Shuo Li ,Jianlin Cao ,Xiang Feng,*,Yupeng Du ,De Chen ,Chaohe Yang,*,Wenhua Wang,Wanzhong Ren

1 College of Chemistry &Chemical Engineering,Yantai University,Yantai 264005,China

2 State Key Laboratory of Heavy Oil Processing,School of Chemical Engineering,China University of Petroleum,Qingdao 266580,China

3 Department of Chemical Engineering,Norwegian University of Science and Technology,Trondheim 7491,Norway

Keywords:C4 alkylation Surface Zeolite Catalysis DFT calculation

ABSTRACT Elucidating the confinement effect harbours tremendous significance for isobutane alkylation with C4 olefin.Herein,the confinement effect over zeolite catalysts was elucidated by combining DFT calculations,experiments (using the novel Beta zeolite exposing only external surfaces (Beta-E) and conventional Beta-I zeolite with both external and internal surfaces) and multi-techniques (e.g.,TGA-DTG,HRTEM,SEM and XRD).It is found that the main active sites for C4 alkylation reaction are located on internal surface rather than external surface.On the external surface,the hydride transfer reaction does not occur because the H-shared intermediate cannot be formed without the confinement effect.Moreover,the external surface has stronger selectivity for C4 olefin adsorption than isobutane,leading to enhanced oligomerization reactions.Therefore,the suitable micropore with confinement effect is essential for zeolite-catalyzed C4 alkylation.The atomic-scale insights of this work are of great referential importance to the design of highly effective zeolite catalyst.

1.Introduction

Automotive exhaust emission is the predominant source of air pollution.With the increasingly stringent environmental protection requirements,the standards of gasoline are constantly upgradingviareducing the content of olefins,sulfur and aromatic[1].However,these measures normally lead to the decline of octane number in gasoline.Therefore,alkylate as the environmentally friendly high-octane gasoline component harbors tremendous potential in industry [2,3].Traditional liquid acid alkylation processes to produce alkylate always suffer from the inherent drawbacks of high acid consumption,corrosion and toxicity [4,5].Hence,the development of alternative solid acid catalysts has attracted considerable attention.

Since the first report by Garwoodet al.[6] that the alkylation process could be catalyzed by faujasite zeolites,various zeolite catalysts have been investigated [7-9].H-Y and H-Beta exhibited promising initial activity and selectivity [10].However,there is still a gap between traditional liquid acid alkylation and solid acid alkylation processes[11-13].The performance of zeolite still needs to be significantly enhanced [14-17].Understanding the relationship between the performance and zeolite structure is of great significance to design more efficient zeolite catalysts for alkylation process.

Iglesiaet al.[18-20] reported that the catalysis of zeolites results from combined effects of acid sites and confinement.Saueret al.[21,22] found that stabilities of alkoxide species and carbonium ion were significantly influenced by confinement effect.Furthermore,Cormaet al.[23] and Speybroecket al.[24] also found that the confinement effect has great influence on the reactivity.Zeolites,as the finite-dimensional crystallites,have both “internal surface”and “external surface”.Notably,the confinement effect only exists on the internal surface inside the zeolite pores.Previous results(e.g.,protonation reaction of styrene,dehydrogenation of nbutane) have confirmed that the confinement effect results from pore structure and the size of reactants [25-29],and the catalytic performances are different over the two types of surfaces [30,31].Nevertheless,the catalytic performance and corresponding mechanism of the internal and external surfaces in the alkylation process still remain unclear.It is highly desirable to elucidate the distinct performance of internal and external surfaces from both experiments and molecular level.

In this contribution,the confinement effect of isobutane alkylation with C4olefin and the intrinsic mechanism were investigated by experiments and density functional theory (DFT).Firstly,the intrinsic alkylation mechanism over the internal and external surface was elucidated by DFT calculation.The protonation,oligomerization and hydride transfer reactions were investigated over the internal surface (67 T) and external surface (52 T) zeolite models.It is found that the hydride transfer reaction can only occur over the internal surface because H-shared intermediate cannot be formed over the external surface.The protonation and oligomerization reactions can occur over both the internal and external surfaces.Furthermore,a novel strategy was proposed to synthesize HBeta zeolite exposing only external surface(i.e.,H-Beta-E).This catalyst is compared with conventional H-Beta zeolite with both internal and external surfaces.Results show that confinement effect is dominating in C4alkylation reaction.H-Beta-E with only external surface has almost no activity for the C4alkylation reaction.Therefore,the external surface has no alkylation catalytic activity,but only rapid deactivation.The essential confinement effect revealed in this work will be crucial to the design of highly efficient zeolite catalysts for C4alkylation.

2.Experimental

2.1.Computational details

Fig.1 shows the 67 T and 52 T cluster models of Beta zeolite.The 67 T cluster model has been successfully used to investigate the C4alkylation mechanism in our previous works [32].Herein,the 67 T and 52 T cluster models represent the internal surface and external surface,respectively.Notably,they have similar deprotonation energy (DPE),indicating that the acid strengths are the same.

The Dmol3module coming from Materials Studio 8.0 was utilized to reveal the reaction mechanism [33].The mGGA-M06L,as the reliable functional in the zeolite catalytic system,was applied in the DFT calculations [34-36].The atomic orbital basis set was set to the double numerical plus polarization(DNP).The core electrons were treated with“All electron”parameter.The threshold of self-consistent field tolerance was 1.0 × 10-6.Complete LST/QST method was utilized to scan the transition states[37].With regard to the transition states,a single imaginary frequency was confirmed with the minimum energy along the reaction path.The convergence criteria were 2.7×10-4eV,0.544 eV·nm-1and 0.0005 nm for energy,maximum force and maximum displacement,respectively.

The adsorption energy (Ead) of the hydrocarbon molecule is defined as follows:

whereEcomplexis defined as the total energy of the optimized adsorption structure,EZOHrepresents the total energy of the zeolite model,andEC4is the total energy of the isolated C4hydrocarbon molecule.

In order to clarify the contribution of the acid site to the adsorption energy,the adsorption energy oftrans-2-butene and isobutane over the pure silicon zeolite model is calculated.Herein,the difference of the adsorption energy (ΔE) is calculated as follows:

whereEad-acidrepresents the adsorption energy over the zeolite surface with acid site,Ead-Siis the adsorption energy over the pure silicon zeolite surface.

2.2.Preparation

The Beta zeolite was synthesized with the hydrothermal method in the absence of alkali metal ion.The molar composition of the precursor gel was 1SiO2:0.02Al2O3:0.56TEAOH:16.67H2O.Typically,17.53 g of deionized water was added to 29.46 g of TEAOH (35% in water).Subsequently,24 ml of aforementioned solution was added to 7.51 g fumed silica (400 m2·g-1,Aladdin),stirred until dissolved (Solution A).Then,1.67 g of aluminum sulfate octadecahydrate (Sinopharm Chemical Reagent Corp.) was added to the rest part of TEAOH solution (Solution B).Finally,the Solution B was dropwise added into the Solution A under vigorous stirring at the temperature of 303 K.After stirring for 1 h,the mixture was transferred into a Teflon autoclave and heated at 413 K for 3 days.After that,the resulting zeolite was washed and centrifuged repeatedly with deionized water,and dried at 353 K for 12 h.Finally,the sample was calcinated for 6 h at 823 and 413 K,and the as-obtained samples were named as Beta-I and Beta-E,respectively.

The crystal phases of the catalysts were measuredviapowder X-ray diffraction(XRD)on a Rigaku D/Max 2550VB/PC(CuKα radiation).The Hitachi S-4800 field-emission scanning electron microscope was utilized to acquire the SEM images.High resolution transmission electron microscope(HRTEM)was taken on the JEOL JEM 2100F (200 kV) [38].N2physisorption (Quantachrome Autosorb-1) was performed to identify the pore volumes and pore diameters for the Beta-I and Beta-E catalysts.Thermogravity(TGA)was performed simultaneously using the TA SDT Q600 simultaneous DSC-TGA analyzer.Powder samples (10 mg) were placed in a quartz basket,and purged 60 min at ambient temperature in N2gas,then heated from ambient to 1173 K at a rate of 2 K·min-1in N2gas.

Fig.1.67 T (represents internal surface) and 52 T (represents external surface) Beta zeolite models.

2.3.Catalytic testing

The liquid phase alkylation reaction between isobutane and 1-butene was carried out in a fixed-bed reactor(Fig.2).3 g of zeolite catalysts with a particle size of 0.250-0.425 mm were pretreated in situ at 423 K for 2 h in flowing nitrogen.When the temperature dropped to 353 K,the reaction system was pressurized to 3 MPa with N2.After that,a mixture of isobutane and 1-butene with the molar ratio of 20 was continuously pumped into the reaction system.The olefin weight hourly space velocity (WHSV) is 0.1 h-1.Owing to the short lifetime of the zeolite catalysts and a long time needed to analyze the reaction product,the reaction effluent was periodically collected and stored in a heated 16-port sampling multi-loop valve.When the reaction was finished,these samples were analyzed by gas chromatography.

3.Results and Discussion

3.1.Alkylation mechanism over the internal and external surface

The C4alkylation process contains reactions such as protonation,oligomerization and hydride transfer.Besides,the competitive adsorption of isobutane and butene also has significant influence on the alkylation performance.Therefore,the experimental method is limited in the microscopic mechanism research due to the complexity of the C4alkylation process.In order to elucidate the intrinsic reason for the confinement effect,the detailed reaction mechanisms over the external and internal surfaces are investigatedviaDFT calculations.The reaction scheme of the calculated elementary steps in this work is exhibited in Fig.3.

3.1.1.Competitive adsorption of isobutane and butene

Isobutane and butene as the two essential reaction species have different roles in the C4alkylation process.Moreover,the adsorption of the reactants has significant effect on the subsequent reaction steps.Therefore,it is necessary to investigate the competitive adsorption properties of isobutane and butene over the zeolite surface.

Fig.2.Schematic of C4 alkylation reaction fixed bed system in liquid phase.

Fig.3.The reaction scheme of the calculated elementary steps in DFT calculations.

The absolute values of the adsorption energies and geometrical parameters of the optimized adsorption structures for isobutane andtrans-2-butene over the internal and external surfaces are exhibited in Fig.4.As shown in Fig.4(a),the adsorption energies of the isobutane andtrans-2-butene over the internal surface are-0.696 and -0.771 eV,respectively.This is consistent with the previous results,in which adsorption energy of the 1-butene in H-FER zeolite is -0.776 eV [39].At the same time,the adsorption energies of the two reactants over the external surface are-0.471 and -0.689 eV,respectively.Notably,the absolute values of adsorption energies for isobutane are lower than those fortrans-2-butene both over the internal and external surfaces.It indicates that the acidic zeolite surfaces are more favorable to adsorb butene molecules.That is due to the stronger interaction between the acid site and the C=C double bond oftrans-2-butene.More interestingly,the difference of the adsorption energies betweentrans-2-butene and isobutane(0.218 eV)over the external surface is much higher than that (0.075 eV) over the internal surface.

The result elucidates that the external acidic surface has stronger selective adsorption capacity of butene than the internal acidic surface.Furthermore,the absolute values of adsorption energies for isobutane andtrans-2-butene over the external acidic surface are lower than those over the internal acidic surface,which suggests that the internal acidic surface has stronger interaction with the adsorbate molecules than external acidic surface.The principal reason is that the whole pore structure of internal surface could provide more van der Waals and static electric interactions for adsorbates,while the external surface solely offers partial van der Waals and static electric interactions [31].

Fig.4.The absolute values of the adsorption energies(a).Geometrical parameters of the optimized adsorption structures for trans-2-butene and isobutane over external(b and d) and internal (c and e) surfaces.

The results will be more clear through the analysis of optimized adsorption structures.As shown in Fig.4(b) and (c),the bond lengths of C1-C2 and O-H intrans-2-butene over the internal surface are slightly longer than those over the external surface.Meanwhile,the distances of the C1-H and C2-H over the internal surface are slightly shorter than those over the external surface.The results show that the interaction betweentrans-2-butene and the internal surface is slightly stronger than that over the external surface,which is consistent with the adsorption energy results.Compared with the adsorption oftrans-2-butene,the structural parameters of isobutane adsorption have more remarkable difference over the internal and external surfaces,as exhibited in Fig.4(d) and (e).It is noteworthy that the distances of H-C2 and H-C3 in isobutane over the internal surface (LH-C1=0.3456 nm,LH-C3=0.2823 nm) are much longer than those over the external surface (LH-C1=0.3100 nm,LH-C3=0.2796 nm).It suggests that the isobutane is much closer to the lower surface over the external surface than that over the internal surface.Due to the complete pore structure,the isobutane over the internal surface has more interaction with the surrounding zeolite framework.As a result,the lengths of the C1-C2 and C1-C4 bonds of isobutane over internal surface are longer than those over the external surface.

In order to clarify the influence of the acid site on the adsorption,the adsorption energies of isobutane andtrans-2-butene over the pure silicon surface are also calculated.As shown in Table S1,the adsorption energy of isobutane (-0.510 eV) is equal to that oftrans-2-butene (-0.512 eV) over the external pure silicon surface.Nevertheless,the adsorption energy of isobutane(-0.741 eV) is much higher than that oftrans-2-butene(-0.574 eV)over the internal pure silicon surface.The results illustrate that the external pure silicon surface has no selective adsorption capacity for isobutane and butene,while the internal pure silicon surface prefers to adsorb the isobutane molecule.Furthermore,the contributions of the acid site to the adsorption energies fortrans-2-butene are-0.177 eV(external surface)and-0.197 eV(internal surface),which for isobutane are 0.039 and 0.045 eV,respectively.The results indicate that the introduction of the acid site promotes the adsorption of butene and hampers the adsorption of isobutane.The adsorption of butene is more sensitive to the introduction of acid site.Moreover,the adsorption energies are mainly contributed to the interaction between the zeolite framework surface and adsorbates.

In summary,micropore pore structure is necessary to maintain high isobutane concentration near the acid sites.High isobutane concentration will promote the hydride transfer and inhibit oligomerization[40].Therefore,the internal surface has higher catalytic activity of alkylation than external surface from the view of adsorption.

3.1.2.Protonation reaction of trans-2-butene

Protonation of the butene is the initial reaction of the alkylation process.Therefore,it is essential to investigate the protonation reaction over the internal and external surfaces.The geometries of reaction species for protonation reaction oftrans-2-butene over the internal and external surfaces are shown in Fig.5.

Firstly,thetrans-2-butene adsorbs over the zeolite surface to form the π-complex species (Fig.4(b) and (c)).Then,the 2-buty cation (Fig.5(b) and (f)) is formed through the proton attacking the C=C double bond.The distances between O1 and H atom are 0.1599 and 0.1646 nm over the external and internal surfaces,respectively.It indicates that the 2-buty cation is much closer to the external surface.Besides,the length of C2-H bond over the external surface (0.1209 nm) is longer than that over the internal surface (0.1198 nm).The results confirm that the proton (H) has stronger interactions with thetrans-2-butene species over the internal surface.Consequently,the C1-C2 bond length over the internal surface (0.1391 nm) is longer than that over the external surface (0.1387 nm).

At the transition state (TS1,Fig.5(a) and (e)),the proton is transferring from the acid site to the C2 atom oftrans-2-butene over both the internal and external surfaces.Differently,the proton is closer to the C2 atom over the external surface(0.1342 nm)than that over the internal surface(0.1372 nm).Notably,the proton is in the middle of the C2 and O1 atom over the internal surface.After the formation of the 2-buty cation,the 2-butoxide (Fig.5(d) and(h)) is generated through the formation of the C1-O2 bond over the two types of surfaces.It can be seen that the bond length of C1-O2 is longer over the internal surface (0.1612 nm) than that over the external surface (0.1601 nm).At transition state (TS2,Fig.5(c) and (g)),the distances between C1 and O2 atom over the internal and external surfaces are 0.2456 and 0.2476 nm,respectively.

Fig.5.Geometries of reaction intermediates and transition states for protonation reaction of trans-2-butene over internal and external surfaces.

Based on the aforementioned discussion,it is found that the reaction intermediates are closer to the active site over the external surface than those over the internal surface,while the contrary results are obtained for the transition states.The difference is due to the confinement effect of the zeolite pore framework.Compared with the external surface,the internal surface offers the more interactions surrounding the C4species.Particularly,the upper internal surface has the attractive force interactions with the C4species.Therefore,the reaction intermediates as the steady state can have longer distance with the lower surface in the zeolite pore,while the transition states as the metastable state have the opposite results.

As shown in Fig.6,the reference value is the total energy of two isolatedtrans-2-butene molecules,an isobutane molecule and the zeolite model.Remarkably,the relative energies of the transition states and reaction intermediates are lower over the internal surface than those over external surface (from 0.082 to 0.168 eV).The result indicates that the transition states and reaction intermediates over the internal surface are more stable than those over the external surface.Herein,the 2-butyl cation is easily to translate into the π-complex over the zeolite surface.Particularly,the relative energy of the 2-butyl cation (-0.116 eV) is almost equal to that of the TS1 (-0.110 eV) over the external surface.Moreover,the rate determining step of the protonation reaction is the formation of the 2-butyl cation over the zeolite surface.The activation energies of this step over the internal and external surfaces are 0.561 and 0.579 eV,respectively.The activation energies of 2-butoxide formation step over the internal and external surfaces are 0.243 and 0.284 eV,respectively.It is noteworthy that the activation energies of the protonation reaction over the internal surface are only slightly lower than those over the external surface.It suggests that the internal and external surfaces have the similar catalytic activity for protonation reaction.In other words,the confinement effect has little influence on the protonation reaction.That is due to the small size of C4species in the protonation reaction compared with the pore structure of Beta zeolite.

Fig.6.The energy profiles for protonation reaction of trans-2-butene over internal(blue) and external surfaces (red).

3.1.3.Oligomerization reaction of trans-2-butene

It is reported that the rapid deactivation of zeolite catalyst is due to the multiple oligomerizations of butene molecules [15].Thus,the oligomerization reactions oftrans-2-butene are investigated in detail over the internal and external surfaces.The geometries of reaction species for oligomerization reaction are exhibited in Fig.7.

As shown in Fig.7,the oligomerization reaction mechanisms over the internal and external surfaces are similar.After the protonation reaction of thetrans-2-butene,anothertrans-2-butene is firstly adsorbed over the 2-butoxide (Fig.7(a) and (f)).Herein,the C-O bonds of the adsorption structure are elongated slightly compared with those of the 2-butoxide in protonation reaction(0.0003 nm for external surface and 0.0001 nm for internal surface).Moreover,the distances of C1-C3 and C2-C3 over the external surface (0.4580 and 0.4619 nm) are much longer than those over the internal surface (0.4196 and 0.4434 nm).

After that,the C3-O2 bond is broken apart to form the C4cation +trans-2-butene reaction intermediate (Fig.7(c) and (h)).The distances of C3-O2,C1-C3 and C2-C3 are changed from 0.1604,0.4580 and 0.4619 nm in the adsorption structure to 0.3552,0.2569 and 0.2680 nm in the C4cation +trans-2-butene structure over the external surface.The corresponding distances over the internal surface are varied from 0.1613,0.4196 and 0.4434 nm to 0.3523,0.2539 and 0.2572 nm.Furthermore,the bond length of C1=C2 is lengthened from 0.1334 nm in the adsorption structure to 0.1349 nm in the C4cation+trans-2-butene structure over the external surface,which is elongated from 0.1333 nm to 0.1351 nm over the internal surface.The results indicate that the C3 atom of the C4cation has a strong interaction with the C1=C2 double bond of thetrans-2-butene in the C4cation +trans-2-butene structure over both the external and internal surfaces.The transition states (TS3) for the formation of C4cation +trans-2-butene are depicted in Fig.7(b) and (g).The distances between the C3 and O2 atom over the external and internal surfaces are 0.2658 and 0.2525 nm,respectively.It indicates that the C3-O2 bonds are broken at transition states over the two types of surfaces,and the C4cations are moving from the surface to the absorbedtrans-2-butene.

Subsequently,the C8cation(Fig.7(e)and(j))is formed through rotation.Herein,the C1-C3 bond is formed.The bond length of C1-C3 over the external surface (0.1694 nm) is larger than that over the internal surface (0.1680 nm).Moreover,the length of the C1-C2 bond over the external surface (0.1408 nm) is shorter than that over the internal surface (0.1418 nm).The results indicate that the two C4species have stronger interactions with each other over the internal surface.Obviously,the zeolite pore compresses the distance between thetrans-2-butene and the C4cation(2-butoxide)resulting in the stronger interactions over the internal surface.

As exhibited in Fig.8,the adsorption energy oftrans-2-butene over the internal surface is -0.564 eV,which is more exothermic compared with the adsorption energy over the external surface(-0.334 eV).That results from the attractive vdW interactions with zeolite pore over the internal surface.The activation energy barriers of the formation for C4cation +trans-2-butene over external and internal surfaces are 0.518 and 0.335 eV,respectively.Besides,the activation energy barriers of the formation for C8cation over external and internal surfaces are 0.368 and 0.362 eV,respectively.Notably,the rate determining step over the external surface is the formation of the C4cation +trans-2-butene,while the rate determining step over the internal surface is the formation of the C8cation.The results illustrate that the confinement effect can change the rate determining step of the oligomerization reaction.Based on the analysis of the activation energy barriers,the oligomerization reaction prefers to occur over the internal surface.The difference of the activation energy barrier for the rate determining step is 0.156 eV over the external and internal surfaces.Furthermore,the relative energies of the reaction intermediates and transition states over the external surface are much higher than those over the internal surface with the difference ranging from 0.370 to 0.652 eV.Based on the parameters of the structures,it is noteworthy that thetrans-2-butene is much closer to the C4cation (2-butoxide) over the internal surface due to the confinement effect of the zeolite pore structure.

Fig.7.Geometries of reaction intermediates and transition states for oligomerization reaction of trans-2-butene over internal and external surfaces.

Fig.8.The energy profiles for oligomerization reaction of trans-2-butene over the internal (blue) and external surface (red).

3.1.4.Hydride transfer reaction of trans-2-butene and isobutane

Our previous work has investigated the hydride transfer reaction of the isobutane and the 2-butoxide over the 67 T Beta zeolite model(internal surface)[32].The detailed reaction process is presented in Fig.S1(Supplementary Material).As shown in Fig.S1,the 2-butoxide is firstly formedviathe protonation reaction.Secondly,isobutane is adsorbed over the 2-butoxide (co-adsorption structure).Thirdly,the H-shared intermediate is generated through the scission of the C-O bond in the co-adsorption structure.Finally,the butane and tert-butyl carbocation are formed through the rotation and desorption.It is noteworthy that the H-shared intermediate with a C-H-C three-centered and two-electron bond is the essential reaction intermediate.In other words,the formation of H-shared intermediate determines whether the hydride transfer reaction takes place successfully.

In this work,the hydride transfer reaction mechanism is investigated over the external surface.It is found that the H-shard intermediate cannot be formed over the external surface.Herein,the species optimized from the C-H-C H-shared intermediate is named by “H-shared intermediate”.As shown in Fig.9(a),the distances of the C1-H,C2-H and C2-O of the “H-shared intermediate”are 0.1101,0.2543 and 0.2611 nm,respectively.However,the distances of the C1-H,C2-H and C2-O of the H-shared intermediate over the internal surface are 0.1219,0.1284 and 0.3214 nm(Fig.9(c)),respectively.Clearly,the H atom belongs to the isobutane molecule over the external surface,while the H atom is in the middle between the C1 and C2 atom over the internal surface.Fig.9(b) and d show the total electron density plots with color mapped iso-surfaces of the “H-shared intermediate”over external surface and H-shared intermediate over internal surface,respectively.It is found that the electron density between the C2 and H atom is very low in“H-shared intermediate”,indicating that there are few electron interactions.The partial Mulliken charge of+0.038e for isobutane also supports this point.As shown in Fig.9(d),the electron density between the C2 and H atom is very high in H-shared intermediate,which is similar to the electron density between the C1 and H atom.Besides,the partial Mulliken charge of isobutane is +0.553e.The results indicate that strong electron interactions exist among the C1,H and C2 atom in H-shared intermediate.All the evidences show that the H-shared intermediate as the essential intermediate for the hydride transfer reaction cannot be formed over the external surface of the Beta zeolite.That is mainly due to the lack of the confinement effect for the external surface.

On the basis of the aforementioned results,it is found that the order of confinement effect on the reactions follows hydride transfer ＞oligomerization ＞protonation.Compared with the protonation reaction,the confinement effect has much stronger influence on oligomerization and hydride transfer reaction.That is mainly due to the larger size of the reactant molecules in the oligomerization reaction,which leads to stronger interaction between the hydrocarbon molecules and the pore framework.Furthermore,the butene,which contains C=C bond,is more active than isobutene.Thus,the interaction betweentrans-2-butene and C4cation (2-butoxide) is stronger than that between isobutane and C4cation (2-butoxide).That leads to the occurrence of oligomerization reaction over the external surface,while hydride transfer reaction cannot occur over the external surface.Therefore,the confinement effect has stronger influence on hydride transfer reaction than oligomerization reaction.

3.2.Characterization and catalytic performance of Beta-I and Beta-E

In order to support the calculation results,the novel Beta zeolite exposing only external surfaces (Beta-E) and conventional Beta-I zeolite with both external and internal surfaces are synthesized.The XRD patterns of the Beta-E and Beta-I zeolites are exhibited in Fig.S2(a).Obviously,Beta-E and Beta-I zeolites have the similar XRD patterns.Typical diffraction peaks corresponding to the two zeolite samples exist at about 7.71°,13.40°,14.55°,21.48°,22.51°,25.40°,27.01°,28.79° and 29.65°,indicating that two zeolite samples exhibit high crystallinity and typical structure of Beta zeolite without the presence of impurities[41].As shown in Fig.S2(b)and(c),Beta-E and Beta-I zeolites have the similar morphology and particle size.The polycrystalline zeolite particles are uniform with size ranging from 90 to 150 nm.Furthermore,both of the zeolite samples have coarse surfaces built with small units,which are agglomerates of orientated building units.As shown in Fig.10 (a)and (b),HRTEM images demonstrate that all the building units are crystalline with clear lattice fringes.Based on the results,the synthesized Beta-I and Beta-E zeolites have similar crystallinity,morphology and sufficient external surfaces.

For the Beta-E and Beta-I zeolites,the calcination temperature is different.Normally,the calcination procedure is employed to decompose the TEAOH template molecules on the internal and external surface of synthesized Beta zeolite.Herein,Beta-E was calcined at low temperature,aiming to remove the external template while maintaining the TEA+inside microporous channels.Therefore,it is essential to investigate the thermal stability of TEAOH template over the zeolite.

Fig.9.The optimized structure(a and c)and total electron density(b and d)of the“H-shared intermediate”over the external surface and the H-shared intermediate over the internal surface.

Fig.10(c)shows the TG-DTG curves of synthesized Beta zeolite.It is found that the whole weight loss curves can be split into four different regions.Previous study [42] reported that the range between 313 and 423 K is associated with desorption of water.The range between 423 and 623 K is ascribed to the decomposition of TEAOH species occluded in the zeolite framework,and the temperature between 623 and 773 K corresponds to the pyrolysis of TEA+species in the zeolite framework.When the temperature is higher than 773 K,the weight loss is the decomposition of residual organic material.The weight losses of the four regions are 1.45%,6.03%,7.19% and 5.25%,respectively.

Based on the above results,the calcination temperature for Beta-E catalyst is determined to be 413 K,which can maintain the stability of template inside micropores.In order to check whether the template on the internal surfaces is stable for Beta-E catalyst,the TG curves of synthesized Beta zeolite at low temperature is further analyzed in Fig.10(d).Firstly,the temperature keeps constant at 313 K for 60 min.Then,the temperature increases from 313 K to 413 K with the heating rate of 2 K·min-1.When the temperature reaches 413 K(i.e.,the calcination temperature of Beta-E),it was maintained for 350 min.It is found that the weight loss between 313 K and 413 K is 1.44%,which is equal to the weight loss of water.The result indicates that the water of the zeolite is thoroughly removed.In addition,the weight continues to drop with the extension of time from 98.56% to 97.00%.This is due to the decomposition of the template over the external surface to CO2,C2H4,H2O and NH3[43].The rate of decline decreases and finally approaches zero,indicating that the template over the external surface is decomposed completely.Simultaneously,the acid sites over the external surface are exposed.The result is consistent with the higher decomposition temperature of the template over the internal surface stem from Fig.10(c).Therefore,the TEAOH molecules over the external surface are decomposed at 413 K while the templates inside micropores remain stable.

The pore structures of Beta-I and Beta-E catalysts are shown in Fig.10(e).The type IV isotherm feature is found in the isotherm of both Beta-I and Beta-E [44].For beta-I sample,it has both the micropores (0.19 cm3·g-1fort-plot micropore volume in Table S2,which is consistent with the typical Beta zeolite [45])and mesopores.However,Beta-E only has the mesoporous structure(0.01 cm3·g-1for t-plot micropore volume in Table S2).Moreover,the hysteresis loops after relative pressure of 0.8 indicate that the mesopore structure of Beta-I and Beta-E results from the aggregation of small crystallites.Therefore,the Beta-E zeolite has only external surface while Beta-I zeolite has both internal and external surfaces.This is further confirmed by the pore size distribution result that no micropores present for Beta-E (Fig.10(f)).

The conversion of butene and product selectivity of C4alkylation at 10 min over Beta-I and Beta-E samples are shown in Table 1.In order to highlight the effect of structure on catalytic perfor-mance,low isobutane/olefin ratio of 20 and olefin WHSV of 0.1 h-1are used.As shown in Table 1,the butene conversions of Beta-I and Beta-E are 100%and 6%,respectively.Table S2 shows that the Beta-E has much lower surface area than that of Beta-I.That must lead to the lower acid amount of the Beta-E.Therefore,the Beta-E zeolite has lower butene conversion.As for the product selectivity,the product for Beta-E are C8olefins andcomponent at 10 min(Table 1).Based on the DFT calculations,the hydride transfer cannot occur over the external surface,so the oligomerization of 1-butene mainly takes place over the external surface.Besides,the reaction temperature is low,and the deprotonation reaction occurs more easily than cracking reaction [9].Thus,there aren’t other products except C8olefin andon Beta-E.Epeldeet al.[46] reported that the product of oligomerization of butene over the conversional zeolite is C8olefin andin a fixed-bed reactor,which is consistent with this work.Remarkably,the C8olefin was found on Beta-E but not on Beta-I catalysts.the conversion of 1-butene is very low on the external surface,which leads to small amount of C8olefin.On basis of the DFT calculations,although oligomerization reaction oftrans-2-butene over internal surface is more favorable than that on external surface,hydride transfer reaction also occurs on the internal surface.The generated C8cation will react with isobutane to form C8alkaneviahydride transfer reaction on the internal surface in the fixed-bed reactor.C8olefins are present in the product only when the catalyst begins to deactivate [9].The result suggests that the Beta-E zeolite with only the external surface has the oligomerization performance rather than alkylation performance.Although the amount of acid affects the distribution of the product,it is not decisive.Therefore,the great difference of product selectivity between Beta-I and Beta-E is mainly decided by the confinement effect.

Table 1 The product selectivity of Beta-E and Beta-I catalysts at 10 min

Based on the above results,the suitable micropore with confinement effect is essential for zeolite-catalyzed C4alkylation.The acid sites on the external surface almost have no C4alkylation activity but only oligomerization performance.Therefore,the C4alkylation performance can be enhanced if the acid site on the external surface of zeolite can be blocked out in the future.It should be noted that the presence of mesopores could cause weak confinement effect than micropores,but facilitate the diffusion of reactants and products.Overmuch mesopores may lead to the lack of confinement effect,and thus worse performance.Therefore,there should be an optimal mesopore size of modified zeolite catalysts.This can also explain the volcano-shaped trends in relation to the mesopore volume in C4alkylation reaction[47].This work is of tremendous importance to the design of more efficient catalysts for C4alkylation reaction over solid-acid catalysts.

4.Conclusions

The confinement effect of zeolite for C4alkylation performance was studied by the experimental and DFT methods using unique Beta-E catalyst with only external surface and traditional Beta-I catalyst.Firstly,the intrinsic reason for the confinement effect was investigated by DFT calculations.It is found that the order of confinement effect on the reactions follows hydride transfer ＞ oligomerization ＞ protonation.Furthermore,the Hshared intermediate cannot be formed over the external surface,and thus the hydride transfer reaction cannot occur over the external surface.The oligomerization and protonation reactions can occur over the two types of surfaces.Besides,the external surface has stronger selectivity for butene.At the temperature of 413 K,the template on external surface was removed while the template inside micropores was maintained.Interestingly,the catalytic performance supported that Beta-E with only external surface had no catalytic activity for C4alkylation due to be absence of confinement effect.Therefore,the external surface has no C4alkylation catalytic activity,but only rapid deactivation.The essential confinement effect is of great significance to the design of highly efficient zeolite catalysts for C4alkylation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21978325,21776312,21908186),the Independent Innovation Foundation of Qingdao (17-1-1-18-jch),the Fundamental Research Funds for the Central Universities(18CX02014A),and the Fundamental Research Funds for the Central Universities and the Opening Fund of State Key Laboratory of Heavy Oil Processing (SKLOP202003002).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.005.