Introduction

1.1 General introduction – from catalyst to solid base catalyst

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Catalysis and Catalyst

Catalysis, from the Greek kata (cata), meaning down, and lyein (lysis) meaning to loosen, to free [ 1], may come from the “philosopher’s stone” or “quinta essential” of the medieval alchemists.

The chemical concept of catalysis was first developed by the great Swedish chemist Berzelius (1779–1848) in 1835 to correlate observations made by other chemists in Europe [2,3], such as the enhanced conversion of starch to sugar by acids; the hastening of gas combustion by platinum; the stability of hydrogen peroxide in acid solution but its decomposition in the presence of alkali and transition metals, such as manganese, silver, platinum, and gold; and the observation that the oxidation of alcohol to acetic acid was accomplished in the presence of finely divided platinum [4]. According to the definition of catalysis introduced by Berzelius and scientifically defined firstly by the German chemist Ostwald (1853–1932) in 1894,catalyst is a substance which alters the rate of approaching of chemical equilibrium without itself being changed or substantially consumed in the process [ 5]. In a catalyzed reaction the catalyst generally enters into chemical combination with the reactants but is ultimately regenerated so that the amount of catalyst remains unchanged. All the remarkable phenomenon involved catalyst can be called catalysis.

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Catalysts may achieve astonishing activities. Very small quantities of catalyst can catalyze the reactions containing thousands or even millions of times their own weight of chemicals. Equally astonishing is just how selective they can be. A catalyst may increase the rate of only one reaction out of many competing reactions [ 6, 7]. This just is the significant phenomenon of catalysis. Whereas normally in nature the law is valid: the cause is equal to the effect, catalytic phenomena are of an entirely different nature. The conceptual importance of catalysis is based on its surprising nature [8]. It is also interesting to note that over 80% of industrial processes involve catalysts and the number is rising; more than half of the elements in the periodic table are involved in catalytic systems.

Heterogeneous (Solid) catalysts

Catalysts are classified roughly according to their phase behavior into homogeneous and heterogeneous catalysts. For heterogeneous catalysis, catalyst and reactants are in different phases. The reactants may be either gases or liquids (or solutions) and usually the catalyst is a solid. Therefore, heterogeneous catalysts are also called as solid catalysts.

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Catalysis takes place always and everywhere. Catalysis by solid materials has been observed quantitatively at temperatures as low as 78 K and as high as 1500 K; at pressures between 10–9 and 103 bar; with reactants in the gas phase or in liquid phase; with or without assistance of photons, radiation, or electron transfer at electrodes; with pure metals as unreactive as gold and as reactive as sodium; with multi-component and multi-phase inorganic compounds and acidic organic polymers; and at site time yields as low as 10–5s–1 (one turnover per day) and as high as 109s–1 (gas kinetic collision rate at 10 bar) [9].

Nowadays the increasing social and environmental pressure on industry to substitute traditional homogeneous-catalyzed reactions by environmentally friendly technologies represents the most important driving force for the development of heterogeneous catalysis. Indeed, the solid catalysts have many advantages over liquid catalysts. They are non-corrosive and environmentally benign, presenting fewer disposal problems. Their reuse is possible and their separation from liquid products is much easier. Furthermore, they can be designed to give higher activity, selectivity and longer catalyst life.….

Today, heterogeneous catalytic processes may be divided into two large groups: redox reactions and acid-base reactions. The first group includes all those reactions in which the catalyst affects the homolytic bond rupture in the reactant molecules with the appearance of unpaired electrons, and formation of homolytic bonds with the catalyst with the participation of catalyst electrons. The second group includes reactions in which the reactants form heterolytic bonds with the catalyst by using the free electron pair of the catalyst or reactants, or the free electron pair formed in the course of reaction by heterolytic rupture of bonds in the reactant molecules [10]. Therefore, heterogeneous catalysts can be divided into redox catalysts and acid-base catalysts.

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Solid acid-base catalysts

In general terms, a solid acid catalyst may be understood as a solid on which the color of a basic indicator changes, or as a solid on which a base is chemically adsorbed. On the contrary, a solid base catalyst may be understood as a solid on which the color of an acidic indicator changes, or as a solid on which an acid is chemically adsorbed. More strictly, following both Brønsted (-Lowry) and Lewis definitions [ 11]: a Brønsted acid is a proton donor and Brønsted base is a proton acceptor; a Lewis acid is an electron-pair acceptor and Lewis base is an electron-pair donor.A solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. These definitions are adequate for an understanding of the acid-base phenomena shown by various solids, and are convenient for a clear description of solid acid and base catalysis. However, it should be noted that the same site could serve as a Brønsted base as well as a Lewis base, depending on the nature of the adsorbate in the reaction [11].

Solid base catalysts

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So far, solid acid catalysts have been extensively studied and applied in numerous reactions due to the demand in the great progress of petroleum and petrochemical industry in the past 40 years. However, in contrast to solid acid catalysts, much fewer efforts have been made to study solid base catalysts. From a statistical survey made by Tanabe and Hölderich [ 12], in industrial processes until 1999, the classification of the types of catalysts into solid acid, solid base, and solid acid-base bifunctional catalysts gives the numbers as 103, 10 and 14, respectively. Obviously, the total number of solid base-related catalysts including solid base and acid-base bifunctional catalysts is much less than that of solid acid catalysts.

Fig.1.1 Number of the solid catalysts in the industrial processes [12]

Actually, numerous reactions such as isomerizations, alkylations, condensations, additions, and cyclizations are carried out industrially by using liquid bases catalysts. The replacement of liquid bases by cleaner catalytic alternatives is quite necessary in the view of environmentally benign. Solid base catalysts are non-stoichiometric, non-corrosive and reusable, which can be a good alternative.

1.2 Types of solid base catalysts

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Table 1.1
Types of solid base catalysts

Type

Typical catalyst

Details of the catalyst

Single metal oxide

Alkaline earth oxide

MgO, CaO, SrO, BaO

Rare earth oxide

La2O3,YbO2

Transition melt oxide

ZrO2

Mixed oxide

Mg-Al mixed oxide

MgO-Al2O3

Mg-Ti mixed oxide

MgO-TiO2

Zeolite

Alkali ion-exchanged zeolite

Cs-exchanged zeolite X, Y

Alkali metal or metal oxide occluded zeolite

Cs-occluded zeolite X, Y

Mesoporous material

Modified mesoporous material

MgO/SBA-15

Functionalized mesoporous

MCM-41 functionalized

with amino groups

Mesoporous silicon oxynitride

-

Supported catalyst

compound: Na, K, KF, KNO3, K2O

Support: C, Al2O3, SiO2, ZrO2, MgO

KF/Al2O3, Na/NaOH/Al2O3,

Na/MgO

Clay and modified clay

Hydrotalcite, calcined and rehydrated hydrotalcite

Mg-Al hydrotalcites

Chrysotile

-

Sepiolite

Magnesium silicate

Oxynitride

Silicon oxynitride

SiON

Aluminophosphate oxynitride

AlPON

Zirconophosphate oxynitride

ZrPON

Other

Modified natural phosphate (NP)

Calcined NaNO3/NP

The first study of heterogeneous basic catalysts, which was pointed out by Hattori [ 13], was that Pines et al. [ 14] studied sodium metal dispersed on alumina acted as an effective catalyst for double bond migration of alkenes in the 1950s. In the following 50 years, until now, the studies of solid base catalysts have been continuous and progressed steadily. From a single metal oxide, such as MgO, to functionalized mesoporous materials, a variety of solid base catalysts have been developed and studied. Here, the solid base catalysts are divided into the types displayed in Table 1.1. Although some excellent reviews [13, 15,16, 17, 18, 19] are available about the solid base catalysts, some selected types of solid base catalysts, such as basic zeolites, functionalized mesoporous basic materials, oxynitrides, KF/Al2O3 and solid superbase catalysts will be briefly discussed in the following. Some other kinds of catalysts will also be further introduced later in the following chapters.

Basic zeolites

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Generally, zeolites are aluminosilicates that are constructed from TO4 tetrahedra (T = tetrahedral atom, e.g., Si, Al) with each apical oxygen atom shared with an adjacent tetrahedron. It is well known that zeolites are usually used as solid acid catalysts [20]. At the beginning of the 1990s, zeolites were used as base catalysts in their ion-exchanged and impregnated forms [19].

There are two main kinds of basic zeolites: alkali ion-exchanged zeolites and metal oxide or metal occluded zeolites. The latter can also be called as modified zeolites, supported zeolites, loaded zeolites or added zeolites. In general, the basic sites in alkali ion-exchanged zeolites are regarded as the framework oxygens and therefore related to the negative charge density on the oxygen atoms, which depend on the zeolite structure and chemical composition. The basic strength and the density of basic sites in alkali ion-exchanged zeolites decrease with an increase in framework Si/Al ratio, while basic strength increases with an increase in electropositivity of the countercation in zeolites [15,17]. Thus, the relatively high aluminum content of zeolite X (Si/Al = 1–1.5) results in a substantial framework negative charge, which makes zeolite X one of the most basic zeolites when in the alkali-exchanged form. Normally, the basic strength of alkali ion-exchanged zeolites decreases in the following order: Cs+ > Rb+ > K+ > Na+ > Li+ [15,17, 21]. This kind of basic zeolites is regarded as weak bases. Therefore, they can be handled in ambient atmosphere, since adsorption of carbon dioxide or water is not too strong, and they can be removed by high-temperature treatment.

Occlusion of alkali metal oxide clusters in zeolite cages via decomposition of impregnated alkali salts results in a further increase in the basicity of basic zeolites.Preparation of fine particles of alkali oxides inside the cavities of zeolites was developed by Hathaway and Davis [ 22, 23,24]. They impregnated CsNaX and CsNaY zeolite with cesium acetate aqueous solution and calcined at 723 K to decompose cesium acetate into cesium oxide occluded in the cavities. Very active basic sites are formed by this method. In the decomposition reaction of isopropanol to produce acetone, the activity of the impregnated CsNaY zeolite was an order of magnitude greater than that found for the impregnated CsNaX zeolite with the identical loadings of cesium acetate. The impregnated CsNaY zeolite was also found to show an order of magnitude greater acetone activity than the parent CsNaY zeolite [22]. The work of Hathaway and Davis was extended by Tsuji et al. [ 25, 26], and they found that potassium and rubidium oxide could be formed in addition to cesium oxide. The resulting zeolite possesses basic sites stronger than those of simple ion-exchanged zeolite and is able to isomerize 1-butene at 273 K with high cis/trans 2-butene ratios. Recently, alkali earth oxides, such as MgO and BaO were also introduced into zeolites to produce strong basic sites [27,28].

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Other basic zeolites containing alkali metals in cages have been studied as strong solid base catalysts. Impregnation of zeolite with NaN3 or CsN3 followed by controlled thermal decomposition of the alkali azide can form either ionic or neutral metal clusters [ 29, 30,31,32]. Martens and co-workers [29,30] first prepared occluded metallic sodium zeolites, by the thermal decomposition of sodium azide adsorbed on the zeolite. These catalysts were active in the isomerization of cis-2-butene and the hydrogenation of acetylene, benzene and cis-2-butene.

Meanwhile, loading of low-valent Yb or Eu species on Y-zeolites by impregnation from Yb or Eu metal dissolved in liquid ammonia also resulted in strong basic catalysts [33,34]. Again, the loaded zeolites had a high catalytic activity for the isomerization of 1-butene at 273 K, when they were heated under vacuum at about 470 K.

It is also worth to mention that the microporous titanosilicate ETS-10 is found to be more basic than alkali ion-exchanged faujasite [35,36]. In the isopropanol conversion, Cs-exchanged X-type zeolite gave a 49.4% selectivity for acetone, while Cs-exchanged ETS-10 gave a 85.8% selectivity at 623 K.

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The basicity (amount and strength) of basic zeolites have been extensively studied by theoretical approaches and experimental characterizations [15], including infrared (IR) spectroscopy of adsorbed probe molecules such as carbon dioxide [26,37], pyrrole [21,38], and chloroform [39], TPD [26, 40], XPS [41,42,43], UV-Vis spectroscopy [44,45], microcalorimetry [ 46,47] and NMR [48,49,50]. Meanwhile, to investigate the active species, 133Cs MAS NMR, ESR and Raman spectroscopy were also used.

By generating framework and/or extra-framework basic sites mentioned above, it is possible to prepare basic zeolites with a very large spectrum of basicities. Then, depending on the reaction to be catalyzed, it should be possible to select the most suitable basic zeolite from the very mild alkaline-exchanged zeolites up to very strong alkali- or alkaline-oxide-cluster containing zeolites. Therefore, until now, basic zeolites have been used as catalysts in a number of base-catalyzed reactions, such as toluene alkylations with methanol [51,52] or ethylene [46], dehydrogenation of alcohols [22], double bond isomerizations [26,40], Knoevenagel condensations [ 53, 54,55], aldol condensations [56,57] and cycloaddition of CO2 to epoxides [58]. However, an industrial process utilizing a basic zeolite has not been commercialized [17].

For an excellent discussion of basic zeolites, a comprehensive review by Barthomeuf [15] is available.

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Mesoporous basic materials

Although basic zeolites have been used in a broad spectrum of reactions, in some cases they are limited in the application of the synthesis of fine chemicals because their small pore openings prevent bulky molecules from reaching the active sites in zeolite cages. New families of mesoporous silicas, such as MCM-41 [ 59 ] and SBA-15 [ 60 ], open the new opportunities for supports due to tuneable larger pore sizes.

Basic mesoporous materials may be prepared by following routes: a) cation exchange with alkali (e.g. Na+ , K+, Cs+) metal ions; b) impregnation with alkali salts solution and calcination; c) functionalization with organic groups. The preparations of the former two kinds of mesoporous materials are similar to those of basic zeolites.

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The use of mesoporous materials MCM-41 as carriers for basic guest species was proposed by Kloetstra and Bekkum [ 61, 62]. They found sodium and cesium cation-exchanged MCM-41s were mild, selective, water-stable and recyclable catalysts for base-catalyzed Knoevenagel condensation [61]. Meanwhile, they obtained finely dispersed cesium oxide clusters in the pores of MCM-41 by impregnation of MCM-41 with cesium acetate in aqueous or methanolic solution and calcination, when the cesium content was not higher than 10 wt%. The CsOy/MCM-41 catalyst had strong base activity in Michael addition [61].However, the catalyst did not show a good thermal and chemical stability. After repeated calcinations or after use, aggregation of the cesium oxide particles and a significant reduction of the specific surface area were observed [63]. Kloetstra and Bekkum further improved the thermal stability substantially and lowered the catalyst moisture sensitivity by addition of equimolar amounts of lanthanum, which gave the MCM-41 supported binary cesium-lanthanum oxide. The resulting catalyst was applied in Michael addition [62] and novel isomerization of ω-phenylalkanals to phenyl alkyl ketones [64]. Recently, basic mesoporous material was also prepared by in-situ coating of SBA-15 with basic MgO in one-step procedure by adding acetate salt into the initial mixture of raw materials for synthesis [ 65].

Functionalized mesoporous basic material can also be prepared by the immobilization of an organic base, such as amino groups, cinchonine, β-aminoalcohols or quaternary organic ammonium hydroxides, on the surface of a carrier material. There have been various attempts to fix functional groups on the surface of mesoporous silica. Macquarrie first reported the one-pot synthesis of surface-modified mono-disperse MCM-type silica [66], and further found MCM-type silicas having aminopropyl groups were effective base catalysts for the Knoevenagel reactions [ 67]. The amino groups can also be introduced by post-synthesis using the reactivity of the OH groups [ 68,69]. Triazabicyclo[4,4,0]dec-5-ene (TBD) is an extremely strong organic base and particularly active both in the free form and immobilized on an organic resin. Subba Rao et al. [ 70] prepared a catalyst based on TBD immobilized on MCM-41. They reported high activity for base-catalyzed reactions such as Michael addition and Knoevenagel condensation.Rodriguez et al. [ 71] prepared strong and stable Brønsted base catalysts by anchoring quaternary organic tetraalkylammonium hydroxide on MCM-41. Corma and co-workers [72] grafted a proton sponge, 1,8-bis(dimethylaminonaphthalene) (DMAN), onto amorphous and pure-silica MCM-41. The results showed that DMAN supported MCM-41 was an excellent base catalyst for the Knoevenagel condensation between benzaldehyde and different active methylene compounds as well as for the Claisen-Schmidt condensation of benzaldehyde and 2′-hydroxyacetophenone to chalcones and flavanones.

The organic-inorganic hybrid materials are less strongly basic than the corresponding free organic molecules and possess a wide distribution of base sites with different strengths, which has been explained by an H-bonding interaction of the amine function with residual silanol groupsAt the same time,these organic-inorganic hybrid materials maintain the advantages of the inorganic support, notably a high surface area and structural stability at elevated temperature and pressure. Thus, mesoporous silicas with a variety of organic bases have been tested for a variety of reactions, including the syntheses of monoglycerides from fatty acids and glycidol [68, 73], Knoevenagel condensations [70,71, 74,75], aldol condensations [71,74], and Michael additions [70,71]. Moreover, it is worth noting that the immobilization of a chiral amine on MCM-41 can give a heterogeneous catalyst for enantioselective reactions [76].

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Oxynitrides

Lednor and Rulter first prepared silicon oxynitride, Si2N2O, by a gas-solid reaction of amorphous silica with ammonia at 1373 K [77] and further found that the material had a solid base character in the Knoevenagel condensations [78,79]. After that, aluminophosphate oxynitrides (AlPON), synthesized by nitridation under NH3 of high-surface-area amorphous aluminophosphate precursors at around 1073 K (the temperature and time of nitridation could be changed to modify the nitrogen content in the final solid), [ 80, 81,82,83, 84] were described as a new family of solid basic catalysts, showing very promising behavior to the synthesis of methyl isobutyl ketone [80] and in Knoevenagel condensations [81,84]. The incorporation of nitrogen in the aluminophosphate anionic framework seems to be an effective way to modify the surface acid-base properties of the precursors and particularly to decrease the number of acid sites and to increase the number of basic sites.Zirconophosphate oxynitride (ZrPON) [85,86,87], galloaluminophosphate oxynitride (AlGaPON) [88,89] and aluminovanadate oxynitride (VAlON) [ 90,91], which were prepared in a similar method, have also been reported as solid base catalysts. Recently, Xia and Mokaya [ 92] extended the preparation method and successfully prepared highly ordered mesoporous silicon oxynitride materials as solid base catalysts.

For oxynitrides, the identification of the basic sites is more difficult than that on oxide basic catalysts because several species present at the surface can act as basic sites. Among them the nitride nitrogen (N3–), the –(NH)–, and the –NH2 group could be candidates, but the oxygen and the hydroxyl whose charge would be modified by the vicinity of the less electronegative framework nitrogen cannot be neglected [90]. Nevertheless, it seems that the acid-base properties of oxynitrides may be tuned by adjusting the O/N ratio.

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In most cases, these oxynitrides are known to catalyze Knoevenagel condensations. However, application of this kind of catalysts to other types of reactions is highly desirable.

KF/Al2O3

Most of heterogeneous basic catalysts are in the form of oxides and the basic sites are O2 ions with different environments depending on their types. If the basic sites are constituted by elements other than O2 , the catalysts are expected to show different catalytic properties [13]. Among the catalysts of the non-oxide type, potassium fluoride supported on alumina (KF/Al2O3) is most widely studied. KF/Al2O3 was introduced by Clark [ 93] as an effective solid catalyst to promote base-catalyzed organic reactions. The reactions using KF/Al2O3 as catalyst include Michael additions, Knoevenagel condensations, aldol condensations and so on. Though KF/Al2O3 has wide application in organic chemistry because of its easy workup after reactions, the idea of its catalytically active species is still controversial and the mechanisms of the appearance of the basicity of KF/Al2O3 are not clarified. Moreover, conflicting conclusions have been reported on its base strength: most authors consider it to be weak or moderate base, but some note high or even super base.

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Hattori and co-workers [94] reported that treating KF/Al2O3 at high temperature 573–673 K under high vacuum is essential for obtaining the high catalytic activity for double-bond isomerization of 1-pentene; the activity showed a sharp maximum at 623 K. The dependence of the catalytic activity on evacuation temperature was also found for the Tishchenko reaction of benzaldehyde [95] and the disproportionation of trimethyl-silylacetylene [96]. Hattori and co-workers also found that the main containing F species on KF/Al2O3 was K3AlF6, which was formed by the reaction of KF with alumina; however, it was not related to the formation of active sites, which gave a peak at -150 ppm in 19F MAS NMR [ 97]. Insufficient coordination of KF only with surface OH groups may result in the formation of the so-called ‘half-naked’ and thus active F ions [98], which was supported by 19F MAS NMR. Ando et al. concluded that there could be three basic species of KF/alumina [99,100]: (a) the presence of active fluoride, (b) the presence of [Al–O] ion which generates OH when water is added, and (c) the cooperation of F and [Al–OH].

Recently, the effect of the support on the properties of KF/Al2O3 was investigated. The basicity of supported KF can be significantly increased by a proper choice of support. The higher basicity is probably due to the dispersion of KF in small crystals [101]. More recently, CsF supported on α-alumina was also found to be an efficient basic catalyst [102].

Solid superbases

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To activate a reactant under mild conditions, a catalyst with very strong basic sites may be needed. Moreover, in the possible commercial industry chemical processes, effective application of superbase catalyst to side chain alkylation for alkyl-aromatics, and high selectivity for double-bond-isomerization of olefinic compounds without causing tar-formation as well as in place of solid acid catalysts having problem of deactivation are highly desirable [103]. Normally, catalysts which possess base sites stronger than H = 26 are called superbases [5,11]. There have been some attempts to prepare those superbase catalysts (see Table 1.2).

Table 1.2
Types of solid superbase catalysts [
104]

Catalyst

Starting material

(preparation method)

Pretreatment

Temperature (K)

Strength

(H )

CaO

CaCO3

1173

26.5

SrO

Sr(OH)2

1123

26.5

NaOH/MgO

(NaOH impregnated)

823

26.5

KNO3/ Al2O3, KNO3/ ZrO2

(dry ground)

873

26.5

Na/MgO

(Na vaporized)

923

35

Na/Al2O3

(Na vaporized)

823

35

Na/NaOH/Al2O3

(NaOH, Na impregnated)

773

37

KNH2/Al2O3

(KNH2 impregnated)

573

>37

K(NH3)/Al2O3

(ammoniacal K impregnated)

523–573

>37

Pines and co-workers [14,105,106] loaded alkali metals on supports by deposition of the metal vapor and used them as highly active catalysts for the isomerization of alkenes and the related compounds. The catalysts were regarded as superbase. Kijienski and Malinowski [107,108] also reported that sodium metal deposited on MgO (Na/MgO) showed a high catalytic activity for the isomerization of alkenes at 293 K and the base sites were stronger than H = 35. Ushikubo et al. [109] also prepared a superbase catalyst by addition of metallic sodium to MgO by decomposing NaN3 to evolve metallic sodium vapor. The resulting catalyst acted as an efficient catalyst for decomposition of methyl formate to CO and methanol. Sun and Klabunde [110] found nanocrystalline MgO doped with potassium metal were capable of alkene isomerization and alkene alkylation, including the conversion of propylene–ethylene mixtures to pentene and heptene.

↓16

Suzukamo et al. [111,112] prepared a superbase catalyst by addition of alkali hydroxides to alumina followed by further addition of alkali metals. The resulting catalyst, Na/NaOH/Al2O3, a pale blue solid, possessed basic sites stronger than H = 37 and catalyzed various base-catalyzed reactions, such as double bond migrations of 5-vinylbicyclo[2.2.1]hept-2-ene to 5-ethylidenebicyclo[2.2.l]hept-2-ene at the reaction temperature 243–373 K, 2,3-dimethylbut-l-ene to 2,3-dimethylbut-2-ene at 293 K, and safrole to isosafrole at 293 K and side chain alkylations of alkylbenzenes at the reaction temperature 293–433 K.

Baba et al. [113,114] obtained a superbase K(NH3)/Al2O3 by loading potassium onto alumina in liquid ammonia and heating the resulting material under vacuum at 523–573K. The resulting catalyst showed extremely high catalytic activity for the isomerization of alkene and had much higher activity than that of Al2O3 loaded with alkali metals by vapor deposition. The active species are not metallic, but probably amide- or imide-like species. However, Al2O3 loaded with KNH2 was found to be more active than K(NH3)/Al2O3 for the isomerization of 2,3-dimethylbut-1-ene. Using KNH2/Al2O3, even toluene can be activated to react with silanes at 329 K [115]. Moreover, the NH2 groups in KNH2/Al2O3 even react with methane (pK a = 50) [116].The basic strength of both catalysts was estimated to be at least H = 37.

Yamaguchi [ 117] and Wang et al. prepared superbases by dispersing potassium salts such as KNO3, K2CO3, KHCO3 on alumina [ 118] or zirconia [ 119] followed by thermal treatments. The dispersed compounds are decomposed partly during the thermal treatment, but the origin of the basic sties is not clear yet. The catalysts possessed a base strength of at least H = 26.5 and have the advantage of easy preparation.

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Acid-base bifunctional catalysts

Acidity and basicity are a pair of concepts and any kind of solid base (or solid acid) possesses more or less acid sites (or base sites), even for the catalysts which are simply regarded to be base (or acid) catalysts. The catalysts having suitable acid-base pair sites sometimes show pronounced activity, even if the acid-base strength of a bifunctional catalyst is much weaker than the acid or base strength of acid or base. ZrO2 was found to be a very important acid-base bifunctional catalyst and has been used in industrial applications [120]. The possible solid acid-base bifunctional catalysts are given in Table 1.3.

Table 1.3 Possible solid acid-base bifunctional catalysts

Type

Details of the catalyst

Typical catalyst

Single metal oxide

Rare earth oxide

La2O3

Transition melt oxide

ZrO2

Mixed oxide

Mg-Al mixed oxide

MgO-Al2O3

Oxynitride

Aluminophosphate oxynitride

AlPON

↓18

Future prospects and problem to be solved

The role of basic active sites and their correlate with the catalytic behavior

Until now, the nature of the basic sites on some solid base catalysts, for example KF/ Al2O3 and KNH2/Al2O3, are not very clear. Moreover, the role of alumina as a support for some catalyst systems is also not clear. It is quite necessary to explore new or proper characterization method to confirm the nature of the active sites. Meanwhile, there are a number of examples of heterogeneous base-catalyzed reactions which cannot be understood only in terms of number and strength of the basic sites, since the catalytic behavior of solid base catalysts are not simple copies of those of homogeneous basic catalysts. In situ spectroscopic techniques (IR, NMR) may be efficient methods to carry out the study. These techniques will furnish new and detailed information on the adsorbates on the working catalysts, thus helping to elucidate reaction mechanisms and find out the active sites. As catalysts, it is also important to define the reaction environment around the active sites to enhance the rate and selectivity of base-catalyzed reactions. On the other hand, theoretical calculations of the surface sites and the reaction mechanisms are quite helpful for exploration of solid base catalysis. So far the results of the quantum chemical calculations which have been done explain well the experimental results, and give us valuable information about the solid base catalysis.Unfortunately, the theoretical calculations have been done only for limited cases. An attempt to calculate for many cases is highly desirable. Keen insight into the surface reaction mechanisms and functions required for the reactions together with the accumulation of the data will enable to design the solid base catalysts active for desired reactions.

↓19

The roles of acid-base cooperative effects

In a lot of base-catalyzed reactions, acid sites on the surface of the catalysts were found to have cooperative effect to enhance the activity. However, the cooperative effect has not been well investigated. It is necessary to find an efficient way to further prove the mechanism of the cooperative effect.

Leaching problem for some kinds of catalysts in liquid-phase reactions

↓20

Some kinds of solid catalysts, such as Na/NaOH/Al2O3, metal or metal oxide occluded zeolites, show good activity in gas-phase reactions. However, when these catalysts were used in liquid-phase reactions, the problem of leaching of the active sites should be considered and studied.

Application of solid base catalysts

Novel reactions should be studied not only from the known homogeneous base-catalyzed reactions, but also from a wider range of reactions which might proceed through “anionic or anion-like” intermediates. Finally, solid bases should be utilized in industrial processes.

1.3 Application of solid base catalysts in liquid-phase reactions

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Traditionally, heterogeneous catalysis has been associated with the production of petrochemicals and bulk chemicals for a long time, whereas fine and speciality chemicals are produced predominantly with non-catalytic organic synthesis or via homogeneous catalysis. Recently, heterogeneous catalysis is beginning to be used in the fine-chemicals industry because of the need for more environmentally friendly production technology. This tendency is assisted by the availability of novel catalytic materials and modern techniques of creating and investigating specific active sites on catalyst surfaces [ 121,122].

In the field of fine chemical production, important steps in the synthesis of relatively large and complex molecules include carbon-carbon bond forming reactions such as Knoevenagel condensations or Michael additions. Bases are usually used in organic reactions to deprotonate and form carbanion intermediates, which are important intermediates in many organic reactions for fine-chemical synthesis. This is why base-catalyzed reactions usually find more applications in intermediates and fine chemical synthesis. Replacing the conventional homogeneous base catalysts, mostly solutions of alkali metal hydroxides and alkoxides, by solids can be desirable for various reasons, e.g., to suppress undesired side reactions (polymerization, self-condensation) or avoid salt formation due to the necessary neutralization of the soluble bases. The amount of by-products (largely inorganic salts) per kilogram of product is generally much larger in fine chemicals and pharmaceuticals than bulk chemicals (see Table 1.4). The use of heterogeneous base catalysts has reached great development in different areas of organic synthesis due to their environmental compatibility combined with the good yield and selectivity that can be achieved.

Table 1.4TheE factor (kg by-product/kg product) [121]

Industry segment

Product tonnage

Kg by-product/kg product

Bulk chemicals

104 – 106

< 1 – 5

Fine chemicals

102 – 104

5 – 50

Pharmaceuticals

10 – 103

25 – > 100

↓22

Therefore, in the following section, application of solid catalysts in liquid-phase reactions, especially in C–C bond formation reactions for fine chemicals will be discussed. Some typical reactions, such as Knoevenagel condensation and aldol condensation, will be detailed below.

Knoevenagel condensation

Knoevenagel condensations are the reactions between a ketone and active methylene compounds and proceed over a variety of basic solid catalysts, including alkali-ion-exchanged zeolites, alkali-ion-exchanged sepiolite, oxynitrides, and hydrotalcite-related catalysts and so forth.

↓23

The Knoevenagel condensations of benzaldehyde and substituted benzaldehydes with ethylcyanoacetate, ethylmalonate, and ethylacetoacetate [Eq.(1.1)] were catalyzed by basic faujasite zeolites [53,54] to obtain intermediates for the production of dihydropyridine derivatives. The catalytic activity increased with the basicity of the zeolite.

(1.1)

Rehydrated hydrotalcite was reported by Kantam et al. to give quantitative yields for a variety of Knoevenagel condensations [Eq.(1.2)] at room temperature using toluene or DMF as solvent in liquid phase [ 123]. Mesoporous silicas modified with amino groups [74] and mesoporous silicon oxynitride [92] were also effective in similar reactions.

↓24

(1.2)

Knoevenagel condensations of malononitrile with cyclohexanone, benzophenone and p-amino acetophenone yield alkenes containing electron withdrawing nitrile groups, which facilitate additions to the double band. These alkenes are useful in anionic polymerization reactions leading to plastics, synthetic fibers or the production of liquid crystals. They can be synthesized using ion-exchanged zeolite X, sepionlite and hydrotalcite as catalysts [124].

Dicyanomethylene derivative dyes could be prepared by two-step synthesis using different solid bases via Knoevenagel condensations [Eq.(1.3)]. The first step is the condensation of acetophenone and malononitrile to give the corresponding α-methylbenzylidene-malononitrile, which was catalyzed by a variety of solid bases such as MgO, calcined hydrotalcites, and AlPONs. Subsequent condensation with benzaldehyde gives the 1,1-dicyano-1,3-butadiene dye, which was catalyzed efficiently by AlPON. Interestingly, the reaction can also be performed in a one-pot system using an optimized AlPON as catalyst [125].

↓25

(1.3)

Knoevenagel condensation is also chosen for the synthesis of unsaturated arylsulfones. For instance, phenylsulfonylacetonitrile and phenylsulfonylacetophenone reacted with benzaldehyde and 4-substituted benzaldehydes using high-surface-area MgO, calcined Mg-Al hydrotalcites or ALPON type materials leading to α-phenylsulfonyl-cinnamononitrileand derivatives as well as α-phenylsulfonylchalcone [Eq.(1.4)] [126].

(1.4)

↓26

Aldol condensation

The aldol condensation is an important reaction for carbonyl compound (aldehyde or ketone) coupling via C–C bond formation. Aldol self-condensation of acetone to diacetone alcohol is catalyzed by a variety of solid bases, such as alkaline earth oxides, La2O3 and ZrO2, and Ba(OH)2 [13]. Alkaline earth oxides are active for the reaction in the following order: BaO > SrO > CaO > MgO [127] and the active sites are suggested to be surface OH- groups. This reaction can also be catalyzed by meixnerite-like hydrotalcite-based catalysts with high selectivity towards the desired product [128].

For aldol condensations, generally, product aldols will undergo dehydration in presence of acid sites besides basic sites.For example, when Choudary et al. used diamino-functionalized mesoporous silica as a catalyst, the reaction products were a mixture of the aldols and their dehydration products [74].

↓27

Properly activated hydrotalcite was used in condensation of citral (a mixture of geranial and neral with a proportion of 25 and 75 wt%, respectively) and acetone into pseudoionone [Eq.(1.5)], which is an intermediate in the commercial production of vitamin A [ 129]. The results showed that even at 273 K this reaction was catalyzed by modified hydrotalcites with a conversion of 65% and a selectivity of 90%, when the citral concentration is not too high (~1 wt%).

(1.5)

However, Climent et al. found both calcined hydrotalcites and rehydrated hydrotalcites could perform the same reaction at 333 K with excellent conversions and selectivities with relatively low acetone to citral ratios. It was worth noting that rehydrated hydrotalcites showed an improved reaction rate [130]. Moreover, under the reaction conditions, it was possible to avoid the inhibiting effect of citral found at 273 K [129]. Using rehydrated hydrotalcites, attractive results have also been reported in the condensation between aromatic aldehydes like benzaldehyde or substituted benzaldehydes and acetone [123].

↓28

In an aldol condensation between a ketone and an aldehyde (Claisen-Schmidt condensation), vesidryl, which is of pharmacological interest owing to its diuretic and choleretic properties, was produced from substituted acetephenone and substituted benyaldehyde [Eq.(1.6)]. By use of calcined hydrotalcite as a catalyst (5 wt%), 85% yield of vesidryl was obtained at 170 °C after 20 h [ 131].

(1.6)

A strongly basic catalyst, which was obtained by impregnation of natural phosphate with a solution of sodium nitrate, followed by calcination at 900 °C, could also catalyze the Claisen-Schmidt condensations [Eq.(1.7)] to produce chalcones with high yields [132]. The catalyst could be easily recovered and efficiently reused.

↓29

(1.7)

Among aldol condensations, nitroaldol condensation (Henry reaction) is the reaction of a nitro compound with a carbonyl compound to form a nitroalcohol under basic conditions. The products, nitroalcohols, can be converted by hydrogenation to β-aminoalcohols, which are then converted to pharmacologically important chemicals; or proceed further to afford nitroalkene. Again, a classical nitroaldol reaction is also routinely performed by use of homogeneous basic catalysts such as alkali metal hydroxides, alkoxides, amines, and ammonium acetate.

The nitroaldol condensation of propionaldehyde and nitromethane gave the product, 1-nitro-2-hydroxybutane, in the presence of different solid bases at 313 K [133]. Among the solid bases studied, MgO was the most active. The activity was not so strongly dependent on the pretreatment temperature and was scarcely retarded by exposure to air. The condensation of aromatic aldehydes with nitroalkanes over alkaline ion-exchanged zeolites affords nitroalkenes directly. Thus, in the reaction of benzaldehyde and chlorobenzene with nitromethane, CsNaX gave the corresponding nitroalkenes in 68% and 80% yields, respectively, at 413 K [134].

↓30

Mg–Al mixed oxides prepared by calcination of hydrotalcite catalyzed the nitroaldol condensation to nitroalcohols with diastereoselective. For example, when 3-nitrobenzaldehyde and nitroethane was refluxed in THF, 1-(3-nitrophenyl)-2-nitropropan-1-ol was obtained in a 95% yield with a threo/erythro ratio of 12.5 [135].

Conjugate addition of alcohols

Conjugate addition of alcohols to α β-unsaturated carbonyl compound forms a new carbon–oxygen bond to yield valuable ethers. The reactions are catalyzed by homogeneous base catalysts such as alkali hydroxides and alkoxides. Conjugate addition of methanol to 3-buten-2-one proceeds to form 4-methoxybutan-2-one over the solid bases such as alkaline earth oxides, strontium hydroxide, barium hydroxide, and KF/Al2O3, KOH/Al2O3 at a reaction temperature of 273 K [136]. MgO treated at 673 K gave the highest activity. The catalytic activities of MgO, CaO, and KF/Al2O3 were not affected by exposure of the catalysts to carbon dioxide or air.

↓31

Among conjugate additions, cyanoethylation of alcohols is a widely used reaction for the synthesis of drug intermediates and organic compounds of industrial interest. Again, high activities for this reaction were reported for high temperature activated MgO catalyst (1073 K in vacuum) [137].

Cyanoethylation of alcohols such as methanol, ethanol, and 2-propanol with acrylonitrile [Eq.(1.8)] proceeds at 273 K over alkaline earth oxides and hydroxides, KF/Al2O3 and KOH/Al2O3. The reaction was not poisoned by adsorption of carbon dioxide at room temperature [138].

(1.8)

↓32

Hydrotalcite by rehydration after calcination was found to be also a highly active catalyst for cyanoethylation of alcohols, such as methanol, ethanol, and 2-propanol, with acrylonitrile at 50 °C. The catalyst was reusable without appreciable loss in activity and was air stable [139].

Michael addition

Michael addition is widely used as C–C bond coupling reaction in the production of pharmaceuticals and fine chemicals. The reaction is also conventionally catalyzed with soluble bases, such as KOH or amines. Normally, it involves nucleophilic addition of a carbanion, formed by abstraction of a proton from a C–H bond of the organic donor molecule by a base, to ,-unsaturated carbonyl compounds. Environmental and economical concerns are driving forces in the replacement of soluble bases by suitable solid catalysts. The latter are easy to separate, recover, and thus, reuse. So far, several solid base catalyst systems, such as Ba(OH)2, MgO, KF/Al2O3, Na/NaOH/Al2O3, and modified Mg-Al hydrotalcite have been used in Michael additions. The efficient catalyst varies with the type of the reactants. Moreover, some transition metal complexes as heterogeneous Lewis acid catalysts instead of conventional strong bases, like montmorillonite-enwrapped scandium, nickel (II) and cobalt (II) complexes were also applied in Michael additions. Some Michael additions of different donors to methyl vinyl ketone (MVK) catalyzed by solid catalysts in liquid phases are listed in Table 1.5.

↓33

Table 1.5Michael additions of different donors to methyl vinyl ketone (MVK) catalyzed by solid catalysts in liquid phase.

Michael donor

Catalyst

Main supposed

active sites

Reference

KF/alumina, KOH/alumina,

MgO

Lewis base or Brønsted base

[97, 140]

Modified Mg-Al hydrotalcite

Brønsted base

[ 141]

Tetraalkylammonium hydroxide and chiral amines on MCM-41

Brønsted base

[142,143]

Organosilicon compounds

Lewis acid and

Brønsted base

[144]

Nickel (II) and cobalt (II) complexes

Lewis acid

[145]

Montmorillonite-enwrapped scandium

Lewis acid

[ 146]

TS-1, Ti-ß molecular sieve

Lewis acid

[ 147]

From Table 1.5, interestingly, not only solid base catalysts but also solid acid catalysts can catalyzed the Michael additions of different donors to methyl vinyl ketone (MVK) catalyzed in liquid phase.

Partially dehydrogenated Ba(OH)2 catalyzes Michael additions of chalcones with active methylene compounds such as ethyl malonate, ethyl acetoacetate, acetylacetone, nitromethane, and acetophenone [148].

↓34

Potassium fluoride supported on alumina (KF/Al2O3) is active for the following Michael additions at room temperature: nitromethane [149], nitroethane, 1,3-diphenyl-2-propen-l-one[150], and dimenone with 3-buten-2-one (methyl vinyl ketone) [151].

(1.9)

For a self-Michael addition of methyl crotonate [Eq.(1.9)], MgO exhibits a higher activity than the other basic catalysts such as CaO, SrO, BaO, KF/alumina, KX zeolite [ 152]. For Michael addition of nitromethane to ,-unsaturated carbonyl compounds, KF/alumina and KOH/alumina exhibit high activities, while MgO and CaO exhibit low activities [140]. The factors to be considered for an efficient catalyst are basic strength of the site, acidity of reactant, and charge on the carbon atom at -position to carbonyl group.

↓35

Mesoporous silica having N ,N-dimethyl-3-aminopropyl groups prepared by a templated sol-gel method are shown to be a good catalyst for Michael addition reactions of nitroalkanes to 3-buten-2-one and 2-cyclohexen-1-one. No leaching of the catalytic component was observed [153].

(1.10)

Na/NaOH/Al2O3 and Zeolite X containing metallic sodium clusters or cesium oxide were applied in Michael addition of ethyl acrylate and acetone forming 5-oxohexanoic acid ethyl ester [Eq.(1.10)]. After 24 h at 90 °C, about 50–80% conversions with 60–70% selectivities were achieved and the catalysts were reusable [154].

↓36

Recently, Choudary et al. found that a rehydrated Mg-Al hydrotalcite with an expected Mg/Al ratio of 2.5, which was obtained by calcination hydrotalcite at 450 °C and then rehydrating at room temperature under a flow of dry nitrogen gas saturated with water vapor, was an efficient and very selective catalyst for Michael additions. For most of the reactions they investigated, more than 88% yield of 1,4-addition products were produced in 2 hours. Moreover, the authors observed that the as synthesized or just calcined hydrotalcite showed no activity for the reactions and ascribed the excellent catalytic behavior of the rehydrated hydrotalcite to Brønsted base sites [141].

1.4 Main reactions investigated in this thesis: Michael additions

The main Michael addition reactions investigated in this thesis are shown in Scheme 1.1. Table 1.6 summarizes the results of the Michael additions using a variety of catalysts in the literature for these reactions.

Scheme 1.1 Michael additions of 2-methylcyclohexane-1,3-dione (1), 2-acetylcyclopentanone (2a), and 2-acetylcyclohexanone (2b) to methyl vinyl ketone (3) to obtain Michael adducts: 2-methyl-2-(3-oxo-butyl)-cyclohexane-1,3-dione (4), 2-acetyl-2-(3-oxo-butyl)-cyclopentanone (5a), and 2-acetyl-2-(3-oxo-butyl)-cyclohexanone (5b).

↓37

As shown in Table 1.6, in most cases, homogeneous catalysts such as KOH and amines or metal complexes were used in these reactions. In the Michael additions of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone, at room temperature, using triethylamine as catalyst and methanol as solvent, only 42% yield of product was obtained. However, increasing the reaction temperature and using hydroquinone as catalyst, about 100% yield of product was obtained. It is worth noting using acetic acid could also perform the reaction.

Table 1.6
Catalysts in the literatures used for the Michael additions

Catalyst

Solvent

Reaction conditions

Yield

Ref.

Reaction 1: Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone

Triethylamine

Methanol

RT

42%

[155]

Et3N

Acetonitrile

20 °C, 24 h, 750.06 Torr

67%

[156]

KOH

Methanol

Heating, 4 h

-

[157]

Hydroquinone

H2O

70–80 °C, 4 h

~100%

[158]

Acetic acid

H2O

75 °C, 1 h

-

[159]

Reaction 2: Michael addition of 2-acetylcyclopentanone to methyl vinyl ketone

Montmorillonite-

enwrapped scandium

H2O

50 °C, 1 h

97%

[146]

-

50 °C, 1 h

99%

[]

Ni(OAc)2⋅4H2O with ligand

CHCl3

23 °C, 16 h

41%a

[160]

Reaction 3: Michael addition of 2-acetylcyclohexanone to methyl vinyl ketone

FeCl3

CH2Cl2

25 °C,18 h

79%a

[160]

Ni(OAc)2⋅4H2O with ligand

CHCl3

23 °C, 16 h

57%a

[160]

P-BEMP b

THF

RT, 23 h

98%

[161]

a: with stereoselective
b: N-phenyl-tris(dimethylamino)iminophosphorane immobilized on polystyrene resin

Although some catalysts including normal liquid base and metal complexes have been used in these Michael additions, few solid base catalysts have been involved in the reactions investigated in this thesis. Solid base catalysts are non-stoichiometric, non-corrosive, reusable and environmentally benign. Therefore, solid base catalysts are good alternative for Michael additions.

1.5 Scope and outline of this thesis

↓38

The aim of this thesis is to study preparation, characterization and application of oxides and modified oxide as solid basecatalysts and find efficient catalysts for the liquid-phase Michael additions, meanwhile, to understand how catalytic performances are influenced by acid-base properties of the catalysts. The catalysts include MgO, potassium-modified ZrO2, calcined Mg-Al hydrotalcites, and a novel catalyst system Mg(O,F), which was prepared by sol-gel method for the first time.

In Chapter 2, general experiment and characterization methods are described. In Chapter 3, MgO catalysts – a common solid base, prepared by different ways are first involved in the Michael addition. In Chapter 4, potassium-modified ZrO2 are studied in both gas phase and liquid phase reactions. The leaching test of potassium-modified ZrO2 in the liquid phase reaction has also been performed. Chapter 5 concentrates on the calcined Mg-Al hydrotalcites. The Michael additions of 1,3-diones with different pK a values to methyl vinyl ketone are examined on calcined commercial Mg-Al hydrotalcites including an Al-rich (Mg/Al = 0.6) sample. Acid-base properties of the catalysts are investigated by FTIR spectroscopy and microcalorimetry. In Chapter 6, based on the results of former chapters, a novel catalyst system oxide/hydroxidefluoride Mg(O,F) is prepared by sol-gel method for the first time and used in the Michael addition. In the final chapter, Chapter 7, results from the previous chapters are briefly summarized.


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