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Please do not adjust margins Chemical Society Reviews REVIEW Aerosol processing: a wind of innovation in the field of advanced heterogeneous catalysts a, a b c Received 00th January 20xx, Damien P. Debecker, * Solène Le Bras, Cédric Boissière, Alexandra Chaumonnot, Clément Accepted 00th January 20xx Sanchezb* DOI: 10.1039/x0xx00000x Aerosol processing is long known and implemented industrially to obtain various types of divided materials and nanomaterials. The atomisation of a liquid solution or suspension produces a mist of aerosol droplets which can then be www.rsc.org/ transformed via a diversity of processes including spray-drying, spray pyrolysis, flame spray pyrolysis, thermal decomposition, micronisation, gas atomisation, etc. The attractive technical features of these aerosol processes make them highly interesting for the continuous, large scale, and tailored production of heterogeneous catalysts. Indeed, during aerosol processing, each liquid droplet undergoes well-controlled physical and chemical transformations, allowing for example to dry and aggregate pre-existing solid particles or to synthesise new micro- or nanoparticles from mixtures of molecular or colloidal precursors. In the last two decades, more advanced reactive aerosol processes have emerged as innovative means to synthesise tailored-made nanomaterials with tunable surface properties, textures, compositions, etc. In particular, the “aerosol-assisted sol-gel” process (AASG) has demonstrated tremendous potential for the preparation of high-performance heterogeneous catalysts. The method is mainly based on the low-cost, scalable, and environmentally benign sol-gel chemistry process, often coupled with the evaporation-induced self-assembly (EISA) concept. It allows producing micronic or submicronic, inorganic or hybrid organic-inorganic particles bearing tuneable and calibrated porous structures at different scale. In addition, pre-formed nanoparticles can be easily incorporated or formed in a “one-pot” bottom-up approach within the porous inorganic or hybrid spheres produced by such spray drying method. Thus, multifunctional catalysts with tailored catalytic activities can be prepared in a relatively simple way. This account is an overview of aerosol processed heteroegneous catalysts which demonstrated interesting performance in various relevant chemical reactions like isomerisation, hydrogenation, olefin metathesis, pollutant total oxidation, selective oxidation, CO2 methanation, etc. A short survey of patents and industrial applications is also presented. Our objective is to demonstrate the tremendous possibilities offered by the coupling between bottom up synthesis routes and these aerosol processing technologies which will most probably represent a major route of innovation in the mushrooming field of catalyst preparation research. Porous materials presenting multimodal or multiscale 1. Introduction porosity are of major interest, for catalysis, fuel cells, batteries, and separation processes, for which optimisation of the Chemists have demonstrated that materials and diffusion and confinement regimes is required.9 In this context, nanomaterials with complex hierarchical structures and the development of bottom-up strategies such as sol-gel textures can be finely tailored through bottom-up and chemistry routes, has open a land of opportunities.10 Various sometimes « bioinspired » chemical strategies.1-5 The resulting templating strategies used to control the materials textural advanced functional materials are playing an important role in properties include cooperative self-assembly (with molecular or the development of energy and environmental sciences and polymeric surfactants), multiple templating with submicronic or nanomedicine. Numerous fields are targeted, such as sensing, micronic objects (e.g.: latex,11-13 biotemplates like bacteria, separation, sorption, electrochemical and viruses, etc.14-17), dynamic templating (breath figures,18, 19 photoelectrochemical devices for energy conversion and organogelation,20, 21 microphase separation,22, 23 nanocasting).24 storage, controlled release, nanobiomaterials, smart However, conventional procedures used to process therapeutic carriers, and of course heterogeneous catalysis.6-8 nanostructured porous nanomaterials mostly rely on multi-step and time-consuming batch operations. 1, 2, 25-27 a. Université catholique de Louvain. Institute of Condensed Matter and Nanoscience. Modern approaches coined “integrative chemistry”, Place Louis Pasteur, 1, box L4.01.09, 1348 Louvain-La-Neuve, Belgium where chemistry and process are strongly coupled, provide the [email protected] ability to design materials at several length scales, with tuned b. Sorbonne Université, Collège de France, PSL University, CNRS, Laboratoire de Chimie de La Matière Condensée de Paris LCMCP, F-75005 Paris, France functions and adjusted complex morphologies. These new [email protected] approaches integrate synergistically template directed sol-gel c. IFP Energies nouvelles, établissement de Lyon, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France chemistry with a large variety of processing methodologies This journal is © The Royal Society of Chemistry 20xx Chem. Soc. Rev., 2018, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins REVIEW Chemical Society Reviews producing efficient materials with multiscale textures. The main organic synthesis, petrochemistry, photocatalysis, processing methods reported to date concern inkjet printing,28- electrocatalysis, etc. Systematically, we cover the details of the 32 electrospinning,33-35 foaming,36-40 dip-pen lithography,41 TPA aerosol preparation procedures and we highlight the decisive lithography,42 multilayers deposition (via dip- or spin- properties of the aerosol-made catalysts which explain their coating),43-46 3-D printing,47-49 and finally, aerosol processing.50- performance. 56 Around the year 2000, the coupling between (i) sol-gel chemistry, (ii) evaporation-induced self-assembly, and (iii) 2. Aerosol processing aerosol processing was envisaged as a practical way to prepare 2.1. The rise of aerosol processes in materials science porous materials.50, 57, 58 In such aerosol-based synthesis, the Aerosol processes encompass all procedures which involve dynamic coupling between chemical and processing conditions the production of an aerosol, i.e. a gas-transported “mist” of is an important factor that controls materials on both the micro liquid particles. The nature and composition of the starting and nanostructures. In contrast to the classical solution material, the subsequent processing of the aerosol and the pathways, the “aerosol-assisted sol-gel process” (AASG) targeted final materials can differ and this gives rise to a involves a very limited number of preparation steps, produces relatively wide range of subclasses of aerosol processes (vide material in a continuous mode, allows for a simple collection of infra). the powder and generates low amounts of waste. Moreover, Spray drying has long been used to dry solid materials that the kinetic quenching associated with droplet formation and are present in a liquid suspension and obtain fine or aggregated fast drying allows for the “freezing” of materials into metastable dry particles or to encapsulate active molecules.70-73 Such states, which are hardly achievable by the usual precipitation techniques are widely used industrially, especially in the food method, because condensation/dissolution equilibrium usually industry, for example for the microencapsulation of flavours favours the formation of the thermodynamically stable and other food ingredients74-78 or for the production of milk or material.59 The very short thermal treatment at relatively low fruit juice powders,79-82 for example. Spray drying is also used temperatures associated with the aerosol process is compatible for the preservation of micro-organisms,83-89 in the with the use of fragile organic moieties.54 Moreover, a control manufacture of pharmaceuticals,90-93 or even in the large scale over particle localisation can be achieved through an adequate production of burning rate modifiers for propellants.94 Spray functionalisation of nanoparticles and accurate control of the drying is a traditional unit operation in chemical engineering spray drying processing parameters.60 with an important know-how about its design, automated Understandably, heterogeneous catalysis is a field which control and scaling up.60 Often, it can be operated at low-cost takes advantage of such preparation technique that offers so and in environmentally benign conditions. many degrees of freedom to tune the properties of the Another particular example of aerosol process is the “gas materials at the nanoscale.61 Continuously, important effort is atomisation” (and derived processes) of metals and metal indeed being made to develop a variety of innovative alloys.95-99 In this case, a melt stream is sprayed through a nozzle nanostructured materials which regularly open up new and disrupted by an inert gas to produce fine liquid droplets, perspectives in catalysis.61-69 In fact, the preparation of which are rapidly cooled and solidified to give small spherical advanced catalytic materials by aerosol processes (spray drying, particles of metals or alloys. Rapid solidification allows flame-aerosol processes, etc.) was already long recognised as a obtaining fine microstructures, avoiding segregation effects and prime method for commercial