Cellulose nanocrystals: preparation, water interactions, and their connection to cell wall ultrastructure

Transcription

1 Cellulose nanocrystals: preparation, water interactions, and their connection to cell wall ultrastructure Eero Kontturi Department of Forest Products Technology School of Chemical Technology Aalto University Finland Polymer and Composites Engineering Department of Chemical Engineering Imperial College London United Kingdom CST FP1105 Workshop, Thessaloniki

2 utline (1) Background: cellulose nanocrystals (2) TEMP-mediated oxidation of microgranular cellulose Three fractions, one of which is cellulose nanocrystals (3) Hydrolysis of cotton fibres by HCl vapour Formation of uncharged nanocrystals in high yield (4) Swelling of cellulose nanocrystal thin films Water interactions in a highly crystalline network

3 Acid hydrolysis of cellulose H Acid hydrolysis involves the breakage of glycosidic bond by addition of water, catalyzed by acid High concentrations are required for complete degradation (e.g., 72% (w/w) H 2 S 4 ) H H H H H H H H H n/2 + H 2 /H + H H H H Glucose

4 Cellulose nanocrystals: principle of preparation Level-off degree of polymerization (LDP): upon acid hydrolysis, cellulose depolymerizes rapidly down to a certain level after which the degradation is minimal. LDP Proposition: amorphous domains are hydrolyzed leaving the crystalline domains intact. Battista Ind. Eng. Chem. 1950, 42, 502.

5 Cellulose nanocrystals: principle of preparation Cellulose microfibril crystalline amorphous Cellulose nanocrystals Strong acid hydrolysis 1µm Acid hydrolysis targets the disordered regions in a cellulose microfibril. Result: cellulose nanocrystals

6 Cellulose nanocrystals: challenges in preparation Main practical challenges - high acid concentrations required for complete hydrolysis - large amounts of water required to wash the sample recovery of the acid is difficult Huge water consumption is not exactly green chemistry.

7 Alternative route to cellulose nanocrystals (and other particles): TEMP-oxidation of microgranular cellulose

8 Microcrystalline cellulose btained by strong acid hydrolysis of native cellulose (down to LDP) Consists of particles of around µm in size Established, commercially available material

9 What happens when microcrystalline cellulose is oxidized with TEMP? Microcrystalline cellulose consists of large particles However, the degree of polymerization is at LDP TEMP-mediated oxidation should be able to penetrate to the areas between the cellulose crystallites or microfibrils, like they do in hornified fibres (Saito et al. 2007) LDP + TEMP-oxidation Charged nanocrystals?

10 TEMP reaction used in this study Activator H N + N H H H H HCl H N H H H H H TEMP-mediated oxidation with Cl 2 as activator* ph 5, adjusted before HCl addition The reaction was carried out for 22h for microgranular cellulose *) Conditions designed by Timo Pääkkönen

11 What happened? Microgranular cellulose was oxidized Three fractions emerged, only one of which was cellulose nanocrystals.

12 Closer look at the fractions (I) Coarse fraction: ~ µm ptical micrograph Cryo TEM Very large particle size distribution Individual particles consist of a swollen network of aligned cellulose crystals

13 Closer look at the fractions (II) Middle fraction: ~ µm ptical micrograph Cryo TEM Large particle size distribution Individual particles consist of a swollen network of aligned cellulose crystals

14 Closer look at the fractions (III) Fine fraction: cellulose nanocrystals ptical micrograph Cryo TEM Narrow particle size distribution Individual particles are clearly rod-like cellulose nanocrystals

15 Particle size in each fraction Width (µm) Width (nm) fine fraction Length (nm) coarse fraction 5 0 middle fraction Length (µm) starting product

16 Summary: TEMP-mediated oxidation of microgranular cellulose TEMP-mediated oxidation of microgranular cellulose resulted in cellulose nanocrystals, yet only with 5% yield Most particles were micron-sized, consisting of a swollen network of aligned cellulose crystals, as revealed by cryo TEM All particles possessed high charge density (~1 meq/g) The swelling and shrinking of the micron-sized particles was directional 0 min drying 100 µm 120 min 100 µm

17 pen question: Why does a fraction (5%) of microcrystals isolate into nanocrystals and most of the microcrystals are retained (yet with very high charge density and in a swollen state)?

18 Hydrolysis of cotton fibres (filter paper) with HCl vapour

19 Idea: do the hydrolysis with acid vapour riginal idea * (CH 3 ) 3 Si Trimethylsilyl cellulose Si(CH 3 ) 3 n Si(CH 3 ) 3 HCl (g) * HCl (g) Cellulose H H H n Hydrolysis of silyl groups by vaporous HCl on supported ultrathin films proceeds with minute HCl vapor concentrations at room temperature. Schaub et al. Adv. Mater. 1993, 5, 919. Quantitative treatise of the reaction: Kontturi and Lankinen J. Am. Chem. Soc. 2010, 132, 3678.

20 Idea: do the hydrolysis with acid vapour Gurnagul, Page, Paice, Nord. Pulp Pap. Res. J. 1992, 7, 152. D. Page usually applied 10% HCl (l) hydrolysis with gaseous acid. Vapour phase HCl HCl (g) concentration in vapour phase is around 0.01 mol-%.

21 Experimental setup seal Cellulose substrate: Whatman 1 filter paper (cotton cellulose) (dry matter content ca. 97%) filter paper HCl vapor Vessel: Vacuum desiccator Acid: HCl in aqueous solution Conditions: Atmospheric (room temperature and pressure) Acid concentration in liquid [w-%] Concentration in vapor phase [volume-%] HCl liquid

22 Development of DP DP v % HCl (2.1% in vapour phase) % HCl (15%) % HCl (38%) 35% HCl (99%) time /min Rapid degradation down to LDP level at room temperature LDP

23 Adsorption of HCl on fibres Because HCl resides originally in vapour phase, it must reach the fibres by adsorption m(hcl) on fibres after 2h / mg x(hcl) DP HCl partial pressure / kpa ΔDP in 2 h

24 Hypothesis: dissociative adsorption Vaporous HCl adsorbs on fibre surface Fibre surface is covered with water HCl dissociates: H 2 +HCl H 3 + +Cl - Absolute amount of water on fibre surface is so low that huge local concentrations of H 3 + occur exceptionally high degradation rate The phenomenon is called dissociative adsorption.

25 Carbohydrate composition of extract 24 h hydrolysis Hydrolysis with HCl gas Neutralization Soxhlet extraction (water) at 100 C for 8h HPLC analysis 35% HCl 25% HCl % % from dry cellulose Glucose Galactose Arabinose Xylose Mannose Total Yield loss after hydrolysis with liquid HCl down to LDP level results in 5-20% yield loss.

26 Crystallinity development during hydrolysis [%] Degree of crystallinity as determined by x-ray diffraction HCl partial pressure / kpa - acid hydrolysis of cellulose usually results in formation of extractable sugars - hot water extraction of the hydrolyzed filter paper failed to extract anything REASN: vapour phase acid causes crystallization of cellulose simultaneously with its degradation.

27 Can we extract nanocrystals from fibres hydrolysed with HCl vapour? If we have degradation down to LDP level, it seems reasonable to expect that we can prepare cellulose nanocrystals with HCl vapour. Hydrolysis with HCl vapour: 35% HCl, 2 h, room temperature crystalline amorphous Grinding the hydrolysed substrate in a Wiley mill Dispersing the powder in formic acid

28 Can we extract nanocrystals from fibres hydrolysed with HCl vapour? Yes, we can extract nanocrystals from the hydrolysate. Hydrolysis with HCl vapour: 35% HCl, 2 h, room temperature 5 5 µm 2 Grinding the hydrolysed substrate in a Wiley mill Dispersing the powder in formic acid 2 2 µm 2

29 Can we extract nanocrystals from fibres hydrolysed with HCl vapour? Yes, we can extract nanocrystals from the hydrolysate. Hydrolysis with HCl vapour: 35% HCl, 2 h, room temperature Measured width of nanocrystals: 7 nm Grinding the hydrolysed substrate in a Wiley mill Dispersing the powder in formic acid

30 Summary of hydrolysis with HCL vapour (1) Degradation of cellulose by acid vapour occurs unusually fast at room temperature via dissociative adsorption. (2) Degradation of cellulose via dissociative adsorption induces simultaneous crystallization within cellulose microfibrils. (3) The process enables effortless preparation of cellulose nanocrystals. (4) Cellulose nanocrystals prepared this way are uncharged. (5) The yield of nanocrystals is >98%.

31 Cellulose nanocrystal films: interactions with water

32 Experimental design Cast ultrathin films from cellulose nanocrystals by spin coating Expose the films to humidity Check the behaviour of films with three methods: ellipsometry, QCM-D, and x-ray reflectivity

33 Thickness development in thin films Initial film thickness, nm QCM - D Ellipsometric porosimetry 5 g/l amorphous cellulose g/l amorphous cellulose g/l amorphous cellulose g/l cellulose nanocrystals g/l cellulose nanocrystals g/l cellulose nanocrystals Film thickness at 97 %RH, nm QCM - D Ellipsometric porosimetry 5 g/l amorphous cellulose 8,7 15,8 10 g/l amorphous cellulose 35,8 37,4 20 g/l amorphous cellulose 118,3 92,2 5 g/l cellulose nanocrystals 5,1 12,3 10 g/l cellulose nanocrystals 16,3 19,0 20 g/l cellulose nanocrystals 26,45 38,6

34 Summary Clear swelling observed for nanocrystal films Unexpected: rigid rods of crystalline cellulose should not swell Water penetrates inside the nanocrystal network, filling the voids between the crystals, but still, the network should not swell (i.e., increase in thickness) The network of nanocrystals is stabilised: it should not swell, either; otherwise it would redisperse when exposed to liquid water Are the joints between the nanocrystals flexible?

35 Acknowledgements Jessie Peyre (Aalto University, Finland) Elina Niinivaara (Aalto University, Finland) Janne Laine (Aalto University, Finland) rlando Rojas (Aalto University, Finland) Laura Taajamaa (Aalto University, Finland) Bruno Jean (CERMAV, France) Ritva Serimaa (University of Helsinki, Finland) Paavo Penttilä (University of Helsinki, Finland) Tekla Tammelin (VTT, Finland) Marco Faustini (Collège de France, France) Niki Baccile (Collège de France, France) Financing: Academy of Finland