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Strategies to Immobilized Catalysts: A Key Tool for Modern Chemistry

Oriana Piermatti1, Raed Abu-Reziq2, and Luigi Vaccaro1

Università di Perugia, Laboratory of Green Synthetic Organic Chemistry, Dipartimento di Chimica, Biologia e Biotecnologie, Via Elce di Sotto, 8, 06123 Perugia, Italy

The Hebrew University of Jerusalem, Casali Institute for Applied Chemistry and Center for Nanoscience and Nanotechnology, The Institute of Chemistry, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel

1.1 Introduction

In all the different cultural and scientific areas, modern era is characterized by the high attention dedicated to the concept of sustainable development and sustainability. In what is nowadays indicated as "circular economy," chemistry plays a pivotal role to steer modern production toward safety, environmental efficiency, reduction of waste, and minimization of CO2 emissions. Both academic and industrial researches are focused in this direction and are, often in collaboration, effectively working at the definition and implementation of innovative solutions [1,2].

In chemistry, sustainability has become synonymous with green chemistry, a term that appeared in the 1980s in the United States and associated to a multidisciplinary area of research aimed at developing innovative approaches to fundamental and applied research that could eventually lead to industrial competitiveness and minimal environmental impact.

The definitions of green chemistry are several, and often they vary according to the most critical chemistry-related issues for specific region of the world. Anyway, Paul T. Anastas recognized the merit of the definition of the 12 Principles of Green Chemistry (Figure 1.1), which simply and in exhaustive manner indicate the most important topics toward which modern research and society need to focus to attain a sustainable development [3]. These principles represent not only a sort of guidelines to the perfect chemical process but also a very useful vademecum to identify the key issues and the key research areas that need to be developed in order to actually achieve sustainability.

An ideal green modern chemical process does not feature one of the different principles. It is instead the combination of all of the principles and the result of a careful process design where strategic political solutions are combined with the development of key strategies and technologies. Therefore, a modern process needs to be based on safer solvents and chemicals, possibly coming from the valorization of waste and renewable resources. Energy-efficient technologies must be developed and used to maximize safety and quality of a chemical process while minimizing the waste and the cost associated to its implementation.

Figure 1.1 Principles of green chemistry.

A central role is played by catalysis [4]. By aiming at the use of safer chemicals and at the same time at the reduction of steps in a chemical process, it is necessary to develop innovative catalytic technologies not only to resolve the use of dangerous highly reactive chemicals but also to minimize the energy consumption and the production of the waste associated [5,6].

The use of effective catalytic systems mainly based on metals has been always crucial in the chemical industry, and homogeneous catalysis has been generally preferred over the use of heterogeneous/solid catalytic systems, especially in the production of fine chemicals and complex active pharmaceutical ingredients (APIs) [7,8].

The design of a modern chemical process should carefully evaluate the actual need for using toxic and exhaustive metal catalysts, and inevitably, it should consider all the available possibilities for their recovery and reuse to consequently minimizing pollution.

Different solutions for the recovery and reuse of a catalytic system are available and all of them need to be implemented in the future. These comprise the phase-transfer/separation techniques, largely already used in industry, and above all, the use heterogeneous/immobilized catalytic systems [9,10].

Heterogeneous catalysts should be effectively recovered and reused at the end of a process simplifying the work-up procedures for the isolation of the desired final target material.

It is also kind of reasonable that current industrial production looks at heterogeneous catalysis skeptically, as homogeneous catalysis is often more effective and the cost of production more easily predictable. In fact, definition of a heterogeneous system featuring a perfectly repetitive catalytic efficiency is truly challenging. Accordingly, the reproducibility of the results for the sufficient number of cycles that justify the use of a heterogeneous catalyst remains a critical point in real industrial cases. Nevertheless, although the difficulty of the challenge, the definition of heterogeneous recoverable catalytic systems is of major importance, and a successful research in this direction is the only manner to pave the route for the ideal chemical processes endowed with the highest innovation and efficiency features.

To completely access the ideal overally efficient green chemical process, heterogeneous catalysts should be developed considering the need for use of novel safer chemicals and solvents deriving from renewable resources and the use of innovative stirring and heating technologies such as flow reactors, microwave, or ultrasounds, which could optimize its reuse and its reproducible catalytic efficiency at the most convenient energy cost [11].

1.2 Catalysis

The word catalysis was first coined by Berzelius in 1836 [12]. It has now grown into a multidisciplinary research field playing a central role in many scientific and industrial activities including chemical, biological, nanotechnology, polymer, energy, pharmaceutical, and agriculture fields. The catalyst alters the reaction course via accelerating the reaction process by decreasing the activation energy without affecting the thermodynamics of the overall reaction. More often, high yields of the desired product are obtained in shorter period of time while consuming less energy compared with the corresponding stoichiometric reactions [13].

Plethora of novel catalysts have been developed over the years and actively employed both in the industrial and academic research communities. Broadly, catalysts are classified into two categories, homogeneous and heterogeneous. In homogeneous catalysis, both the reactants and catalysts are present in the same phase, and active catalytic sites are easily accessible to reactants and generally result in higher activity and reaction selectivity of catalysts. It is possible to fine-tune the regio-, chemo-, and enantioselectivity of reactions through appropriate selection of metals, ligands, and organocatalysts [14].

Despite the impressive achievements in the field of homogeneous catalysis, the recovery and recyclability of the catalysts is a major issue. Substantial production costs and time-consuming purification techniques are employed for the isolation of the catalytic species from the reaction mixture. Recovery and reuse of the catalysts are a vital issue for ecological and economical demands [1]. In heterogeneous catalysis, the catalysts are heterogeneously dispersed in the reactant phase. In the past decades, wide range of methods has been investigated for the development of heterogeneous catalytic systems with inherent ability of being easily separated from the reaction [15-17].

1.3 Heterogenization of Homogeneous Catalysts

Typically, the motive behind the immobilization of compounds onto solid supports is to facilitate their handling and separation. The latter is an especially challenging problem faced when dealing with homogeneous catalysts. Upon heterogenization, the immobilized compounds can be easily separated from the reaction media by simple techniques such as filtration, decantation, and centrifugation, thus enabling multiple reuse and recycling of the immobilized compounds. This is particularly beneficial when working with expensive materials [18-20]. In addition, studies have shown that heterogenization can enhance the stability of embedded compounds and in some cases boost the reactivity and selectivity of catalytic reactions [21].

Nature of the catalyst support and the heterogenization process influence the performance of the heterogenized catalysts. To date, numerous catalytic supports both organic and inorganic, with different methodologies for the immobilization of homogeneous catalysts have been designed and applied in catalysis [22]. The resulting properties and potential application of an immobilized catalyst strongly depend not only on the (i) physicochemical nature, (ii) porosity, and (iii) dimensions of support, but also on the (iv) nature and length of the spacer between the catalytic sites and the surface of matrix, and (v) the density of catalytic sites on the surface of support.

Catalysts immobilization is typically based on the intermolecular interactions between the support and the catalytically active species. These interactions are classified into three types, covalent bonding, non-covalent interactions, and encapsulation. In the covalent bonding the catalysts are covalently tethered to the support; in non-covalent interactions, which are also called...