Thermodynamics calcination reactions

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

Currently, thermal reduction processes have replaced the electrolysis method. The starting material in these methods is limestone, which is calcined to produce calcium oxide. The latter is ground, mixed and compacted with aluminum, and reduced at temperatures between 1,000° to 1,200°C under vacuum. Calcium vapors formed in low yield under such thermodynamic conditions are transferred from the reactor and condensed in cool zones, thus shifting the equilibrium to allow formation of more calcium vapors. The reactions are as follows ... 

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

In some reactant mixtures as many as six crystalline phases were detected, e.g. as in the case of the composition Ce Mo Te 7 8 5 after calcination at 550 C for 8 hours. The presence of more phases than those permitted under thermodynamic equilibrium is a consequence of the incompleteness of the reactions between the components under our experimental conditions. It is also noticed that various areas of the phase diagram are highly sensitive to the preparative conditions e.g. the composition... 

The presence of catalytically active Lewis acid sites in sulfated zirconia catalysts is much debated [1-5]. The conventional preparation of sulfated zirconia catalysts involves reaction of freshly precipitated zirconium hydroxide with diluted sulfiiric acid or impregnation of zirconium hydroxide with sulfuric acid or ammonium sulfate [6,7]. The final solid acid catalyst results by calcination at a temperature of 723 to 873 K. Provided thermodynamic equilibrium has been reached, all water and free sulfuric acid should have evaporated upon calcination at 673 to 873 K and only chemically bonded sulfete groups remain [8]. Above 890 K, bulk anhydrous Zr(S04)2 decomposes [1]. When uptake of water by the calcined catalyst is prevented or after loading of the catalyst in the reactor physisorbed water is removed by thermal treatment, only Lewis acid sites are present. Since it is difficult either to prevent the uptake of water vapor or to remove adsorbed water completely, it is difficult to attribute the acid activity of sulfeted zirconia catalysts unambiguously to Lewis acid sites. 

This solid is the first heterogeneous catalyst yet reported for the aldolization of acetone to diacetone alcohol at 273 K. The products of the batch reaction are diacetone alcohol (selectivity > 96 %) and triacetone dialcohol (4 %) with traces of mesityl oxide (MO). The productivity was 3.7 kg DAA kg solid h and the catalyst could be recycled seven times without any loss of activity. Hydrated HDT reached thermodynamic equilibrium in < 1 h and was more active than MgO, calcined HDT, or even solutions of NaOH (Figure 4). The selectivity is also much better because mesityl oxide was not formed on the rehydrated sample. 

To date, there have been several unsuccessful attempts to fit these results to a simple model—for example, one based on a shrinking unreacted core or on reaction of a porous solid. The apparent role of water in the mechanism suggests that sulfur dioxide may be oxidized to sulfur trioxide on the surface and that sulfur trioxide diffuses through a product layer to react with calcium carbonate. This concept would be consistent with the similar kinetics observed for half- and fully calcined stone since the rate-determining step would presumably be the same in either case. This view is supported by the observation that reactivity in a fluidized bed decreases somewhat above about 850 °C because the thermodynamics of sulfur dioxide oxidation become less favorable. On the other hand, Borgwardt s observations with fully calcined stone (1) suggest that the decreased reactivity is caused by hard-burning of the stone. 

Oxides Compared to silica-based networks, nonsiliceous ordered meso-poious materials have attracted less attention, due to the relative difficulty of applying the same synthesis principles to non-sihcate species and their lower stability (227). Nonsiliceous framework compositions are more susceptible to redox reactions, hydrolysis, or phase transformations to the thermodynamically preferred denser crystalline phases. Template removal has been a major issue and calcination often resulted in the collapse of the mesostracture. This was the case for mesostractured surfactant composites of mngsten oxide, molybdenum oxide, and antimony oxide, and meso-structured materials based on vanadia that were obtained at early stages. Because of their poor thermal stability, none of these mesostructures were obtained as template-free mesoporous solids (85, 228, 229). 

The template removal step, needed to achieve porous materials, is one of the most critical points. In contrast to silica, other compositions are usually more sensitive to thermal treatments and calcination can result in breakdown of the mesostructures. Hydrolysis, redox reactions, or phase transfonnarions to the thermodynamically preferred denser crystalline phases account for this lower thermal stability. Many of the transition metal-based mesostruetured materials synthesized in the presence of cationic surfactants collapse during thermal treatments. The poor thermal stability observed could be due to the different 0x0 chemistry of the metals compared to silicon. Several oxidation states of the metal centers may be responsible for oxidation and/or reduction during calcination. In addition, incomplete condensation of the framewoik is possible. 

Table 20.4 lists examples of salt precursors and their decomposition temperatures. The decomposition of salts to oxides is an example of a solid-state reaction. These reactions are often referred to as calcination and are frequently governed by kinetics rather than thermodynamics. As a consequence, they may be carried out at temperatures much greater than those necessary based on thermodynamic calculations. A feature of the decomposition reactions is that they often result in the production of extremely fine particles. 

Limestone (CaCOs) can be calcined in a kiln to produce solid CaO and CO2 gas. The reaction is endothermic, and so, in every piece of limestone, heat must be supplied to the reaction site. This heat must be transported through the laminar flowing boundary layer in the gas phase, and through the CaO product layer. Simultaneously, the CO2 which is produced must be transported away in the opposite direction. The overall process involves a coupled transport of heat and mass. The relationship between the CO2 partial pressure and the temperature at the reaction site can be determined from thermodynamic data (unless thermodynamic equilibrium is not achieved, in which case additional kinetic data will also be required to determine the relationship). Fuel is burned in the kiln to supply the heat necessary to maintain the reaction. 

In its initial state, a thermodynamic system consists of 2 moles of calcium carbonate CaCOs confined at 25 °C by a frictionless piston. The system is subjected to reversible heating. As a result, the calcium carbonate starts to decompose ( calcination ) at 800 °C according to the following reaction equation... 

Mixed WOj/Al Oj/HY catalysts prepared by calcination of physically mixed WO3, Al Oj and HY zeolite showed unique behavior in the metathesis between ethene and 2-butene to produce propene [147]. Monomeric tetrahedrally coordinated surface tungstate species responsible for the metathesis activity were formed via the interaction with Bronsted acid sites of HY zeolite. Polytungstate clusters are supposed to be less active in the metathesis reaction. The best catalyst demonstrates the 2-butene conversion close to the thermodynamic equilibrium value ( 64%) at 453 K. The catalysts are bifunctional [148] they catalyze first isomerization of 1-butene to 2-butene and then cross-metathesis between 1-butene and 2-butene to produce propene and 2-pentene. 10%W03/Al203-70%HY exhibits the highest propene yield. 

Figure IL8 Calculated changes using thermodynamic modelling as a function of metakaolin content. 70 wt.% portland cement +30 wt.% (metakaolin + limestone) based on the data from Steenberg et al. (2011) assuming complete reaction of metakaolin and A/S = 0.09 in C-S-H. (From Steenberg, M. et al., Composite cement based on portland cement clinker, limestone and calcined clay. Proceedings of the 13th International Congress on the Chemistry of Cement, Madrid, Spain, 97-104, 2011.)...

---