Qin Lihong; Li Haiyan; Li Jun; Sun Famin

(1. China University of Petroleum, Beijing 102249; 2. Daqing Chemical Research Center, Petrochemical Research Institute, PetroChina Company Limited, Daqing 163714; 3. Daqing Petrochemical Company Construction Company, PetroChina Company Limited, Daqing 163714)

Synthesis of Nano-ZSM-5 in Ultra-Concentrated System and Its Performance in Diesel Hydrodewaxing

Qin Lihong1; Li Haiyan2; Li Jun3; Sun Famin2

(1. China University of Petroleum, Beijing 102249; 2. Daqing Chemical Research Center, Petrochemical Research Institute, PetroChina Company Limited, Daqing 163714; 3. Daqing Petrochemical Company Construction Company, PetroChina Company Limited, Daqing 163714)

Nanocrystalline ZSM-5 (with a crystal size in the range of 20—30 nm) has been synthesized in only 20 h via the crystallization of ultra-concentrated homogeneous synthesis mixtures. The infuence of low water content, use of compound surfactant, a proper crystallization time and SiO2/Al2O3molar ratio on the properties of the fnal nanocrystalline ZSM-5 has been studied. Especially, the use of compound surfactant has infuenced some properties of nano-ZSM such as the morphology, crystal size and crystallization greatly. The nano-ZSM-5 zeolite was investigated using XRD, BET, SEM, NH3-TPD, and other techniques. The evaluation results in a 200-mL hydrogenation unit indicated that the synthesized nano-ZSM-5 had excellent catalytic performance in diesel hydrodewaxing.

ultra-concentrated system; nano-ZSM-5; diesel hydrodewaxing

1 Introduction

ZSM-5 zeolite used as the catalyst exhibits excellent features such as thermal stability, resistance to deactivation, high acidic activity and molecular shape selectivity. In many applications particularly FCC[1], gasoline aromatization[2], MTO[3-4], and MTP[5-6], the ZSM-5 zeolite dominates many established and most of the new processes.

ZSM-5 zeolite is generally synthesized in hydrothermal conditions by heating the initially formed hydrogels at various temperatures[7-8]. Nucleation occurs at the interface of the solid and liquid phases, as was reported previously. The pathway of the crystallization process of the MFI-type zeolite is infuenced by different variables: the silicon and aluminum source, the aluminum content, the template, the alkalinity, the temperature of crystallization, the presence of seeds, the water content, etc. However, the production cost of ZSM-5 zeolite is a problem that influences the extensive use of ZSM-5 zeolite. The H2O/SiO2molar ratio in normal hydrothermal method is about 10—500 generally. The high H2O/ SiO2molar ratio adopted in the hydrothermal method has the defects such as low solid content, low synthesis efficiency, and much more wastewater discharge. The synthesis of nano-ZSM-5 in an ultra-concentrated system using a compound surfactant has been studied in this paper. This method has such advantages as shorter crystallization time, less wastewater, and higher synthesis effciency.

2 Experimental

2.1 Material and synthesis

The chemical reagents used in the experiments included NaAlO2(54.5% of Al2O3, 40.9% of Na2O, Tianjin Guangfu Chemical Co.), solid silica (98% of SiO2, Qingdao Baishahe Chemical Co.), NaOH (98% pure, Tianjin Guangfu Chemical Co.), sodium dodecylbenzene sulphonate (SDBS, 99% pure, Tianjin Guangfu Chemical Co.), and sodium alcohol ether sulphate (AES,70% pure, Tianjin Guangfu Chemical Co.). All the above reagents were used without further purifcation.

The starting materials and compositions in ultra-concentrated synthesis system were based on the formula asshown below. The molar compositions of materials were specifed as follows: SiO2:Al2O3＝25—91, Na2O:SiO2＝0.02—0.2, H2O:SiO2＝ 0.5—1.0, and the compound surfactant (SDBS and AES):SiO2＝0.002—0.03.

A typical synthesis procedure was as follows. NaAlO2, NaOH and the compound surfactant were dissolved in distilled water to obtain a mixture solution A. The compound surfactant including SDBS and AES was applied in the prepared solution A. The solution A was loaded into the high pressure atomizer. The solution A was atomized into the solid silica under continuous stirring. Finally, the resulting mixture obtained thereby was charged into a teflon-lined stainless-steel autoclave and was subject to crystallization by thermal treatment under autogenous pressure in a static environment at 110 ℃ for 20—24 h. The solid product was separated by centrifugation, washed several times with distilled water, dried overnight at 110 ℃and calcined in air at 550 ℃ for 4 h.

A synthesis procedure for contrast test was conducted as follows. NaAlO2and NaOH were dissolved in the distilled water to obtain a mixture solution B. The solution B was directly poured into the solid silica without atomizing. Because of the low H2O/SiO2molar ratio in the starting material, the solution B could hardly be dispersed into the solid silica uniformly. Finally, the resulting mixture obtained was charged into a teflon-lined stainless-steel autoclave and was subject to crystallization by thermal treatment under autogenous pressure in a static environment at 110 ℃ for 20—24 h. The solid product was separated by centrifugation, washed several times with distilled water, dried overnight at 110 ℃, and calcined in air at 550 ℃ for 4 h.

2.2 Characterization

The powder XRD patterns of zeolite samples were recorded using a Philips X pert model MPD diffractometer operating at 40 kV and 30 mA using Cu Kα radiation over a scanning angle (2θ)range from 1° to 46°. The crystallinity was determined from the peak area between 2θ=22°—25° using a highly crystalline ZSM-5 sample as reference. The nitrogen isotherms at 77 K were determined using a volumetric adsorption apparatus (Micromeritics, ASAP 2010). The surface area was estimated according to the BET method. The pore size distribution was obtained by applying the BJH model with cylindrical geometry of the pores and using the Harkins and Jura equation for determining the adsorbed layer thickness. The morphology and size of the crystallites were determined from the scanning electron microscopy images taken with a scanning microscope (Cambridge S-360). The FT-IR spectra were recorded using an infrared spectrometer (Nicolet FT-IR-560 plus).

3 Results and Discussion

3.1 Synthesis and characterization of nano-ZSM-5

3.1.1 Effect of surfactant on crystallinity

Figure 1 shows the XRD patterns of samples synthesized in the ultra-concentrated system. Figure 1(A) shows the XRD pattern of the sample of the contrast experiment. There were no XRD diffraction peaks of ZSM-5 in Figure 1(A). Figure 1(B) shows the XRD pattern of the sample synthesized without the surfactant. Figure 1(C) is the XRD pattern of the sample synthesized using compound surfactant such as SDBS and AES. Table 1 illustrates the effect of surfactant on crystallinity. The results showed that the crystallinity of sample using the compound surfactant increased from 82% to 113%.

Table 1 Effect of surfactant on crystallinity

Figure 1 Crystallinity of as-synthesized samples

Figure 2 displays the IR spectra of the as-synthesized sam-ples. The results showed that the absorption bands at about 550 cm-1, 1080 cm-1, 1230 cm-1, 790 cm-1, and 450 cm-1were identifed for the samples. Only a ZSM-5 phase was detected in the IR spectra.

Figure 2 IR spectra of as-synthesized samples

3.1.2 Effect of surfactant on morphology

Figure 3 (C) shows the SEM images of nano-ZSM-5 synthesized by using the compound surfactant. In comparison with the SEM image of the ZSM-5 synthesized without using the compound surfactant (Figure 3(B)), the test results show that the nano-ZSM-5, which was synthesized by using the compound surfactant, was formed into nanoball shaped crystals. The nano-ball shaped crystals adhered to each other. The size of nano-ball shaped crystals was about 20—30 nm. The crystal size of nano-ZSM-5 was much smaller than that of normal ZSM-5 synthesized without using the compound surfactant. The crystal size of normal ZSM-5 was about 8—10 μm (Figure 3(B)). Figure 3(A) shows the SEM image of the sample obtained from the contrast experiment in which the solution containing NaAlO2and NaOH was not uniformly dispersed into solid silica existing in the starting material because of the low H2O/SiO2molar ratio. There were amorphous aluminosilicates without any ZSM-5 crystals in the SEM image.

3.1.3 Effect of surfactant on pore structure

Figure 4(B) shows the BET analysis of nano-ZSM-5 synthesized by using the compound surfactant. It can be seen that there were two kinds of mesopore sizes in the pore distribution image in which one kind of mesopore size was 3 nm and another one was 3.8 nm. The pore distribution was about 2—5 nm which was different from the normal ZSM-5 synthesized without using surfactant (Figure 4(A)). The mesopore size of normal ZSM-5 was only 3.8 nm.

Figure 3 SEM images of the as-synthesized samples

Table 2 illustrates the BET analysis results of nano-ZSM-5. It can be seen that the surface area and pore volume for nano-ZSM-5 were much better than those of thenormal ZSM-5 synthesized without using surfactant. The surface area increased from 299 m2/g to 410 m2/g, while the pore volume increased from 0.23 cm3/g to 0.38 cm3/g.

Table 2 Effect of the surfactant on BET results

Figure 4 BET analysis results of as-synthesized samples

3.2 Mechanism for crystallization of nano-ZSM-5 in ultra-concentrated system

3.2.1 Study of crystallization kinetics

The compound surfactant greatly infuenced the crystallinity of ZSM-5 synthesized in the ultra-concentrated system. But the crystallization time was not affected seriously. Figure 5 shows the crystallization kinetic curves of two different kinds of ZSM-5 synthesized in the ultra-concentrated system. One kind of ZSM-5 was synthesized without surfactant. Another nano-ZSM-5 was synthesized by using the compound surfactant. The crystallization curve of ZSM-5 synthesized without using surfactant shows that the crystallinity of ZSM-5 reached 82%, when the crystallization was carried out for 20 h at 110 ℃. But the crystallinity of ZSM-5 began to decrease, when the crystallization time further increased. If the crystallization time was 24—40 h, the crystallinity of ZSM-5 did not decrease further and was about 78%—79%. However, when the crystallization time increased to 44 h, the crystallinity of ZSM-5 decreased to 75%. The results showed that the crystallinity of ZSM-5 synthesized without using surfactant was the highest when crystallization was performed within 20 h in the ultra-concentrated system.

Figure 5 Crystallization kinetics curves of ZSM-5 obtained from experiment

However, Figure 5 shows the crystallization curve of nano-ZSM-5 synthesized by using the compound surfactant. When crystallization time was 10 h, the crystallinity of nano-ZSM-5 had reached 76%. But the normal ZSM-5 synthesized with-out using the surfactant reached only 56%. Furthermore, the crystallinity of nano-ZSM-5 increased to 112% when its crystallization time was 20 h. But, when the crystallization time was more than 28 h, the crystallinity of nano-ZSM-5 began to worsen. Above all, the results illustrate that the application of compound surfactant in the synthesis of ZSM-5 increased the crystallinity of ZSM-5 greatly over the same crystallization duration.

3.2.2 The effect of SiO2/Al2O3molar ratio in ultraconcentrated system

In order to investigate the aluminum content of nano-ZSM-5 synthesized by using compound surfactant in the ultra-concentrated system, the synthesized nano-ZSM-5 samples were analyzed by XRF spectrometry, with the results summarized in Table 3. Figure 6 shows the SiO2/Al2O3molar ratio of nano-ZSM-5 product that was synthesized using the surfactant. The SiO2/Al2O3molar ratio of product increased as compared to the SiO2/Al2O3molar ratio of starting material. The SiO2/Al2O3molar ratio of the product and that of the starting material varied differently. When the SiO2/ Al2O3molar ratio of starting material increased to 200, the SiO2/Al2O3molar ratio of nano-ZSM-5 product only reached 91.5 by using compound surfactant in the ultraconcentrated systvem. However, the SiO2/Al2O3molar ratio of normal ZSM-5 product during the hydrothermal synthesis process was about 25—65 under the same condition adopted in experiments without using surfactant. Most silica source was dissolved in the water and could not be used fully. The SiO2/Al2O3molar ratio of normal ZSM-5 could hardly reach 91.5 during the hydrothermal synthesis process. The utilization of silica source was better through using the compound surfactant in the ultra-concentrated system.

Table 3 XRF analysis results of synthesized nano-ZSM-5 samples

3.2.3 The exploration of crystallization mechanism

Figure 6 The effect of SiO2/Al2O3molar ratio

Upon taking into account the results of contrast experiment, it is clear that the crystallization and growth of nano-ZSM-5 depended on how the chemical reagents NaAlO2and NaOH could be uniformly dispersed into solid silica of starting material. If the aqueous solution containing NaAlO2and NaOH was not dispersed uniformly into the solid silica of the starting material, the pH value of starting material would be infuenced seriously. The composition of starting material at different position was not uniform. The synthesis of nano-ZSM-5 with different SiO2/Al2O3molar ratios generally required that the pH value should be frequently measured. This explained that the ZSM-5 would not be synthesized when the aqueous solution with NaAlO2and NaOH was not dispersed uniformly into the solid silica of the starting material. Figure 7 shows the crystallization mechanism for synthesis of nano-ZSM-5 in the ultra-concentrated system. Because of the low H2O/SiO2molar ratio, the water was almost absorbed into solid silica and could not fow freely in the starting material. It was different from the hydrothermal synthesis method. The water mainly existed as water vapor at 110 ℃ during crystallization in the ultra-concentrated synthesis method. Because of the absorption action of solid silica, the metal ions such as Na+andspecies could hardly move freely once more during the crystallization stage of the contrast experiment. Most Na+andions could hardly disperse uniformly again. This just explained that the dispersing condition of starting material infuenced the synthesis of ZSM-5 greatly in ultra-concentrated synthesis method. The compound surfactant contributed to the dispersing of Na+andions. Because of the polar functional groups of the surfactant, the Na+andions could stick to the compound surfactant easily. The compound surfactant reduced the surface tension of solid silica too. It was easier for the Na+andions to disperse into the pores of solid silica uniformly. The crystal nuclei of ZSM-5 could be easier subject to synthesis reaction in the presence of compound surfactant. In comparison with the synthesis method without using compound surfac-tant, the amount of crystal nuclei increased greatly in the early stage, which just illustrated that the application of compound surfactant in the ultra-concentrated synthesis of nano-ZSM-5 increased the crystallinity of ZSM-5 product greatly.

3.3 Acidity and catalytic performance of nano-ZSM-5

3.3.1 Acidity of nano-ZSM-5

Figure 8 shows the NH3-TPD curves of the ZSM-5 samples. Two desorption peaks were observed for the ZSM-5 zeolite. The HT peak refected the concentration of the acid sites, while the temperature at which the maximum HT peak occurred could refect the overall acid strength of the sample. There are two desorption peaks identifed in the image of NH3-TPD. The nano-ZSM-5 synthesized by using the compound surfactant had more strong acid sites and weak acid sites as compared with the ZSM-5 synthesized without the surfactant.

Figure 8 The NH3-TPD curves of samples obtained from experiment:

3.3.2 Catalytic performance of nano-ZSM-5 in diesel hydrodewaxing

The properties of nano-ZSM-5 such as the concentration of the acid sites, the surface area and the pore volume could influence the catalytic activity of nano-ZSM-5 greatly. The reaction activity of nano-ZSM-5 is much better than the normal ZSM-5 with large crystal grain (8—10 μm). The catalyst composed of nano-ZSM-5 and nickel oxide was named No. 1. The catalyst composed of commercial ZSM-5 and nickel oxide was named No. 2. The nano-ZSM-5 was the same as the commercial ZSM-5 in terms of its crystallinity and SiO2/Al2O3molar ratio.

The catalytic behavior of the catalysts was tested using FCC diesel fuel as feed, with the results presented in Table 4. Table 5 illustrates the results for evaluation of the catalysts in a 200 ml hydrogenation unit. The evaluation results indicated that the catalyst No. 1 was much better than the contrast catalyst No. 2 in diesel hydroisomerization for reducing the solidification point. It can be seen that the diesel yield reached 93.80% when the solidifcation point reduced to -40 ℃ under the reaction conditions covering a pressure of 8.0 MPa, a hydrogen/oil ratio of 800:1, a LHSV of 1.60 h-1, and a reaction temperature of 350 ℃. The catalytic reaction performance of catalyst No. 1 was better at the temperature of 350 ℃. The properties of diesel product such as cetane number and the content of polycyclic aromatic hydrocarbons achieved by the cata-lyst No. 1 were better than those achieved by the catalyst No. 2. The evaluation results indicate that nano-ZSM-5 had better catalytic performance. The hierarchical pores of nano-ZSM-5 were very important for the diesel hydroisomerization reaction, because they were more easily accessible to diesel molecules of large size that could diffuse into the mesopores of nano-ZSM-5 zeolite.

Table 4 The property of FCC diesel

Table 5 Results for evaluation of diesel hydrodewaxing catalyst in a 200-mL hydrogenation unit

4 Conclusions

An ultra-concentrated system for synthesizing the nano-ZSM-5 by using compound surfactant has been developed. Detailed investigation shows that the compound surfactant could infuence the properties of ZSM-5 including its crystallinity, morphology, crystallite size, and other properties. The crystallinity increased by 30% after using compound surfactant in the course of synthesis. The crystal size of nano-ZSM-5 was much smaller than the normal ZSM-5. The nano-ZSM-5 synthesized by using the compound surfactant performed better during the diesel hydroisomerization reaction. The catalyst evaluation results indicate that the nano-ZSM-5 synthesized by using the compound surfactant was much better than the contrast catalyst composed of the commercial ZSM-5 in diesel hydroisomerization for reducing the freezing point of diesel fraction.

Acknowledgements: Thanks are due to the National Natural Science Foundation (20473039) for the support of this work. We would like to thank Professor Dou Tao for this work.

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Received date: 2016-05-16; Accepted date: 2016-08-04.

Dr. Li Haiyan, Telphone: +86-459-674 3152; E-mail: lhy459@petrochina.com.cn.