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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Synthesis and Catalytic Properties of New Sustainable Aluminosilicate Heterogeneous Catalysts Derived from Fly Ash Mohammad I. M. Alzeer*,†,‡,§ and Kenneth J. D. MacKenzie†,‡ †

School of Chemical and Physical Sciences and ‡MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington 6140, New Zealand S Supporting Information *

ABSTRACT: The field of heterogeneous catalysis has recently become increasingly interested in the utilization of industrial wastes as inexpensive precursors for active catalysts. Here we report a facile synthesis of fly-ash-based geopolymers as new highly reactive sustainable porous aluminosilicate heterogeneous catalysts for acidic and/or redox applications. The catalytic properties of these catalysts for Friedel−Crafts benzylation reactions of benzene and other substituted aromatics were thoroughly investigated. Fly ashes were collected from various sources were investigated, and the impact of the variation of their chemical and physical properties on the resulting catalysts was studied. The catalysts demonstrated excellent catalytic reactivity superior to other commonly used aluminosilicate catalysts such as M-zeolite, mesoporous molecular sieves, mixed oxides, and ionic liquids. This study not only highlights the feasibility of synthesizing active heterogeneous catalysts with predictable features from several different fly ash precursors but also suggests a new and very useful means for effectively utilizing fly ash. KEYWORDS: Geopolymers, Heterogeneous catalysis, Sustainability, Fly ash, benzylation

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valuable metals,3,10 and as a precursor for materials such as ceramics,11−13 antibacterial pigments,14 and thermal insulation monoliths.15 The utilization of fly ash in the field of heterogeneous catalysis has attracted much attention over the last few decades,16,17 since it is mainly composed of SiO2 and Al2O3 and contains metals oxides such as Fe2O3, TiO2, CaO, and Na2O. In this context, fly ash has been investigated either as a support for catalytically active species18−20 or as a precursor for active catalysts such as zeolites21−23 and mesoporous molecular sieves.24,25 Conventional hydrothermal syntheses of zeolites from fly ash fail to produce zeolites with the desired structural crystallinity due to the presence of impurities that are insoluble under alkaline conditions. Therefore, fly-ash-based zeolites and mesoporous silicates are usually synthesized by fusing solid alkali with the fly ash at high temperatures (>500 °C), a common approach to extract silica form fly ash, followed by hydrothermal crystallization and calcination.25,26 The process is time- and energy-consuming which may hinder its large-scale application. Therefore, the use of fly ash in its bulk form for the synthesis of active materials via an energy-efficient process is highly desirable and would provide a useful approach for effective utilization of such a complex anthropogenic material.

INTRODUCTION Effective utilization of large scale industrial wastes and abundant natural materials has recently become a topic of considerable interest in the search for a sustainable future. Hundreds of millions of tonnes of various industrial wastes such as red mud, blast furnace slag, aluminum dross, and coal fly ash, have been accumulating for decades in the ecosystem causing serious environmental problems.1 In particular, fly ash, the solid waste product of coal combustion mainly from thermal power plants, is considered to be one of the most complex and abundantly available anthropogenic materials. In 2010 alone, 780 000 000 tonnes of fly ash were produced worldwide.2 Fly ash is commonly discarded into landfills or dumped into the sea where it poses a serious threat to the environment and to human health, since it contains traces of toxic metals and radioactive elements which could leach out, contaminating the soil and the ground and surface water.3,4 In order to reduce the financial and environmental costs of its disposal, fly ash has been utilized mainly in construction applications such as extenders in concrete, asphalt fillers, and pavement base coursees.5,6 However, the growing production of fly ash is outstripping the demands of the civil engineering sector; ∼50% of the fly ash produced in 2010 was globally utilized,2 with the reminder dumped into the sea or into landfills. Consequently, fly ash has been investigated for a range of other applications including as an adsorbent for flue gases7 and heavy metals from industrial wastewater,8,9 recovery of © XXXX American Chemical Society

Received: December 29, 2017 Revised: February 2, 2018

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DOI: 10.1021/acssuschemeng.7b04923 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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This work presents a facile synthesis of fly-ash-based geopolymers as new sustainable porous heterogeneous catalysts for acid and/or redox catalyzed reactions. Geopolymers are amorphous aluminosilicate inorganic polymers with 3D frameworks consisting of tetrahedral silicate and aluminate units joined through their common oxygen atoms.27 The negative charges on the alumina tetrahedra are compensated for by extra-framework monovalent alkali cations, usually Na+ or K+ that are capable of undergoing ion exchange to produce geopolymers with zeolite-like properties. Thus, geopolymers may be considered as the amorphous analogous of zeolites.28 Geopolymers possess remarkable features such as their porosity; their pores are of different length scales, micro (50 nm), and can be introduced to the geopolymer framework without the need for costly structural directing agents. Furthermore, their ease of synthesis at room temperature from naturally occurring precursors such as clays or industrial wastes including fly ash, makes them environmentally benign and relatively inexpensive materials. In addition, they can be functionalized and thereby tailored for a variety of applications. Therefore, in addition to their usual application as structural materials, geopolymers have recently been investigated for advanced applications such as drug delivery, photoluminescence, separation and catalysis.29 The potential of geopolymers in the field of heterogeneous catalysis, in particular, has recently begun to attract attention. A very few studies have reported the use of clay-based geopolymers as supports for catalytically active species.30−35 More recently, clay-based geopolymers have been reported as a novel class of acidic heterogeneous catalysts with active sites suitable for organic synthesis applications developed within their structure.36,37 However, the potential of fly-ash-based geopolymers as heterogeneous catalysts has been much less investigated; to the best of our knowledge, only two previous papers have reported the use of fly-ash-based geopolymers as heterogeneous photocatalysts for degradation of organic compounds in wastewater.38,39 In these studies, the active sites were metal oxides either present in the original fly ash or incorporated in the synthesized material. In this work, we describe the development of fly-ash-based geopolymers as new porous heterogeneous catalysts with active sites incorporated within the geopolymer framework, acting in conjunction with the active metal oxides present in the original fly ash; this permits their action as bifunctional catalysts in acid and/or redox catalyzed reactions. The catalysts were synthesized at room temperature in an environmentally friendly process without the need for alkali fusion or lengthy hydrothermal treatments, and their catalytic properties was investigated in model Friedel−Crafts benzylation reactions of benzene and other arenes (Scheme 1).

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EXPERIMENTAL SECTION

Materials and Synthesis of Parent Geopolymers. Four different fly ashes (FA) were used as received without further treatment. The fly ashes were from different sources: A C-class (high CaO content) fly ash was from the Huntly power station, New Zealand. The three other fly ashes were F-class (low CaO content) and were from Australian power stations: Gladstone in Queensland as well as Hyrock from Bayswater station and Mount Piper station, both located in New South Wales. The compositions of these fly ashes and the subsequent synthesized catalysts are shown in Table S1. Four different geopolymers were synthesized and designated according to the name of the fly ash, plus “Geo”, as follows: Geo-Hyrock, GeoHuntly, Geo-Gladstone, and Geo-Mt. Piper. The exact weight of each component used to synthesize each of these geopolymers is shown in Table S2. The molar compositions of all the synthesized fly-ash-based geopolymers are summarized in Table S3. The parent geopolymers were synthesized as follows: A weighed amount of analytical-grade NaOH (Panreac) was dissolved in distilled water, and then sodium silicate (Sod-Sil-D) (FERNZ Chemical Co, NZ, Type “D”, Na2O/SiO2 = 0.48, solids content = 41.1 mass %) was added to the mixture, except in the case of the geopolymer synthesized from FA-Hyrock. The solution was cooled to room temperature in an ice bath, and the fly ash was then gradually added to the mixture. Amorphous Al2O3 (Alphabond 300, Alcoa) was simultaneously added with the fly ash to adjust the SiO2/Al2O3 ratio, except in the geopolymer synthesized from FA-Gladstone. After thorough manual mixing for 10 min, the geopolymer resins were cured in covered plastic molds at 80 °C for 6 h, then uncovered and oven-dried at 40 °C overnight. The hardened blocks were then ground in a vibratory mill (Bleuler, Switzerland) fitted with a tungsten carbide pot and milling rings and sieved to pass a 105 μm mesh. Catalyst Preparation. The acidic form of fly-ash-based geopolymers was obtained by ion-exchanging the charge-balancing Na+ ions with NH4+ as reported previously.37 First, 1 g of the geopolymer powder was treated with 100 mL of 0.1 M NH4Cl at room temperature with vigorous stirring (1000 rpm) for 12 h. The powder was then filtered off, washed with fresh 0.1 M NH4Cl solution, and then with distilled water to remove any remaining ions. The powder was then dried at 40 °C overnight. Prior each reaction, the NH4+forms of the fly-ash-based geopolymers were heated to 550 °C for 15 min at a heating rate of 15 °C/min in static air. The heated catalysts were added straight from the furnace to the reaction mixture. Catalyst Characterization. X-ray powder diffraction was collected using a Bruker D8 Avance X-ray diffractometer with Ni-filtered Cu Kα radiation operated at 45 kV and 40 mA. FTIR spectra were acquired from samples suspended in KBr discs using a PerkinElmer Spectrum One FTIR spectrometer in the range 4000−450 cm−1. SEM images were obtained using a JEOL JSM-6610 LA analytical scanning electron microscope operated at 10−20 kV, connected to an energy-dispersive spectrometer (EDS). The sample powder was placed on a carbon tape and coated with a ∼16 nm layer of carbon using a Quorum Q150T turbo-pumped carbon coater. TEM were acquired using a JEOL JSM2100 F transmission electron microscope operated at 200 kV. The samples were dispersed on copper grids (Formvar/carbon coated) and cleaned by exposure to a low-energy plasma using a JEOL EC-2000 IC ion cleaner. The specific surface area (SBET) and the total pore volume (Vtotal) were measured using a Micromeritics ASAP 2010 instrument, and the particle size distribution was determined by laser diffraction using a Malvern Mastersizer 2000 instrument. Qualitative characterization of the surface acidity of the synthesized catalysts was made by FTIR spectroscopy of adsorbed pyridine, while quantitative analysis of the total acidity was determined from the TGA profile of the desorbed pyridine using a Shimadzu TGA-50 thermal analyzer following the procedure described elsewhere.37 Catalytic Reactivity. The catalytic reactions studied here were carried out in liquid-phase systems under atmospheric pressure in a magnetically stirred 50 mL two-necked round-bottomed flask equipped with a reflux condenser placed in a thermostatic bath using silicone oil. In a typical run, 1 mL of BzCl was mixed with 13 mL

Scheme 1. Friedel−Crafts Benzylation of Benzene or Substituted Benzene with Benzyl Halide

B


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ACS Sustainable Chemistry & Engineering of the aromatic compound (analytical-grade benzene, toluene, anisole, p-xylene, or mesitylene). Next, 0.1 g of the catalyst was added to the reaction mixture at 80 °C at a time taken as the starting time of the reaction. Quantitative analysis of the reaction products was performed using conventionally determined calibration curves in which five standards were prepared from the limiting reactant (BzCl) and the product of interest (the monobenzylated product) which were analyzed by GC-MS in each run. The reaction analysis and the catalytic calculations including the conversion %, selectivity %, specific reaction rate, TON, TOF, and the models used to evaluate the mass and heat transfer limitations are described in detail in the Supporting Information. The reusability of the catalysts was evaluated by separating the catalyst from the reaction mixture and reactivation at 550 °C for 1 h prior to each successive run. The spent catalyst was studied by FTIR and TGA.

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RESULTS AND DISCUSSION Catalyst Characteristics. The formation of the flyash geopolymers was investigated by XRD (Figure 1). The broad background hump in the kl range of 25−40° 2θ is typical of a well-formed amorphous geopolymer. Several crystalline phases also identified by XRD were quartz (Powder Diffraction File (PDF) no. 01−070−3755, Joint Committee on Powder Diffraction Standards (JCPDS), [2004]), mullite (PDF no. 15−0776, JCPDS, [2004]), Fe2O3 (PDF no. 04−007−9266, JCPDS, [2004]), and Ca2SiO4 (PDF no. 00−033−0302, JCPDS, [2004]) which are present in the fly ash precursor. In some cases, a crystalline aluminosilicate (sodalite, PDF no. 31−1271, JCPDS, [2004]) was formed after geopolymerization (Figure 1a). The XRD patterns of the raw fly ashes are shown in Figure S1. Figure 2 shows the FT-IR spectra of the synthesized geopolymers. All the parent geopolymers contain a typical

Figure 2. FTIR spectra of geopolymers prepared from different fly ashes. (a) Geo-Hyrock, (b) Geo-Huntly, (c) Geo-Gladstone, (d) GeoMt. Piper.

strong broad peak at ∼1075 cm−1 ascribed to the Si−O−Al stretching vibration, with a shoulder at around ∼1170 cm−1 due to the Si−O−Si stretch.40 Another small peak at 1480 cm−1 arises from carbonate formed by atmospheric carbonation of the geopolymer to form Na2CO3, this also resulted in the broad CO2 asymmetric stretching vibration at about 2347 cm−1.41 A broad peak at about 3500 cm−1 is assigned to hydrogen bonded silanol nests (3500−3400 cm−1),42 while the small peak at 1640 cm−1 is due to the H−OH stretching mode from adsorbed water. The small band at about 795 cm−1 is ascribed to the octahedral Al−O stretching vibration and the band at ∼561 cm −1 is ascribed to the symmetric stretching of Si−O−Al or to the Fe−O stretching vibration which is usually in the range of 540−570 cm−1.43 SEM micrographs showing the morphology of all the fly ashes used in this study, and the corresponding geopolymers are presented in Figure 3. All the fly ashes display a typical spherical shape and consist mainly of Cenospheres with some solid spheres and other amorphous debris. The synthesized geopolymers contain large amorphous particles with diameters ∼20 μm, composed of smaller particle aggregates forming voids within the larger particles. The acidic sites within the catalysts were generated by conventional ion-exchange with NH4+ followed by thermal treatment which was monitored by FTIR as shown in Figure 4. After the ion-exchange step, the geopolymers show the typical peaks of the N−H stretching mode at ∼3200 cm−1 and the corresponding bending mode (the double peak at ∼1400 and 1450 cm−1, Figure 5b). The ion-exchange is accompanied by the disappearance of the carbonate bands at ∼1480 and 2347 cm−1 (Figure 4b), confirming the removal of the Na chargebalancing cations.

Figure 1. XRD of geopolymers prepared from different fly ashes. (a) Geo-Hyrock, (b) Geo-Huntly, (c) Geo-Gladstone, and (d) Geo-Mt. Piper. C


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Figure 3. SEM micrographs the raw fly ashes (a−d) and the corresponding geopolymers (e−h). (a) Hyrock, (b) Huntly, (c) Gladstone, (d) Mt. Piper, (e) Geo-Hyrock, (f) Geo-Huntly, (g) Geo-Gladstone, and (h) Geo-Mt. Piper.

shows the highest surface acidity, as reflected in the intensity of the FTIR bands of the chemisorbed pyridine. These bands were barely present in Geo-Gladstone, indicating its poor surface acidity. A quantitative analysis of the total acidity of the catalysts was obtained from the TGA profile of the pyridine desorption and the results are summarized in Table 1. Table 1 summerizes the chemical and physical properties of the synthesized catalysts. Geo-Huntly contains the highest concentration of acidic sites whereas Geo-Gladstone possess the lowest concentration, consistent with the results obtained from the FTIR spectra of adsorbed pyridine (Figure 5). In addition, the catalysts show an average particle size of ∼20 μm, consistent with the SEM micrographs (Figure 3). Furthermore, all the catalysts show relatively high surface areas (up to ∼140 m2/g in Geo-Hyrock). The very small pore volume of the flyash-based catalysts can be ascribed to the impurities present in the raw fly ash, resulting in the pores of the resulting catalysts being filled with amorphous debris, crystalline phases, and metal oxides. The pore sizes of the catalysts and their morphology were further investigated by TEM (Figure 6). All the catalysts show mesoporosity, with pore diameters varying from ∼5 nm for Geo-Mt. Piper (Figure 6d) up to ∼30 nm for Geo-Hyrock (Figure 6a). In addition, macroporosity is also observed in some of the catalysts (e.g., Geo-Gladstone, Figure 6c). Table 1 also presents the Fe2O3 content of each catalyst, which is proportional to the amount of this oxide in the raw fly ash (see Table S1). Fe2O3 is a catalytically active species for redox catalyzed reactions, including the Friedel−Crafts benzylation which are the target reactions of this study. For this reason, this study also investigated the distribution of the Fe oxide present in the geopolymer particles and the manner in which this oxide is attached to the geopolymer particles. Figure 7 shows a SEM micrograph of a Geo-Hyrock sample (left) with the EDS map showing the Fe distribution over this particular sample (right). High magnification SEM images of some of the bright Fe spots (Figure 7a−d), circled in red in the two source images at the top, show that the Fe species (most probably Fe2O3) present in these areas actually form part of the geopolymer particles, being inserted within the geopolymer matrix. This is very important and highly desirable feature for catalysis applications, where the Fe species are expected to act as the active sites, providing a high degree of reusability and long catalyst lifetime. Catalytic Reactivity. The catalytic reactivity of the fly-ashbased catalysts in the benzylation of benzene and substituted

Figure 4. Representative FTIR spectra of Geo-Hyrock at different preparation stages: (a) Parent Geo-Hyrock. (b) NH4+-form of GeoHyrock, and (c) NH4+-Hyrock-Geo after heating to 550 °C.

After thermal treatment at 550 °C for 15 min (Figure 4c), the N−H stretching and bending modes have completely disappeared, confirming the total decomposition of NH4+ and the formation of the H-form of the geopolymer. Destruction of the silanol groups after the thermal treatment is evidenced by the shrinkage of the hydroxyl band (in the range 3700−3400 cm−1). The shoulder at ∼3625 cm−1 is ascribed to bridging hydroxyls (Bronsted acidic sites). Dealumination after the thermal treatment is also shown by the reappearance of the octahedral Al (EFAl) peak at 795 cm−1 which is present in the FTIR of the raw fly ashes (see Figure S2). These EFAl entities are expected to act as Lewis acidic sites as in the crystalline aluminosilicates (zeolites). A representative XRD pattern of Geo-Hyrock before and after NH4+ ion-exchange is shown in Figure S3. The acidic sites generated within the catalysts were identified by pyridine adsorption as shown in Figure 5. The bands at 1445, 1600, and 1620 cm−1 are usually associated with Lewis acidic sites, while the band at ∼1545 cm−1 represents Bronsted acidic sites, with a combination of both sites represented by the peak at 1490 cm−1. Of the synthesized catalysts, Geo-Huntly D


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Figure 5. FTIR spectra of the fly-ash-based catalysts without pyridine (dashed line), and with desorbed pyridine (solid line): (a) Geo-Hyrock, (b) Geo-Huntly, (c) Geo-Gladstone, and (d) Geo-Mt. Piper.

Table 1. Chemical and Physical Properties of the Fly-Ash-Based Catalysts catalyst

acid content (mmol/g cat.)a

Fe2O3 (mmol/g cat.)b

average particle size distribution (μm)c

SBET (m2/g)d

Vtotal (cm3/g)e

Geo-Hyrock Geo-Huntly Geo-Gladstone Geo-Mt. Piper

0.33 0.39 0.13 0.25

0.284 0.517 0.567 0.053

25 20 14 16

144 59 61 73

0.07 0.09 0.04 0.02

a Determined from the TGA profile of pyridine desorption. bDetermined by XRF. cDetermined by static laser diffraction (see Figure S4). dBy the Brunauer−Emmett−Teller (BET) method over a p/p0 range of 0.05−0.3. eSingle point at p/p0 = 0.985.

aromatics with BzCl as the alkylating agent was initially investigated using the Geo-Hyrock catalyst (Figure 8). Very high reactivity was achieved for substituted benzenes, particularly anisole, p-xylene, and mesitylene in which the benzylation was almost complete within 1 h (Figure 8a) with high selectivity toward the monobenzylated products (>95%) (Figure 8b). Lower reactivity was observed for the benzylation of toluene and benzene, with ∼80 and 40% conversion of BzCl respectively being achieved within the first hour. Nonetheless, after 90 min of reaction, >96% conversion of BzCl was achieved with high selectivity (>95%) in the benzylation of toluene, and up to 75% conversion in the benzylation of benzene with a selectivity of ∼85% to DPM. The raw Hyrock fly ash shows poor reactivity under identical reaction conditions (Figure 8c), being completely inactive for the benzylation of benzene and toluene, and producing only ∼10% conversion of BzCl in the benzylation of p-xylene. However, higher activity was observed for the benzylation of

Figure 6. TEM micrographs of the fly-ash-based geopolymer catalysts: (a) Geo-Hyrock, (b) Geo-Huntly, (c) Geo-Gladstone, and (d) GeoMt. Piper.

E


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Figure 7. High-magnification SEM images of specific spots containing Fe species. Each image (a−d) represents the red circled spots identified at the top two source images.

anisole and mesitylene, with BzCl conversion of ∼70 and 40% respectively. It is interesting to note that the reactivity of the Geo-Hyrock catalyst (Figure 8a) and the raw Hyrock fly ash (Figure 8c) in the benzylation of these aromatics was in the order anisole > mesitylene > p-xylene > toluene > benzene; this order of reactivity suggests a dependence mainly on the electron density of the aromatic ring, with the size of the reactant or product having little or no impact. The absence of the confinement effect when fly-ash-based catalysts are used suggests that the reaction most probably occurs on the surface of the catalyst, since the raw fly ash is a nonporous material and the fly-ashbased geopolymers have very small pore sizes due to their being blocked with other oxides and amorphous debris (Table 1). Figure 9 shows that the present experimental data are fitted well by the Langmuir−Hinshelwood pseudo-first-order kinetic model as expected, since a first step in heterogeneous catalytic reactions is the adsorption process which plays a major role in heterogeneous catalytic reactions. The slopes of the lines in Figure 9 were used to determine the reaction constants of the benzylation reactions of the aromatics, shown in Table 2. The effect of the catalyst/substrate wt % was studied for the benzylation of toluene with BzCl over the Geo-Hyrock catalyst at 80 and 90 °C (Figure 10). A greater degree of conversion was obtained when more of the catalyst was used, which can be understood in terms of the availability of more active sites when more of the catalyst is used. The data obtained in Figure 10 satisfy the Koros−Nowak and Madon−Boudart tests which ensures the absence of any mass or heat transfer limitations (see Table S4). As discussed above, fly ashes collected from different sources possess differing chemical and physical properties which are expected to affect their performance in catalysis applications. For this reason, three further series of geopolymer catalysts were synthesized from different fly ashes collected from

Figure 8. Comparison of the catalytic reactivity of Geo-Hyrock, raw Hyrock fly ash in benzylation of several aromatics: (a) and (b) GeoHyrock, (c) raw Hyrock fly ash. Reaction conditions: 13.0 mL of each aromatic; 1.0 mL of BzCl; 0.1 g of catalyst; T = 80 °C.

Figure 9. Langmuir−Hinshelwood pseudo-first-order kinetic model for the benzylation reactions of benzene, toluene, anisole, p-xylene, and mesitylene over the Geo-Hyrock catalyst.

different sources, and their catalytic reactivities were investigated in the benzylation of the same series of aromatics (Figure 11). Both the Geo-Huntly and Geo-Gladstone fly-ash-based catalysts show high catalytic reactivities in the benzylation of all the substituted aromatics (Figure 11a,c respectively); however, the latter shows no activity toward the benzylation of benzene. The benzylation reactions were almost complete F


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Table 2. Kinetic Parameters of the Catalytic Reactivity of Geo-Hyrock in the Benzylation of the Various Aromatics with BzCl after 30 min of Reaction Timea

a

substituent

conversion (%)

selectivity (%)b

specific reaction rate (mmolBzCl gcat−1 min−1)

k (× 103 min−1)

benzene toluene anisole p-xylene mesitylene

29 60 100 80 95

100 100 98 97 97

0.82 1.70 2.83 2.14 2.75

14.1 32.4 101.3 49.4 114.3

Reaction conditions: 13.0 mL of each aromatic; 1.0 mL of BzCl; 0.1 g of catalyst; T = 80 °C, t = 30 min. bToward the monobenzylated products.

explains the high reactivity of the Geo-Gladstone catalyst (9.05 wt % Fe2O3), whereas Geo-Mt. Piper (0.84 wt % Fe2O3) is inactive. It should also be noted that since these fly ash starting materials for these experiments were as-supplied by commercial producers they can be assumed to be from a stockpile rather than specially sampled fresh from a power plant. Thus, they will reflect more closely the behavior of real commercially available materials. Since the fly ashes were sourced from a range of producers, their degree of oxidation or leaching is expected to be different, yet the present results indicate that the catalytic behavior of the resulting catalysts depends on the iron content rather than their degree of oxidation or leaching. It is possible that the inactivity of Geo-Gladstone in the benzylation of benzene may suggest that in this particular reaction the active sites could be a combination of both Lewis and Bronsted sites arising from the aluminosilicate geopolymer framework and the Fe2O3 present from the original fly ash. This would explain why Geo-Hyrock and Geo-Huntly, with a combination of Lewis, Bronsted, and Fe2O3 sites, are the only active catalysts for the benzylation of benzene. It should be noted that the present catalysts possess different surface areas (see Table 1) which is a crucial element in heterogeneous catalysis. However, the reason for comparing these different fly-ash-based catalysts is to highlight the viability of synthesizing active catalysts derived from fly ashes that were collected from different sources. Fly ashes from different sources have different compositions depending on the coal from which they were obtained; this could potentially influence the catalytic reactivity of the resulting catalysts. The chemical composition of the raw fly ashes used, the parent geopolymers and the corresponding catalysts are summarized in Table S1, which shows that the main components of the fly ashes used in this study are SiO2, Al2O3, and Fe2O3, except in the case of Huntly fly ash which also has high CaO content. Other metal oxides present in the raw fly ash, including Na2O, K2O, MgO, TiO2, MnO, P2O5, and SO3, do not appear to negatively affect the catalytic performance of these fly-ash-based catalysts as these oxides are only present in minor amounts which is further diminished after the ion-exchange process. Table 4 compares the catalytic reactivities of some of the present fly-ash-based catalysts with other Fe-containing solid catalysts and ionic liquids in the benzylation of benzene with BzCl, under identical reaction conditions, unless otherwise stated. The data indicate that the present fly-ash-based catalysts show higher reactivity than Fe-ZSM-5, fly ash-supported sulfated zirconia and ionic liquids. Furthermore, the present catalysts show superior reactivity in the benzylation of other aromatics (toluene, anisole, and xylene) compared with those of the catalysts reported in Table 4. The reusability of the Geo-Hyrock catalyst in the benzylation of toluene with BzCl at 90 °C is shown in Figure 12. The catalyst was tested up to five reaction cycles, with no sign of

Figure 10. Influence of the catalyst: substrate wt % on the outcome of the benzylation reaction of toluene with BzCl over Geo-Hyrock catalyst: (a) at 80 °C and (b) at 90 °C. Aromatic: BzCl wt % = 13.

within 1 h, similar to the Geo-Hyrock catalyst, but during the first 30 min of reaction, Geo-Huntly and Geo-Gladstone show higher reactivity, particularly in the benzylation of mesitylene, xylene, and toluene. The selectivity toward monobenzylated products was usually >95% in the benzylation of all the aromatics except for the benzylation of benzene, where the selectivity decreases at high conversion % to ∼85%, as found in the Geo-Hyrock catalyst. However, the Geo-Mt. Piper catalyst (Figure 11e) is completely inactive in the benzylation of benzene, toluene, and p-xylene, with very poor reactivity toward the benzylation of anisole and mesitylene (95%. Moreover, Figure 12 also shows that the catalytic reactivity of the recycled catalyst is greater than in the first cycle, suggesting that a beneficial change to the catalyst structure may have taken place during use. The FTIR spectra of the spent catalysts (Figure S5) does not show any traces of adsorbed molecules on the surface of the catalyst after activation at 550 °C for 15 min. However, the TGA profile of the recycled catalyst (Figure S6) shows a mass decrease starting at >600 °C (above the reactivation temperature) and continuing to >800 °C, suggesting strong chemisorption is taking place, most probably within the pores of the catalyst, poisoning of some of the less-favorable sites after the first cycle of use; this might explain the higher reactivity obtained in the recycling experiments.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank David Flynn for assistance with the electron microscopy. M.I.M.A. acknowledges the financial support of a Ph.D. scholarship from MacDiarmid Institute for Advanced Materials and Nanotechnology.

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REFERENCES

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CONCLUSION This study reports a facile synthesis of environmentally benign geopolymer-based heterogeneous catalysts derived from industrial waste materials (fly ashes). The excellent catalytic activity of the resulting catalysts shows them to represent a new class of solid catalysts with ecological and economic benefits. Four different fly ashes collected from different sources were used in the synthesis of geopolymer catalysts. Three of the four resulting fly-ash-based catalysts showed excellent catalytic reactivities in the model Friedel−Crafts reaction studied in this work. This is ascribed to the additional benefits arising from the presence of Fe2O3, since the geopolymer catalysts prepared from fly ashes with higher Fe2O3 contents were more reactive toward the target reactions. However, the results suggest that in the benzylation of benzene in particular, the active sites are probably a combination of Lewis and Bronsted acid sites generated within the geopolymer framework, together with Fe2O3, since only the fly-ash-based catalysts that contain a I


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