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Pervaporation-assisted esterification reactions by means of mixed matrix membranes Roberto Castro-Muñoz, Óscar de la Iglesia, Vlastimil Fila, Carlos Téllez, and Joaquin Coronas Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01564 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Industrial & Engineering Chemistry Research

Pervaporation-assisted esterification reactions by means of mixed matrix membranes

Roberto Castro-Muñoza,b, Óscar de la Iglesiac, Vlastimil Fílaa, Carlos Téllezb, Joaquín Coronasb,*

(a) University of Chemistry and Technology Prague. Technická 5, 16628 Prague 6, Czech Republic (b) Department of Chemical and Environmental Engineering and Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50018 Zaragoza, Spain (c) Centro Universitario de la Defensa Zaragoza, Academia General Militar, 50090 Zaragoza, Spain *Corresponding author: [email protected]

Abstract The incorporation of fillers into polymeric membranes, producing mixed matrix membranes (MMMs), is considered a promising way to improve their separation performance. As an alternative method for the dehydration of organics, pervaporation (PV) technology has recently begun to be implemented to assist esterification reactions, in which the water generated is identified as a limitation to further conversion efficiency. In this regard, the present review conveys the evidence from recent literature reports about PV-assisted esterification reactions. Therefore, a particular emphasis will be placed on the enhancements provided by MMMs. Moreover, some key principles regarding the selection of fillers suitable for synergistic effects on water removal are mentioned. In addition, generalities of PV, including the theoretical aspects

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and its role in separation, are discussed. Finally, an outlook on the future directions based on the latest findings on PV-assisted esterification reactions by means of MMMs is provided, as well as a viewpoint concerning the relationship between the “Twelve Principles of Green Chemistry” and PV technology.

Keywords: Mixed matrix membrane (MMM); Porous filler; Pervaporation; Water removal; Esterification.

Abbreviations PV: pervaporation PAN: polyacrylonitrile PVA: polyvinyl alcohol PDMS: polydimethylsiloxane PTMSP: poly(1-trimethylsilyl-1-propyne) PES: polyethersulfone PEBA: polyether-block-amide PBI: polybenzimidazole PBZ: polybenzoxazine PVDF: polyvinylidene fluoride MMM: mixed matrix membrane P: permeability D: diffusivity S: solubility


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K: sorption coefficient JA: permeate flux of component A β: separation factor CA: concentration of component A CB: concentration of component B PSI: pervaporation separation index α: selectivity MOF: metal-organic framework POC: porous organic cage CNT: carbon nanotube GO: graphene oxide MOP: metal-organic polyhedra ZIF: zeolitic imidazolate framework PCMR: pervaporation catalytic membrane reactor

1. Introduction As a membrane-based separation process, pervaporation (PV) technology provides several advantages over typical techniques (e.g., distillation and absorption) during the separation of azeotropic, close boiling-point, diluted and thermally unstable liquid mixtures where a phase change from liquid to vapor occurs. The main advantages are its low energy consumption and lack of any solvent requirements,1,2 as well as the fact that it is not limited by the vapor-liquid equilibrium. In recent years, this technology has attracted strong interest from scientists who consider it as a replacement for typical separation processes. Moreover, the design and



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implementation of “green” processes, according to the “Twelve Principles of Green Chemistry” developed by Anastas and Warner,3 that help preserve the environment and involve green chemistry methods are currently a considerable challenge.4 This development obviously could encourage scientists to investigate PV. For instance, Figure 1 provides an outlook of the evolution concerning the application of this technology to separating different mixtures over the past 20 years, displaying strong evidence of the interest in advances using PV. Moreover, the application of PV is maturing (interestingly, in the past few years, the topic of PV with mixed matrix membranes, MMMs, has appeared), drawing the attention of companies and manufacturers as well. For example, in 2001, the supplier DeltaMem AG (previously known as Sulzer Chemtech) reported the sales of approximately 100 units operating mainly in the purification of chemicals,5 and today more than 130 PV units are known to be commercially operating globally (http://www.deltamem.ch/). Today, there are some other membrane manufacturing companies, like Beroplan GmbH (from Germany, http://www.beroplan.de/) and Mitsui E&S Co. Ltd. (from Japan, www.mes.co.jp), which support the industrial uses of PV. In particular, Beroplan GmbH confirms the current use of PV units within the so-called hybrid production processes, aiming for the dehydration of solvents (mainly isopropanol and ethanol).6– 8

The PV units have been equipped with membranes based on polymeric and supported zeolite

materials. For instance, one of the Mitsui E&S Co. Ltd. plants possesses 16 tubular membrane modules based on 125 pieces of zeolite NaA supported on alpha-alumina, with a thickness of 2030 µm; such plant has an installed capacity of 530 L h-1.6 This trend proves that PV is currently used in industry. Moreover, although its establishment necessitates high investment costs, the progress of DeltaMem AG in the market can encourage other manufacturers and potential users


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to invest in the PV field, producing strong competition and thus decreasing the prices of standard units.

Figure 1. Publications related to the applications of PV technology over past 20 years (until February 28th, 2018). Studies in yellow are those related to PV with mixed matrix membranes (MMMs).

To date, several types of water-organic mixtures have been tested and can be separated by PV for the purpose of dehydration, e.g., mixtures of water with ethanol,9 isopropanol,10 acetone,11 butanol,

acetic

acid,12

N,N-dimethylformamide,

N,N-dimethylsulfoxide,

N,N-

dimethylacetamide, hydrogen peroxide,2 ethylene glycol,13 N-methyl-2-pyrrolidone,14 and tetrahydrofuran.15 Moreover, organic-water mixtures that have been separated to isolate the organic component from the water include butanol-water,16 furfural-water,17 pyridine-water,18 and

ethylene dichloride-water,19

whereas

among organic-organic

mixtures, benzene-

cyclohexane,20 dimethylcarbonate-methanol,21 methanol-methyl tert-butyl ether,22,23 and acetonebutanol24 have been tested. Among all the different mixtures studied, two types of membrane materials in particular can be used depending on the chemical nature of the compounds to be separated: hydrophilic and hydrophobic polymers (as described in Figure 2), whereas inorganic/ceramic materials can be used as well; but especially, their chemical nature has to be crucially known, e.g. zeolites are commonly considered as hydrophilic (i.e. zeolite NaA and mordenite), however, there are some others which can display a hydrophobic nature (i.e. silicalite-1).25 Hydrophilic polymers are generally used for the dehydration of organics; particularly, these polymers enhance the solubility selectivity of water in the membrane through



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hydrogen bonding interactions.2 However, these membranes are susceptible to swelling by water or polar organic molecules. Indeed, polar organics can also diffuse through these hydrophilic membranes. In contrast, hydrophobic polymers, which are used for organic-water and organicorganic mixtures, tend to allow the diffusion the less polar (or non-polar) organic contained in the mixture.26

Figure 2. Graphical description of the separation mechanisms for hydrophilic and hydrophobic polymeric membranes towards mixtures of polar and non-polar compounds.

The versatility of PV technology has allowed its coupling to other processes, for example, to reaction processes. PV has been successfully applied to the esterification of carboxylic acids (acetic acid, lactic acid, etc.), i.e., the reaction of carboxylic acids with alcohols (methanol, ethanol, etc.) to produce esters,27,28 as shown in Figure 3.

Figure 3. Typical esterification reaction of carboxylic acids and alcohols for ester synthesis.

The production of esters is carried out due to their use for manufacturing foods, cosmetics and medicines in the food, pharmaceutical and petrochemical industries. As esterification occurs, water is produced as a byproduct, representing an issue due to the simultaneous hydrolysis of the ester up to a point at which thermodynamic equilibrium is reached. To overcome equilibrium conversion, the addition of excess alcohol has been previously used, as the limiting step of the reaction is the attack of the carboxylic group by the alcohol. However, this strategy requires additional separation of the alcohol. Recently, PV with MMMs comprising the metal-organic


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framework (MOF) MIL-101(Cr) and the polyimide Matrimid® has been used for the removal of water from the reaction environment.29 The use of PV in such reactions is practically in situ; i.e., the membrane constitutes one of the reactor walls, as Figure 4 depicts. For many years, water has been removed using hydrophilic polymeric membranes, and PV has been employed to promote esterification reactions for the synthesis of ethyl lactate,30,31 n-butyl acrylate,32 butyl acetate,33,34

diethyl

succinate,35

and

isobutyl

propionate36

through

the

use

of

chitosan/carbomer/polyacrylonitrile (PAN) composite, Pervap 2201, PervaTech, polyvinyl alcohol (PVA), PVA-polyethersulfone (PES) and polyvinyl acetate membranes.

Figure 4. Pervaporation (PV)-assisted esterification reactions (in situ and ex situ modes).

To overcome selectivity-permeability limitations of those membranes and enhance their separation performances, filler materials have been incorporated into polymeric membranes, leading to the development of MMMs.37 In particular, these MMMs have shown great improvements in selective transport for gas separation applications.38 In fact, the development and application of MMMs for multiple purposes represent one of the new directions currently being pursued by many membrane researchers in their studies. Therefore, this review focuses on the latest findings of using MMMs for PV-assisted esterification reactions. Moreover, a theoretical background on PV technology and key criteria to select fillers suitable for embedding in polymeric membranes are given. Finally, an outlook on the future directions in the field of PV-assisted esterification reactions by means of MMMs is provided.

2. Basic parameters of pervaporation (PV)



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As mentioned previously, PV is a membrane-based separation technique that can separate binary or, in this case, multicomponent mixtures by selective partial vaporization using a perm-selective barrier, i.e., a dense non-porous membrane. Indeed, PV is the coupling of “permeation” and “evaporation” processes.39,40 Typically, the liquid feed mixture is in direct contact with the “selective” side of the membrane, whereas the permeate (the stream collected at the other side of the membrane) is in the vapor phase, which is enriched with the permeating species having a higher affinity for the membrane (see Figure 2). Generally, the transport mechanism through dense polymeric membranes is described by the solution-diffusion model.40 In principle, the mass transfer across a PV membrane can be described by three main steps: i) adsorption of the target component from the mixture to the “selective” layer of the membrane on the basis of its chemical affinity, ii) diffusion of the component through the membrane as a result of the concentration gradient, and iii) desorption of the component at the permeate side of the membrane.26,41 The mass transport is governed by the chemical potential (µi) gradient, the physical properties of the permeating components (i) and their concentrations on the feed and permeate sides of the membrane. The permeability (P) depends on the diffusivity (D) and solubility (S) of the target components,40 as described by Eq. (1). S is a thermodynamic parameter that provides input on the amount of the penetrant adsorbed by the membrane under equilibrium conditions, whereas D is a kinetic parameter that involves the transport rate of the penetrating component through the membrane.26 The two parameters can be represented by the diffusion (D) and sorption (S) coefficients, respectively:

P =D⋅S

………………….. (1)


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The membrane performance is usually described in terms of permeate flux (J) and separation factor (β).42 The permeate flux (JA) of component A, which generally denotes the faster permeating compound, is described by Eq. (2).

JA =

mA Am ⋅ t

………………….. (2)

where mA is the mass of component A transported through the membrane area (Am) to the permeate stream and collected at time t.42,43 The separation factor (β) is defined as the ratio between the concentrations of components A and B in the permeate and retentate (usually feed stream), as described by Eq. (3).

C   A   C B PERMEATE βA B = C   A   CB FEED

………………….. (3)

Another parameter that is typically calculated is the “pervaporation separation index” (PSI),44 which mainly denotes the trade-off relationship between J and β. The PSI, which is defined as the product of both parameters, as shown in Eq. (4), commonly evaluates the overall performance of a membrane.

PSI = J ⋅ β

………………….. (4)



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Nevertheless, in this definition, the PSI can be large if the membrane displays a high flux even when β is equal to 1. Thus, PSI was later defined as the product of J and (β − 1).44 Subsequently, the permeability (PA) of a dense membrane, defined by the solution-diffusion model (see Eq. (1)), can be calculated by Eq. (5).

PA = where

pA,F G

and

pA,P G

JA ⋅l pAF , G − pAP , G

………………….. (5)

are the partial vapor pressures in a hypothetical vapor phase at

equilibrium for the feed and permeate (commonly with vacuum pressure applied), respectively, and l is the dense-selective layer thickness. The selectivity (αAB) of a membrane for component A over component B describes how efficient the two components can be separated and is defined as the ratio of the permeabilities or permeances for components A and B, according to Eq. (6).

P α= A PB where

PA

or

PA PB

 D  K  =  A   A ………………….. (6)  DB   K B 

is the permeance of component A, which is well defined as the ratio of the

permeability (PA) to the dense-selective layer thickness (l). Importantly, the parameters β and J mainly depend on the operating conditions (feed concentration, permeate pressure, feed temperature), but also depend on the intrinsic properties of the membranes, whereas at a given temperature, α depends on the membrane material used and its properties.43,45 Nevertheless, some other phenomena can cause deviation from this typical solution-diffusion model, e.g., the 10 ACS Paragon Plus Environment

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non-uniform swelling of the membrane across its thickness, concentration or temperature polarization or, in this case, the use of fillers or different polymeric materials, which can result in anomalous sorption and diffusion behaviors of the resulting MMM.26 For example, in the case of MOFs, the Fickian model can be used for describing the diffusion if the system is statistically uniform, whereas the Maxwell model is widely applied for MMMs. In this model, the predicted membrane performance is given by Eq. (7).

Peff = Pc

( ) P +2P +2φ ( P − P )

PD + 2PC −2φD PC − PD D

C

D

C

………………….. (7)

D

where Peff is the effective permeability; PC and PD are the permeabilities of the continuous (polymer) and dispersed (filler) phases, respectively; and ΦD represents the volume fraction of the dispersed phase.46 Additionally, some other models, such as the Lewis-Nielsen model, can also be used for predicting the permeability.47

3. Mixed matrix membranes (MMMs): The concept and its basis in PV technology To date, different hydrophilic (e.g. PVA, PAN, chitosan, sodium alginate, PBI and polyimides) and hydrophobic (e.g. PDMS, PEBA, PTMSP, and PVDF) polymers have been used to prepare membranes for PV. Clearly, the polymers most widely used in MMM preparation are PVA, sodium alginate, chitosan and PDMS, and in some cases, these components have been blended to improve the membrane separation properties.24,48 Recently, some other commercial polymers employed in other membrane fields, i.e., gas separation, have started to be implemented, such as Pebax®, PBI, PIM-1, and polyimides (e.g., Matrimid® 5218 and 6FDA-HAB/DABA).49–51 11 ACS Paragon Plus Environment


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However, unlike MMMs prepared for gas separation applications, some specific features have to be carefully taken into account for preparing membranes for PV applications, such as polymer nature (hydrophilic/hydrophobic) linked to the component to be removed out, type of filler and its nature (again hydrophilic/hydrophobic), membrane thickness, solvent resistance (which is related to the chemical stability). Thereby, next section provides a clear outlook regarding the key criteria suggested to prepare compelling MMMs for PV.

3.1.Key criteria to select fillers for embedding in polymeric membranes. As mentioned previously, the concept of MMMs has emerged to overcome the selectivitypermeability trade-off of polymeric membranes.52 Figure 5 shows a graphical description of common MMMs, which were proposed to combine the strengths of inorganic and polymeric membranes2,53 and therefore to ideally achieve a synergistic performance.54 To date, several types of inorganic materials have been embedded in polymeric membranes to carry out PV separations, such as zeolites, MOFs, carbon nanotubes (CNTs), porous organic cages (POCs), silicas (mesoporous MCM-41), graphene oxides (GOs), MOF-silica hybrid particles, metalorganic polyhedra (MOPs), activated carbon, TiO2, magnesium oxide and Ag-based compounds.2,55–58

Figure 5. Graphical depiction of a mixed matrix membrane.

Fundamentally, the nature of the polymer matrix constituting the main continuous phase of an MMM and its properties (e.g., hydrophobicity/hydrophilicity, structure and stability) are crucial for the PV performance of MMMs. In addition, the effective incorporation of the filler will


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depend on the kind of polymer: in principle, a hydrophilic filler should be embedded in a hydrophilic polymeric matrix, whereas a hydrophobic filler should be embedded in a hydrophobic polymeric matrix.59 Following this key principle could help guarantee simultaneous improvements in both the flux and separation factor, besides enhancing the filler-polymer iteraction. Similarly, the features of the inorganic-organic material, e.g., the degree of hydrophobicity/hydrophilicity, surface chemistry and charge, geometry and size and textural properties (i.e., pore diameter and volume),38,59,60 play important roles as well. For example, to select filler particles, their dispersibility, stability, morphology, size, porosity and hydrophobicity/hydrophilicity should be considered:46,60,61 i.

Dispersibility: The dispersion of the filler into the polymer involves the creation of nonselective interfacial defects and thus affects MMM performance. Furthermore, the compatibility between the polymer and filler (its surface functional structure) has an effect as well. For example, excellent filler-polymer compatibility could eliminate the formation of interfacial defects. For MOFs, the elements used for their synthesis, for example are crucial: e.g., the suitable assembly between metal ions and organic ligands and their coupling on the surface of a polymeric matrix could result in a uniform dispersion of the embedded MOF.61 Chemical surface modification of the filler also enhances its dispersion into the membrane,62 but it is important to mention that most filler particles tend to agglomerate when subjected to common drying methods because of the strong capillary forces between porous particles and their high surface energy, particularly when handled as nanoparticles.

ii.

Stability: In MOFs, the metal types and ligands are essential. The properties of the metal species (metal oxidation state, ionic radius, polarizability, etc.) have a considerable effect



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on the metal-ligand bond strength. Similarly, the polarizability of metals and ligands and the similarity of their properties yield a strongly binding coordination complex.46 However, these factors also cause the low stability of MOFs under hydrothermal conditions, where the strength of the bond between the metal cluster and the bridging linker, as well as possibly the framework flexibility, is crucial in determining their hydrothermal stability.63 The MOFs based on group IV metals (Ti, Zr, and Hf in the +4 oxidation state) are characterized by high chemical stability, while using ligands with high pKa values is useful for increased water resistance. Some examples of MOFs that are thermodynamically stable in the presence of water are MIL-101(Cr), MIL-101(Cr)SO3H, ZIF-8, MIL-96(Al), JUC-110, MIL-100(Cr), Bio-MOF-14, UiO-66, and MIL-125NH2.46,61 In contrast to MOFs, fillers such as porous silicates and zeolites, although less compatible with polymers, may present better stabilities in terms of hydrolysis and pH.64,65 For example, regardless of their Si/Al ratios, zeolites tend to be stable under hydrothermal conditions up to 200 °C,66 which can be important when the goal is a long and stable PV operation. In this regard, zirconium-based MOFs may also have potential due to their stability in nitric acid, triethylamine, and boiling water solutions.67 iii.

Hydrophilicity/hydrophobicity: The hydrophobic or hydrophilic nature of MOFs is mainly defined by the ligands and by the presence of open metal sites: generally, hydrophobic MOFs exhibit preferential adsorption selectivity for organic solvents over water, i.e., suitable for the removal of organics in water. Some examples of hydrophobic MOFs are ZIF-89 and ZIF-71.68 By contrast, UiO-66,50 MIL-101(Cr),29 and HKUST-169 are highly hydrophilic, making them suitable for the dehydration of organics. Commonly, these hydrophilic MOFs possess polar sites due to the metal oxygen clusters and very


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non-polar regions due to the organic and mostly aromatic linkers, molecules that constitute the largest fraction of the inner surface. Water molecules preferentially adsorb on the hydrophilic centers, avoiding the hydrophobic areas on the surface, and additional water molecules are bound by hydrogen bridges to these water nucleation sites, resulting in small water clusters.70 In addition, some other inorganic materials, such as zeolites (KA, NaA, CaA, NaX, NaY, silicate-1, and H-ZSM-5), graphene,71 porous silicas,72 TiO273 and CNTs74 tend to be hydrophilic as well. Depending on the type of material (e.g., MOF or zeolite), the filler can augment or overcome the nature of the polymer. For example, Figure 6 shows that in MMMs a hydrophilic zeolite, zeolite 4A, can shift the hydrophobic nature of PVDF,75 whereas a MOF filler with a more similar nature to the polymer can enhance the polymer properties.76

Figure 6. Effect of filler (zeolite 4A and the MOF Zn(BDC)(TED)) loading on the contact angle of MMMs. Data taken from Liu et al.76 and Shen and Lua.75

iv.

Size and morphology: Decreasing the size of the MOFs particle is used to improve the external specific surface and the entry to the interior surface. Nevertheless, in a series of MMMs prepared with 0.35, 0.70 and 2.4 µm particles of the MFI-type zeolite silicalite-1, the smallest particles underperformed in ethanol-water PV due to agglomeration resulting from some PDMS defect healing.77 The particle morphology and pore size also determine MMM features.46,78 For example, if the pore size lies between the molecular kinetic diameters of the target components in the mixture to be separated, the smaller molecule can diffuse into the pores, while the larger molecule is retained, causing a molecular



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sieving effect to occur. However, if the pore size is slightly larger than the diameter of the larger molecule, the separation is based on the difference in diffusion rates. Moreover, if the pore size is vastly larger than both molecules, they can be separated principally by the difference in their adsorption properties.46 Regarding the MOF morphology, core-shell silica-MOF particles (hydrophilic core and MOF shell) can be promising to enhance the water permeability and to achieve better filler-polymer compatibility.9 Certainly, if the agglomeration issues are solved, the use of nanoparticles and nanosheets can be considered a method for preparing MMMs that are more permeable than dense membranes. For example, the use of graphene or graphene oxide nanosheets with narrow nanopores leads to an assembled barrier layer with more channels (or shorter pathways) than those built by non-porous layers. In addition, the single atomic layer structure of graphene-based materials provides the possibility of preparing ultrathin membranes from multiple layers of graphene and/or graphene oxide. Nanoporous surfaces allow higher permeability; thus, the resulting membranes may display higher flux and relatively higher separation efficiencies.79

4. Esterification reactions assisted by PV using MMMs The main point of assisting esterification reactions (between carboxylic acids and alcohols) is the removal of the generated water as soon as possible after it is produced and thus achieving a higher production rate of the desired products.80 The role of PV in the esterification reaction in practice is the dehydration step, which can be carried more efficiently by using MMMs. In principle, the increase in the free volume in the polymeric membrane by incorporating filler particles can definitely create openings in the polymer matrix on a transient basis, facilitating the


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diffusion of penetrant molecules through the polymer.81 In addition, the filler properties can selectively promote water permeation. To date, the esterification reactions that have been assisted include the production of ethyl acetate, ethyl lactate, n-butyl acetate, isobutyl propionate, methyl laurate and methyl acetate from the corresponding alcohols and carboxylic acids. The following sections provide an overview of the influence of PV using MMMs on these reactions. 4.1. Ethyl lactate production PV has been mainly used for the removal of the water from esterification reactions to displace the chemical reaction equilibrium. Certainly, the esterification of lactic acid was the first PVassisted reaction.82–84 The interest in this reaction stems from the production of ethyl lactate (also known as ethyl 2-hydroxypropanoate) by reacting lactic acid and ethanol. Ethyl lactate can be used in the manufacture of foods, cosmetics and flavor chemicals and as a solvent; in fact, it is considered a green solvent and can dissolve cellulose acetate and numerous resins.85 In the first report on enhancing the esterification reaction yield using PV,82,83 the use of zeolite A and zeolite/polyelectrolyte multilayer membranes was proposed. The water removal (≈0.2 kg·m-2·h-1) using the zeolite A membrane improved the ethyl lactate yield to over 90% (from an initial value of 70% at 70 °C).

83

Notably, this membrane was placed in the vapor phase due to the low

stability of zeolite A in acidic environments. Subsequently, other zeolites displaying better resistance than zeolite A were tested (e.g., zeolite T).86 Similarly, the multilayer membrane (comprising chitosan and poly(4-styrenesulfonate) layers coating a zeolite A film) proposed by Budd et al.82 helped increase the ethyl lactate yield from 60% up to 90% (at 70 °C) through the vaporization of water (water fluxes between 0.2 and 0.4 kg·m-2·h-1). Budd et al.82 also pointed out that in the production of ethyl lactate through the esterification of lactic acid, using highly water permeable membranes is more interesting than using selective membranes, especially



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given that the water content increases in the first hours of the reaction because of the high initial reaction rate. More recently, hydrophilic PERVAP® 2201 membranes (a cross-linked PVA selective layer supported on a porous polyester layer) have been proposed.27,87,88 First, a preliminary evaluation of the membrane using model solutions was carried out: this membrane demonstrated very high affinity for water and thus an excellent water permeation flux, which increased as long as the water feed concentration increased for all possible scenarios, e.g., water-ethanol (0.5 kg·m-2·h-1), water-lactic acid (5 kg·m-2·h-1) and water-ethyl lactate (1.2 kg·m-2·h-1), while for the ternary model mixture (water-ethanol-ethyl lactate-lactic acid) the water permeation remained at approximately 1 kg·m-2·h-1.27,87 Subsequently, the PV membrane was coupled to a reactor and tested on a real esterification reaction to help increase the lactic acid conversion. For instance, the non-assisted reaction exhibited a maximum constant lactic acid conversion of 20%, whereas through the use of the PV processes, the lactic acid conversion reached between 70 and 90% (at 75 °C), leading an enhancement in ethyl lactate production.88 Notably, a commercial inorganic membrane made of microporous silica (Pervatech® BV)31 had a high flux and high selectivity for water but reduced ethanol and ethyl lactate permeation and no lactic acid permeation. Specifically, during the PV of water/ethyl lactate mixtures, the membrane displayed waterselective behavior (β=400-800) and a temperature dependency. However, the membrane showed a decrease in water permeation when the feed water concentration increased due to a decrease in the isosteric heat of water adsorption, leading to a higher activation energy of permeation. This is because the activation energy is actually the sum of the diffusion activation energy and heat of sorption;89 therefore, the water permeability decreases as indicated by the negative values of the activation energies of permeation.31 The authors also found a possible saturation effect of water


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diffusion through the silica membrane pores. Finally, the effect of water removal on lactic acid conversion was not studied, but it was likely enhanced. In fact, owing to membrane dehydration, Zhang et al.30 increased the ethyl lactate yield by 28% (at 80 °C). In their study, the yield had a strong relationship with the dehydration rate but was not directly proportional to the water flux. This observation was attributed to the fact that the membranes have to allow not only water but also ethanol to pass through. Moreover, a 94% ethyl lactate yield can be achieved by using a chitosan/carbomer/PAN composite membrane under optimal conditions.30 To the best of our knowledge, the first study using an MMM to assist the esterification of lactic acid was reported by Ma et al.90 Table 1 shows the current findings on esterification reactions assisted by PV using MMMs, as well as the details of the analyzed studies. Ma et al.90 proposed the use of hybrid membranes based on tetraethoxysilane (TEOS)-filled chitosan. This MMM could remove water from the reaction mixture and could substantially enhance the ethyl lactate yield from 66 to 80% (at 80 °C). TEOS allowed the adjustment of the hydrophilicity of the membrane material, thereby enhancing the water removal efficiency. In addition, TEOS improved the resistance of the membrane against swelling by restricting the mobility of the polymer chains. Notably, the β (water-ethanol) increased from 157 to 459 when the mass ratio of TEOS in chitosan increased from 3 to 6%, which was attributed to the increased interactions between water molecules and the TEOS-filled chitosan membranes. This increase in interactions could be related to the greater number of hydrophilic groups generated by the hydrolysis of TEOS.

Table 1. Esterification reactions assisted by PV using MMMs.



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4.2. Ethyl acetate production To date, the reaction most commonly promoted by PV technology is probably the esterification of acetic acid with ethanol. Acetic acid esterification to produce ethyl acetate is important because of the use of ethyl acetate in several applications, including as in inks for flexographic and rotogravure printing, varnishes, lacquers, and nitrocellulose, as well as in the manufacture of perfumes. Considering its predicted growing demand (3-4% per year in the Chinese and Southeast Asia markets), ethyl acetate production has been widely explored for surface coatings and as a replacement for restricted and banned solvents (ICIS Chemical Business, www.icis.com). These potential uses may have encouraged membrane researchers to find a means of overcoming the limitations of the ethyl acetate production reaction or improving the ethyl acetate yield by coupling a reactor with membrane PV. Regarding the initial attempts, inorganic membranes seem to have been proposed first;91 for example, de la Iglesia et al.92 employed zeolite NaA and mordenite membranes as a highly selective barrier to water permeation. The membrane based on zeolite NaA reached a maximum ethyl acetate yield of 73.3% (at 85 °C), whereas the mordenite membrane gave a yield of 72.4%. However, after the maximum value, the conversion with the zeolite NaA membrane decreased, whereas when the mordenite membrane was used the yield increased up to 90%. This behavior was attributed to zeolite NaA containing a larger amount of Al than mordenite, which makes zeolite NaA not only more hydrophilic but also more unstable under acidic conditions. Recently, Zhu et al.93 also corroborated the acid stability of mordenite membranes in the esterification of acetic acid with ethanol. Their membrane greatly enhanced the alcohol conversion (up to 98.1% from an initial conversion of 70% at 85 °C) in the studied esterification reaction.


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In the context of MMMs, Gao et al.94 could be the first scientists who proposed incorporating MMMs based on zeolites (NaA, KA, CaA and NaX) into PVA, which led to a considerable increase in conversion (from 80% in the non-assisted reaction to over 95%) while simultaneously decreasing the reaction time as a result of the continuous removal of water. Recently, zeolite beta particles have been incorporated into sodium alginate and then applied for dehydration during the esterification of acetic acid with ethanol.95 In particular, the filler (aluminum-rich zeolite beta) is hydrophilic, promoting the interaction with sodium alginate (which is also hydrophilic). Therefore, the zeolite beta-sodium alginate MMMs helped increase the ethyl acetate yield through the continuous removal of water from the reaction system: whereas the non-assisted reaction reached a maximum ethyl acetate yield (80% at 70 °C) after 14 h and the assisted reaction with the pristine sodium alginate membrane reached the highest yield (85%) after 13 h, the zeolite-alginate MMM reached a 90% ethyl acetate yield in only 5 h. Hence, the presence of zeolite particles favors the permeation of water molecules (causing an increase in reaction rate), leading to enhanced ethyl acetate conversion. The promising performance of these membranes was attributed to the molecular sieving effect, selective sorption/adsorption and selective diffusion of the zeolites present in the MMMs, which increased the mobility of the preferentially permeating water molecules.95 Similarly, Bhat and Aminabhavi96 added zeolite NaA to crosslinked sodium alginate, showing practically the same tendency as reported by Adoor et al.95 with zeolite beta except with an ethyl acetate yield over 95% (at 70 °C). The authors suggested that the continuous removal of water using the MMMs had a favorable effect by shifting the chemical equilibrium in the forward direction. These zeolite NaA-sodium alginate membranes displayed a lower water flux (0.13 kg·m-2·h-1) than the zeolite beta-sodium alginate membranes (0.22 kg·m-



2

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·h-1), in agreement with the difference in pore sizes (ca. 0.4 and 0.7 nm for zeolites A and beta,

respectively), but a higher β, as shown in Table 1. Recently, relatively new hydrophilic MOF particles, such as Cu3(BTC)2,69 and MIL-101(Cr),29 have been proposed for the PV-aided dehydration in the production of ethyl acetate. Cu3(BTC)2, which is also well known as HKUST-1, is a hydrophilic material containing Cu2 units coordinated by four carboxylate groups, which create a highly porous cubic structure with a 3D network of 0.9 x 0.9 nm2 channels. The most hydrophilic type of HKUST-1 exhibits a bimodal pore size distribution70 and possesses 12 Cu2 (OOC-) paddle-wheel secondary building units carrying two copper coordination sites each.97 Sorribas et al.69 incorporated this MOF into a commercial hydrophilic polyimide (Matrimid® 5218), achieving a 63% reactant conversion (at 50 °C), higher than that of the batch reaction (47% for the non-PV-assisted reaction). These MMMs were able to remove water (0.35 kg·m-2·h-1), shifting the reaction towards the esterification products and increasing the total conversion; in fact, the permeate was nearly 90% water. Even though HKUST-1 is highly selective for water, higher permeation fluxes could be obtained by using other types of MOFs with a higher porosity (pore/cavities of 1.2/2.9 nm), such as MIL-101(Cr). For instance, de la Iglesia et al.29 evaluated the performance of MIL-101(Cr) filled into Matrimid® 5218, demonstrating that these MMMs exhibited higher water stability in the reaction medium than HKUST-1-based MMMs (as shown in Figure 7). There was also no significant reduction in particle size arising from partial dissolution; however, their conversions were relatively similar, i.e., approximately 70.5 and 71.8% (at 70 °C) for the MIL-101(Cr) and HKUST-1 MMMs, respectively. Nevertheless, both MOF-based MMMs afforded better conversions than the pristine polyimide membrane (63.5%) and better water permeation fluxes (0.15-0.18 kg·m-2·h-1), more than two-fold higher than that of the pristine polymer (0.07 kg·m-


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2

·h-1). The enhancement was due to the large pore size of the filled MOF MIL-101(Cr):

compared to HKUST-1 (smaller pore size), MIL-101(Cr) allows high water uptake at the hydrophilic sites near the metal centers and then enables the water molecules to propagate to the hydrophobic organic linkers.97,98 In fact, the MIL-101(Cr) MMMs presented a water uptake of 15.8 wt%, while that of the bare polyimide was approximately 2.5 wt%.29

Figure 7. Effect of water removal on ethyl acetate conversion in a membrane reactor at 70 ° C (Data taken from de la Iglesia et al.29). Solid and hollow symbols correspond to the HKUST-1 and MIL-101(Cr) containing polyimide Matrimid® 5218 MMMs, respectively. The curves are only guides to the eye.

Until recently, some novel composite membranes, which are not precisely considered as MMMs, were used for the separation of water once the esterification reaction had occurred, as Figure 8a depicts. Unlu and Hilmioglu99 proposed the development of a pervaporation catalytic membrane reactor (PCMR) in which the membrane plays a dual role in separation and reaction. In this study, the PCMR consisted of a composite membrane based on cross-linked chitosan coated with a thin layer of zirconium sulfate tetrahydrate (Zr(SO4)2·4H2O) (see Figure 8b).

Figure 8. Comparative depiction of a PCMR with the conventional PV-assisted reactions for the esterification of acetic acid with ethanol.

First, a batch catalytic reaction assisted by pristine chitosan was carried out, obtaining an acetic acid conversion of 41% (approximately 6 h, 70 °C). This result confirms that chitosan itself can



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improve the reaction kinetics and conversion by removing water. In fact, chitosan contains hydroxyl and reactive amine groups that facilitate the diffusion and sorption of water. Nevertheless, during the esterification reaction in the PCMR, the conversion of acetic acid increased up to 98% in less than 6 h with relatively high water permeation fluxes (0.6 kg·m-2·h-1) even with the cross-linking procedure,99 which is crucial to making the membrane more stable, based on the evidence of its swelling and elasticity in aqueous solutions.100 Importantly, the Zr(SO4)2·4H2O-chitosan membranes displayed good reusability and stability.99 In a different approach, the esterification reaction of lauric acid with methanol to produce methyl laurate was performed by Ma et al.101 This reaction was also assisted by using a dual-purpose ceramic composite membrane, which was formed with an α-Al2O3 hollow fiber support coated with a perfluorosulfonic acid solution (containing 10 wt% PVA as an adhesive and nano-TiO2 as a support). These organic-inorganic composite membranes, even if they are not MMMs, were able to reach a lauric acid conversion of approximately 80% (70 °C).

Ma et al.58 also proposed a unique approach to the implementation of MMMs for the production of ethyl acetate: they prepared dual-purpose membranes by incorporating TiO2 into sulfonated poly(phthalazinone ether sulfone ketone)-PES (SPPESK–PES). In this case, the filler was also intended to function as a catalyst. These TiO2–SPPESK–PES nanocomposite membranes showed high catalytic activity, helping reach a maximum ethanol conversion up to 82%. In contrast, a blank (non-membrane-assisted) reaction displayed only 49% ethanol conversion. In conclusion, the filler can not only be optimized and used to enhance the membrane separation performance but also contribute to the esterification reactions as an active catalyst.58,101 A promising strategy for improving the physical properties of chitosan-based membranes was proposed by Lin et


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al.,102 who used graphene oxide (GO) embedded in chitosan affect both the PV and esterification kinetics. GO can function in this manner because it contains several oxygen-containing functional groups (e.g., hydroxyl, epoxide, diol, ketone, and carboxyl), some of which can chemically react with the amine group of chitosan to form a bond between GO and the biopolymer. These GO-chitosan membranes displayed a higher water permeation flux (approximately 0.4 kg·m-2·h-1) than membranes reported in previous studies. The ethanol conversion increased from 64% in the non-assisted reaction up to 84% (at 70 °C) by coupling the MMMs to the reaction. The authors also addressed the potential of these membranes to assist other types of esterification reaction, e.g., the pre-esterification of palmitic acid with methanol for possible biodiesel production.102 Lu et al.103 developed similar bifunctional catalytic membranes through the preparation of multilayer membranes by depositing a thin layer of PES/ perfluorosulfonic acid/SiO2 nanofibers onto a PVA/PAN substrate. First, these membranes were able to remove up to 92% of the produced water by PV, which led to a considerable enhancement in the acetic acid conversion (over 90% at 60 °C) compared to the non-PV-assisted reaction (42%).

4.3. Production of other esters PV technology has recently started to be implemented in the production of other interesting esters such as n-butyl acetate, which is another important organic solvent and raw material that is commonly synthesized through the esterification of acetic acid with n-butanol. n-Butyl acetate is applied as a solvent in the manufacture of acrylic polymers, vinyl resins, and cosmetic formulations and as a synthetic fruit flavoring in foods (candies, chewing gums, cheeses, ice creams).104 For instance, Zhang et al.105 dehydrated n-butyl acetate by using PVA-chitosan blend



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membranes. The performance of these blend membranes was tested on the dehydration of binary (water/n-butyl acetate) and ternary (n-water/butyl acetate/n-butanol) mixtures. Generally, the water permeation of the PVA-chitosan blend membranes was better than that of the membranes made of the pristine polymers; according to Zhang et al.105, this result can be attributed to the hydrogen bonds between chitosan (-OH, -NH2) and PVA (-OH). These bonds are weaker than the intermolecular hydrogen bonds in pristine PVA or chitosan, which therefore may reduce the energy barrier and enhance the water diffusion in the blend membranes. For the binary tests using 25/75% chitosan/PVA membranes, a maximum β of 27,000 with a water flux of 0.4 kg·m2

·h-1 was obtained at 40 °C. For the ternary mixture under the same conditions, the highest water

flux was also approximately 0.4 kg·m-2·h-1 with a strong decrease in β to 1000. This behavior was related to the interaction between n-butanol and n-butyl acetate, which could influence the ability of the membrane to adsorb n-butyl acetate. Indeed, the water flux in the binary mixture was slightly less than that in the ternary mixture, while the n-butyl acetate flux in the binary system was greater than that in the ternary mixture. These promising blend membranes have great potential to improve the esterification yield of n-butyl acetate. The esterification between produces isobutyl propionate, an ester with a characteristic fruity odor similar to apple, apricot and rum. Hence, isobutyl propionate is widely used in the food industry as an artificial flavor and fragrance. The PV-assisted esterification of propionic acid and isobutyl alcohol was performed by Chandane et al.36, who used a commercial hydrophilic PVA-PES blend membrane (supplied by Permionics Membranes) that displayed significant selectivity for water, increasing the conversion from 67 to 88% (at 80 °C). Furthermore, Chandane et al.36 demonstrated that the rate of water removal by the PVA-PES membrane is higher than the rate of water production in the esterification, which was determined through the analysis of the F


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parameter (F > 3.59), i.e., the ratio of the water removal rate to the water production rate. In principle, F > 1 signifies that the rate of water removal is greater than the rate of water production, indicating that conversion can reach up to 100% and is limited by the water production rate.106 Finally, the PVA-PES membranes proposed by Chandane et al.36 also presented good reusability and stability. As previously mentioned, the commercial PERVAP® 2201 membrane is formed by cross-linked PVA. This hydrophilic membrane was employed in the implementation of PV technology in the esterification of acrylic acid with n-butanol to produce n-butyl acrylate,32 which is used in the production of homopolymers and copolymers. The partial water flux of the membrane was higher than the fluxes of the other components (n-butanol, acrylic acid, and n- butyl acrylate). In addition, the water permeation flux increased with temperature, displaying an Arrhenius-type dependency. Therefore, the water affinity of this membrane led to a maximum acrylic acid conversion of 96.3% (at 85 °C). Indeed, the implementation of PV clearly increased the reactant conversion, as the maximum acrylic acid conversion without PV was approximately 36%. Furthermore, the authors revealed that this enhancement could be attributed to the strong waterPVA interaction, resulting in a suitable coupling between water and PV.32 At this point, it is important to note that the solubility parameters (dispersive, polar and hydrogen bonding contributions) for the polymer-solvent pair can govern the affinity or compatibility between the two; i.e., typically, closer solubility parameter values indicate higher compatibility and thus promote the separation of the preferential solvent through the polymeric membrane.107,108 These solubility parameters could be used to choose the optimal filler (MOF)-polymer system in terms of favoring their interaction,38 which would minimize the membrane defects.



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PVA is probably the most widely used hydrophilic polymer for the separation of water from water-organic mixtures. Recently, its use as a continuous phase has been demonstrated in the preparation of MMMs for enhancing the separation of water in several other esterification reactions. For example, Torabi and Ameri109 incorporated organophilic silica into cross-linked PVA for the production of methyl acetate (esterification of acetic acid with methanol). First, the pristine cross-linked PVA membrane increased the acid conversion from 42% for the non-PVassisted reaction up to 52%, while the silica MMMs allowed the authors to reach a 94% acid conversion (at 70 °C). Clearly, the water fluxes were increased (i.e., up to 0.28 kg·m-2·h-1), whereas the permeance rates of methanol, methyl acetate and acetic acid decreased. This result could be attributed to the fact that the presence of hydrophilic silica nanoparticles (featuring polar silanol groups) in the PVA matrix led to a significant increase in water permeation, thereby leading to strong interactions with polar water molecules. In addition, the β values for water were considerably enhanced compared to those for the other components.109 Similarly, Shameli and Ameri110 used cross-linked PVA as the continuous phase for the preparation of MMMs with multiwalled CNTs as the dispersed phase. These MMMs also promoted the esterification of acetic acid with methanol, helping increase the acetic acid conversion from 52.3% (using the pristine membrane at 70 °C) to 99.3% (using the membrane with 2 wt.% filler loading). Once again, the enhanced water removal enabled by the MMMs allowed the realization of this acid conversion, which increased with the filler loading. Indeed, the incorporation of CNTs into PVA imparted more hydrophilic polar groups to the matrix, causing strong hydrogen bonding between the nanotubes and water molecules. Importantly, the CNTs were previously washed with H2SO4 and HNO3 solution for further purification (removal of impurities), providing open-ended CNTs.110 Nevertheless, this chemical treatment can modify


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the surface of the CNTs, generating carboxylic groups (-COOH) and other functional groups (e.g., hydroxyls).111 Moreover, the new polar groups improve the dispersion of the CNTs in organic solvents and polymer matrices.111,112 Over the course of this section, it has been seen that the filler material features provide better membrane performances related to enhanced water removal, resulting in higher conversion efficiencies when compared to the pristine membranes. Indeed, the particle features may mainly influence on the membrane properties according to: i) change on hydrophilicity (see Figure 6)75,76 which influences, as mentioned before, the water permeation fluxes and conversion rates (see Figure 7),29 whereas some other properties can also be beneficed, such as ii) mechanical and thermal properties,69 and iii) the conversion efficiency when the particles are catalytic.58,99,101,102

Concluding outlook The application of mixed matrix membranes (MMMs) to pervaporation (PV)-assisted esterification reactions, where the removal of water from the reactions has been demonstrated, enables the enhancement of the conversion beyond the thermodynamic equilibrium limitations. Therefore, after a careful choice of the suitable inorganic material to be incorporated into the corresponding hydrophilic polymeric material, enhancements in the reactant conversions from 4.4 up to 89% can be obtained (as displayed in Table 1) relative to the values achieved with the pristine polymeric membranes. This enhancement has been reported at least for the production of ethyl lactate, ethyl acetate, methyl laurate and methyl acetate. Henceforth, PV using MMMs could be applied to other esterification reactions (e.g., n-butyl acetate, isobutyl propionate, nbutyl acrylate, diethyl succinate) where only the use of pure polymer membranes has been



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proposed. Most likely, the high added value of the produced esters will encourage membrane researchers to carry out further developments in the field. Similarly, the use of PV in the production of other interesting products, such as biodiesel, can also motivate the expansion of its application,102 even though of its implementation, design, scale-up and optimization represent current challenges.113 The design strategies for the development of new nanomaterials (e.g., MOFs) will play a crucial role in the preparation of novel MMMs. For instance, the preparation of dual-purpose materials would be welcome, as these species can also function as a catalyst, making them an efficient tool for process intensification.58,101,114,115 In the near future, PV will surely play an important role in the replacement of other technologies on the industrial scale. For instance, conventional distillation provides higher productivities (in terms of flux) than PV; however, to reach the high degree of purification obtained by PV, distillation requires the installation of at least two distillation columns,116,117 which directly influences the energy consumption of the overall process and certainly impacts the economic valuation.118 In addition, the production demand in PV can be achieved by modifying the operating parameters (such as membrane area, temperature, driving force).116 For polymeric membranes, since most of the PV-assisted esterifications have been studied with dense membranes, meeting the high productivities in terms of permeate fluxes will necessitate the preparation of highly permeable supports coated with a thin dense-selective layer.119 The main issue of using fillers in MMMs on the industrial scale, however, is their long-term stability, which must still be improved, especially in the case of MOFs. In this regard, the incorporation of new additives such as covalent organic frameworks (COFs) and porous organic frameworks (POFs), which present excellent chemical compatibility


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with the organic polymer phase and chemical stability conferred by irreversible covalent bonding, may be an interesting alternative.56 Moreover, PV-assisted esterification reactions seem to reach most of the “Twelve Principles of Green Chemistry”,3 which are suggested for the design of a greener processes and/or product. For example, the only byproduct of esterifications is water, the removal of which contributes to improving the reactant conversion and overcoming the chemical equilibrium limitations. At this point, this class of reactions meets the following principles: 1st) prevention/minimization of waste; 2nd) synthetic methods should be designed to maximize the incorporation of feedstocks in the final product; and 9th) the catalyst should be as selective as possible or superior to stoichiometric reactants. Importantly, most of the abovementioned esters (e.g., methyl acetate, ethyl lactate, ethyl acetate, and methyl acrylate) are 100% biodegradable, easy to recycle, non-corrosive, and noncarcinogenic, and some of them have been approved for food processing and packaging (e.g., ethyl lactate, ethyl acetate, methyl acrylate) by the U.S. Food and Drug Administration (https://www.fda.gov/). Thus, additional green chemistry principles can be met: 3rd) less hazardous chemical syntheses that generate substances with minimal or non-toxicity to human health and the environment; 4th) designing safer chemicals; 7th) use of renewable feedstocks; and 10th) chemical products should be designed so that at the end of their function, they degrade into innocuous degradation products and do not persist in the environment. Finally, PV is a technology that works without the use of additional solvents and is recognized as a low energy consumption process in comparison to traditional technologies (e.g., distillation), which allows it to meet the remaining principles: 5th) safer chemicals and additives should be minimized and innocuous when need; 6th) energy requirements should be recognized for their



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environmental and economic impacts and should be minimized; and 8th) reduce or avoid derivatives (they could need additional reactants and can generate wastes).

Acknowledgments R. Castro-Muñoz acknowledges the European Commission - Education, Audiovisual and Culture Executive Agency (EACEA) for his PhD scholarship under the program Erasmus Mundus Doctorate in Membrane Engineering – EUDIME (FPA No 2011-0014, Edition V, http:/eudime.unical.it). This work was partially supported by the Operational Program Prague – Competitiveness (CZ.2.16/3.1.00/24501), the “National Program of Sustainability“ (NPU I LO1613) MSMT-43760/2015 and by financial support from specific university (Prague University) research (IGA 2017, MSMT No 20-SVV/2017). Financial support from the Spanish MINECO and FEDER (MAT2016-77290-R) and the Aragón Government (T05) is also gratefully acknowledged.


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Figure 1. Publications related to the applications of PV technology over past 20 years (until February 28th, 2018). Studies in yellow are those related to PV with mixed matrix membranes (MMMs).


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Figure 2. Graphical description of the separation mechanisms for hydrophilic and hydrophobic polymeric membranes towards mixtures of polar and non-polar compounds.


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Figure 3. Typical esterification reaction of carboxylic acids and alcohols for ester synthesis.



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Figure 4. Pervaporation (PV)-assisted esterification reactions (in situ and ex situ modes).


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Figure 5. Graphical depiction of a mixed matrix membrane.



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Figure 6. Effect of filler (zeolite 4A and the MOF Zn(BDC)(TED)) loading on the contact angle of MMMs. Data taken from Liu et al.76 and Shen and Lua.75


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Figure 7. Effect of water removal on ethyl acetate conversion in a membrane reactor at 70 °C (Data taken from de la Iglesia et al.29). Solid and hollow symbols correspond to the PI-HKUST-1 and PI-MIL-101 (Cr) MMMs, respectively. The curves are only guides to the eye.



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Figure 8. Comparative depiction of a PCMR with the conventional PV-assisted reactions for the esterification of acetic acid with ethanol.


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Table 1. Esterification reactions assisted by PV using MMMs. Esterification reaction (reactants)

Ester produced

Reactant conversion/ product yield

MMM

Water flux:

Separation factor

Catalyst

Temperature

Conversion enhancement (%)

Referenc

(kg·m-2·h-1): (%)

Lactic acid

Ethyl lactate

Lactic acid: 80

TEOS-chitosan

0.27

βwater/ethanol: 459

Amberlyst 15

80 °C

21.2*

Ma et al

Ethyl acetate

Acetic acid: 95

NaA, KA, CaA, NaX zeolites-PVA

0.20

βwater/ethanol: 19-40

Sulfonated cationexchange resin

70 °C

28.3*

Gao et al

10.4*

Adoor et

ethanol Acetic acid ethanol Acetic acid

βwater/acetic acid: 200-410 Ethyl acetate

Ethyl acetate: 90

Zeolite beta-sodium alginate

0.22

ethanol

Acetic acid

Dowex-50

βwater/acetic acid: 900

Ethyl acetate

Ethyl acetate: 95

Zeolite 4A-sodium alginate

0.13

ethanol

Acetic acid



Dowex-50

70 °C

4.4**

70 °C

12.5*


Bhat and Aminabh

5.5**

Ethyl acetate

Ethanol: 63

HKUST-1-polyimide

0.35


AmberlystTM 15

50 °C

34.0*

Sorribas

Ethyl acetate

Ethanol: 71.8

HKUST-1-polyimide

0.18

βproduct/reactant: 2.5

Amberlyst 15®

70 °C

13.0**

de la Igle al. 29

Ethyl acetate

Ethanol: 70.5

MIL-101(Cr)polyimide

0.15

βproduct/reactant: 1.8

Amberlyst 15®

70 °C

11.0**

de la Igle al. 29

ethanol Acetic acid ethanol

Acetic acid ethanol



Acetic acid

Ethyl acetate

Acetic acid: 98

ethanol

Top catalytic layer of Zr(SO4)2·4 H2O on chitosan

6x10-5

βwater/total components: 22

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Zr(SO4)2·4H2O

80 °C

139.0*

Unlu and Hilmiogl

Amberlyst 15®

70 °C

123.8*

Torabi an Ameri 10

βethanol/total components: 1 βethyl acetate/total components: 1 βacetic acid/total components: 1

Acetic acid

Methyl acetate

Acetic acid: 94

Silica-cross-linked PVA

0.28

methanol

βwater/methanol: 1107 βwater/acetic acid: 8752

80.7**

βwater/methyl acetate: 6251

Acetic acid

Methyl acetate

Acetic acid: 99.2

Carbon nanotubescross-linked PVA

0.27

methanol


Amberlyst 15®

70 °C

βwater/methanol: 8654

135.0*

Shameli Ameri 11

89.7**

βwater/methyl acetate: 1022

Acetic acid

Ethyl acetate

Ethanol: 82

TiO2-SPPESK–PES

NR

NR

TiO2

80 °C

ethanol

Acetic acid

64.0*

Ma et al.

20**

Ethyl acetate

Ethanol: 84

GO-chitosan

0.4

NR

Amberlyst 15®

74 °C

34.2*

ethanol


Lin et al.

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Acetic acid ethanol

Ethyl acetate

Acetic acid: 92

PES/perfluorosulfonic acid/SiO2 nanofibers on PVA/PAN

0.27

NR

Perfluorosulfonic acid

60 °C

119.0*


Lu et al.


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Notes: Reactions assisted by PV using MMMs compared to the non-PV-assisted reaction (*) and with the PV-assisted reaction with pristine polymeric membranes (**); Amberlyst®, ion-exchange resin; Dowex, sulfonated cation-exchange resin; NR: Not reported.


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GRAPHICAL ABSTRACT


Pervaporation-Assisted Esterification Reactions by ... - ACS Publications

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