- Email: [email protected]
Manipulating the architecture of zeolite catalysts for enhanced mass transfer
Available online at www.sciencedirect.com
ScienceDirect Manipulating the architecture of zeolite catalysts for enhanced mass transfer Kake Zhu and Xinggui Zhou Zeolites belong to microporous solids that are widely used as adsorbents, catalysts and catalyst supports. As diffusion in the microspores is slow, it is highly desirable to develop approaches to enhance mass transfer or to avoid the negative effect from mass transfer resistance. In this perspective, we underline pore architecture control for hierarchical zeolite, and spatial locations of metal nanoparticles on zeolite. For solid acid catalysts, the construction of auxiliary porosity in crystalline zeolites leading to the formation of hierarchical zeolite is a promising solution. Most successful pore-architecture construction strategies are based on templating approaches within the classic LaMer crystallization framework, whereas our strategies are based on the non-classic orientated attachment growth mechanism in dry gel crystallization. Organic additives, such as organosilanes, can be incorporated with protozeolite particles to produce hybrid mesocrystals. Combustion of organics in the mesocrystal affords hierarchical beta zeolites, the pore-architecture of which is tunable with respect to additive selection. Such a crystallization based design of pore-architecture merits small primary crystal size and improved pore-connectivity, which have been verified to be the key factors that affect diffusion. On the other hand, for zeolite supported metal catalysts that suffer from deactivation caused by pore blocking, we propose a strategy to avoid the deactivation problem by depositing metal nanoparticles exclusively on the external surfaces of support. This can be achieved by simply leaving the structure directing agents in the pores before supporting metal particles. Side reactions caused by diffusion resistance are minimized, and moreover, the involatile byproducts, if any, do not produce additional diffusion resistance. Such a simple yet effective architecture control significantly prolongs the lifetime of Au/TS-1 catalyst in direct propene epoxidation with H2 and O2. From these demonstrations, it is important to design purpose-orientated zeolite synthesis to enhance mass transfer, for which the architecture plays a key role. Address State Key Lab of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Corresponding author: Zhou, Xinggui ([email protected]) Current Opinion in Chemical Engineering 2015, 9:42–48 This review comes from a themed issue on Reaction engineering and catalysis Edited by Marc-Olivier Coppens and Theodore T Tsotsis http://dx.doi.org/10.1016/j.coche.2015.07.009 2211-3398/Published by Elsevier Ltd.
Current Opinion in Chemical Engineering 2015, 9:42–48
Introduction Zeolites are three dimensional crystalline porous solids possessing pore channels normally less than 2.0 nm, which lay in the microporous range according to the IUPAC classifications. They are widely used as catalysts, catalyst supports, adsorbents for gas separation/purification, or ion-exchangers in laundry powders or treatment of toxics. On one hand, the successful use of zeolites is largely attributed to their crystalline framework structures that endow them high (hydro) thermal stability, strong acidities, tunable Si/Al ratios, and easy isomorphous substitution by other metals (B3+, Fe3+, Ga3+, Ti4+, Ge4+, among others). Their precise pore apertures and cavity structure make them unique shape-selective catalysts and adsorbents. Henceforth, they are also referred to as molecule sieves. On the other hand, artificial zeolites that are mass produced nowadays at industrial scale are comprised of micron-sized crystals, and the solely presence of micropores brings about mass transfer problems that is associated with slow diffusion in micropores. Moreover, bulky molecules whose sizes exceed the size of micropores are denied access to the micropores and thus the catalytic reactions only take place on the external surface. These drawbacks have limited the applications of zeolites toward treatment of large molecules and also called upon research effort to alleviate such problems. Several synthetic strategies for nanozeolites [1], hierarchically porous zeolites (HPZs) [2–6], extra-large pore zeolite [7], or 2D structured zeolites (through top-down delamination [8–10], bifunctional structural directing agent [11,12], or crystallization control [13]) have been reported so far. Recently, desilication and recrystallization methods to generate HPZs have appeared in commercially viable pilot scale demonstrations [14,15]. From the industrial viewpoint, the appealing fabrication methods should be simple and easy for scaling-up, and use low-cost starting materials. In addition to direct use as solid acid catalysts, zeolites are also applicable as metal supports for bifunctional catalysts. In this case, the spatial distribution of metal centers can affect the mass transfer and accessibility as well. Consequently, the position of metal centers can have substantial influences over catalytic performances. To achieve reactant shape selectivity, it is desirable to synthesize metal@zeolites structure whereby metallic particles core are fully encapsulated by zeolite shells, as recently exemplified by Goel et al. [16] and Bao et al. [17]. For reactions that metal-support synergy is important but no shape-selectivity is required, it is important to enhance the mass transfer by placing metallic particle at the external surfaces of zeolitic crystals. www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 43
In view that the importance of pore-networks for hierarchically nano-porous and macro-porous systems have been rationalized by Rao and Coppens [18,19], herein we underline the importance at the micro-porous and meso-porous scale to catalytic performance. In this perspective article, we intend to give a brief review on the preparation protocols of HPZs that have been achieved recently, with anemphasis on ‘pore-architecture’ engineering through non-classic orientated attachment (OA) crystallization. We also give some successful examples on zeolites supported metal catalysts design, where ‘spatialdistribution’ engineering will be underlined. By both types of catalysts design, we manifest that the ‘architecture’, alternative to composition, is an important way to enhance mass transfer or to reduce mass transfer effect, and can have beneficial consequences on the performance of the catalytic processes.
Hierarchically porous zeolites: porearchitecture engineering The mass transfer and reaction kinetic interplay has been recognized in reaction engineering for a long time and the theoretical basis to understand the effect of mass transfer was established by E.W. Thiele: The Thiele modulus dictating the utility of zeolite catalysts. As the diffusion for a given molecule in a particular micropore channel is fixed, the enhancement of mass transfer for nanozeolites or HPZs stems from a shortened diffusion path [20]. Besides, for HPZs, more and more evidences clearly manifest that it is not the presence of auxiliary pores that is important, it is crucial whether the pores are connected to the external surface or not [21,22,23,24]. Weitkamp et al. [23] have demonstrated through Pulse Field Gradient (PFG) NMR technique that the intracrystalline mesopores in USY zeolites has negligible effect for mass transfer of n-octane and 1,3,5-triisopropylbenzene with respective to conventional Y zeolites. Pe´rez-Ramı´rez et al. [21,24] have reported the use of positron annihilation lifetime spectroscopy (PALS) to measure pore connectivity and correlate it with catalytic behavior. It has been found that the connectivity determines the ‘quality’ of mesopores, and the conclusion is in line with other experimental observations [14,22]. The above considerations highlight the importance of zeolite crystal size and pore-connectivity to the enhancement of mass transfer, as both factors determine to which extent improvement can be made. Retrospectively, the first HPZ ZSM-5 with convincing evidence for two sets of porosity has been reported by Jacobsen et al. [4] using carbon black Black Pearls 2000 as hard templates. Afterwards, several soft templating routes using functionalized silanes, cationic polymers or amphilic silanes have been mapped out to generate HPZs [25– 28]. Meanwhile, Pe´rez-Ramı´rez et al. [29] have reported the low-cost postsynthetic desilication method to generate several HPZs, and the pore-connectivity as well as the www.sciencedirect.com
catalytic merits have been demonstrated recently [2,24]. The top-down methods using layered zeolitic prephase have also been developed over the years, and the method is applicable to a series of framework structures [10]. More recently, a series of designed bifunctional structure directing agents (SDAs) have been found to be able to tailor the thickness of zeolite layers down to unit cell scale by Ryoo et al. [12], and the HPZs obtained have demonstrated superior performance in heterogeneous catalysis, for instance, the prolonged lifetime in reactions. Tsapatsis et al. [13] demonstrated another possibility to control the morphology of intergrowth of MFI and MEL structure by a co-templating strategy, where the branching of growth is tailorable to afford a house-ofcards structure. A method to create mesoporous zeolite composites without the need for separate SDA to generate the mesopores has already introduced by Wang and Coppens in a series of articles [21,22,23,24,25–32]. As more extensive reviews have appeared [3,5,8,33], we will not elaborate the topic of HPZ synthesis. Recently, a series of synthetic protocols have been developed in our lab to generate HPZs with diverse topologies [34,35–38]. An interesting finding from the monitoring of zeolite beta crystallization in a dry gel conversion (DGC) system is that it observes the non-classic OA crystallization pathway. The starting gel undergoes morphology changes during steam assisted crystallization (SAC) treatment, and evolves into discrete tiny particulates of 20–30 nm in 6 h. These particulates aggregate in to nanozeolite assemblage of 90–200 nm in ca. 16 h. Bulky zeolite crystals of >200 nm are finally formed as a result of further alignment. The concurrent X-ray diffraction (XRD), Infrared (IR), Scanning electronic micrographic (SEM) and transmission electronic micrographic (TEM) measurements indicate that a temporal overlap of order development has occurred. The whole process is sketched in Route 1 of Figure 1. Evidently, the crystallization can be attributed to the OA growth mechanism and the tiny particulates can be regarded as building blocks, which are transformed into big crystals after disappearing of grain boundaries. The same phenomenon has also been observed by Matsukata et al. [39] and Bein et al. [40], but the formation mechanism has not been further explored, partly because the OA growth mechanism has not been widely accepted in zeolite community. The OA growth mechanism allows us to design new routes to fabricate HPZs, by interrupting the growth process after the formation of protozeolite particulates and still before their further growth by aggregation. Two types of organosilanes have been utilized for this purpose. When hexadecyltrimethoxysilane has been introduced into the precrystallized dry gel by simple impregnation in ethanol solution and drying in the open, a resumed crystallization has afforded a hierarchical beta with mesopore size between 20 and 60 nm (Route 2 of Figure 1) [34]. Hexadecyltrimethoxysilane resembles surfactants Current Opinion in Chemical Engineering 2015, 9:42–48
44 Reaction engineering and catalysis
Figure 1
1 500 nm
2
500 nm OMe Si OMe
3
OMe OMe
MeO MeO MeO
Si
Si OMe OMe
500 nm
Current Opinion in Chemical Engineering
The crystallization of beta in a dry gel conversion process obeys the non-classic orientated attachment mechanism (1), modification of the process to produce hierarchical beta (2) and nanozeolite beta (3) with the assistance of surfactant-like and bola-type organosilanes, respectively. The SEM images for these zeolites are shown parallel to the cartoons.
in that it can induce the self-assembly of zeolitic particulates, and consequently a 100–160 nm sized assemblage consists of 20–40 nm nanozeolite beta are obtained. When a bola type organosilane, bis(trimethoxysiyl)octane, is charged into the system in a similar way (Route 3 of Figure 1), uniform 35 nm nanocrystals of beta are obtained instead. The attachment of primary nanozeolites into bulky crystals is completely inhibited as the two ends of bis(trimethoxysiyl)octane are condensable with zeolite surfaces. Pore-connectivity information obtained by TEM measurement of metallic Pt-replica synthesized via a nanocasting strategy reveals that the auxiliary mesopores are well-connected, that is, the ‘qualities’ of the mesopores are justified. The small primary crystal size of zeolite and the improved pore-connectivity has contributed to an increase in catalytic activity for Pt/ beta in hydroisomerization of n-heptane. A jump of nheptane conversion from 56.0% to 67.2% and 63.3% has been achieved by Pt supported on conventional beta, assemblage beta and nanocrystal beta, respectively [34]. An improvement in Pt dispersion has also been observed, as a result of larger surface area for HPZ supports. Co¨lfen and Antonietti [41] have categorized the crystallization mechanism in nature into three types: classic LaMer mechanism, OA mechanism and mesocrystal formation mechanism. OA mechanism occurs when large number of nucleates are formed and subsequent growth takes place as a result of mutual alignment of primary particulates. The OA mechanism has been witnessed for a variety of metal, metal oxides, or hybrid system, but examples to synthesize porous solids in such scenario has been rarely reported. Organic additives introduced Current Opinion in Chemical Engineering 2015, 9:42–48
into beta synthesis interact with nanocrystals to form hybrid mesocrystals in an analogous way to the mineralization of some natural materials such as nacre. Seeing from the mineralizing standpoint, the artificial synthesis of zeolite pioneered by Richard Barrer has started from mimicking the mineralization of natural zeolites in hydrothermal conditions [42]. It is herein demonstrated that one can learn from nature to tailor the mesoscale morphology of zeolites in a non-classic crystallization process. It is speculated that zeolite nucleation rates in highly condensed gels are fast as a result of high degree of supersaturation. A large number of uniform tiny crystallites are therefore formed in the early state of crystallization, owing to the fast nucleation rate and short nucleation duration. The low water content limits the complete dissolution of precursors ingredients, and refrains the long distance transport of nutrients to nucleates surfaces. Under such a scenario, OA growth predominates in high supersaturation systems such as DGC. Contribution from classic LaMer mechanism, if there is any, will be negligible for the comparably slow rate. As the DGC route developed by Xu et al. [43] have now been found to be a versatile route to synthesize zeolites and zeotype materials, the OA pathway can be extrapolated to other similar systems, or even to low water (H2O/SiO2 < 10) synthesis of zeolites, where HPZs with controlled architecture can be fabricated by interrupting the attachment process.
Zeolite supported metal: metal position engineering Besides working as catalysts, zeolitic materials are often used as metal support to generate bifunctional or cascade catalysts. Metal elements are often introduced into zeolite www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 45
supports via ion-exchange, incipient wetness, or adsorption of metal precursors. If the architecture building process is not well designed and uncontrolled, formation of ill-defined metal nanoparticles, as well as poor spatial distribution can frequently occur. Confining metal nanoparticles inside the zeolite channels could achieve various objectives, such as (1) distributing highly dispersed ultrasmall metal particles [44], (2) preventing metal nanoparticles from aggregation at high temperatures [45], and (3) selectively allowing reactants to access metal surface to achieve shape selectivity. Several strategies have been postulated to achieve the above purposes. Bein et al. [44] demonstrated that Pd, Pt, and Cu amine complexes could be employed as SDA to synthesize ultrafine nanoparticles encapsulated by EDI zeolites. Wu et al. [45] reported that core/shell-structured TS-1@mesoporous silica synthesized through a layer-by-layer approach could prevent the Au nanoparticles located in the shell (i.e., mesoporous silica) from aggregation. Choi et al. [46] used a 3-mercaptopropyl modified silane to include Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite, by which the accessibility to metal cores are restricted by the molecular sieving effect of zeolites. Goel et al. [16,47] demonstrated that it is possible to fabricate such architectures for small pore zeolites (for instance, LTA, ANA, GIS) via ligand-stabilized precursor control, or for medium pore zeolites via zeolite transformation routes. Core@shell cascade catalysts for syngas conversion in Fischer–Tropsch reaction to break the Anderson–Schulz–Flory (ASF) distribution has been reported by Tsubaki et al. [48], where metal centers convert syngas into alkanes and acid sites on zeolite catalyze isomerization and cracking of primary products into isoparaffins.
Metallic active sites that reside on the external surface of zeolitic crystals can be easily accessible. Side effects from the mass transfer resistance in the micropore channels, such as reduced metal usage and accelerated consecutive side reactions, can be minimized. This concept is critical to enhance the stability of microporous catalysts which easily suffer from deactivation by micropore blocking [49–53]. Pore blocking deactivation is quite common in processes such as oxidation and oligermization since the accessibility for reactants in the interior crystal is hindered by configuration diffusion through the micro-channels, which can cause catalyst deactivation if side reactions leading to coke deposition. For Ni-based zeolites, long chain oligomers could be formed on high density of acid sites, leading to pore blocking and catalyst deactivation [50]. This is also found to be the case in TS-1 supported Au catalysts for direct propene epoxidation with H2 and O2, a green, simple and economic process for the worldwide demanding propylene oxide (PO). After diagnosis of the cause for catalyst deactivation (i.e., micropore blocking), we have designed a catalyst preparation protocol to position Au nanoparticles exclusively on the external surface of TS-1 crystals [52,54,55], as shown in Figure 2. Unlike the frequently used deposition–precipitation method that employs calcined TS-1 as support, we have chosen to use an assynthesized SDA-containing TS-1 for Au deposition, such that all the internal micropores are ‘locked’ and Au nanoparticles could only be deposited on the exterior surface of TS-1. Such a design has been found to be a key factor to enhance the long term stability of the catalyst, as the catalyst has exhibited much higher stability over 30 h time-on-stream tests than traditional Au/TS-1 catalyst
Figure 2
180 PO formation rate (gPO·h–1·kgCat–1)
O
O2,H2
160
140 Au/TS-1-B 120 O
Au/TS-1-O
O2,H2
100 0
5
10
15
20
25
30
Time-on-stream (h) Current Opinion in Chemical Engineering
Schematic diagram of Au locations and performance of Au/Uncalcined TS-1 (high stability) and Au/TS catalysts (severe deactivation by micropore blocking). www.sciencedirect.com
Current Opinion in Chemical Engineering 2015, 9:42–48
46 Reaction engineering and catalysis
with Au clusters inside the micropores that rapidly deactivates.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
Summary In this short perspective, we underline pore-architecture control in producing hierarchical zeolites by orientated attachment crystallization and metal position control in synthesizing zeolite supported metal catalysts, both of which are for enhanced mass transfer and also for minimized side effects from mass transfer resistance. We have shown that it is possible to achieve controlled pore-architectures in non-classic crystallization pathway, with the merits of desirable small primary particle size and preferable pore-connectivity that are crucial for mass transfer. Most previously known methods to generate HPZs are based on the classic LaMer mechanism with the assistance of templates, while the controlling of porosity and morphology by taking advantage of their formation mechanism has been largely unexplored. Efforts to control mesoscale structure of zeolites may be shifted from judicious choice of SDAs to choice of crystallization conditions whereby an OA growth predominates. The selection of organic additives can be used to tune the pore size, pore connectivity or the hierarchical structure of building blocks. The synthetic strategy may also be applied to fabricate other porous crystals that expands our toolkit of porous solids syntheses. The importance of metal particles position on a supported catalyst has frequently been neglected as a factor that influences catalysis, whereas it plays decisive role in a number of reactions. We demonstrate that the metal particle position over zeolite supports can be located exclusively on the external surface, through which the micropore blocking induced deactivation can be prevented. As the use of organic SDAs is a common practice for zeolite synthesis, these protocols are of general value for other similar systems. Further efforts are to be stepped up to enlarge the external surface area of support (by creating auxiliary mesopores or decrease the crystal size of support), so as to increase the effective dispersion of supported metal to maximize its utility. As a final remark, we believe that at the edges where catalysis and reaction engineering meet, tremendous opportunities are available for chemical reaction engineering researchers to explore and exploit. Besides the strategies illustrated in this perspective, there is plenty of room for catalyst architecture manipulation to boost the performance of the catalytic reaction.
Acknowledgements KZ is grateful for the financial support from Fundamental Research Funds for the Central Universities (WB 1213004-1), and New Century Excellent Talents in University (NCET-11-0644). XZ is in grateful for financial support from National Natural Science Foundation of China (91434117). Current Opinion in Chemical Engineering 2015, 9:42–48
1.
Valtchev V, Tosheva L: Porous nanosized particles: preparation, properties, and applications. Chem Rev 2013, 113:6734-6760. This is an updated review on the preparation of nanosized zeolites and zeotype materials.
2.
Verboekend D, Perez-Ramirez J: Design of hierarchical zeolite catalysts by desilication. Catal Sci Technol 2011, 1:879-890.
3.
Serrano DP, Pinnavaia TJ, Aguado J, Escola JM, Peral A, Villalba L: Hierarchical ZSM-5 zeolites synthesized by silanization of protozeolitic units: mediating the mesoporosity contribution by changing the organosilane type. Catal Today 2014, 227:1525.
4.
Jacobsen CJH, Madsen C, Houzvicka J, Schmidt I, Carlsson A: Mesoporous zeolite single crystals. J Am Chem Soc 2000, 122:7116-7117.
5.
Na K, Choi M, Ryoo R: Recent advances in the synthesis of hierarchically nanoporous zeolites. Microporous Mesoporous Mater 2013, 166:3-19.
6.
Zhu Y, Hua ZL, Zhou XX, Song YD, Gong Y, Zhou J, Zhao JJ, Shi JL: CTAB-templated mesoporous TS-1 zeolites as active catalysts in a desulfurization process: the decreased hydrophobicity is more favourable in thiophene oxidation. RSC Adv 2013, 3:4193-4198.
7.
Jiang JX, Jorda JL, Yu JH, Baumes LA, Mugnaioli E, DiazCabanas MJ, Kolb U, Corma A: Synthesis and structure determination of the hierarchical meso-microporous zeolite ITQ-43. Science 2011, 333:1131-1134.
8.
Corma A, Diaz U, Domine ME, Forne´s V: AlITQ-6 and TiITQ-6: synthesis characterization, and catalytic activity. Angew Chem Int Ed 2000, 39:1499-1501.
9.
Corma A, Diaz U, Domine ME, Forne´s V: New aluminosilicate and titanosilicate delaminated materials active for acid catalysis, and oxidation reactions using H2O2. J Am Chem Soc 2000, 122:2804-2809.
10. Wu P, Ruan JF, Wang LL, Wu LL, Wang Y, Liu YM, Fan WB, He MY, Terasaki O, Tatsumi T: Methodology for synthesizing crystalline metallosilicates with expanded pore windows through molecular alkoxysilylation of zeolitic lamellar precursors. J Am Chem Soc 2008, 130:8178-8187. 11. Choi M, Na K, Kim J, Sakamoto Y, Terasaki O, Ryoo R: Stable single-unit-cell nanosheets of zeolite MFI as active and longlived catalysts. Nature 2009, 461:246-249. 12. Na K, Jo C, Kim J, Cho K, Jung J, Seo Y, Messinger RJ, Chmelka BF, Ryoo R: Directing zeolite structures into hierarchically nanoporous architectures. Science 2011, 333:328-332. 13. Zhang XY, Liu DX, Xu DD, Asahina S, Cychosz KA, Agrawal KV, Al Wahedi Y, Bhan A, Al Hashimi S, Terasaki O et al.: Synthesis of self-pillared zeolite nanosheets by repetitive branching. Science 2012, 336:1684-1687. 14. Li K, Valla J, Garcia-Martinez J: Realizing the commercial potential of hierarchical zeolites: new opportunities in catalytic cracking. ChemCatChem 2014, 6:46-66. One perspective article on the industrial applications of hierarchical zeolites with an emphasis over their own contribution. 15. Verboekend D, Mitchell S, Perez-Ramirez J: Hierarchical zeolites overcome all obstacles: next stop industrial implementation. Chimia 2013, 67:327-332. Another perspective article describing the commercial use of hierarchical zeolites, with a focus on desilication based protocols. 16. Goel S, Zones SI, Iglesia E: Encapsulation of metal clusters within mfi via interzeolite transformations and direct www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 47
hydrothermal syntheses and catalytic consequences of their confinement. J Am Chem Soc 2014, 136:15280-15290. An important article demonstrating the possibility to encapsulate metal clusters to achieve reactant shape selectivity. 17. Bao J, He J, Zhang Y, Yoneyama Y, Tsubaki N: A core/shell catalyst produces a spatially confined effect and shape selectivity in a consecutive reaction. Angew Chem Int Ed 2008, 47:353-356. 18. Rao SM, Coppens M-O: Increasing robustness against deactivation of nanoporous catalysts by introducing an optimized hierarchical pore network-Application to hydrodemetalation. Chem Eng Sci 2012, 83:66-76. 19. Rao SM, Coppens M-O: Mitigating deactivation effects through rational design of hierarchically structured catalysts: application to hydrodemetalation. Ind Eng Chem Res 2010, 49:11087-11097. 20. Christensen CH, Johannsen K, Toernqvist E, Schmidt I, Topsoe H, Christensen CH: Mesoporous zeolite single crystal catalysts: diffusion and catalysis in hierarchical zeolites. Catal Today 2007, 128:117-122. 21. Milina M, Mitchell S, Cooke D, Crivelli P, Pe´rez-Ramı´rez J: Impact of pore connectivity on the design of long-lived zeolite catalysts. Angew Chem Int Ed 2014:1591-1594. An article highlights the importance of pore-connectivity in hierarchical zeolites to the catalytic performances. 22. Thomas JM, Leary RK: A major advance in characterizing nanoporous solids using a complementary triad of existing techniques. Angew Chem Int Ed 2014, 53:12020-12021. The authors provide some new progress on the development of techniques to measure pore-connectivity. 23. Kortunov P, Vasenkov S, Karger J, Valiullin R, Gottschalk P, Elia MF, Perez M, Stocker M, Drescher B, McElhiney G et al.: The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales. J Am Chem Soc 2005, 127:13055-13059. 24. Milina M, Mitchell S, Crivelli P, Cooke D, Perez-Ramirez J: Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts. Nat Commun 2014, 5:3922. An interesting article correlates the catalytic lifetime of hierarchical zeolites to the pore-connectivity, together with a positron annihilation lifetime spectroscopy on pore-connectivity determination. 25. Wang H, Pinnavaia TJ: MFI zeolite with small and uniform intracrystal mesopores. Angew Chem Int Ed 2006, 45:76037606. 26. Xiao FS, Wang LF, Yin CY, Lin KF, Di Y, Li JX, Xu RR, Su DS, Schlogl R, Yokoi T et al.: Catalytic properties of hierarchical mesoporous zeolites templated with a mixture of small organic ammonium salts and mesoscale cationic polymers. Angew Chem Int Ed 2006, 45:3090-3093. 27. Serrano DP, Aguado J, Escola JM, Rodriguez JM, Peral A: Effect of the organic moiety nature on the synthesis of hierarchical ZSM-5 from silanized protozeolitic units. J Mater Chem 2008, 18:4210-4218. 28. Choi M, Cho HS, Srivastava R, Venkatesan C, Choi DH, Ryoo R: Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunable mesoporosity. Nat Mater 2006, 5:718-723. 29. Groen JC, Peffer LAA, Moulijn JA, Perez-Ramirez J: Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent. Chem Eur J 2005, 11:4983-4994. 30. Moller K, Bein T: Mesoporosity – a new dimension for zeolites. Chem Soc Rev 2013, 42:3689-3707. 31. Wang J, Vinu A, Coppens M-O: Synthesis and structure of silicalite-1/SBA-15 composites prepared by carbon templating and crystallization. J Mater Chem 2007, 17:42654273. 32. Wang J, Yue W, Zhou W, Coppens M-O: TUD-C A tunable, hierarchically structured mesoporous zeolite composite. Microporous Mesoporous Mater 2009, 120:19-28. www.sciencedirect.com
33. Chal R, Ge´rardin C, Bulut M, van Donk S: Overview and industrial assessment of synthesis strategies towards zeolites with mesopores. ChemCatChem 2011, 3:67-81. 34. Chen J, Hua WM, Xiao Y, Huo QS, Zhu KK, Zhou XG: Tailoring the structure of hierarchically porous zeolite beta through modified orientated attachment growth in a dry gel system. Chem Eur J 2014, 20:14744-14755. We demonstrate that zeolite beta crystallization obeys the non-classic orientated attachment growth mechanism, and it is more convenient to control the hierarchical structure within this framework. As dry gel conversion and low water synthesis are quite common, the mechanism and control method can be a general one in high supersaturation systems. 35. Zhang YC, Zhu KK, Duan XZ, Zhou XG, Yuan WK: The templating effect of an easily available cationic polymer with widely separated charge centers on the synthesis of a hierarchical ZSM-5 zeolite. J Mater Chem A 2014, 2:18666-18676. 36. Zhang YC, Zhu KK, Zhou XG, Yuan WK: Hierarchically porous zeolite beta synthesized via steam-assisted crystallization of silanized dry gel. Mater Lett 2014, 131:214-216. 37. Zhang YC, Zhu KK, Zhou XG, Yuan WK: Synthesis of hierarchically porous ZSM-5 zeolites by steam-assisted crystallization of dry gels silanized with short-chain organosilanes. New J Chem 2014, 38:5808-5816. 38. Zhang YC, Zhu KK, Duan XZ, Li P, Zhou XG, Yuan WK: Synthesis of hierarchical ZSM-5 zeolite using CTAB interacting with carboxyl-ended organosilane as a mesotemplate. RSC Adv 2014, 4:14471-14474. 39. Matsukata M, Osaki T, Ogura M, Kikuchi E: Crystallization behavior of zeolite beta during steam-assisted crystallization of dry gel. Microporous Mesoporous Mat 2002, 56:1-10. 40. Moller K, Yilmaz B, Jacubinas RM, Muller U, Bein T: One-step synthesis of hierarchical zeolite beta via network formation of uniform nanocrystals. J Am Chem Soc 2011, 133:5284-5295. The above two groups have explored the crystallization process of zeolite beta, and the findings are confirmative to the mechanism we disclosed in Ref. [34]. 41. Co¨lfen H, Antonietti M: Mesocrystals and Nonclassical Crystallization. In Mesocrystals and nonclassical crystallization. Edited by John Co¨lfen , Antonietti M. Wiley & Sons Ltd.; 2008:1-6. An comprehensive book describing the crystallization in nature, whereby the major crystallization mechanism and key theories are shown in the first 2 chapters. 42. Ravishankar RA, Martens JA, Jacobs P: The scientific legacy of the late Richard M. Barrer FRS. Microporous Mater 1997, 8:283-284. 43. Xu W, Dong J, Li J, Li J, Wu F: A novel method for the preparation of zeolite ZSM-5. J Chem Soc Chem Commun 1990:755-756. 44. Kecht J, Mintova S, Bein T: Nanosized zeolites templated by metalSamine complexes. Chem Mater 2007, 19:1203-1205. 45. Xu L, Ren Y, Wu H, Liu Y, Wang Z, Zhang Y, Xu J, Peng H, Wu P: Core/shell-structured TS-1@ mesoporous silica-supported Au nanoparticles for selective epoxidation of propylene with H2 and O2. J Mater Chem 2011, 21:10852-10858. 46. Choi M, Wu ZJ, Iglesia E: Mercaptosilane-assisted synthesis of metal clusters within zeolites and catalytic consequences of encapsulation. J Am Chem Soc 2010, 132:9129-9137. 47. Goel S, Wu Z, Zones SI, Iglesia E: Synthesis and catalytic properties of metal clusters encapsulated within small-pore (SOD, GIS ANA) zeolites. J Am Chem Soc 2012, 134:17688-17695. 48. Li X-G, Liu C, Sun J, Xian H, Tan Y-S, Jiang Z, Taguchi A, Inoue M, Yoneyama Y, Abe T et al.: Tuning interactions between zeolite and supported metal by physical-sputtering to achieve higher catalytic performances. Sci Rep 2013, 3:2813. 49. Lallemand M, Finiels A, Fajula F, Hulea V: Catalytic oligomerization of ethylene over Ni-containing dealuminated Y zeolites. Appl Catal A-Gen 2006, 301:196-201. 50. Lallemand M, Rusu OA, Dumitriu E, Finiels A, Fajula F, Hulea V: NiMCM-36 and NiMCM-22 catalysts for the ethylene oligomerization: effect of zeolite texture and nickel cations/ acid sites ratio. Appl Catal A-Gen 2008, 338:37-43. Current Opinion in Chemical Engineering 2015, 9:42–48
48 Reaction engineering and catalysis
51. Finiels A, Fajula F, Hulea V: Nickel-based solid catalysts for ethylene oligomerization – a review. Catal Sci Technol 2014, 4:2412-2426. 52. Feng X, Duan X, Yang J, Qian G, Zhou X, Chen D, Yuan W: Au/ uncalcined TS-1 catalysts for direct propene epoxidation with H2 and O2: effects of Si/Ti molar ratio and Au loading. Chem Eng J 2014. doi:10.1016/j.cej.
54. Feng X, Duan X, Qian G, Zhou X, Chen D, Yuan W: Au nanoparticles deposited on the external surfaces of TS-1: enhanced stability and activity for direct propylene epoxidation with H-2 and O-2. Appl Catal B-Environ 2014, 150:396-401. The controlled preparation of Au nanoparticles exclusively on external surface of zeolite TS-1 is reported, as a proven solution towards enhancement of catalyst stability for the reaction.
53. Lacarriere A, Robin J, S´wierczyn´ski D, Finiels A, Fajula F, Luck F, Hulea V: Distillate-range products from non-oil-based sources by catalytic cascade reactions. ChemSusChem 2012, 5:17871792.
55. Feng X, Duan X, Qian G, Zhou X, Chen D, Yuan W: Insights into size-dependent activity and active sites of Au nanoparticles supported on TS-1 for propene epoxidation with H-2 and O-2. J Catal 2014, 317:99-104.
Current Opinion in Chemical Engineering 2015, 9:42–48
www.sciencedirect.com
ScienceDirect Manipulating the architecture of zeolite catalysts for enhanced mass transfer Kake Zhu and Xinggui Zhou Zeolites belong to microporous solids that are widely used as adsorbents, catalysts and catalyst supports. As diffusion in the microspores is slow, it is highly desirable to develop approaches to enhance mass transfer or to avoid the negative effect from mass transfer resistance. In this perspective, we underline pore architecture control for hierarchical zeolite, and spatial locations of metal nanoparticles on zeolite. For solid acid catalysts, the construction of auxiliary porosity in crystalline zeolites leading to the formation of hierarchical zeolite is a promising solution. Most successful pore-architecture construction strategies are based on templating approaches within the classic LaMer crystallization framework, whereas our strategies are based on the non-classic orientated attachment growth mechanism in dry gel crystallization. Organic additives, such as organosilanes, can be incorporated with protozeolite particles to produce hybrid mesocrystals. Combustion of organics in the mesocrystal affords hierarchical beta zeolites, the pore-architecture of which is tunable with respect to additive selection. Such a crystallization based design of pore-architecture merits small primary crystal size and improved pore-connectivity, which have been verified to be the key factors that affect diffusion. On the other hand, for zeolite supported metal catalysts that suffer from deactivation caused by pore blocking, we propose a strategy to avoid the deactivation problem by depositing metal nanoparticles exclusively on the external surfaces of support. This can be achieved by simply leaving the structure directing agents in the pores before supporting metal particles. Side reactions caused by diffusion resistance are minimized, and moreover, the involatile byproducts, if any, do not produce additional diffusion resistance. Such a simple yet effective architecture control significantly prolongs the lifetime of Au/TS-1 catalyst in direct propene epoxidation with H2 and O2. From these demonstrations, it is important to design purpose-orientated zeolite synthesis to enhance mass transfer, for which the architecture plays a key role. Address State Key Lab of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Corresponding author: Zhou, Xinggui ([email protected]) Current Opinion in Chemical Engineering 2015, 9:42–48 This review comes from a themed issue on Reaction engineering and catalysis Edited by Marc-Olivier Coppens and Theodore T Tsotsis http://dx.doi.org/10.1016/j.coche.2015.07.009 2211-3398/Published by Elsevier Ltd.
Current Opinion in Chemical Engineering 2015, 9:42–48
Introduction Zeolites are three dimensional crystalline porous solids possessing pore channels normally less than 2.0 nm, which lay in the microporous range according to the IUPAC classifications. They are widely used as catalysts, catalyst supports, adsorbents for gas separation/purification, or ion-exchangers in laundry powders or treatment of toxics. On one hand, the successful use of zeolites is largely attributed to their crystalline framework structures that endow them high (hydro) thermal stability, strong acidities, tunable Si/Al ratios, and easy isomorphous substitution by other metals (B3+, Fe3+, Ga3+, Ti4+, Ge4+, among others). Their precise pore apertures and cavity structure make them unique shape-selective catalysts and adsorbents. Henceforth, they are also referred to as molecule sieves. On the other hand, artificial zeolites that are mass produced nowadays at industrial scale are comprised of micron-sized crystals, and the solely presence of micropores brings about mass transfer problems that is associated with slow diffusion in micropores. Moreover, bulky molecules whose sizes exceed the size of micropores are denied access to the micropores and thus the catalytic reactions only take place on the external surface. These drawbacks have limited the applications of zeolites toward treatment of large molecules and also called upon research effort to alleviate such problems. Several synthetic strategies for nanozeolites [1], hierarchically porous zeolites (HPZs) [2–6], extra-large pore zeolite [7], or 2D structured zeolites (through top-down delamination [8–10], bifunctional structural directing agent [11,12], or crystallization control [13]) have been reported so far. Recently, desilication and recrystallization methods to generate HPZs have appeared in commercially viable pilot scale demonstrations [14,15]. From the industrial viewpoint, the appealing fabrication methods should be simple and easy for scaling-up, and use low-cost starting materials. In addition to direct use as solid acid catalysts, zeolites are also applicable as metal supports for bifunctional catalysts. In this case, the spatial distribution of metal centers can affect the mass transfer and accessibility as well. Consequently, the position of metal centers can have substantial influences over catalytic performances. To achieve reactant shape selectivity, it is desirable to synthesize metal@zeolites structure whereby metallic particles core are fully encapsulated by zeolite shells, as recently exemplified by Goel et al. [16] and Bao et al. [17]. For reactions that metal-support synergy is important but no shape-selectivity is required, it is important to enhance the mass transfer by placing metallic particle at the external surfaces of zeolitic crystals. www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 43
In view that the importance of pore-networks for hierarchically nano-porous and macro-porous systems have been rationalized by Rao and Coppens [18,19], herein we underline the importance at the micro-porous and meso-porous scale to catalytic performance. In this perspective article, we intend to give a brief review on the preparation protocols of HPZs that have been achieved recently, with anemphasis on ‘pore-architecture’ engineering through non-classic orientated attachment (OA) crystallization. We also give some successful examples on zeolites supported metal catalysts design, where ‘spatialdistribution’ engineering will be underlined. By both types of catalysts design, we manifest that the ‘architecture’, alternative to composition, is an important way to enhance mass transfer or to reduce mass transfer effect, and can have beneficial consequences on the performance of the catalytic processes.
Hierarchically porous zeolites: porearchitecture engineering The mass transfer and reaction kinetic interplay has been recognized in reaction engineering for a long time and the theoretical basis to understand the effect of mass transfer was established by E.W. Thiele: The Thiele modulus dictating the utility of zeolite catalysts. As the diffusion for a given molecule in a particular micropore channel is fixed, the enhancement of mass transfer for nanozeolites or HPZs stems from a shortened diffusion path [20]. Besides, for HPZs, more and more evidences clearly manifest that it is not the presence of auxiliary pores that is important, it is crucial whether the pores are connected to the external surface or not [21,22,23,24]. Weitkamp et al. [23] have demonstrated through Pulse Field Gradient (PFG) NMR technique that the intracrystalline mesopores in USY zeolites has negligible effect for mass transfer of n-octane and 1,3,5-triisopropylbenzene with respective to conventional Y zeolites. Pe´rez-Ramı´rez et al. [21,24] have reported the use of positron annihilation lifetime spectroscopy (PALS) to measure pore connectivity and correlate it with catalytic behavior. It has been found that the connectivity determines the ‘quality’ of mesopores, and the conclusion is in line with other experimental observations [14,22]. The above considerations highlight the importance of zeolite crystal size and pore-connectivity to the enhancement of mass transfer, as both factors determine to which extent improvement can be made. Retrospectively, the first HPZ ZSM-5 with convincing evidence for two sets of porosity has been reported by Jacobsen et al. [4] using carbon black Black Pearls 2000 as hard templates. Afterwards, several soft templating routes using functionalized silanes, cationic polymers or amphilic silanes have been mapped out to generate HPZs [25– 28]. Meanwhile, Pe´rez-Ramı´rez et al. [29] have reported the low-cost postsynthetic desilication method to generate several HPZs, and the pore-connectivity as well as the www.sciencedirect.com
catalytic merits have been demonstrated recently [2,24]. The top-down methods using layered zeolitic prephase have also been developed over the years, and the method is applicable to a series of framework structures [10]. More recently, a series of designed bifunctional structure directing agents (SDAs) have been found to be able to tailor the thickness of zeolite layers down to unit cell scale by Ryoo et al. [12], and the HPZs obtained have demonstrated superior performance in heterogeneous catalysis, for instance, the prolonged lifetime in reactions. Tsapatsis et al. [13] demonstrated another possibility to control the morphology of intergrowth of MFI and MEL structure by a co-templating strategy, where the branching of growth is tailorable to afford a house-ofcards structure. A method to create mesoporous zeolite composites without the need for separate SDA to generate the mesopores has already introduced by Wang and Coppens in a series of articles [21,22,23,24,25–32]. As more extensive reviews have appeared [3,5,8,33], we will not elaborate the topic of HPZ synthesis. Recently, a series of synthetic protocols have been developed in our lab to generate HPZs with diverse topologies [34,35–38]. An interesting finding from the monitoring of zeolite beta crystallization in a dry gel conversion (DGC) system is that it observes the non-classic OA crystallization pathway. The starting gel undergoes morphology changes during steam assisted crystallization (SAC) treatment, and evolves into discrete tiny particulates of 20–30 nm in 6 h. These particulates aggregate in to nanozeolite assemblage of 90–200 nm in ca. 16 h. Bulky zeolite crystals of >200 nm are finally formed as a result of further alignment. The concurrent X-ray diffraction (XRD), Infrared (IR), Scanning electronic micrographic (SEM) and transmission electronic micrographic (TEM) measurements indicate that a temporal overlap of order development has occurred. The whole process is sketched in Route 1 of Figure 1. Evidently, the crystallization can be attributed to the OA growth mechanism and the tiny particulates can be regarded as building blocks, which are transformed into big crystals after disappearing of grain boundaries. The same phenomenon has also been observed by Matsukata et al. [39] and Bein et al. [40], but the formation mechanism has not been further explored, partly because the OA growth mechanism has not been widely accepted in zeolite community. The OA growth mechanism allows us to design new routes to fabricate HPZs, by interrupting the growth process after the formation of protozeolite particulates and still before their further growth by aggregation. Two types of organosilanes have been utilized for this purpose. When hexadecyltrimethoxysilane has been introduced into the precrystallized dry gel by simple impregnation in ethanol solution and drying in the open, a resumed crystallization has afforded a hierarchical beta with mesopore size between 20 and 60 nm (Route 2 of Figure 1) [34]. Hexadecyltrimethoxysilane resembles surfactants Current Opinion in Chemical Engineering 2015, 9:42–48
44 Reaction engineering and catalysis
Figure 1
1 500 nm
2
500 nm OMe Si OMe
3
OMe OMe
MeO MeO MeO
Si
Si OMe OMe
500 nm
Current Opinion in Chemical Engineering
The crystallization of beta in a dry gel conversion process obeys the non-classic orientated attachment mechanism (1), modification of the process to produce hierarchical beta (2) and nanozeolite beta (3) with the assistance of surfactant-like and bola-type organosilanes, respectively. The SEM images for these zeolites are shown parallel to the cartoons.
in that it can induce the self-assembly of zeolitic particulates, and consequently a 100–160 nm sized assemblage consists of 20–40 nm nanozeolite beta are obtained. When a bola type organosilane, bis(trimethoxysiyl)octane, is charged into the system in a similar way (Route 3 of Figure 1), uniform 35 nm nanocrystals of beta are obtained instead. The attachment of primary nanozeolites into bulky crystals is completely inhibited as the two ends of bis(trimethoxysiyl)octane are condensable with zeolite surfaces. Pore-connectivity information obtained by TEM measurement of metallic Pt-replica synthesized via a nanocasting strategy reveals that the auxiliary mesopores are well-connected, that is, the ‘qualities’ of the mesopores are justified. The small primary crystal size of zeolite and the improved pore-connectivity has contributed to an increase in catalytic activity for Pt/ beta in hydroisomerization of n-heptane. A jump of nheptane conversion from 56.0% to 67.2% and 63.3% has been achieved by Pt supported on conventional beta, assemblage beta and nanocrystal beta, respectively [34]. An improvement in Pt dispersion has also been observed, as a result of larger surface area for HPZ supports. Co¨lfen and Antonietti [41] have categorized the crystallization mechanism in nature into three types: classic LaMer mechanism, OA mechanism and mesocrystal formation mechanism. OA mechanism occurs when large number of nucleates are formed and subsequent growth takes place as a result of mutual alignment of primary particulates. The OA mechanism has been witnessed for a variety of metal, metal oxides, or hybrid system, but examples to synthesize porous solids in such scenario has been rarely reported. Organic additives introduced Current Opinion in Chemical Engineering 2015, 9:42–48
into beta synthesis interact with nanocrystals to form hybrid mesocrystals in an analogous way to the mineralization of some natural materials such as nacre. Seeing from the mineralizing standpoint, the artificial synthesis of zeolite pioneered by Richard Barrer has started from mimicking the mineralization of natural zeolites in hydrothermal conditions [42]. It is herein demonstrated that one can learn from nature to tailor the mesoscale morphology of zeolites in a non-classic crystallization process. It is speculated that zeolite nucleation rates in highly condensed gels are fast as a result of high degree of supersaturation. A large number of uniform tiny crystallites are therefore formed in the early state of crystallization, owing to the fast nucleation rate and short nucleation duration. The low water content limits the complete dissolution of precursors ingredients, and refrains the long distance transport of nutrients to nucleates surfaces. Under such a scenario, OA growth predominates in high supersaturation systems such as DGC. Contribution from classic LaMer mechanism, if there is any, will be negligible for the comparably slow rate. As the DGC route developed by Xu et al. [43] have now been found to be a versatile route to synthesize zeolites and zeotype materials, the OA pathway can be extrapolated to other similar systems, or even to low water (H2O/SiO2 < 10) synthesis of zeolites, where HPZs with controlled architecture can be fabricated by interrupting the attachment process.
Zeolite supported metal: metal position engineering Besides working as catalysts, zeolitic materials are often used as metal support to generate bifunctional or cascade catalysts. Metal elements are often introduced into zeolite www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 45
supports via ion-exchange, incipient wetness, or adsorption of metal precursors. If the architecture building process is not well designed and uncontrolled, formation of ill-defined metal nanoparticles, as well as poor spatial distribution can frequently occur. Confining metal nanoparticles inside the zeolite channels could achieve various objectives, such as (1) distributing highly dispersed ultrasmall metal particles [44], (2) preventing metal nanoparticles from aggregation at high temperatures [45], and (3) selectively allowing reactants to access metal surface to achieve shape selectivity. Several strategies have been postulated to achieve the above purposes. Bein et al. [44] demonstrated that Pd, Pt, and Cu amine complexes could be employed as SDA to synthesize ultrafine nanoparticles encapsulated by EDI zeolites. Wu et al. [45] reported that core/shell-structured TS-1@mesoporous silica synthesized through a layer-by-layer approach could prevent the Au nanoparticles located in the shell (i.e., mesoporous silica) from aggregation. Choi et al. [46] used a 3-mercaptopropyl modified silane to include Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite, by which the accessibility to metal cores are restricted by the molecular sieving effect of zeolites. Goel et al. [16,47] demonstrated that it is possible to fabricate such architectures for small pore zeolites (for instance, LTA, ANA, GIS) via ligand-stabilized precursor control, or for medium pore zeolites via zeolite transformation routes. Core@shell cascade catalysts for syngas conversion in Fischer–Tropsch reaction to break the Anderson–Schulz–Flory (ASF) distribution has been reported by Tsubaki et al. [48], where metal centers convert syngas into alkanes and acid sites on zeolite catalyze isomerization and cracking of primary products into isoparaffins.
Metallic active sites that reside on the external surface of zeolitic crystals can be easily accessible. Side effects from the mass transfer resistance in the micropore channels, such as reduced metal usage and accelerated consecutive side reactions, can be minimized. This concept is critical to enhance the stability of microporous catalysts which easily suffer from deactivation by micropore blocking [49–53]. Pore blocking deactivation is quite common in processes such as oxidation and oligermization since the accessibility for reactants in the interior crystal is hindered by configuration diffusion through the micro-channels, which can cause catalyst deactivation if side reactions leading to coke deposition. For Ni-based zeolites, long chain oligomers could be formed on high density of acid sites, leading to pore blocking and catalyst deactivation [50]. This is also found to be the case in TS-1 supported Au catalysts for direct propene epoxidation with H2 and O2, a green, simple and economic process for the worldwide demanding propylene oxide (PO). After diagnosis of the cause for catalyst deactivation (i.e., micropore blocking), we have designed a catalyst preparation protocol to position Au nanoparticles exclusively on the external surface of TS-1 crystals [52,54,55], as shown in Figure 2. Unlike the frequently used deposition–precipitation method that employs calcined TS-1 as support, we have chosen to use an assynthesized SDA-containing TS-1 for Au deposition, such that all the internal micropores are ‘locked’ and Au nanoparticles could only be deposited on the exterior surface of TS-1. Such a design has been found to be a key factor to enhance the long term stability of the catalyst, as the catalyst has exhibited much higher stability over 30 h time-on-stream tests than traditional Au/TS-1 catalyst
Figure 2
180 PO formation rate (gPO·h–1·kgCat–1)
O
O2,H2
160
140 Au/TS-1-B 120 O
Au/TS-1-O
O2,H2
100 0
5
10
15
20
25
30
Time-on-stream (h) Current Opinion in Chemical Engineering
Schematic diagram of Au locations and performance of Au/Uncalcined TS-1 (high stability) and Au/TS catalysts (severe deactivation by micropore blocking). www.sciencedirect.com
Current Opinion in Chemical Engineering 2015, 9:42–48
46 Reaction engineering and catalysis
with Au clusters inside the micropores that rapidly deactivates.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
Summary In this short perspective, we underline pore-architecture control in producing hierarchical zeolites by orientated attachment crystallization and metal position control in synthesizing zeolite supported metal catalysts, both of which are for enhanced mass transfer and also for minimized side effects from mass transfer resistance. We have shown that it is possible to achieve controlled pore-architectures in non-classic crystallization pathway, with the merits of desirable small primary particle size and preferable pore-connectivity that are crucial for mass transfer. Most previously known methods to generate HPZs are based on the classic LaMer mechanism with the assistance of templates, while the controlling of porosity and morphology by taking advantage of their formation mechanism has been largely unexplored. Efforts to control mesoscale structure of zeolites may be shifted from judicious choice of SDAs to choice of crystallization conditions whereby an OA growth predominates. The selection of organic additives can be used to tune the pore size, pore connectivity or the hierarchical structure of building blocks. The synthetic strategy may also be applied to fabricate other porous crystals that expands our toolkit of porous solids syntheses. The importance of metal particles position on a supported catalyst has frequently been neglected as a factor that influences catalysis, whereas it plays decisive role in a number of reactions. We demonstrate that the metal particle position over zeolite supports can be located exclusively on the external surface, through which the micropore blocking induced deactivation can be prevented. As the use of organic SDAs is a common practice for zeolite synthesis, these protocols are of general value for other similar systems. Further efforts are to be stepped up to enlarge the external surface area of support (by creating auxiliary mesopores or decrease the crystal size of support), so as to increase the effective dispersion of supported metal to maximize its utility. As a final remark, we believe that at the edges where catalysis and reaction engineering meet, tremendous opportunities are available for chemical reaction engineering researchers to explore and exploit. Besides the strategies illustrated in this perspective, there is plenty of room for catalyst architecture manipulation to boost the performance of the catalytic reaction.
Acknowledgements KZ is grateful for the financial support from Fundamental Research Funds for the Central Universities (WB 1213004-1), and New Century Excellent Talents in University (NCET-11-0644). XZ is in grateful for financial support from National Natural Science Foundation of China (91434117). Current Opinion in Chemical Engineering 2015, 9:42–48
1.
Valtchev V, Tosheva L: Porous nanosized particles: preparation, properties, and applications. Chem Rev 2013, 113:6734-6760. This is an updated review on the preparation of nanosized zeolites and zeotype materials.
2.
Verboekend D, Perez-Ramirez J: Design of hierarchical zeolite catalysts by desilication. Catal Sci Technol 2011, 1:879-890.
3.
Serrano DP, Pinnavaia TJ, Aguado J, Escola JM, Peral A, Villalba L: Hierarchical ZSM-5 zeolites synthesized by silanization of protozeolitic units: mediating the mesoporosity contribution by changing the organosilane type. Catal Today 2014, 227:1525.
4.
Jacobsen CJH, Madsen C, Houzvicka J, Schmidt I, Carlsson A: Mesoporous zeolite single crystals. J Am Chem Soc 2000, 122:7116-7117.
5.
Na K, Choi M, Ryoo R: Recent advances in the synthesis of hierarchically nanoporous zeolites. Microporous Mesoporous Mater 2013, 166:3-19.
6.
Zhu Y, Hua ZL, Zhou XX, Song YD, Gong Y, Zhou J, Zhao JJ, Shi JL: CTAB-templated mesoporous TS-1 zeolites as active catalysts in a desulfurization process: the decreased hydrophobicity is more favourable in thiophene oxidation. RSC Adv 2013, 3:4193-4198.
7.
Jiang JX, Jorda JL, Yu JH, Baumes LA, Mugnaioli E, DiazCabanas MJ, Kolb U, Corma A: Synthesis and structure determination of the hierarchical meso-microporous zeolite ITQ-43. Science 2011, 333:1131-1134.
8.
Corma A, Diaz U, Domine ME, Forne´s V: AlITQ-6 and TiITQ-6: synthesis characterization, and catalytic activity. Angew Chem Int Ed 2000, 39:1499-1501.
9.
Corma A, Diaz U, Domine ME, Forne´s V: New aluminosilicate and titanosilicate delaminated materials active for acid catalysis, and oxidation reactions using H2O2. J Am Chem Soc 2000, 122:2804-2809.
10. Wu P, Ruan JF, Wang LL, Wu LL, Wang Y, Liu YM, Fan WB, He MY, Terasaki O, Tatsumi T: Methodology for synthesizing crystalline metallosilicates with expanded pore windows through molecular alkoxysilylation of zeolitic lamellar precursors. J Am Chem Soc 2008, 130:8178-8187. 11. Choi M, Na K, Kim J, Sakamoto Y, Terasaki O, Ryoo R: Stable single-unit-cell nanosheets of zeolite MFI as active and longlived catalysts. Nature 2009, 461:246-249. 12. Na K, Jo C, Kim J, Cho K, Jung J, Seo Y, Messinger RJ, Chmelka BF, Ryoo R: Directing zeolite structures into hierarchically nanoporous architectures. Science 2011, 333:328-332. 13. Zhang XY, Liu DX, Xu DD, Asahina S, Cychosz KA, Agrawal KV, Al Wahedi Y, Bhan A, Al Hashimi S, Terasaki O et al.: Synthesis of self-pillared zeolite nanosheets by repetitive branching. Science 2012, 336:1684-1687. 14. Li K, Valla J, Garcia-Martinez J: Realizing the commercial potential of hierarchical zeolites: new opportunities in catalytic cracking. ChemCatChem 2014, 6:46-66. One perspective article on the industrial applications of hierarchical zeolites with an emphasis over their own contribution. 15. Verboekend D, Mitchell S, Perez-Ramirez J: Hierarchical zeolites overcome all obstacles: next stop industrial implementation. Chimia 2013, 67:327-332. Another perspective article describing the commercial use of hierarchical zeolites, with a focus on desilication based protocols. 16. Goel S, Zones SI, Iglesia E: Encapsulation of metal clusters within mfi via interzeolite transformations and direct www.sciencedirect.com
Manipulating the architecture of zeolite Zhu and Zhou 47
hydrothermal syntheses and catalytic consequences of their confinement. J Am Chem Soc 2014, 136:15280-15290. An important article demonstrating the possibility to encapsulate metal clusters to achieve reactant shape selectivity. 17. Bao J, He J, Zhang Y, Yoneyama Y, Tsubaki N: A core/shell catalyst produces a spatially confined effect and shape selectivity in a consecutive reaction. Angew Chem Int Ed 2008, 47:353-356. 18. Rao SM, Coppens M-O: Increasing robustness against deactivation of nanoporous catalysts by introducing an optimized hierarchical pore network-Application to hydrodemetalation. Chem Eng Sci 2012, 83:66-76. 19. Rao SM, Coppens M-O: Mitigating deactivation effects through rational design of hierarchically structured catalysts: application to hydrodemetalation. Ind Eng Chem Res 2010, 49:11087-11097. 20. Christensen CH, Johannsen K, Toernqvist E, Schmidt I, Topsoe H, Christensen CH: Mesoporous zeolite single crystal catalysts: diffusion and catalysis in hierarchical zeolites. Catal Today 2007, 128:117-122. 21. Milina M, Mitchell S, Cooke D, Crivelli P, Pe´rez-Ramı´rez J: Impact of pore connectivity on the design of long-lived zeolite catalysts. Angew Chem Int Ed 2014:1591-1594. An article highlights the importance of pore-connectivity in hierarchical zeolites to the catalytic performances. 22. Thomas JM, Leary RK: A major advance in characterizing nanoporous solids using a complementary triad of existing techniques. Angew Chem Int Ed 2014, 53:12020-12021. The authors provide some new progress on the development of techniques to measure pore-connectivity. 23. Kortunov P, Vasenkov S, Karger J, Valiullin R, Gottschalk P, Elia MF, Perez M, Stocker M, Drescher B, McElhiney G et al.: The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales. J Am Chem Soc 2005, 127:13055-13059. 24. Milina M, Mitchell S, Crivelli P, Cooke D, Perez-Ramirez J: Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts. Nat Commun 2014, 5:3922. An interesting article correlates the catalytic lifetime of hierarchical zeolites to the pore-connectivity, together with a positron annihilation lifetime spectroscopy on pore-connectivity determination. 25. Wang H, Pinnavaia TJ: MFI zeolite with small and uniform intracrystal mesopores. Angew Chem Int Ed 2006, 45:76037606. 26. Xiao FS, Wang LF, Yin CY, Lin KF, Di Y, Li JX, Xu RR, Su DS, Schlogl R, Yokoi T et al.: Catalytic properties of hierarchical mesoporous zeolites templated with a mixture of small organic ammonium salts and mesoscale cationic polymers. Angew Chem Int Ed 2006, 45:3090-3093. 27. Serrano DP, Aguado J, Escola JM, Rodriguez JM, Peral A: Effect of the organic moiety nature on the synthesis of hierarchical ZSM-5 from silanized protozeolitic units. J Mater Chem 2008, 18:4210-4218. 28. Choi M, Cho HS, Srivastava R, Venkatesan C, Choi DH, Ryoo R: Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunable mesoporosity. Nat Mater 2006, 5:718-723. 29. Groen JC, Peffer LAA, Moulijn JA, Perez-Ramirez J: Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent. Chem Eur J 2005, 11:4983-4994. 30. Moller K, Bein T: Mesoporosity – a new dimension for zeolites. Chem Soc Rev 2013, 42:3689-3707. 31. Wang J, Vinu A, Coppens M-O: Synthesis and structure of silicalite-1/SBA-15 composites prepared by carbon templating and crystallization. J Mater Chem 2007, 17:42654273. 32. Wang J, Yue W, Zhou W, Coppens M-O: TUD-C A tunable, hierarchically structured mesoporous zeolite composite. Microporous Mesoporous Mater 2009, 120:19-28. www.sciencedirect.com
33. Chal R, Ge´rardin C, Bulut M, van Donk S: Overview and industrial assessment of synthesis strategies towards zeolites with mesopores. ChemCatChem 2011, 3:67-81. 34. Chen J, Hua WM, Xiao Y, Huo QS, Zhu KK, Zhou XG: Tailoring the structure of hierarchically porous zeolite beta through modified orientated attachment growth in a dry gel system. Chem Eur J 2014, 20:14744-14755. We demonstrate that zeolite beta crystallization obeys the non-classic orientated attachment growth mechanism, and it is more convenient to control the hierarchical structure within this framework. As dry gel conversion and low water synthesis are quite common, the mechanism and control method can be a general one in high supersaturation systems. 35. Zhang YC, Zhu KK, Duan XZ, Zhou XG, Yuan WK: The templating effect of an easily available cationic polymer with widely separated charge centers on the synthesis of a hierarchical ZSM-5 zeolite. J Mater Chem A 2014, 2:18666-18676. 36. Zhang YC, Zhu KK, Zhou XG, Yuan WK: Hierarchically porous zeolite beta synthesized via steam-assisted crystallization of silanized dry gel. Mater Lett 2014, 131:214-216. 37. Zhang YC, Zhu KK, Zhou XG, Yuan WK: Synthesis of hierarchically porous ZSM-5 zeolites by steam-assisted crystallization of dry gels silanized with short-chain organosilanes. New J Chem 2014, 38:5808-5816. 38. Zhang YC, Zhu KK, Duan XZ, Li P, Zhou XG, Yuan WK: Synthesis of hierarchical ZSM-5 zeolite using CTAB interacting with carboxyl-ended organosilane as a mesotemplate. RSC Adv 2014, 4:14471-14474. 39. Matsukata M, Osaki T, Ogura M, Kikuchi E: Crystallization behavior of zeolite beta during steam-assisted crystallization of dry gel. Microporous Mesoporous Mat 2002, 56:1-10. 40. Moller K, Yilmaz B, Jacubinas RM, Muller U, Bein T: One-step synthesis of hierarchical zeolite beta via network formation of uniform nanocrystals. J Am Chem Soc 2011, 133:5284-5295. The above two groups have explored the crystallization process of zeolite beta, and the findings are confirmative to the mechanism we disclosed in Ref. [34]. 41. Co¨lfen H, Antonietti M: Mesocrystals and Nonclassical Crystallization. In Mesocrystals and nonclassical crystallization. Edited by John Co¨lfen , Antonietti M. Wiley & Sons Ltd.; 2008:1-6. An comprehensive book describing the crystallization in nature, whereby the major crystallization mechanism and key theories are shown in the first 2 chapters. 42. Ravishankar RA, Martens JA, Jacobs P: The scientific legacy of the late Richard M. Barrer FRS. Microporous Mater 1997, 8:283-284. 43. Xu W, Dong J, Li J, Li J, Wu F: A novel method for the preparation of zeolite ZSM-5. J Chem Soc Chem Commun 1990:755-756. 44. Kecht J, Mintova S, Bein T: Nanosized zeolites templated by metalSamine complexes. Chem Mater 2007, 19:1203-1205. 45. Xu L, Ren Y, Wu H, Liu Y, Wang Z, Zhang Y, Xu J, Peng H, Wu P: Core/shell-structured TS-1@ mesoporous silica-supported Au nanoparticles for selective epoxidation of propylene with H2 and O2. J Mater Chem 2011, 21:10852-10858. 46. Choi M, Wu ZJ, Iglesia E: Mercaptosilane-assisted synthesis of metal clusters within zeolites and catalytic consequences of encapsulation. J Am Chem Soc 2010, 132:9129-9137. 47. Goel S, Wu Z, Zones SI, Iglesia E: Synthesis and catalytic properties of metal clusters encapsulated within small-pore (SOD, GIS ANA) zeolites. J Am Chem Soc 2012, 134:17688-17695. 48. Li X-G, Liu C, Sun J, Xian H, Tan Y-S, Jiang Z, Taguchi A, Inoue M, Yoneyama Y, Abe T et al.: Tuning interactions between zeolite and supported metal by physical-sputtering to achieve higher catalytic performances. Sci Rep 2013, 3:2813. 49. Lallemand M, Finiels A, Fajula F, Hulea V: Catalytic oligomerization of ethylene over Ni-containing dealuminated Y zeolites. Appl Catal A-Gen 2006, 301:196-201. 50. Lallemand M, Rusu OA, Dumitriu E, Finiels A, Fajula F, Hulea V: NiMCM-36 and NiMCM-22 catalysts for the ethylene oligomerization: effect of zeolite texture and nickel cations/ acid sites ratio. Appl Catal A-Gen 2008, 338:37-43. Current Opinion in Chemical Engineering 2015, 9:42–48
48 Reaction engineering and catalysis
51. Finiels A, Fajula F, Hulea V: Nickel-based solid catalysts for ethylene oligomerization – a review. Catal Sci Technol 2014, 4:2412-2426. 52. Feng X, Duan X, Yang J, Qian G, Zhou X, Chen D, Yuan W: Au/ uncalcined TS-1 catalysts for direct propene epoxidation with H2 and O2: effects of Si/Ti molar ratio and Au loading. Chem Eng J 2014. doi:10.1016/j.cej.
54. Feng X, Duan X, Qian G, Zhou X, Chen D, Yuan W: Au nanoparticles deposited on the external surfaces of TS-1: enhanced stability and activity for direct propylene epoxidation with H-2 and O-2. Appl Catal B-Environ 2014, 150:396-401. The controlled preparation of Au nanoparticles exclusively on external surface of zeolite TS-1 is reported, as a proven solution towards enhancement of catalyst stability for the reaction.
53. Lacarriere A, Robin J, S´wierczyn´ski D, Finiels A, Fajula F, Luck F, Hulea V: Distillate-range products from non-oil-based sources by catalytic cascade reactions. ChemSusChem 2012, 5:17871792.
55. Feng X, Duan X, Qian G, Zhou X, Chen D, Yuan W: Insights into size-dependent activity and active sites of Au nanoparticles supported on TS-1 for propene epoxidation with H-2 and O-2. J Catal 2014, 317:99-104.
Current Opinion in Chemical Engineering 2015, 9:42–48
www.sciencedirect.com