Most frequently, industrial hydrogenation relies on heterogeneous catalysts. The largest scale application of hydrogenation is for the processing of vegetable oils. Their partial hydrogenation reduces most, but not all, of these carbon-carbon double bonds. The degree of hydrogenation is controlled by restricting the amount of hydrogen, reaction temperature and time, and the catalyst.
Hydrogenation converts liquid vegetable oils into solid or semi-solid fats, such as those present in margarine. Changing the degree of saturation of the fat changes some important physical properties, such as the melting range, which is why liquid oils become semi-solid. Solid or semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product.
Because partially hydrogenated vegetable oils are cheaper than animal fats, are available in a wide range of consistencies, and have other desirable characteristics such as increased oxidative stability and longer shelf life , they are the predominant fats used as shortening in most commercial baked goods.
A side effect of incomplete hydrogenation having implications for human health is the isomerization of some of the remaining unsaturated carbon bonds to their trans isomers. Trans fats resulting from partial hydrogenation have been implicated in circulatory diseases including heart disease. Many countries and regions have introduced mandatory labeling of trans fats on food products and appealed to the industry for voluntary reductions.
In petrochemical processes, hydrogenation is used to convert alkenes and aromatics into saturated alkanes paraffins and cycloalkanes naphthenes , which are less toxic and less reactive. Relevant to liquid fuels that are stored sometimes for long periods in air, saturated hydrocarbons exhibit superior storage properties. On the other hand, alkene tend to form hydroperoxides , which can form gums that interfere with fuel handing equipment. For example, mineral turpentine is usually hydrogenated. Hydrocracking of heavy residues into diesel is another application.
In isomerization and catalytic reforming processes, some hydrogen pressure is maintained to hydrogenolyze coke formed on the catalyst and prevent its accumulation. Hydrogenation is a useful means for converting unsaturated compounds into saturated derivatives. Substrates include not only alkenes and alkynes, but also aldehydes, imines, and nitriles,  which are converted into the corresponding saturated compounds, i. Thus, alkyl aldehydes, which can be synthesized with the oxo process from carbon monoxide and an alkene, can be converted to alcohols. Xylitol , a polyol , is produced by hydrogenation of the sugar xylose , an aldehyde.
Primary amines can be synthesized by hydrogenation of nitriles , while nitriles are readily synthesized from cyanide and a suitable electrophile. The French chemist Paul Sabatier is considered the father of the hydrogenation process. In , building on the earlier work of James Boyce , an American chemist working in the manufacture of soap products, he discovered that traces of nickel catalyzed the addition of hydrogen to molecules of gaseous hydrocarbons in what is now known as the Sabatier process.
For this work, Sabatier shared the Nobel Prize in Chemistry. Wilhelm Normann was awarded a patent in Germany in and in Britain in for the hydrogenation of liquid oils, which was the beginning of what is now a worldwide industry. The commercially important Haber—Bosch process , first described in , involves hydrogenation of nitrogen. In the Fischer—Tropsch process , reported in carbon monoxide, which is easily derived from coal, is hydrogenated to liquid fuels. In , Voorhees and Adams described an apparatus for performing hydrogenation under pressures above one atmosphere.
In Murray Raney developed a finely powdered form of nickel, which is widely used to catalyze hydrogenation reactions such as conversion of nitriles to amines or the production of margarine. In the s, Calvin discovered that copper II complexes oxidized H 2. The s witnessed the development of well defined homogeneous catalysts using transition metal complexes, e. Soon thereafter cationic Rh and Ir were found catalyze the hydrogenation of alkenes and carbonyls.
For most practical purposes, hydrogenation requires a metal catalyst. Hydrogenation can, however, proceed from some hydrogen donors without catalysts, illustrative hydrogen donors being diimide and aluminium isopropoxide , the latter illustrated by the Meerwein—Ponndorf—Verley reduction. Some metal-free catalytic systems have been investigated in academic research.
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One such system for reduction of ketones consists of tert -butanol and potassium tert-butoxide and very high temperatures. A chemical kinetics study  found this reaction is first-order in all three reactants suggesting a cyclic 6-membered transition state. Another system for metal-free hydrogenation is based on the phosphine - borane , compound 1 , which has been called a frustrated Lewis pair. It reversibly accepts dihydrogen at relatively low temperatures to form the phosphonium borate 2 which can reduce simple hindered imines.
The reduction of nitrobenzene to aniline has been reported to be catalysed by fullerene , its mono-anion, atmospheric hydrogen and UV light. The original and still a commonly practised form of hydrogenation in teaching laboratories, this process is usually effected by adding solid catalyst to a round bottom flask of dissolved reactant which has been evacuated using nitrogen or argon gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then supplied from a H 2 -filled balloon. The resulting three phase mixture is agitated to promote mixing. Hydrogen uptake can be monitored, which can be useful for monitoring progress of a hydrogenation.
This is achieved by either using a graduated tube containing a coloured liquid, usually aqueous copper sulfate or with gauges for each reaction vessel. Since many hydrogenation reactions such as hydrogenolysis of protecting groups and the reduction of aromatic systems proceed extremely sluggishly at atmospheric temperature and pressure, pressurised systems are popular. In these cases, catalyst is added to a solution of reactant under an inert atmosphere in a pressure vessel.
Hydrogen is added directly from a cylinder or built in laboratory hydrogen source, and the pressurized slurry is mechanically rocked to provide agitation, or a spinning basket is used. Flow hydrogenation has become a popular technique at the bench and increasingly the process scale. This technique involves continuously flowing a dilute stream of dissolved reactant over a fixed bed catalyst in the presence of hydrogen.
Elevated temperatures may also be used. At the bench scale, systems use a range of pre-packed catalysts which eliminates the need for weighing and filtering pyrophoric catalysts. Catalytic hydrogenation is done in a tubular plug-flow reactor PFR packed with a supported catalyst. The pressures and temperatures are typically high, although this depends on the catalyst. Catalyst loading is typically much lower than in laboratory batch hydrogenation, and various promoters are added to the metal, or mixed metals are used, to improve activity, selectivity and catalyst stability.
The use of nickel is common despite its low activity, due to its low cost compared to precious metals. Gas Liquid Induction Reactors Hydrogenator are also used for carrying out catalytic hydrogenation. From Wikipedia, the free encyclopedia. Chemical reaction between molecular hydrogen and another compound or element. Main article: Bergius process. Carbon neutral fuel Dehydrogenation H-Bio Hydrogenolysis Hydrodesulfurization , hydrotreater and oil desulfurization Josiphos ligands Timeline of hydrogen technologies Transfer hydrogenation Rhodium-catalyzed hydrogenation Trans fats.
Reductions in Organic Chemistry. Washington, D. Of Chem. Journal of Molecular Catalysis.
In fact, this may induce the formation of surface structures not related to the catalytic processes in conjunction with a change in the catalytic activity, yielding an intense debate on the active sites or even the active phase 3 , 4. This approach allows for pointwise spatially resolved measurements but is unable to deliver two-dimensional 2D measurements on a subsecond scale to catch dynamic changes in the gas phase.
Furthermore, probing techniques are inherently intrusive and might affect the gas flow and temperature above the catalyst disturbing the dynamics. However, FTIR is a so-called line-of-sight technique that relies on the absorption along a path, making it unable to resolve the gas phase in the third dimension and when temperatures are inhomogeneous in the integration path, the spectral signal is distorted.
Nonintrusive laser-based techniques, such as Raman scattering, have also been successfully applied to catalysis studies 14 , 15 , 16 , The difficulty with Raman scattering is the low cross-section, limiting the measurements to one-dimension 1D and most often with long collection times because of averaging, ranging from several minutes to an hour although in other fields, and with a very sophisticated experimental set-up, single shot Raman has been achieved This makes it mostly suitable for stationary situations where changes in the gas-phase conditions are minimal or very slow.
The mentioned techniques have the capability for multispecies detection, but are all limited either in the temporal or the spatial domain and are therefore less suitable for measurements of the 2D gas distributions above catalytically active surfaces in a changing environment. Laser-induced fluorescence LIF has been used in numerous research fields, such as combustion and medical research 19 , 20 , to measure temperature, velocity and the concentration of a probed gas.
LIF has high spatial and temporal resolution, is nonintrusive and can be used to perform 2D measurements in situ , without disturbing the flow properties of the gas It is then often referred to as planar laser-induced fluorescence PLIF. However, the technical development during recent years in lasers and infrared detection capabilities 28 together with efficient detection and background suppression schemes developed in an earlier study 29 enables the use of PLIF for several of these previously undetectable gases.
The first detection of CO 2 to study catalysis was made in a recent study by Zetterberg et al. However, with further refinements of the experimental set-up and methodology we have been able to use infrared PLIF to capture instantaneous, 2D images of the probed CO 2 gas formed during oxidation of CO over porous supported noble metal catalysts. A schematic of the process is shown in Fig.
In this report we highlight the advantage of being able to perform spatially resolved, in situ measurements of the gas phase close to catalysts by means of infrared PLIF. Second, the possibility to measure the CO 2 distribution above two catalysts that are simultaneously situated in the chamber is presented, and how the presence of the two samples affects each other is discussed.
Finally, we show that by physically separating the catalytic samples, the differences in activation temperatures can be directly visualized, and activation energies extracted. The study sheds light on several aspects for catalysis research, especially the possibility to visualize an event, in this case the elusive CO 2 product, with high temporal and spatial resolution, at realistic industrial conditions. Schematic of PLIF measurements of the activity of a model catalyst a shows the adsorption and dissociation of O 2 on the catalyst at points 1 and 2 , respectively, and the adsorption of CO and production of CO 2 at points 3 and 4 , respectively.
An example of the CO 2 distribution above an active catalyst at elevated temperature and realistic pressure is shown in c. In our experiments we used infrared PLIF to image the temperature-dependent dynamics of CO 2 close to the surface of catalytic discs during ignition and extinction of catalytic CO oxidation. The samples studied were industrial-like pressed noble metal powder catalysts, and we specifically investigated how the gas distribution around the catalytic discs was changing with sample position, as well as in the presence of two samples in the chamber. In the end we give an example of how to perform combinatorial measurements on three samples simultaneously, without having the samples affecting each other, as a way of screening potential catalysts in the development phase both academically and industrially.
In all our measurements a mass spectrometer, connected to the outlet of the reactor via a leak valve, was used to monitor the global gas composition. The lag originates from the time it takes for the gas to reach the MS and is dependent on the flow speed, pressure and the volume of the gas has to pass to reach the leak valve. Worth noting is that the percentages given is by weight, meaning that the concentration of active atoms is the same in both samples.
The temperatures discussed in the text refer to the thermocouple temperatures. The roman numerals indicate times during the experimental measurement that correspond to the time when the snapshots shown in Fig. This is expected because the CO 2 distribution around the sample is mainly determined by diffusion.
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Above these temperatures, both samples reached a steady-state CO 2 production, which resulted in only small changes in the distribution of the gas. A slight drag towards the right can be seen, which can be attributed to the gas flow passing from left to right in the image. One way to investigate the catalytic activity is to study the spatial gradient vertically away from the Pd-sample, something infrared PLIF gives the opportunity to do on a single-shot basis.
The gradient was followed in time as a function of temperature Fig. For both samples, the concentration plateaus both in the fluorescence and in the MS signal show that the reactions are limited by the CO diffusion to the catalyst, which occurs almost immediately after ignition, and that all the CO molecules that reach the surface are oxidized When the temperature was ramped down the signals concomitantly decreased as the catalytic activity of the samples decreased. The difference in temperatures for ignition and extinction is in agreement with previous studies of supported Pd and Pt catalysts The instant at which the samples became active also coincided with an increase in the surface temperature of the samples because of the exothermic nature of the reaction Fig.
The temperature measurements also did not suffer from the time-lag associated with the MS measurement, thus making a more accurate measurement of the activation temperatures possible. In the present case, the ignition can in principle be monitored by the thermocouple or better the infrared camera; however, for many other reactions or for low concentrations of CO , the reaction is not as strongly exothermic as in the case of CO oxidation, disqualifying the thermocouple reading as a probe for the ignition.
The spatial resolution of the experimental set-up is high enough to spatially distinguish which of the two samples that becomes active by probing the corresponding CO 2 distribution. However, the distributions are still highly localized and separable above each of the samples. The CO 2 concentration in the chamber was inhomogeneous, with a somewhat stronger fluorescence signal detectable on the right-hand side of the reactor, which can be attributed to the gas flowing through the reactor from left-to-right.
When the temperature was ramped down, the Pt—Pd sample was first to extinguish Fig. This experiment shows the potential of laser diagnostics and the advantage of having 2D, spatially resolved, nonintrusive measurements of the gas distribution, opening up for simultaneous characterization of more than one sample at a time and direct in situ comparisons, something very useful, for example, when studying systems where more than one active catalyst is needed to drive different reactions, and how the presence of one affects the other.
Because of the possible interaction of the samples as demonstrated above, we also applied and demonstrated the versatility of the technique by probing the CO 2 production originating from three separate samples simultaneously and, in parallel, by placing these into three separated flow tubes. We pursued this solution for situations in which having more than one sample in the reactor changes the reaction of the adjacent samples and vice versa. To address this problem we developed our reactor to have the possibility to include three tubes see Methods section.
The experimental set-up used the same gas system as in the experiments on one and two samples, the only modification being that the gas was directed to flow through the three tubes. In this way the gas originating from each individual tube could be probed simultaneously by the laser.
We performed similar measurements, as with the one- and two-sample cases, of the infrared PLIF, MS and temperature of the samples, as a function of temperature Fig. In contrast to our previous collection of snapshot images of the infrared PLIF, we here show averaged over 10 laser pulses infrared PLIF images that are measured at the exit of the three tubes Fig.
The signals were not centred in the tubes because of the low flow out from the tubes and because the outlet was placed at the bottom of the reactor. The onset of ignition for each of the samples yielded an abrupt increase in the averaged infrared PLIF signals at the exit of the tubes Fig. This contrasts the MS results where it is not possible to extract the ignition times and temperatures for the different samples Fig. While the MS signal changes with temperature, the changes are not as pronounced as in the infrared PLIF data, which makes it impossible to tell from which sample the signal originates.
This also opens up for the possibility of extracting quantitative data for the individual catalysts. Activation energies are calculated from the three Arrhenius plots Fig. As expected, the activation energy of 0. This is also in good agreement with what is reported in literature 32 , In contrast, the overlapping CO 2 MS signals complicate the analysis of finding the individual reaction rates for each of the three catalysts and thus constrains the possibility to achieve the activation energy with the MS.
The infrared PLIF method could therefore be used to study several catalysts in parallel in a way that has not been possible before. The present report shows that the technical development of lasers, detectors and data treatment during the last 15 years enables direct 2D detection of small molecules relevant for catalysis previously not attainable by LIF. To detect CO 2 at elevated temperatures and realistic gas conditions, it is necessary to excite the molecule with a wavelength of 2.
Further, a fast gateable infrared camera with the ability to detect the fluorescence light at 4. Thus, both laser source and detector used in the present investigations are nonstandard making the combination truly unusual, and have not previously been used for catalysis-related studies. For the direct product of CO oxidation, this has not been possible until now. The results presented here is an important step for a better understanding of gas phase-related phenomena in catalysis, where dynamics can be studied that are otherwise often hidden in averaged data.
The study implies several new aspects for catalysis research, for example, with PLIF it is possible to measure the gas surrounding the sample at realistic pressures, revealing the actual gas composition close to the surface previously not attainable, for a correct interpretation of the active site or phase. The study also shows with images that by following the gas distribution in time it is possible to distinguish whether the entire catalyst or only part of it is active, or by directly comparing two catalysts decide which has the lowest activation temperature.
In this way, it should be possible to determine the surface structure and the gas composition close to the surface of the catalyst, previously simultaneously unattainable properties.
Models for heterogeneous catalysts: studies at the atomic level
Furthermore, our study shows how PLIF applied to catalysis has a potential for simultaneous characterization or combinatorial studies of industrial catalysts in a straightforward way. While infrared PLIF applied to catalysis is just barely developed, a present drawback of optical and laser-based methods is the need for specialized reactors with optical access with at least one, but often as many as three transparent windows. Further, the present relatively complicated experimental set-up and data analysis limits the use to experienced and trained researchers. To this end, we are in the process of developing more standardized reactors, lasers and software suitable for infrared PLIF, with the possibility of applying an additional experimental technique.
An example of a step in this direction is the development of a reactor towards simultaneous synchrotron radiation and laser measurements, where surface and gas phase information can be extracted simultaneously under the same conditions and at the same sample. Using modern laser and detection techniques not limited to the infrared regime , the experiments presented here can be extended to a vast number of gas species, such as CO, CH 4 , NH 3 and NO.
Our present experiment highlights the principle of the experiment and the spatial resolution provided applying infrared PLIF to catalysis, where flows and gas distributions can be measured instantaneously in 2D, something conventional techniques such as MS, GC and FTIR lacks, and provides a tool that in principle can be used simultaneously with synchrotron-based measurements.
The images were collected every 0. A schematic describing the laser set-up, optics and the collection of the images is shown in the Supplementary Fig. The infrared background was subtracted with a scheme presented in ref. An international journal for chemical science. Shopping Cart: empty. Search our journals. Previous Next Contents Vol 66 2.