Figure 7 . (a) Conversion of benzyl alcohol and (b) selectivity
of BTMB with time-on-stream on synthesized zeolites; (c) the proposed
reaction networks and (d) scheme showing the transformation of benzyl
carbocation.
3.5
Coke analysis
A
part of spent catalysts was taken for thermogravimetric analysis (TGA)
to investigate the coke formed in MEL zeolites. Fig. S16 showed the
weight loss curves of spent
H-MEL-31,
P-MEL@Fe and H-MEL@Fe-20 samples, which exhibited three distinct regions
of weight loss for all of samples in temperature ranges of 25‒150oC,
150‒350oC
and
350‒700oC, respecively. Generally, the weight loss below 150oC was attributed to the moisture and adsorbed water
in zeolite cavities and
channels.45 The mass
loss observed in the
temperature
range of 150‒350 oC was assigned to soft coke that
occurred at a moderate temperature and can be extracted by organic
solvent, 46 and the
weight loss of
H-MEL-31,
P-MEL@Fe and H-MEL@Fe-20 in the temperature range of 350‒700oC can be classified as hard coke or insoluble coke.
It was worth noting that the proportion of hard coke in the total coke
for H-MEL-31, P-MEL@Fe and H-MEL@Fe-20 was about 35 %, 27% and 48 %,
respectively. Therefore, we deduced that the
coke
formed during the alkylation of mesitylene with
benzyl
alcohol over the investigated MEL zeolite catalysts was mainly soft
coke.47
To further investigate the composition of the soluble coke on the used
catalyst, the soft coke in H-MEL-31, P-MEL@Fe and H-MEL@Fe-20 were
extracted by 1,3,5-trimethylcyclohexane and subsequently were analyzed
by
GC-MS,47,48as shown in Fig. S17. It was observed that these soft cokes were mainly
composed of raw materials, solvents, products and internal standard
substances (dodecane), the polyaromatics and
muti-methybenzenes
were not detected, which was in agreement with the TGA analysis that the
main coke was soluble
coke.