Ethylbenzene is mainly used for the production ofstyrene, which is then used to produce polystyrene, acrylonitrile- butadiene-styreneresin, styrene-acrylonitrile resin, and other products ^[1]. More than half theworld’s ethylbenzene is produced and consumed by the US, Japan, Korea, ChinaTaiwan, and Mainland China (Fig. 1). The production and consumption ofethylbenzene are growing continuously with the development of economy,especially in Mainland China (Fig. 2). In 2013, the overall ethylbenzenecapacity of Mainland China was about 7 Mt, the largest in the world, and it isexpected to exceed 9 Mt in 2015-2016.

The alkylation of benzene to ethylbenzene has beenstudied and commercially developed for more than 80 years since the 1930s, whena Friedel-Crafts catalyst (AlCl₃-HCl) was used ^[2]. The AlCl₃-HClprocess causes corrosion problems, therefore UOP and Mobil-Badger developedsupported catalysts ^{[3, 4]} and zeolite-based catalysts in the 1960s and 1970s.The third- generation Mobile-Badger process was commercialized using ZSM-5 as acatalyst, and a second reactor was added for transalkylation in the 1990s. Thezeolites used as catalysts for ethylbenzene processes are listed in Table 1.Research institutes and companies in Mainland China have done much work on thedevelopment of their own benzene alkylation techniques in the last two decades.The China Petroleum & Chemical Corporation (Sinopec) has performed a largeamount of research and development on benzene alkylation in recent decades.

In this paper, we summarize the development andindustrial application of benzene alkylation catalysts and processes forethylbenzene production in Mainland China. The structure- activityrelationships of zeolite catalysts in the corresponding processes arediscussed. The results of research on benzene alkylation over catalysts withimproved mass transfer are discussed, and some promising catalytic materialsare proposed.

Ethylbenzene is produced by benzene alkylation,and ethylene is the most frequently used ethylation agent (Scheme 1(1)). Underthe reaction conditions, the produced ethylbenzene further reacts with ethyleneto form polyethylbenzenes (EBs, Scheme 1(2)), such as diethylbenzenes (DEBs)and triethylbenzenes (TEBs). Transalkylation of EBs with benzene is performedto increase the yield of ethylbenzene (Scheme 1(3)). The addition of atransalkylation unit in ethylbenzene production increases the yield of ethylbenzeneby about 10%. AlCl₃-catalyzed benzene alkylation was the firstprocess to be developed for the production of ethylbenzene, and then solid-stateacids (solid phosphoric acid, zeolites) were used as alkylation catalysts.

The AlCl₃-HCl-catalyzed liquid-phaseprocess was developed in the 1930s. It was conducted at low benzene/ethylenemolar ratios (about 2.5/1) and mild temperatures (160-250 °C). As mentionedabove, processes using zeolite-based catalysts have attracted much attention inindustry because of the corrosive effects of the AlCl₃-HCl catalystand the solid phosphoric acid catalysts used since the mid-1960s ^{[2, 3, 4]}.

The ZSM-5 zeolite, which has the MFI structure(Table 1), is most effective in vapor-phase benzene alkylation ^[5]. It is oneof the most important highly siliceous zeolites and was initially developed byMobil ^[6]. There are two types of ten-membered ring (10-MR) channels in thestructure: one straight elliptical pore system (0.53 nm × 0.56 nm) along the (010)axis and one zigzag circular pore system (0.51 nm × 0.55 nm) along the (100)axis ^[7]. ZSM-5 isthermally and hydrothermally stable, with adjustable acid amounts andstrengths. Because of its unique pore structure and acidic properties, ZSM-5zeolite is widely used in petrochemical processes such as fluid catalyticcracking (FCC), methanol-to-propylene conversion, methanol-to-gasolineconversion, MTA, and OCC ^[8].

In the last 20 years, Sinopec has commercializedseveral vapor-phase ethylbenzene processes using different ethylation agents(Table 2), including pure ethylene, dilute ethylene (including FCC off-gas),and ethanol. The Shanghai Research Institute of Petrochemical Technology(SRIPT), a subsidiary research facility of Sinopec, and the Dalian Institute ofChemical Physics (DICP, Chinese Academy of Sciences) have developed alkylationcatalysts incorporating ZSM-5 and ZSM-5/ZSM-11 cocrystalline zeolites,respectively (Table 2).

In the Mobil-Badger ethylbenzene process, whichwas the first to be commercialized, benzene alkylation with pure ethylene wasconducted in a vapor-phase fixed-bed reactor to produce ethylbenzene and EBs(mainly DEBs). Ethylbenzene was separated as the product, and the EBs wererecycled back into the reactor to undergo transalkylation. In the 1990s, theMobil-Badger ethylbenzene process was improved by adding a transalkylationreactor. The service life of the catalyst increased from about 3 months to 1year ^[9].

SRIPT launched research projects on thepure-ethylene process in 1994. There were two main issues to be addressed:catalyst deactivation caused by coke deposits from ethylene polymerization andremoval of the xylene byproduct from ethylbenzene.SRIPT studied ZSM-5 catalysts with different SiO₂/Al₂O₃molar ratios and crystal size for vapor-phase benzene alkylation ^[10]. The datain Table 3 show that ethylene conversion decreased with increasing SiO₂/Al₂O₃molar ratio, but the selectivity for ethylated benzenes (EBs, includingethylbenzene, DEBs, and TEBs) and the ZSM-5 stability were improvedsignificantly. This is because ethylene polymerization decreased because of thedecreased number of active sites resulting from higher SiO₂/Al₂O₃molar ratios in the ZSM-5 zeolite.

As shown in Table 4, ZSM-5 zeolites with smallcrystals are more effective in benzene alkylation, because they give higherethylene conversions and stability, although the selectivity for ethylatedbenzenes is slightly lower. A series of catalysts, i.e., AB-96, AB-97, andAB-05HS (vapor-phase alkylation catalysts), and AB-97-T (vapor-phasetransalkylation catalyst), were developed using highly siliceous nano-sizedZSM-5 zeolite as the active component ^{[11, 12]}; these catalysts werecommercialized by SRIPT. There are several industrial plants in China using thepure-ethylene technique licensed by Sinopec.

Because of the high cost of ethylene derived fromcrude oil and advances in ethanol production by fermentation of crops such asmaize, cassava, and crop straw ^[13], benzene alkylation with ethanol hasattracted much attention recently ^[14]. However, water in either thebio-ethanol raw material or produced in the alkylation reaction causes loss ofactive Al sites, leading to poor catalytic performance ^{[14, 15]}. Combined treatmentswith steam and phosphoric acid solution, based on the SRIPT pure-ethyleneprocess, were therefore introduced to ensure that ZSM-5-zeolite-based catalystshave long lifetimes, high selectivities, and the ability to convert substantialamounts of ethanol to minimize generation of impurities such as xylene ^[16].The zeolitic catalysts AB-AS and DF-AS (alkylation catalysts), and DF-AS-T(transalkylation catalyst) were developed and first used in an industrial plantin 2007; in 2010, they were used in the largest plant in Mainland China, withan annual capacity of 215 kt ^[17].

As early as 1992, the Indian PetrochemicalsCorporation introduced a benzene alkylation process using dilute ethanol ratherthan ethylene, and Fe-ZSM-5 was used as the alkylation catalyst ^[18]. However,the dilute-ethanol process has no distinct advantage over the ethylene process,and no commercial plants using this process have been reported.

FCC off-gas contains 10%-30% ethylene, which canbe used as an ethylene source for ethylbenzene production. In China, about 5.5Mt of FCC off-gas are produced every year, and the ethylene resource isestimated to be 1.0 Mt ^[19], However, only some of the ethylene is recovered,and most of the ethylene is burned. The use of FCC off-gas as an ethylenefeedstock would be of benefit to refineries and decrease carbon emissions.However, when FCC off-gas is directly fed into the alkylation reactor as theethylene resource, impurities such as propylene cause side reactions andcatalyst deactivation. The separation of ethylene from the FCC off-gas wouldfacilitate benzene alkylation, but the overall investment and running costswould increase.

Mobil-Badger modified the pure-ethylene processfor use with an FCC off-gas feed. The dilute-ethylene process involves acomplex system for pretreating the raw material. After water removal,desulfurization, and propylene separation from the FCC off-gas feed, the streamis fed into the alkylation reactor to produce ethylbenzene. This process wasfirst commercialized at Shell’s Stanlow, UK refinery; it had an annualethylbenzene capacity of 160 kt in 1991. The Dow Chemical Company built alarger industrial plant in the Netherlands, with a capacity of 450 kt/year^[20]. There are still five plants using dilute ethylene as a feedstock.

In the early 1990s, DICP found that a ZSM-5/ZSM-11cocrystalline zeolite gave a good catalytic performance in the alkylation ofbenzene with FCC off-gas. In cooperation with Sinopec and the China NationalPetroleum Corporation (CNPC), the DICP ethylbenzene process using FCC off-gasas the ethylation agent was developed and commercialized ^{[19, 21]}. The feedstockpretreatment is simpler than that in the Mobil-Badger dilute-ethylene process.The first-generation technique used only one fixed-bed reactor, in whichbenzene alkylation and transalkylation of EBs took place simultaneously. Thecontent of the main byproduct, xylene, was high, up to 3000 ppm. In the second-generationtechnique, vapor-phase alkylation and vapor-phase transalkylation wereperformed separately in two different reactors. The alkylation temperature waslowered from 410-430 °C to 360-390 °C, and the xylene content was reduced to ~2000 ppm. The third-generationtechnique was further improved by adding a propylene- removal unit and usingβ-zeolite-catalyzed liquid-phase PE-transalkylation instead of vapor-phasetransalkylation ^{[22, 23]}. The alkylation and transalkylation took place at320-360 and 220-260 °C, respectively. The single-pass catalyst lifetimes for alkylationand transalkylation are 6-9 months and 18-24 months, respectively. The productquality is better, the xylene content is lower, <1000 ppm, and theethylbenzene purity is higher than 99.8%. The third-generation technique hasbeen licensed to more than ten industrial plants.

SRIPT’s alkylation catalyst, SEB-08, forvapor-phase benzene alkylation with dilute ethylene was introduced in 2008, andwas first used in an industrial plant in 2009 ^[24]. The SGEB dilute-ethyleneprocess was then developed based on the SEB-08 alkylation catalyst and AEB-1Hliquid-phase EB- transalkylation catalyst ^[25]. Benzene ethylation is performed at320-380 °C and 0.7-1.3 MPa, with a benzene/ethylene feed molar ratio of (6.0-7.0)/1,and transalklylation is performed at 170-235 °C and 2.8-3.0 MPa with abenzene/DEB feed mass ratio of (5.0-8.0):1. The cycling lifetimes of thealkylation catalyst and transalkylation catalyst are more than 1 year and 2years, respectively, and the total catalyst service lives are more than 2 yearsand 4 years, respectively. The ethylbenzene purity is higher than 99.8%, andthe xylene content is lower than 800 ppm.

The SEB-08 alkylation catalyst contains ZSM-5zeolite as the active component. The catalyst has abundant macropores andmesopores. Hydrothermal treatment and acid leaching modification are performedto remove Lewis acid sites to inhibit side reactions. The reactants can easilydiffuse through the macropores and mesopores to reach the active sites locatedin the micropores. The catalyst showed stable performance in terms of ethyleneconversion and selectivity, and the industrial runtime was beyond 12 months.The EB-transalkylation catalyst AEB-1H was developed by the Sinopec ResearchInstitute of Petroleum Processing (RIPP), using an FAU zeolite as the activecomponent. The overall ethylbenzene capacity using the SEB-08 alkylationcatalyst and SGEB dilute-ethylene process is currently about 1.3 Mt annually.In 2014, a new alkylation catalyst, SEB-12, was used in industrial plants. Thefeed molar ratio of benzene/ethylene was lowered to (5.0-6.0)/1, and the energyconsumption decreased.

The energy consumption of vapor-phase benzenealkylation is high because of the high benzene/ethylene molar ratio and highreaction temperature. In addition, more xylene is produced, which lowers theethylbenzene quality. In 1989, the liquid-phase benzene alkylation process wasintroduced by Lummus/Unocal/UOP using a Y-zeolite-based catalyst ^[26]. TheMobil-Raytheon EBMax process and Lummus/UOP EBOne liquid processes are now inoperation. Sinopec has also developed and commercialized a liquid-phase benzeneethylation catalyst and liquid-phase ethylbenzene process (Table 5).

Zeolite Y is a faujasite molecular sieve with athree- dimensional12-MR pore structure ^[27]. The basic structural units of zeolite Y are sodalitecages, which form supercages that are large enough to accommodate spheres ofdiameter 1.2 nm (Table 1). Nanocrystalline zeolite Y has larger intercrystallinevoid spaces, a larger pore volume, more acid sites located on the externalsurface, higher activity, and better stability.

The liquid-phase ethylbenzene process catalyzed byzeolite Y was first commercialized in 1990 ^[18]. Benzene alkylation took placeat a much lower temperature, 240-270 °C, than that of the vapor-phase process,around 400 °C. EB transalkylation was performed in the liquid phase in aseparate reactor using a Y-zeolite-based catalyst. Although the feed ratio ofbenzene/ethylene is high (about 7/1), the process is competitive because it hasthe advantages of better thermal control and a longer catalyst life (about 1year). This process was licensed to several industrial clients, and their totalethylbenzene capacity is about 2.8 Mt annually.

β zeolite has a three-dimensional 12-MR porestructure consisting of perpendicular straight channels (0.66 nm × 0.67 nm),and the interconnections create narrow helical channels with effective porediameters of 0.56 nm × 0.56 nm (Table 1) ^[28]. Since its discovery by Wadlingeret al. ^[29] in 1967, the use of β zeolite for catalytic reactions such asalkylation, alkane hydroisomerization, aromatic acylation, and methyl tert-butylether synthesis ^[30] has been extensively investigated because of itssilica-rich framework and easy access to active sites in the pores.

β zeolite is highly active in benzene alkylationwith ethylene and gives higher ethylene conversion and ethylbenzene selectivitythan Y zeolite ^{[31, 32, 33]}. EBOne was developed and commercialized in 1996 byLummus/UOP, using EBZ-500 as the alkylation catalyst and EBZ-100 as thetransalkylation catalyst ^[34]. The active component of EBZ-500 is a modified βzeolite, which is also active in EB transalkylation ^{[2, 35]}. Zeolite Y was usedas the active component of EBZ-100. The feed ratio of benzene/ethylene isreduced to about 4/1, and the catalyst life is prolonged to 2 years.

RIPP began research on a liquid-phase circulatingprocess for the production of ethylbenzene in 1990, and a highly activealkylation catalyst, AEB-2, with high ethylbenzene selectivity and stabilitywas introduced and commercialized ^{[36, 37]}. The process is operated with benzene/ethylenemolar ratios of (3-6)/1 at 200-250 °C and 3.6 MPa. The catalyst life is 6 years.When operated with a feed ratio of 5, ethylene conversion is almost 100%, theselectivity for ethylbenzene is 86.5%, and the overall ethylbenzene yield is99.9%. AEB-2 is also suitable for transalkylation processes. At 213-221 °C, EB conversionis about 80% and the ethylbenzene selectivity is 100%. After thecommercialization of AEB-2, a new-generation benzene alkylation catalyst,AEB-6, was developed using a much smaller β zeolite as the active component.The alkylation can be operated with a low benzene/ethylene feed ratio of 3.5/1;the ethylbenzene selectivity is about 88% and the total ethylbenzene yield is99.7%. Use of AEB-6 as the catalyst greatly reduces the energy consumption^[38]. A series of AEB catalysts are now being used in several industrialplants.

MWW zeolite is a general term for a series ofzeolites formed by stacking basic layers with the MWW structure, such asMCM-22, PSH-3, SSZ-25, ERB-1, ITQ-1, MCM-49, MCM-56, ITQ-2, UZM-8, MCM-36,EMM-10, EMM-12, EMM-13, SSZ-70, and IEZ-MWW ^{[39, 40, 41, 42, 43]}. They differ in terms ofthe number of layers, stacking manner, and/or spacing between adjacent layers(Fig. 3). MWW zeolite, known as MCM-22, has attracted particular researchattention because of its peculiar structure. It consists of two independentpore systems accessible via 10-MR windows, one of which is defined bytwo-dimensional sinusoidal channels with elliptical ring cross-sections of size0.41 nm × 0.51 nm, and the other contains 12-MR supercages of dimensions 0.71 nm× 0.71 nm × 1.82 nm restricted by 10-MR openings of size 0.45 nm × 0.55 nm(Table 1). In addition, pocket-like semi-supercages are located on the crystalsurfaces. These different types of pore systems play different roles incatalytic reactions ^[44].

MCM-22 zeolite shows comparable activity to thatof zeolite Y in liquid-phase benzene alkylation, but it is less active than βzeolite. The advantage of MCM-22 zeolite is that the ethylbenzene selectivityis higher than those of Y and β zeolites; therefore, smaller amounts of EBs areproduced during benzene alkylation with ethylene using MCM-22 zeolite as thecatalyst ^[2]. Based on experiments, an MCM-22-zeolite-based alkylation catalystand ethylbenzene process (EBMax) were developed by Mobil-Raytheon in 1995. Theliquid-phase process was first commercialized in 1997. The feed ratio ofbenzene/ethylene is lowered to only (3-4)/1, which in turn reduces the energyconsumption. The catalyst is quite stable, and the lifetime is more than 3years. Initially, EB transalkylation in the EBMax process was performed in thevapor phase using a ZSM-5-zeolite-based catalyst. The transalkylation processwas improved by using a new zeolite catalyst, branded as Trans-4, which is alsoused in the liquid phase ^[34].

The performance of the EBMax ethylbenzene processis now even better. The optimum alkylation molar ratio of benzene/ethylene isonly (2-4)/1. There are 28 plants operating with the liquid-phase EBMaxprocess, and the overall annual ethylbenzene capacity is more than 16 Mt.Moreover, the EBMax ethylbenzene process can be easily adapted to accept dilute-ethylenefeedstocks (ethylene accounts for more than 70%). The alkylation reactor islarger than that in the pure- ethylene process, to accommodate a higher catalyst load, and isoperated under partial liquid conditions with a benzene to ethylene feed ratioof 3/1. The per-pass conversion of ethylene is higher than 99.5%.

The commercialized EBOne process was improved byLummus/UOP, using a new catalyst with the UOP trade name EBZ-800TL. The activecomponent is UZM-8 ^[45], a new MWW zeolite synthesized by UOP in 2004 usingdiethyldimethylammonium hydroxide as an organic template ^[40]. UZM-8 is formedthrough stacking of MWW structural layers, but stacking of the adjacent layersis not well organized. It is similar to MCM-56, another MWW zeolite. UZM-8 isefficient in liquid-phase benzene alkylation and gives better product yields^[46].

In addition to the abovementioned ethylbenzeneprocesses, other ethylbenzene production techniques have been reported by theDow Chemical Company, Indian Petrochemicals Corporation, and CD Tech.

CD Tech was the first company to use catalyticdistillation for the production of ethylbenzene ^[2]. In 1997, ABB Lummus Globaland Chemical Research & Licensing (an affiliate of Shell) established apartnership and purchased the CD Tech company. The CD Tech process combines catalyticbenzene alkylation and distillation in one reactor. There are two sections inthe reactor. The liquid-phase benzene alkylation catalyst (Y zeolite) is placedin the upper section, and distillation trays are provided in another section.Ethylene is fed at the bottom of the catalyst bed, and benzene is introducedfrom the top. The heat generated during benzene alkylation is efficientlyremoved by distillation. After the reaction, ethylbenzene is separated from themixture of products, and then EB transalkylation is performed in anotherreactor. The catalyst lifetime is 2 years and the total ethylbenzene yield is99.5%. The energy consumption and equipment investment are low. The CD Techethylbenzene process was commercialized in 1994 at the Mitsubishi ChemicalCorporation’s ethylbenzene plant, with an annual capacity of 260 kt ^[20].

The ethane content of FCC off-gas is about 10%,which is comparable to that of ethylene. However, in the dilute-ethyleneprocess or FCC off-gas process for ethylbenzene production, ethane hardlyreacts with benzene and is emitted as a waste gas. Great efforts have beendevoted to developing catalytic processes for using ethane ^{[47, 48]}. The Dow ChemicalCompany introduced a dual-stage ethylbenzene process that combined ethanedehydrogenation with benzene alkylation ^[18]. Ethane dehydrogenation iscatalyzed by a Zn-, Ga-, or Pt-containing MOR catalyst, and benzene alkylationis catalyzed by β, Y, or ZSM-5 zeolite catalysts. However, there have been noreports of industrial applications of ethane processes.

Zeolites with different framework structures havebeen extensively studied in benzene ethylation, with the aim of developingefficient ethylation catalysts. Among the tested zeolites, including MFI, MEL,MOR, FAU, BEA, MWW, FER, and MTT zeolites ^{[49, 50]}, only ZSM-5, Y, β, and MCM-22zeolites have been successfully used in industrial plants.

ZSM-5 is more active than ZSM-23, ZSM-11, andZSM-5/ZSM-11 cocrystalline zeolites in the vapor-phase alkylation of benzene^[4]. The Mobil-Badger and SRIPT ethylbenzene processes both use ZSM-5 as thecatalyst. However, ZSM-5/ZSM-11 cocrystalline zeolite is also effective inbenzene alkylation ^[19], and several ethylbenzene plants are operated using itas the alkylation catalyst. These results seem almost contradictory. It isdifficult to say which process is better, because each zeolite has its ownoptimum reaction conditions. Justification based on inversion of the catalyticperformance of ZSM-5 and ZSM-5/ZSM-11 under different operating conditions isnot convincing. Moreover, the zeolite composition and morphology are criticalin the catalytic performance. Every zeolite is a potential candidate forindustrial applications, provided the process is profitable. Scientistsidentify possibilities, and engineers realize them.

Vapor-phase benzene alkylation occurs at highertemperatures than liquid-phase processes. More xylene is therefore producedthrough cracking, which reduces the ethylbenzene quality. This is a problembecause xylene is difficult to separate from ethylbenzene. The xylene contentin ethylbenzene produced by vapor-phase processes is difficult to restrict tobelow 600 ppm, whereas it is only about 10 ppm for liquid-phase processes.Moreover, the benzene/ethylene feed ratio is significantly lower inliquid-phase processes. Less excess benzene has to be recycled and circulated.Liquid-phase ethylation of benzene also has other advantages such as a betterthermal control and longer catalyst lifetimes, which allow off-site catalystregeneration, thereby facilitating easier pollution control. The liquid-phasebenzene alkylation process is therefore better than the vapor-phase technique.

However, diffusion resistance to mass transfer isstronger in the liquid-phase process than in the vapor-phase process. WhenZSM-5 zeolite was tested in liquid-phase benzene alkylation with ethylene, itsactivity was rapidly lost. It is therefore necessary to move from medium porezeolites to large pore zeolites such as β and Y.

Zeolite Y has a three-dimensional 12-MR poresystem with supercages at the pore interconnections (Table 1), and the poreopenings are larger than those of ZSM-5 (10-MR). It is suitable forliquid-phase benzene alkylation and was commercialized by UOP/Lummus/Unocal in1989. However, the supercages with 12-MR openings (0.74 nm) favor the formationof bulky byproducts (higher alkylbenzenes such as biphenyl and diphenylethane),and this is a drawback of using zeolite Y as an alkylation catalyst. β zeolitealso has a three-dimensional pore system with 12-MR openings, but no supercagestructures. In liquid-phase benzene alkylation, β zeolite is more active thanzeolite Y, and more DEBs and TEBs are produced. EBs can be transalkylated,therefore the overall selectivity for useful products is higher for β zeolite^[31].

Similar to zeolite Y, MCM-22 zeolite has asupercage structure in the pore system, but MCM-22 is more selective, particularlyin the formation of DEBs and TEBs, the amounts of which are significantly lowerthan with USY or β zeolites. Little heavy byproducts are produced. Benzene andethylene can both diffuse into the supercages through the 10-MR windows, withno limitations ^[51]. EBs are produced without any constraints inside thesupercages, but they can hardly diffuse through the 10-MR openings.

β and MCM-22 zeolites are more suitable thanzeolite Y for liquid-phase operations. A liquid-phase process catalyzed byMCM-22 zeolite can be operated at lower benzene/ethylene feed ratios; β zeolitehas the highest catalytic activity and poison tolerance. MCM-22 and β zeolitesare both widely used in industrial plants. Again, only economic factors decidewhich is more competitive.

Since the first industrial application of azeolite-catalyzed ethylbenzene process, benzene alkylation technology has madesignificant progress. The reaction temperature, benzene/ ethylene feedratio, xylene content, and consumption of energy and materials have all beenlowered, and the operating stability, tolerance of diverse materials, andproduct quality have greatly improved. Further research on catalysts and processeswill undoubtedly promote the development of ethylbenzene technology.

The stoichiometric molar ratio of benzene toethylene in benzene ethylation is 1:1. However, in order to improve theselectivity for ethylbenzene, the feed ratio of benzene/ethylene in industrialplants is much higher (>2/1), especially in vapor-phase processes (>5/1).This is because the selectivity for the desired product is not high enough.Figure 4 shows the influence of the benzene/ethylene molar ratio on the productselectivity in liquid-phase benzene ethylation over MCM-22 zeolite. Theformation of byproducts such as DEBs can be effectively suppressed by raisingthe benzene/ethylene molar ratio. When the reaction is performed at a low benzene/ethylenemolar ratio, dimerization and polymerization of ethylene take place easily,resulting in coke formation, which causes catalyst deactivation. Theethylbenzene selectivity of MCM-22 zeolite catalyst is higher than those of βand Y zeolites, as shown in Fig. 5 ^[52]. The EBMax process, which uses MCM-22as the alkylation catalyst, therefore operates with the lowest benzene/ethylenefeed ratio, about 2/1.

Recently, it has been observed that changes in themorphologies and mesoporosities of the zeolite crystals strongly influence theactivity and selectivity in alkylation reactions ^{[53, 54, 55, 56, 57, 58, 59, 60, 61, 62]}. Wang et al. ^[53]showed that the conversion of ethylene over MCM-22 zeolite nanoplates is higherthan that over MCM-22 zeolite microspheres. When β zeolite with a nanospongemorphology was used in benzene alkylation with benzyl alcohol, the maximumpossible catalytic turnover was increased six-fold compared with that for bulkβ zeolite ^[54]. This high catalytic performance was the result of alkylationreactions occurring on external surfaces. External active sites were able toperform the catalytic function even after the active sites inside the zeolitemicropores were deactivated.

Studies of benzene alkylation over mesoporouszeolites show that zeolites with abundant mesopores are more active thanconventional zeolites ^{[55, 56, 57, 58, 59, 60, 61]}. Figure 6 shows a comparison of ethyleneconversions using conventional MCM-22 and mesoporous MCM-22 prepared at variousSiO₂/Al₂O₃ molar ratios ^[58]. The mesoporousMCM-22 zeolites clearly gave higher ethylene conversions than did theconventional MCM-22 samples. Also, delaminated MWW zeolites and MFI zeolitenanosheets give better catalytic performance than the original zeolites^{[63, 64, 65, 66]}. As well as an improvement in the activity, increased selectivity formonoalkylated products is also observed.

In a typical reaction, several procedures areassumed to occur: a reactant molecule first diffuses to the active site, thenadsorption, activation, and reaction take place, and then the formed productmolecule desorbs and diffuses out to give an available active site. Comparedwith a conventional catalyst, where diffusion restraints are significant, lessmonoalkylbenzene is present in the interiors of the mesoporous zeolite crystalsbecause of the improved mass transfer. The possibility of further reactionbetween monoalkylbenzene and alkylation agent molecules to producepolyalkylbenzenes is therefore reduced. This may be why the activity andselectivity are both improved for mesoporous zeolites.

In liquid-phase benzene alkylation, where masstransport limitations cause much greater problems than in vapor-phaseprocesses, mesoporous zeolites have more potential applications.

Alkylation of benzene over MWW zeolite, which hasinternal 10-MR sinusoidal pores and external 12-MR semi-cages, is more complicatedthan the reactions over Y or β zeolites, which have 12-MR pore systems. Cormaet al. ^[67], and Du and Olson ^[68] showed that the 10-MR pore system of theMCM-22 zeolite contributes little to the overall production of ethylbenzene andEBs in the liquid-phase alkylation of benzene with ethylene. This means thatonly some of the acid sites are active. This may be why MWW zeolite is lessactive than β zeolite.

The synthesis of materials with moresemi-supercages or larger external surface areas is an effective way ofobtaining better MWW-zeolite-based alkylation catalysts. Delaminated ITQ-2^{[63, 64, 69]} and pillared MCM-36 ^[70] zeolites represent such materials. However,the complicated preparation procedures and high costs of ITQ-2 and MCM-36restrict their use. The development of new preparation methods that can be easilyused would benefit progress in ethylbenzene processes.

Because of oil shortages and the requirements forsustainable development, it is necessary to develop new catalytic processesusing a variety of raw materials such as benzene and ethylene from coal,ethanol from biomass, and ethane from FCC off-gas. The challenges are how todesign and synthesize highly active catalysts that are tolerant of theimpurities and poisons in low-quality feedstocks. The development of simple andefficient pretreatment processes is an alternative solution.

There is currently a trend to use unrefinedethylene produced in the methanol-to-olefin process as the ethylation agent toproduce ethylbenzene. The content of ethylene is usually high, 75%-95%, and themain inert impurities are methane (about 3%), ethane (about 2%), CO, and H₂.There are also significant amounts of sulfide and nitrides, which can cause catalystpoisoning. Figure 7 shows the deactivation of ZSM-5 zeolite by anitrogen-containing substance (2-aminoethanol) in vapor-phase benzenealkylation ^[71]. The use of ethanol from biomass faces the same poisoningproblem. The use of poison-tolerant catalytic materials would therefore promotethe use of low-quality raw materials.

The ethane content of FCC off-gas is low (about10%) and it is relatively inert. The development of catalysts that can activateethane or of bifunctional catalysts that have both dehydrogenation andalkylation activity would be worth investigating. Research on Pt/ZSM-5 as acatalyst for benzene alkylation with ethane has been performed for years ^[47].An ethylbenzene process with separate ethane-dehydrogenation and benzene-alkylationprocesses was patented by the Dow Chemical Company. However, there has been noreport of its commercialization.

Industrial catalytic processes using zeolite-basedcatalysts are usually considered to be environmentally benign. However,procedures for the preparation of zeolite-based catalysts cause environmentalpollution ^[72]. Zeolite synthesis is usually performed in the presence of anitrogen-containing organic template, and only some of the used template isrecyclable. Plenty of water is needed to wash the zeolite. NO_xis formed during zeolite calcination. Salt-containing wastewater is producedduring the ion-exchange procedure. Moreover, the catalyst loses activity afteryears of operation, and is no longer suitable for industrial use, and becomesindustrial waste.

It would be useful to produce catalysts in moreeffective ways, to reduce environmental pollution. Template-free or evensolvent-free methods for zeolite synthesis need to be developed ^{[73, 74, 75, 76]}. Directsynthesis of H-form zeolites would avoid the ion-exchange procedure ^[77]. Thedevelopment of ways of using waste catalysts, for example by reusing them asraw materials for zeolite synthesis, would benefit the environment ^{[78, 79]}.

Aromatic hydrocarbons are usually added togasoline as octane enhancers. However, aromatics such as benzene can be toxicto human organs and should be strictly limited or removed from gasoline ^[80].BenzOut is a commercial process for the removal of benzene from gasoline,developed by modern refineries, and involves alkylation with olefins ^{[81, 82, 83]}.The abovementioned alkylation catalysts that have been commercialized, i.e.,ZSM-5, Y, MCM-22, and β zeolites, are all candidates for the BenzOut process.One important factor that needs to be considered is that gasoline rather thanbenzene is fed into the catalyst bed. The feed ratio of benzene/olefin is low,and more multi-alkylated benzene is produced, which causes catalystdeactivation. Also, Lewis acids cause side reactions such as oligomerization.The selection of reaction conditions and modification of the acidic propertiesis therefore critical for the removal of benzene from gasoline throughalkylation.

The development of ethylbenzene processes has beenrapid during the last few decades, especially in China. Various alkylation andtransalkylation catalysts based on different zeolites have been commercializedby Mobil, UOP, DICP, Sinopec, and some other companies.

Along with the development of catalytic materials,vapor- phaseand liquid-phase ethylbenzene processes have been developed and commercialized.Vapor-phase processes are more flexible, because pure ethylene, diluteethylene, ethanol, and even ethane can be used as the feedstock. The dilute-ethyleneprocess is widely used in China, but the production capacity of every plant islimited. Liquid-phase processes have several advantages such as low energyconsumption, low benzene/ethylene feed ratios, low alkylation temperatures, andhigh product quality.

Zeolites with different pore structure and acidicproperties give different catalytic performance. Each has its own distinctivefeatures. ZSM-5 is the most widely used vapor-phase benzene ethylationcatalyst. Among all the liquid-phase catalysts, β zeolite is the most activeand poison tolerant, and MCM-22 zeolite has the highest ethylbenzeneselectivity and longest catalyst lifetime. Use of a combination of two or morezeolite catalysts may therefore provide a significant breakthrough in thedevelopment of ethylbenzene processes.

In the future, the development of ethylbenzeneprocesses will focus on the development and application of new catalyticmaterials and the use of different feedstocks, which would meet therequirements of sustainable development.