A Review: Mesoporous Santa Barbara Amorphous-15, Types, Synthesis and Its Applications towards Biorefinery Production

Problem statement: Santa Barbara Amorphous (SBA-15) is significant me soporous silica with exclusive and important properties of highly o rdered mesopores, hydrothermally stable and thick wall, profusely large surface area and huge pore vo lume which render it as promising catalyst for wide applications. However, the purely siliceous SBA-15 which lacks of acidity characteristic hinders its ideal capabilities as catalyst. Moreover, functiona lization and modification of SBA-15 could enhance and optimize its catalytic activity. Conclusion/Recommendations: Thus, in this review, the various types and different synthesis of modifying SBA-15 a re discussed in detail towards its application in biorefinery production. The catalytic activities in various operating conditions and reactions are als o reviewed for future reference and scope of studies.


INTRODUCTION
Mesoporous material, as defined by IUPAC nomenclature, is material with pore of free diameters in the range of 2-50 nm (McCusker et al., 2003). Generally, mesoporous materials is defined with the term of host, pores and chemical composition of its host and guests (Liebau, 2003). Mesoporous materials is more preferred for its highly ordered mesostructure, apart from high surface area, which allows diffusion and adsorption of larger molecules for wide applications than that of microstructure which only limits to separation and fine chemical catalysis (Fedeyko et al., 2006). Moreover, microporous material has selective thermal stability based on size and shape of cage or channel within lattice of selected organic moieties during synthesis, which restricts its potentiality to limited applications (Morey et al., 1998). Mesoporous materials are widely applied in catalysis (Abdalla et al., 2009), drug control delivery (Pasqua et al., 2009), biosensors (Xu et al., 2009b), biofuel (Ooi and Bhatia, 2007), sorption (Chang et al., 2008) and membrane separation (Zornoza et al., 2009). Typical compounds of mesoporous material include silica (Xia and Mokaya, 2003), alumina (Xu et al., 2009a), carbon (Chen et al., 2009)and transition metal oxides (Rao and Antonelli, 2009).
Silica is widely employed as main building block of mesoporous materials because it is inexpensive, thermally stable, chemically inert, harmless (Nandiyanto et al., 2009) and abundantly available in the Earth's crust (Draggan, 2008). Unlike zeolitic materials which are silica self-assembles reliance upon heating, mesoporous silica is formed in the presence of selfassociating molecules such as cationic surfactant for MCM-41 synthesis and block copolymers for synthesis of SBA-15 (Liebau, 2003). Figure 1 shows typical synthesis of mesoporous silica MCM-41.
Mesoporous silica was first discovered in 1990 by researchers in Japan (Yanagisawa et al., 1990) and later was produced in Mobil Corporation Laboratories (Chen et al., 2009) and named as M41S. MCM-41, MCM-48 and MCM-50 are among the widely known mesoporous silica in M41S family (Vartuli et al., 2008) which have different methods of synthesis and applications based on instability and limitation of mesoporous structure (Tatsumi et al., 1999, Zhang et al., 2007.  (Vartuli et al., 2008) These mesoporous silica are highly ordered, large specific surface area, hexagonal array and uniform mesoporous channel (Chen et al., 2004, Tatsumi et al., 1999. Especially for MCM-41 and MCM-48, the pore size can be controlled and widely used as adsorbent, catalyst and support (Tatsumi et al., 1999). MCM-41 is in hexagonal packed rod-shaped micelle structure, whereas MCM-48 is in cubic and MCM-50 is in lamellar form (Vartuli et al., 2008) which is depicted in Fig. 2. Later in 1995, the research focus was concentrated on mesoporous molecular sieves like HMS (Tanev et al., 1995) and MSU (Bagshaw et al., 1995). In 1998, a prominent research which produces hexagonal array of pores namely as Santa Barbara Amorphous no 15 (SBA-15) with larger pore size from 4.6-30 nm was a research gambit in mesoporous material development (Zhao et al., 1998b). Not only because of its larger pores, but also thermal, mechanical and chemical resistance properties that makes it a preferable choice to other types of catalysts.
According to Zhao et al. (2000) the formation of ordered hexagonal SBA-15 with uniform pores up to 30 nm was synthesized using amphiphilic triblock copolymer in strong acidic media i.e., pH ~1. If pH is more than the isoelectric of silica i.e., at pH 2-6, no precipitation or formation of silica gel would occur. Disordered or amorphous silica would likely to be happened at neutral pH of 7 (Zhao et al., 1998b). Nevertheless, Cui et al. (2005) have proved to synthesize SBA-15 at pH 2-5, which is above isoelectric point of silica. They suggested that the prehydrolyzed of TEOS at pH<3 would interact with template agent to form a mesophase under weak acidic condition (Cui et al., 2005).
The most recent study is the synthesis of highly siliceous SBA-15 from low silica inorganic precursor with the presence of impurities of industrial waste product which typically consist of metal oxide containing 42 wt% SiO 2 . The method used in this study was claimed to be cost effective for bulk production of mesoporous silica since it used industrial waste as silica precursor and no pre-purification of precursor nor material washing steps were applied (Puputti et al., 2009).
Template removal is one of the crucial aspects in ordered mesoporous silica synthesis in which this procedure could modify the final properties of desired porous structure. The usual method of removing template is calcination. According to Zhao et al. (1998a) the calcination of mesoporous structure SBA-15 at 500°C would produce final properties of porous structure with interlattice d spacing of 74.5-320 angstroms between the (100) planes, pore volume fraction up to 0.85 and silica wall thickness of 31-64 angstroms (Zhao et al., 1998a). In preference to studying the final properties of porous structure after calcination, other researchers focused on the calcination behavior during the process of template removal. Kleitz et al. (2003) studied the in situ XRD pattern, intensities reflections and d spacing values of SBA-15 calcination up to 550°C and cooling to room temperature after 5 h (Kleitz et al., 2003). Besides mesoporosity structure, SBA-15 was found to contain micropores which indicated the hierarchical template of material structure (Schmidt-Winkel et al., 1998). These microporous structures are generally disordered and provide interconnectivity between ordered mesopores (Brady et al., 2008).
Modifying SBA-15: Despite promising properties of SBA-15 in catalysis, biomolecule separation, etc, there is still a need to modify or functionalize SBA-15 i.e., to overcome its limitation of low acidity strength (Yue et al., 2000), enhance and optimize its catalytic activity by supporting    (Grieken et al., 2005), reusability (Liu et al., 2009) and enzymatic activity (Takimoto et al., 2008). Besides, SBA-15 is one of the best candidates to be modified and functionalized since it has controllable pore size, pore volume and high surface area (Liu et al., 2008). Deposition of titanium in SBA-15 material render superior mechanical and chemical resistance (Liu et al., 2008) while activation SBA-15 with different types of metal would develop and design large pore catalysts which allow high diffusion rates (Busuioc et al., 2006). Recently, the attachment of organic functional group has caught attention for further development in which this could tune the hydrophilic-hydrophobic character of silica surface to give sorbent chemical selectivity for specific molecules or ions (Li et al., 2008).
From available literature articles, modified and functionalized SBA-15 can be categorized into different types: supported SBA-15 i.e., Pd-Zn, Co (Eswaramoorthi et al., 2009, Prieto et al., 2009 • Different types of metal incorporation into SBA-15 framework i.e., Al, Ce, La, Ti, Mg, Ca , Kim et al., 2004, Yue et al., 2000 Synthesis: Methods: Various methods of synthesis have been employed, which includes direct synthesis, post synthetically impregnation and grafting, sol-gel as well as immobilization (Cui et al., 2005, Grieken et al., 2005, Li et al., 2009, Carrero et al., 2007. Table 1 list down different types of modifying SBA-15 and syntheses. However, only direct synthesis and post grafting method will be discussed in this review. Direct synthesis: Direct synthesis of modifying SBA-15 engaged the co-condensation of silica source i.e. TMOS, TEOS etc. with organotrialkoxysilane with presence of different templating agents. Apart from silane group, this method was then further explored and developed by co-condensing silica source with different groups of metal i.e. transition, alkaline earth, rare earth and poor metals. Phenyl-functionalized-SBA-15 under acidic conditions was concertedly explored by Du et al. (2006). The phenyl-modified-SBA-15 was prepared using direct synthesis method by co-condensation of different molar ratios of tetraethylorthosilicate (TEOS) and phenyltrimethoxysilane (PTMS). It was worthful to note that even at high molar ratio of TEOS: PTMS, the desirable hexagonal structure, large surface area and pore volume were still preserved besides its thermally stable and highly hydrophobic properties (Du et al., 2006) .
A study of Co-SBA-15 was investigated under different pH adjustment using sodium hydroxide, in which under optimum pH of 3, high surface area and pore volume of Co-SBA-15 were demonstrated (Lou et al., 2008). In other work of transition metal incorporation, the effect high loading zirconium in the direct synthesis of Zr-SBA-15 was probed into its catalyst property as well as catalytic activity. The study reported that high ratio of Zr/Si established a high order mesostructure of Zr-SBA-15 and additional sulfation process increased its catalytic activity (Fuxiang et al., 2007).
From the recent studies presented here, they are consistent with the argument reported by Zhao et al. (1998a) that direct synthesis proposes a uniform dispersion of functional groups on material surface and structure which preserve its surface properties (Zhao et al., 1998a).

Post-grafting:
Post-grafting was the first method used to modify mesoporous silica materials. Basically, this method involves reaction between organosilane with silanol group using solvent under reflux condition to form covalent attachment of functional groups on surface of material (Chong et al., 2004).
It has been reported that post-grafting method has its downside aspects. Among disadvantages of this method includes the reductions of pore size and pore volume caused by the attachment of functional group on pore surface (Lim et al., 1999), density limitation of reactive silanol group that would limit the loading of functional group on pore surface in which would further cause cross-linking between functional groups and silanol groups on surface (White and Tripp, 2000), difficulty in achieving uniformity of the functional group and tedious process which involves more than one step of preparation (Mercier and Pinnavaia, 1999). Nevertheless, Ma et al. (2010) has employed postgrafting method in incorporating aluminum into SBA-15 to observe the effect towards structural, textural and acidity of the catalyst. They discovered that, in comparison with direct synthesis method, the acidity and catalytic activity of Al-SBA-15 synthesized by post-grafting method has shown to be higher due to One-step post SBET m −2 g −1 = 524 After functionalization, XRD patterns show the hexagonal structure still remains (aminopropyl-SBA-grafting VP cm −3 g −1 = 0.60 intact. However, the d(100) spacing increased as shown in (100)  Direct synthesis SBET m −2 g −1 = 1004 After various treatment i.e. acidic, basic, neutral and vapor medium, it could be concluded (Yue et al., 2000) VP cm −3 g −1 = 1.53 that Al-SBA-15 is more stable than pure SBA-15 in which the pore size, d(100) spacing, wall Dp nm −1 = 7.4 thickness still remain the same. However, specific surface area and pore volume decrease d (100) (Liu et al., 2009) alcohol from reaction time 10 min -buten-l-ol 96.3% 3-en-2-ol 97.4% from 1allylation-Conversion of benzaldehyde phenyl-3-buten-l-ol isomerization of 98.8% Selectivity of 4-phenyl a.1-phenyl-3-buten--but-3-en-2-ol 95.5% l-ol b. benzaldehyde from bezaldehyde  et al., 2010). In previous work by Zeng et al. (2005) the activity of Al-SBA-15 was proved to be affected by hydrolysis ratio and aluminum concentration which could be correlated to various ratio of Si/Al (Zeng et al., 2005). This was substantiated by research work of the post-grafting in the synthesis of Al-SBA-15 with various ratio of Si/Al which could affect the catalytic activity and stability of the catalyst as well as exhibited better conversion and selectivity (Wu et al., 2006).
The effect on structure and psychochemical properties: Particular interest in synthesizing SBA-15 is also focused on the effect of structure, physical and psychochemical properties of SBA-15. The initiative to improve the adsorption characteristics of hexagonal has been successfully shown by Colilla et al. (2007) by enlarging the surface area of hexagonally ordered SBA-15 (Colilla et al., 2007). The addition of weak acid H 3 PO 4 together with strong acid hydrochloric in varied composition contributed to the increment of surface area, mesopores and micropore volume of SBA-15, which markedly characterized by N 2 adsorption, Brunauer-Emmett-Teller and Small Angle X-ray Diffraction.
The immobilization of Porcine pancreatic lipase on rod-like SBA-15 was investigated to have technical understanding on its adsorption and catalytic activity (Li et al., 2009). The concerted investigations of the effect of pH medium and thermal stability were studied to find the optimum conditions of this biocatalyst activity. It comes to comprehension that the immobilized Porcine pancreatic lipase on rod-like SBA-15 has maximum activity in pH medium of 8, but decreases at the pH medium of 9. This could be suggested by the shifting of protons partitioning between bulk phase and enzyme microenvironment in optimum medium of pH value dependent on the immobilization method, the structure and the charge of the mesoporous carrier.
Incorporation of titanium into SBA-15 with different methods of synthesis, different solvents application and varied amount of titanium deposited have lead to another new research finding of mesoporous material. Das et al.(2010) has successfully synthesized highly ordered 2-D hexagonal titanium incorporated into SBA-15 using non-ionic surfactant pluronic and co-solvent cyclohexane with small amount of titanium deposited (Das et al., 2010). This direct synthesis method has exhibited significant increment surface area of 924 m 2 g -1 , compare to that of pure SBA-15 with 611 m 2 g -1 . However, t his is in contrast to the findings from Kim et al. (2004) who found that the post-synthesis of Ti-SBA-15 has lead to the decrease in surface area compared to pure SBA-15 (Kim et al., 2004).  has been reported to be used widely in biofuel and value-added chemical feedstock from biomass. In the investigations of oil characteristics produced from biomass and tire, SBA-15 has shown to highly reduce the oil density by 3.67% and oil viscosity by 39.4%, significantly better than that of MCM-41 and HZSM-5 (Cao et al., 2009). A study involved different methods of deposition of Al into SBA-15 framework by direct synthesis and post synthesis in the production of biofuel from waste-used palm oil indicated comparable catalytic behavior, but regenerated post-synthesis Al-SBA-15 catalyst gave better catalytic activity and fuel yield due to better thermal stability (Ooi and Bhatia, 2007). Other study which also investigated different methods of Cu-Ni supported on SBA-15 reported that the catalyst prepared by incipient wetness impregnation showed better selectivity and conversion than that of direct synthesis method in the production of hydrogen from steam reforming bioethanol (Carrero et al., 2007). Table 2 summarizes some of the applications and catalytic activity of SBA-15 in biorefinery production.

CONCLUSION
The significant properties of SBA-15 and its modification are remarkably relevant towards the application of biorefinery production. It is important to design modifying SBA-15 with different organic, silane, silanol, sulfonic and metal functional groups by improving its structural, textural and psychochemical properties for better shape selectivity and catalytic activity with different means of syntheses and methods. The remarkable research works presented in this review would definitely assist future research work development of mesoporous SBA-15 as novel catalyst in biorefinery application. study on the performance of mesoporous SBA-15 supported Pd-Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production.