Enhanced ortho-selective t-butylation of phenol over sulfonic acid functionalized mesopore MTW zeolites

Baoyu Liu,Feng Xiong ,Jianwen Zhang ,Manna Wang ,Yi Huang ,Yanxiong Fang,2 ,Jinxiang Dong,2

1 School of Chemical Engineering and Light Industry,Guangdong Provincial Key Laboratory of Plant Resources Biorefinery,Guangzhou Key Laboratory of Clean Transportation Energy Chemistry,Guangdong University of Technology,Guangzhou 510006,China

2 Jieyang Branch of Chemistry and Chemical Engineering Guangdong Laboratory (Rongjiang Laboratory),Jieyang 515200,China

3 School of Engineering,Institute for Materials & Processes,The University of Edinburgh,Robert Stevenson Road,Edinburgh EH9 3FB,United Kingdom

Keywords:MTW Zeolites Acidity Catalysis Alkylation

ABSTRACT Novel organo-inorganic hybrid materials(MTW-x-SO3H)have been fabricated by immobilizing 3-mercap topropyltriethoxysilane onto mesopore MTW zeolites,which is treated via a simple oxidation process with hydrogen peroxide as the oxidant to transform sulfhydryl group into sulfonic acid group.The organic sulfhydryl groups are covalently bonded to the external surface of MTW zeolites through the condensation between siloxane arising from organic fragments with silanol groups on the surface of MTW zeolites,the hybrids contain sulfonic acid group within the external surface of MTW zeolites and an opened mesoporous system in the matrix of MTW zeolites,which provide enough accessible Br?nsted acid sites for the alkylation between phenol with tert-butyl alcohol.Through this methodology it’s possible to prepare multifunctional materials where the plenty of mesopores are benefit for the introduction of larger numbers of sulfonic acid groups that contributes to activity during reactions,resulting in high activity(＞55%)of MTW-4-SO3H and desired selectivity(＞56%)of 2-TBP(2-tert-butyl phenol)in the alkylation between phenol with tert-butyl alcohol.

1.Introduction

The Friedel-Crafts alkylation of phenol withtert-butanol is widely used in industry for the production of resins,antioxidants,dyes,surfactants,rubber chemicals,plastics,petroleum additives and heat stabilizers for polymer materials,etc.[1].Generally,the alkylation of phenol withtert-butanol is catalyzed by various homogeneous catalysts such as liquid acids,BF3and ACl3[2],which inevitably brings problems involving waste disposal,equipment corrosion,difficult separation,and the generation of toxic substances [3].To overcome these issues,the solid acid catalysts are employed for alkylation of phenol withtert-butanol,such as ion-exchange resins [1],heteropoly acids [4] and solid sulfonic acids[5],but these catalysts have the disadvantages of low thermal stability,harsh reaction condition,inferior selectivity and production of a large amount of solid waste,which limits their application in industry[6].In addition,the mesopore porous materials such as MCM-41[7],SBA-15[8],are used as solid acid catalysts to catalyze the alkylation of phenol withtert-butanol,but they exhibited inferior catalytic performances due to the low acidity of mesoporous materials and poor hydrothermal stability.

Zeolite are regarded as one of excellent solid acid catalysts due to their high specific surface area,intrinsic acid sites and high thermal stability [9].According to previous reports,the acidity of zeolite has a significant effect on the product distribution in the alkylation between phenol withtert-butanol[10,11].Thus,it’s crucial to control the acidity of zeolites in order to tune the product distribution,and the selectivity of 4-TBP (4-tert-butylphenol) was greatly improved in our recent work through modulating the degree of defects in the ZSM-12 zeolitic framework[12].However,the 2-TBP(2-tert-butylphenol)is also an valued fine chemicals that can be used in the production of cosmetics,petroleum products,additives in jet fuels,pharmaceuticals and antiseptic [13,14].In addition,2-TBP is commonly used to synthesizetertbutylhydroquinone(TBHQ)as one of the cheapest phenolic derivatives,which is a high-quality edible-oil antioxidant that can effectively slow down the oxidation of oil and improve the shelf life of food,but the 2-tert-butylphenol is difficult to synthesize owing to the strong steric hindrance effect between phenol withtert-butyl carbocation [15].Nowadays,a novel strategy is developed to graft sulfonic acid groups in porous materials that can generate extraordinary synergistic catalytic effect with the porous matrix.For example,Diazetal.[16] successfully grafted sulfonic acid groups on silicon-based and mesoporous material MCM-41,respectively,which presented good catalytic effects in liquid phase acetalization and esterification of glycerol with fatty acids.Therefore,the fabrication of sulfonic acid groups anchored zeolites is a feasible strategy to improve the selectivity of 2-tert-butyl-phenol.

In this work,the sulfonic acid group was grafted into the framework of mesoporous MTW zeolites,in which the 3-mercaptopropyl triethoxysilane was firmly immobilized on the external surface of mesoporous MTW zeolites though the formation of covalent bond with silanol group,then the selective oxidation process was performed with hydrogen peroxide as the oxidant,resulting in the formation of sulfonic acid group,and a series of characterization techniques were employed to confirm the desired structure.As a result,the resultant sulfonic MTW zeolites possessed higher concentration of Br?nsted acid sites while they still maintained the comparable internal diffusion properties with parent MTW zeolites by optimizing the hierarchical structure.In addition,the catalytic performance of MTW-x-SO3H samples in the alkylation between phenol withtert-butyl alcohol were investigated and discussed.

2.Experimental

2.1.Materials

The regents used in the preparation of catalyst were tetraethyl ammonium hydroxide (TEAOH,35% (mass),Energy Chemical,China),fumed silica(99%,J&K,China),sodium aluminate(Aladdin,China),sodium hydroxide (Damao Chemical Reagent Factory,China),trimethoxysilyl propyldimethyl octadecyl ammonium chloride(TPOAC,65%(mass),Macklin,China),toluene(GuangZhou Chemical Reagent Factory,China),hydrogen peroxide (H2O2,30%(mass),GuangZhou Chemical Reagent Factory),sulphuric acid(H2SO4,98%(mass),GuangZhou Chemical Reagent Factory),3-mer captopropyltriethoxysilane(MPTS,97%(mass),Macklin)cyclohexane (Damao Chemical Reagent Factory),phenol (Macklin),tertbutyl alcohol (Damao Chemical Reagent Factory),N-dodecane(Kermel Chemical Reagent company),ammonium chloride(NH4CI,Macklin)and deionized water.All reagents were used without further purification.

2.2.Synthesis of catalysts

MTW-xzeolites were synthesized using fumed silica as the silicon source,TEAOH as the structure directing agent (SDA),TPOAC as the mesopores template agent,and sodium aluminate as the aluminum source.In a typical synthesis,TEAOH,deionized water and sodium aluminate were mixed under vigorous stirring.After complete dissolution,fumed silica was slowly added to the solution at room temperature and kept stirring for 1 h.Then,TPOAC was added to the mixture and kept stirring for 3 h.The gel mixture with molar ratios of 1 SiO2:0.31 TEAOH:0.0125 Al2O3:10.51 H2O:xTPOAC (x=0.02,0.03 and 0.04) was transferred into a Teflonlined stainless-steel autoclave by hydrothermal treatment at 170 °C for 144 h.After hydrothermal treatment,the solid was filtered,washed,and dried at 105°C for 12 h.Finally,the solid were calcined under air at 550°C for 6 h,and the sample was denoted as MTW-x(x=2,3,4)depending on the ratios of(TPOAC/SiO2)×100.To obtain the H+form zeolites,the synthesized MTW-xzeolites were ion-exchanged three times at 80°C for 8 h,using 1.0 mol.L-1of NH4Cl solution.

The synthesized MTW-xand toluene were added into three necks flask at 110 °C and stirring for 0.5 h with the liquid-tosolid ratio of 10 g to 1 g.Then,MPTS was injected into the three necks flask and kept stirring for 24 h.After that,the zeolite suspension was cooled down,filtered,and washed completely with toluene and absolute alcohol.In order to oxidize sulfhydryl groups,the obtained zeolite samples and H2O2aqueous was added into the three necks flask at room temperature and stirring for 24 h with the liquid-to-solid ratio of 10 g to 1 g.After oxidation reaction,the zeolite suspension was filtered and washed with absolute alcohol.Furthermore,the obtained sample was acidified with 0.1 mol.L-1H2SO4for 4 h to guarantee all the sulfonic acid groups was protonated.Finally,the fully acidified sample was filtered and washed with deionized water until the pH was neutral.These samples were dried at 60 °C overnight under vacuum and named as MTW-x-SO3H,wherexwas the previous ratios of (TPOAC/SiO2) × 100%.

2.3.Catalytic tests

The catalytic properties of resultant MTW-xand MTW-x-SO3H zeolites were evaluated in liquid-alkylation between phenol withtert-butyl alcohol,and the detailed information can be found in the experimental section in Supplementary Material.

3.Results and Discussion

3.1.Synthesis of MTW-x-SO3H zeolites

Fig.1 showed the X-ray diffraction (XRD) patterns of different MTW zeolites.It was observed that all of MTW-xand MTW-x-SO3H samples displayed characteristic peak of typical MTW zeolite[17,18],indicating that sulfonation process didn’t change the crystalline structure of MTW zeolites.In addition,the morphology and framework structure of resultant MTW zeolites were further investigated,and the SEM and TEM images of different MTW samples were shown in Figs.2 and 3,respectively.

Fig.1.XRD patterns of different MTW zeolites,for structural comparison,a‘standard’ XRD pattern of MTW is given in the bottom (dark yellow line).

It was seen from Fig.2 that MTW-2(Fig.2(a))and MTW-2-SO3H(Fig.2(b)) exhibited a sphere-like morphology assembled with bulk plates,MTW-3(Fig.2(c))and MTW-3-SO3H(Fig.2(d))showed a starfish-like morphology,while MTW-4 (Fig.2(e)) and MTW-4-SO3H (Fig.2(f)) also presented a starfish-like morphology similar with MTW-3 and MTW-3-SO3H,but surface of MTW-4 and MTW-4-SO3H was a little rough,indicating that the post sulfonation process didn’t influence the morphology of MTW zeolites,and the morphology of MTW zeolites was influenced by the amount of TPOAC in the synthesis gel,which was ascribed to the fact that the organosilane can effectively inhibit the nuclei growth[19].Furthermore,it was observed from Fig.3 that MTW-2 (Fig.3(a))and MTW-2-SO3H(Fig.3(b))presented a condensed structure,MTW-3(Fig.3(c))and MTW-3-SO3H(Fig.3(d))exhibited plate-like morphology,while MTW-4 (Fig.3(e)) and MTW-4-SO3H (Fig.3(f))showed some nanocrystals that were attached on the surface of zeolitic sheets owing to inhibiting effect of TPOAC.

Fig.2.SEM images of MTW-2 (a),MTW-2-SO3H (b),MTW-3 (c),MTW-3-SO3H (d),MTW-4 (e),MTW-4-SO3H (f) zeolites.

Fig.3.TEM images of MTW-2 (a),MTW-2-SO3H (b),MTW-3 (c),MTW-3-SO3H (d),MTW-4 (e),MTW-4-SO3H (f) zeolites.

The porosity of different MTW zeolites were determined by N2adsorption-desorption isotherms,as given in Fig.4.It was observed that the resultant MTW zeolites exhibited typical IV isotherms with obvious hysteresis loop,indicating that the MTW-xand MTW-x-SO3H possessed mesopores [20],but micropore adsorption capacity of MTW-x-SO3H was lower than individual MTW-xowing to grafting sulfonic acid groups on the external surface of MTW-xthat blocked the original micropore and mesopore on a certain degree.Table 1 listed the textural properties of synthesized MTW zeolites,it was noticed that both the surface area and pore volume of MTW-x-SO3H samples decreased compared with individual MTW-xsample due to the additional sulfonic acid groups,and the value ofSextandVmesoincreased in the order of MTW-2 ＜ MTW-3 ＜ MTW-4,indicating that excessive TPOAC can increased the mesopores in the matrix of MTW zeolites,which was beneficial for the introduction of organic sulfonic acid groups.

Table 1 Textural property of different MTW zeolites

Fig.4.N2 adsorption-desorption isotherms of resultant MTW zeolite samples.

The acidity of obtained MTW zeolites were further investigated by pyridine adsorption FT-IR spectra,as presented in Fig.S1 and Table S1 (Supplementary Material).Generally,the adsorption peak at～1545 cm-1and～1455 cm-1was attributed to Br?nsted acid sites and Lewis acid sites,respectively [21].It was noticed from Table S1 that MTW-x-SO3H exhibited raised concentration of Br?nsted acid sites compared with MTW-x,which can be attributed to the fact that a large number of Br?nsted acid sites can be provided by sulfonic (—SO3H) acid groups.Besides,the Br?nsted acidity of MTW-x-SO3H increased in the order of MTW-2-SO3H ＜ MTW-3-SO3H ＜ MTW-4-SO3H,suggesting that the additional TPOAC can effectively increase the external surface area of MTW-x,resulting in more sulfonic acid groups being incorporated into the framework of MTW zeolites with increasing the TPOAC[22].

In order to verify that the additional sulfonic acid groups have been successfully grafted onto the surface of MTW-xsamples,the x-ray photoelectron spectroscopy (XPS) patterns of typical MTW-4 and MTW-4-SO3H samples were conducted,as shown in Fig.5.It was seen that the MTW-4-SO3H samples exhibited an obvious signal of around 169 eV compared with MTW-4 samples,which was attributed to the sulfate species (S6+) that were derived from sulfonic(—SO3H)acid groups,indicating that sulfonic(-SO3H)acid groups were successfully incorporated into the MTW zeolite [22-24].Moreover,the coordinated environment of organotrialkoxysilane on MTW-4-SO3H zeolite was confirmed by29Si MAS NMR,as shown in Fig.6.Obviously,both samples exhibited Q4[Si(OSi)4]and Q3[Si(OH)(OSi)3] peaks at -112 position and -103 position,respectively,but the fraction of Q3peak of MTW-4-SO3H decreased compared with MTW-4 while the fraction of Q4peak of MTW-4-SO3H was larger than MTW-4,the variation in the intensities observed for Q4and Q3silicon atoms of MTW-4-SO3H compared with MTW-4 were assigned to generation of new Q4silicon atoms in the hybrid from surface Q3silanol groups present in the MTW-4 zeolites,because the initial external surface silanol groups of MTW-4 zeolites involved in the covalent insertion with silane groups arising from organotrialkoxysilane.In addition,MTW-4-SO3H samples also exhibited extra T2(C-Si(OH)(OSi)2) and T3(CSi(OSi)3) signals at -59 position and -67 position,respectively,which demonstrated the formation of Si—C bonds and the sulfonic acid groups were successfully grafted in the framework of MTW zeolite [22,25].Besides,the TGA (thermogravimetric analysis)curves of MTW-4 and MTW-4-SO3H were conducted,as shown in Fig.7,it was confirmed that the content of organic sulfonic acid groups incorporated in the MTW zeolites was about 4.82% (mass),and the MTW-4-SO3H can maintain the stability at ＜170 °C.Based on the above analysis,the proposed preparation process of MTWx-SO3H catalysts was presented in Fig.8,it was noted from Fig.8 that the mesopores of MTW-xincreased with increasing the amount of TPOAC in the synthesis gel,leading to the increase of sulfonic acid groups incorporated into the MTW-x-SO3H samples.

Fig.5.XPS patterns of MTW-4 and MTW-4-SO3H zeolite samples.

Fig.6.29Si MAS NMR spectra of MTW-4 and MTW-4-SO3H zeolite samples.

Fig.7.Thermogravimetric analysis of MTW-4 and MTW-4-SO3H zeolite samples.

Fig.8.Proposed sulfonation process MTW zeolites with different TPOAC.

3.2.The catalytic performances of MTW-x-SO3H samples

The catalytic performance of MTW-x-SO3H samples in the alkylation between phenol withtert-butyl alcohol were evaluated,as shown in Fig.9.It was observed from Fig.9 that the MTW-2-SO3H exhibited inferior activity compared with MTW-2 owing to the limited mesopores,in which the additional sulfonic acid groups can seriously block the channels of MTW zeolites,resulting in low activity of MTW-2-SO3H.With increasing the TPOAC,the MTW-3 and MTW-4 achieved enough mesopores with enhanced external surface area and mesopore volume,thus the resultant MTW-3-SO3H and MTW-4-SO3H exhibited excellent activity compared with MTW-3 and MTW-4,respectively,because MTW-3-SO3H and MTW-4-SO3H improve the concentration of Br?nsted acid sites arising from the sulfonic acid groups while the opened channels were maintained since sulfonic MTW zeolites had the similarDeff/r2(diffusion time constant) with parent MTW zeolites (Fig.S2,Table S2),which can greatly enhance the accessibility between acid sites with reactants,leading to raised activity of MTW-3-SO3H and MTW-4-SO3H.

Fig.9.Conversion of phenol over different MTW zeolites.

Moreover,the product distribution over various MTW zeolites was presented in Fig.10,it was noted that TBPE(tertiary butyl phenyl ether) was main product for MTW-xsamples,which was reasonably attributed to the weak Br?nsted acid of MTW-xcatalyst(Table S1) that was beneficial for the formation oftertiary butyl phenyl ether [2,12].When the sulfonic acid groups were incorporated into the framework of MTW-xsamples,the 2-TBP became the main product,which was different from the conventional results that 2-TBP was not easy produced,becauseorthoposition of phenol possessed higher steric hindrance thanmetaandparaposition of phenol owing to the phenolic hydroxyl group[7].Based on the previous reports [15],the alkylation between phenol withtert-butyl alcohol over zeolites followed a Langmuir-Hinshelwood reaction mechanism rather than Rideal-Eley mechanism(Fig.11),where thetert-butyl alcohol and phenol were coadsorbed on the acid sites of zeolites,andtert-butyl carbenium ion was produced by a step of dehydration,then theorthoposition of phenol was closed to the adjacenttert-butyl carbenium ion that leads to the electrophilic reaction betweentert-butyl carbenium ion with phenol,in which the electronic effect is over steric effect,giving preferentially 2-TBP [26].In addition,the obtained MTW-xsamples also exhibited robust stability that effectively inhibited the leaching of organic functional groups in the liquid alkylation(Fig.S3),which is crucial for the practical application of hybrid catalysts in the industrial production.

Fig.10.Product distribution over different MTW zeolite at 9 h.

Fig.11.Schematic diagram of alkylation reaction between phenol with tert-butyl alcohol.

4.Conclusions

In summary,a novel sulfonic acid functionalized MTW-SO3H hybrid material was successfully fabricated through grafting the sulfonic acid group on the external surface of mesopore MTW zeolites.By tuning the amount of TPOAC,the textural properties of MTW-x-SO3H can be systematically modulated,and the sulfonic MTW zeolites exhibited comparable internal diffusion properties compared with parent MTW zeolites by optimizing the textural parameters,which not only improved the concentration of Br?nsted acid sites but also maintained the diffusion properties of mesopore MTW zeolites,resulting in high activity of sulfonic acid group modified MTW zeolites in the alkylation between phenol withtert-butyl alcohol.In addition,MTW-x-SO3H also exhibited a raised selectivity of 2-TBP arising from the Langmuir-Hinshelwood reaction mechanism,in which the electronic effect was superior to steric effect,and ortho position of phenol was closed to neighboringtert-butyl carbenium ion,leading to the formation of preferentially 2-TBP.

Data Availability

Data will be made available on request.

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(21978055,22278090),Natural Science Foundation of Guangdong Province,China (2022A1515012088),the Science and Technology Planning Project of Guangdong Province,China(22A0505050073,2022A0505030013),the Guangdong Provincial Key Laboratory of Plant Resources Biorefinery,China(2021GDKLPRB10),and the ‘‘High-level Talents Program” of the Pearl River,China (2017GC010080).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.02.014.