Thermal conductivity alumina catalysts

The catalytic CO oxidation by pure oxygen was selected as a model reaction. The Pt/alumina catalyst In the form of 3.4 mm spherical pellets was used. The CO used In this study was obtained by a thermal decomposition of formic acid In a hot sulphuric acid. The reactor was constructed by three coaxial glass tubes. Through the outer jacket silicon oil was pumped, while air was blown through the inner jacket as a cooling medium. The catalyst was placed in the central part of the tube. The axial temperature profiles were measured by a thermocouple moving axially in a thermowell. Gas analysis was performed by an infrared analyzer or by a thermal conductivity cell. [7]. 

The TPD apparatus consisted of a stainless steel flow system connected to a thermal conductivity cell. Catalyst samples of 0.1 g were placed in one arm of an L-shaped, 6 mm Vycor tube. A dual adsorption bed containing alumina and Oxy-Trap (Alltech) was placed in the other arm to prevent contamination by water and respectively. Frequent regeneration in and He was required. This in-situ adsorption bed was found necessary despite purification traps on all gas lines coming into the flow system. Pulses of 0.25 cc of a 10% mixture of CO in He were injected into the He carrier gas and passed over the pretreated catalyst at room temperature. All runs were programmed heated at a rate of 20 K min . The Pt catalysts, either commercial or laboratory produced, were prepared by the impregnation of chloroplatinic acid on Cyanamid s Aero 1000 alumina, except for two catalysts which were prepared by platinum diamino dinitrite impregnation. 

Fig. 11-3 Effective thermal conductivity of alumina [boehmite) catalyst pellets at 10 to 25 microns Hg pressure...

Catalyst supports such as silica and alumina have low thermal conductivities so that temperature gradients within catalyst particles are likely in all but the finely ground powders used for infrinsic kinetic studies. There may also be a film resisfance fo heaf fransfer af fhe exfemal surface of the catalyst. Thus the internal temperatures in a catalyst pellet may be substantially different than the bulk gas temperature. The definition of the effectiveness factor, Equation 10.23, is unchanged, but an exothermic reaction can have reaction rates inside the pellet that are higher than would be predicted using the bulk gas temperature. In the absence of a diffusion limitation, rj > 1 would be expected for an exothermic reaction. (The case > 1 is also possible for some isothermal reactions with weird kinetics.) Mass transfer limitations may have a larger... 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

The effective bed conductivity has a static or zero-flow term, which is usually about 5k when the particles are a porous inorganic material such as alumina, silica gel, or an impregnated catalyst, and kg is the thermal conductivity of the gas. The turbulent flow contribution to the conductivity is proportional to the mass flow rate and particle diameter, and the factor 0.1 in the following equation agrees with the theory for turbulent diffusion in packed beds ... 

Sulfur. To assess whether sulfur can be used as a partial oxidant for propane, exploratory experiments have been made at Worcester Polytechnic Institute in which propane and propane-helium mixtures were saturated partially with sulfur at atmospheric pressure and then passed over commercially available chromia-alumina catalysts. No methyl-acetylene was detected by thermal conductivity gas chromatography using a 20-foot squalane column, but significant amounts of methyl and ethyl mercaptans were found. Figure 7 illustrates the nature of the gaseous products obtained. Continued experimentation will establish the system more concretely. No coking data or results from sustained operation are available as yet. The results shown in Figure 7, while preliminary, show that the reaction... 

Fig. 6. Chromatogram obtained on adding 8 cc, of radioactive ethylene and 8 cc. of nonradioactive propylene to a stream of hydrogen carrying gas and passing the mixture over 1 cc. of a silica-alumina cracking catalyst at 400° and through the chromatographic apparatus and analyzers in Fig. 4. The dashed curve is a record of the composition of the exit gas as measured by the chromatographic column and thermal conductivity cell the solid line indicates the radioactivity of the various products passing out of the chromatographic column.

The reaction equipment is operated by means of a data acquisition and control program. The reactor is of stainless steel 316, with 9 mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as inert and operates in isothermal regime. The reaction products are analysed by gas chromatography (Hewlett Packard 6890) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is carried out by means of a system made up of three eolumns 1) HP-1 semicapillary column for splitting the sample into two fi actions a) volatile hydrocarbon components (C4.) and polar components (ethanol, water and diethyl ether) b) remaining products (C5+). 2) SUPEL-Q Plot semicapillary column for individually separating out both volatile components and polar components, which will be subsequently analysed by TCD and FID. 3) PONA capillary column for separation of Cs+ hydrocarbons, which will be analysed by FID. 

The boundary conditions at the external surface of the catalyst are T = Tsurface and Ca = Ca surface, and A effeciive is the effective thermal conductivity of the composite catalyst structure (i.e., 1.6 x 10 J/cm s K for alumina). Initially, the surface temperature and concentration of reactant A in Uie vicinity of a single isolated catalytic peUet are chosen to match the inlet values to the packed reactor. If external mass and heat transfer resistances are minimal, then bulk gas-phase temperature and reactant concentration at each axial position in the reactor represent the characteristic quantities that should be used to calculate the intrapellet Damkohler number for nth-order chemical kinetics ... 

The enthalpy change for reaction is exothermic and varies from 50 to 80 kJ/mol. The activation energy for the forward reaction varies from 25 to 27 kJ/mol. The temperature at the external surface of the pellet is constant at 350 K. The effective thermal conductivity of alumina catalysts is 1.6 x 10 J/cm s K. The chemical reaction is first-order and irreversible and the catalysts exhibit rectangular symmetry. When a(0) 1 in the mass transfer equation, simulations in... 

Reduction of the oxidic precursors resulting from the oxidation of the supported complex cyanides leads to metal or alloy particles. Figure 4 shows temperature-programmed reduction (IPR) profiles measured with the thermal conductivity detector for the oxidic precursors of iron, copper-iron and nickel-iron catalysts. With the pure iron catalyst profiles for the alumina and for the titania supported ones are presented. The reduction profiles of the iron-copper and... 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

The dehydrogenation catalyst must be sufficiently active to allow for very short contact times and the use of low temperatures, to minimize thermal cracking reactions. Carbon deposits are eliminated by beating in the presence of a gas containing oxygen. -This means that the catalyst must be thermally stable to avoid being deactivated during the oxidation of the deposits. The best catalysts contain alumina and chromium oxide, but these cannot be employed in the presence of steam. Operations are conducted at a temperature between 550 and 700 0, and low pressure, less than 0.1.10 Pa absolute. 

Reaction over Base Catalysts. - Masada et al. reported a detailed study on the reaction with methyl propionate over silica-supported various base catalysts. The reaction is conducted in the presence of methanol but in the absence of water vapor. HCHO free from water is obtained by the thermal decomposition of cyclohexanol-hemiformal which was previously prepared from formalin and cyclohexanol. The performances of catalysts are summarized in Table 8. The KOH catalysts are more active than the CsOH catalysts, although the selectivity to methyl methacrylate is lower. Incorporation of halides of alkali metal into the KOH catalyst improves the yield of methyl methacrylate, though the halides by themselves are inactive for the reaction. The best results are obtained with a KOH (1.5 percent) + Csl (0.5 percent) on silica catalyst. The single-pass yield of methyl methacrylate reaches about 59 mol% based on the charged HCHO with a methyl propionate/HCHO/ methanol molar ratio of 10/1/10. It is also found that the selectivity of methyl propionate to methyl methacrylate is very high, nearly 100 mol%. The best support is found to be silica gel. No catalytic activity is observed with the alumina-supported catalysts. 

---