Adsorption
5.4 Commercial and new adsorbents and their properties
The advancement of adsorption technology primarily depends on the development of novel and robust adsorbents. These adsorbents take a broad range of chemical and geometrical structural forms. Table 5.1 shows their general classification.
The following section describes the important properties of some of the commercial and new adsorbents that may have potential application in biorefinery separation processes.
Table 5.1 Types of commercial adsorbents. Reprinted from [1] c2001, with permission from Elsevier
Carbon adsorbents Mineral adsorbents Other adsorbents
Active carbons Silica gels Synthetic polymers
Activated carbon fibres Activated alumina Composite adsorbents:
(complex mineral carbons, X-elutrilithe; X = Zn, Ca) Carbon molecular sieves Oxides of metals
Mesocarbon microbeads Hydroxides of metals
Fullerenes Zeolites
Heterofullerenes Clay minerals Mixed sorbents
Carbonaceous nanomaterials Pillared clays Metal organic frameworks (MOF)
Porous clay hetero-structures (PCHs)
Inorganic nanomaterials
5.4.1 Activated carbon
Activated carbon is the most widely used adsorbent because of its large porous volumes and the resulting high surface area [3]. Activated carbon can be manufactured from any carbonaceous organic material.
Commercial carbons are made from a range of materials, such as sawdust, wood, charcoal, peat, fruit nuts, lignite, petroleum coke, bituminous coal, and coconut shells. The activation steps that are commercially used are steam activation and chemical activation. The steam activation process consists of two steps:
carbonization and activation. Carbonization is done by heating the material in the range of 400–500◦C in an oxygen-free atmosphere to remove the bulk of volatile matter. The carbonized particles are then
“activated” by exposing them to an oxidizing agent, usually steam or carbon dioxide at 800–1000◦C [3].
This technique is used for activation of coal and coconut shell. This forms a porous, three-dimensional graphite lattice and a large surface area by removing the pore blocking pyrolysis materials created during the carbonization step.
Chemical activation is generally used for the activation of peat and wood-based raw materials. The raw material is impregnated with a strong dehydrating agent, typically phosphoric acid (H3PO4) or zinc chloride (ZnCl2), mixed into a paste and then heated to temperatures of 500–800◦C to activate the carbon; the resulting activated carbon is washed, dried, and ground to powder. Activated carbons produced by chemical activation generally exhibit a very “open” pore structure, commonly referred to as “Macroporous,” ideal for the adsorption of large molecules.
The important properties of an adsorbent, which will affect the adsorption characteristics, are: the pore size distribution, surface area, surface qualities (hydrophobic/hydrophilic nature and functional groups), and physical properties such as hardness, bulk density and particle-size distribution. These properties are generally controlled by fine tuning the different parameters of the manufacturing process. Some of these properties of activated carbon are briefly described below.
Activated carbon is found to have polymodal pore size distribution. The International Union of Pure and Applied Chemistry (IUPAC) defines the pore size distribution as:
• micropore radius<1 nm
• mesopore radius 1–25 nm
• macropore radius>25 nm
The macropores are used as the entrance to the activated carbon, the mesopores for transportation and the micropores for adsorption. The total pore volume of activated carbons can be up to 80%. Activated carbons used for gas-phase application are designed to have pore size ranging from 10 to 25 ˚A, whereas for liquid phase application, pore size will be larger than 30 ˚A in order to decrease the mass transfer resistance of large-size dissolved adsorbate molecules.
Surface area is the primary indicator of the activity level. Activated carbons have the largest surface area ranging from 300 to∼4000 m2g−1, as measured by the BET method [3]. The adsorption capacity is typically about 1 to 35 wt%. The surface of activated carbon is essentially nonpolar and a slight polarity may arise because of presence of surface oxide groups. Activated carbons have bulk density between 400 to 640 kg per cubic meter. They are broadly classified based on their physical characteristics as powdered activated carbon (PAC), granulated activated carbon (GAC), extruded activated carbon (EAC), and bead activated carbon (BAC). Careful consideration of particle size can provide significant operating benefits by balancing pressure drop and adsorption kinetics.
5.4.2 Silica gel
Silica gel is an amorphous inorganic adsorbent having mesoporous structure. It is well known for its desiccant property. Commercial silica gels are produced by polymerization of silicic acid. First, a sodium silicate solution is acidified using sulfuric or hydrochloric acid to produce silicic acid. Following this, silicic acid polymerizes into jelly precipitate, which is washed and dried to produce colorless silica gel. By varying the silica concentration, pH and temperature, the properties of silica gel such as pore volume, surface area and shape can be varied [22]. Two common types of silica gel are regular-density and low-density silica gels. The regular-density silica gel has a surface area of 750–850 m2g−1 and an average pore diameter of 22–26 ˚A, whereas the respective values of low-density gel are 300– 350 m2g−1 and 100–150 ˚A [3]. The presence of hydroxyl groups makes its surface hydrophilic. Hence molecules such as water, alcohol, and phenol are adsorbed in preference to non-polar molecules [4]. Common forms are granules, extrudates (2 to 4 mm diameter) and beads (1 to 3 mm diameter).
5.4.3 Zeolites and molecular sieves
Zeolites are microporous crystalline aluminosilicate materials. The basic structural units of a zeolite frame-work are tetrahedra of silicon and aluminum, SiO4 and AlO4, which are cross-linked to each other by oxygen atoms. Clusters of these units form many secondary polyhedral building units, which are further linked to form entire three-dimensional frameworks. There are about 194 unique zeolite frameworks iden-tified so far, and over 40 naturally occurring known zeolite frameworks. The structural formula of a zeolite unit cell can be represented by:
Mx/n AlO2
x SiO2
y
.zH2O
where M is the cation such as Na+, K+, Ca2+, Mg2+and NH4+, x and y are integers and y/x≥1, n is the valence of the cation and z is the number of water molecules in each unit cell [3]. Each aluminum atom introduces a negative charge on the framework, which is compensated by an exchangeable cation. The location of cation on the framework plays a very important role in determining the adsorptive properties.
Aluminum-rich zeolites have hydrophilic nature. The transition from hydrophilic to hydrophobic occurs at a Si/Al ratio of between 8 and 10 [4]. Thus zeolite with a specific adsorptive property can be prepared by appropriate choice of Si/Al ratio and cation type. Commercially significant zeolite types are A, X, Y, beta, ZSM-5, mordenite and silicalite. Type A and X zeolites can selectively adsorb (sieve) molecules depending on their relative sizes and the pore diameter of the adsorbent. Hence they are called molecular
Table 5.2 Commercial zeolites’ characteristics Zeolite
type
Cation type
Nominal pore diameter ( ˚A)
Number of tetrahedra in a ring
Si/Al ratio
3A K 3 8 1
4A Na 4 8 1
5A Ca 5 8 1
10X Ca 8 12 1.2
13X Na 10 12 1.2
Y K 8 12 2.4
Mordenite H 7 12 5
ZSM-5 Na 6 10 31
Silicalite – 6 10 ∞
sieves. The zeolite framework has a very regular structure of cages, which are interconnected by windows in each cage. The window aperture size depends on the number of tetrahedra in a ring and also the type and number of cations present. Typical types of cation present in zeolites are alkali metal such as Na and K, alkaline earths such as Ca and Mg, transition metals such as Ti and V, and rare earths. Table 5.2 shows the characteristics of some of the commercial zeolite types.
Zeolites are usually manufactured by hydrothermal synthesis of sodium aluminosilicate from sodium hydroxide, sodium silicate and sodium aluminate. This is followed by ion exchange with cations and drying of the crystals, which can be pelletized with a binder to form macroporous pellets. By controlling the pH, temperature and concentration, different types of zeolites are produced. There are vast amount of literature reviews available on zeolites synthesis [23–26]. Recently, Chalet al. [27] presented a review on various synthesis strategies towards zeolites with mesopores.
5.4.4 Activated alumina
Activated alumina is produced from hydrated alumina (Al2O3·3H2O) by thermal dehydration and recrys-tallization. The effective surface area of activated alumina varies from 250 to 350 m2g−1 [3]. The surface is more polar than silica gel because of abundant Lewis acid sites (Al3+sites). γ-alumina andη-alumina have very high acid sites (both Lewis and Brønsted) because of spinal defect forms. The pore structure and surface chemistry can be tailored by controlling the heat treatment conditions. Alumina tailored to have high Lewis acidity and low Brønsted acidity are found to be selective adsorbents for oxygenates such as alcohols, aldehydes, ketones, and carboxylic acids [3]. Recently, Candelaet al. [28] patented a process in which activated alumina was found to be a very selective adsorbent for different oxygenate impurities present in tertiary butyl alcohol.
The adsorptive characteristics of zeolites, aluminas, and in some instances silicas are also used very effectively in catalysis to transport desired materials into catalytic sites that are configured deliberately into these materials to enable a reaction to take place. The actual adsorption and desorption steps for reagents are an integral part of the activity of a supported heterogeneous catalyst. Hence the opportunities for process intensification based on adsorptive behavior are exciting as they may apply to biorefineries 5.4.5 Polymeric resins
Polymeric resins are macroporous or macroreticular polymer beads. Most of the commercial resins are made from styrene/divinylbenzene (DVB) copolymers. Other than this, polymers of acrylates, methacrylates,
Table 5.3 Properties of typical commercial polymeric adsorbents (where available) Trade
name
Chemical name
Ionic functionalization
Nominal pore diameter ( ˚A)
Specific surface area (m2g−1)
Supplier
Dowex®Optipore L-493
Poly(styrene-co-DVB)
None 46 1100 Dow
Dowex®Optipore SD-2
Poly(styrene-co-DVB)
Tertiary amine (weak base)
50 800 Dow
Diaion HP-20 Poly(styrene-co-DVB)
None 260 500 Mitsubishi
Chemicals
Diaion HP-2MG Poly(methacrylate) None – 500 Mitsubishi
Chemicals AmberliteTM
XAD-4
Poly(styrene-co-DVB)
None 100 750 Rohm and
Haas AmberliteTM
XAD-16N
Poly(styrene-co-DVB)
None 150 800 Rohm and
Haas AmbersorbTM
XE-563
Carbonaceous – 38 550 Rohm and
Haas Purolite®PD206
Poly(styrene-co-DVB)
Sulfonic acid – – Purolite
and vinylpyridine are also used as adsorbents. Sometimes, functional groups such as sulfonyl groups are attached to the benzene ring of these polymers and they are called ion exchange resins. Polymeric resins are usually available in the form of spherical beads and the size usually ranges from 0.3 to 1 mm in diameter. Each resin bead consists of large number of small “microbeads” joined together forming a macropore structure. These microbeads are made of microgel particles ranging in size between 0.01μm to 15μm [29]. The degree of cross-linking determines the micropore structure of these microbeads and also provides the high surface area and structural strength. The unfunctionalized polymeric resins are more hydrophobic than activated carbon because of presence of aromatic rings on the surface. The properties of some of the commercial resins are given in Table 5.3.
The distinct advantages of polymeric resin adsorbents are: greater phase stability (physically, chemi-cally, and biologically), improved biocompatibility, complete immiscibility with the adsorbate medium, elimination of emulsification, and an increased potential for re-use [30]. There are a few drawbacks: they tend to shrink and swell on cyclic re-use and they are costlier than common available adsorbents. In some cases, better performance compensates for the resin cost.
5.4.6 Bio-based adsorbents
A wide range of agricultural materials are used as adsorbents. These bio-based adsorbents can be classified into starch-based and lignocellulosic adsorbents [31, 32]. Some of the starch-based adsorbents are corn grits, cornmeal, cooked corn, starch and other grains. Examples of lignocellulosic adsorbents include rice straw, bagasse, wheat straw, wood chips, and corn cob. Many of them have been reported to have potential application in biofuel downstream processes such as ethanol dehydration [33]. Bio-based adsorbents have many advantages over molecular sieves; for example, molecular sieves are highly selective, but water is very strongly adsorbed and high temperatures and/or low pressures are required to regenerate them [34], whereas, bio-based adsorbents have lower separation capacity than molecular sieves, but their regeneration
temperature is much lower than molecular sieves. In addition, bio-based adsorbents are much cheaper than molecular sieves. The bio-based adsorbents can also be used as feedstock for upstream fermentation after saturation and thus avoiding pollution through disposal. Some of the disadvantages are the inherent variability of resources, supply fluctuation due to seasonal variation, large bulky nature, and thus constraint over transportation logistics.
5.4.7 Metal organic frameworks (MOF)
The potential of zeolitic imidazolate frameworks (ZIF), a MOF, to be a potential adsorbent for recovering alcohols from aqueous solutions has been recently explored [35]. The structure of ZIF is analogous to Zeolites, where tetrahedral Si (Al) and the bridging O are replaced with transition metal ion and Imidazolate link, respectively. Particularly ZIF-8 has been reported to have an exceptional thermal stability (up to 550◦C) and chemical stability in organic solvents like benzene, water and boiling alkaline water. Yaghi and co-workers synthesized this material by heating a solution of zinc nitrate and 2-methyl imidazole in dimethyl formamide [36]. The pore diameter, pore volume and surface area of ZIF-8 are 11.6 ˚A, 0.663 m3kg−1 and 1 947 000 m2kg−1 respectively.