Mixed-matrix membranes

Mixed-matrix membranes: preparation and characterization for biorefining

3

L. Donato, A. Garofalo, C. Algieri Research Institute on Membrane Technology, ITM-CNR, Rende, Italy

3.1

Introduction

In the past two decades, many researchers have tried to produce energy from renewable sources, considering its enormous demand in the world. The use of biomass may help mitigate the request for different energy sources such as fuels, chemicals, and materials and also reduce the climate change problem. Biomass is a biological material derived from living or recently living organisms. It is used directly via combustion to produce heat or indirectly after its conversion into different biofuel forms (Heinimo and Junginger, 2009; Naik et al., 2010). The term “biofuel” or “biorenewable fuel” refers to any solid, liquid, or gaseous fuel derived from biomass (terrestrial or aquatic carbon-containing matter). In particular, solid biofuel is referred to pellets and wood chips; liquid biofuel refers biodiesel, bioethanol, and oil fuel; biogas and syngas are considered to be gas-biofuel. The biorefinery concept is often considered for the production of fuels from biomass feedstocks (He et al., 2012). Several separation technologies are part of this process. In this field, membrane separation processes are more attractive because of different advantages such as low energy consumption, greater separation efficiency, the reduced number of processing steps, and the high quality of the final product (de Morais Coutinho et al., 2009). In the biorefinery process, when separations are performed by means of membranes, the most commonly used are polymeric. However, they have different drawbacks such as low stability at high temperature and pressure and low permselectivity properties (Ozturk and Demirciyeva, 2013). To solve these problems, different routes are followed such as functionalization of existing polymers, synthesis of new ones, and the development of membranes using more selective materials (eg, carbon, metals, perovskites) (Clarizia et al., 2004). An alternative is represented by the preparation of mixed-matrix membranes (MMMs), in which inorganic fillers are dispersed into the polymeric matrix (Fig. 3.1). These membrane-types combine the easy processability of the polymers and the peculiar properties of the inorganic particles. In this chapter, the main strategic methods used to prepare MMMs will be discussed in depth. Afterward, the application of these membranes in biorefinery processes will be illustrated.

Membrane Technologies for Biorefining. http://dx.doi.org/10.1016/B978-0-08-100451-7.00003-7 Copyright © 2016 Elsevier Ltd. All rights reserved.

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Membrane Technologies for Biorefining

Figure 3.1 Scheme of an MMM.

3.2

Preparation of mixed-matrix membranes

To prepare MMMs, two materials (polymer and filler) are required to be selected for the same gas pairs; in most cases, the inorganic particles should have selectivity superior to the pure polymer. Inorganic fillers used to prepare MMMs can be porous or nonporous types; the most commonly used are various zeolites (Paul and Kemp, 1973; Kulprathipanja et al., 1988, 1992; Mahajan and Koros, 2000, 2002a,b; Suer et al., 1994; Yong et al., 2001; Tantekin-Ersolmaz et al., 2001; Li et al., 2005, 2006; Clarizia et al., 2004; Hasse et al., 2003; Wang et al., 2002; Pechar et al., 2006a,b; Guiver et al., 2003; Sanaeepur et al., 2014), carbon molecular sieves (CMS) (Vu et al., 2003a,b,c; Duval et al., 1993; Rafizah and Ismail, 2008), activated carbons (Anson et al., 2004), nonporous silica (Sanaeepur et al., 2014; Vu et al., 2003a,b,c; Duval et al., 1993; Rafizah and Ismail, 2008), C60 (Anson et al., 2004), and graphite (Merkel et al., 2003a,b; He et al., 2002; Moaddeb and Koros, 1997). A novel class of porous crystalline materials called metaleorganic frameworks (MOFs) has received much attention as good fillers for the preparation of MMMs (Zhang et al., 2008; Perez et al., 2009; Adams et al., 2010; Yang et al., 2011). Metale organic frameworks have a connection of metal ions or cluster through organic bridging ligands (Yaghi et al., 1995). Interaction between the two phases is better than that of traditional fillers owing to the better affinity of the MOF linkers with the polymer chains. Porous particles act as molecular sieving agents into the polymeric matrix. In this case, both permeability and selectivity should increase. However, this occurs when the polymer chains completely wet the inorganic fillers and the membrane is defect-free. When nonporous fillers are used, the separation properties improve, increasing the tortuosity. In addition, the particles disrupt the polymeric chain packing, with an increase in the polymer-free volume. Voids are usually present in the membrane structure as a result of detachment of the polymeric chains from the particle surface. The causes are different: the repulsive force between polymer and particles (Tantekin-Ersolmaz et al., 2000), the different thermal expansion coefficients of polymer and particles (Li et al., 2005), and the high rigidity of the polymeric chains when glassy polymers are used. Rigidification of the polymer near the particles and the partial pore blockage of the particle also contribute to the formation of defects into the polymeric matrix. As an example, Li et al. (2005) studied the effect of polymer rigidification of MMMs using polyethersulfone (PES) and zeolite (Linde type A topology) as polymer and filler, respectively. The authors found a decrease in gas permeability and an increase in gas pair selectivity increasing the zeolite content. They explained these results considering a combination of rigidification and pore blockage. The first effect was confirmed by an increase in the glass

Mixed-matrix membranes: preparation and characterization for biorefining

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transition temperature (Tg) of the MMMs with the zeolite content. Instead, pore plugging remained a supposition for the impossibility of characterizing them. As an example, Clarizia et al. (2004) analyzed the influence of different fillers (NaA, NaX, silicalite, and graphite) on the gas separation performance of MMMs polydimethylsiloxane (PDMS). NaX-based zeolite in the polymeric matrix lowered the permeability of all species investigated. This zeolite type exhibited no molecular sieving effect because the polymeric chains completely filled the zeolitic pore. Thus, they could exert exclusively sorption properties on the external surface of the crystal. In the presence of pore blocking, permeability always decreases, whereas the effect on selectivity depends on the filler type. On the contrary, silicalite exhibited a molecular sieve mechanism by facilitating permeation of smaller molecules and hindering that of larger molecules. Mixed-matrix membranes are prepared via phase inversion using the dry method (Jiang et al., 2005, 2006a,b; Kusworo et al., 2007; Kim et al., 2007; Ahn et al., 2008; Ciobanu et al., 2008; Rafizah and Ismail, 2008; Kim and Marand, 2008; Rezaei et al., 2015; Zhang et al., 2014). In particular, for flat dense membranes, polymerparticle suspension is prepared and casted onto a smooth plate. Subsequently, evaporation of the solvent is carried out. Different methods, reported subsequently, are used to prepare the polymer suspension. 1. Fillers are dispersed in a solvent and stirred; subsequently, the polymer is added in the particle slurry (Pechar et al., 2006a, Jiang et al., 2005, 2006a; Kusworo et al., 2007). 2. A predetermined amount of particles is added to the prepared polymer solution (Kim et al., 2007; Ahn et al., 2008; Ciobanu et al., 2008). 3. The particle suspension and polymer solution are prepared separately. Afterward, the particle slurry is added to the polymer solution (Rafizah and Ismail, 2008; Kim and Marand, 2008).

The first and the third methods are preferred because they permit better particle distribution, because a dilute suspension avoids their agglomeration. In fact, during membrane preparation, a factor to take into account is agglomeration of the particles owing to sedimentation or their migration to the surface. Sedimentation can happen during membrane formation, considering the difference in the density of the polymer and particles. The presence of agglomeration determines the formation of defects. This phenomenon is intense at high particle loading. However, by forming the membrane in a short time it is possible to reduce the formation of large defects (Vu et al., 2003a). Agglomeration on the membrane surface for particle movement occurs when the membrane is formed at a high temperature. In general, there is the formation of convection cells when the liquid is heated or cooled owing to instabilities driven by buoyancy or the surface tension force (Chung et al., 1994; Levich and Krylov, 1969). To improve adhesion between polymer and inorganic particles and avoid filler agglomeration, a priming treatment was proposed by Mahajan (2000). In this protocol, it is important to use a solvent with a lower affinity with polymer and inorganic particles. Mahajan and Koros (2000) used this protocol to prepare MMMs loaded with zeolite NaA and used poly(vinyl acetate) as a polymer. The authors chose toluene as a solvent, considering the Hildebrand solubility parameters and the liquidesolid interaction strength. A small amount of the polymer was added into a zeolite slurry.

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The resulting suspension was stirred overnight and after the bulk of toluene and unadsorbed polymer was decanted. In this way, a thin layer of polymer wrapped the filler. The O2eN2 permselectivity at 35
C was enhanced whereas permeability decreased with respect to the pure polymer (Mahajan and Koros, 2002b). However, membrane performance is not exciting and so not extremely attractive for industrial applications. Shahid and Nijmeijer (2014) prepared MMMs using different MOFs (MIL-53(Al), ZIF8, and Cu3BTC2) and employed a modified priming protocol with thermal annealing. The MMMs exhibited good compatibility and distribution of the fillers into the polymeric matrix. High CO2 permeability and CO2eCH4 selectivity were found at high pressure. Usually, in this condition the pure polymer exhibits plasticization. This behavior can be explained by the presence of MOFs in the matrix, which limited the mobility of the polymeric chains. Another route to improve adhesion between the two phases is chemical modification of the particle surface using coupling agents. Multilayer deposition of coupling agents ensures better adhesion. However, this could cause the filler pore to close. On the other hand, it is possible to change the property of the membrane and create new defects. Therefore, a monolayer of the coupling agent is desirable. Duval et al. (1994) modified the zeolite surface using different silane coupling agents. The chemical modification is reported in Fig. 3.2. However, no significant improvements were found in terms of permselectivity properties. Vankelocom et al. (1996) studied the possibility of improving the incorporation of zeolite in polyimide (PI) using as coupling agent, 3-(aminopropyl)-triethoxysilane (APTES). Density and tensile strength measurements indicated better incorporation of the zeolite after silylation without sorption change. Mahajan and Koros (2002a) used zeolite NaA modified with a silane. Scanning electron microscopy (SEM) analyses showed an improvement in adhesion between the two phases but the transport properties were not attractive. In fact, both permeability and selectivity decreased using modified zeolites. The authors assumed that the coupling agent determined a reduction in the size of the defects but they were not eliminated. This caused an increase in the membrane gas transport resistance.

O OH

HO

R

Δ –3H2O

Si

O

Si HO

R

O Si

O OH

R

O

R

Si

+

OH

OH

HO OH

O R OH

Si O

R OH

Figure 3.2 Zeolite functionalization with silane coupling agent. Elaborated from Mahajan, R., Koros, W.J., 2002a. Mixed matrix membrane materials with glassy polymers. Part 1. Polymer Engineering and Science 42, 1420e1431.

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In another article, the same authors (Mahajan and Koros, 2002b) demonstrated MMMs with improved performance that were prepared by modifying the zeolite by means of the coupling agent and ensuring the flexibility of the polymeric chains during formation of the membrane. Li et al. (2006) first used 3-aminopropyl-dimethylethoxysilane (APDEMS) because it has a lower number of coupling points on the zeolite surface with respect to the APTES. In this way, blocking of the zeolite pores was reduced. In fact, both gas permeability and selectivity increased for MMMs based on modified zeolites with respect to those obtained with the unmodified ones (20 wt% zeolite loading). In other work, APDEMS was used to modify an NaY zeolite surface to investigate the effect on cellulose acetate membranes (Sanaeepur et al., 2014). The best performance was found for membranes prepared with treated zeolites. Another possibility is to operate above the Tg of the polymer during membrane preparation, to have more flexible chains and so to ensure a favorable interaction with the filler. However, the solvents usually used for membrane preparation have a boiling point below the Tg of the most widely used polymers. Considering this aspect, Mahajan et al. (2002) used different plasticizers to lower the Tg of a PI (Matrimid®). Different permeability and permselectivity trends were achieved as a function of the plasticizer type. A different way to prepare MMMs is to employ block copolymers containing both rigid and flexible chains; in this case, modification of the inorganic particles is not required. Kim et al. (2006) synthesized a poly(imide siloxane) using an aromatic dianhydride, an aromatic diamine and amine-terminated PDMS for the siloxane block. This copolymer was mixed with carbon nanotubes (CNTs) to prepare the MMMs. For the O2, N2, and CH4, the permeability values increased in proportion to the amount of CNTs in the polymer matrix. For small gas molecules such as He, H2, and CO2, permeability increased after the addition of 2 wt% of CNTs. On the other hand, there was no difference in permeability between 2 and 10 wt% of CNTs. Other MMMs were prepared for gas separation introducing a diblock copolymer (eg, polystyrene-b-poly(hydroxyl ethyl acrylate)) as a compatibilizer to enhance interfacial adhesion between the poly(imide) matrix and zeolite particles (Patel et al., 2011). The carbonyl and hydroxyl groups present in the diblock copolymer interacted at the same time with the zeolite and PI polymer matrix, as revealed by Fourier transforme infrared (IR) spectroscopy. Experimental results evidenced a reduction in H2, N2, and O2 permselectivity. At the same time, an increase in the permselectivity for CO2 was detected.

3.3

Characterization of mixed-matrix membranes

The top view and cross-section of MMMs are observed using SEM. In particular, a cross-sectional view permits observation of distribution of the particles into the polymeric matrix and fillerepolymer interactions. Fig. 3.3 shows a cross-section of an MMM (polyether ether ketone (PEEK)-WC-NaA). During preparation of this membrane, a plasticizer is used to low the polymer Tg.

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Figure 3.3 Cross-section of PEEK-WC-NaA membrane.

Differential scanning calorimetry measurements are used to determine the Tg of the MMMs at different filler loadings. These values are compared with those obtained for the membrane prepared using pure polymer. When the Tg of the MMMs is much higher than the value of the bare polymer, rigidification of the chains is suggested. X-ray diffraction analysis is used to study the crystallinity of the particles (Sanaeepur et al., 2014; Shajid and Nijmeijer, 2014). In addition, it indicates whether there has been modification of the particles into the polymeric matrix. At the same time, the change in polymer crystallinity can be evaluated. Fourier transformeIR analysis is used to investigate interactions between the polymer and the fillers. Usually, the IR spectra of particles, pure polymer membrane, and MMM are compared to investigate the presence of fillers in the polymeric matrix. Nitrogen adsorptionedesorption experiments are carried out to compare the Brunauer, Emmett, and Teller surface and total pore volume between the unmodified and modified particles (Li et al., 2006). Energy-dispersive X-ray analysis permits a map to be obtained in which the distribution of filler elements (such as Si, Al, and Zn) into the polymeric matrix (Sorribas et al., 2014) is observed. Transport properties (eg, flux and selectivity) require single and mixture separation tests to evaluate the presence of defects in the membrane structure. Usually, gas transport properties were measured using a variable-pressure constant-volume method (Lin et al., 2000). The apparatus used for the single gas permeation tests consists of a permeation cell that represents the heart of the system. Before the gas measurements, the system is kept under constant vacuum to remove residual gas species or air.

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Subsequently, the membrane is put into contact with the gas. The upstream pressure p0 in the input chamber is kept constant while an increase in downstream pressure p of the permeation chamber by a pressure transducer is measured. When steady state is reached, the increase in pressure is linear with time. The gas permeability coefficient is calculated from the curve slope of the pressure on the permeation side versus the time at steady state, using the following equation: P¼

273  1010 VL dp p076
760 AT 14:7 dt

where P is the permeability coefficient in Barrer, V is the volume of the downstream chamber, A is the membrane area, L is the membrane thickness, p0 is the feed pressure, T is the absolute temperature, and dp/dt is the rate of pressure measured in the downstream chamber.

3.4

Mixed-matrix membranes in biorefinery processes

Membrane technology has a relevant part in product recovery and purification in biorefinery and bioenergy production processes. Different up-to-date membrane processes such as microfiltration, ultrafiltration, nanofiltration, diafiltration, distillation, pervaporation, and gas permeation are particularly valuable for biorefining and bioenergy production. The employment of MMMs is promising for obtaining improvement in process performance owing to the joined effects of molecular sieving, selective adsorption, and different diffusion rates of the permeants.

3.4.1

Mixed-matrix membranes for biochemical compound recovery

An important application of MMMs in biorefinery processes is the recovery of furfural from wastewaters. Furfural is a heterocyclic aldehyde used as a platform for the production of chemicals and fuels from biomass. Liu et al. (2013) produced ZIF8-silicone rubber membranes that exhibited excellent stability when the samples were tested more than 120 h at 80
C to recover furfural from water via pervaporation. The separation factor was 53.3 and the flux was 0.90 kg/m2 h. Another interesting application of MMMs loaded with MOFs (ZIF-8) was the selective recovery of iso-butanol from water during pervaporation at 40
C (Liu et al., 2011a). In this case, incorporation of the fillers into the polymer matrix obtained a flexible framework structure that promoted the preferential adsorption of the alcohol. Other MMMs for bio-butanol recovery via pervaporation were prepared introducing Zn(BDC)(TED) 0.5 (in which BDC is benzenedicarboxylate and TED is triethylenediamine) particles into polyether-block-amide (PEBA) (Liu et al., 2014). Introduction of this filler type improved the mechanical properties as well as the flux and separation factor with

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respect to the pure PEBA membrane. In addition, MMMs decreased surface energy, demonstrating good contact at the polymerefiller interface. Pervaporative recovery of bio-butanol from water solutions at 80
C was also achieved with capillary-supported ultrathin homogeneous silicalite-PDMS membranes (Liu et al., 2011b). The authors deposited silicalite nanocrystals onto a porous alumina support; then, PDMS was used to fill the interspaces between the nanocrystals. The membrane exhibited high flux (5.0e11.2 kg/m2 h) and good separation (25.0e41.6). These results are promising for applications in fermentationepervaporation coupled processes. Kudasheva et al. (2015) prepared PI-based MMMs containing different types of molecular sieves (microporous ZIF-8, ordered mesoporous MCM-41 silica spheres of two different sizes, and ordered mesoporous silica-(ZIF-8) coreeshell spherical particles). These prepared membranes were applied to the pervaporation of 10/90 wt% watere ethanol mixtures. The best results in terms of ethanol recovery were obtained using the mesoporous silica spheres (12 wt% (MSS-2)). In particular, pervaporative flux improved from 0.24 to 0.44 kg/m2 h with respect to the PI membrane, even when watereethanol separation remained constant (about 250). Incorporation of H-ZSM-5 zeolite into chitosan (CS) (Sun et al., 2008) obtained MMMs with better pervaporation performance than pure CS for recovering ethanol from water. In particular, the CS membrane had a permeation flux of 0.054 kg/m2 h and a separation factor of 158 for 90 wt% aqueous ethanol solution at 80
C. The MMM loaded with 8 wt% of H-ZSM-5 had a permeation flux of 0.23 kg/m2 h and a separation factor of 153 under the same operating conditions. Others researchers (Ramaswamy et al., 2013) reported separation factors in the range of 5e59 for ethanolewater and 30e145 for butanole water mixtures dispersing silicalite crystals into the PDMS matrix. The wide range of separation performance of these membranes is the result of the different sources of silicalite, particles sizes, and loading and membrane-formation procedures. Kang et al. (2013) also developed CS-based MMMs incorporating ZIF-7 crystal particles to separate watereethanol mixtures. The MMMs containing 5 wt% of ZIF-7 showed 19 times higher separation efficiency than the original membrane. In other work by Amnuaypanich et al. (2009), MMMs adding NaA zeolite crystals into the cross-linked poly(vinyl alcohol) (PVA) were prepared and used to dehydrate watere ethanol mixtures. These membranes exhibited a preferential sorption of water that increased with the zeolite content owing to its hydrophilic nature. In pervaporation studies, an increase in water flux was also observed to increase the amount of zeolite. Instead, ethanol flux slightly decreased with zeolite loading up to 30 wt%. Pervaporative dehydration of ethanolewater mixtures was assessed by Panahian et al. (2015). They employed multilayer MMMs composed of PVA as the active layer and modified CNTs as the inorganic filler. Particle modification permitted obtained better dispersion inside the polymeric matrix and a high water separation factor. Another interesting application was the dehydration of isopropanol employing PI membranes filled with zeolite NaA and ZSM-5 (Mosleh et al., 2012). Zeolite CaA and NaX embedded into the P84 copolyimide membranes were used in the same separation process (Qiao et al., 2006). Another area in which MMMs are employed is ethanol recovery from the fermentation of grain, corn-dry grind, and biomass. Offeman and Ludvik (2011) investigated

Mixed-matrix membranes: preparation and characterization for biorefining

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the performance in pervaporation of PDMS loaded with silicalite. During experimental tests using ethanolewater solutions, MMMs exhibited better performance than pure PDMS. However, membrane performance decreased drastically when a fermentation broth of grains was used as feed. The removal of oleic acid from the broths reduced the MMM deactivation. On the other hand, the performance of the PDMS membranes was not affected by the composition of the fermentation broths. This result indicated that zeolite is a species subject to deactivation. Hennepe et al. (1987) combined a fermentation process with a pervaporation unit employing a silicalite-filled silicone rubber membrane for the continuous recovery of alcohols (methanol, ethanol, and n-propanol). The presence of the zeolite significantly improved membrane performance in terms of flux and selectivity. Experimental data showed a permeate flux in the range of 0.05e0.2 kg/m2 h with selectivity up to 40 for n-propanol. Qureshi et al. (2001) recovered acetone and butanol from the fermentation broth of Clostridium acetobutylicum using a silicalite-silicone MMM. The fermentor was integrated with a pervaporation unit. This configuration increased the produced solvents by eight times with respect to the batch reactor and the MMM was not fouled by the fermentation broth. Vane et al. (2010) investigated factors affecting the alcohol-water pervaporation performance of zeolite-silicone rubber MMMs. Long-term exposure to ethanolewater solutions resulted in slow declines in ethanol permeability, water permeability, and selectivity. Membranes exposed to centrifuged yeast fermentation broth and to clarified acetone/n-butanol/ethanol also resulted in a decline in both ethanol permeability and selectivity. This behavior was more rapid and drastic for the yeast broth. This was attributed to the zeolite pore blockage owing to the adsorption of minor broth constituents such as esters, organic acids, and alcohols. Xue et al. (2014) produced CNTs PDMS membranes to recover bio-butanol from an acetone-butanol-ethanol fermentation broth. The maximum total flux and butanol separation factor were 0.24 and 32.9 kg/m2 h, respectively, adding 10 wt% of fillers. The presence of CNTs improved the flux and the separation factor of the MMM with respect to the PDMS membrane. This was because of the smooth and hydrophobic surface of the CNTs, which provided an alternative route for mass transport through the inner tubes or along the surface, facilitating butanol separation. Mixed-matrix membranes have also been used in organiceorganic separations. For example, Kasik and Lin (2014) prepared and characterized high-quality MOF-5 membranes for the pervaporation of organic solvents. On the other hand, Basu et al. (2009) and Sorribas et al. (2013) developed MMMs for organic solvent nanofiltration. The removal of biorefinery residues (such as humic acids) represents an additional area in which to apply MMMs. Teow et al. (2012) prepared polyvinylidene fluoride (PVDF) membranes loaded with TiO2 nanoparticles to remove humic acid. Membranes were synthesized via in situ colloidal precipitation and the effect of different TiO2 types on membrane morphology and performance was investigated. Ultrafiltration of humic acid solutions revealed an improvement in membrane flux over that of the PVDF. This was attributed to an increase in free volume and membrane hydrophilicity. The highest permeate flux and acid rejection were observed using a smaller nanofiller (X500 type) owing to their better dispersion into the membrane matrix.

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For removal of the same species, Panda et al. (2015) developed new MMMs using amine-stabilized iron oxide nanoparticles (Fe3O4) and polyacrylonitrile coated with CS. In this case, MMMs exhibited an antifouling property and long-term stability. The recovery of amino acids, which are intermediates in the production of bio-based chemicals, represents another area of potential application for MMMs. Amino acids are a power source that can be used to produce chemicals without using fossil resources. This is because of the presence of functional groups (ie, eN and eO) necessary to produce the bio-based substances. As an example, Readi et al. (2014) developed MMMs to separate amino acids with electrodialysis. Using a new approach, they integrated an enzymatic conversion unit with a separation membrane system. Membranes were prepared using Relizyme EP403/S as a carrier and glutamic acid decarboxylase (GAD, EC 4.1.1.15) as the model enzyme. This approach permitted the decarboxylation of L-glutamic acid to geaminobutyric. At the same time, separation of L-aspartic acid and unconverted L-glutamic acid was performed, with an efficiency of 40% at low energy cost. Ion-exchange mixed-matrix adsorber membranes for the separation of proteins were also developed (Avramescu et al., 2003). The membranes were prepared by dispersing different types of Lewatit ion-exchange resins into an ethyleneevinyl alcohol copolymer porous structure. High separation factors were obtained at physiological ionic strength conditions, up to 10 times higher than those reported in the literature.

3.4.2

What is biogas?

The principal source of energy in the world is natural gas, a mixture of hydrocarbons. Currently, it has become increasingly rare and expensive, so it is necessary to find alternative energy resources to overcome these problems. In this context, biogas represents a valid option to obtain new energy carriers and then fresh fossil fuels for use in industrial and commercial fields (Pientka et al., 2007). The term “biogas” commonly refers to a gaseous mixture produced by the biological breakdown of organic matter in the absence of oxygen (Basu et al., 2011). It is a renewable energy source and can be produced from raw materials such as biomass; agricultural, municipal, green, or food waste; manure; plant material; and sewage. The resulting energy can be exploited for heating, electricity, and many other operations. Biogas can be produced by fermenting biodegradable compounds and by anaerobic digestion. The principal substances and their general amount in the biogas are reported in Table 3.1 (Rasi et al., 2007). Biogas has many uses, the most common of which is fuel, owing to energy exploitation released by the combustion or oxidation of principal constituents such as methane, hydrogen, and carbon monoxide with oxygen. It can also be used for heating purposes such as cooking, and to generate both mechanical and electrical power.

Mixed-matrix membranes: preparation and characterization for biorefining

Table 3.1

71

Typical composition of biogas

Substance

%

Methane (CH4)

50e75

Carbon dioxide (CO2)

25e50

Nitrogen (N2)

0e10

Hydrogen (H2)

0e1

Hydrogen sulfide (H2S)

0e3

Oxygen (O2)

0e0

Elaborated from Rasi, S., Veijanen, A., Rintala, J., 2007. Trace compounds of biogas from different biogas production plants. Energy 32, 1375e1380.

Like natural gas, biogas can be compressed and so it can be employed to feed motor vehicles. This type of biogas is becoming widely used in Germany, Switzerland, and Sweden for cars, trucks, and trains. In Europe the use of gas-powered vehicles is gaining growing interest. Researchers in the United States are evaluating the production of biomethane arising from chickens, cattle, and pigs, which may drastically decrease the emissions of greenhouse gas. In addition, it can be employed to produce energy for millions of homes around the world. On other continents, the use of biogas has attracted increasing interest in many fields; eg, in Asia the term “gobar gas” refers to biogas derived from the digestion of manure in the absence of oxygen and in small-scale plants. This type of organization is popular in Pakistan and India owing to the large presence of livestock. One peculiar type of biogas is known as landfill gas (LFG) or digestor gas. Biogas is produced as LFG by the breakdown of biodegradable waste inside a landfill under anaerobic conditions, as a result of the action of microorganisms that produce methane, carbon dioxide, and digestate, in particular, from biomass waste. Usually, an anaerobic digester is commonly called a biogas plant; energy crops or biodegradable waste such as food waste can be used to feed these special plants. Basically, there are two different anaerobic processes for LFG production, depending on the bacteria used: mesophilic bacteria work at a temperature range equal to 20e45
C, and thermophilic bacteria, which operate between 50 and 52
C. If a landfill is not designed to trap the gas, when gas builds up it is slowly released into the atmosphere. For this reason, it is important to check whether landfill gas is released in a controlled way. This avoids an explosion when it combines with oxygen in the air and prevents a negative influence on global warming. In fact, the methane contained in biogas is 20 times more powerful as a greenhouse gas with respect to carbon dioxide. Last but not least, the presence of volatile organic compounds contained in LFG have an important role in the development of photochemical smog.

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The digestate, the part of biomass not converted in biogas, can be employed as a fertilizer in agriculture.

3.4.3

Biogas upgrading

With the exception of methane, other biogas components (especially carbon dioxide) are considered energy dilutors because they decrease the heating power of the biogas. Therefore, they must be upgraded to enhance their calorific value and consequently to be used in different applications; eg, as a motor fuel or component to be injected into the natural gas grid. Biogas upgrading is based on the production of a methane-enriched gas stream by gas separation. Several technologies using easy and more complex methods can be employed to purify biomethane. They are commercially available and have been demonstrated to be technically and economically feasible. Many studies are being conducted to improve these methodologies and to develop new ones in the field of biogas upgrading. The different processes can be influenced by the characteristics and concentrations of energy diluters into the biogas that can be removed by different ways. These routes depend on the energy requirements, additional chemicals, gas quality, methane loss, maintenance, and space and weight demands. Considering the composition of raw biogas, the upgrading processes include mainly the separation of carbon dioxide and the removal of trace substances (eg, oxygen, nitrogen, hydrogen sulfide, ammonia, and siloxanes). Raw biogas is divided into two gas flows during purification. The first is rich in methane whereas the second contains mostly carbon dioxide (Fig. 3.4). In the flow of exhaust gas, there will be also an amount of methane depending on its applied recovery technology, because no separation technology is unfailing.

3.4.4

Mixed-matrix membranes for biogas separation

Currently, numerous methodologies exploited for the biogas upgrading are commercially available. For example, a condenser can easily be used to remove water vapor Biomethane

Raw biogas Gas upgrading unit

Exhaust gas

Figure 3.4 Scheme of biogas upgrading.

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from biogas, whereas hydrogen sulfide may be removed by means of an absorber or an iron-sponge filter (Ravishanker and Hills, 1984; Frare et al., 2010). However, biogas upgrading is especially known to remove carbon dioxide, the principal energy dilutor, from methane (Kayhanian and Hills, 1988; R€ ohr and Wimmerstedt, 1990; Stern et al., 1998; Harasimowicz et al., 2007). Traditional technologies that can be applied to separate the two principal biogas constituents are pressure swing adsorption, physical absorption, chemical absorption, and cryogenics. A new method consisting of gas separation by means of membranes has attracted increasing interest in this field (Iarikov and Oyama, 2011; Mondal et al., 2012). This process is preferred with respect to the others because it offers several benefits such as low cost, ease of handling and processing, and high energy efficiency. The performance of the membrane technology in gas separation is largely influenced by the membrane material type. For instance, polymeric membranes are widely used in this sector owing to their low cost compared with inorganic ones, and to the ease of fabricating and processing and stability at high pressures (Henis and Tripodi, 1983; Merkel et al., 2002; Shiflett and Foley, 1999; Phair and Badwal, 2006). However, considering the tradeoff curve between selectivity and permeability explained by Robeson (2008), many polymeric membranes have values mostly lower than the upper bound of the curve. To overcome this drawback, a great number of innovations were introduced, such as the use of molecular sieve membranes (carbon and zeolite membranes) and especially the development of MMMs (Moore et al., 2004; Hillock et al., 2008; Jia et al., 1991). Generally, the addition of zeolite to the selective skin layer of the polymeric matrix increases both CO2eCH4 selectivity and CO2 permeability compared with pure membranes (Jiang et al., 2006b; Duval et al., 1993; Mirfendereski et al., 2008; Adams et al., 2011; Li and Chung, 2007; Cakal et al., 2012). For example, the low selectivity performances of PDMS were improved using Silicalite, NaX, and KY (Mahajan et al., 2002). However, Tantekin-Ersolmaz et al. (2000) found that increasing the silicalite surface area in the PDMS matrix led to a decrease in permeability. The addition of silica nanoparticles (nonporous filler) into the PS membranes resulted in an increase in both permeability and ideal selectivity for the main constituents of the biogas (CH4, CO2, and N2) (Ahn et al., 2008). Chemical modification of the fillers, as reported in Section 3.2, improves membrane performance. For instance, Ismail et al. (2008) prepared MMMs using PES and the zeolite NaA functionalized with a bifunctional silane (Dynasylan Ameo) and PDMS. The prepared membranes were defect-free and exhibited an enhancement (equal to 38%) of CO2eCH4 selectivity with respect to that of the PES membrane. This behavior indicated the absence of nonselective voids in the membranes owing to the action of both silane and PDMS. Usually, interaction between the polymer matrix and zeolite in MMMs is weaker when glassy polymers are used, inducing in this case an increase in the membrane free volume and, as a consequence, an increase in gas permeability without a significant loss of selectivity (Duval et al., 1994). For example, Ozturk and Demirciyeva (2013) prepared MMMs using both PI and polyetherimide (PEI) as polymer matrices loaded with zeolite KA, NaA, and CaA. These zeolites have the same topology but different pore sizes. For all of the investigated zeolites, the CO2 and CH4 permeabilities increased with the zeolite pore size, filler concentration, and feed pressure for both

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Membrane Technologies for Biorefining

PI and PEI-MMMs. Instead, CO2eCH4 selectivities decreased when the zeolite CaA was loaded in a PI matrix. This behavior is the result of the decrease in CH4 transport resistance with an increase in the filler pore size. In general, PEI-MMMs had better permeabilities and lower ideal selectivity compared with PI-MMMs, although opposite data were found in terms of permeabilities for membranes without filler. These different behaviors could be due to a combination of a weak interaction between PEI and zeolite in the MMMs and PEI and gases in the polymeric membranes. Polyimide-based membranes had higher CO2eCH4 selectivity (Fig. 3.5). However, better results in biogas upgrading were obtained for the PI-NaA membrane. In fact, gas chromatographyemass spectrometry analyses demonstrated that using this membrane it was possible to obtain permeated gas-enriched CO2 with a purity equal to 95% starting from a biogas composition. Sorribas et al. (2014) prepared MMMs using PS as a polymeric matrix loaded with MSS-Z8 at different percentages (from 0 to 32 wt%). The authors studied the effect of filler loading on the separation of CO2eCH4 and H2eCO2 mixtures in the temperature range 30e150
C. Considering an equimolar CO2eCH4 mixture at a fixed temperature of 35
C, they found that permeability increased with the filler loadings, whereas selectivity remained almost constant (Fig. 3.6). In particular, CO2 permeability for membranes with 32 wt% MSS-Z8 was 300% higher than that obtained using membranes without a filler. This behavior can be explained by a combination of two effects. The first is better CO2 adsorption on the loaded membranes compared with membranes without spheres (Scholes et al., 2010). The second is enhancement of the gas diffusivity because the silica core has a mesoporous structure with distribution of two different pores. Further increasing the filler amount, no loss of selectivity was detected. This is because of the good interaction between silica and PS, avoiding the formation of nonselective voids.

CO2–CH4 selectivity

30 PI

25

PEI 20 15 10 5 0 Pure polymer

KA-MMMs NaA-MMMs CaA-MMMs Membranes

Figure 3.5 Biogas separation performances for PI- and PEI-based MMMs with different fillers. Elaborated from Ozturk, B., Demirciyeva, F., 2013. Comparison of biogas upgrading performances of different mixed matrix membranes. Chemical Engineering Journal 222, 209e217.

Mixed-matrix membranes: preparation and characterization for biorefining

Permeability

30

35

25

30 25

20

20 15 15 10

10

5

CO2–CH4 selectivity

CO2 permeability (barrer)

Selectivity

75

5

0

0 0

5

10

15

20

25

30

35

MSS–Z8 loaded (wt%)

Figure 3.6 Comparison of permselectivity data for MSS-Z8 MMMs at different loadings. Elaborated from Sorribas, S., Zornoza, B., Téllez, C., Coronas, J., 2014. Mixed matrix membranes comprising silicas-(ZIF-8) core-shell spheres with ordered meso-microporosity for natural- and bio-gas upgrading. Journal of Membrane Science 452, 184e192.

The same sample exhibited an increase in CO2 permeability with temperature. An explanation for this result is the growth of gas diffusivity and an improvement in polymer chain flexibility. On the contrary, a decrease in CO2eCH4 selectivity with temperature is observed for both better diffusion of CH4 and expansion of the membrane free volume. Instead, for the H2eCO2 mixture, selectivity increased with the temperature, achieving the upper bound of the tradeoff curve (Robeson, 2008) at 120
C. Another class of compounds widely used as a filler in MMMs are the CMS. For example, Vu et al. (2003c) studied the effects of CMS fiber in two different polymer matrices, Matrimid® 5218 and Ultem® 1000. They found improvement in terms of CO2eCH4 selectivity for these MMMs with respect to the corresponding polymeric membranes equal to 45% and 40%, respectively. The use of carbon black as a filler in PS-based MMMs also resulted in an increase in both gas permeability and selectivity, as reported by Bhardwaj et al. (2003). Kim et al. (2006) dispersed CNTs in PDMS and Matrimid® and found an increase in permeability without a loss of selectivity for many gases (eg, CO2 and CH4) compared with pure membranes, owing to the great smoothness of the nanotube pore walls. The performance of membranes loaded with different amounts of single-wall CNTs were investigated by the same authors using PS as polymeric materials. Better permselectivity results were found at 5 wt% of filler (Kim et al., 2007). The authors

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Membrane Technologies for Biorefining

obtained the same experimental data loading brominated poly(2,6-diphenyl1,4-phenylene oxide) polymer with 5 wt% of CNTs. In each filler class, the choice of a suitable particle type, also considering the properties of the polymer, is important. For instance, in loading the Matrimid® with Cu-4,40 -bipyridine-hexafluorosilicate (MOF), an increase in gas permeability and a decrease in CO2eCH4 selectivity were obtained (Zhang et al., 2008). Instead, dispersing another MOF-type (MOF-5) in the same polymer, an improvement in terms of both CO2 permeability and CO2eCH4 selectivity was observed, equal to 55% and 6%, respectively (Perez et al., 2009). Bae et al. (2010) synthesized defect-free MMM using PI 6FDA-DAM and ZIF-90 as polymer and filler, respectively. The high affinity of this MOF toward CO2 enhanced CO2 permeability up to 85% and increased CO2eCH4 selectivity to 37%. Ordonez et al. (2010) obtained good membrane performance in the permselectivity of a CO2eCH4 mixture with ZIF-8-Matrimid membranes. The same results were achieved by Thompson et al. (2012). Rodenas et al. (2014) synthesized membranes using different MOFs (NH2functionalized MIL-53(Al) and MIL-101(Al)) and polymers (PS and PI). Their study focused on the effect of the fillers on CO2eCH4 separation. The experimental results showed better permselectivity for the bare PI compared with PS. However, the respective MMMs membranes increased performance. In addition, using the same amount of filler, the improvement was more pronounced for membranes based on the PS polymer (CO2eCH4 separation increased to 25%). For CO2 permeability, an increase only for PS membranes loaded with NH2-MIL-101(Al) was detected. These data can be explained by the existence of an optimal loading maximum for the MMMs. In fact, further increasing the amount of filler results in the formation of clusters with worse contact between the polymer and particles with the formation of defects (Zornoza et al., 2013).

3.5

Conclusion and future perspectives

Currently, the main challenge to using MMMs in biorefinery processes is the development of defect-free membranes to increase both permeability and selectivity. Researchers have made enormous efforts and significant progress in understanding the procedures of synthesis and the mechanism of formation of MMMs. However, more improvements in the possibility of preparing reproducible defect-free membranes should be achieved. For example, it is necessary to optimize polymerefiller compatibility to ensure good interaction between the two phases. Key strategies followed for future development involve many topics regarding the influence of different filler properties (eg, size and shape) on membrane performance. Moreover, well-defined protocols for functionalizing the filler surface before their use in MMMs could avoid the presence of nonselective voids into the membrane structure. Another important aspect to be considered is a reduction of MMM production costs. Achievement of these goals might place these membranes in the forefront of biorefinery processes at an industrial level.

Mixed-matrix membranes: preparation and characterization for biorefining

77

List of symbols A

Membrane area

L

Membrane thickness

P

Permeability

p

Pressure

T

Temperature

t

Time

Tg

Glass transition temperature

V

Volume

List of acronyms APDEMS

3-Aminopropyl-dimethylethoxysilane

APTES

3-Aminopropyl-triethoxysilane

BET

Brunauer, Emmett, and Teller

BDC

Benzenedicarboxylate

BPY

4,40 -Bipyridine

CMS

Carbon molecular sieves

CNT

Carbon nanotube

CS

Chitosan

DAM

2,4,6-Trimethyl-1,3-diaminobenzene

FDA

4,40 -Hexafluoroisopropylidene diphthalic anhydride

FT-IR

Fourier transformeinfrared spectroscopy

GAD

Glutamic acid decarboxylase

HFS

Hexafluorosilicate

LFG

Landfill gas

LTA

Linde type A

MMM

Mixed-matrix membrane

MOF

Metal organic framework

MSS

Mesoporous silica sphere Continued

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Membrane Technologies for Biorefining

PDMS

Polydimethylsiloxane

PEBA

Polyether block amide

PEI

Polyethylene imine

PES

Polyethersulfone

PI

Polyimide

PS

Polysulfone

PVA

Polyvinyl alcohol

PVDF

Polyvinylidene fluoride

SEM

Scanning electron microcopy

TED

Triethylenediamine

ZIF

Zeolite imidazolate framework

ZSM-5

Zeolite Socony Mobil-5

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