الفهرس 
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Abstract
In an attempt to improve the performance of strontium supported on silica catalyst in methanol transesterfication of vegetable oil to biodiesel different types of silica were used as support. Herein, three different silica supports were prepared by using different types of alkyl amine (Am) surfactants ( viz., n - hexadecylamine, n - octadecylamine, and cetyltrimethyl ammonium bromide (CTAB)) and tetra ethylorthosilicate (TEOS) as precursor, where these silica samples were denoted as Si-Ahexa, Si-Aocta, and Si-Actab, respectively. In addition, fourth traditional silica sample was prepared using sodium silicate and the sample denoted as Si-A0.
Catalyst samples are prepared of strontium oxide (SrO) loaded with 40 wt. % on these four samples of silica by using deposition method to produce a nano-structure strontium particles on the surface. In addition, strontium supported on Si-Aocta catalysts in different weight percentage loads (viz., 20, 40, 60, and 80 w/w %) were prepared using the same deposition procedure. Generally the obtained catalysts were denoted as Sr(wt.%)/Si-Am. Where, Sr(wt.%) indicates strontium oxide weight percentages loaded on different types of silica between 20 to 80%, Si is denote silica (SiO2), and Am referred to alkyl amine group. The main four catalysts were denoted as Sr40/Si-A0, Sr40/Si-Ahexa, Sr40/Si-Aocta, and Sr40/Si-Actab. Furthermore, Sr40/Si-Aocta catalysts were promoted by 10 wt.% alkaline earth metals using dry impregnation. These catalyst was denoted MSr/Si-Aocta , where M is alkaline earth metals (M= Ba,Ca, and Mg).
The physico-chemical characteristics and acid/base properties of different samples were investigated by using different physicochemical techniques (viz., N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), potentiometric acid/base titration and Boehm titration). On the other hand, the catalytic activity of the different catalysts was investigated in methanol transesterfication of sunflower oil to biodiesel (MTSFO), where the reaction was carried out in a batch reactor. The standard reaction conditions are: methanol to oil molar ratio 30:1, a catalyst to oil ratio 1.0 wt. %, a reaction temperature 60 °C and a reaction time 5.0 h. In addition, a series of catalytic reaction tests are carried out over the best catalyst to identify the optimum reaction parameters (viz., temperature, time, catalyst to oil ratio (wt.%) and methanol to oil molar ratio).
Four phases namely, SrO, SrSiO3, α-SrSiO3, and Sr(OH2)xH2O in addition to SiO2 were detected in the diffraction patterns of most of Sr(wt.%)/Si-Am catalysts. However, SrSiO3 is the main phase detected in diffraction pattern of Sr40/Si-Aocta catalyst, in other words SrO phase was not detected in this sample.Which can be attributed to a stronger interaction between strontium and Si-Aocta than other silica supports.
from the surface textural study by low temperature adsorption of N2, different types adsorption isotherms were found for silica supports understudy. Where, the adsorption isotherms obtained are mainly of type IV for Si-A0 sample and type II for Si-ACTAB, Si-AOCTA, Si-Ahexa samples. In addition, different types of adsorption/desorption hystersis were observed, which is H1 type for traditional Si-A0 and there are of type H3 for Si-ACTAB , Si-AOCTA and Si-Ahexa. These results indicated that different pore system and textural characteristics were found to exist in different types of studied silica supports. In the case of Sr(wt.%)/Si-Am and MSr/Si-Aocta catalysts a significant masking effect namely, decrease of BET surface area and pore volume of different types of silica supports were observed. The masking effect of silica supports was interpreted in view of penetration of strontium particles and alkaline metal particles into the pore system beside the possibility of incorporation of some others in the grid of silica structure through bonding with terminal oxygen of silica.
Textural study by SEM indicated that addition of different types surfactants during preparation of different types of silica supports decreased their particles size in comparison with that of traditional silica, it is well known surfactant is well dispersing agent. Deposition of alkaline metals (M = Ba, Ca, and Mg) on Sr40/Si-Aocta sample produced aggregation of Si-Aocta grains and the net structure of these catalysts views as bright clusters of Sr and alkali metals embedded between and associated with the disordered and scattered particles of silica.
The results of potentiometric acid/base titration namely, estimation of amount of base ΔΓH+, amount of acid ΔΓO-, and point of zero charge (P.Z.C.) of samples indicate that the supported Sr(wt.%)/Si-Am catalysts have mainly basic characteristic in comparison with neat silica supports. In addition, when Sr40/Si-Aocta sample doped with different alkaline metals, the basic region in the titration curves is more significant than the acid-region, which can confirm that our samples have higher basicity.
Bohem titration results depicted that, Sr40/Si-A0, Sr40/Si-Aocta, Sr40/ Si-ACTAB catalysts have basic properties. However, Sr40/Si-Ahexa catalyst has both higher amounts of basic and acidic sites. When Sr40/Si-Aocta catalyst modified by alkaline earth metals (M = Ba, Ca, and Mg) the amount of basic sites decreased and small amount of acidic sites was measured. This may due to the interaction of alkaline earth metals with strontium which lead to release of strontium from the silica surface and exposure apart of the acidic silica surface.
In general, Sr40%/Si-Ahexa and Sr40%/Si-Aocta catalysts exhibited the best catalytic activity in sunflower oil transesterfication, giving oil conversion of 74.64 % and 41.82 %, respectively. from the acidity/basicity results, it seems that Sr40/ Si-Ahexa sample is the most activity in MTSFO because it has both higher amounts of basic sites and acidic sites. However, the Sr40%/Si-Aocta catalyst is the most stable under the mentioned above reaction condition. This high stability was verified from low dissolution of strontium in the reaction solution by using ICP analysis and also low homogenous catalytic activity of this sample in comparison to heterogeneous one. The observed higher stability of Sr40/Si-Aocta might be due to it has more basic sites, which can interact strongly with strontium. This suggestion of strong interaction between strontium and Si-Aocta was confirmed from SEM and XRD results. Where, it was observed that strontium overlaid and covered homogeneously Si-A-octa surface and the Sr40/Si-Aocta catalyst contains SrSiO3 as main phase. Thus, the above results recommended the selection of Sr40/Si-Aocta catalyst for further catalytic activity optimization investigation in MTSFO in this work.
On the other hand, MSr/Si-Aocta (M = Ba, Ca, and Mg) catalysts exhibited higher catalytic performance in MTSFO than parent Sr40/Si-Aocta. Where, BaSr40/Si-Aocta catalyst depected the best activity in MTSFO with oil conversion of 97.7 % .The significant higher activity of BaSr40/Si-Aocta towards oil conversion was attributed to that the addition of Ba to Sr40/Si-Aocta sample created acidic sites in addition to the basic sites already exist in the sample. Furthermore, BaSr40/Si-Aocta catalyst could be easily reused for at least four reaction cycles without significant loss in catalytic activity.
Finally, the effect of doping of strontium/silica (Sr40/Si-Aocta) catalyst by barium on its kinetics of methanol transesterification of sunflower oil (MTSFO) to biodiesel was investigated. The experimental data of MTSFO over Sr40/Si-Aocta appear to be good fit into pseudo first order kinetic mechanism. However, the zero-order rate equation is the best fit model for MTSFO over BaSr40/Si-Aocta, compared with the pseudo-first order and second-order rate kinetic models. On the other hand, the intraparticle diffusion kinetics model may also play an important role in the MTSFO over BaSr40/Si-Aocta. In addition, a significant change was observed in Arrhenius activation energy of MTSFO over Sr40/Si-Aocta when it doped by Ba. Where, Arrhenius activation energy values were found to be 16.6 and 49.6 kJ mol-1 for Sr40/Si-Aocta and BaSr40/Si-Aocta , respectively. These values suggested that this catalyzed reactive system was chemically reactive rather than mass transfer controlled. In the other words, the methanol transesterification of SFO reaction over these catalysts are heterogeneous.