Calcium aluminate cement based catalyst

Shaped particles suitable for use as a catalyst, or precursor thereto, particularly for the decomposition of hypohalite ions in aqueous solution, comprising a high alumina cement having an aluminium to calcium atomic ratio above 2.5 and at least one oxide of a Group VIII metal M selected from nickel and cobalt, said particles containing 10 to 70% by weight of said Group VIII metal oxide and having a porosity in the range of 30 to 60%, in which at least 10% of the pore volume is in the form of pores of size in the range 15 to 35 nm and less than 65% of the pore volume is in the form of pores of diameter greater then 35 nm are disclosed.

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 particles, 
e.g. extrudates, containing a calcium aluminate cement, an oxide of cobalt 
and/or nickel, and optionally a finely divided diluent material, the 
shaped particles having specified porosity and specified pore size 
distribution characteristics. 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 
particles was desirable as it allows the reactants to have ready access to 
the active material within the particles. However this had the 
disadvantage that the strength of the particles was decreased. 
We have now found that by employing a calcium aluminate cement having a 
high alumina content, the porosity can be increased without undue loss of 
strength and as a result the activity of the catalyst can be increased. 
Surprisingly, despite the higher porosity, the bulk density of the 
particles can be increased so that a greater mass of particles, 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 particles, with tapping of the vessel to ensure that the 
particles settle, and then determining the weight of particles in the 
vessel.] 
Calcium aluminate cements are hydraulic cements containing one or more 
calcium aluminate compounds of the formula nCaO.mAl.sub.2 O.sub.3 where n 
and m are integers. Calcium aluminate compounds mentioned in the aforesaid 
specification include calcium mono-aluminate CaO.Al..sub.2 O.sub.3, 
tri-calcium aluminate 3CaO.Al.sub.2 O.sub.3, penta-calcium tri-aluminate 
5CaO.3Al.sub.2 O.sub.3, tri-calcium penta-aluminate 3CaO.5Al.sub.2 
O.sub.3, and dodeca-calcium hepta-aluminate 12CaO.7Al.sub.2 O.sub.3. The 
calcium aluminate cement used in the examples of that specification had an 
aluminium to calcium atomic ratio of about 1.4. By the term high alumina 
cement, we mean a calcium aluminate cement having an aluminium to calcium 
atomic ratio above 2.5. Such cements are known and may contain one or more 
of the above calcium aluminate compounds, and or compounds such as calcium 
di-aluminate CaO.2Al.sub.2 O.sub.3, in some cases with additional alumina. 
Accordingly the present invention provides shaped particles suitable for 
use as a catalyst, or precursor thereto, comprising a calcium aluminate 
cement having an aluminium to calcium atomic ratio above 2.5, and 
preferably above 4.0, and at least one oxide of a Group VIII metal M 
selected from nickel and cobalt, said particles containing 10 to 70% by 
weight of said Group VIII metal oxide (expressed as the divalent oxide, 
MO) and having a porosity in the range 30 to 60%, particularly 40 to 55%, 
in which at least 10% of the pore volume is in the form of pores of size 
in the range 15 to 35 nm and less than 65% of the pore volume is in the 
form of pores of diameter greater than 35 nm. 
As described in the aforesaid EP 0 397 342, for some catalytic applications 
the Group VIII metal oxide is the catalytically active species while for 
other catalytic applications the Group VIII metal oxide is a catalyst 
precursor and the catalytically active species is the product of reducing 
the Group VIII metal oxide to the Group VIII metal or is the product of 
oxidising the Group VIII metal oxide in the precursor to a higher 
oxidation state. For example catalysts obtained by reduction of a 
precursor containing nickel and/or cobalt oxide are of use as 
hydrogenation catalysts, e.g. methanation catalysts for the hydrogenation 
of carbon oxides to methane or catalysts for the hydrogenation of aromatic 
compounds such as benzene to cyclohexane. Another use of supported nickel 
and/or cobalt oxides is as catalysts for the decomposition of oxidising 
agents such as hypochlorite ions in aqueous solutions, for example in the 
treatment of effluents containing such ions prior to discharge of into 
rivers, lakes, or estuaries. 
The shaped particles 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. Particles having a greater aspect ratio may be 
liable to suffer from breakage during use. The shaped particles preferably 
have a weight average maximum dimension in the range 2 to 8 mm, 
particularly 3 to 8 mm. This ensures that the particles have a relatively 
high a geometric surface area per unit bed volume, so that a bed of the 
particles has a relatively large external particle area exposed to the 
reactants without the presence of an undue proportion of fines which would 
lead to unacceptable pressure drop on passage of reactants through a bed 
of the particles. 
The particles of the invention have a porosity in the range 30 to 60%, 
particularly 40-55%. By the term porosity we mean the ratio of the volume 
of the pores to the volume of the particle. Porosity may be determined by 
measurement of the mercury and helium densities of the particles: the 
porosity (as a percentage) is given by 
EQU porosity=P.sub.Hg .times.[1/p.sub.Hg -1/p.sub.He ].times.100 
where P.sub.Hg and P.sub.He are respectively the densities of the particles 
measured by displacement of mercury and helium. The mercury density is 
thus a measure of the particle density, while the helium density is a 
measure of the skeletal density. 
The particles of the invention have a particular pore size distribution. 
This may be determined by mercury intrusion porosimetry. In the particles 
of the invention, at least 10%, and preferably 10 to 40%, of the pore 
volume is in the form of pores of average diameter in the range 15-35 nm 
and less than 65% of the pore volume is in the form of pores of average 
diameter above 35 nm. Particles having such a pore size are of particular 
utility where they are used for the decomposition of oxidising agents in 
aqueous media. 
Largely as a result of the porosity and pore size distribution, the 
particles also have a relatively high BET surface area, above 10, and in 
particular in the range 20-100, m.sup.2.g.sup.-1. 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 particles of the 
invention have a bulk density in the range 0.8 to 1.5, preferably 0.9 to 
1.4, g.cm.sup.-3. The bulk density is indicative of the weight of catalyst 
in a bed of given volume. 
During use of the particles as a catalyst for the decomposition of 
oxidising agents, e.g. in effluents, the BET surface area, porosity and/or 
pore size distribution may change: thus the BET surface area, porosity, 
and the proportion of pores of size less than 35 nm may increase. The 
surface area, density, and porosity parameters of the shaped particles 
referred to herein refer to the parameters of the particles in the "as 
made" state, i.e. before use for catalytic purposes. 
Shaped particles having the required porosity and pore volume 
characteristics may be made by a particular pelleting method as described 
hereinafter. 
In addition to the high alumina cement, the composition comprises at least 
one oxide of a Group VIII metal selected from nickel and cobalt. 
Preferably the Group VIII metal is nickel alone, or nickel in admixture 
with cobalt in an amount of up to one mole of cobalt per mole of nickel. 
Calcium aluminate cements are often contaminated with iron compounds, e.g. 
iron oxide. In the aforesaid EP 0 397 342 we indicated that the presence 
of iron oxide was beneficial where the shaped particles were to be used 
for hypochlorite decomposition as the iron exhibited some promoting effect 
on the catalytic activity. In contrast, in the present invention we have 
found that high activity catalysts can be produced with high alumina 
cements of low iron oxide content. In the present invention, the iron 
oxide content (expressed as Fe.sub.2 O.sub.3) of the shaped particles is 
preferably less than 1% by weight. A particularly suitable high alumina 
cement having a low iron oxide content is that known as CA-25 which 
typically contains about 80% alumina, primarily as mono-calcium aluminate 
in admixture with dodeca-calcium hepta-aluminate, calcium di-aluminate, 
and free alumina. 
The Group VIII metal oxide is preferably introduced into the composition by 
precipitation. A preferred route is to precipitate Group VIII metal 
compounds, decomposable to oxides by heating, from an aqueous solution of 
e.g. nitrates by addition of a precipitant such as an alkali metal 
carbonate solution. After precipitation of the Group VIII metal compounds, 
the precipitate is washed free of precipitant. The precipitate may be 
mixed with a finely divided, preferably inert, diluent material, such as 
magnesia and/or a clay, e.g. kaolin. The amount of such diluent material 
employed is conveniently up to twice the weight of the Group VIII metal 
compounds expressed as the divalent oxides. The mixture is then dried, and 
calcined, e.g. to a temperature in the range 200-600.degree. C., 
particularly 400-550.degree. C., to effect decomposition of the Group VIII 
metal compounds to the oxide form. Minor amounts of other ingredients, 
such as co-promoters such as magnesium oxide may be incorporated, e.g. by 
co-precipitation with the Group VIII metal compounds. The resultant 
composition is then mixed with the high alumina cement, optionally 
together with a processing aid such as a little water, a stearate of an 
alkaline earth metal, e.g. magnesium, and/or graphite, and formed into 
pellets. The proportion of cement employed is generally 25 to 100% by 
weight based on the total weight of the Group VIII metal oxide, or oxides, 
and any diluent material, and is such as to give a composition containing 
10 to 70%, particularly less than 50%, and most preferably 20 to 40%, by 
weight of the Group VIII metal oxide or oxides. 
In order to obtain shaped particles of the requisite pore volume 
characteristics, the mixture is conveniently pelletised by means of a 
pellet mill, for example of the type used for pelleting animal feedstuffs, 
wherein the mixture to be pelleted is charged to a rotating perforate 
cylinder through the perforations of which the mixture is forced by a bar 
or roller within the cylinder. The resulting extruded mixture is cut from 
the surface of the rotating cylinder by a doctor knife positioned to give 
pellets of the desired length. It will be appreciated that other extrusion 
techniques may be employed to give shaped particles of the desired 
characteristics. 
After forming the composition into the desired shaped particles, the latter 
are then preferably contacted with water, preferably as steam, to effect 
hydration of the cement and to give the shaped particles adequate 
strength. 
Shaped particles formed by this method have a significantly lower strength, 
e.g. as measured by a crushing test, than pellets prepared by a 
conventional tabletting technique but it is found that, even so, the 
strength is adequate for the applications envisaged and, indeed, the 
strength generally increases where the catalyst is employed for the 
decomposition of oxidising agents in aqueous media, presumably as a result 
of continued hydration of the cement. 
For use for the decomposition of oxidising agents, the catalyst bed is 
contacted with a fluid medium, particularly aqueous, containing the 
oxidising agent to be treated. Examples of oxidising agents that may be 
decomposed using the shaped particles of the invention include hypohalite 
ions, for example hypochlorite and hypobromite ions, and hydrogen 
peroxide. At least some of such oxidising agents are pollutants in various 
industrial processes. In particular hypochlorite ions are a significant 
industrial pollutant. The catalysts may also find utility in the treatment 
of aqueous media containing organic pollutants: thus as hypochlorite may 
be added to an aqueous medium containing an oxidisable organic compound 
and the solution passed through a bed of the catalyst. The catalyst 
catalyses the decomposition of the hypochlorite which effects oxidation of 
the organic compound to more environmentally acceptable products such as 
carbon dioxide and water. 
Conveniently a fixed bed of the catalyst particles is formed and the medium 
containing the oxidising agent, for example hypochlorite ions, is passed 
through the bed. Generally the medium is in the form of an aqueous 
solution which has been filtered prior to contact with the catalyst bed. 
The treatment of aqueous media is conveniently effected under conditions 
such that the pH of the medium is above 7, preferably above 8; it is a 
particularly beneficial aspect of the invention that the particles do not 
physically disintegrate even at pH levels in the range 10 to 14. The 
process can be performed at any convenient temperature, suitably in the 
range 5-100.degree. C., more suitably in the range 20-80.degree. C. 
When the shaped particles are contacted with the oxidising agent in an 
aqueous medium, some or all of the oxides of the particles may become 
hydrated. In addition the Group VIII metal oxides are oxidised to higher 
valency states. For example nickel oxide can be notionally considered to 
be initially present in the particles as NiO. Authorities vary as to 
precisely what higher oxides of nickel are formed but it may be regarded 
that the higher oxides Ni.sub.3 O.sub.4, Ni.sub.2 O.sub.3 and NiO.sub.2 
are formed on contact with the oxidising agent. Such higher oxides are 
active in the process of decomposition of the oxidising agent. In the 
particles of the present invention, the Group VIII metal oxides may be as 
initially formed or in their higher oxidation states, as formed in use. In 
use the oxides may also be present as hydrates. It should be noted, 
however, that the proportions specified herein of the Group VIII metal 
oxide in the particles are expressed on the basis of anhydrous oxides with 
the Group VIII oxides in the divalent state, i.e. NiO and/or CoO. 
In addition to use for the decomposition of oxidising agents as described 
above, the shaped particles of the invention are also of use as precursors 
to hydrogenation catalysts, and may be converted to the catalytically 
active form by reduction, e.g. with a stream of a hydrogen-containing gas 
at an elevated temperature. Such reduction may be effected after charging 
the particles to a vessel in which the hydrogenation is to be effected. 
Alternatively, the reduction may be effected as a separate step prior to 
charging the particles to the hydrogenation reactor and, if desired, the 
reduced particles may be passivated by contact with a gas stream 
containing a small amount of oxygen, or with carbon dioxide followed by a 
gas stream containing a small amount of oxygen, until no further reaction 
occurs when the particles may then be handled in air at ambient 
temperature.

The invention is illustrated by the following example in which all parts 
and percentages are by weight. 
A slurry containing precipitated basic nickel carbonate, and a mixture of 
finely divided magnesia and kaolin as diluent materials, was filtered, 
washed, dried, and then calcined at 400-450.degree. C. The proportions of 
the ingredients were such that the calcined material contained about 14.1 
parts of magnesia and about 113 parts of kaolin per 100 parts of nickel 
oxide. 
100 parts of the calcined material were then mixed with about 2 parts of 
graphite and 41 parts of a high alumina cement, CA-25, having an aluminium 
to calcium atomic ratio of about 4.9 and having an iron content of about 
0.2% to give a dry feed mixture. 
The dry feed mixture was then mixed with water (25 parts per 100 parts of 
the cement-containing mixture), formed into extruded pellets of diameter 
of about 3 mm and lengths in the range of about 3 to 5 mm using a pellet 
mill as described hereinbefore, and then dried to give extrudates A. 
For purposes of comparison the above procedure was repeated using a calcium 
aluminate cement having an aluminium to calcium atomic ratio of about 1.1 
and an iron oxide content (expressed as Fe.sub.2 O.sub.3) of about 14% in 
place of the high alumina cement. The resultant extrudates were termed 
extrudates B. The properties of the extrudates are shown in the following 
table. 
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Extrudates 
A B 
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Calculated NiO 31.3 31.1 
composition Al.sub.2 O.sub.3 38.5 26.9 
(wt %) - after CaO 5.3 11.7 
ignition at MgO 4.5 4.6 
900.degree. C. SiO.sub.2 20.0 21.2 
Fe.sub.2 O.sub.3 0.4 4.5 
Porosity (%) 47 35 
Pore volume 15-35 nm 16 35 
&gt;35 nm 54 38 
Bulk density (g/ml) 1.20 1.05 
BET surface area (m.sup.2 /g) 46 43 
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The activity of the catalysts for the decomposition of hypochlorite was 
assessed by charging 120 ml of the extrudates to a reactor of internal 
diameter 2.5 cm to form a catalyst bed therein. A feed of an aqueous 
solution containing 63.1 g/liter of sodium hypochlorite and having a pH of 
about 12.5 was preheated to about 30.degree. C. was fed to the reactors at 
a space velocity of 0.8 h.sup.-1 so that the hypochlorite solution flowed 
up through the catalyst bed. The exit sodium hypochlorite 10 concentration 
was found to be 0.56 g/l for extrudates A and 1.85 g/l for extrudates B 
indicating that the extrudates had a significantly greater activity. In 
addition, from the above table, it is seen that A had a significantly 
greater bulk density than that of extrudates B.