Zeolite Catalysts

Principles and Applications

Subhash Bhatia

• Éditeur: CRC Press
• Année: 2020
• ISBN:
  9781000141764
• Édition: 1
• Sujet: Technology & Engineering
• Sous-sujet: Applied Sciences

Presented in an easy-to-read form, this book on zeolite catalysis cover all aspects of the subject. It focuses on synthesis, structure, diffusion, deactivation, and industrial applications. This book is an ideal text for courses on catalysis or as a supplementary text for those studying applied or industrial chemistry. It is also a useful resource for anyone who works with zeolites as catalysts in the laboratory, pilot plants, or commercial installations.

Chapter 1

INTRODUCTION

1.1 HISTORICAL DEVELOPMENT

In recent years there have been considerable academic and industrial research efforts carried out in the field of zeolite catalysis.^{1, 2, 3, 4, 5, 6, 7, 8} There are 34 known natural zeolites and about 100 zeolites which do not have natural counterparts have been synthesized. Of this large number of zeolites, only a few have found commercial application: they are mostly synthetic zeolites and synthetic-analog natural zeolites. Since zeolites were first used in petroleum processing in the 1960s, zeolite catalysis has undergone rapid and dynamic advances. The number of zeolite-related U.S. patents published through 1981 exceeds 5,000 and the zeolite scientific and technical literature contains over 25,000 articles.

A zeolite has been defined by Smith⁹ as a “crystalline aluminosilicate with a tetrahedral framework structure enclosing cavities occupied by cations and water molecules, both of which have enough freedom of movement to permit cation exchange and reversible dehydration”. The term zeolite was originally used to describe just such a material. Later, however, the term was broadened to include all ion exchangers — naturally occurring and synthetic inorganic materials as well as organic ones. Because of the widespread use of crystalline aluminosilicate zeolites in industry today, the name zeolite is now fairly well known.

The major industrial processes that use zeolite catalysts are listed in Table 1, together with their respective zeolite usages.

The major part of zeolite catalysis work has been related to reactions where the zeolite is used as solid acid, e.g., isomerization, cracking, hydrocracking etc. The catalyst of choice for catalytic cracking, the heaviest use of zeolites, is usually a rare earth, magnesium, hydrogen, or ultrastable form of zeolite X and Y, or a combination of these. The next heaviest use, hydrocracking, uses catalysts that are dual function in nature and incorporate CO, MO, W, Ni, Pt, or Pd supported on HY, mordenite, erionite, or ultrastable faujasites. Organic and inorganic chemical technologies currently have little use for zeolite catalysts. However, because of the unique intercrystalline pore-channel systems and the excellent maintenance of catalyst activity of the new organic cation containing zeolites, applications in the organic chemical process industry are increasing.

The increased process applications of zeolites in the chemical industry and the virtual explosion of new zeolite synthesis and characterization studies have generated a number of innovations in zeolite catalysis. Review on zeolite catalysis has therefore in general tended to concentrate more on the work related to acid catalysis.^1,4,10

In general, zeolites have four properties that make them especially interesting for heterogeneous catalysis:^11,12

1. They have exchangeable cations allowing the introduction of cations with various catalytic properties.

2. If these cationic sites are exchanged to H⁺, they can have a very high number of very strong acid sites.

3. Their pore diameters are less than 10 Å.

4. They have pores with one or more discrete sizes.

In principle, zeolites offer considerable scope for surface modification. The ion exchange properties and the periodicity (crystallinity) of zeolites, for example, enable functional metal ions or complexes to be stablized in a variety of oxidation states and coordination geometries on specific surfaces in a well-defined pore structure. This represents a valuable starting point in the design of catalytic surfaces. A greater emphasis is given to the most recent and more novel application of zeolites to heterogeneous catalysis.

TABLE 1
Zeolite Usage in the Chemical and Fuel Industries (U.S.) (Based on Approximately 221,500 tons/year)

Process	Wt%	Catalyst examples
Catalytic cracking	90	REY, HY
Hydrocracking	9	CO, MO, W, Ni, HY, mordenite, erionite
Organic chemicals	1	ZSM-5, REY
Inorganic chemicals	1	Mordenite

TABLE 2
Zeolite History¹³

1756	Discovery and naming of first natural zeolite, stilbite
1825	Discovery of natural lecynite
1842	Discovery of natural faujasite
1862	First zeolite synthesis (levynite)
1864	Discovery of natural mordenite
1870—88	First ion exchange studies with zeolites
1890	Discovery of natural erionite
1929	Potential as strong acids described (Pauling)
1930—34	First zeolite structure determinations
1932	Zeolites described as molecular sieves
1942—45	Quantitative separations by molecular sieving
1948	First purely synthetic zeolite made
1948	Synthesis of mordenite
1949	Preparation of acid forms of zeolites
1956—64	Synthesis of zeolites A,X, and Y (Union Carbide)
1962	Introduction of zeolite-based cracking catalysts(Mobil Oil Corp.)
1971—72	Highly-siliceous zeolites (ZSM-5, ZSM-8) synthesized (Mobil Oil Corp.)
1975	ZSM-5 catalysts used in ethyl benzene production
1978	ZSM-5 catalysts used in oil dewaxing
1978	Structure of ZSM-5 and ZSM-11
1980	High resolution electron micros...