Discharging dye contaminants into water is a major concern around the world. Among a variety of methods to treat dye-contaminated water, photocatalytic degradation has gained attention as a tool for treating the colored water. Herein, we review the recent advancements in photocatalysis for dye degradation in industrial effluents by categorizing photocatalyst materials into three generations. First generation photocatalysts are composed of single-component materials (e.g., TiO 2 , ZnO, and CdS), while second generation photocatalysts are composed of multiple components in a suspension (e.g., WO 3 /NiWO 4 , BiOI/ZnTiO 3 , and C 3 N 4 /Ag 3 VO 4 ). Photocatalysts immobilized on solid substrates are regarded as third generation materials (e.g., FTO/WO 3 -ZnO, Steel/TiO 2 -WO 3 , and Glass/P-TiO 2 ). Photocatalytic degradation mechanisms, factors affecting the dye degradation, and the lesser-debated uncertainties related to the photocatalysis are also discussed to offer better insights into environmental applications. Furthermore, quantum yields of different photocatalysts are calculated, and a performance evaluation method is proposed to compare photocatalyst systems for dye degradation. Finally, we discuss the present limitations of photocatalytic dye degradation for field applications and the future of the technology.

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As a subset of the metal-organic frameworks, zeolitic imidazolate frameworks (ZIFs) have potential use in practical separations as a result of flexible yet reliable control over their pore sizes along with their chemical and thermal stabilities. Among many ZIF materials, we explored the effect of thermal treatments on the ZIF-7 structure, known for its promising characteristics toward H2 separations; the pore sizes of ZIF-7 (0.29 nm) are desirable for molecular sieving, favoring H2 (0.289 nm) over CO2 (0.33 nm). Although thermogravimetric analysis indicated that ZIF-7 is thermally stabile up to ~400 °C, the structural transition of ZIF-7 to an intermediate phase (as indicated by X-ray analysis) was observed under air as guest molecules were removed. The transition was further continued at higher temperatures, eventually leading toward the zinc oxide phase. Three types of ZIF-7 with differing shapes and sizes (~100 nm spherical, ~400 nm rhombic-dodecahedral, and ~1300 nm rod-shaped) were employed to elucidate (1) thermal structural transitions while considering kinetically relevant processes and (2) discrepancies in the N2 physisorption and CO2 adsorption isotherms. The largest rod-shaped ZIF-7 particles showed a delayed thermal structural transition toward the stable zinc oxide phase. The CO2 adsorption behaviors of the three ZIF-7s, despite their identical crystal structures, suggested minute differences in the pore structures; in particular, the smaller spherical ZIF-7 particles provided reversible CO2 adsorption isotherms at ~30-75 °C, a typical temperature range of flue gases from coal-fired power plants, in contrast to the larger rhombic-dodecahedral and rod-shaped ZIF-7 particles, which exhibited hysteretic CO2 adsorption/desorption behavior.

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Dehydration and catalytic cracking reactions can be combined to convert glycerol into light olefins using solid acid catalysts. The combination is suitable for a singlestep process to convert glycerol into light olefins at high temperatures (26−36% selectivity at 873 K). However, large quantities of carbon oxides are produced (31−39% CO x selectivity), and catalyst deactivation also occurs. High light olefin selectivity (62−65%) and a smaller quantity of carbon oxides (11−12% CO x selectivity) can be obtained by using a tandem process involving the dehydration of glycerol and subsequent catalytic cracking of the dehydration products (mainly acetol and acrolein). Furthermore, the ratio of propylene to ethylene can be adjusted by changing the dehydration catalysts to favor the production of acetol or acrolein: Acetol forms propylene, and acrolein forms ethylene. To overcome the fast deactivation of acid catalysts in glycerol dehydration, the hydrogenolysis and catalytic cracking reactions can be synchronized to convert glycerol into hydrocarbons using a combination of metal and acid catalysts. The single-step conversion of glycerol over a metal or bifunctional catalyst formed alcohols and paraffin. The highest selectivity for propylene production (approximately 76%) was obtained in a tandem process via the selective hydrogenolysis of glycerol to propanols over Pt/ZSM-5 catalysts followed by the catalytic dehydration/cracking of propanols to propylene over ZSM-5 catalysts at low temperatures (523 K). The selectivity for propylene was improved by increasing the Si/Al ratio of the ZSM-5 catalysts and the reaction time. Under these conditions, economically competitive crude glycerol (mainly mixtures of glycerol and methanol) can be used to synthesize light olefins (approximately 61% selectivity) with a long lifetime (∼500 h) in single-route reactions by increasing the cracking temperature to 773 K, which is suitable for practical methanol to propylene process.

A heterogeneous catalyst was synthesized by supporting copper metal on MCM-41 by chemical vapor deposition (CVD). The usage of oxygen as a carrier gas and oxidizing agent is found to be very important in producing a stable and effective Cu/MCM-41 catalyst. This MCM-41-supported copper catalyst is evaluated in the photo-Fenton-like degradation of a dye pollutant, Orange II. Results show that the Cu/MCM-41 catalyst is effective in mineralizing total organic carbon (TOC) of 80%, 78%, and 70% at pH 3, 5.5, and 7, respectively, successfully overcoming the low efficiency problem of the conventional Fenton reaction at high pH. Moreover, the synthesized catalyst is proved to be durable with a stable TOC removal efficiency after four consecutive cycles. The kinetic study of pollutant degradation using Cu/MCM-41 is also conducted.