Introduction
The benefits from resources and the achievement of self-sufficiency in all aspects are one of the most important requirements for all countries in the current period. Two of the most important topics in this regard, which this paper link together, are the valorization of municipal solid waste (MSW) and the production of fossil fuel alternatives. Continuous population growth and urban development has led to a high level of MSW generation. MSW represents a national wealth when utilized in the right ways and a disaster when neglected. In addition, the high population and high living standards have led to a high rate of energy consumption which makes it necessary to look for alternatives to fossil fuel, especially in non-petroleum countries.
The amount of MSW generated worldwide every year is about 2 billion tons and it is expected to increase to about 3.4 billion tons by 2050 [
1,
2]. The composition of the MSW depends on many factors including country, people’s income, lifestyle/culture, climate, energy sources, etc. [
3,
4]. The MSW in developing countries is mostly organic in nature including food scraps, wood, leaves, and process residues from farms, while the MSW in developed countries is mainly inorganic in nature such as plastic, paper, metal, and e-wastes [
3]. In general, MSW includes kitchen waste, agriculture waste, metals, paper, glass, plastic, electronic waste, inert materials, and miscellaneous thrash [
1,
5]. The treatment approaches of the MSW worldwide include landfilling, which ends up at about 70% of the produced MSW, in addition to classified recycling, incineration, pyrolysis, gasification, composting, and anaerobic digestion [
1,
6,
7]. All these approaches represent a threat to the environment and human health in one way or another. For example, these approaches pose potential risks of infection in the case of manual sorting of MSW or explosion in the case of landfilling, incineration, pyrolysis, gasification, or anaerobic digestion [
7]. Therefore, it is necessary to develop effective approaches to utilize these waste materials as valuable resources, in alignment with the principles of sustainability and environmental preservation. The metallic/metal-containing waste is one of the most important categories of MSW. The proper utilization of this kind of waste needs to be thoroughly examined to conserve natural resources, particularly, considering that these metals represent non-renewable resources. There are many studies in the literature that have investigated the valorization of different kinds of metallic/metal-containing MSW. Liu et al. [
8] presented a novel and effective smelting–-collection process for recovering platinum from waste automotive catalysts (WAC). The process involves mixing different WAC carriers (cordierite and alumina), adding iron powder as a collector metal, and CaO as a flux agent. By optimizing the process conditions, a platinum recovery rate of over 98% was achieved. The recovered platinum was concentrated into Fe–Pt alloy, and environmentally safe glass slag was obtained. Marinato et al. [
9] achieved a 98.4% silver recovery from waste printed circuit boards of obsolete cell phones through hydrometallurgical processing. The recovered silver was converted to silver chloride and used for green synthesis of silver nanoparticles which exhibited strong antibacterial properties against Gram-negative bacteria, particularly,
Escherichia coli. Wang et al. [
10] studied the recovery of high-purity metallic cobalt from NMC-type Li-ion batteries, utilizing lithium nickel manganese cobalt oxide as the cathode material. The recovery process involves the reductive acidic leaching of the cathode material, followed by selective extraction of cobalt. The overall recovery ratio for cobalt was calculated at approximately 93%, with a remarkably high purity of 98.8%. In a study by Roy et al. [
11], a simple and facile strategy was applied to convert waste aluminum cans to nano-alumina which was used as reinforcing filler for the development of natural rubber composites with balanced compact performances. According to Roy and his co-workers, addition of the can waste-based nano-alumina successfully enhances the thermal stability and thermal oxidative aging resistance of natural rubber composites. Tin food packaging cans, primarily composed of iron with a small tin content, constitute a significant portion of the metal category in municipal solid waste (MSW). Tin can waste is generated in a daily basis worldwide due to their extensive use in packaging various food items, particularly low-cost food products. In 2003, the worldwide production of tin cans for food packaging was estimated to be about 80,000 million cans [
12]. Despite the substantial production of this iron-based waste, the studies on the utilization of tin cans waste are limited compared to other types of metal waste. This may be attributed to the abundance of iron as an element, its multiple natural sources, and its cost-effectiveness compared to other metals. However, despite these factors, it is important from the environmental and economical points of view to give the management of iron waste more attention.
Many fossil fuel alternatives have captured the attention of researchers nowadays, and there is hope that these alternatives will contribute to solving the energy problem. Methyl ethyl ketone or 2-butanone C
4H
8O is suggested to be used as a possible fuel for spark-ignition engines and is considered one of the promising alternative energy sources that can contribute significantly to reducing dependence on fossil fuel [
13]. It is an extremely volatile and flammable colorless liquid with a sharp, sweet odor like that of butterscotch and acetone [
14,
15]. MEK has many important industrial applications, including the production of paints, lacquers, varnishes, sticks, resins, gums, nitrocellulose, cells, and artificial leather. In addition, MEK is used in the printing industry and in the manufacture of dyes and as an aerosol surface cleaner [
14]. In general, the production of MEK can be carried out by two routes, first route through compounds produced by oil refining, and the second route which involves intermediate compounds recovered from biomass by biological methods [
14]. In addition, MEK can be produced directly by fermentation of biomass; however, the yields of this process are quite low [
13]. The industrial process to produce MEK is performed through the dehydrogenation of 2-butanol, the alcohol which can be produced by chemical routes from fossil fuel or biologically from biomass fermentation [
13,
16]. The most common catalysts employed for dehydrogenation of 2-butanol to MEK are copper-based catalysts and/or zinc-based catalysts [
17]. Additionally, many different catalytic systems have been studied for the dehydrogenation of alcohols in general including Cu–Ce–Zr-based catalysts, Cu–Ni bimetallic catalysts supported on gamma alumina, Pt supported on different oxides, and solid-state molybdenum sulfide clusters [
18]. Geravand et al. [
15] studied the vapor-phase dehydrogenation of 2-butanol to MEK at 260 °C using Cu/ZnO/Al
2O
3 and Cu/SiO
2 nanocatalysts prepared by three different methods: impregnation, sol–gel, and co-precipitation. They found that the activity and MEK selectivity of the prepared catalysts increased in the following sequence: CuO/ZnO/Al
2O
3 (co-precipitation) > CuO/SiO
2 (sol–gel) > CuO/SiO
2 (impregnation). In addition, the optimum preparation conditions of the catalyst CuO/ZnO/Al
2O
3 (co-precipitation) that can maximize MEK selectivity, according to this study, are precipitation temperature of 67.5 °C, aging temperature of 68.75 °C, pH of precipitation stage of 7.27 and Cu/Zn molar ratio of 1.38. Zhanga et al. [
19] studied the dehydrogenation reaction of 2-butanol to MEK over copper and zinc catalysts supported on Al
2O
3–ZrO
2 composite carriers prepared by the co-precipitation method. They investigated the effect of Al/Zr ratio on the catalytic performance and found that the addition of an appropriate amount of Zr component to the CuO–ZnO–Al
2O
3 catalyst helped disperse CuO species, lower its reduction temperature, and improve its reduction characteristics. According to this study, 2-butanol conversion of 95% and MEK selectivity of 78% was obtained when the CuO:ZnO:Al
2O
3:ZrO
2 molar ratio was 1:1:0.06:0.5 at a reaction temperature of 230 °C. Halawy et al. [
20] studied the production of MEK through the dehydrogenation of 2-butanol over nanocrystalline NiO catalyst with a coral-like structure (38 nm) prepared by the thermal decomposition of nickel galactarate (NiC
6H
8O
8⋅2H
2O). The prepared NiO catalyst exhibited excellent catalytic activity in the synthesis of MEK from 2-butanol which was attributed to the distribution of strong basic sites on the catalyst surface. At a reaction temperature of 300 °C, the catalyst achieved 90% conversion of 2-butanol to MEK with a selectivity of 90%.
The aim of this study is to achieve two important objectives. The first objective is to benefit from tin can waste as a category of MSW, which, despite its large quantities, has not received sufficient attention from researchers compared to other types of MSW. This objective has been achieved by converting tin can waste to catalytically active iron oxide by applying a simple, low-cost precipitation method in which sodium hydroxide or ammonium hydroxide was used as precipitating agents after dissolving the cans in nitric acid. The second objective is to study the dehydrogenation of 2-butanol to MEK over the prepared iron oxide catalysts as a cost-effective alternative to the common catalysts used in this reaction in the industry. In general, iron oxide is a type of transition metal oxide which exists in different stoichiometric and crystalline forms, including wüstite (FeO), hematite (α-Fe
2O
3), maghemite (ν-Fe
2O
3), and magnetite (Fe
3O
4). Iron oxides have a wide range of applications in different fields, such as catalysis, photocatalysis, color imaging, magneto-optical devices, and ferro-fluids. Among all iron oxide phases, hematite (α-Fe
2O
3) is the most stable state under normal conditions and the most favored one due to its beneficial properties, chemical stability, and semiconducting characteristics [
21].
Since the preliminary results showed the preference of the catalysts prepared using NaOH as a precipitating agent, the effect of calcination temperature on the morphological and catalytic properties of this catalyst has been studied. In addition, the catalytic efficiency of the produced iron oxide catalysts was compared with that of a commercial iron oxide catalyst.
Experimental
Materials
Waste tin food cans used in this study are those used in packing cooked fava beans and were collected from the local Egyptian market. All the chemicals were used in the condition as received without any further purification. These chemicals included nitric acid (HNO3, Merck, 55%), ammonium hydroxide (NH4OH, Merck, 35%), sodium hydroxide (NaOH, Merck), commercial hematite (Fe2O3, Merck), and 2-butanol (CH3CH(OH)CH2CH3, Fluka).
Preparation of Fe2O3 catalysts from tin food can waste
Ferric nitrate solution was first prepared from food cans waste. 25 g of clean cans was cut to small pieces and immersed in a beaker containing 1000 mL of 1 M HNO3 solution. The mixture was stirred till complete dissolution of the solid material; then the solution was filtered to remove any insoluble materials. 200 mL of the filtrate was diluted to 600 mL using distilled water and heated to 100 °C, then 2 M NaOH solution was added dropwise with continuous stirring. The reddish-brown precipitate formed was stirred in the mother solution for 12 h at room temperature (pH of the mother solution was 11.2) and then filtered and washed several times with distilled water. The washed precipitate was dried at 100 °C for 24 h and finally calcined in static air for 3 h at 400 °C or higher temperatures. These steps were repeated using another 200 mL of the filtrate and 2 M NH4OH solution as a precipitating agent (pH of the mother solution was 8.18). The sample prepared using NaOH was abbreviated as FeNa and that prepared using NH4OH was abbreviated as FeNH, followed by the drying or calcination temperature, e.g., FeNa100, FeNa400, FeNH100, FeNH400, etc. Commercial Fe2O3 sample was used as a reference catalyst and was abbreviated as FeCm.
Catalyst characterization
Thermogravimetry (TG) was performed in a flow of 40 cm3/min dry nitrogen, using automatically recording model 50 H Shimadzu thermal analyzer from Japan. The thermal analyzer is equipped with a data acquisition and handling system (TA-50WSI). FT-IR spectra of the samples were recorded using a Magna-FT-IR 500 from USA, in the range of 4000–300 cm−1, operating a Nicolet Omnic software, and adopting the KBr disk technique. X-ray powder diffraction analysis (XRD) was carried out using a model D5000 Siemens diffractometer from Germany, equipped with a copper anode generating Ni-filtered Cu Kα radiation (λ = 1.5406 Å), in the 2θ range between 20° and 80°. An online data acquisition and handling system facilitated an automatic JCPDS library search and match using Diffrac software from Siemens for phase identification purposes. The BET surface area measurements were performed at liquid nitrogen temperature of −195 °C using an automatic Gemini VӀӀ Micromeritics Model 2390 P from USA. The catalyst samples were outgassed at 200 °C for 1 h prior to measurements. Scanning electron microscopy (SEM) was used to analyze the morphology of the samples using a JEOL JSM-IT200 SEM. The EDXRF quantitative elemental analysis was carried out using a Jeol JSX-3222 element analyzer system equipped with X-ray tube of Rh anode. The characteristic X-ray radiation was measured using a Si(Li) detector with an energy resolution of 149 eV at 5.9 keV and 1000 cps.
Determination of the surface basicity of the catalysts
The surface basicity of all the samples under investigation was studied quantitatively by means of desorption thermogravimetry using CO
2 as a probe molecule. 50 mg of each sample was pre-heated at 400 °C for 1 h in static air, then all the samples were kept for 2 weeks in a glass chamber fitted with a gas inlet and outlet under a flow of 40 mL/min CO
2 gas. 15–20 mg of sample covered with adsorbed CO
2 was subjected to TG analysis on heating up to 400 °C at a heating rate of 20 °C/min. The mass loss from TG analysis due to desorption of CO
2 molecules from the basic sites was determined as a function of surface basic sites density. Calculation of the density of the basic sites expressed in (site/g) was carried out using the following equation [
22]:
$$\text{Basic\,site\,density\,(site/g)= }\frac{\text{Moles\,of\,C}{\text{O}}_{2}\,\text{ desorped }\times \text{ Avogadro\,Number }(\text{site/mol})}{\text{Weight\,of\,sample }(g)}.$$
(1)
Catalytic activity measurements
The catalytic activity of all the catalysts for the vapor-phase dehydrogenation/dehydration of 2-butanol was performed in a continuous flow system under atmospheric pressure. The reactions were carried out in Pyrex glass reactor (1 cm wide and 16.5 cm long) using nitrogen as a carrier gas. 200 mg of the catalyst was preheated for 1 h at 400 °C under a flow of 100 mL/min nitrogen; then, the temperature was lowered to the reaction temperature, and the catalyst was subjected to the reaction feed (100 mL/min = 0.789% secondary butanol + 99.211% nitrogen). The reactor effluent was analyzed by using a gas chromatograph (Shimadzu GC-14A) equipped with a data processor model Shimadzu chromatopac C-R4AD (Japan). A flame ionization detector (FID) and a stainless-steel column (PEG 20M 20% on chromosorb W, 60/80 mesh, 3 m × 3 mm) at 80 °C were used to identify secondary butanol and the reaction products. Automatic sampling was performed with a heated gas sample cock, type HGS-2 at 140 °C.
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