Figure 6 . Plots of the fractional adsorption uptakes
[(Qt-Q0)/(Qe-Q0)]
of benzene (a),toluene (b) against the square root of adsorption time,
and proposed diffusion path of benzene/toluene molecules during
adsorption over (c) P-MEL@Fe and (d) H-MEL-31/H-MEL@Fe-20 samples.
Schematic illustrations of these two distinct diffusion pathways (i.e.
path 1: intracrystalline diffusion; path 2: surface diffusion) over
either conventional or hierarchical MEL zeolites were shown in Fig. 6c
and Fig. 6d, respectively. In general, the surface diffusion is much
faster than intracrystalline diffusion that is performed through the
complicated porous network due to the limited mass transport. For
P-MEL@Fe with solely micropores (Fig. 6c), molecular diffusion across
the external surface of MEL zeolites (path 2) dominated, together with a
larger D/r2 value, as shown in Table S4. For
hierarchical H-MEL-31 and H-MEL@Fe-20 zeolites with highly
interconnected and open mesopores (Fig. 6d), the intracrystalline
molecular diffusion (path 1) was largely promoted along with a reduced
D/r2 value (Table S4). Interestingly, such a marked
difference in molecular diffusion between conventional and hierarchical
MEL zeolites over the non-polar benzene was totally absent for
experiments carried out over the polar toluene, where enhanced surface
diffusion and a larger D/r2 value was observed. This
indicated that the chemical environment of the internal pore walls of
hierarchical MEL zeolites favored the interaction with non-polar
adsorbates and associated intracrystalline molecular diffusion.
3.4
Catalytic activity
In order to evaluate the catalytic performance of resultant MEL
zeolites, the alkylation of
mesitylene
with benzyl alcohol was carried out, which is an important reaction for
the production of fine chemicals and pharmaceutical
intermediates.1According to the previous
reports,41,42the kinetic diameter of mesitylene (0.87 nm) is obviously larger than
the micropore size of MEL zeolite. For the liquid benzylation of
mesitylene,5 it is well
known that both the alkylation of mesitylene as the target reaction and
self-etherification of benzyl alcohol as a side reaction can be
catalyzed by either Lewis or Brønsted acid sites throughout the
internal/external surfaces of (hierarchical) MEL zeolite. Thus, the
alkylation reaction between mesitylene with benzyl alcohol takes place
exclusively on the external surface of conventional microporous MEL
zeolite particles, while self-etherification occurs on both external
surface and the much more abundant internal surface of pore walls
simultaneously. On the contrary, the hierarchical single-crystalline MEL
zeolite adopts highly open and interconnected mesopores, which allows
the intracrystalline diffusion of the bulky mesitylene molecules. As
shown in Fig. 7a, for the P-MEL zeolite, no conversion of benzyl alcohol
was observed after 20 h, indicating that P-MEL didn’t have any activity
in alkylation reaction owing to the absence of acidity. While H-MEL-31
showed an increasing conversion of benzyl alcohol with time on stream,
suggesting that the acid sites were active species. Interesting, the
integration of both solid acid and Fe-oxy redox sites in such a
hierarchical MEL zeolite could further enhanced the activity against
benzyl alcohol, due to the involvement of
Fe3+/Fe2+ redox pairs in the
well-accepted redox mechanism for benzylation of mesitylene (Scheme
S1).20
From the perspective of product selectivity, the target product
2-benzyl-1,3,5-trimethylbenzene (BTMB) from the benzylation of
mesitylene was greatly favored over the hierarchical H-MEL-31 zeolite
together with an inhibited self-etherification side reaction that
produced dibenzyl ether (Fig. 7b). The explanation was straightforward
and two-fold: i) the highly open and interconnected mesopores with large
surface areas accommodated the bulky mesitylene and BTMB molecules so as
to promote the target alkylation reaction; ii) the remarkably decreased
affinity of pore walls to polar adsorbates further reduced the
concentration of benzyl alcohol in the porous network and inhibited the
self-etherification side reaction. Notably, based on the reaction
cycles20 (Fig. 7c) and
the much faster self-etherification side reaction, a short “induction
period” with a low BTMB selectivity was observed before the
quasi-equilibrium steady state was reached, when dibenzyl ether also
worked as an alkylating
agent3 towards a high
and stable BTMB selectivity (Fig. S12). Interestingly, we observed a
decrease in BTMB selectivity for the acid-redox co-functionalized MEL
zeolite despite the significantly increased conversion of benzyl alcohol
(Fig. 7b, Fig. S13 and Fig. S14). As we all known that Fe-containing
solid catalysts possessed redox properties (Fe3+ +
e-Fe2+), which dominated the
benzylation reaction regardless of catalysts
acidity.19 In other
words, the incorporation of Fe3+ species in the
zeolite framework greatly enhanced the activity of benzyl alcohol,
resulting in the formation of
benzyl carbocation, which was
quickly transformed into dibenzyl ether. However, the dibenzyl ether
must be hydrolyzed to form benzyl alcohol, and then it can participate
in the alkylation with mesitylene to produce BTMB. But the hydrolysis
rate of dibenzyl ether was considered to be slower than the generation
of dibenzyl ether,43leading to the accumulation of dibenzyl ether, which reduced the
selectivity of BTMB for Fe containing MEL zeolites. The whole
transformation process of benzyl
carbocation was depicted in Fig. 7d, which clearly illustrated the
transition pathway of benzyl carbocation arising from the activation of
benzyl alcohol over combined solid acid and redox
sites.44 In addition, a
leaching test of typical catalyst corroborated that the acid-redox
co-functionalized MEL zeolites exhibited excellent stability against
metal leaching (Fig. S15).