RSC Advances
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more than 35.5%. While acetonitrile was used as solvent, the
CPL selectivity was improved to 96.3%.
Effect of zinc compound as catalyst on direct synthesis of
caprolactam. The effect of zinc compound as catalyst on direct
synthesis of CPL was shown in Fig. 3. When Zn(OAc)2, ZnSO4-
$7H2O or ZnO was used, the CPL selectivity was very low. An
interesting nding was that when ZnCl2 was used, most of the
product was CPL, only trace COX was existed. Zhang et al.33
reported that the form of a seven-ring intermediate compound
was probably the reason for the catalytic function of ZnCl2,
which may also be helpful to understand the catalytic activity of
ZnCl2 herein. Hence, ZnCl2 was a better catalyst for the present
Beckmann rearrangement reaction.
Effect of molar ratio of ZnCl2 to CYC, reaction temperature
and time on direct synthesis of caprolactam. The effect of molar
ratio of ZnCl2 to CYC on CPL synthesis was studied and the
result was displayed in Fig. 4(a). As we can see, the CYC
conversion was close to 80% without adding ZnCl2. Most of the
product was COX and the CPL selectivity was only 17.8%.
With increasing of ZnCl2 ratio, the CYC conversions changed
a little. But the CPL selectivity increased greatly and reached
a maximum value of 96.3% when the molar ratio of ZnCl2 to
CYC was 1.5, indicating that ZnCl2 and the [HSO3-b-mim]$HSO4
was an excellent composite catalyst for the Beckmann rear-
rangement. Most of the COX produced from the CYC and
(NH2OH)2$[HSO3-b-mim]$HSO4 could be transformed to CPL.
Nevertheless, when ZnCl2 amount was further increased, the
CPL selectivity decreased a little. As a result, 1.5 molar ratio of
ZnCl2 to CYC was sufficient to achieve a high CYC conversion
and CPL selectivity.
The present reaction includes oximation of CYC to COX,
followed by the Beckmann rearrangement of COX to CPL. Being
a multistep process, the outcome of the reaction depends on the
reactivity of the substrate towards both oximation and Beck-
mann rearrangement.15 As for the rst step, it is believed that
COX is produced through the nucleophilic attack of nitrogen
electron pairs in the NH2OH to the C]O carbon in the CYC.26–28
Free NH2OH,16,27 resulted from the decomposition of hydroxyl-
amine salt, is the nucleophile necessary for conversion of CYC
to the corresponding COX. It was evident that, the aprotic polar
solvents, such as acetonitrile and N,N-dimethylformamide,
demonstrated an obvious promotion on the decomposition of
hydroxylamine salt to form free NH2OH29–31 since high CYC
conversions were obtained in these solvents. This solvent-
promoted decomposition in aprotic polar solvents with high
ability of electron pairs donor was probably due to the forma-
tion of hydrogen bond between the nitrogen electron pairs in
the solvent and hydroxyl in the hydroxylamine salts.29,32
Furthermore, solvent also played an important role in the
subsequent Beckmann rearrangement. Acetonitrile was found
to be a suitable solvent for the rearrangement reaction.33–35 Most
probably the benecial effect of acetonitrile was ascribed to its
high reagent solubility,16 especially that of [HSO3-b-mim]$H2SO4
(an in situ catalyst component for the Beckmann rearrange-
ment) resulting from the decomposition of (NH2OH)2$[HSO3-b-
mim]$H2SO4. Hence, when acetonitrile was used as solvent,
more COX was converted to CPL, which in turn promoted the
oximation step (a reversible process) and improved the CYC
conversion.17 As for N,N-dimethylformamide, it could promote
the oximation reaction. However, its weak basicity probably
poisoned the acid catalyst and hindered the Beckmann rear-
rangement.36–38 Hence, only trace amount of CPL was obtained
when N,N-dimethylformamide was used as solvent. Considering
the inuence on oximation reaction as well as Beckmann rear-
rangement, acetonitrile seems to be a better choice for direct
synthesis of CPL.
The inuence of reaction temperature and time were also
investigated systematically. As depicted in Fig. 4(b) and (c), the
CPL selectivity depended greatly on the reaction temperature
and time. It increased rapidly with rising temperature from 50
to 80 ꢀC, and then increased slightly when further rising the
temperature. Concerning the reaction time, the CPL selectivity
rst increased, passing through a maximum at 4 h, and then
decreased slightly by furtheꢀr prolonging reaction time. Hence,
higher temperature (80–90 C) and longer time (4 h) was suit-
able for CPL selectivity. The CYC conversion was less affected by
the reaction temperature and time. It increased slightly from 50
to 70 ꢀC, and then reduced a little with the temperature above 70
ꢀC. This may be due to easier self-catalysis decomposition of
hydroxylamine ionic liquid salt at higher temperature.39,40 As for
the inuence of reaction time, the CYC conversion was nearly
constant with prolonging reaction time, and a relatively higher
CYC conversion can be obtained at a longer time. Hence,
considering the CYC conversion and CPL selectivity, the
ꢀ
optimum reaction condition was at 80 C for 4 h.
Under the above mentioned optimal conditions, the CYC
conversion and CPL selectivity were 82.5% and 96.3%, respec-
tively. Our previous result was 100% CYC conversion and 91.3%
CPL selectivity under solvent free conditions.18 As we can see,
when acetonitrile was used as solvent, the reaction temperature
was remarkably decreased from 150 to 80 ꢀC. Although the CYC
conversion was decreased a little, the CPL selectivity was
increased to 96.3%. Then the reactivity of the two novel
prepared hydroxylamine ionic liquid salts on direct synthesis of
Fig. 3 Effect of Zinc compound as catalyst on direct synthesis of CPL.
Reaction conditions: CYC 5 mmol, n(CYC) : n((NH2OH)2$[HSO3-b-
mim]$HSO4) : n(zinc compound) ¼ 2 : 1 : 3, acetonitrile 10 mL, 80 ꢀC,
4 h.
83622 | RSC Adv., 2016, 6, 83619–83625
This journal is © The Royal Society of Chemistry 2016