Structured Binder-Free Al‑β Zeolite for Acid-Catalyzed DehydrationJiaxu Liu Shanshan Li Zhenmei Zhang Nicoletta Fusi Chunyan Liu Guang Xiong Gianvito Vileacuteand Ning He

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ABSTRACT It has long remained a challenge in catalysis to synthesizetechnical hierarchical nanosized zeolites without growth modifiers andbinders Herein we report a facile synthetic route to directly preparebinderless hierarchical Al-β zeolite in shaped form with retained acidityand sufficient mechanical strengths The preparation process consists oftwo steps where an Al-β zeolite nanopowder is first shaped usingtetraethyl orthosilicate the extrudate is then recrystallized underhydrothermal conditions with the assistance of tetraethylammoniumhydroxide (TEAOH) solution The alkaline environment leads tomesopores into the zeolite and transforms as well the SiO2 bindersinto a zeolite framework Several characterization methods wereperformed to reveal the structureminusproperty relationships of the materialsin powdered and shaped form It was found that the process of TEAOHtreatment is the combination of desilication mesopore formation andbinder recrystallization to give zeolite Al-β nanoparticles resulting in the obvious increase in microporosity acidity and X-raydiffraction crystallinity of samples Density functional theory calculations were also used to rationalize the recrystallization processFinally the utility of the materials was demonstrated by evaluating their catalytic properties in the dehydration of 2-(4-ethylbenzoyl)-benzoic acid a reaction which is traditionally catalyzed by concentrated sulfuric acid and oleum We found that the catalyst intechnical form exhibits better catalytic performance than the powdered HB nanoparticles and higher product yield than traditional(homogeneous) catalysts

KEYWORDS Al-β zeolites binders full-crystalline monolithic zeolite Al-β mesoporous materials heterogeneous acid catalysis

1 INTRODUCTION

Zeolites have been widely applied in industries because of theirunique shape selectivity excellent catalytic performance andgood hydrothermal stability1 In particular zeolite β (BEAtype) possesses three-dimensional and intersected 12-mem-bered ring pore channels and is importantly used in thepetrochemical and chemical industries However this tradi-tional zeolite with a pore system of precisely narrowedmicrochannels could limit the mass transportation and theaccessibility of molecules to the inner active sites and thiscould lead to potential side reactions and sometimes evenwreck the stability of catalysts23 Hierarchical zeolites with anadditional mesopore andor macropore network on top of andinterconnecting with the micropores can maximize theapplication of porosity of zeolites which hence have arousedwidespread attention4minus6 Typically these methods can bedivided into two categories ldquotop-downrdquo and ldquobottom-uprdquoapproaches Specifically the top-down methods often consistin a secondary treatment of the synthesized zeolite using acidor alkali conditions the bottom-up approaches usually needthe addition of a special template during zeolite synthesis Atpresent most of the research studies have focused on thepreparation of hierarchical powder zeolites However powder

zeolites cause pressure drop inside of the reactor Thus inpractical application structured zeolites (ie extrudatespellets monoliths and granules) with sufficient mechanicalstrength are necessary and this is done by adding binders(usually 30minus50 wt )7minus10 A few synthetic routes have beenproposed to shape zeolite powders into technical form1112

Unfortunately the presence of binders in massive quantity endup covering the surface of the zeolites blocking the channelsand pores and leading to additional diffusional limitation Thissignificantly affects the zeolite acidity and generally has anegative impact on the catalytic performance especially foracid-catalyzed reactions13minus15

In order to solve the abovementioned challenges binderlesszeolites are a technical solution16minus18 For example Wang etal16 successfully prepared a binderless ZSM-5 zeolite applied

Received August 18 2021Accepted October 25 2021Published November 12 2021

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httpsdoiorg101021acsanm1c02559ACS Appl Nano Mater 2021 4 11997minus12005

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in the dehydration of crude methanol to dimethyl etherreaction which exhibited better catalytic performance andlonger lifetime compared with traditional shaped catalysts witha binder Some other groups have also reported the preparationof binderless ZSM-5 zeolite using dry gel conversion methodtransforming the amorphous precursors into a ZSM-5 zeolitephase with the assistance of the organic structure-directingagents1718 These investigations have pointed to the feasibilityof obtaining binderless catalysts Nevertheless the amount ofacid sites of the as-synthesized binderless samples significantlydecreased compared with the parent powder zeolite1920

Therefore exploring a synthetic method to get structuredzeolitic materials that not only maintain the textural and acidicproperties of powdered zeolites but also have adequatemechanical strength has remained an unresolved challengeHowever this is of great importance for industrial applicationBesides ZSM-5 zeolites some other frameworks have also beenexplored for example the shaped binderless ZSM-1121 whichshowed a higher rate of formation of tert-butylamine than didthe binder-containing catalysts binderless zeolites 13X 5Aand ZSM-5 made by 3D printing22 binderless zeolite NaX andA microspheres by using chitosan-assisted synthesis23minus25 andone-pot synthesis route for preparing binderless zeolite A26

and so forthTrying to address this scientific gap we reported herein the

facile preparation method of a binder-free hierarchical Al-βzeolite in shaped form whose size ranged from 200 to 300 nmA series of characterization methods and density functionaltheory calculations were carried out to lift the veil of thestructureminusproperty relationships Finally the nano-sizedmaterials were applied in the dehydration reaction of 2-(4-ethylbenzoyl)-benzoic acid (E-BBA) which traditionally iscatalyzed by concentrated sulfuric acid and oleum Comparedwith the traditionally homogeneous acid catalyst the binder-free hierarchical nano-sized Al-β zeolite showed prominentadvantages such as shape selectivity and stable hydrothermalstability

2 MATERIALS AND METHODS21 Catalyst Preparation NH4minusAl-β zeolites (with particles of

ca 200minus300 nm and total SiAl ratio of 39) were supplied by DalianLigong Qiwangda Chemical Technology HminusAl-β zeolite (hereinindicated as HB) was obtained from the calcination of NH4minusAl-βzeolites under dry air at 540 degC for 6 h HB zeolite (100 g) was mixedwith tetraethyl orthosilicate (833 g 20 aqueous solution)homogenized at room temperature in a speed mixer (SpeedMixer

FlackTek Inc) then extruded at room temperature into a uniformcylindrical body by rapid extrusion molding using a MiniScrewExtruder (Caleva 2 mm cylindrical diameter) and dried at roomtemperature for 24 h Subsequently the extrudate was dried again at110 degC for 12 h and subsequently calcined in air at 540 degC for 5 hgiving HBs

To obtain hierarchical samples the HB zeolite underwent analkaline treatment in a 50 mL PTFE-lined stainless-steel autoclaveusing tetraethylammonium hydroxide solution (TEAOH 40aqueous solution) In particular HBs (5 g) were mixed with avariable concentration of TEAOH solution (25 50 75 and 100 g)and the suspensions were then heated at 150 degC for 48 h to obtainHBs-05 HBs-10 HBs-15 and HBs-20 respectively At thecompletion of this step the product was washed filtered dried at110 degC for 12 h and subsequently calcined in air at 540 degC for 5 hThe preparation process is shown in Scheme 1

22 Catalyst Characterization X-ray diffraction (XRD) patternsof all powder samples (which were grounded into powder if it isshaped) were recorded using a Rigaku Dmax-2004 diffractometer(Rigaku Kyoto Japan) with Cu Kα radiation (40 kV 100 mA) and a001deg minminus1 (2θ) scanning speed To determine the total SiAl ratioinductively coupled plasma optical emission spectrometry (ICP OES)spectra were collected on a Spectro Arcos analyzer We carried outtransmission electron microscopy (TEM) and high-angle annulardark-field scanning transmission electron microscopy (HAADF-STEM) on a FEI (Tecnai F30 G2 The Netherlands) microscopeIn order to study the tendency of the surface area and pore volumeswe conducted nitrogen physisorption on a Micromeritics ASAP 3020instrument (Micromeritics Atlanta USA) at minus196 degC All sampleswere pretreated in vacuum at 350 degC for 5 h before the measurementThe surface area was determined by the BrunauerminusEmmettminusTeller(BET) method using the adsorption branch in the pp0 range from005 to 035 The pore volumes were evaluated at pp0 of 099 whilemicro- and mesoporosity were discriminated using the t-plot methodAmmonia temperature-programmed desorption (NH3-TPD) wasused to qualitatively analyze the tendency of the overall acidity ofsamples The profiles were carried out on a Quantachrome ChemBET3000 chemisorb instrument The samples (014 g ca 20minus40 mesh)were loaded into a quartz U-reactor and degassed in helium for 1 hand then the temperature was cooled to 150 degC for 30 min NH3adsorption Then we removed the weakly adsorbed NH3 by aconstant helium purging after the initial adsorption Taking helium in20 mL minminus1 as a carrier gas desorption was recorded from 150 to600 degC at a rate of 15 degC minminus1 Fourier transform infraredspectroscopy (FTIR) was employed to distinguish the acid sites ofdifferent samples The spectra for surface hydroxyl (OH) vibrationsand pyridine adsorption were recorded with a Nicolet 10 FTIRspectrometer ranged from 4000 to 400 cmminus1 with an opticalresolution of 4 cmminus1 The detailed measurement process has beenreported in our previous work36 Solid-state nuclear magneticresonance experiments were carried out on an Agilent DD2-500

Scheme 1 Preparation Procedure of Hierarchical Nano-Sized Full-Zeolitic Monolith

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MHz spectrometer (Agilent Technologies Inc California USA) Inparticular 29Si magic angle spinning nuclear magnetic resonance(MAS NMR) experiments were performed with a 4 s repetition time4000 scans and contact time of 15 ms The 29Si spectra werereferenced to 44-dimethyl-4-silapentane sulfonate sodium 27Al MASNMR spectra were acquired at 1302 MHz using a 4 mm MAS NMRprobe with 14 kHz spinning speed The 27Al spectra were referencedto 1 Al(NO3)3 aqueous solutions The microcomputationaltomography (microCT) of the shaped bodies was performed in aBIR Actis 130150 The generator and detector were fixed while thesample was rotated the scanning plane was horizontal The X-rayspectra were collected on a flat panel detector which converted thecollected spectra in raw data and sent them to the computer wherethey were processed as blackwhite images The voxel of the imageshere presented is 7 times 7 times 7 μm Segmentation of 2D slices obtainedfrom the microCT technique was carried out by means ofAvizoMercury software The mechanical resistance (or crushingstrength) of the shaped zeolites was measured using the Instron 5965system (Instron) The shaped zeolite was positioned between thecompressing disks of the system and subjected to an increasing loaduntil breakage The force applied at the breaking point was registeredwith an accuracy of 0223 Catalyst Performance Dehydration reaction of E-BBA

(density 11783) was carried out under atmospheric pressure in abatch reactor In a typical experiment E-BBA (2 g) was loaded intothe reactor (25 mL) under neat conditions (ie without any solvent)and heated to reach the reaction temperature (210 degC) At this stageE-BBA (melting point 138 degC boiling point 343 degC) was completelysolubilized Preliminary quantitative high-pressure liquid chromatog-raphy (HPLC) analysis of the liquid ensured no degradation of thestarting materials at this stage Then under vigorous stirring (500rpm) the shaped zeolite catalyst (2 g which was sieved into 20minus40mesh) was added and the suspension was kept at the reactiontemperature for 60 min Upon completion of this step an aliquot ofthe reaction solution was collected and immediately analyzed byHPLC (Waters 1525 SunFire C18 column 5 μm 150 times 46 mm)The liquid phase was the mixture of H2O CH3OH andtetrahydrofuran (111 volumetric ratio) at a flowing rate of 05 mLminminus1 The UV detector of the HPLC was set at 274 nm wavelengthThe calibration curves were linear for E-BBA (R2 = 09995) and 2-EAQ (R2 = 09997)24 Density Functional Theory Calculations To understand

the crystallization process a periodic supercell (1 times 2times1) was used torepresent the zeolite topology (Figure 1) The crystallographic

structure of β was taken from the database maintained by theInternational Zeolite Association All periodic DFT calculations wereconducted with the CASTEP code in Materials Studio We usedgeneralized gradient approximation of PBE form to describe electronexchange and correlation Ultrasoft pseudopotentials and a plane wavecutoff energy of 400 eV were used for core and valence statesrespectively All calculations were performed spin polarized and van

der Waals forces We converged self-consistent field electronicenergies to 5 times 10minus7 eV atomminus1 and atomic forces for geometricrelaxation using the conjugate gradient algorithm to less than 002 eVAringminus1 The Gamma point was used to sample the surface Brillouin zone

3 RESULTS AND DISCUSSION

Table 1 outlines the textural properties of all samples The totalSiAl ratio is 39 over the HB sample this value first increasesover HBs due to the presence of a siliceous binder and thendrops upon increasing the concentration of the base duringalkaline treatment reaching a final SiAl ratio of 32 over HBs-20 The textural properties of the samples are also affected bythe extrusion and TEAOH treatment the increased mesoporesurface area passes from 95 in HB to 138 m2 gminus1 in HBs-15 andto 153 m2 gminus1 in HBs-20 This corresponds to a 60 mesoporesurface area increase between the first and the last sample Inline with previous observations13minus15 the slight drop in Smesoobserved over HBs-05 and HBs-10 is likely due to the presenceof the binder blocking the surface of the catalyst The increasein mesoporosity is obviously observed by the pore volume databecause Vmeso changes from 011 in HB to 018 m2 gminus1 in HBs-20 in accordance with ca 60 increase In addition comparedwith traditional shaped sample HBs the crushing strength ofall shaped samples after TEAOH treatment is increased aftersharing meeting industrial strength requirements (gt10 Nmmminus1)Figure 2ab shows the N2 isotherms and corresponding pore

size distribution of all samples respectively Based on theshape the isotherm of HB can be attributed to a type-Iindicating the material consisting of micropores and limitedmesopores Meanwhile for HBs HBs-15 and HBs-20 acomposite of typical type-I and type-IV with a H1 hysteresisloop can be observed indicating the existence of relativelymore mesopores Moreover the pore size distribution (Figure2b) indicates that the mesopores are bimodal with pore sizes ofca 38 and 6minus12 nm Concerning the peak centered at 38 nmthis has been observed elsewhere and indicates that the zeolitecontains internal mesopores connected to the surface viamicropores (probably those of the β zeolite itself) On theother hand there is a trend that the size of larger mesoporeswas gradually increased from 6 to 12 nm as the concentrationof TEAOH solution increased The formation of mesoporesafter TEAOH treatment is corroborated by the micrographsshown in Figures 3a and S1 Figure 3a in particular shows theslow transformation of the amorphous binder and the presenceof mesopores in the crystals Also Figure 3b highlights theconsistency of the observed average mesopore size with thepore size distribution shown in Figure 2 Finally energy-dispersive system (EDS) maps show the very uniformdistribution of Si and Al species in the HBs-20 samplewhich would be unexpected when a purely siliceous binder(SiO2 came from the calcination of tetraethyl orthosilicate sol)is also present in the sample (Figure 3c) Based on the overallcomposition and textural analysis we can state that the shapingprocess increases the SiAl content and slightly reduces SBETand pore volume due to the existence of the SiO2 binder thatcovers also the surface sites of zeolite After TEAOH treatmentand calcination the micrographs show no areas of higherconcentration of Si together with an increase of mesoporesurface area and SiAl reduction These data indicate a partialdesilication of the zeolite but they are not conclusive whetherthe binder is also removed via desilication or rather

Figure 1 Periodic supercell (1 times 2times1) representing the Al-β zeolitetopology

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recrystallizes in some other forms To address this point wehave conducted additional characterizationsIn particular the XRD patterns of all samples are shown in

Figure 2c Six characteristic diffraction peaks of Al-β zeolite canbe seen in all samples and the crystallinity can thus becalculated by the sum of areas of these peaks2728 Taking thecrystallinity (ξ 100) of HB as a reference the relativecrystallinity (ξrel) of other samples was calculated For thatmatter HBs have the lowest crystallinity of 97 in line withthe fact that an amorphous binder is present Then thecrystallinity linearly increases with the TEAOH treatment Inparticular HBs-15 and HBs-20 have much higher relativecrystallinity than HB This increase in crystallinity is surprisingand if the base treatment is responsible for the observedformation of mesopores and eventually for the destruction ofthe siliceous binder we would expect a reduction of the samplecrystallinity after TEAOH treatment mainly because thealkaline treatment has partially ldquodestroyedrdquo the frameworkInstead the increased crystallinity (and the absence of anyother reflection assigned to other phases such as pure SiO2)

can be explained suggesting that the amorphous silica binderin HBs in the presence of TEAOH and at high temperaturehas recrystallized into an Al-β zeolite phase In fact nosignificant structure damage after the treatment and no otherpeaks assigned to side frameworks can be identified Thisaspect is not surprising considering that under theexperimental conditions TEAOH is known to act as astructure directing agent in the solution and in the alkalineenvironment and the SiO2 binder initiates the secondarycrystallization to give Al-β zeoliteTo obtain deeper insights into the Al distribution and

location34 29Si and 27Al MAS NMR data were obtained(Figure 4) while the 27Al MQ MAS NMR plots are shown inFigure S2 The results further prove that the framework Alspecies increased after TEAOH In particular the spectra showa single broad signal in the isotropic dimension correspondingto four-coordinate Al species in the framework It is interestingto note that the 27Al NMR intensity of HBs-20 is very smallcompared to HB indicating that the TEAOH treatment wasnot only desilication but also dealumination at the same time

Table 1 Composition and Texture Properties of All Samples

catalysta SiAlb SBETc (m2 gminus1) Smeso

d (m2 gminus1) Vporee (cm3 gminus1) Vmicro

d (cm3 gminus1) Vmesof (cm3 gminus1) crushing strength (N mmminus1)

HB 39 614 95 033 022 011HBs 42 557 113 031 018 013 5HBs-05 40 530 70 030 020 010 11HBs-10 34 612 64 030 024 006 12HBs-15 33 687 138 035 023 012 12HBs-2 0 32 671 153 040 022 018 14

aDescriptions of the acronyms are given in the Materials and Methods bSiAl by ICP-OES cBET method dt-plot method applied to the N2isotherm eVpore was calculated from N2 adsorbed at pp0 = 099 fVmeso = Vpore minus Vmicro

Figure 2 Characterization of all samples N2 adsorptionminusdesorption isotherms at minus196 degC (a) corresponding pore size distribution by BJH (b)XRD patterns with relative crystallinity (c) NH3-TPD profile (d) and FTIR-OH (e) and FTIR-pyridine (f)

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31 AcidminusBase Characterization It is often essential tocharacterize the acidity of the samples because most of theindustrial reactions utilize the acidic sites of zeolites and thepurpose of this study is to restore the acidity of shaped zeolitesThe acidity has been determined by NH3-TPD profiles andFTIR spectra As shown in Figure 2d the desorption peaks at200minus220 and 350minus400 degC correspond to weak and strong acidsites respectively29 Compared to HB the intensity of the twodesorption peaks in HBs both decreased because of theaddition of the silica binder which dilutes the acid sitesEspecially compared with the results of NH3 as we can seefrom Table 2 the values of pyridine adsorption decreasedmore significantly indicating the decreased accessibility of acidsites which have been blocked by the binder After TEAOHtreatment the total acidity of zeolite is restored especially forpyridine adsorption Considering the size of molecules (NH3and pyridine) it was deduced that the combination ofdesilication and conversion of the binder into zeolite Al-βstructure mainly improves the unobstructed pore channel ofthe formed nano-sized zeolites and the accessibility of acidsites In HBs-20 the acidity recover and even to the extentthat the adsorption value for NH3 was increased which can beattributed to additional aluminum sources that transform intoAl sites in the framework during TEAOH treatmentAs shown in Figure 2e we have studied the hydroxyls

(minusOH) related to Broslashnsted acidity (3600 and 3550 cmminus1) andthose representative of isolated silanols on the external surface(3740 cmminus1)3031 The structured HBs display a reducedintensity of 3610 cmminus1 related to Broslashnsted acidity compared tothe parent nano-sized HB which correspond to the lower Alcontent resulting from the abundant binders and the formationof Al-rich debris respectively After TEAOH treatment (egHBs-15 and HBs-20) the bands at 3740 cmminus1 becomeprominent indicating the presence of substantial defectsbecause of the hierarchical structure3031 At the same timethe intensity of 3610 cmminus1 has the tendency to grow backindicating that the Broslashnsted sites are gradually recovered dueto the transformation of the SiO2 binder into the zeoliteframework and realumination due to TEAOH3031 To furtherconfirm our conclusions we have used pyridine as a probemolecule which could distinguish the internal and externalsurface acidity to investigate the overall acid sites in differentpositions of zeolites As we can see from Figure 2f all spectradisplay three peaks at 1545 1445 and 1633 cmminus1 which areascribed to Broslashnsted acid sites Lewis acid sites and Broslashnstedacid sites on the external surface respectively Owing to themassive inert binders which could cover the external surfaceand dilute the acid sites all peaks in HBs drastically decreasedcompared with parent powder zeolites On the other hand the

Figure 3 TEM micrographs of all samples (a) HRTEM micrographof HBs-20 with characteristic mesopores (b) STEM and EDSmapping of oxygen aluminum and silicon of HBs-20 (c)

Figure 4 29Si (left) and 27Al (right) MAS NMR spectra of allsamples

Table 2 Amount and Distribution of Acid Sites on the Materials

NH3 adsorption pyridine adsorption

catalyst Cwa (mmol gminus1) Cs

b (mmol gminus1) CwCs (minus) CBSc (μmol gminus1) CLS

d (μmol gminus1) CBSCLS (minus)HB 028 029 097 160 347 046HBs 024 023 104 15 153 010HBs-05 024 034 071 66 235 028HBs-10 031 027 115 109 252 043HBs-15 034 027 126 123 282 044HBs-20 035 030 117 143 263 054

aAmount of NH3 adsorbed on weak acid sites bAmount of NH3 adsorbed on strong acid sites cAmount of pyridine adsorbed on Broslashnsted acidsites dAmount of pyridine adsorbed on Lewis acid sites

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intensity of the samples after TEAOH treatment increasedobviously indicating the recovery of acidity especially forHBs-15 and HBs-20 which confirmed the transformation ofamorphous inert binders into the zeolite phase and theredistribution of aluminum sources in line with above-mentioned conclusions Such redistribution is verified by theEDS micrographs shown in Figure 3c which show nopreferential locations for the Al atoms and homogeneous AldistributionOverall this combination of results over the powdered

materials suggests a combination of desilication mesoporeformation and binder recrystallization to give zeolite Al-βwhose size ranged from 200 to 300 nm In fact if thesecondary crystallization would not happen the microporosityacidity and XRD crystallinity would decrease rather thanincrease Hence the samples are made by the cooperationbetween secondary crystallization and OHminus desilication Insummary TEAOH solutions played a dual role in the post-treatment On the one hand OHminus of TEAOH could dissolveSi species of parent-shaped Al-β on the other hand TEA+ ofTEAOH can promote recrystallization as a template Thesetwo completely reverse effects are in dynamic equilibrium35

The concentration of TEAOH solutions significantly affectsthe pore structure properties of zeolite crystals Moderateconcentration TEAOH solutions tend to perform recrystalliza-tion effects with parent β as seeds and transformed dissolved Sispecies into the zeolite phase contributing to a new zeoliteshell Therefore at appropriate concentration of TEAOHsolutions desilication and recrystallization occurred simulta-neously and the rate of mesopore formation and crystalgrowth would reach to some extent balance Specifically thenumber of micropores exceeds parent powder Al-β while thenumber of mesopores increases At this point if theconcentration of TEAOH continues to increase thedesilication will be deepened resulting in the increasingnumber of mesopores at the expense of micropores35

32 Characterization of the Shaped Bodies Character-ization of the shaped bodies was also undertaken to understandthe distribution of the macropores and analyze the mechanicalstability of the materials In particular microCT images(Figure 5) for HBs-20 reveal irregular macropores (in blueon the left image and in black on the right micrograph) whichare well distributed over the whole sample The pores haveelongated to sub-rounded shape and appear to be accessiblefrom the surface The macroporosity is ca 3 of the wholevolume Recrystallization of amorphous SiO2 binders into

crystalline zeolite improves the mechanical strength of thesamples as well as shown in Table 1 where the crushingstrength goes from 5 N mmminus1 of HBs to 14 N mmminus1 over HBs-20 In summary the shaping and alkaline process with TEAOHimproves the diffusion property due to the formation of macromeso and micropores as well as the mechanical strength

33 Zeolite Recrystallization Mechanism To furtherunderstand the recrystallization process we performed plane-wave supercell DFT calculations studying the position of Si orAl rearranged into the zeolite framework In particular thelocation energies of Si or Al in different T sites and theadsorption energy of NH3 of the obtained different sites todetermine the acidity of the corresponding bridged hydroxylgroup were calculated As shown in Figure 6a the location

energies of Al were more negative than those of Si indicatingthat during the possible recrystallization Al is more favorableto get into the framework of the zeolite More importantly itwas found that for nine different T sites of Al-β the Al locationcan be divided into two groups (i) T1 T2 and T7 located ina five-membered ring linked to two channels of zeolite whichhave relatively high location energies and low Broslashnsted acidity(Figure 6b) (ii) T3minusT6 T8 and T9 most of which located atintersection of channels with good accessibility and have lowerlocation energies and high Broslashnsted acidity Based on thisanalysis the latter T sites (T3minusT6 T8 and T9) tend torearrange aluminum sources into the framework from athermodynamic viewpoint and the resulting acidity is higherThe calculated results corroborate that under the experimentalconditions and in the presence of the siliceous binder thealuminum source in TEAOH can effectively be transformedinto framework Al and the acidity of zeolites is increased

34 Application in Heterogeneous Acid CatalysisThe structured binder-free Al-β zeolite nanoparticles wereapplied in the dehydration reaction of E-BBA to 2-ethyl-anthraquinone (2-EAQ) performed at atmospheric pressure ina flask3233 This reaction is very important for the synthesis ofFigure 5 X-ray tomography micrographs of HBs-20 in shaped form

Figure 6 Location energies of Si or Al (a) and adsorption energies ofNH3 (b) for different T sites in β zeolites during the recrystallizationprocess

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fine chemicals and pharmaceuticals which is traditionallycatalyzed by concentrated sulfuric acid and oleum catalystsleading to severe environmental pollution and unsatisfactoryyield In fact the 2-EAQ product is an important raw materialused in the synthesis of hydrogen peroxide (H2O2) Hydrogenperoxide (H2O2) is a highly efficient and green oxidant becauseit has the highest content of active oxygen As a result thedemand of 2-EAQ worldwide in 2021 is more than 40000 tonsand expected to increase at the rate of 8 The most widelyapplied industrial synthetic route for producing 2-EAQ is thephthalic anhydride method using concentrated sulfuric acid oroleum as a catalyst This process however suffers fromenvironmental post-treatment problem Therefore under thedual pressure of fast-growing demand of 2-EAQ andenvironmental protection producers eagerly need a newenvironmental benign catalyst to replace the concentratedsulfuric acid and oleum catalystsIn general the dehydration reaction of E-BBA catalyzed by

sulfuric acid and the oleum catalyst even though they couldproduce the desired products by intramolecular dehydrationthey also unavoidably give undesired oligomers or otherbyproducts resulting from lack of shape selectivity which thuscause unsatisfied yield (below 30) to 2-EAQ3233 On thecontrary our structured binder-free Al-β zeolite nanoparticlesshowed excellent catalytic performance As shown in Figure 7

the yield to the desired 2-EAQ product dropped dramaticallyfrom approximately 80 in the parent powder HB catalyst to53 in the shaped HBs containing massive binders while forcatalysts hydrothermally treated by TEAOH solutions theyield of 2-EAQ could be gradually improved It was worthmentioning that the catalytic performance of HBs-15 and HBs-20 was improved even better than parent powder HB whichstrongly proved the unique advantages of the abovementionedsamples compared to traditional (homogeneous) catalysts andcould be applied in the industrial reactions catalyzed by acidsites

This result is confirmed plotting the conversion of E-BBAand the selectivity to 2-EAQ (Figure S3) DFT calculationswere carried out to get the energy profile for E-BBAintramolecular dehydration over Al-β and uncover the typeof active sites involved in the process The reaction path isshown in Figure S4 it starts with the E-BBA physicaladsorption followed by a CminusH bond cleavage giving adsorbed2-EAQ and H2O products which desorb In particular the OminusH bond formation takes place between the CO group of theE-BBA molecule and H+ on H-β zeolite The first intermediatehas to go through a large transition state (TS1) through Htransfer from the CminusH bond of ethylbenzene to the O atom ofCO forming a CminusC bond This step is rate-determining dueto its large intrinsic activation barrier (ΔE = 5192 kcal molminus1)However it is crucial to highlight that the acidity of the catalystis critical to the reaction because it plays a key role in the Htransfer from the CminusH bond of ethylbenzene to the O atom ofCO suggesting that the framework aluminum species withstrong Broslashnsted acidity are active sites in E-BBA intra-molecular dehydration

4 CONCLUSIONSIn our report we have developed a facile synthetic route forthe preparation of structured binder-free Al-β zeolite withretained acid sites and sufficient mechanical strength through aprocess that involves mesopore formation and binderrecrystallization to give zeolite Al-β We have studied thephysicochemical properties of the powdered and structuredsamples through a variety of characterization methods such asN2 physisorption IR NMR XRD (S)TEM NH3-TPD andX-ray tomography DFT calculations corroborate that underthe experimental conditions and in the presence of thesiliceous binder the aluminum source in TEAOH caneffectively be transformed into framework Al which wellexplains the reason of the increased acidity of zeolites afterTEAOH treatment We chose the dehydration reaction of E-BBA as probe reaction to evaluate the catalytic performance ofall samples Satisfactorily the as-synthesized nanoparticlesamples showed outstanding catalytic performance withexcellent yield of production 85 which was far higher thanthe traditional acid catalyst due to their unique shapeselectivity retained acidity developed textural structure andsufficient mechanical strength We strongly believe that ourresults provide insights into the preparation and optimizationof zeolite catalysts with practical applications and fundamen-tally uncover the mechanism consisting of binder migrationinto the framework and acidity recovery

ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021acsanm1c02559

Additional characterization data of the materials (PDF)

AUTHOR INFORMATIONCorresponding Author

Jiaxu Liu minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian Chinaorcidorg0000-0003-0815-3979 Email liujiaxu

dluteducn

Figure 7 Reaction scheme for E-BBA dehydration (a) yield to 2-EAQ product over all samples (b) Reaction conditions temperature210 degC 1 bar catalyst 20 g E-BBA 20 g reaction time 60 min

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AuthorsShanshan Li minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian China

Zhenmei Zhang minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian China

Nicoletta Fusi minus Earth and Environmental SciencesDepartment Universita degli Studi Milano-Bicocca 20126Milano Italy

Chunyan Liu minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian China

Guang Xiong minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian China

Gianvito Vileacute minus Department of Chemistry Materials andChemical Engineering ldquoGiulio Nattardquo Politecnico di Milano20133 Milano Italy orcidorg0000-0003-0641-8590

Ning He minus State Key Laboratory of Fine ChemicalsDepartment of Catalytic Chemistry and Engineering DalianUniversity of Technology 116024 Dalian China

Complete contact information is available athttpspubsacsorg101021acsanm1c02559

Author ContributionsThe manuscript was written through contributions from allauthors and everyone gave approval to the final version of themanuscript

FundingJL acknowledges the financial support from the CentralUniversity Basic Research Fund of China (DUT20LAB127)and Dalian High-Level Talent Innovation Program(2017RQ001)

NotesThe authors declare no competing financial interest

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