Questions: Materials Chemistry (Zeolites, MOFs, Perovskites)

Question 1

Zeolites selectively catalyze reactions based on the size of their pores. This shape selectivity means that only molecules smaller than the pore opening can enter and react — what is this specific type called?

• A. Product selectivity — only products that can fit through the pores escape
• B. Reactant selectivity — only molecules small enough to enter the zeolite pores can access the active sites inside
• C. Electronic selectivity — the zeolite selectively activates molecules based on their electron density
• D. Thermodynamic selectivity — the zeolite shifts the equilibrium toward smaller products

Answer: (B)

Reactant shape selectivity means the zeolite acts as a molecular sieve: its uniform pore dimensions (typically 3-12 Å) allow only molecules with the right size and shape to enter. Linear alkanes may enter while branched isomers are excluded, for example. Zeolite catalysis also exhibits product selectivity (only products that can exit the pores form preferentially) and transition-state selectivity (only transition states that fit within the pore geometry are accessible). All three types arise from the same principle: geometric confinement by the crystalline pore structure controls which species can participate in the reaction.

Question 2

Metal-organic frameworks (MOFs) can achieve surface areas exceeding 7000 m²/g — far beyond any conventional porous material — because their porosity is intrinsic to the crystal structure rather than arising from defects or grain boundaries.

• T. True
• F. False

Answer: True

MOFs are constructed by connecting metal cluster 'nodes' (like Zn₄O or Cu₂(COO)₄ units) with organic 'linker' ligands (like terephthalic acid or trimesic acid) in a three-dimensional periodic framework. The resulting crystal structure contains regular, permanent pores and channels — every unit cell is porous by design. By choosing longer linkers or more open topologies, the pore size and surface area can be systematically tuned. MOF-5 (Zn₄O(BDC)₃) has a surface area of ~3500 m²/g; MOF-210 exceeds 6200 m²/g. For comparison, activated carbon reaches ~1000-2000 m²/g, and zeolites typically ~300-800 m²/g. This extraordinary porosity makes MOFs candidates for hydrogen and methane storage, CO₂ capture, and drug delivery.

Question 3

Halide perovskites (like methylammonium lead iodide, MAPbI₃) have achieved solar cell efficiencies exceeding 25% in just over a decade of research. Their success comes in part from a high absorption coefficient, long carrier diffusion lengths, and a tunable band gap.

• T. True
• F. False

Answer: True

MAPbI₃ and related perovskites absorb visible light strongly (requiring only a ~500 nm film, compared to ~200 μm for silicon), have carrier diffusion lengths of 1-10 μm (meaning photogenerated electrons and holes can travel to the electrodes without recombining), and their band gap can be tuned from 1.2-2.3 eV by substituting different halides (I, Br, Cl) or cations. The ABX₃ perovskite structure (where A = organic cation, B = Pb²⁺ or Sn²⁺, X = halide) tolerates substantial compositional variation while maintaining the crystal structure. However, long-term stability (degradation from moisture and heat) and lead toxicity remain challenges for commercialization.

Question 4

Compare the design principles of zeolites and MOFs as porous materials, explaining why MOFs offer greater tunability but zeolites remain dominant in industrial catalysis.

Model answer: Zeolites are aluminosilicate frameworks with SiO₄ and AlO₄ tetrahedra connected through shared oxygen atoms. Their structures are determined by the synthesis conditions (template molecules, temperature, pH) but once formed, the framework is fixed. There are ~250 known zeolite framework types with pore sizes limited to ~3-12 Å. MOFs offer vastly greater tunability: the metal node and organic linker can each be varied independently, and linker length directly controls pore size. Thousands of MOF structures have been made with pore sizes from 5 to 98 Å and surface areas up to 7000+ m²/g. However, zeolites dominate industry because they are thermally stable (>500°C, essential for catalytic cracking), hydrothermally robust (resist steam and water), mechanically strong, and inexpensive. Most MOFs degrade below 300°C, are sensitive to moisture, and are currently too expensive for commodity-scale applications. Zeolites' narrower tunability is offset by decades of optimization and proven reliability.

This comparison illustrates a common pattern in materials science: academic research favors the most tunable system (MOFs), while industry favors the most robust and economical (zeolites). The frontier is developing MOFs that combine tunability with stability — water-stable MOFs based on Zr, Al, or Cr nodes are making progress toward bridging this gap.