Catalyst carriers pore size

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

For the same catalytically active material but with different catalyst carriers, different reaction rates and rate equations can be expected. Consider the hydrogenation of 2,4 DNT as discussed in Section 9.2 for 5% Pd on an active carbon catalyst with an average particle size of 30 (im [3]. These experiments were later repeated but with a Pd on an alumina catalyst [5]. This catalyst consisted of 4 x 4mm pellets, crushed to sizes of lower than 40/um in order to avoid pore diffusion limitations. In Figure 2.9 the measured conversion rates are given as a function of the averaged catalyst particle diameter, showing that above a diameter of 80/im the rate measured diminishes. For small particles they determined the rate equations under conditions where there were no pore diffusion lim-... 

The key problem here is the preparation of these plates. The plates contained one inert hydrophobic part close to the hydrogen gas side, and another part consisting of a catalytically active metal on various types of carrier powder. The hydrophobic layer was made of 30-50-fjLm nonporous PTFE particles. The catalyst carrier particles were porous (mean pore diameter of 10 nm) with a particle size of about 5 p.m. The catalytic material was of three different types 10% Pd on alumina, 10% Pd on carbon, and 1.9% Pd on Ni0/Si02. In addition to these powder materials, the plates contained nets of nickel wire (0.16 mm) or glass fibers (0.2 mm) as reinforcement. The catalytic plates were prepared... 

For special purposes, it is manufactured separately as the so-called microporous glass, whose porosity and pore size can be regulated within certain limits (pore diameter in fractions to tens nm), by adjusting the glass composition and conditions of separation (temperature, time). The glass shows selective absorptive properties its specific surface area amounts to several hundred m g and the pore space takes a third to half the volume. Microporous glass is used as an adsorbent, dessicant and catalyst carrier. 

Experimental methods and techniques for catalyst manufacture are particularly important because chemical composition is not enough by itself to determine activity. The physical properties of surface area, pore size, particle size, and particle structure also have an influence. These properties are determined to a large extent by the preparation procedure. To begin with, a distinction should be drawn between preparations in which the entire material constitutes the catalyst and those in which the active ingredient is dispersed on a support or carrier having a large surface area. The first kind of catalyst is usually made by precipitation, gel formation, or simple mixing of the components. 

Utilisation of sol-gel technique with respect to the preparation of supported bimetallic catalysts allows to produce catalysts with homogeneous distribution of finely dispersed metals. Another advantage includes improved thermal stability of the metals, higher surface areas, well-defined pore size distribution and ability to control the microstructure of the carrier. 

Heterogeneous catalysis is increasingly applied in chemical industries to decrease the raw material consumption, pollutants emission and to inqjrove the product selectivity. Catalytically active metals and metal oxides are usually deposited on a carrier or support. The role of the support is to stabilize the active component in highly dispersed small particles and hereby increase their exposed siuface area. The activity and selectivity of the catalyst can be significantly altered by the particle size of the active metaTmetal oxide and the pore size distribution of the support material. 

Another worthwhile area of endeavor is new support materials, ones which are as single-sited as the catalysts they carry. Eor example, the zeolite carriers discussed above have more uniform pore sizes and volumes than the amorphous silicas hitherto widely used. The importance of single-site catalysts lies in the ability to characterize them comprehensively and alter them in a rational fashion in order to enhance desired polymer properties extending this degree of understanding and control to their interaction with supports and their performance in numerous polymerization processes could only be beneficial. 

The high-specific-surface-area supports (10 to 100 m /g or more) are natural or manufactured materials that normally are handled as fine powders. When processed into the finished catalyst pellet, these materials often give rise to pore size distributions of the macro-micro type mentioned previously. The micropores exist within the powder itself, and the macropores are created between the fine particles when they are compressed together in a pellet press. Diatomaceous earth and pumice (or cellular lava) are naturally occurring low-cost materials that are representative of this class of catalyst support. Among the synthetic carriers that can be created by modem technology are those derived from clays, bauxite, activated carbon, and xerogels of silica gel and alumina gel. 

The catalyst effectiveness factor rji was calculated from the pore network model of Wood and Gladden [15] under the conditions on which capillary condensation was expected. The pore network model was solved over a range of temperatures from 553 to 580 K and for several pressures in the interval 20-40 bar to create a database of effectiveness factors for input to the macroscopic reactor model. The hydrodesulfurization of 1 mole % diethyl sulfide in an inert dodecane carrier was considered, with a molar gas oil ratio of 4. The catalyst was taken to have a connectivity of 6 and a normal distribution of pore sizes with a mean of 136 A and standard deviation of 28 A. By using the results of the pore network simulation as input to the macroscopic fixed bed reactor model, capillary condensation at the scale of the catalyst pellets was accounted for. 

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