Silica-alumina and silica catalyst support bodies

Porous silica particles useful in catalysis are prepared by adding excess alkali to a body of water glass; the excess alkali is responsible for increasing the pore diameter.

REFERENCE TO A RELATED APPLICATION 
This application contains a disclosure related to application Ser. No. 
623,825, filed Oct. 20, 1975, now U.S. Pat. No. 4,039,474 
According to U.S. Pat. No. 2,933,456 a silica-alumina complex useful in 
catalysis is prepared in the following manner: 
1. silica in hydrous form is precipitated by adding a mineral acid 
(sulfuric or hydrochloric) to a body of water glass, Na.sub.2 O 
(SiO.sub.2).sub.3.2, pH 8 to 10.5; 
2. adding to the resultant slurry (silicic acid gel) an acidic aqueous 
solution of an aluminum salt such as aluminum sulfate or aluminum chloride 
having aluminum in the cation portion, lowering the pH to 2 or 3, to 
precipitate alumina in hydrous form on to the silica; 
3. whereafter an aqueous solution of an aluminum salt in which aluminum 
exists in the anion (sodium aluminate or potassium aluminate) is added to 
the silica-alumina slurry to partially neutralize the acidity produced by 
the first aluminum salt, preferably at a pH of about 5, which impregnates 
the silica gel with alumina precipitated from both aluminum salts; 
4. the resultant slurry containing silica impregnated with alumina is 
filtered to remove free moisture and the filter cake is spray dried to 
obtain the catalyst carrier in powder form which may be reslurried and 
refiltered to remove soluble salts. 
In summary: in the known process of U.S. Pat. No. 2,933,456 silica alumina 
catalyst is obtained by precipitating silica in hydrous form from aqueous 
sodium silicate by means of a mineral acid and subsequently impregnating 
the silica gel with alumina precipitated in hydrous form by successive 
additions, first, of a water soluble aluminum salt containing aluminum in 
the cation only and, second, a water soluble aluminum salt in which 
aluminum exists in the anion. Most likely there is co-precipitation of 
silica and alumina during aluminum addition. After successive filtering 
and spray drying steps the final purified product is a powder where the 
silica content is 55 to 95% by weight (dry basis) and alumina the balance. 
The amount of aluminum sulfate (alum) determines whether the product is 
high or low in alumina. 
The particles are microspherical, inherently porous and susceptible to 
impregnation with metallic catalyst precursors such as compounds of 
molybdenum, cobalt, nickel, tungsten, or mixtures thereof. Some reactions 
to be catalyzed take place on the surface of the particles and some 
reactions are promoted by diffusion into the catalyst support particles. 
In the first case, the pores are permissably (and preferably) small so as 
to confine the catalyst to the surface of the particles. In the second 
case the pores should be larger to assure penetration of the catalyst into 
the carrier. These are general cases; there are exceptions. 
The commercial product under U.S. Pat. No. 2,933,456 is represented by 
microspherical particles obtained by spray drying. The microspheres may be 
of 50 micron diameter, employed in a fluidized bed where the particles are 
subjected to a great deal of contact one against another, both impact and 
sliding, resulting in fine subdivisions (as small as say 10 micron 
diameter) which can clog the catalytic reactor. The catalyst bed itself 
shifts, resulting in a grinding action which is also productive of 
fractured particles. 
In some catalytic processes, particles of larger size and of considerable 
strength are required. Such sizes could be obtained by extrusion, 
resulting in pellets composed of numerous microspheres. However, extrusion 
has not been possible on an economic basis principally because the 
material undergoes wall slippage, that is, the bulk to be extrudable must 
be wet and being wet it slips so much inside the chamber behind the 
extrusion nozzle, where the force is applied, that only slow, uneconomic 
rates can be achieved, and even then the pellets are not strong. 
Extrudability of microporous silica-alumina catalyst carriers of 
exceptional strength can be achieved in a practical production sense by 
following the process of the patent except that the starting body of water 
glass, Na.sub.2 O (SiO.sub.2).sub.3.2, is infused with excess alkali. 
Excess alkali is preferably in the form of caustic (NaOH) or sodium 
carbonate, Na.sub.2 CO.sub.3. By employing excess alkali, the resultant 
particles are opaque and more soft compared to the clear, hard, glass-like 
particles obtained under U.S. Pat. No. 2,933,456 where excess alkali does 
not exist. 
Excess alkali means more than is present in water glass. In water glass the 
amount of alkali is precisely as reported in U.S. Pat. No. 2,933,456, 
namely, in the weight proportion of Na.sub.2 O (SiO.sub.2).sub.3.27, that 
is 1:3.27. In orthosilicate, for comparison the mole ratio is 2Na.sub.2 
O:SiO.sub.2 and in meta silicate the mole ratio of alkali to silica is 
1:1. 
Under Example 1 below excess alkali is obtained by adding NaOH to "plant" 
sodium silicate which is water glass.

EXAMPLE 1 
6,720 mls of aqueous sodium silicate containing excess alkali were prepared 
by digesting 3,160 mls plant sodium silicate in 3,560 mls of 50% NaOH at 
170.degree. F for 15 minutes, and added to a tank containing 10.8 gallons 
water (90.degree. F). Then 4,920 mls of plant sodium silicate, water glass 
(9.1% Na.sub.2 O, 28.8% SiO.sub.2), were added and the solution was heated 
to 120.degree. F. To the above solution, 10,930 mls of 35% H.sub.2 
SO.sub.4 were added with agitation over a period of 50 minutes to pH 8.3. 
To the above precipitated silica gel, 6,420 mls of plant concentrated alum 
(25% aqueous solution) were added over a period of 17 minutes. The mixture 
then was adjusted to pH 5.5 with 11,292 mls of diluted sodium aluminate 
(SA) solution (1,830 mls of plant SA diluted in 9,462 mls H.sub.2 O). The 
diluted SA solution was added over a period of 20 minutes. The final batch 
slurry temperature was 108.degree. F. 
The batch slurry was filtered in buchner funnels. The filter cake was 
reslurried in 10 pounds of diethylene glycol and was spray dried. The 
dried product was washed with water and with a dilute NH.sub.4 OH to 
remove the soluble salts. The purified filter cake was oven dried at 
150.degree. F for 16 hours. The dried product was adjusted with water to 
62% FM (free moisture) using a Simpson muller. The wet material was mulled 
for 20 minutes and was extruded. Extrusion was very good. The extrudates 
were calcined at 1100.degree. F for 3 hours. Data on crush strength and 
pore distribution are shown in Table I for the 5/64 inch size. The "free 
moisture" is calculated as the percent weight loss by heating a sample of 
the mulled or extruded material on a Model 6000 OHAUS Moisture 
Determination Balance for 20 minutes at a heater setting of 75.degree.. 
EXAMPLE 2 
The same formulation and procedure were used in preparing this catalyst as 
in Example 1 with the exception the amounts of alum and sodium aluminate 
were increased slightly to obtain 31% Al.sub.2 O.sub.3 on the catalyst. 
The material was extruded at 64.5% FM. Extrusion was very good. Data are 
given in Table I. 
EXAMPLE 3 
To increase the density, the same formulation and procedure were used as in 
Example 2 with the following exceptions, (a) the sodium silicate 
containing excess alkali was prepared in 16 gallons H.sub.2 O and (b) only 
0.1 pound of diethylene glycol was used instead of 1.0 pound per pound of 
catalyst. Extrusion was very good. Data are given in Table I. 
EXAMPLE 4 
This sample is a repeat of Example 2 with the exception that the material 
was extruded at 63% FM instead of 64.5%. Data are given in Table I. 
EXAMPLE 5 
A repeated preparation of Example 2 except the purified spray dried product 
was calcined at 1050.degree. F for 3 hours and prepared in powdered form 
rather than extruded. Data are given in Table I. 
EXAMPLE 6 
This example was prepared by the method of Example 1 herein but using no 
excess alkali thus conforming to Example 1 of U.S. Pat. No. 2,933,456. The 
product could not be extruded to give a material of useable crush 
strength. This example also illustrates the amount of pores below 
100A.degree. which are generally not affected by extrusion, which is to 
say that while the product of Example 6 was not extruded the pore volume 
below 100A.degree. may be validly compared to the corresponding pore 
volume of the extrudates set forth in Table I. 
TABLE I 
______________________________________ 
Pore 
Volume 
(PV) Less 
Crush Than 
Ex- Extruded Strength 1200A 1200-100A 
0-100A 
ample Diameter (pounds) Diameter (1) 
(PV) (PV) 
______________________________________ 
1 5/64 9.5 
2 5/64 9.1 1.15 0.98 0.17 
3 5/64 11.2 0.87 0.71 0.16 
4 5/64 8.5 0.93 0.78 0.15 
5 1.46 1.33 0.13 
6 0.65 0.30 0.35 
______________________________________ 
Diethylene glycol is only an aid to extrusion. It has no appreciable effect 
on strength or pore volume distribution. Each of Examples 2 through 5 was 
of high alumina content (approximately 31% by weight). Example 6 had the 
same alumina content for comparison. 
It will be seen from Table I that extrudates with good crush strength are 
realized. Crush strength is tested simply by applying an increasing force 
until the pellet fails in compression. 
It is believed the advantageous result is because the excess alkali 
generates softer catalyst particles more capable of conforming to the 
extrusion orifices. However, a completely unexpected result is the shift 
in pore volume distribution. Thus, in comparing Example 6, Table I, to the 
others, excess alkalinity results in nearly a fifty percent decrease of 
pore volume below 100A.degree., that is, the known catalyst of Example 6 
had considerably more pore volume in the smaller diameter. 
That increased alkalinity (deemed Na.sub.2 O) is responsible for the shift 
in pore volume is corroborated by sodium carbonate (Na.sub.2 CO.sub.3) 
accomplishing the same thing, as shown by the following examples. 
EXAMPLE 7 
A catalyst was prepared using water glass in accordance with Example 1 of 
U.S. Pat. No. 2,933,456, without excess Na.sub.2 O. This catalyst was low 
in alumina, approximately 13% by weight. It was not possible to extrude 
this catalyst. This example is included as a pore distribution comparison 
catalyst. 
EXAMPLE 8 
This catalyst was a repeat of Example 7 except sodium carbonate (Na.sub.2 
CO.sub.3) was added to the body of water glass at the inception in the 
proportion of 0.32 pounds of Na.sub.2 CO.sub.3 per pound of final 
catalyst. 
EXAMPLE 9 
This catalyst was a repeat of Example 8 except the amount of Na.sub.2 
CO.sub.3 was reduced to 0.16 pounds per pound of final catalyst. 
TABLE II 
______________________________________ 
PV Less Than 
Example 1200A PV 100A or Less 
______________________________________ 
7 0.69 0.47 
8 1.10 0.19 
9 0.98 0.27 
______________________________________ 
The catalyst prepared without excess alkali (Example 7) had nearly 68% of 
the pore volume in the 100A.degree. size or less, compared to only 17% for 
Example 8 and 27.5% for Example 9. Examples 8 and 9 show that as the 
proportion of excess alkali increases the greater the pore volume in the 
larger diameter. The amount of excess alkali in Example 8 is deemed of 
unit value and by that token the amount of excess alkali under Example 9 
is one-half unit. In other words, the greater the proportion of alkali in 
excess of that required to form water glass, the greater the pore 
diameter, 
The preferred example is Example 1 where the amount of sodium hydroxide 
(calculated as pure NaOH) is 0.55 pounds per pound of dry, finished 
catalyst obtained after extrusion and calcining. On a mole basis the 
excess alkali (derived from NaOH) is 2.04 moles Na.sub.2 O per mole of 
water glass. The excess alkali is simply that required to render the 
oven-dried, purified filter cake extrudable (on a commercial scale) in the 
presence of free moisture; or viewed another way the excess over that 
required to form water glass is an amount sufficient to produce pores of 
an appreciably larger diameter compared to the pore size distribution when 
the body of sodium silicate is in water glass proportion. 
It has been further found the phenomonon of varying pore size, in the 
manner disclosed above, is equally applicable to silica particles per se 
precipitated from a sodium silicate solution with acid; also, a variation 
in temperature will alter the pore size. 
The acid neutralizes the sodium ions and causes silica to precipitate as a 
hydrogel. 
In the experimental work with silica, the pore volume was varied from 
0.82cc/gram to 1.53cc/gram and the average pore diameter was varied from 
63A.degree. to 189A.degree.. 
EXAMPLE 10 (Experiment 1991) 
______________________________________ 
Basic Formulation, 
Solution A Pounds Pounds SiO.sub.2 
______________________________________ 
29.1 gallons H.sub.2 O 
243 
5.75 gallons (sodium silicate) 
67 19.2 
Na.sub.2 CO.sub.3 (variable) 
Solution B, 35% H.sub.2 SO.sub.4 
2.16 gallons H.sub.2 O 
18 
0.65 gallons 
98% H.sub.2 SO.sub.4 
10 
Total Batch 338 19.2 
##STR1## 
##STR2## 
______________________________________ 
5.75 gallons of sodium silicate containing 28.6% SiO.sub.2 and 9.1% 
Na.sub.2 O was added to 29.1 gallons of water. This diluted sodium 
silicate solution was heated to 116.degree. F and is designated as 
solution "A". To this solution was added, with agitation, 2.81 gallons of 
35% H.sub.2 SO.sub.4 (solution "B") in 60 minutes to pH 3.8. The silica 
started to gel after 38 minutes of acid addition. 
The precipitated silica gel was aged over a period of 1 hour at 117.degree. 
F. The pH was 5.4 after aging. 
The slurry was filtered on a vacuum rotary filter, the cake was reslurried 
with water to a pumpable mixture and was spray dried. The product was 
purified with water to remove the soluble salts; it was oven dried for 3 
hours at 300.degree. F and then was calcined at 840.degree. F for 4 hours. 
The calcined silica xerogel was evaluated for chemical and physical 
properties. The results are shown in Table II. 
The electrolyte (Na.sub.2 CO.sub.3) was added to the diluted sodium 
silicate solution before acid addition. Using the above basic formulation 
and procedure we evaluated the variables of temperature and concentration 
of Na.sub.2 CO.sub.3 (source of excess alkali) as effecting average pore 
diameter (APD) and pore volume (PV, in cc/gram). 
TABLE III 
______________________________________ 
Experi- Gelation 
ment % Solids #Na.sub.2 CO.sub.3 
Temp. Time 
No. (A-Solution) 
#SiO.sub.2 
.degree. F(1) 
pH (mins.) 
______________________________________ 
1991 6.2 None 116 5.4 38 
2012 6.2 None 130 3.9 35 
2031 6.2 0.15 130 3.5 25 
2051 6.2 0.075 130 3.5 34 
2081 6.2 0.15 131 3.2 27 
PD.sub.av. 
PVD(4) 
ABD(2) PV(3) A 1200-100A 
100A and less 
______________________________________ 
1991 0.448 0.82 62.9 0.18 0.63 
2012 0.390 1.0 67.3 0.33 0.61 
2031 0.284 1.16 87.4 0.76 0.35 
2051 0.315 1.09 78.8 0.61 0.44 
2081 0.304 1.055 83.2 0.69 0.40 
______________________________________ 
(1)Precipitation Temperature 
(2)Average bulk density 
(3)Pore volume in cc/gram; determined by water absorption 
(4)Pore volume distribution by nitrogen absorption; rounded off to two 
decimal places 
It will be seen from the data in Table III that as excess alkali increases 
progressively from zero to 0.15 pounds per pound SiO.sub.2, the average 
pore diameter (PD.sub.av.) increases from 62.9A.degree. to 83.2A.degree., 
the density decreases, pore volume increases and the pore volume 
distribution is altered so that there is a greater proportion in the 
larger diameter. 
Also, the same progression prevails with an increase in precipitation 
temperature. 
EXAMPLE 11 (Experiment 2121) 
______________________________________ 
Basic Formulation, 
Solution A Pounds Pounds SiO.sub.2 
______________________________________ 
29.1 gallons H.sub.2 O 
243 
8.625 gallons (sodium silicate) 
101 28.8 
5.0 pounds Na.sub.2 CO.sub.3 H.sub.2 O 
(4.275# Na.sub.2 CO.sub.3) 
5 
Solution B 
5.1 gallons H.sub.2 O 
42.5 
1.26 gallons 98% H.sub.2 SO.sub.4 
19.5 
Total Batch 411 28.8 
##STR3## 
##STR4## 
______________________________________ 
8.625 gallons of sodium silicate (28.6% SiO.sub.2 and 9.1% Na.sub.2 O was 
added to 29.1 gallons water containing 4.275 pounds of Na.sub.2 CO.sub.3. 
This is solution "A" which was heated to 130.degree. F under agitation. 
62 pounds (6.36 gallon) of 31% by weight of H.sub.2 SO.sub.4 was added to 
the above "A" solution over a period of 56 minutes to pH 5.4. The silica 
started to gel after 26 minutes of acid addition. The final batch 
temperature was 136.degree. F. The batch was aged for 1 hour and after 
aging the temperature was 123.degree. F and the pH was 5.7. 
The precipitated silicic acid gel was filtered and the filter cake solids 
were 17%. The spray dried product was processed as in Example 10. 
Using the above basic formulation and procedure, we investigated the effect 
of variables of temperature and concentration with the results shown in 
Table IV: 
TABLE IV 
______________________________________ 
Experiment 
% Solids #Na.sub.2 CO.sub.3 
Temp. Gelation 
No. (A-solution) 
#SiO.sub.2 
.degree. F 
pH Time 
______________________________________ 
2121 8.3 0.15 130 5.4 26 
2141 8.3 0.11 110 5.7 21 
2171 8.3 0.075 110 5.4 24 
PVD 
ABD PV PD.sub.av. 
1200A 100A and less 
______________________________________ 
2121 0.220 1.46 159.9 (-100) 1.37 0.16 
2141 0.315 1.27 127.1 1.26 1.04 0.22 
2171 0.362 1.10 113.7 1.05 0.81 0.23 
______________________________________ 
The results confirm those presented under Example 10. 
As in the instance of forming silica-alumina particles the source of excess 
alkali is preferably Na.sub.2 CO.sub.3 (but may be NaOH) and again the 
excess alkali is understood as Na.sub.2 O in excess of the stoichiometric 
amount in water glass expressed as Na.sub.2 O(SiO.sub.2).sub.3.2. 
Referring to Table III, no excess alkali was involved with experiment Nos. 
1991 and 2012, evidencing that the temperature change alone was 
responsible: as the temperature of precipitation increases, there is a 
shift to the larger pore diameter and the same pheonomonon is observed in 
the instance of precipitating the silica-alumina particles. Again 
referring to Table III, the first two experiments involve no excess 
alkalinity; experiment Nos. 2031 and 2081 may be viewed as each involving 
one unit of excess alkalinity whilst experiment No. 2051 involved one-half 
unit. 
In the instance of the silica particles, extrudates were not made; here, 
the catalyst (a soluble metal compound) may be used to impregnate the pore 
per se as the carrier.