The Colloid Chemistry of Silica - American Chemical Society

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Surface Structure of Amorphous and Crystalline Porous Silicas Status and Prospects Κ. K. Unger Institut für Anorganische Chemie u n d Analytische Chemie, Johannes Gutenberg-Universität, P . O . Box 3980, D - 6 5 0 0 M a i n z , Germany

Substantial progress in the elucidation of the surface structure of crystalline and amorphous silicas has been achieved by means of high-resolution spectroscopic techniques, for example, Si cross-polarization magic-angle spinning NMR spectroscopy and Fourier transform IR spectroscopy. The results had to a better understanding of the acidity, dehydration properties, and adsorption behavior of the surface. These properties are key features in the design of novel advanced silica materials. The current methods of characterization are briefly reviewed and summarized. 29

A H E SURFACE CHEMISTRY O F SILICA was a subject of intensive study i n the

period between 1960 and 1970 as a consequence of the widespread industrial use of colloidal, pyrogenic, and precipitated silicas, as well as silica hydrogels and xerogels. Chemical surface reactions and IR spectroscopy were the most-applied methods i n surface structure elucidation. Significant contributions to the understanding of the silica surface were made by Fripiat (I), Kiselev and co-workers (2), Hair (3), Little (4), Peri (5) , and others. In contrast to this active period, little progress was since made until about 1980. Advances in surface and materials science caused a search for novel materials with controllable properties, and the surface structure of silica regained considerable interest. Three major developments were important: first, the experience gained i n silicate chemistry (6) , particularly in the area of synthetic zeolites (7); second, the increasing 0065-2393/94/0234-0165$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

166

T H E C O L L O I D CHEMISTRY O F

SILICA

use of surface-modified silicas as packings in high-performance liquid chromatography (HPLC) (8); and third, the progress made in high-resolution spectroscopic techniques applied to surface characterization. This chapter summarizes the most important achievements made in this field and puts the problems involved into perspective.

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Structural Aspects in Silica Chemistry The term silica, as applied to solid forms having the stoichiometric composition of S 1 O 2 , has many meanings. Thus, it is useful to attempt to classify the different solid forms and modifications of silica according to distinct structural characteristics (9). The bulk structures of silicas are classified as crystalline and amorphous polymorphs. More than 35 well-defined crystalline silicas are known, which are well-characterized by the S i - O length, the S i - O - S i bond angle, and the S i - O bond topology and coordination (10). Some of the crystalline polymorphs are collected in Table I. Because of the lack of sufficiently precise methods to assess the long-range structural order, amorphous silicas remain poorly characterized. They can be loosely discriminated according to their dispersity, bulk density, and type of pore structure. The first classifying quantity in the characterization of crystalline silica polymorphs is the framework density dt, which is expressed as the number of S 1 O 2 groups per 1000 À (10). Values of df range from 17 to 43 S 1 O 2 groups per 1000 À (Table I). According to the value of df, crystalline silicas were divided by Liebau (10) into pyknosils (df > 21 S 1 O 2 groups per 1000 Â ) and porosils (df < 21 S 1 O 2 groups per 1000 À ) . Pyknosils are defined as "polymorphs with frameworks too dense to enclose guest molecules that are larger than helium and n e o n " (10). Pyknosils are nonporous. The second quantity is the type of porosity and pore structure. Phases with silica frameworks that have pores wide enough to accommodate larger guest molecules are called porosils, irrespective of whether their pores are filled or empty (10). Porosils have a micropore system of pores with widths between 0.4 and 0.8 nm. In contrast to the well-defined microporous crystalline silicas, porous amorphous silicas lack any longrange structural order. The average pore diameter of amorphous silica materials covers the range between a few to several thousand nanometers. Porosils are further divided into clathrasils and zeosils, depending on whether the pores are closed or open, that is, accessible to adsorption. Clathrasils form cagelike pores that are schematically described by polyhedra. A typical representative of the clathrasil family is Dodecasil 1 H , the crystals of which are depicted in Figure 1 (II). Zeosils can be considered as aluminum-free and zeolites, which are microporous crystal3

3

3

3


8.

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Amorphous and Crystalline Porous Silicas

167

Table I. Crystalline Phases That Have Topologically Distinct S1O2 Frameworks

Name Stishovite Si0 (Fe N-type) Coesite Quartz Moganite Keatite Cristobalite Tridymite Nonasils Melanophlogites Dodecasils 3 C (Silica ZSM-39) Dodecasil 1 H SIGMA-2 Silica sodalites Decadodecasils 3R Decadodecasils 3 H Silica ZSM-23 Silica ZSM-48 Silica ZSM-22 Silica ferrierite Silica ZSM-12 Silica ZSM-50 Silicalite II (Silica ZSM-11) Silicalite I (Silica ZSM-5) Fibrous silica 2


Formula; Unit Cell Content

df

2

43.0 42.8 29.3 26.6 26.3 25.1 23.2 22.9 19.2 19.0 18.6

S1O2 S1O2 S1O2 S1O2 S1O2 S1O2 S1O2 S1O2

88SiO 8m88m94m20 46Si0 2M 6Mi4 136Si0 16Mi58Mi6 2

12

2

2

18.5 17.8 17.4 17.6 17.6 20.0 19.9 19.7 19.3 18.5 18.2 17.9

34Si0 3m 2M lM 64Si0 8M 4M 12Si0 2Mi4 120SiO 6Mi09Mi 6Mi 120SiO 6Mi°9M lM 4M 1 M 24Si0 (CH3) N(CH2)7N (CH ) 48Si0 H2N(CH )8NH2 24Si0 HN(C H )2 36Si0 2H N(CH2)2NH2 28Si0 N(C H )3 112Si0 n-[(CH3)3N(CH ) 6N(CH )3](OH)2 96Si0 n-[N(C4H ) ]OH

17.8

96Si0 4[N(C H )4]F

19.6

S1O2

1 2

2

1 2

9

2

2 0

2 0

2

2

2

9

12

2

2

19

2

2 3

3

2

2

2

2

5

2

2

2

2

2

5

2

2

2

2

15

9

3

3

4

7

NOTE: Abbreviations are as follows: df, framework density in number of S1O2 groups per 1000 À ; M , guest molecule located in a cage that has / = mj faces. SOURCE: Reproduced with permission from reference 10. Copyright 1988. 3

line-network aluminosilicates (10). The end-member of the pentasil ZSM-5 family, for instance, is silicalite-I, with an S i : A l ratio of >1000 (12). Silicalite-I exhibits an open bidirectional pore system of straight and zigzag channels about 0.5-0.6 nm in width (12). Silicalite-I can be synthesized with a defined phase composition, with high purity, and as large crystals with a narrow particle size distribution (13). Because of these properties and its high thermal stability up to 1200 K, silicalite-I is wellsuited as a reference material for crystalline and amorphous porous silicas. A third classifying quantity relates to the surface structure of silicas, which is characterized by the coordination of surface silicon atoms; the resulting functional groups; their density, topology, and distribution; the degree of hydroxylation; the hydration-dehydration behavior; the acidic



168

T H E C O L L O I D C H E M I S T R Y O F SILICA

Figure 1. Crystals of Dodecasil 1H, synthesized with 1-amino adamantan. (Reproduced with permission from reference 11. Copyright 1989.) and basic properties of surface functional groups; and their adsorption behavior and chemical reactivity. The pattern of the surface structure i n terms of these properties is discussed in the section "Current View of the Silica Surface". The following section reviews the methods by which reliable information on these properties is obtained.

Survey of Methods for Characterizing the Silica Surface According to the underlying principle, the methods are grouped into spectroscopic, thermal and calorimetric, adsorption and wetting, isotopic exchange, microscopic, scattering, and chemical reactions techniques (Table II) (14-29). This chapter is not a comprehensive treatment of all methods i n depth. Results of spectroscopic and adsorption methods are discussed for aspects of content of information, validity, applicability, and limitations. Spectroscopic Methods. Fourier Transform Infrared (FUR) Spec troscopy. FTIR is carried out either in the transmission mode or as diffuse reflectance FTIR (14) with pellets, self-supporting sample disks, or loosely packed powders. Spectra are commonly recorded in the frequency range between 400 and 4000 cm- . Figure 2 (30, 31) shows the adsorption bands and band assignments of three different types of silicas: an amorphous highly disperse nonporous silica (Aerosil 200, Degussa), a crystalline nonporous silica (α-quartz), and a microporous crystalline silica (silicalite-I). The band at 3750 cm- is assigned to the stretching vibration of free hydroxyl groups that occur at the surface of amorphous silicas. This band usually overlaps with the absorption bands originating from hydro gen-bonded hydroxyl groups and from adsorbed water. Adsorbed water on amorphous silica is removed under vacuum at 383 to 473 K. Above 473 K, 1

1


8.


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169

Table II. Physical and Chemical Methods for Characterizing the Surface Structure of Silica

Method

Information


Spectroscopic: FTIR Diffuse reflectance FTIR adsorption of probe molecules (pyridine, carbon monoxide, and so forth, monitored by FTIR) S i C P - M A S N M R spectroscopy 29Si C P - M A S N M R spectroscopy, H MAS N M R spectroscopy 29

fl

l

Secondary ion mass spectrometry (SIMS) Extended X-ray absorption fine-structure spectroscopy

Electron spectroscopy for chemical analysis (ESCA) Photophysical studies of direct energy transfer between excited donor and ground-state acceptor molecules Thermal and calorimetric: Microcalorimetry Thermogravimetry Differential thermogravimetry (DTG), differential scanning calorimetry (DSC), and thermoporometry (monitoring liquid-solid phase transitions of a pure liquid capillary condensate in a porous system) Isotopic exchange: Heterogeneous isotopic exchange using deuterated (D , 2

D2O, CH3OD,

C F 3 C O O D , and so forth) and tritiated (HTO) substances, combined with mass spectrometry, IR spectroscopy, and *H N M R spectroscopy

Types of hydroxyl groups, acidity Brônsted and Lewis acid sites, adsorption behavior, and chemical reactivity

Short-range structural order of surface silicon atoms, bond Length, bond angles, acidic properties (protonic sites) of silica, and concentration of protons Chemical composition of the surface as a function of the depth Local structural surrounding, nearest neighbor distances, coordination number, and bond lengths Chemical compositions of the surface Pore and surface morphology

Heat of adsorption, phase transitions of adsorbates Weight loss as a function of temperature Enthalpic (exothermic or endothermic) changes upon heating surface area, pore size distribution, and average pore diameter

Total content of surface hydroxyl groups, content of physisorbed water

Ref

14

15-18 19

20

21

22-24

Continued on next page.


170

T H E C O L L O I D C H E M I S T R Y O F SILICA

Table II.—Continued Information


Method Adsorption and wetting: Adsorption of gases and va pors, combined with IR spectroscopy, microcalorimetry, and so forth Temperature-programmed desorption of absorbed sub stances (ammonia, pyridine, and so forth, coupled with mass spectrometry and IR spectroscopy Adsorption of bases (n-butylamine) in aprotic solvents using Hammett or arylmethanol indicators Wetting with liquids Microscopy: Transmission and scanning electron microscopy, scan ning tunneling microscopy Scattering: Small-angle X-ray and neu tron scattering Chemical Methods: Using chemically reactive substances such as chlorine, metal halides, grignard com pounds, and reactive chloroand alkoxysilanes

Ref.

Heats of adsorption, specific surface area, pore size distri bution, average pore diameter, and fractal dimension Acidic functional groups, rela tive acid strength

Total content of surface acid sites, acidity distribution

25

Surface energy, surface wetta bility, and contact angle Texture of silica, geometrical surface structure, pore shape, and pore homogeneity

26

Pore morphology, roughness of pore surface, and pore size distribution

27, 28

Stoichiometry of surface reac tions, reactivity of surface hy droxyl groups

29

" C P - M A S denotes cross-polarization-magic-angle spinning.

hydrogen-bonded hydroxyl groups condense, and the corresponding absorption band diminishes. Free surface hydroxyl groups still exist after annealing the silica at 1273 Κ at a low content. Figure 3 (32) shows the IR spectra of a porous silica measured at different pretreatment temperatures. Thus, for IR spectroscopic measurements it is essential to control the pretreatment conditions (vacuum, temperature, and moisture) to achieve reproducible results. There is still a discussion on the appearance of the absorption band due to geminal groups. Although Camara et al. (33) suggested that the band at 3710 c n r was specific for geminal groups, M or r ow and Gay (34) claimed that they absorb at 3750 c m . Discrimination between bulk and surface hydroxyl groups is effected by subjecting the silica to deuteration with D 2 O and monitoring the absorption of the corresponding - O D bands (22). Bulk hydroxyls and bulk water do not take part i n the isotopic 1

1


8.

A e r o s i l 200

3,800 cm'

1


1

— 3660 hydrogen-bounded groups, i n t e r n a l h y d r o x y l groups 3,600 cm"

1

3,500 cm"

1

cm

-1

a - quartz

silicalite-I/ZSM-5

I

- 3747 ± 10 f r e e h y d r o x y l groups 3,700 cm" I — 3710 geminal h y d r o x y l groups

3,400

171


UNGER

—

3 690 f r e e h y d r o x y l groups

— 3650 free hydroxyl groups

h - 3615 Brônsted acid sites

—

—

3520 ± 200 h y d r o x y l groups hydrogen-bounded t o a d s o r b e d water

3400 ± 200 adsorbed l i q u i d water

3400 ± 200

I— adsorbed liquid] water

Figure 2. Band assignment of surface silica species in the high-frequency region (2000-4000 cm- ). (Reproduced with permission from reference 30. Copyright 1979.) 1

Figure 3. IR spectra of a porous silica (specific surface area a* = 475 m /g; average pore diameter ρ = 7 nm) obtained after evacuation at (left to right) 473, 673, 773, 873, and 973 K. (Reproduced with permission from reference 32. Copyright 1979.) 2



172

T H E C O L L O I D CHEMISTRY O F SILICA

exchange, and thus their absorption bands remain unaffected. JbTJLK spectroscopy is a technique to identify surface hydroxyl groups and to follow their adsorption behavior toward selected probe substances. Quantitation is difficult to achieve because of the lack of absorption coefficients. Several types of cells have been constructed and are commercially available that allow an in situ measurement or have a movable holder to evacuate and heat the sample. W i t h these cells it is also possible to admit gas or vapor to subsequently measure the frequency shift upon adsorption. This technique can reveal additional information about the surface properties of the material. Carbon monoxide, for instance, has been applied as a probe to detect hydroxyl groups at the silica surface. Upon adsorption of C O at 77 K, the band due to free hydroxyl groups shifts to a lower frequency by 78 or 93 cm- (35, 36, 37). Bronsted and Lewis acid sites are distinguished by the adsorption of pyridine monitored by means of IR spectroscopy (38). Pyridine adsorbed on Bronsted sites gives rise to an absorption band at about 1550 cm- ; pyridine adsorbed on Lewis sites generates a band between 1440 and 1460 cm- , depending on the type of Lewis acid sites. It is interesting to compare the absorption pattern of the three types of silicas that differ in bulk and surface structure (Figure 2). α-Quartz shows two types of free hydroxyl, with absorption bands at 3690 and 3620 cm(32, 39). O n silicalite-I, a band at 3750 cm- with low intensity is assigned to free hydroxyl groups at defect sites of the crystals. Increasing the aluminum content, that is, forming ZSM-5 zeolite, leads to an appearance of a band at 3615 cm- , which is assigned to a B r 0 n s t e d acid of the following type (31): 1

1

1

1

1

1

O \

O

/

O

H

O

O

\ / \ / \ / Si A l S i S i / \ / \ / \ / \ 0 0 0 0 0

Si CP-MAS NMR Spectroscopy. This method is very useful for characterizing silica (15-17). The main information derived from an N M R spectrum is the chemical shift, the intensity, and the line width. The S i chemical shift is determined by the number and type of tetrahedral framework atoms connected to tetrahedral silicon atoms. The spectrum thus allows the detection of the number of structurally inequivalent kinds of silicon atoms of various Si (0-)4-n(OSi) units in silicates and as Q (m Al) for Si (OSi)4-n(OAl)m units in framework alumosilicates (Figures 4 and 5) (40). 29

29

n


4

8.

0" -osicr 0"

0" •oaosj o-

Q°

ο sOSOa o-

-60

Q

Q

1

- 70

S

-

- 80

SiOSiOsi

SiOSiOsi ο-

2

α

0*

3

IQ1 («SiOSi « 1 8 0 !

• Q1 ,


173


UNGER

e

- 90

The Colloid Chemistry of Silica - American Chemical Society

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