Hierarchically‐Ordered Materials

Porous materials, in particular microporous zeolites, are the working horses in industry mainly as catalysts, adsorbents or ion-exchangers. In order to further improve their performance and open new fields of application, one could take advantage of a well and on purpose designed hierarchical porous structures to overcome, i.e., transport limitation into the narrow, interconnected microporous channels with their special reactivity and selectivity properties. To this themed issue 18 reviews and full papers have been contributed from leading research groups in the field of hierarchical materials and in particular zeolites. In this volume, we have collected the latest advances with respect to the synthesis, characterization and application of hierarchical porous materials with a special focus on zeolites. Particular attention is paid to the development of structure-property relationships, which requires the use, and further development of cutting-edge characterization techniques. The idea to this themed issue was born on a workshop on hierarchical materials (HiMat2019) at the FAU Erlangen-Nürnberg in the fall of 2019, which was organized by the guest editors. To this themed issue 18 reviews and full papers have been contributed from leading research groups in the field of hierarchical materials and in particular zeolites. Historically, the expression hierarchy goes back to the Greek words “hieros” (scared) and “archein” (rules) describing the ranking of the sanctifications in churches due to their importance. Thus, the expression “hierarchy” had not been created with respect to materials or porous systems in particular. However, hierarchy is found everywhere; it is a basic organization principle in nature, e.g. in trees, leaves, wood, bones, blood circuits, just to name a few examples. However, it is found also as a more general principle in ecological, social, economic, and even in technical systems. Thus, many materials derive their function throughout a hierarchical organization of their structural elements. Self-organization processes typically lead to these specific structures. Hierarchy is always related to properties such as strength, chemical compositions, or transport related properties, like flux, flow profiles etc. Thus, also the porosity of natural systems like in bones or the structure of blood circle and the lounge are optimized under the aspects of strength or flux profiles, respectively. Porous materials, i.e., in particular microporous zeolites, are the working horses in industry mainly as catalysts, adsorbents or ion-exchangers. In order to further improve their performance, one could take advantage of a well and on purpose designed hierarchical porous structures to overcome, i.e., transport limitation into the narrow, interconnected microporous channels with their special reactivity and selectivity properties.[1, 2] The introduction of hierarchy into the zeolite crystals brought out even better the intrinsic advantages of zeolites for certain reactions. However, the adaptation of hierarchical properties into inorganic, material oriented systems such as zeolite-based catalysts and adsorbents opens new options for the target oriented design of zeolites which started just about 20 years ago initiating a new research area in this field.[3, 4] In Figure 1, the number of papers published in the last 30 years, mentioning the key words “hierarchical materials” and “hierarchical zeolites” are compared starting from the late 90s and early 2000s. An exponentially increasing number of papers is observed starting with the years 2000 and 2005, respectively. The first hierarchical zeolite-containing systems were prepared, like in natural systems, as composites, however, based on preformed diatoms and/or porous glasses as silica supports. Depending on the reaction conditions, both silica-based materials can act as support and/or as raw material for zeolite formation. Nevertheless, in the literature on porous materials, the expression “hierarchy” with respect to their porosity is very often used in a very broad sense for systems which contain just a certain number of different pore types in the form of bi- or multimodal pore size distributions, independently if and how the two (or more) different pore systems are interconnected. However, in the narrow sense, the existence of an interplay between the different pore levels has to be evidenced before one can name the overall pore system “hierarchical”. Accordingly, we can distinguish zeolites, which are just (i) microporous (“zeolites” per definition) and (ii) hierarchical zeolites which can either contain a combination out of interconnected zeolitic microspores with either mesopores (“mesoporous zeolites”, or “zeolites with mesopores”), just macropores (“zeolites with macropores” or also “macroporous zeolites”) or even combinations thereof (multiple systems). From the viewpoint of the guest editors, the interconnectivity of the pore systems is a precondition for a material to be named hierarchical! Materials which contain just pores of two and more different sizes (or size distributions) are not necessarily hierarchical. Therefore a targeted and thorough characterization is very important for this judgement, the classification and the assessment of their applicative properties, which should show more than just an additive effect of the additional pore system. In this context, the topic of this themed issue “Hierarchically-ordered materials” bridges a wide range of aspects of hierarchical materials from a theoretical view on porous systems, their preparation strategies and characterization methods up to their (potential) applications. The idea of the presented topics follows this line to provide a comprehensive and unique overview over all these different aspects of hierarchy in porous materials for the junior scientists – the students and post-docs – as well as for any other interested member of the scientific community in form of tutorial and scientific contributions. The issue compiles new insights into the synthesis and targeted modification of hierarchical materials, selected characterization techniques such as electron tomography, xenon NMR spectroscopy and physical adsorption and mercury porosimetry as well as novel application aspects. In an attempt to bridge this areas, which are often reviewed separately, the guest editors have classified hierarchy and evaluated different preparation strategies providing their view on the different types of zeolite containing hierarchically-ordered structures, see article number 2001841. The authors generalize and discuss these strategies as a “toolbox” consisting of bottom-up and top-down preparation methods to design pure zeolitic or zeolite-containing composite material with tailored hierarchical porosity. In order to establish structure-property relationships, synthetic efforts have to be combined with structural characterization and application testing. In addition to the discussion of hierarchical zeolites that possess only mesopores in addition to the intrinsic micropores, Weissenberger and co-workers review the preparation and catalytic properties of hierarchical materials that possess mainly additional macropores, see article number 2001653. They conclude that materials with additional larger pores than mesopores have some intrinsic advantages that make them attractive for catalytic as well as adsorptive applications. Thus, e.g. unselective catalytic reaction on the unspecific sites at the large surfaces of mesopores and the possibility of a capillary condensation in mesopores can be minimized or completely suppressed in materials with a direct combination of micro- and macropores. Yu and co-workers review the creation of hierarchical titanosilicate TS-1 zeolites with at least two levels of porosities that have aroused growing interest in recent years due to its improved catalytic performance by overcoming steric and diffusional hinderances, see article number 2001095. In their review, recent important progress in the construction of hierarchical TS-1 zeolites as well as effective characterization methods for the Ti species and the hierarchical zeolite structures are described. The impacts of hierarchical structures on the creation of active Ti species and improvement of catalytic performances in the oxidative desulfurization and epoxidation reactions are highlighted. Current challenges are the fine adjustment of the porosities, connectivities, and ordering of the secondary pores which are important for the future development of hierarchical TS-1 zeolite catalysts. In a related original research paper, Přech and co-workers report on the preparation, characterization and catalytic properties of an MFI titanosilicate nanosponge (nsTS-1), see article number 2001288. nsTS-1 is prepared by post-synthesis titanium impregnation of a pure silica MFI nanosponge, producing a fully crystalline and highly open catalyst with a high total pore volume. Due to the high external surface of the nanolayered domains, isolated framework-attached Ti sites are formed rather than extra-framework TiO2. As a result, the structure of nsTS-1 is more defined and uniform than the structure of other layered titanosilicate catalysts (e.g., silica– titania pillared TS-1). The nanosponge TS-1 samples were tested in cis-cyclooctene, norbornene, and linalool epoxidation using hydrogen peroxide and exhibited superior performance with respected to conversion and selectivity to the classical TS-1 catalyst. García-Martínez and co-workers review surfactant-templating as a versatile method to introduce tailored intracrystalline mesoporosity in zeolites, see article number 2001388. This method has been proven to be extremely effective to finely control both the amount of mesoporosity and the size of their mesopores, while maintaining the main features of the original zeolite, such as crystallinity, acidity, and hydrothermal stability. Surfactant-templating has been successfully applied to develop intracrystalline mesoporosity into a variety of zeolite structures including FAU, BEA, MOR, LTL, and MFI. The improvement in both the textural and diffusional properties of the zeolite results in excellent catalytic performances in different industrially relevant processes, ranging from fluid catalytic cracking to the synthesis of active pharmaceutical ingredients. As a result of the excellent catalytic performances shown by the surfactant-templated mesoporous Y zeolite, this catalyst is currently being used in several refineries. The communication by Huang and co-workers proposes a simple and efficient method to synthesize hierarchical ZSM-5 zeolites with additional mesopores of different size by employing water-soluble carbon dots (CDots), see article number 2001846. The size of the carbon dots is varied between 5 and 23 nm, allowing to introduce mesopores of different size into one catalyst. Notably, a hierarchical ZSM-5 catalyst with micropore of 0.55 nm, two small mesopores of 4.8 and 7.4 nm, and one large mesopore of 17.5 nm show excellent catalytic performance in the cracking of 1,3,5-triisopropylbenzene and improved stability compared to the microporous parent zeolite. The enhanced catalytic activity is attributed to the more efficient diffusion of reactants and products in the crystals possessing mesopores of different size, improved coking resistance and increased accessibility of the acid sites. The paper by Valtchev and co-workers discusses chemical etching with aqueous solutions of NH4F and NH4HF2 as an alternate pathway to generate secondary porosity in zeolite L, see article number 2000348. Tuning the etching conditions allows to generate hierarchical zeolites without reducing the micropore volume of the parent zeolite L. The reported experimental data show that an 1 wt.-% NH4HF2 solution is an efficient etchant able to generate mesopores without affecting the parent zeolite properties as lower amounts of fluoride source are required than with concentrated (20–40 wt.-%) NH4F solutions. A new synthetic strategy to prepare nanosized zeolites with faujasite (FAU) and sodalite (SOD) topology is reported by Mintova and co-workers, see article number 2000634. The proposed two-step synthesis procedure includes the formation of colloidal suspensions followed by separation of the amorphous precursor nanoparticles, and their subsequent transformation into nanosized crystals by treatment with alkali suspensions (NaOH) solutions only. In their review, Tiemann and Weinberger address the important topic of selective manipulation of hierarchical surfaces in multimodal ordered mesoporous materials, see article number 2001153. This means that when several distinct, i.e., hierarchical, types of surfaces or pore systems exist in a material, each of them may be chosen for manipulation. Several strategies were proposed to achieve this goal. Molecules or molecule assemblies can be utilized to temporarily protect pores or surfaces (soft protection), while manipulation occurs at the accessible sites. When several distinct functionalization sites are exposed, the reactivity/diffusion of a functionalization agent can be used for targeted surface manipulation. Obviously, the size of the guest species/ reagent also plays an essential role, as access to the pore system may be hindered or prevented entirely. Selective functionalization or loading with guest species may lead to improved functional materials for application, e.g., in adsorption/separation, energy conversion, or catalysis. Reinhard and co-workers report a laser-based procedure for the preparation of metallic hierarchical porous materials, viz. tin, copper, silicon, titanium, and tungsten surfaces, see article number 2000253. The impact of suitably tuned nanosecond laser pulses triggers a process in which laser-induced metal ablation and instantaneous recondensation of partially oxidized metals lead to cauliflower-like superstructures comprising a hybrid micro-/nanopatterning. Repeated scanning with the intense focused beam over the surface creates microstructures of hierarchically tunable porosity in a layer-by-layer design. The introduced concept enables convenient access to a variety of metallic hierarchical porous materials, which are key to performance in environmentally and technologically relevant areas like energy storage, and conversion, as well as sensing and catalysis. Five contributions are devoted to in depth characterization of hierarchically-organized zeolites, which is a prerequisite for understanding of structure-property relationships. Apeleo Zubiri and co-workers explore the capabilities of correlative 360° electron tomography (ET) and lab-based nano X-ray computed tomography (Nano-CT) enabling 3D analyses of volumes of up to (60 µm)³ with down to nm resolution.[5] This is demonstrated for zeolite particles with embedded macropores. By first applying the two techniques to the same particle the higher resolution and fidelity of ET are used to improve the segmentation of pore space in the Nano-CT reconstruction. Extended statistical relevance and access to interparticle pore space are obtained from reconstructions of larger particle agglomerates, using the large-field-of-view mode of the Nano-CT. The presented correlative approach enables real space analyses of important pore characteristics for comparison with complementary pore characterization techniques. Moreover, by investigating samples from different stages of the synthesis, 360°-ET and Nano-CT provide unique insights into the formation mechanism of porous materials. Investigations of guest diffusion in hierarchical porous materials are still in the stage of their very beginning. In this context, the review of Hwang and co-workers discusses mass transfer in hierarchical porous materials as a function of various parameters, notably including the diffusivities in the various pore spaces, their relative populations and the exchange rates, see article number 2000749. Their interplay is shown to be quantified in the two-region model of diffusion (Kärger equations). The underlying formalism is demonstrated to offer an excellent tool for assessing mass transfer in hierarchically-ordered porous materials. A comprehensive description of mass transfer, in parallel with the specification of the various limiting cases and their reflection by experimental measurements is crucial for the development of optimal strategies for performance enhancement for the technological exploitation of such materials. Wisser and Hartmann introduce 129Xe NMR spectroscopy of adsorbed xenon as one of the most powerful methods to probe pore sizes, pore geometry and pore interconnectivity in hierarchically-organized materials, see article number 2001266. Adsorbed to the pore walls, it acts as probe nucleus. In their (tutorial) review, an introduction into the basic principles of 129Xe NMR spectroscopy and the models developed to determine pore sizes in different materials are given. The possibilities and limitations of this method for obtaining insights into hierarchical structures of functional materials are highlighted and a review of recent works is presented. The tutorial review by Schlumberger and Thommes is devoted to the textural characterization of hierarchically-ordered materials such as mesoporous zeolites, see article number 2002181. The focus of this tutorial lies on the application of advanced physisorption methodologies for micro-mesopore analysis, but the paper also addresses the characterization of macroporosity by mercury porosimetry. Fundamental concepts and recent major advances in understanding the underlying mechanisms associated with the adsorption and phase behavior of fluids in porous materials are discussed in much detail. In conjunction with selected case studies, it is demonstrated how the application of advanced physisorption methodologies allows (i) for the determination of reliable surface areas, pore volumes, pore size distributions and (ii) for obtaining information about the pore network characteristics. Rather than serving as a broad review of literature, this tutorial review offers guidance for an advanced characterization of nanoporous materials by physisorption and mercury intrusion/extrusion. The article of Coppens and co-workers reviews multiscale modelling approaches to optimize overall catalytic performance, see article number 2001409. It provides a general methodology and rules of thumb to guide catalyst synthesis with optimal pore network characteristics. Inspiration is taken from nature, such as the structure of leaves and tissues, with similar requirements and associated features. In optimal hierarchically-structured zeolites, the added macro-/mesopore volume fraction, connectivity, crystal size and minimum wide pore size are crucial, while the broad pore size distribution is secondary. 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