Page 232 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPB Final Pages
Encyclopedia of Physical Science and Technology EN009I-420 July 10, 2001 15:8

Mesoporous Materials, Synthesis and Properties 379

Potential areas of application for mesoporous materials in- sites. Such processes include hydrocracking, demetaliza-
clude use as catalysis, adsorbents, and molecular sieves, tion,hydroisomerization,andolefinoligomerization.Acid
and as hosts in the preparation of advanced composite mesoporous materials can also be used as catalysts for or-
materials. Their potential applications therefore span the ganic synthesis and production of fine chemicals in, for
areas of heterogeneous catalysis, molecular sieving, and example, Friedel–Crafts alkylation, acylation, acetalyza-
host/guest chemistry. tion, and Beckmann rearrangement reactions. Common
for all these fine chemical synthesis reactions is the in-
creased activity of mesoporous catalysts when the size
A. Catalysis
of the reacting molecules increases. These reactions of-
The unique structural properties of mesoporous materi- ten involve bulky reactants and products, and in many
als make them highly desirable for catalytic applications. cases they are carried out in liquid phase where diffu-
Their large pore sizes limit diffusion restrictions of re- sion problems can be severe. The use of a mesoporous
actant and product molecules, and thus enable the pro- material reduces or eliminates these diffusion limitations.
cessing of bulky molecules, which are beyond the size An added attraction of mesoporous materials is that the
range of microporous zeolites. Furthermore, unlike in mi- large pore size can produce desired product distribu-
croporous zeolites, their large surface area and pore vol- tions not achievable on microporous zeolites. Organic
ume allows the grafting of large active species within the synthesis is therefore certainly a field in which acidic
mesoporous framework thus increasing the range of pos- mesoporous materials are expected to make a significant
sible reactive sites. Mesoporous materials can therefore contribution.
be used as solid acid catalysts, base catalysts, or reduc-
tion/oxidation (redox) catalysts depending on the nature
of active sites. The versatility of active sites that can be in- 2. Base Catalysis
troduced in these materials greatly expands their catalytic
Mesoporous materials can also be used as base catalysts—
possibilities.
for example, when the negative charge on a mesoporous
aluminosilicate is compensated by metal ions such as
1. Acid Catalysis sodium (Na) or cesium (Cs). Amines anchored on meso-
porous silica can also be used as base catalysts. Successful
Given the absence of active sites and ion-exchange ca-
utilization of basic sites requires the total absence of acid
pacity, purely siliceous mesoporous materials are of lim-
sitessincethetwofunctionstendtodrivereactionsthrough
ited use as catalysts. To generate active and ion exchange
different pathways. There are, however, some cases where
sites, doping by isomorphous substitution with a num-
adjacent acid/base sites are desirable.
ber of metals is performed. The metals used include alu-
minum (Al), zirconia (Zr), gallium (Ga), and titanium (Ti).
The most popular choice of metal dopant for generat-
3. Redox Catalysis
ing ion exchange and acid sites is aluminum. Adding an
acidic ingredient such as a heteropolyacid, an ultrastable Y The ability to introduce transition metals into the walls
(USY) or an Al-containing ZSM-5 zeolite can also gener- of mesoporous materials imparts redox catalytic proper-
ate acid sites in mesoporous silicates. These additions re- ties. Transition metal-substituted mesoporous solids rep-
sult in materials that are essentially physical mixtures. The resent a very useful and versatile group of potential re-
mesoporous silica therefore simply performs the role of a dox catalysts. In particular, they extend redox catalysis
support. For example, by mixing mesoporous silica with by ordered porous solids beyond the microporous (ze-
zeolites it is possible to create a unique composite catalyst olite) range. Zeolites are limited, due to structural con-
in which a mesoporous shell surrounds a zeolite core. Such straints, by the size of transition metal, which can be in-
a catalyst can be used as follows: the mesoporous shell can troduced into their framework. This limitation is largely
provide a high surface area to support metal functional- absent in the case of the ordered mesoporous materials.
ities (e.g., for hydrogenation), and the zeolite core can Though transition metal-containing mesoporous materi-
provide the acid sites for other process such as cracking. als are intrinsically less active than Ti-zeolites, they are
The composite material is essentially a dual (bifunctional) able to process much bulkier substrates. They are there-
catalyst, capable of performing both hydrogenation and fore more active than zeolites for processes involving large
cracking. molecules. The biggest disadvantage however in transition
Although acidic (e.g., aluminosilicate) mesoporous ma- metal-containing mesoporous catalysts is that the activity
terials are only moderately acidic, they can be useful cat- and selectivity strongly decreases when the level of the
alysts for processes that do not require very strong acid metal is increased.