Article pubs.acs.org/Langmuir
Demonstrating the Benefits and Pitfalls of Various Acidity Characterization Techniques by a Case Study on Bimodal Aluminosilicates Cynthia J. Van Oers,† Kinga Góra-Marek,‡ Bénédicte Prelot,§ Jerzy Datka,‡ Vera Meynen,† and Pegie Cool*,† †
Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerpen, CDE, Universiteitsplein 1, 2610 Wilrijk, Belgium ‡ Research Group for Catalysis and Solid State Chemistry I, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland § ICG, AIME-CNRS UMR 5253, Université Montpellier II, Place Eugène Bataillon 2, CC 1502 F-34095 Montpellier Cedex 5, France S Supporting Information *
ABSTRACT: A new combination of a volumetric with a dynamic method to investigate the acidity properties of aluminosilicates is introduced. In the first step, the total acidity is determined volumetrically by the measurement of two-cycle adsorption (TCA) isotherms with ammonia as a probe, directly followed by a dynamic temperature-programmed desorption (TPD) experiment to define the acid strength distribution. Furthermore, the results obtained by the new direct combination of TCA and TPD are validated by comparison with an in-situ FTIR (Fourier transform infrared) study with the same probe molecule on the same materials. Both acidity characterization techniques are compared, and we comment on their complementarity, benefits, and pitfalls. The material under investigation is a new type of bimodal microporous and mesoporous material with zeolitic characteristics, synthesized by a mesotemplate-free method. The acidic nature of the novel material is compared to two reference materials: a crystalline zeolite and a mesoporous aluminum incorporated mesocellular foam (Al-MCF) with amorphous characteristics.
1. INTRODUCTION The catalytic activity of aluminosilicates in acid-catalyzed reactions is highly dependent on its surface acidity characteristics, determined by the nature, number, and strength distribution of acid sites.1−3 Hence, a profound investigation of the acidic features is indispensable in order to obtain good insight into the catalytic behavior of solid-state catalysts. One of the most commonly applied acidity techniques is temperatureprogrammed desorption (TPD) using ammonia as a probe molecule. Although TPD and two-cycle adsorption (TCA) are both known in the art and widely applied to determine properties of acidity, basicity, physisorption, and chemisorption of gases in general, they have until now been applied separately because they normally exist as separate equipment with a different measuring principle (dynamic and static). If applied individually, they both have several limitations because TPD is dynamic and therefore has drawbacks in quantification. TPD cannot be applied quantitatively in a straightforward manner1 because the total number of acid sites is calculated on the basis of the relative contribution of peak areas of the bands to the total sum of the area in the TPD diagrams in combination with a calibration of the TCD. Nevertheless, TPD is mainly applied to deduce information on the strength of adsorption over the whole temperature range chosen (temperature ramping) and © 2014 American Chemical Society
the relative contribution of these strengths. Despite its advantage, this temperature ramping also has pitfalls, as will be shown in this work. TCA, however, is static and operated at one constant temperature per measurement, which makes it a strong method for quantitative work. However, it is more difficult and sample- and time-consuming to deduce information on the strength of adsorption (in comparison to TPD) because it requires separate measurements at different and discrete temperatures. However, by connecting both equipment in such a way that at the end of the TCA experiment the adsorbed NH3 can be directly removed via a TPD experiment on the same sample, one measurement that combines both measuring principle is possible. This can allow us to overcome some of the drawbacks of both techniques, thus opening the opportunity to apply the synergy in their complementary information. We introduce here a new method that couples both techniques and exemplifies the possibility to determine the total concentration of acid sites in a TPD experiment via the more quantitative TCA using gaseous ammonia as a probe molecule in a Received: September 6, 2013 Revised: January 10, 2014 Published: January 28, 2014 1880
dx.doi.org/10.1021/la4034194 | Langmuir 2014, 30, 1880−1887
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and a crystalline full-grown zeolite. Both reference materials have opposing characteristics, making them good comparison models for the mesotemplate-free materials, which can have zeolitic and/or amorphous features combined in one structure or feature even unique properties.
combined volumetric and dynamic measurement. The acidity of the porous structures will be first investigated volumetrically with a two-cycle adsorption (TCA) study, immediately followed by a dynamic temperature-programmed desorption (TPD) experiment. The two-cycle adsorption isotherms give rise to quantitative results for the total acid site concentration, and TPD provides information concerning the acid strength distribution. Thus, the link between the TPD and two-cycle (TCA) data makes the determination of the total acid site concentration based on the peak area unnecessary. Nevertheless, possibly occurring dehydroxylation reactions, which cause interference due to desorbing water in the TPD experiments, cannot be excluded, especially when temperatures above the calcination temperature are applied.4 Moreover, none of these techniques enables the differentiation between Brønsted and Lewis acid sites. Therefore, a classic in-situ FTIR study with ammonia as a probe has been performed. Although it is a much more time-consuming technique, it is valuable to obtain in-depth information on the nature of the acid site, which gives this technique added value compared to TPD or TCA.5 Additionally, in this work it also serves as a reference/validation for the results obtained by the combined TCA-TPD technique. In this case study, a new type of bimodal microporous and mesoporous material with zeolitic characteristics has been tested with respect to its acidity characteristics in comparison to known reference materials. In the past decade, a lot of effort has been devoted to the development of these bimodal microporous and mesoporous materials with zeolitic characteristics, or so-called hierarchical structures, with the intention to combine the best properties of both zeolites (stability, selectivity, and activity) and mesoporous materials (larger pore sizes, 2 < ø < 50 nm).6,7 Intensive research has led to the development of various methods to synthesize these bimodal microporous and mesoporous materials with zeolitic characteristics, often based on zeolite nanoparticles.8 The bimodal materials under investigation have been synthesized by a mesotemplate-free procedure using zeolite β nanoparticle solutions as a silica−alumina source. In a previous publication, it has already been proven that both the porosity and the zeolitic properties of these mesotemplate-free materials can be altered by adapting the synthesis conditions of the initial zeolite β nanoparticle solution.9 Moreover, in the literature, also other bimodal materials based on zeolite β nanoparticles have been described.10−16 Although in some cases their catalytic behavior has been tested,10,11,15,16 researchers have not yet been working on the correlation between the synthesis conditions of the primary zeolite nanoparticles and the acidic characteristics of the final bimodal materials. This knowledge would not only allow the fine tuning of the acidic properties of the materials depending on the required conditions but would also give better insight into their catalytic behavior. To conclude, the main aim of this study is threefold: (i) The use and validation of a new improved acidity characterization technique by combining two-cycle adsorption with temperature-programmed desorption. (ii) A detailed evaluation of the complementarity of the two applied acidity characterization techniques, including a designation of the pitfalls of using one or the other solely. (iii) The determination of the acidity of a novel type of bimodal microporous and mesoporous materials, synthesized by a mesotemplate-free method, in relation to an aluminum-incorporated mesoporous material with amorphous features (Al-MCF, aluminum-incorporated mesocellular foam)
2. EXPERIMENTAL SECTION 2.1. Chemicals. Tetraethylammonium hydroxide (TEAOH (35 wt %, aqueous solution)) was donated by SACHEM Inc. Fumed silica (aerosil) was obtained from Degussa-Hüls. Sodium aluminate (NaAlO2) was purchased from Riedel-de-Haën. Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), sodium chloride (NaCl), silver nitrate (AgNO3), tetraethylorthosilicate (TEOS, 98%), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, 99+%), aluminum isopropoxide (Al(i-PrO)3, 98+%), and mesitylene (C9H12) were purchased from Acros Organics. Pluronic (P123) was obtained from Aldrich, and ammonium chloride (NH4Cl), potassium chloride (KCl), and ammonium fluoride (NH4F) were purchased from Merck. 2.2. Synthesis. In the synthesis procedure of the mesotemplatefree materials, a zeolite β nanoparticle solution with a molar composition of 1:0.028:0.028:0.6:0.2:21 SiO 2 /Al 2 O 3 /Na 2 O/ TEAOH/HCl/H2O is used as the silica−alumina source. The zeolite nanoparticle solution is divided in two equal parts and transferred to Teflon-lined autoclaves. The autoclaves are placed in an oven for hydrothermal treatment for 24 h, one at 373 K and the other at 423 K, resulting in two stock solutions of nanoparticles. Subsequently, the nanoparticle stock solutions are acidified under vigorous stirring (pH
