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Abstract

Porous structure, surface chemistry and catalytic properties of carbon catalysts with various acid groups in ethyl tert-butyl ether synthesis were studied. Carbon catalysts were obtained by phosphoric acid activation followed by sulphonation of lignocellulosic feedstocks of different origin. Porous structure of carbon catalysts was characterized by nitrogen adsorption. Surface chemistry was investigated by potentiometric titration. Carbon catalysts obtained from lignosulphonate and sucrose showed the highest catalytic activity in ethyl tert-butyl ether synthesis (reaction rate at 120℃ is 4.3–5.2 × 10⁻⁶mol·g⁻¹·s⁻¹), is comparable to activity of Amberlyst-15 (5.0 ×10⁻⁶mol·g⁻¹·s⁻¹). Calculated turnover frequency (TOF) of sulphonic groups (pK_a = −2.5) equals 0.00116 s⁻¹, polyphosphate groups (pK_a = 1.9) 0.01019 s⁻¹, strong carboxylic (pK_a = 3.6) 0.00202 s⁻¹ and 0 s⁻¹ for weak carboxylic (pK_a = 6) groups. The activity of acid groups with pK_a lower than 3.6 is higher than the activity of sulphonic groups of Amberlyst-15 (TOF 0.00115 s⁻¹).

Keywords

Carbon catalyst acid strength catalytic activity porous structure surface chemistry

Introduction

The worldwide production and consumption of most popular fuel additive, methyl tert–butyl ether (MTBE), has sharply decreased for environmental reasons, whereas production of oxygenates from bio-resources, particularly, ethyl tert–butyl ether (ETBE), is growing (Yee et al., 2013). ETBE exhibits higher octane rating, higher boiling point, lower flash point, lower blending Reid vapour pressure and reasonably high oxygen contents (Thiel et al., 1997; Westphal et al., 2010; Yang et al., 2000). Moreover, the attraction of the ETBE industrial application is defined by the possibility to use bioethanol, and to obtain gasoline with reduced volatility. Thus, production of ETBE as gasoline fuel oxygenate additive conforms the principles of Green Chemistry (Clark et al., 2012; Sheldon et al., 2007) since it not only increases octane number, but also decreases pollution by reducing exhaust CO and unburned hydrocarbons. Commercially, ETBE is produced by exothermic reversible reaction between iso-butylene (IB) and ethanol (EtOH) over sulphonic ion exchange resins (Badia et al., 2013; Umar et al., 2009; Yee et al., 2013).

However, sulphonic ion exchange resins possess low mass transfer and low thermal stability, which decrease the catalytic activity and cause environmental problems. Furthermore, ion exchange resins tend to swell in polar reaction medium that causes technological difficulties in practical applications.

Sulphonated carbons are considered as promising catalysts in acid–base reactions due to rigid morphology, thermal stability and the lack of swelling in polar reaction medium, They can be inexpensively produced by partial carbonization of sulphopolycyclic aromatic hydrocarbons or sulphonation of partially carbonized organic compounds (Okamura et al., 2006; Toda et al., 2005; Zhao et al., 2010). Carbonaceous catalysts could be prepared from renewable resources as biomass (White, 2015).

Recently, the significance of weak acid surface groups in some catalytic reactions occurring in soft conditions was reported (Bucko et al., 2004; Gianotti et al., 2014; Kim et al., 2003; Vlasenko et al., 2006). The purpose of the present study is revealing the role of surface groups with different acid strength in ETBE synthesis.

Experimental

Carbon catalysts

Carbon catalysts were obtained by phosphoric acid activation (impregnation ratio 0.85, activation temperature 400℃) of lignocellulosic feedstocks of different origin followed by sulphonating with sulphuric acid at 180℃ for 5 h. Carbon catalysts were abbreviated according to the feedstock origin as: AP – apricot stones (Puziy et al., 2005), DW – dogwood stones, CG – coffee grounds, LS – lignosulphonate (Borregaard LignoTech, Sarpsborg, Norway) (Myglovets et al., 2014), SH – sunflower husks.

For comparison purpose, carbon catalyst was obtained by template method using silica gel as hard template and sugar as carbon source (Puziy et al., 2009, 2010a, 2011). Silica gel (for column chromatography, Acros Organics™) was impregnated by sucrose (carbon source) aqueous solution containing phosphoric acid (impregnation ratio 0.25), followed by drying, pre-carbonizing at 230℃ and carbonizing up to 400℃ in flow of argon (0.5 L min⁻¹) with temperature ramp 10° min⁻¹. After carbonization silica template was removed by dissolution in 40% HF. Obtained carbon was sulphonated using the same procedure as for lignocellulosic-derived carbons.

Characterization

Porous structure

Nitrogen adsorption isotherms were measured at −196℃ using Autosorb-6 adsorption analyser (Quantachrome, USA). Pore size distributions were calculated by Autosorb-1 software (Quantachrome, USA) using QSDFT method and slit pore model (Gor et al., 2012; Landers et al., 2013; Neimark et al., 2009). Specific surface area, A_BET, was calculated by BET method using nitrogen adsorption data in the relative pressure range chosen by recently proposed procedure (Rouquerol et al., 2007). The total pore volume, V_tot, was calculated by converting the amount of nitrogen adsorbed at a maximum relative pressure to the volume of liquid adsorbate. The micropore, V_mi, and mesopore, V_me, volumes were calculated from the cumulative pore size distribution as the volume of pores with sizes less than 2 nm and between 2 and 50 nm, respectively.

Surface groups

Surface groups of carbon catalysts were investigated by potentiometric titration method (Lützenkirchen et al., 2012) performed in thermostatic vessel at 25℃ using a 672 Titroprocessor combined with 655 Dosimat (Metrohm, Herisau, Switzerland). The proton concentration was monitored using an LL pH glass electrode (Metrohm). The amount of protons adsorbed at each titration point was calculated using the following equation where V₀ and V_t are the volume of background electrolyte and the volume of added titrant, respectively, and m is the mass of adsorbent. Solution equilibria and a correction for possible carbonate and/or silicate contaminations were calculated using EST software (Del Piero et al., 2006).

Catalytic activity

Catalytic activity of carbon catalysts was investigated in flow reactor with fixed bed (Puziy et al., 2010b; Vlasenko et al., 2006) loaded with 1.0 cm³ of carbon catalyst. Molar ethanol/isobutene ratio was 1.5, liquid hour space velocity – 1 h⁻¹, carrier gas helium. Commercially available ethanol (azeotropic mixture of C₂H₅OH; with 4.43% H₂O), isobutene (99%) (Sigma–Aldrich) and helium were used. The pressure was 1.0 MPa, temperature range 80–160℃.

Reaction products were analysed using gas chromatograph Agat (Mashpriborkomplekt, Russia) equipped with Chromaton N-AW column (3 mm id, 2 m length) with 10% Carbowax 600 and a thermal conductivity detector.

Results and discussion

Porous structure

Nitrogen adsorption isotherms for all carbons (Figure 1(a)) belong to a combination of types I and II of IUPAC classification (Thommes et al., 2015). A steep nitrogen uptake at low relative pressures corresponds to the adsorption in micropores while gradual increasing the adsorption at intermediate and high relative pressures is characteristic of formation of multilayer adsorption.

Figure 1.

Nitrogen adsorption isotherms (a) and pore size distributions (b) for lignocellulosic-derived carbons AP, DW, CG, LS, SH and templated carbon TC.

Templated carbon, TC, shows the lowest specific surface area, the largest fraction of mesopores and the largest average pore size, W_av (Table 1). The other carbons are predominantly microporous having 70–82% of total pore volume in micropore region.

Table 1.

Parameters of porous structure of lignocellulosic-derived carbons AP, DW, CG, LS, SH and templated carbon TC.

Carbon	A_BET (m² g⁻¹)	V_tot (cm³ g⁻¹)	V_mi (cm³ g⁻¹)	V_me (cm³ g⁻¹)	W_av (nm)
TC	198	0.14	0.07 (52%)	0.07	2.79
AP	823	0.39	0.32 (81%)	0.07	1.90
DW	1317	0.63	0.51 (82%)	0.12	1.90
CG	527	0.28	0.20 (70%)	0.09	2.15
LS	1142	0.57	0.45 (78%)	0.12	2.00
SH	558	0.28	0.21 (74%)	0.07	2.00

Main porosity of all carbons lies in the region of pore sizes smaller than 3–4 nm (Figure 1(b)). Templated carbon TC and apricot stone-based carbon AP show bimodal PSD while other carbons exhibit unimodal distribution. The widest micropores are observed for templated carbon TC and lignosulphonate-derived carbon LS, while the narrowest micropores are in apricot stone-based carbon AP (Table 1).

Surface groups

Proton-binding isotherms by all investigated carbons lies in negative region indicating that only proton dissociation occurs in investigated pH range (Figure S1 online). Proton-binding isotherms are relatively flat without steep inflection points that imply highly heterogeneous character of surface groups.

Taking into account engineering history of the carbon catalysts, proton binding isotherms were modelled by dissociation of five types of surface groups. They are sulphonic groups with pK_a −2.5, estimated by Marvin software (ChemAxxon, 2015; Csizmadia, 2000), polyphosphate groups with pK_a 1.9 ± 0.2, strong carboxylic sites with pK_a 3.6 ± 0.3, weak carboxylic sites with pK_a 6 ± 2 and phenolic groups with pK_a 9 ± 1.2. During the fit, the site concentration and the pK_a values were allowed to vary; only pK_a of sulphonic group was fixed at −2.5 since it is far beyond the experimental window. The results of the fit are presented as lines in Figure S1 online and Table 2.

Table 2.

Site concentrations (in mmol g⁻¹) for lignocellulosic-derived carbons AP, DW, CG, LS, SH and templated carbon TC.

Carbon	Total	Sulphonic	Polyphosphate	Strong carboxylic	Weak carboxylic	Phenolic
TC	3.87	0.37	0.38	0.32	0.18	2.61
AP	3.70	0.29	0.13	0.33	0.10	2.84
DW	2.48	0.00	0.27	0.22	0.08	1.92
CG	2.84	0.00	0.11	0.37	0.08	2.29
LS	2.31	0.00	0.21	0.52	0.08	1.50
SH	4.66	0.00	0.02	0.77	0.25	3.61
A-15	4.30	4.30

All investigated carbons show high total amount of surface groups (Table 2). Only templated and apricot stone-derived carbons feature sulphonic groups despite the fact that all carbons were sulphonated using the same experimental procedure. This fact implies that sugar-based (templated) and apricot stone-based carbons offer more reactive carbon material than the rest of the carbons.

Catalytic activity

Investigated carbons show high catalytic activity in ETBE synthesis from ethanol and isobutene (Figure 2). With increasing reaction temperature, the activity of the catalyst increases due to enhancing diffusion of reactants and products. At higher temperatures the reaction rate decreases due to thermodynamic limitations since the formation of ETBE is exothermic (Oudshoorn et al., 1999; Vila et al., 1993; Vlasenko et al., 2011).

Figure 2.

ETBE formation rate over lignocellulosic-derived carbons AP, DW, CG, LS, SH and templated carbon TC at different temperatures.

The activity of carbon catalysts is comparable to the activity of ion exchange resin Amberlyst-15 (A-15), which is widely used as catalyst for commercial production of ETBE (Yee et al., 2013). ETBE selectivity for investigated carbons is in the range 60–90% with a tendency to improve with increasing temperature. Side product of the reaction is tert-butyl alcohol with selectivity 10–30% shows a tendency to decrease with increasing temperature.

To reveal the role of different factors in ETBE synthesis, a correlation between reaction rate at 120℃ and porous structure parameters or surface chemistry was considered. There are no satisfactory correlation between reaction rate and porous structure parameters.

To gain a better insight on impact of surface chemistry on catalytic properties, the total activity of carbon catalyst in ETBE synthesis was modelled by the partial activity of each type of surface groups, turnover frequency (TOF), expressed by the equation

r = \sum_{i = 1}^{n} {TOF}_{i} Q_{i}

(2)

where TOF_i is turnover frequency of surface group i and Q_i is concentration of the surface group.

Fitting the equation (2) to experimental data allows calculating TOF of surface groups. A good correlation between experimentally determined and predicted ETBE formation rate at 120℃ (r@120) was obtained (Figure 3). The coefficient of determination, R², is about 0.8. The following TOF values were obtained: for sulphonic groups (pK_a = −2.5) 0.00116 s⁻¹, for polyphosphate groups (pK_a = 1.9) 0.01019 s⁻¹, for strong carboxylic groups (pK_a = 3.6) 0.00202 s⁻¹ and 0 s⁻¹ for weak carboxylic (pK_a = 6). This fact implies that only surface groups having pK_a lower than 3.6 are active in ETBE synthesis. Weaker acid surface groups are not involved in ETBE synthesis.

Figure 3.

Correlation between experimental reaction rate at 120℃ (r@120) and predicted by equation (2) in the ETBE formation over lignocellulosic-derived carbons AP, DW, CG, LS, SH and templated carbon TC.

Among the catalytically active groups, polyphosphate groups with moderately strong acidity are the most active in ETBE synthesis. High catalytic activity in ETBE synthesis of polymer-based carbons containing polyphosphate groups was also reported (Puziy et al., 2010b). This fact substantiates our previous conclusion on the crucial role of moderately weak acid groups in ETBE synthesis on zeolite catalysts (Vlasenko et al., 2006).

Conclusions

Carbon catalysts have been obtained by phosphoric acid activation followed by sulphonation of lignocellulosic feedstocks of different origin. Carbon catalysts obtained from LS and sucrose (TC) showed the highest catalytic activity in ETBE synthesis (reaction rate at 120℃ is 4.3–5.2 × 10⁻⁶mol·g⁻¹·s⁻¹), is comparable to activity of Amberlyst-15 (5.0 ×10⁻⁶mol·g⁻¹·s⁻¹). Estimated TOF showed inactivity of surface groups with pK_a greater than 3.6. The highest activity showed polyphosphate groups with moderate acidity. Calculated TOF of sulphonic groups (pK_a = − 2.5) equals 0.00116 s⁻¹, polyphosphate groups (pK_a = 1.9) 0.01019 s⁻¹, strong carboxylic (pK_a = 3.6) 0.00202 s⁻¹ and 0 s⁻¹ for weak carboxylic (pK_a = 6) groups. The activity of acid groups with pK_a lower than 3.6 is higher than the activity of sulphonic groups of Amberlyst-15 (TOF = 0.00115 s⁻¹).

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Supplemental Material

Supplementary material is available for this article online.

References

Badia

Fité

Bringué

(2013) Byproducts formation in the ethyl tert-butyl ether (ETBE) synthesis reaction on macroreticular acid ion-exchange resins. Applied Catalysis A: General 468: 384–394.

Bucko

Hafner

Benco

(2004) Active sites for the vapor phase Beckmann rearrangement over mordenite: An ab initio study. The Journal of Physical Chemistry A 108(51): 11388–11397.

ChemAxxon (2015) MarvinSketch 15.9.28.0. Available at: http://www.chemaxon.com (accessed 16 February 2017).

Clark

Luque

Matharu

(2012) Green chemistry, biofuels, and biorefinery. Annual Review of Chemical and Biomolecular Engineering 3(1): 183–207. Available at: http://www.annualreviews.org/doi/10.1146/annurev-chembioeng-062011-081014 (accessed 16 February 2017).

Csizmadia

(2000) JChem: Java applets and modules supporting chemical database handling from web browsers. Journal of Chemical Information and Computer Sciences 40(2): 323–324. Available at: http://pubs.acs.org/doi/abs/10.1021/ci9902696 (accessed 18 March 2013).

Del Piero

Melchior

Polese

(2006) A novel multipurpose Excel tool for equilibrium speciation based on Newton-Raphson method and on a hybrid genetic algorithm. Annali di Chimica 96(1–2): 29–47. Available at: http://www.ncbi.nlm.nih.gov/pubmed/16734021 (accessed 1 June 2012).

Gianotti

Manzoli

Potter

(2014) Rationalising the role of solid-acid sites in the design of versatile single-site heterogeneous catalysts for targeted acid-catalysed transformations. Chemical Science 5(5): 1810. Available at: http://xlink.rsc.org/?DOI=c3sc53088d (accessed 16 February 2017).

Gor

Thommes

Cychosz

(2012) Quenched solid density functional theory method for characterization of mesoporous carbons by nitrogen adsorption. Carbon 50(4): 1583–1590. Available at: http://dx.doi.org/10.1016/j.carbon.2011.11.037 (accessed 10 October 2012).

Kim

Han

(2003) Preparation and characterization of zeolite catalysts for etherification reaction. Catalysis Today 87(1–4): 195–203. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0920586103005455 (accessed 16 February 2017).

10.

Landers

Gor

Neimark

A V.

(2013) Density functional theory methods for characterization of porous materials. Colloids and Surfaces A: Physicochemical and Engineering Aspects 437: 3–32. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0927775713000277 (accessed 16 December 2013).

11.

Lützenkirchen

Preočanin

Kovačević

(2012) Potentiometric titrations as a tool for surface charge determination. Croatica Chemica Acta 85(4): 391–417. Available at: http://hrcak.srce.hr/file/138013 (accessed 10 September 2013).

12.

Myglovets

Poddubnaya

Sevastyanova

(2014) Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions. Carbon 80: 771–783. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0008622314008847 (accessed 22 September 2014).

13.

Neimark

Lin

Ravikovitch

(2009) Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons. Carbon 47(7): 1617–1628.

14.

Okamura

Takagaki

Toda

(2006) Acid-catalyzed reactions on flexible polycyclic aromatic carbon in amorphous carbon. Chemistry of Materials 18(13): 3039–3045. Available at: http://pubs.acs.org/doi/abs/10.1021/cm0605623 (accessed 25 November 2011).

15.

Oudshoorn

Janissen

van Kooten

WEJ

(1999) A novel structured catalyst packing for catalytic distillation of ETBE. Chemical Engineering Science 54(10): 1413–1418. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0009250999000810 (accessed 6 August 2012).

16.

Puziy

Poddubnaya

Gawdzik

(2009) Preparation of nanostructured carbons for solid phase extraction. Annales UMCS, Chemistry 64(5): 64–74. Available at: http://versita.metapress.com/openurl.asp?genre=article&id=doi:10.2478/v10063-008-0006-7 (accessed 14 August 2012).

17.

Puziy

Poddubnaya

Gawdzik

(2010a) Nanostructured carbons for solid phase extraction. Applied Surface Science 256(17): 5216–5220. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0169433209018285 (accessed 3 August 2011).

18.

Puziy

Poddubnaya

Kochkin

(2010b) Acid properties of phosphoric acid activated carbons and their catalytic behavior in ethyl-tert-butyl ether synthesis. Carbon 48(3): 706–713. Available at: http://linkinghub.elsevier.com/retrieve/pii/S000862230900685X (accessed 22 July 2011).

19.

Puziy

Poddubnaya

Martínez-Alonso

(2005) Surface chemistry of phosphorus-containing carbons of lignocellulosic origin. Carbon 43(14): 2857–2868. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0008622305003532 (accessed 22 July 2011).

20.

Puziy

Poddubnaya

Reinish

(2011) One-pot preparation of functionalized nanostructured carbons. Carbon 49(2): 599–604. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0008622310007128 (accessed 22 July 2011).

21.

Rouquerol

Llewellyn

Rouquerol

(2007) Is the BET equation applicable to microporous adsorbents? In: Llewellyn

Rodriquez-Reinoso

Rouqerol

(eds) Characterization of Porous Solids VII, Studies in Surface Science and Catalysis, Amsterdam: Elsevier, pp. 49–56.

22.

Sheldon

Arends

Hanefeld

(2007) Green Chemistry and Catalysis. Green Chemistry and Catalysis, Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA.

23.

Thiel

Sundmacher

Hoffmann

(1997) Residue curve maps for heterogeneously catalysed reactive distillation of fuel ethers MTBE and TAME. Chemical Engineering Science 52(6): 993–1005. Available at: http://linkinghub.elsevier.com/retrieve/pii/S000925099600454X (accessed 16 February 2017).

24.

Thommes

Kaneko

Neimark

AV.

(2015) Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure and Applied Chemistry 87(9–10): 1051–1069. Available at: http://www.degruyter.com/view/j/pac.2015.87.issue-9-10/pac-2014-1117/pac-2014-1117.xml (accessed 16 February 2017).

25.

Toda

Takagaki

Okamura

(2005) Green chemistry: Biodiesel made with sugar catalyst. Nature 438(7065): 178–178. Available at: http://www.nature.com/doifinder/10.1038/438178a (accessed 8 November 2011).

26.

Umar

Patel

Saha

(2009) Kinetic studies of liquid phase ethyl tert-butyl ether (ETBE) synthesis using macroporous and gelular ion exchange resin catalysts. Chemical Engineering Science 64(21): 4424–4432.

27.

Vila

Cunill

Izquierdo

J-F

(1993) Equilibrium constants for ethyl tert-butyl ether liquid-phase synthesis. Chemical Engineering Communications 124(1): 223–232.

28.

Vlasenko

Kochkin

Filippov

(2011) Effect of adsorption–desorption of reaction mixture components on ethyl-tert-butyl ether synthesis over commercial sulfonic acid resins. Catalysis Communications 12(12): 1142–1145. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1566736711001075 (accessed 4 August 2011).

29.

Vlasenko

Kochkin

Puziy

(2006) Liquid phase synthesis of ethyl-tert-butyl ether: The relationship between acid, adsorption and catalytic properties of zeolite catalysts. Journal of Molecular Catalysis A: Chemical 253(1–2): 192–197. Available at: http://www.sciencedirect.com/science/article/pii/S1381116906006662 (accessed 7 August 2012).

30.

Westphal

Krahl

Brüning

(2010) Ether oxygenate additives in gasoline reduce toxicity of exhausts. Toxicology 268(3): 198–203. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0300483X09006386 (accessed 16 February 2017).

31.

White RJ (ed.) (2015) Porous Carbon Materials from Sustainable Precursors. RSC Green Chemistry, The Royal Society of Chemistry.

32.

Yang

B-L

Yang

S-B

Yao

(2000) Synthesis of ethyl tert-butyl ether from tert-butyl alcohol and ethanol on strong acid cation-exchange resins. Reactive and Functional Polymers 44(2): 167–175. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1381514899000929 (accessed 16 February 2017).

33.

Yee

Mohamed

Tan

(2013) A review on the evolution of ethyl tert-butyl ether (ETBE) and its future prospects. Renewable and Sustainable Energy Reviews 22: 604–620.

34.

Zhao

Yang

(2010) Etherification of glycerol with isobutylene to produce oxygenate additive using sulfonated peanut shell catalyst. Industrial & Engineering Chemistry Research 49(24): 12399–12404. Available at: http://pubs.acs.org/doi/abs/10.1021/ie101461g (accessed 16 February 2017).

Ethyl tert -butyl ether synthesis using carbon catalysts from lignocellulose

Abstract

Keywords

Introduction

Experimental

Carbon catalysts

Characterization

Porous structure

Surface groups

Catalytic activity

Results and discussion

Porous structure

Surface groups

Catalytic activity

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

Supplemental Material

References