Chinese Chemical Letters  2017, Vol. 28 Issue (2): 482-486   PDF    
Nitridation: A simple way to improve the catalytic performance of hierarchical porous ZSM-5 in benzene alkylation with methanol
Jing-Hui Lyua,b, Hua-Lei Hua, Jia-Yao Ruia, Qun-Feng Zhanga, Jie Cena, Wen-Wen Hana, Qing-Tao Wanga, Xiao-Kun Chena, Zhi-Yan Panb, Xiao-Nian Lia     
a Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou 310032, China;
b Department of Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, China
Abstract: Nitrided hierarchical porous ZSM-5 was synthesized by nitridation of hierarchical porous ZSM-5 with flowing ammonia at elevated temperature. The samples were characterized by XRD, SEM, Nitrogen sorption isotherms, NH3-TPD and Py-IR, and evaluated in alkylation of benzene and methanol. The result indicated that the high specific surface area of parent ZSM-5 was maintained, while the Brönsted acidity was effectively adjusted by nitridation. Moreover, the high suppression of ethylbenzene was observed on nitrided catalyst and this could be attributed to the decrease of Brönsted acidity which suppressed the methanol to olefins reactions.
Key words: Nitridation     Hierarchical porous ZSM-5     Benzene alkylation with methanol     Ethylbenzene    
1. Introduction

Alkylaromatic compounds including toluene and xylene have been widely used in chemical industry [1]. Utilizing the alkylation reaction of benzene with simple alcohols or alkenes has sparked interest among the petrochemical industry as its potential to be an alternative method for producing alkylaromatic compounds from coal and natural gas [2, 3]. Currently, ZSM-5 zeolite showed good catalytic performance for benzene alkylation with methanol to produce toluene and xylene because of its special shape selectivity and surface acidity [4, 5]. With the introduction of mesopores into zeolite material, hierarchical porous ZSM-5 successfully overcame the limitations prevalent in conventional ZSM-5 caused by the intrinsic micropores which inhibited the diffusivity of larger substrates [6, 7]. However, it should be noted that ethylbenzene, one of the main byproducts generated from benzene alkylation with ethylene, was still challenging to be suppressed on the conventional as well as the hierarchical porous ZSM-5, and this leads to an increased difficulty of product separation [7, 8].

It is of great interest to inhibit the formation of ethylbenzene, and many processes have been developed to modify the ZSM-5 zeolites [8-11]. Modifying the ZSM-5 with Pt was found to have high inhibition of ethylbenzene formation from benzene with ethylene, and this was a result of ethylene into ethane via hydrogenation [8]. In addition, the ethylbenzene and coke was suppressed via introduction of Zn and Mg into hierarchical porous ZSM-5 by impregnation. It was caused by reducing of Brönsted acidity, and this further inhibited the unwanted reaction of methanol to olefins (MTO) [9-11]. It should be noted that the reaction efficiency for methanol is decreased when ethylene is converted to ethane via hydrogenation, since the ethylene produced from MTO could not be converted into toluene and xylene. Specifically, decreasing the Brönsted acidity of the catalyst could inhibit the MTO side reaction and was predicted to fundamentally inhibit the production ethylbenzene. However, introducing metal oxide by impregnating metal nitrates would lead to pore blockage which might produce negative effects on the performance of the catalyst [11-13]. Nitridation was an effective technique to regulate the acidity of ZSM-5 zeolites and the nitrided zeolites could maintain the specific areas and pore-opening sizes of precursors [4, 14]. Although the basic sites also would be introduced into ZSM-5 by nitridation, the basicity of nitrogenincorporated ZSM-5 was insufficient for catalyzing the side chain alkylation of toluene and methanol to form ethylbenzene [14]. Therefore, nitridation might be an appropriate method to modify the acidity of hierarchical porous ZSM-5 along with eliminating the adverse effect of impregnation.

In our present work, we synthesized nitrided hierarchical porous ZSM-5. The influence of nitridation on the textural properties of hierarchical porous ZSM-5 was systematically characterized by various techniques (including XRD, nitrogen adsorption, SEM, NH3-TPD and Py-IR), and the catalytic performances of nitrided catalysts were investigated in the alkylation of benzene with methanol.

2. Results and discussion 2.1. Catalyst characterization and tests

Hierarchical porous ZSM-5 (SiO2/Al2O3 ratio 360) was prepared and nitride. XRD patterns of samples showed (Fig. 1) changes in peak intensity after nitridation at high temperature. Each of the sample exhibited the standard characteristic patterns with peaks observed at 2θ of 7.9°, 8.8°, 23.1°, 23.9°, and 24.3°. These corresponded to the structure of MFI topology [15, 16], indicating that the process of nitridation does not result in the structure collapse of ZSM-5 zeolites. Fig. 2 shows the crystal morphology of nitrided hierarchical porous ZSM-5. The hierarchical porous ZSM-5 in figure is in the shape of approximate ellipsoidal aggregates ranging in size from 200-400 nm. The high magnification TEM image (Fig. 2b) revealed that the samples were crystalline in nature. In addition, the similar morphology of all nitrided samples indicated the nitridation has little influence on the structure of hierarchical porous ZSM-5.

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Figure 1. XRD patterns of nitrided hierarchical porous ZSM-5.

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Figure 2. SEM images of nitrided hierarchical porous ZSM-5 (a: ZSM-5, c: 0.5N-ZSM-5, d: 1N-ZSM-5, e: 1.5N-ZSM-5, f: 2N-ZSM-5) and TEM image of unmodified hierarchical porous ZSM-5 (b).

The textural properties of unmodified and nitrided samples were determined by N2 adsorption/desorption. Table 1 shows the specific surface areas of nitrided HZSM-5 zeolites had no great change, and this also confirmed that no obvious structural collapse occurred during the high temperature nitridation. Moreover, except for a micropore volume of 0.13 cm3/g there was an additional meso-/macro-pore volume of 0.33 cm3/g for hierarchical porous ZSM-5 was observed. Similarly, the other nitrided samples also contained the micropore volume and mesopore volume at the same time, and the difference of the corresponding value was little, suggesting that each of the synthesized samples were a hierarchically porous structure.

Table 1
The textural properties of nitrided hierarchical porous ZSM-5 catalysts

2.2. Catalytic activity evaluation of nitrided hierarchical porous ZSM-5

As shown in Fig. 3, with the time of nitridation increased from 0 to 1.5 h, the conversion of benzene increased from 52.1% to 55.7%, indicating that nitridation could prompt the alkylation of benzene. However, further increasing the time of nitridation to 2 h, the benzene conversion decreased sharply. Therefore, 1.5 h was considered as an adequate nitridation time for the modification of hierarchical porous ZSM-5.

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Figure 3. Benzene conversion of nitrided hierarchical porous ZSM-5 in benzene alkylation with methanol.

Fig. 4 exhibits the selectivity of products on nitrided catalyst; it can be seen that nitridation has a significant influence on the product distribution. The selectivity of ethylbenzene reduced from 2.7% to 0.3% and the selectivity of toluene and xylene increased from 45.9% to 52.4% and 33.6% to 36.9% respectively after increasing the nitridation time from 0 to 1.5h, indicating that nitrided catalyst was with the ability to suppress the production of ethylbenzene and promote the formation of toluene and xylene. In addition, the decrease of ethylbenzene selectivity also proved that the side chain alkylation of toluene and methanol to form ethylbenzene could not occur over nitrided ZSM-5 and this phenomenon was consistent with the literature reported by Zhang et al. [14]. We also noted that on 2.0NZSM-5 catalyst, the selectivity to toluene was increased to 94.4% while the selectivity to other products were greatly decreased. Moreover, a large amount of methanol and dimethyl ether were observed in the reaction products, suggesting that the catalyst was insufficient for catalyzing the alkylation of methanol as well as methanol to olefins reactions.In addition, the catalytic performance of the none NH3 treated sample (treated at 800 ℃ for 2 h in N2 flow alone) was similar to that of untreated ZSM-5 (Fig. S1 in Supporting information), indicating that the change of catalytic performance of nitrided catalyst was caused by nitridation, rather than high-heat treatment.

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Figure 4. Product selectivity of nitrided hierarchical porous ZSM-5 in benzene alkylation with methanol.

The Brönsted acidity of ZSM-5 catalyst has a significant relation to the suppression of methanol to olefins reactions [9-11, 17, 18]. Moreover, Guan et al. and Zhang et al. had reported that nitridation could effectively reduce the strong Brønsted acid sites of conventional ZSM-5 [4, 14]. Therefore, it was necessary to investigate the effect of acidity on the nitridation of hierarchical porous ZSM-5.

In order to understand the chemical environment surrounding the Al atoms of the nitrided catalyst samples, the solid-state 27Al MAS NMR spectra was employed. As shown in Fig. 5, a large signal at ~54 ppm was detected which could be a result of the tetrahedral-coordinated framework of Al with four Al-O-Si bonds. In addition, the peak at 0 ppm which corresponding to the extra-framework octahedral Al was not observed. All the samples showed a similar structure of tetrahedral coordinated framework of Al species, indicating the influence of nitridation on the local chemical environment around the Al atoms was negligible.

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Figure 5. Solid-state 27Al MAS NMR spectra of nitrided hierarchical porous ZSM-5

The NH3-TPD patterns of unmodified and nitrided hierarchical porous ZSM-5 were shown in Fig. 6. The hierarchical porous ZSM-5 forms two peaks of NH3 desorption. The one located below 200 ℃ correlated to the weak acid sites, and the second peak situated above 300 ℃ correlates to strong acid sites. It can be seen that with the increase of nitridation time, the peak areas of both weak and strong acid sites were decreased, demonstrating that the total amount of acid sites were reduced. As indicated by Py-IR spectra of unmodified and nitrided catalyst in Fig. 7, an increase in nitridation time resulted in a continuous decrease of the peak areas at 1540 cm-1 and 1447 cm-1, which represent Brönsted acid sites and Lewis acid sites, respectively, suggesting that the amount of Brönsted and Lewis acid sites were both reduced. According to our previous work [19], the acidity of ZSM-5 zeolite could be regulated via changing the Si/Al ratio of the catalyst, the effect of nitridation on the acidity is similar to the result of increasing the Si/Al ratio. We also found that reducing the strong Brönsted acid sites was a key method for suppressing the side reactions of methanol to olefins [10, 11]. Considering the decrease of total amount of acid sites and the change in the amount of Brönsted and Lewis acid sites, it was reasonable to conclude that the Brönsted acidity of hierarchical porous ZSM-5 could be effectively adjusted by nitridation.

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Figure 6. NH3-TPD profiles of nitrided hierarchical porous ZSM-5

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Figure 7. FT-IR spectra of pyridine adsorption on nitrided hierarchical porous ZSM-5

Since the zeolites were treated with amines to produce a strong basic catalyst, several research groups have investigated the mechanism of nitridation and the acid-base properties of nitrogen-incorporated zeolites [12, 14, 20-24]. Many literatures have reported that nitrogen could be incorporated into the samples via the formation of Si-NH-Si species and Si-NH2 starting from Si-OH-Al and Si-OH, respectively [24-27]. Fig. 8 shows the FTIR spectra ranging from 3200 to 4000 cm-1 of unmodified and nitrided hierarchical porous ZSM-5 which were treated under nitrogen at 673 K for 1 h in order to eliminated the effect of adsorbed water before the IR measurements were taken. The band in the range of 3650-3750 cm-1 was assigned to Si-OH groups. After nitridation, there is little change in the adsorption spectrum for these bands except a new small band at 3406 cm-1 and a shoulder band at 3360 cm-1. According to related literature [27], the band at 3406 cm-1 correlates to the stretching vibration of NH from Si-NH-Si group and the shoulder band at 3360 cm-1 correlates to the stretching vibration of NH from Si-NH-Al group. It indicates that some O atoms in Si-OH-Si and Si-OH-Al groups were replaced by N atoms, forming Si-NH-Al and Si-NH-Si groups. The band at 3406 cm-1 and 3360 cm-1 reveals nearly flat absorption peaks, which indicates the amount of Si-NH-Si and Si-NH-Al formation is very low. The reason for this may be that there are fewer acid sites on the ZSM-5 zeolite with high SiO2/Al2O3 ratio (360), and the nitridation time of the samples is very short. It should be noted that the -NH2 groups were unstable in nitridated samples and could be removed easily during the pretreatment (400 ℃) [27]. As compared to -NH2 groups, the Si-NH-Si species were more stable under the reaction conditions. Moreover, the formation of Si-NH-Al species were at the expense of the Si-OH-Al species which corresponds to the Brönsted acid sites. Therefore, the Brönsted acidity of hierarchical porous ZSM-5 was reduced by nitridation and this inhibited the unwanted reaction of methanol to olefins, thus, the formation of ethylbenzene was suppressed and the alkylation of benzene was promoted in turn. We also noted that the excessive reduction of the Brönsted acid sites was unfavorable for the alkylation reaction due to the insufficient amount of active sites [19]. This explained the significant decrease of benzene conversion on 2.0N-ZSM-5.

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Figure 8. FTIR spectra of nitridated samples treated under nitrogen at 400 ℃ for 1 h before IR measurements.

3. Conclusion

Nitrided hierarchical porous ZSM-5 was obtained by treating hierarchical porous ZSM-5 with ammonia at high temperature. After nitridation, the high specific surface areas and porous structure of hierarchical porous ZSM-5 were maintained. Nitrided catalyst exhibited a high suppression of ethylbenzene formation, and a promotion of benzene alkylation due to the Brönsted acidity of parent ZSM-5 that was reduced via nitridation, and thus the side reaction of methanol to olefins was suppressed in turn.

4. Experimental

Hierarchical porous ZSM-5 was prepared using solvent evaporation assisted dry gel conversion method [28, 29]. During the reaction, tetraethylorthosilicate (TEOS), aluminum isopropoxide (AIP), tetra-n-propylammonium hydroxide (TPAOH), hexadecyltrimethoxysilane (HTS) and ethanol (EtOH) were mixed into a solution with a molar ratio of SiO2: Al2O3: TPAOH: HTS: EtOH (1: 0.0028: 0.2: 0.05: 15). The resultant gel mixture was aged, crystallized and calcined to obtain H-form hierarchical porous ZSM-5. Nitridation of hierarchical porous ZSM-5 took place in an alumina boat, which was placed in a quartz tube furnace. The quartz tube containing the sample was evacuated and flushed with N2 several times before nitridation. The sample was heated by a temperature ramp from ambient temperature to 800 ℃ (heating rate of 10 ℃/min) under a N2 flowing stream, and then the N2 was switched to 10 vol% NH3/Ar and maintained for 0.5, 1.0, 1.5 and 2.0 h, respectively. Once the previous nitridation process was completed, the sample was then gradually returned to ambient temperature under the cooling flow of N2. According to the NH3 treat time, the obtained samples were denoted by ZSM-5, 0.5NZSM-5, 1.0N-ZSM-5, 1.5N-ZSM-5 and 2.0N-ZSM-5, respectively. Detailed experimental parts, characterization data of nitrided hierarchical porous ZSM-5 and the catalytic activity test were described in Supporting information.

Acknowledgments

We acknowledge financial support from the National Natural Science Foundation of China (Nos. 21476207 and 21506189), Zhejiang Postdoctoral Research Funded Projects (No. BSH1502147) and National Basic Research Program of China (973 Program, No. 2011CB710800).

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.2016.10.025.

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