Facile synthesis of new hierarchical aluminosilicate inorganic polymer solid acids and their catalytic performance in alkylation reactions

Accepted Manuscript Facile synthesis of new hierarchical aluminosilicate inorganic polymer solid acids and their catalytic performance in alkylation reactions Mohammad I.M. Alzeer, Kenneth J.D. MacKenzie, Robert A. Keyzers PII:

S1387-1811(16)30577-7

DOI:

10.1016/j.micromeso.2016.12.018

Reference:

MICMAT 8047

To appear in:

Microporous and Mesoporous Materials

Received Date: 24 October 2016 Revised Date:

14 December 2016

Accepted Date: 15 December 2016

Please cite this article as: M.I.M. Alzeer, K.J.D. MacKenzie, R.A. Keyzers, Facile synthesis of new hierarchical aluminosilicate inorganic polymer solid acids and their catalytic performance in alkylation reactions, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2016.12.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile synthesis of new hierarchical aluminosilicate inorganic polymer solid acids and ACCEPTED MANUSCRIPT their catalytic performance in alkylation reactions

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Mohammad I.M. Alzeer a, b *, Kenneth J.D. MacKenzie a, b *, Robert A. Keyzers a a School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600 Wellington, New Zealand b MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand

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Abstract

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This paper reports the synthesis of hierarchical aluminosilicate inorganic polymers (also known as geopolymers) containing both Bronsted and Lewis acid sites that function as a new class of heterogeneous solid acid catalysts. The geopolymers were synthesised from a naturally occurring clay mineral by an energy-efficiently and ecologically friendly process. Their catalytic performance was evaluated in a model liquid phase Friedel-Crafts alkylation of relatively large substituted benzenes (alkylation of toluene, anisole, p-xylene and mesitylene with benzyl chloride). The influence of the geopolymer starting composition on the acidity and porosity of the synthesised catalysts was studied and the impact of post-synthetic demetallation on their catalytic performance was investigated. The geopolymer-based catalysts achieved high catalytic activity which was superior to H-Y zeolite under identical reaction conditions. These results suggest that geopolymers have considerable potential as cost efficient, readily synthesised and environmentally friendly heterogeneous solid catalysts for fine chemical applications.

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Keywords: Friedel-Crafts heterogeneous catalysis.

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1. Introduction

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An increasing awareness of the need for environmental protection lies behind a demand for new heterogeneous solid catalysts, particularly solid acids, for use as environmentally friendly and costeffective alternatives to their homogeneous analogues for a wide range of catalytic processes [1]. Of the heterogeneous catalysts, aluminosilicates, particularly zeolites, are very important solid catalysts with widespread uses, the majority of which are in the bulk industry for cracking, isomerisation, alkylation, etc. [2]. The very limited applications of zeolites in the fine chemical industries is due to the constraints imposed by their strict micropore sizes (≤ 1.2 nm) which hinder their efficiency in liquid phase systems, specifically when dealing with bulk molecules, by diffusional limitations causing deactivation and affecting the catalyst lifetime [3, 4]. Attempts to overcome this limitation in zeolites have resulted in the development of a great diversity of materials with controlled porosities and a variety of functionalities.

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The discovery of a new family of mesoporous silicates (M41S) by Mobil Corporation in 1992 with well-defined pores in the range of 2-50 nm provoked much interest and raised expectations that these materials might be useful catalysts [5]. However, their low acidity and poor hydrothermal stability have hindered their industrial use as catalysts [6]. Another approach was the use of zeolite nanoparticles to overcome diffusional problems by reducing the intracrystaline diffusion path length; however, these still suffer from the difficulty and high cost associated with the separation of fine nanoparticles from the reaction mixture [4]. More recently, the introduction of additional mesopores into zeolite structures to form hierarchical zeolites has attracted attention [7], but despite the highly promising features of these materials, their main disadvantage that may militate against their large1

aluminosilicate

polymers,

geopolymers,

solid

acid,

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alkylation,

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scale production and commercialisation arises from their complicated hydrothermal synthesis that ACCEPTED MANUSCRIPT also requires the use of sophisticated organic structural directing agents (OSDAs) [8, 9].

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Therefore, the development of a new efficient and environmentally benign solid catalyst that could be prepared readily with tailored chemical and physical properties is highly desirable. In this paper we report the synthesis and characterisation of porous aluminosilicate inorganic polymers (geopolymers) as a new class of eco-friendly heterogeneous acid catalysts for liquid phase organic synthesis applications.

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Geopolymers are amorphous aluminosilicate inorganic polymers composed of randomly connected silicate and aluminate tetrahedral units forming a three-dimensional framework [10]. The negative charge on the tetrahedral aluminate is balanced by the presence of alkali cations that undergo ionexchange, providing them with similar properties to their crystalline analogous (zeolites) [11]. Due to their excellent mechanical properties, geopolymers have conventionally been applied as environmentally friendly alternatives to Portland cement [12], while their 3D framework and good porosity has led to their use in waste water treatment and for the storage and immobilization of heavy metals and radioactive wastes [13]. More recently, geopolymers have been investigated as drug delivery media and as supports in heterogeneous catalysis [14].

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The similar chemical and thermal stability of geopolymers to their crystalline zeolite analogues [10] suggests the possibility that they may be suitable candidates for heterogeneous catalysis. Furthermore, geopolymers can readily be prepared at ambient temperature from environmentally friendly natural raw materials. Unlike zeolites or mesoporous silicates, micro, meso or even hierarchical porosity can be introduced into the geopolymer structure by varying the synthesis composition without the use of costly, and sometimes toxic, OSDAs [15]. Acidic active sites can be generated in the geopolymer framework by ion-exchange with ammonium ions followed by thermal treatment, allowing the nature of these acidic sites (Bronsted or Lewis) to be tailored to specific applications [16].

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However, only a few studies have reported the use of geopolymers as catalysts, mainly as supports for catalytic elements. Sazama et. al. [17]. reported the redox catalytic activity of geopolymers containing the transition and noble metals Pt, Fe, Cu or Co. Photocatalytic applications using geopolymers as supports for nanoparticles such as TiO2[18] and CuO [19] have been reported. Geopolymers containing Ca2+ have been reported to act as solid base catalysts for the generation of biofuel [20]. Recently we have reported the synthesis of geopolymers with high surface area and active sites suitable for catalysing the Beckmann rearrangement of cyclohexanone oxime to caprolactam [21], and demonstrated that both weakly acidic silanol groups and minor amounts of Bronsted acid sites can successfully be generated on the surface of the geopolymer, giving it high activity and selectivity for the Beckmann rearrangement reaction. However, no studies have yet been reported of the catalytic properties of mesoporous or hierarchical geopolymers containing both Bronsted and Lewis acid sites generated within the structure, making them suitable for fine chemical applications such as Friedel-Crafts alkylation reactions.

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Aromatic alkylation is an important class of reaction in organic chemistry, and is used to synthesise a number of industrially valuable synthetic intermediates for the production of pharmaceuticals and fine chemicals [22]. Such reactions are catalysed mainly by conventional homogenous Lewis acids such as AlCl3, H2SO4, HF, or ZnCl2; however, these catalysts possess drastic drawbacks in terms of toxicity, corrosiveness, handling difficulty and reusability [23]. These drawbacks are compounded

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by the large amount of toxic wastes generated during the separation process which normally ACCEPTED involves water quenching and constitutes serious MANUSCRIPT environmental problems and high costs for the waste water treatment and disposal [24].

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For this reason, a number of heterogeneous solid acids have been employed as replacements for the homogeneous analogues. Zeolites such as the beta-form have been used and show high reactivity in the alkylation of benzene with benzyl alcohol; however, microporous zeolites show poor reactivity in the alkylation of relatively large molecules such as substituted benzene due to their small micropores [25]. Therefore several other solid catalysts have been developed for this purpose, including mesoporous zeolites [25-27], mesoporous silicates [28-31], supported oxides [32-35], and ionic liquids [36].

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In this paper we report the synthesis and characterisation of inorganic polymers (geopolymers) as novel heterogeneous solid acid catalysts containing both Bronsted and Lewis acidic sites within their structure, as required to catalyse reactions such as Friedel-Crafts alkylations. The acid sites in these materials provide the catalytic activity, rather than merely acting as supports for other catalytic elements. Their catalytic activity was evaluated for the model liquid-phase Friedel-Crafts alkylation of relatively large substituted benzenes (alkylation of toluene, anisole, p-xylene and mesitylene with benzyl chloride (BzCl), scheme 1). The present work also reports the effect of post-synthetic dealumination and desilication treatments on the structural properties and the corresponding improvement of the catalytic activities of these geopolymerbased catalysts.

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Scheme 1: Friedel-Crafts alkylation of benzene or substituted benzene with benzyl halide.

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2. Experimental

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2.1 Catalyst Synthesis

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The geopolymers and their acidic forms were synthesized from halloysite clay as described elsewhere [21]. The geopolymer powder was ion-exchanged with NH4+ following the method of O’Connor et al [11] and the acidic sites were generated by thermal treatment of the NH4+-geopolymer at 550 oC for 15 min. After heating, the catalyst was removed from the furnace straight to the reaction vessel.

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Two Na-based geopolymer catalysts and two K-based geopolymer catalysts, were synthesised each with two different molar compositions; two were of “normal” composition (SiO2/Al2O3 ~3.5, denoted as Na-N and KN respectively), and two with higher silicon contents (SiO2/Al2O3 ~6.5, denoted as Na-hiSi and K-hiSi respectively). See Supporting information (Table SI2) for full composition of the prepared geopolymers.

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2.2 Post Synthetic Treatments

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In order to improve the porosity and acidity of the geopolymer catalysts, they were subjected to sequential dealumination and desilication after ion exchange, following the procedure of Verboekend et al. [8]. The post synthetic procedure was followed as reported by Dealumination of the catalysts was performed by treating one gram of the solid with 20 ml/g 0.11 M Na2H2 EDTA (Merck) for 5 hr. at 85 oC. These samples 3

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were designated DA, e.g. Na-N-DA. Since Na2H2EDTA has no significant effect on dealumination of the ACCEPTED MANUSCRIPT Na-form of high Al content zeolite (e.g. Y zeolite) [8, 37], the dealumination step was applied to the NH4+ion-exchanged-geopolymers. A portion of the dealuminated solid was retained for catalytic testing and the remainder was submitted to desilication by treatment with 30 ml/g of 0.2 M NaOH for 30 min. at 65 oC, followed by acid washing, performed as in the dealumination step, but for only 2 hr. These sequentiallytreated samples were designated Seq e.g. Na-N-Seq. The final step in the post synthetic treatment was NH4+ ion-exchange three times for 6hr each.

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2.3 Catalyst Characterization X-ray powder diffraction patterns were collected using a Bruker D8 Avance X-ray diffractometer (Cu Kα radiation). The 27Al and 29Si MAS NMR spectra were acquired at 11.7 T using a Bruker Avance III 500 spectrometer at frequencies of 130.24 MHz and 99.29 MHz for 27Al and 29Si respectively. The N2 sorption isotherms were obtained using Micromeritics ASAP 2010 equipment, and transmission electron micrographs were obtained using a JEOL JSM-2100 F TEM. FT-IR spectra were acquired using a Perkin Elmer Spectrum One FTIR spectrometer and pyridine adsorption experiments were performed using a micromeritics degasser (VacPrep061), with the profile of the adsorbed pyridine obtained using a Shimadzu TGA-50 thermal analyser. SEM micrographs were obtained using a JEOL JSM-6610 LA analytical scanning electron microscope with an energy- dispersive spectrometer (EDS) operated at 10 keV on samples coated with a ~16 nm layer of carbon using a Quorum Q150T turbo-pumped carbon coater. Further details of these characterization procedures are reported elsewhere [21].

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2.4 Catalytic Reactivity

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Friedel-Crafts alkylation reactions were carried out under atmospheric pressure in a 100-ml magnetic stirred two necked round bottom flask equipped with a condenser in a thermally controlled bath. In a typical run 1ml of BzCl was mixed with 13 ml of the aromatic compound (toluene, anisole, p-xylene or mesitylene). 0.1 g of the catalyst was added to the reaction mixture at 85 oC and the temperature raised to 110 oC, this being taken as the starting time of the reaction. Samples were collected at different time intervals and analysed using a Shimadzu QP20-Plus GC-MS equipped with a 30 meter Rxi-5sil MS capillary column. Quantitative analysis was performed using conventionally-determined calibration curves. Each reaction was repeated at least three times and the error bars represent the standard errors. The degree of conversion was measured to the respect to benzylchloride as the limiting reagent. The conversion, reaction rate and TON [38] were determined as follows:

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number of moles of BzCl converted 100% (1) Total number of moles of BzCl fed

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% =

$% & ' & (&)

012 =

() =

number of moles of BzCl converted (2) amount of catlyst used (g). reaction time(h)

Total number of moles of monobenzylated product formed (3) Total number of moles of acidic sites

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The reusability of the catalyst was evaluated by filtering it off using a micro filter and heating to 550 oC for 1hr. The structural stability of the spent catalyst was investigated by XRD.

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3. Results and Discussion 4

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3.1 Catalyst Characterisation

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The geopolymeric nature of these materials was confirmed by XRD as described in Figure 1. The broad background hump shown in Figure 1b, represents Na-hiSi, in the range of 20-40 o 25 indicates the amorphous nature of the synthesised geopolymers. The sharp reflections are ascribed to crystalline silicates, quartz and cristobalite, present in the clay (Figure 1a). No evidence of recrystallization was detected after thermal treatment (Figure 1c) to generate the necessary acid sites in the catalysts, confirming the excellent thermal stability of these materials. XRD patterns for all the other synthesised geopolymers are attached in the SI (Figure SI1). 27

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Al MAS NMR spectra of all the geopolymers (Figure 2) show chemical shifts of ~60 ppm indicating the coordination of the aluminium is tetrahedral, reflecting a well-formed geopolymer [39, 40]. An additional small, broad 27Al resonances at about 4 ppm in the Na-geopolymer samples (Figure 2c and 2d) is assigned to six-fold AlO6 [40], from unreacted clay. All the as-synthesised geopolymers show a broad resonance at ~90 ppm indicating the Si is present in a tetrahedral form in which each Si is surrounded by 4 Al atoms (Figure 3) [40]. An additional resonance at ~ 94 ppm was observed for The Na-hiSi sample (Figure 3d) representing an additional environment of Si, Si (Q3) units in which Si is surrounded by 3 Al [40].

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Figure 1. Representative XRD patterns with normalised relative intensities; (a) halloysite clay, (b) Na-hiSi, (c) Na-hiSi-NH4+-550 oC. (h: aluminium silicate hydroxide (halloysite); q: quartz; c: cristobalite; ♦ Na2CO3.H2O)

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A well-formed geopolymer structure was also confirmed by FTIR spectroscopy. Figure 4 shows the FTIR spectra of the Na-hiSi geopolymer at different preparation stages. The parent Na-hiSi sample (Figure 4a) shows a typical strong broad peak at 1000 cm-1 with a shoulder at ~1080 cm-1 ascribed to the Si-O-Al stretching vibration, and Si-O-Si stretch respectively.[41] A broad peak at about 3500 cm-1 is assigned to hydrogen-bonded silanol nests (3500-3400 cm-1) [42]. Another small peak at ~1400 cm-1 is ascribed to the carbonation of geopolymer as detected by XRD); this is also associated with the broad CO2 asymmetric stretching vibration at about 2347 cm-1 [43]. The presence of adsorbed H2O is evident by the small peak at around 1640 cm-1 which is ascribed to the H-OH stretching mode [44]. The acidic form of the catalyst was obtained by exchanging the alkali ion with NH4+, using the procedure of O’Connor et. al.[11] followed by thermal treatment. After NH4+ ionexchange (Figure 4b) additional bands appeared at ~3200 cm-1, assigned to the N-H asymmetric stretching vibration, and two other bands at 1455 cm-1 and 1400 cm-1 which are typical bending modes of the ammonium ion.

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Figure 2. 27Al MAS NMR spectra of the as-synthesised geopolymer catalysts. (a) K-hiSi, (b) K-N, (c) NaN, (d) Na-hiSi.

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After heating to 450 oC (Figure 4c), the decomposition of NH3 is evident by the broadening of the N-H asymmetric stretching band and the almost complete loss of the N-H bending modes. The slight broadening of the band at ~3500 cm-1 indicates the removal of silanol nests by the thermal treatment. The generation of acidic hydroxyls after thermal treatment is evident by the appearance of two peaks at 3700 cm-1 and 3620 cm-1, assigned to H-bonded vicinal silanols and Bronsted bridging hydroxyls respectively [45]. In order to generate Lewis acidic sites, the catalysts were heated at 550 oC for 15 min, resulting in the total decomposition of the NH3 and the destruction of the silanol nests and the 5

H-bonded silanol nests because of intensive dehydration. However, the Bronsted OH groups (the ACCEPTED band at ~ 3620 cm-1) was still present in both theMANUSCRIPT Na-hiSi-NH4 and Na-hiSi-Seq catalysts (Figure 4d and 4e respectively).

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The N2 sorption isotherms of the as-synthesised and the sequentially treated catalysts are shown in Figure 5 and the corresponding porosity values are presented in Table 1. Of the parent geopolymers (Figure 5a), only Na-hiSi shows a type I isotherm corresponding to a microporous material. The other geopolymer catalysts show type IV isotherms with unique hysteresis loops indicating their mesoporosity. The hysteresis loop is of type H3 with unlimited gas uptake at high p/po, which is associated with aggregates possessing slit-like pores [46]. This is consistent with SEM micrographs (see Figure SI2) which indicate the presence of small nanoparticles aggregated to form larger particles, creating voids of different sizes within the particles. The pore size distribution of some of the synthesised geopolymers is in the range <10 nm to 100 nm, indicating the presence of macroporosity which was also observed by TEM (Figure 6a and 6c). However, the average pore size distribution of all the catalysts is in the range of 10 – 40 nm.

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Figure 3. 29Si MAS NMR spectra of the as-synthesised geopolymer catalysts. (a) K-hiSi, (b) K-N, (c) Na-N, (d) Na-hiSi. The asterisks denote spinning side bands.

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Table 1. Chemical and textural properties of the as-synthesised catalysts

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SBET (m²/g) 50.00 64.70 40.20 59.60 18.00 42.20 135.00 211.50

Vtotal (cm³/g) b 0.20 0.33 0.32 0.39 0.15 0.25 0.11 0.28

V meso (cm3/g) c 0.20 0.32 0.32 0.39 0.15 0.24 0.01 0.26

Dpore (nm) c 22.07 28.10 42.82 50.20 45.06 47.10 2.70 16.38

obtained from TGA profile of adsorbed pyridine (Figure SI3) single point measurement (p/po = 0.995) c BJH method b

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Acid content (mmol/g) a 0.20 0.23 0.18 0.23 0.14 0.20 0.32 0.37

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Figure 4. IR spectra of selected geopolymers-based catalysts; (a) Na-hiSi-parent, (b) Na-hiSi-NH4+, (c) NahiSi-NH4+-450 oC, (d) Na-hiSi-NH4+-550 oC, (e) Na-hiSi-Seq-550 oC.

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3.2 Influence of the Post Synthetic Treatments

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Around 80 wt. % of the solid catalyst originally used was recovered after the sequential treatment. Figure 5b presents the N2 sorption isotherms of the sequentially treated geopolymers with the corresponding values presented in Table 1. Both the surface area and the pore volume have increased significantly for all the geopolymer catalysts after the sequential treatments. The effect of these treatments is most apparent in the Na-hiSi geopolymer, in which a hierarchical structure is formed (Figure 5b). Moreover, the influence of these treatments on the porosity can also be seen in the TEM images of selected geopolymer catalysts 6

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before and after the sequential treatment (Figure 6). It can be seen that K-hiSi contains slit-like pores ACCEPTED MANUSCRIPT (Figure 6a), whereas Na-hiSi (Figure 6b) exhibits randomly-arranged micropores. Additional new mesopores were introduced into both catalysts after the sequential treatment of K-hiSi-Seq (Figure 6c) and Na-hiSi-Seq (Figure 6d).

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The influence of the sequential treatment on the chemical structure of the geopolymer catalysts was

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also investigated by

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sequential treatment varies with the geopolymer composition (Si/Al molar ratio). Geopolymers with

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the typical Si/Al molar ratio of ~3.5 (e.g. Na-N) show the generation of extra-framework aluminium

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(EFAl) during the dealumination step (Figure 7b), as evidenced by the two resonances at about 30.5

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ppm and 3.1 ppm corresponding to penta-coordinate and octahedral Al respectively [40]. After

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completion of the sequential treatment, Na-N-Seq (Figure 7c) shows similar spectra to the starting

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material in which the EFAl species were removed, in addition to the shift of the tetrahedral peak

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back to ~60 ppm due to desilication. On the other hand, geopolymers with higher Si content (Si/Al

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molar ratio ~6.5, e.g. Na-hiSi) appear more resistant to the acid and the sequential treatments (Figure

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7f-h). However, by heating the catalysts prior use in the catalytic reactions, both sequential treated

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catalysts Na-N-Seq and Na-hiSi-Seq show larger amounts of EFAl (Figures 7e and j) compared with

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catalysts that had not been sequentially treated (Figures 7d and i). These EFAl species usually act as

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Lewis acid sites in zeolites and zeolite-like materials [47]. Therefore, the sequentially-treated

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catalysts are expected to possess higher acidities than their untreated analogues, as was in fact found

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to be the case (see below).

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Al NMR. Figure 7 indicates that the influence of dealumination and the

Figure 5. N2 adsorption-desorption isotherms of the geopolymer-based catalysts. (a) parent catalysts, (b)

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sequentially-treated catalysts

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The types of surface acidity present in the geopolymer catalysts Na-hiSi and Na-hiSi-Seq are shown by their FTIR spectra (Figure 8). The presence of Lewis acid sites is confirmed by the characteristic IR bands at 1448 and 1600 cm-1. In addition, the bands at 1544 and 1638 cm-1 represent adsorbed pyridine on Bronsted acid sites. A distinctive band at 1490 cm-1 is assigned to a combination of both Lewis and Bronsted acid sites [48]. Whereas FTIR spectra of the adsorbed pyridine gives qualitative information about the surface acidity, a quantitative analysis of the total acidity generated within the geopolymer structure was determined from the TGA traces of the desorbed pyridine [21, 49, 50] (see Figure SI3) with the corresponding data shown in Table 1. As shown in Table 1, the sequentiallytreated catalysts have higher acidities than the untreated catalysts. In this respect, geopolymers show similar trends to zeolites in which such demetallation post-synthetic treatments usually enhance the porosity and acidity of the material leading to enhanced catalytic performance [3].

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Figure 6. Representative TEM micrographs of selected geopolymer catalysts before and after sequential treatment; (a) K-hiSi, (b) Na-hiSi, (c) K-hiSi-Seq, (d) Na-hiSi-Seq. 7

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3.3 Catalytic Reactivity

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In the first instance, the catalytic reactivity of the geopolymer catalysts was investigated in the alkylation of toluene with BzCl. The results are shown in Figure 9. The as-synthesised catalysts show poor reactivity, with the conversion of BzCl of less than 30%. The dealuminated geopolymer catalysts, on the other hand, show slightly higher reactivity, with conversions of up to ~50 % in some cases. However, after desilication followed by acid washing (the sequential treatment), a significant improvement of the catalytic reactivity was achieved, where the reaction was almost complete within 3 hr. (Table 2), with 100% selectivity towards monobenzylated toluene. These results indicate that sequential treatment is essential to obtain optimal catalytic performance. This very significant improvement can be attributed to both the enhanced acidity and the improved porosity which provides sufficient access for the reactant to the active sites in the pores of the solid and facilitates the removal of the products from the pores. This highlights the important role of desilication which introduces intracrystaline mesoporosity that cannot be achieved by dealumination [51]. Moreover, the additional step of mild acid treatment after alkaline treatment (desilication) has been reported to further improve the porosity by removing any remaining Al-rich debris that could block the pores [8].

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Of the present catalysts, Na-hiSi-Seq shows superior reactivity, achieving ~90% conversion of BzCl within 1hr. (Figure 9). Na-hiSi-Seq has higher acidity as well as unique hierarchical porosity (Figure 4b) compared with the other geopolymer catalysts. These superior results may be related to its composition (Table SI2) which seems to be optimal for catalyst geopolymers. Table 2 indicates that all the sequentially treated catalysts show comparable catalytic reactivity to H-Y zeolite; however, the synthesis of the geopolymer catalysts is a much more energy-efficient simpler procedure which does not involve lengthy thermal treatments or the use of costly and sometimes toxic, OSDAs that are needed for zeolite synthesis.

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Figure 7. 27Al MAS NMR spectra of selected geopolymer catalysts with different compositions at different stages of post-synthetic treatments. (a-e) Na-N; (a) Na-N parent, (b) Na-N-DA, (c) Na-N-Seq, (d) Na-N asparent heated to 550 oC, (e) Na-N-Seq heated to 550 oC. (f-j) Na-hiSi; (f) Na-hiSi parent, (g) Na-hiSi-DA, (h) Na-hiSi-Seq, (i) Na-hiSi parent heated to 550 oC, (J) Na-hiSi-Seq heated to 550 oC.

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The reaction kinetics were studied for the Na-hiSi-Seq catalyst in the alkylation of toluene (Figure 10). The reaction fits the Langmuir-Hinshelwood pseudo-first-order kinetic model with R2 > 0.996 (Figure 10a). From the slope of the linear plot, the reaction constant was found to be 0.027 min-1. The influence of temperature on the reaction rate is described by the Arrhenius plot (Figure 10b) from which the activation energy was determined to be 152.22 kJ/mol. This plot shows that the reaction rate constant is proportional to the temperature, in agreement with the literature regarding alkylation of benzene or a substituted benzene over various solid catalysts [52, 53].

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Figure 8. IR spectra of adsorbed pyridine over Na-hiSi with and without sequential treatment; (a) background spectrum, (b) Na-hiSi, (c) Na-hiSi-Seq.

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Table 2. Catalytic reactivity of several sequentially treated geopolymer-based catalysts in alkylation of toluene with benzylchloride a

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K-N-Seq

92.90

43.00

57.00

26.94

351

K-hiSi-Seq

94.70

53.90

46.10

27.47

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Na-N-Seq

90.20

47.30

52.70

26.16

392

Na-hiSi-Seq

99.20

52.20

47.80

28.53

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H-Y 92.30 58.90 41.10 26.77 o Reaction condition; 13.0 ml toluene, 1.0 ml BzCl, 0.1 g catalyst; T = 110 C; t = 3 hr.

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When using a porous catalyst, diffusional limitations, whether external (interphase, between the liquid and the solid phases) or, more importantly, internal (intraphase, within the pores of the catalyst) can alter the kinetics of the catalytic system. Several approaches have been used to evaluate the impact of the internal mass diffusion limitations on a catalytic reaction. In this study we used the Weisz-Prater criterion (6W.P.), which states that if the value of the dimensionless 6W.P parameter is < 0.3 for a reaction of 2nd order or less, the internal mass transfer limitations can be eliminated [54]. The value of the 6W.P parameter in the present work was found to be 7.43 x 10-6 (<< 0.3), which on the basis of these criteria indicates the absence of pore diffusion limitations. The complete calculations are described in detail in SI.

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Specific reaction rate TON (mmolBzCl.gcat-1.h-1)

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ACCEPTED MANUSCRIPT p-benzyltoluene o-benzyltoluene

Catalyst

Figure 9. Catalytic reactivity of the various geopolymer catalysts in alkylation of toluene with BzCl. Reaction conditions: 13.0 ml toluene; 1.0 ml BzCl; 0.1 g catalyst; T 110 oC.

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Figure 10. (a) Langmuir-Hinshelwood pseudo-first-order kinetic model and (b) Arrhenius plot for alkylation of toluene with BzCl over Na-hiSi-Seq.

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The influence of the substrate: catalyst weight ratio (wt. %) over the Na-hiSi-Seq catalyst is described in Figure 11. A higher degree of conversion was achieved when a larger amount of the catalyst was used; ~97% conversion of BzCl was achieved with 5:1 BzCl: catalyst wt. % in 1hr., while a conversion of ~90% and ~75% was obtained with 10:1 and 20:1 wt.% respectively. This can be ascribed to the greater number of active sites available when a larger amount of catalyst is present in the reaction mixture. However, the reaction is almost complete at ~3hr. even when BzCl: catalyst wt. % is as high as 20:1.

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Figure 11. The influence of the substrate:catalyst wt. % on the alkylation of toluene with BzCl over NahiSi-Seq catalyst. T = 110 oC.

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The catalytic reactivity of Na-hiSi-Seq in the alkylation of other larger aromatics (anisole, p-xylene, and mesitylene) with BzCl is illustrated in Figure 12 and Table 3. Figure 12 shows that up to 30 min., the reactivity increases in the following order: anisole > p-xylene > mesitylene > toluene. The reaction with anisole was complete in less than 15 min., and was nearly complete after ~3hr when other aromatics were used, with 100% selectivity towards monobenzylated products. This was expected for a typical acid-catalysed electrophilic substitution reaction in which increasing the electron density in the aromatic ring due to attachment of electron donating groups enhances the reaction rate. However, the lower reactivity of mesitylene compared with p-xylene and toluene, after 9

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30 min. reaction time (Table 3) can be ascribed to the confinement effect in which the resulting ACCEPTED MANUSCRIPT monobenzylated mesitylene is larger than monobenzylated p-xylene and benzyltoluene; this could affect the reaction rate.

4 Figure 12. Catalytic reactivity of Na-hiSi-Seq in alkylation of mesitylene, p-xylene, anisole, and toluene with BzCl. Reaction conditions: 13.0 ml of each aromatic; 1.0 ml BzCl; 0.1 g catalyst; T = 110 oC.

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Table 3. Catalytic activity of Na-hiSi-Seq in alkylation of several aromatics at 1 hr. reaction time a Anisole p-Xylene Mesitylene Toluene Conversion % Specific reaction rate (mmolBzCl.gcat-1.h-1) a

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The results of Figure 12 suggest that the performance of the geopolymer catalysts is significantly better than that of regular microporous zeolites which show poor reactivity for the liquid-phase alkylation of such large molecules [25, 52].

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The reusability of the Na-hiSi-Seq catalyst (Figure 13) shows no sign of deactivation after five cycles of use of the same catalyst. These results suggest that geopolymer-based catalysts can readily and simply be regenerated for reuse. Furthermore, these results show that geopolymer catalysts do not suffer the drawbacks of supported catalysts, which suffer from the weak interaction between the active species and the host leading to loss of reactivity due to leaching of these active sites. The stability of the reused geopolymer-based catalyst is illustrated in Figure SI4 which shows little difference between the XRD pattern of the fresh and spent catalyst after three reaction cycles, reflecting the high thermal and hydrothermal stability of geopolymer-based catalysts.

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Figure 13. Reusability of Na-hiSi-Seq catalyst in alkylation of toluene with BzCl. Reaction conditions; 130:10:1 Toluene: BzCl: catalyst wt. %; T 110 oC.

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Thus, the present research illustrates a significant potential of geopolymers in the field of heterogeneous catalysis, representing as they do a new class of solid catalysts with highly desirable features such as ecological friendliness, inexpensiveness, and ease of synthesis. Further research is required to improve the catalytic performance of geopolymers and to investigate their applicability in wide range of catalytic reactions.

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This study reports the synthesis and characterisation of porous inorganic geopolymers as new class of solid acid catalysts. Their heterogeneous catalytic applications were evaluated in the FriedelCrafts alkylation reactions of several substituted benzenes (toluene, anisole, p-xylene, and mesitylene) with benzyl chloride. The starting composition of the geopolymer was found to have a direct influence on the acidity and the porosity of the synthesised catalyst. A very significant improvement in the reactivity of the as-synthesised catalysts was achieved by applying postsynthetic treatments (dealumination and desilication) which enhanced the acidity and porosity,

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resulting in a higher catalytic efficiency. The geopolymer catalyst could readily be regenerated and MANUSCRIPT reused without significant loss ofACCEPTED reactivity, since the acidic sites are part of the geopolymer structure rather than supported on geopolymer framework. The present study indicates the potential of geopolymers as new, cost efficient, readily synthesised and environmentally friendly heterogeneous solid acid catalysts for fine chemical applications, particularly for bulk compounds where regular microporous zeolites show poor reactivity.

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at http://

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Acknowledgment

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We thank Grant O’Sullivan for kindly supplying the halloysite clay. We are also grateful for David Flynn for assisting with TEM experiments. Mohammad Alzeer acknowledges MacDiarmid Institute for Advanced Materials and Nanotechnology for the financial support of a PhD fellowship. References

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Mesoporous and hierarchical geopolymers are presented as solid acid catalysts These catalysts show high activity for Friedel-Crafts alkylation reactions They are synthesised by an energy-efficient and ecologically friendly process These acid sites are integrated within the geopolymers structure Geopolymer catalysts are reusable without loss of activity

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