Determination of the Accessible Hydroxyl Groups in Cellulose by Using
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Determination of the Accessible Hydroxyl Groups in Cellulose by Using Phosphitylation and 31 P NMR Spectroscopy PowerPoint PPT Presentation


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Determination of the Accessible Hydroxyl Groups in Cellulose by Using Phosphitylation and 31 P NMR Spectroscopy. Objectives. To probe the amount of accessible hydroxyls on cellulosic materials including cellulose nanocrystals

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Determination of the Accessible Hydroxyl Groups in Cellulose by Using Phosphitylation and 31 P NMR Spectroscopy

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Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

Determination of the Accessible Hydroxyl Groups in Cellulose by Using Phosphitylation and 31P NMR Spectroscopy


Objectives

Objectives

  • To probe the amount of accessible hydroxyls on cellulosic materials including cellulose nanocrystals

  • To develop a quantitative methodology to follow the surface development (accessible hydroxyls) of cellulose as a function of various treatments

    - Enzymatic

    - Chemical

    - Mechanical


Reactive hydroxyl groups on cellulose

Reactive Hydroxyl Groups on Cellulose

  • Working hypothesis:

    Phosphitylation reagent (2-chloro-4,4,5,5-tetramethyl-1,3,2 -dioxaphospholane) reacts with the hydroxyl groups on cellulosic surface. Therefore, the amount of reactive OH’s can be calculated from the consumption of phosphitylation reagent by using 31P NMR


31 p nmr

Background

Phosphitylation of cellulosic sample for 31P NMR analysis:

31P NMR

O

O

HCl

+

+

Cl

P

R

O

P

R

O

H

O

O

2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane

D.S. Argyropoulos, J. Wood Chem. Technol.14 (1994), pp. 65–82.


Experimental setup

Experimental Setup

  • Schlenk line with four ports (parallel experiments)

  • Distillation apparatus for tetrahydrofuran (THF)


Experimental setup1

Experimental Setup

  • Cellulose sample (from Whatman #1) was suspended in 15 mL of freshly distilled THF, 5 mL of dry chloroform, 5 mL of dry pyridine and 0.03 mmoles of 4-(dimethylamino) pyridine (DMAP)

  • The mixture was kept in 50 mL Schlenk flask equipped with a magnetic stirrer, an Argon inlet and a septum for reagent addition via a steel syringe

  • 600 microliters of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane [P(II)] was then added slowly via the septum under slight agitation


Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

Schlenk flasks were dried by using

a heating gun and a vacuum pump

(cycle repeated 3 times with argon

fillings)

Cellulose sample was transferred

to the Schlenk flask under constant

argon purge


Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

  • Dry THF was injected to the Schlenk flasks (containing cellulose) via a

    steel syringe

  • Samples were stirred 10 minutes before adding the rest of the reagents


Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

An addition of dry chloroform,

Pyridine, DMAP and internal

Standard (guaiacol)

An addition of phosphitylation reagent

(drop-wise)

  • In all experiments one flask with identical amount of reagents, but without

    cellulose, was kept on side (blank)

  • By comparing the amounts of remaining P(II) on reaction flasks to that

    on blank, possible overestimations were avoided (phosphitylation reagent

    decomposes slowly over time)


Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

Sample for analysis

  • This aliquot was then analyzed with quantitative 31P NMR

  • NMR tube contained known amount of relaxation agent (chromium acetylacetonate) dissolved in CDCl3

  • Reaction kinetics were monitored by taking out an aliquot (600 microliters) of reaction mixture containing P(II) that has not reacted with sample.

  • Samples were let to settle down before

    sampling (cellulose falls to the bottom allowing clear sampling)


Typical 31 p nmr spectrum

Typical 31P NMR Spectrum

Phospitylation reagent

The consumption of P(II) is directly proportional for the reactive hydroxyls in the cellulosic surface

Reaction was

followed by

monitoring

the decrease

on this signal

I.S. (Guaiacol)

H2O


Reactivity of oh groups in cellulose

4

3.5

3

2.5

Reactive Hydroxyls (mmol/g)

2

1.5

1

0.5

0

5

30

180

1200

Time (min)

Reactivity of OH Groups in Cellulose

The reaction was first applied to airdry cellulose obtained from whatman #1

filter paper in order to find out the reaction time needed for the complete

phosphitylation. Level off was observed after 30 minutes


Beating of cellulose

Beating of Cellulose

  • Pulp from filter paper were beaten for specified number of revolutions (5000 and 30000), respectively (PFI mill method, T 248 cm-85)

  • Beating fibrillates fibers and it is widely accepted method of simulating commercial refining practices

  • Refined samples were first homogenized and then airdried, ovendried or conditioned at 69% of relative humidity

  • Cellulose samples from different beating intensities with different moisture contents were then subjected for 31P NMR analysis


Ovendry samples

4

3.5

3

2.5

Reactive Hydroxyls (mmol/g)

2

1.5

1

0.5

0

5

30

180

1200

Time (min)

Ovendry Samples

30000 rev.

5000 rev.

Control

Theoretically, the maximum amount of reactive hydroxyls in 1 gram of cellulose

is 18.54 mmol

Control refers to unbeaten sample


Airdry samples

4.5

4

3.5

3

2.5

Reactive Hydroxyls (mmol/g)

2

1.5

1

0.5

0

5

30

180

1200

Time (min)

Airdry Samples

30000 rev.

5000 rev.

Control

Control and 5000 rev. are almost identical whereas 30000 rev. showed

increased reactivity


Relative humidity 69

8

7

6

5

Reactive hydroxyls (mmol/g)

4

3

2

1

0

5

30

180

1200

5760

Time (min)

Relative Humidity 69%

30000 rev.

5000 rev.

Control

Increased moisture content opens up the cellulose matrix


Scale up experiments

Scale-up Experiments

  • The methodology was validated by scaling up the sample size (100mg to 300mg)

  • Reactivity was found to be very similar with maximum deviation of 15%


Determination of the accessible hydroxyl groups in cellulose by using phosphitylation and 31p nmr spectroscopy

3.5

3.5

3

3

2.5

2.5

2

2

Reactive Hydroxyls (mmol/g)

Reactive Hydroxyls (mmol/g)

1.5

1.5

1

1

0.5

0.5

0

0

5

30

180

1200

5760

5

30

180

1200

5760

Time (min)

Time (min)

4.5

4

3.5

3

2.5

2

Reactive Hydroxyls (mmol/g)

1.5

1

0.5

0

5

30

180

1200

5760

Time (min)

Controls

5000 rev.

100 mg of sample

30000 rev.

300 mg of sample


Effect of moisture content

Effect of Moisture Content

  • Individual samples from different pretreatments (beating) were tested to follow the accessibility changes on cellulosic matrix at different moisture levels

  • Increased moisture content was seen to have great influence toward the elevated reactivity of cellulosic hydroxyl groups


Moisture contents

Moisture Contents

  • Determined by using electronic moisture analyzer (Sartorius)

  • Ovendry samples were assumed to have close 0% moisture content

  • 69% RH samples were conditioned 48h at 23°C


Controls

4.5

4

3.5

3

Reactive hydroxyls (mmol/g)

2.5

2

1.5

1

0.5

0

5

30

180

1200

5760

Time (min)

Controls

RH 69%

Airdry

Ovendry

Sample with highest moisture showed highest reactivity


5000 rev beating

6

5

4

Reactive Hydroxyls (mmol/g)

3

2

1

0

5

30

180

1200

5760

Time (min)

5000 Rev. Beating

RH 69%

Airdry

Ovendry

Beaten samples follow the same pathway although at RH 69% values

are slightly higher than those with unbeaten samples


30000 rev beating

8

7

6

5

Reactive Hydroxyls (mmol/g)

4

3

2

1

0

5

30

180

1200

5760

Time (min)

30000 Rev. Beating

RH 69%

Airdry

Ovendry

The most refined samples turned out to be the most reactive ones

(as expected). Furthermore the moisture content seems to have the

greatest effect to the most refined samples.


Conclusions

Conclusions

  • Hydroxyl groups on the cellulose surface can be phosphitylated in heterogeneous system

  • Beating changed the reactivity of surface hydroxyls

  • Different reactivities were observed for dry and moist samples

  • Developed methodology will be further used to monitor changes in enzymatically treated cellulose samples.


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