Aggregation and Peptization Behavior of Zeolite Crystals in Sols and Suspensions

Maurer, Torsten; Müller, Steffen; Kraushaar‐Czarnetzki, Bettina

doi:10.1021/ie0010013

Cited by 30 publications

(19 citation statements)

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“…In addition, a Donnan equilibrium can be established, if the pore system of the zeolite is accessible for anions. As a consequence, the IEP of zeolites with ion-exchange capacity is found at extremely low pH values [4]. The IEP values of the samples under investigation (Table) are in line with these previous results.…”

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confidence: 90%

“…Aiming at a more systematic approach, we recently investigated how the zeta potentials and the corresponding aggregation and peptization behaviors of zeolite crystals are affected by the presence or absence of pore-filling templates, by the amount of framework aluminium, and by the crystal size. This study involved samples of MFI, *BEA, and OFF (IUPAC structure codes of zeolites ZSM-5, beta, and offretite [3]) suspended in aqueous solutions of low ionic strength [4]. One important result concerns the fact that the aggregation of zeolite crystals occurs in a narrow pH range near the respective IEP, which, in many cases, refers to extremely acidic conditions.…”

mentioning

confidence: 99%

“…The liquid is back-mixed, and all particles are kept in suspension. A detailed description of the sample preparation and the pH adjustment in the context of the zeta potential measurements has been given in [4].…”

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Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Maurer¹,

Kraushaar‐Czarnetzki²

2001

HCA

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Dedicated to Professor AndreÂ M. Braun on the occasion of his 60th birthday Recovery of zeolites from aqueous media can be a very difficult operation, when the crystals have colloidal dimensions. The solid-liquid separation can be facilitated by processing at the respective isoelectric point (IEP) where the sol is unstable and the crystals form aggregates. The addition of salts results in a further improvement, because the zeolite agglomerates become larger, and the pH range of flocculation is broadened. This wider operational window, in particular, is of importance for the recovery of zeolites that would dealuminate or collapse at their respective IEP.Introduction. ± Handling of aqueous sols or suspensions of zeolite crystals is a frequently recurring operation in the manufacturing of zeolite-based catalysts or adsorbents. Starting with the zeolite synthesis in aqueous medium, the typical workup comprises repeated ion exchange in aqueous solutions and, finally, a wet shaping process during which bodies of suitable size, shape, strength, pore texture, and site distribution must be formed. Large agglomerates of crystals are beneficial, when zeolites must be recovered from their mother liquor or from ion-exchange solutions, whereas isolated zeolite single crystals, dispersed in the porous matrix of a shaped particle, are highly desirable when catalytic properties are to be optimized. These opposing requirements for efficient solid-liquid separations, on one hand, and optimum product quality, on the other hand, demand for means to control the particle size in a reversible manner.It is well-known that the stability of particles with respect to aggregation depends on the balance between attractive London-Van der Waals and repulsive electrostatic forces. The magnitude of the electrostatic repulsion as a function of the distance from the particle surface can be influenced. It depends on the ionic strength in the diffusive layer and on the surface potential (Nernst potential), which, in turn, can be altered by adjusting the pH value. While the Nernst potential is not experimentally accessible, the electrokinetic potential at the shear plane, the zeta (z) potential, can be monitored. Dispersions can be regarded as stable, when the zeta potential is higher than ca. j 30 mV j , whereas the particles tend to form aggregates near the isoelectric point (IEP), which is defined as the pH at which the zeta potential is zero. The measurement of the zeta potential, therefore, is a widely used tool to characterize the stability of disperse systems. Electrokinetic data of numerous materials, in particular, of inorganics are documented in the literature.

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Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Maurer¹,

Kraushaar‐Czarnetzki²

2001

HCA

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“…During this process it is assumed that the zeolite crystals themselves are homogeneous in nature, that is, only small variations in properties between the zeolite crystals in a batch exist. However, if this assumption is wrong and large variations in, for example, acid site density are present, parts of the shaped catalyst could become more active than others, resulting in undesired coke formation and thus a loss in catalyst activity and lifetime …”

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confidence: 99%

Assessing Inter and Intra‐particle Heterogeneity in Alumina‐poor H‐ZSM‐5 Zeolites

et al. 2017

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We reveal the presence of significant variations in Brønsted catalytic activity within and between individual H-ZSM-5 zeolite crystals. Fluorescence microscopy in combinationw ith a fluorogenic probe was used to resolve the catalytic activity at the nanoscale. The observed variations in catalytic activity could be directly linked to structural parameters and crystal morphology observed in scanninge lectron microscopy and by specifically stainingc rystal defects. The obtained resultsa re directly comparedw ith ensemblea veraged information from techniques such as pyridine IR spectroscopy and nitrogen physisorption, typically used to characterize acid zeolites. The inter-and intra-particle heterogeneities resolved by the employed fluorescence approach remainu naddressed by bulk characterization. Our experimental resultsr elate the heterogeneous catalytic activity to variation in both the Si/Al ratio and mesoporosity inducedd uring the zeolite synthesis.The preparation of zeolite-based industrial catalysts usually involves mixingt he zeolitesw ith silicaa nd or aluminab inders in order to dilute the zeolite properties and to obtain the desired throughput. [1][2][3] During this process it is assumed that the zeolite crystalsthemselves are homogeneous in nature, [2,4,5] that is, only small variations in properties between the zeolite crystals in ab atch exist. However,i fthis assumption is wrong and large variations in, for example, acid site density are present, parts of the shaped catalystc ould become more active than others,r esulting in undesired coke formation and thus al oss in catalystactivity and lifetime. [4,6,7] Twop arameters impacting catalyst activity are the number and strength of acid sites present in az eolite. These are directly relatedt ot he framework Al content, [8,9] hence Al determination is often used to assess zeolite acidity.S everalt echniques can be used to determine the Si/Al ratio, yielding powder averaged information, for example, elemental analysisu sing inductive coupled plasma (ICP), [10,11] magic angle spinning (MAS) 29 Si and 27 Al NMR, [8,12] laser induced break down spectroscopy (LIBS), [13] X-ray photon electron spectroscopy (XPS) [12,14] and time of flight-secondary ion mass spectrometry (TOF-SIMS). [15,16] All of theset echniques lead to the determination of the 'bulk" scale acidity and some are even able to obtain single crystal information given that crystal sizes and aluminum content are sufficiently large; [17] for example, single crystal X-ray diffraction [17,18] and energy dispersive X-ray spectroscopy (EDX) coupled to electron microscopy potentially with focused-ion-beam milling. [14,19] In summary,t he acidic properties of zeolitesa re often deduced indirectly from the aluminum content.F urther,n oi nformation regarding site accessibility is obtained in these results. Direct measurements of acid site density and strengtha re possible via temperature programmed desorption (TPD) [8,14,20] and infrared- [2,21,22] or Raman spectroscopy [23] using basic probe molecules such...

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“…We compare this to related oxide surfaces, such as mica [51] and amorphous silica [52], in aqueous solutions containing inorganic and tetraalkylammonium cations (TAA + ) because very few studies have been conducted on the colloid and surface chemistry of zeolites [53,54] or their precursor particles [55,56]. Force measurements between negatively charged planar mica surfaces with adsorbed TAA + show that surface forces cannot be described using only repulsive diffusedouble layer forces and attractive dispersion forces at surfacesurface distances that approach twice the diameter of TAA + .…”

Section: Zeolite Beta Colloidal Aggregation Growth Rate Limited By Stmentioning

confidence: 99%

Zeolite beta mechanisms of nucleation and growth

Hould

Foster

Lobo

2011

Microporous and Mesoporous Materials

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Aggregation and Peptization Behavior of Zeolite Crystals in Sols and Suspensions

Cited by 30 publications

References 5 publications

Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Assessing Inter and Intra‐particle Heterogeneity in Alumina‐poor H‐ZSM‐5 Zeolites

Zeolite beta mechanisms of nucleation and growth

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