Hilkka I. Kenttämaa

Internal energy distributions of isolated ions after activation by various methods

Abstract A number of activation methods have been compared by approximating internal energy distributions, P(E), of selected activated ions. The ions chosen allow simplifying assumptions to be made concerning the determination of the energy distributions since they fragment by several simple consecutive reactions. Ion abundance data, in combination with known energetics of unimolecular fragmentation, are utilized to estimate the internal energy distributions. Determination of P(E) is therefore not based on a specialized instrument or technique and may be applied to ions which do not have stable neutral counterparts. The data obtained are used to account for several important features of tandem mass spectrometry and to examine and compare different methods of varying ion internal energy. The results show, inter alia, that (i) the general shapes of the energy distributions resulting from collisional activation are relatively insensitive to ion structure; (ii) the average energy of ions activated by collision in the kiloelectron volt or electron volt range of collision energy can be comparable; (iii) in contrast to low energy collisional activation, the distribution of internal energies produced by a kiloelectron volt collision is characterized by a finite probability of depositing very high energies; (iv) the average energy of fragmenting (C2H5)4Si+. ions appears to vary linearly with the collision energy in the 5–28 eV range. As the collision energy increases, the width of the internal energy distribution appears to broaden.

Cleavage and hydrodeoxygenation (HDO) of C–O bonds relevant to lignin conversion using Pd/Zn synergistic catalysis

The development of chemical methods for the direct catalytic conversion of biomass to high value organic molecules is an area of increasing interest. The plant matter component known as lignin is a polymer consisting of aromatic rings that could provide a means of obtaining aromatic materials currently derived solely from petroleum. This report describes a bimetallic Pd/C and Zn catalytic system that can perform selective hydrodeoxygenation (HDO) of monomeric lignin surrogates as well as successfully cleave the β-O-4 linkages found in dimeric lignin model complexes and synthetic lignin polymers with near quantitative conversions and yields between 80–90%. The reaction with lignin polymer was highly selective affording methoxy substituted propylphenol as the major product. These reactions were performed in a Parr reactor operating at relatively mild temperature (150 °C) and pressure (20 bar H2) using methanol as a solvent. Reaction products were characterized using high-pressure liquid chromatography coupled to a linear quadrupole ion trap mass spectrometer equipped with an electrospray ionization source using negative ion mode. Hydroxide ions were doped into the analyte solutions to encourage negative ion formation. This method ionizes all the mixture components to yield a single ion/analyte with no fragmentation. The catalyst is fully recyclable without the need for additional zinc. X-ray absorption spectroscopy (EXAFS) is consistent with Pd nanoparticles (4–5 nm) and no evidence of Pd–Zn alloy formation. A mechanistic hypothesis on the synergy between Pd and Zn is presented.

A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass

Current biomass utilization processes do not make use of lignin beyond its heat value. Here we report on a bimetallic Zn/Pd/C catalyst that converts lignin in intact lignocellulosic biomass directly into two methoxyphenol products, leaving behind the carbohydrates as a solid residue. Genetically modified poplar enhanced in syringyl (S) monomer content yields only a single product, dihydroeugenol. Lignin-derived methoxyphenols can be deoxygenated further to propylcyclohexane. The leftover carbohydrate residue is hydrolyzed by cellulases to give glucose in 95% yield, which is comparable to lignin-free cellulose (solka floc). New conversion pathways to useful fuels and chemicals are proposed based on the efficient conversion of lignin into intact hydrocarbons.

Internal energy distributions acquired through collisional activation at low and high energies

Abstract It is possible to gain information about internal energy distributions of ions knowing their mass spectra and the energetics of fragmentation if one can equate pre-exponential terms in the unimolecular rate expressions. Positively charged triethyl phosphate, which fragments via a linear sequence of unimolecular reactions, has been examined in this way and the results have been checked by back-calculating the mass spectrum. It is found that low energy (28 eV in laboratory frame, argon target) collision-induced dissociation deposits a comparable average internal energy to what occurs at high energy (7 keV). High energy collisions, however, result in a distribution of internal energies which includes a low probability tail representing very highly excited ions. In the low energy regime, it is possible to increase dramatically the effective internal energy deposited by raising the collision gas pressure. The method of estimating internal energy distributions given here is particularly attractive for comparing ions excited by various means.

High-performance liquid chromatography/high-resolution multiple stage tandem mass spectrometry using negative-ion-mode hydroxide-doped electrospray ionization for the characterization of lignin degradation products.

In the search for a replacement for fossil fuel and the valuable chemicals currently obtained from crude oil, lignocellulosic biomass has become a promising candidate as an alternative biorenewable source for crude oil. Hence, many research efforts focus on the extraction, degradation, and catalytic transformation of lignin, hemicellulose, and cellulose. Unfortunately, these processes result in the production of very complex mixtures. Further, while methods have been developed for the analysis of mixtures of oligosaccharides, this is not true for the complex mixtures generated upon degradation of lignin. For example, high-performance liquid chromatography/multiple stage tandem mass spectrometry (HPLC/MS(n)), a tool proven to be invaluable in the analysis of complex mixtures derived from many other biopolymers, such as proteins and DNA, has not been implemented for lignin degradation products. In this study, we have developed an HPLC separation method for lignin degradation products that is amenable to negative-ion-mode electrospray ionization (ESI doped with NaOH), the best method identified thus far for ionization of lignin-related model compounds without fragmentation. The separated and ionized compounds are then analyzed by MS(3) experiments to obtain detailed structural information while simultaneously performing high-resolution measurements to determine their elemental compositions in the two parts of a commercial linear quadrupole ion trap/Fourier-transform ion cyclotron resonance mass spectrometer. A lignin degradation product mixture was analyzed using this method, and molecular structures were proposed for some components. This methodology significantly improves the ability to analyze complex product mixtures that result from degraded lignin.

Maleic acid and aluminum chloride catalyzed conversion of glucose to 5-(hydroxymethyl) furfural and levulinic acid in aqueous media

Maleic acid (MA) and AlCl3 self-assemble into catalytic complexes (Al–(MA)2–(OH)2(aq)) with improved selectivity for converting glucose to HMF, and levulinic acid. The calculated activation energy (Ea) of the MA–aluminum catalyzed glucose-to-fructose isomerization is 95 kJ mol−1 compared to 149 kJ mol−1 for HCl and AlCl3 alone. Furthermore, conversion of fructose to HMF is enhanced. The catalytic conversion of fructose to HMF by MA and AlCl3 at 180 °C is 1.7× faster with 3.3× higher selectivity when compared to HCl with AlCl3. Liquid 13C NMR spectra results indicate that glucose undergoes a ring-opening process in aqueous solution when maleic acid is introduced, which we hypothesize facilitates the hydride shift in glucose for isomerization leading to enhanced rates and selectivity. Improved selectivity of glucose conversion to HMF and levulinic acid could improve the economics of producing these value-added chemicals for use in renewable, sustainable polymers.

Characterization of organosolv switchgrass lignin by using high performance liquid chromatography/high resolution tandem mass spectrometry using hydroxide-doped negative-ion mode electrospray ionization

Lignin is an aromatic biopolymer that may yield valuable chemicals currently obtained solely from petroleum. However, extraction of lignin by using traditional methods, such as organosolv extraction, produces very complex mixtures. Molecular level characterization of the major components is essential to be able to rationally tailor methodology for the conversion of these mixtures to transportation fuel and valuable chemicals. In this study, high performance liquid chromatography/high resolution tandem mass spectrometry (HPLC/MSn) was used to obtain molecular weight, elemental composition and structural information for the major components in an organosolv lignin sample. HPLC/MSn coupled with hydroxide-doped electrospray ionization was used to identify the structures of the major components by using a Thermo Scientific linear quadrupole ion trap-Fourier transform ion cyclotron resonance hybrid mass spectrometer (LQIT/FT-ICR). The results reported here demonstrate that the major products of organosolv extraction are low molecular weight compounds, including monomeric and dimeric lignin units, with various functionalities.

Laser-induced acoustic desorption/chemical ionization in Fourier-transform ion cyclotron resonance mass spectrometry

Abstract Laser-induced acoustic desorption (LIAD) of neutral molecules coupled with electron and chemical ionization was examined as an analysis method for nonvolatile organic and biomolecules in Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. LIAD involved the production of a high amplitude acoustic wave by laser ablation of a copper or titanium foil from the opposite side of where the sample was deposited. The experiment was carried out with a simple probe designed for transmission mode laser desorption. Large amounts of neutral molecules were desorbed this way, but ions were not detected. The desorbed neutral molecules were ionized by 70 eV electron ionization or by reactions with reagent ions that were generated, isolated, and trapped in the ICR cell. Strong, reproducible signals were obtained in these experiments. The applicability of the method was demonstrated for a wide variety of molecules, including an organic salt, steroids, sugars, oligopeptides, nucleic acid bases, nucleosides, and synthetic polymers. For example, the tetrapeptide val-ala-ala-phe was volatilized by LIAD and ionized by a proton transfer reaction. The ions were stored in the FT-ICR cell for up to 30 s before detection. The base peak in the spectrum obtained corresponds to the protonated peptide, which indicates that the neutral peptide was desorbed intact. In contrast, thermal desorption of this peptide leads to substantial degradation. Based on this and other results obtained, LIAD combined with postdesorption ionization in an FT-ICR shows promise as a practical method for the analysis of thermally labile, nonvolatile molecules. A special advantage of this approach is that it allows better control of the ionization step (through selection of reagent ions) and a broader choice of ionization modes (e.g. group transfer, addition/elimination) than laser desorption/ionization methods, such as matrix-assisted laser desorption/ionization (MALDI). A disadvantage is lower sensitivity.

Mechanistic investigation of the Zn/Pd/C catalyzed cleavage and hydrodeoxygenation of lignin

While current biorefinery processes use lignin only for its heat value, the conversion of lignin to high value chemicals is an area of increasing interest. Herein we present a detailed mechanistic study of the hydrodeoxygenation (HDO) of lignin by using a synergistic Pd/C and ZnII catalyst through use of both lignin model compounds and lignocellulosic biomass. Spectroscopic data coupled with the study of lignin model compounds suggest that ZnII activates and facilitates removal of the hydroxyl group at the Cγ position of the β-O-4 ether linkage. Activation is proposed to occur through formation of a six-membered ring complex of ZnII coordinated to the oxygen atoms at Cα and Cγ of the lignin model compound guaiacylglycerol-β-guaiacyl.

Low-energy collisional activation of polyatomic ions with different target gases

Abstract The dependence of collision-activated dissociation spectra on the nature of the target gas was examined by activating the molecular ions of tetraethylsilane, tungsten hexacarbonyl, furan and 2,2′-bithiophene, as well as the chromium pentacarbonyl radical cation, by 5–30 eV collisions with various mono- and polyatomic target gases. Average internal energies were estimated for the activated ions from the collisional activation spectra by utilizing a knowledge of the sequence and energetics of dissociation. For all of the ions studied, the data indicate that the amount of internal energy deposited increases as a function of the kinetic energy of the molecular ion. For some of these systems, the internal energy of the parent ions can also be increased by using a heavier target. However, for ionized furan, ionized tetraethylsilane, and the chromium pentacarbonyl radical cation, smaller internal energy deposition is observed at a given center-of-mass energy (Ecm) when heavier targets are used. This unexpected result appears to be due to significant ion loss caused by scattering of the fragments beyond the acceptance cone of the instrument when the mass of the target (Mt) is large compared to that of the parent ion (Mp). Another result is that the use of polyatomic targets leads to less extensive fragmentation of [(C2H5)4Si]+· and ionized furan than does the use of monatomic targets of similar mass. The relatively large number of internal degrees of freedom in the polyatomic targets appears to favor distribution of a significant amount of the excitation energy into the internal modes of the target. Conversion of translational energy to vibrational energy is observed to be constant (15±5%) over the center-of-mass kinetic energy range 4–16 eV for nearly all targets. Practical consequences of these target effects are discussed.