Review
Inorganic molecular sieves: Preparation, modification and industrial application in catalytic processes

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Abstract

The increasing environmental concern and promotion of “green processes” are forcing the substitution of traditional acid and base homogeneous catalysts by solid ones. Among these heterogeneous catalysts, zeolites and zeotypes can be considered as real “green” catalysts, due to their benign nature from an environmental point of view. The importance of these inorganic molecular sieves within the field of heterogeneous catalysis relies not only on their microporous structure and the related shape selectivity, but also on the flexibility of their chemical composition. Modification of the zeolite framework composition results in materials with acidic, basic or redox properties, whereas multifunctional catalysts can be obtained by introducing metals by ion exchange or impregnation procedures, that can catalyze hydrogenation–dehydrogenation reactions, and the number of commercial applications of zeolite based catalysts is continuously expanding.

In this review we discuss determinant issues for the development of zeolite based catalysts, going from zeolite catalyst preparation up to their industrial application. Concerning the synthesis of microporous materials we present some of the new trends moving into larger pore structures or into organic free synthesis media procedures, thanks to the incorporation of novel organic templates or alternative framework elements, and to the use of high-throughput synthesis methods. Post-synthesis zeolite modification and final catalyst conformation for industrial use are briefly discussed.

In a last section we give a thorough overview on the application of zeolites in industrial processes. Some of them are well established mature technologies, such as fluid catalytic cracking, hydrocracking or aromatics alkylation. Although the number of zeolite structures commercially used as heterogeneous catalysts in these fields is limited, the development of new catalysts is a continuous challenge due to the need for processing heavier feeds or for increasing the quality of the products. The application of zeolite based catalysts in the production of chemicals and fine chemicals is an emerging field, and will greatly depend on the discovery of new or known structures by alternative, lower cost, synthesis routes, and the fine tuning of their textural properties. Finally, biomass conversion and selective catalytic reduction for conversion of NOx are two active research fields, highlighting the interest in these potential industrial applications.

Introduction

Industrial catalysis and the corresponding catalytic processes have evolved during the last 250 years, and have become essential nowadays, with more than 90% of all industrial chemicals being produced by catalytic processes [1], [2]. Catalysis is fundamental for a sustainable industrial society, where it accomplishes a double objective: environmental protection and economic profit. Improved catalytic processes will lower energy requirements, make a better use of natural resources, reduce the amount of subproducts formed and eliminate contaminant effluents. Heterogeneous catalysis provides additional advantages of easier separation and lower salt and waste production [3].

Among the heterogeneous catalysts it is safe to say that zeolites are the most widely used materials [4]. Besides their environmentally benign nature, the combination of a well-defined microporous structure with pore sizes in the range of molecular dimensions and a flexible chemical composition are key factors for their successful applications in fields as different as refining, petrochemistry or fine and speciality chemicals. The main properties of these solids are related to their topology and morphology, and chemical composition, that result in high surface area, the possibility of partitioning reactants from products, high adsorption capacity, possible modulation of the electronic properties of the active sites, and the presence of strong electric fields and confinement effects within the pores, which result in preactivation of the molecules. Last, but not least, zeolites present an outstanding thermal and hydrothermal stability [4].

Concerning their application as heterogeneous catalysts, the shape selectivity effects introduced by zeolites are of paramount importance. Indeed, their microporous channels, with dimensions in the range of many reactant molecules, provide zeolites with shape selectivity towards reactants, products or transition states [5]. The shape selectivity involving reactants and products is due to mass transport discrimination and is related to a true molecular sieve effect [6]. Transition state selectivity occurs when the geometry of the pores can stabilize one transition state among several possible. The implications of the fundamentals of shape selectivity on the development of catalysts for petroleum and petrochemical applications have been overviewed by Degnan in [7]. Despite the advantages that these shape selective properties confer on zeolites, as compared to other heterogeneous catalysts, they may become inadequate when processing reactants with molecular dimensions above those of the pores. Therefore big efforts have been made in order to increase the accessibility of active sites to larger molecules and to reduce the impact of diffusional problems on catalyst life. Possible approaches are: to synthesize extra-large pore zeolites [8], [9], [10], [11], [12], to decrease crystal size by direct synthesis [13], [14], [15], [16], [17], [18], [19] or by zeolite delamination [20], reducing in this way the length of the diffusion path. Another way of decreasing the length of the diffusion path is to generate mesopores in the zeolite crystals by means of carbon templating [21], [22], by chemical or steam postsynthesis treatments [23] or by the use of supramolecular templating [24].

Broadly, the zeolite production directed to catalytic uses is close to 20% of the total zeolite market, the rest being focused to detergents (70%) and adsorbents (10%) [25]. Despite these consumption data, catalytic applications are by far the largest in terms of market value, being this especially so in the oil refining industry. In fact, the catalytic cracking industry alone represents more than 95%, with a maximum catalyst cost of 5$/kg. The rest accounts for specialty zeolites (20–30$/kg), where the catalyst value depends not only on the zeolite synthesis and modifications, but also on the value of the final product [25], [26].

Most of the current large scale commercial processes using zeolite based catalysts are in the petroleum refining and petrochemical industry [27]. Applications for the chemical industry involve mainly oximation, epoxidation, acylations, condensation and amination processes. Moreover, their presence is increasing in emerging fields such as environmental applications, the transformation of raw materials by means of non-conventional processes, such as coal, gas and oil conversion into syngas, olefins, acetylene and aromatics, all of them involved in value added chains, and conversion of methanol to propylene (MTP) and gasoline (MTG) [25]. Their potential application in the conversion of biomass is also gaining interest and has been recently reviewed [28].

If we compare the industrial application of acid and base solid catalysts, only 8% of the processes correspond to solid bases and none of these reactions, as far as we know, is performed with basic zeolite catalysts, although pilot plant trials were conducted in some cases [29], [30], [31]. An important handicap for the industrial application of basic zeolites relies on the fact that inexpensive NaOH and KOH are the competing catalysts. Their low cost and the easy processing of the residues formed, reduce the possibility of zeolite application to cases where special selectivity effects will be needed.

In this manuscript we will introduce some developments in the field of zeolites that go from synthesis and modification to their conformation as heterogeneous catalysts and application in commercial processes. The processes presented range from oil refining and petrochemicals to fine chemicals, and from conversion of alternative raw materials such as natural gas or biomass to the reduction of contaminants in stationary and mobile source emissions. Our aim has been to highlight the most recent advances in all these fields and to direct the reader to more specific revisions recently published.

Section snippets

Synthesis of inorganic molecular sieves

Increasing environmental concern and development of green processes based on heterogeneous catalysts are driving forces, not only for the improvement of conventional zeolites but also for discovering new molecular sieves with novel pore architectures [9], [10], [11], [25], [32], [33]. However, it is important to note that the well-established “big-five” zeolites, i.e. Y, ZSM-5, Mordenite, Beta and Ferrierite, are difficult to beat, due to their good performance and relatively low cost

Post-synthesis modification of inorganic molecular sieves

Two very complete compilations on the most employed post-synthesis modifications of zeolites are given by Kúhl [123] and Szostak [124]. In a more recent study [125], Chen and Zones include two additional modification procedures: the substitution of the original T-atoms present in the as synthesized zeolite, such as B, by the desired atoms, usually Al or Si [126], [127], and the preparation of highly crystalline hydrophobic pure-silica zeolites, such as CIT-1 and SSZ-33, by means of

Catalyst conformation for industrial use

As detailed by Bartholomew and Farrauto in [148], commercial heterogeneous catalysts are chemically and physically complex materials, and their development is a highly multidisciplinary task that requires inputs from chemistry, chemical engineering, material science and physics. In its final state, the catalyst has to accomplish specific dynamic properties which include: activity, selectivity, structure, surface composition or state of the active phase, and physical properties regarding its

Commercial processes based on inorganic molecular sieve catalysts

An interesting overview of the industrial relevance of zeolites and their use as heterogeneous catalysts, especially within refining and petrochemical processes, is given in [150]. Most of the zeolites used in catalytic processes are applied in this field, and other thorough revisions on this topic have also been published recently [2], [25], [32], [151], [152]. Here we will broaden the revision including applications in the field of chemicals and fine chemicals, in conversion of alternative

Future perspectives in commercial application of inorganic molecular sieves as catalysts

The need to replace the currently remaining homogeneously catalyzed commercial processes by heterogeneous alternatives, and to improve existing technologies, offers to new molecular sieves and modified existing ones the possibility to be commercially applied. The use of high-throughput methodologies has already proven to be highly efficient, not only in the discovery of new materials, but also for synthesis optimization to improve the characteristics of already existing zeolite structures, from

Conclusions

More than 80% of all industrial processes are catalytic processes, and many of them use zeolite based catalysts. The number of industrial processes based on heterogeneous catalysts is increasing, not only in refining and petrochemical areas, but also in new emerging fields. Some of the processes are well established, such as catalytic cracking and hydrocracking, light paraffins isomerization or aromatic alkylation. They require relatively inexpensive zeolites and probably only new zeolites with

Acknowledgements

The authors acknowledge financial support from Ministerio de Ciencia e Innovación (project Consolider-Ingenio 2010 MULTICAT).

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