LUDOX® Colloidal Silica in Catalyst Applications

LUDOX® colloidal silica has been used for many years to manufacture catalysts for fluid cracking,

emissions control and a variety of chemical syntheses. In most applications, it offers high

temperature resistance and inertness and does not interfere with the activity of the active catalytic

components of the structure. LUDOX® colloidal silica products are known for their consistent

quality, narrow particle size distribution, purity and, in some grades, low sodium level and are

backed by Grace’s sales and technical support.

What is LUDOX® Colloidal Silica?

LUDOX® colloidal silica products contain discrete, spherical, amorphous silica particles dispersed in water. These dispersions, often called sols, look almost water clear due to the small silica particle size, even with silica concentrations as high as 50%.

When dispersed in water, the silica particles form surface silanol groups. The particles are electrostatically stabilized to keep the particles from reacting with each other and aggregating. In most cases, they are formulated under alkaline conditions so that some silanol groups are de-protonated, imposing a negative charge on the particle surfaces. In this way, the particles repel each other and stay separated. Because the particles are negatively charged, there must be positively charged ions in solution, called counter ions. In grades commonly used in catalysts, the counter ions are usually sodium (Na+) or ammonium (NH4+) ions.

While many other forms of amorphous silica are milled from large aggregates, colloidal silica is grown from sodium silicate to a given target particle size. Conditions of the growth process determine the particle size and distribution. Typical size offerings in Grace’s product line are 5, 7, 12 and 22 nanometers. All sols after particle growth contain sodium counter ions. To make low sodium products, the sodium counter ions are removed by deionization and replaced with ammonium counter ions.

Na+ or NH4+ (Solution Phase)

Figure 1. Particle structure showing surface silanol groups and sodium or ammonium counter ions in solution.

Introduction

1

O

Si Si Si

O O

H

O O

Silica Particle Interior

- -

2

1 2

General Properties

LUDOX® colloidal silica products of potential utility to catalyst manufacture are given in Table 1 along with important properties to consider. Products are distinguished by their nominal particle size, specific surface area, silica concentration (%SiO

2),

and stabilizing counter ion (Na+ or NH4+).

All of these products are designed to be monomodal with narrow particle size distribution, which gives the advantage that all particles will react at the roughly the same rate. The particle size values in Table 1 are nominal values and specific surface area is used as a specification property. In addition to its convenience, specific surface area is related to the number of surface silanol groups on the particle surface and therefore is a measure of the particles’ reactivity.

Product names often contain the target %SiO2

value. For example, LUDOX® AS-40 colloidal silica contains 40% SiO

2 and LUDOX® AS-30 colloidal

silica contains 30% SiO2. The higher silica and

lower water levels in AS-40 might allow a higher solids content in a given catalyst formulation. In other circumstances, the formulator might want the higher specific surface area of AS-30 for a more attrition resistant structure.

In many catalysts, sodium is a poison and must be minimized. While sodium silicate can be used to make some catalysts, its high sodium level (about 21% relative to silica) prevents its use in many others. LUDOX® colloidal silica products with sodium counter ions typically contain 0.5 – 2% Na relative to silica, depending on particle size, and products with ammonium ion typically contain 0.2%.

Table 1. LUDOX® colloidal silica products commonly used in catalyst applications.

Ludox® Colloidal Silica Properties

Product Counter IonParticle

Sizenm

Specific Surface Area

m2/g SiO2

Silica Content%SiO2

Sodium Content (Wet Basis)

%Na

Sodium Content (Dry Basis)

%Na

SM-AS

NH4

7 340 25 0.05 0.2

AS-30 12 220 30 0.06 0.2

AS-40 22 140 40 0.07 0.2

FM

Na+

5 425 15 0.3 2.0

SM 7 340 30 0.5 1.7

LS 12 220 30 0.1 0.5

HS-40 12 220 40 0.4 1.0

TM-50 22 140 50 0.3 0.5

Figure 2. Bonding progression between two colloidal silica particles. Only silanol groups involved in bonding are depicted. Because the particles are multifunctional, each particle can react with more than one particle, giving rise to aggregates and gels. Individual particles eventually fuse together.

Colloidal Silica Particle Consolidation

+ H2O

Paticle ConsolidationContinured Bonding

Initial Bonding

OOH

OHOH

O-H

OH HO

HOHO

H-O

HO

OO

O

3 4

The Role of Colloidal Silica in Catalysts

LUDOX® colloidal silica offers highly pure silica with controlled size that reacts with itself and with other components of the catalyst through silanol-silanol and silanol-metal oxide reactions. Colloidal silica particles can also react with metals through their surface metal oxide/hydroxide groups. Typically, when silica from the colloidal silica constitutes a large percentage of the final catalyst, it will function either as the support for the active catalytic components or it is a component of a zeolite structure. In the latter case, the colloidal silica particles are dissolved in the presence of sodium aluminate to form an aluminosilica channel structure. When the colloidal silica constitutes a small percentage of the final catalyst, it typically functions to bind the other catalyst components together.

Chemically, colloidal silica’s reactivity is analogous to organic ether synthesis from primary alcohols. Silanol groups interact with either surface silanols of other silica particles or with surface metal hydroxides of metal oxides formed in the aqueous slurry. Water is eliminated in the bonding reaction, with drying and subsequent calcining, forming either Si-O-Si or Si-O-Metal bonds. As the reaction progresses, individual particles fuse together as shown in Figure 2.

Similarly, colloidal silica can attach to larger metal oxide particles (e.g., clays, zeolites, etc.) as depicted in Figure 3. In the wet state, the metal oxide particles are surrounded by the colloidal silica particles. On drying, the colloidal silica particles attach and bond to the metal oxide particles and, with heating, fuse together. After the catalyst is made, it can be attached to a substrate by the same mechanism.

3 4

Metal OxideParticle

Colloidal SilicaParticles

Metal OxideParticle

M

M

M

M

OO

O

O

O

M O

M O

OO

Si

SiSi

SiSi

SiSi

Si

Si Si

SiSi

Colloidal Silicabonding withMetal Oxide

particles

Figure 3. Colloidal Silica particles bond to two much larger metal oxide particles (e.g., alumina) or the metal oxide surface of a metal in a similar fashion depicted in Figure 2. Bonding occurs both between silica particles and the metal oxide, binding the metal oxide particles together.

Silica particles will also react with acids and dissolved metal ions, especially those that have divalent or trivalent charge, and are often called “gellants” or “coagulants.” The gellant or coagulant may function, after suitable processing as the active catalyst component, may be inert or may have some other purpose. After the catalyst is made, the silica is often inert under the conditions of use.

LUDOX® Colloidal Silica in Catalyst Applications

LUDOX® colloidal silica is generally used in catalyst applications to:• Provide the silica component of the catalyst

support, zeolite or molecular sieve• Bind the components of the catalyst together• Improve adhesion to substrates (washcoats)• Improve physical properties (hardness,

attrition resistance, etc.)• Stabilize catalytic activity

These functions are not fully distinct from one another. When used as a silica source, for example, colloidal silica also binds both the silica particles and catalytic components together. Colloidal silica in washcoat formulations not only binds the components of the washcoat but additionally will help bind those components to a substrate.

ExamplesThe following examples were taken from U.S. Patents and are presented to illustrate typical applications.

Catalyst SupportOne of the oldest applications using colloidal silica to form a catalyst support is the acrylonitrile catalyst pioneered by Standard Oil of Ohio. U.S. Patent 2,904,580 discloses a preferred catalyst containing bismuth salts of phosphomolybidic and molybdic acids mixed with colloidal silica. LUDOX® AS-40 colloidal silica is recommended for this application. This mixture was evaporated to dryness and calcined. The resulting solid was ground and screened to 40-100 mesh. A mixture of propylene, ammonia, water and air was introduced into a reactor charged with this catalyst heated to 454°C to make acrylonitrile at high yield and selectivity.

Standard Ohio subsequently filed numerous patents in what is often termed the SOHIO process, improving the catalyst for better processing utility, yield, and selectivity. U. S. Patent 3,746,657, for example, describes a catalyst containing 50% Bi

9PMo

12O

52 and

50% silica from colloidal silica produced by spray drying a mixture of ammonium heptamolybdate, colloidal silica (e.g., LUDOX® AS-40 colloidal silica), phosphoric acid, bismuth nitrate and nitric acid. This catalyst was especially suitable for fluidized bed reactors.

Silica Source for ZeolitesZeolites are microporous aluminosilicate crystalline structures that contain channels or cavities such that they can selectively adsorb molecules based on their size and shape. A number of zeolite types are important in catalyst design, some of which may

65

be prepared with colloidal silica. In this application, colloidal silica particles are dissolved in the presence of aluminate to form an aluminosilicate for a target Al

2O

3:SiO

2 ratio and recrystallized to form the desired

zeolite.

In Union Carbide’s U.S. Patent 3,130,007 processes are disclosed to make Zeolite Y in Figure 4. In one example, sodium aluminate solution, sodium hydroxide and colloidal silica were mixed together and heated at 100°C for 21 hours to crystallize the product, which was then separated by filtration and dried. The product was identified as Zeolite Y by its X-Ray diffraction pattern and had a product composition of 0.92 Na

2O: Al

2O

3:4 SiO

2:7 H

2O. LUDOX® HS-40

colloidal silica is recommended for this application. Colloidal silica formed pure Zeolite Y while other silica sources (sodium silicate, silica gel, fused silica) produced Zeolite Y contaminated with other crystalline components.

A method for making SAPO-34 is given in ExxonMobil’s U.S. Patent 6,897,180 in which a

mixture of boehmite powder, water, phosphoric acid, LUDOX® AS-40 colloidal silica, morpholine and zeolite chabazite seeds was prepared. This mixture was crystallized in an autoclave at 175°C, after which the crystals were separated from the mother liquor, washed and dried to make the final zeolite.

Binder Colloidal silica may be used to bind catalytic components in ways similar to the following two examples.

U.S. Patent 3,860,533 from Union Oil describes a hydrocracking catalyst in which a slurry is made from cobalt Zeolite Y (prepared from ammonium Zeolite Y and cobalt chloride solution), molybdic oxide and water. After drying, the powder is mixed with LUDOX® LS colloidal silica and gelled with cobalt nitrate solution. The resulting paste was formed into pellets, dried and calcined to form the catalyst.

A molybdate catalyst useful for converting hydrocarbons to maleic anhydride is described in

Figure 4. Zeolite Y Structure

65

U.S. Patent 4,093,558 (Standard Oil, 1974). In one example, a catalyst containing 80% VFeSb

3Mo

12O

48

and 20% SiO2 was made from ammonium salts of

vanadium and molybdenum, iron(II) nitrate, antimony oxide, nitric acid and LUDOX® AS-30 colloidal silica. The mixture was stirred until it gelled, then dried and calcined to form the final catalyst.

Adhesion to Substrates (Washcoats) Exxon’s U.S. Patent 5,925,800 illustrates the application of a washcoat formulation onto a monolith substrate for the conversion of oxygenated organic molecules (e.g., methanol) to olefins. In one example, a washcoat formulation containing 9 g SAPO-34, 15 g LUDOX® AS-40 colloidal silica, 45 g 2% methylcellulose, and 2 ml 2% polyethylene glycol (MW 2000) was milled, then coated onto an alpha alumina coated cordierite honeycomb. The coating was allowed to dry and the coated honeycomb was calcined.

Attrition Resistance U.S. Patent 2,563,650 is an early example demonstrating how colloidal silica can be used to improve attrition resistance. Bauxite was activated by heating to 600°C, cooled, crushed and sieved to make the starting material. Colloidal silica fitting the description of LUDOX® HS-30 colloidal silica (a lower %SiO

2 variation of LUDOX® HS-40 colloidal

silica) was mixed with the bauxite in sufficient quantity to incorporate 7.5% SiO

2 within the granules,

then calcined. Compared to untreated bauxite, the colloidal silica hardened bauxite exhibited less attrition but with no change in the bauxite surface area. When chromium or molybdenum oxides were added as active catalyst reagents, the colloidal silica hardened bauxite catalyst exhibited similar naptha reforming yield but less attrition compared to untreated bauxite catalyst.

Celanese discloses a somewhat different approach in U.S. Patent 4,251,393 to make an attrition resistant catalyst to oxidize acrolein to acrylic acid. An active catalyst powder was prepared by mixing solutions of

manganous acetate trihydrate and ammonium salts of paramolybdate, metavanadate and paratungstate, drying, then heating at 385°C for 5 hours. The powder’s empirical formula was Mo

12V

3W

1.2M

3O

53.

This powder was blended with porous silicon carbide beads and moistened with LUDOX® AS-40, then dried to make a supported catalyst exhibiting high yield and low attrition loss. While the use of colloidal silica was mainly intended to improve attrition resistance, it almost certainly played a role in binding the active powder to the support.

Stabilizer Henkel’s U.S. Patent 5,294,583 discloses that small amounts of colloidal silica can stabilize a catalyst against loss of activity at high temperatures. In this process, an ammoniated solution of chromium(VI) oxide was precipitated with a solution containing LUDOX® AS-40 colloidal silica and the nitrate salts of barium, manganese and copper. The precipitate was washed free of nitrate, dried and calcined to form the copper(II) chromite catalyst.

References1. Iler, Ralph K., The Chemistry of Silica,

Wiley & Sons (New York, 1979), 346.

2. Iler, Ralph K., The Chemistry of Silica, 223-224.

Figure 5. Grace® Ludox® Colloidal Silica

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We invite you to partner with us. Grace has a strong commitment to open innovation. Our research scientists strive to collaboratively develop tailored solutions to support customers’ needs. We welcome the opportunity to explore the challenges you face and how LUDOX® colloidal silica can help in catalyst applications. Contact Grace today at colloidalsilica@grace.com to schedule a meeting or request a sample.