Ultrafine catalysts

Therefore, when operating in the filter cake mode, the axial velocity should be maintained at a level such that an adequate shear force exists along the filter media to prevent excessive caking of the catalyst that could cause a blockage in the down-comer circuit. For the separation of ultrafine catalyst particles from FT catalyst/wax slurry, the filter medium can easily become plugged using the dynamic membrane mode filtration. Also, small iron carbide particles (less than 3 nm) near the filter wall are easily taken into the pores of the medium due to their low mass and high surface area. Therefore, pure inertial filtration near the filter media surface is practically ineffective. 

Results from constant differential pressure filtration tests have been analyzed according to traditional filtration science techniques with some modifications to account for the cross-flow filter arrangement.11 Resistivity of the filter medium may vary over time due to the infiltration of the ultrafine catalyst particles within the media matrix. Flow resistance through the filter cake can be measured and correlated to changes in the activation procedure and to the chemical and physical properties of the catalyst particles. The clean medium permeability must be determined before the slurries are filtered. The general filtration equation or the Darcy equation for the clean medium is defined as... 

The typical BET surface area of the freshly prepared iron carbide catalyst is approximately 70 m2 g 1. The surface area of the precipitated catalyst and ultrafine catalyst before pretreatment was 140 m2 g 1 and 250 m2 g 1, respectively however, following pretreatment with CO the surface areas dropped to 32 m2 g 1, and 64 m2 g-1, respectively. The particle sizes of the iron carbide and precipitated catalysts after 170 h, determined by X-ray line broadening, were 27 nm and 30 nm, respectively. The ultrafine catalyst had an average particle size of 25 nm after 240 h of synthesis. These particle sizes correspond to a surface area of about 40 m2 g-1. 

Conversion data as a function of time of synthesis for the three catalysts are shown in Figures 19.2-19.4. In general the precipitated catalyst is the most active of the three catalysts studied. The precipitated catalyst had a peak CO conversion of 84% compared to 50% and 40% for the iron carbide and ultrafine catalysts, respectively. All three catalysts deactivated at approximately the same rate, 0.19-0.15% h-1. This suggests that the same deactivation mechanism was responsible for each catalyst. It is unlikely that the deactivation of the catalysts was caused by bulk phase changes in the catalysts because it has been shown that the activity of the ultrafine iron oxide catalyst does not change significantly upon being converted from... 

The phase composition changes for the ultrafine catalyst were also very similar to those seen for the precipitated catalyst. Mossbauer spectroscopy... 

Deng J, Sun Q, Zhang Y, Chen S, Wu D (1996) A novel process for preparation of a Cu/ZnO/ AI2O3 ultrafine catalyst for methanol synthesis from CO2-1-H2 comparison of various preparation methods. Appl Catal Gen 139 75-85... 

This method is one of the dry methods in which no chemical reaction is involved. Preparation of ultrafine particles by physical vapor deposition (PVD) dose not require washing and calcination, which are indispensable for chemical preparation such as in CP and DP methods. As waste water and waste gases are not by-produced, the arc plasma (AP) method is expected to grow in popularity as one of the industrial production methods for gold catalysts and as a clean preparation method. 

Homogeneous deposition of ultrafine metal particles on the surfaces of fine powder is not easy using PVD. A device for stirring the powder support in a vacuum chamber is needed to avoid heterogeneous deposition. Sputter deposition units equipped with stirring powder supports have already been adapted for the industrial production of Ti02 and carbon-supported gold catalysts by 3M [35]. 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

To continuously separate FT wax products from ultrafine iron catalyst particles in an SBCR employed for FTS, a modified cross-flow filtration technique can be developed using the cross-flow filter element placed in a down-comer slurry recirculation line of the SBCR. Counter to the traditional cross-flow filtration technique described earlier, this system would use a bulk slurry flow rate below the critical velocity, thereby forcing a filter cake of solids to form between the filter media and the bulk slurry flow, as depicted in Figure 15.2b. In this mode, multiple layers of catalyst particles that deposit upon the filter medium would act as a prefilter layer.10 Both the inertial and filter cake mechanisms can be effective however, the latter can be unstable if the filter cake depth is allowed to grow indefinitely. In the context of the SBCR operation, the filter cake could potentially occlude the slurry recirculation flow path if allowed to grow uncontrollably. 

A blend of Polywax 500 and 655 (81.3 and 18.7 wt%, respectively) with a CO-activated ultrafine iron catalyst was used for the evaluation of catalyst/ wax slurry filtration performance of the filter module with and without an alcohol compound. All of the filtration tests were conducted with a TMP of... 

As shown in Figure 15.10, the baseline flux, without 1-hexadecene addition, stabilized to 0.30 lpm/m2 at 473 K with a TMP of 1.4 bar. Similar to previous filtration runs using an activated ultrafine iron catalyst slurry, the duration of the induction period for the catalyst particles and membrane was approximately 48 h TOS. Initially, the appearance of permeate was bright white. With the first dosage... 

Sarkar, A., Neathery, J. K., and Davis, B. H. 2006. Separation of Fischer-Tropsch wax products from ultrafine iron catalyst particles. U.S. DOE Final Technical Report, Contract DE-FC26-03NT41965. 

Duff, D.G. and Baiker, A., Preparation and structural properties of ultrafine gold colloids for oxidation catalysis, in Preparation of Catalysts VI, Poncelet, G., Martens, J., Delmon, B., Jacobs, P.A., and Grange, P., Eds., Elsevier, Amsterdam, 1995, p. 505. 

New catalysts such as a Raney Cu-based system containing Zr49 and ultrafine CuB with Cr, Zr, and Th50 exhibit good characteristics. The improved catalyst performance observed for a Cu-ZnO catalyst with added Pd is explained by the relative ease of hydrogen dissociation by the incorporated Pd particles and then spillover to the Cu-ZnO.51... 

Based on these characterizations, a model structure of 0.1 wt% Ni-loaded K4Nb60i7 was proposed as shown in Fig. 16.5. During the loading of the catalyst with nickel, most of the nickel enters the interlayer region I as Ni2+ by replacing K+ ions, leaving a very small fraction on the external surface. During reduction at 700°C, the Ni2+ cations are reduced to metallic nickel in the form of ultrafine particles of about 0.5 nm size. 

Figure 19.4 Synthesis gas conversion as a function of time for the ultrafine iron oxide catalyst pretreated with CO (weight = 10.0 g, Sg = 64 m2 g-1). O, CO , H2 0> CO + H2.

The ratio of C02 produced to CO converted is 0.5 for this case. The C02/ CO ratios presented in Table 19.2 show the water-gas shift activity increased in the order Fe carbide < precipitated < ultrafine oxide. The ability of the Fe carbide catalyst to suppress the water-gas shift enables it to have a high hydrocarbon production. 

The ultrafine iron oxide catalyst run was conducted in a 300 mL autoclave operated as a continuous stirred tank reactor (CSTR) using a configuration similar to that reported previously.12 The catalyst (10 g) was slurried with a C30 oil (ethyl), pretreated with CO (Liquid Carbonic,... 

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