Towards rational design for greener chemicals and processes: expanding the green screening and synthetic toolboxes Open Access
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Green chemistry, a relatively young interdisciplinary field established to promote development of benign chemicals and chemical processes, had its silver anniversary last year in 2016. The core of green chemistry lies in the word “design”, and there are two facets associated with it - design for safer chemicals and development of greener processes. Over the years, tremendous scientific accomplishments and achievements have been made in the field, yet much more are needed to drastically decrease our population’s burden on the environment. This dissertation describes our efforts to expand the green chemistry toolboxes for both facets of green chemistry. For both directions and all sub-topics we have attempted to address the creed of “benign by design” and to show a stepwise approach to building systems and toolboxes rationally. The work is separated into two parts: expanding the green screening toolbox by the application of quantitative spectrometric-data activity relationship (Chapter 2-3) and expanding the green synthetic toolbox by development of tunable heterogeneous catalysts from doped layered double hydroxide (Chapter 4-7). Chapter 1 introduces green chemistry, including its history, status quo, challenges and opportunities. Concepts associated with quantitative spectrometric-data activity relationships (QSDAR) and heterogeneous catalysis are also covered in the chapter.Chapter 2 demonstrates the proof of concept that chemical and physical properties can be estimated from experimental spectra. Predictive tools based on this concept may potentially eliminate the need for a priori knowledge of exact chemical structure and allow the estimation of properties of mixtures. In this chapter, we report that a predictive method for octanol-water partition coefficient (log P) based on 1H-NMR spectra in d3-chloroform is feasible and can yield accuracy comparable to in silico log P models.Chapter 3 presents a QSDAR model for skin permeation rate (log Kp) that is trained on a large data set consisting of structurally diverse chemicals and has been thoroughly externally validated. We also try to address the importance of data selection and curation in this process due to the potential variability within data quality.Starting from Chapter 4, we exploit the development of heterogeneous catalysts for atom-economical processes under mild conditions. To develop tunable, selective and recyclable heterogeneous catalytic system, we identified hydrotalcite (HT)-like materials or layered double hydroxides (LDHs), as potential candidates as catalytic supports due to their systematic tunability for basicity, morphology and electronic properties. However, this application suffers from the fact that the morphology and surface area of LDHs is highly sensitive to the synthetic protocol, which makes it very challenging to produce consistent LDH materials. In Chapter 4, we report a controlled flow synthesis of LDHs doped with various transition metals, which resulted in great reproducibility and high surface area as well as extensive comparative study on the characterizations of these doped LDHs.Chapter 5 describes a tunable catalytic platform consisting of Mg-Al-Pd LDHs which is developed using the controlled method and thoroughly characterized. These catalysts can be electronically tuned by the incorporation of 0-5% compatible transition metals in the cationic LDH layers. Varying the quantity of Pd allows the selective incorporation of Pd only in the LDH cationic layers, or as multi-atom Pd clusters on the LDH surface. The catalysts are potentially very versatile, but we explore their applicability to decarbonylation reactions, which is a very useful yet challenging reaction.Chapter 6 extends the implementation of the Pd-LDH catalysts to the transformation of biomass-derived feedstocks, such as 5-hydroxymethylfurfural (HMF) and furfural. Meanwhile, some other possible routes to tune the catalytic surface are evaluated, including using different Pd sources, reduction and thermal treatment. The recyclability of the catalyst is also further discussed in this chapter.Chapter 7 introduces Pd-LDHs into a different type of reaction, which is dehydrogenative amine coupling. In this chapter, we demonstrated another possible approach to tune the catalysts towards different reactivity, which is to incorporate transition metal (iron in this case) into the LDH matrices. By synchronizing reaction conditions with this approach, the LDH catalysts can show superior activity and selectivity to other Pd catalysts.Chapter 8 evaluates the major challenges and great opportunities associated with the described work in this thesis. These challenges and opportunities are linked with potential furthering of our current approaches and expanding their applications. With the rapid development of modern chemical techniques, computer science as well as data science, our careful stepwise approaches are showing great potential on their way of expansion.