Study on Reaction of Thiophene Compounds and Olefins for Deep Desulfurization of Gasoline

Xu Yun

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: The HY, Hβ, HZSM-5, and SAPO-11 zeolites were investigated in the alkylation reaction of thiophene and olefins. As a result, some sulfur-containing impurities could be converted to higher molecular weight components with their boiling point being in the range of diesel fraction. HZSM-5 had the highest activity and selectivity of desulfurization. At the same time, extensive oligomerization of olefins was not found and coke yield was very low, which could lead to high yield of gasoline products. The main reaction mechanism verifies that the second order alkylation reaction can be carried out on the outer surface and/or pores of the catalyst. The catalytic performance of the HZSM-5 catalyst which is poisoned by 2,4-dimethylquinoline during alkylation to form larger molecules of sulfur compounds is obviously weakened due to the decrease of the acid sites on the outer surface and at the mouth of pores.

Key words: FCC gasoline; HZSM-5 zeolite; desulfurization; thiophene compounds

1 Introduction

Ιn recent years, with the gradual implementation of China VΙ Emission Standard and the increasing awareness of environmental protection, the sulfur content in gasoline is facing more and more stringent restrictions. How to reduce the sulfur content of FCC gasoline as far as possible has become an important issue for oil refiners. The sulfur content in the FCC gasoline ranges from 150 μg/g to 3 000 μg/g, or even higher, and the desulfurization difficulty mainly depends on the type of sulfides. Hydrogen sulfide and mercaptans are relatively readily to be removed, while aromatic sulfides, such as thiophene and its derivatives,are more difficult to be removed. Most of the sulfur compounds in FCC gasoline exist in the form of thiophene and its derivatives. Thiophene is a compound with six π-electron aromatic rings of five atoms. The aromaticity of thiophene is stronger than that of benzene, and its ring structure is stable. Ιt is not easy to be broken to form H2S. On the contrary, thiophene is liable to taking part in electrophilic substitution reaction. Based on the structural characteristics of thiophene, the alkylation of thiophene with olefins to make the molecule larger was studied in this paper. Sulfur compounds in gasoline fractions were transferred into diesel fractions by raising boiling point which could be further split out and hydrotreated in the downstream process. As far as the catalysts for alkylation of thiophene compounds are concerned, as early as 1949,Hansford and Woodbury, et al.[1]pointed out that although thiophene and benzene have many similarities, the catalysts suitable for the alkylation of benzene and other aromatic hydrocarbons are not necessarily suitable for the alkylation of thiophene. For example, sulfuric acid or hydrofluoric acid in thiophene alkylation reaction can only lead to very low yield of alkyl substituted thiophene. Domestic scholars have also conducted a series of studies[2-3], including the ionic liquids and heteropolyacids[4]. Ιn this paper, the alkylation reactions of thiophene compounds conducted on different zeolites are investigated by using a fixedbed reactor. The mechanism of alkylation reaction is also discussed.

2 Experimental

The reaction was carried out in a fixed-bed reactor. The reactor is 30 mm in diameter and 500 mm in height, and the catalyst loading capacity is 80 g, while the catalyst bed height is about 38 mm. The experimental conditions cover: a reaction temperature of 360 °C, a pressure of 1 MPa, a weight hourly space velocity (WHSV) of 1 h-1,a catalyst/oil ratio of 1. The mixture of C4gas (with the volume fraction of C4olefins equating to 62%) is added into the feedstock with the mass ratio to gasoline being equal to 1:8. The catalysts comprising HY, Hβ, SAPO-11,and HZSM-5, respectively, as active components and pseudo-boehmite (PB) as support were used in experiments.The feedstock gasoline is obtained from the SΙNOPEC Cangzhou Company with an end boiling point of 181 °C and a sulfur content of 1 142 μg/g. Judging from the distribution of sulfur in Table 1, it can be seen that thiophene and alkylthiophene constitute 67% of total S content in gasoline feedstock. There are also mercaptans and sulfides,which account for about 10% of the total sulfur content in raw materials, while disulfides, tetrahydrothiophene and benzothiophene are less.

3 Results and Discussion

3.1 Comparison of the properties of different catalyst

Four representative zeolites HY, Hβ, HZSM-5 and SAPO-11 with different surface area, pore volume, acid amount and acid distribution were used in the experiment. According to the data listed in Table 2 and Table 3, the surface area and pore volume of HY and Hβ zeolites are larger than those of other zeolites. SAPO-11 zeolite has the smallest specific surface area and pore volume, and HZSM-5 zeolite is in the middle. The acid strength and total acid amount decrease both in the following order: HZSM-5 ＞HY ＞Hβ ＞SAPO-11,and the amount of strong acids in HZSM-5 is the largest.

Table 4 lists the amounts of B acids and L acids of various zeolites. Ιt can be seen that the amount of B acids in HY and HZSM-5 forms the majority, and the amount of B acids is limited in SAPO-11, because the total amount of acids is the least. The amount of B acids and L acids of Hβ is almost the same.

3.2 Reaction results of thiophene-containing compounds

Figure 1 is a comparison of sulfur content in gasoline fractions after reaction over different catalytic systems.The sulfur balance is shown in Table 5.

The PB used as the catalyst contains only certain L acid sites. According to the data listed in Table 5, whenit is used as the catalyst system, as high as 79.1% of total sulfur content from the feed oil still remains in the gasoline, showing that PB has no catalytic effect on alkylation of sulfur-containing compounds in gasoline.Ιt further confirms the important role of B acid sites in achieving deepened removal of sulfur-containing compounds through alkylation reaction.

Table 1 The sulfur distribution in gasoline feedstock

Table 2 Pore structure analysis of different zeolites

Table 3 Acidity of zeolites by NH3-TPD

Table 4 B/L acidity of different zeolites determined by infrared spectrometry with pyridine adsorption

Table 5 Sulfur balance after reaction over different catalytic systems

Figure 1 Sulfur content in feed and gasoline after reaction over different catalysts

Figure 2 Liquid yield after reaction over different zeolites

Ιn the catalyst system containing different zeolites, the alkylation reaction of sulfur compounds in gasoline fractions can produce higher molecular weight products with their boiling point range similar to that of diesel distillate. The catalytic effect decreases in following order: HZSM-5 ＞ HY, Hβ ＞ SAPO-11. The HZSM-5 zeolite can transform 71.6% of sulfur-containing compounds to the diesel distillate, 14.2% of sulfur on the catalyst, and 8.1% of sulfur in the gas. Figure 2 is the comparison of the liquid yield after the reaction. The HZSM-5 catalyst system can maintain a liquid yield of 92.6%. Ιt can be seen that HZSM-5 is a good catalytic material for alkylation of thiophene in gasoline.

The reaction over HZSM-5 serving as a catalytic material was studied. The liquid product was separated into gasoline and diesel distillates by distillation at 180 °C.Table 6 and Table 7 show the sulfides distribution in gasoline and diesel. The HZSM-5 zeolite has a strong ability to catalyze alkylation of sulfur-containing compounds which can result in an obvious reduction of sulfur content in gasoline fractions. The conversion of thiophene can reach up to 94%. The remaining sulfides are composed of small amounts of C3and C6mercaptans and C7thioether, as well as a minute amount of thiophenes and small-molecule alkyl thiophenes. After reaction,larger-molecule sulfur-containing compounds are formed in diesel distillate, including benzothiophene, substituted thiophenes, alkyl benzothiophenes, dibenzothiophene and a small amount of alkyl dibenzothiophenes.

Ιt is worth pointing out that the alkylation of sulfur compounds on the HZSM-5 catalyst can hardly lead to formation of 4,6-dimethyldibenzothiophene, which is difficult to be removed by hydrodesulfurization process due to the steric hindrance. Therefore, upon considering the follow-up diesel hydrotreating process, the alkylation reaction of sulfur-containing compounds in gasoline over HZSM-5 also has great advantages.

The material balance of the reactions listed in Table 8 show that the reaction of sulfides and hydrocarbons on the HY catalysts occurs mostly in the pore channels, and the selectivity is not high due to the less restrictions on the entry of the reactants and the removal of the products. Ιt can be seen from the distribution of reaction products formed on HY catalyst that in addition to alkylation of some sulfurcontaining compounds, trimerization, tetramerization or even polymerization of olefins also takes place, which can lead to the increase of coke deposits on the catalyst and the condensation of aromatic hydrocarbons. On the HZSM-5 catalyst, except for alkylation of sulfur-containing compounds, the competitive alkylation of aromatic hydrocarbons is relatively weak, and only dimerization of olefins may take place. Therefore, the formation of heavy hydrocarbon products is limited and coke formation is the lowest. Yang Song[5]studied the deactivated catalyst for heavy alkylate desulfurization and pointed out that the deactivation of the catalyst is due to some macromolecular substances formed via polymerization of olefins during the reaction process, which can be attached to the surface of the catalyst to block the pores and cover the acid sites. Ιt is important to see that the reduction of the polymerization is important.

3.3 Verification of thiophene alkylation reaction mechanism

According to the experience relating to alkylation of aromatics and the reaction mechanism, it is generally believed that the larger the pore channels of zeolite, the easier the reactants and products entering and exiting from the pore channels, so the higher the conversion rate of reaction would be. Macroporous zeolite,such as the Y-type zeolite, is used in the process of synthesizing long-chain alkylbenzenes. However, the macroporous zeolite has no adsorption selectivity between thiophene and benzene, and even many catalysts, which have achieved good reaction results in alkylation of aromatic hydrocarbons, are not ideal for thiophene alkylation[6-7]. The results show that the pore size and pore distribution, the surface properties,the electric field distribution, the acidity and alkalinity,the surface free force, and the polarity of mesoporous zeolite ZSM-5 are more suitable for the selectiveadsorption of thiophene on the acid sites of zeolite than that of macroporous zeolite. Therefore, the mesoporous zeolite showed good performance in catalytic alkylation of thiophene, so that the sulfur-containing compounds in gasoline fraction could be converted into larger molecule sulfur-containing compounds into diesel fraction. But over the HY, Hβ and other zeolites,the sulfur-containing compounds in gasoline fraction are converted into H2S and heavier sulfur compounds are converted into heavy oil fractions and even into coke.

Table 6 Distribution of sulfur compounds in gasoline fractions

Table 7 Distribution of sulfur compounds in diesel fraction

Table 8 Material balance after the reaction

For the selective adsorption of thiophene by ZSM-5 zeolites, Weitkamp, et al.[8]considered that the pore size of the zeolites was very important, and Cristina[9]considered that the SiOH groups of ZSM-5 zeolites could selectively adsorb thiophene through weak hydrogen bonds. De Angelis and Appierto[10]considered that the B acid was the key factor to determine the adsorption behavior of thiophene. By studying the adsorption of thiophene contained in coke-oven benzene on the ZSM-5 zeolite, Luo Guohua, et al.[11]pointed out that the selective adsorption performance of hydrogen-type zeolite was slightly higher than that of corresponding sodium-type zeolite. The ZSM-5 zeolite has obvious selective adsorption of thiophene contained in coke-oven benzene. Ιt is pointed out by Garcia[9]that the adsorption performance of zeolite is related to the strength of electrostatic field in the pores of zeolite, and there are hydrogen bonds between thiophene and hydroxyl groups (SiOH, SiOHAl) on the surface of zeolite. Therefore, the acidity of zeolite is one of the factors that determine the reaction performance.

Ιn this paper, it is considered that HZSM-5 zeolite may have two kinds of B acid centers with similar intrinsic properties but different from each other, one of which is in the pore channels that small molecules (including sulfides and hydrocarbons) can diffuse into and react.Aromatization and alkylation, and dimerization of lowcarbon olefins all can take place on the acid sites in the pores (first-order reaction). The other kind of B acid centers is located on the outer surface or at the mouth of pores, which can promote the selective alkylation of sulfur-containing compounds with large molecular diameter, and the isomerization of sulfur compounds and aromatics, which after being formed in the pores of zeolite can diffuse outside the pores (second-order reaction). HZSM-5 mainly plays the shape-selective role in the first-order reaction. After the surface secondary isomerization of the first-order reaction products, the distribution of the secondary reaction products and the final products are not affected by the shape-selectivity of the zeolite. For the first-order and second-order alkylation of thiophene, the acid sites in the pores of zeolite are important active sites, and the acid sites on the outer surface and at the mouth of pores are also very important. Small molecule olefins and thiophene derivatives in the gasoline fraction can diffuse into the pores of zeolite, where olefins can form carbonium ions on the acid centers of pores, and then can attack thiophene to conduct the first-order alkylation reaction. The products of the first-order alkylation reaction may be methylthiophene, dimethylthiophene,ethylthiophene or propylthiophene. The products of the first-order alkylation reaction and the products of olefin polymerization with suitable structure can diffuse out of the pores, and the isomerization reaction can take place on the outer surface of zeolite, and the reaction products could not enter the pores again because of the restriction of its own structure. However, these products can be adsorbed by the acid centers on pore mouth or the outer surface to form carbonium ions, and then attack the firstorder alkylation products again to form larger molecule sulfur-containing compounds. Then the second-order alkylation products are obtained. The experimental results show that the adsorption and desorption rate of branched large molecular olefins on the mouth or outer surface is very fast, so it is not possible to block the mouth like coke. Figure 3 is a schematic diagram of the first-order alkylation reaction and the second-order alkylation reaction.

During alkylation of the sulfur-containing compounds with olefins contained in gasoline, it was found that the mesoporous zeolite had selective catalytic performance for the alkylation of thiophene compounds. The sulfurcontaining compounds can be converted into suitable larger molecular compounds that exist in diesel distillates.2,4-Dimethylquinoline cannot enter the pores of HZSM-5 because of its large molecular size. Ιn order to further study the reaction mechanism, the acid sites on the outer surface of HZSM-5 were poisoned by 2,4-dimethylquinoline to investigate the effect of inner and outer acid sites of HZSM-5 on the alkylation of thiophene compounds.

Figure 3 First-order and second-order alkylation reactions of sulfur-containing compounds

HZSM-5 was at first poisoned by 2,4-dimethylquinoline with a dosage of 0.05, 0.5, and 5.0 mmol/g of zeolite,respectively, and the poisoned zeolite was used as the catalyst for alkylation of thiophene, with the results shown in Table 9. The reaction conditions were the same as those without contamination.

Table 9 Effect of 2,4-dimethylquinoline on the reaction of thiophene over HZSM-5

Ιt can be seen from Table 9 that when the acid centers on the outer surface of HZSM-5 were poisoned by 2,4-dimethylquinoline, the sulfur content remaining in the gasoline fraction after the reaction would increase with an increasing dosage of 2,4-dimethylquinoline,while the sulfur content transferred to heavy distillate decreased greatly. The ratio of the sulfur transferred to H2S increased at first and then decreased with an increasing contamination amount. Zhang Peiqing, et al.[12]used the adsorption capacity ratio of cyclohexylamine to n-butylamine to express the ratio of acid centers at the mouth and on the outer surface to total acid amount.The results showed that in the micron-sized ZSM-5 (with a particle size of 1—2 μm and a Si/Al ratio of 34.18), the acid centers at the mouth and on the outer surface can account for 10% of the total acid amount. Ιn the nanosized ZSM-5 (with a particle size of 20—50 nm and a Si/Al ratio of 25.36), the acid centers at the mouth and the outer catalyst surface can account for more than 20% of the total acid amount. Ιt can be seen that the acid centers at the mouth and on the outer surface of HZSM-5 contribute greatly to the alkylation of thiophenes with olefin. The above results may be ascribed to the poisoning of the acid centers at the mouth and outer surface of zeolite,which leads to the restriction on second-order alkylation reaction. When the first-order alkylation products diffuse out of the pores of zeolite, the transformation of sulfurcontaining compounds into larger molecules is inhibited. Ιt can be seen from the sulfides selectivity that the selectivity change in the sulfur-containing compounds with boiling point higher than that of benzothiophene is not significant after poisoning the HZSM-5 zeolite. The amount of molecules larger than dibenzothiophene decreases rapidly until they disappear. Ιt can be inferred that the first-order alkylation products formed in the inner acid centers of HZSM-5 are sulfur-containing compounds with molecular size smaller than dibenzothiophene. The second-order alkylation of larger sulfur-containing compounds occurs on the mouth and outer acid centers of zeolite.

Ιn addition, the small fluctuation of sulfur distribution in gas and on catalyst also indicates that the cracking desulfurization and deep alkylation (of the products existing on the catalyst) take place at the inner acid sites.Therefore, the poisoning of acid sites of zeolite has little effect on these reactions.

Ιt can be seen from Table 9 that when the dosage of 2,4-dimethylquinoline was 0.05 mmol/g of zeolite,the outer acid centers were not completely poisoned.Therefore, then certain products with molecular mass which was greater than dibenzothiophene could still exist.When the dosage of 2,4-dimethylquinoline increased to 0.5 mmol/g of zeolite, the outer acid centers of zeolite were completely poisoned and the second-order alkylation reaction could not occur. The dosage of 5.0 mmol/g of zeolite was obviously excessive.

4 Conclusions

Among several kinds of zeolite-containing catalysts,the catalyst containing HZSM-5 has a large proportion of strong acids and more B acids. The alkylation of thiophenes with olefins can be achieved to form larger molecule sulfur-containing compounds transferring into higher boiling point distillates. Some sulfur compounds can be cracked down to form H2S and the remainder exists on the catalyst. On the other hand, the unique pore structure of HZSM-5 can effectively reduce the condensation of thiophenes, reduce coke formation, and inhibit the polymerization of olefins, which can maximize the yield of gasoline products.

The 2,4-dimethylquinoline poisoning test results show that the main reaction mechanism of sulfur-containing compounds is the second-order alkylation which occur at the pore mouth and outer acid centers of zeolite.