Furan Production from Glycoaldehyde over HZSM-5

Catalytic fast pyrolysis of biomass over zeolite catalysts results primarily in aromatic (e.g., benzene, toluene, xylene) and olefin products. However, furans are a higher value intermediate for their ability to be readily transformed into gasoline, diesel, and chemicals. Here we investigate possible mechanisms for the coupling of glycoaldehyde, a common product of cellulose pyrolysis, over HZSM-5 for the formation of furans. Experimental measurements of neat glycoaldehyde over a fixed bed of HZSM-5 confirm furans (e.g., furanone) are products of this reaction at temperatures below 300 °C with several aldol condensation products as coproducts (e.g., benzoquinone). However, under typical catalytic fast pyrolysis conditions (>400 °C), further reactions occur that lead to the usual aromatic product slate. ONIOM calculations were utilized to identify the pathway for glycoaldehyde coupling toward furanone and hydroxyfuranone products with dehydration reactions serving as the rate-determining steps with typical intrinsic reaction barriers of 40 kcal mol-1. The reaction mechanisms for glycoaldehyde will likely be similar to that of other small oxygenates such as acetaldehyde, lactaldehyde, and hydroxyacetone. This study provides a generalizable mechanism of oxygenate coupling and furan formation over zeolite catalysts

A Mechanistic Investigation of Acid-Catalyzed Cleavage of Aryl-Ether Linkages: Implications for Lignin Depolymerization in Acidic Environments

Acid catalysis has long been used to depolymerize plant cell wall polysaccharides, and the mechanisms by which acid affects carbohydrates have been extensively studied. Lignin depolymerization, however, is not as well understood, primarily due to the heterogeneity and reactivity of lignin. We present an experimental and theoretical study of acid-catalyzed cleavage of two non-phenolic and two phenolic dimers that exhibit the β-O-4 ether linkage, the most common intermonomer bond in lignin. This work demonstrates that the rate of acid-catalyzed β-O-4 cleavage in dimers exhibiting a phenolic hydroxyl group is 2 orders of magnitude faster than in non-phenolic dimers. The experiments suggest that the major product distribution is similar for all model compounds, but a stable phenyl-dihydrobenzofuran species is observed in the acidolysis of two of the γ-carbinol containing model compounds. The presence of a methoxy substituent, commonly found in native lignin, prevents the formation of this intermediate. Reaction pathways were examined with quantum mechanical calculations, which aid in explaining the substantial differences in reactivity. Moreover, we use a radical scavenger to show that the commonly proposed homolytic cleavage pathway of phenolic β-O-4 linkages is unlikely in acidolysis conditions. Overall, this study explains the disparity between rates of β-O-4 cleavage seen in model compound experiments and acid pretreatment of biomass, and implies that depolymerization of lignin during acid-catalyzed pretreatment or fractionation will proceed via a heterolytic, unzipping mechanism wherein β-O-4 linkages are cleaved from the phenolic ends of branched, polymer chains inward toward the core of the polymer. © 2013 American Chemical Society

A Mechanistic Investigation of Acid-Catalyzed Cleavage of Aryl-Ether Linkages: Implications for Lignin Depolymerization in Acidic Environments

Acid catalysis has long been used to depolymerize plant cell wall polysaccharides, and the mechanisms by which acid affects carbohydrates have been extensively studied. Lignin depolymerization, however, is not as well understood, primarily due to the heterogeneity and reactivity of lignin. We present an experimental and theoretical study of acid-catalyzed cleavage of two non-phenolic and two phenolic dimers that exhibit the β-O-4 ether linkage, the most common intermonomer bond in lignin. This work demonstrates that the rate of acid-catalyzed β-O-4 cleavage in dimers exhibiting a phenolic hydroxyl group is 2 orders of magnitude faster than in non-phenolic dimers. The experiments suggest that the major product distribution is similar for all model compounds, but a stable phenyl-dihydrobenzofuran species is observed in the acidolysis of two of the γ-carbinol containing model compounds. The presence of a methoxy substituent, commonly found in native lignin, prevents the formation of this intermediate. Reaction pathways were examined with quantum mechanical calculations, which aid in explaining the substantial differences in reactivity. Moreover, we use a radical scavenger to show that the commonly proposed homolytic cleavage pathway of phenolic β-O-4 linkages is unlikely in acidolysis conditions. Overall, this study explains the disparity between rates of β-O-4 cleavage seen in model compound experiments and acid pretreatment of biomass, and implies that depolymerization of lignin during acid-catalyzed pretreatment or fractionation will proceed via a heterolytic, unzipping mechanism wherein β-O-4 linkages are cleaved from the phenolic ends of branched, polymer chains inward toward the core of the polymer. © 2013 American Chemical Society

A Device-Independent Evaluation of Carbonyl Emissions from Heated Electronic Cigarette Solvents

To investigate how the two main electronic (e-) cigarette solvents-propylene glycol (PG) and glycerol (GL)-modulate the formation of toxic volatile carbonyl compounds under precisely controlled temperatures in the absence of nicotine and flavor additives.PG, GL, PG:GL = 1:1 (wt/wt) mixture, and two commercial e-cigarette liquids were vaporized in a stainless steel, tubular reactor in flowing air ranging up to 318°C to simulate e-cigarette vaping. Aerosols were collected and analyzed to quantify the amount of volatile carbonyls produced with each of the five e-liquids.Significant amounts of formaldehyde and acetaldehyde were detected at reactor temperatures ≥215°C for both PG and GL. Acrolein was observed only in e-liquids containing GL when reactor temperatures exceeded 270°C. At 318°C, 2.03±0.80 μg of formaldehyde, 2.35±0.87 μg of acetaldehyde, and a trace amount of acetone were generated per milligram of PG; at the same temperature, 21.1±3.80 μg of formaldehyde, 2.40±0.99 μg of acetaldehyde, and 0.80±0.50 μg of acrolein were detected per milligram of GL.We developed a device-independent test method to investigate carbonyl emissions from different e-cigarette liquids under precisely controlled temperatures. PG and GL were identified to be the main sources of toxic carbonyl compounds from e-cigarette use. GL produced much more formaldehyde than PG. Besides formaldehyde and acetaldehyde, measurable amounts of acrolein were also detected at ≥270°C but only when GL was present in the e-liquid. At 215°C, the estimated daily exposure to formaldehyde from e-cigarettes, exceeded United States Environmental Protection Agency (USEPA) and California Office of Environmental Health Hazard Assessment (OEHHA) acceptable limits, which emphasized the need to further examine the potential cancer and non-cancer health risks associated with e-cigarette use

Photocatalytic degradation of chlorinated propenes using TiO2

The photocatalytic degradation of chlorinated propenes using TiO2 was investigated by FTIR spectroscopy. The chlorinated propenes were degraded to HCl, CO2, CO, H2O, and HCOOH during UV irradiation. During the degradation of 3-chloro-1-propene, the concentrations of CO2, CO, and HCOOH increased just after starting the irradiation. The onset of the HCl formation was delayed. On the other hand, the onset of the HCOOH formation was delayed during the degradation of 1-chloro-1-propene. During the degradation of 2-chloro-1-propene, the rate of the HCOOH production was slower than that during the degradation of 3-chloro-1-propene although the HCl production was not delayed. These results indicated that HCOOH was produced by the degradation of the double-bonded carbon bonding to two H atoms during the initial stage. The chlorinated compounds were preferentially produced from the double-bonded carbon bonding to the Cl atom and rapidly degraded to HCl, CO2, and CO during the initial stage. The residual part was degraded in the latter steps. Furthermore, it is suggested that the Cl atom on one of the double-bonded C atoms of the propenes was transferred to the other C atom before the degradation. Consequently, the double-bonded carbon bonding to two H atoms in 2-chloro-1-propene was chlorinated, then degraded to HCl, CO2, and CO during the initial stage.ArticleRESEARCH ON CHEMICAL INTERMEDIATES. 41(10):7641-7654 (2015)journal articl