(208b) Catalytic Strategies to Convert Biomass Into Liquid Hydrocarbons

            Research on lignocellulosic biomass conversion is receiving increased attention as a means to provide a renewable source of carbon which can reduce net CO₂ emissions and replace petroleum as a feedstock for the production of fuels and chemicals.  In this presentation, we describe the conversion of cellulose to liquid hydrocarbon fuels using levulinic acid as a platform chemical intermediate, which is subsequently converted to g-valerolactone (GVL).  In particular, we present an integrated strategy in which GVL is converted to butene, which is then coupled by oligomerization to form hydrocarbons appropriate for use as components in gasoline or jet fuels.

            The catalytic strategy for processing g-valerolactone (GVL) to form liquid hydrocarbon fuels is illustrated in Figure 1. 

Figure 1. Catalytic strategy to convert GVL into liquid hydrocarbons fuels.

In this cascade process, aqueous solutions of GVL are converted to pentenoic acid by ring opening over SiO₂/Al₂O₃ catalysts, and in the same catalytic bed and at the same conditions, pentenoic acid undergoes decarboxylation to produce CO₂ and butene.  In a second catalytic reactor, butene is converted to C₈₊ alkenes by oligomerization over an acid catalyst, such as HZSM-5 or Amberlyst 70.  The decarboxylation reaction is favored at lower pressures, but we found that it can be carried out at pressures appropriate for olefin oligomerization (e.g., 500 psi), providing several important advantages.  First, we obtain a stream of butene and CO₂ at conditions that favor the downstream oligomerization reaction.  Second, working at elevated pressures allows removal of water as a condensed liquid in a simple gas-liquid separator, while butene remains in the gas phase to be fed into the second reactor. And third, we obtain a high purity stream of CO₂ at elevated pressure, suitable for use in other processes, such as reduction to methanol or liquefaction for transportation to sites for sequestration.

            Table 1 presents results obtained in the integrated reactor system at different conditions and using HZSM-5 and Amberlyst as oligomerization catalysts¹.  All the reactions and the separation step can be carried out at high yields. Entry 6 shows that 77% of the GVL fed to the reactor system can be converted to C₈₊ alkenes.

Table 1. Performance of integrated catalytic system consisting of two flow reactors in series with an inter-reactor separator. Second reactor operated at 36 bar.

^* Reactor 1: 2.7 g SiO₂-Al₂O₃.WHSV = 0.68 h^-1. First separator at 373 K.

^? Reactor 1: 10g  SiO₂-Al₂O₃.WHSV = 0.18 h^-1. First separator at 383 K.

^? Reactor 1: 10 g SiO₂-Al₂O₃.WHSV = 0.18 h^-1. First separator at 388 K.

^µ Reactor 1: 10 g SiO₂-Al₂O₃.WHSV = 0.22 h^-1. First separator at 398 K.

^♯ Reactor 1: 8 g SiO₂-Al₂O₃.WHSV = 0.22 h^-1. First separator at 398 K.

            The approach described here provides a complementary strategy for biomass conversion to liquid fuels with several interesting advantages.  First, it offers a route for production of branched, fuel range hydrocarbons, which are requisite in gasoline and jet fuels.  Further, owing to extensive hydrogen integration, this processing strategy can be executed with minimal demand for external (petroleum derived) hydrogen, ensuring a greater degree of independence from fossil fuels.  Finally, we offer a strategy by which one equivalent of CO₂ can be reclaimed from GVL as a pure, high pressure product appropriate for sequestration or chemical transformation. 

¹ J.Q. Bond, D. Martin Alonso, D. Wang, R. M. West, J.A. Dumesic, Science, 327, (2010) 1110-1114