Studies Of Synthesis Of Medium Pore SAPO Molecular Sieves And Their Effect On Hydroisomerization Of N-Octane Over Pt Loaded Catalyst

In 1990s, Pt/SAPO-11 for isomeization dewaxing was successfully developed by Chevron Co., that changed traditional process for lubricating oil. In the bifunctional catalyst, SAPO-11 molecular sieve with medium strength acid and monodimentional pore system plays an important role. Now, there still are different explanations for high isomeization selectivity of long chain paraffins over Pt/SAPO-11. It is important for developing new isomerization catalyst to explore the cause of high isomeization selectivity of Pt/SAPO-11. Hydroisomerization of n-octane over some molecular sieves representative of different acid strength and channel structure were studied in order to clarify characteristics of the molecular sieves that can exhibit high selectivity to isomerization of long chain paraffins, the synthesis methods of medium pore SAPOs (SAPO-11, -31, -41) were also developed for industrial manufacture. The main contents of this paper are as follows:1. Effects of channel structure and acidity of molecular sieves on hydroisomerization of n-octane.SAPO-5, SAPO-11, SAPO-31, SAPO-41, SAPO-34, ZSM-5, ZSM-22, ZSM-23 and MCM-22 were synthesized, and crystalline phase, crystallinity, crystalline morphology, elemental composition, acidity of these molecular sieves were characterized by XRD, SEM, XRF, NH3-TPD, Py-FTIR. Hydroisomerization of n-octane over these molecular sieves loaded with 0.5 wt % Pt were examined. Firstly, the effcts of pore size and channel shape of molecular sieve on hydroisomerization of n-octane were determined. It is shown that hydroisomerization of n-octane is not ideal over the large pore molecualr sieve (such as SAPO-5) with 12-member ring pore and the small pore molecular sieve (such as SAPO-34) with 8-member ring pore, hydrocracking of n-octane is also obvious over the medium pore molecular sieve containing zigzag channel system (such as ZSM-5) and cage space in channel system (such as MCM-22). The molecular sieves containing monodimentional channel system (such as SAPO-11, -31, -41, ZSM-22, -23) exhibt good isomerization performance of n-octane, over which high isomerization selectivity (>90%) is obtained until high n-octaneconversion (-80%). In the monodimentional medium pore channel, the formation of dibranched isomers and multibranched isomers susceptible of cracking is restriced, and diffusional restriction of monobranched isomers is in the absence, so high isomerization selectivity is obtained owing to lower occurance of 3 scission of isomers. In addition, the effect of acid strength on hydroisomerization of n-octane was determined through comparision between medium pore SAPOs and medium pore zeolites, it is shown that the strong acid sites over medium pore zeolite do not result in acceptation of 3 scission. So it is further demonstrated that the channel shape of molecular sieves plays a key role in isomerization selectivity of n-octane, and acid properties have close relations with n-octane conversion activity. On the basis of above results, it is proposed that according to the curve concerning isomerization selectivity varied with n-octane conversion and the ratio of normal paraffins to its isomers in cracking products as n-octane conversion of about 70%, the information concerning inchannel shape of medium pore molecular sieves can be obtained, such as straight shape, zigzag shape or channel shape containing cage. 2. The study on synthesis of SAPO-11 for hydroisomerization of n-octane.On the basis of the standard crystalline conditions described in the paper, the effects of crystallization temperature, crystallization time, type of Al source, content and type of Si source, content and type of template on synthesis of SAPO-11 and its isomerization performance were studied. In the synthesis system, crystallization temperature of 170°C and crystallization time of 48h are suitable to high crystallinity SAPO-11. If crystallization temperature was 190°C, either SAPO-5 or SAPO-11 would be of frequent occurrence. By comparision with Gibbsite, aluminum isopropoxide and Pseudo-Bohmite are two types of active Al source for synthesis of SAPO-11. The n-octane conversion activity over the SAPO-11 synthesized with Pseudo-Bohmite is higher than the SAPO-11 synthesized with aluminum isopropoxide, it is possible to be due to difference of aggregation situation and morphology between two samples. The number of acid sites over SAPO-11 can be enhanced by increasing silica content in the gels when fumed silica is used as Si source. By comparision with fumed silica, the range of Si content in the gels is narrower when Si(OC2H5)4 and silica sol are used as Si source, because high Si content in gels can lead to formation of SAPO-11 with lower crystallinity. In five typical dialkylamines with different alkyl chain, diethylamine, dipropylamine and diisopropylamine have been shown to be effective template for synthesis of SAPO-11. Compared with dipropylamine, diethylamine and diisopropylamine are moresusceptible to Si content in the gels.3. The study on synthesis of SAPO-31 and SAPO-41 for hydroisomerization of n-octane The SAPO-31 was successfully synthesized wih di-n-hexylamine as new template. The starting materials were fumed silica, aluminum isopropoxide, orthophosphoric acid. The gels had the following composition expressed in molar ratio: 1.0Al2O3l.05P2O5l.0SiO2-1.0Template-35H2O. The two SAPO-31 samples respectively synthesized with di-n-hexylamine and dipropylamine were characterized by XRD, SEM, XRF, 29Si CP/MAS NMR, NH3-TPD, it is found that when di-n-hexylamine is used as template, other phases (such as SAPO-11) at final products and transformation of SAPO-31 into SAPO-11 during synthesis are suppressed, and higher isomerization selectivity is obtained. In addition, the SAPO-41 was synthesized in mixed solvent of H2O and ethanol. The starting materials were Si(OC2Hs)4, Pseudo-bohmite, orthophosphoric acid. The gels had the following composition expressed in molar ratio: 1.0Al2O3l.0P2O50.4SiO2-1.0Template-8.0EtOH-40H2O. By comparision with the conventional synthesis method of SAPO-41, the content of template in the gels is reduced and the content of Si in SAPO-41 is enhanced in the mixed solvent method.4.Effects of reaction conditions on n-octane hydroisomerization over Pt/SAPO-11 On the basis of the standard reaction conditions described in the paper, the effects of the four reaction parameters on n-octane hydroisomerization were studied, i.e., the reaction pressure (0.5~2.0Mpa) , H2/n-octane molar ratio (0.125-2.0) , the reaction temperature ( 270-365 °C ) , WHSV of n-octane (0.5-4.0 h"1) . The results indicate that the increase of the reaction pressure and IVn-octane molar ratio are beneficial to good stability of Pt/SAPO-11 but not beneficial to n-octane conversion activity, higher reaction temperature and lower WHSV of n-octane can result in increase of n-octane convertion. Pt/SAPO-11 exhibits good stability during the on-stream time of 400 h under the following reaction conditions: the reaction temperature of 340 °C, the reaction pressure of 1.5Mpa, tt/n-octane molar ratio of 1.0, WHSV of n-octane of 1.5h"'. Distribution of products over Pt/SAPO-11 is explained by taking advantage of thermodynamic equilibrium and size of product molecules.