7 Heterogeneous Catalysis by Frustrated Lewis Pairs
273
split H 2 with low-energy barriers, and be able to promote the reduction of acetone,
HCN and ethylene. Given the findings that it is possible to experimentally access
certain doped phosphorene materials [139], it will be interesting to see whether
these materials can be synthesised in the future, and whether they can act as effective
heterogeneous FLP catalysts.
7.4 Concluding Remarks
This chapter has showcased the myriad possibilities of developing heterogeneous
FLP systems. Semi-immobilised FLPs consist of either a solid Lewis acid or base
paired with a complementary soluble molecular Lewis base or acid, respectively.
The semi-immobilised systems are typically easy to access, and can effectively
carry out heterogeneous catalysis, but due to one of the FLP components being
soluble they often still suffer from issues involving work-up of the product and catalyst recyclability. The fully immobilised systems feature both the Lewis acid and
base on (or within) the same solid support, and therefore tend to exhibit enhanced
catalyst stability and recyclability. The solid supports have taken a wide range of
guises, including silica, zeolites, MOFs, polyoxometalate clusters, metal oxides and
graphene. As well as the usual advantages of heterogeneous over homogeneous catalysis, there are often added benefits to these systems over simple molecular FLPs.
For example, carrying out the FLP reactions inside the porous cavity of a zeolite
or MOF enables an additional level of control over selectivity, as larger substrates
cannot access the reactive site. It will be interesting to see in the future how far this
concept can be extended; can chiral cavities be used as hosts for FLPs to promote
asymmetric catalysis?
This chapter has also highlighted how far the definition of FLPs has evolved since
the original discovery in 2006, which involved the preclusion of Lewis acid/base
adduct formation due to sterics [2, 140]. The contemporary chemical literature
demonstrates that a wide array of reactions and interactions can be related to FLP
chemistry, including transition metal reactivity, metal–ligand cooperativity and cooperative surface–ligand interactions. The example of surface chemistry is an interesting
one, as the importance of the acidic and basic properties of surface sites (due to
defects, dopants, vacancies, etc.) had been known and studied for decades before the
advent of FLP chemistry [141]. It is arguably unnecessary to invoke FLPs to explain
the catalytic activity of graphene, for example; however, that completely misses the
point. By drawing parallels between the two fields, and in fact between FLPs and any
other fields mentioned in this chapter and elsewhere, it allows the coming together
of two otherwise separate research communities to share knowledge and advance
both areas of study. The detailed mechanistic knowledge of molecular FLPs can be
used to tailor the reactive sites in metal oxide surfaces, and to fine-tune their ability
to promote small-molecule activation or catalysis. Alternatively, the heterogeneous
materials that are capable of promoting reactivity that is currently unattainable by
homogeneous FLPs, such as methane activation, can serve as inspiration for the
273
split H 2 with low-energy barriers, and be able to promote the reduction of acetone,
HCN and ethylene. Given the findings that it is possible to experimentally access
certain doped phosphorene materials [139], it will be interesting to see whether
these materials can be synthesised in the future, and whether they can act as effective
heterogeneous FLP catalysts.
7.4 Concluding Remarks
This chapter has showcased the myriad possibilities of developing heterogeneous
FLP systems. Semi-immobilised FLPs consist of either a solid Lewis acid or base
paired with a complementary soluble molecular Lewis base or acid, respectively.
The semi-immobilised systems are typically easy to access, and can effectively
carry out heterogeneous catalysis, but due to one of the FLP components being
soluble they often still suffer from issues involving work-up of the product and catalyst recyclability. The fully immobilised systems feature both the Lewis acid and
base on (or within) the same solid support, and therefore tend to exhibit enhanced
catalyst stability and recyclability. The solid supports have taken a wide range of
guises, including silica, zeolites, MOFs, polyoxometalate clusters, metal oxides and
graphene. As well as the usual advantages of heterogeneous over homogeneous catalysis, there are often added benefits to these systems over simple molecular FLPs.
For example, carrying out the FLP reactions inside the porous cavity of a zeolite
or MOF enables an additional level of control over selectivity, as larger substrates
cannot access the reactive site. It will be interesting to see in the future how far this
concept can be extended; can chiral cavities be used as hosts for FLPs to promote
asymmetric catalysis?
This chapter has also highlighted how far the definition of FLPs has evolved since
the original discovery in 2006, which involved the preclusion of Lewis acid/base
adduct formation due to sterics [2, 140]. The contemporary chemical literature
demonstrates that a wide array of reactions and interactions can be related to FLP
chemistry, including transition metal reactivity, metal–ligand cooperativity and cooperative surface–ligand interactions. The example of surface chemistry is an interesting
one, as the importance of the acidic and basic properties of surface sites (due to
defects, dopants, vacancies, etc.) had been known and studied for decades before the
advent of FLP chemistry [141]. It is arguably unnecessary to invoke FLPs to explain
the catalytic activity of graphene, for example; however, that completely misses the
point. By drawing parallels between the two fields, and in fact between FLPs and any
other fields mentioned in this chapter and elsewhere, it allows the coming together
of two otherwise separate research communities to share knowledge and advance
both areas of study. The detailed mechanistic knowledge of molecular FLPs can be
used to tailor the reactive sites in metal oxide surfaces, and to fine-tune their ability
to promote small-molecule activation or catalysis. Alternatively, the heterogeneous
materials that are capable of promoting reactivity that is currently unattainable by
homogeneous FLPs, such as methane activation, can serve as inspiration for the