The error-prone estimation that is the current benchmark for characterizing the porosity of a material could be replaced by new, much more accurate software. The method, already tested by 61 leading laboratories, increases reliability and reproducibility, and redefines the gold standard of adsorption measurements – surface area calculations.
Surface area provides a good estimate of the amount of fluid entering the pores of activated carbons, zeolites, metal-organic structures (MOFs), and other porous materials. Traditionally, scientists estimated these values by entering experimental data into the Brunauer-Emmett-Teller (Bet) equation, which dates back to 1938. However, the team of David Fairén Jiménez from the University of Cambridge, UK , discovered some disparities.
“It was pure coincidence,” explains Fairén Jiménez. ‘We were analyzing the same material in two devices, and Bet’s equation estimated totally different adsorption areas,’ he adds. To confirm the findings, the team shared their data with other international experts, who reported identical inconsistencies. “Bet’s equation is the bread and butter of adsorption studies, but experiments suggest it sometimes fails.”
This happens because, to apply Bet’s equation, scientists have to choose their best experimental data. “We have to define the [most] linear region of the adsorption isotherm, … this choice influences the result, because the calculated area depends on it [data] says Michelle Ernst, who studies MOFs at the Institute for Theoretical Studies in Heidelberg, Germany. Therefore, the French physicists Jean and Françoise Rouquerol proposed a list of criteria, seeking to standardize the results. “However, even these criteria remain ambiguous, [and] the reported surfaces of porous materials vary between research groups”, adds Ernst.
After collecting results from more than 60 different labs, Fairen Jiménez’s team observed big discrepancies. In some cases the differences were fivefold, which is equivalent to estimating the area of Glasgow and finding London. “We decided to introduce new criteria into the Rouquerol list and to design software to implement them autonomously,” he explains. This program – baptized Betsi – is freely accessible, accessible to all surface scientists. “Betsi, for the first time, makes unambiguous assignments for surfaces.”
Tina Düren, an adsorption and simulation expert at the University of Bath, UK, explains that Bet’s equation was not originally designed for microporous materials, but for flat surfaces. “Nevertheless, everyone uses it because it provides a good reference, alongside pore size,” she adds. The manual input required for the original betting procedure prompted cherry picking and often inflated betting surfaces. “People could play games with the data points used…to get the best results,” she says. “The Betsi tool has great strength, because it identifies the ideal measurement and removes any ambiguity.
Russell Taylor, a zeolite expert at Durham University, UK, agrees. “The algorithm reduces imprecision, removing human error and enabling better ways to analyze data.” Betsi measures surfaces – and therefore adsorption capacity – more precisely. “It will set new standards in adsorption, a key process in many technological applications such as catalysis, carbon capture and water recovery,” he says.
Although the analysis is different, the data remains the same. Researchers could perform new studies on existing materials, increasing reproducibility and transparency. “The geometric surface is already a descriptor in calculation programs, this Betsi number could become one too,” explains Düren. This could lead to better predictive machine learning models and better comparisons between materials and labs. Düren hopes that equipment manufacturers will also adopt Betsi and provide updates for their instruments soon. “I think people are going to embrace it, try it in their labs,” confirms Russell. “It’s simple, self-checking, and eliminates human error – it just spits out the numbers you’re looking for.”