Process for producing a hydrogenation catalyst

Hydrogenation catalyst production by calcining a calcium aluminate cement catalyst support at 900.degree.-1050.degree. C. followed by impregnation with a palladium salt solution of pH below 1.3.

This invention relates to a method of making a material suitable for use as 
a catalyst for the selective hydrogenation of highly unsaturated 
hydrocarbons in the presence of less unsaturated hydrocarbons. 
The manufacture of unsaturated hydrocarbons usually involves cracking 
saturated and/or higher hydrocarbons and produces a crude product 
containing, as impurities, hydrocarbons that are more unsaturated than the 
desired product but which are very difficult to separate by fractionation. 
The commonest example is ethylene manufacture in which acetylene is a 
contaminant. In a similar way, the formation of propylene is accompanied 
by hydrocarbons of the empirical formula C.sub.3 H.sub.4 (methyl acetylene 
and/or allene), and the formation of butadiene by vinyl acetylene. Such 
highly unsaturated hydrocarbons can be removed by hydrogenation using 
process conditions and a carefully formulated catalyst such that no 
significant hydrogenation of the desired hydrocarbon takes place. 
Two general types of gaseous selective hydrogenation processes for 
purifying unsaturated hydrocarbons have come into use. One, known as 
"front-end" hydrogenation, involves passing the crude gas from the initial 
cracking step, after removal of steam and condensible organic matter, over 
a hydrogenation catalyst. The crude gas normally contains a relatively 
large amount of hydrogen, far in excess of that required to hydrogenate 
the acetylenes and sufficient in fact to hydrogenate a substantial part of 
the olefin present. Despite this hydrogen excess, operation with 
sufficient selectivity to give olefins of polymerisation quality is well 
established and catalyst lives of many years are obtained. In the other 
type, known as "tail-end" hydrogenation, the crude gas is fractionated and 
the resulting product streams are reacted with hydrogen in slight excess 
over the quantity required for hydrogenation of the highly unsaturated 
hydrocarbons present. The tail-end hydrogenation is less critical than 
front-end hydrogenation in that at the low hydrogen excess a runaway 
reaction is not possible; however, there is a greater tendency to 
deactivation of the catalyst and formation of polymers from the highly 
unsaturated hydrocarbons may occur as an alternative to the hydrogenation 
thereof. Consequently periodic regeneration of the catalyst is required. 
Catalysts that have been found to be suitable for such selective 
hydrogenation reactions include palladium supported on certain alumina 
substrates, see for example UK Pat. No. 916,056 and U.S. Pat. No. 
4,126,645. The catalyst is normally employed in the form of shaped pieces 
such as pellets, e.g. small cylindrical particles: heretofore pellets of 
about 3 mm diameter and 3 mm height have been used. Such shaped pieces are 
made by shaping, e.g. pelletising, a suitable catalyst support 
composition: the resultant shaped pieces are then converted to the desired 
support form by calcining, e.g. at temperatures in the range 1000.degree. 
C. to 1200.degree. C. The calcining temperature affects the physical 
properties of the catalyst support, notably its porosity and surface area. 
After calcining the palladium is deposited on the support by, for example, 
dipping or spraying. 
One disadvantage of the alumina substrates (other than those of 
.alpha.-alumina) is that they often have poor strength and may tend to 
crumble. A further disadvantage of the alumina (including .alpha.-alumina) 
catalyst supports is that the precursors thereto tend to be difficult to 
fabricate, e.g. pelletise, into the desired shaped pieces. 
In U.S. Pat. No. 4,239,530 there are described selective hydrogenation 
catalysts formed from a support composition containing a calcium aluminate 
cement and having a calcium:aluminium atomic ratio of between 1:4 and 
1:10. This support composition is easier to fabricate than alumina and 
gives catalyst pieces that are of similar strength to those made from 
.alpha.-alumina and stronger than pieces formed from other grades of 
alumina. 
After forming the support composition into the desired shaped pieces they 
are calcined and then palladium is applied to the calcined support. 
While the palladium may be applied by a dry procedure such as sputtering, a 
catalyst precursor is preferably formed by a wet process by applying a 
solution of a palladium compound, for example a salt such as the chloride 
or nitrate, to the calcined catalyst support pieces e.g. by dipping or 
spraying, followed by drying. The resultant catalyst precursor is 
subsequently converted to the active catalyst by reducing the palladium 
compound to the active metal as described hereinafter. 
For good selectivity the palladium should be present essentially only at or 
near the surface of the catalyst pieces. Thus the average depth of 
penetration of the palladium into the catalyst pieces should be less than 
300 .mu.m, preferably below 210 .mu.m. 
The depth of penetration may be determined by cutting a catalyst piece 
which has previously been reduced by dipping in a hydrazine solution and 
observing the extent of the darker palladium containing region. 
Where a `wet` process is employed for the incorporation of the palladium 
compound, the degree of penetration depends on a number of factors, e.g. 
the pore volume of the calcined support, the pH of the solution of the 
palladium compound, and the alkalinity of the calcined support. Where the 
calcining temperature is relatively high, e.g. 1100.degree. C. or more, 
the desired penetration can be achieved by using a palladium compound, 
e.g. nitrate, solution having a pH of between about 1.7 and 1.9. However 
palladium nitrate solutions in this pH range are unstable and so this 
technique is not satisfactory for full scale operation, the use of 
solutions of lower pH, below 1.3, being desirable. The desired degree of 
penetration with solutions of pH below about 1.3 can be achieved by 
treatment of the calcined catalyst support pieces by dipping in an 
alkaline solution, e.g. aqueous solutions of hydroxides or carbonates of 
potassium or sodium, followed by drying, prior to dipping in the palladium 
salt solution. Such an alkali dip prior to calcining does not, however, 
give the desired control on the depth of palladium penetration. 
It has now been found that if lower calcining temperatures are employed, 
the desired degree of penetration can be achieved with palladium solutions 
of pH below 1.3 without the need for an alkaline pretreatment. It is 
thought that the use of such lower calcining temperatures results in the 
calcium oxide in the cement being less strongly bound and so renders the 
support more alkaline. Indeed the desired penetration can be achieved 
using palladium solutions of pH below 1.0. 
Accordingly the present invention provides a process for the manufacture of 
a precursor to a material suitable for use as a catalyst for the selective 
hydrogenation of highly unsaturated hydrocarbons in the presence of less 
unsaturated hydrocarbons comprising forming shaped pieces from a 
composition containing a calcium aluminate cement, said composition having 
a calcium:aluminium atomic ratio within the range 1:4 to 1:10, calcining 
said shaped pieces at a temperature in the range 900.degree. to 
1050.degree. C., and applying an aqueous solution of a reducible palladium 
compound having a pH below 1.3 to said calcined shaped pieces. 
The present invetion also provides a process for the manufacture of a 
material suitable for use as a catalyst for the selective hydrogenation of 
highly unsaturated hydrocarbons in the presence of less unsaturated 
hydrocarbons comprising heating a precursor obtained by the above process 
to decompose the palladium compound to the active metal. 
The calcining temperature is preferably within the range 930.degree. to 
1020.degree. C. 
By the term calcium aluminate cement we include those 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. Examples of such 
calcium aluminate compounds include calcium monoaluminate (CaO.Al.sub.2 
O.sub.3), tricalcium aluminate (3CaO.Al.sub.2 O.sub.3), penta calcium 
trialuminate (5CaO.3Al.sub.2 O.sub.3), tricalcium penta aluminate 
(3CaO.5Al.sub.2 O.sub.3), and dodeca calcium hepta aluminate 
(12CaO.7Al.sub.2 O.sub.3). Some calcium aluminate cements, e.g. the 
so-called "high alumina" cements, may contain alumina in admixture with, 
dissolved in, or combined with, such calcium aluminate compounds. For 
example, a well known commercial high alumina cement has a composition 
corresponding to about 18% calcium oxide, 79% alumina and 3% water and 
other oxides. This material has a calcium:aluminium atomic ratio of about 
1:5, i.e. 2CaO.5Al.sub.2 O.sub.3. 
The support composition employed in the present invention has a 
calcium:aluminium atomic ratio within the range 1:4 to 1:10, preferably 
1:5 to 1:8. Where the calcium aluminate cement is a "high alumina" cement, 
no additional alumina may be necessary but, in general, the support is 
made from the calcium aluminate cement to which an additional amount of 
alumina, conveniently in the form of the trihydrate, has been added. 
Hence the support will usually be a refractory composition consisting of a 
calcined mixture of alumina and one or more of said calcium aluminate 
compounds. 
The refractory composition should be relatively free of silica or 
silicates: preferably the silicon content is below 3% by weight of the 
composition. 
The catalyst support pieces may be made by forming a calcium aluminate 
cement, with additional alumina as necessary, into the desired shape and 
subsequently calcining the shaped pieces. 
Fabrication aids, such as graphite, may be incorporated into the 
composition prior to fabrication: typically the proportion of graphite is 
1 to 5% particularly 2 to 4%, by weight of the composition. 
To accelerate setting, a small amount of lime, e.g. up to 2% by weight of 
the composition, may also be incorporated into the fabrication 
composition. 
A typical composition suitable for pellet formation comprises 50 to 70% by 
weight of a calcium aluminate cement (comprising 75 to 85% by weight of 
alumina and 15 to 25% by weight of lime) mixed with 24 to 48% by weight of 
alumina trihydrate, 0 to 2% by weight of lime, and 2 to 4% by weight of 
graphite. 
We have found that the use of the calcium aluminate cement reduces 
considerably the pressure required to form catalyst support pieces of 
comparable strength to catalyst support pieces formed from alumina. This 
results in less wear and breakage of the pelletising machinery, e.g. 
punches and dies, and hence fabrication costs are reduced. 
The catalyst conveniently is of the fixed-bed type, that is, in the form of 
shaped pieces whose largest dimension is in the range 2 to 12 mm and whose 
shortest dimension is at least one third of their largest dimension. 
Cylindrical compressed pellets or extrusions or approximate spheres are 
very suitable. Shaped pieces such as extrusions or pellets are especially 
preferred because they can be cheaply and readily made. A useful 
alternative catalyst support is in the form of a honeycomb, the specified 
calcium aluminate material forming the whole of the honeycomb or, less 
preferably, a coating on the surface of a honeycomb made from another 
material. Where the specified calcium aluminate material is present as a 
coating (secondary support) on another material (primary support), the 
primary support should be one that will not cause undesirable reaction of 
the hydrocarbons or else should be coated thickly and coherently enough to 
prevent access by the hydrocarbons. Preferably the palladium is present 
only in the secondary support. In view of the advantages conferred by the 
use of the calcium aluminate material, viz ease of fabrication and 
strength, it is preferred that the whole of the catalyst support is made 
from the calcium aluminate material. 
Prior to calcining, the catalyst support pieces may be subjected to a water 
soaking step, followed by drying, to increase their strength. Preferably 
they are soaked in water for at least 12 hours and then dried, preferably 
at 100.degree. to 150.degree. C. Prior to such a soaking step, the 
catalyst support pieces may be fired at temperatures between 400.degree. 
and 500.degree. C., preferably for at least 2 hours. 
As is known in the art, the calcining conditions affect the surface area 
and pore size of the catalyst support pieces. The calcining conditions are 
preferably such that the calcined catalyst support, before incorporation 
of the palladium has a pore volume of at least 0.2 cm.sup.3 g.sup.-1 and a 
surface area of between 15 and 35 m.sup.2 g.sup.-1. 
The pore volume is defined as the difference between the reciprocal of the 
"mercury" density and the reciprocal of the "helium" density of the 
sample. These densities, and the surface area, are determined by the 
following methods which are applied to samples which have been dried in 
air at 110.degree. C.: 
1. Mercury density. The density of the catalyst immersed in mercury at 
20.degree. C. and 900 mm Hg pressure is determined after allowing 15 
minutes for the system to equilibriate. This measurement represents the 
density of the solid containing pores not penetrated by mercury, i.e. 
pores of radius small than about 6.times.10.sup.4 A. 
2. Helium density. The density of the catalyst immersed in helium at room 
temperature is determined: this represents the true density of the 
ultimate solid material. 
3. Surface area. This is determined, after degassing the sample in flowing 
nitrogen for 40 minutes at 150.degree. C., by the method of Brunauer, 
Emmett, and Teller (JACS 60 309 (1938)) by measuring the quantity of 
nitrogen absorbed on the catalyst at the boiling point of liquid nitrogen; 
in calculating the surface area, the cross sectional area of the nitrogen 
molecule is taken as 16.2 square Angstrom units. 
It has been found that generally the surface area increases on 
incorporation of the palladium. It is thought that this increase in 
surface area results from some rehydration of the catalyst support during 
palladium incorporation followed by the formation of alternative 
crystalline forms upon reheating. The surface area of the catalyst pieces, 
i.e. after incorporation of the palladium is preferably less than 50 
m.sup.2 g.sup.-1. 
The activity and selectivity of the catalyst depends on the amount of 
palladium incorporated, its distribution within the catalyst support, and 
its physical form. The overall palladium content of the catalyst pieces is 
preferably in the range 0.02 to 0.06% by weight. 
Throughout the palladium containing region of the catalyst pieces the 
palladium content should be at least 0.005% by weight while the average 
palladium content of this palladium containing region should be 0.05 to 2% 
by weight. At depths greater than the palladium containing region, small 
amounts, less than 0.005% by weight, of palladium may be present. 
As mentioned hereinbefore, after application of the palladium compound to 
the shaped pieces, they are dried, for example at a temperature within the 
range 25.degree. C. to 150.degree. C., conveniently at about 100.degree. 
C. and the catalyst precursor may, with or without a distinct drying step, 
be heated to decompose the palladium compound, suitably at a temperature 
up to 500.degree. C., especially in the range 150.degree. C. to 
450.degree. C. The catalyst precursor pieces may be treated with hydrogen 
to complete reduction to palladium metal, for example during the heating 
step just mentioned and/or during an additional heating step (in which 
case the temperature should be in the range 25.degree. to 450.degree. C.) 
after the first heating step but before use. If there is no preliminary 
reduction step, the reduction of the catalyst precursor to the active 
catalyst may be effected when it is first used in the selective 
hydrogenation process. If the catalyst is reduced before use it may be 
stored under an inert atmosphere but should preferably not be kept for 
prolonged periods in hydrogen. 
The palladium metal in the active catalyst is preferably in the form of 
crystallites having a size of less than 40 A. The crystallite sizes may be 
determined by electron microscopy. If the crystallite size is above 40 A, 
the catalyst becomes less selective. 
When using relatively small catalyst support pieces, e.g. cylinders 
approximately 3 mm diameter and 3 mm length, a palladium crystallite size 
below 40 A may be readily achieved when the palladium compound is applied 
by dipping. However dipping larger pieces e.g. 5.4 mm diameter and 3.6 mm 
length results in larger palladium crystallites. With such larger pieces, 
a spraying technique enables the water uptake (which affects the 
crystallite size) to be reduced giving crystallites below 40 A. 
Spraying is thus a preferred method of application of the palladium 
compound and this technique also allows the desired uptake and penetration 
of palladium to be more carefully controlled. The catalysts made in 
accordance with the present invention are of use for the selective 
hydrogenation of highly unsaturated hydrocarbons in the presence of less 
unsaturated hydrocarbons. 
When the process is a "front-end" hydrogenation, the temperature is 
suitably up to 250.degree. C., for example 60.degree. to 150.degree. C.; 
the pressure is suitably in the range 1 to 70, for example 8 to 40, 
atmospheres absolute; and the space velocity is suitably in the range 
100-20,000 for example 5000-15,000 hour.sup.-1, that is, liters of gas per 
liter of catalyst filled space per hour, calculated for 20.degree. C., 1 
atmosphere absolute pressure. The volume percentage composition of the gas 
fed to the catalyst is suitably as follows for a process producing 
ethylene and/or propylene as main products: 
ethylene or propylene: 10 to 45 or each up to 20 when both are present 
higher hydrocarbons: up to 2 
acetylene and/or C.sub.3 H.sub.4 : 0.01 to 2 
hydrogen: 5 to 40 
unreactive gases (alkanes, nitrogen): balance 
For long catalyst life without regeneration, the hydrogen content is 
preferably at least 5 times by volume as much as the content of acetylene 
and C.sub.3 H.sub.4. 
When the process is a "tail-end" hydrogenation the temperature is suitably 
in the range 40.degree.-150.degree. C.; the pressure is suitably in the 
range 1-70, for example, 8-40 atmospheres absolute; and the space velocity 
is suitably in the range 500-7000 hour.sup.-1. The hydrogen content should 
be at least sufficient to hydrogenate to mono-olefin all the highly 
unsaturated hydrocarbons present and is preferably 1.5 to 3 times that 
content for acetylene and 1.1 to 3 times that content for C.sub.3 H.sub.4. 
The life of the catalyst between regenerations is longer the higher the 
hydrogen content of the gas, but this advantage is counter balanced by the 
expense of separating and recycling greater quantities of saturated 
hydrocarbon. The gas passed over the catalyst typically contains up to 
about 6% (for example 0.1 to 3) of highly unsaturated hydrocarbons and at 
least 50%, commonly over 95% of the desired mono-olefin or conjugated 
diolefin. 
When the process is a "tail-end" liquid-phase selective hydrogenation, the 
temperature is typically 0.degree.-50.degree. C., the pressure up to about 
50 atmospheres absolute, and the space velocity typically 5-40 kg per hour 
per liter of catalyst filled space. The liquid hydrocarbon suitably 
trickles downwards over the catalyst in a substantially stationary 
hydrogen atmosphere. 
Whichever type of hydrogenation is used, it appears to be advantageous to 
have a small quantity of carbon monoxide present. In a front-end 
hydrogenation the proportion of carbon monoxide is suitably 0.03 to 3% by 
volume of the total gas mixture. Such a content commonly enters in as a 
by-product of the initial cracking reaction. In a tail-end hydrogenation 
the proportion is suitably in the range 4 to 500 ppm by volume; it may be 
added deliberately if fractionation of the crude gas has removed it or 
left too little of it.

The invention is illustrated by the following examples. 
EXAMPLE 1 
Preparation of catalyst support 
600 g of a calcium aluminate cement containing 79% by weight of alumina and 
18% by weight of lime, 396 g of alumina trihydrate, 10 g of lime, and 36 g 
of graphite were mixed and then fabricated, using a pelletiser, into 
cylindrical pellets of 3.2 mm diameter and 3.2 mm height. These raw 
pellets had a bulk density of 1.54 g cm.sup.-3 and a vertical crushing 
strength of about 50 kg. (The vertical crushing strength is the 
compression load in the direction of the cylinder axis of a pellet that 
has to be applied to cause the pellet to break). By way of comparison to 
achieve a raw alumina pellet of similar strength, a considerably higher 
load, 40-60% greater, was required on the pelletising machinery. The use 
of such higher loads reduces the life of the punches and dies employed. 
The raw pellets were then fired for 4 hours at 450.degree. C. and then 
soaked in water for 16 hours. They were then dried for 5 hours at 
120.degree. C. The dried pellets then had a vertical crushing strength of 
100 kg. The dried pellets were then calcined for 6 hours at 1000.degree. 
C. whereupon they gave pellets having a water absorption of 25% by weight 
and a vertical crushing strength of about 70 kg. By way of comparison 
standard calcined alumina supported selective hydrogenation catalyst 
pellets had a vertical crushing strength of only 23 kg. 
The catalyst support pieces had the following characteristics: 
surface area: 24.2 m.sup.2 g.sup.-1 
helium density: 3.25 g cm.sup.-3 
mercury density: 1.81 g cm.sup.-3 
pore volume: 0.24 cm.sup.3 g.sup.-1 
pore radius: 200 A 
(The pore radius, assuming the pores are cylindrical and of the same size, 
is twice the pore volume divided by the surface area). 
Impregnation of the catalyst support 
The calcined pellets were impregnated with palladium by dipping 35.5 g of 
the pellets for 2 minutes in 100 ml of an aqueous solution of pH 0.98, 
containing 1.5 ml of a 10% by weight palladium nitrate solution. The 
impregnated pellets were dried for 6 hours at 450.degree. C. which also 
served to decompose the palladium nitrate. 
The overall palladium content was 0.049% by weight while the extent of the 
palladium containing region, (i.e. the penetration) was 100 .mu.m. The 
surface area of the impregnated catalyst was 35.2 m.sup.2 g.sup.-1. 
Use of the catalyst 
The activity of the catalyst was assessed by using it for the laboratory 
selective hydrogenation of a gas stream containing: 
15% v/v hydrogen 
35% v/v ethylene 
0.1% v/v acetylene 
0.1% v/v carbon monoxide 
balance nitrogen 
The catalyst was first pretreated at 150.degree. C. for 4 hours with a 
nitrogen gas stream containing 5% by volume hydrogen, and then used for 
the hydrogenation reaction at a space velocity of 10,000 hr.sup.-1 at 
various temperatures. 
The activity and selectivity of the catalyst at each temperature was 
assessed by measuring the acetylene and ethane contents of the exit gas. 
The activity is the ratio of the exit and inlet acetylene contents while 
the ethane content gives an indication of the selectivity. 
By way of comparison a commercially available alumina supported palladium 
selective hydrogenation catalyst (ICI Hydrogenation catalyst 38-1) was 
tested under the same conditions. The catalyst made according to the 
present invention had superior activity and selectivity to the 
commercially available hydrogenation catalyst. 
EXAMPLE 2 
Similar results were obtained when Example 1 was repeated but using an 
impregnation solution of pH 0.78. The penetration was 182 .mu.m, the 
overall palladium content was 0.046%, and the surface area of the 
impregnated catalyst was 37.1 m.sup.2 g.sup.-1. 
EXAMPLE 3 
Example 1 was repeated but employing a calcining temperature of about 
940.degree.-950.degree. C. and a palladium nitrate solution of pH 1.06. 
The surface area of the calcined support, before impregnation, was 27 
m.sup.2 g.sup.-1. 
EXAMPLE 4 
Example 1 was repeated but using a calcining temperature of 980.degree. C. 
and applying the palladium by spraying with a palladium nitrate solution 
of pH 1.03. The surface area of the calcined support, before impregnation, 
was 17.2 m.sup.2 g.sup.-1. 
EXAMPLES 5-8 (COMATIVE) 
A catalyst support was made as in Example 1 but was calcined at 
1100.degree. C. The support had the following characteristics: 
Surface area: 6.4 m.sup.2 g.sup.-1 
helium density: 3.2 g cm.sup.-3 
mercury density: 1.72 g cm.sup.-3 
pore volume: 0.27 cm.sup.3 g.sup.-1 
pore radius: 844 A. 
The calcined support pieces were impregnated and tested as in Example 1, 
but using impregnating solutions of various pH values and containing 1.7 
ml of the 10% palladium nitrate solution. The results are shown in Table 
1. 
EXAMPLE 9 (COMATIVE) 
Example 5 was repeated except that the impregnating solution had a pH of 
about 1.1-1.2 and the calcined support was dipped for 2 minutes in an 
aqueous sodium hydroxide solution containing 15 g l.sup.-1 of NaOH, 
allowed to drain for 20 minutes and dried at 125.degree. C. before 
impregnating with the palladium nitrate solution. The results are shown in 
Table 1. 
EXAMPLE 10 (COMATIVE) 
Example 9 was repeated except that the sodium hydroxide solution contained 
30 g l.sup.-1 NaOH. The results are shown in Table 1. 
EXAMPLE 11 (COMATIVE) 
Example 10 was repeated except that the treatment with the sodium hydroxide 
solution was effected before calcining. The results are shown in Table 1. 
In Table 1 the activity (A) and selectivity (S) are compared with the 
alumina supported catalyst 38-1. 
In the Table: 
+ indicates that the property is superior to that of the alumina supported 
catalyst, 
- indicates that the property is inferior to that of the alumina supported 
catalyst, 
= indicates that the property is similar to that of the alumina supported 
catalyst. 
TABLE 1 
______________________________________ 
Overall 
Calcining 
Impreg- Pd Pd 
Ex- Temper- nating content 
pene- Surface 
am- ature Solution % by tration 
Area 
ple .degree.C. 
pH weight .mu.m m.sup.2 g.sup.-1 
A S 
______________________________________ 
1 1000 0.98 0.049 100 35.2 + + 
2 1000 0.78 0.046 182 37.1 + + 
3 940-950 1.06 0.048 94 42 + + 
4 980 1.03 0.05 92 48 + = 
5 1100 0.9 0.048 863 35.7 - - 
6 1100 1.4 0.036 295 12.4 - - 
7 1100 1.7 0.031 224 11.5 = = 
8 1100 1.9 0.044 158 14.7 + + 
9* 1100 1.1-1.2 0.033 107 24.7 + + 
10** 1100 1.1-1.2 0.029 82 = + 
11*** 
1100 1.1-1.2 0.032 444 5.1 - - 
______________________________________ 
*treatment with 15 g l.sup.-1 NaOH before impregnation 
**treatment with 30 g l.sup.-1 NaOH before impregnation 
***treatment with 30 g l.sup.-1 NaOH before calcining 
From the table it is seen that, if high calcination temperature, e.g. 
1100.degree. C., are used, satisfactory catalysts can be obtained (e.g. 
Example 8), if impregnating solutions of relatively high pH are employed. 
However such solutions have a poor stability: thus after a short period of 
time, e.g. 1-2 hours, the palladium tends to precipitate from these 
solutions as a hydroxide and hence the solutions are unsuitable for making 
catalysts on a large scale. 
It is also seen, from Examples 9 and 10, that, by using an alkali dip, 
after calcining but before palladium impregnation, satisfactory catalysts 
can by obtained using low pH impregnating solutions. However such a 
process has the disadvantage that an additional processing step, i.e. the 
alkali dip, is required.