Support pelletization

Implementation of MRI to quantify holdup and wetting in packings of porous packing elements (e.g., catalyst support pellets) must be performed with care. Difficulties in data acquisition and analysis arise because the signal we wish to... 

The most common method for preparing samples for pretreatment and subsequent measurement of IR spectra is the self-supporting pellet technique. In this... 

Figure 8. Gas treatment cell for transmission XAS. The sample is prepared as a pressed self-supporting pellet in the sample holder, diluted with BN. The liquid nitrogen dewar enables data collection at 77 K, and the connection to gas-flow or a vacuum system enables control of the sample environment. (Reproduced with permission from ref 154. Copyright 1997 B. L. Mojet).

This technique was successfully used to monitor the deposition of catalyst within a support pellet and clearly distinguish the amount of penetration there was into the pellet. Therefore, one can quickly obtain a time required to produce a catalyst pellet that is of the egg-shell type or completely saturated. Moreover, it is possible to very finely resolve the thickness of the catalyst penetration into the particle, resulting in good control of the finished catalyst. 

Clearly by working with typical spatial resolutions of approximately 30-50 pm, individual pores within the material are not resolved. However, a wealth of information can be obtained even at this lower resolution (53,54,55). Typical data are shown in Fig. 20, which includes images or maps of spin density, nuclear spin-lattice relaxation time (Ti), and self-diffusivity of water within a porous catalyst support pellet. In-plane spatial resolution is 45 pm x 45 pm, and the image slice thickness is 0.3 mm. The spin-density map is a quantitative measure of the amount of water present within the porous pellet (i.e., it is a spatially resolved map of void volume). Estimates of overall pellet void volume obtained from the MR data agree to within 5% with those obtained by gravimetric analysis. 

Fig. 20. Spin density, and water diffusion images for a 2.2-inm-diameter, spherical silica catalyst support pellet. In-plane pixel resolution was 45 pm x 45 pm image slice thickness was 0.3 mm. (a) Spin-density map lighter shades indicate higher liquid content, (b) map (150 00 ms) lighter shades indicate longer values of Ti. (c) Diffusivity map ((0-1.5) x 10 m s ) lighter shades indicate higher values of water diffusivity within the pellet.

Extension of this methodology to porous packing elements (e.g., catalyst support pellets) is not straightforward. The challenge arises because the signal we wish to measure is associated with the liquid (water) in the bed. However, the signal intensity acquired from a specific region of water depends on its local environment, because the nuclear spin relaxation times of water in different physical environments will vary. In this system, the different environments will be (i) free water in the bulk of the inter-pellet space, (ii) water within the intra-pellet pore space, and... 

From the different characterization techniques, it follows that catalysts have been prepared that display a homogeneous distribution of the supported phase on the support pellets, with an increased interaction as compared with a physical mixture of iron oxide and titania. Tn the case of a pure anatase support, the interaction leads to the formation of a mixed oxide of iron and titanium. 

For the FTIR measurements (Bruker IFS28 resolution 2 cm-1 MCT detector), the samples, pressed into self-supporting pellets, were placed into a quartz IR cell equipped with KBr windows attached to a vacuum line (residual pressure 1 x 10 6 Torr). Thermal treatments were carried out in situ. To decrease the scattering of the IR beam, severely affecting the transparency of the samples in the in the 4000-3000 cm-1 range, a system of condenser mirrors were placed between the IR cell and the detector. 

The coverage equation for A, the crystallite temperature, and the gas-phase concentration in the void volume of the support pellet were modeled with partial differential equations. The temperature of the pellet was assumed to be uniform throughout the pellet, and thus description with an ordinary differential equation was sufficient. In-depth analysis of this model yielded rich phenomenology, which, because of its complexity, will be discussed in Section V. 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Analysed in the form of a self-supported pellet. Pure pellets are produced by pressing a mass of 6 to 20 mg, chosen to give a sample which is transparent in a given spectral domain. This type of technique is routinely used for the in situ study of catalysts. When the sample is too absorbent, even for pellet weights of the order of 6 to 7 mg, another method is used in which the sample is deposited via friction on a KBr plate. This technique only allows approximately 0.5 to 4 mg of product to be deposited and may result in detection problems. 

The hydroxy oxides, especially y boehmite, have excellent coalescing properties so they are particularly good for forming large support pellets and granules. They are also used as wash coats to provide porous surfaces for catalyst adhesion to non-porous materials such as metals and ceramics. 

Simulations of sorption experiments on model representations of catalyst support pellet macroscopic structure derived from NMR images... 

Catalyst powders as self-supporting pellets, were put into an IR cell connected to a conventional vacuum line. The catalysts were reduced in the IR cell in the conditions reported above. CO (99.99 %, Air Liquide) was adsorbed at room temperature after liquid nitrogen trapping. Spectra were recorded with a Bruker IFS 66V spectrometer, using a 4 cm 1 resolution and 128 accumulating scans. 

Fig. 10.1.3 [Kop3] Moisture transport in cylindrical catalyst support pellets from alumina with a diameter of 3.5 mm. (a) Drying profiles of water along the diameter of an initially wet piellet. Dotted lines Experimentally detected profiles. Solid lines simulated profiles. The first profile was acquired when the dry gas flow was turned on. The delay between the detection of the successive profiles is 60s. Profiles 1-7,9, 11,14, and 18 are shown, (b) Experimental profiles for water vapour sorption by an initially dry pellet containing CaC with uniform salt distribution.

The IR spectra were obtained with a Bruker IFS 48 spectrometer. The catalyst powders, as self-supporting pellets, were placed in an infrared cell allowing adsorption-desorption experiments to be carried out in situ. Prior to the adsorption of toluene, the cell was evacuated (1.0x10 Torr) at room temperature. 

The catalyst pellets and the processes taking place on them represent the heart of this important and troublesome reactor. Modem catalysts for this reaction are usually non-porous with active material coating the external surface of the support pellets. Only few steady state cases are presented in order to show the complexity of the behaviour of the catalyst pellets. The behaviour is quite complex although the governing mathematical equations are simple. The only mass and heat transfer resistances in this case are the external mass and heat transfer resistances which are evaluated using J-factor correlations. The effect of the different parameters on the effectiveness factors are shown on the effectiveness factors vs. bulk temperature diagrams. 

To minimize this problem, self-supporting pellets are usually prepared, and the minimum pressure required to form a mechanically stable pellet is used. Although much information related to surface acidity has been obtained with transmission techniques, there remains some question regarding the extent of surface alteration due to sampling procedures. 

The adsorption of NO and CD3CN were studied by FT-IR (Nicolet Mx-IE). Heatable, high vacuum IR cuvette with samples in the form of self supporting pellets( 10 mg/cm2) were used. Before measurements, all samples were evacuated at 350°C overnight. In all the experiments the equilibrium amount of either NO (2 Torr) or CD3CN ( 1 Torr) was adsorbed at 25°C for 15 minutes and spectrum recorded. The normalized absorbance of the respective bands was measured after desorption at room temperature and 100 °C its variation was selected to... 

Each of the prepared supported metal catalysts (40 mg) was pressed into self-supporting pellet of 20 mm in diameter, and was placed in an infrared cell, which was connected to an iso-volumetric system equipped with a vacuum line. The pressure of this system can be measured to the order of 1 x 10 Torr by a capacitance manometer. The catalysts were pretreated with O2 at 723 K for ten hours and then reduced under H2 at 723 K for ten hours, followed by evacuation at the same temperature for one hour before use. Adsorption of CO was carried out at 298 K. 

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