Interest in utilizing biorenewable feedstocks to produce fuels and chemicals has risen greatly in the past decade due to the economic, political and environmental concerns associated with diminishing petroleum reserves. A fundamental challenge lying ahead in the development of efficient processes to utilize biomass feedstock is that, unlike their petroleum counterparts, biomass contains an excess amount of oxygen. Therefore, catalytic strategies such as dehydration and hydrogenolysis amongst others have been extensively studied as platform technologies for deoxygenation. In this review, we primarily discuss the catalytic dehydration of C 6 carbohydrates to 5-hydroxymethylfurfural, which has attracted much attention due to the versatility of using furanic compounds as an important platform intermediate to synthesize various chemicals. The emphasis is on the fundamental mechanistic chemistry so as to provide insights for further catalyst/catalytic system design. After separately discussing fructose and glucose dehydration, this review summarizes recent progress with bi-functional catalyst systems for tandem glucose/fructose isomerization and subsequent fructose dehydration, thereby realizing highly selective HMF production directly from the more abundant and cheaper C 6 sugar feedstock, glucose.

Fast pyrolysis of lignocellulosic biomass, utilizing moderate temperatures ranging from 400 to 600 °C, produces a primary liquid product (pyrolytic bio-oil), which is potentially compatible with existing petroleum-based infrastructure and can be catalytically upgraded to fuels and chemicals. In this work, experiments were conducted with a micropyrolyzer coupled to a gas chromatography–mass spectrometry/flame ionization detector system to investigate fast pyrolysis of neat cellulose and other glucose-based carbohydrates. A detailed mechanistic model building on our previous work was developed for fast pyrolysis of neat glucose-based carbohydrates by integrating updated findings obtained through experiments and theoretical calculations. The model described the decomposition of cellulosic polymer chains, reactions of intermediates, and formation of a range of low molecular weight compounds at the mechanistic level and specified each elementary reaction step in terms of Arrhenius parameters. The mechanistic model for fast pyrolysis of neat cellulose included 342 reactions of 103 species, which included 96 reactions of 67 species comprising the mechanistic model of neat glucose decomposition.

As biomass pyrolysis is a promising technology for producing renewable fuels, mechanistic descriptions of biomass thermal decomposition are of increasing interest. While previous studies have demonstrated that glucose is a key primary intermediate and have elucidated many important elementary mechanisms in its pyrolysis, key questions remain. For example, there are several proposed mechanisms for evolution of an important product and platform chemical, 5-hydroxymethylfurfural (5-HMF), but evaluation with different methodologies has hindered comparison. We evaluated a host of elementary mechanisms using a consistent quantum mechanics (QM) level of theory and reveal a mechanistic understanding of this important pyrolysis pathway. We also describe a novel route as a target for catalyst design, as it holds the promise of a more selective pathway to 5-HMF from glucose. We further demonstrate the effect of conformational and structural isomerization on dehydration reactivity. Additionally, we combined QM and experimental studies to address the question of whether only the reactions of β-D-glucose, the cellulose monomer, are relevant to biomass pyrolysis, or if α-D-glucose needs to be considered in mechanistic models of glucose and cellulose pyrolysis. QM calculations show notable differences in elementary mechanisms between the anomers, especially in levoglucosan formation, which provide a means for evaluating experimental yields of α-D-glucose and β-D-glucose pyrolysis. The combined data indicate that both anomers are accessible under pyrolysis conditions. The kinetic and mechanistic discoveries in this work will aid catalyst design and mechanistic modeling to advance renewable fuels from nonfood biomass.

A computational framework based on continuous distribution kinetics was constructed to solve the mechanistic model that was developed for fast pyrolysis of glucose-based carbohydrates in the first part of this study [Zhou et al. Ind. Eng. Chem. Res. 2014, 53.

The primary reactions and secondary effects resulting from cellulose fast pyrolysis were investigated using a micropyrolyzer system by changing sample weight and length scale. To exclude the catalytic effects from metal ions, all cellulose samples were demineralized prior to pyrolysis. Heat transfer calculations estimated the characteristic time scale for heat transfer to be 1 order of magnitude smaller than the pyrolysis reaction time when the sample weight was less than 800 μg. It was found that mass transfer limitations existed when the sample weight of the powder cellulose was larger than 800 μg or when the cellulose particles were pyrolyzed at a larger characteristic length scale. The mass transfer limited system led to secondary reactions including secondary char and gas formation from volatile products and decomposition/dehydration of levoglucosan into low molecular weight products, furans, and dehydrated pyranose. The secondary reactions were found to be catalyzed by the char from cellulose pyrolysis. The pyrolysis of powder celluloses of differing crystallinity, degree of polymerization, and feedstock type were studied. Over 87 wt % mass balance closure was achieved for each type of cellulose. Similar product distributions were obtained for all of the different celluloses, implying that the primary products from cellulose were not influenced by these factors.

An oak bio-oil was aged at 90 °C using various times and methods. A novel method for aging bio-oils under shear is introduced and compared to standard (quiescent) aging experiments. In a hermetically sealed concentric cylinder rheometer, aging with shear for 8, 16, and 24 h showed increases in viscosity of 57, 300, and 720%, respectively. A similar increase in viscosity was observed after quiescently aging of sealed samples in a forced air oven (100, 120, and 740% after 8, 16, and 24 h, respectively). Another aging experiment under shear consisted of three 8 h aging steps with intermediate viscosity measurements. Viscosity increases were comparable to the 8, 16, and 24 h tests. A control experiment in the rheometer without shear found the increase in viscosity to be 30–50% less than the sheared experiments. The number-average molecular weight increased as samples were heat-treated at 90 °C for longer times. The water content showed small increases and decreases with aging, which was attributed to the heterogeneity of the sample. Real-time viscosity measurements during the 90 °C aging step found that the rate of viscosity growth decreased over time. An exponential decay function estimated the viscosity to be 90% of the steady-state viscosity after ∼3 days at 90 °C.

Vapors from corn stover pyrolysis were deoxygenated to aromatics and alkanes with low pressure hydrogen over MoO3.

Bio-oil is a renewable energy source that is produced from the pyrolysis of lignocellulosic biomass. The pyrolysis oils are emulsion-like fluids, containing aqueous and phenolic phases, and can be more than 400 times more viscous than water at 25 °C. A series of rheological tests were performed on a set of bio-oils from different feedstocks and pyrolysis conditions. In general, the viscosity of the oils was independent of the shear rate (i.e., Newtonian). However, some of the hardwood samples shear thin at lower temperatures (−5 °C) and high shears (>100 s−1). Oscillatory frequency sweeps were also performed. All of the oil samples were found to be viscous liquids, and the loss modulus (G′′) was orders of magnitude greater than the storage modulus (G′). A strong dependence of viscosity upon the temperature showed that the viscosity of poplar and oak 500 °C oils increased over 220-fold between 55 and −5 °C. Water content and acidity were also measured and compared to viscosity. The water content was found to have a stronger effect on viscosity than acidity. Generally, the oils that had higher water contents had lower viscosities. Viscosity does not correlate with the acid number or pH. While the acid number and pH are independent measurements of the acidity of the bio-oils, no correlation between the acid number and pH was observed. The microstructure of the oils was investigated using optical microscopy and small-angle neutron scattering. Optical microscopy did not show discrete boundaries between the aqueous and organic phases. The neutron-scattering profiles showed that a fractal structure is present in two of the three oils studied.