H2 feedstocks

NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis, Vol. 119 A. Parmaliana et al. (Editors) o 1998 Elsevier Science B.V. All rights reserved.

485

I s o a l c o h o l synthesis from CO/H2 feedstocks C. R. Apesteguia a, S. Miseo b, B. De Rites b and S. Soledb

aINCAPE,UNL-CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina bExxon Research and Engineering Company, Corporate Research Science Laboratories, Annandale, New Jersey 08801, USA.

The synthesis of methanol and isobutanol from synthesis gas over copper-containing MgyCe(Y)Ox catalysts was studied. The influence of the catalyst composition, temperature and contact time on isoalcohol productivity was established. The reaction pathway for isoalcohol synthesis was studied by adding methanol, ethanol or propanol to the reactant feed.

I. INTRODUCTION The selective production of isobutanol-methanol mixtures from synthesis gas is a potentially attractive technology for the synthesis of methyl-tert-butil ether (MTBE). Modified methanol synthesis catalysts, such as Mn(Zn)O/Cr203/alkali operated at high temperatures and CuO/ZnO/alkali operated at low temperatures, exhibit promising catalytic performances for a one step synthesis of isobutanol [1-4]. Catalysts consisting of palladium supported on a coprecipitated manganese, zinc, zirconium, lithium oxide support show high isobutanol synthesis productivity at high temperatures (>400~ and pressures (> 100 atm) [5]. However, even at the high reactor temperatures and pressures required, current catalytic approaches for isoalcohol synthesis produce relatively low yields and selectivities. Hence, practical application of such catalytic process still requires significant improvements, and one approach involves developing novel catalytic materials capable of more selectively producing methanol and isobutanol mixtures at moderate reactor temperature and pressure. Recently [6,7], we have found that catalysts based on coprecipitated mixtures or solid solutions of alkaline earth oxides and rare earth oxides (also including yttrium oxide), which may contain copper oxide and an alkali dopant, are active and selective for isoalcohol synthesis at less severe conditions than those required by prior art catalysts. In this paper we intentionally introduce reaction products (methanol, ethanol, 1-propanol) into the CO/H2 feed streams and study their effect on isoalcohol selectivity. The catalytic tests were performed on coppercontaining MgyCe(Y)Ox mixed oxides. 2. EXPERIMENTAL Catalysts were prepared by coprecipitation of rare earth oxides and alkaline earth oxides

486 under controlled pH conditions as described elsewhere [7]. Copper oxide was either coprecipitated or added by impregnation. The precursor was then decomposed in air at 400600~ for 4 h. The supported catalysts are designated by a slash, e.g. Cu/MyNzOx designates Cu supported on a MyNzOx mixed oxide with Cu loading given in wt % and y, z, and x represent g-atom quantities. Coprecipitated catalysts are designated without a slash, e.g. CupMyNzOx,indicates a coprecitated oxide where p, y, z, and x represent g-atom quantities of the respective elements. Powder X-ray diffraction patterns (XRD) were collected on a Rigaku diffractometer using monochromatic CuK,, radiation. The chemical composition of the samples was measured by an Inductively Coupled Plasma (ICP) spectrometer (Jarrel Ash). BET surface areas (Sg) were measured by N2 adsorption at 77 K in a Omnisorp sorptometer. Samples were tested in a plug-flow packed-bed reactor using a 1:1 CO/H2 feed. Both online and off-line samples were analyzed by GC-MS for CO2, methanol, ethanol, linear C3-C6 primary alcohols, branched C4-C6 primary alcohols, secondary alcohols (isopropanol, 2butanol), methane, ethane, linear C3-C~2 aliphatic hydrocarbons, branched C4-C6 hydrocarbons, ethylene, linear C3-C5 olefinic hydrocarbons, dimethylether (DME) and esters (methyl acetate, methyl propanoate, methyl i-butanoate). 3. RESULTS AND DISCUSION Table 1 shows the chemical composition and BET surface areas of impregnated and coprecipitated catalysts studied here. All the catalysts contained less than 10 wt% of copper and had surface areas between 75 and 125 mE/g. Table 1 Catalysts: Chemical composition and BET surface areas Catalyst Elemental Analysis (wt%) a Cu M8 Ce Y Cu/CeO2 6.83 72.15 Cu0.sMgsCeOx 7.71 28.40 32.82 K/Cu0.sMgsCeOx 7.71 28.40 32.80 Cu/MgsYOx 9.62 35.03 23.81 Cu0.sMsYCeOx 7.81 6.63 37.12 25.84

K 0.86 -

Sg m2/8 75 102 90 125 88

The XRD patterns, which are not shown here, indicate that sample Cu0.sMgYCeOx was a solid solution which contains the CuO and MgO substituted into a crystalline CeO2 structure. In samples with a high magnesium loading (Cu0.~MgsCeOx and Cu/Mg~YOx catalysts), the solubility of the MgO in the rare earth oxide host was exceeded, so we observed both crystalline MgO and doped cerium (yttrium) oxide. Copper oxide was not detected in any sample, thereby suggesting that copper was finely dispersed into the mixed oxide matrix. Catalytic tests were carried out at 50 atm. During the 250 hour catalytic runs, the temperature was varied between 260 and 360~ and the space velocity (GHSV) between 460 and 1850 cma(sTP)/gcath. In Table 2 we present the catalytic results obtained on Cu/MgsYOx, Cu0.sMgsCeOx, and Cu0.sMgYCeO• catalysts after 120 h on stream, at 320~ and 50 atm. Methanol and isobutanol were the major products in the oxygenate fraction. Neither the alcohols nor the hydrocarbons followed a Schulz-Flory distribution; branched alcohols were readily formed as indicated by the high branched alcohol/linear alcohol ratio. Table 2 shows

487 Table 2 Alcohol synthesis (productivities, expressed in g/kgcath) 320~ GHSV = 1832 cc/lgCat-h, CO:H2 = 1, P = 50 atm. Data measured after 120 on stream Product Cu/MgsYOx Cu0.sMgsCeOx Cu0.sMgYCeOx Methanol Ethanol 1-propanol 1 -butanol 1-pentanol 1 -hexanol

Isobutanol 2-m- 1-butanol 2-m-l-pentanol DME Methane Xco(%) a

Alc/Hyd b (%C) BraJLin ~

64.0 0.77

58.8 1.87

60.8 2.90 2.32 0.32 017 015 4.35 0.54 0 26 1 34 10.0

1.02

1.44

0.07 0.33 0.18 7.07 0.42 0.22 2.82 9.98

025 0 26 0 09 5.76 0.58 0.23 0.33 10.3

19.9 1.9 1.2

20.8 2.3 3.7

18.7 1.6 2.0

a CO conversion b Total alcohols/Total hydrocarbons ratio Branched C4-C6 alcohols/Linear C2-C6 alcohols that, at a similar CO conversion level, the Cu0.sMgsCeOx catalyst yields the highest productivity to isobutanol and the Alc/Hyd and Bra/Lin ratios. Thus, additional studies were performed using catalysts containing copper, ceria, magnesia and potassium. Fig. 1 presents the catalytic selectivities obtained on Cu/CeO2, Cu0.sMgsCeOx and K/Cu0.sMgsCeOx catalysts. The Cu/CeO2 catalyst promoted selectively the formation of isobutanol but the production of short-chain linear alcohols was significant, leading to a relatively low Bran/Lin ratio of 0.9. When copper oxide was coprecipitated with magnesia and ceria, the catalytic activity as well as selectivity and productivity to isobutanol clearly increased. Compared with Cu/CeO2, CO conversion increased from 16.4 to 20.8% and isobutanol productivity from 5.1 to 7.1 g/kgcat'h. As shown in Fig. 1, the selectivity to isobutanol also increased and, on a methanol-free basis, isobutanol represented about 75% of the alcohol fraction. The Bran/Linear ratio increased from 0.9 (with the. Cu/CeO2 catalyst) to 3.7. The addition of 0.86% potassium to Cu0.sMgsCeOx catalyst blocks surface acid sites and selectively decreases the formation rate of methane and DME. Fig. 1 shows that the selectivity to methane on K/Cu0.sMgsCeOx was substantially lower as than on undoped Cu0.sMgsCeOx. The decrease in methane formation was accompanied by a simultaneous increase of the Alc/Hyd ratio from 2.8 to 4.1. The K-doped Cu0.sMgsCeOx catalyst had similar isobutanol and methanol productivities as the undoped catalyst. Over all the catalysts, increasing the temperature from 260 to 340~ increased the isoalcohol formation rate while methanol yield decreased due to thermodynamic constraints. Therefore, the C2§ linear alcohol selectivity passed through a maximum. As a consequence, the oxygenate fraction became depleted in intermediates (ethanol, propanol) and enriched in

488 290oc 12

!!1211;~:~S~:

54

g,

55

61

Kig3i!ii!!i!il

[] O t h e r Alc.

~ l~butanol Methanol ~ Methane

320"C P =

+O's-

5 77.5~

atm

I

50.4 I

High nyd

50

i:~11.4!iil :i ::~.........::::::,: .. CktSV= 1832 cc/gh

!ii ................ 9............... ,

70.7

m Other Alc ] 0 Isolmtan~ I [] Methanol

I[[]N Methane Hig Hyd

0~

,, m

,

[

i

_

---- -----alON

M16j Z Z

NS~

N

N

N

Figure 1. Alcohol synthesis on the C u / C e / M g ~ system. T = 320~ P = 50 atm

5

!

~4.~

Figure 2. Effect of temperature on catalyst selectivity. Cu0.sMgsCeOx; selectivity in %C

branched alcohols. However, the production of CO2 and hydrocarbons also increased at higher temperatures. In Fig. 2 we have presented the selectivities to carbon dioxide, total alcohols, total hydrocarbons, methane, methanol, and isobutanol using the Cu0.sMgsCeOx catalyst at 290 and 320~ The selectivity to CO2 at 290~ (22.5%) increased to 35% at 320~ On a CO2-free basis, selectivities to methane and higher hydrocarbons increased from 8.4 and 4.2 respectively, at 290~ to 17 and 15.5 at 320~ On the other hand, increasing contact time favored isobutanol and branched alcohol formation while decreasing methanol slightly. Thus, the isobutanol/methanol ratio can be somewhat controlled by the temperature and space velocity of the reaction. Table 3 shows how adding methanol, ethanol or propanol to the reactant feed influences alcohol productivity. Addition of propanol selectively increases the formation of isobutanol; the production of linear and branched C3+ alcohols is also promoted. The productivity to 1butanol drastically increases when ethanol is added. Finally, the addition of methanol Table 3" Effect of the alcohol addition to CO/H2 feedstocks on isoalcohol synthesis Productivities in g/kg cat/h. Alcohol added: 1-propanol, 0.774 mmol C/h; methanol, 1.29 mmol C/h; ethanol, 1.68 mmol C/h. Cu0.sMgsCeOx, CO:H2 = 1, P = 50 atm Product Alcohol added None Propanol None Methanol None Ethanol 290~ 300~ 310~ 69.4 51.5 110.4 Methanol 153.0 143.4 6.14 2.39 2.40 3.80 Isobutanol 4.94 15.9 0.48 0.90 2-me- 1-butanol 0.59 0.71 0.51 0.70 0.86 021 0 40 0.26 2-me-l-pentanol 0.21 1.00 0 80 0 78 110 Ethanol 1.17 1.10 0 86 5.51 1-propanol 1.24 091 119 011 26.21 1-butanol 0.11 0.28 0.08 011 1.30 010 0.07 011 1-pentanol 0.08 0.44 005 1.85 0.05 O05 1-hexanol 0.04 0.08

489 ptomotes linear and branched chain growth pathways, and thereby increases the productivities to C2, alcohols. Alcohol mixtures of methanol/1-propanol and methanol/ethanol where also added to the CO:H2 feed; results are given in Table 4. It is shown that the addition of Table 4 Effect of the addition of alcohol mixtures to CO/H2 feedstocks on isoalcohol synthesis Productivities in g/kg cat/h. Alcohol mixture added: (methanol + 1-propanol), 3.970 mmol C/h; (methanol + ethanol), 2.064 mmol C/h Cu0.sMgsCeOx, CO:H2 = 1, P = 50 atm, GHSV = 1832 cc/g/h Product Alcohol mixture added After 230 h in run Aider 48 h in run Methanol + ethanol None None Methanol + 1-propanol 142.9 Methanol 157.2 5.32 5.76 Isobutanol 8.08 45.8 1.78 0.56 2-me- 1-butanol 0.92 1.02 0 34 0.39 2-me- 1-pentanol 0.35 3.32 139 Ethanol 1.72 1.80 6.23 155 1-propanol 1.87 16.6 012 1-butanol 0.14 0.54 0.53 0.23 1-pentanol 0.16 3.22 0.39 0.06 1-hexanol 0.06 0.64 methanol/1-propanol selectively promotes the formation of isobutanol while the mixture methanol/ethanol mainly increases the productivity to 1-butanol. The formation of higher alcohols from CO/H2 occurs through a sequential mechanism involving four main steps [7,8]: i) synthesis of methanol and formation of a C1 intermediate species, ii) formation of the primary carbon-carbon bond, probably via the coupling of two C1 intermediates, followed by a linear chain-building process (L) dominated by aldol coupling and C1 insertion pathways which produce a Cn+l alcohol from a C, alcohol, iii) aldol-type addition of the C I intermediate to the 13 carbon of a linear Cn alcohol to produce a 2-methyl branched Cn+l alcohol (A-C1), iv) addition of C2 (A-C2) and C3 (A-C3) intermediates via selfcondensation and cross-coupling reactions to produce linear C2+n and C3+, alcohols from a Cn alcohol. Fig. 3 presents a simplified reaction network of synthesis of methanol and higher alcohols from synthesis gas. In Fig. 3, isobutanol is formed by the aldol-type C~ addition to C3 intermediate species. The addition of 1-propanol to the reactants increases the concentration of surface C3 species and, as a consequence, increases isobutanol productivity. The predominant formation of isobutanol via the direct reaction between C1 and C3 species is confirmed by the results in Table 4 which show that the addition of a methanol/1-propanol mixture selectively increases the production of isobutanol. The addition of propanol also increases the formation of higher 2-methyl alcohols and linear Ca+ alcohols. The increase in 2-me-l-butanol and 2-me-1pentanol therefore results from a higher 1-butanol and 1-pentanol selectivity and by A-C2 propanol/ethanol addition or self-condensation of 1-propanol (A-C3 addition). As expected, the addition of methanol increases both linear and aldol condensation chain growth rates by CI. Nevertheless, the ratio of increase in isobutanol productivity is significantly higher than the increase in 1-butanol. This shows that the aldol A-C1 reactions are the predominant chain growth pathways on Cu0.sMgsCeOx. However, Table 3 shows that

490

C O/I-I2

p-~ ~'

CH3OH I

C2HsOH

T

C2"

CI*

y-

C3H7OH

~

C3"

C4H9OH

~

T

C4"

CsHllOH

~v-

T

C5"

A-C2 A-C3 ,A-CI eN4 ~

C2H6

~ C3H8

~H3 CH3CHCH2OH

A-C1

A-C1

2m-butanol 2m-pentanol

Figure 3. Simplified reaction network of synthesis of methanol and higher alcohols L: Chain growth A-C1, A-C2, A-C3: 1-, 2-, and 3- carbon addition, respectively the addition of ethanol selectively increases the formation of 1-butanol via an A-C2 condensation mechanism. Similar qualitative result was obtained when a methanol/ethanol mixture was added to the reactants (Table 4). The self-condensation of ethanol is a bimolecular reaction between adjacent adsorbed species and requires a high density of basic sites. The selective formation of 1-butanol therefore reflects a high concentration of surface C2 species derived from the addition of ethanol. The high coverage in C2 intermediates blocks the C~/C3 aldol condensation reactions and, as a result, the isobutanol productivity is not significantly changed by adding ethanol in relatively high concentrations. Results show that our copper-containing MgyCe(Y)O• catalysts selectively catalyze the low-temperature isoalcohol synthesis from CO/H2. The catalyst formulation combines the hydrogenation function required to form methanol with the basic/aldol condensation function needed to promote branching. Magnesium oxide provides the basic sites needed for the formation of 2-methyl branched alcohols via aldol condensation reactions. Copper promotes methanol formation and hydrogenation-dehydrogenation reactions. Ce(Y)Ox is a high surface area matrix which contains finely dispersed metallic copper. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

R. di Pietro and A. Paggini, Fr. Patent 2,490,215 (1982). P. Forzatti and E. Tronconi, Catal. Rev. Sci. Eng., 33 (1991) 109. M. Schneider, K. Kochloefl and O. Bock, Eur. Pat. Appl. 152, 809 (1985). J.G. Nunan, R.G. Herman and K. Klier, J. Catal., 116 (1989) 222. C.R. Apesteguia, S.L. Soled and S. Miseo, U.S. Patent 5,387,570 (1995); US Patent 5,508,246, (1996), Eur. Pat. Appl. 94303184.9 (1994). C.R. Apesteguia, B. De Rites, S. Miseo, and S.L. Soled, Catal Lett., 44 (1997) 1. T.J. Mazanec, J. Catal., 98 (1986) 115. J.G. Nunan, C.E. Bogdan, K. Klier, K.J. Smith, C.-W. Young and R. Herman, J. Catal., 113 (1988) 410.