Chem. Rev. 1995, 95. 537-550 537

Heterogeneous Basic Catalysis

Hideshi Hattori Center for Advanced Research of Energy Technology (CARET). Hokkaido Universiw, Kita-Ku, Kita 13, Nishi 8. Sapporo 060, Japan

Recei{bed August 16, 1994 (Revised Manuscript Received Februafy 27, 1995)

Contents I. Introduction 537 11. Generation of Basic Sites 538 Ill. Characterization of Basic Surfaces 540 Ill-1. Indicator Methods 540 111-2. Temperature-Programmed Desorption (TPD) 541 of Carbon Dioxide 111-3. UV Absorption and Luminescence 541 Spectroscopies 111-4. Temperature-Programmed Desorption of 542 Hydrogen 111-5. XPS 542 111-6. IR of Carbon Dioxide 543 111.7. IR of Pyrrole 543 ial Hideshi Hattori was born on Dec 18. 1939 in Tokyo, Japan. He graduated Ill-8. Oxygen Exchange between Carbon Dioxide 544 from the Tokyo Institute of Technology in 1963, and received a Ph.D. in and Surface engineering in 1968, whereupon he began his academic career at the IV. Catalysis by Heterogeneous Basic Catalysts 545 Oepaltment 01 Chemistry Hokkaido University. In 1971-1973. he did IV-1. Double Bond Migration 545 post-doctoral work at Rice University. He then moved to the Graduate IV-2. Dehydration and Dehydrogenation 546 School of Environmental Eanh Science, Hokkaido University, and is presently a Professor at the Center for Advanced Research of Energy IV-3. Hydrogenation 546 Technology, Hokkaido University. His special field of interest is solid acid IV-4. Amination 548 and base catalysis. IV-5. Meerwein-Ponndorf-Verley Reduction 548 catalyzed reactions, on the contrary, reactants act as IV-6. Dehydrocyclodimerization of Conjugated 548 acids toward catalysts which act as bases. Dienes In homogeneous systems, a huge number of acid- IV-7. Alkylation 549 catalyzed reactions and base-catalyzed reactions are IV-8. Aldol Addition and Condensation 549 known. In heterogeneous systems, a limited number IV-9. The Tishchenko Reaction 550 of reactions are recognized as acid- or base-catalyzed IV-10. Michael Addition 550 reactions. In particular, base-catalyzed reactions IV-11. The Wittig-Horner Reaction and 551 have been studied to a lesser extent as compared to Knoevenagel Condensation acid-catalyzed reactions in heterogeneous systems. IV-12. Synthesis of ag-Unsaturated Compound by 551 Heterogeneous acid catalysis attracted much at- Use of Methanol tention primarily because heterogeneous acidic cata- IV-13. Ring Transformation 552 lysts act as catalysts in petroleum refinery and are IV-14. Reactions of Organosilanes 552 known as a main catalyst in the cracking process V. Characteristic Features of Heterogeneous Basic 553 which is the largest process among the industrial Catalysts of Different Types chemical processes. Extensive studies of heteroge- V-1 . Single Component Metal Oxides 553 neous cracking catalysts undertaken in the 1950s V-2. Zeolites 554 revealed that the essential nature of cracking cata- V-3. Basic Catalysts of the Non-Oxide Type 555 lysts are acidic, and generation of acidic sites on the solids was extensively studied. As a result, amor- V-4. . Heterogeneous Superbasic Catalysts 556 phous silica-alumina was utilized as a cracking VI. Concluding Remarks 556 catalyst, and then crystalline aluminosilicate (zeolite) was used afterward. 1. Introduction In contrast to these extensive studies of heteroge- neous acidic catalysts, fewer efforts have been given Acid and base are paired concepts; a number of to the study of heterogeneous basic catalysts. The chemical interactions have been understood in terms first study of heterogeneous basic catalysts, that of acid-base interaction. Among chemical reactions sodium metal dispersed on alumina acted as an which involve acid-base interactions are acid- effective catalyst for double bond migration of alk- catalyzed and base-catalyzed reactions which are enes, was reported by Pines et al.' Considering the initiated by acid-base interactions followed by cata- strong tendency of Na to donate electrons, it seems lytic cycles. In acid-catalyzed reactions, reactants act natural that Na dispersed on alumina acts as a as bases toward catalysts which act as acids. In hase- heterogeneous basic catalyst. 0M)9-2665/95/0795-0537515.50/0 0 1995 American Chemical Socieh, 538 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori Table 1. Types of Heterogeneous Basic Catalysts (W,IR, XPS,ESR, etc.) indicate that basic sites exist (1)single component metal oxides on the surfaces. alkaline earth oxides (2) There is a parallel relation between catalytic alkali metal oxides activity and the amount and/or strength of the basic rare earth oxides sites: The catalytic activities correlate well with the ThOz, ZrOn, ZnO, Ti02 (2) zeolites amount of basic sites or with the strength of the basic alkali ion-exchanged zeolites sites measured by various methods. Also, the active alkali ion-added zeolites sites are poisoned by acidic molecules such as HC1, (3) supported alkali metal ions H20, and C02. alkali metal ions on alumina (3) The material has similar activities to those of alkali metal ions on silica alkali metal on alkaline earth oxide homogeneous basic catalysts for “base-catalyzed re- alkali metals and alkali metal hydroxides on alumina actions” well-known in homogeneous systems: There (4)clay minerals are a number of reactions known as base-catalyzed hydrotalcite reactions in homogeneous systems. Certain solid chrysotile materials also catalyze these reactions to give the sepiolite same products. The reaction mechanisms occurring (5) non-oxide KF supported on alumina on the surfaces are suggested to be essentially the lanthanide imide and nitride on zeolite same as those in homogeneous basic solutions. (4)There are indications of anionic intermediates participating in the reactions: Mechanistic studies Following the report by Pines et al., certain metal of the reactions, product distributions, and spectro- oxides with a single component were found to act as scopic observations of the species adsorbed on certain heterogeneous basic catalysts in the absence of such materials indicate that anionic intermediates are alkali metals as Na and K. In the 1970s, Kokes et involved in the reactions. al. reported that hydrogen molecules were adsorbed The studies of heterogeneous catalysis have been on zinc oxide by acid-base interaction to form proton continuous and progressed steadily. They have never and hydride on the ~urface.~,~They proved that the been reviewed in the Chemical Reviews before. It is heterolytically dissociated hydrogens act as interme- more useful and informative to describe the studies diates for alkene hydrogenation. In the same period, of heterogeneous basic catalysis performed for a long Hattori et al. reported that calcium oxide and mag- period. In the present article, therefore, the cited nesium oxide exhibited high activities for 1-butene papers are not restricted to those published recently, isomerization if the catalysts were pretreated under but include those published for the last 25 years. proper conditions such as high temperature and high vacu~m.~The 1-butene isomerization over calcium oxide and magnesium oxide was recognized as a base- 11. Generation of Basic Sites catalyzed reaction in which the reaction was initiated One of the reasons why the studies of heteroge- by abstraction of a proton from 1-butene by the basic neous basic catalysts are not as extensive as those site on the catalyst surfaces. of heterogeneous acidic catalysts seems to be the The catalytic activities of basic zeolites were re- requirement for severe pretreatment conditions for ported also in early 1970s. Yashima et al. reported active basic catalysts. The materials which are now that side chain alkylation of toluene was catalyzed known as strong basic materials used to be regarded by alkali ion-exchanged X and Y type zeo1ites.j The as inert catalysts. In the long distant past, the reaction is a typical base-catalyzed reaction, and the catalysts were pretreated normally at relatively low activity varied with the type of exchanged alkali temperatures of around 723 K. The surfaces should cation and with type of zeolite, suggesting that the be covered with carbon dioxide, water, oxygen, etc. basic properties can be controlled by selecting the and showed no activities for base-catalyzed reactions. exchanged cation and the type of zeolite. Generation of basic sites requires high-temperature pretreatment to remove carbon dioxide, water, and, In addition to the above mentioned catalysts, a number of materials have been reported to act as in some cases, oxygen. heterogeneous basic catalysts. The types of hetero- This can be understood with the data in Figure 1 geneous basic catalysts are listed in Table 1. Except in which decomposition pressures are plotted against for non-oxide catalysts, the basic sites are believed reciprocal temperature for carbonates and peroxides to be surface 0 atoms. Oxygen atoms existing on any of alkaline earth elements.6 In addition to carbonates materials may act as basic sites because any 0 atoms and peroxides, hydroxides are formed at the surface would be able to interact attractively with a proton. layers of the oxides. The decomposition pressures are The materials listed in Table 1 act as a base toward very low at room temperature. On exposure to the most of the reagents and, therefore, are called atmosphere, alkaline earth oxides adsorb carbon heterogeneous basic catalysts or solid base catalysts. dioxide, water, and oxygen to form carbonates, hy- droxides, and peroxides. Removal of the adsorbed Four reasons for recognizing certain materials as species from the surfaces is essential to reveal the heterogeneous basic catalysts are as follows. oxide surfaces. Therefore, high-temperature pre- (1)Characterization of the surfaces indicates the treatment is required to obtain the metal oxide existence of basic sites: Characterizations of the surfaces. surfaces by various methods such as color change of The evolutions of water, carbon dioxide, and oxygen the acid-base indicators adsorbed, surface reactions, when Mg(OH)2 and BaO are heated under vacuum adsorption of acidic molecules, and spectroscopies at elevated temperatures are shown in Figures 2 and Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 539

9 A 8

7

.--x6 .-> $5

4

3

I 2 T-' x i03/~-1 Figure 1. Equilibrium pressure for decomposition: 1 0 nI I d I mi I 1 I I (a) 2Sr0, * 2Sr0 + 0,, (b) 2Ba0, t 2Ba0 + 0,, 600 800 1000 1200 1400 (c) MgCO, tMgO + CO,, (d) CaCO, t CaO + CO,, Pretreatment temperature/K (e) SrCO, t SrO + CO,, (0 BaCO, t BaO + CO,. Figure 4. Variations of activity of MgO for different types of reactions as a function of pretreatment temperature: 0, 1-butene isomerization at 303 K (3.5 x lo3 mmHg min-' g-') A,CHd-D, exchange at 673 K (4.3 x lo3% s-l g-l); A, amination of 1,3-butadienewith dimethylamine at 273 K (5 x lo1' molecules mix1 g-l); 0, 1,3-butadienehydroge- nation at 273 K (2.5 x 10% min-l g-l); U, ethylene hydrogenation at 523 K (0.3%min-l g-l). catalytically active sites for several reaction types. The nature of the basic sites generated by removing the molecules covering the surfaces depends on the severity of the pretreatment. The changes in the nature of basic sites are reflected in the variations of the catalytic activities as a function of pretreat- ment temperature. In many cases, the variations of 500 700 900 1100 1300 the activity are dissimilar for different reaction types. Pretreatment temperature I K The activity variations of MgO for different reactions are shown in Figure 4.8 The activity maxima appear Evolution of H,O and CO, from Mg(OH), at different catalyst-pretreatment temperatures for Figure 2. Evolution of HzO and COz from Mg(0H)Z. different reaction types: 800 K for 1-butene isomer- ization, 973 K for methane-Dz exchange and ami- 1.2, , I , , I I nation of 1,3-butadiene with dimethylamine, 1273 K for hydrogenation of 1,3-butadiene, and 1373 K for hydrogenation of ethylene. As the pretreatment temperature increases, the molecules covering the surfaces are successively desorbed according to the strength of the interaction with the surface sites. The molecules weakly inter- acting with the surfaces are desorbed at lower 0 pretreatment temperatures, and those strongly in- teracting are desorbed at higher temperatures. The sites that appeared on the surfaces by pretreatment a at low temperatures are suggested to be different 500 600 700 800 900 1000 1100 1200 1300 from those that appeared at high temperatures. If Pretreatment temperature / K simple desorption of molecules occurs during pre- treatment, the basic sites that appeared at high Evolution of H,O,CO,,and 0,from BaO temperatures should be strong. However, rearrange- Figure 3. Evolution of HzO, CO,, and 02from BaO. ment of surface and bulk atoms also occurs during pretreatment in addition to the desorption of the 3.4,7 For MgO, evolution of water and carbon dioxide molecules, which is evidenced by a decrease in the continues up to 800 K. For BaO, evolution of these surface area with an increase in the pretreatment gases continues to much higher temperatures. In temperature. addition, oxygen evolves above 900 K. Evolution of Coluccia and Tench proposed a surface model for carbon dioxide, water, and oxygen results in genera- MgO (Figure There exist several Mg-0 ion pairs tion of basic sites on the surfaces which act as of different coordination numbers. Ion pairs of low 540 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori

0.0.

Figure 5. Ions in low coordination on the surface of MgO. (Reprinted from ref 9. Copyright 1981 Kodansha.) Pretreatment temperature/K coordination numbers exist at corners, edges, or high Figure 6. Variations of activity of La203 for different types of reaction as a function of pretreatment temperature: 0, Miller index surfaces of the (100) plane. Different l-butene isomerization at 303 K (1 unit: 6.4 x 1020 basic sites generated by increasing the pretreatment molecules min-l g-l); A, CH4-D2 exchange at 573 K (1 temperature appear to correspond to the ion pairs of unit: s-l g-l); 0,1,3-butadienehydrogenation at 273 different coordination numbers. However, the cor- K (1 unit: 1.2 x lozomolecules min-l g-l). respondence between the catalytically active sites for different reaction types and the coordination number single method. Integration of the results obtained of the ion pairs is not definite yet. by different characterizations leads us to understand Among the ion pairs of different coordination the structures, reactivities, strengths, and amounts numbers, the ion pair of 3-fold Mg2+-3-fold 02- of the basic sites on the surfaces. In this section, (Mg2+3,-02-3,) is most reactive and adsorbs carbon representative methods for characterization of the dioxide most strongly. To reveal the ion pair Mg2+3,- surface basic sites are described. It is emphasized 02-3c,the highest pretreatment temperature is re- what aspect of the basic sites is disclosed by each quired. At the same time, the ion pair Mg2+3c-02-3c characterization method. is most unstable. The Mgz+gcand 02-3, tend to rearrange easily at high temperature. The appear- 111-1. Indicator Methods ance of such highly unsaturates sites by the removal Acid-base indicators change their colors according of carbon dioxide and the elimination by the surface to the strength of the surface sites and PKBHvalues atom rearrangement compete. Such competition of the indicators. The strength of the surface sites results in the activity maxima as the pretreatment are expressed by an acidity function (H-) proposed temperature is increased. by Paul and Long. The H- function is defined by the Although the surface model shown in Figure 5 is following equation:13J4 proposed for MgO, the other metal oxide heteroge- neous bases may be in a situation similar to that of MgO. The nature of basic sites varies with the severity of the pretreatment conditions for most where [BHI and [B-I are, respectively, the concentra- heterogeneous basic catalysts. tion of the indicator BH and its conjugated base, and The surface sites generated on rare earth oxides, PKBHis the logarithm of the dissociation constant of however, behave differently from those of the other BH. The reaction of the indicator BH with the basic heterogeneous base catalysts. The sites of rare earth site (E) is oxides do not seem to vary in nature with pretreat- ment temperature. Variations of the activities of BH + = B- + BH+ La203 as a function of the pretreatment temperature is shown in Figure 6 for l-butene isomerization, 1,3- The amount of basic sites of different strengths can butadiene hydrogenation, and methane-Dz ex- be measured by titration with benzoic acid. A sample change.1°-12 Pretreatment at 923 K results in the is suspended in a nonpolar solvent and an indicator maximum activity for all reactions. The surface sites is adsorbed on the sample in its conjugated base form. generated by removal of water and carbon dioxide The benzoic acid titer is a measure of the amount of seem to be rather homogeneous in the sense that the basic sites having a basic strength corresponding to same surface sites are relevant to all the reactions the ~KBHvalue of the indicator used. Using this mentioned above. method, Take et al. measured outgassed samples of MgO, CaO, and SrO. The results are shown in Ill. Characterization of Basic Surfaces Figure 7.15 Magnesium oxide and CaO possess basic sites stronger than H- = 26. The surface properties of the heterogeneous basic The indicator method can express the strength of catalysts have been studied by various methods by basic sites in a definite scale of H-, but this has which existence of basic sites has been realized. disadvantages too. Although the color change is Different characterization methods give different assumed to be the result of an acid-base reaction, information about the surface properties. All the an indicator may change its color by reactions dif- properties of basic sites cannot be measured by any ferent from an acid-base reaction. In addition, it Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 541

0.3 - -07 U E" - 0.2 a w c +- D U 4 2 0.1

2z Desorption temperature/K W m Figure 8. TPD plots of carbon dioxide desorbed from alkaline earth oxides. (Reprinted from ref 16. Copyright 0.0 1988 Elsevier.) BASE STRENGTH (H-) - r ___ csx Figure 7. Benzoic acid titer vs base strength of (A) MgO, (B) CaO, and (C) SrO. (Reprinted from ref 15. Copyright 1971 Academic.) requires a long time for benzoic acid to reach an adsorption equilibrium when titration is carried out in a solution. In some cases, the surface of hetero- geneous basic catalysts may dissolve into a titration solution. If this happens, the number of basic sites should be overestimated. Therefore, special care should be taken with the indicator method.

111-2. Temperature-Programmed Desorption (TPD) . - NaxOiNaX of Carbon Dioxide __ .I._ __--- This method is frequently used to measure the 273 373 473 573 673 number and strength of basic sites. The strength and Temperature / K amount of basic sites are reflected in the desorption temperature and the peak area, respectively, in a Figure 9. TPD plots of COz adsorbed on alkali ion- TPD plot. However, it is difficult to express the exchanged and alkali ion-added zeolites. strength in a definite scale and to count the number of sites quantitatively. Relative strengths and rela- 111-3. UV Absorption and Luminescence tive numbers of basic sites on the different catalysts Spectroscopies can be estimated by carrying out the TPD experi- W absorption and luminescence spectroscopies ments under the same conditions. If the TPD plot give information about the coordination states of the gives a sharp peak, the heat of adsorption can be surface atoms. estimated. High surface area MgO absorbs W light and emits TPD plots of carbon dioxide desorbed from alkaline luminescence, which is not observed with MgO single earth oxides are compared in Figure 8 in which crystal. Nelson and Hale first observed the absorp- adsorption of carbon dioxide and the following treat- tion at 5.7 eV, which is lower than the band gap (8.7 ment before the TPD run were done under the same eV, 163 nm) for bulk MgO by 3 eV.18 Tench and Pott conditions.16 The strength of basic sites is in the observed photolumines~ence.~~~~~The W absorption increasing order of MgO < CaO < SrO < BaO. The corresponds to the following electron transfer process number of basic sites per unit weight that can retain involving surface ion pair.21,22 carbon dioxide under the adsorption conditions in- creases in the order BaO < SrO < MgO < CaO. Mg2+02-+ hv - Mg'O- Enhancement of basic strength by addition of alkali ions to X-zeolite in excess of the ion exchange capacity Absorption bands were observed at 230 and 274 was demonstrated by TPD plots of carbon dioxide as nm, which are considerably lower in energy than the shown in Figure 9.17 The peak areas are larger for band at 163 nm for bulk ion pairs. The bands at 230 the alkali ion-added zeolites (solid lines) than for the and 274 nm are assigned to be due to the surface 02- ion-exchanged zeolites (dotted lines). In particular, ions of coordination numbers 4 and 3, respectively. desorption of carbon dioxide still continues at the Luminescence corresponds to the reverse process desorption temperature of 673 K for ion-added zeo- of W absorption, and the shape of the luminescence lites. spectrum varies with the excitation light frequency 542 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori

Table 2. Coordination Numbers of Active Sites on Coordination 15- no - MgO and Their Concentration Obtained from TPD W, 02- MQ" for Hydrogen Adsorbed J w, W] 4 3 w. w, 3 4 w,-w, 3 3 number of sites/1015m-2 at m to- - coordination no. pretreatment temperature activesite Olc MgLc 673 K 823 K 973 K 1123K WzandW3 4 3 4.0 11.6 29.3 32.4 - W4andW5 3 4 0.0 4.9 22.1 26.5 W6andW7 3 3 0.0 0.3 1.3 4.1 W, WS 3 3 1.2 4.2 i _.__._

5I- 5321 o\

531, 0 0.2 0,4,-\\\;I 0.6 0.8 1.0 AI/Si Atomic Ratio Figure 11. Correlation between the binding energy of the 01, band and the AVSi atomic ratio. (Reprinted from ref 25. Copyright 1988 Academic.)

TPD of hydrogen supports the surface model of MgO illustrated. Heterolytic dissociation of hydrogen on the MgO surface is also demonstrated by IR spectroscopy. The IR bands for 0-H and Mg-H stretching vibration were ~bserved.~ 111-5. XPS The XPS binding energy (BE) for oxygen reflects the basic strength of the oxygen. As the 01, BE decreases, electron pair donation becomes stronger. 111-4. Temperature-Programmed Desorption of Okamoto et al. studied the effects of zeolite composi- Hydrogen tion and the type of cation on the BE of the constitu- This method gives information about the coordina- ent elements for X- and Y-zeolites ion-exchanged with tion state of the surface ion pairs when combined a series of alkali cations as well as H-forms of A, X, with the other methods such as W absorption and Y, and m~rdenite.~~The BE values of 01, are plotted luminescence spectroscopies. The number of each ion against the AVSi atomic ratio in Figure 11. The BE pair could be counted if TPD is accurately measured of 01, decreases as the Al content increases. with a proper calibration method. This method has The effect of an ion-exchanged cation on the 01, been applied only to the MgO surface. BE is shown in Figure 12 as a function of the Hydrogen is heterolytically dissociated on the electronegativity (x)of the cation. With increasing surface of MgO to form H+ and H-, which are x, the 01, BE increases. The 01, BE of zeolite is adsorbed on the surface 02- ion and Mg2+ ion, directly delineated to the electron density of the respectively. TPD plots of hydrogen adsorbed on framework oxygen. On the basis of XPS features of MgO pretreated at different temperatures are shown zeolite, Okamoto et al. proposed a bonding model of in Figure 10.23)24Seven desorption peaks appear in zeolite as shown in Figure 13.25Configurations I and the temperature range 200-650 K, and appearance I1 are in resonance. In configuration I, extra frame- of the peaks varies with the pretreatment tempera- work cations form covalent bonds with framework ture. Appearance of the peaks at different temper- oxygens, while in configuration 11, the cations form atures indicates that several types of ion pairs with fully ionic bondings with the negatively charged different coordination numbers exist on the surface zeolite lattice. As the electronegativity of the cation of MgO. The adsorption sites on MgO pretreated at increases and approaches that of oxygen, the contri- different temperatures and the coordination numbers bution of configuration I increases to reduce the net of each ion pair are assigned as summarized in Table charges on the lattice. This explains the dependences 2. The assignment of the surface ion pairs are based of the 01, BE on the electronegativity of the cation on the surface structure model of MgO (Figure 5). as shown in Figure 12. Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 543 533 1 I 0 Y H+ M+ / M' ($

Figure 14. Model for pyrrole chemisorbed on a basic site. Scheme 1. Adsorbed Forms of Carbon Dioxide

i I bidentate unidentate 0.5 1.0 1.5 2.0 2.5 Electronegativity 111-6, IR of Carbon Dioxide Figure 12. Binding energy of the 01, band for cation exchanged zeolite as a function of the cation electronega- This method gives information about the adsorbed tivity (x): (0)Y-zeolite and (0)X-zeolites. (Reprinted from ref 25. Copyright 1988 Academic.) state of C02 on the surface. Carbon dioxide interacts strongly with a basic site and, therefore, the surface M M+ structures including basic sites are estimated from the adsorbed state of COZ. Carbon dioxide is adsorbed on heterogeneous basic catalysts in different forms: bidentate carbonate, unidentate carbonate, and bicarbonate (Scheme 1). (1) (11) On the MgO surface, the adsorbed form varies with Figure 13. Schematic bonding model of zeolite. the coverage of the adsorbed carbon dioxide. Biden- tate carbonate is dominate at low coverage, and Although the relation between electron density and unidentate carbonate at high coverage.28 Evans and the basic strength of 0 is not theoretically estab- Whately reported the adsorption of carbon dioxide on lished, a good correlation between the BE of the N1, Mg0.29 In addition to unidentate and bidentate band and basicity is well established for a wide carbonates, bicarbonate species were also detected. variety of organic compounds containing N. It may For CaO, carbon dioxide is adsorbed in the form of be acceptable that the BE of the 01, band changes bidentate carbonate regardless of the coverage. monotonously with the basic strength of 0 when comparison is made within a same series of ex- In the adsorption state of unidentate carbonate, changed cations. only surface oxygen atoms participate, while the XPS measurement of the probe molecule adsorbed metal ion should participate in the adsorption state on basic sites gives information about the strength of bidentate. In other words, the existence of only a of the basic sites. Huang et al. measured the N1, BE basic site is sufficient for unidentate carbonate, but of the pyrrole adsorbed on alkali cation-exchanged the existence of both a basic site and a metal cation X- and Y-zeolites.26 The NI, envelopes were decon- is required for bidentate carbonate. voluted into three peaks. One of the peaks was assigned to pyrrole adsorbed on the framework 111-7. IR of Pyrrole oxygen adjacent to the alkali cations other than the sodium cation. The BE of the peak varies with the Pyrrole is proposed to be a probe molecule for exchanged cation in such a way that the NI, BE measurement of the strength of basic sites.17 The IR decreases as the basic strength of the zeolite in- band ascribed to the N-H stretching vibration shifts creases as Li < Na < K < Rb < Cs. The deconvolu- to a lower wavenumber on interaction of the H atom tion of XPS N1, peaks into three peaks indicates that in pyrrole with a basic site. Barthomeuf measured the basic strength of the framework oxygen is inho- the shifts of N-H vibration of pyrrole adsorbed on mogeneous in the zeolite cage and that the cation alkali ion exchanged zeolite^.^^,^^ The results are exerts an influence only on the adjacent framework given in Table 3. The charges on the oxygen calcu- atoms. These suggest that electrons are localized lated from Sanderson's electronegativities are also significantly on M+(Al02)- units. A proposed model listed in Table 3. The shift increases when the for pyrrole chemisorbed on a basic site of alkali the cation-exchanged zeolite is shown in Figure 14. negative charge on oxide ion increases. The As described later, the basic strength is reflected negative charge is associated closely with the strength in the N-H stretching vibration frequency of pyrrole of the basic site. The basic strengths of alkali ion- in the IR ~pectrum.~'The N1, BE in XPS correlates exchanged zeolites are in the order CsX > NaX > KY linearly with the N-H vibration frequencies of > Nay, KL, Na-mordenite, Na-beta. chemisorbed pyrrole. As the exchanged cation changes The N-H vibration frequencies observed by IR are in the sequence Li, Na, K, Rb, and Cs, both the N1, plotted against the NI, BE observed by XPS as shown BE and the frequency of the N-H stretching vibra- in Figure 15.26 For both X- and Y-zeolites, linear tion decrease for X- and Y-zeolites.26 relations are observed; strengths of the basic sites 544 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori

Table 3. Shifts of N-H Vibration of Pyrrole Adsorbed 1.2 , I 0 on Zeolites and Calculated Average Charge on 0 Oween 1.0 - I A/ To'a' co2 1 zeolite AVNH" qob zeolite AVNH" qob - 0.8} CsX 240 -0.461 Na-MOR 30 -0.278 NaX 180 -0.413 Na-beta 30 -0.240 KY 70 -0.383 CS ZSM-5 0 -0.236 NaY 30-40 -0.352 NaZSM-5 0 -0.225 KL 30 -0.356

a Shift of N-H from the liquid. Charge on oxygen calcu- lated from Sanderson electronegativity. a l,8,#,#,\.l YNH 200 400 600 800 1000 1200 cml Desorption temperature / K 3450.00 Figure 16. TPD plots for ClSOz adsorbed on MgO.

0-c4 /a' pmcess ( I1 ) II I 16 -- 04-Mg-O-Mg- I6 I -Mg -0-Mg -0'-

0' 8 Ho9 0-c \\C-OlO pmcen ( Ill ) Nls Binding Energy (eV) II I 110 -O'-Mg- - -Mg-0- Figure 15. Relationship between N1,binding energy and Figure 17. Proposed processes for the mechanisms of N-H stretching vibration frequencies of chemisorbed pyr- migration of the surface bidentate carbonate. role on (0)X-zeolites and (0)Y-zeolites. (Reprinted from ref 26. Copyright 1992 Academic.) processes I and 11, for the adsorbed carbonate species to migrate over the surface. In process I, carbon are in the order CsX > RbX > KK X NaX > LiX and dioxide rolls over the surface in such a way that the CsY > RbY > KY > NaY > LiY. free oxygen atom in the bidentate carbonate ap- proaches the adjacent Mg atom on the surface. In 111-8. Oxygen Exchange between Carbon Dioxide process 11, the carbon atom approaches the adjacent and Surface 0 atom on the surface. This method gives information about the dynamic In process I, the carbonate species always contains nature of interaction of adsorbed C02 with the two l80 atoms. Therefore, repetition of process I surface ion pair. As described above, carbon dioxide results in the exchange of one oxygen atom, but not is used as a probe molecule for the basic properties the exchange of two oxygen atoms in the desorbed in IR and TPD. If 180-labeled,carbon dioxide is used, C02. The repetition of process I1 also results in the additional information about the nature of basic sites exchange of one oxygen atom. For evolution of Cl6O2, is obtained. both processes I and I1 should be involved. In Yanaasawa et al. reported that oxygen exchange addition to processes I and 11, process I11 is possible. between adsorbed COZ and the MgO surface takes This process is essentially the same as the mecha- place to a considerable extent.31 They observed a nism proposed for the oxygen exchange between TPD desorption peak consisting mainly of Cl6O2 and bidentate carbonate and oxide surface. The carbon- Cl60l8O after Cl802 adsorption on MgO and sug- ate species are able to migrate on the surface over a gested that the adsorbed Cl8O2 interacts with the long distance by a combination of process 1-111 peroxy ion (160,)22- on a defect in the MgO surface. without leaving the surface, if process I11 exists. IR Essentially the same result was independently re- spectra of the adsorbed C02 changes with increasing ported by Shishido et al.32 The interpretation of the temperature. It is suggested that the bidentate exchange mechanisms, however, was not the same carbonate formed on room temperature adsorption as that of Yanagisawa et al. of CO2 migrates over the surface as the temperature Tsuji et al. reported the oxygen exchange in de- is raised in the TPD run. The migration occurs TPD plots for Cl80z adsorbed on MgO are mostly in the temperature range from room temper- shown in Figure 16 in which 41 x mol PO2 ature to 473 K. g-I (one COZ molecule per 670 A2) was adsorbed. The results of the oxygen exchange between C02 Extensive oxygen exchange was observed; no C1*02 and MgO surface suggest an important aspect of the was desorbed. Proposed processes for the mechanism nature of surface basic sites. The basic sites are not of migration of the surface bidentate carbonate are fixed on the surface but are able to move over the shown in Figure 17. There are at least two ways, surface when carbon dioxide is adsorbed and de- Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 545 Scheme 2. 1-Butene Isomerization

__ctH+ ,CH=CH CHTCH, \ cH3 -H+ a3 a3 -."-;, cis-2-butene CH4H-CH2-CH3

1-butene F3 CH- CH tH+ ,CH=CH / _--- - &;- Q -H+ k3 trans-2-butene sorbed. The position of the basic site (surface 0 H (or D) transfer is involved, and the products will atom) changes as CO2 migrate over the basic site. In be composed of do and dg isotopic species. addition, it became clear that not only 02-basic sites Since an H+ is abstracted first for base-catalyzed but also adjacent Mg2+ sites participate in C02 isomerization to form allyl anions to which the H+ adsorption. Therefore, it is reasonable to consider returns at a different C atom, an intramolecular H that the metal cations adjacent to the basic site (or D) transfer is expected. Therefore, an intra- participate in the base-catalyzed reactions. molecular H (or D) transfer and a high cidtrans ratio are characteristic features for 1-butene double bond IV. Catalysis by Heterogeneous Basic Catalysts isomerization over heterogeneous basic catalyst^.^'^^^ The fundamental studies of 1-butene double bond In this section, selected examples of heterogeneous isomerization over heterogeneous basic catalysts base-catalyzed reactions are described. Some of them were extended to the double bond migration of olefins aim at elucidating the reaction mechanisms. The having more complex structures such as pinene (l), others are applications to various organic syntheses carene (2), protoilludene (4), illudadiene (S), as shown to show the potential use of heterogeneous catalysts. bel~w.~~-~l IV-1. Double Bond Migration 1-Butene isomerization to 2-butenes has been extensively studied over many heterogeneous basic catalysts to elucitade the reaction mechanisms and to characterize the surface basic properties. The reaction proceeds at room temperature or below over most of heterogeneous basic catalysts. Over MgO, 9-q5 5' for example, the reaction occurs even at 223 K if the +-+ catalyst is properly activated. The reaction mechanisms for 1-butene isomeriza- tion are shown in Scheme 2.34 The reaction is initiated by abstraction of allylic H 3 3' 6 6 by basic sites to form cis or trans forms of the allyl anion. In the form of the allyl anion, the cis form is These olefins contain three-membered and four- more stable than the trans form. Therefore, cis-2- membered rings. If acidic catalysts were used, the butene is predominantly formed at the initial stage ring-opening reactions would easily occur, and the of the reaction. A high cidtrans ratio observed for selectivities for double bond migration should mark- the base-catalyzed isomerization is in contrast to the edly decrease. A characteristic feature of heteroge- value close to unity for acid-catalyzed isomerization. neous basic catalysts is a lack of C-C bond cleavage The cis to trans ratio in 2-butenes produced could ability. The double bond migration selectively occurs be used to judge whether the reaction is a base- without C-C bond cleavages over heterogeneous catalyzed or acid-catalyzed one. Tsuchiya measured basic catalysts. the ratio cidtrans in 1-butene isomerization, and As mentioned above, the heterogeneous basic cata- found a high value for R~zO.~~ lysts are highly active for double bond migration, the Coisomerization of butene-do and -dg is a useful reactions proceed at a low temperature. This is method to determine the reaction mechanism^.^^ In advantageous for olefins which are unstable at high the coisomerization, a mixture containing equal temperature. Because of this advantage, the hetero- amounts of nondeuteriobutene (do) and perdeuterio- geneous basic catalyst, Na/NaOWA1203,is used for butene (dg) is allowed to react, and the isotopic an industrial process for the selective double bond distributions in the products and reactant are ana- migration of 5-vinylbicyclo[2.2.llheptene (6).41,42The lyzed. If the reaction proceeds by hydrogen addi- reaction proceeds at the low temperature of 243 K. tion-abstraction mechanisms, an intermolecular H Heterogeneous basic catalysts have another ad- (or D) transfer is involved and the products will be vantage in double bond migration. For the double composed of do, dl, d7, and dg isotopic species. On bond migration of unsaturated compounds containing the other hand, if the reaction proceeds by hydrogen heteroatoms such as N and 0, heterogeneous basic abstraction-addition mechanisms, an intramolecular catalysts are more efficient than acidic catalysts. 546 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori

Scheme 3. Double Bond Migration of Allylamines 01 01 to Enamines I I I N- N- N- / -Ht 1 tHt / CH -CH &,CH=CHZ CH=CHCH, LCH- CH2 - /,,,;;. - /CH=CH safrole isosafrole

to aldehydes or ketones over basic catalysts. In some cases, however, heterogeneous basic catalysts pro- mote dehydration of alcohols in which the mecha- nisms and product distribution differ from those for acid-catalyzed dehydration. The characteristic fea- Acidic catalysts interact strongly with heteroatoms, tures of base-catalyzed dehydration are observed for became poisoned, and show no activity. On the other 2-butanol dehydration. The products consist mainly hand, the active sites of heterogeneous basic catalysts of 1-butene over the rare earth Th02,46,47and interact weakly with heteroatoms and, therefore, act Zr02.48 This is in contrast to the preferential forma- as efficient catalysts. tion of 2-butenes over acidic catalysts. The initial Allylamines undergo double bond migration to step in the base-catalyzed dehydration is the abstrac- enamines over alkaline earth oxides (Scheme 3).43For tion of an H+ at C-1 and 2-butanol to form anion. instance, 1-N-pyrrolidino-2-propeneisomerizes to Dehydration of 1-cyclohexylethanol to vinylcyclo- 1-N-pyrrolidino-1-propeneover MgO, CaO, SrO, and hexane has been industrialized by use of ZrO2 as a BaO at 313 K. The reaction mechanisms are es- catalyst.49 In the dehydration of 2-alcohols to the sentially the same as those for 1-butene isomeriza- corresponding 1-olefins over ZrO2, the selectivity for tion. The basic sites abstract an H+ from the 1-olefins depends on the amount of Si contained in reactant to form allyl anions as an intermediate as ZrO2 as an impurity. Si contaminants in ZrO2 shown below. In this scheme too, the cis-form of the generate acidic sites. By treatment of ZrOa with intermediate of the allyl anion is more stable than NaOH to eliminate the acidic sites, the byproducts the trans-form, and the products are mostly in the of 2-olefins are markedly reduced and the selectivity thermodynamically less stable cis-form. for 1-olefins is increased. The ZrO2 treated with Similarly, 2-propenyl ethers undergo double bond NaOH is used for the industrial process for the migration to 1-propenyl ethers.44 The reaction mech- production of vinylcyclohexane. anisms are the same as those for 1-butene and allylamines in the sense that the intermediates are allyl anions and mostly in the cis-form. Among heterogeneous basic catalysts, CaO exhibits the high- est activity, and La203, SrO, and MgO also show high activities. The reaction temperatures required to initiate the reactions are different for each reactant, as shown below. 3-Methoxycyclohexene is unreac- Intramolecular dehydration of monoethanolamine c=c-c-0-c-c c-c=c-0-c-c o'c to ethylenimine has also been industralized by use - of the mixed oxide catalyst composed of Si, alkali metal, and P. The catalyst possesses both weakly acidic and basic sites.50 Because monoethanolamine ooc has two strong functional groups, weak sites are sufficient to interact with the reactant. If either acidic sites or basic sites are strong, the reactant l0O0C interacts too strongly with the sites and forms undesirable byproducts. It is proposed that the acidic and basic sites act cooperatively as shown in Scheme 4. The composition of the catalyst is adjusted to o-loooc control the surface acidic and basic properties. A selectivity of 78.8% for ethylenimine was obtained for the catalyst composed of Si/Cs/P/O in the atomic ratio c-00 - GOO No reaction 1/0.1/0.08/2.25. tive, which is explained as being due to the fact that IV-3. Hydrogenation the adsorbed state is such that the allylic H points Kokes and his co-workers studied the interaction away from the surface, and cannot be abstracted by of olefins with hydrogen on ZnO, and reported het- the basic sites on the surface. erolytic cleavages of H2 and C-H bond^.^^^ The Double bond migration of safrole to isosafrole was negatively charged n-allyl anions are intermediate reported to proceed at 300 K over Na/NaOW&03:41 for propylene hydrogenation. Participation of het- erolytically dissociated H+ and H- in the hydrogena- IV-2. Dehydration and Dehydrogenation tion is generally applicable in base-catalyzed hydro- In general, alcohols undergo dehydration to olefins genation. The observation that MgO pretreated at and ethers over acidic catalysts, and dehydrogenation 1273 K exhibited olefin hydrogenation activities was Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 547

Scheme 4. Intramolecular Dehydration of Scheme 6. Hydrogenation of Carbon Monoxide Monoethanolamine H I I T-7N HObNHz [cp],2-7' y- Ill I Ill -Mg-0-Mg- -HzO P-o-M+--f HCHO +

-Mg-0-Mg-

D+ to form butenes. Since the electron density of the allyl anions is highest on the terminal C atom, the positively charged D+ selectively adds to the terminal C atom to complete 1,4-addition of D atoms to yield Scheme 5. Hydrogenation of 1,a-Butadiene 2-butene. On alkaline earth oxides, the interconversion be- /CH2D / CHzD tween the trans-allyl anion and cis-allyl anion is - ,CH=CH CH2D faster than the addition of D+. As a result, cis-2- butene-& is preferentially formed. On the other it hand, the addition is faster than the interconversion on Zr02,54,55and rare earth trans- 2-butene-& being a main product. A large difference in the reactivity between dienes and monoenes is caused by difficulty of alkyl anion a clear demonstration of heterogeneous base-cata- formation compared to allyl anion formation. Alkyl lyzed hydr~genation.~~The hydrogenation occurring anions are less stable than allyl anions; thus, the on heterogeneous basic catalysts has characteristic reactions of monoenes with H- to form alkyl anions features which distinguish heterogeneous basic cata- require high temperature. lysts from conventional hydrogenation catalysts such as transition metals and transition metal oxides. Feature 3 arose from the location of the active sites. The characteristic features of base-catalyzed hy- Both D+ and D- on one set of active sites are assumed drogenation are as follows. not to migrate to other sites, and each set of active sites is isolated from the others. This happens (1)There is a large difference in the hydrogenation rate between monoenes and conjugated dienes: Con- because the basic hydrogenation catalysts are metal oxides. jugated dienes undergo hydrogenation much faster than monoenes. For example, 1,3-butadiene under- The active sites for hydrogenation on alkaline earth goes hydrogenation at 273 K over alkaline earth oxides are believed to be metal cation-02- ion pairs oxides, while butenes need a reaction temperature of low coordination, as described in the preceding above 473 K. The products of diene hydrogenation section. In the surface model structure of MgO, it is consist exclusively of monoenes, with no alkanes plausible that the Mg2+3c-02-3cpairs act as hydro- being formed at 273 K. genation sites. (2) There is a predominant occurrence of 1,4- Dissociatively adsorbed H+ and H- also hydroge- addition of H atoms in contrast to 1,2-addition which nate CO on MgO, La203, ZrO2, and Th02.57358TPD is commonly observed for conventional hydrogenation study and IR measurement indicate that the reaction catalysts: In 1,3-butadienehydrogenation, 2-butenes proceeds by the following mechanism shown in are preferentially formed over heterogeneous basic Scheme 6. catalysts, while 1-butene is the main product over 1,3-Butadiene undergoes transfer hydrogenation conventional hydrogenation catalysts. with 1,3-cyclohexadieneover La203, CaO, ThO2, and (3) There is retention of the molecular identity of Zr02.59,60 The product distributions are similar to H atoms during reaction: While a hydrogen molecule those for hydrogenation with H2 except for ZrO2, on dissociates on the catalyst surface, two H atoms used which a relatively large amount of 1-butene is for hydrogenation of one reactant molecule originate formed. from one hydrogen molecule. Direct hydrogenation (or reduction) of aromatic Features 1 and 2 are characteristic of hydrogena- carboxylic acids to corresponding aldehydes has been tion in which anionic intermediates are involved.52 industrialized by use of Zr02.61,62 Although the The reaction (Scheme 5)of 1,3-butadiene hydrogena- reaction mechanism is not clear at present, the tion is shown below, where H is replaced by D for hydrogenation and dehydration abilities, which are clarity. The products contain two D atoms at the associated with the basic properties of ZrOz, seem to terminal C atoms if D2 is used instead of H2. be important for promoting the reaction. The cata- Deuterium 1is dissociatively adsorbed to form Df lytic properties are improved by modification with the and D-. 1,3-Butadiene consists of 93% s-trans con- metal ions such as Cr3+and Mn4+ions. Crystalliza- former and 7% s-cis conformer in the gas phase at tion of ZrO2 is suppressed and coke formation is 273 K. At first, D- attacks 1,3-butadiene to form the avoided by addition of the metal ions. allyl anion of the trans form which undergoes either interconversion to form cis allyl anion or addition of AKOOH t Hz - ArCHO + H2O 548 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori Scheme 7. Amination of 1,bButadiene Scheme 8. Hydrogen Transfer from 2-Propanol to Aldehydes and Ketones

(CH3),NCH2-CH=CH-CH3 t Ca2+02' 'Cat. OH 0 IV-4. Amination Scheme 9. Meerwein-Ponndorf-Verley Reduction of Ketones or Aldehydes with 2-Propanol Amines undergo an addition reaction with conju- gated dienes over heterogeneous basic catalyst^.^^ Primary and secondary amines add to conjugated dienes to form unsaturated secondary and tertiary amines, respectively. Amination with monoenes scarcely proceeds over basic catalysts. The reaction mechanisms for amination with conjugated dienes are essentially the same as those for the hydrogena- tion in the sense that heterolytic dissociation of hydrogen (Hz << H+ + H-) and amine (RNH:! << H+ + RNH-) are involved in the reaction. The sequence R R' H3C CH3 R, R' that the anion and H+ successively add to the 1,4 '$I4 'C' F position of conjugated dienes is common to hydroge- OH - b 0d- rid+ nation and amination. As an example, the reaction M+ HM mechanisms for addition of dimethylamine to 1,3- od-- ~ od- 00, / ,/ \ /o, butadiene are shown in Scheme 7. The reaction over ,Si, ,AI, ,Si, CaO takes place at 273 K. nature of the active sites has not been elucidated. The IV-5. Meerwein-Ponndorf-Verley Reduction reaction mechanisms involving the active sites are Meenvein-Ponndorf-Verley reduction is the hy- not clear for the hydrous zirconium oxide catalyzed drogenation in which alcohols are used as a source reactions. In particular, the reason why the hydrous of hydrogen and is one of the hydrogen transfer form of zirconium oxide is more efficient than the reactions. Aldehyde and ketones react with alcohols anhydrous form of zirconium oxide is unclear. to produce corresponding alcohols by Meerwein- Meerwein-Ponndorf-Verley reduction of ketones Ponndorf-Verley reduction: or aldehydes with 2-propanol proceeds over x-zeolites ion-exchanged with Cs+ and Rb+.67The mechanisms 0 proposed for the zeolites are shown in Scheme 9. The R-( tCH3-CH-CH3 - reaction is initiated by abstraction of an H+ from X I R-P+oH.t CH3-fm3 0 2-propanol by the basic sites of the catalyst. In OH X addition to the basic sites, exchanged cations play a role of stabilizing the ketone by the hydride trans- Shibagaki et al. studied Meenvein-Ponndorf- fering from adsorbed 2-propanol to the ketone. Verley reduction of various kinds of aldehydes and ketones in the temperature range 273-473 K.64 IV-6. Dehydrocyclodimerization of Conjugated They found hydrous zirconium oxide as a highly active catalyst for the reactions. 2-Propanol was used Dienes as a hydrogen source. On the basis of the isotope Conjugated dienes such as 1,3-butadiene and effect and kinetic analysis, it was suggested that the 2-methyl-1,S-butadiene (isoprene) react over ZrOz slow step is hydride transfer from the adsorbed and MgO to yield aromatics at 643 K.68,69Heteroge- 2-propanol to the adsorbed carbonyl compound neous basic catalysts other than ZrOz and MgO (Scheme 8). scarcely exhibit appreciable activities. For the for- The reactions of carboxylic acids with alcohols to mation of aromatics from dienes, two kinds of mech- form esters are also catalyzed by hydrous zirconium anisms are possible. One involves the Diels-Alder oxide at a reaction temperature above 523 K.65 reaction followed by double bond migration and Similarly, amidation of carboxylic acids or esters with dehydrogenation. The other involves anionic inter- amines or ammonia proceeds over hydrous zirconium mediates. oxide in the temperature range 423-473 K.66 Over MgO, 1,3-butadiene mainly produces o- and Although hydrous zirconium oxide is proposed to p-xylenes, which will not be formed via the Diels- be an effective catalyst for the above reactions, the Alder reaction. Over ZrOz, the main product from Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 549 Scheme 10. Dehydrocyclodimerization of 1,3-Butadiene c=c Diels-Alder mechanisms e Isomerization Dehydrogenation* (yc-c

Anionic mechanisms

1,3-butadiene is ethylbenzene which will be formed the simple ion-exchanged zeolite^.'^ The high activi- via the Diels-Alder reaction. Two mechanisms for ties are caused by the generation of strong basic sites dehydrocyclodimerization are shown in Scheme 10. by addition of alkali ions which are located in the The mechanisms involving the Diels-Alder reaction zeolite cavities in the form of alkali oxides. take place over ZrO2, and the anionic mechanisms mow&o3 is an efficient catalyst for alkylation take place over MgO. of isopropylbenzenewith olefins such as ethylene and pr~pylene.~~The reaction occurs at 300 K. In this IV-7. Alkylation reaction, too, alkylation occurs selectively at the side In general, alkylation of aromatics occurs at a ring chain. The selective occurrence of the side chain position over an acidic catalyst, while side chain alkylation is due to the anionic mechanisms as alkylation takes place over a basic catalyst. Toluene proposed by Suzukamo et. al. The basic sites of undergoes side chain alkylation with methanol to WKOWAl203 are sufficiently strong to abstract an produce ethylbenzene and styrene over Csf ion- H+ from isopropylbenzene to form an unstable ter- exchanged X-~eolite.~ tiary anion at a low temperature. IV-8. Aldol Addition and Condensation Aldol addition of acetone to form diacetone alcohol is well known to be catalyzed by Ba(OH)2. Alkaline earth oxides, La203, and ZrO2 are also active for the reaction in the following order: BaO > SrO > CaO The first step in this reaction is dehydrogenation > MgO > La203 > With MgO, addition of a of methanol to formaldehyde, which undergoes aldol small amount of water increases the activity, indicat- type reaction with toluene to form styrene. Ethyl- ing that the basic OH- ions either retained on the benzene is formed by hydrogenation of styrene. The surface or formed by dehydration of diacetone alcohol basic sites in the zeolite catalyst participate in both are active sites for aldol addition of acetone. By the the dehydrogenation of methanol and the aldol type tracer experiments in which a mixture containing reaction. equal amount of acetone-& and -dg was allowed to Alkylation of toluene was studied by computer react, the slow step was elucidated to be step B in graphics on the basis of quantum chemical calcula- Scheme ll.74 The calculation also suggests that the high By use of the catalysts possessing both acid and activity results from copresence of acidic and basic base sites, the product diacetone alcohol undergoes sites in a cavity of zeolite. dehydration to mesityl oxide. If hydrogenation abil- The zeolites having alkali ions in excess of their ity is further added to the catalyst, mesityl oxide is ion-exchange capacity exhibit higher activities than hydrogenated to methyl isobutyl ketone (MIBK). 550 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori Scheme 11. Aldol Addition of Acetone Scheme 12. Esterification of Benzaldehyde

Step A CH3-C-CH3tB CH3-C-?H2 +HfB 76% I1 - II 76% 0 0-F-H 0 0 O=C-H + -Ca.O- -t -Ca-O-

(1)

a3 (343 I I Step c CH,-C-CH~-C-O’+ H+B CH,-C-CH,-C-OH + B II I - I1 I 0 a3 0 (343

When the aldol condensation of acetone is per- formed over A-, X-, Y-, or L-zeolites containing alkali metal clusters at 623 K, mesityl oxide__ and isophorone are produced as main products.‘” The ratio of the two products is dependent on the types of zeolites. A-type zrolites favor the formation of the smaller molecule of mesityl oxide. With X- or Y-zeolite, For the aldehydes with a-hydrogen, such as isophorone is preferentially produced. For the smaller benzaldehyde and pivalaldehyde, Tishchenko reac- pore sized L-zeolite, the formation of mesityl oxide tions take place selectively to produce corresponding is about twice as great as that of is~phorone.~~These esters. For the aldehydes having a-hydrogen, Tish- catalysts possess acidic sites in addition to basic sites. chenko reactions compete with aldol condensations. Controlling the acid-base properties and choice of This was typically observed in the reaction of the zeolite pore size results in obtaining each product butyraldehyde. selectively. In self-condensation of butyraldehyde, the dimers Aldol condensation of formaldehyde with methyl resulting from aldol condensation and the trimers propionate to form methyl methacrylate is catalyzed resulting from Tishchenko reaction of the dimer with by X- and Y-zeolites having a basic property. The butyraldehyde were formed by use of alkaline earth highest conversion was obtained with the zeolite ion- oxides as catalysts, as shown in Scheme 13. exchanged with K followed by being impregnated By use of aluminas modified with alkalis as cata- with potassium hydroxide.77 lysts, the reaction was selective for the formation of Hydrotalcite (Mg6A12(OH)~6C03*4H~0)and chryso- dimer by aldol condensation, and the Tishchenko tile (Mg3(OH)4Siz05)act as efficient catalysts for the reaction scarcely occurred. For the aldol condensa- production of methyl vinyl ketone (MVK) through tion, the presence of only basic sites is sufficient, but aldol condensation between acetone and formalde- for the Tishchenko reaction, the presence of both hyde at 673 K.78 Synthetic Co2+ion-exchange chryso- basic sites and acidic sites is required. By modifica- tile, CO,M~~-~(OH)~S~~O~,produces methyl vinyl ke- tion of alumina with alkali ions, basic sites are tone from acetone and methanol. By addition of Co2+, generated and the acidic sites are suppressed. There- dehydrogenation sites are generated. Methanol is fore, only the aldol condensation takes place. On the dehydrogenated to formaldehyde which undergoes other hand, a considerable amount of trimer was aldol condensation with acetone to produce MVK. formed on alkaline earth oxides. It is suggested that not only basic sites but also acidic sites participate - H2 CH3COCH3 in the reaction taking place on alkaline earth oxides. CH30H -HCHO *H~C-~-CHCHZ - HzO 0 IV-10. Michael Addition IV-9. The Tishchenko Reaction Michael additions are conjugate additions of carb- anions and are catalyzed by bases such as sodium The Tischenko reaction is a dimerization of alde- hydroxide, sodium ethoxide, and piperidine. The hydes to form esters. Since the reaction mechanisms reactions have special value since they serve to form are similar to those of the Cannizzaro reaction, the carbon-carbon bonds. However, only limited types Tischenko reaction is through to be a base-catalyzed of heterogeneous catalysts have been applied to reaction. Michael additions. In heterogeneous system, the Benzaldehyde converts to benzylbenzoate over basic sites are responsible for forming the carbanion alkaline earth oxides.79 This reaction proceeds by a by abstraction of an H+ from the molecule having an Tishchenko type reaction as shown in Scheme 12. a-hydrogen. In this reaction, not only basic sites (02-ion) but Partially dehydrogenated Ba(0H)z catalyzes Michael also acidic sites (metal cation) participate. The slow additions of chalcones with active methylene com- step is H- transfer from I to 11. The activities of the pounds such as ethyl malonate, ethyl acetoacetate, alkaline earth oxides were reported to be in the order acetylacetone, nitromethane, and acetophenone.80 MgO < CaO < SrO < BaO, indicating that basic Potassium fluoride supported on alumina (KF- strength is important among alkaline earth oxides. AlzO3) is active for the following Michael additions Tishchenko C3H7-WO

at room temperature: nitromethane with 3-buten- Horner reaction and the Knoevenagel condensation 2-one and 1,3-diphenyl-2-pr0pen-l-one,~~nitroethane as shown below.85 with 3-buten-2-0ne,~~and dimenone with methyl vinyl ketone.83 Dimerization of methyl crotonate proceeds by self- Michael addition to form methyl diesters of (2)-and (E)-2-ethylidene-3-methylglutalicacid (Z-MEG and K-reaction W-H reaction E-MEG).84 A 0 II CH < /C-0-cH3 /c=c\ CH-CH2-C-O-CH3 H I II CH3 0 E-MEG For the Knoevenagel condensation of benzaldehyde CH~-CH=CH-C-O-CH~ II - t with the compounds possessing a methylene group, 0 0 hydrotalcite and alkali ion-exchange zeoliteP and I/ alkali ion-exchange sepiolitesS7act as catalysts. For H 'c=c /C-o-cH3 the zeolites and sepiolites, the aldol condensation /\ CHJ CH-CH~-C-O-CHJ which would occur as a side reaction is suppressed I I1 due to weak basic properties, and Michael addition CHJ 0 producing bulky products is also suppressed due to 2-MEG the small space of the cavities where the basic sites are located. The activities of the zeolite are enhanced Among various types of basic catalysts, only MgO by replacing framework Si by Ge, which causes a exhibits a high activity. The reasons why only MgO change in the basic properties.88 is active are not clear yet, but the other base catalysts such as CaO, La203, ZrOz, KOWAl203, and KF/Al203 IV-12. Synthesis of a@Unsaturated Compound show negligible activities as compared to MgO. The by Use of Methanol reaction is initiated by abstraction of allylic H of The methyl and methylene groups at the a-position methyl crotonate by the basic site to form an allyl of saturated ketones, esters, and nitriles are con- carbanion. The carbanion attacks a second methyl verted to vinyl groups by reactions with methanol crotonate at the /3-position to form the methyl diester over MgO modified by Mn ion or Cr ion.89-93 Ueda of 3-methyl-2-vinylglutalic acid, which undergoes et al. reported that acetonitrile reacts with methanol double bond migration to form the final product. to produce acrylonitrile over Mn-MgO at the reac- tion temperature of 648 K. IV-11. The Wittig-Horner Reaction and Knoevenagel Condensation CH3CN t CH30H -CHz= CHCN + Hz + H2O Aldehydes react with nitriles over basic catalysts Magnesium oxide is required to be modified by such as MgO, ZnO, and Ba(OH)2 to yield the Wittig- addition of transition metal ions to exhibit high 552 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori

Scheme 14. Formation of Acrylonitrile Table 4. Requirements on Zeolites in O/S and O/S Transformations -0-Mg-O-M-O-Mg-O- f-- reaction basic acidic saturated 5-member ring + NH3 - + saturated 5-member ring + H2S + + saturated 6-member ring + NH3 - ++ unsaturated 5-member ring + NH3 + - unsaturated 5-member ring + H2S + - CH3OH 93CN saturated 5-member ring lactone + NH3 + + I I saturated 5-member ring lactone + H2S + - -0-Mg-O-M-O-Mg-O- saturated 6-member ring lactone + NH3 - +

the intermediate methylene anion, which is also adsorbed on the added metal ion. Ueda et al. expanded the reaction of acetonitrile with methanol to the formations of a,P-unsaturated n compounds generally expressed as follows: R-azZ + CH30H R-C-2 t H, + H20 .. II -0-Mg-O-M-O-Mg-O- a,

0 I1 :: 2: -C-R' -C-0-R' -cN Phenyl ( ketones ) ( esters ) ( nitriles )

H, H R: Alkyl, -H IV-I 3. Ring Transformation

\, An oxygen atom in a ring position can be replaced -0-Mg-O-M-O-Mg-O- by N or S with NH3 or H2S over zeolite catalysts for which the acid and base properties are adjusted by ion ex~hange.~~-~~

c) + H2X -0 + H,O 0 X

no0 +H2X -OX X + H20 activity and selectivity for the reaction. The addition X: S,-NH of a metal ion with an ionic radius larger than Mg2+ ion increases the amount of basic sites, while the For the reaction of y-butyrolactone and H2S, the addition of a metal ion with an ionic radius smaller activity order is CsY > RbY > KY =- NaY > LiY, than Mg2+ion induces the surface acid sites without which coincides with the strength of basicity. Hoel- any appreciable change in the amount of surface derich summarizes the relation between acid-base basic sites. The active catalysts are obtained in the properties and the selectivities as given in Table 4.99 latter case. The question as to what properties the catalysts On the basis of isotopic tracer studies for reaction should possess, i.e. basicity or acidity, for O/N and mechanisms, Scheme 14 is elucidated. O/S exchange of heterocyclic compounds cannot be The reaction mechanisms consist essentially of answered definitely. However, there is a tendency dehydrogenation and aldol type condensation. Metha- that increasing the basic properties enhances the nol is dehydrogenated to form formaldehyde, which activity and selectivity for ring transformation of 0 undergoes aldol condensation with acetonitrile. At into S with H2S. The basic sites that exist in the first, methanol is dissociated on the basic site to form zeolite cavities should play an important role for the H+ and CH30-. The methoxy anion (CHSO-) is ring transformation reactions. adsorbed on the added metal ion site because the metal ion is a stronger Lewis acid than Mg2+ ion. IV-I 4. Reactions of Organosilanes Then, an H- is abstracted from the methoxy anion Recently, reactions involving organosilanes have by the metal ion to form formaldehyde. On the other been reported to be catalyzed by heterogeneous basic hand, an H+ is abstracted from acetonitrile to form catalysts. Onaka et al. reported that heterogeneous Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 553 basic catalyst such as MgO, CaO, and hydroxyapatite catalyze cyanosilylations of carbonyl compounds and unsaturated ketones like 2-cyanohexenone with cya- nomethylsilane.lo0J02 Basic sites interact strongly with Si in silanes so that the nucleophilicity of the silanes increases. In the cyanosilylation of unsatur- ated ketones, 1,2-addition products are selectively formed by use of basic catalysts, while 1,4-addition products are obtained by use of acidic catalyst such as ion-exchanged mortmorillonites. For the formation of silane by disproportionation of trimethoxysilane, heterogeneous basic catalysts are used:lo3

4(CH30),SiH SiH, 3(CH30),Si - + Figure 18. Catalytic activities of rare earth oxides for l-butene isomerization (O), 1,3-butadiene hydrogenation High activities were reported for hydrotalcite and (O), and aldol addition of acetone (A). alumina-supported fluorides such as KF/Al203. 18 for l-butene isomerization, 1,3-butadiene hydro- V. Characteristic Features of Heterogeneous genation, and acetone aldol condensation.lo4 The Basic Catalysts of Different Types activity sequence is the same for l-butene isomer- ization and 1,3-butadiene hydrogenation, which is The catalytic properties of heterogeneous basic different from that of aldol condensation. For the catalysts are closely associated with the amount and isomerization and the hydrogenation, the oxides of strength of the basic sites existing on the surfaces. sesquioxide stoichiometry show activity while the However, the amount and strength of the basic sites oxides with metal cations of higher oxidation states are not whole measures to determine the catalytic are entirely inactive. The situation is different in properties. The other factors to be taken into account acetone aldol condensation. The oxides with high are not clear at present. It appears that there are oxidation state, CeO2, Tb407, and Pr6011, show con- characteristic features commonly observed for a siderable activity. The oxides with metal cations of certain type of heterogeneous basic catalysts. In this oxidation state higher than 3 possess weak basic sites section, catalytic features of different types of het- which are sufficient to catalyze the aldol condensa- erogeneous catalysts are described. tion but not strong enough to catalyze hydrogenation and isomerization. V-1. Single Component Metal Oxides As described in the preceding section, rare earth Alkaline earth oxides such as MgO, CaO, SrO, and oxides show characteristic selectivity in dehydration BaO are most extensively studied. They possess of alcohols. 2-Alcohols undergo dehydration to form strong basic sites. The order of the basic strength is l-olefins. The formation of thermodynamically un- BaO > SrO > CaO > MgO. As described in the stable l-olefins contrasts with the formation of stable earlier section, the surfaces are covered with C02 and 2-olefins in the dehydration over acidic catalysts. The H2O before pretreatment. To be active catalysts, they selectivity is the same as that observed for ZrO2. need pretreatment at a high temperature to remove Zirconium oxide is a unique heterogeneous basic adsorbed C02 and H2O. In addition, the active sites catalyst in the sense that two industrial processes are easily poisoned by even small amounts of impuri- have been established recently that use ZrOz as a ties like C02 and H2O contained in the reactants. To catalyst. One is reduction of aromatic carboxylic obtain full capabilities of alkaline earth oxides, the acids with hydrogen to produce aldehydes.61 The reaction system should be kept free of the impurities, other is dehydration of l-cyclohexylethanol to vinyl- which makes the industrial uses of the alkaline earth cy~lohexane.~~In addition, the production of diiso- oxides difficult, especially at low reaction tempera- butyl ketone from isobutyraldehyde has been indus- tures. At high reaction temperatures, the poisoning trialized for more than 20 years.lo5 The reaction effects are reduced, and certain alkaline earth oxides scheme for the production of diisobutyl ketone in show catalytic activities for the reactions from which which the Tishchenko reaction is involved is shown poisons like H20 are liberated. in Scheme 15. One of the features of alkaline earth oxides is a One of the difficulties of most of the heterogeneous high ability to abstract an H+ from an allylic position. basic catalysts for industrial uses arises from rapid This feature is revealed in the double bond migration poisoning by CO2 and HzO. This is not the case with of olefinic compounds. Butene, for instance, under- ZrO-2. Zirconium oxide retains its activity in the goes double bond migration even at 223 K. presence of water, which is one of the products for Rare earth oxides have been studied to a lesser the reactions of carboxylic acid and l-cyclohexyl- extent as compared to alkaline earth oxides. The ethanol. reactions for which basic sites of rare earth oxides The catalytic features of ZrO2 are understood in are relevant are hydrogenation of olefins, double bond terms of bifunctional acid-base propertie~.~~J~~J~' migration of olefins, aldol condensation of ketones, Although the strengths of the basic and acid sites are and dehydration of alcohols. The activity sequences low, a cooperative effect makes ZrO2 function as an of a series of rare earth oxides are shown in Figure efficient catalyst. Because of weakly acidic and basic 554 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori Scheme 15. Formation of Ketone from Aldehyde

2RCH20H Tishchenko 1 -4H2, reaction 4RCH20H 4RCHO -2RCH200CR

2RCOOH I RCOR t'C0, t H20

Table 5. Activities of Ion-Exchanged and Ion-Added sites, the active sites are not poisoned by C02 and Zeolites for l-Butene Isomerization water. Zirconium oxide shows not only basic properties reaction rate/mmol g-I min-l but also acidic properties, depending on the reac- catalysta 273 K 423 K tant.lo6 The acid-base bifunctionality of ZrO2 is NaX E 0 0 clearly revealed in the reaction of alkylamine to NaX A 0 1.1 x 10-2 nitrile.lo8 The conversion of secondary amines and KXE 0 0 tertiary amines to nitriles requires both acidic and KXA 2.4 x 7.8 x RbX E 0 0 basic sites as shown below. RbX A 3.2 x 1.3 CsX E 8.6 x 1.3 x lo-' CsX A 1.4 x lo-' 1.1

a E, ion-exchanged; A, ion-added.

base\ Preparation of fine particles of alkali oxides inside H2 C2H4 CH3CN t the cavities of zeolites was developed by Hathaway and They impregnate NaY zeolite with cesium acetate aqueous solution and calcine at 723 By use of acidic catalyst like Si02-Al203, ethylene and ammonia are formed. Over ZrOa, dehydrogena- K to decompose cesium acetate into cesium oxide tion to produce nitrile occurs in preference to the placed in the cavities. The resulting zeolite possesses formation of ethylene and ammonia. basic sites stronger than those of simple ion-ex- Although ZrO2 shows interesting catalytic proper- changed zeolite. ties, the structure of the active sites is still unclear. Tsuji et al. prepared the zeolites containing a series Clarification of the active sites is desired. of alkali metal ions in excess of the ion-exchanged capacities and compared their catalytic activities and V-2. Zeolites basic sites with simply ion-exchanged zeolites. TPD plots of adsorbed carbon dioxide are shown in Figure The characteristic features of zeolites result from 9. The TPD peaks appear at higher temperatures their ion-exchange ability and specific pore structure. for "ion-added" zeolites than for ion-exchanged zeo- The acid-base properties are controlled by selecting lites, indicating generation of new basic sites that are the types of ion-exchanged cations and by the SUN stronger than sites of ion-exchanged zeolites. ratio of the zeolite framework. Wide variation of The results of the catalytic activities of the ion- acid-base properties can be achieved by ion-ex- added zeolites and the ion-exchanged zeolites for change and ion-addition, while relatively small change l-butene isomerization are summarized in Table in acid-base properties is yielded by changing the 5.l7Jlo Except for CsX, ion-exchanged zeolites did not SUN ratio. exhibit any activities at 273 K and 423 K. The ion- To prepare basic zeolites, two approaches have added zeolites showed considerable activities, and the been undertaken. One approach is to ion-exchange order of the activities for different alkali ions was Cs with alkali metal ions, and the other is to impregnate > Rb > K > Na. the zeolite pores with fine particles that can act as Zeolites often collapse during preparation proce- bases themselves. The former produces relatively dures. Yagi et al. prepared Cs ion-added zeolites to weak basic sites, while the latter results in the strong establish the preparative conditions to retain the basic sites. zeolite framework during preparative procedures.lll With alkali ion-exchanged zeolites, the type of It was found that the crystalline structures of zeo- alkalis used affects the basic strength of the resulting lites, in particular alkali ion-added zeolites, are easily zeolites. Effects of the alkali ions on basic strength destroyed by exposure to water vapor at high tem- are in the following order: Cs+ > Rb+ > K+ > Na+ > peratures and that zeolites of high SUM ratio are Li+. the basic sites are framework oxygen. The unstable to alkali treatment. bonding of the framework oxygen is rather covalent Besides alkali metal oxides, the fine particles of in nature. This causes the basic sites of ion- MgO were placed in the zeolite cavities.l1° The exchanged zeolites to be relatively weak as compared resulting zeolites also showed strong basic properties, to, for example, those of alkaline earth oxides. though the basic sites on the fine particles of MgO Heterogeneous Basic Catalysis Chemical Reviews, 1995, Vol. 95, No. 3 555 are not as strong as those of bulk MgO. The ionicity of the Mg-0 bond is reduced for a fine particle of 100 MgO as compared to bulk MgO, and therefore, the basic strength of the 02-ion is reduced. The depen- dence of the particle size on the strength of basic site was studied for ultrafine MgO particles by Itoh et a1.112 It was also concluded that smaller particles exhibit weaker basicity. One of the important objects for preparation of basic zeolites is to realize the shape selectivity in base-catalyzed reactions. Corma et al. reported the shape selectivity of alkali ion-exchanged zeolites in the reaction of benzaldehyde with ethyl cyano- 0' acetate.l13 Lasperas et al. prepared zeolite contain- 300 400 500 600 700 800 900 1000 evacuation temperature/K ing cesium oxide in the cavities by the "postsynthetic method", which is similar to the methods by Hartha- Figure 19. Fraction of Yb3+ (0)and Yb2' (0)plotted way and Davis72and Tsuji et al.17J10 The reaction against evacuation temperature and the catalytic activities of Yb/Na-Y for (- -) l-butene isomerization, (- -) ethylene of benzaldehyde with ethyl cyanoacetate proceeded hydrogenation, and (0)Michael reaction of cyclopent-2- as shown below.l14J15 enone with dimethyl malonate. l-Butene isomerization was carried out at 273 K over Yb/NaY. Ethylene hydro- CN genation was carried out at 273 K over Yb/LY. Michael Ph\ Ph\ I P + HZO reaction was carried out at 323 K over YdNaY. (Reprinted c=o + p - ,c=c\ from 133. Copyright 1993 Chemical Society of London.) H COOEt H COOEt Knoevenagel condensation with benzaldehyde,124alkylations at C, 0, N, and S with aldehydes and dimethyl s~lfate,~~~J~~-~~~and Ph\ ,CN CN Ph, ,CN disproportionation of alky1~ilanes.l~~ ,c=c, +p'- "-cy In contrast to many applications to organic syn- NC-a COOEt theses as a base catalyst, KF/Al203 has not been H COOEt COOEt I COOEt studied extensively for the surface properties, and the structures of basic sites have not been clarified yet. Michael addition At the beginning, the basic sites were considered to be F- ions dispersed on the alumina support. Insuf- Knoevenagel condensation proceeded, but the product ficient coordination only with surface OH groups may did not undergo the following Michael addition result in the formation of active F- ions. This was because of the limited space in the zeolite cavities. supported by 19FMASNMR.128-130 Tsuji et al. reported the shape selectivity of the On the other hand, it was proposed on the basis of zeolite containing Mg0.llo Nonsupported MgO cata- IR and XRD studies that the basic sites originate lyzes double bond migrations of both l-butene and from KOH and/or aluminate produced by the follow- allylbenzene, while the zeolite containing MgO in the ing reacti~ns:~~~J~~ cavities catalyzes the former but fails to catalyze the latter. i-M203 3H20 2K3MF6 6KOH The studies of basic zeolites, in particular, those 12KF + - + of strongly basic zeolites have started quite recently. 6KF 2M20, 3K3MF6 3m02 To reveal the potential of basic zeolites, establish- + - + ment of preparative methods, identification of basic sites, and application of the basic zeolites to a wide Taking account of the above results and the results variety of the base-catalyzed reactions are required. of titrating the water soluble base on the surface together with the results of IR study, thermogravim- V-3. Basic Catalysts of the Non-Oxide Type etry, and SEM, Ando et al. concluded that there are three basic species or mechanisms of the appearance Most of heterogeneous basic catalysts are in the of the basicity on the surface of KF/A1203.130J32These form of oxides. The basic sites are 02-ions with are (i) well-dispersed and incompletely coordinated different environments depending on their type. If F- ions, (ii) [Al-0-1 ions which generate OH- ions the basic sites are constituted by elements other than when water is present, and (iii) cooperation of F- and 02-,the catalysts are expected to show catalytic [Al-OH] which can behave as an in situ-generated properties different than those of the catalysts of the base during the course of the reaction. oxide form. For the other catalysts of the non-oxide type, Baba Potassium fluoride supported on alumina (KF/ et al. prepared low-valent lanthanide species intro- Al203) was introduced by Clark116and by Ando and duced into zeolite ~avities.l~~J~~They impregnated Yama~akill~J~~as a fluorinating reagent and a base K-Y with Yb and Eu dissolved into liquid ammonia catalyst. As a base catalyst, KF/Al203 has been followed by thermal activation. The variations of the applied to a number of organic reactions. The reac- catalytic activities of the YbK-Y catalyst as a tions for which KF/Al203 acts as a catalyst include function of the thermal activation temperature are Michael Wittig-Honner reac- shown in Figure 19 for l-butene isomerization, eth- tions,121J22Knoevenagel condensations,121J22Darzen ylene hydrogenation, and Michael addition of cyclo- condensations,81J21condensation of phenyl acetylene penten-2-one with dimethyl ma10nate.l~~ 556 Chemical Reviews, 1995, Vol. 95, No. 3 Hattori The chemical states of Yb were studied by TPD, be due to the enhancement of basic strength caused IR, XAFS, and XPS as a function of evacuation by the addition of Na to MgO. temperature. The states of Yb changed from Yb(I1, 111) amides, Yb(I1, 111) imides, to Yb(II1) nitride as follows: VI. Concluding Remarks

Yb(NH2)3 YbNH NH3 '/2N2 H2 Heterogeneous basic catalysts have been investi- - + + + gated for almost 40 years during which a number of Yb(NH,), YbNH NH3 reactions have been found to proceed on the basic - + catalysts. Nevertheless, the reactions for which and heterogeneous basic catalysts have been used are only a part of a great number of organic reactions. 2YbNH 2YbN H2 Use of heterogeneous basic catalysts in organic - + syntheses has been increasing in recent years. There As for the catalytically active sites, it was con- should be many reactions which heterogeneous basic cluded that the Yb(I1) imide species catalyze 1-butene catalysts can efficiently promote, but have not been isomerization and the Michael addition and that the used for. One reason for the limited use of hetero- Yb(II1) nitride species catalyzes ethylene hydrogena- geneous basic catalysts arises from a rapid deactiva- tion. tion while being handled under the atmosphere; the In the above reactions, characteristic features catalysts should be pretreated at high temperatures which distinguish the non-oxide catalysts from the and handled in the absence of air prior to use for the metal oxides are not obvious. However, it is expected reaction. If this care is taken, heterogeneous cata- that the features will become apparent for certain lysts should promote a great number of reactions. base-catalyzed reactions if the applications of the It was found that some of the reactions specifically non-oxide catalysts to various kinds of reactions are proceed on the heterogeneous basic catalysts. The expanded. catalytic actions of heterogeneous basic catalysts are not simple copies of those of homogeneous basic V-4. Heterogeneous Superbasic Catalysts catalysts, though it is not clearly understood where the features of heterogeneous basic catalysts origi- To activate a reactant under mild conditions, a nate from. To clarify this point, characterizations of catalyst possessing very strong basic sites is desired the surface sites together with elucidation of the to be prepared. There have been some attempts to reaction mechanisms occurring on the surfaces should prepare those superbasic catalysts. be extended. Suzukamo et al. prepared a superbasic catalyst by Although the theoretical calculations of the surface addition of alkali hydroxides to alumina followed by sites and the reaction mechanisms are not described further addition of alkali metals.41 To a calcined in this article, there have been efforts on these alumina, sodium hydroxide was added at 583-593 p~ints.l~~-'~lThe results of the quantum chemical K with stirring under a nitrogen stream. In 3 h, calculations explain well the experimental results, sodium metal was added and the mixture was stirred and give us valuable information about the hetero- for another 1 h at the same temperature to give a geneous basic catalysis. Unfortunately, the theoreti- pale blue solid. The resulting catalyst possesses basic cal calculations have been done only for the MgO sites stronger than H- = 37 and catalyzes various catalyst. An attempt to calculate the other catalyst base-catalyzed reactions such as double bond migra- systems is highly desirable. tions of 5-vinylbicyclo[2.2.1lhept-2-eneto 5-ethyli- The methods of preparing heterogeneous catalysts denebicyclo[2.2.llhept-2-ene at the reaction temper- and the characterizations of the surfaces have been ature 243-373 K, 2,3-dimethylbut-l-ene to 2,3-di- developed. 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