Heterogeneous Catalytic Methoxycarbonylation of 1,6-Hexanediamine

Article pubs.acs.org/IECR

Heterogeneous Catalytic Methoxycarbonylation of 1,6Hexanediamine by Dimethyl Carbonate to Dimethylhexane-1,6dicarbamate Hui-Quan Li,* Yan Cao, Xin-Tao Li, Li-Guo Wang, Feng-Jiao Li, and Gan-Yu Zhu National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A nonphosgene route for synthesis of dimethylhexane-1,6-dicarbamate (HDC) by methoxycarbonylation of 1,6hexanediamine (HDA) with dimethyl carbonate (DMC) using AlSBA-15 as a heterogeneous catalyst was proposed. Catalyst was characterized, and effects of catalyst concentration, DMC to HDA molar ratio, reaction temperature, and reaction time were investigated. Results showed that AlSBA-15 exhibited high catalytic activity because of its large surface area and average pore diameter as well as acidity because of impregnation with Al. Under the optimum conditions of 5% Al incorporation, 5% catalyst concentration, 6:1 DMC to HDA molar ratio, 70 °C reaction temperature, and 35 h reaction time, the HDC yield reached 84.2%. Catalyst was easily separated, and no obvious deactivation was found even after the catalyst was used four times. Interactions of AlSBA-15 with the substrates were also studied by quasi in situ Fourier transform infrared (FTIR), and a possible reaction mechanism was proposed.

1. INTRODUCTION Hexamethylene diisocyanate (HDI) is one of the most important aliphatic isocyanates, most of which are used to synthesize aliphatic polyurethanes. Compared with aromatic isocyanates such as MDI and TDI, the structure of saturated straight-chain paraffin confers HDI derivatives with excellent properties (e.g., bright color, moderate hardness, oil and wear resistance, and nondiscoloration). Thus, HDI derivatives are widely applied in Original Equipment Manufacturer coatings, repair coatings, top-grade furniture paints, good stability binders, and so on.1 However, HDIs are traditionally synthesized using phosgene as the starting material,2−4 which have the drawbacks of extreme toxicity and formation of corrosive HCl as a side product. Thus, an environmentally friendly synthesis route for HDI must be developed. Among the reported phosgene-free methods of HDI production (e.g., onepot transesterification, carbamate anion dehydration, oxidative carbonylation, and thermal decomposition),5−8 thermal decomposition of dimethylhexane-1,6-dicarbamate (HDC) is the most promising route. Therefore, synthesis of the intermediate HDC is crucial to the entire process and thus receiving considerable attention. In recent years, HDC synthesis has mainly focused on reductive carbonylation of nitro compounds,5 oxidative carbonylation of amines,9 and alcoholysis of substituted urea.10,11 However, most of these methods require high temperatures and pressures, use expensive noble metal catalysts, and produce more byproducts, which lead to low selectivity. Methoxycarbonylation of amines with dialkyl carbonates, especially dimethyl carbonate (DMC), has recently become an attractive route.12−18 DMC is one of the most important fundamental raw materials for green organic synthesis and can replace the highly toxic phosgene, dimethyl sulfate, and chloromethane. DMC can also realize the clean reaction of carbonylation, © XXXX American Chemical Society

methoxycarbonylation, and methylation. Another advantage of using DMC as a raw material is that the byproduct of the methoxycarbonylation of 1,6-hexanediamine (HDA) with DMC is CH3OH, whereas DMC can be synthesized in the environmentally friendly method of oxidative carbonylation of CH3OH at the industrial scale. Thus, the above two processes can be coupled to realize “zero emission” and atom economy, consistent with the green chemistry concept. Although DMC has many advantages, the effective catalyst is crucial to this reaction in terms of increasing the reaction rate and selectivity toward the target compound. Therefore, researchers are focusing on developing a catalyst that is low cost, easy to separate, and recyclable. Several homogeneous catalysts can reportedly increase the selectivity for this reaction, but problems involving the recovery of homogeneous catalysts remain unsolved.13−17 Development of heterogeneous catalysts is a good approach to overcome these disadvantages. Unfortunately, only two heterogeneous catalysts for synthesis of HDC from HDA and DMC have been reported.12,19 However, the recyclability of these catalysts has not yet been tested. Mesoporous materials are known to be a special type of nanomaterial with ordered arrays of uniform nanochannels. SBA-15 is a large-scale mesoporous material with highly ordered hexagonally arranged mesochannels, thick walls, adjustable pore sizes from 3 to 30 nm, and high hydrothermal and thermal stabilities.20,21 SBA-15 is attracting special attention in many fields, including adsorption, catalysis, separation, nanoscience, and solid templates for other materials. Received: September 11, 2013 Revised: November 14, 2013 Accepted: December 8, 2013

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dx.doi.org/10.1021/ie4029945 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis of HDC from HDA and DMC

Scheme 2. Methylation Side Reaction in the Reaction of HDA and DMC

2.3. Catalyst Characterization. Active compound loadings in the catalysts were determined by X-ray fluorescence PANalytical AXIOS. X-ray diffraction (XRD) patterns of the catalysts were obtained using a Siemens D/max-RB powder Xray diffractometer with Cu Kα radiation and a liquid-nitrogencooled germanium solid-state detector scanned from 5° to 40°. Samples were also observed by transmission electron microscopy (TEM) recorded on a JEM-2100 (UHR) TEM system. The specific surface area of the catalysts was measured by N2 physisorption at liquid nitrogen temperature with a Quantachrome Autosorb-1. Samples were degassed at 200 °C in a vacuum for 6 h before N2 physisorption measurements. The specific surface area was determined using the standard Brunauer−Emmett−Teller method based on adsorption data. Pore size distributions were calculated from both adsorption and desorption branches of isotherms using the Barrett− Joyner−Halanda method and the corrected Kelvin equation. Total pore volume values were estimated from the amount adsorbed at a relative pressure (P/Po) of 0.995 determined using the t-plot method of De Boer.20 Acidity of the sample was measured by temperatureprogrammed desorption (TPD) of adsorbed NH3 (Micromeritics, Autochem 2910).20 The standard procedure for TPD measurements involved activation of the sample in flowing helium at 500 °C for 3 h, cooling to 25 °C, adsorbing NH3 from a stream of He−NH3 (10% NH3 in He), removing the physically adsorbed NH3 by desorbing at 100 °C for 1 h in He, and finally carrying out the TPD experiment by increasing the catalyst temperature at 10 °C/min. 2.4. Reaction Procedure and Product Analysis. All experiments were performed in a 100 mL stainless steel autoclave equipped with a magnetic stirrer. About 4.84 g of

However, pure silica SBA-15 mesoporous materials possess neutral Si frameworks that limit their broad applicability. Incorporation of aluminum induces Bronsted and Lewis acid properties in silica SBA-15 mesoporous materials, thereby showing great potential in moderate acid-catalyzed reactions of large molecules. However, only a few applications of AlSBA-15 in acid-catalyzed reactions have been realized so far. In this study, application of aluminum mesoporous silica was extended to synthesis of the aliphatic dicarbamate HDC. Catalyst structure was characterized, and reaction conditions (i.e., reaction temperature, reaction time, molar ratio of reactants, and catalyst content) were investigated. Furthermore, the interaction between catalyst and substrates was investigated using quasi in situ FT-IR spectroscopy, and a possible reaction mechanism was proposed. This work presented a promising way of cleanly synthesizing HDC.

2. EXPERIMENTAL SECTION 2.1. Chemicals. HDA, Al(NO3)3, Pb(OAc)2, Zn(OAc)2, and Fe(NO3)3 were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. DMC was purchased from BP and dried by molecular 5A before use. All chemicals were analytical grade. γ-Al 2 O 3 and the mesoporous silica materials SBA-15, MCM-41, APO-5 and ZSM-5 were purchased from Nanjing XFNANO Materials Tech Co., Ltd. 2.2. Catalyst Preparation. Several catalysts with different Al loadings were prepared by volumetric immersion. In a typical sample preparation, 0.785 g of Al(NO3)3 was added to 2.740 g of SBA-15 and maintained at room temperature for 10 h. The obtained product was washed, dried at 100 °C for 4 h, and calcined at 600 °C for 4 h in air (heating rate of 10 K min−1). B



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HDA, 30.00 g of DMC, and 1.74 g of catalyst were charged into the autoclave and reacted at 70 °C for 10 h. After reaction, the catalyst was separated by filtration. The filtrate was analyzed by gas chromatography (GC) Shimadzu GC-2014 equipped with an FID detector. GC coupled with mass spectrometry (MS) was run on a GC (Agilent 6890 series)-MS system with an EI ion source and a mass-selective detector (Agilent 5975C) (see Supporting Information). Quasi in situ FT-IR spectroscopy analysis was performed on a Bruker NC-97 FT-IR equipped with a cell (Φ = 13 mm) having ZnSe windows. Scanning was performed from 4000 to 650 cm−1 with 256 scans and a resolution of 4 cm−1. About 1 mg of sample and 200 mg of KBr were ground and pressed into a thin disk. Then, the disk was placed in the cell, and 2 mL of DMC was injected into the cell. The cell was heated to 70 °C, and spectra were collected once every 3 min. Three types of experiments were performed, namely, (a) sample including HDA and catalyst, (b) sample including catalyst with 2 mL of DMC injected into the cell, and (c) sample including HDA and catalyst with 2 mL of DMC injected into the cell.

Figure 1. Activities of different heterogeneous catalyst on reaction between HDA and DMC.

3. RESULTS AND DISCUSSION After reaction, the filtrate was analyzed by GC-MS. Dimethylhexane-1,6-dicarbamate (HDC) was detected as the object product, dimethylhexane-1,6-monocarbamate (HMC) was the intermediate, N-methyl HDA (N-HDA), N-methyl HMC (N-HMC), and N′,N-methyl HMC (N′N-HMC) were the three methylation byproducts (MBP). Products and byproducts may have been generated as shown in Schemes 1 and 2, as reported by Toshiba et al.17 DMC is known to have two active groups, i.e., carbonyl and methyl. If the carbonyl group was attacked by one amino moiety of HDA, HMC was first generated by the carbonylation reaction (Scheme 1, reaction 1), and HDC was finally synthesized by attack of the other amino moiety of HDA on the carbonyl group of DMC (Scheme 1, reaction 2). Meanwhile, Nmethylated products were formed when the methyl moiety group of DMC was attacked (Scheme 2, reactions 3−6) by the amino moiety of HDA. 3.1. Catalyst Screening. This reaction can be carried out without a catalyst, but with a low selectivity. Thus, a highefficiency catalyst is important in improving the selectivity of the object product HDC. Accordingly, a catalytic activity study of commonly used metallic oxides as well as mesoporous silica materials including ZSM-5, APO-5, MCM-41, and SBA-15 as heterogenerous catalysts for synthesis of HDC from DMC was conducted. Experiments were carried out under the conditions of n(DMC):n(HDA) = 6:1, n(catalyst):n(HDA) = 1:20, t = 10 h, and T = 80 °C. Figure 1 shows that the reaction rate was slow, and the HDC yield was only 1.2% without any catalyst (blank experiment). The HDC yield slightly increased after adding the metallic oxide catalysts ZnO, PbO, Fe2O3, and ZrO2. However, the highest HDC yield of 19.2% from ZrO2 was too low for application. The mesoporous molecular sieve material was then tested. Figure 1 shows that the mesoporous molecular sieves APO-5 and ZSM-5 with small average diameters showed relativity low catalytic activity such that the highest HDC yield was only 16.0%, which approached the value of metallic oxides. By contrast, mesoporous molecular sieves MCM-41 and SBA15 with relatively large diameters and surface areas showed higher catalytic activity than the others such that the HDC yield was higher than 33.0%. In particular, AlSBA-15 catalyst revealed

excellent catalytic activity such that the HDC yield reached as high as 55.4% under the same conditions. The largest average pore diameter (data not shown) of SBA15 was the main reason for the high conversion and yield. Notably, MCM-41 also had a high surface area (850 m2/g), even higher than that of AlSBA-15 (603 m2/g). However, the catalytic activity of MCM-41 was lower than that of SBA-15 because of the smaller pore size of MCM-41 (3.7 nm) than AlSBA-15 (6.23 nm), which hindered accessibility to acidic sites. Another reason for choosing SBA-15 was that SBA-15 had thicker pore walls and higher hydrothermal stability than MCM-41, and the hexagonal channels of SBA-15 did not rupture with enlarged pore diameter. By contrast, mesoscopic ordering diminished with enlarged pore diameter of MCM41;22 hence, the difference in catalytic activity can be attributed to the difference in textural properties of SBA-15 and MCM-41. All these findings showed the superiority of SBA-15 to MCM41 and explained why AlSBA-15 was chosen as the effective catalyst in this study. 3.2. Characterization of Catalysts. The surface area and pore volumes of the samples with different loadings are presented in Table 1. AlSBA-15 catalyst was found to have a large surface area (527−789 m2/g) and average pore diameter (5.92−6.41 nm), which favored carbonation of HDA to HDC. Methoxycarbonylation of alkyl diamine HDA with DMC was an acid catalytic reaction, and an acidic catalyst can promote this reaction. The acid sites on AlSBA-15 catalysts are reportedly Lewis type.20 Thus, catalyst acidity was measured by TPD, and the influences of catalyst loading and catalyst acidity on the reaction were studied. The total acidity of catalyst always increased with increased catalyst loading, as shown by the TPD results in Table 1. HDA conversion also increased with increased catalyst acidity rapidly at first and slower thereafter. When the reaction was carried out without any catalyst, HDA conversion reached only 32% and the selectivity of the intermediate HMC and the MBP reached 47% and 50%, respectively. This finding indicated that serious side reactions occurred, and the intermediate HMC was not effectively converted to HDC. After adding pure SBA-15 with a total acidity of 0.08, HDA conversion and HDC selectivity slightly increased to 39% and 8% compared with the blank. Meanwhile, C



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Table 1. Physicochemical Property of Catalysts and Their Catalytic Activitya selectivity (%) sample blank SBA-15 AlSBA15(1)b AlSBA15(2.5) AlSBA15(5) AlSBA15(10) AlSBA15(5)c

loadings Al (XRF)

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

total acidity (mmol NH3/g)

HDA conversion (%)

HDC

HMC

0 0.93

789 740

1.15 1.09

6.36 6.41

0.08 0.10

32 39 76

3 8 42

47 50 39

50 42 19

6 86

2.33

708

1.07

6.23

0.12

81

56

26

18

124

4.2

603

0.94

6.03

0.20

90

61

22

17

150

8.5

527

0.84

5.92

0.23

93

44

36

20

111

4.2

580

0.92

5.93

0.19

89

60

23

17

146

MBP TONd

Conditions: reaction temperature 80 °C, reaction time 10 h, molar ratio of DMC to HDA 6:1, and catalyst concentration 5 wt % of total reaction mixture weight (TRM). bNumbers in parentheses are wt of Al/wt of SBA-15 and represent theoretical active compound impregnated. cCatalysts have been recycled four times. dTON: wt of HDC/wt of catalyst.

a

(2D) hexagonal P6mm hexagonal symmetry in the materials.20,23 Diffraction patterns indicated preservation of the SBA15 mesoporous structure after Al incorporation. Moreover, compared with pure SBA-15, a gradual shift was observed toward lower angles for the (100), (110), and (200) peaks of AlSBA-15, which can be attributed to adulterated aluminum.24 Figure 3a shows TEM images of SBA-15 sample. Well-ordered hexagonal arrays of mesochannels indicated typical 2D P6mm hexagonal symmetry. Figure 3b shows the TEM patterns of AlSBA-15 sample, which indicated that the uniformly ordered mesoporous structure of SBA-15 was preserved after impregnation with Al. Moreover, no aluminum balls were observed from AlSBA-15 by TEM, clearly indicating that they were completely consumed during the preparation procedure. 3.3. Catalyst Study. 3.3.1. Effect of Catalyst Concentration. The effect of catalyst concentration on HDC synthesis is shown in Figure 4. HDA conversion increased and the selectivity of HMC continually decreased with increased catalyst weight from 1% to 12.5%. More active sites were provided by increasing the catalyst concentration, which then facilitated conversion of HDA and HMC. The selectivity of HDC initially increased and then decreased, reaching the highest value of 61.4% at a catalyst concentration of 5 wt %. The trends of MBP were opposite to that of HDC and reached the lowest value of 18.0% also at 5 wt % catalyst concentration. Appropriate active sites and acid strength promoted synthesis of HDC, but a Lewis acid that was too strong increased the formation rate of N-methylated byproducts. Thus, the optimum catalyst concentration was 5 wt % 3.3.2. Effect of MC to HDA Molar Ratio. The effect of DMC to HDA molar ratio on HDC synthesis is shown in Figure 5. HDA conversion slightly increased with increased molar ratio from 2:1 to 12:1 and approached 100% when the molar ratio was 6:1. The selectivity of HDC increased sharply at first and then decreased, reaching a maximum of 81.2% at 6:1 molar ratio. Trends for HMC were opposite to that of HDC, with the selectivity of HMC reaching a minimum of 12.8% at the same molar ratio. This phenomenon can be explained by the DMC concentration in the system. When the molar ratio was lower than 6:1, the increase in DMC promoted reaction to the positive direction such that HMC was quickly converted to HDC. Consequently, the yield of HMC decreased and that of HDC increased. When the molar ratio was higher than 8:1,

the catalyst showed obviously higher activity after impregnation with Al. HDA conversion sharply increased from 39% to76% when the total acidity was increased only from 0.08 to 0.10. MPB also sharply decreased from 42% to 19%, which indicated that impregnation of acid Al restrained side reactions as well as promoted conversion of HDA and intermediate HMC to HDC. With increased catalyst acidity from 0.10 to 0.20, the activity increment was smaller and the highest activity for synthesis of HDC was obtained when the total acidity reached 0.20 (AlSBA15(5)), with HDA conversion of 90% and HDC yield of 61%. With further increased loading to 10%, although the total acidity increased to 0.23, HDC selectivity decreased and HMC and MBP selectivity increased. These findings indicated that too strong acidity promoted side reactions. Thus, with comprehensive consideration of HDA conversion and HMC and HDC selectivity, the optimum catalyst loading was deemed to be 5 wt %. Powder XRD was used to assess the structural ordering of AlSBA-15 materials. Figure 2 shows the XRD patterns of SBA15 and calcined AlSBA-15 with different loadings. In agreement with previous reports, the small-angle XRD profiles of SBA-15 materials showed three well-resolved peaks within the 2θ range from 0.8° to 2° corresponding to the (100), (110), and (200) reflections. These peaks can be indexed to two-dimensional

Figure 2. Low-angle XRD patterns of (a) SBA-15, (b) AlSBA-15(1), (c) AlSBA-15(2.5), (d) AlSBA-15(5), and (e) AlSBA-15(10). D



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Figure 3. TEM images of the catalysts: (a) SBA-15, (b) AlSBA-15(5), and (c) AlSBA-15(5) used four times.

than 6:1. Thus, the optimum molar ratio of DMC to HDA was 6:1. 3.3.3. Effect of Reaction Temperature. The effect of reaction temperature on HDC synthesis was studied, and results are shown in Figure 6. HDA conversion increased and

Figure 4. Effect of catalyst concentration on HDC synthesis. Reaction conditions: reaction temperature 80 °C, reaction time 10 h, molar ratio of DMC to HDA 6:1.

Figure 6. Effect of reaction temperature on HDC synthesis. Reaction conditions: reaction time 10 h, molar ratio of DMC to HDA 6:1, catalyst concentration of 5 wt % of TRM.

selectivity of HMC decreased with increased temperature from 50 to 90 °C, indicating that high temperatures favored conversion of HDA to HMC and then to HDC. The selectivity of HDC also slightly increased from 45 to 70 °C and then decreased with increased temperature, reaching the highest of 52.3% at 70 °C. Meanwhile, MBP selectivity was extremely sensitive to temperature such that with increased temperature from 70 to 90 °C, MBP selectivity sharply increased from 1.75% to 42.4%. This finding can be attributed to the fact that high temperatures favored attack of amines on the methyl moiety group of DMC, thereby increasing the formation rate of N-methylated byproducts. Thus, the favorable temperature for this reaction was 70 °C. 3.3.4. Effect of Reaction Time. The effect of reaction time on synthesis of HDC is shown in Figure 7. The figure shows that HDA conversion consistently increased with increased reaction time, but the reaction rate slowed down. Only 5 h was needed to reach 89.5%, whereas another 20 h was needed to reach 100%. The formation rate of HDC linearly increased from 5 to 35 h, and the HDC yield reached the highest value of 84.2% after 35 h. At this time, nearly no HMC was detected, which meant that HMC was converted to HDC with sufficient time. Moreover, the selectivity of MBP more slowly increased

Figure 5. Effect of molar ratio of DMC to HDA on HDC synthesis. Reaction conditions: reaction temperature 70 °C, reaction time 24 h, catalyst concentration 5 wt % of TRM.

HDA conversion remained almost constant, and increasing the DMC amount only decreased the catalyst concentration, thereby decreasing the acid sites. In turn, the conversion rate of HMC and the generation rate of HDC decreased. The selectivity of MBP also rapidly increased with molar ratio higher than 6:1, which also demonstrated that a decrease in catalyst concentration promoted the side reaction at molar ratios higher E



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Quasi in situ FT-IR experiments conducted in section 2.3 were performed by collecting spectra at different times to explore the interaction between substrates and catalyst. The catalytic activity of KBr was initially tested, and no obvious activity was observed. Figure 9a shows the IR spectrum evolution of the reaction mixture of HDA with AlSBA-15. No obvious spectrum changes were observed with increased reaction time from 0 to 226 min, which indicated that no obvious interaction occurred between HDA and AlSBA-15. This experiment result was in accordance with theory. Methoxycarbonylation of amines by DMC proceeded only when the basicity of the attacking amine was significantly higher than the leaving methoxy group.25−28 Meanwhile, if HDA coordinated with Al3+ and acted as an active species, although the aliphatic diamine possessing a potential chelating N donor may coordinate with Al3+, the basicity of HDA coordinated to Al decreased compared with free HDA. Consequently, methoxycarbonylation failed to occur. Figure 9b shows the IR spectrum evolution of the reaction mixture of DMC and AlSBA-15. Characteristic peaks for DMC were 1765, 1292, and 974 cm−1, which were assigned to the CO, C−O−C, and O−O stretching modes.29 Only one peak was observed at 1765 cm−1 at 0 min, whereas the carbonyl band split into two peaks at 1781 and 1765 cm−1 with prolonged time. In addition, the relative intensity diversity of the peaks from 1292 to 1765 cm−1 increased, and the peak at 974 cm−1 disappeared with prolonged time. All of the above changes were direct evidence of reaction of the carbonyl group of DMC. This phenomenon can be attributed to the interaction of Al3+ of AlSBA-15 catalyst with the oxygen atom of CO of DMC, which decreased the carbonyl bond and caused a blue shift. The metal atom Al of AlSBA-15 catalyst was coordinated with the oxygen atom of carbonyl group and formed the intermediate species I (Scheme 3). In turn, the carbonyl polarity decreased and the force constant increased, thereby leading to the blue shift of the carbonyl group. Meanwhile, the excess DMC in the system led to an interaction of the blue shift peak and unblue shift peak with the intermediate species I. Consequently, the peak at 1765 cm−1 split. In fact, transition metal ions often possess high oxophilicity in the presence of both N- and Odonor species. The unusually high ability of Al in AlSBA-15 to attract electrons facilitated the activation of DMC by partially transferring an electron from the DMC carbonyl oxygen to Al3+.30 The conclusion of carbonyl oxygen of DMC coordinate with Al3+ in this study can also be supported by Distaso’s study, in which they proved the central role of DMC coordinate with Sc3+ in the catalytic process by IR and NMR spectroscopy and fully demonstrated by the isolation.31 Figure 9c shows the IR spectrum evolution of the reaction mixture of HDA, AlSBA-15, and DMC. Compared with Figure 9b, a similar blue shift and peak split appeared at 1762 cm−1 and the relative intensity diversity of the peaks at 1288−1762 cm−1 increased with prolonged time. All these findings indicated obvious interaction between the carbonyl groups of DMC and AlSBA-15 catalyst. Meanwhile, the band at 1701 cm−1 assigned to the carbamate of HDC emerged at 45 min, and the intensity increased with prolonged time, meaning that the object product HDC was produced. Methoxylcarbonylation of HDA with DMC is known to be a nucleophilic addition− elimination reaction. Interaction of metal Al with the oxygen of carbonyl led to an electropositive increase in carbonyl carbon, which enabled easy reaction with the attacking group NH2.

Figure 7. Effect of reaction time on HDC synthesis. Reaction conditions: reaction temperature 70 °C, molar ratio of DMC to HDA 6:1, and catalyst concentration 5 wt % of TRM.

with increased reaction time than with increased reaction temperature. This result indicated that production of MBP was more sensitive to reaction temperature. 3.3.5. Catalyst Recycling. Investigating the stability and recyclability of AlSBA-15 catalyst for industrial applications is very important. After each experiment, the catalyst was repeatedly washed with methanol, dried at 120 °C for 2 h, calcined at 600 °C in air for 4 h, and then used for another cycle. Results are shown in Figure 8. HDA conversion and

Figure 8. Recyclability of the catalyst. Reaction conditions: reaction temperature 70 °C, reaction time 35 h, molar ratio of DMC to HDA 6:1, catalyst concentration 5 wt % of TRM.

HDC yield were found to be nearly the same in all four recycles. Meanwhile, no obvious change was observed from the TEM image of the used catalyst, which indicated that the ordered structure of AlSBA-15 was retained after usage for four times. 3.4. Reaction Mechanism. 3.4.1. Interaction between Substrates and Catalyst. Table 1 shows that the HDC yield was lower than 10% without catalyst or only with SBA-15 catalyst, whereas the yield rapidly increased to 42% with only 0.93 wt % Al impregnation onto SBA-15 (AlSBA-15(1)). This finding revealed that either HDA or DMC had some interaction with Al of the catalyst to some extent. F



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3.4.2. Possible Reaction Mechanism Underlying the Methoxycarbonylation of HDA in the Presence of AlSBA15. As described in Scheme 1, HDC was formed by two similar steps (Scheme 1, reactions 1 and 2). Here, conversion of HDA to HMC (Scheme 1, reaction 1) is mainly discussed because formation of HDC from HMC is the same methoxycarbonylation process. On the basis of the quasi in situ FT-IR spectra, the mechanism of methoxycarbonylation of HDA with DMC using AlSBA-15 catalyst is proposed in Scheme 3. First, the carbonyl group of DMC was activated by attack of Al3+ on the oxygen atom and formation of active intermediate species I. The activated carbonyl carbon was then attacked by the nucleophile HDA to form reaction intermediate species II with a tetrahedral structure. Considering the electrophilic effect of the attacking nucleophilic agent, the carbonyl electron shifted to the carbon atom. Consequently, the coordinate bond that formed between catalyst and the oxygen atom ruptured, and the catalyst was regenerated. At the same time, nucleophilic addition reaction occurred between the attacked −NH2 and CO of DMC. Then, HMC was generated by a substitute reaction wherein the methoxy group was substituted by amines to generate the −NH−CO−OCH3 group because the methoxy group was more easily eliminated than the amine group. Therefore, methoxycarbonylation of amines was accomplished. The amines of HMC attacked intermediate species I again, and then the final product HDC was synthesized the same as the above methoxycarbonylation. This mechanism fit the Eley− Rideal mechanism, in which only one of the molecules adsorbs and the other one reacts with it directly from the gas phase, without adsorbing. Notably, the ambident electrophilic nature of DMC enabled it to react with a nucleophile at the carbonyl group to form carbamate and at the methyl moiety to form the N-methylated product. Table 1 shows that the HDC and MBP selectivities were 47% and 50%, respectively, without any catalyst, which meant that the carbonyl carbon and methoxyl carbon possessed similar electrophilicities. When AlSBA-15 was added, the electrophilic nature of the carbonyl carbon activated by coordination of Al with carbonyl oxygen increased and became more reactive with the nucleophile amine than the methoxyl carbon. For this reason, the amine more easily attacked the carbonyl carbon to form HDC, and only a very limited amount of N-methylated product was formed during the reaction between HDA and DMC over AlSBA-15 catalyst.

4. CONCLUSIONS A nonphosgene route to heterogeneous catalytic synthesis of HDC through reaction between HDA and DMC was proposed. A series of heterogeneous catalysts was screened, and AlSBA-15 was found to exhibit high catalytic activity ascribed to its large surface area and average pore diameter as well as high acidity ascribed to impregnation with Al. Under the optimum conditions of 5% Al incorporation, 5% catalyst concentration, 6:1 DMC to HDA molar ratio, 70 °C reaction temperature, and 35 h reaction time, the HDC yield reached 84.2%. The mechanism of this reaction involved activation of DMC because of coordination of the carbonyl oxygen with Al3+ and nucleophilic attack by the amino group of HDA on the carbonyl carbon of the activated DMC.

Figure 9. FTIR spectra of (a) Cat + HDA, (b) Cat + DMC, and (c) Cat + HDA + DMC.

Moreover, the emergence time for the carbamate peak was close to that of the blue shift peak of carbonyl, which indicated that the reaction was catalyzed by formation of the intermediate of catalyst with DMC, as shown in Scheme 3. G



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Scheme 3. Proposed Mechanism for Methoxycarbonylation of HDA with DMC over AlSBA-15 Catalyst

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ASSOCIATED CONTENT

S Supporting Information *

Details of the GC-MS procedure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 010 82544825. Fax: +86 010 62621355. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Science and Technology Ministry of China (No. 2013BAC11B03), the Knowledge Innovation Fund of Chinese Academy of Science (No. KGCX2-YW-215-2), and National Natural Science Foundation of China (No. B061201).

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Heterogeneous Catalytic Methoxycarbonylation of 1,6-Hexanediamine

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