Synthesis of hierarchical SAPO-11 for hydroisomerization reaction in refinery processes
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- Ma, Z., Liu, Z., Song, H. et al. Appl Petrochem Res (2014) 4: 351. doi:10.1007/s13203-014-0071-0
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A series of SAPO-11 molecular sieves with hierarchical structure (Meso-SAPO-11) were synthesized by adding certain amount of carbon particles. The co-existing micropore and mesopore feature of Meso-SAPO-11 was confirmed by N2 adsorption–desorption isotherm, TEM and SEM. XRD, TEM, SEM and Py-FTIR were employed to examine the crystallization, morphology and acidity properties of the resulting meso-SAPO-11 prepared from two typical kinds of carbon material and different contents of template. The hydroisomerization performance of meso-SAPO-11 as catalyst support and acid active site, with loading 0.5 wt% Pt as metal active site, was also tested to evaluate the mesoporous effects on catalytic activity and product selectivity.
KeywordsSAPO-11Hierarchical structureHard templateCarbon materialHydroisomerization
With the environmental regulations becoming more stringent and people’s environmental awareness getting stronger, the demand for clean production of gasoline, diesel and lubricants and other products from crude oil is increasing. However, because of that crude oil is becoming heavier and poorer in quality, the deep upgrading and full utility of heavy oil attract extensive attention and also become more of a disposal challenge. Hydroisomerization, as a technology to produce higher quality petroleum products, is getting more and more attentions. In the production of gasoline, the octane number of the product can be improved by hydroisomerization. In the production of diesel and lubricants, the pour point and viscosity nature of the product can be ameliorated by hydroisomerization, while maintaining the efficiency of carbon conversion and yield target oil product for maximum . The hydroisomerization catalyst is a kind of bifunctional catalyst, which includes both a metal and an acid active site. The acid function is usually provided by acidic support, such as amorphous oxide or oxide mixture (Al2O3, SiO2–Al2O3, ZrO2/SO42− treated by HF) [2, 3], aluminosilicate molecular sieves (Y, Beta, Mordenite, ZSM-5, ZSM-22) [4–6], or silicoaluminophosphate molecular sieves (SAPO-11, SAPO-31, SAPO-41) [7–10].
SAPO-11 molecular sieve, which has one-dimensional 10-ring pore structure and suitable surface acidity, has been widely used in refinery process because of the good stability and selectivity in alkane isomerization [11–15]. However, the application of traditional SAPO-11 is restricted by its microporous structure, especially for the long-chain alkane isomerization process. The reactant conversion and isomer product selectivity are significantly reduced by low mass transfer rate. As we know, hierarchical porosity by generating mesopores inside microporous molecular sieve is a feasible approach to reduce the mass transfer resistance of the reactants and construct more reactive sites inside microporous molecular sieve, furthermore to improve the reactivity . Therefore, SAPO-11 with hierarchical structure (meso-SAPO-11) has been attracted more and more attention [17, 18].
Template method is commonly used to generate mesopores inside crystallized molecular sieve [19, 20]. The template commonly used to construct hierarchical structure is organic macromolecular surfactant, i.e. soft template strategy. However, specially designed organic macro molecular surfactant used to act as soft template is usually expensive, which restricts to be used in large scale, especially for industrial application. The account relative to soft template is hard template strategy, which uses a structural rigid organic or inorganic material as template. Normally, carbon material is a best choice as hard template for aluminosilicate or silicoaluminophosphate synthesis, which could be removed by simple thermal treatment in the presence of oxygen or air. As an example, hard template strategy has been applied to produce hierarchical structured zeolite, such as macroporous MFI zeolite crystals by applying polystyrene beads  as the macropore template or nanosized ZSM-5 crystals by carrying out the crystallization in the confined space of amorphous carbon black . However, little research has been reported to construct hierarchical structure in SAPO-11 molecular sieve using hard template . Herein, an approach to synthesize SAPO-11 molecular sieve with hierarchical structure (Meso-SAPO-11) using carbon nanoparticles as hard template is demonstrated. The effects of carbon template on the mesoporosity, crystallization, morphology and acidity properties of the resulting molecular sieve are examined with N2 adsorption/desorption isotherm, XRD, TEM, SEM and Py-FTIR. The catalytic performance of the as-synthesized Meso-SAPO-11 on n-dodecane hydroisomerization are investigated to evaluate the mesoporous effects on catalytic activity and products selectivity.
Synthesis of hierarchical SAPO-11 and catalyst
Based on the preparation method of traditional SAPO-11, aluminum isopropoxide, phosphoric acid and acidic silica sol were chosen as aluminum source, silicon source and phosphorus source, respectively. In a typical synthesis of hierarchical SAPO-11, the precursors were dispersed into proper amount of distilled water in a certain order under vigorously stirring. The mixture of di-n-propylamine and di-iso-propylamine was then slowly added into the solution as microporous template (SDA). The pH value of the resultant gel was adjusted to 6.86 by sulfuric acid after sufficient stirring. Carbon nanoparticles, which act as hard template, were added under vigorous stirring. The gel was then transferred into a stainless steel autoclave lined with polytetrafluorethylene, and crystallized at 473 K for 24 h. Finally, the products were washed, dried at 373 K for 12 h and calcined at 873 K for another 12 h to remove the template.
Carbon material templates used in the experiment are commonly used in industry, for instance, carbon black (Degussa Company, FW200, denoted as FW200), conductive carbon black (Carbot Company, XC72R, denoted as Carbot), carbon nanotubes (denoted as CNTS) and graphitic carbon (denoted as Graphite).
Bifunctional Pt/meso-SAPO-11 catalyst was prepared by incipient-wetness impregnation method. Chloroplatinic acid solution, as a source of platinum, was prepared according to 0.5 wt% Pt loading amount. Then, chloroplatinic acid solution was added into the pretableted meso-SAPO-11 in 20–40 mesh. The as-prepared meso-SAPO-11 with Pt precursor was deposited for 2 h. Finally, the products were dried at 373 K for 2 h and calcined at 753 K for another 4 h to transform into the final bifunctional Pt/meso-SAPO-11 catalyst.
The crystallinity of the prepared SAPO-11 were analyzed by XRD, which was carried out on X’Pert PRO MPD (Netherlands, Cu target, Kα radiation λ = 0.1542 nm, 35 kV, 40 mA), using molecular sieve phase analysis method, scan angle of 5°–45°. The pore structure and morphology of molecular sieves were characterized by TEM, which was carried out on JEOL-2010 with acceleration voltage of 200 kV. The particle morphology and particle size of the samples were observed by SEM, which was carried out on S-4800 (Hitachi Company, acceleration voltage of 0.5–20 kV). N2 adsorption/desorption isotherms were recorded at 77 K with a Micromeritics TriStar 3000. The total surface area of the samples was calculated by BET method, the mesopore surface area and pore volume were calculated by BJH method based on the N2 desorption branch, and specific surface area of micropores was determined by substraction of total specific surface area from mesoporous one. The surface acidity analyses were carried out on a NEXUS FTIR (Thermo Fisher Scientific Company).
N-dodecane hydroisomerization was conducted in a continuously flowing tubular fixed-bed micro-reactor at ambient pressure with 1.2 g catalyst loading. The catalyst was activated in situ in an H2 flow at 673 K for 4 h prior to reaction. The reaction temperature was 603 K at the WHSV feeding condition of 1.5 h−1 with H2/n-dodecane molar ratio of 25. Products were analyzed on-line by a Agilent 7820A GC (HP-PONA capillary column size of 50 m × 0.2 mm, FID detector). The reaction conversion, selectivity and yield were calculated by the program of GC.
Results and discussion
Short survey on carbon template
Pore structure properties of meso-SAPO-11 by different carbon templates
According to N2 adsorption/desorption isotherms of meso-SAPO-11, all the meso-SAPO-11 samples present type-IV isotherm and H3 hysteresis loop according to the IUPAC classification, which indicate that samples have the characteristics of both microporosity and mesoporosity. According to the pore size distribution of meso-SAPO-11 by different carbon templates, all the samples possess abundant of mesopores concentrated at 10–30 nm. The pore size at 3.8 nm is artifact because of the Tensile Strength Effect for a forced closure of the H3 hysteresis loop  The hierarchical structure of as-synthesized SAPO-11 is more remarkable when taking Carbot as hard template. N2 adsorption/desorption isotherm of the sample rises most significantly at high relative pressure P/P0, and the jump segment of its hysteresis loop is the steepest. Correspondingly, the result of Table 1 shows that both the mesoporous specific surface area and pore volume of the meso-SAPO-11 templated by Carbot are relatively the largest, indicating a large content of mesopore.
Effects of template content
Pore structure properties of meso-SAPO-11 by different FW200 content
Pore structure properties of meso-SAPO-11 by different Carbot content
Catalytic evaluation of meso-SAPO-11
Isomerization evaluation of meso-SAPO-11 templated by different FW200 content
Isomerization evaluation of meso-SAPO-11 templated by different Carbot content
The SAPO-11 with hierarchical structure (Meso-SAPO-11) was successfully prepared with the aid of carbon materials as hard template. Compared with the traditional SAPO-11, both the mesoporous specific surface area and volume of meso-SAPO-11 were increased significantly, while the crystal structure was properly preserved. After brief survey on different kinds of carbon materials, it was found that both FW200 and Carbot nanoparticles could be used to generate hierarchical structure effectively, by which 10 wt% FW200 and 15 wt% Carbot showed the best nature of mesoporosity. The increase of mesoporosity could significantly improve the hydroisomerization performance. Correspondingly, the meso-SAPO-11 by 10 wt% FW200 and 15 wt% Carbot showed the best performance on n-dodecane isomer product yield.
This work was supported by Science and Technology Development Project of CNPC (No. 11-13-01-05); National Science Foundation China (U1362202 and 21206196), Innovation Foundation of CNPC (2013D-5006-0404), and Funds for Distinguished Young Scientists of Shandong Province (BS2012NJ013).
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