Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Published by Elsevier B.V.
Hydroisomerization and hydrocracking of long chain n-alkane and Fischer-Tropsch wax over bifunctional Pt-promoted AI-HMS catalysts Yanyong Liu, Toshiaki Hanaoka, Kazuhisa Murata and Kinya Sakanishi Biomass Technology Research Center, National Institute of Advanced Industrial Science and Technology (~lIST), Hirosuehiro 2-2-2, Kure, Hiroshima 737, Japan
In this manuscript, the hydroconversion of long chain n-alkanes
n-C36) and Fischer-Tropsch waxes (including hydroisomerization and hydrocracking) over bifunctional Pt-promoted and Al-containing hexagonal mesoporous silica (HMS) were investigated. A1/HMS prepared by a post-modified method showed stronger acidity than that of AI-HMS prepared by a sol-gel method. Because the n-alkane hydroconversion undergoes via a bifunctional mechanism, the balance of Pt sites and acid sites in Al-containing HMS is crucial for the catalyst performance and final product distribution. 1. Introduction
Fischer-Tropsch (FT) synthesis produces clean n-alkanes (> 90%) from syngas, while the latter is easily obtained from biomass, coal and natural gas. As consequence of the chain growth mechanism, a large fraction of FT products has a boiling point higher than 370°C (C22+, i.e. FT waxes). Thus hydroisomerization and hydrocracking of FT waxes are necessary for improving the yield and the quality of middle distillates. Mesoporous silica possesses high thermal stability (up to 850°C), large surface area (above 1000 m2g-1) and uniform-sized pores (about 40 A). Moreover, hexagonal mesoporous silica (HMS) has thicker framework walls, small crystallite size of primary particles and complementary textural porosity [1, 2]. These advantages render HMS-based materials very interesting in the catalysis filed as catalysts and supports [3-7].
782 2. Experimental Section A1-HMS was prepared by a sol-gel method . The mixture of Al(iso-OC3H7)3 in isopropyl alcohol and Si(OC2H5)4 in ethanol (AI: Si = 1/100 (molar ratio)) heated with vigorous stirring at 70°C for 4 h, and then the solution was added to a dodecylamine water/ethanol solution. The resultant gel mixture reacted at room temperature for 20 h. Then the solid products were filtrated, air dried and calcined in air at 650°C for 4 h to remove the template. A1/HMS was prepared by a post-modified method. The HMS support after calcination at 650°C for 4 h was impregnated with AI(iso-OC3H7)3 in isopropyl alcohol (AI : Si = 1/100 (molar ratio)), following by adding 25 ml H20 to precipitate aluminum oxide. Then the solid products were filtrated, air dried and calcined at 650°C K for 4 h. The 0.5 wt% of Pt was loaded on A1-HMS and A1/HMS using Pt(NH3)4C12 solution. After impregnation, the product was dried at 110°C for 5 h, calcined at 400°C for 2 h, and reduced by flowing H2 at 350°C for 1 h. The hydroconversion reaction was carried out using a 50 ml stainless steel autoclave reactor. The catalyst and long chain n-alkanes or F-T waxes were added into the reactor. The reaction started at the range from 250 to 350°C (mainly at 300°C) with stirred speed at about 300 rpm. After reacted, the products were analyzed by gas chromatographys. 3. Results and Discussion In XRD pattern (Fig. 1), each HMS-based sample exhibits an intense reflection corresponding to the (100) plane at 2-3 degrees. The d~00 spacing calculated from the degree of (100) plane was 35.3 A for HMS, and the value of
° v,,,~ (D
A1-HM S I
4 6 8 1 0 2 10 4 6 8 Pore diameter/A 20/degree Fig. 1. XRD pattern and pore-size distribution of various samples after calcination.
783 dloo spacing decreased by incorporating A1 in the HMS framework (A1-HMS). On the other hand, A1/HMS showed a dl00 spacing similar to that of HMS. The BET surface areas were over 1000 m 2 g-r for all HMS-based simples. The average pore sizes were about 26 A for HMS and about 24 A for A1-HMS. Introducing A13+ into the framework of HMS caused the reduction of pore size. In the FT-IR spectra, HMS exhibits no band at around 960 cm -~ [ 1], while A1-HMS exhibits a band at 960 cm -~. Because this band has been widely used to characterize the incorporation of metal ions in the silica framework as the stretching Si-O vibration mode perturbed by the neighboring metal ions, A13+ ions entered into the HMS framework in A1-HMS prepared by sol-gel method. A1/HMS did not show a band at -~ A1/HMS 960 cm -~, indicating A13+ ions in =o A1/HMS prepared by postmodified method exist at the A1-HMS outside of the HMS framework. In the NH3-TPD profiles (Fig. HMS 2), HMS did not show any peaks. Both the desorption amount and i I I I I the maximum temperature of 600 100 200 300 400 500 AI/HMS were much higher than Desorption temperature/°C those of AI-HMS, suggesting that AI/HMS contains much more and Fig. 2. NH3-TPD profiles of various stronger acid sites comparing to samples after calcination. A1-HMS. AI 3+ located uniformly in the HMS fremework in L" Lewis acid site A1-HMS since A13+ was B" Bronsted acid site introduced in the preparing step of hydrogel. On the other hand, A13+ L L introduced in A1/HMS by postB L+B modified mainly existed at the ,.Q extra-framework. The extrao'1 framework A13+ ions are strong acid sites while the intraframework A13÷ ions are weak acid sites. FT-IR spectra of chemisorbed 1650 1600 1550 1500 1450 1400 pyridine (Fig. 3) indicate that both Wave number/cm -~ A1-HMS and A1/HMS possess two types of acid sites: Lewis acid Fig. 3. FT-IR spectra of chemisorbed sites and Bronsted acid sites. The pyridine over (a) A1/HMS and (b) AI-HMS after treated at 473 K for 1 h. intensities of bands for chemi-
784 sorbed pyridine over A1/HMS were much stronger than those over AI-HMS, indicating that the acidic strength of A1/HMS is stronger than that of AI-HMS with the same A1 amount, coinciding with the results obtained from the NH3-TPD. While using Pt/AI-HMS and Pt/AI/HMS as catalysts for the hydroconversion of long chain n-alkanes (r/-C16, r/-C28, n-C36), at the same conversion level, higher reaction temperatures lead to the cracking products with a lower ratio of iso-alkane/n-alkane. The rate constants showed a considerable increase between r/-C16 and n-C28, whereas a slight decrease between n-C28 and r/-C36 was observed. The increase in reaction temperature leads to a small decrease in isomerization selectivity. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C28 conversion was observed. Pt/A1/HMS is effective for hydroisomerization of n-C16. As for the hydrocracking of FT wax over Pt/AI-HMS, although the selectivity to naphtha distillate increased with increasing the conversion of C22+, the maximum yield achieved to middle-distillate was obtained over 70% over 0.5wt% Pt/AI-HMS (A1/Si = 1/100 (molar ratio)). The hydroconversion (including hydrocracking and hydroisomerization) of n-alkanes over Pt/A1-HMS and Pt/AI/HMS undergoes a bifunctional mechanism . Pt site achieves the function of dehydrogenates and hydrogenates and acid site achieves the function of isomerization or cracking. The balance of the acid catalyst and dehydrogenation/hydrogenation catalyst is very important for preparing a bifunctional catalyst for the hydroconversion of n-alkanes . Moreover, hydrocracking and hydroisomerization of n-alkane are competitive reactions which occur in parallel sharing a common intermediate (carbenium cation). Pt sites decide the amounts of carbenium cation in the system. Whether the carbenium cations absorbed on the acid sites undergo an isomerization process or undergo a 13-scission process, determined the occurrence of hydrocracking or hydroisomerization of n-alkane. The carbenium intermediate is not stable and cannot exist in the system for a long time. If strong acid sites exist in the system, the n-carbenium can be isomerized to iso-carbenium quickly Thus it is reasonable to assume that strong acid sites would promote n-alkane isomerization and weak acid sites would promote n-alkane cracking. As shown in NH3-TPD profiles (Fig. 2), A1-HMS possesses weak acid sites because A13÷ ions entered the framework of HMS and AI/HMS possesses strong acid sites because AI 3+ ions existed at the outside of HMS framework, which causes that Pt/A1-HMS is a good catalyst for hydrocracking of n-alkanes while Pt/AI/HMS is a good catalyst for hydroisomerization of n-alkanes. 4. References
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