An Overview on Zeolite Shaping Technology and Solutions to Overcome Diffusion Limitations
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catalysts Review An Overview on Zeolite Shaping Technology and Solutions to Overcome Diffusion Limitations Rogéria Bingre 1, Benoît Louis 1,* and Patrick Nguyen 2 1 ICPEES—Institute de Chimie et Procédés pour l’Energie, l’Environment et la Santé, Energy and Fuels for a Sustainable Environment Team, UMR 7515 CNRS—Université de Strasbourg—ECPM, 25 rue Becquerel, F-67087 Strasbourg CEDEX 2, France; [email protected] 2 Saint-Gobain C.R.E.E, 550 Avenue Alphonse Jauffret, BP 224, 84306 Cavaillon CEDEX, France; [email protected] * Correspondence: [email protected]; Tel.: +33-3-6885-2766 Received: 19 March 2018; Accepted: 16 April 2018; Published: 18 April 2018 Abstract: Synthetic zeolites are widely used as catalysts/carriers for many petrochemical reactions and in refining processes. These materials are usually synthesized in a powder form and must be shaped prior to use in industrial reactors. This review presents the state-of-the-art of the zeolite shaping technology describing the main modifications induced by the interactions between the zeolite and the binder. Additionally, a strategy is presented to overcome the diffusion limitations associated to the microporous structure of zeolites, consisting in the introduction of hierarchical porosity in the binder. Several developments in the field of hierarchical aluminas are summarized in this article, highlighting the possibility to design different ordered/disordered mesoporous and macroporous structures. Keywords: zeolites; shaping; hierarchical porosity; alumina; binder 1. Zeolite Properties The term zeolite was defined in 1756 by the Swedish mineralogist Axel F. CrØnstedt [1] after heating the mineral stilbite with a blowpipe flame, while observing the release of large amounts of steam previously adsorbed in the material [2]. The term “zeolite” comes from the Greek ζ ! (zeo), meaning “to boil” and λ θoσ (l thos), meaning “stone”. Normally, raw zeolite is naturally formed in the Earth’s crust under particular hydrothermal and geological conditions, but it is rarely pure, often being contaminated by other minerals, metals, quartz, or other zeolite structures. For this reason, their industrial application is excluded, being mostly used in geological museums, jewellery, or as construction materials. Fortunately, these materials can also be synthesized. Last century, the industry has been using synthetic zeolites as catalysts/carriers in several petrochemical reactions and in refining processes since the 1950s. Their framework is constructed by the association of TO4 tetrahedra, most commonly 4− 5− [SiO4] or [AlO4] linked by shared oxygen atoms; however, other elements, such as B, P, Ge, Ga, Ti, or Fe may also be incorporated. Zeolites are crystalline microporous materials, and the presence of water and cations (alkalis, alkaline-earths) allows the compensation of the negative charges generated by the presence of Al, giving birth to the general formula: Mx/n(AlO2)x(SiO2)y·zH2O with M being the compensating cation of the negatively charged framework (as H+ or Na+), y/x the Si/Al ratio (SAR), and z the number of molecules of water. Based on Löewenstein’s rule [3] Al-O-Al linkages are not allowed and, therefore, y/x ≥ 1. Catalysts 2018, 8, 163; doi:10.3390/catal8040163 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 163 2 of 18 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 FigureCatalysts1 shows 2018, 8, x ANAFOR PEER zeolite REVIEW crystals which can be found in nature, but also synthesized2 of 18 in Figure 1 shows ANA zeolite crystals which can be found in nature, but also synthesized in laboratories; it is worthy to mention that the crystal sizes differ, at least, from two orders of magnitude laboratories;Figure it 1 isshows worthy ANA to zeolitemention crystals that the which crystal can sizesbe found differ, in atnature, least, butfrom also two synthesized orders of in due to geological time scales on one hand, and a few days on the other. magnitudelaboratories; due toit geologicalis worthy timeto mention scales on that one thehand, cr ystaland asizes few daysdiffer, on atthe least, other. from two orders of magnitude due to geological time scales on one hand, and a few days on the other. A A B B Figure 1. Analcime zeolite, natural mineral (A); as-synthesized in our laboratory (B) [4]. Figure 1. Analcime zeolite, natural mineral (A); as-synthesized in our laboratory (B)[4]. Figure 1. Analcime zeolite, natural mineral (A); as-synthesized in our laboratory (B) [4]. These peculiarities render zeolites so important in industry, since it creates well-defined Thesechannels peculiaritiesThese and cagespeculiarities of render molecular render zeolites dimensions. zeolites so important so Typically, important in industry, zeolite in industry, pores since have it since creates diameters it well-definedcreates up towell-defined 2 nm channels [5] and cagesand,channels so-called of molecular and micropores, cages dimensions. of molecular and due Typically, dimensions. to their zeoliteregula Typically,r pores openings havezeolite of diameters determinedpores have up diameterssize to 2 they nm [ 5allowup] and, to 2small so-callednm [5] micropores,moleculesand, so-called and to duediffuse tomicropores, theirstraight regular through, and openingsdue but to traptheir of larger determinedregula ones,r openings acting size theyasof moleculardetermined allow small sieves, size molecules theyleading allow to to this small diffuse other common name for zeolites. Normally, those pores are filled with water and other cations, but straightmolecules through, to but diffuse trap straight larger ones, through, acting but astrap molecular larger ones, sieves, acting leading as molecular to this sieves, other leading common to this name theyother can common be easily name exchanged for zeolites. by other Normally, positively-charged those pores are ions. filled There with arewater currently and other 235 cations, distinct but for zeolites. Normally, those pores are filled with water and other cations, but they can be easily zeolitethey structurescan be easily known exchanged (228 or bydered other structures positively-charged and seven ions. structures There arewith currently partial 235disorder) distinct exchanged by other positively-charged ions. There are currently 235 distinct zeolite structures known possessingzeolite structures peculiar physical known and(228 chemical ordered properties, structures which and haveseven been structures allocated with unique partial three-letter disorder) (228 orderedcodespossessing by structuresthe International peculiar and physical seven Zeolite and structures Association, chemical with properties, such partial as MFI, which disorder) BEA, have and been possessing FAU allocated (Figure peculiar 2). unique Among physicalthree-letter them, and chemicalnearlycodes properties, twenty by the areInternational which implemented have Zeolite been at the Association, allocated industrial unique such scale, as three-letter butMFI, only BEA, five and codes constitute FAU by (Figure the the International 2).so-called Among “big them, Zeolite Association,five”:nearly MFI, such twenty BEA, as MOR, MFI,are implemented BEA,FAU, and FER. FAU at the (Figure industrial2). Among scale, but them, only nearly five constitute twenty arethe implementedso-called “big at the industrialfive”: MFI, scale, BEA, but MOR, only FAU, fiveconstitute and FER. the so-called “big five”: MFI, BEA, MOR, FAU, and FER. a a b c b c Figure 2. Three members of the “Big Five”: (a) MFI a 10-MR zeolite with a pore diameter of 5.5 Å; (b) BEA a 12-MR zeolite with a pore diameter of 6.0 Å; and (c) FAU a 12-MR zeolite with pore diameter of 7.4 Å [6]. Catalysts 2018, 8, x FOR PEER REVIEW 3 of 18 Figure 2. Three members of the “Big Five”: (a) MFI a 10-MR zeolite with a pore diameter of 5.5 Å ; (b) BEA a 12-MR zeolite with a pore diameter of 6.0 Å ; and (c) FAU a 12-MR zeolite with pore diameter of 7.4 Å [6]. 2. Applications of Zeolites Zeolites are applied in several processes, such as cracking, isomerization, hydrocarbon synthesis, synfuel production, refining, and production of chemicals. These processes involve the use of fixed-bed or fluidized-bed reactor technologies. Fluidized-bed reactors are required for processes likeCatalysts fluid2018 catalytic, 8, 163 cracking (FCC) and oxychlorination of ethane to 3chloroethane, of 18 for example, where it is necessary to lift the catalyst material in its fluid state. Although synthetic zeolites are obtained2. Applications in the of form Zeolites of a fine powder the crystallites are too small to be used directly in such kinds of reactorZeolites technology. are applied in In several this processes, way, the such catalyst as cracking, is isomerization,shaped by hydrocarbon spray-dry synthesis,ing methods to give synfuel production, refining, and production of chemicals. These processes involve the use of fixed-bed birth to abrasionor fluidized-bed-resistant reactorparticles technologies. of 60 Fluidized-bedto 100 µm reactors diameter. are required Additionally for processes, like in fluid cases where the process requirescatalytic a fixed cracking-bed (FCC) reactor and oxychlorinationtechnology, of the ethane problem to chloroethane, relies on for example,the fact where that itpowder is exhibits necessary to lift the catalyst material in its fluid state. Although synthetic zeolites are obtained in the poor mechanicalform strength of a fine powder that the can crystallites lead to are toodamage small to beor used degradation directly in such of kinds the of catalyst, reactor technology. thus creating fines at the reactor outlet.In this way, Indeed, the catalyst the is presence shaped by spray-drying of small methodsparticulates to give birth can to lead abrasion-resistant to dramatic particles consequences on both the reactionof 60 toand 100 µthem diameter. equipment. Additionally, The inrequired cases where mechanical the process requires strength a fixed-bed may reactor be conferred by technology, the problem relies on the fact that powder exhibits poor mechanical strength that can lead shaping the zeoliteto damage (Figure or degradation 3), for of the example catalyst, thus in creating a pellet fines ator the extrudated reactor outlet.