Moreover, to reveal the role of iron species in the reaction, the catalytic
performance of H-M-Mor was also evaluated under the same reaction
conditions (benzyl chloride as the reactant). As a result, the
reaction can not be carried out even after a reaction time of 2 h. This
means that Fe-M-Mor had a much stronger reactivity than H-M-Mor
in the reaction. The introduction of iron species can significantly enhance
the catalytic performance of the catalyst. To make the point
clear, the state and content of iron species in Fe-Mor and Fe-M-Mor
were determined by UV–vis spectroscopy and X-ray photoelectron
spectroscopy. The UV-spectra of both samples showed a strong absorption
band below 300 nm and absorption bands above 300 nm (Fig. 4c).
Generally, the absorption band below 300 nm is attributed to isolated
iron species and the absorption band between 300 and 400 nm is corresponding
to iron oxide nanoclusters. Bulk iron oxide exhibits the absorption
band between 400 and 600 nm [22]. Obviously, Fe-M-Mor
exhibited a relatively strong adsorption band around 350 nm corresponding
to the iron oxide nanoclusters besides the absorption band
below 300 nm. This means that Fe-M-Mor contained not only isolated
iron species but also many iron oxide nanoclusters. It has been
suggested that nano-sized Fe2O3 clusters with a high degree of coordinative
unsaturation should be much more reactive iron species for the
reaction and even that isolated iron species are inactive [18,19]. Thus,
the presence of more iron oxide nanoclusters could contribute to the
improvement of the catalytic activity. The nanoclusters may be formed
during the calcination of the ion-exchanged samples in air at high temperature.
It can be imagined that mesoporosity could be helpful for the
formation of iron oxide nanoclusters because they may be located in the
mesopore channels. The result from XPS showed that Fe-M-Mor possessed
higher iron content than Fe-Mor, nearly two times as that in
Fe-Mor (Table 2). Considering that both samples were obtained by the
same ion-exchanged method, the relatively high iron content in
Fe-M-Mor could be attributed to the improvement of the exchangeability
due to the presence of mesopores. Additionally, the surface state of
iron species in both samples from XPS analysis is similar and the iron
valence is mainly tri-valence. Considering this fact that the apparent reaction
rate constant for Fe-M-Mor (155.4×10−3 min−1
) is ten times as
that for Fe-Mor (14.7×10−3 min−1
) and the iron content in Fe-M-Mor
is about two times as that in Fe-Mor (Table 2), the high catalytic activity
for Fe-M-Mor can not be solely attributed to the increase of iron content.
The mesoporosity and more active iron sites in Fe-M-Mor should
be mainly responsible for the enhancement of catalytic activity. Based
on the above results, it can be concluded that such a big difference in
catalytic activity should be mainly attributed to the existence of
mesopores that greatly overcame the diffusion limitation from the
microporous network and more active iron sites in Fe-M-Mor.