Chemistry Reference
In-Depth Information
Caba˜as, Pollikoff and co-workers have deposited Pd on
mesoporous SBA-15 using SC-CO
2
as a solvent with palladium hexa-
fluoroacetylacetonate (pd(hfac)
2
) as the precursor salt.
69,70
The process
involved the addition of pd(hfac)
2
and SBA-15 into a sealed stainless-steal
reactor, which was then heated to the required temperature before the
vessel was charged with CO
2
to the desired pressure. The system was then
left under the required conditions for 3-16 h to allow for adsorption of
pd(hfac)
2
onto the SBA-15. Decomposition of pd(hfac)
2
to metallic Pd was
carried out using two different procedures; one involved adding H
2
to the
SC-CO
2
after the pd(hfac)
2
deposition step and the other involved de-
pressurisation of the SC-CO
2
system followed by reduction under pure H
2
at 60 bar and 40 1C. Adsorption isotherms of pd(hfac)
2
onto SBA-15 were
measured at conditions between 40-80 1C and 85-140 bar. It was found
that when the temperature was kept constant, but the density of the SC-
CO
2
was increased, by increasing system pressure, pd(hfac)
2
adsorption
onto SBA-15 decreased. Adsorption also decreased when the temperature
was increased and the density was maintained constant. These results
indicated that pd(hfac)
2
adsorption onto SBA-15 was relatively weak with
the solubility of pd(hfac)
2
in the SC-CO
2
dictating the maximum attain-
able adsorption. This knowledge and control of the adsorption process
then enabled the authors to successfully target preparation with various
Pd loadings.
Pd/SBA-15 catalysts prepared with Pd loadings of 1.1, 4.3 and 9.6 wt%
on SBA-15 by this method were tested for the hydrogenation of iso-
phorone to trimethylcyclohexanone in SC-CO
2
at 100 bar over the tem-
perature range of 50-250 1C. Pd crystallite sizes, calculated by application
of the Scherrer equation, were broadly similar given the errors associated
with the calculations. Average crystallite sizes were in the range 5-8 nm
for the three samples. Analysis of the porosity indicated that at low
loadings Pd blocked the micropores of SBA-15, whilst at higher loadings
Pd agglomerates were formed in the mesoporous channels.
Light off experiments showed that T
50
(temperature required to achieve
50% conversion of isophorone) was
160 1C for 4.3 wt% Pd/ SBA-15 and as
low as 55 1C for 9.6 wt% Pd/ SBA-15, whilst isophorone conversion only
reached 35% by the maximum 250 1C temperature investigated for 1.1 wt%
Pd/ SBA-15. Selectivity towards trimethylcyclohexanone was reported to be
100 % for the two lower loadings, while the 9.6 wt% Pd/ SBA-15 catalyst was
reported to produce unspecified amounts of 3,3,5-trimethylcyclohexanol.
In addition to the lower selectivity towards trimethylcyclohexanone the
9.6 wt% Pd/ SBA-15 catalyst did not display a conventional light off curve, as
conversion oscillated as the temperature was increased, implying a degree of
catalyst instability. The catalysts prepared by supercritical deposition of
pd(hfac)
2
were compared against a commercial 2 wt% Pd/SiO
2
-Al
2
O
3
catalyst
for isophorone hydrogenation, which had a T
50
value of
B
210 1C. The
ca. 50 1ClowerT
50
value of the stable 4.3 wt% Pd/ SBA-15 catalyst, relative to
the commercial catalyst, shows the viability of the supercritical deposition
technique to make ecient hydrogenation catalysts.
Platinum deposition onto activated carbon and carbon nanotubes, using
dimethyl(1,5-cyclooctadiene)platinum(II) (PtMe
2
COD) as a precursor was
B
