An Philippaerts

Zeolite-catalysed conversion of C3 sugars to alkyl lactates

The direct conversion of C3 sugars (or trioses) to alkyl lactates was achieved using zeolite catalysts. This reaction represents a key step towards the efficient conversion of bio-glycerol or formaldehyde to added-value chemicals such as lactate derivatives. The highest yields and selectivities towards the desired lactate product were obtained with Ultrastable zeolite Y materials having a low Si/Al ratio and a high content of extra-framework aluminium. Correlating the types and amounts of acid sites present in the different zeolites reveals that two acid functions are required to achieve excellent catalysis. Bronsted acid sites catalyse the conversion of trioses to the reaction intermediate pyruvic aldehyde, while Lewis acid sites further assist in the intramolecular rearrangement of the aldehyde into the desired lactate ester product. The presence of strong zeolitic Bronsted acid sites should be avoided as much as possible, since they convert the intermediate pyruvic aldehyde into alkyl acetals instead of lactate esters. A tentative mechanism for the acid catalysis is proposed based on reference reactions and isotopically labelled experiments. Reusability of the USY catalyst is demonstrated for the title reaction.

Catalytic Production of Conjugated Fatty Acids and Oils

The reactive double bonds in conjugated vegetable oils are of high interest in industry. Traditionally, conjugated vegetable oils are added to paints, varnishes, and inks to improve their drying properties, while recently there is an increased interest in their use in the production of bioplastics. Besides the industrial applications, also food manufactures are interested in conjugated vegetable oils due to their various positive health effects. While the isomer type is less important for their industrial purposes, the beneficial health effects are mainly associated with the c9,t11, t10,c12 and t9,t11 CLA isomers. The production of CLA-enriched oils as additives in functional foods thus requires a high CLA isomer selectivity. Currently, CLAs are produced by conjugation of oils high in linoleic acid, for example soybean and safflower oil, using homogeneous bases. Although high CLA productivities and very high isomer selectivities are obtained, this process faces many ecological drawbacks. Moreover, CLA-enriched oils can not be produced directly with the homogeneous bases. Literature reports describe many catalytic processes to conjugate linoleic acid, linoleic acid methyl ester, and vegetable oils rich in linoleic acid: biocatalysts, for example enzymes and cells; metal catalysts, for example homogeneous metal complexes and heterogeneous catalysts; and photocatalysts. This Review discusses state-of-the-art catalytic processes in comparison with some new catalytic production routes. For each category of catalytic process, the CLA productivities and the CLA isomer selectivity are compared. Heterogeneous catalysis seems the most attractive approach for CLA production due to its easy recovery process, provided that the competing hydrogenation reaction is limited and the CLA production rate competes with the current homogeneous base catalysis. The most important criteria to obtain high CLA productivity and isomer selectivity are (1) absence of a hydrogen donor, (2) absence of catalyst acidity, (3) high metal dispersion, and (4) highly accessible pore architecture.

Unprecedented Shape Selectivity in Hydrogenation of Triacylglycerol Molecules with Pt/ZSM-5 Zeolite†

Vegetable oils are made up of triacylglycerol moleculemixtures, which comprise three fatty acids esterified to aglycerol backbone. The fatty acids can differ in chain length,number, and double-bond configuration. Common vegetableoils contain fatty acid chains with 16 to 20 carbon atoms and 0to 3 double bonds, all of which are in a cis configuration(Table 1).

Is there still a Future for Hydrogenated Vegetable Oils

Catalytic hydrogenation of vegetable oils, viz. soybean oil, is an established technology in food industry. Its major concern is the hydrogenation of autoxidation-sensitive polyunsaturated fatty acid tails to stabilize the vegetable oil. Flash-hydrogenated oils find application in cold dressings and cooking oils. Somewhat deeper partial hydrogenation converts the vegetable oils into (semi-) solid fat products with different organoleptic and melting characteristics for use in various food applications, such as margarines and shortenings. The catalytic hydrogenation of vegetable oils has a history of more than 100 years with fluctuating popularity, depending not only on economical factors but also on nutritional findings. For more than a decade, the use of industrially hydrogenated fats in food products was controversial because of the presence of trans fatty acid isomers. The presence of geometrical isomers is an inherent consequence of the Horiuti-Polanyi hydrogenation mechanism, which describes the stepwise and reversible addition of adsorbed hydrogen atoms to the adsorbed double bond on the catalyst surface. After reaction with one adsorbed hydrogen atom, a halfhydrogenated intermediate is formed, which is either reduced by the reaction with a second hydrogen atom, leading to the formation of a saturated bond, or it loses a hydrogen atom prior to desorption, potentially leading to the formation of geometric (trans) and positional isomers of the starting unsaturated fatty acid. Although triglycerides that contain trans fatty acids have ideal melting characteristics, they are associated with a negative health impact, thus imposing the replacement of catalytic hydrogenation by other technologies, such as fractionation and catalytic interesterification, which yield “zero trans” fatty products. The hydrogenation technology experiences industrial interest, because it allows the modification of fatty acid and triglyceride compositions in a single step. As hydrogenation is a catalytic process, novel catalyst design is key to the production of (semi-solid) fats with acceptable organoleptic, melting, and nutritional properties, provided the “trans issue” can be circumvented. Other fat/oil applications, such as the production of essentially trans-free biolubricants and many renewable chemicals, could also benefit from new advances of catalytic vegetable oil hydrogenation. The present essay reviews briefly the history of the catalytic hydrogenation of vegetable (edible) oils and illustrates how trends in food chemistry research converge with new nutritional insights. Certain novel promising catalytic developments, which were recently reported in this journal, are highlighted, such as the trans-free chemocatalytic hydrogenation, which shows unprecedented types of shape selectivity, and trans-selective enzymatic hydrolysis. This pioneering work uses new concepts, which could pave the way to alternatives to the nowadays prominently present palm-based shortenings. It illustrates that, 100 years after the first industrial production of hydrogenated fats for food products, selective hydrogenation of vegetable oils is still an exciting research domain of utmost importance for the food industry and beyond (Scheme 1).

Towards biolubricant compatible vegetable oils by pore mouth hydrogenation with shape-selective Pt/ZSM-5 catalysts

Pt/ZSM-5 catalysts with various crystal sizes were prepared via competitive ion-exchange, followed by a slow activation procedure. Even when using very large ZSM-5 crystals, highly dispersed Pt nano-clusters were contained within the zeolite crystals voids, as ascertained by 2D pressure-jump IR spectroscopy of adsorbed CO and focussed ion-beam transmission electron microscopy. The shape-selective properties of the Pt/ZSM-5 catalysts were evaluated in the partial hydrogenation of soybean oil. Unique hydrogenation selectivities were observed, as the fatty acids located at the central position of the triacylglycerol (TAG) molecules were preferentially hydrogenated. The resulting oil has therefore high levels of intermediately melting TAGs, which are compatible with biolubricants due to their improved oxidative stability and still appropriate low-temperature fluidity. The TAG distribution in the partially hydrogenated soybean oil samples was independent from the zeolite crystal size, while the hydrogenation activity linearly increases with the crystals external surface area. This trend was confirmed with a Pt loaded mesoporous ZSM-5 zeolite, obtained via a mild alkaline treatment. These observations imply and confirm a genuine pore mouth catalysis mechanism, in which only one fatty acid chain of the TAG is able to enter the micropores of ZSM-5, where the double bonds are hydrogenated by the crystal encapsulated Pt-clusters.

Catalyst design by

Hierarchical zeolites are a class of superior catalysts which couples the intrinsic zeolitic properties to enhanced accessibility and intracrystalline mass transport to and from the active sites. The design of hierarchical USY (Ultra-Stable Y) catalysts is achieved using a sustainable postsynthetic room temperature treatment with mildly alkaline NH4OH (0.02 m) solutions. Starting from a commercial dealuminated USY zeolite (Si/Al = 47), a hierarchical material is obtained by selective and tuneable creation of interconnected and accessible small mesopores (2–6 nm). In addition, the treatment immediately yields the NH4+ form without the need for additional ion exchange. After NH4OH modification, the crystal morphology is retained, whereas the microporosity and relative crystallinity are decreased. The gradual formation of dense amorphous phases throughout the crystal without significant framework atom leaching rationalizes the very high material yields (>90%). The superior catalytic performance of the developed hierarchical zeolites is demonstrated in the acid-catalyzed isomerization of α-pinene and the metal-catalyzed conjugation of safflower oil. Significant improvements in activity and selectivity are attained, as well as a lowered susceptibility to deactivation. The catalytic performance is intimately related to the introduced mesopores, hence enhanced mass transport capacity, and the retained intrinsic zeolitic properties.

NH_{4}OH

Partial and full hydrogenation of vegetable oils are extremely important for the food and chemical industries. The selectivity of the catalytic process determines the chemical and physical properties of the hydrogenated products, defining the application potential, and therefore the product value. In the partial hydrogenation, the conversion of unstable polyunsaturated fatty acids into more stable monounsaturated fatty acids is highly desired, without a significant increase in the content of saturated fatty acids. Nowadays, the challenge is the control of the cis/trans isomerization, occurring as a side reaction of the hydrogenation process, since trans fatty acids are suspected of increasing the risk of cardiovascular diseases. Accordingly, in many countries, there are specific regulations banning the use of trans fats in food products. Since catalyst properties largely influence hydrogenation selectivity and formation of trans isomers, catalyst development is crucial in providing products showing superior functionality, i.e. good stability, suitable physical properties, and low levels of trans isomers.