Temperature-programmed desorption (TPD) of basic probe molecules is a widely used technique for characterizing the acid properties of zeolites […]
Introduction
Temperature-programmed desorption (TPD) of basic probe molecules is a widely used technique for characterizing the acid properties of zeolites. By adsorbing a base onto the zeolite surface, then linearly increasing the temperature under inert gas flow, the desorption of the base can be monitored.
Quantitative analysis of the desorbed species provides information about:
Extrinsic acidity (number of acid sites)
Intrinsic acidity (acid strength, based on desorption temperature)
This approach allows both types of acidity to be evaluated in a single experiment (Figure 1).
Principles of TPD for Acidity Measurement
The area under the desorption peak corresponds to the quantity of acid sites, while the peak temperature (Tₘₐₓ) reflects the strength of those sites.
Figure 1. TPD experiment: Tₘₐₓ reflects acid strength (intrinsic acidity); peak area reflects number of acid sites (extrinsic acidity).
Common Probe Molecules
Ammonia (NH₃) is the most commonly used probe due to:
Small kinetic diameter (0.26 nm), allowing access to virtually all acid sites
Strong adsorption on sites of varying strength
Thermal stability over a broad temperature range
Example: Ammonia TPD on H-Y Zeolite
Desorption patterns typically show:
<150°C: Physically adsorbed ammonia (physisorption). This signal can be minimized by conducting adsorption at elevated temperatures (∼100°C).
200–500°C: Chemisorbed ammonia on acid sites. Multiple peaks may appear, reflecting a distribution of acid strengths.
Literature Example
Zi et al. (1) observed that increasing the Si/Al ratio in H-Y zeolites resulted in a stronger high temperature desorption peak, indicating a higher number of acid sites.
Shakhtakhtinskaya et al. (2) correlated desorption signals between 600–900 K (327–627°C) to
Brønsted acid sites, which disappeared upon dehydroxylation.
Correlating Acidity with Catalytic Activity TPD data can provide insights into catalytic performance.
Example
For H-Y zeolites, the highest ammonia desorption temperature correlated with the cracking activity of n-pentane, as shown by turnover frequency (TOF) data (3).
Figure 3. Correlation between n-pentane cracking activity (TOF) and the highest ammonia desorption temperature.
Alternative Probe Molecules While ammonia is versatile, other probe molecules offer advantages in selectivity and sensitivity to acid site type.
Pyridine
Adsorbs on both Brønsted and Lewis acid sites.
Allows differentiation using infrared (IR) spectroscopy (4, 5).
Adsorption parameters (temperature and time) are critical to ensure complete coverage, especially for larger pore zeolites like mordenite (6).
Other Probes
A variety of bases can be employed, chosen based on acid strength and pore accessibility.
Table 1. Common Probe Bases for Acidity TPD
Note: Weak bases are generally used to probe only the strongest acid sites.
Practical Considerations
Adsorption Temperature: Elevated temperatures reduce physisorption artifacts.
Adsorption Time: Sufficient to ensure pore diffusion and full surface coverage.
Reaction Risk: For strong acid sites, probe molecules may undergo side reactions.
Selection should consider thermal and chemical stability.
Summary
Temperature-programmed desorption (TPD) of basic probe molecules is a powerful and flexible technique for characterizing the acidity of zeolites and related materials. By selecting the appropriate adsorbate and optimizing adsorption conditions, users can reliably quantify both the number and strength of acid sites—critical parameters that directly influence catalytic performance.
All AMI chemisorption analyzers are equipped to perform these TPD experiments with precision. Whether using ammonia for total acidity measurements or larger probe molecules like pyridine to selectively assess stronger or Brønsted versus Lewis acid sites, AMI systems offer the flexibility and control required for high-quality acidity analysis.
With robust temperature programming, sensitive detection options, and easy-to-use software, AMI’s chemisorption product line enables researchers and catalyst developers to accurately measure acidity and apply these insights to optimize catalyst design, performance, and longevity.
Temperature-programmed desorption (TPD) of basic probe molecules is a widely used technique for characterizing the acid properties of zeolites […]
