Elsevier

Catalysis Today

Volume 246, 15 May 2015, Pages 81-91
Catalysis Today

Role of shaping in the preparation of heterogeneous catalysts: Tableting and slip-casting of oxidation catalysts

https://doi.org/10.1016/j.cattod.2014.08.030Get rights and content

Highlights

  • Shaping of bismuth molybdate and vanadium–aluminum mixed (hydr)oxide catalysts was investigated.

  • Graphite was demonstrated to be an effective shaping agent for tableting.

  • Graphite can have a promotional effect in ODHP.

  • A method to prepare catalyst pellets by slip-casting was developed.

  • Colloidal silica, used as a slip-casting binder, had a masking effect on catalyst active sites.

Abstract

The process and impact of shaping mixed vanadium aluminum (hydr)oxides, VAlOH and VAlO, respectively, and BiMo catalysts by tableting and slip-casting were examined. Graphite (G) was employed as a shaping agent for tableting. Without it tableting was impracticable. Graphite was found to enhance the mechanical resistance of VAlOH-xG and BiMo-xG and changed the surface area by increasing it for the non-porous BiMo and by decreasing it for the mesoporous VAlOH. In addition, graphite modified the catalytic performance despite changing neither the chemical nor the structural state of the base VAlO(H) and BiMo catalysts. A positive effect on the performance of VAlO-xG in propane oxidative dehydrogenation was found. It was proposed that catalytic active sites are formed on graphite during calcination. Conversely, graphite was harmful for non-calcined VAlOH-xG and BiMo-xG. On the other hand, the preparation of chemically and physically stable VAlO(H) suspensions for slip-casting was accomplished. Chemical stability was achieved at pHs near the isoelectric point of these catalysts. For physical stability, the use of a dispersing agent, poly(acrylic acid), combined with a control of the solids concentration was necessary. A simple and reliable method for preparing VAlOH and BiMo pellets by slip-casting was thus developed with the use of colloidal silica as binding agent. The so prepared pellets were mechanically resistant and kept the surface area of the base materials. A decrease in the surface concentration of the active metals due to surface active site masking by silicon for VAlOH-xSi and BiMo-xSi led to an inferior catalytic performance.

Introduction

Simply put, catalyst shaping, or forming as it is often also called, is the process by which a catalytic powder is transformed into a structured body such as a tablet or pellet, an extrudate or spaghetti, a bead or a monolith (Fig. 1). Most academic researchers skip the subject and consider it a matter of practical interest but not a research topic per se. Conversely, shaping is often conducted in industry by mere trial and error. Furthermore, as an author admitted [1], secrecy is the rule: “The patent literature gives some information, but catalyst manufacturing technologies are often not patented but kept secret.” Therefore, in most cases, there is a huge gap between the fundamentals of the process, the phenomena occurring during catalyst shaping, and the correlation of the latter with the performance of real life catalysts, the so-called technical catalysts. The literature for shaping is scarce despite several interesting contributions dating back to the 70s; all of them belonging to the first “International Symposium for Scientific Bases for the Preparation of Heterogeneous Catalysts” [2], [3], [4], [5]. The two most popular methods for catalyst shaping are tableting and extrusion. For tableting, the few information available consists on a general description of the process; including lists of the most preferred tableting additives; binders, lubricants, and fillers, and their general role on the mechanical resistance, the porous structure, and, ironically, marginally the catalytic performance [6], [7], [8]. Among the different tableting additives, graphite (G) is often employed although with great caution. According to Stiles [9]: “to emphasize once more – graphite is harmful; the more graphite, the more harm.”

For extrusion, which is basically a wet plastic molding process for powder shaping, a very recent review by Mitchell et al. [10] addressed how important is controlling the physicochemical characteristics of catalyst extrudates for producing efficient and durable technical catalysts such as the zeolites employed in the FCC process. It must be said that, it is indeed FCC catalysts that have drawn most of the attention in this sense given the enormous economic stake of this process. Most studies concern the effect of the nature of extrusion additives; peptizers and binders, on the catalytic activity from a phenomenological approach disregarding the fundamentals of the phenomena therein involved. Although, a high volume of information on the wet processing shaping of ceramic powders is available in journals not normally consulted by the catalytic community, see for example Refs. [11], [12] from the Journals of the American and European Ceramic Societies, respectively, and more recently, even in videos posted in youtube [13]. It is somehow amazing that the catalytic community has kept its look away from such knowledge.

This work was made within the frame of a project that aims establishing molecular basis for understanding the impact of shaping on the physicochemical and catalytic properties of materials as well the evolution of catalysts during the shaping process [14], [15], [16]. Herein, particular attention was paid to: first, the effect of the loading of the “objectionable” graphite on the properties of tableted bismuth molybdate (BiMo) and vanadium–aluminum mixed (hydr)oxides (VAlO(H)) catalysts. Second, the process of preparing pellets of BiMo and VAlO(H) by slip-casting and to the physicochemical and catalytic properties of the thus obtained materials. BiMo and VAlO(H) are catalysts employed in selective oxidation reactions [14], [17], [18], [19]. In this case, the oxidative dehydrogenation of propane to propylene (ODHP) and the oxidation of propylene to acrolein were studied.

Section snippets

Preparation of powdered catalysts

Both the hydroxides of bismuth molybdate [14] and of vanadium–aluminum [19] were prepared, at the 1 kg scale, by coprecipitation. For BiMo, the concentration of the metallic precursors was adjusted as to obtain a bulk Bi/Mo molar ratio of 1.3. A typical synthesis consisted in preparing a solution of 121.25 g of bismuth nitrate (Bi(NO3)3·5H2O, Riedel-de Haën) in 0.0185 dm3 of concentrated HNO3 and 0.375 dm3 of distilled and de-ionized H2O. This solution was mixed with another of 33.95 g of ammonium

Results and discussion

This section is divided into two parts. In the first part, the process of tableting will be described and discussed, followed by a discussion of the physicochemical and catalytic properties of the tableted materials. In the second part, the same aspects will be dealt with but for slip-casted materials.

Conclusions

The purpose of this contribution was to study both the process and impact of shaping VAlO(H) and BiMo catalysts by tableting and slip-casting. In the case of tableting, graphite was used as binding agent. Due to its lubricating effect, tableting without graphite was basically impracticable. Graphite was found to be an effective shaping agent with a positive effect on the mechanical resistance of the VAlOH-xG and BiMo-xG tablets. Such an effect did not depend on the graphite loading. Regarding

Acknowledgements

Authors acknowledge the financial support from IUAP – Inanomat, Belspo-Belgium, FNRS and the “DGTRE, Région Wallonne”, project “DEPOLAIR”. The authors especially thank Pascal Valvassori, Michel Genet, Marc Sinnaeve, and Mohammed N. Ghazzal without whose collaboration this work could not have been done.

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