Papers by Danny Verboekend
ChemSusChem, Jan 3, 2015
In the valorization of α-pinene, which is an important biomass intermediate derived from turpenti... more In the valorization of α-pinene, which is an important biomass intermediate derived from turpentine oil, hierarchical (mesoporous) zeolites represent a superior class of catalysts. Hierarchical USY, ZSM-5, and beta zeolites have been prepared, characterized, and catalytically evaluated, with the aim of combining the highest catalytic performance with the most sustainable synthetic protocol. These zeolites are prepared by alkaline treatment in aqueous solutions of NH4 OH, NaOH, diethylamine, and NaOH complemented with tetrapropylammonium bromide. The hierarchical USY zeolite is the most attractive catalyst of the tested series, and is able to combine an overall organic-free synthesis with an up to sixfold activity enhancement and comparable selectivity over the conventional USY zeolite. This superior performance relates to a threefold greater activity than that of the commercial standard, namely, H2 SO4 /TiO2 . Correlation of the obtained benefits to the amount of solid lost during t...
, and SAPO-34 silicoaluminophosphates (SAPOs) are exposed to various acid and base treatments aim... more , and SAPO-34 silicoaluminophosphates (SAPOs) are exposed to various acid and base treatments aimed at mesopore formation and investigation of associated physicochemical modifications. SAPOs amorphize strongly in aqueous NaOH, requiring the use of organic bases (e.g., tetrapropylammonium hydroxide or diethylamine) to preserve the crystallinity during base treatment. In acid media (HCl, H 4 EDTA, and Na 2 H 2 EDTA), SAPO-11 remains fully crystalline, while SAPO-34 strongly amorphize. No clear influence of the framework topology is established. The high resistance in alkaline media and low stability in acid media of SAPO-34 is attributed to its relatively high silicon content. Base treatment of SAPOs leads predominately to the formation of intercrystalline porosity. Still, an up to 4-fold increase in external surface and pore volume in SAPO-11 are achieved. Besides the formation of secondary porosity, base treatment of SAPOs induces a variety of (correlated) physicochemical changes. For example, the silicon distribution clearly influences the dissolution behavior of SAPO-11 in alkaline media, as zeolitic-like Si-domains are more resistant than AlPO domains. As a result, base leaching is selective to phosphorus and leads, depending on the silicon distribution, to either aluminum or silicon enrichment. The resulting changes in bulk composition can be directly related to the secondary porosity, as the Si and Al enrichment takes place predominately on the external surface. Acidity characterization (TPD of ammonia and IR spectroscopy of pyridine or 2,6-di-tert-butylpyridine adsorbed) shows that base treatment of SAPO-11 slightly reduces the concentration of Brønsted sites, while the number of Lewis sites is substantially increased. Moreover, the amount of Brønsted acid sites associated with the external surface is largely enhanced. The behavior of zeotypes, that is, AlPOs, SAPOs, and zeolites, in acid and basic aqueous solutions is generalized, highlighting the role of charge balancing cations. Catalytic evaluation of SAPO-11 shows the potential of base-treated samples in the alkylation of benzyl alcohol with toluene.
Hydroisomerization and hydrocracking using bifunctional zeolite catalysts with hydrogenationdehyd... more Hydroisomerization and hydrocracking using bifunctional zeolite catalysts with hydrogenationdehydrogenation next to the Brønsted acid functionality are at the heart of key refinery processes converting heavy petroleum fractions to high grade lubricants and fuels. Hierarchical zeolites presenting a network of auxiliary mesoporosity in addition to their native microporosity are known to improve accessibility to their active sites. In this paper we assess the impact of hierarchization by demetallation of the well-established hydroisomerization catalyst Pt/ZSM-22 on the reaction pathways of the n-decane, n-nonadecane and pristane (2,6,10,14-tetramethylpentadecane) model molecules. Detailed analysis of reaction products and assessment of acid site accessibility in conventional and hierarchical ZSM-22 samples highlight the contributions of acid sites in pore mouths and micropores to the skeletal rearrangement and cracking reactions.
The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate spe... more The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate species during desilication are examined to better understand the properties of alkaline-treated hierarchical zeolites and their catalytic performance. This is achieved using a systematic experimental strategy, starting from treating the filtrate of alkaline-treated silicalite-1 in the presence of various external additives. No recrystallization is evidenced upon addition of tetrapropylammonium (TPA + ) and/or aluminum hydroxide ions [Al-(OH) 4 − ], confirming the low probability of zeolite nucleation and/or growth during desilication. Conversely, ordered mesoporous materials (OMMs) form upon addition of cetyltrimethylammonium (CTA + ) to the filtrate. By using other silicon sources, i.e., tetramethyl orthosilicate or the organosilane dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium, we verify the facile formation of amorphous materials during alkaline treatment of USY zeolites in the presence of hydrophobic micelle-forming alkyl moieties. A systematic characterization by X-ray diffraction, transmission electron microscopy, N 2 and Ar adsorption, inductively coupled plasma optical emission spectroscopy, and Fourier transform infrared spectroscopy of pyridine adsorbed, demonstrates that zeolites exposed to base solutions containing CTA + display weaker zeolitic properties, compared to those prepared using TPA + , and should be considered as hierarchical zeolite/OMM composites. Catalytic tests in the alkylation of toluene with isopropyl alcohol or benzyl alcohol evidence that CTA + -derived composites do not outperform the conventional USY zeolite. Only the hierarchical USY zeolite prepared by alkaline treatment in the presence of TPA + yielded a superior catalytic performance.
Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination−desi... more Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination−desilication treatments. The main challenge in the postsynthetic modification of these zeolites is the high Al content, requiring a tailored dealumination prior to the desilication step. For natural clinoptilolite, sequential acid treatments using aqueous HCl solutions were applied, while for L a controlled dealumination using ammonium hexafluorosilicate is required. Subsequent desilication by NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite, 64 m 2 g −1 ; L, 135 m 2 g −1 ) relative to the parent zeolite (clinoptilolite, 15 m 2 g −1 ; L, 45 m 2 g −1 ). A thorough characterization sheds light on the composition, crystallinity, porosity, morphology, coordination, and acidity of the modified clinoptilolite and L zeolites. It is elaborated that, besides the degree of dealumination, the resulting Al distribution is a critical precondition for the following mesopore formation by desilication. Adsorption experiments of Cu 2+ and methylene blue from aqueous solutions and the catalytic evaluation in alkylations and Knoevenagel condensation evidence the superiority of the hierarchical zeolites, as compared to their purely microporous counterparts. Finally, the postsynthetic routes for clinoptilolite and L are generalized with other recently reported modification strategies, and presented in a comprehensive overview.
Andrew J. deMello, Joshua B. Edel et al. Rapid cell extraction in aqueous twophase microdroplet s... more Andrew J. deMello, Joshua B. Edel et al. Rapid cell extraction in aqueous twophase microdroplet systems PERSPECTIVE Barry M. Trost et al. Catalytic asymmetric allylic alkylation employing heteroatom nucleophiles: a powerful method for C-X bond formation 5
Hierarchical USY zeolites obtained by scalable and affordable post-synthetic modifications (PSM) ... more Hierarchical USY zeolites obtained by scalable and affordable post-synthetic modifications (PSM) are stabilized by means of REO ion exchange and/or hydrothermal treatments, leading to FCC catalysts with improved hydrothermal stability, increased bottoms conversion capacity and improved product selectivity, as compared to a conventional commercial USY based catalyst of comparable activity. The stabilized mesoporous USY yields more and better quality diesel with a reduced content of polyaromatic compounds, while producing lower amounts of gases but with a LPG fraction enriched in propene and butenes. The obtained selectivity slate is attributed to the combination of appropriate Brønsted acidity and reduced diffusion pathway in the zeolite crystals. A detailed characterization relates the physicochemical, textural and morphological changes induced by the PSM to the resulting catalytic behavior.
The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilicatio... more The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17 Â 4 Â 4 lm 3 ). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m 2 g À1 of mesopore surface area, respectively, in contrast to the 62 and 5 m 2 g À1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.
The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially re... more The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially reduced Ni-Al mixed oxides derived from takovite, a hydrotalcite-type compound. The unique attributes of the hydrotalcite route leads to more active and selective catalysts compared to conventional Ni/Al 2 O 3 prepared by impregnation. Tuning calcination and reduction conditions of the catalyst precursor is essential to optimize the hydrogenation performance. The best catalyst, calcined and reduced at 773 K, rendered stable propene yields up to ca. 65% and consisted of a Ni(Al)O x solid solution with 55% of the total bulk nickel in reduced form and surface enrichment by aluminum. Sintering of NiO and crystallization of NiAl 2 O 4 at high calcination temperature induce lower activity. The alkyne or diene conversion increases with the percentage of metallic Ni in the samples, while an optimal degree of nickel reduction maximizes the monoalkene selectivity. Below the optimum, oligomer formation is favored and above the optimum, alkane production increases. A similar pattern was found for the H 2 /HC ratio. The alkene selectivity experiences a dramatic increase in early stages of the reaction, which correlated with the build-up of C-containing species on the catalyst (sub-)surface. These selectivity-enhancing species are formed at specific reaction temperatures, highlighting the relevance of the testing procedure on assessing hydrogenation catalysts. The catalytic performance is strongly influenced by the hydrocarbon substrate. In contrast to propyne and propadiene, ethyne hydrogenation led to a C 2 H 4 yield of only 6%.
Hierarchical (or mesoporous) zeolites have received an ever-increasing attention due to their imp... more Hierarchical (or mesoporous) zeolites have received an ever-increasing attention due to their improved performance in catalysed reactions with respect to conventional (purely microporous) zeolites. Desilication in alkaline media has become a widely applied preparation method to tailor these modified zeolites, due to an optimal combination of efficiency and simplicity. This review presents recent developments that have expanded its general understanding and turned this top-down treatment highly versatile, controllable, and scalable. Design aspects of mesoporous zeolites for catalytic applications are emphasised, encircling the establishment of synthesis-property-function relationships. Alkaline treatment is a key step in strategic combinations with other post-synthesis modifications towards superior zeolite catalysts. The outlook of the field, pinpointing present needs and short-term priorities, is discussed.
Catalysis Science & Technology, 2011
A hierarchical (mesoporous) ZSM-22 zeolite displays a greatly enhanced sorption capacity for n-oc... more A hierarchical (mesoporous) ZSM-22 zeolite displays a greatly enhanced sorption capacity for n-octane, compared to its purely microporous parent. In n-octane hydro-isomerisation, the mesoporous bi-functional Pt/ZSM-22 catalyst clearly outperforms its microporous parent, judged by the higher monobranched isomer yield. This is attributed to an increased number of accessible micropore mouths in the mesoporous zeolite.
Catalysis Science & Technology, 2012
Desilication of conventional zeolites in alkaline medium generates variable intracrystalline meso... more Desilication of conventional zeolites in alkaline medium generates variable intracrystalline mesoporosity, but inevitably changes other properties such as the Si/Al ratio and aluminium distribution. Assessing the individual effects of porosity and composition on the catalytic performance of desilicated zeolites is relevant for their optimal design. Herein, we decouple the respective impacts in the acid-catalysed alkylation of toluene (or cyclohexylbenzene) with benzyl alcohol. These reactions experience strong accessibility constraints to the micropores, providing high sensitivity to the properties of the developed mesopore surface. Through strategic comparison of alkaline-treated ZSM-5 zeolites prepared with and without subsequent acid treatment, we show that while acidity is important, the alkylation activity is dominated by the mesoporous surface area. The selectivity to (methylbenzyl)benzene does not depend on the available external surface. Excessively large mesopore volumes are detrimental to the activity, and variation in micropore volume has a minimal effect. Acid-treated mesoporous zeolites exhibit higher catalytic activities primarily due to textural enhancements by removal of aluminium-rich amorphous debris. The catalytic results are rationalised on the basis of extensive characterisation (AAS, N 2 sorption, XRD, TEM, 27 Al MAS NMR, FTIR, NH 3 -TPD, and adsorption of toluene and cyclohexylbenzene).
The Journal of Physical Chemistry C, 2011
We demonstrate that desilication in alkaline medium is a suitable post-synthetic method to introd... more We demonstrate that desilication in alkaline medium is a suitable post-synthetic method to introduce intracrystalline mesoporosity in MFI zeolites independent of the Si/Al ratio in the parent material. By systematic screening of the influence of both base concentration (0.1À1.8 M NaOH) and Si/Al ratio (10À1000) on the properties of the treated zeolites, we reveal that effective mesoporosity introduction (>200 m 2 g À1 ) may be achieved in the Si/Al range of 12À200. The use of descriptors like the "indexed hierarchy factor" and the "desilication efficiency" enable the rational categorization of the solids obtained. The highest desilication efficiencies, estimated by correlating the introduced mesoporosity with the yields after NaOH treatment, are obtained in the previously established Si/Al range of 25À50. We identify the crucial role of a subsequent acid treatment for removing amorphous Alrich debris from alkaline-treated samples in the case of low Si/Al ratios (<20). The latter acid wash uncovers the complete micro-and mesopore network, enabling full compositional flexibility of desilication. The removal of these debris concomitantly enabled restoration of both the acidity and the chemical composition of the hierarchical zeolite to that of the starting (purely microporous) zeolite. Catalytic evaluation of selected Al-rich zeolites in the alkylation of toluene with benzyl alcohol, confirmed the superiority of the mesoporous alkaline-treated samples with respect to the parent material. Hierarchical ZSM-5 after acid washing stands as the most active sample, which stresses the relevance of the additional post-synthetic treatment step.
Soft Matter, 2009
Direct visual observations on how grain boundaries are pinned between multiple large spherical im... more Direct visual observations on how grain boundaries are pinned between multiple large spherical impurities during colloidal hard sphere crystallisation are presented. The fluid is stabilized between impurities and acts as a precursor for grain boundary formation. The range of fluid stabilisation by a single impurity is characterized by the frustration length, which goes through a maximum as a function of the impurity-to-particle size ratio. Grain boundaries are more strongly confined to the area between two impurities as the ratio between the impurity-to-impurity spacing and the combined frustration lengths decreases. Our results identify the key parameters in grain boundary formation in doped systems, which may lead to a better control of the grain boundary density in materials.
Journal of Physics: Condensed Matter, 2005
We report how large spherical impurities affect the nucleation and growth of hard sphere colloida... more We report how large spherical impurities affect the nucleation and growth of hard sphere colloidal crystals. Both the impurities and the colloids are fluorescently labelled polymethylmetacrylate particles and are dispersed in an optically and density matching solvent mixture. Crystal growth, initiated either at the impurity surface, or at the sample bottom, was studied by imaging sequences of two-dimensional x y-slices in the plane of the impurity's centre of mass with a laser scanning confocal microscope. At least two factors determine whether a large impurity can function as a seed for heterogeneous nucleation: timescales and impurity curvature. The curvature needs to be sufficiently low for crystal nuclei to form on the impurity surface. If bulk crystal growth has already approached the impurity, bulk growth is dominant over growth of crystallites on the impurity surface. Such surface crystallites eventually reorient to adapt to the overall bulk crystal symmetry.
Journal of Catalysis, 2008
The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially re... more The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially reduced Ni-Al mixed oxides derived from takovite, a hydrotalcite-type compound. The unique attributes of the hydrotalcite route leads to more active and selective catalysts compared to conventional Ni/Al 2 O 3 prepared by impregnation. Tuning calcination and reduction conditions of the catalyst precursor is essential to optimize the hydrogenation performance. The best catalyst, calcined and reduced at 773 K, rendered stable propene yields up to ca. 65% and consisted of a Ni(Al)O x solid solution with 55% of the total bulk nickel in reduced form and surface enrichment by aluminum. Sintering of NiO and crystallization of NiAl 2 O 4 at high calcination temperature induce lower activity. The alkyne or diene conversion increases with the percentage of metallic Ni in the samples, while an optimal degree of nickel reduction maximizes the monoalkene selectivity. Below the optimum, oligomer formation is favored and above the optimum, alkane production increases. A similar pattern was found for the H 2 /HC ratio. The alkene selectivity experiences a dramatic increase in early stages of the reaction, which correlated with the build-up of C-containing species on the catalyst (sub-)surface. These selectivity-enhancing species are formed at specific reaction temperatures, highlighting the relevance of the testing procedure on assessing hydrogenation catalysts. The catalytic performance is strongly influenced by the hydrocarbon substrate. In contrast to propyne and propadiene, ethyne hydrogenation led to a C 2 H 4 yield of only 6%.
CrystEngComm, 2011
Mesoporous ZSM-22 zeolite (TON structure) is prepared by controlled silicon extraction in aqueous... more Mesoporous ZSM-22 zeolite (TON structure) is prepared by controlled silicon extraction in aqueous NaOH under different conditions of concentration, temperature, and time. The first challenge is to synthesize a pure ZSM-22 sample, since ZSM-5 and cristobalite impurities are hard to avoid. For this purpose, hydrothermal syntheses in autoclaves of 30-1000 cm 3 using tumbling or magnetic mixing were conducted. The parent (calcined) and alkaline-treated samples are characterised by XRD, AAS, N 2 and Ar adsorption, TEM, 27 Al MAS NMR, and FTIR. The introduction of mesoporosity in ZSM-22 crystals is not straightforward due to their peculiar characteristics: the rod-like morphology of their small crystallites, the one-dimensionality of the ellipsoidal micropore system, and an uneven Al distribution. Compared to other zeolite frameworks such as MFI, the generation of up to 95 m 2 g À1 of (both inter-and intracrystalline) mesopore surface area by NaOH treatment leads to a sizeable drop of the micropore volume (down to 0.006 cm 3 g À1 ), attributed to blocking by re-deposited Al species. A subsequent mild acid treatment in aqueous HCl restores ca. 90% of the original micropore volume and increases the mesopore surface area to 114 m 2 g À1 . However, due to the particular Al distribution in the parent ZSM-22 crystals, only 37% of the original Brønsted acidity is recovered. A new descriptor 'desilication efficiency' is introduced to relate the mesopore area generated to the mass of zeolite dissolved. In the case of ZSM-22 nanorods and ferrierite platelets, the desilication efficiency is relatively low compared to ZSM-5, most likely due to the crystal morphology of the former two zeolites. The auxiliary mesoporosity developed in ZSM-22 brings new prospects to catalytic applications of the zeolite due to the extensive creation of pore mouths in the hierarchical sample.
Crystal Growth & Design, 2012
Molecular criteria for the selection of organic pore-directing agents (PDAs) in NaOH leaching, th... more Molecular criteria for the selection of organic pore-directing agents (PDAs) in NaOH leaching, that is, desilication, were investigated on USY and beta zeolites of distinct aluminum contents (Si/Al = 15−385). PDAs prove particularly useful for FAU and BEA topologies since they serve a dual purpose: tailoring the mesopore structure while preventing realumination and amorphization of the crystals. An efficient PDA is positively charged and contains ca. 10−20 carbon atoms, for example, TPA + or CTA + . Compositional, textural, morphological, structural, and acidity studies performed on selected hierarchical zeolites confirmed the presence of extensive secondary porosity coupled to well-preserved zeolitic properties. Inclusion of TPA + in the alkaline solution led to the largest preservation of the intrinsic zeolite properties, whereas CTA + facilitates the reassembly of dissolved species during alkaline treatment. Finally, we report the preparation of mesoporous zeolites in a continuous-mode using a tubular reactor and a high-shear reactor, attaining productivities up to 100 times higher than in conventional batch preparation.
CHIMIA International Journal for Chemistry, 2013
This review emphasizes key recent accomplishments towards the industrial exploitation of hierarch... more This review emphasizes key recent accomplishments towards the industrial exploitation of hierarchically structured zeolites in catalytic processes. A major milestone comprises the demonstration that affordable post-synthetic modifications enable the transformation of any conventional zeolite into hierarchical analogues with tunable porosity and functionality. Through specific examples, belonging to the transformation of fossil fuel and renewable feedstocks, we quantitatively illustrate the spectacular benefits attained upon application of hierarchical zeolite catalysts due to improved accessibility or modification of the type and distribution of active sites. A crucial step for these exciting lab-designed materials to be implemented in industrial processes is to shape them into technical forms. Accordingly, we studied the synthesis, characterization, and catalytic evaluation of millimeter-sized hierarchical zeolite bodies, enriching the fundamental understanding on scale-up and representing an additional solid step towards the commercial application of these materials.
Uploads
Papers by Danny Verboekend