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INDUSTRIAL ALKYLATION OF AROMATIC HYDROCARBONS

CATALYZED BY NANOSTRUCTURE ZEOLITES: MECHANISMS

AND INTERMEDIATES

INTRODUCTION

In recent years, there have been considerable academic and industrial

research efforts carried out in the field of zeolite catalysis. The word zeolite is

Greek in origin and means boiling stones as natural zeolites visibly losing

water when heated. Today the word, as strictly defined, is reserved for

crystalline aluminosilicate materials (either natural or synthetic). There are

over 720 zeolite materials known at the moment (2003) and the number is

growing. 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 1950s,

zeolite catalysis has undergone rapid and dynamic advances. The number of

zeolite-related U.S. patents published though 1981 exceeds 5,000 and the

zeolite scientific and technical literature contains over 25,000 articles.

Zeolites are three-dimensional, microporous, crystalline aluminosilicates

with frameworks made of SiO4 and AlO4 tetrahedra (those tetrahedral atoms

are often referred as T atoms). Insertion of trivalent Al3+ in place of

tetrahedrally coordinated Si4+ creates negative charge on the lattice, which is

compensated by extraframework cations. If the charge compensating cation is

H+, a bridged hydroxyl group, (SiO(H)Al), is formed, which functions as a

strong Brnsted acid site. Due to these acid sites, zeolites are solid acids and

are used as catalysts. The catalytic activity of zeolites is often related to

strength of the acid sites, which depends on chemical composition and

topology of zeolite frameworks. There are numerous naturally occurring and

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Faujasite (FAU) Channels: 12 (7.4 )

Mordenite (MOR) Channels: 12 (6.5 x7.0 ) 8 (6.0 x 5.7 )

Beta (BEA) Channels: 12(5.5 x 5.5 )

Ferrierite (FER) Channels: 10 (4.2 x 5.4 ) 8 (3.5 x 4.8 )

ZSM-5 (MFI) Channels: 10(5.3 x 5.6 ) 10(5.1 x 5.5 )

A (LTA) Channels: 8 (4 )

synthetic zeolites. However, most zeolites used commercially are produced

synthetically, each with a unique structure. Zeolites have void and space

(cavities or channels) that can host cations, water or other molecules. The pore

sizes commercially available range from approximately 3 to approximately

8 . Some of the commercial materials are: A, beta, mordenite, faujasite,

ZSM-5, ferrierite.

Figure 1 Various types of zeolite pores and channels.