The Reaction Mechanism of Acetaldehyde Ammoximation to Its Oxime in the TS-1/H2O2 System

catalysts

Article The Reaction Mechanism of Acetaldehyde Ammoximation to Its Oxime in the TS-1/H2O2 System

Chaoqun Meng 1,2, Suohe Yang 1, Guangxiang He 1, Guohua Luo 1, Xin Xu 1 and Haibo Jin 1,*

1 Department of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China; [email protected] (C.M.); [email protected] (S.Y.); [email protected] (G.H.); [email protected] (G.L.); [email protected] (X.X.) 2 College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China * Correspondence: [email protected]; Tel.: +86-184-1822-9069

Academic Editor: Keith Hohn Received: 11 May 2016; Accepted: 16 July 2016; Published: 22 July 2016

Abstract: A qualitative analysis for the ammoximation of acetaldehyde to its oxime in the TS-1(Titanium Silicalite-1)/H2O2 system was investigated using an in situ infrared spectrometer (ReactIR15). NH3 is ﬁrst oxidized to NH2OH by TS-1/H2O2; then, CH3CH=NOH forms after NH2OH reacts with CH3CHO. That means the intermediate of this reaction is NH2OH instead of CH3CH=NH. Experiments have been conducted to verify the mechanism, and the results are in good agreement with the infrared ﬁndings.

Keywords: TS-1; H2O2; ReactIR15; ammoximation; acetaldehyde

1. Introduction Acetaldoxime is one of the simplest oxime-containing compounds, and it has a wide variety of uses in chemical synthesis processes as an important intermediate [1,2]. It is especially notable for its commercial application as an intermediate in the production of pesticides and cyanogenic glucosides [3] or as boiler chemicals to remove oxygen with its limited toxicity and strong reduction [2]. Initially, acetaldoxime can be prepared using hydroxylamine sulfate or hydroxylamine hydrochloride with sodium nitrite and sulfur dioxide, which has a low utilization rate, a high level of low value by-products and serious environmental pollution effects [4–6]. Therefore, it is extremely important to develop a new synthesis process for acetaldoxime. Acetaldehyde ammoximation to its oxime using TS-1 (Titanium Silicalite-1) as a catalyst and H2O2 as an oxidant offers a better approach [7–10]. Moreover, the utilization of carbon atoms is up to 100%, and water is a unique byproduct [11] that meets the development requirements of green chemical industry [12–14]. Due to its excellent performance, the TS-1/H2O2 system has been widely used in the hydroxylation of benzene, ammoxidation of ketone and epoxidation of alkenes, etc. Wang et al. investigated the correlation between the membrane structure and reaction efficiency; additionally, the effects of the conditions on the benzene conversion, product yield, hydrogen conversion and water production rate were examined in detail [15]. Somnath Nandi et al. evaluated the efficacy of the hybrid formalism for modeling and optimizing the zeolite (TS-1)-catalyzed benzene hydroxylation to phenol reaction whereby several sets of optimized operating conditions were obtained [16]. Tsai et al. reported the effects of the TS-1 crystal size and post-treatment on the porous structure and catalytic performances of TS-1 in phenol hydroxylation [17]. Liu et al. investigated the influences of essential process parameters on ammoximation and the reaction conditions [18]. Jiang et al. developed a new approach for phenol production by one-step selective hydroxylation of benzene with hydrogen peroxide over an ultrafine TS-1 in a submerged ceramic membrane reactor [19]. Li et al. investigated the feasibility of continuous ammoximation of acetone to acetone oxime over TS-1, and the effects of operation

Catalysts 2016, 6, 109; doi:10.3390/catal6070109 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 109 2 of 10 conditions were examined in the tubular membrane reactor [20]. Bars et al. made comparisons between TS-1, Ti-A1-Beta and Ti-ZSM-48 in the liquid-phase ammoximation of cyclohexanone [21]. Sang Baek Shin and David Chadwick studied the kinetics of the heterogeneous catalytic epoxidation of propene in the TS-1/H2O2 system and reported on the apparent activation energy [22]. V. Russo et al. performed a kinetic investigation of both the main and side reactions, aiming to find general kinetic expressions and related parameters in the reaction of propene oxide production via hydrogen peroxide with TS-1 [23]. Jernej Stare assessed various aspects of propene epoxidation using hydrogen peroxide and then elucidated some of the important factors that govern its mechanism [24]. Most researchers studied the reaction conditions, catalysis preparation, catalytic performance and more. In addition, some researchers considered the mechanism of the TS-1/H2O2 system. However, until now, only two main reaction amination mechanisms have been proposed. One is the hydroxylamine pathway; its intermediate is hydroxylamine, such that NH3 is first oxidized to hydroxylamine by H2O2 on the site of Ti. Then, hydroxylamine reacts with reagent, forming a target product. Sirijaraensre and Limtrakul illustrated the presence of hydroxylamine in the liquid phase using adsorption microcalorimetry IR, XANES and EXAFS techniques, as well as DFT (Density Functional Theory) calculations with the ONIOM scheme [25]. Chu et al. demonstrated that Ti-OOH is the oxidant in the TS-1/H2O2 system and that the formation of NH2OH passed through a lower reaction energy barrier according to Gibbs free energies and DFT [26]. Pozzo et al. used an experimental approach instead of calculating the values to explain the existence of hydroxylamine by determining the nitrate and nitrite concentrations [27]. The other mechanism is the imine pathway. In this pathway, NH3 reacts with reagent via TS-1, forming imine, which is then oxidized to a target product with the collective effect of H2O2 and TS-1. Zhang et al. studied the ammoximation of cyclohexanone with H2O2 and NH3 on TS-1 using DRIFTS and concluded that the peroxytitano complex—Ti-OOH was formed after TS-1 was treated with H2O2. Additionally, it reacted with imine to produce oxime [28]. Tvarłzˇková et al. analyzed the adsorption of ammonia and cyclohexanone on TS-1 according to the IR spectra and observed the presence of cyclohexylimine [29]. In addition, Yip and Hu suggested the two proposed reaction models using kinetic analysis, namely, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms [30]. Liu et al. proposed a new possible mechanism in which the intermediate had strong absorption of the C–O bond with the aid of on-line ATR–FTIR spectroscopy [31]. However, most articles did not clearly describe the catalytic mechanism in the TS-1/H2O2 System. Thangaraj et al. reported the possible mechanisms for the formation of oxime, including the imine and hydroxylamine pathways [32]. Xia et al. demonstrated that hydrogen peroxide could be adsorbed and activated at the Ti sites of TS-1 by combing DFT calculations with experimental studies [33]. Bolis et al. also computed the absorption of ammonia on TS-1 using adsorption micro calorimetry IR, XANES and EXAFS techniques, and the authors showed that a minor level of ammonia was irreversibly held at the TS-1 surface [34]. Zecchina et al. clarified the structure of the titanium silicon molecular sieve at different reaction times and reported on the presence of an intermediate TS-1 structure, (”SiO)3(H2O)2TiOOH, with IR, Raman, UV-Vis and XAFS spectroscopy methods [35]. Zhang et al. inferred that acetone, butanone, cyclohexanone and 3-methylcyclohexanone could enter the TS-1 cavity, which was not the case for 4-butylcyclohexanone with TGA (Thermal Gravity Analysis) [36]. From the above reviews, it is very complex and difficult to study the mechanism of ammoximation in the TS-1/H2O2 system through the reaction between acetaldehyde and its oxime. In the area of theoretical study, we used an in-situ infrared spectrometer (ReactIR15, METTLER-TOLEDO, Zurich, Switzerland). FTIR is an effective way to identify substances and analyze the structure of the material, and it is now commonly used. However, this method still has some shortcomings. First, it is troublesome to make the sample with solid compression and a liquid membrane. Moreover, it is possible to cause morphological changes or surface contamination when adding infrared inert material or making the self-support tablet. This results in concealing the true nature of the material. Inconveniently, the instrument can only detect the substance off-line. Because it cannot track changes Catalysts 2016, 6, 109 3 of 10 Catalysts 2016, 6, 109 3 of 10 whichin the was reaction utilized system, in this we paper, cannot can see support the entire a scanning reaction liquid process.‐phase ReactIR15, reaction and which instantly was utilized generate in infraredthis paper, spectrum can support results. a ReactIR15 scanning liquid-phasecan analyze the reaction material and on instantly‐line without generate damaging infrared the spectrummaterial. Basedresults. on ReactIR15 the analysis can of analyze the characteristic the material peaks, on-line we without can identify damaging whether the material.a specific Basedsubstance on the is presentanalysis and of the then characteristic perform qualitative peaks, we cananalysis identify of whetherthe reaction a specific mechanism. substance On is the present basis and of thenthe experimentalperform qualitative analysis, analysis we conducted of the reaction the experiments, mechanism. and On some the basis useful of information the experimental was obtained analysis, fromwe conducted the comparison the experiments, of the two reaction and some mechanism useful information pathways. was We obtained illustrated from the themechanism comparison using of twothe twoapproaches, reaction mechanismand the results pathways. were mutually We illustrated supportive the mechanism and complementary. using two approaches, and the results were mutually supportive and complementary. 2. Results and Discussion 2. Results and Discussion 2.1. Analysis of Imine Reaction Mechanism 2.1. Analysis of Imine Reaction Mechanism Ammonia, acetaldehyde and TS‐1 were added to the reactor in accordance with the order of reactantsAmmonia, with a constant acetaldehyde proportion and TS-1 under were agitation added conditions. to the reactor Then, in hydrogen accordance peroxide with the was order added of afterreactants all peaks with visible a constant on the proportion computer under stabilized agitation (see conditions. Figure 1). To Then, eliminate hydrogen overlapping peroxide waspeaks, added the characteristicafter all peaks peaks visible were on processed. the computer stabilized (see Figure1). To eliminate overlapping peaks, the characteristic peaks were processed.

(a) (b)

−1 FigureFigure 1. 1. WithWith the the addition addition of of reactants reactants in inorder, order, (a) (thea) the peak peak value value of 1667 of 1667 cm cm and´ 1(band) the ( bpeak) the value peak −1 ofvalue 985 ofcm 985 changed. cm´1 changed.

As in Figure 1, there was no absorption peak near 1670 cm−1 without hydrogen peroxide. As in Figure1, there was no absorption peak near 1670 cm ´1 without hydrogen peroxide. However, the C=N bond stretching vibration absorption peak appeared at 1667 cm−1 after hydrogen However, the C=N bond stretching vibration absorption peak appeared at 1667 cm´1 after hydrogen peroxide was added; additionally, the peak intensity was high, and the peak increased rapidly. At peroxide was added; additionally, the peak intensity was high, and the peak increased rapidly. At the the same time, there was an N–O bond absorption peak at 985 cm−1, indicating that there was no same time, there was an N–O bond absorption peak at 985 cm´1, indicating that there was no intermediate product containing a C=N bond; instead, acetaldehyde imine was formed. Additionally, intermediate product containing a C=N bond; instead, acetaldehyde imine was formed. Additionally, the C=N and N–O absorption peaks coalesced after the addition of hydrogen peroxide, which showed the C=N and N–O absorption peaks coalesced after the addition of hydrogen peroxide, which showed that acetaldoxime is generated. that acetaldoxime is generated. 2.2. Analysis of Hydroxylamine Reaction Mechanism 2.2. Analysis of Hydroxylamine Reaction Mechanism Ammonia, hydrogen peroxide and TS‐1 were added to the reactor in a guaranteed proportion Ammonia, hydrogen peroxide and TS-1 were added to the reactor in a guaranteed proportion under agitation conditions. Additionally, acetaldehyde was added after all peaks visible on the under agitation conditions. Additionally, acetaldehyde was added after all peaks visible on the computer stabilized (see Figure 2). Similarly, all peaks were separated to avoid overlap with each other. computer stabilized (see Figure2). Similarly, all peaks were separated to avoid overlap with each other.

Catalysts 2016, 6, 109 4 of 10 Catalysts 2016, 6, 109 4 of 10

Catalysts 2016, 6, 109 4 of 10

(a) (b)

(a) (b) −1 FigureFigure 2. 2.With With the the reactants reactants added added in in the the same same order, order, (a) ( peaka) peak curve curve at 1227at 1227 cm cm´1 and and (b ()b Peak) Peak curve curve −1 atat 985 Figure985 cm cm´ 12.began Withbegan the to to changereactants change in inadded a a different different in the pattern.same pattern. order, (a) peak curve at 1227 cm−1 and (b) Peak curve at 985 cm−1 began to change in a different pattern. As in Figure 2, the specific peak of hydroxylamine at 1227 cm−1 was significantly higher after the As in Figure2, the specific peak of hydroxylamine at 1227 cm ´1 was significantly higher after additionAs of in TS Figure‐1, indicating 2, the specific hydroxylamine peak of hydroxylamine was generated. at 1227 After cm−1 wasthe peak significantly stabilized, higher the after peak the at 1227 the addition of TS-1, indicating hydroxylamine was generated. After the peak stabilized, the peak at cmaddition−1 gradually of TS ‐decreased1, indicating with hydroxylamine the addition was of generated. acetaldehyde, After the and peak a peak stabilized, at 985 the cm peak−1 attributed at 1227 to 1227 cm´1 gradually decreased with the addition of acetaldehyde, and a peak at 985 cm´1 attributed oximecm−1 simultaneouslygradually decreased appeared. with the This addition indicated of acetaldehyde, that acetaldehyde and a peakwas atformed 985 cm after−1 attributed the addition to of to oxime simultaneously appeared. This indicated that acetaldehyde was formed after the addition of oxime simultaneously appeared. This indicated that −acetaldehyde1 was formed after the addition of acetaldehyde. However, the peak value of 1227 cm´1 was not reduced to the original one before the acetaldehyde. However, the peak value of 1227 cm −1 was not reduced to the original one before the additionacetaldehyde. of TS‐ 1,However, which may the peak be caused value of by 1227 catalytic cm was activity, not reduced reaction to theconditions original oneand before other thereasons additionaddition of TS-1, of TS which‐1, which may may be causedbe caused by by catalytic catalytic activity, activity, reaction reaction conditions conditions and and other other reasons reasons and and may lead to incomplete transformation of hydroxylamine to oxime. Based on Figure 2, we can mayand lead may to incomplete lead to incomplete transformation transformation of hydroxylamine of hydroxylamine to oxime. to oxime. Based Based on Figure on Figure2, we can 2, we conclude can conclude that hydroxylamine was generated and then formed acetaldoxime throughout the reaction thatconclude hydroxylamine that hydroxylamine was generated was and generated then formed and then acetaldoxime formed acetaldoxime throughout throughout the reaction the reaction process. process.process. 2.3. The Comparative Test and Results 2.3.2.3. The The Comparative Comparative Test Test and and ResultsResults Based on the above infrared results, we designed experiments to confirm the mechanism. The entireBasedBasedreaction on on the the wasabove above divided infraredinfrared into results, two we steps.we designed designed First, experiments ammonia experiments reactedto confirmto confirm with the hydrogen mechanism.the mechanism. peroxide The The inentire theentire presence reaction reaction ofwas was TS-1. divided divided Second, intointo two the steps. reactionsteps. First, First, solution ammonia ammonia was reacted reacted filtered with with off hydrogen TS-1 hydrogen and peroxide then peroxide continued in the in the presence of TS‐1. Second, the reaction solution was filtered off TS‐1 and then continued to react with topresence react with of TS acetaldehyde.‐1. Second, the Additionally, reaction solution the reaction was filtered with off TS-1 TS in‐1 and two then steps continued was performed to react as with a acetaldehyde. Additionally, the reaction with TS‐1 in two steps was performed as a contrast contrastacetaldehyde. experiment. Additionally, the reaction with TS‐1 in two steps was performed as a contrast experiment. experiment.In the above two groups of experiments, the acetaldehyde selectivity was similar, but it was not In Inthe the above above two two groups groups of experiments, the the acetaldehyde acetaldehyde selectivity selectivity was was similar, similar, but it but was it not was not high.high. The The acetaldehyde acetaldehyde conversion conversion rate rate waswas muchmuch lower than than the the contrasting contrasting experiment experiment for forthe the high. The acetaldehyde conversion rate was much lower than the contrasting experiment for the designdesign experiment experiment in Figurein Figure3. 3. design experiment in Figure 3.

FigureFigure 3. 3.Comparison Comparison of of the the twotwo groups of of experimental experimental results. results.

Figure 3. Comparison of the two groups of experimental results.

Catalysts 2016, 6, 109 5 of 10 Catalysts 2016, 6, 109 5 of 10

TheCatalystsThe acetaldehyde acetaldehyde 2016, 6, 109 solutions solutions were were analyzed analyzed by by LC-MS LC‐MS andand GC-MS.GC‐MS. We foundfound many byproducts 5 of 10 in thein system, the system, which which were identifiedwere identified as self-condensation as self‐condensation products products of acetaldehyde of acetaldehyde in Figure in 4Figure. Although 4. The acetaldehyde solutions were analyzed by LC‐MS and GC‐MS. We found many byproducts manyAlthough substances many cannotsubstances be precisely cannot be identified, precisely weidentified, could makewe could conclusions make conclusions that these that byproducts these in the system, which were identified as self‐condensation products of acetaldehyde in Figure 4. contributedbyproductsAlthough to manycontributed the low substances acetaldoxime to thecannot low selectivity. be acetaldoximeprecisely Monitoring identified, selectivity. we the could hydroxylamine Monitoring make conclusions the concentration hydroxylamine that these using ReactIR15,concentrationbyproducts we found usingcontributed that ReactIR15, the to hydroxylamine the we low found acetaldoxime that concentration the hydroxylamineselectivity. was uniformlyMonitoring concentration reduced the hydroxylamine was during uniformly filtration. Thisreduced isconcentration because during hydroxylamine usingfiltration. ReactIR15, This is is generatedwe because found hydroxylaminethat in the the TS-1 hydroxylamine channels is generated by concentration catalysis in the andTS was‐1 was channelsuniformly transferred by intocatalysis thereduced reaction and during was solution. transferred filtration. Because Thisinto theis of because reaction the excess hydroxylamine solution. ammonia Because inis thegeneratedof the system, excess in theammoniathe solutionTS‐1 channelsin the was system, alkaline. by However,thecatalysis solution in and anwas was alkalescent alkaline. transferred However, environment, into the inreaction an hydroxylaminealkalescent solution. Because environment, is of extremely the excess hydroxylamine ammonia unstable, in and theis extremely system, it is easily decomposedunstable,the solution and into it was ammonia,is easily alkaline. decomposed nitrogen, However, nitrate intoin an ammonia, andalkalescent water nitrogen, [ 37environment,–40]. nitrate After hydroxylamine the and addition water [37–40]. of is acetaldehyde extremely After the in theaddition secondunstable, of step, acetaldehyde and the it hydroxylamineis easily in decomposed the second concentration step, into theammonia, hydroxylamine decreased nitrogen, rapidly nitrateconcentration to and a low water value.decreased [37–40]. Then, rapidlyAfter it remained the to a stable,lowaddition value. as in Then, Figureof acetaldehyde it5 remained. Because in thestable, of second the as decomposition in step, Figure the hydroxylamine5. Because of hydroxylamine, of the concentration decomposition there decreased of was hydroxylamine, rapidly a much to lower a low value. Then, it remained stable, as in Figure 5. Because of the decomposition of hydroxylamine, concentrationthere was a thatmuch reacted lower withconcentration acetaldehyde that reacted in the design with acetaldehyde experiment comparedin the design to the experiment contrasting comparedthere was to athe much contrasting lower concentration one, which ledthat to reacted the low with conversion acetaldehyde rate ofin acetaldehyde.the design experiment The entire one, which led to the low conversion rate of acetaldehyde. The entire progress is presented in Figure6. progresscompared is presented to the contrasting in Figure one, 6. whichThese ledresults to the demonstrated low conversion that rate the of procedureacetaldehyde. for The generating entire These results demonstrated that the procedure for generating hydroxylamine was the control step of hydroxylamineprogress is presented was the incontrol Figure step 6. These of the results reaction. demonstrated Moreover, thatthe thesecond procedure step is for a nongenerating‐catalytic the reaction. Moreover, the second step is a non-catalytic reaction such that the entire reaction passes reactionhydroxylamine such that thewas entire the control reaction step passes of the through reaction. hydroxylamine. Moreover, the second step is a non‐catalytic throughreaction hydroxylamine. such that the entire reaction passes through hydroxylamine.

FigureFigureFigure 4. 4.The 4.The The comprehensive comprehensive comprehensive results results of of acetaldehydeacetaldehyde inin in GCGC GC-MS‐‐MSMS and and and LC LC‐ LC-MS.MS.‐MS.

Figure 5. The change of the peak value of hydroxylamine in the entire process of the design Figureexperiment, 5. The includingchange of the the separation peak value step ofof the hydroxylamine catalyst. in the entire process of the design Figure 5. The change of the peak value of hydroxylamine in the entire process of the design experiment, experiment, including the separation step of the catalyst. including the separation step of the catalyst.

Catalysts 2016, 6, 109 6 of 10 Catalysts 2016, 6, 109 6 of 10

Figure 6. InIn the the TS TS-1/H‐1/H2O2O2 2system,system, the the reaction reaction pathway pathway of of acetaldehyde acetaldehyde ammoximation to acetaldehyde and the byproducts are presented in the experiments.

3. Experimental Experimental

3.1. Materials and Measurements The industrial industrial catalyst catalyst TS TS-1‐1 (average (average particle particle size size of ofapproximately approximately 0.2 μ 0.2m;µ m;surface surface area area of 417 of m4172∙g m−1)2 ¨wasg´1 provided) was provided by RIPP, by SINOPEC. RIPP, SINOPEC. Analytical Analytical grade ammonia grade ammonia (25%, AR, (25%, Sinopharm, AR, Sinopharm, Shanghai, China),Shanghai, hydrogen China), hydrogenperoxide peroxide(30%, AR, (30%, Sinopharm, AR, Sinopharm, ShangHai, ShangHai, China), China), acetaldehyde acetaldehyde (40%, (40%, AR, GuangFu,AR, GuangFu, Tianjin, Tianjin, China), China), hydroxylamine hydroxylamine hydrochloride hydrochloride (98.5%, (98.5%, AR, GuangFu, AR, GuangFu, Tianjin, Tianjin, China) China) and acetaldoximeand acetaldoxime (HPLC, (HPLC, TCI, Tokyo, TCI, Tokyo, Japan) Japan) were all were obtained all obtained commercially. commercially. Acetaldehyde Acetaldehyde was purified was bypurified distillation by distillation and stored and at stored 0 °C. at 0 ˝C. The GC GC-2014C‐2014C (SHIMADZU, Tokyo, Japan) was equipped with a TCD detector.detector. The packed column (3 m ˆ× 44 mm mm ×ˆ 0.250.25 mm) mm) was was filled filled with with 10% 10% PEG6000, PEG6000, which which was was applied applied to to the the 101 101 pickling pickling ˝ white diatomite carrier, andand thethe ovenoven temperaturetemperature waswas keptkept atat 100100 °C.C. H22 was used as carrier gas at a flowflow rate of 5555 mLmL¨∙min−´1.1 .The The detector detector temperature temperature was was 120 120 °C˝C and and injection injection port port temperature was 140 ˝°C.C. The injection quantity was 0.4 μµL. GCGC-MS‐MS analysis analysis was was accomplished accomplished using using a gas a chromatograph/mass gas chromatograph/mass spectrometer spectrometer (7890A GC/5975C(7890A GC/5975C MS, Agilent, MS, Agilent,CA, USA). CA, The USA). separationThe separation column for GC column‐MS was for DB GC-MS‐5MS (30 was m × 0.25 DB-5MS mm ×(30 0.25 m μˆm).0.25 Helium mm ˆ was0.25 usedµm). as Helium a carrier was gas used with as a flow a carrier rate of gas 1.0 with mL amin flow−1. The rate oven of 1.0 temperature mL min´1. wasThe oveninitiallytemperature 40 °C and waskept initiallyfor 2 min. 40 Then,˝C and it increased kept for 2 to min. 290 Then, °C at a it rate increased of 4 °C to∙min 290−1 ˝andC at was a rate kept of for4 ˝C 4¨ min.min´ The1 and injection was kept port for temperature 4 min. The injectionwas 280 °C. port temperature was 280 ˝C. LCLC-MS‐MS analysis was achievedachieved usingusing aa liquidliquid chromatograph/masschromatograph/mass spectrometer (Alianle2695+2Q2000, Waters, Waters, Milford, Milford, MA, MA, USA). USA). Liquid Liquid chromatographic separation was optimized and established on a reverse phase ODSC18ODSC18 columncolumn (150(150 mmmm ˆ× 2.1 mm ˆ× 55 μµm), which was kept at room temperature, and the mobile phase was acetonitrile acetonitrile-water‐water solution (80:20). The The flow flow rate was 0.2 mL¨∙min−´1,1 and, and the the detection detection wavelength wavelength was was 254 254 nm. nm. Mass Mass spectrometer spectrometer detection detection and quantificationquantification were performed in negative mode. The The curtain curtain gas gas was was 0.5 0.5 MPa MPa and and the ion spray voltage was 35 V. TheThe scanningscanning rangerange waswas 20–18020–180 mm//zz,, and the the dryer flow flow rate was 500 L ∙¨minmin−´1. 1. A ReactIR15 reaction analysis system (METTLER (METTLER-TOLEDO,‐TOLEDO, Zurich, CHE, Switzerland, 2012), 2012), equipped with a light conduit and diamond insertion probe, was used to collect the mid mid-FTIR‐FTIR spectra of the condensation components. Additionally, Additionally, the system of MCT (Mercury cadmium telluride) was cooled using liquid nitrogen. FTIR spectra were collected in the wave number range between 3000 and 650 cm−1.

Catalysts 2016, 6, 109 7 of 10 cooled using liquid nitrogen. FTIR spectra were collected in the wave number range between 3000 and ´ Catalysts650 cm 20161. ,, 6,, 109 7 of 10

3.2. Experimental Procedures We obtainedobtained the infraredinfrared spectrumspectrum of aldehyde,aldehyde, deionized water, ammonia,ammonia, acetaldoxime and ˝ hydroxylamine using ReactIR15. The experiments were performed at 60 °CC inin a a 250 250-mL,‐mL, four four-necked‐necked flaskﬂask with a condenser, thermometer,thermometer, andand magneticmagnetic stirrer. The molar ratio of each substance was 3 2 2 3 2 2 CH3CHO:HCHO:H22OO2:NH:NH2:NH3 =3 1:1:2.5.= 1:1:2.5. The The acetaldehyde acetaldehyde levellevel level was was 0.1 mol, 0.1 mol, and H and2O2 H was2O2 addedwas added using a using micro a −1 ´1 pumpmicro pumpfor 30 min for 30 at mina rate at of a 0.14 rate mL of 0.14∙∙min mL−1.. The¨ min catalyst. The amount catalyst was amount 0.5 g. wasThe ReactIR15 0.5 g. The probe ReactIR15 was stretchedprobe was intointo stretched thethe liquidliquid into system the liquid and system scanned and every scanned 15 s. every The experimental 15 s. The experimental device isis shown device inin is Figure shown 7.in FigureThe addition7. The addition order of order thethe of reactants the reactants according according toto thethe to the separate separate mechanisms mechanisms of of imineimine andand hydroxylamine are listed as followsfollows (as(as shownshown inin FigureFigure8 8).).

Figure 7. DiagramDiagram of of thethe experiment experiment setup; 1. Data Data analysis analysis of of computer; computer; 2. ReactIR15; ReactIR15; 3. 3. Dector Dector part; part; 4. Electric mixer; 5. Reflux Reﬂux condensation; condensation; 6. Oil Oil bath bath pot. pot.

Figure 8. The different sequences of reactants in two mechanisms. Figure 8. The different sequences of reactants in two mechanisms. 3.3. Spectral Analysis 3.3. Spectral Analysis To analyze thethe typicaltypical IR spectra of hydroxylamine and acetaldoxime, thethe absorption band of To analyze the typical IR spectra of hydroxylamine−1 and acetaldoxime, the absorption band of C=N bond inin acetaldoxime isis locatedlocated at 1667 cm´−11,, and thethe N–O bond stretching vibration absorption C=N bond in acetaldoxime is located at 1667 cm , and the N–O−1 bond stretching vibration absorption isis a strong band, which isis locatedlocated between 1055 and 870 cm´−11 and sometimes split intointo a few peaks. is a strong band, which is located−1 between 1055 and 870 cm and sometimes split into a few peaks. Because thethe 2250–1950 cm´−1 1 region for thethe diamond isis thethe infraredinfrared shielding area, we selected Because the 2250–1950−1 cm region for the diamond is the infrared shielding area, we selected−1 approximately 1670 cm−1 as the C=N bond stretching vibration absorption peak and 985 cm−1 as the approximately 16701670 cmcm´1 asas the the C=N C=N bond bond stretching vibration absorptionabsorption peakpeak andand 985 985 cm cm´1 as the N–O bond absorption peak by comparing to the pure material infrared spectrum. Both characteristic N–O bond absorption peak by comparing to the pure material infrared spectrum. Both characteristic peaks of thethe acetaldoxime absorption are shown inin Figure 9a. In addition, we similarly chose 1220´1 peaks−1 of the acetaldoxime absorption are shown in Figure9a. In addition, we similarly chose 1220 cm cm−1 as a characteristic peak of hydroxylamine in Figure 9b. ascm a characteristic as a characteristic peak peak of hydroxylamine of hydroxylamine in Figure in Figure9b. 9b.

Catalysts 2016,, 6,, 109109 8 of 10

(a) (b)

Figure 9.9. TheThe infrared infrared spectrum of pure substances acquired by ReactIR15:ReactIR15: ( a)) IR spectrum of acetaldoxime and (b)) IRIR spectrum of hydroxylamine.

4. Conclusions 4. Conclusions Based on the experimental results of the infrared spectrum and two step reactions, we can Based on the experimental results of the infrared spectrum and two step reactions, we can confirm that the hydroxylamine route may be the most important catalytic mechanism for the liquid confirm that the hydroxylamine route may be the most important catalytic mechanism for the liquid phase ammoximation of acetaldehyde to its oxime in the TS‐1/H2O2 system by the appearance of the phase ammoximation of acetaldehyde to its oxime in the TS-1/H2O2 system by the appearance of hydroxylamine infrared characteristic peak. At the same, the experiments confirmed that the process the hydroxylamine infrared characteristic peak. At the same, the experiments confirmed that the by which hydroxylamine reacts with acetaldehyde was a non‐catalytic reaction. Furthermore, process by which hydroxylamine reacts with acetaldehyde was a non-catalytic reaction. Furthermore, hydroxylamine is confirmed to be the intermediate in the reaction of ammoximation with TS‐1 in the hydroxylamine is confirmed to be the intermediate in the reaction of ammoximation with TS-1 in the liquid phase. However, this work is not completely finished. In the future, we can also explain the liquid phase. However, this work is not completely finished. In the future, we can also explain the reaction mechanism according to the dynamics and more or less achieve a quantitative determination reaction mechanism according to the dynamics and more or less achieve a quantitative determination of the intermediate products. of the intermediate products. Acknowledgements: This research was supported by the National Natural Science Foundation of China under Acknowledgments:agreement number 21073020,This research and The was Importation supported by and the Development National Natural of High Science‐Caliber Foundation Talents Project of China of Beijing under agreement number 21073020, and The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions Agreement Number CIT&TCD 20130325. Municipal Institutions Agreement Number CIT&TCD 20130325. Author Contributions:Contributions: C.M., S.Y. and H.J.H.J. conceivedconceived andand designeddesigned thethe experiments;experiments; C.M. performed the experiments; C.M., G.L. and X.X. analyzedanalyzed thethe data;data; G.H.G.H. contributedcontributed materials;materials; C.M.C.M. andand H.J.H.J. wrotewrote thethe paper.paper. ConflictsConflicts ofof Interest:Interest: TheThe authorsauthors declaredeclare nono conflictconflict ofof interest.interest.

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