Recent advances in zeolite-encapsulated metal catalysts: A suitable catalyst design for catalytic biomass conversion

Journal Pre-proofs Review Recent Advances in Zeolite-Encapsulated Metal Catalysts: A Suitable Catalyst Design for Catalytic Biomass Conversion Mutjalin Limlamthong, Alex C.K. Yip PII: DOI: Reference:

S0960-8524(19)31718-3 https://doi.org/10.1016/j.biortech.2019.122488 BITE 122488

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Bioresource Technology

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24 September 2019 9 November 2019 12 November 2019

Please cite this article as: Limlamthong, M., Yip, A.C.K., Recent Advances in Zeolite-Encapsulated Metal Catalysts: A Suitable Catalyst Design for Catalytic Biomass Conversion, Bioresource Technology (2019), doi: https://doi.org/ 10.1016/j.biortech.2019.122488

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Recent Advances in Zeolite-Encapsulated Metal Catalysts: A Suitable Catalyst Design for Catalytic Biomass Conversion Mutjalin Limlamthong, Alex C. K. Yip* Department of Chemical and Process Engineering, The University of Canterbury, Christchurch 8041, New Zealand.

Corresponding author: [email protected]

Abstract Metal clusters and nanoparticles, which have been used to tune the acidity of zeolite support, are beneficial for promoting the catalytic performance of various reaction processes, including biomass conversion. However, catalytic instabilities resulting from metal coalescence, sintering and leaching are major problems that need to be resolved. Therefore, metal encapsulation within the zeolite structure has been proposed as a feasible solution for this issue, particularly for biomass conversions that require high temperatures. In this current review, recent developments in metal confinement techniques are described along with experimental examples of biomass upgrading reactions. The present and future perspectives of zeolite-encapsulated metal catalysts in biomass conversions are also given.

Keywords: Metal encapsulation; Zeolite; Biomass conversion; Metal stability; Inter-zeolite transformation.

1.

Introduction Biomass waste has led to a number of negative effects on the environment and human

health. Air pollution and emission of chemicals, such as methane which also contribute to the greenhouse effect (Hobbs et al., 2018; Lisboa & Lansing, 2013), have been unpreventably escalated due to the exponential growth of global population and industrial development. It was statistically reported that more than 340 million tonnes of dry lignocellulosic residues were generated in the United States in 2012 (Ge et al., 2016), while approximately 25 to 50% of food produced was disposed of as waste via the supply chain (Lundqvist et al., 2008). Despite the disadvantages of biomass waste, volarisation of biomass has received much attention because of its benefit to sustainability; therefore, methods to reduce waste to mitigate the abovementioned problems and utilise these renewable resources have been proposed. Given the significance of the problems, many attempts have been made to reduce organic waste (Ilham & Saka, 2009; Lu et al., 2012; Marda et al., 2009; Zhang et al., 2009). However, several drawbacks regarding the non-catalytic process have been reported. It is clear that the production of low-value light hydrocarbons, e.g., methane, has been significantly improved through the non-catalytic pyrolysis of lignin (Patwardhan et al., 2011; Wang et al., 2009). A decline in the fuel quality of the products has also been detected during the non-catalytic autothermal gasification reaction of woody biomass (Yoon et al., 2011). In addition to the biomass conversion process without a catalyst, the instability of homogeneous catalysts is still a major challenge despite their unique reactivity and excellent selectivity (Deuss et al., 2014). During recent decades, microporous substances such as zeolites have been promoted as one of the most prevalently used materials for a variety of applications. Zeolites are either natural or synthetic aluminosilicates constructed from forming a linkage between the oxygen atoms of silicate and aluminate tetrahedral, resulting in more than two hundred crystalline lattice structures (Baerlocher & McCusker, n.d.). The electroneutrality of cations is achieved by replacing the

negative charge in the framework where Al3+ is substituted with Si4+ (van Bekkum et al., 2001), making zeolites an excellent candidate for a wide range of applications, for example, as adsorbents (Bae et al., 2013) and ion-exchangers (Malekian et al., 2011; Xue et al., 2012). Furthermore, zeolites are of interest as catalysts (Jia et al., 2018; Jia et al., 2019) and molecular sieves (Huang & Caro, 2011) owing to their uniform zeolitic arrangements that endow them with unique characteristics, such as high thermal and mechanical stability, high specific surface area, tuneable acidity, shape selectivity, and corrosion resistance (Cheng et al., 2001; García-Martínez et al., 2012; Kanezashi et al., 2007; Olsbye et al., 2012; Seo et al., 2013; Smit & Maesen, 2008; Teketel et al., 2011). Considering their advantages, zeolites have been applied as catalysts in the conversion of biomass into more valuable resources. Because of their acidic properties, zeolites have been reported to play an important role in deoxygenating condensable components in biomass, which increases the instability of bio-oil and promotes production of aromatics through a fast pyrolysis process (Mihalcik et al., 2011). Multiple acid sites on zeolite supports have attracted attention for dealing with different types of biomass sources, including lignocellulose and lipids. The number of Brønsted and Lewis acid sites can be tuned by the introduction of metals into the zeolites for different reactions (Heo et al., 2011; Huang et al., 2012; Kim et al., 2013). For instance, a Brønsted acid site is necessary for accelerating polycyclic aromatic hydrocarbon (PAH) formation from aromatic molecules, whereas a Lewis acid site is responsible for catalysing oligomerisation and cracking reactions. It has been suggested that metal-incorporated zeolites have fewer Brønsted acid sites than zeolites without metals. On the other hand, irreversible deactivation is generally observed as a result of metal coalescence, sintering and structural deformation (Luo et al., 2016). The type of zeolite used for biomass conversion should also be carefully considered, as previously mentioned, as the transformation may involve a diverse set of substrates, each with different favourable reaction

conditions. Thus, heterogeneous catalysts with multifunctional properties are required for such a conversion process (Khan et al., 2019). Many efforts have been made to maintain the zeolitic structure under harsh reaction conditions. One way to address this issue is to synthesise more stable zeolite crystals by mild dealumination of a pristine Y zeolite (Verboekend et al., 2012). The stability of ultra-stable Y (USY) for catalytic cracking of waste cooking oils into liquid hydrocarbon fuels has been addressed in the literature (Li et al., 2016a). This zeolite was used up to six times without any significant change in its X-ray diffraction (XRD) patterns, surface area, and pore properties. However, the problems arising upon metal agglomeration or metal loss have become a more difficult challenge to solve. The conventional methods of introducing metals into zeolites, such as ion-exchange, impregnation, and deposition-precipitation, have been published elsewhere (Che et al., 2019; Eibner et al., 2015; Fodor et al., 2015; Kikhtyanin et al., 2016; Kumar et al., 2019; Lee & Valla, 2017; Marakatti & Halgeri, 2015; Singh & Ekhe, 2015; Vichaphund et al., 2015). The substitution of metals, including Ti, Sn, and Zr, on the silicon atoms in BEA zeolite increased the unit cell volume of modified BEA, changed the bond lengths and distorted the angles between the metal and oxygen atoms, thereby decreasing the framework stability (Yang et al., 2013). A substantial loss (approximately 10-60%) in metal dispersion on Pd catalysts was also observed after performing deoxygenation of palmitic and stearic acids (Simakova et al., 2009). Accordingly, metal encapsulation into the zeolitic structure has been considered the most plausible approach for curbing the catalytic deactivation issues. The objective of this review, therefore, is to demonstrate recent advances to immobilise metal species in porous materials. The range of synthesis methods used to encapsulate metal precursors within zeolites will be intensively explored in terms of their advantages and limitations. The perspective of the upcoming trend of embedding metal complexes into the material will also be discussed in the current review, together with some experimental reports to confirm the catalytic ability and stability of those modified

zeolites towards the conversion of biomass into value-added resources. The traditional and new synthesis approaches will also be compared.

2.

Encapsulation of metal via post-modification The post-modification approach is the easiest and most convenient way to introduce metals

into porous materials. This strategy, mainly including ion-exchange, impregnation, and depositionprecipitation, has the main advantage of being applicable to any type of substance having permeable accessibility, regardless of its structure. However, the process, which normally occurs by immersing metal precursors into the materials, can result in either random distribution of the metal on the outer surface of the support or dispersion of the metal inside the zeolitic cages (Singh & Ekhe, 2015), most likely leading to metal agglomeration after heat treatment. Low-framework-density zeolites, such as FAU, show the high possibility of embedding metal complexes inside zeolitic supercages, which is attributed to their large cage size (FAU: 1.12 nm) and small pore aperture (FAU: 0.74 × 0.74 nm) (Baerlocher & McCusker, n.d.). Hence, the encapsulation of metal into FAU-topology zeolites was conducted and described in the literature (Bagherzadeh & Zare, 2012; Bania & Deka, 2012; Bania & Deka, 2013; Cai et al., 2013; Iida et al., 2017; Kuźniarska-Biernacka et al., 2011; Kuźniarska-Biernacka et al., 2013; Razavi & Loghman-Estarki, 2012; Yang et al., 2011). In the typical synthesis, metal complexes were introduced into the zeolitic structure via the flexible ligand method, which started from the suspension of zeolite Y in an aqueous solution consisting of a metal precursor. The mixture was stirred for a certain period, followed by filtration and drying under controlled temperature conditions that do not cause metal agglomeration during the whole process. Thereafter, the ligand solution was treated with the metal-exchanged Y zeolite and stirred at room temperature. Soxhlet extraction in ethanol was performed after another

filtration step to remove uncomplexed ligands and the species adsorbed on the external surface until the solution obtained from the elution showed no colour. Due to the complicated synthesis procedure, as described above, the ligand and metal were simultaneously introduced into zeolite Y, and the metal encapsulation capacity was compared with that of the flexible ligand method (Kuźniarska-Biernacka et al., 2013). Both synthesis strategies demonstrated successful encapsulation of metal complexes in the Y zeolitic framework. The metalmodified zeolite prepared with this technique is beneficial for catalysing the liquid-phase reaction because of its high stability against leaching of the metal species into the solution. However, the applications of this zeolite as a heterogeneous catalyst are limited to the catalytic process, which requires a low reaction temperature because of the susceptibility of the ligand to high temperatures. Another widespread approach for metal encapsulation involves demetallation of the zeolitic framework by acidic-basic solutions to form a hierarchical structure prior to metal introduction. In this method, the metal precursor has a high permeability, allowing it to preferentially enter the interior structure of the zeolites rather than dispersing on the surface. In addition, this strategy can be applied with a wide range of zeolite types, e.g., MFI, MOR, FAU, BEA, and LTA (Fodor et al., 2015; Goodarzi et al., 2018; Hernando et al., 2017; Mielby et al., 2014; Verboekend et al., 2013; Wang et al., 2018; Zhang et al., 2018a), in contrast to the utilisation of a ligand to fix the metal inside the zeolite structure, as previously mentioned. As an example, monometallic Pt@hollow ZSM-5 and Co@hollow ZSM-5 as well as bimetallic Fe/Pt@hollow ZSM-5 were synthesized by Fodor et al. (2015). The parent ZSM-5 was treated with NaOH to promote Si leaching from the interior zeolitic structure. After washing, centrifugation, and drying, the resultant zeolite was washed again with HCl to remove the Al-rich deposit on the surface (Beers et al., 2003), which ultimately increased the Si/Al ratio of the product. The incorporation of a metal into post-synthetic ZSM-5 was achieved by wetness impregnation to obtain Fe/hollow ZSM-5, Pt@hollow ZSM-5, Co@hollow ZSM-5, and Fe/Pt@hollow ZSM-5.

One factor that contributes to successful metal encapsulation is the rate of evaporation of water from the zeolite. Drying the zeolite at higher temperatures caused the metal to sit on the external surface, whereas low-temperature drying did not trigger agglomeration of the metal outside the structure, leading to successful metal encapsulation even after calcination. The adjustable pore mouth and cavity of the zeolite obtained from acidic-basic treatment solve the mass transfer limitation and deactivation problem during the metal introduction and some catalytic reactions involving large substrate molecules. As described, although treatment of the parent zeolite to produce a larger pore aperture can improve the metal dispersion of the whole zeolite, especially inside the channel, the synthesis conditions should be carefully controlled to prevent metal agglomeration on the external surface.

3.

Encapsulation of metal via co-crystallization synthesis Co-crystallization is performed by introducing a metal precursor and silica source into the

same initial media before typical hydrothermal synthesis of a zeolite. This technique overcomes the problems associated with post-synthetic encapsulation as it can be applied to deposit metal clusters inside zeolitic confinement. Goel et al. (2012) demonstrated successful deposition of Pt, Pd, Ru and Rh clusters into SOD-type (pore aperture: 0.28 × 0.28 nm) and GIS-type (pore aperture: 0.45 × 0.31 nm) zeolites through the co-crystallization process. Despite the small channels of both zeolite networks, TEM images and XRD patterns illustrated a good metal dispersion over both zeolites without any change in their original structures. The limited accessibility of isobutanol (kinetic diameter: 0.55 nm) via oxidative dehydrogenation in both SOD and GIS proved the success in metal incorporation, while a significantly higher reaction turnover rate was obtained through oxidative dehydrogenation of smaller reactant molecules such as methanol (kinetic diameter: 0.37 nm).

Nevertheless, metal precipitation problems can frequently be observed during cocrystallization synthesis. The high pH of the solution (pH>12) required for crystallization during heat treatment leads to the formation of colloidal metal hydroxides larger than the zeolite channel, thereby promoting premature precipitation of the metal precursors (Choi et al., 2010). In such cases, ammonia and organic amine ligands have been used as metal cation stabilisers to solve the mentioned issue. Table 1 summarises all ligands and conditions used for metal encapsulation through the hydrothermal co-crystallization process. It is noted that cetyltrimethylammonium bromide (CTAB), which is normally introduced to create mesoporous voids in microporous zeolites, was suggested to act as a metal anchor to produce Pd@mesoporous MCM-41 (Lin et al., 2015). This metal encapsulation resulted in a more stable framework under heat treatment with no change in metal particle size up to 1073 K, whereas large Pd nanoparticles were found in the Pd-SiO2 sample after heating to 873 K. The polymer-assisted strategy has recently been applied to incorporate metal precursors inside the zeolite framework (Cho et al., 2018; Cui et al., 2016). Polydiallyldimethylammonium chloride (PDDA) was used as a cationic polymer to bind the anionic metal precursor, which in this case is PtCl62-, with the zeolite crystallization domain through electrostatic interactions (Cho et al., 2018). In addition, Cui et al. (2016) reported the benefit of polyvinyl pyrrolidone (PVP, K30) as an effective polymer for preventing metal agglomeration based on the shape selectivity of Pd@mesoporous S-1 during hydrogenation, oxidation and carbon-carbon coupling of nitrobenzene and 1-nitronaphthalene. Alternatively, encapsulation via co-crystallization without the presence of ligands is also possible if the synthetic medium compositions or hydrothermal conditions are modified. The utilisation of HCl was reported for producing Co-encapsulated SBA-15 instead of a basic solution (Reddy et al., 2009). However, it is obvious that the acidic mixture required a much longer

crystallization time. Therefore, in this case, the hydrothermal process had to be performed at a higher pressure to reduce the synthesis time. The alcoholic solvent was applied to stabilise the metal precursor and prevent alkaline degradation (Lai et al., 2015). The results showed an insignificant change in TEM images after heat treatment at 973 K in an air environment. Another method of encapsulating metals through co-crystallization of zeolites is to use less aqueous solution during the hydrothermal synthesis. Zhang et al. (2015) reported the use of bimetallic Au-Pd@S-1 synthesized by grinding metal nanoparticles encapsulated with amorphous SiO2 with tetrapropylammonium hydroxide (TPAOH). Hydrothermal crystallization was then carried out without further addition of water, resulting in a zeolite with high metal dispersion throughout its interior cavity. Similar to the abovementioned work, Cho et al. (2018) also conducted hydrothermal synthesis via steam crystallization to prevent metal segregation from the polymer during the process, with a substantially high zeolite product yield calculated from the mass of starting mother gel. The diffusion constraint of fixing large metal complexes inside the zeolitic framework leads to the production of metal-encapsulated zeolite via co-crystallization. However, dispersing metal precursors in the initial gel becomes challenging as a result of metal precipitation in a basic environment. Thus, either the starting synthesis composition or ligand introduction must be adjusted and carefully considered.

4.

Encapsulation of metal via re-crystallization approach The incorporation of metal inside the zeolite structure through a re-crystallization process

or the so-called core-shell/hollow crystal method is similar to the strategy mentioned in the previous section with regard to the involvement of heat treatment for crystalline zeolite production. Nevertheless, the major difference between these two methods is the use of a silica source before proceeding through the hydrothermal synthesis. In detail, while the re-crystallization process is

performed mainly with a highly crystalline zeolite to trap a metal precursor in its cavity and maintain its topology after heat treatment, co-crystallization starts with amorphous silica that transforms to a more highly crystalline structure during the hydrothermal process. Generally, impregnation of a metal precursor into a crystalline silica source is performed prior to the re-crystallization approach. The hydroxy ion used in alkaline treatment (mostly TPAOH) after metal introduction creates large voids via desilication, followed by the formation of thin zeolite shells via the re-crystallization of Si leaching on the surface with a cation (TPA+). The syntheses can be conducted either in an aqueous solution with or without a ligand to facilitate metal incorporation (Dai et al., 2015a; Dai et al., 2017; Dai et al., 2015b; Li et al., 2014; Li et al., 2016b) or by the use of several techniques, as previously mentioned, including hierarchical meso/microporous formation, dry gel conversion (DGC), steam-assisted techniques and some recently developed approaches, such as the seed-directed route and the conversion of a metal-containing solid crystal to a metal-encapsulated hollow crystal, to increase the capability of metal encapsulation (Dai et al., 2016; Gu et al., 2017; Gu et al., 2015; Li et al., 2015; Prates et al., 2018; Zhang et al., 2017; Zhang et al., 2018b). One of the interesting points for the impregnation-dissolution-re-crystallization approach is that all of the literature, except that describing the zeolite production via the seed-directed route, focuses only on metal encapsulation on MFI-type zeolites, which begs the question of whether other zeolitic structures are able to trap metals in their channels through the re-crystallization technique. One reason for this focus might be that MFI-topology zeolites are well-known for their high stability (framework density: 18.4, according to Baerlocher and McCusker (n.d.)) and assorted applications compared to those with a high framework density, leading to more research work relating to its modification. It is noted that an alkaline solution treated with zeolites not only enables the desilication process but also accelerates the leaching of metals deposited on the surface to the solution during the dissolution process. Dai et al. (2016) proved the success of Fe leaching

from the zeolitic structure after TPAOH introduction in the hierarchical Fe-S-1 zeolite. Fe was later found to be encapsulated together with Pt, which is first impregnated on the zeolite surface, in the zeolite hollow crystal during the re-crystallization procedure. Hereby, it is likely that the metal will both disperse over the zeolitic shell and becomes trapped inside the core. This indicates that zeolites with high structural stability are required for metal encapsulation via the recrystallization strategy. Low zeolitic stability, however, shows limitations for enabling metal incorporation via this method, possibly as a result of structural degradation after heat treatment in basic environments.

5.

Inter-zeolite transformation Inter-zeolite transformation can occur upon rearrangement of one zeolite structure to

another type of zeolite when subjecting the initial framework to heat treatment under suitable conditions. This alternative method for zeolite production has been widely used, as it can be conducted under milder conditions and/or requires a shorter synthesis time than the traditional method (Geng et al., 2018; Goel et al., 2015). Although, to date, there is no certain rule on how the mother zeolite transforms to another structure due to the involvement of many variables, such as the initial gel composition and reaction conditions, the conversion normally follows two main basic concepts: (1) the framework density (FD) of the substrate and product zeolites and (2) the relationship between their building units and ring sizes. Table 2 displays the FD; the basic building units, including composite building units (CBUs) and secondary building units (SBUs); and the ring sizes of some common zeolites that were reported to undergo inter-zeolite transformation. Generally, the conversion occurs in the sequence of low to high framework density because of the thermodynamic driving force. Transformation of the FAU topology, which is reported to be one of the lowest-density zeolite

frameworks, has resulted in a wide variety of products obtained, as shown in Table 3. In contrast, other zeolite types resulted in fewer zeolitic structures via this process, as shown in Table 4. Nevertheless, the success of inter-zeolite transformation is dependent not only on the FD but also on the basic building units of the zeolite structure. Inter-zeolite transformation is favourable for initial and targeted zeolites that have common building units or ring sizes. This can be explained by the MFI syntheses from FAU and BEA (Goel et al., 2015). From Table 5, considering the similar Si/Al ratios of the parent zeolites, BEA can be easily converted into MFI without any assistance of a structure-directing agent (SDA) or seeds, whereas it is necessary for FAU to have either SDAs or MFI seeds in the synthesis gel to undergo transformation to the MFI zeolite. Regarding all building units and ring sizes of the abovementioned zeolites, which are listed in Table 3, it is obvious that BEA and MFI have most of the units in common. FAU, which displayed more differences in the building units, was more difficult to convert into MFI; thereby, it was transformed into zeolites with more common building units, such as CHA or ANA, instead (Itakura et al., 2011a; Kim & Kim, 2018; Takata et al., 2016; van Tendeloo et al., 2013; Wang et al., 2010; Xiong et al., 2017; Yamanaka et al., 2012). The ring sizes of zeolites are determined from the number of silicon and aluminium atoms contained in one closed ring. This number normally decreases when the daughter zeolite is formed owing to the dissolution of the mother zeolite. Alternatively, it is noticeable from Goel et al. (2015) that parent zeolites with lower Si/Al ratios cannot transform into the targeted zeolite under the same synthesis conditions and times as those with higher Si/Al ratios. One of the main reasons for this difference is that the daughter zeolite, in this case, has MFI topology and showed characteristics of high-silica-content materials due to the pentasil unit within its framework (Suhendar et al., 2018). In addition, a lower Si/Al ratio in the products than in the substrates is observed in the literature as a result of Si leaching during alkaline treatment (Funase et al., 2017; Geng et al., 2018; Goel et al., 2015), leading to

higher difficulty or no possibility of MFI formation from the low-silica-content FAU and BEA. One exception was the conversion of the inter-zeolite of FAU (Si/Al ratio: 30) to MFI without the use of SDA (Qin et al., 2019). In this respect, the conversion is viable due to the use of high-silica substrates even if the building units of both zeolites are completely different. The transformation may not follow the two mentioned approaches when the types of chemicals involved, the pH of the solution or the synthesis conditions, e.g., time and temperature, are changed. One example is the inter-zeolite transformation of LEV to CHA, which mainly occurred from the dissolution of the mother zeolite, as demonstrated in Table 6 (Goto et al., 2012). Reducing the temperature by 35 K yielded an amorphous phase, as it required a much longer synthesis time (10.5 h more) to be completely converted to CHA. Moreover, it is clear that while the synthesis temperature of 443 K was still too low to form a new type of zeolite, the types of daughter zeolites changed from CHA to ANA when the hydrothermal temperature increased from 443 K to 473 K. This can be explained by the phase transformation kinetics, which drove the reaction to form a more stable product. Each type of starting zeolite also required different alkalinities for hydrothermal conversion. Generally, the inter-zeolite transformation consisted of two stages: dissolution of the parent zeolite and re-crystallization of the daughter zeolite. While a weakly basic solution is adequate for less stable zeolites such as FAU to dissolve, BEA and MOR demand stronger alkalinity to undergo transformation. It is noted in Table 5 that an increase in the alkalinity of the initial solution resulted in a lower yield of the target zeolite (Goel et al., 2015). Understanding the inter-zeolite transformation offers us great potential to convert one zeolite into more beneficial structures, which satisfies the demands of industry. It can also play an important role in adding value to a renewable resource such as natural zeolites by reconstruction of their frameworks into commercial types. However, currently, it is still challenging to proceed with the process as a result of contamination of the zeolite.

6.

Strategy of metal encapsulation via inter-zeolite transformation Fixation of metal precursors within zeolites having packed structures and small cages by

the use of aforementioned techniques is impractical. Considering each method, the post-synthetic modification mostly causes dispersion of the deposited species on the zeolite surface rather than incorporation into the framework interior, leading to coalescence and sintering of the metal during heat treatment. Moreover, high hydrothermal temperatures and strong basicity (pH > 12) are required to produce highly crystalline zeolite from amorphous silica. Under these conditions, however, metal precipitation is often observed. Given metal encapsulation is detected only upon re-crystallization of the target zeolite using alkaline solutions (pH >12), metal precipitation is almost inevitable which results in low metal loading in the zeolite. Recently, the inter-zeolite transformation has become a key method to load metal species into small-aperture daughter zeolites by re-crystallising the framework construction of metal-modified parent zeolites. The exploration of metal-incorporated zeolites via inter-zeolite transformation is still limited. Iglesia’s research group pioneered the work in this field starting from the encapsulation of noble metal clusters within ANA using GIS as a starting material (Goel et al., 2012). Since harsher crystallization conditions are demanded for synthesizing ANA, the decomposition or precipitation of metal complexes as colloidal hydroxides unpreventably occurred. In this respect, a GIS-topology zeolite, which was reported to be an intermediate for ANA synthesis (Kohoutková et al., 2007), was used to confine the metal precursor inside its cavity before transforming to ANA. TEM images and XRD patterns illustrated a clear metal dispersion without interfering with the crystallization process. The successful metal encapsulation inside the ANA zeolite (0.42 × 0.16 nm) was verified via two reactions: the oxidative dehydrogenation of methanol (kinetic diameter: 0.37 nm) and isobutanol (kinetic diameter: 0.55 nm) and the hydrogenation of ethene (kinetic diameter: 0.39 nm) and toluene (kinetic diameter: 0.59 nm) in the presence and absence of

thiophene (kinetic diameter: 0.46 nm). The results exhibited shape selectivity of the catalyst in segregating larger molecules (isobutanol and toluene) from the smaller ones (methanol and ethene) in terms of reaction turnover rate. The metal/support also maintained the same reaction turnover rate of ethene via hydrogenation as before thiophene poisoning, confirming that metal species were trapped inside the ANA zeolite. The experiment was extended to the encapsulation of metal clusters within MFI zeolite via inter-zeolite transformation of FAU and BEA (Goel et al., 2014). Ion-exchange was conducted to deposit metal precursors into the parent zeolites. After metal cluster formation by H2 and O2 treatments, both zeolites were proceeded through hydrothermal re-crystallization to create MFI structures. Selective hydrogenation of toluene and 1,3,5-trimethylbenzene (1,3,5-TIPB) was performed to probe the metal location. As expected, a substantially low reaction turnover rate was observed when using the latter substrate, proving the success of the metal encapsulation.

7.

New opportunities for metal-modified zeolites in biomass conversions Generally, metal addition causes a loss of Brønsted acid sites and increase in the number

of Lewis acid sites (Heo et al., 2011; Huang et al., 2012; Kim et al., 2013). The variation in the Brønsted acid/Lewis acid ratio in zeolites allows the resulting catalysts to be utilised in chemical processes such as biomass conversion. Although many researchers have reported the use of metalmodified zeolites in this reaction, most works still depend on the addition of metal via postmodification techniques. As mentioned above, the metal accessibility of post-modified zeolites is restricted by pore size. Considering small-pore-aperture zeolites, e.g., MFI (0.51 × 0.55 nm along the [100] direction and 0.53 × 0.56 nm along the [010] direction, according to Baerlocher and McCusker (n.d.)), this method mainly brings about the dispersion of metal precursors on the external surface of zeolite

rather than inside the crystals, contributing to metal coalescence and degradation during the chemical process. Thus, the introduction of low metal loading is preferred to avoid metal loss or sintering after heat treatment. Considering the applications of post-synthetic metal/zeolite on biomass conversion, Mullen and Boateng (2015) prepared Fe/HZSM-5 with various Fe loadings (0-4.2 wt%) for catalytic pyrolysis of cellulose, cellobiose and lignin. The aromatic hydrocarbon selectivity was enhanced upon Fe addition. However, the excess Fe significantly reduced the percentage of aromatic hydrocarbon yield attained, which is most likely attributed to the agglomeration of Fe on the surface, leading to blockage of the pores. The optimum yield of benzene, toluene, ethylbenzene and xylene (BTEX), in this case, was achieved via Fe/HZSM-5 with a low metal loading of 1.4 wt%. The BTEX yield decreased substantially by approximately 40%, similar to the yield of the unmodified HZSM-5 zeolite when the Fe composition was doubled, indicating the existence of an inactive metal active site in 2.8 wt% Fe/HZSM-5. The accumulation of metal on the surface, which resulted in catalytic deactivation as coke deposition, was also observed on the Mo-impregnated HZSM-5 zeolite via the conversion of rapeseed oil with high metal loadings (4-7 wt%) (Botas et al., 2012). Both cases above confirmed the disadvantages of excess metal contents in the microporous zeolite towards biomass processing. On the other hand, a large-pore zeolitic structure, e.g., FAU (0.74 × 0.74 nm along the <111> direction), can be occupied by a large number of metal active sites, as its accessible volume was noted to be approximately three times higher than that of MFI (Baerlocher & McCusker, n.d.). This contributes to high metal dispersion along the zeolite, which prevents metal agglomeration and improves its stability via the chemical reaction. It can be seen from the reaction of carbohydrates to methyl levulinate (ML) via the Zr-Y zeolite that the catalyst exhibited superior catalytic activity and reusability over five cycles, with a less than 10% decrease in glucose conversion in total (Li et al., 2017). However, the use of these large-pore framework zeolites may

still be restricted to only biomass processes that do not require high reaction temperatures (e.g., sugar conversion, as mentioned above). The use of an alkaline solution (e.g. NaOH) to enlarge the pores of the restricted zeolitic structure not only can fix the metal dispersion problem but also can maintain the zeolitic stability at high reaction temperatures. Ni-hierarchical ZSM-5 zeolites showed an improvement in aromatic production at 823 K and an excellent interaction with the metal precursors. These are the results of increased accessibility from mesopores and macropores generated by NaOH treatment, which enhanced the metal loading up to 11 wt% without a significant decrease in the yield of aromatics (mainly toluene and xylene) (Dai et al., 2019). However, modification of zeolite via decreasing metal content or pore enlargement does not prevent metal leaching, especially in reactions that require high temperature, such as biomass conversion. The solubility of metal in liquid media may lead to a permanent loss of catalytic activity in successive cycles in liquid-phase reactions. In contrast, the deactivation of metal/zeolite caused by sintering and coke deposition could be significantly reduced by having a low metal loading and calcination under an oxidative environment. The stability of metal on the catalyst support, however, is strongly subjected to the reaction conditions, e.g., temperature, pressure, types of solvent, and the pH (Sádaba et al., 2015). For example, the loss of metal from zeolite-supported vanadia catalyst during the oxidation of 5-hydroxymethylfurfural (HMF) was only detected under high-pressure systems, whereas no leaching was observed under atmospheric pressure (Sádaba et al., 2013). Although optimizing the reaction conditions may minimize irreversible deactivation caused by metal leaching, it often compromises the product selectivity owing to the formation of undesired by-products under inappropriate conditions. Hence, the encapsulation of metal active sites inside the zeolite framework could be an ideal solution for isolating and protecting the metal. Several works have reported the utilisation of metal-encapsulated zeolites on biomass conversion. The confinement of metals through hydrothermal crystallization without channel size

adjustment can enhance the metal stability but limits the reactants and product to small molecules and causes mass transfer issues. It is obvious that small products are favourable in furfural hydrogenation over the Pd@S-1 zeolite, as no selectivity towards bulky molecules is observed (Wang et al., 2016a). The Pd@Na-ZSM-5 zeolite maintained a high catalytic performance in furfural hydro-conversion without significant change in metal cluster size after five reaction cycles (Chai et al., 2018). The introduction of a hierarchical structure on the metal@zeolite has been applied to improve metal accessibility and mass transfer of metal-encapsulated zeolites. Neumann and Hicks (2012) conducted catalytic fast pyrolysis of lignocellulosic biomass using a ceriumincorporated hierarchical zeolite. The mesoporosity of Ce-encapsulated HZSM-5 promoted both catalytic activity and stability by decreasing coke formation, leading to more valuable oxygenated chemicals were produced. To better understand the effect of different catalyst synthesis techniques on the catalytic efficiency, Wang et al. (2017) compared the performance of metal/zeolite catalysts prepared from post-synthetic technique and encapsulation in hydrogenation of furfural. It was found that all the Pd-loaded S-1 zeolites gave comparable furfural conversion over the studied temperature. The encapsulation of metal, however, resulted in a significant increase in selectivity to furan (82.697.1% and > 82.5% by Pd@S-1 and Pd@S-1-OH-10, respectively), while conventional Pd-doped S-1 zeolite (Pd/S-1) showed a wide range of product distribution with furan selectivity lower than 12.6%. To confirm the positive impact of metal encapsulation on product selectivity, Pd@S-1OH-10 was treated in 0.5 wt % of hydrofluoric acid (HF) solution to partially damage the zeolitic shell before catalytic reaction. The resulting catalyst, denoted as Pd@S-1-OH-10-HF, clearly demonstrated a decrease in both furfural conversion and furan selectivity to approximately 80% and 60%, respectively. The results confirmed the importance of hydrophilic zeolite shell in protecting the Pd particles from sintering or deactivaction during furfural hydrogenation. More importantly, Pd-encapsulated S-1 zeolite also showed a slight increase in the mean diameter of Pd

nanoparticle (from 0.6 to 0.65 nm) without negligible amount of Pd in the reaction media after a 90-h reaction, confirming the capability of the encapsulation technique on the prevention of sintering and leaching. To conclude, it can be clearly seen that metal active sites are preferred for the production of many valuable chemicals from biomass. Since conventional modification techniques often lead to unfavorable problems, such as the inefficacy of catalysts and their deactivation from metal leaching, sintering, and coke deposition, the encapsulation method becomes a promising solution for eliminating these issues. Nevertheless, some adjustments to the structure accessibility may be needed for the improvement in mass transfer. Metal immobilization through encapsulation in zeolites via inter-zeolite transformation will yield promising benefits for biomass conversion. The method can eliminate the adverse effects of post-modification, such as metal loss, leaching, and sintering, in any zeolites. The emerging metal-loading technique also improves catalytic stability and performance since the postmodification of metal precursors has to occur in the parent zeolite prior to the inter-zeolite transformation process. Furthermore, inter-zeolite transformation can convert the mother zeolite into structures that are more suitable for industrially relevant reactions in terms of a longer catalytic lifetime, higher metal accessibility and a higher mass transfer rate. Therefore, the integration of metal encapsulation and inter-zeolite transformation can be considered a potential technique to prepare metal-zeolites for the biomass conversion process.

8.

Conclusions The paradigm of maintaining the stability of the metal active sites within zeolitic

framework is shifting from the distribution of metal species on the support surface to

immobilisation/encapsulation of metals inside the structure. Although the latter may not favour the mass transfer of (bulky) reactants and/or products, metal encapsulation via inter-zeolite transformation will produce the metal/zeolite catalysts suitable for biomass conversions. Detailed understanding of the relationship between the starting and the end-product zeolites, gel compositions, and reaction conditions are expected to be a major trend of study in zeolite catalysis towards sustainable bioresource technologies.

Acknowledgement The authors would like to thank KiwiNet for the financial support through the Tier One PreSeed Accelerator Fund and the Emerging Innovator Programme. This paper is also supported by the Ministry of Business, Innovation & Employment in New Zealand under the MBIE Endeavour “Smart Ideas” grant (UOCX1905).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at (link to be confirmed by BITE).

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etal type Co

Au , Ir, Rh, Ag

Pd, Ru, Rh

Ce Pd, Rh, Ir, Re, Ag O, Fe2O3 Pd Au-Pd Au Pt Au Pt, Pd, Auu-Pt, Pd-Pt Pd Pt Pd Pd Ni(OH)2, Ni(OH)2, Co(OH)2 Pd Pt Pt

Table 1 Summary information of metal encapsulation through co-crystallisation strategy. Hydrothermal Zeolite product Ligand conditions 373 K, 12 h, SBA-15 1MPa S-1 373 K, 72 h (3-mercaptopropyl)trimethoxysilane FAU 373 K, 12 h (3-mercaptopropyl)trimethoxysilane 373 K, 7 h (SOD) SOD, GIS Ethylenediamine, Ammonia 363 K, 72 h (GIS) ZSM-5 448 K, 18 h Ammonia (Pt, Ir), LTA 373 K, 16 h Ethylenediamine (Pd, Rh, Re, Ag) ZSM-5 453 K, 48 h MCM-41 333 K, 2 h Cetyltrimethylammonium bromide S-1 453 K, 48-72 h MCM-22, ZSM-5 431 K, 72 h Hexamethyleneimine SOD 373 K, 7 h Ammonia LTA, ZSM-5 393 K, 15 h (3-mercaptopropyl)trimethoxysilane

References

Reddy et al. (2009

Laursen et al. (2010 Choi et al. (2010)

Goel et al. (2012)

Neumann and Hicks (2 Wu et al. (2014)

Lai et al. (2015) Lin et al. (2015) Zhang et al. (2015 Saxena et al. (2016 Sibi et al. (2016) Otto et al. (2016b)

LTA

373 K, 12 h

(3-mercaptopropyl)trimethoxysilane

Otto et al. (2016a)

S-1 S-1 S-1 S-1

393 K, 48 h 423 K, 17-96 h 453 K, 72 h 443 K, 96 h

Polyvinyl pyrrolidone (3-mercaptopropyl)trimethoxysilane Polyvinyl pyrrolidone Ethylenediamine

Cui et al. (2016) Kim et al. (2016) Wang et al. (2016a Wang et al. (2016b

S-1

443 K, 96 h

Ethylenediamine

Sun et al. (2017)

ZSM-5 ZSM-5 BEA

443 K, 72 h 443 K, 72 h 413 K, 10 h

(3-mercaptopropyl)trimethoxysilane Polydiallyldimethylammonium chloride -

Chai et al. (2018) Cho et al. (2018) Dai et al. (2018)

Table 2 Framework density (FD/T.1000A-3), composite building unit (CBU), secondary building unit (SBU) and ring size of zeolite types involved in inter-zeolite transformation. Ring sizea Type FDa/T.1000A-3 CBUsa SBUsa (T-atomsb) EMT 13.3 d6r, sod 6-6 / 6-2 / 6 / 4-2 / 1-4-1 / 4 12, 6, 4 FAU 13.3 d6r, sod 6-6 / 6-2 / 6 / 4-2 / 1-4-1 / 4 12, 6, 4 LTA 14.2 d4r, sod, lta 8 / 4-4 / 6-2 / 6 / 1-4-1 / 4 8, 6, 4 KFI 15.0 d4r, pau, lta 6-6 / 6-2 / 8 / 6 / 4-2 / 4 8, 6, 4 AEI 15.1 d6r 6-6 / 4-2 / 6 / 4 8, 6, 4 CHA 15.1 d6r, cha 6-6 / 6 / 4-2 / 4 8, 6, 4 GME 15.1 d6r, gme 12 / 6-6 / 8 / 6 / 4-2 / 4 12, 8, 6, 4 *BEA 15.3 mor, bea, mtw 5-1c 12, 6, 5, 4 LEV 15.9 d6r, lev 6 8, 6, 4 MWW 15.9 d6r, mel 1-6-1 / 6-1 (1:4) 10, 6, 5, 4 ERI 16.1 d6r, can 6/4 8, 6, 4 OFF 16.1 d6r, can, gme 6 / 4-2 12, 8, 6, 4 GIS 16.4 gis 8/4 8, 4 MER 16.4 d8r, pau 8-8 / 8 / 4 8, 4 PHI 16.4 phi 8/4 8, 4 LTL 16.7 d6r, can, ltl 6 / 4-2 12, 8, 6, 4 MAZ 16.7 gme 5-1 / 4-2 12, 8, 6, 5, 4 SOD 16.7 sod 6 6, 4 CAN 16.9 can 12 / 6 / 4 12, 6, 4 STF 16.9 stf, cas 5-3 10, 6, 5, 4 MOR 17.0 mor 5-1 12, 8, 5 ,4 MTN 17.2 mtn N/A 6, 5 HEU 17.5 bre 4-4=1 10, 8, 5, 4 ABW 17.6 abw 8/4 8, 6, 4 FER 17.6 mor, fer, pcr 5-1 10, 8, 6, 5 ITW 17.7 d4r 1-4-1 / 4-[1,1] 8, 6, 5, 4 DDR 17.9 mtn N/A 8, 6, 5, 4 RUT 18.1 rte, rut 6 6, 5, 4 TON 18.1 jbw, mtt, bik, ton 5-1 10, 6, 5 MTW 18.2 jbw, cas, bik, mtw 5-[1,1] 12, 6, 5, 4 BRE 18.3 bre 4 8, 6, 5, 4 MFI 18.4 mor, cas, mfi, mel 5-1 10, 6, 5, 4 *STO 18.7 ats, afs, mel, imf, doh N/A 12, 6, 5, 4 ANA 19.2 N/A 6-2 / 6 / 4-[1,1] / 1-4-1 / 4 8, 6, 4 a data obtained from Baerlocher and McCusker (n.d.) b aluminium or silicon atoms c data obtained from Suhendar et al. (2018)

Table 3 All possible zeolite topology obtained from the inter-zeolite conversion of FAU as component(s) of initial gel. Daughter Daughter References References zeolite zeolite EMT - Matsuda et al. (2019) PHI - Chiyoda and Davis (1999) KFI - Kim and Kim (2018) LTL - Honda et al. (2013) - Maruo et al. (2014) MAZ - Honda et al. (2014) AEI - Sonoda et al. (2015) - Shi et al. (2014) SOD - Itakura et al. (2011a) - Buhl (2016) - Yamanaka et al. (2012) CAN - Buhl (2016) - van Tendeloo et al. (2013) - Yashiki et al. (2011) CHA - Takata et al. (2016) MOR - Honda et al. (2014) - Xiong et al. (2017) - Kim and Kim (2018) - Kim and Kim (2018) MTN - Sasaki et al. (2009) - Chiyoda and Davis (1999) HEU - Chiyoda and Davis (1999) GME - Kim and Kim (2018) ABW - van Tendeloo et al. (2013) - Mitani et al. (2019) FER - Shi et al. (2014) *BEA - Honda et al. (2011) DDR - Shibata et al. (2011) - Shibata et al. (2011) - Jon et al. (2008) LEV - Yashiki et al. (2011) RUT - Sasaki et al. (2009) - Funase et al. (2017) - Itakura et al. (2011b) - Shi et al. (2014) BRE - Chiyoda and Davis (1999) MWW - Xing et al. (2016) - Goel et al. (2015) MFI - Itakura et al. (2010) - Qin et al. (2019) ERI - Yashiki et al. (2011) - Sonoda et al. (2015) - Itakura et al. (2010) - Wang et al. (2010) OFF - Yashiki et al. (2011) - van Tendeloo et al. (2013) ANA - Yashiki et al. (2011) - Kim and Kim (2018) GIS - Honda et al. (2013) - van Tendeloo et al. (2013) MER - Kim and Kim (2018)

Table 4 Parent-daughter zeolites data via inter-zeolite transformation. Parent Daughter References zeolite zeolite CHA - Geng et al. (2018) - Walton et al. (2001) SOD - Ding et al. (2010) LTA MWW - Xing et al. (2015) GIS - Subotić et al. (1982) GME ANA - Joshi et al. (1991) AEI - Xu et al. (2019) CHA - Tang et al. (2019) MAZ - Honda et al. (2014) - Honda et al. (2014) MOR - Zhang et al. (2016) *BEA RUT - Itakura et al. (2011b) MTW - Hari Prasad Rao et al. (1998) - Honda et al. (2014) MFI - Goel et al. (2015) *STO - Hari Prasad Rao et al. (1998) LEV CHA - Goto et al. (2012) MWW FER - Xie et al. (2014) FAU - Azizi et al. (2013) - Zones and van Nordstrand (1988) CHA GIS - Zones (1990) - Goel et al. (2012) ANA - Azizi et al. (2013) LTL CHA - Tang et al. (2019) SOD CAN - Barnes et al. (1999) TON ITW - Zicovich-Wilson et al. (2010) MTW ITW - Rojas et al. (2013) MFI AEI - Xu et al. (2019)

Table 5 Initial synthesis molar compositions, product phase, yield and final pH of samples for synthesis of MFIa. (Adapted with permission from Goel et al. (2015). Copyright 2015 American Chemical Society.) Sample Parent Additional Product NaOH/SiO2b H2O/SiO2b Time (h) name zeolite (Si/Al) (OSDA/seed)c phased MFIB-D1 BEA (12.5) 0.35 65 24 Am. MFIB-D2 BEA (37.5) 0.35 65 24 MFI MFIB-T BEA (37.5) 0.35 65 24 TPABr (0.05)f MFI MFIB-S BEA (37.5) 0.35 65 24 10 wt% MFI seeds MFI MFIF-D1 FAU (6) 0.50 95 40 Am. MFIF-D2 FAU (40) 0.50 95 40 Am. MFIF-T FAU (40) 0.50 95 40 TPABr (0.05)f MFI MFIF-S1 FAU (40) 0.50 95 40 10 wt% MFI seeds MFI MFIF-S2 FAU (40) 0.23 95 40 10 wt% MFI seeds MFI + Am. MFIF-S3 FAU (40) 0.85 95 40 10 wt% MFI seeds MFI a T = 423 K for all the syntheses. b Reported values excludes the SiO2 amount present in seed materials. c Seed (wt%) = (seed material (g)/parent zeolite (g)) × 100. d Am. = amorphous. e Yield (%) = [Product (g)/(Parent zeolite (g) + Seed (g))] × 100. f Values in parentheses show molar composition of TPABr relative to SiO2 amount of parent zeolite.

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Table 6 Hydrothermal conversion of LEV and product obtained. (Adapted from Goto et al. (2012). Copyright 2012 Elsevier.) Synthesis conditionsa Product Sample Starting Bulk Temp. Time Yieldc no. Si/Al NaOH/SiO2 Phaseb Si/Al (K) (h) (%) ratio ratio 1 9 3.0 398 1.5 CHA 2.8 19 2 6 3.0 398 1.5 LEV 3 12 3.0 398 1.5 CHA 2.6 21 No 4 22 3.0 398 1.5 product 5 9 3.0 443 1.5 CHA 3.1 20 6 9 3.0 473 1.5 ANA 7 9 3.0 363 1.5 Am 8 9 3.0 363 12 CHA 4.9 20 9 9 3.0 (LiOH) 398 1.5 Am.+Un. 10 9 3.0 (KOH) 398 1.5 Am. 11 9 3.0 (RbOH) 398 1.5 Am. a H2O/SiO2 = 80. b Am. = amorphous; Un. = unknown. c Yield (%) = [Product (g)/Starting LEV zeolite (g)] × 100

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Highlights 

Conventional metal modification is limited by the pore aperture of zeolite.



Encapsulation offers the potential way to trap metal inside zeolite framework.

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The effect of sintering at high temperature is lessened by encapsulation.

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Encapsulation controls shape selectivity and mass transfer of chemicals.



Inter-zeolite conversion is applied for metal encapsulation.

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