Supported molybdenum oxides as effective catalysts for the catalytic fast pyrolysis of lignocellulosic biomass

The catalytic fast pyrolysis (CFP) of pine was investigated over 10 wt% MoO[subscript 3]/TiO[subscript 2] and MoO[subscript 3]/ZrO[subscript 2] at 500 °C and H[subscript 2] pressures ≤0.75 bar. The product distributions were monitored in real time using a molecular beam mass spectrometer (MBMS). Both supported MoO[subscript 3] catalysts show different levels of deoxygenation based on the cumulative biomass to MoO[subscript 3] mass ratio exposed to the catalytic bed. For biomass to MoO[subscript 3] mass ratios <1.5, predominantly olefinic and aromatic hydrocarbons are produced with no detectable oxygen-containing species. For ratios ≥1.5, partially deoxygenated species comprised of furans and phenols are observed, with a concomitant decrease of olefinic and aromatic hydrocarbons. For ratios ≥5, primary pyrolysis vapours break through the bed, indicating the onset of catalyst deactivation. Product quantification with a tandem micropyrolyzer–GCMS setup shows that fresh supported MoO[subscript 3] catalysts convert ca. 27 mol% of the original carbon into hydrocarbons comprised predominantly of aromatics (7 C%), olefins (18 C%) and paraffins (2 C%), comparable to the total hydrocarbon yield obtained with HZSM-5 operated under similar reaction conditions. Post-reaction XPS analysis on supported MoO[subscript 3]/ZrO[subscript 2] and MoO[subscript 3]/TiO[subscript 2] catalysts reveal that ca. 50% of Mo surface species exist in their partially reduced forms (i.e., Mo5[superscript +] and Mo3[superscript +]), and that catalyst deactivation is likely associated to coking.BP (Firm) (MIT Energy Initiative. Advanced Conversion Research Program)National Science Foundation (U.S.) (Award 1454299

Spectroscopy and Dynamics of the Predissociated, Quasi-linear S2 State of Chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S2 state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S1 state. This study, which represents the first observation of the S2 state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S2 state vibrational levels with angular momentum ℓ = 1 were observed for CHCl, and 34 levels for CDCl. The range of ℓ in this case was restricted by the pronounced Renner-Teller effect in the low-lying S1 levels, which severely reduces the fluorescence lifetime for levels with Ka \u3e 0. Nonetheless, by exploiting different intermediate S1 levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S1 state. Plots of the predissociation linewidth (lifetime) vs. energy for various S2 levels show an abrupt onset, which lies near the calculated threshold for elimination to form C(3P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold

Vapor-Phase Stabilization of Biomass Pyrolysis Vapors Using Mixed-Metal Oxide Catalysts

© 2019 American Chemical Society. Mixed-metal oxides possess a wide range of tunability and show promise for catalytic stabilization of biomass pyrolysis products. For materials derived from layered double hydroxides, understanding the effect of divalent cation species and divalent/trivalent cation stoichiometric ratio on catalytic behavior is critical to their successful implementation. In this study, four mixed-metal oxide catalysts consisting of Al, Zn, and Mg in different stoichiometric ratios were synthesized and tested for ex-situ catalytic fast pyrolysis (CFP) using pine wood as feedstock. The catalytic activity and deactivation behavior of these catalysts were monitored in real-time using a lab-scale pyrolysis reactor and fixed catalyst bed coupled with a molecular beam mass spectrometer (MBMS), and data were analyzed by multivariate statistical approaches. In the comparison between Mg-Al and Zn-Al catalyst materials, we demonstrated that the Mg-Al materials possessed greater quantities of basic sites, which we attributed to their higher surface areas, and they produced upgraded pyrolysis vapors which contained less acids and more deoxygenated aromatic hydrocarbons such as toluene and xylene. However, detrimental impacts on carbon yields were realized via decarbonylation and decarboxylation reactions and coke formation. Given that the primary goals of catalytic upgrading of bio-oil are deoxygenation, reduction of acidity, and high carbon yield, these results highlight both promising catalytic effects of mixed-metal oxide materials and opportunities for improvement

Emission Spectroscopy of Halocarbenes: Spin-Orbit Mixing, the Singlet-Triplet Gap and Rovibrational Structure of ̃X¹A\u27 and ã³A‪ States

We recorded Single Vibronic Level (SVL) emission spectra following excitation of excited state vibrational levels in the à 1A [arrow left] X 1 A\u27 systems of the monohalocarbenes CHF, CDF, CHCl, CDCl, CHBr, and CDBr and the Ã1B 1 [arrow left] X1A1 system of CCl2 . Spectra were measured with a 0.3 m spectrograph equipped with a gated intensified CCD detector and obtained under jet-cooled conditions using a pulsed discharge source. The emission spectra reveal rich detail concerning the vibrational structure of the X state, spin-orbit mixing with the low lying triplet state, and the singlet-triplet gap. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with previous experimental and high level ab initio studies, are reported. The Dunham expansion works well in reproducing the ground state term energies for all the molecules except CHF, where Fermi resonances are observed, and CHBr and CDBr, which feature extensive spin-orbit interaction. In C35 Cl2 we measured [Special characters omitted.] -sorted emission spectra in order to test the previously established hypothesis by M.-L. Liu, et al , that unassigned lines lying above ∼ 5000 cm-1 belong to the ã 3B1 state. Further, we obtained high resolution spectra of singlet-singlet transitions for CHF and both singlet-singlet and singlet-triplet transitions in CHCl using Stimulated Emission Pumping (SEP) Spectroscopy. In CHF, an anharmonic effective Hamiltonian model poorly reproduces the term energies even with the improved set of data, due to the extensive interactions among levels in a given polyad (p ) related by p =2ν1 +ν2 +ν3 . The precise A rotational constants determined from the SEP data were invaluable in clarifying the assignments for these strongly perturbed levels, and the data are well reproduced using a multiresonance effective Hamiltonian model. For CHCl, spectra reveal detailed information on the rovibrational structure of the X 1A\u27 and ã3A states, and for the triplet state they show a pronounced vibrational state dependence of the spin-spin splitting, which is a sensitive probe of spin-orbit coupling with nearby singlet levels. For the ground state we observed forbidden Ka = 0 levels due to axis switching, which allowed us to determine the A rotational constants. The parameters derived from our spectra are in excellent agreement with recent ab initio calculations

SINGLE VIBRONIC LEVEL EMISSION SPECTROSCOPY OF CHCl AND CDCl: VIBRATIONAL STRUCTURE OF THE XX1A′^{1}A^{\prime} AND aa3A′′^{3}A^{\prime\prime} STATES

Author Institution: Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881We report on single vibronic level (SVL) emission spectra from bands in the progressions 2$_0^{n}$, 2$_0^{n}$3$_0^{m}$, 1$_0^{1}$2$_0^{n}$, and 1$_0^{1}$2$_0^{n}$3$_0^{1}$ in the $A$$^{1}A^{\prime\prime}$-$X$$^{1}A^{\prime}$ system of CH$^{35}$Cl, CH$^{37}$Cl, CD$^{35}$Cl and CD$^{37}$Cl. As in previous studies of CHF/CDF and CHBr/CDBr, the carbenes were generated using a pulsed discharge source, and SVL emission spectra obtained using a 0.3 m spectrograph in combination with a gated, intensified CCD detector. These spectra reveal rich new detail regarding the vibrational structure of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states, and spin-orbit induced mixing between them, up to 9000 cm$^{-1}$ above the vibrationless level of the $X$$^{1}A^{\prime}$ state. For CHCl, we observe around three times the number of $X$$^{1}A^{\prime}$ levels previously reported,}, and a number of new $a$$^{3}A^{\prime\prime}$ state levels. The results of Dunham expansion fits to the vibrational term energies, and comparisons with previous experimental and recent high quality theoretical studies}$^{,}$} will be reported. Overall, the derived vibrational parameters of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states are in excellent agreement with $ab$ $initio$ predictions, including our own DFT calculations

Automated Construction of Potential Energy Surfaces

Methods to construct molecular potential energy surfaces through automated generation of ab initio electronic structure data are reviewed. Given a chosen method for fitting ab initio data (electronic energies represented at particular geometries) into an analytic surface, the questions of how best to select the data point locations and how to interface an electronic structure software package with fitting codes in parallel on a high-performance computing cluster are addressed. It is shown that methods based on interpolating moving least squares fitting are useful as they lend themselves to an algorithm which iteratively refines the fitted surface towards arbitrary accuracy. Several variants of the method are illustrated through examples including spectroscopic potentials for van der Waals systems, systems with high permutation symmetry, reactive systems, and systems with multiple coupled electronic states. An outlook identifying areas for future development is given

LASER SPECTROSCOPY OF A HALOCARBOCATION: OBSERVATION OF THE ELECTRONIC SPECTRUM OF CH2_{2}I+^{+}

Author Institution: Department of Chemistry, Marquette University, Milwaukee, WI 53233We report the first observation of the gas-phase electronic spectrum of a simple halocarbocation, CH$_{2}$I$^{+}$. Fluorescence excitation and emission spectra were measured under jet-cooled conditions using a pulsed discharge source with CH$_{2}$I$_{2}$ as precursor. The spectral carrier was identified by: (1) comparison of experimentally derived ground state vibrational frequencies for various isotopomers with the predictions of Density Functional Theory (DFT) calculations, and (2) simulation of the observed rotational contour in our excitation spectra, which indicated an a-type transition, consistent with an excited state of A$_{1}$ symmetry. Calculations predict a triplet A$_{1}$ excited state with transition energy in the visible region, and the observed transition is therefore assigned as $^{3}$A$_{1}$-$^{1}$A$_{1}$, with band origin at 15180 cm$^{-1}$