New method for the study of surface acidity of zeolites by NH3-TPD, using a pH-meter equipped with an ion selective electrode

Abstract

Desorbed ammonia is captured by an acetic acid solution whose concentration of ammonium ions formed by neutralization, is continuously recorded by a pH-meter equipped with an ion-selective electrode (ISE). This technique allows the study of both the acidity density and the distribution of acid site strengths. Such ''strength profile'' was obtained by plotting the relative rate of desorbed NH 3 versus the temperature. Results obtained can be fairly well correlated with those already published. It is shown that by investigating several H-ZSM-5 zeolites with different SiO 2 =Al 2 O 3 ratios, the data of acid density obtained by the ISE method are very close to those calculated from the Al content. The slight shift of the desorption peaks towards higher temperature suggests a slightly higher strength for more isolated acid sites. Zeolite structures, which are richer in Al, contain weaker acid sites, as evidenced by the H-Y when compared to the H-mordenite, while the acid sites density of the former is much higher. The ultra-stabilized Y zeolite (H-USY) shows a lower acidity density than the parent H-Y owing to the removal of some of the tetrahedral Al atoms from the framework during the preparation. The strength profile of the H-USY is also quite different from that of the H-Y. The new technique appears to be much simpler, more rapid, more informative and quite accurate when compared to the technique using a gas chromatograph equipped with a thermal conductivity detector (GC-TCD).

Integrated Cascade Process for the Catalytic Conversion of 5‐Hydroxymethylfurfural to Furanic and TetrahydrofuranicDiethers as Potential Biofuels

“…In particular, the two characteristic peaks of ammonia desorption are observed, the first one centered at 262 °C and the second one between 470 and 500 °C, corresponding to weak and strong acid sites, respectively. A higher amount of weak acid sites was recorded for the zeolite HZSM‐5 with Si/Al=15 compared to that with Si/Al=25, representing 66 and 54 % of the total acidity, respectively, in agreement with the literature results [31] …”

“…A higher amount of weak acid sites was recorded for the zeolite HZSM-5 with Si/Al = 15 compared to that with Si/Al = 25, representing 66 and 54 % of the total acidity, respectively, in agreement with the literature results. [31] Regarding the DRIFTS of pyridine performed on the zeolites HZSM-5 with Si/Al = 15 and 25, the spectra recorded in function of the increasing temperature of the sample are shown in Figure S11. All investigated samples showed bands at around 1640, 1491, and 1541 cm À 1 , characteristic of pyridinium ions (Brønsted acid sites), and bands at around 1600, 1575, and 1444 cm À 1 , characteristic of coordinatively bound pyridine over Lewis acid sites.…”

General aspects of zeolite acidity characterization

“…The desorption profile can also be followed by thermogravimetry (TG), simultaneous differential scanning calorimetryethermogravimetry (DSC-TG), and transmission infrared spectroscopy at variable temperature. A simpler detection method consists in bubbling the desorbed gases through an excess of acidic solution and determining the concentration of NH 4 þ with a selective sensor [135].…”

Mixed Naphtha/Methanol Feed Used in the Thermal Catalytic/Steam Cracking (TCSC) Process for the Production of Propylene and Ethylene

“…(2) BET total surface area and pore size distribution by using nitrogen adsorption/desorption (apparatus: Micromeretics ASAP 2000). (3) Surface acidity by the technique of ammonia adsorption and temperature-programmed desorption (TPD) using a system based on a pH meter equipped with an ionselective electrode [20,22]. (4) Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) that were carried out to determine the amounts of bound species and/or coke deposited onto the catalyst surface (apparatus: PL Thermal Science model STA-1500 DTA/TGA; flow rates of argon (inert gas) and air (oxidative gas) = 30 mL/ min, rate of temperature-programmed heating = 15°C/ min from ambient temperature to 700°C).…”