Renewable Energy 147 (2020) 275e283
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Function promotion of SO2 4 /Al2O3eSnO2 catalyst for biodiesel production from sewage sludge Rongyan Zhang a, Fenfen Zhu a, *, Yi Dong a, Xuemin Wu b, Yihe Sun a, Dongrui Zhang a, Tao Zhang a, Meiling Han a a Department of Environmental Engineering, School of Environment & Natural Resources, Renmin University of China, No. 59 Zhongguancun Street, Beijing, 100872, China b Beijing Drainage Group Huaifang Water Reclamation Plant, Intersection of Huaifang Road and Tongjiu Road, Beijing, 100076, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2019 Received in revised form 21 August 2019 Accepted 30 August 2019 Available online 31 August 2019
Catalysts are critical materials for biodiesel production using sewage sludge feedstock. In this research, different SO2 4 /Al2O3eSnO2 catalysts were prepared and characterized. The catalysts prepared at different conditions were characterized through Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), NH3temperature-programmed desorption (NH3-TPD), NH3 adsorption FT-IR, and TGA. Then, esteriﬁcation/ transesteriﬁcation reaction was performed with the lipid extracted from sewage sludge to verify the activity of these catalysts. Results showed that the catalysts with n(Al):n(Sn) ¼ 1:10 prepared by 79 wt% H2SO4 possessed the best physical properties with a large number of active catalytic sites, which led to the biodiesel yield of 73.3% (based on dried extracted crude fat) under the optimized reaction conditions at 130 C as reaction temperature, 0.8 g as catalyst loading for lipids extracted from 10 g freeze-dried sludge, 4 h as reaction time. Catalysts characterization results showed that acidity, acid sites and Al/Sn molar ratio play an important role in the activity of catalysts. The thermal properties showed that 450 C was the suitable calcination temperature for catalyst preparation. The optimized SO2 4 /Al2O3eSnO2 catalyst in this research will have a bright future in the ﬁeld of sludge production of biodiesel. © 2019 Published by Elsevier Ltd.
Keywords: Sewage sludge SO2 4 /Al2O3eSnO2 Biodiesel
1. Introduction As the by-product of wastewater treatment plant (WWTP), sewage sludge has once been ignored and has not been environmentally treated. However, with the increase disposal rate of wastewater, the amount of sewage sludge rapidly grew up [1e3]. Sewage sludge from WWTP normally contains about 80% water, considerable organic compounds, pathogenic bacteria, some heavy metals, persistent organic compounds, etc [4,5]. There are many technologies which can treat sewage sludge with merits and shortcomings [6e9]. This research would like to talk about recycling sewage sludge as one of the source for biodiesel. As a sustainable and green fuel, biodiesel has gained increasing attention. Considering the environmental ethics and cost, the raw material for biodiesel production has changed from normal biomass to organic waste [10e12]. Sewage sludge contains relatively high concentration of lipid (>10%)  and has no or negative
* Corresponding author. E-mail address: [email protected]
(F. Zhu). https://doi.org/10.1016/j.renene.2019.08.141 0960-1481/© 2019 Published by Elsevier Ltd.
cost of acquirement and abundant with a steady stream. The lipids can be extracted from sewage sludge and then be transferred into biodiesel by esteriﬁcation and transesteriﬁcation with the function of catalyst [10,14]. Our previous research found that the lipids in sewage sludge contain more than 14 big category of cellular lipids, >30 kinds of fatty acid, wax, and so on . Lipids in sewage sludge has complicated origin and contains not only glyceride and fatty acid, which were the classic reactant for esteriﬁcation/transesteriﬁcation, but also considerable ceramide, coenzyme, phosphatidylcholine, phosphatidylethanolamine (PE), phosphatidy linositol (PI), cardiolipin (CL) and so on [15,16], which have seldomly been reported to be the reactant of esteriﬁcation and transesteriﬁcation. As catalysts play a very important role in the esteriﬁcation and transesteriﬁcation reaction, many scholars have focused their research on the development of high-efﬁciency catalysts in the ﬁeld of biodiesel production from sludge sewage. Some related studies have conﬁrmed that H2SO4 is an excellent catalyst , Qi et al.  of our research group used concentrated H2SO4 to obtain 16.6% (based on total dry weight of activated sludge) yield of biodiesel from A2/O sludge, and the purity of FAMEs was 96.7%.
R. Zhang et al. / Renewable Energy 147 (2020) 275e283
Although H2SO4 exhibits good function, it can't be recovered and have to be neutralized, thereby increasing the cost [15,18]. Alkaline catalysts have been successfully applied in biomass-originated oil/ lipids (such as palm oil, rapeseed oil, etc.) in esteriﬁcation/transesteriﬁcation reaction, but they have less function in sewage sludge-originated lipids [19e22]. Our previous research tested the traditional alkaline solid catalysts such as KOH, solid KOH/activated carbon (KOH/AC) and KOH/CaO , the maximum yields of biodiesel were only 1.2%, 6.8% and 6.0% (based on total dry weight of activated sludge), respectively. According to our previous experimental data, the yield of biodiesel obtained by anion exchange resin as catalyst was only 8.1%. As a result, this research started to look for suitable catalysts in solid acid catalysts. Solid acid catalysts generally refer to the carrier materials that can load various acids. They can be broadly classiﬁed as supported solid acids, solid super acids, metal oxides and composites, cation exchange resins, and zeolite molecular sieves. These catalysts have several advantages, such as high stability, strong acidity, easy recycling, low corrosion, and simple product separation [23e26]. SiO2, Al2O3, ZrO2, SBA-15, and attapulgite are catalyst carriers that are commonly used due to their relatively large speciﬁc surface area and high catalytic efﬁciency [8,27e29]. Table 1 summarizes the catalyst function of different solid acid catalysts for biodiesel production with vegetable oil or animal fat feedstocks. It is reported that some sulfated metal oxides produce super acid materials with surface acidity and much larger surface areas than those of metal oxides without sulfate . Table 1 also reveals that solid acid catalyst with SO2 4 normally performs better functions. In 1979, Hino and Arata  ﬁrst synthesized SO2 4 / MxOy-type (M ¼ Zr, Fe, Ti, etc.) solid super acids, which show remarkable catalytic performances in the isomerization of alkanes. Since then, these solid super acids receive considerable amount of attention . 2 SO2 4 /SnO2 is one kind of SO4 /MxOy solid super acid, which has the highest acid strength; it also exhibits superior catalytic performance in various reactions . It's reported that the introduction of Al can stabilize the S species on the catalyst surface, increase the acidity of the catalysts, and improve the catalytic activity of the SO2 4 /MxOy solid super acid [51,52]. However, studies on SO2 4 /Al2O3eSnO2 catalysts used for the esteriﬁcation/transesteriﬁcation of lipids extracted from sewage sludge, as well as their own structure and other applications, are lacking. This
research mainly investigates the freeze-dried sludge obtained from the A2/O process in Beijing to produce biodiesel via SO2 4 / Al2O3eSnO2 solid acid catalysts. One purpose of this research is to optimize the preparation condition of the SO2 4 /Al2O3eSnO2 by adjusting the SO2 4 loading and the Al/Sn molar ratio, which are the most important factors to inﬂuence the performance of that catalyst. The other purpose is to optimize the reaction condition of esteriﬁcation/transesteriﬁcation of lipids extracted from sewage sludge with SO2 4 /Al2O3eSnO2 as catalysts such as reaction temperature, reaction time and catalyst loading. 2. Materials and methods 2.1. Materials The sewage sludge samples were collected from a typical big scale WWTP that had undergone A2/O process in Beijing, China. These samples are the mixed sludge of primary sedimentation and excess sludges. The basic characteristics of sewage sludges are shown in Table 2. 2.2. Catalyst preparation and characterization 2.2.1. Catalyst preparation SO2 4 /Al2O3eSnO2 catalysts were prepared via a coprecipitation method. SnCl4$5H2O and Al2(SO4)3 were dissolved in 600 ml of deionized water with n(Al):n(Sn) ¼ 1:5, 1:10, 1:15. Then, 30% NH3$H2O was added to the beaker under room temperature until the solution pH adjusted 8.0 in a fume hood. The liquid mixture was gradually changed to suspended state. After ﬁltration, the separated precipitates were suspended in 4 wt% of CH3COONH4 solution and stirred for 1 h. After secondary ﬁltration, the precipitates were Table 2 The basic characteristics of sewage sludge. Components
Moisture content(%) VS(%) Crude fat * (%) Crude protein * (%) TOC(%)
83.0 ± 0.5 65.3 ± 0.6 16.1 ± 0.1 26.2 ± 8.2 32.7 ± 0.2
Table 1 Results of biodiesel production by solid acid catalysts. Catalyst
H-type faujasite zeolite Modernite zeolite H3PW12O40/SiO2 Cs2.5H0.5PW12O40 WO3eZrO2 ZrO2eAl2O3 Al2O3/TiO2/ZnO Ar-SBA-15 Anion/cation exchanged resin SiO2eSO3H/COFe2O4 AlCl3,6H2O SO2 4 /ZrO2
Soybean Soybean Palm fatty acid distillate Sesame oil unspeciﬁed Jatropha Rapeseed oil Palm oil Glycerides Rambutan tree oil Brown grease Sea Lime Tree Palm oil Mango oil Lauric acid Jatropha Moringa Large fruit croton Palm fatty acid distillate Acidiﬁed cottonseed oil Kitchen waste oil
5:1,0.5 wt%,60 C,1 h 5:1,0.5 wt%,60 C,1 h 12:1,15 wt%,85 C,15 h 40:1,3 wt%,260 C,1 h 19.4:1, 75 C,20 h 9:1,7.61 wt%,150 C,4 h 5 wt%,200 C,8 h 20:1,6 wt%,140 C,2 h 10:1,4 wt%,50 C,4 h 20:1,5 wt%,65 C,5 h 10:1,2 wt%,42 C,<4 h 8:1,6 wt%,180 C,3 h 8:1,8 wt%,60 C,2 h 12:1,8 wt%,150 C,3 h 3:1,3 wt%,180 C,1 h 15:1,3 wt%,180 C,2 h 5:1,3 wt%,150 C,2.5 h 15:1,3 wt%,180 C,2 h 5.85:1,2.97 wt%,150 C,3.12 h 9:1,3 wt%,200 C,6 h 1.4:1,0.3 wt%,200 C,4 h
75 80 96.7 92 85 90.32 93.7 90 98.8 95 90 84 82.8 94.1 96 97 84 95 93.3 92 98.4
                    
SO2 4 /SnO2eSiO2
SO2 4 /TiO2eSiO2 SO2 4 /ZrO2eAl2O3
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dried at 100 C for 24 h to obtain an AleSn mixture. The mixture was added with an appropriate amount of H2SO4 solution to obtain 79, 82, and 85 wt% sulfates, which were stirred for 1 h, ﬁltered, dried at 100 C for 2 h, calcined in a mufﬂe furnace at 450 C for 3 h, and cooled to regular temperature. Finally, different SO2 4 / Al2O3eSnO2 catalysts were obtained. 2.2.2. Catalyst characterization The speciﬁc surface areas were measured using BET method. The pore volumes and size distributions were checked using the Barrett-Joyner-Halenda (BJH) method. The 3He2000PS2 static volume method with speciﬁc surface pore size analysis instrument (BeiShiDE Instrument-SAT) was used to analyze the BET of catalysts. The XRD patterns were obtained from the sample powders utilizing a Bruker D8 Advance X-ray diffractometer by using Cu Ka radiation. The distinction between Lewis acid and Bronsted acid center on the sample surface was mainly analyzed by NH3 adsorption FT-IR. When the basic molecule was chemisorbed on the sample surface, it can react with the surface Bronsted acid to form NHþ 4 and react with the Lewis acid center to form NH3*. In terms of NHþ 4 and NH3*, different absorption spectra can be used to distinguish the acid type of the sample, and the acid center content can be compared and analyzed according to the absorption peak area. NH3 adsorption FT-IR was carried out on a Nicolet 6700 infrared spectrometer (American Thermo Fisher Scientiﬁc Co., Ltd.). The samples were placed on a TP-5079TPD/TPR dynamic adsorber ﬁrst and heated to 300 C under He atmosphere. The temperature was kept for 1 h, and the ﬂow rate of He was controlled at 20 ml/min. After the end of the activation, the samples were scanned for the skeleton pattern. Then, these samples were adsorbed at 60 C for 1 h in a NH3 atmosphere at a ﬂow rate of 20 ml/min and heated to 120 C subsequently. NH3 that was physically adsorbed on the sample surface was purged with He for 1 h, and the samples were subjected infrared spectrum acquisition ﬁnally. The TPD measurements were performed on an AutoChem 2910 instrument (Micromeritics, USA). A thermal conductivity detector was used for continuous monitoring of the desorbed NH3, and the areas under the peaks were integrated. Prior to TPD measurements, samples were pretreated at 200 C for 1 h in ultrapure He gas at a ﬂow rate of 50 ml min1. After the pretreatment, the sample was saturated with 10% ultrapure anhydrous NH3 gas (balanced He, 75 ml min1) at 80 C for 2 h and subsequently ﬂushed with He (60 ml min1) at 110 C for 1 h to remove the physiosorbed NH3. The heating rate for the TPD measurements at a temperature ranging from 110 C to 800 C was 10 C/min. TGA analysis was carried out by a TG (DTG-60, Shimadzu, Japan) instrument to determine the catalysts’ thermal behaviors. The sample (15 ± 2 mg) in the Al crucible was heated at a rate of 10 C/ min from 25 C to 800 C. The N2 ﬂow rate was 40 ml/min. 2.3. Lipid extraction from sewage sludge The dehydrated sewage sludge samples were freeze-dried at 50 C for 48 h (Beijing Tianlin Hengtai Technology Co., Ltd., China), ground on a vibratory mill for 2 min (ZDM-50ML vibration grinding mill; Tianjin KEQI and New Technology Co., Ltd., China), and stored in a brown bottle at 4 C for use. Then, weighed 10 g dried sludge into a ﬁlter paper tube and placed into a connected Soxhlet extractor . A total of 200 ml nhexane and 200 ml of ethanol were added, the extractor was placed into a water bath, and the extraction reaction was kept for over 10 h at 80 C. Finally, the extractor was transferred into a rotary evaporator and weighed after evaporation, and the extracted crude fat was obtained. The above lipid extraction conditions were based
on our previous research, in which the extraction method and extraction solution have been optimized . 2.4. Catalytic reaction/esteriﬁcation/transesteriﬁcation reaction The reaction process of the SO2 4 /Al2O3eSnO2 solid acid catalyzed sewage sludge samples to produce biodiesel is shown in Fig. 1. Speciﬁcally, different amounts of solid acid catalysts (0.4, 0.6, 0.8, 1.0, and 1.2 g), 50 ml of n-hexane, 128 ml of methanol, and extracted crude fat were reacted together at 130 C for 4 h in a reactor. After the reaction was completed, the mixture was cooled to room temperature, transferred it into a 500 ml beaker, added with 10 ml saturated NaCl and shaken vigorously for 3 min, and then centrifuged at 3000 rpm for 5 min. The supernatant was transferred into a 250 ml separating funnel and rinsed with 20 ml of distilled water. After the solution was kept, the bottom and intermediate ﬂuids were removed after delamination. Then, the water in the biodiesel layer was removed through a ﬁlter device containing anhydrous Na2SO4. Afterward, n-hexane was removed at 40 C by using a rotary evaporator, and one biodiesel product was obtained lastly. Throughout the process, catalysts and solvents, such as methanol, ethanol, and n-hexane, can be recycled. 2.5. Analyses for fatty acid methyl esters (FAMEs) Biodiesel samples were analyzed quantitatively and qualitatively to determine the biodiesel yield and FAMEs composition. The biodiesel yield from the sludge sample was calculated based on the weight of the product, as shown in the following equation:
Biodiesel yieldcrude ð%Þ ¼
A 100% B
where A is the weight of biodiesel product after removal of water and n-hexane after catalytic reaction (g); and B is the weight of the crude fat dried after the Soxhlet extraction (g). The purity of the crude biodiesel will be checked by GC-MS. The FAMEs in the hexane phase were analyzed using SHIMADZU GCMS-QP2010 (Japan). A DB-5ms (30 m 0.25 mm) column was used. Column temperature was set 80 C at the start, kept for 2 min, then raised to 250 C with a rate of 10 C$min1, and ﬁnally held at 250 C for 20 min. The sample injection volume was 1 ml with a split ratio of 10:1. The inlet line to MS was kept at 250 C, whereas the MS source temperature was maintained at 200 C. Standard 70 eV electron ionization spectrum was recorded from 20 m/z to 650 m/z. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Physical characteristics Table 3 shows the physical properties, including the BET surface area, aperture, and pore volume of the SO2 4 /Al2O3eSnO2 catalysts. The SO2 4 /Al2O3eSnO2 catalysts shown in Table 3(a) were prepared by 79 wt% H2SO4. The results showed that with the increase of Sn, the BET value and pore volume of these catalysts increased sharply and then decreased rapidly, whereas the aperture showed in the opposite trend. Hence, Al can do good functions but it should be added in a proper amount. Considering that the active component SO2 4 mainly combines with Sn to generate a strong acid site , when the Al content is appropriate, the bonding strength and quantity of SO2 4 and Sn on the catalyst surface can be effectively promoted . However, if the Al content is exceedingly high, the speciﬁc surface area and acid amounts of the catalysts will decrease . The maximum value of BET (60.53 m2/g) in this research
R. Zhang et al. / Renewable Energy 147 (2020) 275e283
Fig. 1. Diagram of reaction process.
Table 3 Physical properties of the different catalysts. (a) Physical properties of the catalysts prepared by 79 wt%H2SO4 Catalysts (79 wt%H2SO4)
n(Al):n(Sn) ¼ 1:5 n(Al):n(Sn) ¼ 1:10 n(Al):n(Sn) ¼ 1:15
8.95 60.53 6.81
10.97 7.23 15.58
0.025 0.47 0.026
(b) Physical properties of the catalysts had n(Al):n(Sn) ¼ 1:10 Catalysts (n(Al):n(Sn) ¼ 1:10)
76 wt%H2SO4 79 wt%H2SO4 82 wt%H2SO4 85 wt%H2SO4
21.40 60.53 26.80 34.82
4.77 7.23 2.03 7.28
2.55 0.47 1.36 0.07
appeared at n(Al):n(Sn) ¼ 1:10. As shown in Table 3(b), all SO2 4 /Al2O3eSnO2 catalysts had the same Al/Sn molar ratio of 1:10, and the catalysts prepared by 79 wt % H2SO4 possessed the best physical properties. Appropriately increasing the H2SO4 concentration showed that the sulfate groups will remain bounded at the sample surface and inhibit the SnO2 crystallite growth. This result agrees with the ﬁndings on other transition metal oxides, such as TiO2, ZrO2, and Fe2O3, which increase the speciﬁc surface area and the formation of active centers . However, when the concentration of H2SO4 is further increased, some large crystals, such as Al2(SO4)3, may attach to the catalyst surface, thereby lowering the physical properties of the SO2 4 / Al2O3eSnO2 catalysts . Therefore, the addition of appropriate amount of Al improves the catalytic properties of SO2 4 /Al2O3eSnO2 catalysts remarkably. 3.1.2. XRD analysis The XRD results of the SO2 4 /Al2O3eSnO2 are shown in Fig. 2. Fig. 2 shows that all the catalysts contained diffraction peaks of
Al2(SO4)3 and SnO2. As n(Al):n(Sn) changed from 1:5 to 1:15, the diffraction peaks of Al2(SO4)3 in catalysts (a), (b), and (c) gradually weakened and the SnO2 diffraction peaks were enhanced. All of the catalysts have signiﬁcant diffraction peaks at 2q ¼ 27, 34 , 38 , 52 , indicating that the SnO2 on the surface of the sample is mainly tetragonal. However, former research reported that SnO2 crystallizations are possibly incomplete in SO2 4 /Al2O3eSnO2 catalyst, which may resulted from the introduction of Al and lead to the existence of defect sites on the surface. The defect sites can provide catalytically active centers . Comparing the crystal forms of catalysts (b) and (d), with the increase of SO2 4 loading, the intensity of Al2(SO4)3 is strengthened. It is reported to reduce the effective catalytic area . In summary, the catalyst (b) has relatively better surface properties, the above results indicate that the XRD results are consistent with the surface area results. 3.1.3. NH3 adsorption FT-IR results The active sites of Lewis acid and Bronsted acid play key roles in
R. Zhang et al. / Renewable Energy 147 (2020) 275e283
seen that all the four kinds of catalysts possessed Lewis and Bronsted acid sites. The vibrations at 1244 and 1539 cm1 of the catalysts (b) were due to chemical adsorption of NH3 on the Lewis acid site. Simultaneously, the peak at 1381 cm1, which is characteristic of the Bronsted acid site, was induced by the interaction of sulfuric group with Sn. In Fig. 3, catalysts (b) had the biggest absorption peak area, it shows that this kind of catalyst had the largest amount of Lewis acid and Bronsted acid sites, this result was consistent with its catalytic performance. In summary, for the SO2 4 /Al2O3eSnO2 solid acid catalysts in our research, the best catalytic activity is obtained when n(Al):n(Sn) ¼ 1:10 and the H2SO4 concentration is 79 wt%.
Fig. 2. XRD patterns of the catalysts. Preparation conditions for all catalysts were: a. n(Al):n(Sn) ¼ 1:5, 79 wt%H2SO4; b. n(Al):n(Sn) ¼ 1:10, 79 wt%H2SO4; c. n(Al):n(Sn) ¼ 1:15, 79 wt%H2SO4; d. n(Al):n(Sn) ¼ 1:10, 85 wt%H2SO4.
Fig. 3. NH3 adsorption FT-IR spectra of different SO2 4 /Al2O3eSnO2 catalysts. Preparation conditions for all catalysts were: a. n(Al):n(Sn) ¼ 1:5,79 wt%H2SO4; b. n(Al):n(Sn) ¼ 1:10,79 wt% H2SO4; c. n(Al):n(Sn) ¼ 1:15,79 wt% H2SO4; d. n(Al):n(Sn) ¼ 1:10,85 wt% H2SO4,respectively.
the esteriﬁcation/transesteriﬁcation reaction , and the distribution of both acid sites on SO2 4 /Al2O3eSnO2 solid acid catalysts was checked by NH3 adsorption FT-IR. Fig. 3 shows the FT-IR spectra of NH3 that was adsorbed on the catalyst samples. The ﬂexion vibration band of the H2O molecule occurs at 1630 cm1 . In the SO2 4 /MxOy-type solid super acid, the metal ions on the sulfate and metal oxide surfaces are generally combined by chelation and bridge double coordination . If the highest S]O anti-symmetric stretching vibration peak appears below 1200 cm1, then it is the bridge type double-coordination adsorption bonding . As shown in Fig. 3, there are evident adsorption in the range of 1000e1200 cm1, which indicate the sulfate and Sn4þ are bonded in the form of bridge type. The samples exhibited expected bands at 1241 and 1597 cm1 due to the adsorbed NH3 on the Lewis acid sites and on Bronsted acid sites at 1421 cm1 . It can be clearly
3.1.4. NH3-TPD analysis The acidity of sulfate groups deposited on the catalyst surface was measured by TPD. The TPD proﬁles of desorbed NH3 on different samples of catalysts are shown in Fig. 4. The NH3 desorption peaks of four catalysts both appeared at high temperatures and were all strong acid peaks. The desorption peak area of NH3 showed that the number of strong acid sites displayed in Fig. 4(b) was the highest, followed by the one shown in Fig. 4(d), which is more conducive to improving the catalytic productivity. As shown in Fig. 4(a), a trend of occurrence of the second peak was observed at approximately 800 C, and the peak appearing later may be the desorption peak of other strong acid sites. Fig. 4(c) shows two desorption peaks, and a high temperature peak appeared at 786.8 C, thereby indicating that other acids may be present in the catalysts. However, the desorption peaks were relatively dispersed. Hence, achieving a concentrated catalytic reaction is difﬁcult. Some desorption peaks of NH3 also appeared at nearly 200 C in Fig. 4(a), (c), and (d), thereby illustrating that some weak acid sites are still present in these catalysts. These abovementioned results proved that the SO2 4 /Al2O3eSnO2 solid acid catalysts with n(Al):n(Sn) ¼ 1:10 prepared by 79 wt% H2SO4 showed high catalytic activity due to the increase in acid sites on the catalysts, which are consistent with the catalyst activities. 3.1.5. TGA analysis The TGA analysis of SO2 4 /Al2O3eSnO2 catalysts is shown in Fig. 5. The thermogravimetric curve showed three distinct mass loss intervals. The ﬁrst mass loss zone was between 25 C and 250 C (stage I), which was caused by the physically adsorbed water on the catalyst surface. Most catalysts showed the second signiﬁcant mass loss between 400 C and 500 C (stage II), possibly due to the removal of free SO2 4 on the catalyst surface during the heating process. During the second signiﬁcant mass loss period, catalysts (b) showed signiﬁcant decrease in weight, reaching a maximum of 53.7%. Meanwhile, catalysts (c) and (d) only showed 20.8% and 21.4%. Hence, the highest amount of SO2 4 was loaded on the surface of catalysts (b), which played an important role on catalyst performance. The third mass loss of the catalysts ranged from 600 C to 800 C due to the decomposition and desorption of the sulfurcontaining components on the surface in the form of H2S and SOx (stage III) , which can lead the structural collapse and a sharp drop in the acidity of the catalysts. These aforementioned results are consistent with those of other analyses. In order to retain a larger amount of SO2 4 , the catalyst calcination temperature cannot exceed 500 C. Thus, the temperature of 450 C should be selected as the suitable calcination condition of the catalyst preparation method provided in this research. 3.2. Esteriﬁcation/transesteriﬁcation reaction Based on literature review, the main factors affecting the conversion rate of esteriﬁcation/transesteriﬁcation reaction are
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Fig. 4. NH3-TPD spectra of different SO2 4 /Al2O3eSnO2 catalystsa. n(Al):n(Sn) ¼ 1:5, 79 wt%H2SO4 b. n(Al):n(Sn) ¼ 1:10, 79 wt%H2SO4 c. n(Al):n(Sn) ¼ 1:15, 79 wt%H2SO4 d. n(Al):n(Sn) ¼ 1:10, 85 wt%H2SO4.
reaction temperature, molar ratio of methanol to oil, loading and composition of catalyst, reaction time and other factors. However, in order to maximize the function of catalyst itself or to discuss solely the inﬂuence of catalyst on the reaction, reaction time and reaction temperature was ﬁrst optimized. The reaction time was set as 0.5, 1, 2, 4, and 6 h, respectively. From 0.5 to 4 h, the yield gradually increased from 33.7 ± 3.8% to 73.3 ± 5.3%. However, when the reaction time was 6 h, the yield decreased to 72.1 ± 2.6%. This is because the amount of active sites participating in the reaction is limited under the conditions of the speciﬁc catalyst dosage, the esteriﬁcation/transesteriﬁcation process is substantially completed in the initial rapid reaction, but the side reaction causes the yield to decrease, and it also reduces the fuel properties of the product . In addition, experiments at 130, 150 and 170 C were performed to investigate the effect of reaction temperature on biodiesel yield, and obtained yields of 57.0 ± 2.6%, 50.3 ± 4.2% and 50.5 ± 3.7%, respectively. This can be explained by the fact that unsaturated
fatty acids and their glycerides polymerized at high temperatures, and at the same time, evaporation of methanol was accelerated, which resulting in a decrease in yield . From the perspective of energy conservation and full use of catalysts, reaction time ¼ 4 h, reaction temperature ¼ 130 C is chosen for the production of biodiesel from SO2 4 /Al2O3eSnO2 solid acid catalyst in this research.
3.2.1. Effect of catalyst loading Catalyst loading is an important parameter that needs to be optimized to increase the biodiesel yield. Different loading amounts of several SO2 4 /Al2O3eSnO2 catalysts were utilized in the reaction process. All experiments were carried out thrice, and the arithmetical averages, as well as the standard deviations, were calculated for all results. The performances of all catalysts for biodiesel production with lipids from sewage sludge as feedstock are shown in Fig. 6. The values of catalysts loaded into the reaction
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amount of catalysts (b) loading exceeded 0.8 g in the reaction, an insigniﬁcant effect was observed on the FAMEs yield . Although the highest FAMEs yield of 74.2% was observed at 1.0 g loading amount at the condition that the catalysts with n(Al):n(Sn) ¼ 1:10 and prepared by 79 wt% H2SO4, considering the cost and efﬁciency, the most suitable SO2 4 /Al2O3eSnO2 catalyst loading amount had better be 0.8 g for 10 g freeze-dried sludge. 3.2.2. Effect of catalyst composition 1) Al/Sn molar ratio
Fig. 5. TG spectra of different SO2 4 /Al2O3eSnO2 catalysts. Preparation conditions for all catalysts were: a. n(Al):n(Sn) ¼ 1:5,79 wt%H2SO4; b. n(Al):n(Sn) ¼ 1:10,79 wt% H2SO4; c. n(Al):n(Sn) ¼ 1:15,79 wt% H2SO4; d. n(Al):n(Sn) ¼ 1:10,85 wt% H2SO4; e. n(Al):n(Sn) ¼ 1:10,82 wt% H2SO4, respectively.
The surface SO2 in the SO2 4 4 /Al2O3eSnO2 catalyst mainly combines with Sn to produce strong acid sites. Al can enhance the stability of the catalyst. However, when the Al content is excessively high, the bonding of surface SO2 and Sn may be remarkably 4 reduced. As shown in Fig. 6, if the catalysts had the same loading of SO2 4 , such as catalysts (a), (b), (c), the highest FAMEs yield almost appeared at n(Al):n(Sn) ¼ 1:10 along with the changes in catalyst dosage. Therefore, in addition to other results of this research, the catalyst may have the best reactivity when the Al/Sn molar ratio is 1:10. 2) SO2 4 loading According to XRD analysis, SO2 4 bonded to the metal oxide carrier Al2O3eSnO2 and not simply physically adsorbed on the catalyst surface. With the electron-withdrawing effect of sulfate group, the Sn4þ Lewis acidity increased. The Bronsted acid sites derived from sulfates were ﬁxed on the catalyst surface. However, excessively high SO2 4 content showed a negative function. When the H2SO4 concentration was excess, some large crystals, such as Sn(SO4)2 and Al2(SO4)3, may block the catalyst surface, thereby decreasing the catalytic activity. As shown in Fig. 6, catalysts (b), (d), (e) both had n(Al):n(Sn) ¼ 1:10, the highest crude biodiesel yield was almost obtained by 79 wt% H2SO4. This result agrees with the catalyst characteristics analysis in Section 3.1. 4. Conclusion
Fig. 6. Esteriﬁcation/transesteriﬁcation reaction by different catalysts. Preparation conditions for all catalysts were: a. n(Al):n(Sn) ¼ 1:5,79 wt%H2SO4; b. n(Al):n(Sn) ¼ 1: 10,79 wt% H2SO4; c. n(Al):n(Sn) ¼ 1:15,79 wt% H2SO4; d. n(Al):n(Sn) ¼ 1:10,85 wt% H2SO4; e. n(Al):n(Sn) ¼ 1:10,82 wt% H2SO4; f. n(Al):n(Sn) ¼ 1:15,85 wt% H2SO4; g. n(Al):n(Sn) ¼ 1:5,82 wt% H2SO4, respectively.
were set at 0.4, 0.6, 0.8, 1.0, and 1.2 g. Fig. 6 shows that FAMEs yield increased with the addition of loading amount ranging from 0.4 g to 0.8 g and then increased slowly or decreased. Almost all catalysts except catalysts (b) had a maximum yield when the loading amount was 0.8 g. During this process, the total number of available active sites increased with more catalysts addition, thereby resulting in a fast reaction rate to reach the reaction equilibrium . For catalysts (a), (c) and (d), when the loading amount exceeded 0.8 g, the presence of the catalysts affected the mass transfer process, thereby decreasing the FAMEs yield. However, for catalysts (b), further increase in catalyst loading amount beyond its optimum value caused negligible increase in FAMEs yield. This can be interpreted as the ﬂuidity and mass transfer ability of the liquid phase were declined [61,62], which affected the catalytic activity in the reaction, and further leaded to a decrease in biodiesel yield. Hence, when the
This research investigated the preparation and characterization of SO2 4 /Al2O3eSnO2 as a promising solid acid catalyst in biodiesel production with lipids from sewage sludge. SO2 4 /Al2O3eSnO2 catalysts had the best catalytic performance when they had the Al/Sn ratio of 1:10 and prepared by 79 wt% H2SO4, as well as calcined at 450 C. Catalysts characterization results showed that acidity, acid sites and Al/Sn molar ratio play an important role in the activity of catalysts. When these catalysts were prepared under the optimal conditions, the relatively highest FAMEs yield of 73.3% was obtained under the optimized reaction conditions at 130 C as reaction temperature, 0.8 g as catalyst loading for lipids extracted from 10 g freeze-dried sludge, 4 h as reaction time. In summary, SO2 4 / Al2O3eSnO2 might have a bright future in the ﬁeld of biodiesel production with sewage sludge as feedstock. Acknowledgement This research was ﬁnancially supported by Beijing Natural Science Foundation (grant number: 8172029), the National Natural Science Foundation of China (grant number: 51308538). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.08.141.
R. Zhang et al. / Renewable Energy 147 (2020) 275e283
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