catalysts Review Catalytic Membrane Reactors: The Industrial Applications Perspective Catia Algieri 1, *, Gerardo Coppola 2 , Debolina Mukherjee 2 , Mahaad Issa Shammas 3 , Vincenza Calabro 2 , Stefano Curcio 2 and Sudip Chakraborty 2, *   Citation: Algieri, C.; Coppola, G.; Mukherjee, D.; Shammas, M.I.; Calabro, V.; Curcio, S.; Chakraborty, S. Catalytic Membrane Reactors: The Industrial Applications Perspective. Catalysts 2021, 11, 691. https://doi.org/10.3390/ catal11060691 Academic Editor: Jose Antonio Calles Martín Received: 23 April 2021 Accepted: 27 May 2021 Published: 29 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institute on Membrane Technology, National Research Council of Italy (ITM–CNR), Cubo 17C, Via Pietro Bucci, 87036 Rende, Italy 2 Department of DIMES, University of Calabria, Via Pietro Bucci, Cubo 42A, 87036 Rende, Italy; coppola0gerardo@gmail.com (G.C.); DEBOLINA.MUKHERJEE@unical.it (D.M.); vincenza.calabro@unical.it (V.C.); stefano.curcio@unical.it (S.C.) 3 Department of Civil & Environmental Engineering at Dhofar University, Salalah 211, Sultanate of Oman; Mahad@du.edu.om * Correspondence: author: c.algieri@itm.cnr.it (C.A.); sudip.chakraborty@unical.it (S.C.) Abstract: Catalytic membrane reactors have been widely used in different production industries around the world. Applying a catalytic membrane reactor (CMR) reduces waste generation from a cleaner process perspective and reduces energy consumption in line with the process intensification strategy. A CMR combines a chemical or biochemical reaction with a membrane separation process in a single unit by improving the performance of the process in terms of conversion and selectivity. The core of the CMR is the membrane which can be polymeric or inorganic depending on the operating conditions of the catalytic process. Besides, the membrane can be inert or catalytically active. The number of studies devoted to applying CMR with higher membrane area per unit volume in multi- phase reactions remains very limited for both catalytic polymeric and inorganic membranes. The various bio-based catalytic membrane system is also used in a different commercial application. The opportunities and advantages offered by applying catalytic membrane reactors to multi-phase systems need to be further explored. In this review, the preparation and the application of inorganic membrane reactors in the different catalytic processes as water gas shift (WGS), Fisher Tropsch synthesis (FTS), selective CO oxidation (CO SeLox), and so on, have been discussed. Keywords: catalysis; membrane reactor; process intensification; inorganic catalyst; enzyme; environmental applications 1. Inorganic Membrane Reactor 1.1. Introduction to Catalytic Inorganic Membrane Reactors Since the conventional methodologies of chemical processes have been successfully applied to synthesize enormous numbers of sophisticated products, researchers worldwide have been searching for alternative methods, which are more efficient concerning energy consumption, space reduction, and environmental safety [1]. One of the most promising strategies to achieve these challenging goals is utilizing catalysts. Catalysts play an essential role in numerous environmental transformations with high reaction regioselectivity and stereospecificity. These peculiar characteristics allow scientists to apply them in modern chemistry and organic synthesis processes. Anyway, the rapid growth of the chemical industries for producing different chemicals requires improving manufacturing and pro- cessing by reducing the equipment size, energy consumption, and waste production to achieve sustainable and cheaper technologies (process intensification strategy) [1]. Membrane reactors, a process intensification technology, combine the membrane sepa- ration process with chemical or biochemical reactions in a single unit [2]. This combination determines higher conversion and improved selectivity and a compact and cost-effective Catalysts 2021, 11, 691. https://doi.org/10.3390/catal11060691 https://www.mdpi.com/journal/catalysts