Establishing ZIF-8 as a reference material for hydrogen cryoadsorption: An interlaboratory study.

Hydrogen storage by cryoadsorption on porous materials has the advantages of low material cost, safety, fast kinetics, and high cyclic stability. The further development of this technology requires reliable data on the H2 uptake of the adsorbents, however, even for activated carbons the values between different laboratories show sometimes large discrepancies. So far no reference material for hydrogen cryoadsorption is available. The metal-organic framework ZIF-8 is an ideal material possessing high thermal, chemical, and mechanical stability that reduces degradation during handling and activation. Here, we distributed ZIF-8 pellets synthesized by extrusion to 9 laboratories equipped with 15 different experimental setups including gravimetric and volumetric analyzers. The gravimetric H2 uptake of the pellets was measured at 77 K and up to 100 bar showing a high reproducibility between the different laboratories, with a small relative standard deviation of 3-4 % between pressures of 10-100 bar. The effect of operating variables like the amount of sample or analysis temperature was evaluated, remarking the calibration of devices and other correction procedures as the most significant deviation sources. Overall, the reproducible hydrogen cryoadsorption measurements indicate the robustness of the ZIF-8 pellets, which we want to propose as a reference material.

Hydrogen Storage in Porous Materials: Status, Milestones, and Challenges.

In this account the most relevant advancements in hydrogen storage in porous materials are presented. These include the current state-of-the-art, the challenges which have been overcome, and the hurdles which still remain. The most important milestones which will be discussed in this work will be the development of new apparatuses capable of delivering reliable results under a broad range of operational conditions, in which analysis temperature and pressure are critical parameters. Other aspects such as the materials storage capacity in gravimetric and volumetric terms will be critically discussed to identify the conditions required from an ideal material. Finally, different upgrade possibilities from modifying the adsorbate-adsorbent interaction to using rigid or flexible materials will be presented and put into perspective with current literature.

Biotechnological Applications of Proteases in Food Technology.

This review presents some of the hottest topics in biotechnological applications: proteases in biocatalysis. Obviously, one of the most relevant areas of application is in the hydrolysis of proteins in food technology, and that has led to a massive use on proteomics. The aim is to identify via peptide maps the different proteins obtained after a specific protease hydrolysis. However, concepts like degradomics are also taking on a more relevant importance in the use and study of proteases and will also be discussed. Other protease applications, as seem in cleaning (detergent development), the pharmaceutical industry, and in fine chemistry, will be analyzed. This review progresses from basic areas such as protease classification to a discussion of the preparation of protease-immobilized biocatalysts, considering the different problems raised by the use of immobilized proteases due to the peculiar features of the substrates, usually large macromolecules. Production of bioactive peptides via limited hydrolysis of proteins will occupy an important place in this review.

Synthesis of TiO2 with Hierarchical Porosity for the Photooxidation of Propene.

The elimination of volatile organic compounds (VOCs) at low concentration is a subject of great interest because these compounds are very harmful for the environment and human health. In this work, we have developed a synthesis methodology of TiO2 that allows obtaining meso-macroporous materials with hierarchical porosity and with high thermal stability for their application as photocatalysts in the removal of VOCs, specifically propene. The materials synthesized in this work were characterized by Scanning electron microscope (SEM), Transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Thermogravimetric Analysis (TG), and nitrogen adsorption. It is observed that the samples calcined at 250 °C and 500 °C present a high photoactivity for the photooxidation of propene, which is similar to the benchmark material P25 (commercial TiO2). Moreover, the textural properties are better than those for P25, indicating that the samples are interesting for the preparation of photocatalysts with different conformations, such as in the form of coatings and fillings in different size scales.

Evolution of the PVP-Pd Surface Interaction in Nanoparticles through the Case Study of Formic Acid Decomposition.

Palladium nanoparticles (Pd NPs) were synthesized by the reduction-by-solvent method using polyvinylpirrolidone (PVP) as capping agent. The nonstatic interaction between PVP and the metallic surface may change the properties of the NPs due to the different possible interactions, through either the O or N atoms of the PVP. In order to analyze these effects and their repercussions in their catalytic performance, Pd NPs with various PVP/Pd molar ratios (1, 10, and 20) were prepared, deposited on silica, and tested in the formic acid decomposition reaction. The catalytic tests were conducted using catalysts prepared by loading NPs with three different time lapses between their purification and their deposition on the silica support (1 day, 1 month, and 6 months). CO adsorption, FTIR spectroscopy, XPS, and TEM characterization were used to determine the accessibility of the Pd NPs surface sites, the electronic state of Pd, and the average NPs size, respectively. The H2 production from the formic acid decomposition reaction has a strong dependence on the Pd surface features, which in turn are related to the NPs aging time due to the progressive removal of the PVP.

Chemical Modification in the Design of Immobilized Enzyme Biocatalysts: Drawbacks and Opportunities.

Chemical modification of enzymes and immobilization used to be considered as separate ways to improve enzyme properties. This review shows how the coupled use of both tools may greatly improve the final biocatalyst performance. Chemical modification of a previously immobilized enzyme is far simpler and easier to control than the modification of the free enzyme. Moreover, if protein modification is performed to improve its immobilization (enriching the enzyme in reactive groups), the final features of the immobilized enzyme may be greatly improved. Chemical modification may be directed to improve enzyme stability, but also to improve selectivity, specificity, activity, and even cell penetrability. Coupling of immobilization and chemical modification with site-directed mutagenesis is a powerful instrument to obtain fully controlled modification. Some new ideas such as photoreceptive enzyme modifiers that change their physical properties under UV exposition are discussed.

Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters.

Supported gold nanoparticles have excited much interest owing to their unusual and somewhat unexpected catalytic properties, but the origin of the catalytic activity is still not fully understood. Experimental work on gold particles supported on a titanium dioxide (110) single-crystal surface has established a striking size threshold effect associated with a metal-to-insulator transition, with gold particles catalytically active only if their diameters fall below approximately 3.5 nm. However, the remarkable catalytic behaviour might also in part arise from strong electronic interaction between the gold and the titanium dioxide support. In the case of industrially important selective oxidation reactions, explanation of the effectiveness of gold nanoparticle catalysts is complicated by the need for additives to drive the reaction, and/or the presence of strong support interactions and incomplete understanding of their possible catalytic role. Here we show that very small gold entities ( approximately 1.4 nm) derived from 55-atom gold clusters and supported on inert materials are efficient and robust catalysts for the selective oxidation of styrene by dioxygen. We find a sharp size threshold in catalytic activity, in that particles with diameters of approximately 2 nm and above are completely inactive. Our observations suggest that catalytic activity arises from the altered electronic structure intrinsic to small gold nanoparticles, and that the use of 55-atom gold clusters may prove a viable route to the synthesis of robust gold catalysts suited to practical application.

Modifying enzyme activity and selectivity by immobilization.

Immobilization of enzymes may produce alterations in their observed activity, specificity or selectivity. Although in many cases an impoverishment of the enzyme properties is observed upon immobilization (caused by the distortion of the enzyme due to the interaction with the support) in some instances such properties may be enhanced by this immobilization. These alterations in enzyme properties are sometimes associated with changes in the enzyme structure. Occasionally, these variations will be positive. For example, they may be related to the stabilization of a hyperactivated form of the enzyme, like in the case of lipases immobilized on hydrophobic supports via interfacial activation. In some other instances, these improvements will be just a consequence of random modifications in the enzyme properties that in some reactions will be positive while in others may be negative. For this reason, the preparation of a library of biocatalysts as broad as possible may be a key turning point to find an immobilized biocatalyst with improved properties when compared to the free enzyme. Immobilized enzymes will be dispersed on the support surface and aggregation will no longer be possible, while the free enzyme may suffer aggregation, which greatly decreases enzyme activity. Moreover, enzyme rigidification may lead to preservation of the enzyme properties under drastic conditions in which the enzyme tends to become distorted thus decreasing its activity. Furthermore, immobilization of enzymes on a support, mainly on a porous support, may in many cases also have a positive impact on the observed enzyme behavior, not really related to structural changes. For example, the promotion of diffusional problems (e.g., pH gradients, substrate or product gradients), partition (towards or away from the enzyme environment, for substrate or products), or the blocking of some areas (e.g., reducing inhibitions) may greatly improve enzyme performance. Thus, in this tutorial review, we will try to list and explain some of the main reasons that may produce an improvement in enzyme activity, specificity or selectivity, either real or apparent, due to immobilization.

Proteases immobilized on nanomaterials for biocatalytic, environmental and biomedical applications: Advantages and drawbacks.

Proteases have gained significant scientific and industrial interest due to their unique biocatalytic characteristics and broad-spectrum applications in different industries. The development of robust nanobiocatalytic systems by attaching proteases onto various nanostructured materials as fascinating and novel nanocarriers has demonstrated exceptional biocatalytic performance, substantial stability, and ease of recyclability over multiple reaction cycles under different chemical and physical conditions. Proteases immobilized on nanocarriers may be much more resistant to denaturation caused by extreme temperatures or pH values, detergents, organic solvents, and other protein denaturants than free enzymes. Immobilized proteases may present a lower inhibition. The use of non-porous materials in the immobilization prevents diffusion and steric hindrances during the binding of the substrate to the active sites of enzymes compared to immobilization onto porous materials; when using very large or solid substrates, orientation of the enzyme must always be adequate. The advantages and problems of the immobilization of proteases on nanoparticles are discussed in this review. The continuous and batch reactor operations of nanocarrier-immobilized proteases have been successfully investigated for a variety of applications in the leather, detergent, biomedical, food, and pharmaceutical industries. Information about immobilized proteases on various nanocarriers and nanomaterials has been systematically compiled here. Furthermore, different industrial applications of immobilized proteases have also been highlighted in this review.

A review on the immobilization of bromelain.

This review shows the endeavors performed to prepare immobilized formulations of bromelain extract, usually from pineapple, and their use in diverse applications. This extract has a potent proteolytic component that is based on thiol proteases, which differ depending on the location on the fruit. Stem and fruit are the areas where higher activity is found. The edible origin of this enzyme is one of the features that determines the applications of the immobilized bromelain to a more significant degree. The enzyme has been immobilized on a wide diversity of supports via different strategies (covalent bonds, ion exchange), and also forming ex novo solids (nanoflowers, CLEAs, trapping in alginate beads, etc.). The use of preexisting nanoparticles as immobilization supports is relevant, as this facilitates one of the main applications of the immobilized enzyme, in therapeutic applications (as wound dressing and healing components, antibacterial or anticancer, mucus mobility control, etc.). A curiosity is the immobilization of this enzyme on spores of probiotic microorganisms via adsorption, in order to have a perfect in vivo compatibility. Other outstanding applications of the immobilized enzyme are in the stabilization of wine versus haze during storage, mainly when immobilized on chitosan. Curiously, the immobilized bromelain has been scarcely applied in the production of bioactive peptides.

Nanoarchitectures based on layered titanosilicates supported on glass fibers: application to hydrogen storage.

This work reports on the synthesis of nanosheets of layered titanosilicate JDF-L1 supported on commercial E-type glass fibers with the aim of developing novel nanoarchitectures useful as robust and easy to handle hydrogen adsorbents. The preparation of those materials is carried out by hydrothermal reaction from the corresponding gel precursor in the presence of the glass support. Because of the basic character of the synthesis media, silica from the silicate-based glass fibers can be involved in the reaction, cementing its associated titanosilicate and giving rise to strong linkages on the support with the result of very stable heterostructures. The nanoarchitectures built up by this approach promote the growth and disposition of the titanosilicate nanosheets as a house-of-cards radially distributed around the fiber axis. Such an open arrangement represents suitable geometry for potential uses in adsorption and catalytic applications where the active surface has to be available. The content of the titanosilicate crystalline phase in the system represents about 12 wt %, and this percentage of the adsorbent fraction can achieve, at 298 K and 20 MPa, 0.14 wt % hydrogen adsorption with respect to the total mass of the system. Following postsynthesis treatments, small amounts of Pd (<0.1 wt %) have been incorporated into the resulting nanoarchitectures in order to improve their hydrogen adsorption capacity. In this way, Pd-layered titanosilicate supported on glass fibers has been tested as a hydrogen adsorbent at diverse pressures and temperatures, giving rise to values around 0.46 wt % at 298 K and 20 MPa. A mechanism of hydrogen spillover involving the titanosilicate framework and the Pd nanoparticules has been proposed to explain the high increase in the hydrogen uptake capacity after the incorporation of Pd into the nanoarchitecture.

Heterofunctional supports in enzyme immobilization: from traditional immobilization protocols to opportunities in tuning enzyme properties.

A heterofunctional support for enzyme immobilization may be defined as that which possesses several distinct functionalities on its surface able to interact with a protein. We will focus on those supports in which a final covalent attachment between the enzyme and the support is achieved. Heterofunctionality sometimes has been featured in very old immobilization techniques, even though in many instances it has been overlooked, giving rise to some misunderstandings. In this respect, glutaraldehyde-activated supports are the oldest multifunctional supports. Their matrix has primary amino groups, the hydrophobic glutaraldehyde chain, and can covalently react with the primary amino groups of the enzyme. Thus, immobilization may start (first event of the immobilization) via different causes and may involve different positions of the enzyme surface depending on the activation degree and immobilization conditions. Other "classical" heterofunctional supports are epoxy commercial supports consisting of reactive covalent epoxy groups on a hydrophobic matrix. Immobilization is performed at high ionic strength to permit protein adsorption, so that covalent attachment may take place at a later stage. Starting from these old immobilization techniques, tailor-made heterofunctional supports have been designed to permit a stricter control of the enzyme immobilization process. The requirement is to find conditions where the main covalent reactive moieties may have very low reactivity toward the enzyme. In this Review we will discuss the suitable properties of the groups able to give the covalent attachment (intending a multipoint covalent attachment), and the groups able to produce the first enzyme adsorption on the support. Prospects, limitations, and likely pathways for the evolution (e.g., coupling of site-directed mutagenesis and thiol heterofunctional supports of enzyme immobilization on heterofunctional supports) will be discussed in this Review.

Clay-supported graphene materials: application to hydrogen storage.

The present work refers to clay-graphene nanomaterials prepared by a green way using caramel from sucrose and two types of natural clays (montmorillonite and sepiolite) as precursors, with the aim of evaluating their potential use in hydrogen storage. The impregnation of the clay substrates by caramel in aqueous media, followed by a thermal treatment in the absence of oxygen of these clay-caramel intermediates gives rise to graphene-like materials, which remain strongly bound to the silicate support. The nature of the resulting materials was characterized by different techniques such as XRD, Raman spectroscopy and TEM, as well as by adsorption isotherms of N2, CO2 and H2O. These carbon-clay nanocomposites can act as adsorbents for hydrogen storage, achieving, at 298 K and 20 MPa, over 0.1 wt% of hydrogen adsorption excess related to the total mass of the system, and a maximum value close to 0.4 wt% of hydrogen specifically related to the carbon mass. The very high isosteric heat for hydrogen sorption determined from adsorption isotherms at different temperatures (14.5 kJ mol(-1)) fits well with the theoretical values available for hydrogen storage on materials that show a strong stabilization of the H2 molecule upon adsorption.

Binderless thin films of zeolite-templated carbon electrodes useful for electrochemical microcapacitors with ultrahigh rate performance.

Controlled nanozeolite deposits are prepared by electrochemical techniques on a macroporous carbon support and binderless thin film electrodes of zeolite-templated carbon are synthesized using the deposits as templates. The obtained film electrodes exhibit extremely high area capacitance (10-12 mF cm(-2)) and ultrahigh rate capability in a thin film capacitor.

Biocatalytic production of biolubricants: Strategies, problems and future trends.

The increasing worries by the inadequate use of energy and the preservation of nature are promoting an increasing interest in the production of biolubricants. After discussing the necessity of producing biolubricants, this review focuses on the production of these interesting molecules through the use of lipases, discussing the different possibilities (esterification of free fatty acids, hydroesterification or transesterification of oils and fats, transesterification of biodiesel with more adequate alcohols, estolides production, modification of fatty acids). The utilization of discarded substrates has special interest due to the double positive ecological impact (e.g., oil distillated, overused oils). Pros and cons of all these possibilities, together with general considerations to optimize the different processes will be outlined. Some possibilities to overcome some of the problems detected in the production of these interesting compounds will be also discussed.

A review on the immobilization of pepsin: A Lys-poor enzyme that is unstable at alkaline pH values.

Pepsin is a protease used in many different applications, and in many instances, it is utilized in an immobilized form to prevent contamination of the reaction product. This enzyme has two peculiarities that make its immobilization complex. The first one is related to the poor presence of primary amino groups on its surface (just one Lys and the terminal amino group). The second one is its poor stability at alkaline pH values. Both features make the immobilization of this enzyme to be considered a complicated goal, as most of the immobilization protocols utilize primary amino groups for immobilization. This review presents some of the attempts to get immobilized pepsin biocatalyst and their applications. The high density of anionic groups (Asp and Glu) make the anion exchange of the enzyme simpler, but this makes many of the strategies utilized to immobilize the enzyme (e.g., amino-glutaraldehyde supports) more related to a mixed ion exchange/hydrophobic adsorption than to real covalent immobilization. Finally, we propose some possibilities that can permit not only the covalent immobilization of this enzyme, but also their stabilization via multipoint covalent attachment.

Nanoparticulate PdZn--pathways towards the synthetic control of nanosurface properties.

This paper reports an in-depth structural investigation of PdZn nanoparticulates prepared over an entire compositional range. By using a combination of HRTEM, ICP-OES, EDX and XPS alongside PXRD, we are able to show how a liquid-type reduction process can be exploited to target different PdZn bimetallic structures while maintaining reproducibly narrow particle size distributions and average particle diameters of approximately 3 nm. Samples have been further analyzed by quantitative phase analysis of the Rietveld refined diffraction data, providing indications as to how variations in specific surface compositions are obtained when Zn is used as the alloying metal. The influence of nanolattice strain is investigated by geometric analysis of TEM data. Results suggest, in conjunction with previously published catalytic data, how different compositions of this specific bimetallic system may be exploited in catalytic processes to control substrate/product affinity. We thus demonstrate a new and simplified approach to PdZn bimetallics, which may offer novel perspectives for applications in industrial catalysis.

Stabilization of enzymes via immobilization: Multipoint covalent attachment and other stabilization strategies.

The use of enzymes in industrial processes requires the improvement of their features in many instances. Enzyme immobilization, a requirement to facilitate the recovery and reuse of these water-soluble catalysts, is one of the tools that researchers may utilize to improve many of their properties. This review is focused on how enzyme immobilization may improve enzyme stability. Starting from the stabilization effects that an enzyme may experience by the mere fact of being inside a solid particle, we detail other possibilities to stabilize enzymes: generation of favorable enzyme environments, prevention of enzyme subunit dissociation in multimeric enzymes, generation of more stable enzyme conformations, or enzyme rigidification via multipoint covalent attachment. In this last point, we will discuss the features of an "ideal" immobilization protocol to maximize the intensity of the enzyme-support interactions. The most interesting active groups in the support (glutaraldehyde, epoxide, glyoxyl and vinyl sulfone) will be also presented, discussing their main properties and uses. Some instances in which the number of enzyme-support bonds is not directly related to a higher stabilization will be also presented. Finally, the possibility of coupling site-directed mutagenesis or chemical modification to get a more intense multipoint covalent immobilization will be discussed.

Immobilization of papain: A review.

Papain is a cysteine protease from papaya, with many applications due to its broad specificity. This paper reviews for first time the immobilization of papain on different supports (organic, inorganic or hybrid supports) presenting some of the features of the utilized immobilization strategies (e.g., epoxide, glutaraldehyde, genipin, glyoxyl for covalent immobilization). Special focus is placed on the preparation of magnetic biocatalysts, which will permit the simple recovery of the biocatalyst even if the medium is a suspension. Problems specific to the immobilization of proteases (e.g., steric problems when hydrolyzing large proteins) are also defined. The benefits of a proper immobilization (enzyme stabilization, widening of the operation window) are discussed, together with some artifacts that may suggest an enzyme stabilization that may be unrelated to enzyme rigidification.

Enzyme co-immobilization: Always the biocatalyst designers' choice or not?

The increasing relevance of cascade reactions in biocatalysis has sparked a great interest for enzyme co-immobilization. Enzyme co-immobilization allows access to some kinetic advantages that in some instances are necessary to get the desired product, avoiding side-reactions. However, the kinetic effect is very relevant mainly at the initial reaction rates, while it may be less relevant in the whole reaction course, depending on the kinetic parameters of the involved enzymes. This review not only critically discusses the advantages but also the drawbacks of enzymes co-immobilization: immobilization on the same support and surface, under similar conditions, discarding the whole biocatalyst when one of the co-immobilized enzymes is inactivated. We will discuss when co-immobilization is almost compulsory, when the advantages of co-immobilization may not be enough to compensate their problems and when it should be fully discarded. The co-immobilization of cofactors and enzymes bears special interest, as this can open up the opportunity to the building of artificial cells and extremely complex one-pot transformations. Finally, some recent strategies to overcome some the co-immobilization problems will be presented.