Zeolites characterization

The objective of this work is to synthesize and characterize zeolite-bentonite hybrid catalysts and perform test reactions in the pyrolysis of woody biomass in a dual-fluidized bed reactor. The aim is to produce catalytic materials which have good mechanical strength and are still able to de-oxygenate the pyrolysis oil. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

H. (2001) Characterizing zeolite acidity by spectroscopic and catalytic means a comparison. J. Phys. Chem. B, 105 (18), 3917. 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

When metal centers act in conjunction with acid sites on the zeolite, bifunctional catalysis can occur (e.g., Pd/HY). This type of catalysis is used mainly for the hydrocracking and isomerization of long-chain n-alkanes. For example, the rates of formation of 2- and 5-methylnonane isomers obtained from n-decane isomerization over bifunctional zeolite catalysts depend on the size and structure of the zeolites used. This reaction has been developed as a test reaction to characterize zeolite structures (17-19). 

Techniques for the characterization of acid sites in zeolites have progressed much in the past decade. Advances have also been made in the understanding of factors contributing to acidity in such catalysts. No technique can claim superiority in its ability to characterize zeolitic acid sites and, indeed, the technique of choice is most likely to be dictated by the particular catalyst of problem at hand. 

In particular, the Si/Al ratio of the FAU-polytype is higher than that characterizing zeolite Y but lower than that measured for the more siliceous ZSM-20. We conclude that under our temperature and compositional conditions, Aerosil provokes the formation of such a structure that accomodates the appropriate amount of A1 to get stabilized. At 170°C, the same gel yields the thermodynamically more stable zeolite Beta. This observation again goes in line with the general idea that the development of nuclei of metastable zeolites is favoured by a low temperature aging /nucleation of gels (51). 

N. Rajic, N.Z. Logar, and V. Kaucic, A Novel Open Framework Zincophosphate Synthesis and Characterization. Zeolites, 1995, 15, 672-678. 

In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-speclroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. 

High resolution MAS multinuclear NMR is a very valuable tool to characterize zeolitic catalysts in the as-synthesized form, to detect changes that occur during their pretreatment and subsequent modifications and to follow the conversion of adsorbed reactants. 

The hypotheses and interpretations proposed here need further testing with other zeolite frameworks and additional spectral analysis. Infrared spectroscopy must be used as a supplemental tool to characterize zeolites, along with the more common methods of determining structure and properties e.g., x-ray crystallography, chemical analysis, adsorption, and other characterizations. 

We have successfully utilized our integrated system to synthesize and characterize zeolites, AlPOs, microporous oxides and other relevant inorganic oxides. We find that although we are working with microliters of reagents, that the results in this multiautoclave are scalable to the laboratory scale (125 ml). In terms of applicability, we have utilized this system for the discovery, optimization, and the scale-up of new materials. We will present four examples throughout this paper to demonstrate this capability. 

This chapter has presented a brief overview of various studies in which calorimetry has been used to characterize zeolites, oxides, and metallic catalysts. It is apparent from the results in the literature that microcalorimetry is a very powerful technique even if it does not provide direct information about the molecular nature of the adsorbed species. 

X-ray powder diffraction (XRPD), thermo gravimetric (TGA) analysis, solid-state nuclear magnetic resonance (NMR), and measurements of adsorption isotherms are key methods for characterizing zeolite-like behavior. However, a simple proof for observing structural changes during the sorption processes is XRPD. 

W.E. Pameth, R.J. Gorte, Methods for characterizing zeolite acidity, Chem. Rev. 95... 

Llewellyn, P.L. and Maurin, G. 2005. Gas adsorption microcalorimetry and modelling to characterize zeolites and related materials. C. R. Chim. 8 283-302. 

This work reviews the use of pyrrole, chloroform, methanol, as well as methoxy and nitrosonium groups generated in situ , as infrared and NMR probe molecules to characterize zeolites basicity. ... 

Tricoli and Nanetti [17] prepared a novel zeolite-Naflon composite manbrane by embedding zeolite fillers in Nafion. The zeolites used in this study were chabazite and clinoptilolite. The presence of zeolites in the membranes caused notable changes in conductivity, methanol permeability, and selectivity with respect to pure Nafion. In another interesting study, Holmberg et al. synthesized and characterized zeolite-Y nanocrystals for Nafion-zeolite-Y composite proton exchange membranes. The composite membranes were found to be more hydrophilic and proton conductive than the base-unmodified membranes at high temperatures [18]. 

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