Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1999-03-18
2001-07-17
Griffin, Steven P. (Department: 1754)
Liquid purification or separation
Processes
Chemical treatment
C502S326000, C502S337000, C502S327000
Reexamination Certificate
active
06261465
ABSTRACT:
This invention relates to catalysts and in particular to catalysts, or precursors thereto, containing an inert support material and at least one oxide of a metal of Group VIII of the Periodic Table and selected from nickel and cobalt.
In our EP 0 397 342 we describe catalysts in the form of shaped units, e.g. extrudates, containing a calcium aluminate cement, an oxide of cobalt and/or nickel, and optionally a finely divided diluent material, the shaped units having specified porosity and specified pore size distribution characteristics. Inter alia, the units had a porosity of 25-50% and less than 40% of the total pore volume was in the form of pores of size greater than 35 nm. These catalysts were of particular utility for the decomposition of hypochlorite ions in an aqueous medium.
In that specification we indicated that increasing the porosity of the units was desirable as it allows the reactants to have ready access to the active material within the units. However this had the disadvantage that the strength of the units was decreased.
Methanation catalysts having a high porosity and based upon nickel oxide, alumina and a calcium aluminate cement are described in GB 1 278 424. Those materials were granules made by a dish granulation technique and had a high porosity (55-75%) and at least 80% of the total pore volume was formed from pores of size greater than 35 nm. The bulk density of the catalysts was relatively low, namely 0.4 to 0.656 g/ml. As shown in the aforesaid EP 0 397 342, granules made in accordance with the procedure of GB 1 278 424 had a low crush strength and, when employed for hypochlorite decomposition, rapidly disintegrated and were leached away.
In our PCT application GB 96/01698 (now published as WO 97 04870) we showed that by employing a calcium aluminate cement having a high alumina content, the porosity of catalysts of the type described in the aforesaid EP 0 397 342 can be increased without undue loss of strength and as a result the activity of the catalyst can be increased.
We have now devised catalysts of still greater activity. In the aforesaid EP 0 397 342 and PCT application GB 96/01698, although described as being optional, all the exemplified materials contained a significant amount of kaolin as a clay diluent. As a result the catalysts contained a significant amount of silica. We have found that if the day is omitted and alumina, alone or in combination with magnesia, is employed as a diluent, catalysts of greater macroporosity and activity, but still having adequate strength, can be obtained.
Surprisingly, despite the higher porosity, a relatively high bulk density can be achieved so that a significant mass of units, and hence active material, can be accommodated in a catalyst bed of given volume. [The bulk density is determined by filling a vessel of known volume with the catalyst units, with tapping of the vessel to ensure that the units settle, and then determining the weight of units in the vessel.]
Accordingly the present invention provides shaped units suitable for use as a catalyst, or precursor thereto, comprising a compacted particulate mixture of at least one oxide of a Group VIII metal M selected from nickel and cobalt, a calcium aluminate cement, and alumina and/or magnesia, said shaped units having (after ignition in air at 900° C.) a content of said Group VIII metal oxide of 10 to 50% by weight (expressed as the divalent oxide, MO), a calcium oxide content of 1 to 10% by weight, an alumina content by weight that is at least four times the weight of calcium oxide, a total content of alumina plus magnesia of at least 40% by weight, and a silica content of less than 1% by weight, and said shaped units having a pore volume in the range 02 to 0.5 ml/g and having a pore size distribution such that from 5 to 25% of the total pore volume is in the form of pores of diameter in the range 15 to 35 nm, at least 30% of the total pore volume is in the form of pores of diameter greater than 35 nm, and 10 to 20% of the total pore volume is in the form of pores of diameter greater than 1500 nm.
Thus in the shaped units of the invention, there are substantial amounts of porosity in the form of pores in the size ranges 15 to 35 nm and 35 to 1500 nm, and a significant, but not excessive, amount of macroporosity in the form of pores of size above 1500 nm.
As used herein the porosity of the units is determined by mercury intrusion porosimetry, i.e. by measuring the volume of mercury intruded into a sample of the units at increasing applied pressures. As defined herein the total pore volume of a sample of the units is the volume of mercury intruded into the sample at a pressure corresponding to intrusion of all open pores of diameter greater than 3.5 nm. The proportion of pores of size above 35 nm is the ratio between a) the volume of mercury intruded into the sample at a pressure corresponding to intrusion of all open pores of diameter greater than 35 nm and b) the total pore volume. Correspondingly the proportion of pores of size above 1500 nm is the ratio between a) the volume of mercury intruded into the sample at a pressure corresponding to intrusion of all open pores of diameter greater than 1500 nm and b) the total pore volume, and the proportion of pores of size in the range 15-35 nm is the ratio between a) the difference between the volume of mercury intruded into the sample at a pressure corresponding to intrusion of all open pores of diameter greater than 15 nm and the volume of mercury intruded into the sample at a pressure corresponding to intrusion of all open pores of diameter greater than 35 nm and b) the total pore volume.
In the units of the invention, in order to give a high activity, the pore volume is in the range 02 and 0.5 ml/g, preferably 02 to 0.4 ml/g, and at least 30%, preferably 35 to 80%, of the total pore volume is in the form of pores of diameter above 35 nm, and at least 10% of the total pore volume is in the form of pores of size above 1500 nm. The presence of a substantial proportion of the pore volume in the form of the large pores, i.e. above 35 nm and above 1500 nm permits ready access of the reactants to the active material within the shaped units, and, where, the shaped units are used as catalysts for the decomposition of oxidants, facilitates the passage of gaseous oxygen formed from such decomposition out of the shaped units.
However, in order that the units have adequate strength, it is important that not more than 20% of the pore volume is in the form of pores of diameter above 1500 nm and that substantial proportion of the porosity is given by pores of size less than 35 nm diameter. Thus in the units of the invention 5 to 25%, preferably 10 to 20%, of the total pore volume is given by pores of diameter in the range 15 to 35 nm. Indeed it is preferred that at least 10% of the total pore volume is provided by pores of diameter below 15 nm. The presence of a substantial proportion of pores of diameter below 35 nm also ensures that the BET surface area of the shaped units is relatively high. It is preferred that the BET surface area is at least 10, and in particular in the range 20-100, m
2
/g. As a result the active material is present in a finely divided state. Such a BET surface area may be achieved by introducing the Group VIII metal oxide into the composition by a precipitation route as described hereinafter.
As a result of their composition and porosity, the shaped units of the invention have a bulk density in the range 0.8 to 1.5, preferably 0.9 to 1.4, g.cm
−3
. The bulk density is indicative of the weight of catalyst in a bed of given volume.
The shaped units are preferably in the form of granules, extrudates, or pellets and preferably have an aspect ratio, by which we mean the ratio of the weight average maximum geometric dimension, e.g. length, to weight average minimum geometric dimension, e.g. diameter, of less than 3, particularly less than 2. Shaped units having a greater aspect ratio may be liable to suffer from breakage during use. The shaped units preferably have a weight averag
Griffin Steven P.
Ildebrando Christina
Imperial Chemical Industries plc
Pillsbury & Winthrop LLP
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