A. Hamaed et al. / Journal of Organometallic Chemistry 694 (2009) 2793–2800
2799
uct was then filtered and washed with a second 100 ml portion of
ethanol. This extraction procedure was repeated twice, and the
crystalline product air-dried at 80 °C. Another three batches of
HMS silica were prepared according to the procedure below, but
using different molar ratios of tetraethyl orthosilicate to dodecyl-
amine (3:1, 2:1, and 1:0.75). Also C -HMS, C -HMS and C10-HMS
6 8
were prepared as per the same procedure, but employing hexyl-
amine, octylamine and decylamine template, respectively.
Samples were run at liquid nitrogen temperature (77 K) and liquid
argon temperature (87 K) to 60 atm.
The skeletal density was measured using a Quantachrome Ul-
tra-pycnometer. When the skeletal density is used for the hydro-
gen uptake measurement, the compressed hydrogen within the
pores is treated as part of the sample chamber volume. Therefore,
only the hydrogen physisorbed to the walls of the structure is re-
corded by the PCI instrument as the adsorption capacity of the
2
material. Thus, when H adsorption is measured, the skeletal den-
sity of the porous materials derived from the pycnometer is em-
ployed to determine the adsorbed amount of hydrogen. However,
when the total storage is measured, a value that is dependent on
the compression level of the solid that contains both the hydrogen
physisorbed to the walls of the pores and the hydrogen existing in
the pores as a second phase, the total desktop density is used in-
stead of the skeletal density. This apparent density ignores the
presence of pores inside the material, and therefore the void space
inside the material is not subtracted out in the measurement.
Taking both the 77 and 87 K hydrogen adsorption data, enthal-
pies of adsorption were calculated using a variant of the Clausius–
Clapeyron equation [22]:
4
.4. Synthesis of Ti grafted mesoporous silicas
0
.2 equiv. of tetrabenzyl titanium (or tribenzyl titanium), as cal-
culated on the basis of 33% Si in the porous oxide was added to a
suspension of HMS mesoporous silica in dry toluene under nitro-
gen. After 1 day of stirring to ensure complete absorption of the
organometallic, the reduced material was collected by suction fil-
tration under nitrogen and washed several times with toluene. In
the case of tetrabenzyl titanium only, the reduced material was
activated by heating at 120 °C for 6 h under vacuum at 10 Torr
on a Schlenk line until all volatiles had been removed. This mate-
rial was used as either a powder or a 1.3 cm diameter pellet com-
ꢀ
3
ꢀ
ꢁ
pressed in a commercial pellet press at 4 metric tones. Assuming
1
P
P
1
2
T
RT
2
ꢀ T
1
2
ln
¼
D
Hads
ð1Þ
g
of 0.2TiBz
4
-HMS, with
a
surface area of 1165 m /g
1
T
2
2
1
2
(
1.165 ꢁ 10 nm ) and 4.12 wt.% Ti for 0.2TiBz
4
-HMS by elemental
ꢀ
4
where P is the pressure for isotherm n, T is the temperature for the
analysis, 1 g of 0.2TiBz
4
-HMS contains 41.2 mg Ti (8.60 ꢁ 10
n
n
isotherm n, and R is the gas constant.
moles Ti). The Ti coverage can then be calculated as follows: Ti
ꢀ
4
23
Pressure as a function of the amount adsorbed was determined
by using exponential fit for each isotherm; the first 10 points of the
isotherms were picked up and fit to the exponential equation. This
coverage = (8.60 ꢁ 10
moles Ti ꢁ 6.02 ꢁ 10 Ti/mole)/(1.16 ꢁ
2
1
2
2
1
0
nm )=0.4443 Ti/nm .
exponential equation gives an accurate fit over the pressure up to
4.5. Synthesis of trimethyl Ti grafted silica [26]
2
1
0 atm with the goodness of fit (R ) above 0.99. The corresponding
1 2 2
P and P values at a certain amount of H adsorbed at both tem-
TiMe
THF at ꢀ40 °C drop wise over 3 h, followed by stirring for another
h. A deep green solution was obtained. The TiMe solution was
3
was prepared by adding TiCl
3
ꢂ 3THF in THF to MeMgCl in
peratures can be obtained by the simulated exponential equation.
Inputting these numbers into Eq. (1), we then calculate the enthal-
pies of the adsorption.
3
3
treated immediately with mesoporous silica placed into a side
adaptor connected to the 3-neck flask reaction compartment. The
temperature was then allowed to warm to ambient over several
hours followed by further stirring at room temperature for 4 h.
At this stage the solution was colorless and the silica had taken
on a green color, suggesting complete absorption of the Ti. The
solution was then filtered under inert atmosphere in the glove
box and washed 3 times with THF in order to purge the sample
Acknowledgments
The authors wish to acknowledge the Natural Science and Engi-
neering Research Council of Canada for funding and Evonic-Degus-
sa Chemicals for providing the 380-Aeorosil sample.
of MgCl
gen before use.
2
. The solid was then dried in vacuo and stored under nitro-
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4
.6. Synthesis of (Allyl)
3
Ti grafted silica [27,28]
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[
[
[
2
the H adsorption–desorption experiments complete reversibility
was observed for all samples across the whole range of pressures.