Experimental Methods in Catalytic Research. Preparation and Examination of Practical Catalysts
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The relative amount of pyridine adsorbed at the different positions in the different samples is quantified by determining the ratio of the pyridine to mordenite band intensity from the Raman spectra. The latter is achieved by measuring single catalytic turnovers using the fluorescence microscope after addition of an aqueous furfuryl alcohol FFA solution. Localization and accumulation of the individual fluorescent reaction products results in nanoscale activity maps.
The different MOR crystals that make up the aggregate are pseudocolored as a guide for the eye and to simplify the later discussion. Here, the small blue H-MOR crystal protrudes from the larger orange crystal, and on the opposite side a large defect is present. Judging from the shape of the defect, it likely results from another intergrown crystallite that has broken off. After this detailed morphological characterization, FFA was added to the SP-MOR cluster, enabling the visualization of single acid-catalyzed turnovers by recording the fluorescent FFA oligomer formation.
Since the optical focus was centered around the middle of the aggregate, the recorded catalytic events originate from an optical slice around the center of the aggregate of approximately nm thickness Scheme S1. This is also apparent from the location of the recorded catalytic events with respect to the scanning electron micrograph. The majority of the observed catalytic events are clustered in a zone about nm wide, delineating the outer surface of the aggregated particle.
This is indicative of small port behavior as the size of the FFA reagent molecule should enable diffusion through the 12MR channels molecular diameter is about 6. The observed small-port behavior is therefore assumed to be an effect of mass transport limitations rather than pore blocking, which might gradually occur by accumulation of oligomers. In the latter case, the catalytic activity would decrease during the experiment. Second, varying numbers of turnovers are observed within the active zone.
In the main crystal orange , most of the acid-catalyzed oligomerization reactions are limited to two opposite crystal facets. On the basis of the crystallographic structure, in combination with the morphology of the crystals, these slightly curved and roughened facets are identified as the crystallographic facets where the 12MR pores are surfacing. At the other facets, the microporous zeolite framework, and, hence, the catalytically active sites, are not accessible toward the FFA substrate.
Note that the sparse turnovers detected at these facets can be related to acid sites at the outer surface or minute crystal imperfections. Surprisingly, also a significant number of catalytic turnovers is observed away from the outer surface and inside the main crystal, in the zone underneath the intergrown crystal at the top. The width of this catalytically active zone is comparable to that at the facets at the crystal exterior. Judging from the shape of this catalytically active area, it can be assigned to the interface between the intergrown crystals, i. Note that the activity of this region is similar to that observed at the readily accessible facets and that no activity gradient is observed from the outside of the crystal toward the most deeply buried part of the intergrowth.
It can therefore be concluded that the extra-framework porosity at this interface is sufficient to allow reagent molecules to readily reach the active sites along the intergrowth. A direct comparison clearly demonstrates that the structural complexity of small particles in typical zeolite samples cannot be captured by diffraction limited optical imaging. The crystal at the top part green is a nearly complete crystal with an obvious defect at the top. Again, from the shape of the defect, it seems to have resulted from a broken off intergrown crystallite.
Judging from the morphology of the particle vide supra , the 12MR micropores run perpendicular to the rough top facet along the optical z axis. In the center of the aggregate, another large crystal orange is present, perpendicular to the top green crystal.
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The 12MR pores in this crystal run parallel to the imaging plane, connecting the curved facets where two more protruding crystals red and blue are present. The slightly different orientation of these two protruding crystals suggests that both are independent of each other. The interface of the central crystal orange with the intergrown crystals red and blue results in a highly active x-shaped zone of activity.
Additionally, significant catalytic activity is observed at the interface between the central orange and top green crystals. The width of the catalytically active zones, measuring about nm, and the absence of an activity gradient within this zone is in line with earlier observations on the first aggregate. The absence of catalytic activity in the upper green crystal can be explained by the orientation of the crystal; the 12MR micropores run parallel to the propagation direction of the excitation light optical z axis.
As the fluorescent product molecules are spatially constrained within the microporous structure, the molecules will be aligned along the micropore orientation. This results in a minimal overlap between the electric field vector of the excitation light and the transition dipole moment of the fluorescent product molecules. Clearly, the absence of a perfect coalignment between the crystal lattices of the intergrown crystals results in a local discontinuity of the zeolite framework at the boundary between these SP-MOR crystals.
The resulting extra-framework porosity facilitates molecular diffusion and leads to an enhanced molecular accessibility. As such, this results in a zone of high catalytic activity at the interface between the individual crystallites, away from the outer surface. These data clearly underline the importance of intergrowths on the overall performance of these H-MOR zeolites. At the same time, these results, showing activity away from the outer surface, also indicate that an aluminum gradient cannot be the origin of the observed activity profile, i.
On the other hand, it cannot be deduced from these observations whether this catalytic activity is only linked to acid sites within the SP-MOR micropores or if some portion of the activity is linked to defect sites at the highly defected interface. This is important, since the latter would have a negative impact on the shape selectivity. The orientation of fluorescent reaction products can be straightforwardly visualized by implementing linear polarized excitation light.
In this cluster, the 12MR pores of the respective crystals are oriented almost perpendicular to each other. The aggregate itself was oriented in such a way that the 12MR pores of the individual crystals are along the optical x , y , and z axes. As the catalytic activity map is a 2D projection of single turnovers taking place within a nm-thick optical slice vide supra , the width of the projection will depend on the relative orientation of the intergrowth structure to the optical section.
Clearly, this interface does not follow any of the crystallographic axes of the other two intergrown crystals. This investigation reveals that the formed fluorescent product molecules at the facets of the respective crystals are mostly confined to their respective 12MR pores. When the excitation polarization is oriented along the direction of the 12MR pores of the orange crystal i. Even though the catalytic reaction still occurs, the fluorescent product molecules are no longer efficiently excited, as the polarization direction of the excitation light is perpendicular to the orientation of the molecular excitation transition dipole moment.
In this case, the blue crystal exhibits the majority of the catalytic activity, and only a fraction remains excitable and detectable in the orange crystal. Surprisingly, the same observation holds for the fluorescent product molecules formed at the intergrowth regions of the two crystals with the middle green crystal.
As the 12MR pores of the latter crystal are oriented along the optical z axis, no significant contribution to the catalytic activity map is expected from the molecules confined within the microporous structure. Therefore, this is direct proof of the pore-confinement of reaction products near crystalline defects such as intergrowth interfaces. The color intensity of each pixel provides a qualitative indication of the relative number of turnovers occurring in this zone.
This value is obtained by determining the ratio of the number of fluorescent product molecules oriented along the local microporous structure, to the total number of catalytic turnovers observed over the catalyst particle. As such, this number represents the shape selectivity of the individual catalyst particle based on the FFA probe reaction.
The reaction-pore confinement determined during this experiment reveals that active sites are predominantly confined within the microporous structure as reaction products are preferentially oriented along the one-dimensional 12MR pores. However, catalytic activity is observed on all intergrowth structures, regardless of the crystallographic facets that are included. The formation of the crystallographic surface is therefore assumed to be incomplete where intergrowth formation has taken place. This leads to an opening up of the microporous structure. Additionally, active sites accessible through intergrowth structures are as reactive as those accessible through the intrinsic pore mouths on the facets.
This is an indication that mass transport inside intergrowths is unrestricted. As such, intergrowth structures are regarded as microporous voids that facilitate mass transport toward catalytically active sites located within the opened up crystallographic framework. One method commonly applied to enhance molecular transport in SP-MOR catalysts is acid leaching, removing both framework and extra-framework aluminum species hindering molecular transport along the 12MR pores. Already in , Raatz et al.
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N 2 -physisorption revealed a small increase in micropore- and external surface area. Finally, also a bulk scale naphthalene isopropylation reaction was performed on the mordenite catalysts. As such, the reactivity trends and shape selectivity properties, observed at the single particle level, before and after acid leaching, could be validated using an industrially relevant bulk scale process.
A summary of these bulk scale experiments can be consulted in the Supporting Information. A combination of the newly developed iFlEM approach with the analysis tools introduced above provides a novel way to investigate the effects of acid leaching beyond the bulk scale. It is generally accepted that bases are required for either substrate activation exemplified by transmetallation in the Suzuki cross-coupling , or HX capture e.
Ni NHC 2 X 2 complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O—NHC coupling to give azolones and metallic nickel under the action of t -BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M II into "ligandless" M 0 species.
A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including 18 O labeling experiments. Inorganic and organic "solvent-in-salt" SIS systems have been known for decades but have attracted significant attention only recently. SIS with organic components for example, ionic liquids containing small amounts of water demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents.
Water molecules tend to form nano- and microstructures droplets and channel networks in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.
A complete cycle from cellulose to printed object has been performed. The studied PEF polymer has shown key advantages for 3D printing: optimal adhesion, thermoplasticity, lack of delamination and low heat shrinkage. The high thermal stability of PEF and relatively low temperature that are necessary for extrusion are optimal for recycling printed objects and minimizing waste. Several successive cycles of 3D-printing and recycling were successfully demonstrated. The suggested approach for extending additive manufacturing to carbon neutral materials opens a new direction in the field of sustainable development.
In the present review, we consider the transformations of molecular catalysts, leaching, aggregation and various interconversions of metal complexes, clusters and nanoparticles that occur during catalytic processes. The role of catalyst evolution and the mechanistic picture of "cocktail"-type systems are considered from the perspective of the development of a new generation of efficient, selective and re-usable catalysts for synthetic applications.
Rational catalyst development and the improvement of catalyst performance cannot be achieved without an understanding of the dynamic nature of catalytic systems. Ionic liquids are remarkable chemical compounds, which find applications in many areas of modern science. Because of their highly tunable nature and exceptional properties, ionic liquids have become essential players in the fields of synthesis and catalysis, extraction, electrochemistry, analytics, biotechnology, etc. Apart from physical and chemical features of ionic liquids, their high biological activity has been attracting significant attention from biochemists, ecologists, and medical scientists.
This Review is dedicated to biological activities of ionic liquids, with a special emphasis on their potential employment in pharmaceutics and medicine. The accumulated data on the biological activity of ionic liquids, including their antimicrobial and cytotoxic properties, are discussed in view of possible applications in drug synthesis and drug delivery systems. Dedicated attention is given to a novel active pharmaceutical ingredient-ionic liquid API-IL concept, which suggests using traditional drugs in the form of ionic liquid species.
The main aim of this Review is to attract a broad audience of chemical, biological, and medical scientists to study advantages of ionic liquid pharmaceutics. Overall, the discussed data highlight the importance of the research direction defined as "Ioliomics", studies of ions in liquids in modern chemistry, biology, and medicine. Environmental profiles for the selected metals were compiled on the basis of available data on their biological activities. Analysis of the profiles suggests that the concept of toxic heavy metals and safe nontoxic alternatives based on lighter metals should be re-evaluated.
Comparison of the toxicological data indicates that palladium, platinum, and gold compounds, often considered heavy and toxic, may in fact be not so dangerous, whereas complexes of nickel and copper, typically assumed to be green and sustainable alternatives, may possess significant toxicities, which is also greatly affected by the solubility in water and biological fluids. Overall, the available experimental data seem insufficient for accurate evaluation of biological activity of these metals and its modulation by the ligands.
Boğaziçi University - Department of Chemical Engineering
Spectral studies revealed the presence of a specific arrangement of 5-hydroxymethylfurfural 5-HMF molecules in solution as a result of a hydrogen—bonding network, and this arrangement readily facilitates the aging of 5-HMF. A highly selective synthesis of a 5-HMF derivative from glucose was achieved using a protecting group at O 6 position. Water-containing organic solutions are widespread reaction media in organic synthesis and catalysis. Research on catalysts can also aim to identify novel catalysts that replace toxic metal oxides with environmentally benign, catalytically active metals; and materials that selectively remove hydrogen from the reaction milieu by using hydrogen-permeable membranes.
With the shift from naphtha as a feedstock in the United States, butadiene production may not be able to meet future demand, so an area that Lemonidou believes merits special attention is butadiene synthesis from light alkanes. There is a desire, she said, to exploit new processes for butadiene production, and she suggested two potential routes: ethylene dimerization followed by oxidative dehydrogenation and a one-step oxidation of butane to butadiene.
She also highlighted the lack of research on alternative oxidants for oxidative dehydrogenation, including the use of carbon dioxide as a mild oxidant Ascoop et al. Lemonidou concluded her remarks with her perspective on opportunities in this area. For propane to propylene, the immediate target should be to increase yield given that high selectivity is difficult with the known catalyst and using oxygen as the oxidant. Current yields of butane to butadiene are low, so near-term work should focus on identifying catalysts that can boost yields to acceptable levels.
The goal of all of this research on oxidative dehydrogenation, she said, should be to minimize the deep oxidation of alkanes and sequential oxidation of the resulting olefins. Approaches to achieving this goal include designing catalyst surfaces that adsorb weakly to the formed olefin, controlling active site density, and keeping the partial pressure of oxygen low, perhaps through the design of membrane-based reactors, or by decoupling the reduction and oxidation steps. A major obstacle is that the technologies currently used are mature with acceptable selectivity, and the capital costs of implementing a new technology are considerable.
For PDH, one challenge is to develop enough of a knowledge base to enable the rational design of selective and stable catalysts, which Marks added is an overarching theme for the entire workshop. Another challenge is to achieve similar selectivity but with a lower carbon footprint than oxidative dehydrogenation, and to do so with simpler reactors requiring smaller capital expenditures.
For dienes, the discussion focused on direct routes from butane to butadiene. With regard to the top two to three well-established research approaches to making alkenes and dienes, Marks reported that the group discussed identifying catalysts that would improve the carbon selectivity of oxidative dehydrogenation while maintaining acceptable turnover rates and frequencies.
Alternative oxidants might be able to address this challenge, as might novel reactor designs such as circulating fluid bed and short-contact-time reactors. The discussion on promising but higher-risk novel approaches produced a long list of ideas that Marks characterized as a good guide for developing a research program.
The list included. This group also identified a long list of new tools and scientific advances that are creating important research opportunities. This list included. With regard to the industrial requirements and environmental constraints that researchers are responsible for knowing when developing new approaches to utilizing natural gas, an overarching impediment as reported by Marks is reducing the energy requirements of any process. With oxidative dehydrogenation reactor design, safety is a critical issue given how much heat these reactions produce.
The group noted that the national laboratories have facilities to test novel reactor designs safely and that researchers could collaborate with those laboratories when it comes to testing design prototypes. When incorporating oxidative dehydrogenation chemistry with other processes, or considering the use of alternative oxidants, Marks added that there is value for researchers to think about scalability and environmental viability. According to Marks, the last constraint that researchers should consider is critically important in the real world of the chemical industry—the cost of capital.
Specifically, this group suggested that lower capital costs in implementing a new technology compared with an existing technology could reduce the risk and produce a 15 to 20 percent return on investment. In his opening presentation to this working group, Bruce Gates, distinguished professor of chemical engineering and materials science at the University of California, Davis, said that the conversion of propane to aromatics is less uphill thermodynamically than conversion of methane to aromatics, which had been discussed by one of the first four working groups.
Various groups have reported success converting light alkanes to aromatics using acidic zeolites and zeolite-supported metals such as zinc, gallium, and molybdenum, but in his opinion these have been incompletely characterized. The metal atoms, for example, may be present as carbides or oxycarbides, or they may not be in a metallic state.
It also appears, he said, that the metal atoms are both inside and outside of the zeolite pores. These catalysts, he added, coke rapidly and require frequent regeneration, which might contribute to catalyst deactivation. Numerous authors, said Gates, have suggested that alkane dehydrogenation is a slow reaction catalyzed by molybdenum and that the subsequent oligomerization and cyclization are catalyzed by the acidic zeolite sites. The resulting product stream of benzene, toluene, and mixed xylenes can be recovered without an extraction unit or sent to an aromatics complex for conversion of the toluene and mixed xylenes into benzene and p-xylene.
The yield of this process, said Gates, is reported to be 58 to 60 percent. Rapid catalyst. Gates noted that this reaction resembles naphtha reforming but without the acidic function in the catalyst. Gates noted there are opportunities for discovering improved catalysts, including the use of metal-containing molecular sieves that have been shown to catalyze reactions such as hydrogenation and dehydrogenation. This is a large and growing class of catalytic materials, he explained, though many of these materials have not been well characterized and are not uniform structurally.
He also explained that catalytic performance in any alkane-to-aromatic reaction scheme developed so far depends strongly on the structure of the metal-containing species. As a result, there is an opportunity for chemists to explore that structure—activity relationship with an eye on improving catalyst design through the many synthetic routes that have been developed to tune catalyst structure and other properties that influence activity. These synthetic routes include organometallic syntheses and atomic layer deposition, the latter. Gates wondered if there were opportunities to use that type of approach for synthesizing well-defined catalysts containing metals such as zinc, gallium, and molybdenum in zeolites, and to create single-site catalysts.
As a conclusion to his presentation, Gates enumerated several possible directions for research. One approach would be to vary the metal or combination of metals in molecular sieves of different pore structures and sizes. Another avenue for research would be to attempt to tailor metal-containing catalytic sites on or in a molecular sieve framework, either as single sites or multi-atom clusters.
He also suggested a research effort aimed at understanding the chemistry of catalyst synthesis and at relating catalytic activity, selectivity, and stability to structure using theory and spectroscopy with functioning catalysts. Gates noted the lack of, repeated frequently during the workshop, developing a deeper characterization of catalysts. He also thought it worthwhile to investigate processes that would use methane in combination with other feedstocks to produce aromatics.
Research approaches that could make production of aromatics from alkanes viable included varying the metal in the zeolite, which the discussion noted has been the subject of several patents involving the use of rhenium and tungsten. Other approaches would be to vary the zeolite structure and to balance the metal and acid function in the zeolite. With regard to the second of these, the working group discussed the possibility of speeding up the rate-limiting dehydrogenation step by using a gallium or zinc catalyst, but then it speculated that perhaps it was important for this step to be slow so that too much olefin did not accumulate in the zeolite pores so as to prevent higher oligomerization and runaway reactions.
Promising but higher-risk novel approaches described by this working group included the use of confinement-based catalysts to steer selec-. With regard to this last possibility, Lercher noted there has been conflicting data in the literature so it was not clear whether this approach was a real possibility for industrial application or merely an interesting research project. This working group discussed a long list of research opportunities, many of which, said Lercher, reiterate what other groups have proposed:.
While electrochemical catalysis has potential as a means of converting hydrocarbons into value-added products, Pez said, one of its main limitations is the high relative cost of using electricity as a reagent to drive endothermic hydrocarbon conversion processes. There are a number of thermodynamically feasible fuel cells for chemicals and energy cogeneration, said Pez. These include ethane plus oxygen to ethylene and water; methane coupling in the presence of oxygen to produce ethane and water or ethylene and water; and methane in the presence of oxygen to produce methanol.
One group Liu et al. The published fuel cell was a 0. This system produced no carbon dioxide, so in a zero-carbon environment, it is possible to consider such as a system as a replacement for steam cracking of ethane, said Pez. A methane to ethane and ethylene fuel cell has also been reported Kiatkittipong et al. This system achieved 91 percent selectivity for ethane and ethylene, with the relative amount of these two products varying with temperature. At 1, K, this fuel cell produces ethylene almost exclusively, with only trace amounts of ethane, carbon monoxide, and carbon dioxide. There are also published reports of electrocatalytic conversion of methane to methanol Fan, ; Lee and Hibino, ; Spinner and Mustain, , but these systems required energy input.
Electrocatalysis does not have to happen solely in the context of a fuel cell. It is possible, said Pez, for electrochemistry to promote catalysis or modify catalytic activity Katsaounis, Others have used spark discharge Kado et al. Turning to the subject of biocatalysis, Koffas said that methane is an excellent source of carbon and energy for microorganisms known as methanotrophs, which historically have been used for producing feed-grade biomass. These bacteria are capable, he explained, of converting methane into protein, alkanes, alcohols, sugars, dicarboxylic acids, and other higher-value chemicals such as carotenoid pigments and vitamins.
Currently, a plant in Norway is producing , tons per year of methanol and 10, tons per year of protein for animal feed from crude methane using the microorganism Methylococcus capsulatus. One area of industrially motivated research aims to produce carotenoid pigments and antioxidants using a microorganism known as Methylomonas sp. The genome of this organism has been sequenced, said.http://sivamont.hu/tcnicaejercicio-de-cmo-mantenerse-y-lucir-37-permanentemente.php
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Today, production is dominated by chemical synthesis, but researchers have engineered the organism to produce a variety of these valuable compounds and are now working to boost production to economically viable levels. The discussion on electrocatalysis covered conversion of C1, C2, and C3 hydrocarbons to chemicals by use of electricity including limiting process economics. Methods covered were direct hydrocarbon fuels cells, electrically promoted catalysis, methane to syngas via electroreforming and methane to C2 hydrocarbons via electrical plasma processes.
In recapping the discussion on electrocatalysis, Monty Alger from Pennsylvania State University said that the outcome of discussion was straightforward: Researchers are working on fuel cells, others are doing work on materials development, a third group is studying electrocatalysis, and none of these groups is talking to one another, a point that he noted had been raised throughout the workshop. As far as specifics, the working group voiced interest in these electrochemical processes but the concern was that these technologies may not be viable at an industrial scale because of the difficulty in scaling the electrocatalytic systems and operating them at scale.
Another barrier to commercial viability is the high expected cost of building industrial scale electrocatalytic reactors, whether they are fuel cells or systems based on non-Faradaic electrochemical modification of catalytic activity. At one point in the discussion, Alger recounted, that it was mentioned that there has been a substantial body of work on ceramic membrane technology developed in recent years that could present an opportunity to advance an integrated solution for overall chemical conversion using electrochemical means.
He noted that the opportunities that could result from merging membrane research and electrocatalysis are substantial and could lead to entirely new processes for chemical conversion. The discussion also pointed out that the drivers for fuel cell development are different than for catalysis. Some in this working group stressed that the fuel cell community will not solve the challenges to developing industrial scale processes without collaborating with the catalyst, materials, and engineering communities.
However, the group also recognized that funding is not available today for critical research on materials, such as transport membranes and mixed conductors, which will present challenges to collaboration. Others in. The discussion about biocatalysis began with working group members pointing out that biocatalysis is being used in small-scale commercial processes. The group also noted that, at least theoretically, anything that can be made biologically could be made from methane given enough time and money to do the necessary metabolic engineering. The resulting challenge, then, will be to select the best opportunities to pursue.
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However, one qualifier for that selection would be that the resulting biocatalytic process converts methane into chemicals with no carbon dioxide generation. That would be a unique outcome with a unique value proposition, Alger reported. A possibility the group mentioned was to couple biocatalysis with electrocatalysis to invent processes that convert methane to chemicals without generating carbon dioxide or water. A challenge for research in this area is addressing overall economics to be viable for long-term commercialization.
One of the biggest impediments to commercialization of biocatalytic routes is their poor yield of product. Therefore, improving yields, kinetics, reaction rates, and process costs related to separations will be critical for any commercially viable process to come out of biocatalysis research. Another confounding issue for biological systems is the potential impact of natural gas impurities on the microorganisms.
One potential advantage of biosynthetic approaches to alkane modification is the possibility of making materials not currently accessible in high volumes or entirely new materials for which markets could be developed. Such systems may also be more economically viable at smaller scales than current industrial chemical processes, which could be important for utilizing stranded and flared gas. Biocatalytic systems may also have lower energy demands, though the cost of separating product from a biological reactor could negate any energy-related savings. One of the main drivers of developing nontraditional oxidants for activating natural gas is the benefit of eliminating carbon dioxide emissions associated with electricity production, transportation, and chemical, agricultural, and other industrial processes, said Eric McFarland, professor of chemical engineering at the University of California, Santa Barbara.
In the area of alkane conversion, it has so far proven impossible to partially oxidize alkanes with oxygen at high rates and low cost without producing carbon dioxide, he noted. As an example, converting methane to syngas for the production of methanol and other chemicals produces between 0. Aside from the issue of carbon dioxide emissions, McFarland said there is another reason to look at alternative oxidants for hydrocarbon conversion, which is to make the best use of the chemical potential stored in the carbon—hydrogen bond. Among the potential alternative oxidants McFarland listed were sulfur Zhu et al.
The halogens—chlorine, bromine, and iodine—are quite effective, he said, at oxidative dehydrogenation, which is why they are used as flame retardants. This has been known, said McFarland, since the late s Rust and Vaughan, In the s, Shell developed a dehydrogenation process using molten iodine salts Sanborn et al. While working with halogens presents some engineering challenges, halogen chemistry is practiced safely and profitably on massive scales, McFarland noted.
In fact, said McFarland, the catalytic reoxidation of the hydrogen halide by oxygen to produce the molecular halogen can be used to generate heat or electricity. This mixture then passes over a solid metal. The resulting solid metal bromide is regenerated with oxygen to produce metal oxide plus bromine for reuse Lorkovic et al. This reaction scheme can be used to convert ethane to ethylene, propane to propylene, and butane to butene at greater than 95 percent selectivity. The unresolved issues with these processes include the ability to regenerate the solid metal oxide, the reactive capacity of the solid metal oxide, and hydrocarbon stability over the solids.
To address these and other challenges, McFarland and his collaborators changed their approach, using a molten halogen salt to generate the halogen in situ see Figure , to control heat exchange, and to absorb and transport bromine and oxygen. This approach reduced the system complexity, he explained. Group rapporteur James Stevens, recently retired as the Dow Distinguished Fellow at The Dow Chemical Company, began the discussion with the comment that over the years he had seen numerous examples of methane activation with nontraditional oxidants involving the conversion of methane to methyl-X, where X is a leaving group, and that if he were to poll the workshop participants, each one could probably identify one leaving group that someone in industry or academia had tried and.
Having said that, he reported that the working group discussed a number of impediments for nontraditional oxidants to become commercially viable, including the lack of a tax on carbon dioxide emissions and the risk-aversive nature of the chemical industry with regard to new technology. The group also noted that current technology can work well in an environment where there is no penalty for emitting carbon dioxide. The group noted that dealing with corrosion issues in strong electrolyte environments raises the engineering and material demands and unknown safety issues, but these are not insurmountable if the economics become favorable.
For example, the chemical industry has extensive experience in handling halogens and strong acids, and so the use of bromine to form methyl bromide and HBr, followed by coupling the methyl bromide to ethylene or other hydrocarbons, would probably not be a technical challenge for industry. There was a discussion on the use of oxygen in a cycle to form more selective oxidants, thereby moderating the selectivity. However, the relative lack of research on how to use oxygen to form more selective oxidants at industrial scales was also noted in the discussion as an obstacle to progress.
With regard to well-established research approaches for activating natural gas with nontraditional oxidants, this group pointed out that processes using halogens to make, for example, methyl chloride and methyl bromide, which would serve as intermediates to make value-added hydrocarbon products have been piloted by numerous companies. The use of bromine as a nontraditional oxidant for methane coupling has several advantages, particularly because the heat of reaction of methane with bromine is much lower than that with oxygen, while still being an exothermic reaction, which has the potential to make the bromination reaction more selective.
In addition, the use of halogens as nontraditional oxidants has the potential to make product separation easier. The use of halogens to convert methane to ethylene and other hydrocarbons has not been commercialized yet, primarily because of economic rather than technical reasons.