Applied Catalysis A: General 254 (2003) 327–338 Organized mesoporous alumina: synthesis, structure and potential in catalysis Jiˇi Cejka∗ r ˇ J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague 8, Czech Republic Received 19 December 2002; received in revised form 4 March 2003; accepted 10 March 2003 Abstract Organized mesoporous alumina represents a very interesting molecular sieve exhibiting a narrow pore size distribution with higher surface areas compared to conventional aluminas, used as a support for catalytically active species in numerous large-scale industrial processes. This review encompasses various synthesis approaches to organized mesoporous aluminas, description of their structures and properties, and characterization by various experimental techniques. The potential of the mesoporous alumina with respect to use in catalysis is also outlined. Surface areas up to 800 m2 /g and pore sizes ranging from 2.0 to more than 10 nm are characteristic for organized mesoporous aluminas prepared by neutral, anionic and cationic synthesis routes. Although utilization of mesoporous aluminas as a support in catalysis has not been reported frequently, they have a certain potential in hydrodesulfurization and metathesis reactions. © 2003 Elsevier B.V. All rights reserved. Keywords: Organized mesoporous alumina; Mesoporous molecular sieves; Synthesis; Characterization; Catalytic applications 1. Introduction all-silica or aluminosilicate mesoporous molecular sieves were synthesized with well-deﬁned pore sizes The ﬁrst successful synthesis of mesoporous ranging from 1.5 to 10 nm and having surface ar- molecular sieves opened a new era in the investiga- eas greater than 1000 m2 /g. These molecular sieves tion of inorganic molecular sieves all over the world have been synthesized using a new approach where, [1,2]. This discovery revealed exciting possibilities instead of employing single molecules as templates for new types of molecular sieves with signiﬁcantly (as for the synthesis of microporous zeolite and zeo- larger pores than zeolites and narrow pore size dis- type materials), self-assembled molecular aggregates tribution, with applications not only in catalysis but or supramolecular assemblies were employed as also in other areas of chemistry. The invention of structure-directing agents. Since then, numerous pa- mesoporous molecular sieves by the researchers at pers and reviews have appeared covering many aspects the Mobil Research and Development Corporation, of the synthesis, structural characterization and appli- enabled escape from the “1 nm prison” imposed by cation of mesoporous molecular sieves, e.g. [3–7]. zeolite-based microporous molecular sieves. Initially, While incorporation of aluminum into mesoporous silica materials, providing for the catalytic function- ∗ Tel.: +420-2-66053795; fax: +420-2-86582307. ing of silica materials, was reported immediately af- ˇ E-mail address: email@example.com (J. Cejka). ter the invention of this new type of material, the ﬁrst 0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00478-2 328 ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 successful synthesis of mesoporous aluminum oxide the synthesis of mesoporous aluminas. The authors (alumina) was published several years later. Many re- reported that these synthesized mesoporous aluminas search groups paid considerable attention to silica or did not contain “zeolitic” micropores. Dodecyl sulfate aluminosilicate materials, which are rather stable in surfactant, used by Yada et al. [17,18], led to the syn- various respects  and seemed to be more promis- thesis of mesostructures with lamellar and hexagonal ing for modiﬁcation to incorporate the catalytically arrangement, which, however, exhibited low stabil- active phases. This was also probably connected to ity after calcination. Cabrera et al. described another the fact that synthesis of mesoporous aluminas rep- approach, similar to that one to synthesize MCM-41 resents a much more complex problem compared to . Mesoporous alumina with continuously ad- the synthesis of mesoporous silicas. Thus, an alter- justable pore size has been synthesized by carefully native but indirect approach for preparation of meso- changing the synthesis parameters. Further synthe- porous alumina-like material was reported by Landau sis routes included the use of tartaric acid  or et al. . The procedure was based on the synthesis “atrane” complexes as structure-directing agents . of siliceous MCM-41, which was covered by grafted The main objective of this contribution is to describe alumina to form a monolayer in the second step. the state of the art of the synthesis and potential of Alumina is a very interesting material with broad mesoporous aluminas and to stimulate further research applicability as a support for various catalytically in this area. Various synthetic procedures for produc- active phases, which are employed in industry in a tion of organized mesoporous aluminas described to high number of large-scale technological processes date, the means of characterization of these molecu- [10,11]. Usually, conventional aluminas with surface lar sieves, their main properties and also the potential areas of 50–300 m2 /g are manufactured by precip- for use of mesoporous aluminas for catalysis are ad- itation . As alumina represents very important dressed in this contribution. support, particular attention was devoted in the liter- ature to the description of the properties of alumina materials . Various transition aluminas ( , , , 2. Synthesis of organized mesoporous aluminas and ) were prepared by heat treatment of different aluminum oxide-hydroxide precursors (e.g. boehmite, For the synthesis of organized mesoporous alu- pseudoboehmite, bayerite, nordstrandite) and their mina, it was necessary to modify and optimize the structures and transformations were reported [13,14]. procedures, that were well-described and understood The aluminas usually exhibit surface areas lower than for the synthesis of mesoporous silicas. While the 350–400 m2 /g and their main disadvantage is in their original synthesis of the family of M41S mesoporous broad pore size distribution, sometimes even with molecular sieves was based on electrostatic interaction more than one maximum in the 3–15 nm range. For between a cationic surfactant (hexadecyltrimethyl am- these reasons, the successful synthesis of organized monium salts or hydroxide) and a negatively charged mesoporous aluminas with surface areas exceeding inorganic precursor [1,2], further syntheses have been 500 m2 /g and having narrow pore size distribution extended to charge-reversed and counter-mediated appeared very challenging both from material and pathways [22–24]. In addition, Pinnavaia and cowork- application points of view. ers [25,26] demonstrated that mesoporous molecular For the synthesis of organized mesoporous alumi- sieves can be successfully synthesized from neu- nas, it was necessary to modify and optimize proce- tral inorganic precursors and neutral alkyl amines or dures that have been well described and understood non-ionic polyethylene (polypropylene) oxides. All for the synthesis of mesoporous silicas. Bagshaw these approaches were also tested for the synthesis of et al.  Bagshaw and Pinnavaia  have shown mesoporous aluminas. that non-ionic templating can provide a potential A “neutral” synthesis pathway employing electri- pathway for the synthesis of mesoporous aluminas by cally neutral polyethylene oxide surfactants and alu- hydrolysis of aluminum alkoxides. Vaudry et al. used minum alkoxide as an inorganic precursor was used also the same source of aluminum ; however, by the group of Pinnavaia and coworkers [3,15]. The long chain carboxylic acids were employed to direct mesoporous alumina formed exhibited a wormhole ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 329 channel motif and surface areas of up to 500 m2 /g. size diameter can be tuned through the volume of the Signiﬁcant differences in the pore size distribution surfactant head (Triton X-114, Tergitol 15-S-9, Ter- of these mesoporous molecular sieves were found to gitol 15-S-15, cf. Table 1). The authors also claimed depend on the evaluation procedure. The pore sizes that the pore size diameter can be further tuned by obtained according to the Horvath–Kawazoe model addition of di-propylamine . The extension of the  were substantially overestimated (between 4 block copolymer templating approach was described and 8 nm), while the pore diameters obtained from by Yang et al. , showing that not only alumina but Barrett–Joyner–Hallender (BJH) model  provided also other oxides (TiO2 , ZrO2 , Nb2 O5 , Ta2 O5 , SnO2 , values in favorable agreement with the TEM images WO3 and various mixed oxides) can be synthesized (2.5–4.5 nm). Various templates, namely Tergitols, with Pluronic P123 copolymer (PEO20 PPO70 PEO20 ), Tritons and Pluronics, were used. A relationship has where PPO represents the poly(propylene oxide) unit, been found between the size of poly(ethylene ox- exhibiting pore size diameters up to 14 nm. ide) (PEO) units in Tergitols and the resulting pore An “anionic” route for the synthesis of meso- sizes . Addition of small amounts of Ce3+ or porous aluminas was described by Vaudry et al. . La3+ cations to the reaction mixture signiﬁcantly Caproic, lauric and stearic acids were employed as increased the thermal stability of mesoporous alu- structure-directing agents and the syntheses were per- minas prepared via the “neutral” synthesis pathway formed in alcohols, formamide, chloroform or diethyl . This ﬁnding conﬁrms that the incorporation ether. Aluminum alkoxides were used as a source of of rare earths into transition aluminas stabilizes the aluminum. The surface areas of the calcined aluminas metastable phases against sintering and further struc- were in the range between 500 and 700 m2 /g with a tural transformations. The neutral synthesis route for narrow pore size distribution centered around 2.0 nm. mesoporous aluminas using triblock copolymers was ˇ In contrast to these results, Cejka et al. [35,36] have also conﬁrmed by Luo et al.  and Deng et al. shown that the pore size diameter of mesoporous alu- . Recently, González-Pena et al.  have shown minas prepared with stearic acid is larger compared that 1,4-dioxane can be effectively used as a medium to that of the shorter lauric acid. The addition of do- for the synthesis of mesoporous aluminas via the decane, to ensure expansion of the micelles formed, “neutral” pathway. In contrast to alcohols, the pore led to an increase in the mean pore size diameter; Table 1 List of surfactants and textural parameters of organized mesoporous aluminas synthesized according to different route in different laboratories Template Surfactant formulae Pore size (BJH) (nm) Surface area (m2 /g) Reference Tergitol 15-S-9 C11–15 [PEO]9 3.3 490  Tergitol 15-S-12 C11–15 [PEO]12 3.5 425  Triton X-114 C8 Ph[PEO]8 3.6 445  Pluronic 64L [PEO]13 [PPO]30 [PEO]13 2.4 430  Pluronic P123 [PEO]20 [PPO]69 [PEO]20 10.3 487  Pluronic 64L [PEO]13 [PPO]30 [PEO]13 8.7 470  Triton X-114 C8 Ph[PEO]8 3.5 490  Tergitol 15-S-9 C11–15 [PEO]9 4.0 525  Tergitol 15-S-15 C11–15 [PEO]15 4.5 510  Caproic acid C5 COOH 2.1 530  Lauric acid C11 COOH 1.9 710  Stearic acid C17 COOH 2.1 700  Lauric acid C11 COOH 3.3 489  Stearic acid C17 COOH 3.7 758  Dibenzoyl-l-tartaric acid [C6 H5 CO2 CH(CO2 H)–]2 3.3 386  Dibenzoyl-l-tartaric acid [C6 H5 CO2 CH(CO2 H)–]2 4.2 425  CTMABr C16 N(CH3 )4 Br 3.3 340  CTMABr C16 N(CH3 )4 Br 6.0 250  330 ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 however, the resulting pore size distribution was only example of successful removal of dodecyl sulfate signiﬁcantly broader. was reported by Valange et al. . After calcination The controlled synthesis of aluminum-based sur- in a stream of nitrogen at 450 ◦ C, the resulting surface factant mesophases by the homogeneous precipitation area of mesoporous alumina was about 450 m2 /g with method using urea and sodium dodecyl sulfate was a pore diameter of 3.4 nm. Thus, for the ﬁrst time, it described by Yada et al. [17,18,37]. It was suggested was shown that mesoporous alumina can be prepared that dodecyl sulfate surfactant initially forms a lay- in aqueous media. The pore characteristics were con- ered mesophase with an interlayer spacing depending trolled by the nature of the soluble aluminum precursor on the amount and type of surfactant. The surfactants (aluminum nitrate or Keggin type of Al13 polycationic form a layer, which links the sheets of aluminum oxy- species) and by the nature of the surfactant micelle hydroxide species. As a result of further hydrolysis as structure-directing agent (hexadecyl trimethyl am- of urea, the layered mesophase is transformed into a monium bromide, sodium dodecyl sulfate, long-chain hexagonal form through interlayer condensation and carboxylic acids, Triton X or their combinations) . cross-linking of the Al–OH groups in any adjacent Recent results report the investigation of phase be- aluminate sheets . The proper method of removing havior of the system: sodium dodecyl sulfate–alumi- dodecyl sulfate from the alumina mesophase seems to num nitrate–water, using NMR, small angle X-ray be the crucial point in the preparation of mesoporous scattering and optical microscopy . Since the alumina by this method. Thermal decomposition of valency of counterions inﬂuences dramatically the dodecyl sulfate from as-synthesized alumina, prepared electrostatic interactions, replacement of sodium with by Yada’s method, studied by sample-controlled ther- aluminum as predominant species leads to the for- mal analysis, was reported by Sicard et al. . The mation of the isotropic solution and hexagonal and alkyl chain of the surfactant was completely removed lamellar liquid crystalline phases. It was found that below 200 ◦ C, while the sulfate groups were removed the isotropic solution can uptake relatively small at temperature between 400 and 550 ◦ C. These results amount of aluminum nitrate and then coagulation indicate that there is a strong interaction between of the micelles occurs. The large coagulate is com- the sulfate head groups and the alumina framework. pletely re-dispersed by further addition of the nitrate Study of the mechanism of mesostructured hexagonal into a new isotropic solution consisting of very large alumina by ﬂuorescence probing techniques [39,40] thread-like micelles. While hexagonal phase ﬁlls a revealed that the polymerization process was induced narrow region, lamellar phase was found in the di- by the hydroxyl ions formed by decomposition of lute part region at very large aluminum content. It urea, and occurred at the surface of the dodecyl sulfate should be mentioned that the precursor system used micelles. The results indicate that the organization of by Yada et al. [17,18] for the synthesis of mesostruc- the complex, formed by the growing alumina polymer tured alumina is located in the re-dissolution area and dodecyl sulfate micelles, occurs just before the of this ternary system diagram. The structure of the precipitation of the solid material. While Yada et al. aggregates in this region was investigated in detail by [17,18] did not investigate the calcination of these time-resolved ﬂuorescence techniques . The ﬂuo- mesophases and, therefore, did not report the quality rescence quenching data have shown that in the entire of the calcined aluminas, Sicard et al.  showed that re-dissolution area the micelles are very long, being alumina mesophases are transformed during a calci- considered inﬁnite on the time scale of the experiment, nation procedure into microporous aluminas with pore and therefore aggregation numbers could be evaluated. diameter between 1.5 and 2.0 nm. Therefore, it can be The “cationic” route for synthesis of mesoporous inferred that the mesophase structure (despite hexag- alumina was described by Cabrera et al.  using onal or lamellar) is stabilized by the strong interaction hexadecyl trimethyl ammonium bromide in combi- between the alumina rods or sheets and the removal of nation with triethanol amine in water. The authors sulfate head groups leads to the structural collapse of claimed that varying the ratio among surfactant, wa- the alumina mesophase. On the other hand, it has been ter and triethanol amine (the “atrane” route) made shown that mesoporous alumina or gallium oxide can it possible to adjust the pore size between 3.3 and be stabilized by an addition of yttrium [41–43]. The 6.0 nm. This synthesis approach was extended to other ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 331 mesoporous oxide systems . This procedure would precursors, can be hydrolyzed in a controlled way to be extremely useful for tailoring the pore size diame- start the synthesis. In addition, surfactant removal is ter; however, it seems not to be readily reproducible. signiﬁcantly easier compared to that for the anionic Dibenzoyl-l-tartaric acid and aluminum sec- route. In addition, the possibility to change the length butoxide were used by Liu et al. . Depending of the individual oxide parts of block copolymers can on the concentration of surfactant, the surface area be used to tailor the pore size diameter. Practically, was varied between 380 and 430 m2 /g and the pore all synthesis approaches to mesoporous aluminas de- size increased from 3.8 to 5.0 nm with increasing scribed in this section provide relatively well-deﬁned concentration of the surfactant. mesoporous materials sometimes even with a different degree of microporosity. In contrast, sol–gel methods, 2.1. Washing which were also used to prepare mesoporous alumi- nas, demonstrate the formation of substantially less After each synthesis, the solid material should be deﬁned materials with lower surface areas and pore recovered by ﬁltration and washing. It was observed size volumes, e.g. . that washing in distilled water led to the collapse of the mesoporous structure and, thus, ethanol or propanols are recommended. Structural collapse caused by the 3. Characterization of the structure of organized presence of water was also observed after calcination mesoporous aluminas or extraction. Therefore, it is suggested that all the post-synthesis treatments and modiﬁcations should be In a similar way to mesoporous silicas and alu- carried out in non-aqueous solutions. minosilicates, X-ray powder diffraction, sorption isotherms of nitrogen and transmission electron mi- 2.2. Removal of surfactants croscopy were mainly employed to characterize orga- nized mesoporous aluminas. Surfactant removal represents a critical step in the preparation of mesoporous aluminas and is probably 3.1. X-ray powder diffraction even more important than the synthesis itself. The proper removal of the surfactant is highly dependent In the case of X-ray diffraction, it is necessary to on the type of surfactant. While non-ionic surfactants measure the diffraction lines at low values of 2θ and it like polyethylene oxide–polypropylene oxide types of is recommended to use chromium as an X-ray source block copolymers can be easily removed during cal- due to its longer wavelength compared to commonly cination in air or oxygen at relatively low tempera- employed copper or cobalt. Fig. 1 depicts the charac- tures, even under static conditions or via extraction teristic X-ray powder diffraction pattern of organized in alcohols, it is far more complicated to remove car- mesoporous aluminas synthesized with stearic acid boxylic acids. We have optimized this calcination pro- (Fig. 1A) and triblock copolymer Pluronic PE10400 cedure as follows: calcination in a stream of nitrogen (BASF) (Fig. 1B). It is clearly seen that the diffrac- (60 ml/min) at 410–420 ◦ C for 2 h with a temperature togram usually consists of only one broader diffrac- ramp of 1 ◦ C/min, followed by further calcination in tion line without further reﬂections. This is in contrast air (oxygen) at 430–450 ◦ C for at least 6–8 h . to, e.g. MCM-41 or MCM-48 mesoporous molecular Small differences in calcination procedures can result sieves, where also further diffraction lines observed in major differences in the ﬁnal product. indicate hexagonal or cubic symmetry, respectively. All the syntheses (with one exception ) of In some cases, one or two additional diffraction lines organized mesoporous aluminas were performed in with a low intensity can be found in the as-synthesized non-aqueous media (preferable alcohols) and only samples . However, these lines are missing after a small amount of water was added to control the calcination. Calcination leads to a signiﬁcant increase hydrolysis of aluminum alkoxides. It seems that the in the intensity of the main diffraction lines due to re- neutral synthesis route has some advantages over elec- moval of the organic phase located inside the channels, trostatic pathways as aluminum alkoxides, as neutral exhibiting lower adsorption and scattering properties. 332 ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 80000 70000 (A) 60000 50000 Intensity 40000 (B) 30000 20000 10000 0 0 1 2 3 4 5 6 7 8 9 10 2 theta (degree) Fig. 1. X-ray powder diffraction patterns of organized mesoporous aluminas prepared with stearic acid (A) and Pluronic PE10400 (B) as structure-directing agents, respectively. This intensity increase is accompanied by a shift of the description of this comparative analysis of adsorption maximum of this line to lower angle values. As will isotherms can be found, e.g. in [48–50]. With orga- be shown later, this shift depends on the calcination nized mesoporous aluminas, it has been proven that temperature. The higher the calcination temperature, Degussa Aluminiumoxid C is suitable for the compar- the larger shift of the maximum to low 2θ values . ative analysis of nitrogen adsorption isotherms . Due to the narrow pore size distribution, the typical 3.2. Nitrogen sorption isotherms nitrogen adsorption isotherms recorded on organized mesoporous aluminas are characterized by a rela- Due to limited information on mesoporous aluminas tively steep increase in the adsorbed amount starting (or mesoporous molecular sieves in general) provided at p/p0 = 0.4–0.8 depending on the mean pore di- by X-ray powder diffraction, gas adsorption appears ameter ranging from about 3 to more than 10 nm, to be very important technique for identiﬁcation of e.g. [29,36]. Since larger pores are absent, all the porous structure of these materials. Based on the de- isotherms end with a nearly horizontal plateau. Char- termination of surface areas, pore volume and pore acteristic nitrogen adsorption isotherms of organized size diameters including pore size distribution, the mesoporous aluminas prepared via “anionic‘ route adsorption data provide accurate and reliable evalua- with stearic acid and “neutral” route with triblock tion of the structural properties of porous solids. One copolymer Pluronic PE10400 are depicted in Fig. 2. approach stems from the comparison of the adsorp- It can be clearly seen that, with increasing pore di- tion isotherm a(p/p0 ) of the material under study with ameter, a larger partial pressure is needed to start the the adsorption isotherm aref (p/p0 ) of an appropriate steep increase in the adsorbed amount. reference solid, on which adsorption occurs like on a ﬂat open surface. In this so-called “comparison plot” 3.3. Transmission electron microscopy the adsorption isotherm a(p/p0 ) is transformed to a function a(aref ) of the amount aref adsorbed on a ref- Transmission electron microscopy provides an- erence solid at the same relative pressure. A detailed other important technique to characterize the pore ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 333 25 (B) 20 -1 a / mmol g (A) 15 10 5 0 0.0 0.2 0.4 0.6 0.8 1.0 p/po Fig. 2. Nitrogen sorption isotherms of organized mesoporous aluminas prepared with stearic acid (A) and Pluronic PE10400 (B), respectively. size diameter and long-range channel ordering. The channel motif, however, without clear indication of results should agree with the results of X-ray powder the long-range channel packing order. Similar TEM diffraction and nitrogen adsorption. Fig. 3 depicts the pictures were also reported by the other authors TEM image of organized mesoporous alumina after [15,29], who claimed that the packing of the chan- calcination at 420 ◦ C . It is clearly shown that nel system is more or less random, in spite of the this mesoporous material exhibits a regular worm-like presence of one X-ray diffraction line. Fig. 3. High-resolution transmission electron micrograph image of organized mesoporous alumina after calcination at 420 ◦ C, according to . 334 ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 6 ppm ate region was discernible in the spectra of extracted mesoporous aluminas, prepared using, e.g. Triton X-114 with dipropyl amine. This ﬁnding suggests the absence of this type of aluminum species in this meso- porous material. It can be inferred that the changes in the aluminum coordination prior to and after template removal probably depend on the template removal 35 ppm procedure. While calcination procedures usually re- sulted in more profound changes in the coordination (B) of aluminum, less “severe” conditions of template 70 ppm removal via extraction caused a smaller extent in this transformation. Further, calcination between 500 and (A) 800 ◦ C did not signiﬁcantly change the coordination of different aluminum species. As reported by Deng 200 100 0 -100 -200 et al. , practically no further changes in alu- δ 27Al (ppm) minum coordination were observed for mesoporous aluminas prepared with Pluronic 64L and synthesized Fig. 4. Typical 27 Al MAS NMR spectra of as-synthesized and at temperatures ranging between 25 and 90 ◦ C. On calcined mesoporous alumina, prepared by “anionic” route. the basis of high-resolution multiple quantum MAS NMR spectroscopy, the authors have shown that the 3.4. 27 Al MAS NMR amount of octahedrally coordinated aluminum ranges from 65 to 70%, penta-coordinated aluminum from 2 MAS NMR is an indispensable tool in charac- to 6% and tetrahedrally coordinated aluminum from terization of the coordination of aluminum in orga- 25 to 33%. In addition, Vaudry et al. reported that the nized mesoporous aluminas. The 27 Al MAS NMR dehydration of already calcined mesoporous aluminas spectra of the as-synthesized and calcined organized also resulted in an increase in the amount of four- mesoporous aluminas, prepared with long-chain car- and ﬁve-fold coordinated aluminum at the expense of boxylic acids, are depicted in Fig. 4. In agreement six-coordinated aluminum . with other papers [15,33], the most intense signal in the spectra lies at about 5–6 ppm, which has been unambiguously assigned to the aluminum atoms in 4. High-temperature behavior of mesoporous a octahedral coordination. In addition, a small sig- aluminas nal at about 70 ppm appeared in the spectrum of the as-synthesized alumina; this signal is usually at- X-ray powder diffraction indicated that the walls of tributed to the tetrahedrally-coordinated aluminum. organized mesoporous aluminas are X-ray amorphous Following calcination, the intensity of this signal and no distinct diffraction lines can be observed in the substantially increased, indicating that during the as-synthesized samples or following calcination up to template removal a considerable portion of aluminum 500 ◦ C. The thermal stability of organized mesoporous changed its coordination from octahedral to tetra- aluminas represents one of the key points in the future hedral during the template removal. The occurrence application as a catalyst support and, therefore, several of penta-coordinated aluminum in the mesoporous research groups investigated the structural changes in aluminas is the subject of debate. In Fig. 4, a low mesoporous aluminas induced by thermal treatment. intensity signal is visible at about 35 ppm, which can There is a good agreement among the results of dif- be assigned to penta-coordinated aluminum species. ferent groups describing the high temperature behav- This signal was also reported by Bagshaw and Pin- ior of organized mesoporous aluminas [20,31,33,51]. navaia  for mesoporous alumina synthesized using High temperature treatment, without respect to the Pluronic 64L surfactant as the structure director. On synthesis procedure, resulted in a continuous decrease the other hand, no resonance signal in an intermedi- in the surface area and inner channel volume, and ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 335 simultaneously in an increase in the pore size diame- at 1000 ◦ C (Fig. 5). This indicates that transformation ter. These conclusions are consistent with the results of organized mesoporous alumina, synthesized with of X-ray powder diffraction (shift of the diffraction carboxylic acids as a structure-directing agent, pro- line to lower 2θ values), sorption isotherms for nitro- ceeding between temperatures 420 and 1000 ◦ C, leads gen (surface area, channel volume and pore diame- to the formation of crystalline alumina material. A de- ter) and also transmission electron microscopy. The tailed X-ray diffraction study revealed the formation surface areas following heat treatment at 1000 ◦ C are of -alumina particles . In contrast to these results, still higher than 100 m2 /g. The resulting pore diam- formation of -alumina by high-temperature treat- eter probably depends on the synthesis procedure or ment of mesoporous alumina, synthesized with tartaric on the wall thickness of the original samples. While acid, was described by Liu et al. . It is worth not- Liu et al.  reported pore size following calcination ing that the high-temperature behavior of organized at 1000 ◦ C of about 9.0 nm, Cejka et al. claimed that ˇ mesoporous alumina differs completely from that of the pore diameter increase is substantially lower (up mesoporous silicas. Ryoo and coworkers  reported to 6.0 nm) . The increase in the pore size diameter that, following calcination at 900 ◦ C, the intensity of is, however, accompanied by broadening of the pore the diffraction line (1 0 0) of (Al)MCM-41 was not size distribution. changed and the decrease in the BET area was less The structural transformations proceeded simul- than 10% compared to MCM-41 calcined at 500 ◦ C. taneously with the signiﬁcant changes in the tex- tural properties of organized mesoporous alumina during the high-temperature treatment. While a 5. Potential of mesoporous aluminas for catalytic high-resolution TEM image of mesoporous alu- applications mina calcined at 420 ◦ C did not show any crys- talline particles (Fig. 4), well-developed crystallo- To date, only several reactions have been investi- graphic planes were observed following calcination gated over modiﬁed mesoporous aluminas functioning Fig. 5. High-resolution transmission electron micrograph image of organized mesoporous alumina after calcination at 1000 ◦ C, according to . 336 ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 as a support for the catalytically active phases. The main reason is probably that the synthesis of meso- Thiophene conversion (%) porous aluminas is not as straightforward compared to mesoporous silicas and that mesoporous alumina is 60 still not easily available. Wieland et al.  tested mesoporous alumina, synthesized according to Vaudry et al. , using car- 40 boxylic acids as structure-directing agents, and mod- iﬁed by cesium and boron, in toluene alkylation with methanol. It was found that the catalyst was inactive 20 in toluene methylation but methanol was decomposed to carbon monoxide. The authors proposed that the physical constraints imposed by mesopores together 0 with the less than optimum proximity of acid–base 300 350 400 o sites within molecular sieve environment did not fa- Temperature / C cilitate the side-chain toluene alkylation reaction . Mesoporous alumina modiﬁed with copper was Fig. 6. Temperature dependence of thiophene conversion of on tested as a catalyst for selective hydrogenation of organized mesoporous alumina modiﬁed with 30 wt.% of MoO3 via thermal spreading method ( ) and commercial catalyst of cinnamaldehyde. The catalyst was prepared by di- BASF with 15 wt.% of MoO3 ( ), modiﬁed from . rect synthesis using aluminum Keggin polycations, copper nitrate, palmitic acid and hexadecyl trimethyl ammonium bromide. The remarkable selectivity of tically the same relative pseudo ﬁrst-order rate con- this catalyst towards formation of unsaturated alco- stants normalized to the weight of dried catalyst or hol was observed compared to conventional alumina. to one mole of molybdenum. This indicates compa- This ﬁnding was attributed to the particularly strong rable activity of the two catalysts related to one Mo interaction of the nanometer sized Cu0 particles with atom . Only negligible intensities of characteristic the mesoporous alumina walls, while the conjugated diffraction lines of MoO3 were observed after support- C=C bond was more readily hydrogenated on larger ing MoO3 on mesoporous alumina. This indicates that Cu0 clusters exhibiting weaker interaction with the practically no bulk MoO3 was present on the alumina support . support and probably a monolayer of this oxide was Hydrodesulfurization reactions represent one of the formed. most important application of conventional alumina as Recently, it has been reported that rhenium oxide a support for various Co–Mo or Ni–Mo phases in sul- ﬁnely dispersed on mesoporous alumina, prepared ﬁde forms . We have tested organized mesoporous by the Vaudry procedure  and possessing pores alumina for hydrodesulfurization of thiophene, and of 3 nm in a diameter, is signiﬁcantly more active compared it to the commercial catalyst [55,56]. Orga- and selective in metathesis of terminal and inner nized mesoporous alumina prepared by the “anionic” oleﬁns compared to -alumina . The equilibrium route with long-chain carboxylic acids was modiﬁed concentrations of 7-tetradecene and 9-octadecene by conventional impregnation with a solution of am- in metathesis of 7-hexadecene were easily achieved monium heptamolybdate and by thermal spreading of on Re2 O7 supported on mesoporous alumina under molybdenum oxide. Due to the signiﬁcantly larger mild reaction conditions (50 ◦ C) without formation surface area of different mesoporous aluminas com- of any side-products. This catalyst was prepared by pared to commercial Mo catalyst (BASF M8-30), it impregnation of mesoporous alumina with ammo- was possible to spread about 30 wt.% of MoO3 on nium perrhenate solution. Another approach was used mesoporous alumina. This resulted in a signiﬁcantly ˇ by Cejka and Balcar . A mixture of ammonium higher thiophene conversion compared to the con- perrhenate with mesoporous alumina was heated at ventional catalyst possessing only 15 wt.% of MoO3 500 ◦ C for several hours to prepare a highly active (Fig. 6). The conversion was almost doubled with prac- catalyst for metathesis of linear 1-oleﬁns. ˇ J. Cejka / Applied Catalysis A: General 254 (2003) 327–338 337 6. Summary and outlook Acknowledgements (i) The variety of developed synthesis proce- This work was carried out with the ﬁnancial sup- dures, the differences in the textural properties port by the Grant Agency of the Academy of Sciences as well as structural changes caused by the of the Czech Republic (A4040001), Grant Agency high-temperature treatment indicate that or- of the Czech Republic (104/02/0571), Ministry for ganized mesoporous aluminas are interesting Education, Youth and Sport of the Czech Republic molecular sieve materials not only from the (ME404), and NATO in the framework of “Science material science point of view but also for the for Peace” (SfP-974217). I would like also to thank possibilities of their application, e.g. as a support B. Wichterlová and A. Zukal for helpful discussions. in heterogeneous catalysis. 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