Konstantinos Alexopoulos

Insights into the Reaction Mechanism of Ethanol Conversion into Hydrocarbons on H‐ZSM‐5

Ethanol dehydration to ethene is mechanistically decoupled from the production of higher hydrocarbons due to complete surface coverage by adsorbed ethanol and diethyl ether (DEE). The production of C3+ hydrocarbons was found to be unaffected by water present in the reaction mixture. Three routes for the production of C3+ hydrocarbons are identified: the dimerization of ethene to butene and two routes involving two different types of surface species categorized as aliphatic and aromatic. Evidence for the different types of species involved in the production of higher hydrocarbons is obtained via isotopic labeling, continuous flow and transient experiments complemented by UV/Vis characterization of the catalyst and ab initio microkinetic modeling.

Selective etherification of β-citronellene catalyzed by zeolite beta

The etherification of β-citronellene with bioalcohols over zeolite beta was performed in a continuous flow liquid phase reactor. At 80 °C, the catalyst exhibits 50% β-citronellene conversion with a high selectivity for the etherification reaction. High chemoselectivity (90%) at the β-double bond of β-citronellene was observed, while β-citronellene isomers were formed as minor products. In order to rationalize the observed chemoselectivity, the relative stabilities of the protonated reaction intermediates were estimated using theoretical calculations. The zeolite beta catalyst exhibits high stability as well as low coke formation. Considering the industrial importance of terpene ethers as sophisticated solvents, fragrance or flavor additives, a novel and environmentally friendly synthesis is presented as an alternative to homogeneous catalysis using strong Bronsted or Lewis acids in solution.

Mechanistic insights into the formation of butene isomers from 1-butanol in H-ZSM-5: DFT based microkinetic modelling

Besides being a renewable energy source, the catalytic conversion of bio-alcohols can serve as a sustainable means of producing high-value chemicals. Butenes produced by dehydration of 1-butanol could serve as building blocks for several essential compounds such as fuels and polymers. This study provides theoretical insights into the competing pathways for the formation of butene isomers (1-butene, cis/trans 2-butenes and iso-butene) during catalytic dehydration of 1-butanol in H-ZSM-5. As di-1-butyl ether (DBE) is one of the key products during low temperature dehydration of 1-butanol, a new mechanism for direct formation of trans-2-butene from DBE via E1 elimination is also envisaged along with the direct dehydration of 1-butanol to trans-2-butene. A 2-butoxide mediated stepwise mechanism and a concerted mechanism involving simultaneous protonation of the double bond by the Bronsted acid site and abstraction of the Hγ by the zeolite oxygen are considered for the double bond isomerization in H-ZSM-5. A monomolecular 2-butoxide and iso-butoxide mediated mechanism is considered for the skeletal isomerization. The transformation of 2-butoxide to iso-butoxide occurs via a π-bonded propene–methyl carbocationic transition state. DFT based microkinetic simulations show that, except for very low conversion levels where 2-butenes are produced via E1 elimination of 1-butanol from the protonated di-1-butyl ether (DBE*), the formation of 2-butenes occurs essentially via double bond isomerization mechanisms with comparable contributions of both concerted and 2-butoxide mediated stepwise mechanisms. Owing to the higher activation barrier for the skeletal isomerization, isobutene is not observed in the simulated temperature range of 450–500 K. Simulation results indicate that low reaction temperature, low site time and high butanol pressure favor production of 1-butene and DBE, while high temperature and site time and low butanol pressure favor the consecutive reactions leading to production of butene isomers.

Effect of zeolite confinement on the conversion of 1-butanol to butene isomers: mechanistic insights from DFT based microkinetic modelling

Ab initio based microkinetic modelling of 1-butanol dehydration to butene isomers is used to obtain mechanistic insights into the effect of a zeolite framework. A detailed microkinetic model including double bond isomerization, skeletal isomerization and mechanisms for the direct formation of 2t-butene from 1-butanol dimer and di-1-butyl ether (DBE) is considered for the dehydration in H-ZSM-5, H-ZSM-22 and H-FER. H-FER favors the production of 2t-butene and H-ZSM-22 achieves thermodynamic equilibrium composition for linear butenes even at low conversion levels, while H-ZSM-5 maximizes 1-butene selectivity. Significant differences are observed in the reaction mechanism leading to formation of 2t-butene. For H-ZSM-5 and H-ZSM-22, the formation of 2-butenes occurs via double bond isomerization of 1-butene produced from butanol dehydration. For the double bond isomerization of 1-butene to 2t-butene, both concerted and 2-butoxide mediated stepwise mechanisms contribute significantly in H-ZSM-5, while only the concerted mechanism is operative in H-ZSM-22. On the other hand, for H-FER, 2t-butene is mainly produced from the butanol dimer via an E1 elimination accompanied by a 1,2-hydride shift. This in turn can be attributed to an increase in enthalpic stabilization of the E1 elimination transition state for the direct formation of 2t-butene from 1-butanol dimer when moving from H-ZSM-5 to H-FER. Isobutene formation is not observed in all three zeolites at the investigated temperature range of 450–500 K.

Role of anharmonicity in the confinement effect in zeolites: structure, spectroscopy, and adsorption free energy of ethanol in H-ZSM-5

Phononic structures (composite materials) in which a periodic distribution of elastic parameters facilitates control of the propagation of phonons, hold the promise to enable transformative material technologies in areas ranging from acoustic and thermal cloaking to thermoelectric devices. This requires strategies to deliberately engineer the phononic band structure of materials in the frequency range of interest. Phononics, the acoustic equivalents of the photonics are controlled by a larger number of material parameters, as phonon cannot propagate in vacuum. The study of hypersonic phononics (hPnC) imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at length scales commensurate with the wave length of hypersonic phonons and hence visible photons. The key quantity is the dispersion ω(q) of high frequency (GHz) acoustic excitations with wave vector q which is measured by the noninvasive high resolution Brillouin light scattering. The approach involves the exploitation of Bragg-type band gaps (BGs) that result from the destructive interference of waves in periodic media. However, the sensitivity of BG formation to structural disorder limits the application of self-assembly methods that are susceptible to defect formation. Hybridization gaps (HG), originating from the anticrossing between local resonant and propagating modes, are robust to structural disorder and occur at wavelengths much larger than the size of the resonant unit. Here, examples based on hierarchical structures will be highlighted: 1D-hPnC to acquire comprehensive understanding, while the incorporation of defects holds a wealth of opportunities to engineer ω(q); in colloid based phononics, ω(q) has revealed both types of band gabs; particle brush materials with controlled architecture of the grafted chains enable a new strategy to realize HG’s and; hierarchically nanostructured matter can involve unprecedented phonon phono propagation mechanisms. Phononic crystals, the acoustic equivalents of the photonic crystals, are controlled by a larger number of material parameters. The study of hypersonic crystals imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at mesoscopic length scales commensurate with the wave length of hypersonic phonons and hence photons of the visible light. Polymerand colloid-based phononics is an emerging new field at the interface of soft materials science and condensed matter physics with rich perspectives ahead. The key quantity is the dispersion of high frequency (GHz) acoustic excitations which is nowadays at best measured by high resolution spontaneous Brillouin light scattering. Depending on the components of the nanostructured composite materials, the resolved vibration eigenmodes of the individual particles sensitively depend on the particle architecture and their thermo-mechanical properties [T. Still et al., Nano Lett. 10, 3194 (2008)]. In periodic structures of polymer based colloids, the dispersion relation ω(k) between the frequency and the phonon wave vector k has revealed hypersonic phononic band gaps of different nature. Colloid and polymer science allows the engineering of acoustic and optical material functionalities of hierarchical structures on various length scales commensurate with and well below the characteristic length scales of phonons and photons. Periodic structures act as both hypersonic phononic and visible light photonic crystals (phoxonics). We recently extended the decade-old field to hypersonic phononics. Many important questions in this young field are just being raised and require new conceptual and technical approaches to address them. Powerful synthesis and assembly methods are able to create novel structures to host unconventional properties of flexibility and multi-functionality, locally resonant hypersonic soft metamaterials and topological phononic insulators. To complement our best world-wide Brillouin spectroscopy for retrieving the dispersion relations in transparent structures, two new experimental techniques based on laser-induced high frequency phonons and tapered fiber optomechanics will be implemented to engineer strong wavematter interactions. Band structure calculations will be used as tools to model and predict the acoustic wave propagation in composite structures of varying symmetry, architecture and topology of the building components. Our novel approach, together with intricate methods of processing such materials at a large scale, shows the outline of the emerging field of polymer-and colloid-based phononics. Promising applications range from tunable responsive filters and one way phonon waveguides to compact acousto-optic devices and sensors and from hypersonic imaging to materials and devices, which allow for directed heat flow and recovery. To access such fundamental concepts a detailed understanding of phonon propagation in nanostructured media is a precondition. This proposal ensures that we will hear much more about currently unknown and unexpected properties and functions of soft phononics and will open up many new lines of research.Z is one of promising solid acid catalysts for the conversion of renewable biomass-derived alcohols into fuels and chemicals. Dehydration of alcohols to alkenes is a well-known prototypical acid catalyzed reaction, where confinement and entropic effects impact the rates of these reactions. For such conversions, HZSM-5 zeolite is commonly used as a platform for acid catalyzed reactions due to its strong acidity and enhancement of reaction rates due to confinement in pores. In this talk, we present the structure and thermochemistry of ethanol adsorption on the Brønsted acid site of the HZMS-5 by means of ab inito molecular dynamics (AIMD) simulations directly compared with in situ IR spectroscopy and thermochemical measurements on the same material. Simulations were performed using two different ethanol loadings (with/without deuterium substitution) at different temperatures (100 ≤ T ≤ 700). This enables us to take into account enthalpic and entropic effects caused by the dynamics of the motion of the reaction intermediates. AIMD simulations show that hydrogen transfer from the zeolite scaffold to ethanol occurs as temperature increases. In the simulations with higher ethanol loading, proton transfer occurs via relay between H-bonded ethanol molecules. Calculated projected vibrational density of states (VDOS) obtained from velocity autocorrelation function show a broad peak around 1600 cm-1 related to H-O-H bending mode which is also observed experimentally. We estimated entropy and enthalpy of adsorption using the computed VDSO along with a quasi-harmonic approximation, which shows good agreement with experimental measurement conversely, the more commonly employed harmonic vibrations lead to free energy estimates that deviate from experiment substantially. Overall, this study exemplifies how enharmonic effects, as capture by AIMD, are critical for the quantitative modeling of the free energetics of zeolitecatalyzed processes.