solid base catalyzed depolymerization (liquifaction/valorization) of lignin into low molecular...

P r e s e n t e d b y ,R icha Chaudhary

R e s e a r c h G u i d e : D r . P a r e s h L . D h e p e

C a t a l y s i s & I n o r g a n i c C h e m i s t r y D i v i s i o nC S I R - N a t i o n a l C h e m i c a l L a b o r a t o r y , P u n e , I n d i a

T e l . + 9 1 - 2 0 - 2 5 9 0 2 0 2 4 , F a x . + 9 1 - 2 0 - 2 5 9 0 2 6 3 3E m a i l : p l . d h e p e @ n c l . r e s . i n

G r o u p W e b p a g e : h t t p : / / a c a d e m i c . n c l . r e s . i n / p l . d h e p e

Solid base catalyzed depolymerization (liquefaction / valorization) of lignin into low molecular weight aromatic products

Keywords: Biomass, Lignin, Hydrolysis, Depolymerization, Solid base catalyst, Aromatic compounds

Schematic representation of lignin

Alkaline lignin: (A) XRD, (B) TGA-DTA, (C) 13C NMR, (D) FT-IR

10 20 30 40 50 60 70

Inte

nsity

(a.u

)

2q°

(A)

Cleavage of α- and β-aryl-alkyl-ether linkages

Aliphatic chainsplitting

Aromatic ringdecomposition

(B)

(D)

260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -40 -60Chemical Shift (ppm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Norm

alize

d Inte

nsity

21.42

45.48

55.50

69.98

83.23

114.6

312

3.12

129.4

1

147.4

9

182.1

3

208.0

2

228.1

2

Carbonyl groups (Ar-CHO)

Ester groups(Ar/R-CO-R/Ar)

sp2 carbon (C=C) in aromatics and alkenes

Methoxyl groups attached to the aromatic rings

Acetyl and alkanes(CH3-CO/ R3CH)

sp3 carbon next tooxygen

(Ar/R-CH2-O-)

(C)

TPD CO2 profile of various solid base catalysts

100 200 300 400 500 600 700

Temperature (°C)

NaX

NaY

NaP

KLTL

HAP

HT

MgO

CaO

v Order of basicity (mmol/g):

CaO (1.96) > NaX (0.42) ≈ HT (0.40) >NaY (0.34) > MgO (0.24) > KLTL (0.14) ≈ HAP (0.12) > NaP (0.09).

v Order of pH in reaction mixture:

CaO (pH ≥ 13.4) > HT (pH 12.5) > MgO (pH 10.5) > KLTL (pH 9.4) > NaX (pH 9.2) > NaY = NaP (pH 8.7) ≈ HAP (pH 8.5) > Millipore water (pH 6.8).

Characterization of lignin and various solid base catalysts

CatalystN2 sorption TPD-CO2

BET Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Total Basicity(mmol/g)

Peak maxima (°C)

NaX 586 0.32 1.10 0.42 404NaY 575 0.33 1.15 0.34 292NaP 14 0.10 1.8 0.09 171 & 486KLTL 220 0.13 nd 0.14 188HT 207 0.95 9.2 0.40 294 & 603HAP 39 0.18 9.08 0.12 166 & 587CaO 12.4 0.05 8.2 1.96 408 & 673MgO 9.2 0.02 4.8 0.24 604

Substrate M. Wt. (Da) Elemental analysis (%)

TGA-DTA(Residue %)

EDAX(element)

Monomer molecular formula

C H S N2 Air

AlkalineLignin

60,000 52 5 2.05 44 17 C, O, Na, S C8.7H9.1O5.1S0.13

Methodology for separation of products

Extraction with organic solvents

Reaction Mixture

Centrifugation

Solid(Catalyst + Solid)

Solution (EtOH + H2O soluble)

Acidified mixture

HCl (pH 1-2)

Reaction Charge

Depolymerization

Liquid

InsolubleSoluble*

Evaporation

SolidExtraction with organic solvents

InsolubleSoluble*

Evaporation

Aromatic products Aromatic products

v Reaction Condition: Lignin(0.5 g), Catalyst (0.5 g),Solvent (EtOH:H2O = 30 mL,1:2 v/v), 250°C, 1 h

v Solvents used for theextraction of products: diethylether (DEE) & ethyl acetate(EtOAc)

v Organic solvent solubleproducts(*) were analyzed byusing GC, GC-MS and HPLC.

Effect of various solid base catalysts on lignin depolymerization

0

10

20

30

40

50

60Pr

oduc

t yie

ld (w

t%)

Catalysts

DEE EtOAc SR Mix. Sol

24%

34%

51%

18% 17.4%14%

38%34%

30%

10.2%

Reaction condition: Lignin:Catalyst = 1:1 wt/wt, EtOH:H2O (1:2 v/v) 30 mL, 250 oC, 1 h.

Products adsorption study

v GC chromatographs of mixture of

Phenol, p-cresol, Guaiacol, Eugenol and

Vanillin in EtOH:H2O (1:2 v/v) 30 mL.

(A) Before reaction with CaO

(B) After reaction with CaO at 30 oC

(C) After reaction with CaO at 250 oC

(D) Before reaction with NaX

(E) After reaction with NaX at 250 oC

Reaction Condition: Monomers (0.1 g each), Catalyst (0.5 g), 1 h.

Minutes6 7 8 9 10 11 12 13 14 15

pA

25

50

75

100

125

150

175

pA

25

50

75

100

125

150

175Back SignalPVGEpC (1-2)

Back SignalPVGEpC (1-2) RC30C

Back SignalPVGEpC RC(1-2) 250C

Minutes7 8 9 10 11 12 13 14 15

pA

40

60

80

100

120

140

pA

40

60

80

100

120

140

Back SignalPVGEpC@RT1-2

Back SignalPVGEpC@2501-2

(A)

(B)

(C)

(D)

(E)

Inte

nsity

(a.u

.)

Products adsorption study with single molecules

v GC chromatographs of adsorption

study with single molecules at 30 oC.

(A) Eugenol in EtOH:H2O (1:2 v/v) 6 mL

(B) Eugenol:CaO (1:1 wt/wt) in EtOH:H2O

(1:2 v/v) 6 mL

(C) Eugenol:CaO (2:1 wt/wt) in EtOH:H2O

(1:2 v/v) 6 mL

(D) Vanillin in EtOH:H2O (1:2 v/v) 6 mL

(E) Vanillin:CaO (1:1 wt/wt) in EtOH:H2O

(1:2 v/v) 6 mL

(F) Vanillin:CaO (2:1 wt/wt) in EtOH:H2O

(1:2 v/v) 6 mL

(A)

(B)

(C)

(D)

(E)

(F)

Inte

nsity

(a.u

)

Eugenol

Eugenol + CaO (1:1 wt/wt)

Eugenol + CaO (2:1 wt/wt)

Vanillin

Vanillin + CaO (1:1 wt/wt)

Vanillin + CaO (2:1 wt/wt)

Optimization of pH for lignin depolymerization


Effect of pH on lignin depolymerization

0

10

20

30

40

50

60

9.2 8.7 8.7 8.7 ≥ 13.4 9.2 10.5 9.2

Prod

uct y

ield

(wt%

)

pH

DEE EtOAc SR Mix. Sol.

NaX NaY NaP CaO MgO

Reaction condition: Lignin, Catalyst, EtOH:H2O (1:2 v/v) 30 mL, 250 oC, 1 h.

Depolymerization of lignin using CaO catalyst

0

10

20

30

40

50

60

0

10

20

30

40

50

0.5 0.1 0.05

Tim

e (m

in.)

Prod

uct Y

ield

(wt%

)

Catalyst weight (g)

DEE EtOAc Time (B)

0

10

20

30

40

50

0.5 0.05 0.005

Prod

uct Y

ield

(wt%

)

Catalyst weight (g)

DEE EtOAc(A)

v Effect of CaO loading on lignin depolymerization

Reaction condition: (A) Lignin (0.5 g), EtOH:H2O (1:2 v/v) 30 mL, 250 oC, 1 h. (B) Lignin (0.5 g), EtOH:H2O (1:2 v/v) 30 mL, 250 oC.

Optimization of reaction condition

0

10

20

30

40

50

60

0.33 0.5 0.66 0.83 1 2P

rodu

ct y

ield

(wt%

)

Time (h)

DEE EtOAc (B)

0

10

20

30

40

50

60

230 240 250 260 240 + 0.3 MPa N2

Pro

duct

yie

ld (w

t%)

Temperature (oC)

DEE EtOAc(A)

Reaction condition: Lignin:NaX = 1:1 wt/wt, EtOH:H2O (1:2 v/v) 30 mL.

(A) Effect of temperature (B) Effect of time

GC-MS spectrum of DEE & EtOAc Soluble products

Reaction condition: Lignin:NaX = 1:1 wt/wt, EtOH:H2O (1:2 v/v) 30 mL, 250 oC, 1 h.

(a) DEE soluble (liquid) (b) EtOAc soluble (liquid)

(c) DEE soluble (solid) (d) EtOAc soluble (solid)

HPLC Spectrum of products

Minutes0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

nRIU

0

5000

10000

15000

20000

25000

30000

nRIU

0

5000

10000

15000

20000

25000

30000

RID: RI SignalULDEE-C


Quantification of identified products


Table1Quantificationofidentifiedlignindepolymerisationproducts.Reactioncondition:Lignin:NaX=1:1wt/wt, EtOH:H2O (1:2 v/v) 30 mL,250oC,1h

Monomer Structure Yield(wt%)

Guaiacol

1

Pyrocatechol

0.3

2-methoxy-4-methylphenol

3.7

Resorcinol

0.8

4-hydroxybenzylalcohol

1.4

2,6-dimethoxyphenol

0.03

Eugenol

0.4

1,2,4-trimethoxybenzene

0.03

Vanillin

3.6

3,4-dimethoxyphenol

0.02

4-hydroxy-3-methoxybenzylalcohol

0.3

Acetoguaiacone

4.1

2,4-ditert-butylphenol

0.5

Hexadecane 0.5

Homovanillicacid

0.3

Table1Quantificationofidentifiedlignindepolymerisationproducts.Reactioncondition:Lignin:NaX=1:1wt/wt, EtOH:H2O (1:2 v/v) 30 mL,250oC,1h

Monomer Structure Yield(wt%)

Guaiacol

1

Pyrocatechol

0.3

2-methoxy-4-methylphenol

3.7

Resorcinol

0.8

4-hydroxybenzylalcohol

1.4

2,6-dimethoxyphenol

0.03

Eugenol

0.4

1,2,4-trimethoxybenzene

0.03

Vanillin

3.6

3,4-dimethoxyphenol

0.02

4-hydroxy-3-methoxybenzylalcohol

0.3

Acetoguaiacone

4.1

2,4-ditert-butylphenol

0.5

Hexadecane 0.5

Homovanillicacid

0.3

Lignin and products correlation


FT-IR of alkaline lignin

FT-IR of (a) DEE and (b) EtOAc soluble products

Characterization Fresh & Spent Catalyst (NaX)


10 20 30 40 50 60 70

Inte

nsity

(a.u

)

2q°

Fresh

Spent

ARTICLE JournalName

8 |J.Name.,2012,00,1-3 Thisjournalis©TheRoyalSocietyofChemistry20xx

Pleasedonotadjustmargins

Pleasedonotadjustmargins

reaction was carried out in absence of nitrogen pressure at240oC.This suggests thatpressureplaysan important role inachieving higher yields. In our earlier work on solid acidcatalyzeddepolymerization,effectofpressureonthereactionwas studied in detail and it was established that with anincrease in pressure yield of products increases.12,13Subsequently, reaction was performed at highertemperature (260 oC) believing that higher pressures wouldbenefit the reaction but, again slight decrease in yield (46%)was seen, which was against expectation. Thus, 250 oC is anoptimumtemperature tocarryoutdepolymerizationof ligninusingNaXcatalyst.SinceonNaXcatalystunlikeCaOandothercatalysts, only minimal quantity of products were adsorbedunderreactionconditions,itisassumedthatdecreaseinyieldathigher reaction temperatures isdue todegradationand/orrepolymerization reactions. Nevertheless, detailed study isrequiredtocommentfurtheronthismatter. The effect of reaction time on the product yield wasinvestigatedat250oCoverNaXasacatalyst inethanol:water(1:2 v/v) solvent system (Fig. S8(B), ESI). The products yieldwasincreasedupto51%forareactiontimeof1handbeyondthis time products yield started declining.However, a carefullookattheresultsimplythatalthoughoverallyielddecreasedin 2 h, but contribution from DEE soluble products wasincreased in 2 h reaction. This implies that EtOAc solubleproducts like oligomers are over the period of time wereconvertedintolowmolecularweightproducts. Further, tomakesure that there isnota significanteffectfrom series degradation reactions, we have performed thedepolymerizationreactionsbynormalizingthereactiontimetothemassofcatalyst.Detailsarerepresented inFigureSxx.AsseenfromFigureSxx(ESI),whenreactionswereperformedfor1 h, with the decrease in CaO loading, increase in yield wasobserved.ThismaybeduetopHorproductadsorptioneffect.Later, reactionswereperformedwithvaryingcatalyst loadingand accordingly change in reaction time. For e.g. with 0.5 gCaOcatalystloading,reactionwasperformedfor1hhowever,when 0.1 g of CaOwas charged in the reactor, reactionwascarriedout for 20min (i.e. 5 timesdecrease inboth, catalystandtime).Subsequently,reactionwascarriedoutwith0.05gCaOfor10min.Theresults(FigureSxx,ESI)showthatthoughwith decrease in time according to decrease in catalystloading, slight improvement in theyieldswas seen.However,

thisincreaseismarginalconsideringwhenreactionwascarriedout for 1 h with 0.05 catalyst (Figure Sxx, ESI). This suggeststhat pH (9.2) along with time is important to achieve betteryields. Mostly,lignindepolymerizationreactionsarecarriedoutinalcohol:water mixture for achieving better yields due tosolubility of lignin in the solvent system which makes thereaction system homogeneous (whenwater soluble catalystsareused).Sincealkalineligninusedinthisstudyiscompletelysolubleinwater(TableS1,ESI),reactionwascarriedoutat250oCwithNaX as a catalyst for 1 h in onlywater. However, noproduct formationwasseen in this reaction.This resultcouldnotbeexplainedonthebasisofpressureeffectaswithwaterasasolventat250oC,totalpressureof4.0MPawasobservedwhilewhenreactioniscarriedoutwithethanol:water(1:2v/v)solventsystem,4.2MPapressurewasmonitoredyet51%yieldwas achieved. So, it can be considered that presence ofethanol in subcritical condition is helpful in forwarding thereaction. Subsequently, reactionswere performed by varyingthe ratio of ethanol-water (2:1, 1:1, 1:5 v/v) and maximumyield was observed in 1:2 (v/v) solvent system. One of thereasonsforreactionstoproceedinpresenceofethanolmightbeduetotheformationofsodiumethoxide(reactionbetweenNa+oncatalystandethanol),whichcanactasstrongbase.Thisscenario is not possible when only water is used in thisreaction. Though, the in-situ formation of sodium ethoxidecould not be confirmed under reaction conditions. Yet, it isexpected that when reaction is completed Na+ willcompensate the negative charge on zeolites. This is becauseduring spent catalyst characterization studies no loss of Na+wasobserved(formoredetailspleaseseesection2.3.9,ESI).3.1.6.Catalystrecyclability

Inthecatalystrecyclabilitystudy,NaXwasrecoveredfromthereactionmixturebycentrifugationand laterwaswashedwithethanol:water (1:2 v/v), and used in the next reaction (fordetails see section 2.3.8, ESI). With fresh catalyst, 51% yieldwasobservedandlaterinthe1strecyclerundecreaseinyieldto 34% was seen. In subsequent recycle run almost similaryieldwas seen (35% in 2nd run, 33% in 3rd run) (Fig. S9, ESI).Nevertheless, it is interesting to note here that though inrecycle runs total yield was decreased but formation of DEE

TPD-CO2,N2sorptionandICP-OESanalysisoffresh&spentNaXcatalyst.

Catalyst TPD-CO2 Nitrogensorption ICP-OES

NaX Totalbasicity(mmol/g)

BETsurfacearea(m2/g)

Poresize(nm)

Porevolume(cm3/g)

Si/AlratioActual

Si/AlratioObserved

NaContent(mg/g)

Fresh 0.42 582 1.1 0.3 1.2 1.2 17.3

Spent 0.41 586 1.1 0.3 - 1.2 20.5

XRD of fresh & spent NaX TPC CO2 of fresh (a) & spent (b) NaX

Conclusions

1. Depolymerization efficiencies of various solid base catalyst were evaluated.

• Shows that NaX is capable to depolymerize high molecular weight lignin (60,000

Da) into low molecular weight aromatic products (51% yield).

2. The optimization of reaction conditions and detailed studies on catalyst properties

was done.

• Reveal that pH 9.2 is optimum to catalyze depolymerization.

• Solid base are effective for lignin depolymerization at milder conditions (T≤ 250 °C,

atmospheric pressure).

3. An unique product adsorption studies was done.

• Indicates that strong bases have more tendency to adsorb the products.

Further readings

1. Solid base catalyzed depolymerization of lignin into low molecular weight

products.

Richa Chaudhary, Paresh L. Dhepe, Green Chemistry, 2016,

DOI:10.1039/C6GC02701F.

http://pubs.rsc.org/en/content/articlelanding/2017/gc/c6gc02701f#!divAbstract

2. Solid base catalyzed depolymerization of lignin.

P. L. Dhepe and Km. Richa, Council of Scientific and Industrial Research, 2016,

INDIAN Patent Application no. 201611007650.

3. Group Webpage: http://academic.ncl.res.in/pl.dhepe

solid base catalyzed depolymerization (liquifaction/valorization) of lignin into low molecular...

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