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Robichaud, David J.

Publications and source records attributed to Robichaud, David J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Prediction of Hydroxymethylfurfural Yield in Glucose Conversion through Investigation of Lewis Acid and Organic Solvent Effects

Hydroxymethylfurfural (HMF) is one of the important renewable platform compounds that can be obtained from biomass feedstocks through glucose conversion catalyzed by Brønsted and Lewis acids. However, it is challenging to enhance the HMF yield due to side reactions. In this study, a systematic approach combining theory and experiment was performed to investigate the influence of Lewis acids and organic solvents on the HMF yield. For the Lewis acid effect, a relationship between chemical hardness and experimental HMF yields was found in the rate-limiting step of glucose-to-fructose isomerization for six metal chlorides; HMF production was promoted when the metal chloride and a substrate had a similar chemical hardness. To study the organic solvent effect, a multivariate model was developed based on the insights gained from the mechanistic study of fructose dehydration, to predict HMF yields in a given water-organic cosolvent system. It showed a reliable accuracy in evaluating HMF yields with a mean absolute error (MAE) of 3.0% with respect to experimental HMF yields for 13 solvents, and also predicted HMF yields with a MAE of 10.7% for four new solvents. Chemical interpretation of the model revealed that it is desirable to use a solvent capable of stabilizing the carbocation intermediates with low proton transfer activity and high hydrogen bond basicity, to maximize the HMF yield. This multivariate model informs experimentalists about rational selection of solvents with very low computational costs needed to calculate only six variables for each solvent. It can be expanded to other catalytic systems such as heterogeneous Brønsted–Lewis bifunctional catalysts and enables optimization of reaction conditions to obtain other useful platform molecules through biomass conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Elucidating the Chemical Pathways Responsible for the Sooting Tendency of 1 and 2-phenylethanol

Yield Sooting Index (YSI) measurements have shown that oxygenated aromatic compounds (OACs) tend to have lower YSI than aromatic hydrocarbon (AHC) compounds. For example, typical AHCs such as toluene and ethyl benzene have a YSI of 170 and 216, respectively, in contrast, OACs such as phenol and anisole have a YSI of 81 and 111, respectively. However, this trend is not always true as was observed for the structural isomers 1-phenylethanol (1PE, YSI=142) and 2-phenylethanol (2PE, YSI=207), where 2PE contains a YSI more representative of AHCs than OACs. We applied flow reactor experiments and density functional theory (DFT) calculations to examine how oxygen functionality present in 1PE and 2PE alters the reaction pathways leading to the observed difference in soot formation. It was determined that the proximity of the oxygen functional group to the aromatic ring determines whether the oxygen remains attached to the primary reacting species (for 1PE) or is eliminated early in the combustion sequence (for 2PE). For these alcohols, preservation of the oxygen in the molecule leads to further OACs, while loss of the oxygen leads to AHCs and benzyl radical. The direct pathways to AHCs and benzyl radical result in the higher YSI observed for 2PE.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Theoretical Determination of Size Effects in Zeolite-Catalyzed Alcohol Dehydration

In the upgrading of biomass pyrolysis vapors to hydrocarbons, dehydration accomplishes a primary objective of removing oxygen, and acidic zeolites represent promising catalysts for the dehydration reaction. Here, we utilized density functional theory calculations to estimate adsorption energetics and intrinsic kinetics of alcohol dehydration over H-ZSM-5, H-BEA, and H-AEL zeolites. The ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) calculations of adsorption energies were observed to be inconsistent when benchmarked against QM (Quantum Mechanical)/Hartree–Fock and periodic boundary condition calculations. However, reaction coordinate calculations of adsorbed species and transition states were consistent across all levels considered. Comparison of ethanol, isopropanol (IPA), and tert-amyl alcohol (TAA) over these three zeolites allowed for a detailed examination of how confinement impacts on reaction mechanisms and kinetics. The TAA, seen to proceed via a carbocationic mechanism, was found to have the lowest activation barrier, followed by IPA and then ethanol, both of which dehydrate via a concerted mechanism. Barriers in H-BEA were consistently found to be lower than in H-ZSM-5 and H-AEL, attributed to late transition states and either elevated strain or inaccurately estimating long-range electrostatic interactions in H-AEL, respectively. Molecular dynamics simulations revealed that the diffusivity of these three alcohols in H-ZSM-5 were significantly overestimated by Knudsen diffusion, which will complicate experimental efforts to develop a kinetic model for catalytic fast pyrolysis.

09 BIOMASS FUELS↗
DOE OSTI2019

110th Anniversary: Microkinetic Modeling of the Vapor Phase Upgrading of Biomass-Derived Oxygenates

Bio-oil produced from fast pyrolysis of biomass is a complex mixture of more than 200 compounds, including oxygenates and acids. As these species are highly undesirable in fuels, catalytic upgrading of biomass pyrolysis product vapors, also known as catalytic fast pyrolysis, is performed to upgrade the vapors to valuable fuels and chemicals. This work presents a detailed microkinetic model, composed of elementary steps, of the catalytic upgrading of acetic acid and acetone, two common oxygenates present in bio-oil. An automated network generator was utilized to construct a reaction network composed of 580 unique species and 2160 unique reactions. The kinetic parameters for each reaction in the network were estimated using transition state theory, the Evans-Polanyi relationship, and thermodynamic data. The resulting mechanistic model is able to describe experimental data presented in the literature for the transformation of acetic acid and acetone on HZSM-5 in a fixed-bed reactor, which is modeled as a plug-flow reactor. Additionally, the model solutions reveal vital information regarding the mechanism by which acetic acid and acetone are upgraded to valuable fuels and chemicals. In the first phase of the mechanism, acetic acid is converted to acetone via acylium ion addition to acetic acid; this is followed by decarboxylation of acetoacetic acid. The second phase is dominated by the self-aldol condensation of acetone, which is shown to occur predominantly through the keto form of acetone rather than the enol form, and subsequent deoxygenation reactions leading to olefins and aromatics. Finally, net rate analysis shows that aromatics are primarily formed via a pathway including aldol condensation of mesityl oxide, whereas olefins are produced from the addition of isobutene and subsequent cracking.

09 BIOMASS FUELS↗
DOE OSTI2018

Experimental and theoretical insight into the soot tendencies of the methylcyclohexene isomers

The formation of soot precursors during combustion of three positional isomers of methylcyclohexene was investigated in flow reactor experiments and through density functional theory simulations. As evidenced by a recently published structure-property model, the sooting tendencies of these compounds differ from those of structurally similar molecules - suggesting new or unusual reaction chemistry. It was demonstrated that 1-methyl-1-cyclohexene and 4-methyl-1-cyclohexene preferentially react via a retro-Diels-Alder pathway leading to ring opening and molecular weight reduction. 3-methyl-1-cyclohexene, which exhibits much higher yield sooting index, preferentially reacts via dehydrogenation to cyclohexadienes and toluene - consistent with higher soot formation. It was demonstrated that the relative stability of the first radical intermediate plays a considerable role in determining the branching ratio between formation of soot precursors and ring opened retro-Diels-Alder reaction products. This study underscores the importance that small structural features can have in determining the ultimate fate of carbon during combustion processes.

09 BIOMASS FUELS↗
DOE OSTI2018

Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

Here, the nascent steps in the pyrolysis of the lignin components salicylaldehyde (o-HOC 6 H 4 CHO) and catechol (o-HOC 6 H 4 OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID x 3 cm long); transit times through the reactors are about 100 us. Temperatures in the microreactors can be as high as 1600 K, and pressures are typically a few hundred torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: o-HOC 6 H 4 CHO (+ M) → H 2 + CO + C 5 H 4 -C-O (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HC=C-C=C-CH 3 , HC=C-CH 2 -C=CH, and HC=C-CH-C-CH 2 . These alkynes decompose to a mixture of radicals (HC=C-C=C-CH 2 and HC=C-CH-C=CH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: o-HOC 6 H 4 CHO + H → C 6 H 5 OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: o-HOC 6 H 4 OH (+ M) → H 2 O + C 5 H 4 -C-O. These findings have implications for the pyrolysis of lignin itself.

09 BIOMASS FUELS↗
DOE OSTI2018

Diffusion of aromatic hydrocarbons in hierarchical mesoporous H-ZSM-5 zeolite

Hierarchical mesoporous zeolites exhibit higher catalytic activities and longer lifetime compared to the traditional microporous zeolites due to improved diffusivity of substrate molecules and their enhanced access to the zeolite active sites. Understanding diffusion of biomass pyrolysis vapors and their upgraded products in such materials is fundamentally important during catalytic fast pyrolysis (CFP) of lignocellulosic biomass, since diffusion makes major contribution to determine shape selectivity and product distribution. However, diffusivities of biomass relevant species in hierarchical mesoporous zeolites are poorly characterized, primarily due to the limitations of the available experimental technology. In this work, molecular dynamics (MD) simulations are utilized to investigate the diffusivities of several selected coke precursor molecules, benzene, naphthalene, and anthracene, in hierarchical mesoporous H-ZSM-5 zeolite. The effects of temperature and size of mesopores on the diffusivity of the chosen model compounds are examined. The simulation results demonstrate that diffusion within the microspores as well as on the external surface of mesoporous H-ZSM-5 dominates only at low temperature. At pyrolysis relevant temperatures, mass transfer is essentially conducted via diffusion along the mesopores. Furthermore, the results illustrate the heuristic diffusion model, such as the extensively used Knudsen diffusion, overestimates the diffusion of bulky molecules in the mesopores, thus making MD simulation a powerful and compulsory approach to explore diffusion in zeolites.

09 BIOMASS FUELS↗
DOE OSTI2017

Consideration of the Aluminum Distribution in Zeolites in Theoretical and Experimental Catalysis Research

Research efforts in zeolite catalysis have become increasingly cognizant of the diversity in structure and function resulting from the distribution of framework aluminum atoms, through emerging reports of catalytic phenomena that fall outside those recognizable as the shape-selective ones emblematic of its earlier history. Molecular-level descriptions of how active-site distributions affect catalysis are an aspirational goal articulated frequently in experimental and theoretical research, yet they are limited by imprecise knowledge of the structure and behavior of the zeolite materials under interrogation. In experimental research, higher precision can result from more reliable control of structure during synthesis and from more robust and quantitative structural and kinetic characterization probes. In theoretical research, construction of models with specific aluminum locations and distributions seldom capture the heterogeneity inherent to the materials studied by experiment. In this Perspective, we discuss research findings that appropriately frame the challenges in developing more predictive synthesis-structure-function relations for zeolites, highlighting studies on ZSM-5 zeolites that are among the most structurally complex molecular sieve frameworks and the most widely studied because of their versatility in commercial applications. We discuss research directions to address these challenges and forge stronger connections between zeolite structure, composition, and active sites to catalytic function. Such connections promise to aid in bridging the findings of theoretical and experimental catalysis research, and transforming zeolite active site design from an empirical endeavor into a more predictable science founded on validated models.

09 BIOMASS FUELS↗
DOE OSTI2017

Improving biomass pyrolysis economics by integrating vapor and liquid phase upgrading

Partial deoxygenation of bio-oil by catalytic fast pyrolysis with subsequent coupling and hydrotreating can lead to improved economics and will aid commercial deployment of pyrolytic conversion of biomass technologies. Biomass pyrolysis efficiently depolymerizes and deconstructs solid plant matter into carbonaceous molecules that, upon catalytic upgrading, can be used for fuels and chemicals. Upgrading strategies include catalytic deoxygenation of the vapors before they are condensed (in situ and ex situ catalytic fast pyrolysis), or hydrotreating following condensation of the bio-oil. In general, deoxygenation carbon efficiencies, one of the most important cost drivers, are typically higher for hydrotreating when compared to catalytic fast pyrolysis alone. However, using catalytic fast pyrolysis as the primary conversion step can benefit the entire process chain by: (1) reducing the reactivity of the bio-oil, thereby mitigating issues with aging and transport and eliminating need for multi-stage hydroprocessing configurations; (2) producing a bio-oil that can be fractionated through distillation, which could lead to more efficient use of hydrogen during hydrotreating and facilitate integration in existing petroleum refineries; and (3) allowing for the separation of the aqueous phase. In this perspective, we investigate in detail a combination of these approaches, where some oxygen is removed during catalytic fast pyrolysis and the remainder removed by downstream hydrotreating, accompanied by carbon–carbon coupling reactions in either the vapor or liquid phase to maximize carbon efficiency toward value-driven products (e.g. fuels or chemicals). The economic impact of partial deoxygenation by catalytic fast pyrolysis will be explored in the context of an integrated two-stage process. In conclusion, improving the overall pyrolysis-based biorefinery economics by inclusion of production of high-value co-products will be examined.

09 BIOMASS FUELS↗
DOE OSTI2017

Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions

Bio-oil produced from biomass fast pyrolysis often requires catalytic upgrading to remove oxygen and acidic species over zeolite catalysts. The elementary reactions in the mechanism for this process involve carbenium and oxonium ions. In order to develop a detailed kinetic model for the catalytic upgrading of biomass, rate constants are required for these elementary reactions. The parameters in the Arrhenius equation can be related to thermodynamic properties through structure-reactivity relationships, such as the Evans-Polanyi relationship. For this relationship, enthalpies of formation of each species are required, which can be reasonably estimated using group additivity. However, the literature previously lacked group additivity values for oxygenates, oxonium ions, and oxygen-containing carbenium ions. In this work, 71 group additivity values for these types of groups were regressed, 65 of which had not been reported previously and six of which were newly estimated based on regression in the context of the 65 new groups. Heats of formation based on atomization enthalpy calculations for a set of reference molecules and isodesmic reactions for a small set of larger species for which experimental data was available were used to demonstrate the accuracy of the Gaussian-4 quantum mechanical method in estimating enthalpies of formation for species involving the moieties of interest. Isodesmic reactions for a total of 195 species were constructed from the reference molecules to calculate enthalpies of formation that were used to regress the group additivity values. The results showed an average deviation of 1.95 kcal/mol between the values calculated from Gaussian-4 and isodesmic reactions versus those calculated from the group additivity values that were newly regressed. Importantly, the new groups enhance the database for group additivity values, especially those involving oxonium ions.

09 BIOMASS FUELS↗
DOE OSTI2017

A Quantitative Model for the Prediction of Sooting Tendency from Molecular Structure

Particulate matter emissions negatively affect public health and global climate, yet newer fuel-efficient gasoline direct injection engines tend to produce more soot than their port-fuel injection counterparts. Fortunately, the search for sustainable biomass-based fuel blendstocks provides an opportunity to develop fuels that suppress soot formation in more efficient engine designs. However, as emissions tests are experimentally cumbersome and the search space for potential bioblendstocks is vast, new techniques are needed to estimate the sooting tendency of a diverse range of compounds. In this study, we develop a quantitative structure-activity relationship (QSAR) model of sooting tendency based on the experimental yield sooting index (YSI), which ranks molecules on a scale from n-hexane, 0, to benzene, 100. The model includes a rigorously defined applicability domain, and the predictive performance is checked using both internal and external validation. Model predictions for compounds in the external test set had a median absolute error of ~3 YSI units. An investigation of compounds that are poorly predicted by the model lends new insight into the complex mechanisms governing soot formation. Predictive models of soot formation can therefore be expected to play an increasingly important role in the screening and development of next-generation biofuels.

09 BIOMASS FUELS↗
DOE OSTI2017

Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules

Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalities on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C-H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R-OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. In conclusion, these findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups.

09 BIOMASS FUELS↗
DOE OSTI2017

Deactivation of Multilayered MFI Nanosheet Zeolite during Upgrading of Biomass Pyrolysis Vapors

Here, the catalytic fast pyrolysis (CFP) of biomass is a promising technology for producing renewable transportation fuels and chemicals. MFI-type catalysts have shown promise for CFP because they produce gasoline range hydrocarbons from oxygenated pyrolysis compounds; however, rapid catalyst deactivation due to coking is one of the major technical barriers inhibiting the commercialization of this technology. Coke deposited on the surface of the catalysts blocks access to active sites in the micropores leading to rapid catalyst deactivation. Our strategy is to minimize rapid catalyst deactivation by adding mesoporosity through forming MFI nanosheet materials. The synthesized MFI nanosheet catalysts were fully characterized and evaluated for cellulose pyrolysis vapor upgrading to produce olefins and aromatic hydrocarbons. The data obtained from pyrolysis-GCMS (py-GCMS), showed that fresh MFI nanosheets produced similar aromatic hydrocarbon and olefin yields compared to conventional HZSM-5. However, MFI nanosheets demonstrated a longer lifetime than HZSM-5 even though coke contents were also higher than for HZSM-5 because the mesopores enabled better accessibility to active acid sites. This conclusion was supported by results from post-reaction analysis of various spent catalysts collected at different points during the deactivation experiments.

09 BIOMASS FUELS↗
DOE OSTI2017

Computational Studies of Pyrolysis and Upgrading of Bio-oils: Virtual Special Issue

As research activities continue, our understanding of biomass pyrolysis has been significantly elevated and we sought to arrange this Virtual Special Issue (VSI) in ACS Sustainable Chemistry & Engineering to report recent progress on computational and experimental studies of biomass pyrolysis. Beyond highlighting the five national laboratories' advancements, prestigious researchers in the field of biomass pyrolysis have been invited to report their most recent activities.

09 BIOMASS FUELS↗
DOE OSTI2017

Predictive Model for Particle Residence Time Distributions in Riser Reactors. Part 1: Model Development and Validation

Here in this computational study, we model the mixing of biomass pyrolysis vapor with solid catalyst in circulating riser reactors with a focus on the determination of solid catalyst residence time distributions (RTDs). A comprehensive set of 2D and 3D simulations were conducted for a pilot-scale riser using the Eulerian-Eulerian two-fluid modeling framework with and without sub-grid-scale models for the gas-solids interaction. A validation test case was also simulated and compared to experiments, showing agreement in the pressure gradient and RTD mean and spread. For simulation cases, it was found that for accurate RTD prediction, the Johnson and Jackson partial slip solids boundary condition was required for all models and a sub-grid model is useful so that ultra high resolutions grids that are very computationally intensive are not required. Finally, we discovered a 2/3 scaling relation for the RTD mean and spread when comparing resolved 2D simulations to validated unresolved 3D sub-grid-scale model simulations.

09 BIOMASS FUELS↗
DOE OSTI2017

Catalyst Residence Time Distributions in Riser Reactors for Catalytic Fast Pyrolysis. Part 2: Pilot-Scale Simulations and Operational Parameter Study

Here, wsing the validated simulation model developed in part one of this study for biomass catalytic fast pyrolysis (CFP), we assess the functional utility of using this validated model to assist in the development of CFP processes in fluidized catalytic cracking (FCC) reactors to a commercially viable state. Specifically, we examine the effects of mass flow rates, boundary conditions (BCs), pyrolysis vapor molecular weight variation, and the impact of the chemical cracking kinetics on the catalyst residence times. The factors that had the largest impact on the catalyst residence time included the feed stock molecular weight and the degree of chemical cracking as controlled by the catalyst activity. Lastly, because FCC reactors have primarily been developed and utilized for petroleum cracking, we perform a comparison analysis of CFP with petroleum and show the operating regimes are fundamentally different.

09 BIOMASS FUELS↗
DOE OSTI2016

Diffusion of biomass pyrolysis products in H-ZSM-5 by molecular dynamics simulations

Diffusion of biomass pyrolysis vapors and their upgraded products is an essential catalytic property of zeolites during catalytic fast pyrolysis and likely plays a critical role in the selectivity of these catalysts. Characterizing the diffusivities of representative biofuel molecules is critical to understand shape selectivity and interpret product distribution. Yet, experimental measurements on the diffusivities of oxygenated biofuel molecules at pyrolysis temperatures are very limited in the literature. As an alternative approach, we conducted MD simulations to measure the diffusion coefficients of several selected molecules that are representative of biomass pyrolysis vapors, namely water, methanol, glycolaldehyde, and toluene in H-ZSM-5 zeolite. The results show the diffusion coefficients calculated via MD simulations are consistent with available NMR measurements at room temperature. The effect of molecular weight and molecular critical diameter on the diffusivity among the chosen model compounds is also examined. Furthermore, we have characterized the diffusivities of representative biofuel molecules, namely xylene isomers, in H-ZSM-5. Our calculations determined that the ratio of the diffusion coefficients for xylene isomers is p -xylene: o -xylene: m -xylene ≈ 83:3:1 at 700 K. Furthermore, our results also demonstrate the different diffusivity between p-xylene and toluene is due to the molecular orientations when the molecules diffuse along the channels in H-ZSM-5 and provide deep insight into the effect of molecular orientation on its diffusivity.

09 BIOMASS FUELS↗