Solvent-free semihydrogenation of acetylene alcohols in a capillary reactor coated with a Pd-Bi/TiO2 catalyst

© 2016 Elsevier B.V. All rights reserved. A solvent-free semihydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol was performed in a capillary reactor (10 m long, 0.53 mm i.d.) coated with a titania supported Pd-Bi catalyst. Several coatings with different Pd/Bi ratio have been prepared. The catalysts have been characterized with SEM, TEM, EDX, XRD analysis and N 2 adsorption-desorption measurements. The maximum alkene yield of 90% was obtained at a molar Pd/Bi ratio of 11. The yield was increased to 95% in the presence of 10 mol.% pyridine in the reaction mixture. The alkene selectivity decreased with time due to leaching of Bi. The leaching was fully suppressed in the presence of 1 vol.% acetic acid in the reaction mixture. The catalyst remained stable for 100 h of continuous operation. The results demonstrate that capillary reactors provide alkene selectivity the same compared to ideal stirred tank batch reactors

Liquid-phase adsorption : common problems and how we could do better

Adsorption plays a vital role in many applications from adsorbents for concentrating valuable compounds or removing pollutants to catalysts. In gas and liquid phases, the adsorption phenomena may look similar and the results are often transferred. But solvents play a role and may change the adsorption behaviour even for strong adsorbates – liquid phase adsorption is different. The review covers kinetics and thermodynamics of adsorption processes and focuses on several areas that receive only minor attention despite being crucial for obtaining reliable results. Such underappreciated areas include the analysis of how to maximise experimental accuracy of adsorption studies and analyse the model parameters and their confidence intervals; the effect of the mathematical representation and model linearization on the results; the possibility of processes other than adsorption during the experiments. The experiments based on disappearance of the adsorbate from the equilibrium solution shall be performed to ensure at least 10% decrease in concentration for reasonable accuracy. Regression analysis and analysis of the confidence intervals of the parameters merit particular attention as well as an independent validation of the model assumptions. Even an excellent data fit may provide results differing by several times from the correct values. Adding to the dispute on dimensions in the adsorption constants in van't Hoof equation, the review adds arguments in favour of using constants in L/mol units. The review concludes with the proposed workflow in the analysis of liquid-phase adsorption data from the data acquisition to data analysis and modelling and offers a Matlab app for Langmuir adsorption data analysis

Process intensification of alkynol semihydrogenation in a tube reactor coated with a Pd/ZnO catalyst

Semihydrogenation of 2-methyl-3-butyn-2-ol (MBY) was studied in a 5 m tube reactor wall-coated with a 5 wt % Pd/ZnO catalyst. The system allowed for the excellent selectivity towards the intermediate alkene of 97.8 ± 0.2% at an ambient H 2 pressure and a MBY conversion below 90%. The maximum alkene yield reached 94.6% under solvent-free conditions and 96.0% in a 30 vol % MBY aqueous solution. The reactor stability was studied for 80 h on stream with a deactivation rate of only 0.07% per hour. Such a low deactivation rate provides a continuous operation of one month with only a two-fold decrease in catalyst activity and a metal leaching below 1 parts per billion (ppb). The excellent turn-over numbers (TON) of above 10 5 illustrates a very efficient utilisation of the noble metal inside catalyst-coated tube reactors. When compared to batch operation at 70 °C, the reaction rate in flow reactor can be increased by eight times at a higher reaction temperature, keeping the same product decomposition of about 1% in both cases

Scale up study of capillary microreactors in solvent-free semihydrogenation of 2‐methyl‐3‐butyn‐2‐ol

A 2.5 wt.% Pd/ZnO catalytic coating has been deposited onto the inner wall of capillary reactors with a diameter of 0.53 and 1.6 mm. The coatings were characterised by XRD, SEM, TEM and elemental analysis. The performance of catalytic reactors was studied in solvent-free hydrogenation of 2-methyl-3-butyn-2-ol. No mass transfer limitations was observed in the reactor with a diameter of 0.53 mm up to a catalyst loading of 1.0 kg (Pd) m −3 . The activity and selectivity of the catalysts has been studied in a batch reactor to develop a kinetic model. The kinetic model was combined with the reactor model to describe the obtained data in a wide range of reaction conditions. The model was applied to calculate the range of reaction conditions to reach a production rate of liquid product of 10–50 kg a day in a single catalytic capillary reactor

Palladium-bismuth intermetallic and surface-poisoned catalysts for the semi-hydrogenation of 2-methyl-3-butyn-2-ol

The effects of poisoning of Pd catalysts with Bi and annealing in a polyol (ethylene glycol) were studied on the semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY). An increase in the Pd:Bi ratio from 7 to 1 in the Bi-poisoned catalysts decreased the hydrogenation activity due to blocking of active sites, but increased maximum alkene yield from 91.5% for the Pd catalyst to 94–96% for all Bi-poisoned Pd catalysts, by decreasing the adsorption energy of alkene molecules and suppressing the formation of β-hydride phase. Annealing of the catalysts induced the formation of intermetallic phases and decreased its activity due to sintering of the catalytic particles and low activity of intermetallic compounds. Langmuir–Hinshelwood kinetic modelling of the experimental data showed that poisoning of Pd with Bi changed the relative adsorption constants of organic species suggesting ligand effects at high Bi content

Ultrasound- and microwave-assisted preparation of lead-free palladium catalysts: effects on the kinetics of diphenylacetylene semi-hydrogenation

The effect of environmentally benign enabling technologies such as ultrasound and microwaves on the preparation of the lead-free Pd catalyst has been studied. A one-pot method of the catalyst preparation using ultrasound-assisted dispersion of palladium acetate in the presence of the surfactant/capping agent and boehmite support produced the catalyst containing Pd nanoparticles and reduced the number of pores larger than 4 nm in the boehmite support. This catalyst demonstrated higher activity and selectivity. The comparison of kinetic parameters for diphenylacetylene hydrogenation showed that the catalyst obtained by using the one-pot method was seven times as active as a commercial Lindlar catalyst and selectivity towards Z-stilbene was high. Our work also illustrated that highly selective Pd/boehmite catalysts can be prepared through ultrasound-assisted dispersion and microwave-assisted reduction in water under hydrogen pressure without any surfactant

Direct amide synthesis over core-shell TiO2@NiFe2O4 catalysts in a continuous flow radio frequency-heated reactor

Core-shell composite magnetic catalysts TiO2@NiFe2O4 with a titania loading of 9–32 wt. % have been synthesised by sol-gel method for direct amide synthesis in a radiofrequency (RF)-heated continuous flow reactor. The catalyst calcination temperature was optimised in the range of 350-500 ºC and the highest activity was observed for the catalyst calcined at 500 ºC due to conversion of titania into catalytically active anatase phase. No reaction between the magnetic core and the titania shell was observed up to the calcination temperature of 1000 ºC and no sintering of titania shell was observed after calcination at 500 ºC. The comparison of direct amide synthesis in a continuous flow fixed bed reactor under conventional and RF heating demonstrated that the RF heating mode increased the apparent reaction rate by 60 % and decreased the deactivation rate due to a better temperature uniformity. The titania weight normalised reaction rate in the RF-heated reactor was constant for titania loadings above 17 wt. %, while it decreased by a factor of 3 at lower titania loadings because of interactions between the ferrite core on the thin layer of the catalyst. The catalyst deactivation study showed that the deactivation rate could be accurately described by a first order kinetics and that the main reason of deactivation was coking. The catalyst regeneration via calcination at 400 ºC resulted in the catalyst sintering, while a treatment with a hydrogen peroxide solution at 90 ºC fully recovered catalytic activity

Gas-liquid hydrogenation in continuous flow – The effect of mass transfer and residence time in powder packed-bed and catalyst-coated reactors

Catalyst-coated tube reactors have been compared with the reactors packed with catalyst powder in alkyne semi-hydrogenation over a 5 wt% Pd/ZnO catalyst and cinnamic ester full hydrogenation over a 2.4 wt% Pd/C catalyst. The “powder packed-bed” reactors (packing with catalyst powder below 30 μm) showed irreproducible performance in time due to mobility of the catalyst layer in the bed which altered the fluidic path and therefore affected the mean liquid residence time and the dispersion. The catalyst-coated tube reactors demonstrated an ideal plug-flow behaviour (Péclet number > 120), while the powder packed-bed showed a considerable back-mixing (Péclet ~ 25). Under all conditions studied, the reaction rate in the powder packed-bed was limited by external mass transfer, while in the coated tube – by the intrinsic kinetics. The coated tubes demonstrated a much lower pressure drop, an improved alkene selectivity, and a 5 times higher throughput compared to the powder packed-bed. The dilution of the catalyst bed with glass beads improved the throughput 4-fold at the expense of 4-fold increase in the pressure drop. In full hydrogenation reaction, the catalyst-coated tube showed a 14 times higher throughput than in the powder packed-bed at the full alkyne conversion. A reactor model for the catalyst-coated tube has been proposed that takes into account the change in the fluid velocity during the reaction. The model described the reaction kinetics demonstrating that the catalyst-coated tubes can be used as a tool to obtain kinetic data in gas-liquid reactions in flow

Counting bubbles : precision process control of gas-liquid reactions in flow with an optical inline sensor

Quality by Design encouraged by the US Food and Drug Administration (FDA) in the continuous flow synthesis requires tight monitoring of all the reaction input and output parameters to improve reproducibility and eliminate the process rejects. The reaction monitoring, however, relies on costly (above 10,000$) process analytical technology (PAT) – one of the factors that prevents a wider utilisation of continuous processes. In the work, we show that gas-liquid reactions can be monitored using low-cost (10$) hardware – optical liquid inline sensors – that allows instantaneous analysis of gas fraction in the moving stream. We discuss the application of the sensor for various gas-liquid reactions. The gas-consuming reactions such as hydrogenation are the easiest to implement because the sensor without calibration provides accurate readings close to complete consumption of the gas. The gas-evolving reactions can be monitored but require sensor calibration to determine the gas fraction accurately. Operation of the sensor was demonstrated for various hydrogenation reactions self-optimised using a proportional-integral (PID) algorithm which adjusted the substrate concentration to provide high (but not full) pre-defined hydrogen consumption. The optimised hydrogen consumption agreed with the product analysis for a range of the substrates hydrogenated under various pressures and with different selectivities. The optical sensor was also proven to be an efficient tool in adapting the reaction condition to the catalyst deactivation in the reaction of 2-methyl-3-butyn-2-ol semi-hydrogenation – the autonomous reactor allowed reaching a turn-over number (TON) of 2.7·106 with the value of 1.5·107 expected till a twofold decrease in the catalyst activity. The TON values demonstrated are significantly higher than those observed in batch reactors (~103) even in case of catalyst re-use (105) demonstrating a substantial improvement of process sustainability operating with the process control

Influence of precursors on the induction period and transition regime of dimethyl ether conversion to hydrocarbons over ZSM-5 catalysts

ZSM-5 catalysts were subjected to step response cycles of dimethyl ether (DME) at 300 °C in a temporal analysis of products (TAP) reactor. Propylene is the major olefin and displays an S-shaped profile. A 44-min induction period occurs before primary propylene formation and is reduced upon subsequent step response cycles. The S-shaped profile was interpreted according to induction, transition-regime and steady-state stages to investigate hydrocarbon formation from DME. The influence of precursors (carbon monoxide, hydrogen, dimethoxymethane, and 1,5-hexadiene) was studied using a novel consecutive step response methodology in the TAP reactor. Addition of dimethoxymethane, carbon monoxide, hydrogen or 1,5-hexadiene reduces the induction period of primary olefin formation. However, while dimethoxymethane, carbon monoxide and hydrogen accelerate the transition-regime towards hydrocarbon pool formation, 1,5-hexadiene attenuates it. Heavier hydrocarbons obtained from 1,5-hexadiene compete for active sites during secondary olefin formation. A phenomenological evaluation of multiple parameters is presented