The origin of life is a fundamental question in science, and understanding the synthesis of prebiotic molecules, such as carbohydrates, is crucial. Carbohydrates serve as both building blocks and energy sources, essential for any life form. The formose reaction, discovered by Butlerow in 1861, presents a plausible pathway for prebiotic sugar formation, where formaldehyde and glycolaldehyde react in a cascade of reactions to generate a variety of sugars. However, this reaction typically suffers from low yields and the formation of numerous unwanted byproducts in aqueous environments. This is problematic because the early Earth's conditions are largely debated, and the presence of water is still uncertain. Previous research has suggested that solid-state mechanochemical reactions offer a more selective and efficient route for prebiotic sugar synthesis, potentially circumventing the limitations of aqueous-phase formose reactions. This study builds on these prior findings by exploring the catalytic role of a wide range of minerals in mechanochemical formose reactions and examining the influence of different atmospheric conditions on the reaction's efficiency and product distribution. This research aims to provide a comprehensive understanding of the feasibility of prebiotic sugar formation under various primordial conditions, potentially offering a water-free and robust pathway for monosaccharide synthesis on early Earth or in extraterrestrial settings. The discovery of ribose and other sugars in meteorites further fuels this line of investigation, suggesting that mechanochemical formose reactions could have played a crucial role in the delivery of prebiotic molecules to early Earth.

Previous studies have explored the formose reaction extensively, focusing primarily on aqueous-phase catalysis. The use of calcium hydroxide as a catalyst is well-established, but other mineral catalysts have also been proposed for prebiotic pathways. Research on the aqueous formose reaction has highlighted the challenges posed by side reactions and the instability of the products under alkaline conditions. These side reactions significantly lower the yield of desired monosaccharides. The Eschenmoser’s glycoylate scenario is another plausible pathway for the prebiotic formation of sugars. Several studies have explored different catalysts, such as thiazolium salts (analogous to vitamin B1), to enhance the selectivity and efficiency of the reaction. The discovery of ribose and other sugars in meteorites has raised the possibility of an extraterrestrial formose reaction, which is highly relevant for this study as the findings help support this hypothesis. However, the precise conditions under which these reactions might have occurred, specifically the role of various mineral catalysts and the impact of atmospheric conditions, remain largely unexplored, thus the importance of this research.

The study employed both oscillatory and planetary ball mills to perform mechanochemical reactions. The oscillatory ball mill was used to test the catalytic activity of a wide range of minerals. A total of 20 different minerals representing various classes (hydroxides, carbonates, sulfates, silicates, micas, zeolites, clays, olivines, phosphates, phosphides, and borates) were assessed. Glycolaldehyde was used as the starting material in these experiments, along with 20 mol% of each mineral catalyst. The reaction was carried out at 30 Hz for 90 minutes at room temperature or under liquid nitrogen cooling. The planetary ball mill was used for experiments involving formaldehyde and different gas atmospheres. In these experiments, glycolaldehyde and calcium hydroxide (20 mol%) were milled for 90 min at 400 rpm under various atmospheres (air, nitrogen, methane, carbon dioxide). Formaldehyde was adsorbed onto zeolites and sheet silicates and subsequently reacted with glycolaldehyde in the planetary ball mill to investigate the incorporation of formaldehyde into the reaction. Experiments using thiazolium salts as umpolung catalysts were also performed to investigate the potential use of prebiotic forms of vitamin B1. For the analysis of the reaction products, a two-step derivatization protocol was employed, which uses O-ethyl hydroxylamine and N-/O-bistrifluoroacetamide to create silylated oximes. This protocol was selected based on the work of Haas et al. (2018). Gas chromatography-mass spectrometry (GC-MS) was used to identify and quantify the different sugars and byproducts. A separate method involving a gas chromatograph coupled with a thermal conductivity detector (TCD) was used to measure more volatile components (formaldehyde, methanol, formic acid). The data reported in the study typically represents mean values with standard deviations from multiple experiments.

The study revealed that a broad range of minerals are capable of catalyzing the mechanochemical formose reaction. All minerals, except for anhydrite and colemanite, showed catalytic activity, leading to the formation of tetroses and hexoses from glycolaldehyde. The product distribution was significantly affected by the choice of mineral catalyst, with variations observed in the ratios of tetroses to hexoses and aldoses to ketoses. For instance, portlandite yielded over 40% tetroses while quartz produced only trace amounts. The chromium-rich fuchsite showed nearly double the conversion compared to muscovite, highlighting the effect of mineral composition on the reaction. The reaction's robustness was demonstrated by its tolerance to different atmospheric conditions (air, nitrogen, methane) and low temperatures (liquid nitrogen cooling). The experiments incorporating formaldehyde, using zeolites and sheet silicates as catalysts, demonstrated the synthesis of glyceraldehyde, dihydroxyacetone, pentoses (including ribose), and heptoses. While side products such as lactic acid and apiose were produced, their amounts remained low. The mechanochemical reaction also showed tolerance for different gas phases such as methane, nitrogen, air, or carbon dioxide. Only carbon dioxide led to a slight decrease in conversion, presumably due to carbonate formation that affects the reaction's alkalinity. The attempted umpolung reaction using thiazolium salts (precursors of vitamin B1) also produced traces of sugars, demonstrating another plausible catalytic route for prebiotic sugar formation. The methyl-substituted thiazolium salt, however, showed no activity which aligns with previous findings on this catalyst's diminished performance in aqueous solutions.

The findings of this study significantly advance our understanding of prebiotic carbohydrate synthesis. The demonstration that various minerals can catalyze mechanochemical formose reactions, yielding monosaccharides with minimal side products, provides compelling evidence for a plausible abiotic pathway for sugar formation in various geochemical settings. The reaction's robustness and insensitivity to atmospheric conditions suggest that it could have readily occurred on early Earth, regardless of the prevailing atmospheric composition. Moreover, the possibility of extraterrestrial sugar synthesis through mechanochemical processes offers a potential explanation for the presence of sugars in meteorites. This work enhances previous models suggesting the formation of prebiotic molecules in solid-state mechanochemical reactions. The demonstrated ability to alter the product distribution by choosing specific minerals offers possibilities for selective sugar synthesis. The observation that the reaction proceeds under different gases and temperatures shows the robustness of this pathway for prebiotic sugar formation, implying that the reaction may have been prevalent in various early Earth scenarios. Moreover, the use of readily available minerals as catalysts increases the likelihood of this being a prevalent prebiotic process.

This research demonstrates a robust and versatile pathway for prebiotic monosaccharide synthesis via mechanochemical formose reactions using various mineral catalysts. The reaction's efficiency, broad applicability across diverse minerals and atmospheric conditions, and the low levels of side products significantly strengthen the hypothesis of abiotic sugar formation on early Earth and other celestial bodies. Future research could focus on exploring a wider range of minerals and investigating the influence of other environmental factors, such as pressure and radiation, on the reaction's efficiency and product distribution. Investigating the role of other potential prebiotic catalysts could also provide further insights into this important process. The findings could also be extended to study the synthesis of other essential prebiotic molecules using similar mechanochemical approaches.

While the study demonstrates the feasibility of mechanochemical sugar synthesis, several limitations need to be considered. The study focuses primarily on the formation of simpler sugars; more complex sugars might require additional reaction steps or catalysts. The reaction conditions in the laboratory setting might not perfectly replicate the conditions of early Earth or extraterrestrial environments. While the study explores a wide range of minerals, it does not encompass all possible minerals present in primordial settings. Furthermore, the exact mechanisms behind the mineral catalysis are not fully understood, and future investigations could be focused on this aspect.