Formation of Nanodiamonds during Pyrolysis of Butanosolv Lignin

Abstract

The preparation of artificial diamonds has a long history driven by decreased costs compared to naturally occurring diamonds and ethical issues. However, fabrication of diamonds in the laboratory from readily available biomass has not been extensively investigated. This work demonstrates a convenient method for producing nanodiamonds from biopolymer lignin at ambient pressure. Lignin was extracted from Douglas Fir sawdust using a butanosolv pretreatment and was pyrolyzed in N₂ at 1000 °C, followed by a second thermal treatment in 5% H₂/Ar at 1050 °C, both at ambient pressure. This led to the formation of nanodiamonds embedded in an amorphous carbon substrate. The changes occurring at various stages of the pyrolysis process were monitored by scanning electron microscopy, Fourier transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy. High resolution transmission electron microscopy revealed that nanodiamond crystallites, 4 nm in diameter on average, formed via multiple nucleation events in some carbon-containing high density spheres. It is proposed that highly defected graphene-like flakes form during the pyrolysis of lignin as an intermediate phase. These flakes are more deformable with more localized π electrons in comparison with graphene and join together face-to-face in different manners to form cubic or hexagonal nanodiamonds. This proposed mechanism for the formation of nanodiamonds is relevant to the future fabrication of diamonds from biomass under relatively mild conditions.

Keywords: electron microscopy; lignin; multiple nucleation; nanodiamond; pyrolysis.

Figure 1

(a) FT-IR spectra of butanosolv lignin and residue after pyrolysis at various temperatures. Regions of absorption frequencies corresponding to relevant structural features in lignin based on literature assignments⁻ are highlighted: A—3600–3000 cm^–1, O–H stretch; B—2950–2850 cm^–1, aromatic and alkyl C–H stretch; C—1730 cm^–1, C=O stretch in oxidized units; D—1600 cm^–1, aromatic C=C stretch; E—1510 cm^–1, aromatic skeletal vibrations associated with C–H deformations; F—1460 cm^–1, aromatic C–H deformations; G—1360 cm^–1, aliphatic C–H bend (not in OMe); H—1260 cm^–1, guaiacyl breathing vibrations; I—1220 cm^–1, aromatic C–O stretch; J—1120 cm^–1, aromatic C–H in-plane deformation; K—1030 cm^–1, aliphatic C–O deformation. HSQC NMR (700 MHz, DMSO-d₆) spectra of the linkage region of (b) butanosolv lignin before pyrolysis; (c) DMSO-soluble component of residue after pyrolysis at 200 °C; and (d) DMSO-soluble component of residue after pyrolysis at 300 °C. (e) Structures of relevant lignin interunit linkages that correspond to signals shown in HSQC NMR spectra. The cross peak signal corresponding to the β–β Hα-C position is shown in (b) with the threshold decreased five times due to low abundance. Structural features that change upon increasing pyrolysis temperature are highlighted by dashed black circles in the HSQC NMR spectra.

Figure 2

(a) Typical TEM image of carbon specimen showing embedded nanocrystallites. (b) Size distribution of nanocrystallites based on measurement of ∼200 nanocrystallites in HRTEM images.

Figure 3

HRTEM images of typical individual nanoparticles embedded in amorphous carbon in the produced carbon material after a two stage heat treatment initially at 1000 °C in a N₂ gas atmosphere and then at 1050 °C in a 5% H₂/Ar mixed gas atmosphere. (a) Example of a cluster of very small crystallites. The arrows point to individual crystallites. (b) Spherical particle showing two single crystalline hemispherical sections with an amorphous region between them. Label A indicates the d-spacing of the fringes marked by the double line with d_A = 2.10 Å. The single line shows that the fringes in the two parts have an antiphase relationship. (c) Cubic diamond particle with d-spacings of d_B = 2.06 and d_C = 1.79 Å and an interplanar angle of 55°. (d) Another cubic diamond particle with d-spacings of d_D = d_E = 2.05 Å and an interplanar angle of 70°. The inset is an enlarged image of the central area of the crystallite, and the arrows point to dark spots with local lattice distortion.

Figure 4

(a) Structural model of a cubic diamond. The orange spheres in the unit cell represent partial atomic vacancies. (b) Structural model of hexagonal diamond (lonsdaleite). The dashed red lines show a cyclohexane ring, which can be generated during the face-to-face connection of the graphene-like flakes.

Figure 5

HRTEM image of a crystallite of a hexagonal diamond. The d-spacings of the fringes are measured to be A = 2.18 Å and B = 2.06 Å. The corresponding interplane angle is 90°.

Figure 6

(a) Schematic representation of the experimental procedure used to prepare the nanodiamonds. See Figure S2 for an exploration of the impact of heating rate on lignin pyrolysis. (b) Representative structure of butanosolv lignin chain. (c) Representative structure of increasingly aromatic material resulting from the thermal decomposition of lignin. (d) Face-to-face interaction between two partially overlapped aromatic rings to form cubic diamond. (e) Full overlapped aromatic rings link together to form a hexagonal diamond.