Contents:

1. Introduction
2. Experimental
3. Results and Discussion
4. Conclusions
5. References

1. Introduction

Microwave synthesis represents one of the important dimensions of modern chemistry. Microwave heating allows substantially improved productivity of many chemical processes with reduced formation of by-products caused by overheating. The basic research is focused on fundamental aspects of microwave chemistry such as effects of microwave energy parameters (pulse duration, frequency, power) as well as chemical parameters (structure of starting materials, nature of solvents, type of reactions) on the outcome of microwave-assisted chemical processes. The technical advantages of microwave techniques [1] are:

Rapid heating is often easily obtained;

Volumetric deposition of energy within the material avoids surface limitations;

Economy of energy can be realized in that it is not necessary to heat the environment as well as the product (although in many important applications, such as drying, the environment must be at the product temperature so no energy advantage may be realized);

Electromagnetic heating is non-polluting – at least it removes the source of pollution from the processing plant to the electric generating station – and non-contact, so food processing may be simplified;

In many important cases electromagnetic heating is easy to apply;

Electromagnetic heating can be automated.

Our best estimate based upon MW catalytic experiments was about 3 times the thermodynamic requirement. We have therefore redesigned the reactor system by taking advantage of the pulsed MW as electromagnetic radiation and we have achieved much better results at lower power and very low overall surface temperature of the catalyst.

2. Experimental

2.1. Reagents and catalysts

Methane (purity 99.99) was obtained from Air Products Canada and it was used as received. In all the microwave experiments the methane continuously flow was set-up at 10 ml/min.

Microwave catalysts were specially developed and designed to facilitate the electric discharge. Three configurations of the catalyst were used as shown in Fig. 1. Catalyst A was a 120 mm Cu tube, 6 mm inner diameter, wrapped with 70 mm nickel (Ni) or tungsten (W) wire. On the surface of catalyst B - 120 mm Cu tube, 6 mm inner diameter - 10 Cu needles were welded in order to produce a uniform distributed and long lasting electric discharge. Catalyst C was a Ni wire cloth 40 x 40 mesh. The microwave reactor was a quartz tube 18 mm diameter and 450 mm in length.
 

Catalyst A

Catalyst C: Ni wire cloth 40 x 40 mesh

2.2. MW system

The microwave system was described by detail elsewhere [2]. The microwave power pulses are generated from a Cober magnetron with the maximum 4 kW of power at 2.45 GHz fixed frequency. In our experiments, pulses were set-up for 5 s control gate period, the cycle on/off of the gate was 30%, the frequency of pulses 120 Hz and the width 1 ms – Fig. 2. Taking into account all these parameters, the effective irradiation time is about 4% of processing time. As an example, considering 10 min of total processing time, the irradiation time will be 24 s. The power was ranged between 50 and 130 W and the processing time was 10 and 40 min.
 
 

2.3. Analysis

The products from decomposition reaction were analysed online using a Hewlett Packard 5890A gas chromatograph fitted with Porapak N packed stainless steel column 1.8 m x 3 mm, with helium as carrier gas and flame ionisation detector.

The conversion of methane was calculated with the following equation:

(1)

Where is the molar concentration of methane in products.

The selectivity of acetylene was calculated:  (2)

Where is the molar concentration of acetylene in products.

Deactivation of the catalyst D_activity was calculated:

(3)

Where and represents the methane conversion after 10 and 40 min of reaction.

3. Results and discussion

 

 
 
 
 
 
 
 

As most if not all, industrial processing requires some input of energy often in the form of thermal energy via heat transfer. Also, the chemical reactions involving breaking and forming of chemical bonds, are energy driven and temperature does not play the traditional role in the rate parameters when the energy applied is not in the form of thermal energy.

For the decomposition of the CH₄ to C₂H₂, lower power microwave was used to create a surface discharge of electrons on the metal; these electrons acquire a great deal of kinetic energy through being accelerated in the MW high frequency electric field. This phenomenon is very similar to the gas phase plasma – the major difference is that the discharge is not in the gas phase but on the metal surface where methane absorbs. Without adequate control of the kinetic energy of electrons, it is likely that they will fragment the methane molecule by breaking three or four of the C-H bonds instantly; subsequent reactions of hydrogen atoms and CH radicals leading to acetylene do not require further input of energy and by minimizing the undesirable heating effect, carbon deposition on the reactor wall can be brought under control.

1. Type of catalyst
Three types of catalyst were developed and examined from the point of view of methane conversion as well as acetylene selectivity and they are shown in Fig. 1.

In accordance with the observation that for a scale-up process - most likely involving multiple stages of the chemical process and separation of the products - it is important to get the sum (C+S) between the selectivity of the desired product and conversion of the reactant higher than 120 [4], our results were also estimated as the total of the methane conversion (C) and acetylene selectivity (S).

From Table 1 and Fig. 3, it is obvious that the use of the catalyst B gives the highest conversion of methane and selectivity of acetylene, therefore the highest C+S number.

At the end of 40 minutes processing time, the lowest dropping down in the activity of the catalyst was observed by using the catalyst B. This better activity could be explained taking into account the configuration of catalyst B that enables a longer lasting and uniform distribution of the electron discharge on the surface.

Table 1. Microwave catalytic conversion of CH₄ to C₂H₂ - Dependence on the configuration of the catalyst. Microwave power 130 W; gate on/off cycle 30%
 

Catalyst type	Time min	CH4 conv.%	PRODUCTS SELECTIVITY, %								C+S %
			Aliphatics							C6H6
			C2H6	C2H4	C2H2	C3	C4	C5	C6+
a B	10	37.70	2.12	0.53	85.94	1.6	7.69	0.27	0	1.33	123.64
	40	29.80	2.68	0.67	84.56	2.0	7.38	0.34	0	2.35	114.36
a A/ Wwire	10	21.70	3.23	1.38	81.57	2.8	8.29	0.46	0.46	1.84	103.27
	40	14.10	4.96	3.55	74.47	3.6	9.22	0.71	0	2.84	88.57
a A / Niwire	10	16.50	3.64	1.82	76.36	3.0	7.88	3.64	1.21	2.42	92.86
	40	9.70	5.15	5.15	73.20	4.1	9.28	1.03	1.03	3.09	82.90
b C	10	7.00	5.71	5.71	75.71	5.7	4.29	2.86	0	1.43	82.71
	40	11.60	6.90	6.90	69.63	5.2	8.62	0.86	0	2.59	81.23

^{a Cooling with water at 4 0C; b No cooling of the catalyst}

^{Fig. 3 - Methane conversion and acetylene selectivity dependence on the catalyst type and microwave irradiation time}

^{3.2. Effect of temperature of the cooling water through the catalyst}

^{By our previous experiments [4] it was demonstrated that high temperatures at the surface of the catalyst led to the formation of carbon deposits on the glass reactor wall and on the catalyst surface and therefore, to the inhibition of the activity of the catalyst. In order to decrease this temperature, we took the advantage of working with pulsed microwave at lower power levels and of using a catalyst configuration (tube) that makes easier to circulate a cooling agent through it. The cooling agent was water at 4, 55, and 68 0C. The flow of water through the catalyst was set enough fast to get no difference between the inlet and outlet temperatures of the cooling agent thus, uniform temperature at the surface of the catalyst. The activity of the catalyst lasted longer when cooling water was used. This effect seems to increase with the increasing of water temperature as shown in Table 2 and Fig. 4.}

^{Table 2. Microwave catalytic conversion of CH₄ to C₂H₂ - Dependence on the temperature of cooling water. Microwave power 130W; gate on/off cycle = 30%}
 

Catalyst type	T*, 0C	Time, min	CH4 conv.%	PRODUCTS SELECTIVITY, %								C+S %
				Aliphatics							C6H6
				C2H6	C2H4	C2H2	C3	C4	C5	C6+
B	No	10	41.20	2.18	0.49	86.41	1.70	7.52	0.24	0.24	1.70	127.61
		40	9.60	2.08	2.08	79.17	3.13	8.33	1.04	0	3.13	88.71
	4	10	37.70	2.12	0.53	85.94	1.59	7.69	0.27	0	1.33	123.64
		40	29.80	2.68	0.67	84.56	2.01	7.38	0.34	0	2.35	114.36
	55	10	33.00	2.12	0.61	84.85	1.82	8.18	0.30	0.30	2.12	117.85
		40	30.10	1.72	1.72	81.03	2.59	8.62	0.30	1.72	2.59	111.13
	68	10	33.60	2.08	0.60	85.41	1.79	7.74	0.30	0.30	2.08	119.01
		40	38.00	2.89	0.53	83.68	2.11	7.11	0.53	0.26	2.63	121.68

^{* Temperature of water circulated through the catalyst}

^{Although the highest methane conversion and acetylene selectivity was observed with catalyst B not cooled, the activity of the catalyst had the highest dropping down in activity - D_activity = 31.60% - after 40 min of reaction, Fig. 4. The inhibition of the catalyst activity decreased with the increasing of the temperature of the cooling agent; at 4 0C it was 7.90%, at 55 0C was 2.90% and showed a slight increase at 68 0C.}

^{Fig. 4 - Methane conversion, acetylene selectivity and catalyst activity dependence on the temperature of cooling agent and irradiation time}

^{3.3. Microwave parameters}

^{Table 3 presents conversion of methane, selectivity of acetylene and total product distribution for methane decomposition over catalyst B, using 2 values of the gate on/off cycle (GOF), at the microwave power 130 W.}
 
 
 
 
 
 
 
 

^{Table 3. Microwave catalytic conversion of CH₄ to C₂H₂ - Dependence on the microwave parameters - Gate on/off cycle. Catalyst B; microwave power 130W.}
 

GOF* %	T 0C	Time Min	CH4 conv.%	PRODUCTS SELECTIVITY, %								C+ S %
				Aliphatics							C6H6
				C2H6	C2H4	C2H2	C3	C4	C5	C6+
20	No	10	26.50	2.26	0.75	84.15	1.89	8.30	1.13	0	1.89	110.65
		40	19.60	2.04	1.02	84.18	2.04	7.65	0	0	2.04	103.78
	4	10	28.40	2.11	0.70	85.21	1.76	8.10	0.35	0	1.41	113.61
		40	23.10	2.16	0.87	84.42	2.16	7.36	0	0.87	1.73	107.52
	68	10	24.10	2.07	0.83	84.65	2.07	8.30	0.45	0.45	2.07	108.75
		40	23.50	2.13	0.85	84.26	2.13	7.66	0.85	0	2.13	107.76
30	No	10	41.20	2.18	0.49	86.41	1.70	7.52	0.24	0.24	1.70	127.61
		40	9.60	2.08	2.08	79.17	3.13	8.33	1.04	0	3.13	88.71
	4	10	37.70	2.12	0.53	85.94	1.59	7.69	0.27	0	1.33	123.64
		40	29.80	2.68	0.67	84.56	2.01	7.38	0.34	0	2.35	114.36
	68	10	33.60	2.08	0.60	85.41	1.79	7.74	0.30	0.30	2.08	119.01
		40	38.00	2.89	0.53	83.68	2.11	7.11	0.53	0.26	2.63	121.68

* Gate on/off cycle

It could be seen that decreasing 1.5 times the incident irradiation power per pulse (26 W/pulse for 20% GOF compared to 36 W/pulse at 30%) led to the lower values of methane conversion. However, it should be pointed that the selectivity of acetylene was ~ 84 % in all experiments. In addition, in the tests carried out using 20% GOF the decrease in the activity of the catalyst was lower than for 30%, no matter of the temperature of the cooling water through the catalyst. In both cases, D_activity was also direct dependent on the temperature of cooling water, Fig. 5.



Fig. 5 - Catalyst activity dependence on the gate on/off cycle (GOF) and temperature

of cooling water

4. Conclusions

Low microwave power levels (50 - 130 W) with gate on/off period of the pulses properly set to allow the cooling down of the catalyst surface between 2 pulses were found to have a great influence on the activity of the catalyst and amount of coke. The continuous flowing of a cooling agent through the catalyst was proved to help in maintaining the catalyst activity and to control the coke deposition inside the glass reactor.

5. References

1. Roussy, G., Pearce, J.A., Foundations and industrial applications of microwaves and radio frequency fields, Physical and Chemical Processes, John Willey & Sons, 1995.
2. Wan, J.K.S., U.S. Patent 5,472,581, December 5, 1995.
3. Ioffe, M.S., Pollington, S.D., Wan, J.K.S., J. Catal., 151, 349-55, 1995.
4. Wan, J.K.S, Depew, M.C., Microwaves: theory and Application in materials processing V, Ceramic Transactions vol.III, 241-7, 2001, published by the American Ceramic Society, 735 Ceramic Place, Westerville, Ohio 43081 (on-line book catalog at www.ceramics.org).