482
Energy & Fuels 1999, 13, 482-484
Dehydrogenative Methane Homologation to C2 Hydrocarbon and Aliphatic Oil by a Thermal Diffusion Column Reactor with Platinum-Loaded Pyrogen Kenichi Suzuki, Rei Takahashi, Kaoru Onoe, and Tatsuaki Yamaguchi* Department of Industrial Chemistry, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan Received August 10, 1998. Revised Manuscript Received November 23, 1998
By using a surface-modified carbon-rod loaded with platinum as the pyrogen of a thermal diffusion column reactor, the dehydrogenative methane homologation was investigated. The supplied energy using platinum-loaded pyrogen has dropped in value by 50% (150 K depression expressed in reaction temperature) compared with the use of a platinum-unloaded one. Furthermore, both higher fractions of aliphatic oil (63.5% at 1173 K) and a very high selectivity of ethane (almost 100% from 1073-1173 K) were obtained.
Introduction Methane homologation is a potential alternative technology soon to obtain vital feedstocks for the petrochemical industry and liquid fuels for energy use from natural gas. It is evident that the dehydrogenative coupling of methane is advantageous compared to the oxidative one, because the former has produced hydrogen efficiently. When methane is supplied downwardly to a vertically held thermal diffusion column (TDC) reactor with a hot wire, a higher selectivity of C2 hydrocarbons has been established by the thermal diffusion effect promoted synergistically by convection in this temperature-gradient system together with the apparent methane conversion, which is higher than the thermodynamic calculation.1,2 However, to create a satisfactorily higher methane conversion by this process, the upward supply of methane to the TDC with a higher temperature of the pyrogen is necessary due to the risk of coke formation. The required lowering of the reaction temperature for this purpose causes the development of catalysts. In this study, the surface-modified carbon-rod loaded with platinum was developed as the pyrogen of a TDC, and the loading effect of platinum on the degradation of the supplied energy and the product selectivity was examined. Experimental Section A carbon-rod pyrogen (195 mm in length and 3 mm in diameter, Daiichi Carbon Co.) molded of graphite and carbon black was used as the platinum support. To enlarge the affinity of the pyrogen surface for platinum solution, the rod was pretreated by dipping it into the novolac-type phenolic resin solution at 298 K for 48 h, where commercial-grade resin (TD2090, Dainippon Ink & Chemicals Co.) was dissolved in
Figure 1. Thermal diffusion column (TDC) reactor upward supply of methane. Table 1. Physical Properties of Platinum-Loaded Carbon Rod
pyrogen type C C(Phe) Pt/C(Phe)-I Pt/C(Phe)-II a
(1) Yamaguchi, T.; Kadota, A.; Saito, C. Chem. Lett. 1988, 681682. (2) Gesser, H. D.; Morton, L. A. Catal. Lett. 1991, 11, 357-364.
pyrogen Pt conc. of surface moles of crystal H2PtCl6 area Pt loaded size pretreatment [mM] [m2/g] [µmol/m2] [nm] N.P.R.a N.P.R.a N.P.R.a
32 64
1.3 3.5 11.8 13.2
4.5 11.9
17.6 18.3
Novolac-type phenol resin.
tetrahydrofuran. After drying, the rod was carburized in a nitrogen atmosphere at 973 K for 2 h and soaked in an aqueous
10.1021/ef980168u CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
Dehydrogenative Methane Homologation
Energy & Fuels, Vol. 13, No. 2, 1999 483
Table 2. Effect of Gas Feed Conditions on Products Using Untreated Carbon Rod rod supplied surface methane expt flow rate pyrogen energy temp conv. no direction [mmol/min] type Q [MJ] Ts [K] XCH4 [%] methane feed
101 102 103 201 202 203 204 205 206 a
down down down up up up up up up
0.89 0.89 0.89 0.89 0.89 0.89 0.22 0.45 1.79
C C C C C C C C C
3.2 4.4 6.9 3.2 4.4 6.9 6.9 6.9 6.9
1220 1300 1400 1220 1300 1400 1400 1400 1400
0.4 9.3 30.8 6.3 30.3 46.3 61.7 66.8 42.5
product selectivity [%] oil fractiona [%] gas fraction [C-mol %] oil gas carbonaceous aromatic aliphatic C2H6 C2H4 C2H2 C3+ tri. tri. 0.2 tri. 12.9 59.3 53.3 68.1 54.4
99.8 93.8 94.6 95.2 79.7 38.1 31.5 24.6 42.8
tri. 6.2 5.2 4.8 7.4 2.6 15.2 7.3 2.8
90.0
10.0
78.1 73.0 88.7 87.0 93.2
21.9 27.0 11.3 13.0 6.8
39.2 19.1 1.2 32.6 6.9 7.3 tri. 1.3 3.2
60.8 63.9 65.4 67.4 59.0 69.9 87.0 70.4 44.2
tri. 10.1 31.1 tri. 34.1 22.8 13.0 28.3 52.6
tri. 6.9 2.3 tri. tri. tri. tri. tri. tri.
Calculated from 1H NMR.
solution of chloroplatinic acid for 5 h at 323 K. The concentration of chloroplatinic acid was varied from 32 to 64 mM. The rod was then set in a thermal diffusion column and electrochemically reduced at 1220 K by heating with an electrical current for 2 h under a hydrogen atmosphere. The surface temperature, Ts, of the pyrogen was measured pyrooptically by a radiation pyrometer (Minolta IR-630). The platinumunloaded carbon rod was also prepared by phenol resin pretreatment, carbonization, and hydrogen reduction under the same conditions. The net change in the specific surface area of the rod pyrogen before and after dehydrogenative coupling of methane was examined by means of liquid nitrogen adsorption. The platinum-loaded carbon rod was ground into a powder, and 100 mg of the sample was set on the cell in a thermogravimetric tube. After the desired temperature was obtained, a controlled flow of hydrogen was passed through the tube and the weight change was recorded continuously. The platinum crystal size was determined by an X-ray line-broadening measurement of the platinum peak at 2θ ) 39.8° (Cu KR) using Scherrer’s equation. Figure 1 schematically shows the thermal diffusion reactor used for the dehydrogenative methane homologation. The reactor was modified with a Clusius-Dickel-type thermal diffusion column, consisting of a steel inner tube (stainless steel of 27 mm i.d. with 300 mm length) and outer tube (glass of 62 mm i.d. with 250 mm length). The platinum-loaded carbon rod was attached in the center of the cross-sectional area of the inner tube. The coolant temperature at the inlet was 293 K. Pure methane was fed in at a flow rate of 0.221.79 mmol/min, either downward or upward to the vertically held reactor. The reactions were carried out under atmospheric pressure in the temperature range from 1073 to 1400 K. During the reaction, the pyrogen was kept at the desired temperature by controlling the voltage and the current of the electric apparatus. The reaction products were analyzed by TCD and FID gas chromatography.
Results and Discussions Table 1 shows the comparison of the physical properties of prepared pyrogen, where the terms C, C(Phe), and Pt/C(Phe) represent the uncarbonized carbon rod, the carbonized one using phenol resin but platinum unloaded, and both the carbonized and platinum-loaded one, respectively. All the rods were pretreated under hydrogen at 1220 K. It was confirmed that the increase in specific surface area was observed due to carbonization of impregnated phenol resin. The number of moles of platinum loaded was calculated from the reduced platinum weight and the BET surface area as 4.5-11.9 µmol/m2, and the platinum crystal size was about 18 nm, independent of the concentration of chloroplatinic acid. It is known that the treatment of phenol resin
introduces different functional groups on the carbon surface.3 These results suggest that the surface porosity of the carbon rod is changed accompanied with the reduction of the platinum compounds to metals. Table 2 shows the effect of gas feed conditions (mainly flow direction and feed rate) on products obtained during the dehydrogenative methane homologation, where the uncarbonized carbon rod was used. With the downward flow of methane (experiment nos. 101-103), methane conversion (XCH4) was 9.3% at 1300 K and the selectivity of gaseous products was >93%, independent of reaction temperature. Ethylene was the major product, accounting for 60-66% of the molar fraction, and ethane and acetylene were minor products under these conditions. The selectivity for C3 hydrocarbons (mainly propylene) was
