Temperature-programmed desorption (TPD) of basic probe molecules is a widely used technique for characterizing the acid properties of zeolites […]
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
Temperature-programmed desorption (TPD) of basic probe molecules is a widely used technique for characterizing the acid properties of zeolites […]
