CHEM F337: Green Chemistry & Catalysis Lecture 05 PDF

CHEM F337: Green Chemistry & Catalysis Lecture 05 Green Chemistry: Sustainability, LCA, Evaluating Reactions BITSPilani, Pilani Campus What is SUSTAINABILITY? A sustainable society is one that...

CHEM F337: Green Chemistry & Catalysis Lecture 05 Green Chemistry: Sustainability, LCA, Evaluating Reactions BITSPilani, Pilani Campus What is SUSTAINABILITY? A sustainable society is one that.meets the needs of the current generation without sacrificing the ability to meet the needs of future generations. Sustainable development is a strategic goal. There are several kinds of relationships between the strategic goals, the practical approaches, and the operational and monitoring tools. Thus, green chemistry is just one step (albeit an important one) along the road to sustainability. Thus sustainability can be reached using various approaches, and this is where green chemistry comes in. BITSPilani, Pilani Campus Need for SUSTAINABILITY BITSPilani, Pilani Campus See the bigger picture …. Atom economy is not the single best pointer to understand sustainability Example 1: Adding Br2, HF, or HCN to a double bond is ‘clean’ from the atom economy perspective, but storing and/or transporting these highly toxic reagents is problematic. Example 2: Hydrogen fuel cell: Comparison of the efficiency of the conventional fuels yields important facts. H2 must be manufactured from non-fossil fuels, - biomass, or water. This costs time, capital, and energy. Then H2 must be transported and stored needing dedicated infrastructure. H2 manufacture generates waste All these factors must be taken into account when evaluating the overall efficiency of fuel cells. BITSPilani, Pilani Campus  Life-Cycle Thinking Figure 16.04: The elements of a product’s life cycle. BITSPilani, BITS Pilani, Deemed to be University under Section Pilani Act, 3 of UGC Campus 1956 Product and Process Life Cycle Analysis (LCA) Life Cycle Analysis helps us evaluate the environmental impact of a chemical product or process. The main advantage of LCA is that it gives an overall view. This includes Raw-material extraction and acquisition, Chemical reactions, Processing, Manufacture, Packaging, Transportation, distribution, consumer use, and End-of-life management LCA is not limited to chemistry only Lankey, R.L. and Anastas, P.T. (2002) Lifecycle approaches for assessing green chemistry technologies. Ind. Eng. Chem. Res., 41, 4498. McDonough, W., Braungart, M., Anastas, P.T. and Zimmerman, J.B. (2003) Applying the principle of green engineering to cradle-to-cradle design. Environ. Sci. Technol., 37, 434. BITSPilani, Pilani Campus Product and Process Life Cycle Analysis (LCA) LCA has mainly 4 stages (1) Defining the assessment scope and boundaries; (2) Quantifying energy and materials flows (inventory analysis); (3) Impact analysis, determining the effects on the environment and human health (4) Improvement analysis, which can include methods such as green chemistry and green engineering, and environmental design BITSPilani, Pilani Campus  Life Cycle Analysis (LCA) What is Done (1) Defining scope: In this stage, functional units are defined, so that products can be compared on the basis of the functions they fulfill, rather than by their amount (2) Inventory analysis: Making an inventory of all the environmental interventions, such as emissions to air and water, and the acquisition of raw materials. This is done using mass- and energy-balances. The interventions are then clustered by type, and totaled for all the processes. Such inventory tables can contain over 100 entries. (3) Impact analysis: First, impact categories are classified. These usually pertain to common environmental threats, such as global warming, acid rain, or ozone depletion. The environmental interventions from step 2 are then translated into scores in each impact category, used for calculating an overall environmental impact profile for the original product or process. This is often the most problematic step, because quantifying the environmental impact of a process is complex and subjective. (4) Improvement analysis: Results are interpreted and an improvement analysis tries to pinpoint the process elements that can be changed by using a different technology or a different design - Green chemistry and green engineering scope BITSPilani, Pilani Campus  Life-Cycle Thinking: Life-Cycle Assessment of sandwich spread A life cycle map for the production of sandwich spread made from soy beanBITS oil.Pilani, Deemed to be University under Section BITSPilani, Pilani Act, 3 of UGC Campus 1956  Leapfrog Technology Leapfrogging means abandoning traditional development steps to get to advanced technology faster. BITSPilani, BITS Pilani, Deemed to be University under Section Pilani Act, 3 of UGC Campus 1956 Evaluating Reaction Types Simple approach is to identify the classes of reactions in use industrially SIX broad types of reaction/ process are generally involved , classically(~1990s) but at present several different classes are also practiced, Rearrangements Addition Substitution Elimination Pericyclic Oxidation/reduction Evaluative understanding of the nature of reaction Do they require additional chemicals? Do they generate waste? BITSPilani, Pilani Campus Evaluation methods for safer chemicals design “Form follows function” Structural form required for performing certain function is first conceptualized, then we design routes to obtain it – sometimes redesigning is necessary to obtain a GREENER route to the same. Sometimes a new but similarly effective structure is designed altogether Once the desired function is decided correlated to a structure, efforts are made to minimize the hazards and toxicity associated, by using basic methods such as , Mechanism of action analysis SAR Avoidance of toxic functional groups Minimizing bioavailability Minimizing auxiliary substances BITSPilani, Pilani Campus Evaluation methods for safer chemicals design Mechanism of action analysis Some chemicals are inert in atmosphere, but may become active once inside biological systems Some chemicals are intrinsically toxic Some chemicals generate secondary toxic end-points after metabolism process and passed out of itself Elucidation of mechanism of action/ reaction is essential to map all activities Finding the alternative pathways (mechanisms) is also important R-CH2-CN R-CH2 + CN Formation of a radical at the a-position to the CN group is known as R-ĊH-CN Thus, blocking the a-position by a substitution will prevent the release of cyanide. E.g. if we use R-C(CH3)2-CN BITSPilani, Pilani Campus Evaluation methods for safer chemicals design SAR – Structure Activity Relationships Used mostly in cases where the mechanism of action is not known Subtle changes in structure can change toxic potency (or minimize/ remove) Mostly based on known examples to create the change in structures thus affecting the toxic behavior Avoiding Toxic Functional groups Used mostly in cases where BOTH the mechanism of action as well as SAR is not known Identify the functional group and avoid using it Replacement functional group also needs to be identified e.g. Adhesives for car windshields – made of isocyanates which crosslinks to form polyurethanes. Alternatively acetoacetate esters can be used to form crosslinking polymers Masking technique – the FG is changed to a derivative which is non-toxic to the contacts BITSPilani, Pilani Campus Evaluation methods for safer chemicals design Minimizing Bioavailability The ability if a chemical to enter the various biological systems is called bio- availability Lack of information of mechanism, SAR can be treated by JUST AVOIDING CONTACT Also, modifying the chemical moieties to change the physical properties such as solubility, surface tensions etc. on the molecule to avoid being bio- available Minimizing auxiliary substances Certain inert chemicals required the use of associates/ auxiliary substances to become toxic and be bio-available e.g. paints involve VOCs for getting applicability advantage, which are hazardous. Design of water-based paints/ components will reduce the use of auxiliaries. BITSPilani, Pilani Campus