Abstract:
Zeolite NaY was successfully synthesized by a dynamic hydrothermal crystallization method with water glass and kaolin from Yunnan Provice of China. The products were modified to serve as hydrocracking catalysts. The influences of aging conditions of the structure-directing agent and crystallization methods were examined. The compositions and microstructures of zeolites were characterized by field emission scanning electron microscopy, X-ray diffraction and Brunauer-Emmett-Teller (BET). The catalytic performance for hydrocracking was briefly evaluated. The results showed that appropriate aging temperature and time of structure-directing agent were beneficial to nucleation. Combined with static crystallization, the yield of zeolite NaY prepared by the dynamic crystallization method was significantly improved, relative crystallinity increased from 46.91% to 74.18%, and conversion in hydrocracking accordingly increased from 39.58% to 55.93%.
1 Introduction
Zeolite Y is a crucial active component in catalysts for its uniform aperture, high activity and good stability. It is widely used in the catalytic cracking, hydrocracking and isomerization process [1, 2]. Currently, the hydrothermal synthesis method is used in the industrial production of zeolite NaY. The general raw materials of alkaline aluminum-silica gel are sodium aluminate, sodium silicate, sodium hydroxide and aluminum sulfate, leading to higher production cost [3]. To improve this situation, cheap kaolin clay and industrial water glass are used as ingredients. Kaolin resource is widely distributed in China [4] with the characteristics of easy mining and good quality, which is conducive to deep processing. Kaolin used to prepare zeolites in the experiment is from Yunnan of China instead of Suzhou [5, 6] by other researchers. Kaolin from Suzhou contains 39.0% active Al2O3. It is mainly used in catalyst carriers and chemical raw materials, but its output cannot meet the growing demand of oil refining and the petrochemical industry. Searching for and research on a new variety of kaolin is urgent. Kaolin reserves in Yunnan are 40 million tons with little developed and utilized at present. The purity of kaolinite and natural whiteness are higher and the content of impurities (iron and titanium) is less, which fits the standard of catalyst synthesis. Overall, the design has a contribution to reduce costs and efficiently use resources.
Many researchers are attracted to natural minerals. The synthesis of various kinds of molecular sieves with kaolin have been studied, for instance zeolite A [7, 8], zeolite X [9], zeolite Y [10], beta zeolite [11] and zeolite ZSM-5 [12]. In addition, many researchers demonstrated that overgrowing zeolite NaY on kaolin microspheres as catalysts had much better catalytic performance due to better dispersion and improved accessibility of active sites [13, 14]. However, integrity degree of crystallization reflected as relative crystallinity was lower, and the main difficulty focused on the mass transfer between the solid phase and liquid phase in the system. Liu et al. [13] mixed two kinds of calcined kaolin at different temperatures to prepare molecular sieves according to the different properties of high temperature calcined kaolin and low temperature calcined kaolin. Zhou et al. [15] adopted the two-step crystallization method, suggesting that pre-crystallization was beneficial to form a nucleus and improve the relative crystallinity. Some researchers paid close attention to avoid the heterogeneous reaction. The alkali fusion method is often used to activate silicon and aluminum species of kaolin, resulting in soluble silico-aluminate solution generating [16, 17]. The technique of adding an organic solvent (such as polyvinyl alcohol) [18] or an intercalating reagent (such as urea) [19] could disperse the solvent, reduce the grain size, and increase the crystallinity. Being a complex process or expensive reagents are the drawbacks to the above methods.
In view of the existing technology problems of zeolite preparation by kaolin, a dynamic crystallization method was designed to synthesize zeolite NaY. It is based on the feature that reaction kettles were rotated by the axis on the homogeneous reactor. The textural and structural properties of zeolites are discussed, and catalyst performance on hydrocracking was simply evaluated. The study provides a theoretical and experimental basis for the feasibility of industrialization.
2 Materials and methods
2.1 Synthesis of zeolite Y
The kaolin used in experiments is from the Menglong mining area in Yunnan of China. The chemical components of raw kaolin are shown in Table 1. Its Brunauer-Emmett-Teller (BET) specific surface area and mean pore size were 18.40 m2/g and 14.91 nm, respectively (Table 2). Industrial water glass (producted in Tianjin, China) contains SiO2≥24.6 wt%, Na2O≥7.0 wt%. Other reagents were provided by Sinopharm Chemical Reagent Co., Ltd. (China). After washing, crushing and grinding, screened kaolin mineral with particle size in 300–500 mesh was activated at a high temperature. The structure-directing agent solution Na2O:Al2O3:SiO2:H2O=(16~17):1:(15~17):(320~330) aged at a certain temperature for some time was obtained after water-cooling to room temperature. The structure-directing agent includes an octahedral structure unit and crystal nucleus, which are beneficial in accelerating the synthesis reaction process. This is necessary and commonly used. According to definite proportion of Na2O:Al2O3:SiO2:H2O=(3~5):1:(5~12):(100~300), the activated kaolin, sodium hydroxide, water glass, distilled water was mixed as sequence, and structure-directing agent was added. Then, sol-gel was placed in a hydrothermal reaction kettle (namely hydrothermal synthesis reactor) to crystallize for some time at 373 K. Zeolite NaY was obtained after filtering, washing, drying and calcining. The static crystallization referred to that reaction kettle always placed in horizontal, corresponding product was denoted by M-NaY-1. The reaction kettles were rotated by the axis on the homogeneous reactor in dynamic crystallization, and the product was labeled as M-NaY-2. In particular, the industrial product with chemical synthesis was marked as C-NaY.
Component | SiO2 | Al2O3 | K2O | Fe2O3 | Na2O | MgO | CaO | Others |
---|---|---|---|---|---|---|---|---|
Content (wt%) | 56.91 | 39.63 | 1.95 | 0.47 | 0.46 | 0.18 | 0.11 | 0.29 |
Samples | SBETa (m2/g) | VPb (cm3/g) | DAc (nm) | SiO2/Al2O3d | Crystallinitye (%) |
---|---|---|---|---|---|
Kaolin | 18.40 | 0.04 | 14.91 | 2.34 | – |
Metakaolin | 16.84 | 0.04 | 13.94 | – | – |
C-NaY | 783.57 | 0.24 | 3.89 | 5.01 | 99.27% |
M-NaY-1 | 572.76 | 0.44 | 5.96 | 4.08 | 46.91% |
M-NaY-2 | 743.11 | 0.33 | 4.84 | 4.80 | 74.18% |
aBrunauer-Emmett-Teller (BET) surface area.
bTotal pore volume.
cThe average pore diameter.
dFramework SiO2/Al2O3: calculation by Eq. (1) from X-ray diffraction (XRD), except kaolin from quantitative analysis.
eCalculation from XRD.
2.2 Characterizations
The chemical compositions were determined by a Bruker S4 Explorer X-ray fluorescence spectrometer. The thermal analysis was tested with the STA 449 F3 Jupiter type synchronous thermal analyzer produced by NETZSCH-Gerätebau GmbH, with conditions of air atmosphere at a temperature ranging from 298 K to 1473 K, and heating rate of 10 K/min. X-ray diffraction patterns were recorded on a Dutch X’pert HighScore Plus X-ray diffractometer using Cu Kα radiation in the range of 5°–60° with a scan rate of 2°/min. Lattice parameters (a0) were obtained by the least square method. The silica alumina ratio of zeolite wascalculated with empirical formula:
A simple method was adopted to compute the relative crystallinity of zeolite, namely relative crystallinity was equal to the ratio of the sum of some peak heights in the sample and that corresponding in the standard sample. Characterizations of the microstructures were performed with an Ultra Plus field emission scanning electron microscope (Zeiss, Germany). N2 adsorption-desorption isotherms were measured at 77 K with a Mike ASAP 2010 multifunctional adsorption instrument, and samples were located in vacuum for 4 h at 673 K as pretreatment. The specific surface area was determined by the BET method. The pore volume and aperture size distribution were calculated from the Barrett-Joyner-Halenda formula.
2.3 Evaluation of catalytic performance
Modified zeolites, activated alumina powder, adhesive, sesbania powder and distilled water were mixed and shaped by single-screw extrusion. Hydrogenation activity metal (Ni and W) was loaded with the isovolumetric impregnation method. Finally, hydrocracking catalysts were prepared after dipping, drying and calcining. The catalysts were denoted by C-Y, M-Y-1 and M-Y-2 according to different molecular sieves, respectively. Catalyst presulfurization was carried out before the hydrocracking. The n-heptane was used as raw material. Evaluation criteria were that 10 ml catalyst reacted at 573 K under 4 MPa reaction pressure with the conditions of 1500 hydrogen-oil ratio and 1.5 h−1 space velocity. Analysis of the liquid product was obtained by GC122 gas chromatography (Pona chromatographic column and flame ionization detector) after condensation. The liquid yield was the ratio of the quality of liquid after and before the reaction. The n-heptane conversion was defined as follows:
where wr(nC7) and wp(nC7) are the mass fraction of n-heptane in reactants and products, respectively.
3 Results and discussion
3.1 The activation of kaolin
The metakaolin was obtained from thermal activation of kaolin in the Menglong mining area of China. The crystal structure of kaolinite collapsed after roasting at high temperature. One part converted into an active component that canparticipate in a reaction under hydrothermal conditions, and others transformed to inert components as the catalyst supports. The contents of active components (active SiO2 refers to alkali soluble silicon and active Al2O3 refers to acid soluble aluminum) at different calcination temperatures are shown in Figure 1. It was concluded that the content of active SiO2 was higher at high temperatures and active Al2O3 was higher at low temperatures. To save raw materials, only when the temperature was 1173 K were the contents of active SiO2 and Al2O3 both suitable. It contained 19.25% active Al2O3 and 9.69% SiO2. The BET specific surface area and mean pore size of metakaolin produced by raw kaolin calcinating at 1173 K were 16.84 m2/g and 13.94 nm, respectively (Table 2), which were inconspicuous changes from those of the original kaolin. As seen from Figure 3, calcination had little impact on pore structure.
3.2 Effect of aging conditions of structure-directing agent on zeolite NaY production
The crystallization cannot be promoted by a structure-directing agent without aging. The octahedral structure unit and crystal nucleus in the structure-directing agent are beneficial to accelerate the synthesis reaction process and restraint for foreign crystals [20]. So, sol-gel could be fully crystallized in 24 h at 373 K. The relative crystallinities of zeolite NaY at different aging conditions of structure-directing agents were obviously different, as shown in Figure 2.
The relative crystallinities of zeolite NaY at 20 min (as a symbol of nucleation rate) by different aging temperatures followed the sequence 343 K>353 K>333 K>323 K>313 K. The nucleation rate Iv can be signified though Factor P affected by the nucleation barrier and Factor D affected by atomic diffusion as Eq. (3) [21]. The relationship between nucleation rate and temperature is presented in the following formulas:
In the range of 343–313 K, due to increased liquid viscosity, the diffusion rate of atomic or molecular reduced as temperature decreased. ΔGm in Eq. (5) rose and Factor D reduced, so Iv declined. That is to say, more crystal nuclei were generated at high temperature in the same aging time, and integrity degree of crystallization was higher. However, as the temperature increased, the degree of supercooling (ΔT) declined, and the nucleation rate decreased as the result of nucleation barrier in Eq. (4) rising (ΔGc∝1/ΔT2). Thus, the relative crystallinity did not rise, but decreased at 353 K. From 323 K to 353 K, aging time extension eventually led to the decrease of crystallinity. This might be attributed to structure-directing agent inactivation. In addition, there were points of intersection in the effective range (before inactivation) between curves of 323 K and 333 K, or 333 K and 343 K. It explained that the relationship between the aging time and temperature should be a comprehensive result of Factor P and Factor D. In other words, Factor D suppressed the growth of the nucleation rate at lower temperature, and Factor P was the main control factor when the temperature was higher. Only at the appropriate temperature did nucleation rate and relative crystallinity reach a maximum. The best aging temperature of the structure-directing agent was 333 K, and optimal aging time was 50 min.
3.3 Effect of different crystallization methods on synthesis of zeolite NaY
X-ray diffraction spectrums of zeolites synthesized by different crystallization methods are shown in Figure 3. Under identical experimental conditions, the diffraction peaks of zeolite Y were both complete. There was a characteristic diffraction peak of the quartz at 26.6°, and a peak of mica at 27.8°. This was because small amounts of quartz and mica in kaolin were relatively stable [12] and could not be effectively dissolved in the crystallization process. However, the peak intensity of M-NaY-2 was remarkably higher than that of M-NaY-1, and mottle peaks in M-NaY-2 were hardly detected which existed in M-NaY-1. The relative crystallinity of M-NaY-2 was 74.18%, which was higher than that of M-NaY-1 (46.91%). This demonstrated that the solution could contact well with solid reactants to mix and distribute evenly in the dynamic crystallization method, which was more conducive to synthesis and effectively restrained mottle crystals.
Scanning electron microscopy photos of zeolites synthesized in the dynamic and static conditions are shown in Figure 4C and D. The morphologies of crystals in Figure 4C were clear and dispersed uniformly on kaolin substrate. The crystals in Figure 4D were not of uniform size, dispersion was also uneven, and there were parts of the micelles without complete crystallization. The comparison illustrated that crystal shape of molecular sieves by dynamic synthesis was better than that of static condition. It proved directly why the crystallinity of M-NaY-2 was higher.
The textural properties and structural parameters of samples are listed in Table 2. The part of metakaolin with mesoporous structure could not be involved in the reaction and used as a catalyst carrier, so the synthetic product M-NaY was a composite material of zeolite NaY and metakaolin. Compared with C-NaY, the specific surface area and SiO2/Al2O3 of M-NaY were both lower, and the average pore diameter was higher. However, the specific surface area of M-NaY-2 was obviously enhanced compared with the M-NaY-1, and it was close to the value of C-NaY. The result indirectly demonstrated that the yield of NaY in M-NaY-2 was higher. It further confirmed the advantage of uniform contact between the solid phase and liquid phase in the dynamic crystallization method, which was in conformity with previous analysis.
3.4 Evaluation of catalysts
An equal amount of product was added in the experiment to evaluate catalysis ignoring the inert components that could be used as carriers. The results of the n-heptane hydrocracking reaction by different catalysts at 573 K are presented in Table 3. The conversion of M-Y was lower than that of C-Y, but the liquid yield was higher. The catalytic activity was closely related to the structure and acid sites of the molecular sieve in the catalyst. Because of the composite structure of M-Y containing a part of metakaolin with weak acid sites [10, 14], conversion rate decreased and the secondary cracking phenomenon was suppressed at the same time. Accordingly, the gas yield reduced and the liquid yield increased. However, the conversion rate of M-Y-2 was improved obviously contrastedwith M-Y-1 and was close to that of C-Y. The reason was that the reaction degree of n-heptane in the hydrocracking process was enhanced by increased acidity of the catalyst [22, 23]. Itfurther illustrated that the yield of zeolite Y by dynamic hydrothermal synthesis was higher. In addition, compared with product distribution of C-Y, C4-C6 production rate of M-Y-2 was relatively higher. This might be due to a higher content of weak acid sites in composite materials reducing the secondary cracking, and larger pore size that could make C4-C6 macromolecules pass in and out of the holes.
Catalysts | Conversiona(%) | YLb(wt%) | Cc (wt%) | ||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Product yield (mole fraction %) | C2 | C3 | C4 | C5 | C6 | C7 | |||||||||||||||
C-Y | 59.64 | 61.46 | 0.918 | 12.11 | 19.41 | 23.09 | 16.64 | 6.38 | 22.37 | ||||||||||||
M-Y-1 | 39.58 | 72.79 | 0.669 | 8.94 | 15.03 | 19.07 | 11.36 | 4.09 | 41.50 | ||||||||||||
M-Y-2 | 55.93 | 63.37 | 0.812 | 9.24 | 18.26 | 24.07 | 17.10 | 7.69 | 23.64 |
aConversion of n-heptane by Eq. (2).
bLiquid yield of products.
cCarbon deposition amount.
In principle, the catalyst does not change in the reaction and it can be restored to the original state after the catalytic reaction finishes, thus it can be constantly recycled. However, composition, structure and texture organization of the catalyst are changed as a result of the addition or loss of certain substances in the reaction. This will make the activity of the catalyst gradually decrease, and eventually lead to deactivation. Carbon deposition is one of the main reasons for catalyst deactivation. The formation of the carbon deposit correlates with total acid sites of the catalyst. It is generally believed that strong acid sites are conducive to the carbon deposition formation. The likely reason is that steady carbenium ions formed by strong acid centers on the catalyst surface are not easy to strip and then develop into the carbon deposition. It can be seen from Table 3 that the carbon deposition amount of M-Y was almost the same as that of C-Y. The comparison explained that the lifetime of zeolite Y synthesized with kaolin as a hydrocracking catalyst was capable of achieving the standard industrial product, which lays the foundation of feasibility for practical applications.
4 Conclusions
In this paper, the synthesis of zeolite Y was based on abundant, cheap kaolin as one of the main raw materials to reduce the production cost. Meanwhile, the synthetic process was improved to achieve the aim of reducing production time and increasing production rate. The synthetic product had feasibility of practical application through the analysis of characterizations and catalysis.
High temperature aging of structure-directing agents could save the aging time, but too high a temperature would lead to inactivation. The best aging temperature was 333 K, and aging time was 50 min. Zeolite Y with better performance was synthesized at 373 K in 24 h with guide agent by the dynamic crystallization method. Its relative crystallinity was 74.18%, specific surface area was 743.11 m2/g, and average pore diameter was 4.84 nm. The dynamic crystallization method contributed to uniform contact between the solid and liquid phase in the synthesis system. So, yield of zeolite Y in M-NaY-2 increased significantly compared with M-NaY-1, and conversion rate of n-heptane accordingly increased from 39.58% to 55.93%. C4-C6 yield of M-Y-2 increased slightly than C-Y owing to the weak acid and large aperture of metakaolin. Carbon deposition amount of M-Y-2 was almost the same as of C-Y, which showed that its service life could reach the standard of industrial products.
About the authors
Yuting Bai obtained a Master’s degree in 2014. She studied at the School of Metallurgy, Northeastern University in China for a Doctors degree, working on catalyst synthesis and application of rare earth elements.
Wenyuan Wu is a doctoral supervisor at Northeastern University. His main research has focused on green preparation technology of catalysts and efficient utilization ofresources.
Xue Bian is an Associate Professor at Northeastern University. His research has focused on the application of rare metal elements and green preparation technology of resources.
Acknowledgments
Financial support of this work was provided by the National Key Basic Research Program of China (no. 2012CBA01205). The authors also thank Yunnan Jinghong Wanxiang Kaolin Co., Ltd. located in the kaolin mining area of MengLong town, Jinghong City, who supplied the kaolin to conduct the experiments.
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