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Ind. Eng. Chem. Res. 2007, 46, 7838-7843
RESEARCH NOTES Preparation of a Solid Phosphoric Acid Catalyst from Low-Quality KieselguhrsParameters Controlling Catalyst Quality and Performance Nicolaas M. Prinsloo* Fischer-Tropsch Refinery Catalysis, Sasol Technology, Research & DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa
High-quality Kieselguhr is normally employed as the support for solid phosphoric acid (SPA), the catalyst used for the oligomerization of C3-C5 alkenes in a Fischer-Tropsch refinery. In this paper, we demonstrate that an SPA catalyst of suitable activity can be prepared from a low-quality Kieselguhr provided that the Kieselguhr bulk density is e0.3 g/mL. Several catalyst samples were evaluated on microreactor and pilotplant scale using propene mixtures as feed. An increase in acid-to-Kieselguhr ratio used during catalyst preparation was shown to improve catalyst activity, although this is detrimental to catalyst particle strength under similar calcination conditions. By studying the effect of impurities on catalyst performance, it was confirmed that high impurity levels lower oligomerization activity. It was also established that a high Kieselguhr quartz content (>10% by mass) reduces the catalyst particle crushing strength. 1. Introduction Solid phosphoric acid (SPA) catalysis has been applied for the oligomerization of light olefins since the early 1930s.1 Some of the early processes involved quartz chips coated with phosphoric acid,2 whereas the modern version consists of phosphoric acid impregnated onto a porous support. SPA is produced nowadays by mixing of phosphoric acid with Kieselguhr (also referred to as diatomite or diatomaceous earth) and extrusion of the mixture, followed by calcination at high temperature.3 Kieselguhr or diatomite is a naturally occurring mineral that consists mostly of the skeletons of diatoms.4 The structure of these skeletons, consisting mainly of amorphous silica (SiO2‚xH2O), provides the porosity and surface area required from the catalyst support. Kieselguhr processing involves drying, gentle milling, and screening. During processing, care is taken to minimize the mechanical damage to the structure of the diatoms, i.e., the hollow forms should not be destroyed and the pores should not be blocked, e.g., by sintering during calcination. The resulting catalyst consists of several catalytically active, phosphoric acid species that are supported on a mixture of silica and silicon phosphates.5,6 At Sasol Synfuels (South Africa), SPA is used to produce petrol, diesel, and jet-fuel blending components from a mixture of light olefins (C3-C5).7 From a catalyst consumption perspective, Sasol is one of the largest SPA users in the world with 1000 metric ton of catalyst loaded at any given time. The lifetime of SPA is comparatively short ((500 (kg of product)/ (kg of catalyst)),3 and the catalyst cost must, therefore, be kept as low as possible. As there is a significant price differential between catalyst grade and other Kieselguhr grades (e.g., those used in filtration-aid production), SPA catalyst production cost can, thus, be lowered significantly if it is not required to use high-quality Kieselguhr. It is, therefore, vital to show that it is possible to prepare an active SPA catalyst using natural Kieselguhr with high impurity levels. * E-mail:
[email protected]. Tel.: +27 16 960 2749. Fax: + 27 11 522 4388.
Aomura and Ohtsuka showed that the selection of the Kieselguhr source is important, with the most active SPA catalyst resulting from a Kieselguhr with low bulk density, large micropore volume, high acid holding power, and high heat stability.8 Apart from differences in the structure of the diatoms, different Kieselguhr sources also contain variable amounts of impurities, e.g., feldspar, mica, clay minerals, calcium carbonate, quartz (
