Format And Type Fonts CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545216 Please cite this article as: Ayoub M., Inayat A., Rashid U., Ullah S., Lal B., 2015, Synthesis of stable lithium modified mesoporous catalyst for oligomerization of biodisel-drive glycerol to diglycerol, Chemical Engineering Transactions, 45, 1291-1296 DOI:10.3303/CET1545216 1291 Synthesis of Stable Lithium Modified Mesoporous Catalyst for Oligomerization of Biodisel-Drive Glycerol to Diglycerol Muhammad Ayoub* ,a,b , Abrar Inayat a , Umer Rashid c , Sami Ullah a , Bhajan Lal a a Chemical Engineering Department, Universiti Teknologi PETRONAS, 31750, Tronoh, Perak, Malaysia b Petroleum Engineering Department, Universiti Teknologi PETRONAS, 31750, Tronoh, Perak, Malaysia c Institute of Advanced Technology, University Putra Malaysia, 43400, Serdang, Selangor, Malaysia muhammad.ayoub@petronas.com.my Biodiesel is an alternative fuel option for future, which comes from pure renewable resources. The rapid growth of the biodiesel industry will result in overproduction of low value glycerol and create a superfluity of this impure by-product. The synthesis of lithium modified mesoprous catalyst for dilyglycerol production via oligomerization of low value biodiesel drive glycerol is reported. This reaction of selective glycerol conversion to diglycerol was studied in a heterogeneous catalysis under solvent free system, using alkaline mesoporous catalysts. Mesoporous materials SBA-15 was synthesized by using P123 and TEMOS as the template. Lithium in the form of LiOH was loaded over prepared SBA-15. The basic strength of prepared samples of SBA-15 was found increasing but structure of mesoporous was found destroying after LiOH loading. The mesoporous structure of prepared SBA-15 was found stable after doping it with 20 % alumina before loading with LiOH. This stability of mesoporous structure was observed very sensitive with alumina doping. These samples were well characterized by surface area, pore volume, pore size, basic strength, and transmission electron microscope (TEM) measurement of prepared samples. The glycerol conversion and dilyglycerol production yield was noted maximum 92 % and 74.5 % over prepared stable lithium modified SBA-15 catalysts. Industrially, the findings attained in this study might contribute towards promoting the biodiesel industry through utilization of its by-products. 1. Introduction The development of value added chemicals production requires new basic heterogeneous catalysts as well as optimisation of the materials used at present. The great majority of studies involving the catalytic activity of basic catalysts have used either alkali-exchanged zeolites or zeolites impregnated with sodium metal clusters or alkali oxides (Dartt and Davis, 1994). Ordered mesoporous silicas, such as MCM-41 (Kresge et al., 1992) and SBA-15 (Zhao et al., 1998) are attractive materials because of their high surface area and ordered arrays of uniform channels. There are many research reports on their separation, catalysis, etc. Among the ordered mesoporous materials, SBA-15 is a unique; it has ordered mesopores as well as disordered interchannel micropores in the mesopore wall (Yang et al., 2003). In addition, Zhang and co-workers were also found these type of structural observations related to micropore and mesopore walls with in the material of SBA-15 (Zhang et al., 2006). Current research is directed towards the design of mesoporous material SBA-15 for basic components (Glocheux et al., 2013). The main problem with these materials is their poor alkali-resistance correlated with different factors. Alkaline metal oxides are often used to improve the base strength of a solid catalyst due to their strong basic behaviour. Mesoporous solid strong bases can be prepared by impregnation of mesoporous material with alkali metals or alkaline earth metals. Some previous researchers (Kloestra et al., 1997), worked on preparation of strong bases mesoporous solid especially MCM-41 with cesium acetate solution. Although, the prepared bases materials showed poor stability because cesium oxide can react with the silica host and damage the mesoporous frame works (Kloestra et al., 1997). As lithium is strongest alkali metal in first alkaline metal group and stand at top position due to lighter. It can be used as the guest to generate strong basicity on various porous hosts, such as MCM-41 and zeolite (Clacens et al., 2002). Aiming at forming 1292 strong basic strength on mesoporous silicas, SBA-15 was introduced as the host to disperse LiOH. The obtained material exhibited considerable basic strength but the mesostructure of SBA-15 was destroyed completely during preparation process of sample as our previous study (Muhammad Ayoub and AZ Abdullah, 2011). Hence, generation of strong basicity on mesoporous silicas with stable structure of mesoporous material is still an open question up until now. There are two main factors which are considered to hinder the generation of strong basicity on mesoporous silicas. First factor is the weak host-guest interaction between silica and the base precursors, which leads to the difficulty in the decomposition of the base precursors to strongly basic species in the form of their oxides. It is already reported that only a small amount of alkali salt can be decomposed on silica even if the sample was activated at the high temperature of 600 °C (Sun et al., 2008a). The second factor is the poor resistance of mesoporous silicas against alkali. These are factors which results in the collapse of the mesoporous structure after the formation of strongly basic species (Sun et al., 2008b). The objective of this work is to demonstrate the effects of strong metal hydroxide on the porous structures of SBA-15. The basic and structural properties of the obtained materials were well characterized by various approaches. It is verified that the mesoporous structure of SBA-15 was totally destroyed by adding 10 wt % of LiOH over SBA-15 but basic strength of material was enhanced on this newly prepared fluffy structure of SBA-15. This structure was stabilized with alumina coating of suitable amount prior to lithium loading. This prepared basic catalyst was observed for selective conversion of glycerol to diglycerol over oligomerization process. 2. Experimental Methods Materials Synthesis: Mesoporous silica SBA-15 was synthesized according to the reported method as follows Zhao et al., (1998). Briefly, 4 grams of triblock copolymer P123 (EO20PO70EO20, M = 5,800, Aldrich) was dissolved in 90 ml of water and 60 ml of 4M HCl aqueous solution with stirring at 40°C for 2 h. A 8.5 g portion of tetraethyl orthosilicate (TEOS) was then added to the homogeneous solution and stirred at this temperature for 22 h. Finally, the temperature was heated to 100 °C and held at this temperature for 24 h under a static condition. The prepared sample was recovered by filtration, washed with water, and air- dried at room temperature. The removal of the template was carried out at 550 °C in air for 6 h. Alumina coating over prepared material SBA-15 was performed using an impregnation method. The required amount of alumina precursor, i.e. aluminium nitrate was dissolved in 10 mL of deionised water and 10 mL of ethanol. After complete dissolution of magnesium nitrate, 2 g of the previously prepared SBA-15 support was added and kept under stirring for 24 h. Then, it was allowed to settle for 4 h. The mixture was subsequently dried using a rotary evaporator and then air dried in an oven at 100 °C for 4 h. The prepared sample was then calcined at 550 °C in air for 6 h. The resulting samples are denoted as Alx/SBA-15 where x represents the calculated mass percentages 10 or 20 wt% of alumina. The alkali metal LiOH, was introduced by wet impregnation. An identical amount of LiOH (namely 10 wt %) was used for all samples. The required amount of LiOH was dissolved in deionized water, followed by addition of host SBA-15 or Alx-SBA-15. After stirring at room temperature for 24 h, the mixture was evaporated at 80 °C and subsequently dried at 100 °C for 4 h. The obtained solid was calcined at 550 °C for 5 h in air. The resulting samples are denoted as LiOH10/Alx-SBA-15. The catalysts were characterized using BET surface area, pore volume, basic strength and TEM images techniques. The N2 adsorption- desorption isotherms were measured using a Belsorp II system at - 196 °C. The samples were degassed at 300 °C for 4 h prior to analysis. The BET surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The pore diameter was calculated from the adsorption branch by using the Barrett-Joyner-Halenda (BJH) method. Base strengths of the catalysts (H_) were determined using Hammett indicators. The TEM images of the prepared samples were analysed using a Phillips CM 12 transmission electron microscope equipped with an image analyser and operated at 120 kV. 3. Results And Discussion Nitrogen adsorption-desorption isotherms of the LiOH on SBA-15 in different metal loadings are given in Figure 1. These data were obtained directly from the nitrogen adsorption-desorption isotherms characterized by a sharp nitrogen uptake at high relative pressure. The isotherm showed that the prepared SBA-15, Al10/SBA-15 and LiOH10-Al20/SBA-15 samples exhibited nitrogen adsorption-desorption behaviours that are in agreement with uniform mesoporous ordering, all in terms of type IV with H1 hysteresis loop at the high relative pressure. With hysteresis loop of type H1, the two branches are almost vertical and nearly parallel. Such loops are often associated with porous materials which are known to 1293 have very narrow pore size distributions or agglomerates of approximately uniform spheres in a fairly regular array (Ayoub and Abdullah, 2014). On the other hand, the shapes of the curves for other LiOH10/SBA-15 and LiOH10-Al10/SBA-15 samples do not agree with the type IV isotherm but more of type I that is a characteristic of microporous material. Hence, LiOH loading led to the collapse of some mesoporous structures of SBA-15 which causes a drop in porosity and exhibiting a decrease in the hysteresis loop (Chris et al., 2003). This porosity was not sustained even 10 % alumina coating before lithium loading. Although, after 20 % alumina coating prior to lithium loading sustained the structure of SBA-15 and showed comparative high porosity. Therefore, it can be concluded from this figure that 20 % alumina coated SBA-15 support successfully preserved its mesoporous structure after loading of LiOH. Relative Pressure (P/Po) 0.4 0.5 0.6 0.7 0.8 0.9 1.0 V o lu m e A d s o rb e d ( c c S T P /g ) 0 100 200 300 400 500 SBA-15 LiOH10/SBA-15 Al10/SBA-15 LiOH10/Al20-SBA-15 LiOH10/Al10-SBA-15 Figure 1: Nitrogen adsorption-desorption isotherms for different prepared materials The physical characteristics of prepared materials are shown in Table 1. It can be seen in this table that the basic strength of the prepared samples increased after loading alkali metal LiOH (10 wt%). The basic strength of SBA-15 was noted H_ < 4.0, however after loading 10 wt % LiOH, it was noted 4.8 < H_ < 9.3. This basic strength of prepared materials was found further increased by coating alumina prior to lithium loading (LiOH10-Al10/SBA-15) which was noted 9.3 < H_ < 15. The basic strength of Al10/SBA-15 was noted in the range of 4.8 < H_ < 6.8. In the same table it is also clear that surface areas of prepared samples were sharply decreased after loading alkali metal over SBA-15. A decrease in surface area, pore size and pore volume after coating with alumina and loading LiOH can be further observed from the data given in this Table 1. As clearly noted, the BET surface area, mesoporous area and pore dia of the prepared samples LiOH10/SBA-15 and LiOH10-Al10/SBA-15 was sharply decreased while it was significantly persistent by LiOH10-Al20/SBA-15. The decrease in surface area and pore size might happen due to destruction of mesoporous structure of SBA-15 support in the presence of strong alkali lithium which cannot preserve even 10% coating of alumina prior to this lithium loading (Ayoub and Abdullah, 2011). The surface area of sample Al10/SBA-15 was found to slightly decreased after coating with alumina. Sample LiOH10-Al10/SBA-15 also showed sufficiently high surface area, mesoporousity and pore dia which was found to be sufficient compare to the parent SBA-15. It is determined that mesoporous area and pore volume also sharply decreased along with decreasing surface area of these samples. Obviously it is indicating that there is no anymore mesoporous structure present inside these samples. Overall, the surface area analysis of these samples revealed that the mesoporous structure of samples LiOH10/SBA-15 and LiOH10-Al10/SBA-15 were totally destroyed while this structure seemed to be still intact in LiOH10- Al20/SBA-15 sample. 1294 Table 1: Physical characteristics of prepared materials with different metal loadings Sample Basic Strength (H_ ) BET surface area (m 2 /g) Mesopore area (m 2 /g) Pore size (A) Pore volume (cc/g) SBA-15 H_ < 4.0 674 571 59.3 0.73 LiOH10/SBA-15 6.8