# H-ZSM-5 Zeolite Synthesis by Sourcing Silica from the Wheat Husk Ash: Characterization and Application as a Versatile Heterogeneous Catalyst in Organic Transformations including Some Multicomponent Reactions

Authors: Isak Rajjak Shaikh; Rafique Ahmed Shaikh; Alamgir Abdulla Shaikh; Javeed Ahmad War; Shankar Poshetti Hangirgekar; Ahmad Lalahmad Shaikh; Parveen Rajjak Shaikh; Rafik Rajjak Shaikh
Journal: Journal of Catalysts
 (2015)

---

## Abstract

ZSM-5 zeolite was synthesized by sourcing silica from wheat husk ash and was converted into its protonated form following careful,
conventionally accepted ion-exchange method and calcinations. The H-ZSM-5 zeolite (Si/Al ratio = 30) prepared was appropriately characterized by
XRD, FT-IR, SEM, TEM, TG-DTA, CV, and so forth, and evaluated as recyclable catalyst in selective organic transformations including three- and four-component reactions.
The catalytic activity of the zeolite sample supposedly relates to increased density of the accessible acid sites.


---

## Body

## 1. Introduction

Zeolites and related nanoporous materials are widely used as adsorbents, ion-exchangers, and catalysts [1]. In oil and chemical industries, the employment of zeolites as acid cracking catalysts [2] is such that some 350,000 tonnes are used annually for this purpose. Zeolites are a well-defined class of crystalline aluminosilicates [3]. By definition, the zeolite synthesis generally takes place under high pH conditions and involves crystallization from an inhomogeneous gel of a silica source and an alumina source combined with water. Scientific literature and trends in industrial applications established synthetic zeolites as a new commodity.Today, the growing environmental concerns stemming from the large-scale material syntheses and related industrial activities necessitate the development of “sustainable or environmentally benign” methods. Off late, researchers have recognized that high-value nanoporous materials and zeolites, identical to that prepared using chemical sources, can be synthesized in the presence of organic templates by utilizing low-value rice husk ash as a sustainable silica source [4]. Sustainability is a widely used term and encompasses many aspects including careful development and utilization of resources without severely jeopardizing the ecological and environmental health. Biomass and renewable raw materials can be assigned important roles in strategies for sustainable chemistry [5]. Nowadays, for example, the need for generating heat and electricity from renewables is more than ever. Ash production is bound to increase with the increase in the biomass combustion. Neither producers nor end-users know the value of ash. As a result, the ash produced is not treated properly and therefore disposed of. Utilization of biomass ashes has been so far limited to soap production and glass making [6]. In view of the issues with regard to the pollution, there is a need for research on developing innovative products and processes from ash utilization. The ashes are usually categorized into fly ash and bottom ash. Currently biomass (fly) ashes especially from grain polish plant/power stations or fuel pellet combustion [7] are recognized as raw material with soil remediation properties and fertilizers [8, 9] due to their constituents such as potassium (as KCl or K2SO4), magnesium (as MgO), or phosphorus (as phosphates). However, in some countries, regulation is in place to prevent environmental and ecotoxicological effects of biomass ash to be used directly on farmland due to its high content of cadmium and other trace elements. Biomass ashes, in particular the bottom ashes, are rich in silica [10] and so recovering value added products or preparing materials from bottom ashes, especially from grain milling plants/grain polish stations, or power stations is desirable. The ashes from rice husk/hulls [4] andMiscanthus grass [11] have shown the potential for forming silicate solutions by alkaline extraction and that can further be used to synthesize high-value materials.Wheat husk is one of the highest-volume bioresidues worldwide, particularly in India. Open-air burning of wheat husk is still a common practice in Brazil and India causing CO2 emission and related pollution. It was Javed et al. who firstly reported extraction of amorphous silica from the wheat husk [12]. Inspired researchers studied not only the chemical value of extraction residues but also the product distribution and their generation mechanisms through lab-scale combustion and microwave pyrolysis of wheat straw [13]. Terzioglu and Yucel published magnesium silicate synthesized from wheat husk ash [14]. An attractive and potential scalable methodology would be the conversion of the wheat husk based power station bottom ashes into high-purity and useful siliceous ion-exchangers, sorbents, and catalysts. We were the first to report the utilization of silica extracted from the wheat husk ash in synthesizing MCM-41 (Mobil Composite Material number 41) type mesoporous silica [15]. Furthermore, this material was found to have interesting catalytic properties suitable for the Knoevenagel condensation. To the best of our knowledge, the result documented herein is the first synthesis of H-ZSM-5 zeolite by sourcing silica from the wheat husk ash. We once again proved the wheat husk ash as low-cost alternative source to prepare high-cost and high-value zeolite materials, making it possible to close production life cycles. In addition to that, we also present the applications of this zeolite as a heterogeneous catalyst in various types of organic reactions.During the course of this research, researchers, namely, Pinar Terzioglu, Sevil Yücel, and Mehmet Öztürk, published a nice short paper [16] entitled “Synthesis and Characterization of ZSM-5 Zeolite from Wheat Hull Ash” on pages 292–296 ofThe Proceedings of the 1st International Porous and Powder Materials Symposium and Exhibition PPM 2013 (ISBN: 978-975-6590-05-8). Our manuscript reports the synthesis of ZSM-5 and H-ZSM-5 zeolite by sourcing silica from the wheat husk ash. Except the source of silica, all the reagents used and stepwise procedure followed during the synthesis of ZSM-5 are similar to those of the previous report [17].Zeolite Socony Mobil-5 (ZSM-5), first reported [18] by Mobil Co. in 1972, is the end-member of high-silica pentasil zeolite family. It is recognized by the International Zeolite Association of MFI topological category. ZSM-5 is one of the best working heterogeneous catalysts at industrial level due to its varied applications [3, 19, 20], for example, in gas phase ethylation (the Mobil/Badger process), xylene isomerization, benzene alkylation, hydrodewaxing, methanol to gasoline conversion, and catalytic breakdown of natural oils. It is also found that ZSM-5 can be modified and used for catalyzing some organic transformations under solvent-free conditions [21]. The structure of ZSM-5 zeolite has 10-membered oxygen ring and two types of channel systems with similar size: straight channels (5.3 × 5.6 Å) and sinusoidal channels (5.1 × 5.5 Å). These two different channels are perpendicular to each other and generate intersections with diameters of 8.9 Å. The zeolite has high silicon and low aluminium content that consequently makes its framework hydrophobic in nature. H-ZSM-5 is the protonic type or H form of ZSM-5 and has been extensively employed in acid catalyzed industrial chemical processes [22]. It is learnt from the literature that usually the as-synthesized H-ZSM-5 zeolites are known to exhibit low selectivity due to the presence of unselective acid sites located on the external surface and in the pore mouth regions. Therefore, post-synthesis modification mostly described as pore-size engineering [23] is chosen to tailor the properties of zeolites, including the concentration of acid sites. Modification of ZSM-5 to H-ZSM-5 can be achieved with a broad range of Si/Al ratio (from 6 to infinity). This, in turn, could help zeolite catalysts achieve selectivity by influencing diffusion and overall properties [24]. Transition metal ions can also be substituted into the zeolite framework to arrange active site(s) required for specific reactions [25]. Chemical modification of H-ZSM-5 zeolites by impregnation with acids (e.g., phosphoric/boric acid) led to their catalytic properties in n-hexane cracking [26], methanol to hydrocarbon (MTH) conversion [27], toluene disproportionation [28, 29], toluene alkylation with methanol [28–30], alkylation of benzene with ethanol [31], and so forth.We present herein (i) the synthesis of ZSM-5 by sourcing silica from the wheat husk ash, (ii) the modification of ZSM-5 to H-ZSM-5 with Si/Al ratio of 30, (iii) the physicochemical characterizations of the sample, and (iv) the catalytic applications. Except the source of silica, all the reagents used and stepwise procedure followed during the synthesis of ZSM-5 are similar to those published by W. Panpa and S. Jinawath (2009) in their research paper titled “Synthesis of ZSM-5 Zeolite and Silicalite from Rice Husk Ash” [17]. Our paper also reports the catalytic behavior of H-ZSM-5 in various types of organic reactions, such as cyclocondensation and some multicomponent reactions (MCRs) that have not so far been proved possible to achieve in a zeolite catalyst such as this. Isak Rajjak Shaikh is the principal investigator of this research and the lead author of this communication. Here, no claim is being made for first publishing ZSM-5 sourcing silica from wheat waste stream because during the course of this research, Pinar Terzioglu, Sevil Yücel, and Mehmet Öztürk published a short paper entitled “Synthesis and Characterization of ZSM-5 Zeolite from Wheat Hull Ash” on pages 292–296 ofThe Proceedings of the 1st International Porous and Powder Materials Symposium and Exhibition PPM 2013 (Editors: Sevgi Kilic Ozdemir, Mehmet Polat, and Metin Tanoglu; ISBN: 978-975-6590-05-8). Herein, the protonated form ofZSM-5, that is, H-ZSM-5 with various Si/Al ratios, was prepared by modifying the ZSM-5 synthesized by sourcing silica from the wheat husk ash. We report the first synthesis of H-ZSM-5 by utilizing wheat husk as a source of silica. As far as highlighting the importance of this original research paper is concerned, it should be stressed that little research was carried out to applications of this catalytic system in the area of multicomponent reactions [32, 33]. The novel catalytic applications of H-ZSM-5 are interesting because the organic transformations over this neat zeolite sample are achieved without having to introduce isomorphously substituted or extraframework or impregnated metal ions or clusters.


## 2. Materials and Methods

The emphasis of this work was to prepare ZSM-5 zeolite material identical to that prepared by using chemical sources of silicates. Accordingly, silica was sourced by the extraction of wheat husk bottom ash and was further used to prepare the microporous aluminosilicates, ZSM-5, and H-ZSM-5 zeolite materials.Here, in the first part of this paper, the stepwise synthetic methodology involving the preparation of the ZSM-5 and its modification to the H-ZSM-5 is described.(a)
The wheat husk ash was collected from a grain milling & power plant and firstly boiled in 1 M hydrochloric acid (HCl) at 363 K. It was then washed with double distilled water. The material was slowly subjected to thermogravimetric decomposition and calcined around 823 K for minimum 6 hours in muffle furnace. Then, the silica thus extracted was utilized as the raw materials for the preparation of ZSM-5.(b)
The tetrapropyl ammonium bromide (TPABr) templated synthesis of ZSM-5 was carried out [17] by hydrothermal crystallization of reactants NaOH, H2SO4, HCl, n-propyl amine, and aluminum sulfate (Al2(SO4)3·16H2O) along with the material extracted from the wheat husk ash as SiO2 source. Firstly, gel comprising silica, extracted from wheat husk ash, and NaOH were dissolved in double distilled water at 353 K. In another beaker, TPABr solution was prepared in double distilled water at 323 K and then added to the silica solution. Few drops of n-propyl amine solution were added and the mixture was stirred for 10 minutes. The aqueous solution of the aluminium sulfate was prepared in concentrated H2SO4 and was poured to the wheat husk ash silica + NaOH or (in-situ formed) sodium silicate and TPABr template solution at pH 11. The pH of the mother liquor was maintained in the range of 11 to 12. The reactants were then subjected to hydrothermal crystallization in Teflon-lined stainless steel autoclaves for one week at around 423 K. Then, the solid produced was filtered out and washed at least five times with deionized water. The solid was dried overnight at 383 K. Finally, the sample was calcined in dry air at 823 K for 5 h in furnace to decompose the organic amine template.(c)
H-ZSM-5 was prepared by ion-exchange of sodium form of ZSM-5 with NH4Cl. This is how Na+ in Na-ZSM-5 was replaced with NH4+ by impregnating the solution of 1 mol/L NH4Cl solution at 363 K to give NH4+ form of ZSM-5. Subsequent calcination of NH4+ form of ZSM-5 at 773 K overnight gave its protonated form, H-ZSM-5.The H-ZSM-5 is preferred as solid acid catalysts in industries because they are noncorrosive, easy to handle, stable, relatively selective, and recyclable and generate less waste as compared to homogeneous catalysts. Two types of acidic sites are usually present in H-ZSM-5: (i) Brønsted acid sites are related to aluminum located in the zeolite framework; (ii) Lewis acid sites presumably stem from the extraframework aluminum or distorted aluminum in the framework. In order to adjust acidity, Jin et al. [34] reported an environmentally benign, timesaving, and improved dealumination method caused by (utilizing) the self-adsorbed water in H-ZSM-5. When treated at around 400°C, about 80% acidic amount was removed from parent H-ZSM-5 and more Lewis acid sites were generated after treatment at 500°C. The dealumination method was found to decrease the acidity and weaken acid strength. Similarly, the H-ZSM-5 solid sample which we calcined at 823 K for 5 h could influence its acid sites and their density and strength. A discussion on appropriate characterization of the H-ZSM-5 sample especially including the characterization of acid sites of this sample by using infrared (IR) spectroscopy is included in the later part of this paper.


## 3. Results and Discussion

### 3.1. Characterization

In this section of the paper, characterization of the H-ZSM-5 is described. X-ray diffraction (XRD) measurements were performed with a D8 Advance Bruker diffractometer, 40 KV, 40 mA current to check if they have the characteristic microporous zeolite structure of the ZSM-5. Transmission Electron Microscopy (TEM) was performed on a Tecnai F20 200 kV microscope from Philips to confirm the structure and microporous H-ZSM-5. Simultaneous DSC/TGA were recorded using NETZSCH STA 449F1. DSC and TG analyses were carried out using 10 mg of the sample at a heating rate of 10°C min−1 up to 1000°C, in nitrogen under a flow of 60 cm3 min−1. Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrophotometer (Shimadzu model 8400 S) with a resolution of 2 cm−1 and in the range 400–4000 cm−1. Cyclic voltammetry (CV) measurements were done on 7G7 VA Computrace (Swiss model) instrument with glassy carbon electrode as the working electrode, tetramethyl ammonium bromide as a supporting electrolyte, Ag/AgCl as the reference electrode, and Pt electrode as counter electrode. DMF was used to dissolve the sample before the actual measurements were made.A single crystalline phase of the MFI-structure was confirmed by observing the typical reflection peaks at 2θ of 7.94, 9.07, 13.96, 23.98, 24.57, 26.02, 27.05, 29.22, 29.46, 45.14, 29.42, 30.10, and 45.44 degrees, and so forth (Figure 1). The H-ZSM-5 zeolite was hydrothermally and chemically stable and the XRD patterns were similar to that of the usual H-ZSM-5 reported earlier in the scientific literature [35]. Scanning Electron Microscopy (SEM) deduced the morphology of the zeolite. Figure 1 (SEM image as inset) shows spherical morphology of the structures.Figure 1
XRD pattern of H-ZSM-5 (with SEM image as inset).The silicon to aluminium ratio (Si/Al = 30) was determined by elemental analysis and the atomic absorption spectroscopy (AAS). Elemental analysis of the H-ZSM-5 catalyst used in this study was performed by the electron dispersive spectroscopy (EDS) as shown (Table1) in weight% and atomic%. The spectrum (Figure 2) contains strong characteristic peaks of Si, O, Al, and C as expected, together with additional peak corresponding to Cu. This is either an impurity or artifact arising from the Cu grid on which the sample is supported.Table 1
Elemental composition in weight% and atomic wt%.
Element
Weight%
Atomic%
C K
7.47
12.82
O K
42.12
54.28
Al K
1.64
1.25
Si K
38.39
28.19
S K
0.28
0.18
Cu K
10.11
3.28
Total
100
100Figure 2
EDS.The examination of defects and phases of zeolite character was made by the TEM (Figure3). The aggregates observed at couple of places in TEM images need explanation and could correspond to the crystalline aluminium oxide phases or A1 oxidic systems, isolated in some significant stages of the surface dehydration. These aggregates might have been dispersed on the surface and also inside the pores of zeolite by solid-state reactions at calcination temperature via a surface process. During this process, aluminum oxide can react with the silanol and bridging hydroxyl groups and also with extraframework Al species of the zeolite. And also small amount can enter into the pores and react with the bridging hydroxyl groups. Such a modification could partially passivate unselective Brønsted acid sites of the medium pore H-ZSM-5 zeolite framework and might thereby influence their overall catalytic behaviour.Figure 3
TEM images of H-ZSM-5 obtained from different locations within the sample under study.We know that the thermal treatment can cause transformations of one zeolite structure to another, structure collapse to an amorphous phase, recrystallization to non-zeolitic materials, and so forth. Thermogravimetric-diffractometry thermal analysis (TG-DTA) was performed to investigate the mass loss of the zeolite as the temperature increased. TG-DTA plot of synthesized H-ZSM-5 (Figure4) shows that the structure of the zeolite is clearly sustained up to about 723 K (600°C). It also revealed distinct mass losses of about 2.63% (40 to 100°C), 3.77% (100 to 700°C), and 8.12% (up to 800°C; residual mass 92.88).Figure 4
TG-DTA curves of H-ZSM-5.In principle, the IR spectroscopy could help us understand the active sites and subsequently the mechanisms invoked in zeolite catalysis. The surface acidity of polycrystalline solids is a complex but technologically important property. In this section, only few preliminary considerations with regard to the acidity of catalytic system and its determination with IR spectroscopy will be introduced. We know that the lattice vibrations characteristic to H-ZSM-5 zeolite usually appear in the range of 400–1200 cm−1, while the bands corresponding to OH bond vibrations appear at wave numbers higher than 3200 cm−1. Fourier Transform Infra Red (FT-IR) spectrum of the neat H-ZSM-5 zeolite sample, in the range of 400–4000 cm−1, is studied here and shown in Figure 5. The sharp peak at 545 cm−1 and also the bands at 549 cm−1, 585 cm−1, and 600 cm−1 could be assigned to the structurally sensitive double five-member ring tetrahedral vibrations, and it is typical for the crystalline ZSM-5 zeolite. The characteristic band of MFI structure corresponding to the bending of the TO4 (T = Si, Al) tetrahedra is found shifted from 450 cm−1 to 425 cm−1 and the IR bands at 780 and 1100 cm−1 can be assigned to the symmetric and asymmetric stretching vibrations of the Si–O–Si linkages of the zeolite framework, respectively [24, 36–38]. The vibration modes of 1046 and 793 cm−1 are assigned to the internal vibration of SiO4 and AlO4 tetrahedra and a small IR band near 1235 cm−1 is attributed to their asymmetric stretching vibration. The H–OH bending vibrations of the adsorbed water molecules are observed at 1625 cm−1. The IR spectroscopy carefully observes the acidic hydroxyl OH groups of solid catalysts [36–38]. Sharp features appearing at 3235, 3605, 3425, and 3444 cm−1 correspond to hydroxyl groups of our H-ZSM-5 whereas in case of the spectrum of a neat H-ZSM-5, the aforementioned bands usually appear at 3235 cm−1 and 3425 cm−1. The IR band at 3605 cm−1 is a characteristic of protonated form of zeolite and its intensity is correlated with framework aluminium (FAL) [39]. Previous works in the literature ascribed the absorption bands at 3745 and 3680 cm−1 to the nonacidic hydroxyl groups and the hydroxyl groups of extraframework aluminum oxide, respectively [36–38]. These IR bands are not observed in our H-ZSM-5 spectrum. The IR band at 3606 cm−1 corresponds to bridging hydroxyl groups of the Brønsted acid sites that are found responsible for catalyzing cracking [40, 41]. Overall intrinsic activity depends on the local environment [42] and the presence of extraframework aluminium (EFAL) caused by mild steaming is also known to enhance the catalytic activity per H+ [43]. Some reports postulated that the extralattice Al is tetrahedrally coordinated and has at least one OH group with an IR vibration at 3656 cm−1 while others believed that due to the transformation of tetrahedrally coordinated lattice Al3+ to extraframework AlOHx species, the additional IR band at 3656 cm−1 is seen [44, 45]. In Figure 5, the IR band around 3660–3690 cm−1 could be attributed to EFAL, bound to the zeolite framework via one or two oxygen bonds, and is consistent with the literature [46]. During the course of this paper preparation, we come across an original research article where the IR band appearing at 3656 cm−1 was assigned to the extraframework aluminium species in proximity of the strongest Brønsted acid site and was found to impact the catalytic activity of ZSM-5 [47]. It is therefore clear that the high specific activity stems from cooperative interaction between Brønsted acid site and an adjacent EFAL. In the IR spectrum of our H-ZSM-5, the band at 3610 cm−1 is attributable to the strongest Brønsted acid sites Si(OH)Al [48]. The IR signal at 3740 cm−1 corresponding to silanol groups (SiOH) in amorphous silica was not seen there. The bands at 3720 cm−1 and 3745 cm−1 related to defect site and terminal silanol group, respectively, are also absent. It can also be said that the OH group associated with octahedral EFAL is not observed because no IR band appeared around 3780 cm−1 as it was reported in MFI-15 [45].Figure 5
FT-IR spectrum of H-ZSM-5.EFAL is a term that describes chemically different aluminium (Al) species, besides the framework Al. The EFAL could be (i) a charged aluminum oxide, (ii) neutral species such as AlOOH and Al(OH)3, (iii) aluminum oxo- and hydroxyl clusters, (iv) bulk aluminum oxide aggregates [49–51], (v) a partially dealuminated Al (O3Si–O–Al–OH), and so forth, [51, 52]. Lewis acid sites are generated due to the presence of EFAL and due to electronic defaults in the framework aluminium (FAL). Marques et al. [53] proposed EFAL as a weaker Lewis acid site than FAL; the IR spectrum of H-ZSM-5 in Figure 5 assigns the band around 1400–1415 cm−1 to Lewis acid sites related to plausible defects. Interestingly, the usual bands at 1444 and 1454 cm−1 corresponding to EFAL and FAL, respectively, are missing. From the preliminary spectroscopic studies, we conclude that the EFAL is identified as the main Lewis acid sites of H-ZSM-5 zeolite sample and is weak. The band at 1400 cm−1 is in agreement with the atomic oxygen linked to strong Lewis acid sites [54]. This unique IR measurement deserves further investigation because this could be the strongest Lewis acid site due to an electronic defect. We were initially tempted to assign the sharp absorption band at 800 cm−1 to the tetrahedral coordination of Al ions present, as in the transition alumina phase due to stretching vibrations of a lattice of interlinked tetrahedral Al, but there was no peak around 3775 cm−1 which is attributed to the most reactive OH species on all types of the transition alumina. So, as far as envisaging active sites of H-ZSM-5 is concerned, no one can deny the role of defective crystal configurations, if any. Methodology involving infrared and adsorption of selected probe molecules should be adopted for confirming the surface acidity and the catalytic active sites of this H-ZSM-5 sample [37].Though the cyclic voltammetry cannot confirm the nature of the redox couple, the peak potential when correlated with standards is an indicator of the possible electroactive and electroinactive species in solid catalyst. The point should also be noted that the synergistic effect of various acid sites might be facilitating a better charge delocalization in the zeolite to result in a more delocalized proton required for a successful protonation of reactant. This is consistent with the result obtained by cyclic voltammetry (Figure6). An electrode was prepared by incorporation of H-ZSM-5 zeolite in the carbon paste matrix. At this stage, it suffices to say that the cations located inside the pores or extra-framework exhibit electrochemical behavior [55]. This could be due to the mononuclear and or dinuclear oxygenated aluminium species formed on the protonic ZSM-5 surface [56].Figure 6
Cyclic voltammetry of H-ZSM-5.


### 3.2. Catalytic Applications

At industrial level, many organic syntheses are catalyzed by conventional strong acids such as AlCl3, BF3, and FeCl3. Handling hazards, toxicity, difficulty in recovering the catalyst, difficulty in separation of product, and so forth are the problems associated with corrosive acid catalysts under homogeneous conditions and therefore necessitate the development of heterogeneous catalysts. The zeolites present an environmental advantage over conventional acid catalysts for various synthetic methodologies [57, 58]. There exist correlations between zeolite properties and catalytic functionality. Catalysts like silica, alumina, and zeolites are known to catalyze condensation, addition, and some other useful methodologies in fine chemicals [58–60]. In the light of the synthetic conditions and the abundant protons present in the high surface area aluminosilicate framework, we investigated how H-ZSM-5 (Si/Al = 30) is capable of supporting acid catalyzed reactions. We examined the shape-selective catalysis by carrying out the nitration of toluene with n-propyl nitrate as a test reaction. The acidic nature of the catalyst formed nitronium ions from n-propyl nitrate required for nitration reaction. The reaction over this particular catalyst yielded only thepara-isomer. This result was in line with the literature [61].In view of the possible accessibility of acidic sites, we investigated whether H-ZSM-5 with Si/Al ratio 30 is capable of supporting some organic transformations.Scheme1. Quinoxalines are a class of nitrogen containing bicyclic heterocyclic compounds and are currently attracting widespread attention due to their wide range of therapeutic applications. Quinoxaline moiety forms the basic skeleton for various antibiotics [62]. The H-ZSM-5 was found to catalyze the classical methodology (Scheme 1) for synthesis of quinoxalines by condensing 1,2-aromatic diamines with 1,2-diketones. The classic synthetic transformation took place at room temperature stirring within 6 to 10 minutes in methanol.Scheme 1
Classical synthesis of 2,3-diphenyl quinoxaline by cyclocondensation of benzil and benzene-1,2-diamine.A reaction mixture of 1,2-diaminobenzene (2 mmol), benzil (2 mmol), and catalytic amount of H-ZSM-5 was taken in methanol and stirred at room temperature. The reaction was monitored by TLC solvent system: n-hexane/ethyl acetate (7 : 3) till its completion. After 10 minutes, the mixture was then poured into ice-water. Solid reaction product was filtered out (yield: 88%). The quinoxaline product thus formed was purified by solvent recrystallization using ethanol as solvent. The product thus obtained was confirmed by measuring its melting point (125–127°C) first. The product of the reaction in Scheme1 is identified appropriately as 2,3-diphenylquinoxaline (molecular formula: C20H14N2).The product was characterized using1H NMR (Figure 7) and FT-IR (Figure 8), as 1H NMR (CDCl3, 300 MHz) in Figure 7: δ 8.18 (m, 2H), 7.78 (m, 2H), 7.48 (m,4H), 7.34 (m, 4H), IR in Figure 8 (KBr) max/cm−1: 3057, 1602, 1442, 1346, 1246, 852, 767, and 698. The described methodology offers advantages of 100% conversion as well as 100% selectivity, short reaction time, safe and mild reaction conditions, easy workup, recyclable and efficient catalyst, and so forth.(a) Normal view and (b) the expanded view of1H NMR spectrum of 2,3-diphenylquinoxaline.
(a)
(b)Figure 8
FT-IR spectrum of 2,3-diphenylquinoxaline.Despite the tremendous growth in industrial catalysis, the fundamental issues in heterogeneous catalysts with regard to their reactivity, efficiency, and selectivity in catalyzing multicomponent reactions (MCRs) and regioselective syntheses remained unanswered. MCR produces organic compounds in a single step by the breaking and making of several chemical bonds without isolation of any intermediate [63]. This catalyst has found widespread applications in the following MCRs.Scheme2. Imidazoles are an important class of nitrogen containing heterocycles. They are attractive compounds for organic chemists due to their useful biological activities [64]. Synthetic methodologies offering green chemistry and involving recyclable as well as cost-effective catalyst are in demand [65]. Accordingly, the catalytic methodology demonstrates the synthesis of 2,4,5-triphenyl-1H-imidazole over H-ZSM-5 catalyst by three-component reaction under refluxing ethanol with 100% conversion within one hour.Scheme 2
MCR for the synthesis of 2,4,5-triphenyl-1H-imidazole.Scheme3. According to the literature reports, organic compounds with pyrimidine and furan as a core unit are known to induce a wide range of biological activities. A three-component Biginelli reaction [66] was carried out over H-ZSM-5 (Si/Al ratio 30) catalyst using urea, ethyl acetoacetate, and benzaldehyde as starting materials for one-pot construction of pyrimidin-2(1H)-one derivative as a valuable heterocyclic scaffold.Scheme 3
MCR for the synthesis of ethyl 1,2,3,4-tetrahydro-6-methyl-2-oxo-4-phenyl pyrimidine-5-carboxylate.Scheme4. An MCR using benzene-1,2-diamine, ethyl 3-oxobutanoate, and benzaldehyde as reactants was catalyzed by H-ZSM-5 in methanol at room temperature stirring for about 48 hours to yield ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate.Scheme 4
MCR for the synthesis of ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate.The reaction of diamino benzene in methanol took place by addition of diethylacetylene-dicarboxylate and benzaldehyde while stirring at room temperature to yield corresponding ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate in almost quantitative 55% yield. These results might generate interest for undertaking unprecedented multi-component organic reactions over this zeolite.Scheme5. H-ZSM-5 has demonstrated a simple and greener catalytic protocol for the one-pot four-component reaction, forming a methyl 2-(acetamido(phenyl)methyl)-3-oxobutanoate by combining methyl 3-oxobutanoate, benzaldehyde, and acetonitrile and acetyl chloride. The catalyst exhibited the activity which led to 100% selectivity for the product with a quantitative yield of about 60%. The procedure has the advantages of easy workup and recyclability of the catalyst in this reaction. Furthermore, this synthetic methodology requires inexpensive catalyst, short time, and mild reaction conditions as compared to previously reported methods [67].Scheme 5
MCR for the synthesis of methyl 2-(acetamido(phenyl) methyl)-3-oxobutanoate.Usually, cations generate small volumes of high electrostatic field (active sites) to which a reactant molecule can be adsorbed to cause molecular rearrangements and reactions. The chemical nature, density, strength, and location of the active acid sites in this zeolite could be the important parameters which might have affected multicomponent reactions’ activities and selectivities. We assume that this catalyst, in principle, possesses high density of well-distributed Brønsted acid sites along with few external Lewis acid sites. Such active sites can exert synergetic effect due to their cooperative interactions. Furthermore, the neighbouring Brønsted acid sites could enhance the catalysis by performing multiple steps in single pot in a sequential manner with high conversion and selectivity. The interaction of heterocycles with H-ZSM-5 zeolite might be taking place via the formation of hydrogen bonded species involving its Brønsted acid sites and the pi-electron system of heterocyclic molecules [68]. There is a saying in the research community of zeolite science that zeolites can be made to catalyse any reaction on the basis of their high surface area alone. For example, the crystal voidage and channels within zeolite provide extensive internal surface to encourage catalytic processes. Most of the zeolite catalysis occurs inside the pores but heterogeneous reactions at the external crystal surfaces can never be ruled out. During the course of this paper preparation, we came across an original research article which reported the influence of the SiO2/Al2O3 ratio (30, 80, and 280) on properties of protonic ZSM-5 zeolite and that paper, in its author’s words, showed that the relative weak/strong acidities, as expressed by the ratio of desorption kinetic constants, augment consistently for the H-ZSM-5 studied, with the reduction of the SiO2/Al2O3 ratio [22]. In view of the industrial applications of H-ZSM-5, such knowledge [22] could be applied to further our understanding of the H-ZSM-5 zeolite surface and their observations could be taken into account to study mainly the acidic sites and their influence on catalytic properties (stemming from the “Si to Al ratio”) of this H-ZSM-5 zeolite material which, here in our case, is prepared by utilizing silica sourced from the wheat husk ash.A comprehensive account of this research is well beyond the scope of this communication so only the major results are described in brief. It suffices to note that the synthetic approaches for H-ZSM-5 catalytic material using low-cost and relatively environmentally benign silica source of wheat husk ash seem to be not yet published to date. In summary, hydrothermal synthesis of ZSM-5, a microporous aluminosilicate, by sourcing silica from wheat husk ash was followed by its modification to the protonated form. Though such a catalyst might have limited use due to its activity and acid strength declining in the presence of water, it still offers technological advantages such as eco-friendly and efficient organic transformations with catalyst reusability. Isak R. Shaikh and his collaborators reported herein their investigations on accessibility of acidic sites of H-ZSM-5 (with Si/Al ratio 30) for catalyzing some organic transformations including some multicomponent reactions. It is worth taking such a methodology further for preparing relevant aluminosilicates with other functionalities or applications.


## 3.1. Characterization

In this section of the paper, characterization of the H-ZSM-5 is described. X-ray diffraction (XRD) measurements were performed with a D8 Advance Bruker diffractometer, 40 KV, 40 mA current to check if they have the characteristic microporous zeolite structure of the ZSM-5. Transmission Electron Microscopy (TEM) was performed on a Tecnai F20 200 kV microscope from Philips to confirm the structure and microporous H-ZSM-5. Simultaneous DSC/TGA were recorded using NETZSCH STA 449F1. DSC and TG analyses were carried out using 10 mg of the sample at a heating rate of 10°C min−1 up to 1000°C, in nitrogen under a flow of 60 cm3 min−1. Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrophotometer (Shimadzu model 8400 S) with a resolution of 2 cm−1 and in the range 400–4000 cm−1. Cyclic voltammetry (CV) measurements were done on 7G7 VA Computrace (Swiss model) instrument with glassy carbon electrode as the working electrode, tetramethyl ammonium bromide as a supporting electrolyte, Ag/AgCl as the reference electrode, and Pt electrode as counter electrode. DMF was used to dissolve the sample before the actual measurements were made.A single crystalline phase of the MFI-structure was confirmed by observing the typical reflection peaks at 2θ of 7.94, 9.07, 13.96, 23.98, 24.57, 26.02, 27.05, 29.22, 29.46, 45.14, 29.42, 30.10, and 45.44 degrees, and so forth (Figure 1). The H-ZSM-5 zeolite was hydrothermally and chemically stable and the XRD patterns were similar to that of the usual H-ZSM-5 reported earlier in the scientific literature [35]. Scanning Electron Microscopy (SEM) deduced the morphology of the zeolite. Figure 1 (SEM image as inset) shows spherical morphology of the structures.Figure 1
XRD pattern of H-ZSM-5 (with SEM image as inset).The silicon to aluminium ratio (Si/Al = 30) was determined by elemental analysis and the atomic absorption spectroscopy (AAS). Elemental analysis of the H-ZSM-5 catalyst used in this study was performed by the electron dispersive spectroscopy (EDS) as shown (Table1) in weight% and atomic%. The spectrum (Figure 2) contains strong characteristic peaks of Si, O, Al, and C as expected, together with additional peak corresponding to Cu. This is either an impurity or artifact arising from the Cu grid on which the sample is supported.Table 1
Elemental composition in weight% and atomic wt%.
Element
Weight%
Atomic%
C K
7.47
12.82
O K
42.12
54.28
Al K
1.64
1.25
Si K
38.39
28.19
S K
0.28
0.18
Cu K
10.11
3.28
Total
100
100Figure 2
EDS.The examination of defects and phases of zeolite character was made by the TEM (Figure3). The aggregates observed at couple of places in TEM images need explanation and could correspond to the crystalline aluminium oxide phases or A1 oxidic systems, isolated in some significant stages of the surface dehydration. These aggregates might have been dispersed on the surface and also inside the pores of zeolite by solid-state reactions at calcination temperature via a surface process. During this process, aluminum oxide can react with the silanol and bridging hydroxyl groups and also with extraframework Al species of the zeolite. And also small amount can enter into the pores and react with the bridging hydroxyl groups. Such a modification could partially passivate unselective Brønsted acid sites of the medium pore H-ZSM-5 zeolite framework and might thereby influence their overall catalytic behaviour.Figure 3
TEM images of H-ZSM-5 obtained from different locations within the sample under study.We know that the thermal treatment can cause transformations of one zeolite structure to another, structure collapse to an amorphous phase, recrystallization to non-zeolitic materials, and so forth. Thermogravimetric-diffractometry thermal analysis (TG-DTA) was performed to investigate the mass loss of the zeolite as the temperature increased. TG-DTA plot of synthesized H-ZSM-5 (Figure4) shows that the structure of the zeolite is clearly sustained up to about 723 K (600°C). It also revealed distinct mass losses of about 2.63% (40 to 100°C), 3.77% (100 to 700°C), and 8.12% (up to 800°C; residual mass 92.88).Figure 4
TG-DTA curves of H-ZSM-5.In principle, the IR spectroscopy could help us understand the active sites and subsequently the mechanisms invoked in zeolite catalysis. The surface acidity of polycrystalline solids is a complex but technologically important property. In this section, only few preliminary considerations with regard to the acidity of catalytic system and its determination with IR spectroscopy will be introduced. We know that the lattice vibrations characteristic to H-ZSM-5 zeolite usually appear in the range of 400–1200 cm−1, while the bands corresponding to OH bond vibrations appear at wave numbers higher than 3200 cm−1. Fourier Transform Infra Red (FT-IR) spectrum of the neat H-ZSM-5 zeolite sample, in the range of 400–4000 cm−1, is studied here and shown in Figure 5. The sharp peak at 545 cm−1 and also the bands at 549 cm−1, 585 cm−1, and 600 cm−1 could be assigned to the structurally sensitive double five-member ring tetrahedral vibrations, and it is typical for the crystalline ZSM-5 zeolite. The characteristic band of MFI structure corresponding to the bending of the TO4 (T = Si, Al) tetrahedra is found shifted from 450 cm−1 to 425 cm−1 and the IR bands at 780 and 1100 cm−1 can be assigned to the symmetric and asymmetric stretching vibrations of the Si–O–Si linkages of the zeolite framework, respectively [24, 36–38]. The vibration modes of 1046 and 793 cm−1 are assigned to the internal vibration of SiO4 and AlO4 tetrahedra and a small IR band near 1235 cm−1 is attributed to their asymmetric stretching vibration. The H–OH bending vibrations of the adsorbed water molecules are observed at 1625 cm−1. The IR spectroscopy carefully observes the acidic hydroxyl OH groups of solid catalysts [36–38]. Sharp features appearing at 3235, 3605, 3425, and 3444 cm−1 correspond to hydroxyl groups of our H-ZSM-5 whereas in case of the spectrum of a neat H-ZSM-5, the aforementioned bands usually appear at 3235 cm−1 and 3425 cm−1. The IR band at 3605 cm−1 is a characteristic of protonated form of zeolite and its intensity is correlated with framework aluminium (FAL) [39]. Previous works in the literature ascribed the absorption bands at 3745 and 3680 cm−1 to the nonacidic hydroxyl groups and the hydroxyl groups of extraframework aluminum oxide, respectively [36–38]. These IR bands are not observed in our H-ZSM-5 spectrum. The IR band at 3606 cm−1 corresponds to bridging hydroxyl groups of the Brønsted acid sites that are found responsible for catalyzing cracking [40, 41]. Overall intrinsic activity depends on the local environment [42] and the presence of extraframework aluminium (EFAL) caused by mild steaming is also known to enhance the catalytic activity per H+ [43]. Some reports postulated that the extralattice Al is tetrahedrally coordinated and has at least one OH group with an IR vibration at 3656 cm−1 while others believed that due to the transformation of tetrahedrally coordinated lattice Al3+ to extraframework AlOHx species, the additional IR band at 3656 cm−1 is seen [44, 45]. In Figure 5, the IR band around 3660–3690 cm−1 could be attributed to EFAL, bound to the zeolite framework via one or two oxygen bonds, and is consistent with the literature [46]. During the course of this paper preparation, we come across an original research article where the IR band appearing at 3656 cm−1 was assigned to the extraframework aluminium species in proximity of the strongest Brønsted acid site and was found to impact the catalytic activity of ZSM-5 [47]. It is therefore clear that the high specific activity stems from cooperative interaction between Brønsted acid site and an adjacent EFAL. In the IR spectrum of our H-ZSM-5, the band at 3610 cm−1 is attributable to the strongest Brønsted acid sites Si(OH)Al [48]. The IR signal at 3740 cm−1 corresponding to silanol groups (SiOH) in amorphous silica was not seen there. The bands at 3720 cm−1 and 3745 cm−1 related to defect site and terminal silanol group, respectively, are also absent. It can also be said that the OH group associated with octahedral EFAL is not observed because no IR band appeared around 3780 cm−1 as it was reported in MFI-15 [45].Figure 5
FT-IR spectrum of H-ZSM-5.EFAL is a term that describes chemically different aluminium (Al) species, besides the framework Al. The EFAL could be (i) a charged aluminum oxide, (ii) neutral species such as AlOOH and Al(OH)3, (iii) aluminum oxo- and hydroxyl clusters, (iv) bulk aluminum oxide aggregates [49–51], (v) a partially dealuminated Al (O3Si–O–Al–OH), and so forth, [51, 52]. Lewis acid sites are generated due to the presence of EFAL and due to electronic defaults in the framework aluminium (FAL). Marques et al. [53] proposed EFAL as a weaker Lewis acid site than FAL; the IR spectrum of H-ZSM-5 in Figure 5 assigns the band around 1400–1415 cm−1 to Lewis acid sites related to plausible defects. Interestingly, the usual bands at 1444 and 1454 cm−1 corresponding to EFAL and FAL, respectively, are missing. From the preliminary spectroscopic studies, we conclude that the EFAL is identified as the main Lewis acid sites of H-ZSM-5 zeolite sample and is weak. The band at 1400 cm−1 is in agreement with the atomic oxygen linked to strong Lewis acid sites [54]. This unique IR measurement deserves further investigation because this could be the strongest Lewis acid site due to an electronic defect. We were initially tempted to assign the sharp absorption band at 800 cm−1 to the tetrahedral coordination of Al ions present, as in the transition alumina phase due to stretching vibrations of a lattice of interlinked tetrahedral Al, but there was no peak around 3775 cm−1 which is attributed to the most reactive OH species on all types of the transition alumina. So, as far as envisaging active sites of H-ZSM-5 is concerned, no one can deny the role of defective crystal configurations, if any. Methodology involving infrared and adsorption of selected probe molecules should be adopted for confirming the surface acidity and the catalytic active sites of this H-ZSM-5 sample [37].Though the cyclic voltammetry cannot confirm the nature of the redox couple, the peak potential when correlated with standards is an indicator of the possible electroactive and electroinactive species in solid catalyst. The point should also be noted that the synergistic effect of various acid sites might be facilitating a better charge delocalization in the zeolite to result in a more delocalized proton required for a successful protonation of reactant. This is consistent with the result obtained by cyclic voltammetry (Figure6). An electrode was prepared by incorporation of H-ZSM-5 zeolite in the carbon paste matrix. At this stage, it suffices to say that the cations located inside the pores or extra-framework exhibit electrochemical behavior [55]. This could be due to the mononuclear and or dinuclear oxygenated aluminium species formed on the protonic ZSM-5 surface [56].Figure 6
Cyclic voltammetry of H-ZSM-5.


## 3.2. Catalytic Applications

At industrial level, many organic syntheses are catalyzed by conventional strong acids such as AlCl3, BF3, and FeCl3. Handling hazards, toxicity, difficulty in recovering the catalyst, difficulty in separation of product, and so forth are the problems associated with corrosive acid catalysts under homogeneous conditions and therefore necessitate the development of heterogeneous catalysts. The zeolites present an environmental advantage over conventional acid catalysts for various synthetic methodologies [57, 58]. There exist correlations between zeolite properties and catalytic functionality. Catalysts like silica, alumina, and zeolites are known to catalyze condensation, addition, and some other useful methodologies in fine chemicals [58–60]. In the light of the synthetic conditions and the abundant protons present in the high surface area aluminosilicate framework, we investigated how H-ZSM-5 (Si/Al = 30) is capable of supporting acid catalyzed reactions. We examined the shape-selective catalysis by carrying out the nitration of toluene with n-propyl nitrate as a test reaction. The acidic nature of the catalyst formed nitronium ions from n-propyl nitrate required for nitration reaction. The reaction over this particular catalyst yielded only thepara-isomer. This result was in line with the literature [61].In view of the possible accessibility of acidic sites, we investigated whether H-ZSM-5 with Si/Al ratio 30 is capable of supporting some organic transformations.Scheme1. Quinoxalines are a class of nitrogen containing bicyclic heterocyclic compounds and are currently attracting widespread attention due to their wide range of therapeutic applications. Quinoxaline moiety forms the basic skeleton for various antibiotics [62]. The H-ZSM-5 was found to catalyze the classical methodology (Scheme 1) for synthesis of quinoxalines by condensing 1,2-aromatic diamines with 1,2-diketones. The classic synthetic transformation took place at room temperature stirring within 6 to 10 minutes in methanol.Scheme 1
Classical synthesis of 2,3-diphenyl quinoxaline by cyclocondensation of benzil and benzene-1,2-diamine.A reaction mixture of 1,2-diaminobenzene (2 mmol), benzil (2 mmol), and catalytic amount of H-ZSM-5 was taken in methanol and stirred at room temperature. The reaction was monitored by TLC solvent system: n-hexane/ethyl acetate (7 : 3) till its completion. After 10 minutes, the mixture was then poured into ice-water. Solid reaction product was filtered out (yield: 88%). The quinoxaline product thus formed was purified by solvent recrystallization using ethanol as solvent. The product thus obtained was confirmed by measuring its melting point (125–127°C) first. The product of the reaction in Scheme1 is identified appropriately as 2,3-diphenylquinoxaline (molecular formula: C20H14N2).The product was characterized using1H NMR (Figure 7) and FT-IR (Figure 8), as 1H NMR (CDCl3, 300 MHz) in Figure 7: δ 8.18 (m, 2H), 7.78 (m, 2H), 7.48 (m,4H), 7.34 (m, 4H), IR in Figure 8 (KBr) max/cm−1: 3057, 1602, 1442, 1346, 1246, 852, 767, and 698. The described methodology offers advantages of 100% conversion as well as 100% selectivity, short reaction time, safe and mild reaction conditions, easy workup, recyclable and efficient catalyst, and so forth.(a) Normal view and (b) the expanded view of1H NMR spectrum of 2,3-diphenylquinoxaline.
(a)
(b)Figure 8
FT-IR spectrum of 2,3-diphenylquinoxaline.Despite the tremendous growth in industrial catalysis, the fundamental issues in heterogeneous catalysts with regard to their reactivity, efficiency, and selectivity in catalyzing multicomponent reactions (MCRs) and regioselective syntheses remained unanswered. MCR produces organic compounds in a single step by the breaking and making of several chemical bonds without isolation of any intermediate [63]. This catalyst has found widespread applications in the following MCRs.Scheme2. Imidazoles are an important class of nitrogen containing heterocycles. They are attractive compounds for organic chemists due to their useful biological activities [64]. Synthetic methodologies offering green chemistry and involving recyclable as well as cost-effective catalyst are in demand [65]. Accordingly, the catalytic methodology demonstrates the synthesis of 2,4,5-triphenyl-1H-imidazole over H-ZSM-5 catalyst by three-component reaction under refluxing ethanol with 100% conversion within one hour.Scheme 2
MCR for the synthesis of 2,4,5-triphenyl-1H-imidazole.Scheme3. According to the literature reports, organic compounds with pyrimidine and furan as a core unit are known to induce a wide range of biological activities. A three-component Biginelli reaction [66] was carried out over H-ZSM-5 (Si/Al ratio 30) catalyst using urea, ethyl acetoacetate, and benzaldehyde as starting materials for one-pot construction of pyrimidin-2(1H)-one derivative as a valuable heterocyclic scaffold.Scheme 3
MCR for the synthesis of ethyl 1,2,3,4-tetrahydro-6-methyl-2-oxo-4-phenyl pyrimidine-5-carboxylate.Scheme4. An MCR using benzene-1,2-diamine, ethyl 3-oxobutanoate, and benzaldehyde as reactants was catalyzed by H-ZSM-5 in methanol at room temperature stirring for about 48 hours to yield ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate.Scheme 4
MCR for the synthesis of ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate.The reaction of diamino benzene in methanol took place by addition of diethylacetylene-dicarboxylate and benzaldehyde while stirring at room temperature to yield corresponding ethyl 4-(2-aminophenylamino)-2,5-dihydro-5-oxo-2-phenylfuran-3-carboxylate in almost quantitative 55% yield. These results might generate interest for undertaking unprecedented multi-component organic reactions over this zeolite.Scheme5. H-ZSM-5 has demonstrated a simple and greener catalytic protocol for the one-pot four-component reaction, forming a methyl 2-(acetamido(phenyl)methyl)-3-oxobutanoate by combining methyl 3-oxobutanoate, benzaldehyde, and acetonitrile and acetyl chloride. The catalyst exhibited the activity which led to 100% selectivity for the product with a quantitative yield of about 60%. The procedure has the advantages of easy workup and recyclability of the catalyst in this reaction. Furthermore, this synthetic methodology requires inexpensive catalyst, short time, and mild reaction conditions as compared to previously reported methods [67].Scheme 5
MCR for the synthesis of methyl 2-(acetamido(phenyl) methyl)-3-oxobutanoate.Usually, cations generate small volumes of high electrostatic field (active sites) to which a reactant molecule can be adsorbed to cause molecular rearrangements and reactions. The chemical nature, density, strength, and location of the active acid sites in this zeolite could be the important parameters which might have affected multicomponent reactions’ activities and selectivities. We assume that this catalyst, in principle, possesses high density of well-distributed Brønsted acid sites along with few external Lewis acid sites. Such active sites can exert synergetic effect due to their cooperative interactions. Furthermore, the neighbouring Brønsted acid sites could enhance the catalysis by performing multiple steps in single pot in a sequential manner with high conversion and selectivity. The interaction of heterocycles with H-ZSM-5 zeolite might be taking place via the formation of hydrogen bonded species involving its Brønsted acid sites and the pi-electron system of heterocyclic molecules [68]. There is a saying in the research community of zeolite science that zeolites can be made to catalyse any reaction on the basis of their high surface area alone. For example, the crystal voidage and channels within zeolite provide extensive internal surface to encourage catalytic processes. Most of the zeolite catalysis occurs inside the pores but heterogeneous reactions at the external crystal surfaces can never be ruled out. During the course of this paper preparation, we came across an original research article which reported the influence of the SiO2/Al2O3 ratio (30, 80, and 280) on properties of protonic ZSM-5 zeolite and that paper, in its author’s words, showed that the relative weak/strong acidities, as expressed by the ratio of desorption kinetic constants, augment consistently for the H-ZSM-5 studied, with the reduction of the SiO2/Al2O3 ratio [22]. In view of the industrial applications of H-ZSM-5, such knowledge [22] could be applied to further our understanding of the H-ZSM-5 zeolite surface and their observations could be taken into account to study mainly the acidic sites and their influence on catalytic properties (stemming from the “Si to Al ratio”) of this H-ZSM-5 zeolite material which, here in our case, is prepared by utilizing silica sourced from the wheat husk ash.A comprehensive account of this research is well beyond the scope of this communication so only the major results are described in brief. It suffices to note that the synthetic approaches for H-ZSM-5 catalytic material using low-cost and relatively environmentally benign silica source of wheat husk ash seem to be not yet published to date. In summary, hydrothermal synthesis of ZSM-5, a microporous aluminosilicate, by sourcing silica from wheat husk ash was followed by its modification to the protonated form. Though such a catalyst might have limited use due to its activity and acid strength declining in the presence of water, it still offers technological advantages such as eco-friendly and efficient organic transformations with catalyst reusability. Isak R. Shaikh and his collaborators reported herein their investigations on accessibility of acidic sites of H-ZSM-5 (with Si/Al ratio 30) for catalyzing some organic transformations including some multicomponent reactions. It is worth taking such a methodology further for preparing relevant aluminosilicates with other functionalities or applications.


## 4. Conclusions

This research paper reported the hydrothermal synthesis of ZSM-5, a microporous aluminosilicate, by sourcing silica from wheat husk ash and its further modification to its protonated form (H-ZSM-5) having Si to Al ratio 30. The H-ZSM-5 zeolite sample was appropriately characterized by XRD, SEM, TEM, EDS, FT-IR, TG-DTA, and CV. The zeolite sample of H-ZSM-5 is found to have characteristic crystallinity, good thermal as well as chemical stability, unique catalytic character, and so forth. The synthesis and characterization of H-ZSM-5 were followed by its evaluation as recyclable catalyst in selective organic transformations including three- and four-component reactions. The efficacy of H-ZSM-5 for these organic reactions suggests the availability of supposedly increased density of the accessible acid (catalytic) sites associated with the MFI-structured zeolite framework Al and or Si atoms. Lessons learnt from scientific literature [34, 37] and, if required, further in-depth investigation of the zeolite structure and the adsorption behaviour will be required before one can explain the activity and selectivity for each and every organic reaction studied. This research demonstrates (i) an attractive and potential scalable and industrially viable methodology for the wheat husk ash based high-purity and useful zeolite H-ZSM-5 synthesis and (ii) the principles of green chemistry such as mild reaction conditions, recyclability of the solid acid catalyst, and selectivity of desired product from organic reactions. On a philosophical note, a methodology based on valorization of a significant agricultural waste stream and also the relevant efforts to diminish the consumer dependency on the chemical reagents (from petroleum or fossil resource) could become important aspects for establishing a sustainable system of catalytic material syntheses for chemical industries in the future.



---

*Source: 805714-2015-01-29.xml*