7 Heterogeneous Catalysis by Frustrated Lewis Pairs
259
methanol, by direct analogy with the homogeneous system explored by O’Hare et al.
[82]. Note that this impregnation of a porous polymer network containing Lewis
basic sites with the strong Lewis acid B(C 6 F 5 ) 3 is conceptually similar to the previously discussed examples from Thomas and Rose (Figs. 3 and 4) [33, 35]. However,
in the case of MOF-545/B(C 6 F 5 ) 3 combination, the acid–base interaction is sufficiently strong enough to enable recycling of the heterogeneous catalyst for at least
three cycles without the need to add additional Lewis acid for each run. PXRD characterisation of the MOF after these reaction cycles is essentially unchanged relative
to the starting material, which highlights the stability of the system under relatively
harsh conditions (40 bar H 2 /CO 2 , 100 °C, 20 h).
This section has highlighted the benefits that can be achieved by combining the
metal-free reactivity of FLPs with the vast structural diversity of MOFs. There are
many different approaches that can be taken, whether that is incorporating either a
Lewis acidic site or a Lewis basic site in the framework of the MOF, or both, or by
impregnating/grafting Lewis pairs into the pores of MOFs. The research on this topic
is very much in its infancy, and there are innumerable opportunities for exploiting
the topology and size of pores to achieve selectivities in FLP-mediated reactions that
are unattainable in solution; for example, carrying out reactions in a chiral cavity to
effect asymmetric catalysis by an FLP.
7.3.4 Mesoporous Silica
In addition to MOFs, other porous structures have been explored as hosts for FLP
reactivity. Kleitz et al. designed mesoporous solid-supported Lewis acid–base pairs
and studied their adsorption behaviour towards CO 2 [83]. The solid Lewis acids
were synthesised by the metalation of silica SBA-15 with appropriate precursors to
afford Ti
4+ -, Zr
4+ - and Al
3+ -deposited mesoporous materials M-SBA-15 (M=Ti, Zr,
Al), according to a previously reported protocol [84]. These were then impregnated
with a range of Lewis bases, including 2,2,6,6-tetramethylpiperidine (TMP), triethylamine and tris(tert-butyl)phosphine to yield 28, 29 and 30, respectively (Fig. 7.15a),
although certain combinations could not be fully characterised. For comparison
purposes, systems were also developed where the amine or phosphine Lewis base
was grafted onto the surface of the silica (31 and 32 in Fig. 7.15b). The porosity of the
materials was measured by low temperature N 2 physisorption, and as expected the
surface area for each system decreases after impregnation/grafting of the Lewis base;
and the compounds were further characterised using solid-state NMR spectroscopy
and EDX (energy-dispersive X-ray) spectroscopy.
The Lewis acidity of the systems was probed by adsorption of pyridine, and it was
shown that the Lewis acidity of the metal surfaces was preserved even in the presence
of the other Lewis bases. CO 2 binding was quantified by measuring the low-pressure
adsorption isotherms at three different temperatures, and it was shown that generally
the systems with the grafted Lewis base (31 and 32) had a higher CO 2 sorption ability
than the impregnated systems (28-30). The Ti Lewis acid sites interact too strongly
259
methanol, by direct analogy with the homogeneous system explored by O’Hare et al.
[82]. Note that this impregnation of a porous polymer network containing Lewis
basic sites with the strong Lewis acid B(C 6 F 5 ) 3 is conceptually similar to the previously discussed examples from Thomas and Rose (Figs. 3 and 4) [33, 35]. However,
in the case of MOF-545/B(C 6 F 5 ) 3 combination, the acid–base interaction is sufficiently strong enough to enable recycling of the heterogeneous catalyst for at least
three cycles without the need to add additional Lewis acid for each run. PXRD characterisation of the MOF after these reaction cycles is essentially unchanged relative
to the starting material, which highlights the stability of the system under relatively
harsh conditions (40 bar H 2 /CO 2 , 100 °C, 20 h).
This section has highlighted the benefits that can be achieved by combining the
metal-free reactivity of FLPs with the vast structural diversity of MOFs. There are
many different approaches that can be taken, whether that is incorporating either a
Lewis acidic site or a Lewis basic site in the framework of the MOF, or both, or by
impregnating/grafting Lewis pairs into the pores of MOFs. The research on this topic
is very much in its infancy, and there are innumerable opportunities for exploiting
the topology and size of pores to achieve selectivities in FLP-mediated reactions that
are unattainable in solution; for example, carrying out reactions in a chiral cavity to
effect asymmetric catalysis by an FLP.
7.3.4 Mesoporous Silica
In addition to MOFs, other porous structures have been explored as hosts for FLP
reactivity. Kleitz et al. designed mesoporous solid-supported Lewis acid–base pairs
and studied their adsorption behaviour towards CO 2 [83]. The solid Lewis acids
were synthesised by the metalation of silica SBA-15 with appropriate precursors to
afford Ti
4+ -, Zr
4+ - and Al
3+ -deposited mesoporous materials M-SBA-15 (M=Ti, Zr,
Al), according to a previously reported protocol [84]. These were then impregnated
with a range of Lewis bases, including 2,2,6,6-tetramethylpiperidine (TMP), triethylamine and tris(tert-butyl)phosphine to yield 28, 29 and 30, respectively (Fig. 7.15a),
although certain combinations could not be fully characterised. For comparison
purposes, systems were also developed where the amine or phosphine Lewis base
was grafted onto the surface of the silica (31 and 32 in Fig. 7.15b). The porosity of the
materials was measured by low temperature N 2 physisorption, and as expected the
surface area for each system decreases after impregnation/grafting of the Lewis base;
and the compounds were further characterised using solid-state NMR spectroscopy
and EDX (energy-dispersive X-ray) spectroscopy.
The Lewis acidity of the systems was probed by adsorption of pyridine, and it was
shown that the Lewis acidity of the metal surfaces was preserved even in the presence
of the other Lewis bases. CO 2 binding was quantified by measuring the low-pressure
adsorption isotherms at three different temperatures, and it was shown that generally
the systems with the grafted Lewis base (31 and 32) had a higher CO 2 sorption ability
than the impregnated systems (28-30). The Ti Lewis acid sites interact too strongly