Selection of binder recipes for the formulation of MOFs into resistant pellets for molecular separations by fixed-bed adsorption

https://doi.org/10.1016/j.micromeso.2019.02.009Get rights and content

Highlights

  • 55 binder recipes for the formulation of the ZIF-8 MOF into pellets were evaluated.

  • The mechanical, acid-base, hydrothermal and long term stability were verified.

  • Separation performance of ZIF-8 is maintained upon formulation.

  • PVF binder results in the most resistant ZIF-8 composite particles.

Abstract

The formulation of metal-organic frameworks (MOFs) materials into resistant structures is a key step towards their use in adsorptive separation processes. In this study, ZIF-8 crystals have been formulated into resistant pellets with various binders by a simple extrusion-crushing-sieving (ECS) approach. 55 recipes were evaluated with a series of stability tests – mechanical stability upon friction, acid/base stability, hydrothermal stability and long-term stability for storage over several years – as well as with a set of adsorption and separation experiments. The stability tests have highlighted that PVF (polyvinylformal) is the most promising binder to generate resistant pellets with ZIF-8 crystals for the adsorptive recovery of n-butanol in the acetone-butanol-ethanol (ABE) fermentation process, while other binders, including methylcellulose, may find their use in gas/vapor adsorption processes. Adsorption of ethanol vapor has shown that the formulation results into a decrease of adsorption capacity, which is proportional to the binder fraction, while having no effect on the adsorption kinetics. On the other hand, packed bed experiments in liquid phase have indicated that ZIF-8 composite materials maintain the separation ability of the unformulated ZIF-8, but also that the binder results into an unfavorable additional mass transfer resistance. These results have been supported by complementary porosimetry measurements with Ar and Hg, indicating that the composite materials are very porous with narrow pore-size distributions. From this experimental work, various formulation recipes have been identified, which may lead to resistant MOF composite materials for adsorptive separations.

Introduction

Within the family of Metal-organic frameworks (MOFs), the Zeolitic Imidazolate Frameworks (ZIFs) have gained a lot of attention due to their stability together with a large chemical and structural tunability, offering new opportunities for adsorption applications [1,2]. Recently, we identified a promising ZIF for the recovery of renewable organic compounds from aqueous mixtures [3]. In particular, the ZIF-8 MOF material showed remarkable properties for the separation of bio-butanol from ABE (acetone-butanol-ethanol) fermentation broths, outperforming traditional materials [3,4]. Towards its application in an adsorptive separation process, ZIF-8 must be shaped into a resistant structure, which, moreover, (if possible) enhance mass and heat transfer, while maintaining the interesting properties of the unformulated material [5].

In practice, many adsorptive separation processes are operated using fixed beds packed with adsorbent particles through which mixtures are pumped and separated [5]. Unformulated porous materials, commonly being in the form of small crystals or powder, are generally not used in fixed beds due to the large pressure drop such fine particles generate [5]. For that reason, porous materials are shaped into larger particles, usually made of crystals or powder strongly agglomerated together by a binder, such as a polymer or clay [[5], [6], [7], [8]]. Although MOF materials containing binders were made in a limited number of studies [[9], [10], [11], [12], [13], [14], [15], [16]], no clear criteria are available to select a binder for a given application and which, moreover, results into resistant pellets. Furthermore, only very limited work on the formulation of MOFs is present in the literature and formulation recipes remain often unpublished and secret within companies.

Therefore, in this work, an extensive screening and evaluation of various binders and recipes to make ZIF-8 pellets is presented, specifically towards their use in the recovery of biobutanol from aqueous mixtures by fixed-bed adsorption. Starting from the selection of the binders, based on criteria related to the abovementioned application of the ZIF-8 MOF, pellets are made by a simple extrusion-crushing-sieving (ECS) approach. In order to identify suitable binder formulation procedures, pellets are evaluated with stability tests, including mechanical strength upon friction, storage over several years, or dissolution in acid/base environment, as well with various adsorption and separation experiments.

Section snippets

Material

ZIF-8 powder was bought from Sigma-Aldrich. Various binders and solvents (see Supporting information, Table S1 for more details) were used to produce ZIF-8 composite particles (see below for more details). An overview of the different solvents and binders is provided in Fig. 1, with the binder fraction, given in weight percent (wt%), relative to the total solid mass (i.e. binder and ZIF-8 powder). Adsorption isotherm measurements and fixed-bed separation experiments were carried out with

Formulation

The formulation of porous materials relies a lot on trial-error. In order to reduce the number of chemicals to be tested, binders were selected based on criteria related to the application of ZIF-8 for the separation of n-butanol from aqueous mixtures [3,4]. Consequently, binders were chosen depending on their hydrophobicity, thermal stability for regeneration of the adsorbent (up to 353 K) [3,4] and ease of dissolution in the organic solvent used in the formulation recipes (see Fig. 1, vide

Conclusions

The shaping of metal-organic frameworks (MOFs) materials into resistant structures, while maintaining their adsorption and separation performance, is an important aspect towards their implementation into adsorptive processes. In this work, a simple extrusion-crushing-sieving (ECS) approach was used to formulate ZIF-8 crystals into resistant pellets. 55 recipes, with different binders, binder fractions, and solvents, were evaluated with a series of stability tests and adsorption and separation

Conflicts of interest

The authors declare they have no conflicts of interest, nor competing financial interests.

Acknowledgements

Julien Cousin-Saint-Remi and Joeri Denayer are grateful for the financial support of FWO Vlaanderen (12P2217N and 1512118N).

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