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Selective synthesis of dimethyl ketone oxime through ammoximation over Ti-MOR catalyst
Applied Catalysis A: General 488 (2014) 86–95
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Selective synthesis of dimethyl ketone oxime through ammoximation over Ti-MOR catalyst Jianghong Ding, Peng Wu ∗ Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 7 August 2014 Received in revised form 20 September 2014 Accepted 22 September 2014 Available online 28 September 2014 Keywords: Dimethyl ketone Dimethyl ketone oxime Titanosilicate Ti-MOR Liquid-phase ammoximation.
a b s t r a c t Titanosilicate with the MOR topology (Ti-MOR), postsynthesized from highly dealuminated mordenite and TiCl4 vapor through a solid–gas reaction, was highly active and selective for the liquid-phase ammoximation of dimethyl ketone (DMK) with ammonia and hydrogen peroxide. The parameters effecting the formation of the ammoximation product of dimethyl ketone oxime were investigated systematically in a batch-type reactor, and the optimized conditions were further applied to continuous ammoximation of DMK in a slurry reactor. Ti-MOR was superior to other titanosilicates in terms of activity and lifetime. TS-1 was not suitable for the ammoximation of DMK, whereas Ti-MWW required a higher catalyst loading to reach a reasonable activity, and they both easily produced a main byproduct of oxidative coupling of dimethyl ketone oxime. The deactivation behavior of Ti-MOR was investigated. Ammoniainduced structural desilication and accompanied Ti sites migration altered a more serious influence on the catalyst duration than coke deposition during continuous ammoximation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Acetoxime or dimethyl ketone oxime (DMKO), with a low toxicity, low environmental risk and a high deoxidization effect, is widely used as a corrosion inhibitor and passivator in boilers instead of carcinogenic N2 H4. Conventionally, DMKO is produced through non-catalytic oximation of dimethyl ketone (DMK) with hydroxylamine derivatives. First, nitride reacts with acid to give hydroxylamine salt such as hydroxylamine sulfate, then the oxime is recovered from the aqueous oximation mixture by distilling after neutralizing the formed acid with ammonia. The whole process includes multi-steps, using a great amount of poisonous oximation agents, and also producing hazardous byproducts such as oleum, halides and oxides of nitrogen. In addition, a large quantity of lowvalue byproduct such as ammonium sulfate is coproduced [1]. Thus, it is urgent to develop environmentally friendly and routes for synthesizing DMKO cleanly. The discovery of titanosilicate TS-1 with the MFI topology opens up a new prospect of developing greener route for oxime production by employing NH3 and H2 O2 as the ammoximation agents [2,3]. This zero-emission process would overcome the shortcomings of serious pollution problems and uncertain factors encountered in traditional technologies. Many problems still need
∗ Corresponding author. Fax: +86 21 62232292. E-mail address: [email protected] (P. Wu). http://dx.doi.org/10.1016/j.apcata.2014.09.038 0926-860X/© 2014 Elsevier B.V. All rights reserved.
to be solved. TS-1 is highly active for cyclohexanone ammoximation [4], exhibiting a high cyclohexanone conversion and a cyclohexanone oxime selectivity. This innovative catalytic technology has already been applied in industrial process. However, TS-1 is poor for the ammoximation of linear ketones like DMK in terms of oxime selectivity. In contrast to cyclic ketones, the linear ketones themselves are chemically more active. Several researches have been carried out on the ammoximation of DMK [1,5,6], but the results are far from satisfactory. TS-1 over-oxidizes easily DMKO to 2,3-dimethyl-2,3-dinitrobutane (DMNB) [7] and shows a poor selectivity to DMKO. Thus, more efficient and selective titanosilicates are required to develop a clean ammoximation process for DMKO production. With a unique pore structure, Ti-MWW shows a good performance in ammoximation of cyclohexanone [8,9] and methyl ethyl ketone [10], especially it could suppress the over-oxidation of oxime to the corresponding nitro-hydrocarbon, giving a high selectivity for linear oximes. Ti-MWW could be a promising catalyst for DMK ammoximation. However, the preparation of Ti-MWW requires a hydrothermal synthesis using organic structure-directing agent (SDA) and a large quantity of boric acid, in which a post-acid treatment is necessary to get rid of the nonframework titanium and together boron species [15]. The whole process is relatively complicated. Thus, we focused our effects on another titanosilicate with the MOR topology (Ti-MOR), which can be prepared by secondary synthesis from mordenite synthesized without organic SDAs. Ti-MOR possesses larger-pore channels
J. Ding, P. Wu / Applied Catalysis A: General 488 (2014) 86–95
restricted by 12-membered ring (MR) windows together with inner connected 8-MR channels. It severs as an efficient catalyst in the ammoximation of cyclohexanone [11], methyl ethyl ketone [12], and acetaldehyde [13], but not yet applied to DMK ammoximation. In this work, post-synthesized Ti-MOR was used as the catalyst for the ammoximation of DMK. The experimental parameters governing the activity have been systematically studied, and the optimized conditions were further applied to a continuous slurry reactor. By comparing the catalytic behaviors among Ti-MOR, Ti-WWW and TS-1, we found Ti-MOR possessed great advantages over other titanosilicates in terms of selective production of DMKO. The deactivated Ti-MOR was investigated to give a better understanding of the deactivation mechanism of Ti-MOR in DMK ammoximation. 2. Experimental 2.1. Titanosilicate materials Following previously reported methods [11–13], Ti-MOR catalysts were prepared by gas–solid isomorphous substitution between highly dealuminated mordenite (Del-MOR, Si/Al molar ratio of 160) and TiCl4 vapor. Del-MOR (2 g) placed in a quartz tube reactor (ø 3 cm) was pretreated at 673 K in a dry N2 stream for 2 h. TiCl4 vapor was then brought into the reactor by N2 flow, treating dehydrated Del-MOR at 673 K for 2 h. After the treatment, the sample was purged with pure N2 at the same temperature for 1 h to remove any residual TiCl4 from the zeolite powder. After cooling to room temperature in N2 , the treated sample was washed with deionized water and dried in air at 353 K overnight, giving rise to MOR-type titanosilicate Ti-MOR (Si/Ti = 60). For control experiment, TS-1 (Si/Ti = 30) [14] and Ti-MWW (Si/Ti = 32) [15] were hydrothermally synthesized following literature methods. With the purpose of enhancing the catalytic performance, TS-1 was further re-crystallized with tetrapropylammonium hydroxide (TPAOH) following the literature method [16]. All catalysts were finally calcined at 823 K in air to remove the organic species. 2.2. Characterization methods Powder X-ray diffraction (XRD) was employed to check the structure and crystallinity of the zeolites. The XRD patterns were collected on a Rigaku Ultima IV diffractometer using Cu K␣ radiation at 35 kV and 25 mA in the 2Â angle range of 5–35◦ . Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 microscope to determine the morphology. The Ti, Al and Si contents were determined by inductively coupled plasma emission spectrometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer after dissolving the samples in HF solution. The specific surface area was measured by N2 adsorption at 77 K on a BELSORP-MAX instrument equipped with a precise sensor for lowpressure measurement. The samples were activated at 573 K under vacuum for at least 10 h. The specific surface area was determined using Langmuir method. The UV–visible diffuse reflectance spectra were recorded on a Shimadzu UV-2550 spectrophotometer using BaSO4 as a reference. The IR spectra were collected on Nicolet Nexus 670 FT-IR spectrometer in absorbance mode at a spectral resolution of 2 cm−1 using KBr technique (25 wt% wafer). Transmission electron microscope (TEM) (FEI-Tecnai G2F30) was used to investigate the crystalline structure of the deactivated Ti-MOR catalyst. 2.3. Catalytic reactions The liquid-phase ammoximation of DMK was first carried out in a batch-type reactor. For typical reactions, the mixture
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of catalyst (0.05 g), 20 mmol DMK, 27 wt% H2 O2 (20–36 mmol), NH3 (20–36 mmol) and water or 85% t-BuOH as solvent (0–10 g) were heated and stirred at 333 K in a 50 mL flask for 1.5 h. After cooling and removing the solid catalyst, the reaction mixture was subjected to GC analysis on a gas chromatograph (Shimadzu 2014, FID detector) equipped with a 50 m FFAP capillary column. The by-products were extracted with CH2 Cl2 and further identified by gas chromatography–mass spectrometry (Agilent 6890 series GC system, 5937 network mass selective detector). The continuous ammoximation of DMK was carried out in a 160 mL glass slurry reactor equipped with a glass sand filter and a magnetic stirrer. For a typical run, 2.0 g of catalyst powder and 120 mL of t-BuOH aqueous solution (85 wt%) were added in the reactor and heated under stirring at 333 K. The mixture of DMK and 85 wt% t-BuOH aqueous solution (weight ratio of 1:3) and 27 wt% H2 O2 were then fed into the reactor separately with a micro-pump. The feeding rate of DMK was always kept constant at 0.21 mol h−1 . Meanwhile, ammonia gas (99.9%) was charged into the reaction system with a mass flowmeter. The molar ratios of H2 O2 /ketone and NH3 /ketone were kept at 1.05 and 1.7, respectively. With the reaction proceeding, the reaction mixture overflowed from the outlet filter and the catalyst powder remained in the reactor. The ammonia unconverted and not soluble in the reaction mixture was exhausted through a condenser vent. The organic products were analyzed with GC to calculate the conversion of DMK and the selectivity of DMKO. The content of unconverted H2 O2 was determined by iodometric titration. 3. Results and discussion 3.1. A summary of catalyst characterization The crystalline structures of three titanosilicates (Ti-MOR, TS1 and Ti-MWW) were investigated with XRD technique. The XRD patterns indicate that all the samples possessed a high crystallinity without any impurity phase (see supporting information Fig. S1). The UV–visible and IR spectra of these samples showed characteristic adsorption bands at 220 nm and 960 cm−1 , respectively, which are assigned to the tetrahedral Ti species isolated in the zeolite framework [17]. Base on the N2 adsorption isotherms, Ti-MOR, TS-1 and Ti-MWW had a specific surface area of 565, 534 and 525 m2 g−1 , respectively. These physicochemical properties verified that these titanosilicates were qualified as liquid-phase oxidation catalysts with aqueous H2 O2 as an oxidant. 3.2. Reaction pathways in DMK ammoximation Scheme 1 summarizes the reaction pathways in the ammoximation of DMK. With a relatively active nature, DMK undergoes easily non-catalytic condensation with NH3 and H2 O2 . NH3 reacts with DMK in ambient temperature and form 2,2,6,6tetramethylpiperidin-4-one (TMPDO). When the temperature is higher, a mixture of acetone amines such as diacetone amine, triacetone diamine, triacetone amine and other products can also be formed [18,19]. Bearing a carboxyl group, TMPDO could also go through ammoximation reaction like other ketones to form 2,2,6,6-tetramethylpiperidin-4-one oxime (4-hydroxyiminoTMPD). H2 O2 may react with organic solvents to produce dangerous peroxides. In the ammoximation, a high-concentration mixture of DMK and H2 O2 is an especially hazardous mixture which may form various explosive peroxides. The primary explosive peroxide synthesized by DMK and H2 O2 is 3,3,6,6,9,9-hexamethyl1,2,4,5,7,8-hexaoxonane (TATP), which could further decompose into O2 and DMK, causing a rapid increase in reaction system pressure [20,21].
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Scheme 1. A summary of reaction pathways involved in DMK ammoximation.
The ammoximation takes place with an intermediate of hydroxylamine which is in situ produced by catalytic oxidation of NH3 with H2 O2 on the Ti active sites. Hydroxylamine reacts with DMK to DMKO via non-catalytic oximation either inside zeolite channels or in the reaction mixture. As hydroxylamine is formed on the active Ti sites located inside zeolite channels, it is possibly decomposed into nitrogen oxide as a result of deep oxidation by H2 O2 /titanosilicate. The large pores of titanosilicates would be helpful for DMK molecules diffusing into channels to react the hydroxylamine molecules formed therein, and they are beneficial for hydroxylamine and oxime molecules diffusing out of zeolite channels smoothly as well. DMKO can also go through deoximation by H2 O2 /titanosilicate. The major byproduct of ammoximation is 2,3-dimethyl-2,3-dinitrobutane (DMNB), which is formed by deep oxidative coupling DMKO over Ti active site [22]. 3.3. A comparison of ammoximation among various catalysts The adding methods for the reactants are of great importance for the ammoximation of ketones. In particular, Ti-MWW preferred dropwise addition of H2 O2 in the ammoximation of cyclohexanone [8] and methyl ethyl ketone [10]. Adding H2 O2 at once resulted in an extremely low conversion ketone on Ti-MWW. On the other hand, the adding method of H2 O2 or NH3 had little influence on the
catalytic performance of Ti-MOR in the ammoximation of ketones and aldehydes, which is probably due to its unique oxidation property [11–13]. However, to avoid forming explosive TATP, it is favorable to keep either DMK or H2 O2 at a low concentration in reaction system. When DMK was added dropwise, it was difficult to well control the formation procedure of hydroxylamine since the hydroxylamine intermediate was further decomposed by titanosilicates. Thus, H2 O2 was chosen to add dropwise in this study. Table 1 compares the results of DMK ammoximation on three representative titanosilicate catalysts. The titanosilicate-catalyzed liquid-phase oxidations have been reported to depend greatly on the nature of the solvents used, in the ammoximation of ketones like cyclohexanone and methyl ethyl ketone, both Ti-MWW and Ti-MOR were highly performed well in water as the solvent, while TS-1 favored the co-solvent of H2 O and t-BuOH [8,10]. When H2 O2 was added dropwise into the reactor (Table 1, Nos. 1 and 2), both the DMK conversion and the DMKO selectivity reached high values (>98%) over Ti-MOR at a relatively low catalyst loading whatever in water or 85% t-BuOH. In agreement to our previous works that the solvents made no significant difference for the ammoximation of methyl ethyl ketone and acetaldehyde TiMOR [12,13], Ti-MOR also served as an efficient catalyst for DMK ammoximation irrespective of solvent. Ti-MWW and TS-1 performed a poor activity for the ammoximation of DMK at a low catalyst loading of 0.05 g (Table 1, Nos. 3, 4, 7 and 8). Especially, Ti-MWW produced easily a large quantity of condensation product of NH3 and DMK, the reason of which is assumed to its high acidity related to the framework boron and induced defect sites like hydroxyl nests [10]. TS-1 usually needs a relatively high weight ratio of catalyst to substrates to achieve an ideal conversion level [2]. Moreover, the competitive reactions between ammoximation and other side reactions exist parallelly, so it is essential to fasten the main reaction in order to achieve a high oxime selectivity by increasing the catalyst amount. When the catalyst amount was increased to 0.5 g, Ti-MWW showed a high DMK conversion in water or 85% t-BuOH (Table 1, Nos. 5 and 6). The DMKO selectivity for TS-1 was still poor even 85% t-BuOH was chosen as the solvent, as it produced a lot of DMNB byproduct (Table 1, Nos. 9 and 10). In addition to ketone ammoximation, the TS-1/H2 O2 system could effectively oxidize a large number of organic substrates under mild conditions such as epoxidation of alkenes, oxidation of alkanes and alcohols and hydroxylation of aromatics [23–26]. Here, in the ammoximation of DMK, the DMKO product was converted relatively easily into DNMB via coupling oxidization on the Ti active sites of TS-1. Based on the results achieved with three titanosilicates, the high activity and DMKO selectivity of Ti-MOR could be ascribed to its large porosity and moderate oxidative ability. Compared with TiMWW and TS-1, the reactant and product molecules could diffuse
Table 1 The results of liquid-phase ammoximation of DMK over different titanosilicates.a No.
Catalyst
Si/Tib
Cat. amount (g)
Solvent
DMK conv. (%)
DMKO sel.c (%)
1 2 3 4 5 6 7 8 9 10
Ti-MOR
60
Ti-MWW
32
TS-1
30
0.05 0.05 0.05 0.05 0.5 0.5 0.05 0.05 0.5 0.5
H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH
98.3 99.2 9.3 13.5 99.6 98.9 22.8 17.5 83.9 97.8
99.2 99.5 28.9 22.2 94.3 97.9 78.8 79.4 55.3 78.1
a
Reaction conditions: DMK, 20 mmol; DMK: H2 O2 : NH3 = 1: 1.2: 1.5 (molar ratio); solvent, 3 g; temp., 333 K; time, 1.5 h. H2 O2 added dropwise at a constant rate within
1 h. b c
Given by ICP. The main byproducts were DMNB, TMPDO and 4-hydroxyimino-TMPD.
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Table 2 The deoximation of DMKO and formation of DMNB via coupling oxidation.a No. 1 2 3 4
Cat.
Yield b (%)
Yield c (%)
None TS-1 Ti-MWW Ti-MOR
DMK 3.6 29.9 20.0 10.1
DMNB ndd 7.5 5.4 0.3
DMK 24.0 0.4 0.9 0.2
DMNB 0.8 30.8 nd nd
a Reaction conditions: cat., 0.05 g; DMKO, 20 mmol; DMKO:H2 O2 = 1:1.2 (molar ratio); H2 O, 3 g; temp., 333 K; time, 1.5 h. H2 O2 added dropwise at a constant rate within 1 h. b Without adding ammonia. c With ammonia addition, DMKO:NH3 = 1:1.5 (molar ratio). d Not detected.
freely into or out of 12-MR channels of Ti-MOR. Meanwhile, Ti-MOR was characteristic of a weaker oxidative ability, which suppressed a deep oxidization of DMKO. 3.4. Deoximation of DMKO and coupling oxidation of DMNB Possessing a relatively inert chemical property and more importantly having a bulky molecular dimension not easy for entering the micropores of zeolites, cyclohexanone oxime is usually achieved at a selectivity of >99% on various titanosilicates independent of crystalline topologies. In contrast, the ketone or aldehyde oximes with linear molecular shapes may undergo over-oxidation to give corresponding byproducts, lowing the oxime selectivity in ammoximation. The oxidative deoximation of oximes by H2 O2 is reported to take place on titanosilicates or aluminosilicates [26]. Nevertheless, this kind of over-oxidation closely depends on the titanosilicate structure. Thus, we have carried out the oxidation of DMKO with H2 O2 on TS-1, Ti-MOR and Ti-MWW. As shown in Table 2, DMKO was hydrolyzed to DMK by H2 O2 in the absence of any catalyst, and more easily with ammonia addition (Table 2, No. 1). However, DMNB, the couple oxidation product of DMKO, was almost not detected, which implied that over-oxidation of DMKO to DMNB required a catalytic reaction. Among three titanosilicates, TS-1 showed the highest yields of DMK (29.9%) and DMNB (7.5%) in the absence of ammonia (Table 2, No. 2), indicating it possessed a high oxidative ability. Ti-MWW was less active to the oxidation of DMKO, while Ti-MOR gave the lowest DMK yield (10.1%) together with a negligible DMNB yield (0.3%) (Table 2, Nos. 3 and 4). To simulate the basic condition in ammoximation reaction, an adequate amount of ammonia was added. With ammonia addition, the DMK yield increased to 24.0% without catalyst, which means that DMKO was preferred to hydrolyze in an alkaline media. In the presence of catalyst, DMK was few produced, mainly because DMK was further converted into DMKO with ammonia and H2 O2 via ammoximation. DMNB was still achieved on the TS-1 catalyst with a yield of 30.8%, even much higher than that in the absence of ammonia. On the other hand, the formation of DMNB was suppressed totally on Ti-MWW and Ti-MOR with the presence of ammonia, suggesting that the ammoximation of DMK on these two titanosilicates took place more easily than the over-oxidation of DMNB. Once H2 O2 was consumed to form hydroxylamine within a short time, the coupling oxidation of DMKO to DMNB would be avoided. As a result, Ti-MOR appeared to be the catalyst with a weaker catalytic ability towards the oxidation of DMKO. 3.5. Effects of reaction parameters on the ammoximation of DMK over Ti-MOR 3.5.1. Effect of solvent and reaction temperature As is shown in Table 1, the solvents of water and t-BuOH had little effects on the ammoximation of DMK on Ti-MOR; we thus
Fig. 1. Effect of the amount of solvent (A) and temperature (B) on the ammoximation over Ti-MOR.
chose water as the solvent to investigate other parameters affecting the ammoximation of DMK. Fig. 1A shows a suitable amount of water was desirable for achieving a high DMK conversion and a high DMKO selectivity. When the reaction was carried out without additional water except for the water from aqueous hydrogen peroxide, the initial concentrations of DMK and NH3 were high in reaction mixture. They tended to condensate forming a small number of TMPDO. The DMK conversion and DMKO selectivity increased with the amount of water at first, and both reached 99% at 3 g of water. With further increasing amount of water, the DMK conversion dropped slightly because the solvent diluted the reactants and catalyst. The temperature played a more significant role than other reaction parameters. When the reaction was carried out at 313 K (Fig. 1B), TATP was produced and further degraded into O2 , making the pressure of reaction system increase very fast in a short time. The DMK conversion increased notably when the temperature increased and reached the maximum at 333 K. The DMK conversion decreased significantly when further raising the temperature to 353 K, which was proper because the decomposition of H2 O2 and evaporation of DMK and NH3 . More importantly, the oxime was readily hydrolyzed to DMK at high temperatures. Consequently, the ammoximation proceeded most effectively at an optimum reaction temperature of 333 K, which was similar to the ammoximation of acetaldehyde and methyl ethyl acetone [12,13]. The key step of ammoximation was the hydroxylamine formation
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Fig. 2. Effect of H2 O2 /DMK ratio (A) and NH3 /DMK ratio (B) on the ammoximation over Ti-MOR.
reaction which was demonstrated to take place effectively at an optimized temperature 333 K [27]. Once hydroxylamine was produced at an optimized temperature, other competitive reactions would be well suppressed, although the physicochemical properties of aldehydes or ketones are different.
3.5.2. Effect of H2 O2 /DMK and NH3 /DMK ratios The ammoximation of DMK to MEKO requires stoicheometrically equal moles of DMK, NH3 and H2 O2 . Fig. 2A shows the effect of H2 O2 /DMK molar ratio on the DMK ammoximation. The conversion reached a flat in the H2 O2 /DMK ratio range of 1.2–1.4. When the H2 O2 /DMK ratio was further increased, the DMK conversion and DMKO selectivity both decreased slightly. This is because the excessive H2 O2 would accelerate the hydrolysis of DMKO back to DMK and the deep oxidation of intermediate of hydroxylamine, which was not helpful for achieving a high selectivity of DMKO. The DMK conversion increased with increasing NH3 /DMK ratio, and it reached >99% at 1.5 (Fig. 2B), which implied that an excessive amount of NH3 was in favor of DMK conversion. Meanwhile, a high selectivity to oxime was obtained (>99.5%). No obvious changes were observed for either DMK conversion or the oxime selectivity when the NH3 /DMK ratio was further increased to 1.8. These phenomena were in agreement with previous reports [28]. NH3 may be partly vaporized and decomposed during ammoximation. And the ammoximation generally needs a suitable basic environment. Thus, an excess of NH3 was useful for leading the main reaction to DMKO.
Fig. 3. The kinetics of DMK ammoximation on Ti-MOR (A) and TS-1 (B).
3.6. The kinetics of ammoximation of DMK The kinetics of DMK ammoximation were investigated in the temperature range of 313–353 K. Neglecting the impact of reactant concentration and mass transfer resistance, good linear relations between initial DMKO yield and reaction time were obtained (Fig. 3), suggesting that the DMK ammoximation could be considered as a zeroth-order reaction for both Ti-MOR and TS-1. Table 3 lists the reaction rate constants that were calculated from the slopes of the DMKO yield plots depending on the reaction time at various temperatures. The Arrhenius plots showed good linear correlations of −ln k versus T−1 (Fig. 4). The apparent activation energy (Ea ) estimated from the slopes of the plots was 48.0 kJ mol−1 for Ti-MOR and 55.2 kJ mol−1 for TS-1, which were very comparable. Different greatly in catalytic ability and product selectivity between TS-1 and Ti-MOR (Table 1), the ammoximation of DMK took place fundamentally via the same reaction mechanism independent of the crystalline structures of titanosilicates. However, the apparent Table 3 Ammoximation rate constants on Ti-MOR and TS-1 at different temperature. a Temp. (K)
313
323
333
343
353
Constant of Ti-MOR (s−1 )b Constant of TS-1 (s−1 )c
0.0066 0.0108
0.0123 0.0201
0.0215 0.0368
0.0364 0.0627
0.0596 0.1202
a Reaction conditions: 20 mmol DMK; DMK:H2 O2 :NH3 = 1:1.2:1.5; water, 50 g; temp., 333 K. b Catalyst amount, 0.05 g. c Catalyst amount, 0.5 g.
J. Ding, P. Wu / Applied Catalysis A: General 488 (2014) 86–95
Fig. 4. Arrhenius plots for Ti-MOR and TS-1.
activation energy of cyclohexanone ammoximation in 85% t-BuOH was reported to be 95 kJ mol−1 for TS-1 [29]. The difference should be attributed to the natures of solvent and ketone molecules. Easily dissolved in water with a high polarity, NH3 and H2 O2 have a good opportunity reaching the Ti active sites to produce NH2 OH. Moreover, DMK is also soluble in water and has a molecular size smaller than cyclohexanone. The DMK molecules would easily diffuse into the zeolite channels to react with NH2 OH formed therein, resulting in a lower apparent active energy. 3.7. Comparison of the ammoximation lifetime among various titanosilicates In addition to a high activity in bath-type reactor, the real applicability of titanosilicates relies on their lifetime and reusability. So we applied three titanosilicates to the MEK ammoximation in a continuous slurry reactor. The ammoximation were carried out under the same condition except that the H2 O2 /DMK ratio was controlled as low as possible without affecting the DMK conversion and DMKO selectivity, because the remaining H2 O2 unconverted would cause possible explosion during the separation of DMKO by distillation in an industrial process. Xing and co-workers [30] studied the catalytic mechanism and the reaction pathway of DMK ammoximation on TS-1 in a bath-type reactor, and further applied it to a continuous ceramic membrane reactor system with a lifetime of 21 h. Xu and co-workers [5] systematically investigated the operation conditions of TS-1 in a tubular membrane reactor system. Under optimized conditions, the reaction process maintained a steady production of DMK over 30 h. TS-1 with a hollow structure (HTS) has also been applied in acetone ammoximation in a slurry bed reactor, where the conversion was maintained at 82% at the beginning [22]. The conversion slowly decreased during 30 h, and then dropped quickly. In this study, when TPAOH-treated TS-1 was used as the catalyst to the DMK ammoximation in a continuous slurry reactor, the conversion of DMK was 64% at initial stage, and decreased to 53% at 4 h when the reaction was performed using 2.0 g of TS-1 loading (Fig. 5). When the catalyst loading was increased to 3.0 g, the initial DMK conversion reached 95%, but the conversion dropped rapidly to 57% with prolonging reaction time to 9 h. This indicated that TS-1 was not a stable catalyst for the ammoximation of DMK. The rapid deactivation of TS-1 in continuous DMK ammoximation is presumed to be closely related to its oxidative ability. This made DMKO undergo deep oxidative coupling to DMNB, retarding the ammoximation reaction. When the unconverted DMK is high in concentration in
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Fig. 5. The DMK conversion (, ) and DMKO selectivity (,•) in a continuous slurry reactor using different loading of TS-1 catalyst.
the reaction system, it may further react with H2 O2 to produce explosive TATP. Fig. 6 shows the DMK conversion and DMKO selectivity in the continuous ammoximation of DMK over Ti-MWW. Ti-MWW gave a DMK conversion of >98% and a DMKO selectivity of >99% within a lifetime of 34 h at 2 g of catalyst loading, which was obviously superior to TS-1. The lifetime was fairly shorter in comparison to that of previously reported cyclohexanone ammoximation [30,31]. The difference in hydrophilic/hydrophobic property of ketones could be a possible reason. DMK is more hydrophilic than cyclohexanone, while Ti-MWW, synthesized in the presence of boric acid, was of less hydrophobic as contained a relatively high amount of boron species (Si/B = 30). It is beneficial for the enrichment of reactants, but unfavorable for hydrophilic DMKO diffusing out of pores. The lifetime of Ti-MWW could be enhanced by increasing the hydrophobic property and reducing the framework defects after a structural rearrangement with hexamethyleneimine or piperidine [32,33]. Another method is to simply increase the catalyst loading to maintain the DMK conversion and DMKO selectivity high, suppressing the side reaction. Exactly, with a catalyst loading of 3.0 g, the lifetime of Ti-MWW was prolonged to 128 h (Fig. 6). On the other hand, Ti-MOR was demonstrated to be a more stable catalyst, giving rise to a much longer lifetime at 2 g of catalyst loading (Fig. 7). It maintained a high DMK conversion and DMKO selectivity within 180 h, and then deactivated rapidly in a short time. The deactivation behavior was very similar to that of
Fig. 6. The DMK conversion (, ) and DMKO selectivity (,•) in a continuous slurry reactor using different loading of Ti-MWW catalyst.
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Fig. 7. The DMK conversion, DMKO selectivity and unconverted H2 O2 in a continuous slurry reactor of Ti-MOR catalyst.
Ti-MWW. H2 O2 conversion was kept fairly stable during the whole process, with the unconverted H2 O2 in the reaction mixture below 0.2%. Only when the catalyst began to deactivate, the amount of H2 O2 consumed in producing DMKO also decreased, resulting in an increase in reaming H2 O2 . 3.8. Deactivation behavior of Ti-MOR catalyst The deactivated Ti-MOR catalyst after use for 170 h was first investigated by XRD (Fig. S2). The used catalyst still maintained the typical MOR structure, but the crystallinity decreased obviously in comparison to fresh Ti-MOR. UV–visible spectroscopy was employed to characterize the coordination states of the Ti species of fresh and deactivated Ti-MOR (Fig. S3). They both showed a dominated band at approximately 210 nm, which is assigned to the
ligand-to-metal charge transfer from O2− to Ti4+ [11]. Most of the Ti species still occupied the tetrahedral sites in the MOR framework after ammoximation. However, the intensity of the 210 nm band was reduced after deactivation due to transformation of a part of tetrahedral Ti sites. Actually, a new band assigned to octahedral Ti species appeared at 260 nm. With the desilication of the silica matrix, that is, the scaffolds where the Ti active sites existed, the coordination states of the leached Ti species were changed to octahedral ones. The distribution of Ti species was verified by SEM-EDS, the Si/Ti ratio of different areas varies in the range of 95–104, suggesting that the Ti was well-distributed without aggregate Ti species which was in agreement with UV–visible spectroscopy. After deactivation, the Si/Ti ratio varied from 85 to 185 in different area, implying that a part of tetra-coordinate Ti species on crystal surface probably has suffered leaching or aggregation problem. ICP analysis determined the fresh Ti-MOR had a bulk Si/Ti ratio of 47, while the deactivated catalyst had a same ratio increased of 43, suggesting that the content of Ti increased slightly in comparison to Si as a result of desilication. Combing EDS and ICP investigation, we can conclude that most of Ti was still in the framework without aggregation even after deactivation. Ti-MOR was robust against Ti leaching, but the coordination states of a part of Ti sites were changed as their scaffolds of silica matrix experienced desilication. Fig. S4 shows the 29 Si MAS NMR spectra of the Ti-MOR samples. The resonances at −101 and −106 ppm are attributed to the internal SiOH groups and Si(OSi)3 OH (Q3 ) groups [34]. In comparison to fresh Ti-MOR, these resonances increased in intensity for the deactivated catalyst. These results also verified that the framework desilication occurred during the ammoximation of DMK, resulting in a loss of crystallinity by forming defect sites like hydroxyl nests. The SEM images indicated that the crystals of fresh Ti-MOR had a smooth surface (Fig. 8A and C). After being used in DMK ammoximation for 170 h, the morphology of Ti-MOR was almost
Fig. 8. SEM images of fresh Ti-MOR (A, C) and deactivated Ti-MOR (B, D).
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Fig. 9. TEM images of fresh Ti-MOR (A, B) and deactivated Ti-MOR (C–F).
maintained, but a serve erosion was observed, leading to cavity on the crystals (Fig. 8B and D). The deactivated Ti-MOR showed an increased in N2 adsorption capacity at P/P0 > 0.8 in the N2 adsorption–desorption isotherm (Fig. S5), which was due to the formation of mesopores by desilication in basic media. Although the erosion is not preferable, the mesopores introduced by desilication could enlarge the external surface, improve the accessibility
reactant molecules to the Ti active sites, minimize the diffusing limitation of molecules and depress the coke formation as well. However, a deep dissolving of zeolite framework can hydrolyze the Ti-O-Si bonds, making the active Ti sites leach or change in coordination state. The TEM images provided more detailed information about structural changes and Ti species of Ti-MOR (Fig. 9). No crystalline
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4. Conclusions Ti-MOR is capable of catalyzing the liquid-phase ammoximation of DMK to corresponding oxime at a DMK conversion and DMKO selectivity >98%. H2 O2 was chosen to add dropwise in bath-type reactor. When further applied to the continuous ammoximation of DMK in a slurry reactor, Ti-MOR gives a long lifetime while maintaining the DMKO selectivity over 99%, indicating that TiMOR is a promising catalyst for clean synthesis of DMKO though liquid-phase ammoximation. TS-1 is not suitable for the DMK ammoximation in terms of activity and MEKO selectivity, as it catalyzes the oxidative coupling of DMKO easily, giving a main byproduct of DMNB. Ti-MWW shows a long catalyst life only at a high catalyst loading. Ti-MOR deactivates mainly due to the desilication in basic reaction media rather than coke deposition, as the desilication decreases the crystallinity of Ti-MOR and changes the coordination states of Ti sites as well. Fig. 10. TG curves of deactivated titanosilicates in continuous ammoxiamtion DMK with different catalyst loading.
TiO2 species were detected for both fresh and deactivated Ti-MOR. The Ti active sites were highly dispersed but not aggregated even after deactivation. In contrast, the TS-1 catalyst had the aggregated titanium oxide after it deactivated in the ammoximation of cyclohexanone [35]. The fresh Ti-MOR showed distinctive channels corresponding to the 12-MR micropores arraying in a highly ordered manner without any interruption (Fig. 9A and B). A small fraction of large mesopores of 20–40 nm occurred on the lattice planes, which were probably created in the process of dealumination. These mesopores were distributed randomly with mordenite crystals. After a long run and deactivation in ammoximation, the desilication of Ti-MOR by ammonia was clearly observed (Fig. 9C–F). The crystals of Ti-MOR were dissolved into small fragments, most of which were long strips along the 12-MR channel direction. This means that the desilication occurred across the c-axis. Some hollows were eroded in the centre of the crystals. In the initial stage of ammoximation, the desilication may be helpful for opening the channels of large crystals and achieving a high activity. Nevertheless, a deep desilication with reaction made the Ti-MOR framework lose crystalline structure gradually, and the scaffolds dissolve into reaction mixture. The coordination states of Ti then were varied, resulting in leaching and deactivation the Ti active sites. Fig. 10 shows the TG curves of deactivated Ti-MWW and TiMOR. At 2.0 g of catalyst loading, Ti-MWW deactivated at 32 h and gave a coke deposition of 12.0%. DMK with a small molecular size could diffuse into zeolite channels, and the heavy byproducts formed from side reactions deposited and blocked the channels. Once the main reaction step of ammoximation was slowed down, the non-catalytic side reactions of residual reactants dominated the reaction, making the catalyst deactivate within a short time. In contrast, when the reaction was carried out with 3.0 g of catalyst loading, ammoximation reaction was accelerated and the side reactions were suppressed. The coke deposition on Ti-MWW was then reduced to 8.2% after it deactivated at a longer time of 127 h. Ti-MOR with a relatively high hydrophobicity has shown a good performance in the ammoximation of cyclohexanone and acetaldehyde [11,13]. In this study, it also showed an excellent activity and lifetime in DMK ammoximation. The 12-MR large pores of TiMOR were beneficial for the diffusion of molecules, and its catalytic ability for the oxidative coupling DMKO to DMNB was relatively low. The coke deposition on Ti-MOR (5.2%) after ammoximation for 170 h was much lower than that on Ti-MWW, suggesting that the deactivation of Ti-MOR was mainly due to the desilication rather than the coke deposition.
Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (21373089, U1162102), PhD Programs Foundation of Ministry of Education (2012007613000), the National Key Technology R&D Program (2012BAE05B02) and the Shanghai Leading Academic Discipline Project (B409). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2014.09.038. References [1] X. Liang, Z. Mi, Y. Wang, L. Wang, X. Zhang, React. Kinet. Catal. Lett. 82 (2004) 333–337. [2] P. Roffia, M. Padovan, G. Leofanti, M.A. Mantegazza, G. De Alberti, G.R. Tamazik, U.S. Patent 4,794,198 (1998). [3] X. Liang, Z. Mi, Y. Wang, L. Wang, X. Zhang, T. Liu, Chem. Eng. Technol. 27 (2004) 176–180. [4] L. Dal Pozzo, G. Fornasari, T. Monti, Catal. Commun. 3 (2002) 369–375. [5] Z. Li, R. Chen, W. Xing, W. Jin, N. Xu, Ind. Eng. Chem. Res. 49 (2010) 6309–6316. [6] R. Chen, Z. Bu, Z. Li, Z. Zhong, W. Jin, W. Xing, Chem. Eng. J. 156 (2010) 418–422. [7] S. Zhang, Y. Zhao, H. Zhang, U.S. Patent 20,130,184,504 (2013). [8] F. Song, Y. Liu, H. Wu, M. He, P. Wu, T. Tatsumi, J. Catal. 237 (2006) 359–367. [9] S. Zhao, W. Xie, J. Yang, Y. Liu, Y. Zhang, B. Xu, J. Jiang, M. He, P. Wu, Appl. Catal. A 394 (2011) 1–8. [10] F. Song, Y. Liu, L. Wang, H. Zhang, M. He, P. Wu, Appl. Catal. A 327 (2007) 22–31. [11] P. Wu, T. Komastu, T. Yashima, J. Catal. 168 (1997) 400–411. [12] J. Ding, L. Xu, H. Xu, H. Wu, Y. Liu, P. Wu, Chin. J. Catal. 34 (2013) 243–250. [13] J. Ding, L. Xu, Y. Yu, H. Wu, S. Huang, Y. Yang, J. Wu, P. Wu, Catal. Sci. Tech. 3 (2013) 2587–2595. [14] M. Taramasso, G. Perego, B. Notari, U.S. Patent 4,410,501 (1983). [15] P. Wu, T. Tatsumi, T. Komatsu, T. Yashima, J. Phys. Chem. B 105 (2001) 2897–2905. [16] Y. Wang, M. Lin, A. Tuel, Micro. Meso. Mater. 102 (2007) 80–85. [17] G. Bellussi, M.S. Rigguto, Stud. Surf. Sci. Catal. 137 (2001) 911. [18] J. Cao, X. Zhao, K. Morishitaa, L. Lia, X. Xiao, R. Obara, X. Wei, T. Takaradaa, Bioresour. Technol. 101 (2010) 4242–4245. [19] B. Hirai, N. Kubota, Y. Nakahara, U.S. Patent 4,252,958 (1981). [20] J.C. Oxley, J. Brady, S.A. Wilson, J.L. Smith, J. Chem. Health Saf. 19 (2012) 27–33. [21] F. Dubnikova, R. Kosloff, J. Almog, Y. Zeiri, R. Boese, H. Itzhaky, A. Alt, E. Keinan, J. Am. Chem. Soc. 127 (2005) 1146–1159. [22] Y. Wang, S. Zhang, Y. Zhao, M. Lin, J. Mole. Catal. A 385 (2014) 1–6. [23] G. Bellussi, M.S. Rigutto, Stud. Surf. Sci. Catal. 85 (1994) p177. [24] B. Notari, Adv. Catal. 41 (1996) p253. [25] F. Bonino, A. Damin, G. Ricchiardi, M. Ricci, G. Spano, R. D’ Aloisio, A. Zecchina, C. Lamberti, C. Prestipino, S. Bordiga, J. Phys. Chem. B 108 (2004) 3573–3583. [26] N. Vijayakumari, B. Balakrishna Reddy, L. Nagarapu, Heterocycl. Commun. 12 (2006) 407–410. [27] L. Xu, J. Ding, Y. Yang, P. Wu, J. Catal. 309 (2014) 1–10. [28] M. Oikawa, M. Fukao, U.S. Patent 7,067,699 (2006).
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Selective synthesis of dimethyl ketone oxime through ammoximation over Ti-MOR catalyst Jianghong Ding, Peng Wu ∗ Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 7 August 2014 Received in revised form 20 September 2014 Accepted 22 September 2014 Available online 28 September 2014 Keywords: Dimethyl ketone Dimethyl ketone oxime Titanosilicate Ti-MOR Liquid-phase ammoximation.
a b s t r a c t Titanosilicate with the MOR topology (Ti-MOR), postsynthesized from highly dealuminated mordenite and TiCl4 vapor through a solid–gas reaction, was highly active and selective for the liquid-phase ammoximation of dimethyl ketone (DMK) with ammonia and hydrogen peroxide. The parameters effecting the formation of the ammoximation product of dimethyl ketone oxime were investigated systematically in a batch-type reactor, and the optimized conditions were further applied to continuous ammoximation of DMK in a slurry reactor. Ti-MOR was superior to other titanosilicates in terms of activity and lifetime. TS-1 was not suitable for the ammoximation of DMK, whereas Ti-MWW required a higher catalyst loading to reach a reasonable activity, and they both easily produced a main byproduct of oxidative coupling of dimethyl ketone oxime. The deactivation behavior of Ti-MOR was investigated. Ammoniainduced structural desilication and accompanied Ti sites migration altered a more serious influence on the catalyst duration than coke deposition during continuous ammoximation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Acetoxime or dimethyl ketone oxime (DMKO), with a low toxicity, low environmental risk and a high deoxidization effect, is widely used as a corrosion inhibitor and passivator in boilers instead of carcinogenic N2 H4. Conventionally, DMKO is produced through non-catalytic oximation of dimethyl ketone (DMK) with hydroxylamine derivatives. First, nitride reacts with acid to give hydroxylamine salt such as hydroxylamine sulfate, then the oxime is recovered from the aqueous oximation mixture by distilling after neutralizing the formed acid with ammonia. The whole process includes multi-steps, using a great amount of poisonous oximation agents, and also producing hazardous byproducts such as oleum, halides and oxides of nitrogen. In addition, a large quantity of lowvalue byproduct such as ammonium sulfate is coproduced [1]. Thus, it is urgent to develop environmentally friendly and routes for synthesizing DMKO cleanly. The discovery of titanosilicate TS-1 with the MFI topology opens up a new prospect of developing greener route for oxime production by employing NH3 and H2 O2 as the ammoximation agents [2,3]. This zero-emission process would overcome the shortcomings of serious pollution problems and uncertain factors encountered in traditional technologies. Many problems still need
∗ Corresponding author. Fax: +86 21 62232292. E-mail address: [email protected] (P. Wu). http://dx.doi.org/10.1016/j.apcata.2014.09.038 0926-860X/© 2014 Elsevier B.V. All rights reserved.
to be solved. TS-1 is highly active for cyclohexanone ammoximation [4], exhibiting a high cyclohexanone conversion and a cyclohexanone oxime selectivity. This innovative catalytic technology has already been applied in industrial process. However, TS-1 is poor for the ammoximation of linear ketones like DMK in terms of oxime selectivity. In contrast to cyclic ketones, the linear ketones themselves are chemically more active. Several researches have been carried out on the ammoximation of DMK [1,5,6], but the results are far from satisfactory. TS-1 over-oxidizes easily DMKO to 2,3-dimethyl-2,3-dinitrobutane (DMNB) [7] and shows a poor selectivity to DMKO. Thus, more efficient and selective titanosilicates are required to develop a clean ammoximation process for DMKO production. With a unique pore structure, Ti-MWW shows a good performance in ammoximation of cyclohexanone [8,9] and methyl ethyl ketone [10], especially it could suppress the over-oxidation of oxime to the corresponding nitro-hydrocarbon, giving a high selectivity for linear oximes. Ti-MWW could be a promising catalyst for DMK ammoximation. However, the preparation of Ti-MWW requires a hydrothermal synthesis using organic structure-directing agent (SDA) and a large quantity of boric acid, in which a post-acid treatment is necessary to get rid of the nonframework titanium and together boron species [15]. The whole process is relatively complicated. Thus, we focused our effects on another titanosilicate with the MOR topology (Ti-MOR), which can be prepared by secondary synthesis from mordenite synthesized without organic SDAs. Ti-MOR possesses larger-pore channels
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restricted by 12-membered ring (MR) windows together with inner connected 8-MR channels. It severs as an efficient catalyst in the ammoximation of cyclohexanone [11], methyl ethyl ketone [12], and acetaldehyde [13], but not yet applied to DMK ammoximation. In this work, post-synthesized Ti-MOR was used as the catalyst for the ammoximation of DMK. The experimental parameters governing the activity have been systematically studied, and the optimized conditions were further applied to a continuous slurry reactor. By comparing the catalytic behaviors among Ti-MOR, Ti-WWW and TS-1, we found Ti-MOR possessed great advantages over other titanosilicates in terms of selective production of DMKO. The deactivated Ti-MOR was investigated to give a better understanding of the deactivation mechanism of Ti-MOR in DMK ammoximation. 2. Experimental 2.1. Titanosilicate materials Following previously reported methods [11–13], Ti-MOR catalysts were prepared by gas–solid isomorphous substitution between highly dealuminated mordenite (Del-MOR, Si/Al molar ratio of 160) and TiCl4 vapor. Del-MOR (2 g) placed in a quartz tube reactor (ø 3 cm) was pretreated at 673 K in a dry N2 stream for 2 h. TiCl4 vapor was then brought into the reactor by N2 flow, treating dehydrated Del-MOR at 673 K for 2 h. After the treatment, the sample was purged with pure N2 at the same temperature for 1 h to remove any residual TiCl4 from the zeolite powder. After cooling to room temperature in N2 , the treated sample was washed with deionized water and dried in air at 353 K overnight, giving rise to MOR-type titanosilicate Ti-MOR (Si/Ti = 60). For control experiment, TS-1 (Si/Ti = 30) [14] and Ti-MWW (Si/Ti = 32) [15] were hydrothermally synthesized following literature methods. With the purpose of enhancing the catalytic performance, TS-1 was further re-crystallized with tetrapropylammonium hydroxide (TPAOH) following the literature method [16]. All catalysts were finally calcined at 823 K in air to remove the organic species. 2.2. Characterization methods Powder X-ray diffraction (XRD) was employed to check the structure and crystallinity of the zeolites. The XRD patterns were collected on a Rigaku Ultima IV diffractometer using Cu K␣ radiation at 35 kV and 25 mA in the 2Â angle range of 5–35◦ . Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 microscope to determine the morphology. The Ti, Al and Si contents were determined by inductively coupled plasma emission spectrometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer after dissolving the samples in HF solution. The specific surface area was measured by N2 adsorption at 77 K on a BELSORP-MAX instrument equipped with a precise sensor for lowpressure measurement. The samples were activated at 573 K under vacuum for at least 10 h. The specific surface area was determined using Langmuir method. The UV–visible diffuse reflectance spectra were recorded on a Shimadzu UV-2550 spectrophotometer using BaSO4 as a reference. The IR spectra were collected on Nicolet Nexus 670 FT-IR spectrometer in absorbance mode at a spectral resolution of 2 cm−1 using KBr technique (25 wt% wafer). Transmission electron microscope (TEM) (FEI-Tecnai G2F30) was used to investigate the crystalline structure of the deactivated Ti-MOR catalyst. 2.3. Catalytic reactions The liquid-phase ammoximation of DMK was first carried out in a batch-type reactor. For typical reactions, the mixture
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of catalyst (0.05 g), 20 mmol DMK, 27 wt% H2 O2 (20–36 mmol), NH3 (20–36 mmol) and water or 85% t-BuOH as solvent (0–10 g) were heated and stirred at 333 K in a 50 mL flask for 1.5 h. After cooling and removing the solid catalyst, the reaction mixture was subjected to GC analysis on a gas chromatograph (Shimadzu 2014, FID detector) equipped with a 50 m FFAP capillary column. The by-products were extracted with CH2 Cl2 and further identified by gas chromatography–mass spectrometry (Agilent 6890 series GC system, 5937 network mass selective detector). The continuous ammoximation of DMK was carried out in a 160 mL glass slurry reactor equipped with a glass sand filter and a magnetic stirrer. For a typical run, 2.0 g of catalyst powder and 120 mL of t-BuOH aqueous solution (85 wt%) were added in the reactor and heated under stirring at 333 K. The mixture of DMK and 85 wt% t-BuOH aqueous solution (weight ratio of 1:3) and 27 wt% H2 O2 were then fed into the reactor separately with a micro-pump. The feeding rate of DMK was always kept constant at 0.21 mol h−1 . Meanwhile, ammonia gas (99.9%) was charged into the reaction system with a mass flowmeter. The molar ratios of H2 O2 /ketone and NH3 /ketone were kept at 1.05 and 1.7, respectively. With the reaction proceeding, the reaction mixture overflowed from the outlet filter and the catalyst powder remained in the reactor. The ammonia unconverted and not soluble in the reaction mixture was exhausted through a condenser vent. The organic products were analyzed with GC to calculate the conversion of DMK and the selectivity of DMKO. The content of unconverted H2 O2 was determined by iodometric titration. 3. Results and discussion 3.1. A summary of catalyst characterization The crystalline structures of three titanosilicates (Ti-MOR, TS1 and Ti-MWW) were investigated with XRD technique. The XRD patterns indicate that all the samples possessed a high crystallinity without any impurity phase (see supporting information Fig. S1). The UV–visible and IR spectra of these samples showed characteristic adsorption bands at 220 nm and 960 cm−1 , respectively, which are assigned to the tetrahedral Ti species isolated in the zeolite framework [17]. Base on the N2 adsorption isotherms, Ti-MOR, TS-1 and Ti-MWW had a specific surface area of 565, 534 and 525 m2 g−1 , respectively. These physicochemical properties verified that these titanosilicates were qualified as liquid-phase oxidation catalysts with aqueous H2 O2 as an oxidant. 3.2. Reaction pathways in DMK ammoximation Scheme 1 summarizes the reaction pathways in the ammoximation of DMK. With a relatively active nature, DMK undergoes easily non-catalytic condensation with NH3 and H2 O2 . NH3 reacts with DMK in ambient temperature and form 2,2,6,6tetramethylpiperidin-4-one (TMPDO). When the temperature is higher, a mixture of acetone amines such as diacetone amine, triacetone diamine, triacetone amine and other products can also be formed [18,19]. Bearing a carboxyl group, TMPDO could also go through ammoximation reaction like other ketones to form 2,2,6,6-tetramethylpiperidin-4-one oxime (4-hydroxyiminoTMPD). H2 O2 may react with organic solvents to produce dangerous peroxides. In the ammoximation, a high-concentration mixture of DMK and H2 O2 is an especially hazardous mixture which may form various explosive peroxides. The primary explosive peroxide synthesized by DMK and H2 O2 is 3,3,6,6,9,9-hexamethyl1,2,4,5,7,8-hexaoxonane (TATP), which could further decompose into O2 and DMK, causing a rapid increase in reaction system pressure [20,21].
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Scheme 1. A summary of reaction pathways involved in DMK ammoximation.
The ammoximation takes place with an intermediate of hydroxylamine which is in situ produced by catalytic oxidation of NH3 with H2 O2 on the Ti active sites. Hydroxylamine reacts with DMK to DMKO via non-catalytic oximation either inside zeolite channels or in the reaction mixture. As hydroxylamine is formed on the active Ti sites located inside zeolite channels, it is possibly decomposed into nitrogen oxide as a result of deep oxidation by H2 O2 /titanosilicate. The large pores of titanosilicates would be helpful for DMK molecules diffusing into channels to react the hydroxylamine molecules formed therein, and they are beneficial for hydroxylamine and oxime molecules diffusing out of zeolite channels smoothly as well. DMKO can also go through deoximation by H2 O2 /titanosilicate. The major byproduct of ammoximation is 2,3-dimethyl-2,3-dinitrobutane (DMNB), which is formed by deep oxidative coupling DMKO over Ti active site [22]. 3.3. A comparison of ammoximation among various catalysts The adding methods for the reactants are of great importance for the ammoximation of ketones. In particular, Ti-MWW preferred dropwise addition of H2 O2 in the ammoximation of cyclohexanone [8] and methyl ethyl ketone [10]. Adding H2 O2 at once resulted in an extremely low conversion ketone on Ti-MWW. On the other hand, the adding method of H2 O2 or NH3 had little influence on the
catalytic performance of Ti-MOR in the ammoximation of ketones and aldehydes, which is probably due to its unique oxidation property [11–13]. However, to avoid forming explosive TATP, it is favorable to keep either DMK or H2 O2 at a low concentration in reaction system. When DMK was added dropwise, it was difficult to well control the formation procedure of hydroxylamine since the hydroxylamine intermediate was further decomposed by titanosilicates. Thus, H2 O2 was chosen to add dropwise in this study. Table 1 compares the results of DMK ammoximation on three representative titanosilicate catalysts. The titanosilicate-catalyzed liquid-phase oxidations have been reported to depend greatly on the nature of the solvents used, in the ammoximation of ketones like cyclohexanone and methyl ethyl ketone, both Ti-MWW and Ti-MOR were highly performed well in water as the solvent, while TS-1 favored the co-solvent of H2 O and t-BuOH [8,10]. When H2 O2 was added dropwise into the reactor (Table 1, Nos. 1 and 2), both the DMK conversion and the DMKO selectivity reached high values (>98%) over Ti-MOR at a relatively low catalyst loading whatever in water or 85% t-BuOH. In agreement to our previous works that the solvents made no significant difference for the ammoximation of methyl ethyl ketone and acetaldehyde TiMOR [12,13], Ti-MOR also served as an efficient catalyst for DMK ammoximation irrespective of solvent. Ti-MWW and TS-1 performed a poor activity for the ammoximation of DMK at a low catalyst loading of 0.05 g (Table 1, Nos. 3, 4, 7 and 8). Especially, Ti-MWW produced easily a large quantity of condensation product of NH3 and DMK, the reason of which is assumed to its high acidity related to the framework boron and induced defect sites like hydroxyl nests [10]. TS-1 usually needs a relatively high weight ratio of catalyst to substrates to achieve an ideal conversion level [2]. Moreover, the competitive reactions between ammoximation and other side reactions exist parallelly, so it is essential to fasten the main reaction in order to achieve a high oxime selectivity by increasing the catalyst amount. When the catalyst amount was increased to 0.5 g, Ti-MWW showed a high DMK conversion in water or 85% t-BuOH (Table 1, Nos. 5 and 6). The DMKO selectivity for TS-1 was still poor even 85% t-BuOH was chosen as the solvent, as it produced a lot of DMNB byproduct (Table 1, Nos. 9 and 10). In addition to ketone ammoximation, the TS-1/H2 O2 system could effectively oxidize a large number of organic substrates under mild conditions such as epoxidation of alkenes, oxidation of alkanes and alcohols and hydroxylation of aromatics [23–26]. Here, in the ammoximation of DMK, the DMKO product was converted relatively easily into DNMB via coupling oxidization on the Ti active sites of TS-1. Based on the results achieved with three titanosilicates, the high activity and DMKO selectivity of Ti-MOR could be ascribed to its large porosity and moderate oxidative ability. Compared with TiMWW and TS-1, the reactant and product molecules could diffuse
Table 1 The results of liquid-phase ammoximation of DMK over different titanosilicates.a No.
Catalyst
Si/Tib
Cat. amount (g)
Solvent
DMK conv. (%)
DMKO sel.c (%)
1 2 3 4 5 6 7 8 9 10
Ti-MOR
60
Ti-MWW
32
TS-1
30
0.05 0.05 0.05 0.05 0.5 0.5 0.05 0.05 0.5 0.5
H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH H2 O 85% t-BuOH
98.3 99.2 9.3 13.5 99.6 98.9 22.8 17.5 83.9 97.8
99.2 99.5 28.9 22.2 94.3 97.9 78.8 79.4 55.3 78.1
a
Reaction conditions: DMK, 20 mmol; DMK: H2 O2 : NH3 = 1: 1.2: 1.5 (molar ratio); solvent, 3 g; temp., 333 K; time, 1.5 h. H2 O2 added dropwise at a constant rate within
1 h. b c
Given by ICP. The main byproducts were DMNB, TMPDO and 4-hydroxyimino-TMPD.
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Table 2 The deoximation of DMKO and formation of DMNB via coupling oxidation.a No. 1 2 3 4
Cat.
Yield b (%)
Yield c (%)
None TS-1 Ti-MWW Ti-MOR
DMK 3.6 29.9 20.0 10.1
DMNB ndd 7.5 5.4 0.3
DMK 24.0 0.4 0.9 0.2
DMNB 0.8 30.8 nd nd
a Reaction conditions: cat., 0.05 g; DMKO, 20 mmol; DMKO:H2 O2 = 1:1.2 (molar ratio); H2 O, 3 g; temp., 333 K; time, 1.5 h. H2 O2 added dropwise at a constant rate within 1 h. b Without adding ammonia. c With ammonia addition, DMKO:NH3 = 1:1.5 (molar ratio). d Not detected.
freely into or out of 12-MR channels of Ti-MOR. Meanwhile, Ti-MOR was characteristic of a weaker oxidative ability, which suppressed a deep oxidization of DMKO. 3.4. Deoximation of DMKO and coupling oxidation of DMNB Possessing a relatively inert chemical property and more importantly having a bulky molecular dimension not easy for entering the micropores of zeolites, cyclohexanone oxime is usually achieved at a selectivity of >99% on various titanosilicates independent of crystalline topologies. In contrast, the ketone or aldehyde oximes with linear molecular shapes may undergo over-oxidation to give corresponding byproducts, lowing the oxime selectivity in ammoximation. The oxidative deoximation of oximes by H2 O2 is reported to take place on titanosilicates or aluminosilicates [26]. Nevertheless, this kind of over-oxidation closely depends on the titanosilicate structure. Thus, we have carried out the oxidation of DMKO with H2 O2 on TS-1, Ti-MOR and Ti-MWW. As shown in Table 2, DMKO was hydrolyzed to DMK by H2 O2 in the absence of any catalyst, and more easily with ammonia addition (Table 2, No. 1). However, DMNB, the couple oxidation product of DMKO, was almost not detected, which implied that over-oxidation of DMKO to DMNB required a catalytic reaction. Among three titanosilicates, TS-1 showed the highest yields of DMK (29.9%) and DMNB (7.5%) in the absence of ammonia (Table 2, No. 2), indicating it possessed a high oxidative ability. Ti-MWW was less active to the oxidation of DMKO, while Ti-MOR gave the lowest DMK yield (10.1%) together with a negligible DMNB yield (0.3%) (Table 2, Nos. 3 and 4). To simulate the basic condition in ammoximation reaction, an adequate amount of ammonia was added. With ammonia addition, the DMK yield increased to 24.0% without catalyst, which means that DMKO was preferred to hydrolyze in an alkaline media. In the presence of catalyst, DMK was few produced, mainly because DMK was further converted into DMKO with ammonia and H2 O2 via ammoximation. DMNB was still achieved on the TS-1 catalyst with a yield of 30.8%, even much higher than that in the absence of ammonia. On the other hand, the formation of DMNB was suppressed totally on Ti-MWW and Ti-MOR with the presence of ammonia, suggesting that the ammoximation of DMK on these two titanosilicates took place more easily than the over-oxidation of DMNB. Once H2 O2 was consumed to form hydroxylamine within a short time, the coupling oxidation of DMKO to DMNB would be avoided. As a result, Ti-MOR appeared to be the catalyst with a weaker catalytic ability towards the oxidation of DMKO. 3.5. Effects of reaction parameters on the ammoximation of DMK over Ti-MOR 3.5.1. Effect of solvent and reaction temperature As is shown in Table 1, the solvents of water and t-BuOH had little effects on the ammoximation of DMK on Ti-MOR; we thus
Fig. 1. Effect of the amount of solvent (A) and temperature (B) on the ammoximation over Ti-MOR.
chose water as the solvent to investigate other parameters affecting the ammoximation of DMK. Fig. 1A shows a suitable amount of water was desirable for achieving a high DMK conversion and a high DMKO selectivity. When the reaction was carried out without additional water except for the water from aqueous hydrogen peroxide, the initial concentrations of DMK and NH3 were high in reaction mixture. They tended to condensate forming a small number of TMPDO. The DMK conversion and DMKO selectivity increased with the amount of water at first, and both reached 99% at 3 g of water. With further increasing amount of water, the DMK conversion dropped slightly because the solvent diluted the reactants and catalyst. The temperature played a more significant role than other reaction parameters. When the reaction was carried out at 313 K (Fig. 1B), TATP was produced and further degraded into O2 , making the pressure of reaction system increase very fast in a short time. The DMK conversion increased notably when the temperature increased and reached the maximum at 333 K. The DMK conversion decreased significantly when further raising the temperature to 353 K, which was proper because the decomposition of H2 O2 and evaporation of DMK and NH3 . More importantly, the oxime was readily hydrolyzed to DMK at high temperatures. Consequently, the ammoximation proceeded most effectively at an optimum reaction temperature of 333 K, which was similar to the ammoximation of acetaldehyde and methyl ethyl acetone [12,13]. The key step of ammoximation was the hydroxylamine formation
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Fig. 2. Effect of H2 O2 /DMK ratio (A) and NH3 /DMK ratio (B) on the ammoximation over Ti-MOR.
reaction which was demonstrated to take place effectively at an optimized temperature 333 K [27]. Once hydroxylamine was produced at an optimized temperature, other competitive reactions would be well suppressed, although the physicochemical properties of aldehydes or ketones are different.
3.5.2. Effect of H2 O2 /DMK and NH3 /DMK ratios The ammoximation of DMK to MEKO requires stoicheometrically equal moles of DMK, NH3 and H2 O2 . Fig. 2A shows the effect of H2 O2 /DMK molar ratio on the DMK ammoximation. The conversion reached a flat in the H2 O2 /DMK ratio range of 1.2–1.4. When the H2 O2 /DMK ratio was further increased, the DMK conversion and DMKO selectivity both decreased slightly. This is because the excessive H2 O2 would accelerate the hydrolysis of DMKO back to DMK and the deep oxidation of intermediate of hydroxylamine, which was not helpful for achieving a high selectivity of DMKO. The DMK conversion increased with increasing NH3 /DMK ratio, and it reached >99% at 1.5 (Fig. 2B), which implied that an excessive amount of NH3 was in favor of DMK conversion. Meanwhile, a high selectivity to oxime was obtained (>99.5%). No obvious changes were observed for either DMK conversion or the oxime selectivity when the NH3 /DMK ratio was further increased to 1.8. These phenomena were in agreement with previous reports [28]. NH3 may be partly vaporized and decomposed during ammoximation. And the ammoximation generally needs a suitable basic environment. Thus, an excess of NH3 was useful for leading the main reaction to DMKO.
Fig. 3. The kinetics of DMK ammoximation on Ti-MOR (A) and TS-1 (B).
3.6. The kinetics of ammoximation of DMK The kinetics of DMK ammoximation were investigated in the temperature range of 313–353 K. Neglecting the impact of reactant concentration and mass transfer resistance, good linear relations between initial DMKO yield and reaction time were obtained (Fig. 3), suggesting that the DMK ammoximation could be considered as a zeroth-order reaction for both Ti-MOR and TS-1. Table 3 lists the reaction rate constants that were calculated from the slopes of the DMKO yield plots depending on the reaction time at various temperatures. The Arrhenius plots showed good linear correlations of −ln k versus T−1 (Fig. 4). The apparent activation energy (Ea ) estimated from the slopes of the plots was 48.0 kJ mol−1 for Ti-MOR and 55.2 kJ mol−1 for TS-1, which were very comparable. Different greatly in catalytic ability and product selectivity between TS-1 and Ti-MOR (Table 1), the ammoximation of DMK took place fundamentally via the same reaction mechanism independent of the crystalline structures of titanosilicates. However, the apparent Table 3 Ammoximation rate constants on Ti-MOR and TS-1 at different temperature. a Temp. (K)
313
323
333
343
353
Constant of Ti-MOR (s−1 )b Constant of TS-1 (s−1 )c
0.0066 0.0108
0.0123 0.0201
0.0215 0.0368
0.0364 0.0627
0.0596 0.1202
a Reaction conditions: 20 mmol DMK; DMK:H2 O2 :NH3 = 1:1.2:1.5; water, 50 g; temp., 333 K. b Catalyst amount, 0.05 g. c Catalyst amount, 0.5 g.
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Fig. 4. Arrhenius plots for Ti-MOR and TS-1.
activation energy of cyclohexanone ammoximation in 85% t-BuOH was reported to be 95 kJ mol−1 for TS-1 [29]. The difference should be attributed to the natures of solvent and ketone molecules. Easily dissolved in water with a high polarity, NH3 and H2 O2 have a good opportunity reaching the Ti active sites to produce NH2 OH. Moreover, DMK is also soluble in water and has a molecular size smaller than cyclohexanone. The DMK molecules would easily diffuse into the zeolite channels to react with NH2 OH formed therein, resulting in a lower apparent active energy. 3.7. Comparison of the ammoximation lifetime among various titanosilicates In addition to a high activity in bath-type reactor, the real applicability of titanosilicates relies on their lifetime and reusability. So we applied three titanosilicates to the MEK ammoximation in a continuous slurry reactor. The ammoximation were carried out under the same condition except that the H2 O2 /DMK ratio was controlled as low as possible without affecting the DMK conversion and DMKO selectivity, because the remaining H2 O2 unconverted would cause possible explosion during the separation of DMKO by distillation in an industrial process. Xing and co-workers [30] studied the catalytic mechanism and the reaction pathway of DMK ammoximation on TS-1 in a bath-type reactor, and further applied it to a continuous ceramic membrane reactor system with a lifetime of 21 h. Xu and co-workers [5] systematically investigated the operation conditions of TS-1 in a tubular membrane reactor system. Under optimized conditions, the reaction process maintained a steady production of DMK over 30 h. TS-1 with a hollow structure (HTS) has also been applied in acetone ammoximation in a slurry bed reactor, where the conversion was maintained at 82% at the beginning [22]. The conversion slowly decreased during 30 h, and then dropped quickly. In this study, when TPAOH-treated TS-1 was used as the catalyst to the DMK ammoximation in a continuous slurry reactor, the conversion of DMK was 64% at initial stage, and decreased to 53% at 4 h when the reaction was performed using 2.0 g of TS-1 loading (Fig. 5). When the catalyst loading was increased to 3.0 g, the initial DMK conversion reached 95%, but the conversion dropped rapidly to 57% with prolonging reaction time to 9 h. This indicated that TS-1 was not a stable catalyst for the ammoximation of DMK. The rapid deactivation of TS-1 in continuous DMK ammoximation is presumed to be closely related to its oxidative ability. This made DMKO undergo deep oxidative coupling to DMNB, retarding the ammoximation reaction. When the unconverted DMK is high in concentration in
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Fig. 5. The DMK conversion (, ) and DMKO selectivity (,•) in a continuous slurry reactor using different loading of TS-1 catalyst.
the reaction system, it may further react with H2 O2 to produce explosive TATP. Fig. 6 shows the DMK conversion and DMKO selectivity in the continuous ammoximation of DMK over Ti-MWW. Ti-MWW gave a DMK conversion of >98% and a DMKO selectivity of >99% within a lifetime of 34 h at 2 g of catalyst loading, which was obviously superior to TS-1. The lifetime was fairly shorter in comparison to that of previously reported cyclohexanone ammoximation [30,31]. The difference in hydrophilic/hydrophobic property of ketones could be a possible reason. DMK is more hydrophilic than cyclohexanone, while Ti-MWW, synthesized in the presence of boric acid, was of less hydrophobic as contained a relatively high amount of boron species (Si/B = 30). It is beneficial for the enrichment of reactants, but unfavorable for hydrophilic DMKO diffusing out of pores. The lifetime of Ti-MWW could be enhanced by increasing the hydrophobic property and reducing the framework defects after a structural rearrangement with hexamethyleneimine or piperidine [32,33]. Another method is to simply increase the catalyst loading to maintain the DMK conversion and DMKO selectivity high, suppressing the side reaction. Exactly, with a catalyst loading of 3.0 g, the lifetime of Ti-MWW was prolonged to 128 h (Fig. 6). On the other hand, Ti-MOR was demonstrated to be a more stable catalyst, giving rise to a much longer lifetime at 2 g of catalyst loading (Fig. 7). It maintained a high DMK conversion and DMKO selectivity within 180 h, and then deactivated rapidly in a short time. The deactivation behavior was very similar to that of
Fig. 6. The DMK conversion (, ) and DMKO selectivity (,•) in a continuous slurry reactor using different loading of Ti-MWW catalyst.
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Fig. 7. The DMK conversion, DMKO selectivity and unconverted H2 O2 in a continuous slurry reactor of Ti-MOR catalyst.
Ti-MWW. H2 O2 conversion was kept fairly stable during the whole process, with the unconverted H2 O2 in the reaction mixture below 0.2%. Only when the catalyst began to deactivate, the amount of H2 O2 consumed in producing DMKO also decreased, resulting in an increase in reaming H2 O2 . 3.8. Deactivation behavior of Ti-MOR catalyst The deactivated Ti-MOR catalyst after use for 170 h was first investigated by XRD (Fig. S2). The used catalyst still maintained the typical MOR structure, but the crystallinity decreased obviously in comparison to fresh Ti-MOR. UV–visible spectroscopy was employed to characterize the coordination states of the Ti species of fresh and deactivated Ti-MOR (Fig. S3). They both showed a dominated band at approximately 210 nm, which is assigned to the
ligand-to-metal charge transfer from O2− to Ti4+ [11]. Most of the Ti species still occupied the tetrahedral sites in the MOR framework after ammoximation. However, the intensity of the 210 nm band was reduced after deactivation due to transformation of a part of tetrahedral Ti sites. Actually, a new band assigned to octahedral Ti species appeared at 260 nm. With the desilication of the silica matrix, that is, the scaffolds where the Ti active sites existed, the coordination states of the leached Ti species were changed to octahedral ones. The distribution of Ti species was verified by SEM-EDS, the Si/Ti ratio of different areas varies in the range of 95–104, suggesting that the Ti was well-distributed without aggregate Ti species which was in agreement with UV–visible spectroscopy. After deactivation, the Si/Ti ratio varied from 85 to 185 in different area, implying that a part of tetra-coordinate Ti species on crystal surface probably has suffered leaching or aggregation problem. ICP analysis determined the fresh Ti-MOR had a bulk Si/Ti ratio of 47, while the deactivated catalyst had a same ratio increased of 43, suggesting that the content of Ti increased slightly in comparison to Si as a result of desilication. Combing EDS and ICP investigation, we can conclude that most of Ti was still in the framework without aggregation even after deactivation. Ti-MOR was robust against Ti leaching, but the coordination states of a part of Ti sites were changed as their scaffolds of silica matrix experienced desilication. Fig. S4 shows the 29 Si MAS NMR spectra of the Ti-MOR samples. The resonances at −101 and −106 ppm are attributed to the internal SiOH groups and Si(OSi)3 OH (Q3 ) groups [34]. In comparison to fresh Ti-MOR, these resonances increased in intensity for the deactivated catalyst. These results also verified that the framework desilication occurred during the ammoximation of DMK, resulting in a loss of crystallinity by forming defect sites like hydroxyl nests. The SEM images indicated that the crystals of fresh Ti-MOR had a smooth surface (Fig. 8A and C). After being used in DMK ammoximation for 170 h, the morphology of Ti-MOR was almost
Fig. 8. SEM images of fresh Ti-MOR (A, C) and deactivated Ti-MOR (B, D).
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Fig. 9. TEM images of fresh Ti-MOR (A, B) and deactivated Ti-MOR (C–F).
maintained, but a serve erosion was observed, leading to cavity on the crystals (Fig. 8B and D). The deactivated Ti-MOR showed an increased in N2 adsorption capacity at P/P0 > 0.8 in the N2 adsorption–desorption isotherm (Fig. S5), which was due to the formation of mesopores by desilication in basic media. Although the erosion is not preferable, the mesopores introduced by desilication could enlarge the external surface, improve the accessibility
reactant molecules to the Ti active sites, minimize the diffusing limitation of molecules and depress the coke formation as well. However, a deep dissolving of zeolite framework can hydrolyze the Ti-O-Si bonds, making the active Ti sites leach or change in coordination state. The TEM images provided more detailed information about structural changes and Ti species of Ti-MOR (Fig. 9). No crystalline
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4. Conclusions Ti-MOR is capable of catalyzing the liquid-phase ammoximation of DMK to corresponding oxime at a DMK conversion and DMKO selectivity >98%. H2 O2 was chosen to add dropwise in bath-type reactor. When further applied to the continuous ammoximation of DMK in a slurry reactor, Ti-MOR gives a long lifetime while maintaining the DMKO selectivity over 99%, indicating that TiMOR is a promising catalyst for clean synthesis of DMKO though liquid-phase ammoximation. TS-1 is not suitable for the DMK ammoximation in terms of activity and MEKO selectivity, as it catalyzes the oxidative coupling of DMKO easily, giving a main byproduct of DMNB. Ti-MWW shows a long catalyst life only at a high catalyst loading. Ti-MOR deactivates mainly due to the desilication in basic reaction media rather than coke deposition, as the desilication decreases the crystallinity of Ti-MOR and changes the coordination states of Ti sites as well. Fig. 10. TG curves of deactivated titanosilicates in continuous ammoxiamtion DMK with different catalyst loading.
TiO2 species were detected for both fresh and deactivated Ti-MOR. The Ti active sites were highly dispersed but not aggregated even after deactivation. In contrast, the TS-1 catalyst had the aggregated titanium oxide after it deactivated in the ammoximation of cyclohexanone [35]. The fresh Ti-MOR showed distinctive channels corresponding to the 12-MR micropores arraying in a highly ordered manner without any interruption (Fig. 9A and B). A small fraction of large mesopores of 20–40 nm occurred on the lattice planes, which were probably created in the process of dealumination. These mesopores were distributed randomly with mordenite crystals. After a long run and deactivation in ammoximation, the desilication of Ti-MOR by ammonia was clearly observed (Fig. 9C–F). The crystals of Ti-MOR were dissolved into small fragments, most of which were long strips along the 12-MR channel direction. This means that the desilication occurred across the c-axis. Some hollows were eroded in the centre of the crystals. In the initial stage of ammoximation, the desilication may be helpful for opening the channels of large crystals and achieving a high activity. Nevertheless, a deep desilication with reaction made the Ti-MOR framework lose crystalline structure gradually, and the scaffolds dissolve into reaction mixture. The coordination states of Ti then were varied, resulting in leaching and deactivation the Ti active sites. Fig. 10 shows the TG curves of deactivated Ti-MWW and TiMOR. At 2.0 g of catalyst loading, Ti-MWW deactivated at 32 h and gave a coke deposition of 12.0%. DMK with a small molecular size could diffuse into zeolite channels, and the heavy byproducts formed from side reactions deposited and blocked the channels. Once the main reaction step of ammoximation was slowed down, the non-catalytic side reactions of residual reactants dominated the reaction, making the catalyst deactivate within a short time. In contrast, when the reaction was carried out with 3.0 g of catalyst loading, ammoximation reaction was accelerated and the side reactions were suppressed. The coke deposition on Ti-MWW was then reduced to 8.2% after it deactivated at a longer time of 127 h. Ti-MOR with a relatively high hydrophobicity has shown a good performance in the ammoximation of cyclohexanone and acetaldehyde [11,13]. In this study, it also showed an excellent activity and lifetime in DMK ammoximation. The 12-MR large pores of TiMOR were beneficial for the diffusion of molecules, and its catalytic ability for the oxidative coupling DMKO to DMNB was relatively low. The coke deposition on Ti-MOR (5.2%) after ammoximation for 170 h was much lower than that on Ti-MWW, suggesting that the deactivation of Ti-MOR was mainly due to the desilication rather than the coke deposition.
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