T. Alexander Nijhuis

Preparation of monolithic catalysts

Monolithic catalysts can be attractive replacements for conventional catalysts in randomly packed beds or slurry reactors. The conventional procedures for preparing catalysts, however, cannot simply be applied to monolithic catalysts. Different procedures are discussed on how to put a coat layer of a catalyst support material like alumina, silica, or carbon on a monolith body by either filling the pores in that support or by putting a layer on that support. Different methods to apply an active phase to the support are discussed as well. Finally, methods to convert ready-made catalysts into monolithic catalysts are presented.

Fructose Dehydration to 5-Hydroxymethylfurfural over Solid Acid Catalysts in a Biphasic System

Different acidic heterogeneous catalysts like alumina, aluminosilicate, zirconium phosphate, niobic acid, ion-exchange resin Amberlyst-15, and zeolite MOR have been studied in fructose dehydration to 5-hydroxymethylfurfural (HMF). The acidity of these materials was characterized using temperature-programmed desorption of NH₃ and IR spectroscopy of adsorbed pyridine. The nature and strength of acid sites was shown to play a crucial role in the selectivity towards HMF. Brønsted acid sites in the case of zeolites and ion-exchange resin led to high selectivities in the dehydration of fructose with an increase in selectivity with the addition of an organic phase. Lewis acidity in the case of phosphate and oxides resulted in the intensive production of humins from fructose at the initial stages of the process, whereas organic phase addition did not affect selectivity.

Glucose Dehydration to 5-Hydroxymethylfurfural in a Biphasic System over Solid Acid Foams

A solid acid foam-structured catalyst based on a binderless zirconium phosphate (ZrPO) coating on aluminum foam was prepared. The catalyst layer was obtained by performing a multiple washcoating procedure of ZrPO slurry on the anodized aluminum foam. The effect of the pretreatment of ZrPO, the concentration of the slurry, and the amount of coating on the properties of the foam was studied. The catalytic properties of the prepared foams have been evaluated in the dehydration of glucose to 5-hydroxymethylfurfural (HMF) in a biphasic reactor. The catalytic behavior of ZrPO foam-based catalysts was studied in a rotating foam reactor and compared with that of bulk ZrPO. The effect of a silylation procedure on the selectivity of the process was shown over bulk and foam catalysts. This treatment resulted in a higher selectivity due to the deactivation of unselective Lewis acid sites. Addition of methylisobutylketone leads to extraction of HMF from the aqueous phase and stabilization of the selectivity to HMF over bulk ZrPO. A more intensive contact of the foam with the aqueous and organic phases leads to an increase in the selectivity and resistance to deactivation of the foam in comparison with a bulk catalyst.

Hydrogen Production through Aqueous‐Phase Reforming of Ethylene Glycol in a Washcoated Microchannel

Aqueous-phase reforming (APR) of biocarbohydrates is conducted in a catalytically stable washcoated microreactor where multiphase hydrogen removal enhances hydrogen efficiency. Single microchannel experiments are conducted following a simplified model based on the microreactor concept. A coating method to deposit a Pt-based catalyst on the microchannel walls is selected and optimized. APR reactivity tests are performed by using ethylene glycol as the model compound. Optimum results are achieved with a static washcoating technique; a highly uniform and well adhered 5 μm layer is deposited on the walls of a 320 μm internal diameter (ID) microchannel in one single step. During APR of ethylene glycol, the catalyst layer exhibits high stability over 10 days after limited initial deactivation. The microchannel presents higher conversion and selectivity to hydrogen than a fixed-bed reactor. The benefits of using a microreactor for APR can be further enhanced by utilizing increased Pt loadings, higher reaction temperatures, and larger carbohydrates (e.g., glucose). The use of microtechnology for aqueous-phase reforming will allow for a great reduction in the reformer size, thus rendering it promising for distributed hydrogen production.

Monolithic Catalysts and Reactors

Abstract Structured catalysts and reactors offer high precision in catalysis at all relevant scales of the catalytic process, from that of the catalytic species up to that of the reactor. Monoliths are the prime example of such catalysts because of their wide practical applications. Thus, monoliths are emphasized in this review, but most of the text is also relevant to all structured reactors, including microreactors. Conceptually, monoliths exhibit more degrees of freedom in design than conventional reactors, such as fixed-bed and slurry reactors. The flow in monoliths is laminar, and as a consequence, they are associated with high efficiency and minimum chaotic characteristics. The hydrodynamics of single-phase and multiphase flow reactors are remarkably simple. Under most conditions in multiphase systems, Taylor flow (segmented flow) prevails, associated with high rates of mass transfer notwithstanding low energy consumption, but under other conditions, the film flow regime can be realized either in cocurrent or in countercurrent flow of gas and liquid streams, making the monolith a good structure for novel technologies such as catalytic distillation. Monoliths offer freedom in the design of reactor configuration. Examples are loop reactors for strong exo- and endothermic reactions, which allow a combination with separate heat exchange without the penalty of a large energy consumption, which otherwise is usually unavoidable for the large recycle ratios needed. For applications in fine chemistry and in the laboratory, a convenient monolithic stirred reactor is presented. The principal bottleneck for practical application of monolith reactors is the synthesis rather than the design of the catalytic monolith. When a monolith reactor is considered as an alternative to a fixed-bed reactor packed with commercially available catalyst particles, the grim reality is that a development program is needed to producing the catalytic monolith. Therefore, preparation methods including synthesis of various coating layers and the deposition of active catalytic species are described in detail here. This chapter also includes an exhaustive review of practical applications of monolith reactors. In applications in which high gas flow rates have to be accommodated, monoliths monoliths are the state of the art in many cases, exemplified by automobile exhaust abatement reactors—because of the popularity of automobiles, more monolithic reactors are being used than fixed-bed reactors. Applications in processes with liquid-phase and gas–liquid-phase reactants are scarce, but one well-known commercial process (the reduction step in the production of hydrogen peroxide) shows the feasibility of monoliths. Several processes are in the development stage. Included in the review are an assessment of the impact of these reactors on process intensification and applications in biotechnology and photocatalysis.

How metallic is gold in the direct epoxidation of propene: an FTIR study

Unraveling the oxidation state of gold is important to understand the role of gold in direct propene epoxidation on gold–titania catalysts. A Fourier transform infrared study of low-temperature carbon monoxide adsorption was performed over Au/TiO2 and Au/Ti–SiO2 under an atmosphere of the reaction mixture of oxygen, hydrogen, and propene. Data reveals that the active gold sites treated by the reaction mixture are fully covered by reaction intermediates and deactivating species. Oxidation at 573 K removes these carbonaceous species on gold. Oxygen adsorption at the reaction temperature leads to positively charged gold, which can be reduced to metallic gold in the presence of hydrogen. Propene acts as an electron donor to the gold atoms resulting in negatively charged gold with the carbonyl band at 2079 cm−1. The results in this study may provide a general scheme of electron transfer via gold on the gold–titania catalysts for direct propene epoxidation.

Aqueous phase reforming in a microchannel reactor: the effect of mass transfer on hydrogen selectivity

Aqueous phase reforming of sorbitol was carried out in a 1.7 m long, 320 μm ID microchannel reactor with a 5 μm Pt-based washcoated catalyst layer, combined with nitrogen stripping. The performance of this microchannel reactor is correlated to the mass transfer properties, reaction kinetics, hydrogen selectivity and product distribution. Mass transfer does not affect the rate of sorbitol consumption, which is limited by the kinetics of the reforming reaction. Mass transfer significantly affects the hydrogen selectivity and the product distribution. The rapid consumption of hydrogen in side reactions at the catalyst surface is prevented by a fast mass transfer of hydrogen from the catalyst site to the gas phase in the microchannel reactor. This results in a decrease of the concentration of hydrogen at the catalyst surface, which was found to enhance the desired reforming reaction rate at the expense of the undesired hydrogen consuming reactions. Compared to a fixed bed reactor, the selectivity to hydrogen in the microchannel reactor was increased by a factor of 2. The yield of side products (mainly C3 and heavier hydrodeoxygenated species) was suppressed while the yield of hydrogen was increased from 1.4 to 4 moles per mole of sorbitol fed.

Zirconium Phosphate Coating on Aluminum Foams by Electrophoretic Deposition for Acidic Catalysis

The electrophoretic deposition method has been applied for the formation of an amorphous zirconium phosphate layer on the surface of open‐cell aluminum foam. The aluminum foam was fully and uniformly covered by the zirconium phosphate layer with a good mechanical adherence to the support. The obtained composites were characterized by using XRD, SEM and nitrogen adsorption. The coated aluminum foams showed high catalytic activity in the dehydration of fructose to 5‐hydroxymethylfurfural. This method of foam coating is much more convenient and effective than the traditional washcoating procedure, avoiding the anodization pretreatment of the foam to increase adherence.

Direct Synthesis of Hydrogen Peroxide over Au-Pd Catalyst—The Effect of Co-Solvent Addition

Direct synthesis represents a green alternative, an environmentally benign process for the production of hydrogen peroxide. This is a three‐phase reaction, a solid catalyst with gas reactants and a liquid phase, which collects hydrogen peroxide formed from the catalyst surface. The choice of solvent and/or addition of promoters has a significant effect on the reaction rates observed, as well as selectivity towards peroxide. In addition to non‐toxic water, short chain alcohols are very attractive, due to good solubility of reacting gases. Here we report an extensive study on the influence of different groups of solvents on the direct synthesis reaction when either applied alone or as co‐solvent under non‐explosive and conventionally explosive reaction conditions.

Preparation of ZSM-5 zeolite coatings within capillary microchannels

A method is described to incorporate a zeolite ZSM-5 layer as a potential catalytic coating or membrane into tubular supports by the in situ crystallization of a previously deposited silica layer. The silica layer simultaneously provides nucleation sites and functions as a nutrient source giving coatings with excellent uniformity and controllable coating thickness. The ability to control the simultaneous dissolution of the silica layer in conjunction with nucleation and growth processes was found important in controlling the morphology of the zeolite crystals contained in the final zeolite coating. The formation of a ±4 μm coating containing ±300 nm crystals was obtained at high silica dissolution and nucleation rates. A continuous intergrown zeolite layer of ±7.6 μm was obtainable at high silica dissolution rates in the presence of sodium.