Summary of Research Progress on Pore Volume Modification of Carrier Silica Gel

4.1 Research Background and Significance

1.Development History of Silica Gel: As the earliest mesoporous silica product, silica gel was reviewed for its technical level at that time in the academic paper of A.Patrick from the United States in Göttingen, Germany in 1914. Later, Patrick obtained a preparation patent in the United States, and silica gel was put into production in 1920.

2.Uses of Silica Gel: It has a wide range of uses, such as adsorbents, desiccants, thickeners, fillers, and carriers for chromatography. With the development of the petroleum and petrochemical industries, its application as a catalyst carrier has attracted increasing attention.

3.Existing Problems and Demands: Most domestic carrier silica gels are of small pore size and large specific surface area, which have limitations in use. In the catalytic field, catalysts supported on small-pore-size carriers will cause deep oxidation due to hindered product diffusion, reducing the required yield; while catalysts supported on large-pore-size carriers have significantly higher selectivity. Therefore, modifying the pore volume of carrier silica gel to obtain appropriate pore size and specific surface area is crucial for the petroleum, petrochemical, and especially catalytic fields.

4.2 Key Influencing Factors of Carrier Silica Gel Performance and Optimal Pore Size Design

4.2.1 Key Influencing Factors

The performance of the carrier is mainly affected by the pore volume, pore size, and specific surface area of the particles. The relationship between the three is: average pore size R=5Vp/(2S) (Vp is pore volume, S is specific surface area). The pore size distribution directly affects the mass transfer of reactants and products in the catalyst particles. After carrier loading, the pore size is divided into 4 categories:

• ① Coarse pores, leading to the loss of active components, which should be minimized;
• ② Too small pores, causing intermediate compounds or products to form coking due to spatial constraints, which should be avoided;
• ③ Appropriate pores, enabling active components to give full play to their performance, which should be the main focus;
• ④ Fine pores, which are prone to blockage and coking when the active component content is too high, which should be avoided.

4.2.2 Principles of Optimal Pore Size Design

After the active component is determined for a first-order catalytic reaction, the selection of the optimal pore size shall follow the following principles:

6.Pressurized operation should choose a single-peak pore size distribution, and normal pressure should choose a double-peak pore size distribution;

7.The pore size should be larger than the maximum molecular diameter of the reaction components to ensure that the reaction molecules can effectively diffuse into the pores for reaction;

8.Within the range of ensuring the mechanical strength, selectivity, and toxicity resistance of the catalyst, and allowing activity, appropriately increasing the pore size is more beneficial. When the carrier particle size is 4-20mm, internal diffusion usually occurs.

4.3 Main Pore-Expanding Methods and Characteristics

4.3.1 Hydrothermal Treatment Method

9.Principle: Under the action of hydrothermal steam, large particles are formed through particle dissolution and condensation, thereby forming larger pore sizes.

10.Operation Process: Place the carrier silica gel in an autoclave, add water several times the mass of the carrier, heat to make the pressure in the autoclave reach several megapascals, keep it at a constant temperature for a period of time, cool to room temperature, wash the silica gel with water until the surface is clean, dry it, and then calcine it at a high temperature in an electric furnace.

11.Effects and Disadvantages: As shown in Table 1, after hydrothermal treatment, the pore size and pore volume of silica gel increase significantly (for example, the relative pore volume of Sg-2 is 1.33 and the relative pore size is 1.75, which are significantly higher than those of Sg-1 without steam treatment). However, it will greatly damage the mechanical strength of silica gel, making it easy to crush.

4.3.2 Ammonia Solution Treatment Method

12.Pore-Expanding Mechanism: First, ammonia increases the degree of polymerization of water glass molecules, so that the macromolecules formed combine with acids to generate larger initial basic particles; second, ammonia destroys the hydration film and double electric layer structure of the basic particles in the sol, promoting particle combination and growth; third, ammonia slows down the formation rate of basic particles, prolongs the gelation time, and is conducive to particle growth; fourth, ammonia reduces the hydrophilicity of the gel skeleton, reduces the shrinkage pressure, and is conducive to the formation of large-pore-size silica gel.

13.Effects and Precautions: As shown in Table 2, ammonia solution treatment can improve the pore size (for example, when sample 4 is added with 60ml of ammonia, the average pore radius reaches 14.50nm, which is significantly higher than that of sample 1 without ammonia). However, the amount of ammonia will show a peak value with time. Excessive ammonia (higher than the peak value) will make the colloidal particles carry reverse charges, re-form a stable double electric layer, hinder particle growth, and finally obtain silica gel with small pores and large specific surface area. Therefore, the amount of ammonia and gelation time must be strictly controlled.

4.3.3 Mixed Solution Treatment Method

14.Reason for Adoption: Ammonia solution has strong singularity and is difficult to meet the needs of all links in the test, which limits the improvement of silica gel pore volume. Mixed solutions can make up for this defect and improve the pore volume more effectively.

15.Selection of Mixed Solution: Mainly composed of weakly alkaline substances, supplemented by other substances.

16.Mechanism of Action: Alkaline substances can destroy the hydration film and double electric layer structure of the basic particles in the sol, slow down the particle formation rate, prolong the gelation time, and facilitate particle growth to increase the pore size.

4.3.4 Combined Physicochemical Treatment Method

17.Classification and Function of Pore-Expanding Agents:

• • Physical Pore-Expanding Agents: Such as carbon black, high molecular organic compounds, which are oxidized into gas and escape during calcination, releasing the occupied space and facilitating the formation of coarse pores.
  • Chemical Pore-Expanding Agents: React with substances to change the particle size and dispersion state, generate coarser secondary particles, and increase the pore size.

18.Advantages of Combination: Using a single type of pore-expanding agent requires a large dosage, which easily leads to dispersed pore distribution, reduced bulk density, and decreased crushing strength. The combination of the two can play a synergistic role, reduce the total dosage of pore-expanding agents, lower the catalyst cost, and make up for the shortcomings of a single pore-expanding agent.

19.Example: Tests by Wang Xiaoyong from Hebei University of Science and Technology showed that 单独 steam pressure pore expansion will damage the mechanical strength of silica gel, and after some silica gel is crushed, the active components in the impregnation solution cannot be effectively adsorbed, affecting the catalyst performance; while adding the chemical agent m-phenylenedimethylamine not only increases the pore size but also maintains the mechanical strength. At the same time, m-phenylenedimethylamine neutralizes the acidity on the silica gel surface, which is conducive to the uniform distribution of active components, suitable for mass transfer in oxidation reactions, and avoids excessive oxidation of reactants.

4.3.5 New Template Pore-Expanding Agent Treatment Method

1.R&D Background: The demand for microscopically ordered large-pore-size porous materials in fields such as nanoengineering and heterogeneous catalysis is increasing, and traditional pore-expanding technologies can no longer meet the needs, so more effective pore-expanding methods are required.

2.Common Template Agents: High molecular weight triblock surfactants, Gemini-type gemini amine neutral surfactants, polymer latex microspheres, etc.

3.Mechanism and Advantages of Typical Template Agents (Gemini): Gemini is formed by connecting two single-chain ordinary surfactants at the ionic part through chemical bonds. After adding ester and water, the silicon source and the template agent convert the bilayer structure into a mesoporous structure through electrostatic interaction. Its unique advantage is that when the pore size increases, the wall thickness also increases (ionic pore wall: 30-70nm), and the stability is extremely excellent. Even when the calcination temperature is increased to 1000℃ or hydrothermal treatment is performed at 100℃, the structure remains intact and undamaged, solving the contradiction between increased pore size and decreased stability.

4.4 Influence of Pore-Expanding Treatment on Carrier Silica Gel Performance

4.4.1 Influence of Physical Pore-Expanding Treatment

The principle is that ions or acid radicals diffused into the pores release space through calcination and drying to achieve pore expansion. However, the colloidal particles undergo a pressure release process from filling to evacuation, which will cause deformation of the skeleton structure and reduce mechanical properties (such as wear resistance, surface strength). During treatment, the crystal type, skeleton structure, and shape of the colloidal particles must be considered.

4.4.2 Influence of Chemical Pore-Expanding Treatment

Secondary particles are generated through chemical reagent immersion or chemical reactions to change the stacking state and increase the pore size. However, chemical reagents will corrode the colloidal particles or react with them, destroying the unit cell walls of the colloidal particles, leading to a significant decrease in mechanical properties. In addition, the dosage of chemical reagents has clear limitations; excessive dosage not only fails to expand pores but also reduces the performance of colloidal particles.

4.5 Conclusion

1.Essence of Carrier Silica Gel Pore Volume Modification: First, using carbon compounds that can be oxidized into gas to expand pores through the pressure release process before and after entry; second, using chemical reagents to react with colloidal particles to change the bulk density of colloidal particles to achieve pore expansion.

2.Technology Development Trend: With the advancement of science and technology and the emergence of new materials, the research on carrier silica gel modification is continuously deepened. While improving the original treatment methods, new treatment methods such as template agents, high molecular polymer surfactants, and Gemini gemini amines have been developed.

3.Existing Problems: During the modification of carrier silica gel, other properties of the colloidal particles (especially mechanical properties) will be damaged to a certain extent, which is a key problem to be solved in subsequent research.