(543d) Computer-Aided Fuel Design with Generative Graph Machine Learning

Fuel components that enable higher engine efficiency are of crucial importance for reducing carbon emissions in the transportation sector and their identification is actively pursued by means of computer-aided molecular design (CAMD), e.g., in [1-3]. CAMD employs computational methods for the generation of candidate molecules and the prediction of application-relevant properties from molecular structure. For the latter, quantitative structure-property relationships (QSPRs) are typically employed which first decompose the molecular structure into structural groups or compute molecular descriptors and then correlate the number of occurrences of these groups or the molecular descriptor values to the physicochemical properties of interest. The generation of candidate structures in CAMD usually builds on the concept that structural groups can be combined into chemically feasible molecules with desired properties, e.g., by using concepts from evolutionary theory [4] or by formulating and solving a mathematical program [5,6].

Recently, novel machine learning (ML) methods in the form of generative graph-ML models and graph neural networks have been utilized for CAMD [7,8]. These ML methods directly operate on a representation of molecules as graphs and thus circumvent the need for selecting meaningful structural groups or molecular descriptors. Instead, generative graph-ML models are trained on a data set of molecules in an unsupervised manner to generate new molecules not included in the data set. Specifically, they learn to generate molecules from a continuous latent space, overcoming discrete representations in the form of structural groups used in well-established CAMD methods and enabling the application of continuous optimization approaches [7,8]. Furthermore, graph neural networks are able to learn molecular properties directly from the molecular graph [9,10] and thus allow for end-to-end property prediction. However, applications of graph-ML-based CAMD have mainly focused on drug discovery so far; chemical engineering applications, including model-based fuel design, are widely missing.

We present a graph-ML CAMD framework for designing fuel components with desirable research octane number (RON) and octane sensitivity (OS) for increased engine efficiency. Our framework utilizes generative graph-ML models, graph neural networks, and optimization for strategic sampling of candidate structures from a continuous latent molecular space. We analyze the effect of different generative graph-ML models, including variational autoencoders and generative adversarial networks, cf. [11-13], on the identified candidate molecules. Further, we explore different strategies for optimization within the continuous molecular space, i.e., Bayesian optimization and genetic algorithms. The results show that our framework can find well-established fuel components as well as new candidate molecules that require further investigation. By experimentally assessing one novel candidate, we demonstrate the importance of experimental validation in model-based fuel design and highlight the need for additional RON/OS training data for a broader range of molecular classes. Extension of our graph-ML CAMD framework to other chemical engineering applications is straight-forward. Publication of the models and codes is currently in preparation.

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