Crystals size deeply affects catalytic performances for it
Table 5 Catalytic performances for 1793-111 catalyst after one to
ꢀ
three regeneration cycles. Catalytic tests are carried out at 350 C,
values are referred to the fourth hour of reaction
modifies the ratio between internal and external surface.
Caprolactam selectivity increases with increasing crystal size
for -111 samples, whereas cyclohexanone oxime conversion
decreases. On the contrary, other samples (-121,-212,-222)
show a decrease of both cyclohexanone oxime conversion
and caprolactam selectivity and a deactivation rate consis-
tently higher. Such a behavior has been explained by the differ-
ent acid sites distribution between internal and external
surface. As a matter of fact silanol nests which are located in
the channels of silicalite-1 undergo a slower deactivation for
the small channels dimensions, hinder tars formation through
a shape selective activity. Terminal silanols are mainly located
at the external surface of the catalysts, promote cyclohexanone
oxime non-selective conversion thus they quickly deactivate.
After regeneration upon 1793-111 catalyst, the catalytic activ-
ity is unchanged for thermal treatment in air removes tars and
leaves the silicalite-1 structure unchanged, we propose that
Si–NH2 bondings give to the silicalite-1 structure a higher
thermal stability.
Fresh
99.9
I cycle
99.9
II cycle
99.9
III cycle
99.9
Cyclohexanone oxime
conversion (%)
Caprolactam
94.0
91.8
94.0
91.3
selectivity (%)
Catalytic results for sample 1793-111 are rather high and we
do not observe any relevant difference between catalytic per-
formances. In table 5 we report the results for catalyst 1793-
1
3
11 after three regeneration cycles with operating temperature
ꢀ
50 C.
The stability of catalytic activity is attributed to the Si–NH2
bondings which are co-ordinated to hydroxyl groups and leads
to a higher stability of silanol-nests since they are unaltered
ꢀ
after thermal treatment in air at 450 C for 10 h.
We verified the effect of temperature and we found that for
ꢀ
silicalite-1 at T ¼ 350 C the best equilibrium between capro-
lactam reaction activation energy and minimization of by-
products formation is reached, at this temperature the higher
caprolactam yield is obtained and the longer catalyst lifetime
achieved.
Conclusions
High silica MFI zeolite lattice is formed by tetrahedral silicon
oxides and are characterized by defective sites namely H-
bonded silanols (vicinal or nests of silanols) whose amount
and distribution is modified by the different thermal treatment
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1847