Gas-phase dehydration of glycerol over commercial Pt/γ-Al2O3 catalysts

Sergey Danov ,Anton Esipovich ,2,Artem Belousov ,2,*,Anton Rogozhin ,2

1 Nizhny Novgorod State Technical University n.a.R.E.Alekseev,49 Gaidar Street,Dzerzhinsk,606026,Russian Federation

2 Lobachevsky State University of Nizhni Novgorod,National Research University,23,Gagarin Avenue,Nizhny Novgorod,603950,Russian Federation

Keywords:Glycerol Dehydration Acrolein Zeolite Gamma alumina

ABSTRACT Gas-phase dehydration of glycerol to produce acrolein was investigated over commercial catalysts based onγ-Al2O3,viz.A-64,A-56,I-62,AP-10,AP-56,AP-64 and KR-104.To understand the effect of Cl-anions,HCl-impregnated supports have been investigated in the dehydration reaction of glycerol at 375°C.For comparison,various H-zeolites were also examined.It was found that the glycerol conversion over the solid acid catalysts was strongly dependent on their acidity and surface area.And the relationship between the catalytic activity and the acidity of the catalysts was discussed.The outstanding properties of Pt/γ-Al2O3 catalyst systems for the dehydration of glycerol were revealed.Pt/γ-Al2O3 catalyst(AP-64)showed the highest catalytic activity after 50 h of reaction with an acrolein selectivity of 65%at a conversion of glycerol of 90%.Based on these results,catalysts based on γ-Al2O3 appear to be most promising for gas phase dehydration of glycerol.

1.Introduction

In recent years,the increasing production of biodiesel has resulted in a price decline of crude glycerol,making aqueous glycerol an attractive compound for the synthesis of fine and crude chemicals[1–4].The production of biodiesel is still increasing and is even forecasted to nearly double from 22.7 million metric tons in 2012 to 36.9 million metric tons in 2020[5].Therefore,the use of glycerol as a starting material becomes economically and environmentally feasible.Finding value-added alternatives to glycerol incineration would improve economic viability of biodiesel manufacture and the biofuel supply chain[6].

Among the glycerol derivatives,acrolein is an important chemical[7,8]used as a feedstock for production of acrylic acid and 1,3-propanediol,which are important monomers of industrial polyesters[9,10].Acrolein is currently produced by gas-phase oxidation of propene with a Bi–Mo–V mixed oxide catalyst[11,12].Production of acrolein from surplus glycerol by catalytic dehydration leads to oil saving and cost lowering[13].

Dehydration of glycerol is carried out in liquid or gas phase.Several catalyst systems have been investigated,for example,zeolites[14,15],heteropolyacids[16,17],mixed oxides[18],WO3/TiO2[19],WO3/ZrO2[20],unsupported[21]and supported Nb2O5[22,23],Ta2O5[24],bifunctional MOx–Al2O3–PO4catalysts[25,26],rare earth pyrophosphates[27],zirconium phosphate[28]and other classes such as silica-supported ionic liquids[29].

Kim et al.[14,30]studied the dehydration of glycerol over various H-zeolites.These catalysts did not show a good performance due to quick deactivation.Microporous materials with low external surface such as H-ferrierite,H-mordenite and H-Y exhibited low conversion of no more than 30%.This correlation supported the hypothesis of internal mass transfer limitations,indicating that the active sites inside the catalyst particle did not contribute to the reaction[14].

Tsukuda et al.[31]tested heteropolyacids supported over SiO2and found that the introduction of mesopores in silica support significantly affected the catalytic activity.However,such strongly acidic catalysts were deactivated quickly as carbonaceous deposits blocked the catalyst surface,in particular in the presence of large amounts of micropores.

Atia et al.[32]and Haider et al.[33]studied supported alkaline salts of silicotungstic acid.In details,silica-supported lithium silicotungstate gave 70%of yield in acrolein,whereas the corresponding potassium salt yielded 65%and the cesium salt only 55%[32].

Silica-supported niobium oxides were used by Shiju et al.[22].The authors prepared catalysts with different amounts of niobium oxide(5%–40%in mass)and calcined the as-prepared samples at 400–800°C.The performances over catalysts prepared using different niobia loadings increased with the amount of active phase with the best result over the catalyst containing 20%(in mass)niobium oxide(68%yield in acrolein).

Although most catalytic systems lead to a high selectivity to acrolein at total glycerol conversion,very few maintain their catalytic properties for more than 5–10 h.The catalyst deactivation occurs due to extensive coke deposition on its surface,and attempts to limit the rate of coking by adding inert gas and modifying of the catalysts by introducing of expensive additives(e.g.,Nb2O5,rare-earth elements)have not been convincingly successful.Furthermore,in most published studies dehydration of glycerol was carried out on the catalyst powder(particle size less than 1 mm),which may cause some technical problems in industrial application and will increase the cost of production of acrolein,making these methods practically inapplicable.

In this study the role of the acidic sites of various catalysts on the catalytic behavior in the dehydration of glycerol was investigated in more detail.From these results,it was possible to establish acidity–activity–deactivation relationships to explain the behavior of the catalysts.The strength and the type of the catalyst acidity were of utmost importance for obtaining high catalytic performances.

A new type of catalyst systems based on γ-Al2O3in the gas-phase dehydration of glycerol was also examined.Many of these commercial catalysts showed excellent catalytic performance.Not only its catalytic performance,but also the low price of gamma alumina increased its attractiveness as catalyst carrier.

2.Experimental

2.1.Catalysts

In this work various types of commercially available zeolites,e.g.,FAU(HX,HY),MFI(HZSM-5),BEA(Hβ), fluorinated(A-56,I-62)and nonfluorinated(A-64)supports for the catalysts reforming and reforming catalysts(AP-56,AP-64,KR-104)were used.

In order to convert the acid form of the NaX zeolite samples were treated with 5%aqueous NH4Cl solution at 80°C for 2 h under stirring.After that,obtained samples were dried at 100–110 °C for 3 h and calcined for 2 h at 400°C.

Zeolites CBV-760/HY and CP811C-300/Hβ were formed with kaolin.

HZSM-5 samples,А-64,А-56,I-62 AP-56,AP-64,KR-104 were used without any treatment.

To understand the effect of Cl-anions,two samples of chlorinated γ-alumina have been prepared by impregnating the γ-Al2O3support(A-64)with aqueous solution of HCl following the incipient wetness method with 1.0 and 5.0%Cl(in mass).After impregnation,the solids were dried at 110°C for 3 h.The impregnation and drying procedure were repeated a few times to obtain the desired total loading.Before use of the sample in glycerol dehydration,calcination of the samples were conducted during 2 h at 400°C.The samples obtained were denoted as A-64 Clxwith x%of chlorine loading(in mass).

To understand the effect of F-anions in reforming catalyst on catalyst deactivation,the sample AP-10 was treated by aqueous solution of NH4F.After impregnation,the solid was dried at110°C for 3 h.Before use of the sample in glycerol dehydration,calcination of the sample was conducted during 2 h at 400°C.The sample obtained was denoted as AP-10 F0.1,with 0.1%of fluorine loading(in mass).

2.2.Catalysts characterization

The acidity determination of all catalysts was carried out by modified method of Tamele[34,35].The catalyst suspended in n-hexane was titrated with n-butylamine in the presence of p-dimethylaminoazobenzene(p Ka=+3.3).

The KBr pellet technique was applied for determining IR spectra of the samples.IR spectra were recorded on a Shimadzu IR Affinity-1 spectrometer in the region of wave numbers from 1000 to 4000 cm-1with a resolution of 4 cm-1.

The surface area of the catalysts was determined by using the adsorption desorption method at 77 K by the standard Brunauer–Emmett–Teller(BET)method using NOVA 1200e(Quantachrom Instruments)equipment.

X-ray powder diffraction data have been recorded with an XRD-6100 diffractometer(Shimadzu)using Cu Kαradiation.The X-ray tube worked at 30 kV.The measurements were done from 20°to 70°(2θ).

2.3.Catalytic test and products analysis

Catalytic tests on the glycerol dehydration were carried out using a continuous flow stainless reactor(internal diameter 10 mm,length 90 mm,catalyst particles diameter 2–3 mm)under atmospheric pressure.Catalyst was sandwiched in the middle of the reactor with inert packing.This reactor was placed in a heated chamber maintained at the reaction temperature.Prior to the reaction,the catalyst was pretreated atreaction temperature for1 h in the flowing of dry nitrogen.The reactor was fed with an aqueous solution of glycerol by means of HPLC pump.The aqueous glycerol solution was vaporized in the evaporator and passed over the catalyst.

Products were condensed in a cryostated(-20°C)condenser and collected hourly for analysis on a Chromos GC 1000 gas chromatograph equipped with a VB-1701 capillary column(0.25 mm i.d.,60 m long)and a flame ionization detector(FID).The reaction was usually run for 10 h.

3.Results and Discussions

3.1.Acid and textural properties of catalysts

The BET surface areas and the total pore volumes of are presented in Tables 1 and 2 together with the total acidity of the catalysts.

Structure properties and surface acidity of catalysts are important factors for the dehydration of glycerol to acrolein.As seen in Table 1,H-zeolites display a surface area in the range of 221–616 m2·g-1with the specific pore volume in the range of 0.16–0.46 cm3·g-1.Тhe pore volumes and surface areas of the catalysts based on γ-Al2O3are very similar(Table 2).

In the case of H-zeolites(Table 1),total acidity decreases in the following order:HY＞Hβ＞HZSM-5＞HX.Alumina is an active catalyst for several reactions,such as olefin isomerization and alcohol dehydration.However,gamma alumina can be treated with halogenated compounds(HCl,HF)[36]to increase acidity as can be seen from Table 2.

Metal function properties in bifunctional metal-acid catalyst were provided by Pt.The acid properties were supplied by the support itself and in this case it was alumina promoted with chlorine[37].The oxidation state of Pt in the reduced Pt-Re/γ-Al2O3catalysts is currently accepted to be zero[38].The oxidation state of Re is however still a matter of unresolved controversy.Metallic Re would join to Pt to form Pt–Re microcrystals,small tom groups or quasi-alloys known as“clusters”or“ensembles”[39].As seen in Table 2,total acidity increases with the chlorine content of the alumina supported catalysts[40].

3.2.Activity comparison of H-zeolites and catalysts based on γ-Al2O3

As can be seen from Fig.1 the main products are acrolein,acetol(hydroxyacetone)and acetaldehyde.Furthermore,carbon oxides and coke are formed during the process as demonstrated by the material balance divergence from glycerol.According to Suprun et al.[41],coke was formed during the course the polymerization of acrolein and oligomerization of glycerol.

Deactivation of acid sites and blocking of the catalyst surface occur due to the coke formation.As a result a sharp reduction of glycerol conversion is observed in the first hours(Fig.2).

Table 1 Physical and textural properties of H-zeolites

Table 2 Physical and textural properties of catalysts based on γ-Al2O3

At the same time,it can be seen from Fig.3 that selectivity of acrolein decreases when Hβ and HZSM-5 zeolites are used.

According to reference[42],the TPD profile for Hβ and HZSM-5 zeolites was characterized by a wide distribution of centers with two peaks corresponding to the medium and strong acid sites.Apparently,the presence of strong acid centers in Hβ and HZSM-5 zeolites(the average heat of desorption of ammonia is equal to 135 and 155 kJ·mol-1,respectively[43])leads to a high selectivity towards acrolein at the initial stage(Fig.3).However,the strong acidic sites are quickly deactivated by carbonaceous deposits and selectivity is reduced.Zeolites X and Y are characterized by a narrow distribution of the acid sites in strength with the maximum of the distribution corresponds to the acid sites of medium strength(the average heat of desorption of ammonia is equal to 105 and 130 kJ·mol-1,respectively[43]).Thus,selectivity towards acrolein remains relatively constant(Fig.3).

Influences of H-zeolites acidity on their deactivation and selectivity to acrolein are shown in Fig.4.

Deactivation of the catalyst was calculated as the difference between the conversion of glycerol to 1 and 10 h.Increasing of the number of acid sites leads to increase in the catalyst deactivation and reduction of its selectivity.Increasing of the surface area also reduces selectivity towards acrolein and enhances the catalyst deactivation(Fig.5).

It is demonstrated that the acrolein selectivity is equal to 62.2%at complete glycerol conversion after 10 h over non- fluorinated A-64.The developed mesopore system based on gamma alumina with relatively low acidity gamma alumina provides high selectivity and stability in a course of glycerol dehydration(Figs.6 and 7).

Application of catalysts based on fluorinated γ-Al2O3characterized by a higher concentration and strength of acid sites leads to decrease in selectivity towards acrolein and increase in deactivation of the catalyst.As shown in Figs.6 and 8,deactivation of A-56(0.7%F)catalyst is not observed after 10 h.On the other hand,increasing the fluorine content(I-62,3.1%F)leads to about 3.5%of deactivation.The selectivity to acrolein decreases with increasing concentration in acid sites(Figs.7 and 8).

Fig.1.Formation of the main dehydration products from glycerol.

The surface chlorination[44–46]as well as the surface fluorination[47–49]changed the acid-basic properties of transition aluminas in so far as the acid strength increase.The addition of chlorine to γ-Al2O3creates new stronger acid sites and leads to deterioration of catalyst properties(Figs.6 and 7).

However,the acrolein selectivity is equal to 50%at75%conversion of glycerol over A-64 after 50 h.

The deactivation phenomenon can be explained by the formation of coke.It shows that increasing the number of acid sites leads to rapid deactivation of catalysts due to the formation of carbonaceous deposits,and the formed coke blocks the surface active sites for further reactant adsorption.A medium acid site strength combined with a high resistance to deactivation by coke formation is a necessary parameter for good catalyst performance.

Furthermore,increasing surface area also reduces selectivity towards acrolein and increases the catalyst deactivation.This may be due to the decreasing of desorption rate from the catalyst surface.This causes an increase residence time of reaction products and leads to the formation of carbonaceous deposits.

Then it is attempted to enhance catalyst lifetime and reduce coke deposition by using γ-Al2O3doped with Pt.This approach has been effectively used in the industrial hydroisomerisation of alkanes,which is carried out on Pt-doped alumina or zeolite.

In glycerol to acrolein dehydration,coke is likely to form by acrolein polymerization on strong acid sites.Platinum has a high affinity to olefins,which results in migration of olefin from the acid sites to the metal sites.These improve catalyst stability to deactivation due to the coke formation.

Fig.2.Conversion of glycerol for the different H-zeolites with time on stream:●HY,▲Hβ,▼ HX,■ НZSM-5.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375°C.

Fig.3.Acrolein selectivity over H-zeolites with time on stream:● HY,▲ Hβ,▼ HX,■НZSM-5.Reaction conditions:20%glycerol solution(in mass)at 12 ml·h-1,amount of catalyst 1 g,T=375°C.

The dehydration of glycerol was carried out over AP-56,AP-64,KR-104,AP-10 and AP-10 F1.The variations of the glycerol conversion and acrolein selectivity with the time on stream are measured as shown in Figs.9 and 10.The deactivation phenomenon can be explained by the formation of coke.This effect is more pronounced for KR-104(1.20%Cl)which is due to the high total acidity of the sample.In the case of AP-64(0.70%Cl),the decrease in the glycerol conversion is very slow.The lower rate of deactivation and the higher acrolein selectivity in this case are likely due to the lower acidity in comparison to the KR-104(Table 2).The effect of total acidity on the acrolein selectivity and the catalyst deactivation are very similar for AP-10 and AP-10 F0.1 samples.

Furthermore,the activity and stability of reforming catalysts in gasphase dehydration of glycerol is increased with increasing Pt content from 0.1(AP-10)to 0.62%(AP-64)(in mass)(Figs.9 and 10).The Pt doping is found to reduce the amount of coke deposited on the catalyst surface,which is in line with the enhancement of catalyst stability to deactivation.The promotion effect of platinum was also investigated by Alhanash et el.[50].

Fig.4.Correlation between the amount of acid sites and acrolein selectivity(●)and zeolites deactivation(■)obtained after 10 h of the reaction.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

Fig.5.Correlation between the specific surface area and acrolein selectivity(●)and zeolites deactivation(■)obtained after 10 h of the reaction.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

Fig.6.Conversion of glycerol over fluorinated and chlorinated alumina with time on stream:□ A-64,● A-56,▲ I-62,▼ A-64Cl1,? A-64Cl5.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

Fig.7.Acrolein selectivity over fluorinated and chlorinated alumina with time on stream:□ A-64,● A-56,▲ I-62,▼ AlCl-1,? AlCl-5.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

Fig.8.Correlation between the amount of acid sites and acrolein selectivity(●)and supports for the catalysts reforming deactivation(■)obtained after 10 h of the reaction.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375°C.

Our experiments demonstrate that among the tested catalysts AP-64 is shown the highest stability and selectivity.Furthermore,doping catalysts with Pt improves their regeneration by reducing the combustion temperature to ～350 °C although traditional regeneration by coke combustion is typically carried out at～500 °C[50,51].

3.3.Influence of the reaction temperature on conversion and selectivity over AP-64

In the presence of catalyst AP-64, five experiments at different reaction temperatures were conducted.As usual,the glycerol conversion significantly increases with increasing reaction temperatures as shown in Fig.11.

The acrolein selectivity also increased with increasing reaction temperatures(Fig.12).

The acrolein yield is equal to 65%at complete glycerol conversion after 10 h at375°C.At lower temperatures the intermolecular dehydration,yielding oligomers of glycerol,is thermodynamically favored over the desired intramolecular dehydration forming acrolein.In addition to that,at temperatures lower than 300°C,desorption of the reaction products is slowed down,leading to coke formation.At temperatures higher than 375°C,the formation of CO and CO2is dominated due to cracking processes.These results demonstrate that the reaction temperature can be important for obtaining high catalytic performances over AP-64.

Fig.9.Conversion of glycerol over catalysts based on γ-Al2O3 with time on stream:■ AP-64,● KR-104,▲ AP-56,? AP-10,○ AP-10 F0.1.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

Fig.10.Acrolein selectivity over catalysts based on γ-Al2O3 with time on stream:■ AP-64,● KR-104,▲ AP-56,? AP-10,○ AP-10 F0.1.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g,T=375 °C.

3.4.Influence of the glycerol concentration on conversion and selectivity over AP-64

As shown in Fig.13 the glycerol conversion decreases noticeably with increasing glycerol content in the feed.

When the glycerol content in the feed increases,the side reactions such as bimolecular condensation reactions leading to the formation of larger molecules are dominated which results in rapid deactivation of catalyst.On the other hand,the acrolein selectivity increases noticeably with increasing glycerol content in the feed(Fig.14).

Thus,the acrolein selectivity is equal 72%and 59%after 10 h with 80%(in mass)and 10%(in mass)glycerol content in the feed,respectively.

3.5.Influence of the residence time on conversion and selectivity over AP-64

The effect of the residence time on the dehydration of glycerol over AP-64 is shown in Fig.15.The glycerol conversion generally increases with increasing residence time.The maximum acrolein selectivity(69%)is reached with residence time at 17.1 s after 10 h(Fig.16).

As can be seen from Fig.17 the acrolein selectivity seems to pass through a maximum by further increase of residence time.This might be due to consecutive reactions like acrolein degradation or polymerization and glycerol polycondensation.

Fig.11.Glycerol conversion over AP-64 with time on stream at different reaction temperatures:■ 375 °C,○ 350 °C,▲ 330 °C,? 300 °C,▼ 280 °C.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g.

Fig.12.Acrolein selectivity over AP-64 with time on stream at different reaction temperatures:■ 375 °C,○ 350 °C,▲ 330 °C,? 300 °C,▼ 280 °C.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,amount of catalyst 1 g.

3.6.Long-term stability and regeneration of AP-64

Long-term catalytic test on the glycerol dehydration over AP-64 was carried out at375°C under atmospheric pressure.Catalyst(1 g,particles diameter 2–3 mm)was sandwiched in the middle of the reactor with inert packing.The reactor was fed with an aqueous solution contained 20%(in mass)of glycerol at a feed flow rate of 12 ml·h-1by means of HPLC pump.The reaction was run for 50 h.

The variations of the glycerol conversion and acrolein selectivity with the time-on-stream over AP-64 are shown in Fig.18.

Although the AP-64 catalyst is stable for at least 10 h of reaction,a 10%decrease in activity is observed after 50 h(Fig.18).A regeneration step is therefore still necessary and it is important to determine whether the thermal stability of the catalyst is high enough for regeneration by burning off the coke formed on the catalyst.In the literature,the cause of deactivation is frequently reported to be the formation of coke[22,31,50,52,53].The integration of a regeneration step in an industrial process is discussed in some of the articles on glycerol dehydration[7,8,54,55].

After 50 h,the reaction flow was switched to an air flow of 50 ml·min-1,and the reactormaintained for5 h at the reaction temperature to regenerate the catalyst.After that the reactant flow was reintroduced into the reactor.Properties of the fresh,spent and regenerated AP-64 are shown in Table 3.The glycerol conversion and acrolein selectivity after regeneration are shown in Fig.19.

Fig.13.Conversion of glycerol over AP-64 with time on stream at different glycerol concentration in the feed:▲ 10%C3H8O3(in mass),□ 20%C3H8O3(in mass),● 40%C3H8O3(in mass),?80%C3H8O3(in mass).Reaction conditions: flow rate of glycerol 12 ml·h-1,amount of catalyst 1 g,T=350 °C.

Fig.14.Acrolein selectivity over AP-64 with time on stream at different glycerol concentration in the feed:▲ 10%C3H8O3(in mass),□ 20%C3H8O3(in mass),● 40%C3H8O3(in mass),? 80%C3H8O3(in mass).Reaction conditions: flow rate of glycerol 12 g·h-1,amount of catalyst 1 g,T=350°C.

It can be seen(Fig.19)that the catalytic activity is completely recovered by the heat treatment and there is a similar initial period of selectivity increase and a similar deactivation behavior.This result suggests that coke formation is the main reason for catalyst deactivation during the dehydration of glycerol.

Fig.20 shows the powder X-ray diffraction pattern of the fresh and the used AP-64 catalyst.

The fresh sample consists mainly of three phases:γ-Al2O3phase,Pt phase and AlO(OH)phase[56–58].The peaks confirm that the main crystalline matrix is gamma alumina.The three peaks of gamma alumina can be indexed.In AP-64 catalyst small diffractions lines are detected at 39.46°,46.54°and 67.24°.These lines can be attributed to a Pt metallic phase.It is observed that the AP-64 samples contain a small amount of AlO(OH)phase in addition to the γ-Al2O3phase.This oxy-hydroxide phase AlO(OH)commonly termed as pseudoboehmite is generally the precursor to γ-Al2O3phase and completely converts to the alumina phase at higher temperatures[59,60].X-ray diffraction pattern of the used AP-64 catalystsample is very similaron XRD pattern of the fresh catalyst.However,δ-Al2O3phase is detected in the spent sample[61].Any type of phase transformations of the support alumina has a significant effect on the catalytic properties of alumina supported metal catalysts[62],in some cases reported as being the major cause for the catalyst deactivation[63]while in others the phase of the alumina used has a major impact on the catalyst stability[64].

Fig.15.Glycerol conversion over AP-64 with time on stream at different residence time:▲28.5 s,□ 17.1 s,● 5.7 s,? 2.8 s.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,T=350 °C.

Fig.16.Acrolein selectivity over AP-64 with time on stream at different residence time:▲28.5 s,□ 17.1 s,● 5.7 s,? 2.8 s.Reaction conditions:20%(in mass)glycerol solution at 12 ml·h-1,T=350 °C.

Fig.17.Conversion of glycerol(□)and acrolein selectivity(○)versus residence time with AP-64(TOS=4 h).

The IR spectrum of the fresh and used samples(Fig.21)clearly shows a broad OH stretching mode from 2500 to 3700 cm-1and from 2800 to 3700 cm-1respectively,which stands for surface hydroxyls and adsorbed water.The observed decrease in the OH peak region is a good indication of decreasing of AP-64 acidity(Table 3)in a course of glycerol dehydration.

Fig.18.Conversion of glycerol(□)and acrolein selectivity(○)with time on stream over AP-64.

Table 3 BET surface area and acid strength of the fresh,used and regenerated AP-64

Fig.19.Evolution of glycerol conversion(□)and acrolein selectivity(○)as a function of time at 375°C after in situ regeneration of AP-64 at reaction temperature for 5 h in an air flow of 50 ml·min-1.

The bands at 2800–3000 cm-1are associated with the Csp3-H stretching vibration.These bands are characteristic to the glycerol adsorbed on the metal oxide surface[65,66].

SEM analysis of AP-64[Fig.22(a)]reveals the presence of large Pt particles.Deactivated AP-64 is black in color and the natural visual impression is that the catalysts are totally covered by coke.Fig.22(b)presents the SEM images of the AP-64 catalyst which is deactivated during the dehydration of glycerol.

4.Conclusions

The influence of acid and surface properties of various catalysts on selectivity towards acrolein and catalysts deactivation in glycerol dehydration was investigated in this paper.Acidity of the catalyst was important factor for conversion and product distribution.A sensitive tuning of this property may be the key to further improvement of activity,selectivity and stability of catalysts for the catalytic conversion of glycerol to acrolein.

Fig.20.XRDpatterns of the fresh(a)and spent(b)AP-64:○γ-Al2O3 phase;●Ptphase;□AlO(OH)phase;■ δ-Al2O3 phase.

Fig.21.IR spectrum of the fresh(a)and spent(b)AP-64.

Fig.22.SEM analysis of fresh(a)and spent(b)AP-64.

It was shown that increase in concentration of acid sites resulted in increase in catalyst deactivation.The increase in surface area also led to deterioration of catalyst properties.

It was demonstrated that γ-Al2O3catalyst exhibited high initial activity with a glycerol conversion of 100%at 62%acrolein selectivity after 10 h.However,the acrolein selectivity was equal to 50%at 75%conversion of glycerol over A-64 after 50 h.

It was found that doping γ-Al2O3with Pt improved catalyst stability to deactivation.Pt/γ-Al2O3commercial catalyst(AP-64)showed the highest catalytic activity after 50 h of reaction with an acrolein selectivity of 65%at a conversion of glycerol of 90%and a reaction temperature of 375°C under ambient pressure.The AP-64 catalyst can therefore be regenerated by simple coke burning with air at reaction temperature.Based on these results,Pt/γ-Al2O3catalysts appear to be most promising for gas phase dehydration of glycerol.

Acknowledgments

This work was performed in Lobachevsky State University of Nizhni Novgorod and was supported by the Ministry of Education and Science of the Russian Federation(contract№02.G25.31.0119).This work was also performed as a project part of the state task in the field of scientific activity(№10.1686.2014/K).