Tungsten Keeps Acid Sites Working: A Tougher Solid Acid Catalyst for Greener Chemistry

Solid acid catalysts promise cleaner, solvent-free routes to industrial chemistry, but they share a stubborn weakness: the acid sites that drive reactions tend to disappear during operation. A team at the University of Science and Technology Beijing and the Chinese Academy of Sciences reports a practical fix. By adding tungsten to sulfated titania, the researchers created a catalyst that holds onto far more of its working acid sites, keeping reactions running for longer. The work was published in Catalysis Letters.

The catalyst was tested on the transesterification of ethyl acetate, a model reaction relevant to biodiesel production and other industrial processes. In this reaction, a solid acid catalyst helps swap chemical groups between molecules without needing corrosive liquid acids. The problem has always been durability. As the reaction proceeds, the catalyst loses accessible acid sites, and its activity fades.

To measure this loss directly, the team developed a new diagnostic called Dynamic Brønsted Acid Site Evaluation. The method uses pyridine, a probe molecule that binds to acid sites, to count how many Brønsted acid sites remain accessible after the reaction. When adsorbed on Brønsted acid sites, pyridine produces a distinctive infrared signal near 1540 cm⁻¹. By comparing this signal before and after reaction, the researchers could quantify acid site retention rather than just inferring deactivation from falling conversion.

The numbers tell a clear story. The unmodified sulfated titania retained 48.2 percent of its Brønsted acid sites after reaction. The tungsten-modified version retained 72.6 percent. That improvement translated into better catalytic performance, with the modified catalyst achieving 86.4 percent conversion of ethyl acetate compared with 71.3 percent for the original.

Structural analysis confirmed the mechanism behind this durability. X-ray diffraction showed that the anatase framework of titania remained intact after tungsten modification, with no bulk tungsten oxide phases forming. Nitrogen adsorption measurements revealed that the modified catalyst preserved its mesoporous structure and surface area. The tungsten appears to anchor the sulfate groups that generate Brønsted acidity, preventing them from leaching or collapsing during operation.

The researchers also found that tungsten modification increased the concentration of Lewis acid sites, which can cooperate with Brønsted sites in transesterification reactions. This dual acidity, combined with better acid site retention, explains the improved performance. Temperature-programmed desorption of ammonia confirmed that the modified catalyst held acid sites of appropriate strength for the reaction.

This work addresses a fundamental challenge in heterogeneous catalysis: measuring and preventing catalyst deactivation. Rather than accepting that solid acid catalysts fade, the team showed that a simple modification can dramatically slow the loss of working sites. The approach could extend to other sulfate-based solid acids used in biodiesel production, biomass conversion, and fine chemical synthesis, where catalyst durability determines whether greener processes are economically viable.

The distinction between Brønsted and Lewis acidity is central to understanding why the tungsten-modified catalyst behaves differently from its unmodified counterpart. Brønsted acid sites donate protons and are typically generated in sulfated metal oxides by surface sulfate groups bonded to the oxide framework. Lewis acid sites, by contrast, accept electron pairs and are associated with coordinatively unsaturated metal centers on the oxide surface. In transesterification chemistry, both types of sites can participate, often acting in concert: the Lewis site polarizes the carbonyl bond of the ester while the Brønsted site activates the alcohol nucleophile. Pyridine-adsorbed infrared spectroscopy distinguishes the two cleanly, because pyridine bound to Brønsted sites produces the characteristic ring vibration near 1540 cm⁻¹, while pyridine on Lewis sites gives a band near 1450 cm⁻¹. This spectral separation is what allows the retention of each site population to be tracked independently across a reaction cycle.

The choice of tungsten as the modifying element has precedent in solid acid chemistry. Tungsten oxide supported on zirconia, for example, is a well-studied superacid system in which surface tungsten oxide species stabilize the underlying oxide phase and generate strong acidity. Similar logic applies to tungsten-promoted titania and related group 4 oxides, where dispersed tungsten species can enhance both thermal stability and acid strength without forming bulk crystalline tungsten oxide. The absence of detectable WO₃ phases in the present catalyst is significant, because crystalline tungsten oxide is generally far less active as an acid catalyst than highly dispersed surface tungsten species. High dispersion maximizes the interfacial contact between tungsten and the sulfated titania surface, which is where the stabilizing interaction is thought to occur.

The dynamic nature of the retention measurement deserves emphasis. Conventional catalyst characterization is typically performed on fresh materials, and deactivation is inferred indirectly from declining conversion over time. This approach conflates several possible causes of activity loss, including site poisoning, sintering, sulfate leaching, and pore blockage. By measuring accessible Brønsted acid sites on the spent catalyst directly, the retention descriptor isolates one specific deactivation channel: the loss of sites that remain physically reachable by probe molecules after reaction. A site may exist in the bulk but be inaccessible due to deposits or structural collapse, and only the accessible fraction contributes to catalysis. This ex situ probe-based method therefore provides a more mechanistically meaningful picture of what the catalyst retains under working conditions.

The transesterification of ethyl acetate with n-butanol serves as a convenient model for a broader class of industrially relevant reactions. Ester-alcohol exchange underlies the production of biodiesel from triglycerides, the synthesis of plasticizers, and the manufacture of solvents and fragrance intermediates. In each case, the commercial process often relies on homogeneous acids or bases that must be neutralized and washed from the product stream, generating salt waste and corroding equipment. Heterogeneous solid acids avoid these drawbacks because the catalyst can be filtered off and potentially regenerated. The economic case for solid acids, however, depends on reuse cycles, and a catalyst that loses its acid sites after a few hours of operation cannot compete with cheap homogeneous alternatives regardless of its initial activity.

Sulfated titania itself has a long history as a solid superacid, valued for acid strengths approaching or exceeding those of concentrated sulfuric acid. Its weakness has equally long been recognized: the sulfate groups that confer acidity are vulnerable to hydration, reduction, and thermal degradation, all of which strip sulfur from the surface and eliminate Brønsted sites. Various strategies have been explored to slow this degradation, including calcination optimization, promotion with other oxides, and control of sulfate loading. The tungsten approach fits within this lineage but adds a quantitative dimension, because the retention metric allows different modification strategies to be compared on the specific property that matters most for durability rather than on initial activity alone.

The apparent activation energy extracted from kinetic analysis offers an additional window into the reaction. A change in apparent activation energy between the modified and unmodified catalysts would suggest that the rate-determining step or the dominant active site population differs between the two materials. Combined with the Lewis acid concentration data and the ammonia desorption profiles, such kinetic information helps distinguish between a scenario in which tungsten merely preserves more of the same sites and one in which it creates a qualitatively different catalytic environment. The regression quality of the kinetic fits, reported through correlation coefficients, indicates how confidently the activation energies can be assigned.

Looking forward, the retention descriptor could serve as a screening tool across catalyst families. If the fraction of accessible Brønsted sites surviving a standardized reaction cycle correlates reliably with long-term performance, then a single post-reaction pyridine measurement could predict catalyst lifetime without lengthy stability trials. This would be particularly valuable for sulfate-based systems, where deactivation mechanisms are diverse and difficult to disentangle. Extending the method to other probe molecules or to in situ measurements under reaction conditions would further strengthen the link between the measured retention and real operating behavior, and could reveal whether site loss occurs gradually during operation or in an initial burst as the catalyst equilibrates with the reaction medium.

The broader significance lies in reframing catalyst durability as a designable property rather than an inherent limitation. Solid acid catalysis has advanced enormously in site strength and selectivity, but the operational lifetime of the sites themselves has often been treated as fixed. Demonstrating that a single compositional modification can raise retention from roughly thirty percent to nearly half again as much, while simultaneously improving initial activity, suggests that stability and activity need not be traded against each other. For biodiesel and biorefinery applications, where feedstocks are impure and reaction conditions are demanding, catalysts that combine strong acidity with durable site retention could finally make heterogeneous processes the default rather than the exception.

Subject of Research: Role of Tungsten Modification in Regulating Brønsted Acid-Site Retention and Catalytic Stability of Sulfated Titania

Article Title: Role of Tungsten Modification in Regulating Brønsted Acid-Site Retention and Catalytic Stability of Sulfated Titania

Article References: Khan, K., Wang, W., Li, X., Chang, Z., & Dong, B. (2026). Role of Tungsten Modification in Regulating Brønsted Acid-Site Retention and Catalytic Stability of Sulfated Titania. Catalysis Letters, 156(11), 306. https://doi.org/10.1007/s10562-026-05550-x

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05550-x

Keywords: tungsten, sulfated titania, solid acid catalyst, Brønsted acid sites, transesterification, catalyst deactivation, pyridine FTIR, heterogeneous catalysis, Role, Modification, Regulating, nsted