Recovery of n-butanol using ionic liquid-based pervaporation membranes

Recovery of n-butanol using ionic liquid-based pervaporation membranes

                                                                                                           Biobutanol (n-butanol) offers the possibility of expanding the production of bulk chemicals and fuels based on renewable resources. A drawback in the microbial production of n-butanol is the energy-intensive product recovery process, making biobutanol expensive. One method for overcoming this limitation is the application of supported ionic liquid membranes (SILMs) for continuous product removal.

 

In this work, the pervaporation performance of SILMs with tetracyanoborate and tris(pentafluoroethyl)trifluorophosphate ionic liquids (ILs) was investigated. Pervaporation was carried out at 37 °C using binary mixtures of n-butanol and water withn-butanol concentrations lower than 5 wt.%. Two concepts for immobilisation of ILs were tested using nylon or polypropylene as support material. ILs were immobilised by inclusion between silicone layers or by dissolution in poly(ether block amide). It was observed that a higher affinity of the IL for n-butanol increases the permeability of the membrane for more than three times, whereas no changes in the selectivity occurred. Furthermore it was shown, that fluxes increased with an increasing IL content in the membrane. The maximum permeate flux achieved was 560 g/(m² h), and the highest concentrations of n-butanol in the permeate was found to be 55 wt.%. In future thickness of SILMs needs to be reduced to make these membranes competitive with respect to conventional pervaporation membranes.

The shrinking of fossil fuel resources and growing environmental awareness have driven the search for new routes for the synthesis of fuels from renewable resources. One example is the microbial production of biobutanol (n-butanol). Drawbacks of this process are the high toxicity of n-butanol to the production organisms and the resulting low concentrations of n-butanol in the fermentation broth, limiting the productivity of the process [1] and [2]. In China several large-scale fermentation processes have been established for the production of n-butanol since 2007. Distillation processes are currently used as “end-of-pipe” technologies for the recovery of n-butanol [3]. Unfortunately, these processes generate large wastewater streams and require high energy consumption, which can be reduced by the continuous separation of n-butanol from the fermentation broth. In particular, the in situ extraction of n-butanol and separation via pervaporation were found to be promising [4] and [5].

Oleyl alcohol has been considered in the past as a potential extractant of n-butanol from the fermentation broth [6] and [7]. In comparison to oleyl alcohol, novel extraction solvents such as ionic liquids (ILs) show similar distribution coefficients and selectivities[8], and the ILs’ properties, such as the melting point, viscosity and density, can be adjusted based on the process requirements. These properties can be changed, for example, by combining different cations and anions or by the introduction of functional groups into the ions [9]. In particular, ILs containing tetracyanoborate anions such as 1-decyl-3-methylimidazolium tetracyanoborate and trihexyltetradecylphosphonium tetracyanoborate are reported to be suitable for the extraction of n-butanol [10]. However, extraction with ILs might have some shortcomings, such as toxicity issues [11] or a large demand for ILs for extraction. One possibility to overcome these potential limitations is the recovery of n-butanol by pervaporation through membranes in which ILs are immobilised in pores. Such supported ionic liquid membranes (SILMs) are often investigated for use in gas permeation processes [12] and [13]. The negligible vapour pressure of ILs is a favourable property to prevent leaching in gas permeation. Additionally, the separation characteristics of the membranes can be tuned by varying the type and quantity of the immobilised IL. Substituting IL-based extraction with a pervaporation with SILMs reduces the IL demand for broth purification and avoids direct contact between the production organisms and the IL, as long as no leaching of the IL occurs. In this case the potential toxicity of ILs becomes less important. Hence, permanent IL immobilisation is crucial for the technical application of SILMs.

The issue of immobilisation has already been addressed in catalysis research, in which ILs are used as supported ionic liquid phases (SILPs) [14], in investigations of polymer electrolytes used in fuel cells [15] and in the production of solar cells [16]. An overview of some methods for utilising ILs as an active separation layer in membranes is given in Fig. 1. The first approach is the linking of polymerisable groups to the IL molecules and the direct cross-linking of the ILs by covalent bonding (a). In this case, the IL’s properties are changed, and the resulting SILM exhibits a permeation behaviour that is different from that of neat ILs, as shown by the Noble research group [12]. Thus, the choice of a suitable IL based on extraction and the transfer of the IL properties to SILMs may not always be appropriate. In another approach, anions or cations of ILs might be immobilised in ion exchange membranes (b). Next to these concepts for immobilisation, ILs can be solidified by the use of gelling agents [17] and [18]. Gelators allow IL contents higher than 90 wt.% in gels, but low chemical and mechanical stability limit their suitability for pervaporation (c). In related systems, ILs are often solidified by dissolving these liquids in a polymer (d). In the literature, a wide range of so-called host polymers, such as poly(vinylidenefluoride)-hexafluoropropylene (PVDF-HFP), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), poly(dimethyl siloxane) (PDMS), poly(ether block amide) (PEBA) and others, has been reported [19], [20], [21], [22] and [23].

 

 

The easiest way to prevent leaching of ILs into the liquid feed during pervaporation is probably the inclusion of ILs in membranes with an additional coating with a polymer such as PDMS or silicone (e). These polymers are usually suitable for the pervaporative separation of organic compounds from water [24]. It seems obvious that an additional IL layer added to the PDMS membrane or the combination of IL and PDMS layers will result in a higher mass transfer resistance of the whole membrane. However, Yu et al. observed that this combination results in higher fluxes of acetic acid compared to neat PDMS membranes [25]. A similar approach combining PDMS layers with ILs was also used by Izák et al., who immobilised tetrapropylammonium tetracyanoborate in a ceramic nanofiltration membrane [26]. Izák et al. observed that without immobilisation, the IL was flushed out of the pores by the feed solution, although the operating temperature was far below the melting point of the IL. To overcome this problem, a ceramic membrane impregnated with IL was coated with a layer of PDMS. In the separation of 1,3-propanediol from an aqueous mixture, permeate concentrations of approximately 60 wt.% and permeate fluxes smaller than 10 g/(m² h) were obtained using this so-called multi-phase membrane [27].

In another work by Izák et al., a different approach was used to immobilise ILs in membranes. Two ILs were included into silicone as host polymer [22]. The maximum total permeate flux achieved by the resulting membrane containing 50 wt.% of tetrapropylammonium tetracyanoborate was 90 g/(m² h) at a concentration of 18 wt.% n-butanol in the permeate. In general, the addition of an IL to silicone decreased the permeate flux slightly but increased the permeate concentration of n-butanol. In a similar work, Kohoutová et al. examined the influence of the IL content on the permeation properties when the IL was included in a silicone matrix [28]. The IL content was varied between 0 and 30 wt.%, resulting in permeate fluxes of approximately 55 g/(m² h), which were nearly independent of the IL content. In contrast to the permeate flux, the concentration of n-butanol in the permeate increased by 50% with an increasing IL content in the membrane. In 2011, Matsumoto et al. described polymer inclusion membranes containing up to 70 wt.% of different ILs in PVC [21]. In the pervaporation ofn-butanol and isopropyl alcohol, the inclusion of ILs resulted in n-butanol fluxes of 27 g/(m² h) with permeate concentrations of n-butanol lower than 10 wt.%. Unfortunately, a direct comparison of different SILMs described in literature is difficult because primary factors influencing pervaporation, e.g., feed concentrations, permeate pressures, temperatures and membrane thicknesses, vary.

Future promising applications of SILMs in technical separation processes can only be possible if SILMs are able to compete with conventional membranes in terms of stability, flux and separation efficiency. An overview of membranes used for the pervaporation ofn-butanol can be found in papers of Oudshoorn et al. and Liu et al. [24] and [29]. According to Oudshoorn et al., it is reasonable to assume a standard n-butanol flux of 20–100 g/(m² h) when handling fermentation broth. For model solutions consisting of only n-butanol and water, higher n-butanol fluxes have been reported [24]. Generally, fluxes of SILMs are often lower than fluxes through polymeric, ceramic and several types of composite membranes.

To make SILMs competitive, the permeate fluxes have to be increased. This increase can be achieved in different ways. The easiest way is to reduce the thickness of the active separation layer. Unfortunately, reducing the thickness can result in lower selectivities at a certain value. The use of additional coatings should be avoided or minimised. The selection of more suitable ILs might result in increased fluxes. Furthermore, a high content of IL in the membrane could result in higher fluxes [21]and/or selectivities [28].

In this work, two different concepts for the pervaporation of n-butanol out of aqueous solutions were investigated, both of which involved membranes with immobilised ILs. The influence of different ILs on the permeation properties of the membranes was tested using 1-decyl-3-methylimidazolium tetracyanoborate (Im_10,1 tcb), trihexyltetradecylphosphonium tetracyanoborate (P_6,6,6,14 tcb) and 1-decyl 3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (Im_10,1 fap) (Fig. 2). For this purpose, the ILs were incorporated in a porous membrane using two polymer layers of PDMS, as described by Izák et al. [27], to prevent possibly leaching of the ILs. It was also determined if inclusion of the ILs in a polymer matrix yields functional membranes suitable for the pervaporation of n-butanol. In this case, the use of additional polymer layers and thus the corresponding additional mass transfer resistances can be avoided. PEBA was chosen as the host polymer because of its favourable pervaporation characteristics for hydrophobic pervaporation and because silicone was not suitable for the dissolution of Im_10,1 tcb.