Georgios D. Stefanidis

• Professor
• Professor (Full) at National Technical University of Athens

Microwave irradiation can significantly speed up solvent evaporation in crystallization processes, resulting in rapid crystallization and reduced particle size. A single-mode microwave setup was used for evaporative crystallization of the model pharmaceutical compound, niflumic acid. Production of crystals by microwave irradiation offers a novel wa...

In the context of microwave enhanced chemistry, usually a narrow set of single and multimode cavities are considered as microwave field applicators. This study aims to widen this current scope by exploring the potential of rectangular waveguides as a basis for microwave applicator systems. It is demonstrated that such systems offer microwave field...

This study reports for the first time on the application of microwave (MW) heating under ambient pressure reflux conditions to intensify the transesterification reaction of ethylene glycol (EG) with dimethyl carbonate (DMC) for the synthesis of ethylene carbonate (EC), a relevant industrial compound for polycarbonate products, lubricants and li‐ion...

Hydrocarbon chains produced as byproduct of natural gas extraction and petrochemical processing can be valorised into syngas/H2 and oxygenated fuels in a modular fashion through electrified modular plasma reactors. A plethora of configurations is available for light hydrocarbons reforming, with cold plasma assemblies emerging as the favourite optio...

Efficient particle processing during and downstream of a crystallization process is a paramount task in pharmaceutical industry regarding production of Active Pharmaceutical Ingredients (APIs). Due to mass transfer limitations, supersaturation is often not uniformly controlled in reactive crystallization processes generating an excessive amount of...

We study the selective catalytic hydrogenation of C2H2, the main product from non-oxidative CH4 coupling in gas-phase plasmas, to C2H4, a cornerstone of the global chemical industry, by experiments and temperature-dependent micro-kinetic modelling. The model is validated against new experimental data from a nanosecond pulsed plasma reactor integrat...

Continuous crystallization has gained substantial interest due to its high product reproducibility, high labor efficiency, and low capital and production costs. Continuous seeding is preferable and often even required in the application of pharmaceuticals, which presents a bottleneck in continuous crystallization. This work proposes to apply ultras...

Ultrasound intensifies a wide range of processes involving solids, enhancing control over their formation and post-treatment. This review summarizes key ultrasonic mechanisms and effects in solids' synthesis and processing.

Ultrasound has shown its benefits in the manufacturing processes of many pharmaceuticals and fine chemicals. This study focused on the reactive crystallization system of an aromatic amine and explored the potential uses of ultrasound in both batch and continuous modes. In batch experiments, we studied the effects of different sonication conditions...

In this work, novel numerical models were developed and validated to offer new strategies in modelling sonochemical reactors. More specifically, in our original approach the non-linear Louisnard model was coupled with heat and mass transport equations to predict gradients in temperature and species concentration in a sonicated reactor. Additionally...

Chemical valorization of methane (CH4) via modular electrified reactors could represent a profitable avenue for biogas producers. Ethylene (C2H4) is the most valuable product due to its large demand that results in high energy cost and carbon emissions. However, alternative electrified processes proposed so far cannot compete with the state-of-the-...

After decades of consistent growth, U.S. revolving credit declined drastically post 2009. We study the Ability to Pay provision of the Credit CARD Act of 2009, a policy that restricts credit card limits, as a contributing factor. Extending a model of revolving credit lines, we find that the policy accounts for 54–60% of the decline in revolving cre...

Carbon-negative electrified production of methanol (e-MeOH) can play a central role in sustainable chemical manufacturing in the coming years. In this context, CO 2-based methanol synthesis routes solely based on renewable electricity have been proposed. However, the production route via direct air-captured (DAC) CO 2 and green H 2 from water elect...

Plasma-assisted conversion of ethane (C2H6) can produce value-added chemical building blocks using green electricity. Here we employ a simple packed-bed coaxial dielectric barrier discharge (DBD) reactor to convert C2H6 at mild operating conditions unattainable by conventional thermocatalysis. Ethylene (C2H4), acetylene (C2H2), and methane (CH4) ar...

Electrification of ethylene production via methane coupling could represent a considerable step toward decarbonization of the chemical industry. Several promising electrified technologies can drive the reaction via different energy delivery modes (e.g. Joule and dielectric heating, mechanical/rotational energy, and plasma discharge). However, only...

The catalytic upgrading of post-consumer mixed polyolefinic waste pyrolysis vapors has been studied over five different solid acid catalysts to understand the effect of vapor-catalyst contact time and catalyst properties on the recovery of light olefins (C2-C4). HZSM-5 with Si/Al ratio of 11 and 27 produced the highest light olefins yields (83.4 wt...

In this work, mixtures of increasing viscosity (from 0.9 to ≈720 mPas) are sonicated directly using an ultrasonic horn at 30 kHz to investigate the effect of viscosity on the ultrasound field both from an experimental and numerical point of view. The viscosity of the mixtures is modified by preparing water-polyethylene glycol solutions. The impact...

The role of carbon dioxide in our changing climate is now hard to ignore, and many countries are making pledges to reduce or eliminate their carbon output. Chemical valorisation of carbon dioxide, as an alternative to sequestration, is likely to play an important part in reaching these targets, and as such is one of the fastest developing areas of...

The role of carbon dioxide in our changing climate is now hard to ignore, and many countries are making pledges to reduce or eliminate their carbon output. Chemical valorisation of carbon dioxide, as an alternative to sequestration, is likely to play an important part in reaching these targets, and as such is one of the fastest developing areas of...

Plasma processing can be one of the drivers of low-carbon footprint chemical manufacturing using renewable electricity. Several established commercial processes already rely on this technology in different production sectors, such as metallurgy, waste-to-energy, olefin production, and ozone production. Emerging plasma applications such as activatio...

Advanced plastic waste recycling via pyrolysis and subsequent steam cracking of pyrolysis oils has the potential to partly close the cycle between the petrochemical production of plastics and current end-of-life waste management (i.e., downcycling, incineration, landfilling). However, the greatest obstacle is the complex composition of real plastic...

In this work, we report for the first time on the influence of the quality of reactants and reaction conditions on the production of hydrophobically modified ethoxylated urethanes (HEURs) and selected prepolymers without the use of solvents. We show that the polyol water concentration is detrimental to the progress of the main urethane forming reac...

Artificial intelligence and specifically machine learning applications are nowadays used in a variety of scientific applications and cutting-edge technologies, where they have a transformative impact. Such an assembly of statistical and linear algebra methods making use of large data sets is becoming more and more integrated into chemistry and crys...

Lignocellulosic biomass conversion to renewable, carbon-neutral materials, fuels, and chemicals is the cornerstone of the transition to a sustainable future bioeconomy. Green energy in the form of electricity needs to be coupled with or substitute conventional thermally driven processes to realize small-scale, economically viable and environmentall...

Catalytic fast pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals at high selectivity. HZSM-5 zeolite with different properties, affected by Si/Al, mesoporosity, phosphorus stabilization, and steaming, were tested and thoroughly characterized. Different feeds, catalyst/feed ratios and reaction temperatures w...

The sustainability of novel electrified processes for methanol production, including plasma-assisted and electrically heated thermocatalytic dry methane reforming based processes, is assessed. Conceptual process design is applied to obtain the life cycle inventory data to perform the ex-ante life cycle assessment, with a focus on the climate change...

Microfluidic devices bring new opportunities in vast applications, in which the temperature control of nanoliter droplets is often required. Due to the inherit size of employed miniature volumes, several challenges exist related to temperature measurements. In this work, we present a non-contact and label-free dielectric-based temperature measureme...

Following the global trend towards increased energy demand together with requirements for low greenhouse gas emissions, Adaptable Reactors for Resource- and Energy-Efficient Methane Valorisation (ADREM) focused on the development of modular reactors that can upgrade methane‐rich sources to chemicals. Herein we summarise the main findings of the pro...

A novel continuous flow reactive crystallization process for the in situ on-demand access of single enantiomer crystals is reported and exemplified for a chiral pharmaceutical intermediate that crystallizes as a racemic conglomerate. During this process, a nearly racemic feed suspension with a racemization catalyst is circulated through a tubular c...

Revolving credit in the U.S. declined drastically in the last decade after several years of upward trending growth. We show that the Ability to Pay provision of the Credit CARD Act of 2009, which places restrictions on credit card limits, accounts for this decline. Extending a model of revolving credit to analyze this policy, we account for changes...

The search for sustainable alternatives for use in chemical synthesis and catalysis has found an ally in non-conventional energy sources and widely available green solvents. The use of glycerol, an abundant natural solvent, as an excellent “sacrificial” hydrogen source for the copper-catalyzed microwave (MW)-promoted transfer hydrogenation of nitro...

Herein, the pivotal role of secondary nucleation in a crystallization‐enhanced deracemization process is reported. During this process, complete and rapid deracemization of chiral conglomerate crystals of an isoindolinone is attained through fast microwave‐assisted temperature cycling. A parametric study of the main factors that affect the occurren...

Herein, the sustainability of plasma-assisted processes for ethylene production from rich-in-methane gas streams namely, natural and shale gas, is investigated by performing life cycle assessment (LCA). Two plasma-assisted process alternatives, a direct gas conversion to ethylene (one-step) and a stepwise gas conversion to acetylene followed by ace...

Widespread use of microwave technology requires quantitative scale-up models. Currently, microwave models are typical qualitative in nature. This work focuses on rendering models quantitative and elucidating the scale-up behavior of microwave heating using numerical simulations. A commonly used bench-scale microwave reactor from the CEM Corp. (CEM...

In this work, we present a novel electrical, non-intrusive and label-free temperature measurement technique for nanoliter water volumes. A planar interdigital capacitor (IDC) volumetrically interrogates the loaded water sample under test. A vector network analyzer (VNA) measures the reflection coefficient of the IDC, which senses temperature-depend...

Non-thermal microwave plasma reactors can efficiently split the CO2 molecule. However, big challenges remain before this technology becomes a feasible industrial technology. Computer modelling can be very useful to tackle such challenges. Detailed kinetic modelling is commonly used to get insights into the complex vibrational kinetics of CO2 as vib...

In this work, we demonstrate a semi-batch solid-state deracemization process for N-(2-chlorobenzylidene)-phenylglycine amide (NCPA), a complex chiral polymorphic system that involves three types of crystalline racemates (racemic compound and conglomerate forms I and II). In this process, gradually fed metastable racemic compound crystals are conver...

The aim of this review is to discuss the applications of microwave heating to separation and purification processes in chemical engineering from the perspective of its three unique features, namely rapid heating, selective heating and specific microscopic effects. These features render microwaves an alternative clean energy source that can replace...

The aim of this review is to discuss the applications of microwave heating to separation and purification processes in chemical engineering from the perspective of its three unique features, namely rapid heating, selective heating and specific microscopic effects. These features render microwaves an alternative clean energy source that can replace...

In this work, we establish a set of reactor operation guidelines for efficient non-oxidative methane coupling (i.e., high olefins yield) in nanosecond pulsed discharges and plate-to-plate electrode configuration. The reactor performance analysis suggests that the operating conditions directly affect the bulk gas temperature, which in turn determine...

In this work, we report on air/N2 gasification of a byproduct stream from an industrial fermenter in a tubular microwave plasma reactor to investigate the feasibility of the technology for organic compounds valorization, given the limited number of relevant works in the literature. In this context, an operating window regarding air/N2/biomass flow...

Crystallization is an important and widely used separation technique in the chemical and pharmaceutical industry. Control of the final particle properties is of great importance for these industries. The application of ultrasound in these crystallization processes, also referred to as sonocrystallization, has shown to impact nucleation, crystal gro...

We present a study on the reaction pathways involved in non-oxidative methane coupling in a nanosecond pulsed discharge reactor using isotope analysis. Specifically, plasma-assisted reactions with isotopes, serving as tracers, are performed in two ratios (CH4:D2=1:1 and CH4:D2=1:3) and elevated pressures (up to 5 bar). Acetylene hydrogenation react...

The electrification of the petrochemical industry, imposed by the urgent need for decarbonization and driven by the incessant growth of renewable electricity share, necessitates electricity-driven technologies for efficient conversion of fossil fuels to chemicals. Non-thermal plasma reactor systems that successfully perform in lab scale are investi...

Modelling approach that comprises a 2D self-consistent plasma model for discharge characterization in the first step and a 0D global plasma model for performance analysis in the second step.

It has been recently observed that a suspension of conglomerate crystals of both chiralities in contact with a solution where racemization takes place can undergo symmetry breaking due to periodic fluctuations in temperature. The interplay between the mechanisms behind this process is still a matter of debate, although there is consensus that growt...

Front Cover: The cover artwork illustrates the interaction of argon plasma with supported metal catalyst surfaces in a co‐axial nanosecond‐pulsed dielectric barrier discharge reactor. The impingement of the active plasma species onto the catalyst surface increases the active metal surface area, resulting in higher catalytic activity for selective a...

In this work, we report on low energy cost methane conversion to ethylene in a hybrid plasma-catalytic reactor system. Methane is first converted to acetylene, reaching up to 23.5% yield per pass, by a nanosecond pulsed discharge (NPD), and subsequently, acetylene is hydrogenated to ethylene using a Pd-based catalyst, which is placed in the post-pl...

We report that gas phase plasma-assisted non-oxidative methane coupling can lead to formation of ethylene as major product at ∼20% yield per pass. This is attained by using a nanosecond pulsed discharge (NPD) reactor, featuring rapid product quenching rates, (recyclable) hydrogen co-feeding (CH4:H2 = 1:1) and elevated pressures (5 bar) at which NPD...

In this work, we report on a Johnson Matthey-developed palladium (Pd)-based catalyst activity enhancement using nanosecond pulsed discharge treatment. The impact of treatment energy via pulse voltage, pulse frequency, and treatment time individual variation on acetylene-to-ethylene hydrogenation reaction performance is investigated. Up to 11.2% inc...

Rapid cooling and microwave heating substantially speed up temperature cycling-enhanced deracemization, while limiting the concomitant side reactions. During fast cooling, secondary nucleation is shown to enable deracemization.

In this study, we report on experiments designed to deconvolute the particle breakage kinetics and mechanism from the parallel phenomena (growth-dissolution, agglomeration) in attrition-enhanced deracemization processes. Through such experiments, we derived the specific breakage rates and cumulative breakage distribution functions for three grindin...

A series of ruthenium-doped strontium titanate (SrTiO3) perovskite catalysts were synthesized by conventional and microwave-assisted hydrothermal methods. The structure was analyzed by X-Ray diffraction (XRD) confirming the formation of the perovskite phase with some TiO2 anatase phase in all the catalysts. Microwave irradiation decreases the tempe...

Industrial synthesis of enantiopure compounds is nowadays heavily based on the separation of racemates through crystallization processes. Although the application of ultrasound in solution crystallization processes (sonocrystallization) has become a promising emerging technology, offering several benefits (e.g. reduction of the induction time and n...

Microwave plasma (MWP) technology is currently being used in application fields such as semiconductor and material processing, diamond film deposition and waste remediation. Specific advantages of the technology include the enablement of a high energy density source and a highly reactive medium, the operational flexibility, the fast response time t...

The aim of this paper is to produce spherical encapsulates of wheat gluten in a food‐grade biopolymer for preparing sheared meat analogs, to prevent instant fibrilization of the gluten during a pre‐mixing step. The hydrogel should release the gluten inside the Couette Cell, as a result of the higher temperature and shear in the process. Both sodium...

The growing surplus of green electricity generated by renewable energy technologies has fueled research towards chemical industry electrification. By adapting power-to-chemical concepts, such as plasma-assisted processes, cheap resources could be converted into fuels and base chemicals. However, the feasibility of those electrified processes at lar...

A community based sanitation system that processes human waste at an omnigasification plant is presented. The concept aims to destroy pathogens and generate energy from waste. The waste is dried, converted to syngas in a microwave assisted plasma gasifier and the gas produced is fed into a solid oxide fuel cell to produce electric power. In additio...

The complexity and challenges in non-contact temperature measurements inside microwave- heated catalytic reactors are presented in this paper. A custom-designed microwave cavity has been used to focus the microwave field on the catalyst and enable monitoring of the temperature field in 2D. A methodology to study the temperature distribution in the...

Synthesis of enantiopure compounds at industrially relevant scales is heavily based on the separation of racemates by means of crystallization processes, which are typically limited to a maximum 50% yield of the desired enantiomer. In cases for which the racemate crystallizes as a racemic mixture of conglomerate crystals, near equilibrium crystal r...

It has been recently observed that coupling Viedma ripening with a seeded in situ metastable racemic crystal to conglomerate transformation leads to accelerated and complete deracemization: crystal transformation-enhanced deracemization. By means of a simple kinetic model, we show that the mechanistic pathway of this new process depends profoundly...

The release of hydrogen from solid hydrides by thermolysis can be improved by nanoconfinement of the hydride in a suitable micro/mesoporous support, but the slow heat transfer by conduction through the support can be a limitation. In this work, a C/SiO2 mesoporous material has been synthesized and employed as matrix for nanoconfinement of hydrides....

A novel surface-wave microwave discharge reactor configuration to generate syngas via gaseous CO2 reduction with H2 (non-catalytic Reverse Water-Gas Shift reaction) is studied in the context of power-to-chemicals concept. Improvement of CO2 conversion to maximize CO production is explored by adding an external cylindrical waveguide downstream of th...

A systematic study of the conventional and microwave (MW) kinetics of an industrially relevant demethylation reaction is presented. In using industrially relevant reaction conditions the dominant influence of the solvent on the MW energy dissipation is avoided. Below the boiling point, the effect of MWs on the activation energy Ea and k0 is found n...

New insights into the obscure mechanisms of solid-state deracemization phenomena are obtained by crystal ripening experiments that, contrary to standard techniques, exclude attrition enhancement (grinding). The results point out that small particles and an initial size imbalance between the two enantiomeric crystal populations can intensify the rat...

The use of renewable energy (RE) to transform CO2 into commodities (i.e.CO2 valorization) will pave the way towards a more sustainable economy in the coming years. But how can we efficiently use this energy (mostly available as electricity or solar light) to drive the necessary (catalytic) transformations? This paper presents a review of the techno...

Plasma reactor technologies have the potential to enable storage of green renewable electricity into fuels and chemicals. One of the major challenges for the implementation of these technologies is the energy efficiency. Empirical enhancement of plasma reactors' performance has proven to be insufficient in this regard. Numerical models are therefor...

Photo-flow chemistry has become an important research area due to the ability of the technology to boost reaction rates and productivity. This field unites the mass transfer enhancement of flow chemistry with the high energy field density of microstructure geometries. However, even though the space-time yield increases dramatically with microphotor...

Available online xxx Keywords: CO 2 utilization Microwave plasma reactor Reverse water-gas shift a b s t r a c t In the context of converting electricity into value-added chemicals, the reduction of carbon dioxide (CO 2) with hydrogen (H 2) in a surface-wave-induced microwave plasma discharge, so-called surfatron, was investigated. The effect of di...

Microwave chemistry has been investigated for nearly thirty years with many notable results being published on apparent process enhancement due to microwave exposure. Conclusive proof of beneficial microwave-chemical interactions is lacking though, as are design rules for successful implementation of microwave-chemical processing systems. In this c...