INTRODUCTION
1.3 Adsorbents of Industrial Importance
The adsorbent is the heart of any adsorption process and the success of the process largely depends upon its selection. From a large number of adsorbents that are reported in the literature, only a few have survived the technological progress. Silica gel, activated carbon, carbon molecular sieve, activated alumina and zeolites are the most studied adsorbent materials. Based on the elemental composition, conventional adsorbents can be divided into two categories.
(i) Organic frameworks: Activated carbons are a typical example in this category. They are usually synthesized by the pyrolysis of carbon-rich materials and generally have higher pore volume which results into large saturation capacities [6]. However, these materials have disadvantages such as unordered structures and poor selectivities. To overcome the issue of poor selectivity, carbon adsorbents with narrow distribution of micropore sizes are prepared using special activation procedures and are known as carbon molecular sieves. Carbon sieves with pore diameter ranging from about 4 to 9 Å [6] are synthesized to obtain enhanced selectivities;
however, porosity and hence saturation adsorption capacity for these materials is lower. Another disadvantage of carbon molecular sieves is the lack of reproducibility of properties between different batches [9].
(ii) Inorganic frameworks: In these frameworks, porous structure is formed entirely by inorganic elements. Silica gel, activated alumina and zeolites are the common examples of inorganic frameworks.
Silica Gel. Partial dehydrated form of polymeric colloidal silicic is known as silica gel (SiO2.nH2O) [6]. The presence of hydroxyl group in the structure yields polarity and it exhibits selectivity for polar adsorbates such as water, alcohol, amine etc. over non-polar adsorbates.
However, the pore size distribution is quite large for this material and thus its adsorption capacity is lower than that of carbon molecular sieves at low pressures.
Activated Alumina. Activated alumina is a porous high-area form of aluminium oxide. The surface of this material is more polar than that of silica gel and has both acidic and basic characters, indicating the amphoteric nature of aluminium [6]. This material has similar affinity for water at room temperature as that of silica gel; however, at higher temperature capacity of activated alumina is higher and therefore this material was earlier used as desiccant before its replacement by molecular sieves.
Zeolites. Zeolites are crystalline aluminosilicates of alkali or alkali earth elements, such as sodium, potassium and calcium. The chemical composition of zeolites can be represented by the following formula [6].
O H z SiO AlO
Mz/n[( 2)z( 2)z ]. 3 2
2 1
1
Where z1 and z2 are the integers with z2/z1 equal to or greater than 1, n is the valancy of the metal cation M, and z3 is the number of water molecules in each unit cell [4]. In these materials, silica and alumina tetrahedra are connected together in various arrangements through shared oxygen atoms to form an open crystal lattice [6]. Since the micropore structure in these materials is determined by crystal lattice, the pore size in these materials is fixed [6]. Change in Aluminium to Silicon ratio leads to a systematic shift in the adsorption properties of these materials.
Aluminium rich zeolitic frameworks are hydrophilic and have very high affinities for polar molecules (such as water) whereas microporous silicas (that are Si rich) such as silicalite are hydrophobic. The transition from hydrophilic to hydrophobic character generally occurs at a Si/Al ratio ranging from 8 to10. Some zeolites (Zeolite 13X) have shown very promising CO2 adsorption capacities [10] and selectivities at lower pressures, but they have limited high
pressure capacity compared to activated carbons due to low pore volume. In addition, due to strong affinity for CO2 at low pressure, the regeneration energy requirement of Zeolite 13X is also high.
Some features of adsorbents have a strong effect on their adsorption characteristics and are worthy to be discussed.
(a) Surface Polarity: Some adsorbents exhibit surface polarity due to the presence of charged ions in the structure. Polar adsorbents such as silica gel, activated alumina and zeolites are generally hydrophilic in nature and show enormous affinity for polar adsorbates (such as H2O).
On the other hand, activated carbons have non-polar surface; however, a slight polarity may arise from surface oxidation. As a result these materials exhibit hydrophobic and organophilic nature [6]. The feature of hydrophilicity or hydrophobicity can be judicially exploited for many separation processes based on adsorption equilibrium.
(b) Porosity: Since adsorption is a surface phenomenon, porosity is one of the important characteristics for an adsorbent. Higher porosity indicates higher specific surface area and pore volume. Higher surface area and pore volume generally results into higher saturation adsorption capacity and thus can extremely be useful for adsorptive storage of gases (such as methane and hydrogen) at high pressures. However, extremely porous materials usually tend to have low/poor volumetric uptake capacities and hence the adsorbent bed sizes might be large. A reasonable balance should be obtained between these two contradicting necessities. In addition to porosity, pore size distribution also affects the adsorption characteristics of adsorbents. Based on International Union of Pure and Applied Chemistry (IUPAC) standards, porous materials can be divided into three different category: (i) microporous (pore size < 2 nm), (ii) mesoporous (pore size from 2 to 50 nm) and (iii) macroporous (pore size > 50 nm). It is of great interest to study
the adsorption characteristics of microporous materials due to their potential for adsorptive storage, separations, and catalysis applications. Wilmer et al. [11] reported that among best adsorbents for CH4 storage, the most frequent pore sizes are between 4 and 8 Å, exactly big enough for one or two CH4 molecules.