Formation of carbyne-like materials during low temperature pyrolysis of lignocellulosic biomass: A natural resource of linear sp carbons

The exploration, understanding and potential applications of ‘Carbyne’, the one-dimensional sp allotrope of carbon, have been severely limited due to its extreme reactivity and a tendency for highly exothermic cross-linking. Due to ill-defined materials, limited characterization and a lack of compelling definitive evidence, even the existence of linear carbons has been questioned. We report a first-ever investigation on the formation of carbyne-like materials during low temperature pyrolysis of biobased lignin, a natural bioresource. The presence of carbyne was confirmed by detecting acetylenic –C≡C– bonds in lignin chars using NMR, Raman and FTIR spectroscopies. The crystallographic structure of this phase was determined as hexagonal: a = 6.052 Å, c = 6.96 Å from x-ray diffraction results. HRSEM images on lignin chars showed that the carbyne phase was present as nanoscale flakes/fibers (~10 nm thick) dispersed in an organic matrix and showed no sign of overlapping or physical contact. These nanostructures did not show any tendency towards cross-linking, but preferred to branch out instead. Overcoming key issues/challenges associated with their formation and stability, this study presents a novel approach for producing a stable condensed phase of sp-bonded linear carbons from a low-cost, naturally abundant, and renewable bioresource.

Conventionally, the spectra of all the carbon species are acquired by turning on 1 H decoupling during the acquisition period. However, turning off the 1 H decoupling for a short period of 40 μs (i.e., gated decoupling) suppresses the signal of the protonated, nonmobile carbon species, due to dipolar dephasing of the 13 C signal, and yields the signal for only the non-protonated or methyl carbon species. This step can identify and suppress signals from hydrogen bonded carbon atoms.
The spectra for lignin chars after heat treatment for 30 minutes at 350 °C and 400 °C are shown in Fig. S1a. The spectra in red, acquired without gated 1 H decoupling, represents the spectra of all carbon species, whereas the spectra in black, acquired after 40 μs of gated 1 H decoupling, represents the signals from non-protonated carbon species only. The baseline is represented by the horizontal dashed line in these spectra. The signals from the -C≡Cspecies (the sp hybridized alkyne carbons) are expected to be found in the chemical shift region between 100-60 ppm (see insets). No signal was observed in this region for 350 °C char after suppressing the signal from C-H species thereby indicating the absence of -C≡Cspecies at this temperature. However, a significant signal for the −C≡C− species was observed in the nonprotonated carbon species spectra in the black curve for the 400 °C char. The signal to noise 4 ratio in this region was measured to be 4.25, thereby indicating the presence of a 'real peak' clearly above the noise level.  Fig. S1c shows the NMR spectra of raw lignin powder, which contained contributions from three main constituents: cellulose (42%), lignin (35%), and hemicellulose (23%). The presence (or absence) of -C≡Cspecies could not be ascertained due to large contributions from several functional groups in the chemical shift region between 100-60 ppm. It is important to note that the NMR signal for the -C≡Cspecies was clearly absent for the 350 °C chars, and was unambiguously present in 400°C chars. The initial presence (or absence) of the -C≡Cspecies in the starting material may therefore not be an essential requisite for the formation of carbyne phase at higher temperatures.

Peak identification and phase characterization
We had previously reported detailed X-ray diffraction investigations using Cu Kα radiation on lignin chars heat treated in the temperature range 200-800°C 21 . The diffraction pattern for the 400°C char had contributions from three distinct phases. The first set included lignin based fibers labelled as Phase 'B' that were present in 200°C as well as 400°C chars; but this phase was no longer present at 600°C. The second set included additional peaks, that made their initial appearance at 400 °C and were labelled as the phase 'C'. The XRD peaks for the 'C' phase increased in intensity at 600°C, increasing further upon heating to 800 °C. The third set included peaks that did not follow any such well-defined pattern. The peaks for the carbyne phase were not identified unambiguously due to their relatively low intensities.

Spectroscopic investigations (NMR, Raman and FTIR) on 400 °C lignin chars had
indicated the presence of small amounts of the 'carbyne' phase along with different structural forms of sp 2 carbon, mineral impurities etc. The small domain size of the 'carbyne' phase is expected to give rise to broad XRD peaks; this aspect was alleviated to some extent by using a longer wavelength radiation CoKα (1.789 Å) instead of the standard CuKα (1.54 Å) and highresolution optics and beam focusing. Data collection was carried out over a longer period using step-scan with up to 30 seconds per angular step.
The XRD peaks for the carbyne phase were identified using the following criteria (see main text): (a) all relevant peaks should have similar peak shapes and profile clearly distinct from other peaks, and (b) these peaks must only be present in 375°C and 400°C chars and be absent in 600°C chars. Six peaks, marked with the symbol '▲', met these criteria and were used for 'carbyne' structure determination. Peaks located at 24°, 51° and 53.5° (marked with '*') did not belong to the carbyne phase, as these were present at all three temperatures.
Diffraction peaks at 31°, 49.5°, 76° and 81° were found to be relatively too sharp. 8 Figure S2a| XRD spectrum for the 600°C lignin char using Co radiation The XRD spectrum for the 600°C lignin char is shown in Fig. S2a. With the peak at 31° peak (marked with an '*' in the figure) attributed to impurity silica, indexing and structural characterization of other peaks have been detailed in Table S2. The structural phase of carbon present in 600°C chars was very similar to the phase 'C' (a=8.83Å; c=6.9Å) reported previously 21 .

Spatial distribution of the 'Carbyne' phase
In addition to determining the structure of 'carbyne', an attempt was made to determine its spatial distribution as well. After identifying the peaks for the 'carbyne' phase, a microdiffraction investigation was carried out for preparing a spatial map for this phase. Micro XRD measurements were performed on Philips X'pert MRD PRO system with a horizontal high resolution Ω-2θ Goniometer (320 mm radius) with a minimum step size of 0.  is presented in Fig. S3a. Key constituents in this char were found to be predominantly carbon with a very small peak for oxygen.

Figure S3a|
SEM/EDS images for 600°C chars indicating carbon to be the key constituent, along with a small oxygen peak. White regions in the SEM image indicate ash impurities.

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All these results indicate that the key element present in lignin chars was carbon along with a small amount of oxygen, nitrogen, sulphur and mineral impurities. The presence of hydrogen, which cannot be measured accurately during above analysis was indicated in the NMR spectra from all carbon species (Fig. S1a) and proximate analysis.
Spatial distribution of the 'Carbyne' phase: Fig. S3b shows an HRSEM image of the 400°C char showing the spatial distribution of cavities containing the carbyne phase (Fig. 4F). Several such cavities were found distributed across the specimen and were not localized in a specific area. This result on the spatial distribution of the 'carbyne phase' is in good agreement with the epitaxial mapping results from x-ray diffraction (Fig. S2).

Figure S3b|
HRSEM images for 400°C chars indicating the spatial distribution of cavities and the carbyne phase across the specimen.