Chemtrails - Aerosol Studies - Organized Mesoporous Alumina - HAARP Psychotronics by FlavioBernardotti1


									                                        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

   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-
E-mail address: (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.
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 [8] 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                 [19]. 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 [20] or
et al. [9]. The procedure was based on the synthesis            “atrane” complexes as structure-directing agents [21].
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 [12]. 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 [13]. 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. [3] Bagshaw and Pinnavaia [15] 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 [16]; 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
                                        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 [33]. The extension of the
[27] were substantially overestimated (between 4                             block copolymer templating approach was described
and 8 nm), while the pore diameters obtained from                            by Yang et al. [34], showing that not only alumina but
Barrett–Joyner–Hallender (BJH) model [28] 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 [15]. Addition of small amounts of Ce3+ or                             porous aluminas was described by Vaudry et al. [16].
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
[29]. 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. [30] and Deng et al.                             shown that the pore size diameter of mesoporous alu-
[31]. Recently, González-Pena et al. [32] 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                           [15]
Tergitol 15-S-12                   C11–15 [PEO]12                      3.5                            425                           [15]
Triton X-114                       C8 Ph[PEO]8                         3.6                            445                           [15]
Pluronic 64L                       [PEO]13 [PPO]30 [PEO]13             2.4                            430                           [15]
Pluronic P123                      [PEO]20 [PPO]69 [PEO]20            10.3                            487                           [29]
Pluronic 64L                       [PEO]13 [PPO]30 [PEO]13             8.7                            470                           [31]
Triton X-114                       C8 Ph[PEO]8                         3.5                            490                           [32]
Tergitol 15-S-9                    C11–15 [PEO]9                       4.0                            525                           [32]
Tergitol 15-S-15                   C11–15 [PEO]15                      4.5                            510                           [32]
Caproic acid                       C5 COOH                             2.1                            530                           [16]
Lauric acid                        C11 COOH                            1.9                            710                           [16]
Stearic acid                       C17 COOH                            2.1                            700                           [16]
Lauric acid                        C11 COOH                            3.3                            489                           [35]
Stearic acid                       C17 COOH                            3.7                            758                           [35]
Dibenzoyl-l-tartaric acid          [C6 H5 CO2 CH(CO2 H)–]2             3.3                            386                           [20]
Dibenzoyl-l-tartaric acid          [C6 H5 CO2 CH(CO2 H)–]2             4.2                            425                           [20]
CTMABr                             C16 N(CH3 )4 Br                     3.3                            340                           [19]
CTMABr                             C16 N(CH3 )4 Br                     6.0                            250                           [19]
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. [44]. 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) [44].
cross-linking of the Al–OH groups in any adjacent                   Recent results report the investigation of phase be-
aluminate sheets [17]. 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 [45]. 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. [38]. 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 [46]. 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. [38] 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. [19] 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
                                  J. Cejka / Applied Catalysis A: General 254 (2003) 327–338                           331

mesoporous oxide systems [21]. 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. [20]. 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. [47].
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 [35].              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 [44]) 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 [44]. 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.
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                                             J. Cejka / Applied Catalysis A: General 254 (2003) 327–338








                                         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 [35].                       ative analysis of nitrogen adsorption isotherms [35].
                                                                                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
                                          J. Cejka / Applied Catalysis A: General 254 (2003) 327–338                                   333


                      a / mmol g




                                        0.0          0.2           0.4          0.6           0.8               1.0


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 [51]. 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
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                                          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
                                                                          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. [31], 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 [16].
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 [15] 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
                                     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 [51]. 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. [20]. It is worth not-
Liu et al. [20] 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 [52] reported
to 6.0 nm) [51]. 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 [51].
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. [53] tested mesoporous alumina,
synthesized according to Vaudry et al. [16], 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
sites within molecular sieve environment did not fa-                                                        Temperature / C
cilitate the side-chain toluene alkylation reaction [53].
   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 [55].
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 [55]. 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 [54].                                                    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 [11]. We have tested organized mesoporous              by the Vaudry procedure [16] 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 [57]. 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 [58]. 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.
                                   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. The modification and
      adoption of a synthesis procedure applied to syn-
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