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Reducing chain ends in various hydrolyzed cellulose samples were quantitatively analyzed by solid-state 13C NMR. Cellulose di- and tetramers served as model systems to demonstrate that distinct signals at 97 ppm and 92.7 ppm can be detected quantitatively and assigned unambiguously to reducing chain ends in β and α anomers, respectively. In standard Avicel microcrystalline cellulose, the same signals were detected at the expected low intensity; their strength increased significantly upon further hydrolysis. The assignment of the 97 and 92.7 ppm signals to reducing chain ends was confirmed by their absence after ethanolysis, which instead produced an α-ethyl-ether end group signal at 99 ppm. Due to moderate-amplitude chain-end mobility, chain-end signals were relatively enhanced in direct-polarization 13C NMR with heteronuclear Overhauser enhancement. In 13C-enriched hydrolyzed cellulose, the assignment of the C–OH chain-end signals was further corroborated by hydroxyl-proton selection and two-dimensional 13C-13C NMR. From the fractional chain-end signal intensity, the number-average degree of polymerization (DPn) was determined. For Avicel, this yielded DPn = 43 (+ 50, -6), consistent with gel-permeation chromatography but significantly lower than deduced from a more indirect optical method likely hampered by limited chain-end accessibility. After 60 min of hydrolysis of ball-milled Avicel or cellulose from maize, highly reliable values of DPn = 18 ± 3 and 15 ± 3, respectively, were obtained. Solid-state NMR completely avoids the potential loss of low-molar-mass chains in solution-based approaches. The accurate, solvent-free solid-state NMR method introduced here can serve as a primary standard to calibrate other methods for molar-mass determination in hydrolyzed cellulose. [ABSTRACT FROM AUTHOR]

A systematic and structure-agnostic method for identifying heterogeneous activity of solid acids for catalyzing cellulose hydrolysis is presented. The basis of the method is preparation of a supernatant liquid by exposing the solid acid to reaction conditions and subsequent use of the supernatant liquid as a cellulose hydrolysis catalyst to determine the effects of in situ generated homogeneous acid species. The method was applied to representative solid acid catalysts, including polymer-based, carbonaceous, inorganic, and bifunctional materials. In all cases, supernatant liquids produced from these catalysts exhibited catalytic activity for cellulose hydrolysis. Direct comparison of the activity of the solid acid catalysts and their supernatants could not provide unambiguous detection of heterogeneous catalysis. A reaction pathway kinetic model was used to evaluate potential false-negative interpretation of the supernatant liquid test and to differentiate heterogeneous from homogeneous effects on cellulose hydrolysis. Lastly, differences in the supernatant liquids obtained in the presence and absence of cellulose were evaluated to understand possibility of false-positive interpretation, using structural evidence from the used catalysts to gain a fresh understanding of reactant--catalyst interactions. While many solid acid catalysts have been proposed for cellulose hydrolysis, to our knowledge, this is the first effort to attempt to differentiate the effects of heterogeneous and homogeneous activities. The resulting supernatant liquid method should be used in all future attempts to design and develop solid acids for cellulose hydrolysis. [ABSTRACT FROM AUTHOR]

Efficient cellulose depolymerization is a major bottleneck for economical production of second‐generation biofuels. In this work, crystalline cellulose was subjected to sequential ball milling and ethanolysis as a mild and selective depolymerization approach. Ball milling and ethanolysis resulted in 38±1 % cellulose conversion, with 24 % ethyl‐glucopyranoside as the main identified and quantified product and negligible side reaction of the ethanol solvent to form diethyl ether. In comparison, ethanolysis of the original cellulose resulted in only 3±1 % conversion. Additional soluble products from cellulose ethanolysis included carbohydrate isomers and oligomers, differing from the products obtained from hydrolysis. X‐ray diffraction and nuclear magnetic resonance spectroscopy revealed increased crystallinity post‐reaction, retarding further depolymerization. Hot liquid water extracted soluble oligomers from the ethanolyzed cellulose, suggesting formation of a nanoscale barrier of crystalline cellulose that traps soluble products during ethanolysis. Use of cellulose‐swelling co‐solvents and repeated mechanical decrystallization both proved effective at increasing cellulose conversion and soluble product yields. Repeated ball milling and ethanolysis resulted in 62±1 % cellulose conversion. Ethanolysis of decrystallized cellulose has potential for rapid (<2 h) de‐polymerization at mild conditions. [ABSTRACT FROM AUTHOR]

Mechanical decrystallization and water-promoted recrystallization of cellulose were studied to understand the effects of cellulose crystallinity on reaction engineering models of its acid-catalyzed hydrolysis. Microcrystalline cellulose was ball-milled for different periods of time, which decreased its crystallinity and increased the glucose yield obtained from acid hydrolysis treatment. Crystallinity increased after acid hydrolysis treatment, which has previously been explained in terms of rapid hydrolysis of amorphous cellulose, despite conflicting evidence of solvent promoted recrystallization. To elucidate the mechanism, decrystallized samples were subjected to various non-hydrolyzing treatments involving water exposure. Interestingly, all non-hydrolyzing hydrothermal treatments resulted in recovery of crystallinity, including a treatment consisting of heat-up and quenching that was selected as a way to estimate the crystallinity at the onset of hydrolysis. Therefore, the proposed mechanism involving rapid hydrolysis of amorphous cellulose must be incomplete, since the recrystallization rate of amorphous cellulose is greater than the hydrolysis rate. Several techniques (solid-state nuclear magnetic resonance, X-ray diffraction, and Raman spectroscopy) were used to establish that water contact promotes conversion of amorphous cellulose to a mixture of crystalline cellulose I and cellulose II. Crystallite size may also be reduced by the decrystallization-recrystallization treatment. Ethanolysis was used to confirm that the reactivity of the cellulose I/cellulose II mixture is distinct from that of truly amorphous cellulose. These results strongly point to a revised, more realistic model of hydrolysis of mechanically decrystallized cellulose, involving recrystallization and hydrolysis of the cellulose I/cellulose II mixture. [ABSTRACT FROM AUTHOR]

• Product yields of biomass constituents were determined in high temperature pyrolysis. • Lignin samples showed greater soot yields than during fast pyrolysis of holocelluloses. • K content and temperature have the dominating influence on char, and soot yields. • Simple empirical models can successfully capture observed char and gas yields. The effects of lignocellulosic biomass composition on product yields and distributions were studied under high-temperature pyrolysis conditions (800–1250 ° C) in a drop tube reactor. Several types of biomass were studied along with xylan, cellulose, and two types of lignin as model feeds. Among the model feeds, soot yields obtained from lignin pyrolysis were greater than those obtained from cellulose or xylan. Cellulose pyrolysis produced mostly gaseous products, along with small amounts of tars. Impregnation of lignin with alkali metals greatly reduced tar and soot formation, simultaneously increasing the hydrogen content of the syngas product. An empirical model predicted with reasonable accuracy trends in the product yields obtained from pyrolysis of whole biomass samples using as input data obtained from model feeds composition data and the pyrolysis temperature. Reaction temperature and ash content both have a strong influences on char yield, whereas gas yields were mostly affected by the reaction temperature. [ABSTRACT FROM AUTHOR]