DBT oxidative desulfurization (ODS)
In order to investigate the activity of PW/KSF and optimize the reaction conditions, deep desulfurization of 1000 ppm DBT in n-hexane was carried out, with EtOH as extractant, at different temperatures, O/S molar ratios and amounts of the catalyst (Table 1). As the temperature increased from 55 °C to 75 °C, the conversion of DBT increased greatly from 31 % to 99 % at an O/S molar ratio of 10:1. The corresponding TON = Nproduct/NPW was calculated (where TON is the mole number of converted DBT per mole of PW supported on KSF). The TON value increased with an increase in reaction time at each reaction temperature (Table 1, Nos. 5, 6 and 8). With an increase in temperature, the TON value for ODS increased at each reaction time. Above an O/S molar ratio 10:1, TON value increased very little.
Table 1 Optimization of reaction conditions for oxidation of DBT (1000 ppm) in n-hexane, using EtOH as extractant and H2O2 as oxidant
Table 1 shows that at 75 °C, the conversion of DBT and corresponding reaction time changed very little when the amount of the catalyst increased from 0.03 g to 0.04 g. In addition, the ODS activity of PW/KSF increased only slightly when the O/S molar ratio was higher than 10:1. Based on the above experimental results, it was found that optimum conditions for ODS were O/S (10:1), temperature (75 °C) and amount of the catalyst (0.03 g). These optimum conditions were used in the next experiments. Prior to investigating the effect of the catalyst (PW/KSF) loading on ODS, two more experiments were conducted for comparison: one with O/S molar ratio 10:1 in the presence of 0.03 g of KSF and another without using any catalyst at optimum reaction conditions. The results showed that the conversion of DBT was 46 % and 28 %, respectively, which did not meet the requirement of deep desulfurization. It was due to the lack of a suitable reaction medium, and DBT could not effectively contact the H2O2 (Zhang et al. 2013). In both cases, noticeable transformation of DBT into sulfone was not observed on the TLC plate.
During the reaction, it was found that the mixture (EtOH, n-hexane and DBT) tended to form two phases. The active polyoxoperoxo (peroxo-metallate complexes) species converted DBT to DBTO2 and the oxidized DBT moved into the EtOH phase. The catalyst was a separate phase during the entire reaction process (Scheme 1).
The experiment was also conducted with 500 ppm and 100 ppm DBT in n-hexane with EtOH as extractant. Figure 1 shows the catalytic performance of PW/KSF for 1000, 500 and 100 ppm of DBT in n-hexane at optimum reaction conditions. The results showed that the catalyst was active for all the concentrations of DBT in n-hexane with EtOH as extractant. It can be seen that DBT was almost fully converted by PW/KSF. Table 2 shows the comparison of catalytic activity of PW/KSF with that of other reported catalysts. The results indicated that PW/KSF was quite reliable for ODS in comparison with other catalysts. There are some disadvantages of these reported systems (compared with our system) such as long reaction time, high reaction temperature or large amount of catalyst and using ionic liquids as additives.
Table 2 Comparison of the reaction data in this work with that using other catalyst
The mechanism of the catalytic desulfurization process (Scheme 2) is assumed to include the following steps: (1) transformation of [PW12O40]3− (denoted as PW) into its peroxide form [PO4{WO(O2)2}4]3− (denoted as PW4) in the presence of H2O2; (2) oxidation of the extracted DBT to DBTO (corresponding sulfoxide) and DBTO2 (corresponding sulfone) by PW4, and reduction of PW4 to [PO4{WO(O2)}4]3−; (3) regeneration of PW4 via oxidizing its reduced form [PO4{WO(O2)}4]3− by H2O2; (4) transformation of PW4 into PW12 species with free tungsten species, after the desulfurization (Zhang et al. 2012a, b).
Scheme 2Probable mechanism for catalytic desulfurization process using KSF/PW and H2O2
Desulfurization of mixed thiophenic model oil
Another model oil was prepared by dissolving required amounts of DBT, BT and T in n-hexane to get mixed model oil with sulfur concentration of 1000, 500 and 100 ppm, respectively. The experiments were performed under the conditions of O/S molar ratio 10:1, 0.03 g of PW/KSF, 75 °C and reaction time of 80 min (Figure 2). The result showed that the catalyst was effective for mixed thiophenic compounds models. It was clear that the conversion rate of 1000, 500 and 100 ppm mixed model could reach 97 %, 95 % and 93 %, respectively. As shown in Figs. 1 and 2, the initial S-compounds concentration did not have much more effect on ODS after 80 min.
It is instructive to compare the catalytic activity of PW/KSF with different extractants (EtOH, DMF and MeCN) in the oxidation process (Fig. 3). In general, the performance of the extractant solvent depends considerably on the solubility of the sulfur oxidation products in the reaction mixture. During ODS process, the oxidized S-compounds are transferred to the extractant solvent, and the solvent can influence the mass transport and subsequently have diffusional problems, especially with porous catalysts (Caero et al. 2005). As shown in Fig. 3, EtOH was the best extractant for removing oxidized thiophenic compounds in the model oil. It can be found that the catalytic activity of PW/KSF depended on the kind of the extractant (Fig. 3). To study the recyclability of the catalyst, 1000 ppm mixed model oil was used successively seven times and the catalytic activity decreased a little (Fig. 4). The recycled catalyst showed 83 % ODS removal after seventh recycling. The average recovery weight was ~93 % and ~92 % for PW/KSF and model oil, respectively. The FTIR spectrum of the recovered PW/KSF after fourth run confirmed no significant change in the Keggin structure of the catalyst (Fig. 5). The PW12O
−340
Keggin ion structure consists of a PO4 tetrahedron surround by four W3O13 groups formed by edge-sharing octahedral. These groups are connected with each other by corner-sharing oxygens. This structure consists of four types of oxygens, being responsible for the fingerprint bands of the Keggin ion between 700 cm−1 and 1200 cm−1. Bulk PW shows the typical bands for absorptions of P–O (1080 cm−1), W = Ot (985 cm−1), W–Oc–W (890 cm−1) and W–Oe–W (814 cm−1). For PW supported on KSF, the bands at 1078, 986, 899 and 802 cm−1 are attributed to stretching vibrations of P–O, W=Ot, W–Oc–W corner-shared bonds and W–Oe–W edge-shared bonds, respectively, which indicates the encapsulation of PW in the KSF frameworks, respectively (Rafiee et al. 2009; Zhang et al. 2011a, b). After catalytic experiments on 1000 ppm mixed model oil using ethanol as extractant solvent, the PW/KSF exhibits a TON = 8.69 at optimum conditions.
ODS desulfurization of crude oil
For investigating the industrial performance of PW/KSF, the ODS was tested using 1000 ppm crude oil as a real model oil with different extractants (MeCN, DMF and EtOH). The results showed that the extractability of S-compounds decreased in the order of MeCN > DMF > EtOH (Fig. 6). The conversion of S-compounds of 1000 ppm crude oil only reached 60 %, 53 % and 47 % by employing MeCN, DMF and EtOH, respectively, as extractant.
Desulfurization of crude oil has been a challenge for a long time, and ODS can be a promising complementary method for hydro-treated crude oil. So, the HDS-treated crude oil was selected for testing performance of the catalyst. By using EtOH, MeCN and DMF as extractants at optimum conditions, the conversion of S-compounds of 300 ppm HDS crude oil only reached 41 %, 38 % and 39 %, respectively (Fig. 6). Indeed, when crude oil was first desulfurized with HDS process, thiols, sulfides and disulfides were removed but aromatic S-compounds (thiophenic compounds) were remained in crude oil. It should be noted that EtOH is the most effective extractant for crude oil after HDS process but it is not as efficient for crude oil without HDS.
Effect of composition of mixed model oil on the ODS conversion
With respect to the considerable decrease in ODS conversion of crude oil in comparison with simulated model oil (see Fig. 6), the effect of crude oil composition on ODS conversion was investigated. Crude oil is the most important source of aromatics and olefins, and they may have a significant effect on ODS (Abdalla et al. 2009a, b; Yan et al. 2013; Xiao et al. 2014). In these systems (PW catalysts), the active sites of the catalyst with H2O2 exhibited considerable activity in epoxidation of alkenes (Wenjia et al. 2013; Aoto et al. 2014). For investigation of the effects of composition of crude oil and epoxidation of alkenes, 5 vol%, 15 vol% and 25 vol% (according to crude oil properties of west Iranian oil wells) of o-xylene, cyclohexene and 1,7-octadiene were, respectively, added to the 1000 ppm model oil (DBT/BT/T, 500:250:250 ppm) to examine their effects on ODS using the PW/KSF catalyst with H2O2 at optimum reaction conditions. Figure 7 shows the ODS conversion of the model oil with addition of o-xylene at various concentrations. The ODS conversion (97 %) of model oil decreased by 10 %, 24 % and 28 %, respectively, after 80 min of reaction time when 5 vol%, 15 vol% and 25 vol% of o-xylene were added to the model oil.
Figure 8 shows that when 5 vol%, 15 vol% and 25 vol% of cyclohexene were added to the model oil, the ODS conversion decreased to 63 %, 52 % and 47 %, respectively, compared to that of model oil. Under the same reaction conditions, the ODS conversion of model oil decreased to 84 %, 72 % and 66 % by the addition of 5 vol%, 15 vol% and 25 vol% of 1,7-octadiene (Fig. 9). The results of olefins and aromatics addition showed strong negative effects on ODS. Therefore, the presence of aromatics and olefins in crude oil will decrease ODS conversion.
The high electron-donating ability of the olefins and aromatics double bonds is considered to be the problem factor in ODS of crude oil (Te et al. 2001; Xiao et al. 2014). Figures 7, 8 and 9 show that the inhibiting effect on ODS conversion increased in the order of o-xylene < 1,7-octadiene < cyclohexene. To explain this trend, the electronic and steric effects should be taken into consideration. The partial electron charge on the alkenes and aromatics plays a detrimental role for oxidation reactivity of the catalyst (Xiao et al. 2014; Yan et al. 2009a).