Determination of Microporous and Mesoporous Surface Areas and Volumes of Mesoporous Zeolites by Corrected t‐Plot Analysis

Abstract

Zeolites are well-known microporous catalysts involved in many industrial applications. Their microporosity yet impedes diffusion of bulky molecules. To overcome this drawback, zeolites with hierarchical bimodal porosity, e. g. micropores and mesopores, are developed. In this context, it is important to develop/use methods able to evaluate accurately the textural properties of mesoporous zeolites. t-Plot can be in theory used for determining the micro-and mesopore surface areas and pore volumes. However, classical t-plot analysis overestimates the mesopore surface areas and underestimates the micropore volumes for hierarchical micro-/mesoporous materials. Measurements performed with mechanical mixtures of FAU-Y and MCM-41 allows to provide corrections, which are up to date limited to mesoporous FAU-Y. The present work describes specific corrections of the t-plot for a broader series of zeolites including MFI, *BEA and MOR, based on a set of mechanical mixtures of zeolites and MCM-41. Minor correction factors are required for MOR, whilst for MFI and *BEA similar corrections as for the FAU-Y are inferred for the micropore volume. As far as the surface area is concerned higher correction factors are required for MFI compared to *BEA. This corrected t-plot analysis is applied to characterize a great variety of mesoporous zeolites prepared by micelle-templating.

Hierarchical MFI Zeolite Catalysts: How to Determine Their Textural Properties?-A Comparative Study

“…The application domain of the BET equation (p/p0)/[V(1-p/p0) = f(p/p0) was determined based on the superior limit reflected by the maximum of the Rouquerol curve V(1-p/p0) = f(p/p0). This allows the determination of the linear BET range in an unambiguous way [42,43,[47][48][49][50]. The BET area (SBET) calculated based on the cross-sectional area of the nitrogen molecule (16.2 Å 2 ) for MFI is in agreement with the nitrogen accessible surface area obtained by simulating the crystal framework [49].…”

“…Mesopore diameters were calculated following the Broekhoff and De Boer (BdB) desorption method [51] at the inflection point of the desorption step, as it was demonstrated to be one of the most appropriate methods for the analysis of mesoporous materials such as MCM-41 [52]. The results obtained using the BdB method are in quite good agreement with the results obtained using the NLDFT method [43,53].…”

“…Results obtained by conducting computational studies revealed that the t-plot method underestimates the micropore volumes and overestimates the mesopore surface areas of hierarchical materials [38][39][40]. This was confirmed experimentally using mechanical mixtures of zeolites and MCM-41 [41][42][43]. The t-plot method was corrected to accurately calculate the micro-and mesopore volumes and the mesopore plus external surface areas of hierarchical zeolites [41][42][43].…”

“…This was confirmed experimentally using mechanical mixtures of zeolites and MCM-41 [41][42][43]. The t-plot method was corrected to accurately calculate the micro-and mesopore volumes and the mesopore plus external surface areas of hierarchical zeolites [41][42][43]. The micropore surface area was obtained by subtracting the mesopore plus external surface area from the total surface area given by the BET area (SBET).…”

“…The micropore surface area was obtained by subtracting the mesopore plus external surface area from the total surface area given by the BET area (SBET). As the efficiency of using SBET for zeolites is under debate, we demonstrated [43] that the calculated SBET value (calculated based on the Rouquerol criteria and the cross-sectional area of nitrogen (0.162 nm 2 )) can be used for several zeolites. The BET area matches the theoretically obtained N2 accessible surface area of FAU-Y (SBET = 975 m 2 /g, Vmic = 0.355 mL/g).…”