Hai-Long Liao,Bao-Ju Wang,Ya-Zhao Liu,Yong Luo,Jie-Xin Wang,Guang-Wen Chu,Jian-Feng Chen

State Key Laboratory of Organic-Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,China

Research Center of the Ministry of Education for High Gravity Engineering and Technology,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Monolithic catalyst Rotating packed bed reactor Hydrogenation Selectivity

ABSTRACT Selective hydrogenation plays an important role in chemical industries,yet its selectivity is usually limited by the mass transfer.In this work,the enhanced hydrogenation selectivity was achieved in a rotating packed bed(RPB)reactor with excellent mass transfer efficiency.Aiming to be used under the centrifugal filed,a monolithic catalyst Pd/γ-Al2O3/nickel foam suiting for the shape and size of the rotor of RPB reactor was prepared by the electrophoretic deposition method.The mechanical strength of the catalyst can meet the requirement of high centrifugal force in the RPB.The hydrogenation selectivity in the RPB reactor using the 3-methyl-1-pentyn-3-ol hydrogenation system was 3–8 times higher than that in a stirred tank reactor under similar conditions.This work proves the feasibility of intensifying the selectivity of hydrogenation process in the RPB reactor.

1.Introduction

Selective hydrogenation of organic compounds accounts for a big share in chemical reactions that are involved in petrochemical and fine chemical industries [1,2].In particular,the selective hydrogenation of alkynes is industrially significant,because alkene-enriched streams are necessarily required for the synthesis of various polymers [3].Extensive studies have paid attention to catalyst design to improve the intrinsic activity of hydrogenation [4–6].However,the mass transfer resistance of reactor can also result in the decreased hydrogenation selectivity.Increasing the mass transfer rate can reduce the contact time between liquid solution and hydrogen,and avoid excessive hydrogenation,thus improve the selectivity of the target product.Efficient reactors such as microreactor [7] and magnetically stabilized bed [8] have been explored to minimize overhydrogenation.

The rotating packed bed(RPB)is regarded as one of the promising reactors for process intensification [9].Due to the existence of centrifugal filed in the rotor,the liquid is teared into numerous thin films or tiny droplets,and their interactions with the solid packing are enhanced.The mass transfer rate of RPB can be increased by 1–3 orders of magnitude,compared with the conventional fixed bed reactor and fluidized bed reactor [10,11].The advantages of the RPB reactor in heterogeneously catalytic reactions gradually reached prominence these years[12,13].For example,the enhanced mass transfer in the RPB reactor significantly improved the ozone utilization efficiency [13–15].In addition to the increment of the reaction rate,the RPB reactor can also intensify the selectivity of complex reactions.For example,Chen et al.[16] tailored the product distribution of Fischer-Tropsch synthesis by choosing the optimum rotational speed of the RPB reactor.Aside from the gas–solid reaction,the RPB reactor also offered a comparable performance in gas–liquid–solid phases hydrogenation process [12].

The catalyst used in the RPB reactor shall has both functions of catalysis and packing,having shearing action on the liquid.The rotor rotating at high speed suffers different centrifugal forces at different radial positions.The long-term operation of classic particle catalyst in the rotor may result in a loose inside and a dense outside distribution in the radial direction.This uneven distribution will cause the vibration and shorten the life of RPB reactor,and catalyst particles may be crushed under high centrifugal force.Hence,monolithic catalyst with good mechanical strength is supposed to be a candidate for the RPB reactor compared with particle catalyst.Nickel foam(NF)has been proved to be a promising packing in the RPB reactor which can strengthen both gas-side and liquid-side volumetric mass-transfer coefficients [17,18].As a result,nickel foam acting as the catalyst substrate may enhance the mass transfer of insoluble hydrogen during the reaction.Nickel foam structured catalyst has already been prepared by the wetness impregnation method [19],yet the nature of the nickel foam provides limited specific surface area for the active component,compared to alumina.Consequently,further improvement needs to be conducted on the nickel foam to optimize its catalytic performance to be used in the RPB reactor.

Electrophoresis has high dispersion capacity.It can form uniform coating even in the internal pores of the substrate.As a result,electrophoretic deposition(EPD)can achieve higher homogeneous distribution of the microstructure of the material and more reasonable compaction density.In this work,the EPD was applied to prepare a coating layer of porous γ-Al2O3to increase the specific surface area of nickel foam,and then palladium (Pd) was loaded onto the γ-Al2O3/nickel foam (NF) by impregnation.Characterizations of the Pd/γ-Al2O3/NF catalyst were carried out to illustrate its morphology,crystal structure,specific surface area,and mechanical strength.The hydrogenation of 3-methyl-1-pentyn-3-ol (C6H10O) was chosen as the model reaction for a typical hydrogenation of functionalized alkyne.Its intrinsic reaction rate is considerably high,and it is a commonly-used heterogeneous catalytic reaction system to evaluate reactor performance[20,21].The selective hydrogenation performance was investigated by the above structured Pd/γ-Al2O3/NF catalyst in the RPB reactor under different rotational speeds.Furthermore,the selectivity in the RPB reactor and a stirred tank reactor (STR) was compared under similar conditions.

2.Experimental

2.1.Catalyst preparation

The basic principle to prepare Pd/γ-Al2O3/NF was to coat γ-Al2O3onto nickel foam as the first step to increase its surface area,and then load Pd onto the γ-Al2O3/NF.Accordingly,the alumina coating was prepared by EPD [22].The EPD cell used in this work was depicted in Fig.1.Graphite sheet was employed as anode and bare NF was acted as cathode.The electrodes were connected to the output end of a power supply.Slots on the side walls of the cell can be used to adjust the distance of electrodes.

All chemicals were purchased form Macklin and used without purification.The alumina sol was prepared as follows.Pseudo boehmite (AlOOH·nH2O) was added into deionized water which was heated to 358 K.After the above mixture was stirred with recirculation cooling for 2 h,nitric acid (HNO3) was dripped into the solution.The molar ratio of AlOOH·nH2O and HNO3was kept as 10:1.The stirring process continued for another 4 h.The sol was then cooled to ambient temperature and kept stationary for 24 h before it was diluted with ethanol and used in the deposition.The pH value of the above solution was adjusted by HNO3,and the zeta potential and average diameter of the suspension were measured by a laser particle size analyzer(Malvern Mastersizer 2000).

Fig.1.Diagram of electrophoretic deposition for γ-Al2O3 deposition.

The volume fraction of the sol (Vx),deposition time (t),deposition voltage (U),and electrode distance (D) were the key parameters that were needed to be characterized before EPD.The EPD process was presented at ambient temperature.The γ-Al2O3/NF was subsequently rinsed with ethanol and acetone at the end of the EPD process.Afterwards,it was calcined in a tube furnace at 773 K for 2 h under air atmosphere,and the heating rate was 5 K·min-1.

The Pd/γ-Al2O3/NF was prepared by the method of impregnation [19].The calcined γ-Al2O3/NF was first soaked into a 10 mmol·L-1SnCl2solution for 1 h,and then the γ-Al2O3/NF was immersed into the 11 mmol·L-1PdCl2solution for another 1 h.The catalyst was taken out of the above solution and rinsed it carefully with deionized water.Finally,after the catalyst was dried at a temperature of 393 K for 2 h,Pd/γ-Al2O3/NF was obtained.

2.2.Catalyst characterization

The surface morphologies of NF,γ-Al2O3/NF and Pd/γ-Al2O3/NF were characterized by a scanning electron microscope (SEM,FEI Nova NanoSEM 450).The Pd loading amount on the substrate was acquired by an inductively coupled plasma-optical emission spectrometer (ICP-OES,Thermo iCAP-6300).The crystal structures of the catalysts were investigated by an X-ray diffractometer(XRD,Rigaku MiniFlex 600).The specific surface area and pore size distribution of the γ-Al2O3/NF were analyzed by a surface characterization analyzer (Micrometrics ASAP 2020).The mechanical strength stabilities of NF and γ-Al2O3/NF were tested by a universal testing machine (MTS Industrial Systems CMT6503).

2.3.Experimental setup and procedure for hydrogenation

The schematic diagram of hydrogenation setup by the RPB reactor was displayed in Fig.2.The volume of the reactor cavity was 500 ml.The axial height,inner diameter,and outer diameter of the structured Pd/γ-Al2O3/NF catalyst were 20,8,and 18 mm,respectively.Such structured catalyst was installed inside the rotor and the rotor was driven by a motor and its rotational speed can be adjusted by a frequency converter.There was a spiral riser to lift the liquid into the rotor.The liquid was then teared into tiny droplets or film under centrifugal force and reacted with hydrogen that was fulfilled in the whole RPB.The product was collected by the wall and then flew down to the bottom for the circulation by the spiral riser.The temperature was controlled by the heating jacket.The pressure was stabilized by the gas inlet and outlet valves.Liquid samples were extracted out of the reactor by system pressure.

Fig.2.Schematic diagram of the RPB reactor for hydrogenation.(1) gas cylinders;(2) gas filter;(3) gas check valve;(4) gas pressure reducing valve;(5) RPB reactor(5-1:temperature transducer,5-2:motor,5-3:pressure transmitter,5-4:gas outlet,5-5:sampling point,5-6:rotor,5-7:riser,5-8:heating jacket);(6) control box(temperature and rotational speed).

Fig.3.Hydrogenation of 3-methyl-1-pentyn-3-ol.

Fig.4.Zeta potential and average diameter as a function of pH value.

The reaction pathway for hydrogenation of C6H10O was presented in Fig.3.The initial concentration of C6H10O was 300 mol·m-3in ethanol,and 250 ml solution was charged into the RPB reactor before the reaction.The reaction temperature and pressure were set as 298 K and 0.2 MPa,respectively.The liquid samples were analyzed by a gas chromatograph (Shimadzu,GC-2014C) equipped with a flame ionization detector (FID) and a HP-INNOWAX column to determine the composition.Nitrogen was used as carrier gas.The temperatures of the injector and FID were 438 K and 453 K,respectively.The column temperature was maintained at 323 K for 1 min and stepwise increased to 388 K at 20 K·min-1.

Fig.5.Effects of (a) volume fraction of the sol,(b) deposition time,(c) deposition voltage,and (d) electrode distance on coating ratio of γ-Al2O3.

3.Results and Discussion

3.1.Characteristics of EPD process

The initial pH value of the EPD solution was 5,and the solutions with pH values of 2,3,3.5,and 4 were analyzed as shown in Fig.4.The zeta potential was high enough (～+30 mV) so that the Coulomb force could prevent particles from agglomeration and form a stable dispersion for weeks.The average particle diameter in the suspension was 120 nm and did not change significantly with the pH value.In order to maintain a high zeta potential without excessive corrosion of the NF substrate,the final pH value of the EPD solution was set as 4.

Fig.6.SEM micrographs of (a) NF,(b)γ-Al2O3/NF,(c) enlarged view of γ-Al2O3/NF,(d)γ-Al2O3 coating layer,and (e) Pd/γ-Al2O3/NF.

Small pieces of NF(diameter of 12 mm and thickness of 5 mm)were employed to study the coating ratio in the EPD process.The coating ratio (x) was calculated by weighing the mass of the NF before (m1) and after (m2) the EPD process expressed by Eq.(3):

The effects of Vx,t,U,and D on coating ratio of γ-Al2O3were presented in Fig.5.The coating ratio increased with the increase of Vx,t,and U,while it decreased with the increase of D.The morphology of the γ-Al2O3/NF was initially observed with a digital microscope(Dino-lite AD4113T),and typical morphologies of the coating on NF were illustrated in Fig.5(d).Cracking of the layer occurred when the coating ratio was high.Correspondingly,the incomplete layer was formed when the coating ratio was low.The superior morphology was obtained under conditions of Vx=35%,t=6 min,U=10 V,and D=2 cm,and the coating ratio was approximately 12% (mass).

3.2.Structural analysis of catalyst

The NFs before and after EPD were presented in Fig.6(a) and 6(b).The original surface of the NF was evenly coated with a layer of coating without changing its initial geometry structure.Fig.6(c)was the enlarged view of the coated strut,and the rugged surface was caused by the accumulation of alumina on the cathode during the EPD process.The side view of the coating layer could be photographed near the edge of the strut and was demonstrated in Fig.6(d).The average thickness of the coating layer was approximately 12 μm.The magnified view of the surface of the Pd/γ-Al2O3/NF was shown in Fig.6(e),and the chemical composition of the catalyst was also determined by an energy dispersive X-ray spectroscopy (EDS).Element Pd was detected in the EDS spectrum,demonstrating the successful deposition of Pd on the γ-Al2O3/NF.The element loading amount of Pd evaluated by ICP-OES analysis was 1.20% (mass).

The XRD patterns of NF,γ-Al2O3/NF,γ-Al2O3(scraped off the γ-Al2O3/NF),and Pd/γ-Al2O3/NF were presented in Fig.7.All samples except γ-Al2O3exhibited the nickel structure with major peaks at 2θ about 44°,52°,and 76°.The wide peaks of γ-Al2O3may be caused by the moderate calcination temperature.The major peaks at 2θ about 40.1°and 46.7°were crystal faces of Pd(111)and(200).

The results of nitrogen adsorption–desorption analyses of NF,γ-Al2O3/NF,and Pd/γ-Al2O3/NF were depicted in Fig.8(a1)–(c1).The typical Type IV adsorption–desorption isotherm revealed that the catalyst support was mesoporous [23].The Barrett-Joyner-Halenda pore size distribution of NF showed a broad peak between 10 and 20 nm (Fig.8(a2)),while the pore size distribution of γ-Al2O3/NF was in the range of 3–10 nm (Fig.8(b2)).The results of specific surface areas(SBET)and average pore diameters(Davg)were listed in Table 1.The SBETof γ-Al2O3/NF increased nearly 10 times than that of the NF.The loading of γ-Al2O3on NF greatly contributed to increase the BET surface area and decrease the pore size of supports.Since the weight of the NF substrate contributed to the major proportion of the γ-Al2O3/NF,the SBETof γ-Al2O3/NF was much lower than that of the γ-Al2O3scraped off the NF.In addition,the SBETand Davgof Pd/γ-Al2O3/NF changed little compared with that of γ-Al2O3/NF (Fig.8(c1),(c2) and Table 1).

The structured catalyst was designed to be used in the RPB reactor with considerable centrifugal force,therefore the mechanical strength stabilities of NFs before and after the Al2O3loading need to be determined.Fig.9 indicates the compressive strengthstrain curves of NF and γ-Al2O3/NF.The two curves were similar and the yield limits (σs) of NF and γ-Al2O3/NF were both 0.41 MPa,which suggested that the loaded NF preserved the mechanically robust nature of the untreated NF.

3.3.Hydrogenation of 3-methyl-1-pentyn-3-ol

The catalytic performance of Pd/γ-Al2O3/NF in the RPB reactor was evaluated.The concentration profiles of C6H10O,C6H12O,and C6H14O were presented in Fig.10.The conversion profiles of C6H10O were shown in Fig.11.Thegradually decreased with the reaction time,and bothand C6H14O were simultaneously generated.The experimentally ultimateat rotational speeds of 900 and 1200 r·min-1was 69% and 74%,respectively.The former studies had pointed out that the increase of the rotational speed in the RPB reactor can boost not only the wetting efficiency of the catalyst[24],but also the gas–liquid and liquid–solid mass transfer rate[17,25].Consequently,thewas larger at a higher rotational speed.Theat 1200 r·min-1was also higher than that at 900 r·min-1.

The catalytic performance of Pd/γ-Al2O3/NF in the STR was also investigated.The STR was reformed from the above RPB reactor.The riser was removed and a long shaft was required.And the catalyst was fixed at the end of shaft to ensure the submersion of the catalyst.The stirring speed was 900 r·min-1,and other reaction conditions were kept the same as those in the RPB reactor.The concentration profiles were presented in Fig.12.Theat 40 min was 61%,which was slightly lower than that in the RPB reactor.The comparison ofbetween the RPB reactor and STR were summarized in Fig.13 and Fig.14.The selectivity towards C6H12O in the RPB reactor was 3–8 times higher than that in the STR.

According to the data in Table 2,the intrinsic kinetics parameter in the alkyne-alkene reaction is about 30 times higher than the alkene-alkane reaction.Since the contact time between hydrogen and alkynes is short in the RPB reactor,there might not be sufficient time to finish the hydrogenation process before the liquid left the rotor.Additionally,according to the definition of the instantaneous selectivity of consecutive hydrogenation reaction,the instantaneous selectivity of the hydrogenation of C6H12O can be calculated as Eq.(6).The ratio of rate constants (k2/k1) was only affected by temperature.Therefore,it could be seen from Eq.(6)that increasing CEor decreasing CYwas beneficial to increasing the instantaneous selectivity.

Table 2 Intrinsic kinetics parameters in the hydrogenation of 3-methyl-1-pentyn-3-ol

Fig.8.(a1) (b1) (c1) N2 adsorption–desorption isotherm and (a2) (b2) (c2) pore size distribution of NF,γ-Al2O3/NF,and Pd/γ-Al2O3/NF,respectively.

Table 1 The physical properties of NF,γ-Al2O3,γ-Al2O3/NF and Pd/γ-Al2O3/NF

The mass transfer performance of the reactor has an explicit effect on the maximum concentration of intermediate in the reaction.In STR,the high mass transfer resistance leads to a decrease in hydrogenation selectivity,which reduces CEand increases CY.However,in the RPB reactor,the intensified mass transfer can shorten the residence time of the liquid in the rotor and avoid further hydrogenation of olefin.Therefore,the selectivity of target product is improved.The problem of low selectivity caused by the limitation of mass transfer in STR is solved.

Fig.9.Compressive strength-strain curves of NF and γ-Al2O3/NF.

Fig.10.Concentration profiles in the RPB reactor at (a) N=900 r·min-1 and (b)N=1200 r·min-1.

Fig.11.Conversion profiles in the RPB reactor at N=900 r·min-1 and 1200 r·min-1.

Fig.12.Concentration profiles in the STR at N=900 r·min-1.

Fig.13.Selectivity towards C6H12O in the RPB reactor and STR.

Fig.14.Conversion profiles in the RPB reactor and STR.

4.Conclusions

A Pd/γ-Al2O3/NF monolithic catalyst has been prepared for selective hydrogenation of 3-methyl-1-pentyn-3-ol in the RPB reactor.A layer of uniform coating was formed on the NF employing the EPD method.The specific surface area of the γ-Al2O3/NF increased nearly 10 times than that of the NF,and the mechanical strength remained unchanged.The increase of rotational speed in the RPB reactor was beneficial to increasing the selectivity of the desired intermediate product.The selectivity in the RPB reactor was 3–8 times higher than that in the STR under similar conditions.This work provides an attractive approach to intensify the selectivity of hydrogenation processes in the RPB reactor.

DeclarationofCompetingInterest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22022802 and 91934303).

Nomenclatures

Ccatcatalyst concentration (catalyst/liquid),m3·m-3

Ciconcentration,mol·m-3

D electrode distance,cm

Davgaverage pore diameter of the catalyst,nm

Kiadsorption constant,m3·mol-1

kireaction rate constant,m3·s·mol-1

m mass,g

N rotational speed,r·min-1

rikinetics parameters,m3·mol-1

SBETspecific surface area of the catalyst,m2·g-1

t time,min or s

U deposition voltage,V

Vxvolume fraction of the sol in the EPD solution,%

x coating ratio,%

σ compressive strength,MPa

ε strain,%

Subscripts

A alkane

E alkene

Y alkyne

1 alkyne-alkene reaction

2 alkene-alkane reaction