Benzothiophene desulfurization

Desulfurization of highly substituted alkyl and aryl DBTs 

Some of the recent work from Japan has identified strains capable of desulfurizing dipentyl DBTs and other larger alkyl DBTs. Research in the area of substrate transport and identification of any active transport proteins would be greatly helpful in developing effective biocatalysts for desulfurization of larger molecules. Second, strains capable of desulfurization of benzonaphthothiophenes have also been identified [11]. Desulfurization of whole crude oils will require desulfurization of not just benzonaphthothiophenes 

The overall rate of desulfurization for enabling commercialization has to be greater than 1.2mmol/g dcw/h [1,12,13], The rates of desulfurization of model compounds such as DBT is usually higher than that achievable for real feedstocks such as diesel and other middle distillates. Thus, it is estimated that a more than 100-fold improvement in rates for removal of total sulfur (not just DBT) over wild-type organisms is necessary to achieve rates suitable for commercialization. 

The use of immobilized cell reactors have shown improved biocatalyst stability, however, the specific rates of desulfurization have been much lower than for suspended cell (stirred) reactors. Mass transfer limitations have been significant resulting in lower rates. Thus, the activity is sacrificed to achieve stability. Further work in this area and improved immobilization matrices can help improve the activity along with the stability. 

The ongoing work on sludge-blanket and draft-tube reactors requires demonstration of sufficient gas-liquid mass transfer to provide the necessary oxygen needed in high cell density reactors. 

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Figure 6. The postulated pathway of benzothiophene desulfurization by Paenibacillus sp. Strain A-112 [31].

The weight percents in Table XI demonstrate that the thiophenes were very reactive toward sulfur removal under all experimental conditions. The absence of partially hydrogenated thiophenes in the products is consistent with the mechanism of di-benzothiophene desulfurization (18,22). 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

Another strain R. erythropolis KA2-5-1 was found to desulfurize alkyl derivatives of benzothiophene in addition to DBT and its alkyl derivatives [36], Specifically, it was able to desulfurize various methyl, ethyl, and multi-alkyl derivatives of DBT and of BT however, it could not desulfurize BT itself or 2-methyl, 5-methyl, 7-methyl, or 5,7-dimethyl derivatives of BT. The product of desulfurization of 3-methyl BT was... 

The sulfur-specific pathway for desulfurization of benzothiophene (BT) has been reported in Gordonia sp. Strain 213E. The metabolites of BT conversion were determined by ethylacetate extraction of the culture medium followed by GC-MS analysis [33,34], The reaction mechanism was proposed to be very similar to that of DBT for the first two steps (Fig. 4) however, the third step was quite different. 

Rhodococcus sp. Strain WU-K2R A Rhodococcus strain capable of sulfur-specific desulfurization of benzothiophene, naphthothiophene (NT), and some of their alkyl derivatives was reported [35]. The metabolites of BT desulfurization were BT sulfone, benzo[c][l,2]oxanthiin S-oxide, benzo[c][l,2]oxanthiin S,S-dioxide, o-hydroxystyrene, 2,(2 -hydroxyphenyl)ethan-l-al, and benzofuran. The NT metabolites were NT sulfone, 2 -hydroxynaphthyl ethene, and naphtho[2,l-b]furan [35], The exact biochemical pathway was not determined, however, part of the pathway for BT desulfurization was speculated to be similar to Paenibacillus All-2. 

A strain N. asteroides KGB1, ATCC 202089 was discovered by EBC during its search for catalysts capable of desulfurizing thiophene and BT [126], The strain was patented and that patent covered the nucleic acid molecules encoding one or more enzymes catalyzing DS of thiophene/BT. The ability of the biocatalyst to desulfurize benzothiophene, makes it a suitable biocatalyst for gasoline treatment. The strain was also capable of converting... 

Pseudomonas has been disclosed [233], The method consists of mutating the genes by UV irradiation or other methods and separating the desired protein. A specific amino acid sequence, present in that protein, shows the function for regulating the expression of benzothiophene oxidase gene. The fact that the protein is thought to be useful for both, desulfurization and for purification of sulfur contaminated soil or waste waters indicates a probable destructive pathway. 

Recently, several thermophilic organisms have been reported to be capable of sulfur-specific biodesulfurization. These include the Paenibacillus [87,151], Mycobacterium [30,31,85,94,294,295], etc. The ability to desulfurize sulfur compounds other than DBT derivatives, including benzothiophene, naphthothiophene, and benzonaphthothio-phene derivatives has also been demonstrated, thus widening the substrate specificity of the biodesulfurization process. Second, the thermophilic ability of the organisms offers temperature and operational advantages to further improve the commercialization potential of the BDS process. 

Konishi, J. Onaka, T. Ishii, Y., and Suzuki, M., Demonstration of the Carbon-Sulfur Bond Targeted Desulfurization of Benzothiophene by Thermophilic Paenibacillus Sp Strain Al 1-2 Capable of Desulfurizing Dibenzothiophene. Ferns Microbiology letters, 2000. 187(2) pp. 151-154. 

Oldfield, C., Microorganism which can desulfurize benzothiophenes Patent No. EP0917563. 2001, July 19. 

Matsui, T. Hirasawa, K. Konishi, J., et al., Microbial Desulfurization of Alkylated Diben-zothiophene and Alylated Benzothiophene by Recombinant Rhodococcus Sp. Strain, T09. Appl. Microbiol. Biotechnol., 2001. 56(1-2) pp. 196-200. 

The research was oriented towards the development of biocatalysts for removal of recalcitrant sulfur heterocyclic compounds including benzothiophenes, naphthothio-phenes, and alkylbenzothiophenes. To begin with, they focused on asymmetric sulfur compounds in this class and developed a method for desulfurization of these compounds present in petroleum products [108], The identity of the microorganisms was not disclosed in the abstract but they do claim use of the enzymes as well in the application. 

Two patents were awarded on microbial desulfurization of sulfur-containing heterocyclic compound [155,156], the first targeting DBT and alkylated DBTs and the other benzothiophenes and alkylated benzothiophenes. In both cases, the selective cleavage of the C—S bonds is reported as the main mechanism. The claimed bacteria strains are Mycobacterium G3 strain (PERM P-16105) and R. erythropolis KA2-5-1 strain (PERM P-16277), respectively. Special emphasis was made to the desulfurization of the recalcitrant 4,6-dimethyl-dibenzothiophene. The main product from DBT... 

The researchers from Waseda University have been following the most recent GE strategy for developing improved biocatalysts with desulfurization activity. They hold two patents, the first concerns with the use of Mycobacterium frei WU-0103 strain in a method for decomposing heterocyclic sulfur compounds [169], This bacterium has the ability for desulfurizing DBTs, benzothiophenes, naphthothiophene, and their alkyl derivatives, at 50°C. Details about this strain and the method can be found in Section 2.2.3 in Chapter 3. 

As mentioned earlier, the desulfurization rates of tetra- and hexahydrodi-benzothiophenes are high relative to rates of other reactions in the overall HDS reaction scheme. Because of this, these intermediates are often not observed experimentally. Thus, the origins of cyclohexylbenzene and bicyclohexane are confused. When attempting to deconvolute all of the rate... 

As mentioned earlier, the MOPAC-PM3 calculations also helped to determine the importance of bond order in the hydrogenative route to desulfurization. Figure 28 shows the calculated bond orders of all bonds in a wide variety of thiophenes, benzothiophenes, and dibenzothiophenes (38). These values were correlated with the rates of desulfurization of sterically hindered alkyl-substituted benzothiophenes and alkyl-substituted 1,1 -diox-... 

These compounds were believed to undergo desulfurization primarily by hydrogenation of the thiophene ring prior to desulfurization, and so the overall rates of desulfurization would be expected to relate to the ease of the hydrogenation of the first unsaturated bond in the molecule. A higher bond order is expected to correspond to a position that is more labile to hydrogenative attack. The correlation shown in Fig. 30 supports this assumption and indicates that, for benzothiophenes, enhancement of the bond order by 0.05 units will increase the rate of hydrogenation by a factor of about 10. 

Perhaps the largest discrepancies in reported results are the relative values for the adsorption constants of H2S and thiophene molecules (THs, including thiophene, benzothiophene, and dibenzothiophene). The reported preference for adsorption on the direct desulfurization site ranges from H2S THs (122,123,125) to about the same (104) to H2S < c THs (125). 

With the exception of quinoline HDN, all catalysts are active for the reactions studied. All three catalysts are found to attain complete sulfur removal in the form of H2S (100% HDS) from benzothiophene after 1 h of reaction time (Figure 27.6(a)). The desulfurization pathway is found to go through the rapid hydrogenation of benzothiophene to dihydrobenzo-thiophene, followed by cleavage of the C-S bond in dihydrobenzo-thiophene, to yield ethyl benzene as the main product of the reaction. As seen in Table 27.3, the oxynitride and the oxycarbide are almost twice as active for HDS than the sulfated hematite. In terms of the turnover frequency, the oxycarbide is more active than the oxynitride for all reactions tested. 

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