DOI: 10.1002/cctc.201300345

Realizing the Commercial Potential of HierarchicalZeolites: New Opportunities in Catalytic CrackingKunhao Li,[a] Julia Valla,[a, b] and Javier Garcia-Martinez*[a, c]

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The field of hierarchical zeolites in general and mesoporouszeolites in particular has seen tremendous growth over thepast decade, as part of the ongoing efforts to alleviate the dif-fusional limitations that are imposed by the micropores of con-ventional zeolites. Many new approaches, herein classified aseither “bottom-up” or “top-down”, have been reported and arebriefly surveyed in this Review. Zeolite Y is the most widelyused zeolite in catalysis and the developments in mesoporousY reflect the general “landscape” of mesoporous zeolites. Thepreparation of mesoporous Y, the materials’ properties, andtheir catalytic application in fluid catalytic cracking (FCC) andhydrocracking are critically reviewed. Finally, the scale-up anduse of mesostrutured zeolite Y on an industrial scale are de-scribed, which constitute the first commercial application of hi-erarchical zeolites.

1. Introduction

Zeolites are crystalline microporous aluminosilicates that arecomposed of TO4 tetrahedra (T = mostly Al or Si) as primarybuilding units that are connected through the corner-sharingof oxygen atoms to form 3D frameworks that encompasspores of diameters of molecular dimensions (micropores of di-ameters smaller than 2 nm).[1] The framework structure andchemical composition determine the unique properties ofa specific zeolite and its use in important areas, such as watertreatment, adsorption and separation, and catalysis. Amongthe different properties of zeolites, their pore structure, that is,the size, shape, and interconnectivity of the pores, have anoverarching effect across all applications. On one hand, theyimpart different zeolites with unique and important capabili-ties, such as molecular sieving and shape/size selectivity.[2, 3] Onthe other hand, they not only exclude certain large molecules,such as some of the larger molecules in heavy petroleum feed,from accessing the active sites that are located within the mi-cropores of a zeolite, but they also impose significant diffu-sional limitations to molecules that can only tightly fit withinthe pores, for example, reactants, intermediates, and productsin many reactions that are catalyzed by zeolites (Figure 1 a).The diffusion of tightly fitting molecules into the micropores ofzeolites, referred as “configurational diffusion”, is often therate-limiting step of a catalyzed reaction because, as the sizeof the molecules approaches the dimension of the pores, themolecular diffusivity drops sharply to orders of magnitude

lower than, for example, the Knudsen-diffusion (often thedominating diffusion mechanism in mesopores) and molecu-lar-diffusion regimes (Figure 1 b).[4]

Since the early days of utilizing zeolites in catalytic process-es, there have been ongoing efforts to alleviate the diffusionchallenges/limitations through a wide variety of approaches.They typically align with one of the following three directions:1) The synthesis of zeolites with larger micropores;[5–9] 2) reduc-tion of the crystal size of the zeolite down to nanoscales in atleast one dimension;[10–13] 3) introduction of additional porosi-ties of larger sizes, typically mesopores or even macropores,into the crystals of microporous zeolites or into the particlesthat comprise the zeolite crystals. Whilst impressive progresshas been made in the first two approaches, technical challeng-es, including the use of expensive templates, low hydrothermalstability, and difficulty in handling of the nanosized materials,have limited their large-scale commercial application. In thisReview, we first provide a critical overview of the main strat-egies for introducing mesoporosity into zeolites and then un-dertake a more-detailed comparison of the different ap-proaches to mesoporous zeolite Y in particular, as well as theircatalytic applications and commercialization prospects. Nota-bly, the term “hierarchical zeolite” encompasses any zeolitewith at least a secondary pore-structure system and, therefore,“mesoporous zeolite” should be considered as a subclass ofthe former because it defines the size of the additional porosi-ty as falling within mesopore range, that is, between 2 and50 nm.

Figure 1. a) Schematic representation of the effect of pore size on the diffu-sion of large (red) and small (black) molecules. b) Effects of pore diameteron molecular diffusivity (D) and of the energy of activation (Ea) on diffusion.Adapted and reprinted with permission from Ref. [4] . Copyright 1995 Ameri-can Chemical Society.

[a] K. Li, J. Valla, J. Garcia-MartinezRive Technology, Inc.1 Deer Park Drive, Suite AMonmouth Junction, NJ 08852 (USA)

[b] J. VallaDepartment of Chemical and Biomolecular EngineeringUniversity of ConnecticutStorrs, CT 06269 (USA)

[c] J. Garcia-MartinezInorganic Chemistry DepartmentUniversity of AlicanteCampus de San Vicente E-03690, Alicante (Spain)E-mail : j.garcia@ua.es

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2. Preparation Strategies of MesoporousZeolites

Since the beginning of the 21st century, much progress hasbeen made in the synthesis, characterization, and applicationof mesoporous zeolites.[14–19] Typically, these methods can bedivided into two categories: “Top-down” or post-syntheticmodification, which may or may not involve the use of tem-plates, and “bottom-up” or primary syntheses, which mostly in-volve the use of templates (Figure 2).

2.1. “Bottom-up” approaches

2.1.1. Hard-templating

Jacobsen et al. published pioneering work on the preparationof ZSM-5[20] (and TS-1)[21] single crystals with intracrystallinemesoporosity (Figure 3) as an extension of their earlier ef-forts[22–24] in the synthesis of nanosized zeolites ZSM-5, beta, X,and A in the confined space within the pores of carbon black.This approach, typically referred to as “hard-templating”, wasquickly adopted by many researchers. In addition to carbonblack, many other hard templates, such as carbon nanotubesor nanofibers,[25, 26] ordered mesoporous carbons,[27, 30] non-or-dered carbon aerogels or mesoporous carbons,[31–34] pyrolyzedwood or carbonized rice husk,[35, 36] CaCO3 nanoparticles,[37] andpolystyrene (and other polymeric) microspheres,[38–40] havebeen utilized to synthesize various mesoporous (or macropo-rous in a few cases) zeolites with various topologies, includingMFI, MEL, MWT, BEA, AFI, and CHA. Because all of these strat-

Dr. Kunhao Li joined Rive Technology,

Inc. as a Project Leader in late 2008.

Since then, he has been heavily in-

volved in the improvement of Rive’s

core technology in zeolite mesostruc-

turing processes, zeolite and fluid cata-

lytic cracking catalyst characterization,

testing, and evaluation, as well as ex-

ploration and extension of the applica-

tion areas of Rive’s mesoporous zeo-

lites in chemical separation and other

petroleum cracking and petrochemical

processes. He obtained his doctorate in chemistry in 2006 at The

George Washington University in Washington, D.C. , USA. He then

moved to Rutgers University, New Brunswick, USA, for his postdoc-

toral research on microporous metal-organic framework materials

for applications such as hydrogen storage, gas adsorption and sep-

aration, and chemical sensing. His work at The George Washington

University, Rutgers, and Rive has resulted in many publications in

the form of original papers and reviews, book chapters, technical

reports, patent applications, and patents.

Dr. Julia Valla is an Assistant Professor

in the Chemical and Biomolecular En-

gineering Department at the Universi-

ty of Connecticut. Prior to her position

in academia, she worked as a Project

Leader at Rive Technology, Inc. on the

development and catalytic evaluation

of novel zeolites with ordered meso-

porous structure for refinery applica-

tions. Her studies were focused on the

diffusion limitations of zeolites and the

kinetics and reaction pathways of

heavy hydrocarbons in the micro/mesopore network within zeo-

lites. She received her PhD in Chemical Engineering in 2005 from

the Aristotle University of Thessaloniki in Greece in the field of in

situ sulfur reduction in gasoline and diesel in the fluid catalytic

cracking unit. Today, her research focuses on the modification of

zeolite structure and the application of hierarchical pore zeolites in

catalysis, adsorption, and energy. She has authored several patents

and scientific papers in peer-reviewed journals and book chapters.

Dr. Javier Garcia-Martinez is the found-

er and Chief Scientist of Rive Technolo-

gy, Inc. (Boston, USA), a venture capi-

tal-funded Massachusetts Institute of

Technology (MIT) spin-off commercial-

izing hierarchical zeolites for refining

applications. He is also Director of the

Molecular Nanotechnology Lab and

a faculty member at the University of

Alicante, Spain. He has published ex-

tensively in the areas of zeolites and

nanotechnology and is co-inventor of

more than 25 patents. His latest books are Nanotechnology for the

Energy Challenge (2010, Wiley-VCH), The Chemical Element: Chem-

istry’s Contribution to Our Global Future (2011, Wiley-VCH) and

Mesoporous Zeolites (2014, Wiley-VCH). He received the Europe

Medal in 2005, the Silver Medal of the European Young Chemist

Award in 2006, and the TR35 Award from MIT’s Technology Review

magazine. Since 2010, he has been a member of the Council on

Emerging Technologies of the World Economic Forum. He is

a Fellow of the Royal Society of Chemistry and member of the

Global Young Academy and IUPAC Bureau.

Figure 2. “Bottom-up” and “top-down” approaches to mesoporous zeolitesas categorized herein.

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egies start from typical or modified zeolite-synthesis gels, theyare all classified as “bottom-up” approaches (Figure 2).

2.1.2. Soft-templating

“Bottom-up” approaches also include “soft-templating” meth-ods, which involve the use of cationic surfactants,[41, 42] meso-scopic cationic polymers,[43] silylated polymers or surfac-tants,[44–48] polymeric aerogels,[49] starch,[50, 51] bacteria,[52] etc.[53]

Many of the early efforts at synthesizing mesoporous zeolites,often referred to as “dual-templating” methods, involved thesimultaneous use of both mesopore templates, such as surfac-tants, and structure-directing agents (SDA, for example, short-chain alkylammonium salts), to form certain zeolite phaseswhilst introducing mesoporosity. In some earlier works, this ap-proach often led to physical mixtures of amorphous mesopo-rous material and conventional (non-mesoporous) zeolite crys-tals.[54–56] More recently, mesoporous zeolites with MFI, BEA,FAU, MOR, LTA, etc. , topologies have been obtained by build-ing both the mesopore template and the SDA function intothe same molecule. For example, Na et al.[41] designed and syn-thesized a gemini surfactant that contained both a mesopore-templating function and a structural moiety that was similar tomolecular SDAs that are typically used in zeolite synthesis(Figure 4). By having the long aliphatic chain and the SDAfunctions in the same molecule (18-N3-18), phase segregationwas avoided and nanocrystalline zeolite with hexagonally or-dered intracrystalline mesoporosity was obtained.

2.1.3. Zeolitization of mesoporous materials or the hierarchi-cal assembly of nanozeolites

Also included in the “bottom-up” approaches, as shown inFigure 2, are the approaches that involve either converting theamorphous pore walls of mesoporous-silica-containing materi-als (such as silicates or aluminosilicates, silica monoliths, dia-tomite) into zeolites or assembling protozeolitic units intomesoporous materials with nanocrystalline pore walls.[57–76]

Some of these approaches appear to be quite simple and ver-satile, for example, the TUD-C and TUD-M approaches only useTPAOH (tetrapropylammonium hodoxide) as a template forboth micropores and mesopores in the preparation of compo-sites of ZSM-5 nanocrystals that are embedded into well-con-nected mesoporous matrices.[74–76]

Increased acidity and hydrothermal stability, relative to ma-terials such as MCM-41 and SBA-15 with amorphous porewalls, are typically associated with these materials; however,only intercrystalline mesoporosity (in between the nanozeolitecrystals) can be introduced by using these approaches. Thepore walls often can only be partially crystallized, with veryfew exceptions.[58] Therefore, the acidity of these materials are,in general, inferior to those of conventional zeolites.[15]

The “bottom up” approaches mostly involve the use of or-ganic or inorganic templates in the zeolite crystallization pro-cess. While a main advantage of this approach could be thesynthesis of mesoporous zeolites with identical (or similar)chemical compositions to conventional zeolites, the key to thisgeneral approach is to optimize the interactions between thetemplates and the aluminosilicate species in the reaction mix-ture to avoid phase segregation during the hydrothermal crys-tal-growth process. How well the templates are dispersed inthe synthesis gels and become occluded during the hydrother-mal synthesis will affect the phase purity and morphology ofthe zeolite crystals, as well as the location, distribution, shape,size, and interconnectivity of the mesopores. The cost of thetemplates would be of concern to the scalability and large-scale application of mesoporous zeolites that are prepared byusing these approaches, especially if, for example, orderedmesoporous carbons are used, which are typically formed fromtedious processes that involve the use of another template, forexample, either SBA-15 mesoporous silica or carefully arrangedcolloidal silica particles.

Figure 3. Growth of zeolite crystals around carbon particles as hard tem-plates. Adapted with permission from Ref. [20] . Copyright 2000 AmericanChemical Society.

Figure 4. Dual templating: A) 18-N3-18 surfactant (white spheres: hydrogen,gray spheres: carbon, red spheres : nitrogen). B) SEM, C), D) TEM, and E) XRDpatterns of the mesostructured ZSM-5. Insets in (C) and (D): Fourier diffracto-grams. For structural comparison, an MFI framework model is shown in thebottom-right inset of (D). Reprinted with permission from Ref. [41] . Copy-right 2011 American Association for the Advancement of Science.

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2.2. “Top-down” approaches

2.2.1. Dealumination

Before the turn of the century, mesoporosity in zeolites wastypically generated in a limited number of ways, which mostlyinvolved dealumination through post-synthetic calcination, hy-drothermal treatment (steaming), acid leaching, or chemicaltreatments.[77] Originally, dealumination treatments were per-formed to control the concentration and strength of the acidsites by increasing the Si/Al ratio of low-silica zeolites. Howev-er, it was observed that, during the hydrothermal dealumina-tion process, secondary pores at mesoscale may form. Becausedealumination unavoidably alters the Si/Al ratio, the acidicproperties of the zeolite will also be different from those ofthe original zeolite.[78, 79] Nowadays, the hydrothermal dealumi-nation techniques, alone or in combination with dealuminationby acid, are widely practiced industrially, mainly to manufac-ture ultrastable Y (USY) zeolite for FCC and hydrocracking ap-plications.[77] Their limitations and drawbacks, discussed inmore detail below, are the driving forces for the discovery ofthe new methods described in this Review.

2.2.2. Desilication

Desilication is another well-known demetalation approach forthe creation of mesoporosity. The selective hydrolysis of Si�O�Si bonds can be traced back to as early as the 1960s. Youngand Linda[80] observed that, at high silica-to-alumina ratios,MOR zeolites could be treated in aqueous caustic solution toleach out a minor portion of the structural silica, thereby im-proving their adsorption capacity. At that point, the mecha-nism of desilication was not clearly understood. Mao et al.[81]

studied the desilication of ZSM-5 zeolites by using NaOH andNa2CO3 and they reported that the resulting zeolite had a ho-mogeneous pore system of 0.56 nm and a higher density ofion-exchange sites.

Ogura et al.[82] first described how the desilication of ZSM-5zeolite in NaOH could generate intracrystalline mesoporosity.By using a 0.2 m NaOH aqueous solution, they observed meso-pores of 4 nm in the zeolite and a loss of microporosity com-pared to the original zeolite.[83] They also reported that a largeportion (about 40 %) of the zeolite dissolved during the alkalitreatment.[83] Later on, they described how the NaOH concen-tration, desilication temperature, and treatment time couldgreatly affect the mesoporosity, and structural and acidic char-acteristics of the resulting zeolite. The mesopores size waslater corrected to be a broad distribution approximately10 nm.[84]

Groen et al.[85] identified that the presence of Al gradients inthe zeolite crystals and, specifically, the concentration of theframework Al species played key roles in the mechanism ofmesopore formation in MFI (ZSM-5) zeolites in alkalinemedium. They showed that the presence of high Al concentra-tions in the MFI zeolite framework (Si/Al<20) prevented theextraction of Si and, thus, limited the pore formation by desili-cation, whereas highly siliceous zeolites (Si/Al>50) showed ex-cessive and unselective Si dissolution, thus leading to the crea-

tion of relatively large pores. A framework Si/Al ratio of 25–50was optimal for obtaining substantial intracrystalline meso-porosity and generally preserved Al centers (Figure 5). A mech-anism for the directing role of the framework aluminum spe-

cies has also been proposed.[86] Besides the concentration ofthe framework aluminum, the nature of these species andtheir location can impact desilication by alkaline treatment.Thus, alkaline treatment of a steam-calcined ZSM-5 zeolite(which contained a high degree of non-framework aluminum)led to minor silicon extraction and, consequently, limited mes-oporosity development. The same group demonstrated the ad-vantages of mesoporous ZSM-5 derived from desilication byusing neopentane-adsorption experiments.[87] They further dis-covered that the use of organic bases, such as TPAOH orTBAOH (tetrabutylammonium hydroxide) instead of NaOH, oruse of tetraalkylammonium salts (the so-called “pore-growthmoderators”) together with NaOH helped to better preservemicroporosity.[88] The same group also reported a few othervariations to the typical NaOH-desilication process, for exam-ple, NaAlO2 treatment followed by acid washing to removedebris aluminum species[89] and the addition of aluminum ni-trate as a pore-directing agent to the NaOH solution.[90] Byusing these different approaches, they effectively expandedthe Si/Al ratio from about 10 to 1 for the generation of meso-porosity in ZSM-5 by desilication (Figure 6).[91]

The desilication approach has also been applied to otherzeolite topologies, such as MOR, BEA, FER, FAU, and CHA.[15] Al-though both the chemical composition and structure of thezeolites seem to dictate the optimal desilication conditionsand the properties of the resultant mesoporous zeolites, thisapproach clearly shows high versatility. However, there areissues that are associated with desilication. First, owing to thehigh usage of NaOH, material loss can be very significant, inaddition to a loss of microporosity. Silica leaching from zeolite

Figure 5. Schematic representation of the influence of Al content on the de-silication of MFI zeolites in NaOH solution and the associated pore-formationmechanism. Reprinted with permission from Ref. [85]. Copyright 2004 Ameri-can Chemical Society.

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crystals and the less-crystalline regions in between the zeolitecrystals (which typically serve as a binder to hold a few zeolitecrystals together as larger particles) could cause a significantdecrease in the size of the zeolite crystals and disintegration ofthe particles, which may lead to significant difficulties in filtra-tion and further loss of yield.

2.2.3. Surfactant templated “top-down” approaches

A surfactant-templated method to introduce mesoporosityinto zeolites was first filed as a patent application in 2004 andhas been described in a few recent publications by Garcia-Mar-tinez and co-workers.[92–94] This approach is based on the useof significantly milder conditions (e.g. , dilute NH4OH solution)than those for desilication. Surfactants such as cetyltrimethyl-ammonium bromide or chloride (CTAB or CTAC) are used to in-troduce well-controlled mesoporosity into various zeolites(e.g. , Y, mordenite, ZSM-5). The technique does not suffer fromthe typical drawbacks of the desilication approach, i.e. signifi-cant loss of silica or damage of the zeolite crystals.

About the same time, Ivanova et al.[95] published a prepara-tion of composite micro/mesoporous mordenite that also in-volved the use of CTAB as a mesopore template, which wasa follow-up of a procedure reported by Goto et al.[96] There isan important difference between this approach and the surfac-tant-templating approach by Garcia-Martinez. The former pro-cess involves two steps: 1) Partial-to-total dissolution of thezeolite in NaOH solution, followed by, 2) hydrothermal treat-ment in the presence of CTAB at lower pH values, to producea zeolite/mesoporous molecular-sieve composite (ZMC) of desi-licated mesoporous zeolite and another mesoporous silica-richamorphous phase that is deposited onto the surface of thecrystals. The latter approach is a single-step process that com-bines surfactant templating with base treatment to producesingle-phase mesostructured zeolites with intracrystalline mes-oporosity. These two different approaches lead to the forma-tion of markedly different materials, as illustrated in Figure 7,which will be discussed in details in the third section.

In 2006, Pacheo-Malag�n et al.[97] reported an interesting de-polymerization-recrystallization (DR) approach to hierarchicalzeolites that first involved the depolymerization of zeolites(USY or ZSM-5) in glycerol at about 200 8C to form an amor-phous gel (by X-ray Diffraction, XRD) and subsequent recrystal-

lization of the amorphous gel in the presence of tetramethy-lammonium or tetrapropylammonium cations (TMA+ or TPA+)under hydrothermal conditions. Hierarchical zeolites that ex-hibited Y or ZSM-5 diffraction patterns were composed of re-crystallized zeolite nanocrystals that were embedded in a meso-porous phase. Later, this approach was extended to silicalite-1[98] and Y zeolites.[99] CTAB was also used in the preparation ofhierarchical Y zeolite;[100] however, nitrogen-adsorption/desorp-tion analysis did not show the mesopore-templating effect ofthe surfactant.

2.3. Summary

Many new synthetic approaches for the generation of mesopo-rous zeolites have been discovered over the past decade or soand they can generally be categorized as either “bottom-up”or “top-down” approaches (Figure 2). It is also clear that “notall mesopores are created equal”, that is, qualitatively differentmesoporosity results from different preparation methods. Insome cases, tiny variations in the process conditions couldresult in quite different materials. The differences in materialproperties could lie in the amount (measured by the pore vol-umes or surface areas), location (intra- or inter-crystalline), sizedistribution, or interconnectivity (with adjacent mesopores andmicropores) of the secondary mesopores. In addition, differentmethods also have different effects on the remaining micro-porosity, crystallinity, acid-site strength and distribution, andhydrothermal stability (of both the meso- and microporosity),because porosity is clearly not the only change to the originalzeolites during the different chemical processes. Some effortshave attempted to make more-generalized comparisons of ma-

Figure 7. Schematic representation of the differences between the materialsprepared by using single-step and two-step approaches with cationic surfac-tants and NaOH; blue “snowflakes” represent the surfactant micelles, largered rectangle represents a zeolite crystal, small red squares represent sili-cate/aluminosilicate species that may contain zeolitic subunits.

Figure 6. Schematic representation of the procedures for the desilication ofMFI zeolites with different framework Si/Al ratios. Adapted with permissionfrom Ref. [91] . Copyright 2011 Royal Society of Chemistry.

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terials that were prepared by using different techniques. Perez-Ramirez et al.[101] defined the hierarchical factor (HF) from theporosity results as the product of Smeso/Stotal and Vmicro/Vtotal

(Figure 8), which was suggested as a tool to look at how effec-

tively the mesopore surface area was generated by using dif-ferent approaches at the expense of micropore volume, re-gardless of zeolite type. However, cross-laboratory comparisonsshould be treated very carefully, because HF is sensitive tohow gas-adsorption experiments and the BET, t-plot, and total-pore-volume analyses are performed. Although there is someindication that HF can be used to rank the catalytic perfor-mance of different mesoporous zeolites,[101] its generality is notyet confirmed.[18] The optimal balance between the degree ofmesoporosity and microporosity may be very different for dif-ferent applications and the assumption that a higher HF willlead to better catalytic performance is questionable.

3. Mesoporous Zeolite Y

Much progress has also been made in the field of hierarchicalzeolite Y over the last decade. Owing to the great relevance ofzeolite Y to industrial catalysis, this section will try to providea detailed comparison of the different preparation approaches,the catalytic applications that have been demonstrated in lab-oratories, and the most up-to-date progress in the commercial-ization efforts.

Zeolite Y is a synthetic zeolite with the faujasite (FAU) frame-work structure, which features 3D interconnected micropores.The pore openings are delimited by 12 tetrahedrally coordinat-ed Si or Al atoms (linked through O atoms), which afford a rela-tively large diameter of about 7.4 �. The inner cavity, which issurrounded by 10 sodalite cages, has a diameter of about12 �. Because of its large pore size, strong Brønsted acidity,and high hydrothermal stability, zeolite Y quickly superseded

zeolite X in FCC in the early 1960s and still remains the activecomponent of FCC and many other petroleum processing cat-alysts, for example, hydrocracking catalysts.[1]

3.1 Dealumination

Despite the supremacy of Y zeolites in petroleum refining,soon after its debut in FCC catalysis, diffusion limitations wererecognized as a major problem that needed to be resolved tounleash its full potential.[102–105] Post-synthetic (“top-down”)modifications of zeolite Y, such as steaming and acid or chemi-cal dealumination techniques, as briefly mentioned in the pre-vious section, have been used to improve the hydrothermalstability of zeolite Y and may also have generated secondarymesoporosity. The ultrastabilization of ammonium-ion-ex-changed zeolite Y to prepare USY is a well-known example ofsuch a process. Heating NH4Y in the presence of steam (eitheradded or evolved from the adsorbed water in the zeolite)causes hydrolysis of the Al�O�Si bonds. Then, Al atoms are ex-pelled from the framework and atom-sized vacancies are creat-ed. Subsequently, some of these vacancies are filled (or“healed”) by mobile Si atoms from the amorphous region ofzeolite crystals or particles, while the other vacancies coalesceto form larger cavities or pores, which typically appear as“mesopores” by N2 or Ar gas-adsorption analysis (Figure 9).[106]

Although some of the microporosity and framework Al speciesare lost during the ultrastabilization process, the resultant USYis much more hydrothermally stable. The extra-framework Alspecies are deposited onto the internal and external surfacesof USY zeolite, which can be removed by mild acid leachingusing either inorganic or organic acids to enhance the porosityof the USY zeolite. Although this process is not typically usedin the making of USY zeolite for FCC catalysis, repetitive steam-ing and acid leaching have been commonly utilized to prepareUSY zeolites with much higher mesoporosity for other applica-tions, such as hydrocracking. The extra pore volume and sur-face area serve as favorable support for the noble-metal cata-lyst particles that provide hydrogenation function of the bi-functional hydrocracking catalyst.[107–109]

Although there are indications that mesoporosity generatedby steaming may lead to enhanced accessibility and higherconversions of larger molecules,[110] electron-tomography stud-ies,[111–113] as well as a combination of nitrogen-physisorptionand mercury-intrusion studies, have shown that some of themesoporosity generated by steaming was, in fact, cavities thatwere entrapped within the zeolite crystals and were only con-nected to the crystal surface through micropores. Such meso-scale cavities do not help to improve the intracrystalline diffu-sion of molecules through the crystals. Furthermore, pulsedfield gradient (PFG)-NMR studies showed that the intracrystal-line diffusion of n-octane and 1,3,5-triisopropylbenzenethrough USY were not affected by the presence of the “meso-pores” that were generated by steaming.[114] The discrepancybetween these studies may lie in the subtle differences in thesteaming processes and the following acid-leaching processesthat are used for the preparation of samples that lead to differ-ent types of mesoporosity (open mesopores or entrapped

Figure 8. Contour plot of the hierarchical factor (HF) of different zeolitetypes prepared by using different methods. Reprinted with permission fromRef. [101] .

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meso-scale cavities), which suggests that steaming techniquesmay not be a reliable approach to the desirable openmesopores.

More recently, there have been efforts to combine frame-work desilication with dealumination by ammonium hexafluor-osilicate (AHFS) or steaming to generate USY zeolite withdefect-guided mesoporosity.[115, 116] The cracking of 1,3,5-triiso-propylbenzene and vacuum gas oil has shown improvementsin product selectivity, which was attributed to the introducedmesoporosity.

3.2 Surfactant-templated Y with intrachrystalline mesopores

As briefly mentioned in the previous section, surfactant-tem-plated post-synthetic modification is a unique “top-down” ap-proach that involves the single-step treatment of zeolites witha surfactant in mildly basic solution.[92, 117] Specifically, the origi-nal invention described the hydrothermal treatment of a com-mercial USY with a Si/Al ratio of about 15, that is, ZeolystCBV720, with a solution of CTAB in 0.37 m aqueous NH4OH (or0.09 m TMAOH, that is, tetramethylammonium hydroxide) at150 8C for 10–20 h. Removal of the surfactant templates bycareful calcination first in nitrogen then in air exposes the uni-formly distributed intracrystalline mesopores, as shown in theTEM images of the treated zeolite (Figure 10).[93] Nitrogen-phys-isorption isotherms also show distinct uptake slightly belowP/P0 = 0.4, which corresponds to a sharp Barrett–Joyner–Halen-da (BJH) pore-size distribution[118] at about 4 nm (the size ofCTAB micelles).[93] The validity of both the synthesis and the in-tracrystalline nature of the mesoporosity were later verified byan independent group.[119] The direct introduction of well-con-

trolled mesoporosity into zeolite crystals may happen througha crystal-rearrangement mechanism (Figure 11).[93] This structur-al reorganization is only possible if a cationic surfactant is pres-ent when the base opens the Si�O�Si bonds to form negative-ly charged Si�O� species. This process allows the needed inter-actions between the surfactant and the zeolite and preventsthe dissolution of the crystals. Typically, almost complete re-covery is observed during this mesostructuring process. SEMimages (Figure 12 and the Supporting Information, Figure S1,in Ref. [93]) showed that there was only one phase and no no-table morphology changes were observed. The co-existence ofboth mesoporosity and crystallinity within the crystal bounda-ries is evident in the TEM images (Figure 10 and Figure 12).The remarkably high hydrothermal stability of the mesoporosi-ty also supports the intracrystalline nature of the surfactant-templated mesopores.[93]

The cracking of triisopropylbenzene over the proton-formmesoporous zeolite Y (Meso-HY),[92, 117] the starting CBV720(HY), and Al-containing MCM-41 (AlMCM-41) suggested that,whereas both Meso-HY and HY showed much higher activitiesthan AlMCM-41, owing to their higher zeolitic acidity, Meso-HYmaintained higher activity for a longer period of time than HY,

Figure 9. Schematic representation of the formation of mesopores in a hy-drothermal dealumination process. Adapted with permission from Ref. [106].

Figure 10. TEM images of a) zeolite CBV720 and b) mesostructured zeolite Y.The higher-magnification micrographs of various mesostructured zeolite-Ycrystals clearly show intracrystalline mesoporosity and crystal lattice fringes.Reprinted with permission from Ref. [93] . Scale bars: a) 100 (left) and 20 nm(right), b) 500 (center) and 50 nm. Copyright 2012 Royal Society of Chemis-try.

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because the extra mesoporosity that was introduced by surfac-tant templates helped to slow down the deactivation by cokebuild-up. The products selectivity (Meso-HY versus HY) wasalso shifted to higher yields of 1,3-diisopropylbenzene andlower yields of cumene and benzene, which was attributed tothe suppression of over-cracking by allowing more of thelarger product (1,3-disiopropylbenzene) to diffuse out of the

zeolites through the open mesopores before they werecracked further into the lower-molecular-weight products(cumene and benzene).

Strong interest in the catalytic and separation applicationsof mesoporous zeolites that were developed by using the sur-factant-templated post-synthetic modification approachprompted the foundation of Rive Technology, Inc. in 2006 toscale up and commercialize this new technology (referred toas “molecular highway” technology).[94] Since then, much prog-ress has been made. Firstly, the technique has been extendedfrom high Si/Al-ratio USY zeolite (and mordenite, ZSM-5) toNaY,[120, 121] NaX, and NaA[122] zeolites with much lower Si/Alratios (1–3) by the incorporation of a pre-treatment step witha mild acid (with either organic or inorganic acids or mixturesof both). Careful acid treatment to selectively open some ofthe Al�O bonds with limited removal of Al atoms from thezeolite framework creates some defects and weakens the ri-gidity of the structure. Subsequent treatment of the acid-treat-ed zeolite with cationic surfactants such as CTAB in a basic so-lution (e.g. , NH4OH or NaOH solution) at elevated temperatures(typically below 100 8C) for as short as 1 h affords mesoporousNaY (or NH4Y) with very similar characteristics to Meso-HY(Figure 10 and Figure 12).[93] Electron-diffraction patterns andfield-emission scanning electron microscopy (FE-SEM) imagesfurther confirmed the co-existence of mesoporosity within thezeolite crystals and the well-controlled size and shape of themesopores, as well as their uniform distribution throughoutthe crystals (Figure 12 and 13). Remarkably, the mesoporouszeolite Y showed high hydrothermal stability that was verysimilar to that of conventional zeolite Y. After steam treatmentat 788 8C and 100 % steam for 4 h, the mesoporous zeolite Ystill retained about 63 % of its original micropore volume andthe total mesopore volume remained the same at 0.16 cc g�1

(Table 1).[93] Temperature-programmed ammonia desorption(TPAD) confirmed that the mesoporous USY (as prepared by ul-trastabilization of the ammonium-exchanged meso-NaY) hada total acidity (1.18 mmol) that was very close to that of con-ventional USY (CBV500, 1.25 mmol).[93]

One of the significant advantages of the surfactant-templat-ing approach over other methods, such as desilication, is thatthe degree of mesoporosity can be increased without signifi-

Figure 11. Schematic representation of the speculated mechanism for the formation of surfactant-templated mesopores in zeolite: a) Original zeolite Y; b) Si�O�Si bond-opening/reconstruction in basic media; c) crystal rearrangement to accommodate the surfactant micelles ; and d) removal of the template toexpose the introduced mesoporosity. Reprinted with permission from Ref. [93] . Copyright 2012 Royal Society of Chemistry.

Figure 12. a) SEM and b–f) TEM images of the mesostructured Y zeolites. In(c), a single crystal of mesostructured zeolite shows crystalline lattice fringesand mesoporosity (holes) and, in (d), an ultramicrotomed mesostructuredzeolite crystal shows both crystallinity and mesoporosity. e, f) Two TEMimages of the same area of a mesostructured zeolite as obtained at two dif-ferent foci to better visualize the two features of this material, that is, crys-tallinity and mesoporosity. Scale bars : a) 1 mm, b) 500 nm, c, d) 50 nm, e) 20nm, f) 20 nm. Reprinted with permission from Ref. [93] . Copyright 2012Royal Society of Chemistry.

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cantly compromising some important zeolite features, such asthe Si/Al ratio, mesopore-size distribution, and the physical in-tegrity of zeolite crystals, with retention of high recoveryyields. The introduced mesopores have a very sharp meso-pore-size distribution, the modal size of which varies in excel-lent accordance with the size of the surfactants that are used(Figure 14). Non-ordered mesostructures are typically observedin the highly crystalline meso-NaY (and the meso-USY deriva-tive).[93] However, if enough mesoporosity is introduced for theCTAB micelles to self-assemble, low-angle X-ray diffractionpeaks that are similar to those observed in MCM-41 can be ob-served in the low 2q range (1.5–58), in addition to the typicaldiffraction peaks of zeolite Y at higher angles (Figure 15).[92, 117]

Subsequently, the process of producing the mesostructuredzeolite Y by the surfactant-templated method was scaled up

from the gram level to the kilo-gram level in a pilot catalyst-de-velopment plant and the meso-structured USY was also formu-lated in a FCC catalyst matrix(that comprised a binder anda clay, representative of commer-cial FCC catalysts) by using thespray-drying technique to createthe first FCC catalyst with meso-structured zeolite Y. Then, thecatalysts were deactivated byfluidized steaming at 788 8C for8 h to simulate deactivation ina FCC unit. The unit-cell con-stants of the steam-deactivatedcatalysts that contained conven-tional and mesostructured USYzeolites were almost identical(24.26 and 24.27 �, respectively).However, the latter material hada higher external surface area, asestimated by t-plot analysis (70versus 51 m2 g�1). The additional20 m2 g�1 external surface areacame from the mesopores in themesostructured zeolite becauseboth catalysts had the same for-

mulation (zeolite, binder, and matrix contents). The catalyticperformance of the FCC catalyst was tested against a FCC cata-lyst that was made from conventional USY and deactivatedunder the same conditions for the cracking of vacuum gas oil(VGO) in an Advanced Catalyst Evaluation (ACE) test unit. Sig-nificantly higher yields of the valuable transportation fuels(gasoline and diesel) were obtained by using the FCC catalystwith the mesostructured USY zeolite although the yields of un-desirable coke and unconverted bottoms were reduced(Figure 16).[93, 123] These results are in line with what would beexpected from the alleviated diffusion limitation in the micro-porous structure of the conventional Y zeolite. Thus, the openmesoporous network of the new zeolite provided better acces-sibility for heavy hydrocarbon molecules, thereby allowingthem to diffuse in and out of the pores faster and more easily,

Table 1. Micropore volume, mesopore volume, and unit-cell size of the samples described in Figure 8 and Figure 9 of ref. [93] . Adapted with permissionfrom Ref. [93] . Copyright 2012 Royal Society of Chemistry.

Micropore volume Mesopore volume[a] BET surface External surface Unit-cell(pore size 0–20 �) [cc g�1] (pore size 20–135 �) [cc g�1] area [m2 g�1] area [m2 g�1] size [�]

zeolite NH4Y 0.38 0.03 970 22 24.70mesostructured zeolite Y[b] 0.37 0.16 916 243 24.67mesostructured zeolite USY[b] 0.27 0.16 812 152 24.55steamed mesostructured zeolite USY[b,c] 0.24 0.16 661 136 24.35conventional USY (CBV500) 0.32 0.04 857 75 24.55

[a] The mesopore range 20–135 � was chosen to capture the characteristic mesoporosity that was introduced by using this technique. [b] The zeolites con-tained about 5 % rare-earth oxides. [c] Steaming was performed at 1450 8F (788 8C) under 100 % steam for 4 h.

Figure 13. FE-SEM images of the untreated NaY zeolite (top) and the mesostructured Y-zeolite crystals (bottom).Scale bars: a) 1 mm, b, c) 500 nm, d) 200 nm.

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increasing their conversion into more-valuable intermediate-molecular-weight hydrocarbons, and decreasing their deposi-tion as coke.

Thirdly, the process has been successfully scaled up furtherto the tonnage level in a commercial zeolite manufacturing

plant by using existing equipment, starting from NaY zeo-lite.[123] So far, about 500 tons of FCC catalysts with mesopo-rous zeolite Y have been made in a commercial catalyst manu-facturing plant over three separate production runs.[94, 124, 125]

Concurrently, major improvements have been made to boththe raw materials and operating cost structures of the surfac-tant-based process. A larger than 10-fold decrease in manufac-turing costs has been realized without capital investment. Asa result, past reservations[77, 126] about the high costs of the sur-factant-based process were overcome. In addition, research atRive Technology also showed that it was possible to recoverand reuse the surfactant to further decrease the cost.[127] Nota-bly, the flexibility of this process allowed easy optimization ofthe zeolite properties for targeted applications. The first twobatches of FCC catalyst with mesoporous zeolite Y were sup-plied to two refineries and showed successful operation intheir FCC units, which confirmed both the hydrothermal andmechanical stability of the mesoporous zeolite and the yieldsof the cracking products observed in the lab catalytic testing.

Specifically, during 2011, the new mesostructured zeolite-Y-based FCC catalysts were successfully introduced into a NorthAmerican refinery.[94, 123, 124] Comparing the performance of theincumbent equilibrium catalyst (containing conventional zeo-lite) before the trial and the equilibrium catalyst in the unitafter the catalyst inventory was replaced with the new catalystcontaining the mesostructured Y zeolite at 66% change-out,an increase in bottoms upgrading and significant coke reduc-tion were observed. The projected economic uplift for the re-finery was estimated to be between $0.60–1.17/bbl (barrel) ofthe FCC feed.

Recently, a second refinery trial of a second-generation mes-ostructured zeolite Y was completed with outstanding suc-cess.[125] Prior to this commercial trial, both the fresh incum-bent catalyst and the FCC catalyst with the mesostructuredY zeolite were impregnated with same levels of nickel and va-nadium and then deactivated by cyclic propylene steaming(CPS)[128] and then tested in an ACE unit under identical condi-

Figure 14. a) Nitrogen-adsorption isotherms, b) nonlocal density functionaltheory (NLDFT) pore-size distributions (with normalized peak heights tobetter demonstrate the correspondence between the modal mesopores sizeand surfactant size), and c) a linear correlation between the modal mesoporediameters and the carbon numbers of the long alkyl chains in the surfac-tants that were used to prepare a series of mesostructured NH4-Y zeolitesunder the same reaction conditions. C8, C10, C12, C14, C16, and C18 denoteoctyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl trimethylammoni-um bromides, respectively. Adapted with permission from Ref. [93] . Copy-right 2012 Royal Society of Chemistry.

Figure 15. XRD pattern of a mesostructured CBV720 (the one with thelowest micropore volume shown in the inset), which shows the (100), (110),and (210) peaks that are owing to hexagonal ordering of the mesopores(2q<58) and peaks that are characteristic of zeolite Y (2q>58). Inset showsthat the microporosity/mesoporosity parameters in the surfactant-templatedmesoporous Y zeolite can be fine-tuned in a linear trend by varying themesostructuring conditions.

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tions using the refinery FCC feed. The results at a constantconversion of 75 wt. % are shown in Table 2.[125] The mesostruc-tured zeolite Y was able to: 1) Increase the yields of gasolineand diesel [light cycle oil (LCO)]; 2) increase the production ofvaluable light olefins (propylene and butenes) ; and 3) remark-ably decrease the yield of coke. Most importantly, these advan-tages were associated with almost no penalty in catalyst activi-ty (as evidenced by the very close catalyst-to-oil ratios thatwere necessary to achieve 75 % conversion). The estimatedeconomic uplift based on the laboratory test results as shownin Table 2 was about $2.00/barrel of FCC feed for the specificrefinery. At the end of the commercial trial, the additional reve-nue that was delivered to the refinery by replacing the incum-bent catalyst with the FCC catalyst that contained the newmesostructured zeolite Y, was estimated to be over $2.50/bblof the FCC feed (Figure 17).[125]

In April 2013, Rive Technology began the ongoing supply ofa commercial FCC catalyst with mesoporous zeolite Y that wasmade by surfactant-templated post-synthetic modification,which represents the first large-scale industrial catalyticapplication of hierarchical zeolites.

3.3 Surfactant-templated zeolite/mesophase composites

As mentioned in the first section, another surfactant-templatedpost-synthetic modification approach was first reported byGoto et al. in 2002[96] and later by Ivanova et al. in 2004.[95] Thisprocess involves two steps: 1) Partial or complete destructionof the ZSM-5 or mordenite structure, followed by 2) hydrother-mal treatment with CTAB and NaOH over a longer period oftime, during which a pH adjustment was performed. Com-

Table 2. Comparison of the FCC catalyst (MH-1) with the new mesostruc-tured zeolite Y to the incumbent catalyst that is used in a commercial re-finery. Reprinted with permission from Ref. [125]. Copyright 2013 Ameri-can Fuels & Petrochemical Manufacturers.

Catalyst Incumbent catalyst Rive catalyst (MH-1)

C/O ratio 6.2 6.4Conversion [wt. %] 75.0 75.0Yield [wt. %]dry gas 3.37 3.19liquefied petroleum gas 15.64 15.94

propane 0.83 0.83propylene 4.67 4.77butanes 3.85 3.95butenes 6.29 6.39

gasoline 50.39 51.33LCO 19.09 19.14bottoms 5.91 5.86coke 5.60 4.54

Figure 16. Catalyst evaluation of two FCC catalysts that were prepared anddeactivated under the same conditions (788 8C in 100 % steam for 8 h), oneof which contained a conventional zeolite USY (c^c), whereas theother contained a mesostructured zeolite USY (a~a). The catalyst eval-uation was performed in an ACE unit at 527 8C by using a VGO feedstock.The lines were fitted by a kinetic lump model. Adapted with permissionfrom Ref. [93] . Copyright 2012 Royal Society of Chemistry.

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pared to the single-step process described above, the materialsthat resulted from the two-step process were very different. Assuggested by the proposed mechanism of the two-step ap-proach (Figure 18),[129] the dissolved zeolite species are re-con-

densed and re-assembled around the surface of the surfactantmicelles and are deposited back onto the crystal surface asa separate mesoporous molecular sieve phase, if the pH valueof the reaction mixture is adjusted to about 8.5. The first stepalso generates some mesoporosity in the zeolite crystals by de-silication, similar to the other desilicated mesoporosity as de-scribed in the previous section. SEM images show the co-exis-tence of both zeolite crystals and a mesoporous phase that istemplated by the surfactant (Figure 19).[129] The bimodal meso-porosity, as suggested by the two distinct nitrogen uptakes atP/P0�0.35 and 0.95 in the isotherms (Figure 20), is also consis-tent with the existence of two types of mesoporosity, that is,well-controlled mesopores with sizes of about 4 nm (templatedby the surfactant) in the deposited mesophase and broadmesopores that are created by desilication in the remainingzeolite.[95] Presumably, the surfactant-templated mesophase issimilar to mesoporous materials that are formed by condensa-tion from the filtrate that is obtained from the alkaline dissolu-tion of the zeolite (first step), which may contain fragments ofthe zeolite and, therefore, shows enhanced Brønsted aciditycompared to conventional Al-MCM-41.[130] The depolymeriza-tion/recrystallization (DR) approach (as mentioned in Section 2)

Figure 17. Observed trends during a trial at Alon’s Big Spring, Texas refinery:a) Increased feed rate by 700 BPSD (barrel per stream day); b, c) increasedproduction of gasoline and LCO (the big spikes in the plant data wereowing to process interruptions and not to the catalyst) ; d) an incrementalvalue uplift owing to the change-out of the incumbent catalyst for Rive’sMH-1 catalyst that contained mesostructured Y zeolite. Adapted with per-mission from Ref. [125]. Copyright 2013 American Fuels & PetrochemicalManufacturers. *) Rive catalyst, ^) conventional catalyst.

Figure 18. Schematic representation of the recrystallization procedure thatleads to different types of materials. Reprinted with permission fromRef. [129] . Copyright 2013 Royal Society of Chemistry.

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shares many characteristics with the dissolution/re-crystallization approach described above, except thatglycerol is used for the depolymerization of zeoliteinstead of NaOH. Another major difference is that, ifa surfactant, such as CTAB, was used in the hydrolysisstep of DR approach before recrystallization, no mes-opore-templating effect was observed. The hierarchi-cal Y zeolite showed a broad mesopore-size distribu-tion from 10–100 nm.[100] TEM analysis clearly showedthe composite nature of the materials, that is, a mix-ture of recrystallized Y-zeolite nanocrystals and anamorphous mesoporous phase.[100]

3.4 Desilicated zeolite Y

There has also been progress inthe template-free desilicationapproach to introduce meso-porosity into zeolite Y. Desilica-tion had only been applicable tozeolites such as ZSM-5, morden-ite, and ferrite, with optimal Si/Al ratios of between 25–50.Later, the Si/Al range was ex-panded to 10–1. USY zeolitestypically have Si/Al ratios withinthis range, for example, ZeolystCBV720 (about 15), CBV760(about 30), and CBV780 (about40). In 2010, de Jong et al.[131]

published the first examples ofmesoporous zeolite Y that wereprepared by desilication. Start-ing from a commercial USY(CBV760) at a Si/Al ratio of 30(named HY-30), room-tempera-

ture treatment of the zeolite with dilute NaOH solution (0.05 m

and 0.10 m) for a short period of time, followed by quenchingof the reaction by pH neutralization with 1 m H2SO4 solution,yielded the desilicated mesoporous Y zeolite. However, it is notclear whether pH neutralization with H2SO4 caused re-precipi-tation of the dissolved silicates (similar to the re-deposition ofdissolved silicates/aluminosilicates in the two-step approach ofIvanova et al. as described above). The dried zeolite was fur-ther subjected to mild steam calcination, the purpose and ef-fects (synergistic or antagonistic) of which on the desilicatedzeolite properties were not described. Nonetheless, the finalproducts exhibited significantly enhanced mesoporosity overthe starting HY-30. Bimodal mesoporosity was evident fromboth nitrogen-physisorption characterization and 3D TEM (orelectron tomography, ET) analysis. Desilication generated moresmall mesopores (about 3 nm) than larger mesopores (about30 nm), which led to a significant increase in the mesoporesurface area from 213 m2 g�1 in the starting HY-30 to339 m2 g�1 in HY-A and 443 m2 g�1 in HY-B, at the expense ofsignificant drops in micropore volume and, possibly more so,of the crystallinity (by XRD; Table 3 and Figure 21).[131] Desilica-

Figure 20. Nitrogen-adsorption/desorption isotherms at 77 K over start-ing (^,*,&) and recrystallized (^,*,&) mordenites. Reprinted with permis-sion from Ref. [95] . Copyright 2004 IUPAC.

Table 3. Textural properties of the zeolite Y samples. Reprinted with permission fromRef. [131].

Sample Smeso[a] Vmicro

[b] Vmeso[c] Vs-meso

[d] Vl-meso[e] Vtotal

[f] Pore diameter[g]

[m2 g�1] [cm3 g�1] [cm3 g�1] [cm3 g�1] [cm3 g�1] [cm3 g�1] [nm]Small Large

HY-30 213 0.21 0.16 0.07 0.09 0.45 – 28HY-A 339 0.16 0.25 0.14 0.11 0.51 2.7 27HY-B 443 0.07 0.37 0.23 0.14 0.55 3.1 27

[a] Mesopore surface area. [b] Micropore volume. [c] Mesopore volume (2–50 nmpores). [d] Volume of the small mesopores (2–8 nm). [e] Volume of the large meso-pores (8–50 nm pores). [f] Total pore volume. [g] Determined from the pore-size distri-bution (PSD); see the Supporting Information, Experimental Section of Ref. [131].

Figure 19. Morphology and texture of recrystallized zeolites a) RMOR-1, b) RMOR-2, and c) RMOR-3 by using TEMand of d) RMFI-1, e) RMFI-2, and f) RMFI-3 by using SEM. Reprinted with permission from Ref. [129]. Copyright2013 Royal Society of Chemistry.

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tion also caused a decrease in the Si/Al ratio from about 28 to25 and 21, respectively, which suggested that there was be-tween 10 % and 25 % yield loss to the solution, respectively, as-suming that only silica dissolved from the zeolite and that nosmaller crystals were lost during the filtration process. Theunit-cell constant a0 increased from 24.28 to 24.30 and 24.31 �,suggesting that some silica had dissolved from the framework.Notably, the total acidity, as measured by TPAD, stayed thesame as that of the starting HY-30 at about 0.30 mmol.

To establish the hydrocracking performance of the zeolites,HY-30 and HY-A were loaded with 0.3 wt. % Pt and used to hy-drocrack the model compound n-hexadecane. Over mesopo-rous Pt/HY-A, the product yields showed an ideal C6/C10 ratioof about 1:1 over the whole temperature range tested, where-as, for conventional Pt/HY-30, the ratio deviated quickly awayfrom 1, owing to overcracking. Hydrocracking of anothermodel compound, squalane (branched C30 alkane), at 230 8Cshowed that better product symmetry was achieved over Pt/HY-A than over Pt/HY-30, thus also suggesting that secondarycracking was alleviated, possibly through improved productdiffusion out of the zeolite before overcracking into the lighterproducts (Figure 22).[131] The authors took another step closerto real applications by forming NiMoS2/HY-A/alumina catalystextrudates and testing them with a pretreated commercialvacuum gas oil feed. Compared to a state-of-the-art commer-cial catalyst, the catalyst that was made from the HY-A zeoliteyielded dramatically higheramounts of middle distillates(diesel and kerosene), with sig-nificantly smaller amounts of theless-desirable naphtha and coke.The activity of the catalyst withmesoporous zeolite was onlyslightly lower than that of thecommercial catalyst.

In early 2011, Garcia-Martinezet al.[132] filed a patent applica-tion for the desilication of zeoli-te Y with a Si/Al ratio of below10 by incorporating a pre-treat-

ment step with mild acid prior to desilication. Examples includ-ed the desilication of NaY (Zeolyst CBV100) zeolite with a Si/Alratio of about 2.5. Mild dealumination by citric acid slightly in-creased the Si/Al ratio of the zeolite to about 3–5 and subse-quent desilication restored the Si/Al ratio to almost the originallevel. Significant mesoporosity (with good retention of micro-porosity) was observed by TEM analysis and argon-physisorp-tion (Figure 23 and Figure 24), albeit with much larger and

Figure 22. Catalytic testing of the parent HY-30 and base-leached HY-A thatwere loaded with 0.3 wt. % Pt. a) Conversion and b) selectivity for the hydro-cracking of n-hexadecane over Pt/HY-30 ( � ) and Pt/HY-A (~). c) Hydrocrack-ing of squalane over Pt/HY-30 (&) and Pt/HY-A (&). Reprinted with permis-sion from Ref. [131].

Figure 21. XRD patterns of the parent HY-30, base-leached HY-A (0.05 m

NaOH), and HY-B (0.10 m NaOH). Reprinted with permission from Ref. [131].

Figure 23. TEM images of mesoporous zeolite Y that was treated with citric acid and then desilicated. Scale bars:a) 200 nm, b) 50 nm, c) 20 nm.[132]

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less-controlled mesopore sizes compared to the surfactant-templated mesoporosity described above.

Later that year, Verboekend et al.[133] also published the desi-lication of zeolite Y after first performing an acid treatment onthe starting material. A series of post-synthetic treatmentswere applied to pristine zeolite NH4Y (Zeolyst CBV300, named“P-Y”) and three dealuminated USY zeolites (CBV500, named“USY1-P”; CBV720, named “USY2-P”; and CBV760, named“USY3-P”) by using a variety of chemicals, such as H4EDTA(EDTA = ethylenediaminetetraacetic acid), Na2H2EDTA, citricacid, HCl, NaOH, and TPAOH, as well as the sequential combi-nation of some of these treatments. Their main findings werethat, for pristine Y zeolite, sequential dealumination by theacid, followed by desilication in alkaline solution, resulted inthe introduction of mesoporosity at the expense of a decreasein XRD crystallinity and material loss; however, the microporevolumes stayed relatively high, thus suggesting that the treat-ments disrupted the long-range ordering of the atoms in thezeolite crystals, but retained much of the coordination of thelocal zeolitic subunits, for example, sodalite cages and supercages. Subsequent mild treatment of the mesoporous Y zeolitewith Na2H2EDTA solution helped to remove the Al-rich debris,thereby freeing the porosity and enhancing crystallinity(Figure 25).[133] The authors also reported that dealuminatedUSY zeolites were more sensitive to the desilication conditionsand that severe acid treatments of the USY zeolites allowedthe introduction of more mesoporosity into the USY zeolites.However, the loss of crystallinity and microporosity were signif-icant. The addition of tetraalkylammonium salts to the alkalinesolution helped to preserve more micropore volume.

Catalytic evaluation of the proton forms of the parent zeo-lite, termed “P-H”, and two modified zeolites, termed “P-DA4-AT1-H” and “P-DA4-AT1-AW1-H”, was performed by the liquid-phase alkylation of benzyl alcohol with toluene, which showedthat, upon the introduction of mesoporosity (P-DA4-AT1-H),there was a slight increase in activity. After the final AW1 wash,to remove the Al-rich debris that possibly blocked the meso-

pores, the activity increased by about 55 % (from 53 % to 84 %conversion) over 40 min reaction time. Pyrolysis of low-densitypolyethylene was also performed over the modified USY zeo-lites to demonstrate the beneficial effects of additional meso-porosity and also the importance of maintaining adequateactivity (Figure 26).

Verboekend et al.[134] also studied the effects of the so-called“pore-directing agents” (PDAs) on the desilication of a USYzeolite, that is, CBV760 (named “USY30” by the authors).Whereas negatively charged surfactants were found to be inef-fective as PDAs, non-ionic surfactants, in particular amines,were able to enhance mesoporosity whilst preserving more ofthe microporosity of the modified zeolites. Alkylammoniumcations, including the CTA+ cation that was used in the surfac-tant-templated approach described above, were found to bemost effective as pore-directing agents. This result was attrib-uted to the strong electrostatic affinity of the positivelycharged head groups with the negatively charged zeolite sur-face and the soluble species in the alkaline reaction mixture,which had also been recognized as one of the driving forcesfor the formation of surfactant-templated mesopores(Figure 20).[93] Among the many alkylammonium salts thatwere tested, two stood out in the PDA screening experiments:TPA+ and CTA+ showed the highest indexed hierarchical factors(IHFs) of 0.67 and 0.54, respectively. Further studies revealedthat, at proper concentrations, both TPA+ and CTA+ not onlyhelped to preserve the microporosity whilst dramatically en-hancing mesoporosity, but they also maintained the Si/Al ratioof the products and improved the yields. Such beneficial ef-fects of CTA+ had previously been observed in the surfactant-templated approach described by Garcia-Martinez. Notably, theauthors demonstrated the possibility of using a continuousprocess to achieve much higher productivity than the batch-wise lab preparations. In 2013, the catalytic cracking ofvacuum gas oil over dealuminated-desilicated mesoporousY zeolite on a fixed-bed Microactivity Test (MAT)[135] reactor was

Figure 24. Ar-sorption isotherms of starting NaY (&), surfactant-templatedmesoporous NaY (~), and acid-treated/desilicated mesoporous NaY (&).[132]

The latter two samples were prepared under the same reaction conditions(e.g. , temperature, time, NaOH dose), except for the presence of CTAB in theformer but not in the latter.

Figure 25. Strategies for the design of hierarchical FAU zeolites by usingpost-synthetic modifications. After the desilication of Al-rich zeolites, the re-moval of any remaining debris by washing with mild acid is crucial. On theother hand, upon alkaline treatment of Si-rich zeolites, the inclusion of pore-growth moderators is highly beneficial. Reprinted with permission fromRef. [133] .

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reported.[126] After ammonium exchange, the as-preparedmesoporous MY zeolite showed a micropore volume of0.163 cc g�1, which was significantly lower than that(0.30 cc g�1) of the starting CBV300, and a mesoporosity(0.218 cc g�1) that was only marginally higher than that(0.209 cc g�1) of an equivalent conventional USY, that is,CBV760. Then, steaming at 750 8C for 5 h and 100 % steam wasperformed on MY. After further ammonium exchange and cal-cination at 500 8C for 3 h to convert the zeolite into its protonform, the zeolite was steamed again at 750 8C to afford HMY-S750C for testing. HMY-S750C showed almost zero microporos-ity (0.028 cc g�1) and similar mesoporosity (0.214 cc g�1) to MY,owing to the very severe hydrothermal treatments. The lantha-num-exchanged MY (with about 3.5 wt. % La2O3) was first con-verted into the proton-form zeolite by double calcination at500 8C for 3 h and then steamed at 780 8C and 800 8C for 5 h in100 % steam to result in LaHMY-S780 and LaHMY-S800, both ofwhich showed slightly higher micropore volumes (0.053 and0.045 cc g�1, respectively) than HMY-S750, possibly owing tothe stabilization effect of lanthanum in the zeolite. However,the unit-cell constants (a0) of LaHMY-S780 (24.55 �) andLaHMY-S800 (24.54 �) were significantly higher than those ofHMY-S750 (24.31 �) and CBV760 (24.26 �), which were alsohigher than that of a typical steam-deactivated rare-earth sta-bilized conventional USY (about 24.4 �). It is unknown whetherthese unusually high steamed unit-cell constants for theLaHMY zeolites are the result of measurement issues, lower-

than-believed deactivation severity, or of implausibly highsteam stability for the desilicated mesoporous Y zeolites.

MAT testing of the above-described zeolite samples repre-sents the first published example of evaluation of the desilicat-ed Y zeolites for catalytic cracking. Although the over 20 % gapin mass balance in the reported MAT test results (Table 4 of ref.[126]) raises some questions about the quality of the tests,taken at face value, the data corroborate the advantages inbottoms upgrading and improved selectivity, such as higherdiesel yield, less gases, and higher olefinicity in the liquefiedpetroleum gas fraction, that have been attributed to the pres-ence of mesoporosity within zeolite crystals. The results also il-lustrate the well-known effects of rare-earth ion exchange (i.e. ,higher steamed zeolite unit-cell parameter) on the quality andyield of LCO (diesel). Prior unpublished work at Rive Technolo-gy by using more-realistic evaluation conditions on the desili-cated mesoporous zeolite Y described elsewhere[132] demon-strated similar trends in product yields, plus significantimprovement in coke selectivity.

3.5 “Bottom-up” approaches to mesoporous Y

Since early 2000, there has also been some progress in thetemplated “bottom-up” approaches to mesoporous zeolite Y.In 2003, Tao et al.[31] reported a synthesis of mesoporous zeoli-te Y by using carbon aerogels as templates, prepared first fromthe pyrolysis of the supercritically dried resorcinol-formalde-hyde (RF) polymer aerogels. The synthesis of the zeolite in-volves three steps: 1) Infiltration of the precursors into themesopores of the carbon aerogels; 2) synthesis of zeolite Yinside the inert mesopores of the templates; and 3) removal ofthe carbon aerogel templates by calcination to expose themesoporous zeolite Y (Figure 27).[32]

In 2010, Gu et al.[136] reported another “bottom-up” approachto mesoporous zeolite Y or, more accurately, mesoporous silicathat contains zeolite Y or sodalite fragments. By using a mixtureof CTAB, tert-butanol (TBA), and trimethylbenzene (TMB) as thetemplates, the authors assembled nanocrystals with the zeo-lite-Y structure around the swollen micelles of CTAB to forma mesoporous silica that contained zeolite-Y or sodalite frag-ments (Figure 28). Fine-tuning of the synthesis conditions,such as silica source, TMB/CTAB molar ratio, and the amount of

Figure 26. Catalytic evaluation of Y and USY zeolites in a) the alkylation oftoluene with benzyl alcohol (BA) and b) the pyrolysis of low-density polyeth-ylene (LDPE). Inset in (b) shows derivative of the thermogravimetric (DTG)profiles. Reprinted with permission from Ref. [133] .

Figure 27. Growth of zeolite crystals in the uniform mesopores of carbonaerogel that consist of interconnected uniform carbon particles. The meso-pores are large enough to allow the gel to be sufficiently concentrated andto allow growth to continue until the mesopores are filled. Adapted withpermission from Ref. [32] . Copyright 2003 American Chemical Society.

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TBA, is crucial for the formation of the mesostructure. Al-though the results show that this method is applicable to a rel-atively wide range of bulk Si/Al ratios, the XRD crystallinity andmicropore volumes of the synthesized materials are significant-ly lower than those of conventional NaY. Interestingly, the re-sults of catalytic dehydration of 2-propanol and the TPAD datasuggest much stronger acidity of one of the mesoporoussamples than conventional NaY.

More recently, in 2011, Xiao and co-workers published a syn-thesis of mesoporous zeolite Y by using a silylated quaternaryammonium surfactant, that is, N,N-dimethyl-N-octadecyl-(3-tri-ethoxy-silylpropyl)ammonium bromide (TPOAB), as tem-plate.[137] TPOAB was found to be well-dispersed in the synthe-sis mixture, owing to strong interactions of the silyl groupsand the quaternary ammonium groups with the soluble alumi-nosilicate species in the crystal-growth gel. This is critical tothe success of the “bottom-up” approach to mesoporous zeoli-te Y. Mesoporous material NaY-M showed very good crystallini-ty and micro-/mesoporosity. The TEM images showed the mes-opores templated by the silylated surfactant (Figure 29). Then,palladium nanoparticles of similar size were loaded onto theH-form zeolites (mesoporous HY-M, conventional HY, mesopo-rous HBeta-M, and HZSM-5M) and the hydrodesulfurization(HDS) of a bulky model compound, 4,6-dimethyldibenzothio-phene (4,6-DM-DBT), over the Pd-loaded zeolites (or g-Al2O3)was tested on a fixed-bed continuous-flow reactor. Pd/HY-Mshowed the highest activity, 97.3 % conversion at 6 h versus61.8 % conversion over Pd/HY and 34.1 % conversion overa conventional Pd/g-Al2O3 catalyst. The higher activity of Pd/HY-M compared to those of Pd/HBeta-M and Pd/HZSM-5M wasattributed to the larger micropore size of Y zeolite, which al-lowed the bulky sulfur-containing compound to access theacid sites inside the mesopores.

4. Summary and Outlook

The discovery of surfactant-templated amorphous mesoporousmaterials, such as MCM-41, about 20 years ago raised many ex-

pectations because their open and tunable structuresare ideal for the fast diffusion and processing of largemolecules. However, their weak acidity and limitedhydrothermal stability has restricted their use in ap-plications for which such properties are required,such as in FCC and other important refining process-es. Many efforts have been made to improve theiracidity and hydrothermal stability by using variousapproaches and yet only limited success has beenachieved.

Since the turn of the 21st century, a tremendousamount of effort has been devoted to introducinglarger pores, typically mesopores of 2–50 nm in diam-eter, into conventional zeolites. The main motivationis to alleviate the diffusional limitation of moleculesthrough the micropore system, in which strong acidsites catalyze many important reactions, such as the

cracking of petroleum, isomerization, and desulfurization.Many preparation approaches have emerged, which can begenerally categorized as either “bottom-up”/primary synthesesor “top-down”/post-synthetic modifications (Figure 2). Most“bottom-up” approaches involve the use of mesoscale tem-plates of some sort, either hard templates, such as carbonblack, carbon nanotubes or nanofibers, ordered or non-orderedmesoporous carbon, and carbon aerogels, or soft templates,such as cationic surfactants, silylated surfactants or polymers,cationic polymers, polymer aerogels, starch, and bacteria. Opti-mal interactions between the templates and the silicate or alu-minosilicate species in the zeolite crystal-growth gels are es-sential to the success of the “bottom-up” approaches. Efforts

Figure 28. Proposed route for the synthesis of hierarchical mesoporous zeolites. Reprint-ed with permission from Ref. [136]. Copyright 2010 American Chemical Society.

Figure 29. a) XRD pattern, b) N2 isotherm, c) SEM image, and d) TEM imageof NaY-M; the mesoporosity in the crystal is marked by red lines and whitearrows. Reprinted with permission from Ref. [137]. Scale bars : c) 1 mm,d) 20 nm. Copyright 2011 American Chemical Society.

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to either convert preformed amorphous mesoporous silicate oraluminosilicate materials (or other precursors) into zeolitic ma-terials or the assembly of protozeolitic sub-units or nanozeolitecrystals into mesostructures have also achieved some success.In the “top-down” category, hydrothermal treatment andchemical dealumination have been known for about half a cen-tury. However, recent studies have suggested that not all ofthe “mesopores” are open to the external surface of the zeolitecrystals, thus limiting the usefulness of these treatments to en-hance the diffusion through zeolites. Desilication of zeoliteswith suitable Si/Al ratios could generate intracrystalline meso-porosity at the penalty of significant material loss, compro-mised zeolite integrity, and difficulty in material handling.Some “top-down” approaches also involve the use of tem-plates, such as cationic surfactants. The single-step surfactant/alkaline treatments lead to single-phase mesoporous zeoliteswith intracrystalline mesoporosity, possibly through a crystal-rearrangement mechanism, whereas the two-step processeslead to the formation of a composite of desilicated mesopo-rous zeolite with amorphous surfactant-templated mesostruc-tures through dissolution/re-deposition or recrystallizationmechanism. The single-step approach bridges the gap be-tween conventional zeolites and surfactant-templated amor-phous mesoporous materials, which realizes the long-sought-after goal of combining the desirable properties of both typesof materials, that is, large and tunable mesopores, high crystal-linity, strong Brønsted acidity, and excellent hydrothermal sta-bility, within a single-phase mesoporous zeolite.

The developments in the field of mesoporous zeolite Ynicely represent the “big picture” described above. Althoughmany studies have shown the beneficial effects of secondarymesoporosity on the catalytic applications of mesoporous zeo-lites Y, most of which remain as academic research. Owing tothe great importance of zeolite Y in industrial catalysis, signifi-cant developments and commercialization work have beenperformed at Rive Technology and its partners to commercial-ize surfactant-templated mesoporous zeolite Y (and other zeo-lites, such as ZSM-5, mordenite, X, and A) for applications inpetroleum cracking, as well as other catalytic and non-catalyticapplications. The zeolite-modification process has been scaledup to the commercial scale. FCC catalysts with proper chemicalcompositions, as well as physical and mechanical properties,have also been manufactured on the commercial scale, sup-plied to two separate commercial refineries, and have under-gone successful trials that delivered significantly improvedproduct yields and increased economic value to the refiners.Rive Technology began the ongoing supply of the very firstcommercial FCC catalyst with surfactant-templated mesopo-rous zeolite Y in early April 2013. The era of mesostructuredzeolites in industrial catalysis has arrived!

Acknowledgements

The authors express their sincere appreciation to Dr. Barry Spero-nello (Rive Technology) for the many helpful discussions duringthe preparation of this Review.

Keywords: aluminosilicates · cracking · heterogeneouscatalysis · mesoporous materials · zeolites

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Received: May 3, 2013

Published online on September 5, 2013

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