Preparation of Porous Cellulose Beads via Introduction of Diamine

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Preparation of Porous Cellulose Beads via Introduction of Diamine Spacers Jonas Lindh, Changqing Ruan, Maria Stromme, and Albert Mihranyan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01288 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 21, 2016

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Preparation of Porous Cellulose Beads via Introduction of Diamine Spacers Jonas Lindh,* Changqing Ruan, Maria Strømme and Albert Mihranyan* Nanotechnology and Functional Materials, Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden. KEYWORDS. Nanocellulose, Cladophora, reductive amination, porous beads, cross-linking.

The current work presents a synthesis route based on reductive amination of 2,3-dialdehyde cellulose beads with diamines to render micrometer-sized beads with increased specific surface area (SSA) and porosity in the mesoporous range. Specifically, the influence of reductive amination of 2,3-dialdehyde cellulose (DAC) using aliphatic and aromatic tethered mono- and diamines on bead microstructure was investigated. Aliphatic or aromatic tethered monoamines were found to have limited utility for producing porous beads while introduction of diamines provided beads with a porous texture and a SSA increasing from 30 m2/g. Both aliphatic and aromatic diamines were found useful for producing porous beads having a pore size distribution range between 10 and 100 nm, as verified by N2 gas adsorption and mercury intrusion porosimetry analyses. The true density of the functionalized DAC beads decreased to an average of about 1.36 g/cm3 as compared to 1.48 g/cm3 for the unfunctionalized, fully

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oxidized DAC beads. The total porosity of the beads was, according to mercury porosimetry, in the range of 54-64%. Reductive amination with 1,7-diaminoheptane provided beads that were stable under alkaline conditions (1 M NaOH). It was concluded that introduction of tethered diamines to DAC beads is a facile method to produce mesoporous beads.

Introduction Cellulose beads are important in a vast number of applications as described in an excellent recent review1 and provide a sustainable alternative to synthetic polymer-based beads relying on a petroleum-based feedstock e.g. polystyrene. The relatively high mechanical- and chemical stability of cellulose beads in combination with the appropriate shape has led to their widespread use as matrix material in e.g. protein purification,2 liquid chromatography,3 size exclusion chromatography,4 solid-phase synthesis,5 metal adsorption6 and as drug delivery vehicle.7 The preparation of cellulose beads has hitherto been a multistep procedure, relying on consecutive dissolution, shaping and regeneration of cellulose. This multistep approach can be a tedious procedure and often involves environmentally harmful solvents and additives but also has its benefits as alterations to the dissolution, shaping and coagulation steps can alter properties e.g. size and porosity of the beads. To form beads in the low micrometer range is, however, demanding using this approach as formation of the micron sized droplets is energy demanding and requires dedicated droplet formation equipment. Another approach to produce bead shaped cellulose is by mechanical processing of cellulose suspensions, which requires dedicated and often expensive equipment.1 A recently developed method has, however, facilitated the production of 2,3-dialdehyde cellulose (DAC) beads. The method relies on periodate oxidation of highly crystalline cellulose, e.g. Cladophora cellulose, under aqueous conditions to produce


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highly oxidized DAC beads, ranging in size from 1 to 20 µm, in a one-pot procedure.8 With this method the beads are spontaneously formed in the reaction mixture without the need of additional procedures e.g. dissolution, droplet formation etc. Reductive amination of DAC with polyamines is an efficient method, which has been previously employed for cross-linking cellulose.9 In particular, it was observed that by Schiff base coupling aldehyde groups with amines the texture and porosity of dried DAC beads can be altered.8 Introducing a high porosity in such beads is deemed to be important, as this will increase the area available for interactions with the target molecules beyond that of the outer surface of the beads.10 Furthermore, the pores can be beneficial when separating analytes based on their size, i.e. size exclusion chromatography.10a Hence, in order to take full advantage of DAC beads, routes for controlling the porosity and the surface area are essential, as these properties will control the available surface area and determine the capacity of the material in many of the conceivable applications. This work presents an attempt to control the above-mentioned properties by functionalizing the DAC with different mono- and diamines,9,

11

featuring different alkyl and aryl tethers, and

producing porous DAC beads with increased surface areas. DAC functionalized beads have previously also been prepared by oxidizing cellulose nanospheres.12 By oxidizing preformed cellulose beads only moderate dialdehyde content was achievable as the formation of DAC resulted in disruption of the beads morphology at higher dialdehyde content.13

Our group has dedicated research efforts towards finding useful applications for nanocellulose originating from the green algae Cladophora,14 which is often considered a pollutant in costal areas,15 and the work described in this article constitute important steps towards making it useful in separation applications.


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Experimental Materials Nanocellulose from Cladophora algae was provided by FMC Biopolymer. Sodium metaperiodate,

sodium

borohydride,

1,2-diaminoethane,

1,3-diaminopropane,

1,4-

diaminopropane, 1,6-diaminopropane, 1,7-diaminoheptane, 6-aminohexan-1-ol, 6-aminocaproic acid, 1,2-phenylenediamine, 1,3-phenylenediamine, 1,4-phenylenediamine, aniline and 4,4’oxydianiline were acquired from Sigma-Aldrich and were used as received (all chemicals had a purity ≥ 98%). Deionized water was used throughout the experimental procedures.

Preparation of DAC beads As previously described,8 Cladophora cellulose, 20 g dispersed in 1000 mL of water, was mixed with 132 g sodium metaperiodate (about 5 mol per mol of anhydroglucose units) dissolved in 1000 mL water. The periodate-containing reaction mixture was wrapped in aluminum foil to avoid the reaction mixture being exposed to light. The reaction mixture was vigorously stirred at room temperature in the dark for 10 days. The reaction was quenched via the addition of ethylene glycol, and the DAC product was washed repeatedly with water to provide pure DAC.

Determination of aldehyde content The DAC samples were transformed to aldoximes via Schiff base reactions with hydroxylamine according to literature procedure,8, 16 and analyzed for elemental composition (C, H and N). To a 100 mL RB-flask was added never-dried DAC (corresponding to a dry weight of 100 mg), 40 mL acetate buffer (pH 4.5) and 1.65 mL hydroxylamine solution (aq. 50 wt%). The


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reaction mixture was stirred at room temperature for 24 h. The product was thoroughly washed with water and dried under reduced pressure prior to elemental analysis. The term “degree of oxidation” represents the degree of 2,3-alcohols in the anhydroglucose units that has been transformed into their corresponding aldehydes. The highest degree of oxidation, i.e. 100%, corresponds to all anhydroglucose units being converted to the corresponding non-cyclic 2,3dialdehyde structure, which corresponds to approximately 12.5 mmol of aldehyde groups per gram of cellulose.

Reductive amination of linear alkyl diamines to DAC beads Never-dried DAC (corresponding to a dry weight of 100 mg), 40 mL acetate buffer (pH 4.5) and 0.25 equiv. diamine were added to a 100 mL RB-flask. The reaction mixture was stirred at room temperature for 24 h. The product was washed with water followed by EtOH. The resulting imine product was dispersed in MeOH and treated with sodium borohydride (1.2 equiv.) and reduction was performed during 2 h. The crude product was washed with water followed by EtOH and dried in ambient air at room temperature for 48 h.

Reductive amination of aryl diamines to DAC beads The same process steps as for the reductive amination of linear alkyl diamines were employed with 40 mL methanol and 0.25 equiv. aryl diamine replacing the 40 mL acetate buffer and the 0.25 equiv. diamine, respectively.

Reductive amination of monoamines to DAC beads The same process steps as for the reductive amination of aryl diamines were employed with 0.5 equiv. monoamine replacing the 0.25 equiv. diamine.


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Alkaline stability evaluation To a 15 mL tube containing 10 mL 1 M NaOH (aq) was added 100 mg of amine functionalized never-dried DAC beads. The tube was fitted in a tube carousel and the mixture was rotated at room temperature. After 1 h the material was repeatedly washed with water to produce pure material.

FTIR FTIR spectra were collected on a Bruker Tensor 27 (Germany) with KBr pellets (1 wt% dry sample in KBr and pressed into a pellet using a hydraulic press at 7 ton/cm3). The resolution was 4 cm-1 and 128 scans were averaged. Two analyses were made with each sample. For viewing and comparison of spectra, all spectra were normalized with respect to the absorption at 2897 cm-1, corresponding to a C-H stretching vibration.17

SEM Scanning electron micrographs were recorded on a LEO1550 field emission SEM instrument (Zeiss, Germany). Samples were dried from EtOH in ambient air and were further dried at 60 °C under reduced pressure prior to analysis and were then mounted on aluminum stubs with doublesided adhesive carbon tape and sputtered with a mixture of Au/Pd to reduce charging effects.

Nitrogen sorption isotherms Nitrogen sorption isotherms were acquired using an ASAP 2020 instrument (Micromeritics, USA). Samples were dried from EtOH in ambient air and were further dried under reduced pressure using the ASAP 2020 instrument prior to analysis. BJH pore size distributions18 were


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determined from the isotherm desorption branch using the instrument software (ASAP 2020 v4.01, Micromeritics, USA). The BET specific surface area19 was assessed from the adsorption branch of the isotherm using the instrument software.

Pycnometry The true density, σt, was measured using helium pycnometry (AccuPyc 1340, Micromeritics, USA). Samples were dried from EtOH in ambient air and were further dried at 60 °C under reduced pressure prior to analysis. All measurements were repeated 6 times and the standard deviation was 95% corresponding to

12.5 mmol aldehyde/g cellulose) and

formed a relatively compact and non-porous surface upon drying in ambient air at room temperature (specific surface area (SSA) < 1 m2/g),8 see Figure 1 and Figure S1.

Figure 1. SEM micrograph of unfunctionalized DAC bead. We were interested in investigating if the introduction of cross-linking agents is a feasible method to increase the surface area of the dried beads and make them porous, which could possibly be achieved via stabilizing the porous structure of the material during the drying process and inhibiting the structure from collapsing into a compact, non-porous structure. As the DAC beads are rich in aldehyde groups, a facile way to achieve a stable covalent cross-linking would be by reductive amination using diamines. Thus, never-dried DAC beads were reacted with mono- and diamines, featuring 2 to 7 carbon atom tethers, in reductive amination reactions between the aldehyde groups of the DAC beads and the amine groups (see Scheme 1).11 The resulting products were carefully washed and dried in ambient air.


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Scheme 1. Reaction scheme for reductive amination of DAC.11

In Table 1 the characteristics of the products obtained by reductive amination with various alkyl and aryl tethered mono- and diamines are summarized. It was found that diamines were particularly suitable to produce high surface area beads whereas the use of monoamines was less beneficial. The SSA of the diamine-grafted beads ranged from 16 to 32 m2/g, as assessed with the BET method from nitrogen adsorption data (see Table 1).

Table 1. Beads functionalized with different types of amines via reductive amination and their resulting SSA according to gas sorption (BET), density according to He pycnometry and pore volume according to gas sorption (BJH). Amine

SSA (m2/g)

Density (g/cm3)

Beads

Unmodified DAC bead

0.3

1.48

Yes

1,2-diaminoethane

27

1.42

Yes

1,3-diaminopropane

32

1.38

Yes

1,4-diaminobutane

22

1.35

Yes

1,6-diaminohexane

16

1.35

Yes

1,7-diaminoheptane

20

1.37

Yes

6-aminohexan-1-ol

n.d.*

n.d.*

No

6-aminocaproic acid

n.d.*

n.d.*

No

1,2-phenylenediamine

22

1.37

Yes


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28

1.38

Yes


28

1.37

Yes

aniline

3

1.26

Yes

4,4’-oxydianiline

24

1.37

Yes

*n.d. = not determined

The SEM micrographs confirmed the presence of beads with a porous-looking surface when diamines were used (see Figure 2, 3 and S2). To compare the influence of the diamines with other types of grafting compounds two different monoamines (6-aminohexan-1-ol and 6aminocaproic acid) containing different functional groups, i.e. alcohol and carboxylic acid, were coupled to the DAC via reductive amination. Interestingly, none of the attempted reactions yielded the desired material and the bead shape disappeared, probably due to the increased solubility of the formed product.


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Figure 2. SEM micrographs of diamine functionalized beads under 30K magnification.


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Figure 3. Close-up SEM micrographs of diamine functionalized beads under 60K magnification.

Compared to the unmodified fully oxidized DAC beads, which featured a true density of around 1.48 g/cm3,8 the true density of aminated DAC beads was significantly lower with an average value around 1.36 g/cm3, see Figure 4.


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1,2-diaminoethane

Density (g/cm3)

1.40 1,3-diaminopropane 1,4-phenylenediamine

1.35

1,6-diaminohexane

1,3-phenylenediamine 4,4’-oxydianiline 1,2-phenylenediamine 1,4-diaminobutane

1.30

aniline

1.25

Density

Figure 4. Dot plot of true densities from helium pycnometry for aminated DAC beads. The horizontal bar corresponds to the average value for the total set. No correlation between the length of the alkyl tether of the diamine and SSA or the pore size distribution was observed (see Figure 5a). Based on the gas adsorption results, most of the pores for linear alkyl diamines appeared to be in the mesoporous range (2-50 nm) with some pores in the macroporous range covering a broad size range between 10 and 100 nm with the largest

a

b

0.3

1,2-diaminoethane 1,3-diaminopropane 1,4-diaminobutane 1,6-diaminohexane 1,7-diaminoheptane

0.2

0.1

0.0

1

10

100

Differential pore volume (cm3/g)

mode around 40-50 nm.

Differential pore volume (cm3/g)

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 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.20

1,3-phenylenediamine 1,4-phenylenediamine

0.15

aniline 0.10

1,2-phenylenediamine 4,4'-oxydianiline

0.05 0.00 1

Pore size (nm)

10

100

Pore size (nm)

Figure 5. Pore size distribution of beads cross-linked with a) aliphatic diamines and b) aryl mono- and diamines according to BJH pore size distributions18 from N2 gas adsorption measurements.


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Further, the possibility of coupling more rigid aryl diamines was explored and the produced materials were characterized with respect to their SSA (see Table 1) and pore size distribution (see

Figure

5b).

In

particular,

1,2-phenylenediamine,

1,3-phenylenediamine,

1,4-

phenylenediamine and 4,4’-oxydianiline were employed. The cellulose cross-linked with the 1,3phenylenediamine and 1,4-phenylenediamine both provided a SSA of ~28 m2/g whereas 1,2phenylenediamine and 4,4’-oxydianiline gave a SSA of 22 and 24 m2/g, respectively. Interestingly, aryl monoamine, i.e. aniline, grafted beads provided a much lower SSA of 3 m2/g (see Table 1). The pore size distribution for the aryl diamine modified beads was similar to that of linear alkyl diamine modified beads, covering the size range between 10 and 100 nm.

The gas adsorption analysis only probes pores with diameters below 100 nm, and mercury porosimetry is more suitable to probe macropores above 100 nm in size. Therefore, in order to assess the presence of macropores in the amine modified DAC beads, mercury porosimetry was performed on a selected number of samples, i.e. 1,3-diaminopropane, 1,4-phenylenediamine and 4,4’-oxydianiline. Figure 6a-c shows the mercury porosimetry results, which are further detailed in Table 2. The apparent (skeletal) density of the beads obtained from mercury porosimetry was within the same range as that obtained with He gas pycnometry. The results of the mercury porosimetry suggested a total porosity of the beads in the range of 54-64%, which was in the range expected for cellulose beads.20

The pores with diameters above 1000 nm seen in Figure 6 correspond to voids between the beads. The pore size distribution representative of the samples therefore is confined to the region between 10 and 300 nm. As seen from the inserts in all plots of Figure 6, the pore size distribution was rather broad with the mode around 40 nm, which agrees well with the N2 gas


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adsorption analysis. The SSA values from Hg porosimetry were of the same order of magnitude as those obtained from N2 gas adsorption although there was a slight difference in the actual values obtained from the different methods. Such variation is not uncommon considering that during mercury porosimetry liquid metal is intruded into pores under pressure, which may

Log Differential Intrusion (mL/g)

5 4 3 2 1

a

0.20 0.15 0.10 0.05 0.00

1

10

100

1000

Pore size Diameter (nm)

0 1

10 100 1000 10000 100000 Pore size diameter (nm)

15




inadvertently alter the pore structure.

10

5

0.20

b

0.15 0.10 0.05 0.00 1

10

100

1000


0 1

10

100

1000

10000 100000

Pore size Diameter (nm) 1.5



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 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

0.5

0.20

c

0.15 0.10 0.05 0.00

1

10

100

1000


0.0 1

10

100

1000

10000 100000



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Figure 6. Pore size distribution of beads cross-linked with a) 4,4’-oxydianiline b) 1,4phenylenediamine c) 1,3-diaminopropane according to mercury porosimetry measurements. Table 2. Summary of the results of mercury porosimetry. The results are the average of two measurements (pressure: 0.0007 to 420 MPa). Sample 4,4’-oxydianiline

1,4-phenylene diamine

1,3-diamino propane

Total intrusion 1.32 volume (mL/g)

1.19

0.83

Total pore wall area 12.3 (m2/g)

9.6

45.8

Bulk density at 0.1 0.49 MPa (g/cm3)

0.49

0.65

Apparent density 1.37 (skeletal) (g/cm3)

1.16

1.41

Porosity (%)

58

54

64

The inherent alkaline sensitivity of DAC21 may limit its usefulness in many applications, e.g. biomolecule purification where cleaning of columns used for purification is usually performed by rinsing the columns with ~1 M NaOH. The latter is performed to regenerate the column for the next cycle and to mitigate the risk of viral contamination. In order to assess if the reductive amination of the DAC beads had any effect on the base stability the unfunctionalized DAC beads and beads functionalized with different diamines, the samples were suspended in 1 M NaOH for 1 h. The samples functionalized with 1,3-diaminopropane, 1,4-phenylene diamine or 4,4’oxydianiline provided beads with insufficient alkaline stability. In contact with alkaline solution, the characteristic bead-shape of the latter samples disappeared and non-porous, compact material was formed (Figure 7). The sample functionalized with 1,4-diaminobutane showed some beads


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present after alkaline treatment but mostly compact and non-porous material. Alkaline treatment of 1,6-diaminohexane provided mostly bead shaped particles and small amounts of compact material. Satisfyingly, the 1,7-diaminoheptane functionalized DAC beads were apparently not affected by the alkaline solution (Figure 7).

Figure 7. SEM micrographs of diamine functionalized beads under 30K magnification. Left column: dried, diamine functionalized beads. Right column: dried, diamine functionalized beads after submission to 1 M NaOH for 1 h. Interestingly, the FTIR-spectra of the corresponding materials showed that the reduction of the 1,3-diaminopropane grafted DAC with NaBH4 clearly reduced the aldehydes as seen in the band


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corresponding to the hemiacetals (~880 cm-1)22 and less clearly at the weaker C=O stretching band of the dialdehydes (~1735 cm-1)11,

23

in Figure 8. The same effect was however not

observed in the spectra for the 1,7-diaminoheptane-, 1,4-phenylene diamine- or 4,4’-oxydianiline grafted DAC materials, where no clear effect on the hemiacetal signal nor the C=O stretching band of the dialdehydes could be detected after reduction with NaBH4. Unfortunately, it was very hard to detect the effect of the reduction with NaBH4 on the imines, as the band for the C=N stretching vibrations of the imine overlap extensively with the C–N bending band (~1565 cm-1)11 of the resulting amine. Based on the FTIR analysis the improved base stability of the 1,7-diaminoheptane grafted DAC beads cannot be explained by lack of aldehydes, which are prone to β-elimination in alkaline media.21 It is possible that the propyl chain of the 1,3-diaminopropane is too short to achieve efficient cross-linking and the same might be valid for the rigid 1,4-phenylenediamine. The longer, 4,4’-oxydianiline is very rigid, which might hamper its cross-linking abilities. The 1,7-diaminoheptane, on the other hand, is probably both sufficiently long and flexible to function as an efficient cross-linking agent for the adjacent aldehydes on the DAC beads. The ability of the 1,7-diaminoheptane functionalized beads to tolerate strongly alkaline conditions is probably a result of covalent cross-linking stabilizing the beads. The apparent inability of NaBH4 to reduce aldehydes in the 1,7-diaminoheptane-, 1,4-phenylene diamine- or 4,4’-oxydianiline grafted DAC materials might be a result of sterical hindrance and increased hydrophobicity of the materials.


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800

1200

1600

2000

2400

2800

3200

3600

Normalized absorbance (Arb. units) 4000

400

a

Cladophora (unmod.) DAC

Wavenumber (cm-1)

b

400

800

1200

1600

2000

2400

2800

3200

3600

4000

Normalized absorbance (Arb. units)

1,3-diaminopropane 1,3-diaminopropane (reduced)

Wavenumber (cm-1)

800

1200

1600

2000

2400

2800

3200

3600


400

c

1,7-diaminoheptane 1,7-diaminoheptane (reduced)

Wavenumber (cm-1)

800

1200

1600

2000

2400

2800

3200

3600


400

d

4,4'-oxydianiline 4,4'-oxydianiline (reduced)

Wavenumber (cm-1)

e

400

800

1200

1600

2000

2400

2800

3200

4000

3600

1,4-phenylenediamine 1,4-phenylenediamine (reduced)

Normalized absorbance (Arb. units)

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 48 49 50 51 52 53 54 55 56 57 58 59 60

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Wavenumber (cm-1)


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Figure 8. FTIR spectra of selected samples a) unmodified Cladophora (black line) and DAC (grey line) b-e) before (black line) and after reduction with NaBH4 (grey line). To further establish the proposed cross-linking of the DAC beads with 1,7-diaminoheptane via reductive amination, three samples; unmodified DAC, 1,3-diaminopropane- and 1,7diaminoheptane functionalized DAC, were subjected to thermogravimetric analysis under a nitrogen atmosphere (Figure 9). Thermogravimetric analysis has been previously employed for demonstrating cross-linking of cellulose.24 The first derivative plot (Figure 9b) shows a two-step degradation of the unmodified DAC and an additional low temperature weight loss for the functionalized DAC samples corresponding to the loss of water. The second distinct weight loss occurred at 160-240 °C for all samples and was attributed to reduced inter chain hydrogen bonding interactions in the lower range and to covalent bond cleavage in the higher range.24a The third distinct weight loss occurred at different temperatures for the different samples and is attributed to covalent bond cleavages. The 1,3-diaminopropane functionalized DAC beads, which were not stable in alkaline solution, were less thermally stable than the unmodified DAC beads, corresponding to insufficient cross-linking and disruption of hydrogen bonds found in the unmodified DAC due to the introduction of the amine ligand. The DAC beads functionalized with the 1,7-diaminoheptane, however, were more stable and in the temperature range from 250440 °C they were more stable than the unmodified DAC beads, which is a strong indication that the beads display a considerable degree of cross-linking.

a) 100

DAC 1,3-diaminopropane 1,7-diaminoheptane

Mass%

80 60 40 20 0 200

400

600

800

T (°C)

b) Derivative weight (mass%)

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0.0

-0.2

DAC 1,3-diaminopropane 1,7-diaminoheptane

-0.4

-0.6 0

200

400

600

800

T (°C)


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Figure 9. TGA curves a) and their derivative b) of unmodified DAC (black), 1,3diaminopropane functionalized DAC (red) and 1,7-diaminoheptane functionalized DAC (blue).

Conclusions By introducing alkyl- and aryl- tethered diamines as reductive amination agents, porous beads with high specific surface area (20-30 m2/g) in the dry state can be formed. The beads produced using linear alkyl- and aryl- tethered diamines are predominantly mesoporous featuring an average pore size around 40-50 nm with a detected pore size range of 10-100 nm. The total porosity of the beads was estimated to be in the range around 54-64% with the average true density value around 1.36 g/cm3 in the dry state. Grafting substances with monoamine groups was not beneficial to produce porous beads: aryl monoamines, such as aniline, produced beads of low porosity, whereas the linear monoamines of hexanol or caproic acid did not produce any beads. These initial studies demonstrate that reductive amination with diamines is an efficient way to produce mesoporous cellulose beads. Furthermore, the aminated beads still provide good opportunities for additional functionalizations via utilization of the remaining aldehyde groups (>6 mmol/g beads). The 1,7-diaminoheptane functionalization provides beads that are both highly porous and stable under alkaline conditions, making them promising candidates for further development. The presented results are expected to facilitate development of environmentally friendly alternatives to petroleum-based beads in a range of biotechnological applications.

Supporting Information SEM micrographs depicting amine functionalized beads at lower magnification are available.


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Acknowledgements The Knut and Alice Wallenberg Foundation, the Göran Gustafsson Foundation and Ollie and Elof Ericssons Foundation are gratefully acknowledged for their financial support. One of the authors (A.M.) is Wallenberg Academy Fellow and acknowledges the program for their longterm support. C. R. thanks the China Scholarship Council (CSC) for financial support.

Corresponding Author e-mail: [email protected], [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Preparation of Porous Cellulose Beads via Introduction of Diamine

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