One commonly cited factor that contributes to the recalcitrance of biomass is cellulose crystallinity. The present study aims to establish the effect of several pretreatment technologies on cellulose crystallinity, crystalline allomorph distribution, and cellulose ultrastructure. The observed changes in the cellulose ultrastructure of poplar were also related to changes in enzymatic hydrolysis, a measure of biomass recalcitrance. Hot-water, organo-solv, lime, lime-oxidant, dilute acid, and dilute acid-oxidant pretreatments were compared in terms of changes in enzymatic sugar release and then changes in cellulose ultrastructure measured by 13C cross polarization magic angle spinning nuclear magnetic resonance and wide-angle X-ray diffraction. Pretreatment severity and relative chemical depolymerization/degradation were assessed through compositional analysis and high-performance anion-exchange chromatography with pulsed amperometric detection. Results showed minimal cellulose ultrastructural changes occurred due to lime and lime-oxidant pretreatments, which at short residence time displayed relatively high enzymatic glucose yield. Hot water pretreatment moderately changed cellulose crystallinity and crystalline allomorph distribution, yet produced the lowest enzymatic glucose yield. Dilute acid and dilute acid-oxidant pretreatments resulted in the largest increase in cellulose crystallinity, para-crystalline, and cellulose-Iβ allomorph content as well as the largest increase in cellulose microfibril or crystallite size. Perhaps related, compositional analysis and Klason lignin contents for samples that underwent dilute acid and dilute acid-oxidant pretreatments indicated the most significant polysaccharide depolymerization/degradation also ensued. Organo-solv pretreatment generated the highest glucose yield, which was accompanied by the most significant increase in cellulose microfibril or crystallite size and decrease in relatively lignin contents. Hot-water, dilute acid, dilute acid-oxidant, and organo-solv pretreatments all showed evidence of cellulose microfibril coalescence.

The structural changes occurring to hardwood Alcell™ lignin as a result of fiber devolatilization/extrusion, oxidative thermo-stabilization and carbonization are investigated in this study by solid-state and solution nuclear magnetic resonance (NMR) spectroscopy techniques. Solution based 1H–13C correlation NMR of the un-spun Alcell™ lignin powder and extruded lignin fiber detected modest changes occurring due to fiber devolatilization/extrusion in the type and proportion of aliphatic side-chain carbons or monolignol inter-unit linkages. Molecular weight analysis by gel permeation chromatography (GPC), along with an additional 31P NMR method used to indicate changes in terminal hydroxyl functionality, suggest fiber devolatilization/extrusion causes both chain scission and condensation reactions. 1H CRAMPS (combined rotation and multiple-pulse spectroscopy) and 13C cross-polarization/magic angle spinning (CP/MAS) spectra of extruded and stabilized lignin fibers indicate stabilization severely reduces the proportion of methoxy groups present, while also increasing the relative proportion of carbonyl and carboxyl-related structures, typically associated with cross-linking chemistries. 13C direct-polarization/magic angle spinning (DP/MAS) analysis of stabilized and carbonized fibers shows an increased relative amount of carbon–carbon bonds on aryl structures and a relative decrease of aryl ethers. DP/MAS dipolar dephasing experiments suggest that a majority of non-protonated carbons convert from carbonyl to aryl and condensed aryl structures during carbonization.

The pyrolysis of softwood (SW) kraft lignin in the presence of various H-ZSM-5 zeolites with different SiO2/Al2O3 mole ratios from 23/1–280/1 as additives were examined at 600 °C. Nuclear magnetic resonance (NMR), including quantitative 13C, 31P NMR, and heteronuclear single-quantum correlation (HSQC)-NMR, and gel permeation chromatography (GPC) were used to characterize various pyrolysis oils. On the basis of the results of the 13C and 31P NMR for pyrolysis oils, the use of H-ZSM-5 zeolites during the pyrolysis process caused the near complete decomposition of aliphatic hydroxyl and carboxyl groups. With the exception of carboxylic acid, the H-ZSM-5 zeolite with a relatively higher SiO2/Al2O3 mole ratio was more effective at the elimination of methoxyl groups, ether bonds, and aliphatic C–C bonds, and dehydration of aliphatic hydroxyl groups during pyrolysis. However, the H-ZSM-5 zeolite with a very large SiO2/Al2O3 mole ratio, such as 280, has only limited effects on the properties of upgraded pyrolysis oil. After the use of zeolite, the pyrolysis oils contain some polyaromatic hydrocarbons, the content of which decreased with an increasing SiO2/Al2O3 mole ratio of zeolite. GPC results show that the molecular weight decreased by 8–16% after the use of H-ZSM-5 zeolites.Keywords: H-ZSM-5 zeolite; Pyrolysis oil; Si/Al ratio; Upgrading

This study demonstrates the use of isotopic labelling and NMR to study the HDO process. As far as we know, this is the first reported effort to trace the incorporation of hydrogen in the HDO process of lignin pyrolysis oil thereby providing key fundamental insight into its reaction mechanism.

Wheat straw and corn stover are promising biorenewable resources that can be utilized in the pulp and paper industry. This study examines changes in the physical strength of pulps made from blends of agricultural residue and hardwoods. Wheat straw and corn stover were substituted for hardwood chips in the amounts of 10, 15, and 20 wt % in kraft pulping experiments while keeping the H-factor constant. This substitution allows a maximum 29% increase in tensile index and 12%increase in tear index for unrefined samples containing wheat straw. Pulp yields and kappa numbers changed slightly with increasing woodchip replacement levels. Three substituted pulps were bleached using a relatively mild OD(E + P + O)D sequence, and the viscosities and sugar profiles were traced throughout the process. The strength improvement can be attributed to the increased xylan contents of pulps made with agricultural residue. Fully bleached pulps preserved the improved mechanical properties, and an attempt was made to correlate the xylan content with the degree of strength development.

The average spatial dimensions between major biopolymers within the plant cell wall can be resolved using a solid-state NMR technique referred to as a 13C cross-polarization (CP) SELDOM (selectively by destruction of magnetization) with a mixing time delay for spin diffusion. Selective excitation of specific aromatic lignin carbons indicates that lignin is in close proximity to hemicellulose followed by amorphous and finally crystalline cellulose. 13C spin diffusion time constants (TSD) were extracted using a two-site spin diffusion theory developed for 13C nuclei under magic angle spinning (MAS) conditions. These time constants were then used to calculate an average lower-limit spin diffusion length between chemical groups within the plant cell wall. The results on untreated 13C enriched corn stover stem reveal that the lignin carbons are, on average, located at distances ∼0.7–2.0 nm from the carbons in hemicellulose and cellulose, whereas the pretreated material had larger separations.

One step thermal conversion of lignin to gasoline range liquid products was accomplished by pyrolyzing softwood (SW) kraft lignin with select zeolites at 600 °C. Gel Permeation Chromatography (GPC) and Nuclear Magnetic Resonance (NMR) including quantitative 13C, 31P-NMR and Heteronuclear Single-Quantum Correlation (HSQC)-NMR were used to characterize various pyrolysis oils. By employing a zeolite catalyst aliphatic hydroxyl groups decreased by 70–100% in the resulting bio-oil and the content of carboxylic acid groups also decreased by 44–85% in comparison to a control pyrolysis oil generated with no additive. Of the additives studied MFI and MOR zeolites provided the best in situ decarboxylation of bio-oils. The results of 13C-NMR indicated after the use of FAU and BEA zeolites, the pyrolysis oils contained ∼80% less methoxy groups than the native pyrolysis oil, and almost all the oxygen (up to ∼87%) belonged to phenolic hydroxyl groups. In addition, the average molecular weight of these two upgraded pyrolysis oils decreased by ∼60% with respect to the control pyrolysis oil and they had a molecular weight profile in the gasoline range (80–120 g mol−1). By adding MFI, FAU and BEA zeolites, the pyrolysis oils contained some polyaromatic hydrocarbons (PAH). In contrast, there were very limited amount of PAH in FER and MOR upgraded pyrolysis oils and almost no PAH in the native pyrolysis oil.

Kanlow switchgrass was subjected to aqueous ethanol organosolv pretreatment, and ethanol organosolv lignin (EOL) was recovered through precipitation of pretreatment effluents. Ball-milled switchgrass lignin was also isolated from unpretreated and pretreated switchgrass. The obtained switchgrass lignins were characterized using Fourier transform infrared (FTIR) spectroscopy, 31P nuclear magnetic resonance (NMR), two-dimensional (2D) heteronuclear single-quantum coherence (HSQC) NMR spectroscopy, and gel permeation chromatography (GPC). The results showed that the molecular weight of switchgrass EOL lignin decreased while polydispersity increased. The pretreatment resulted in an increase of free phenolic guaiacyl and syringyl units and a decrease of the β-O-4 linkage content.

Biomass is one of the most abundant potential sustainable sources for fuel and material production, however to fully realize this potential an improved understanding of lignocellulosic recalcitrance must be developed. In an effort to appreciate the underlying phenotypic, biochemical and morphological properties associated with the reduced recalcitrance observed in tension stress-induced reaction wood, we report the increased enzymatic sugar yield and corresponding chemical and ultrastructural properties of Populus tension wood. Populus tremula x alba (PTA) was grown under tension and stem segments containing three different wood types: normal wood (NW), tension wood (TW) from the elongated stem side and opposite wood (OW) from the compressed stem side were collected. A variety of analytical techniques were used to describe changes occurring as a result of the tension stress-induced formation of a gelatinous cell wall layer (G-layer). For example, gel permeation chromatography (GPC) and 13C solid-state nuclear magnetic resonance (NMR) revealed that the molecular weight and crystallinity of cellulose in TW is greater than that of cellulose acquired from NW. Whole cell ionic liquid and other solid-state NMR analysis detailed the structure of lignin and hemicellulose in the samples, detecting the presence of variations in lignin and hemicellulose sub-units, linkages and semi-quantitatively estimating the relative amounts of syringyl (S), guaiacyl (G) and p-hydroxybenzoate (PB) monolignol units. It was confirmed that TW displayed an increase in PB or H-like lignin and S to G ratio from 1.25 to 1.50 when compared to the NW sample. Scanning electron microscopy (SEM) and coherent anti-Stokes Raman scattering (CARS) were also used to evaluate the morphology and corresponding spatial distribution of the major lignocellulosic components. We found changes in a combination of cell wall properties appear to influence recalcitrance more than any single factor alone.

A common goal in present and future forestry, biofuels and biomaterials practices, is the need to valorize lignocellulose processes to maximize value and optimize autonomic economy. Consequently, a key focus of modern biorefining is the on-site utilization of all residual materials generating products of the highest possible value. The LignoBoost process, recently demonstrated on the pilot-scale at Kraft pulp mills, injects CO2 into pulping liquors which results in a lower solution pH and thereby precipitates lignin. The present paper compares and evaluates the pyrolysis of pulping liquor lignins precipitated by sulfuric acid (pH 3) and the aforementioned CO2 method (pH 10.5 and 9.5). The CO2 based process yielded lignin that showed superior pyrolysis properties including low gas formation and increased bio-oil yields, close to 40%, consisting primarily of low (∼150 g mol−1) molecular weight compounds. Subsequent NMR analysis showed that the oils exhibit favorable changes in functionalities, e.g. loss of aromatic and gain in aliphatic carbon percentages as well as decrease in carboxyl and methoxyl (oxygen containing) groups. Moreover, NMR results further confirmed previously hypothesized lignin pyrolysis reactions, while at the same time showed the potential of CO2 precipitated lignin for pyrolysis and subsequent liquid biofuel production.

The pyrolysis of softwood (SW) kraft lignin was examined at 400, 500, 600, and 700 °C. The yields of pyrolysis oil, char, and gas were determined to be 35−44%, 57−38% and 8−18%, respectively. The pyrolysis oil has a comparable heating value with ethanol and coal. The elevated temperature of 700 °C was found as the point of primary decomposition of lignin and the secondary decomposition of pyrolysis oil. Gel permeation chromatography (GPC) and quantitative 13C and 31P NMR were used to characterize the pyrolysis oil. A 13C NMR database was created to provide a more accurate chemical shift assignment database for analysis of pyrolysis oils. On the basis of the results of 13C and 31P NMR for the pyrolysis oil, aliphatic hydroxyl, carboxyl, and methoxyl groups are eliminated during pyrolysis. Cleavage of ether bonds in lignin was also shown to be a primary decomposition reaction occurring during thermal treatment. The results of GPC analysis indicated that lower pyrolysis temperatures yielded a bio-oil that had a lower molecular weight and lower polydispersity value.

Switchgrass is currently being developed as a sustainable bio-energy crop due to its broad adaptability, high mass yield and low agricultural input. Its current conversion to biofuels is detrimentally impacted by its native recalcitrance which is typically addressed using chemical and/or biological pretreatments. In this study, extractives free switchgrass was pretreated with steam, dilute H2SO4 and lime at 160 °C for 1 h. The degradation and impact of pretreatment was estimated semi-quantitatively by 13C–1H HSQC (heteronuclear single quantum coherence) NMR analysis of ball milled untreated and pretreated switchgrass samples in perdeuterated pyridinium chloride–DMSO-d6 solvent system. As a result of steam pretreatment the resulting switchgrass was depleted of xylan and a slight degradation of lignin were observed. This was confirmed by the relative decrease of cross peak intensity for β-aryl ether, phenylcoumaran, resinol and dibenzodioxocin units. Significant structural changes observed due to the lime pretreatment of switchgrass was deacetylation/dissolution of hemicellulose and the extent of delignification was less however, a preferential removal p-hydroxy of benzoyl ester, ferulate and coumarate type linkages were notified from the HSQC studies. Finally the most significant degradation resulted in acid pretreatment involving ∼90% loss of hemicellulose and a substantial degradation of various lignin sub-units. These results are further supported by the composition analysis of the respective switchgrass samples.

The pyrolysis of softwood kraft lignin, cellulose, and Loblolly pine wood was examined at 400, 500, and 600 °C. The analysis of the yields of pyrolysis products indicated that lignin yielded the largest amount of a heavy oil and char and only trace levels of a light oil. In contrast, cellulose produced minor amounts of a heavy oil and char and more light oil. All of the pyrolysis oils were analyzed by heteronuclear single-quantum correlation–nuclear magnetic resonance (HSQC–NMR) to analyze the structural components of the bio-oils, and three chemical-shift databases of compounds reported to be presented in pyrolysis oils produced from lignin, cellulose, and pine wood were employed for data analysis. On the basis of databases, analysis of the HSQC–NMR spectral data provides chemical-shift assignment of 27 different types of C–H bonds presented in the pyrolysis oils. The HSQC–NMR analysis of these pyrolysis oils indicated that there are two different types of methoxyl groups presented in the pyrolysis oils produced from lignin and pine wood, which indicated that the native methoxyl group in the lignin rearranges during the thermal treatment. The content of aromatic C–H and aliphatic C–H bonds in the pyrolysis oils produced from lignin and pine wood was increased with increasing pyrolysis temperatures. Levoglucosan was shown to be one of the major components in the pyrolysis oils produced from cellulose and pine wood, and furfurals and phenols were also found as the major components in the cellulose pyrolysis oils. Most aromatic C–H and aliphatic C–H bonds in the pine wood pyrolysis oils were produced from the lignin component. The results demonstrate the capability of HSQC–NMR to provide in-depth analysis of pyrolysis oils.

The pyrolysis of softwood (SW) kraft lignin in the presence of NiCl2 and ZSM-5 zeolite as an additive was examined at 700 °C. Gel permeation chromatography (GPC), quantitative 13C and 31P NMR were used to characterize the pyrolysis oil. Based on the results of 13C and 31P NMR for the pyrolysis oils, the use of zeolite during pyrolysis caused the near complete loss of aliphatic hydroxyl and carboxyl groups in the bio-oil and about 80% of methoxyl groups were also eliminated. The zeolite was shown to improve the decomposition of aliphatic hydroxyl groups, carboxyl, methoxyl groups, and ether bonds in the lignin during pyrolysis. In addition, as determined by 13C NMR, the oxygen content in the bio-oil decreased after the use of zeolite. The results of GPC analysis indicated that the addition of H-ZSM-5 zeolite with lignin provided a bio-oil that had ∼10% lower average molecular weight than the pyrolysis product acquired without the additive.

Perdeuterated pyridinium chloride–DMSO-d6 is an effective solvent system for whole cell biomass dissolution and NMR characterization. Employing this solvent system, semi-quantitative 13C–1H heteronuclear single quantum correlation (HSQC) spectroscopy of untreated, steam, dilute acid and lime pretreated poplar biomass samples was readily accomplished. In an effort to demonstrate the efficacy and usefulness of this fairly new characterization technique, relative spectral intensities of the untreated and pretreated biomass samples were evaluated and compared. From the relative signal intensities of hemicelluloses in each system it was observed that hemicelluloses are being removed in various pretreatment conditions, but complete dissolution of hemicellulose was observed only with acid pretreatment. The relative changes in lignin subunits after pretreatment were estimated from the volume integration of resolved cross peaks of various lignin subunits. The degradation of lignin was observed in all pretreatments, though more significant changes were noticed after dilute acid and lime pretreatment. HSQC analysis results were in agreement with the composition analysis of pretreated biomass samples. Thus, this methodology broadens the application of whole cell NMR analysis in biofuel research.Highlights► Poplar was subjected to steam, lime and dilute acid pretreatments. ► Perdeuterated pyridiniumchloride-DMSO-d6 used for biomass dissolution followed by HSQC NMR analysis. ► HSQC analysis estimated steam pretreatment significant lignin and hemicellulose degradation occurred. ► Lime pretreatment resulted mainly hemicelluloses dissolution and selective PB lignin degradation. ► Dilute sulfuric acid pretreatment completely removed the hemicelluloses and harsh lignin degradation.

Converting pulp mills into forest biorefineries to produce biopower and biomaterials can decrease their environmental impact and increase feasibility at the same time. One of the key challenges to reach this goal is the recovery of lignin from process streams for subsequent utilization in a variety of innovative green processes. This study examines the fundamental chemical structure of lignin recovered from Kraft pulping streams by an acid precipitation/washing methodology. Functional group analysis and molecular weight profiles were determined by NMR and SEC with promising results for future conversions; such as low hydroxyl (oxygen) contents and low molecular weights (∼3000 g mol−1).

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze the molecular constituents on cross sections of juvenile poplar (Populus deltoids) stems before and after dilute acid pretreatment (DAP). Bulk analysis of milled and 50 μm thick cross sections of poplar before and after DAP was shown to be chemically equivalent by FT-IR and carbohydrate analysis. ToF-SIMS analysis of dilute acid pretreated material indicated significant changes in relative contents of cellulose, xylan, and lignin occurred upon pretreatment. The relative content of xylan after DAP increased by 30% on the surface of the poplar stem by ToF-SIMS, while bulk carbohydrate analysis showed that the relative concentration of xylose decreased 10-fold in comparison with untreated poplar wood. The relative content of cellulose and G-lignin units doubled on the surface of the poplar stem sections, while the bulk glucose concentration and Klason lignin increased 40% and 5%, respectively, as determined by bulk carbohydrate and Klason lignin analysis. The spatial distributions of the major lignocellulosic components on the surface of juvenile poplar stem before and after DAP were examined by SIMS and this data was processed into mapping images. Scanning electron microscopy (SEM) was used to evaluate the morphological changes of the cell wall layers before and after DAP, which was also correlated with the results of ToF-SIMS analysis.

Dilute acid pretreatment (DAP) is a frequently employed technique in biofuel production to increase overall sugar and subsequent ethanol yields from downstream fermentation. This is done prior to enzymatic deconstruction of cellulose to increase accessible surface area as well as to remove or redistribute hemicellulose and lignin, which have an inhibitory effect on enzymatic hydrolysis. In this study, the effect of DAP on the supramolecular and ultrastructure of lignocellulosic biomass was evaluated by both 1H and 2H NMR techniques. A series of DAPs were conducted on Populus using ∼0.10 M H2SO4 at ∼160 °C for varying residence times. 2H spin−lattice (T1) times of deuterium oxide (D2O) adsorbed within the lignocellulosic biomass were measured on untreated and pretreated Populus, and an inverse Laplace transform of the T1 decays was then used to generate pore size distributions. The resulting distributions indicate that substantial pore expansion within the cellulose fibril bundles occurs during pretreatment. 1H Goldman−Shen (GS) spin−diffusion experiments which were also conducted, qualitatively supporting the magnitude of observed pore expansion obtained from 2H NMR relaxation profiles. 1H Carr−Purcell−Meiboom−Gill (CPMG) and pulse field gradient (PFG) experiments were used to investigate the altering supramolecular structure of the lignocellulose and the self-diffusive behavior of water adsorbed within the biomass as a function of DAP. Spin−spin (T2) relaxation times indicate the nature of cellulose−water interactions change during DAP. Inverse Laplace distributions of the resulting T2 decays demonstrate not only a shift in T2 times to longer relaxation or a more mobile state but also indicate that the population of water with longer relaxation times increase, indicating that pretreatment begins to break down and loosen the cellulosic ultrastructure within the biomass. Lastly, the water self-diffusion experiments demonstrates that DAP increases pore tortuosity within the biomass.

A key property involved in plant recalcitrance is cellulose crystallinity. In an attempt to establish the typical diversity in cellulose ultrastructure for poplar, the variation and distribution of supramolecular and ultrastructural features, including the fraction of crystalline cellulose forms \( {\text{I}}_{\alpha } \) and \( {\text{I}}_{\beta } \), para-crystalline cellulose and amorphous cellulose content were characterized. In this study, the percent crystallinity (%Cr) and lateral fibril dimensions of cellulose isolated from poplar were determined for 18 poplar core samples collected in the northwestern region of the USA.

The main step during biodiesel production is the catalytic transesterification of triglycerides. Glycerol and fatty acids are by-products of the biodiesel production and considered as contaminants in the final product. By selectively measuring the amount of fatty acids and glycerol with different substitution levels, the yield of this step and the quality of the final biodiesel can be determined.This study examines the application of phosphitylation of glycerol hydroxyl groups with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane followed by 31P NMR analysis to provide a rapid quantitative analytical technique for the determination of substitution patterns on partially esterified glycerols, alcohols and the detection of fatty acids. 31P NMR chemical shift data was established with a series mono and di-substituted esters of glycerol, fatty acids and alcohols, then utilized to characterize commercial glycerol and biodiesel samples.

Dilute acid pretreatment (DAP) is commonly employed prior to enzymatic deconstruction of cellulose to increase overall sugar and subsequent ethanol yields from downstream bioconversion processes. Typically optimization of pretreatment is evaluated by determining hemicellulose removal, subsequent reactivity towards enzymatic deconstruction, and recoverable polysaccharide yields. In this study, the affect of DAP on the supramolecular and ultrastructure of lignocellulosic biomass was evaluated. A series of dilute acidic pretreatments, employing ∼0.10–0.20 mol/m3 H2SO4 at ∼160–180 °C, for varying residence times were conducted on both Populus and switchgrass samples. The untreated and pretreated biomass samples were characterized by carbohydrate and lignin analysis, gel permeation chromatography (GPC) and 13C cross polarization magic angle spinning (CPMAS) NMR spectroscopy. GPC analysis shows a reduction in the molecular weight of cellulose and change in its polydispersity index (PDI) with increasing residence time, indicating that pretreatment is actually degrading the cellulose chains. 13C CPMAS and non-linear line-fitting of the C4 region in the carbon spectrum of the isolated cellulose not only showed that the crystallinity index increases with residence time, but that the lateral fibril dimension (LFD) and lateral fibril aggregate dimension (LFAD) increase as well.

This study demonstrates the use of isotopic labelling and NMR to study the HDO process. As far as we know, this is the first reported effort to trace the incorporation of hydrogen in the HDO process of lignin pyrolysis oil thereby providing key fundamental insight into its reaction mechanism.