Chinese Chemical Letters  2017, Vol. 28 Issue (7): 1583-1589   PDF    
Zn-Co bimetallic supported ZSM-5 catalyst for phosgene-free synthesis of hexamethylene-1, 6-diisocyanate by thermal decomposition of hexamethylene-1, 6-dicarbamate
Muhammad Ammara,b, Yan Caoa, Peng Hea, Li-Guo Wanga, Jia-Qiang Chena, Hui-Quan Lia    
a Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, CAS, Beijing 100190, China;
b University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: A set of mono-and bimetallic (Zn-Co) supported ZSM-5 catalysts was first prepared by PEG-additive method. The physicochemical properties of the catalysts were investigated by FTIR, XPS, XRD, N2 adsorption-desorption measurements, SEM, EDS and NH3-TPD techniques. The physicochemical properties showed that the ZnCo2O4 spinel oxide was formed on the ZSM-5 support and provided effectual synergetic effect between Zn and Co species for the bimetallic catalyst. Furthermore, bimetallic supported ZSM-5 catalyst exhibited weak, moderate and strong acidic sites, while the monometallic supported ZSM-5 catalyst showed only weak and moderate or strong acidic sites. Their catalytic performances for thermal decomposition of hexamethylene-1, 6-dicarbamate (HDC) to hexamethylene-1, 6-diisocyanate (HDI) were then studied. It was found that the bimetallic supported ZSM-5 catalysts, especially Zn-2Co/ZSM-5 catalyst showed excellent catalytic performance due to the good synergetic effect between Co and Zn species, which provided a suitable contribution of acidic sites. HDC conversion of 100% with HDI selectivity of 91.2% and by-products selectivity of 1.3% could be achieved within short reaction time of 2.5 h over Zn-2Co/ZSM-5 catalyst.
Key words: Hexamethylene-1, 6-dicarbamate (HDC)     Hexamethylene-1, 6-diisocyanate (HDI)     Thermal decomposition     Bimetallic supported ZSM-5 catalyst     Synergetic effect    
1. Introduction

HDI is one of the most extensively used aliphatic diisocyanate in the manufacture of polyurethane, paint, insulator, synthetic rubber, adhesives [1, 2]. Traditionally, the synthesis route of HDI involves extremely toxic phosgene as a reagent at industrial scale [3]. This route to produce HDI not only causes of environmental hazards due to toxicity of phosgene, but also produces seriously corrosive byproduct HCl [2]. The drawbacks of this phosgene route divert attention to the development of phosgene-free route. Among other phosgene-free routes for HDI synthesis, thermal decomposition of HDC to HDI is one of the most effective, environmentally benign and green route [4]. Thermal decomposition of HDC to HDI involves two steps, HDC decomposes to hexamethylene-1-carbamate-6-isocyanate (HMI) as intermediate and then HMI decomposes to HDI as shown in Scheme 1.

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Scheme 1. Thermal decomposition of HDC to HDI.

Thermal decomposition of HDC to HDI is still in an immature state and so far it has faced the problems such as low HDC conversion, low HDI selectivity and polymerization of highly active HDI. In order to improve this phosgene-free route and overcome its problems, many researchers have focused on the use of catalysts, e.g. di-n-butyltin oxide [5], montromrillonite K-10 [6], ZnAlPO4 [7], ZnO [8], CuO/ZnO [9] and Co2O3 [10]. Concerning all the catalysts reported up to date, zinc and cobalt based catalysts have found relatively active and selective to HDI. However, HDI yield is still not ideal (i.e. less than 90%) and the route suffers with long reaction time, which results in high energy cost. Therefore, it is vital to modify the catalyst which can efficiently improve this route.

The catalytic properties of the catalyst can be improved by depositing active species on a support [11, 12]. The choice of the support plays an important role, since support not only acts as a carrier of the active species, but also contributes to the performance of the catalyst. It has been reported that the interactions between metal and support significantly affect the physicochemical properties of the catalyst such as dispersion, size and metal crystallite distribution or acidity [13, 14]. ZSM-5 is a suitable choice to use as a catalyst support due to its unique channel structure, large surface, thermal stability, shape-selectivity and acidity. The strength and distribution of acidic sites in ZSM-5 have been shown to be altered upon loading metal species on the ZSM-5 [15]. Therefore, modified ZSM-5 as catalyst is widely used in various moderate acid-catalyzed reactions [16]. Furthermore, it is reported that ZSM-5 modified by bimetallic cations found much more active and selective catalyst as compared to monometallic modified ZSM-5 catalyst [17, 18]. It is clear from previous work that both Zn and Co are active and selective to HDI. Therefore, it could be expected that Zn-Co bimetallic catalyst might give better performance in the thermal decomposition of HDC. However, the catalytic performance of Zn-Co bimetallic supported ZSM-5 catalyst for the thermal decomposition of HDC has not been reported and the influence of Zn and Co synergetic effect on the selectivity of HDI has not been studied.

In this work, a phosgene-free route for the thermal decomposition of HDC to HDI was developed over Zn-Co bimetallic supported ZSM-5 catalyst. For this, the monometallic (Co, Zn) and bimetallic (Zn-Co) supported ZSM-5 catalysts were prepared by PEG-additive method and used in the thermal decomposition reaction using low boiling point solvent. The physicochemical properties of the catalysts were thoroughly characterized by different techniques. The catalytic performances of the monometallic and bimetallic supported ZSM-5 catalysts were studied. In addition, a correlation was explored between the contribution of acidic sites and catalytic performance of the catalysts. The reusability of the catalyst was also evaluated.

2. Results and discussion 2.1. Catalyst characterization

Fig. 1 shows the FTIR spectra of the ZSM-5, monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts. The band at 3654, 3442 and 3230 cm-1 were ascribed to terminal hydroxyl groups SiOH, hydroxyl groups on extra framework aluminum and strong Bronsted acid sites Si-OH-Al groups on the zeolite's surface [20]. The band at 1630 cm-1 was assigned to water bending vibration and observed in ZSM-5 and all monometallic and bimetallic supported ZSM-5 catalysts. The characteristic bands of MFI structure at 1220, 1083, 795, 544 and 452 cm-1 were observed in all catalysts, which ascribed to external asymmetric stretch, internal asymmetric stretch, external symmetric stretch, double five ring, and T-O bending vibration of internal tetrahedral respectively [21, 22]. These results confirmed that the deposition of mono and bi metal cations on the ZSM-5 had no influence on the framework of ZSM-5.

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Fig. 1. FTIR spectra of bare ZSM-5 (a), 2Co/ZSM-5 (b), Zn-2Co/ZSM-5 (c), Zn-Co/ZSM-5 (d), 2Zn-Co/ZSM-5 (e) and 2Zn/ZSM-5 (f) catalyst.

Compared with ZSM-5 spectrum (Fig. 1, curve a), two new bands at 579 and 670 cm-1 were observed in the 2Co/ZSM-5 spectrum (Fig. 1, curve b). These bands can be associated with the metal-oxygen bonding which resulted in after the metal loading on the ZSM-5. The band at 579 cm-1 was ascribed to the Co3+ octahedral coordination, while the band at 670 cm-1 was ascribed to the Co2+ tetrahedral coordination in cubic spinel Co3O4 structure [23]. As can be seen, all the bimetallic supported ZSM-5 spectra (Fig. 1, curves c-e) also showed the two new bands at 579 and 670 cm-1 same as the 2Co/ZSM-5 spectrum. The FTIR results apparently suggested the deposition of cubic spinel oxide of metals on bimetallic supported ZSM-5 catalysts. However, there was no new band appeared in the 2Zn/ZSM-5 spectrum (Fig. 1, curve f) as compared to ZSM-5 spectrum, since the band of ZnO at 450 cm-1 was overlapped with the intensive adsorption band of T-O bending vibration in MFI structure at 452 cm-1 [24].

Fig. S1 in Supporting information shows the surface composition and oxidation state of monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts analyzed by XPS. The XPS full-range survey spectra of monometallic and bimetallic supported ZSM-5 catalysts are shown in Fig. S1(A). The characteristic peaks of Zn and/or Co, O, Si and Al elements were detected in all the catalysts. Fig. S1(B) shows the Zn2p spectra of monometallic (Zn) and bimetallic supported ZSM-5 catalysts. Two main peaks at 1022-1022.3 and 1045.2-1045.4 eV were detected in Zn2p spectra, which were ascribed to Zn2p3/2 and Zn2p1/2, signifying Zn2+ oxidation state. Fig. S1(C) shows the Co2p spectra of monometallic (Co) and bimetallic supported ZSM-5 catalysts. The Co2p spectra displayed two peaks at 780.3-780.7 eV and 795.3-795.7, which were attributed to Co2p3/2 and Co2p1/2, indicating Co2+ and Co3+ oxidation state. Fig. S1(D) shows the O1s spectra of monometallic and bimetallic supported ZSM-5 catalysts and could be deconvoluted to three components. The first component at 529.9 eV corresponded to the surface lattice oxygen of the catalysts (Oα), the second component at 531.1 eV corresponded to the adsorbed oxygen of the catalysts (Oβ) and the last component at 532.7 eV corresponded to the lattice oxygen of the ZSM-5 support (Oγ) [25, 26].

Fig. 2 shows the XRD patterns of the ZSM-5, monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts. It can be seen that all the catalysts showed peaks at range of 2θ = 7-9° and 2θ = 23-25°, which were corresponded to the typical peaks of ZSM-5 zeolite [27]. It implied that the crystalline structure of ZSM-5 remained intact after Zn and/or Co supported on ZSM-5. The XRD pattern of 2Zn/ZSM-5 catalyst revealed the deposition of hexagonal phase of ZnO on the ZSM-5 support. The diffraction peaks of hexagonal phase of ZnO found at 2θ = 37.7°, 34.7°, 36.2°, 47.5°, 56.5°, 62.7° and 67.9° correspond to the (100), (002), (101), (102), (110), (103) and (112) crystal planes (JCPDS 89-1397) in the 2Zn/ZSM-5 catalyst respectively (pattern f). While, the diffraction peaks of the cubic spinel phase of Co3O4 found at 2θ = 19.0°, 31.2°, 36.8°, 38.5°, 44.8°, 59.4° and 65.2° correspond to the (111), (220), (311), (222), (400), (511) and (440) crystal planes (JCPDS 42-1467) in the 2Co/ZSM-5 catalyst respectively (pattern b). It is noteworthy that the XRD pattern of Zn-2Co/ZSM-5 catalyst showed the position of diffraction peaks almost same as the position of diffraction peaks of cubic spinel Co3O4 in XRD pattern of 2Co/ZSM-5 catalyst (pattern b and c). As shown earlier by XPS results that Zn was presented on the surface of Zn-2Co/ZSM-5 catalyst. However, there was no characteristic peak of ZnO phase detected in the XRD pattern of Zn-2Co/ZSM-5 catalyst. Therefore, it clearly revealed the deposition of cubic spinel phase of ZnCo2O4 on the ZSM-5 support in Zn-2Co/ZSM-5 catalyst and was in good agreement with JCPDS 23-1390. According to literature [28], cubic spinel ZnCo2O4 is isomorphic to the cubic spinel Co3O4 crystal structure wherein Zn2+ has tetrahedral coordination and Co3+ has octahedral coordination in the lattice. Therefore, it can be expected that interaction between divalent and trivalent metal cations and their respective position in the lattice of the cubic spinel structure may be beneficial in improving the catalytic performance. In contrast, the diffraction peaks of the hexagonal phase of ZnO appeared in the XRD patterns of Zn-Co/ZSM-5 and 2Zn-Co/ZSM-5 (pattern d and e) indicated the co-existence of cubic spinel phase of ZnCo2O4 and hexagonal phase of ZnO.

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Fig. 2. XRD diffraction patterns of bare ZSM-5 (a), 2Co/ZSM-5 (b), Zn-2Co/ZSM-5 (c), Zn-Co/ZSM-5 (d), 2Zn-Co/ZSM-5 (e) and 2Zn/ZSM-5 (f) catalyst.

Surface texture properties of the ZSM-5, monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts were measured by N2 adsorption-desorption measurements and the data are listed in Table 1. It can be seen that the bare ZSM-5 revealed a high BET surface area and total pore volume. With the addition of Zn and/or Co species on the ZSM-5 support, the resultant monometallic and bimetallic (Zn-Co) supported catalysts showed a decrease in the BET surface area and total pore volume, which can be attributed to the blocking of some pores and channels of ZSM-5 support by metal species [29]. In contrast, an increase in the average pore diameter was observed for monometallic and bimetallic (Zn-Co) supported catalysts, indicating the location of some metal oxide species inside ZSM-5 pores caused an enlargement [30]. It is noteworthy that the deposition of Co3O4 or ZnCo2O4 on the ZSM-5 support led to a large increase in average pore diameter compared with bare ZSM-5. This observation revealed that more Co3O4 or ZnCo2O4 species accommodated inside ZSM-5 pores as compared to ZnO. While, ZnO was mostly deposited on the exterior surface of the ZSM-5 support. As shown in Fig. S2 in Supporting information, N2 adsorption-desorption isotherms for ZSM-5, monometallic supported ZSM-5 and Zn-2Co/ZSM-5 catalysts exhibited type Ⅵ isotherms according to the IUPAC classification. The type of isotherms was not changed after the deposition of metal species on the ZSM-5, which revealed the presence of still open mesopores on the outer surface. However, a decrease in the quantity of adsorbed N2 was observed, indicating the pore blockage by metal species. Therefore, it could be deduced that Co3O4 or ZnCo2O4 with cubic spinel structure blocked a large number of pores, resulting in the drastic decrease in BET surface.

Table 1
Physicochemical properties of catalysts and their catalytic performances for thermal decomposition of HDC.a

Fig. 3 shows the surface SEM images of ZSM-5, monometallic supported ZSM-5 and Zn-2Co/ZSM-5 catalysts. The bare ZSM-5 had typical orthogonal crystal shape. While for monometallic and Zn-2Co/ZSM-5 catalyst, all the catalysts consisted of agglomerate particles. Compared to 2Co/ZSM-5 and Zn-2Co/ZSM-5 catalyst, more loose metal oxide particles were observed on the surface of 2Zn/ZSM-5 catalyst which resulted in high surface area of 2Zn/ ZSM-5 catalyst than other catalysts. The surface EDS images of monometallic supported ZSM-5 and Zn-2Co/ZSM-5 catalysts showed the signal of Zn and/or Co, O, Si and Al, suggesting that metal species are also present on the exterior surface of ZSM-5 support as shown in Fig. S3 in Supporting information. The weight percent of Zn and/or Co obtained by EDS analysis was in accordance with the surface texture properties. As the weight percent of metal species increased on the exterior surface of the ZSM-5, BET surface area and total pore volume also increased and followed the order of 2Zn/ZSM-5 > 2Co/ZSM-5 > Zn-2Co/ZSM-5. This observation further confirmed that Co3O4 and ZnCo2O4 species were mostly deposited inside ZSM-5 pores as compared to ZnO and ZnO species existed on the exterior surface of ZSM-5 which caused less decrease in BET surface area.

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Fig. 3. Surface SEM images of bare ZSM-5 (a), 2Co/ZSM-5 (b), Zn-2Co/ZSM-5 (c) and 2Zn/ZSM-5 (d) catalyst.

Fig. 4 shows the NH3-TPD profiles of the ZSM-5, monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts and the quantitative results are listed in Table 1. According to the literature [31], the desorption peaks centered in the temperature range of 100-150 ℃, 225-350 ℃ and 400-500 ℃ could be attributed to weak, moderate and strong acidic sites respectively. NH3-TPD profiles of the ZSM-5 and monometallic supported ZSM-5 catalysts showed two major desorption peaks, whereas the bimetallic (ZnCo) supported ZSM-5 catalysts revealed three major desorption peaks. The ZSM-5 revealed two major desorption peaks at 112 and 425 ℃, ascribing to weak and strong acidic sites on the surface (profile a). As can be seen in Fig. 4, the TPD profiles were varied significantly with the introduction of metal species on the ZSM-5. Interestingly, the introduction of Co species on the ZSM-5 demolished the weak acidic sites and facilitated the formation of new strong acidic sites (profile b). No shift in the desorption peak of 2Co/ZSM-5 catalyst indicated that the acidic strength was unchanged compared with the ZSM-5 and acidic sites distribution was only affected by Co loading. In a contrast, the introduction of Zn species on the ZSM-5 poisoned the strong acidic sites of the ZSM-5 and facilitated the formation of new moderate acidic sites in 2Zn/ZSM-5 catalyst (profile f).

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Fig. 4. NH3-TPD profiles of bare ZSM-5 (a), 2Co/ZSM-5 (b), Zn-2Co/ZSM-5 (c), Zn-Co/ZSM-5 (d), 2Zn-Co/ZSM-5 (e) and 2Zn/ZSM-5 (f) catalyst.

Compared to the NH3-TPD profile of the ZSM-5, the NH3-TPD profiles of bimetallic (Zn-Co) supported ZSM-5 catalysts exhibited an extra desorption peak at 270 ℃, suggesting the presence of weak, moderate and strong acidic sites on the surface (profile c-e). This reflects that the coexistence of Zn and Co species on the ZSM-5 effectively affected the acidic sites distribution of the catalyst through a synergetic effect of both metal species. It is worth noting that a significant shift in desorption peak centered at 425-465 ℃ for the bimetallic supported catalysts, suggesting that major change in the strong acidic sites strength occurred. The desorption peaks were found to be shifted in high temperature with increasing Zn and decreasing Co proportion in bimetallic supported catalysts. Following the order Zn-Co/ZSM-5 < 2Zn-Co/ZSM-5 < Zn-2Co/ZSM-5, the contribution of moderate and strong acidic sites increased and the contribution of weak acidic sites decreased in the catalysts as shown in Table 1.

2.2. Catalyst performance evaluation

The catalytic performances of the as-prepared catalysts for the thermal decomposition of HDC to HDI were evaluated and the results are listed in Table. 1. A low HDC conversion of 89.2% with 49.0% HDI selectivity and 39.2% HMI selectivity was achieved in the presence of ZSM-5. Interestingly, the introduction of cobalt and/or zinc species on the ZSM-5 gave dramatic increment of strongly active sites on the catalysts, which resulted in an increase of not only HDC conversion but also HDI selectivity. Comparing both monometallic supported ZSM-5 catalysts, the catalytic performance of 2Co/ZSM-5 catalyst with 81.5% HDI selectivity was inferior to the 2Zn/ZSM-5 catalyst with 85.0% HDI selectivity. These results apparently suggested that Zn species was comparatively more selective to HDI than Co species for the thermal decomposition of HDC. Thus, it could be expected that the synergetic effect between Zn and Co species in bimetallic supported ZSM-5 catalyst as confirmed by the characterization, might be beneficial to improve the catalytic performance. Therefore, different asprepared bimetallic supported ZSM-5 catalysts were investigated in the thermal decomposition of HDC to HDI under the same reaction conditions.

The bimetallic supported ZSM-5 catalyst showed obviously better selectivity to HDI as compared to monometallic supported ZSM-5 catalysts. The HDI selectivity was about 83.2% with 10.0% by-products selectivity over Zn-Co/ZSM-5 catalyst. Compared with Zn-Co/ZSM-5 catalyst, HDI selectivity increased and by-products selectivity further decreased after increasing Zn species and reached 87.1% and 7.4% over 2Zn-Co/ZSM-5 catalyst respectively. On the other hand, when Co species were increased in the Zn-Co/ ZSM-5 catalyst, the Zn-2Co/ZSM-5 catalyst revealed superior selectivity to HDI and inferior selectivity to by-products. The HDI selectivity reached 91.2% with only 1.3% by-products selectivity after 2.5 h.

According to the literature [6], the modification of acidic properties of the catalyst for thermal decomposition of carbamate to isocyanate could be beneficial for improving its selectivity to desired product. Therefore, the acidic properties of the catalysts were correlated with HDI selectivity to gain further insight into the possible reason for performance. As can be seen in Table 1, the 2Zn/ ZSM-5 catalyst with the contribution of moderate acidic sites revealed higher HDI selectivity and slightly lower by-products selectivity than 2Co/ZSM-5 catalyst with contribution of strong acidic sites. As discussed earlier, the synergetic effect between Zn and Co led the simultaneous formation of weak, moderate and strong acidic sites in bimetallic supported ZSM-5 catalysts. It was observed that bimetallic supported ZSM-5 catalysts were more selective to HDI than monometallic supported ZSM-5 catalysts due to the combined contribution of weak, moderate and strong acidic sites. All the bimetallic supported catalysts tested in this study, the selectivity of HDI increased and selectivity of by-products decreased as the contribution of moderate and strong acidic sites increased and contribution of weak acidic sites decreased. This finding indicated that the co-contribution of moderate and strong acidic sites of the bimetallic supported ZSM-5 catalysts played a crucial role for the HDI selectivity. Thus, bimetallic supported ZSM-5 catalysts revealed better catalytic performance than monometallic supported catalysts. Particularly, Zn-2Co/ZSM-5 catalyst showed superior catalytic performance in the thermal decomposition of HDC to HDI, which might be due to the relatively average acidity and greater contribution of moderate and strong acidic sites.

2.3. Catalyst reusability

The recycling of the Zn-2Co/ZSM-5 catalyst was also investigated and results are shown in Fig. 5. The catalyst was collected after the reaction by simple centrifugation, washed with chlorobenzene and dried at 120 ℃ for 12 h then reused directly for the next run. During the recycling experiments, it can be seen that the HDC conversion was unchanged. In contrast, a slight decrease in the HDI and HMI selectivity and increase in by-products selectivity was observed in the second and third run. However, the catalyst revealed low performance after being used in three consecutive runs. The HDI selectivity gradually decreased and HMI and byproducts selectivity increased in fourth and fifth run. The decrease in the performance of the Zn-2Co/ZSM-5 catalyst was possibly due to loss of active sites on the catalyst surface during the thermal decomposition reaction. Therefore, XPS characterization of the Zn-2Co/ZSM-5 catalyst was used to investigate the changes of the used catalyst. Fig. S4 in Supporting information shows the Zn2p and Co2p spectra of fresh and used Zn-2Co/ZSM-5 catalyst. Compared with fresh Zn-2Co/ZSM-5 catalyst, the Zn2p and Co2p peaks of used Zn-2Co/ZSM-5 catalyst shifted to higher binding energy, i.e. 1021.9-1045.1 eV to 1023.7-1046.9 eV and 780.7-795.7 to 782.3-797.3 eV, respectively. The higher binding energies of both Zn and Co could be attributed to the weak interaction inside ZnCo2O4 phase due to electron-deficient state of metal species in used Zn-2Co/ZSM-5 catalyst. It is reported that methoxy group (CH3O--) can adsorbed on the Zn-site and atomic hydrogen (H) can bonded to the O-site in the system containing ZnO species and transferred crystalline ZnO to amorphous form [8, 32]. Therefore, it could be deduced that ZnO formed intermediate species with methoxy group and atomic hydrogen during the thermal decomposition reaction and leached out into the reaction mixture. The leaching of ZnO caused structural defects in the ZnCo2O4 cubic spinel structure and resulted in loss of active sites on the surface of the catalyst and performance of the Zn-2Co/ZSM-5 catalyst.

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Fig. 5. Reusability of Zn-2Co/ZSM-5 catalyst in the thermal decomposition of HDC to HDI. Reaction condition: HDC concentration in chlorobenzene, 3.5%; catalyst, 0.25 g; temperature 230 ℃; nitrogen flow rate, 800 mL/min; pressure, 0.68 MPa; time 2.5 h.

3. Conclusion

In summary, a phosgene-free route for the thermal decomposition of HDC to HDI was developed over Zn-Co bimetallic supported ZSM-5 catalyst, using low boiling point solvent. The monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts with different Zn and Co weight composition were prepared by the PEG-additive method. Their physicochemical properties were characterized by various techniques and found that the introduction of Co and/or Zn on the ZSM-5 had no significant influence on the framework of ZSM-5. The synergetic effect between Zn and Co over bimetallic supported ZSM-5 catalyst significantly affected the contribution of acidic sites. The catalytic performances of different as-prepared monometallic and bimetallic supported ZSM-5 catalysts were investigated in the thermal decomposition of HDC. The results showed that bimetallic supported ZSM-5 catalysts were more selectivity to HDI than monometallic supported ZSM-5 catalysts. The catalytic performance of bimetallic supported ZSM-5 catalyst depended upon the contribution of weak, moderate and strong acidic sites. Among all the catalysts, Zn-2Co/ZSM-5 catalyst revealed excellent performance with 91.2% HDI selectivity and 1.3% by-products selectivity.

4. Experimental 4.1. Materials

The ZSM-5 zeolite (SiO2/Al2O3 = 25) was purchased from the Catalyst Plant of Nankai University and used as support. Cobalt nitrate hexahydrate (98.5%), Zinc nitrate hexahydrate (98.5%) Chlorobenzene (≥99.0%), Methanol (≥99.0%) and Biphenyl were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Polyethylene glycol (PEG) (MW = 400) was purchased from Aladdin Chemical Co., Ltd., Shanghai, China. HDI (>98%) was purchased from Tokyo Chemical Industry Co., Ltd., Japan. All these reagents were used as received without further purification. HDC was synthesized by the reaction of HDI with methanol (m(HDI):m (Methanol) = 1:5) at 35 ℃ for 12 h. HDC (>99%) was finally obtained by purification and confirmed by Mass Spectrometry and FTIR Spectroscopy. HMI was obtained in the form of HDC, HMI and HDI mixture, by the reaction of HDI with methanol (m(HDI):m (Methanol) = 3:1) at 35 ℃ for 1.5 h., since HMI was not commercially available.

4.2. Preparation of catalysts

The monometallic and bimetallic (Zn-Co) supported ZSM-5 catalysts were prepared by PEG-additive method, according to literature reported [19]. Prior to metals loading, ZSM-5 zeolite was calcined at 500 ℃ for 3 h under static air in a muffle furnace. The desired amounts of Co(NO3)2·6H2O and/or Zn(NO3)2·6H2O and PEG (metal/PEG ratio 0.16) were added into a solvent containing methanol (30 mL) and deionized water (90 mL). The resultant solution was refluxed at 90 ℃ for 2 h. 3 g of ZSM-5 support was added into the solution subsequently. The obtained suspension was heated at 100 ℃ with continuous stirring to evaporate the solvent. All the catalysts were dried at 120 ℃ for 12 h and calcined at 600 ℃ for 6 h under static air in a muffle furnace. The monometallic supported ZSM-5 catalyst was denoted as xZn/ZSM-5 and yCo/ZSM-5 and bimetallic supported ZSM-5 catalyst were denoted as xZn-yCo/ZSM-5, where x and y denotes the weight composition of zinc and cobalt ('1' refers to 10 wt% and '2' refers to 20 wt% relative to the weight of the support). For example, the catalyst with 10 wt% Zn and 20 wt% Co was named Zn-2Co/ZSM-5.

4.3. Characterization of catalysts

Fourier-transform infrared spectra (FTIR) were recorded on a Bruker Tensor 27 spectrometer at room temperature. For the FTIR characterizations, the catalysts were dispersed in KBr to make pellets. The spectra resolution was 4 cm-1. All the spectra were recorded in the range of 4000-400 cm-1. Surface composition of the catalysts was examined by X-ray photoelectron spectroscopy (XPS) analysis. XPS was performed under an ultrahigh vacuum using an ESCALAB 250Xi spectrometer with Al Kα radiation (1486.6 eV) and a multichannel detector. The collected binding energies were calibrated using the C1s peak at 284.6 eV as the reference. The XRD diffraction patterns of all the catalysts were obtained with a PANalytical Empyrean diffractometer using Cu Kα radiation and recorded in the 2θ range of 5°-90°. N2 adsorptiondesorption isotherms of the catalysts at 77 K were measured by using Quantachrome Autosorb-1. The samples were outgassed at 523 K under vacuum for 3 h before recording their isotherms. The BET surface area of the samples was calculated according to the Brunauer-Emmett-Teller theory (BET). Total pore volumes were evaluated at relative pressures (P/Po) closed to unity. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were conducted using Hitachi SU 8020 field-emission scanning electron microscope. The acidity of the catalysts was detected using the temperature-programmed desorption of ammonia (NH3-TPD) using the Micrometrics Autochem Ⅱ 2920 unit equipped with the thermal conductivity detector. NH3-TPD was carried out from 50 ℃ to 600 ℃ with a ramping rate of 10 ℃/min, and the amount of desorbed NH3 was monitored by a thermal conductivity detector (TCD).

4.4. Reaction procedure and product analysis

The thermal decomposition reaction was carried out in a 1 L stainless steel autoclave. In a typical reaction, HDC (19.6 g) and chlorobenzene (560.0 g) were charged into the autoclave containing catalyst (0.25 g). Then, the autoclave was purged with N2 (99.9%) at the flow rate of 800 mL/min for 15 min to ensure the complete removal of inner oxygen. The reaction mixture was then heated to 230 ℃ at the heating rate of 10 ℃/min and held at the temperature with continuous mechanical stirring for 3 h. The reaction time started once the autoclave reached the desired temperature. During the reaction, the methanol produced by the decomposition of HDC to HDI, was continually removed from the reaction system using N2 flow at 800 ml/min. The pressure of the autoclave was 0.68 MPa. After the reaction, the autoclave was cooled to room temperature and the product mixture sample was taken for analysis. The product mixture collected was quantitatively analyzed by Shimadzu GC-2010 equipped with a Rtx-5 (30 m × 0.25 mm × 0.25mm) capillary column and flame ionization detector (FID). The injection and detector temperatures were 260 ℃ and 280 ℃, respectively. The temperature program for GC analysis was held at 150 ℃ for 6 min., from 150 ℃ to 230 ℃ at 10 ℃/min and then held at 230 ℃ for 5 min. Nitrogen and Biphenyl were used as a carrier gas and internal standard for quantitative analysis, respectively. HDC, HMI and HDI standard curves were conducted under the same conditions. The HMI standard curve was accomplished by the mass balance technique. The relative standard deviation (RSD) for the same sample through five sampling analyses by GC was less than 0.3%. HDC conversion (XHDC), HDI yield (YHDI) and HMI Yield (YHMI) was calculated according to the Eqs. (1)-(3):

(1)
(2)
(3)

where CHDC is the concentration of HDC in the product sample, ppm; CHDI is the concentration of HDI in the product sample, ppm; CHMI is the concentration of HMI in the product sample, ppm; mHDC is the mass of HDC fed to the autoclave, g; mps is the mass of product sample in the solution for quantitative analysis, g; ms is the total mass of the solution for quantitative analysis, g; mp is the total mass of product, g; MHDC is the molecular weight of the HDC, g/mol; MHDI is the molecular weight of the HDI, g/mol; MHMI is the molecular weight of the HMI, g/mol.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos. 21476244 and 21406245) and Youth Innovation Promotion Association CAS.

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.cclet.2017.03.015.

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