Selective conversion of butane into liquid hydrocarbon fuels on alkane metathesis catalysts

Abstract

We report a selective direct conversion of n-butane into higher molecular weight alkanes (C₅₊) by alkane metathesis reaction catalysed by silica–alumina supported tungsten or tantalum hydrides at moderate temperature and pressure. The product is unprecedented, asymmetrically distributed towards heavier alkanes.

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Liquid hydrocarbons, especially gasoline (alkanes typically in the range from C₅ to C₁₂) and diesel (C₁₂–C₂₀), are essential in everyday life and thus are highly desired products.¹ These liquid fuels are mainly isolated from distillation of crude oil, an important, but unfortunately a restricted resource.² To circumvent the rarefaction of conventional crude oil, transformation of gaseous alkanes to liquid hydrocarbons is important and represents a continuing scientific challenge in petrochemical research.³

Extended research efforts have been made in high temperature conversion of gaseous hydrocarbons, but the obtained products are generally aromatic compounds,^4,5 which are less suitable as fuels with respect to environmental and health hazards. Unfortunately, there is still no existing process to transform light alkanes directly at moderate temperature into higher homologues in high selectivity without aromatics. Nevertheless, it is well known that any alkanes can be transformed into longer chains by metathesis reaction (eqn (1)).^6–8 Thus, development of an effective catalytic system based on metathesis reaction to convert butane to liquid fuels in high selectivity is a promising approach to supply the liquid fuel reservoir. Production of hydrocarbons using alkane metathesis is attractive since the product is highly paraffinic (not aromatic). These fuels have shown to release less NO_x during combustion and are cleaner than crude oil distilled gasoline and diesel as light alkanes after a convenient processing are essentially free of heteroelements (mainly sulphur and nitrogen).^3,9

2C_nH_2n+2 ⇌ C_(n−i)H_2(n−i)+2 + C_(n+i)H_2(n+i)+2 n > 2 i = 1,2…(n−1) (1)

Direct upgrade of butane to liquid hydrocarbons via metathesis reaction has been exemplified already in the 1970's (known as the Chevron process) for the disproportionation of paraffins.¹⁰ But this system requires high temperature and high pressure of butane to obtain enough concentration of alkene intermediates. In fact, the reaction took place in a dynamic reactor, at 400 °C and 62 bar, with a dual function catalyst system comprising a dehydrogenation/hydrogenation (Pt/Al₂O₃) and an olefin metathesis catalyst (WO₃/SiO₂). The weight selectivity in liquid hydrocarbons (C₅₊) is moderate (55%).¹⁰ The principle of the dual catalyst methodology has been further developed by replacing the dehydrogenation catalyst with a more selective γ-alumina-physisorbed iridium pincer catalyst and the olefin metathesis catalyst with Mo(NAr)–(CHCMe₂Ph)(ORF₆)₂ (Ar = 2,6-i-Pr₂C₆H₃; ORF₆ = OCMe(CF₃)₂) or Re₂O₇ on alumina.^11–13 This system indeed yields a promising selectivity to heavier hydrocarbons from n-heptane, and more recently from n-octane and n-decane.^14,15 These reported dual catalysts require two separate active phases having different functions with divergent working temperatures for optimal activity. In practical and industrial contexts, it is important to develop a simple catalytic system, comprising only one active phase in a single reactor that can perform both functionalities at the same temperature and pressure. With the development of surface organometallic chemistry, a solid catalyst with well-defined multifunctional active sites can readily be obtained.¹⁶ In particular, tantalum or tungsten hydrides supported on oxides have shown the ability to catalyse alkane metathesis reactions at low temperature (150 °C).^6,8 Further investigations revealed that the reaction occurs on a single site metal–carbene-hydride, M(=CHR)(H), and undergoes: (i) dehydrogenation, via C–H activation on metal hydride (M–H), (ii) olefin metathesis on the carbene moiety (M=CHR) with metallocyclobutane as a key intermediate, and (iii) hydrogenation on metal hydride (M–H) of the formed olefins.¹⁷

Herein, we report an efficient heterogeneous catalytic system to produce clean liquid fuel from n-butane in high weight selectivity by using a simple fixed-bed reactor under mild reaction conditions. This reaction is catalysed by tungsten carbene hydride supported on silica–alumina, which functions as a “trisfunctional single site” catalyst. Furthermore, the catalytic performance of Ta–H supported on silica–alumina for the same reaction is compared. The precursors, supported tungsten hydride (W–H/SiO₂–Al₂O_3-(500))¹⁸ and tantalum hydride (Ta–H/SiO₂–Al₂O_3-(500))¹⁹ were obtained by first grafting W(≡C^tBu)(CH₂^tBut)₃²⁰ and Ta(=C^tBu)(CH₂^tBut)₃ on silica–alumina (Akzo-Nobel, 375 m² g⁻¹) pre-treated at 500 °C under vacuum. Secondly, treatment of the W and Ta perhydrocarbyl surface species under H₂ at 150 °C leads to the corresponding hydrides.

Evaluation of the catalytic performance in n-butane conversion was carried out in a continuous flow reactor (P_C4H10 = 20 bar, T = 150 °C, flow rate = 4 ml min⁻¹, 0.32 mol_C4H10 mol_W⁻¹ min⁻¹). The reaction undergoes a steep maximal conversion of 39% at the start of reaction accompanied by a gradual deactivation until a stable value of 14% is reached after 40 h (Fig. 1a). According to the classical mechanism of alkane metathesis, the main products expected are propane and pentane.¹⁷ However, the current experiment shows an asymmetrical product distribution toward liquid alkanes (C₅₊), as depicted in Fig. 1b. Note that no aromatic compounds were observed after extraction of the catalyst with pentane followed by GC analysis. In addition, elemental analysis of the catalyst gives a C/W ratio of 4.5. These results suggest that no significant amount of coke is formed under these reaction conditions. After 100 h, the analysis of the reaction product shows that the weight selectivity in C₅₊ reaches 85% (Fig. 1b). The obtained liquid product consists mainly of linear alkanes in the range C₅–C₁₂. Less than 1% of typical diesel composites (C₁₃–C₂₀) were also registered.

Fig. 1 (a) Conversion of n-butane and (b) the weight selectivity of the product at 100 h catalysed by W–H/SiO₂–Al₂O₃ at 150 °C, 20 bar.

Previous investigation showed that alkane metathesis proceeds via olefin metathesis which is consistent with the fact that 1-butene is the primary product of butane conversion.¹⁷ The study of the selectivities obtained during 1-butene conversion on W–H/SiO₂–Al₂O_3-(500) would allow us to explain the unexpected selectivities to higher hydrocarbons observed in butane metathesis at high pressure. Therefore, we study the 1-butene conversion at 150 °C and 20 bar, in the presence of W–H/SiO₂–Al₂O_3-(500), in a continuous flow reactor. Due to the higher activity of supported tungsten hydride in olefin metathesis in comparison to alkane metathesis, this study was carried out using a molar flow rate of 4.8 mol_C4H8 mol_W⁻¹ min⁻¹. The reaction shows a conversion of 69% (Fig. 2a), which is remarkably higher than butane conversion. Noteworthy, in contrast to butane conversion, the catalyst does not show any deactivation during the course of the reaction (33 h) (Fig. 2a). The selectivities observed show an asymmetric distribution towards heavier alkenes of 86 wt% (Fig. 2b). The global reaction mechanism for the formation of the different products is rather complex, since multiple consecutive reactions are involved.²¹ However, it is known that a tungsten hydride precursor can act as a multifunctional catalyst,²² like isomerisation and metathesis. Thus, the mechanism of the selective transformation of 1-butene to higher homologues involves a “bi-functional single active site”: a tungsten carbene-hydride.²³ The tungsten hydride moiety is responsible for 1-butene isomerisation to 2-butenes, and the tungsten carbene moiety performs the 1-butene/2-butenes cross-metathesis to give pentenes and propene as well as 1-butene self-metathesis to hexenes and ethene. Due to low flow rate (residence time in the reactor), these newly formed products can readily undergo isomerisation and metathesis reactions to give heavier alkenes (C₇–C₁₁).

Fig. 2 (a) Conversion of 1-butene and (b) the weight selectivity of the product at 33 h catalysed by W–H/SiO₂–Al₂O₃ at 150 °C, 20 bar.

The observed asymmetric product distribution can thus be explained by the consumption of the relatively small and more reactive olefins (ethylene and propene) by metathesis reaction with heavier disubstituted alkenes. These asymmetric selectivities in alkenes products are similar with those observed for n-butane conversion where the selectivity in C₅₊ is majority (85 wt%).

Replacing the active tungsten site with tantalum (Ta–H/SiO₂–Al₂O₃) shows a similar trend in catalytic performance as observed in the former experiment. The initial conversion for Ta–H/SiO₂–Al₂O₃ is 40% (versus 39% for W–H) and decreases to 12% (versus 14% for W–H) after 36 h (Fig. 3a). Again, the products are asymmetrically distributed toward liquid alkanes (Fig. 3b), affording 71 wt% (versus 85 wt% for W–H) of C₅₊ after 36 h.

Fig. 3 (a) Conversion of butane and (b) the weight selectivity at 36 h catalysed by Ta–H/SiO₂–Al₂O₃ at 150 °C, 20 bar.

The W–H catalyst appears to be more selective to heavier hydrocarbons and resistant than the Ta–H catalyst, as reflected by a slight deactivation in the conversion curve. A similar trend has also been observed in the selective non-oxidative coupling of methane to ethane and hydrogen.^24,25 Moreover, the yield of liquid hydrocarbons is also higher when the reaction is catalysed by W–H/SiO₂–Al₂O₃. The slight difference in the product distribution can be explained by the enhanced activity of supported Ta–H in comparison to W–H in the C–C bond cleavage by hydrogenolysis reaction.¹⁹

Conclusion

Silica–alumina supported tantalum and tungsten hydrides catalyse the metathesis of n-butane under moderate conditions (150 °C, 20 bar) giving an asymmetric product distribution towards heavier alkanes. This phenomenon is due to consecutive reactions which are strongly assisted by the multifunctional nature of the single site catalyst. Crucially, a promising selectivity of liquid alkanes or alkenes in a typical gasoline range has been obtained from n-butane or 1-butene by passing the feed through a fixed-bed reactor and thereby providing a simple methodology to convert C₄ feeds directly to liquid hydrocarbons.

Experimental

The metal hydrides were prepared according to the literature procedures, by H₂ treatment at 150 °C of the grafted W(≡C^tBu)(CH₂^tBut)₃ or Ta(=CH^tBu)(CH₂^tBu)₃ complexes as previously described (metal loading determined by elemental analysis: W = 7.6 wt%; Ta = 7.8 wt%).¹⁹

General procedure for n-butane and 1-butene metathesis

The catalytic reaction was performed in a Microactivity reactor purchased from PID Eng&Tech, equipped with a hot box which allowed to gasify the feed. Liquid n-butane was purified and injected into the reactor via a high pressure pump obtained from TOP Industrie. The n-butane and 1-butene metathesis was carried out at 150 °C and 20 bar, the stainless steel reactor was connected to a Varian CP-3800 GC equipped with separate injectors, KCl/Al₂O₃ and HP5 columns and two FIDs for product determination.

Acknowledgements

This work was financed by BP.

Notes and references

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Footnote

† Current address: KCC, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia.

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