Ammoximation of ketones on titanium silicalite. A study of the reaction byproducts

Sustainable chemical technology through catalytic multistep reactions

“…This process was developed by EniChem (Mantegazza et al, 1996) and titanium silicalite molecular sieve was found to display high activity and selectivity at ambient conditions. It should be, however, kept in mind, that cyclohexanone oxime should undergo further Beckmann rearrangement, which was typically conducted in the presence of sulfuric acid and only recently solid acids started to be used commercially (Ichihashi and Sato, 2001).…”

Insights into the key to highly selective synthesis of oxime via ammoximation over titanosilicates

“…Scheme 1 shows the overall reaction behavior of cyclohexanone ammoximation following the hydroxylamine mechanism, which has been fully confirmed [6,7]. This network consists of three parts: (a) the inorganic side reactions involving the unproductive decomposition of H 2 O 2 as well as the consecutive oxidation of NH 2 OH and NH 3 to form nitrogen derivatives [14,[24][25][26] (steps 1-4), which would decrease ketone conversion but not affect product selectivity; (b) the organic side reactions containing aldol condensation of ketone [20], the formation of imine and its consecutive oxidation [4,21], together with the hydrolysis and deep oxidation of oxime [2,6,20,22] (steps 5-10), which reasonably have a great influence on product selectivity; (c) the main reactions, consisting of the catalytic formation of NH 2 OH as a result of oxidation of NH 3 and H 2 O 2 on Ti sites and the noncatalytic oximation of ketone with NH 2 OH to oxime [6,7] (steps 11 and 12), which are able to influence both the ammoximation activity and selectivity. It is surprising to find that the oxidant H 2 O 2 is involved in most reaction steps, so the corresponding reaction network adopting H 2 O 2 as reactive center is summarized in Scheme 2.…”

“…However, no further studies were performed to correlate these properties with the ammoximation mechanism to elucidate the inner reasons. Furthermore, many studies supported the conclusion that oxime selectivity could be noticeably improved by adopting an excess of NH 3 and deep oxidation reactions would be intensified by lowering the NH 3 concentration [2][3][4]20], but this positive role of excessive NH 3 is limited and depends on the type of titanosilicates. For instance, the linear oxime selectivity gained by TS-1 (around 95%) still fails to reach the level obtained by Ti-MWW (>99.5%) even if the mole ratio of NH 3 /ketone is as high as 4 [2].…”

“…It was reported that catalyst properties related to diffusion resistance [14], hydrophilicity/hydrophobicity [15], and Ti active species [16], as well as the additives [17], oxidant [18], and solvent [19] used for selective oxidation processes, can significantly influence the product selectivity during typical titanosilicate-catalyzed propene epoxidation and phenol or benzene hydroxylation reactions. In terms of the ammoximation of ketones or aldehydes over titanosilicates, it has been widely accepted that the typical side reaction is deep oxidation [2,[20][21][22]. Nevertheless, the factors that determine the selectivity of various titanosilicates in specific ketone and aldehyde ammoximation reactions are not yet clear, even though considerable progress toward this has been made.…”

“…In the case of diffusion resistance, the low methyl ethyl ketone oxime (MEKO) selectivity obtained over TS-1 with medium pores was considered to derive from the catalytic oxidation of linear oxime to 2-nitrobutane over Ti active sites located inside the zeolite channel [2], whereas previous reports demonstrated that, in cyclohexanone ammoximation, nitrocyclohexane could be produced from the homogeneous oxidation of oxime without the participation of Ti active sites [20,21]. Additionally, although TS-1 showed better selectivity for cyclohexanone oxime than for linear oximes such as MEKO, which was due to the fact that large cyclic molecules were unable to enter the pore for further oxidation, it was proved that the pores owned by TS-1 zeolite were also able to absorb cyclohexanone molecules in the liquid phase, which indicates that cyclic oximes would be generated inside the TS-1 channels and be oxidized successively on Ti active sites as well [23].…”