"1er ciclo de seminarios de Adsorción: Dr. Juan Antonio Cecilia"

Scientific Concepts, Discoveries, and Nature/Phenomena Discussed

CO₂ environmental problem and mitigation context

• Fossil-fuel combustion releases CO₂, which accumulates in the atmosphere and intensifies the greenhouse effect, driving global warming and downstream impacts (e.g., drought and ice melting).
• Mitigation can involve replacing fossil fuels with solar, wind, biomass, and hydro. However, because renewables are not yet fully mature, CO₂ capture/absorption is emphasized as a necessary complementary strategy.
• Two linked approaches are highlighted:
  • CO₂ capture–transport–storage
  • CO₂ valorization (turning CO₂ into commercially valuable products) A central challenge is that CO₂ is chemically inert, and valorization routes often have low yields.

CO₂ capture process types and cost drivers

• Capture methods mentioned include:
  • Post-combustion: absorbing CO₂ from flue gas (e.g., chimney filters).
  • Pre-combustion: absorbing CO₂ in hydrogen production streams.
  • Oxy-combustion: referenced as part of the standard set (with fewer details provided).
• Cost breakdown emphasized:
  • Capture can be up to ~70% of total cost, making low-cost, high-capacity adsorbents especially important.
• Traditional solvent-based capture:
  • Amine or ammonia-based absorption is effective but expensive because of:
    • Corrosion (CO₂ and other components such as H₂S; bending/elbow regions are especially prone; supercritical/acidic conditions worsen corrosion).
    • Expensive regeneration, where CO₂ is released and the solvent is restored.

Alternative capture strategy: solid adsorbents

• A promising alternative is solid absorbents that selectively retain CO₂ and can be regenerated using pressure and/or temperature changes.
• Adsorbent classes surveyed include examples such as:
  • Microporous carbons, graphene, activated carbon
  • MOFs/KOFs (e.g., “KOF MOV” mentioned)
  • Nanostructured silicas
  • Alkali hydroxides
  • Double-layered materials
  • Clay-derived materials and derivatives

Performance tradeoffs: capacity vs. regeneration and synthesis cost

• Example tradeoff: some porous/carbon materials can have high CO₂ uptake but are costly to regenerate due to high temperature requirements.
• The goal is low-cost, widely available adsorbents with:
  • High CO₂ uptake
  • Preferably easy regeneration
  • High selectivity against N₂/O₂
  • In some contexts: low affinity for H₂ (pre-combustion) and CH₄ (natural gas mixtures)

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Methodology / Material Design Approaches for “Low-Cost CO₂ Absorbents”

A) Clay minerals (phyllosilicates) and their modification

Mineral framework

Phyllosilicates are described as:

• A tetrahedral layer plus an octahedral layer

Clay classification mentioned:

• Kaolin/serpentine group: no cations between layers
• Talc/pyrophyllite: differing interlayer content/structure
• Mica group: interlayer cations; hydrated cations increase spacing
• Chlorite group: an additional “interlayer” witchite-like layer

Key clay targets for CO₂ capture

• Kaolin group
  • CO₂ absorption occurs mainly on the surface via silanol groups, forming bicarbonate-like species.
  • Raw kaolin shows low uptake values.
  • Intercalation attempts can increase spacing, but CO₂ still tends not to enter well—uptake remains largely surface-driven.
• Smectite (mesquite)
  • Charge deficiency is balanced by interlayer cations.
  • Swelling from hydrated cations gives cation exchange capacity.
  • Raw uptake remains limited; improvement strategies include:
    • Replacing interlayer alkali cations (Na/K) with more CO₂-affine polarizable cations (e.g., Ca)
    • Inserting organic cations (e.g., dendrimers, polyols with OH/NH₂) to increase uptake via swelling and stronger interactions
• Fibrous clays: sepiolite and palygorskite (“paleskite”)
  • Tetrahedral inversion generates nanocavities.
  • Uptake primarily occurs in these cavities, with > an order-of-magnitude improvement over some other clays.
  • CO₂ interacts with silanol groups, but cavity-driven uptake dominates overall capacity.

Strategies to improve clay adsorption capacity

• Acid treatment
  • Leaches octahedral cations (Al/Mg), partially collapses lamellae, and increases surface area/microporosity.
  • Can increase uptake for some clays.
  • Drawback: requires washing to remove residues, increasing cost.
  • Less beneficial for fibrous clays since cavities are already well-sized; acid can enlarge cavities and reduce effectiveness.
• Pillaring / pillar formation
  • Replace interlayer Na⁺ with bulky polyoxocations (commonly Al polyoxocations, also zirconium polyoxide).
  • Calcination forms oxide “pillars” (Al₂O₃ or ZrO₂), increasing interlayer spacing and microporosity and often significantly raising uptake.
  • PCH / porous clay heterostructures
    • Replace Na⁺ with bulky organic cations, then introduce a silica precursor that polymerizes into silica pillars.
• Amino-functionalization (increasing chemical adsorption capacity)
  • Chemical anchoring: amino-alcosilane groups react with silanols, leaving amino groups on the surface.
    • CO₂ forms carbamate species (sorbent–CO₂ chemical interaction).
    • Typical anchored nitrogen: ~3–5 wt%.
    • Side effects may include reduced textural properties; physical uptake centers can be affected.
  • Amino-rich polymer impregnation
    • Creates composites with mainly chemical adsorption, but can drastically reduce surface area/microporosity (making uptake more surface-limited).
    • Can yield high uptakes (sometimes >100–200 mg CO₂/g).
    • Uptake may increase with temperature due to improved CO₂ diffusion.

Tradeoff principle

Physical adsorption tends to benefit from microporosity. Chemical adsorption depends strongly on surface functional groups (e.g., amino groups), even if microporosity decreases.

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B) Zeolites derived from clay precursors

Zeolite structure

• Zeolites are aluminosilicates with ordered frameworks and defined micropores.
• Framework charge balance:
  • When Al substitutes for Si, charge deficiency is balanced by cations (Na, K, Ca, H⁺, etc.).
• Pore types/structures referenced include:
  • Type A, sodalite, and Type X/Y zeolites (with examples of narrow pore diameters).

Synthesis from clay minerals

• Uses clay precursors such as:
  • Kaolinite
  • Mordenite-like (“morillonite”) precursors chosen for suitable Si/Al ratio
• Processing:
  • Strongly alkaline medium
  • Hydrothermal conditions
• Mechanism (high level):
  • Solubilize aluminosilicate into a gel
  • Silicate/aluminate species polymerize
  • Rearrangement forms ordered zeolite crystal structures
• Thermal pretreatment:
  • Heat kaolinite above ~500–550°C for dehydroxylation and structural collapse, enabling zeolite crystallization under basic/hydrothermal conditions.

Performance

• Clay-derived zeolites exhibit much higher CO₂ uptake than raw clays (often cited in the ~100–300 mg/g range).

Modifying zeolites

• Cation exchange with more CO₂-binding, polarizable cations increases binding strength.
• Amino functionalization can increase chemical adsorption, but:
  • Since zeolites already have microporosity, amino groups can block/consume micropores, reducing physical adsorption.

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C) Geopolymers (clay-derived amorphous aluminosilicates)

• Presented as “disordered siblings” of zeolites:
  • Aluminosilicates like zeolites but highly amorphous
  • Lower microporosity → lower CO₂ uptake than crystalline zeolites
• Prior roles mentioned:
  • cement industry and wastewater treatment (via cation exchange) before gas-capture exploration
• Synthesis route:
  • kaolin → metakaolin (heat treatment) → solubilize into a gel → disordered polymerization (analogous overall steps to zeolite synthesis but without the crystalline ordering)
• Enhancement options:
  • Porogens: add small compounds that decompose at low temperatures to generate microporosity
  • Incorporate microporous materials (carbon, zeolite) into macroporous geopolymer matrices to raise uptake, though composites may underperform compared with standalone powders and introduce morphology tradeoffs.

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D) Circular economy: valorizing “floating ash” waste for CO₂ capture

Source and motivation

• “Floating ash” comes from coal-fired power plants and cement industries.
• Historically it has had low-value uses (road fill, bedding, construction blocks).
• The talk focuses on transforming it into:
  • Luminosilicates
  • Activated carbons
  • Alkali silicates
  • Mesoporous silicas
• Focus targets emphasized: luminosilicates and activated/carbon materials.

Raw ash properties and limitations

• Composition is mainly Al and Si (similar in spirit to clays/zeolites), with impurities such as:
  • Fe
  • Ca
  • variable carbon content
• Uptake depends strongly on particle size (smaller particles → higher uptake).
• Ca can bind CO₂ strongly, but regeneration may require ~700–800°C, reducing practical feasibility.

Turning floating ash into improved adsorbents

1. Carbon-based materials

   • Remove inorganic species via acid/basic treatment to leave a carbonaceous base.
   • Thermal activation:
     • inert gas activation (N₂/He) or CO₂ activation to create porosity
     • chemical activation using KOH, H₃PO₄, ZnCl₂ to generate more microporosity
   • Physical adsorption dominates.
   • Reported carbon uptakes are in a lower range (e.g., ~26–68 mg/g) compared with industrial carbons (>~200 mg/g).
   • Further functionalization (polarizable cations/amino groups) could enhance chemical adsorption, but increases cost and regeneration demands.

2. Zeolites from ash (luminosilicate-derived)

   • Treat ash to solubilize aluminosilicate → gel → polymerize and route to zeolite crystallization.
   • Methods named (examples):
     • hydrothermal method
     • hydrothermal fusion
     • alkaline fusion
     • Hollmann method
   • Key challenges:
     • impurity effects (Ca and Fe insolubility under basic conditions; quartz/glass and residual carbon)
   • Example performance:
     • X-type zeolite from floating ash reported uptake around ~220 mg/g, approaching commercial zeolites.

Practical selection (speaker’s preference)

• If using raw low-cost material directly: sepiolite (microporous nanocavities).
• If willing to modify/synthesize: clay/floating-ash-derived zeolites.

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Researchers / Sources Featured (as named in subtitles)

• Dr. Juan Antonio Cecilia (University of Malaga)
• Dr. Diana Acevedo (Federal University of Sarah)
• Dr. Karim Sapad (Argentina)
• Dr. Mildred Quintana (Mexico)
• Dr. Armando Domínguez
• Dr. Enrique Rodríguez (Spain)
• Dr. Raúl López (Argentina)
• Dr. Carlos Felipe
• Dr. Marcos (Dr. Marcos Esparza mentioned as laboratory/head later)
• Dr. Jorge Garza
• Dr. Salomón
• Dr. Juan Marcos Esparza
• Dr. Enrique Vilarza García
• Isaac Pérez
• Pedro Augusto
• Anabel (only first name given)