Formulation of Multifunctional Materials Based on the Reaction of

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Formulation of multifunctional materials based on the reaction of glyoxalated lignins and a nanoclay/nanosilicate Pedro Luis De Hoyos Martinez, Eduardo Robles, Abdel Khoukh, Fatima Charrier - El Bouhtoury, and Jalel Labidi Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00799 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Biomacromolecules

Formulation of multifunctional materials based on the reaction of glyoxalated lignins and a nanoclay/nanosilicate Pedro L. de Hoyos-Martínez1,2, Eduardo Robles1, Abdel Khoukh2’, Fatima Charrier - El Bouhtoury2, Jalel Labidi1* *Corresponding author: Jalel Labidi: [email protected] 1

Chemical and environmental engineering department, University of the Basque Country UPV/EHU, Plaza Europa, 1, 20018, Donostia-San Sebastián, Spain.

2

CNRS/UPPA PAU & PAYS ADOUR/ E25 UPPA, Institute of Analytical Sciences and Physico-Chemistry for Environment and Materials (IPREM), IUT des Pays de l’Adour, 371 Rue de Ruisseau, 40004, Mont de Marsan, France

2’

CNRS/UPPA PAU & PAYS ADOUR/ E25 UPPA, Institute of Analytical Sciences

and Physico-Chemistry for Environment and Materials (IPREM), IUT des Pays de l’Adour, 371 Rue de Ruisseau, 40004, Mont de Marsan, France, 2 avenue du Président Angot, Pau F-64053, France

1 ACS Paragon Plus Environment

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Abstract:

Two organosolv lignins from different origin, namely almond shells and maritime pine, were modified by using a nanoclay and a nanosilicate. Prior to modification, they were activated via glyoxalation to enhance the reactivity of the lignins and thus ease the introduction of the nanoparticles into their structure. The lignins were characterized by several techniques (FT-IR, HPSEC, 1H-NMR, XRD and TGA) before and after modification to elucidate the main chemical and structural changes. The reaction with glyoxal proved to increase the amount of hydroxyl groups and methylene bridges. This tendency was more pronounced, as the percentage of glyoxal was incremented. On the other side, the addition of the nanoclay and nanosilicate particles improved the thermal stability of the lignins compared to the original unmodified ones. This trend was more evident for the lignin derived from maritime pine, which displayed better results regarding the thermal stability, indicating a more effective combination of the nanoparticles in the lignin structure during the modification process.

Keywords: lignin, glyoxalation, nanocomposite, nanoclays, nanosilicates


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Biomacromolecules

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1. INTRODUCTION

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Lignin is a natural and renewable organic polymer, which can be used as raw material

3

for diverse applications in several fields. Industrially, it is known for being a by-product,

4

especially from the pulp and paper industry but also from different biomass conversion

5

processes. In fact, 70 million tons of lignin are produced every year worldwide from the

6

pulping processes according to a recent report1. Despite its great availability, the major

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part of lignin is currently burnt and used as fuel. Only about the 2% of the produced lignin

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is actually utilized for value-added applications like biomaterials or biocomposites, as

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well as other chemical products2. Consequently, the development of lignin-derived

10

polymers for different applications constitutes a desirable option to overcome this

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situation. Moreover, it is known that currently there is a growing interest on the

12

replacement of synthetic chemical products by renewable ones. In this sense, the

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polyphenolic nature of lignin makes it a good substitute for phenol in the formulation of

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different polymers such as phenolic resins in the chemical industry3. Nevertheless, lignin

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presents less active sites within its structure compared to phenol, thus having lower

16

reactivity4. For this reason, the lignin modification with different reagents is needed prior

17

to its utilization for the synthesis of new polymers. In the recent years, several works have

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studied the enhancement of lignin reactivity via different methods such as methylolation

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or hydroxymethylation5, demethylation6, amination7 and phenolation8. From the previous

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techniques, methylolation is one of the most employed especially for lignin used in bio-

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based resins9–11. This method consists in the reaction of lignin with formaldehyde in

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alkaline medium to introduce hydroxymethyl groups (-CH2OH) into its structure. The

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major problem of this modification lies on the non-renewable nature of formaldehyde and

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its toxicity. Accordingly, there have been efforts towards the utilization of a more

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environmentally friendly aldehyde. Glyoxal, which is composed of two aldehyde groups, 3 ACS Paragon Plus Environment


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is a non-toxic aldehyde classified as non-volatile (NTIS2005). Furthermore, it can be

2

directly obtained from various natural sources e.g. lipids oxidation and as side product of

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some biological processes12. Taking this into consideration, there has been a tendency

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towards the replacement of formaldehyde by glyoxal for lignin modification2,13,14.

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Lignin has proved its efficiency as a matrix for bio based materials. However, in certain

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areas like building and construction industry, certain properties such as mechanical

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strength, thermal stability or fire resistance need to be improved15. The combination of

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lignins with inorganic compounds like silicates or silicate clays appears as an option to

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overcome this issue. Moreover, this kind of modification presents various advantages

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such as the combination of properties of organic and inorganic materials into one material

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and the introduction of several functional groups, creating multifunctional materials16.

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The synergistic effect of the different components in organic-inorganic nanocomposites

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has been widely reported17.

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Lately the process of grafting of inorganic compounds in the nanoscale to different

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organic matrices has been used in the elaboration of clay or silicate-polymer

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nanocomposites. In fact, the dispersion of nanoscale clay layers into polymeric matrices

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is said to block the diffusion of volatile decomposition products during thermal

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degradation, improving the thermal resistance of the materials18. Besides, this type of

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nanocomposites show enhanced mechanical properties, flame retardancy and barrier

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properties at the nanoscale level19 compared to the typical only organic-based materials.

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One of the most used nanoclays in this type of nanocomposites is montmorillonite

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(MMT), a natural 2:1 sheet phyllosilicate belonging to the family of smectites20. It

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displays various advantages such as low price and rich intercalation chemistry, providing

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the possibility of being chemically modified to increase the compatibility with the

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polymeric matrices21. 4 ACS Paragon Plus Environment

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Biomacromolecules

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On the other side, polyhedral oligomeric silsesquioxanes (POSS) is another type of

2

silicon-based nanoparticles receiving a great deal of attention in the recent years22.

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Structurally, POSS are characterized by a silica cage core with organic functional groups

4

attached to the corners of the structure. Hence, they present the general formula (RSiO1.5)n

5

where R can be an hydrogen atom or any organic functional group23. In this case again,

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the availability of several R groups within the structure allows the introduction of various

7

functional groups to increase the compatibility with different polymers.

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In this study, two organosolv lignins obtained from different raw materials namely

9

almond shell and maritime pine wood residues were employed. Those substrates were

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chosen because they represent a significant and abundant source of lignocellulosic

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residues from the countries where this work was carried out. On the one hand, Pinus

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pinaster (maritime pine) is the most abundant species in the forest from the region of

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Landes (France). On the other side, Spain is the second major producer of almonds of the

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world24, resulting in a high amount of residue (almond shells) available to be utilized.

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This residue is normally burnt for energy purposes owing to its high heating value (HHV),

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leading to environmental problems. For this reason, its lignin valorization is a more

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interesting and environmentally friendly alternative, since lignin displays a variety of

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potential applications. The two isolated lignins were glyoxalated prior to their

19

modification with inorganic nanoparticles to improve their reactivity. The mentioned

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modification was carried out by using organically modified montmorillonite (OMMT)

21

and a polyhedral oligomeric silsesquioxane (POSS). The result was the synthesis of

22

lignin-based organic-inorganic nanocomposites. To our knowledge, these specific

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nanocomposites have been obtained for the first time. The mentioned compounds

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displayed enhanced thermal properties, owing to the synergistic effect of the OMMT and

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POSS®. This is modification would be a first step for their utilization as a core component 5 ACS Paragon Plus Environment


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of biosourced resins with fireproofing properties. This is of great importance since they

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can represent an environmentally friendly and efficient alternative to the current flame

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retardant products in the industry based on petrol-derived compounds.


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Biomacromolecules

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2. MATERIALS AND METHODS

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2.1 Materials

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The organosolv lignins employed as starting material were extracted from almond

4

shells and maritime pine wood residues. In the former case, the almond shells were

5

coming from almond trees (Prunus amygdalus) of the variety Marcona. In later case, the

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maritime pine residues mainly consisted in leftovers coming from the wood industry

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activities (small pieces of wood, parts of wood panels, wood chips etc.). In both cases,

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the residues were milled and sieved to chips of 5 mm prior to the isolation of the lignins.

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The process of extraction was carried out using a mixture of water-ethanol (70%) as

10

solvent, a ratio 1:6 solid: liquid and 200 ºC during 90 min as reported in a previous work25.

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The organosolv liquor obtained from the reactor was added two volumes of acidified

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water with sulfuric acid (pH=2) to achieve the lignin precipitation by shifting the pH. The

13

process of precipitation was left overnight to allow the lignin precipitate and separate

14

from the liquid phase properly. Then the liquor with the lignin precipitated was filtered

15

and eventually the lignin filtered was dried during 3 days in an oven at 50ºC.

16 17

The glyoxal (40% aqueous solution) and the sodium hydroxide used in this work were purchased from Sigma Aldrich.

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The nanoclay employed was Dellite® 43B, which was kindly provided by LAVIOSA

19

Advance Mineral Solutions (Italy). It was derived from naturally occurring

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montmorillonite

21

(dimethylbenzylhydrogenated tallow ammonium) as presented in figure 1.

purified

and

modified

with

a

quaternary

ammonium

salt



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1 2

Figure 1. Structure of the organically modified montmorillonite employed in this work

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(Dellite 43B).

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The nanosilicate used was SO1458-TriSilanolPhenyl POSS®, purchased from Hybrid

5

Plastics® Inc. (USA). It consisted of a polyhedral oligomeric silsexquioxane core

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modified with organic phenyl groups and three active silanol functionalities as showed in

7

figure 2.

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Figure 2.Structure of the nanosilicate trisilanol phenyl POSS® 8 ACS Paragon Plus Environment

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Biomacromolecules

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2.2 Lignins modifications

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2.2.1 Glyoxalation of lignins

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The reaction of lignin with glyoxal was carried out according to a procedure published

4

in another previous work with some modifications26. In this work, different formulations

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of glyoxalated lignins were prepared by varying the amounts of water and glyoxal used

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(Table 1). Thereby the influence of these components in the process of lignin modification

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can be studied and the lignin formulation with the highest activated structure selected.

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Shortly, 15 g of organosolv lignin were added to different amounts of water. Then, an

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aqueous solution of sodium hydroxide (30%) was added to the suspension until a pH in

10

the range 12-12.5 was obtained. When the desired pH was reached, different amounts of

11

glyoxal (40% in water) were added and the mixture was heated at 58 ºC during 8 h. During

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the reaction, mechanical agitation was employed (450 rpm). Afterwards, small samples

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of the liquid glyoxalated lignins were dried following the procedure described in the

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ASTM D4426-96 standard, to determine the solid content of the modified lignins. The

15

values obtained for this parameter ranged between 32-35±1.70.

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Table 1. Molar ratios of the components for the different formulations of glyoxalated

17

lignins Formulation Origin

Lignin

Water

Glyoxal

LG1

1

13.93

0.25

1

13.93

0.58

LG3

1

10.44

0.58

LG4

1

13.93

0.25

1

13.93

0.58

1

10.44

0.58

LG2

LG5 LG6

Almond shells

Maritime pine wood

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2.2.2 Modification of the lignins with the inorganic nanoparticles

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The glyoxalated lignins obtained from the previous step, were homogeneous brown

3

solutions to which the organically modified montmorillonite (OMMT) and polyhedral

4

oligomeric silsesquioxanes (POSS®) were added. The amount of these nanoparticles used

5

for the reaction was 5% (w/w) respect to solid lignin in solution. The mixture was left

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overnight at 40 ºC with magnetic agitation (550 rpm). After this period, it was observed

7

that no particles were remaining floating on the surface of the solution and that no particle

8

agglomerates were settled at the bottom. Hence, a homogeneous solution was obtained

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where the nanoparticles were properly dissolved and dispersed. Once the lignin

10

modification was carried out, the samples were dried at 50ºC during 24h to remove the

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liquid phase. Then the solid was recovered and crushed into powder (mesh size ≤ 0.25

12

mm) to be ready for further analyses. This procedure was also employed with the

13

glyoxalated lignins for the same purpose. The formulations containing OMMT were

14

designated with the letter A (LGiA) while those with POSS® were labeled as B (LGiB).

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The conditions of the different lignins modified with these inorganic compounds are

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described in table SI1 in supplementary information.

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2.3 Characterization of lignins

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2.3.1 Analysis of chemical composition of lignins

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The used lignins were analyzed chemically prior to modification to elucidate their

20

major components and the differences between them. Several physicochemical

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parameters of lignin were analyzed to evaluate the composition and purity of the samples.

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Thereby, the Klason lignin (KL) and acid soluble lignin (ASL), ash and sugar content

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were determined as described in the TAPPI standard methods T222-om-15, T211-om-16

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and T249-cm-00 respectively. All the experiments for these parameters were done in

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triplicate. The S/G ratios of the lignins were obtained by pyrolysis-gas chromatography10 ACS Paragon Plus Environment

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Biomacromolecules

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mass spectroscopy (Py-GC-MS). The equipment used was a 5150 Pyroprobe pyrolyzer

2

branched to a gas chromatograph (Agilent 6890), which was coupled to a mass

3

spectrometer (Agilent 5973). The GC column size dimensions were 30 m × 0.25 mm ×

4

0.25 µm. A sample amount in the range 400-800 μg was pyrolyzed at 600 °C for 15 s

5

using a heating rate of 20 °C/ms. Then the pyrolyzates were purged from the pyrolysis

6

interface into the GC injector under inert conditions (He). The GC oven was programmed

7

in the following intervals: start at 50 °C and hold for 2 min; temperature increase to 120

8

°C at 10 °C/min holding it for 5 min; increment of temperature to 280 °C at 10 °C/min

9

hold during 8 min and a final temperature raise to 300 °C at 10 C/min hold for another 10

10

min. The identification of the compounds derived from the analysis was carried out by

11

comparing the obtained mass spectra to the National Institute of Standards Library (NIST)

12

and the mass spectra of other compounds reported in the literature, especially through

13

their m/z numbers.

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2.3.2 Fourier Transformed Infrared (FTIR) spectroscopy

15

The FT-IR analysis of lignins before and after each modification was performed on a

16

Spectrum Two FT-IR Spectrometer with a L1050231 Universal Attenuated Total

17

Reflectance (ATR) accessory, Perkin-Elmer (USA) to assess the main occurring

18

structural changes. A number of 64 scans were accumulated in transmission mode with a

19

resolution of 4 cm-1. The spectrum was obtained from a range of 4000 to 400 cm-1.

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2.3.2 High performance size exclusion chromatography (HPSEC)

21

This analysis was carried out to evaluate the molecular weight of the unmodified and

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modified lignin samples. The equipment employed was a Jasco instrument equipped with

23

an interface LC Net II/ADC, a reflex index detector RI-2031Plus and two PolarGel-M

24

(300 mm x 7.5 mm) columns displayed in series. The analyses were performed using

25

dimetylformamide with 0.1% lithium bromide as mobile phase and a 0.7 mL min-1 flow 11 ACS Paragon Plus Environment


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at 40 ºC. The equipment was calibrated using polystyrene standards from Sigma-Aldrich

2

ranging between 70000-266 g mol-1. All the samples were analyzed in duplicate and if

3

still considerable divergences were observed a triplicate was carried out.

4

2.3.3 1H-NMR spectroscopy analysis of the lignins

5

The 1H-NMR analysis was performed for the lignin samples before and after

6

glyoxalation. Thereby the main structural differences between the pristine lignins could

7

be elucidated as well as the main changes induced by the reaction with glyoxal. All the

8

NMR spectra were recorded in DMSO as solvent in a 400 MHz Bruker Avance

9

spectrometer (equipped with a 5 mm BBFO probe) at ambient temperature. The chemical

10

shifts are quoted in ppm relative to tetramethylsilane (δH = 0.00 ppm). The experimental

11

conditions consisted in a pulse width 8.1 ms (zg pulse program), an acquisition time of

12

1.9 s, a pre-scan delay of 6.5 μs and a relaxation delay between scans of 5 s. The spectrum

13

width was 16.0 ppm (6400 Hz) and 128 scans were accumulated.

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2.3.4 X-ray diffraction analysis (XRD)

15

X-ray powder diffraction was measured to evaluate the introduction and substitution of

16

the inorganic nanoparticles in the modified lignin structure. Diffraction scatters were

17

collected with a Panalytical Phillips X’Pert PRO multipurpose diffractometer (Almelo,

18

the Netherlands) using monochromatic Cu Kα radiation (λ = 1.541874 Å) in the 2θ range

19

from 3° to 70° with step size of 0.026 at room temperature and a counting time of 148.92

20

s.

21

2.3.5 Thermogravimetric analysis (TGA)

22

Dynamic thermo-gravimetric measurements were carried out to evaluate the thermal

23

resistance of the lignin samples before and after modification, especially after addition of

24

the inorganic nanoparticles into the lignin structure. The analyses were performed in a

25

TA instruments TGA Q5000 IR equipment, under dynamic nitrogen flow with a flow rate 12 ACS Paragon Plus Environment

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Biomacromolecules

1

of 40 mL min-1. Lignin samples of 5-10 mg were placed in a platinum crucible and heated

2

in a temperature range of 30 to 800 ºC at a constant heating rate of 10 ºC min-1 under N2

3

atmosphere. For the quantitative calculations, the response factors between the weight

4

gain (TG) and the mass loss rate (DTG) were determined.

5



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3. RESULTS AND DISCUSSION

2

3.1 Chemical composition of lignins

3

Two organosolv lignins from different origin were employed as starting materials, i.e.

4

lignin from almond shells (LAS) and lignin from maritime pine (LMP). Their main

5

chemical characteristics were analyzed prior to any further modification process. These

6

parameters are displayed in table 2.

7

Table 2. Major parameters regarding the chemical composition of the lignins. Lignin type

Klason lignin

Acid soluble lignin

Sugars

Ashes

Ratio S/G

(%)

(%)

(%)

(%)

Almond shells

89.30±0.44

2.18±0.25

1.49±0.18

4.04±0.49 1.51

Maritime pine

90.69±0.60

1.98±0.18

0.98±0.06

4.80±0.23 0.01

8 9

It was seen from the previous table that both lignins present similar compositions, since

10

they were extracted using the same process and conditions. The content of insoluble lignin

11

in both cases is high and in the typical range of organosolv lignins27,28. Concerning the

12

sugar content, both display low values, a bit higher in the case of LAS. The amount of

13

inorganic compounds was slightly high (4-5%). This behavior has also been observed in

14

another work by Fernández-Rodríguez et al29 for almond shells organosolv lignin (≈4%).

15

This ash contents may be due to some remaining sulfuric acid from the acidified water

16

used to precipitate the lignin, which was not washed completely after the filtration. The

17

major and most important difference between the lignins is observed in the S/G ratio. The

18

value is considerably higher in the LAS compared to the LMP, as it was expected. This

19

is because maritime pine (Pinus pinaster), is a softwood species which generally presents

20

a very low content or even lacks syringyl units, typical from hardwoods30. Thereby, the 14 ACS Paragon Plus Environment

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Biomacromolecules

1

guaiacyl units are predominant within the LMP lignin structure, resulting in low S/G

2

ratios. On the other hand, the almond shells, which do not strictly belong to hardwood or

3

softwood, present a higher amount of syringyl groups and therefore a higher S/G ratio.

4

This difference in the prominence of guaiacyl groups between the two lignins (reflected

5

on the S/G ratio) has a crucial influence in the further lignins modifications as it will be

6

described in the following sections.

7

3.2 FTIR spectroscopy analysis

8

The two lignins were analyzed by FTIR spectroscopy before the reaction with glyoxal

9

to elucidate their major structural differences. Both lignins are characterized by a broad

10

band at 3400 cm-1 of hydroxyl groups, another intense band around 3000 cm-1 linked to

11

methyl and methylene groups and two narrow bands between 1600-1500 cm-1 typical of

12

aromatic rings vibrations of lignin. In the range 1500-1400 cm-1, bands corresponding to

13

C-H bonds of methylene and aromatic rings are found as well. Nevertheless, several

14

differences are also found within the structure of the lignins depending on their

15

provenance. In fact, it is known that the origin of the lignin influences its structure31.

16

Hence, the lignin obtained from almond shells (LAS), which presents a higher S/G ratio,

17

displays an intense band related to syringyl structures at 1119 cm-1. This peak do not

18

appear in the lignin extracted from maritime pine (LMP), as it is softwood with prevalence

19

of guaiacyl groups. Two bands related to guaiacyl rings appear in LMP as well (1266,

20

1139 cm-1), which are not present in LAS. Thus, the major difference between the lignins

21

is the prevalence of syringyl groups in LAS and of guaiacyl groups in LMP. This is

22

concurrent with the chemical characterization of lignins by their S/G ratios. The

23

assignations to all the bands presented before are provided in section 2 of supplementary

24

information in table SI2.



Page 16 of 47

1

FTIR spectroscopy was also employed to assess the main structural changes in the

2

lignin samples after glyoxalation. In figure 3, the spectra corresponding to the pristine

3

lignins (LAS and LMP) and the different formulations of glyoxalated lignins (LG1-LG6)

4

are shown. Some differences are observed in the structure of the lignins before and after

5

the reaction with glyoxal, as it can be seen in the spectra from figure 3. First, the band of

6

hydroxyl groups at 3400 cm-1 increases after glyoxalation becoming wider, especially as

7

the amount of glyoxal employed is incremented. A band at 1700 cm-1 linked to carbonyl

8

groups appears in both lignins. Nevertheless, the behaviors observed after glyoxalation

9

concerning this band are different. On the one hand, in the glyoxalated LAS the intensity

10

of the band decreases considerably and becomes broader and less intense. On the other

11

side, in the case of glyoxalated LMP the band intensity decreases in a small extent. This

12

may be associated to a different reaction path of the carbonyl groups present in both types

13

of pristine lignins during glyoxalation. Another band around 1325 cm-1 is seen, which is

14

related to phenolic hydroxyl groups. Nevertheless, after glyoxalation the major changes

15

are observed in the region between 1120 and 1030 cm-1. Here three main bands appear at

16

1117 cm-1, 1060-1080 cm-1 and 1030 cm-1, corresponding to C-O stretching of secondary

17

alcohol and C-O stretching and deformation of primary alcohol respectively. The major

18

band is located in the region between 1060 and 1080 cm-1, which becomes broader as the

19

amount of glyoxal in the reaction increases.


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Biomacromolecules

1 2

Figure 3. Comparison of the spectra from pristine and glyoxalated lignins: a) almond

3

shells lignin b) maritime pine lignin

4

To calculate the glyoxalation index, the absorbance spectra were obtained and

5

normalized to the maximum absorbance. Then, the resulting curve was fitted using

6

PeakFit 4.12 signal processing software, to elucidate the absorbance wavelengths of each

7

band by integrating the area under the fitted curves. The numerical fitting was performed

8

with a correlation of 0.999 ± 0.015. With the values of absorbance of each band, relative

9

absorbance ratios (equations below) were calculated based on the work by Malutan et

10

al.32. The glyoxalation index (GI) was calculated as the quotient of the means of these

11

ratios.

12

OH-total groups = Average (A3400, A1325, A1117, A1076, A1030)

13

Phenolic OH-groups = A1325

14

GI =OH-Aliphatic / OH-Aromatic = [(OH-total / OH-Aromatic)-1]

15

The ratios previously defined are included in table SI3 from section 2 of supplementary

16

information. With regard to the results, it is observed that both the total hydroxyl groups

17

and the glyoxalation index follow the same increasing tendency. This is because during

18

the glyoxalation reaction, hydroxymethyl groups (-CH2OH ) were introduced into the

19

lignin structure as reported by Younesi-Kordkheili and Pizzi33. Thereby, the GI is taken 17 ACS Paragon Plus Environment


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1

as indicator to assess the degree of activation of the lignins. Regarding the amount of

2

aromatic hydroxyl groups (OH-aromatic), two trends are observed. In the glyoxalation of

3

LAS, this ratio increase (from 1.95 to 3.44), whereas it remains within a smaller range in

4

the case of LMP (from 1.78 to 2.13). This indicates that LMP presents a higher amount

5

of aliphatic OH compared to LAS after glyoxalation. For this reason, the GI of

6

glyoxalated LMP is considerably higher than that of LAS (4.29 and 2.53 respectively in

7

the most favorable cases), since the GI is defined as the ratio between aliphatic and

8

aromatic OH. This index outlines that the introduction of hydroxymethyl groups (–

9

CH2OH) in the lignin structure is more evident in the LMP, and therefore the degree of

10

modification is higher. The reason for that is related to the S/G ratios of both pristine

11

lignins. The fact that LMP has a low S/G ratio underlines a majority of guaiacyl units

12

within its structure (one methxoyl group in the aromatic ring), whereas the higher S/G

13

ratio of LAS highlights a preponderance of syringyl moieties (two methoxyl groups in

14

the aromatic ring). Thus, it is clear that more free positions in the aromatic rings will be

15

available in LMP compared to LAS. In the reaction between lignins and glyoxal and

16

formaldehyde, the hydroxymethyl (-CH2OH) groups are normally introduced in the C3

17

and/or C5 position of the aromatic rings34,35. Accordingly, the LMP lignins with more

18

free spaces in their aromatic rings would facilitate the process of activation, leading to a

19

higher extent of glyoxalation. Besides, it is evidenced in both cases that the formulations

20

with higher amounts of glyoxal added (LGS3 and LGS6) are the ones with a higher degree

21

of glyoxalation and thus more activated. Accordingly, these formulations were the ones

22

being tested and analyzed later in the process of modification with inorganic

23

nanoparticles.

24

The reaction of lignin with the nanoparticles was evaluated by FTIR analysis as well to

25

determine the main structural differences between the activated lignins before and after 18 ACS Paragon Plus Environment

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1

the addition of the inorganic compounds. Thus, it is possible to elucidate whether the

2

nanoclay (OMMT) and the nanosilicate (POSS®) were introduced into the lignin structure

3

and whether the substitution of the functional groups happened. In figure 4 the spectra

4

from the different formulation of lignins modified with the inorganic nanoparticles are

5

shown. Additionally, both nanoparticles are displayed as well and presented as reference.

6

The selected spectra range is between 1900 and 400 cm-1 to highlight the bands showing

7

the most significant changes.

8 9

Figure 4. Comparison of the spectra from glyoxalated lignins before and after addition

10

of inorganic nanoparticles: a) almond shells lignin with OMMT b) maritime pine lignin

11

with OMMT, c) almond shells lignin with POSS® and d) maritime pine lignin with

12

POSS®.

13

It is seen in the previous figure that new bands derived from OMMT and POSS®

14

appeared in the lignins after the process of modification. Nevertheless, some differences 19 ACS Paragon Plus Environment


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1

are regarded depending on the type of nanoparticles used. In the case of lignins modified

2

with OMMT, two main bands are highlighted at 460 cm-1 and 520 cm-1. The first one

3

corresponds to deformation of Si-O-Si bonds36 whereas the second band to Si-O-Al

4

linkages of montmorillonite layered-structure37. On the other hand, the lignins modified

5

with POSS® show four new bands at 499 cm-1, 695 cm-1, 890-895 cm-1 and 1423 cm-1

6

associated to Si linkages. These are linked to Si-O bending38, Si-C stretching39, Si-OH

7

bending40 and Si-Ph stretching41 vibrations respectively. Another new band is seen in the

8

modified lignin spectra at 740-745 cm-1 related to C-H deformations of aromatic rings

9

from POSS® structure. In all the lignins modified with inorganic nanoparticles, both

10

OMMT and POSS ®, a strong band is observed at 1100-1000 cm-1. This band, which

11

corresponds to Si-O-Si symmetric stretching, is overlapped with that of glyoxalated

12

lignins attributed to C-O stretching and deformation of primary and secondary alcohols.

13

This phenomenon was also observed in another work by Zhang et al.42

14

Considering the mentioned variations in the spectra of the lignins modified with

15

inorganic nanoparticles, a possible reaction between the glyoxalated lignins and the

16

inorganic nanoparticles is presented. Thus, it is proposed that components are reacting

17

through their hydroxyl groups by means of a condensation reaction. This is based in

18

several observations from the spectra of the different lignins, OMMT and POSS®. On the

19

one hand, the OMMT shows two small peaks at 917 and 885 cm-1 attributed to the in-

20

plane vibration of the hydroxyl groups of AlMgOH and AlAlOH respectively43 and those

21

peaks disappear from the original spectra to the lignin modified with this nanoparticle. A

22

similar tendency is seen for the band of POSS® related to the bending vibrations of

23

hydroxyl groups linked to Si, whose intensity considerably decreases from the original

24

POSS® spectra to the lignins modified with this nanoparticle.


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Biomacromolecules

1

In another work by An et al. 201844 an analogous explanation to the previous ones is

2

described regarding the reaction of the hydroxyl groups between montmorillonite and a

3

lignocellulosic compound to form a nanocomposite.

4

Besides these facts, it is observed that the intensity of the band corresponding to the

5

total hydroxyl groups (≈3400 cm-1) decreases from the spectra of the glyoxalated lignins

6

to the spectra of the lignins modified with OMMT and POSS® (as shown in section 2 of

7

supplementary information in figure SI1). This decrease in the intensity of the mentioned

8

band from one spectra to the other was attributed to the above-mentioned condensation

9

reaction between the lignins and the inorganic nanoparticles through the hydroxyl groups.

10

Thus, when the OMMT and POSS® were linked to the lignin structure they were attached

11

to the different types of hydroxyl groups present in the glyoxalated lignins, lowering the

12

amount of these functional groups and accordingly reducing the intensity of the band of

13

the infrared spectra.

14

From these results, it was confirmed that both OMMT and POSS® were introduced into

15

the lignins structure and therefore the formation of organic-inorganic nanocomposites

16

based on activated lignins and inorganic nanoparticles was achieved.

17

3.3 Molecular weight determination

18

The molecular weight of the pristine lignins and that of the lignins after glyoxalation

19

and modification with inorganic nanoparticles was elucidated to evaluate the size

20

variations of the lignin structures. The results derived from this analysis are displayed in

21

figure 5.



Page 22 of 47

1 2

Figure 5. Molecular weights of the lignins before modification, after glyoxalation and

3

after modification with inorganic nanoparticles. PL: pristine lignins, G1: glyoxalation

4

conditions of LG1 and LG4, G2: glyoxalation conditions of LG2 and LG5, G3:

5

glyoxalation conditions of LG3 and LG6, M1: modification with OMMT, conditions of

6

LG3A and LG6A and M2: modification with POSS ®, conditions of LG3B and LG6B.

7

From the previous results, it is observed a clear difference between the molecular

8

weights of the pristine lignins (PL) and those of glyoxalated (G1-3) and modified lignins

9

(M1-2). After the reaction with glyoxal, the molecular weights increase considerably

10

since they are almost doubled in both cases. This is attributed to the condensation between

11

cleaved and uncleaved lignin units through glyoxylene bridges as reported by Navarrete

12

et al.45. As it was stated in the previous section, during the glyoxalation reaction,

13

hydroxylmethyl groups are introduced in the free positions of the aromatic rings from the

14

lignin units. Then those lignin units can follow a condensation reaction through the

15

hydroxymethyl groups forming methylene bridges, as reported by Hussin et al.34. A

16

possible reaction path for these reactions is proposed in figure 6. 22 ACS Paragon Plus Environment

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Biomacromolecules

1 2

Figure 6. Mechanism proposed for the reaction between lignin units and glyoxal

3

Concerning the different glyoxalated lignins, no significant differences are regarded

4

between them in terms of molecular weights. Thus, the increment of the glyoxal content

5

along the glyoxalated lignins might contribute mainly to the introduction of

6

hydroxymethyl groups in the lignin units (e.g. blue groups in figure 6) rather than to

7

further condensations, which could grow the molecular weights of the lignins. This would

8

be in agreement with the increase of the OH groups as proved with FTIR analysis.

9

Regarding the modified lignins with inorganic nanoparticles, none of the molecular

10

weights showed any big differences compared to the glyoxalated lignins. This is because

11

during the process of modification a 5% of nanoparticles were added based on the amount

12

of lignin. Since these nanoparticles do not have big molecular weights, the values for

13

these lignins remain almost constant.

14

3.4 1H-NMR spectroscopy analysis of the lignins

15

The structural changes of the lignins before and after the glyoxalation were regarded

16

by analyzing their 1H-NMR spectra. Before this analysis, the peaks were normalized to

17

the highest intensity. In figure 7, the spectra of the lignin samples before and after

18

modification are shown. The assignations to the main signals detected in the spectra are

19

included in section 3 of supplementary information in table SI4.

20

3.4.1 Unmodified or pristine lignins

21

In the spectra of the lignins LAS and LMP different signals appear in the range 0.7-3.0

22

ppm, which correspond majorly to CH3 and CH2 from saturated aliphatic chains. 23 ACS Paragon Plus Environment


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1

According to Oliveira et al.46, this suggests the presence of aliphatic compounds linked

2

to the lignin structure. At 3.7 ppm a strong signal is seen in both pristine lignins, which

3

is related to the protons of methoxyl groups. In this signal, a significant difference was

4

found between the spectra of LAS and LMP. Thus, its intensity in the case of the former

5

lignin is higher compared to the later, indicating that a higher abundance of this type of

6

protons is present in LAS respect to LMP (supplementary information figure SI2). This

7

was attributed to the fact that LAS displays syringyl units within its structure, whereas in

8

LMP there was almost an absence of these moieties. Since each syringyl unit is composed

9

of two methoxyl groups with their corresponding protons, it can be expected that the

10

intensity of the peak for these protons would be lower in the lignin where syringyl

11

moieties are not present or are present in a lower extent.

12

Some mild to small signals are found in the range between 4-6 ppm attributed to the

13

protons from the typical lignin interlinkages -O-4, -, -5. In the range 6-8 ppm, more

14

intense signals are present corresponding to aromatic protons as reported in other

15

works47,48 and differences are encountered between LAS and LMP. The former lignin

16

displays higher intensity within this region compared to LMP and its band is also wider,

17

highlighting a bigger amount and a greater variety of aromatic protons. In LAS, proton

18

signals corresponding to syringyl and guaiacyl moieties are seen, whereas in LMP there

19

are only guaiacyl-derived signals. This is in accordance with the S/G ratios previously

20

discussed in section 3.1. Between 8 and 9 ppm protons derived from phenolic hydroxyl

21

groups are normally reported49,50. To confirm the presence of the mentioned groups, the

22

lignin samples were added trifluoroacetic acid (TFA) prior to the 1H-NMR analysis. TFA

23

is known to react with alcohols through the hydroxyl groups, leading to the esterification

24

of the polymer51. By comparing the spectra of the samples without any TFA added and

25

the spectra after addition of a small volume of TFA, the position of the peaks due to the 24 ACS Paragon Plus Environment

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Biomacromolecules

1

hydroxyl groups can be determined. Thereby, it is seen that some peaks present in the

2

spectra of the pristine lignins, disappear after adding TFA. These signals can be

3

undoubtedly define as hydroxyl derived protons. The comparison of the spectra from the

4

pristine lignins before and after addition of TFA is presented in section 3 of

5

supplementary information in figure SI3. Following this procedure, several hydroxyl

6

signals are detected in the range 8-9 ppm. Again, differences are found regarding the

7

content of guaiacyl and syringyl units. In LAS samples, signals related to hydroxyl groups

8

from syringyl and guaiacyl moieties are detected while in LMP only the later ones are

9

seen. Beyond 9 ppm, few signals are observed such as a couple of peaks related to protons

10

from cinammaldehyde and benzaldehyde and a weak broad peak around 12 ppm

11

associated to protons from carboxylic acids.

12

3.4.2 Modified or glyoxalated lignins

13

During the glyoxalation, some of the bands previously described were modified owing

14

to the structural changes induced by the reaction of lignins with glyoxal. The main

15

differences in the spectra of the modified lignins are regarded specially in the range

16

between 4-5.5 ppm. Within this region, a moderate to strong signal is observed at 4.5

17

ppm, which could be associated to the hydroxyl proton of hydroxymethyl groups (-

18

CH2OH). To confirm that this signal is due to a hydroxyl group, the same methodology

19

based on the addition of TFA and described before was employed (spectra in

20

supplementary information figure SI4). Following this method, the previous hypothesis

21

was proved. This is in agreement with the introduction of hydroxyl groups mentioned in

22

section 3.2 and observed in the infrared spectra as well. The rest of the peaks present in

23

this section of the spectra, are related to the methylene or glyoxalene groups, both linked

24

and non-linked to hydroxyl groups. The later ones would be associated to linkages

25

between lignin small fractions to form a bigger structure. This assumption is in 25 ACS Paragon Plus Environment


Page 26 of 47

1

accordance with the condensation of lignin moieties through glyoxalene bonds stated in

2

section 3.3.

3

The other major divergences are seen in the last part of the spectra (6-10 ppm). Here it

4

is observed that the width of the peak due to aromatic protons is reduced after reaction

5

with glyoxal. A similar tendency is regarded for the signals of the aromatic hydroxyl

6

groups, but in this case, they disappear after glyoxalation. This can indicate that some of

7

the linkages formed between lignin moieties via glyoxalene bridges may occur through

8

the H and some of the OH of the aromatic rings.

9

On the other hand, a narrow and intense peak appears in both cases after glyoxalation

10

at 8.4-8.5 ppm. This peak is related to the reaction of glyoxal and lignin. Capraru et al.52

11

pointed out an analogous trend in the same range for the reaction of lignin and

12

formaldehyde. Finally, peaks at 9.3 ppm and 9.4 ppm appear after the lignin modification.

13

These peaks, which have the typical shift of aldehyde protons47, could be attributed to

14

aldehyde compounds derived from some residual glyoxal or glyoxal oligomers remaining

15

unreacted.


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Biomacromolecules

1 2

Figure 7. H-NMR curves of the original (LAS and LMP) and glyoxalated lignins (LG3

3

and LG6) focusing in the different parts of the spectra: a) aliphatic H+ (range 0-3 ppm),

4

b) methoxyl and lignin interunits linkages H+ (range 3-6 ppm), and c) aromatic and

5

hydroxyl H+ (range 6-10 ppm).

6

3.5 XRD analysis of the lignins

7

The X-ray diffraction analysis was carried out for the lignin samples modified with

8

inorganic nanoparticles to evaluate the introduction of the nanoparticles into the lignin

9

structure. The diffractograms obtained from the XRD analysis were normalized to highest

10

intensity prior to any assessment. Then the FitPeak 4.12 signal processing software was

11

used to perform the fit of the curves from the diffractograms. The numerical fitting was

12

done with a correlation of 0.999±0.012. 27 ACS Paragon Plus Environment


Page 28 of 47

1

In figure 8, the normalized diffraction patterns of nanoparticles, pristine and modified

2

lignins are displayed. Results from indexing and phase identification are presented in

3

supplementary information (tables SI5 and SI6).

4

Concerning the organically modified montmorillonite, the peaks with the highest

5

intensities are regarded in the range 2 between 0-10º. Here it can be highlighted the

6

peaks at 3.29º and 4.96º related to the silicate structure and the organic modifier present

7

in the nanoclay (dimethylbenzylhydrogenated tallow ammonium) respectively53,54. In the

8

next range (2 between 10-30º) considerable intensities are observed as well. Within this

9

region, the peaks at 20.04º and 22.04º are remarked, which are attributed to the planes

10

020 and -111 respectively. They are typical from the montmorillonite as reported in

11

various works55,56. Beyond 2=35º, other important planes at -201 and -131 of monoclinic

12

montmorillonite are seen.

13

In both modified lignins with OMMT (LG3A, LG6A), the contribution of the first

14

peaks in the range 2=0-10º (owing to the silicates) is clearly visible. However, the peaks

15

corresponding to the planes in the range 2=15-30º are only present in the modified lignin

16

from maritime pine (LG6A) and not in that from almond shells (LG3A). This is observed

17

specially in the peak of OMMT at 2=20.04º, which is shifted in the modified lignin (2

18

≈18.5º). This may be caused by a better interaction between the lignin from maritime pine

19

and the OMMT, owing to its structure with more free positions in the aromatic rings thus

20

having higher activation. This tendency is also observed and explained in the lignin

21

modified with POSS® presented afterwards. In the range. In the range 2=30-50º, the

22

contribution of planes of the monoclinic montmorillonite are present in both modified

23

lignins but it is more perceptible in the case of LG6A.


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Biomacromolecules

1

Regarding the polyoligomeric silsesquioxane, several peaks are observed in the region

2

2=0-10º. Weak signals at 6.54º and 8.99º and strong one at 7.53º57, corresponding to the

3

triclinic structure of trisilanol phenyl (TS-Ph), are identified. Besides a peak at 7.96º

4

attributed to the rhombohedral structure of POSS® is seen. In the range 2=10-20º, strong

5

to weak signals are highlighted at 14.53º, 17.00º, 19.43º and 19.98º, which are associated

6

to the trisilanol phenyl structure (TS-Ph) as well. Beyond 2=20º, the majority of the

7

signals are attributed to rhombohedral planes of POSS® 58.

8

Regarding the lignins modified with POSS (LG3B and LG6B), in the range 2=0-10º

9

the contribution of the signal corresponding to rhombohedral POSS structure is seen in

10

both lignins. Nevertheless in the next region (2=10-20º), the contributions due to

11

trisilanol phenyl (TS-Ph) at 19.43º and 19.98º are more evident (more pronounced signals

12

at those scattering angles) in the case of modified lignin from maritime pine (LG6B).

13

Furthermore, the relative intensity of the broad curve associated to the amorphous

14

structure of lignin is reduced around 15% in this same modified lignin. Thereby, in the

15

case of the modified lignin from maritime pine (LG6B) there is a higher predominance

16

of the organic-inorganic modified structure of lignin compared to the modified lignin

17

from almond shells (LG3B). This would mean that the extent of insertion of the inorganic

18

nanoparticles was higher in the LG6B than in LG3B.

19

This tendency was attributed to the fact that pristine lignin from maritime pine presents

20

more free positions in the aromatic rings than the lignin from almond shells; owing to the

21

prevalence of guaiacyl units, as commented in the section 3.1. Thereby a higher degree

22

of activation was achieved during the glyoxalation for those lignins. For this reason, the

23

introduction of the inorganic nanoparticles into the lignin matrix was higher compared to

24

the almond shells lignin, which presents a lower degree of activation.

25 29 ACS Paragon Plus Environment


Page 30 of 47

1 2

Figure 8. Diffractograms of the different lignins modified with inorganic nanoparticles:

3

a) LAS modified with OMMT, b) LMP modified with OMMT, c) LAS modified with

4

POSS® and d) LMP modified with POSS®.

5

3.6 Thermal stability assessment

6

The thermal stability and degradation of the different lignin samples was evaluated by

7

means of thermogravimetric analysis. The thermogravimetric and derivative

8

thermogravimetric curves from the pristine, glyoxalated and lignins modified with

9

inorganic nanoparticles are showed in figure 9. Additionally, the main parameters

10

extracted from these curves are attached to the section 5 of supplementary information in

11

table SI7.

12

In general, the pristine lignins present two main steps of degradation. The first event

13

(137-141ºC) is related to the vaporization of the moisture remaining in the samples and

14

the volatiles decomposition. The second stage is attributed to the degradation of lignin 30 ACS Paragon Plus Environment

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Biomacromolecules

1

structure, since it corresponds to the major percentage of weight loss (≈50%). Moreover,

2

it appears at the typical range of lignin thermal decomposition (380-400 ºC). In the case

3

of LMP, another small extra step is observed before 300ºC. Since lignins are known for

4

having a heterogeneous and complex structure, this small extra step may be attributed to

5

the degradation of lignin moieties of lower molecular weight or lignin-carbohydrate

6

complexes (derived from hemicellulose fractions linked to the lignin structure), which are

7

degraded at lower temperature (more easily degraded).

8

After glyoxalation, it is clear that the structure of the lignins is changed since new

9

degradation stages, which were not present in the pristine lignins, appear. Thereby, in

10

glyoxalated lignins four main events are regarded during the thermal degradation. The

11

first one is again associated to the moisture and volatiles and appears at the same range

12

of temperatures described before. Nevertheless, the rate of degradation of this step is

13

increased compared to that of unmodified lignins. Accordingly, the initial degradation

14

temperature (T5%) considerably decreases compared to that of pristine lignins (table 5).

15

This diminution observed in the glyoxalated lignins could be attributed to unreacted

16

glyoxal and glyoxal dimers or trimers present in a small percentage, which were not in

17

the pristine lignins. The second stage of degradation is observed between 295-310 ºC.

18

Since this step is present only in the glyoxalated samples, it is related to the reaction

19

between lignin and glyoxal. Thus, this step can be attributed to the formation of new

20

lignin condensates of smaller molecular weight after the crosslinking between the lignin

21

moieties by means of methylene bridges as reported by Ang et al.59 The third event (360-

22

363 ºC) is linked to degradation of the glyoxalated lignin structure. In comparison to the

23

pristine lignins, this temperature of degradation is decreased. This may be due to the

24

introduction of more hydroxyl groups, which would increase the susceptibility to thermal

25

decomposition60. The last stage of degradation (450-500 ºC), is derived from the 31 ACS Paragon Plus Environment


Page 32 of 47

1

glyoxalation reaction and therefore it is again present only in the lignin samples modified

2

with glyoxal. An extra step of decomposition at 240 ºC is regarded in LG6 following the

3

same tendency of LMP mentioned before.

4 5

Figure 9. Curves from the thermal analysis of the different lignin samples before and

6

after modification: a) thermogravimetric curves of lignins from almond shells, b)

7

derivative thermogravimetric curves of lignins from almond shells, c) thermogravimetric

8

curves of lignins from maritime pine, and d) derivative thermogravimetric curves of

9

lignins from maritime pine.

10

The lignins modified with inorganic nanoparticles display some differences concerning

11

the thermal behavior, compared to the glyoxalated ones. Regarding the initial degradation

12

temperature (T5%), an increment is observed after the addition of inorganic nanoparticles

13

(5.3% from LG3 to LG3A,B and 12.7% from LG6 to LG6A, B). This was due to the

14

hydrophobicity of the integrated inorganic nanoparticles, which increments the

15

temperatures of the first stage of degradation (moisture evaporation). Since T5% is located 32 ACS Paragon Plus Environment

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Biomacromolecules

1

within this stage an increase is observed in the values of these temperatures after the

2

addition of the nanoparticles. This tendency is also seen for the temperature at which the

3

half of the mass was lost (T50%). In this case, the increase in the T50% was associated to

4

the attachment of the inorganic nanoparticles to the lignin structure, which improved the

5

thermal resistance of the modified lignins (up to 30ºC more in the most favorable case).

6

These trends are observed in both types of lignins. The residues remaining after thermal

7

degradation are in the range 33-42% and slightly higher in the modified lignins than in

8

the pristine ones. Concerning the stages of degradation, lignins modified with inorganic

9

nanoparticles show the same ones described for the glyoxalated lignins. Nonetheless, not

10

the same results are obtained for both type of lignins. In the case of LAS, the temperatures

11

of degradation of each step in glyoxalated lignin (LG3) and lignins modified with

12

inorganic nanoparticles (LG3A and LG3B) remain constant or slightly increase. On the

13

other hand, in the case of LMP the stages of degradation are delayed in the lignins

14

modified with inorganic nanoparticles (LG6A and LG6B) compared to the glyoxalated

15

one (LG6). This tendency is more noticeable in the last stages of degradation. In the step

16

of degradation associated to the lignin structure (≈360 ºC), the temperature of

17

decomposition is increased between 10-15 ºC. In addition to that, in the last stage of

18

degradation (due to lignin modification with glyoxal) the degradation temperature is

19

delayed 15-20 ºC. Thereby it is confirmed that the introduction of the nanoclay and

20

nanosilicate particles do improve the thermal behavior of the lignins specially that of

21

LMP. These results concur with those observed in the XRD patterns, linking directly

22

proportionally the higher thermal stability to the presence of inorganic agents.

23

4. CONCLUSIONS

24

The results presented in this work display the structural modification of lignins from

25

two different origins via glyoxalation and modification with inorganic nanoparticles. On 33 ACS Paragon Plus Environment


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1

the one hand, the glyoxalation reaction results in an increase of the amount of hydroxyl

2

groups within the lignin structure and in the formation of condensates between lignin

3

small units by means of methylene bridges. These phenomena were verified mainly by

4

the FTIR and 1H-NMR analyses. On the other hand, the introduction of the nanoparticles

5

and their substitution into the lignin structure was confirmed as well, as shown in the

6

results from the FTIR and XRD analyses. Accordingly, it is observed that the activation

7

of the lignin through the reaction with glyoxal succeeded aiding the incorporation of the

8

inorganic phases into the organic polymer matrix. These inorganic phases are proved to

9

enhance the thermal behavior of the nanocomposite by delaying the temperatures of

10

degradation, especially in the case of lignin derived from maritime pine.


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Biomacromolecules

5. SUPPLEMENTARY INFORMATION Section 1. Conditions of the process of modification with inorganic nanoparticles for the different lignin formulations 

Conditions used for the different formulations of the lignins modified with inorganic nanoparticles: Table SI1.

Section 2. FTIR spectroscopy analysis of the lignins 

Assignments to the main bands found in the spectra of pristine lignins: Table SI2



Comparison of the band corresponding to OH groups between glyoxalated lignins (LG3, LG6) and lignins modified with inorganic nanoparticles (LG3A, B and LG6A, B): Figure SI1



Results of the different ratios calculated namely OH-aromatic, OH-total and GI for the process of glyoxalation: Table SI3

Section 3. 1H-NMR spectroscopy analysis of the lignins 

Assignation to the main peaks determined in the 1H-NMR spectra of the pristine and glyoxalated lignins: Table SI4



Comparison of the pristine lignins (LAS and LMP) regarding the peaks corresponding to methoxy groups: Figure SI2.



Determination of the peaks of protons from hydroxyl groups for pristine and glyoxalated lignins: Figures SI3 and SI4.

Section 4. DRX analysis of the lignins 

Diffractograms obtained for the inorganic nanoparticles: Figure SI5 and SI6.



Insets of the diffractograms showed in the main manuscript in figure 7 (range 2=3-10 degrees): Figure SI7. 35 ACS Paragon Plus Environment




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Indexing of the different peaks obtained for the inorganic nanoparticles: Tables SI5 and SI6.

Section 5. Thermogravimetric analysis of the lignins 

Thermal parameters determined from the curves of thermogravimetric and derivative thermogravimetric analyses and stages of degradation: Table SI7


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Biomacromolecules

6. ACKNOWLEDGMENTS The authors would like to thank to the University of the Basque Country UPV/EHU and the University of Pau and Pays de l’Adour UPPA (Predoctoral fellowshipPIFPAU15/01) for financially supporting this work.



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8. TABLE OF CONTENTS GRAPHIC


Formulation of Multifunctional Materials Based on the Reaction of

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