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).