Preparation of MCM-41 supported phosphoric acid catalyst for thiophenic compounds alkylation in FCC gasoline
Introduction
New regulations aim to achieve very low sulfur concentration (less than 10 ppm by weight) that will be in full force in 2011. However, fluid catalytic cracking (FCC) gasoline represents almost 40% of the total gasoline pool and it is the major sulfur contributor to the environment, with up to 85–95%. Consequently, many approaches to deep desulfurization for ultra-clean gasoline have been proposed [1]. Olefinic alkylation of thiophenic sulfur (OATS) process as one of these approaches can be handled under relatively mild conditions with a minimal loss of octane number and without any hydrogen consumption, it can be seen as a good alternative to the conventional catalytic hydrodesulfurization process [2], [3].
Many publications have proposed different catalysts to catalyze the alkylation reaction of thiophenic sulfurs with olefins, such as zeolites (Hβ, HY, USY etc.) [4], [5], [6], [7] and supported heterogeneous acids [8], [9]. However, most of them were carried out in a simple simulation system which could not fully represent the complex real gasoline. Furthermore, the catalyst life was greatly reduced by products generated by side reactions (olefins polymerization or aromatics alkylation) in OATS process. Therefore, it is essential to evaluate the catalyst reactivity in real gasoline and choose carefully the catalyst nature to limit the secondary reactions and improve the catalytic stability.
A solid phosphoric acid (SPA) catalyst has been used industrially for polymer gasoline through the oligomerization of light olefins, owing to its higher acid strength, thermal stability and environmentally benign properties [10], [11]. Considering that olefins polymerization and thiophene alkylation are both acid catalyzed reactions. It seems to be potential to utilize it as the acidic catalyst in OATS process by improving preparation methods. Virginie et al. [12] synthesized a Grace Davison silica (specific surface area 320 m2/g) supported phosphoric acid with a loading of 11 wt.% (SPA-11) to evaluate its catalytic properties in the alkylation of 3-methylthiophene with 2-methyl-2-butene. SPA-11 appeared to be less active than USY zeolite but better selective towards monoalkylated products to reduce the olefins consumption.
In this work, we selected a purely siliceous MCM-41 zeolite as the support of SPA catalyst in view of its enormous specific surface area (954 m2/g), and aimed to obtain a MCM-41 supported phosphoric acid catalyst (SPAM), with the maximum catalytic activity for thiophene and its derivatives alkylation in FCC gasoline, by optimizing preparation conditions. And the optimal SPAM was compared with the previously studied HY zeolite by designed activity tests. Additionally, the active structures of SPAM under different conditions were studied by characterizations to study the effect of preparation on reactivity. And the relationship between the active structures and reactivity was also investigated, for it was beneficial for the further improvement and industrial application of SPAM in catalytic distillation process [13].
Section snippets
Catalysts and feedstock
The SPAM was prepared by impregnating the MCM-41 silica zeolite made by ourselves [14] (specific surface area 954 m2/g and mean pore diameter 3.54 nm) with a mixture of H3PO4 and H4P2O7 in an appropriate proportion. After impregnated at 353 K for 18 h, the admixture was dried at 473 K until it could be crushed into particles between 20 and 60 mesh. Finally, the catalyst used in this experiment was obtained by roasting the particles at a setting temperature for 4 h. HY zeolite (Si/Al = 8) was obtained
The distribution of sulfur compounds before and after catalyzed by SPAM
As shown in Fig. 1, the major sulfurs in FCC gasoline are thiophene (T), 2-methylthiophene (2MT), 3-methylthiophene (3MT), and small amount of tetrahydrothiophene (THT), C2- and C3-thiophenes. For the alkylation reaction will redistribute the sulfur compounds in gasoline fraction. Fig. 1 displays the distribution of thiophenic compounds after the activity test, revealing that most of sulfur compounds in the feed were transformed into much heavier formulation, except C3-T.
Moreover, the boiling
Conclusions
To obtain the excellent catalyst used in the alkylation reaction between thiophenic compounds and olefins in gasoline for the industrial desulfurization with catalytic distillation, the MCM-41 supported phosphoric acid catalyst must be prepared under the optimal conditions, namely, weight ratio of total acid/MCM-41 was 6:1, calcination temperature was 773 K, and weigh ratio of H3PO4/H4P2O7 was 1:2.5. The catalytic efficiency of the superlative SPAM was higher than 95% and there was just minimal C
Acknowledgement
This project was supported by the National Petroleum Company Limited of China (0603A010103).
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