WO2020075075A1 - Process for preparation of cellulose based carbon molecular sieve membranes and membranes thereof - Google Patents

Process for preparation of cellulose based carbon molecular sieve membranes and membranes thereof Download PDF

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Publication number
WO2020075075A1
WO2020075075A1 PCT/IB2019/058573 IB2019058573W WO2020075075A1 WO 2020075075 A1 WO2020075075 A1 WO 2020075075A1 IB 2019058573 W IB2019058573 W IB 2019058573W WO 2020075075 A1 WO2020075075 A1 WO 2020075075A1
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Prior art keywords
cellulose
precursor
molecular sieve
ionic liquid
film
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PCT/IB2019/058573
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French (fr)
Inventor
Adélio Miguel MAGALHÃES MENDES
Sandra Cristina VALE RODRIGUES
Márcia Rafaela SILVA DE ANDRADE
Tiago Jorge TORRES BESSA MENEZES ARAÚJO
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Universidade Do Porto
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Priority to EP19790293.5A priority Critical patent/EP3863754A1/en
Publication of WO2020075075A1 publication Critical patent/WO2020075075A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/021—Carbon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0067—Inorganic membrane manufacture by carbonisation or pyrolysis
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/18—Manufacture of films or sheets
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2301/00—Characterised by the use of cellulose, modified cellulose or cellulose derivatives
    • C08J2301/02—Cellulose; Modified cellulose

Definitions

  • the present invention relates bo a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) , including the preparation of an adequate polymeric film precursor and to the obtained membranes thereof.
  • CMSMs cellulose based carbon molecular sieve membranes
  • the process of the present invention is performed in a single carbonization step and produces CMSMs that exhibit no pore blockage in the presence of water vapor up to 80% RH and displays high ideal permeselectivites and permeabilities in gas separation process, in particularly when using humid gas streams.
  • the present invention is thus useful in applications where gas separation is required, particularly useful in separation process using humid gas streams.
  • the present invention lays in the field of separation methods, more specifically gas separation methods, even more specifically in separation processes using humid gas streams in high HR conditions.
  • Membrane processes for gas separation have been increasingly improved over the past decades.
  • Membrane based gas separation is very attractive compared with conventional separation techniques due to its high adaptability, high reliability, low energy consumption and low capital cost, operation and maintenance, which makes it often a more energy-saving and environmentally friendly technology.
  • CMSMs carbon molecular sieve membranes
  • CMSMs are typically prepared from carbonization of a polymeric precursor under controlled conditions, in particular of atmosphere and temperature conditions.
  • the precursor material plays a key role in the production of CMSMs since different precursor materials carbonized under the same conditions result in carbon membranes with different properties.
  • the applied carbonization conditions such as carbonization end temperature, heating rate, atmosphere and soaking time, are also crucial for tailoring the final structure of the CMSM, namely pore size and structure.
  • CMSMs Despite all attractive characteristics displayed by known CMSMs, the permeability of all reported membranes decreases abruptly when contacting with humidified streams with a relative humidity above a threshold; this threshold is typically in the range of 30 % to 40 % of relative humidity.
  • Many polymeric precursors have been widely studied for obtaining CMSM with excellent separation performances and stability. Polyimides, polyacrylonitrile, poly ( furfuryl alcohol), phenolic resins, resorcinol-formaldehyde resin and cellulose include some of these precursors.
  • Cellulose is a biodegradable material with a moderate carbon yield and low-cost and is emerging as a very good candidate for CMSM preparation.
  • Document CN102527156 (A) relates to the preparation of carbon membranes from linear and branch polymers network.
  • Document WO2017068517 Al relates to the production of carbon molecular sieve membranes having as precursor material regenerated cellulose produced by viscose-process for obtaining CMSMs displaying no pore blockage when permeating humidified gas streams.
  • viscose technology originates hazardous by ⁇ products and a large quantity of waste. Furthermore, this process presents limited dissolution capacity, toxicity, high cost, uncontrollable side reactions and instability during cellulose procession, which limits its use to three days. In fact, preparing cellulose precursors with tailor-made properties using the viscose technology is rather difficult.
  • the present invention proposes the preparation of a new precursor material based on ionic liquid (ILs) solvents and cellulose films.
  • ILs ionic liquid
  • the present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs), including the preparation of an adeguate polymeric film precursor and to the obtained membranes thereof.
  • CMSMs cellulose based carbon molecular sieve membranes
  • the present invention relates to a process for preparing a precursor system based on cellulose and ionic liquid (ILs) solvents according to claim 1.
  • This precursor system is advantageously used in processes for preparing carbon molecular sieve membranes (CMSMs) .
  • the present invention relates to a precursor cellulose film for preparing a carbon molecular sieve membrane (CMSM) according to claim 5.
  • CMSM carbon molecular sieve membrane
  • the present invention relates to a process for prepacation of a carbon molecular sieve membrane (CMSM) using a cellulose and IL precursor system according to claim 6.
  • CMSM carbon molecular sieve membrane
  • This process allows the preparation of improved CMSMs in a single carbonization step.
  • the present invention relates to improved carbon molecular sieve membranes (CMSMs) according to claim 10.
  • CMSMs are selective and have tuneable properties, thus present high gas separation performance and further avoiding blockage under humid gas streams separation conditions as high as up to 80% of RH.
  • the present invention is thus useful in applications where gas separation is required, particularly advantageously in separation processes using humid gas streams in high HR conditions .
  • Fig.l Refers to a graphic showing the temperature history to prepare carbon molecular sieve membranes (CMSMs) .
  • the graphic plots the carbonization process-heating rate accomplished by dwells of 30 min for avoiding a quick release of volatile matter.
  • the slow heating rate prevents the damage of the carbon matrix, i.e., avoids the formation of cracks/defects on the membrane.
  • the graphic depicts two different carbonization end temperatures; 550°C and 600°C. After reaching the end temperature, the system cools naturally until room temperature.
  • Fig.2 Shows surface and cross-sectional views of scanning electron images of a CMSM carbonized at 550°C, wherein:
  • Fig.3 Shows the micropore size distribution of a carbon molecular sieve membrane (CMSM) prepared at 550°C.
  • CMSM carbon molecular sieve membrane
  • the CMSM displays ultramicropores (0.4-0.7 nm range) and micropores (0.7-1.2 nm range) .
  • Fig.4 Shows, the gas permeation results for 0 2 /N 2 in carbon membranes carbonized at different temperatures and comparison with the respective upper bound limit, wherein:
  • Fig.5 Refers to FTIR spectra of regenerated cellulose precursor and resultant carbon molecular sieve membranes (CMSMs) prepared at 500°C and 550°C.
  • CMSMs carbon molecular sieve membranes
  • Fig.6 Shows the permeability values of CMSMs in function of the gas kinetic diameter for regenerated cellulose precursor systems, wherein
  • ⁇ represents based membranes carbonized at 600°C.
  • the present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) , including the preparation of an adequate polymeric film precursor and to the obtained membranes thereof.
  • CMSMs cellulose based carbon molecular sieve membranes
  • the present invention relates to a process for preparation of a precursor system based on cellulose and ionic liquid (ILs) solvents that allows to prepare improved CMSMs in a single carbonization step.
  • CMSMs are produced by regenerated cellulose films through an ionic liquid process in a single carbonization step.
  • the molecular sieving process of CMSMs occurs at the ultramicropores levels, which are constrictions in larger pores pathways. These larger pores connect ultramicropores that are responsible for the molecular sieve mechanism allowing higher permeabilities .
  • the precursor system selection and the carbonization process are crucial steps since it influences directly the performance for the production of CMSMs.
  • the precursor material By tailoring the precursor material, it is possible to control CMSMs properties and increase the performance for a given gas separation.
  • the precursor should be selected from a thermosetting polymer to avoid melting or softening during the carbonization process and should display a high carbon yield.
  • the present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) .
  • CMSMs cellulose based carbon molecular sieve membranes
  • CMSMs Cellulose based carbon molecular sieve membranes
  • Cellulose is a biodegradable material with a moderate carbon yield and low-cost.
  • Cellulose polymer originates a gate sieving morphology suitable for the separation of small spherical/spheroid molecules.
  • Cellulose materials suitable to be used in the scope of the present invention can be selected from cellulose wood pulp. Other cellulose materials such as microcrystaline cellulose (MCC) , cellulose acetate or nitrocellulose can be used.
  • the cellulose material is wood pulp having a degree of polymerization of 450 and water content is about 10%
  • Ionic liquids are considered "environmentally-friendly" salts with a wide range of melting temperatures, excellent dissolution ability, high thermal stability (up to 400°C), non-inflammable, presenting chemical stability, and easiness of recyclability.
  • the ionic liquid 1-ethyl- 3-methyl imidazolium acetate (EMIMAc) is liquid solvent at room temperature having high dissolving power even in the presence of 10 %wt . of water, and relatively low viscosity, when compared with other ionic liquids and low toxicity. Additionally, the ability as high hydrogen bond acceptor, namely, hydrogen bond acceptor sites in the anion structure and lack of hydrogen bond donors in the ionic liquid cation favours the dissolution of cellulose.
  • EMIMAc Due to these characteristics, EMIMAc revealled to be a very suitable IL solvent for the production of tailor-made regenerated precursor films that enables the preparation of improved carbon membranes .
  • the selected IL is mixed with another organic solvent, DMSO (Dimethyl sulfoxide) for improving the cellulose dispersion.
  • DMSO Dimethyl sulfoxide
  • DMSO is added to EMIMAc in a proportion of 70:30 %wt . of DMSO to EMIMAc.
  • the precursor solution is prepared by mixing the selected cellulose material with the ILs solvent solution prepared as described above. This solution is added to the cellulose material, preferably to cellulose pulp, to form a 8-10 %wt . preferably a 9-9.5 %wt. cellulose precursor solution, under stirring and heating conditions with a variable temperature of 70 to 100°c, preferably of 80 to 90°C, more preferably of 75 to 80°C until the cellulose material is completely dissolved.
  • the precursor solution is advantageously filtered, for example with a wire mesh and placed in a vacuum oven at a variable temperature of 30 to 50°C, preferably of 35 to 45 °C for degassing during 1 to 4hours, preferably 2 to 3hours.
  • the precursor solution prepared as described above is then subject to a casting process.
  • the casting process is performed by spin coating on rectangular glass plates with a suitable spin-coater at a spinning speed of 2000 rpm, spin acceleration of 1000 rpm/ s and a spinning time of 10 seconds.
  • the films are coagulated in distilled water, at a temperature of 20 to 30°C, preferably of 22 to 25°C, for obtaining a regenerated cellulose film.
  • the obtained film is washed to remove the excess of ionic liquid, for example with distilled water for around 60 min.
  • the washed film is advantageously dipped in a softener bath containing 5 %wt . of propylene glycol for 1 min and then dried in an oven at 100°C for 10 min.
  • a softener bath containing 5 %wt . of propylene glycol for 1 min and then dried in an oven at 100°C for 10 min.
  • the present invention relates to cellulose precursor films suitable for preparing CMSMs .
  • a plasticizer is added to the precursor film and preferably has a concentration of 3-10 wt %.
  • the plasticizer confers flexibility to the film acting as a polymeric chain-spacer.
  • Cellulose films can be plasticized with a series of biopolymers' plasticizers according to the described by Vieira, M.G.A., et al . , in Natural-based plasticizers and biopolymer films: A review. European Polymer Journal, 2011. 47(3) : p. 254-263) .
  • elongated molecules containing -OH groups typically, elongated molecules containing -OH groups.
  • the plasticizer addition can be made in the last step in the precursor film preparation.
  • Carbon molecular sieve membranes according to the present invention are prepared by a process that allows to control their pore size distribution independently by tailor the precursor material and varying the carbonization end temperature .
  • the precursor material is heated until the desired temperature in a controlled atmosphere at a specific heating rate, originating an amorphous carbon membrane with a very narrow porosity that is responsible for the molecular sieve properties of the carbon membrane.
  • the molecular sieving process of CMSM occurs at the ultramicropores which are constrictions in larger pores pathways. These larger pores connect ultramicropores that are responsible for the molecular sieve mechanism allowing higher permeabilities .
  • the carbonization temperature is situated between the decomposition temperature of the carbonaceous precursor material and its graphitization temperature.
  • the optimum carbonization end temperature strongly depends on the precursor material.
  • the precursor selection and the carbonization process are crucial steps since it influences directly the performance of the produced CMSM. In fact, the choice of a suitable precursor is crucial to guarantee the production of defect-free CMSM.
  • the precursor should be thermosetting polymer to avoid melting or softening during the carbonization process and should display a high carbon yield.
  • the carbonization of the ionic liquid-regenerate cellulose film has a heating rate of 0.1-
  • the ionic liquid-regenerate cellulose film has a heating rate of 0.5-5°C-min , preferably of 1°C -min
  • the carbonization of the ionic liquid- regenerate cellulose film has one or more dwells, in particular comprising a first dwell at 110°C.
  • the carbonization of the ionic liquid- regenerate cellulose film is in a controlled atmosphere, preferably of nitrogen or vacuum atmosphere.
  • the carbonization of the ionic liquid- regenerate cellulose film is in an atmosphere comprising a
  • the ionic liquid process according to the present invention allows the tailoring of the precursor films and consequently the optimization of preparation of improved CMSMs . Accordingly, in another embodiment the present invention relates to improved carbon molecular sieve membranes obtained by the above described process.
  • CMSMs Carbon molecular sieve membranes
  • Carbon molecular sieve membranes produced from an ionic liquid- regenerate cellulose precursor in a single carbonization step displayed high performances and stability when exposed to humidity. Namely, the CMSMs obtained show no pore blockage to the permeation of humidified gas mixtures up to relative humidities of 80 % and display high ideal permselectivities and permeabilities for industrially relevant gas separations.
  • the carbon molecular sieve membrane pore size distribution is controlled independently by tailor the precursor material and varying the carbonization end temperature.
  • amorphous carbon membranes are obtained with a very narrow porosity that is the features responsible for the molecular sieve properties of each carbon membrane.
  • the carbon molecular sieve membrane present large pores with size of 0.5-1 nm interconnected to smaller pores with size of 0.4- 0.5 nm .
  • CMSMs often suffer oxidation when in contact with ambient air resulting in a loss of permeability and renders the surface more hydrophilic and then prone to retain water and other vapor clusters.
  • the water vapor adsorption isotherm exhibits a S- shape behaviour. This shape is related to the hydrophobic nature of CMSM; for RH lower than ca. 30 % water molecules adsorb in hydrophilic sites and desorb permeating the membrane. For higher humidity values, water still adsorb in hydrophilic sites but when the vapor concentration is high enough several water molecules stablish sequentially H-bonding resulting in water clusters. When these clusters gain enough dispersion energy to detach from the hydrophilic site they move over the hydrophobic path towards pore constrictions, blocking them.
  • CMSM present substantial advantages for application several industries, such as separation of nitrogen and oxygen from air, separation of hydrogen from syngas, removal of C0 2 from natural gas wells, recover of hydrogen from natural gas, C0 2 separation from flue gas, air dehumidification, gas natural dehydration, helium recovery/separation, ethene/ethane or xenon from gas mixtures.
  • CMSMs are selective and have tuneable properties, thus present high gas separation performance and further avoiding blockage under humid gas streams separation conditions as high as up to 80% of RH.
  • the mixture was heated at 90 °C under magnetic stirring until cellulose was completely dissolved.
  • the solution was filtered with a wire mesh and placed in a vacuum oven at 40 °C for degassing for 2 hours. After the obtained solution was spin coated on rectangular glass plates with a spin-coater (POLOSTM, SPIN150i) at a spinning speed of 2000 rpm, spin acceleration of 1000 rpm/ s and a spinning time of 10 seconds. After coating, the films were coagulated in distilled water (25 °C) for obtaining a regenerated cellulose film.
  • POLOSTM, SPIN150i spin-coater
  • the film was then intensely washed with distilled water for 60 min to remove the excess of ionic liquid. After that, the washed film was dipped in a softener bath containing 5 wt . % of propylene glycol for 1 min and then dried in an oven at 100 °C for 10 min.
  • the precursor Prior to the carbonization step, the precursor was cut in disks with 48 mm in diameter.
  • the ionic liquid-regenerate cellulose films carbonization was accomplished in a quartz tube (80 mm in diameter and 1.5 m in length) inside a tubular horizontal Termolab TH furnace.
  • the protocol had an end temperature ranging between 500 °C and 900 °C, preferably between 525 °C and 575 °C, and more preferably 550 °C, without soaking time, a nitrogen flow rate
  • FIG.2 shows the surface and cross- sectional views of an obtained CMSM at 550 °C.
  • the carbon membranes were glued to steel O-rings. Epoxy glue (Araldite® Standard) was applied along the interface of the steel 0- ring and the CMSM. Also, and for support the film in the test cell, a sintered metal disc with a filter paper was used. Single gases were tested at 25 °C, feed pressure of 1 bar and vacuum at the permeate side. Tests were performed in a standard pressure- rise setup LabView® data logging.
  • Permeation system included the membrane module connected to a vessel with a calibrated volume at the permeate side and connected also to a gas cylinder at the feed side. Feed gas could be dry or humidified by passing through a bubbler with distilled water prior to the membrane module. The relative humidity was measured by a RH meter at an exit port.
  • the permeability, Pg of the CMSM towards to pure component i was determined accordingly to: where Fluxi is the flux of the species i , DRi the partial pressure of the species i across the membrane and d the membrane thickness.
  • permselectivity or ideal selectivity [7] The ratio of two gases permeability coefficients is termed permselectivity or ideal selectivity [7] as described:
  • Table 1 shows the obtained permeabilities and ideal selectivities for several dry gases on CMSM produced at 550°C and 600 °C.
  • Carbon molecular sieve membranes are prepared from ionic liquid-regenerated cellulose according to Example 1 and Example 2, varying the carbonization end temperature between 550°C and 600°C.
  • FIG.6 shows the permeation results of 02 and N2 in carbon membranes carbonized at 550°C and 600°C end temperatures and the upper bound limit. This figure shows that the prepared CMSM have a separation performance well above the referred upper bound limit.
  • the pore size distribution and the porosity volume were obtained for carbon molecular sieve membranes based on the adsorption equilibrium isotherm of C02 at 0°C using the volumetric method.
  • FIG. 3 plots the pore size distribution for a sample prepared at 550 °C and Table 2 the micropore volume, the mean pore width and the characteristic energy for adsorption.
  • CMSM 550 presents micropores situated in the range of 0.7-1 nm, mainly responsible for surface diffusion mechanism, and interconnected ultramicropores in the range of 0.4-0.7 nm responsible for the sieving process of carbon membranes .
  • Table 3 shows the permeation results for the prepared CMSM when exposed to different levels of relative humidity for several hours .

Abstract

The present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs), including the preparation of an adequate polymeric film precursor and to the obtained membranes thereof. The use of an innovative film precursor of ionic liquid-regenerated cellulose allows the preparation of CMSMs in a single carbonization step. The obtained CMSM are selective and have tuneable properties, thus present high gas separation performance and further avoiding blockage under humid gas streams separation conditions as high as up to 80% of RH. The present invention is thus useful in applications where gas separation is required, particularly advantageously in separation processes using humid gas streams in high HR conditions, in the field of separation methods, more specifically gas separation methods, even more specifically in separation processes using humid gas streams in high HR conditions.

Description

DESCRIPTION
PROCESS FOR PREPARATION OF CELLULOSE BASED CARBON MOLECULAR SIEVE
MEMBRANES AND MEMBRANES THEREOF
Technical field
The present invention relates bo a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) , including the preparation of an adequate polymeric film precursor and to the obtained membranes thereof.
In particular, it is herein proposed a tailor-made ionic liquid-regenerated cellulose precursor that is useful in the preparation of a selective CMSM.
In result, the process of the present invention is performed in a single carbonization step and produces CMSMs that exhibit no pore blockage in the presence of water vapor up to 80% RH and displays high ideal permeselectivites and permeabilities in gas separation process, in particularly when using humid gas streams.
The present invention is thus useful in applications where gas separation is required, particularly useful in separation process using humid gas streams.
Therefore, the present invention lays in the field of separation methods, more specifically gas separation methods, even more specifically in separation processes using humid gas streams in high HR conditions. Background
Membrane processes for gas separation have been increasingly improved over the past decades. Membrane based gas separation is very attractive compared with conventional separation techniques due to its high adaptability, high reliability, low energy consumption and low capital cost, operation and maintenance, which makes it often a more energy-saving and environmentally friendly technology.
Furthermore, carbon molecular sieve membranes (CMSMs) display high corrosion resistance, high thermal stability and excellent permeabilities / permselectivities making them very promising candidates for gas separation processes.
CMSMs are typically prepared from carbonization of a polymeric precursor under controlled conditions, in particular of atmosphere and temperature conditions. The precursor material plays a key role in the production of CMSMs since different precursor materials carbonized under the same conditions result in carbon membranes with different properties. Also, the applied carbonization conditions, such as carbonization end temperature, heating rate, atmosphere and soaking time, are also crucial for tailoring the final structure of the CMSM, namely pore size and structure.
Despite all attractive characteristics displayed by known CMSMs, the permeability of all reported membranes decreases abruptly when contacting with humidified streams with a relative humidity above a threshold; this threshold is typically in the range of 30 % to 40 % of relative humidity. Many polymeric precursors have been widely studied for obtaining CMSM with excellent separation performances and stability. Polyimides, polyacrylonitrile, poly ( furfuryl alcohol), phenolic resins, resorcinol-formaldehyde resin and cellulose include some of these precursors. Cellulose is a biodegradable material with a moderate carbon yield and low-cost and is emerging as a very good candidate for CMSM preparation.
Document CN102527156 (A) relates to the preparation of carbon membranes from linear and branch polymers network.
Document WO2009129984 (Al) relates to the preparation of carbon molecular sieve membranes from cellulose acetate as precursor material dissolved in an organic solvent such as DMSO .
Document WO2017068517 Al relates to the production of carbon molecular sieve membranes having as precursor material regenerated cellulose produced by viscose-process for obtaining CMSMs displaying no pore blockage when permeating humidified gas streams.
However, said viscose technology originates hazardous by¬ products and a large quantity of waste. Furthermore, this process presents limited dissolution capacity, toxicity, high cost, uncontrollable side reactions and instability during cellulose procession, which limits its use to three days. In fact, preparing cellulose precursors with tailor-made properties using the viscose technology is rather difficult.
To overcome the above described drawbacks of the art, the present invention proposes the preparation of a new precursor material based on ionic liquid (ILs) solvents and cellulose films. By using the proposed process, it is possible to obtain improved CMSMs .
Summary of the invention
The present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs), including the preparation of an adeguate polymeric film precursor and to the obtained membranes thereof.
In a first embodiment, the present invention relates to a process for preparing a precursor system based on cellulose and ionic liquid (ILs) solvents according to claim 1. This precursor system is advantageously used in processes for preparing carbon molecular sieve membranes (CMSMs) .
In a second embodiment, the present invention relates to a precursor cellulose film for preparing a carbon molecular sieve membrane (CMSM) according to claim 5.
In a third embodiment, the present invention relates to a process for prepacation of a carbon molecular sieve membrane (CMSM) using a cellulose and IL precursor system according to claim 6. This process allows the preparation of improved CMSMs in a single carbonization step.
In a fourth embodiment, the present invention relates to improved carbon molecular sieve membranes (CMSMs) according to claim 10. The obtained CMSMs are selective and have tuneable properties, thus present high gas separation performance and further avoiding blockage under humid gas streams separation conditions as high as up to 80% of RH. The present invention is thus useful in applications where gas separation is required, particularly advantageously in separation processes using humid gas streams in high HR conditions .
Description of the figures
Fig.l Refers to a graphic showing the temperature history to prepare carbon molecular sieve membranes (CMSMs) . The graphic plots the carbonization process-heating rate accomplished by dwells of 30 min for avoiding a quick release of volatile matter. The slow heating rate prevents the damage of the carbon matrix, i.e., avoids the formation of cracks/defects on the membrane. The graphic depicts two different carbonization end temperatures; 550°C and 600°C. After reaching the end temperature, the system cools naturally until room temperature.
Fig.2 Shows surface and cross-sectional views of scanning electron images of a CMSM carbonized at 550°C, wherein:
(A) Surface view of CMSMs carbonized at 550°C; the surface appears to be very smooth, with no visible defects.
(B) Cross-sectional view of CMSMs carbonized at 550°C; the micrograph allows the membrane thickness measurement.
Fig.3 Shows the micropore size distribution of a carbon molecular sieve membrane (CMSM) prepared at 550°C. The CMSM displays ultramicropores (0.4-0.7 nm range) and micropores (0.7-1.2 nm range) .
Fig.4 Shows, the gas permeation results for 02/N2 in carbon membranes carbonized at different temperatures and comparison with the respective upper bound limit, wherein:
- The dashed line represents the best result and was added for comparison.
- The solid line represents Robeson upper bound (2008) .
Fig.5 Refers to FTIR spectra of regenerated cellulose precursor and resultant carbon molecular sieve membranes (CMSMs) prepared at 500°C and 550°C.
Fig.6 Shows the permeability values of CMSMs in function of the gas kinetic diameter for regenerated cellulose precursor systems, wherein
• represents based membranes carbonized at 550°C, and
■ represents based membranes carbonized at 600°C.
Description of the invention
The present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) , including the preparation of an adequate polymeric film precursor and to the obtained membranes thereof.
In a first embodiment, the present invention relates to a process for preparation of a precursor system based on cellulose and ionic liquid (ILs) solvents that allows to prepare improved CMSMs in a single carbonization step. In particular, CMSMs are produced by regenerated cellulose films through an ionic liquid process in a single carbonization step.
The process for preparation of a precursor system according to the present invention can be described as comprising the following steps:
a) Dissolving a cellulose material in an ionic liquid solvent , b) Casting the cellulose solution of (a) to obtain a cellulose based film,
c) Regenerating the cellulose film of (b) ,
d) Drying the regenerated cellulose film of (c) .
The molecular sieving process of CMSMs occurs at the ultramicropores levels, which are constrictions in larger pores pathways. These larger pores connect ultramicropores that are responsible for the molecular sieve mechanism allowing higher permeabilities .
Therefore, the precursor system selection and the carbonization process are crucial steps since it influences directly the performance for the production of CMSMs. By tailoring the precursor material, it is possible to control CMSMs properties and increase the performance for a given gas separation. For originating high quality carbon membranes, the precursor should be selected from a thermosetting polymer to avoid melting or softening during the carbonization process and should display a high carbon yield. Also, there are chemical structures, such as benzene rings, which upon carbonization originate graphene platelets.
When these structures display an accurate arrangement in the precursor, they borough this structure to the carbonized membrane. Because of that, it is possible to obtain a gate or tubular sieving microporous morphologies with two extreme morphologies adapted to different gas separations, or an intermediate morphology. The chemistry of the precursor as well as its 3D configuration is critical on obtaining tailor made inproved CMSMs , suitable for a given separation.
Therefore, in a seoond srbodiment, the present invention relates to a process for preparation of cellulose based carbon molecular sieve membranes (CMSMs) .
In general, the process of the present invention can be described as comprising the following steps:
a) Providing a precursor system based on a cellulose film with an ionic liquid solvent,
b) Carbonizing the film of (a) under specific carbonization conditions.
1. Process for preparing a precursor film based on cellulose wi th ionic liquid solvents
Cellulose based carbon molecular sieve membranes (CMSMs) are prepared by using cellulose based precursor film. This precursor film is prepared with a cellulose material dissolved in an ionic liquid solvent, casting to obtain a cellulose film, regeneration and drying said film.
1.1. Cellulose materials
Cellulose is a biodegradable material with a moderate carbon yield and low-cost. Cellulose polymer originates a gate sieving morphology suitable for the separation of small spherical/spheroid molecules. Cellulose materials suitable to be used in the scope of the present invention can be selected from cellulose wood pulp. Other cellulose materials such as microcrystaline cellulose (MCC) , cellulose acetate or nitrocellulose can be used.
In a preferred embodiment, the cellulose material is wood pulp having a degree of polymerization of 450 and water content is about 10%
1.2 Ionic liquid solvents
Ionic liquids are considered "environmentally-friendly" salts with a wide range of melting temperatures, excellent dissolution ability, high thermal stability (up to 400°C), non-inflammable, presenting chemical stability, and easiness of recyclability.
The ionic liquid 1-ethyl- 3-methyl imidazolium acetate (EMIMAc) is liquid solvent at room temperature having high dissolving power even in the presence of 10 %wt . of water, and relatively low viscosity, when compared with other ionic liquids and low toxicity. Additionally, the ability as high hydrogen bond acceptor, namely, hydrogen bond acceptor sites in the anion structure and lack of hydrogen bond donors in the ionic liquid cation favours the dissolution of cellulose.
Due to these characteristics, EMIMAc revelled to be a very suitable IL solvent for the production of tailor-made regenerated precursor films that enables the preparation of improved carbon membranes .
The selected IL is mixed with another organic solvent, DMSO (Dimethyl sulfoxide) for improving the cellulose dispersion.
In a preferred embodiment DMSO is added to EMIMAc in a proportion of 70:30 %wt . of DMSO to EMIMAc.
1.3 Precursor solution
The precursor solution is prepared by mixing the selected cellulose material with the ILs solvent solution prepared as described above. This solution is added to the cellulose material, preferably to cellulose pulp, to form a 8-10 %wt . preferably a 9-9.5 %wt. cellulose precursor solution, under stirring and heating conditions with a variable temperature of 70 to 100°c, preferably of 80 to 90°C, more preferably of 75 to 80°C until the cellulose material is completely dissolved.
The precursor solution is advantageously filtered, for example with a wire mesh and placed in a vacuum oven at a variable temperature of 30 to 50°C, preferably of 35 to 45 °C for degassing during 1 to 4hours, preferably 2 to 3hours.
1.4 Film casting
The precursor solution prepared as described above is then subject to a casting process. This can be done by a known technique in the art. Preferably, the casting process is performed by spin coating on rectangular glass plates with a suitable spin-coater at a spinning speed of 2000 rpm, spin acceleration of 1000 rpm/ s and a spinning time of 10 seconds. After coating, the films are coagulated in distilled water, at a temperature of 20 to 30°C, preferably of 22 to 25°C, for obtaining a regenerated cellulose film.
The obtained film is washed to remove the excess of ionic liquid, for example with distilled water for around 60 min.
After that, the washed film is advantageously dipped in a softener bath containing 5 %wt . of propylene glycol for 1 min and then dried in an oven at 100°C for 10 min. The above described process allows to obtain tailor made polymeric precursor films, namely cellulose precursor films that enable the preparation of improved carbon sieve membranes .
It is also possible to add functionalising substances to the precursor film such as plasticizers for giving flexibility and to change the final CMSM microporous morphology and chemistry .
Therefore, in another embodiment, the present invention relates to cellulose precursor films suitable for preparing CMSMs .
These cellulose precursor films obtained by the ionic-liguid process are far more stable than the ones obtained by the viscose process .
In a preferred embodiment, a plasticizer is added to the precursor film and preferably has a concentration of 3-10 wt %. The plasticizer confers flexibility to the film acting as a polymeric chain-spacer.
Cellulose films can be plasticized with a series of biopolymers' plasticizers according to the described by Vieira, M.G.A., et al . , in Natural-based plasticizers and biopolymer films: A review. European Polymer Journal, 2011. 47(3) : p. 254-263) . Typically, elongated molecules containing -OH groups. The plasticizer addition can be made in the last step in the precursor film preparation.
2. Process far pc^arat n c£ a carbon molecular sieve membrane (CMSM)
Carbon molecular sieve membranes according to the present invention are prepared by a process that allows to control their pore size distribution independently by tailor the precursor material and varying the carbonization end temperature .
In the carbonization step, the precursor material is heated until the desired temperature in a controlled atmosphere at a specific heating rate, originating an amorphous carbon membrane with a very narrow porosity that is responsible for the molecular sieve properties of the carbon membrane.
The molecular sieving process of CMSM occurs at the ultramicropores which are constrictions in larger pores pathways. These larger pores connect ultramicropores that are responsible for the molecular sieve mechanism allowing higher permeabilities .
The carbonization temperature is situated between the decomposition temperature of the carbonaceous precursor material and its graphitization temperature. The optimum carbonization end temperature strongly depends on the precursor material.
The precursor selection and the carbonization process are crucial steps since it influences directly the performance of the produced CMSM. In fact, the choice of a suitable precursor is crucial to guarantee the production of defect-free CMSM.
By tailoring the precursor material, it is possible to control CMSM properties and increase the performance for a given gas separation .
As mentioned before, for originating high quality carbon membranes, the precursor should be thermosetting polymer to avoid melting or softening during the carbonization process and should display a high carbon yield.
Therefore, in a general way the process for preparing CMSMs may be described by comprising the following steps:
a) Providing a precursor system based on a cellulose film with an ionic liquid solvent,
b) Carbonizing the film of (a) at a temperature of 500- 575°C, more preferably at a temperature of 525-550°C.
In a preferred embodiment, the carbonization of the ionic liquid-regenerate cellulose film has a heating rate of 0.1-
10°C -min , preferably the ionic liquid-regenerate cellulose film has a heating rate of 0.5-5°C-min , preferably of 1°C -min
In another embodiment, the carbonization of the ionic liquid- regenerate cellulose film has one or more dwells, in particular comprising a first dwell at 110°C.
In another embodiment, the carbonization of the ionic liquid- regenerate cellulose film is in a controlled atmosphere, preferably of nitrogen or vacuum atmosphere.
In another embodiment, the carbonization of the ionic liquid- regenerate cellulose film is in an atmosphere comprising a
Figure imgf000014_0001
nitrogen flowrate ranging between 100- 200 ml. min
The ionic liquid process according to the present invention allows the tailoring of the precursor films and consequently the optimization of preparation of improved CMSMs . Accordingly, in another embodiment the present invention relates to improved carbon molecular sieve membranes obtained by the above described process.
3. Carbon molecular sieve membranes (CMSMs)
Carbon molecular sieve membranes produced from an ionic liquid- regenerate cellulose precursor in a single carbonization step displayed high performances and stability when exposed to humidity. Namely, the CMSMs obtained show no pore blockage to the permeation of humidified gas mixtures up to relative humidities of 80 % and display high ideal permselectivities and permeabilities for industrially relevant gas separations.
The carbon molecular sieve membrane pore size distribution is controlled independently by tailor the precursor material and varying the carbonization end temperature.
During the carbonization step, according to the process described above, and by using a cellulose precursor film prepared by ionic liquid solvent process, amorphous carbon membranes are obtained with a very narrow porosity that is the features responsible for the molecular sieve properties of each carbon membrane.
Therefore, in an embodiment of the present invention, the carbon molecular sieve membrane present large pores with size of 0.5-1 nm interconnected to smaller pores with size of 0.4- 0.5 nm .
The measurement of the membrane pore size distribution was performed according to the method described by Do et al . [4,5] for carbonaceous materials.
CMSMs often suffer oxidation when in contact with ambient air resulting in a loss of permeability and renders the surface more hydrophilic and then prone to retain water and other vapor clusters.
However, up to a certain point this event can favour the selectivity of the membranes without impairing the surface chemistry .
Typically, the water vapor adsorption isotherm exhibits a S- shape behaviour. This shape is related to the hydrophobic nature of CMSM; for RH lower than ca. 30 % water molecules adsorb in hydrophilic sites and desorb permeating the membrane. For higher humidity values, water still adsorb in hydrophilic sites but when the vapor concentration is high enough several water molecules stablish sequentially H-bonding resulting in water clusters. When these clusters gain enough dispersion energy to detach from the hydrophilic site they move over the hydrophobic path towards pore constrictions, blocking them.
These improved CMSM present substantial advantages for application several industries, such as separation of nitrogen and oxygen from air, separation of hydrogen from syngas, removal of C02 from natural gas wells, recover of hydrogen from natural gas, C02 separation from flue gas, air dehumidification, gas natural dehydration, helium recovery/separation, ethene/ethane or xenon from gas mixtures.
Further, the obtained CMSMs are selective and have tuneable properties, thus present high gas separation performance and further avoiding blockage under humid gas streams separation conditions as high as up to 80% of RH.
EXAMPLES
Example 1. Precursor film formation
Wood pulp provided by Innovia Films Ltd, having a degree of polymerization of 450 and water content of about 10% was dispersed in DMSO and EMIMAc (70:30 wt . % of DMSO : EMIMAc ) to prepare a 9.2 wt . % cellulose solution. The mixture was heated at 90 °C under magnetic stirring until cellulose was completely dissolved.
The solution was filtered with a wire mesh and placed in a vacuum oven at 40 °C for degassing for 2 hours. After the obtained solution was spin coated on rectangular glass plates with a spin-coater (POLOSTM, SPIN150i) at a spinning speed of 2000 rpm, spin acceleration of 1000 rpm/ s and a spinning time of 10 seconds. After coating, the films were coagulated in distilled water (25 °C) for obtaining a regenerated cellulose film.
The film was then intensely washed with distilled water for 60 min to remove the excess of ionic liquid. After that, the washed film was dipped in a softener bath containing 5 wt . % of propylene glycol for 1 min and then dried in an oven at 100 °C for 10 min.
Example 2. Carbonization procedure
Prior to the carbonization step, the precursor was cut in disks with 48 mm in diameter. The ionic liquid-regenerate cellulose films carbonization was accomplished in a quartz tube (80 mm in diameter and 1.5 m in length) inside a tubular horizontal Termolab TH furnace.
The protocol had an end temperature ranging between 500 °C and 900 °C, preferably between 525 °C and 575 °C, and more preferably 550 °C, without soaking time, a nitrogen flow rate
_ g
ranging between 100-200 ml -mm and a slow heating rate with some dwells for avoiding a quick release of residual solvents and volatile matter that could damage the carbon matrix. The protocol is pictured in FIG.l. After the end carbonization temperature was reached, the system was allowed to cool naturally until room temperature and the carbon membranes could be removed from the tubular furnace.
Micrographs of the produced CMSM were obtained by scanning electron microscopy (SEM) . FIG.2 shows the surface and cross- sectional views of an obtained CMSM at 550 °C.
Example 3 . Permeation tests
For performing the permeation experiments, the carbon membranes were glued to steel O-rings. Epoxy glue (Araldite® Standard) was applied along the interface of the steel 0- ring and the CMSM. Also, and for support the film in the test cell, a sintered metal disc with a filter paper was used. Single gases were tested at 25 °C, feed pressure of 1 bar and vacuum at the permeate side. Tests were performed in a standard pressure- rise setup LabView® data logging.
Permeation system included the membrane module connected to a vessel with a calibrated volume at the permeate side and connected also to a gas cylinder at the feed side. Feed gas could be dry or humidified by passing through a bubbler with distilled water prior to the membrane module. The relative humidity was measured by a RH meter at an exit port. The permeability, Pg , of the CMSM towards to pure component i was determined accordingly to:
Figure imgf000019_0001
where Fluxi is the flux of the species i , DRi the partial pressure of the species i across the membrane and d the membrane thickness.
The ratio of two gases permeability coefficients is termed permselectivity or ideal selectivity [7] as described:
Figure imgf000019_0002
Carbon molecular sieve membranes prepared from ionic liquid regenerated cellulose in agreement with the Example 1 and Example 2, varying the carbonization end temperature between 525 °C and 625 °C, more preferably, between 550 °C and 600 °C.
Table 1 shows the obtained permeabilities and ideal selectivities for several dry gases on CMSM produced at 550°C and 600 °C.
Table 1. CMSMs permeabilities and ideal selectivities for several dry gases
CMSM 550°C CMSM 600°C
Gas Permeab. Perm Permeab. Perm species (barrer) selectivity (barrer) selectivity X/N2 X/N2
N2 0.16 ~ 0.09 ~
02 5.16 32.3 2.19 24.3
C02 13.4 83.8 4.18 46.4
He 126 788 174 1993
H2 206 1288 121 1344
Carbon molecular sieve membranes are prepared from ionic liquid-regenerated cellulose according to Example 1 and Example 2, varying the carbonization end temperature between 550°C and 600°C.
The performance of a carbon molecular sieve membrane towards a gas separation is defined by the gas permeability and the correspondent selectivity. Robeson [46] proposed a selectivity/permeability upper bound for representative binary gas separations for the best performing polymer membranes .
FIG.6 shows the permeation results of 02 and N2 in carbon membranes carbonized at 550°C and 600°C end temperatures and the upper bound limit. This figure shows that the prepared CMSM have a separation performance well above the referred upper bound limit.
The pore size distribution and the porosity volume were obtained for carbon molecular sieve membranes based on the adsorption equilibrium isotherm of C02 at 0°C using the volumetric method.
For performing the experiments, the sample was fragmented in flakes. FIG. 3 plots the pore size distribution for a sample prepared at 550 °C and Table 2 the micropore volume, the mean pore width and the characteristic energy for adsorption.
TABLE 2
Parameter CMSM 550
Micropore volume {cm3-kg i) 250.2
Characteristic energy {kJ-moH) 11,79
Mean pore width (nmj 0.711
In an embodiment, CMSM 550 presents micropores situated in the range of 0.7-1 nm, mainly responsible for surface diffusion mechanism, and interconnected ultramicropores in the range of 0.4-0.7 nm responsible for the sieving process of carbon membranes .
Example 4 . Effect of humidi ty on CMSM performance
Table 3 shows the permeation results for the prepared CMSM when exposed to different levels of relative humidity for several hours .
TABLE 3
Pre-exposure Post-exposure
Permeability Permeability Permeability to Permeability to Sample RH (%) to O; to Nj humidified Oj humidified j
(barrer) (barrer) {barrer} (barrer)
CMSM 550 75-77 S.16 0.16 8.47 1.33
CMSM 600 75-77 2.19 0.09 S.96 0.85
After exposure to humidified oxygen stream (RH humidity relative- of 75-77 %), the overall permeability increased in the range of ca . 1.6-2.7 times (permeability to oxygen stays roughly constant) . General disclaimer
The term "comprising" whenever cited in this document aims to indicate the presence of the stated features, integers steps, components, but not exclude the presence or addiction of one or more other features, integers, steps. Components or groups thereof.
All references recited in the present document are incorporated herein in their entirety by reference, as if each and every reference had been incorporated by reference individually.
Where ranges are given, endpoints are included.
Also, it is to be understood that unless otherwise indicated or otherwise evident from the context and/or the understanding of one of ordinary skill in the state of art, values are expressed as ranges can assume any specific value within the stated ranges in different embodiments of the invention, to the tenth of the unit of the lower limit of the range, except if the context clearly dictates otherwise .
It is also important to understood that unless otherwise indicated or otherwise evident from the context and/or understanding of one of ordinary skill in the art, values expressed as ranges can assume any subrange within the given range, wherein the endpoints of the subrange are expressed to the same degree of accuracy as the tenth of the unit of the lower limit of the range.
The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.
Where ranges are given, any value within the range may explicitly be included from any one or more of the claims.
Any embodiment, element, feature, application, or aspect of the compositions and/or methods of the invention, can be excluded from any one or more claims .
The present disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.
The above described embodiments are combinable. The following claims further set out particular embodiments of the disclosure. References
[1] Q. Liu, T. Wang, H. Guo, C. Liang, S. Liu, Z. Zhang, Y. Cao, D.S. Su, J. Qiu, Controlled synthesis of high-performance carbon/zeolite T composite membrane materials for gas separation, Microporous Mesoporous Mater., 120 (2009) 460-466.
[2] W. Wei, G. Qin, H. Hu, L. You, G. Chen, Preparation of supported carbon molecular sieve membrane from novolac phenol-formaldehyde resin, J. Membr . Sci., 303 (2007) 80-85.
[3] A.M.M. Mendes, M.R.S. de Andrade, M.F. da S. Boaventura, S.C.V. Rodrigues, A carbon molecular sieve, method of preparation and uses thereof, WO 2017/068517 Al, 2016.
[4] C. Nguyen, D.D. Do, Adsorption of Supercritical Gases in Porous Media: Determination of Micropore Size Distribution, J. Phys . Chem. B. 103 (1999) 6900-6908. doi : 10.1021/jp9906536.
[5] C. Nguyen, D.D. Do, K. Haraya, K. Wang, The structural characterization of carbon molecular sieve membrane (CMSM) via gas adsorption, J. Memb. Sci. 220 (2003) 177- 182. doi : 10.1016/S0376- 7388 (03) 00219-9.
[6] S.C.V. Rodrigues, Non-aging disruptive carbon molecular sieve membranes: preparation and characterization, 2017.
[7] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 62 (1991) 165- 185.

Claims

1. A process for preparing a precursor cellulose film, for preparing a carbon molecular sieve membrane (CMSM) , comprising the following steps:
a) Preparing a cellulose solution by dissolving a cellulose material, preferably cellulose pulp in a solvent mixture comprising an ionic liquid (ILs) solvent, preferably 1-ethyl- 3-methyl imidazolium acetate (EMIMAc) , under stirring and heating conditions with a variable temperature of 70 to 100°c, preferably of 80 to 90°C, more preferably of 75 to 80°C until the cellulose material is completely dissolved,
b) Casting the cellulose solution of (a) to obtain a cellulose based film,
c) Regenerating the cellulose film of (b) ,
d) Drying the regenerated cellulose film of (c) .
2. A process according to claim 1, wherein the preparation of the cellulose solution of step (a) :
a) the cellulose material is cellulose pulp material, b) the solvent mixture comprises 1-ethyl- 3-methyl imidazolium acetate (EMIMAc) and dimethyl sulfoxide (DMSO) in a ratio of 70:30 %wt . of DMSO to EMIMAc, and
c) the solvent mixture of (b) is added to the cellulose pulp of (a) to form a 8-10 %wt . preferably a 9-9.5 %wt. cellulose solution.
3. A process according to claim 1 or 2, wherein the casting step (b) is performed by spin coating the cellulose solution of step (a) on a rectangular glass plates with a suitable spin-coater at a spinning speed of 2000 rpm, spin acceleration of 1000 rpm/s and a spinning time of 10 seconds .
4. A process according to any of the claims 1 or 2, wherein after coating, the cellulose film resulting of step (b) is coagulated in distilled water, at a temperature of 20 to 30°C, preferably of 22 to 25°C, for obtaining a regenerated cellulose film, washed to remove the excess of ionic liquid with distilled water for around 60 min and optionally dipped in a softener bath containing 5 %wt . of propylene glycol for 1 min and then dried, at 100°C, for 10 min .
5. A precursor cellulose film based on cellulose and ionic liquid (ILs) solvents far preparing a carbon molecular sieve membrane (CMSM) characterized for comprising a cellulosic material, preferably cellulose pulp, and an ionic liquid
(ILs) solvent, preferably 1-ethyl- 3-methyl imidazolium acetate (EMIMAc) obtainable by the process as described in any of the claims 1 to 4.
6. A process far preparing a carbon molecular sieve membrane (CMSM) characterized for comprising the following steps:
c) Providing a precursor cellulose film with an ionic liquid solvent as described in claim 5, and
d) Carbonizing the precursor cellulose film of (a) at a temperature of 500-575°C, more preferably at a temperature of 525-550°C.
7. A process according to claim 6 wherein the precursor cellulose film of (a) has a heating rate of 0.1-10°C -min i
, preferably the ionic liquid-regenerate cellulose film
Figure imgf000025_0001
has a heating rate of 0.5-5°C-min , preferably of 1°C· i
mm and the carbonization of the ionic liquid- regenerate cellulose film has one or more dwells, in particular comprising a first dwell at 110°C.
8. A process according to claim 6 or 7 wherein the carbonization of the precursor cellulose film is performed in a controlled atmosphere, preferably of nitrogen or vacuum atmosphere.
9. A process according to claim 8 wherein the carbonization of the precursor cellulose film is performed in an atmosphere i
comprising a nitrogen flowrate of 100- 200 ml -min .
10. A carbon molecular sieve membrane (CMSM) obtainable by the process as described in any of the claims 6 to 9.
11. A carbon molecular sieve membrane (CMSM) according to claim 10 characterized by presenting large pores with size of 0.5- 1 nm interconnected to smaller pores with size of 0.4-0.5 nm.
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