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Magnesium Alcoholates as Precursors for Magnesia
Page 1
J. MATER. CHEM., 1995, 5(4), 589-594
589
Magnesium Alcoholates as Precursors for Magnesia
Heiko Thorns, Matthias Epple, Heiko Viebrock and Armin Reller*
lnorganic and Applied Chemistry Institute, University of Hamburg, Martin-Luther-King- Platz 6,
0-20746 Hamburg, Germany
The synthesis, structural aspects and thermal behaviour of six different magnesium alcoholates are reported. The
magnesium oxides obtained by decomposition of the alkoxides show different particle morphologies, as shown by
scanning electron microscopy. The mechanism of the thermal decomposition of magnesium methanolate was studied
in detail. The substance decomposed in three steps at 90, 330-350 and 360-400 "C. Two intermediates were charac-
terized by elementary analysis (C, H) and IR spectroscopy.
Metal alcoholates [general formula MJOR),,] are important
substances for several industrial applications. As precursors
they can be used for the low-temperature synthesis of metal
oxides in different
The alcoholates can often be
sublimated and used for MOCVD., Metal alcoholates have
been known for over a century, and the topic has been
extensively reviewed.4- ' The last authors discussed the new
area of heterometallic alcoholates with the general formula
M,M',(OR),. Bradley et a1.' have also discussed many differ-
ent aspects of metal alcoholate chemistry.
We were interested in magnesium alcoholates, Mg(OR),,
because they are easily prepared in high
and because
the magnesium oxide MgO ('magnesia') which is obtained
after thermal decomposition is an important substance in
industrial applications. For example, it is used as a catalyst
in metal organic synthesis, a good insulator in electrical
devices, a neutralising agent in medicine and a flame retardant.
An interesting aspect for such applications is the possibility
of changing the porosity, morphology and microcrystallinity of
MgO particles. This can be done by using magnesium
alcoholates with varying organic groups as precursors. In this
context we examined magnesium oxides obtained from six
different precursors by scanning electron microscopy (SEM).
The influence of different inorganic precursors (e.g. magnesium
acetate and magnesium nitrate) upon the synthesis of MgO
has been described by Choudhary et a1.' The crystal structures
of magnesium alcoholates are often difficult to determine
because of the polymeric character of the alcoholates.
Additionally, extensive structural disorder is frequently
observed. Here we report on the results derived from the crystal
structure determination of a magnesium methanolate solvate
complex Mg(OCH,),-2CH30H. We also studied the thermo-
chemical reactivity of six different alkoxides under different
atmospheres by combined thermogravimetry-differential ther-
mal analysis with coupled mass spectrometry (TG-DTA-MS).
The purpose of these investigations was to identify
decomposition temperatures and decomposition mechanisms
in order to define optimal conditions for the synthesis of MgO
samples with specific morphologies. Magnesium oxide is
prepared industrially by the decomposition of magnesium
carbonate at 800-900 "C. The much lower decomposition
temperatures of magnesium alcoholates (400-500 "C) than
magnesium carbonate are attractive for the synthesis of MgO.
Heating energy can be saved and high-temperature corrosion
of the oven material can be reduced. Such low-temperature
syntheses are gaining increasing interest in materials science."
Experiment a1
Synthesis
Six magnesium alcoholates were prepared via different
methods. The sufficiently high reactivity of methanol, ethanol
and propan- 1-01 allows direct reaction between magnesium
metal and the alcohol by heating the suspension under reflux.
Eqn. (1) describes the exothermic reaction:
Mg + 2ROH % Mg(OR), + H2
(1)
R = CH3, CH3CH2, CH3CH2CH2
The lower reactivity of the other three alcohols, propan-2-01,
tert-butyl alcohol and octan-2-01, requires the more reactive
diethyl magnesium, prepared by standard methods," to obtain
the alcoholates. The reason for this lower reactivity is the
acidity of the hydroxy group owing to the longer hydrocarbon
chains in the alcohols. Increasing steric hindrance for bulkier
alkyl groups is expected to be another reason for the lower
reactivity. Eqn. (2) describes the exothermic reaction which is
complete within a few minutes:
Mg(CH,CH3), + 2R'OH + Mg(OR')2 + 2CzH6 (2)
R' = CH,),CH, (CH3)3C, (CH,)(C,H,3)CH
All alcoholates were characterized by elemental analysis (C,
H), thermogravimetry and IR spectroscopy. No magnesium
hydroxide was detected in the IR spectra.
All procedures were carried out under a dry argon atmos-
phere. All solvents were dried by standard methods.
Synthesis of Mg(OCH,),
Magnesium (4 g, 165 mmol) was dried in oucuu for 30 min at
80 "C. The reaction began after the addition of 4 ml of absolute
methanol. The mixture was heated for 10min. After the
addition of 50 ml of methanol the reaction mixture was cooled
in a water bath to room temperature and stirred for 24 h.
Then the solution, which contained a grey-white precipitate,
was heated under reflux for 10 min and slowly cooled to room
temperature.
The product Mg(OCH3),-2CH,0H crystallized as colour-
less needles. Yield: 100% (29 g). Anal. calc.: C, 31.9 mass;
H, 9.4%. Found: C, 32.7; H, 10.1%. Mass loss in TG for
decomposition to MgO: calc.: 73.2%; found: 72.3%.
When the product was heated to 40°C in uucuo for 3 h, a
white powder was obtained which was characterized as pure
Mg(OCH,),. Anal. calc.: C, 27.8; H, 7.0%. Found: C, 27.7;
H, 7.1%. Mass loss in TG for decomposition to MgO: calc.:
53.3%; found: 55.0%. IR (KBr) v/cm-':2936, 2874, 2808 (s,
C-H str.); 1102, 1052 (vs, C - 0 str.); 546, 476 (s, Mg-0).
Synthesis of Mg(OC,H, ),
Magnesium (2 g, 83 mmol) was dried in vacua for 30 min at
80 "C. Absolute ethanol (45 ml) was added, and the mixture
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590
J. MATER. CHEM., 1995, VOL. 5
heated under reflux for lOmin, then stirred for 24 h. The
product, a grey-white precipitate, was filtered off and dried in
uacuo. Yield: 90% (8.54 8). Anal. calc.: C, 42.0; H, 8.8%. Found:
C, 37.3; H, 8.0%. Mass loss in TG for decomposition to MgO:
calc.: 64.7%; found: 62.9%. IR (KBr) v/cm-l:2964, 2858 (s,
C-H str.); 1443, 1385, 1318 (m, C-H def.); 1129, 1100, 1068
(vs, C - 0 str.); 884 (m, C-C str.); 557-405 (s, Mg-0).
Synthesis o ~ M ~ ( O C , H ~ ) ~
Magnesium (0.15 g, 6.17 mmol) was dried in uacuo for 30 min
at 80°C. Propan-1-01 (50 m) was added and the reaction
mixture heated under reflux for 24 h. The grey-white precipi-
tate was filtered off and dried in uucuo for 3 h, giving the
product as a white powder. Yield: 90% (0.79 8). Anal. calc.:
C, 50.6; H, 9.9%. Found: C, 49.4; H, 9.3%. Mass loss in TG
for decomposition to MgO: calc.: 71.7%; found: 69.7%. IR
(KBr) v/cm-' : 2959, 2872 (s, C-H str.); 1461 (m, C-H def.);
1117, 1094 (vs, C - 0 str.); 537 (m, Mg-0).
Synthesis ofMg(0 i-C3H,)2
Diethylmagnesium (0.5 g, 6.07 mmol) was suspended in 20 ml
dry n-hexane. Propan-2-01 (4.5 ml, 59 mmol) was slowly
added. After 10 min the exothermic reaction was complete
and the mixture was stirred for another 24 h. The solvent was
removed and the white powder dried in uacuo. Yield: 96%
(0.83 8). Anal. calc.: C, 50.6; H, 9.9%. Found: C, 49.0; H, 10.1%.
Mass loss in TG for decomposition to MgO: calc.: 71.7%;
found: 70.4%. IR (KBr) v/cm-':2962, 2929, 2864, 2844 (s,
C-H str.); 1464, 1377, 1364, 1347 (m, C-H def.); 1165, 1132,
989 (vs, C - 0 str.); 883 (m, C-C str.); 582-404 (s, Mg-0).
Synthesis ofMg(OC(CH,),),
Diethylmagnesium (0.65 g, 7.88 mmol) was suspended in 50 ml
dry n-hexane. tert-Butyl alcohol (1.7 ml, 18 mmol) was then
slowly added to the mixture. After the mixture had been
stirred for 24 h, the solvent was removed under vacuum, and
the white powder dried in uucuo for 5 h. Yield: 97% (1.3 g).
Anal. calc.: C, 56.3; H, 10.6%. Found: C, 53.6; H, 11.0%.
Mass loss in TG for decomposition to MgO: calc.: 76.3%;
found: 74.0%. IR (KBr) v/cm-':2972 (s, C-H str.); 1473,
1385, 1361 (s, C-H def.); 1205 (vs, C-0 str.); 574, 499
(m, Mg-0).
Synthesis of Mg(0CH ( CH3)C6H13)2
Diethylmagnesium (0.5 g, 6.07 mmol) was suspended in 50 ml
dry n-hexane. Octan-2-01 (2.1 ml, 13 mmol) was added and
the mixture stirred for 24 h. The gelatinous product was dried
in uucuo by heating it (ca. 60-70°C) for 5 h. Yield: 93%
(1.59g). Anal. calc.: C, 68.0; H, 12.1%. Found: C, 67.3;
H, 12.5%. Mass loss in TG for decomposition to MgO: calc.:
86.8%; found: 85.9%. IR (KBr) vlcm-' : 2961, 2927, 2858 (s,
C-H str.); 1467, 1377 (s, C-H def.); 1141 (vs, C-0 str.);
586 (m, Mg-0).
Methods of Characterization
IR Spectroscopy
IR spectra of the six magnesium alcoholates were measured
on a Perkin-Elmer FT-IR 1720. In all cases the moisture-
sensitive alcoholates were mixed with dry KBr under nitrogen
and pressed at 125 bar.
Crystal Structure Analysis
Single-crystal data of Mg(OCH,)2.2CH,0H were measured
on an Enrafo Nonius CAD4 diffractometer (Cu-Ka radiation,
A = 1.54178 A). The structure solution was carried out using
the program SHELXS-86,12 the structure was refined with
SHELXL-93.13
Scunning Electron Microscopy
The SEM analysis of MgO from Mg(OCH3)2 and
Mg(OC2H5)2 was performed using a Philips XL20 scanning
electron microscope; the other four MgO types were examined
with a JEOL JSM 6400 scanning electron microscope.
Thermal Analysis
The thermoanalytical behaviour of the alcoholates was meas-
ured with a simultaneous thermal analyser (Netzsch STA
409C/MS) which simultaneously measures DTA, thermogravi-
metry and evolved reaction gases by mass spectrometry. The
apparatus is equipped with a quadrupole mass spectrometer
(Balzers QMS 421). The analysis of the six alcoholates was
made in a dynamic air atmosphere with a flow rate of
50 ml min-', unless otherwise specified. The typical sample
quantity was between 30 and 70 mg. The samples were heated
in alumina crucibles with heating rates ranging from 0.2 to
10 K min-'.
Results and Discussion
The crystalline magnesium methanolate solvate complex was
examined by elemental (C, H) and thermogravimetric analysis
to determine the crystal methanol content. Combination of
the results led to the formula Mg(OCH,)2.2CH,0H for the
moisture-sensitive solvate crystals.
We attempted to determine the crystal structure of
Mg(OCH3),-2CH3OH by single-crystal X-ray structure analy-
sis. However, owing to the extensive disorder which frequently
occurs among alcohol ate^,'^ it was not possible to obtain a
good structure refinement. We were able to determine some
structural elements after refining the structure in the ortho-
rhombic space group Ccca with 2 = 64. Crystallographic data
are given in Table 1. The main structural elements are isolated
heterocubane units with Mg and OCH, ions in the corners
(Fig. 1). In addition, the Mg ions are surrounded by a
distorted oxygen octahedron (Fig. 2). The oxygen atoms in
the heterocubane belong to methoxy groups CH,. The mol-
ecules which are fixed in the octahedron but not in the
heterocubane are CH,OH molecules.
The structure can also be described as a cube composed of
four Mg ions linked together by four p,-methoxy oxygen
atoms. From outside the cube, each Mg ion is coordinated by
one terminal methanolate and two methanol molecules, which
leads to eight methanolate and eight methanol molecules in
the structure. It is impossible to describe the superlattice
Table 1 Crystallographic data for Mg(OCH3),.2CH30H
formula
mol mass/g mol-'
space group
Z
9/oc
cell parameters/A
VIA3
Pcalclg cm -
2Oddegrees
Q/A
crystal size/mm3
p/cm
observed reflections
significant reflections
R
diffractometer
MgC4H
150.31 14O4
Ccca (International Tables: no. 68)
64
- 100
u = 24.21 1 _+ 0.005
b = 31.857 f 0.006
c = 23.434 f 0.005
18074.40
0.884
2.25-76.50
1.54178 (CU-Ka)
0.5 x 0.2 x 0.1
9.82
19183
18317
0.135
CAD4, Enraf Nonius
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J. MATER. CHEM., 1995, VOL. 5
59 1
,CH3
0
Fig. 1 Schematic drawing of the magnesium-oxygen heterocubane in
Mg(OCH3),-2CH30H. The cube is composed of four Mg ions linked
together by four p3-methoxy oxygen atoms. One Mg-0 octahedron
is also shown.
0
I0
Fig. 2 SCHAKAL drawing of the oxygen octahedral coordination of
Mg2+ in Mg(OCH3),.2CH,0H. The oxygen atoms (small spheres) in
the octahedron are part of methoxy groups or of methanol molecules.
structure of magnesium methanolate because of the high
degree of disorder.
The heterocubane structure was recently observed for some
barium alcohol ate^.'^ It is known for alkyl magnesium
alcoholates'6 and many metal organic compounds, e.g.
met hylli t hium. l7
Thermochemical Reactivity
The thermal behaviour of the six alcoholates under a dynamic
air atmosphere (heating rate 10 K min-') is different for
alcoholate groups of different size. All alcoholates decompose
in steps. The product gases are generally water and the
corresponding alcohol at lower temperature, and carbon
dioxide due to combustion at higher temperature.
Additionally, with the higher alcoholates a number of
hydrocarbons were detected, especially C2 and C3 fragments,
as there is a high tendency towards p-elimination for
longer carbon chains. Eqn. (3) describes the reaction for
Mg(°CH2CH3 )2.
H
I
-Mg-OCH2CH2-+ -Mg- OH + C2H4
(3)
Final decomposition temperatures to MgO for all alkoxides
are listed in Table 2. Mg(OCH3)2 decomposes to MgO at the
lowest temperature (400°C) and therefore seems to be the
most suitable precursor for magnesia. The preparation of
magnesium methanolate is also very easy.
We also examined Mg(OCH3)2 in a dynamic argon atmos-
phere with a heating rate of 10K min-'. It decomposes in
Table 2 Final decomposition temperatures for six magnesium alk-
oxides under a dynamic air atmosphere as determined by thermo-
gravimetry; the samples were heated at a rate of 10 K min-'
alcoholate
decomposition temperature/"C
methanolate
ethanolate
n-propanolate
propan-2-olate
2-methylpropan-2-olate
octan-2-olate
400
448
48 5
44 1
474
450
three steps at 80, 390 and 420 "C. The main reaction products
are methanol, water and ethene. The absence of oxygen leads
to pyrolysis:
CH3-0-Mg-O-CH3
MgO+H,O+C2H, (4)
The MgO obtained in this way is a black solid. Obviously,
some unidentified side reactions lead to small amounts of
magnesium carbide and/or carbon which precipitate on the
white magnesium oxide. This could not be clarified because
X-ray diffraction showed no patterns other than those of
MgO. Elemental analysis gave 7.6% C and 12.5% H. Thermal
decomposition of the 'black MgO' in a dynamic air atmos-
phere showed the formation of C02 which confirms the
presence of carbon.
Mechanistic Studies: Thermal Decomposition of
Mg(OCH3),.2CH,OH
To learn more about the decomposition process itself, we
studied the decomposition of magnesium methanolate in
detail. This was found to be much more complex than expected
because the reaction intermediates are amorphous and moist-
ure-sensitive. Therefore, X-ray diffractometry is not applicable.
During an in situ decomposition in a heated X-ray chamber
only the final product MgO could be detected. The consider-
able moisture-sensitivity of the precursor and the intermedi-
ates further complicated the analysis. By combination of IR
spectroscopy, elemental analysis and TG-DTA-MS, it was
possible to derive probable formulae for the intermediates.
Mg(OCH3),.2CH30H decomposed in three steps (Fig. 3
and 4). The first step between 80 and 90°C gave a mass loss
of 50.9% with the reaction products water and methanol and
the loss of solvate methanol. The IR spectrum of the substance
shows a new absorption band at 1623cm-1 which can be
attributed to bridging OH groups. This leads to the assump-
tion that this intermediate A must have a formula like
Mg(OCH3)x(OH),.
E.....
I .... I .
. 1 . . . . I . . . . , . . . I .... I ....
0
50
100
150
200
250
tlmin
Fig. 3 TG-DTA diagram of Mg(OCH3),.2CH30H (63.28 mg) in air;
heating rate 2 K min-'
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h -2OF
s
F
v
d
-40
-60
J. MATER. CHEM., 1995, VOL. 5
1
L
Comparative SEM Analysis of the Magnesium Oxides
SEM examination of the different magnesium oxides prepared
by thermal decomposition of the six alkoxides in air showed
significant differences in size, porosity and microcrystallinity
of the product particles. This may be an important result for
possible applications of MgO. Fig. 5(a)-(e) shows images of
MgO particles obtained by decomposition of different
alcoholates with a heating rate of 10 K min-' . The cubic MgO
particles formed from Mg(OCH,), [Fig. S(a)] have a size of
ca. 300 nm; those from Mg(OC,H5), [Fig. 5(b)] have only a
size of ca. 100nm. This trend seems to continue for larger
alcoholates [Fig. 5(c)-(e)]. For the largest alkyl chain used,
0 !
magnesium 2-octanolate, no discrete particles are present in
the MgO, only a system of cavities with different sizes
.
100
200
300
400
TI'C
[Fig. S(f)]. However, the reason for this may also be the
different morphology (gelatinous instead of powdery) of this
alcoholate.
This leads to the conclusion that the particle size of
Fig. 4 TG-MS diagram of Mg(OCH3)2.2CH,0H (63.28 mg) in air;
heating rate 2 K min '. Same experiment as in Fig. 3.
Calculations which combine the results of TG data and
elemental analysis led to the following formal formula for
intermediate A:
Mg02Cl .5H5
Mg(°CH3)1.5 (OH),.,
The following formal structure with bridging OH groups
could be a possibility for intermediate A:
H
I
CH3
1"
magnesium oxide can be controlled by choice of the appro-
priate alcoholate. Three different aspects are responsible for
the particle size and porosity: (i) the use of precursor crystals
of variable size and shape; (ii) the route of thermal decompo-
sition of the precursor, especially the variation of heating rate,
final temperature and annealing time of the sample; (iii) the
molecular size of the alcoholate group.
The size of the alcoholate group obviously plays an import-
ant role in determining the particle size. When comparing X-
ray powder patterns for MgO from magnesium methanolate
and magnesium ethanolate, it is observed that the X-ray
reflections are broader in the latter case, due to the smaller
particle size. This is understandable because the larger etha-
nolate group leads to larger distances between the molecular
units in the alcoholate crystal. When the alcoholate group is
removed by thermal treatment, the remaining domains are
smaller for magnesium ethanolate. These results demonstrate
the possibility of preparing magnesia in different forms and
porosities for a wide range of possible applications.
Conclusions
(Anal. calc.: C, 22.7; H, 6.3%. Found: C, 23.3; H, 6.7%).
Between 150 and 200°C we found an exothermic DTA
peak without any mass loss. This leads to the assumption
that intermediate A recrystallizes or forms a new phase.
The decomposition of the substance continues in the range
360-400°C where we find two TG steps. In both steps the
reaction products are water and now additionally carbon
dioxide. The first mass loss of 16.5% (350°C) leads to the
second intermediate B. The IR spectrum of this air-sensitive
substance shows an enlarged absorption in the range of
1620 cm-' and a decrease of the C-H and C - 0 absorptions.
This suggests that more OH groups and fewer OCH3 groups
are now present in the sample. Calculations which combine
TG data and results from elemental analysis show that
intermediate B has the following formal formula:
This work has shown that magnesium alcoholates are a group
of promising precursor substances for the preparation of
tailor-made magnesia. The synthesis is easy because the lower
alcoholates can be prepared by direct reaction of magnesium
metal and alcohol. Higher alcoholates can be obtained by
reaction of diethyl magnesium and the corresponding alcohol.
The simple preparation with no byproducts is an important
aspect for possible applications of magnesia.
Magnesium alcoholates decompose to MgO at compara-
tively low temperatures (400-500 "C). The six examined
alcoholates show different decomposition temperatures
depending on the alcoholate group. Magnesium methanolate
Mg(OCH,), seems to be the best precursor for MgO because
it has the lowest decomposition temperature of all the exam-
ined alcoholates (400 "C).
We also studied the morphology of the resulting magnesium
Mg02C0.65H3 Mg(°CH3)0.65(0H)1.35
(Anal. calc.: C, 11.6; H, 4.5%. Found: C, 11.8; H, 4.6Oh.)
The last TG step with a mass loss of 4.9% (390 "C) leads
to pure magnesium oxide MgO. Magnesium oxide (as the
product in all thermoanalytical experiments) is characterized
by IR spectroscopy, which shows only the strong Mg-0
absorption between 500 and 400crn-', and also by X-ray
diffraction (Guinier method).
We did not perform mechanistic studies on the other
alcoholates because of the increasing complexity of the
decomposition. This is obvious from the increasing number
of decomposition steps and gaseous reaction products.
oxides when different alcoholates were decomposed. SEM
analysis showed significant differences in the morphology,
microcrystallinity and porosity of the MgO particles
depending on the alcoholate used. The particle size of MgO
decreases with increasing alcoholate group size. This is an
important result for the possible applications of MgO as
a catalyst.
We attempted to determine the crystal structure of
Mg(OCH3),.2CH30H. Owing to the extensive structural dis-
order, it was not possible to obtain a good structure refine-
ment. However, we were able to determine a heterocubane
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J. MATER. CHEM., 1995, VOL. 5
593
Fig. 5 Scanning electron micrographs of magnesium oxides prepared from different precursors: (a) precursor Mg(OCH,), (30 000 x ); (b) precursor
Mg(OC2H5), (50000 x); (c) precursor Mg(0 i-C3H7)2 (5000 x); (d) precursor Mg(OC,H,), (10000 x); (e) precursor Mg(OC(CH,),), (5000 x);
(f) precursor Mg(OCH(CH3)C6HI3), (2000 x)
unit with Mg2+ and OCH,- ions in the corners as the main
structural element.
We studied in detail the thermal decomposition of mag-
nesium methanolate in air. The substance decomposes in three
steps at 90, 330-350 and 360-400°C. We identified two
intermediates which were characterized by thermogravimetry,
elemental analysis and IR spectroscopy. As the reaction
progressed we found an increase in the number of OH groups
and a decrease in the number of OCH, groups in the
intermediates. Because of the amorphous nature of the inter-
mediates, no X-ray diffraction data could be obtained.
We thank Dr. P. M. Wilde (Berlin) and Prof. Dr. H. K.
Cammenga (Braunschweig) for SEM analyses of the different
magnesium oxides. We also thank Dr. M. Morf for the
SCHAKAL drawings of the crystal structure of Mg(OCH,),.
References
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Paper 4/05424E; Received 6th September, 1994
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