In addition to the adsorption mechanism, the heat released during exothermic adsorption influences the chemical reactions that follow during heterogeneous catalysis. Both steps depend on the structure and surface chemistry of the catalyst. An example of a typical catalyst is the faujasite zeolite. For faujasite zeolites, the influence of the Si/Al ratio and the number of Na⁺ and Ca²⁺ cations on the heat of adsorption was therefore investigated in a systematic study. A comparison between a NaX (Sodium type X faujasite) and a NaY (Sodium type Y faujasite) zeolite reveals that a higher Si/Al ratio and therefore a smaller number of the cations in faujasite zeolites leads to lower loadings and heats. The exchange of Na⁺ cations for Ca²⁺ cations also has an influence on the adsorption process. Loadings and heats first decrease slightly at a low degree of exchange and increase significantly with higher calcium contents. If stronger interactions are required for heterogeneous catalysis, then the CaNaX zeolites must have a degree of exchange above 53%. The energetic contributions show that the highest-quality adsorption sites III and III’ make a contribution to the load-dependent heat of adsorption, which is about 1.4 times (site III) and about 1.8 times (site III’) larger than that of adsorption site II.

The aim of this study is to unravel the influence of single, double and triple bonds in hydrocarbons on the mechanisms of adsorption on zeolites. Therefore, the adsorption of the C2 hydrocarbons ethane, ethene and ethyne on different adsorbents is studied by adsorption calorimetry. As adsorbents pure sodium LTA (NaA) and FAU (NaX) zeolites and calcium exchanged CaNaA and CaNaX zeolites are used. Based on experimental loadings and heat of adsorption, the influence of the number and distribution of cations on different cation positions are discussed in detail. With increasing degree of exchange the increasing number of Ca²⁺-cations introduce energetically more valuable adsorption sites into the zeolites. On the other hand, the decreasing total number of cations has a negative effect on saturation loading. The impact of these opposing effects and the different occupation of cation positions on the interactions and mechanisms occurring are discussed. The loading increases from ethane to ethene to ethyne and shows higher values on FAU compared to LTA. In terms of interactions, due to the single bond, in ethane only dispersion and induction interactions are formed. In ethene and ethyne due to the double and triple bonds, respectively, additional quadrupole cation and π-interactions occur. In this study, for the first time the formation of a π-complex with Ca²⁺-cations at position I in LTA and at positions II, III, and III’ in FAU is demonstrated. For ethyne, additional π-complex formation with Na⁺-cations on the identical positions is also detected, which was previously unknown in literature.

Increasing environmental concerns, stricter legal requirements, and a wide range of industrial applications have led to growing demand for activated carbon worldwide. The energy-intensive production of fresh activated carbon causes significant CO₂ emissions and contributes to global competition for renewable carbon-based raw materials. Although (thermal) reactivation of spent activated carbon can drastically reduce the demand for fresh material, the reactivation process itself is still mostly based on experience and empirical knowledge locked into activated carbon companies. Despite the vast number of papers published in the field, practically relevant, systematic, and quantitative knowledge on the thermal reactivation process is barely available. This paper presents a simple and robust methodology for the development of a predictive model for the production of reactivated carbon with a defined product quality under energetically optimized conditions. An exhausted activated carbon sample was subjected to 26 reactivation experiments in a specially designed laboratory rotary kiln, whereas the experiments were planned and evaluated with statistical design of experiments. The influence of the reactivation conditions (heating rate, heating time, H₂O/N₂ volume ratio, and CO₂/N₂ volume ratio) on the specific surface area, energy consumption, yield, and adsorption capacity for diatrizoic acid were evaluated. The BET surface of the reactivated carbons ranged between 590 m²/g and 769 m²/g, whereas the respective fresh carbon had a BET surface of 843 m²/g. The adsorption capacity for diatrizoic acid measured as the maximum solid phase concentration q_m derived from the Langmuir equation varied between 24.4 g/kg and 69.7 g/kg (fresh carbon: 59.6 g/kg). It was possible to describe the dependency of the quality criteria on different reactivation parameters using mathematical expressions, whereas the response surface methodology with nonlinear regression was applied to build the models. A reactivation experiment under statistically optimized conditions resulted in energy savings up to 65%, whereas the properties of the reactivated sample were close to the predicted values.