Sustainable Production of Acrylic Acid: Catalytic Performance of Hydroxyapatites for Gas-Phase Dehydration of Lactic Acid

Design Strategies for Hydroxyapatite-Based Materials to Enhance Their Catalytic Performance and Applicability

Hydroxyapatite (HAP) is a green catalyst that has a wide range of applications in catalysis due to its high flexibility and multifunctionality. These properties allow HAP to accommodate a large number of catalyst modifications that can selectively improve the catalytic performance in target reactions. To date, many studies have been conducted to elucidate the effect of HAP modification on the catalytic activities for various reactions. However, systematic design strategies for HAP catalysts are not established yet due to an incomplete understanding of underlying structure-activity relationships. In this review, tuning methods of HAP for improving the catalytic performance are discussed: 1) ionic composition change, 2) morphology control, 3) incorporation of other metal species, and 4) catalytic support engineering. Detailed mechanisms and effects of structural modulations on the catalytic performances for attaining the design insights of HAP catalysts are investigated. In addition, computational studies to understand catalytic reactions on HAP materials are also introduced. Finally, important areas for future research are highlighted.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

Multifunctional Amine Modifiers for Selective Dehydration of Methyl Lactate to Acrylates

Dehydration of methyl lactate to acrylic acid and methyl acrylate was experimentally evaluated over a Na-FAU zeolite catalyst impregnated with multifunctional diamines. 1,2-Bis(4-pyridyl)ethane (12BPE) and 4,4'-trimethylenedipyridine (44TMDP), at a nominal loading of 40 wt % or two molecules per Na-FAU supercage, afforded a dehydration selectivity of 96 ± 3% over 2000 min time on stream. Although 12BPE and 44TMDP have van der Waals diameters approximately 90% of the Na-FAU window opening diameter, both flexible diamines interact with internal active sites of Na-FAU as characterized by infrared spectroscopy. During continuous reaction at 300 °C, the amine loadings in Na-FAU remained constant for 12BPE but decreased as much as 83% for 44TMDP. Tuning the weighted hourly space velocity (WHSV) from 0.9 to 0.2 h^-1 afforded a yield as high as 92% at a selectivity of 96% with 44TMDP impregnated Na-FAU, resulting in the highest yield reported to date.

Sustainable production of bio-propionic acid: synergy between vacancy and thermoelectron in MoS2/MoO3 composite-enhanced hydrodeoxygenation of lactic acid

The MoS2/MoO3 composite offered superior conversion (90.4%) of lactic acid and propionic acid selectivity (88.4%) since the hydroxyl-specific site in the lactic acid molecule was accurately activated by a sulfur vacancy.

Development of a New Ternary Al2O3-HAP-Pd Catalyst for Diethyl Ether and Ethylene Production Using the Preferential Dehydration of Ethanol

This study aims to convert ethanol to higher value-added products, particularly diethyl ether and ethylene using the catalytic dehydration of ethanol. Hence, the gas-phase dehydration of ethanol over Al2O3-HAP catalysts as such and modified by addition of palladium (Pd) in a microreactor was evaluated. The commercial Al2O3-HAP catalyst was first prepared by the physical mixing method, and then, the optimal ratio of the Al2O3-HAP catalyst (2:8 by wt %) was impregnated with Pd to develop a new functional catalyst to alter surface acidity. Based on the results, the combination of Al2O3 and HAP catalysts generated significant quantities of weak acid sites which demonstrates an enhancement in catalytic activity. In addition, Pd modification in the optimal composition ratio of the Al2O3-HAP catalyst extremely increased the amount of weak acid sites as well as weak acid density due to the synergistic effect between the Pd and Al2O3-HAP catalyst that are supposed to suggest the active sites in the reaction. Among all catalysts, the Al20-HAP80-Pd catalyst displayed brilliant catalytic performance in the course of diethyl ether yield (ca. 51.0%) at a reaction temperature of 350 °C and ethylene yield (ca. 75.0%) at a reaction temperature of 400 °C having an outstanding stability under time-on-stream for 10 h. This is recognized to the combination of the effects of weak acid sites (Lewis acidity), small amount of strong acid sites, and structural characteristics of the catalytic materials used.

The Role of the Surface Acid-Base Nature of Nanocrystalline Hydroxyapatite Catalysts in the 1,6-Hexanediol Conversion

Hydroxyapatite is known to have excellent catalytic properties for ethanol conversion and lactic acid conversion, and their properties are influenced by the elemental composition, such as Ca/P ratio and sodium content. However, few reports have been examined for the surface acid–base nature of hydroxyapatites containing sodium ions. We prepared nanocrystalline hydroxyapatite (Ca-HAP) catalysts with various Ca/P ratios and sodium contents by the hydrothermal method. The adsorption and desorption experiments using NH₃ and CO₂ molecules and the catalytic reactions for 2-propenol conversion revealed that the surface acid–base natures changed continuously with the bulk Ca/P ratios. Furthermore, the new catalytic properties of hydroxyapatite were exhibited for 1,6-hexanediol conversion. The non-stoichiometric Ca-HAP(1.54) catalyst with sodium ions of 2.3 wt% and a Ca/P molar ratio of 1.54 gave a high 5-hexen-1-ol yield of 68%. In contrast, the Ca-HAP(1.72) catalyst, with a Ca/P molar ratio of 1.72, gave a high cyclopentanemethanol yield of 42%. Both yields were the highest ever reported in the relevant literature. It was shown that hydroxyapatite also has excellent catalytic properties for alkanediol conversion because the surface acid–base properties can be continuously controlled by the elemental compositions, such as bulk Ca/P ratios and sodium contents.

Continuous Flow Upgrading of Selected C2-C6 Platform Chemicals Derived from Biomass

The ever increasing industrial production of commodity and specialty chemicals inexorably depletes the finite primary fossil resources available on Earth. The forecast of population growth over the next 3 decades is a very strong incentive for the identification of alternative primary resources other than petro-based ones. In contrast with fossil resources, renewable biomass is a virtually inexhaustible reservoir of chemical building blocks. Shifting the current industrial paradigm from almost exclusively petro-based resources to alternative bio-based raw materials requires more than vibrant political messages; it requires a profound revision of the concepts and technologies on which industrial chemical processes rely. Only a small fraction of molecules extracted from biomass bears significant chemical and commercial potentials to be considered as ubiquitous chemical platforms upon which a new, bio-based industry can thrive. Owing to its inherent assets in terms of unique process experience, scalability, and reduced environmental footprint, flow chemistry arguably has a major role to play in this context. This review covers a selection of C₂ to C₆ bio-based chemical platforms with existing commercial markets including polyols (ethylene glycol, 1,2-propanediol, 1,3-propanediol, glycerol, 1,4-butanediol, xylitol, and sorbitol), furanoids (furfural and 5-hydroxymethylfurfural) and carboxylic acids (lactic acid, succinic acid, fumaric acid, malic acid, itaconic acid, and levulinic acid). The aim of this review is to illustrate the various aspects of upgrading bio-based platform molecules toward commodity or specialty chemicals using new process concepts that fall under the umbrella of continuous flow technology and that could change the future perspectives of biorefineries.

Lactic Acid Conversion to Acrylic Acid Over Fluoride-Substituted Hydroxyapatites

One of the most interesting intermediates for the chemical industry is acrylic acid, which can be derived from lactic acid by catalytic dehydration in the gas phase. The realization of this reaction is complex due to a strong thermal activation leading to the formation of undesired by-products (acetaldehyde, propanoic acid…) as well as polymerization. We studied this reaction over hydroxyapatites modified by substitution of the hydroxyl groups by fluoride. This notably enabled increasing the selectivity to acrylic acid while reducing the formation of the undesired acetaldehyde. Introduction of fluoride induced a modification of the phosphate (PO 4 3 - ) groups. In the presence of water, fluoride prevented the formation of hydrogenophosphate species (HPO 4 2 - ), which are well-known acid sites responsible for the formation of acetaldehyde by decarboxylation/decarbonylation. Further, we evidenced an important impact of fluoride substitution on crystallinity, specific surface area and on the surface Ca/P ratio. This latter is known to be a key parameter to control the acidity and the basicity of the hydroxyapatites. Using FT-IR spectroscopy with propyne as a probe molecule, we could show that lactic acid was concertedly adsorbed on basic and acid sites, which might be at the origin of the observed superior performances.

Hydrodeoxygenation of lignocellulose-derived oxygenates to diesel or jet fuel range alkanes under mild conditions

This work provided an efficient way to produce diesel or jet fuel range alkanes under mild conditions from lignocellulose-derived oxygenates.

Advances in catalytic production processes of biomass-derived vinyl monomers

This review provides a summary and perspective for three bio-derived vinyl monomers – acrylic acid, methacrylic acid and styrene.

Sustainable production of pyruvic acid: oxidative dehydrogenation of lactic acid over the FeMoO/P catalyst

Novel redox of FeMoO/P by electron transfer between Fe and Mo is favorable for the oxidative dehydrogenation of lactic acid.

Irreversible adsorption of acidic, basic, and water gas molecules on calcium-deficient hydroxyapatite

Hydroxyapatite [Ca10(PO4)6(OH)2, HAP] has P-OH Brønsted acidic sites, Ca2+ Lewis acidic sites, and OH- and O2- basic sites on which acidic and basic gas molecules can be selectively adsorbed, and has no micropore onto which various molecules adsorb regardless of the chemical properties of gas molecules. The interaction between the surface sites and acidic and basic gas and water molecules has been investigated by evaluating the adsorption properties of various molecules on the surfaces of calcium-deficient HAP. The specific adsorption sites were assessed by examining the reversible and irreversible adsorption of NH3, CO2, aldehydes, and water vapor on HAP at the temperature of 298 K, using two HAP samples with different Ca/P ratios, but similar structures and surface areas: Ca-deficient HAP with an extreme lower Ca/P ratio (named P-HAP) and one with a higher Ca/P ratio (named C-HAP). Irreversible adsorption of NH3 on C-HAP is attributed to the adsorption on both Ca2+ Lewis acidic and P-OH Brønsted acidic sites. Irreversible adsorption on P-HAP is attributed to the adsorption on P-OH Brønsted acidic sites only. Irreversible adsorption of CO2 occurred on C-HAP only, and preferentially on OH- basic sites. Acetaldehyde undergoes a catalytic reaction over both OH- basic sites and surface P-OH Brønsted acidic sites at 298 K. Water irreversible adsorption was extensively observed for P-HAP, and water was barely desorbed at low pressures. In situ powder X-ray diffraction showed an asymmetric expansion of the lattice in the [100] direction, indicating that water was incorporated into P-HAP crystals, especially on structural OH- sites. Irreversible adsorption of acidic and basic molecules was therefore less observed on P-HAP than on C-HAP, but P-HAP had considerable irreversible adsorption of water vapor with associated asymmetric lattice expansion. The incorporation of water vapor was first observed and could be useful to improve adsorption or catalytic performance with the mediation of water vapor and/or hydration.

A hybrid composite of hydroxyapatite and Ca-Al layered double hydroxide supported Au nanoparticles for highly efficient base-free aerobic oxidation of glucose

In this work, a new hybrid composite of hydroxyapatite and Ca-Al layered double hydroxide (HAP-LDH) was successfully assembled via an in situ growth route, by which large quantities of small needle-like HAP crystals in situ grew over the lateral surface of large platelet-like CaAl-LDH particles, and applied to immobilize Au nanoparticles for base-free aerobic glucose oxidation in water to produce gluconic acid using molecular oxygen. A combination of characterization techniques and catalytic experiments revealed that the activity of supported Au catalysts was strongly associated with the composition of supports, and the hybrid HAP-LDH supported one with a Au loading amount of about 0.2 wt% delivered a high gluconic acid yield of >98% under optimal reaction conditions, along with a quite high turnover frequency value of ∼20 225 h-1. High efficiency of the as-formed Au/HAP-LDH was mainly ascribed to cooperation between favorable surface Au species (Au0/Auδ+) and abundant basic sites. Furthermore, the present catalyst also presented good structural stability, because of the novel hybrid three-dimensional nano/microstructure of the HAP-LDH composite support facilitating the stabilization of active Au species and components of the support. The present synthesis strategy of employing a hybrid composite support provides a new way to design stable and high-performance supported metal nanocatalysts for a variety of advanced heterogeneous catalytic processes.

In Situ-Generated Supported Potassium Lactate: Stable Catalysis for Vapor-Phase Dehydration of Lactic Acid to Acrylic Acid

We have studied unsupported, silica gel- and amorphous silica-alumina-supported catalysts derived from K salts for the vapor-phase dehydration of lactic acid (LA) to acrylic acid (AA). A catalytic study shows that the supported catalysts improve the activity and selectivity for the production of AA and decrease the selectivity for the production of propionic acid (PA). The silica-alumina-supported catalysts remain fairly stable in the catalytic performance during 90 h of reaction. The IR spectroscopic characterization combined with the catalytic study demonstrates that potassium lactate (C₃H₅KO₃) in situ generated from LA and a K salt is an important reaction intermediate for the production of AA and the catalytic stability is associated with the chemical stability of C₃H₅KO₃ and the activity for the regeneration of C₃H₅KO₃ in the catalytic cycle. On silica-alumina, C₃H₅KO₃ is well stabilized and smoothly regenerated during the reaction, leading to the good catalytic stability. This work suggests for the first time that lactate salt acts as the true catalytic active species for the dehydration of LA to AA. We also propose a predominant reaction pathway for the vapor-phase dehydration of LA to AA with K salt catalyst systems.

Optimising surface d charge of AuPd nanoalloy catalysts for enhanced catalytic activity

Understanding the catalytic mechanism of bimetallic nanocatalysts remains challenging. Here, we adopt an adsorbate mediated thermal reduction approach to yield monodispersed AuPd catalysts with continuous change of the Pd-Au coordination numbers embedded in a mesoporous carbonaceous matrix. The structure of nanoalloys is well-defined, allowing for a direct determination of the structure-property relationship. The results show that the Pd single atom and dimer are the active sites for the base-free oxidation of primary alcohols. Remarkably, the d-orbital charge on the surface of Pd serves as a descriptor to the adsorbate states and hence the catalytic performance. The maximum d-charge gain occurred in a composition with 33-50 at% Pd corresponds to up to 9 times enhancement in the reaction rate compared to the neat Pd. The findings not only open an avenue towards the rational design of catalysts but also enable the identification of key steps involved in the catalytic reactions.

Highly Selective Synthesis of Acrylic Acid from Lactide in the Liquid Phase

A new reaction system for the highly selective, hydrobromic acid catalyzed conversion of lactide into acrylic acid under mild conditions is reported. The applied liquid reaction system consists of a temperature-stable bromide-containing ionic liquid and 2-bromopropionic acid as a source of dry HBr, with no volatile organic solvent being used. This allows for the in situ removal of the formed acrylic acid, leading to an unmatched acrylic acid selectivity of over 72 % at full lactide conversion. Accounting for leftover reaction intermediates on the way to acrylic acid, which could be recycled in an elaborate continuous process, the proposed reaction system shows potential for acrylic acid yields well above 85 % in the liquid phase. This opens new avenues for the effective conversion of biogenic lactic acid (e.g., obtained by fermentation from starch) to acrylic acid. The resulting bio-acrylic acid is a highly attractive product for, for example, the diaper industry, where we expect consumers to be especially sensitive to aspects of sustainability.

Hydrogenation of Lactic Acid to 1,2-Propanediol over Ru-Based Catalysts

The catalytic hydrogenation of lactic acid to 1,2-propanediol with supported Ru catalysts in water was investigated. The influence of catalyst support (activated carbon, γ-Al2O3, SiO2, TiO2, and CeO2) and promoters (Pd, Au, Mo, Re, Sn) on the catalytic performance was evaluated. Catalytic tests revealed that TiO2 yields the best Ru catalysts. With a monometallic Ru/TiO2 catalyst, a 1,2-propanediol yield of 70 % at 79 % lactic acid conversion was achieved at 130 °C after 20 h reaction. Minor byproducts of the hydrogenation reaction were propionic acid, ethanol, 1-propanol, and 2-propanol. For the bimetallic catalysts, the addition of Pd and Au slightly enhanced the performance of Ru/TiO2, whereas the addition of common hydrogenation promoters such as Re, Mo, and Sn impaired the activity.

Production of Ethylene through Ethanol Dehydration on SBA-15 Catalysts Synthesized by Sol-gel and One-step Hydrothermal Methods

The present work deals with the catalytic performance of SBA-15 supported catalysts in the gas phase catalytic dehydration of ethanol in the temperature range of 200 to 400°C. The SBA-15 support was incorporated on a zirconium (Zr) and bimetal of zirconium and lanthanum (Zr-La) prepared by sol-gel (SG) and hydrothermal (HT) methods. The catalysts were characterized by means of N₂ physisorption, SEM/EDX, and NH₃-TPD. The experimental results demonstrated that the Zr-La/SBA-15-HT exhibited the highest catalytic activity. Ethanol conversion and ethylene selectivity were found to increase with increased reaction temperature. The best catalytic results were achieved for Zr-La/SBA-15-HT indicating values of ethanol conversion and ethylene yield of ca. 84% and 80%, respectively at 400°C. The most important parameter influencing their catalytic properties appears to be the interaction between metal and support depending on different methods. The metal dispersion inside the siliceous matrix of SBA-15 has a direct influence on their surface acidity. Meanwhile, the performance of these SBA-15 supported catalysts in ethanol dehydration is also related with the alteration of surface acidity caused by the introduction of Zr and Zr-La.

Incorporation of vanadium into the framework of hydroxyapatites: importance of the vanadium content and pH conditions during the precipitation step

Even though vanadium-modified hydroxyapatite (V-HAp) samples are very promising systems for oxidative dehydrogenation of propane, the incorporation of vanadium into the hydroxyapatite framework was reported to be limited and to lead to over-stoichiometric compounds. Here, the synthesis of a Ca₁₀(PO₄)_6-x(VO₄)_x(OH)₂ stoichiometric solid solution using a co-precipitation method is monitored in the whole composition range (0 ≤ x ≤ 6) by controlling the pH of the precipitation medium, with continuous (the first series of samples) or periodic (the second series of samples) addition of NH₄OH during the precipitation step or during the maturation step, respectively. It is demonstrated that the changes in pH conditions result in materials of a substantial difference in terms of the final composition. From XRD patterns and Rietveld refinements, a solid solution V-HAp phase was found to be exclusively obtained for the first series of samples for x varying from 0 to 6. This also occurred in the second series of samples but only for x lower than 4. For 4 ≤ x ≤ 5.22, the materials were composed of a mixture of V-HAp and Ca₂V₂O₇, whereas for a x value of 6 only Ca₂V₂O₇ was formed. The predominance of polymeric V species in solution at a high vanadium concentration deduced from the diagram of speciation of vanadium accounts for the preferential formation of Ca₂V₂O₇ under these particular conditions. However, provided that a higher pH value was maintained, isolated VO₃(OH)^2- species are predominant, which accounts for the incorporation of isolated vanadates into the hydroxyapatite framework and for the well-controlled stoichiometry with Ca/(P + V) ratios found to be close to 1.67. Such a very good accommodation of vanadium in the hydroxyapatite framework is illustrated by the characterization of the local surrounding of phosphorus and vanadium species using ³¹P and ⁵¹V NMR, Raman and UV-vis spectroscopies.

Efficient production of acrylic acid by dehydration of lactic acid over BaSO4with crystal defects

Crystal defects in BaSO₄catalyst provide acid sites that catalyze the dehydration of lactic acid to acrylic acid.

Selective conversion of lactic acid to acrylic acid over alkali and alkaline-earth metal co-modified NaY zeolites

A high AA selectivity of 84% was achieved through an environmentally friendly and sustainable catalytic process.

Ammonia promoted barium sulfate catalyst for dehydration of lactic acid to acrylic acid

Defects were formed in a BaSO₄catalyst by controlling its synthesis and it offered excellent activity for the dehydration of lactic acid to acrylic acid.