\r\n\tThis book aims to expose the recent advances in the research and development of chemical and biochemical processes to obtain bio-based chemical compounds and fuels from glycerol.\r\n
\r\n\tChapters dealing with the synthesis and characterization of catalysts (single and mixed hydroxides and oxides, supported catalysts, zeolites, heteropolyacids, pillared-clays, and metal-organic frameworks) and biocatalysts (novel microbial and fungi cultures, immobilized cells, immobilized enzymes, and nanobiocatalysts) to carry out the conversion of glycerol, as well as their testing in discontinuous and continuous stirred reactors, fixed-bed, fluidized-bed, trickle-bed, bubble column, airlift and membrane (bio)reactors are welcome.\r\n
\r\n\tThe book will comprise, but will not be limited to, the homogeneous and heterogeneous chemical reactions of glycerol such as dehydration, hydrogenolysis, partial oxidation, steam- and dry-reforming, glycerol to hydrocarbon fuels and aromatics, (trans)esterification, etherification, halogenation, ammoxidation, as well as supercritical, and photocatalytic processes.\r\n
\r\n\tAdditionally, we hope to cover the bioprocessing of glycerol, including microbial and fungal fermentation and enzymatic reactions to obtain C2-C4 alcohols, diols, hydrogen, methane, organic acids, dihydroxyacetone, biopolymers, and others.
\r\n\tThe book will also deal with the engineering aspects of glycerol processing, such as chemical equilibrium of glycerol reactions, reaction kinetics, (bio)reactor modeling, as well as process simulation and optimization of process variables and reactors.
A variety of processes of crystal growth proceeds via the gas phase. A short comparative overview on gas phase transports is given here. However, in the main we deal with the concept of Chemical Vapor Transport Reactions [1, 2]. The term “
Chemical vapor transport reactions address the formation process of pure and crystalline solids. Especially, the growth of single-crystalline material is of particular value because, among other things, it allows the determination of the crystal structure by diffraction methods. Beyond the aspect of basic research, chemical vapor transport reactions have also gained practical significance: they form the basis of the operating mode of halogen lamps. Furthermore, an industrial process is based on a chemical transport reaction, the Mond-Langer-Process for the production of ultrapure nickel . Chemical vapor transports likewise occur in nature forming minerals without human influence, in particular at places of high temperatures. Bunsen was the first who observed and described it . He noticed that the formation of crystalline Fe2O3 is associated with the presence of volcanic gases which contain gaseous hydrogen chloride. Van Arkel and de Boer were the first scientists who carried out specific transport reactions in the laboratory from 1925 onwards . They were motivated by the huge interest in finding a process to fabricate pure metals like titanium at that time . Van Arkel and de Boer used the so called
A systematic research and description of chemical transport reactions was carried out by Schäfer in the 1950s and 1960s . It became apparent that pure and crystalline species of various solids could be made with the help of chemical transport reactions: metals, metalloid, and intermetallic phases as well as halides, chalcogen halides, chalcogens, pnictides and many others. The current knowledge comprises of thousands of different examples for chemical vapor transport reactions. The results of different periods of investigations are recorded in some review articles [7-13]. Besides, the monographs [1, 2] and an extensive book chapter  give an overview on principles and applications of chemical transport reactions referred to the pertinent period of knowledge. To date the chemical vapor transport method developed to be an important and versatile preparative method of solid state chemistry.
Schäfer’s endeavour also showed that chemical transport reactions follow thermodynamic regularities ; kinetic effects are rarely observed which makes a general description easier. Subsequently, the thermodynamic approaches for detailed description of chemical vapor transports became more sophisticated [16-22]. As a result, complex models for the description of vapor transports of phase mixtures, phases with variable composition, and transports with deposition sequences were established - the “Extended transport model” [18-21] and the “Co-operative transport model” . Thus, the understanding of chemical vapor transport reactions is well developed; predictions on alternative transport agents, optimal reaction conditions and the amount of transported substance are possible and fairly easy accessible via computer programs [23, 24]. Indeed, the proper handling of these programs requires a profound knowledge on thermodynamic data (enthalpy, entropy, heat capacity) of all condenses and gaseous substances that are involved.
The following section shall provide an extensive overview on both principles and mechanisms of chemical vapor transport reactions and on characteristic examples of crystal growth of different substance classes by CVT. A simple thermodynamics basis is given in order to set you in ability to estimate the conditions of vapor transport experiments by own calculations; more complex calculations methods are presented for advanced investigations. Not at least, a short introduction for performing different CVT experiments (ampoule technique, oven setup, determination of transport rates, investigation of transport sequences,…) is given.
A vast number of reactions involving gas phases hardly differ from each other: If a condensed substance encounters a temperature gradient, it moves from the place of dissolution via the gas phase to the place of deposition, from
The other possible gas species in the system, I(g) is of less importance due to the significantly lower partial pressures at the temperature of sublimation.
Saline solids can sublime, too. A well-known example is aluminum(III) chloride which is present in the gas phase in large proportion in form of dimeric molecule Al2Cl6, Figure 2. The additional systems gas species, such as AlCl3(g), Cl2(g), and Cl(g) show significantly lower partial pressures at the given temperature and thus not take part in the evaporation process.
In a generalized form, the sublimation of a compound
The gas phase transport of bimuth(III) selenide - Bi2Se3, an important constituent for thermoelectric materials - shows the characteristic of that. It decomposes into stoichiometric amounts of BiSe(g) and Se2(g) in the vapor phase, the molecule of the initial composition Bi2Se3(g) does not occur in evaporation process, Figure 3. During cooling, the gas phase condenses completely and solid solid bimuth(III) selenide is formed (4).
The gas phase transport of Bi2Se3 by decomposition sublimation gives an example for congruent dissolution and condensation. Thus, a solid of always the same, constant composition is deposited. Nevertheless, a decomposition sublimation can be incongruent, too. Often, the product of an incongruent decomposition sublimation has the same composition as the initial solid. A simple example of this is copper(II) chloride. If heated at a running pump to several hundred degrees, the steam that is built in subsequent equilibria (7) and (8) contains the molecules CuCl, Cu3Cl3, Cu4Cl4 and Cl2.
The mechanism of decomposition sublimation is even more complex in the case of Bi6Cl7 [25, 26]. The initial solid is decomposed into a second solid – here elemental bismuth – and the dominating gas species BiCl3 in equilibrium (9). Thereby, the compositions of the gas phase and thus the “solubility” of all components is unequal the initial composition of the solid, Figure 4.
Nevertheless, the gas phase transport of Bi6Cl7 is realized by a second, subordinated equilibrium (10). In this case, the composition of the deposited solid strongly depends on the experimental conditions: Congruent deposition of Bi6Cl7 only occurs with low temperature gradients between source and sink. At higher temperature gradients pure BiCl3 is deposited.
As has been shown, the mechanism of gas phase transport will get more complicated if the decomposition leads to a further condensed solid and a reactive gas phase. Subsequent, auto transport processes can result.
Based on this example, one can formulate the course of the auto transport in general terms : A compound
Auto transports are generally endothermic reactions like sublimation and decomposition sublimation (deposition in the direction form source to sink: hot to cold). The transport equilibrium can only be effective if two conditions are met: First, the partial pressure of
There may be a smooth transition of the described phenomena of sublimation or decomposition sublimation to the mechanism of auto transport. The dissolution of CrCl3 in the gas phase represents such a complex behavior. One can find congruent sublimation, the formation of gaseous chromium(III) chloride, and of an incongruent decomposition at the same time. In a consecutive reaction chlorine can react with the primary solid CrCl3, thus becoming the transport agent. The transport effective gaseous molecule is CrCl4, Figure 6. In case of the chlorides MoCl3 or VCl3, the gas molecules
The principle of auto transport is to apply also for other substance classes, such as oxides, chalcogenides and above all chalcogenide halides. As an example of the auto transport of an oxide, the crystallization of IrO2 is presented. At temperatures of about 1050 °C the phase decomposes into the metal and molecular oxygen (18). In a subsequent heterogeneous equilibrium (19), oxygen reacts with the primary solid to form the transport effective gas species IrO3, Figure 7. The back reaction takes place at lower temperature and IrO2 is deposited.
Generally, all the auto transports are feasible as “regular” chemical vapor transport reactions. In these cases, the transport is possible as well through the addition of the transport agent without the preceding decomposition reaction. An important difference in both experiments can be observed: As the auto transport is based on a decomposition reaction, crystals of a different (metal rich) composition can be deposited. Thus the auto transport of IrO2 leads to the deposition of IrO2−x (depleted by oxygen). Otherwise, the regular vapor transport in an open systems oxygen stream (
If at least one of the components of
The chemical vapor transport of ZnO is also possible by addition of hydrogen chloride. Likewise for the transport with chlorine ZnCl2 is formed as the transport effective species for the transfer of zinc from source to sink, Figure 10. Otherwise the used transport agent HCl can react with oxygen, too. Thus the oxygen transferring species H2O is formed in equilibrium (21).
Here, the general principle of transport reactions can be seen clearly: The source material is transformed reversibly into gaseous products by the use of the transport agent. The transfer of the solid can be realized in different ways by formation of both heteronuclear species (like ZnCl2 and H2O) and atomar or homonuclear species (O2).
In principle, two working methods are applied for the practical realization in the laboratory: the transport in open or closed systems. An open system is applied with an on both sides opened tube made of glass or ceramic material. Inside, a continuous flow of the transport agent is led over the source material; the solid, which is kept at a certain temperature, deposits at a different place with another temperature under the release of the transport agent. In a closed system, typically a sealed ampoule, the transport agent remains in the system and constantly re-enters the reaction. Thus, in a closed system, a much smaller amount of the transport agent is needed. In some cases only few milligrams of the transport agent are sufficient to cause a transport effect. In the laboratory one predominantly works with closed systems. An easy closed system is a sealed glass tube. Such a
It is of prime interest for preparative working chemists whether a certain solid can be prepared with the aid of chemical vapor transport reactions, which transport agents are suitable and under which conditions a transport can be expected. At this point, we want to appoint some general qualitative considerations.
The vapor transport reaction has to realize, that all formed products are gaseous under the reaction conditions. Thus a suitable transport agent is to select, which can transfer all components of the initial solid into the gas phase.
The equilibrium position of the transport reaction must not be extreme, so that dissolution into the gas phase and re-condensation of the solid are possible under slightly changed experimental conditions. In cases of an extreme equilibrium no dissolution occurs (evaporation reaction unfavored) or the formation of gaseous products is not reversible (back reaction under re-condensation unfavored). In both cases no vapor transport is observed.
The temperature at which the numerical value of the equilibrium constant
The transport is caused in almost every case by different temperatures and therefore changed equilibrium position in source and sink. It is common to characterize the volatilization (source) and the deposition temperature (sink) with
A chemical vapor transport reaction can be divided into three steps: the
To intensify the theoretical understanding of chemical vapor transport reactions in a comprehensible way the representative experiment of the transport of tungsten(IV) oxide is illustrated. With the help of the clear example of the transport of WO2, the mentioned general considerations can be tackled:
Also, adding mercury halides, which are solid at room temperature, is potentially suitable to transport both components of the solid phase – tungsten as well as oxygen – into the gas phase (26).
At temperatures above 300 °C the mercury halides evaporate completely. Afterwards the gas species WO2
A highly exergonic reaction Δr
The calculations’ results give a realistic outlook on the prospective results of transport experiments: Using halogens the transport with iodine seems to be promising (29). In the case of bromine, the transport seems at least possible (28) whereas chlorine causes an extreme equilibrium position under the formation of WO2Cl2(g) – a transport should not be possible (27). With the hydrogen halides equilibria are far on the side of the reaction products (30 – 32). This is due to clearly higher gain of entropy during the reaction. Although one can observe gradations in the equilibrium position for transports with HI and HBr compared to HCl, transports are principally not expected.
The transport of WO2 with mercury halides seems possible for all three transport agents Hg
For the precondition of balanced equilibrium position at
Through differences in the temperatures of the source and sink side, the equilibrium position is brought towards the gaseous products when dissolving and shifted towards the solid when deposing. Calculations of the equilibrium constants were first made for an average temperature of 1000 K. If the temperatures vary, one will get the typical courses of the curve (see Figure 11). If the temperature is decreased, the equilibrium position in the transport system with HgCl2 becomes less extreme. In contrast, the equilibrium position for the transport with HgI2 becomes more favorable when the temperature is increased above 1000 K. The optimum, average temperature resulting from the quotient of the reaction enthalpy and entropy for the transport with HgCl2 is at about 700 K respectively 400 °C; with HgBr2 at about 1100 K respectively 800 °C and with HgI2 1400 K (1100 °C, respectively). In this case, the calculation of the temperature on the basis of the standard values at 298 K as well as of the derived values for 1000 K lead to the same results; which means that an estimation is possible with simple calculations, (39 – 41).
In a reaction with negative reaction enthalpy (exothermic dissolving reaction), the equilibrium constant
In a reaction with positive reaction enthalpy (endothermic dissolving reaction),
The transport direction results only from the reaction enthalpy which is why the conclusion of all three investigated transport systems of WO2 is clear: The reaction enthalpy is positive in each case – a transport to the cooler zone results. The total amount of the reaction enthalpy does not affect the decision if a transport is carried out. If the reaction enthalpy is close to zero one has to check the accuracy of the used data as they can contain errors of 10 to 20 kJ mol−1.
The term “
: Transport rate /mol h−1
average temperature along the diffusion path /K (practically results as an average of
In most cases instead of the diffusion factor 0.6 10−4 a value of 1.8 10−4 is given which found entrance to the literature . According to recent findings the factor 0.6 10−4 results in a smaller numerical value of the diffusion coefficient and corrects a mathematical error.
The calculation of transport rates for WO2 by Schornstein and Gruehn [29, 30] at first show a clear dominance of transports with HgBr2 in the average temperature range: The expected transport rates are ten times higher than for transports with HgCl2 and HgI2. Due to the balanced position of the equilibrium, high differences of partial pressures occur between the source and the sink. This way, the driving force for diffusion of the gas particles is high and thus for the substance transport as well. For the transport with HgCl2 the transport rate decreases with increasing temperatures. As we have already seen, the equilibrium position, which is far to the right side, is responsible for it. Only if the temperature decreases, the equilibrium position can move to the left. The resulting, higher differences of partial pressures between dissolution and deposition side cause increasing transport rates at low temperatures. Using mercury iodide as transport agent, the equilibrium position is on the side of the source material at low temperatures. By increasing the temperature the equilibrium position is shifted to the side of the reaction products, the transport rate increases, Figure 12.
Corresponding to the simple estimation of the transport behavior of WO2 with mercury halides, one gets the best results with the addition of HgBr2. The chemical vapor transport of mercury bromide is possible in a wide temperature range. Transport rates above 30 mg h–1 are achievable, Figure 13. Temperatures of the source side of about 800 °C and of the sink side of 720 °C prove optimum. This result confirms the estimations of the optimum transport temperature. Due to the shift of the equilibrium position, the transport rate decreases at both, rising temperatures (880 → 800 °C respectively 960 → 880 °C; and falling temperatures (720 → 640 °C) . Transports with HgCl2 and HgI2 clearly show smaller transport rates. Experiments with mercury iodides must be realized with higher temperatures according to the estimation. Temperatures up to 1000 °C are practicable; above, the silica glass ampoule will be heavily damaged by re-crystallization. Using an average transport temperature of 940 °C, transport rates of up to 15 mg h–1 can be achieved, Figure 13. The transport rate decreases drastically with falling temperatures. With an average temperature of 640 °C the rate is even lower than 1 mg h–1. Transport experiments with HgCl2 show worst results as far as the transport rate is concerned: According to the calculation, lower temperatures are principally more favorable, however, in the range from 500 to 700 °C the transport rates are only in the range of 1 mg h–1. The transport almost grinds to a halt at higher temperatures.
One can come to a completely different evaluation if the quality of the crystals instead of the transport rate is given prominence. Relatively high transport rates cause uncontrolled nucleation and crystal growth. As a consequence, one gets highly epitaxial and rose-shaped crystal agglomerations for transports with HgBr2. Frequently
Finally, in selecting the transport agent, the temperature, and the temperature gradient, respectively, one should consider the aim of the transport. A high transport rate is undoubtedly advantageous for the synthesis of a compound or the purification of it. If crystals are to be grown, keep in mind the crystal quality and therefore rather choose a smaller transport rate.
It is of prime interest for preparative working chemists whether a certain solid can be prepared by chemical transport reactions, which transport agents are suitable and under which conditions a transport can be expected. If one wants to use transport reactions only in a preparative way – without the purpose of understanding the course of the reaction in detail – often it is sufficing to check on an empiric basis which solid can be transported by using what kind of transport agent. A further, quantitative description of the transport reaction requires knowledge of the thermodynamic data of the condensed phases and gaseous molecules that are involved. In this section, we will provide a short overview of the different kinds of gaseous inorganic molecule that can occur during chemical vapor transport reactions. Under the precondition of formation of only gaseous species, transport agents and transport effective species share the property of high volatility under experimental conditions. Thus, especially halogens and halogen compounds are qualified. Some elements, hydrogen compounds, and oxygen compounds are suitable as transportable species, too.
Hydrogen halides are versatile transport agents. The oxidation levels of the metal in the solid and in the transport effective gas species are generally equal because hydrogen halides do not have an oxidizing effect. Hydrogen halides are often used during the transport of oxides. Here, the gaseous metal halide and water vapor are formed. Halogen compounds, such as TeCl4, PCl5, NbCl5 or TaCl5 are also useful transport agents, especially for metal oxides. Reactions of the mentioned chlorides lead on the one hand to the formation of gaseous metal chloride or metal oxide chloride, on the other hand oxygen is fixed in form of volatile oxides (TeO2, P4O6, P4O10) or oxide chlorides (TeOCl2, POCl3, NbOCl3, TaOCl3). Oppermann was able to show that tellurium(IV) chloride is a particular versatile transport agent . According to the basic works of Schäfer, gaseous metal respectively semi-metal halides are formed as transport effective species during the reaction of different solids with halogens or halogen compounds .
The vapor of metal halides can consist of monomeric, dimeric and/or oligomeric molecules. With
Some gaseous oxide halides are known of main group metals. Elements of group 13 form oxide halides, such as AlOCl, at very high temperatures around 2000 °C. Phosphorus forms several oxide chlorides and -bromides that are stable at high temperatures: PO
In some cases, gaseous elements can work as transport agents. Hence, oxygen can cause the transport of some platinum metals . Sulfur can transport a series of transition metal sulfides . Here, gaseous polysulfides, such as TaS3, are assumed transport effective species. There are similar observations for the chemical transport of some selenides. Sulfur is an effective transport agent for tellurium as well . Compounds in which tellurium atoms were integrated in the different ring-shaped sulfur molecules were detected as transport effective species. Phosphorus can transport gallium phosphide, GaP, and indium phosphide, InP, probably via GaP5 respectively InP5 as transport effective species . With the help of arsenic, the transport of gallium arsenide, GaAs, and indium arsenide, InAs, succeeded in a similar way .
Metal vapors predominantly consist of the atoms. The fraction of bi- or polyatomic molecules in the saturated vapor is between 10−5 and 10 % . In contrast, the vapors of non-metals, apart from noble gases, consist of very stable polyatomic molecules which appear in the gas phase in great amounts, atoms appear only subordinated: N2, P4, P2, As4, As2, Sb4, Sb2, O2, S2, S3 … S7, S8, Se2, Se3 … Se7, Se8, Te2, Cl2, Br2, and I2. The ratio of different molecular species in the vapors of non-metals depends on the temperature and the pressure. Higher temperature and lower pressures abet the formation of small molecules respectively atoms.
The transport reaction of tungsten with water and iodine is an important one in daily life. This reaction provides the basis of the operating mode of halogen lamps.