Paper
RSC Advances
heteroatom MFI zeolite (Experimental section, ESI†), the
ꢀ
ꢀ
temperature of 80 C and 170 C are dened as low temperature
zone and high temperature zone, respectively. As shown in
Fig. 1 and S1 (ESI†), the rise rate of internal FO temperature is
faster than external IR temperature in the Pyrex vessel system,
whereas the former is slower than the latter in the SiC vessel. As
illustrated in Fig. 1c and f, the reaction mixture directly absorbs
the microwave in Pyrex vessel, and then the heat generates and
conducts to the outside wall. Therefore, it will take some time
for the vessel to be warmed “from the inside”. On the contrary,
in the SiC vessel, SiC converts the completely absorbed micro-
wave into heat and then conduct inside, leading to the delayed
response of FO probe. However, the ramping time of reaction
system in Pyrex and SiC vessels are basically the same whether
in low or high temperature zone (Table S1, ESI†). Furthermore,
the specic experiment parameters are depicted in Fig. S2
(ESI†), which shows the stable reaction process. All above
conrm that the investigation can be operated in parallel in the
dedicated microwave reactor equipped with FO probe and the
two vessels.
The synthesis processes of Sn- and Ti-MFI zeolites in two
vessels are monitored by an approximate in situ dynamic light
scattering (DLS) measurement. The effective diameters of as-
prepared zeolites at different crystallization time are obtained
from number-weighted particle size distribution. Variations of
particle diameter as a function of crystallization time of Sn- and
Fig. 2 Variation of particle diameter as a function of crystallization
time of Sn-MFI (a) and Ti-MFI (b) zeolites prepared under microwave
irradiation in Pyrex and SiC systems (the time interval before 0 at the x
ꢀ
axis represents that of the microwave pre-treatment at 80 C for
90 min, whereas that after 0 represents the microwave treatment time
Ti-MFI are shown in Fig. 2 (termed as growth curves). Clearly, at different temperatures). FESEM images of Sn- and Ti-MFI synthe-
sized in Pyrex (c, d) and SiC (e, f) vessels, respectively (high temperature
zone: 170 C). The average particle size and Si/M ratios of corre-
sponding products are shown in the insets, which are obtained based
on their FESEM images and energy dispersive X-ray (EDX) data,
there is a sudden pronounced rise in particle diameter observed
in each curve. We regard the time before the inection point as
nucleation period, and that aer the inection point as crystal
ꢀ
growth period. In detail, Sn-MFI zeolites in two vessels experi-
ence a long nucleation period and a slow growth process to
achieve their stable sizes (Fig. 2a). However, the nucleation and
respectively.
growth of Sn-MFI in SiC vessel are slower than those in Pyrex much larger than that in SiC. Although Ti-MFI zeolites, by
vessel. Moreover, the nal stable size of the product in Pyrex is contrast, display a comparatively quick nucleation period and
growth rate to reach their stable sizes, the average size of Ti-MFI
obtained in Pyrex is also bigger than the one in SiC nally
(Fig. 2b). Moreover, this difference of crystallization rate and
nal particle size between reaction vessels exists at different
crystallization temperatures (Fig. 2a and b). Furthermore, the
size differences between two vessels can be further conrmed
by their average solid sizes calculated by a size-calculated so-
ware on the basis of eld emission scanning electron micro-
scope (FESEM) images (Fig. 2c–f), although they are slightly
smaller than those in the solution. These results imply that the
direct interaction between the electromagnetic eld and reac-
tion medium, except for its thermal effect, possesses positive
inuence on the preparation process of Sn- and Ti-MFI zeolites.
However, it is worthy to note that when no any heteroatom
(
Sn or Ti) species is added during MFI zeolite synthesis, nearly
Fig. 1 Temperature (TIR, TFO) and power (P) profiles in the ramping
no difference between two synthesis systems can be observed
phase recorded for the reaction system for Ti-MFI in the Pyrex ((a) from
ꢀ
ꢀ
ꢀ
ꢀ
room temperature to 80 C, (b) from 80 C to 170 C) and SiC vessel even if the crystallization temperature is decreased to 130 C
ꢀ ꢀ ꢀ
(
(d) from room temperature to 80 C, (e) from 80 C to 170 C),
(Fig. 3). Therefore, we deduce that this microwave effect is
respectively, heated with microwave under sealed-vessel conditions in
the Monowave 300 reactor. Schematic diagram of heat transfer mode
in Pyrex (c) and SiC (f) vessels (the vial photos show in the insets, and
the red colour and blue colour denote high and low temperature,
respectively).
derived from the interaction between microwave eld and
heteroatom species. Moreover, taking microwave heating prin-
ciple and previous reports into consideration,
effect to the microwave selective heating for Sn–O and Ti–O
17,24
we assign this
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35252–35256 | 35253