Zeolites are aluminosilicate materials that can occur naturally and provide functionality as acid catalysts and molecular sieves. New zeolites developed through synthetic processes provide optimal catalysis for specific emerging applications, especially in the biofuel, plastic reuse, and pharmaceutical industries. These synthetic zeolites require a highly pure source of silica with controlled particle sizes to form the precise, crystalline structures necessary to be the most effective catalysts in each application.

Leveraging LUDOX® Colloidal Silica in Catalysts

LUDOX® colloidal silicas are electrostatically stabilized, aqueous dispersions of discrete nano-sized spherical silica particles with very narrow particle size distributions. They are extremely stable, durable, and heat-tolerant particles that provide several important functionalities in catalyst applications:

 

• Provide a source of highly pure silica for catalyst formation
• Act as a binder for catalyst particles
• Promote adhesion for catalysts onto substrates
• Increase hardness and attrition resistance
• Stabilize catalytic activity

Silica Source

As mentioned previously, aluminosilicate catalysts, like zeolites and other silica-based catalysts, require silica that has a uniform particle size and is highly pure. LUDOX® colloidal silica provides an optimal raw material source of silica for catalyst creation. In general, zeolite catalysts benefit greatly from the narrow particle size distribution of LUDOX®; this allows for greater control over zeolite pore sizes, more uniform reactivity, and higher catalytic efficiency. In terms of purity, some silica-based catalysts can use simpler materials like sodium silicate, but many others cannot use it due to its very high sodium level. There are several options of LUDOX® that are highly pure and minimize the content of sodium, which can act as a poison to catalysts and cause partial or total deactivation. 

Binder

In addition to being an ideal source of silica for catalysts like zeolites, LUDOX® may also be used to bind catalytic components together. In this scenario, the high surface area of LUDOX® silica particles is leveraged. The surface of LUDOX® is rich in silanol groups that can interact with surface metal hydroxides of catalyst components as well as other silanol groups on LUDOX® particles. After drying and calcination, water is eliminated via a condensation reaction to form stable Si-O-metal or Si-O-Si chemical bonds. From this mechanism, LUDOX® and catalytic particles are bound together to make catalysts that are more chemically and thermally stable.

Below is a depiction of the bonding mechanism between LUDOX® colloidal silica particles to themselves and to catalyst particles. 

 

Adhesion Promotion onto Substrates

The binding mechanism described in the previous section is also used to promote the adhesion of catalysts to substrates. As LUDOX® colloidal silica binds very well to itself and metal oxide catalyst particles, it will also bind exceptionally to ceramic substrate materials. Catalyst washcoats can also be formed easily with the use of LUDOX®. In this process, a washcoat containing catalysts and other particles forms a film after calcination. The resultant washcoat has a very high specific surface area and thermal stability. This makes LUDOX® especially useful in applications where catalysts need to be deposited onto monolithic flow-through supports, such as catalytic converters for automobiles. Below is a rendering of the adhesion mechanism of LUDOX® within the context of washcoat formation. 

 

Increasing Hardness, Attrition Resistance, and Thermal Stability 

In many catalytic processes, attrition is an issue that degrades catalyst particles and leads to the generation of fines that can escape into the atmosphere and present health and environmental issues. LUDOX® colloidal silica can be used to improve attrition resistance by increasing the hardness of the catalyst without impacting its surface area. This helps to prolong the longevity of catalyst particles while preserving their yield and efficiency. A small amount of LUDOX® added to catalysts can also help increase thermal stability and resistance to thermal shock, similar to the mechanism in which it functions as a binder for refractory materials.
 

LUDOX® Colloidal Silica Grades for Catalysts

LUDOX® Grade	Counterion	Nominal Particle Size (nm)	Specific Surface Area (m2/g SiO2)	Silica Content, % SiO2	Typical Sodium Content (Wet Basis), % Na
SM-AS	NH4+	7	360	25	0.05
AS-30	NH4+	12	230	30	0.06
AS-40	NH4+	22	140	40	0.07
HSA	H+	12	230	30	0.10
TMA	H+	12	130	35	0.10
FM	Na+	5	435	15	0.30
SM	Na+	7	360	30	0.50
LS	Na+	12	230	30	0.10
HS-30	Na+	12	230	30	0.30
HS-40	Na+	12	230	40	0.40
TM-50	Na+	22	130	50	0.30

Specific choices from the above table may be selected based on the desired functionality of LUDOX® and tolerance for sodium content in the catalyst. LUDOX® colloidal silica grades with smaller nominal particle sizes have a larger surface area. Larger surface areas are ideal for maximizing the binding properties of LUDOX®. If LUDOX® is to be used as a silica source to form catalysts such as zeolites, grades should be selected based on the desired silica particle size and the structure of the catalyst dimensions or geometry. Also, if sodium is to be minimized in the application, grades that are stabilized via ammonium are recommended. 

Summary 

With the growing demand for the development of silica-containing catalysts, such as synthetic zeolites, LUDOX® stands out as an ideal silica source and additive to increase the value and functionality of catalysts. When added to catalysts like zeolites, LUDOX® can serve as a highly pure and uniform-particle-size silica–raw material source. In addition, LUDOX® is an effective binder for catalyst particles that can also increase adhesion to substrates to assist in the final catalytic product’s formation or deposition. Lastly, LUDOX® improves resistance to attrition and strengthens hardness and temperature stability in catalysts. With many options available and some that can even be customized, LUDOX® colloidal silica is the best choice to help you create the catalyst you need with optimized performance. Click below to discuss the ideal option for your application and request a sample.