Alejandro Montoya

Biocrude yield and productivity from the hydrothermal liquefaction of marine and freshwater green macroalgae

Six species of marine and freshwater green macroalgae were cultivated in outdoor tanks and subsequently converted to biocrude through hydrothermal liquefaction (HTL) in a batch reactor. The influence of the biochemical composition of biomass on biocrude yield and composition was assessed. The freshwater macroalgae Oedogonium afforded the highest biocrude yield of all six species at 26.2%, dry weight (dw). Derbesia (19.7%dw) produced the highest biocrude yield for the marine species followed by Ulva (18.7%dw). In contrast to significantly different yields across species, the biocrudes elemental profiles were remarkably similar with higher heating values of 33-34MJkg(-1). Biocrude productivity was highest for marine Derbesia (2.4gm(-2)d(-1)) and Ulva (2.1gm(-2)d(-1)), and for freshwater Oedogonium (1.3gm(-2)d(-1)). These species were therefore identified as suitable feedstocks for scale-up and further HTL studies based on biocrude productivity, as a function of biomass productivity and the yield of biomass conversion to biocrude.

CO2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO2 chemisorption on carbonaceous surfaces. The differential heat of CO2 adsorption at low and high coverages was determined in the temperature range 553–593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO2 chemisorption on carbonaceous surfaces was performed. Several of the carbon–oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO2 adsorption. In the high-coverage region, our theoretical results suggest that CO2 molecules are likely to adsorb on surface oxygen complexes and on graphene planes.

Formation of CO precursors during char gasification with O2, CO2 and H2O

The nature of some of the carbon–oxygen complexes formed after chemisorption of O2 ,C O2 and H2O on carbonaceous surfaces was determined. The analysis was done by means of density functional theories. Among the three reactions studied, CO2 chemisorption is the less exothermic. The nature of carbon–oxygen complexes depends on the oxidant agents. However, surface transformations of those complexes produce common surface oxygen groups that can desorb CO. Therefore, new data are presented to get insight into an unified mechanism of uncatalyzed carbon gasification. D 2002 Elsevier Science B.V. All rights reserved.

Adsorption on carbonaceous surfaces: cost-effective computational strategies for quantum chemistry studies of aromatic systems

We present a systematic analysis of the accuracy and efficiency of several computational quantum chemistry models for studying reactions involving aromatic systems. In particular, we have examined different multi-layer ONIOM models in which the whole system is divided into subsystems that can be treated at different levels of theory. The carbonaceous surface is modeled by a graphene layer that has unsaturated carbon atoms to represent active sites and has different shapes to simulate the local environment of the active sites of a carbonized material. We emphasized the model performance in predicting geometrical parameters, interaction energies and infrared spectra of carbon-oxygen complexes. We found that any attempt to partition the graphene layer into subsystems for employing different levels of theory yields considerable errors. However, it is possible to obtain reasonable accuracy by using the same level of theory for the whole system at different basis sets. This computational strategy can predict accurate geometrical parameters, interaction energies and infrared spectra of common oxygen complexes at lower computational cost.  2002 Elsevier Science Ltd. All rights reserved.

CO2 strong chemisorption as an estimate of coal char gasification reactivity

Abstract In this article, coal char gasification reactivity was correlated with the strong chemisorption of CO 2 at 300°C. Chars of as-received, demineralized, K and Fe loaded coals were prepared at 800°C, under high purity nitrogen. The CO 2 chemisorption method described in this article allows differentiation between two types of chemisorption that takes place at low temperatures: strong CO 2 chemisorption (irreversible) which is related to the presence of the active inorganic components of the char, and weak CO 2 chemisorption (reversible) which is because of the organic matter of the char. The char doped with K showed the highest CO 2 strong chemisorption and at the same time the highest reactivity in the CO 2 gasification, while the char loaded with Fe had the highest amount of weak chemisorption. It was found that total chemisorption (weak+strong) at 300°C depends on the CO 2 pressure of the analysis. The reactivity of the CO 2 gasification of the char was normalized using the value of the amount of CO 2 strongly chemisorbed at 300°C.

DFT Analysis of the Reaction Paths of Formaldehyde Decomposition on Silver

Periodic density functional theory is used to study the dehydrogenation of formaldehyde (CH(2)O) on the Ag(111) surface and in the presence of adsorbed oxygen or hydroxyl species. Thermodynamic and kinetic parameters of elementary surface reactions have been determined. The dehydrogenation of CH(2)O on clean Ag(111) is thermodynamically and kinetically unfavorable. In particular, the activation energy for the first C-H bond scission of adsorbed CH(2)O (25.8 kcal mol(-1)) greatly exceeds the desorption energy for molecular CH(2)O (2.5 kcal mol(-1)). Surface oxygen promotes the destruction of CH(2)O through the formation of CH(2)O(2), which readily decomposes to CHO(2) and then in turn to CO(2) and adsorbed hydrogen. Analysis of site selectivity shows that CH(2)O(2), CHO(2), and CHO are strongly bound to the surface through the bridge sites, whereas CO and CO(2) are weakly adsorbed with no strong preference for a particular surface site. Dissociation of CO and CO(2) on the Ag(111) surface is highly activated and therefore unfavorable with respect to their molecular desorption.

Kinetics of nitric oxide desorption from carbonaceous surfaces

We carried out a molecular modeling study of an important reaction in the combustion of nitrogen-containing chars, the evolution of nitric oxide, (CNO) !NO+(C*). Density functional theory at the B3LYP level was used to provide potential energy surface information and transition state theory was used to provide temperature dependant rate constants. Desorption of NO from nitrogen-containing carbonaceous surfaces is modeled from a pyridine-N-oxide model. The fitted Arrhenius expression for the NO desorption process is k(T)=8.84 � 10 13 exp[ � 42132 K/T ]( s � 1 ). We found that the rate constants to release CO are slightly larger than rate constants to release NO from the carbonaceous surface. The NO desorption activation energy is 10 kcal/mol lower than that of the CO desorption. D 2002 Elsevier Science B.V. All rights reserved.

Lovastatin and (+)-geodin production by Aspergillus terreus from crude glycerol.

The use of pure substrate represents a significant proportion of the cost of manufacturing a drug such as lovastatin. This study explores the production of lovastatin and (+)‐geodin by Aspergillus terreus ATCC 20542 using biodiesel‐derived crude glycerol (CG) as a feedstock. Shake flask experiments showed reduced lovastatin production and glycerol consumption in the presence of 10–50 g/L CG with respect to pure glycerol controls. At 50 g/L, lovastatin and (+)‐geodin production was significantly reduced by 82 and 73%, respectively. The lowest lovastatin inhibition was detected in 30 g/L of CG (48%), which was accompanied by a significant rise in (+)‐geodin production (338%). Further investigation was performed on three major impurities found in CG, namely methanol (MeOH), sodium chloride (NaCl), and fatty acids (oleic acid and palmitic acid (PA), soap). None was particularly inhibitory for lovastatin, except soap and PAs, which reduced its production by more than 50% at all concentrations tested. In contrast, (+)‐geodin was inhibited in the presence of MeOH and PA by up to 46 and 91%, respectively. These observations indicate that partial purification of CG would be potentially useful in improving production of lovastatin and (+)‐geodin by A. terreus.

Conformational and Thermodynamic Properties of Gaseous Levulinic Acid

Molecular modeling is used to determine low-energy conformational structures and thermodynamic properties of levulinic acid in the gas phase. Structure and IR vibrational frequencies are obtained using density functional and Møller-Plesset perturbation theories. Electronic energies are computed using G3//B3LYP and CBS-QB3 model chemistries. Computed anharmonic frequencies are consistent with reported experimental data. Population analysis shows a boat- and a chainlike structure to be most abundant at 298 K, with increasing proportions of two other conformers at higher temperatures. Population mean distribution values for thermodynamic quantities are derived. At 298 K and 1 atm, the enthalpy of formation, entropy, and heat capacity are -613.1 ± 1.0 kJ·mol(-1), 407.4 J·mol(-1)·K(-1), and 132.3 J·mol(-1)·K(-1), respectively.

Molecular dynamics study of Acid-catalyzed hydrolysis of dimethyl ether in aqueous solution.

The acid-catalyzed hydrolysis of dimethyl ether (DME) to methanol was examined using ab initio density functional metadynamics simulations. Diffusion of the acid proton from the aqueous medium, leading to the formation of a protonated DME, is 12.3 kcal mol(-1) activated and 9.3 kcal mol(-1) endothermic, indicating a greater affinity of the acid proton to water than to the ether group. Subsequent scission of the protonated ether bond is found to be 30.7 kcal mol(-1) activated, leading to the formation of a solvated methyl-carbocation, which is thermodynamically unstable. The methyl-carbocation reacts readily to form methanol and regenerate the acid proton.