Question

In the following three instances, which choice is greener in a chemical process? Explain. (a) A reaction that can be run at \(350 \mathrm{~K}\) for \(12 \mathrm{~h}\) without a catalyst or one that can be run at \(300 \mathrm{~K}\) for \(1 \mathrm{~h}\) with a reusable catalyst. (b) A reagent for the reaction that can be obtained from corn husks or one that is obtained from petroleum. (c) A process that produces no by-products or one in which the by-products are recycled for another process.

Short answer

In the given instances, the greener choices for a chemical process are: (a) using a reusable catalyst at 300 K for 1 h, as it reduces energy consumption and time, (b) using a reagent obtained from corn husks, as it is more sustainable and has lower environmental impact, and (c) a process that produces no by-products as it may have a lower environmental impact due to absence of waste production, though both options with no by-products and recyclable by-products indicate environmentally friendly approaches.

Step-by-step solution

(a) Temperature and Time without/with Catalyst

: The greener process in this case will be the one that requires less energy input and has less environmental impact. Comparing the two reactions: 1. Without a catalyst, the reaction takes place at 350 K for 12 h. 2. With a catalyst, the reaction can occur at 300 K in just 1 h. The usage of a reusable catalyst makes this process more sustainable because it reduces the overall energy consumption and time for the reaction. Thus, the reaction with a reusable catalyst at 300 K for 1 h is greener.

(b) Reagent Origin: Corn Husks vs. Petroleum

: In this case, we need to consider the sustainability of the reagents and their environmental impact from the source. Comparing the two sources: 1. Corn husks are renewable resources and are repurposing what would otherwise be considered an agricultural waste. 2. Petroleum is a non-renewable resource, and its extraction and use are associated with higher greenhouse gas emissions and environmental pollution. Therefore, the reagent derived from corn husks has a lower environmental impact and is a more sustainable choice.

(c) By-products: None vs. Recyclable

: The greener process will be determined by how the production process affects the environment. Let's compare the two options: 1. A process which produces no by-products means that there is no waste generated or any materials that need to be disposed of or recycled. This can lessen the environmental impact. 2. A process with recyclable by-products can be sustainable if the recycling process's energy consumption and environmental impact are minimal and the recycled by-products are repurposed effectively. Comparing both options, while both are potentially "green," the first option which does not produce any by-products may have a lower environmental impact due to the absence of any waste production. However, this also depends on the environmental cost of each of the processes in their entirety. Without more information on the specific chemical processes, both options indicate an environmentally friendly approach, with the by-products free process being potentially greener.

Key concepts

Catalysis

A catalyst is a substance that speeds up a chemical reaction without being consumed or altered in the process. It lowers the activation energy required for the reaction. This means that the reaction can occur under less extreme conditions, such as lower temperatures or pressures. By doing so, it reduces the energy needed for the reaction, making the process more energy-efficient and environmentally friendly.
In the example provided, using a reusable catalyst allows the reaction to proceed at a lower temperature and for a shorter period (300 K for 1 hour) compared to when no catalyst is used (350 K for 12 hours). This not only conserves energy but also decreases the carbon footprint associated with energy production.
Additionally, reusable catalysts offer significant advantages in terms of sustainability since they can be used multiple times without losing their effectiveness. This minimizes waste and reduces the need for producing new materials, aligning with green chemistry principles. The overall impact is a more sustainable and eco-friendly chemical process.

Renewable resources

Renewable resources are natural resources that can be replenished over time, making them a sustainable option for industrial use. They include resources like plant materials, solar energy, wind, and water. Using renewable resources helps reduce dependence on finite non-renewable resources such as fossil fuels.
In the scenario discussed, using a reagent derived from corn husks is considered greener than one derived from petroleum. Corn husks are a byproduct of corn production and can be continuously regenerated as long as corn is cultivated. This transforms what could be agricultural waste into a useful material. By contrast, petroleum is a finite resource that contributes to pollution and global warming when extracted and utilized.
Opting for renewable resources also aligns with efforts to decarbonize chemical processes and minimize overall environmental footprints, making it a fundamental aspect of sustainable chemistry practices. It's about making the choice that supports a circular economy where materials are constantly cycled and reused.

Waste management

In sustainable chemical processes, waste management is essential to minimize environmental impact. Effective waste management involves reducing waste generation, reusing materials, and recycling by-products effectively.
In the context given, a process that generates no by-products is preferable since it means no waste is produced, negating the need for disposal or recycling. This also streamlines operations by reducing the steps involved in treating or managing waste, directly lowering the environmental burden.
However, if by-products are inevitable, recycling them can also be a sustainable approach. Effective recycling processes can convert what might be waste into valuable input for other processes, promoting a circular economy. It's important, though, for the recycling process to be efficient with minimal energy input and environmental impact itself.
The key is to adopt processes that either limit waste creation or efficiently manage and repurpose it, reflecting the holistic approach of green chemistry to safeguard the environment while maintaining economic viability.