Zheng Shuqin; Chen Ou; Liu Sicheng; Zhang Peiqing

(1. Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006;2. Hunan province Key Laboratory of Speciality Petrochemicals Catalysis and Separation, Yueyang 414000)

Abstract: In this paper, the kaolin/urea intercalation composites prepared by direct intercalation method and the catalysis composites containing ZSM-5 molecular sieve synthesized based on the kaolin/urea intercalation composites by an in-situ crystallization technique were investigated. The effects of the intercalation ratios and de-intercalation rate and the amounts of added kaolin/urea intercalation composite on the synthesis of the catalysis composites containing the ZSM-5 molecular sieve were studied. The samples were characterized by X-ray diffraction, FT-IR, TG-DTA, N2 adsorption-desorption, and SEM, respectively. The results showed that the structure of the samples prepared by kaolin/urea intercalation composite was pure ZSM-5 molecular sieve. The crystallinity of ZSM-5 molecular sieve increased at first and then decreased with the increase of intercalation ratio of kaolin/urea intercalation composite. When the intercalation ratio was 62%, the crystallinity of ZSM-5 molecular sieve was lower. When the amount of added kaolin/urea intercalation composite with an intercalation ratio of 22% was 3%, the crystallinity of ZSM-5 zeolite was improved to reach 65%. Compared to the crystallization product formed without adding kaolin/urea intercalation composite, the crystallinity of ZSM-5 molecular sieve has increased by 54.8%.The catalytic composites containing ZSM-5 molecular sieve had better thermal stability with a wide pore structure, featuring a particle diameter of about 2.5 μm, a BET speci fic surface area of 236 m2/g, and a pore size of 10.6 nm.

Key words: intercalation composite; urea; kaolin; ZSM-5 zeolite; in-situ technique

1 Introduction

Zeolites are microporous aluminosilicate materials with a 3-dimensional pore structure that plays an important role in many industrial and chemical processes, acting as molecular sieves. Their shape selective properties permit the control of product distribution in chemical processes and have made them invaluable as catalysts in the petrochemical industry[1].

The ZSM-5 zeolite is a synthetic molecular sieve with high silica ratio, excellent thermal stability, unique pore structure and good shape-selective catalytic properties,serving as the preferred catalytic material in petrochemical industry[2-4]. In recent years, researchers have explored various factors such as raw materials, templating agents,and initiator in order to further improve the crystallinity and stability of ZSM-5 molecular sieve synthesized by insitu technology.

It is economical to synthesize zeolite from cheap raw materials. Kaolin is mainly composed of the mineral kaolinite with a chemical composition of Al2Si2O5(OH)4.Kaolin is the hydrated silica of alumina with a composition of about 46% of SiO2, 40% of Al2O3and 14% of H2O, and is abundant worldwide. Since kaolinite has a Si/Al ratio similar to that of zeolites, it can be converted into zeolites by hydrothermal reaction.

The kaolinite is a kind of layered structure and layers are bound by reactive hydrogen bonds[5-6]. According to the layered structure characteristics, the kaolin can be intercalated by suitable organic compound to form kaolin/ organic intercalation composite[7-8]. The composite material not only has the unique adsorption, dispersion,rheology, porosity and surface acidity, but also possesses the changeable functional groups and reaction activity of organic compounds, which have good properties to be used in the fields of polymers nano-composites, heterogeneous catalytic chemistry, polymers, medicine, advanced ceramic materials, environmental engineering materials, etc.

The kaolin/organic intercalation composites with high performance have been attracting the attention of researchers, so that more and more kaolin/organic intercalation composites have been prepared[9-12]. Frost,et al.[13]have demonstrated by Raman spectra that the intercalation molecules form strong hydrogen bond with interlayer and the inner surface hydroxyl of kaolinite.Olejnik[14]studied kaolinite/DMSO by using infrared spectroscopy. It was found that the oxygen atoms in DMSO molecules between kaolinite layers formed strong hydrogen bonds with the hydroxyl groups on the inner surface of kaolinite. The research from Frost, et al.[15]and Clifford, et al.[16]has also reached the similar conclusions.Since the kaolin/urea intercalation composite may change more of the chemical and thermal behaviors, it has potential applications in the industry. This paper has focused on the kaolin/urea intercalation composites prepared by direct intercalation method, and the catalytic composites containing ZSM-5 molecular sieve were synthesized by the in-situ crystallization technique using the kaolin/urea intercalation composites. The effects of the intercalation ratios, and the de-intercalation rate, and the amount of added kaolin/urea intercalation composite on the synthesis of the ZSM-5 molecular sieve have been investigated.

2 Experimental

2.1 Materials

Kaolin (China Kaolin Company), Urea (Tianjin Komeo Chemical Reagent Co. Ltd., AR), water glass (Changsha Wanfang Chemical Co. Ltd., wSiO2=20.7%, wNaO2=6.8%),sodium hydroxide (Tianjin Chemical Reagent Factory,reagent purity ≥98%, AR), sulfuric acid (Tianjin Chemical Reagent Factory, reagent purity ≥98%, AR).

2.2 Preparation of kaolin/urea intercalation composite

According to the following ratio of murea:mkaolin:mH2O=1:2(1.5, 1):0.9, kaolin, urea and water were mixed thoroughly under stirring, and then the kaolin/urea intercalation composite was obtained by filtration, washing and drying after reaction at 70 ℃ for 3 hours (labeled as KUM). The intercalation ratios of the samples can be calculated by the X-ray diffraction method. The intercalation ratio was calculated according to the formula：

in which Q is the intercalation ratio；Ii(001)and Ik(001)are the intensity of special diffraction peak d(001) of kaolin with intercalation composite and the non-intercalated kaolin, respectively.

2.3 De-intercalation of kaolin/urea intercalation composite

The de-intercalation reaction of the above intercalation composites was carried out. The samples were calcined at 40 ℃, 60 ℃, 70 ℃, 80 ℃, 90 ℃, 100 ℃, 110 ℃, 120 ℃,and 130 ℃ for 2 hours, respectively (labeled as DKUM).

2.4 Synthesis of ZSM-5 zeolite by in-situ technology

(1) The kaolin was calcined at 950 ℃ for 2 h to obtain the active kaolin;

(2) Water glass, sodium hydroxide solution, H2O, crystal seed, and active kaolin were placed in a beaker and mixed uniformly so that the ratio of materials in the system was: nSiO2/nAl2O3=15, nNa2O/ nSiO2=0.30, and nH2O/nNa2O=200.The pH value of the system was adjusted to 11—12 with sulfuric acid solution, and the materials were mixed uniformly prior to being transferred into a PTFE-lined stainless steel reactor, and then the reaction was carried out at 150 ℃ for 72 hours.

(3) After the reaction was terminated, the product was filtered, washed, and dried to obtain the ZSM-5 crystallization product (labeled as NZSM-5).

2.5 ZSM-5 zeolite molecular sieve synthesized by introducing KUM

According to the experimental procedure mentioned in Section 2.4, besides the introduction of KUM in step(2), the other steps were the same. The amount of added KUM was 1%, 3%, 6%, 9%, and 18%, respectively.(The percentage refers to the KUM/active kaolin ratio.) The ZSM-5 molecular sieve synthesized by introducing KUM was recorded as UZSM-5.

2.6 Characterization

X-ray diffraction analysis: The crystallinity and crystal shape of samples were recorded on a Rigaku Ultimi IV diffractometer using Cu-Kα radiation (λ = 1.54056 ?)operating at a tube voltage of 40 kV and a tube current of 30 mA. The samples were scanned at a speed of 0.2(°)/min.N2adsorption-desorption methods: The specific surface areas, pore volumes, and pore size distributions were measured on an ASAP2020 sorptometer using adsorption and desorption isotherm plots at -196 ℃. Prior to the measurement, the samples were degassed at 100 ℃ for 12 h.FT-IR analysis: The samples were recorded on an AVATAR 370 FT-IR spectrometer.

SEM analysis: The morphology and size of the samples were determined using scanning electron microscopy(SEM) (JEOL JSM-6360) after the samples were coated with an Au evaporated film.

3 Results and Discussion

3.1 Properties of the intercalation composite materials

3.1.1 XRD characterization

The interlayer distance of kaolin is 0.72 nm. When the organic molecules are inserted into the interlayers of kaolin,the interlayer expansion will take place, and the interlayer distance will increase correspondingly. The XRD value of d(001) can directly re flect this change. After the formation of kaolin intercalation composite, the diffraction peak at d(001) = 0.72 nm will become weak, and the diffraction intensity of a new peak will become strong.

Figure 1 XRD patterns of samples(1)—Kaolin; (2)—Intercalation ratio of 22%;(3)—Intercalation ratio of 50%; (4)—Intercalation ratio of 62%

The XRD patterns of raw kaolin and kaolin/urea intercalation composite with different intercalation ratios are given in Figure 1. Figure 1(1) displays the intensity of the original diffraction peak at 2θ =12.4°(with a d value of 0.72 nm) of kaolin, and in Figure 1(2)—(4), a new diffraction peak appears at 2θ=8.2°(with a d value of 1.07 nm) for the kaolin/urea intercalation composites. After kaolin is intercalated by urea, the intensity of the original diffraction peak at 2θ=12.4° decreases greatly, and the intensity of a new diffraction peak at 2θ=8.2° gradually strengthens, which indicates that the interphase of urea molecules into kaolin increases the distance between kaolin layers. The intensity of the 0.72 nm peak indicates that intercalation is not fully completed. At the same time,the diffraction peak of urea appears at 2θ=22.4°(with the d value equating to 0.397 nm) in Figure 1(3)—(4).When the mass ratios of kaolin and urea are 1:2, 1:1.5,and 1:1, respectively, the intercalation ratios of kaolin/urea intercalation composites are 62%, 50%, and 22%,respectively. Because urea is a macromolecular organic compound, compared with indirect intercalation method,the intercalation ratio of kaolin/urea intercalation composite formed by the direct intercalation method is lower.

3.1.2 FTIR spectroscopy

Figure 2 is the infrared spectra of kaolin and kaolin/urea intercalation composite (KUM), respectively. It can be seen that there are four hydroxyl groups in the kaolin crystal cell, among which three are the external hydroxyl groups, and the other is the inner hydroxyl group. The wave numbers of the external hydroxyl groups are 3 694 cm-1, 3 668 cm-1, and 3 652 cm-1, respectively. The wave number of the internal hydroxyl is 3 620 cm-1. The remaining wave numbers and attribution are as follows:950—900 cm-1are Al-O-OH bending vibration, while 800—600 cm-1are caused by Si-O-Al vibration. The wave numbers at 500—400 cm-1are related to Si-O vibration.After intercalation reaction, the position and intensity of the infrared absorption peak of the external hydroxyl groups will be affected. When urea is inserted into the interlayer of kaolin, the strength of wavenumbers at 3694.54 cm-1and 914.65 cm-1has decreased, respectively.This phenomenon is attributed to the interaction between aluminum hydroxyl and urea in the kaolin. The expansion vibration peaks of Si-O and Si-O-Al are not affected much because they are all the vibration peaks of the silicon oxygen surface, which forms the framework of kaolin.Combined with XRD results, it can be observed that the urea molecules have been intercalated into the kaolin layers, forming kaolin/urea intercalation composite.

Figure 2 FT-IR spectra of samples

3.1.3 SEM analysis

Figure 3 shows the SEM micrographs of kaolin and KUM, respectively. Compared with kaolin, the particle size distribution of KUM has obvious changes. After intercalation reaction, the particles become smaller and the content of fine particles increases in kaolin, which is advantageous to the dispersion, and consequently the pH value of kaolin slurry changes from acidic to neutral sign,leading to the increase of solid content of kaolin particles with an increasing pH value coupled with the occurrence of dealumination reaction. Therefore, after intercalation,the dispersion of kaolin particles becomes better, the viscosity of slurry decreases, and the negative charges increase.

3.1.4 Pore structure

Figure 4 shows the N2adsorption-desorption isotherms of kaolin and KUM. It can be seen that the isotherm of KUM exhibits the representative characteristics of type IV adsorption-desorption. The hysteresis loop that occurs in a pressure range 0.70 ＜P/P0＜ 1.0 is ascribed to the presence of mesopores and macropores. The hysteresis ring belongs to the H3 hysteresis ring, indicating that the material has a wedge-shaped hole with loose accumulation of flake particles.

Figure 5 shows the pore size distributions of kaolin and KUM. It can be seen that the pore size distribution of KUM is approximately at 4.0 nm and 30 nm, while the pore size distribution of kaolin is approximately at 4.0 nm, 15 nm, and 30 nm. Hence, these pore size distributions indicate the existence of mesopores.

Figure 3 SEM micrographs of the samples

Figure 4 N2 adsorption-desorption isotherms of kaolin/urea intercalation composite

Figure 5 Pore size distribution of kaolin/urea intercalation composite

3.2 De-intercalation behavior of the intercalation composite

In the synthesis process of ZSM-5 molecular sieve, the reaction temperature of the system is higher, and the intercalation composite materials will change at this temperature. It is meaningful to study the de-intercalation behavior of the intercalation composite materials and its effect on the synthesis of molecular sieves.

Figure 6 shows the de-intercalation behavior of kaolin intercalation composites with different intercalation ratios obtained at different temperatures. It can be seen that the de-intercalation behavior of the kaolin/urea intercalation composite with different intercalation ratios is basically the same, and they start to be subject to de-intercalation at 100 °C, but the de-intercalation rate of the intercalation composites with lower intercalation ratios is obviously faster, and they are completely de-intercalated at about 125 °C. When the temperature reaches 175 °C, the kaolin/urea composite with an intercalation ratio of 62% is completely subjected to de-intercalation.

Figure 6 The de-intercalation behavior of KUMs Intercalation ratio: (1)—22%; (2)—50%; (3)—62%

3.3 Properties of the synthesized ZSM-5 zeolite

3.3.1 XRD characterization

The de-intercalation behavior of kaolin/urea intercalation composites with different intercalation ratio is different.In the system for synthesis of ZSM-5 molecular sieve,different de-intercalation behaviors will produce different heat energy and pressure.

Figure 7 shows the effect of kaolin/urea intercalation composite with different intercalation ratios on the ZSM-5 molecular sieve synthesis system. It can be seen that the intercalation composite with different intercalation ratios has certain influence on the molecular sieve synthesis system. The crystallinity of ZSM-5 molecular sieve increases at first and then decreases with an increasing intercalation ratio of kaolin/urea intercalation composite.When the intercalation ratio is 62%, the crystallinity is lower. According to the de-intercalation behaviors of kaolin/urea intercalation composite, the completely deintercalation temperature of the kaolin/urea composite with an intercalation ratio of 62% is 175 °C. Since the synthesis temperature is 150 °C, the intercalation composite continues to be de-intercalated in this process,which takes up the heat energy of the system. Therefore,at this reaction temperature and reaction time, the kaolin/urea intercalation composite with higher intercalation ratio shows an inhibition effect on the synthesis of ZSM-5 molecular sieve. So only a suitable intercalation ratio has a positive effect on the synthesis of ZSM-5 molecular sieve. When the intercalation ratio of kaolin / urea composite is 22%, the crystallinity of ZSM-5 molecular sieve is higher.

Figure 7 XRD patterns of ZSM-5 molecular sieve synthesized by KUM with different intercalation ratios(1)—0%; (2)—22%; (3)—50%; (4)—62%

Figure 8 shows the effect of different amount of the added kaolin/urea intercalation composite (with an intercalation ratio of 22%) on the synthesis of molecular sieve. It can be seen that the crystallinity of ZSM-5 molecular sieve increases at first and then decreases with the increase of added amounts of the kaolin/urea intercalation composite.When the amount of the added kaolin/urea intercalation composite is 6%, the crystallinity of ZSM-5 molecular sieve decreases gradually. Because the kaolin/urea intercalation composite with higher intercalation ratio has a certain inhibition effect on the crystal growth, only an appropriate amount of added intercalation composite has a positive impact on the synthesis of ZSM-5 molecular sieve[17]. When the amount of added kaolin/urea intercalation composite is 3%, the crystallinity of ZSM-5 molecular sieve is higher, which reaches 65%. Compared to the crystallization product formed without adding kaolin/urea intercalation composite, the crystallinity of ZSM-5 molecular sieve increases by 54.8%.

Figure 8 XRD patterns of ZSM-5 molecular sieve synthesized by KUM with different addition amounts

3.3.2 DTA analysis

Figure 9 shows the DTA curves of ZSM-5 molecular sieve synthesized by using KUM and without addition of kaolin/urea intercalation composite. It can be seen that the DTA curves of the synthesized ZSM-5 molecular sieves are basically the same, and there is a large endothermic peak at about 300 °C, which belongs to the desorption of adsorbed water on the surface. The NZSM-5 molecular sieve has an exothermic peak at about 1 050 °C, which indicates that the sample begins to decompose, and new substances are generated. The exothermic peak of UZSM-5 molecular sieve occurs at about 1 200 °C, which indicates that the ZSM-5 molecular sieve has a higher thermal stability. The intercalation composite improves the skeleton stability of molecular sieve. It is further confirmed that ZSM-5 molecular sieve synthesized by using the intercalation composite has better performance.

3.3.3 SEM analysis

Figure 9 DTA curve of synthesized ZSM-5 molecular sieve

Figure 10 shows the SEM patterns of NZSM-5 and UZSM-5 molecular sieves, respectively. It can be seen that the synthesized ZSM-5 molecular sieve are mainly rhombohedral. It can be seen from Figure 10 (a) and(b) that the grain size of the NZSM-5 molecular sieve is about 20 μm, and there is almost no agglomeration except for a few small crystal grains on the surface. It can be learned from Figure 10 (c) and (d) that the UZSM-5 molecular sieve has good crystal growth, with the crystal size equating to about 2.5 μm. The particle size of the UZSM-5 molecular sieve is obviously reduced. The smaller particle size means that the activation centers of zeolite and the catalytic performance are better. It can be seen that the intercalation composite is not only bene ficial to reducing the grain size and making the grains on the surface smooth and pure, but can also prevent the aggregation of ZSM-5 crystal in the synthesis process.

3.3.4 Pore structure

Figure 11 shows the N2adsorption desorption isotherms of synthesized ZSM-5 molecular sieve. It can be seen that the N2adsorption-desorption isotherms of the NZSM-5 and UZSM-5 molecular sieve belong to type IV, which contains mesoporous pore structure. The hysteresis loop that exists in a pressure range 0.45 ＜P/P0＜ 1.0 is asribed to the presence of mesopores. The ZSM-5 molecular sieve with rich mesoporous pore structure has higher performance and wider application.

Figure 12 shows the pore size distributions of UZSM-5 molecular sieve and NZSM-5 molecular sieve. It can be seen that the pore size distributions of these samples are basically the same. The pore size distribution of UZSM-5 molecular sieve is approximately at 2.7 nm, 4.0 nm, and 28 nm, and this distribution indicates the existence of mesopores. Compared with NZSM-5 molecular sieve, UZSM-5 possesses a wide pore structure with a trimodal distribution.

Figure 10 SEM of synthesized ZSM-5 molecular sieve(a) and (b): NZSM-5; (c) and (d): UZSM-5

Figure 11 N2 adsorption-desorption isotherms of synthesized ZSM-5 molecular sieve

Figure 12 Pore size distribution of synthesized ZSM-5 molecular sieve

Table 1 BET surface area and pore structure of ZSM-5 molecular sieve

Table 1 shows BET specific surface area and pore structure of ZSM-5 molecular sieve. Compared with NZSM-5 molecular sieve, BET specific surface area,pore volume and average pore diameter of UZSM-5 have been improved to increase by 20 m2/g, 0.010 cm3/g, and 0.9 nm, respectively. The larger the speci fic surface area is, the stronger the adsorption capacity would be. The increase of pore volume and pore diameter improves the selectivity of the molecular sieve.

4 Conclusions

This paper focused on the catalytic composites containing ZSM-5 molecular sieve synthesized using the kaolin/urea intercalation composites by an in-situ crystallization technique. The effects of the intercalation ratios and deintercalation rate and the amounts of added kaolin/urea intercalation composite on the synthesis of the ZSM-5 molecular sieve were studied. The results showed that the particles become smaller and the content of fine particles increases in kaolin after intercalation. When the temperature reached 175 °C, the kaolin/urea composite with an intercalation ratio of 62% was completely subjected to de-intercalation. The kaolin/urea intercalation composite with higher intercalation ratio has an inhibition effect on the synthesis of ZSM-5 molecular sieve. Only a suitable intercalation ratio has a positive effect on the synthesis of ZSM-5 molecular sieve. When the optimum added amount of kaolin/urea intercalation composite was 3%, the crystallinity of ZSM-5 zeolite was 65%. The catalytic composites containing ZSM-5 molecular sieve with a wide pore structure had better thermal stability,featuring a particle diameter of 2.5 μm, and BET speci fic surface area of 236 m2/g, and a pore size of 10.6 nm.

Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (No.21371055),and the Key Project of Scientific Research Project of Hunan Education Department (No.18A313).