Continuous amination of propanediols in supercritical ammonia

Article abstract of DOI:10.1002/(SICI)1521-3773(19990201)38:3<351::AID-ANIE351>3.0.CO;2-0

Enhancements in selectivity by a factor of 4 to 18 were observed in the near critical regions of ammonia during the Co- and Ni-catalyzed synthesis of 1,3-diamino-propanes. These results are attributed to the higher concentration of ammonia at the surface of the catalyst, which favors the amination and suppresses degredation reactions.

Full text of DOI:10.1002/(SICI)1521-3773(19990201)38:3<351::AID-ANIE351>3.0.CO;2-0

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amide/ester group undergoing reaction,[6] and converts react-
ing groups into neighboring groups. Based on the assumption
that, under the conditions of the experiments listed in Table 1,
only half of the catalyst is in the active form of a 1:1 ethoxide
complex, rate enhancements for reactions catalyzed by the
bimetallic vs. monometallic catalyst translate into an effective
barium ion transforms an electron-donating (rate-retarding) carbox-
ylate into an electron-withdrawing (rate-enhancing) carboxylate ±
metal ion pair.
[
7] The close similarity of the two values would indicate that the higher
�
affinity for the monometallic complex of the more basic AcO ion is
substantially counterbalanced by the statistical factor which
2
operates in the binding of the less basic 3 to the bimetallic complex.
The fact that the ligand is not exactly the same in the two cases is
presumably unimportant.
8] The effective molarity (EM) for an intramolecular reaction is defined
by the ratio kintra/kinter, where kinter refers to the intermolecular model
reaction. For a discussion of structure effects on the EM value see:
a) A. J. Kirby, Adv. Phys. Org. Chem. 1980, 17, 183 ± 278; b) L.
Mandolini, Adv. Phys. Org. Chem. 1986, 22, 1 ± 111.
[
8]
molarity (EM) value of 0.08m for the reaction of 3 and one
of 0.04m for the reaction of 5. We believe that the loss of
entropy associated with torsional motions around two C�C
and two C�N bonds is largely responsible for these relatively
[
[
[
8]
low EM values. Current work is aimed at the construction of
more preorganized, and hopefully more efficient, catalysts.
9] M. R. Johnson, I. O. Sutherland, R. F. Newton, J. Chem. Soc. Perkin
Trans. 1 1980, 586 ± 600.
[
10] M. R. Johnson, I. O. Sutherland, R. F. Newton, J. Chem. Soc. Perkin
Experimental Section
Trans. 1 1979, 357 ± 371.
Ligands 1[9] and 2[10] and compounds 3 ´ H ,
were prepared according to published procedures (melting points and/or
spectral data in agreement with literature data). The compound 8 ´ H was a
‡ [11]
4,[12] 5 ´ H‡,[13] and 7 ´ H
‡[13]
[11] A. Rosowsky, J. H. Freisheim, H. Bader, R. A. Forsch, S. A. Susten,
C. A. Cucchi, E. Frei, J. Med. Chem. 1985, 28, 660 ± 667.
[12] E. J. Bourne, S. H. Henry, C. E. M. Tatlow, J. C. Tatlow, J. Chem. Soc.
1952, 4014 ± 4019.
‡
‡
‡
‡
‡
commercial sample. Acids 3 ´ H , 5 ´ H , 7 ´ H , and 8 ´ H were converted
‡
[13] F. D. Chattaway, J. Am. Chem. Soc. 1931, 2495 ± 2496.
in situ into the Me
4
N
salts by neutralization with Me
4
NOEt. Spectropho-
tometric rate measurements were carried out in the thermostatted cell
compartment of a Hewlett Packard HP 8452A diode array spectrometer.
For initial rate measurements, a fast mixing accessory, HI-TECH SCIEN-
TIFIC SFA-12, was connected to the spectrometer and the process was
2‡
monitored at the isosbestic point for the free and the Ba -paired reaction
product (l ˆ 276 nm). Other materials, apparatus and techniques were as
reported previously.[4, 5]
Continuous Amination of Propanediols in
Supercritical Ammonia**
Received: July 31, 1998 [Z12235IE]
German version: Angew. Chem. 1999, 111, 359 ± 362
Achim Fischer, Tamas Mallat, and Alfons Baiker*
Keywords: alkaline earth metals
macrocyclic ligands ´ supramolecular chemistry
´
enzyme mimetics
´
The heterogeneously catalyzed amination of alcohols has
been established as the most important industrial process for
the manufacture of a variety of aliphatic and aromatic
amines.[
1±6]
However, the yields and selectivities are usually
[
1] a) R. Breslow, Acc. Chem. Res. 1995, 28, 146 ± 153; b) Y. Murakami, J.
Kikuchi, Y. Hisaeda, O. Hayashida, Chem. Rev. 1996, 96, 721 ± 758;
c) A. J. Kirby, Angew. Chem. 1996, 108, 770 ± 790; Angew. Chem. Int.
Ed. Engl. 1996, 35, 707 ± 724; d) D. J. Cram, J. M. Cram, Container
Molecules and Their Guests, The Royal Society of Chemistry, Cam-
bridge, 1994, pp. 65 ± 84.
rather low in the synthesis of primary aliphatic diamines from
the corresponding diols and ammonia. The transformation of
a simple aliphatic alcohol to the corresponding amine on a
metal catalyst includes three major reaction steps (Scheme 1):
1) dehydrogenation to a carbonyl compound; 2) condensation
[
2] R. Cacciapaglia, L. Mandolini, Chem. Soc. Rev. 1993, 22, 221 ± 231.
3] a) K. D. Karlin, Science 1993, 261, 701 ± 708; b) N. N. Murthy, M.
Mahroof-Tahir, K. D. Karlin, J. Am. Chem. Soc. 1993, 115, 10404 ±
with ammonia or an amine to form an imine or an enamine;
[
and 3) hydrogenation to the corresponding amine.[
7, 8]
Each
intermediate and the product amine can undergo various side
1
1
0405; c) D. H. Vance, A. W. Czarnik, J. Am. Chem. Soc. 1993, 115,
2165 ± 12166; d) M. Yashiro, A. Ishikubo, M. Komiyama, J. Chem.
reactions such as condensation, decarbonylation, dispropor-
Soc. Chem. Commun. 1995, 1793 ± 1794; e) M. W. Göbel, Angew.
Chem. 1994, 106, 1201 ± 1203; Angew. Chem. Int. Ed. Engl. 1994, 33,
[4, 9±11]
tionation, and hydrogenolysis.
The direct transformation
of an aliphatic diol to the corresponding diamine requires the
repetition of steps 1 ± 3, which favors the formation of the by-
product. In addition, the bifunctional intermediates extend
1
2
141 ± 1143; f) H. Steinhagen, G. Helmchen, Angew. Chem. 1996, 108,
489 ± 2492; Angew. Chem. Int. Ed. Engl. 1996, 35, 2339 ± 2342; g) M.
Wall, R. C. Hynes, J. Chin, Angew. Chem. 1993, 105, 1696; Angew.
Chem. Int. Ed. Engl. 1993, 32, 1633 ± 1635; h) N. H. Williams, J. Chin,
Chem. Commun. 1996, 131 ± 132; i) P. Hurst, B. K. Takasaki, J. Chin, J.
Am. Chem. Soc. 1996, 118, 9982 ± 9983; j) W. H. Chapman, Jr., R.
Breslow, J. Am. Chem. Soc. 1995, 117, 5462 ± 5469; k) T. Koike, M.
Inoue, E. Kimura, M. Shiro, J. Am. Chem. Soc. 1996, 118, 3091 ± 3099;
l) K. G. Ragunathan, H.-J. Schneider, Angew. Chem. 1996, 108, 1314 ±
the scope of possible side reactions (for example, oligomeri-
[12, 13]
zation and cyclization).
However, reasonable yields have
still been reported for the amination of alkanediols with
secondary amines, as the tertiary amine product is moderately
reactive.[
7, 14]
Unfortunately, the situation is the reverse in
1316; Angew. Chem. Int. Ed. Engl. 1996, 35, 1219 ± 1221; m) P.
aminations with ammonia, as the reactivities of the inter-
Molenveld, S. Kapsabelis, J. F. J. Engbersen, D. N. Reinhoudt, J. Am.
Chem. Soc. 1997, 119, 2948 ± 2949; n) S. Liu, Z. Luo, A. D. Hamilton,
Angew. Chem. 1997, 109, 2794 ± 2796; Angew. Chem. Int. Ed. Engl.
[*] Prof. Dr. A. Baiker, Dipl.-Chem. A. Fischer, Dr. T. Mallat
Laboratory of Technical Chemistry
1997, 36, 2678 ± 2680.
Swiss Federal Institute of Technology, ETH-Zentrum
Universitätstrasse 6, CH-8092 Zurich (Switzerland)
Fax.: (‡41)1-632-11-63
[
[
[
4] R. Cacciapaglia, S. Di Stefano, E. Kelderman, L. Mandolini, F.
Spadola, J. Org. Chem., 1998, 63, 6476 ± 6479.
5] R. Cacciapaglia, L. Mandolini, V. Van Axel Castelli, J. Org. Chem.
E-mail: baiker@tech.chem.ethz.ch
1997, 62, 3089 ± 3092.
6] Reactions of 3 and 5 are more sensitive to the presence of the metal
ion than the corresponding reactions of 4 and 6 because binding to the
[**] Financial support from Lonza Ltd, Visp (Switzerland) is kindly
acknowledged.
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On the basis of literature data[22±25] it is expected that at
35 bar or above, where the selectivity to the diamine is
1
constant (Figure 1), the reaction mixture is in the supercritical
Figure 1. Influence of pressure on the conversion X (
and the selectivity to 3-amino-1-propanol ( ) and 1,3-diaminopropane (
Conditions: catalyst: 95 wt% Co and 5 wt% Fe, 1958C, space time
&
) of propanediol,
&
&
).
�
1
1
2
1 ghmol , molar ratio of alcohol/H /ammonia: 1/2/60.
(
sc) region. To confirm this assumption the phase composition
Scheme 1. Important reactions that occur in the amination of alkanediols
with ammonia. The continuous and dashed line frames indicate the
detected main and by-products, respectively.
of the reaction mixture was investigated in a 50 mL quartz
autoclave. The presence of a homogeneous fluid at 130 bar
and 2008C was confirmed by visual inspection. Accordingly,
we propose that the remarkable improvement in amination
selectivity with increasing pressure is connected with the
change of the medium in the near critical region of ammonia
mediate and product primary amines are significantly higher
than that of ammonia. The above considerations account for the
difficulties in the selective synthesis of primary diamines from
diols. A practical solution is the separate preparation of the ami-
(110 ± 120 bar) and at the reaction temperature of 608C above
the critical temperature.
noalcohol intermediate and its further amination with ammo-
nia. This method provides good selectivities to diamines.[15±19]
It seems from Figure 1 that the amination selectivity
increased in both consecutive steps, namely in the formation
and in the further amination of the aminoalcohol intermedi-
ate. In a control experiment the intermediate 3-amino-1-
propanol was fed into the reactor under similar conditions.
Figure 2 shows the effect of increasing pressure: likewise a
Intrigued by the obvious advantages of the one-step
process, we reinvestigated the amination of alkanediols with
ammonia in a continuous fixed-bed reactor at pressures where
the ammonia forms a supercritical fluid (critical data of
[
20]
ammonia: T ˆ 132.48C, P ˆ 114.8 bar ). The amination of
c
c
1
,3-propanediol and 2,2-dimethyl-1,3-propanediol were chos-
en as test reactions. Two types of catalysts were used: an
unsupported Co catalyst stabilized by 5 wt% Fe and a
commercial silica-supported Ni catalyst. Preliminary experi-
ments indicated thatÐin agreement with the literature
data[ Ða rather high ammonia/alcohol molar ratio in the
range of 10/1 to 100/1 was necessary to suppress the
dimerization and oligomerization of the intermediate and
product amines. A small ratio of hydrogen in the feed (1 ±
21]
5
mol% of the reaction mixture) was sufficient to prevent the
undesired dehydrogenation reactions and the formation of
nitriles and carbonaceous deposit.
Figure 1 illustrates the role of the total pressure in the
amination of 1,3-propanediol over the unsupported Co ± Fe
catalyst. The conversion varied only from 85 to 99% over the
whole pressure range studied. However, a pressure increase
from 50 to over 100 bar had a striking effect on the amination
selectivity. The technically important cumulative selectivity to
Figure 2. Effect of pressure on the conversion X (
&
) of 3-amino-1-
propanol and the selectivity to 1,3-diaminopropane (*). Conditions:
catalyst: 95 wt% Co and 5 wt% Fe, 1958C, space time 11 ghmol , molar
ratio of alcohol/H /ammonia: 1/2/40.
3-amino-1-propanol and 1,3-diaminopropane increased from
� 1
about 1% to 43 ± 48%.
2
3
52
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small drop in conversion and a significant improvement in
diamine selectivity was observed in the near critical region.
Below and above this region the variations in conversion and
selectivity were minor.
The striking effect of supercritical conditions in the
amination of alkanediols was not limited to the use of the
Co ± Fe catalyst or 1,3-propanediol as reactant. Figure 3
along the catalyst bed), and a gas phase that contains
dominantly ammonia. In this pressure range the influence of
total pressure on conversion and product composition was
minor as illustrated in Figures 1 ± 3. In the near critical region
the transformation of the two-phase system to a homogeneous
sc fluid generally results in a significant improvement of mass
transport because of the elimination of the phase transfer
resistance, and the higher diffusion coefficients and lower
viscosity, relative to the situation in the liquid phase. Besides,
the effect of pressure on the chemical equilibria and reaction
rates can also be crucial.[
26±28]
Unfortunately, for a network of
consecutive and parallel reactions the effect of pressure is
rather complex because of the different reaction volumes
=
(
DV ) and activation volumes (DV ). We assume that in the
r
supercritical region the surface ammonia concentration is
significantly higher than under two phase conditions, which
govern in the subcritical region. Although this change is
mainly a consequence of the enhanced mass transfer in the sc
fluid, a change in adsorption equilibrium (solvent effect) can
also contribute. There are some recent examples in the
literature on the enhancement of stereoselectivity (cis/trans
ratio) in the hydrogenation of vegetable oils in sc CO2.[ The
improvement was attributed to the significantly better hydro-
gen availability on the metal surface that arises from the
elimination of mass transport limitations in the sc fluid phase.
A clearer picture is obtained by analysis of the influence of
the subcritical ± supercritical transition on the main reaction
steps and the side reactions. The major side reactions that
produce monofunctional alcohols and amines are illustrated
in Scheme 1. Decarbonylation of the intermediate aldehydes
on the metal surface results in primary alcohols and amines.
Amination of the alkanol by-product via the aldehyde
intermediate leads to alkylamines. This aldehyde intermedi-
ate can also form by the (acid-base catalyzed) retro-aldol
reaction.[ The variation of the total amount of alkylamines
detected in the liquid product is illustrated in Figure 3 for the
amination of 2,2-dimethyl-1,3-propanediol. The main by-
product was isobutylamine. A significant drop in the amount
of this by-product in the near critical and supercritical region
corroborates the picture discussed above. In the supercritical
region of ammonia, the dimerization and oligomerization side
reactions were not important.
29]
Figure 3. Influence of pressure on the conversion X (
,3-propanediol, and the selectivity to 2,2-dimethyl-3-amino-1-propanol
*) and 2,2-dimethyl-1,3-diaminopropane (~) and alkylamine (Â). Con-
ditions: catalyst: 56 wt% Ni on a support (Engelhard Ni-6458), 2108C,
&
) of 2,2-dimethyl-
1
(
�
1
2
space time 11 ghmol , molar ratio of alcohol/H /ammonia: 1/2/60.
presents another example: the amination of 2,2-dimethyl-1,3-
propanediol. In this case the reaction was carried out with the
supported Ni catalyst. Again, the increasing pressure had only
minor influence on the diol conversion, but favored the
formation of the aminoalcohol intermediate and especially
the desired diamine product. The changes in selectivity were
negligible below 90 bar, and only minor above the critical
pressure of ammonia.
Before the possible reasons for the observed selectivity
enhancement could be discussed we had to ascertain that in
the amination of various alkanediols and aminoalcohols the
conversion above 60% had only a small influence on the
selectivity to the corresponding diamines. For example, the
selectivity in the amination of 2,2-dimethyl-1,3-propanediol at
13]
We suggest that the changes in amination selectivity and
alkanediol conversion in the near critical region arise from the
increased ammonia concentration at the surface. This change
favors the amination and suppresses the degradation-type
(hydrogenolysis) side reactions. The situation is rather similar
2
278C to 2,2-dimethyl-1,3-diaminopropane decreased from 58
to 53% when the conversion increased (by increasing the
contact time) from 61 to 89%. A minor variation in selectivity
of about Æ2% was observed in the amination of 1,3-propane-
diol in a similar conversion range. The likely interpretation for
the small effect of increasing conversion is that not only is
more diamine formed from the aminoalcohol intermediate at
higher contact time, but also more is consumed by consecutive
side reactions such as disproportionation or hydrogenolysis
degradation). Accordingly, the changes in selectivity with
increasing pressure (Figures 1 ± 3) cannot be traced to the
small variations in conversion.
At medium pressures (<90 bar) the reaction mixture
consists of two phases: a liquid phase that is rich in the non-
volatile alkanediol and amines (depending on the conversion
to the well-known, and widely applied, selective poisoning of
metal catalysts.[
30±32]
For example, the presence of strongly
adsorbing amines in the hydrogenation of carbonyl com-
pounds hinders the C�O bond hydrogenolysis, and the
corresponding alcohols can be prepared in very high yields.
In the amination of alkanediols and alkanolamine (Fig-
ures 1 and 2), the small drop in conversion and the strong
decrease in the amount of alkylamines by-products in the
liquid phase (Figure 3) are clear indications of selective
poisoning: the degradation-type reactions are effectively
slowed down but also the first step, the dehydrogenation of
the alcohol to the carbonyl compound, is affected. On the
(
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other hand, the higher ammonia concentration at the surface
favors the fast addition of ammonia to the aldehyde-type
intermediates, and suppresses the reaction of the alcoholic
OH group with the intermediate or product amines that leads
to dimerization products. That is, the weaker basicity of
ammonia, relative to the product amines, is partly compen-
sated by its higher concentration at the surface. All these
effects can contribute to the remarkable enhancements in the
overall amination selectivities (Figures 1 ± 3).
[7] R. E. Vultier, A. Baiker, A. Wokaun, Appl. Catal. 1987, 30, 167.
[
[
8] J. Kijenski, P. J. Niedezielski, A. Baiker, Appl. Catal. 1989, 53, 107.
9] A. Baiker, J. Kijenski, Catal. Rev. Sci. Eng. 1985, 27, 653.
[
[
10] J. Kijenski, J. Burger, A. Baiker, Appl. Catal. 1984, 11, 295.
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[12] G. Sirokm a n, A. Moln a r, M. Bartok, J. Mol. Catal. 1983, 19, 35.
Â
[
[
[
13] A. Moln a r, G. Sirokman, M. Bartok, J. Mol. Catal. 1983, 19, 25.
14] J. Runeberg, A. Baiker, J. Kijenski, Appl. Catal. 1985, 17, 309.
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17] A. Moriya (Nippon Shokubai Co.), EP-B 0652207, 1994, [Chem.
Abstr. 1995, 123, 9064].
In conclusion we can state that the application of scNH as a
3
[
solvent and reactant in the amination of simple alkanediols
and aminoalcohol affords significant improvement in selec-
tivities to primary diamines, relative to the procedure at
subcritical pressure. Most of the changes in selectivities occur
in a narrow pressure range in the near critical region of the
medium. The selectivity improvement is attributed to the
higher concentration of ammonia at the surface, which favors
the amination with ammonia and suppresses the hydrogenol-
ysis-type (degradation) side reactions. It seems that the
phenomenon is not limited to a specific catalyst or reactant,
though the final selectivity is a function of the structure of the
reactant aminoalcohol or diol. We are presently working on
the extension to other amination reactions where the low
reactivity of ammonia relative to the product amine prevents
the efficient synthesis of primary amines.
[
18] T. Hironaka, N. Nagasaka, Y. Hara (Tosoh Corp.), EP-B 0526851,
1
992, [Chem. Abstr. 1993, 118, 212475].
[19] F. Merger, A. Segnitz (BASF AG), US-A 4158017, 1979.
20] Encyclopedie de Gaz, Elsevier, 1976, p. 951.
21] H. Feichtinger, H. Aschmann, H. Birnkraut (Ruhrchemie AG), US-A
[
[
4
078003, 1978, [Chem. Abstr. 1975, 83, 78566x].
[
22] E. Brunner, J. Chem. Thermodynamics 1988, 20, 1397.
[23] E. Brunner, J. Chem. Thermodynamics 1988, 20, 273.
[24] J. C. Giddings, M. N. Myers, L. McLaren, R. A. Keller, Science 1968,
1
62, 67.
[
[
[
25] S. Borman, Sci. Technol. 1995, 33.
26] A. Baiker, Chem. Rev. 1999, in press.
27] C. Eckert, B. L. Knutson, P. G. Debenedetti, Nature 1996, 383, 313.
[28] P. E. Savage, S. Gopalan, T. I. Mizan, C. J. Martino, E. E. Brock,
AIChE J. 1995, 41, 1723.
[
29] T. Tacke, S. Wielander, P. Panster in High Pressure Chemical
Engineering, Vol. 12 (Eds.: R. von Rohr, C. Trepp), Elsevier, Zurich,
Switzerland, 1996, p. 17.
[30] P. N. Rylander, Catalytic Hydrogenation over Platinium Metals,
Academic Press, New York, 1967.
Experimental Section
The Co ± Fe catalyst was prepared by co-precipitation. Aqueous solutions
of cobalt nitrate, iron nitrate, and ammonium carbonate were mixed at
room temperature and the pH adjusted to 7. The precipitate was filtered off,
dried at 1208C in a vacuum, and calcined at 4008C for 4 h. The BET surface
[
31] P. Rylander, Catalytic Hydrogenation in Organic Synthesis, Academic
Press, New York, 1979.
[32] M. Freifelder, Practical Catalytic HydrogenationÐTechniques and
Applications, Wiley-Interscience, New York, 1971.
2
� 1
3
� 1
area was 8 m g and the pore volume 0.1 cm g
.
The commercial Ni catalyst (Engelhard No. 6458) contained 56 wt% of
2
� 1
pre-reduced Ni. The BET surface area was 180 m g and the pore volume
3
� 1
0
.3 cm g .
The amination experiments were carried out isothermally in a continuous
tubular reactor with an inner diameter of 13 mm. The reactor was loaded
with crushed and sieved catalyst particles of 140 to 400 mm. The liquid
ammonia and 1,3-propanediol or the solution of 2,2-dimethyl-1,3-propane-
diol in ammonia were dosed into the reactor by ISCO D500 syringe pumps.
The total pressure in the reaction system was set by a TESCOM back-
pressure regulator. Details of the reaction conditions are indicated in the
figure captions. The liquid product was separated from the gas and
analyzed on an HP 5890 gas chromatograph (HP 1701 column). The
products were identified by GC-MS analysis.
Calcium, Strontium, and Barium AcetylidesÐ
New Evidence for Bending in the Structures of
Heavy Alkaline Earth Metal Derivatives**
David C. Green, Ulrich Englich, and
Karin Ruhlandt-Senge*
Since their development in the early 1900s, Grignard
reagents have proven to be immensely useful in synthetic
Received: June 26, 1998 [Z12057IE]
German version: Angew. Chem. 1999, 111, 362 ± 365
chemistry and are among the most common organometallic
reagents.[
1, 2]
In contrast, information about beryllium, calci-
Keywords: aminations ´ ammonia ´ diols ´ heterogeneous
catalysis ´ supercritical fluids
[1c, 2, 3]
um, strontium, and barium analogues is scarce.
Organo-
[
*] Prof. Dr. K. Ruhlandt-Senge, D. C. Green, Dr. U. Englich
Department of Chemistry
-014 Center for Science and Technology
[
[
1] M. G. Turcotte, T. A. Johnson, Kirk-Othmer, Encycl. Chem. Technol.,
Vol. 2, Wiley, New York, 1992, p. 369.
2] G. C. Carter, A. R. Doumaux, S. W. Kaiser, P. R. Umberger, Kirk-
Othmer, Encycl. Chem. Technol., Vol. 8, Wiley, New York, 1992, p. 74.
3] D. M. Roundhill, Chem. Rev. 1992, 92, 1.
1
Syracuse University
Syracuse NY 13244-4100 (USA)
Fax: (‡1)315-443-4070
E-mail: kruhland@syr.edu
[
[
4] A. Baiker in Catalysis of Organic Reactions (Eds.: J. R. Kosak, T. A.
Johnson), Marcel Dekker, New York, 1994, p. 91.
[**] This work was supported by the National Science Foundation (CHE-
9702246) and the Deutsche Forschungsgemeinschaft (Postdoctoral
stipend for U. E.). Purchase of the X-Ray diffractometer was made
possible with grants from NSF (CHE-95 ± 27898), the W. M. Keck
Foundation, and Syracuse University.
[
[
5] A. Fischer, T. Mallat, A. Baiker, Catal. Today 1997, 37, 167.
6] T. Mallat, A. Baiker in Handbook of Heterogeneous Catalysis, Vol. 5
(Eds.: G. Ertl, H. Knözinger, J. Weitkamp), WILEY-VCH, Weinheim,
1997, p. 2334.
3
54
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