Figure 12b) and characterize these materials with DR-UV-Vis spectroscopy and Nb K-edge XANES to study the synthesis-structure-function relationships of silica supported Nb catalysts for alkene epoxidation. They apply in situ chemical titration with phenylphosphonic acid in the epoxidation of cyclooctene by H2O2 to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO2 catalysts. By this method, the fraction of catalytically active NbOx species ranges from ∼15% to ∼65%, which correlates with spectroscopic evaluation of the NbOx sites. This titration leads to a single value for the average TOF, on a per active site basis rather than a per Nb atom basis, of 1.4 ± 0.52 min-1 across the 21 materials in the series. These quantitative maps of structural properties and kinetic consequences illustrates that Nb-SiO2 can be an intrinsically good catalyst for electrophilic activation of H2O2, but only if its synthesis facilitates high preponderance of site-isolated, undercoordinated Nb centers. Bulky molecular precursors and supramolecular-templated co-condensation with SBA-15 are highlighted as routes that give an abundance of such sites, with fractions of active Nb > 50%. Grafting most commercially available precursors such as NbCl5 or niobium ethoxide (Nb(OC2H5)5 can result in catalysts with a significantly smaller fraction of active Nb species.

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Coelho et al.184 proposed a modification in the Nb-doped silica nanoparticles as a means to increase the number of oxidizer groups on the catalyst surface through the formation of the peroxo species by previous treatment of Nb-doped mesoporous silica with H2O2.23,233 The H2O2 treatment strongly affected the morphology of the catalysts, but EDX (Energy dispersive X-ray spectroscopy) and XPS measurements showed that the Nb is still maintained in the silica structure even after H2O2 treatment. The isomorphic substitution of Si4+ by Nb5+ in the silica framework can generate acidic sites on the Nb-doped silica, which can contribute to the electron transfer in the oxidation process of organic compounds. Catalytic tests using rhodamine B as a model molecule of textile effluents showed that both processes of Nb doping and treatment with H2O2 are essential to obtain a highly active and oxidizing system. The better performance of Nb catalysts treated with H2O2 was assigned to formation of niobium-peroxo groups, which act as excellent oxidizing sites. Ziolek et al.101 described that when H2O2 was applied as an oxidizing agent in the presence of Nb containing amorphous materials, effective catalysts were produced because NbOx in such catalysts strongly interacts with H2O2 resulting in the formation of active O=Nb(V)O2• radicals. Deshmane et al.234 demonstrated the epoxidation of cyclooctene over mesoporous Ga (X=12%, S=100%) and bimetallic Ga-Nb (X=31%, S=100%) and Ga-Mo

oxides

(X=41%,

S=100%)

synthesized

53


via

self-assembly


hydrothermal-assisted approach. These mesoporous catalysts displayed high epoxide selectivity at moderate cyclooctene conversion. The incorporation of Nb led to an increase in the acidity of Ga-Nb mixed oxides. These mesoporous Ga-Nb oxides displayed 80-100% selectivity for the epoxide at conversion levels of cyclooctene in the range of 17-30% range after 2 h of reaction at 60 oC. Surface characterization found that active epoxidation catalysts should be both a weak oxidative property and more strong Lewis acid sites. Thus, it is not surprising that (1) Ga3+–Nb5+ oxides are more active than Ga2O3, (2) Ga3+–Mo6+ oxides are more active than Ga2O3 and (3) Ga3+–Mo6+ oxides are more active than Ga3+–Nb5+ oxides. The higher oxidation state of Nb5+ as compared to Ga3+, and Mo6+ as compared to Nb5+ results in the superior activity on bimetallic catalysts.

54


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55



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Figure 12. Schematic description of immobilization of homogeneous Nb catalysts on solid SiO2 supports and possible oxide surface structures. (a) grafting Nb(CP)2Cl2 onto SiO2 215, (b) grafting

NbCl5 ， Nb(OC2H5)5,

Nb(N-(CH3)2)5,

Nb(C2H5)Cl4

onto

SiO2

189

,

(c)

synthesis-structure-function relationships of silica supported Nb catalysts for alkene epoxidation189

Other Supports TUD-1 which has a sponge-like, 3D, and irregular pore structure is straightforward to prepare. Yan et al.190 observed that Nb incorporated mesoporous silicate materials Nb-KIT-5, Nb-MCM-48, and Nb-TUD-1 showed significant ethylene epoxidation activity with H2O2 as oxidant and MeOH as solvent under mild operating conditions (35 °C and 50 bar). No CO2 as by-product was detected at these conditions. The measured EO productivity over Nb-TUD-1 materials (342-2539 gEO/h/kgNb) spans a greater range than those observed with Nb-KIT-6 (234-794 56


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gEO/h/kgNb), Nb-KIT-5 (273-867 gEO/h/kgNb) and Nb-MCM-48 (71-219 gEO/h/kgNb) materials at similar operating conditions. However, significant H2O2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Brønsted acidity may lead to these detrimental pathways and which was confirmed by Density Functional Theory (DFT) calculation (Figure 13).235 Indeed, lowering the metal loading to significantly reduce the Brønsted acidity results in a dramatic increase in H2O2 utilization toward EO formation (4304 gEO/h/kgNb). The increased EO productivity either matches or surpasses what was observed on the conventional Ag-based heterogeneous catalyst (with O2 as oxidant) as well as a Re-based homogeneous catalyst (with H2O2 as oxidant). These results are paving the way for further computational and experimental investigations aimed at the rational design of improved epoxidation catalysts that reduce H2O2 decomposition and metal leaching to practically viable levels.

Figure 13. Optimized model structures of (a) Nb=O and (b) Nb-OH190 structures

57



Experimental studies by Liu et al.236 for Nb-HMS (Hexagonal Mesoporous Silica) using n-dodecylamine show that Nb-HMS material exhibited excellent catalytic activity and selectivity for PO with H2O2 as oxidant in MeOH medium. A propylene conversion of 41.3% and PO selectivity of 98.3% were obtained at 50 oC. The efficiencies of the H2O2 utilization were larger than 98%. Under the same conditions, Nb-HMS showed higher activity and selectivity than Nb-MCM-41 (X=33%, S=97%). Catalyst stability was not reported in this work. Maiti et al.237 have demonstrated for the first time that capping of the Brønsted acid sites in Nb-TUD-1 catalysts with bases can improve H2O2 utilization. It can also enhance resistance of catalyst to Nb leaching during the liquid phase epoxidation of ethylene (Figure 14). Their first strategy was to create hydrophobic ion pairs through the base treatment of Nb-TUD-1 using organic bases. Although these systems significantly enhanced H2O2 utilization toward EO formation (up to 90%), resistance to metal leaching was only moderately improved. Therefore, they investigated an alternative strategy using four different covalently bound capping groups, methyl, trimethylsilyl, tert-butyl, and benzyl. The catalyst capped using the benzyl group, showed the best performance in terms of significantly improved H2O2 utilization toward EO formation (∼60-71%) and reduced metal leaching (∼3%). The EO selectivity was >98%. Further, the structure of the Nb catalyst was found to be stable

58


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under reaction conditions even after several recycle runs, thus confirming its potential as a viable epoxidation catalyst.

Figure 14. Capping groups for Nb-TUD-1 catalysts237

The structure and reactivity of reactive intermediates derived from H2O2 and the mechanism for olefin epoxidation on transition metal substituted zeolites are debated. Bregante et al.191 calculated cyclo-HO formation and H2O2 decomposition rates along with in situ infrared and DR-UV-Vis spectroscopy to probe the intervening elementary steps for cyclohexene epoxidation and the identity of the reactive intermediates on a Nb-β catalyst. IR and DR-UV-Vis spectra acquired in situ show that the reactive intermediates are predominantly superoxide species (Nb4+-O2), 59



observed also by XPS), which form by the irreversible activation of H2O2 over Nb centers. Similar M4+-O2 (M = Ti or Ta) intermediates were previously assumed to form via reversible processes; however, in situ IR and DR-UV-Vis measurements directly show that M4+-O2 forms irreversibly in both H2O and ACN. Activation of H2O2 to form Nb5+-OOH with the interconversion of Nb5+-OOH, Nb5+-(O2)2-, and Nb4+-O2 are shown in Figure 15. Activation enthalpies for C6H10 epoxidation are 27 kJ/mol higher than for H2O2 decomposition, while activation entropies (∆S) for epoxidation are 56 J/mol/K lower than for H2O2. These comparisons show that the selectivity for epoxidation, via primary reaction pathways, increase with increasing reaction temperatures. Collectively, these data, when combined with the observed dependencies of reaction rates on C6H10, H2O2, and C6H10O provide a complete mechanistic understanding of olefin epoxidation over Nb sites that is consistent with an Eley-Rideal mechanism and indicate that the reactive form of oxygen (i.e., Nb4+-O2) forms irreversibly under reaction conditions. Additionally, this mechanism also accurately describes the dependence of epoxidation rates on reactant concentrations observed in previous studies on similar Ti- and Ta-based catalysts, which had assumed the quasi-equilibrated formation of the reactive intermediate. Calculated values of ∆H and ∆S for C6H10O formation and H2O2 decomposition demonstrate that epoxide formation is enthalpically disfavored, which suggests that H2O2 selectivity toward epoxidation is maximized at higher temperatures. These

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findings will aid the rational design and study of alternative metal oxide catalysts for olefin oxidation reactions.

Figure 15. Activation of H2O2 to form Nb5+-OOH with the interconversion of Nb5+-OOH, Nb4+-(O2)2-, and Nb4+-(O2)2-. The oxidation state of each Nb center is depicted beneath the atom191

Evaporation-induced self-assembly (EISA) is a simple and inexpensive technique which can be applied for the preparation of mesoporous materials in the form of fibers, powders and ordered thin silica films.238 Ivanchikova et al.186 prepared the

hydrothermally

stable

mesoporous

Nb

silicates

Nb-MMM-E

(Microporous/Mesoporous Materials) for the first time following the convenient and versatile EISA methodology. They use Nb(OC2H5)5 modified with acetylacetone as Nb source. Acetylacetone has been widely employed as hydrolysis retarding additive in the sol-gel and template syntheses of mesoporous metal-silicates.239-241 The Nb-MMM-E materials catalyzed selectively oxidation of various unsaturated compounds of interest for fine and specialty chemistry using the green oxidant-aqueous hydrogen peroxide. For cyclooctene epoxidation, the catalysts 61



Page 62 of 114

showed moderate conversion (X=40%) and epoxide selectivity (S=90%). Importantly, the activity could be completely restored after regeneration of the catalyst by calcination, although epoxide selectivity slightly decreased. The niobium-silicates prepared by the EISA technique behaved as truly heterogeneous catalysts, did not suffer from Nb leaching and could be easily recovered and reused several times with maintenance of the catalytic properties. Epoxidation of both electron rich and deficient bonds could be accomplished over Nb-MMM-E. While catalysts with isolated Nb centers are preferable for the selective formation of epoxides which are sensitive to ring opening and overoxidation, both single site and oligomerized Nb centers were equally effective for the production of relatively stable epoxides. Trejda et al.242 prepared new Nb containing mesoporous catalysts based on MCF structure by incorporating Nb species via grafting and co-precipitation approaches. The materials were characterized by N2 adsorption, XRD, DR-UV-Vis, pyridine adsorption followed by FTIR and test reactions. Three various Nb precursors were applied: ammonium oxalate complex, ethoxide and chloride. It is documented by DR-UV-Vis

that

the

best

isolation

of

Nb

species

is

reached

when

NH4[NbO(C2O4)2(H2O)2]·3H2O is applied for grafting and the same sample reveals the highest acidity determined by the test reaction-liquid phase epoxidation and pyridine adsorption combined with FTIR study. Nb-containing MCF materials reveal both acidic and oxidative properties. The redox centers are active in the presence of

62


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O2. For liquid phase oxidation of cyclohexene with H2O2 both kinds of centers are active. Oxidative one causes the formation of epoxide, and acidic sites lead to the ring opening and diol production. Periodic ordered mesoporous organosilicas (PMOs) materials were synthesized in 1999 by integrating organic groups into silica framework using a bistrialkoxysilyl precursor directly.243 Feliczak et al.244 prepared Nb-PMO by the acid catalyzed hydrolysis and condensation of bridged silsesquioxane precursors containing two different organic bridging groups [(C2H5O)3Si-R-Si(OC2H5)3], R=ethylene or octylene) in the presence of nonionic template P123. The retention of Nb and organic groups in the organosilica framework was ensured by slow template removal. A successful synthesis of Nb-PMO was confirmed by XRD (high periodicity), N2 adsorption (surface area: 900m2/g and narrow pore size distribution with maximum at about 4-11nm), TEM (hexagonal symmetry), DR-UV-Vis spectroscopy (characteristic absorption band for Nb in the framework), H2-temperature programmed reduction (TPR, localization of Nb in the framework) and FTIR spectroscopy (presence of CH2-Si(OSi≡)3 groups). Experiments show that increasing the content of the organic chain length functionality in the precursors is found to significantly lower the mesostructural ordering of the materials. It is shown that Nb can act as a co-template in making nanoporous organosilica materials. The synthesized materials were found to show excellent catalytic activity and selectivity for direct oxidation of methyl

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oleate (X=99%, S=100%) and sunflower oil (X=96%, S=79%) with H2O2 as oxidant and hexane as solvent at 70 °C. The proposed catalysts can be reused four times with small loss in conversion and no significant changes on the selectivity. They also synthesized Nb-SBA-15 and tested it for methyl oleate epoxidation under the same conditions as those employed for Nb-PMO. A relatively lower olefin conversion (X=69%) and epoxide selectivity (S=81%) were obtained. MSU materials (Michigan State University Material) was synthesized using nonionic surfactants method which is attractive due to the characteristics like low price, nontoxicity, and biodegradability. MSU materials has porous structure that represents a 3D interconnecting network of ‘worm-like’ channels. Feliczak et al.245 prepared supermicroporous and/or mesoporous molecular sieves of the Nb-MSU type by the reaction between low cost Nb precursors and biodegradable surfactant in different medium (strong acidic, acidic or almost neutral medium). The nanostructure of these materials can be monitored by the accurate control of the synthesis parameters. The selective oxidation of geraniol (X=35%, S=98%), limonene and alpha-terpineol (X=15%, S=82%) using H2O2 as oxidant agent is studied in details. A variety of Nb-MSU materials prepared under different conditions has been employed (supermicroporous/mesoporous

molecular

sieves)

for

this

purpose.

The

diffuse-reflectance UV–Vis and H2-TPR measurements additionally confirmed that Nb was incorporated into the MSU framework.

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Hydroxyapatites (HAPs) have been extensively used in heterogeneous catalysis. HAP has the unusual feature of containing both acid sites and basic sites in a single crystal lattice. The application of HAP in catalysts exploit its acid-base and redox properties which can be easily adjusted by controlling the apatite structure.246,247 Carniti et al.248 synthesized a new class of Nb-containing catalysts, Nb-HAP, which exhibit both acid and base functionalities and provides the first evidence that Nb centers can be successfully added into a HAP scaffold. The resulting Nb-HAP samples can be used as a new family of amphoteric catalysts with tunable acid-base properties. This catalyst has been studied in the epoxidation of limonene (X=39%, S=62%) at 90 °C using H2O2 as oxidant and MeOH as solvent. This type of material is found to be effective and its activity is strongly influenced by Nb dispersion in the framework and the overall acidity of the catalyst.101,105,249-251 The presence of Nb is mainly associated with Lewis acid sites, and which gives a promising catalytic activity limonene epoxidation in the presence of aqueous H2O2. Hot-filtration tests have shown no further reaction activity of the solid-free reaction mixture. Somma et al.207 reported the preparation of a series of Nb-based aerogel samples under acidic [Nb2O5-SiO2-acidic, Nb2O5-Al2O3, Nb2O5-ZrO2] and basic conditions [Nb2O5-SiO2-basic]. The pyridine TPD profiles showed that the samples show the acidity order Nb2O5-ZrO2>Nb2O5-SiO2-basic>Nb2O5-Al2O3>Nb2O5-SiO2-acidic. With respect to the total amount of pyridine adsorbed, the two silica-based samples possess

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only weak or moderate acid sites, while in the case of Nb2O5-ZrO2 and Nb2O5-Al2O3 the acidity is mainly due to strong sites. The samples were evaluated for the epoxidation of cyclooctene, geraniol, nerol and trans-2-pentene-1-ol with H2O2 as oxidant and MeOH as solvent at 70-90 ºC. The results indicate that the silica-based samples are the best catalysts for the cyclooctene epoxidation reaction, while ZrO2 support exhibits poor performances, as it leads to extensive decomposition of H2O2 with very little epoxide selectivity. However, the epoxidation of cyclooctene in view of the stability of the corresponding epoxide, does not allow to address selectivity issues related to the different acidities of the catalysts. For this reason, they also tested geraniol in the epoxidation with H2O2. The results show that Nb2O5-SiO2 maintain 100% selectivity for epoxide for about 50-100 min. As the concentration of epoxide in the system further increases, the selectivity decreases with the presence of small amounts of diols (by-products). The case of Nb2O5-Al2O3 and Nb2O5-ZrO2 is quite different due to the high content of strong acid sites evidenced by pyridine desorption. The conversion observed on these catalysts is poor and selectivity towards epoxide drops dramatically along with the reaction. They also proposed a mechanism similar to the one suggested by Kumar et al.223 for the epoxidation of geraniol with H2O2 (Figure 15a). The mechanism consists of (i) the activation of the oxidant at the metal center, (ii) the pivotal role of the allylic alcoholic function in bringing the C=C double bond close to the peroxy oxygen, and (iii) the concerted transfer of oxygen via the so-called butterfly transition state, which is responsible for the stereochemistry of the 66


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epoxide. The recycle study show that after four cycles there is no loss of activity (conversion) and the utilization efficiency of H2O2 remains almost unchanged. FDU-1 is cage-like ordered mesoporous silica with a face-centered cubic structure. Feliczak et al.185 synthesized highly ordered, 3D caged cubic Nb containing mesoporous silicates, Nb-FDU-1 materials (Figure 10d) and tested for epoxidation of cyclohexene (X=65%, S=55%). H2O2 utilization efficiency of 90% were obtained at 45 °C. The efficiency and activity for the utilization of H2O2 greatly depended on the Nb center isolation/localization. The catalyst was reused twice and lost part of their activity.

However, such loss of activity does not have a parallel Nb leaching, which

is lower than 3% taking into account the gain in weight, shown by the carbon analysis. The catalytic activity and selectivity were dependent on the type of Nb source used and the location of Nb species. The high catalytic activity of Nb-FDU-1 prepared from NbCl5 can be rationalized by taking into account the fact that more Nb was incorporated and accessible in this material. Moreover, the extra-framework Nb sites tend to be Brønsted acidic in nature, promoting epoxide hydrolysis to form a diol. Proper understanding of the oxidation state and acidity of the surface Nb species is critical to understand the observed selectivity.119,244,245 As shown schematically in Figure 16b, the reaction pathway of cyclohexene epoxidation over Nb-FDU-1 catalysts include several steps: (i) H2O2 insertion at the Nb center accompanied by formation of an active hydroperoxy intermediate,252 (ii) cyclohexene sorption near the

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Nb/hydroperoxy intermediate site, (iii) oxygen abstraction to form cyclo-HO, and (iv) desorption of water.

Figure 16. Reaction mechanism on Nb catalysts, (a) Plausible reaction mechanism over Nb2O5-SiO2 catalysts (redrawn from ref207), (b) Cyclohexene epoxidation pathway over Nb-FDU-1 catalyst (redrawn from ref185);

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Both experimental and theoretical studies have confirmed that stronger Lewis sites possess lower activation enthalpies for olefin and H2O2 activation.102,103,230,232 This is because strong Lewis acid pull electron density away from O2-, making oxidants more electrophilic and reactive in aqueous phase. As a result, catalyst surface would be more attractive towards electron-rich functional molecules such as olefins and lone pairs of O in H2O2. Chagas et al.253 utilized a surface modified niobium oxyhydroxide (NbO2OH) catalyst for the selective liquid phase oxidation of cyclohexene in the presence of H2O2. A new class of Nb compounds was developed by pretreatment of a synthetic niobia with H2O2 to generate a modified catalyst.254,255 The synthetic NbO2OH obtained can be modified by surfactant (e.g. CTAB) treatment generating a new catalyst bearing hydrophobic tail on the surface. After surface modification with a surfactant, the very active NbO2OH, presented hydrophobic characteristics and good activity for the oxidation of cyclohexene over the heterogeneous catalyst. The partial hydrophobization of the catalyst provides it the possibility to react with compounds present in polar or nonpolar solvent (amphiphilic properties). Furthermore, the hydrophobic material presented a high selectivity toward the formation of the epoxide species. The mass spectrometry analysis of the reaction showed that epoxide species was produced with a selectivity of 60% and a conversion of 65%. The results strongly suggest that the reaction involves oxidizing species generated after the reaction with

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H2O2. The surfactant anchored over the Nb catalyst promotes a better interaction with the nonpolar substrate. These novel results describe, for the first time, the use of a synthetic Nb2O5 in liquid phase at low temperature (25 °C) in the presence of H2O2 without metal impregnation. Furthermore, the catalyst can be reused without deactivation, which is an advantage compared to others catalytic systems.

Conclusion In this paper, we have critically reviewed most recent progress on W and Nb nanostructured catalysts for liquid phase epoxidation of light olefins to value-added epoxides. Since homogeneous W and Nb catalysts have been widely used in facile epoxidation of olefins to epoxides, recent research efforts have been focused on developing effective techniques for immobilization of homogeneous catalytic species to heterogeneous supports. In this context, mesoporous materials with tunable surface acidity/basicity, large surface area and pore volume have been considered as one of most promising heterogeneous supports for W and Nb catalysts. Both experimental and computational studies have confirmed that such unique properties with mesoporous materials display synergistic effect on promoting W and Nb for selective activation of C=C bond in epoxides using H2O2 as green oxidant. Despite significant advances in improving catalyst activity, poor catalyst selectivity and stability is still a major issue plaguing further applications of existing nanostructured W and Nb catalysts. Future research attention will be paid to sophisticated manipulation of 70


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interaction of homogeneous W and Nb species and solid supports for selective activation of C=C and H-O bond while retaining robust structures in aqueous phase reaction medium. Based on the discussion above, it is clear that the addition of Nb often enhances surface redox properties in mesoporous materials. It can also act as structural promoters for structured materials and prevent agglomeration of noble metal species. The addition of W is known to facilitate electron transfer between surface –OH group and transition metals in a way changing electrophilic features on catalyst surface. However, both catalytic systems still suffer significant deactivation due to surface hydrolysis reactions converting Lewis acid sites into Bronsted acid ones. Therefore, future research efforts should be focused on stabilizing Lewis acid sites while maintaining good catalytic activity and selectivity in liquid phase epoxidation.

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Table 1. Ethylene and propylene epoxidation using W and Nb catalysts

Catalyst

Substrate

Oxidant

L

(%)b

UH2O2 (%)c

t

T

X

S

(h)

(°C)

(%)a

Solvent

(%)d

ref

W-KIT-6

Ethylene

H2O2

MeOH

5

35

10

81

3.6

75%

45

Nb-KIT-5

Ethylene

H2O2

MeOH

5

35

26

48

14

40

190

Nb-MCM-48

Ethylene

H2O2

MeOH

5

35

5

79

11

n.m.

190

Nb-TUD-1

Ethylene

H2O2

MeOH

5

35

22

79

12

12

190

Ethylene

H2O2

MeOH

5

35

0.65

98

71%

99

n.m.

n.d.

99

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WO3/TS-1

1-Octene

H2O2

Acetone

3

70

7

96

7

n.m.

161

W-Zn/SnO2

1-Octene

H2O2

DMC

4

60

87

93

n.m.

n.m.

59

WO3-SBA-15

1-Octene

H2O2

ACN/NaHCO3

3

30

82

100

n.m.

85

80

80

Liquid-Phase Epoxidation of Light Olefins over W and Nb

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